Physicochemical Aspects of Metal-Organic Frameworks: A New Class of Coordinative Materials 3031186745, 9783031186745

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Physicochemical Aspects of Metal-Organic Frameworks: A New Class of Coordinative Materials
 3031186745, 9783031186745

Table of contents :
Preface
Contents
About the Editors
Overview of Metal Organic Frameworks
1 Introduction
2 Chemical Composition
3 Porosity
4 Metal Biomolecule Frameworks (Bio-MOFs)
5 MOFs Synthesis
6 Conclusion
References
Classification of the MOFs Based on the Secondary Building Units (SBUs)
1 Introduction
2 MOFs with Their Secondary Building Units
2.1 Ti-Base MOFs
2.2 Zr-Based MOFs
2.3 Sc-Based MOFs
2.4 Mn-Based MOFs
2.5 Alkali Metal-Based MOFs
3 Conclusion
References
MOFs Preparation and Synthetic Approaches
1 New Synthetic Approaches to Provide Metal–Organic Frameworks
2 Ionothermal Method
3 Deep Eutectic Solvent Usage
4 Surfactant-Thermal process
5 Mechanochemistry
6 MOFs Synthetic Approaches and Potential Usage
7 Synthesis of MOFs
8 The Synthetic Strategies of 2D-MOFs
9 D-Metal Organic Frameworks
10 Synthetic Methods of 2D-MOFs
11 Langmuir–Blodgett Technique
12 Sonication Exfoliation Method
13 Mechanical Exfoliation Method
14 Modulated Strategy
References
MOFs Functionalization Approaches
1 Introduction
2 Functionalization of MOFs Produces Unique Materials with Multiple Properties for Different Applications
2.1 Functionalization of MOF for Producing Photoactive Materials
2.2 Functionalization of MOFs in Drug Delivery
2.3 Functionalization of MOF for Catalytic Applications
2.4 Functionalization of MOF for Removal of HG2+
2.5 Functionalization of MOF as Phase Transfer Catalyst
2.6 Functionalization of MOF for Sensitive Fluorescent Probe of S2O82− and Fe3+
2.7 Functionalization of MOF for CO2 Adsorption
3 Conclusion
References
MOFs Structural Morphologies
1 Solvent Effect
2 PH Effect
3 Effect of Metal Ions
4 Time Effect
5 Additive Effect
6 Effect of Synthesis Method
6.1 Synthesis of Deprotonation Regulation
6.2 Synthesis of Coordinate Modulation
7 Temperature Effect
8 Effect of Molar Ratio of Reactants
9 Conclusion
References
MOFs Bandstructure
1 Semiconducting MOFs
2 Band Gap Investigation
3 Band Energy Values (in Electronvolts) Calculated from Periodic Systems and Linker Molecules
4 Semiconductor Metal–Organic Framework (MOF) Photocatalyst
5 The Band Gap Value of Different Linkers of MOFs
References
Evolution in MOF Porosity, Modularity, and Topology
1 Introduction
2 Porosity of MOFs
2.1 Surface Area and Distribution of Size and Volume of Pores
2.2 Methods for Porous MOF Designing
2.3 Controlling Porosity Using Isoreticular Expansion and/or Contraction
3 Topology
4 Conclusion
References
MOF Scaffolds Tunability and Flexibility
1 Tunable Nanomicrostructure
1.1 Tunable Mechanical Properties
1.2 Morphologically tunable
2 Flexibility
2.1 Temperature and Gust Molecules Affect Flexibility
2.2 Flexible MOF Nanorod
2.3 Flexible MOF-Aminoclay Nanocomposites
References
MOF Scaffolds Defects and Disorders
1 Introduction
2 Structural Defect Generation in MOFs
2.1 De Novo Synthesis
2.2 Post-synthetic Modification
3 Defect Characterization in MOFs
4 Applications of Defective MOFs
4.1 Applications in Gas Adsorption and Separation
4.2 Applications in Catalysis
4.3 Decontamination Applications
4.4 Bio-Applications
4.5 Smart Applications
5 Conclusion and Future Prospects
References
Composition States of MOFs
1 Introduction
2 Generation of Basic Sites
3 MOFs with Intrinsic Basicity
3.1 Basicity from Alkaline Earth Metal Sites
3.2 Basicity from Hybrid Metal Nodes
3.3 Basicity from N-Containing Ligands
3.4 Basicity from Structural Phenolates
4 MOFs with Modified Basicity
4.1 Functionalization of Metal Sites
4.2 Functionalization of Ligands
References
Identification and Analytical Approaches
1 Fourier Transform Infrared (FT-IR)
2 Energy Dispersive X-ray (EDS)
3 Dynamic Light Scattering (DLS)
4 Scanning Electron Microscopy (SEM)
5 Transmission Electron Microscopy (TEM)
6 Brunauer–Emmett–Teller (BET)
7 X-Ray Diffraction (XRD)
8 Thermogravimetric Analysis (TGA)
References
Coordination Chemistry of MOFs
1 Molecular Orbital
1.1 Ionic Size and Crystal Environment
1.2 Metal–Ligand Bonds
2 Synthesis of Coordination Compounds
2.1 The Reaction of a Metal Salt with a Ligand
2.2 Ligand Replacement Reactions
2.3 The Reaction of Two Metal Compounds
2.4 Oxidation–Reduction Reactions
2.5 Partial Decompositions
2.6 Precipitation Making Use of the Hard-Soft Interaction Principle
2.7 Reactions of Metal Compounds with Amine Salts
3 Coordination in MOFs
3.1 The Charge Density
3.2 Thermal Stability
3.3 Chemical Stability
References
Applications of MOFs
1 Catalysts and Photocatalysts
1.1 Introduction
1.2 MOF Catalyst Modifications
1.3 Possible Routes and Syntheses
1.4 Physical–chemical Property Modification
1.5 Modification of Morphology
1.6 Modifying of Metal Ions and Ligands
1.7 Defect Engineering
1.8 Functional Modification
2 Optics
2.1 Introduction
2.2 Photonic MOF Enterprise and Manufacturing Strategies
2.3 MOFs for Luminescent Sensors
2.4 MOFs for Lighting and Info Display in Solid-State
3 Sensors and Biosensors
4 Batteries and Supercapacitors
4.1 Introduction
4.2 MOFs as Electrode Materials
4.3 MOFs as Host Material for Li–O2, Zn–air, Li–S, and Li–Se Batteries
4.4 Supercapacitors
5 Solar Cells
5.1 Introduction
5.2 Generation of Solar Cells
5.3 Perovskite Solar Cells
5.4 Effects of MOF on Perovskite Solar Cells
6 Fuel Cells
6.1 Introduction
6.2 H2 Production from Water Splitting Using MOFs
6.3 H2 Production from Ammonia Borane Andorganosilanes Using MOFs
6.4 MOFs as Oxygen Reduction Reaction Catalysts
6.5 MOFs as Proton-Conducting Polymer Electrolyte Membranes
6.6 MOFs as H2 Storage Medium
7 Energy Storage and Conversion
8 Molecular Transport
9 CO2 and N2 Reduction
9.1 Introduction
9.2 MOFs that Have not Been Changed
9.3 Linker Modification of MOFs
9.4 Amine Functionalization
10 Water and Alcohol Oxidation
10.1 Alcohol Oxidation
10.2 Water Oxidation
11 Water Electrolysis and Splitting
12 Environmental Remediation
12.1 Degradation of Organic Dyes
13 Environmental Contaminants Adsorption
14 Environmental Contaminants Degradation
15 Membranes
15.1 Introduction
15.2 Advanced MOF Materials for Mixed Matrix Membranes
15.3 MOF Glasses for Membranes
15.4 Neat MOF Membranes
16 Separation
16.1 The Requirement for Energy-Effectual Gas Separations
16.2 Current and Emerging Technologies for Gas Separations
16.3 Existing Status of Membrane-Constructed Gas Separations
16.4 MOF as an Adsorbent
17 Drug Delivery
17.1 Introduction
17.2 Functionalization for Drug Delivery
17.3 Applications in Drug Delivery
18 Antibacterial and Antimicrobial Scaffolds
19 Tissue Engineering
19.1 Introduction
19.2 Physiology of Bone Healing
19.3 Applications of Nano-MOFs in Bone Tissue Engineering
20 Wound Healing
20.1 Physiology of Wound Healing
20.2 Applications of Nano-MOFs in Wound Healing
References
Industrialization of MOFs
1 Introduction
2 MOFs from Academia to Industrial Applications
3 Industrial Synthetic Routes of MOFs
3.1 Microwave Synthesis
3.2 Continuous Flow Chemistry
3.3 Electrochemical Synthesis
3.4 Mechanochemistry Synthesis
3.5 Ultrasonic Synthesis
3.6 Supercritical CO2
3.7 Solvothermal/Hydrothermal
4 Conclusions
References
Computational Studies
1 Introduction
2 Enhancement of Computational Approaches and MOF’s Conceptions
2.1 Atomic Partial Charge Estimation
2.2 Extended Charge Equilibration Approach
3 Conclusion
References
Future Outlook
References
Conclusion
1 Synthetic Work Focusing on Metal-Containing Nodes or Coordination Bonds
2 Ligand Design and Post-synthetic Modification on Linkers
3 Symmetry-Guided Synthesis and Structural Characterization of MOFs from Micro-, Meso- to Macro-scale
4 MOF Interdisciplinary Research
5 Potential Application of MOFs
References

Citation preview

Engineering Materials

Ali Maleki Reza Taheri-Ledari   Editors

Physicochemical Aspects of Metal-Organic Frameworks A New Class of Coordinative Materials

Engineering Materials

This series provides topical information on innovative, structural and functional materials and composites with applications in optical, electrical, mechanical, civil, aeronautical, medical, bio- and nano-engineering. The individual volumes are complete, comprehensive monographs covering the structure, properties, manufacturing process and applications of these materials. This multidisciplinary series is devoted to professionals, students and all those interested in the latest developments in the Materials Science field, that look for a carefully selected collection of high quality review articles on their respective field of expertise. Indexed at Compendex (2021)

Ali Maleki · Reza Taheri-Ledari Editors

Physicochemical Aspects of Metal-Organic Frameworks A New Class of Coordinative Materials

Editors Ali Maleki Catalysts and Organic Synthesis Research Laboratory Department of Chemistry Iran University of Science and Technology Tehran, Iran

Reza Taheri-Ledari Catalysts and Organic Synthesis Research Laboratory Department of Chemistry Iran University of Science and Technology Tehran, Iran

ISSN 1612-1317 ISSN 1868-1212 (electronic) Engineering Materials ISBN 978-3-031-18674-5 ISBN 978-3-031-18675-2 (eBook) https://doi.org/10.1007/978-3-031-18675-2 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

Metal-Organic Frameworks (MOFs) are the recent class of coordinative materials, and this research field has garnered much attention. At the same time, the MOFs’ synthesis approaches, nodes and ligands adjustments, and different secondary building units’ utilization have enhanced the MOFs’ structural characterizations. In this regard, addressing porosity developments, MOF scaffold tunability, structural stability, and structural defects and disorders were crucial. Also, various physicochemical facets of MOFs and the interactions between the MOF precursors and the MOFs and other chemical moieties have been highlighted. According to the practical experiences gained in the laboratory, the effective characteristics of MOFs applied in a vast array of applications, such as catalytic systems, gas adsorption, air purification, water remediation, biosensing, energy storage, solar cells, tissue engineering, wound healing, and drug delivery, have been reviewed by experienced members in the following 17 chapters. Initially, an overview of MOFs and a logical classification for MOFs based on the secondary building units (SBUs) is submitted. Subsequently, preparation and identification approaches, and the nature and chemistry of the interactions in the aforementioned systems, are discussed in detail. Furthermore, the composition states and MOF’s defects were discussed. Next, the role of these materials in the various facets is investigated. Later, theoretical characterizations through Density Functional Theory (DFT) calculations are discussed. Also, MOF’s industrialization and scaling up in the different applications are reviewed. Finally, future perspectives of these types of coordinative materials and possible developments in the field are reviewed. In this text, our experienced Ph.D. -graduated authors, with the seniority

v

vi

Preface

of Mrs. Fatemeh Ganjali and the accompaniment of our young experts, provide an advantageous collaboration by passing their knowledge to everyone interested in MOFs as a recent matter of coordinative materials. All the best. Tehran, Iran

Ali Maleki Reza Taheri-Ledari

Acknowledgements Special thanks of Dr. Reza Taheri-Ledari to Miss Fereshteh Rasouli Asl, Mrs. Simindokht Zarei-Shokat, and Mr. Peyman Ghorbani for their effective accompaniment.

Contents

Overview of Metal Organic Frameworks . . . . . . . . . . . . . . . . . . . . . . . . . . . . Simindokht Zarei-Shokat and Fatemeh Ganjali Classification of the MOFs Based on the Secondary Building Units (SBUs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Maryam Kamalzare

1

15

MOFs Preparation and Synthetic Approaches . . . . . . . . . . . . . . . . . . . . . . . . Fatemeh Ganjali, Simindokht Zarei-Shokat, and Farinaz Jalali

31

MOFs Functionalization Approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Maryam Kamalzare

45

MOFs Structural Morphologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Simindokht Zarei-Shokat, Fatemeh Ansari, Mohadeseh Forouzandeh-Malati, and Ana Zamani

61

MOFs Bandstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zoleikha Hajizadeh and Mohammad Mehdi Salehi

79

Evolution in MOF Porosity, Modularity, and Topology . . . . . . . . . . . . . . . . Fatemeh Ganjali, Peyman Ghorbani, Nima Khaleghi, and Maryam Saidi Mehrabad

91

MOF Scaffolds Tunability and Flexibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 Fereshteh Rasouli Asl, Fatemeh Ganjali, and Zahra Rashvandi MOF Scaffolds Defects and Disorders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 Fatemeh Ganjali, Peyman Ghorbani, and Nima Khaleghi Composition States of MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 Fereshte Hassanzadeh-Afruzi and Mohammad Mehdi Salehi Identification and Analytical Approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 Simindokht Zarei-Shokat, Mohadeseh Forouzandeh-Malati, Fatemeh Ansari, and Reihane Dinmohammadi vii

viii

Contents

Coordination Chemistry of MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 Zahra Rashvandi, Fereshteh Rasouli Asl, and Fatemeh Ganjali Applications of MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 Mohammad Mehdi Salehi, Farhad Esmailzadeh, and Fereshte Hassanzadeh-Afruzi Industrialization of MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 Jamal Rahimi and Fatemeh Ganjali Computational Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327 Fatemeh Ganjali, Peyman Ghorbani, and Nima Khaleghi Future Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 Fereshteh Rasouli Asl and Fatemeh Ganjali Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337 Fatemeh Ganjali

About the Editors

Prof. Dr. Ali Maleki was born in Mianeh, East Azerbaijan in 1980. He received his Ph.D. in Chemistry in 2009. He started his career as an Assistant Professor in Iran University of Science and Technology (IUST) in 2010, where he is currently Full Professor. His research interests focus on design and development of novel catalysts, nanomaterials, and green chemistry. He has hundreds of ISI-JCR publications. Some of his honors include: Distinguished Researcher of IUST within 2010–2020; IUPAC Prize for green chemistry in 2016; and Top 1% International Scientists in ESI (Web of Science) in 2018, 2019, and 2020. Dr. Reza Taheri-Ledari was born in Tehran-Iran in 1988. He was graduated from University of Tehran (UT) in B.Sc. (in 2012) and M.Sc. (in 2015) in Pure and Organic Chemistry, respectively. Since 2015, he became a Ph.D. student in Organic Chemistry at Chemistry Department of Iran University of Science and Technology (IUST). He accomplished his Ph.D. in 2021, and currently works at IUST as a Senior Researcher. His research interests focus on drug development, drug delivery, high-tech pharmaceutical compounds like antibody-drug conjugates, and other different types of micro- and nanomaterials. So far, Reza has succeeded to publish several ISI publications involving foreign researchers from China, USA, Spain, and Canada. He was Top Researcher of IUST between 2019 and 2020. Also, he was introduced as a Top 2% Scientists in the World (ESI, Stanford, 2021).

ix

Overview of Metal Organic Frameworks Simindokht Zarei-Shokat and Fatemeh Ganjali

Abstract A new class of hybrid material known as a metal–organic framework (MOF) is created when an organic molecule coordinately bonds with an inorganic molecule (often high nuclearity metal clusters or transition metal ions) to form a framework or cage-like structure. The MOFs are a particular kind of coordinate polymers (CPs). The remarkable crystallinity of MOFs, which distinguishes them from noncrystalline CPs, is provided by the periodic arrangement of metal and organic linkers. MOFs are typically classified as crystalline, amorphous, luminescent, nano-, and bio-MOFs depending on the crystalline shape and synthetic methods. MOFs have received substantial attention as a highly flexible substrate for functional applications in a variety of research domains due to their extraordinarily high surface area, enormous porosity, tunable pore size, and flexible functionality. Keywords Metal–organic frameworks · Synthesis · Ligand · Coordination · Porosity

1 Introduction Porous materials have drawn a lot of interest during the past 50 years, including zeolites, coordination polymers,1 and metal–organic frameworks.2 Their porosity, which enables the migration of guest molecules into the bulk structure, is an intriguing property. The selection of the guests to be incorporated depends on the form and size of the pores. According to Yaghi et al., MOFs are porous structures made of coordinative bonds between organic bridging ligands and metal ions [1]. 1 2

CPs. MOFs.

S. Zarei-Shokat (B) · F. Ganjali Catalysis and Organic Synthesis Research Laboratory, Department of Chemistry, Iran University of Science and Technology, 16846-13114 Tehran, Iran e-mail: [email protected] F. Ganjali e-mail: [email protected]; [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 A. Maleki and R. Taheri-Ledari (eds.), Physicochemical Aspects of Metal-Organic Frameworks, Engineering Materials, https://doi.org/10.1007/978-3-031-18675-2_1

1

2

S. Zarei-Shokat and F. Ganjali

As secondary building units3 or metal-containing units4 are anchored with organic linkers by coordination, open frameworks with extraordinary properties including permanent porosity, a stable framework, a large surface area, and large pores are produced. Long organic linkers give MOFs a lot of storage space and a lot of adsorption sites, which results in porosity. Additionally, they possess the capacity to purposefully alter and functionalize their pore structure [2]. The synthesis of MOF-5 (Zn4 O (bdc)3 ; bdc = terephthalate) and HKUST-1 (Cu3 (btc)2 ; btc = 1,3,5-benzenetricarboxylate) with high porosity and low pressure gas sorption, followed by the formation of chromium (III) terephthalate (MIL-101) with high chemical stability, served as benchmarks in the history of MOFs. Several isostructural analogs of Mg-MOF-74, referred to as IRMOF-74I to IRMOF-74XI, have large pore apertures to accommodate protein, and NU110E with acetylene expanded hexatopic linker has the highest experimental Brunauer–Emmett–Teller (BET) surface area of any porous material reported to date (7140 m2 g−1 ) among porous materials. Table 1 provides some illustrations of MOFs and their applications.

2 Chemical Composition The kind of the linker is one fundamental factor to take into account when determining what qualifies as a MOF and what does not. The name “metal–organic framework” implies the need for a metal, but the nature of the organic components has caused considerable misunderstanding, and some people have chosen to ignore the problem in favor of maintaining ambiguity [16]. Typically, substances having carbon–carbon and/or carbon–hydrogen bonds are referred to as organic compounds. Prussian blue with cyanide ligands is not a MOF in light of this argument. Prussian blue with cyanide ligands is not a MOF in light of this argument. Inclusion of polymeric metal carbonates, etc. follows an argument that it happens swiftly. In contrast, because the linkers are regarded as organic compounds even in the absence of C–H bonds, framework structures comprising metal-oxalates, metal tetrafluoroterephthalates, etc. belong to the class of MOFs. To distinguish MOFs from simply organic or inorganic polymers, they have also been referred to as organic–inorganic hybrids [17, 18]. However, the term “hybrid” describes a species that is created by fusing two or more creatures with various characteristics. By combining two or more phases, usually at the nanoscale, hybrid materials are created. According to some reports, porous CPs and MOFs have been used as hosts to enclose or graft discrete molecular species, nanoparticles, organic polymers, etc., creating composite or hybrid materials that benefit from the functional traits of both the host and guest components [19, 20]. However, a straightforward MOF should not be referred to as an inorganic– organic hybrid material since it is a chemically separate entity that is produced by coordination bonding between metal ions and organic linkers [21]. 3 4

SBUs. MCUs.

Zn

HKUST (Hong Kong University of Science Cu and Technology)-1 Cu2 (H2 O)2 (CO2 )4 Zn In

In

Cu

MOF-5 Zn4 (1,4-bdc)3

IRMOF-9 Zn4 O (bpdc)3

MOF-74, Zn2 (C8 H2 O6 )

(In) MIL-68-NH2 or IHM-2

metal–organic Zn (bix) sphereswith encapsulated DOX [DOX/Zn(bix)], SN-38 [SN-38/Zn(bix)], CPT [CPT/Zn(bix)] and DAU [DAU/Zn(bix)] Doxorubicin (DOX), SN-38, camptothecin (CPT) and daunomycin (DAU)

(In) MIL-68-NH-ProFmoc and (In) MIL-68-NH-Ala-FmocIn *fluorenylmethyloxycarbonyl group(Fmoc), abase-label protecting group for amines

Cu-BTC(MOF-199)

Co-TDM

MIL-100(Fe or Cr) and MIL-127(Fe)

Ag2 (O-IPA) (H2O)·(H3O) and Ag5 (PYDC)2 (OH)

Mn3 (HCOO)6 ·DMF

Methane Storage

Adsorption and storage

Adsorption and storage

Adsorption and storage



Drug delivery

Soft coupling– deprotection sequence

Antibacterial

Highly potent bacteriocidal activity

Delivery of nitric oxide

Antibacterial

Adsorption of CO2 over N2

Mn

Ag

Fe, Cr or Fe

Co

Zn

Zn

Cr

MIL101[Cr3 O(OH, F, H2 O)3 (1,4-bdc)3 and MIL-100

Drug delivery

Metal

MOF

Application

Table 1 Some examples of MOFs and their applications

2014

2014

3-nitrophthalic acid (H2 npta) and 4.4' -bipyridine (4.4' - bipy) 2014

2014 HO-H2 ipa = 5- hydroxyisophthalic acid and H2 pydc = pyridine-3, 5- dicarboxylic acid

2012

2014

2011

2010

2011

Tricarboxylate or tetracarboxylate

H8 tdm: tetrakis [(3.5- dicarboxyphenyl)oxamethyl] methane

H3 btc

Amino acid such as L-proline (Pro-OH) and D-alanine (Ala-OH)

Bix: 1.4-bis (imidazol-1- ylmethyl) benzene

bdc-NH2: 2- aminoterephthalates

2006

2006

4,4' -biphenyldicarboxylate (bpdc) 2,5-dihydroxybenzene-1,4- dicarboxylic acid

2006

2002

2006

Year

H3btc

Bdc

1,4-benzenedicarboxylate moieties (bdc) or H3btc: Benzene-1,3,5-tricarboxylate

Ligand

[15]

[14]

[13]

[12]

[11]

[10]

[9]

[8]

[7]

[7]

[7]

[5, 6]

[4]

References

Overview of Metal Organic Frameworks 3

4

S. Zarei-Shokat and F. Ganjali

3 Porosity The amount of holes in a material is related to its porosity, which is commonly expressed by the volume of solvent-accessible spaces or by the surface area derived from gas adsorption isotherms. The IUPAC concept of “potential void” has been proposed as a prerequisite for classifying a coordination-based polymer as a MOF [22]. The phrase “potential void” describes the breathing behavior (or flexibility) of certain compounds’ frameworks, but not that of nonporous frameworks. The size and chemical build of the guests determine the porosity, which is not an absolute feature (close-packed spheres occasionally contain spaces that can be filled). Positrons are always able to locate porosity, therefore a substance that is porous to one gas may act as nonporous to other gases. In order to balance the charges on the ionic framework structures of several of these solids, guests with opposing charges are present in their cavities. These compounds have the ability to exchange ionic species for counterionic guests, although they may act impermeable to any neutral guests or ions carrying the same charges as the frameworks [23]. The next generation of functional materials in the disciplines of energy materials, optoelectronics, ferroelectrics, multiferroics, batteries, magnets, etc. are presently developing as nonporous MOFs with high density [24]. Since an arbitrary pore size would have to be specified and none can be justified on the basis of science, porosity or “potential void” should not be used as a parameter to determine whether or not a coordination molecule is a MOF.

4 Metal Biomolecule Frameworks (Bio-MOFs) Biomolecules are plentiful and spontaneously occurring. They produce structurally varied, physiologically compatible MOFs because they are flexible, stiff, and cost-effective with various coordination sites. Additionally, non-toxic endogenous cations (such Ca, Mg, Fe, and Zn) and ligands made of naturally occurring compounds or biomolecules have both been used to make MOFs [25]. Typically, these Bio-MOFs are biocompatible and appropriate for use in biomedical applications. Several therapeutic benefits (anti-allergic, anti-inflammatory, antibacterial, and anticarcinogenic actions) are also linked to these combinations of natural ligands and endogenous cations [26]. Examples of Bio-MOFs and their uses are provided in Table 2. These nontoxic, ecologically friendly, and biologically compatible metal ions and organic linkers serve as the building blocks for these biologically and environmentally compatible MOFs, which are created and developed based on particular composition requirements. Biomolecules like amino acids, peptides, proteins, nucleobases, carbohydrates, and other naturally occurring substances like cyclodextrins, porphines, and some carboxylic acids are emerging building blocks for the creation of metal–biomolecule frameworks with novel and intriguing properties and applications that cannot be achieved through the use of conventional organic linkers.

Ni

[Ni7 (suc)6 (OH)2 (H2 O)2 ] 2H2 O

[Ni7 (suc)4 (OH)6 (H2 O)3 ] 7H2 O

Reversible H2 O sorption/desorption

Ligand

Ni

Co Ni

[Ni2 o(L-Asp)H2 o] 4H2 O

Zn2 (bdc)(L-lac)(DMF)

[Ni2 (L-Asp)2 (4,4' -bipy)] 2H2 O

CO2 (L-Asp)2 (4,4' -bipy)]· 2H2 O

Ni2 (L-Asp) (4,4' -bipy)·(HCl)1.8(MeOH)

1.3-Butanediol sorption

Enantioselective separation and catalytic

CO2 sorption

H2 sorption

Heterogeneous Asymmetric catalysts for the methanolysis of rac-propylene oxide

Zn

Fe3 O (MeOH)3 (fum)3(CO2 CH3 )]0.4.5MeOH

Adsorption Ni

Fe

Mn

[Mn3 (HCOO)6 ] (CH3 OH) (H2 O)

Selective CO2 and H2 sorption

L-Asp and 4,4' -bipy

L-Asp and 4,4' -bipy

L-Asp and 4,4' -bipy: 1,2-bis (4-pyridyl)ethane

bdc: 1,4 benzendicarboxylic acid and Llac:Lactic acid

Amino acid LAsp:L-aspartic acid

Fum

Formicacid

Suc Formic acid

Ni Mn

Suc: Succinic acid

Fum: Fumaric acid

[Mn3 (HCOO)6 ]·(CH3 OH)·(H2 O)



Metal Cu

Bio-MOF

[Cu(trans-fum)]

Application

Ar and CH4 sorption

Table 2 Some examples of Bio-MOFs and their applications Year

2008

2008

2006

2006

2004

2004

2004

2004

2003

2002

2001

References

(continued)

[37]

[36]

[35]

[34]

[33]

[32]

[31]

[30]

[29]

[28]

[27]

Overview of Metal Organic Frameworks 5

Cu

Cu2 (L-Asp)2 (bpe) (HCl)2 (H2O)2

Zn8 (Ade)4 (bpdc)6 O 2Me2 NH2 8DMF 11H2 O

Co2 (Ade)2 (CO2 CH3 )2 2DMF 0.5H2 O

Fe3 O (MeOH)3 (fumarate)3 -(CO2 CH3 )] 4.5 MeOH and [Fe3 O(MeOH) (C6 H4 O8 )3 C1] 6MeOH

BioMIL-1

[Zn(glyAla)2 ] (solvent)

Heterogeneous asymmetric catalysts for the methanolysis of rac-propylene oxide

Cation exchange capabilities, including cationic drugs and lanthanide ions

Selective CO2 . sorption

Drug delivery and imaging

Therapeutic agent

Reversible flexible structure; CO2 , MeOH and H2 O sorption

Zn

Fe

Co

Zn

Metal

Bio-MOF

Application

Table 2 (continued)

Peptide,Glycine-adenine

Nicotinic acid (pyridine-3carboxylic acid, also called niacin or vitamin B3)

Fumarate and C6 H4 O8 is galactarate

Ade

Nucleobases Adenine:Ade and bpdc: biphenyldicarboxylate

L-Asand bpe: 1,2-bis(4-pyridyl)ethane

Ligand

2010

2010

2010

2010

2009

2008

Year

(continued)

[42]

[41]

[40]

[39]

[38]

[37]

References

6 S. Zarei-Shokat and F. Ganjali

Fe,Mn, Co and Ni Rb

K, Rb and Cs γ-CD

M(II/III)Gallates

α-CD-MCF

CD-MOF-1and CD-MOF-2 CD-MOF-3

Porous

Adsorption

α-CD R-cyclodextrin (R-CD),comprised of sixR-1,4D-Glupresidues portrayed in their stable 4C1 conformations

H4 gal: gallic acid

Ade and bpdc

Zn and lanthanide

Zn8 (Ade)4 (bpdc)6 O 2Me2 NH2 ] loadedwith lanthanidecations (Tb(III), Sm(III), Eu(III) and Yb(III))

2012

2012

2011

2011

(continued)

[48]

[47]

[46]

[45]

[44]

2011

γ-CD

Photostable O2 sensor

Rb

CD-MOF-2

[43]

γ-CD 2010 γ-CD is a (chiral) cyclic oligosaccharide composed of eightR-1,4- linkedDglucopyranosyl (R-1,4-D-Glup)

References [43]

Year 2010

Saccharides γ -CD: cyclodextrins

Ligand

Highly selective adsorption of CO2

Rb

K

(γ-CD) (KOH)2

(γ-CD) (RbOH)2

Metal

Bio-MOF

Inclusion of several molecules (e.g. Rhodamine B,4-phenylazoplenol,etc.)

Application

Table 2 (continued)

Overview of Metal Organic Frameworks 7

CD-MOF-1

MOF-525

Al-PMOF

[Zn(ain) (atz)]n

Inclusion and loading the drug molecules

Electrochemical Nitrite detection

Ammonia uptake

Highly active anti-diabetic activity Zn

Al

Zr

Na

Zn Mg

Bio-MIL-5

Mg(H4 gal)

Zr

MIL-151 to -154

Antibacterial

Zn

Bio-MOF-100

Drug storage and release or for the immobilization and organization of large biomolecules

Antioxidant carrier

Metal

Bio-MOF

Application

Table 2 (continued)

H4 tcpp Hatz: 5aminotetrazole and Hain: 2amino-4-isonicotinic

2016

2015

2015

2015

β-CD:cyclodextrins H4 tcpp: meso-tetra (4-carboxyphenyl)

2015

2014

2014

2012

Year

H4 gal

AzA: azelaic acid

H4 gal

Ade

Ligand

[56]

[55]

[54]

[53]

[52]

[51]

[50]

[49]

References

8 S. Zarei-Shokat and F. Ganjali

Overview of Metal Organic Frameworks

9

5 MOFs Synthesis The creation or breaking of bonds requires energy, and this phenomena is true for the synthesis of MOFs as well because it involves the bonding of organic linkers and metal oxides. The primary goal of MOF synthesis is to produce distinct inorganic building components that do not include organic linker breakdown [58]. For the synthesis of MOFs, any device that can provide a controlled quantity of heat will work. For instance, heat may be delivered from ovens, microwaves, mechanical ultrasound, electric potential, and electromagnetic wave radiation, among other sources. The sort of energy or source used determines the nature of the MOFs that are produced, and the synthesis process determines the MOFs’ characteristics. Multiple MOF synthesis pathways are essential due to their variety of characteristics. The main characteristics of the generated MOFs, such as size distribution, shape, and particle size, determine their suitability or application. As a result, the type of heat energy generated during its formation and the heat source employed both affect the porosity of the MOFs that are produced. Even though the coordination of metal clusters with organic ligands to create MOFs looks simple, getting the desired structure is really difficult. The kind of MOFs manufactured is influenced by process variables (time, temperature, and pressure) as well as compositional characteristics (solvent, pH, linker substituent, and metal ion concentration). High surface area, high porosity, and common starting materials, especially for their ligands, are a few essential characteristics that make MOFs incredibly attractive to researchers working in multidisciplinary fields. Even though the same reactive mixture (a metal source, an organic ligand, and a solvent) is employed to create MOFs, reaction duration, particle size, yield, and morphology all have an impact on the final structure. Therefore, the various MOF synthesis techniques are much required. Some synthesis methods are excellent choices for the large-scale process. Conventional solution, diffusion synthesis, solvothermal synthesis, microwave synthesis, sono-chemical synthesis, electrochemical synthesis, and iono-thermal synthesis are a few of the approaches employed for MOF synthesis (Fig. 2).

6 Conclusion MOFs have a wide range of uses, such as catalysts, sensors, storage and separation devices, and drug delivery systems. Nanodrug carriers for antitumor and anti-HIV medications may be made from non-toxic nano-MOFs with customized cores and surfaces (biomedicine, nontoxic, drug). In contrast to their counterparts containing conventional organic linkers, MOFs with biomolecules as organic linkers are still in the nascent stage. However, biomolecules give MOFs easy recyclability and biological compatibility. They also provide structures with distinctive qualities including

10

S. Zarei-Shokat and F. Ganjali

Fig. 1 Illustration of MOF formation and factors affecting MOF synthesis. This figure was adapted by permission from: Trends in Food Science and Technology, (2020), 104, 102–116 [3]

Fig. 2 Different methods of MOF synthesis, a Sono-chemical synthesis method, b Conventional solution method, c Diffusion synthesis method, d Iono-thermal process method, e Microwave synthesis method, f Electrochemical synthesis method, and g Solvothermal synthesis method. This figure was adapted by permission from: Trends in Food Science & Technology, (2020), 104, 102–116 [3]

Overview of Metal Organic Frameworks

11

chirality and particular recognition, separation, ion exchange, and catalytic capabilities, as well as bio-inspired features. Future potential for MOFs’ structures, characteristics, and applications in various domains may arise from a greater knowledge of the chemistry involved in their creation.

References 1. Yaghi, O-M., Kalmutzki, M-J., Diercks, C-S.: Introduction to reticular chemistry: metalorganic frameworks and covalent organic frameworks. JWS (2019). ISBN 9783527345021, 3527345027 2. Redfern, L.-R., Farha, O.-K.: Mechanical properties of metal–organic frameworks. Chem Sci 10, 10666–10679 (2019). https://doi.org/10.1039/C9SC04249K 3. Sharanyakanth, P.-S., Radhakrishnan, M.: Synthesis of metal-organic frameworks (MOFs) and its application in food packaging: A critical review. Trends. Food. Sci. Technol. 104, 102–116 (2020). https://doi.org/10.1016/j.tifs.2020.08.004 4. Zhu, G., Ren, H.: Porous organic frameworks: Design, synthesis and their advanced applications. Springer. Sci. Rev. (2014). https://doi.org/10.1007/978-3-662-45456-5 5. Li, H., Eddaoudi, M., O’Keeffe, M., Yaghi, O-M.: Design and synthesis of an exceptionally stable and highly porous metal-organic framework. Nature 402, 276–279 (1999). https://doi. org/10.1038/46248 6. Chae, H.-K., Siberio-Pérez, D.-Y., Kim, J., Go, Y., Eddaoudi, M., Matzger, A.-J., O’Keeffe, M., Yaghi, O.-M.: A route to high surface area, porosity and inclusion of large molecules in crystals. Nature 427, 523–527 (2004). https://doi.org/10.1038/nature02311 7. Rowsell, J.-L., Yaghi, O.-M.: Effects of functionalization, catenation, and variation of the metal oxide and organic linking units on the low-pressure hydrogen adsorption properties of metal− organic frameworks. JACS 128, 1304–1315 (2006). https://doi.org/10.1021/ja056639q 8. Savonnet, M., Farrusseng, D.: PCT Appl. WO2011048284 (2011) 9. Imaz, I., Rubio-Martínez, M., García-Fernández, L., García, F., Ruiz-Molina, D., Hernando, J., Puntes, V., Maspoch, D.: Coordination polymer particles as potential drug delivery systems. Chem Comm 46, 4737–4739 (2010). https://doi.org/10.1039/C003084H 10. Canivet, J., Aguado, S., Bergeret, G., Farrusseng, D.: Amino acid functionalized metal–organic frameworks by a soft coupling–deprotection sequence. Chem Comm 47, 11650–11652 (2011). https://doi.org/10.1039/C1CC15541E 11. Rodríguez, H-S., Hinestroza, J-P., Ochoa-Puentes, C., Sierra, C-A., Soto, C-Y.: Antibacterial activity against Escherichia coli of Cu-BTC (MOF-199) metal-organic framework immobilized onto cellulosic fibers. J. Appl. Polym. Sci. 131 (2014). https://doi.org/10.1002/app.40815 12. Zhuang, W., Yuan, D., Li, J.-R., Luo, Z., Zhou, H.-C., Bashir, S., Liu, J.: Highly potent bactericidal activity of porous metal-organic frameworks. Adv. Healthc. Mater. 1, 225–238 (2012). https://doi.org/10.1002/adhm.201100043 13. Eubank, J.-F., Wheatley, P.-S., Lebars, G., McKinlay, A.-C., Leclerc, H., Horcajada, P., Daturi, M., Vimont, A., Morris, R.-E., Serre, C.: Porous, rigid metal (III)-carboxylate metal-organic frameworks for the delivery of nitric oxide. APL. Mater. 2, 124112 (2014). https://doi.org/10. 1063/1.4904069 14. Indumathy, R., Radhika, S., Kanthimathi, M., Weyhermuller, T., Nair, B.-U.: Cobalt complexes of terpyridine ligand: crystal structure and photocleavage of DNA. J. Inorg. Biochem. 101, 434–443 (2007). https://doi.org/10.1016/j.jinorgbio.2006.11.002 15. Zhao, Y.-P., Yang, H., Wang, F., Du, Z.-Y.: A microporous manganese-based metal–organic framework for gas sorption and separation. J. Mol. Struct. 1074, 19–21 (2014). https://doi.org/ 10.1016/j.molstruc.2014.05.033

12

S. Zarei-Shokat and F. Ganjali

16. Batten, S.-R., Champness, N.-R., Chen, X.-M., Garcia-Martinez, J., Kitagawa, S., Öhrström, L., O’Keeffe, M., Suh, M.-P., Reedijk, J.: Coordination polymers, metal–organic frameworks and the need for terminology guidelines. CrystEngComm 14, 3001–3004 (2012). https://doi. org/10.1039/C2CE06488J 17. Cheetham, A-K., Rao, C-N., Feller, R-K.: Structural diversity and chemical trends in hybrid inorganic–organic framework materials. ChemComm., 4780-4795 (2006). https://doi.org/10. 1039/B610264F 18. Du, M., Zhang, Z.-H., Tang, L.-F., Wang, X.-G., Zhao, X.-J., Batten, S.-R.: Molecular tectonics of metal-organic frameworks (MOFs): A rational design strategy for unusual mixed-connected network topologies. Eur. J. Chem. 13, 2578–2586 (2007). https://doi.org/10.1002/chem.200 600980 19. Gamage, N.-D., McDonald, K.-A., Matzger, A.-J.: MOF-5-Polystyrene: direct production from Monomer, improved hydrolytic stability, and unique guest adsorption. Angew. Chem., Int. Ed. Engl. 55, 12099–12103 (2016). https://doi.org/10.1002/anie.201606926 20. Cohen, S.-M.: The postsynthetic renaissance in porous solids. JACS 139, 2855–2563 (2017). https://doi.org/10.1021/jacs.6b11259 21. Wu, H., Chua, Y.-S., Krungleviciute, V., Tyagi, M., Chen, P., Yildirim, T., Zhou, W.: Unusual and highly tunable missing-linker defects in zirconium metal–organic framework UiO-66 and their important effects on gas adsorption. JACS 135, 10525–10532 (2013). https://doi.org/10. 1021/ja404514r 22. Batten, S.-R., Champness, N.-R., Chen, X.-M., Garcia-Martinez, J., Kitagawa, S., Öhrström, L., O’Keeffe, M., Suh, M.-P., Reedijk, J.: Terminology of metal–organic frameworks and coordination polymers (IUPAC Recommendations 2013). Pure. Appl. Chem. 85, 1715–1724 (2013). https://doi.org/10.1351/PAC-REC-12-11-20 23. Nugent, P., Belmabkhout, Y., Burd, S.-D., Cairns, A.-J., Luebke, R., Forrest, K., Pham, T., Ma, S., Space, B., Wojtas, L., Eddaoudi, M.: Porous materials with optimal adsorption thermodynamics and kinetics for CO2 separation. Nature 495, 80–84 (2013). https://doi.org/10.1038/nat ure11893 24. Tominaka, S., Hamoudi, H., Suga, T., Bennett, T.-D., Cairns, A.-B., Cheetham, A.-K.: Topochemical conversion of a dense metal–organic framework from a crystalline insulator to an amorphous semiconductor. Chem. Sci. 6, 1465–1473 (2015). https://doi.org/10.1039/ C4SC03295K 25. Imaz, I., Rubio-Martinez, M., An, J., Sole-Font, I., Rosi, N.-L., Maspoch, D.: Metal– biomolecule frameworks (MBioFs). ChemComm 47, 7287–7302 (2011). https://doi.org/10. 1039/C1CC11202C 26. Cai, H., Huang, Y.-L., Li, D.: Biological metal–organic frameworks: structures, host–guest chemistry and bio-applications. Coord. Chem. Rev 378, 207–221 (2019). https://doi.org/10. 1016/j.ccr.2017.12.003 27. Kitagawa, S., Kitaura, R., Noro, S.-I.: Functional porous coordination polymers. Angew. Chem., Int. Ed. Engl. 43, 2334–2375 (2004). https://doi.org/10.1002/anie.200300610 28. Forster, P.-M., Cheetham, A.-K.: Open-framework nickel succinate, [Ni7 (C4H4O4) 6 (OH) 2 (H2O) 2]· 2 H2O: a new hybrid material with three-dimensional Ni− O− Ni connectivity. Angew. Chem. Int. Ed. Engl. 41, 457–459 (2002). https://doi.org/10.1002/1521-3773(200202 01)41:3%3c457::AID-ANIE457%3e3.0.CO;2-W 29. Guillou, N., Livage, C., van Beek, W., Noguès, M., Férey, G.: A layered nickel succinate with unprecedented hexanickel units: structure elucidation from powder-diffraction data, and magnetic and sorption properties. Angew. Chem. Int. Ed. Engl. 42, 643–647 (2003). https:// doi.org/10.1002/anie.200390177 30. Wang, Z., Zhang, B., Fujiwara, H., Kobayashi, H., Kurmoo, M.: Mn 3 (HCOO) 6: a 3D porous magnet of diamond framework with nodes of Mn-centered MnMn 4 tetrahedron and guestmodulated ordering temperature. ChemComm, 416-417 (2004). https://doi.org/10.1039/b31 4221c 31. Dybtsev, D.-N., Chun, H., Yoon, S.-H., Kim, D., Kim, K.: Microporous manganese formate: a simple metal− organic porous material with high framework stability and highly selective gas sorption properties. JACS 126, 32–33 (2004). https://doi.org/10.1021/ja038678c

Overview of Metal Organic Frameworks

13

32. Serre, C., Millange, F., Surblé, S., Férey, G.: A route to the synthesis of trivalent transition-metal porous carboxylates with trimeric secondary building units. Angew. Chem. Int. Ed. Engl. 43, 6285–6289 (2004). https://doi.org/10.1002/anie.200454250 33. Anokhina, E.-V., Jacobson, A.-J.: [Ni2O (l-Asp)(H2O) 2]⊙ 4H2O: A homochiral 1D helical chain hybrid compound with xtended Ni− O− Ni bonding. JACS 126, 3044–3045 (2004). https://doi.org/10.1021/ja031836f 34. Han, X., Zhou, S.-J., Tan, Y.-Z., Wu, X., Gao, F., Liao, Z.-J., Huang, R.-B., Feng, Y.-Q., Lu, X., Xie, S.-Y., Zheng, L.-S.: Crystal structures of saturn-like C50Cl10 and pineapple-shaped C64Cl4: geometric implications of double-and triple-pentagon-fused chlorofullerenes. Angew. Chem. Int. Ed. Engl. 47, 5340–5343 (2008). https://doi.org/10.1002/anie.200503023.14 35. Vaidhyanathan, R., Bradshaw, D., Rebilly, J.-N., Barrio, J.-P., Gould, J.-A., Berry, N.-G., Rosseinsky, M.-J.: A family of nanoporous materials based on an amino acid backbone. Angew. Chem. Int. Ed. Engl. 45, 6495–6499 (2006). https://doi.org/10.1002/anie.200602242 36. Zhu, P., Gu, W., Cheng, F.Y., Liu, X., Chen, J., Yan, S.P., Liao, D.Z.: Design of two 3D homochiral Co(II) metal–organic open frameworks by layered-pillar strategy: structure and properties. Cryst. Eng. Commun. 10, 963–967 (2008). https://doi.org/10.1039/B801177J 37. Ingleson, M-J., Barrio, J-P., Bacsa, J., Dickinson, C., Park, H., Rosseinsky, M-J.: Generation of a solid Brønsted acid site in a chiral framework. ChemComm. 1287–1289 (2020). https:// doi.org/10.1039/B718443C 38. Horcajada, P., Serre, C., Vallet-Regí, M., Sebban, M., Taulelle, F., Férey, G.: Metal–organic frameworks as efficient materials for drug delivery. Angew. Chem. 118, 6120–6124 (2006). https://doi.org/10.1021/ja902972w 39. An, J., Geib, S.-J., Rosi, N.-L.: High and selective CO2 uptake in a cobalt adeninate metal− organic framework exhibiting pyrimidine-and amino-decorated pores. JACS 132, 38–39 (2010). https://doi.org/10.1021/ja909169x 40. Horcajada, P., Chalati, T., Serre, C., Gillet, B., Sebrie, C., Baati, T., Eubank, J.-F., Heurtaux, D., Clayette, P., Kreuz, C., Chang, J.-S.: Porous metal–organic-framework nanoscale carriers as a potential platform for drug delivery and imaging. Nat. Mater. 9, 172–178 (2010). https:// doi.org/10.1038/nmat2608 41. Miller, S.-R., Heurtaux, D., Baati, T., Horcajada, P., Grenèche, J.-M., Serre, C.: Biodegradable therapeutic MOFs for the delivery of bioactive molecules. ChemComm 46, 4526–4528 (2010). https://doi.org/10.1039/C001181A 42. Rabone, J., Yue, Y.-F., Chong, S.-Y., Stylianou, K.-C., Bacsa, J., Bradshaw, D., Darling, G.-R., Berry, N.-G., Khimyak, Y.-Z., Ganin, A.-Y., Wiper, P.: An Adapt. Pept.-Based Porous Mater. Sci. 329, 1053–1057 (2010). https://doi.org/10.1126/science.1190672 43. Smaldone, R.-A., Forgan, R.-S., Furukawa, H., Gassensmith, J.-J., Slawin, A.-M., Yaghi, O.M., Stoddart, J.-F.: Metal–organic frameworks from edible natural products. Angew. Chem. Int. Ed. Engl. 49, 8630–8634 (2010). https://doi.org/10.1002/anie.201002343 44. Gassensmith, J.-J., Furukawa, H., Smaldone, R.-A., Forgan, R.-S., Botros, Y.-Y., Yaghi, O.M., Stoddart, J.-F.: Strong and reversible binding of carbon dioxide in a green metal–organic framework. JACS 133, 15312–15315 (2011). https://doi.org/10.1021/ja206525x 45. An, J., Shade, C.M., Chengelis-Czegan, D.A., Petoud, S., Rosi, N.L.: Strong and reversible binding of carbon dioxide in a green metal organic framework. J. Am. Chem. Soc. 133, 1220– 1223 (2011). https://doi.org/10.1021/ja206525x 46. Saines, P.-J., Yeung, H.-H.-M., Hester, J.-R., Lennie, A.-R., Cheetham, A.-K.: Detailed investigations of phase transitions and magnetic structure in Fe(III), Mn(II), Co(II) and Ni(II) 3,4,5 trihydroxybenzoate (gallate) dihydrates by neutron and X-ray diffraction. Dalton. Trans. 40, 6401–6410 (2011). https://doi.org/10.1039/c0dt01687j 47. Gassensmith, J.-J., Smaldone, R.-A., Forgan, R.-S., Wilmer, C.-E., Cordes, D.-B., Botros, Y.Y., Slawin, A.-M., Snurr, R.-Q., Stoddart, J.-F.: Polyporous metal-coordination frameworks. Org. Lett. 14, 1460–1463 (2012). https://doi.org/10.1021/ol300199a 48. Forgan, R.-S., Smaldone, R.-A., Gassensmith, J.-J., Furukawa, H., Cordes, D.-B., Li, Q., Wilmer, C.-E., Botros, Y.-Y., Snurr, R.-Q., Slawin, A.-M., Stoddart, J.-F.: Nanoporous carbohydrate metal–organic frameworks. JACS 134, 406–417 (2012). https://doi.org/10.1021/ja2 08224f

14

S. Zarei-Shokat and F. Ganjali

49. An, J., Farha, O.-K., Hupp, J.-T., Pohl, E., Yeh, J.-I., Rosi, N.-L.: Metal-adeninate vertices for the construction of an exceptionally porous metal-organic framework. Nat. Commun. 3, 1–6 (2012). https://doi.org/10.1038/ncomms1618 50. Cooper, L., Guillou, N., Martineau, C., Elkaim, E., Taulelle, F., Serre, C., Devic, T.: ZrIV coordination polymers based on a naturally occurring phenolic derivative. Eur. J. Inorg. Chem. 2014, 6281–6289 (2014). https://doi.org/10.1002/ejic.201402891 51. Tamames-Tabar, C., Imbuluzqueta, E., Guillou, N., Serre, C., Miller, S.R., Elkaïm, E., Horcajada, P., Blanco-Prieto, M.-J.: A Zn azelate MOF: combining antibacterial effect. CrystEngComm 17, 456–462 (2015). https://doi.org/10.1039/C4CE00885E 52. Cooper, L., Hidalgo, T., Gorman, M., Lozano-Fernández, T., Simón-Vázquez, R., Olivier, C., Guillou, N., Serre, C., Martineau, C., Taulelle, F., Damasceno-Borges, D.: A biocompatible porous Mg-gallate metal–organic framework as an antioxidant carrier. ChemComm 51, 5848– 5851 (2015). https://doi.org/10.1039/C5CC00745C 53. Lu, H., Yang, X., Li, S., Zhang, Y., Sha, J., Li, C., Sun, J.: Study on a new cyclodextrin based metal–organic framework with chiral helices. Inorg. Chem. Commun. 61, 48–52 (2015). https:// doi.org/10.1016/j.inoche.2015.08.015 54. Kung, C.W., Chang, T.-H., Chou, L.-Y., Hupp, J.-T., Farha, O.-K., Ho, K.-C.: Porphyrinbased metal–organic framework thin films for electrochemical nitrite detection. Electrochem. Commun. 58, 51–56 (2015). https://doi.org/10.1016/j.elecom.2015.06.003 55. Wilcox, O.-T., Fateeva, A., Katsoulidis, A.-P., Smith, M.-W., Stone, C.-A., Rosseinsky, M.-J.: Acid loaded porphyrin-based metal–organic framework for ammonia uptake. ChemComm 51, 14989–14991 (2015). https://doi.org/10.1039/C5CC06209H 56. Briones, D., Fernández, B., Calahorro, A.-J., Fairen-Jimenez, D., Sanz, R., Martínez, F., Orcajo, G., Sebastián, E.-S., Seco, J.-M., González, C.-S., Llopis, J.: Highly active anti-diabetic metal– organic framework. Cryst. Growth. Des. 16, 537–540 (2016). https://doi.org/10.1021/acs.cgd. 5b01274 57. Keskin, S., Kızılel, S.: Biomedical applications of metal organic frameworks. Ind. Eng. Chem. Res 50, 1799–1812 (2011). https://doi.org/10.1021/ie101312k 58. Bag, P-P., Singh, G-P., Singha, S., Roymahapatra, G.: Synthesis of metal-organic frameworks (MOFs) and their biological, catalytic and energetic application: a mini review. Eng 13, 1-0 (2020). https://doi.org/10.30919/es8d1166

Classification of the MOFs Based on the Secondary Building Units (SBUs) Maryam Kamalzare

Abstract One of the most important and useful subsets of hybrid porous materials is metal–organic frameworks (MOFs). The structures of metal–organic frameworks are composed of metal ions/clusters and organic linkers. The term secondary building units (SBUs) is related to the metal-containing nodes. The properties of both parts (node and linker) can determine MOF characteristics like network structure (net) topology, physical, mechanical, and morphological features. The alteration of node and organic linker results in a huge number of MOFs with special properties in many fields. MOFs are unique materials in terms of their high porosity, large surface area, structural and functional tolerability, and highly ordered crystalline. Keywords Metal cluster · Crystal structure · Organic ligand · Metal · Linker · Node

1 Introduction Secondary building units are the key components of metal–organic frameworks that help to build potentially porous periodic networks by linking multitopic organic ligands. Hence, metal SBUs are critical for determining the underlying topology of MOFs. Moreover, SBUs are the main MOF research topic nowadays, because of the simplicity of their synthesis, diverse directionality, and their ability to easily harness open metal sites [1, 2]. The SBUs are principally composed of monovalent (Na+ , Cu+ , K+ , Ag+ , etc.) divalent (Mg2+ , Mn2+ , Fe2+ , Co2+ , Ni2+ , Cu2+ , Zn2+ , Cd2+ , etc.), trivalent (Al3+ , Sc3+ , Cr3+ , Fe3+ , In3+ , lanthanides3+ , etc.), or tetravalent (Ti4+ , Zr4+ , Hf4+ ) metals. Due to the infinite number of organic linkers and secondary building units, boundless frameworks can be designed and synthesized with different properties, geometries, and structures [3, 4]. In the following, some MOFs with their secondary building units with a view to monovalent, divalent, trivalent, and tetravalent metals are summarized. M. Kamalzare (B) Catalysis and Organic Synthesis Research Laboratory, Department of Chemistry, Iran University of Science and Technology, 16846-13114 Tehran, Iran e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 A. Maleki and R. Taheri-Ledari (eds.), Physicochemical Aspects of Metal-Organic Frameworks, Engineering Materials, https://doi.org/10.1007/978-3-031-18675-2_2

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2 MOFs with Their Secondary Building Units 2.1 Ti-Base MOFs Among the thousands of MOFs, the titanium-based MOFs have attracted great and consecutive attention because of the amazing inherent features of titanium elements, including high earth’s crust abundance, low toxicity, and novel photo redox property. Titanium is considered one of the most attractive candidates for the synthesis of MOFs with high chemical stability and structural diversity. Titanium is just above zirconium in the periodic table, thus also a typical tetravalent element in ionic form, while the much smaller ionic radius of Ti4+ compared to Zr4+ causes its stronger tendency to oxygen [5]. Because of the special properties of Ti-based MOFs, there are numerous articles about this field but in comparison with other MOFs, the number of Ti-MOFs is less than other MOFs. The reason is that the complexity of Ti chemistry in solution brings difficulties in the design and synthesis of titanium-based MOFs, which can be distinctly seen in the reported structures, where their inorganic building units range from discrete Ti octahedra to infinite Ti-oxo-chains. The first Ti-MOF, MIL-22 was constructed by a direct synthetic method, using phosphorus ligand and TiO2 under high temperature in 1999 by Serre and Férey. The titanium-oxo-clusters can be seen as TiO2 nanoclusters, which can be served as building blocks for MOFs construction and at the same time, confer materials photocatalytic activity [6, 7]. In the following, some of the Ti-based MOFs and their properties and applications are described. Amandine Cadiau and colleagues introduced new photo catalytically active Ti-based MOFs by utilizing 4,4' ,4'' ,4''' -(pyrene-1,3,6,8-tetrayl)tetrabenzoic acid (H4 TBAPy) as a photosensitizer ligand. Due to the extended π-electron system of H4 TBAPy, it could be displayed a broad visible-light absorption spectrum. The presence of Ti and H4 TBAPy ligand enhances the photocatalytic features of MOF [8]. Sujing Wang et al. have reported a new Ti-based MOF denoted as MIP-208. This MOF consists of Ti8 oxo clusters and in situ acetylation of the 5-NH2 -IPA linker. In isophthalic acid (IPA), the unique angle (120°) and the medium distance between the two carboxylate groups cause a large degree of manipulation in the synthesis of MOFs. In this work with the mixed solid-solution linkers strategy, multi-faceted MIP-208 structures were synthesized with different size porosity and tunable chemical environment. Among the pure MOF catalysts, MIP-208 represents the best efficacy for the photocatalytic methanation of carbon dioxide. For enhancing the photocatalytic activity of MOF, ruthenium oxide nanoparticles were photo-deposited on MIP-208, forming a highly active and selective composite catalyst. MIP-208@RuOx has some properties such as notable visible-light response, recyclability, and great stability [9]. Yayong Sun and colleagues reported three novel Ti-based MOFs with singlecrystal structures denoted FIR-125, FIR-126, and FIR-127. These new Ti-MOFs were synthesized by using a large Ti44 -oxo cluster as the precursor to assembly with the organic ligand. In the synthesis process, the large Ti44 -oxo cluster is transferred to small Ti8 O8 (CO2 )16 building units in the Ti-MOF. FIR-125 was synthesized by

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utilizing the soluble Ti44 -oxo cluster as the metal source to react with the V-shaped 4,4/ - oxydibenzoic acid (Fig. 1a). FIR-126 and FIR-127 were synthesized, respectively, with 4,4/ -sulfonyldibenzoic acid and 1,5-naphthalenedisulfonic acid. In addition, The as-prepared FIR-125 exhibits high stability and photocatalytic activity with constant porosity [10]. Ha L. Nguyen et al. have reported a new Ti-based MOF termed MOF-901 with a two-dimensional structure, as a novel photocatalyst. The crystal structure of MOF-901 is composed of hexagonal porous layers that are likely stacked in staggered conformation (hxl topology). MOF-901 was applied in the photocatalyzed polymerization of methyl methacrylate and the result of the polymerization product is better than the commercially available photocatalyst. The geometry of the proposed SBU in MOF-901 is composed of a trigonal prismatic Ti6 O6 inner core, in which the octahedral Ti atoms are divided evenly over two separate triangular planes and bound to one another, in a corner-sharing fashion, through 6 μ3 -O atoms (Fig. 1b). The inner core is surrounded by six equatorial, planar, and chelating terminal 4-aminobenzoate ligands. This arrangement affords the SBU with six points of extensions in the shape of a hexagon. Finally, capping the top and bottom

Fig. 1 a Synthesis of big Ti-MOF crystals from high nuclear titanium-oxo-clusters precursors. This figure was adapted by permission from ACS Materials Lett, 2021, 3, 64–68 [10]; b synthetic scheme depicting the generalized formation of a discrete hexameric titanium cluster, which can be appropriately functionalized with amine groups to affect imine condensation reactions. Atom colors: Ti, blue; C, black; O, red; R groups, pink; H atoms and capping isopropoxide units are omitted for clarity. This figure was adapted by permission from Journal of the American Chemical Society, 2016, 138, 4330–4333 [11]

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of the inner core are six methoxide moieties. When linked together through imine condensation reactions with BDA (benzene-1,4-dialdehyde), the resulting extended structure contains triangular windows, 14 Å in width, and an interlayer spacing of ca. 3.47 Å. With respect to the topology, MOF-901 has one vertex, represented by the Ti6 O6 (−CO2 )6 SBU, and one edge (linker) leading to a 36 faced hxl topology [11]. Ha L. Nguyen et al. in another work reported isoreticular structure to MOF-901 named MOF-902. In MOF-902 4,4' -biphenyldicarboxaldehyde used as a linker with two benzene rings cause the high conjugation backbone, which results in the visiblelight absorption in the red-shifted region. MOF-902 was synthesized solvothermally with slight modifications to MOF-901 (Fig. 2). The 2D structure of MOF-902 is prepared by imine linking units, which are produced by imine condensation reaction from six terminal ligands chelating to a trigonal prismatic Ti6 O6 inner core of six points of extension SBU. Each second layer of hxl eclipsed network in MOF-902 is translated to generate the Ti6 O6 SBUs to the center of triangular windows of the first layer resulting in the staggered version, in which a ca. 16 Å diameter of the hexagonal window was found to be larger than in the case of MOF-901. MOF-902 represents better photocatalytic activity compared to commercial catalyst P25-TiO2 , MOF-901, as well as other photoactive MOFs such as UiO-66, UiO-66-NH2 , MIL-125(Ti), and MIL-125-NH2 (Ti) [12].

2.2 Zr-Based MOFs Considering that in the last two decades there is considerable attention to the design and synthesis of new MOFs with special properties, Zr-based MOFs have attracted great attention in this field. Taking into account that Zr-based MOFs have some properties including Zr-MOFs have a high oxidation state of Zr(IV) in comparison with M(I), M(II), and M(III)-based MOFs (M stands for metal elements). In addition, because of the high charge density and bond polarization, there is a strong connection between Zr(IV) and carboxylate O atoms in most carboxylate-based ZrMOFs. Zr(IV) ions and carboxylate ligands are introduced as hard acid and hard base, respectively, and their coordination bonds are strong. This is in the direction of hard/soft acid/base (HSAB) theory. Because of what has been mentioned most ZrMOFs are stable in organic solvents and water, and even most of them could tolerate acidic aqueous solution. Concerning this matter, Zirconium is extensively available in nature and it has low toxicity. In regard to these features, Zr-based MOFs could be utilized in many fields such as catalysis, molecule adsorption and separation, drug delivery, fluorescence sensing, and as porous carriers [13]. Zr6 O8 cluster in Zr-MOFs is the Zr6 (μ3 -O)4 (μ 3 -OH)4 octahedral cluster which is the most commonly observed in Zr-SBUs. In Zr6 O8 cluster the six vertices of the octahedron are occupied by Zr(IV) centers and eight triangular faces are alternatively capped by four μ3 -OH and four μ3 -O groups. Every Zr(IV) is eight-coordinated by O atoms in a square-antiprismatic coordination geometry. The connectivity and symmetry of SBUs are the most important factors influencing the net topology of MOFs. When the Zr6 (m3 -O)4 (μ3 -OH)4

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Fig. 2 Synthetic procedure depicting the strategy to produce MOF-901, and MOF-902 based on a the in situ Ti−oxo cluster generation and aldehyde functionalities and b the space-filling model of the crystal structure of MOF-901 and isoreticular MOF-902, which contains the longer aldehyde linking unit. Atom colors: Ti, blue; C, black; O, red; N, green; H, pink; and second layer, orange. Capping methoxide moieties have been removed for clarity. This figure was adapted by permission from ACS Catalysis, 2017, 7, 338–342 [12]

cluster is fully coordinated by 12 carboxylate groups, a Zr6 (μ3 -O)4(μ3 -OH)4 (CO2 )12 SBU is produced with the Oh -symmetry, which is observed in most of the reported ZrMOFs. The Oh -symmetry consists of many subgroups. From a topological point of view, there are a larger probability of producing three-dimensional (3D) frameworks from the highly symmetric Zr6 (μ3 -O)4 (μ3 -OH)4 (CO2 )12 SBU and organic ligands by decreasing the symmetry and/or connectivity. Volodymyr Bon et al. reported DUT-69, which involved 10-connected SBU and this cluster implies four additional ligands, occupied by two acetates and two water molecules (Fig. 3a) [14]. Dawei Feng et al. reported that in PCN-222 only eight edges of the Zr6 octahedron are occupied by carboxylates groups from ligands, while the remaining positions are bridged by terminal OH− groups. As a result, the symmetry of the Zr6 carboxylate unit is changed to one of Oh subgroups which is D4h , which potentially makes additional space for the formation of mesopores [15]. In another work reported by Dawei Feng et al., they synthesized new MOF donated PCN-224 with six-connected Zr6 cluster SBU and metalloporphyrins by a linkerelimination strategy. In this structure, just six edges of the Zr6 octahedron are occupied by carboxylate groups with D3d symmetry. The presence of fewer carboxylate

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Fig. 3 a Linker orientation in the cluster (left) and local environment of Zr/Hf clusters (right) in the DUT-69 structure from the topological point of view (gray circles symbolize the clusters and the edges correspond to the 2,5-thiophenedicarboxylate ligands). This figure was adapted by permission from Crystal Growth & Design, 2013, 13, 1231–1237 [14]. b left: Structure of the SBU found in Zrbtbp-a. ZrO6 octahedra are represented in purple, PO3 C tetrahedra are represented in green; right: view of a single layer of Zrbtbp-a along and down: perpendicular to the c-axis. This figure was adapted by permission from chemical communication, 2014, 50, 5737 [19]

linkers on the Zr6 cause more space for catalysis. Besides, the insert of OH groups improves the hardness of the Zr6 core, which cause strong bonding between the bridging ligands and the Zr6 clusters. Hence the as-prepared MOF could represent new features including high stability. PCN-224 displays considerable stability in a wide range of pH in aqueous solution and particularly PCN224(Co) is a novel reusable heterogeneous catalyst for the CO2 /epoxide coupling reaction [16]. Feng et al. reported the existence of Zr8 O6 cluster SBU in PCN-221. In the Zr8 (μ4 O)6 cluster, each Zr atom coordinates with three O atoms from carboxylates and three μ4 -O atoms, forming a distorted octahedral coordination environment. Eight Zr atoms connecting six μ4 -O atoms lead to the construction of a cubic cluster based on the Zr arrangement, in which eight vertices are occupied by Zr atoms and six faces

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are capped by μ4 -O atoms. In the structure of PCN-221, each edge of the Zr8 O6 cube cluster is bridged by a carboxylate group to produce a Zr8 (μ4 -O)6 (OH)8 (CO2 )12 SBU with Oh -symmetry. In a study by Charlotte Koschnick et al. in 2021, they found that Zr8 O6 SBU in MOF PCN-221 is basically a superposition of four statically disordered Zr6 O4 (OH)4 clusters revealed on average as a truncated cube, which can wrongly interpret as a Zr8 O6 cluster [17, 18]. In a study by Marco Taddei et al., they have reported a new Zr-based MOF by rigid tritopic phosphonic ligand. This new MOF has a honeycomb-like structure with considerable thermal stability and hydrolysis resistance. Every layer of this compound is based on the connection of separate secondary building units, each containing three ZrO6 octahedra. The SBUs are connected in the ab plane via the bridging organic ligands. As illustrated in Fig. 3b the ZrO6 in the center is connected with two crystallographically equivalent terminal ZrO6 octahedra by six bidentate PO3 C which connect the central octahedron to the terminal ones and six monodentate PO3 C groups occupied the remaining vertices of the terminal octahedra. Each SBU is joined to its six nearest neighbors by three stacks of four ligands, which cause the layers to have a remarkable thickness (about 14 Å) and represent large open regions, with an available space of about 10 Å diameter [19]. Guillerm et al. reported that ZrO7 clusters were discerned in an infinite chain type SBU of the MOF ZrO(O2 C-R-CO2 ) (R = C6 H4 (MIL-140A), C10 H6 (MIL140B), C12 H8 (MIL-140C), and C12 N2 H6 Cl2 (MIL-140D)) (Fig. 4). The zirconium atoms demonstrate a seven-coordination mode with three μ3 -O oxygen atoms and four oxygen atoms from the dicarboxylate ligands. SBU chains can be contemplated either as resulting from the linkage of two parallel corner-sharing chains or chains of edge-sharing dimers of zirconium polyhedral [20]. In some phenolic Zr-based MOFs, there is a ZrO8 core as an SBU chain. In these MOFs Zr ions basically are coordinated with eight O atoms. For instance in MIL-153 Zr(IV) is coordinated by eight O atoms of four pyrogallates in a chelated way and each ligand is chelated to two Zr(IV) atoms. In MIL-154 the Zr(IV) atom is coordinated by five O atoms of three gallate ligands, two O atoms of salicylate ligands, and one from the oxygen of DMF [21].

2.3 Sc-Based MOFs The lightest member of the rare earth family is scandium which is the first transition metal in the periodic table. The position of scandium causes multiple characteristics for this metal. There is a little number of Sc-based MOFs compared with other metals because of the high cost and the scarcity of the element. Contrarily to these reasons scientists have a great tendency to design and synthesize Sc-based MOFs due to their high thermal stability and porosity. Following, there are some examples of Sc-base MOFs [22]. In a study by Ram R. R. Prasad et al., they reported STA-27 a porous Lewis acidic scandium MOF with an unexpected topology as a reusable catalyst for organic

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Fig. 4 Left: view of the crystal structure for the series of MIL-140(Zr) solids along the c-axis; Right: view of the inorganic subunit of MIL-140. Zr atoms or polyhedra blue, C black, and Cl green. Oxygen atoms from the linker are red and oxo groups are gray. Cl atoms in MIL-140D are disordered with a 50% site occupancy. This figure was adapted by permission from Angewandte Chemie, 2012, 124, 1–6 [20]

C–C and C = N forming reactions. The ScO6 octahedra in STA-27 are linked by bridging carboxylate groups from the tetratopic carboxylate linker. Each Sc3+ atom is coordinated by four carboxylate O atoms and a terminal O ligand. STA-27 possesses a 1D rod SBU which is new in trivalent metal MOFs (Fig. 5a–e). In this rod, cornersharing dimers of two ScO6 octahedra (formula Sc2 O11 ) are linked to give a MOF that has two distinct types of parallel diamond-shaped channels [23]. J. Antonio Zárate et al. reported Sc-based MOF as a favorable adsorbent for SO2 capture with special chemical stability toward SO2 , the ability to recycle involving a considerable facile regeneration at room temperature. In MFM-300(Sc), each Sc(III) center is octahedrally coordinated to six O-donors, four from different carboxylate groups of BPTC ligand (BPTC = biphenyl-3,3’,5,5’-tetracarboxylate), and two from two different μ-OH groups. MFM-300(Sc) represents an overall 3D-framework structure with a channel of 8.1 Å [24]. In a study by Ilich A. Ibarra and colleagues, they synthesized NOTT-400 and NOTT-401 based on a binuclear [Sc2 (μ2 -OH)(O2 CR)4 ] SBU. NOTT-400 crystallizes in the chiral tetragonal space group, Each Sc(III) center is octahedrally coordinated with six O-donors, four from four different carboxylate ligands and two from μ-OH

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Fig. 5 Crystal structure of STA-27. a Sc2O11 dimer of STA-27, b 1D rod SBU of STA-27 made up of connected Sc2 O11 dimers, c two distinct diamond-shaped pore channels of STA-27 viewed along the z-axis, d STA-27 connected via TCPP viewed along x-axis, e rod SBUs of STA-27 connected via TCPP viewed along the y-axis. Hydrogen atoms are omitted for clarity. Color codes: Sc, lavender; N, light blue; C, black; O, red. This figure was adapted by permission from Journal of Materials Chemistry A, 2019, 7, 5685–5701 [23]

groups that bridge two Sc(III) centers. There are several reports about the applications of NOTT-400 as gas capture and drug delivery. NOTT-401 crystallizes in the tetragonal crystal system. The Sc(III) center adopts a similar octahedral environment to NOTT-400, with four O-donors from four different thiophene carboxylate ligands and two μ-OH groups. The difference between NOTT-400 and NOTT-401 is their ligands. In NOTT-400, they employed H4 BPTC (biphenyl-3,3’,5,5’-tetracarboxylic acid) as a ligand and in NOTT-401 they used H2 TDA (thiophene-2,5-dicarboxylic acid) as a ligand (Fig. 6a) [25].

