Handbook of Catalysis 9781774694053

Table of contents :
Cover
Half Title
Handbook of Catalysis
Copyright
About the Editor
Table of Contents
List of Figures
List of Tables
List of Abbreviations
Preface
1. Fundamentals of Catalysis
Contents
1.1. Introduction
1.2. What Is Catalysis?
1.3. Molecules, Atoms, Solid Surfaces, and Enzymes as Catalysts
1.3.1. Homogeneous Catalysis
1.3.2. Biocatalysis
1.3.3. Heterogeneous Catalysis
1.4. Why Is Catalysis Important?
1.4.1. Catalysis and Green Chemistry
1.4.2. E Factors, Atom Efficiency, and Ecological Affability
1.4.3. The Chemical Industry
1.5. Multidisciplinary Field of Catalysis
1.5.1. Length Gages (Scales) of a “Catalyst”
1.5.2. Time Gages (Scales) in Catalysis
References
2. Basics of Catalysis Reactions and Chemical Kinetics
Contents
2.1. Introduction
2.2. Order of a Reaction
2.2.1. Reaction Rate Determination
2.2.2. Initial Rates Technique
2.2.3. Integral Technique
2.2.4. Differential Technique
2.2.5. Different Variation in Reaction Order
2.2.6. Zero Order Reaction
2.2.7. First Order Reaction
2.2.8. Pseudo-First Order Reaction
2.2.9. Second Order Reaction
2.2.10. Complex Order Reaction
2.2.11. Factors Affecting Reaction Rate
2.2.12. Rate Constant Calculations
2.3. Integrated Reaction Rate Laws
2.4. Half-Life
2.5. Rate Law Determination From Experimental Information
2.5.1. Isolation Method
2.5.2. Differential Methods
2.5.3. Integral Methods
2.6. Experimental Methods
2.6.1. Methods for Blending the Reactant Materials and Instigating the Reaction
2.6.1.1. Flow Techniques
2.6.1.2. Laser Pump-Probe and Flash Photolysis Techniques
2.6.1.3. Relaxation Methods
2.6.2. Methods for Observing Concentrations with Respect to Time
2.6.2.1. Absorption Spectroscopy
2.6.2.2. Resonance Fluorescence
2.6.2.3. Laser-Induced Fluorescence
2.6.3. Temperature Measurement and Regulation
2.7. Complex Reactions
2.8. Catalysis
2.8.1. Illustration
2.8.2. Units
2.8.3. Catalytic Reaction Mechanisms
2.8.4. Reaction Energetics
2.9. Significance
2.9.1. Energy Processing
2.9.2. Bulk Chemicals
2.9.3. Fine Chemicals
2.9.4. Food Processing
2.9.5. Environment
2.10. Inhibitors, Poisons, and Promoters
References
3. Homogeneous Catalysis
Contents
3.1. Introduction
3.2. Metal Compound Catalysis in the Liquid Phase
3.2.1. Elementary Phases in Homogeneous Catalysis
3.2.1.1. Ligand Exchange: Dissociation and Coordination
3.2.1.2. Oxidative Addition
3.2.1.3. Reductive Elimination
3.2.1.4. Insertion and Migration
3.2.1.5. De-Insertion and b-Elimination
3.2.1.6. Nucleophilic Attack on an Organized Substrate
3.2.1.7. Other Reaction Types
3.2.2. In Homogeneous Catalysis, the Structural Relationships
3.2.2.1. Steric Effects: Symmetry, Flexibility, and Ligand Size
3.2.2.2. Electronic Effects of Solvents, Substrates, and Ligands
3.2.3. Asymmetric Homogeneous Catalysis
3.2.4. Industrial Examples
3.2.4.1. SHOP (The Shell Higher Olefins Process)
3.2.4.2. The Wacker Oxidation Process
3.2.4.3. The Du Pont Synthesis of Adiponitrile
3.2.4.4. The Ciba–Geigy Metolachlor Method
3.3. Homogeneous Catalysis Without Metals
3.3.1. Classic Acid/Base Catalysis
3.3.2. Organocatalysis
3.4. Scaling Up Homogeneous Reactions: Cons and Pros
3.4.1. Catalyst Recycling and Recovery
3.4.2. Hybrid Catalysts: Bridging the Homogeneous/Heterogeneous Gap
3.5. “Click Chemistry” and Homogeneous Catalysis
References
4. Heterogeneous Catalysis
Contents
4.1. Introduction
4.2. Classic Gas/Solid Systems
4.2.1. The Concept of the Active Site
4.2.2. Model Catalyst Systems
4.2.3. Real Catalysts: Poisons, Modifiers, and Promoters
4.3. Preparation of Solid Catalysts: Black Magic Revealed
4.3.1. Alloy Leaching and High-Temperature Fusion
4.3.2. Slurry Precipitation and Co-Precipitation
4.3.3. Impregnation of Porous Supports
4.3.4. Hydrothermal Synthesis
4.3.5. Drying, Calcination, Activation, and Forming
4.4. Selecting the Right Support
4.4.1. Specific Surface Area
4.4.2. Substrate Accessibility
4.4.3. Catalyst Stability
4.5. Catalyst Characterization
4.5.1. Traditional Surface Characterization Methods
4.5.2. Determining the Surface Area
4.5.3. Temperature-Programed (TP) Techniques
4.5.4. Spectroscopy and Microscopy
4.5.4.1. Solid-State Nuclear Magnetic Resonance Spectroscopy (SS-NMR)
4.5.4.2. Infrared (IR) Spectroscopy
4.6. The Catalytic Converter: An Instance from Everyday Life
References
5. Environmental Catalysis
Contents
5.1. Introduction
5.2. Attributes of Environmental Catalysis
5.3. Catalysis All Over
5.4. Environmental Catalysis as a Driver for Innovation
5.5. Sustainable Catalytic Materials
5.6. Environmental Electrocatalysis
References
6. Fundamentals of Biocatalysis
Contents
6.1. Introduction
6.2. Advantages and Disadvantages of Biocatalysts
6.3. Strategies to Improve the Performance of Biocatalysts
6.4. Biocatalysts: An Interdisciplinary Science
6.5. The Effect of Biocatalysis on Teaching Natural Science
References
7. Pharmaceutical Applications of Catalysis
Contents
7.1. Introduction
7.2. Biocatalysts in Pharmaceutical Industry
7.3. Over the Counter and Remedy Drugs
7.4. Hydrogenation of C=C Double Bonds
7.5. Semi-Hydrogenation of C=C Triple Bonds
7.6. Improved Methods for Catalyst Recovery in Biopharmaceutical Production
7.6.1. Synthesis of Cefprozil
7.6.2. Catalyst Recovery
7.7. The Problem With Carbon
7.8. A More Selective Method: Ion Exchange Resins
References
8. Applications of Catalysis in Nanotechnology and Energy
Contents
8.1. Introduction
8.2. Nanoparticles in Catalysis
8.3. Nanocatalysis
8.3.1. Effect of Size
8.3.1.1. On Catalytic Properties
8.3.1.2. Size-Dependent Coordination Environment
8.3.1.3. Size-Dependent Electronic State
8.3.1.4. Size-Dependent Adsorption Energy
8.3.2. Impact of Shape on Catalytic Properties
8.3.3. Composition Effect
8.3.4. Highly Selective Nanocatalysts
8.4. Solar-Driven Water Splitting
8.4.1. Electrocatalysts
8.4.2. Light Absorbers
8.4.3. Developments in Science and Technology to Solve Problems
References
Index
Cover back

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本书版权归Arcler所有

Handbook of Catalysis

本书版权归Arcler所有

HANDBOOK OF CATALYSIS

Praveen Bhai Patel

ARCLER

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www.arclerpress.com

Handbook of Catalysis Praveen Bhai Patel

Arcler Press 224 Shoreacres Road Burlington, ON L7L 2H2 Canada www.arclerpress.com Email: [email protected]

e-book Edition 2023 ISBN: 978-1-77469-604-0 (e-book)

This book contains information obtained from highly regarded resources. Reprinted material sources are indicated and copyright remains with the original owners. Copyright for images and other graphics remains with the original owners as indicated. A Wide variety of references are listed. Reasonable efforts have been made to publish reliable data. Authors or Editors or Publishers are not responsible for the accuracy of the information in the published chapters or consequences of their use. The publisher assumes no responsibility for any damage or grievance to the persons or property arising out of the use of any materials, instructions, methods or thoughts in the book. The authors or editors and the publisher have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission has not been obtained. If any copyright holder has not been acknowledged, please write to us so we may rectify.

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© 2023 Arcler Press ISBN: 978-1-77469-405-3 (Hardcover)

Arcler Press publishes wide variety of books and eBooks. For more information about Arcler Press and its products, visit our website at www.arclerpress.com

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ABOUT THE AUTHOR

Praveen Bhai Patel is presently working as an Assistant Professor at Dept. of Chemical Engineering, University Institute of Engineering and Technology Chhatrapati Shahu Ji Maharaj University, Kanpur, India. He has over 15 years of teaching and research experience. He has completed his Ph.D from Motilal Nehru National Institute of Technology, Allahabad, UP, India. He has published several research papers in international and national journals and has also contributed chapters in books.

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TABLE OF CONTENTS

List of Figures ................................................................................................xi List of Tables ...............................................................................................xvii List of Abbreviations ....................................................................................xix Preface ........................................................................................................xxi Chapter 1

Fundamentals of Catalysis ........................................................................ 1 1.1. Introduction ....................................................................................... 2 1.2. What Is Catalysis? .............................................................................. 2 1.3. Molecules, Atoms, Solid Surfaces, and Enzymes as Catalysts ............. 5 1.4. Why Is Catalysis Important? ............................................................... 9 1.5. Multidisciplinary Field of Catalysis .................................................. 17 References .............................................................................................. 20

Chapter 2

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Basics of Catalysis Reactions and Chemical Kinetics .............................. 25 2.1. Introduction ..................................................................................... 26 2.2. Order of a Reaction ......................................................................... 28 2.3. Integrated Reaction Rate Laws ......................................................... 32 2.4. Half-Life .......................................................................................... 33 2.5. Rate Law Determination From Experimental Information ................. 33 2.6. Experimental Methods ..................................................................... 35 2.7. Complex Reactions .......................................................................... 44 2.8. Catalysis .......................................................................................... 45 2.9. Significance ..................................................................................... 49 2.10. Inhibitors, Poisons, and Promoters ................................................. 52 References .............................................................................................. 53

Chapter 3

Homogeneous Catalysis ......................................................................... 63 3.1. Introduction...................................................................................... 64 3.2. Metal Compound Catalysis in the Liquid Phase ............................... 64 3.3. Homogeneous Catalysis Without Metals .......................................... 98 3.4. Scaling Up Homogeneous Reactions: Cons and Pros ..................... 101 3.5. “Click Chemistry” and Homogeneous Catalysis ............................. 104 References ............................................................................................. 108

Chapter 4

Heterogeneous Catalysis ...................................................................... 119 4.1. Introduction.................................................................................... 120 4.2. Classic Gas/Solid Systems ............................................................... 121 4.3. Preparation of Solid Catalysts: Black Magic Revealed .................... 129 4.4. Selecting the Right Support ............................................................. 138 4.5. Catalyst Characterization ................................................................ 140 4.6. The Catalytic Converter: An Instance from Everyday Life ................ 150 References ............................................................................................. 153

Chapter 5

Environmental Catalysis ....................................................................... 165 5.1. Introduction.................................................................................... 166 5.2. Attributes of Environmental Catalysis ............................................. 168 5.3. Catalysis All Over ........................................................................... 169 5.4. Environmental Catalysis as a Driver for Innovation ........................ 172 5.5. Sustainable Catalytic Materials ...................................................... 174 5.6. Environmental Electrocatalysis ....................................................... 178 References ............................................................................................. 180

Chapter 6

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Fundamentals of Biocatalysis ............................................................... 185 6.1. Introduction.................................................................................... 186 6.2. Advantages and Disadvantages of Biocatalysts ............................... 189 6.3. Strategies to Improve the Performance of Biocatalysts .................... 190 6.4. Biocatalysts: An Interdisciplinary Science ...................................... 191 6.5. The Effect of Biocatalysis on Teaching Natural Science.................... 193 References ............................................................................................. 195

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Chapter 7

Pharmaceutical Applications of Catalysis ............................................ 199 7.1. Introduction .................................................................................. 200 7.2. Biocatalysts in Pharmaceutical Industry ........................................ 201 7.3. Over the Counter and Remedy Drugs ........................................... 204 7.4. Hydrogenation of C=C Double Bonds .......................................... 205 7.5. Semi-Hydrogenation of C≡C Triple Bonds .................................... 206 7.6. Improved Methods for Catalyst Recovery in Biopharmaceutical Production ................................................... 207 7.7. The Problem With Carbon ............................................................. 209 7.8. A More Selective Method: Ion Exchange Resins ............................ 209 References ........................................................................................... 210

Chapter 8

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Applications of Catalysis in Nanotechnology and Energy .................... 215 8.1. Introduction .................................................................................. 216 8.2. Nanoparticles in Catalysis ............................................................. 217 8.3. Nanocatalysis ............................................................................... 220 8.4. Solar-Driven Water Splitting .......................................................... 228 References ........................................................................................... 233 Index ................................................................................................... 239

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LIST OF FIGURES Figure 1.1. Each catalytic reaction is composed of a series of simple stages in which reactant molecules connect to the catalyst, react, and then detach from the catalyst, freeing it for the next cycle Figure 1.2. Diagram depicting the potential energy of a heterogeneous catalytic process with products and reactants in gaseous form along with a solid catalyst Figure 1.3. An enzyme-catalyzed reaction is depicted schematically. Enzymes frequently conform to the geometry of their substrates or the transition state of the process that they catalyze. Enzymes are extremely efficient catalysts and provide a wealth of inspiration for the development of technological catalysts Figure 1.4. The structure of a nanosized catalyst Figure 1.5. Diagram illustrates the catalytic oxidation of CO by oxygen, showing the reaction cycle and potential energy Figure 1.6. Structure of the chemical industry’s most significant intermediate: ethylene epoxide Figure 1.7. Europe is the biggest manufacturer of bulk chemicals Figure 1.8. Pertinent molecular sizes in catalysis vary from the subnano-scale and molecular level to the macro-level of an industrial vessel Figure 2.1. Rate constant determination via slope method Figure 2.2. Reaction chambers for flow techniques Figure 2.3. Resonance fluorescence Figure 2.4. Laser-induced florescence Figure 2.5. Energy diagram of a reaction Figure 2.6. (a) Partially burned sugar cube; (b) sugar cube burning with a catalyst of ash Figure 2.7. Catalytic decomposition of Ti-Cr-Pt to release oxygen after immersing in H2O2 Figure 2.8. Levofloxacin synthesis from hydroxyacetone Figure 3.1. (a) 1-hexene to hexane in hydrogenation in the influence of Wilkinson’s catalyst, Rh(P(Ph)3)3Cl.; (b) 1-octene is hydroformylation to form the branched 2-methyloctanal and linear nonanal Figure 3.2. The key basic steps in homogeneous catalysis

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Figure 3.3. (a) In enzymatic systems and heterogeneous catalysis, dynamic adsorption happens both at active sites both here and on the substrate; (b) coordination on a similar scale occurs in homogenous complexes as well Figure 3.4. (a) Dissociative; and (b) associatory ligand exchange from Ni(CO)4 Figure 3.5. (a) The conventional method for oxidative addition; (b) oxidative addition of CH3I to Vaska’s compound Figure 3.6. The oxidative addition of benzyl bromide to PtII(bipy)(CH3)2 results in the formation of a trans PtIV complex that isomerizes to the cis compound Figure 3.7. (a) Standard equations for cyclometallation; (b) development of a ruthenacyclopentadiene compound via cyclometallation of a diacetylene Figure 3.8. (a) Common reductive exclusion reaction; (b) reductive removal of HCN from a nickel compound Figure 3.9. Carbon-carbon bond arrangement through reductive elimination from a palladium compound Figure 3.10. Instances of a CO insertion and b CH3 trek in a square-planar Pd compound with a PN chelating ligand Figure 3.11. Examples of (1,1) and (1,2) migratory insertion reactions Figure 3.12. Elimination of b-hydride from a transition metal–alkyl combination using a generic equation Figure 3.13. (a) Nucleophilic hit of water on organized ethene in the Wacker oxidation cycle; (b) damage of ethoxide on organized CO Figure 3.14. Common instances of (a) α-elimination; and (b) α-abstraction Figure 3.15. (a) Generic formulation; and (b) example of a-bond metathesis Figure 3.16. Orthometallation of a Pd–organoimine compound Figure 3.17. 3D structure of Ni(P(Ph)3)4 and Graphic drawing, also displaying the reaction pocket generated by the detachment of one of the triphenylphosphine ligands Figure 3.18. (a) space-filling model and schematic drawing showing the calculation of the cone angle for nonsymmetric and symmetric ligands; (b) instances of some ligands with their resultant cone angle values Figure 3.19. Graphic illustration of the sphere work parameter Socc, the solid angle Omax, and bulk radius Rmax Figure 3.20. (a) Ligand detachment equilibrium in the presence of a nickel complex, as well as the associated constructions and space-filling equations of the o-polyphosphateand bp-tolyl ligands. The three-dimensional models demonstrate the increased cone angle induced by an ortho methyl group Figure 3.21. (a) Instances of bidentate phosphine ligands and their resultant bite angles; (b) a plasticity profile for the Xantphos ligand, demonstrating how the energy shifts with the bite angle

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Figure 3.22. The structures of four identical titanium catalyst precursors with varying proportions were employed in the polymerization of 1-hexene, as well as the yields of isotactic poly(hexene) Figure 3.23. How do the ligand acceptor/donor properties impact the CO bonded trans to this ligand? Figure 3.24. Instances of organometallic compounds of Ni, W, and Co exhibit agostic interactions Figure 3.25. Chemical shapes and improved geometries of (S)- and (R)-thalidomide Figure 3.26. The idea of asymmetric catalysis applying an organometallic compound with a chiral ligand Figure 3.27. (a) Chemical structure and 3D interpretation of L-DOPA; (b) of the chiral ligands tested through Monsanto with their equivalent ee values; (c) the Rh compound employed in the large-scale industrial process Figure 3.28. The asymmetric isomerization stage in the production of (-)menthol from myrcene is catalyzed by the An(S)-BINAP–Rh complex. The chemical structure and three-dimensional depiction of the (R)-BINAP ligand are shown in the inset Figure 3.29. (a) Two instances of nickel catalyst precursors, emphasizing the “chelate part” and the “organic part;” (b) a basic catalytic cycle for the SHOP oligomerization step Figure 3.30. (a) a simplified representation of the Pd and the Cu catalytic cycles; and (b) the net reaction of the Wacker oxidation system; (c) the three stoichiometric redox reactions Figure 3.31. The palladium Wacker catalytic cycle for oxidizing ethene to acetaldehyde in a simplified schematic. The broken rings depict the redox cycles of copper and oxygen Figure 3.32. The two-step Ni-catalyzed hydrocyanation of butadiene Figure 3.33. The catalytic cycle of hydro cyanation, commencing with a NiL4 catalyst precursor. The dashed lines illustrate the development of the 2M3BN by-product Figure 3.34. (a) The secondary hydrocyanation step, which is co-catalyzed by Lewis acids, results in the formation of the different products (LA); (b) Lewis acids with a large molecular weight, such as BPh3, alter the equilibrium of linear/branched products more towards the desired linear product Figure 3.35. (a) Chemical configurations of the four stereoisomers of metolachlor; (b) 3D depiction of the isomer, displaying the chiral axis; and (c) the asymmetric imine hydrogenation phase Figure 3.36. (a) The initial Josiphos ligand; (b) the Ir–xyliphos compound employed in the Ciba–Geigy metolachlor method Figure 3.37. Piperidine, an organocatalyst, catalyzes the Knoevenagel concentration among butyraldehyde and dimethyl maleate

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Figure 3.38. (a) The usual enamine catalytic cycle in the existence of L-proline; (b) an instance of an asymmetric Mannich-type accumulation of cyclohexanone to iminoethyl glyoxalate, catalyzed through a proline tetrazole derivative Figure 3.39. (a) The structure and planned mode of action of the altered Cinchona alkaloid catalyst; (b) an instance of the catalytic Henry reaction among nitromethane and benzaldehyde Figure 3.40. Plenio’s multisite phosphine ligand centered on poly(methyl styrene) Figure 3.41. The two routes for the Lewis-acid-catalyzed ring-opening of aziridines Figure 3.42. (a) Common reaction method for the thermal Huisgen cycloaddition; (b) the copper-catalyzed reaction among benzyl azide and phenyl propargyl ether. The catalytic reaction is performed in the existence of a reductant and gives just one of the product isomers in the high harvest Figure 3.43. The proposed catalytic cycle for the Huisgen (3 + 2) cycloaddition, showing the “direct” and “indirect” routes Figure 4.1. A plug-flow catalytic reactor at several zoom-in levels Figure 4.2. The two major processes in solid-gas/ heterogeneous catalysis: (a) Langmuir–Hinshelwood; (b) Eley–Rideal Figure 4.3. Graphic illustration of a solid catalyst crystal surface Figure 4.4. On distinct surfaces of Fe single crystals, relative rates of ammonia production and dissociative N2 adsorption Figure 4.5. The antibonding LUMO of the external N2 molecule is reduced by an activity promoter, making it easier for it to dissociate on the catalytic surface Figure 4.6. Solid catalyst types and examples, organized by preparation technique (tm= transition metal) Figure 4.7. The key unit operations and phases in solid catalyst production are summarized in this flowchart Figure 4.8. The Raney method involves fusing an aluminum alloy and then dissolving the aluminum in aqueous NaOH, resulting in a “metallic sponge.” Figure 4.9. A lab-scale parallel reactor with vacuum pore impregnation (photo and diagram) Figure 4.10. Hydrothermal production of ZSM-5 with successive ion exchange Figure 4.11. A computer replication of the MFI-type zeolite HZSM5 (left) and a structure is depicting the shape-selective synthesis of p-xylene in the cage (right). The simulation snapshot was provided by Dr. Edith Beerdsen Figure 4.12. Extrusion templates allow for the creation of a wide range of catalyst pellet forms and sizes Figure 4.13. Zoom-in picture of a single active site on a porous alumina support particle with well-distributed active sites (left)

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Figure 4.14. (a) Channel/cage structure; (b) polygonal capillaries; (c) “ink bottle” pores; (d) laminae; (e) slit pores are examples of pores that vary in size, shape, and connectivity Figure 4.15. The following are the six different kinds of adsorption isotherms: Type I displays a monolayer; types II and III display multilayer adsorption; and type IV displays a monolayer initially, afterward mesopore filling. The point of monolayer formation (point “B”) is shown by the “knee” in isotherms I, II, and IV, which is shown by a black dot Figure 4.16. (a) TPR profile of CeO2, CuO, along with a copper-doped ceria model, Cu0.1O2, Ce0.9 is illustrating the effect of Cu doping on the decrease temperature; (b) TPR profile of CeO2, CuO, and a copper-doped ceria sample, Cu0.1O2, Ce0.9 observe the impact of Cu doping on the reduction temperature Figure 4.17. (a) When an electron beam strikes the catalyst surface, it triggers a series of reactions; the electrons that pass via the sample are used to generate; (b) the TEM image (Wang et al., 2004) Figure 4.18. SEM image of “polyelectrolyte onions,” which can block active metal clusters inside their shell thickness, revealing the porous three-dimensional structure at the capsule surface Figure 4.19. The IR activity of the four distinct CO adsorption arrangements on metal surfaces may be identified Figure 4.20. (a) The channel washcoat, monolith, and supporting active metals in the TWC are depicted in this cartoon; (b) for transforming the three primary pollutants, reduction reactions and chemical oxidation is used Figure 5.1. Global catalyst market prospects through the general industry sector Figure 5.2. The tree depicts the function of catalysis in environmental protection Figure 5.3. Sustainable (photo/electro) catalytic materials, catalyst synthesis techniques, emerging environmental technologies, and pertinent applications in the field of ecological catalysts and green chemistry are represented schematically Figure 5.4. (A) Photos of raw plant biomass; (B) carbonized biomass; (C) SEM microstructure of carbonized biomass after oxidation at 350°C; (D) their porous surface morphology; and (E) controlled pyrolysis of enlarged starch yields mesoporous carbon in this overview Figure 5.5. (A) Overview of the preparation of iron oxide and polysaccharide nanocomposites; (B) demonstration of the synthesis of N-HPCMs using a hybrid dual template technique; (C) formation of humin byproducts and their valorization as nanocomposite catalysts Figure 5.6. (A) Nitroarenes hydrogenative coupling catalyzed by Co–N–C catalysts; (B) schematic representation of the creation of metallic transition NPS covered by thin carbon layers (NNM@C) (B1); and its usage in acid valorization (B2) Figure 6.1. The organic structure of phosphoric acid-ester compound

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Figure 6.2. The conventional biocatalysts application domains are joined by a slew of new industrial industries (right) (left) Figure 6.3. Some of the scientific disciplines that contribute to biotechnology are also relevant to biocatalysts and include classic subjects and some new ones, such as ‘material science and ‘bioinformatics.” Figure 7.1. Interpretation of enzyme structure making multiple points of contact with a substrate for outstanding selectivity Figure 7.2. Rhodium price variations over time Figure 7.3. Every 5-year period over the previous 50 years, the number of papers and patents mentioning “pharmaceutical biocatalysis” has increased Figure 8.1. Artist’s perspective of complete water splitting by the semiconductor-metal hybrid nanoparticle Figure 8.2. Dependency of catalytic activity on shape, size, and composition Figure 8.3. Different kinds of anisotropic nanoparticles Figure 8.4. Various options of bimetallic nanostructures were examined: (a) requested alloy; (b) arbitrary alloy; (c) Janus-like; and (d) core-shell Figure 8.5. The shape and size-controlled Pt nanoparticles were set up by the colloidchemistry-controlled method Figure 8.6. (Upper panel) The kinked Pt surface of the model. At the kink and step sites, the C-C and C-H bonds are detached, respectively. (Panel on the lower left) The two-pathway reaction’s schematic free energy potential surface. Breaking the C-C bond produces Product 1, and disrupting the C-H bond produces Product 2 Figure 8.7. (a) The selectivity of pyrrole hydrogenation as a function of nanoparticle size at the specific reaction conditions: 4 Torr pyrrole, 400 Torr H2, 413 K. Small nanoparticles have a strong pyrrolidine selectivity. (b) Pyrrole hydrogenation selectivity is affected by nanoparticle shape underneath the following conditions: 400 Torr H2, 4 Torr pyrrole nanopolyhedra particles exhibit a more excellent selectivity for pyrrolidine than nanocubes at lower temperatures

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LIST OF TABLES

Table 1.1. The environmental friendliness of several chemical industry products Table 1.2. Major processes involving heterogeneous catalysis Table 1.3. Organic chemical synthesis in the USA and annual change in production over the years 1995–2005 Table 1.4. Inorganic chemical synthesis in the USA and annual change in production over the years 1995–2005 Table 1.5. Organic chemical (plastics and polymers) synthesis in the USA and annual change in production over the years 1995–2005 Table 1.6. Top chemical manufacturers in the world Table 4.1. Concentrations of exhaust gas constituents and the EU’s regulatory limitations (2005) Table 6.1. The primary benefits and drawbacks of biocatalysts are their potential use in biotransformations on a laboratory or industrial scale

