Metal-Organic Framework: From Design to Applications [1st ed.] 9783030473396, 9783030473402

Table of contents :
Front Matter ....Pages i-ix
Why design matters: From decorated metal-oxide clusters to functional metal-organic frameworks (Alexander Schoedel, Sahar Rajeh)....Pages 1-55
State of Art and Prospects of Biomolecules-Incorporation in Functional Metal–Organic Frameworks (Wenjie Duan, Zhengfeng Zhao, Hongde An, Zhenjie Zhang, Peng Cheng, Yao Chen et al.)....Pages 57-87
Regulation of the degree of interpenetration in metal-organic frameworks (Gaurav Verma, Sydney Butikofer, Sanjay Kumar, Shengqian Ma)....Pages 89-133
Functionalized Dynamic Metal-Organic Frameworks as Smart Switch for Sensing and Adsorption Applications (Binbin Qian, Ze Chang, Xian-He Bu)....Pages 135-173
Metal–Organic Frameworks Towards Desulfurization of Fuels (Leiduan Hao, Matthew J. Hurlock, Guodong Ding, Qiang Zhang)....Pages 175-202
Synthesis and Applications of Porous Organosulfonate-Based Metal-Organic Frameworks (Guiyang Zhang, Honghan Fei)....Pages 203-214
Insights into the Gas Adsorption Mechanisms in Metal–Organic Frameworks from Classical Molecular Simulations (Tony Pham, Brian Space)....Pages 215-279
Theoretical Exploration and Electronic Applications of Conductive Two-Dimensional Metal-Organic Frameworks (Jia Gao, Shubo Geng, Yao Chen, Peng Cheng, Zhenjie Zhang)....Pages 281-304
Current Status of Microporous Metal–Organic Frameworks for Hydrocarbon Separations (Jiyan Pei, Kai Shao, Ling Zhang, Hui‑Min Wen, Bin Li, Guodong Qian)....Pages 305-338
Mechanical Properties of Shaped Metal-Organic Frameworks (Bhuvan B. Shah, Tanay Kundu, Dan Zhao)....Pages 339-372
MOFs-based catalysts supported chemical conversion of CO2 (Ying Shi, Shengli Hou, Xiaohang Qiu, Bin Zhao)....Pages 373-426

Citation preview

Topics in Current Chemistry Collections

Xian-He Bu Michael J. Zaworotko Zhenjie Zhang  Editors

Metal-Organic Framework From Design to Applications

Topics in Current Chemistry Collections

Journal Editors Massimo Olivucci, Siena, Italy and Bowling Green, USA Wai-Yeung Wong, Hong Kong, China Series Editors Hagan Bayley, Oxford, UK Greg Hughes, Codexis Inc, USA Christopher A. Hunter, Cambridge, UK Seong-Ju Hwang, Seoul, South Korea Kazuaki Ishihara, Nagoya, Japan Barbara Kirchner, Bonn, Germany Michael J. Krische, Austin, USA Delmar Larsen, Davis, USA Jean-Marie Lehn, Strasbourg, France Rafael Luque, Córdoba, Spain Jay S. Siegel, Tianjin, China Joachim Thiem, Hamburg, Germany Margherita Venturi, Bologna, Italy Chi-Huey Wong, Taipei, Taiwan Henry N.C. Wong, Hong Kong, China Vivian Wing-Wah Yam, Hong Kong, China Chunhua Yan, Beijing, China Shu-Li You, Shanghai, China

Aims and Scope The series Topics in Current Chemistry Collections presents critical reviews from the journal Topics in Current Chemistry organized in topical volumes. The scope of coverage is all areas of chemical science including the interfaces with related disciplines such as biology, medicine and materials science. The goal of each thematic volume is to give the non-specialist reader, whether in academia or industry, a comprehensive insight into an area where new research is emerging which is of interest to a larger scientific audience. Each review within the volume critically surveys one aspect of that topic and places it within the context of the volume as a whole. The most significant developments of the last 5 to 10 years are presented using selected examples to illustrate the principles discussed. The coverage is not intended to be an exhaustive summary of the field or include large quantities of data, but should rather be conceptual, concentrating on the methodological thinking that will allow the non-specialist reader to understand the information presented. Contributions also offer an outlook on potential future developments in the field. More information about this series at http://www.springer.com/series/14181

Xian-He Bu • Michael J. Zaworotko Zhenjie Zhang Editors

Metal-Organic Framework From Design to Applications

With contributions from Hongde An • Xian‑He Bu • Sydney Butikofer • Ze Chang • Yao Chen Peng Cheng • Guodong Ding • Wenjie Duan • Honghan Fei Jia Gao • Shubo Geng • Leiduan Hao • Shengli Hou • He Huang Matthew J. Hurlock • Sanjay Kumar • Tanay Kundu • Bin Li Shengqian Ma • Jiyan Pei • Tony Pham • Binbin Qian Guodong Qian • Xiaohang Qiu • Sahar Rajeh • Alexander Schoedel Bhuvan B. Shah • Kai Shao • Ying Shi • Brian Space • Gaurav Verma Hui‑Min Wen • Bin Zhao • Dan Zhao • Zhengfeng Zhao Guiyang Zhang • Ling Zhang • Qiang Zhang • Zhenjie Zhang

Editors Xian-He Bu College of Materials Science and Engineering Nankai University Tianjin, China

Michael J. Zaworotko Department of Chemical Sciences and Bernal Institute University of Limerick Limerick, Ireland

Zhenjie Zhang College of Chemistry Nankai University Tianjin, China

Partly previously published in Topics in Current Chemistry Volume 377 (2019); Topics in Current Chemistry Volume 378 (2020). ISSN 2367-4067 Topics in Current Chemistry Collections ISBN 978-3-030-47339-6 © Springer Nature Switzerland AG 2020 This work is subject to copyright. All rights are reserved 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

Contents

Preface ................................................................................................................ Why Design Matters: From Decorated Metal Oxide Clusters to Functional Metal–Organic Frameworks ..................................................... Alexander Schoedel and Sahar Rajeh: Topics in Current Chemistry 2020, 2020:19 (3, February 2020) https://doi.org/10.1007/s41061-020-0281-0 State‑of‑the‑Art and Prospects of Biomolecules: Incorporation in Functional Metal–Organic Frameworks ..................................................... Wenjie Duan, Zhengfeng Zhao, Hongde An, Zhenjie Zhang, Peng Cheng, Yao Chen and He Huang: Topics in Current Chemistry 2019, 2020:34 (30, October 2019) https://doi.org/10.1007/s41061-019-0258-z Regulation of the Degree of Interpenetration in Metal–Organic Frameworks ........................................................................ Gaurav Verma, Sydney Butikofer, Sanjay Kumar and Shengqian Ma: Topics in Current Chemistry 2020, 2020:4 (2, December 2019) https://doi.org/10.1007/s41061-019-0268-x

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Functionalized Dynamic Metal–Organic Frameworks as Smart Switches for Sensing and Adsorption Applications ........................ 135 Binbin Qian, Ze Chang and Xian‑He Bu: Topics in Current Chemistry 2020, 2020:5 (11, December 2019) https://doi.org/10.1007/s41061-019-0271-2 Metal–Organic Frameworks Towards Desulfurization of Fuels ................... 175 Leiduan Hao, Matthew J. Hurlock, Guodong Ding and Qiang Zhang: Topics in Current Chemistry 2020, 2020:17 (29, January 2020) https://doi.org/10.1007/s41061-020-0280-1 Synthesis and Applications of Porous Organosulfonate‑Based Metal–Organic Frameworks ............................................................................. 203 Guiyang Zhang and Honghan Fei: Topics in Current Chemistry 2019, 2020:32 (26, October 2019) https://doi.org/10.1007/s41061-019-0259-y

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Insights into the Gas Adsorption Mechanisms in Metal–Organic Frameworks from Classical Molecular Simulations....................................... 215 Tony Pham and Brian Space: Topics in Current Chemistry 2020, 2020:14 (13, January 2020) https://doi.org/10.1007/s41061-019-0276-x Theoretical Exploration and Electronic Applications of Conductive Two‑Dimensional Metal–Organic Frameworks ..................... 281 Jia Gao, Shubo Geng, Yao Chen, Peng Cheng and Zhenjie Zhang: Topics in Current Chemistry 2020, 2020:25 (18, February 2020) https://doi.org/10.1007/s41061-020-0288-6 Current Status of Microporous Metal–Organic Frameworks for Hydrocarbon Separations ........................................................................... 305 Jiyan Pei, Kai Shao, Ling Zhang, Hui‑Min Wen, Bin Li and Guodong Qian: Topics in Current Chemistry 2019, 2020:33 (29, October 2019) https://doi.org/10.1007/s41061-019-0257-0 Mechanical Properties of Shaped Metal–Organic Frameworks ................... 339 Bhuvan B. Shah, Tanay Kundu and Dan Zhao: Topics in Current Chemistry 2019, 2020:25 (16, September 2019) https://doi.org/10.1007/s41061-019-0250-7 MOFs‑Based Catalysts Supported Chemical Conversion of CO2 ................. 373 Ying Shi, Shengli Hou, Xiaohang Qiu and Bin Zhao: Topics in Current Chemistry 2020, 2020:11 (6, January 2020) https://doi.org/10.1007/s41061-019-0269-9

Preface

We are now in the “Age of Gas” means that energy efficient approaches for gas purification and storage must be developed in order to further enable the utility of gases as fuels, therapies or feedstock chemicals. Porous solids such as zeolites and activated carbons are already used industrially in this context but they are ill-suited for many separation and storage applications, especially those that require ultrahigh selectivity or extra-large surface area. These limitations are at least partly due to their narrow range of composition and difficulties with respect to design of pore size and/or pore chemistry. In short, it is difficult to fine-tune the properties of zeolites and activated carbons for a particular application. Fortunately, several new families of porous materials have emerged in the past two decades. These families are exemplified by a class of porous coordination networks known as metal-organic frameworks (MOFs), which are the subject of this topical collection. MOFs are typically comprised of a metal or metal cluster (the “node”) that is coordinated to a multi-functional organic ligand(s) (the “linker”). It is now well recognized that MOFs represent a new frontier in materials science because the “node and linker” approach to their design has afforded materials with the following features: unprecedented levels of permanent porosity; crystallinity that brings uniformity, process scale-up and reproducibility; modular compositions that can offer control over both structure and properties. This combination of features makes it unsurprising that MOFs have captured the imagination of chemists worldwide and they are primed to solve societal challenges in the areas of energy sustainability and environmental remediation. Today there are already tens of thousands of MOFs so the challenge is no longer how to make them. Rather, it is how to customize MOFs for a given application. As detailed below, the contributions in this topical collection are focused upon two main themes: design strategies for creation of new MOFs; structure-property relationships in MOFs in the context of topical applications of porous materials. With respect to design strategies, there are three contributions herein. Rajeh & Schoedel discuss the use of metal oxide clusters as building blocks for functional MOFs. Duan & Chen et al., detail how to incorporate biomolecules into MOFs. The topic of interpenetration and how to control it is addressed by Verma and Ma et al..

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The broad scope of properties that can be exhibited by MOFs is addressed in the remaining eight contributions. Qian and Bu et al. report on the switching properties of MOFs and how this phenomenon can be exploited for sensing and adsorption. Hao & Zhang et al. cover the use of MOFs for desulfurization of fuels. The uses of organosulfonate MOFs are presented and discussed by Zhang & Fei et al.. The application of molecular simulations to understanding and insight into the sorption properties of MOFs is detailed by Pham and Space. Gao and Zhang et al. summarized the progress on theoretical exploration and electronic applications of conductive 2-dimentional MOFs. Li & Qian et al. address hydrocarbon separations using MOFs whereas Shah and Zhao et al. present a review on the mechanochemical properties of MOFs. The final contribution by Shi & Zhao et al. covers the use of MOFs for catalytic conversions of CO2. This topical collection highlights the two key reasons that interest in MOFs has grown exponentially to the point where there are over 5000 publications annually. The modularity of MOFs, which makes them amenable to systematic study of structure-function relationships, also allows chemists to exert control over pore size and pore chemistry in a way that is not possible in existing classes of porous materials. The outcome of such control is unprecedented properties and performance as detailed herein.

Xian-He Bu College of Materials Science and Engineering, Nankai University, Tianjin, China

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Michael J. Zaworotko Department of Chemical Sciences and Bernal Institute, University of Limerick, Limerick, Ireland

Zhenjie Zhang College of Chemistry, Nankai University, Tianjin, China

Topics in Current Chemistry (2020) 378:19 https://doi.org/10.1007/s41061-020-0281-0 REVIEW

Why Design Matters: From Decorated Metal Oxide Clusters to Functional Metal–Organic Frameworks Alexander Schoedel1   · Sahar Rajeh1 Received: 30 June 2019 / Accepted: 14 January 2020 / Published online: 3 February 2020 © Springer Nature Switzerland AG 2020

Abstract The opportunity to generate functional solids with defined properties by deliberate design has not been materialized in traditional solid-state chemistry over many decades. The emergence of metal–organic frameworks (MOFs), permanently porous, crystalline solids with defined metrics, has allowed for studying design, synthesis, and properties, which then translated into new applications. Aggregates of metal ions stitched together by multidentate functional groups form such metal oxide clusters and represent the nodes of MOFs. These clusters, termed secondary building units (SBUs), are decorated with organic moieties that provide directionality and can be linked through geometric principles into extended nets using organic molecules (spacers). This concept of reticular chemistry has afforded permanently porous MOFs, and has resulted in over 20,000 structures over the past 20 years. However, there are still only a limited number of symmetric, discrete SBUs commonly used to design and synthesize MOFs. We herein introduce the most important SBUs that have emerged over time together with prototypal MOF structures and their fundamental applications. Both the discovery and the scientific impact will be highlighted alongside advantages and/or drawbacks. In addition, an outlook will be given on how the combination of multiple SBUs can lead to heterogeneous but ordered materials with higher complexity and functionality. Keywords  Metal-oxide clusters · Secondary building units · Framework design · Reticular chemistry · Topology

Chapter 1 was originally published as Schoedel, A. & Rajeh, S. Topics in Current Chemistry (2020) 378: 19. https://doi.org/10.1007/s41061-020-0281-0. * Alexander Schoedel [email protected] 1



Department of Biomedical and Chemical Engineering and Sciences, Florida Institute of Technology, 150 W University Blvd, Melbourne, FL 32901, USA

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1 Introduction “One of the continuing scandals in the physical sciences is that it remains in general impossible to predict the structure of even the simplest crystalline solids from a knowledge of their chemical composition” (John Maddox,1988) [1]. In this statement, Sir John Maddox, then the editor of Nature, referred to the enormous challenges of achieving structures by design associated with solidstate materials in general. Only 1  year later, Hoskins and Robson detailed the first example of deliberately designed and synthesized frameworks using geometrically defined building units [2]. A combination of tetrahedral ­CuI centers together with rigid organic nitrile linkers enabled the prediction and synthesis of a diamond (dia) net. Although, metal–organic crystals composed of single metal ion units, i.e., Cu(–CN)4 and dinitrile linkers were discovered in earnest in the late 1950s, their enormous potential has long remained unnoticed [3–5]. Those and related 1D, 2D, and 3D nets were later termed coordination polymers and enabled characterization at the atomic level by means of single-crystal X-ray diffraction. During the 1990s, other coordination polymers emerged, mainly built from single metal ions and linear pyridyl linkers, e.g., 4,4′-bipyridine. In these cases, interpenetration has often precluded the formation of cavities and the establishment of permanent porosity. However, these sometimes-called “first-generation” metal–organic frameworks (MOFs) have set the groundwork for development of more robust and complex materials. Nonetheless, metal–pyridine-based frameworks still represent a very active research area today, particularly with respect to gas adsorption and separation applications [6, 7]. Metal–organic frameworks (MOFs) have since attracted considerable scientific interest due to their many potential applications, such as gas storage and separation, catalysis, and chemical sensing, among others. Their potential impact on environmental issues in energy-related fields holds great promise towards carbon-neutral cycles, including the sequestration of carbon dioxide or the storage of methane and hydrogen for vehicular applications [8]. MOFs are composed of metal cluster entities, often referred to as secondary building units (SBUs) [9], that are in turn joined with multifunctional, branched organic molecules, the linkers. The inorganic and organic units are connected into extended, porous structures by virtue of crystal engineering [10] or reticular chemistry [11]. These concepts provide many opportunities for making robust metal–organic crystals by design and for translating molecular functionality and reactivity into the solid state [12]; this is in contrast to inorganic zeolites [13], mesoporous silica [14], and porous carbon [15] that are useful materials because of their permanent porosity and architectural stability, but lack of a modular nature and the amenability to fine-tuning of properties. Herein, we focus on what is referred to as “second-generation MOFs.” They are composed of discrete high-symmetry metal–carboxylate clusters, the SBUs.