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Fig. 6 a Views of the coordination environment of binuclear Sc(III) centers with [BPTC]4− and [TDA]2− ligands (scandium: green; sulfur: yellow; oxygen: red; carbon: gray; hydrogen: small gray). This figure was adapted by permission from Chemical Communications, 2011, 29, 8304– 8306 [25]. b Left: connectivity of ScO6 octahedral nodes by bridging carboxylate groups; right: connectivity of dF-BTC3− ligand to six Sc3+ cations; color code Sc, O, C and F are presented as white, red, dark gray and green spheres respectively. This figure was adapted by permission from Z. Anorg. Allg. Chem., 2021, 674, 490–495 [26]

Tim Mattick et al. reported a new Sc-based MOF by solvothermal reaction process in an ethanol/water solvent mixture. They utilize 2,4-difluoro-1,3,5benzenetricarboxylate (dF-BTC3− ) as a linker. In UoC-4, two Sc(III) centers coordinate with each of the three carboxylate groups of the linker, resulting ScO6 octahedron. The connection between ScO6 and dF-BTC3− building units provided a 3D-framework structure in the tetragonal space group (Fig. 6b) [26].

2.4 Mn-Based MOFs Ru-Jin Li and colleagues introduced a highly stable MOF with a rod SBU called ROD6 with CO2 uptake behavior. They utilized 1,3,6,8-tetrakis(p-benzoic acid)pyrene

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(H4 TBAPy), as a tetratopic linker with a pyrene core. The 3-periodic framework of ROD-6 consists of 1-periodic rod SBUs linked by the tetratopic linker, with nanoscale one-dimensional channels viewed along the direction. One of the Mn(II) ions is coordinated with two H2 O. All the pyrene cores are arranged within two sets of intersecting planes (dihedral angle 88.31°), giving a much closer contact (4.32 Å) between the pyrene cores. The 1-periodic rod SBU can be observed as a zigzag ladder, and the tetratopic linker with two branch points is simplified into two linked triangles (Fig. 7a) [27]. Erpan Zhang and his colleagues reported new Mn-based MOFs with a twodimensional structure with H3 bidc (1H-benzimidazole-5,6-dicarboxylic acid) as a ligand. In the as-prepared MOF, Mn(II) is five coordinated and each MN(II) is coordinated with three carboxylate oxygen atoms from three different ligands, one oxygen atom from the coordinated water molecule, and one nitrogen atom from the benzimidazole causing distorted tetragonal pyramidal geometry (Fig. 7b). The Hbidc2− ligand represents a kind of scarce coordination, the nitrogen atom and two oxygen atoms coordinate to four different Mn(II) centers, while the protonated nitrogen atom and the carbonyl oxygen atom in each linker is not coordinated to any Mn(II). This MOF with five-coordinated Mn(II) represents a layered arrangement, in which the hydrogen bonding interactions between uncoordinated carboxylic oxygen atoms and N–H groups in the imidazole rings cause the final 3D supramolecular architecture [28]. In a study by Yike Huang and colleagues, they introduced Mn-based MOF which could be used as a great Pb2+ sensor in an aqueous solution. A flexible Mn-based MOF named Mn-sdc-1 converts to a new phase (Mn-sdc-2) with a totally different structural geometry when induced by a little amount of water at room temperature. By checking the effect of temperature and the volume of water in the conversion of Mn-sdc-1 and Mn-sdc-2, the first H2 O-temperature phase diagram in MOFs was introduced. Mn-sdc-1 presents trinuclear Mn3 (RCOO)6 SBUs with one central octahedrally coordinated Mn2+ ion and two terminal symmetry equivalent sevencoordinated Mn2+ ions. With its two carboxylate groups every sdc2− ligand coordinates with two separate SBUs. Interlaced sdc2− ligands coordinate neighbor SBUs to make a 1D chain along the b-axis, which extends in a 2D hexagonal pinwheel layer along the c-axis. These chains also stack together to consist of a staggered ABC pattern geometry along the c-axis. In Mn-sdc-2 all Mn2+ atoms are six-coordinated with four O atoms from four sdc2− ligands and two O atoms from guest molecules to complete the octahedral geometry. Each sdc2− ligand coordinates with four individual Mn2+ ions and two adjacent Mn2+ octahedrons which cause different inclination angles in the space. Viewed along the a-axis, these separate Mn2+ octahedrons are linked by sdc2− to form 2D planes, plus connected by sdc2− ligands along the a-axis leading to a 3D framework. The structures of Mn-sdc-1 and Mn-sdc-2 are entirely different. When induced by water, the trinuclear Mn2+ clusters in Mn-sdc-1 are demolished and the lattice endures a rearrangement through the rotation of H2 sdc and the breakage and reconstruction of Mn–O bonds, causing the mononuclear Mn2+ octahedral polyhedron of Mn-sdc-2 (Fig. 7c) [29].

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Fig. 7 a Structure of ROD-6. Left: Deconstruction of the Mn-based rod SBU into a zigzag ladder and the tetracarboxylate into two linked triangles. Right: The overall framework of ROD-6. This figure was adapted by permission from Chemical Communications, 2014, 50, 4047–4049 [27]. b The crystal structure of MOF with a view along c-axis. SBUs are emphasized by blue tetrahedron. Nitrogen atoms are depicted as green spheres, carbon atoms as black spheres, and oxygen atoms as red spheres. Hydrogen atoms are omitted for clarity. The ligands connecting with the same SBU are highlighted in red color. This figure was adapted by permission from Chinese Journal of chemistry. 2016, 34, 233–238 [28]. c left: Coordination environments of Mn2+ ions in Mn-sdc-1, showing the trinuclear Mn2+ clusters bridged by six sdc2− ligands; and Central projection view of Mn-sdc-1 along the c-axis, showing a staggered ABC structural pattern stacked by hexagonal pinwheel layers. The hydrogen atoms are omitted for clarity; Right: Coordination environments of Mn2+ ions in Mnsdc-2, showing the octahedral coordination mode; and 3D structure of Mn-sdc-2 along c-axis. The hydrogen atoms are omitted for clarity. This figure was adapted by permission from Chemistry-A European Journal. 2018, 24, 13,231–13,237 [29]

2.5 Alkali Metal-Based MOFs Among diverse metal centers for designing and synthesizing new MOFs, alkali metals gained considerable attention due to their inexpensive, nontoxic, and biocompatibility. The following there are some examples of alkali-metal-based SBUs [30].

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José Manuel Seco and colleagues introduced a new K-based MOF and study its photoluminescence activity at room temperature. For synthesizing this threedimensional metal–organic framework they used 3,4,9,10-perylenetetracarboxylic acid (H4 ptca) as a linker and potassium as the metallic center. The as-prepared MOF crystallizes in the trigonal space group P-3. The 3D-MOF structure is due to potassium atoms bridged by (ptca)4− linkers and hydronium cations. In this MOF, potassium ions are coordinated by the eight oxygen atoms relating to the four carboxylate groups. As illustrated in Fig. 8a linker represents a coordination mode that bridges 12 potassium atoms. With a view to other articles, this coordination mode related to the (ptca)4− linker is observed for the first time [31]. Andrew Clough and his colleagues reported two new kinds of lithium clusters, Li4 tetramer and Li2 dimer as the building blocks of novel MOFs named CPM-45 and CPM-46. CPM-45 crystallizes in the noncentrosymmetric cubic with a squareplanar Li4 cluster. Square-planar clusters consist of an oxo (O2− ) or hydroxyl (OH− ) species at the center that helps to collect metal ions together. Each edge of the square is coordinated with a carboxyl group which causes a neutral Li4 (COO)4 unit. The Li4 (COO)4 unit in CPM-45 still has two residual coordination sites per Li for a total of eight coordination sites which provide just two additional − COO groups, set above and below the Li4 square, to complete all these eight coordination sites, creating [Li4 (COO)6 ]2− building blocks (Fig. 8b). In [Li4 (COO)6 ]2− , four edges connected H3 BTC(1,3,5-benzene tricarboxylic acid) ligands alternate above and below the Li4 plane, while two face-connected BTC ligands are orthogonally oriented to each other. On the basis of these findings, Li4 clusters act as monomeric 6-connected M4+ ions, which are not often seen in MOFs. In CPM-46, every Li2 dimer operates as a 6connected node linked by tritopic BTB (1,3,5-tri(4-carboxyphenyl)benzene) ligands to make a new (3,6)-connected 3D framework with 1D open channels along the caxis (Fig. 8c). Four carboxylic groups around each Li2 SBU are protonated and they bond to Li+ ions in the monodentate mode, while the residual two carboxylic groups are deprotonated and they bond to Li+ ions in the bidentate mode, resulting in the construction of the dimeric Li2 SBU in CPM-46 [32].

3 Conclusion In conclusion, the structure, physical, mechanical, morphological properties, and application of MOFs could be changed with the variation of SBUs. SBU is the metal parts of MOFs that are placed in the node category. In this chapter, we discuss several SBUs with monovalent, divalent, trivalent, and tetravalent metals. Each SBU with a special linker has a specific characteristic that causes a huge number of possible MOFs with multiple applications and usage in lots of fields.

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Fig. 8 a Coordination mode exhibited by 3,4,9,10-perylene-tetracarboxylate linker. This figure was adapted by permission from Scientific Reports. 2018, 8, 14,414 [31]. b left: View of Li4 SBUs in CPM-45; Right: The orientation of six btc molecules around each Li4 SBU in CPM-45. c left: Li2 SBUs in CPM-46; Right: The orientation of six btc molecules around each Li2 SBU in CPM-46. This figure was adapted by permission from Crystal Growth & Design, 2014, 14, 897–900 [32]

References 1. Zhou, H.-C., Kitagawa, S.: Metal-organic frameworks (MOFs). Chem. Soc. Rev. 43, 5415–5418 (2014). https://doi.org/10.1039/C4CS90059F 2. Ha, J., Lee, J.H., Moon, H.R.: Alterations to secondary building units of metal–organic frameworks for the development of new functions. Inorg. Chem. Front. 7, 12–27 (2020). https://doi. org/10.1039/C9QI01119F

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3. Zhou, H.-C., Long, J.R., Yaghi, O.M.: Introduction to metal-organic frameworks. Chem. Rev. 112, 673–674 (2012). https://doi.org/10.1021/cr300014x 4. Bosch M, Yuan S, Zhou H-C (2016) Group 4 metals as secondary building units: Ti, Zr, and Hf-based MOFs. In: Chemistry of Metal-Organic Frameworks: Synthesis, Characterization, and Applications, pp. 137–170. https://doi.org/10.1002/9783527693078.ch6 5. Zhu, J., Li, P.-Z., Guo, W., Zhao, Y., Zou, R.: Titanium-based metal–organic frameworks for photocatalytic applications. Coord. Chem. Rev. 359, 80–101 (2018). https://doi.org/10.1016/j. ccr.2017.12.013 6. Li, L., Wang, X., Liu, T., Ye, J.: Titanium-based MOF materials: from crystal engineering to photocatalysis. Small Methods 4, 2000486 (2020). https://doi.org/10.1002/smtd.202000486 7. Serre, C., Férey, G.: Hybrid open frameworks. 8. hydrothermal synthesis, crystal structure, and thermal behavior of the first three-dimensional Titanium(IV) diphosphonate with an open structure: Ti3 O2 (H2 O)2 (O3 P−(CH2 )−PO3 )2 ·(H2 O)2 , or MIL-22. Inorg. Chem. 38, 5370–5373 (1999). https://doi.org/10.1021/cm980781r 8. Cadiau, A., Kolobov, N., Srinivasan, S., Goesten, M.G., Haspel, H., Bavykina, A.V., Tchalala, M.R., Maity, P., Goryachev, A., Poryvaev, A.S., Eddaoudi, M., Fedin, M.V., Mohammed, O.F., Gascon, J.: A titanium metal-organic framework with visible-light-responsive photocatalytic activity. Angew. Chem. 132, 13570–13574 (2020). https://doi.org/10.1002/anie.202000158 9. Wang, S., Cabrero-Antonino, M., Navalón, S., Cao, C., Tissot, A., Dovgaliuk, I., Marrot, J., Martineau-Corcos, C., Yu, L., Wang, H., Shepard, W., García, H., Serre, C.: A robust titanium isophthalate metal-organic framework for visible-light photocatalytic CO2 methanation. Chem 6, 3409–3427 (2020). https://doi.org/10.1016/j.chempr.2020.10.017 10. Sun, Y., Lu, D.-F., Sun, Y., Gao, M.-Y., Zheng, N., Gu, C., Wang, F., Zhang, J.: Large TitaniumOxo clusters as precursors to synthesize the single crystals of Ti-MOFs. ACS Mater. Lett. 3, 64–68 (2020). https://doi.org/10.1021/acsmaterialslett.0c00456 11. Nguyen, H.L., Gándara, F., Furukawa, H., Doan, T.L.H., Cordova, K.E., Yaghi, O.M.: A titanium-organic framework as an exemplar of combining the chemistry of metal– and covalentorganic frameworks. J. Am. Chem. Soc. 138, 4330–4333 (2016). https://doi.org/10.1021/jacs. 6b01233 12. Nguyen, H.L., Vu, T.T., Le, D., Doan, T.L.H., Nguyen, V.Q., Phan, N.T.S.: A Titanium-organic framework: engineering of the band-gap energy for photocatalytic property enhancement. ACS Catal. 7, 338–342 (2016). https://doi.org/10.1021/acscatal.6b02642 13. Bai, Y., Dou, Y., Xie, L.-H., Rutledge, W., Li, J.R., Zhou, H.-C.: Zr-based metal–organic frameworks: design, synthesis, structure, and applications. Chem. Soc. Rev. 45, 2327–2367 (2016). https://doi.org/10.1039/C5CS00837A 14. Bon, V., Senkovska, I., Baburin, I.A., Kaskel, S.: Zr- and Hf-based metal-organic frameworks: tracking down the polymorphism. Crystal Growth Des. 13, 1231–1237 (2013). https://doi.org/ 10.1021/cg301691d 15. Feng, D., Gu, Z.-Y., Li, J.-R., Jiang, H.-L., Wei, Z., Zhou, H.-C.: Zirconium-Metalloporphyrin PCN-222: mesoporous metal-organic frameworks with ultrahigh stability as biomimetic catalysts. Angew. Chem. Int. Ed. 51, 10307–10310 (2012). https://doi.org/10.1002/anie.201 204475 16. Feng, D., Chung, W.-C., Wei, Z., Gu, Z.-Y., Jiang, H.-L., Chen, Y.-P., Darensbourg, D.J., Zhou, H.-C.: Construction of Ultrastable Porphyrin Zr metal-organic frameworks through linker elimination. J. Am. Chem. Soc. 135, 17105–17110 (2013). https://doi.org/10.1021/ja408084j 17. Feng, D., Jiang, H.-L., Chen, Y.-P., Gu, Z.-Y., Wei, Z., Zhou, H.-C.: Metal-organic frameworks based on previously unknown Zr8/Hf8 cubic clusters. Inorg. Chem. 52, 12661–12667 (2013). https://doi.org/10.1021/ic4018536 18. Koschnick, C., Stäglich, R., Scholz, T., Terban, M.W., Mankowski, A., Savasci, G., Binder, F., Schökel, A., Etter, M., Nuss, J., Siegel, R., Germann, L.S., Ochsenfeld, C., Dinnebier, R.E., Senker, J., Lotsch, B.V.: Understanding disorder and linker deficiency in porphyrinic zirconiumbased metal–organic frameworks by resolving the Zr8O6 cluster conundrum in PCN-221. Nat Commun 12(1), 3099 (2021). https://doi.org/10.1038/s41467-021-23348-w

30

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19. Taddei, M., Costantino, F., Vivani, R., Sabatini, S., Cohen, L.-H., SM,: The use of a rigid tritopic phosphonic ligand for the synthesis of a robust honeycomb-like layered zirconium phosphonate framework. Chem. Commun. 50, 5737–5740 (2014). https://doi.org/10.1039/c4c c01253d 20. Guillerm, V., Ragon, F., Dan-Hardi, M., Devic, T., Vishnuvarthan, M., Campo, B., Vimont, A., Clet, G., Yang, Q., Maurin, G., Férey, G., Vittadini, A., Gross, S., Serre, C.: A series of isoreticular, highly stable, porous zirconium oxide based metal-organic frameworks. Angew. Chem. 124, 9401–9405 (2012). https://doi.org/10.1002/anie.201204806 21. Cooper L, Guillou N, Martineau C, Elkaim E, Taulelle F, Serre C, Devic T (2014) Zr IV coordination polymers based on a naturally occurring phenolic derivative. Eur. J. Inorg. Chem. 6281–6289. https://doi.org/10.1002/ejic.201402891 22. Gupta, N.K., Osorio-Toribio, G., Hernández, M., Percástegui, E.G., Lima, E., Ibarra, I.A.: Sc(III)-based metal–organic frameworks. Chem. Commun. 58, 4116–4131 (2022). https://doi. org/10.1039/D1CC05768E 23. Prasad, R.R.R., Seidner, S.E., Cordes, D.B., Lozinska, M.M., Dawson, D.M., Thompson, M.J., Düren, T., Chakarova, K.K., Mihaylov, M.Y., Hadjiivanov, K.I., Hoffmann, F., Slawin, A.M.Z., Ashbrook, S.E., Clarke, M.L., Wright, P.A.: STA-27, a porous Lewis acidic scandium MOF with an unexpected topology type prepared with 2,3,5,6-tetrakis(4-carboxyphenyl)pyrazine. J. Mater. Chem. A 7, 5685–5701 (2019). https://doi.org/10.1039/C8TA10610J 24. Zárate, J.A., Sánchez-González, E., Williams, D.R., González-Zamora, E., Martis, V., Martínez, A., Balmaseda, J., Maurin, G., Ibarra, I.A.: High and energy-efficient reversible SO2 uptake by a robust Sc(III)-based MOF. J. Mater. Chem. A 7, 15580–15584 (2019). https://doi.org/10. 1039/C9TA02585E 25. Ibarra, I.A., Yang, S., Lin, X., Blake, A.J., Rizkallah, P.J., Nowell, H., Allan, D.R., Champness, N.R., Hubberstey, P., Schröder, M.: Highly porous and robust scandium-based metal–organic frameworks for hydrogen storage. Chem. Commun. 47, 8304–8306 (2011). https://doi.org/10. 1039/C1CC11168J 26. Mattick, T., Smets, D., Christoffels, R., Körtgen, L., Tobeck, C., Ruschewitz, U.: UoC-4: a MOF based on octahedral ScO6 nodes and fluorinated trimesate ligands. Z. Anorg. Allg. Chem. 647, 490–495 (2021). https://doi.org/10.1002/zaac.202000432 27. Li, R.-J., Li, M., Zhou, X.-P., Li, D., O’Keeffe, M.: A highly stable MOF with a rod SBU and a tetracarboxylate linker: unusual topology and CO2 adsorption behaviour under ambient conditions. Chem. Commun. 50, 4047–4049 (2014). https://doi.org/10.1039/C3CC49684H 28. Zhang, E., Zhang, L., Tan, Z., Ji, Z., Li, Q.: Mn-based two dimensional metal-organic framework material from Benzimidazole-5,6-dicarboxylic acid. Chin. J. Chem. 34, 233–238 (2016). https://doi.org/10.1002/cjoc.201500636 29. Huang, Y., Zhang, J., Yue, D., Cui, Y., Yang, Y., Li, B., Qian, G.: Solvent-triggered reversible phase changes in two manganese-based metal-organic frameworks and associated sensing events. Chem. Eur. J. 24, 13231–13237 (2018). https://doi.org/10.1002/chem.201801821 30. Ye, G., Chen, C., Lin, J., Peng, X., Kumar, A., Liu, D., Liu, J.: Alkali /alkaline earth-based metal–organic frameworks for biomedical applications. Dalton Trans. 50, 17438–17454 (2021). https://doi.org/10.1039/D1DT02814F 31. Seco, J.M., San Sebastián, E., Cepeda, J., Biel, B., Salinas-Castillo, A., Fernández, B., Morales, D.P., Bobinger, M., Gómez-Ruiz, S., Loghin, F.C., Rivadeneyra, A., Rodríguez-Diéguez, A.: A potassium metal-organic framework based on Perylene-3,4,9,10-tetracarboxylate as sensing layer for humidity actuators. Sci Rep 8, 14414 (2018). https://doi.org/10.1038/s41598-018-328 10-7 32. Clough, A., Zheng, S.-T., Zhao, X., Lin, Q., Feng, P., Bu, X.: New lithium ion clusters for construction of porous MOFs. Cryst. Growth Des. 14, 897–900 (2014). https://doi.org/10. 1021/cg4016756

MOFs Preparation and Synthetic Approaches Fatemeh Ganjali , Simindokht Zarei-Shokat, and Farinaz Jalali

1 New Synthetic Approaches to Provide Metal–Organic Frameworks Metal–organic frameworks (MOFs) have generated a great deal of interest in research because of their rich chemical structure and potential industrial usages in fields such as gas segregation and storage, catalysis, drug delivery, chemical sensors, and magnetics. The synthesis of MOF materials through practical and eco-friendly approaches is strongly recommended for the improvement of chemistry in metal organic frameworks [1]. Porous materials are of great significance for the advancement of modern industry because of their significant tasks in a variety of technical usages [2]. Apart from inorganic zeolites or zeolite analogs widely studied, metal– organic frameworks (MOFs), as a new class of crystalline porous organic–inorganic hybrid substances, have received considerable attention during recent decades [3, 4]. Organic linkers provide more options for changing the structure and MOF physicochemical features [5, 6]. Schematic illustration of the unique features, common application, synthesis mechanism, and methods of MOFs can be seen in Fig. 1. Because of these features, many MOF materials were synthesized [7–12], notably some classical MOFs such as MOF-5, HKUST-1 MIL-101, UiO-66 that is shown in Fig. 2 [13–16]. Innovation in synthetic methodology with the aim of advancement of new MOF materials remains basic research in chemistry [17]. Hydrothermal and solvothermal techniques are two synthetic methods in order to prepare metal organic frameworks that involved the thermal treatment of metallic salts and organic ligands in water or organic solvents in autoclaves under autogenic pressure which is demonstrated in Fig. 3 [18].

F. Ganjali (B) · S. Zarei-Shokat · F. Jalali Catalysts and Organic Synthesis Research Laboratory, Department of Chemistry, Iran University of Science and Technology, 16846-13114 Tehran, Iran e-mail: [email protected]; [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 A. Maleki and R. Taheri-Ledari (eds.), Physicochemical Aspects of Metal-Organic Frameworks, Engineering Materials, https://doi.org/10.1007/978-3-031-18675-2_3

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Fig. 1 Schematic illustration of the unique features, common application, synthesis mechanism, and methods of MOFs. This figure was adapted by permission from Materials 2020, 13(12), 2881; [https://doi.org/10.3390/ma13122881]

Certain organic solvents may be broken down or hydrolyzed into other chemicals under high-temperature situations [17]. Many solvent systems utilized to make crystalline organic metal frames, including ionic fluids, deep eutectic solvents, and surfactants over the past 10 years. The melting point of ionic fluids (ILs) is less than 100 °C which differs significantly from conventional molecular solvents because of their excellent features such as low vapor pressure, non-flammability, high thermal stability, and high ionic conductivity [19–23]. Deep eutectic solvents consist of blends of various tetramerous ammonium halide salts as well as different hydrogen bond givers. Although they are inexpensive, their features are the same as ILs [24, 25]. Surfactants were utilized as a reaction media for the synthesis of novel MOFs. Because of surfactants multi-functional types, containing acidic, basic, neutral, cationic, and anionic they prepare more options to the reaction systems [26]. The most environmentally friendly reaction is solventless mechanochemistry, there is not any solvent during the synthesis which is lead to being affordable and contamination decline [27, 28].

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Fig. 2 Some examples of different structures of MOFs. This figure was adapted by permission from Elsevier 2017, 2(3), 218–245; [https://doi.org/https://doi.org/10.1016/j.gee.2017.05.003]

2 Ionothermal Method Ionic liquids have many exceptional physicochemical characteristics which are very diverse from traditional molecular solvents, like slight vapor pressure, nonflammability, superior thermal stability, great ionic conductivity, and excellent polarity for dissolving inorganic salts and organic ligands. These properties make them a good replacement for molecular solvents and then they are often used in the fields of electrochemistry, lubrication, exploitation of solvent, and catalysis [19]. In the ionothermal synthesis of MOFs, the mechanisms of reaction among ligands and ions can be distinguished from molecular environments [29]. ILs have a multi-purpose role in forming MOF structures including: (1) convey reactants; (2) counterions, constructional agents, or templates for both cations and anions of ILs; (3) ILs anions may be used as reagents by co-ordination with metallic

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Fig. 3 Illustration of different MOF techniques; conventional and microwave-assisted hydrothermal methods are the most commonly applied approaches, while electrochemical, mechanochemistry, and sonochemistry are quite new methods in the synthesis of MOFs (a). A variety of temperatures from room conditions to solvothermal situations are used in the fabrication of MOFs (b). This figure was adapted by permission from J. Compos. Sci. 2020, 4(2), 75; https:// doi.org/10.3390/jcs4020075

ions; and (4) ILs chirality and hydrophilicity/hydrophobicity features have a significant impact on framework structures as-formed [30]. Because of great conductivity and polarity, ILs can be utilized as a kind of high-quality absorbers for microwave synthesis [29]. ILs components also influenced on the thermal stability and fluorescence features of the MOFs for as-synthesized [31, 32] (Fig. 4 and Table 1).

Fig. 4 Species that are usually considered as cations, anions, and substituents for designing taskspecific ionic liquids. This figure was adapted by permission from Int. J. Mol. Sci. 2014, 15(9), 15,320–15,343; https://doi.org/10.3390/ijms150915320

MOFs Preparation and Synthetic Approaches Table 1 Unique properties of ionic liquids. This table was adapted by permission from Int. J. Mol. Sci. 2014, 15(9), 15,320–15,343; https://doi. org/10.3390/ijms150915320

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Properties

Values

Melting point

Preferably below 100 °C

Liquidus range

Often > 200 °C

Thermal stability

Usually high

Viscosity

Normally < 100 cP, workable

Dielectric constant

Implied < 30

Polarity

Moderate

Ionic conductivity

Usually < 10 mS/cm

Molar conductivity

4 V

Vapor pressure

Usually negligible

3 Deep Eutectic Solvent Usage Ammonium halide salts and various hydrogen bond donors are the composition of deep eutectic solvents and the interactions between them are because of the melting point decline of the deep eutectic combination [24]. Some physicochemical characteristics of deep eutectic solvents like low vapor pressure, high ionic conductivity, and polarity are the same as ILs. There are remarkable benefits to deep eutectic solvents based on choline chloride: (1) constituents such as urea and choline chloride are significantly less expensive than some ILs. (2) Blending two components of deep eutectic mixtures with no regard for purity; (3) These solvents are pretty unaffected by air humidity; (4) They are green because of their biodegradable and non-toxic essence [25, 33]. In fact, deep eutectic solvents applications are in the area of material synthesis, electrochemistry, dissolution, and extraction processes [33–36].

4 Surfactant-Thermal process Surfactants are typically made from organic combinations which consist of both hydrophilic and hydrophobic groups. Polar groups are soluble in water and insoluble in oil, whereas hydrophobic groups prefer to be soluble in oil [37]. In a proven fact, surfactants not only have many notable solvent features, like minor vapor pressure, high thermal stability but also have multifunctional properties, such as acidic, basic, neutral, cationic, and anionic as a result it prepares more options for the reaction systems of MOFs. In comparison with ILs, surfactants are inexpensive, making them suitable for large-scale synthetic approaches [38]. The thermal surfactant environments differed greatly from the solvothermal situations [39] (Fig. 5).

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Fig. 5 Structures of some halide salts (HBAs) and hydrogen bond donors (HBDs) used in the formation of deep eutectic solvents. This figure was adapted by permission from Molecules 2020, 25(24), 6026 [https://doi.org/10.3390/molecules25246026]

5 Mechanochemistry Mechanochemistry linked to a chemical reaction driven by mechanical force that may be obtained by grinding two or more solid ingredients into a pestle and mortar [28– 40]. However, in order to guarantee reproducibility and to control the speed and the grinding time, different electric grinders are utilized in mechanochemical synthesis. It is also an environmentally friendly synthetic method in polymer science, organic and inorganic synthesis [41, 42]. Because of some aspects like short reaction times (5–60 min), low cost and perform at room temperature synthesis a lot of researchers chose this method [43].

6 MOFs Synthetic Approaches and Potential Usage MOFs are a class of porous materials that construct from metal ions or oligonuclear metal and organic ligand structures. They may be regarded as subcategories of coordinating polymers and may be developed into one dimension and two or three dimensions. Metal organic frameworks are active systems that are sensitive to structural shifts caused by external stimuli like temperature, pressure and cannot be crystalline.

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The synthesis of metal organic frameworks needs full control of the ingredients and implements to be utilized, which is supported by numerous researchers who think that full control over all the parameters of a chemical reaction is impossible [44].

7 Synthesis of MOFs The synthesis of metal organic frameworks is specified via numerous factors associated with reaction time, temperature and the solvent usage, the character of organic ligands and metallic ions, the size and structure of the nodes, and crystallization which is expected for crystal growth and nucleation. In many cases, their synthesis is carried out in a liquid phase by blending ligand and metallic salt solutions. Mainly, metal–organic frameworks are synthesized via the classic procedure in solvothermal situations at elevated temperatures and pressure. There are other synthetic approaches like mechano-chemical, electrochemical, microwave, and sono-chemical in the last few years which are inexpensive, quicker, and ecofriendly [45, 46] (Fig. 6). (A) Slow evaporation and diffusion techniques These methods run at room temperature and do not require any power supply. In the slow evaporation system, the reagent solutions are blended and evacuated for this purpose, and crystals are organized when an acute concentration is attained. The blend of solvents with a downward boiling point frequently expedites the procedure [47, 48].

Fig. 6 Synthetic methods for the production of MOFs. This figure was adapted by permission from Materials 2021, 14(2), 310 [https://doi.org/10.3390/ma14020310]

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During the diffusion technique, reagent solutions are positioned over each other, detached from a substrate of solvent, or progressively spread through physical obstacles. Gels are sometimes utilized for crystallization and dissemination [49]. In addition, this method is used for products in cases of little solubility [13]. (B) Solvo(Hydro)-Thermal and Iono-Thermal technique Solvo(hydro) thermal reactions are performed in containers with an autogenic pressure over the boiling point of the solvent which is used for many MOFs [18]. Reactions are generally made in polar solvents with closed containers (autoclaves) at temperatures between 50 and 260° centigrade and need a long time (hours or days). In fact, for temperatures over 400° centigrade, teflon-coated autoclaves are utilized. Furthermore, temperature influences crystal morphology, whereas extended reaction times can cause the final product to break down [50, 51]. Solvents with high boiling point are most common which includes dimethyl formamide (DMF), diethyl formamide (DEF), MeCN, MeOH, EtOH, H2 O, Me2 CO, or their blends. Under solvo(hydro)thermal conditions, the original reagents can endure accidental chemical changes, which are not carried out beneath softer synthetic status, leading to the foundation of novel ligands in situ. Ionothermal synthesis relies on the usage of ionic liquids as solvents and models and may be regarded as a subcategory of hydrothermal approaches. Ionic liquids are eco-friendly reagents due to their particular properties such as low vapor pressure, high solubility, and thermal stability which forms them a great substitute for metal organic frameworks synthesis [1]. (C) Microwave-Assisted technique It is frequently utilized to synthesize inorganic and organic compounds and also metal clusters and metal organic frameworks [52–54]. There are some benefits that could be taken into account like the reaction time needed, the high efficiency, and the low cost [55]. Indeed, microwaves simplify the molecules’ movement, resulting in nucleation and crystal formation with a controlled form and size by correctly adjusting the concentration and temperature [56]. (D) Mechanochemical technique This method needs mechanical power in order to bonding formation through manual grinding or auto ball mills reagents. This procedure simplifies mass transition, diminishes the size of the particle, heats, and melts the reactants, as a result, boosting the reaction time (RT). It is an ecofriendly technique that generates substances with high purity and yield in a short RT. The isolation of amorphous products is not appropriate for the structure of a single X-ray survey which is the greatest drawback of this method [57]. (E) Electrochemical technique The MOF powders are industrially utilized via this method. The metallic ion is supplied through anodic dissolution in reaction blends that contain organic ligands and electrolytes. The main benefits of this way are the lower temperatures and extremely rapid synthesis over softer situations, compared with the hydrothermal

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one. Many MOFs, namely, HKUST-1, ZIF-8, MIL-100(Al), MIL-53(Al), and NH2-MIL-53(Al), were synthesized in an electrochemical cell via this approach [58]. (F) Sonochemical Method It focuses on the chemical molecule’s changes subjected to high-energy ultrasound (20 kHz–10 MHz) [59]. (G) Microemulsion Method It is used to provide nanoparticles and synthesis of MOFs. This is a beneficial approach since the nanoscale substances’ dimensions may be checked, however, it is expensive and surfactants are not green (drawbacks) [60]. (H) Post-Synthetic Modification It is primarily a procedure of MOFs chemical conversion after they have been isolated. The method has been extensively utilized in order to provide isostructural MOFs with various physical and chemical features [61]. (I) Template Strategies The template molecules usage in the reaction blend makes new MOFs, which are hard to achieve through conventional synthetic approaches. They are small organic molecules containing organic solvents and amines, carboxylic acids, N-heterocyclic aromatic compounds, ion liquids, surfactants, etc. Every compound influences the MOFs crystallization and synthesis in a various way [62].

8 The Synthetic Strategies of 2D-MOFs Organic metal frameworks (MOFs) are now increasingly absorbing because of their diverse features and unique tasks. They are considered potential candidates for gas segregation, adsorption, chemical detection, catalysis, and much more. 2D designs based on MOFs are extended, including MOF membranes, thin films, and 2D MOFs [63]. Supramolecular building blocks and secondary building units (SBU) may be utilized to construct the MOFs structure at the building unit level instead of the main building block level [64]. In recent years, 2D architectural substances like graphene, metallic nano-sheets, and 2D hexagonal boron have distributed hierarchically structured MOF architectures into four dimensions. 2D-MOFs thickness is mostly at the atomic level, and the MOF thin films thickness may vary from nanometer to millimeter, whereas MOF membranes focus primarily on membranes having adsorption, segregation, or segregation functions. 2D-MOFs may be delaminated from thin MOF films that are made up of nano-MOF layers [65].

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9 D-Metal Organic Frameworks 2D-MOFs have been noted for their features like high conductivity, chemical tunability, and high porosity of structure, as a novel sort of 2D materials [66, 67], that typically has three, four, or six topologies of connected networks [68]. Typically, three metal coordination sites are generally included in MOF metal complexes to build the 2D structure [69, 70]. 2D polymers are now hotspots in research, especially graphene and 2D-MOFs [71].

10 Synthetic Methods of 2D-MOFs It could usually be broken down into bottom-up and top-down strategies. The bottomup strategy designates the direct assembly of metal ions and organic ligands to build MOF layers, whereas top-down focuses on exfoliating from bulk MOFs. A variety of 2D-MOF layers are related through the weak interactions, like Van der Waals forces, π-π stacking, etc. [72].

11 Langmuir–Blodgett Technique It is a workable procedure for preparing thin nanosheets with a large area on the liquid surface [73]. This technique could result in single or multiple layers of metal organic frameworks that provide a pliable way to the development of 2D-MOFs. Actually, this method is typically mixed with the layer-by-layer way to collect multiple MOF nanosheets in order to create a nanofilm. 2D MOF is initially created on the surface of the solution by the LB method, afterward a second monolayer is built to produce a multilayer nanofilm. The monolayers are joined together via linked H2 O molecules [74, 75].

12 Sonication Exfoliation Method With regard to 2D-MOF crystals, various layers are linked via weak interactions, containing hydrogen bonding and Van der Waals forces. Since weak interactions are cleaved, single-layer 2D-MOF may be achieved. And it could be done using a short sonic(liquid) exfoliation technique, that may result in 2D-MOF nanosheets like 2D MOF-2 [76]. This MOF (dried powder) has been sonicated at room temperature to perform acetone delamination. Following the sedimentation of the laminated suspension, a colloidal suspension was produced. Finally, 2D MOF-2 nanosheets may be acquired through acetone evaporation [77].

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Fig. 7 Scheme showing different bottom-up and top-down methodologies. This figure was adapted by permission from RSC, 2018, 6, 16,292–16,307 [https://doi.org/10.1039/C8TA03159B]

13 Mechanical Exfoliation Method This technique is another great way for making 2D materials, including 2D MOFs. Figure 7 demonstrates the differences between sonic and mechanical exfoliation procedures. Approximately every 2D coordination polymer may be built according to this approach. Bulk crystals may be delaminated easily by a plastic tape, and the monolayers crusts may be collected and seen [78].

14 Modulated Strategy To achieve 2D-MOFs, monodentate ligands may operate as a modulator and be used to respond with metallic ions to withstand the growth of MOF crystals. It may lead to the anisotropic growth of MOF crystals and encourage the foundation of 2D-MOFs. In order to obtain single 2D-MOF nanosheets, using monodentate ligands to get the noncoplanar haptos of metallic ions and left the coplanar ones, may result in the foundation of MOF nanosheets. This method could lead to 2D-MOFs that have the original 3D design [72].

References 1. Li, P. et al.: New synthetic strategies to prepare metal–organic frameworks. Inorganic Chem. Front. 5(11), 2693–2708 (2018) 2. Férey, G.: Hybrid porous solids: past, present, future. Chem. Soc. Rev. 37(1), 191–214 (2008) 3. Kitagawa, S., Kitaura, R., Noro, S.-I.: Functional porous coordination polymers. Angew. Chem. Int. Ed. 43(18), 2334–2375 (2004)

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4. Rowsell, Jesse, L.C., Yaghi, O.M.: Metal–organic frameworks: a new class of porous materials. Microporous Mesoporous Mater. 73(1–2), 3–14 (2004) 5. Gao, W.-Y., Chrzanowski, M., Ma, S.: Metal–metalloporphyrin frameworks: a resurging class of functional materials. Chem. Soc. Rev. 43(16), 5841–5866 (2014) 6. Yuan, S., et al.: Stable metal–organic frameworks: design, synthesis, and applications. Adv. Mater. 30(37), 1704303 (2018) 7. Long, J.R., Yaghi, O.M.: The pervasive chemistry of metal–organic frameworks. Chem. Soc. Rev. 38(5), 1213–1214 (2009) 8. Zhou, H.-C., Long, J.R., Yaghi, O.M.: Introduction to metal–organic frameworks. Chem. Rev. 112(2), 673–674 (2012) 9. Furukawa, H., et al.: The chemistry and applications of metal-organic frameworks. Science 341(6149), 1230444 (2013) 10. Kitagawa, S.: Metal–organic frameworks (MOFs). Chem. Soc. Rev. 43(16), 5415–5418 (2014) 11. Peplow, M.: The hole story. Nature 520(7546), 148–151 (2015) 12. Freund, R., et al.: Multifunctional efficiency: extending the concept of atom economy to functional nanomaterials. ACS Nano 12(3), 2094–2105 (2018) 13. Li, H., et al.: Design and synthesis of an exceptionally stable and highly porous metal-organic framework. Nature 402(6759), 276–279 (1999) 14. Chui, Stephen, S.-Y., et al.: A chemically functionalizable nanoporous material [Cu3 (TMA) 2 (H2O) 3] n. Science 283(5405), 1148–1150 (1999) 15. Férey, G., et al.: A chromium terephthalate-based solid with unusually large pore volumes and surface area. Science 309(5743), 2040–2042 (2005) 16. Cavka, J.H., et al.: A new zirconium inorganic building brick forming metal organic frameworks with exceptional stability. J. Amer. Chem. Soc. 130(42), 13850–13851 (2008) 17. Qiu, S., Xue, M., Zhu, G.: Metal–organic framework membranes: from synthesis to separation application. Chem. Soc. Rev. 43(16), 6116–6140 (2014) 18. Stock, N., Biswas, S.: Synthesis of metal-organic frameworks (MOFs): routes to various MOF topologies, morphologies, and composites. Chem. Rev. 112(2), 933–969 (2012) 19. Welton, T.: Room-temperature ionic liquids. Solvents for synthesis and catalysis. Chem. Rev. 99(8), 2071–2084 (1999) 20. Ma, Z., Yu, J., Dai, S.: Preparation of inorganic materials using ionic liquids. Adv. Mater. 22(2), 261–285 (2010) 21. Freudenmann, D., et al.: Ionic liquids: new perspectives for inorganic synthesis? Angewandte Chemie Int. Edition 50(47), 11050–11060 (2011) 22. Rogers, R.D., Seddon, K.R.: Ionic liquids–solvents of the future? Science 302(5646), 792–793 (2003) 23. Karmakar, A., Desai, A.V., Ghosh, S.K.: Ionic metal-organic frameworks (iMOFs): Design principles and applications. Coord. Chem. Rev. 307, 313–341 (2016) 24. Ruß, C., König, B.: Low melting mixtures in organic synthesis–an alternative to ionic liquids? Green Chem. 14(11), 2969–2982 (2012) 25. Zhang, Q., et al.: Deep eutectic solvents: syntheses, properties and applications. Chem. Soc. Rev. 41(21), 7108–7146 (2012) 26. Evans, Fennell, D., Mitchell, D.J., Ninham, B.W.: Oil, water, and surfactant: properties and conjectured structure of simple microemulsions. J. Phys. Chem. 90(13), 2817–2825 (1986) 27. Beyer, M.K., Clausen-Schaumann, H.: Mechanochemistry: the mechanical activation of covalent bonds. Chem. Rev. 105(8), 2921–2948 (2005) 28. Kaupp, G.: Mechanochemistry: the varied applications of mechanical bond-breaking. Cryst. Eng. Comm. 11(3), 388–403 (2009) 29. Morris, R.E.: Ionothermal synthesis—ionic liquids as functional solvents in the preparation of crystalline materials. Chem. Commun. 21, 2990–2998 (2009) 30. Jin, K., et al.: [Cu (i)(bpp)] BF 4: the first extended coordination network prepared solvothermally in an ionic liquid solvent. Chem. Commun. 23, 2872–2873 (2002) 31. Zhang, Z.-H., et al.: Combination effect of ionic liquid components on the structure and properties in 1, 4-benzenedicarboxylate based zinc metal–organic frameworks. Dalton Trans. 44(41), 17980–17989 (2015)

MOFs Preparation and Synthetic Approaches

43

32. Zhang, Z.-H., Xu., Ling, Jiao, H.: Ionothermal synthesis, structures, properties of cobalt-1, 4-benzenedicarboxylate metal–organic frameworks. J. Solid-State Chem. 238, 217–222 (2016) 33. Francisco, M., van den Bruinhorst, A., Kroon, M.C.: Low-transition-temperature mixtures (LTTMs): a new generation of designer solvents. Angewandte Chemie Int. Edn. 52(11), 3074– 3085 (2013) 34. Jhang, P.-C., et al.: A fully integrated Nanotubular yellow-green phosphor from an environmentally friendly eutectic solvent.. Angewandte Chemie Int. Edn. 48(4), 742–745 (2009) 35. Jhang, P.-C., Chuang, N.-T., Wang, S.-L.: Layered zinc phosphates with photoluminescence and photochromism: chemistry in deep eutectic solvents. Angew. Chem. 122(25), 4296–4300 (2010) 36. Tang, B., Zhang, H., Row, K.H.: Application of deep eutectic solvents in the extraction and separation of target compounds from various samples. J. Separat. Sci. 38(6), 1053–1064 (2015) 37. Winsor, P.A.: Binary and multicomponent solutions of amphiphilic compounds. Solubilization and the formation, structure, and theoretical significance of liquid crystalline solutions. Chem. Rev. 68(1), 1–40 (1968) 38. Gao, J., et al.: Tuning metal–carboxylate coordination in crystalline metal–organic frameworks through surfactant media. J. Solid-State Chem. 206, 27–31 (2013) 39. Gao, J., et al.: A surfactant-thermal method to prepare four new three-dimensional heterometal– organic frameworks. Dalton Trans. 42(32), 11367–11370 (2013) 40. Trask, A.V., Samuel Motherwell, W.D., Jones, W.: Solvent-drop grinding: green polymorph control of cocrystallisation. Chem. Commun. 7, 890–891 (2004) 41. Frišˇci´c, T.: New opportunities for materials synthesis using mechanochemistry. J. Mater. Chem. 20(36), 7599–7605 (2010) 42. James, S.L., et al.: Mechanochemistry: opportunities for new and cleaner synthesis. Chem. Soc. Rev. 41(1), 413–447 (2012) 43. Braga, D., et al.: Mechanochemical preparation of molecular and supramolecular organometallic materials and coordination networks. Dalton Trans. 10, 1249–1263 (2006) 44. Raptopoulou, C.P.: Metal-organic frameworks: Synthetic methods and potential applications. Materials 14(2), 310 (2021) 45. Dey, C., et al.: Crystalline metal-organic frameworks (MOFs): synthesis, structure and function. Acta Crystallographica Sect. B Struct. Sci. Crystal Eng. Mater. 70(1), 3–10 (2014) 46. Safaei, M., et al.: A review on metal-organic frameworks: synthesis and applications. TrAC Trends Analyt. Chem. 118, 401–425 (2019) 47. Halper, S.R., et al.: Topological control in heterometallic metal− organic frameworks by anion templating and metalloligand design. J. Amer. Chem. Soc. 128(47), 15255–15268 (2006) 48. Du, M., Li, C.-P., Zhao, X.-J.: Metal-controlled assembly of coordination polymers with the flexible building block 4-pyridylacetic acid (Hpya). Cryst. Growth Des. 6(1), 335–341 (2006) 49. Lazarou, K.N., et al.: Network diversity and supramolecular isomerism in copper (II)/1, 2-bis (4-pyridyl) ethane coordination polymers. Polyhedron 30(6), 963–970 (2011) 50. Biemmi, E., et al.: High-throughput screening of synthesis parameters in the formation of the metal-organic frameworks MOF-5 and HKUST-1. Microporous Mesoporous Mater. 117(1–2), 111–117 (2009) 51. Millange, F., et al.: A time-resolved diffraction study of a window of stability in the synthesis of a copper carboxylate metal–organic framework. CrystEngComm 13(1), 103–108 (2011) 52. Hu, Y., et al.: Microwave-assisted hydrothermal synthesis of nanozeolites with controllable size. Microporous Mesoporous Mater. 119(1–3), 306–314 (2009) 53. Zhang, S.-H., et al.: Microwave-assisted synthesis, crystal structure and properties of a disclike heptanuclear Co (II) cluster and a heterometallic cubanic Co (II) cluster. CrystEngComm 11(5), 865–872 (2009) 54. Lin, Z.-J., et al.: Microwave-assisted synthesis of a series of lanthanide metal–organic frameworks and gas sorption properties. Inorganic Chem. 51(3), 1813–1820 (2012) 55. Seo, Y.-K., et al.: Microwave synthesis of hybrid inorganic–organic materials including porous Cu3 (BTC) 2 from Cu (II)-trimesate mixture. Microporous Mesoporous Mater. 119(1–3), 331– 337 (2009)

44

F. Ganjali et al.

56. Ni, Z., Masel, R.I.: Rapid production of metal−organic frameworks via microwave-assisted solvothermal synthesis. J. Am. Chem. Soc. 128(38), 12394–12395 (2006) 57. Frišˇci´c, T., et al.: Real-time and in situ monitoring of mechanochemical milling reactions. Nat. Chem. 5(1), 66–73 (2013) 58. Martinez Joaristi, A., et al.: Electrochemical synthesis of some archetypical Zn2+, Cu2+, and Al3+ metal organic frameworks. Crystal Growth Design 12(7), 3489–3498 (2012) 59. Kim, J., et al.: Control of catenation in CuTATB-n metal–organic frameworks by sonochemical synthesis and its effect on CO2 adsorption. J. Mater. Chem. 21(9), 3070–3076 (2011) 60. Zheng, W., et al.: Controllable preparation of nanoscale metal–organic frameworks by ionic liquid microemulsions. Indus. Eng. Chem. Res. 56(20), 5899–5905 (2017) 61. Cohen, S.M.: Postsynthetic methods for the functionalization of metal–organic frameworks. Chem. Rev. 112(2), 970–1000 (2012) 62. Zhao, N., Cai, K., He, H.: The synthesis of metal–organic frameworks with template strategies. Dalton Trans. 49(33), 11467–11479 (2020) 63. Zhu, H., Liu, D.: The synthetic strategies of metal–organic framework membranes, films and 2D MOFs and their applications in devices. J. Mater. Chem. A 7(37), 21004–21035 (2019) 64. Choi, J., et al.: A 2D Layered Metal–Organic Framework Constructed by Using a Hexanuclear Manganese Metallamacrocycle as a Supramolecular Building Block, 5465–5470 (2008) 65. Furukawa, S., et al.: Structuring of metal–organic frameworks at the mesoscopic/macroscopic scale. Chem. Soc. Rev. 43(16), 5700–5734 (2014) 66. Sun, L., Campbell, M.G., Dinc˘a, M.: Electrically conductive porous metal–organic frameworks. Angew. Chem. Int. Ed. 55(11), 3566–3579 (2016) 67. Li, W., et al.: High temperature ferromagnetism in π-conjugated two-dimensional metal– organic frameworks. Chem. Sci. 8(4), 2859–2867 (2017) 68. Yang, Q.-Y., et al.: A simple topological identification method for highly (3, 12)-connected 3D MOFs showing anion exchange and luminescent properties. Chem. Commun. 47(14), 4234– 4236 (2011) 69. Kurth, D.G., Higuchi, M.: Transition metal ions: weak links for strong polymers. Soft Matter 2(11), 915–927 (2006) 70. Spokoyny, A.M., et al.: Infinite coordination polymer nano-and microparticle structures. Chem. Soc. Rev. 38(5), 1218–1227 (2009) 71. Rodríguez-San-Miguel, D., Amo-Ochoa, P., Zamora, F.: MasterChem: cooking 2D-polymers. Chem. Commun. 52(22), 4113–4127 (2016) 72. Zhao, M., et al.: Two-dimensional metal–organic framework nanosheets: synthesis and applications. Chem. Soc. Rev. 47(16), 6267–6295 (2018) 73. Dong, R., et al.: Large-area, free-standing, two-dimensional supramolecular polymer singlelayer sheets for highly efficient electrocatalytic hydrogen evolution. Angewandte Chemie Int. Edn. 54(41), 12058–12063 (2015) 74. Motoyama, S., et al.: Highly crystalline nanofilm by layering of porphyrin metal−organic framework sheets. J. Amer. Chem. Soc. 133(15), 5640–5643 (2011) 75. Makiura, R., et al.: Surface nano-architecture of a metal–organic framework. Nat. Mater. 9(7), 565–571 (2010) 76. Wang, Z., et al.: Two-dimensional light-emitting materials: preparation, properties and applications. Chem. Soc. Rev. 47(16), 6128–6174 (2018) 77. Li, P.-Z., Maeda, Y., Xu, Q.: Top-down fabrication of crystalline metal–organic framework nanosheets. Chem. Commun. 47(29), 8436–8438 (2011) 78. Abhervé, A., et al.: Graphene related magnetic materials: micromechanical exfoliation of 2D layered magnets based on bimetallic anilate complexes with inserted [Fe III (acac 2-trien)]+ and [Fe III (sal 2-trien)]+ molecules. Chem. Sci. 6(8), 4665–4673 (2015)

MOFs Functionalization Approaches Maryam Kamalzare

Abstract Metal–organic frameworks (MOFs) are a class of crystalline materials with high porosity and three-dimensionality that have attracted considerable attention from scientists. MOFs structure are consist of inorganic (node) and organic (linker) parts. These two parts could specify the MOF properties. As a class of porous materials, MOFs have some features including high surface area, low density, high porosity, thermal stability, and adjustable chemical functionality. With regard to the application of MOFs, the design and synthesis of MOFs could be done in different ways. One of the most important and practical procedures for constructing new MOFs with special characteristics is to functionalize the existing MOFs. Keywords Pre-synthetic functionalization · Post synthetic modification · Node · Linker · Special properties

1 Introduction Metal–organic frameworks (MOFs) are a class of porous materials that have received great attention due to their considerable structural and chemical tunability. Their synthetic versatility, long-range order, and rich host–guest chemistry make MOFs perfect platforms for identifying design features for advanced functional materials. Considering that MOFs contain an organic component (the ligand or linker), they could functionalize more conveniently with various functional groups when compared to those other inorganic solids. MOFs could potentially functionalize via metal ions/clusters, organic bridging ligands, and pores (Fig. 1a). Functionalization of MOFs has some advantages including effects on the structural properties such as crystallinity, porosity, flexibility, stability, and topology of MOFs. Therefore, with the functionalization of MOFs researchers could design MOFs for specific applications [1]. M. Kamalzare (B) Catalysts and Organic Synthesis Research Laboratory, Department of Chemistry, Iran University of Science and Technology, 16846-13114 Tehran, Iran e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 A. Maleki and R. Taheri-Ledari (eds.), Physicochemical Aspects of Metal-Organic Frameworks, Engineering Materials, https://doi.org/10.1007/978-3-031-18675-2_4

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Fig. 1 a Functionalizable parts of MOFs. b Classification of organic functional groups based on their chemical characteristics and structural properties

Functionalization of MOFs could be obtained in two main ways including the self-assembly process by a selection of desirable functional building blocks (presynthesis functionalization strategy) or after synthesis of the frameworks (postsynthesis functionalization strategy). Most MOFs are synthesized via solvothermal synthetic methods which restrict the preparation of highly functionalized MOFs. Post-synthetic modification (PSM) of MOFs has attracted great attention from researchers as an alternative plan to expand the functional group scope of MOFs. Post-synthetic modification (PSM) has attracted high attention as an effective and flexible procedure to change and improve the structure and properties of MOFs. PSM methods simplify the design and synthesis of unique functional MOFs which is not easily possible to achieve via direct synthesis. PSM techniques can be classified into two main categories including coordinative PSM and covalent PSM. There are multiple functional groups that could change the chemical and physical properties of MOFs due to the functionalization process. The infinite ways for the functionalization of MOFs is an important point to control their chemical properties and host–guest interactions [2–4]. Taking into account that MOFs have special features including regular crystalline structure, high porosity, and surface area, hybrid inorganic–organic nature, structural stability, and adjustability in chemical functionality, MOFs could be applied for multiple applications of gas storage and separation [5, 6], heterogeneous catalysis [7], drug delivery [8, 9], electrochemical applications [10, 11], and photocatalysis [12], biosensing [13], removal and separation of hazardous chemicals [14], designing MOF-based energetic materials [15], and sensing [16]. On the basis of related literature most functional groups could be sorted according to their chemical properties and structural similarities including nitrogen-based functions, oxygen-based functions, sulfur-based functions, carbonyl-based functions, halogen-based functions, alkoxy-based functions, alkyl-based functions, and other

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Table 1 Some examples of functional groups and their applications Functional groups

MOFs

Applications

References

Urea

Zr-UiO-67

Friedel−Crafts Alkylation

[18]

Urea

UiO-68

Henry reactions

[19]

Amide

In-MOF

Hydrocarbon Separation and CO2 Catalytic Fixation

[20]

Amide

In-soc-MOF

Carbon dioxide adsorption

[21]

Ketone

Gd (III)-MOF

Drug Delivery

[22]

Imide

NH2 -UiO-66

Photocatalytic degradation of tetracycline

[23]

Imide

In-MOF

Adsorption of Carcinogenic Dyes

[24]

Hydroxy

mfj-type

Adsorption of CO2

[25]

Thiol

HKUST-1

Adsorption of HG2+

[26]

Imidazole

MOF-801

Proton exchange membrane

[27]

Amine

Zr-MOF/CNTs

Photocatalyst for removing organic contaminants

[28]

Amine

Cu-MOF

Electrochemical immunosensor

[29]

Chlorine

MIL-53-Fum

Gas adsorption

[30]

Alkoxy

IRMOFs

Gas storage or separation applications

[31]

Alkyl

HKUST-1

Gas adsorption and separation [32] performance

Nitro

Zn (II)-paddlewheel MOFs

Gas adsorption

[33]

functions such as phosphonate and nitro groups [1, 17] (Fig. 1b). Some examples of various functional groups and their applications are summarized (Table 1).

2 Functionalization of MOFs Produces Unique Materials with Multiple Properties for Different Applications 2.1 Functionalization of MOF for Producing Photoactive Materials Solar energy could be used to break down water and generate hydrogen and oxygen using photocatalytic materials, which is desirable for converting solar energy into chemical energy in an eco-friendly manner. When a semiconductor absorbs visible light energy, electrons are excited to the conduction band (CB), while holes are present in the valence band (VB). The electrons present in the CB combine with

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protons to produce hydrogen, and the holes combine with reductive reactants to create oxidative products. MOFs also follow a similar type of catalytic reaction. The energy levels correspond to the HOMO and LUMO [34]. Functionalization of MIL-101 (Cr) which was introduced as one of the most attractive MOFs was first synthesized by Ferey and co-workers. Hui Chang et al. designed and synthesized a series of low toxicity, environmental-friendliness photocatalysts ZIS/MIL-101/PTCs (ZIS = ZnIn2S4, MIL = Materials of Institute Lavoisier, PTCs = Polyoxo-titanium clusters) with high photocatalytic activity for H2 generation by step-by-step in situ reaction method. They integrated PTCs into ZIS/ MIL-101 and La-ZIS/MIL-101 systems and the result demonstrates that both ZIS/ MIL-101/PTCs and La-ZIS/MIL-101/PTCs photocatalysts represent enhancement in photocatalytic H2 evolution activity and this result was higher than ZIS/ MIL-101 and La-ZIS/MIL101. Important factors to boosting photocatalytic splitting water reaction are utilizing PTCs can well match energy gap and appropriate LUMO energy level for accelerating to transfer photogenerated charge. The proposed mechanism for H2 production. The photo-induced electrons produced under visible light jump from VB to CB, then the photo-induced electrons migrate to the LUMO of PTCs, in the following these electrons reach the surface of MIL-101 for splitting water. Cyclic durability experiments showed that the hydrogen production rates by all investigated photocatalysts have no significant decrease after 6 cycle experiments, showing these photocatalysts are resistant during the photocatalytic reactions [35].

2.2 Functionalization of MOFs in Drug Delivery Drug delivery is focused on the progress of unique materials or carrier systems for the effectual therapeutic delivery of drugs and minimizing side effects. Successful delivery systems must be nontoxic carriers and the carrier’s systems should have some characteristics including, being controlled release not burst release, high capacity as well as high efficiency for drug load, controllable degradation, possible surface modification of carriers, and the ability to detect by various imaging techniques. Recently applications of MOFs in drug delivery system has attracted the great attention of researchers because of the properties of MOFs such as low toxicity, high stability, controllable size and shape, and easy functionalization [36]. Irlene M.P. Silva and colleagues modified MIL-101 with the NH2 group as a functional group to increase the interaction of MIL-101(Cr) with Ibuprofen and Nimesulide. The NH2 group facilitates the interaction between drugs and matrix via the formation of hydrogen bonds. The result represented that functionalization of MIL-101(Cr) could prolong the release time of a drug [37]. Angshuman Ray Chowdhuri and co-workers introduced carbon dots embedded magnetic nanoparticles@chitosan @metal organic framework for targeted drug delivery. In this study, they synthesize superparamagnetic iron oxide nanoparticles and coated them with O-carboxymethyl chitosan. IRMOF-3 was developed with

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49

encapsulation of folic acid in one-pot on the surface of the Fe3 O4 @OCMC nanoparticle (Fe3 O4 @OCMC@ IRMOF-3/FA) (Fig. 2). They conjugated doxorubicin into the magnetic NMOFs by a physical encapsulation. To display the optical imaging, carbon dots (CDs) are conjugated into the synthesized magnetic NMOF, thereby causing fluorescence features. Chitosan was utilized to control PH and being pHresponsive drug release inside the acidic tumor endosomes for several days. Fe3 O4 core could be used as a T2 - weighted magnetic resonance imaging (MRI) contrast agent and the presence of magnetic nanoparticles could cause drugs delivered usefully to the target location [38]. Vandana Gupta and colleagues reported PEG functionalized zirconium dicarboxylate MOFs as drug carriers. They utilize Docetaxel (DTX), which is an anti-cancer drug. The drug release time of UiO-66 particles after coating with PEG is observed to be increased which confirmed the high performance of MOFs as a drug carrier as well as decreased the dosage of the drug (Fig. 3). In addition, the presence of PEG in the final product cause it to be biocompatible. The drug alone has more cytotoxicity than DTX @UiO-66 because of the sustainable release of DTX from the nanocarrier [39]. Mengru Cai and colleagues functionalized MOF-5 with different functional groups including: –NH2 , –CH3 , –Br, –OH, and –CH2 =CH and studied the effect of these functional groups on drug delivery features. These substituents were chosen because the length of the ligand and the nature of the functional group are different, which causes a better investigation of drug delivery behavior. The size and nature of the substituents have a great influence on the performance of the carrier systems. Oridonin (ORI), with a wide range of biological properties,

Fig. 2 Schematic presentation of the synthetic procedure for the folic acid encapsulated magnetic NMOFs as a targeted DOX carrier. This figure was adapted by permission from: ACS Applied Materials & Interfaces, 2016, 8, 16,573–16,583 [38]

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Fig. 3 The schematic representation comprises of drug carrier PEG@DTX@UiO-66 composite, their cellular uptake, and drug release. This figure was adapted by permission from: Journal of Drug Delivery Science and Technology, 2019, 52, 846–855 [39]

including anti-tumor, anti-inflammatory, antibacterial effects, and prevention of liver fibrosis selected as a model drug. Zn–O clusters, large conjugation of ligands, and the presence of hydrogen bonds bring about multiple potential host–guest interactions between IRMOFs and ORI. Each IRMOF displayed a different drug-loading capacity and in all groups, the amount of drug-loading decreased in comparison with MOF5. The IRMOFs could be classified according to drug-loading capacity: MOF-5 > HO–MOF-5 > H3 C–MOF-5 = Br–MOF-5 > H2 N–MOF-5 > CH2 = CH–MOF-5. CH2 = CH–MOF represented the smallest drug-loading and this fact is due to the CH2 =CH–group being the larger group among others and leads to stronger steric hindrance which prevents the entry of ORI into the cavity of IRMOFs. coordination binding, interactions among groups, hydrogen bond formation, π–π packing between the ORI and IRMOFs, and electrostatic interactions all affect drug-loading capacity. With a view to the presence of hydroxyl groups in HO–MOF-5, the OH group has some interaction with MOF-5 including coordination binding, hydrogen bonding, p– p packing, and electrostatic interaction which decreased the effect of steric hindrance of the group and cause the highest loading capacity in compared with other groups. considering that there are close and numerous interactions between the ORI molecule and MOF-5 the releasing process could be categorized into 3 stages. free ORI at the edge of the frameworks is first released, and there is a weak interaction between ORI and the skeleton. In the following, the free ORI in the center of the pores is released. Due to the molecular interaction between the drugs and the steric hindrance, they

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are released slowly in the second stage. The ORI loaded to the frame is difficult to be released due to its hydrogen bond, π–π conjugation, and binding with the frameworks. This makes the most gentle drug release rate in the third step (Fig. 4a). Release rate of IRMOFs is in the following order: ORI@MOF-5 > ORI@Br–MOF-5 > ORI@H3 C–MOF-5 > ORI@H2 N–MOF-5 > CH2 = CH–MOF-5 > ORI@ HO– MOF-5. This order is similar to the drug loading capacity, except for HO–MOF-5. HO–MOF-5 shows the lowest drug release because of complex interactions [40]. Caixue Lin, et al have synthesized acetaldehyde-modified-cystine functionalized Zr- MOFs (Zr-MOF/AM) for pH/GSH dual-responsive drug delivery and determine the concentration of glutathione (GSH) in living cells in order to study the cells to detect abnormal GSH concentrations for early diagnosis. Acetaldehyde-modifiedcystine (AMC) is a self-fluorescing substance that is induced by the n–π* transition of the two –C=N–bonds. When the disulfide bond in AMC encounters a thiol in GSH, there is a thiol-disulfide exchange reaction and fluorescence increases due to disulfide bond cleavage. Moreover, methotrexate (MTX), is an anti-folate antitumor drug loaded on the Zr-MOF/AMC as a drug carrier. Releasing of the drug happened with the cleavage of –S–S– bond at high concentrations of GSH, and –C=N–bonds hydrolyze and partially break under acidic conditions (Fig. 4b) [41]. Hong Dong et al. synthesized folic acid-functionalized Zr-based metal–organic frameworks as drug carriers for targeted drug delivery of 5- fluorouracil (5-FU) as an anti-cancer drug. Two Zr-based MOFs, MOF-808, and NH2 -UiO-66 were selected as models as a drug carrier, then the Zr6 cluster on the surface of the nanoparticle was further coordinated by terminal carboxylate in folic acid molecule to substitute the previous formate or terminal −OH ligands. The pH-dependent release behavior of the carrier was determined by 5-fluorouracil as an anticancer drug (Fig. 5). The final results show that the surface modifications of NMOFs are a promising route for controllable loading and releasing of the drugs. FA-NMOFs were introduced as a biocompatible nanocarrier for effective targeted drug delivery [42].

2.3 Functionalization of MOF for Catalytic Applications Thanthapatra Bunchuay et al. have functionalized the metal–organic framework MIL-53(Al)-NH2 with copper(II) complexes and utilized this metal–organic framework as a novel catalyst for the oxidation of olefins. They introduced rapid, one-pot post-synthetic modification of MIL-53(Al)-NH2 with salicylaldehyde and copper(II) ions via microwave-assisted functionalization. The one-pot post-synthetic modification was based on the Schiff base reaction between the aldehyde added and the primary aryl amino group in the organic linker of MIL-53(Al)-NH2 , which supplied coordination sites for copper(II) [43]. Transesterification is a chemical reaction used for conversion both in the industry for biodiesel production and in laboratories for organic synthesis, which could be carried out by acid or base catalysis. Jinzhu Chen and co-workers have designed and synthesized an amine-functionalized MOF-based catalyst for the transesterification

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Fig. 4 a Schematic illustration of the release process of ORI from MOF-5 s through three states. This figure was adapted by permission from: RSC Advances, 2020, 10, 36,862–36,872 [40]. b Schematic illustration of the synthetic procedure for Zr-MOF/AMC/MTX and the proposed mechanisms by which Zr-MOF/AMC nanoparticles act as a GSH probe and dual-responsive drug carrier. This figure was adapted by permission from: RSC Advances. 2020, 10, 3084–3091 [41]

of triglycerides and methanol, with the triglyceride conversions exceeding 99%. The amine-functionalized MOFs were gained by two distinct chemical processes: (1) dative modification of unsaturated metal sites located at the secondary building units (SBUs) of MOFs, and (2) covalent modification of the organic linkers within the MOF [44].

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Fig. 5 Schematic illustration of the synthesis of FA-NMOFs by Postmodifying FA on the surface of NMOFs and Encapsulating 5-FU into the pores of the framework and targeting delivery 5-FU. This figure was adapted by permission from: Chemistry-A European Journal, 2018, 24, 17,148–17,154 [42]

2.4 Functionalization of MOF for Removal of HG2+ Lijin Huang et al have reported functionalization of magnetic MOF composite by coordination-based post-synthetic strategy for the increasing removal of Hg2+ from water. The magnetic MOF was effectively prepared via a green method, the nano Fe3 O4 @SiO2 core was first coated by a Cu(OH)2 shell as copper sources and benzene tricarboxylic acid (H3 BTC) as the organic ligand in the water–ethanol mixture solvent to synthesize Fe3 O4 @SiO2 @HKUST-1. Besides, a facile coordination-based postsynthesis strategy was employed for the functionalization of magnetic HKUST-1 with bismuthiol (Bi-I). The prepared functionalized magnetic MOF composites could easily and effectively remove Hg2+ from environmental water samples in a fast and selective procedure. One of the important features of the as-prepared MOF is the ability to sorption of Hg2+ from water in different PH. As can be seen from (Fig. 6a), the Bi-I-functionalized Fe3 O4 @SiO2 @HKUST-1 could adsorb Hg2+ in a wide PH range which could be ascribed to the strong interaction between the –SH functional group and the mercury species [45].

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Fig. 6 a Schematic diagram of the preparation of Bi-I-functionalized magnetic HKUST-1 composites. This figure was adapted by permission from: Journal of Materials Chemistry, 2015, 3, 11,587– 11,595 [45]. b TEPA-functionalization (in blue) of Mg-MOF-74 and its effect on CO2 (in red) adsorption depending on the degree of saturation (s-TEPA being saturated particle). The native Mg-MOF allows both H2 O and CO2 to enter and thus damage the crystal structure, whereas in the TEPA case the amine layer protects the bulk MOF against H2 O entry. This figure was adapted by permission from: ACS Applied Materials & Interfaces, 2017, 9, 11,299–11,306 [48]

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2.5 Functionalization of MOF as Phase Transfer Catalyst Jian-Cheng Wang et al. have functionalized Cd(II)-MOF as a triphase transfer catalyst with imidazolium salt via post-synthetic functionalization. The as-prepared Cd(II)MOF-IM could be a highly active solid phase transfer catalyst (PTC) to contribute to the azidation and thiolation of bromoalkane under PTC conditions. The functionalized MOF could be easily recovered and reused under PTC conditions. Cd(II)-MOFIM was introduced as solid-phase transfer catalysts for promoting a broad scope of reactions carried out in a biphasic mixture of two immiscible solvents [46].

2.6 Functionalization of MOF for Sensitive Fluorescent Probe of S2 O8 2− and Fe3+ Shu-Yin Zhu and Bing Yan have synthesized functionalized MOF (UiO-66-NH2 IM) as a novel sensitive fluorescent probe of S2 O8 2− and Fe3+ . First, a zirconium metallic organic framework UiO-66-NH2 has been synthesized and is further functionalized by imidazole-2-carboxaldehyde in a covalent post-synthetic route which is based on the Schiff-base reaction. This study introduces a facile way to develop the luminescent MOFs and further expand the area of sensing anions and metal ions with high sensitivity and selectivity [47].

2.7 Functionalization of MOF for CO2 Adsorption Xiao Su and co-workers have introduced a novel modified Mg-MOF-74 with tetraethylenepentamine as a functional group for increasing the capacity of CO2 adsorption in dry and humid conditions. Magnesium 2,5-dihydroxyterephthalate (Mg-MOF-74) is a substantial and practical MOF for carbon dioxide adsorption due to its very high dynamic and equilibrium CO2 uptake, favorable structural characteristics, and higher surface areas. Despite the significant ability of Mg-MOF-74 for adsorption of CO2 uptake modifying and increasing the ability of the MOFs is very significant and valuable. Post synthetic functionalization of magnesium-based MOFs with tetraethylenepentamine enhancing CO2 adsorption. The degree of functionalization with the amines was found to be important for increasing the ability of CO2 capture. Suitable surface coverage makes better efficiency and stability under both pure CO2 and CO2 /H2 O coadsorption. Despite its higher surface amine loading, the amine-saturated s-TEPA-MOF species displayed a significantly lower CO2 uptake capacity than did TEPA-MOF (Fig. 6b). On the basis of findings, this result is due to the steric hindrance to gas transport offered by the dense TEPA film on the surface of the saturated MOF [48].

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3 Conclusion A wide range of applications, the ability to change the structure with functionalization and different properties caused MOFs to receive special attention from researchers in various fields. One of the most important and unique properties of MOFs that distinguish them is their ability to be functionalized. MOFs could be functionalized via their pore, ligands, and metal ions and make valuable and useful changes for specific characteristics. In conclusion, MOFs could be modified with pre-synthetic methods and post-synthetic methods. Moreover, there are multiple groups for the functionalization process. In this chapter, there are some examples of MOFs functionalization with different applications.