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LIST OF ABBREVIATIONS API

active pharmaceutical ingredient

CN

coordination number

ESN

ethyl succinonitrile

IR

infrared

Ir

iridium

MGN

2-methylglutaronitrile

Os

osmium

Pd

palladium

PGM

platinum group metals

Pt

platinum

Rh

rhodium

Ru

ruthenium

SEM

scanning electron microscopy

SEO

selective ethylene oxidation

SPE

solid-phase extraction

SS-NMR

solid-state nuclear magnetic resonance spectroscopy

TOF

turn over frequency

TON

turnover number

TP

temperature-programmed

TPR

temperature-programmed reduction

TPS

temperature-programmed sulfiding

UHV

ultra-high vacuum

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PREFACE

Catalysis is the method of using a catalyst to influence a chemical reaction. This procedure is only applicable to substances that already have a reaction and is used to expedite the reaction for business reasons. Catalysis is quicker than conventional chemical reactions because catalysts need less energy of activation, which is the very minimum amount of energy required to activate a chemical reaction. The name “catalyst” comes from the Greek word “katalúō,” which means “to untie or to loosen.” Elizabeth Fulhame, a chemist, created the notion of catalysis based on her groundbreaking work in oxidation-reduction studies. Catalysts are not utilized during the reaction and so remain unaltered afterward. When the reaction is fast and the catalyst recycles rapidly, extremely little quantities of catalyst are often sufficient; mixing condition, temperature, pressure, and surface area, all have a role in the pace of the reaction. Catalysts often interact with one or multiple reactants to create intermediates that eventually yield the ultimate reaction product, renewing the catalyst in the process. Catalysis may be characterized as homogeneous, in which the reactant’s components are scattered in the same phase (often gaseous or liquid), or heterogeneous, in which the reactant’s components are not distributed in the same phase. Enzymes along with some other biocatalysts are categorized in the third category of catalysts. Catalysis is used extensively in all branches of the chemical industry. Catalysts are estimated to be used in the manufacturing of 90% of all commercially generated chemical compounds. Gottlieb Kirchhoff first discovered the starch conversion to glucose by acid-catalysis in 1811, which was the first chemical process in organic chemistry to use a catalyst. Jöns Jakob Berzelius used the word catalysis in 1835 to refer to processes that are hastened by chemicals that stay unaltered after the reaction. Fulhame, who before Berzelius, conducted reduction experiments using water rather than metals. Eilhard Mitscherlich, who used the term “contact processes,” and Johann Wolfgang Döbereiner, who used the term “contact action,” were two other 18th-century chemists who worked with catalysis. He invented Döbereiner’s lamp, a hydrogen-based lighter with a platinum (Pt) sponge that was an industrial success in the 1820s and continues to be used today. Humphry Davy pioneered platinum’s application in catalysis. In the 1880s, Wilhelm Ostwald of Leipzig University began a systematic examination of chemical processes catalyzed by acids and bases. He discovered that chemical interactions take place at limited speeds and that these rates may be used to calculate the strengths of bases and acids. Ostwald

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was given the Chemistry Nobel Prize in 1909 for this achievement. Vladimir Ipatieff pioneered many early industrial processes, such as the discovery and industrialization of oligomerization as well as the invention of hydrogenation catalysts. Because catalysts are utilized in the manufacture of the majority of chemicals, catalysis has transformed the chemical industry, resulting in a multibillion-euro sector. This advanced textbook is required reading for all Master’s and PhD students studying catalysis, since it takes a unique multidisciplinary approach to the subject. It is organized as a series of chapters that describe the principles of catalysis as the field has evolved over the last several decades and introduces novel catalytic systems which are gaining growing contemporary significance. It covers all fundamental concepts, from molecular catalysis through catalytic reactor design, and contains various case studies demonstrating the critical role of catalysis in the chemical sector. The book is mainly divided into eight chapters. Chapters 1 and 2 primarily deal with the fundamentals of catalysis, catalysts, and catalytic reactions. Chapters 3 and 4 discuss the essential concepts of homogenous and heterogeneous catalysis, respectively. Chapter 5 focuses on the importance of catalysis and catalytic reactions for environmental sustainability. Chapter 6 explains the basics of biocatalysis and the use of catalysts in biological processes. Finally, Chapters 7 and 8 shed light on the applications of catalysis in pharmaceutical industries and nanotechnology-based commercial sectors, respectively. This book provides fundamental yet comprehensive insights into the essentials of catalysis, catalysts, and catalytic reactions. The book is equally beneficial for students, professors, and chemical engineers working in the field of catalysis.

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—Author

CHAPTER

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FUNDAMENTALS OF CATALYSIS

CONTENTS

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1.1. Introduction ...................................................................................... 2 1.2. What Is Catalysis? .............................................................................. 2 1.3. Molecules, Atoms, Solid Surfaces, and Enzymes as Catalysts ............ 5 1.4. Why Is Catalysis Important? ............................................................... 9 1.5. Multidisciplinary Field of Catalysis .................................................. 17 References .............................................................................................. 20

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1.1. INTRODUCTION When asked what a catalyst is, the average person on the street would likely say that it is what is inserted underneath the car to unsoil the exhaust. Certainly, the automotive exhaust converter is a very successful use of catalysis; it successfully eliminates the bulk of pollutants from vehicle engine exhaust. However, catalysis has a far larger use than pollution control (Roduner, 2018). For instance, living organisms depend on enzymes, which are the most precise catalysts known to humankind. Furthermore, the chemical sector would not survive without catalysis, which is a critical instrument in the manufacturing of bulk chemicals, fuels, and fine chemicals (Ranocchiari and Bokhoven, 2011). Catalysis is an extremely difficult and multidisciplinary field of study for scientists and engineers. To begin, let us define catalysis and then describe why it is vital to mankind (Armor, 1992).

1.2. WHAT IS CATALYSIS? A catalyst is a material that speeds up the pace of a chemical reaction. This is accomplished by forming bonds with the reactive molecules and allowing them to react, resulting in the formation of a product that separates from the catalyst, leaving it unaltered and ready for the next reaction. Indeed, the catalytic reaction may be thought of as a cyclic event in which the catalyst participates and is restored to its initial condition at the cycle’s completion (Armor, 1992). Consider the catalytic reaction that occurs between two molecules A and B, resulting in the product P. (see Figure. 1.1). When bonds form between molecules (i.e., A and B) and the catalyst, the cycle begins (Erkkilä et al., 2007). Then, within this complex, A and B react to produce P, which is likewise linked to the catalyst. P then separates from the catalyst, resuming the chemical cycle’s initial condition (Watson and Parshall, 1985).

Figure 1.1. Each catalytic reaction is composed of a series of simple stages in which reactant molecules connect to the catalyst, react, and then detach from the catalyst, freeing it for the next cycle. Source: https://slideplayer.com/slide/10751001/.

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To understand how the catalyst speeds up the reaction, we must first look at the potential energy graph as shown in Figure 1.2, which contrasts noncatalytic and catalytic processes. The figure depicts the Arrhenius equation for the non-catalytic reaction in a familiar manner: the reaction occurs when A and B meet with enough energy to surpass the activation barrier in Figure 1.2. Differential of Gibbs free energy among the reactants (i.e., A + B) and the product (i.e., P) is denoted by the symbol DG (Ho, 1988). The catalytic process begins when the reactants spontaneously bind to the catalyst. As a result, the process of creating this compound is exothermic, lowering the free level of energy. Then, while coupled to the catalyst, A and B react (Seiser et al., 2011). Although there is an activation energy in this phase, it is much lower than in the uncatalyzed process. Finally, the catalyst is separated from the product P by the endothermic process.

Figure 1.2. Diagram depicting the potential energy of a heterogeneous catalytic process with products and reactants in gaseous form along with a solid catalyst.

Note that the uncatalyzed process must overcome a significant energy barrier, while the catalytic pathway has significantly lower hurdles. Source: https://application.wiley-vch.de/books/sample/3527316728_c01.pdf.

The following are significant properties of catalysts and catalysis (Hattori, 1995):

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• •

The catalyst creates an additional reaction route that is obviously more difficult but substantially more energy-efficient. Because the catalytic process has a lot lower activation energy than the uncatalyzed reaction, the catalytic reaction advances at a much quicker rate.

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•

The overall variation in the free energy of the catalytic reaction is equal to the total variation in the free energy of the uncatalyzed process. As a consequence, the catalyst has no influence on the total A + B to P reaction’s equilibrium constant. Thus, if the thermodynamics of a process are unfavorable, a catalyst will have no influence on the result. Catalysts affect the kinetics of a process but not its thermodynamics. • The catalyst speeds both forward and reverse reactions greatly. In other words, if a catalyst accelerates the formation of P from A and B, it also accelerates P’s decomposition into A and B. Thus far, it is self-evident that the combination of catalyst and reactants or products will fail in several instances (Beletskaya et al., 2018): •

If the reaction between the reactants and the catalyst is insufficiently strong, just a little amount of A and B will be transformed to products. • On the other hand, if the catalyst’s affinity for one of the reactants, say A, is excessively great, the catalyst will be mostly filled by species A, leaving no space for the synthesis of the product. If both A and B form strong bonds with the catalyst, the intermediate state containing either A or B may become very stable, rendering the reaction implausible. According to Figure 1.2, the second level is so deep that the activation energy needed to generate P on the catalyst becomes too large. The catalyst, it is suggested, has been poisoned by (one of the) reactants. Similarly, the product P may be too closely associated with the catalyst to permit separation. The product serves as a poison to the catalyst in this case. Thus, we intuitively feel that an efficient catalyst-reaction combination is one in which the catalyst-reacting species interaction is neither too weak nor too strong. This is a condensed version of Sabatier’s Principle. Given that the catalyst has so far been a nameless, abstract material, let us first look at the many forms of catalysts (Armor, 2011).

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1.3. MOLECULES, ATOMS, SOLID SURFACES, AND ENZYMES AS CATALYSTS Catalysts come in a range of forms and sizes, from atoms and molecules to bigger structures such as zeolites and enzymes. Additionally, they are applicable in a number of settings, including liquids, gasses, and at the surfaces of solids. Developing the best condition of a catalyst and investigating its precise composition and form is a crucial specialty that we shall address in coming chapters (Marsh and Warburton, 1970). Catalysis is often classified into three subfields: homogeneous, heterogeneous, and biocatalysis. Each is illustrated with a case study.

1.3.1. Homogeneous Catalysis All molecules in homogeneous catalysis are in the same phase, whether it is the gas phase or the liquid phase (which occurs more often). One of the most straightforward examples may be found in the field of atmospheric chemistry. A variety of mechanisms exist in the atmosphere for ozone decomposition, one of which is interaction with chlorine atoms (Centi et al., 2002).

or overall:

Ozone naturally decomposes and also under the influence of light, while the addition of a Cl atom significantly accelerates the process. Because the Cl atom has no effect on the chemical cycle, it functions as a catalyst. The reaction cycle demonstrates homogeneous catalysis since both reactants and catalysts are in the same phase, i.e., the gas phase. (In the past, this reaction was critical in anticipating the ozone hole (Panov et al., 1998). In a range of chemical processes, industry use a variety of homogeneous catalysts. [Rh(CO)2I2]– complexes in solution catalyze the methanol to acetic acid carbonylation, which is a good example of catalytic carbonylation.

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CO + CH3OH = CH3COOH Homogeneous catalysis, which is widely employed to synthesize sensitive pharmaceuticals, results in the formation of organometallic complexes under molecular control, with the ligands steering the reactions toward the preferred products (Boltes et al., 2001).

1.3.2. Biocatalysis Nature’s catalysts are enzymes. For the time being, consider an enzyme to be a large protein with a highly specialized active site structure (Figure 1.3). Because enzymes’ geometries are optimized for directing reactant molecules (commonly referred to as substrates) into the right configuration for reaction, they are exceptionally selective and efficient catalysts. For example, catalase catalyzes the breakdown of hydrogen peroxide into water and oxygen at an amazing rate of up to 107 molecules of hydrogen peroxide (H2O2) per second (Fu et al., 2010).

Figure 1.3. An enzyme-catalyzed reaction is depicted schematically. Enzymes frequently conform to the geometry of their substrates or the transition state of the process that they catalyze. Enzymes are extremely efficient catalysts and provide a wealth of inspiration for the development of technological catalysts. Source: https://application.wiley-vch.de/books/sample/3527316728_c01.pdf.

Enzymes allow biological processes to occur at the rates necessary for life to exist, such as protein and DNA synthesis or chemical breakdown

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and energy storage in carbohydrates. A particularly intriguing example for children may be the body’s alcohol dehydrogenase enzyme’s breakdown of alcohol to acetaldehyde. The acetaldehyde is subsequently converted to acetate by aldehyde hydrogenase. Certain people are unable to handle alcohol (as demonstrated by facial flushing after a little quantity) due to a lack of the enzyme responsible for acetaldehyde degradation (Kawasaki et al., 2002).

1.3.3. Heterogeneous Catalysis During heterogeneous catalysis, solids catalyze reactions involving molecules in gaseous or solution form. Because solids—with the exception of porous ones – are often impermeable, catalytic reactions occur near the surface. Catalysts are generally nanometer-sized particles supported on an inert and porous framework to maximize the use of commonly costly materials (e.g., platinum). Heterogeneous catalysts are often regarded as the workhorses of the petrochemical and chemical industries, and we will cover a range of heterogeneous catalysis applications throughout this book (Figure 1.4) (Sherwood et al., 2003).

Figure 1.4. The structure of a nanosized catalyst. Source: https://application.wiley-vch.de/books/sample/3527316728_c01.pdf.

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Handbook of Catalysis

As a starting point, consider one of the most important processes in automobile exhaust cleaning: the catalytic oxidation of carbon monoxide on the surface of inert metals such as platinum, palladium (Pd), and rhodium (Rh). Let us suppose that the metal surface has active sites, which are symbolized by the symbol “*,” in order to better comprehend the process. Later on, they will be given the right classification. Initially, the adsorption of oxygen and carbon monoxide on the surface of platinum results in the dissociation of the oxygen molecule into two oxygen atoms (the X* symbol denotes that it is surface-adsorbing) and the formation of CO (Billeter et al., 2000).

In turn, the adsorbed oxygen atom and carbon monoxide molecule combine on the surface to form CO2. Because of its great stability and low reactivity, CO2 interacts poorly with platinum’s surface and desorbs very immediately.

Particularly noteworthy is that the later phase liberates the adsorption sites on the catalyst, so making them available for use in following reaction cycles. On the right-hand side of Figure 1.5, you can see the chemical cycle and a potential energy graph (Vries et al., 1999). In which part of the loop does the catalyst have the most influence? Assume that the reaction happens in a gaseous state without the aid of a catalyst to accelerate the process. As long as the temperature is elevated enough to produce dissociation of the O2 molecule into two oxygen atoms, the process will continue (radicals). In the presence of radicals, the reaction between CO and CO2 takes place very fast. The activation energy for the gas phase processes will be about equal to the energy necessary to break the very strong O–O bond in oxygen, which is around 500 kJ mol–1 at room temperature (Metz et al., 2014).

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Figure 1.5. Diagram illustrates the catalytic oxidation of CO by oxygen, showing the reaction cycle and potential energy. Source: https://slideplayer.com/slide/10751001/.

However, in a catalytic process, the O2 molecule dissociates rapidly on the catalyst surface—in fact, it does so without the need for any activation energy. The activation energy is in the range of 50–100 kJ mol–1 and is connected to the reaction between adsorbed Carbon monoxide and oxygen atoms in the atmosphere. The desorption of the product molecule CO2 has a cost of around 16–32 kJ/mol, reliant on the temperature. By comparison the catalyzed reactions with the uncatalyzed reactions, we can see that the catalyst is very efficient at performing the most crucial stage of the gas phase homogeneous process, namely the dissociation of the O–O bond. Therefore, the efficiency with which the Carbon dioxide molecule grows controls the rate at which the full interaction between CO and O2 occurs. For catalyzed reactions, this is a very common occurrence, which is why the phrase “a catalyst breaks bonds and allows for the production of new ones” is used to describe what is taking place. Because the catalyst’s beneficial function is in dissociating a strong link, it is possible that the subsequent processes will go more swiftly even if the catalyst is not present (Sherwood et al., 1997).

1.4. WHY IS CATALYSIS IMPORTANT? Because of non-stoichiometric reactions in the laboratory, the industrial sector of the 20th century could never have evolved to its present degree of growth on its own. Several factors in chemical reactions may be controlled

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to a certain degree, including temperature, concentration, pressure, and time of contact with the reactant. It is necessary to elevate the temperature and pressure in order for stoichiometric reactions to occur at a pace that is sustainable for the manufacturing process (Field et al., 1990). Constructing reactors where such circumstances can be effectively maintained, on the other hand, is becoming more and more expensive and difficult to do as time goes on. It is also bound by thermodynamic constraints, which preclude the manufacture of certain products at temperatures higher than 600°C due to thermodynamic restraints. For example, at temperatures more than 600°C, the transformation of H2 and H2 into ammonia is very hard to do. It is true that greater temperatures are required to break the extraordinarily strong N-N bond in nitrogen dioxide, but that this is not always the case. The absence of catalysts would make many reactions that are frequent in the chemical industry impossible to complete, and several processes would be unproductive if they were not carried out with the help of catalysts (Quesne et al., 2014). In addition to allowing operations to be carried out in the most favorable thermodynamic domain possible, catalysts also allow them to be carried out at far lower pressures and temperatures than would otherwise be possible in the absence of catalysts. In order for chemical processes to be more cost-effective in respect of both original investment and ongoing operating expenditures, it is necessary to use effective catalysts in conjunction with optimal reactor design and overall plant structure. This, however, is not the case at all (Cornils and Herrmann, 2003).

1.4.1. Catalysis and Green Chemistry If a technology properly utilizes raw materials while avoiding the use of dangerous and hazardous chemicals and solvents, it is referred to be “green,” and waste and undesirable byproducts are kept to a minimum. These criteria are commonly satisfied by catalytic techniques, which are described below. Ethylene glycol (antifreeze), as well as other polyethers and polyurethanes, is produced by the process of selective hydrogenation of ethylene-to-ethylene epoxide. This is an outstanding example of selective ethylene oxidation (SEO) (Figure 1.6).

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Figure 1.6. Structure of the chemical industry’s most significant intermediate: ethylene epoxide. Source: https://slidetodoc.com/kinetics-rates-and-mechanisms-of-chemicalreactions-kinetics/.

The epichlorohydrin procedure (the traditional, non-catalytic technique) is a three-step synthesis:

As a consequence, 1 molecule of salt is produced for every molecule of ethylene oxide produced, resulting in a waste issue that was formerly dealt with by disposing of the trash in a river. Of course, such conduct is today considered to be quite unacceptable. In contrast, while the catalytic technique is easy and environmentally friendly, it does produce a little amount of CO2. C2H4 and O2 are used to form ethylene oxide, which is produced with a selectivity of around 90%, with only about 10% of the ethylene being converted to CO2. Silver is used as a catalyst, and it is given a boost by trace amounts of chlorine. The use of catalysts in ethylene oxide production facilities is now mandatory in all of them (Cole-Hamilton, 2003).

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1.4.2. E Factors, Atom Efficiency, and Ecological Affability In a number of organic syntheses, stoichiometric oxidations of organic compounds using potassium permanganate and sodium dichromate, as well as hydrogenations with borohydrides, alkali metals, or zinc, are used. Some chemical reactions, such as acylations with AlCl3 or aromatic nitrations with HNO3 and H2SO4, result in significant amounts of inorganic salts being produced as byproducts. Homogeneous catalysis is mostly (but not exclusively!) limited to fine chemicals, with solvents constituting an extra environmental hazard due to their toxicity. The ideal solvent, according to Sheldon, is none at all, but if one is essential, water is a reasonable alternative (Wang et al., 2012). Sheldon has developed a variety of metrics for evaluating the efficiency of a response as well as its impact on the environment. When it comes to atom efficiency, it is defined as the ratio of the molecular weight of the goal product to the aggregate molecular weight of all output products. For instance, secondary alcohol’s conventional oxidation:

has an atomic efficiency of 360/860 = 42%. Comparatively, the catalytic route: provides an atomic efficiency of 120/138 = 87.0%, with H2O as the only byproduct. The reverse step (i.e., catalytic hydrogenation) continues with 100% atomic efficiency: Similarly catalytic carbonylation also shows 100% efficiency: C6H5–CHOH–CH3 + CO → C6H5–CH(CH3)COOH The E factor, which is known as the ratio of the weight of waste to the weight of the intended product, is another important indicator of environmental acceptability in manufacturing. As seen in Table 1.1, pharmaceutical industries produce the greatest amount of waste per unit weight of product produced. 1. Although tom efficiencies and E factors may be calculated from one another, E factors can be much bigger in practice due to lower-than-optimal yields and excessive reagent use. Losses of solvents, as well as energy consumption and CO2 emissions, should all be taken into consideration (Villa et al., 2000).

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Table 1.1. The Environmental Friendliness of Several Chemical Industry Products Industry Segment

E Factor (kg of Waste/kg of Product)

Product Tonnage

Bulk chemicals

99% (Connon, 2008). This catalyst is a promising contender for practical applications because it also functions very in organic solvents like acetonitrile, THF, and dichloromethane (Figures 3.38 and 3.39).

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Figure 3.38. (a) The usual enamine catalytic cycle in the existence of L-proline; (b) an instance of an asymmetric Mannich-type accumulation of cyclohexanone to iminoethyl glyoxalate, catalyzed through a proline tetrazole derivative. Source: https://onlinelibrary.wiley.com/doi/10.1002/9783527621866.ch3.

Figure 3.39. (a) The structure and planned mode of action of the altered Cinchona alkaloid catalyst; (b) an instance of the catalytic Henry reaction among nitromethane and benzaldehyde. Source: https://www.researchgate.net/publication/285398199_Catalysis_Concepts_and_Green_Applications.

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3.4. SCALING UP HOMOGENEOUS REACTIONS: CONS AND PROS Considering their relevance in the industry, scale-up concerns are always so often overlooked by academics. This is unfortunate since assessing the practical feasibility of large-scale catalyst manufacturing and recovery soon on in the scheme improves the prospects of a positive application.

3.4.1. Catalyst Recycling and Recovery Ligand design and organometallic production provide the catalyst precursor complexes with a great degree of molecular control. As a result, homogeneous catalysts are typically more selective and active than heterogeneous parts. Furthermore, homogeneously catalyzed processes are not limited by surface effects, phase-transfer constraints, or mass-transfer issues. Every catalytic site is available, and every metal atom has the potential to be an active site. Despite these benefits and the growing importance of homogeneous catalysis in academic and business communities (Hamilton, 2003), several homogeneous catalytic systems remain uncommercialized because of challenges in catalyst isolation, recycling, and recovery. Product distillation, the very frequent extraction process in large-scale processes, necessitates high temperatures. The majority of homogeneous catalysts are heat difficult and disintegrate under 150°C. As a result, even at low pressure, distillation will result in catalyst breakdown. Indeed, the majority of commercial processes that use homogeneous catalysis either employ unstable substrates and products or do not include thermosensitive organic ligands (Bhanage Arai, 2001). Low-pressure extraction is particularly difficult since a catalyst designed for high-pressure process situations may suffer unwanted side reactions below lower pressure. Catalyst extraction and recovery are critical process economic variables for two purposes. Most importantly, the catalysts are quite cheap in comparison to the compounds. In the bulk chemical sector, operating profit margins are normally 200–300%. It shows that the finished product is worth around up to 3 times the worth of the raw materials. If the catalyst is 1,000 significantly more costly than the result, as is frequently the case with valuable transition metals, losing just as a percentage of it makes the whole method uneconomical. Second, any catalyst that remains in the product may cause issues later in the process. It has the potential to alter the selectivity of future processes, bind to and so poison adjacent catalysts, or corrode apparatus. Even though the catalyst is not costly, the presence of catalyst contaminants

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in the product may be objectionable. One common example is the production of biodiesel through the esterification of free fatty acids using methanol. In the addition of 1 mol percent sulfuric acid, an inexpensive and commonly accessible catalyst, this reaction occurs smoothly. The problem is that the existing standards More than 20 ppm sulfur are prohibited for diesel due to the environmental consequences of the ensuing SOx emissions. Decreasing the sulfur concentration from 1% to < 20 ppm is a near-impossible operation, prompting the hunt for solid-acid catalysts. Several strategies are used by industry to solve these difficulties (Jessop, 1999). The most frequent are precise product crystallization, which leaves the catalyst and extra reagents and substrates in the liquid state, and catalyst deposition and filtration, which removes the catalyst as salt as of the organic reaction medium. Other methods contain a high-vacuum flash distillation of the product and liquid/liquid separation of the accelerator from the reaction medium. Supercritical solvents, particularly supercritical CO2, are an intriguing option (Pereda et al., 2005). Supercritical fluids are gasses that have been compressed to temperatures and pressures above their critical values. They dissolve a wide range of medium-polar and polar organic compounds and are highly soluble with all producer gas, preventing any phase-transfer issues. The supercritical solvent can easily be decompressed into gas and discarded at the end of the process (118). CO2’s low critical pressure and temperature make it excellent for this application (Jessop et al., 1999). Even though several metal-containing compounds are difficult to dissolve in scCO2, introducing trialkyl phosphines or fluorocarbon chains can alleviate this problem (Webb et al., 2005). This, nevertheless, does not address the basic issue of removing the catalyst from the product. This was accomplished by researchers using an ingenious method in scCO2 whereby an Ir hydrogenation catalyst is soluble in the vicinity of the substrate however precipitates as soon as the substrate is exhausted (Koch and Leitner, 1998). Alternative approaches use pressure and temperature turns to precipitate the catalyst, followed by decompression to remove the product (Pereda et al., 2005). High-molecular-weight transition-metal compounds and nanoporous membranes are combined in a novel concept (Sellin et al., 2002). Ceramic, as well as organic nanofiltration and ultrafiltration membranes, are now commercially accessible as a result of improvements in membrane technology. Such membranes are capable of filtering species with sizes ranging from just a few nanometers to tens to 100 nanometers. These dimensions correspond to those of macromolecules and dendrimers, accordingly. Furthermore, the

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membranes can be designed to resist organic solvents, varying pH levels, and a broad temperature range. As a result, homogeneous catalyst extraction and constant treating are physically conceivable. If the homogeneous catalyst is large, a sufficient amount is maintained through the membrane. Allylic amination, hydroxylation, and Kharasch inclusion are some of the applications (Webb et al., 2005). There are various methods for creating high-molecular-weight homogeneous catalysts. One easy and successful method is to covalently attach the ligand to a soluble polymer or oligomer. Plenio and colleagues determined this method by attaching phosphine ligands to poly(methyl styrene) or poly monomethyl ether. They next reacted this polyfunctional ligand with Pd0 or PdII precursors to produce a multisite homogeneous Pd catalyst that could be maintained during liquid/liquid extraction or nanofiltration. These catalysts were successfully employed in a variety of CC cross-coupling processes. Other methods include using shape-constant dendrimers or the attachment of huge “exterior units” to single metal-ligand compounds. Another frequent technique is to anchor the catalyst to strong support. Even though these systems are no more actually “homogeneous,” they are included in this chapter because they operate on the very same principles as their homogeneous equivalents. Section 3.3.2 describes these “hybrid catalysts.” Another hybrid strategy is to create heterogeneous catalysts via surface organometallic chemistry (Figure 3.40) (Bhanage and Arai, 2001).