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Their architectural stability allows for the evacuation of guests from the cavities or channels to create permanently porous materials. The difference to metal–pyridine coordination polymers arises from the bond energy between the building units, i.e., the neutral M–N bond compared to the charged M–carboxylate bond. However, the bond energy is still low enough to enable reversible reactions and therefore the growth of single crystals [16]. Metal–carboxylate MOFs had gained traction by the end of the 1990s following seminal contributions by Yaghi, Williams, and Kitagawa [17–19]. The first report on a microporous (pore diameter 0.2 bar. Indeed, at 77 K/1 atm, the simulated uptake for this model was more than double that the experimental value at the same state point. Including Feynman–Hibbs quantum corrections for simulations using the DL model did not decrease the theoretical uptakes by much, especially when compared to the experimental isotherm. As anticipated, incorporating electrostatic interactions within the simulations led to higher calculated uptakes for H2 in the material. However, in this case, it appears that the addition of such effects produced uptakes that notably deviate from the experimental values at the majority of the pressures considered. From this, the authors concluded that the DL model can be a highly attractive potential for simulating H2 adsorption in MOFs. Garberoglio et  al. obtained simulated H2 adsorption isotherms that somewhat underestimated experimental measurements for Mn-formate and IRMOF-8 at nearly all considered pressures at 77 K [73]. In addition, their simulated uptakes for IRMOF-1 and Cu-MMOM, a MOF that consists of Cu2+ ions coordinated to 4,4′ -(hexafluoroisopropylidene)bis(benzoate) linkers [131], at 298 K were drastically lower than those for the corresponding experimental measurements at all pressures. From these results, the authors concluded that it is not always possible to obtain simulated results that are in good agreement with experiment for gas adsorption in MOFs. Nevertheless, it was still shown that simulations could be useful for examining the adsorption trends in different MOFs. Thus, the authors utilized classical molecular simulations to predict the H2 adsorption isotherms for MOF-2, MOF-3, and the different IRMOFs that they considered at 77 and 298 K and pressures up to 100 bar. We note that no experimental high-pressure H2 adsorption data at these temperatures were available for such MOFs at the time. The simulations were performed using the Buch model with Feynman–Hibbs quantum corrections omitted for computational efficiency. Figure  18a displays the simulated absolute and excess H2 adsorption isotherms for all considered MOFs at 77 K and high pressures. The predicted trend in both the absolute and excess H2 uptakes at 77 K/100 bar was the following: IRMOF-14 > IRMOF-8 > IRMOF-1 > IRMOF-6 > MOF-2 > MOF-3. Interestingly, the simulated uptakes for MOF-2 and MOF-3 are greater than those for any of the IRMOFs within 0.01–1 bar, but lower than those for such MOFs at pressures > 3 bar. Moreover, the overall shapes of the isotherms for the two MOFs are relatively flat, which is distinct from that observed for the IRMOFs. This implies that MOF-2 and MOF-3 display a different H2 adsorption mechanism compared to that for the IRMOFs at this temperature. IRMOF-14 exhibits the highest H2 uptake at 77 K/100 bar out of all MOFs investigated, presumably because it possesses the largest surface area and pore volume and contains more aromatic C atoms within the linkers than the other materials. Furthermore, a smaller pore volume for IRMOF-6 relative to that for IRMOF-1 can potentially explain why the simulated H2 uptake for the former is lower than that for the latter at 77 K/100 bar. At 298 K, the shapes of the simulated absolute and excess H2 adsorption isotherms for all considered MOFs are very similar to each other, which indicates a common H2 adsorption mechanism for these materials at this temperature (Fig. 18b). Reprinted from the journal

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Fig. 18  Absolute (top) and excess (bottom) H2 adsorption isotherms in MOF-2 (closed circles), MOF-3 (squares), IRMOF-1 (open circles), IRMOF-6 (diamonds), IRMOF-8 (up-pointing triangles), and IRMOF-14 (down-pointing triangles) at a 77 K and b 298 K and pressures up to 100 bar for simulations using the Buch model with Feynman–Hibbs quantum corrections excluded according to the GCMC simulations performed in Ref. [73]. These figures were reproduced from Ref. [73] within the guidelines provided by the American Chemical Society. Copyright 2005 American Chemical Society

 

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Indeed, at high temperatures, the H2 molecules exhibit weak interactions with the framework regardless of the MOF environment, thus explaining the similarity in the shapes of the isotherms between the different materials. It can be observed that IRMOF-14 displayed the largest calculated absolute and excess H2 uptakes over the entire pressure range at 298 K. Thus, consistent with the results at 77 K, this MOF has the highest predicted H2 uptake at 298 K/100 bar out of all considered MOFs according to the simulations. This could be due to the fact that IRMOF-14 contains more aromatic C atoms on the organic linkers than the other MOFs, which can serve as potential sites for H2 at this temperature. However, the calculated H2 uptake for IRMOF-14 at this state point was not sufficient for the purposes of potentially using the material for vehicular applications. Thus, the results from these simulations suggested that none of the MOFs investigated were able to meet the challenging Department of Energy (DOE) target for onboard H2 storage at room temperature and high pressures at the time [135]. In general, the work by Garberoglio et al. showed that using different classical potential energy functions can lead to distinct results for simulations of gas adsorption in various prototypical MOFs. The authors further demonstrated that molecular simulations could still provide valuable information on the adsorption mechanisms in MOFs even if the theoretical results are not necessarily in quantitative agreement with the experimental data. 4.5  CO2 in IRMOF‑1 Another molecular simulation study that demonstrated the importance of stationary electrostatic interactions for simulations in MOFs was performed by Walton et  al. in 2008 [136]. The authors simulated CO2 adsorption in IRMOF-1 using GCMC methods over a wide range of temperatures and compared their results to the experimental data reported therein. The simulations were executed with a rigid crystal structure for the MOF, with all framework atoms being treated with Lennard–Jones parameters from DREIDING to model repulsion/dispersion interactions. The authors utilized the three-site electrostatic CO2 potential from the TraPPE force field, but carried out the simulations in cases in which electrostatic interactions were both included and excluded. Although no partial charges were assigned to the MOF atoms, electrostatic interactions were still considered between the CO2 molecules for simulations that included such effects. The authors claimed that including partial charges to the framework atoms did not significantly alter the results. Figure 19 shows the simulated CO2 adsorption isotherms that were obtained for the two “models” compared to experiment [137] in IRMOF-1 at 298 K and high pressures. When only Lennard–Jones interactions were considered, the simulations produced an isotherm that both underestimated experimental results at pressures > 103 kPa and did not reproduce the notable inflection point that was observed in the experimental isotherm. In contrast, simulations in which electrostatic interactions were included generated an isotherm that was in very good agreement with experiment over the considered pressure range. Furthermore, incorporating electrostatic effects allowed for the reproduction of the inflection point that was obtained experimentally. These results show the importance of Reprinted from the journal

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Fig. 19  CO2 adsorption isotherms in IRMOF-1 at 298 K and high pressures for experiment [137] (black) and simulations using the TraPPE CO2 potential with electrostatic interactions included (blue) and excluded (red) according to the theoretical study performed in Ref. [136]. This figure was reproduced from Ref. [136] within the guidelines provided by the American Chemical Society. Copyright 2008 American Chemical Society

electrostatic interactions between the CO2 molecules in reproducing the experimental adsorption isotherm in this MOF in terms of both CO2 uptake and shape of the isotherm. The authors also simulated CO2 adsorption with electrostatic effects added in IRMOF-1 at different temperatures ranging from 195 to 273 K and compared their results to the corresponding experimental data. Figure  20 shows the comparison of the experimental and simulated CO2 adsorption isotherms in IRMOF-1 at various temperatures. Even though the experimental isotherms exhibit dramatic steps with decreasing temperature, the simulations were still able to reproduce the complex shapes of these isotherms with a rigid MOF crystal structure. In addition, it can be observed that such simulated isotherms were in excellent agreement with the corresponding experimental measurements at these temperatures. Inspection of the simulated CO2 molecule positions in IRMOF-1 revealed that, below the sharp rise in the isotherms in Fig.  20, the CO2 molecules mostly adsorb near the corners of the cavities in the structure. As the pressure increases, the adsorbate molecules essentially fill in the pores of the MOF, leading to a type V isotherm with no hysteresis [138]. Walton et al. employed the same electrostatic model to simulate CO2 adsorption in IRMOF-3 [124] and MOF-177 [139] at 298 K and high pressures and obtained simulated isotherms that were in very good agreement with experiment for the respective MOFs in terms of both CO2 uptake and isotherm shape [136]. Overall, the study showed that including adsorbate–adsorbate electrostatic interactions in simulation can be essential for predicting the inflection points and steps in the gas adsorption isotherms in MOFs.

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Fig. 20  CO2 adsorption isotherms in IRMOF-1 at various temperatures and pressures up to 100 kPa for experiment (195 K = circles, 208 K = triangles, 218 K = squares, 233 K = stars, and 273 K = diamonds) and simulation (blue lines) according to the theoretical study performed in Ref. [136]. This figure was reproduced from Ref. [136] within the guidelines provided by the American Chemical Society. Copyright 2008 American Chemical Society

4.6  CO2 in Prototypical MOFs In 2008, Yang et  al. [140] performed a comprehensive simulation study of CO2 adsorption in nine different prototypical MOFs: IRMOF-X (X = 1, 8, 10, 11, 14, and 16) [124], MOF-177 [139], Cu-BTC [122], and Mn-formate [130]. As with their earlier theoretical studies [97, 99, 141], the authors used the three-site electrostatic CO2 potential from the TraPPE force field for the GCMC simulations. The OPLS-AA force field served as the basis for assigning the Lennard–Jones parameters to the atoms of the MOFs, although some of these parameters were refined in order to better reproduce the experimental CO2 adsorption results in simulation. The partial charges for the framework atoms were determined through DFT calculations on model fragments that were extracted from the crystal structure of the individual MOFs. The authors obtained simulated excess CO2 adsorption isotherms that were in very good agreement with the corresponding experimental measurements for most of the considered MOFs. By the time the work of Yang et al. was published, it was well-known in the literature that MOFs that possess small pore sizes display higher low-pressure gas uptake and Qst than those containing large pore sizes [142, 143]. This is because smaller pore sizes in MOFs allow for the adsorbate molecules to interact with multiple portions of the framework simultaneously. Overall, it was observed from the simulations that the smaller the pore size in the material, the higher the CO2 uptake at 298 K and low pressures [140]. To confirm this behavior, the authors calculated the theoretical CO2 Qst values for each MOF from GCMC simulation using fluctuation theory [87]. Figure 21a shows the relationship between the simulated CO2 Qst at zero loading and volumetric uptakes at 0.02 MPa in five IRMOFs that have the Reprinted from the journal

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Fig. 21  Simulated volumetric CO2 uptakes at 0.02 MPa plotted against Qst at zero loading for: a IRMOFs with the same pcu topology and b MOFs with different topologies according to the GCMC simulations performed in Ref. [140]. This figure was reproduced from Ref. [140] within the guidelines provided by the American Chemical Society. Copyright 2008 American Chemical Society

same primitive cubic (pcu) topology. The following trend can be observed in the Qst for CO2 in the considered IRMOF series: IRMOF-1 > IRMOF-14 > IRMOF-8 > IRMOF-10 > IRMOF-16. With IRMOF-14 excluded, the trend is consistent with the patterns in the pore size of these MOFs (i.e., Qst decreases as pore size increases). Even though IRMOF-14 possesses larger pore sizes than IRMOF-8, it probably displays a higher Qst value because this variant contains more aromatic C atoms within the organic linker. Figure  21b shows the analogous volumetric uptake vs. Qst plot for five MOFs exhibiting different topologies (IRMOF-1, IRMOF-11, MOF-177, Cu-BTC, and Mn-formate). The Qst for CO2 in IRMOF-11 is significantly higher than that for IRMOF-1 because it contains small pore sizes as a result of interpenetration of two equivalent frameworks. MOF-177 has a larger Qst than all IRMOFs studied with the exception of IRMOF-11 because of its different topology with smaller pore sizes in its structure. Cu-BTC and Mn-formate display much higher CO2 Qst than the other seven MOFs because of their even smaller pore sizes. Cu-BTC has the highest Qst out of all considered MOFs because it is the only MOF in this work that possesses open-metal sites. Overall, it can be concluded that smaller pore sizes in MOFs generally lead to a higher Qst value. Yang et al. also investigated the importance of electrostatic interactions for simulations of CO2 adsorption in three MOFs: IRMOF-10, IRMOF-14, and MOF-177 [140]. They examined these effects by turning off the electrostatic interactions between the CO2 molecules and the framework atoms for the considered MOFs in another series of GCMC simulations. By comparing the simulated results that they obtained in these control cases to those that were produced normally, the authors were able to determine the electrostatic contribution for CO2 adsorption in these MOFs at different pressures. Thus, Fig.  22 shows the averaged percent contribution from electrostatic interactions plotted as a function of pressure for simulations of CO2 adsorption in IRMOF-10, IRMOF-14, and MOF-177 at 298 K. It can be observed that for IRMOF-1 and IRMOF-14, both of which have the same pcu

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Fig. 22  Averaged percent contribution from electrostatic interactions in IRMOF-10 (black squares), IRMOF-14 (blue circles), and MOF-177 (red triangles) at 298 K and pressures up to 6.0 MPa according to the GCMC simulations performed in Ref. [140]. This figure was reproduced from Ref. [140] within the guidelines provided by the American Chemical Society. Copyright 2008 American Chemical Society

topology, electrostatic interactions contribute to about 20% of the total energy for CO2 adsorption at the lowest pressure evaluated. For MOF-177, the electrostatic contribution is enhanced to approximately 30% at the same pressure, presumably because it exhibits a different topology, with smaller pore sizes relative to the other two MOFs. For all three MOFs, the percentage from electrostatic contributions decreases with increasing pressure. This is likely because repulsion/dispersion interactions become more important between the CO2 molecules in the MOF at higher loadings, especially under conditions approaching CO2 saturation where the MOF acts like a container. Overall, Yang et al. demonstrated the effects of pore size and topology as well as the role of electrostatic interactions on CO2 adsorption in nine prototypical MOFs through their classical simulation studies.

5 Importance of Classical Polarization While the majority of reported classical simulation studies of gas adsorption in MOFs include solely repulsion/dispersion and stationary electrostatic interactions, some previous studies have shown that it was necessary to incorporate explicit many-body polarization effects for proper modeling of the MOF–adsorbate interactions, especially in MOFs that contain open-metal sites. Notably, it has been shown that it is possible to capture the expected adsorption of the gas onto the open-metal sites in such MOFs using a classical potential energy function rather than employing ab initio or DFT methods. The inclusion of induced dipole interactions was also requisite to reproduce the experimental gas adsorption isotherms and Qst in these materials through GCMC. 5.1  H2 in In‑soc‑MOF The first theoretical study that demonstrated the importance of classical polarization for simulations in MOFs was performed by Belof et al. in 2007 [56]. The authors utilized CMC methods to investigate H2 adsorption in In-soc-MOF, a highly charged Reprinted from the journal

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MOF that consists of cationic In3 O trimers coordinated to 5,5′-azobis-1,3-benzenedicarboxylate linkers [144]. This MOF has a rare square octahedral (soc) topology and is notable for the presence of open-metal sites through the In3+ ions of the In3 O trimers and uncoordinated NO−3 counterions that reside in the framework. Experimental studies have shown that In-soc-MOF can impressively adsorb ca. 2.50 wt% of H2 at 78 K/1 atm, with the density of adsorbed H2 in the pores being close to that for liquid H2 at this state point. Belof et al. carried out CMC simulations of H2 adsorbed in the MOF at 78 K/1 atm using three different H2 potentials of increasing complexity: a model including only Lennard–Jones interactions, an electrostatic (nonpolarizable) potential, and a Thole–Applequist many-body polarizable potential [56]. Figure 23 shows the g(r) of the COM of adsorbed H2 molecules about the In3+ ions and azobenzene N atoms in In-soc-MOF. It can be observed that simulations using only the polarizable model generated a large peak at ca. 3.25 Å in the g(r) about the In3+ ions (Fig.  23a). This peak corresponds to a significant quantity of H2 molecules adsorbed within the vicinity of the open-metal sites in the MOF. On the other hand, simulations using a nonpolarizable model produced a distribution in which the nearest-neighbor peak is both decreased in magnitude and shifted to a higher distance. This indicates that there are fewer H2 molecules localized near the In3+ ions in the modeled structure. Simulations using the potential that includes only Lennard–Jones interactions did not reveal any well-defined binding sites near the In3+ ions as exemplified by the broad distribution and lack of a noticeable nearestneighbor peak in the g(r) plot. Overall, these results indicate that the inclusion of explicit many-body polarization interactions was necessary to capture the binding of H2 onto the open-metal sites in the MOF. Simulations using both the electrostatic and polarizable potentials produced a large peak at ca. 3.4 Å in the g(r) about the azobenzene N atoms in In-soc-MOF (Fig.  23b). In contrast, the g(r) plot for simulations using the Lennard–Jones-only model reveals a smaller and broader distribution for the nearest-neighbor peak, which indicates that there are some H2 molecules adsorbed near the azobenzene N atoms, but located at farther distances. The nearest-neighbor peak for simulations using the polarizable model is both larger in magnitude and shifted to a lower distance compared to that for the electrostatic model. This suggests that the inclusion of explicit polarization interactions in simulation allowed for more H2 molecules to bind onto the azobenzene N atoms at shorter distances in the material. Thus, while the effect is less dramatic compared to the binding of H2 onto the In3+ sites, it can be deduced that polarization effects allow for increased interactions between the H2 molecule and the azobenzene N atoms. In 2015, Pham et al. performed a follow-up computational study of H2 adsorbed in In-soc-MOF using GCMC methods [145]. The authors utilized a five-site polarizable potential [116] to generate simulated H2 adsorption isotherms that were in very good agreement with experimental measurements at 77 and 87 K and pressures up to 1 atm. This study reasserted the notion that polarization interactions were important to capture the adsorption of H2 onto the In3+ ions in the MOF. Overall, the GCMC simulations carried out by Pham et al. reproduced the three expected H2 binding sites in the material: (1) the NO−3 counterions, (2) the In3+ ions of the In3 O

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Fig. 23  The g(r) of the COM of H2 molecules about a the In3+ ions of the In3 O trimers and b the azobenzene N atoms in In-soc-MOF for simulations using three different H2 potentials (Lennard–Jones-only model = red, electrostatic model = orange, polarizable model = red) at 78 K and 1 atm according to the CMC simulations performed in Ref. [56]. These figures were reproduced from Ref. [56] within the guidelines provided by the American Chemical Society. Copyright 2007 American Chemical Society

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trimers, and (3) the azobenzene N atoms. A close-up view of these binding sites as obtained from the MC simulation history is shown in Fig.  24. Furthermore, twodimensional quantum rotation calculations for H2 adsorbed about these sites using a classical potential energy function generated rotational transitions that were in good agreement with those that appeared in the INS spectra for the material. We note that while the inclusion of explicit polarization effects was necessary to properly model the In3+–H2 interaction in In-soc-MOF, charge–quadrupole interactions governed the binding of H2 onto the NO−3 counterions and azobenzene N atoms. This can explain why a different simulation study that included electrostatic interactions but neglected polarization was able to reproduce the adsorption of H2 onto these sites in this MOF [18]. 5.2  H2 O in MIL‑53(Cr) In 2012, Cirera et  al. revealed the importance of explicit polarization effects on H2 O adsorption in MIL-53(Cr) through classical MD simulation studies [57]. MIL-53(Cr) is a MOF that consists of CrO4 (OH)2 clusters that are interconnected by BDC linkers [146]. Experimental and theoretical studies have shown that this material undergoes a reversible structural transition upon the adsorption of H2 O [24, 147]. For parametrizing MIL-53(Cr), the authors employed the same bonded and nonbonded interaction parameters in the classical flexible force field that was developed for the MOF by Salles et  al. [24], but added atomic point polarizabilities to the framework atoms to model induced dipole interactions. Polarization effects were implemented in the simulations via the approach of Thole [61], and the polarizabilities for the MOF atoms were taken from the works of van Duijnen et al. [67] and Shannon [70].