References 1. Razavi, S.A.A., Morsali, A.: Linker functionalized metal-organic frameworks. Coord. Chem. Rev. 399, 213023 (2019). https://doi.org/10.1016/j.ccr.2019.213023 2. Kalaj, M., Cohen, S.M.: Postsynthetic modification: an enabling technology for the advancement of metal–organic frameworks. ACS Cent. Sci. 6, 1046–1057 (2020). https://doi.org/10. 1021/acscentsci.0c00690 3. Yin, Z., Wan, S., Yang, J., Kurmoo, M., Zeng, M.-H.: Recent advances in post-synthetic modification of metal–organic frameworks: new types and tandem reactions. Coord. Chem. Rev. 378, 500–512 (2019). https://doi.org/10.1016/j.ccr.2017.11.015 4. Yan, B.: Lanthanide-functionalized metal-organic framework hybrid systems to create multiple luminescent centers for chemical sensing. Acc. Chem. Res. 50(11), 2789–2798 (2017). https:// doi.org/10.1021/acs.accounts.7b00387 5. He, Y., Zhou, W., Qian, G., Chen, B.: Methane storage in metal–organic frameworks. Chem. Soc. Rev. 43, 5657–5678 (2014). https://doi.org/10.1039/C4CS00032C 6. Sumida, K., Rogow, D.L., Mason, J.A., McDonald, T.M., Bloch, E.D., Herm, Z.R., Bae, T.-H., Long, J.R.: Carbon dioxide capture in metal-organic frameworks. Chem. Rev. 112, 724–781 (2011). https://doi.org/10.1021/cr2003272 7. Liu, M., Wu, J., Hou, H.: Metal-organic framework (MOF)-based materials as heterogeneous catalysts for C−H bond activation. Chem. Eur. J. (2018). https://doi.org/10.1002/chem.201 804149 8. Mallakpour, S., Nikkhoo, E., Hussain, C.M.: Application of MOF materials as drug delivery systems for cancer therapy and dermal treatment. Coord. Chem. Rev. 451, 214262 (2022). https://doi.org/10.1016/j.ccr.2021.214262 9. Ni, W., Zhang, L., Zhang, H., Zhang, C., Jiang, K., Cao, X.: Hierarchical MOF-on-MOF Architecture for pH/GSH-controlled drug delivery and Fe-based Chemodynamic therapy. Inorg. Chem. 61, 3281–3287 (2022).https://doi.org/10.1021/acs.inorgchem.1c03855 10. Yang, M., Sun, Z., Jin, H., Gui, R.: Sulfur nanoparticle-encapsulated MOF and boron nanosheetferrocene complex modified electrode platform for ratiometric electrochemical sensing of adriamycin and real-time monitoring of drug release. Microchem. J. 177, 107319 (2022). https:// doi.org/10.1016/j.microc.2022.107319 11. Morozan, A., Jaouen, F.: Metal organic frameworks for electrochemical applications. Energy Environ. Sci. 5, 9269 (2012). https://doi.org/10.1039/C2EE22989G 12. Li, T., Jin, Z.: Unique ternary Ni-MOF-74/Ni2P/MoSx composite for efficient photocatalytic hydrogen production: role of Ni2P for accelerating separation of photogenerated carriers. J. Colloid Interface Sci. 605, 385–397 (2022). https://doi.org/10.1016/j.jcis.2021.07.098

MOFs Functionalization Approaches

57

13. Pandey, M.D.: Luminescent metal–organic frameworks as biosensors. Mater. Lett. 308, 131230 (2022). https://doi.org/10.1016/j.matlet.2021.131230 14. Ke, F., Jiang, J., Li, Y., Liang, J., Wan, X., Ko, S.: Highly selective removal of Hg2+ and Pb2+ by thiol-functionalized Fe3 O4 @metal-organic framework core-shell magnetic microspheres. Appl. Surf. Sci. 413, 266–274 (2017). https://doi.org/10.1016/j.apsusc.2017.03.303 15. Yoon, J., Kim, J., Kim, C., Jang, H.W., Lee, J.: MOF-based hybrids for solar fuel production. Adv. Energy Mater. 11, 2003052 (2021). https://doi.org/10.1002/aenm.202003052 16. Stassen, I., Burtch, N., Talin, A., Falcaro, P., Allendorf, M., Ameloot, R.: An updated roadmap for the integration of metal–organic frameworks with electronic devices and chemical sensors. Chem. Soc. Rev. 46, 3185–3241 (2017). https://doi.org/10.1039/C7CS00122C 17. Moghadam, P.Z., Li, A., Liu, X.-W., Bueno-Perez, R., Wang, S.-D., Wiggin, S.B., Wood, P.A., Fairen-Jimenez, D.: Targeted classification of metal–organic frameworks in the Cambridge structural database (CSD). Chem. Sci. 11, 8373–8387 (2020). https://doi.org/10.1039/D0SC01 297A 18. Das, A., Anbu, N., SK, M., Dhakshinamoorthy, A., Biswas, S.: Highly active ureafunctionalized Zr(IV)-UiO-67 metal–organic framework as hydrogen bonding heterogeneous catalyst for Friedel–Crafts Alkylation. Inorg. Chem. 58(8), 5163–5172 (2019).https://doi.org/ 10.1021/acs.inorgchem.9b00259 19. Zhang, H., Gao, X.-W., Wang, L., Zhao, X., Li, Q.-Y., Wang, X.-J.: Microwave-assisted synthesis of urea-containing zirconium metal–organic frameworks for heterogeneous catalysis of Henry reactions. Cryst. Eng. Comm. 21, 1358–1362 (2019). https://doi.org/10.1039/ C8CE02153H 20. Ma, L.-N., Zhang, L., Zhang, W.-F., Wang, Z.-H., Hou, L., Wang, Y.-Y.: Amide-functionalized In-MOF for effective hydrocarbon separation and CO2 catalytic fixation. Inorg. Chem. 61(5), 2679–2685 (2022). https://doi.org/10.1021/acs.inorgchem.1c03821 21. Liu, H.-Y., Gao, G.-M., Bao, F.-L., Wei, Y.-H., Wang, H.-Y.: Enhanced water stability and selective carbon dioxide adsorption of a soc-MOF with amide-functionalized linkers. Polyhedron 160, 207–212 (2019). https://doi.org/10.1016/J.POLY.2018.12.048 22. Sun, L.-L., Li, Y.-H., Shi, H.: A Ketone Functionalized Gd(III)-MOF with low cytotoxicity for anti-cancer drug delivery and inhibiting human liver cancer cells. J. Clus. Sci. 30, 251–258 (2018). https://doi.org/10.1007/s10876-018-1482-3 23. Wang, J., Liu, X., Li, C., Yuan, M., Zhang, B., Zhu, J., Ma, Y.: Fabrication of perylene imidemodified NH2 -UiO-66 for enhanced visible-light photocatalytic degradation of tetracycline. J. Photochem. Photobio. A 401, 112795 (2020). https://doi.org/10.1016/j.jphotochem.2020. 112795 24. Wan, S., Li, L., Liu, J., Liu, B., Li, G., Zhang, L., Liu, Y.: An imide-decorated indium-organic framework for efficient and selective capture of carcinogenic dyes with diverse adsorption interactions. Crys. Growth Des. 20(5), 3199–3207 (2020). https://doi.org/10.1021/acs.cgd.0c0 0066 25. Lu, Z., Xing, Y., Du, L., He, H., Zhang, J., Hang, C.: Isostructural functionalization by –OH and –NH2: different contributions to CO2 adsorption. RSC Adv. 7, 47219–47224 (2017). https:// doi.org/10.1039/C7RA10369G 26. Ke, F., Qiu, L.-G., Yuan, Y.-P., Peng, F.-M., Jiang, X., Xie, A.-J., Shen, Y.-H., Zhu, J.-F.: Thiolfunctionalization of metal-organic framework by a facile coordination-based postsynthetic strategy and enhanced removal of Hg2+ from water. J. Hazard. Mat. 196, 36–43 (2011). https:// doi.org/10.1016/j.jhazmat.2011.08.069 27. Zhang, Z., et al.: Adjust the arrangement of imidazole on the metal-organic framework to obtain hybrid proton exchange membrane with long-term stable high proton conductivity. J. Memb. Sci. 607, 118194 (2020). https://doi.org/10.1016/j.memsci.2020.118194 28. Abdi, J., Banisharif, F., Khataee, A.: Amine-functionalized Zr-MOF/CNTs nanocomposite as an efficient and reusable photocatalyst for removing organic contaminants. J. Mol. Liq. 334, 116129 (2021). https://doi.org/10.1016/j.molliq.2021.116129 29. Rezki, M., Septiani, N.L.W., Iqbal, M., Harimurti, S., Sambegoro, P., Adhika, D.R., Yuliarto, B.: Amine-functionalized Cu-MOF nanospheres towards label-free hepatitis B surface antigen

58

30.

31.

32.

33. 34.

35.

36.

37.

38.

39.

40.

41.

42.

43.

M. Kamalzare electrochemical immunosensors. J. Mater. Chem. 9, 5711–5721 (2021). https://doi.org/10. 1039/D1TB00222H Matemb Ma Ntep, T.J., Wu, W., Breitzke, H., Schlüsener, C., Moll, B., Schmolke, L., Buntkowsky, G., Janiak, C.: Halogen functionalization of aluminium fumarate metal-organic framework via in situ Hydrochlorination of Acetylenedicarboxylic acid. Aust. J. Chem. 72, 835 (2019). https://doi.org/10.1071/CH19221 Jhon, Y.H., Cho, M., Jeon, H.R., Park, I., Chang, R., Rowsell, J.L.C., Kim, J.: Simulations of methane adsorption and diffusion within alkoxy-functionalized IRMOFs exhibiting severely disordered crystal structures. J. Phys. Chem. C 111, 16618–16625 (2007). https://doi.org/10. 1021/jp0749470 Zhou, G., Du, Z., Ma, Y., Zhang, Y., Wu, H., Sun, X., Song, W., Zhang, X., Jiao, Y., Lu, G.: Molecular simulation study on gas adsorption and separation performance of alkylfunctionalized HKUST materials. Comput. Mater. Sci. 181, 109755 (2020). https://doi.org/ 10.1016/j.commatsci.2020.109755 Dau, P.V., Cohen, S.M.: The influence of nitro groups on the topology and gas sorption property of extended Zn(ii)-paddlewheel MOFs. Cryst. Eng. Comm. 15, 9304 (2013) Reddy, C.V., Reddy, K.R., Harish, V.V.N., Shim, J., Shankar, M.V., Shetti, N.P., Aminabhavi, T.M.: Metal-organic frameworks (MOFs)-based efficient heterogeneous photocatalysts: synthesis, properties and its applications in photocatalytic hydrogen generation, CO2 reduction and photodegradation of organic dyes. Int. J. Hydrog. Energy 45, 7656–7679 (2020). https:// doi.org/10.1016/j.ijhydene.2019.02.144 Chang, H., Wu, H., Yang, Y., Xie, L., Fan, W., Zou, M., Ma, G., Jiang, Z., Zhang, Y.: Polyoxotitanium clusters dually functionalized ZnIn2 S4 /MIL-101 catalyst for photocatalysis of aquatic hydrogen production. Int. J. Hydrog. Energy 45, 30571–30582 (2020). https://doi.org/10.1016/ j.ijhydene.2020.08.115 Abánades Lázaro, I., Haddad, S., Rodrigo-Muñoz, J.M., Marshall, R.J., Sastre, B., del Pozo, V., Fairen-Jimenez, D., Forgan, R.S.: Surface-Functionalization of Zr-fumarate MOF for selective cytotoxicity and immune system compatibility in nanoscale drug delivery. ACS Appl. Mater. Interfaces 10, 31146–31157 (2018). https://doi.org/10.1021/acsami.8b11652 Silva, I.M.P., Carvalho, M.A., Oliveira, C.S., Profirio, D.M., Ferreira, R.B., Corbi, P.P., Formiga, A.L.B.: Enhanced performance of a metal-organic framework analogue to MIL101(Cr) containing amine groups for ibuprofen and nimesulide controlled release. Inorg. 70, 47–50 (2016). https://doi.org/10.1016/j.inoche.2016.05.020 Chowdhuri, A.R., Singh, T., Ghosh, S.K., Sahu, S.K.: Carbon Dots Embedded magnetic nanoparticles @Chitosan @Metal organic framework as a Nanoprobe for pH sensitive targeted anticancer drug delivery. ACS Appl. Mater. Interfaces 8, 16573–16583 (2016). https://doi.org/ 10.1021/acsami.6b03988 Gupta, V., Mohiyuddin, S., Sachdev, A., Soni, P.K., Gopinath, P., Tyagi, S.: PEG functionalized zirconium dicarboxylate MOFs for docetaxel drug delivery in vitro. J. Drug Deliv. Sci. Technol. 52, 846–855 (2019). https://doi.org/10.1016/j.jddst.2019.06.003 Cai, M., Qin, L., You, L., Yao, Y., Wu, H., Zhang, Z., Zhang, L., Yin, X., Ni, J.: Functionalization of MOF-5 with mono-substituents: effects on drug delivery behavior. RSC Adv. 10, 36862– 36872 (2020). https://doi.org/10.1039/D0RA06106A Lin, C., He, H., Zhang, Y., Xu, M., Tian, F., Li, L., Wang, Y.: Acetaldehyde-modified-cystine functionalized Zr-MOFs for pH/GSH dual-responsive drug delivery and selective visualization of GSH in living cells. RSC Adv. 10, 3084–3091 (2020). https://doi.org/10.1039/C9RA05741B Dong, H., Yang, G., Zhang, X., Meng, X., Sheng, J., Sun, X., Feng, Y., Zhang, F.: Folic acid functionalized zirconium-based metal-organic frameworks as drug carriers for active tumortargeted drug delivery. Chem. Eur. J. 24, 17148–17154 (2018). https://doi.org/10.1002/chem. 201804153 Bunchuay, T., Ketkaew, R., Chotmongkolsap, P., Chutimasakul, T., Kanarat, J., Tantirungrotechai, Y., Tantirungrotechai, J.: Microwave-assisted one-pot functionalization of metal– organic framework MIL-53(Al)-NH2 with copper(II) complexes and its application in olefin oxidation. Catal. Sci. Technol 7, 6069–6079 (2017). https://doi.org/10.1039/C7CY01941F

MOFs Functionalization Approaches

59

44. Chen, J., Liu, R., Gao, H., Chen, L., Ye, D.: Amine-functionalized metal-organic frameworks for the transesterification of triglycerides. J. Mater. Chem. A 2, 7205–7213 (2014). https://doi. org/10.1039/C4TA00253A 45. Huang, L., He, M., Chen, B., Hu, B.: A designable magnetic MOF composite and facile coordination-based post-synthetic strategy for the enhanced removal of Hg2+ from water. J. Mater. Chem. A 3, 11587–11595 (2015). https://doi.org/10.1039/C5TA01484K 46. Wang, J.-C., Ma, J.-P., Liu, Q.-K., Hu, Y.-H., Dong, Y.-B.: Cd(II)-MOF-IM: post-synthesis functionalization of a Cd(II)-MOF as a triphase transfer catalyst. Chem. Commun. 52, 6989– 6992 (2016). https://doi.org/10.1039/C6CC00576D 47. Zhu, S.-Y., Yan, B.: A novel sensitive fluorescent probe of S2O82− and Fe3+ based on covalent post-functionalization of a zirconium(IV) metal–organic framework. Dalton Trans. 47, 11586– 11592 (2018). https://doi.org/10.1039/C8DT02051E 48. Su, X., Bromberg, L., Martis, V., Simeon, F., Huq, A., Hatton, T.A.: Postsynthetic functionalization of Mg-MOF-74 with Tetraethylenepentamine: structural characterization and enhanced CO2 adsorption. ACS Appl. Mater. Interfaces 9, 11299–11306 (2017). https://doi.org/10.1021/ acsami.7b02471

MOFs Structural Morphologies Simindokht Zarei-Shokat, Fatemeh Ansari, Mohadeseh Forouzandeh-Malati, and Ana Zamani

Abstract The intriguing class of crystalline porous materials known as metal– organic frameworks (MOFs) is made up of metal ions and organic ligands. This chapter set out to rigorously pinpoint the variables that impact MOF properties and how they affect biological and structural traits. The size and structure of MOFs can therefore be influenced by a variety of elements, including additives and organic ligands. The nucleation, growth, and, ultimately, particle size processes can all be affected by additives, which are substances that can compete with the ligand. The size and structure of MOF are significantly influenced by the kind and structure of the ligand. In order to control the size and structure, it is crucial to optimize synthesis parameters such as the reaction duration and initial reagents ratio. It is noteworthy that the kind of ligand and the use of an appropriate addition can regulate also the porosity of MOF. Generally, by optimizing these affecting factors, MOFs with desirable properties and morphologies will be obtained. Keywords MOF · Morphology · Size · pH · Organic ligand · Synthesis · Solvent · Temperature Nanoparticle morphology (size-shape-composition) and surface chemistry are the determining factors that affect the efficiency of materials. The size, shape, and surface chemistry of a nanoparticle can greatly affect key features such as interaction with biologically diverse fluids and interfaces and delivery of bioactive cargo, and modulating therapeutic performance. Morphology is an important factor in the design of drug delivery systems due to its effect on behaviors such as cell uptake, drug release kinetics, and diffusion in complex environments [1]. Due to the unique properties of MOF, these compounds have received a lot of attention. Different synthesis methods and agents cause particles with different morphology and particle size distributions. Because of their high surface area, structural diversity, and flexibility, these compounds have several applications in the fields S. Zarei-Shokat (B) · F. Ansari · M. Forouzandeh-Malati · A. Zamani Catalysts and Organic Synthesis Research Laboratory, Department of Chemistry, Iran University of Science and Technology, 16846-13114 Tehran, Iran e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 A. Maleki and R. Taheri-Ledari (eds.), Physicochemical Aspects of Metal-Organic Frameworks, Engineering Materials, https://doi.org/10.1007/978-3-031-18675-2_5

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of catalysts, storage, separation of gases, biomedicine, etc., each of which has a different morphology and structure size for a separate application [2]. There are various strategies for resizing and morphology of MOF crystals at the micro and nanoscale, including compositional and process parameters, temperature programs, and additives. Combined parameters that can be adjusted to manipulate the size and shape of MOF crystals include solvent, pH, metal source, reactant concentration, or molar ratio of reactants. Process parameters, such as time, temperature, pressure, and heating source, can also have a large impact on the size and shape of the crystal [3].

1 Solvent Effect The choice of the appropriate reaction solvent is an important parameter in MOF synthesis and morphology because they directly or indirectly affect the coordination behavior of the metal and the ligand. A number of synthesized MOF samples show that each solvent system plays an important role in the regulation and formation of different coordinate media. The solvents used may participate in coordination with metal ions or may also act as guest molecules and fillers of MOF space in the final lattice structure. Various MOF structures may be obtained with differences in the central metal ion coordinate environments controlled by the solvent system [4]. Although most of the solvents used may not be included in the synthesized MOF, they act as a guiding factor in the final or environmental structure of the crystal growth process. Sometimes a mixture of several solvents can be used for synthesis. Among the solvents used in MOF synthesis can be mentioned dimethylformamide (DMF), diethylformamide (DEF), dimethyl sulfoxide (DMSO), dimethyl acetate (DMA), alcohols, acetone, acetonitrile, etc., which are used in the reaction. The pore size of the synthesized MOF is influenced by different solvent systems and the pore size is related to the size of the solvent molecules [5]. To better understand the mentioned concepts, we will give some examples: Figure 1 shows the FE-SEM, Co-MOF-based ZIF-67 images prepared using various solvents. In the present study, when methanol (Fig. 1a) is used as a solvent, relatively uniform particles and a dodecahedral rhombic morphology with an average size of 344 nm are observed and when ethanol is used as the solvent (Fig. 1b), a filament-like morphology with a size of 249 nm is visible. Use of water as solvent (Fig. 1c), leaf-like morphology is observed in two dimensions with larger dimensions than the previous two cases and smooth surface. In the case of a mixture of water and methanol (Fig. 1d) and water and ethanol (Fig. 1e), the hexagonal morphology of the agglomerate and the smaller hexagon are observed, respectively. As a result, the particle size distribution varies with different solvents, ethanol, and methanol as solvents showed that the average size distributions are 250 and 350 nm, respectively, indicating the fact that there are fewer nuclei and more growth, and the resulting particles are larger in size. When a mixture of ethanol and water is used, the resulting particles have a sheet-like morphology with a thickness of less than 100 nm [6].

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Fig. 1 Scanning electron microscope images of Co-MOF-based ZIF-67 prepared using a Methanol, b Ethanol, c Water, d Methanol–water mixture and e Ethanol–water mixture. This Figure was adapted by permission from: RSC Advances, 2021, 11(5), 2643–2655 [https://doi.org/10.1039/ D0RA09298C]

A number of reactions are also performed in the mixed solvent system. The combination of two or more solvents can greatly contribute to the different coordination states of ligands. Two zinc-based quinine polymers called [Zn2 (tfbdc)2 (DMF)2 (EtOH)]n and [Zn(tfbdc)(MeOH)4 ]n were synthesized by reaction of zinc nitrate,1 tetraflouroteraphthalic acid,2 and hexamethylenetetramine in various solvent systems. The first complex prepared with a mixture of anhydrous ethanol and DMF enabled different bridging modes of the tfbdc ligand, resulting in the formation of a threedimensional columnar framework with a rare topology. The second coordinated polymer, on the other hand, has a one-dimensional zigzag chain structure with cross-chain hydrogen bond interactions using anhydrous methanol solvent instead of DMF/EtOH mixture. Here, differences in solvent polarity play an important role in changing dimensions from one-dimensional to three-dimensional structures by affecting ligand bridging modes (Fig. 2) [7].

2 PH Effect Crystallization and growth of inorganic–organic hybrids are strongly influenced by the acidity and alkalinity of the reaction medium. The rate of protonation of an organic ligand and sometimes the production of an OH ligand in an aqueous solution due to 1 2

Zn(NO3 )2 .6H2 O. H2 tfbdc.

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Fig. 2 Effect of solvent on metal–organic frameworks. This Figure was adapted by permission from: Inorganic Chemistry Communications, 2011, 14(1), 300–303 [https://doi.org/10.1016/j.ino che.2010.11.020]

the pH of the reaction medium causes the binding of the polycarboxylate ligand to the metal ion based on the acid–base concept. Some interesting effects of pH on MOF synthesis have been studied. The pH of the reaction, as one of the external factors, has a significant effect on determining the coordination states of carboxylic acid ligands involved in the synthesis of MOFs. For example, by changing the pH values, two MOFs were synthesized using CdCl2.2.5H2O, 4-carboxy-4,2' ,6' ,4'' -terpyridine (CTPY), and oxalic acid. Frameworks with structure [Cd2(CTPY)4]n.2nH2O] and [Cd2(CTPY)2(ox)]n were obtained at pH, 7.5 and 5.5, respectively, and oxalate in structure [Cd2(CTPY) 4]n.2nH2O] did not appear. The mentioned frameworks create different coordination modes depending on the pH of oxalic acid. The presence of oxalic acid as a template also depends on the pH. Oxalic acid may act as a factor in the formation of the structure [Cd2(CTPY)4] n.2nH2O] at pH above 7.5, leading to the formation of three-dimensional frames, whereas oxalic acid with the Cd(II) atom Coordinate in the structure of [Cd2(CTPY) 4]n.2nH2O] at pH below 5.5. Hence, higher pH values have contributed to the formation of permeable networks and lower pH values tend to form a simple three-dimensional frame without intrusion [8]. In another example, Fig. 3 shows the effect of pH on the various compounds of a flexible ligand, cyclohexane-5,4,2,2-tetracarboxylic acid (H4CTA) in reaction with the metal ion Zn(II). Coordination polymers formulated as n {Zn4 (µ7 −CTA)(µ3 −OH) (µ2 −OH)3 (H2 O)2 ]n 2nH2 O} and [Zn2 (µ7 −CTA)(H2 O)3 ]n at pH 7 and 4.5 respectively. Of the four possible compounds of H4 CTA, in the structure {Zn4 (µ7 −CTA) (µ3 −OH) (µ2 −OH) 3 (H2 O)2 ]n 2nH2 O the ligand uses the compound (a, e, e, a), whereas in the structure [Zn2 (µ7 −CTA) (H2 O)3 ]n , when the pH is changed to 4.5, the ligand composition becomes more stable to forms (e, a, e, e). The final structure is also from the twodimensional network for {Zn4 (µ7 −CTA) (µ3 −OH) (µ2 −OH)3 (H2 O)2 ]n 2nH2 O} to the 3D framework for [Zn2 (µ7 −CTA) (H2 O)3 ]n has changed [9].

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Fig. 3 Synthetic conditions for different conformations and coordination modes of the CTA ligand in {Zn4 (µ7 −CTA)(µ3 −OH) (µ2 −OH)3 (H2 O)2 ]n 2nH2 O}and [Zn2 (µ7 −CTA)(H2 O)3 ]n This Figure was adapted by permission from: Inorganica Chimica Acta, 2011, 367(1), 127–134 [https://doi.org/10.1016/j.ica.2010.12.023]

One more illustration is Al3+ ions with increased pH to form various structures, including MIL−121 (pH = 1.4), MIL−118 (pH = 2), and MIL−120 (pH = 12.2) that different coordinate states of the linkers may occur in different pH ranges and the degree of deprotonation increases with increasing pH value (Fig. 4) [10].

3 Effect of Metal Ions Herein, by replacing the metal ion source, the coordinated reactions between the metal ion and the organic binder change, causing a morphological change. according to the different interaction paths between different sources of cobalt, Co2+ and Cl− , SO4 2− and NO3− , the nucleation rate changes and as a result, particles of different sizes are produced. Another factor is the rate of hydrolysis between cobalt and imidazole, which affects the size and morphology of the various crystals produced. Figure 5 is a cube-shaped image obtained from different samples of cobalt sources. The size and morphology of crystalline structures change with the change of cobalt source. In addition, the average size of ZIF−67 nanostructures prepared with Co (Cl)2 as well as CoSO4 as cobalt sources has been reduced from 404 to 287 nm.

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Fig. 4 Various structures of MOFs in different ranges of PH. This table was adapted by permission from: Inorganic chemistry, 2010, 49(21), 9852–9862 [https://doi.org/10.1021/ic101128w]

Fig. 5 FE-SEM images of Co-MOF-based ZIF−67 prepared with different cobalt sources: a Co(NO3 )2 , b CoCl2 , and c CoSO4 This Figure was adapted by permission from: RSC Advances, 2021, 11(5), 2643–2655 [https://doi.org/10.1039/D0RA09298C]

4 Time Effect Another important factor that affects the morphology of the MOF structure is the reaction time. In this research, the MOF structure has been imaged at different times. As the reaction time increases, so does the particle size, which can be attributed to Ostwald’s growth behavior. When the reaction time is 30 min (Fig. 6a), Co-MOFbased ZIF−67 particles accumulate, but in 6 h hexagonal particles with a diameter of 1 µm are obtained (Fig. 6b). By increasing the reaction time to 48 h (Fig. 6c), sponge-like hollows are obtained [6].

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Fig. 6 FE-SEM images of Co-MOF-based ZIF−67 prepared with different reaction times a 30 min, b 6 h, and c 48 h This Figure was adapted by permission from: RSC Advances, 2021, 11(5), 2643–2655 [https://doi.org/10.1039/D0RA09298C]

5 Additive Effect Some of the surfactants used in the synthesis of MOFs are steel trimethylammonium bromide (CTAB), polyvinyl pyrrolidone (PVDF), oleic acid, etc. In the crystallization of MOF crystals, surfactant molecules can be selectively adsorbed on one or more specific dimensions of MOF crystals, thus stopping and altering their growth, leading to changes in crystal morphology and size. To better understand the mentioned concepts, we will give some examples: HKUST-1, also known as Cu3 (BTC)2 , is a known MOF made from copper dimer units attached to benzene-1,3,5-tricarboxylic acid (BTC). The liquid–solid solution (LSS) strategy, which effectively prepares high-performance nanomaterials, was used to synthesize this metal–organic framework. Nano-organic metal–organic frameworks have received a lot of attention due to their small size and features. The LSS method is mainly based on the phase transfer mechanism and general separation that occurs in the interface. The LSS approach is used to precisely control the size of HKUST-1 nanocrystals. The oleic acid (OA) molecules used can not only be used to coordinate with copper ions to form copper (II) oleates, but also act as a surfactant to control the growth of HKUST-1 at the nanoscale. According to the results of Fig. 7, as the amount of oleic acid increases, the size of HKUST-1 nanoparticles gradually increases [11]. For example, in the hydrothermal synthesis of ZIF-67 crystals, MOF CTAB surfactant is added as an additive to the reaction and changes the crystal morphology and size of MOFs. In Fig. 8, by changing the CTAB value from 0.0025 to 0.025 wt%, cubic or rhombic ZIF-67 rhombic crystals with sizes between 150 nm to 1 µm are produced [12]. As another instance, a porous coordinated polymer [Cu3 (btc)2 ] with controllable particle size and morphology from nano cube to fine octagon was synthesized by adjusting the surfactant concentration. By decreasing and increasing the amount of CTAB to form MOF [Cu3 (btc)2 ], by placing the surfactant in the structural aspects, it has formed different morphologies. The properties of [Cu3 (btc)2 ] crystals depend on the morphology and particle size (Fig. 9).

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Fig. 7 SEM images of HKUST-1 nanocrystals synthesized with different OA a 30-nm with 0.20 mL of OA, b 50-nm with 0.25 mL of OA, c 70-nm with 0.30 mL of OA, d 100-nm with 0.35 mL of OA, e 140-nm with 0.40 mL of OA This Figure was adapted by permission from: Crystal Growth & Design, 2019, 19(2), 556–561 [https://doi.org/10.1021/acs.cgd.8b01695]

Fig. 8 SEM and TEM images of ZIF-67 with a 0 wt %, b 0.0025 wt %, c 0.01 wt %, and d 0.025 wt % CTAB concentrations. This Figure was adapted by permission from: ACS Sustainable Chemistry & Engineering, 2017, 5(9), 7824–7831[https://doi.org/10.1021/acssuschemeng.7b01306]

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Fig. 9 Proposed [Cu3 (btc)2 ] crystal formation mechanism. This Figure was adapted by permission from: Chemical Communications, 2012, 48(70), 8814–8816 [https://doi.org/10.1039/C2CC34 192A]

Figure 10 shows the morphological development that occurs with increasing CTAB concentration: cube, truncated cube, cuboctahedron, truncated octahedron, and octahedron. Also, the size of crystals has increased from 300 nm to 1 µm [13].

Fig. 10 SEM images of [Cu3 (btc)2 ] samples prepared with different concentrations of CTAB: a 0; b 0.005; c 0.01; d 0.05; e 0.1 and f 0.5 M. The insets in the SEM pictures are schematic sketches of the crystal morphologies. This Figure was adapted by permission from: Chemical Communications, 2012, 48(70), 8814–8816 [https://doi.org/10.1039/C2CC34192A]

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6 Effect of Synthesis Method The synthesis of MOFs involves the process of crystallization during which the nucleus and the growth of crystals occur. Nucleation and growth of MOF crystals involve self-assembly between metal–oxygen clusters and organic bonds. Understanding the factors affecting the nucleus and the growth of MOF crystals during their synthesis makes it possible to precisely control the morphology and size of the crystal. The morphology and size development of MOF crystals during differently modulated syntheses are discussed below.

6.1 Synthesis of Deprotonation Regulation Different types of synthesis conditions: such as temperature, time, solvent type, and reactant concentration, play an important role in the morphology and size of the MOF crystals obtained. For example, an NH2 -MIL-125(Ti) MOF organic metal–organic framework can be synthesized by the thermal solvent method in a mixed solvent of DMF and methanol. Figure 11 shows the SEM images of NH2 -MIL-125(Ti) crystals synthesized at different concentrations of the reactant with the total volume of the solvent. By changing the total volume of the solvent while maintaining a constant ratio between DMF and methanol and the amount, of reagents, the morphology of the NH2 -MIL-125(Ti) crystals can be changed from circular to hexagonal and octagonal plates. By changing the crystal morphology, the NH2 -MIL-125(Ti) light-absorbing band can be adjusted from 480 to 533 nm, making it useful for photocatalytic applications. Reactive concentration has been shown to have a significant effect on the protonation of organic bonds during the synthesis of NH2 -MIL-125(Ti) crystals. The degree of protonation plays an important role in the nucleus and growth of NH2 -MIL125(Ti) crystals. Adjusting the crystal morphology and size of MOFs by changing the protonation rate is called protonation adjustment synthesis [14].

6.2 Synthesis of Coordinate Modulation In addition to changing the synthesis conditions, the introduction of additives during crystallization is another method of controlling the morphology and size of MOF crystals. Figure 12 shows the coordination modulation of acetate ions as additives in the growth of metal–organic framework crystals, NiFe-MOFs. In this work, a certain number of acetate ions are added to NiFe-MOFs. Acetate ions prevent coordination between metal clusters and organic bonds through selective coordination with metal clusters. Specifically, acetate ions possess the same coordination groups as the participating terephthalic ligand. Acetate ions prevent

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Fig. 11 SEM images of NH2 -MIL-125(Ti) crystals synthesized with different total solvent volumes of a 40 mL, b 30 mL, c 20 mL, d 15 mL, e 14 mL, and f 13.5 mL. This Figure was adapted by permission from: CrystEngComm, 2014, 16(41), 9645–9650 [https://doi.org/10.1039/C4CE01 545B]

Fig. 12 a Coordination modulation method, b TEM image, c SEM image. This Figure was adapted by permission from: ACS Sustainable Chemistry & Engineering, 2021, 9(32), 10,892–10901 [https://doi.org/10.1021/acssuschemeng.1c03385]

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Fig. 13 SEM images of the HKUST-1 crystals obtained with different amounts of the lauric acid in mmol: a 0, b 2.34, c 4.75, d 7.13, e 9.5, and f 11.88. This Figure was adapted by permission from: Journal of the American Chemical Society, 2011, 133(39), 15,506–15,513 [18]

coordination between metal clusters and organic bonds through selective coordination with metal clusters, which affects the expansion of the lattice structure and causes a change in the growth of MOF crystals [15]. Also, we can refer to HKUST-1 crystals which were synthesized under ultrasonic conditions using three additives of monocarboxylic acid, acetic acid, dodecanoic acid, and lauric acid [16, 17]. Figure 13 shows the effect of additive quantity on the morphology of HKUST-1 crystals. As can be seen, as the amount, of additive increases, not only does the size of HKUST-1 crystals increase from tens of nanometers to several micrometers, but the shape of the crystals also changes from cube to octahedron, truncated cube, and gradually to tetrahedron. As a result, it was found that the coordination modulation resulting from the use of carboxylic acids is unique and effectively controls the morphology and crystal size of several MOFs including [{Cu2 (ndc)2 (dabco)}n ], Zr-based HKUST-1. The amount of additive used varies from 2 to 100. Carboxylic acid additives do not affect the crystal structure of the resulting MOFs [16].

7 Temperature Effect In addition to the combined parameters mentioned, the process parameter such as temperature is also a key parameter. Which should be analyzed when developing synthetic methods for the preparation of metal–organic frameworks. The morphology of the synthesized MOFs can be changed by changing the temperature of the reaction medium. For example, there are samples of MOFs obtained from succinate ligand at

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different temperature conditions due to their flexibility and different modes of coordination, which were synthesized in an aqueous solution at two different temperatures. The reaction of TmCl3 .6H2 O with succinic acid in an aqueous solution at 100 °C gave a three-dimensional MOF {[Tm2 (L)3 (H2 O)].H2 O}. At 180. It again adopts a threedimensional MOF with the same formula. Although they obtained two compounds with the same experimental formula and dimensions, they crystallized in two different spatial groups (Fig. 14). Here the coordination of succinate with metal ions is in two different ways, that is, the succinate anion accepts different structures under two different temperature conditions. Thus, temperature control enabled them to obtain monoclinic or triclinic structures by affecting the crystal growth of both compounds. As a result, the two, lanthanide succinate had the same experimental formula but different spatial groups [19]. The next example is the combination of nickel acetate and 1,4-cyclohexane dicarboxylate (CDC) in the presence of cyclohexanol and aqueous solvents. As shown in Fig. 15, at 140 °C it leads to the formation of a three-dimensional structure {Ni3 (OH)2 (CDC)2 (H2 O)4 ]0.4H2 O} and at a temperature of 170 °C a two-dimensional structure {[Ni6 (OH)6( CDC)3 (H2 O)6 ]0.2H2 O}[20]. In Fig. 16, the size of the crystals at no temperature (room temperature) is larger than the crystals at 100 °C. Due to the high solubility of the reactants at high temperatures, it leads to the formation of a large number of nuclei and the formation of small crystals. On the other hand, nuclei grow at a relatively low rate at room temperature, leading to the formation of large crystals. Therefore, under temperature conditions, size and morphology are more uniform and regular in structure [6].

Fig. 14 Two Tm-succinates obtained under different temperature conditions. This Figure was adapted by permission from: Journal of Solid State Chemistry,2013, 197, 7–13 [https://doi.org/ 10.1016/j.jssc.2012.08.036]

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Fig. 15 Temperature effect. This Figure was adapted by permission from: Crystal growth & design, 2006, 6(3), 664–668 [https://doi.org/10.1021/cg050363g]

Fig. 16 FE-SEM images of ZIF-67 based on Co-MOF formation at 25 and 100 °C with two different solvents. This Figure was adapted by permission from: RSC Advances, 2021, 11(5), 2643–2655 [https://doi.org/10.1039/D0RA09298C]

8 Effect of Molar Ratio of Reactants The molar ratio of the reactants is also an important factor in the synthesis of MOFs because the topological pattern of the MOFs depends on the stoichiometry of the reactants. For example, the morphology of the synthesized MIL-101(Cr) surface is shown in Fig. 17. By increasing the molar ratio of chromium to the 1,4-benzene decarboxylate (BDC) organic bond, an increase in the surface roughness of MIL-101(Cr) can be observed. Increasing surface roughness is also associated with decreasing particle size. Based on the SEM images in Fig. 17a–c, we see that the MIL-101(Cr) particle size was reduced using a higher Cr to BDC molar ratio. The SEM image of

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Fig. 17 SEM images of MIL-101(Cr) with different Cr: BDC molar ratios: a 0.2:1, b 0.25:1, c 0.5:1, d 1:1, e 1.5:1, f 2:1, and g 2.5:1. This Figure was adapted by permission from: Sustainability, 2022, 14(3), 1152 [https://doi.org/10.3390/su14031152]

the sample with a Cr: BDC molar ratio of 1.5:1 (Fig. 17e) shows that it is composed of small, irregular particles. As the molar ratio decreases from 1.5:1 to 1:1, we see the growth of small, irregular, granular particles in the aggregate state (Fig. 17d). MIL101(Cr), can grow well using a solvent-free method with a molar ratio of chromium to BDC organic bond. The surface of MIL-101(Cr) with a molar ratio of 2:1 and 2.5:1 chromium to BDC has a large number of regular round particles. The surface of MIL-101(Cr), with a molar ratio of 2:1 and 2.5:1 chromium to BDC has a large number of regular round particles. Changing the molar ratios is necessary to achieve a suitable morphology [21].

9 Conclusion This chapter examines the formation and structure of a series of MOFs. Structural differences When each parameter is variable, each reaction condition has a large effect on the structure or dimensions of the metal–organic framework. Studies show that there is no universal rule for selecting parameters for any MOF synthesis. The effect of the parameters varies depending on the reactants involved in crystal formation. From the examples of solvent effect mentioned above, the effect of the solvent

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selection is reflected in all the structures obtained. It is suggested that different solvent systems may regulate crystallization through a process of dissimilar self-assembly, certain reagents are affected by the solvents used in the reactions in incompatible ways. Some studies show that the size of solvents plays an important role in the structural self-assembly process of alkaline-organic frameworks. The size of the channels increases with the size of the solvents. Also, in the mentioned examples, it was shown that changing the pH creates different chlorination states. Temperature also acts as a guiding factor in the structure by affecting the coordination modes of the organic ligand and the coordination number of the central metal ion. Thus, multidimensional structures can be purposefully obtained by adjusting the reaction temperature. However, the construction of preferred structures is still a major challenge because each factor has its role to play in the synthesis of MOF structures.

References 1. Suresh, K., Kalenak, A.-P., Sotuyo, A., Matzger, A.-J.: Metal-organic framework (MOF) morphology control by design. Eur. J. Chem. 28, e202200334 (2022). https://doi.org/10.1002/ chem.202200334 2. Li, Z., Ning, S., Zhu, H., Wang, X., Yin, X., Fujita, T., Wei, Y.: Novel NbCo-MOF as an advanced peroxymonosulfate catalyst for organic pollutants removal: growth, performance and mechanism study. Chemosphere 288, 132600 (2022). https://doi.org/10.1016/j.chemosphere. 2021.132600 3. Stock, N., Biswas, S.: Synthesis of metal-organic frameworks (MOFs): routes to various MOF topologies, morphologies, and composites. Chem. Rev. 112, 933–969 (2012). https://doi.org/ 10.1021/cr200304e 4. Seetharaj, R., Vandana, P.-V., Arya, P., Mathew, S.: Dependence of solvents, pH, molar ratio and temperature in tuning metal organic framework architecture. Arab. J. Chem. 12, 295–315 (2019). https://doi.org/10.1016/j.arabjc.2016.01.003 5. Soni, S., Bajpai, P.-K., Arora, C.: A review on metal-organic framework: synthesis, properties and application. Characterization Appl. Nanomaterials 3, 87–106 (2020). https://doi.org/10. 24294/can.v3i2.551 6. Davoodi, M., Davar, F., Rezayat, M.-R., Jafari, M.-T., Shalan, A.-E.: Cobalt metal–organic framework-based ZIF-67 for the trace determination of herbicide molinate by ion mobility spectrometry: investigation of different morphologies. RSC Adv. 11, 2643–2655 (2021). https:// doi.org/10.1039/D0RA09298C 7. Cheng, M.-L., Zhu, E., Liu, Q., Chen, S.-C., Chen, Q., He, M.-Y.: Two coordinated-solvent directed zinc (II) coordination polymers with rare gra topological 3D framework and 1D zigzag chain. Inorg. Chem. Commun. 14, 300–303 (2011). https://doi.org/10.1016/j.inoche. 2010.11.020 8. Yuan, F., Xie, J., Hu, H.-M., Yuan, C.-M., Xu, B., Yang, M.-L., Dong, F.-X., Xue, G.-L.: Effect of pH/metal ion on the structure of metal–organic frameworks based on novel bifunctionalized ligand 4' -carboxy-4, 2' : 6' , 4'' -terpyridine. CrystEngComm 15, 1460–1467 (2013). https://doi. org/10.1039/C2CE26171E 9. Liu, B., Yang, G.-P., Wang, Y.-Y., Liu, R.-T., Hou, L., Shi, Q.-Z.: Two new pH-controlled metal–organic frameworks based on polynuclear secondary building units with conformationflexible cyclohexane-1, 2, 4, 5-tetracarboxylate ligand. Inorg. Chim. Acta 367, 127–134 (2011). https://doi.org/10.1016/j.ica.2010.12.023 10. Volkringer, C., Loiseau, T., Guillou, N., Ferey, G., Haouas, M., Taulelle, F., Elkaim, E., Stock, N.: High-throughput aided synthesis of the porous metal− organic framework-type aluminum

MOFs Structural Morphologies

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

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pyromellitate, MIL-121, with extra carboxylic acid functionalization. Inorg 49, 9852–9862 (2010). https://doi.org/10.1021/ic101128w Cai, X., Xie, Z., Pang, M., Lin, J.: Controllable synthesis of highly uniform nanosized HKUST1 crystals by liquid–solid–solution method. Cryst. Growth Des. 19, 556–561 (2019). https:// doi.org/10.1021/acs.cgd.8b01695 Li, W., Zhang, A., Jiang, X., Chen, C., Liu, Z., Song, C., Guo, X.: Low temperature CO2 methanation: ZIF-67-derived Co-based porous carbon catalysts with controlled crystal morphology and size. ACS Sustain. Chem. Eng. 5, 7824–7831 (2017). https://doi.org/10.1021/acssusche meng.7b01306 Liu, Q., Jin, L.-N., Sun, W.-Y.: Facile fabrication and adsorption property of a nano/microporous coordination polymer with controllable size and morphology. ChemComm 48, 8814–8816 (2012). https://doi.org/10.1039/C2CC34192A Hu, S., Liu, M., Li, K., Zuo, Y., Zhang, A., Song, C., Zhang, G., Guo, X.: Solvothermal synthesis of NH 2-MIL-125 (Ti) from circular plate to octahedron. CrystEngComm 16, 9645–9650 (2014). https://doi.org/10.1039/C4CE01545B Zhao, H., Yu, L., Zhang, L., Dai, L., Yao, F., Huang, Y., Sun, J., Zhu, J.: Facet engineering in ultrathin two-dimensional NiFe metal–organic frameworks by coordination modulation for enhanced electrocatalytic water oxidation. ACS Sustain. Chem. Eng. 9, 10892–10901 (2021). https://doi.org/10.1021/acssuschemeng.1c03385 Tsuruoka, T., Furukawa, S., Takashima, Y., Yoshida, K., Isoda, S., Kitagawa, S.: Nanoporous nanorods fabricated by coordination modulation and oriented attachment growth. Angew. Chem. 121, 4833–4837 (2009). https://doi.org/10.1002/ange.200901177 Diring, S., Furukawa, S., Takashima, Y., Tsuruoka, T., Kitagawa, S.: Controlled multiscale synthesis of porous coordination polymer in nano/micro regimes. Chem. Mater 22, 4531–4538 (2010). https://doi.org/10.1021/cm101778g Umemura, A., Diring, S., Furukawa, S., Uehara, H., Tsuruoka, T., Kitagawa, S.: Morphology design of porous coordination polymer crystals by coordination modulation. J. Am. Chem. Soc. 133, 15506–15513 (2011). https://doi.org/10.1021/ja204233q de Oliveira, C.-A., da Silva, F.-F., Malvestiti, I., Malta, V.-R., Dutra, J.-D., da Costa Jr, N.-B., Freire, R.-O., Júnior, S.-A.: Effect of temperature on formation of two new lanthanide metalorganic frameworks: synthesis, characterization and theoretical studies of Tm (III)-succinate. J. Solid State Chem. 197, 7–13 (2013). https://doi.org/10.1016/j.jssc.2012.08.036 Chen, J., Ohba, M., Zhao, D., Kaneko, W., Kitagawa, S.: Polynuclear core-based nickel 1, 4-cyclohexanedicarboxylate coordination polymers as temperature-dependent hydrothermal reaction products. Cryst. Growth Des. 6, 664–668 (2006). https://doi.org/10.1021/cg050363g Chong, K.-C., Ho, P.-S., Lai, S.-O., Lee, S.-S., Lau, W.-J., Lu, S.-Y., Ooi, B.-S.: Solvent-free synthesis of MIL-101 (Cr) for CO2 gas adsorption: the effect of metal precursor and molar ratio. Sustainability 14, 1152 (2022). https://doi.org/10.3390/su14031152

MOFs Bandstructure Zoleikha Hajizadeh and Mohammad Mehdi Salehi

Abstract As photocatalysts, a variety of MOFs have been developed, with pristine MOFs being introduced from UV light response to visible-light response, and MOFs modifications being developed in the following areas: decoration of organic linker or metal center, combination with semiconductors, MNPs loading, decoration with RGO, sensitization, pyrolyzation, and incorporation with other functional materials. Even yet, UiO-66(Zr), MIL-125(Ti), and ZIF-8(Zn) have received the majority of attention because they are strong enough to withstand difficult catalytic conditions and have ideal crystal structures. This is despite the fact that much effort has been done to examine MOFs for photocatalysis. However, the absence of redox activities and visible-light response in these MOFs restricted their use and photocatalytic efficacy when exposed to visible light. Keywords Semiconductors · Photocatalytic · Band gap · Linkers · Visible light

1 Semiconducting MOFs Metal–organic frameworks (MOFs) as a semiconductor material have been exclusively utilized and studied for various applications like gas storage, sensing, chemical separations, catalysis, drug delivery, biomedical imaging, and oxides [1]. MOFs’ magnetic, optical, and ferroelectric behaviors have been a topic of scientific interest for more than a decade. So, the semiconducting behavior of MOFs is still in its initial stages, and further investigations are highly desired. The use of semiconductor MOFs as an active component in the design of transistors for various integrated circuits [2]. The presence of a functional organic component along with an inorganic counterpart, the easy tunability of the structural properties, and their multifunctionality make MOFs more versatile than organic or inorganic semiconductors for developing efficient semiconducting materials [3]. Z. Hajizadeh (B) · M. M. Salehi Catalysis and Organic Synthesis Research Laboratory, Department of Chemistry, Iran University of Science and Technology, 16846-13114 Tehran, Iran e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 A. Maleki and R. Taheri-Ledari (eds.), Physicochemical Aspects of Metal-Organic Frameworks, Engineering Materials, https://doi.org/10.1007/978-3-031-18675-2_6

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The linking organic groups are mostly insulators with small π-orbital conjugation; therefore, metal–organic frameworks are generally considered poor conductors. However, in recent years, considerable efforts have been expended to incorporate electrical conductivity in MOFs and follow the roadmap for implementing MOFs in microelectronic devices [4]. MOFs can show proton-conducting action through welldesigned pores and metal-oxide chain networks [5]. The highest proton conduction for MOFs has been reported to be around the order of 10–2 S cm−1 at 90% relative humidity and 85 °C [6]. The interaction between the water molecules and metallic bridges by hydrogenbonding in MOFs can also display significant roles in proton conduction. However, the proton conduction in MOFs can be supported by solvent molecules and does not make much assistance in providing electrical conductivity [7]. Electrical conduction paths in MOFs can be achieved by replacing isomorphic ally with sulfur atoms. This change gives rise to infinite metal–sulfur chains with the potential to enable charge transport [7]. The tunable electrical conductivity in HKUST-1 thin films by incorporating a 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecule inside the crystal framework was suggested in 2013 [8]. In the progress toward developing semiconductor MOFs, semiconductor@MOF core–shell structures were reported to have various properties. For example, Cu3 (BTC)2 @TiO2 core–shell and ZnO@ZIF-8 nanorod structures were designed for photovoltaic applications [9]. It is commonly assumed that the semiconductor@MOF heterostructures are favorable in many electronic devices. Semiconductor Mn-based MOFs have been reported with activation energy Ea = 0.70 (±0.03) eV [10]. the delocalization and excitation of the π-electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the π-molecular orbital lead to having the semiconducting property in the Mn-MOFs, [Mn(μ-Pz) (μ-Cl)2 ] n (Pz = pyrazine). Lu and co-workers also synthesized a 3D Sr-based semiconductor MOF [11]. Designing MOFs by s-block element strontium (Sr) as an inorganic node is applied as a non-radioactive element. Functioning Sr-MOF with organic ligand as a bridge between two metallic oxide layers shows a 3D structure. This 3D crystal structure with an organic linker between the layers resembles the famous semiconductor MoS2 . The electrical conductivity of this compound was reported to be in the range of 10–6 (S cm–1 ) [11].

2 Band Gap Investigation MOFs are insulating materials with a high bandgap. So, to optimize their bandgap to the lower values that are needed for semiconductors, some simulation techniques have been used to optimize their band gap [11]. Based on the hybrid crystalline structure of MOFs with both organic and inorganic components, choosing a suitable simulation basis set is essential. Simulation basis set with the describing the electronic structure of the organic linkers and the metallic nodes. The energy for the HOMO–LUMO levels for the periodic structure of MOFs can be calculated by density functional

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theory (DFT) [11]. To investigate the band gap relationship with the crystal structure of the MOFs, MFU-4-type MOFs have been investigated [11]. Three techniques for engineering the band gap of MOFs were reported by Volkmer et al. [12]. Increasing the conjugation of the linkers will cause higher valence band (HOMO) energy; hence, the band gap can be reduced. The band gap would be decreased by selecting appropriate metal nodes at octahedral coordination sites with their unoccupied d-orbitals below the LUMO of the organic linker. So, the band gap can be controlled by modification of organic linkers with various functional groups (−NH2 , −OH, −CH3 , −Cl) that can lead to band shift by donating 2p electrons to the aromatic linker [13]. DFT calculations had shown that the bandgap for Zr-UiO-66 MOFs decreased from 3.1 to 2.2 eV when the benzene dicarboxylate (bdc) linker was functionalized by nitro (bdc–NO2 ) and amino (bdc– NH2 ) groups [12]. The theoretical data for a 3D semiconductor Sr-based MOF was investigated by Lu et al. [14]. Starting with the experimentally determined unit cells of the Sr-based MOF, DFT calculations were performed with generalized gradient approximations (GGA), as parameterized by Perdew, Burke, and Ernzerhof (PBE) for exchange–correlation functions and projector augmented-wave (PAW) potentials. The smallest band gap was found to be indirect (Γ → X) between the Γ point in the valence band and the X point in the conduction band, being 2.04 eV. The bandgap energy of the Sr-based semiconductor MOF was comparably lower than the reported values for MOF-5 (Eg = 3.4 eV) and ZIF-8 (Eg = 5.5 eV) and very close to the reported value for IRMOF-M2a [15, 16]. For having the MOF with an efficient semiconductor feature, the band gap can be decreased by (i) increasing the conjugation in the linker, (ii) selecting electron-rich metal nodes and organic molecules, and (iii) functionalizing the linker with nitro and amino groups. In addition, by using halogen atoms as functional groups in MOFs, the valence band maximum (VBM) and tune can be modified [13]. To reduce the band gap and increase the VBM value, iodine is the best candidate among halogens. In the presence of antiaromatic linker DHPDC (1,4-dihydropentalene-2,5-dicarboxylic acid), the energy gap (0.95 eV) is even lower than other aromatic linkers like TTDC (thieno[3,2-b] thiophene-2,5-dicarboxylic acid) and FFDC (furo[3,2-b] furan-2,5dicarboxylic acid). The signification role of organic linkers in the MOF band gap was highlighted by analyzing the calculations of the lowest unoccupied molecular orbital−highest occupied molecular orbital gaps at the molecular level. The difference in band gaps in MOF crystals can be analyzed by changing the dihedral angle of C−C−C = O in the organic linker [13]. The reticular MOFs (IRMOFs) with different kinds of linear linkers (Fig. 1), namely, IRMOF-20C (DHPDC), IRMOF-20O (FFDC), IRMOF-20S (TTDC), IRMOF-2X (BDC-X; X = F, Cl, Br, I), and IRMOF-F4, were studied by Pham and co-workers. The determination of the band gap was done by the energy difference between the valence band maximum (VBM) and the conduction band minimum (CBM) at Γ (gamma) point. The study of the energy gap by the DFT-B3LYP method for MOF-5 (or IRMOF-1) shows that it is significantly higher than the experimental value, 5.0

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Fig. 1 Left panel: isoreticular pcu-MOF model. Orange cylinders are organic linkers (A, B, C, and D), and blue polyhedra are metallic oxide clusters (Zn4 O(CO2 )6 ). Right panel: 12 organic linkers investigated in this work. FFDC (linker A with Y = O), TTDC (linker A with Y = S), DHPDC (linker B), BDC-X (linker C with X = H, F, Cl, Br, I), and BDC-X4 (linker D with X = H, F, Cl, Br, I). J. Phys. Chem. C 2014, 118, 9, 4567–4577 [17]

and 3.4 eV, respectively [18]. Also, the band gap for MOF-5 is 3.5 eV using generalized gradient approximation Perdew-Burke-Ernzerhof (GGA-PBE) functional for XC energy. This result is in excellent agreement with the experiment. This level of theory shows reasonable results for other MOFs like IRMOF-8, IRMOF-10, and IRMOF-14. The results of DFT calculations confirm the excellent agreement of the GGA-PBE function with the existing experimental data (Table 1).

3 Band Energy Values (in Electronvolts) Calculated from Periodic Systems and Linker Molecules The band edge position at the Γ point of the IRMOF-2X (X = F, Cl, Br, I) series is shown in Fig. 2a. The predicted values of semiconductor band gaps in these materials varied from 2.65 to 3.20 eV, which are lower than that obtained for the IRMOF-1 crystal at 3.37 eV. Using the halogen atoms in the aromatic ring leads to the reduction of the band gap. By increasing the atomic size of the halogen from F to I, the energy gap systematically decreases interestingly. This variation results from a significant increase in VBM due to the difference in CBMs.

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Table 1 Band energy values (in Electronvolts) calculated from periodic systems and linker molecules Sample

VBM positiona

CBM positiona

Ega

EHOMO b

ELUMO b

ELUMO − EHOMO b

IRMOF-2F

−6.49

−3.29

3.20

−6.90

−3.45

3.45

IRMOF-2Cl

−6.44

−3.35

3.09

−6.27

−3.46

3.26

IRMOF-2Br

−6.29

−3.33

2.96

−6.58

−3.46

3.26

IRMOF-2I

−5.96

−3.31

2.65

−6.35

−3.95

2.76

IRMOF-20C

−5.19

−4.24

0.95

−5.69

−4.66

1.03

IRMOF-20O

−5.59

−2.94

2.65

−6.20

−3.39

2.80

IRMOF-20S

−5.90

−3.38

2.52

−6.19

−3.44

2.75

IRMOF-F4

−6.39

−3.62

2.77

−6.76

−3.84

3.34

IRMOF-1

−6.59

−3.13

3.46

−6.86

−3.30

3.56

a b

Calculated from crystalline systems Calculated from linker molecules

Fig. 2 a Band edge position of MOFs (Black segment) calculated from electronic band structure of crystalline systems and HOMO−LUMO energy levels obtained from linker molecules (HOMO, filled blue circle; LUMO, filled red circle) for a IRMOF-2X and b IRMOF-20Y. J. Phys. Chem. C 2014, 118, 9, 4567–4577 [17], b HOMO−LUMO energies calculated for BDC-X4 linker (HOMO, solid blue triangles; LUMO, solid red triangle) and BDC-X linker (HOMO, open blue circle; LUMO, open red circle). J. Phys. Chem. C 2014, 118, 9, 4567–4577 [18]

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Band edge position of MOFs (Black segment) calculated from electronic band structure of crystalline systems and HOMO−LUMO energy levels obtained from linker molecules (HOMO, filled blue circle; LUMO, filled red circle) for (a) IRMOF2X and (b) IRMOF-20Y. The band gap energies of MOF compounds can be predicted by investigating the electronic structure of their organic linkers. The energy gaps from the HOMO−LUMO of the linker were calculated and are denoted by filled red and blue circles in Fig. 2a. It should be notable that during the geometry optimization, for having a structure similar to the linker configuration in the crystal, the dihedral angle between two carboxylic groups of linkers was constrained at zero value [17]. Although the exact values of band edge positions are not the same in the two cases, the trend of HOMO−LUMO is in amazing agreement with those of the corresponding band gap energies for all the crystal systems. In addition, the difference between the energy gap estimated from the linker and the crystalline phase is relatively small (less than 0.25 eV). Also, the presence of metal elements in the topological structure contributes to this slight difference in band gap energies [19]. The effect of the metals was investigated by Yang and co-workers [20]. In this study, the properties of IRMOFs are based on different kinds of metal elements: Cd, Be, Zn, Mg, Ba, Sr, and Ca, and two linkers, HPDC (4,5,9,10-tetrahydropyrene-2,7-dicarboxylate) and PDC (pyridine-3,5-dicarboxylate) were examined. The results confirm the negligible effect of changing the metal compared to the impact of the linker on the band gap. Also, by replacing the Zn atoms in IRMOF-1 with Co atoms, the electronic structure can be tuned from a semiconducting to a metallic state. Based on reports, the most popular structure among octahedral secondary building units (SBUs) is Zn4 O(CO2 )6 cluster [21]. Using organic linkers is recommended due to being more diverse than those of the metallic oxide cluster and straightforward to control because of the power of retrosynthesis. Crystal net control can be achieved through the rational design of the geometric structure of the linker. So, engineering the band gap of the material by the rational design of the chemical structure of the linked is possible [22, 23]. The agreement between the band gap energy calculated from MOF crystal structures with those obtained from the HOMO−LUMO energy of the corresponding linker opens an efficient strategy for screening promising linkers for semiconductor applications. The HOMO−LUMO gap implies the energy value calculated from the linker cluster. In contrast, the band gap energy is the terminology used for those values calculated from the periodic system [24]. The halogen atoms as a substitute group in the benzene ring can reduce the energy gap by increasing the HOMO energy. Figure 2b shows the gap estimated from HOMO−LUMO energy of linker BDC-X and BDC-X4 (X = F, Cl, Br, I). The estimated energy gap for the BDC-F4 linker is 3.34 eV, which is lower than the value of BDC-F (3.45 eV). The difference between the dihedral angle of CAr − CAr−C = O in the two systems can be the main reason for this discrepancy. The dihedral angle value that is obtained from fully relaxed calculations for linker BDCF4 is 46°. Meanwhile, 0° is constrained by the symmetry of the crystalline system. So, to find the result of this limitation, the zero value of the dihedral angle was fixed within the linker optimization. The HOMO−LUMO gap for this configuration and

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Fig. 3 HOMO −LUMO energies of two kinds of conformers of BDC-Cl4 (left panel, planar conformer; right panel, vertical conformer). Gray, red, blue, and white represent C, O, Cl, and H, respectively. J. Phys. Chem. C 2014, 118, 9, 4567–4577 [17]

IRMOF-F4 are 2.92 and 2.77 eV, respectively [25, 26]. So, the result is in closer agreement. In the case of other halogens (Cl, Br, and I), the lowest energy structure is the configuration where the dihedral angle is 90° (Table 1). Accordingly, the energy gap of BDC-X4 increased instead of decreased compared to that of the BDC-X linker. The changing of the HOMO−LUMO energy of the BDC-Cl4 linker by varying the CAr−CAr−C = O dihedral angle is shown in Fig. 3 [17]. Due to the decrease of HOMO energy and the increase of LUMO energy, the number of nodes of HOMO−LUMO was changed by conversion angle from 0° to 90°. So, the CAr −CAr−C = O dihedral angle value can be considered an essential factor in predicting the energy gap.

4 Semiconductor Metal–Organic Framework (MOF) Photocatalyst Based on the above information and optical and tunable structure, the MOFs with Photocatalysis have attracted extensive attention. A semiconductor photocatalyst has its band gap. When the energy provided by light illumination is higher than the band gap energy, the negative electrons (e-) in the valance band (VB) will move to the conduction band (CB), generating positive holes (h+ ) in the VB. Then the reduction and oxidation half-reactions with the electrons and holes will be taken place, respectively (Fig. 4a) [10, 27]. As for MOFs, the organic linker is regarded as VB, and the metallic cluster plays the role of CB.

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Fig. 4 a The possible mechanism of oxidative photocatalytic degradation by MOF-5. (Chem. Eur. J. 2007, 13, 5109) [10], b Band gap values were observed for the different MOFs arranged in decreasing order with the linker groups’ structures. (ChemSusChem 2008, 1, 981) [28]

MOFs with organic components could be versatile. For instance, MOF-5 has an absorption spectrum with an onset at 450 nm, while pure aluminosilicates and zeolites generally could not absorb UV radiations of wavelengths longer than 220 nm [10]. This material can go through photochemical processes by photoexciting the organic linker. MOF-5 behaves as a semiconductor and undergoes charge separation (electrons and holes) by decaying in the microsecond time scale upon light excitation. Gascon and co-workers investigated the influence of organic linkers on the photocatalytic activity of MOF-5. As shown in Fig. 4b, the bandgap energy of metal–organic frameworks can be tuned by changing the organic linker [28]. They found that using the 2,6-naphthalenedicarboxylic acid as a linker performed the best catalytic activity in the photooxidation of propene.

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5 The Band Gap Value of Different Linkers of MOFs The band gap of MOF-5 as a semiconductor was simply calculated from the plot of the reflectance versus the radiation energy giving a value of 3.4 eV. The energy value of the conduction band or valence band in contact with the neutral aqueous solution could not be obtained by electrochemical measurements due to the limitation of the available potential window in water. These energy values were estimated by constructing a photovoltaic cell using MOF-5 as a semiconductor. A photovoltaic cell constructed with I2/I-electrolyte and TiO2 or MOF-5 as the photoactive semiconductor was applied to determine the conduction band’s position by comparing the open-circuit voltage [29]. The conduction band energy of MOF-5, 0.2 V versus NHE, has been estimated, taking into account that the photovoltaic cell with MOF-5 gives a value of open current voltage that is 0.3 V lower than the one obtained for a solar cell with TiO2 [30]. The estimated energy values for conduction and valence bands of MOF-5 were shown in Fig. 4b concerning these values in comparison to the TiO2 standard; the similarity of the band gap is remarkable, which allows excitation of MOF-5 in a similar UV spectral window as TiO2 [29]. The conduction and valence band in MOF-5 is shifted to more positive potentials than TiO2 . In other words, the charge-separated state of TiO2 has a more reducing conduction band electron and somewhat less oxidizing valence band hole than MOF-5. In many respects, MOF-5 is a microporous semiconductor that is stable to light exposure giving rise to charge-separated states. Therefore, charge separation occurs by light absorption on the ligand to the metal charge transfer band. Delocalized electrons living in the microsecond time scale and most probably occupying conduction bands have been detected by laser flash photolysis. Although a growing number of MOFs appeared for photocatalysis, their application was limited due to their large band gaps under visible light. So, reducing the bandgap to increase their activity under visible-light irradiation has stimulated researchers. There are several different strategies, including the decoration of linker or metal center [16, 18], combination with semiconductors [27, 31], and sensitization by such dyes [32, 33]. This is particularly important because it extensively functionalizes MOFs for visible-lightinduced photocatalysis, thereby leading to a far-reaching development of MOFs as photocatalysts. Das and co-workers synthesized a doubly interpenetrated semiconducting MOF Zn4 O(2,6-NDC)3- (DMF)1.5 (H2 O)0.5 ·4DMF·7.5H2 O (UTSA-38) for the methyl orange (MO) degradation [24]. The UV–vis diffuse reflective spectra (DRS) of UTSA-38 presented that it is a semiconductor MOF with a band gap of 2.85 eV. MIL-53 is a series of isostructural materials with the same organic linkers but different metal centers, including Fe, Al, and Cr. Among them, the MIL-53(Fe) possesses a narrow band gap of about 2.72 eV [34]. It has stimulated researchers to develop novel MOFs with a reduced band gap to enhance the activity under visiblelight irradiation. The fundamental structure–property relations between the organic linker and the band gap of MOFs, Flage-Larsen, and co-workers realized band gap modulations of the UiO-66-R (R=H, NH2 , NO2 ) and the band gap changes could be quantified. They demonstrated that the NH2 introduction reduced the band gap

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significantly and the changes related to the alters in the linker and the nonbonding oxygen near the metalloid cluster [35]. Later, the band gap modulation by the same three linker designs (BDC, BDC-NO2, and BDC-NH2 ) of UiO-66 was investigated via the computation and experimental perspectives [35]. The reduced band gaps of UiO-66-NH2 (2.75 eV) and UiO-66-NO2 (2.93 eV) were highly influenced by the bonding nature between the functional group and the aromatic carbon ring, which was confirmed by the time-dependent density functional calculations. The two studies as mentioned above guaranteed the linker decoration by NH2 group for UiO-66 possesses the smallest band gap, which makes the UiO-66-NH2 a popular material for the various catalytic studies [35]. Recently, a Zr-MOF-FePC decorated by 2-pyridinecarboxaldehyde (PC) and FeCl3 ·6H2 O based on UiO-66-NH2 was synthesized through combined covalent and dative post-synthetic modification [36].