Figure 3.40. Plenio’s multisite phosphine ligand centered on poly(methyl styrene). Source: https://www.worldcat.org/title/catalysis-concepts-and-green-applications/oclc/652505580.

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3.4.2. Hybrid Catalysts: Bridging the Homogeneous/ Heterogeneous Gap Mixing the benefits of heterogeneous catalysts with those of heterogeneous catalysts (simple separation and excellent heat stability) is a significant trial for the chemical industry. Numerous clever and elegant ways to “heterogenization of similar catalysts” have been devised. Incapacitation of transition metals on solid supports is one example, as is the construction of “ship-in-a-bottle catalysts.” The most frequent method of heterogenization is to bind the homogeneous catalyst to a solid substrate (Cobb et al., 2005). There are various approaches to this. The easiest method is ion exchange, which involves replacing an active cation on the surface of metal ions (Dalko and Moisan, 2004). However, this simple substitution has a two-way effect, with the metal complex frequently leaking back to the solution. Truly, leaching is an issue with hybrid catalysts, and establishing that a mixed catalyst does not seep is difficult (Mishra et al., 2016). A new possibility is to encase the homogeneous compound in a cage, resulting in a ship-in-a-bottle hybrid catalyst. Large organometallic complexes are frequently trapped within the crystalline zeolite cage by zeolites (Leadbeater et al., 2003). Although the catalyst is confined, small substrate and product molecules can pass through the zeolite pores. A number of reactions have been described, such as alkene hydrogenation and selective oxidation of terpenes.

3.5. “CLICK CHEMISTRY” AND HOMOGENEOUS CATALYSIS “Click chemistry” is a perfect instance about just exactly how homogeneous catalysis can promote green chemistry ideas. It is a chemical reaction family that is used to connect organic compounds with heteroatom linkages. Sharpless and colleagues invented the term, which is now generally used to describe quick and high-yielding catalytic processes (Togni, 1996). According to Sharpless’ definition, a clicking reaction must meet numerous severe characteristics. It should be modular, broad in scale, provide extremely high harvests, be stereospecific, and produce mainly in aggressive

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by-products that may be simply eliminated using nonchromatographic techniques. Furthermore, the reactions should be easy, ideally without the need for any solvents. Product separation must also be straightforward, utilizing crystallization or distillation. Because of this large list of conditions, many reactions are excluded from the click repertory, but those that remain are particularly well aligned with green chemistry principles. In fact, the objective of click chemistry is to generate diversified chemical functionality from “some good reactions,” with several of these reactions exhibiting 100% atom economy. Click reactions are prominent in drug discovery because of their modularity and convenience of use (Togni, 1996). Thermodynamics governs click reactions. The substrates contain “springloaded” energy emitted during the reaction. In click reactions, the energy difference among substrates and products is typically greater than 20 kcal/ mole. Because of the huge energy difference, just one product is frequently generated. A catalyst is not always required due to the high energy “laden” in the substrates – a few of these reactants create autocatalytic combinations that should be managed cautiously! In several circumstances, however, the activation threshold is reasonably high that no reaction occurs till a catalyst is included. The ring-rearrangement of acylaziridine is a famous instance. This instance demonstrates how selecting the correct homogeneous catalyst can result in a significant change in the reaction trail. Because of the strain in the three-membered ring, acylaziridines require a lot of energy. They can easily undergo ring-opening and isomerization in the existence of Lewis acid catalysts. Researchers demonstrated that the Lewis acid catalyst regulates the reaction mechanism (Kwan, 2005). There is no reaction in the existence of a nucleophile but not in the absence of a catalyst. By using an “oxophilic” Lewis acid catalyst, like Ti(OiPr)4, the quick ring-opening to carry out trans-disubstituted cyclohexene is accelerated. Remarkably, including an “azaphilic” Lewis acid catalyst, like Cu(OTf)2, results in an altogether distinct reaction: The acylaziridine undergoes rearrangement to form a 5-membered ring that is cis-attached to the cyclohexane core. Nothing happens if there is no Lewis acid (Figure 3.41).

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Figure 3.41. The two routes for the Lewis-acid-catalyzed ring-opening of aziridines. Source: https://onlinelibrary.wiley.com/doi/10.1002/9783527621866.ch3.

The Huisgen-type (3+2) cycloaddition of alkynes and azides to triazoles is perhaps the most well-known click reaction. This reaction exemplifies two fundamental benefits of catalysis: better reaction situations and increased product selectivity. The noncatalytic form typically necessitates high temperatures and produces a 1:1 combination of the 1,4 and 1,5 regioisomers. To use a CuI catalyst, the reaction can be performed at room temperature in a water/t-BuOH combination, yielding the 1,4 regioisomer exclusively in good yield. Surprisingly, nearly any copper species can catalyze this process. Nanoparticles, powder, and copper shavings are dissolved in situ to CuI, but a variety of CuII salts can be utilized as a catalyst in the absence of a mild reduction agent like ascorbic acid (Cheong et al., 1990). The reaction bears a wide variety of functional groups and circumstances and has been used in solid-phase production, dendrimer synthesis, polymer synthesis, or still peptide bond surrogate construction (Figures 3.42 and 3.43) (Freixa and Leeuwen, 2003).

Figure 3.42. (a) Common reaction method for the thermal Huisgen cycloaddition; (b) the copper-catalyzed reaction among benzyl azide and phenyl propar-

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gyl ether. The catalytic reaction is performed in the existence of a reductant and gives just one of the product isomers in the high harvest. Source: https://www.worldcat.org/title/catalysis-concepts-and-green-applications/oclc/652505580.

Figure 3.43. The proposed catalytic cycle for the Huisgen (3 + 2) cycloaddition, showing the “direct” and “indirect” routes. Source: https://www.amazon.com/Catalysis-Concepts-Applications-GadiRothenberg/dp/3527318240.

The catalytic cycle is an intriguing example of how computational chemistry might supplement mechanistic studies. The reaction between the CuI salt and the terminal acetylenic carbon “begins.” This produces copper acetylide, a well-known species from the Stephens–Castro, Glaser, and Sonogashira reactions (Miller and Wayner, 1990). This intermediary is expected to react immediately with the azide in a one-step (3+2) cycloaddition. DFT simulations hint at a sequential reaction in which the azide initially binds to the copper center and then creates a six-membered heterocyclic intermediary that relocates to the product.

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CHAPTER

4

HETEROGENEOUS CATALYSIS

CONTENTS

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4.1. Introduction .................................................................................. 120 4.2. Classic Gas/Solid Systems .............................................................. 121 4.3. Preparation of Solid Catalysts: Black Magic Revealed .................... 129 4.4. Selecting the Right Support ............................................................ 138 4.5. Catalyst Characterization ............................................................... 140 4.6. The Catalytic Converter: An Instance from Everyday Life ............... 150 References ............................................................................................ 153

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4.1. INTRODUCTION Heterogeneous catalysis had a significant impact on our lives in the 20th century. Fritz Haber, a German chemist, succeeded in manufacturing ammonia in 1908 by pouring H2 and N2 at elevated pressures above catalyst. Alwin Mittasch and Carl Bosch of BASF followed up on this discovery and examined almost 2,500 dissimilar metals till they discovered an ironbased complex that was cheap active sufficient to act as a commercialized catalyst. The Haber–Bosch ammonia production has developed one of the world’s several essential chemical methods, gaining Haber the Nobel Prize in chemistry in 1918. Nitrogen fixation provides humans with necessary fertilizer, increasing crop production for the world’s expanding population. Paradoxically, a similar technique also generated raw materials for the manufacture of explosives, which aided Germany’s side in World War I. Mittasch’s magnetite catalyst is still employed on a massive scale today—a whopping 110 million tons of ammonia were generated in 2005, providing for 1% of worldwide energy use. In the 1930s, three key varieties of refinery catalysts were developed: those for dehydrogenation, hydrocarbon cracking, and alkylation. Once again, heterogeneous catalysis was important in warfare, this time during World War II. Allied troops manufactured higher-octane aviation fuel using innovative cracking and alkylation catalysts, giving the Spitfires a performance advantage against the Messerschmitts in the famed Battle of Britain. Likewise, catalytic dehydrogenation of methyl-cyclohexane provided toluene to both sides for the production of TNT. The Fischer–Tropsch synthesis was another major catalytic technique that evolved from political instability. Germany and Japan have plenty of coal; however, no consistent cause of petroleum. The Co/Fe-catalyzed Fischer–Tropsch procedure transformed coal to syngas, which was then further processed to produce a liquid mixture rich in paraffin and C5C11 olefins. During the apartheid era, South Africa employed this technique to convert coal to offset a lack of petroleum supply. Surprisingly, Fischer– Tropsch technologies are currently again in need as administrations pursue sulfur-free fuels and solutions to petroleum (Agrawal et al., 2007). Heterogeneous catalysis now controls the petrochemical and mass chemical industries (Gary and Kaiser, 2009). Refinery catalyst sales surpassed €2 billion in 2005, with an estimated yearly growth of 3.24% (Bryner and Scott, 2006). Not unexpectedly, the highest need for these catalysts is being driven by tougher sulfur content regulations in gasoline

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and diesel. Table 4.1 illustrates a few of the essential products, catalysts, and processes at work. With the growth of green chemistry over the last two decades, heterogeneous catalysis has migrated into the pharmaceuticals and fine-chemicals industries as well (Goede et al., 1994). Solid catalysts are also employed in sustainable applications of energy, including energy storage cycles, solar energy conversion, fuel cells (Carrette et al., 2000; Kamat and Dimitrijević, 1990). At first appearance, heterogeneous catalysis appears to be a daunting task. It combines organometallic and organic chemistry, inorganic chemistry, physical chemistry, materials science, and surface science to form the most diverse and intricate of the three catalysis sub-disciplines. There are several books and magazines on numerous areas of heterogeneous catalysis, as well as approximately 18,000 research articles. Fortunately, these numerous uses are founded on a few basic ideas. This chapter presents the scientific basics by using a variety of industry examples to illustrate them. I will concentrate on material science and physical chemistry challenges rather than engineering. The thorough textbook by Weitkamp and Kn€ozinger (1997) contains an outstanding full description of the latter. It is worth noting that the difference between “homogeneous catalysis” and “heterogeneous catalysis” or “biocatalysis” is arbitrary. A unique enzyme is significantly larger than some of its substratum, and the reaction atmosphere it creates is so different from the neighboring solvent that it is classified as a “heterogeneous catalyst.” Likewise, there are decipherable catalysts that exist on the cusp of “heterogeneous” and “homogeneous” systems. To prevent ambiguity, this chapter concentrates solely on three instances: liquid/solid systems, liquid/liquid systems, and conventional gas/ solid systems (Thathagar et al., 2002).

4.2. CLASSIC GAS/SOLID SYSTEMS The majority of catalytic processing is done via conventional gas/solid heterogeneous catalysis. Solid catalysts are used in the production of more than 90% of the chemicals in the world. The gaseous reactants are typically supplied on the catalyst bed, normally at elevated temperatures often at elevated pressures. These settings are appropriate for constant processes, such as those utilizing plug-flow reactors. Externally, the process appears straightforward: reactants approach the reactor, and products exit (Figure 4.1). Inner, things are a lot more complex: reactants must diffuse across the

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pores of the catalyst, adsorb on its surface, move to the effective site, react, and desorb in return to the gas phase. All of these processes take place at the molecular level. The catalyst, on the other hand, is a macroscopic entity with a complicated surface composition, and interaction at the macroscopic level also influences the reaction result (Bernal et al., 1999).

Figure 4.1. A plug-flow catalytic reactor at several zoom-in levels.

The distinction between heterogeneous and homogeneous catalysis is what makes heterogeneous catalysis so difficult. In each and all gas/solid catalytic cycle, however, a few of the reactants should be retained on the catalyst surface. Let us look at the A+B+C reaction. There are two possibilities: On the catalyst, both reactants A and B are adsorbed, move to each other, and react on the surface, yielding product C, which is desorbed into the gas phase. Since several reactants are initiated by means of adsorption on the catalyst surface, the Langmuir–Hinshelwood process is far more common (Taylor, 1921). The stability, selectivity, and stability of the catalyst are all determined by surface contacts. Unlike molecular catalysts, bulk factors including mechanical strength, particle size, and shape are critical in this case. The available active surface area is proportional to particle size. Platinum, for instance, is a good catalyst for alkane dehydrogenation. However, it is also quite pricey (about D32/g or $44/g in February 2007) (Figure 4.2) (Otto et al., 1992).

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Figure 4.2. The two major processes in solid-gas/ heterogeneous catalysis: (a) Langmuir–Hinshelwood; (b) Eley–Rideal.

Let us say your catalyst is a cubic centimeter of Pt. This cube would be 21.3 grams in weight and cost around €800 ($1,000). Since one side would have to be coupled to the reactor, the active surface area of a cube-like this would be just 5 cm2. If you divide this cube into 1012 smaller cubes, with each 1 mm side, the total surface area accessible is 5 × 1012 μm2 or 50,000 cm2. Modern catalyst manufacturing techniques can create particles as small as 2 nm in diameter, with correspondingly large surface areas. Surprisingly, lowering particle size can have unintended consequences due to foreign atoms penetrating the surface (Otto, 1992; Bernal et al., 1999). The fact that O2 is rapidly adsorbed on Pt particles and dissociates into two O atoms, which react with a range of substrates, is one of the reasons why Pt is an effective oxidation catalyst. When the particles are quite thin (less than 10 nanometers), oxygen reacts with them to generate platinum oxide. Despite the increased metal surface area, the ensuing strong PtO bond reduces catalytic activity. Putting all of these little particles in a single reactor at high temperatures would result in extremely high back pressures and agglomeration. Coating the active metal particles on a substrate with a high surface area, such as silica, alumina, zeolites, or carbon, can prevent this. Catalyst particle form and mechanical strength are also significant, particularly in big applications and reagents. They influence reactor packing and, as a result, the flow of gaseous products to and from the catalyst. Mass transmission and heat transfer are two other critical factors. There is no guarantee that a molecule will react further once it approaches the macroscopic catalyst particle. The reactant should initially diffuse within the pores of porous materials. The molecule may require to traverse across the surface once adsorbed to get to the active site. A similar is true for the

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product molecule’s exit as well as heat transport to and from the reaction location. The product is “hot” when it leaves the catalyst in several gas/ solid systems, and it takes the extra energy with it. This energy should be dissipated via the catalyst particles and reactor wall. Sintering, hotspots, and runaway reactions can all be caused by uneven heat transport.

4.2.1. The Concept of the Active Site Because surfaces are not quite as smooth and uniform as we like to assume, the point that heterogeneous catalysis acts at a surface further confuses things. Metal crystal surfaces have varied steps and kinks at the microscopic level (Figure 4.3). Surprisingly, catalysis occurs most frequently at these irregular spots. This is due to the fact that the surface atoms at certain locations are not entirely aligned, giving them extra opportunities to interact with substrate molecules. Hugh Taylor, a British chemist, proposed the idea of active sites in the 1920s, along with the idea that active sites on surfaces can be small in number (Taylor, 1921, 1926).

Figure 4.3. Graphic illustration of a solid catalyst crystal surface.

Nowadays, we understand that Taylor was correct, and so in several instances, the “catalyst” is primarily invented by inactive surfaces with some very active places. Figure 4.3 is a graphic depiction of a catalyst particle, highlighting the different surface imperfections. It is worth noting that the active catalytic site is not always the most energy-efficient location for adsorption. The most preferred adsorption site may be “too good,” in the idea that any variety that becomes adsorbed on it will remain there indefinitely. The active site fulfills Sabatier’s law: reactants

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should be allowed to be adsorbed there. However, they should also be ready to detach after the reaction is completed. The active locations should ideally be all the same and segregated from one another. Because commercial heterogeneous catalysts are frequently multiphase, multicomponent, and amorphous solids having a variety of active sites, this is rarely the case. There is just one notable omission, however: zeolites. These very crystalline materials can be made to very strict requirements, as well as the placement of the same active sites at consistent distances. It is just the significant benefits of zeolites that are now the chosen catalysts for several industrial methods, as we will see.

4.2.2. Model Catalyst Systems Every year, some eight lac tons of solid catalysts are made and employed around the world, yet we still do not know how they operate. Catalyst treatment and synthesis methods are frequently depending on empirical investigations that have been passed down the generations like traditional remedies. Because chemists despise ignorance much more than nature despises a vacuum, there is a lot of research into how solid catalysts work. Because genuine commercial catalysts are typically nonuniform, multicomponent solids, much of this study is conducted on basic model systems (Freund et al., 2003). The gap between real and model catalysts is a key issue of contention in the catalysis community. Porous, high-surface-area materials (generally 50–400 m2 g–1) are used in industrial catalysts. They can be microcrystalline or amorphous, and they can have several phases. Moreover, because of ion diffusion from the mass, high-pressure impacts, or contaminants in the supply, their surface composition can alter during the installation period. Model catalysts, on the other hand, are well-described materials that are frequently observed in “clean” laboratory environments. Does it make sense to examine such model systems in light of these differences? Yes, as much as you understand the model’s limits. However, significant work is put into “bridging the gap” among real catalysts and model systems, with catalytic characterization carried out as close to the real process environments as feasible (Banares and Wachs, 2002; Weckhuysen, 2003). Single crystals are the most typical model methods for heterogeneous catalysis. These are very pure and homogenous materials. Both in simulated method conditions and under ultra-high-vacuum settings, they are simple to characterize and investigate in detail (Ertl, 2002). In spite of the differences

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between single-crystal and actual catalysts, single-crystal systems can teach us a lot regarding the catalytic cycle (Ertl, 1994). The model systems offer a solid scientific foundation for the development of novel ideas and concepts, which can then be evaluated in real-world scenarios (Strongin et al., 1987). For determining structure/activity connections, single crystals are extremely valuable. The temperatures of desorption and absorption, the adhering prospects, and the rate of surface reactions can all be measured using a variety of characterization methods. Indeed, tests like these reveal that various crystal facets can have drastically distinct catalytic characteristics! On Fe single crystals, Figure 4.4 depicts the rate of nitrogen dissociative adsorption. This separation is a crucial step in the production of ammonia. The substantial dissociative adsorption discrepancies between the (1,1,1) and (1,1,0) planes match well with their activity in ammonia production below simulated industrial circumstances (Strongin et al., 1987). You may adjust the type and number of kinks and leaps on the crystal’s surface by cutting it in specified directions. You can investigate the act of various surface imperfections under precise settings in this way. Davis et al. (1982), for example, found that in platinum-catalyzed hydrogenolysis, the kink spots on the Pt surface were much more active than the sites (Davis et al., 1982). Even though the kinks on the surface were just 5% of the total surface area, they accounted for nearly 90% of the catalytic activity, proving Taylor’s active-site concept. Single crystals can also be employed to investigate the impacts of multicomponent promoters and catalysts, which are more representative of complicated industrial formulations. For example, catalytic reforming is conducted in the existence of a S/Re/ Pt catalyst. Somorjai and Kim identified the part of each of the catalyst’s components by coating S and Pt above Re crystals and the other way around (Kim and Somorjai, 1992). Thin metal and supported bimetallic and homometallic oxides, glassy metals, and clusters, supported catalysts depend on chemical vapor deposition, and oxide coatings are all common model catalyst systems (Frank and Bäumer, 2000). These model systems, like single crystals, are generally tightly defined, allowing researchers to analyze distinct phases in the catalytic cycle (Wallace et al., 1997).

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Figure 4.4. On distinct surfaces of Fe single crystals, relative rates of ammonia production and dissociative N2 adsorption.

4.2.3. Real Catalysts: Poisons, Modifiers, and Promoters Commercial catalysts, in comparison to the well-defined and clean model methods, frequently comprise many phases containing a variety of species. Impurities and also minor amounts of modifiers or promoters, which are included purposely during catalyst manufacturing, are to blame. The stability, selectivity, activity, and/or accessibility of the catalyst are all improved by adding these chemicals. Alkali metals like K and Na are frequently utilized as activity modifiers and electrical boosters. Small concentrations of alkali increase dissociative chemisorption of molecules like CO and N2 on the surface of transition metals via electron donation, as shown in Figure 4.5. The transition metal receives electron density from the electropositive alkali atom. This enhances substrate breakdown by increasing back-donation into the p antibonding orbital of the substrate. It is worth noting that while the promoter activates the sites around it, it is not an active catalyst, so it essentially blocks the site it is on. Alkali ions separate to the higher layers of the catalyst since their surface energy is less than that of transition metals due to high surface intensities: Adding up 0.5% Na to the catalyst during production can result in up to 10% Na upon the surface. For example, ammonia is synthesized using a magnetite catalyst and two promoters: K2O and Al2O3.

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Figure 4.5. The antibonding LUMO of the external N2 molecule is reduced by an activity promoter, making it easier for it to dissociate on the catalytic surface.

Alumina boosts the catalyst’s stability by preventing Fe particles from sintering. The orbitals of arriving N2 molecules interact with the potassium oxide, enhancing N2 dissociation on the surface (Fastrup, 1994). The rate of ammonia synthesis increased by 300 when potassium is introduced to the (1,0,0) face of a Fe crystal, showing that the rate-determining phase in ammonia production is really the dissociative chemisorption of N2. Poisons, nothing like promoters, are often electronegative elements like Cl, S, P, and C. A poisoned atom or ion obstructs surface locations while also reducing metal/substrate interaction. Because they are electronegative, at the active site, they reduce the electron density, which decreases backdonation and slows the dissolution of substrates like N2 and CO. Poisons are not all terrible; by reducing side effects, they can improve product selectivity. When 1–2 ppm ethane dichloride is added to the Ag-catalyzed ethene epoxidation procedure, Cl atoms are deposited on the catalyst surface. This slows the entire oxidation of ethene to H2O and CO2, improving ethene oxide selectivity (epoxyethane, oxirane). In the chemical business, promoters, toxins, and modifiers are all common. They are frequently used at the catalyst optimization phase or perhaps the pilot stage. A catalyst may require a promoter that has little immediate impact; however, improves long-term stability. Tighter laws often necessitate the adaption of current catalysts and processes, which is another common cause for introducing promoters and modifiers. Promoter impacts are relied on empirical findings and trial-and-error studies, in general. They are hardly comprehended. Furthermore, several academic scientists prefer to deal with distinct and simplified catalytic systems rather than promoters.

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4.3. PREPARATION OF SOLID CATALYSTS: BLACK MAGIC REVEALED Solid catalyst synthesis techniques are often intricate and diverse, giving the sense of alchemy rather than 21st-century chemistry. Every step of the preparation, along with the purity of the starting materials, has a significant impact on the final catalytic characteristics. Small variations in drying temperatures, age durations, solvent compositions, and stirring speeds can all have an impact on the catalyst’s performance. As a result, most corporations never provide the specifics of their catalyst production processes (Figure 4.6).

Figure 4.6. Solid catalyst types and examples, organized by preparation technique (tm= transition metal).

To make things easier, I will break down the synthesis processes into a few fundamental unit activities and the solid catalysts into two important preparation classes: impregnated/supported catalysts and bulk catalysts (Schwarz et al., 1995; Perego and Villa, 1997). The key stages and unit operations in the production of solid catalysts are summarized in Figure 4.7, which are detailed in the subsequent subsections. Precipitation, hydrothermal synthesis, and fusion are the most

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common methods for producing bulk catalysts. They are oxides or metals, and the whole catalyst is formed of active material, as the name implies. This category includes zeolites, the ammonia synthesis catalyst, alumina/ silica hydrocracking catalysts, and Raney metals. In the instance of precious metals or unstable chemicals, impregnated catalysts are typically utilized. On porous bulk support, the active metal precursor is deposited. An oxide, hybrid polymer, or just as an organic or activated carbon resin can be used as a support.

Figure 4.7. The key unit operations and phases in solid catalyst production are summarized in this flowchart.

The majority of the time, bulk approaches are used to create the supports. Pt/Sn/Al2O3 dehydrogenation catalysts, Pd/C hydrogenation catalysts, and automobile 3-way catalysts are examples of impregnated catalysts. Catalysts can also be created using hybrid synthesis techniques, such as mixing active precursors with powdered support and then agglomerating the combination.

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4.3.1. Alloy Leaching and High-Temperature Fusion Fusion of metal and co-melting precursors at high temperatures can be used to make metallic alloys as well as some mixed-oxide catalysts. The melting allows the precursor atoms and clusters to combine closely, resulting in very pure, well-dispersed solids. Furthermore, by adjusting the cooling of the melt, a variety of distinct phases can be created. Mixed-oxide catalysts and alloys are commonly identified by adding a dash to the formulae of their predecessors, such as V2O5MoO3 or simply VMoO. The fabrication of metallic glasses as a novel form of catalyst is an intriguing alternative (Armbruster et al., 1986). The melting/fusion method has the disadvantage of being an energy-intensive procedure that necessitates specialized equipment, especially in comparison to the precipitation–calcination procedures (Figure 4.8).

Figure 4.8. The Raney method involves fusing an aluminum alloy and then dissolving the aluminum in aqueous NaOH, resulting in a “metallic sponge.”

The creation of skeletal catalysts is particularly noteworthy. Murray Raney, an American engineer, found in 1924 that metal alloy leakage produced excellent hydrogenation catalysts. He fused a 50:50 Al/Ni alloy and then leaked the Al off with aqueous NaOH. The subsequent “nickel sponge,” also called Raney nickel, was far extra active than commercial catalysts at the time. Surprisingly, Raney figured out the ideal alloy composition straight away—extensive experiments revealed that the 50:50 mix was the best like some alumina was kept within to support the catalyst (Raney, 1940). Raney’s inventions and trademarks were purchased. In today’s reductive

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alkylation, hydrogenation, ammonolysis, and skeleton catalysts (mainly Ni and Cu) are utilized. The catalysts are prepared to utilize and have a large metal surface area, needing no reduction or other activation. Because the catalyst particles are heavy, settling/decantation is an easy way to separate them from the products. Raney metals, unlike several other catalysts, are persistent in alkaline liquids. They are also quite pyrophoric. Thus, they are kept underneath water (Smith and Trimm, 2005).