Fig. 24  Molecular illustration of a H2 molecule (orange) about a the NO−3 counterions, b the In3+ ions, and c the azobenzene N atoms in In-soc-MOF as determined from the GCMC simulations performed in Ref. [145]. Atom colors: C = cyan, H = white, N = blue, O = red, In = yellow. These figures were reproduced from Ref. [145] within the guidelines provided by the American Chemical Society. Copyright 2014 American Chemical Society

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Three different H2 O models were used for the simulations: the anharmonic Simple Point-Charge Flexible water (aSPC/Fw) model [148], the Transferable Intermolecular Potential with 4 Points (TIP4P/2005) model [149], and an extension of the flexible, polarizable Thole-type model potential (TTM3-F) [150]. Only the last model includes explicit many-body polarization interactions, while the other two models are nonpolarizable. Cirera et  al. [57] carried out MD simulations of H2 O adsorption in MIL-53(Cr) using all three models for loadings ranging from 1 to 20 molecules per unit cell at a temperature of 300 K and a pressure of 1 atm within the constant stress and constant temperature ( N 𝜎T  ) ensemble [151]. Figure 25a shows the g(r) between the O atoms of the H2 O molecules (Ow) and certain atoms of the MOF–adsorbate system for all three models at different loadings for the thermodynamic condition considered (label of atoms shown in Fig. 25b). It can be observed that simulations using only the polarizable TTM3-F model produced a sharp peak at ca. 2.0 Å in the g(r) about the hydroxyl H atoms (Ho) for all considered loadings. This peak corresponds to a close hydrogen bonding interaction between the H2 O molecules and the OH− groups of the framework. On the other hand, this nearest-neighbor peak is both smaller in magnitude and located at larger distances for simulations using the aSPC/Fw and TIP4P/2005 potentials. This particular peak is shifted to even longer distances as the number of adsorbate molecules within the simulations increases for these models. This indicates that simulations using the nonpolarizable models do not properly describe the expected Ow–Ho interactions. Negligible differences can be observed in the g(r) of adsorbate O atoms about the aromatic C1 atoms of the linker as well as other Ow atoms for all three models at different loadings. The authors also performed MD simulations utilizing the TTM3-F model with the polarizability of the framework turned off and obtained g(r) plots that were very similar to those obtained from simulations using the aSPC/ Fw and TIP4P/2005 models. Overall, Cirera et  al. showed that including classical polarization in simulation was essential for capturing the formation of hydrogen bonds between the H2 O molecules and the hydroxyl groups in MIL-53(Cr). 5.3  H2 in PCN‑61 Since 2012, the Space group has demonstrated the importance of including explicit many-body polarization interactions for simulations of gas adsorption in various rht-MOFs [64, 108, 111–113, 152–156]. These MOFs consist of a metal ion (usually Cu2+ ) coordinated to a hexatopic linker that contains three coplanar isophthalate-based moieties [106]. All rht-MOFs contain open-metal sites through the [ M2 (O2 CR)4 ] clusters, with both metal ions projecting into chemically distinct environments in the structure. The first study that utilized classical polarization for simulations in an rht-MOF was performed by Forrest et al. on PCN-61 [64]. This MOF has a relatively simple linker and was therefore chosen for baseline computational studies. The classical force field for PCN-61 consisted of Lennard–Jones parameters from UFF, partial charges that were determined through fragment-based charge fitting, and point polarizabilities from the work of van Duijnen and Swart [67] for all light atoms (C, H, and O). The polarizability for Cu2+ was determined Reprinted from the journal

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through quantum mechanical calculations on fragments containing copper paddlewheel units. Forrest et al. performed GCMC simulations of H2 adsorption in PCN61 using three different, increasing realistic potentials: the Buch model [132], the five-site electrostatic model developed by Belof et  al. (denoted BSS, where BSS stands for Belof Stern Space) [116], and the five-site polarizable model that was also developed by Belof et al. (denoted BSSP, where BSSP stands for Belof Stern Space Polar) [116]. Figure  26a shows a comparison of the experimental [105] and simulated absolute H2 adsorption isotherms for the three different potentials in PCN-61 at 77 K and pressures up to 1 atm [64]. It can be observed that the uptakes produced by the Buch model underestimated experimental results for all pressures considered. This indicates that pure repulsion/dispersion interactions are insufficient to accurately model H2 adsorption in this MOF. At 77 K/1 atm, the uptake for the Buch model was approximately 28% lower than that for the experiment. Simulations using the BSS model generated a H2 adsorption isotherm that was somewhat higher than that for the Buch model for the considered pressure range. This implies that permanent electrostatic interactions are making an important contribution to the H2 adsorption mechanism in the MOF. However, as seen in Fig.  26a, the addition of such interactions was not enough to reproduce the experimental isotherm, as the calculated uptake for the BSS model at 77 K/1 atm was 11% lower than that for the experiment. In contrast, simulations using the BSSP model resulted in an isotherm that was in outstanding agreement with experiment to within joint uncertainties at all pressures, thus signifying the importance of polarization effects for simulations of gas adsorption in MOFs with exposed metal centers. Analogous trends can be observed in the simulated H2 Qst values for the three models compared to experiment (Fig. 26b). Specifically, only the polarizable BSSP model produced Qst values that were in very good agreement with experiment for all loadings considered, while such values for the Buch and BSS models underestimated experimental results at all uptakes. In general, the results displayed in Fig. 26 demonstrate that incorporating induced dipole interactions in simulation was required to model H2 adsorption in PCN-61 properly and reproduce experimental observables in this MOF containing open-metal sites. Analysis of the adsorption sites in PCN-61 through plotting the g(r) and dipole distribution revealed that the H2 molecules initially bind onto the Cu2+ ions that face away from the center of the linker in the material [64]. Only simulations using the BSSP model were able to reproduce the adsorption of H2 onto these open-metal sites. A close-up view of this site as obtained from the MC simulation history is shown in Fig. 27. The optimal distance that was captured between the COM of the H2 molecule and this Cu2+ ion from the simulations was ca. 2.5 Å, which is comparable to that for the corresponding interaction observed in HKUST-1 through NPD studies (2.39(1) Å) [6] and ab initio calculations (2.47 Å) [157] and within another rht-MOF through NPD (2.23(1) and 2.41(1) Å) [158]. We note that a similar theoretical study of H2 adsorption in an isostructural analogue of PCN-61 in which the alkyne groups in the linker arms are replaced with amide groups produced the same general trends described here for the different potentials [108]. However, the primary binding site in this MOF was different relative to that in PCN-61 as it was

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Fig. 25  a The g(r) of the Ow atoms of H2 O molecules about the Ho (left) and C1 (middle) atoms of MIL-53(Cr) and other adsorbate Ow atoms (right) in the MOF–adsorbate system for simulations using three different H2 O models (aSPC/Fw = blue, TIP4P/2005 = orange, and TTM3-F = red) at different loadings (N = 3, 5, 7, 10, 14, and 20) according to the MD simulations performed in Ref. [57]. b Label of the atoms used in the definition of the g(r) plots shown in a. These figures were reproduced from Ref. [57] within the guidelines provided by the American Institute of Physics. Copyright 2012 American Institute of Physics

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Fig. 26  a Absolute H2 adsorption isotherms at 77 K and pressures up to 1 atm and b Qst for H2 plotted as a function of loading in PCN-61 for experiment [105] (black) and simulations using the Buch [132] (blue), BSS [116] (green), and BSSP [116] (red) models according to the theoretical study involving explicit many-body polarization performed in Ref. [64]. These figures were reproduced from Ref. [64] within the guidelines provided by the American Chemical Society. Copyright 2012 American Chemical Society

 

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Fig. 27  Molecular illustration of a H2 molecule (orange) adsorbed onto one of the unique types of Cu2+ ions of the [ Cu2 (O2 CR)4 ] units in PCN-61 as determined from the GCMC simulations involving explicit many-body polarization performed in Refs. [64, 108]. Atom colors: C = cyan, H = white, O = red, Cu = tan. This figure was reproduced from Ref. [108] within the guidelines provided by the American Chemical Society. Copyright 2013 American Chemical Society

discovered that the H2 molecules prefer to adsorb onto the “other” type of Cu2+ ion of the copper paddlewheel in the material at low loading. 5.4  CO2 in PCN‑61 The addition of explicit polarization interactions was also shown to be critical for reproducing the expected metal–adsorbate interaction for simulations of CO2 adsorption in PCN-61. This was revealed in the work of Pham et al. in 2015 [152], where the authors performed GCMC simulations of CO2 adsorption in the MOF using two different potentials for the adsorbate: a five-site polarizable potential developed by Mullen et al. (denoted CO2-PHAST*, where PHAST stands for Potentials with High Accuracy, Speed, and Transferability, and the asterisk (*) denotes the inclusion of explicit polarization), [159] and the well-known three-site electrostatic potential from the TraPPE force field [160]. For these simulations, the authors used the same classical force field that was developed for the MOF in one of their previous theoretical studies [108]. Figure 28a shows the simulated excess CO2 adsorption isotherms in PCN-61 at 298 K and pressures up to 1 atm for both models compared to experiment [161]. Simulations using the polarizable CO2-PHAST potential generated an isotherm that is in excellent agreement with experimental results within the pressure range considered. The TraPPE model yielded an isotherm that slightly underestimates the experiment at most pressures, but it is still considered to be in very good agreement with the experimental data. However, in the case of the TraPPE model, producing results in good agreement with experimental results does not necessarily correspond to capturing the most important MOF–adsorbate interactions in simulation, as explained below.

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Fig. 28  a Excess CO2 adsorption isotherms at 298 K and pressures up to 1 atm and b g(r) of the C atoms of CO2 molecules about one of the unique types of Cu2+ ions of the [ Cu2 (O2 CR)4 ] units in PCN-61 at 298 K/0.2 atm for experiment (black) and simulations (green) using a polarizable CO2 potential [159] (solid) and the TraPPE potential [160] (dashed) according to the theoretical study involving explicit many-body polarization performed in Ref. [152]. Note that the g(r) plots were normalized to a total magnitude of unity over the distance examined. The inset of b shows a molecular illustration of a CO2 molecule adsorbed about this Cu2+ ion in the MOF as determined from the GCMC simulations. Atom colors: C = cyan, H = white, O = red, Cu = tan. These figures were reproduced from Ref. [152] within the guidelines provided by Wiley-VCH (Germany). Copyright 2015 Wiley-VCH (Germany)



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The g(r) of the C atoms of CO2 molecules about the preferential Cu2+ ions of the copper paddlewheel clusters in PCN-61 at 298 K/0.2 atm as obtained from simulations using both the CO2-PHAST* and TraPPE potentials are plotted in Fig. 28b [152]. A sharp peak can be observed at ca. 3.3 Å in the g(r) plot for the polarizable model, which represents a significant quantity of CO2 molecules adsorbing onto these Cu2+ ions in the MOF with this particular Cu2+–C(CO2 ) interaction distance (inset of Fig. 28b). This distance is actually comparable to that for the corresponding interaction observed in HKUST-1 through NPD and ab initio studies (3.0–3.2 Å) [162]. As expected, simulations involving explicit many-body polarization interactions captured a considerable amount of CO2 molecules binding onto the open-metal sites in PCN-61 at low pressures. On the other hand, simulations using the nonpolarizable TraPPE model resulted in a nearest-neighbor peak that was shifted to ca. 3.6 Å, which is farther away than the Cu2+–C(CO2 ) distance observed experimentally for metal paddlewheel adsorption. Thus, the CO2 molecules do not adsorb as closely to the Cu2+ ions for simulations using the TraPPE model compared to that for the CO2-PHAST model. In addition, the population of CO2 molecules about the open-metal sites was reduced for simulations using the TraPPE model as exemplified by the notably smaller nearestneighbor peak in the g(r) plot for this model. Overall, these results demonstrate that the TraPPE potential may not describe the adsorption of CO2 molecules onto the open-metal sites as well as a model that accounts for explicit polarization. Indeed, simulations using the TraPPE potential in PCN-61 mostly generated the adsorption of CO2 into regions that are dominated by repulsion/dispersion and electrostatic interactions, such as the corners of the truncated tetrahedral cages [152, 159]. Ultimately, it was shown by Space et al. that induced dipole effects were required to capture the binding of adsorbate molecules onto the highly charged and polar Cu2+ ions in rht-MOFs [64, 108, 111–113, 152–156]. Utilizing a model that has only Lennard–Jones parameters and partial charges such as the TraPPE model can be insufficient for capturing the most salient metal–adsorbate interactions in this platform of MOFs. 5.5  H2 in M‑MOF‑74 The M-MOF-74 (or CPO-27-M or M-DOBDC) series is one of the most popular classes of MOFs within the literature [163–168]. These MOFs are synthesized by combining metal ions in the 2+ oxidation state with 2,5-dioxido-1,4-benzenedicarboxylate (DOBDC) linkers to afford a honeycomb-like structure with cylindrical pores. Pham et al. have shown that it is possible to reproduce the metal–adsorbate interaction within different members of this series of MOFs through classical GCMC simulations [69, 169, 170]. Specifically, they utilized classical polarization to simulate H2 adsorption within the M-MOF-74 series, beginning with the Mg analogue in 2014 [169]. Prior to this, most theoretical studies of gas adsorption within this class of MOFs employed some version of targeted ab initio or DFT modeling [171–177]. Pham et  al. developed a polarizable force field for these MOFs by

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Fig. 29  a H2 adsorption isotherms at 77 K and pressures up to 100 kPa, b Qst for H2 plotted as a function of loading, and c g(r) of the COM atoms of H2 molecules about the Mg2+ ions in Mg-MOF-74 at 77 K/0.01 atm for experiment [178] (black) and simulations using three different models (Model 1 (DL) [133] = orange, Model 2 (BSS) [116] = green, and Model 3 (BSSP) [116] = red) according to the theoretical study involving explicit many-body polarization performed in Ref. [169]. Note that the g(r) plots were normalized to a total magnitude of unity over the distance examined. The inset of c shows a molecular illustration of a H2 molecule adsorbed about a Mg2+ ion in the MOF as determined from the GCMC simulations. Atom colors: C = cyan, H = white, O = red, Mg = gray. These figures were reproduced from Ref. [169] within the guidelines provided by the American Chemical Society. Copyright 2014 American Chemical Society

 

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treating all framework atoms with Lennard–Jones parameters from UFF, partial charges that were determined through electronic structure calculations on various model fragments, and point polarizabilities that were taken from the training set of van Duijnen and Swart [67] for all C, H, and O atoms and calculated separately for different metal ions [69, 169, 170]. Figure 29a shows the experimental [178] and simulated H2 adsorption isotherms in Mg-MOF-74 at 77 K and pressures up to 100 kPa [169]. The simulated results were generated using three different models for the adsorbate: (1) the DL model [133], (2) the BSS model [116], and (3) the BSSP model [116]. It can be observed that the isotherm produced using the DL model underestimated experimental results by a significant amount at low pressures. Although the simulated isotherm for this model comes into line with experiment starting at around 60 kPa, it still does not reproduce the shape of the experimental isotherm, thus implying an adsorption mechanism that is inconsistent with experimental expectations. Simulations using the BSS model generated uptakes that are lower than experiment for all pressures considered. Consistent with the DL model, the isotherm produced by the BSS model does not reflect the shape of the experimental H2 adsorption isotherm, which therefore indicates the lack of binding onto the Mg2+ ions. The DL model yielded higher uptakes than the BSS model in Mg-MOF-74 at all pressures because the magnitudes of the partial charges are greater for the former. Simulations using the polarizable BSSP model, however, resulted in a H2 adsorption isotherm that is in very good agreement with experiment at all pressures. Indeed, this model was able to

Fig. 30  Normalized distribution of the induced dipoles on H2 molecules adsorbed in Mg-MOF-74 (red), Ni-MOF-74 (blue), Co-MOF-74 (violet), Zn-MOF-74 (green), and Cu-MOF-74 (orange) at 77 K and 0.20 atm according to GCMC simulations involving explicit many-body polarization performed in Refs. [69, 170]. This figure was reproduced from Ref. [170] within the guidelines provided by the American Chemical Society. Copyright 2016 American Chemical Society Reprinted from the journal

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reproduce the exceptionally sharp increase in uptake at very low loading (< 5 kPa) within the experimental isotherm. This indicates that the BSSP model was able to capture the adsorption of H2 onto the exposed Mg2+ ions with high accuracy, thus further demonstrating the importance of induced dipole interactions for simulating H2 adsorption in MOFs with open-metal sites. The theoretical Qst values that were calculated for the three models in MgMOF-74 using GCMC methods are shown in Fig.  29b and compared to those obtained experimentally [178]. Simulations using the DL and BSS potentials produced nearly constant Qst values at about 6.5 and 6.0 kJ mol−1 , respectively, which are significantly lower than experiment for loadings below 1.0 mol mol−1 . In contrast, simulations using the BSSP model resulted in a Qst plot that is very close to experiment in both magnitude at all considered uptakes and general shape. Notably, simulations involving classical polarization allowed for the reproduction of the noticeable inverse sigmoidal shape of the experimental Qst plot. This implies that simulations using the BSSP model captured two well-known H2 adsorption sites within the material, with the first corresponding to binding onto the Mg2+ ions. Figure 29c displays the g(r) of the COM of the adsorbed H2 molecules about the Mg2+ ions in Mg-MOF-74 at 77 K/0.01 atm for all three models. Only simulations employing the BSSP model produced a large peak at ca. 2.60 Å, which corresponds to a sizeable quantity of H2 molecules binding onto the Mg2+ ions in the MOF with a Mg2+–COM(H2 ) distance of roughly 2.60 Å (inset of Fig. 29c). This distance is very close to that observed for the material through NPD studies (2.45(4) Å) [179] and DFT calculations (2.54 Å) [171]. In contrast, the g(r) plots for the other two models revealed a nearest-neighbor peak that is both reduced in magnitude and shifted to higher distances (ca. 3.20 Å). This indicates that simulations using these two electrostatic potentials mostly captured the adsorption of H2 onto the DOBDC linkers and not the open-metal sites. Pham et al. also reproduced experimental observables [178, 180, 181] and captured the binding of H2 onto the open-metal sites in the Ni, Co, Zn, and Cu analogues of the M-MOF-74 series through classical GCMC simulations that included explicit many-body polarization interactions [69, 170]. A significant outcome from their theoretical studies was that the authors were able to reproduce the experimental trend in the metal–H2 interaction strength within the series, which is Ni-MOF-74 > Co-MOF-74 > Mg-MOF-74 > Zn-MOF-74 > Cu-MOF-74. This was supported through plotting the normalized distribution of induced dipoles for H2 molecules from simulations involving explicit polarization in all five analogues at 77 K/0.2 atm, which are shown in Fig.  30. Two notable peaks can be observed within the dipole distribution for all MOFs, with the peak located on the far right correlating to the adsorption of H2 onto the open-metal sites in the material. Since the exposed metal ions in such MOFs are highly charged and polar, these sites should induce significantly high dipoles on the adsorbate molecules. Even though all M-MOF-74 variants are isostructural, each analogue induces dipole magnitudes of different ranges that depend on the strength of the metal–H2 interaction. Overall, it can be concluded that higher induced dipole magnitudes on the H2 molecules from the metals correspond to stronger energetic interactions between the metal ions and the adsorbate within the M-MOF-74 series.