References 1. Lee, J., Farha, O.K., Roberts, J., Scheidt, K.A., Nguyen, S.T., Hupp, J.T.: Metal–organic framework materials as catalysts. Chem. Soc. Rev. 38, 1450–1459 (2009). https://doi.org/10.1039/ B807080F 2. Usman, M., Mendiratta, S., Batjargal, S., Haider, G., Hayashi, M., Rao Gade, N., Chen, J.W., Chen, Y.F., Lu, K.L.: Semiconductor behavior of a three-dimensional strontium-based metal– organic framework. ACS Appl. Mater. Interfaces 7, 22767–22774 (2015). https://doi.org/10. 1021/acsami.5b07228 3. Mei, J., Diao, Y., Appleton, A.L., Fang, L., Bao, Z.: Integrated materials design of organic semiconductors for field-effect transistors. J. Am. Chem. Soc. 135, 6724–6746 (2013). https:// doi.org/10.1021/ja400881n 4. Allendorf, M.D., Schwartzberg, A., Stavila, V., Talin, A.A.: A roadmap to implementing metal– organic frameworks in electronic devices: challenges and critical directions. Chem. Eur. J. 17, 11372–11388 (2011). https://doi.org/10.1002/chem.201101595 5. Rajendran, S., Naushad, M., Ponce, L.C., Lichtfouse, E.: Green photocatalysts for energy and environmental process. Springer Nature 22, 13–7114 (2019). https://doi.org/10.1007/978-3030-17638-9 6. Shimizu, G.K., Taylor, J.M., Kim, S.: Proton conduction with metal-organic frameworks. Science 341, 354–355 (2013). https://doi.org/10.1126/science.1239872 7. Sun, L., Miyakai, T., Seki S., Dinc˘a, M.: Mn2 (2, 5-disulfhydrylbenzene-1, 4-dicarboxylate): a microporous metal–organic framework with infinite (− Mn–S−)∞ chains and high intrinsic charge mobility. J. Am. Chem. Soc. 135, 8185–8188 (2013). https://doi.org/10.1021/ja4037516 8. Talin, A.A., Centrone, A., Ford, A.C., Foster, M.E., Stavila, V., Haney, P., Kinney, R.A., Szalai, V., Gabaly, F.E.l., Yoon, H.P.: Tunable electrical conductivity in metal-organic framework thin-film devices Science 343, 66–69 (2014). https://doi.org/10.1126/science.1246738 9. Gao, S.T., Liu, W.H., Shang, N.Z., Feng, C., Wu, Q.H., Wang, Z., Wang, C.: Integration of a plasmonic semiconductor with a metal–organic framework: a case of Ag/AgCl@ ZIF-8 with enhanced visible light photocatalytic activity. RSC Adv. 4, 61736–61742 (2014). https://doi. org/10.1039/C4RA11364K 10. Silva, C.G., Corma, A., García, H.: Metal–organic frameworks as semiconductors. J. Mater. Chem. 20, 3141–3156 (2010). https://doi.org/10.1039/B924937K 11. Karthikeyan, M., Bhagyaraju, B., Mariappan, C.R., Mobin, S.M., Manimaran, B.: Novel semiconducting metal-organic framework: Synthesis, structural characterisation and electrical conductivity studies of manganese based two-dimensional coordination polymer. Inorg. Chem. Commun. 20, 269–272 (2012). https://doi.org/10.1016/j.inoche.2012.03.024

MOFs Bandstructure

89

12. Hendon, C.H., Tiana, D., Fontecave, M., Cm, S., D’arras, L., Sassoye, C., Rozes, L., MellotDraznieks, C., Walsh, A.: Engineering the optical response of the titanium-MIL-125 metal– organic framework through ligand functionalization. J. Am. Chem. Soc. 135, 10942–10945 (2013). https://doi.org/10.1021/ja405350u 13. Chen, X., Shen, S., Guo, L., Mao, S.S.: Semiconductor-based photocatalytic hydrogen generation. Chem. Rev. 110, 6503–6570 (2010). https://doi.org/10.1021/cr1001645 14. Musho, T., Li, J., Wu, N.: Band gap modulation of functionalized metal–organic frameworks. Phys. Chem. Chem. Phys. 16, 23646–23653 (2014). https://doi.org/10.1039/C4CP03110E 15. Alvaro, M., Carbonell, E., Ferrer, B., Llabrés i Xamena, F.X., Garcia, H.: Semiconductor behavior of a metal-organic framework (MOF), Chem. Eur. J. 13, 5106–5112 (2007). https:// doi.org/10.1002/chem.200601003 16. Butler, K.T., Hendon, C.H., Walsh, A.: Electronic chemical potentials of porous metal–organic frameworks. J. Am. Chem. Soc. 136, 2703–2706 (2014). https://doi.org/10.1021/ja4110073 17. Pham, H.Q., Mai, T., Pham-Tran, N.N., Kawazoe, Y., Mizuseki, H., Nguyen-Manh, D.: Engineering of band gap in metal-organic frameworks by functionalizing organic linker: a systematic density functional theory investigation. J. Phys. Chem. C 118(9), 4567–4577 (2014). https:// doi.org/10.1021/jp405997r 18. Fu, Y., Sun, D., Chen, Y., Huang, R., Ding, Z., Fu, X., Li, Z.: An amine-functionalized titanium metal–organic framework photocatalyst with visible-light-induced activity for CO2 reduction. Angew. Chem. Int. Ed. 51, 3364–3367 (2012). https://doi.org/10.1002/anie.201108357 19. Yang, L.M., Ravindran, P., Vajeeston, P., Tilset, M.: Ab initio investigations on the crystal structure, formation enthalpy, electronic structure, chemical bonding, and optical properties of experimentally synthesized isoreticular metal-organic framework-10 and its analogues: MIRMOF-10 (M = Zn, Cd, Be, Mg, Ca, Sr and Ba). RSC Adv. 2, 1618−1631 (2012). https:// doi.org/10.1039/C1RA00187F 20. Yang, R.L.M., Vajeeston, P., Tilset, P.: Properties of IRMOF-14 and its analogues M-IRMOF-14 (M = Cd, Alkaline Earth Metals): electronic structure, structural stability, chemical bonding, and optical properties. Phys. Chem. Chem. Phys. 14, 4713–4723 (2012). https://doi.org/10. 1039/C2CP24091B 21. Corey, R.R., Lecture, E.J.: Retrosynthetic thinkingessentials and examples. Chem. Soc. Rev. 17, 111–133 (1988). https://doi.org/10.1039/CS9881700111 22. Yaghi, O.M., O’Keeffe, M., Ockwig, N.W., Chae, H.K., Eddaoudi, M., Kim, J.: Reticular synthesis and the design of new materials. Nature 423, 705–714 (2003). https://doi.org/10. 1038/nature01650 23. Choi, J.H., Choi, Y.J., Lee, J.W., Shin, W.H., Kang, J.K.: Tunability of electronic band gaps from semiconducting to metallic states via tailoring Zn ions in MOFs with coIons. Phys. Chem. Chem. Phys. 11, 628–631 (2009). https://doi.org/10.1039/B816668D 24. Ockwig, N.W., Delgado-Friedrichs, O., O’Keeffe, M., Yaghi, O.M.: Reticular chemistry: occurrence and taxonomy of nets and grammar for the design of frameworks. Acc. Chem. Res. 38, 176–182 (2005). https://doi.org/10.1021/ar020022l 25. Meek, S.T., Perry, J.J., Teich-McGoldrick, S.L., Greathouse, J.A., Allendorf, M.D.: Complete series of monohalogenated isoreticular metal−organic frameworks: synthesis and the importance of activation method. Cryst. Growth Des. 11, 4309−4312 (2011). https://doi.org/10.1021/ cg201136k 26. Rowsell, J.L.C., Yaghi, O.M.: Effects of functionalization, catenation, and variation of the metal oxide and organic linking units on the low-pressure hydrogen adsorption properties of metal−organic frameworks. J. Am. Chem. Soc. 128, 1304–1315 (2006). https://doi.org/10. 1021/ja056639q 27. Serpone, N., Emeline, A.: Semiconductor photocatalysis past, present, and future outlook. ACS Publ. 35, 673–677 (2012). https://doi.org/10.1021/jz300071j 28. Gascon, J., Hernandez-Alonso, M.D., Almeida, A.R., Gerard, P.M., Klink, V., Kapteijn, F., Mula, G.: Isoreticular MOFs as efficient photocatalysts with tunable band gap: an operando FTIR study of the photoinduced oxidation of propylene. Chemsuschem 1, 981–983 (2008). https://doi.org/10.1002/cssc.200800203

90

Z. Hajizadeh and M. M. Salehi

29. Hagfeldt, A., Graetzel, M.: Light-induced redox reactions in nanocrystalline systems. Chem. Rev. 95, 49–68 (1995). https://doi.org/10.1021/cr00033a003 30. Bordiga, S., Lamberti, C., Ricchiardi, G., Regli, L., Bonino, F., Damin, A., Lillerud, K.P., Bjorgen, M., Zecchina, A.: Electronic and vibrational properties of a MOF-5 metal–organic framework: ZnO quantum dot behavior. Chem. Commun. 2300–2301 (2004). https://doi.org/ 10.1039/B407246D 31. Guo, D., Wen, R., Liu, M., Guo, H., Chen, J., Weng, W.: Facile fabrication of g-C3N4/MIL-53 (Al) composite with enhanced photocatalytic activities under visible-light irradiation. Appl. Organomet. Chem. 29, 690–697 (2015). https://doi.org/10.1002/aoc.3352 32. He, J., Wang, J., Chen, Y., Zhang, J., Duan, D., Wang, Y., Yan, Z.: A dye-sensitized Pt@ UiO66 (Zr) metal–organic framework for visible-light photocatalytic hydrogen production. Chem. Commun. 50, 7063–7066 (2014). https://doi.org/10.1039/C4CC01086H 33. Kuang, P.Y., Su, Y.Z., Chen, G.F., Luo, Z., Xing, S.Y., Li, N., Liu, Z.Q.: g-C3N4 decorated ZnO nanorod arrays for enhanced photoelectrocatalytic performance. Appl. Surf. Sci. 358, 296–303 (2015). https://doi.org/10.1016/j.apsusc.2015.08.066 34. Du, J.J., Yuan, Y.P., Sun, J.X., Peng, F.M., Jiang, X., Qiu, L.G., Xie, A.J., Shen, Y.H., Zhu, J.F.: New photocatalysts based on MIL-53 metal–organic frameworks for the decolorization of methylene blue dye. J. Hazard. Mater. 190, 945–951 (2011). https://doi.org/10.1016/j.jhazmat. 2011.04.029 35. Flage Larsen, E., Røyset, A., Cavka, J.H., Thorshaug, K.: Band gap modulations in UiO metal– organic frameworks. J. Phys. Chem. C 117, 20610–20616 (2013). https://doi.org/10.1021/acs. chemmater.6b05444 36. Azarifar, D., Ghorbani-Vaghei, R., Daliran, S., Oveisi, A.R.: A multifunctional zirconiumbased metal-organic framework for the one-pot tandem photooxidative passerini threecomponent reaction of alcohols. ChemCatChem 9, 1992–2000 (2017). https://doi.org/10.1002/ cctc.201700169

Evolution in MOF Porosity, Modularity, and Topology Fatemeh Ganjali, Peyman Ghorbani, Nima Khaleghi, and Maryam Saidi Mehrabad

Abstract From the time metal–organic frameworks (MOFs) emerged, this class of substances has received increasing attention due to their crystallinity and porosity which led them to be applied in a wide spectrum of applications, including gas storage, catalysis, drug delivery, water entrapment, gas separation, and sensors. Porosity is one of the crucial characteristics of MOFs that renders micro and meso scale voids to enclose and accomplish the functionalities. Thus, MOF design with high porosity and enhancing the appropriate activation approaches to preserve and access the pores have been a highlighted subject in the MOF field. Highly adjustable metal nodes and organic linkers in a convenient MOFs’ design refer to the reticular chemistry, providing various MOF pore structures, topologies, and functions. Based on the developing research on MOF porosity, the experiments on adjustable pores and easy access to their pore space will carry on. Keywords Activation · Catenation · Isoreticular contraction · Isoreticular expansion · Reticular chemistry

F. Ganjali (B) · M. S. Mehrabad Catalysis and Organic Synthesis Research Laboratory, Department of Chemistry, Iran University of Science and Technology, 16846-13114 Tehran, Iran e-mail: [email protected]; [email protected] P. Ghorbani School of Chemistry, College of Science, University of Tehran, Tehran, Iran e-mail: [email protected] N. Khaleghi Endocrinology and Metabolism Research Center, Endocrinology and Metabolism Clinical Sciences Institute, Tehran University of Medical Sciences, Tehran, Iran e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 A. Maleki and R. Taheri-Ledari (eds.), Physicochemical Aspects of Metal-Organic Frameworks, Engineering Materials, https://doi.org/10.1007/978-3-031-18675-2_7

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1 Introduction The porous materials enhancements have been evaluated to be one of the most crucial factors for technologies in daily life and different industrial applications, including filters, masks, adsorbents, foams, and catalysts. One of the first and foremost characteristics of the porous materials relates to their low density since they have abundant structural voids, through which the modifications and functionalization of the porous materials would be designed due to the demanded application. Porous substances with long-range ordered crystallinity have garnered attention based on their various and convenient structure control [1]. The first reports of crystalline porous materials were named metal–organic frameworks (MOFs) and demonstrated permanent porous structures with a high surface area which are assigned to the strong bonds between metal ions and organic ligands [2, 3]. The adjustability of the MOF’s building units, i.e., inorganic metal nodes or clusters and organic linkers, has shed light on the arena of porous materials’ design and application. In spite of diverse possibilities in combining organic and inorganic building units with various geometries, MOFs provide unparalleled adjustability in isoreticular manipulation [4]; while preserving the structural design and topology, resulting in a huge number of structural alterations prepared through synthesis steps or post-synthetic modifications. Due to the potential to create a wide range of visually appealing structures that could also be extremely valuable for applications in a number of sectors connected to porous materials, the synthesis of metal–organic frameworks (MOFs) has drawn enormous focus over the last two decades. MOFs may fill a need in searching for novel porous materials since they have advantages over organic and inorganic building blocks. The concept and progress of reticular chemistry are enormous in synthesizing and applying frameworks like MOFs with high porosity and long-ranged crystallinity [5, 6]. The nanoporous materials have exhibited enhanced applicability in various fields, such as drug delivery [7–12], catalysis [13–20], supercapacitor [21], solar cells [22, 23], and wound engineering [24–26], water remediation [27, 28]. Based on the definitions, MOFs are categorized as nanoporous materials with spectacular features, such as tunability, low density, gas, and liquid permeability. The MOF chemistry field moves forward to reach new evolutions, focusing on the porosity and how to reach pores for further modifications and applicabilities.

2 Porosity of MOFs In terms of pore size, porous materials like MOFs are divided into three groups by the International Union of Pure and Applied Chemistry (IUPAC): microporous, mesoporous, and microporous [29]. The pore width in microporous materials is less than 2 nm, while the pore diameters of mesoporous materials range from 2 to 50 nm, and more than 50 nm of pore space is found in macroporous materials. Probe molecules such as nitrogen (N2 ) and argon (Ar) are commonly used to evaluate the porosity

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of MOFs by conducting adsorption isotherms. However, due to its accessibility and low cost, N2 adsorption studies are commonly used by research teams to examine the porosity of MOF materials. Although the quadrupole moment of carbon dioxide (CO2 ) is even stronger than that of nitrogen (N2 ), the IUPAC advises against using it to analyze porous materials with polar surfaces. However, CO2 adsorption is occasionally used to obtain the porosity of MOFs with small pore sizes (smaller than 0.45 nm), when N2 and Ar molecules are difficult to diffuse into the pores. Adsorption data are typically given as a physisorption isotherm, which is the “quantity of gas adsorbed” plotted versus “relative pressure.” Physisorption isotherms were divided by IUPAC into six traditional kinds (types I through VI) [29]. While numerous isotherms of flexible MOFs display geometries that are different from the six main kinds, the majority of physisorption isotherms of rigid MOFs are based on type I (microporous) or type IV (mesoporous) [30, 31]. Flexible MOFs undergo structural changes during the adsorption process, which makes it particularly challenging to interpret their isotherms. Therefore, it is imperative to continue to develop cuttingedge methods for the analysis and classification of new varieties of physisorption isotherms seen in flexible MOFs [32]. It is important to note that various adsorbates, such as water, frequently exhibit different isotherm types during the adsorption process, and the isotherm’s form can rely on the complex characteristics of the MOF adsorbents [33, 34].

2.1 Surface Area and Distribution of Size and Volume of Pores Collecting physisorption isotherms has become a standard way to characterize MOF materials since the materials’ persistent porosity was established. Surface area and distribution of pore sizes and volume can all be determined from an isotherm’s interpretation. The Brunauer–Emmett–Teller (BET) method is often used to calculate the surface area of porous MOFs based on a multilayer adsorption model. When micropores are present, as is the case for the majority of MOFs, the IUPAC recommends using the BET equation with extreme caution. It is also applicable to many type II and type IV isotherms for porous materials with pore widths greater than 4 nm [29]. C −1 P P/P0 1 + ( ) = N mC N mC P0 N (1 − PP0 ) Finding a linear range for BET calculations is challenging since it is difficult to distinguish between monolayer adsorption, multilayer adsorption, and micropore filling in the case of MOFs containing micropores. To that aim, it is advised that the terms “apparent BET surface area” or “estimated BET area” be extensively utilized. To determine the linear range needed to calculate the estimated BET area, strict criteria should be used. N(1–P/P0 ) growing monotonically with P/P0 is the first

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criterion in the equation above, where N denotes the adsorbate loading and P/P0 denotes the relative pressure. To begin with, the linear regression’s C value must be a positive number. If P/P0 falls within the given linear range, the monolayer loading must be within the selected range of P/P0 . Using BET theory, the P/P0 for monolayer loading should be close to the predicted relative pressure in the third criterion [35]. Practically, it is advised to minimize variation from these criteria when it is hard to choose an area from experimental isotherms that satisfies all four BET consistency criteria [35]. It is significant to note that many published BET area values only satisfy the first two BET consistency requirements; nevertheless, we think the trend will be toward using all four BET consistency criteria, especially for highly porous MOFs with large surface areas [36, 37]. Despite these drawbacks, the BET area offers a reliable way to compare the surface area of MOFs and can be a useful fingerprint for MOF adsorbents. Another important factor for determining the porosity of MOFs is pore volume. Ideally, a MOF should have a nearly horizontal plateau in the isotherm if it has a small exterior surface area and doesn’t have any big pores, such as macropores. When the adsorption isotherm is saturated with adsorbed molecules, type I isotherms for a microporous MOF and type IV isotherms for a mesoporous MOF reach the plateau. The observed pore volume can be determined from the adsorbed capacity by applying the Gurvich rule and assuming that the condensed adsorbate filled the MOF pores in the liquid state. The conventional method for calculating total pore volume is to absorb at a relative pressure close to unity. The theoretical pore volume of MOFs, on the other hand, may be derived from a crystal structure and is a useful benchmark for the experimental pore volume. It is important to take into account both pore geometries (such as slit, cylinder, and spherical) and kernels when evaluating the pore size distribution from isotherms. Depending on the chosen shape models and kernels, the total pore volume estimated from MOF isotherms and divided into the various pore widths can vary greatly. For the purpose of determining the pore size distribution, a number of techniques, including Horvath Kawazoe (HK), density functional theory (DFT), and Monte Carlo simulation (MC), have been developed over time [38]. Despite theoretical restrictions that assume a molecularly flat surface, the DFT pore size distribution provides a pretty accurate assessment for the majority of porous MOFs. In order to do this, it is critical to improve pore size distribution models for MOFs, especially flexible MOFs.

2.2 Methods for Porous MOF Designing MOFs can have a variety of topologies and porosities depending on the rigidity of metal–ligand connections, the geometry of metal clusters, and the organic struts. MOF design is sometimes compared to making tinker toys. Reticular chemistry and the molecular building block method have made MOFs accessible to a huge number of researchers all over the world, notwithstanding the oversimplification of this concept. For the rational design of MOFs, theoretical topologies can be developed by matching the geometry of the metal nodes and organic linkers. MOF discovery

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can be aided by the development of a library of suitable geometries and MBBs [39]. Because MOF building blocks have a well-defined structure and geometry by the reciprocal combination of experimental and computational investigations, this geometry matching technique has been frequently used in MOF design. Several general strategies for the design of MOFs with distinct pore systems are presented, along with illustrative examples.

2.3 Controlling Porosity Using Isoreticular Expansion and/or Contraction By changing the size or chemical functions of the organic linkers while keeping the same underlying MOF topology, isoreticular expansion/contraction is one frequently used technique to modify MOF porosity [40, 41]. It is possible to produce higher porosity systems and identify the ideal pore sizes for a variety of applications, including gas storage and catalysis, by controlling the isoreticular synthesis of MOFs [42]. In this line, the implementation of alternative ditopic carboxylic-acid-based linkers presenting various lengths and functionalities results in the establishment of an isoreticular series of sixteen IRMOFs. In this regard, each series pore volume can be adjusted to reach ca. 90% free volume, considering each MOF’s functionalization or catenation. Catenation is mathematically restricted via framework design, as represented by the “infinite SBU” method introduced by Yaghi et al. The MOF-69A is comprised of infinite Zn–O–C SBUs that are connected via ditopic linkers. Therefore, the distance between the linkers in the direction of [0 0 1] plane is small, resulting in an “impenetrable wall” of the phenyl rings’ π–π stacking (Fig. 1); while the distance between carbon atoms in carboxylate is larger in the [1 1 0] direction. According to this design, large pores can be formed utilizing long linkers without catenation risk because the impenetrable wall in the [0 0 1] direction restricts catenation [43]. Except for the de novo synthesis, by metal ion alteration to the harder metal ions, the kinetically unstable metal-carboxylate coordination bonds strengthen, the kinetically inert metal–O bonds form, and hence the framework stabilizes. For instance, Zhou et al. have performed a post-synthesis metathesis and oxidation (PSMO) technique for synthesizing MOFs with high stability from MOFs with kinetically unstable connections, like Mg-carboxylate [44]. At first, the ions of the parent frameworks were substituted with hard metal ions with lower oxidation states like Fe2+ and Cr2+ ; then, the resulting isostructural MOFs with new metal ions were exposed to oxidation to form kinetically more inert Fe3+ – or Cr3+ –O bonds (Fig. 2).

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Fig. 1 The structure of MOF-69A a. The rod-packing Zn-carboxylate SBU columns b. The open channels viewed along the c-axis c. The “impenetrable wall” of linkers is viewed along the a-axis. This figure was adapted with permission from Chemical Society Reviews, 49, 7406–7427 [6]

3 Topology Defining and Comprehending the matter is the main part of the chemical sciences. For finite units like molecules, this is mainly negligible; nonetheless, crystalline structure description of solid-state substances is importantly more challenging, and abundant methods can be applied to describe such structures. The crystal structure is the most informative definition and consists of information about the overall structure’s atomic composition, connectivity, spatial arrangement, and symmetry. This definition seems to be complicated, and thus, in the inorganic solid-state chemistry field, a conception of describing crystal structures as a packing of one type of atom where other atoms occupy the vacancies of the structure is conventional practice. Although this is practical for ionic solids, this is less beneficial for extended solids, including metal–organic frameworks (MOFs), zeolitic imidazolate frameworks (ZIFs), and covalent organic frameworks (COFs). The extended structures definition in topology is more often utilized [45]. This concept permits simplifying structures by contemplating constituents’ linkages, not their chemical character. This method importantly diminishes the complex description of the desired structure [46, 47]. Except for the simplified definition of crystal structures, this conception permits the reverse engineering of crystal matter [48]. Nets are defined as nodes collection,

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Fig. 2 a Schematic diagram of metathesis-oxidation strategy of M3-cluster. b Structures of NU 1500-Cr (left) and Cr-soc-MOF-1 (right). Color scheme: C (gray), O (red), Fe3+ (green), Cr2+ (blue), and Cr3+ (viridian). This figure was adapted with permission from Chemical Society Reviews, 49, 7406–7427 [6]

which is connected by linkages (edges). They are a unique kind of graph, an abstract mathematical object. Mathematical graphs are determined between infinite and finite graphs and categorized due to their features into (i) graphs containing loops, meaning vertices linked to themselves, (ii) graphs comprised of vertices that are attached by multiple links, (iii) graphs that are directional, and (iv) graphs with loose ends. Due to the definition of the MOF’s crystal structures in the case of nets, there are only infinite 2- and 3-periodic graphs, which means they do not have any of the features mentioned above (loops, multiple connections, directionality, and loose ends). It is crucial to note that periodicity is not synonymous with dimensionality. While every polyhedron is a 3D object, it is not periodic.

4 Conclusion In the last two decades, MOF research had a huge growth in the enhancement of new materials and using these programmable substances in a wide application area. The majority of reports focus on the design and control of the porosity as the porosity is one of the most fundamental features of MOFs and several applications rely on these characteristics. Looking forward, the efforts will go on to better control the reach of the MOF’s voids and the functionalities inside the framework. One of the

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challenges in this area is to prepare MOFs with high amounts of porosity and balanced gravimetric and volumetric surface areas to be employed as adsorbents in on-board storage and clean fuel delivery. From a historical point of view, the MOF research field has been successful along with challenges, but with a bright future with plenty of opportunities.

References 1. Davis, M.E.: Ordered porous materials for emerging applications. Nature 417(6891), 813–821 (2002) 2. Yaghi, O., Li, H.: Hydrothermal synthesis of a metal-organic framework containing large rectangular channels. J. Am. Chem. Soc. 117(41), 10401–10402 (1995) 3. Yaghi, O.M., Li, G., Li, H.: Selective binding and removal of guests in a microporous metal– organic framework. Nature 378(6558), 703–706 (1995) 4. Eddaoudi, M., Kim, J., Rosi, N., Vodak, D., Wachter, J., O’Keeffe, M., Yaghi, O.M.: Systematic design of pore size and functionality in isoreticular MOFs and their application in methane storage. Science 295(5554), 469–472 (2002) 5. Gropp, C., Canossa, S., Wuttke, S., Gándara, F., Li, Q., Gagliardi, L., Yaghi, O.M.: Standard practices of reticular chemistry. ACS Publications (2020) 6. Zhang, X., Chen, Z., Liu, X., Hanna, S.L., Wang, X., Taheri-Ledari, R., Maleki, A., Li, P., Farha, O.K.: A historical overview of the activation and porosity of metal–organic frameworks. Chem. Soc. Rev. 49(20):7406–7427 (2020) 7. Zhang, W., Taheri-Ledari, R., Ganjali, F., Afruzi, F.H., Hajizadeh, Z., Saeidirad, M., Qazi, F.S., Kashtiaray, A., Sehat, S.S., Hamblin, M.R.: Nanoscale Bioconjugates: A review of the structural attributes of drug-loaded nanocarrier conjugates for selective cancer therapy. Heliyon e09577 (2022) 8. Parvaz, S., Taheri-Ledari, R., Esmaeili, M.S., Rabbani, M., Maleki, A.: A brief survey on the advanced brain drug administration by nanoscale carriers: with a particular focus on AChE reactivators. Life Sci. 240, 117099 (2020) 9. Taheri-Ledari, R., Fazeli, A., Kashtiaray, A., Salek Soltani, S., Maleki, A., Zhang, W.: Cefiximecontaining silica nanoseeds coated by a hybrid PVA-gold network with a Cys-Arg dipeptide conjugation: enhanced antimicrobial and drug release properties. Langmuir 38(1), 132–146 (2021) 10. Taheri-Ledari, R., Zhang, W., Radmanesh, M., Mirmohammadi, S.S., Maleki, A., Cathcart, N., Kitaev, V.: Multi-stimuli nanocomposite therapeutic: docetaxel targeted delivery and synergies in treatment of human breast cancer tumor. Small 16(41), 2002733 (2020) 11. Taheri-Ledari, R., Maleki, A., Zolfaghari, E., Radmanesh, M., Rabbani, H., Salimi, A., Fazel, R.: High-performance sono/nano-catalytic system: Fe3 O4 @Pd/CaCO3 -DTT core/shell nanostructures, a suitable alternative for traditional reducing agents for antibodies. Ultrason. Sonochem. 61, 104824 (2020) 12. Taheri-Ledari, R., Zhang, W., Radmanesh, M., Cathcart, N., Maleki, A., Kitaev, V.: Plasmonic photothermal release of docetaxel by gold nanoparticles incorporated onto halloysite nanotubes with conjugated 2D8-E3 antibodies for selective cancer therapy. J. Nanobiotechnol. 19(1), 1–21 (2021) 13. Ghafuri, H., Ganjali, F., Hanifehnejad, P.: Cu. BTC MOF as a novel and efficient catalyst for the synthesis of 1, 8-Dioxo-octa-hydro Xanthene. Chem. Proc. 3(1):2 (2020) 14. Doustkhah, E., Mohtasham, H., Farajzadeh, M., Rostamnia, S., Wang, Y., Arandiyan, H., Assadi, M.H.N.: Organosiloxane tunability in mesoporous organosilica and punctuated Pd nanoparticles growth; theory and experiment. Microporous Mesoporous Mater. 293, 109832 (2020)

Evolution in MOF Porosity, Modularity, and Topology

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15. Farajzadeh, M., Alamgholiloo, H., Nasibipour, F., Banaei, R., Rostamnia, S.: Anchoring Pd nanoparticles on dithiocarbamate-functionalized SBA-15 for hydrogen generation from formic acid. Sci. Rep. 10(1), 1–9 (2020) 16. Mohammadi, R., Alamgholiloo, H., Gholipour, B., Rostamnia, S., Khaksar, S., Farajzadeh, M., Shokouhimehr, M.: Visible-light-driven photocatalytic activity of ZnO/g-C3 N4 heterojunction for the green synthesis of biologically interest small molecules of thiazolidinones. J. Photochem. Photobiol. A 402, 112786 (2020) 17. Nouruzi, N., Dinari, M., Mokhtari, N., Farajzadeh, M., Gholipour, B., Rostamnia, S.: Selective catalytic generation of hydrogen over covalent organic polymer supported Pd nanoparticles (COPPd). Mol. Catal. 493, 111057 (2020) 18. Mohtasham, H., Gholipour, B., Rostamnia, S., Ghiasi-Moaser, A., Farajzadeh, M., Nouruzi, N., Jang, H.W., Varma, R.S., Shokouhimehr, M.: Hydrothermally exfoliated P-doped g-C3 N4 decorated with gold nanorods for highly efficient reduction of 4-nitrophenol. Colloids Surf. A Physicochem. Eng. Asp. 614, 126187 (2021) 19. Nouruzi, N., Dinari, M., Gholipour, B., Mokhtari, N., Farajzadeh, M., Rostamnia, S., Shokouhimehr, M.: Photocatalytic hydrogen generation using colloidal covalent organic polymers decorated bimetallic Au-Pd nanoalloy (COPs/Pd-Au). Mol. Catal. 518, 112058 (2022) 20. Doustkhah, E., Farajzadeh, M., Mohtasham, H., Habeeb, J., Rostamnia, S.: Exfoliated graphene-based 2D materials: synthesis and catalytic behaviors. Handbook of Graphene Set 1, 529–558 (2019) 21. Eivazzadeh-Keihan, R., Taheri-Ledari, R., Mehrabad, M.S., Dalvand, S., Sohrabi, H., Maleki, A., Mousavi-Khoshdel, S.M., Shalan, A.E.: Effective combination of rGO and CuO nanomaterials through poly (p-phenylenediamine) texture: utilizing it as an excellent supercapacitor. Energy Fuels 35(13), 10869–10877 (2021) 22. Valadi, K., Gharibi, S., Taheri-Ledari, R., Akin, S., Maleki, A., Shalan, A.E.: Metal oxide electron transport materials for perovskite solar cells: a review. Environ. Chem. Lett. 19(3), 2185–2207 (2021) 23. Taheri-Ledari, R., Valadi, K., Maleki, A.: High-performance HTL-free perovskite solar cell: an efficient composition of ZnO NRs, RGO, and CuInS2 QDs, as electron-transporting layer matrix. Prog. Photovolt. 28(9), 956–970 (2020) 24. Eivazzadeh-Keihan, R., Ganjali, F., Aliabadi, H.A.M., Maleki, A., Pouri, S., Mahdavi, M., Shalan, A.E., Lanceros-Méndez, S.: Synthesis and characterization of cellulose, β-cyclodextrin, silk fibroinbased hydrogel containing copper-doped cobalt ferrite nanospheres and exploration of its biocompatibility. J. Nanostruct. Chem. 1–11 (2022) 25. Eivazzadeh-Keihan, R., Choopani, L., Aliabadi, H.A.M., Ganjali, F., Kashtiaray, A., Maleki, A., Cohan, R.A., Bani, M.S., Komijani, S., Ahadian, M.M.: Magnetic carboxymethyl cellulose/silk fibroin hydrogel embedded with halloysite nanotubes as a biocompatible nanobiocomposite with hyperthermia application. Mater. Chem. Phys. 126347 (2022) 26. Ganjali, F., Eivazzadeh-Keihan, R., Aghamirza Moghim Aliabadi, H., Maleki, A., Pouri, S., Ahangari Cohan, R., Hashemi, S.M., Mahdavi, M.: Biocompatibility and antimicrobial investigation of agar-tannic acid hydrogel reinforced with silk fibroin and zinc manganese oxide magnetic microparticles. J. Inorg .Organomet. Polym. Mater. 1–13 (2022) 27. Hassanzadeh-Afruzi, F., Esmailzadeh, F., Asgharnasl, S., Ganjali, F., Taheri-Ledari, R., Maleki, A.: Efficient removal of Pb (II)/Cu (II) from aqueous samples by a guanidine-functionalized SBA-15/Fe3 O4 . Sep. Purif. Technol. 291, 120956 (2022) 28. Ganjali, F., Kashtiaray, A., Zarei-Shokat, S., Taheri-Ledari, R., Maleki, A.: Functionalized hybrid magnetic catalytic systems on micro-and nanoscale utilized in organic synthesis and degradation of dyes. Nanoscale Adv. (2022) 29. Singh, J., Kalamdhad, A.: Effects of heavy metals on soil. Plants Human Health and Aquatic Life 1, 1051–1069 (2011) 30. Lee, J.H., Jeoung, S., Chung, Y.G., Moon, H.R.: Elucidation of flexible metal-organic frameworks: research progresses and recent developments. Coord. Chem. Rev. 389, 161–188 (2019)

100

F. Ganjali et al.

31. Zhang, J.-P., Zhou, H.-L., Zhou, D.-D., Liao, P.-Q., Chen, X.-M.: Controlling flexibility of metal–organic frameworks. Natl. Sci. Rev. 5(6), 907–919 (2018) 32. Cychosz, K.A., Thommes, M.: Progress in the physisorption characterization of nanoporous gas storage materials. Engineering 4(4), 559–566 (2018) 33. Hanikel, N., Prévot, M.S., Yaghi, O.M.: MOF water harvesters. Nat. Nanotechnol. 15(5), 348– 355 (2020) 34. Coudert, F.-X.: Water adsorption in soft and heterogeneous nanopores. Acc. Chem. Res. 53(7), 1342–1350 (2020) 35. Gómez-Gualdrón, D.A., Moghadam, P.Z., Hupp, J.T., Farha, O.K., Snurr, R.Q.: Application of consistency criteria to calculate BET areas of micro-and mesoporous metal–organic frameworks. J. Am. Chem. Soc. 138(1), 215–224 (2016) 36. Chen, Z., Li, P., Anderson, R., Wang, X., Zhang, X., Robison, L., Redfern, L.R., Moribe, S., Islamoglu, T., Gómez-Gualdrón, D.A.: Balancing volumetric and gravimetric uptake in highly porous materials for clean energy. Science 368(6488), 297–303 (2020) 37. Chen, Z., Li, P., Wang, X., Otake, K-I., Zhang, X., Robison, L., Atilgan, A., Islamoglu, T., Hall, M.G., Peterson, G.W.: Ligand-directed reticular synthesis of catalytically active missing zirconium based metal–organic frameworks. J. Am. Chem. Soc. 14, 12229–12235 (2019) 38. Rouquerol, J., Rouquerol, F., Llewellyn, P., Maurin, G., Sing, K.S.: Adsorption by powders and porous solids: principles, methodology and applications. Academic Press (2013) 39. Boyd, P.G., Chidambaram, A., García-Díez, E., Ireland, C.P., Daff, T.D., Bounds, R., Gładysiak, A., Schouwink, P., Moosavi, S.M., Maroto-Valer, M.M.: Data-driven design of metal–organic frameworks for wet flue gas CO2 capture. Nature 576(7786), 253–256 (2019) 40. Wang, T.C., Bury, W., Gómez-Gualdrón, D.A., Vermeulen, N.A., Mondloch, J.E., Deria, P., Zhang, K., Moghadam, P.Z., Sarjeant, A.A., Snurr, R.Q.: Ultrahigh surface area zirconium MOFs and insights into the applicability of the BET theory. J. Am. Chem. Soc. 137(10), 3585–3591 (2015) 41. Maldonado, R.R., Zhang, X., Hanna, S., Gong, X., Gianneschi, N.C., Hupp, J.T., Farha, O.K.: Squeezing the box: isoreticular contraction of pyrene-based linker in a Zr-based metal–organic framework for Xe/Kr separation. Dalton Trans. 49(20), 6553–6556 (2020) 42. Jiang, H., Zhang, W., Kang, X., Cao, Z., Chen, X., Liu, Y., Cui, Y.: Topology-based functionalization of robust chiral Zr-based metal–organic frameworks for catalytic enantioselective hydrogenation. J. Am. Chem. Soc. 142(21), 9642–9652 (2020) 43. Rosi, N.L., Eddaoudi, M., Kim, J., O’Keeffe, M., Yaghi, O.M.: Infinite Secondary Building Units and Forbidden Catenation in Metal-Organic Frameworks. Angew. Chem. Int. Ed. Engl. 41(2):284–287 (2002) 44. Liu, T.-F., Zou, L., Feng, D., Chen, Y.-P., Fordham, S., Wang, X., Liu, Y., Zhou, H.-C.: Stepwise synthesis of robust metal–organic frameworks via postsynthetic metathesis and oxidation of metal nodes in a single-crystal to single-crystal transformation. J. Am. Chem. Soc. 136(22), 7813–7816 (2014) 45. Öhrström, L.: Designing, describing and disseminating new materials by using the network topology approach. Chem. Eur. J. 22(39), 13758–13763 (2016) 46. Wells, A.: The geometrical basis of crystal chemistry. Part 1. Acta Crystallogr. 7(8–9):535–544 (1954) 47. Wells, A.F.: Structural inorganic chemistry. Oxford University Press (2012) 48. Yaghi, O.M., O’Keeffe, M., Ockwig, N.W., Chae, H.K., Eddaoudi, M., Kim, J.: Reticular synthesis and the design of new materials. Nature 423(6941), 705–714 (2003)

MOF Scaffolds Tunability and Flexibility Fereshteh Rasouli Asl, Fatemeh Ganjali , and Zahra Rashvandi

Abstract Metal–organic frameworks (MOFs) appeared as a subclass of highly crystalline inorganic–organic materials, with huge surface areas, tunable pores, and magnetic nanostructures. Heterostructure MOF composites are adsorbed attention to the field of chemistry and materials science. Recently studies have focused on assembling homogeneous or heterogeneous MOFs with various structures and morphologies. The difference between inorganic metals and organic ligands structures and direction in the single-crystal cells could bring small or large structural differences, and these differences illustrate new properties. It could lead the compounds with distinct growth methods to obtain secondary MOF growth from the initial MOF. This chapter investigated the tunability and flexibility of MOF. Keywords MOF nanorod · Guest molecules · F-MOF1 · Zn-based MOF · ZIF-C · HKUST-1

1 Tunable Nanomicrostructure MOFs derived from M@C composites with unique micro/nanoscale structures and morphology are used in industry. Ni@C is a magnetic and tunable MOF fabricated via carbothermal reduction and solvothermal reaction progress. Co (NO3 )2 ·6H2 O and Ni(NO3 )2 ·6H2 O salts with 0.15 g p-benzene dicarboxylic acid and 0.1 g PVA were dissolved in the mixture of deionized water and ethanol (1:1). It stirred for 30 min and moved to a 50 mL autoclave for 12 h at 150 °C temperature. After centrifugation and washing, the Co–Ni-MOF products dried at a heated 600 °C. To the ratio F. R. Asl (B) · F. Ganjali · Z. Rashvandi Catalysis and Organic Synthesis Research Laboratory, Department of Chemistry, Iran University of Science and Technology, 16846-13114 Tehran, Iran e-mail: [email protected] F. Ganjali e-mail: [email protected]; [email protected] Z. Rashvandi e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 A. Maleki and R. Taheri-Ledari (eds.), Physicochemical Aspects of Metal-Organic Frameworks, Engineering Materials, https://doi.org/10.1007/978-3-031-18675-2_8

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Fig. 1 a Illustration of the MA process, b multiple reflection, c conduction loss, d interfacial polarization, e, f magnetic coupling, and g magnetic resonance of the Ni@C absorber. This figure was adapted with permission from Inorganic Chemistry Communications, 2020, 12.1, 1–17 [1]

of metals used, Nix Cox @Carbon magnetic products were obtained Ni0.8 Co0.2 @C, Ni0.5 Co0.5 @C, Ni0.2 Co0.8 @C, and CoO@C, respectively [1]. SEM and TEM analysis investigated and displayed the morphology and uniformity of the products; Ni@C microspheres have a particle size of 1.5–2 μm. The Ni0.8 Co0.2 @C microspheres were observed to have a particular hollow structure and size distribution of about 3 μm. Ni0.5 Co0.5 @C composites with larger nanoscale units and about 8 μm have a more similar structure to Ni@C MOFs. In the Ni0.2 Co0.8 @C composites, the structure of obtained microspheres has many microscopic particles connected to the surface of the MOFs and a size of about 10 μm. For more studies, Co-MOF without Ni was constructed and exhibited the cause of the increasing size in the shape of MOF in the presence of Co. The XRD pattern demonstrated that Ni@C composites exhibited controllable electromagnetic properties, and magnetic and catalytic Ni:Co content ratio tunability regularly increased in the same condition [2, 3] (see Fig. 1).

1.1 Tunable Mechanical Properties In other studies, a new three-dimensional (3D) printing approach exhibits two compounded stretchable and tough MOF hydrogel structures with tunable mechanical properties. The HKUST-1 MOF has grown with copper ions and ligands and simultaneous cross-linking of alginate and acrylamide in situ, resulting in high MOF dispersity in the double network (DN) hydrogel matrix with high pore accessibility. In preparing this structure, a homogenous solution with acrylamide and other polymers is created, then Cu2+ with the ligand is added to the polymers solution, forming the HKUST-1 in situ and allowing adequate diffusion of the Cu2+ into the hydrogel matrix for 24 h. These 3D-printed MOF hydrogels could

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tune the mechanical features by covalent and ionic cross-linkers by varying the concentrations of the AAm monomer [4]. The 3D printed HKUST-1 hydrogel was investigated dumbbell-shaped and mechanical properties by the ASTM D412 standard tests. It was observed the strength, tensile modulus, and toughness of 3D printed HKUST-1 increased by increasing AAm concentration. This improvement is attributed to higher pAAm content in the hydrogel substrate, which leads to elastomer-like features like other DN hydrogels [5, 6]. In this article, 3D printed HKUST-1 hydrogels with 3.6 M AAm display 453% elongate of length, the highest failure strain before breaking (Strength of 277.6 kPa, modulus of 152.3 kPa, and toughness of 744.7 kJ/m3 ). Additionally, the Cu2+ concentration affects the mechanical properties of the HKUST-1 hydrogel 3D printed. The tensile modulus increased by increasing Cu2+ concentration, which could be caused by a great extent of ionic cross-linking in the matrix. Also observed, the strain at break decreased with increasing Cu2+ concentration. This optimal concentration is due to the formation of the MOF layer on the surface, and decreased cross-linking in the interior core leads to lower strength and toughness. This explanation is consistent with SEM images which investigate MOF particle size distribution and density [6, 7] (Fig. 2 a).

Fig. 2 a Schematic showing the three critical steps in the 3D printing process including printing, UV curing, and ionic cross-linking. This figure was adapted with permission from ACS Appl. Mater. Interfaces, 2020, 12, 29, 33,267–33,275 [7]. b Schematic showing the difference between two kinds of ZIF-C c mixed matrix membrane and ZIF-C. This figure was adapted with permission from Chemistry of Materials, 2020, 32, 10, 4174–4184 [8]

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Fig. 3 SEM images of the 3D printed hydrogel samples. a Cross-section SEM image of the 3D printed and photocured hydrogel matrix. b SEM image of the surface of the 3D printed HKUST-1 hydrogel. (c,d) Cross-section SEM images of the 3D printed HKUST-1 hydrogel, c toward the outer region, and d at the core region. This figure was adapted with permission from ACS Appl. Mater. Interfaces, 2020, 12, 29, 33,267–33,275 [7]

The HKUST-1 3D printed hydrogel structure represents high MOF particle dispersity and excellent mechanical efficiency. The MOF could be stretched up to 453.0% of primary length, with a strength of 277.6 kPa, modulus of 152.3 kPa, and toughness of 744.7 kJ/m3 . Moreover, the mechanical feature tunable by the concentration of Cu2+ and polymer. SEM images exhibit uniform macropores morphology consistent with previously reported AAm gels. After ionic cross-linking and in situ MOF growth, octahedron-shaped HKUST-1 particles were distributed. The high uniformity of structure is related to the dispersity of MOF microporous structure. In SEM images, the structure’s higher density and smaller crystals were observed (Size about 5.5 -10 μm) [8, 9] (Fig. 3 a–d).

1.2 Morphologically tunable The studies investigated methods to synthesize zeolitic imidazolate framework cuboid (ZIF-C) nanosheets with tunable thicknesses from 70 to 170 nm. The synthesized ZIF-C nanosheets are applied as nanofillers to prepare Pebax-based mixed matrix membranes (MMMs) for studying the efficacy of morphology on membrane

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properties and CO2 /N2 separation. The CO2 permeability of 387.2 Barr with a CO2 /N2 selectivity of 47.1 has been reported, double in CO2 [10]. PVA was dissolved in DI water with reflux to prepare 1wt.% PVA solution. Zn(NO3 )2 ·6H2 O added to PVA solution. Then ZIF-C was separated by a centrifuge and redispersed with DI water and centrifuged three times to remove the residual PVA. Finally, ZIF-C dries in a vacuum oven for one night. PVA used with three different molecular weights is obtained ZIF-C XX [11, 12]. ZIF-C 85–124, ZIF-C 72, and ZIF-C 30–70 with different percentages of present polymer investigated. FT-IR, TGA, and XRD analysis to study the structure performed. The surface of the membrane has smooth, defect-free, and cross-section. ZIF-C 85–124 of up to 20 wt. %, the ZIF-C 85–124 is uniform without agglomeration. The same morphology for MMMs with nanofillers was observed; ZIF-C 30–70 has smaller nanofillers. In contrast, ZIF-C 85–124 has the most significant size among these three composites and accommodates ZIF-C morphology [9, 13, 14]. The mixed gas of CO2 and N2 was used to study the effects of different ZIF-C nanosheets on the CO2 separation. The permeability of CO2 is around 30, which is in accord with the reported values. The maximum of the CO2 and CO2 /N2 separation by adding ZIF-C is 10 wt.% to MMMs. The permeability CO2 of MMMs with 10 wt. % ZIF-C 85–124 is 141.7 Barr, twice the free Pebax membrane. There are two explanations for this: first, the interlayers inside the ZIF-C transfer fast the CO2 permeability. Otherwise, ZIF-C in the polymer rigidifies the polymer’s chains, thus increasing CO2 /N2 selectivity. Moreover, the increase in CO2 selectivity leads to the lack of voids between polymer and filler. In contrast, interface voids were observed with the addition of ZIF-C, leading to decreasing CO2 permeability and CO2 /N2 in MMMs. For fillers with a high ratio, the nanoscale plays much more role in the properties of the neat fillers. In this project, only the thickness is on the nanoscale, so the effect of one nanoscale dimension was investigated. The membranes with ZIF-C 85–124 display the maximum CO2 permeability in this article, and ZIF-C 30–70 and ZIF-C 72 following levels, respectively. In this work, the simple preparation method for controllable cubic ZIF-C nanosheet morphology, three different weights of PVA were used to investigate its effect on morphology. The results demonstrate that the PVA molecular weight affects the thickness and size of the synthesized ZIF-C nanosheets. The crystal structure of ZIF-C is evidenced by N2 adsorption and XRD analysis [15, 16]. The shape and size of nanosheets could be tunable during the growth of crystals. PVA 85–124 shows the best thermal stability and highest crystallinity among the synthesized ZIF-C nanosheets. Moreover, the gas permeation results show the ZIF-C nanosheets increase the permeability and CO2 /N2 selectivity (Fig. 2 b, c) [17].

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2 Flexibility Incorporating the bimetal cation affected MOFs’ intrinsic properties (e.g., thermal stabilities and band gap). Adsorptive separation by porous solids is energyefficient for purifying critical chemical species. This report is about a MOF [Zn5 (μ3 -OH)2 (adtb)2 (H2 O)5 ·5 DMA] (Zn-adtb) compounded from a butterflyshaped carboxylate linker. Preparation for this MOF 32 mg Zn(NO3 )2 ·6 H2 O was dissolved in N' , N' -dimethylacetamide (DMA). Afterward, 1 mL of water was added to the solution. Then H4 adtb (30 mg, 0.044 mmol) was added to the solution. After 48 h, rod-shaped single crystals were filtrated and washed with DMA. 20 mg (40.13% yield based on Zn) of crystal sample was obtained; lastly, samples were active under dynamic vacuum for 8 h at 373 K [18–20]. The bulk phase purity of Zn-adtb was corroborated by powder X-ray diffraction (PXRD) analysis. The porosity of Zn-adtb was approximated by CO2 adsorption isotherm data (this MOF has no sensitivity to N2 at 77 K) [21]. Washing by DMA and drying are activated MOF. CO2 isotherms uptake amount of 120 cm3 /g of CO2 at 195 K. The theoretical and experimental BET surface areas do not accommodate because solvent exchange and activation transformed PXRD patterns. The thermogravimetric analysis (TGA) demonstrated MOFs before and after activation are stable. This MOF was investigated in solution with various pH values (1–12) for 72 h, and the PXRD pattern showed which compound demonstrated high stability in harsh chemical environments [22]. This Zn-based MOF connected with a bowl-shaped butterfly linker has a microporous structure and SCU topology, a rare topology for Zn-based MOFs. This MOF adsorption C6 alkane isomers and column chromatography experiments demonstrate separating the linear from branched isomers at 2, 8, and 31 min. The kinetic diameter of this column is 23 DMB and is too large to diffuse through the pore of MOF [23].

2.1 Temperature and Gust Molecules Affect Flexibility Zr-MOF is regarded as rigid MOFs, but this article presents flexible topology and responsibility to temperature and solvent molecules with different sizes and polarities. This MOF explores the dynamic responses of external and based on the Zr6 cluster (abbreviated as TCBP-F-Zr). Also, possess a significant anisotropic thermal feature from 100 to 298 K. For the investigating mechanism of TCBP-F-Zr, the single-crystal structures from 100 to 298 K were collected and compared. The OZr-O bond angles (ε) and Zr-O bond (dZr-O) lengths do not change with increasing temperature, which displays that the Zr6 cluster contributed little to the structure of TCBP-F-Zr. However, the flexibility of this structure imputes to stretching of the ligand. To observe the changes of ligands, obviously described ligands in 3D and the benzene rings’ figuration in space. Single-crystal X-ray diffraction analyses at room temperature, and the framework contains a classical Zr6 cluster and TCBP-F4 ligand.

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Each Zr6 cluster links to 8 TCBP-F4 ligands to construct Zr6 O4 (OH)8 (H2 O)4 (COO)8 to build SBU MOF units and the size of the cages is 23.38 Å × 14.38 Å × 29.05 Å. The free spaces for solvent-accessible TCBP-F-Zr are about 75.3% of crystal unit cells and are calculatable by PLATON [24–27]. As mentioned, Zr-MOFs have rigid and close tetrahedrons, like TCHB4–in MFM133, MTB4–in MOF-841, and MTBC4–in PCN-521. However, in the TCBP-FZr framework, the biphenyl rings have small steric hinder, and the C–C bonds to connected two phenyl rings could rotate and make Zr-MOF flexible and respond to external stimuli. Crystal X-ray diffraction analyses proved the anisotropic thermal expansion feature in Zr-MOF in various temperatures. Generally, the volume of unit cells increased by about 7.6% from 100 to 298 K. In the Zr6 cluster, the flexibility of structure attributes to the rotating or stretching of the organic ligand. The solvents molecules affected crystals, so to explore the flexible nature of the Zr-MOF, the synthesis MOF submerged in Methanol (MeOH), Ethanol (EtOH), Dimethyl sulfoxide (DMSO), N, N' -Dimethylacetamide (DMA), and N, N' -Dimethyl formamide (DMF), respectively. Then collected, the results of crystallographic data at 298 K, the single-crystal X-ray diffraction analyses exchange to solvent molecules with different sizes and polarity of molecular. As shown in Fig. 1, the unit cell volume increases when the solvents change from MeOH to EtOH, DMSO, DMA, and DMF (Fig. 4a). A similar phenomenon due to the size and priority of solvent molecules could be observed in MCF-18, MIL-88, and FJI-H11-R MOFs series. In conclusion, a Zr-based flexible MOF and tetracarboxylic ligands build tetrahedral MOF, which responds to temperature (Fig. 4b) [28, 29].

2.2 Flexible MOF Nanorod In another study, the flexible Cu-CAT-1 MOF connected to the gelatin membrane and, with solar absorption, performed seawater purification. For preparation, Cu(NO3 )2 solution with wt.% gelatin aqueous solution was added to the causing CHN dispersion. After stirring and filtering the solution mixture slowly, using glutaraldehyde (GA) as a cross-linker, the paper-based Cu-CAT-1 nanorod arrays/gelatin composite membrane (PCG) is obtained. Gelatin was uniformly mixed and filtered onto the filter paper. The GA cross-linker synthesizes a CHN/gelatin-based membrane. SEM imaging exhibited the formation of a dense coating of gelatin on the surface of the paper. The Cu2+ formed the Cu-CAT-1 on the paper-based CHN/gelatin membrane. The SEM images and XRD pattern demonstrated that the u-CAT-1 nanorods with a length of 400 nm and diameter of 60 nm were compounded. N and Cu are the elements of Cu-CAT-1 and gelatin, respectively, illustrating the connection between nanorod and gelatin and the filter paper. This membrane has stout and flexible mechanical features, even when it folds with no pop. The excellent mechanical characteristics and stable structure are due to the presence of gelatin. Gelatin is the linker between the filter paper and the Cu-CAT-1 nanorods. Cause of the presence of the gelatin, the new membrane is super hydrophobic. The PSG under the sun irradiation could

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Fig. 4 a Guest-dependent parameter changes of TCBP-F-Zr. b Change of TCBP-F4− ligand in TCBP-F-Zr flexibility. This figure was adapted with permission from Inorganic Chemistry Communications, 2021, 128, 108,597 [24], c The structure of AC and schematic of the stepwise formation of F-MOF@AC composites. This figure was adapted with permission from Journal of Materials Chemistry A, 2017, 5.18, 8423–8430 [30]

increase to 70.9 °C. The zeta potential of the PCG membrane in the different pH solutions was investigated; the zeta potential of the PCG membrane is −41.5 mV and increased in alkaline solution and basic conditions. The seawater has a weak basic condition and pH value between 8.5 and 8.5 that is important to the PCG membrane [31–37].

2.3 Flexible MOF-Aminoclay Nanocomposites The flexible metal–organic framework (F-MOF) nanoscale synthesized and studied the tunability, gas adsorption, and separation features. The difference between synthesized F-MOF1@AC composite and absorbance molecules (CO2 /C2 H2 ) could be tunable. To synthesize this MOF, the first amino clay (AC) is dispersed in water/ethanol (60:40). Then Cu (NO3 )2 .2.5H2 O was added to the AC solution and stirred. The ligand was added to the solution dropwise and stirred for 6 h, filtered and washed several times, and F-MOF1@AC was obtained. Different amounts for AC were investigated, and the composites named F-MOF1@AC-1, F-MOF1@AC-2,

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F-MOF1@AC-3, and F-MOF1@AC-4 included 20, 30, 50, and 60 mg, respectively [30]. The F-MOF1 composites has {[Cu(pyrdc)(bpp)] (5H2 O)}n structure. It has a 2D pillared-bilayer framework linked by a bpp linker, and the bpp linker exhibits flexibility along -(CH2 )- chains. F-MOF1 display study by single-crystal structure and determine dehydrated formula {[Cu(pyrdc)(bpp)]2}n and decreased the cell volume. The dehydrated frameworks contain two different 2D layers linked by one bpp linker. The MOF is connected to the polymer by Cu–N(bpp) bond. The activated framework of F-MOF1 investigated the adsorption of MeOH, EtOH, and CO2 . The crystal structure of the framework display C-H–-O interactions stabilized CO2 molecules inside the MOF. Four composites with four various AC amounts synthesize and study XRD patterns. In the calculations of the MOF1@AC-1, F-MOF1@AC-2, F-MOF1@AC-3, and F-MOF1@AC-4, the AC contents are 9.7, 2.3, 32.7, and 32.8 wt.%, respectively. Energy Dispersive X-ray spectroscopy (EDX) analyses of MOF1@AC-1 mentioned the presence of Cu, Mg, and Si elements related to the AC. The SEM image exhibits a micrometer size for the layer of MOF on the polymers. The TEM images show the crystals are spherical with a 10–14 nm size range. F-MOF1@AC-2 and FMOF1@AC-3 investigate spherical NPs with 5–8 nm and 2–5 nm, respectively. The Cu@AC MOF suggests Cu (II) ions to the amine groups of AC and is proven with analysis. The observations show the AC sheets are the platform for the nucleation and growth of F-MOF1 nanocrystals. The F-MOF1@AC-1 shows the significant adsorption of N2 . The MOF NPs compared with the composite showed enhancement of the accessibility and porosity. The pore size of the F-MOF1@AC-1 perused in micro and macro (pore diameter ~16–18 Å and ~24–30 Å) enhanced the uptake. In F-MOF1@AC-1, accessibility of guest molecules and adsorption increase. The gas adsorption studies of F-MOF1@AC composite investigated increasing while MOF has different behavior (Fig. 4c) [31, 38–40].

References 1. Wang, L., Huang, M., Yu, X., You, W., Zhang, J., Liu, X., Wang, M., Che, R.: MOF-derived Ni1− x Cox@carbon with tunable nano–microstructure as lightweight and highly efficient electromagnetic wave absorber. Nanomicro. Lett. 12(1), 1–17 (2020) 2. Zhu, H., Jiao, Q., Fu, R., Su, P., Yang, C., Feng, C., Li, H., Shi, D., Zhao, Y.: Cu/NC@ Co/NC composites derived from core-shell Cu-MOF@Co-MOF and their electromagnetic wave absorption properties. J. Colloid Interface Sci. 613, 182–193 (2022) 3. Hussain, I., Iqbal, S., Hussain, T., Cheung, W.L., Khan, S.A., Zhou, J., Ahmad, M., Khan, S.A., Lamiel, C., Imran, M.: Zn–Co-MOF on solution-free CuO nanowires for flexible hybrid energy storage devices. Mater. Today Phys. 23, 100655 (2022) 4. Lieu, W.Y., Fang, D., Tay, K.J., Li, X.L., Chu, W.C., Ang, Y.S., Li, D.S., Ang, L.K., Wang, Y., Yang, H.Y.: Progress on 3D-printed metal-organic frameworks with hierarchical structures. Adv. Mater. Technol. 2200023 (2022) 5. Pal, S., Su, Y.-Z., Chen, Y.-W., Yu, C.-H., Kung, C.-W., Yu, S.-S.: 3D printing of metal–organic framework-based ionogels: wearable sensors with colorimetric and mechanical responses. ACS Appl. Mater. Interfaces (2022)

110

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6. Zheng, C., Wang, J., Jiang, H., Ma, Y., Shao, Z.: Green synthesis of polyacrylamide/polyanionic cellulose hydrogels composited with Zr-based coordination polymer and their enhanced mechanical and adsorptive properties. Polym. J. 54(4), 515–524 (2022) 7. Liu, W., Erol, O., Gracias, D.H.: 3D printing of an in situ grown MOF hydrogel with tunable mechanical properties. ACS Appl. Mater. Interfaces 12(29), 33267–33275 (2020) 8. Yan, C., Dong, J., Chen, Y., Zhou, W., Peng, Y., Zhang, Y., Wang, L.-N.: Organic photocatalysts: from molecular to aggregate level. Nano Res. 1–24 (2022) 9. Deng, J., Dai, Z., Hou, J., Deng, L.: Morphologically tunable MOF nanosheets in mixed matrix membranes for CO2 separation. Chem. Mater. 32(10), 4174–4184 (2020) 10. Xiong, S., Yuan, C., Zhang, X., Xi, B., Qian, Y.: Controllable synthesis of mesoporous Co3 O4 nanostructures with tunable morphology for application in supercapacitors. Eur. J. Chem. 15(21), 5320–5326 (2009) 11. Qin, M., Lan, D., Wu, G., Qiao, X., Wu, H.: Sodium citrate assisted hydrothermal synthesis of nickel cobaltate absorbers with tunable morphology and complex dielectric parameters toward efficient electromagnetic wave absorption. Appl. Surf. Sci. 504, 144480 (2020) 12. Jo, Y.-M., Kim, T.-H., Lee, C.-S., Lim, K., Na, C.W., Abdel-Hady, F., Wazzan, A.A., Lee, J.-H.: Metal–organic framework-derived hollow hierarchical Co3 O4 nanocages with tunable size and morphology: ultrasensitive and highly selective detection of methylbenzenes. ACS Appl. Mater. Interfaces 10(10), 8860–8868 (2018) 13. Li, B., Liu, J., Liu, Q., Chen, R., Zhang, H., Yu, J., Song, D., Li, J., Zhang, M., Wang, J.: Core shell structure of ZnO/Co3 O4 composites derived from bimetallic-organic frameworks with superior sensing performance for ethanol gas. Appl. Surf. Sci. 475, 700–709 (2019) 14. Liang, C., Lin, H., Guo, W., Lu, X., Yu, D., Fan, S., Zhang, F., Qu, F.: Amperometric sensor based on ZIF/g-C3 N4 /RGO heterojunction nanocomposite for hydrazine detection. Mikrochim. Acta 188(2), 1–9 (2021) 15. Ding, M., Chen, J., Jiang, M., Zhang, X., Wang, G.: Ultrathin trimetallic metal–organic framework nanosheets for highly efficient oxygen evolution reaction. J. Mater. Chem. A 7(23), 14163–14168 (2019) 16. Buddin, M.S., Ahmad, A.: A review on metal-organic frameworks as filler in mixed matrix membrane: Recent strategies to surpass upper bound for CO2 separation. J. CO2 Util. 51, 101616 (2021) 17. Zhang, X., Chen, A., Zhong, M., Zhang, Z., Zhang, X., Zhou, Z., Bu, X.-H.: Metal–organic frameworks (MOFs) and MOF-derived materials for energy storage and conversion. EER 2(1), 29–104 (2019) 18. Velasco, E., Xian, S., Wang, H., Teat, S.J., Olson, D.H., Tan, K., Ullah, S., Osborn Popp, T.M., Bernstein, A.D., Oyekan, K.A.: Flexible Zn-MOF with rare underlying scu topology for effective separation of C6 alkane isomers. ACS Appl. Mater. Interfaces 13(44), 51997–52005 (2021) 19. Agarwal, R.A., Gupta, A.K., De, D.: Flexible Zn-MOF exhibiting selective CO2 adsorption and efficient Lewis acidic catalytic activity. Cryst. Growth Des. 19(3), 2010–2018 (2019) 20. Chaudhari, A.K., Souza, B.E., Tan, J.-C.: Electrochromic thin films of Zn-based MOF-74 nanocrystals facilely grown on flexible conducting substrates at room temperature. APL Mater. 7(8), 081101 (2019) 21. Kim, H.-C., Huh, S., Kim, Y.: Selective carbon dioxide sorption by a new breathing threedimensional Zn-MOF with Lewis basic nitrogen-rich channels. Dalton Trans. 47(14), 4820– 4826 (2018) 22. Zasada, F., Piskorz, W., Grybos, J., Sojka, Z.: Periodic DFT+ D molecular modeling of the Zn MOF-5 (100)/(110) TiO2 interface: electronic structure, chemical bonding, adhesion, and strain. J. Phys. Chem. C 118(17), 8971–8981 (2014) 23. Yang, Y., Shen, K., Lin, J.-z, Zhou, Y., Liu, Q.-y, Hang, C., Abdelhamid, H.N., Zhang, Z.-q, Chen, H.: A Zn-MOF constructed from electron-rich π-conjugated ligands with an interpenetrated graphene-like net as an efficient nitroaromatic sensor. RSC Adv. 6(51), 45475–45481 (2016)

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24. Ji, Z., Di, Z., Li, H., Zou, S., Wu, M., Hong, M.: A flexible Zr-MOF with dual stimulus responses to temperature and guest molecules. Inorg. Chem. Commun. 128, 108597 (2021) 25. Qin, J.-S., Yuan, S., Alsalme, A., Zhou, H.-C.: Flexible zirconium MOF as the crystalline sponge for coordinative alignment of dicarboxylates. ACS Appl. Mater. Interfaces 9(39), 33408–33412 (2017) 26. Zhang, X., Zhai, Z., Wang, J., Hao, X., Sun, Y., Yu, S., Lin, X., Qin, Y., Li, C.: Zr-MOF combined with nanofibers as an efficient and flexible capacitive sensor for detecting SO2. ChemNanoMat. 7(10), 1117–1124 (2021) 27. Chen, C.-X., Wei, Z.-W., Cao, C.-C., Yin, S.-Y., Qiu, Q.-F., Zhu, N.-X., Xiong, Y.-Y., Jiang, J.J., Pan, M., Su, C.-Y.: All roads lead to rome: tuning the luminescence of a breathing catenated Zr-MOF by programmable multiplexing pathways. Chem. Mater. 31(15), 5550–5557 (2019) 28. Xue, J., Xu, M., Gao, J., Zong, Y., Wang, M., Ma, S.: Multifunctional porphyrinic ZrMOF composite membrane for high-performance oil-in-water separation and organic dye adsorption/photocatalysis. Colloids Surf. A Physicochem. Eng. Asp. 628, 127288 (2021) 29. Lin, C.-C., Huang, Y.-C., Usman, M., Chao, W.-H., Lin, W.-K., Luo, T.-T., Whang, W.-T., Chen, C.-H., Lu, K.-L.: Zr-MOF/polyaniline composite films with exceptional seebeck coefficient for thermoelectric material applications. ACS Appl. Mater. Interfaces 11(3), 3400–3406 (2018) 30. Chakraborty, A., Roy, S., Eswaramoorthy, M., Maji, T.K.: Flexible MOF–aminoclay nanocomposites showing tunable stepwise/gated sorption for C2 H2 , CO2 and separation for CO2 /N2 and CO2 /CH4 . J. Mater. Chem. A 5(18), 8423–8430 (2017) 31. Ma, X., Li, Z., Deng, Z., Chen, D., Wang, X., Wan, X., Fang, Z., Peng, X.: Efficiently cogenerating drinkable water and electricity from seawater via flexible MOF nanorod arrays. J. Mater. Chem. A 9(14), 9048–9055 (2021) 32. Xie, W., Wang, Y., Zhou, J., Zhang, M., Yu, J., Zhu, C., Xu, J.: MOF-derived CoFe2 O4 nanorods anchored in MXene nanosheets for all pseudocapacitive flexible supercapacitors with superior energy storage. Appl. Surf. Sci. 534, 147584 (2020) 33. Lee, C.S., Moon, J., Park, J.T., Kim, J.H.: Highly interconnected nanorods and nanosheets based on a hierarchically layered metal–organic framework for a flexible, high-performance energy storage device. ACS Sustain. Chem. Eng. 8(9), 3773–3785 (2020) 34. Ma, X., Wan, X., Fang, Z., Li, Z., Wang, X., Hu, Y., Dong, M., Ye, Z., Peng, X.: Orientational seawater transportation through Cu (TCNQ) nanorod arrays for efficient solar desalination and salt production. Desalination 522, 115399 (2022) 35. Yue, Y., Yang, S.-Y., Huang, Y.-L., Sun, B., Bian, S.-W.: Reduced graphene oxide/polyester yarns supported conductive metal-organic framework nanorods as novel electrodes for allsolid-state supercapacitors. Energy Fuels 34(12), 16879–16884 (2020) 36. Ma, H., Zhang, X.-F., Wang, Z., Song, L., Yao, J.: Flexible cellulose foams with a high loading of attapulgite nanorods for Cu2+ ions removal. Colloids Surf. A Physicochem. Eng. Asp. 612, 126038 (2021) 37. Xu, Z., Wang, Q., Zhangsun, H., Zhao, S., Zhao, Y., Wang, L.: Carbon cloth-supported nanorodlike conductive Ni/Co bimetal MOF: A stable and high-performance enzyme-free electrochemical sensor for determination of glucose in serum and beverage. Food Chem. 349, 129202 (2021) 38. Zeng, H., Xie, M., Huang, Y.L., Zhao, Y., Xie, X.J., Bai, J.P., Wan, M.Y., Krishna, R., Lu, W., Li, D.: Induced fit of C2 H2 in a flexible MOF through cooperative action of open metal sites. Angew. Chem. Int. Ed. Engl. 58(25):8515–8519 (2019) 39. Qiu, Q.-f, Chen, C.-X., Wei, Z.-W., Cao, C.-C., Zhu, N.-X., Wang, H.-P., Wang, D., Jiang, J.-J., Su, C.-Y.: A flexible Cu-MOF as crystalline sponge for guests determination. Inorg. Chem. 58(1), 61–64 (2018) 40. Shi, C., Zhou, X., Liu, D., Li, L., Xu, M., Sakiyama, H., Muddassir, M., Wang, J.: A new 3D high connection Cu-based MOF introducing a flexible tetracarboxylic acid linker: photocatalytic dye degradation. Polyhedron 208, 115441 (2021)

MOF Scaffolds Defects and Disorders Fatemeh Ganjali , Peyman Ghorbani, and Nima Khaleghi

Abstract It is now widely accepted that defects could be used to modify metal– organic frameworks’ characteristics (MOFs). Defects formation can lead to advantageous alterations in catalytic, thermal, electronic, and adsorbing facets, as well as the pore structure of MOFs; thus, many applications have been found for the structural defects in MOFs in catalytic or anti-pollution systems, bio-applications, adsorption, separation, etc. arena. A study of MOFs’ structural defects is the initial concern of this chapter, followed by an examination of the most recent developments in MOF defect detection methods. Afterward, how the defects can be controlled for various procedures was discussed, and an outline of potential prospective research areas for MOFs’ defects. Keywords Metal–organic frameworks · Defect engineering · Missing-linker defects · Missing-cluster defects · Defect characterization

1 Introduction It is impossible to find “perfect crystals” in nature with an endless number of repetitions or orders of the same atomic groupings in space. Crystal abnormalities may result from structural deformations, and the term “defects” is frequently used interchangeably with this concept. Chemically varied MOFs are employed in several diverse uses. While most research have concentrated on ideal structures, a distinct F. Ganjali (B) Catalysis and Organic Synthesis Research Laboratory, Department of Chemistry, Iran University of Science and Technology, 16846-13114 Tehran, Iran e-mail: [email protected]; [email protected] P. Ghorbani School of Chemistry, College of Science, University of Tehran, Tehran, Iran e-mail: [email protected] N. Khaleghi Endocrinology and Metabolism Research Center, Endocrinology and Metabolism Clinical Sciences Institute, Tehran University of Medical Sciences, Tehran, Iran e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 A. Maleki and R. Taheri-Ledari (eds.), Physicochemical Aspects of Metal-Organic Frameworks, Engineering Materials, https://doi.org/10.1007/978-3-031-18675-2_9

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trend is now shifting toward the defective forms of this material. To modify MOFs’ physical–chemical features, including adsorption capacity [1], band structures [2], conductivity [3], mechanical responses [4], and catalytic capabilities [5], defects are now acknowledged as a valuable tool. Research has shown that MOF materials’ physical and chemical properties can benefit from structural defects in experiments and theoretical calculations [6]. However, various MOF features, including gas adsorption, diffusion, and optical properties, might be negatively affected by defects [7]. Furthermore, structural defects are frequently linked to a loss in the framework’s chemical, thermal, and mechanical stability. Stability and tolerance for such defects are therefore critical when designing the framework. The practical importance of maximizing the unique features of MOFs conferred through defects in the structure can’t be overstated. MOF defects are referring to the “sites that locally break the regular periodic arrangement of atoms or ions of the static crystalline parent framework because of missing or dislocated atoms or ions.” [8]. Regarding symmetrical scale, defects are divided into point, line, and planar. Defects of point have received much attention in MOFs and were studied in various fields based on their significant effects on MOF characteristics [9, 10]. The main kinds of MOF structure point defects are missing-linker and missing-cluster defects (Fig. 1) [11]. Missing-cluster defects are caused by a critical level of missing-linker defects as well as the related spatial distribution [10]. Missing-linker and missing-cluster defects are usually obtainable obliquely using current approaches. The UiO-66’ (Universitet I Oslo 66, Node: Zr6 (OH)4 O4 , Linker: benzene-1,4-dicarboxylate) structural defects were detected at a subunit-cell resolution recently [12]. In this chapter, the origin of MOF defects including a missing-linker/cluster is addressed. Several MOFs, including those that have been meticulously prepared, have structural defects. Even though the engineering of defects in MOFs is now in the infant stages, many attempts have gone into discovering and attempting to control defects [9]. There’s been increasing attention to figuring out how these defects form and how to use them in practical uses [6]. With the progress of advanced analysis techniques and a better understanding over MOFs’ defect characteristics, the MOFs’ defect engineering in is predicted to progress in the future. This chapter brings to light and summarizes current advancements in forming, detecting, and controlling MOF defects for application-relevant purposes.

Fig. 1 Representation of the structural discrepancies between the ideal UiO-66 unit cell and those with missing-cluster/linker defects. This figure was adapted with permission from Inorganics, 2019, 7(10), 123 [11]

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2 Structural Defect Generation in MOFs In spite of the difficulty of exhibiting the defect distribution inside initial frameworks, introducing defects into MOFs might be a helpful technique to achieve desirable qualities and promote their uses. The primary components for building a framework are metals and organic ligands. The fundamental step in constructing MOFs is the design of the coordination between them. As a result, specific general synthetic methods for introducing defects into MOF structures have been investigated and distinguished from those described in the literature [6]. Two synthetic techniques are used in the procedures: the “de novo” synthesis and the post-synthetic modification. Figure 2 depicts an illustration of all techniques used for producing MOF defects.

2.1 De Novo Synthesis 2.1.1

Modulation Method

Defective MOFs are most commonly formed using the modulation approach. In this method, extra-high amounts of monocarboxylic acids besides the linkers must be added during MOF formation [13]. Initially, the approach attempts to slow the crystallization rate of MOFs to generate larger levels of crystallinity during the synthesis step. The monocarboxylic acids, known as modulators, affect the equilibrium reaction and challenged the linkers to slow crystallization. Meanwhile, coordination sites can be occupied by the modulator and cause a synthesis defect. Several common monocarboxylic modulators have been used in the manufacture of defective MOFs, namely formic acid (FA) [14, 15], acetic acid [16, 17], difluoroacetic acid (DFA)

Fig. 2 Representation of all main methods for generating MOF defects

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[18], trifluoroacetic acid (TFA) [19], benzoic acid (BA) [20], etc. MOFs’ defects can be influenced by a multitude of parameters, such as the solvent, the modulators’ type and amounts, and the crystal growth rate of the MOFs [21, 22]. For the Ti-MOFs of the MUV-10 family, Lazaro et al. investigated the effect of monoand dicarboxylic modulators (benzoic and isophthalic acid) on the formation of defects [23]. They showed that the application of a dicarboxylic modulator can facilitate the propagation of defects in titanium-organic frameworks containing benzene 1,3,5-tricarboxylic linkers. Unlike monocarboxylic benzoic acid, which has poor efficiency, isophthalic acid is sufficient to promote the enhancement of missing linkers (up to 40%) in the MUV-10 structure. With the elimination of the BTC units, the enthalpy gain was greater in the latter and allows it to reduce the defective phase’s structural distortion. By adjusting the acidity or the modulator’s concentration, it is possible for concentration tunability of missing-linker and cluster defects [16, 18, 24]. Besides monocarboxylic modulators, nitrogenous heterocyclic modulators have been reported [25]. Temperature modulation also appears to be playing a significant character in the defective MOFs’ de novo preparation. DeStefano et al. used the modulator acetic acid in conjunction with temperature regulation to adjust the concentration of UiO-66’ defect sites [26]. MOFs’ defect sites could be reduced using changing the synthesis temperature, with the highest quantity of defect sites (around 1.3 missing linkers per Zr6 node) obtained when the temperature is 45 °C (Fig. 3a).

2.1.2

Mixed-Linker Method

An alternate method to design and construct a novel MOFs’ structure is to use mixed linkers instead of homologous ligands in synthesizing MOFs. The linkers are integrated and co-crystallized or copolymerized in the final structure of MOFs using the mixed-linker method [27]. As a result of the differences between the two organic linkers, a novel MOF structure can emerge that combines the functions of both ligands. This strategy’s results inspired the development of a method to produce defective MOFs. It is feasible to create defects by synthesizing MOFs, including isoreticular ligand series that can control pore size with varied lengths [28]. To conclude, the MOF’s unexpected breathing behavior was assigned reversibly loss of long-range crystalline organization via discharge after binding a mixture of organic molecules. This way, the MOFs can be introduced with a defect structure using this synthesis method. On the other hand, at the final synthesis phase, the iso-reticular ligands (extra ligands) were eliminated, leading to structural missing-linkers defects in the materials. Furthermore, mixed-ligand inclusion allows easy manipulation of features like pore size/volume, functional chemical features, and fine-tuning defect density. Several research groups have looked at and reported on the mixed-ligand technique for forming the defect structure on MOFs. For example, in the ruthenium MOF RuHKUST-1, Epp et al. induced point defects at the different Ru2 paddlewheel nodes [29]. The mixed linker strategy used a combination of pyridine-3,5-dicarboxylate and benzene 1,3,5-tricarboxylate linkers.