4.3.2. Slurry Precipitation and Co-Precipitation Several essential catalysts and assistance materials, such as alumina, silicon, and the Cu/ZnO/Al2O3 methanation catalyst, are made via precipitation processes. High-purity materials can be obtained through precipitation, while stoichiometric mixes with well-defined blended crystallites can be obtained through co-precipitation. Precipitation, on the other hand, needs a precipitating and a solvent agent, unlike fusion, which requires no further reagents. This means that catalyst separation and waste management will be more expensive. Precursor solutions are typically combined, then the required salt is precipitated as a gel by adding an acid/base. Nucleation, supersaturation, and growth are the three stages of precipitation. The gel is then aged, filtered, washed, dried, and then calcined. There are a variety of precipitator combinations that can be employed. For instance, co-precipitating the nitrate predecessors in the existence of NaOH or mixing an alkaline NaAlO2 solution with just an acidic nickel nitrate solution can be utilized to prepare Ni/Al2O3 steam reforming catalysts. Slurry mixing is used to make several bulk catalysts straight from their active components, with either alcohol or as a tiny amount of water a solvent. For instance, the FeSbO4 catalyst utilized in the ammoxidation of propane to acrylonitrile is made by mixing Sb2O3 powder with a heated FeNO3. 9H2O solution and then adding aqueous ammonia to raise the pH of the slurry (Bowker et al., 1996). The rutile-structured solid mixed oxide is formed after the precipitate is dried, calcined, and filtered at high temperatures.

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4.3.3. Impregnation of Porous Supports The transforming catalyst Pt/Al2O3 was made via impregnation, which is a typical way of manufacturing supported catalysts (Komiyama, 1985). The (porous) substrate is dissolved in a mix of the catalyst precursor in wet impregnation. The precursor is either spontaneously adsorbed on the support or caused, thereby altering the pH, or triggering a chemical reaction. Following that, the catalyst is calcined, dried, and filtered. For example, porous alumina is mixed with a chloroplatinic acid solution to make Pt/ Al2O3 (Eqn. (1)). The solute is disseminated on the surface once the solvent has evaporated. The supporting PtIV ions are decreased to Pt0 with hydrogen gas.

The high amounts of watery waste involved in this approach are one disadvantage. This can be circumvented employing the incipient wetness approach, which involves adding a solution of the active precursor to the dry support powder till it grows to be “slightly sticky,” suggesting that the liquid has filled the pores of the support. Another version is vacuum pore fertilization, which involves drying the support initially and then vacuuming the pores to remove the air. The support is then combined with a volume of precursor solution equal to the pore volume beneath a vacuum, and the solution is absorbed in the pores of the apparently dry powder. Drying the catalyst and continuing the impregnation procedure can result in high active material loadings. Figure 4.9 depicts a basic vacuum pore impregnation reactor that can prepare gram amounts of catalyst (Zande, 2002).

4.3.4. Hydrothermal Synthesis Hydrothermal treatments entail heating precipitates, flocculates, or gels in the presence of moisture, as the name implies. Textural and/or structural changes emerge from these treatments, which are commonly executed in an autoclave at 100–300 C and include crystal and/or particle development, variations in the crystal structure, and the conversion of amorphous solids to crystalline solids.

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Figure 4.9. A lab-scale parallel reactor with vacuum pore impregnation (photo and diagram).

The production of zeolites is the essential feature of hydrothermal synthesis (Cundy and Cox, 2003). Zeolites are typically manufactured at 100–150°C from sodium aluminate and sodium silicate precursors. The exchange of Al3+ in place of the tetrahedral Si4+ necessitates the existence of an additional cation in a supersaturated solution of the forerunners, resulting in a microporous crystalline material. Zeolite creation, like several other crystallizations, is a gradual process. Furthermore, the zeolite type is greatly influenced by the synthesis circumstances. Researchers at Mobil Oil discovered in the early 1970s that combining modest amounts of alkylammonium salts to the precursor solution produced new and unique zeolites with odd pore diameters. The organic salts serve as templates for the zeolite crystals, controlling their growth. Remarkably, the addition of the alkylammonium salts followed in a greater Al/Si ratio, resulting in so-called high-silica zeolites. Zeolites are also famous as molecular sieves because their pores are cavities and nanometric channels. The processes in the hydrothermal production of ZSM-5, the most well-known synthetic zeolite, are depicted in Figure 4.10. The solid acid H-ZSM-5 is produced by ion exchange of Na+ cations with NH4+, followed by drying and calcination. It is employed in synthetic

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gasoline production and selective isomerization techniques. Compared to HF, HCl, or H2SO4, solid Brnsted acids have a significant advantage: they are easily extracted from the product blend and do not need neutralization (Okuhara, 2002). It shows that no salt waste, which, given the petrochemical industry’s massive output volumes, is a significant benefit still when the acid is employed in catalytic amounts (Corma et al., 2001). The pharmaceuticals and fine-chemicals industries are similar, with smaller production volumes, but acids are frequently used as stoichiometric reagents.

Figure 4.10. Hydrothermal production of ZSM-5 with successive ion exchange.

Figure 4.11 demonstrates how ZSM-5 is used as a catalyst in the synthesis of xylene. The zeolite has two kinds of channels: horizontal and vertical, which link in a zigzag 3D structure (Olson et al., 1998). In the existence of Brnsted acid sites, methanol and toluene react to produce a blend of xylenes within the zeolite cages. Benzene, p-xylene, and toluene, on the other hand, can easily flow in and out of the channels, while the bulkier-and-xylene is caught inside the cages and eventually isomerizes for additional information on computer simulations in zeolite research.

4.3.5. Drying, Calcination, Activation, and Forming Most of the formulation processes call for the removal of solvents (mainly water). It is simple to dry crystalline solids. However, hydrogels and flocculates are more difficult. The cause for this is that these gels can have up to 90% water, and eliminating it could cause the porous structure to collapse. The mixture is a step-by-step drying method in which water is eliminated from the hydrogels outside surface at a constant rate. The gel

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mass diminishes, leaving a dry xerogel with a water content of 25–30% in its pores. This xerogel is either completely transported to the developing stage, or it is calcined initially at a high temperature. Calcination eliminates all water as of the catalyst, decays the carbonate precursors, and creates metaloxide bonds with the support. Calcination can also dry out surface hydroxyl groups, depending on the temperature. This is critical because it changes the hydrophobicity of the catalyst surface. Calcination, such as drying, should be done at a regulated time to prevent pore collapse.

Figure 4.11. A computer replication of the MFI-type zeolite HZSM5 (left) and a structure is depicting the shape-selective synthesis of p-xylene in the cage (right). The simulation snapshot was provided by Dr. Edith Beerdsen.

Several industrial catalysts are shaped into macroscopic pellets or spheres once the required microscopic characteristics are established (Fulton, 1986). Every year, the chemical industry consumes thousands and thousands of tons of catalysts, the majority of which are used in extremely large reactors. The specialized macroscopic properties of the catalyst are critical for these applications. Mechanical strength, particle size, filtration qualities, bulk thermal expansion coefficients, flocculation properties, and mass density are all factors to consider. The flow of gasses in the catalyst bed, or even the pressure drop on the reactor, is determined by the size and form of the catalyst particles. The catalyst particles will be crushed and broken down to a powder if they are not well-formed and strong, blocking the reactor

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and diminishing efficiency. Catalyst shape and size are a trade-off: smaller particles provide more activity. However, they also boost pressure drop across the catalyst bed. Pellets, granules or beads, spheres, and extrudates are the four basic types of catalysts. Spray drying is used to make small spherical particles (10–100 mm) by spraying the hydrogel or sol solution into a hot chamber (Figure 4.12) (Lukasiewicz, 1989).

Figure 4.12. Extrusion templates allow for the creation of a wide range of catalyst pellet forms and sizes.

The solvent evaporates, allowing the catalyst particles to clump together. Typically, these catalyst spheres are utilized in fluidized-bed reactors. Granulation, “snowball rolling,” are commonly used to make more spheres in the millimeter range (Holt, 2004). Pelleting, also known as tableting, is the process of compacting catalyst powder and a binder below high pressure. The powder is initially formed inside a paste using a binder before being extruded. Based on the mechanical and rheological qualities required, alumina, starch, and clays are widely employed as binders. This paste is driven over with a former that resembles a spaghetti machine and determines the extrudates’ macroscopic shape. The final catalyst particles are made by cutting, drying, and calcining the extrudates. Coating the active catalyst at monolith support is an attractive alternative to pellets. The latter is often constructed of stainless steel or ceramic and has a honeycomb structure of 1 mm2 channels. The catalytic converter is the most well-known use of catalytic monolith reactors (Forzatti et al., 1998).

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The catalyst may need to be activated before the reaction, depending on the eventual use. For instance, if you create Pd/silica by injecting the silica support with a Pd(NO3)2 predecessor and then calcining and burning away the nitrates as nitrogen oxides, the Pd will also be oxidized during the calcination. At the active site, the oxides are normally reduced back to Pd0 by treating the catalyst with H2.

4.4. SELECTING THE RIGHT SUPPORT The catalyst help should allow for excellent active component dispersion, accessibility, and sintering resistance. The active component of many solid catalysts is a tiny metal cluster, whereas the support is a stiff and inert substance along with a high melting point. Examples include alumina, titania, silica, and porous carbon. Hybrid organic/inorganic and organic polymers, on the other hand, can serve as solid supports. It is worth noting that the support you use can affect a variety of catalytic factors (Roozeboom et al., 1980). Even though most supports are chemically passive, active supports are beneficial in specific situations. Solid oxygen exchangers like ceria-based mixed oxides and cerium oxide are extremely flexible. The Ce3+ ↔ Ce4+ + e– redox cycle promotes oxygen storage capacity and release as of the fluorite lattice at temperatures above 350 C. As a result, they are perfect for direct oxidation applications, for example, automobile hydrocarbon fuel cells and three-way catalysts (Murray et al., 1999; Park et al., 2000). A bifunctional catalytic system is created when the support is also a catalyst. For example, to convert n-heptane to isoheptane, an isomerization catalyst and a dehydrogenation catalyst are required. By infusing a porous alumina/ silica support with bifunctional, a Pt particles system is created that can accelerate both processes at the same time.

4.4.1. Specific Surface Area In general, the greater the surface area, the greater the scattering and the most catalyst for your money you will receive. The majority of catalyst supports are porous, which signifies that the surface is “within the catalyst” or in the pores. Metal oxides, for instance, alumina or silica, have specific surface areas of 100–300 m2 g–1, but several porous carbons have surface areas of up to 3,000 m2 g–1. To understand what these figures signify, we must compare them to the active site and substrate/product molecule sizes. Assume you are

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working with chromium oxide on alumina to make an oxidation catalyst. Your alumina has a specific surface area of 200 m2 g–1. If the active sites must be kept separate, how much Cr should you fill on your support? In this sense, “separate sites” refers to the distance between each Cr atom and its neighbor, such that reactions at one site do not affect reactions at the other. If the product and substrate molecules are tiny, such as simple aromatics or diterpenes, a single active site with a surface area of 1 nm2 will be sufficient. Every gram of your alumina can accept around 0.1 mmol of Cr precursor on different sites because 200 m2 = 2 × 1020 nm2.

4.4.2. Substrate Accessibility Porous materials are classified as per the size of their pores. This can be determined by the use of adsorption methods. The IUPAC defines microporous materials as those with pores less than 2 nm in diameter, mesoporous materials as those with pores between 2–50 nm in diameter, and macroporous materials as those with an average pore width greater than 50 nm (Rouquerol et al., 1994). The pores should be significantly large to allow for the entry of substrates and the outflow of products. Pores are available in a variety of shapes and sizes. Certain materials feature enormous holes with narrow pore mouths, colloquially referred to as “ink bottle” pores. Others, such as zeolites, have a structure resembling a channel and a cage. Enormous product molecules may become caught within the pores in such instances. By determining the adsorption of molecular probes of various sizes, one can measure the fraction of a substrate’s surface area that can be accessed (Figure 4.13).

Figure 4.13. Zoom-in picture of a single active site on a porous alumina support particle with well-distributed active sites (left).

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4.4.3. Catalyst Stability The deactivation of catalysts is a critical issue. Certain deactivation mechanisms, like poisoning or cooking, are more dependent on the reaction itself than on the type of support. Sintering, on either hand, is highly dependent on the active material’s degree of dispersion as well as the support’s stability at elevated temperatures. Solid/Gas catalytic processes are commonly used in large-scale constant operations, with catalysts that stay stable for months or even ages on the stream being preferred. Temperature constancy is significant since a greater temperature frequently equates to a larger space-time yield and, as a result, higher earnings. At temperatures below 130 C, polymeric ion-exchange resins like Nafion and Amberlyst, for example, are effective solid acid catalysts used for fatty acid esterification (Kiss et al., 2006). The support decays and lacks its catalytic action above this temperature. Inorganic solid acids are used instead in high-temperature transesterification and esterification applications, like biodiesel generation from mixed oil/fat, feeds (Figure 4.14) (Kiss et al., 2006).

Figure 4.14. (a) Channel/cage structure; (b) polygonal capillaries; (c) “ink bottle” pores; (d) laminae; (e) slit pores are examples of pores that vary in size, shape, and connectivity.

4.5. CATALYST CHARACTERIZATION Theoretical types depend on fundamental principles, like Langmuir’s adsorption pattern, which assist us in comprehending what occurs at the surface of the catalyst. Nevertheless, there is no alternative for scientific proof, and most heterogeneous catalysis articles contain surface and surfacebound varieties characterization. Chemists must choose from a variety of characterization techniques, ranging from micrometer-scale particle size dimension to angstrom-scale atomic force microscopy (Brazdil, 1999). Some systems need UHV and room temperature, whereas others operate at 200 bar and 750°C. Real industrial catalysts are used in some ways, while single-crystal model catalysts are required in others. I will cover four

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primary topics in this book: traditional surface characterization techniques, temperature-programmed (TP) approaches, microscopy and spectroscopy, and macroscopic property analysis. See the sources in each section, and also the books by Thomas (1997) for more information on the individual procedures.

4.5.1. Traditional Surface Characterization Methods The reactant must be adsorbed on the catalyst surface in any gas/solid catalytic system. It is for this reason why surface characterization is so critical. The pore size distribution, pore volume, and surface area of a catalyst can be determined by exploring the adsorption of several compounds under precise conditions (Meyer et al., 1994). Accessibility is the most important factor here. Advanced spectroscopic assessment of single-crystal types can reveal a great deal about what happens at the active site. However, the molecules should first get there. Adsorption can be divided into two categories. The molecules are connected to the surface because of Van der Waals interfaces in physical adsorption, also known as physisorption. The corresponding adsorption temperatures are minimal, around 3–10 kcal mol1. Chemisorption or chemical adsorption, on the other hand, requires the breaking and formation of chemical bonds, with adsorption heats varying from 20 to 100 kcal mol1. In characterization investigations, both forms of adsorption are applied. Physisorption is a technique for determining pore volume and total surface area at temperatures near the adsorbate’s boiling point. Many DIN and ISO guidelines, for example, use nitrogen adsorption at 77 K as a reference comparison. Chemisorption, on the other hand, is used to measure specific chemical entities on the surface (such as Brnsted acid sites).

4.5.2. Determining the Surface Area At a particular temperature and pressure, the total surface area of a solid is proportional to the volume of gas adsorbed on it. At constant temperature, an adsorption isotherm is a graph that depicts how the amount adsorbed varies with the gas’s equilibrium pressure. There are just six major kinds of isotherms (Figure 4.15) based on empirical evidence, independent of the gas or solid in the issue. Adsorption on microporous materials, like a few activated carbons and, molecular filters are characterized by Type I isotherms.

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Figure 4.15. The following are the six different kinds of adsorption isotherms: Type I displays a monolayer; types II and III display multilayer adsorption; and type IV displays a monolayer initially, afterward mesopore filling. The point of monolayer formation (point “B”) is shown by the “knee” in isotherms I, II, and IV, which is shown by a black dot.

Multilayer adsorption on the macroporous medium is described by Type II isotherms. We detect no monolayer development in isotherm types III and V because the arriving molecule is adsorbed primarily on some other adsorbed molecule instead of on an “empty” site. On a hydrophobic material, this is what occurs when H2O is adsorbed, for example. The very significant materials for heterogeneous catalysis activities are those with type IV isotherms. This is due to the fact that the majority of the products and substrates we work with have mesopore diameters of 2–50 nm. Adsorption isotherms can be derived theoretically in a variety of ways. The generation of a monolayer on a surface is described by the basic Langmuir (1915). This is the type I isotherm. Here, V represents the quantity adsorbed, Vm represents the amount adsorbed in some monolayer, p represents pressure, and b represents the adsorption constant, which is proportional to the heat of adsorption.

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Langmuir’s model consists of three strong interpretations: • • •

1st, on the solid, the adsorption sites are all the same; 2nd, molecules are only adsorbed on the surface of each other, not on top of one another; 3rd, there are no horizontal interactions.

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Real catalysts are frequently more difficult to come by. Adsorption sites vary in terms of energy and availability, and lateral interactions are common, particularly at high surface coverage. The overall adsorption can be described as a sum of many Langmuir isotherms, which helps to bridge these gaps. The Brunauer–Emmett–Teller equation, released in 1938 (Brunauer et al., 1938), is another typical model that also covers multilayer adsorption. Here, Vm is the sum adsorbed in one monolayer, V is the quantity adsorbed, p is the pressure, p0 is the saturation pressure, and N is the determined number of layers that can develop in a pore C is a constant related to the net heat of adsorption.

We can predict the number of particles in a monolayer using these equations for substances of type I, II, and IV isotherms. We can calculate the solid’s monolayer capability using this information. This is how much adsorbate is required to cover 1 gram of solid. This solid’s specific surface area is calculated by dividing its capacities by the average area occupied by one adsorbable molecule. The parameters of the adsorbate should be tiny in comparison to the pore diameter in order to achieve relevant values. Small spheres, such as Ar or Kr, produce the finest effects. Despite the fact that N2 adsorption at 77 K is not a spherical molecule, it is regarded as the IUPAC standard for practical and historical reasons. Gas adsorption can be employed to estimate pore size distribution and pore volume, whereas the hysteresis of the adsorption isotherms can provide insight into pore shape (Thommes et al., 2015). Gas will be condensed to a liquid in the pores at pressures lower than its saturated vapor pressure due to the confined volume. This pressure ratio is given by the Kelvin equation (Eqn. (5)), where R is the gas constant (2 cal mol–1 K–1), V is the molar volume of the liquid, g is the liquid surface tension and T is the temperature. Several approaches for generating pore-size distributions are based on this equation (Cranston and Inkley, 1957).

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Mercury porosimetry is another method for assessing the pore size distribution of macro-and mesoporous. In this experiment, the volume of mercury, a non-wetting liquid, pushed within the pores of a catalyst sample absorbed in mercury is measured. The pore size is inversely related to the pressure needed to encroach mercury keen on the sample’s pores (Thompson et al., 1987). The Washburn equation (Washburn, 1921) gives this pressure for cylindrical g is the liquid’s surface tension, where r is the pores of radius and an is the contact angle. Regardless of the fact that true pores are hardly cylindrical, this approach provides accurate pore size distribution predictions. The measurements are restricted to pores bigger than 3 nm because of the surface tension and high contact angle of mercury.

4.5.3. Temperature-Programed (TP) Techniques TP (temperature-programmed) procedures are used to determine a solid’s reactivity as a function of temperature in a regulated environment (Washburn, 1921). The temperature upon which species react on the surface is a reflection of the species’ binding strength. Various chemical species bond at varied temperatures, resulting in TP profiles with different maxima. At lower temperatures, the more reactive species react, and the other way around. Temperature programmed desorption, sulfiding (TPS), reduction (TPR), and oxidation are the most used TP methods (TPO). Despite the fact that these methods are older and less sophisticated than modern spectroscopic techniques, they can provide valuable info on the types of functional groups and bonds on the surface. They also have the benefit of being able to operate under actual process conditions. You can track the variations in the catalyst precursor through the activation and calcination phases, for example, using TPO and TPR. A typical design of a TP setup, along with samples of TPR profiles, is shown in Figure 4.16. While the temperature is raised, a gas reagent is pushed over the catalyst. The consumption of reactants and/or the creation of products are measured as the temperature rises. This is frequently accomplished by utilizing a thermal conductivity detector to compare the thermal conductivity of the gas after and before the reactor (TCD). Alternatively, using mass spectrometry, gas chromatography, or other methods, one can identify the structure of the gas phase near the reactor exit (Baker et al., 1999).

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4.5.4. Spectroscopy and Microscopy In the field of catalyst characterization, this is the quickest area. With photons, ions, and electrons, samples are bombarded, using a variety of methods, providing scientists with a wealth of information about what is going on at the catalyst surface. Several of these approaches were previously limited to low temperatures/or UHV and, resulting in pressure and temperature difference among the research situations and the actual procedure. In situ and operand spectroscopic approaches, on the other hand, are rapidly decreasing this difference, and online spectroscopic examining of solid/gas catalytic reactions is developing more widespread (Hunger, 2005). I will go over the fundamentals of 5 distinct approach “families” here. Specialist textbooks and reviews provide a complete view of these and many other strategies (Anderson et al., 2004).

Figure 4.16. (a) TPR profile of CeO2, CuO, along with a copper-doped ceria model, Cu0.1O2, Ce0.9 is illustrating the effect of Cu doping on the decrease temperature; (b) TPR profile of CeO2, CuO, and a copper-doped ceria sample, Cu0.1O2, Ce0.9 observe the impact of Cu doping on the reduction temperature.

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For distinguishing calculating particle sizes and crystalline phases, XRD is employed. It is centered on atoms in a periodic lattice scattering X-ray photons elastically. A beam of X-rays with a wavelength of 0.5–2 A strikes the catalyst sample and is diffracted by the crystalline phases in the catalyst as per Bragg’s rule, where y is the diffraction angle, and d is the spacing among atomic planes in the crystalline phase and n = 1, 2, …,

The intensity of diffracted X-rays is displayed as a result of the sample orientation and diffraction angle. This diffraction pattern is employed to detect crystalline phases as well as to measure crystallite size and lattice spacing. By evaluating the X-ray diffraction pattern of unknown samples to recognize patterns of metals and oxides, crystalline phases of unidentified samples can be determined. Because XRD relies on the interference of revealing X-rays from lattice planes, it necessitates a high level of longrange order. Noncrystalline catalysts, like alumina/silica gels employed in hydrocarbon cracking, have weak and broad diffraction lines or no diffraction to some extent. SAXS is comparable to XRD, except instead of covering the scattering range of 10–180, SAXS deals with 2y angles < 2. This allows the size of catalyst particles in the 50–500 nm range to be determined. In some circumstances, SAXS can be employed to determine the surface area of particles, yielding findings that are equivalent to BET adsorption experiments (Xiang et al., 2000). The Nobel laureate Kai Siegbahn and Swedish physicist developed XPS in the mid-1960s. XPS is a simple and effective method for detecting atoms on the catalyst surface (Brinen, 1976). It depends on the photoelectric impact, which occurs when photons as of a “soft” X-ray source like Mg Ka or Al Ka diffuse the first few atomic layers beneath the surface and intermingle with inner-shell electrons. As demonstrated in Eqn. (1), the sample absorbs the photon’s energy, hn, and dislodges an electron with a kinetic energy of Ekin (Eqn. (8)). The difference in energies is essentially equal to the electron’s bonding energy, Eb, with an alteration factor j that accounts for the disparity between the sample’s work function and the spectrometer’s work function. Because each element has its own set of bonding energies, the XPS peak regions can be utilized to identify the material’s surface composition (with the proper sensitivity factors).

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XPS can be used to determine relative elemental concentrations at the surface as well as the oxidation states of elements. Reliant on the photoelectron energy, material, and measuring angle, the probe depth varies from 2 to 20 atomic layers. The approach necessitates ultra-high vacuum (UHV) and can detect all elements except helium and hydrogen. The composition, shape, and size of supported particles are determined using electron microscopy (Anderson et al., 2004). A TEM is an optical microscope with electromagnetic lenses in place of the optical lenses. The transmitted electrons are enlarged by the electromagnetic lenses after an electron beam impacts the sample. The optics transport the scattered electrons from a similar location in the sample to a similar location in the image. A standard TEM has a magnification of 300,000 and a resolution of 0.5 nm, but a high-resolution instrument has a magnification of 1000,000 and atomic resolution (Figure 4.17).

Figure 4.17. (a) When an electron beam strikes the catalyst surface, it triggers a series of reactions; the electrons that pass via the sample are used to generate; (b) the TEM image (Wang et al., 2004).

The image’s contrasts represent the various scattering mechanisms as well as interactions among transmitted electrons and various atoms in the

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experiment. As a result, supported metal particles look like dark spots on a lighter background, whereas the support seems like a lighter background. A high-energy (usually 10 keV) electron beam is scanned above a narrow rectangular area of the model in scanning electron microscopy (SEM). Lowenergy secondary electrons are produced, which some are escaping from the surface. These secondary electrons are detected by attracting them to a phosphor screen and using a photomultiplier to measure the light intensity. Few of the beam’s electrons “bounce back” after striking atomic nuclei. The back-scattered primary is electrons that provide info on the average atomic number and surface topography in the scanned area. The back-scattered and secondary electrons are recorded as a function of the beam position by the microscope. Surfaces that face the detector seem lighter than those that are arranged at an angle, resulting in picture distinction. SEM produces a 3D image of the catalyst, however, at a lesser resolution than TEM because secondary electrons derive mostly from the surface, although back-scattered electrons originate from the mass. Figure 4.18 shows a scanning electron micrograph of hollow and porous polyelectrolyte microcapsules employed as supports for noble metal clusters inside their onion-like shell structure by Isbester et al. (1999).

Figure 4.18. SEM image of “polyelectrolyte onions,” which can block active metal clusters inside their shell thickness, revealing the porous three-dimensional structure at the capsule surface.

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4.5.4.1. Solid-State Nuclear Magnetic Resonance Spectroscopy (SS-NMR) Solid-state NMR works on the same principles as liquid-phase NMR. Solid-state NMR spectra, on the other hand, are more complicated because there is no isotropic averaging of NMR connections. The nuclei’s mobility is constrained, and the spectra show orientation-dependent anisotropic features. This, along with the magnetic effects of nearby atoms, affects the peaks to expand. Solid-state NMR spectra, on the other hand, contain far more detail than solution spectra. J-coupling, dipolar coupling, chemical shielding, and quadruple interactions provide useful chemical information about solid materials when their magnitude and orientation rely on each other. For gaining high-resolution NMR spectra of materials, special procedures have been devised. The very significant is magic angle spinning, which involves rapidly spinning the sample at a cos–1 (1/3) angle to the magnetic field, or 54° 54ˈ. The widening of the chemical shift anisotropy is canceled by spinning at this angle (Isbester et al., 1999). MASNMR is currently the industry requirement for acquiring high-resolution spectra of solids, and it has a variety of applications in heterogeneous catalysis (Han et al., 2001).

4.5.4.2. Infrared (IR) Spectroscopy The most extensively utilized approach for examining the surface chemistry of heterogeneous catalysts is infrared (IR) spectroscopy (Ryczkowski, 2001). It can give info on the structure of the catalyst along with the species adsorbed on the catalyst surface. Information regarding the environment and nature of atoms and ions visible on the surface is collected using probe molecules such as NH3, NO, and CO. The process works by IR radiation being absorbed, transmitted, or reflected by a catalyst, which causes molecular vibrations to occur (Figure 4.19).