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Fig. 31  a CO2 adsorption isotherms at 298 K and pressures up to 107 Pa and b Qst for CO2 plotted as a function of loading in Mg-MOF-74 for experiment (Herm et al. [187] = white diamonds, Queen et al. [189] = yellow left-pointing triangles, Yu et al. [188] = orange circles, and Dietzel et al. [186] = brown squares) and simulations using a polarizable force field (solid black line with down-pointing triangles), UFF (dashed blue line with circles), and the DFT-derived nonpolarizable force of Mercado et al. [190] (dotted green line with × ) according to the theoretical study involving explicit many-body polarization performed in Ref. [182]. Note, Herm et al. report results at 313 K rather than 298 K. These figures were reproduced from Ref. [182] within the guidelines provided by the American Chemical Society. Copyright 2017 American Chemical Society

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5.6  CO2 in M‑MOF‑74 Vlugt et al. have also implemented classical polarization for GCMC simulations of gas adsorption in MOFs. Notably, they developed polarizable force fields for different gases for simulations within various members of the M-MOF-74 series [59, 182–184]. One of the adsorbates that Becker et al. studied within this class of MOFs starting in 2016 was CO2 [59]. Consistent with the work of Pham et al. [169], the authors developed a polarizable force field for Mg-MOF-74 by treating the framework atoms with Lennard–Jones parameters, partial charges, and point polarizabilities. The Lennard–Jones parameters were taken from UFF, although the value of 𝜖 for each atom was scaled using an equation that relates this parameter to the individual and largest point polarizabilities [59]. For modeling electrostatic interactions, the authors used the MOF partial charges that were reported in the study of Lin et al. [175]. The point polarizabilities were taken from the works of van Duijnen and Swart [67] for the light atoms and Shannon [70] for the metal and scaled with a factor of 0.09. Becker et al. used the CO2 potential from the TraPPE force field as a starting point to develop a polarizable potential for the adsorbate [59]. Specifically, they adjusted the Lennard–Jones parameters for the TraPPE model and added point polarizabilities to the atomic sites. As executed in the Space group, the authors calculated the many-body polarization energy of the MOF–adsorbate system via Eq.  (10). However, instead of using an iterative method to calculate the induced dipole at each site, the authors employed the assumption of Lachet et  al., which considers only the polarization between the framework and the adsorbate molecules, not those caused by the induced dipoles in the system [185]. As a result, the contribution of the induced electric field is neglected and the calculation of the polarization energy becomes the following: N

Upol = −

1 ∑ ◦ ⃗ stat 2 𝛼 |E | . 2 i i i

(22)

In 2017, Becker et  al. [182] performed GCMC simulations of CO2 adsorption in Mg-MOF-74 using polarizable potentials for both the MOF and the adsorbate and compared the resulting theoretical isotherms and Qst values to those obtained from different experimental measurements that were reported for the material [186–189]. The authors also carried out the simulations using the normal TraPPE CO2 potential with unmodified UFF parameters for the MOF (denoted the UFF force field) and the DFT-derived nonpolarizable potential developed by Mercado et al. [190] for comparison. The isotherms obtained from four different experimental measurements and those calculated from simulations using the three distinct force fields in MgMOF-74 at 298 K and pressures up to 107 Pa are plotted in Fig. 31a. Simulations using the UFF force field, which includes only repulsion/dispersion and stationary electrostatic interactions, produced uptakes that were significantly lower than those for any of the experimental measurements within the low-pressure region ( < 104 Pa ). This signifies the lack of CO2 molecules adsorbing onto the Mg2+ ions in the MOF, thereby demonstrating that electrostatic effects are insufficient to

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capture this interaction in a classical simulation. On the other hand, simulations using the polarizable force field resulted in an isotherm that was in very good agreement with the experimental isotherms at essentially all pressures [182]. Notably, by reproducing experimental CO2 uptakes within the low-pressure region, simulations involving explicit many-body polarization generated the expected adsorption of CO2 molecules onto the open-metal sites in the material. In addition, simulations using this force field reproduced the notable inflection point that was observed in all four experimental adsorption isotherms. The simulated isotherm produced by the force field of Mercado et  al. is also in good agreement with the experimental measurements. This is because this nonpolarizable force field consists of parameters that were fitted to reproduce the Mg2+ −C(CO2 ) interaction in Mg-MOF-74 through DFT calculations [190]. Simulations using all three force fields generated uptakes that were in close agreement with the experimental data at high pressures ( > 105 Pa ). This is because the MOF acts like a simple container under such conditions, as the adsorption of CO2 is dominated by the remaining accessible volume. Figure 31b shows a comparison of the CO2 Qst plots that were reported from three different experimental studies [186, 188, 189] and those that were obtained from simulations using the polarizable and UFF force fields in Mg-MOF-74. All experimental Qst plots exhibit an initial Qst value within the range of 40 to 45 kJ mol−1 . These values remain constant up until a loading of about 0.70 CO2 ∕Mg , at which the Qst starts to drop. At an uptake of roughly 1 CO2 ∕Mg , all experimental Qst plots are within the range of 25 to 30 kJ mol−1 , which indicates that all metal sites are occupied in the MOF at this loading. Simulations using the UFF force field produced a zero-loading Qst value of about 28 kJ mol−1 , which is notably lower than any of the corresponding experimental values. Indeed, simulations using this force field did not yield a Qst plot that is representative of experiment as such Qst values increased gradually as the loading increases. In contrast, the Qst plot obtained from simulations using the polarizable force field is in outstanding agreement with the experimental plots in both magnitudes and shape. Notably, the inverse sigmoidal shape that is displayed by all three experimental Qst plots was reproduced from simulations utilizing classical polarization. The theoretical initial CO2 Qst value that was generated by the polarizable force field was 39 kJ mol−1 , which is close to the analogous experimental values. These results support the notion that simulations using a polarizable force field captured the binding of CO2 onto the exposed metal ions in Mg-MOF-74, whereas simulations using the UFF force field did not. After utilizing the aforementioned force field to simulate CO2 adsorption in MgMOF-74, Becker et  al. extended their methods to carry out analogous simulations within different metal variants of the M-MOF-74 series [182]. The simulated CO2 adsorption isotherms at 298 K and Qst values that were obtained using the polarizable force field in Co-MOF-74, Cu-MOF-74, Ni-MOF-74, and Zn-MOF-74 are compared to the corresponding experimental data [189] in Fig.  32. Similar to the results for the Mg analogue, simulations involving explicit many-body polarization resulted in CO2 uptakes that were in excellent agreement with experiment for all four variants at all considered pressures (Fig. 32a). Particularly, the distinct shapes that are observed in the experimental isotherms for these MOFs were reproduced from Reprinted from the journal

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Fig. 32  a CO2 adsorption isotherms at 298 K and pressures up to 107 Pa and b Qst for CO2 plotted as a function of loading in Co-MOF-74 (squares), Cu-MOF-74 (circles), Ni-MOF-74 (right-pointing triangles), and Zn-MOF-74 (diamonds) for experiment [189] (yellow) and simulations using a polarizable force field (black) according to the theoretical study involving explicit many-body polarization performed in Ref. [182]. These figures were reproduced from Ref. [182] within the guidelines provided by the American Chemical Society. Copyright 2017 American Chemical Society

 

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such simulations. The simulated Qst values for these analogues, however, are only in decent agreement with the corresponding experimental data (Fig. 32b). The authors attributed the difference between the experimental and simulated Qst values for these variants to the presence of residual solvent molecules in the structure. Nevertheless, by using a polarizable force field to simulate CO2 adsorption within the M-MOF74 series, the authors were able to reproduce the experimentally observed trend in the metal–CO2 interaction within this class of MOFs, which is the following: MgMOF-74 > Ni-MOF-74 > Co-MOF-74 > Zn-MOF-74 > Cu-MOF-74. Utilizing a force field that includes only repulsion/dispersion and electrostatic parameters did not reproduce this pattern among the M-MOF-74 analogues.

6 Conclusions and Outlook Ever since Kawakami et al. [119] published the first molecular simulation study of gas adsorption in a MOF through GCMC methods, a vast number of subsequent classical simulation studies have been reported that provided detailed insights into the gas adsorption mechanisms and binding sites in different MOFs. The groups of Snurr [123], Johnson [73], Greathouse [22], Zhong [97], Sholl [191], Space [56], Smit [192], and Jiang [17] are some of the major theoretical research groups that have consistently reported classical MOF–adsorbate simulation studies to contribute to our increasing understanding of the mechanism of gas adsorption in these materials. Simulations of gas adsorption in MOFs are widely performed using GCMC methods, for which a general overview of this technique was provided in this review. In order to carry out classical molecular simulations, a force field for both the MOF and the adsorbate must be established. As described in this review, the majority of classical MOF force fields contain Lennard–Jones parameters and partial charges assigned to the framework atoms to model repulsion/dispersion and stationary electrostatic interactions, respectively. As pioneered by the Space group, the MOF atoms can also be treated with point polarizabilities to handle explicit manybody polarization interactions in MC simulations [56]. Common adsorbate potentials that have been utilized in various MOF–adsorbate simulation studies include the Buch, DL, BSS, and BSSP models for H2 [116, 132, 133], as well as potentials from the TraPPE force field [125, 160] and the recently emerging PHAST potentials [35, 155, 159, 193, 194] for CO2 , CH4 , N2 , and different hydrocarbons. Theoretical techniques that can be used to probe the adsorbate binding sites in MOFs include viewing the GCMC snapshots and probability distributions, plotting the density contours and g(r), and performing simulated annealing calculations. The normalized dipole distribution for an adsorbate in the MOF could also be plotted from simulations involving explicit polarization to identify the binding sites. The choice of the method to utilize can depend on factors such as the MOF–adsorbate system that is under investigation and the general scope of the work. Each method has its own advantages and drawbacks for determining the locations of the adsorption sites. For example, while examining the GCMC snapshots and probability distributions are relatively straightforward processes, these methods may not provide a clear portrayal of the most favorable binding site in the material if there is a large Reprinted from the journal

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number of adsorbate molecules in the system. Moreover, while plotting the g(r) can provide quantitative information on the relative population and interaction distances for an adsorbate molecule about a particular MOF atom, such plots do not reveal specific regions of occupancy in the structure. The inclusion of explicit many-body polarization effects in simulation has been shown to be critical for the proper modeling of gas adsorption in highly charged and polar MOFs, including those that contain open-metal sites. While calculating the polarization energy can be computationally expensive, certain iterative methods [65, 66] can be employed to solve the many-body equations in a timely fashion. The groups of Space [64] and Vlugt [182] have demonstrated that using a polarizable force field for the MOF and the adsorbate was essential to reproduce the experimental gas adsorption isotherms and Qst values in such MOFs through GCMC simulations. Notably, these groups have shown that it is possible to accurately capture the metal–adsorbate interaction within the popular M-MOF-74 series using classical polarization [59, 69, 169, 170, 182–184]. In addition, Cirera et al. revealed that polarization effects played an important role in hydrogen bond formation within a MOF–adsorbate system through MD simulations [57]. Although utilizing currently available methods can provide sufficient insights into the MOF–adsorbate interactions, future efforts could involve developing more sophisticated classical modeling techniques and force fields in order to better reproduce experimental observables and explain more complicated gas adsorption mechanisms in certain MOFs. For example, it is actually desirable to compute more realistic repulsion/dispersion parameters for the MOF atoms rather than using Lennard–Jones parameters from known force fields. Since repulsion/dispersion interactions are usually the main contributor to the total energy for many MOF–adsorbate systems, the quantitative results obtained from these classical simulations can be highly sensitive to the choice of the parameters utilized. One possible alternative for handling van der Waals interactions for simulations in MOFs is through the coupled-dipole model [195]. Furthermore, performing simulations of gas adsorption in flexible MOFs can be quite challenging since such MOFs have their own unique structural behavior in the presence of certain gases. As a result, a special force field must be established for each flexible MOF in order to model the flexibility upon adsorbate binding properly. In order to make modeling gas adsorption in flexible MOFs more convenient, the development of a general force field that is transferable for simulations in these MOFs is required [10]. Overall, the results from classical simulation studies in MOFs can greatly supplement experimental measurements by providing molecular-level details on the gas adsorption mechanism and rationalizing certain data that are collected from such experimental studies. This information can help guide scientists and engineers to gain new perspectives into creating new MOFs that display improved gas uptake and Qst . This is needed in order to design and synthesize novel porous materials that can target specific energy-related applications, such as the onboard storage of H2 [3] and CH4 [4] and capturing CO2 from flue gas or directly from the atmosphere [5]. If the interplay between experimental and theoretical MOF–adsorbate studies continues, then tackling certain significant problems within this energy economy could become a reality in the future.

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Acknowledgements The authors acknowledge the National Science Foundation (Award No. DMR1607989), including support from the Major Research Instrumentation Program (Award No. CHE1531590). B.S. also acknowledges support from an American Chemical Society Petroleum Research Fund Grant (ACS PRF 56673-ND6).

Compliance with Ethical Standards  Conflict of interest  The authors declare that they have no conflict of interest.

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Affiliations Tony Pham1   · Brian Space1  1



Department of Chemistry, University of South Florida, 4202 East Fowler Avenue, CHE205, Tampa, Fl 33620‑5250, USA

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Topics in Current Chemistry (2020) 378:25 https://doi.org/10.1007/s41061-020-0288-6 REVIEW

Theoretical Exploration and Electronic Applications of Conductive Two‑Dimensional Metal–Organic Frameworks Jia Gao1,2,4 · Shubo Geng1,2,4 · Yao Chen1,3 · Peng Cheng1,2,4 · Zhenjie Zhang1,2,4  Received: 30 June 2019 / Accepted: 5 February 2020 / Published online: 18 February 2020 © Springer Nature Switzerland AG 2020

Abstract Two-dimensional (2D) metal–organic frameworks (MOFs) belong to a subgroup of MOFs reminiscent of graphite and covalent organic frameworks (COFs). In the past decade, conductive 2D MOFs have received increasing attention due to their relatively high charge carrier mobility and low resistivity that originate from in-plane charge delocalization and extended π  conjugation within the layers. This review comprises the current state-of-the-art of the representative progress in theoretical exploration and electronic applications of conductive 2D MOFs. Special emphasis is placed on the intrinsic relations between the structural factors and the electronic properties of conductive 2D MOFs. This review will provide guidance for researchers to design and synthesize conductive 2D MOFs for advanced applications. Keywords  Metal–organic frameworks · Two-dimensional · Electronic · Conductivity

Chapter 8 was originally published as Gao, J., Geng, S., Chen, Y., Cheng, P. & Zhang, Z. Topics in Current Chemistry (2020) 378: 25. https://doi.org/10.1007/s41061-020-0288-6. * Zhenjie Zhang [email protected] 1

College of Chemistry, Nankai University, Tianjin 300071, China

2

College of Pharmacy, Nankai University, Tianjin 300071, China

3

State Key Laboratory of Medicinal Chemical Biology, Nankai University, Tianjin 300071, China

4

Key Laboratory of Advanced Energy Materials Chemistry (MOE), Nankai University, Tianjin 300071, China



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1 Introduction As an emerging class of porous materials, metal–organic frameworks (MOFs) have undergone a bright development during the past decade. Numerous MOFs with ordered crystalline, tunable pore size and various shapes can be prepared from a wide range of organic linkers and metal ions [1–8]. Many functional MOF series materials, such as IRMOF (isoreticular metal–organic framework) series, ZIF (zeolitic imidazolate framework) series, PCN (porous coordination network) series, MIL (Materials of Institute Lavoisier) series, UiO (University of Oslo) series, and CPL (coordination pillared-layered) series, have been reported with great potential in many fields [9–16]. MOFs are widely utilized in a variety of territories, including gas storage and separation [17–19], catalysis [20–22], chemical sensing [23–25], conductivity [26, 27], drug delivery and separation [28–34], etc. According to their structural characteristics, MOFs can be divided into twodimensional (2D) MOFs and three-dimensional (3D) MOFs. Up to now, MOFs have been dominated by 3D structures, while 2D MOFs are relatively less studied. 2D MOFs are composed of layers of extended conjugated planes that bind with metal ions or clusters bridged with organic ligands, which is similar to the metal-complex nanosheets [35–37]. Multiple monolayers then further stack to form layered bulk structures in a manner analogous to graphene and graphite. Conductive 2D MOFs have received increasing attention as a novel class of electronic materials owing to their relatively high charge carrier mobility and low resistivity, which cannot be achieved in 3D MOFs. This review article focuses on highlighting the latest fundamental and applied advances in research on the electronic conductivities of 2D MOFs. We review several key breakthroughs in exploring the unique properties of conductive 2D MOFs in terms of theoretical aspects and summarize recent progress in the application of conductive 2D MOFs for the development of electronic devices and sensors.