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Fig. 3 a The amount of missing linkers per Zr6 unit correlates with the synthesis temperature (red). This figure was adapted with permission from Chemistry of Materials, 2017, 29(3), 1357– 1361 [33]. b Controllable linker thermolysis’ versatility in constructing HP-MOFs with diverse linkers. This figure was adapted with permission from Journal of the American Chemical Society, 2018, 140(6), 2363–2372 [32]. c VCN -mediated Ni–Fe PBA preparation is depicted schematically. N2 plasma bombardment causes VCN to develop in Ni–Fe PBA material. This figure was adapted with permission from Nature communications, 2019, 10(1), 1–9 [34]

Over the last few years, a potent strategy to construct the MOFs’ structural defects and hierarchical pores has been created by integrating the mixed-linker and unstable linker decomposition methods [30, 31]. Using a mixed-linker strategy, the stable and labile linkers are initially inserted within the structure of MOFs, followed by the post-synthetic deconstruction of the labile linkers. Rich missing-linker defects were produced due to the degradation of labile linkers. By applying this methodology to conventional linkers and nodes, Zhou et al. illustrated the diversity of labile linker degradation [32]. As shown in Fig. 3b, the correlating sets of multivariate MOFs (MTV-MOFs) can be formed by combining two appropriate linkers (thermolabile and thermostable). MTV-MOFs with various nodes (e.g., MIL-125(Ti), MOF-5(Zn), UiO-66(Hf), and MIL-53(Fe)) have been synthesized as well precisely. Selective removal thermolabile linkers could convert the porosity of all MTV-MOFs from micro to hierarchical without crystallinity loss. MTV-MOF structures produced missing- linker and/or cluster defects.

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Fast Crystal Growth

In order to create the defects, a rapid crystallization procedure could be inspired by understanding the modulator’s behavior. Microwave-assisted synthesis and high precursor concentrations were two of the fast crystallization methods. Defect sites can be taken up by other species (counter anions) in the reaction mixture, increasing the overall functionality of OH groups in MOFs [22].

2.2 Post-synthetic Modification MOF defects can also be introduced through post-synthetic treatment and the “de novo” synthetic approaches [35]. The mechanisms by which pre-fabricated MOFs are subjected to solid-solution reactions or surface alterations to incorporate extra functionalization are known as post-synthetic modifications. Instead of generating new MOF synthesis conditions, post-synthetic modification of pristine MOFs can modify MOF structure to include a variety of chemical functions [36]. Also, various active defect sites can be produced in the MOF structures using post-synthetic treatments [37].

2.2.1

Post-synthetic Exchange

Post-synthetic exchange (PSE), which involves metal ion or linker metathesis from perfect MOFs, is the most well-known post-synthetic treatment approach [38–40]. The metal ion exchange process in MOFs includes replacing initial metal ions with various coordination numbers compared to the originating metal ions [6, 41]. A heterogeneous process utilizing a suitable solvent can exchange the initial linkers of the MOF with other functionalized linkers by linkers’ post-synthetic exchange, also referred to as solvent-assisted linker exchange. Using changed ligands (such as ligands with inserted catalyst species or functionalized), PSE can inject a variety of chemical functions and defect locations into MOF structures [42, 43]. Importantly, PSE opens up a new path for creating unique MOF substances that are hard to create using standard approaches [43]. Cai et al. [44] established a varied postsynthetic ligand exchange technique to transform standard microporous MOFs and their relative composites to hierarchically porous MOFs (HP-MOFs) and their relative composites in a simple reflux system at 10 g and beyond. The resulting HP-MOFs have innate micropores and an abundance of defective mesopores that considerably aid the large substrates’ transport and activation for reliable and efficient heterogeneous catalysis. Besides, defects in the HP-MOF composites mesopores improve the one-pot successive catalysis activity and selectivity involving big molecules.

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Etching Methods

In a post-synthesis treatment, etching agents (such as acids, bases, salts, and plasma) can also generate defects in MOFs [45–49]. In microporous MOFs, the etching method can introduce not just missing-linker and cluster defects but also microporosity and even microporosity. For instance, Chang et al. employed tetrafluoroborate as an etching reagent and a functional site simultaneously to induce defects and mesopores in microporous Al-MOF [46]. The defect-induced mesopores are formed when copper (II) tetrafluoroborate breaks the bonds between Al3+ and the carboxylate groups. By meticulously adjusting the salt etching agent loading amount, the size of the mesopore and active sites may be appropriately controlled. The crystal structure was only slightly etched by a low salt concentration, whereas a significant salt content resulted in the framework being deeply etched. To produce defects in MOF materials, plasma etching, an eco-friendly and simple method, has recently been created. After treatment, the plasma’s bulk gas temperature is close to ambient temperature even though it includes highly reactive particles, such as electrons, ions, and free radicals. Plasma technology is promising in generating imperfections in MOFs without destroying their framework structures due to the advantages of high reactivity and low heat influence. Yu et al. broke the bonds of Fe–C-N-Ni units in nickel–iron Prussian blue counterparts using an ionized nitrogen plasma, resulting in unusual carbon–nitrogen vacancies (Fig. 3c) [34]. This research paves the way for new ways to create vacancy defects in nanomaterials.

3 Defect Characterization in MOFs Because of their various and distinct chemical structures, MOFs are a good platform for studying defect chemistry. Increased crystal structure cavities, lower density, and deteriorated thermal and mechanical capabilities are the most obvious defects’ impacts on MOFs’ Physical and chemical characteristics [50]. MOFs’ structural and missing-linker defects were initially detected by a decrease in the thermal stability of what was previously thought to be a “defect-free” MOF structure [51]. Conventional characterization approaches face a hurdle when attempting to characterize defects in great detail, such as determining the concentration, steric distribution, electrical properties, and defect chemistry. Determining essential relationships between defects and the features of the resulting defective materials is an additional significant problem. In order to identify the MOFs’ structural defects, researchers have used a variety of techniques, either singularly or in combination. These methods can be commonly classified into two categories: the spectrum and microscopy approaches, which include powder X-ray diffraction (PXRD), ultraviolet–visible spectroscopy (UV–vis), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, acid–base titration, electron paramagnetic resonance (EPR), BET surface area (SBET ) and volume (Vpore ), extended X-ray absorption fine structure (EXAFS), positron annihilation lifetime

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spectroscopy (PALS), high-resolution transmission electron microscopy (HR-TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), Confocal fluorescence microscopy (CFM), Scanning electron diffraction technique (SED), etc. One of the most frequent techniques for calculating the MOF defect amounts is thermogravimetric analysis (TGA). Data from TGA is consistent with the idea that some linkers are removed from the frameworks, which can show non-stoichiometric MOF structures. In this scenario, the actual losing weight that occurs during thermal degradation is frequently a considerable amount less than the weight loss predicted to occur theoretically, showing that the frameworks are substantially lighter than predicted by the idealized equations. Hardian et al. used the TGA method to assess the quantity of missing linkers in defective and Pt-encapsulated MOF-808 (MP: pristine MOF and MD: defective MOF) [33]. TGA was performed in the air and showed three distinct weight loss stages. Nearly all samples show a discernible mass loss below 373 K, which can be attributed to the elimination of water or other loosely bound species adsorbed on the MOFs’ exterior surface. The mass loss between 373 and 623 K can be due to the elimination of coordinated -OH and H2 O, which replace the formic acid utilized as a modulator during the synthesis after activation. Between 723 and 873 K, the TGA curve’s last significant section can be found. It’s the decomposition of the BTC linker. The amount of missing linkers in each sample can be determined by measuring the mass loss in this location (Fig. 4a). The electron paramagnetic resonance (EPR) technique effectively detects oxygen vacancies because it can identify unpaired electrons in paramagnetic species [52]. Because missing-linker defects in MOFs cause oxygen vacancies (open metal defects), EPR is now commonly used to identify missing-linker defects [53]. Xu et al. for example, determined the oxygen vacancy density in hierarchically porous UiO-66 MOF (HP-UiO-66) using the EPR technique [53]. According to the authors’ report, the amount of coordinatively unsaturated Zr atoms could be represented by the concentration of oxygen vacancies. As shown in Fig. 4b, the concentration of defects increases a symmetrical signal at g-factor = 2.003 in the HP-UiO-66 sample’s EPR spectra, which is attributed to oxygen vacancies. HP-UiO-66 has abundant and adjustable coordinatively unsaturated Zr atoms, according to EPR studies. MOFs’ local structure can be investigated via the EXAFS technique [54, 55]. Xue et al. revealed that when missing-linker defects were inserted into CoBDC MOF, Fourier transforms of EXAFS showed that the local coordination geometry of Co2+ ions varied [24]. There were essentially no differences between the Co–O distance of pristine CoBDC (2.07 Å) and the defective CoBDC (termed CoBDCFc0.17 ) (2.08 Å) as determined by the fitting curve (Fig. 4c). Co–O for CoBDC-Fc0.17 , on the other hand, had a lower coordination number than CoBDC (4.4 versus 6.2). These findings indicate that the missing-linkers’ introduction leads to open Co2+ sites. Another method for determining the characteristics and types of defects in robust MOFs is potentiometric acid–base titration. The measurement’s limited applicability is partly due to the MOF’s requirement for high acid and base stability. As is common knowledge, potentiometric titration enables the measurement and discrimination of Brønsted acid sites and their corresponding pKa values. DeStefano et al. observed

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Fig. 4 a TGA of the activated samples performed under airflow. This figure was adapted with permission from Chemistry–A European Journal, 2021, 27(22), 6804–6814 [33]. b EPR spectra of HP-UiO-66. This figure was adapted with permission from Journal of Materials Chemistry A, 2020, 8(16), 7870–7879 [53]. c CoBDC and CoBDC-Fc0.17 Fourier transformed EXAFS spectra. This figure was adapted with permission from Nature communications, 2019, 10(1), 1–8 [24]. d Acid–base titration (red) and first derivative (blue) curves from an Hf-UiO-66 sample. This figure was adapted with permission from Chemistry of Materials, 2017, 29(3), 1357–1361 [26]

that incorporating defects into the very stable Zr-MOFs generated two new protons (Zr-OH and Zr-OH2 ) [26]. The three inflection points were presented in the titration curves, while the only distinctive point that should be exhibited in defect-free UiO-66 was assigned to the μ3-OH groups (Fig. 4d). The remaining two pKa values, which were associated with the acidity of metal-bound hydroxo and aqua ligands (Zr-OH and Zr-OH2 ), were discovered in defects caused by linker vacancies. Fourier transform infrared (FT-IR) analysis was utilized to confirm the C = O break in UiO-66 due to defects caused by linker vacancies [32, 56]. The crystallization of Zr-fumarate MOF in systems based on H2 O and DMF was observed using in-situ IR [57]. Only in the system based on DMF did the modulator formic acid participate in the structure formation of Zr-fumarate MOF, causing structural defects to arise as a result. In addition, FT-IR of adsorbed particular probe molecules, such as CO [58, 59], CD3 CN [48, 60], and pyridine [61, 62], has been employed to describe unsaturated metal sites that are a result of a linker vacancy. With the help of FT-IR and Raman spectroscopy, as well as a study of the vibrational spectra using ab initio theory, Gentile et al. recently discovered that the HKUST-1 MOF has spectroscopic features of framework defects [63]. PALS gives indications of the inner pores in porous materials as an analytical tool [64–66]. The major electron concentration in a MOF is found in the framework. Therefore, positron lifetime is proportional

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to pore size. For instance, when defective UiO-66 was contrasted to perfect UiO66, the positron lifetime and annihilation intensity were obviously increased [66]. A linker vacancy defect was determined to be the cause of the framework’s pore size development, which was discovered during the synthesis of UiO-66 with the modulator. When synthesizing defective MOFs, other species (competitive linkers, modulators, and solvent molecules, for example) typically cap metal sites in MOF structures. A common method for determining the MOF structure’s linker proportions is NMR spectroscopy, which has also widely been used to identify the linkers and the modulator [67, 68]. Direct measurement of the defect distribution at a spatial resolution is highly desired. If defects in MOFs could be seen, determining local arrangements for defects would be a lot simpler at the microscopic level. It is now possible to see defects in MOFs utilizing high-resolution transmission electron microscopy (HR-TEM) [12, 19], atomic force microscopy (AFM) [69], confocal fluorescence microscopy (CFM) [70], and scanning electron diffraction techniques (SED) [71], which have been developed in recent years. To see individual local structures in real space, HR-TEM is a valuable technique. Due to the sensitivity of MOFs to electron beams, the HR-TEM electron beam can cause considerable structural damage to MOFs [72]. For imaging MOFs at atomic resolution, new low-dose electron microscopy technology has just recently been established [73]. HR-(S)TEM has already revealed some MOF bulk and local structures [73]. Using HR-TEM, Wang et al. [19] investigated both normal and defective UiO-66 (UiO-66(1d)) for their precise defect structures. HR-TEM images generated prospective mappings and predicted structural models are shown in Fig. 5a. The horizontally aligned BDC linkers in UiO-66(1d) cannot be seen in comparison to UiO-66 (red arrows are used to indicate) in Fig. 5a (images of the first column, top and middle), indicating the existence of linker vacancy defects in UiO-66 (1d). Ordered defects due to cluster vacancies in UiO-66(1d) are readily obvious from the visible variation in image contrast (Fig. 5a, images of the first column, top and bottom). CFM is another technology for 3D imaging of defects in MOF single crystals. Furfuryl alcohol probe molecules can be captured by MOF lattice defects acting as Lewis acid sites [70]. To identify the fluorescence signal in Lewis acid-catalyzed furfuryl alcohol oligomerization, CFM images can be used. Self-aggregation was used to create the mesoporous defective Cu(II)-MOF MFM-100. CFM examination demonstrates the creation of uncoupled Cu(II) centers crystal defects (Fig. 5b). SED, a four-dimensional scanning transmission electron microscopy (4DSTEM) analysis, has recently been useful for studying the location of defects or microstructure in functionalized MOFs [71]. Figure 5c shows images of defect domains at the nanoscale on the MOF UiO-66(Hf) with a spatial resolution of around 5 nm over an area of about 1000 nm. SED makes it possible to expand the idea of defect engineering to microstructure engineering in order to enhance MOF efficiency.

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Fig. 5 a HR-TEM images of UiO-66, missing-linker and cluster defect (left column from top to bottom, respectively) regions of UiO-66(1d) along the [110] zone axis; and related representations of simulated potential maps (middle column from top to bottom, respectively) and structural models (right column from top to bottom, respectively). This figure was adapted with permission from Journal of Materials Chemistry A, 2020, 8(8), 4464–4472 [19]. b MFM-100a CFM images. In all images, the scale bar is 5 m. The existence of crystal defects is indicated by fluorescence (shown in red). This figure was adapted with permission from Nature communications, 2019, 10(1), 1–9 [70]. c High defect density UiO-66(Hf) particle imaged by STEM, with integrated electron diffraction patterns (top picture) chosen in magenta and yellow (image below, left and right). This figure was adapted with permission from Journal of the American Chemical Society, 2020, 142(30), 13,081–13,089 [71]

4 Applications of Defective MOFs The physico-chemical features of crystalline materials are greatly impacted by defects, including surface, point, line, and volume defects, as is commonly understood in the material science field. The MOFs’ pores are expected to be uniform in size, shape, and function because MOFs are composed of repeating crystalline

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structures. This conduct establishes connections between structures and characteristics [74]. The normal porosity structure of MOFs is interrupted by the insertion of defects, and the considered applications can be significantly different from the parent MOFs. MOFs’ structural defects are widely known to have a significant impact on their features, including porous structure, thermal–mechanical features, electronic functionality, and Lewis acidity, all of which may have an impact on their applications [10]. For instance, more open metal sites (OMSs) could be produced as active sites for adsorption and catalysis due to missing-linker defects [49]. Mass-transport channels inside the pores, critical for adsorption and separation operations, could be affected by the linker and/or metal vacancies. Essential advancements in using defective MOFs are covered in the following sections.

4.1 Applications in Gas Adsorption and Separation Due to their great porosity, MOFs are viewed as promising gas adsorbents as a subclass of porous solids. The ability of MOF defects to change pore molecular functionalization has long been known. Interaction of pores with adsorbed molecules, and hence the MOFs’ gas adsorption features, are influenced by their microenvironment [75, 76]. Additional adsorption sites may be provided by MOF defects, as discussed above. For microporous HKUST-1, Kim et al. [77] used acetic acid-fragmented linkers to create mesoporous defects in the structure. With its large surface area and pores, the acid-fragmented version of HKUST-1 had an advantage over the pristine version when it came to methane uptake (storage capacity was raised by 13% at 65 bar, and realizable capacity improved by 16% between 5 and 65 bar.). Defects in the MOF structure are also crucial in gas separation. Wu et al. [78] recently demonstrated that data-driven insights into how defects govern the function of defective UiO-66 (UiO-66-Ds) in the separation of ethane-ethylene might be obtained by machine learning. 425 samples of UiO-66-Ds with a wide variety of missing-linker defects were developed as a modeling library as a result of this research (in terms of density and distribution). Figure 6a depicts the overall performance of ethane-ethylene separation utilizing UiO-66-Ds. Compared to the perfect UiO-66, the structures in each panel’s upper right corner have a higher working capacity, selectivity, and equivalent mechanical stability. These favored structures have relatively modest defect densities (linker vacancy ratios of 0.2–0.3); however, the defects are scattered randomly (SRO = 0.02) at 1.0 bar. According to the authors managing defects’ density was more critical in tweaking the overall qualities than regulating the defects’ dispersion.

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Fig. 6 a Scatter plots for UiO-66-Ds structures at 1.0 bar for ethane work capacity and selectivity. Data points’ size determines the bulk modulus. Defects’ density and distribution can be quantified using a variety of missing-linker ratios and short-range orders (SRO). This figure was adapted with permission from Chemistry of Materials, 2020, 32(7), 2986–2997 [78]. b Imaginative diagram of Ti-BDC-A formation process. c Suggested reaction mechanism of DBT over Ti-BDC-A catalytic ODS processes. This figure was adapted with permission from ACS Catalysis, 2020, 10(3), 2384– 2394 [17]. d The perfect and defective UiO-66 structures generated by argon plasma bombardment are shown schematically and crystallographically. This figure was adapted with permission from Applied Energy, 2020, 277, 115,560 [49]

4.2 Applications in Catalysis There is a variety of reactions that can benefit from the usage of MOF catalysts [79]. MOF pores are uniform in size and functionality, making them simple to construct and customize for specific catalytic applications. Immobilization of catalytic species in the immediate microenvironment of structural defects and the formation of defectbased Bronsted (when an additional -OH/H2 O group occupies a node) or Lewis acid sites (when the ligands that cap the sites are eliminated) have been the three main approaches to altering MOFs’ catalytic behavior.

4.2.1

Catalysis Due to Lewis Acid Sites

The Lewis acid sites are inherent in some MOFs; however, they must be induced in others. Inducing defects in the MOF structure is one method that can be utilized to generate Lewis acid sites. The existence of free coordination sites in MOFs causes Lewis acid sites. MOFs are crystals that can be characterized as an infinitely repeated sequence of pieces in space. However, natural crystals are far from perfect and might have several imperfections. The overall qualities of the crystal aren’t affected if the

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number of defects isn’t excessive. Defects can define a crystal’s general properties if there are a high number of them. If part of the linkers is missing during the synthesis of a MOF, additional Lewis acid sites will result since the metal will not be properly coordinated. These defects can occur naturally in all crystallization processes but can also be purposefully manufactured. There is a crucial role for these Lewis acid sites in heterogeneous catalysis in the defective MOFs. In order to create titanium terephthalate MOF (Ti-BDC)25 with several defect sites, the acid modulator techniques and solvent-free synthesis were applied [17]. According to the analysis results, the defect sites in Ti-BDC are mostly Ti–OH sites. The oxidative desulfurization (ODS) reaction was used to test the MOFs’ catalytic activity. The defective sample had excellent catalytic activity in the ODS reaction than the defect-free sample, which is caused by the development of abundantly accessible active sites and defect sites in these MOFs. Defective MOFs are superior in the deep ODS of oil fuels, according to these findings (Fig. 6b and c). It has recently been demonstrated that defect-engineered HKUST-1 type MOFs can be generated by oxidative decarboxylation, which results in Cu+ /Cu2+ dimer defects [80]. The decarboxylation reduces the intact framework’s pure Cu2+ /Cu2+ pairs, thereby producing pairs of Cu+ /Cu2+ defects. It has been shown by density functional theory (DFT) simulations that the generated defects within HKUST-1 not only provide increased adsorption sites’ binding energy for CO, but also open up space to permit the CO and dioxygen simultaneous binding on Cu+ /Cu2+ single dimers. During CO oxidation at relatively low temperatures, this cooperative result adds to the increased catalytic performance. Plasma treatment can be applied to control the missing-linker defects’ amount in UiO-66. UiO-66’s catalytic performance is significantly influenced by the linker defects’ increasing amount of open metal sites [49]. The high CO2 cycloaddition activity of defect-rich UiO-66 with epoxides was confirmed (Fig. 6d).

4.2.2

Catalysis Due to Bronsted Acid Sites

The major active sites for catalysis are Bronsted acid sites, although due to their labile character, they haven’t been used much in MOFs. Because of their better resilience, Zr-MOFs are suitable MOF substrates for introducing Bronsted acidic sites. In addition to the clusters’ natural μ3 -OH groups acting as weak Bronsted acids, defect generation can result in the formation of Bronsted acid sites [58]. Feng et al. [48] used a hemilabile (Hl) linker to introduce up to six defects for every cluster in UiO-66. They created hemilabile UiO-66 (Hl-UiO-66) by combining terephthalate (BDC) as a linker and 4-sulfonatobenzoate (PSBA) as a hemilabile linker. The PSBA performs two roles: one as a co-ligand that increases the resulting defective frameworks’ stability, and the other as a modulator that induces defects. Additionally, after a post-synthetic modification in H2 SO4 , the average defect amount grows to an optimal of six missing-BDC ligands for every cluster (three for each unit of the formula), retaining the Zr-nodes six-fold coordinated on average.

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Moreover, the produced materials’ catalytic performance in the acid-catalyzed αpinene oxide isomerisation is examined. The Lewis or Bronsted acid sites of the catalyst have a significant impact on this reaction. The Hl-UiO-66 and post-synthetically modified Hl-UiO-66 showed more Lewis acid sites as well as increased activity and selectivity when compared to parent UiO-66, which predominantly contains Bronsted acid sites. CD3 CN spectroscopic sorption tests are being conducted to investigate this further. They demonstrated that by manipulating the number of UiO-66 defects utilizing PSBA as the hemilabile linker, they could generate extremely defective and robust MOFs and easily manipulate the Bronsted to Lewis acid ratio in the materials, hence controlling their selectivity and catalytic performance.

4.2.3

Catalysis Due to Loading Materials

In recent years, defective MOFs have proven as influential hosts for several active guest species, in addition to the catalytic properties generated from metal nodes [81]. Defective MOFs’ large surface areas and hierarchical porosity allow reactive molecules to reach active sites even when they have been loaded with active guest molecules [82]. Various guest species have been effectively integrated into the defective MOFs, including polyoxometalates (POMs) and metal nanoparticles (NPs) [82–85]. For instance, Keggin-type POMs encapsulated in defective UiO-66 were synthesized in a one-pot approach with in-situ thiourea addition [83]. Thiourea can hydrolyze into ammonia and hydrogen sulfide in an acidic solution when heated. In creating UiO-66, these produced species generate missing-linker defects or open metal sites. The ODS reaction revealed the cooperative effect of defective UiO-66 and POM.

4.3 Decontamination Applications In combination with their high porosity, MOFs’ defect-induced active sites make them ideal decontamination media. Defective MOFs have been used to study chemical warfare agent (CWAs) decontamination [86, 87] and the contaminant adsorption. Zr-based MOFs with lewis acid sites and exceptional stability have recently demonstrated efficient CWA uptake and improved organophosphorus-based CWAs’ chemical detoxification during a hydrolysis [88, 89]. The nerve agent binding on Zr-MOF secondary building units (SBUs), followed by the phosphoester bond hydrolysis, appears to be the mechanism of the hydrolysis reaction, according to DFT simulations (Fig. 7a) [90, 91]. Increasing access to Zr6 active sites has been discovered to improve the hydrolysis rates of nerve agents and their mimics. When defects are introduced into Zr-MOF, they form hierarchically porous structures with more active sites for hydrolysis. Defective MOFs’ defects, pore size, and electronegativity are crucial factors in regulating the adsorption capacity of organic dyes and organo-arsenic chemicals

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Fig. 7 a Sarin hydrolysis on Zr-MOF uncapped faces: a mechanistic model. This figure was adapted with permission from Chemistry of Materials, 2018, 30(13), 4432–4439 [91]. b Formation of defect pores in UiO-66 by means of in-situ doping of pyrrole. c RhB adsorption on UiO-66 and NX-UiO66 and the resulting equilibrium selectivity (RhB/ST). This figure was adapted with permission from Chemical Engineering Journal, 2019, 356, 329–340 [94]. d Drug-modified MOF synthesis and drug loading after synthesis. Chemical structures of the drug (coordinating groups in red) and abbreviations are indicated in the inset. This figure was adapted with permission from Angewandte Chemie, 2020, 132(13), 5249–5255 [95]

when employing defective MOFs [92, 93]. Furthermore, the defective UiO-66’s surface alkalinity has a significant impact on its preferential adsorption of cationic dyes (Rhodamine B (RhB) and ST) of identical sizes [94]. The coordination of alkaline N-compounds (pyrrole, dopamine, and 2-methylimidazole) was shown to modulate pore diameters and amplify the surface alkalinity of the pristine UiO66 (Fig. 7b), reducing the adsorption interaction with basic colors substantially. The

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pyrrole coordinated UiO-66 (NP -UiO-66) outperformed the original UiO-66 in terms of selectivity (56 times) for the equimolar RhB/ST binary system and adsorption capacity (three times) for RhB (282 mg g−1 ), as shown in Fig. 7c.

4.4 Bio-Applications Because of their low toxic effects, good removal, and high loadings, MOFs have been suggested as potential drug and biomolecule carriers. Creating defective MOFs also opens up a world of possibilities for developing new pharmacological or biomoleculeloaded MOFs. In defective MOFs, the unimodal pore size distribution can help substrates access encapsulated active molecules [96–98]. As a result of the slight pore collapse in MOFs caused by the increasing temperature, Teplensky and coworkers [99] created a thermal treatment method to postpone the release of model drug chemical calcein and the anticancer therapy alpha-cyano-4 hydroxycinnamic acid (-CHC). Anticancer drugs were more effective when put into defective MOFs that had been heated, although the authors ignored the impact of MOF defects. This study demonstrates that defect-engineered MOFs hold significant promise for long-term controlled drug release. Forgan et al. recently used defect generation in a one-pot synthesis to insert up to three medicines into the UiO-66, in which several drugs with carboxylate and phosphonate-containing groups were utilized as modulators and drug molecules (Fig. 7d) [95]. By coordinating with the metal clusters instead of being pore-loaded, these drugs were able to remain in both crystallinity and porosity while being disseminated at various defect sites across the MOF. The fourth drug, 5-FU, was post-synthetically loaded into the MOFs using the residual porosity, resulting in NPs loaded with drug combinations that exhibit enhanced selective anticancer cytotoxicity in vitro against MCF-7 breast cancer cells. The use of MOFs in biomedicine has greatly advanced thanks to multivariate modulation, which can also be applied to other branches of MOF chemistry.

4.5 Smart Applications Recent research indicates that the physical properties of MOFs may differ from those of their native counterparts if defects are present. Defects in MOF materials can alter the electronic band structure [100–102]. For example, modulators of hydroxyisophthalic acid (Iso) and fluoro were used to engineer the titanium heterometallic MOF MUV-10 to improve its photocatalytic efficiency for producing hydrogen [103]. The samples’ MUV-10-Iso-OH band gap reduces with the addition of Iso-OH, from 3.3 eV for parent MUV-10 to 2.3 eV for MUV-10-Iso-OH(33). In order to cause UiO-66’s missing-linker defects, Xiang et al. used plasma etching [49]. According to Fig. 8a, the band gap of the defective UiO-66 decreased as the number of defects increased, while altering the etching duration can adjust the defect

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density. According to Xue et al. [24] and Yu et al. [34] reports, altering the electronic structure of CoBDC-NF through the production of missing-linker defects could improve CoBDC-NFs’ activity in an oxygen evolution reaction (OER). CoBDC-FcNF has dramatically increased OER activity after adding missing linkers (Fc), with an ultralow overpotential of 178 mV for the production of 10 mA, rendering 74 and 57 mV lower than CoBDC-NF and commercial RuO2, respectively (Fig. 8b). When MOFs are put under a lot of mechanical strain in commercial extrusion procedures, their mechanical properties or stability must be considered. Consequently, the commercialization of defective MOFs depends on an in-depth knowledge of how they react to stress. Experimental data supporting the effect of defects on the UiO-66 s’ compression was published by Dissegna et al. [4]. Different equivalents of trifluoroacetic acid (TFA) were used as modulators to create defective UiO-66(Zr) samples. The bulk modulus decreases with increasing TFA equivalents from 0 to 5. However, as shown in Fig. 8c, the UiO-66 s’ bulk modulus seemed to rise at higher defect densities (10 eq TFA). As a result of their inherent chemical bonding (chemistry of coordination bond), MOFs are primarily electrical insulators, despite their many promising applications

Fig. 8 a The number of linker defects and computed band gaps for each Zr6 node vs. plasma treatment period. This figure was adapted with permission from Applied Energy, 2020, 1(277), 115,560 [49]. b Overpotential at different current densities. This figure was adapted with permission from Nature communications, 2019, 10(1), 1–8 [24]. c The pressure-dependent relative volumes of UiO-66 with varying defect densities. Lower bulk modulus values are associated with steeper slopes. This figure was adapted with permission from Journal of the American Chemical Society, 2018, 140(37), 11,581–11,584 [4]. d A diagram of ORR turnover frequency (TOF) (blue) and Doping (black) as Hemin surface loading function. This figure was adapted with permission from The Journal of Physical Chemistry C, 2019, 123(9), 5531–5539 [104]

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[10]. As a result of meticulous framework design and intentional incorporation of guest species, functionalized groups, and defects, charge transfer in MOFs can be tuned [3, 104]. A post-synthetically inserted molecular catalyst’s (Hemin) spatial distribution was modulated by Hod et al. by methodically controlling the number of MOF defect sites [104]. Therefore, variations in counterion diffusion rates toward electroactive Hemins installed in bulk or on the surface have changed the rate of charge transport by a factor of 10. Also, the oxygen reduction reaction activity was found to be directly related to the tuned redox-based conductivity (Fig. 8d).

5 Conclusion and Future Prospects After reviewing the recent advancements in the field, it is clear that the MOFs’ defect chemistry offers a wealth of potential for developing and adjusting applicationrelevant features and is expanding our understanding of MOFs’ basic qualities. MOF defect engineering is a potent tool for designing and tuning physical–chemical features like electronic band structures, conductivity, and mechanical stability. For the production of defective MOFs, both de novo synthesis and post-synthetic modification are widely utilized and effective methods. During the de novo synthesis of MOFs, the modulator method, mixed-linker procedure, and fast crystal growth can enhance defects’ development. The parent MOFs are heterogeneously treated with changed ligands, etching reagents, or other activation techniques once they have been synthesized. The investigation of defects in MOFs has proved robust using an established spectrum and microscopy characterization approach combined with specific developing strategies. With the rapid development of defective MOFs, these cutting-edge characterization approaches will become even more important in gaining a unique perspective on defect engineering. In addition to catalysis, adsorption, separation, detoxification, and biological therapy, defective MOFs and their composites have demonstrated many uses. Defective MOFs provide an effective link between homogeneous and heterogeneous catalysts. We anticipate the expanding use of defective MOFs and their composites. In recent years, defect engineering of MOFs has emerged as a hot research issue. MOF defects have a serious influence on a number of diverse uses. A basic knowledge of MOF and related composite materials defects is still far away. There are still a lot of unknowns when it comes to MOF defects. In order to close the gap in our understanding of MOF imperfections, we need to conduct additional fundamental research. Defective MOFs studies that could contribute to progress in the future will be discussed in the following. MOF characteristics can be fine-tuned through defect engineering, which has been proven to be effective. Researchers have found that the characteristics of MOFs are influenced by the density and spatial configuration of MOFs’ missing-linker defects. MOFs can now be tuned experimentally by employing appropriate synthetic techniques to control the density of defects. However, managing the distribution of defects in MOFs is still difficult. Creating new

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synthetic approaches for producing controllable structural defects in MOFs is necessary. Radiation-based local spot treatments in MOFs may allow for efficient defect development and spatial layout. The production of MOFs with a single missinglinker and cluster would pique the interest of researchers in defect chemistry and lead to additional novel uses. Defects detection and characterization are essential for the research of MOF defects. Common characterization techniques are insufficient to comprehend the nature of faults and their link to MOF characteristics. Due to the complexity of their coexistence, it remains uncertain whether the missinglinker/cluster defects can be separated completely from one another. To learn more about the links between defect structure and property, novel and potent characterization techniques are sought. Neutron diffraction, SED, and anomalous diffraction, currently nonstandard characterization techniques, are anticipated to produce useful information in the future [10, 71]. Missing-linker defect generation and subsequent defect healing are predicted to be used soon to include active guest molecules of substantial size in microporous MOFs [105, 106]. This preparation allows for the avoidance of guest molecules leaching from the pores of MOF. Furthermore, MOF structural defects create specific local conditions for the anchoring of guest species and even the change of metal NPs’ electronic states [107]. In the future, it is envisaged that MOF defects will be utilized entirely from various perspectives. Finally, defects have a vital role in improving the performance of MOFs. With the recent results of characteristics produced by structural defects in diverse MOFs and MOF-based composites, defective MOFs will have more potential applications. Rather than catalysis, gas adsorption, decontamination, and drug delivery, new uses of defective MOFs are envisaged in disciplines such as optics, magnetism, sensing, photothermal therapy, and electrochemical energy storage.

References 1. Vo, T.K., Nguyen, V.C., Quang, D.T., Park, B.J., Kim, J.: Formation of structural defects within UiO-66 (Zr)-(OH) 2 framework for enhanced CO2 adsorption using a microwave-assisted continuous-flow tubular reactor. Microporous Mesoporous Mater. 312, 110746 (2021) 2. Kondo, Y., Kuwahara, Y., Mori, K., Yamashita, H.: Dual role of missing-linker defects terminated by acetate ligands in a zirconium-based MOF in promoting photocatalytic hydrogen peroxide production. J. Phys. Chem. C 125(51), 27909–27918 (2021) 3. Liu, Q.-Q., Liu, S.-S., Liu, X.-F., Xu, X.-J., Dong, X.-Y., Zhang, H.-J., Zang, S.-Q.: Superprotonic conductivity of UiO-66 with missing-linker defects in aqua-ammonia vapor. Inorg. Chem. 61(8), 3406–3411 (2022) 4. Dissegna, S., Vervoorts, P., Hobday, C.L., Duren, T., Daisenberger, D., Smith, A.J., Fischer, R.A., Kieslich, G.: Tuning the mechanical response of metal–organic frameworks by defect engineering. J. Am. Chem. Soc. 140(37), 11581–11584 (2018) 5. Luo, R., Wu, J., Zhao, J., Fang, D., Liu, Z., Hu, L.: ZIF-8 derived defect-rich nitrogen-doped carbon with enhanced catalytic activity for efficient non-radical activation of peroxydisulfate. Environ. Res. 204, 112060 (2022) 6. Feng, Y., Chen, Q., Jiang, M., Yao, J.: Tailoring the properties of UiO-66 through defect engineering: a review Ind. Eng. Chem. Res. 58(38), 17646–17659 (2019)

MOF Scaffolds Defects and Disorders

133

7. Wang, T., Zhu, H., Zeng, Q., Liu, D.: Strategies for overcoming defects of HKUST-1 and its relevant applications. Adv. Mater. 6(13), 1900423 (2019) 8. Fang, Z., Bueken, B., De Vos, D.E., Fischer, R.A.: Defect-engineered metal–organic frameworks. Angew. Chem. Int. Ed 54, 7234–7254 (2015) 9. Ren, J., Ledwaba, M., Musyoka, N.M., Langmi, H.W., Mathe, M., Liao, S., Pang, W.: Structural defects in metal–organic frameworks (MOFs): formation, detection and control towards practices of interests. Coordi. Chem. Rev. 349, 169–197 (2017) 10. Dissegna, S., Epp, K., Heinz, W.R., Kieslich, G., Fischer, R.A.: Defective metal-organic frameworks. Adv. Mater. 30(37), 1704501 (2018) 11. Gatto, G., Macchioni, A., Bondi, R., Marmottini, F., Costantino, F.: Post synthetic defect engineering of UiO-66 metal–organic framework with An iridium (III)-HEDTA complex and application in water oxidation catalysis. Inorganics 7(10), 123 (2019) 12. Wei, R., Gaggioli, C.A., Li, G., Islamoglu, T., Zhang, Z., Yu, P., Farha, O.K., Cramer, C.J., Gagliardi, L., Yang, D.: Tuning the properties of Zr6O8 nodes in the metal organic framework UiO-66 by selection of node-bound ligands and linkers. Chem. Mater. 31(5), 1655–1663 (2019) 13. Dissegna, S., Epp, K., Heinz, W.R., Kieslich, G., Fischer, R.A.: Metal–organic frameworks: defective. Adv. Mater. 30(37), 1870280 (2018) 14. Koutsianos, A., Kazimierska, E., Barron, A.R., Taddei, M., Andreoli, E.: A new approach to enhancing the CO2 capture performance of defective UiO-66 via post-synthetic defect exchange. Dalton Trans. 48(10), 3349–3359 (2019) 15. Iacomi, P., Formalik, F., Marreiros, J., Shang, J., Rogacka, J., Mohmeyer, A., Behrens, P., Ameloot, R., Kuchta, B., Llewellyn, P.L.: Role of structural defects in the adsorption and separation of C3 hydrocarbons in Zr-fumarate-MOF (MOF-801). Chem. Mater. 31(20), 8413– 8423 (2019) 16. Ma, X., Wang, L., Zhang, Q., Jiang, H.L.: Switching on the photocatalysis of metal–organic frameworks by engineering structural defects. Angew. Chem. Int. Ed 131(35), 12303–12307 (2019) 17. Ye, G., Gu, Y., Zhou, W., Xu, W., Sun, Y.: Synthesis of defect-rich titanium terephthalate with the assistance of acetic acid for room-temperature oxidative desulfurization of fuel oil. ACS Catal. 10(3), 2384–2394 (2020) 18. Shearer, G.C., Chavan, S., Bordiga, S., Svelle, S., Olsbye, U., Lillerud, K.P.: Defect engineering: tuning the porosity and composition of the metal–organic framework UiO-66 via modulated synthesis. Chem. Mater. 28(11), 3749–3761 (2016) 19. Wang, J., Liu, L., Chen, C., Dong, X., Wang, Q., Alfilfil, L., AlAlouni, M.R., Yao, K., Huang, J., Zhang, D.: Engineering effective structural defects of metal–organic frameworks to enhance their catalytic performances. J. Mater. 8(8), 4464–4472 (2020) 20. Zhang, H.-Y., Shi, R.-H., Fan, H.-L., Yang, C., Zhang, C.-N., Wang, Y.-S., Tian, Z.: Defect creation by benzoic acid in Cu-based metal−organic frameworks for enhancing sulfur capture. Microporous Mesoporous Mater. 298, 110070 (2020) 21. Hu, Z., Castano, I., Wang, S., Wang, Y., Peng, Y., Qian, Y., Chi, C., Wang, X., Zhao, D.: Modulator effects on the water-based synthesis of Zr/Hf metal–organic frameworks: quantitative relationship studies between modulator, synthetic condition, and performance. Cryst. Growth Design 16(4), 2295–2301 (2016) 22. Chaemchuen, S., Luo, Z., Zhou, K., Mousavi, B., Phatanasri, S., Jaroniec, M., Verpoort, F.: Defect formation in metal–organic frameworks initiated by the crystal growth-rate and effect on catalytic performance. J Catal 354, 84–91 (2017) 23. Lázaro, I.A., Almora-Barrios, N., Tatay, S., Martí-Gastaldo, C.: Effect of modulator connectivity on promoting defectivity in titanium–organic frameworks. Chem. Sci. 12(7), 2586–2593 (2021) 24. Xue, Z., Liu, K., Liu, Q., Li, Y., Li, M., Su, C.-Y., Ogiwara, N., Kobayashi, H., Kitagawa, H., Liu, M.: Missing-linker metal-organic frameworks for oxygen evolution reaction. Nat. Commun. 10(1), 1–8 (2019)

134

F. Ganjali et al.

25. Chen, Y., Zhang, S., Chen, F., Cao, S., Cai, Y., Li, S., Ma, H., Ma, X., Li, P., Huang, X.: Defect engineering of highly stable lanthanide metal–organic frameworks by particle modulation for coating catalysis. J. Mater. 6(2), 342–348 (2018) 26. DeStefano, M.R., Islamoglu, T., Garibay, S.J., Hupp, J.T., Farha, O.K.: Room-temperature synthesis of UiO-66 and thermal modulation of densities of defect sites. Chem. Mater. 29(3), 1357–1361 (2017) 27. Lee, J.S., Kapustin, E.A., Pei, X., Llopis, S., Yaghi, O.M., Toste, F.D.: Architectural stabilization of a gold (III) catalyst in metal-organic frameworks. Chem 6(1), 142–152 (2020) 28. Fan, Z., Wang, J., Wang, W., Burger, S., Wang, Z., Wang, Y., Wöll, C., Cokoja, M., Fischer, R.A.: defect engineering of copper paddlewheel-based metal-organic frameworks of type NOTT-100: implementing truncated linkers and its effect on catalytic properties. ACS Appl. Mater. 12(34), 37993–38002 (2020) 29. Epp, K., Luz, I., Heinz, W.R., Rapeyko, A., Llabrés i Xamena, F.X., Fischer, R.A.: Defect engineered ruthenium MOFs as versatile heterogeneous hydrogenation catalysts. ChemCatChem 12(6), 1720–1725 (2020) 30. Yuan, S., Zou, L., Qin, J.-S., Li, J., Huang, L., Feng, L., Wang, X., Bosch, M., Alsalme, A., Cagin, T.: Construction of hierarchically porous metal–organic frameworks through linker labilization. Nat. Commun. 8(1), 1–10 (2017) 31. Bueken, B., Van Velthoven, N., Krajnc, A., Smolders, S., Taulelle, F., Mellot-Draznieks, C., Mali, G., Bennett, T.D., De Vos, D.: Tackling the defect conundrum in UiO-66: a mixed-linker approach to engineering missing linker defects. Chem. Mater. 29(24), 10478–10486 (2017) 32. Feng, L., Yuan, S., Zhang, L.-L., Tan, K., Li, J.-L., Kirchon, A., Liu, L.-M., Zhang, P., Han, Y., Chabal, Y.J.: Creating hierarchical pores by controlled linker thermolysis in multivariate metal–organic frameworks. J. Am. Chem. Soc. 140(6), 2363–2372 (2018) 33. Hardian, R., Dissegna, S., Ullrich, A., Llewellyn, P.L., Coulet, M.V., Fischer, R.A.: Tuning the properties of MOF-808 via defect engineering and metal nanoparticle encapsulation. Chem. Eur. J. 27(22), 6804–6814 (2021) 34. Yu, Z.-Y., Duan, Y., Liu, J.-D., Chen, Y., Liu, X.-K., Liu, W., Ma, T., Li, Y., Zheng, X.S., Yao, T.: Unconventional CN vacancies suppress iron-leaching in Prussian blue analogue pre-catalyst for boosted oxygen evolution catalysis. Nat. Commun. 10(1), 1–9 (2019) 35. Lee, B., Moon, D., Park, J.: Microscopic and mesoscopic dual postsynthetic modifications of metal-organic frameworks. Angew 59(33), 13793–13799 (2020) 36. Lin, H., Xu, Y., Wang, B., Li, D-S., Zhou, T., Zhang, J.: Postsynthetic modification of metal− organic frameworks for photocatalytic applications. Small Struct. 2100176 (2022) 37. Gao, F., Yan, R., Shu, Y., Cao, Q., Zhang, L.: Strategies for the application of metal–organic frameworks in catalytic reactions. RSC Adv. 12(16), 10114–10125 (2022) 38. Wang, Y., Peng, C., Jiang, T., Li, X.: Research progress of defect-engineered UiO-66 (Zr) MOFs for photocatalytic hydrogen production. Front Energy 15(3), 656–666 (2021) 39. Tan, J., He, X., Yin, F., Liang, X., Li, G.: Post-synthetic Ti exchanged UiO-66-NH2 metal organic frameworks with high faradaic efficiency for electrochemical nitrogen reduction reaction. Int. J. Hydrog. 46(62), 31647–31658 (2021) 40. Cui, P., Wang, P., Zhao, Y., Sun, W.-Y.: Fabrication of desired metal–organic frameworks via postsynthetic exchange and sequential linker installation. Cryst Growth Design 19(2), 1454–1470 (2019) 41. Cohen, S.M.: The postsynthetic renaissance in porous solids. J. Am. Chem. Soc. 139(8), 2855–2863 (2017) 42. Tan, C., Han, X., Li, Z., Liu, Y., Cui, Y.: Controlled exchange of achiral linkers with chiral linkers in Zr-based UiO-68 metal–organic framework. J. Am. Chem. Soc. 140(47), 16229– 16236 (2018) 43. Zhang, X., Zhang, Z., Boissonnault, J., Cohen, S.M.: Design and synthesis of squaramide based MOFs as efficient MOF-supported hydrogen-bonding organocatalysts. ChemComm 52(55), 8585–8588 (2016) 44. Cai, G., Ma, X., Kassymova, M., Sun, K., Ding, M., Jiang, H.-L.: Large-scale production of hierarchically porous metal-organic frameworks by a reflux-assisted post-synthetic ligand substitution strategy. ACS Cent. Sci. 7(8), 1434–1440 (2021)

MOF Scaffolds Defects and Disorders

135

45. Rodríguez-Albelo, L.M., López-Maya, E., Hamad, S., Ruiz-Salvador, A.R., Calero, S., Navarro, J.A.: Selective sulfur dioxide adsorption on crystal defect sites on an isoreticular metal organic framework series. Nat. Commun. 8(1), 1–10 (2017) 46. Chang, G.G., Ma, X.C., Zhang, Y.X., Wang, L.Y., Tian, G., Liu, J.W., Wu, J., Hu, Z.Y., Yang, X.Y., Chen, B.: Construction of hierarchical metal–organic frameworks by competitive coordination strategy for highly efficient CO2 conversion. Adv. Mater. 31(52), 1904969 (2019) 47. Koo, J., Hwang, I.-C., Yu, X., Saha, S., Kim, Y., Kim, K.: Hollowing out MOFs: hierarchical micro-and mesoporous MOFs with tailorable porosity via selective acid etching. Chem. Sci. 8(10), 6799–6803 (2017) 48. Feng, X., Hajek, J., Jena, H.S., Wang, G., Veerapandian, S.K., Morent, R., De Geyter, N., Leyssens, K., Hoffman, A.E., Meynen, V.: Engineering a highly defective stable UiO-66 with tunable Lewis-Brønsted acidity: the role of the hemilabile linker. J. Am. Chem. Soc. 142(6), 3174–3183 (2020) 49. Xiang, W., Ren, J., Chen, S., Shen, C., Chen, Y., Zhang, M., Liu, C.-j: The metal–organic framework UiO-66 with missing-linker defects: a highly active catalyst for carbon dioxide cycloaddition. Appl. Energy 277, 115560 (2020) 50. Taddei, M.: When defects turn into virtues: the curious case of zirconium-based metalorganic frameworks. Coord Chem. Rev. 343, 1–24 (2017) 51. Healy, C., Patil, K.M., Wilson, B.H., Hermanspahn, L., Harvey-Reid, N.C., Howard, B.I., Kleinjan, C., Kolien, J., Payet, F., Telfer, S.G.: The thermal stability of metal-organic frameworks. Coord Chem. Rev. 419, 213388 (2020) 52. Zhang, L., Guo, C., Chen, T., Guo, Y., Hassan, A., Kou, Y., Guo, C., Wang, J.: Effects of different defective linkers on the photocatalytic properties of Cu-BTC for overall water decomposition. Appl. Catal. 303, 120888 (2022) 53. Xu, R., Ji, Q., Zhao, P., Jian, M., Xiang, C., Hu, C., Zhang, G., Tang, C., Liu, R., Zhang, X.: Hierarchically porous UiO-66 with tunable mesopores and oxygen vacancies for enhanced arsenic removal. J. Mater. 8(16), 7870–7879 (2020) 54. Smolders, S., Willhammar, T., Krajnc, A., Sentosun, K., Wharmby, M.T., Lomachenko, K.A., Bals, S., Mali, G., Roeffaers, M.B., De Vos, D.E.: A Titanium (IV)-based metal-organic framework featuring defect-Rich Ti-O sheets as an oxidative desulfurization catalyst. Angew. Chem. Int. Ed 131(27), 9258–9263 (2019) 55. Waitschat, S., Fröhlich, D., Reinsch, H., Terraschke, H., Lomachenko, K., Lamberti, C., Kummer, H., Helling, T., Baumgartner, M., Henninger, S.: Synthesis of M-UiO-66 (M= Zr, Ce or Hf) employing 2, 5-pyridinedicarboxylic acid as a linker: defect chemistry, framework hydrophilisation and sorption properties. Dalton Trans. 47(4), 1062–1070 (2018) 56. Zhang, X., Yang, Y., Song, L., Chen, J., Yang, Y., Wang, Y.: Enhanced adsorption performance of gaseous toluene on defective UiO-66 metal organic framework: equilibrium and kinetic studies. J. Hazard. 365, 597–605 (2019) 57. Ren, J., Musyoka, N.M., Langmi, H.W., Walker, J., Mathe, M., Liao, S.: In-situ IR monitoring to probe the formation of structural defects in Zr-fumarate metal–organic framework (MOF). Polyhedron 153, 205–212 (2018) 58. Cirujano, F.G., Llabrés i Xamena, F.X.: Tuning the catalytic properties of UiO-66 metal organic frameworks: from lewis to defect-induced Brønsted acidity. J. Phys. Chem. Lett. 11(12)4879–4890 (2020) 59. Driscoll, D.M., Troya, D., Usov, P.M., Maynes, A.J., Morris, A.J., Morris, J.R.: Characterization of undercoordinated Zr defect sites in UiO-66 with vibrational spectroscopy of adsorbed CO. J. Phys. Chem. C 122(26), 14582–14589 (2018) 60. Chakarova, K., Strauss, I., Mihaylov, M., Drenchev, N., Hadjiivanov, K.: Evolution of acid and basic sites in UiO-66 and UiO-66-NH2 metal-organic frameworks: FTIR study by probe molecules. Microporous Mesoporous Mater. 281, 110–122 (2019) 61. Xu, Y.-P., Wang, Z.-Q., Tan, H.-Z., Jing, K.-Q., Xu, Z.-N., Guo, G.-C.: Lewis acid sites in MOFs supports promoting the catalytic activity and selectivity for CO esterification to dimethyl carbonate. Catal. Sci. Technol. 10(6), 1699–1707 (2020)

136

F. Ganjali et al.

62. Insyani, R., Verma, D., Cahyadi, H.S., Kim, S.M., Kim, S.K., Karanwal, N., Kim, J.: One-pot di-and polysaccharides conversion to highly selective 2, 5-dimethylfuran over Cu-Pd/Aminofunctionalized Zr-based metal-organic framework (UiO-66 (NH2))@ SGO tandem catalyst. Appl. Catal. B Environ. 243, 337–354 (2019) 63. Gentile, F.S., Pannico, M., Causà, M., Mensitieri, G., Di Palma, G., Scherillo, G., Musto, P.: Metal defects in HKUST-1 MOF revealed by vibrational spectroscopy: a combined quantum mechanical and experimental study. J. Mater. 8(21), 10796–10812 (2020) 64. Muldoon, P.F., Venna, S.R., Gidley, D.W., Baker, J.S., Zhu, L., Tong, Z., Xiang, F., Hopkinson, D.P., Yi, S., Sekizkardes, A.K.: Mixed matrix membranes from a microporous polymer blend and nanosized metal–organic frameworks with exceptional CO2 /N2 separation performance. ACS Mater. Lett. 2(7), 821–828 (2020) 65. Mondal, S.S., Dey, S., Attallah, A.G., Krause-Rehberg, R., Janiak, C., Holdt, H.-J.: Insights into the pores of microwave-assisted metal–imidazolate frameworks showing enhanced gas sorption. Dalton Trans. 46(14), 4824–4833 (2017) 66. Yuan, L., Tian, M., Lan, J., Cao, X., Wang, X., Chai, Z., Gibson, J.K., Shi, W.: Defect engineering in metal–organic frameworks: a new strategy to develop applicable actinide sorbents. ChemComm 54(4), 370–373 (2018) 67. Wang, K.Y., Feng, L., Yan, T.H., Wu, S., Joseph, E.A., Zhou, H.C.: Rapid generation of hierarchically porous metal–organic frameworks through laser photolysis. Angew. Chem. Int. Ed 132(28), 11445–11450 (2020) 68. Chen, X., Lyu, Y., Wang, Z., Qiao, X., Gates, B.C., Yang, D.: Tuning Zr12O22 node defects as catalytic sites in the metal-organic framework hcp UiO-66. ACS Catal. 10(5), 2906–2914 (2020) 69. Weckhuysen, B.M., Öztürk, Z., Brand, R.P., Boereboom, J.M., Meirer, F.: Vibrational fingerprinting of defects sites in thin films of zeolitic imidazolate frameworks. Chem. Eur. J. 25(34), 8070–8084 (2019) 70. Kang, X., Lyu, K., Li, L., Li, J., Kimberley, L., Wang, B., Liu, L., Cheng, Y., Frogley, M.D., Rudi´c, S.: Integration of mesopores and crystal defects in metal-organic frameworks via templated electrosynthesis. Nat. Commun. 10(1), 1–9 (2019) 71. Johnstone, D.N., Firth, F.C., Grey, C.P., Midgley, P.A., Cliffe, M.J., Collins, S.M.: Direct imaging of correlated defect nanodomains in a metal–organic framework. J. Am. Chem. Soc. 142(30), 13081–13089 (2020) 72. Shen, B., Chen, X., Shen, K., Xiong, H., Wei, F.: Imaging the node-linker coordination in the bulk and local structures of metal-organic frameworks. Nat. Commun. 11(1), 1–8 (2020) 73. Liu, L., Zhang, D., Zhu, Y., Han, Y.: Bulk and local structures of metal–organic frameworks unravelled by high-resolution electron microscopy. Commun. Chem. 3(1), 1–14 (2020) 74. Fang, Z., Bueken, B., De Vos, D.E., Fischer, R.A.: Defect-engineered metal–organic frameworks. Angew. Chem. Int. Ed. 54(25), 7234–7254 (2015) 75. Ji, Z., Wang, H., Canossa, S., Wuttke, S., Yaghi, O.M.: Pore chemistry of metal–organic frameworks. Adv. Funct. 30(41), 2000238 (2020) 76. Ohsaki, S., Nakazawa, R., Teranishi, A., Nakamura, H., Watano, S.: Control of gate adsorption characteristics of flexible metal-organic frameworks by crystal defect. Microporous Mesoporous Mater. 302, 110215 (2020) 77. Kim, S.-Y., Kim, A.-R., Yoon, J.W., Kim, H.-J., Bae, Y.-S.: Creation of mesoporous defects in a microporous metal-organic framework by an acetic acid-fragmented linker co-assembly and its remarkable effects on methane uptake. Chem. Eng. J. 335, 94–100 (2018) 78. Wu, Y., Duan, H., Xi, H.: Machine learning-driven insights into defects of zirconium metal– organic frameworks for enhanced ethane–ethylene separation. Chem. Mater. 32(7), 2986– 2997 (2020) 79. Yang, D., Gates, B.C.: Catalysis by metal organic frameworks: perspective and suggestions for future research. ACS Catal. 9(3), 1779–1798 (2019) 80. Wang, W., Sharapa, D.I., Chandresh, A., Nefedov, A., Heißler, S., Heinke, L., Studt, F., Wang, Y., Wöll, C.: Interplay of electronic and steric effects to yield low-temperature co oxidation at metal single sites in defect-engineered HKUST-1. Angew. Chem. Int. Ed. 59(26), 10514–10518 (2020)

MOF Scaffolds Defects and Disorders

137

81. Xiang, W., Zhang, Y., Lin, H., Liu, C.-j: Nanoparticle/metal–organic framework composites for catalytic applications: current status and perspective. Molecules 22(12), 2103 (2017) 82. Gutterød, E.S., Pulumati, S.H., Kaur, G., Lazzarini, A., Solemsli, B.G., Gunnæs, A.E., AhobaSam, C., Kalyva, M.E., Sannes, J.A., Svelle, S.: Influence of defects and H2O on the hydrogenation of CO2 to methanol over Pt nanoparticles in UiO-67 metal-organic framework. J. Am. Chem. Soc. 142(40), 17105–17118 (2020) 83. Chang, X., Yang, X.F., Qiao, Y., Wang, S., Zhang, M.H., Xu, J., Wang, D.H., Bu, X.H.: Confined heteropoly blues in defected Zr-MOF (Bottle Around Ship) for high-efficiency oxidative desulfurization. Small 16(14), 1906432 (2020) 84. Wang, Y., Wan, J., Tian, W., Hou, Z., Gu, X., Wang, Y.: Theoretical screening of VSe2 as support for enhanced electrocatalytic performance of transition-metal single atoms. J. Colloid Interface Sci. 590, 210–218 (2021) 85. Dong, M.J., Wang, X., Wu, C.D.: Creation of redox-active PdSx nanoparticles inside the defect pores of MOF UiO-66 with unique semihydrogenation catalytic properties. Adv. Funct. 30(7), 1908519 (2020) 86. Kirlikovali, K.O., Chen, Z., Islamoglu, T., Hupp, J.T., Farha, O.K.: Zirconium-based metal organic frameworks for the catalytic hydrolysis of organophosphorus nerve agents. ACS Appl. Mater. 12(13), 14702–14720 (2020) 87. Giannakoudakis, D.A., Bandosz, T.J.: Defectous UiO-66 MOF nanocomposites as reactive media of superior protection against toxic vapors. ACS Appl. Mater. 12(13), 14678–14689 (2019) 88. Cho, K.Y., Seo, J.Y., Kim, H.-J., Pai, S.J., Do, X.H., Yoon, H.G., Hwang, S.S., Han, S.S., Baek, K.-Y.: Facile control of defect site density and particle size of UiO-66 for enhanced hydrolysis rates: insights into feasibility of Zr (IV)-based metal-organic framework (MOF) catalysts. Appl. Catal. 245, 635–647 (2019) 89. Islamoglu, T., Ortuño, M.A., Proussaloglou, E., Howarth, A.J., Vermeulen, N.A., Atilgan, A., Asiri, A.M., Cramer, C.J., Farha, O.K.: Presence versus proximity: the role of pendant amines in the catalytic hydrolysis of a nerve agent simulant. Angew. Chem. Int. Ed. 57(7), 1949–1953 (2018) 90. Momeni, M.R., Cramer, C.J.: Dual role of water in heterogeneous catalytic hydrolysis of sarin by zirconium-based metal–organic frameworks. ACS Appl. Mater. 10(22), 18435–18439 (2018) 91. Momeni, M.R., Cramer, C.J.: Structural characterization of pristine and defective [Zr12 (μ 3-O)8 (μ3-OH)8 (μ2-OH)6] 18+ double-node metal-organic framework and predicted applications for single-site catalytic hydrolysis of sarin. Chem. Mater. 30(13), 4432–4439 (2018) 92. Bui, T.T., Nguyen, L.T., Pham, N.P., Tran, C.C., Nguyen, L.T., Nguyen, T.A., Nguyen, H.N., Nguyen, M.V.: A new approach for ultra-high adsorption of cationic methylene blue in a Zr-sulfonic-based metal–organic framework. RSC Adv. 11(58), 36626–36635 (2021) 93. Qiu, J., Feng, Y., Zhang, X., Jia, M., Yao, J.: Acid-promoted synthesis of UiO-66 for highly selective adsorption of anionic dyes: adsorption performance and mechanisms. J. Colloid Interface Sci. 499, 151–158 (2017) 94. Hu, P., Zhao, Z., Sun, X., Muhammad, Y., Li, J., Chen, S., Pang, C., Liao, T., Zhao, Z.: Construction of crystal defect sites in N-coordinated UiO-66 via mechanochemical in-situ Ndoping strategy for highly selective adsorption of cationic dyes. Chem. Eng. J. 356, 329–340 (2019) 95. Abánades Lázaro, I., Wells, C.J., Forgan, R.S.: Multivariate modulation of the Zr MOF UiO-66 for defect-controlled combination anticancer drug delivery. Angew. Chem. Int. Ed. 132(13), 5249–5255 (2020) 96. Hu, C., Bai, Y., Hou, M., Wang, Y., Wang, L., Cao, X., Chan, C.-W., Sun, H., Li, W., Ge, J.: Defectinduced activity enhancement of enzyme-encapsulated metal-organic frameworks revealed in microfluidic gradient mixing synthesis. Sci. Adv. 6(5):eaax5785 (2020) 97. Yang, P., Mao, F., Li, Y., Zhuang, Q., Gu, J.: Hierarchical porous Zr-based MOFs synthesized by a facile monocarboxylic acid etching strategy. Chem. Eur. J. 24(12), 2962–2970 (2018)

138

F. Ganjali et al.

98. Wang, Z., Hu, S., Yang, J., Liang, A., Li, Y., Zhuang, Q., Gu, J.: Nanoscale Zr-based MOFs with tailorable size and introduced mesopore for protein delivery. Adv. Funct. 28(16), 1707356 (2018) 99. Teplensky, M.H., Fantham, M., Li, P., Wang, T.C., Mehta, J.P., Young, L.J., Moghadam, P.Z., Hupp, J.T., Farha, O.K., Kaminski, C.F.: Temperature treatment of highly porous zirconiumcontaining metal–organic frameworks extends drug delivery release. J. Am. Chem. Soc. 139(22), 7522–7532 (2017) 100. Zhang, L., Guo, Y., Guo, C., Chen, T., Feng, C., Qiao, S., Wang, J.: Construction of defective zeolitic imidazolate frameworks with improved photocatalytic performance via vanillin as modulator. Chem. Eng. J. 421, 127839 (2021) 101. Chen, W., Wang, T., Pino-Yanes, M., Forno, E., Liang, L., Yan, Q., Hu, D., Weeks, D., Baccarelli, A.: Cronfa-Swansea University open access repository (2016) 102. Wang, S.-Q., Gu, X., Wang, X., Zhang, X.-Y., Dao, X.-Y., Cheng, X.-M., Ma, J., Sun, W.-Y.: Defectengineering of Zr (IV)-based metal-organic frameworks for regulating CO2 photoreduction. Chem. Eng. J. 429, 132157 (2022) 103. Lázaro, I.A., Szalad, H., Valiente, P., Albero, J., García, H., Martí-Gastaldo, C.: Tuning the photocatalytic activity of Ti-based metal-organic frameworks through modulator defectengineered functionalization. ACS Appl. Mater. 14(18), 21007–21017 (2022) 104. Shimoni, R., He, W., Liberman, I., Hod, I.: Tuning of redox conductivity and electrocatalytic activity in metal–organic framework films via control of defect site density. J. Phys. Chem. C 123(9), 5531–5539 (2019) 105. Idrees, K.B., Chen, Z., Zhang, X., Mian, M.R., Drout, R.J., Islamoglu, T., Farha, O.K.: Tailoring pore aperture and structural defects in zirconium-based metal–organic frameworks for krypton/xenon separation. Chem. Mater. 32(9), 3776–3782 (2020) 106. Taddei, M., Wakeham, R.J., Koutsianos, A., Andreoli, E., Barron, A.R.: Post-synthetic ligand exchange in zirconium-based metal-organic frameworks: beware of the defects! Chem. Int. Ed. 57(36), 11706–11710 (2018) 107. Wang, X.-X., Meng, S., Zhang, S., Zheng, X., Chen, S.: 2D/2D MXene/g-C3N4 for photocatalytic selective oxidation of 5-hydroxymethylfurfural into 2, 5-formylfuran. Catal. Commun. 147, 106152 (2020)

Composition States of MOFs Fereshte Hassanzadeh-Afruzi and Mohammad Mehdi Salehi

Abstract The current interest in solid bases constructed from metal–organic frameworks (MOFs) is well-founded. In addition to this, their Catalytic structure allows them to perform admirably in a wide variety of chemical reactions. Because of their unique skeletal system, MOFs cannot support the large array of critical capabilities that traditional solid bases are capable of accommodating. Hence, it is challenging to combine all of these capabilities into a MOF. On the other hand, MOFs for heterogeneous basic catalysis have never been investigated in any study. According to MOFs’ structure, metal ions and ligands can cause intrinsic basicity. In this rapidly expanding scientific subject, significant advancement has been achieved over the last 10 years. As a result, in this part, the possibility of using MOFs to manufacture basic catalysts has been discussed. Keywords Basicity · Metal sites · Catalytic · Ligands · Functionalization

1 Introduction A group of compounds that include metal ions and clusters connected with organic ligands to form one or some dimensional structures are called metal–organic frameworks (MOFs). The bands between inorganic and organic sections in MOFs are resilient. Moreover, a purposeful arrangement of components in MOFs can affect their properties, such as thermal and chemical stability and crystal porosity. Besides, the shape of units and their chemical properties in MOFs lead these compounds to have synergistic character. This feature is notable about MOFs, and other solids have not exhibited any change with modification in their structure [1]. There has been reported different approaches for fabricate the basic MOFs as summarized in Fig. 1 [2].

F. Hassanzadeh-Afruzi (B) · M. M. Salehi Catalysis and Organic Synthesis Research Laboratory, Department of Chemistry, Iran University of Science and Technology, 16846-13114 Tehran, Iran e-mail: [email protected]; [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 A. Maleki and R. Taheri-Ledari (eds.), Physicochemical Aspects of Metal-Organic Frameworks, Engineering Materials, https://doi.org/10.1007/978-3-031-18675-2_10

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Fig. 1 Classification of basic MOFs and typical methods for generation of basicity on MOFs. Chem. Rev. 2017, 117, 12, 8129–8176 [2]

2 Generation of Basic Sites There are different routes for synthesizing basic sites on MOFs which produce MOFsderived solid bases. As reported in other research, the MOFs with intrinsic basicity and MOFs with modified basicity are two common classes of solid bases. Selfassembly of special metal ions and/or functional ligands forms the first group while the second group obtains by modifying synthetic MOFs that are not basic [2].

3 MOFs with Intrinsic Basicity According to the structure of MOFs, both metal ions and ligands can cause intrinsic basicity. Taking into account metal center types, they can act as Lewis’s acid sites and basic sites. Two main kinds of metal centers should be considered. The alkaline earth metal such as Mg, Ca, Sr, and the hybrid metal node are two groups that may provide a starting point for imparting basic catalytic activity to MOFs and enhance the activity of the unsaturated metal sites in order. Moreover, there are two kinds of organic ligands that give basicity to MOFs. Ligands like NH2 -BDC are called N-containing ligands and the structural phenolates are the mentioned ligands [2].

3.1 Basicity from Alkaline Earth Metal Sites Nowadays synthesizing MOFs from alkali metal oxides and alkaline earth metals as nodes has attracted a lot of attention because of their attribute basic property. Though MOFs obtained from alkali metal ions are not so popular because their stability is not enough, MOFs based on alkaline earth metal ions such as Mg, Ca, Sr, and Ba specially MgO as a benchmark solid base are fabricated successfully. For example, Mg3 (PDC)(OH)3 (H2 O)2 as an Mg-based carboxylate framework was obtained from Mg (NO3 )2 and 3,5-pyrazoledicarboxylic acid (H3 PDC) in water at

Composition States of MOFs

141

170 °C after 3 days. The obtained product showed an asymmetric unit with three Mg centers which have distorted octahedral geometry. A secondary building unit (SBU) is the Mg3 triad. The arrangement of Mg centers forms a three-dimensional (3D) framework. Some analyses like N2 adsorption, the Brunauer–Emmett–Teller (BET), the aldol condensation reactions (for evaluating the basicity), and the heterogeneous process were done to demonstrate and examine the structure of this 3D MOF. The result of all these evaluations was identical to expectations [3]. The same reaction and addition of the same ligand, H3 PDC, were done about Ba for synthesizing a novel 3D alkaline earth MOF. The unit structure in this Ba-based MOF was completely from the Mg-based one. Also, the same evaluations were done and the results were noticeable. Both basicity and the yield of β-aldol product in Aldol condensation reactions were higher in contrast to its Mg-based counterpart. The alkaline earth metal oxides are known as classic solid bases so gathering these metals with appropriate ligands can be a good way to construct basic MOFs. The basic property of metal oxides comes from low-coordination sites located at the corner, edge, and surface while the basicity of alkaline earth MOFs is because of structural anions transformation to create defect sites. It should be noticed although there is proper knowledge about the coordination chemistry of transition metals, studies about the alkaline earth metal coordination frameworks and alkali metalsbased MOFs are not enough. However, a large number of alkaline earth MOFs were synthesized but the experiments show their basicity is relatively scarce. Two reasons are explained for their behavior, the first one is that more studies were done about their preparation rather than applications, and the latter is that the basicity of MOFs is strongly dependent on the activation conditions [4]. So, for more information, it is necessary to increase the studies on the mentioned directions. De Vos’s group studied the formation of basic sites in the alkaline earth MOFs. This study was done on the Ba2 (BTC)(NO3 ) (DMF) which is a Ba-based MOF. The H3BTC ligand is 1,3,5-benzenetricarboxylic acid and DMF is N, N-dimethylformamide that acts as a solvent. The investigations demonstrated the Ba–O–Ba bonds are expanded in three dimensions and the arrangement of inorganic SBUs and organic ligands configures a hexagonal prism-shaped crystal possessing three-leaf-clover-shaped pore channels. The size of pores is variable from 6.5 to 13 Å. Besides, it should be stated that nitrate anions have an important role in the structure. They keep charge neutrality in the final structure [5].

3.2 Basicity from Hybrid Metal Nodes In past decades, hybridization has attracted more attention in fabricating catalytically active sites in MOFs [6]. Although most studies were done on ligand hybridization, some efforts were taking place on the possibility of hybrid metal nodes in MOFs which the self-assembly process has become a well-known route and concludes frameworks with more diverse performance compared to the use of only one type of metal [7]. In order to generate basic properties via hybridization on

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Cu3 (BTC)2 (H2 O)3 which is known as Cu–BTC or HKUST-1 tungsten (W) ions are picked because of their special attributes. Since W ions are in the same group as Mo and Cr, and the Mo–BTC and Cr–BTC were synthesized, W ions can be a suitable candidate for hybridization. Moreover, W ions are active in CO2 activation and are inexpensive. The W–Cu–BTC was a Hybrid MOF designed by using density functional theory (DFT) calculations. In this calculation, Cu–BTC frameworks were the original structure, and Cu ions were replaced by W ions. After completing 50% of the process, the triclinic crystalline structure was broadened. The explanation for this outcome is the length of bonds between W–Cu and Cu–Cu. It is obvious that the W–Cu bond is longer than the Cu–Cu bond. On the other hand, an increase happened in atomic binding energy, demonstrating that stability was enhanced. When the W– Cu–BTC was heated at about 500 K open W ions were obtained. As result, the six coordination of each W cation was reduced to five coordination. So the unsaturated W ions can be points that act as catalytically active sites. The CO adsorption experiment was done, and the results demonstrated that a few electrons from exposed W ions were adsorbed by CO molecules. This behavior is the reason that W ions can be Lewis’s base. However, it is not true about the primary Cu–BTC and some other MOFs which lose electrons in CO adsorption. Further investigations like electron density difference (EDD) [8], IR, and XPS were done and more detailed results were obtained. As a good result for constructing hybrid metal, nodes can be mentioned basicity of Cu–BTC and 100% W-substituted Cu–BTC (namely, W–BTC) and 100% W-substituted Cu–BTC. The parent and 100% W-substituted have no basic catalytic activity but hybrid MOF owns basicity. The charge polarization is the reason for Lewis’s base sites on the hybrid MOFs while the parent MOFs reveal the Lewis acid behavior from metal ions. There is a difference between the mechanism that provide basicity in hybrid MOFs and the alkaline earth MOFs. The metals used for hybridization like Mo, Cr, and W are crucial and the obtained frameworks have various actions in CO2 activation. Also, the number of used metals in hybridization can affect the basic properties of hybrid MOFs [9].