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Figure 4.19. The IR activity of the four distinct CO adsorption arrangements on metal surfaces may be identified.

The IR beam can be transmitted across a thin portion of the solid or reflected from the surface to perform IR spectroscopy of solid catalysts. The type and bonding of the molecules determine the energy of these vibrations. Surface groups and probing molecules’ vibrational frequencies are detected by matching their “thumbprints” to literature databases. The advantages of IR spectroscopy are twofold: It is non-destructive and non-invasive, and it can be modified to high-temperature and high-pressure measurements. This implies that IR research can be carried out in real-world processes, and even online analysis can be carried out with today’s powerful computers (Clark, 1998). You can classify and measure the number and type of active sites by utilizing reactive probe molecules. For instance, pyridine vapor is employed to titrate Brnsted acid sites on solids, and DRIFTS may easily monitor differences in the acid absorption bands (Craciun et al., 1996). Even the way particles are adsorbed on the catalyst surface can be detected using IR spectroscopy. CO can form a variety of complexes on metal surfaces: linear, spanned among two metal atoms, triply bridged, and even quadruply bridged (Sheppard and Nguyen, 1978). Most group VII metals have a linear configuration, but group VIII metals have more bridging forms.

4.6. THE CATALYTIC CONVERTER: AN INSTANCE FROM EVERYDAY LIFE If you inquire the ordinary person for an instance of catalysis, they will probably say, “the item on top of the discharge pipe that cuts engine discharges.” As we will see, the catalytic converter’s fascinating narrative is not only a great example of catalysis, but also demonstrates the link between sustainable development, green chemistry, and heterogeneous catalysis.

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Unburned fuel, CO, incomplete oxidized gasoline, hot particles, and nitrogen oxides are all harmful pollutants emitted by today’s automobile engines. Table 4.1 depicts the usual dilutions of exhaust gas elements in gasoline engines, as well as the EU’s 2005 emission limitations (McCune and Keon, 2002). Table 4.1. Concentrations of Exhaust Gas Constituents and the EU’s Regulatory Limitations (2005) Compound

Hydrocarbons

N2

O2

CO

NOx

H2

CO2

H2O

Concentration/ vol.%

0.075

72.5

0.51

0.680

0.105

0.230 13.5

12.5

Legal limit/ gkm–1

0.10

n/a

n/a

1.0

0.08

n/a

n/a

n/a

There are three different approaches to lowering car emissions. The first is to drive less and rely on cycling, public transport, and walking instead. This may appear unfeasible, particularly in California. Elevated gasoline charges, sophisticated urban design, and the capacity to operate from the house are all helping to advance this option (Mosammam et al., 2017). The catalytic converter is built of cordierite or stainless steel and has monolithic support along with a honeycomb structure of 1 mm2 channel (Cybulski and Moulijn, 1994). Since the temperature can change by up to 500°C when the engine is started, the monolith should have good influence and thermal shock resistance. The monolith is initially wash-coated with a 20–60 mm coating of gAl2O3 that has a surface area of 200 m2 g–1 due to its nonporous nature. After that, the entire catalyst is filled with a solution of Pd, Rh, and Pt salt precursors, as well as a CeO2 layer that serves as an oxygen reservoir (Heck et al., 2001). The overall mass loading of noble metals is around 0.25%, with Pt accounting for the majority. Since it converts the three primary pollutants into harmless chemicals, the last catalyst is known as a 3-way catalyst. Pd and Pt are efficient oxidation catalysts, but H2 catalyzes CO with NO and Rh. While the majority of the noble metal remains on the catalyst, some do end up on the road, creating a variety of environmental impacts (Juan et al., 2007). Pt metal concentrations in urban road dust can be 100 times greater than the average background absorption (particularly on roundabouts). There is also a corporation in the United Kingdom that collects this dust in order to extract the Pt group metals! (Figure 4.20).

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Figure 4.20. (a) The channel washcoat, monolith, and supporting active metals in the TWC are depicted in this cartoon; (b) for transforming the three primary pollutants, reduction reactions and chemical oxidation is used.

The air-to-fuel (A/F) ratio determines the fraction composition of exhaust gasses. The exhaust gasses include more reduced reactants than oxidizing reactants during low A/F ratios. High A/F ratios encourage catalytic oxidation of hydrocarbons and CO, on the other hand. Modern automobiles mix the catalytic convertor with an electronically monitored air management system that keeps the A/F ratio in the combustion chamber close to stoichiometric. CO, HC, and NOx conversions of 90% are obtained simultaneously at this ratio (Muraki and Zhang, 2000). The engine switches among “leaner” and “richer” modes of operation. The ceria/zirconia or ceria oxygen reserve comes into play at this point. The storage component absorbs oxygen during the lean cycle and releases it during the rich cycle. Ceria can retain and release around 25% of its lattice oxygen dependent on lattice dopants and operation requirements (Rothenberg et al., 2003). The qualitative impact of adding up an oxygen storage element to the TWC is shown in Figure 4.20.

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36. Eley, D. D., & Rideal, E. K., (1940). Parahydrogen conversion on tungsten. Nature, 146(3699), 401, 402. 37. Ertl, G., (1994). Reactions at well-defined surfaces. Surface Science, 299, 742–754. 38. Ertl, G., (2002). Heterogeneous catalysis on atomic scale. Journal of Molecular Catalysis A: Chemical, 182, 5–16. 39. Fagherazzi, G., Tauszik, G. R., Cocco, G., Schiffini, L., Enzo, S., Benedetti, A., & Passerini, R., (1978). Average particle size, particlesize distribution, and surface area of supported metal catalysts. Chim. Ind. (Milan);(Italy), 60(11). 40. Fastrup, B., (1994). Temperature programmed adsorption and desorption of nitrogen on iron ammonia synthesis catalysts, and consequences for the microkinetic analysis of NH3 synthesis. Topics in Catalysis, 1(3), 273–283. 41. Feller, A., & Lercher, J. A., (2004). Chemistry and technology of isobutane/alkene alkylation catalyzed by liquid and solid acids. ChemInform, 35(49), 12. 42. Forzatti, P., Ballardini, D., & Sighicelli, L., (1998). Preparation and characterization of extruded monolithic ceramic catalysts. Catalysis Today, 41(1–3), 87–94. 43. Frank, M., & Bäumer, M., (2000). From atoms to crystallites: Adsorption on oxide-supported metal particles. Physical Chemistry Chemical Physics, 2(17), 3723–3737. 44. Freund, H. J., Rupprechter, G., Bäumer, M., Risse, T., Ernst, N., & Libuda, J., (2003). From real-world catalysis to surface science and back: Can nanoscience help to bridge the gap?. In Metal-Ligand Interactions, 1(3), 65–92. 45. Friend, C. M., Queeney, K. T., & Chen, D. A., (1999). Structure and reactivity of thin-film oxides and metals. Applied Surface Science, 142(1–4), 99–105. 46. Fulton, J. W., (1986). Selecting the catalyst configuration. Chemical Engineering, 93(9), 97–101. 47. Gellings, P. J., & Bouwmeester, H. J., (2000). Solid state aspects of oxidation catalysis. Catalysis Today, 58(1), 1–53. 48. Goodman, D. W., (1996). Correlations between surface science models and “real-world” catalysts. The Journal of Physical Chemistry, 100(31), 13090–13102.

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49. Guzman, F., Singh, R., & Chuang, S. S., (2011). Direct use of sulfurcontaining coke on a Ni− yttria-stabilized zirconia anode solid oxide fuel cell. Energy & Fuels, 25(5), 2179–2186. 50. Han, X., Yan, Z., Zhang, W., & Bao, X., (2001). Applications of in situ NMR in catalytic processes of organic reactions. Current Organic Chemistry, 5(10), 1017–1037. 51. Heck, R. M., & Farrauto, R. J., (2001). Automobile exhaust catalysts. Applied Catalysis A: General, 221(1, 2), 443–457. 52. Hodge, V. F., & Stallard, M. O., (1986). Platinum and palladium in roadside dust. Environmental Science & Technology, 20(10), 1058– 1060. 53. Holt, E. M., (2004). The properties and forming of catalysts and absorbents by granulation. Powder Technology, 140(3), 194–202. 54. Huang, J., & Rempel, G. L., (1995). Ziegler-Natta catalysts for olefin polymerization: Mechanistic insights from metallocene systems. Progress in Polymer Science, 20(3), 459–526. 55. Hunger, M., (2005). Applications of in situ spectroscopy in zeolite catalysis. Microporous and Mesoporous Materials, 82(3), 241–255. 56. Isbester, P. K., Kaune, L., & Munson, E. J., (1999). Magic-angle spinning NMR: A window into flow catalytic reactors. Chemtech, 29(11), 40–47. 57. Jones, C. W., McKittrick, M. W., Nguyen, J. V., & Yu, K., (2005). Design of silica-tethered metal complexes for polymerization catalysis. Topics in Catalysis, 34(1), 67–76. 58. Juan, W., Zhu, R. H., & Shi, Y. Z., (2007). Distribution of platinum group elements in road dust in the Beijing metropolitan area, China. Journal of Environmental Sciences, 19(1), 29–34. 59. Kaiser, M. J., & Gary, J. H., (2009). Refinery cost functions in the US gulf coast. Petroleum Science and Technology, 27(2), 168–181. 60. Kamat, P. V., & Dimitrijević, N. M., (1990). Colloidal semiconductors as photocatalysts for solar energy conversion. Solar Energy, 44(2), 83–98. 61. Kamatani, A., & Riley, J. P., (1979). Rate of dissolution of diatom silica walls in seawater. Marine Biology, 55(1), 29–35. 62. Kašpar, J., Fornasiero, P., & Graziani, M., (1999). Use of CeO2-based oxides in the three-way catalysis. Catalysis Today, 50(2), 285–298.

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63. Kim, C., & Somorjai, G. A., (1992). The effects of rhenium and sulfur on the reactivity and selectivity of Pt single-crystal catalysts. Journal of Catalysis, 134(1), 179–185. 64. Kim, S. H., & Somorjai, G. A., (2006). Surface Science of single-site heterogeneous olefin polymerization catalysts. Proceedings of the National Academy of Sciences, 103(42), 15289–15294. 65. Kiss, A. A., Dimian, A. C., & Rothenberg, G., (2006). Solid acid catalysts for biodiesel production–‐towards sustainable energy. Advanced Synthesis & Catalysis, 348(1, 2), 75–81. 66. Kiss, A. A., Omota, F., Dimian, A. C., & Rothenberg, G., (2006). The heterogeneous advantage: Biodiesel by catalytic reactive distillation. Topics in Catalysis, 40(1–4), 141–150. 67. Klier, K., (1982). Methanol synthesis. Advances in Catalysis, 31, 243– 313. 68. Knozinger, H., & Weitkamp, J., (1997). In: Ertl, G., Knozinger, H., & Schuth, F., (eds.), Handbook of Heterogeneous Catalysis (pp. 555– 571). 69. Köhnke, T., Elder, T., Theliander, H., & Ragauskas, A. J., (2014). Ice templated and cross-linked xylan/nanocrystalline cellulose hydrogels. Carbohydrate Polymers, 100, 24–30. 70. Kokotailo, G. T., Lawton, S. L., Olson, D. H., & Meier, W. M., (1978). Structure of synthetic zeolite ZSM-5. Nature, 272(5652), 437–438. 71. Komiyama, M., (1985). Design and preparation of impregnated catalysts. Catalysis Reviews Science and Engineering, 27(2), 341–372. 72. Kruissink, E. C., Van, R. L. L., & Ross, J. R., (1981). Coprecipitated nickel–alumina catalysts for methanation at high temperature. Part 1.—Chemical composition and structure of the precipitates. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 77(3), 649–663. 73. Langmuir, I., (1915). Modulization of adsorption. Phys. Rev., 6, 79–80. 74. Lefebvre, F., & Basset, J. M., (1999). Recent applications in catalysis of surface organometallic chemistry. Journal of Molecular Catalysis A: Chemical, 146(1, 2), 3–12. 75. Lukasiewicz, S. J., (1989). Spray‐drying ceramic powders. Journal of the American Ceramic Society, 72(4), 617–624.

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CHAPTER

5

ENVIRONMENTAL CATALYSIS

CONTENTS

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5.1. Introduction .................................................................................. 166 5.2. Attributes of Environmental Catalysis ............................................ 168 5.3. Catalysis All Over ......................................................................... 169 5.4. Environmental Catalysis as a Driver for Innovation ........................ 172 5.5. Sustainable Catalytic Materials ..................................................... 174 5.6. Environmental Electrocatalysis ...................................................... 178 References ............................................................................................ 180

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5.1. INTRODUCTION Over the last two decades, environmental catalysis has become increasingly important, not just in terms of the global catalyst business but also as a driver of developments in the entire field of catalysis. The creation of innovative “environmental” catalysts is thus a critical component of creating a new, more sustainable industrial chemistry. The traditional area of environmental catalysis has seen a significant increase in the previous decade. Unexplored areas comprise: (i) catalytic methods for the reduction or purification of liquid or solid waste; (ii) Catalysts are used in energy-saving catalytic processes and technologies; (iii) mitigation of the environmental effects of catalyst usage or disposal; (iv) novel eco-compatible refinery, non-chemical or chemical catalytic methods; (v) Using catalysis to reduce greenhouse gas emissions; (vi) Catalysts for manageable technology and indoor pollution reduction; (vii) Catalysis methods for environmentally friendly chemistry; (viii) lowering of transportation’s environmental effect. As a result, there has been a substantial shift in the fields of interest for environmental catalysts and the research methodology in recent times (Delmon et al., 1997; Centi et al., 2002). Catalysis is an essential commercial opportunity and a key technology for providing realistic answers to many environmental challenges. Currently, the worldwide economy for catalysts is worth around $9 billion, with environmental catalysis accounting for nearly a third of that. Figure 5.1 depicts the global catalyst market’s expected growth through 2005, broken down by general industrial segment. The potential for development in the field of environmental catalysis is very considerable. Environmental catalysis is one of the best five areas of US business that combine technological challenges and economic rewards, according to the US NIST “White Paper on Catalysis and Biocatalysis” from 1999. Furthermore, a few crucial developing scientific systems in environmental catalysis are predicted to have a significant cross-industry influence (Centi and Perathoner, 1995). Catalytic technologies for lowering emissions of environmentally undesirable substances are referred to as environmental catalysis. Mobile emission control, NOx removal from stationary sources, sulfur VOC and compounds conversion, solid and liquid waste treatment, and greenhouse gas abatement or conversion are some of the issues addressed by these catalytic cleanup technologies.

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Figure 5.1. Global catalyst market prospects through the general industry sector. Source: https://pubs.rsc.org/en/content/articlelanding/2016/sc/c6sc02105k.

Catalysis for new eco-friendly refineries, non-chemical or chemical catalytic methods, catalytic technologies for waste minimization, and modern catalytic paths to essential products without generating undesired pollutants are all examples of environmental catalysis. Catalysis in the lessening of the ecological effect of transportation, catalysis in refineries to yield new fuels, catalysis in the decrease of the environmental impact of transportation not just use of catalysts in emissions control, nevertheless also inside the engine to recover combustion, on the radiator to decrease ozone, etc. Other catalysts applications include the development of userresponsive technologies, the reduction of indoor pollution, and catalytic technologies for decontamination of polluted sites. Finally, as part of the environmental catalysis aims, the reduction of environmental effects in the usage or removal of catalysts should be mentioned. As seen by the predominant influences in this area at the inaugural meetings, scientists operating in environmental catalysis were first concentrated mainly on NOx management, sulfur, and VOC reduction. Scientific interest gradually shifted away from the cleaning technique and into the other stated topics. However, new difficulties and questions

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have recently rekindled interest in catalytic cleanup technology. Three fundamental driving forces may be mentioned: (i) the necessity to develop catalytic technology beyond gaseous releases to include liquid and solid waste treatment, (ii) novel post-treatment equipment for mobile sources is required, and (iii) the importance of rethinking post-treatment technology in terms of a procedure or system incorporation. As a result, in the recent decade, there has been a substantial shift in the area of interest for environmental catalysts and the method of conducting research. This necessitates a re-evaluation of the entire field of environmental catalysis to decide future research directions and identify the most pressing challenges to be addressed and basic and applied research.

5.2. ATTRIBUTES OF ENVIRONMENTAL CATALYSIS Various definite characteristics distinguish environmental catalysis from additional areas of catalysis (Armor, 1994): Except for catalysis for chemical production and refining, environmental catalysis frequently necessitates the development of technologies that can operate efficiently under the parameters set by upstream units. • Environmental catalysis is used in various applications for the treatment of discharges in other kinds of production, residence, or indoor applications and auto, ship, and flight emissions control. Environmental catalysis extends the concept of catalysis beyond the confines of chemistry to the broader realms of industrial production and everyday life. As a result, catalysis can be defined as a crucial technology for improving human quality of life and ensuring a sustainable future. • Environmental catalysts are frequently used in more harsh reaction settings than chemical or refinery catalysts. It, on occasion, should be able to work along with a variety of supplies or in the face of rapid variations in feed composition. While sharing an everyday basis of knowledge, environmental catalysis addresses difficulties distinct from those of heterogeneous catalysis. As a result, novel techniques for solving specific challenges in ecological catalysis are being researched. As a result, this field is marked by a high level of innovation in catalytic technologies and materials. Several discoveries could be applied to other domains of catalysis in the future, so environmental

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•

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catalysis serves as a catalyst for research in all fields of industry and catalysis (Misono and Nojiri, 1990).

5.3. CATALYSIS ALL OVER Catalysis has long been involved with refinery and chemical manufacturing. Catalytic converters for the treatment of automobile discharges were the initial large-scale application of catalysis outside of refinery and chemical production, considerably assisting in the diffusion of understanding about the advantages of catalysis for developing environmental and human health. Catalytic environmental technologies have swiftly grown into various new domains in recent years, providing new prospects: •

for a variety of industrial areas that have historically avoided using catalysis; • to make life better and the indoor environment through userfriendly gadgets. M. Berndt and P. Landry gave the workshop’s plenary session, which centered on market demands and current for non-standard environmental catalysis and the underlying technical and scientific challenges raised by these applications. In comparison to the traditional centralized areas, nontraditional applications are a rapidly growing market (Höller et al., 2000). Non-standard products frequently need a new customized and low perunit costs and flexible approach. Interesting instances are: •

Hydrogen recombiners made of thin-walled stainless steel plates covered with a catalytic coating; • In self-cleaning domestic ovens, metal ring-nets covered with catalysts or ceramic honeycomb will be used; • Ozone smog reducers. The mentioned instances show a few fascinating concepts which are worth highlighting:

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•

In these non-standard applications, designed catalysts in the form of metallic or ceramic monoliths are required. This is because of the need for a minimal pressure drop and quick heating and the need for a catalytic device that can be quickly replaced. The need for occasionally unique shapes and increased gas-solid contact while retaining a minimal pressure drop drives study into alternate

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structural supports. Glass fiber sections are an excellent example of this type of material (Höller et al., 2000). They have higher gas-solid contact efficiency than ceramic monoliths, a lower vacancy fraction of the catalyst, and great flexibility to adapt to diverse geometries. However, thermal stability and dispersion of active catalytic components must be enhanced. • While the progress of these devices frequently necessitates extensive and interdisciplinary studies, which may seem incongruent with the original small volume of projected applications, the catalysts are commonly found to be helpful in a much broader range of applications later on. • Customer needs have mainly motivated research into non-standard products of environmental catalysts in catalyst manufacturers. Researchers and practitioners in this field have been stifled in the past, owing to a lack of dedicated research money. As a result, there is a shortage of basic background information on the individual challenges and a broader approach to searching for possible applications more systematically. However, public awareness of the potential benefits of catalysis is limited. Because most research is customer-driven (see above), several potential areas of use for environmental catalysts are not being discovered, necessitating public inducements to rectify this problem. More non-standard environmental catalyst applications were reviewed at the seminar, and some of these are included in this edition of Catalysis Today. The pattern of cross-flow-channel monoliths for the management of smoke and odor generated by culinary procedures was described by Fu and Chen (Engelhard, US). Berge and Berg presented the outcomes of field testing of catalysts utilized for domestic wood-fired boilers. The behavior of several TiO2 catalysts for photocatalytic oxidation of hydrocarbons was presented by different researchers (Anpo, 2000). Since the photocatalytic function of TiO2 permits for the removal of pollutants and also has an antibacterial impact, titanium-coated tiles are currently being tested in various civic structures in Japan, including as hospitals, and in self-cleaning soundproofing highway-walls. The behavior of low-temperature CO oxidation catalysts based on Pt–SnO2/alumina was discussed by Whitehead

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(2010). These catalysts were primarily found for the oxidation of dashes of CO produced in a high-power CO2 laser; nevertheless, their use can be prolonged to a variety of applications where room-temperature CO oxidation is required, for example, indoor air purification (the catalyst is efficient not just at room temperature but also at high temperatures). Gas masks, catalytic converters, and underwater ventilation systems for cold-start emissions control of car discharges are other applications (Li et al., 2013). The distillation of H2 in proton exchange fuel cells applies to lowtemperature CO oxidation catalysts (PEFCs). CO concentrations of more than roughly 100 ppm poison the fuel cell’s Pt electrocatalyst. However, H2 formed by auto thermal reforming or steam of traditional fuels like natural gas, gasoline, or methanol, followed by water gas shift conversion, creates a 1% CO or more mixture. Because H2 oxidation should be reduced, an effective CO oxidation catalyst with excellent selectivity is required. Catalysts for this application must also be robust to Carbon dioxide and Hydrogen deactivation. Scientists address the performance of supported noble metal catalysts, mainly supported gold nanoparticles on changeover metal oxides (Nishino, 1991; Singh et al., 2019). Further instances of non-standard uses of environmental catalysts contain several domestic applications (Anpo, 2000), for example (i) water cleanser catalysts for HClO and Cl2 removal, (ii) mosquito killers and catalytic hair cutters, (iii) deodorization device for fridges, (iv) internal catalytic combustors, (v) catalytic lighters intended for outdoor use, (vi) moveable warmers and Kerosene heaters are two types of portable heaters. And (vii) catalytic deodorization systems. These examples show how environmental catalysis is broadening the variety of catalysis applications from energy domains to non-chemical, transportation, and everyday life applications. This approach is summarized in Figure 5.2, built on a unique idea by Misono and Nojiri (1990). This is significant in terms of a larger marketplace for catalysis and novel research challenges to overawed the often-difficult restraints imposed by this increased usage of catalysts and improved social responsiveness that catalysis is a way to advance the quality of life (Miller and Grassian, 1995).

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Figure 5.2. The tree depicts the function of catalysis in environmental protection. Source: https://www.researchgate.net/figure/The-role-of-catalysis-in-improving-environment-1_fig2_267973240.

5.4. ENVIRONMENTAL CATALYSIS AS A DRIVER FOR INNOVATION The expansion of the usage of catalysis outside of outdated fields, combined with the fundamental problem that in many environmental knowhows, it is not likely to select the ideal reaction conditions, which are resolute by feed constraints and energy, and circumstances defined by challenging units, necessitates a significant amount of innovation in the development of solutions, devices, and new catalytic materials (Nishino, 1991). The whole area of heterogeneous catalysis, like other commercial sectors, would profit from this study endeavor, not just the specialized subject of environmental catalysis. Environmental catalysts necessity sometimes operates at lower temperatures or very high temperatures (Chen et al., 2021). Extremely high temperatures need the development of catalytic materials with excellent strength and strength at high reaction temperatures. In the catalysis workshops, several contributions focus on this issue, including the advancement of new support materials for catalytic ignition in power production, (ii) realizing the changeable transformation of palladium (Pd) in Pd-created catalysts designed for gas turbine combustors.

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Low-temperature action necessitates the creation of innovative, highly active catalytic elements or, otherwise, new methods for activating reactants or supplying the reaction’s energy. During the event, some exciting new developments in this area were presented. NTP (Non-thermal plasma) was used to help clean catalytic gas flows comprising low quantities of pollutants. A dielectric discharge reactor can synthesize NTP by producing chemically active species including excited nitrogen and oxygen molecules, radicals, atoms, and like HO2• and OH•. Even though the application to mobile emissions needs the resolution of various technical problems, plasma aided catalytic elimination of NOx at temperatures < 100°C is a significant achievement intended for evolving low temperature (150 C) catalysts for NOx decrease in diesel engine use emissions (Labhsetwar et al., 2003). Xu et al. (2001) noted the use of dielectric and solid catalysts barrier flows to support low-temperature activity while reducing VOC emissions. In general, the area of application is VOC conversion. Still, unique value can be discovered in eliminating fragrant or poisonous chemicals that appear in room temperature surroundings or emissions, and in general, when the VOC content is under the matter to get an autothermic procedure (Di Monte and Kašpar, 2005). Another option, as suggested by Emerson et al. (1982) is to use other oxidants such as ozone. Although catalyst deactivation is an issue, supported MnO2 catalysts provide adequate reduction of toluene at temperatures lower than 100°C. Centi et al. (1999) investigated the question of a natural gas explosion at room temperature to utilize NOx catalytic burners. They discovered that hydrogen-supported combustion permits room temperature light off of methane and greater use of hotspot temperatures, both of which significantly impact catalyst life. The creation of diesel particulate reduction devices is likewise centered on increasing low-temperature activity. A new idea of an electrochemical reactor consists of a highly porous, oxygen ion-conducting electrolyte protected by a catalytically active, electron conductive perovskite-based electrode. The porous reactor functions as a mechanical filter, trapping soot particles in the exhaust flow. The combustion method of the collected dust particles can be made to happen at low temperature by polarizing the reactor with an external power supply, wherever the lower temperature limit (250 C) is set through the ionic conductivity of the electrolyte material (Serwicka and Bahranowski, 2004). Because the catalytic activity may be adjusted based on the potential users of the electrochemical device, the idea of an electrochemically controlled catalytic machine can be used not just to

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encourage low-temperature activity but also to design systems with tunable and variable activity. This enables the creation of “smart” catalytic devices with changeable activity controlled by a sensor. As previously stated, the food composition in diverse environmental applications can change fast. In these cases, temperature control is inadequate in following these quick variations, and general efficiency can be below. The concept of a “smart” catalytic device could solve this dilemma, though the study is still in its early phases. The growing concern in building “intelligent materials” among scientists may spur study in this area (Hoelderich, 2000). While not inclusive of all current technological breakthroughs in environmental catalysis, these instances demonstrate the exploration for unconventional answers to the difficult obstacles that environmental catalysis addresses. Because of the interdisciplinary approach to the research, the information generated will have a favorable effect on all areas of catalysis and an inter-industry high impact (Yang et al., 2019).

5.5. SUSTAINABLE CATALYTIC MATERIALS A slew of studies has been devoted to the creation of new materials having catalytic potential. Supported is one of the most well-known catalytic materials (Figure 5.3).

Figure 5.3. Sustainable (photo/electro) catalytic materials, catalyst synthesis techniques, emerging environmental technologies, and pertinent applications in the field of ecological catalysts and green chemistry are represented schematically.