2 Construction of Conductive 2D MOFs 2D MOFs with thin-layered structure have attracted great attention in the past decade owing to their advantages of good charge transport together with high porosity and excellent electrical conductivity [38]. As shown in Fig.  1, 2D layered MOFs can be fabricated by two distinct approaches: bottom-up and topdown [39]. Bottom-up approaches are considered as a self-assembly process in building up 2D nanostructures on an atomic scale by assembling the materials from atoms, ions, or molecules. Typical bottom-up approaches are wet chemical synthesis [40], chemical vapor deposition, and physical vapor transport [41]. Topdown approaches are generally based on techniques that can exfoliate the bulk 3D MOFs with a layered structure into monolayer or multiple layers, including mechanical exfoliation [41], expansion and separation of layers by intercalation

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Fig. 1  Illustration of methods used to prepare 2D layered MOFs: (upper) bottom-up assembly and (bottom) top-down exfoliation

[42], and chemical etching [43]. Compared with the top-down approaches, bottom-up approaches exhibit greater structural diversity in design and choice of molecular and ionic components, allowing the tailoring of nanosheets for specific applications, especially for electronic uses. To the best of our knowledge, most of the conductive 2D MOFs are synthesized through bottom-up approaches. According to the “node and linker” design principle reported by Robson and the “reticular chemistry” strategy reported by Yaghi and O’Keeffe, MOFs can be considered as the combination of metallic nodes (e.g., Ni, Cu, Co, Pd) and organic linkers within coordination groups (e.g., –OH, –NH2, –SH). Achieving high electrical conductivity requires maximizing the concentration and mobility of charge carriers. In 2D conductive MOFs, the graphene-like planar structure confines the charge transport in the direction of the plane as a result of improved conjugation and enhanced π–d orbital coupling, and both metallic nodes and organic linkers can serve as the source of charge carriers. Hence, those organic linkers with either stable radicals or redox-active molecules together with metal nodes possessing unpaired electrons can achieve high charge delocalization through continuous π conjugation in 2D MOFs. Another approach is to introduce inherently redox-active linkers that exhibit numerous accessible redox states (Fig. 2). The highly π-stacked and extended π-conjugated network enables high electrical conductivity. In conductive 2D MOFs, metal nodes coordinate with conjugated ligands to form 2D layers with sql and hcb topology. The adjacent layers can stack in different ways including eclipsed (AA) and slipped-parallel (AB) orientations to form 3D supramolecular structure via weak interactions such as π–π interaction and electrostatic interaction. In some cases, subtle changes in the substitution of the ligand and the metal node can form distinct layer stacking patterns. For example, as shown in Fig.  3, 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) coordinated with ­Ni2+ or ­Co2+ to generate a 2D hcb network in an AB stacking mode, while Reprinted from the journal

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Fig. 2  Representative ligands used in construction of conductive 2D layered MOFs

Fig. 3  AA and AB stacking pattern of 2D layered MOFs

HHTP coordinated with C ­ u2+ to form a 2D hcb network in an AA stacking pattern [44]. The difference of stacking mode can affect the spatial and orbital overlap and change the through-space charge transport between the two adjacent layers, which can be predicted with theoretical study.

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3 Theoretical Exploration of Conductive 2D MOFs Conductive 2D MOFs usually exhibit high electrical conductivity due to full charge delocalization in the 2D plane or π–π stacking along the stacked columns. The delocalization of π electrons in conductive 2D MOFs not only leads to high electrical conductivity but also gives rise to some unique physical properties including topological insulator behavior [45, 46], the quantum anomalous Hall effect [47, 48], and flat-band ferromagnetism [49]. Owing to the periodic essence of the 2D MOFs, all the structural factors of bulk materials can be obtained in high accuracy and reliability by applying ab  initio or density functional theory (DFT) calculations on a single crystal lattice. Theoretical methods play a critical role in predicting the electronic, optical, and mechanical properties of conductive 2D MOFs. On the basis of theoretical studies in spintronics, conductive 2D MOFs are predicted to possess a non-zero bandgap and even behave as a topological insulator (TI) [50–58]. TIs represent a class of functional materials with insulating bulk electronic states but conducting boundary states distinguished by nontrivial topology. TIs exhibits nontrivial bulk band topology around a global spin–orbit coupling (SOC) gap in both a Dirac band and a flat band and hence demonstrates some unique quantum transport properties with potential applications in spintronics and quantum computing. Electronic structures of 2D TIs are characterized by a bulk SOC gap and an odd number of Dirac-like edge states connecting the conduction and valence edge at certain k  points. Without SOC, the electronic band structure displays a Dirac cone with the Fermi level located exactly at the Dirac point. Turning on the SOC will open a small bandgap at the K point. Hence, the two key factors for searching and designing a TI material are the lattice symmetry and the SOC. As a branch of TI materials, conductive 2D MOFs could provide some extra advantages over their inorganic counterparts. Firstly, conductive 2D MOFs are less sensitive against oxidation, which greatly simplifies the device fabrication. Secondly, the wide variety of metal ions and organic ligand choices make the tailored design of the electronic properties of 2D MOF TIs possible. In 2013, the first identification of nontrivial topological states in a structure of conductive 2D MOF, ­Ni3(BHT)2, was reported by Liu et al. [59]. First-principles calculations of band structure, edge state, Chern number, and spin Hall conductance showed that the structure exhibited nontrivial topological states of both a Dirac band and a flat band in a lattice structure. A typical kagome band exists in the ­Ni3(BHT)2 lattice (single layer) with SOC around the Fermi level, consisting of one flat band above two Dirac bands. Besides N ­ i3(BHT)2, a theoretical derivative of triphenyl-lead [Pb(C6H5)3] lattice was also proposed, which consists of a Pb atom bonded to three benzene rings with threefold rotational symmetry [60]. The 2D lattice of [Pb(C6H5)3] (single layer) is slightly bulked with the paraPb atoms moving alternately up and down out of the plane of the benzene rings to satisfy the Pb 6s26p2 electronic configuration that favors sp3 hybridization (Fig. 4). The gap of the [Pb(C6H5)3] lattice is located exactly in the middle of the Dirac cone and is relatively small (ca. 8.6 meV). One option to increase the gap

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Fig. 4  A Top and side view of the 2D organometallic superlattice B, D Band structures of triphenyl-lead (TL) lattice without and with spin–orbit coupling (SOC), respectively. C, E Zoom-in view of band structures around the Dirac point without and with SOC, respectively. Reprinted with permission from [60]. Copyright 2013 Macmillan Publishers Limited

is to choose a different metal atom with larger SOC, e.g., by replacing Pb with Bi in the design of the [Pb(C6H5)3] lattice. The Dirac cone gap of [Pb(C6H5)3] is enlarged (ca. 43 meV), together with an increase of the Fermi level above the Dirac point without SOC. Doping of holes in the unit cell will move the Fermi level back into the Dirac cone gap, which is possibly attributed to a gate effect. These theoretical studies suggest a high tunability of conductive 2D MOFs with metal substitution. Metal substitution in the lattice of conductive 2D MOFs can result in changes from both electronic properties and structural factors. Zeng et al. studied the effect of substitution of coordinated metal ions from Ni to Cu in ­M3(HITP)2 on the structural

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and electronic properties of the MOF bulks and 2D sheets [61]. ­Ni3(HITP)2 and ­Cu3(HITP)2 single sheets exhibit the same kagome lattice pattern. Both these kagome lattices possess Dirac bands in the vicinity of the Fermi level. In ­Ni3(HITP)2, each Ni atom adopts dsp2 hybridization, whereas in ­Cu3(HITP)2, each Cu atom adopts sp3 hybridization. The computed electronic band structures, density of state (DOS), and the charge density isosurface for the bands crossing the Fermi level are presented in Fig. 5. The bulk structures of ­Ni3(HITP)2 and ­Cu3(HITP)2 crystals are metallic with a band crossing the Fermi level, which is consistent with the excellent bulk and thin-film conductivity of 2 and 40  S/cm in experiments [62]. The band structures exhibit relatively strong dispersions indicating strong π–π interaction between metal–organic sheets along their stacking direction. The ­M3(HITP)2 bulk structure, assembled from M ­ 3(HITP)2 sheets, is stabilized by strong π–π interaction and weak metal–metal interaction. However, when reducing dimensionality from 3D ­M3(HTIP)2 bulk structure to 2D sheets, the ­Ni3(HTIP)2 sheets transform into indirect semiconductors with a small bandgap of 0.13 eV, while the C ­ u3(HTIP)2 sheets are still metallic. On the basis of the natural bond orbital (NBO) analysis, the Ni atoms in the ­Ni3(HITP)2 sheet are known to adopt dsp2 hybridization to form the square-planar geometry and then form a perfectly 2D conjugated plane. Each Cu atom in the ­Cu3(HITP)2 adopts sp3 hybridization, resulting in slightly distorted 2D sheets. This theoretical study provides new insights into the tunability of electronic and structural properties of conductive 2D MOFs. Theoretical calculations predict that conductive 2D MOFs are metallic in bulk form. Intriguingly, 2D sheets of the ­Ni3(HITP)2 are predicted to be semiconducting and have a nonzero bandgap. The conductivity of ­Ni3(HITP)2 bulk sample in experiments is observed to increase with temperature, consistent with a nonzero bandgap. However, the exponential increase expected for a semiconductor is not displayed in experiments [62]. Foster et al. explained this abnormal phenomenon of ­Ni3(HITP)2 [63]. Monolayers of ­Ni3(HITP)2 possess a quantum spin Hall state in their band structure and shows a small gap. By contrast, the 3D material is metallic, and its calculated band structure exhibits appreciable dispersion, not only in-plane but also perpendicular to the stacking plane. As shown in Fig. 6, ­Ni3(HITP)2 bulk structure yields a metallic band structure and nonzero density of states at the Fermi energy in consideration of spin polarization. A nonzero magnetic moment is predicted, but the results are qualitatively the same and the systems are still predicted to be metallic. This result is consistent with previous calculations, which showed that the inclusion of SOC changes only the fine details of the band structure. The large band dispersion perpendicular to the stacking planes reveals that there are strong interplane interactions among the electronic wave functions. These interactions should decrease to zero as the interlayer spacing increases, corresponding to the evolution of the material from 3D to 2D. Grain boundaries and defects introduce barriers to transport, causing charge carriers to be localized and requiring thermal energy to surmount the activation barrier. Electronic structure calculations are also used to explore the evolution of the band structure with increasing interlayer distance. The calculations show appreciable band dispersion not only in-plane but also perpendicular to the stacking planes, which suggests that the material may exhibit appreciable conductivity in all crystallographic directions. Reprinted from the journal

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Fig. 5  Computed band structures, DOS (A, C, left) and charge density isosurface of bands crossing the Fermi level (B, D, left) are for the 3D ­M3(HITP)2 (M = Ni and Cu) bulk structures; computed band structures, DOS (A, D, right), charge-density isosurface (B, E, right) and a zoom-in view of kagome bands with SOC gaps (C, F, right) obtained from the SOC calculations of 2D ­M3(HITP)2 (M = Ni and Cu) sheets. Reprinted with permission from [61]. Copyright 2015 Owner Societies

 

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Internal interface defects can also greatly influence the electronic structure. Some experimental facts have been proven: (1) ­Ni3(HITP)2 depicts a monotonic increase in conductivity with increasing temperature, which is a signature of the presence of a charge-hopping barrier; (2) ­Ni3(HITP)2 sample under scanning electron microscopy and atomic force microscopy shows a large density of grains with diameters of tens of nanometer [62]. Foster et  al. evaluated the electronic structure of ­Ni3(HITP)2 structural models within three kinds of interface defects: (1) a perpendicular grain boundary in which two grains meet with their layers perpendicularly; (2) a strike–slip fault in which the material is fractured, and one grain is slipped in-plane with respect to the other; (3) a layer–layer displacement defect (stacking fault) [64]. Their studies demonstrate that interface defects can introduce a transport barrier by breaking the π conjugation and decreasing the dispersion of the electronic bands near the Fermi level (Fig. 7). Although the ideal ­Ni3(HITP)2 model of extended conjugated sheets is predicted to be highly conducting and has a metallic electronic structure [46, 61, 63], the overall conductivity is governed by charge hopping within the presence of defects such as grain boundaries and nonoptimal layer–layer displacements. The delocalized electronic states in conductive 2D MOFs may also give rise to some exotic magnetic properties of the lattice. In 2017, Dincă et al. designed a series of metal–phthalocyanine (MPc)-based 2D MOFs using first-principles calculations [65]. The result demonstrated that NiMn-OIPc, a charge-neutral derivative made from octaamino-phthalocyanines metallated with M ­ n2+ ions and bound to N ­ i2+ ions, exhibits a half-metallic and ferromagnetic ground state with large exchange energy. In the 2D MOF of NiMPc, magnetic moments are predominantly localized on the transition metals atoms in the MPc moieties and the square-planar Ni ions connecting the MPc moieties do not carry magnetic moments. Analysis of the partial density of states (PDOS) reveals that the NiMnPc is half-metal since the electronic states in the vicinity of the Fermi level are completely spin polarized (Fig.  8). By comparing the energies of the systems with ferromagnetic (FM) or antiferromagnetic (AFM) coupling between the transition metal (TM) atoms in the MPc moieties, NiMn-OIPc exhibits an FM metallic ground state with larger exchange energy than other NiMPc-based 2D MOFs. The mechanism of the strong FM coupling in NiMnPc may originate from the strong hybridization between the metal d orbitals and the p electrons of the ligand in octaamino-MnPc.

4 Electronic Applications of Conductive 2D MOFs Two-dimensional MOFs have been drawing considerable interest for their outstanding conductivity values, as a result of in-plane charge delocalization and extended π  conjugation in the 2D sheets. The emergence of 2D MOFs with high intrinsic charge mobility or electrical conductivity provides an opportunity for the development of a new class of electronic devices such as active electrodes, chemiresistive sensors, and field effect transistors (FETs) that are suitable for a broad range of energy and environmental applications. Reprinted from the journal

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Fig. 6  A Primitive unit cell of 2D (monolayer) ­ Ni3(HITP)2. B Primitive unit cell of 3D (bulk) ­Ni3(HITP)2. Band structure of ­Ni3(HITP)2 of C monolayer, D bulk parallel stacked, and E parallel displaced to minimum-energy structure. Reprinted with permission from [63]. Copyright 2016 American Chemical Society

The frameworks of 2D MOFs are assembled from multichelating ligands such as multitopic, dithiolene, o-semiquinone, and o-phenylenediamine aromatic organic moieties coordinated with square-planar metal ions. The multichelating ligands play a conclusive role in determining the band structure, π-conjugation distribution, and intralayer charge mobility of conductive 2D MOFs. On the basis of the variety of chelating sites and central parts, the multichelating ligands used in the synthesis of 2D MOFs can be categorized into three groups: (a) o-semiquinone ligand, HHTP; (b) o-phenylenediamine ligands, HIB and HITP; (c) dithiolene ligands, BHT and HTT. In this section, we review and summarize the representative fundamental and applied advances in the research of conductive 2D MOFs constructed with different multichelating ligands. 4.1 Conductive 2D MOFs with o‑Semiquinone‑Based Ligands The first conductive 2D MOFs were prepared by combining HHTP, highly conjugated tricatecholate (CAT), with metal(II) acetate hydrated in an aqueous solution to form 2D porous extended frameworks. In 2012, Yaghi et al. successfully obtained a series of such 2D MOFs and named them metal–catecholates (M-CATs) [44]. The

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crystalline structure of Co-CAT-1 was determined using synchrotron radiation and features two distinct types of alternatively stacked layers with the HHTP molecules in high planarity, which led to good conductivity (Fig. 9). The electrical conductivity of CATs was measured at room temperature on single-crystal samples by using a four-point probe method. The conductance of Cu-CAT-1 was measured to be as high as 2.0 × 10−1 S/m, which is ten times higher than any previously reported value for an iodine-loaded MOFs [66]. After 50 charge–discharge cyclic voltammetry measurements, Cu-CAT-1 exhibits a lithium ion capacity of 284 C/g (80 mA h/g), greater than a previously reported MOF (MIL-53) at similar timescales. All these promising results reveal that conductive 2D MOFs can be utilized as an electrochemical energy storage material. Moreover, the large bulk capacitance of conductive 2D MOFs and their low contact resistance at the interface make them a good candidate as ion-to-electron transducers with promising utility in potentiometric detection. In 2018, Mirică et al.