3.3 Basicity from N-Containing Ligands Besides the mentioned advantages of MOFs, there is another advantage about them that is noteworthy and useful in generating basic MOFs. The variability of organic ligands in the formation of MOFs is the mentioned advantage. One of the common ways to obtain basic MOFs is by using N-containing ligands, and one of the highusage N-containing ligands is NH2 -BDC that assembles with metals like Zn, Zr, Al, Fe, Ti, and Cu [10]. The pores and pore size of porous materials are the factors that influence their application and active sites. It should be mentioned that some N-containing ligands cause structure lacking in porosity for intraporous catalysis to MOFs, so their basic active sites are in lattice-terminating surfaces. Zeolite imidazolate frameworks (ZIFs) with significant chemical stability and tetrahedral coordination are usual examples of the described MOFs [11].

AlCl3

FeCl3

NH2 -MIL-101(Al)

NH2 -Mil-101(Fe)

NH2 -Mil-53(Al)

3

4

5 AlCl3

ZrCl4

NH2 -Uio-66

2

Zn (NO3 )2

2-Aminoterephthalic acid (NH2 -BDC)

2-Aminoterephthalic acid (NH2 -BDC)

2-Aminoterephthalic acid (NH2 -BDC)

2-Aminoterephthalic acid (NH2 -BDC)

2-Aminoterephthalic acid (NH2 -BDC)

Metal salt Ligand

IRMOF-3

1

Entry MOF name and/or formula

Table 1 Summary of basic MOFs constructed directly from various N-containing ligands Ligand structure

(continued)

[13, 26–29]

[23–25]

[21, 22]

[14–20]

[12, 13]

References

Composition States of MOFs 143

Tb (NO3 )2

(continued)

[35]

[25, 30, 31]

4,4’,4'' -tricarboxytriphenylamine (H3 TCA)

Tb-TCA

9

References [24]

[32–34]

Ligand structure

2-Methylimidazole (H-MeIM)

ZIF-8

8 Zn (NO3 )2

Ti 2-Aminoterephthalic acid (NH2 -BDC) (OC4 H9 )4

NH2 -Mil-125(Ti)

7

2-Aminoterephthalic acid (NH2 -BDC)

Metal salt Ligand FeCl3

NH2 -Mil-53(Fe)

6

Entry MOF name and/or formula

Table 1 (continued)

144 F. Hassanzadeh-Afruzi and M. M. Salehi

[38]

4,4’,4'' -S-triazine-1,3,5-triyltri-p-aminobenzoate(H3 TATAB)

PCN-100

12 Zn (NO3 )2

(continued)

[37]

1,4-bis(3-phenol)-2,3-diaza-1,3-butadiene (BPDB)

Cd (SCN)2

[Cd (BPDB)(SCN)2 ].CH2 CL2

References [36]

11

Ligand structure

1,3,5,-benzene tricarboxylic acid tris[N-(4-pyridyl)amide] (4-BTAPA)

Metal salt Ligand

[Cd(4-BTAPA)2 (NO3 )2]0.6H2 O.2DMF Cd (NO3 )2

10

Entry MOF name and/or formula

Table 1 (continued)

Composition States of MOFs 145

Cu (NO3 )2

Zn (NO3 )2

PCN-124

MIXMOF

14

15

Benzene-1,4-dicarboxylate (H2 BDC) and NH2 -BDC

5,5’-((Pyridine-3,5-dicarbonyl) bis-(azanediyl)) diisophthalate (H4 PDAI)

4,4’,4'' -S-triazine-1,3,5-triyltri-p-aminobenzoate(H3 TATAB)

Metal salt Ligand Zn (NO3 )2

PCN-101

13

Entry MOF name and/or formula

Table 1 (continued) Ligand structure

References

(continued)

[40]

[39]

[8]

146 F. Hassanzadeh-Afruzi and M. M. Salehi

Oxalic acid (H2 OA) and 3-amino-1,2,4-triazole (ATZ)

Metal salt Ligand ZnCO3

16

Zn2(OA)(ATZ)2 .(H2 O)0.5

Entry MOF name and/or formula

Table 1 (continued) Ligand structure

References [41]

Composition States of MOFs 147

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The following Table 1 shows the Summary of Basic MOFs Constructed Directly from Various N-Containing Ligands. Another study was carried out about MIXMOF materials. The studied MOF was Zn4 O(BDC)x (NH2 -BDC)3−x and the used moieties were BDC which were substituted by NH2 -BDC. The main purpose of this study was to demonstrate Vegard’s law which focuses on the random distribution of the two mentioned ligands. It is remarkable in phase-pure material the synthetic condition is important. So, in the reaction process, an additional base was not added because the addition of an extra-base causes selective deprotonation of the stronger acid. By controlling the temperature and using various analyses, the random distribution was proved [40].

3.4 Basicity from Structural Phenolates The M2 DHTPs are a group of MOFs that have hybrid frameworks. They are exhibited by the general name of CPO-27 or MOF-74 and are composed of DHTP = 2,5dihydroxyterephthalate, and M2+ = Mg2+ , Co2+ , Ni2+ , Cu2+ , and Zn2+ . The metal ions in these MOFs have the main role in the adsorption and separation of various gases like CO, CO2 , CH4 , ethane, acetylene, propane, and propylene. On the other hand, some reports showed their catalytic activity; however, it is very scarce [42]. Valvekens et al. [43] have reported that these hybrid MOFs have catalytic influence in the Knoevenagel condensation and the other base-catalyzed reactions. In the case of the reaction between malononitrile (or cyanoacetate) and benzaldehyde, the main role of metal ions especially Ni in Ni2 DHTP as a catalyst was clear. To understand more about the basic source, the structure of M2 DHTP materials was studied in more detail. The structure includes metal oxide chains and the DHTP ligands which form a honeycomb-like arrangement that has 1D hexagonal pores with a diameter of ∼1.2 nm. For constructing the final framework, the DHTP as linker is fully deprotonated and oxygen atoms from carboxylate and phenolate coordinate the metal centers [42]. Regarding the basic potency of different oxygen atoms in these MOF structures, it should be mentioned that the carboxylate oxygen atoms are weak basic sites, while phenolate oxygens are more basic. So, oxygens of phenolate act as a deprotonating agent and activate the compound for the rest of the reaction. The CUS is located near the phenolate oxygen atom. Its application is a docking site that deprotonates substrate molecules. In the Knoevenagel reaction by adsorbing substrate like benzaldehyde, coupling of the other substrates occurs. This process is like to what occurs in alkaline earth metal binaphtholate catalysts. This means the basic sites existing in M2 DHTP were also confirmed by the chemisorption of pyrrole. Proton affinity (PA) is a numerical value that describes the basicity of a substance. PA1 and PA2 explain the proton affinity of phenolate and carboxylate oxygen atoms in order. Table 2 shows PA1 is larger than PA2 because the protonation is promoted on phenolate oxygen atoms, consequently phenolate oxygen atoms are proper sites for the deprotonation of donor molecules. This explanation is true about the phenolate in

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Table 2 Proton Affinities (PAs) of M2 DHTP for the Phenolate Oxygen Atom (PA1) and Carboxylate Oxygen Atom (PA2) as well as the Calculated Adsorption Energies of Acetonitrile (ΔEads ). Chem. Rev. 2017, 117, 12, 8129–8176 [2] M

PA1 (KJ.mol−1 )

PA2 (KJ.mol−1 )

ΔEads (KJ.mol−1 )

Mg

930

913

−91

Co

947

918

−83

Ni

946

757

−91

Cu

923

905

−47

Zn

944

917

−79

Ni2 DHTP; however, PA values are not enough to rationalize all basicity tendencies [43].

4 MOFs with Modified Basicity As discussed in previous sections, in addition to the direct solvothermal synthesis method, there is another way of synthesizing the basic MOFs. In this method, after the formation of MOFs, a modification in a heterogeneous condition occurs. As an advantage of this manner that can be introduced is the great controllability of the type and number of a basic functional group on MOFs. So post-synthetic method has been the most interesting way of preparing functional MOFs because it produces different types of materials with various properties. Besides, there are lots of MOFs which are topologically identical but functionally diverse. The reason for these kinds of structures is the impressibility of functionalized MOFs from reagents that are used in functionalization processes. When the post-synthetic method is used, the overall structure does not change, although both the metals and the organic linkers become functional. The CUSs in MOFs are proper basic points for chemical bonding with electron-rich amine molecules. These sites should be open; for this purpose, suitable treatment is used for removing solvents and/or water coordinated on metal. For instance, IRMOF-1, IRMOF-10, MIL-101(Cr), Mg2DHTP, and several basic MOFs are ones fabricated by the explained method. The amines used in the above MOFs can be aliphatic, aromatic, and even chiral amines [44].

4.1 Functionalization of Metal Sites The CUSs in MOFs are proper basic points for chemical bonding with electron-rich amine molecules. These sites should be open; for this purpose, suitable treatment is used for removing solvents and/or water coordinated on metal. For instance, IRMOF1, IRMOF-10, MIL-101(Cr), Mg2 DHTP, and several basic MOFs are ones fabricated

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by the explained method. The amines used in the above MOFs can be aliphatic, aromatic, and even chiral amines [40].

4.2 Functionalization of Ligands Functionalization of ligands via basic groups is another way of synthesizing basic MOFs. In general, there are two main routes for this approach. The first one is the pre-functionalization of ligands, and the second is post-synthetic functionalization. In the case of the pre-functionalization method, ligands are functionalized at first, then the fabrication of MOFs occurs, while in the post-synthetic method, MOFs are synthesized first, and then basic functional groups are immobilized on ligands. On the other hand, direct amination of aromatic rings, amination of the aromatic amine/aldehyde tags, and exchange of pending hydroxyl protons to alkaline/alkali earth metal cations are three approaches that are adapted to attach basic functionality to ligands until now [45]. Stock’s group reported a direct amination of aromatic rings in ligands which is an example of the second-mentioned approach. In fact, they developed a post-synthetic approach to MIL-101(Cr). However, it is possible to directly fabricate aminecontaining MOFs by replacing the unfunctionalized ligands with corresponding amine-functionalized ligands, but it is not always possible. The coordination of amino groups with metal sites and the stability of amine-functionalized ligands are two issues that make the difficulties [45].

References 1. Safaei, M., Foroughi, M.M., Ebrahimpoor, N., Jahani, S., Omidi, A., Khatami, M.: A review on metal-organic frameworks: Synthesis and applications. TrAC Trends Anal. Chem. 118, 401–425 (2019). https://doi.org/10.1016/j.trac.2019.06.007 2. Zhu, L., Liu, X.Q., Jiang, H.L., Sun, L.B.: Metal–organic frameworks for heterogeneous basic catalysis chemical reviews. Chem. Rev. 117(12), 8129–8176 (2017)https://doi.org/10.1021/ acs.chemrev.7b00091 3. Platero Prats, A.E., de la Peña-O’Shea, V.A., Iglesias, M., Snejko, N., Monge, A., GutiérrezPuebla, E.: Heterogeneous catalysis with alkaline-earth metal-based MOFs: a green calcium catalyst. ChemCatChem 2, 147−149 (2010).https://doi.org/10.1002/cctc.200900228 4. Chevreau, H., Devic, T., Salles, F., Maurin, G., Stock, N., Serre, C.: Mixed-linker hybrid superpolyhedra for the production of a series of large-pore iron (III) carboxylate metal−organic frameworks. Angew. Chem Int. Ed. 52, 5056–5060 (2013). https://doi.org/10.1002/anie.201 300057 5. Garibay, S.J., Cohen, S.M.: Isoreticular synthesis and modification of frameworks with the UiO-66 topology. Chem. Commun. 46, 7700–7702 (2010). https://doi.org/10.1039/C0CC02 990D 6. Zhang, C., Xiao, Y., Liu, D., Yang, Q., Zhong, C.: A hybrid zeolitic imidazolate framework membrane by mixed-linker synthesis for efficient CO2 capture. Chem. Commun. 49, 600–602 (2013). https://doi.org/10.1039/C2CC37621K

Composition States of MOFs

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7. Marx, S., Kleist, W., Huang, J., Maciejewski, M., Baiker, A.: Tuning functional sites and thermal stability of mixed-linker MOFs based on MIL-53 (Al). Dalton Trans. 39, 3795–3798 (2010). https://doi.org/10.1039/C002483J 8. Dietzel, P.D., Panella, B., Hirscher, B., Blom, M., Fjellvåg, R.: Hydrogen adsorption in a nickel based coordination polymer with open metal sites in the cylindrical cavities of the desolvated framework. Chem. Commun. 959−961 (2006).https://doi.org/10.1039/B515434K 9. Gotthardt, M.A., Grosjean, S., Brunner, T.S., Kotzel, J., Ganzler, A.M., Wolf, S., Brase, S., Kleist, W.: Synthesis and post- synthetic modification of amine-, alkyne-, azide- and nitrofunctionalized metal-organic frameworks based on DUT-5. Dalton Trans. 44, 16802–16809 (2015). https://doi.org/10.1039/C5DT02276B 10. Aguilera-Sigalat, J., Bradshaw, D.A.: Colloidal water-stable MOF as a broad-range fluorescent pH sensor via post-synthetic modification. Chem. Commun. 50, 4711–4713 (2014). https:// doi.org/10.1039/C4CC00659C 11. Dong, X.W., Liu, T., Hu, Y.Z., Liu, X.Y., Che, C.M.: Urea postmodified in a metal-organic framework as a catalytically active hydrogen-bond-donating heterogeneous catalyst. Chem. Commun. 49, 7681−7683 (2013). https://doi.org/10.1039/C3CC42531B 12. Kim, J., Kim, S.N., Jang, H.G., Seo, G., Ahn, W.S.: CO2 cycloaddition of styrene oxide over MOF catalysts. Appl. Catal. A 453, 175−180 (2013). https://doi.org/10.3389/fenrg.2014.00063 13. Toyao, T., Saito, M., Horiuchi, Y., Matsuoka, M.: Development of a novel one-pot reaction system utilizing a bifunctional Zr-based metal-organic framework. Catal. Sci. Technol. 4, 625– 628 (2014). https://doi.org/10.1039/C3CY00917C 14. Long, J.L., Wang, S.B., Ding, Z.X., Wang, S.C., Zhou, Y.E., Huang, L., Wang, X.X.: Aminefunctionalized zirconium metal- organic framework as efficient visible-light photocatalyst for aerobic organic transformations. Chem. Commun. 48, 11656–11658 (2012). https://doi.org/10. 1039/C2CC34620F 15. Vermoortele, F., Ameloot, R., Vimont, A., Serre, C., De Vos, D.: An amino-modified Zrterephthalate metal-organic framework as an acid-base catalyst for cross-aldol condensation. Chem. Commun. 47, 1521–1523 (2011). https://doi.org/10.1039/C0CC03038D 16. Shen, L.J., Liang, S.J., Wu, W., Liang, M., Wu, R.W.: Multifunctional NH2 -mediated zirconium metal-organic framework as an efficient visible-light-driven photocatalyst for selective oxidation of alcohols and reduction of aqueous Cr (VI). Dalton Trans. 42, 13649–13657 (2013). https://doi.org/10.1039/C3DT51479J 17. Yang, Y., Yao, H.F., Xi, F.G., Gao, E.Q.: Amino- functionalized Zr(IV) metal-organic framework as bifunctionalacid-base catalyst for knoevenagel condensation. J. Mol. Catal. A Chem. 390, 198−205 (2014). https://doi.org/10.1016/j.molcata.2014.04.002 18. Timofeeva, M.N., Panchenko, V.N., Jun, J.W., Hasan, Z., Matrosova, M.M., Jhung, S.H.: Effects of linker substitution on catalytic properties of porous zirconium terephthalate UiO-66 in acetalization of benzaldehyde with methanol. Appl. Catal. A 471, 91−97 (2014). https://doi. org/10.1021/acsanm.9b01403 19. Srirambalaji, R., Hong, S., Natarajan, R., Yoon, M., Hota, R., Kim, R., Ko, Y., Kim, Y.H.: Tandem catalysis with a bifunctional site- isolated Lewis acid-bronsted base metal-organic framework, NH2 -MIL-101(Al). Chem. Commun. 48, 11650–11652 (2012). https://doi.org/10. 1039/C2CC36678A 20. Hartmann, M., Fischer, M.: Amino-functionalized basic catalysts with MIL-101 structure. Microporous Mesoporous Mater. 164, 38–43 (2012). https://doi.org/10.1016/j.micromeso. 2012.06.044 21. Wang, D.K., Huang, R.K., Liu, W.J., Sun, D.R., Li, Z.H.: Fe- Based MOFs for photocatalytic CO2 reduction: role of coordination unsaturated sites and dual excitation pathways. ACS Catal. 4, 4254–4260 (2014). https://doi.org/10.1021/cs501169t 22. Wang, D., Li, Z.: Bi-functional NH2 -MIL-101(Fe) for one-pot tandem photooxidation/knoevenagel condensation between aromatic alcohols and active methylene compounds. Catal. Sci. Technol. 5, 1623–1628 (2015). https://doi.org/10.1021/cs501169t

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F. Hassanzadeh-Afruzi and M. M. Salehi

23. Rodenas, T., van Dalen, M., Garcia-Perez, E., Serra-Crespo, P., Zornoza, B., Kapteijn, F., Gascon, J.: Visualizing MOF mixed matrix membranes at the nanoscale: towards structureperformance relationships in CO2 /CH4 separation over NH2 -MIL-53(Al)@PI. Adv. Funct. Mater. 24, 249–256 (2014). https://doi.org/10.1002/adfm.201203462 24. Zhang, F., Zou, X.Q., Gao, X., Fan, S.J., Sun, F.X., Ren, H., Zhu, G.S.: Hydrogen selective NH2 -MIL-53(Al) MOF membranes with high permeability. Adv. Funct. Mater. 22, 3583–3590 (2012). https://doi.org/10.1002/adfm.201200084 25. Rodenas, T., van Dalen, M., Serra-Crespo, P., Kapteijn, F., Gascon, J.: Mixed matrix membranes based on NH2 -functionalized MIL-type MOFs: influence of structural and operational parameters on the CO2 /CH4 separation performance. Microporous Mesoporous Mater. 192, 35−42 (2014). https://doi.org/10.1016/j.micromeso.2013.08.049 26. Chen, X.Y., Hoang, V.T., Rodrigue, D., Kaliaguine, S.: Optimization of continuous phase in amino-functionalized metal- organic framework (MIL-53) based co-polyimide mixed matrix membranes for CO2 /CH4 separation. RSC Adv. 3, 24266–24279 (2013). https://doi.org/10. 1039/C3RA43486A 27. Sun, D.R., Ye, L., Li, Z.H.: Visible-light-assisted aerobic photocatalytic oxidation of amines to imines over NH2 -MIL-125(Ti). Appl. Catal. B 164, 428−432 (2015). https://doi.org/10.1016/ j.apcatb.2014.09.054 28. Vaesen, S., Guillerm, V., Yang, Q.Y., Wiersum, A.D., Marszalek, B., Gil, B., Vimont, A., Daturi, M., Devic, T., Llewellyn, P.L., Serre, C., Maurin, G., Weireld, G.A.: Robust amino- functionalized titanium (IV) based MOF for improved separation of acid gases. Chem. Commun. 49, 10082–10084 (2013). https://doi.org/10.1039/C3CC45828H 29. Chizallet, C., Lazare, S., Bazer-Bachi, D., Bonnier, F., Lecocq, V., Soyer, E., Quoineaud, A.A., Bats, N.: Catalysis of transesterification by a nonfunctionalized metal-organic framework: acido-basicity at the external surface of ZIF-8 probed by FTIR and ab initio calculations. J. Am. Chem. Soc. 132, 12365–12377 (2010). https://doi.org/10.1021/ja103365s 30. Tran, U.P.N., Le, K.K.A., Phan, N.T.S.: Expanding applications of metal-organic frameworks: zeolite imidazolate framework ZIF-8 as an efficient heterogeneous catalyst for the knoevenagel reaction. ACS Catal. 1, 120–127 (2011). https://doi.org/10.1021/cs1000625 31. Jin, R.Z., Bian, Z., Li, J.Z., Ding, M.X., Gao, L.X.: ZIF-8 crystal coatings on a polyimide substrate and their catalytic behaviours for the knoevenagel reaction. Dalton Trans. 42, 3936– 3940 (2013). https://doi.org/10.1039/C2DT32161K 32. Wu, P., Wang, J., Li, Y., He, C., Xie, Z., Duan, C.: Luminescent sensing and catalytic performances of a multifunctional lanthanide- organic framework comprising a triphenylamine moiety. Adv. Funct. Mater. 21, 2788–2794 (2011). https://doi.org/10.1002/adfm.201100115 33. Hasegawa, S., Horike, S., Matsuda, R., Furukawa, S., Mochizuki, S., Kinoshita, K., Kitagawa, Y.: Three-dimensional porous coordination polymer functionalized with amide groups based on tridentate ligand: selective sorption and catalysis. J. Am. Chem. Soc. 129, 2607–2614 (2007). https://doi.org/10.1021/ja067374y 34. Xiao, J., Chen, C.X., Liu, Q., Ma, Q.X., Dong, J.P.: Cd (II)- schiff-base metal organic frameworks: synthesis, structure, and reversible adsorption and separation of volatile chlorocarbons. Cryst. Growth Des. 11, 5696–5701 (2011). https://doi.org/10.1021/cg201226t 35. Fang, Q.R., Yuan, D.Q., Sculley, J., Li, J.R., Han, Z.B., Zhou, H.C.: Functional mesoporous metal−organic frameworks for the capture of heavy metal ions and size-selective catalysis. Inorg. Chem. 49, 11637–11642 (2010). https://doi.org/10.1021/ic101935f 36. Park, J., Li, J.R., Chen, Y.P., Yu, J., Yakovenko, A.A., Wang, Z.U., Sun, L.B., Balbuena, P.B., Zhou, H.-C.: A versatile metal- organic framework for carbon dioxide capture and cooperative catalysis. Chem. Commun. 48, 9995−9997 (2012)https://doi.org/10.1039/C2CC34622B 37. Kleist, W., Jutz, F., Maciejewski, M., Baiker, A.: Mixed-linker metal-organic frameworks as catalysts for the synthesis of propylene carbonate from propylene oxide and CO2 . Eur. J. Inorg. Chem., 3552−3561 (2009).https://doi.org/10.1002/ejic.200900509 38. Dietzel, P.D.C., Morita, Y., Blom, R., Fjellvåg, H.: An in-situ high-temperature single-crystal investigation of a dehydrated metal−organic framework compound and field-induced magnetization of one-dimensional metal−oxygen chains. Angew. Chem. Int. Ed. 44, 6354−6358 (2005). https://doi.org/10.1002/anie.200501508

Composition States of MOFs

153

39. Rosi, N.L., Kim, J., Eddaoudi, M., Chen, B., O’Keeffe, M., Yaghi, O.M.: Rod packings and metal−organic frameworks constructed from rod-shaped secondary building units. J. Am. Chem. Soc. 127, 1504–1518 (2005). https://doi.org/10.1021/ja045123o 40. Geier, S.J., Mason, J.A., Bloch, E.D., Queen, W.L., Hudson, M.R., Brown, C.M., Long, J.R.: Selective adsorption of ethylene over ethane and propylene over propane in the metal-organic frameworks M2(dobdc)(M = Mg, Mn, Fe Co, Ni, Zn). Chem. Sci. 4, 2054–2061 (2013). https:// doi.org/10.1039/C3SC00032J 41. Liu, J., Tian, J., Thallapally, P.K., McGrail, B.P.: Selective CO2 capture from flue gas using metal−organic frameworks–a fixed bed study. J. Phys. Chem. C 116, 9575−9581 (2012). https://doi.org/10.1021/jp300961j 42. Remy, T., Peter, S. A., Van der Perre, S., Valvekens, P., De Vos, D.E., Baron, G.V., Denayer, J.F.M.: Selective dynamic CO2 separations on Mg-MOF-74 at low pressures: a detailed comparison with 13X. J. Phys. Chem. C 117, 9301−9310 (2013).https://doi.org/10.1021/jp4 01923v 43. Valvekens, P., Vandichel, M., Waroquier, M., Van Speybroeck, V., De Vos, D.: MetalDioxidoterephthalate MOFs of the MOF-74 type: microporous basic catalysts with well-defined active sites. J. Catal. 317, 1−10 (2014) 44. Couck, S., Gobechiya, E., Kirschhock, C.E., Serra-Crespo, P., Juan-Alcañiz, J., Martinez Joaristi, A., Stavitski, E., Gascon, J., Kapteijn, F., Baron, G.V.: Adsorption and separation of light gases on an amino-functionalized metal−organic framework: an adsorption and in situ XRD study. ChemSusChem 5, 740−750 (2012). https://doi.org/10.1039/c5dt02276b 45. Liu, J., Tian, J., Thallapally, J., McGrail, P.K.: Selective CO2 capture from flue gas using metal−organic frameworks—a fixed bed study. J. Phys. Chem. C 116, 9575−9581 (2012). https://doi.org/10.1021/jp300961j

Identification and Analytical Approaches Simindokht Zarei-Shokat, Mohadeseh Forouzandeh-Malati, Fatemeh Ansari, and Reihane Dinmohammadi

Abstract Metal–organic frameworks (MOFs) are a new class of composite materials in which organic molecules bind to inorganic molecules (usually high-core metal clusters or intermediate metal ions) by bonding coordinates and forming a cage-like framework or structure and have unique features. Therefore, several types of crystalline or flat structural arrangements are feasible and are studied by the X-ray diffraction method (XRD). Energy dispersive X-ray (EDX) analysis is used to determine the atomic makeup of MOFs since they contain both organic and inorganic types of molecules. Numerous techniques, such as Fourier transform infrared spectroscopy (FT-IR) analysis, Dynamic light scattering (DLS), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), etc., are used to examine the functionality following MOF creation. Some of these techniques, which are mostly critical, are described in this chapter. Keywords MOFs · Structure · Dynamic light scattering · XRD · Catalyst

1 Fourier Transform Infrared (FT-IR) Fourier transform infrared (FT-IR) spectroscopy is one of the most widely used spectroscopic methods in organic and inorganic chemistry to determine the structure of compounds, which is able to radiant the entire spectral range simultaneously to sample by using an interferometer system and then analyze it. Fourier transform spectrometers generally use an interferometer called a Michelson interferometer or similar instruments. These devices are also equipped with microprocessors for spectrum analysis. The main applications of infrared spectroscopy include qualitative applications for the identification of functional groups and the determination of the structure of organic species. Due to the fact that different compounds have different properties due to the presence of different functional groups, the structure of these compounds and the types of functional groups can be determined with the help of S. Zarei-Shokat (B) · M. Forouzandeh-Malati · F. Ansari · R. Dinmohammadi Catalysis and Organic Synthesis Research Laboratory, Department of Chemistry, Iran University of Science and Technology, 16846-13114 Tehran, Iran e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 A. Maleki and R. Taheri-Ledari (eds.), Physicochemical Aspects of Metal-Organic Frameworks, Engineering Materials, https://doi.org/10.1007/978-3-031-18675-2_11

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infrared spectroscopy. Different functional groups in the infrared range have specific wavelengths (such as fingerprints) and their absorption peaks in this range are sharper and more obvious than in the ultraviolet and visible regions, so infrared spectroscopy is very effective for determining the structure of compounds. This method provides images of cell structure through a variety of computational algorithms by passing spectral data. Because the images obtained from IR are from the fingerprint spectra of the functional groups, these images can determine the health status of the analyzed sample [1]. In this regard, infrared spectroscopy has found many other applications in various fields such as chemistry, biological and pharmaceutical sciences, environment, polymers, various industries, identification of mineral molecules, etc. A list of some applications of this spectroscopy method can be seen below 1. Determining the accuracy of a chemical reaction according to the spectrum of the product created 2. Determining the progress of different reactions according to the intensity of peaks related to the raw material at different times after the start of the reaction 3. Detect the presence of hydrogen bonds in different products 4. Detection of functionalization of a species by comparing the spectrum of species before and after the process of functionalization 5. Help in determining the structure of heterocyclic and metal–organic species. The applications of infrared spectroscopy are often in the form of comparing several species with each other that are in line with the intended purpose. The Fourier transform spectroscopy method is used for non-destructive analysis of biological samples, which identifies the texture of different compounds by creating spectral images. The Fourier transform spectroscopy method is used for non-destructive analysis of biological samples, which identifies the texture of different compounds by creating spectral images. In general, compounds that have molecular bonds with electric dipole moment are active in IR, which can be changed by atomic displacement due to natural vibrations. These vibrational states can be quantified by IR spectroscopy and are used to study molecular composition [2]. To better understand the content, we will give some examples: The following Fig. 1a shows the IR spectroscopy of the metal–organic framework MOF-5 in the spectral region of wave numbers from 400 to 1400 cm−1 . MOF-5 is made of terephthalic acid and zinc nitrate hexahydrate in dimethylformamide and is in the form of white crystals. The peaks in region of 1381 and 1573 cm−1 show asymmetric stretch of C–O bond in zinc metal by binding of carboxylate ligand to Zn4 O center, which is related to MOF-5. Peak values between 900 and 1250 cm−1 range have different small peaks that indicate the C–H stretching band of benzene dicarboxylate. The width peak shown in the range of 3161 cm−1 includes the IR bands of the OH group in the structure of the MOF-5 framework [3]. In another example, infrared spectrum data is shown in Fig. 1b. There were some adsorption peaks in the UiO-66 infrared spectrum that come from aromatic rings and carboxyl groups. For example, the peak at 1420 cm−1 is attributed to the C–C vibrational state, and the peak at 1580 cm−1 was related to the stretch vibration of

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Fig. 1 a Fourier transform infrared (FT-IR) spectrum of MOF-5. This figure was adapted by permission from Separation Science and Technology, (2019), 54.3, 434–446, b FT-IR spectrum of UIO-66 and UIO-66-NH2. This figure was adapted by permission from Journal of Chemical & Engineering Data, (2016), 61.11, 3868–3876

the C–O bond in the carboxyl group. For UiO-66-NH2 , C–N stretch band and N–H vibration were reported at 1258 and 764 cm−1 , respectively [4]. As discussed in this section, FT-IR analysis is a useful way to identify organic and inorganic compounds and their functional groups. Using this method, even covalent bonds of metal ligands can be identified. In the end, it should be noted that about 95% of the use of this device is quality, which is done from the peak of the chart.

2 Energy Dispersive X-ray (EDS) Energy Dispersive X-ray (EDS) microanalysis is an elemental analysis technique based on the production of specific X-rays in sample atoms by the collision of electron beams. After X-rays strike atoms, elastic and inelastic scattering occur, which are the two basic events of physics. Elastic scattering is a change in the direction of electrons without significant energy loss, usually due to interaction with the nucleus of the material but inelastic scattering is the loss of energy without changing the direction of the electrons, which usually results from the interaction with specific electrons and the nuclei in the atoms. Elastic scattering events are the main determinants of the shape of the volume of interactions and inelastic scattering is the main determinant of the volume size of the interactions. The atoms are then ionized, and when they return to their ground state, they emit certain X-rays, and the X-ray photon energy is the potential energy resulting from the difference between the two orbitals involved in the transmission, which characterizes the element. X-ray emission at different wavelengths can be measured by a photon energy sensitive detector. X-rays for the element from which they originate give information about the elements in the sample. X-ray energy depends on the atomic number of the element in which the interaction occurs. Heavy elements produce more cohesive intensity than light elements. The

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EDS detector system simultaneously displays all medium-energy X-rays (1–20 kV) collected during each analysis period, and the X-ray energy is reproduced as a spectrum. The histogram is the number of counts against X-ray energy and this range includes semi-qualitative and semi-quantitative information. The position of a peak in the spectrum identifies the energy of that element, and the area under the peak is proportional to the number of element atoms in the irradiated region. When the electron beam is attenuated by the electrostatic fields of the atomic nuclei of the elements in the sample, X-rays are generated, forming a continuous radiation that appears below the peaks of the spectrum. EDS analysis has a high sensitivity in detecting different elements. This technique is also an important tool for the detection of nanoparticles in that metal–organic frameworks are synthesized by mixing a metal salt and one or more types of organic binders in a solvent or without a solvent by various methods. Therefore, after the formation of MOFs, the elements in MOFs are analyzed by using EDS analysis [5]. Here are some instances to help you grasp the aforementioned: The first example is the EDS analysis of the compounds MIL-53-(Al)-NH2 and MIL-53(Al)-N(CH2 PO3 H2 )2 . Metal–organic framework MIL-53-(Al)-NH2 through functionalization of MIL-53-(Al) compound using organic binder 2-amino-1,4benzene1 dicarboxylate and metal–organic compound MIL-53(Al)-N(CH2 PO3 H2 )2 contains components N(CH2 PO3 H2 )2 , which have global environmental effects in agricultural, chemical, and pharmaceutical applications. As shown in Fig. 2a, b, the elemental peaks show the presence of elements C, O, N, and Al in the structure of MIL-53 (Al)-NH2 and element P due to the presence of phosphate group in the structure of MIL-53 (Al)-N(CH2 PO3 H2 )2 confirmed. In addition, EDS mapping images show a uniform and homogeneous distribution of elements [6]. In another one (2c-d), the chemical composition of UiO-66-NH2 and UiO-66biguanidine/Cu nanocomposites was analyzed. The metal–organic framework Uio66-biguanidine/Cu is formed by the functionalization of the metal–organic framework Uio-66-NH2 , in which biguanidine acts as a chelating ligand and on which copper nanoparticles act as an active catalyst. In Fig. 2c the EDS spectrum is related to the metal–organic framework UiO-66-NH2 , where the peaks of atoms O, N, C are related to the BDC ligand and the peak of the Zr atom is related to the octahedral secondary building units (SBU) of Zr and as shown in the picture, Zr and C have a much higher density than other elements due to the uniform and homogeneous distribution of the elements. Figure 2d examines the UiO-66-biguanidine/Cu assay. The presence of a copper atom peak indicates the binding of Cu nanoparticles to UiO-66-biguanidine, derived from UiO-66-NH2 [7]. In the following figures, the EDS analysis of the Zn-MOF-82 and Zn-MOF @AgQD3 metal–organic frameworks is also investigated. Figure 3a confirms the presence of Zn, O, and C atoms in Zn-MOF-8, and Fig. 3b shows the Ag, Zn, C, and O atoms, and shows nanocomposite formation, respectively. These EDS images 1

NH2 -H2 BDC. Zinc-metal Organic Framework-8. 3 zinc–metal organic framework-8 and silver quantum dot composite. 2

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Fig. 2 a EDX analysis and elemental mapping of electro-synthesized MIL-53(Al)-NH2 , b MIL53(Al)-N(CH2 PO3 H2 )2 . This figure was adapted by permission from Scientific reports, (2021), 11.1, 1–20. EDX spectra of UiO-66-NH2 (c) and Cu@bigua/UiO-66 nanocomposite (d). This figure was adapted by permission from Journal of Chemical & Engineering Data, Scientific reports, (2021), 11.1, 1–20

are proof that silver quantum dots are well enclosed by Zn-MOF-8 also the atomic percentage of silver was also determined in Zn-MOF-8 [8].

3 Dynamic Light Scattering (DLS) The size distribution of nanoparticles can be measured using the dynamic light scattering (DLS) technique. In this analysis, the powder sample is homogeneously suspended in solution, usually water is used for MOFs, and the hydrodynamic diameter is used to measure the average particle size. The DLS instrument can only measure nanometer particles with dimensions ranging from 1 to 1000 nm. Samples to be analyzed in laser light scattering devices as – Are solid particles that are suspended in the liquid. – Are solid or liquid particles that are suspended in a gaseous environment.

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Fig. 3 EDX analysis of a Zn-MOF-8, b Zn-MOF-8@AgQDs. This figure was adapted by permission from: Applied Sciences, (2019), 9.22, 4952

In order to be able to have an accurate size distribution, sample preparation must be done carefully. Any aggregation and agglomeration affect the final particle size distribution. In the case of suspended solids in a liquid medium, because the device cannot distinguish between agglomerated particles or particles that are separated, the preparation process must be performed. Dispersion, however, is done by various methods such as ultrasonic, dispersant addition, and pH stability. Of course, it should be noted that the dispersion process does not cause bubbles because it causes errors in the particle size distribution. Sample concentration is also an issue that should be considered. Since this device is based on light scattering, if the concentration of suspension is high, double scattering occurs and if the concentration is low, the light does not hit a particle. Particles are subjected to random thermal motion in suspension, known as Brownian motion (Fig. 4). One of the characteristics of Brownian motion is that small particles move fast and the large particles move slowly. Particle size can be measured from this random motion using the Stokes–Einstein equation: Dh = KB T/3π ηDt where Dh is the hydrodynamic diameter, Dt is the transfer diffusion coefficient, KB is the Boltzmann constant, T is the thermodynamic temperature, and η is the dynamic viscosity. But from DLS analysis, the actual particle size cannot be obtained instead of the hydrodynamic diameter [9]. There are some examples: In Fig. 5, after measuring the results of the DLS analysis for the metal–organic frameworks UiO-66-NH2 , CuS@UiO-66-NH2 , and OXA-CuS-UiO-66-NH2 signs of particle aggregation observed throughout the solution. In addition, greater amounts of particle aggregation were observed at sizes that were more than 10 times larger than the crystal size. The probable cause of this phenomenon can be attributed to the interaction of solvent molecules with the surface of nanoparticles and the presence of amine agents on the MOF, it can also be assumed that hydrogen bonds are the most important interaction in nanoparticle sizes. As shown in Fig. 5b, there was no apparent increase in particle size after the modification process, therefore, any

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Fig. 4 Schematic showing the instrumentation of DLS

Fig. 5 DLS distribution in water and growth medium of a UiO-66-NH2 , b CuS@UiO-66-NH2 , and c OXA-CuS@UiO-66-NH2 . This figure was adapted by permission from Journal of Materials Science: Materials in Medicine, (2022), 33.3, 1–10

conclusions related to the tendency to increase or decrease the size after the changes are rejected [10]. In another example, the DLS analysis of the hydrodynamic diameters of the MOF5 and ZnO@MOF-5 metal–organic frameworks were investigated and the spectra are shown in Fig. 6. The size distribution analysis of MOF-5 and ZnO@MOF-5 shows that the particles are dispersed and because of this, there is a wide range of

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Fig. 6 Diameter distribution diagrams of a MOF-5 and b ZnO@MOF-5. This figure was adapted by permission from ACS omega, (2022), 7.15, 13, 031–13, 041

particles of different sizes in the sample. The average diameters of the MOF-5 as well as the final composite (ZnO@MOF-5) are 773.6 and 1036.9 nm, respectively. The diameters obtained are much larger than the size obtained from other analyzes. This may be because the DLS instrument only detects larger particles with larger diameters. The increase in diameter when changing the MOF to the composite may be due to the accumulation of particles or due to the adhesive nature of water solvent molecules during the analysis [11]. DLS analysis actually examines the behavior and size of particles in a liquid medium; It also measures the diameter of the clusters where agglomeration causes the particles to cluster.

4 Scanning Electron Microscopy (SEM) Among the various methods for identifying and analyzing materials, we can mention the scanning electron microscopy (SEM). One of the most famous microscopic methods that are used both for imaging (magnified images) and for chemical identification and as an effective method in the analysis of metal–organic materials such as MOFs and polymers, etc., at the nanoscale or micrometer with a magnification of × 5 to × 300,000 are sometimes used in advanced SEMs up to × 1,000,000 to create highly accurate images in a variety of contexts. The basis of SEM work depends on the emission of electrons, which usually uses a heat source to emit electrons. In general, SEM analysis is performed by applying a beam of energetic electrons, the clearer and less scattered the electron beam, the clearer the images [12].

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There are generally three types of SEM: (1) CSEM4 (2) ESEM5 (3) LVSEM.6 CSEM and LVSEM operate at lower pressures than ESEM. (1) CSEM: In Conventional scanning electron microscopy, an electron beam (beam) interacts with the sample in a high vacuum (6–10 Torr), resulting in the release of low-energy electrons from the sample, which have the least contact with the gas molecules in the chamber. But this large vacuum causes dehydration, resulting in cracking (breaking) of concrete. However, this method can be used in cases that are not changed by vacuum, such as detecting the reactivity of alkaline silica. (2) ESEM: In Environmental scanning electron microscopy, the electron beam and the sample interact at high pressures (0.2–20 Torr), which can have both positive and negative effects. One of its positive effects is that, for example, increasing the gas pressure, the surface charge is discharged by ionization of gas molecules, which ionization of gas molecules increases the strength of the electron signal. One of the negative effects, for example, is that these collisions can cause the electron beam to scatter, and as a result, the electron beam will not hit the sample properly and will be problematic in X-ray microanalysis. In general, ESEM does not provide accurate results for X-ray microanalysis and has limited capabilities in this area. (3) LVSEM: In Low vacuum scanning electron microscopy, the electron beam interacts with the sample in vacuum (similar to CSEM), and is compatible at high pressures (0.2–20 Torr). The effect of LVSEM on cracking is very slow during a typical analysis [13]. In general, MOFs have a specific morphology that depends on organic metal ions. The ligand or solvent used in the synthesis can be altered by changing any of these precursors. FE-SEM analysis can be observed with any change in surface morphology. For example, the metal–organic framework Al-MIL-53-RSO3 H and AL-MIL-53-ArSO3 H, which are derived from the metal–organic framework AlMIL-53 and developed as heterogeneous acid catalysts. In these compounds, sulfonic acid functional groups are incorporated into metal–organic frameworks by postsynthetic modification. As can be seen in the scanning electron microscope images below in section a, the Al-MIL-53-NH2 crystals appear as uniform nanorods 35 nm wide and 210 nm long. According to SEM images, the structural morphology of the crystals has been preserved during post-synthetic modifications of Al-Mil-53-NH2 and AL-MIL-53-ArSO3 H crystals (Fig. 7) [14].

4

Conventional scanning electron microscopy. Environmental scanning electron microscopy. 6 Low vacuum scanning electron microscopy. 5

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Fig. 7 Scanning Electron Microscope analysis a AL-, IL-53-NH2 , b AL-MIL-53-RSO3 H, c ALMIL-53-ArSO3 H, d AIL-53-ArSO3 H. This figure was adapted by permission from RSC advances, (2017), 7.55, 34, 591–34, 597

Also, in the next example the SEM images below show the morphology, texture, and particle size of the two metal–organic framework MIL-53-Al and MIL-53Al/Ag/AgCl NC. Figure 8a shows the MIL-53-Al, which is synthesized as a semicubic layer with the shape of irregular crystals and the thickness of the layers is up to about 20–60 nm in some parts, and Fig. 8b shows the MIL-53-Al/Ag/AgCl NC. The spherical particles in Fig. 8b may be due to the formation of Ag/AgCl clusters at the surface of the MIl-53-Al framework. Ag/AgCl nanoparticles are dispersed on the surface of MIL-53 (Al) and their size is less than 30 nm [15]. Finally, we can refer to pure ZIF-8 scanning electron microscopy images and ZIF-8/CNT composite composition. Figure 8c shows a spherical morphology corresponding to the structure of ZIF-8, and Fig. 8d shows a grape-like morphology for the synthesis combined with ZIF-8, and the ZIF-8 crystals are completely outside the surface of carbon nanotubes, which indicates a strong interaction between ZIF-8 crystals and (CNTs), and ZIF-8 crystals with a size of about 50 nm grow on the outer surface of the CNT [16]. SEM is one of the most widely used microscopic methods. Like other electron microscopes, a very high-resolution limit can be achieved due to the use of electron beam in sem. This microscope has undergone significant advances since its invention. The basis of the SEM function is the interaction of the electron beam with matter, which results in the emission of electrons and photons from matter. The most important of these beams used to study matter are secondary electrons (to study morphology and topography) and return electrons (to study fuzzy distribution).

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Fig. 8 Scanning electron Microscope images of a MIL-53-Al, b MIL-53-Al/Ag/AgCl NC. This figure was adapted by permission from Journal of Solid State Chemistry, (2021), 297,122,087. Scanning electron Microscope images of c ZIF-8, d ZC. This figure was adapted by permission from Journal of Materials Chemistry A, (2016), 4.16, 6084–6090

5 Transmission Electron Microscopy (TEM) Transmission electron microscopy (TEM) is a method for determining the morphology and structure of materials with high magnification and resolution, as well as the study of crystal structures. Morphology and particle size are the most important macroscopic structural aspects and have been studied since the early years of network chemistry. Transmission electron microscopy played a key role in the imaging tools used to detect morphological features. This analysis provides information on the determination of size, morphology, and crystal phase of metal–organic frameworks. In this way, objects the size of several angstroms can be observed, in which electrons are collected and concentrated using electromagnetic lenses and are emitted as an electron beam, these electron beams pass through the sample and TEM images are obtained. In these microscopes, equipment is used such as electron guns, focusing lenses, object lenses, diffraction lenses, intermediate lenses, vacuum systems, cameras, etc. (Fig. 9). Images generated by the transmission electron microscope include light wallpapers and dark wallpapers. Some of the primary electrons that hit the sample pass through it, but some of them are affected by their scattering angle range of elastic and inelastic scattering. Electrons scattered at larger angles

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Fig. 9 Schematic drawing of transmission electron microscope (TEM) (by S. Pusz) [24]. This figure was adapted by permission from International Journal of Coal Geology, (2019), 211, 103, 203

are stopped by the valve. When in the absence of the sample, the valve is in the focal axis, a clear background is seen. In bright wallpapers, the aperture of the object is generally used to block all diffused rays, allowing only non-deflected electrons to participate in the image. However, if the aperture is moved to be used only for specific diffraction beams, an image is obtained which, because it is in the absence of the sample, leaves the background dark and the wallpaper dark [17, 18]. We will provide a few illustrations to help you comprehend the content: Figure 10 shows the TEM analyzes of the metal–organic frameworks UiO-66, Pd-UiO-66, UiO-66-NH2 , and Pd-UiO-66-NH2 . Figure 10a, b and e, f shows that UiO-66 has a more specific crystal structure than UiO-66-NH2 , which may be due to the negative effect of amino groups on the crystal structure of the metal–organic framework. Comparing Fig. 10c, d with g, h, it appears that Pd is located in the pores of Pd-UiO-66, while on the surface of Pd-UiO-66-NH2 due to the presence of groups The amine and its nitrogen pair, which are coordinated with metal ions, have been proven. These observations are positive that Pd loading on Pd-UiO-66-NH2 has very little interaction with Zr but strong interaction with N. In addition, some Pd accumulation on the surface of UiO-66-NH2 can be observed, indicating that Pd nanoparticles are too large to enter the pores of UiO-66-NH2 [19].

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Fig. 10 TEM images of UiO-66 (a, b), Pd-UiO-66 (c, d), UiO-66-NH2 (e, f), and Pd-UiO-66-NH2 (g, h); (i, j) for the Pd-UiO-66 catalyst after the recycle use. This figure was adapted by permission from Energies, (2020), 13.3, 521

In the subsequent instance, TEM images were performed with low magnification of the samples to investigate the structural changes at the sample level. Figure 11 shows high-quality monolithic crystal with a significant dodecahedral rhombus shape and an average crystal size of about 100 μm, which corresponds to ZIF-8 and Zr/ZIF-8, respectively. Figure 11b is proof that there is no accumulation or change in particle size and morphology from Fig. 11a. The TEM image of the recycled catalyst (Fig. 11e) shows that the catalyst crystals were very stable during the reaction [20]. Figure 12 is also obtained to further confirm the preparation of ZIF-8@ SiO2 crystals, ZIF-8 particles and those modified with SiO2 have been carefully identified by TEM. Figure 12a shows well the twelve-sided rhombic morphology of ZIF-8 beads with a size of about 500 nm. In contrast, when combined with SiO2 , the rugged ZIF-8 edge is clearly visible (Fig. 12b). This indicates that the SiO2 nanoscale structure has been successfully formed [21]. According to the description in this section, TEM analysis is a special tool to determine the structure and morphology of materials. Today, TEM analysis can be used to study the structure of crystals, symmetry and orientation, and crystal defects.

Fig. 11 Transmission electron microscopy (TEM) image of a pristine ZIF-8 crystals, b Zr/ZIF-8, and c recycled Zr/ZIF-8 synthesized with 10% dopant of Zr. This figure was adapted by permission from ACS applied materials & interfaces, (2017), 9.12, 11, 106–11, 115

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Fig. 12 TEM images of a ZIF-8 and b ZIF-8@SiO2 particles. This figure was adapted by permission from Environmental Science and Pollution Research, (2015), 22.21, 17, 238–17, 243

6 Brunauer–Emmett–Teller (BET) Accurate measurement of surface area and porosity is of great importance in many applications such as catalysts, nanosorbents, metal–organic frameworks, etc. Surface area is one of the main physical properties of a MOF and accurate determination of surface area in MOF is important for applications such as gas storage or separation processes [22]. Among the all methods for calculating the surface area of MOFs is the method based on nitrogen adsorption isotherms, which is based on Brunauer– Emmett–Teller’s theory. BET theory is a standard method that allows comparisons between different materials and was first developed to describe the adsorption of multiple layers of gas molecules on a solid surface [23]. BET analysis is based on isothermal lines for the adsorption of non-reactive gas molecules (such as nitrogen at 77 K or argon at 87 K). BET analysis assumes that adsorption occurs by the formation of multiple layers and that the number of adsorbed layers at saturation pressure is infinite, for example, adsorption occurs freely. Significant overlap of single-layer and multi-layer layers ignores this assumption. It has been suggested that adsorption on MOFs is mediated through the pore-filling mechanism rather than layer formation [24]. Thus, while the BET method allows the ranking of different materials, it is not yet clear whether the BET surface area values reported for these materials are really significant. One of the issues with the BET method is the selection of the linear region of the BET linear diagram. Typically, the linear region in BET analysis is from a relative pressure range (P/P0) between 0.05 and 0.30, known as the standard BET pressure range, was selected on the assumption that monolayer formation occurs in this pressure range [25]. The BET calculation can significantly overestimate the actual monolayer load, especially at MOFs that combine mesopores (d ≥ 20 A) and large pores (d = 10–20 A) with overlap. An important problem in using the available surface to BET results from experimental isothermal lines is that real materials may be defective. For example, parts of a crystal may decompose or retain solvent molecules from synthesis. Such defects naturally reduce the amount of nitrogen absorbed and thus the area of BET surface that has been experimentally reported. As a result, we use molecular modeling to calculate nitrogen adsorption

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isotherms for a series of MOFs. The BET method is then applied to these simulated isothermal lines to calculate surface areas that can be geometrically compared to the available surface areas calculated from the crystal structures. In this way, exactly the same known crystal structure is used to calculate the available surface area and BET surface area (via simulated isotherms) to avoid the problem of defective materials in real experiments. For example, the adsorption–desorption isotherm of N2 and the structural properties of the metal–organic structures UIO-66, Pd-UiO-66, UiO-66NH2, and Pd-UiO-66-NH2 . As reported in Fig. 13 and Table 1, all isotherms have a Type I pattern. UiO-66, UiO-66-NH2 , Pd-UiO-66, and Pd-UiO-66-NH2 have the high surface area and fine porosity. The specific surface area of UiO-66 and UiO-66-NH2 is 1470 and 1047 m2 /g, respectively, indicating that the addition of amine groups leads to a decrease in the specific surface area of UiO-66-NH2 . After deposition of Pd nanoparticles on the bases of UiO-66 and UiO-66-NH2 , the specific surface area and pore volume are reduced, which is a reason for Pd penetration into the cavities. However, the average pore diameter increases, mainly due to the fact that Pd nanoparticles can occupy or block UiO-66 and UiO-66-NH2 pores [19]. Finally, we can mention the specific surface areas of MIL-53 and Au@MIL-53 (NH2 ), which were analyzed by N2 adsorption/desorption measurements. MIL53 (NH2 ) has a type I isotherm as shown in Fig. 13. As can be seen in Table 2, MIL53 (NH2 ) has a specific surface area of 439.6 m2 g−1 . After gold deposition, Au @ MIL-53 (NH2 ) BET level decreases to 325.3 m2 g−1 . The reduction in surface area indicates that the pores of the MIL-53 (NH2 ) framework may be occupied by gold nanoparticles or blocked by gold nanoparticles deposited on the surface [26].

Fig. 13 Nitrogen adsorption–desorption isotherms at 77 K for a UiO-66, b Pd-UiO-66, c UiO66-NH2 , d Pd-UiO-66-NH2 . This figure was adapted by permission from Fuel, (2017), 205, 130– 141. Adsorption/desorption isotherms N2 , e MIL-53 (NH2 ), f Au@MIL-53 (NH2 ). This figure was adapted by permission from European Journal of Inorganic Chemistry 2015, (2015), 30, 5099–5105

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Table 1 Physical structural properties of UiO-66, Pd-UiO-66, UiO-66-NH2, and Pd-UiO-66-NH2 . This Table was adapted by permission from Fuel, (2017), 205, 130–141 Sample UiO-66 Pd-UiO-66 UiO-66-NH2 Pd-UiO-66-NH2

SBET (m2 /g)

Total pore volume (cm3 /g)

278

0.71

1305

0.64

45

0.45

101

0.43

Table 2 Adsorption/desorption levels N2 MIL-53(NH2 ) and Au@MIL-53 (NH2 ). This Table was adapted by permission from European Journal of Inorganic Chemistry 2015, (2015), 30, 5099–5105 SBET (m2 /g)

Sample

MIL-53(NH2 )

439.6

Au@MIL-53(NH2 )

325.3

According to the explanations of this section, BET analysis is one of the most important methods for accurate measurement of surface area and porous samples, which is based on the adsorption of certain molecular species in the gas state on their surface. This method also makes it possible to determine the type, amount, and shape of porosity in the material from the type of adsorption isotherm.

7 X-Ray Diffraction (XRD) X-Ray Diffraction (XRD) analysis is an old and widely used technique for interpreting the intensity of X-rays scattered on crystal plates and is considered as one of the most effective methods for describing the microstructure of materials. By studying XRD peaks, the type of materials and their phase can be identified qualitatively. In XRD, the sample can be in the form of layers, thin sheets or powder, and the size of the powder particles must be less than 50 μm. If the sample has smaller particles, it leads to the widening of the peaks in the diffraction diagram, and if the sample has larger particles, they are more prominent in a certain direction than the plates. When an X-ray hits electrons in the capacitance of atoms, it diffuses from different atomic layers. Depending on the angle of radiation and the relative position of the atoms in the crystal lattice, diffused X-rays can cause both constructive and destructive interference with a relative phase change. Although the maximum scattering of elastic X-rays indicates destructive interference, there is a sufficient amount of scattering beams that combine constructively to form a stronger wave. This law was first used by William Henry Bragg and his son William Lawrence Bragg to study the structural properties of crystals, which won them the 1915 Nobel Prize in Physics.

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Bragg’s equation basically explains the principle of X-ray diffraction by a set of grid plates: nλ = 2d sinθ n: In the above formula is the reflection coefficient and is an integer. λ: is the wavelength of the X-ray that displaces the electron or neutron. d: The empty space between the atoms. θ: The angle of the reflected rays in the direction of the initial rays. Following this principle, the diffraction gauge provides the characteristic peaks of the sample that provide information about the crystallinity of the material. In addition, XRD analysis can be used to show the crystal size (100 nm) of the sample [27]. Currently, the powerful XRD tool can be used directly to visualize accurate and structural information of metal–organic frameworks, characterize the crystal structure of matter, evaluate the purity and crystallization of phase, especially MOF particles in various states. X-ray diffraction techniques generally include single crystal X-ray diffraction (SCXRD), powder diffraction (PXRD), and Grazing Incidence XRay Diffraction (GIXRD). The SCXRD technique regulates the exact structure of the crystal at the atomic level. However, the crystals involved must be large enough to be identified. Therefore, the SCXRD method mainly shows the structure of MOF and MOF defects at the molecular level. The PXRD technique is the most important identification technique for detecting material crystallinity and phase purity by comparing diffraction peaks with a simulated pattern from single-crystalline data (Fig. 14). However, common specimens and PXRD data often suffer from preferential orientation, low resolution, low intensity, and peak overlap, which prevent the observation of small changes in atomic positions or functional groups that may play important roles in properties. Also, due to the severe overlap of diffraction peaks, powder X-ray diffraction patterns can only provide much less structural information with less accuracy than single crystal X-ray diffraction. Indeed, the initial determination of unknown and complex crystal structures from PXRD data is still a very difficult or even impossible challenge [28, 29]. In contrast, solving structures from SCXRD data has become a fairly common method. Therefore, single crystals are always preferred to determine the crystal structure. The GIXRD technique is used to describe new unstructured materials. To improve these definitions, we will give a few examples: Figure 15a shows the X-ray diffraction patterns of MOF MIL-53(Al) through a center not modulated with dimethyl formaldehyde in the pores. MIL-53 consists of octahedral transformers of MO4 (OH)2 , containing the metal centers Fe, In, Ga, V, or Al, which are joined together by hydroxyl groups and terephthalate ligands. As can be seen in the figure, the index peaks of this compound are reported at 2θ: 9.1°, 10.0°, 18.3°. In contrast, MOF MIL-68 (Al) (Fig. 15b) was formed by synthesis with formic acid modulated at different concentrations (1.5, 2.5, 3.5, 5 M). The addition of carboxylic acid modifiers during MOF synthesis is due to the fact that it affects

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Fig. 14 Schematic tool of PXRD technique

and controls the particle size and shape, and sometimes increases the reproducibility, crystallinity, and surface area of the desired MOF structures. A concentration of 2.5 M was reported as the optimal concentration for MOF MIL-68. Modulation with 2.5 M formic acid leads to a significant increase in particle size. As can be seen in the figure, the index peaks at 2θ: 9.7°, 8.4°, 4.8o indicate the formation of this compound. After determining the optimum concentration, it was found that when the reaction temperature decreased from 100 to 55 °C, the MOF MIL-68 fraction in the product increased continuously and as a result pure MIL-86 was formed at 55 °C (Fig. 15c) [30]. Another example showed X-ray diffraction patterns UiO-66-NH2 , Pd-UiO-66NH2 , UiO-66, and Pd-UiO-66 (Fig. 15d). UiO-66 is a zirconium (Zr)-based MOF with high surface area and good thermal stability. After loading of Pd nanoparticles, the peak positions of Pd-UiO-66 are in accordance with UiO-66, which indicates that the frame structure of UiO-66 is well preserved. This is true for UiO-66-NH2 and PdUiO-66-NH2 . However, the peak intensity of Pd-UiO-66 decreases after the presence of Pd, which may be due to the deposition of Pd nanoparticles and a slight change in the structural order of the framework. Compared to Pd-UiO-66, the Pd-UiO-66-NH2 peak has a lower crystallization intensity and this suggests that the peak intensity of UiO-66-NH2 decreased significantly after the addition of Pd nanoparticles, which may be due to some interactions between Pd particles and amino groups and to some extent affect the structure of UiO-66-NH2 . Since the amine groups in UiO-66-NH2 can act as ligands to stabilize Pd nanoparticles, the interaction between Zr and Pd in Pd-UiO-66-NH2 is less than that of Pd-UiO-66. Pd appears to be located in the pores of Pd-UiO-66 while it is fixed at the level of Pd-UiO-66-NH2 due to the presence of amino groups and electron-amine pairs. Different experimental conditions can also affect the stability of UiO-66-NH2 , such as temperature, synthetic solvent and preparation methods, etc. No peaks characteristic of Pd species were found in the diffraction patterns Pd-UiO-66 and Pd-UiO-66-NH2 . The absence of palladium peaks

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Fig. 15 a X-ray diffraction of MIL-53, b XRD patterns of MOF product were synthesized with formic acid modulation of 0, 1.5, 2.5, 3.5, and 4 M at 80 °C. § = MIL-68, c X-ray diffraction pattern MIL-68. This figure was adapted by permission from RSC advances, (2020), 10.13, 7336– 7348. d X-ray diffraction patterns UiO-66-NH2 , Pd-UiO-66-NH2 , UiO-66, and Pd-UiO-66. This figure was adapted by permission from Fuel, (2017, 205, 130–141. e X-ray diffraction patterns (1) recycled Zr/ZIF-8, (2) Zr/ZIF-8, and (3) ZIF-8 catalysts. This figure was adapted by permission from Energies, (2020), 13.3, 521

may mean that palladium species are well dispersed or the Pd concentration is too low to generate a signal. The absence of palladium peaks may mean that the palladium species are well dispersed and the Pd load is too low to generate a signal. There is also some Pd accumulation on the surface of UiO-66-NH2 , indicating that Pd nanoparticles are too large to enter the pores of UiO-66-NH2 . With -NH2 , the crystal structure of UiO-66-NH2 was somewhat damaged, and the interaction between Zr and Pd in Pd-UiO-66-NH2 is less than that of Pd-UiO-66 [19]. In this example, the XIF-8, Zr/ZIF-8, and Zr/ZIF-8 recycled X-ray diffraction patterns are shown in Fig. 15. Diffraction peaks appear at small 2θ angles with eight diffraction peaks at 7.31, 10.31, 12.71, 14.71, 16.41, 18.01, 24.61, and 26.71 and were indexed on pages (011), (002) (112), (022), (013), (222), (233), and (134), respectively. The XRD patterns of both the recycled Zr/ZIF-8 and Zr/ZIF-8 catalysts are the same as shown in the figure, confirming that Zr/ZIF-8 has high crystal stability under normal reaction conditions. Decreasing the peak intensity of these diffractions (θ = 28–352) also indicates the effect of additional Zr increase in the ZIF-8 metal–organic framework. Because increasing zirconium to ZIF-8 can enlarge its pore spaces, resulting in crystallographic defects in the Zr/ZIF-8 catalyst. The XRD Zr/ZIF-8 pattern also shows the characteristic peak of ZIF-8 without peak diffraction of zirconium nitrate. Guest molecules (such as zirconium) that occupy the MOF pore

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spaces may degrade the pattern and consequently reduce the gas adsorption capacity in the MOF. Guest molecules (such as zirconium) that occupy the MOF pore spaces may degrade the pattern and consequently reduce the gas adsorption capacity in the MOF [31]. X-rays are used to study the structure of crystalline materials, and spectral regions can be used to obtain information about the structure, material, and quantification of elements. It is also used to detect crystal phases and their position and to measure thickness, thin films and multilayers. Therefore, X-ray diffraction methods are widely used in chemistry.

8 Thermogravimetric Analysis (TGA) Thermogravimetric analysis (TGA) is used as a quantitative analytical method to control the mass of a sample from 1 mg to several grams from a minimum temperature of 25 °C to a maximum temperature of 1600 °C under a constant or variable gas flow. TGA analysis controls and records sample mass, time, and temperature. This analysis is used to study the thermal stability of metal–organic frameworks. TGA thermal analysis data have been investigated for most MOF structures. TGA analyzes record mass loss in a sample after heating at a constant rate in a specific atmosphere for a specified time. For MOFs, this process usually occurs in several separate stages, beginning with separation at relatively low temperatures (1000 m2 g−1 , up to 7000 2 −1 m g ) encourages the adsorption and enrichment of substrate molecules near the active sites, which aids in subsequent activation and catalytic conversion. MOFs have well-defined structures due to their crystalline nature, which is critical for understanding the underlying mechanism and link between structure and catalytic performance. Furthermore, various pore structure design techniques, such as pore space partition, are crucial for MOF catalytic characteristics [2]. Smaller holes in MOFs following pore partition may increase host–guest interactions, resulting in higher catalytic efficiency. The pore partition technique, in particular, may be used to introduce uncommon heterometallic species into MOFs, which would improve their catalytic characteristics. As a result, MOFs, as unique solid catalysts, have great promise, particularly in fundamental catalysis. In virgin MOFs, the catalytic centers are often confined to coordinatively unsaturated metal sites (CUSs)-Lewis acid centers and/or the active group on the organic linkers (generally acid/base sites). Because of this, MOFs can only be used in a limited number of catalytic processes. Fortunately, there are at least two ways that may be used to solve the problem: (a) functionalized modification—using the multivariate approach or post-synthetic modification, desirable active sites can be grafted onto metal ions/clusters or organic linkers; and, (b) pore encapsulation/containment MOFs’ pore space may host a range of other active species as guests (e.g., organic molecules, inorganic nanoparticles, metal complexes, enzymes, etc.) and operate as a nanoreactor to host catalytic processes [3].

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Apart from heterogeneous organic reactions over MOFs or MOF-based composites as a result of the aforementioned modification or inclusion, MOFs and MOFbased materials have also been shown to be promising in photocatalysis and electrocatalysis for clean energy in the face of the global energy crisis and environmental issues. Many MOFs with semiconductor-like properties have demonstrated mainly their benefits in photocatalysis, such as water splitting and CO2 reduction. Due to the short transit distance of charge carriers to the pore surface throughout the structure for reactions, the high porosity of MOFs would considerably inhibit classical volume recombination of electron and hole; the high crystallinity also prevents defects that may form recombination sites. Adding more electron acceptors can boost charge separation and photocatalytic performance even more. Furthermore, most MOFs are prone to disassembly under the harsh conditions necessary in some catalytic systems, particularly electrocatalysis (strong acidic/alkaline solutions). Only a few are directly usable as electrocatalysts. In this example, the conversion of MOFs as precursors/templates, together with any necessary additives, to a variety of nanocomposites with pore characteristics inherited partly from their parent MOFs has been thoroughly investigated. The vast surface area and widely scattered active sites of the materials produced were shown to be relevant in both heterogeneous and electrocatalysis [4].

1.2 MOF Catalyst Modifications The shape and structure of the MOFs catalyst were enhanced by altering it, resulting in a stronger connection between the catalyst’s active site and the carrier. The redox capability of the catalyst, thermal permanency, and mechanical characteristics of the MOFs Catalysts in Fig. 1 were all enhanced using these approaches. Fig. 1 The graphic diagram for adjustment-based Arrangement of MOFs Catalysts. This figure was adapted by permission from Elsevier, 2021, 375, 20 [5]

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1.3 Possible Routes and Syntheses Chemical processes need some types of energy input, and they only come to a halt at temperatures close to 0 K. MOF is typically synthesized in a solvent at temperatures ranging from 250 °C to 350 °C. The most common method of introducing energy from the heat source is by conventional electric heating (oven). Other methods of introducing energy include electric potential, interfacial diffusion, electromagnetic radiation (microwave synthesis, mechanical waves (sonochemical synthesis), and mechanical means. It is critical to employ a variety of synthesis techniques. Different procedures, obviously, yield novel molecules that cannot be produced in any other manner. Alternative routes may also affect compounds with varied particle dimensions and morphologies, which might impact material characteristics. Dissimilar particle dimensions in porous substance, for example, might alter guest molecule diffusion, which has a straight impact on the adsorption or separation of catalytic processes or molecules [6]. Currently, numerous synthetic approaches for preparing catalytic substance with homogeneity, improved crystal shape, and great crystallization are of great scientific importance. At low temperatures (250 °C), the solvothermal technique is commonly employed to manufacture MOFs. Interactivity diffusion, microwave synthesis, ultrasonic synthesis, and mechanochemical techniques are all typical ways of producing MOFs. Aside from metal ions and ligands, the crystal structure is influenced by parameters, for example solvent, solution concentration, temperature, and reaction time. Microwave products have physicochemical and structural features that are quite comparable to hydrothermal synthesis products [7]. The microwave approach offers the benefits of extensive synthesis, quick response (5–10 min), phase choosiness (Synthesis of unique crystals from similar reaction components), and crystallite size reduction to increase catalytic performance. Microwave-assisted largescale synthesis of a variety of microporous lanthanide metal–organic frameworks was employed by Cao et al. (Ln-MOFs). Sonochemical synthesis provides a short crystallization time, low reaction temperature, and a large particle dimension reduction. Catalytic performance increases when more catalytic sites are exposed. Sung et al. synthesized Cu-BTC in 1 min in DMF at room temperature using an ultrasonic wave (N, N-dimethylformamide) [8]. The hydrothermal/solvothermal approach is still the most prevalent MOF preparation method today, and it works well. It has the capability to synthesize a homogenous catalytic material with ease, and it is simple to enhance the catalytic site. Kun et al. employed Cr (NO3 )3 , TPA, HF, and H2 O as raw materials and carried out the hydrothermal synthesis of MIL-101 (Cr) at 220 °C, according to the literature. Masood et al. employed HNO3 instead of HF in hydrothermal synthesis to produce MIL-101-HNO3 with a higher specific surface area and used for H2 S adsorption [9]. Zhao et al. synthesized HF-free MIL-101 (Cr) and utilized it to remove Hg° from the environment. The inclusion of additional ions increased the catalytic performances, and the altered procedures did not employ hydrofluoric acid, resulting in a smaller risky synthesis protocol. The most crucial aspect of mechanically activated MOF

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production is environmental considerations. In the absence of a solvent, the reaction can be carried out at ambient temperature with a short reaction time (typically between 10 and 60 min). Furthermore, the metal salt might be substituted with a metal oxide as a starting material in some circumstances [9]. Zn-MOF-74, for example, was synthesized directly from ZnO without the need of any solvent. At room temperature, ZIF-90 nanopolyhedron (Zn (ICA)-1) and ZIF-90 amorphous network (Zn (ICA)-2) were both effectively constructed. These catalytic materials have the potential to be used in large-scale catalytic activity for air pollution management.

1.4 Physical–chemical Property Modification Each molecule’s energy, pressure, and energy are all linked. Each of these factors in the system may have a substantial influence on the morphology and shape of the product being created [10].

1.5 Modification of Morphology The Shape and structure of the material have a considerable influence on MOF catalytic performance, and numerous investigators have worked to enhance this area. M-MIL-101(Cr), a modified MIL-101(Cr) with a different linker to cluster percentage, was synthesized. In comparison to MIL-101(Cr), a large ratio of crystals in M-MIL-101(Cr) samples were linked with each other, resulting in increased porosity and surface area. The average particle size, homogeneity, and octahedral shape of the M-MIL-101 were improved by using modulators (i.e., HNO3 and HF) (Cr). The impact of solvothermal temperature and co-solvents on the characteristics of MOF-74 materials has been investigated based on the findings of previous researchers. The Cu-MOF-74 illustration synthesized at 80 °C with isopropanol as a co-solvent had a rod-like morphology, whereas the sample synthesized at 65 °C and 80 °C with ethanol as a co-solvent had one-way and two-way bouquet structures. CoMOF-74 with a flower-like shape and greater specific surface area and pore volume was synthesized at 100 °C; at other temperatures, a hexagonal prism structure was formed. Simultaneously, the hydrothermal approach was used to successfully create Mn-MOF-74 with a hollow spherical structure. Researchers stated that they were able to adjust the crystallinity and adsorption characteristics of the graded microporous mesoporous Zn-MOF-74, while the process was taking place at room temperature by manipulating time and solvent. Cu-BTC was produced and employed for NH3 -SCR in few investigations. Many discrepancies in the described Cu-BTC properties due to the change in reaction temperature and reaction pressure. According to recent research, acetic acid can assist generate a lattice-defect porous structure by modulating crystal structure and morphology through the synthesis process.