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Blunt needles, on either hand, have garnered considerable attention. Plant biomass residues, such as bamboo, have been effectively used as carbon precursors to manufacture porous carbonaceous catalyst macroalgal species, chitosan, citrus peel-derived pectin, and other supports using a chemical-free approach (Zhang et al., 2020). Others, generally metal nanoparticles, metal-organic frameworks, metal oxide nanoparticles, composite materials, and more recently bioconjugates all benefit from this family of carbonaceous materials, also known as biomass leftovers. From an environmental standpoint, the production via template of biomassdeveloped catalysts and materials with specialized features for applications such as photo- and electrocatalysis has garnered significant attention. BioDerived wastes, starch, aquatic cultures, and lignocellulosic materials are all examples of biomass (Misono, 1998). Combining biomass and associated carbon products is a promising way to make high-end materials from regrowing resources. It is common knowledge that catalytic effectiveness is closely linked to textural qualities, particularly surface area, and is thus influenced by the support material. HTC (Hydrothermal carbonization) methods, which permit for increased or changed solubility, accelerated physical and chemical interactions, and ultimately the development of carbonaceous erections, have been used to create these types of supports. Carbons have acquired outstanding textural features with tunable surface functions, making them suitable for use as catalysts and catalytic supports (Hoelderich and Dahlhoff, 2001). A straightforward process centered on producing an aqueous gel, after that thermal carbonization and solvent exchange, was used to create the materials above (Figure 5.4(B)). In addition, numerous biomass residues, such as cornstalks, pinecone hulls, rice straws, and pine resulted in carbons, have been used to create alternative carbonaceous stands with large surface areas that can operate as attaching sites for additional metal deposition. Despite the numerous advantages of biomass-derived catalysts, it is essential to note that the materials mentioned above frequently have limitations that limit their industrial applicability. Biomass-derived materials frequently have many undesirable chemicals, making it hard to manage their shape, porosity, and surface chemistry, restricting their commercial use (Kamiya et al., 2008).

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Figure 5.4. (A) Photos of raw plant biomass; (B) carbonized biomass; (C) SEM microstructure of carbonized biomass after oxidation at 350°C; (D) their porous surface morphology; and (E) controlled pyrolysis of enlarged starch yields mesoporous carbon in this overview. Source: https://pubmed.ncbi.nlm.nih.gov/16671136/.

The template approach is a simple and successful way of creating nanostructures with various morphologies and enormous surface areas. The majority of described methods in this regard contain two steps: initially, the chosen materials or precursors organize around the surface of a pattern such as through physical or chemical observance to form transitional composites; and second, the preferred precursors or materials contained around the surface of a way through the use of physical or chemical adherence to form intermediate composites. The templates are then deleted from the composite forms selectively. The sacrificial template approach is used when the template is completely transformed to the desired materials. Via biomass valorization, a broad range of composite materials has been manufactured using the method mentioned above. In this comment, a typical example of iron oxide-based catalysts has been produced using humins, a byproduct of acid-catalyzed biomass conversion, both as composite materials and templates. Iron oxide species and oxygenated carbon from humans’ byproducts remain interesting synergistic impacts on the oxidation of isoeugenol, a lignin model molecule. These findings show that biomass waste could be a viable replacement for traditional carbonaceous materials, developing cheaper and more efficient, customizable catalytic systems. Iron

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oxide nanocatalysts relying on the same principles have been developed, using polysaccharides produced from fungal microorganisms as a sacrificial template (Misono, 2001). The substitution of noble metal catalysts with non-noble metal catalysts is another promising technique for developing more ecological chemistry and, in particular, catalysis, which also has an economic advantage. Even though there is a long way to attain this goal, a few research have described promising results using transition metal nanoparticles as hydrogenation catalysts. For example, Abe and colleagues investigated the possibility of replacing Pt-based catalysts with a supporting Cu-Ni nanomaterial that may be used in gas phase processes (Centi and Perathoner, 2003). Like MOF metal-organic frameworks, emerging materials have a vast catalytic potential due to their inherent properties. Chemical mutability, readily available interior surface regions, customized pore architectures, and the existence of Lewis acid metal sites are only a few of these characteristics. The porosity shape and size tunability opens up new opportunities for reagent discrimination and, as a result, catalytic reaction reactivity and selectivity. MOF-based materials, on the other hand, have a bit of a way to go instead of being used as efficient catalytic systems (Figure 5.5) (Hetes et al., 1995).

Figure 5.5. (A) Overview of the preparation of iron oxide and polysaccharide nanocomposites; (B) demonstration of the synthesis of N-HPCMs using a hybrid dual template technique; (C) formation of humin byproducts and their valorization as nanocomposite catalysts. Source: https://pubs.rsc.org/en/content/getauthorversionpdf/C5TA09323F.

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Bioconjugates built on enzymes and metal oxide nanoparticles, in particular, are emerging materials with encouraging catalytic outcomes in oxidative polymerization reactions. New frontiers for the design of enhanced catalysts can be opened by merging the environmental benefits of enzyme-catalyzed processes, for example, mild operating circumstances and good selectivity, with the facile reusability and retrieval of metal oxide nanoparticles (Figure 5.6) (Singh et al., 2020).

Figure 5.6. (A) Nitroarenes hydrogenative coupling catalyzed by Co–N–C catalysts; (B) schematic representation of the creation of metallic transition NPS covered by thin carbon layers (NNM@C) (B1); and its usage in acid valorization (B2). Source: https://pubs.rsc.org/en/content/articlelanding/2016/sc/c6sc02105k.

5.6. ENVIRONMENTAL ELECTROCATALYSIS Traditional energy sources produced from fossil fuels produce hazardous air pollutants that harm the environment. As a result, developing clean technologies for producing fuels via low-cost and efficient routes has become a critical goal for an innovative society. Electrocatalysis, which is typically described as a type of catalysis that affects the modulation of electrochemical reaction rates at electrolyte interfaces, is proving to be a valuable tool in achieving this goal. Because of the finding of viable electrocatalysts with increased stability and activity for the synthesis of fuels, this branch of

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electrochemistry has piqued the interest of the green scientific community. In this way, electrochemical reactions may efficiently generate hydrogen from water and electro-reduce CO2 to produce sustainable carbon-based fuels from renewable resources (Zhang et al., 2020). It is worth noting that one of the many advantages of electrocatalytic technologies is that: it is possible to combine different approaches to gain more profound perceptions into the systems, as the method is regulated by the potential at electrode/electrolyte interfaces. Natural sources like wind, solar, hydropower, thermoelectric tidal, and geothermal can be employed to initiate electrocatalytic processes, and the electrochemical system is simple, small. It can be utilized for accessible scale-up applications. The development of ecologically acceptable methodologies for synthesizing green electrocatalysts has piqued the scientific community’s interest in recent years, owing to the growing demand for renewable energy conversion and storage to ensure global sustainability. Most present methods for fabricating prospective electrocatalysts necessitate the use of expensive and hazardous precursors, external heteroatom-containing compounds, and different templates and chemicals, all of which raise prices and limit their practical applicability. Converting the national environment into carbon-based materials, along with microbial production and the creation of electrocatalytic enzyme systems, have started up new frontiers as viable ways for fabricating more sustainable raw materials with superior electrocatalytic capabilities (Armor, 1992; Centi et al., 1999).

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12. Centi, G., Perathoner, S., & Vazzana, F., (1999). Catalytic control of non-CO2 greenhouse gases. ChemTech, 29(12), 48–55. 13. Chen, L. X., Chen, Z. W., Jiang, M., Lu, Z., Gao, C., Cai, G., & Singh, C. V., (2021). Insights on the dual role of two-dimensional materials as catalysts and supports for energy and environmental catalysis. Journal of Materials Chemistry A, 9(4), 2018–2042. 14. Cui, T., Li, L., Ye, C., Li, X., Liu, C., Zhu, S., & Wang, D., (2021). Heterogeneous single atom environmental catalysis: Fundamentals, applications, and opportunities. Advanced Functional Materials, 2108381. 15. Dai, Y., Liu, W., Formo, E., Sun, Y., & Xia, Y., (2011). Ceramic nanofibers fabricated by electrospinning and their applications in catalysis, environmental science, and energy technology. Polymers for Advanced Technologies, 22(3), 326–338. 16. Delmon, B., Ruiz, P., Part, B., Delmon, C. B., Froment, G. F., Marin, G. B., & Brazdil, J. F., (1997). Elsevier catalysis today 35, Al-A7. Recent Advances in Catalysis Over Sulfides, 3(4). 17. Di Monte, R., & Kašpar, J., (2005). Heterogeneous environmental catalysis–a gentle art: CeO2–ZrO2 mixed oxides as a case history. Catalysis Today, 100(1, 2), 27–35. 18. Emerson, S., Kalhorn, S., Jacobs, L., Tebo, B. M., Nealson, K. H., & Rosson, R. A., (1982). Environmental oxidation rate of manganese (II): Bacterial catalysis. Geochimica et Cosmochimica Acta, 46(6), 1073–1079. 19. Farrauto, R. J., & Heck, R. M., (2000). Environmental catalysis into the 21st century. Catalysis Today, 55(1, 2), 179–187. 20. Gai, P. L., (2002). Developments in in situ environmental cell highresolution electron microscopy and applications to catalysis. Topics in Catalysis, 21(4), 161–173. 21. Hao, M., Qiu, M., Yang, H., Hu, B., & Wang, X., (2021). Recent advances on preparation and environmental applications of MOFderived carbons in catalysis. Science of the Total Environment, 760, 143333. 22. Harling, A. M., Demidyuk, V., Fischer, S. J., & Whitehead, J. C., (2008). Plasma-catalysis destruction of aromatics for environmental clean-up: Effect of temperature and configuration. Applied Catalysis B: Environmental, 82(3, 4), 180–189.

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23. Hetes, R., Moore, M., & Northeim, C., (1995). Office Equipment: Design, Indoor Air Emissions, and Pollution Prevention Opportunities (Vol. 1, pp. 1–22). US environmental protection agency, Air and Energy Engineering Research Laboratory. 24. Hoelderich, W. F., & Dahlhoff, G., (2001). The greening of nylon. Chemical Innovation, 31(2), 29–40. 25. Hoelderich, W. F., (2000). ‘One-pot’ reactions: A contribution to environmental protection. Applied Catalysis A: General, 194, 487–496. 26. Höller, V., Wegricht, D., Yuranov, I., Kiwi‐Minsker, L., & Renken, A., (2000). Three‐phase nitrobenzene hydrogenation over supported glass fiber catalysts: Reaction kinetics study. Chemical Engineering & Technology: Industrial Chemistry‐Plant Equipment‐Process Engineering‐Biotechnology, 23(3), 251–255. 27. Kamiya, Y., Okuhara, T., Misono, M., Miyaji, A., Tsuji, K., & Nakajo, T., (2008). Catalytic chemistry of supported heteropolyacids and their applications as solid acids to industrial processes. Catalysis Surveys from Asia, 12(2), 101–113. 28. Kandathil, V., & Patil, S. A., (2021). Single-atom nanozymes and environmental catalysis: A perspective. Advances in Colloid and Interface Science, 102485. 29. Kim, H. S., Pastén, P. A., Gaillard, J. F., & Stair, P. C., (2003). Nanocrystalline todorokite-like manganese oxide produced by bacterial catalysis. Journal of the American Chemical Society, 125(47), 14284–14285. 30. Kundu, S., Mukadam, M. D., Yusuf, S. M., & Jayachandran, M., (2013). Formation of shape-selective magnetic cobalt oxide nanowires: Environmental application in catalysis studies. CrystEngComm, 15(3), 482–497. 31. Labhsetwar, N. K., Watanabe, A., & Mitsuhashi, T., (2003). New improved syntheses of LaRuO3 perovskites and their applications in environmental catalysis. Applied Catalysis B: Environmental, 40(1), 21–30. 32. Li, J., He, H., Hu, C., & Zhao, J., (2013). The abatement of major pollutants in air and water by environmental catalysis. Frontiers of Environmental Science & Engineering, 7(3), 302–325.

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33. Miller, T. M., & Grassian, V. H., (1995). Environmental catalysis: Adsorption and decomposition of nitrous oxide on zirconia. Journal of the American Chemical Society, 117(44), 10969–10975. 34. Misono, M., & Nojiri, N., (1990). Recent progress in catalytic technology in Japan. Applied Catalysis, 64, 1–30. 35. Misono, M., (1998). Catalytic reduction of nitrogen oxides by bifunctional catalysts. Caltech, 2, 183–196. 36. Misono, M., (2001). Unique acid catalysis of heteropoly compounds (heteropolyoxometalates) in the solid-state. Chemical Communications, (13), 1141–1152. 37. Nishino, A., (1991). Household appliances using catalysis. Catalysis Today, 10(1), 107–118. 38. Piram, A., Li, X., Gaillard, F., Lopez, C., Billard, A., & Vernoux, P., (2005). Electrochemical promotion of environmental catalysis. Ionics, 11(5), 327–332. 39. Roes, A. L., & Patel, M. K., (2011). Ex-ante environmental assessments of novel technologies–Improved caprolactam catalysis and hydrogen storage. Journal of Cleaner Production, 19(14), 1659–1667. 40. Sahiner, N., Butun, S., & Ilgin, P., (2011). Hydrogel particles with core-shell morphology for versatile applications: Environmental, biomedical and catalysis. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 386(1–3), 16–24. 41. Serwicka, E. M., & Bahranowski, K., (2004). Environmental catalysis by tailored materials derived from layered minerals. Catalysis Today, 90(1, 2), 85–92. 42. Singh, D. V., Bhat, R. A., Dervash, M. A., Qadri, H., Mehmood, M. A., Dar, G. H., & Rashid, N., (2020). Wonders of nanotechnology for remediation of polluted aquatic environs. In: Fresh Water Pollution Dynamics and Remediation (Vol. 1, pp. 319–339). Springer, Singapore. 43. Singh, M., Dwivedi, P., Mott, D., Higashimine, K., Ohta, M., Miwa, H., & Maenosono, S., (2019). Colloid chemical approach for fabricating Cu–Fe–S nanobulk thermoelectric materials by blending Cu2S and FeS nanoparticles as building blocks. Industrial & Engineering Chemistry Research, 58(9), 3688–3697. 44. Wang, Y., Pan, C., Chu, W., Vipin, A. K., & Sun, L., (2019). Environmental remediation applications of carbon nanotubes and graphene oxide: Adsorption and catalysis. Nanomaterials, 9(3), 439.

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45. Whitehead, J. C., (2010). Plasma catalysis: A solution for environmental problems. Pure and Applied Chemistry, 82(6), 1329–1336. 46. Xu, Z., Ao, Z., Yang, M., & Wang, S., (2021). Recent progress in singleatom alloys: Synthesis, properties, and applications in environmental catalysis. Journal of Hazardous Materials, 127427. 47. Yang, Y., Wu, M., Zhu, X., Xu, H., Ma, S., Zhi, Y., & Ma, J., (2019). 2020-Roadmap on two-dimensional nanomaterials for environmental catalysis. Chinese Chemical Letters, 30(12), 2065–2088. 48. Zhang, N., Ye, C., Yan, H., Li, L., He, H., Wang, D., & Li, Y., (2020). Single-atom site catalysts for environmental catalysis. Nano Research, 1–18.

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CHAPTER

6

FUNDAMENTALS OF BIOCATALYSIS

CONTENTS

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6.1. Introduction .................................................................................. 186 6.2. Advantages and Disadvantages of Biocatalysts ............................... 189 6.3. Strategies to Improve the Performance of Biocatalysts .................... 190 6.4. Biocatalysts: An Interdisciplinary Science ..................................... 191 6.5. The Effect of Biocatalysis on Teaching Natural Science ................... 193 References ............................................................................................ 195

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6.1. INTRODUCTION The argument that biocatalysts are extremely important appears insignificant when considered because biocatalysts are a precondition for the existence of any life at all. If you count the unknowing employment of the underlying factors in ancient times, such as fermentation in conjunction with baking bread or beer brewing, the application of biocatalysts has a history that dates back more than 8,000 years. In today’s world, biocatalysts’ research has an increasingly broad impact on all aspects of daily life. This includes medical and pharmaceutical research as well as nutritional items, analytics, and environmental technologies. Thousands of years of evolution have resulted in an unfathomable variety of creatures still being discovered. Biocatalysts are enzymes that regulate the entire metabolic reactions in animals, plants, and microorganisms in a highly selective manner and under circumstances that are either mild or adapted to the specific requirements of the environment in which the organism is growing and developing its activities. For example, bacteria have been discovered to survive in strange environments such as abandoned chemical plants by inventing enzyme machinery that allows them to feed on the chemicals they uncover. The so-called extremophiles (Berger et al., 2014; and literature cited therein) inhabit deep-sea habitats where they expand at temperatures just above 0°C and pressures greater than 1,000 bar nevertheless perish when the temperature rises and the pressure falls; psychrophiles metabolize at even lower temperatures, such as those found in the permafrost of Siberia; and basophils grow at temperatures just above 0°C and pressures more significant than. More importantly, thermophilic bacteria that live in so-called ecological niches, such as those discovered in the hot springs of Yellowstone National Park over 60 years ago, are of greater biotechnological significance. They produce enzymes capable of functioning at temperatures as high as 130°C and under a variety of pH settings (alkaliphiles or acidophiles). Enzymes with such high stability are of tremendous interest for a wide range of biotechnological applications. Not even all enzymes isolated from (hyper)thermophiles are highly heat-stable, which is surprising. As a result, these organisms developed alternative strategies to survive under adverse conditions. For instance, they developed the synthesis of low-molecular-mass metabolites that exert an invivo stabilizing effect on protein unfolding by strengthening intramolecular interactions inside the protein molecule, as demonstrated by Roy Choudhury and colleagues (2013) by using atomic force microscopy. The disaccharide trehalose, amino acids- and -glutamate, di-myoinositol phosphate, and

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1-glyceryl-1-myoinositol phosphate were found in the hyperthermophiles bacterium Aquifer pyrophilus in reply to both heat and osmotic stresses, are all examples of so-called compatible (Lamosa et al., 2006). These solutes are of interest because they can be used in a range of biotechnology applications. Overall, there should be an infinite number of distinct enzymes (the vast majority of which have not yet been discovered) that can catalyze nearly all the chemical reactions taught in Organic Chemistry classes. Clouthier and Pelletier have recently published a review of the future applications of biocatalysts in chemical synthesis, which is available online (2012) (Figure 6.1).

Figure 6.1. The organic structure of phosphoric acid-ester compound. Source: https://www.worldscientific.com/doi/ pdf/10.1142/9781783269099_0001.

Biocatalysts are the most environmentally friendly kind of catalysis available. According to the researchers, the reaction conditions are mild, water is used as a solvent, and the catalysts are entirely biodegradable and biocompatible. While enzymes such as nitrogenase can remove nitrogen from the air and convert it to ammonia at room temperature, the Haber– Bosch method needs temperatures greater than 700°C. Enzymes are particularly effective catalysts because of their high catalytic efficiency. It is not uncommon for them to have TOFs in the 102–104 s1 range, with values as high as 108 not being out of the question. They are also highly regioselective, discriminating among functional groups that are similar on a single substrate molecule, and so eliminate the demand for additional measures such as protecting and deprotecting. This translates into fewer byproducts, low Q-values, and low E-factors, among other things. Yet another significant advantage of enzymes is that many of their reactions are enantiospecific, resulting in the formation of chirally pure compounds. The issue arising from all these advantages is not “Why to utilize biocatalysts?” but instead “How come there are still no biocatalytic methods?”

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The explanation is not that biocatalysts are a recent finding; rather, it is the opposite. Instead, at the end of the 20th century, very chemists would have stated that enzymes are usually costly, rigid in their substrate selection, unstable at high temperatures, and perform inadequately in organic media, to name a few characteristics (Hu et al., 2019). However, the times have changed. The 21st century is expected to be the golden age of biotechnology, and with it, the age of biocatalysts as well. Genetic engineering has already made things that were thought impossible in the 1980s and 1990s but are now a reality. Several new applications are emerging because of the massive global investments in biotechnology, and commercial biocatalysts are pushing this wave’s crest. Today, more than 150 biocatalytic processes are in operation in various industrial areas, such as the fine chemicals and bulk chemicals industries, among others (Rozzell, 1999; Devine et al., 2018). This chapter covers the introduction to biocatalysts, including definitions of terms and an outline of the fundamental mechanisms. In this presentation, I will address the many approaches for producing novel biocatalysts and the advantages and disadvantages of substituting “conventional catalysis” with biocatalytic alternatives. Following the review of these fundamentals, we will cover the current state of the art in enzyme development and engineering and some industrial processes that employ biocatalysts for the environmentally friendly production of products and specialty chemicals (Figure 6.2) (Laane et al., 1987; Clouthier and Pelletier, 2012).

Figure 6.2. The conventional biocatalysts application domains are joined by a slew of new industrial industries (right) (left). Source: https://onlinelibrary.wiley.com/doi/book/10.1002/9783527621866.

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Enzymatic applications and reactions provide the core of this chapter’s discussion. Keep in mind that this chapter will be concentrating on biocatalysts applications in chemical chemistry instead of biological transformations in this presentation. Biochemistry textbooks provide a fair overview of the latter subject matter (Sheldon and Brady, 2019; Wohlgemuth, 2021).

6.2. ADVANTAGES AND DISADVANTAGES OF BIOCATALYSTS Evolution over millions of years has resulted in the creation of superb catalysts by nature. Mostly proteins, but there are also nucleic acids with catalytic capabilities that are comparable to those of enzymes discovered in the early 1980s that have been found. Only enzymes have been used in applied biocatalysts up to this point, out of all of the naturally occurring catalysts. Enzymes catalyze chemical reactions in a single cell or across an organism, and they are critical for the survival and reproduction of that cell or organism. A biocatalyst can be either a whole-cell in any stage of viability or a single enzyme in any state of activity. Alternatively, they can increase the rate at which equilibrium is reached without impacting the equilibrium constant by offering another reaction path with lower activation energy than the correlating uncatalyzed reaction, thereby increasing the rate at which equilibrium is reached of affecting the equilibrium constant. What is remarkable is the degree to which the pace of acceleration has increased (Majumder et al., 2008; Dias and Woodley, 2019). The enzyme catalase, which catalysis the breakdown of hydrogen peroxide, is an excellent example. The activation energies for the uncatalyzed and catalyzed reactions, 75 kJ/mol and 8 kJ/mol, respectively, yield a rate enhancement factor of roughly 1,015. This value is at the top end of the range; typically, these factors for enzyme-catalyzed reactions are around 108 and 1,012. The tremendous rate of acceleration permits reactions to occur below physiological conditions in a fraction of a second, but they would take longer to achieve equilibrium without a catalyst. Because of their regiospecificity, enzyme-catalyzed reactions are frequently very substrate-specific, not including producing byproducts. Furthermore, because enzymes are asymmetric molecules, they can distinguish between stereoisomers with

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precision, resulting in chiral products. However, chemical synthesis frequently results in racemic mixtures. These characteristics make enzymes attractive candidates for use as industrial catalysts (Hernandez and Fernandez-Lafuente, 2011; Beers et al., 2012). However, enzymes also have drawbacks (Table 6.1) restricting their use. Table 6.1. The Primary Benefits and Drawbacks of Biocatalysts are Their Potential Use in Biotransformations on a Laboratory or Industrial Scale Advantages

Disadvantages

• Very effective catalysis of most • Several enzymes are cofactor well-known chemical reactions High regio- and stereoselectivity Mild reaction situations and therefore low energy use • The amount of byproducts is low • They are biodegradable • Preparation on a large scale is achievable through fermentation • Recycle is possible • They can be constructed to a certain amount • They are non-toxic if properly applied

dependent • Allergic reactions are viable Protein molecules are very unstable in aqueous ways • Enzymes may be deactivated by: o extreme pH-values higher temperatures o (polar) organic solvents o higher salt concentrations • Inactivation may further occur because of inhibition by: o product o substrate o inhibitors metal ions

6.3. STRATEGIES TO IMPROVE THE PERFORMANCE OF BIOCATALYSTS Not long ago, enzymes had to be used as nature intended. The introduction of new biological and molecular tools made it possible to manipulate biocatalyst features such as catalytic activity, selectivity, and stability. To make a competitive alternative to existing chemical techniques, a biocatalyst’s high stability below process conditions is a requirement for its economical application in the industrial synthesis of high-value fine chemicals along with bulk compounds. This does not mean that biotechnological methods will always be able to replace existing chemical processes completely. Still, they will likely be combined with conventional chemical technologies to a greater extent in the future, helping to reduce the use of hazardous substances, reduce energy consumption, and reduce waste generation. Another critical

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issue is to use renewable raw materials in production processes whenever possible. In the last two decades, several biotechnological tools have been created to improve the efficiency of enzyme- or whole-cell-catalyzed reactions under specific process circumstances (Bornscheuer et al., 2012). Recent advances in what is known as systems biology have benefited metabolic engineering of production strains such as the commonly used Escherichia coli by insertion of a foreign gene into its genome or transformation with a plasmid containing the desired gene, allowing, among other things, a holistic view on biocatalysts. This technique is based on one of Aristotle’s famous statements: “The whole is greater than the sum of its parts.” As Kitano (2002) points out in his summary of systems biology, a thorough understanding of biology necessitates comprehension of the structure and dynamics of cellular and organismal function. This implies that the characteristics of a biological system cannot be adequately described by a single look at the genes and proteins present but must also consider gene interactions and metabolic pathways, the influence of time-varying parameters, mechanisms involved in the control of a cell’s state, and questions about system design to change their properties. As a result, several relatively new subdisciplines of modern biology have emerged, dealing with the entirety of, for example, genetic information (genome), mRNA species (transcriptome), proteins (proteome), and so on, as analyzed by omics techniques such as genomics, transcriptomics, and proteomics, respectively. Köhler et al. (2010); de Mara et al. (2019) generated a large amount of data analyzed using bioinformatics tools designed to create novel biological knowledge.

6.4. BIOCATALYSTS: AN INTERDISCIPLINARY SCIENCE The European Federation of Biotechnology defines biocatalysts as “the inclusion of natural sciences and engineering sciences to accomplish the application of cells, organisms, parts thereof, and molecular analogs for services and products,” according to EuropaBio (2003), “White Biotechnology is the application of Nature’s toolset to industrial production.” Biotechnology, and hence biocatalysts, are both viewed as interdisciplinary disciplines in both definitions. A basic understanding of the domains depicted in Figure 6.3 is required for biotechnology application (Lee et al., 2008; Rodrigues et al., 2014).

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Traditional sciences such as biology, chemistry, physics, mathematics, and more recent ones such as bioinformatics and material science are all relevant to biotechnology. Biotechnology research necessitates that anyone working in the field, regardless of specialization, such as enzymology, have at least a basic understanding of nearby fields to facilitate collaboration. Simultaneously, this interdisciplinarity reflects the complexity of several scientific challenges and must be considered while teaching natural sciences (Lee et al., 2008).

Figure 6.3. Some of the scientific disciplines that contribute to biotechnology are also relevant to biocatalysts and include classic subjects and some new ones, such as ‘material science’ and ‘bioinformatics.’ Source: https://www.worldscientific.com/doi/ pdf/10.1142/9781783269099_0001.