Fig. 7  Schematic diagrams of the three interface defect structures in ­Ni3(HITP)2: A perpendicular grain boundary; B strike–slip fault between grains; C layer–layer displacement defect. Interface band structure: D perpendicular interface; E strike–slip fault. Reprinted with permission from [64]. Copyright 2018 American Chemical Society Reprinted from the journal

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successfully fabricated ­M3(HHTP)2 2D MOF (M = Cu, Ni, and Co)-based multilayer potentiometric sensing devices for ­K+ and ­NO3− ion detection. A potentiometric sensor was formed by drop casting ­M3(HHTP)2 onto a glassy carbon electrode and the resulting device exhibited excellent signal stability, low long-term drift, and high sensitivity to ­K+ and ­NO3−, respectively [67]. 4.2 Conductive 2D MOFs with Phenylenediamine‑Based Ligands Phenylenediamine-based 2D MOFs are the largest family among the conductive 2D MOFs. In 2017, a family of hexaiminobenzene-based 2D MOFs, M ­ 3(HIB)2, were synthesized through liquid–liquid and air–liquid interfacial reactions by Louie and co-workers [68]. Using HIB as the coordinating ligand and metal ions such as N ­ i2+, 2+ 2+ ­Cu , and ­Co as the metal linkers, a series of ­M3(HIB)2 thin film samples with not only large lateral dimension (1–2 μm) and ultrathin ( 10,000

90

52

25

48.7

48i

355h

> 20,000h

70 h

279

~ 110j

5.9

6320h

44.54h

10.72h

46.9

6409.1

Molecular sieving

Molecular sieving

Thermodynamics driven

Thermodynamics driven

Thermodynamics driven

Thermodynamics driven

Molecular sieving

Kinetics driven

Molecular sieving

Thermodynamics driven

Thermodynamics driven

Thermodynamics driven

Thermodynamics driven

Molecular sieving

Thermodynamics driven

Thermodynamics driven

Thermodynamics driven

Kinetics driven

Molecular sieving

Thermodynamics driven

Thermodynamics driven

Thermodynamics driven

Thermodynamics driven

Selectivityb Separation mechanism

[80]

[79]

[76]

[75]

[74]

[73]

[71]

[70]

[69]

[68]

[45]

[60]

[59]

[58]

[57]

[56]

[55]

[54]

[53]

[52]

[51]

[42]

[41]

Reference

Topics in Current Chemistry (2019) 377:33  

13

13

310

 Selectivity is for a 1/99 mixture

 Value is only for the qualitative comparison purposes

j

 Selectivity is for a 0.5/99.5 mixture

i

h

 At 0.6 bar

g

 At 0.45 bar

 At 303 K

 At 293 K

d

e

OX > EB > MX > PX

 Selectivity is calculated by IAST for an equimolar mixture at 298 K and 1 bar

 At 296 K

c

b

f

11

Co2(dobdc)

PX > MX > OX > EB

nHEX > 3MP > 23DMB

8.5

MAF-X8

Zr6O4(OH)4(bptc)3 ~ 4.5

 Ssingle gas adsorption data collected at 298 K and 1 bar

a

Aromatic ­C8 isomers

nHEX > 2MP > 3MP > 22DMB ≈ 23DMB

nHEX > 3MP > 22DMB

4.9–5.8

C4H6 > n-C4H8 > iso-C4H8

i-C4H8 > n-C4H8 > C4H10 > C4H6

5.5–6

Ca(H2tcpb)

C5–C6 alkane isomers Fe2(BDP)3

4.2

3.6

GeFSIX-14-Cu-i

Zn-BTM

C4 olefins

The former

Aperture size (Å Adsorption uptake (mmol g−1)a

MOF

Gas separation

Table 1  (continued) The latter

Thermodynamics driven

Kinetics driven

Molecular sieving

Molecular sieving

Thermodynamics driven

Thermodynamics driven

Molecular sieving

Selectivityb Separation mechanism

[93]

[92]

[91]

[90]

[89]

[85]

[84]

Reference

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mesh-adjustable molecular sieve N ­ i8(5-bbdc)6(μ3-OH)4 (MAMS-1) to separate C ­ 2H4 from ­CH4 by controlling separation temperatures [39, 40]. Recently, Peng et al. [41] developed two stable ultramicroporous MOFs, NKMOF1-M (Cu[M(pdt)2], M=Cu or Ni), which exhibit highly selective adsorption of C ­ 2H2 over ­CH4 owing to the specific binding site for ­C2H2. As shown in Fig. 1a,b, it was found that [M(pdt)2]− building units formed by Cu or Ni ions with four sulfur atoms, can generate three-dinemsional (3D) networks with one-dimensional (1D) square channels (5.75  Å) in NKMOF-1-M by linking with four connected square planar Cu centers. Single-component adsorption isotherms revealed that NKMOF-1-Ni adsorbs 61.0 cm3 g−1 and 33.7 cm3 g−1 of ­C2H2 at 298 K under 1 bar and 0.003 bar, respectively (Fig.  1c). This high ­C2H2 capture capacity, combined with high isosteric enthalpy of ­C2H2 adsorption (60.3 kJ mol−1), indicates the existence of strong interactions between ­C2H2 molecules and the framework. Based on dispersion-corrected density functional theory (DFT-D) calculations, NKMOF-1-M possesses two binding sites (I and II) for C ­ 2H2 (Fig. 1d). In site I, C ­ 2H2 molecules interact with the pyrazine units through hydrogen bonding [HC≡CH···S(MOF)] and π–π interactions, which is stronger than site II (namely OMS), where ­C2H2 molecules are between two adjacent ­MS4 units. The ideal absorbed solution theory (IAST) selectivities of NKMOF-1-Ni for ­C2H2/CH4 (1/1, v/v) and ­C2H2/CH4 (2/1, v/v) were calculated to be 6409.1 and 4949.2 at 298  K, respectively, which are the benchmark selectivities reported so far (Fig. 1e). The exceptional C ­ 2H2/CH4 separation performance of

Fig. 1  a [M(pdt)2]− metalloligands (M=Cu, Ni) in the ultramicroporous metal-organic framework (MOF) NKMOF-1-M. b Crystal structure of NKMOF-1-M viewed along the c axis. c Single-component gas adsorption isotherms of NKMOF-1-M at 298  K. d Two ­C2H2 binding sites (I, II) determined by single-crystal structure of C ­ 2H2@NKMOF-1-Ni. e, f Ideal absorbed solution theory (IAST) adsorption selectivities (e) and breakthrough curves (f) of various ­C2H2 gas mixtures at 298 K for NKMOF-1-Ni. Reprinted with permission from [41] Copyright 2018, Wiley–VCH Reprinted from the journal

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NKMOF-1-Ni was further confirmed by breakthrough experiments, producing pure ­CH4 with the concentration of ­C2H2 under 1 ppm (Fig. 1f). In 2019, Xing and colleagues [42] reported the first ultramicroporous MOF based on the closo-dodecaborate cluster, ­[CuB12H12(bpa)2] (BSF-1), which interconnected via B–H···M interactions and B–Hδ-···Hδ+–C interactions, showing excellent separation performance for ­C2H2/CH4 mixtures. At 298 K and 1 bar, the C ­ 2H2 uptake of BSF-1 is 52.6  cm3  g−1, whereas the C ­ H4 uptake is only 10.5  cm3  g−1. Modeling studies using first-principles DFT-D calculations revealed that ­C2H2 molecules had strong interactions with BSF-1 through B–H···H–C dihydrogen bonds (1.99 Å), and the binding energy was −35.7  kJ  mol−1. In contrast, ­CH4 molecules in the BSF-1 channels showed a lower binding energy (−25.5  kJ  mol−1). As a result, the IAST selectivity for equimolar ­C2H2/CH4 mixture of BSF-1 reach a high value (46.9) at 298 K and 1 bar. Moreover, its good cyclability and stability make BSF-1 a possible separation material for industrial processes. 2.2 C2H2/C2H4 Separation To obtain polymer-grade C ­ 2H4, ­C2H2/C2H4 separation is one of the most important but challenging industrial processes. The commercial technologies used currently— partial hydrogenation and solvent extraction—are highly cost and energy-intensive. Adsorption-based porous materials offer great promise to create cost-effective and energy-efficient separation technologies. In 2011, the M’MOF series was the first example used for ­C2H2/C2H4 separation [43]; thereafter, many MOFs have been developed for this separation [37, 44–50]. However, most porous MOFs reported so far suffer from a trade-off between ­C2H2 adsorption capacity and ­C2H2/C2H4 selectivity. For example, although the open metal sites (OMSs) within Fe-MOF-74 can notably enhance its ­C2H2 uptake capacity, the large nanopores lead to quite a low selectivity of 2.08 [37, 44]. The same situation was also observed in NOTT-300, which has high ­C2H2 uptake but low selectivity of 2.17 [45]. In contrast, with high selectivities up to 24, the M’MOF series features narrow pore space and so exhibits low ­C2H2 uptake capacities [46]. Tremendous efforts have been made in recent years to address this trade-off. In 2014, Chen and colleagues developed a MOF, UTSA-100a, featuring dual functionalities, which could achieve ­C2H2 removal from ­C2H2/C2H4 mixtures [51]. Due to the narrow windows of 3.3 Å and suitable cages of 4.0 Å, UTSA-100a exhibits high ­C2H2/C2H4 selectivity (10.72). In addition, the –NH2 groups on the pore surfaces enhance affinity towards ­C2H2 molecules, leading to high ­C2H2 uptake. Combining both high selectivity and ­C2H2 uptake, UTSA-100a is superior to other reported MOFs such as MOF-74, NOTT-300 and M’MOF-3. Separation performance was further confirmed by breakthrough experiments. Despite some progress, C ­ 2H2/C2H4 selectivity is still not satisfactory for practical applications and the above trade-off is not fully addressed. In 2016, Cui et al. [52] reported a series of SIFSIX materials with their adsorption and separation properties of ­C2H2 and ­C2H4, in which the trade-off has been remarkably minimized. All these SIFSIX materials (SIFSIX-1-Cu, SIFSIX-2-Cu, SIFSIX-2-Cu-i, SIFSIX-3-Cu,

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SIFSIX-3-Zn and SIFSIX-3-Ni) feature C ­ 2H2/C2H4 separation performance, contributing to much higher uptake of ­C2H2 than ­C2H4. Particularly, SIFSIX-2-Cu-i shows both high ­C2H2/C2H4 selectivity (39.7–44.8) and extremely high ­C2H2 uptake of 2.1  mmol  g−1 at 298  K and 0.025  bar. DFT calculations and neutron powder diffraction experiments revealed that each ­C2H2 molecule could bond with two F atoms from different nets. It was the strong interactions between C ­ 2H2 molecules and framework that led to the outstanding adsorption and separation properties of SIFSIX-2-Cu-i. The breakthrough experiments on SIFSIX materials further confirmed their excellent separation performance for ­C2H2/C2H4 mixtures. In the former case of SIFSIX-2-Cu-i, the trade-off between adsorption capacity and selectivity for ­C2H2/C2H4 separation was significantly minimized; however, the pore size of SIFSIX-2-Cu-i is still larger than that of both ­C2H2 and ­C2H4 molecules, which cannot preclude coadsorption of the larger ­C2H4 molecule, resulting in moderate selectivity. To fully address the trade-off on ­C2H2/C2H4 separation, Li et  al. [53] used a shorter organic linker of azpy (9.0 Å) instead of dpa (9.6 Å) to construct an ideal molecular sieve, namely SIFSIX-14-Cu-i/UTSA-200 (Fig. 2a) with a contracted pore size (3.4 Å). Structural and modeling studies showed that the pore size in UTSA-200 is smaller than the larger ­C2H4, and can completely block C ­ 2H4. Thus, UTSA-200a exhibits a steep and high C ­ 2H2 uptake of 116 cm3 cm−3 at 298 K and 1 bar (Fig. 2b). Conversely, entry of the ­C2H4 molecule can be prevented completely by UTSA-200a below 0.2 bar, with very little uptake (≈ 0.25 mmol g−1) up to 0.7 bar

Fig. 2  a Structure description of UTSA-200a, indicating the channel structure with a pore size of ≈ 3.4  Å; dispersion-corrected density functional theory (DFT-D)-calculated ­C2H2 adsorption models in UTSA-200a. b Adsorption isotherms of C ­ 2H2 (circles) and ­C2H4 (triangles) for UTSA-200a and SIFSIX-2-Cu-i at 298  K and 1  bar. c Plots of the captured C ­ 2H2 amount as a function of τbreak in the simulated column breakthrough for UTSA-200a and the other indicated materials. d Neutron crystal structure of UTSA-200a·C2D2 at 200  K viewed along the c axis. e Experimental column breakthrough curves of UTSA-200a for ­C2H2/C2H4 (1/99) separation at 298 K and 1.01 bar, compared with the indicated MOFs. Reprinted with permission from [53]. Copyright 2017, Wiley–VCH Reprinted from the journal

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at 298  K. Neutron powder diffraction data indicated that the stronger C≡D···F H-bonding (1.921 Å) compared to SIFSIX-2-Cu-i (2.015 Å) is the main reason for the ultrastrong C ­ 2H2 adsorption of UTSA-200 (Fig.  2d). As a result, UTSA-200a exhibits record selectivity of 6000 at 1 bar and 298 K for a 1/99 mixture (Fig. 2c). In addition, UTSA-200a also exhibits the highest C ­ 2H2 uptake (1.74 mol kg−1) for adsorption from this gas mixture. UTSA-200a thus set new benchmarks for both adsorption capacity and selectivity, fully addressing the trade-off on C ­ 2H2/C2H4 separation. Experimental breakthrough curves confirmed its benchmark separation performance for removal of trace ­C2H2 from C ­ 2H4 from a 1/99 C ­ 2H2/C2H4 mixture (Fig. 2e). The aforementioned MOFs for C ­ 2H2/C2H4 separation are dependent mainly on thermodynamics (enthalpic)-driven mechanisms where the MOF adsorbent expresses a relatively higher affinity toward C ­ 2H2 over ­C2H4. Recently, Zhou and co-workers reported a novel MOF, (NH4 ){CuII3 ⋅ [CuII CuI6 (OH)6 (Ad)6 ]2 } ⋅ (H2 O)x (NbU-1), which has planar metal-binding sites to separate ­C2H2/C2H4 efficiently via a kinetics-driven mechanism [54]. The separation selectivity values for NbU-1 are 12.1 and 5.9 under 100 kPa at 273 K and 298 K, respectively. Abnormally, the adsorption enthalpies for ­C2H2 and ­C2H4 are almost identical, with values of 38.3 and 37.9, indicating that the ­C2H2/C2H4 separation mechanism in NbU-1 is not thermodynamic driven. The Lewis-basic sites such as amino groups of adenine, combined with the planar metal-binding sites on the pores, coordinately lead to strong interaction between NbU-1 and gas molecules. To test the equilibrium time for ­C2H2 and ­C2H4, the former is much shorter than the latter, manifesting the fast diffusion rate within NbU-1 for ­C2H2. The factors above endow NbU-1 with the longest dimensionless breakthrough time of ­C2H2 for the 50/50 mixtures. Furthermore, cheap starting materials and high stability make NbU-1 a potentially practical adsorbent material for ­C2H2/C2H4 separation. 2.3 C3H4/C3H6 Separation Propylene ­(C3H6) is one of the most common prime olefin raw materials for petrochemical production, second only to ethylene. The industrial production of ­C3H6 by steam cracking in petroleum refining is unavoidably mixed with a trace impurity of ­C3H4, which must be removed to meet the criterion of polymer-grade C ­ 3H6 (the ­C3H4 concentration is required to be below 5 ppm). Adsorptive C ­ 3H4/C3H6 separation based on porous materials is a more cost-effective and energy-efficient method than traditional cryogenic distillation. However, due to the similar size of C ­ 3H4 and ­C3H6 molecules (6.2 × 3.8 × 3.8 vs. 6.5 × 4.0 × 4.2 Å3), their separation is very challenging. To date, only a handful of MOFs have been realized to show excellent separation performance on this challenging separation [55–57]. Chen and co-workers reported the first example of flexible-robust MOF, [Cu(bpy)2(OTf)2] (ELM-12), for C ­ 3H4/C3H6 (1/99, v/v) separation [55]. This flexible-robust framework is formed by a rigid square-grid copper bipyridine scaffold with dynamic dangling ­OTf− groups, resulting in two kinds of cavities (I and II) with a size of 6.1 × 4.3 ×  4.3 Å3 and 6.8 × 4.0 ×  4.2 Å3, respectively. Compared with

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­C3H6, ­C3H4 molecules could match better with the cavities due to suitable size and shape. Therefore, ELM-12 exhibits notably higher uptake of ­C3H4 (1.83 mmol g−1) than of C ­ 3H6 (0.67 mmol g−1) at 0.01 bar and 298 K, leading to high selectivities of both 1/99 (83) and 50/50 (279) C ­ 3H4/C3H6 mixture. Neutron powder diffraction data showed that specific sites in the two cavities provide affinities towards ­C3H4 molecules. Particularly, the two ­OTf− groups form short C−H···O hydrogen bonds with two ­C3H4 molecules in cavity I, and the longer C−H···O hydrogen bonds in cavity II exhibit relatively weaker interactions with C ­ 3H4 molecules. The breakthrough experiments confirmed that ELM-12 can efficiently realize ­C3H4/C3H6 separation to produce 99.9998% pure ­C3H6. Subsequently, Xing and co-workers found that three SIFSIX materials, including SIFSIX-1-Cu, SIFSIX-2-Cu-i and SIFSIX-3-Ni, had the potential to realize ­C3H4/ C3H6 separation efficiently [56]. All these SIFSIX materials could not only adsorb ­C3H4 steeply at low pressure, but also exhibited larger amounts of ­C3H4 uptake than ­C3H6 at 298 K and 1 bar. In particular, due to the strong interactions between ­C3H4 molecules and frameworks, the ­C3H4 uptake of SIFSIX-3-Ni is nearly saturated at 0.003 bar (2.65 mmol g−1). DFT calculations further demonstrated that the small pore space (7.5 × 4.6 × 4.6  Å3) and geometric disposition of ­SiF62− anions in SIFSIX-3-Ni fits well with the guest ­C3H4 molecule in terms of both molecular shape and hydrogen bonding interacting sites, endowing it with the properties of a highly efficient single-molecule trap for capture of C ­ 3H4 over C ­ 3H6. The efficient removal of trace ­C3H4 from ­C3H4/C3H6 mixtures by SIFSIX-3-Ni was further confirmed by column breakthrough experiments. Detailed structural studies have indicated that the pore space of SIFSIX-3-Ni is slightly larger than both ­C3H4 and ­C3H6 molecules, allowing this material to adsorb large amounts of both ­C3H4 and ­C3H6. Wen et al. [57] used pyz-NH2 with functional sites and larger size as the ligand instead of pyz to construct ZJUT-1, which features an isoreticular network with SIFSIX-3-Ni and improved C ­ 3H4/C3H6 separation performance. Compared with SIFSIX-3-Ni, the pore size of ZJUT-1 contracted to 7.5 × 3.7 × 3.7  Å3 because of the amino groups incorporated on the pore surfaces. This fine-tuned pore size is more suitable for trapping single ­C3H4 molecules, while ­C3H6 cannot be accommodated. As a result, the ­C3H6 uptake of ZJUT-1 decreased notably from 88 cm3 cm−3 in SIFSIX-3-Ni to 28 cm3 cm−3, while the ­C3H4 uptake was comparable to that of SIFSIX-3-Ni (89 vs. 91 cm3 cm−3). In addition, ZJUT-1 showed the benchmark uptake ratio of 3.06 at 298 K and 1 bar, with relatively high selectivity for a 1/99 C ­ 3H4/C3H6 mixture of 70. Combined with the fine-tuned nanocages and multiple hydrogen bonds formed by ­SiF62−/–NH2 and C ­ 3H4 molecules, ZJUT-1 exhibited good ­C3H4/C3H6 adsorption and separation performance. Breakthrough experiments confirmed the efficient removal of trace C ­ 3H4 from 1/99 ­C3H4/C3H6 mixtures and the production of 99.9995% pure ­C3H6 in ZJUT-1. Through comprehensive screening of large number of MOFs, Chen’s group found that UTSA-200a is the best material for the removal of trace C ­ 3H4 from ­C3H4/C3H6 mixtures [58]. Structural analysis revealed that UTSA-200 possesses channels of 3.4 Å much smaller than both ­C3H4 and ­C3H6, which might induce selective sieving toward the larger C ­ 3H6 molecules when the framework flexibility, and thus slightly enlarged pore sizes, are taken into the account. Indeed, gas sorption isotherms found Reprinted from the journal