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1.6 Modifying of Metal Ions and Ligands Depending on the kind of metal and ligand, numerous non-porous MOFs have additional essential uses, including magnetism, luminescence, and sensors. The adsorption and catalysis of reactant molecules can be aided by treating metal centers. It may be classified into two categories: metal center replacement and coordination unsaturated metal sites. For example, Ni was employed to replace Fe’s active sites in MIL-100(Fe) and to hydrothermally create a crystalline porous Ni-MOF catalyst. Its stability in the NH3-SCR system was superior to the earlier reported Cu-BTC or MIL-100 systems (Fe) [11]. The structure of the final MOF is determined by the ligand’s geometry and connectivity. The dimensions, morphology, and chemistry of the MOF are all influenced by the ligand’s geometry, length, and functional group. For example, employing the novel ligands AIPA, a new Cu-MOF was created in DMF by solvothermal technique. Concluded non-classical hydrogen bond interaction, [Cu (AIPA)DMF] n produced a 3D skeleton from a two-dimensional crystal structure. Various ligands were examined for MOFs with the same activity center but different topologies, pore topography, and catalytic center size and distribution. The goal of this research was to learn more about the link between structure and catalytic arrangement. Dissimilar pore diameters and separation performance for C3 H8 /CH4 were observed in Zr-MOFs, which were produced using a variety of ligands (for example, H2-2, 6-NDC, H2 Fum, and so on). Spencer et al. used a solvothermal approach to make two kinds of Zn-MOFs, with inorganic structure units bridged to create a 3D network. Microwave-assisted hydrothermal approach was used to produce a novel style Ni+ -1,3,5-tribenzyl-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione coordinated polymeric MOF in a recent work [11].

1.7 Defect Engineering Designing MOF materials using catalysis at the nodes by introducing defects has become a viable option. For particularly active catalytic systems, generating flaws by direct techniques or post-synthesis processing is a frequent tactic. The method of thermally pre-treating catalysts to liberate catalyst sites is quite prevalent. Lewis acidity in MOFs is linked to a low-coordination-number-accessible metal site; this is a coordinately unsaturated site (CUSs). MOF activation enhanced CUSs, easing reactant adsorption and catalysis. A mesoporous iron triacetate MIL100(Fe) with Fe (III)/Fe (II) mixed state was synthesized by vacuum behavior at around 150 °C based on porous MIL-100(Fe). For H2 S selective catalysis and NH3 -SCR, a mixed valence state of MIL-100(Fe) was employed. Furthermore, by removing material weaknesses in a synthetic way, metal ion coordination may be modified and new active sites can be generated throughout the process. The weak

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combination was identified as the coordination bond that bound the linker to the metal [12]. This flaw might lead to a selective ligand or metal substitution in MOF materials, as well as defect-based catalytic sites. Two neighboring Cu2+ CUS sites, for example, can interact synergistically with the consistent chemical middle. Likewise, the frequency of a pair voids in UiO-66 increased with the number of washes due to hydrolysis. A typical approach of defect engineering on MOF was metal cation isomorphous substitution. By using the direct ion exchange pre-assembly approach, a succession of A-Cu-BTC (A=Fe, Ni, Co, Mn, Sr, La, Ce, Al) was created. When the crystallite’s outer surface approached the active region, the catalytic activity was boosted by shrinking the crystallite or creating voids inside it. The nucleation rate of the [Cu3 (BTC)2 ] skeleton was dramatically modified by altering the additive concentrations to regulate the crystal size. Finally, previous research has shown that nanocrystals with sizes ranging from limited tenths of a nanometer to limited microns may be synthesized in a controlled way [13].

1.8 Functional Modification Aside from the benefits of high porosity, huge specific surface area, and structural variation, the most intriguing characteristic of MOFs is the ability to manipulate their chemical composition. The functional group may be incorporated into the MOF and has a stable reaction with the entering molecule. When comparing the traditional amino-functionalized changed MOF NH2 -MIL53 (Fe) to its parent skeleton MIL-53 (Fe), the addition of an amino group lowered the reaction’s initiation energy and impacted a moderately basic region on the catalyst’s surface. Adding amine functional groups to a series of Fe-based MOF materials (MIL-101 (Fe), MIL-53 (Fe), and MIL-88 (Fe) considerably increased their photocatalytic carbon dioxide reduction activity [14]. Similarly, adding the thiol (−SH) functional group to UiO-66 lowered the adsorption energy considerably. The thiol skeleton demonstrated high adsorption ability for Hg (II) in both the aqueous and gas phases, according to the findings. Zhang et al. created a bromine-containing metal skeleton (Br-MOF) with a high Hg° capture effectiveness. UiO-66 (Zr) and its functionalized by-products had superior adsorption performance in sulfur-containing binary mixed gases than original UiO-66 (Zr) and its functionalized by-products [15]. Table 1 shows the application of metal–organic structures in their use in catalysts and photocatalysts.

(±)-cis-4-ethoxy-2-phenyl-1,2,3,4-tetrahydroquinoline 2-Benzylidenemalononitrile

Three-component Povarov reactions

Knoevenagel reaction

Knoevenagel condensation–Michael addition–cyclization

One-pot direct Knoevenagel condensation–Michael addition–cyclization

One-pot direct Knoevenagel condensation–Michael addition–cyclization

Esterification reaction of butyric acid and butanol

Acetylation of glycerol

Glycerol Dehydration

Acetylation of glycerol

One-pot deacetalization–Knoevenagel condensation

Zr6 OTf-BTB

MF-ZIF-8 sponge

ChCl@UiO-66-Urea

UiO-66@Schiff-Base-Cu(II)

UiO-66@Schiff-Base-Cu(II)

MTV-UiO-66

AC/UiO-66

MOF-808-S

AC@MIL-1015

PCN-222-Co@TpPa-1 2-Benzylidenemalononitrile

Monoacetin, Diacetin, Triacetin

Acrolein

Monoacetin, Diacetin, Triacetin

Butyl butyrate

1H-Pyrazolo[1,2-b] phthalazine-5,10-diones

Dihydropyrano[2,3-c]chromenes

2-amino-4H-chromenes

(R)-3-phenyl-3,4-dihydro-2H benzo[b][1,4]oxazinemethyl

Asymmetric transfer hydrogenation & Hantzsch ester reduction

CMOF

Synthesized product

Type of reaction

Catalyst

Table 1 The application of metal–organic structures in their use in catalysts and photocatalysts

600

15

570

180

1440

10

10

180

150

1440

600

Time (min)

0.005 g

4 g.L−1

0.2 g

6 g.L−1

0.05 g

0.035 g

0.03 g

0.01 g

0.01 g

1.0 mol %

5.0 mol%

Catalyst loading

96.8

100

92.1

92

93

92

92

>99

>99

97

Yield (%)

DMSO, 50 °C, under 99.3 N2 atmosphere

Acetic acid/glycerol, 110 °C

Gly/H2 O, 170 °C

Gly/AcAc: 1/3, 90 °C

1 wt % H2 SO4

Solvent-free, 80 °C

Solvent-free, 90 °C

Solvent-free, 100 °C, under N2 atmosphere

Toluene, r.t

CH3 CN, r.t

CHCl3 , 40 °C

Conditions

[25]

[25]

[24]

[23]

[22]

[21]

[20]

[19]

[18]

[17]

[16]

References

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2 Optics 2.1 Introduction Photonics is the study of producing, regulating, and sensing photons and light waves, and it is used in a variety of sectors such as communication, information processing, military, and medical [26]. Because photons outperform electrons as info transporters in relation to rapidity, bandwidth, also valency, photonics offers a possible alternative to the present bottlenecks that prevent conventional electronics from improving further. The blooming growth of optical fiber technology is a great illustration of how photonic devices are having a major influence on the world today. Luminescent detectors, illumination, nonlinear optical devices, micro/nanolasers, a three-dimensional modeling and data storing, and photoconductors have all been shown using inorganic or organic materials, for example, ZnO, CdSe, LiNbO3 , and p-conjugated macromolecules [27]. In this regard, developing innovative photonic materials with required characteristics for functional submissions is critical, In the realm of photonics, The dominant forces will be these materials. MOFs have received a lot of attention and have had a lot of experimental success in the quest to build better photonic functional materials, prompting us to propose a novel concept of “multiple photonic units” (MPUs) to emphasize the unique characteristic of MOFs in this burgeoning field. MPUs are photo-responsive components that come in a variety of shapes and sizes, such as inorganic metal ions/clusters, organic bridging ligands, and guest species. Choosing photo-responsive metal ions/clusters and organic ligands as building blocks allows photonic MOFs to be easily produced. In addition, the pores in MOFs may be used to encapsulate a huge range of photonic guests, such as nanoparticles, metal complexes, and organic dyes, in order to create photonic MOFs [28]. It is easy to see how MOF materials might benefit photonic functional applications in a variety of ways. First, the number of possible synthetically accessible MOFs is tremendous when considering the broad mix of inorganic and organic photonic components. Luminescence detecting, adjustable light emission, visible light-driven photocatalysis, and photodynamic/photothermal theragnostic have all remained effectively shown in MOFs thanks to the purposeful design and selection of photonic units. Second, MOFs may not solitarily retain the fundamental qualities of each photonic part, but likewise boost and enhance photonic functioning, owing to the synergistic influence among MPUs. Lanthanide ions, for example, have parity forbidden 4f–4f transitions that may be sensitized by organic ligands through ligand–metal energy transfer. Because of this advantage, MOFs have long been used in luminescence sensing and light emission applications. Finally, it is shown that both the aggregation morphology and the molecular structure impact the photonic features of aggregated organic molecules. MOFs’ coordination geometry and pore constriction lead organic ligands and/or guest molecules to pack in a preferred route, resulting in unique combination morphologies that differ dramatically from their bulk or solution counterparts. As a

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result, the controlled and ordered arrangements of MPUs inside MOFs, as compared to traditional amorphous porous materials and polymers, provide an unquestionable possibility to produce unique photonic functions. In fact, coordination-induced emission, lasing, and nonlinear optical response in MOFs have all been achieved using this method. MOFs will also provide a lot of flexibility in the implementation of complicated photonic functional applications since their pore size and pore shape can be readily modified to fit the guest substrates and vary the intermolecular aloofness, and so regulate the energy/charge transfer process between MPUs [29]. MOFs are an ideal platform for designing and synthesizing functionally customized photonic materials due to their wide combination possibilities, synergistic effects, and controlled and ordered arrangements of MPUs. MOFs have unlocked a world of potential uses in photonics domains where traditional inorganic or organic supplies may be insufficient [29].

2.2 Photonic MOF Enterprise and Manufacturing Strategies Lanthanide metal ions, metal clusters, dyes, quantum dots (QDs), organic chromophores, luminous organic groups, perovskites, and other MPUs abound. Some of them can be constructed or selected as supplementary building units (SBUs) and linkers for the framework, while others can be enclosed inside the MOF channels as photonic guests to compose photonic MOFs [29].

2.2.1

Photoreactive Metal Ions/clusters

Metal ions/clusters serve as important link nodes in the construction of MOFs by coordinating with organic ligands. SBUs are metal–oxygen/nitrogen–carbon clusters that form as a result of the process [29]. Since the 4f–4f transition’s sharp emission spectra and insusceptibility to the environment provide for very bright luminescence, lanthanide ions are a class of photo-responsive metal ions. Furthermore, the emission wavelengths of lanthanide ions span from blue to near-infrared: Tm3+ , Tb3+ , Sm3+ , and Eu3+ produce blue, green, orange, and red light, respectively, while Yb3+ , Nd3+ , and Er3+ emit nearinfrared light. The banned f–f transitions, on the other hand, make it impossible to directly excite lanthanide ions. The lanthanide ions can be sensitized by the “antenna effect,” in which light is absorbed by the ligands, the absorbed energy is transferred to the excited energy level of the lanthanide ions, and the lanthanide ions emit luminescence as a result. The incorporation of lanthanide ions into MOFs results in Ln-MOFs, which have been generously studied through the previous decade [30]. The Ln-SBUs may readily be substituted by each other due to the chemical similarities of lanthanide ions, resulting in mixed-Ln-MOFs that may exhibit the distinctive emissions of singly lanthanide ion in the meanwhile. For instance, an embattled self-referencing luminous thermometer constructed on the emissions of

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Tb3+ and Eu3+ was produced across an extensive temperature variety from 10 to 300 K by doping Eu3+ into an isostructural Tb-MOF to generate a mixed-Ln-MOF EuxTb1xDMBDC (DMBDC = 2,5-dimethoxy-1,4-benzenedicarboxylate). Color tuning and up-conversion materials may now be made with even more lanthanide ions integrated into MOFs [31]. Mixed-Ln MOF hierarchical single crystals were also formed via a solutionmediated epitaxial growth technique, as opposed to the normal solid-solution type outlined above. Each component in the resulting MOF crystals can keep its unique luminescence property, and multicolor to white light emissions are achieved via a color mixing process that manipulates the domain and orientation of hierarchical crystals. The 5d–4f transitions of lanthanide ions can also be used to make photonic MOFs, despite the fact that they have a far larger dependency on the coordination environment of the metal ions than f–f transitions due to the presence of d states. Because the 5d–4f transitions are parity permitted, they enable wider absorption and emission bands lacking the antenna significance. Muller-Buschbaum et al. [32] provided several instances in this field. The metals Eu and Sr were combined through a melt of 1H-imidazole (Im) to produce 3 ∞ N [Sr1-x Eux (Im)2 ], in which together metal ions were bivalent. A wide emission band attributable to divalent europium was found after excitation, demonstrating transitions among the 5d and 4f 8 S7/2 energy planes. Likewise, the same group discovered that a Ce3+ -based MOF 3 ∞ N [Ce (Tz)3 ] (Tz = 1,2,3-triazolate) emitted a strong blue d–f emission. The use of photo-responsive metal clusters to make photonic MOFs has recently piqued researchers’ curiosity. Li et al. built the 0D-Cu4 I4 cluster into 1D, 2D, and 3D frameworks via solid Cu–N bonds, generating high-presentation hybrid lighting phosphors, inspired by the exceptional luminous capabilities of copper bidentate ligands. It’s the organic components that make up C 2p and N 2p the conduction band minimum, whereas Cu 3d and I 5p make up the valence band maximum (VBM) for those with aromatic ligands (VBM). The luminescence is caused by MLCT or XLCT (metal-to-ligand charge transfer). Both CBM and VBM are inorganic atomic states in aliphatic ligand-based compounds; hence, the emission is cluster-centered. Zang, Mak, and colleagues say silver(I) chalcogenide/chalcogenolate clusters may be employed as MOF photonic nodes. In air, the luminous Ag12 cluster degraded rapidly; though, when MOFs with the organic ligand bpy (4,40-bipyridine) were formed, the Ag12 bpy crystals sustained their crystallinity for 1 year at room temperatures. In air, Ag12 emits faint red light (620 nm), but Ag12 bpy emits at 507 nm, indicating that MOF production may affect the excited state [33]. Calculations using the density functional theory (DFT) revealed that Ag12 ’s luminescence is a combination of LMCT and metal-centered transitions. The enthusiastic states of Ag12 bpy, on the other hand, are MLCT (Ag(I)-to-bpy) with approximately ligand (S, O)-to-bpy mixing.

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Functionalization of Organic Ligands

Efficient sites including exposed metal sites, Lewis acidic sites, and Lewis basic pyridyl sites container increase MOF compassion to environmental variations and analyte choosiness by including them. Chen, Qian, and collaborators developed one of the first samples using open metal sites for small molecule luminescence detection. Eu (BTC) was calcined at 140 C under vacuum for 24 h to get the open Eu3+ site [34]. It was discovered that the weakly coordinated 1-propanol molecules on Eu3+ open sites were progressively replaced by DMF or acetone when the N,Ndimethylmethanamide (DMF) or acetone solvent was gradually introduced to the Eu(BTC) 1-propanol emulsion. Two MOFs, Zn2 (TCPE) and Mg(H2 DHBDC) (H2 DHBDC = 2, 5-dihydroxybenzene-1,4-dicarboxylic acid), were developed by Dinca et al. to selectively detect gaseous ammonia at elevated temperatures by interacting with NH3 with open metal sites. Using Lewis basic sites and coordinated water molecules as signal transmission mediums, Zhang and colleagues developed an Eu-MOF that can detect acetone, aliphatic alcohol, explosive 2,4,6-trinitrophenol (TNP), and Cu2+ . Despite the fact that the functional site performance plays a major part in fluorescence detection capabilities, the greatest early MOF sensor research investigations failed to give a comprehensive picture of the sensing process. However, given the rapid advancement of MOF sensors, a thorough examination of the sensing process is critical. Jiang and colleagues recently disclosed Tb(BTC) nanocrystals that use the quenching phenomenon to detect picric acid (PA). The excited electrons in Tb(BTC) are more likely to transfer into PA molecules owing to PA’s lower LUMO level compared to other analytes and the strong supramolecular connection between Tb(BTC) and PA molecules, such as hydrogen bonding. Li et al. studied electron transport between MOF sensors and analytes for Aflatoxin B1 detection in 2015. The MOF LMOF-241 was produced using mixed-ligand H2 BPDC and 1,1,2,2-tetrakis(4(pyridin-4-yl) phenyl) ethane (TPPE), and its 16.6 nm channel diameter is excellent for concentrating aflatoxin molecules. The addition of Aflatoxin B1 decreased the emission intensity of LMOF-241 and lowered the limit of detection (LOD) to 46 ppb, significantly below the FDA’s 300 ppb acceptable level (FDA) [35]. Because LMOF-241’s estimated LUMO energy state is greater than Aflatoxin B1’s, electron transfer from LMOF-241 to Aflatoxin B1 explains the quenching effect. Banerjee and colleagues created a solvatochromic MOF for detecting organic amines from electron-deficient N,N0-bis(5-isophthalic acid)naphthalenediimide (H4 BINDI). The MOF’s highest occupied molecular orbital (HOMO) energy is 6.02 eV, lower than that of organic amine analytes, suggesting donor–acceptor electron transfer between MOF and amines may occur, dampening the MOF’s emission intensity. Yan and colleagues developed luminescence quenching-based MOF sensors that leverage fluorescence resonance energy transfer [36]. Based on fluorescence quenching, a 3D framework Ga (OH)(btec)0.5H2 O was functionalized with Eu3+ , and the resulting material showed excellent sensing to 1-hydroxypyrene. The quenching impact of

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Eu3+ emission was induced by a considerable overlap between 1-hydroxypyrene’s absorption spectrum and the MOF ligand’s emission spectrum, which restricts energy transfer to Eu3+ . The ability to construct low LOD MOF sensors has considerably increased as a result of a better knowledge of the luminescence sensing mechanism of MOFs. Zhao and colleagues produced a heterometallic cationic MOF [Ln2 Zn(L2 )3(H2 O)4 ](NO3 )2 .12H2 O (L2 = 4,40-dicarboxylate-2,20-dipyridine) that can oxidize I- into I2, permitting I-detection at 0.001 ppm in aqueous solutions. Konar et al. created a high-choosiness MOF sensor for sensing picric acid at 1.5 ppm by using resonance energy transfer and electron transfer [37].

2.2.3

Photonic Organisms Are Encapsulated in MOFs

Separately from the inherent luminescence of the frameworks, encapsulating photonic class is a key technique for increasing the luminescence modes of MOFs by utilizing their vast and organized pore structure. Lanthanide ions, quantum dots (QDs), organic dyes, and other light emitters have all been effectively incapacitated into MOFs. MOFs lacking specific strategy might not firmly encapsulate photonic species into pores through basic adsorption procedures, though this does not influence the greatest photonic applications. Falcaro and colleagues used a one-pot approach to insert highly luminous semiconductor QDs into MOF-5, resulting in QD@MOF-5 composites [38]. The QD@MOF-5 framework’s QD dispersion was uniform. Combining the MOF’s sieving properties with the QD’s luminous probe quenched ethanethiol and n-isopropyl acrylamide/acrylic acid/t-butyl acrylamide mercaptan. Yan et al. studied the optical characteristics of micrometer-sized isostructural Tb-BTC, Eu-BTC, and Eu-doped Tb-BTC. All three Ln-MOFs showed the optical waveguide result, with a low waveguide loss constant and a significant photoluminescence quantum yield, showing that Eu3+ in the pores performs alike photonic purposes as Eu3+ in the SBUs. Though, in demand to actualize around unique structures or applications, an intricate host–guest system design must be considered. This is mostly studied in COFs, supramolecular organic polymers, and porous organic polymers. Since MOFs have a lot of structural flexibility, the architecture of the host–guest classification should be considered as well. For example, Huo and colleagues discovered that PVP-changed nanoparticles may be effectively disseminated and totally contained within ZIF-8 crystals [39]. Furthermore, by altering the period of nanoparticle addition during the MOFformation process, the spatial distribution of nanoparticles inside ZIF-8 may be regulated. Further experiments revealed that the PVP-changed nanoparticles may be sequentially adsorbed on the expanding coordination-polymer spheres’ continually generating fresh surfaces. Qian et al. gave another example: the dye 4-[p-(dimethylamine) styryl]-1-methylpyridinium (DMASM) was tightly confined and highly oriented in single solid-state microcrystals because the pore dimensions

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of the as-synthesized ZJU-68 was precisely coordinated with the dye’s dimensions, resulting in a polarized three-photon-pumped laser in single solid-state microcrystals [40]. Redundant coordination sites on the frameworks are frequently used to increase the loading effectiveness of photonic class within MOFs. The coordination of guest species and frameworks may also result in extra effective energy transmission among hosts and visitors. Qian et al. produced CPM-5 and MIL-100(In) films, respectively, and subsequently loaded Tb3+ in similar conditions. CPM-5 container only physically absorbs Tb3+ , but MIL-100(In) possesses uncoordinated carboxylate groups that container chelate with Tb3+ . Tb3+ coordinated MIL-100(In) has a quantum yield of 16.8%, but Tb3+ absorbed CPM-5 has a quantum yield of just 1.1%. The shorter donor–acceptor distance of the former material is attributed to the coordination structure, which results in a substantially larger quantum yield. Yan and coworkers created a fluorescent MOF-based ammonia sensor by coordinating Eu3+ on the N,N-chelating sites of the ligand biphenyl-5,50-dicarboxylate (bpydc) [41]. Researchers have studied the electric force between hosts and guests. DMASM emits red light at 628 nm. It can be ion-exchanged into bio-MOF-1, and electrostatic force helps connect dye molecules to MOF pores. 3D framework ZJU-21 with polarized benzothiadiazole ligand moieties showed efficient DMASM absorption. Li and colleagues recently described the primary sample of perovskite nanoparticles being integrated into MOFs through a direct conversion process driven by a halide salt, n-butanol solution containing CH3NH3Br, broadening photonic guest encapsulation approaches inside MOFs [42]. Zhang et al. also succeeded in encapsulating carbon nanodots (CDs) in MOFs by saturating porous MOFs with glucose and calcining the glucose at a low temperature. Glucose may be introduced into MOF holes from the liquid phase and carbonized at low temperatures. Afterward, glucose penetrates the MOF pores, and it is heated at 200 C in nitrogen. Since the MOF structure is unchanging at this temperature, calcination creates CD-loaded MOF with well-defined pores. CD-loaded MOF thin layer on quartz glass demonstrated satisfactory optical limiting characteristics at 532 nm.

2.3 MOFs for Luminescent Sensors Luminescence sensing, as opposed to classical sensing, can provide a quicker response, advanced sensitivity, superior tolerance to electromagnetic interfering, harmless operation in a volatile or comburent environment, and the ability to monitor remotely. The luminescence possessions of MOFs are highly penetrating to their structural characteristics, coordination environment, nature of the pore surfaces, and connections with guest species via coordination bonds, p–p interactions, hydrogen bonding, and other mechanisms, providing a solid rationale for developing luminescent MOF sensors. Furthermore, MOF MPUs may be used to construct and develop ratiometric sensors that are free of external interference [43].

Applications of MOFs

2.3.1

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MOFs Containing Functional Sites for Targeted Luminescence Detection

By incorporating uncluttered metal sites, Lewis acidic sites, and Lewis basic pyridyl sites, MOF compassion to environmental variations and analyte choosiness may be increased. Chen, Qian, and coworkers used open metal sites for small molecule luminescence detection. Eu (BTC) was calcined at 140 C for 24 h under vacuum to open Eu3+ . The 5 D0 → 7 F2 emission intensity of Eu3+ rose or reduced when N,Ndimethylmethanamide (DMF), or acetone was introduced to the Eu(BTC) 1propanol emulsion, showing that 1-propanol molecules on Eu3+ uncluttered sites were progressively replaced by DMF or acetone molecules. Dinca et al. proposed 2 MOFs, Zn2 (TCPE) and Mg(H2 DHBDC), for the discriminating recognition of gaseous ammonia at elevated temperatures owing to NH3 interaction with open metal sites. Zhang and colleagues presented a Eu-MOF with multi-luminescence sensing of acetone, aliphatic alcohol, explosive 2,4,6-trinitrophenol (TNP), and Cu2+ . Lewis basic sites and coordinated water molecules operate as signal transmission mediums [44]. Despite the fact that functional site performance plays a major part in fluorescence detection capabilities, most early MOF sensor research investigations failed to give a comprehensive picture of the sensing process. However, given the rapid advancement of MOF sensors, a thorough examination of the sensing process is critical. Jiang and colleagues recently disclosed Tb(BTC) nanocrystals that use the quenching phenomenon to detect picric acid (PA). The excited electrons in Tb(BTC) are more likely to transfer into PA molecules owing to PA’s lower LUMO level compared to other analytes and the strong supramolecular connection between Tb(BTC) and PA molecules, such as hydrogen bonding. Li et al. studied electron transport between MOF sensors and analytes for Aflatoxin B1 detection in 2015. The MOF LMOF-241 was produced using mixed-ligand H2BPDC and 1,1,2,2-tetrakis (4-(pyridin-4-yl) phenyl) ethane (TPPE). Its 16.6 nm channel diameter is excellent for concentrating aflatoxin molecules. The addition of Aflatoxin B1 decreased the emission intensity of LMOF-241 and lowered the limit of detection (LOD) to 46 ppb, significantly below the FDA’s 300 ppb acceptable level (FDA). Because LMOF-241’s estimated LUMO energy state is greater than Aflatoxin B1’s, the quenching effect is due to efficient electron transport. Banerjee and colleagues reported a solvatochromic MOF for detecting organic amines produced from N, N0-bis (5-isophthalic acid) naphthalenediimide (H4BINDI). The MOF’s highest occupied molecular orbital (HOMO) energy is 6.02 eV, which is lower than the energy of organic amine analytes, suggesting that donor–acceptor electron transfer may occur, dampening the MOF’s emission intensity. Yan and colleagues developed MOF sensors that leverage fluorescence resonance energy transfer from ligand to analytes. Based on fluorescence quenching, a three-dimensional framework Ga (OH)(btec)0.5H2 O was functionalized with Eu3+ , and the resulting material presented outstanding sensing to 1-hydroxypyrene. The quenching impact of Eu3+ emission was induced by a considerable overlap between

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1-hydroxypyrene’s absorption spectrum and the MOF ligand’s emission spectrum, which restricts energy transfer to Eu3+ .

2.3.2

Active Luminescence Detection

Until recently, the bulk of luminescence-based MOF devices has relied on the luminescence quenching effect’s turn-off mechanism. For luminescent quenching-based sensors, several distracting effects, for example, the nonradiative energy transition and luminous self-absorption, can lead to false detection findings. Turn-on detection, in which MOFs interact with analytes to transform a comparatively “dark” material into a luminous one, can minimize false discovery results caused by nonradiative transition, improving the indication to noise relation and making them more subtle to lower analyte concentrations. The application of MOFs for turn-on luminescence sensing, on the other hand, is still in its infancy [45]. The MOFs must be purposefully constructed for the exclusive energy transfer mechanism that can be favorably adjusted by the analytes in order to achieve such turn-on sensing. Most organic chromophores are extremely emissive in solution, but when the molecules are aggregated, they become weak or even completely destroyed. As Briks demonstrated in 1970, most aromatic hydrocarbons and their derivatives have an aggregation-caused-quenching (ACQ) effect. [399] Because organic chromophores have a similar structure based on p-conjugation, the large conjugate ring enhanced the possibility that such luminophores would form excimers or exciplexes, which would effectively quench chromophores’ emission. However, certain organic compounds can have the reverse effect: they do not emit while dissolved in excellent solvents, but when accumulated in week solvents or in the solid state, they become extremely emissive. Aggregation-induced emission (AIE) is a technique that allows you to energetically use the accumulation process rather than passively working against it. Dinca et al. described the novel matrix coordination-induced emission effect (MCIE) in stiff porous MOFs by coordinative immobilization of an AIE ligand TCPE. The MOFs Zn2 (TCPE)(H2 O)2 .4DEF (DEF = N,N-diethylformamide) and Cd2 (TCPE) (DEF)(C2 H5 OH)0.2DEF (DEF = N,N-diethylformamide) have significant “turn-on” emission at 480 nm and 455 nm, respectively, identical to the solid ligand H4 TCPE, whose maximum emission Furthermore, these MOFs have biexponential fluorescence decay characteristics with values comparable to solid H4 TCPE. This finding shows that anchoring AIE-type molecules within MOFs may effectively switch on fluorescence by constraining phenyl ring rotation, which could be beneficial in the development of novel turn-on luminous MOF sensors. Manos et al. published an Mg2+ -based MOF in 2015 that used turn-on luminescence to detect amounts of water in organic solvents [46]. The 3D AEMOF-1DMAc (DMAC = N, N-dimethylacetamide) as created has a 1D channel and can breathe by removing/readsorbing DMAc guest molecules. The guest-free compound AEMOF-10 demonstrated two forms of signal transduction for water in THF (tetrahydrofuran): luminescence increases and emission peak shift.

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Surprisingly, even at a water content of 0.05% v/v, a significant shift in the emission spectra of AEMOF-10 may be detected. They calculated the sensing mechanism and discovered that the energy transfer process inside the framework was impacted by the occurrence of redistribution electron density after the hydrogen-bond creation.

2.3.3

Dual-Emitting MOF Luminescence Sensing

Despite the fact that much research has gone into MOF luminescent sensors, most instances of turn-on sensors that have been described are based on the intensity fluctuations of a single emission. However, because incorrect reactions or erroneous observations might be caused by changes in the external environment, intensity changes may not always be the most exact manner of identification. Even if the fundamental different parameters remain constant throughout the tests, the luminescence intensity may vary from sample to sample, resulting in inaccurate findings. Taking all of this into account, ratiometric luminescent devices, which use the ratio of the power of two MPUs inside similar luminescent material, were created to overcome the single emission-based luminescence sensors’ fundamental shortcomings. A MOF thermometer based on a mixed-Ln-MOF, Eu0.0069Tb0.9931-DMBDC, was reported in 2012 and is the first ratiometri luminescent MOF thermometer. Spectra of the 7 F6 → 5 D4 transition from the Tb3+ ions stimulated at 488 nm show that the emission of Eu3+ ions in mixed-Ln-MOFs was sensitized by Tb3+ ions in the same frameworks. From 10 to 300 K, the emission intensity of 5 D4 → 7 F5 (Tb3+ , 545 nm) of Eu0 .0069 Tb0.9931 –DMBDC decreased, while the emission intensity of 5 D0 → 7 F2 (Eu3+ , 613 nm) increased, which is attributed to the temperature-dependent energy transfer process from Tb3+ to Eu3+ ions, as established by luminescence lifetime measurements. In order to create the self-referenced luminous thermometer, the same compound’s luminescence emission changed depending on the temperature. The intensity ratio (ITb/IEu) of the 5 D4 → 7 F5 (Tb3+ , 545 nm) to the 5D0 → 7 F2 (Eu3+ , 613 nm) transitions is linearly related to temperature from 50 to 200 K, with an extreme relative understanding (Sm) of 1.15% K at 200 K, indicating that the Eu0 .0069 Tb0 .9931 -DMBDC can measure temperature with a maximum relative sensitivity aside from that, the green-yellow to red temperature variations may be seen immediately and clearly thanks to the changeable luminosity colors from 10 to 300 K. It is possible to use a Tb3+ /Eu3+ mixed-Ln MOF with temperature-reliant on luminescence as a thermometer [47].

2.4 MOFs for Lighting and Info Display in Solid-State Consequently, MOFs may be tailored to fulfill the needs of solid-state lighting supplies in a variety of applications. MOFs have been created for luminescence adjusting, barcode patterns, info encryption, and white light-generating devices

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throughout the last decade. Their adjustable luminescence and various architectures enable emission wavelengths spanning the whole visible spectrum as well as white light production from a variety of processes. This example’s quantum yield depends on the efficiency with which energy is transferred from the singlet state to the ligands’ lowest triplet state and from the triplet state of ligands’ lowest triplet state to the excited energy level of the lanthanide ions, the most common building blocks of lighting materials. To get a high quantum yield, it is critical to use organic ligands with appropriate excited energy levels. Furthermore, ligand-based emission is used as one of the key blue color sources in the majority of reported MOFs for white light-producing devices. This suggests that the ligand transfers less energy to the lanthanide ions, resulting in a decreased quantum yield. In order to boost the quantum yield of solid-state lighting MOF materials, several researchers have proposed numerous unique techniques to implement them.

2.4.1

Luminescence Tuning

Because of the potential uses in lasers, optoelectronic devices, and light display devices, much care has been taken to fabricate luminous tunable MOFs under single wavelength stimulation. With the prohibited 4f–4f transitions, lanthanide ions can release long-lived narrow and distinctive emissions. Furthermore, the same chemical characteristics allow for a wide range of mixing and co-doping of various lanthanides, resulting in easy luminescence tuning possessions. As a result, lanthanide ions are frequently used in luminescence tuning. Zhang and colleagues published a Eu1-x Tbx MOF film whose luminescence can be controlled by changing the ratio of Eu3+ and Tb3+ in 2010. The 5 D4 → 7 F5 transition of Tb3+ and the 5 D0 → 7 F1 and 5 D0 → 7 F2 transitions of Eu3+ ions are ascribed to the intensity of three photoluminescence peaks situated at 540, 589, and 615 nm, respectively. The luminescence strength at 540 nm may be controlled by altering the Tb3+ /Eu3+ ratio due to the occurrence of energy transfer among Eu3+ and Tb3+ , resulting in color tuning of this MOF film. According to Guillou and coworkers, introducing more types of Ln3+ ions into MOFs may result in a wider variety of color coordinates and intensity adjustment. They created a sequence of mixed-Ln-MOFs, [Tb2x Eu2y La2 -2x-2y(bdc)3 (H2 O)4 ], by adding La3+ as dilution ions and adjusting the Tb3+ /Eu3+ ratio. The color of emission can be changed by changing the Tb3+ /Eu3+ ratio, and the brightness of emission may be changed by using optical dilution with the addition of La3+ . Zhang and colleagues created a new MOF, Tb(1,3,5-BTC) (H2 O)0.3H2 O, with various Eu3+ ion concentrations. The photoluminescence hue may be changed from green to green-yellow, yellow, orange, and red–orange by adjusting the doping concentration of Eu3+ ions. Color tunable mixed-Ln-MOFs have freshly been presented as a feasible use for barcode production [48].

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Barcode Pattern and Information Encryption

Bioanalytical science and anti-counterfeiting applications have both focused on barcoded materials. Anti-forgery may be done using luminous barcoded materials by monitoring the emission readout. Good stability, ease of production, a large number of encoding combinations, high repeatability, and simple recognition are all factors in the demand for barcoded items. Based on mixed-Ln-MOFs that emit numerous distinct NIR signals coming from various lanthanide cations, White, Rosi, and coworkers produced the first luminous MOF barcode. [407] ErxYb1x-PVDC-1 (x = 0.32, 0.58, 0.70, and 0.81, H2 PVDC = 4,40-[(2,5-dimethoxy-1,4-phenylene) di2,1-ethenediyl] biosensorics acid), the resulting mixed-Ln-MOFs, may be sensitized by the same ligand PVDC. Both Er3+ (1530 nm) and Yb3 + (980 nm) emitted strong signals in these MOFs, and the NIR emission strength of Er3+ and Yb3+ changes linearly with their concentration in the MOF. This includes the possibility of using them as NIR barcodes. Su et al. and Yuan et al. both reported on the usage of mixed-Ln-MOFs for barcode applications. Mix-Ln-MOFs allow for the creation of as many code combinations as desired, and the readily controllable color tuning of mix-Ln-MOFs matches the need for barcoded products. Yan et al. devised an encoding approach that involves encapsulating a screen layer containing various quantities of organic dyes and modulating the emission intensity of the Ln3+ @MIL-100(In) film. Due to the reabsorbed light process, the emission intensities of Eu3+ /Tb3+ @MIL-100(In) decrease in the presence of the filtered dyes, fluorescein isothiocyanate, and methylene blue. Furthermore, the filtered emission intensities may be varied by changing the organic dye loading concentration, resulting in unique ratio metric optical codes [49].

2.4.3

MOF-Based White LEDs

CIE (0.333, 0.333) coordinates are required for optimal white light illumination, as are color rendering index (CRI) values above 80 and correlated color temperature (CCT) ranges between 2500 and 6500 K. Multichip white LEDs (also known as MC-WLEDs) and phosphor-converted white LEDs are now the two main types of commercially accessible white LEDs, based on how they were made (PC-WLEDs). There are certain advantages to using MC-WLEDs in color-rendering applications; however, it may be difficult to balance the brightness of individual chips. As a result, it is possible to overcome these limitations by using a single-component method (PC-WLEDs) and integrating trichromatic MPUs with MOFs [50]. There are a number of commercially-available phosphors on the market, including cerium-doped yttrium–aluminum garnet (YAG:Ce). In addition, the absence of red light in the spectrum results in narrow spectral widths, resulting in a poor CRI, and the emission intensity is also volatile at high temperatures. For this reason, research into novel phosphors continues apace. Because of the wide variety of MPUs available, MOF-based phosphors are particularly appealing in this industry. However, the energy transfer mechanism inside MOFs

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may become very complicated if numerous construction units are included, therefore it is still necessary to increase the quantum yield and spectrum width of MOFbased white LEDs. In the early stages of this study, white light quality is the primary emphasis. [Ag(4-cyanobenzoate)] n H2 O, a new direct white light-emitting MOF, was discovered by Guo and colleagues in 2009. The emission peaks at 427 and 566 nm are equivalent in intensity, resulting in direct white light when stimulation is approximately 350 nm. When stimulated by 349 nm light, the white light emission’s CIE coordinates are (0.33, 0.34) and (0.31, 0.33), respectively, and the outside quantum yield is 10.86%. As with the pure ligand 4-cyanobenzoate, [Ag(4-cyanobenzoate)] n H2 O has an emission peak at 427 nm that is identical to the intraligand l–l * transition of the ligand. Theoretical simulations suggest that metal-to-ligand transitions are the primary cause of the cluster of low-energy emission peaks [51].

3 Sensors and Biosensors Because the greatest MOFs comprehend aromatic sub-units for luminous emission, UV or visible adsorption, optical MOFs have gotten a lot of attention in sensing applications (Table 2). Solvatochromism, a visible color change in a substance, is one of the simplest and greatest influential detection mechanisms. A nanotubular MOF, [(WS4 Cu4 )I2 (dptz)3 ]DMFn (dptz = 3,6-di-(pyridin-4-yl)-1,2,4,5-tetrazine, DMF = N,N-dimethylformamide), was described for detection tiny solvent molecules, for example. When dissimilar solvent molecules were accommodated as guests, the resulting inclusion compounds showed diverse colors based on the solvent guests, demonstrating a new method of signal transduction as a fresh type of device [52].

4 Batteries and Supercapacitors 4.1 Introduction Renewable power made from solar and wind energies has expanded dramatically in recent decades, helping to alleviate worldwide climate modification. The mismatch among variable and irregular sources and power grid demand, on the other hand, leads to a massive loss of renewable energy. A new generation of electrical energy-storing devices with high theoretical measurements and energy density is needed to meet the growing petition for high-energy storage systems. Environmentally friendly production processes should be available as part of the development and implementation of new energy storage technologies. To solve energy and environmental challenges, sensible use of clean and renewable energy, as well as the development of sustainable energy storage systems (EES), have become a worldwide priority. Exploring large-scale EES for stable

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Table 2 Particular samples of metal–organic frameworks (MOFs) for detection applications MOF formula

Mechanism of detection Analytes

Linear range/DL References

MIL-53(Al)

Fluorescence quenching Fe3+ based on cation exchange

3–200 lM 0.9 lM

[53]

H2 dtoaCu

Fluorescence recovery after forming a rigid triplex structure

4–200 nM 1.3 nM

[54]

Adenine-based lanthanide MOF

Fluorescence Hg2+ enhancement via suppression of the PET process by coordination of Hg2+ with adenine

0.2–100 nM 0.2 nM

[55]

UiO-66

Visual detection (Olive green)

Bi3+

0.95–470 nM 5.31 nM

[56]

Fe-MIL-88NH2

Colorimetric method using MOF as peroxidase mimic

Glucose

2.0–300 lM 0.48 lM

[57]

ZIF-70

Two-electron reduction Glucose of methylene green into fluoroethylene green

0.1–2 mM Not available

[58]

1–50 nM 0.79 nM

[59]

DNA

MWCNTs@Cu3 (BTC)2 Differential pulse anodic stripping voltammetry in a lab-on-valve format

Pb2+

Au-SH-SiO2 @Cu-MOF Electrocatalytic oxidation

L-cysteine 0.02–300 lM 8.0 nM

[60]

Au-SH-SiO2 @Cu-MOF Electrocatalytic oxidation

Hydrazine 0.04–500 lM 0.01 lM

[61]

SWNT@Zn MOF

Electrochemical reduction

Methyl Parathion

0.034–6.9 lM 7.9 nM

[62]

ZIF-8 film

Refractive index

Ethanol

Not available 100 ppm

[63]

Fe-BTC

Impedance

Water

0 to 2.5 vol% Not available

[64]

and sustainable energy supply with low cost and great performance. Lithium-ion batteries (LIBs) have been widely used in portable electronics devices, drones, robotics, and electric vehicles due to their superior properties of high energy density, high output voltage, minimal memory effect, and extended cycle life [65]. Everyone knows that electrode materials are one of the most important aspects determining the electrochemical activity of batteries. Graphite has long been thought to be a potential anode material for lithium and sodium storage. Nonetheless, graphite’s theoretical specific capacity is insufficient to fulfill the battery’s rising demand for increased energy and power density. Researchers should focus more on developing

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novel anode materials that aren’t graphite. Because of their strong networks, quick electron transfers, open channels, and structural stability, microporous organic polymers have recently been widely employed as electrodes of LIBs to facilitate the movement of electrolyte ions. Since Yaghi’s group initially defined metal–organic frameworks (MOFs) in 1995 as a typical crystalline porous organic material made of metal ions/clusters and organic ligands through coordination bonds, there has been a lot of study interest in them. MOFs’ diverse structure and excellent properties, such as large surface area, porosity, tunable molecular structure, active redox activity, and rich reaction sites, enable them to be used in a wide range of research fields, including gas storage and separation, sensors, water cleaning, drug delivery, catalysis, and so on. Various MOFs and derivatives have been used as electrodes in energy storage and conversion devices in recent years summarizing the advancement of coordination chemicals in nanoarchitecture. MOFs’ electrochemical uses, on the other hand, are constrained by their weak electrical conductivity, side reactions, and low initial Coulombic efficiency. As a result, using MOFs and derivatives in LIBs remains a substantial difficulty. MOFs are a new type of hybrid crystal with luge-accessible surface areas and customizable architectures that self-assemble through coordination bonds with metal ions and organic ligands. MOF materials are widely employed in a variety of applications, including chemical sensing, drug delivery, and catalysis. Because of the appropriate pore size and shape of MOFs, Li+ and the electrolyte may easily reach the electroactive sites [66].

4.2 MOFs as Electrode Materials Also, aqueous or organic electrolytes pores the oxidation–reduction of metal ion clusters in MOFs might provide an electron mobility channel. Chen’s group was the first to use MOF-177 as an electrode material for Li supplement, demonstrating a permanent high preliminary capacity followed by a low-capacity following cycle. Then Tarascon et al. exhibited MIL-53(Fe), a conscious-type material, as an electrode material for LIBs, with an initial capacity of 80 mAh g−1 that stabilized at 70 mAh g−1 after 50 cycles. The charge and discharge curves were nicely overlaid, suggesting an impressive level of capacity retention. This result pointed to a highly reversible mechanism of Li+ ion uptake/removal and FeIII /FeII oxidation/reduction. Doublet’s group has extensively explained the lithium storage mechanism of MIL-53 (Fe)H2 O using computational calculations (DFT). Before the full dissolution of the coordination bonds inside the porous MOF architecture, there is a two-step insertion/conversion process and an unchanging Fe3+ /Fe2+ mixed-valence state. Then Tarascon’s group developed this concept to MIL-68 (Fe), though the battery presentation was disappointing. Meng’s group created MIL-101 (Fe) as electrode materials in the future, as illustrated in Fig. 2. The MIL-101(Fe) SBU is made up of carboxylate ligands and trinuclear Fe3+ metal ions, and it has three different widths of windows (6, 12, and 15), as well as two different pore diameters (29 and 34) (Fig. 2a, b). The contour plot and in operando and ex situ XANES spectra

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show the lithiation and de-lithiation process. The redox chemistry (Fe2+ /Fe3+ ) was, however, only partially reversible (Fig. 2c–e). As a result, MIL-101(Fe) had unfortunate capacity retention, with a primary coulombic effectiveness of only 79% and a coulombic effectiveness of just 23% after 72 h of rest. In this experiment, it was discovered that lithiated MIL101(Fe) had a permanent reduction mechanism that reduced reversibility. Additional sample is Vittal’s group’s discovery of the oxalatophosphate framework of Li2 (VO)2 (HPO4 )2 (C2 O4 ). This novel MOF has been evaluated for reversible lithium storage and includes lithium ions. There are extractible lithium ions in the interlayer space, as well as a V4+ /V5+ redox pair, indicating that this chemical is suited for LIB cathodes. Subsequently, ten cycles, the primary capacity of 75 mAh g−1 was determined to be stable. The second and consequent charge/discharge curves had a alike plateau, indicating that lithium insertion/extraction was reversible. With a coulombic efficiency of 97%, the charge/discharge capacities were 80 mAh g−1 after 25 cycles. Since of the limited lithium-ion insertion quantity, the permanent

Fig. 2 a MIL-101’s architecture (Fe). The SBU, to begin with (blue, Fe; red, O; stick, C; green, Cl; H is omitted for clarity). b MIL-101(Fewindows)’s and pores with the Cl and O atoms removed for simplicity’s sake. XANES of MIL-101 in operation and out of it (Fe). c–e Voltage profile as a function of scanning time for XAS in operando For the in operando XAS XANES area, the timedependent contour plot shows a change in the Fe oxidation state that may be reversed. Comparing XAS XANES spectra in operation (red) and ex situ (black) No change in the binding energy of the ex situ lithiated sample implies a time-dependent fading process. This Figure was adapted by permission from Journal Pre-proof, 2020, 31, 115–134 [67]

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reduction mechanism throughout oxidation–reduction reactions, and notably unwelcomed conductivity, MIL series are not yet suitable for lithium storage as compared to the direct usage of MOFs as cathode materials. As a result, the particular battery performance specifications, such as reversible capacity and rate capability, are not met. Direct conductive MOF and conductive substrate materials and MOF composites are widely explored to increase the conductivity of the electrode system to address the aforementioned concerns. MOFs are also being used in potassium-ion batteries (KIBs) for stationary energy storage, due to the abundance of K resources on Earth compared to Li and the redox potential of K+ /K and Li+ /Li being equivalent. Li et al. were the first to disclose L-Co2 (OH)2BDC as a KIB anode material. With values of 246 (100 mA g−1 ) and 214 mAh g−1 (200 mA g−1 ) and 100% coulombic efficiency, the as-fabricated MOF electrode demonstrated the best cycling stability. They exhibited a high capacity of 188 mAh g−1 after 600 cycles at 1 A g−1 with a high capacity of 188 mAh g−1 . They then used ex situ FT-IR/XRD characterization and theoretical calculations to study the potassium-storage mechanism in a K-MOF ([C7 H3 KNO4 ] n) based on H2 PDA. Because of the abundance of N–K/O–K coordination bonds, it demonstrated a reversible three-step redox reaction process with good rate capabilities and exceptional long-cycle capacities. Huang et al. also looked at new PBAs with ternary metal sites dubbed K2 NixCo1–x Fe(CN)6 . In KIBs, the effects of metal-incapacitating and nitrogen-containing ligands were validated.

4.3 MOFs as Host Material for Li–O2 , Zn–air, Li–S, and Li–Se Batteries 4.3.1

Li–O2 or Zn–air Batteries

The exceptionally high theoretical energy density (11,400 Wh kg−1 ) of rechargeable Li–O2 batteries has drawn greater interest, surpassing the limitations of traditional LIBs. The sole distinction between Li–O2 batteries and other lithium-based batteries is that the interaction between lithium (contained in the anode) and oxygen (existing in the air) produces the insoluble chemicals LiO2 and Li2 O2 . The following are the reaction formulae for anodes and cathodes: (1) Li–metal side (Li ↔ Li+ + e− ) Discharging: Li → Li+ + e− Charging: Li+ + e− → Li

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(2) Oxygen electrode side (2Li+ + 2e− + O2 ↔ Li2 O2 ) Discharging: (a) (b) (c) (c' )

2O2 + 2e− → 2O2(− ) + + 2Li ( ++ 2O2−)→ 2 Li − O2− ↔ 2LiO2 2 Li − O2− ↔ 2LiO*2 → Li2 O2 + O2 LiO∗2 + Li+ + e− → Li2 O2

(a) 2O2 + 2e− → 2O2 − (b)

2Li+ + 2O2 − → 2(Li+ –O2 − ) ↔ 2LiO2 (c). 2(Li+ -O2 − ) ↔ 2LiO* 2 → Li2 O2 + O2 (c’) LiO2 * + Li+ + e− → Li2 O2

Charging: Li2 O2 → 2Li+ + O2 Because the cathode uses oxygen from the air we breathe as an active substance, the density of the cathode’s capacity is potentially limitless. Furthermore, the theoretical capacity of the lithium metal electrode as the anode increases by one digit when compared to a lithium-ion rechargeable battery. There are, however, a few hurdles that must be surmounted before Li–O2 batteries may be commercialized: Oxygen gas supply is limited due to the poor interaction among oxygen gas molecules and the electrode; the electrolyte and product deposit easily block the path for oxygen gas diffusion and reaction; the reactive oxygen radical, carbon variabilities, and the reversibility of Li2 O2 formation have a negative impact on the last presentations of the devices. During a Li–O2 battery’s charge/discharge cycle, oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) cause these problems. [131] High-performance ORR and OER catalysts are essential to solving these problems. There are a number of noble metal catalysts that may increase the cycle life of Li– O2 batteries and their reversibility but they cannot guarantee the long-term stability of the batteries themselves. A further drawback of these precious metals is their prohibitively expensive price. As a result of these efforts, researchers have been focusing on developing low-cost noble metal substitutes. MoS2 , Co3 S4 , NiZnC, and Co4 N are only a few of the many metal nitrides and carbides that have been intensively explored [68]. MOFs and COFs are also being explored as electrode materials. MOFs and COFs are appropriate for exact connections with guests because of their many pores, which implies that O2 binding and electrolyte transport and product deposition may be significantly boosted. The dangling bonds at those coordinatively unsaturated locations, meantime, are critical in catalysis. When it comes to studying the topological topologies and accessible metal sites on battery performance, many MOFs have been

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employed as O2 electrode materials, for example, MOF-5 [Zn4 O(BDC)3 ], HKUST-1 [Cu3 (BTC)2 ], and M-MOF-74 (M = Mg, Mn, Co). With 3622 m2 g−1 , MOF-5 has the biggest surface area of all of the MOFs. Pores containing Cu (II) coordination centers in HKUST-1 are likely to act as binding sites for visitors. Several divalent metal ions may be found in the 1D M-MOF-74 structure (Mg, Mn, Co). Figure 3a, d MOF-5, although having the largest surface area among these MOFs, is found to have the lowest O2 absorption at 273 K at low pressure of 6.6 mg g−1 at 1 A. These pores are decorated with open metal sites, which means that these MOFs have greater O2 uptake capabilities than MOF-5, demonstrating that open accessible metal sites in these MOFs are the primary factor in boosting their O2 uptake capabilities. Since the M-MOF-74 series has the same structure, it has similar adsorption capabilities (15.0–18.4 mg g−1 at 1 A). See Fig. 3e for an illustration of what I mean). With a primary discharge capacity of 9420 mAh g1 at 50 mA g−1 , the Mn-MOF-74-Super P composite as the O2 electrode outperformed the Super P electrode. It is possible to build batteries with a discharge capacity of 4170 mAh g-1 by utilizing HKUST1 (Mg-MOF-74), Co-MOF-74 (Super P), and Mg-MOF-74 (Super P). For further information, see Fig. 3f. Super P has ample deposition sites for electrolyte as well as product in the as-prepared MOF–Super P composites electrode to offer sufficient O2 supply for Li–air batteries without considerably collective the weight and volume of the electrode [69].

Fig. 3 a The isostructural MOF-74, MOF-5, and HKUST-1 crystal structures, as well as a perspective view of the 1D channel (yellow cylinder) in the isostructural M-MOF-74 crystal structure. Blue polyhedrons and spheres represent metal atoms, red spheres represent oxygen atoms, and gray spheres represent carbon atoms. Isothermal isotherms for low-pressure adsorption of O2 for MOFs at 273 K, with the adsorption quantity stated in parentheses for 1 atm. For the Li–O2 cells utilizing MOF–Super P composites or Super P solely, f shows the discharge patterns of the cells under O2 environment at room temperature, with a current of 50 mA g−1 . This figure was adapted by permission from Journal Pre-proof, 2020, 31, 115–134 [70]

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Materials like carbonization and the combination with conductive materials are now being investigated intensively due to the low conductivity of metal–organic frameworks (MOFs). Increasing conductivity by reducing the size of pure MOFs is also an option. Two-dimensional materials have developed a hot topic in academia because of their atomic layer thickness, broadband absorption, and ultrafast optical response. They have applications in ultrafast laser generation, optical switching and modulators, optoelectronics devices, biosensor and biotherapy, and energy. Furthermore, 2D nano-sized materials have shorter mass transport pathways than their counterparts with varied dimensionalities, as has been shown in a recent study. Aside from their bigger surface area, 2D MOFs feature unsaturated metal sites, which are crucial for electrocatalysis processes. 3D metal–organic frameworks (2D Co-MOF; Ni-MOF; Mn-MOF) are manufactured utilizing a simple ultrasonic technique to improve the performance of Li–O2 batteries. Batteries using MOF air electrodes outperformed bulk Li–O2 batteries and those without MOFs in terms of rechargeability. The 2D Mn-MOF exhibited the maximum discharge specific capacity of 9464 mAh g−1 , which was substantially larger than the 2D Co-MOF (6960 mAh g−1 ) and 2D Ni-MOF (5367 mAh g−1 ). For more than 200 cycles, it was likewise stable at 100 milliamperes-per-gallon (mA-per-gallon). On TDOS basis, the 2D Mn-MOF seems to have the widest band gap among the three 2D MOFs in terms of electrochemical conductivity, which might lead to an increase in the activity of donating or receiving electrons in the OER and ORR. The OER and ORR activities of 2D Mn-better MOF bolster this argument even further. Each MOF has a high activity because of its unique internal structure. As a result of the synergistic integration of bimetal clusters, MOFs utilized in Li–O2 batteries may be limited in their performance if just a single metal atom is present in the MOF. Hydrothermal synthesis of bimetallic MnCo-MOF 74 as an electrocatalyst for Li–O2 batteries resulted in a 1:4.24 Mn to Co ratio and stable hexagonal porous rod structure for the cathode electrode. When used as an O2 electrode, the MnCo-MOF-74 cathodes had a high discharge capacity of 11,150 mAh g−1 and an excellent cyclability of 44 cycles. Co–metal clusters dissolving Li2 O2 in a discharge product test were shown to be an effective solvent for Mn–metal clusters that converted Li2 O2 to LiOH. The synergistic catalytic activity of Mn– and Co–metal clusters in bimetallic MnCo-MOF-74 significantly improved battery performance.

4.3.2

Li–S and Li–Se Batteries

The high theoretical explicit capacity (approximately 1672 mAh g−1 for sulfur and 3860 mAh g−1 for metallic Li), high theoretical energy density (around 2600 Wh kg−1 ), cheap cost, and environmental friendliness of Li–S batteries have shown promise in next-generation energy storage systems (due to the use of abundant and nontoxic sulfur). However, due to drawbacks such as the insulating nature of sulfur and polysulfide species, large volume expansion, the severe tendency of intermediate polysulfides (Li2 S2 and Li2 S) to dissolve in organic electrolytes, and low active material utilization, Li–S batteries are not used in large-scale commercial applications.

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Selenium, like Li, belongs to the same group as sulfur and has a two-electron electrochemistry. Though Li–Se batteries have a lower theoretical capacity (675 mAh g−1 ) than Li–S batteries, their volume-specific capacity (3253 mAh cm3 ) is equivalent, and the Se element has substantially greater electrical conductivity than the S element, giving Li–Se batteries a major practical advantage. Se cathode, like S cathode, has two major disadvantages: (1) The dissolution and shuttle effect of the generated polyselenides cause Se runoff, large irreversible capacity, low coulombic efficiency, and poor cycling performance; (2) The electrodes undergo a large volume expansion during the charging state due to the large difference in densities of Li2 Se (2.07 g cm3 ) and Se (4.81 g cm3 ), resulting in the pulverization of the electrode material, which further leads to low coulombic efficiency and poor cycling performance. Many studies have been conducted in trying to develop acceptable cathode materials to address such challenges, particularly the transport of soluble polysulfides/polyselenides [71]. Because of their structured pore structure and organic–inorganic contemporaneous feature, MOFs and their derivatives are regarded to be suitable sulfur host materials. Meso-MOFs, on the other hand, have a larger specific surface area than porous carbon materials and, as a consequence, a more complicated pore structure. Sulfur is equally dispersed in the micropores of mesoporous MOFs, allowing for maximal sulfur consumption and relief from volume expansion. Furthermore, considerable adsorption causes sulfur and polysulfide ions to get stuck in the mesopores of meso-MOF, preventing them from moving around. To boost electrical contact when utilized as a sulfur host material, pure MOFs are typically combined with other conductive materials. A mesoporous MIL(Cr)@rGO composite was created utilizing a two-step liquid process when employed as a cathode material in a Li–S battery, for example. In terms of discharge capacity (650 mAh g−1 ) and capacity retention rate (66.6% retention at the 50th cycle under 335 mA g−1 ) it beat the contrast of simply mixed MIL-101(Cr)/S cathode (458 mAh g−1 and 37.3% retention). In another case, MOFs/CNT (weight ratio 3:2) were used to make a foldable interpenetrated thin film with a good layered structure, which was then loaded with sulfur by a simple confinement conversion and used as a free-standing cathode for enhanced Li–S batteries. CNTs are uniformly interpenetrated into MOF crystals and interwoven into a thin film, in which CNTs improve electroconductivity and provide structural integrity, while abundant pores in MOFs and hierarchical 3D conductive networks of the composites facilitate them to well-confined S8 (the sulfur species with the highest power density) to obtain higher sulfur loading, high volumetric energy density, and good cyclability. The ability of sulfur confinement was explored using three kinds of MOFs with varied entrance sizes based on this understanding. S8 can easily pass-through entrances larger than its molecule size, as evidenced by various characterizations, and thus can be found in the cavities of MOF-5 and HKUST-1 (entrances of 0.8 nm and 0.9 nm, respectively), but not ZIF-8 due to the small entrance (0.34 nm), which is consistent with theory and confirmed by electrochemical tests. (For further information, Fig. 4a, d.) The self-supporting HKUST1/CNT as the S cathode had an extremely high capacity (1263 mAh g−1 , 0.2 C),

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225

good cyclability (0.08% capacity loss per cycle after 500 cycles at 0.2 C), and excellent rate capability (preferentially capacity of 880 mAh g−1 at 2 C, 449 mAh g−1 at 10 C), indicating good integrity and fast reaction kinetics. (For further detail, see Fig. 4c) Furthermore, increasing HKUST-1 concentration in composites increases cycle stability by confining more sulfur species but reduces capacity owing to the reduced conductivity of the insufficient conductive CNT network. Cai et al., on the other hand, described a high-conductivity MOF-Ni3(HITP)2, which they built as a cathode material for Li–S batteries, employing CNT as a conductive binder to aid electron/ion transport. The S@Ni3 (HITP)2 composite, as developed, has a high initial capacity (1302.9 mAh g−1 ), as well as better rate and cycle performance (reversible discharge capacity of 807.4 mAh g−1 ). Another good way to improve battery performance is to add functionality to it. MOFs provide unique platforms for inducing defects into the crystalline structure, resulting in additional routes for ion transport in the cathode. Thiophosphate materials can conduct Li+ , increasing the quantity of sulfur available for redox chemistry once they’re introduced into Li2 S cathodes, resulting in a higher specific capacity. In prior research, Zr-based UiO-66 was changed to have different amounts of defects while maintaining its shape and overall connectivity, with the extra labile protons introduced which can be replaced for lithium ions and get adjustable content under moderate circumstances. The performance of a Li–S battery with lithiated UiO66 as the cathode was dramatically enhanced. Even though the amount of lithium thiophosphate in Zr-MOFs is low, Baumann et al. discovered that the sulfur usage rate and polysulfide encapsulation may be greatly increased, resulting in a longterm cycle with high capacity. Furthermore, the higher the lithium concentration in lithiated Zr-MOFs, the higher the maximal release capacity. Furthermore, because to restrict influence of functionalized MOF additives, cells would not be harmed even under harsh cycling circumstances, which may also increase rate performance, which is critical for future energy storage devices.

4.3.3

MOFs as Functional Separators

Because of their extremely ordered porosity, vast surface area, and ionic selectivity based on their adjustable porosity and structure, MOFs are likewise thought to be suitable candidates as ionic sieves to selectively govern ion transit. As a result, the use of MOF materials has been studied extensively and investigated as battery separators, particularly for Li–S or Li–Se batteries. The “shuttle effects” (as noted above, induced by extremely soluble polysulfides or polyselenides) are significantly hampered, resulting in improved capacity loss of the sulfur/selenium active materials from the cathode side and enhanced battery cycle stability. Coating CNTs, graphene, or metal oxides (alleviating the shuttle effect of polysulfide compounds), adding hydroxyapatite and polyacryl (reducing flammability problems), overview of ceramic particles such as alumina or zirconia (improving wettability, thermal stability), and so on have all been developed to improve separators. However, such membranes’ porosity is frequently uneven, and they lack the capacity to transport ions quickly,

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Fig. 4 a S8-loaded MOFs/CNT composite thin films are synthesized. In the SEM photos, the scale bars are 1 mm. a The cycling and b rate presentations of S@HKUST-1/CNT electrode; c The cycling performances of S@HKUST-1/CNT, S@MOF-5/CNT, and S@ZIF-8/CNT electrodes, respectively; d The cycling performances of S@HKUST-1/CNT, S@MOF-5/CNT, and S@ZIF8/CNT electrodes, respectively. This figure was adapted by permission from Journal Pre-proof 2020, 31, 115–134 [70]

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227

resulting in slow reaction kinetics. Discovering separators that can decrease the shuttling impact but still speeding up ion transport is one of the current investigated topics. One hundred and eighty Zhou’s group was the first to publish a MOF-based ionic sieve for the Li–S battery that selectively passed lithium ions while blocking polysulfides. They developed a Cu3 (BTC)2 (HKUST-1)@GO separator that effectively prevents polysulfides from shuttling. Because Cu3 (BTC)2 has a huge number of micropores with a size window of around 9 μm. Finally, without extensive synthesis or chemical surface modification of the cathode materials, such separator-based Li–S batteries demonstrated excellent stability during long-term cycling in a Li–S battery (capacity loss rates of roughly 0.019% per cycle over 1500 cycles). The researchers then looked at an isostructural zinc framework with the same organic ligand and high-ordered micropores. The Li–S battery displayed 657 mAh g−1 beyond 1000 cycles at 1C after coating on the Celgard separator using vacuum filtering. Furthermore, the IR spectra of a cycled MOF@GO separator revealed the production of Zn–S bonds after charging, which might lower the energy barrier and increase the skeleton’s stability [72]. On the anode side, MOFs have been found to restrict the production of Li dendrites. Li and colleagues created a bifunctional separator consisting of ultrathin MOF nanosheets that suppressed Li dendrite growth while simultaneously attaching polysulfides, resulting in very safe and long-lasting Li–S batteries. Using regularly ordered cobalt atoms coupled with oxygen atoms, they employed a layer-by-layer (LBL) assembly strategy to produce a bifunctional separator (bacterial cellulose/2D MOFCo) on the surface of ultrathin MOF nanosheets (Co-O4 moieties). They discovered that the Co single-atom element mimic can efficiently anchor polysulfides via Lewis acid–base interaction, significantly inhibiting the polysulfide shuttle effect at the cathode side, and can homogenize lithium ions flux due to the adsorption of the surface O atoms, resulting in stable Li stripping and plating at the anode side. Finally, they use the bifunctional separator to produce excellent cycling stability beyond 600 cycles and a high reversible areal capacity of 5.0 mAh cm−2 (200 cycles, high sulfur loading of 7.8 mg cm−2 ). MOF-based functional separators have been demonstrated to boost Li+ transport efficiency while suppressing Li dendrites in high-energy–density lithium batteries. Shen et al. reported in situ growth of UiO-66NH2 on GF composite separators. They conclude that the UiO-66-NH2 /GF separator immobilized liquid electrolyte anions owing to unsaturated metal sites in MOFs, and that the improved Li+ transit was shown by the doubled Li+ value. Even at high current density, large overpotential, poor coulombic efficiency, and dendritic penetration issues on the anode side were handled, with a lifetime of over 1000 cycles at 4 mA cm−2 . Wang et al. developed a sulfonate (SO3 ) anionic Ui-O66 and PVDF nanoporous separator that effectively suppressed shuttle effects and promoted homogeneous Li deposition [73].

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4.3.4

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MOFs as Solid-State Electrolytes

In comparison to traditional liquid electrolytes, solid-state electrolytes (SSEs) have been a popular topic in recent years. Because of the severe flammability, poor thermal and electrochemical stability of organic liquid electrolytes, SSEs can alleviate safety problems while also providing improved power density. Polymers and inorganic ceramics are two major areas that have received a lot of attention in recent articles. Polymer SSEs include poly (ethylene oxide) (PEO), polyacrylonitrile (PAN), and poly (methyl methacrylate) (PMMA) and have strong electrode interfacial contact. Ionic conductivity, on the other hand, is inefficient unless the operating temperature window is extended to 60–80 °C. Inorganic SSEs, on the other hand, include perovskite, garnet LISICON, NASICON, and sulfides, which have similar ionic conductivity as liquid electrolytes. Poor contact and high interfacial resistance between electrodes and SSEs continue to be a significant concern. MOFs are still intriguing possibilities for SSEs materials because of their enormous surface area, and chemical, thermal, and mechanical stability. Because of the sturdy organic frameworks and open channels with ample capacity for accommodation, solid ionic conductors were constructed using a mix of MOFs and metallic salts. Long’s group pioneered the use of a lithium alkoxide within an Mg2 (dobdc) matrix, resulting in a pressed pellet with a high ionic conductivity of 3.1 10–4 S cm−1 at ambient temperature. They took the concept and applied it to solid Mg2+ electrolytes, achieving a conductivity of 2.5 10–4 S cm−1 at room temperature by cumulative the pore size of the MOF matrix or adjusting the types and concentrations of magnesium salts. Park et al. revealed a remarkable Li+ , Na+ , and Mg2+ ionic conductivity in a Cu (II)–azolate MOF with a reversible two-phase transition. MOF solid electrolytes have a conductivity of 4.8 10–4 S cm−1 when LiBF4 was added. Zou et al. incorporated LiPF6 into a porous aromatic framework (PAF1 ) as the SSEs to illustrate the mechanism of lithium salt stability inside the pores of MOFs. The LiPF6 @PAF1 produced has a good bulk conductivity of 4.0 10–4 S cm−1 . The Li+ is positioned between two phenyl rings of the tetra phenyl methane node of the PAF1 structure, according to X-ray photoelectron spectroscopy (XPS) studies and molecular dynamic simulations. Beyond 1000 cycles, the constructed LiFePO4 /LiPF6 @PAF 1 /Li demonstrated an outstanding cycling routine of 94.2 mAh g−1 .

4.4 Supercapacitors Because of rising global energy consumption and pollution, the development of an efficient and environmentally friendly energy conversion and storage technology is critical. Supercapacitors (SCs) stand out among energy storage devices because of their high-power density, quick charge and discharge capabilities, and good cycle performance. The inadequate energy density of SCs, on the other hand, restricts their practical applicability [74].

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229

Because of their outstanding porosity architectures, which may provide plentiful redox active sites and adjustable characteristics, MOFs have recently attracted a lot of attention as electrode materials for SCs. Their poor conductivity, on the other hand, obstructs electron transmission during the electrochemical reaction process, limiting their direct use in SCs. Massive attempts have been undertaken to address the aforementioned issue. Incorporating conductive additives, such as conducting polymers, is one method. Another method is to control morphology. Synthesized materials with a shape-controlled method are widely employed in a variety of applications. By regulating the solvent, varying the reaction temperature, adding the surfactant, or using the template, it has been shown that designing MOFs with unique morphologies presenting abundant exposed active sites and short distances for ion diffusion can improve the electrochemical performance of electrode materials. Zhang et al. discovered that the majority of their produced Ni-BTC had a heterogeneous morphology of spherical structures with smooth surfaces, whereas NiCo-BTC (mole ratio of Ni:Co of 2:1) had a spherical form made up of ultrathin nanosheets. At 1 A g−1 , the latter had a higher specific capacitance than the former (568 versus 407 C g −1 ). Yan et al. found that a synthesized accordion-like Ni-BTC electrode with a similar loosely packed layer structure can reach a specific capacitance of 988 F g− 1 at 1.4 A g −1 , and Zhao et al. discovered that NiCo-BTC with a similar loosely packed layer structure can reach a specific capacitance of 1067 F g −1 at 1 A g −1 . The rod-like NiCo-BTC with etched edges (mole ratio of Ni:Co = 1.5:1) had a specific capacitance of 565 F g −1 at 1 A g −1 . Zhang et al. discovered that adding BTC to Ni-PTA alters the structure of the resulting MOFs. They created an albizia flower-like spheres@nanosheets structure at an 8:2 mol ratio of PTA:BTC by altering the ratio of BTC and PTA. The nanowires atop the spheres are thought to provide plenty of redox reaction sites, while the spheres between layers operate as a spacer to keep the 2D nanosheets, which can provide rapid ion transport pathways, from clumping together. Two-dimensional vertical Co-BTC nanoplate arrays applying the polyvinylpyrrolidone (PVP) surfactant as the controller have recently been reported to have an aerial capacity of 8.56 C cm −2 at 5 mA cm−2 , which is much greater than that of faveolated Co-BTC nanosheets (2.39 C cm−2 at 5 mA cm−2 ) [75]. Furthermore, with SCs, it is assumed that the large bulk loading of active materials on the electrode promotes its area-specific capacitance. Because of the enormous mass loading, it may be able to supply massive reactive sites, which is critical for SC performance. Maintaining an ultra-fine morphology while maintaining a high mass loading of the active material, however, is a technological challenge. The high mass loading dual NiCo-MOFs nanosheets are made using Co (OH)2 as the template and PTA and BTC as mixed ligands in this study. The shape and mass loading of the produced MOFs are mostly controlled by Co (OH)2 . In addition, the mole ratio of combined PTA and BTC linkers influences the morphologies of synthesized MOFs. With an 8:2 mol ratio of PTA and BTC, the NiCo-(PTA)0.8(BTC)0.2 nanosheet electrode material has a superior areal capacitance of 5.84 F cm −2 at 1 mA cm−2 . The NiCo-(PTA)0.8 (BTC)0.2 nanosheets as the positive electrode and the produced rGO as the negative electrode of an ASC device offer an energy density of 40 Wh kg−1 at a power density of 800 W kg−1 , with a cycle stability of 97.7% after 10,000 cycles.

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Tables 3 and 4 show the application of metal–organic structures in their use in batteries and supercapacitors, respectively.

5 Solar Cells 5.1 Introduction Renewable and clean energy resources, such as wind power, biomass power, hydropower, and solar energy, have been regarded as a potential answer to meet everincreasing energy demands in recent years. Solar energy, which is the most plentiful and environmentally friendly of these energy sources, may be used to generate power without emitting greenhouse gases, toxic by-products, or noise pollution. There are three primary varieties of solar cells, depending on the photosensitive material layer utilized.