Designing a biocatalyst-based process demonstrates interdisciplinarity in this subject. The search for the necessary catalyst necessitates a screening process using many analytical approaches, some more advanced than others. If this is successful, it must be determined whether its attributes (kinetic behavior, stability, and so on) are compatible with the process conditions. For improving these features, there are a plethora of approaches accessible. To improve its operational stability, the biocatalyst can be immobilized via a variety of methods. An enzyme’s temperature stability, solvent tolerance, and other properties can be enhanced through rational design, which requires knowledge of the structure obtained through X-ray diffraction and NMR spectroscopy; conversely, evolutionary means or the use of the catalyst in the form of designer bugs relying on metabolic engineering can also be used. Additional factors, such as possible cofactor regeneration and

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environmental precautions, must be considered when designing a process that can compete with current conventional ones, as demonstrated by the biocatalysis cycle by Schmid et al. (2001). Meanwhile, well-established methods include the production of high-fructose syrup through xylose isomerase, the use of penicillin amidase for the synthesis of semisynthetic penicillins, and the use of nitrile hydratases for the hydration of the substrate to convert acrylonitrile to acrylamide (Hernandez and Fernandez-Lafuente, 2011; Chen et al., 2016).

6.5. THE EFFECT OF BIOCATALYSIS ON TEACHING NATURAL SCIENCE Emil Fischer’s renowned comment regarding the connections between chemistry and biology (Fischer, 1907) is as pertinent today as it was a century ago. “…the separation of biology and chemistry was required whereas experimental theories and methods were being utilized,” he noted in his Faraday Lecture to the Chemical Society on Synthetic Chemistry and its Relationship to Biochemistry. Chemistry may once again reestablish its partnership with biology, not just for the benefit of biology but also for the glory of chemistry now that our field has a tremendous arsenal of analytical and synthetic weaponry. But, unlike at the beginning of the 20th century, the advancement of (natural) sciences has a growing impact on our society in terms of social dimensions and decision-making processes. Biofuel production, for example, assumes that the carbon dioxide Released by its burning is equivalent to the amount absorbed by the crop during its growth. However, there are significant disadvantages to biofuel development. Forests, which act as CO2 sinks, are being razed to make way for large-scale agriculture. As successful biofuel sources, maize (bioethanol) and rapeseed (biodiesel) require considerable amounts of nitrogen fertilizers, resulting in additional N2O emissions that are projected to be equal to 70% greater than those arising from fossil fuels (Crutzen et al., 2007). Worst of all, people in developing nations are suffering because of the biofuel craze, as prices for staple goods have risen considerably in recent years, leaving less money for education and health care. As a result, there is a shift toward advanced— second-generation—biofuels made from agricultural waste, non-food biomass, microalgae, and other sources (Carriquiry et al., 2010; Sheldon and Brady, 2019). Suppose it is agreed that all future citizens should comprehend natural sciences’ fundamental economic and ethical problems. In that case, it is self-

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evident that the relevant school and university courses must be tailored to the advancement of natural sciences. Different issues exist in this regard—aside from some still existing conservatism among those in charge of revising, such as chemistry courses; two significant factors are the extremely increasing knowledge inside ever shorter time intervals combined with the fact that the time teaching remains constant. As a result, what should be left out of the existing curriculum and what should be added arises? David Samuel proposed a far-reaching proposal for chemistry curricula 30 years ago in a talk given at the 29th IUPAC meeting in 1983. David Samuel suggested that the content of chemistry curricula be geared toward Life Sciences; some topics that should be covered include lipid membranes, lipids, and phosphorous chemistry, the properties and structure of macromolecules, the chemistry of glycoproteins, and so on, as well as the fundamental concepts of Physical Chemistry (Guo and Berglund, 2017; Dias Gomes and Woodley, 2019). A similar approach to solving some of the challenges that today’s chemistry teachers (at both the high school and university levels) confront is to base the course content, however in part, on the chemistry of daily life. This not just encourages students to pursue Natural Sciences. Still, it also enables the study of real-world issues while also teaching students the basics of chemistry relevant to that topic. Another notable instance is ‘Biocatalysts,’ which is commonly explained in biochemistry classes and labs. On the other hand, biocatalysts are well-suited to treating catalysis in conjunction with kinetics in a Physical Chemistry practical course. Suppose this is done, for example, with urease-catalyzed urea hydrolysis. In that case, conductivity measures can be employed to verify the reaction rate, allowing for the simultaneous teaching of principles of electrolyte behavior – a time-saving teaching strategy. Urease may also be immobilized using simple preparative chemistry. Comparing data from immobilized urease tests with those from related studies into the behavior of the native enzyme inevitably leads to the rules of heterogeneous catalysis. Moreover, optimizing features such as storage and operational stability in immobilized biocatalysts includes a link to Materials Sciences. This approach of tying issues in Common Chemistry to features of Biochemistry, particularly Biocatalysts, has the added benefit of corresponding curricula reflecting scientific development and thus assisting in keeping instruction up to date (Rosenbaum et al., 2006; Kadisch et al., 2017).

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REFERENCES 1.

Beers, M., Archer, C., Feske, B. D., & Mateer, S. C., (2012). Using biocatalysis to integrate organic chemistry into a molecular biology laboratory course. Biochemistry and Molecular Biology Education, 40(2), 130–137. 2. Berger, E., Ferreras, E., Taylor, M. P., & Cowan, D. A., (2014). Extremophiles and their use in biofuel synthesis. Industrial Biocatalysis (Vol. 1, pp. 239–282). Pan Stanford Publishing Pte Ltd.: Singapore. 3. Bornscheuer, U. T., Huisman, G. W., Kazlauskas, R. J., Lutz, S., Moore, J. C., & Robins, K., (2012). Engineering the third wave of biocatalysis. Nature, 485(7397), 185–194. 4. Busch, R., Hirth, T., Liese, A., Nordhoff, S., Puls, J., Pulz, O., & Ulber, R., (2006). The utilization of renewable resources in German industrial production. Biotechnology Journal: Healthcare Nutrition Technology, 1(7, 8), 770–776. 5. Carriquiry, M., Du, X., & Timilsina, G. R., (2010). Second-generation biofuels: Economics and policies. World Bank Policy Research Working Paper, 1, 5406. 6. Chen, Z., Zhao, C., Ju, E., Ji, H., Ren, J., Binks, B. P., & Qu, X., (2016). Design of surface‐active artificial enzyme particles to stabilize pickering emulsions for high‐performance biphasic biocatalysis. Advanced Materials, 28(8), 1682–1688. 7. Clouthier, C. M., & Pelletier, J. N., (2012). Expanding the organic toolbox: A guide to integrating biocatalysis in synthesis. Chemical Society Reviews, 41(4), 1585–1605. 8. Crutzen, P. J., Mosier, A. R., Smith, K. A., & Winiwarter, W., (2016). N2O release from agro-biofuel production negates global warming reduction by replacing fossil fuels. In: Paul J. Crutzen: A Pioneer on Atmospheric Chemistry and Climate Change in the Anthropocene (Vol. 1, pp. 227–238). Springer, Cham. 9. De María, P. D., De Gonzalo, G., & Alcántara, A. R., (2019). Biocatalysis as useful tool in asymmetric synthesis: An assessment of recently granted patents (2014–2019). Catalysts, 9(10), 802. 10. Devine, P. N., Howard, R. M., Kumar, R., Thompson, M. P., Truppo, M. D., & Turner, N. J., (2018). Extending the application of biocatalysis to meet the challenges of drug development. Nature Reviews Chemistry, 2(12), 409–421.

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11. Dias, G. M., & Woodley, J. M., (2019). Considerations when measuring biocatalyst performance. Molecules, 24(19), 3573. 12. Fischer, E., (1907). Faraday lecture. Synthetical chemistry in its relation to biology. Journal of the Chemical Society, Transactions, 91, 1749–1765. 13. Grunwald, P., (2009). Biocatalysis: Biochemical Fundamentals and Applications, 1, 1–20. 14. Grunwald, P., (2013). Teaching catalysis by means of enzymes and microorganisms. In: Chemistry Education and Sustainability in the Global Age (Vol. 1, pp. 131–144). Springer, Dordrecht. 15. Guo, F., & Berglund, P., (2017). Transaminase biocatalysis: Optimization and application. Green Chemistry, 19(2), 333–360. 16. Hanss, M., & Rey, A., (1971). Application of conductimetry to the study of butyrylcholinesterase enzymatic reactions. Biochimica et Biophysica Acta (BBA) -Enzymology, 227(3), 618–629. 17. Hernandez, K., & Fernandez-Lafuente, R., (2011). Control of protein immobilization: Coupling immobilization and site-directed mutagenesis to improve biocatalyst or biosensor performance. Enzyme and Microbial Technology, 48(2), 107–122. 18. Hu, Y., Liu, X., Ren, A. T. M., Gu, J. D., & Cao, B., (2019). Optogenetic modulation of a catalytic biofilm for the biotransformation of indole into tryptophan. ChemSusChem, 12(23), 5142–5148. 19. Illanes, A., (2008). Enzyme biocatalysis. Principles and Applications (pp. 1–56). Editorial Springer-Verlag New York Inc., United States. 20. Kadisch, M., Julsing, M. K., Schrewe, M., Jehmlich, N., Scheer, B., Von, B. M., & Bühler, B., (2017). Maximization of cell viability rather than biocatalyst activity improves whole‐cell ω‐oxyfunctionalization performance. Biotechnology and Bioengineering, 114(4), 874–884. 21. Kitano, H., (2002). Systems biology: A brief overview. Science, 295(5560), 1662–1664. 22. Köhler, V., Bailey, K. R., Znabet, A., Raftery, J., Helliwell, M., & Turner, N. J., (2010). Enantioselective biocatalytic oxidative desymmetrization of substituted pyrrolidines. Angewandte Chemie, 122(12), 2228–2230. 23. Laane, C., Boeren, S., Vos, K., & Veeger, C., (1987). Rules for optimization of biocatalysis in organic solvents. Biotechnology and Bioengineering, 30(1), 81–87.

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24. Lamosa, P., Gonçalves, L. G., Rodrigues, M. V., Martins, L. O., Raven, N. D., & Santos, H., (2006). Occurrence of 1-glyceryl-1-myoinosityl phosphate in hyperthermophiles. Applied and Environmental Microbiology, 72(9), 6169–6173. 25. Lawrence, A. J., & Moores, G. R., (1972). Conductimetry in enzyme studies. European Journal of Biochemistry, 24(3), 538–546. 26. Lee, J., Lee, Y., Youn, J. K., Na, H. B., Yu, T., Kim, H., & Hyeon, T., (2008). Simple synthesis of functionalized superparamagnetic magnetite/silica core/shell nanoparticles and their application as magnetically separable high‐performance biocatalysts. Small, 4(1), 143–152. 27. Majumder, A. B., Mondal, K., Singh, T. P., & Gupta, M. N., (2008). Designing cross-linked lipase aggregates for optimum performance as biocatalysts. Biocatalysis and Biotransformation, 26(3), 235–242. 28. Naessens, M., Cerdobbel, A. N., Soetaert, W., & Vandamme, E. J., (2005). Leuconostoc dextransucrase and dextran: Production, properties and applications. Journal of Chemical Technology & Biotechnology: International Research in Process, Environmental & Clean Technology, 80(8), 845–860. 29. Rodrigues, R. C., Barbosa, O., Ortiz, C., Berenguer-Murcia, Á., Torres, R., & Fernandez-Lafuente, R., (2014). Amination of enzymes to improve biocatalyst performance: Coupling genetic modification and physicochemical tools. RSC Advances, 4(72), 38350–38374. 30. Rosenbaum, M., Zhao, F., Schröder, U., & Scholz, F., (2006). Interfacing electrocatalysis and biocatalysis with tungsten carbide: A high‐performance, noble‐metal‐free microbial fuel cell. Angewandte Chemie International Edition, 45(40), 6658–6661. 31. Roychoudhury, A., Bieker, A., Häussinger, D., & Oesterhelt, F., (2013). Membrane protein stability depends on the concentration of compatible solutes–a single-molecule force spectroscopic study. Biological Chemistry, 394(11), 1465–1474. 32. Rozzell, J. D., (1999). Commercial-scale biocatalysis: Myths and realities. Bioorganic & Medicinal Chemistry, 7(10), 2253–2261. 33. Samuel, D., (1984). Chemistry and the life sciences. Chemistry in Britain, 20, 515. 34. Schmid, A., Dordick, J. S., Hauer, B., Kiener, A., Wubbolts, M., & Witholt, B., (2001). Industrial biocatalysis today and tomorrow. Nature, 409(6817), 258–268.

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35. Schoemaker, H. E., Mink, D., & Wubbolts, M. G., (2003). Dispelling the myths--biocatalysis in industrial synthesis. Science, 299(5613), 1694–1697. 36. Schulze, B., & Wubbolts, M. G., (1999). Biocatalysis for industrial production of fine chemicals. Current Opinion in Biotechnology, 10(6), 609–615. 37. Sheldon, R. A., & Brady, D., (2019). Broadening the scope of biocatalysis in sustainable organic synthesis. ChemSusChem, 12(13), 2859–2881. 38. Sheldon, R. A., & Woodley, J. M., (2018). Role of biocatalysis in sustainable chemistry. Chemical Reviews, 118(2), 801–838. 39. Wohlgemuth, R., (2010). Biocatalysis—key to sustainable industrial chemistry. Current Opinion in Biotechnology, 21(6), 713–724. 40. Wohlgemuth, R., (2021). Biocatalysis–key enabling tools from biocatalytic one-step and multi-step reactions to biocatalytic total synthesis. New Biotechnology, 60, 113–123. 41. Zechendorf, B., (1999). Sustainable development: How can biotechnology contribute?. Trends in Biotechnology, 17(6), 219–225.

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CHAPTER

7

PHARMACEUTICAL APPLICATIONS OF CATALYSIS

CONTENTS

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7.1. Introduction .................................................................................. 200 7.2. Biocatalysts in Pharmaceutical Industry ........................................ 201 7.3. Over the Counter and Remedy Drugs ........................................... 204 7.4. Hydrogenation of C=C Double Bonds .......................................... 205 7.5. Semi-Hydrogenation of C≡C Triple Bonds .................................... 206 7.6. Improved Methods for Catalyst Recovery in Biopharmaceutical Production ................................................... 207 7.7. The Problem With Carbon ............................................................. 209 7.8. A More Selective Method: Ion Exchange Resins ............................ 209 References ........................................................................................... 210

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7.1. INTRODUCTION This chapter focuses on the ever-increasing significance of catalysts in the industry to produce value-added yields in the chemical industry, emphasizing the pharmaceutical business. Where a high number of pharmaceuticals are manufactured each year, the role of catalysts is always in play. Several reactions would not happen devoid of the use of a catalyst. Catalysts are very profitable because of their cost-effectiveness, yield, selectivity, and environmental easiness. Asymmetric catalysts are the future trend and backbone in the pharmaceutical sector, which will obliterate outdated multi-step synthesis, which requires extended reaction times, complicated situations, and expensive and user-friendly catalysts. Industrial procedures have been created utilizing catalysts in the pharmaceutical area, for example, K1 Vks, Vitamin K, Naproxin, L-Dopa, Cefixime, Ceprozil, Paracetamol, Brufen, and Erythromycin, to name some. Nevertheless, there is however an opportunity for catalyst improvement in the chemical industry. To restrict pollutants through the development of medication synthesis methods, the catalytic community should create more innovative catalysts that answer growing concerns about international climate change. J. Roebuck utilized a catalyst for the first time in the industry in 1746 when he made lead chamber sulfuric acid (Sheldon and van Bekkum, 2001). Generally, a catalyst is a chemical substance that accelerates a reaction. Moreover, many reactions do not occur in the absence of catalysts. As a result, it is critical to accelerate reactions in a short period. Catalysts have been employed in a considerable section of the chemical industry since then. Initially, just pure components were utilized as catalysts, but multicomponent catalysts were explored after 1,900 and are now widely utilized. Catalysis is crucial in the industrial research and chemical industry. To comprehend the issues of catalyst application in the fine chemicals sector, one must first grasp not just the fundamental industrial needs but also how procedure development occurs and which situations influence the applicability of the best catalyst. Pharmaceutical compounds are often complicated multifunctional molecules that are created through a series of chemical processes. Catalysis is seen and used as an organic synthesis process in this setting. It is employed chiefly to transform a functional group of a specified molecule into a new one selectively to obtain the starting material for the subsequent synthetic step. As a result, regio-, chemo-, and stereoselectivity of catalysts are of significant interest. Catalysis is also used synthetically to generate new chemical bonds or cleave existing ones (Sheldon, 2000; Cabri, 2009).

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Various catalysts are constantly being developed to meet economic, political, and environmental objectives. When utilizing a catalyst, it is feasible to substitute a polluting chemical reaction with a very ecologically friendly option. This is critical for the chemical industry today and in the future. Furthermore, it is critical for a researcher to keep track of market developments. If a firm’s catalyst isn’t improved regularly, an additional company can advance in a study on that catalyst and win marketplace share. Many chemical transformations in the pharmaceutical and fine chemicals industries require stoichiometric volumes of reagents, resulting in enormous waste. This contrasts with the bulk chemical industry, which is heavily reliant on catalysis. The higher complexity of fine chemicals and pharmaceuticals explains this discrepancy, making catalysis more complex and the development of the process more costly. It was around 1835 when bromine was first used for medical purposes. Balá et al. (2000); Stahl and Alsters (2016) employed it as a tranquilizer in the 19th and early 20th centuries, and it is today used in both drugs and as a catalyst in the pharmaceutical industry (Balá et al., 2000; Stahl and Alsters, 2016).

7.2. BIOCATALYSTS IN PHARMACEUTICAL INDUSTRY Because of enhanced access to enzymes and the ability to modify those enzymes to match the demands of industrial methods, the usage of biocatalysts in the pharmaceutical business endures growing. However, we are only beginning to scratch the surface of biocatalytic applications. Pharmaceutical method development time pressures are unsuited with the considerable lead times necessary to build an appropriate biocatalyst. To meet the ever opportunities for commercial biocatalytic processes, dramatic advances in protein engineering speed are required (Elazab, 2019). The usage of enzymes in catalytic reactions is known as biocatalysts. These enzymes can be employed as isolated arrangements or complete cells, and they can be generated in their natural cells or as recombinantly stated proteins in other host cells. Biocatalysts have had a considerable impact on chemical synthesis in various industries, such as fine chemicals, pharmaceuticals, and food, over the earlier few decades. My contribution to this issue of Novelty will focus on the use of biocatalysts in the pharmacy sector, although several of the core ideas apply to other industries as well (Busacca et al., 2012).

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The pharmaceutical business is interested in biocatalysts for a variety of causes. Enzyme-based catalysis satisfies the growing demand for highly safe, selective, and long-lasting industrial methods. Biocatalysts, contrasting chemo catalysts, have a broad 3-D structure with various points of contact with the substrate of attention, permitting exceptional selectivity. Protein engineering allows simple changes to the protein sequence and, thus, structure to modify the biocatalyst’s skills. Enzyme catalysts’ high regioand stereoselectivity and their capability to perform below mild reaction conditions allow transformations to take place deprived of the need for repeated defense and deprotection stages during synthesis (Figure 7.1) (Gawande et al., 2016).

Figure 7.1. Interpretation of enzyme structure making multiple points of contact with a substrate for outstanding selectivity. Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5430392/.

Furthermore, when compared to chemocatalytic approaches, biocatalysis has both environmental and economic advantages. Enzymes are made from low-cost renewable materials and are biodegradable, allowing them to meet the following essential principles of green chemistry and ecological development (Gutiérrez et al., 2012):

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• •

Using resources at a rate that do not deplete supplies unacceptably over time; Producing and dispersing wastes at rates no greater than those that the natural environment can easily digest.

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When likened to synthetic catalytic methods that use precious metals, the economic and sustainability advantages are evident. In chemical synthesis, metals such as rhodium are frequently utilized for asymmetric reactions. Nevertheless, this is one of the rarest metals on the planet. Aside from the environmental impact of mining for the scarcity, precious metal and competitive need from other industries cause considerable variations in the market price of metals like rhodium, possibly upsetting cost-of-goods and supply chains predictions. In comparison, the costs of creating biocatalysts are steady, easy, and predictable modellable using standard techniques (Figure 7.2) (Douglas et al., 2016).

Figure 7.2. Rhodium price variations over time. Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5430392/figure/fig2/.

Despite the obvious advantages of biocatalytic reactions, the number of commercial applications has historically been small, with a substantial surge in biocatalyst use occurring only in the last two decades. The reason for this is due to one critical criterion sought by the pharmaceutical industry: SPEED. Put, until recently, the capacity to find, obtain, test, and optimize biocatalysts to produce pharmaceutical intermediates was too slow to have a significant influence. Two important variables have boosted our capability to discover and use biocatalytic pathways in pharmaceutical syntheses on a timely basis. The initial is that enzymes are readily available, and the second is that we can create those proteins (Figure 7.3).

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Figure 7.3. Every 5-year period over the previous 50 years, the number of papers and patents mentioning “pharmaceutical biocatalysis” has increased. Source: https://pubmed.ncbi.nlm.nih.gov/25747291/.

7.3. OVER THE COUNTER AND REMEDY DRUGS Although significant emphasis has been dedicated to designing catalysts for substrates, the impact of substrate preparation on the overall process efficacy has received less attention (Angrick, 2006). As a result, the catalytic step’s benefits may be negated if the substrate’s synthesis and purification require further stages. Purification of imines, for example, can be challenging, and the creation of pure E- or Z-vinyl compounds can be problematic. This issue requires further attention to avoid wasting time and money optimizing the catalytic phase. Dow pharma has developed a range of bis-phosphate ligands, primarily one called BiPhePhos, that offer reasonable costs and rising yields of linear aldehydes for a broad range of substances after acquiring Union Carbide’s expertise. Reactions with a substrate: catalyst ratio of roughly 1,000 can be conducted at 3 bar pressure and 80°C. The active ingredient in many medicinal medications is a unique enantiomorph. Chemical reactions usually produce enantiomeric combinations as a result. Purification after that, such as small crystallization to separate the active enantiomer, is an added step that raises the cost of manufacture.

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The single active enantiomer can be created using chiral- or asymmetriccatalysts. In the 1970s, William Knowles discovered that rhodium linked to chiral phosphine ligands might catalyze asymmetric hydrogenation. The process was quickly used in the industrial manufacturing of L-dopa, an anti-Parkinson medication. In 2001, Knowles (2002); Ryoji (2002); and Barry Sharpless shared the Noble Prize in Chemistry for their work on asymmetric catalysis. With the rapid growth of the pharmaceutical business, chiral compounds are in high demand, and other chiral catalysts are being produced (Govan and Gun’ko, 2014).

7.4. HYDROGENATION OF C=C DOUBLE BONDS Hydrogenation is a chemical reaction utilized in various commercial applications, ranging from food to petrochemicals and medicines. Heavy metals, like palladium (Pd) or platinum, are typically used to catalyze the chemical reaction in this procedure. These metals are highly effective catalysts, but they are non-renewable, expensive, and vulnerable to significant price variations on global markets. The hydroxylation of carboncarbon double bonds is by far the most typical hydrogenation reaction in industry. Catalysts are offered from a variety of commercial sources. Wide range of antioxidant and biological qualities – tocopherol is the most cheaply important vitamin E chemical family member. This fat-soluble vitamin is manufactured in large quantities at a rate of more than 30,000 tons per year. TMHQ (Trimethylhydroquinone), which is transformed into (all-rac)-tocopherol through condensation with (all-rac)-isophytol, is one of the fundamental building blocks for the chemical manufacture of synthetic vitamin E. This transition, which leads to (all-rac)-tocopherol and ultimately Vitamin E, is regarded as a strong and atom-economical reaction. Several complete syntheses approach to the water-soluble vitamin (+)-biotin used stereoselective hydrogenation of a tri-substituted olefinic C=C double bond (De Clercq, 1997; Netscher, 2007). Just the stereoisomer demonstrates total biological activity, and it is generated on a scale of roughly 100 tons yearly. Catalytic hydrogenation of the exocyclic olefin having indefinite double-bond stereochemistry on Pd/C or other heterogeneous catalysts can introduce stereo center C-4 of the thiophane ring, providing the required all-cis relative configuration at centers C-4, C-3a, and C-6a. Under those conditions, the N-benzyl protecting groups are stable. Goldberg and Sternbach (1949a, b) invented this method, which other groups afterward adopted in other racemic and optically active intermediary syntheses.

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As per Sheldon’s classification, most vitamins are fine compounds with annual production levels ranging from 100 to 10,000 tons (Sheldon, 2000). The class of mass chemicals includes a variety of vitamins. In general, multistep synthesis with high total yields has been used to manufacture these chemicals in the industrial sector for decades. Due to price pressure on these products, the employment of catalytic techniques in the highly economical market of vitamins has expanded considerably in recent years. As a result, the unavoidable need to decrease waste, utilize less hazardous solvents and reagents, increase energy competence, recycle reagents and catalysts, and mix unit activities to lower costs and attain more sustainable methods drives research and development (Isler, 1979).

7.5. SEMI-HYDROGENATION OF C≡C TRIPLE BONDS One of the most valuable hydrogenations for producing vitamins is the semihydrogenation of carbon-carbon ring structure to alkenes; nevertheless, good selectivity requires careful selection of catalyst and reaction conditions (Lindlar, 1952, 1954). The completely saturated alkane product is formed when acetylenes are hydrogenated with a metal catalyst, but the second hydrogenation is often more rapid than the first. Nevertheless, selectivity can be high because the alkynes join more clearly to the metal surface when some initial alkynes persist in the reaction mixture (Lindlar and Dubuis, 1966). The selectivity of the metal catalyst can be increased by using catalyst poisons that change the activity of the metal catalyst. The catalyst created by Lindlar is one of the most used and selective catalysts (1952). The Pd-plated on calcium carbonate is doped along with a lead acetate solution during the manufacturing process. This catalyst can then be employed precisely in hydrogenation or further improved with an organic component like an amine. Because the hydrogen is transferred from the metal surface to the alkyne, the Z-(cis)-alkene product usually has high selectivity. Depending on the application, “Lindlar catalysts” typically have a Pd loading of 5% and a lead loading of 2–5%. The semi hydrogenation of a vitamin A primary medium (2) to give tetraene (2) was one of the first applications of the catalyst developed by Lindlar (1954). While this could be accomplished with poisoned palladium on palladium or charcoal on calcium carbonate, selectivity was significantly higher with the lead-doped catalyst. The reaction could be stopped just after the uptake of just one equal of hydrogen gas.