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that UTSA-200a shows the highest C ­ 3H4 uptake (95 cm3 cm−3) at 0.01 bar for all the materials reported (Fig. 3c), while the fine-tuned pore size of UTSA-200a can efficiently exclude the ­C3H6 molecule at low pressures, indicating its bright promise for ­C3H4/C3H6 separation. The adsorption selectivity of UTSA-200 exhibits an extraordinary high selectivity of over 20,000 for a 1/99 (v/v) C ­ 3H4/C3H6 mixture at 298 K (Fig.  3d), far exceeding the previous benchmark ELM-12 (83) and SIFSIX-3-Ni (76). This exceptional separation performance is attributed to the framework flexibility originated from the rotation of pyridine rings inside the pores and the strong binding sites that can selectively block the larger ­C3H6 but capture large amount of the preferred smaller ­C3H4 at low-pressure regions, as supported by DFT calculations and NPD measurements (Fig.  3b). Breakthrough experiments confirmed that UTSA-200a can efficiently remove trace ­C3H4 from 1/99 and 0.1/99.9 ­C3H4/C3H6 mixtures with record-high ­C3H6 production scales (Fig. 3e). Similarly, Yang et  al. [59] reported two flexible MOFs (GeFSIX-14-i and TIFSIX-14-i) to separate trace C ­ 3H4 from ­C3H6, whose structures are isostructural to the nets in UTSA-200a. Different central coordination atoms (Ge and Ti) instead of Si endow these two MOFs with slightly different C–H···F hydrogen bond distances, called “F sliding” behavior, leading to the precise control of threshold pressures. TIFSIX-14-i has a longer C–H···F hydrogen bond distance (2.47 Å), compared with GeFSIX-14-i (2.46 Å), so that pyridine rings can rotate more easily for C ­ 3H4 adsorption. As a consequence, TIFSIX-14-i, with ultra-low threshold pressure and a strong interaction with ­C3H4, has a ­C3H4 uptake capacity of up to 2.18  mmol  g−1 and 3.88 mmol g−1 under 0.01 bar and 1 bar. Conversely, the larger-sized C ­ 3H6 molecule

Fig. 3  a Pore aperture and pore chemistry of SIXSIF materials. b DFT-D calculated structure and binding site of UTSA-200 ⊃ C3H4. c Experimental ­C3H4 and ­C3H6 adsorption isotherms of SIFSIX-2-Cu-i (black), SIFSIX-3-Ni (blue), and UTSA-200 (red) at 298 K in the region of 0–0.05 bar. d IAST selectivity of the indicated MOFs from 1:99 (v/v) gas mixtures. e Experimental breakthrough curves for 1:99 (v/v) mixture under a flow of 2.0 mL min−1 at 298 K and 1.01 bar. Reprinted with permission from [58]. Copyright 2018, Wiley–VCH

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is still excluded from TIFSIX-14-i, and thus the ­C3H6 capacity is only 1.4 mmol g−1 under 1 bar at 298 K. The IAST selectivity of TIFSIX-14-i is ~355 under 1 bar at 298 K for 1/99 ­C3H4/C3H6 mixture, and the breakthrough experiment confirmed the high performance for separation of a 1/99 ­C3H4/C3H6 mixture. The simultaneous removal of trace propyne and propadiene from propylene was also realized by several groups recently. Yang et  al. [60] reported a novel anionpillared MOF, ZU-62/NbOFFIVE-2-Cu-i, featuring asymmetric O/F node coordination for simultaneous removal of trace propyne and propadiene from propylene. Compared with the uniform pore cavities in SIFSIX-2-Cu-i, a narrow distribution of multiple binding sites (Site I 6.75 Å, Site II 6.94 Å, Site III 7.20 Å) was generated within ZU-62 (Fig.  4a), caused by the shrinking pendant F atoms and the difference in bond length between Nb=O (1.75 Å) and Nb-F (2.01 Å). DFT-D calculated results revealed that Site III is the more energy favorable binding site (− 0.59 eV) for propyne molecules through strong C≡H···F H-bonding, whereas Site I is the most energy favorable binding site (− 0.48 eV) for propadiene through multiple C=H···F weak H-bonding. The multisite-adsorbent contributed to the high affinity for both propyne and propadiene, as confirmed by the adsorption isotherms. As shown in

Fig. 4  a The structure of ZU-62 and SIFSIX-2-Cu-i. Red F, dark blue Nb, gray-40% C; gray-25% H, bright green O; turquiose N; yellow Cu. b Adsorption isotherm results on ZU-62 and SIFSIX-2-Cu-i. c IAST results of ZU-62 and SIFSIX-2-Cu-i. d Experimental column breakthrough curves for propyne/ propadiene/propylene (0.5:0.5:99) separation. Reprinted with permission from [60]. Copyright 2018, Wiley–VCH Reprinted from the journal

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Fig.  4b, ZU-62 exhibits record propadiene uptake (1.74  mmol  g−1) and excellent propyne uptake (1.87  mmol  g−1) under ultra-low pressure (5000  ppm) at 298  K, while only 0.05 mmol g−1 for propylene uptake. The IAST selectivities for propyne/ propylene and propadiene/propylene (0.5/99.5) reached high values of 48 and 34 (Fig.  4c), respectively. Breakthrough experiments for 0.5/0.5/99 propyne/propadiene/propylene mixtures revealed that high-purity propylene (over 99.9999%) could be captured through a one-step multicomponent removal (Fig. 4d). Similarly, Chen and colleagues [61] reported a class of microporous MOFs, NKMOF-1-M (M=Cu, Ni), which also show benchmark selectivities for ternary propyne/propadiene/propylene (0.5/0.5/99.0) mixtures. 2.4 Olefin–Paraffin Separation Separations of olefin–paraffin mixtures, such as C ­ 2H4/C2H6 and ­C3H6/C3H8, are very important processes, because olefins are the prime raw materials for petrochemical production. The industrial separation of paraffin from olefin typically relies on highpressure cryogenic distillation at temperatures as low as −160  °C. Purification of ethylene and propene alone accounts for 0.3% of global energy use. The discovery of new materials for this separation by adsorption, instead of using cryogenic distillation, is a key milestone for molecular separations because of the widely extended uses of these molecules in industry. In the past two decades, large amounts of porous MOFs have shown great potential as adsorbents for olefin–paraffin separations based on three types of mechanisms: (1) an equilibrium-based mechanism, (2) a gateopening effect, and (3) a kinetic-based mechanism. With detailed examples to illustrate the above mechanisms, several reviews have highlighted recent developments in this field [26–28]. For example, with specific OMSs, HKUST-1 and MOF-74 showed equilibrium olefin/paraffin separation [62, 63]. In the flexible zeolitic imidazolate frameworks (ZIFs) ZIF-7 and ­Zn2(bpdc)2(bpee), separations of olefin/paraffin mixtures are based on gate-opening effects [64, 65]. ZIFs were reported by Li et al. [66] as the first example of kinetic separation of C ­ 3H6/C3H8 mixtures. Next, we will give an update on recent developments in novel MOFs for this important separation. 2.4.1 C2H4/C2H6 Separation C2H4/C2H6 separation is of prime importance in the production of polymer-grade ­C2H4 for industrial manufacturing. Yang et al. developed a hydroxyl-functionalized Al-MOF, NOTT-300, exhibiting high ­C2H4/C2H6 selectivity [45]. The ­C2H4 and ­C2H6 uptakes of NOTT-300 at 293 K and 1 bar are 4.28 and 0.85 mmol g−1, respectively. The selectivity of an equimolar C ­ 2H4/C2H6 mixture is 48.7, which is much higher than that of other outperforming MOFs, such as Fe-MOF-74 (13.6), CoMOF-74 (6), HKUST-1 (4), and PAF-1-SO3Ag (26.9) [67]. Synchrotron X-ray and neutron powder diffraction experiments showed that supramolecular interactions between ­C2H4 molecules and the framework in NOTT-300 are provided by M–OH groups, aromatic –CH groups and phenyl rings. In addition, the C···HO distance between ­C2H4 molecules and hydroxyl groups is shorter than that of ­C2H6 and –OH

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group (4.62 vs. 5.07 Å), leading to stronger interactions with C ­ 2H4 and the preferred adsorption of C ­ 2H4 in NOTT-300. Due to the difference in affinities towards C ­ 2H4 and ­C2H6, NOTT-300 exhibits high selectivity of ­C2H4/C2H6 mixtures. In 2017, Long and colleagues [68] examined several MOFs M ­ 2(m-dobdc) (M=Mn, Fe, Co, Ni), which are structural isomers of M-MOF-74, with their separation performance of olefin/paraffin mixtures. Compared with M-MOF-74, the ­C2H4/C2H6 and C ­ 3H6/C3H8 selectivities of M ­ 2(m-dobdc) are both increased due to their enhanced affinities towards olefins over paraffins. In particular, F ­ e2(m-dobdc) exhibits both high olefin uptakes over 7  mmol  g−1 and highest ­C2H4/C2H6 (> 25) and ­C3H6/C3H8 (> 55) selectivity at 298 K and 1 bar. Single-crystal X-ray diffraction indicated that increased charge density at the metal site in ­M2(m-dobdc) enhanced the interactions with olefins, leading to notably improved adsorption and separation performance. In addition, the separation mechanism does not rely on molecular exclusion, so that high purity olefins can be obtained without a temperature swing, contributing to fast adsorption kinetics. Combined with high selectivity, large uptakes, fast kinetics and low cost, ­M2(m-dobdc) are becoming very promising adsorbents for realization of olefin/paraffin separations in industrial processes. Bao et  al. [69] developed a family of gallate-based MOFs, M-gallate [M(C7O5H4)·2H2O, M = Ni, Mg, Co], exhibiting high selectivity for ­C2H4/C2H6 separation. M-gallate features 3D interconnected zigzag channels (3.47 × 4.85, 3.56 × 4.84, 3.69 ×  4.95 Å2 for M=Ni, Mg, Co, respectively) (Fig. 5a). Interestingly, the aperture sizes in M-gallate are slightly larger than the minimum cross-section of the ­C2H4 molecule (3.28 × 4.18 Å2) but smaller than that of ­C2H6 (3.81 × 4.08 Å2) (Fig. 5b). The optimum size endows on M-gallate an absolutely exclusion for C ­ 2H6 molecules while ­C2H4 gas can enter the pores to be adsorbed, resulting in molecular

Fig. 5  a Coordination environment of gallate ligand and M ­ O6. b Perspective view of the structure along the c axis showing the triangular main channels and the regular branched channels leaning against the main ones. c Single-component adsorption isotherms of ­C2H4 and ­C2H6 in Co-gallate at 298 K. d Comparison of ­C2H4/C2H6 adsorption selectivity and volumetric ­C2H4 uptake at 1 bar in M-gallate with other indicated materials. e Experimental breakthrough curves of M-gallate for the equimolar ­C2H4/C2H6 mixture at 273 K and 1 bar. f Adsorption binding sites (Site I) in Mg-gallate. Reprinted with permission from [69]. Copyright 2018, Wiley–VCH Reprinted from the journal

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sieving effects. This was precisely confirmed by the adsorption isotherms, wherein Co-gallate exhibits the highest C ­ 2H4 uptake of 3.37 mmol g−1 but a low uptake of −1 ­C2H6 (less than 0.31 mmol g ) at 298 K under 1 bar (Fig. 5c). As a consequence, Co-gallate exhibits high IAST selectivity, up to 52 at 298 K and 1 bar for the equimolar ­C2H4/C2H6 mixtures (Fig. 5d), setting a new benchmark for ­C2H4/C2H6 separation. Breakthrough experiments for equimolar ­C2H4/C2H6 mixtures confirmed that M-gallate is highly selective for C ­ 2H4 (Fig. 5e). High-resolution neutron powder diffraction (NPD) experiments showed that C ­ 2H4 molecules could be located preferentially at the intersections of the straight and zigzag channels through cooperative supramolecular interactions (Fig. 5f). Combined with readily available ligand, high stability and excellent separation performance, M-gallate is capable of being used as a ­C2H4/C2H6 separation adsorbent in industrial practice. In 2017, Casty and colleagues discovered a new flexible pure silica zeolite, called ITQ-55, featuring a tortuous monodirectional small-pore system with relatively large cavities [70]. The size of the smallest pore window is about 5.9 × 2.1 Å2. ITQ-55 can separate ­C2H4 from ­C2H6 with an unprecedented selectivity of ~ 100, due to its structural flexibility and the differential kinetics between C ­ 2H4 and ­C2H6. The flexibility of ITQ-55 can be observed through ab initio molecular dynamics (AIMD) simulations. The mean window size of the empty structure is 2.38  Å. When ­C2H4 molecules enter the cavities, the mean window size can expand to 3.08 Å. The “braces the window open” effect accelerates the diffusion of ethylene molecules. However, the molecular diffusion of slightly larger ethane molecules is hindered by nearly two orders of magnitude. As shown in single-component gas adsorption kinetics experiments, the rate of ethylene adsorption on ITQ-55 is much higher than that of ethane, and equilibrium is attained on ITQ-55 for ethylene only, but not for ethane adsorption. This absolutely indicated that diffusion kinetics, namely a preferential diffusion of ethylene over ethane, is the separation mechanism in ITQ-55. Realization of ideal molecular sieves can enable ultrahigh selectivity and working capacity for diverse gas separations, and thus improve product purity and productivity in the adsorption-based separation process. In this regard, Chen and colleagues recently reported an ultramicroporous MOF, Ca(C4O4)(H2O) (UTSA-280), possessing extremely high selectivity for ­C2H4/C2H6 due to molecular sieving of ­C2H6 from ­C2H4 [71]. As shown in Fig. 6a, UTSA-280 features rigid 1D open cylindrical channels, whose aperture sizes are 3.2 × 4.5  Å and 3.8 × 3.8 Å. The cross-sectional areas of apertures in UTSA-280 (14.4 Å2) are larger than the minimum cross-sectional area of ­C2H4 (13.7 Å2), but smaller than that of ­C2H6 (15.5  Å2), leading to the complete exclusion of ethane molecules by the rigid pore channels. In contrast, the tilted C ­ 2H4 molecules fit the narrow pore channels well, since the oriented C ­ 2H4 molecules interact with ligands and coordinated water molecules through C−H···O hydrogen bonding (3.32–3.44 Å), π···π stacking (3.31 Å), and van de Waals interactions (shortest C−H···π distance of 3.32 Å) (Fig. 6b, c). Pure-component equilibrium adsorption isotherms show that the adsorption capacity for C ­ 2H4 and ­C2H6 in UTSA-280 are 2.5 mmol g−1 −1 and 0.098 mmol g at 298 K and 1 bar, well consistent with the expected exclusion for ­C2H6 (Fig. 6e). In addition, the IAST selectivity for an equimolar ­C2H4/ C2H6 mixture can reach up to 10,000 at 298 K and 1 bar (Fig. 6f), which is much

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Fig. 6  a Crystal structure of guest-free UTSA-280, showing one-dimensional (1D) channels viewed along the [001] direction. b Side views of the packing diagram of the ­C2H4 adsorbed structure. c Preferential binding site for ­C2H4 molecules and their close contacts with the framework. d Scheme diagram of the size/shape sieving of ­C2H4 and ­C2H6 molecules. e Gas sorption isotherms of ­C2H4 and ­C2H6 at 298 K for UTSA-280. f Qualitative comparison of IAST adsorption selectivities of different MOFs for an equimolar C ­ 2H4/C2H6 mixture at 298 K. g Breakthrough curves for UTSA-280 from different scales for an equimolar binary mixture of ­C2H4/C2H6 at 298 K and 1 bar. Reprinted with permission from [71]. Copyright 2018, Nature Materials

higher than that of Fe-MOF-74 (13.6), NOTT-300 (48.7) and so on. Breakthrough experiments proved that C ­ 2H4 can be enriched from an equimolar C ­ 2H 4/ C2H6 mixture (up to 1.86  mol  kg−1) (Fig.  6g). Besides, UTSA-280 is waterstable and easily synthesized, combined with excellent separation performance, rendering it one of the most promising energy-efficient separation materials. Recently, Zhao and colleagues synthesized two zirconium MOFs, UiO-66ADC and NUS-36, which are constructed from ­Zr6 clusters and acetylenedicarboxylate (ADC) but featuring different topologies [72]. Compared with the fcu network in UiO-66-ADC, NUS-36 exhibits a bcu topology and the contracted pore size smaller than 3.6 Å. The C ­ 2H4 uptake of NUS-36 is 1.46 mmol g−1 at 298  K and 105.8  kPa, which is comparable to that of UiO-66-ADC. However, the ­C2H6 uptake is significantly decreased from 1.6  mmol  g−1 in UiO-66-ADC to 1.01  mmol  g−1 in NUS-36. In addition, the IAST selectivity of NUS-36 for an equimolar ­C2H4/C2H6 mixture is 4.1 at 298  K and 100  kPa, which is much higher than that of UiO-66-ADC (1.8). The calculated isosteric heat (Qst) and entropy change (ΔSads) of ­C2H4 and ­C2H6 sorption proved that the constricted pores in NUS-36 provide stronger affinities to ­C2H4 over ­C2H6, while there is nearly no penalty of ­C2H4 adsorption entropy. As a result, the pore size reduction strategy can be used to design and synthesize MOFs for challenging gas separations.