5.2 Generation of Solar Cells Solar cells of the first generation are crystalline silicon cells, which have a highpower conversion efficiency (PCE) and stability but are costly to manufacture. The second generation includes thin-film solar cells, which are more cost-effective but less efficient than first-generation cells. Third-generation solar cells include organic thinfilm/polymer solar cells, desensitized/perovskite solar cells (PSCs), and quantum dot solar cells. Because of their high PCE, availability of elemental components, low-cost, low-temperature, and scalable production process, PSCs have gained a lot of attention recently. A single PSC typically consists of a fluorine-doped tin oxide (FTO) substrate, hole block layer, electron transport layer (ETL), perovskite layer, hole transport layer (HTL), and metal electrode structure. ETLs may be made from a variety of materials, including TiO2 , SnO2 , Al2 O3 , ZnO, and ZrO2 [96].

5.3 Perovskite Solar Cells Perovskite solar cells (PSCs) have gotten a lot of interest from the material research community in recent years because of their high efficiency, low production cost, and inherent optoelectronic properties. PSCs’ power conversion efficiency (PCE) has recently grown from 3.8 to 25.5%, indicating a promising commercial future. A perovskite layer is frequently placed between an electron transport layer (ETL) and a hole transport layer (HTL). The perovskite layer absorbs photons and generates free electron–hole pairs, which are subsequently transmitted to ETL and HTL,

138.3 95.4 273 575 534.5

LMB3 SMB4 Li–Br2 5 LSB6 LIB7

Solution mixing & UV-curing

Solvothermal & Solution mixing

Solution mixing

One-step method

HKUST-1-supported PEO-based gel electrolyte

ZIF-8

Ni-MOF-1D

Mg-MOF-74/Cu

2

1

current density (mA.cm−2 ), CN: cycle number. (* aqueous redox flow batteries). Vanadium redox flow battery. 3 lithium metal batteries. 4 sodium metal batteries. 5 Secondary lithium-bromine (Li–Br ) batteries. 2 6 lithium–sulfur batteries. 7 Lithium-Ion Batteries.

87.2

11.3 Ah. L−1

VRFB2

ISE, Electrospinning & solvothermal

PAN/ZIF-8

89.1

82

88.0

98.6 90.3

Capacity retention (%)

Performance capacity (mAh.g−1 )

Batteries

Synthesis method

MOF

Table 3 The application of metal–organic structures in their use in batteries

(2000 mA. g−1 –300)

(3.0C–1000)

(1.0C/100)

(1.0C/700) (1.0C/800)

(200/600)

Cycling stability (CD/CN)1

0.01–3.0

1.7–2.8

3.0–3.8

2.5–4.3

0.4–1.2

Voltage range(V)

(continued)

[80]

[79]

[78]

[77]

[76]

References

Applications of MOFs 231

Solution mixing & Melting-diffusion

Solution mixing

MIL-101(Cr)

ZIF-67@CNF9

9

Ketjen Black (KB). Cellulose nanofiber.

LSB

Solvothermal & Solution mixing

MIL-125(Ti)

8

LSB

Water solution reaction at room temperature

Co–N/KB8

LIB

LSB

LSB

Solvothermal & Solution mixing

MIL-101(Fe)–NH2

Batteries

Synthesis method

MOF

Table 3 (continued)

142.12

635

726

705

705

Performance capacity (mAh.g−1 )

88.41

83

60

81.1

94.8

Capacity retention (%)

(0.5C–100)

(3.0C–500)

(0.2C–200)

(0.5C–200)

(0.5C–200)

Cycling stability (CD/CN)

2.5–3.8

1.7–2.8

1.7–2.7

1.7–2.8

1.8–2.8

Voltage range(V)

[85]

[84]

[83]

[82]

[81]

References

232 M. M. Salehi et al.

Solution mixing

ZIF-67/PEDOT 1926

Liquid–liquid NA interfacial reactions

Zn-pPDA MOF

258.81

{Cu2 SiW12 O40 }@HKUST-1 One-step solution method

Template-controlled NA in situ growing

1063.2

Solution mixing & Electrochemical deposition

C-ZIF-8@MWCNTs

NiCo-MOF//rGO

Solvothermal & 583.0 Electrospinning followed by thermal annealing

ZnO/P-PAN@ZIF-8 nanofiber

PVA/1.0 M H2 SO4

1.0 M KOH

1.0 M KOH

1.0 M KOH

1.0 M H2 SO4

1.0 M H2 SO4

Surface Electrolyte area (m2 .g−1 )

Method of Synthesis

Electrode material (MOF-based)

1.0 A. g−1

1.0 A. g−1

1.0 A. g−1

1.0 A. g−1

0.5 A. g−1

10 A. g−1

106.8 F. g−1

200.86 F. g−1

113 F. g−1

5096.5 F. g−1

259.2 F. g−1

NA

1.0

93% (4000) 1.6

∼11 Wh. kg−1 & 200 W. kg−1

0.6

96% (2000) 1.5

97.7% (10,000)

92% (6000) 1.0

92% (5000) 1.0

98.8% (100,000)

(continued)

[91]

[90]

[89]

[88]

[87]

[86]

Capacitance Potential References retention window(V) (w.r.t cyclic stability)

62.8 Wh. kg−1 & 4500 W. kg−1

40 Wh. kg−1 & 800 W. kg−1

15.31 Wh. kg−1 & 510.81 W. kg−1

NA

NA

Current Specific Energy density capacitance density and power density

Table 4 The application of metal–organic structures in their use in supercapacitors

Applications of MOFs 233

Method of Synthesis

Scalable templated method

Hydrothermal & Solution mixing

“Bottle around ship” method

Solvothermal & electrochemical method

Electrode material (MOF-based)

ZNC ZIF-67

Ni doped ZIF-67/rGO

NiV LDH@ZIF-67

Ce-UiO-66/TNF

Table 4 (continued)

1021.3

267

369.1

65.9

1.0 A. g−1

1.0 A. g−1

0.1 A. g−1

42.3 Wh. kg−1 & 520.6 W. kg−1

21.5 Wh. kg−1 & 1000 W. kg−1

27.94 Wh. kg−1 & 1300 W. kg−1

120% (5000)

2.0

1.8

87% (4500) 2.0

99% (5000) 1.5

[95]

[94]

[93]

[92]

Capacitance Potential References retention window(V) (w.r.t cyclic stability)

1385 F. g−1 123.11Wh. 95% kg−1 & (10,000) 6400 W. kg−1

530.5 F. g−1

304 F. g−1

247 F. g−1

Current Specific Energy density capacitance density and power density

Fe(CN)6 3− /4− 4.0 A. g−1

6.0 M KOH

1.0 M H2 SO4

6.0 M KOH

Surface Electrolyte area (m2 .g−1 )

234 M. M. Salehi et al.

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235

respectively, and a photocurrent is created. Because it may improve carrier extraction efficiency and minimize interface charge recombination by optimizing hole transport materials, the HTL is critical in planar PSCs (HTMs).

5.4 Effects of MOF on Perovskite Solar Cells MOFs are porous organic–inorganic hybrid materials having metal ion centers and organic ligands. MOFs, as a composite material, may be able to take use of the synergetic effects of each element. First, metal ions or functional groups (such as carbonyl groups, amino groups, and so on) may efficiently passivate perovskite films; second, most MOFs can filter ultraviolet (UV) light owing to the conjugation of the framework. MOFs can be used at the substrate surface to direct film growth, hence influencing residual stress in the resulting film from a mechanical standpoint. Sargent et al. recently showed that releasing residual tensile strain might minimize the rate of perovskite disintegration, and our group also reported that strain engineering enhanced the carrier transportation dynamic and increased device photovoltaic performance [97]. PSCs doped with MOFs have been created to further increase chemical and thermal stability. MOFs, also known as porous coordination polymers (PCPs), are crystalline materials that develop a periodic infinite network structure by the selfassembly of metal ions or metal clusters with organic ligands. As a result, it possesses both organic polymer and inorganic compound properties. MOF compounds have demonstrated their unique physical and chemical characteristics and tremendous prospective application value in various areas such as magnetism, fluorescence, nonlinear optics, adsorption, separation, catalysis, hydrogen storage, and so on in recent decades as a new study topic. By altering the kinds of ligands, adjusting the functional groups in the ligands, and doping with various metal ions, researchers may create functionalized MOFs for specific applications. As a result, researchers in a variety of domains have embraced MOF materials. The use of MOF materials in PSC has also received a lot of attention. The four kinds of structures present in perovskite cells include mesoporous n-type semiconductor structures, mesoporous insulating metal oxide structures, frontal planar heterojunction structures, and trans planar heterojunction structures. Perovskite is infiltrated into a mesoporous skeleton, which can be an n-type semiconductor or an insulating metal oxide, in mesoporous structures. The perovskite HTL and electrode are located above the mesoporous framework, while a dense hole barrier layer and the conductive substrate are located below it. The mesoporous structure aids in the formation of a homogenous perovskite coating and may also aid in electron transport. PSCs doped with MOFs have been created to further increase chemical and thermal stability. MOFs, also known as porous coordination polymers (PCPs), are crystalline materials that develop a periodic infinite network structure by the selfassembly of metal ions or metal clusters with organic ligands. As a result, it possesses

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both organic polymer and inorganic compound properties. MOF compounds have demonstrated their unique physical and chemical characteristics and tremendous prospective application value in various areas such as magnetism, fluorescence, nonlinear optics, adsorption, separation, catalysis, hydrogen storage, and so on in recent decades as a new study topic. By altering the kinds of ligands, adjusting the functional groups in the ligands, and doping with various metal ions, researchers may create functionalized MOFs for specific applications. As a result, researchers in a variety of domains have embraced MOF materials. The use of MOF materials in PSC has also received a lot of attention (Table 5.).

6 Fuel Cells 6.1 Introduction By 2050, it is anticipated that the world’s energy supply would have to double. The demand for clean and alternative energy sources is and will continue to be the most pressing challenge facing science in the twenty-first century. Fuel cells fueled by hydrogen are one of the finest alternatives to fossil fuels since they provide a clean and carbon-free way of transforming chemical energy into electrical energy. Furthermore, they are nearly twice as efficient as fossil fuels (60% for fuel cells vs. 34% for fossil fuels). The low efficiency of fossil fuel combustion engines can be attributed to their compliance with the Carnot cycle regulations—fuel cells are not subject to such laws. If H2 is to become the ideal fuel, working fuel cells must be available to help it realize its full potential. Polymer electrolyte membrane fuel cells (PEMFCs), direct methanol fuel cells (DMFCs), alkaline fuel cells, solid oxide fuel cells, phosphoric acid fuel cells, and molten carbonate fuel cells have all been proposed so far. The exact technologies of these fuel cells differ, but the core working concept—turning chemical energy straight into electricity—remains the same. The majority of these fuel cells use hydrogen as a fuel, although some, such as the DMFCs, use different substrates. This article will concentrate on fuel cells that use hydrogen as fuel, with a concentration on PEMFCs. Because the water—hydrogen cycle is closed, the H2 and O2 formed from water splitting can be recombined into water with the release of energy, thus refilling the water required in their synthesis [108]. However, such sustainability can only be maintained if renewable methods for water splitting, such as photocatalysis and electrocatalysis, are used; fossil fuels cannot be used in the process. Inevitably, more energy is required to split water than can be created during its recombination—current approaches can split water with an efficiency of approximately 80% but only generate electricity with an efficiency of about 60%. However, it should be emphasized that this is still far more efficient than the 34% efficiency achieved by fossil fuels, making H2 a very appealing option. PEMFCs are the most prevalent type of fuel cell, both in research and commercial applications. They are low-temperature fuel cells that run between 85 and 105 degrees Celsius. The protons

power conversion efficiency.

short-circuit current density.

open-circuit voltage.

fill factor.

12

13

Room-temperature ultrasonic, spin-coating & ZIF-8 were synthesized via a simple one-pot reaction in water at room temperature

FTO/c-TiO2 /ZIF-8/Cs/MA/FA/Spiro-OMeTAD/Au

11

Several steps of spin-coating & thermal evaporation

FTO/c-TiO2 /mp-TiO2 /ZIF-8/MAPbI3 /Spiro-OMeTAD/Au

10

23.71

Several steps of spin-coating & thermal evaporation



16.8

16.99

22.16

22.90

Thermal 21.28 evaporation, spin-coating & 2D-functionalized MOFs through a solvothermal method

FTO/PEDOT: PSS/MAPbI3 + Zn-cbpp/Spiro-OMeTAD/Au

Jsc 11 (mA/cm2 )

21.8

22.82

22.58

PCE10 (%) 22.02

Fabrication Method Solvothermal & Several steps of spin-coating

Device structure

ITO/PTAA/Cs0.05( FA0.85 MA0.15 )0.95 Pb(I0.85 Br0.15 )3 /PC61 BM/ZrL3: bis-C60/silver (Ag)

Table 5 The application of metal–organic structures in their use of solar cells Voc 12 (V)

1.23

1.02

1.14

1.108

1.20

FF13

59%

73%

82%

77.2%

81.28%

References

(continued)

[102]

[101]

[100]

[99]

[98]

Applications of MOFs 237

Fabrication Method Several steps of spin-coating, thermal evaporation & ZIF-8 were synthesized in an aqueous solution at room temperature Spin-coating & MOF was synthesized through a solvothermal method Spin-coating & MOF was synthesized through a solvothermal method Several steps of spin-coating, thermal evaporation & ZIF-8 were synthesized through a hydrothermal method NH2 -MIL-101(Fe) was synthesized by hydrothermal reaction, Several steps of spin-coating & thermal evaporation

Device structure

FTO/ZIF-8-derived porous carbon layer/TiO2 /MAPbI3 /Spiro-OMeTAD/Au

FTO/c-TiO2 /Cs/FA/MAPbI2 Br /Spiro-OMeTAD/Zn-CBOB/Au

FTO/c-TiO2 /PEIE-2D MOF /FA0.25 MA0.75 PbI3 /Spiro-OMeTAD/Au

FTO/NiOx/Cr-MOF-CsPbI2 Br/ZnO/ZnO@C60 /Ag

FTO/C-TiO2 /PC61 BM/Cs0.05 FA0.81 MA0.14 PbI2.55 Br0.45 /Spiro-OMeTAD/Li-TFSI@NH2 -MIL-101/Au

Table 5 (continued)

19.01

17.02

22.22

20.64

17.32

PCE(%)

23.41

16.51

25.36

23.17

22.13

Jsc (mA/cm2 )

1.073

1.30

1.111

1.135

1.06

Voc (V)

75.7%

79%

79%

78.4%

72%

FF

[107]

[106]

[105]

[104]

[103]

References

238 M. M. Salehi et al.

Applications of MOFs

239

pass through an electron-insulating but proton-conducting polymer membrane at the cell’s center, while the cathode moves via an electrical circuit carrying an external load. The electrons and protons are accepted by O2 at the cathode, and it is converted to water. There are currently a few restrictions with PEMFCs. Because both the anode and cathode reactions are intrinsically slow, fuel cells are compelled to use expensive noble metal catalysts like platinum or platinum alloys to speed them up. The prohibitively high cost of noble metal catalysts, on the other hand, prevents their broad commercial use. Furthermore, carbon monoxide, which can be present in the fuel if it originates from fossil fuels, can easily poison platinum-based catalysts. As a result, non-platinum electrocatalysts are receiving a lot of attention. In PEMFCs, the polymer electrolyte membrane (PEM) also plays an important role. It must be proton-conducting while also being electron-insulating, allowing protons to flow from anode to cathode while forcing electrons to pass through the external circuit. In order to be commercially successful, the membrane must be inexpensive, corrosionresistant, and an excellent proton conductor. Another concern is the supply of H2 and O2 , which are needed as reactants in PEMFCs. H2 must not come from fossil fuels for fuel cells to be fully carbon-free and environmentally friendly. Furthermore, using fossil fuels as a source of H2 will open the closed water–hydrogen cycle, which was one of the primary benefits that prompted the use of H2 as a fuel in the first place. As a result, renewable techniques of creating H2 such as photocatalysis or electrochemical water splitting are appropriate. The goal of current research is to develop effective catalysts for such water-spitting reactions. However, a number of obstacles stand in the way of large-scale commercialization of fuel cells, including the high cost of electrocatalysts, the need for substantial fuel output, and portability concerns. As a result, fuel cells must be made more affordable and have better performance before they can be used widely. Safe storage and effective distribution of H2 have been intensively pursued in fuel cell technologies, as it is a key enabling technology in fuel cells. Because H2 is an exceedingly volatile gas in ambient settings, its volumetric energy density is insufficient for practical applications. If H2 is to be employed as a source of energy for mobile applications, greater focus must be made to denser storage technologies. Commercial H2 storage currently uses two technologies: high-pressure compression and liquefaction. The energy efficiency of liquefaction and compression of hydrogen, as well as safety issues with compressed hydrogen, significantly limit the use of these two H2 storage technologies.

6.2 H2 Production from Water Splitting Using MOFs In the recent year’s MOFs, Researchers have recently succeeded in using as photocatalysts to create H2 . Kataoka et al. published one of the earliest reports. Ru-based MOFs have been shown to be capable of creating H2 from water via photocatalysis. In the presence of Ru(bpy)3 2 (bpy = 2,2 -bipyridine) as a photosensitizer, methyl viologen (MV2+ ) as an electron relay, and EDTA as a sacrificial donor, the RuMOFs served as activity sites for the photochemical reduction of water into H2 .

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The Ru-MOFs were further modified to boost photocatalytic efficiency by altering the counter ions inside the framework. Three heterogeneous (Ru2 (bdc)2 X), bdc = 1,4-benzenedicarboxylate (X=Cl, Br-BF4 ) were used to study the photochemical generation of H2 from water. The Ru–Cl and Ru–Br complexes feature a 3D jungle gym-like structure, in which the Clor Brcounter ions bridge a 2D square grid sheet of Di nuclear RuII,III paddle wheel motif including abdc linker, extending the 2D structure into a 3D motif. H2 generation was impacted when Ru-MOFs were combined with Ru(bpy)3 2+ , MV2+ , and EDTA. The MV2 + molecules were adsorbed on the Ru-MOFs’ surfaces rather than in the voids. As a result, the variation in catalytic performance between the three MOFs is due to surface alteration, which can be traced back to counter-ion selection. Ru2 (bdc)2 Br, the highest active catalyst reported in this study, has a turnover number (TON) of 18.7. This activity was 16.5 times higher than anatase TiO2 and 1.37 times higher than Pt(bpy)Cl2 . Photochemical generation of H2 from water under visible light irradiation was reported using an Rh2 (bdc)2 MOF in the presence of Ru(bpy)3 2+ , MV2+ , and EDTA, similar to the Ru-MOFs. The MOF was stable in the catalytic process, and the reaction was terminated due to other variables such as MV2+ hydrogenation.

6.3 H2 Production from Ammonia Borane Andorganosilanes Using MOFs MOFs are used to produce H2 from ammonia, borane, and organ silanes. Apart from water, ammonia borane (AB) is gaining popularity and attracting researchers’ interest as a viable on-board fuel for fuel cells. Because it is stable in air and has a mild dehydrogenation temperature, AB qualifies as an on-board H2 storage medium. In addition, it contains a high hydrogen concentration of 19.6% or 140 g/L. The stored hydrogen in AB can be released by thermolysis or chemical interaction with water. Surprisingly, at the fuel cell working temperature of 100 °C, AB has sluggish H2 release kinetics, as well as the generation of undesired side products such as ammonia, borazine, and diborane during its decomposition. The electrocatalysts are poisoned by these volatile by-products. Reducing the dehydrogenation temperature to 85 °C, dramatically boosting the H2 release rate, and avoiding the generation of hazardous by-products are all technological issues that must be handled. Several studies have shown that MOFs have the potential to solve the problems outlined above and improve the practicality of AB as an on-board fuel. The employment of a MOF in combination with the nanoconfinement of AB improved H2 release kinetics and prevented the generation of hazardous by-products, according to Li et al. Xray diffraction experiments showed that the structure of JUC-32-Y (Y-btc) remained intact when AB was infused into it to create AB/JUC-32-Y. Thermal degradation of the confined AB took place at temperatures as low as 50 °C, compared to the 112 °C necessary for pure AB decomposition. At 85 °C, AB/JUC-32-Y emitted 8% of hydrogen in 10 min and 11% in 3 h. When the temperature was increased to 95 °C,

Applications of MOFs

241

AB/JUC-32-Y released all of its hydrogen (13%) in 3 h. The development of the undesired by-product ammonia was reported to be absent. Unsaturated Y3+ metal centers in JUC-32-Y are thought to interact with the NH3 moiety in AB, strengthening the BN bond and preventing ammonia production. As a result, MOFs are natural candidates for research into this synergistic impact. This is due to the inherent nature of MOFs.H2 release from AB-JUC-32-Y and tidy AB is time-dependent. Ref. is reprinted with permission. American Chemical Society, 2010. permeable, making it ideal for AB nanoconfinement. Furthermore, their metal nodes might act as catalysts, fostering the previously discovered synergistic collaboration between nanoconfinement and catalyst to improve H2 release kinetics. Small molecules like AB may easily enter the inner surface of microporous MOFs and react with the active metal sites thanks to the uniform pathways present. The porosity of MOFs also encourages homogeneous dispersion of reactant AB molecules across the framework, hence increasing the reaction surface area.

6.4 MOFs as Oxygen Reduction Reaction Catalysts The electrons lost at the cathode are rejoined with the H+ ions produced during the oxidation of H2 at the anode. They make water when they interact with O2 . This is the oxygen reduction process (ORR) that takes place at a fuel cell’s cathode. Because of ORR’s slow kinetics, it’s usually catalyzed by Pt-based electrocatalysts. In H2 fuelcells, ORR is the rate-determining step in comparison to H2 oxidation at the anode. As a result, sluggish ORR might be regarded the Achilles heel of fuel cells, preventing them from obtaining their maximum efficiency. Furthermore, the exorbitant cost of Pt-based ORR electrocatalysts used in modern fuel cells accounts for half of the fuel-stack cost. As a result, much work has gone into developing new electrocatalysts for the ORR in fuel cells in order to improve their practical application. Non-platinum group metals (non-PGM) materials are more attractive than other suggested electrocatalysts, primarily because they are more easily accessible and less expensive. However, non-PGM materials are often less effective, necessitating the employment of larger quantities of catalysts to reach a good catalytic efficiency, resulting in a thick electrode layer with poor mass transfer. As a result, the current challenges are to attain larger catalytic site densities and better catalytic powers utilizing non-PGMORR electrocatalysts. Electrocatalysts should also be equally dispersed across the electrode and conveniently accessible by gaseous and liquid substrates [109].

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6.5 MOFs as Proton-Conducting Polymer Electrolyte Membranes The effectiveness of PEM is required for PEMFCs to work. Aside from minimizing dangerous fuel crossover, which results in voltage loss, the PEM must also be proton conducting while being electrically insulating. The Grotthuss mechanism and the vehicle mechanism are thought to be responsible for proton conduction across the PEM. Both include proton carriers, however, the proton carriers in the vehicle mechanism are mobile (as the name implies), whereas the proton carriers in the Grotthuss mechanism are stationary [110].

6.6 MOFs as H2 Storage Medium H2 storage is another area of study that is significant to fuel cell technology. Commercial H2 storage currently uses two technologies: high-pressure compression and liquefaction. The high energy consumption of compression and liquefaction, the constant boil-off of liquid hydrogen, and the safety considerations associated with compressed H2 restrict the applicability of these two H storage systems. As a result, researchers have focused on solid-state storage technologies. If H2 is to be extensively used as a clean source of energy, new materials capable of holding it at high gravimetric and volumetric densities are needed. The storage of hydrogen in solid materials can be divided into two categories: (1) chemical storage, in which hydrogen is stored in a chemical compound and released via a chemical reaction and (2) physical storage, in which hydrogen is either adsorbed onto the surface of a solid material or captured by cages. The ability of a porous substance to store H2 is mostly determined by its surface area and pore volume. MOFs have lately emerged as one of the most promising candidate materials for H2 storage because of their unusually large surface areas, highly consistent pore densities and dimensions, and diverse and highly programmable chemical characteristics and topologies. Only representative examples are mentioned in this section to illustrate the progress and prospects of the field, the relationships between structural features and the enthalpy of H2 adsorption, and strategies for improving storage capacity, in light of the exploding research activities and several excellent reviews published recently on this topic [111]. Rosi et al. were the first to investigate the cubic carboxylate-based framework MOF-5 (Zn4 O(BDC)3 ) as an H2 storage material in 2003. MOF-5 proved to be the greatest cryogenic storage material presently available. It was also shown that H2 may be injected into a cold compound sample in under 2 min and can be totally desorbed and re-adsorbed for at least 24 cycles without losing capacity. Hundreds of different MOFs have subsequently been investigated, and several computational studies have been conducted to try to influence the design of MOFs for H2 storage and simulate H2 adsorption data in MOFs. Surface area, pore size, and pore volume of MOFs have been widely explored since H2 absorption is directly related to the surface area of a

Applications of MOFs

243

material in physisorption. The physisorption of gases is known to be pressure- and temperature-dependent. H2 uptake seems to be linked to the heat of adsorption at low pressures, surface area at middle pressures, and free volume at high pressures in MOFs. Designing MOFs with the same topology but various organic ligand lengths is a systematic way to study the influence of pore size and shape on surface area and H2 uptake. For example, tetracarboxylate ligands were used to build a variety of NbO-based MOFs. The window size and surface area (BET surface area) are raised when the ligand length is increased. As a consequence, H2 uptake decreased somewhat at low pressures but increased dramatically above 20 bar [112].

7 Energy Storage and Conversion Huge quantities of fossil fuels have been used to fuel the economy and provide unparalleled riches to human civilization since the industrial revolution. Fossil fuels including coal, natural gas, and petroleum still account for a majority of the world’s overall energy consumption (more than 80%). These fuels must be burned in order to release a significant quantity of CO2 , which is one of the primary greenhouse gases responsible for global warming. A low-carbon emission economy that increases the use of renewable and environmentally friendly energy sources and storage technologies is a comprehensive approach against climate change. As a consequence, enormous efforts are being made to enhance environmentally friendly energy storage and conversion technologies, including fuel cells, lithium-ion batteries, hybrid supercapacitors, and water-splitting electrolyzes. These technologies must, however, continue to be improved upon in order to be used on a big scale and at cheap cost. Nanostructured electrodes or catalysts, especially in porous and/or hollow forms, have garnered a lot of interest for their ability to facilitate electrochemical processes and maintain structural stabilities. Due to their high surface-to-mass/volume ratio, interparticle pore channel/void, tiny grain size, and sturdy secondary architecture, these materials have various structure-dependent benefits. High surface area specifically offers a lot of active areas for activities including surface redox reactions, insertion/deinsertion, adsorption/desorption, and heterogeneous catalysis. For loading guest species, tolerating mechanical stresses, and enabling mass movement, large pore volume, and open pore structure is crucial. In porous and hollow particles, the solid-state diffusion distance (i.e., the thickness of the wall or shell) is substantially lower than the size of the particles, overcoming the diffusion constraint in nonporous materials. The confinement capabilities of pore channels and cavities also show considerable potential for specific applications. More crucially, unlike their bulk counterparts, nanostructured materials do not have exceptional size-dependent physicochemical and chemical characteristics. There is a great deal of interest in both fundamental research and practical applications due to the increasing complexity of nanostructures in terms of architecture, structural subunits, and chemical composition. Complex nanostructures may have properties that are needed for a variety of present and future applications with a careful and logical design. [44, 45] Complex

244

M. M. Salehi et al.

architectures often perform better than simple nanoparticle-assembled systems in electrochemical applications. For instance, it is anticipated that highly exposed active sites on complicated porous structures made of secondary subunits would optimize the electrocatalytic activity. Furthermore, compared to their single-shelled counterparts, multishelled hollow particles as charge storage materials may increase the volumetric energy density owing to the optimized unoccupied space and relatively high active component percentage. However, much more difficult synthetic hurdles arise when trying to build complex nanostructures in a controlled manner. MOFs, a significant subset of coordination polymers (CPs) with crystalline structures and possible porosity, have developed into a quickly growing study field during the last two decades [113]. The most notable benefit of MOF materials is the ability to create frameworks with different functional species of metal ions or clusters and organic linkers using a modular selfassembly technique. Additionally, MOF-derived nanostructured functional materials have proved crucial in energy-related applications. MOFs may be readily transformed into inorganic functional materials by pyrolysis in an inert environment or chemical reactivity with the right reagents because of their inherent thermal and chemical instability. MOF precursors might be converted into various porous or hollow nanostructures, carbon-based chemicals, and their composites, depending on the conversion technique. In addition to employing straightforward MOF nanoparticles as precursors, complex nanostructured materials with customizable structures and compositions might be created by carefully designing and integrating MOFs and functional nanomaterials into unique multi-compositional MOF-based nanocomposites. These MOF-derived complex nanostructures may provide a variety of benefits depending on their composition and structure, providing possible answers to the major difficulties in electrochemical applications for renewable energy [114].

8 Molecular Transport Transparency is a common design aspect in today’s world, and it’s also a major property of many everyday materials like glasses, plastics, and crystals. As the need for intelligent consumer items grows, there is a trend toward designing transparent electronics that are compatible with lens display panels, French windows, and automobile windshields, among other things. Integrating transparent electronics into personal gadgets to provide real-time monitoring of biological signals such as pulse, respiration, and blood pressure, as well as environmental factors such as temperature, humidity, and gaseous pollutants in the air, is very appealing. Transparent electronics has two important components: supporting materials like substrates and electrodes, and core functional units that provide electronic responses to external inputs. Despite the fact that a few materials are easily accessible and match the industry criteria for substrates and electrodes, namely, transparency of 90% at 550 nm and sheet resistance of less than 100 Ω sq−1 ; progress on transparent core functional units is modest.

Applications of MOFs

245

This is due to the restricted number of appropriate materials that are highly conductive, sensitive, and operate at room temperature without losing transparency. MOFs are a type of porous crystalline material made up of functional molecular building pieces, and many of them are transparent to visible light by nature. MOFs are utilized in systems for monitoring the gaseous environment because of their unique interactions with gas molecules. MOFs are difficult to produce into centimeter size without cracks, bubbles, or irregular grain boundaries, which reduce their transparency. The recent discovery of glassy MOFs is a possible answer, and some of them have persistent porosity. Although the transparency of these MOF glasses can theoretically be improved, their brittleness and low intrinsic electrical conductivity limit their use in transparent electronics [115]. Jie Wu and his colleagues show that by utilizing MOFs, they can achieve the requirements for transparency, conductivity, and electrochemical responsiveness at room temperature simultaneously (Fig. 5). This is accomplished by epitaxially growing MOFs on single-layer graphene (SLG), resulting in a MOF-on-SLG construct. The Ni-CAT-1-on-SLG construct was created using a MOF consisting of triphenylene units, Ni-CAT-1, which has a modest lattice misfit to graphene (1.08%), demonstrating its capabilities as transparent electronics for the detection of NH3 , CO, and O2 . The thickness of the MOF layer was precisely controlled, progressively decreasing from 170 to 10 nm, resulting in the desired transmittance of 95.7% for the complete Ni-CAT-1-on-SLG-10 nm build. The electrical conductivity of MOFs guided by SLG was 4.0 × 104 S m−1 , 7 orders of magnitude greater than that of pristine MOFs and 6 orders of magnitude higher than that of randomly oriented MOF on SLG. A linear signal range of 10–108 ppb enabled a direct and precise electrochemical readout of gas concentration on a ppb level at ambient temperature. Due to their sensitive gas identification, conductivity, and transparency, these MOF-on-SLG constructions were exhibited in transparent electronics as functional units for real-time gas monitoring at ambient temperature. Furthermore, these structures worked with various transparent substrates, including two flexible ones, polyethylene terephthalate (PET) and polydimethylsiloxane (PDMS), which are perfect for personal gadgets. The devices illustrated here have a maximum size of 3,5 cm and could fold continuously at 3 mm radii more than 200 times without compromising conductivity or transparency. Although MOFs have been successfully incorporated into substrates to produce excellent transparency, these composites are not electrically conductive. Although conductive elements such as graphene oxide, carbon nanotubes, graphite, and polymers have been utilized to increase the conductivity of MOFs, their transparency to visible light has scarcely exceeded 70%. With strong conductivity and transparency of 97.3% at 550 nm, SLG is a good contender for transparent electronics. The matching of lattice characteristics and symmetry, in this case, enables the epitaxial development of MOFs on SLG with fine thickness control. Furthermore, transparent electronics based on these structures can be flexible and capable of real-time gaseous environment monitoring at ambient temperature, indicating MOFs’ possibilities in transparent personal devices. Table 6 shows the application of metal–organic structures in their use in molecular transport.

246

M. M. Salehi et al.

Fig. 5 Three criteria for transparent electronics for the resistance-based monitoring of a gaseous environment: conductivity, response, and room temperature operation, were fulfilled by MOF-onSLG constructs. This figure was adapted by permission from Advance Science News, 2020, 7, 2 [116]

9 CO2 and N2 Reduction 9.1 Introduction Fossil fuels account for 86.4% of worldwide energy use, according to estimates. Carbon emissions (e.g., CO2 , CO, and CH4 ) have grown as a result of the usage of fossil fuels, resulting in major environmental pollution challenges. Every year, billions of tons (21.3) of CO2 are emitted into the atmosphere, with only half of that CO2 being absorbed by natural processes, resulting in a net increase of 10.65 billion tons of CO2 in the atmosphere. Furthermore, the amount of fossil fuels available on the planet is finite and will most certainly be depleted in the near future, if necessary, precautions are not taken. As a result, climate change is one of the most pressing issues confronting humanity today. This is due to rising global temperatures caused by rising CO2 levels in the atmosphere, which have crossed 400 ppm (a 40% increase since 1750) and are increasing at a rate of 2 ppm/year. As a result, global research efforts are focused on the development of alternative energy sources. To fulfill future energy demands,

3.641 Å

10.4 Å

1610



2248.69

627

1029.1

700

MnP3nF714

PHZ15 /Pebax MMMs

MKP-PVAm/mPSf MMM

CAU-10-NH2

JXNU-6

Ni(tzba)0.5 (F)(bpy)].DMF. 0.5H2 O

15

14

3.6 Å

460

GEFSIX-2-Cu-i

C2 H2 /CH4 C2 H4 /CH4 C2 H6 /CH4

C3 H4 /C3 H6

C2 H2 /CO2

CO2 /N2

CO2 /N2

CO2 /CH4

CO2 /H2 CO2 /CH4 CO2 /N2

Separated of molecules

Matrimid/NH2 -PVDF (97/3) + 7 wt% NH2 -MIL-101(Cr). polystyrene-acrylate (PSA) modified hollow ZIF-8 (PHZ).



0.997 nm





Pore Size

Surface area (m2 .g−1 )

MOF

44.8 10.0 16.9

99

10.9

242

103.9

65.6

17 18 57

Selectivity (50/50)



C2 H2 (171 cm.cm−3 ) C2 H4 (101 cm.cm−3 ) C2 H6 (115 cm.cm−3 ) CH4 (35 cm.cm−3 )





C2 H2 (0.46cm3 . g−1 ) C3 H4 (90.7cm3 . g−1 ) C3 H6 (128.8 cm3 . g−1 )

CO2 (823 Barrers)

CO2 (236.5 Barrers)

CO2 (19.01 Barrers) CH4 (0.29 Barrers)

CO2 (460 Barrers)

Permeability

CO2 (2366 cm3 . g−1 ) N2 (1867 cm3 . g−1 )





CO2 (5.02) H2 (0.11) N2 (0.25) CH4 (0.51)

Uptake (mmol. g−1 )

Table 6 The application of metal–organic structures in their use in molecular transport

C2 H2 (36.7) C2 H4 (31.9) C2 H6 (23.1) CH4 (17.8)

C3 H4 (39.9) C3 H6 (27.1)

C2 H2 (31.3)







CO2 (34.5)

Qst (kJ. mol−1 )

273

298

298

298

298

308

298

T (K)

(continued)

[122]

[121]

[120]

[119]

[118]

[117]

[116]

References

Applications of MOFs 247

23.87 Å

3.9 Å

667

694

360

ATC-Cu

Co(pyz)[Ni(CN)4 ]

Ce(IV)-MIL-140-4F

3.4–4.3 Å

Pore Size

Surface area (m2 .g−1 )

MOF

Table 6 (continued)

CO2 /C2 H2

C2 H2 /CO2 C2 H2 /C2 H4 C2 H2 /CH4

C2 H2 /CO2

Separated of molecules

41.5

36.5 24.2 1312.9

53.6

Selectivity (50/50)

CO2 (151.7 cm3 .cm−3 )

C2 H2 (3.35)

C2 H2 (5.01)

Uptake (mmol. g−1 )





Permeability

C2 H2 (27.4)

C2 H2 (65)

C2 H2 (79.1)

Qst (kJ. mol−1 )

273

296

298

T (K)

[125]

[124]

[123]

References

248 M. M. Salehi et al.

Applications of MOFs

249

a clean, inexpensive, safe, viable, and renewable source of energy that is environmentally friendly is required. Solar power is a limitless, renewable energy source that generates far more energy than the total quantity of energy produced annually by humans. Solar energy conversion to chemical energy is thought to be particularly advantageous for achieving long-term energy generation. Innumerable materials and devices have been developed since the first discovery of UV light-driven water splitting in 1972, but it is extremely difficult to develop novel, eco-friendly, and highly stable systems at low cost that can use the entire solar spectrum for solar energy transformation to recognize the global energy crisis and pollution problems [126]. Most well-known homogeneous photocatalysts incorporate organic ligand-tuned transition metal (TM) complexes that are precisely sensitive. By adjusting the mechanism, degradation rate, and photocatalytic techniques employed, they may be customized to catalyze a wide range of processes. Heterogeneous catalysts are longlasting and simple to remove from the reaction solution. Traditional heterogeneous photocatalysts, on the other hand, are significantly inefficient due to their active sites, which restricts the photocatalytic spectrum of processes. Methods for developing innovative materials that combine the benefits of homogeneous and heterogeneous catalysis are being researched in order to deposit TM photocatalysts onto a solid-state substrate. Amorphous silica, organic polymers, porous materials, and, more recently, new MOFs are common solid-state supports. MOFs are a relatively new type of porous-structured material that has seen fast development in recent years. MOFs are often made by joining metal centers together with organic linkers to create lengthy 1D, 2D, or 3D coordination networks [127]. Because of the infinite number of metal and ligand combinations, MOFs may be produced with a wide range of physical characteristics, adjustable porosity properties, and a wide range of chemical functions. The high absorbency and huge surface area of MOFs are two of the most essential qualities that make them ideal for heterogeneous photocatalysis. MOFs have sparked a lot of interest in a variety of applications, such as membrane separation and gas storage, energy conversion and storage, sensors, medication distribution, and photocatalysis [128]. The efficacy of a MOF-based catalyst in practical applications is determined by I metal energetic sites, (ii) ligands attached to the MOF during the preparation technique or by the introduction of a dopant by post-synthetic modification, and (iii) organic linker modification. Organic connecting linkers can be utilized as supports for capturing or excluding biomolecules, as well as to separate catalytic complexes and as homogeneous catalysts. As a result, MOF-based materials have been successfully employed as photocatalysts for oxidation processes, epoxidation, and a variety of other reactions. MOFs can be used as supports to load active nanomaterials for diverse catalytic applications or as sacrificial templates to generate a variety of functional nanomaterials using different treatment techniques. Because of their long-range ordering and exceptionally high porosity with diverse organic structures, MOFs, for example, can be rehabilitated into extraordinarily porous carbon. MOFs may also easily dissolve to create metal oxides, which are suitable for heterogeneous catalysis due to a straightforward process of self-assembly with organic ligands and metal ions.

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As photocatalysts, semiconductors have been extensively studied. They are suitable materials since they lack a range of electrical states but do have a band gap. Absorption of the photon creates charge–carrier pairs, with an electron in the CB and a hole in the VB, when the incident photon energy is equal to or higher than the band gap energy. MOFs have recently been employed as photocatalysts for CO2 reduction to address the aforementioned difficulties. MOFs often have enormous surface areas (thousands of m2 .g−1 ), which is beneficial for CO2 reduction. MOFs can cover catalytic sites in metal clusters, inside pore hollows, or in the organic linker. MOFs have shown promising results in photocatalytic CO2 reduction since the initial publication, owing to their appealing properties for photocatalytic CO2 reduction due to their high porosity. The absorption of CO2 at the reactive sites has been significantly enhanced by most MOFs. Furthermore, because of their tunability, they may be utilized to make solar light photocatalysts, posing a challenge to commonly used and studied semiconductors like TiO2 .

9.2 MOFs that Have not Been Changed Several pure MOFs have been reported for photocatalytic CO2 reduction. For example, an unaltered Zr-based MOF (NNU-28) was employed for CO2 reduction with TEOA as a sacrificial agent under visible-light illumination. The synthesized MOF was effective in converting CO2 to HCOO. Because of its multiple catalytic routes, this MOF had better catalytic performance: I direct ligand excitation and (ii) charge transfer from the ligand to the metal cluster. The anthracene (C14H10)-based ligand in NNU-28 acted as an active visible-light harvester for Zr6 oxo-clusters during visible-light irradiation, transferring charge from the ligand to the metal cluster while also producing photo-induced charges through the generation of radicals. These two photocatalytic pathways contributed to photo-reduction simultaneously, resulting in a higher photocatalyticCO2 reduction. The MOF’s optical performance was improved because of the strong conjugacy of the anthracene-based ligand. Xu et al. investigated the charge transport features of a Zr-based porphyrin (PCN-222) MOF and its usage for photocatalytic CO2 reduction. The 3D network of PCN-222, CO2 and N2 sorption isotherms, and UV–vis spectra. A deep electron trap state in PCN-222 successfully avoided charge carrier recombination, resulting in a significant improvement in photocatalytic CO2 conversion.

9.3 Linker Modification of MOFs Chen et al. reported the synthesis of a new rod-like A deposited Co-ZIF-9 photocatalyst for CO2 reduction under visible-light irradiation via linker modification of MOFs. Because of its vast specific surface area and short energy transfer channel

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length, the unusual rod-like shape considerably increased photocatalytic CO2 reduction. Under 30 min of illumination, the Ag@Co-ZIF-9 MOF had 20% higher catalytic activity (28.4 mmol CO) and 20% higher selectivity (22.9 mmol H2 ) than the pristine MOF (Co-ZIF-9). Furthermore, the Ag@Co-ZIF-9 MOF catalyst’s photostability was dramatically improved by the rod-like morphological, significantly improving the photostability and repeatability of the catalytic systems.

9.4 Amine Functionalization For photocatalytic CO2 reduction, the addition of amine groups to MOF organic linkers was investigated, with the amine groups potentially promoting interactions with acidic CO2 molecules. For example, iron-based MOFs [MIL-101(Fe), MIL-53(Fe), and MIL-88B(Fe) catalysts were examined for photocatalytic CO2 reduction in the presence of TEOA as a sacrificial agent under visible-light illumination. Because of the presence of coordinated unsaturated Fe sites, the MIL-101(Fe) photocatalyst had the best photocatalytic activity for CO2 reduction among the three iron-based MOFs. Charge separation can occur in unaltered Fe-based MOF systems under visible-light illumination due to the transfer of an electron from O2 in the Fe–O clusters to Fe3 . To complete the photocatalytic cycle, the Fe3 state is reduced to Fe2 , which has a great capacity to decrease CO2 , while TEOA functions as a hydrogen donor and electron giver. A second approach is provided for an amino-functionalized MOF material, in addition to the Fe–O cluster path. A Fe2 state can be created by excitation of the NH2 functional group followed by electron transport from the organic linker to the metal center. These two pathways might be crucial in NH2 -functionalized ironbased MOF-catalyzed CO2 reduction. As a result, Fe-based amino-functionalized MOFs have excellent CO2 reduction photocatalytic capabilities. For CO2 conversion, composites containing ZIF-8 (zeolite imidazole framework-8) coated with Ti/TiO2 nanotubes (NTs) were created [129]. Table 7 shows the application of metal–organic structures in CO2 and N2 reduction.

10 Water and Alcohol Oxidation 10.1 Alcohol Oxidation One fundamental and significant functional group change in the synthesis of organic compounds is the oxidation of alcohols to the corresponding aldehydes and carboxylic acids. Chromium trioxide (Jones oxidation) and potassium permanganate procedures are only two of the many effective oxidation techniques that have been

Solution mixing

In situ hydrothermal assembly

reflux-solvothermal

Cu SAs/UiO-66-NH2

Bi2 S3 /UiO-66

TiO2 /UiO-66

0.05 g

0.025 g

-

photothermalmagnetic coupled reaction system

0.02 g

Hydrothermal

60

60

720

UV–visible light (300 W Xe lamp)

60



Eg (1.31 eV), ECB(-1.09 eV)



Eg(3.20 eV)

78.9 cm3 .g−1

[131]

[130]

References

CH4 (17.9 μmolg−1 .h−1 ) CO (1.9 μmolg−1 .h−1 )

(continued)

[134]

[133]

C2 H5 OH (5.33 [132] μmolg−1 .h−1 ) CH3 OH (4.22 μmolg−1 .h−1 ) 2.65 mmol. g−1 CO (25.60 μmolg−1 .h−1 ) CH4 (0.25 μmolg−1 .h−1 )



0.79 mmol. g−1 CH4 (270.02 μmolg−1 .h−1 ) CO (14.67 μmolg−1 .h−1 )

CO (2.09 μmolg−1 .h−1 ) CH4 (0.1 μmolg−1 .h−1 ) H2 (0.19 μmolg−1 .h−1 )

CO2 adsorption Hydrocarbon ability (or) CO2 fuel yield uptake

Eg(1.85 eV), – ECB(-0.583 eV), EVB(1.197 eV)

Time Fermi energy (min) (Ef), Band gap (Eg), conduction band potential (ECB)

PLS-SXE300 60 Xe lamp

UV–visible light (300 W Xe lamp)

UV–visible light (300 W Xe lamp)

UV–visible light (300 W Xe lamp)

Catalyst Light source loading

Synthesis method

NF@ZnO/Au@ZIF-8

MOF

Table 7 The application of metal–organic structures in their use of CO2 and N2 reduction

252 M. M. Salehi et al.

-

Antisolvent Method & Solution mixing

Solution mixing & Hydrothermal

Solvothermal-assisted in situ interfacial reaction

WO3/CsPbBr3 /ZIF-67

CdS-P25/ZIF-67

ZnO/Pt@ZIF-8

-

0.1

0.01 g

CsPbBr3 QDs/UiO-66(NH2) Hot-injecting & Solvothermal

-

Solution mixing & Solvothermal

PTPA/UIO-66-NH2

UV light (Xe lamp irradiation)

UV–visible light (300 W Xe lamp)

150-W Xe lamp (Zolix)

300 W Xe lamp with a UV-cut filter (420 nm)

UV–visible light (300 W Xe lamp)

Catalyst Light source loading

Synthesis method

MOF

Table 7 (continued)

60

360

180

60

30

ZnO/Pt@ZIF-8 (Eg = 3.21 eV) Ef(0.2 eV)

CdS (Eg = 2.4 eV) P25 (Eg = 2.74 eV) ZIF-67 (Eg = 1.84 eV)

CsPbBr3 (Eg = 2.35 eV) WO3 (Eg = 2.64 eV)

UiO-66(NH2 ) (Eg = 2.89 eV) CsPbBr3 (Eg = 2.27 eV)

Eg (2.94 eV)

Time Fermi energy (min) (Ef), Band gap (Eg), conduction band potential (ECB)

0.570 mmol. g−1



18.21 cm3 .g−1





CH3 OH (1.13 μmolg−1 .h−1 )

CH4 (1.58 μmolg−1 .h−1 ) CO (1.49 μmolg−1 .h−1 )

CO (99.38 μmolg−1 )

CH4 (0.26 μmolg−1 .h−1 ) CO (8.21 μmolg−1 .h−1 )

HCOOH (3.06 μmolg−1 )

CO2 adsorption Hydrocarbon ability (or) CO2 fuel yield uptake

[139]

[138]

[137]

[136]

[135]

References

Applications of MOFs 253

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documented. Although powerful transition metal-based oxidants have excellent efficiency for oxidizing alcohols, it is difficult to selectively convert alcohols to aldehydes as opposed to carboxylic acids using conventional techniques. Additionally, the use of chromium trioxide and potassium permanganate results in the production of significant quantities of metal by-products, making these procedures unsustainable. Aerobic oxidations, which only produce water as a by-product and use molecular oxygen as the ultimate oxidant, were created as a solution to these problems. Two or three redox cycles typically make up the aerobic oxidation process. For instance, by combining the molecular oxygen–water cycle, the redox cycle of a metal, and the redox cycle of a radical species like (2,2,6,6-tetramethylpiperidin1-yl)oxyl (TEMPO),4 alcohols may be efficiently oxidized. For the effective aerobic oxidation of alcohols, a number of techniques have been developed during the last three decades, and their mechanistic features have been researched. As a consequence, the aerobic oxidation of alcohols has produced complete selectivity for aldehydes. Inorganic–organic hybrid materials with three dimensions and pores are known as MOFs. Metal clusters (or ions) in secondary building units (SBUs) and multichelating ligands like benzene-1,4dicarboxylic acids (BDCs) or 4,40-bypyridyl repeatedly create coordination bonds to make MOFs (bpy). Molecular adsorption, separation, storage, shuttling, sensing, and electrical energy storage are only a few of the many MOF applications that have been explored, but catalytic applications are also a crucial area of MOF study. MOFs themselves are effective catalysts for Lewis acid-catalyzed organic conversions because the MOF structure includes metal ions (often transition metal ions). When used in organic processes, MOFs’ perfect heterogeneity simultaneously enables simple separation and excellent reusability. Additionally, MOFs’ superb tunability may be leveraged to add more catalytic species to their structures. MOFs are regarded as significant multifunctional catalytic platforms for organic transformations as a result. In-depth research has been done on MOF-based catalysts for the aerobic oxidation of alcohols. According to where the catalytic species are located, these catalysts may generally be divided into four groups: catalysts in SBUs, metal catalysts coordinated to the ligands, organic catalysts integrated into the ligands, and metal nanoparticles (NPs) in the pores. Furthermore, molecular level characteristics of MOFs may be altered to affect the catalytic activity. The four catalyst-loading techniques in this account are both of these writers equally contributed to this work. The aerobic oxidation of alcohols is examined, and current systems are emphasized. Due to their metal–ligand coordination structures, which offer repeating and controllable pores that facilitate size selectivity, low density, and perfect heterogeneity that are advantageous for recycling, and organic ligands that provide excellent tunability through simple functionalization, MOFs and related materials are emerging catalytic platforms. Numerous MOFs have been used in significant research on the aerobic oxidation of alcohols to the corresponding aldehydes (or ketones) under a range of reaction conditions. The methods for creating radical (ligand)-functionalized MOFs, metal cation-incorporated MOFs, metal-NP-installed MOFs, and metal catalysts in the MOF structure (SBU) may all be classified as forms of MOF-based catalysts for the aerobic oxidation of alcohols. In general, a combination of the metal, radical species, and O2/water redox cycles is needed for successful aerobic

Applications of MOFs

255

oxidation. As a result, metal catalysts or MOFs with metal ions inserted need an outside radical species like TEMPO, while MOFs with TEMPO functionality need a redox-active metal or an extra-active radical species like TBN. The creation of multifunctional catalysts and the use of synergistic effects to achieve increased efficiency have been made possible by the structural flaws in MOFs and their fine tunability. Metal cations and TEMPO moieties placed in the pores of the same MOF may influence the activity of Lewis acid defect sites, which can function as catalytic sites. The most significant benefit of MOF-based catalysts over other porous inorganic catalytic platforms is their simplicity of fine-tuning. The catalytic state under water is crucial to maximizing the benefit of MOF-based catalyst for aerobic oxidation. The extremely effective, recyclable MOF-based catalysts for aerobic oxidation greatly benefit from the absence of radical oxidant consumption in aqueous conditions [140].

10.2 Water Oxidation One of the most promising methods for producing green hydrogen from renewable energy is water electrolysis. But in order to get meaningful catalytic currents, the necessary oxygen evolution reaction (OER), which is thermodynamically unfavorable, often requires high overpotentials. At this time, catalysts made of precious metals (such Ru and Ir) have shown high OER activity and stability in both acidic and basic conditions. The lack of these valuable metals, however, limits the amount that can be produced on a big scale. A lot of work has been put into developing OER catalysts employing earth-abundant metals such as Co, Ni, Fe, and Mn. Recent research has concentrated in particular on the synergistic manipulation of activity and stability by metal cation doping and their impacts on electrocatalytic performance. MOFs, among other things, have drawn more attention in relation to electrocatalysis. Because of the adaptability of their composition and crystal structure, MOFs may provide a large number of reactant-accessible catalytically active sites. Nevertheless, the bulk of MOFs have low electronic conductivity, which limits the overall efficiency of converting electrical energy into chemical energy, and the organic linkers in MOFs often have poor chemical stability, which leads to subpar long-term performance. Engineering extremely thin MOF catalysts, such as 2D, is a viable strategy for overcoming these difficulties. For instance, OER activity was considerably increased in ultrathin Ni–Co MOF nanosheets containing coordinatively unsaturated metal sites compared to bulk Ni–Co MOFs. Achieving a current density of 10 mA cm2 at a cell voltage of 1.55 V, Duan et al.’s development of an ultrathin nickel–iron-based MOF array on Ni foam demonstrated promising electrocatalytic performance for both the OER and the hydrogen evolution process (HER). In order to create 2D Co-MOFs, Huang et al.used an electrochemical/chemical exfoliation approach, proving their effectiveness as electrocatalysts for OER. However, when grouped in larger scale electrodes, 2D MOFs still struggle with difficulties related to their inherently poor conductivity due to their huge active surface area.

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Engineering composite structures out of thin MOF grains on porous supports that carry electricity is an option. MOF/graphene composites have been investigated for use in supercapacitors and Li–S batteries. Despite these successes, the fundamental structural factors governing the activity and stability of MOF electrocatalysts are still poorly understood, which leads to inconsistent performances [141].

11 Water Electrolysis and Splitting Global energy markets are thriving to explore alternative and sustainable energy sources as a result of depleting fossil fuel supplies, an expanding global population, and fast industrialization. The world population is expected to expand by a factor of 26% to around 9.7 billion people by the year 2050, while primary energy consumption will jump by about 50%. In order to fulfill these escalating energy needs, fossil fuels are being used unrestrictedly; yet, it won’t be long until they run out of fuel. Fossil fuel usage also contributes to serious environmental issues such as air pollution, water contamination, and greenhouse gas emissions, which in turn cause global warming and dangerous climate change. If global warming continues to progress at its present pace, it is predicted that between 2030 and 2052, the average global temperature would rise by causing the sea level to rise by roughly 3.8 m. In order to address these issues, there is significant interest in researching clean and renewable alternative energy sources for a sustainable future. Clean energy solutions work toward achieving a number of important goals, including (a) increased efficiency, (b) better resource usage, (c) cost-effectiveness, (d) a cleaner environment, (e) greater energy security, and (f) efficient design and analysis. 6 In this period, it is vitally important to create alternative energy supplies in order to fulfill the population’s ever-expanding needs without jeopardizing the future of subsequent generations. However, obtaining effective, plentiful, and affordable sustainable energy systems that can rival the usage of fossil fuels today remains a significant issue. The most effective and long-lasting green process among the projected clean energy supply options is hydrogen as a clean fuel. Solar energy that is efficiently captured may meet our energy demands via photoelectrocatalytic water splitting, which uses sunlight to split water into hydrogen, and solar energy cells. A comforting alternative energy source with limitless promise is hydrogen. Since hydrogen energy affords a chance to utilize a wide range of raw materials to create a fuel with high energy efficiency, unbeatable environmental and social advantages, and the ability to be used for all practical energy uses, it provides a carbon-free energy source. Making the switch to a hydrogen-based economy from one based on fossil fuels has a number of important benefits. When compared to gasoline, which has an energy density of 44 MJ/kg, hydrogen has a high energy density of 120 MJ/kg. Hydrogen also has a high combustion efficiency, is non-toxic, produces clean exhaust products, can be stored, and is renewable. The majority of hydrogen generation technologies are still in the development stage for commercial applications, despite the fact that hydrogen economy is anticipated to be the most

Applications of MOFs

257

popular sustainable energy alternative. At the moment, the direct source of industrialscale hydrogen generation is fossil fuels. Today, the chemical industry or on-site oil refineries are where the majority of commercial hydrogen is produced. Most often, hydrogen is utilized as a feedstock for oil refineries and other processes, including the production of polymers, resins, and solvents. Around 96% of the world’s supply of hydrogen is produced by the steam reforming of oil and natural gas as well as the gasification of coal which negates the goal of not depending on fossil fuel reserves. Ali et al. Production of hydrogen should come from plentiful, clean sources using eco-friendly procedures in order to reduce reliance on fossil fuels. Water splitting is a well-known method of producing hydrogen from renewable sources. The most plentiful resource on Earth is water, which covers 70% of the planet’s surface. As water is used as a feedstock, which can be regenerated back into nature endlessly, the water-splitting process can create hydrogen with essentially no negative effects on the environment. Oxygen, a by-product of the process, has no adverse effects on the environment and may be utilized in different ways to increase the electrolysis process’s economic viability. Only 4% of the world’s hydrogen is now produced through water electrolysis. High energy consumption (considerable energy is required to break the hydrogen bonds in a water molecule) and significant expenditure are to blame for this. The oxygen evolution reaction (OER) and the hydrogen evolution reaction are the two half-cell processes that make up the water-splitting process (HER). However, these processes have slow kinetics. For the process to proceed favorably, a catalyst must be used to break the powerful chemical bonds that hold the oxygen and hydrogen in a water molecule together. Ir/Ru, Pt, and other noble metals are often the basis for the catalysts employed in water splitting. The utilization of the electrolysis method on a broad scale is limited by the disadvantages of these catalysts, including their high cost, scarcity of reserves, and short lifespan. As they are easily accessible, and have outstanding activity and stability, transition metals found in abundance on Earth (such as Fe, Ni, and Co) may serve as appropriate substitutes for noble metal-based catalysts. The catalyst activity for water splitting is significantly influenced by shape in addition to catalyst composition. Therefore, for effective hydrogen generation, catalyst material and structural modification are crucial. Due to simple access to pores, low charge transfer distances, exposed active sites, and smooth diffusion of reactants and products from the catalytic sites, porous micro-nanostructures exhibit high photocatalytic activity. A useful strategy for enhancing photocatalytic activity is the production of semiconducting materials based on heterojunctions, such as metal semiconductors. This enhancement is the result of bandgap tunability, efficient separation, movement of photo-induced electrons/holes, and effective use of the whole light spectrum. A lot of effort has been put into designing the porous structure of photocatalysts in recent years. However, an effective and simple to make photocatalyst with excellent activity is required owing to the complex procedures of the current synthesis method. MOFs, which combine metal cores with organic linkers to form a structure, are being heralded as a brand-new class of materials. Due to their high porosity (90% free volume), dynamic structure, design tunability, ultrahigh surface area, and crystalline nature, they have remarkable optical, electrical, and catalytic capabilities. Their potential has recently been used in a wide range of

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applications, including catalysis, CO2 collection, sensing, and adsorption. The use of MOFs as an active catalyst for photoelectrochemical (PEC) H2 generation has also been thoroughly studied. MOFs are attractive catalyst materials for water splitting because of their inherent ability to preserve their structural integrity and functional features after synthetic alteration procedures. In order to enable and maintain watersplitting processes, MOFs exhibit amazing physical, spatial, chemical, and electrical diversity. MOFs may be designed for efficient and improved catalytic sites in water electrolysis, introducing light sensitization, band gap adjustment, and maintaining a photo-generated charge. As templates and/or precursors for the production of functionalized materials (such as porous carbon, metal sulfides, metal oxides, and carbon metal/metal oxide hybrids), MOFs have certain distinct advantages over typical semiconductor catalysts. The majority of the time, the structural morphologies of the original MOFs are maintained in the derivatives, and increased catalytic performance is often seen. For the creation of well-designed nanocatalysts, MOFs and their derivatives may also be used as supports. Several researches have been conducted in the last few years on the use of MOFs as photoelectrocatalysts and electrocatalysts for water splitting. This field of study will continue to get more attention. In 1972, Fujishima and Honda developed the PEC water-splitting idea. In 1998, Turner and Khaselev showed the conversion of solar energy to hydrogen in PEC cells with an efficiency of 12.4%. This successfully illustrated PEC’s capability to collect solar energy for water electrolysis. When submerged in an aqueous electrolyte, a PEC catalyst with the right characteristics may transform photon energy from sunlight into electrochemical energy, which causes water to split into its components, hydrogen and oxygen [142].

12 Environmental Remediation 12.1 Degradation of Organic Dyes Organic pollutants found in industrial effluent, of which dye is one of the primary sources and has a negative impact on the environment, are present in large numbers. In the treatment of industrial wastewater, photocatalysis has been employed extensively as an effective technique. The majority of research focuses on dye degradation, and the QD-MOF materials with high light-catching capacity and quick electron separation rate are appropriate photocatalysis for wastewater treatment. Rhodamine 6G (Rh 6G), RhB, and MB are the primary substances that are now degraded. By using the bottle-around-the-ship approach, Kaur and colleagues encapsulated the produced CdTe QDs inside the Eu-MOF and NTU-9 matrix. Due to improved photodegradation kinetics, CdTe QDs/NTU-9 demonstrated a quicker rate of deterioration than CdTe QDs/Eu-MOF. Compared to Eu-MOF, NTU-9 had a greater specific surface area and bigger pore volume, which meant that more dye molecules could bind to it and be more easily degraded. Lower pH settings were beneficial to increase

Applications of MOFs

259

the photocatalysis activity of dye degradation, according to research on the impact of pH on Rh 6G degradation efficiency. The cause was identified as the fact that hydrogen peroxide in acidic circumstances stimulated the creation of hydroxyl radicals, speeding up the breakdown of colors. Huang et al. used N-doped TiO2 /MIL-100 to study the impact of pH on the degradation of MB and RhB. (Fe). Under contrast, the results demonstrated that dye desorption and catalyst active site corrosion caused the catalysis effectiveness to be compromised in acidic or alkaline conditions. Incorporating CdSe QDs with UiO-66, Gan et al. found that under acidic circumstances, RhB degraded to zero within 50 min. CdSe QDs sensitized MIL-125(Ti)/TiO2 @SiO2 two-level type-II heterostructure was also created by Huang et al. and the composite demonstrated significant RhB degradation. Unfortunately, RhB’s limited oxidation capacity prevented it from fully oxidizing into CO2 and H2 O, instead producing tartaric acid and 2-hydroxyhexanedial. Wang et al. modified MIL-125(Ti) with amino groups to create NH2-MIL-125 in order to improve the stability and capacity of MIL-125(Ti) to catch light (Ti). Then, 2 nm-sized CQDs were applied to the surface of NH2-MIL-125 (Ti). Due to the narrowing of the band gap of NH2-MIL125(Ti) following the addition of CQDs, the produced CQDs/NH2 -MIL125(Ti) composite demonstrated a superior response in the visible region compared to pure NH2 -MIL-125(Ti) and even responded to nearinfrared light area. The CQDs activated NH2 -MIL-125(Ti) by upconverting nearinfrared light to visible light after receiving photon radiation. As a result, the efficient absorption of light energy caused more photogenerated charges to be produced, which enhanced RhB’s photodegradation capability. The key determinants of the pace at which dyes degrade during photocatalytic dye degradation are the adsorption of dye molecules and the activity of the photocatalyst. The huge specific surface area of MOFs creates the ideal environment for dye adsorption, whereas QDs improve light trapping capacity and slow down the pace at which photogenerated charges recombine to effectively degrade dye. Additionally, one of the elements that cannot be disregarded in dye degradation is pH. The structure of the dyes, the electrostatic interaction between dye molecules, the functionality of the catalysts, and the kind of reactive radicals will all be impacted by pH, which will then have an impact on the photocatalytic activity [143].

13 Environmental Contaminants Adsorption MOFs are attractive sensing elements in chemical sensors because of their reversible adsorption, high catalytic activity, adjustable chemical functionalization, and varied structure. In recent years, the chemical, physical, and structural changes in a MOF as a result of the adsorption of guest molecules have been used to detect environmental pollutants such as heavy metals, organic compounds, and toxic gases. Heavy metals, anions, organics, antibiotics, and bacteria are all contaminants in water that are damaging to human health and the environment. As a result, sensitive and reliable sensors for determining water contamination are extremely important.

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Because of their unique characteristics for selective capture and identification of analytes, MOFs have been widely used for chemical detection in aqueous solutions. The reversible adsorption and release of target molecules are enabled by their porosity and wide surface area. MOF-based sensors have been employed in a variety of luminescent, electrochemical, and colorimetric sensors. Despite the fact that the sensors use distinct detecting techniques, their sensing capabilities in water pollution detection are promising. Table 8 shows the application of metal–organic structures in their use in environmental contaminants adsorption [144].

14 Environmental Contaminants Degradation Every year, a wide range of hazardous materials, including NOx , SOx, CO, nitrogen-containing organic compounds (e.g., hydrogen cyanide), sulfur-containing compounds (e.g., organothiols), and volatile organic compounds, are discharged into water and air. The majority of these toxic pollutants come from anthropogenic activities, such as fossil fuel combustion, the leaking of harmful industrial gases and vapors, chemical warfare agents, as well as industrial sewage discharge. The release of toxic gases/vapors into the air can have serious consequences for the environment and human health [156]. The rapid expansion of the economy and technology has put immense strain on the environment, but human civilization’s enormous demand for energy is equally difficult to meet. In the case of water pollution, the current discharge of industrial wastewater is vast, which not only has a detrimental impact on the environment and poses a health risk to people but also has a significant restraining effect on the industry’s expansion. A wide spectrum of pollutants can be found in industrial effluent. Organic dyes, pharmaceuticals, and personal care products, as well as other organic pollutants, are all substantial polluters These chemicals are extremely toxic and difficult to break down using typical methods, posing a major threat to the environment’s long-term viability as well as human health. Finding low-energy water pollution treatment options that are also environmentally friendly has become a global concern in this setting. MOFs, a fascinating type of porous crystalline material composed of metal ions and multitopic organic linkers, have recently received a lot of attention due to their excellent porosity, tunable pore size, and applications in magnetism, heterogeneous catalysis, sensing, gas adsorption, and drug delivery, among other things. Metalcontaining nodes are connected by strong chemical bonds between organic linkers to form MOFs. Certain MOFs behave like semiconductors when exposed to light, implying that they could be employed as photocatalysts. Not only has recent research established porous MOF materials as a new class of photocatalysts capable of catalytic degradation of organic pollutants under UV/visible/UV–visible irradiation but also sparked a flurry of interest in investigating the use of MOFs as photocatalysts in other applications. Because of the abundance of metal-containing nodes and organic bridging

678.3

730 105

two-step post-synthetic methods (PSMs)

Hydrothermal & solution mixing

Immersion grinding & in situ polymerization

Solvothermal

Solvothermal

Solvothermal & solution mixing

Solution mixing-two-step method

sodalite topology 1370 in continuous flow

UiO-66-NH-(AO)

UiO-66-TABC

P[C4 (VIM)2 ]Cl2 @MIL-101

UiO-66-EDA

NH2 -MIL-125(Ti)@TpPa-1

ZIF-8-EGCG

β-CD@ZIF-8/PVDF

ZIF-8

1066

113.02

743.1

-

Surface area (m2 .g_1 )

Synthesis method

Name

243.90 217.39 208.33 593.97 536.73 79.24 123.58 232.97 82.90

Pb2+ Cd2+ Cu2+ Eu3+ UO2 2+ Cr6+ Cr2+ Cu2+ Ni2+

1780.91 1271.27

673

TcO4 −

Pb2+ Hg2

137.0

Co 2+

708.130 651.379

47.2

UO2 2+

Pb2+ Cu2

Adsorption capacity (mg. g−1 )

Target

17.5

120

240

90 60

60

240

180

1500

Adsorption Equilibration Time (min)

Table 8 The application of metal–organic structures in their use environmental contaminants adsorption

6.5

5.5

4

9 6

6

4

9

6

pH

0.01

0.0035

0.05

-

0.04

0.7

0.01

0.002

Absorbent dosage(g)

20

200

25 200 200 200

100

300

1000

38.0

150

Concentration of (ppm)

(continued)

[152]

[151]

[150]

[149]

[148]

[147]

[146]

[145]

References

Applications of MOFs 261

Sequential Hydrothermal process

Co-precipitation & 178.28 Solvothermal

Co-Al-LDH@Fe2 O3 /3DPCNF

Fe3 O4 /ZIF-67@AmCs

-

1889

Solvothermal

ZIF-67

Surface area (m2 .g_1 )

Synthesis method

Name

Table 8 (continued) Adsorption capacity (mg. g−1 ) 1978.63 1436.11 400.40 426.76 119.05

Target

Pb2+ Hg2 Cr6+ Pb2+ Cr6+ 60

60

17.5

Adsorption Equilibration Time (min)

2

2 6

6.5

pH

0.01

0.005

0.01

Absorbent dosage(g)

100

250

20

Concentration of (ppm)

[155]

[154]

[153]

References

262 M. M. Salehi et al.

Applications of MOFs

263

linkers, as well as the controllability of the synthesis, it is simple to construct MOFs with a tailorable capacity to absorb light, thereby initiating desirable photocatalytic properties for specie applications in the degradation of organic pollutants. Photodegradation via the photosensitization process occurs when the bandgap of many MOFs/semiconductors is not low enough to absorb in the visible region. Dye absorbs visible light through the excitation of electrons from the HOMO to the LUMO levels. If the energy level of LUMO of dye molecules is higher than CB of MOFs/semiconductors, photoexcited electrons can be transferred from LUMO of dye molecules to CB of MOFs/semiconductors. The photodegradation of dyes over MOFs progresses via stages that are similar to those seen in the advanced oxidation process. For example, photoexcited electrons in MOFs’ CB can be transferred to molecular oxygen dissolved in water and oxidized to superoxide radicals (O2 ). H+ /H2 O/H2 O2 may react with O2 − , forming hydroxyl radicals as a result (OH). Due to the strong oxidizing capacity of both hydroxyl radicals (OH) and superoxide radicals, surface-adsorbed dye molecules may be reduced to carbon dioxide, water, and other small molecules (O2 − ). Both of the above-mentioned mechanisms for photocatalytic dye degradation (direct photodegradation/advanced oxidation processes (AOPs) or sensitizationmediated degradation process) show that photocatalytic decomposition is predominantly mediated by the generation of radicals such as O2 and OH. Only a few articles address the complete mechanistic investigation of dye degradation across MOFs [157]. Studies on photocatalytic decomposition of dyes over other semiconducting materials, on the other hand, suggest that the proposed mechanism involves the formation of various radicals, followed by conversion to a simple carboxylic acid, and finally to carbon dioxide and water, as well as a low-molecular-weight compound. It’s inescapable that different MOFs degrade dyes in different ways, or that the same MOFs degrade dyes in diverse ways. The degradation pathway is largely caused by the band-gap of the MOF, which has led to many strategies for the creation of active radical species. The dye breakdown mechanism is mostly determined by the bandgap, pore size, volume, and structure of SBU. Table 9 shows the application of metal–organic structures in their use in environmental contaminants degradation.

15 Membranes 15.1 Introduction Membranes, as a disruptive technology, can cut global energy consumption in raw chemical separations while also actively lowering greenhouse gas emissions, laying the groundwork for a sustainable future. Membrane technology alone in the petrochemical sector could replace distillation procedures and save up to 80% of energy in separation processes, resulting in an 8% reduction in world energy consumption. Gas separations account for more than half of the separations. [538–540] From the earliest

ultrasonic & self-assembled biomineralization

solvothermal

in situ

OPP18 -(ZIF-8)

Co/NH2 -MIL-88B(Fe)

ZIF-9@GEL & ZIF-12@GEL22

Reaction systems

activation of peroxymonosulfate

H2 O2 , Heterogeneous Electro-Fenton

dye solutions were prepared in phosphate buffer and stirred with OPP–MOF at room temperature

H2 O2 , visible light

17

2D/2D FeNi-layered double hydroxide/bimetal-organic frameworks nanosheets. Tetracycline hydrochloride. 18 Orange peel peroxidase. 19 Methylene blue. 20 Congo red. 21 Tetracycline. 22 Aerogels. 23 P-nitrophenol.

16

Synthesis method

in situ semi-sacrificial template strategy

MOF

FeNi-LDH/BMNSs16 85.67% 89.95%

91% 90%