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2.4.2 C2H6/C2H4 Separation As mentioned above, most reported MOF materials show preferential sorption of ­C2H4 over ­C2H6, because unsaturated hydrocarbons ­(C2H4) typically have stronger interactions with the host framework due to the π-electron density and larger polarity of unsaturated hydrocarbons. In this context, the desired highpurity ­C2H4 product needs to be obtained through a two-step “adsorption and desorption” process, which is obviously more energy intensive. This issue can be overcome by exploring a ­C2H6-selective adsorbent, so that pure ­C2H4 can be produced directly from the outlet during the fixed-bed adsorption operations. Despite great challenges, a handful of ­C2H6-selective MOF materials have been reported in the past few years through the strategies of avoidance of built-in high polarity (e.g., OMS) or the immobilization of ­C2H6-affinity sites. In this regard, Zhang and colleagues reported a porous metal-azolate framework [Zn(batz)] (MAF-49), exhibiting rarely ­ C2H6-selective adsorption [73]. MAF-49 features a 3D coordination network with narrow 1D zigzag channels (3.3 × 3.0 Å2), with four uncoordinated nitrogen sites, a pair of free amino groups and a pair of methylene groups on the pore surfaces. Uptake of C ­ 2H6 was significantly higher than that of C ­ 2H4 at low pressure in MAF-49, resulting in an inverse ­C2H6/C2H4 selectivity of 9 at 316  K. Computational simulations and singlecrystal structures of MAF-49·C2H6 and MAF-49·C2H4 showed that three strong C–H···N hydrogen bonds and three weak C–H···N electrostatic interactions could be formed with ­C2H6 molecules and MAF-49. In contrast, both C–H···N hydrogen bonds and electrostatic interactions between ­C2H4 molecules and framework were fewer and weaker, leading to the preferred adsorption of ­C2H6 over ­C2H4. In 2018, Chen and colleagues reported two isoreticular ultramicroporous MOFs, Cu(ina)2 and Cu(Qc)2, featuring low-polarity pore surfaces that lead to stronger binding affinity toward C ­ 2H6 over C ­ 2H4 (Fig. 7a, b) [74]. The uptake capacities for ­C2H6 and C ­ 2H4 in Cu(Qc)2 are 1.85  mmol  g−1 and 0.78  mmol  g−1 at 298  K and 1  bar, respectively (Fig.  7c). The IAST selectivity for an equimolar ­C2H6/C2H4 mixture is 3.4, which is higher than that of Cu(ina)2 (1.3) and many MOFs selective for ethane. Compared with Cu(ina)2, Cu(Qc)2 has a smaller pore aperture size (3.3  Å), which can easily accommodate ­C2H6 molecules through multiple interactions, showing self-adaptive sorption behavior for ­C2H6. High-resolution neutron powder diffraction (NPD) measurements revealed the binding conformations of [Cu(Qc)2]·0.41C2H6 and [Cu(Qc)2]·0.16C2H4 (Fig.  7d, e). Due to the optimized cavity of Cu(Qc)2, ­C2H6 molecules could fit well with this staggered conformation, resulting in five of six hydrogen atoms on the C ­ 2H6 molecule being involved in C–H···π interactions with aromatic rings on the pore surfaces. In contrast, ­C2H4 molecules can provide only four hydrogen atoms to form C–H···π interactions, leading to a lower adsorption heat. More C–H···π interactions and higher occupancy in Cu(Qc)2 both contribute to a higher binding affinity for ethane over ethylene. Breakthrough experiments indicate a ­C2H4 productivity of 587  mmol  L−1 sorbent with purity of over 99.9% at 298 K, confirming efficient separation of a ­C2H6/C2H4 mixture (Fig. 7f).

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Fig. 7  Comparison of crystal structures and channels between a Cu(ina)2 and b Cu(Qc)2. c ­C2H6 and ­C2H4 sorption isotherms for Cu(ina)2 and Cu(Qc)2 at 298 K. Neutron diffraction crystal structures of d [Cu(Qc)2]·0.41C2D6 and e [Cu(Qc)2]·0.16C2D4. f Experimental column breakthrough curves for equimolar ­C2H6/C2H4 (orange/purple) mixture (298  K, 1  bar) in an adsorber bed packed with Cu(Qc)2. Reprinted with permission from [74]. Copyright 2018, American Chemical Society

Telfer and co-workers reported a ­C2H6-selective MOF, ­[Co3(μ3-OH)(ipa)2.5(H2O)] (MUF-15), featuring three narrow zigzag 1D pores decorated by benzene rings [75]. Because of the large pore volume (0.51 cm3 g−1), MUF-15 exhibits a high uptake capacity for ­C2H6 (4.69  mmol  g−1), but lower uptake for ­C2H4 (4.15  mmol  g−1) at 293  K and 1  bar, offering a C ­ 2H6/C2H4 selectivity of 2. Based on first-principles DFT-D calculations, the C ­ 2H6 molecule offers all six hydrogen atoms to form C−H···π interactions with three adjacent phenyl rings, while the ­C2H4 molecule exhibits only short contacts with two parallel edges in the cavity. The more C−H···π interactions and stronger polarity endow ­C2H6 molecules a higher binding affinity with MUF-15. Besides, MUF-15 could realize selectively adsorption for C ­ 2H6 from the ­C2H2/C2H4/C2H6 mixture for the first time. Breakthrough experiments show the productivity of ethylene, with purity in excess of 99.95%. High capacity and C ­ 2H6/ C2H4 selectivity, combined with inexpensive precursors, good regenerability and structural stability, make MUF-15 a significant addition to known C ­ 2H6-selective MOFs. In 2018, Chen and colleagues reported a microporous MOF, ­[Fe2(O2)(dobdc)], possessing highly selective separation of C ­ 2H6 from ­C2H4 [76]. Iron(III)-peroxo sites on the pore surfaces in ­Fe2(O2)(dobdc), as specific sites strongly interacting with alkanes over alkenes, play a key role in recognition of ethane over ethylene. ­Fe2(O2)(dobdc) exhibits a large uptake capacity for C ­ 2H6 (74.3  cm3 ­g−1) and relatively lower amounts of ­C2H4 adsorption, giving a new benchmark selectivity for an equimolar ­C2H6/C2H4 mixture (4.4) at 298  K and 1  bar (Fig.  8b, c). This ­ 2H6-selective MOFs. selectivity is absolutely higher than the previously reported C Reprinted from the journal

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Fig. 8  a Structures of ­Fe2(dobdc), ­Fe2(O2)(dobdc), and ­Fe2(O2)(dobdc) ⊃ C2D6 at 7  K. b Adsorption (solid) and desorption (open) isotherms of ­C2H6 (red circles) and ­C2H4 (blue circles) in ­Fe2(O2)(dobdc) at 298  K. c Comparison of IAST selectivities of F ­ e2(O2)(dobdc) with those of the previously reported best-performing materials for C ­ 2H6/C2H4 (50/50) mixtures. d Predicted productivity of 99.95% pure ­C2H4 from ­C2H6/C2H4 (50/50 and 10/90) mixtures in fixed-bed adsorbers at 298 K. e Experimental column breakthrough curves for a ­C2H6/C2H4 (50/50) mixture. Reprinted with permission from [76]. Copyright 2018, Science

High-resolution NPD measurements reveal that ­C2H6 molecules interact with peroxo sites through C−H···O hydrogen bonds (H···O, ~ 2.17 to 2.22 Å, Fig. 8a), whose distance is much shorter than the sum of van de Waals radii of oxygen (1.52  Å) and hydrogen (1.20 Å) atoms, confirming the extremely strong interaction with the Fe-peroxo sites. In addition, compared with planar ­C2H4 molecules, the nonplanar ­C2H6 molecules fit better into the uneven pore surfaces in ­Fe2(O2)(dobdc), causing stronger van de Waals interactions between ­C2H6 molecules and the ligand surfaces. As a consequence, ­Fe2(O2)(dobdc) features higher binding affinity to ­C2H6 over ­C2H4 and efficient separation of ­C2H6/C2H4, as validated by a breakthrough operation that could readily produce ­C2H4 with ≥ 99.99% purity from a ­C2H6/C2H4 mixture (Fig. 8e). In 2019, Wang et  al. [77] reported a series of ­ C2H6-selective MOFs, Ni(BDC)1−x(TMBDC)x(DABCO)0.5 (x = 0.2, 0.45, 0.71, 1), whose pore size and pore environment can be tuned precisely through the introduction of TMBDC ligand. With the increase in the TMBDC/BDC ratio, the number of methyl groups in the framework increases, contributing to a decrease in the Brunauer–Emmett–Teller (BET) surface area and pore volume of Ni(BDC)1−x(TMBDC)x(DABCO)0.5. As a result, Ni(TMBDC)(DABDO)0.5 features the most methyl groups and the smallest pore size of ~ 5.9 Å, providing stronger affinities with gas molecules, thereby lead­ 2H6 and C ­ 2H4 uptakes of 5.45 and 5.02 mmol g−1 at 298 K and 1 bar, ing to higher C

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respectively. Due to the greater number of C−H bonds, Ni(TMBDC)(DABDO)0.5 could form more interactions with ­C2H6 over C ­ 2H4 molecules. The IAST selectivity of the ­C2H6/C2H4 (1:15 v/v) mixture for Ni(TMBDC)(DABDO)0.5 at 298 K and 100 kPa reaches up to 1.985. The breakthrough experiment further confirms the separation performance of Ni(TMBDC)(DABDO)0.5. Recently, Hao et  al. synthesized a stable porous MOF, ­ (Me2NH2) [Co3(DCPN)2(μ3-OH)(H2O)]·11H2O (TJT-100), as an adsorbent trapping ­ C2H2 and ­C2H6 from a ternary mixture of C ­ 2H2/C2H4/C2H6 [78]. TJT-100 exhibits a 3D porous structure with uncoordinated carboxylate oxygen atoms and coordinated water molecules on the pore surfaces. The adsorption capacities in TJT-100 are, in order, C ­ 2H2 > C2H6 > C2H4, with selectivities for C ­ 2H2/C2H4 and C ­ 2H6/C2H4 at 298 K and 1 atm of 1.8 and 1.2, respectively. Breakthrough experiments for a ternary mixture of ­C2H2/C2H4/C2H6 (0.5:0.5:99 v/v/v) confirm the production of ethylene with a purity higher than 99.997%. Through grand canonical Monte Carlo (GCMC) simulations, ­C2H2 molecules form C−H···O hydrogen-bonding interactions with two uncoordinated carboxyl oxygen atoms, simultaneously make intermolecular van de Waals interactions with M ­ e2NH2+ cations and dicarboxylphenyl-nicotinic acid (DCPN) linkers, while ­C2H6 molecules form four C−H···O interactions aided by polarization through M ­ e2NH2+ cations. In contrast, C ­ 2H4 molecules provide only weak van de Waals interactions with ­Me2NH2+ cations, which TJT-100 shows obviously the lowest binding affinity towards, consist with the adsorption capacities and breakthrough experiments results. 2.4.3 C3H6/C3H8 Separation Propylene is a prime olefin raw material for petrochemical production, second in importance only to ethylene, which is an essential raw for the manufacturing of various chemicals such as polypropylene. The removal of ­C3H8 from ­C3H6 is a critical separation for the chemical industry to produce high-purity propylene with 99.5% minimum (polymer-grade specifications). In 2016, Eddaoudi’s group reported a fluorinated MOF, NbOFFIVE-1-Ni, which could exclude ­C3H8 from ­C3H6 completely under ambient conditions [79]. NbOFFIVE-1-Ni was isoreticular with SIFSIX-3-Ni, constructed by bridging Ni(II)-pyrazine square-grid layers with ­(NbOF5)2− pillars. Instead of ­(SiF6)2−, ­(NbOF5)2− as the inorganic pillars in NbOFFIVE-1-Ni led to a longer metal-fluorine distance (1.95  Å), thereby tilting the pyrazine. As a result, the pore size of NbOFFIVE-1-Ni is 3.0471  Å, which is much smaller than that of SIFSIX-3-Ni (4.965 Å, Fig. 9a). When ­C3H6 molecules were adsorbed, it was found that the pyrazine tilted with a greater angle, leading to gate opening and a theoretical maximum aperture size of 4.752 Å, so that ­C3H6 molecules could pass into the pores. In contrast, the extra tilting of pyrazine did not occur when ­C3H8 molecules passed in. Gas adsorption isotherms showed that NbOFFIVE-1-Ni exhibited the rare molecular exclusion of C ­ 3H8 from C ­ 3H6 at 298 K up to ~ 1 bar (Fig. 9b). Breakthrough experiments further confirmed the complete molecular sieving of ­C3H8 from a 50/50 ­C3H6/C3H8 mixture, with a ­C3H6 productivity of ~ 0.6 mol kg−1 (Fig. 9c). Reprinted from the journal

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Fig. 9  a Structural description of NbOFFIVE-1-Ni, highlighting the building block arrangement and pore sizes compared with the parent SIFSIX-3-Ni. b Pure C ­ 3H8 (pink), pure ­C3H6 (purple), and equimolar mixture of ­C3H6/C3H8 50/50 (orange) isotherms of NbOFFIVE-1-Ni at 298 K. c ­C3H6/C3H8 (50/50) mixed-gas experiment using a packed column bed at 298 K and 1.0 bar. Reprinted with permission from [79]. Copyright 2016, Science

Using a topology-guided design strategy, Wang et  al. [80] reported a designer microporous MOF ­ Y6(OH)8(abtc)3(H2O)6(DMA)2 (Y-abtc), with cage-like pores that exhibits ­C3H8 molecular exclusion behavior. The framework of Y-abtc has ftw topology, with large cages and small windows that are suitable for molecular separation (Fig. 10a). The confined pore size (4.72 Å) is just between the kinetic diameter of ­C3H6 (4.68 Å) and C ­ 3H8 (5.1 Å), endowing Y-abtc with full exclusion for C ­ 3H8

Fig. 10  a Crystal structure of Y-abtc. Y-abtc is built on 12-connected hexanuclear secondaary building unit (SBU), forming cage-like pores interconnected by small windows. b Adsorption–desorption isotherms of propane and propylene at 25 °C for Y-abtc. c Propylene adsorption for as synthesized Y-abtc and after thermal and hydrothermal treatments. Breakthrough curve for a mixture of propane/propylene with a feed ratio of 50/50 (d) and 5/95 (e). Red propylene, blue propane. Reprinted with permission from [80]. Copyright 2018, Wiley–VCH

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molecules. In addition, as the charge-balancing cations in the channels, dimethylammonium provides strong interactions with ­C3H6 molecules, contributing to appreciable amount of ­C3H6 uptake (2 mmol g−1 under 1 bar at 298 K) and high isosteric heat of adsorption (Qst = 50  kJ  mol−1) (Fig.  10b). Multicomponent column breakthrough experiments show that Y-abtc could produce C ­ 3H6 with a purity of 99.5%, and the retention time (12.9  min) is twice as long as that of KAUST-7 (5.7  min) (Fig. 10d, e). Combined with the ­C3H8 molecular exclusion, high stability and scalable synthesis, Y-abtc is expected to be a potential separating material for industrial implementation. Recently, Zhang and colleagues reported a porous MOF ­[Zn2(btm)(btk)] (namely MAF-23-O) obtained by heating [­ Zn2(btm)2] (MAF-23) under oxygen flow, thereby oxidizing half of the ­btm2− ligands to form ­btk2− with C=O bonds [81]. On the one hand, the carbonyl groups in MAF-23-O provide additional recognition sites for ­C3H6 molecules by forming hydrogen bonds, while ­C3H8 molecules could not be interacted with, leading to higher thermodynamic selectivity. On the other hand, oxidation reduces the flexibility of the framework. Because the carbonyl groups are in conjugation with the aromatic rings, ­C3H6 molecules need more energy to diffuse into MAF-23-O, resulting in improved kinetic selectivity. Breakthrough experiments show that the adsorption selectivity of the equimolar C ­ 3H6/C3H8 mixture for MAF23-O is 15 at 298 K and 1 atm, much higher than that of MAF-23 (1.5). Thus, high adsorption selectivity can be achieved by simultaneously improving both thermodynamic and kinetic selectivity.

3 Separation of Linear and Branched Alkane Isomers 3.1 Separation of ­C4 Olefins C4 olefins are important raw materials for the production of a variety of synthetic rubbers and chemicals, including 1,3-butadiene (­ C4H6), 1-butene (n-C4H8) and isobutene (iso-C4H8). However, due to the similar structures of ­C4 olefins, their separation presents one of the great challenges among hydrocarbon purifications. In this respect, Cui et  al. [82] reported a flexible MOF, MnINA, with large pocket-like cages and narrow bottlenecks, leading to benchmark high n-C4H8/iso-C4H8 selectivity (327.7). Eddaoudi’s group reported a novel rare-earth (RE) metal fcu-MOF, RE-1,4-NDC-fcu-MOF, featuring optimized aperture sizes and leading to the steric adsorptive separation of branched paraffins from normal ones [83]. As the ligand, 1,4-NDC was shorter and bulkier so that the aperture size of RE-1,4-NDC-fcu-MOF is narrowed to about 4.7 Å. This size made this MOF suitable for the complete sieving of branched paraffins from normal ones. As a result, no iso-C4H10 adsorption was observed in this MOF, while it showed typically type-I gas adsorption behavior for n-C4H10. Breakthrough experiments further confirmed the molecular exclusion of iso-C4H10 from a n-C4H10/iso-C4H10/N2 (5/5/90) mixture at 298 K and 1 bar. The results indicated that iso-C4H10 was not retained in the column, and the production of n-C4H10 was about 0.8 mmol g−1. Reprinted from the journal

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Xing and colleagues reported a series of SIFSIX materials (namely GeFSIX2-Cu-i, NbFSIX-2-Cu-i and GeFSIX-14-Cu-i), exhibiting different C ­ 4 olefins separation performance [84]. The pore size of these MOFs can be finely tuned within a 0.2 Å scale by precise control of the anion pillars and organic linkers. Among them, due to the azpy ligand, GeFSIX-14-Cu-i featured a pore size of 4.2  Å, which was just larger than the smallest cross-section of ­C4H6 but smaller than those of n-C4H8 and iso-C4H8. As a result, GeFSIX-14-Cu-i could realize the molecular exclusion of specific ­C4 olefins. Although GeFSIX-14-Cu-i adsorbed 1,3-butadiene ­(C4H6) or 1-butene (n-C4H8) significantly, experimental breakthrough data confirmed its separation properties of ­C4H6/n-C4H8, ­C4H6/iso-C4H8 and n-C4H8/iso-C4H8 with high capacities, combining the molecular recognition of ­C4H6 through H-bonding interactions with inorganic pillared anions. Due to the strongest coordination ability, most reported MOFs with functional sites commonly absorb ­ C4H6 preferentially over other ­ C4 hydrocarbons. This adsorptive separation leads to a more energy-intensive two-step “desorption” process, which means that, to produce high-purity C ­ 4H6, release of C ­ 4H6l following the capture process is essentia. Very recently, Zhang’s group realized a hydrophilic MOF ­[Zn2(btm)2] (Zn-BTM) with flexible quasi-discrete pores, featuring inverse selective adsorption of n-C4H8, iso-C4H8 and ­C4H10 over ­C4H6 (Fig. 11a) [85]. Gas adsorption isotherms revealed that the uptake of C ­ 4 olefins in Zn-BTM followed the sequence ­C4H6