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7.6. IMPROVED METHODS FOR CATALYST RECOVERY IN BIOPHARMACEUTICAL PRODUCTION Catalytic organic synthesis is used to produce active pharmaceutical ingredients (APIs), resulting in cleaner procedures and thus greener chemistries. Transition metal catalysts are widely used in these processes, and they have various advantages over standard reaction chemistries. They typically allow for faster reactions with small iterations than stoichiometric mixtures. They work at lower temperatures and pressures, lowering capital costs while increasing safety. The Ullman process, which employs copper to link two aryl halides to generate the biaryl molecule, is a single example. Heck, Suzuki, and Fukuyama’s reactions are three other well-known coupling chemistries that utilize transitional metal catalysts. The PGM (platinum group metals) are extensively employed for pharmaceutical syntheses in transitional metals. Palladium (Pd), rhodium (Rh), Ruthenium (Ru), platinum (Pt), iridium (Ir), and osmium (Os) are all PGMs (Pt). Selected instances of PTC applications for the synthesis of physiologically active chemicals, mainly from recent publications. The O-alkylation method, which is carried out on a 100-kilogram scale in the PTC system, is used to synthesize Sibenadet hydrochloride, a strong medication used to treat chronic obstructive pulmonary disease (Cosgrove et al., 2005). Banwell and colleagues (2007) reported using widely obtainable gemdihalogenocyclo-propane as building blocks in the synthesis of NPS (or their analogs) and other biologically active chemicals. Synthesis of the alkaloid-erythramine has cardiovascular and molluscicide activities and has been obtained from a range of plant sources.

7.6.1. Synthesis of Cefprozil Using a witting reaction of the triphenyl phosphoranyl alternate derivetaldehyded from the 33-chloromethylcephem compound with the acetaldehyde, Hdeaki Hoshi and Ichikawa (1985) revealed an alternative way of the combination of Cefprozil by presenting a propenyl group at the C3 position of Cephem complex by using 10 equivalents of lithium halides, such as In dichloromethane with a co-solvent selected from isopropyl alcohol or dimethylformamide at a temperature ranging from 0 to 25°C, the process produces a yield of 71%. After further diacylation with PCl5 in the presence of the organic base pyridine, followed by alcoholic deprotection

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with TFA in anisole at 0°C, the cephem compound was converted into the transitional 7—Amini-3-(propenyl-1-yl)-3-cephem-4-carboxylic acid, which was then converted into the final product, Cefprozil, by condensation with D-p-hydroxyphenyl (Hoshi et al., 1987).

7.6.2. Catalyst Recovery Catalyzed reactions can be divided into two major types: homogeneous reactions and heterogeneous reactions. In heterogeneous reactions, the catalyst phase differs from the phase of the reactants, which might be liquid, solid, or gaseous. However, the miscibility of the catalyst may vary from that of the reaction mixture because of this. The catalyst and the other reactants will be in a similar phase and flammable with inhomogeneous reactions. The kind of reaction will impact the extraction of the catalyst post-reaction, which is a time-consuming procedure compared to heterogeneous catalysts, which are recovered by filtering the reaction mixture during the reaction (Truppo, 2017). Catalyst recovery techniques such as chemical precipitation, filtration, nanofiltration, solvent extraction, and adsorption are commonly used in industrial and medicinal processes, among other applications. Nanofiltration recovery is just employed inhomogeneous procedures, and catalysts are promptly re-employed in the liquid phase after being recovered by nanofiltration. As a result, nanofiltration is not particularly suitable for incineration and the recovery of catalysts for future use. Chemical precipitation and solvent removal are non-specific processes that might result in yield losses of the API (Martin et al., 2020). Materials including activated carbon and ion-exchange resins can be used in adsorption recovery procedures to recover either homogeneous ligand or homogeneous catalysts, depending on the application. If the catalyst is a homogeneous mixture, the carbon or resin will selectively absorb the catalyst. The catalyst is removed from the adsorbent and either recycled or transferred to a recovery specialist for recovery and incineration in the following step. The homogeneous leachate and ligand are formed when a piece of a heterogeneous catalyst is discarded and then dissolves in the phase where the transformation of the chemical is taking place, as described above. After the catalyst has been filtered from the batch, it is necessary to remove any remaining ligand and leachate in solution using a resin or carbon treatment (Rosenthal and Lütz, 2018).

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7.7. THE PROBLEM WITH CARBON Activated carbon has traditionally been used in adsorption procedures for catalyst recovery. However, this has changed recently. Carbon is wellcharacterized, reasonably affordable, and comparatively successful at removing wasted catalysts from process steps when used in conjunction with other methods. Carbon, on the other hand, has several disadvantages. For starters, it can be hard to handle and filter out of the process stream once introduced. This is usually avoided by using pre-packaged carbon cartridges in the removal process, which are then removed. The non-specific adsorptive characteristic of carbon is the second and more challenging to overcome disadvantage. Carbon will eliminate the catalyst, but it will also bind a portion of the API. As a result, yields are reduced, and production expenses are increased. Adsorption with selective catalyst extraction would have been a more appealing adsorption strategy (Sun et al., 2018).

7.8. A MORE SELECTIVE METHOD: ION EXCHANGE RESINS Ionic resin technology has been in use in the biopharmaceutical sector for more than 60 years, particularly for purifying APIs. Ion exchange resins have been used to purify a wide range of molecules, including oligonucleotides, antibiotics, and monoclonal antibodies. Specific ion exchange resins are also scavengers in the chemical synthesis process, eliminating byproducts and unreacted elements from the reaction mixtures. It is possible to combine solid-phase extraction (SPE) methods with combinatorial libraries in various applications ranging from bench-top operations in the laboratory to full-scale production. The usage of iminodiacetic functionalized resin for heavy metals removal in industrial processing is an instance of a rummager resin being used at a large scale in industrial managing. Before discharge, the waste sewage still includes trace amounts of heavy metals, ranging from 5 to 20 parts per million (ppm). It is possible to reduce the overall metal concentration to less than 0.1 parts per million by using a polishing component that contains the chelating resin and is mounted in a packedcolumn merry-ground arrangement, even in the existence of high calcium or sodium salt concentrations (Rasor and Voss, 2001).

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REFERENCES 1.

Anastas, P. T., & Eghbali, N., (2010). Green chemistry: Principles and practice. Chem. Soc. Rev., 39, 301–312. 2. Angrick, M., (2006). Sustainable Chemistry: Experiences and Perspectives (Vol. 66, pp. 1–20). Metropolis-Verlag GmbH. 3. Baláž, M., Boldyreva, E. V., Rybin, D., Pavlović, S., Rodríguez-Padrón, D., Mudrinić, T., & Luque, R., (2020). State-of-the-art of eggshell waste in materials science: Recent advances in catalysis, pharmaceutical applications and mechanochemistry. Frontiers in Bioengineering and Biotechnology, 8, 1522. 4. Baldenius, K. U., Hunnefeld, L., Hilgemann, E., Hoppe, P., & Stürmer, R., (1996). Chapter 4: Vitamin E (tocopherols, tocotrienols). In: Ullmann’s Encyclopedia of Industrial Chemistry, A (Vol. 6, pp. 478– 488). Weinheim, Germany: VCH. 5. Banwell, Stanislawski, Anthony, & Banwell (2007). Gemdihalocyclopropanes as building blocks in natural-product synthesis: Enantioselective total syntheses of ent-erythramine and 3-epierythramine. Chemistry An Asian Journal, 2, 1127–1136. 6. Bonrath, W., Eggersdorfer, M., & Netscher, T., (2007). Catalysis in the industrial preparation of vitamins and nutraceuticals. Catal. Today, 121, 45–57. 7. Bouvier, A., Chapline, J., Boerner, R., Jeyarajah, S., Cook, S., Acharya, P. S., & Shepard, S. R., (2003). Identifying and modulating disulfide formation in the biopharmaceutical production of a recombinant protein vaccine candidate. Journal of Biotechnology, 103(3), 257–271. 8. Busacca, C. A., Fandrick, D. R., Song, J. J., & Senanayake, C. H., (2011). The growing impact of catalysis in the pharmaceutical industry. Advanced Synthesis & Catalysis, 353(11, 12), 1825–1864. 9. Busacca, C. A., Fandrick, D. R., Song, J. J., & Senanayake, C. H., (2012). Transition metal catalysis in the pharmaceutical industry. Applications of Transition Metal Catalysis in Drug Discovery and Development: An Industrial Perspective, 52, 1–24. 10. Cabri, W., (2009). Catalysis: The pharmaceutical perspective. Catalysis Today, 140(1, 2), 2–10. 11. Chen, D., Sirkar, K. K., Jin, C., Singh, D., & Pfeffer, R., (2017). Membrane-based technologies in the pharmaceutical industry and

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continuous production of polymer-coated crystals/particles. Current Pharmaceutical Design, 23(2), 242–249. Cosgrove, S. D., Steele, G., Austin, T. K., Plumb, A. P., & Stensland, B., (2005). Understanding the polymorphic behavior of sibenadet hydrochloride through detailed studies integrating structural and dynamical assessment. Journal of Pharmaceutical Sciences, 94, 2403– 2415. De Clercq, (1997). Biotin: A timeless challenge for total synthesis. Chem. Rev., 97, 1755–1792. Douglas, J. J., Sevrin, M. J., & Stephenson, C. R., (2016). Visible light photocatalysis: Applications and new disconnections in the synthesis of pharmaceutical agents. Organic Process Research & Development, 20(7), 1134–1147. Elazab, Dr. H. A., (2019). Optimization of the Catalytic Performance of Pd/Fe3O4 Nanoparticles Prepared via Microwave-Assisted Synthesis for Pharmaceutical and Catalysis Applications, 1, 1–30. Ellis, H., (1900). The dictionary of national biography. The Argosy: A Magazine of Tales, Travels, Essays, and Poems, 72, 336–344. Gawande, M. B., Goswami, A., Felpin, F. X., Asefa, T., Huang, X., Silva, R., & Varma, R. S., (2016). Cu and Cu-based nanoparticles: Synthesis and applications in catalysis. Chemical Reviews, 116(6), 3722–3811. Goldberg, M. W., & Sternbach, L. H., (1949a). Synthesis of Biotin (pp. 1–10). US patent 2489232. Goldberg, M. W., & Sternbach, L. H., (1949b). Synthesis of Biotin (pp. 1–10). US patent 2489235. Govan, J., & Gun’ko, Y. K., (2014). Recent advances in the application of magnetic nanoparticles as a support for homogeneous catalysts. Nanomaterials, 4(2), 222–241. Gutiérrez, L. F., Hamoudi, S., & Belkacemi, K., (2012). Lactobionic acid: A high value-added lactose derivative for food and pharmaceutical applications. International Dairy Journal, 26(2), 103–111. Hansen, E. C., Pedro, D. J., Wotal, A. C., Gower, N. J., Nelson, J. D., Caron, S., & Weix, D. J., (2016). New ligands for nickel catalysis from diverse pharmaceutical heterocycle libraries. Nature Chemistry, 8(12), 1126–1130.

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23. Hawkins, J. M., & Watson, T. J., (2004). Asymmetric catalysis in the pharmaceutical industry. Angewandte Chemie International Edition, 43(25), 3224–3228. 24. Hayler, J. D., Leahy, D. K., & Simmons, E. M., (2018). A pharmaceutical industry perspective on sustainable metal catalysis. Organometallics, 38(1), 36–46. 25. Hoshi, H., & Ichikawa, (1985). United States Patent (pp. 1–10). US patent 4,520,022. 26. Hoshi, H., Ichikawa, & Jun, K., (1987). Dose-Response Study of Caudal Neostigmine for Postoperative Analgesia. US 4, 699, 979. 27. Isler, O., (1979). History and industrial application of carotenoids and vitamin A (1). In: Carotenoids℃ 5 (Vol. 1, pp. 447–462). Pergamon. 28. Jozala, A. F., Geraldes, D. C., Tundisi, L. L., Feitosa, V. D. A., Breyer, C. A., Cardoso, S. L., & Pessoa, A., (2016). Biopharmaceuticals from microorganisms: From production to purification. Brazilian Journal of Microbiology, 47, 51–63. 29. Knowles, W. S., (2002). Asymmetric hydrogenations. Angew. Chem. Int. Ed., 41, 1998–2007. 30. Krska, S. W., DiRocco, D. A., Dreher, S. D., & Shevlin, M., (2017). The evolution of chemical high-throughput experimentation to address challenging problems in pharmaceutical synthesis. Accounts of Chemical Research, 50(12), 2976–2985. 31. Lindlar, H., & Dubuis, R., (1966). Palladium catalyst for partial reduction of acetylenes. Organic Syntheses, 45, 89–92. 32. Lindlar, H., (1952). A new catalyst for selective hydrogenation. Helvetica Chimica Acta, 35(2), 446–450. 33. Lindlar, H., (1954). Hydrogenation of Acetylenic Bond Utilizing a Palladium-Lead Catalyst (pp. 1–10). US2681938. 34. Martin, L. L., Peschke, T., Venturoni, F., & Mostarda, S., (2020). Pharmaceutical industry perspectives on flow chemocatalysis and biocatalysis. Current Opinion in Green and Sustainable Chemistry, 25, 100350. 35. Netscher, T., (2007). Synthesis of vitamin E. Vitamins and Hormones, 76, 155–202. 36. Noyori, R., (2002). Asymmetric catalysis: Science and opportunities. Angew. Chem. Int., 41, 2008–2022.

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37. Peterson, A. A., Vogel, F., Lachance, R. P., Fröling, M., Antal, Jr. M. J., & Tester, J. W., (2008). Thermochemical biofuel production in hydrothermal media: A review of sub-and supercritical water technologies. Energy & Environmental Science, 1(1), 32–65. 38. Pollard, D. J., & Woodley, J. M., (2007). Biocatalysis for pharmaceutical intermediates: The future is now. Trends in Biotechnology, 25(2), 66– 73. 39. Rasor, J. P., & Voss, E., (2001). Enzyme-catalyzed processes in pharmaceutical industry. Applied Catalysis A: General, 221(1, 2), 145–158. 40. Rosenthal, K., & Lütz, S., (2018). Recent developments and challenges of biocatalytic processes in the pharmaceutical industry. Current Opinion in Green and Sustainable Chemistry, 11, 58–64. 41. Ruffert, C., Bigall, N. C., Feldhoff, A., & Rissing, L., (2014). Investigations on the separation of platinum nanoparticles with magnetic beads. IEEE Transactions on Magnetics, 50(11), 1–4. 42. Sabatier, P., & Sendrens, B., (1900). Hydrogenation of ethylene in the presence of different metals. Weekly Reports J of Sessions of the Academy of Weekly Sessions Accounts, 130, 1781. 43. Sabatier, P., & Sendrens, J. B., (1899). Action of hydrogen on acetylene in the presence of nickel. Weekly Reports of Sessions of the Academy of Weekly Sessions Accounts, 128, 1173. 44. Sheldon, R. A., & Van, B. H., (2001). Fine Chemicals Through Heterogeneous Catalysis (Vol. 1, pp. 1–20). Weinheim, Germany: Wiley-VCH. 45. Sheldon, R. A., (2000). Atom efficiency and catalysis in organic synthesis. Pure Appl. Chem., 72, 1233–1246. 46. Stahl, S. S., & Alsters, P. L., (2016). Liquid Phase Aerobic Oxidation Catalysis: Industrial Applications and Academic Perspectives (Vol.1, pp. 1–22). John Wiley & Sons. 47. Sun, H., Zhang, H., Ang, E. L., & Zhao, H., (2018). Biocatalysis for the synthesis of pharmaceuticals and pharmaceutical intermediates. Bioorganic & Medicinal Chemistry, 26(7), 1275–1284. 48. Truppo, M. D., (2017). Biocatalysis in the pharmaceutical industry: The need for speed. ACS Medicinal Chemistry Letters, 8(5), 476–480.

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49. Vogl, T., Hartner, F. S., & Glieder, A., (2013). New opportunities by synthetic biology for biopharmaceutical production in Pichia pastoris. Current Opinion in Biotechnology, 24(6), 1094–1101. 50. Woodley, J. M., (2008). New opportunities for biocatalysis: Making pharmaceutical processes greener. Trends in Biotechnology, 26(6), 321–327.

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CHAPTER

8

APPLICATIONS OF CATALYSIS IN NANOTECHNOLOGY AND ENERGY

CONTENTS

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8.1. Introduction .................................................................................. 216 8.2. Nanoparticles in Catalysis ............................................................. 217 8.3. Nanocatalysis ............................................................................... 220 8.4. Solar-Driven Water Splitting .......................................................... 228 References ........................................................................................... 233

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8.1. INTRODUCTION The capacity to optimize and customize the electronic band structure of nanoparticles by composition, shape, and size, as well as the vast surfaceto-volume ratio and the ability to use tiny particles straight in solution or fixed in a medium, have controlled the investigation of several systems for water excruciating, primarily by separating the dual half-cell reactions. Furthermore, the tunability of their properties permitted systematic studies to be carried out to uncover the essential parameters that regulate performance and unravel their mode of action (Bayón et al., 1999; Hutchings, 2013). The main phases in the photocatalytic cycle are as follows: The absorption of light, the movement of transporters of charge to the catalytic reaction, and the reaction site are all factors to consider. All of them would be discovered to be dependent on numerous parameters, necessitating finetuning to counter all the impacts and maximize the system’s efficiency (Haruta, 2002). The following categorical parameters have been originated to impact these stages: •

The dimensions and composition of the photocatalysts, which fundamentally regulate their chemical and electronic properties, such as chemical stability and band offset, over-potentials to the wanted reactions, or the adsorption energy and reactivity of the mixtures to the catalytic site (Shukla and Sinha, 2018); • The surface coating of the NCs, which should offer enough dispersion. Nanoscience and nanotechnology have seen the most rapid growth among the subjects that have seen significant advancements. In the past, nanomaterials were not completely unknown in the various domains of surface research. Chemical conversion methods, for example, have long used heterogeneous catalysts in the shape of nanoscale transition metal particles spread on microporous substrates. Our increased capability to control and design nanomaterials, their chemical composition, shape, size, and fabrication structure for developed applications reflects the amazing breakthroughs in modern nanotechnology (Figure 8.1) (Ozin and Arsenault, 2015).

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Figure 8.1. Artist’s perspective of complete water splitting by the semiconductor-metal hybrid nanoparticle. Source: https://iopscience.iop.org/article/10.1088/1361-6528/abbce8.

8.2. NANOPARTICLES IN CATALYSIS Nanoparticles, particularly gold nanoparticles, have been employed by women in Egypt and China for millennia for esthetic and medicinal purposes and through artists for the decoration and creation of glassware and ceramics like the famous Lycurgus cup. Dissertations on colloidal nanoparticles have been published since the early 17th century. Faraday’s famous publication was the first scientific study on the influence of light, followed by Mie’s original explanation of the plasmon barely half a century later (Somorjai and Park, 2008). The first articles on platinum nanoparticle hydrogenation and poly(vinyl) alcohol-protected Pd catalysts produced by reducing metal salts through H2 were published in 1941. This research was like that of Paul Sabatier, who found catalyzed hydrogenation utilizing thinly dispersed nickel particles made by reducing nickel oxide or hydroxide with H2. In 1987, Haruta reported that the catalytic commotion of gold nanoparticles for the oxidation of CO to CO2 via O2 was increased when the gold nanoparticles were < 5 nm. This was another breakthrough in the 20th century. Even though the nanoworld in nanomedicine, optics, nanoscience, and nanotechnology cover 1 to 100 nm and beyond,

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Haruta’s groundbreaking discovery focused nano catalytic research on the tiniest nanoparticles (Grass et al., 2008). However, there are exemptions; for example, plasmonic excitation of massive gold nanoparticles by visible light irradiation produces hot electrons that activate semiconductors like TiO2 for substrate change. We also understand that sub nanoparticles of early transition metals are quite active, albeit the peak of catalytic activity is most likely moved for particles with 12 to 20 metal atoms, i.e., with diameters close to 1 nm or somewhat further down. Advanced methods like X-ray absorption spectroscopy and aberrationcorrected electron microscopy, which now permit the viewing of single metal atoms and sub nanoclusters as well as their surrounding surroundings, have made these characterizations conceivable. Even though there is a variety of conditions from molecules to solid-state among tiny clusters described by molecular orbitals and more significant nanoparticles identified by energy band structures, small nanoparticles are commonly dubbed nanoclusters or sub nanoclusters for those < one nm. The number of metal atoms, the design of the ligands, and the dispersion of these diverse types differ. Molecularly same polymetallic molecules with ligands for which the X-ray crystal structure is famous are referred to as groups or nanoclusters. In contrast, nanoparticles refer to mixtures of large polydisperse nanoclusters distinct by the histogram revealed by transmission electron microscopy capacity. Metal-metal bound groups typically only some metal atoms, though some massive ones have been discovered. Since the late 1970s, X-ray crystal structures of metal clusters have been available for theoretical research, followed by catalytic experiments that revealed the cluster frame’s loss of structural integrity during catalysis. As the discovery of structurally perfect specified nanoclusters containing thiolate ligands identical to those found in the most well-known gold nanoparticles, this subfield has experienced a rebirth over the last decade. The cluster framework is well described for hypothetical results and calculations employed in structure reactivity interactions; hence catalytic investigations of these nanoclusters have just now been produced. Tiny metal nanoparticles at the intersection of heterogeneous and homogeneous catalysis, which are just loosely stabilized by ineffective ligands like inorganic molecules polymers, ionic liquids, dendrimers, solvents, and solids, are the prevalent subjects of catalytic research. This is because of the ease with which they can be made

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and the strong catalytic performance that results from the substrates’ easy access to the nanoparticle surfaces due to mild ligand dislocation. Even though many effective high-temperature methods were established over the 20th century, a large section of the old heterogeneous catalysis society is currently combining into the nanoparticle group. The significance of catalytic nanoparticle supports is a unifying idea for both diverse and homogeneous societies. Templates with zeolites were expanded to microstructure alumina, silica, and other oxides by heterogeneous catalysis, permitting sub nanoclusters to be stabilized down to the nanoscale. New 3D, 2D, and 1D supports, such as graphene byproducts, nanotubes, and metal-organic structures, have recently appeared with considerable success. The supports alleviate the nanoparticles and work with nanoparticle surfaces to initiate substrates like the positive synergy seen in alloys among two transition metal atoms or among the central group atom and a transition metal. Because of these synergistic effects, N-doped carbon supports were found to be superior to undoped equivalents. Catalytic nanoparticle families include ternary and binary particles with main group elements, as well as mono- and bimetallic nanoparticles. Nanocomposites are complex nanoparticle-supported ensembles. The complexity of the nanocatalyst design is sometimes reminiscent of Mother Nature’s organization of complexity. Supported MoS2 sub nanoparticles with only a few atoms, for example, resemble MoS-based nanoclusters that serve as cofactors for nitrogenase enzymes that catalyze N2 fixation. These biomimetic features are helpful in the layout of nano catalytic CO2, N2, and H2O reduction processes, for example. Light has a critical role in activating tiny molecules, hydrogen generation, and water splitting in this environment. The reduction of nanocatalyst size to a single atom has gotten a lot of interest recently because no atoms in the nanoparticle’s interior are missing. Furthermore, supported single-atom catalysts have a superior definition than polydisperse subnanocatalysts, which are less selective. This section refers to single-site organometallic catalysts in which ligands are available in addition to the single metal site’s oxide support. In summary, homogeneous and heterogeneous catalysis, materials science, organometallic chemistry, bioinorganic chemistry, and surface science, and catalysis are all needed for nanocatalysis. These advances toward “green” nanocatalysis, which now priorities safe Earth-copious

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elements more than rare, costly, and occasionally poisonous noble metals, solve our society’s most pressing synthetic and energy challenges.

8.3. NANOCATALYSIS A catalyst is a substance that runs up a chemical reaction devoid of even being absorbed by it. Due to the limits of both homogeneous and heterogeneous catalysts, new catalytic paradigms are required to overcome these constraints. The features of homogeneous and heterogeneous catalysis should be combined in the model catalyst. As a result, the catalyst for a specific reaction must have high selectivity, activity, and yield. It must be detachable from the reaction, moderate, stable, and recyclable at the same time. Customized nanostructures have shown the ability to join these demanding criteria. Transition metals, particularly valuable noble metals like Rh, Pd, Pt, Ag, Au, Ru, and Cu, are widely utilized as heterogeneous and homogeneous catalysts in chemical reactions. The fundamental explanation for this is that they provide a variety of oxidation states. They also have excellent adsorption capabilities, which are necessary for heterogeneous catalysis. The transition metal nanoparticles’ combination of these two features allows them to operate as electron channels for the reactants adsorbed on the catalyst’s surface. Ag nanoparticles in Pt and photography used in the breakdown of hydrogen peroxide were the first instances of nanoparticles in catalysis (H2O2). Noble metal nanoparticles have since been widely utilized as catalysts for various organic processes, including carbon-carbon combinations in the Heck, Stille, and Suzuki reactions, oxidation, dehydrogenation, hydrogenation, and so on (Somorjai, 1981; Calle-Vallejo et al., 2015). Because of their high surface energy, nanoparticles be likely to clump together, developing in larger particle sizes with less surface area. The latter means that the catalyst has fewer surface-active sites. Surfactants and polymers, which can also function as functionalizing agents, are widely utilized to keep the surfaces of nanoparticles from aggregation. The electrical structure of the nanoparticle changes because of these surface-changing activities, as does its catalytic activity. Another way to get over this issue is to implant these NPs on low density, large surface area insoluble solids support such as carbon-based materials, zeolites, and so on. The support material may be relatively innocuous. Instead, the support could alter the catalyst’s adsorption and chemical capabilities. By tweaking the electron density of NPs, active supports like these can improve or hinder the effectiveness of

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the catalyst for a specific reaction. Another possibility is that the aid is a superior absorbent for a few reactants, improving the catalyst’s performance (Figure 8.2) (Miller et al., 2006; Somorjai and Li, 2010).

Figure 8.2. Dependency of catalytic activity on shape, size, and composition. Source: https://www.intechopen.com/chapters/67023.

Nanoparticles are increasingly being used to replace traditional heterogeneous catalysts (Astruc, 2008; Ozin and Arsenault, 2015). Nanoparticles have a larger surface area and more exposed active areas due to their tiny size. As a result, nanoparticles have a higher surface area in contact with reactants and are more catalytically active than traditional heterogeneous catalysts. Nanocatalysts come in a variety of shapes and compositions, allowing contact with different sorts of catalytic spots. A specific kind of site has a higher selectivity for a particular chemical pathway. As a result, nanocatalysts exhibit features like homogeneous catalysts in terms of improved selectivity and activity. Nanocatalysts, in contrast, are heterogeneous catalysts because they are comparatively easy to distinguish from reaction mixtures. Furthermore, each nano catalyzed reaction requires the adsorption of reactant(s) on the nanocatalyst as a prerequisite. This, too, is typical of a heterogeneously catalyzed reaction. By manipulating nanomaterials’ size, shape, and content, nanocatalysts with improved activity, resilience, and selectivity can be developed and manufactured (Ishida and Haruta, 2007). The usual cause and effect relationship is depicted in Figure 8.2. Metal nanoparticles of similar form but varied sizes are used in a reaction to explore the size impact of catalyst. It is, therefore, possible to examine the effect of nanoparticle size on catalytic selectivity and activity.

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8.3.1. Effect of Size 8.3.1.1. On Catalytic Properties Nanocatalysts often have a significantly higher surface-to-volume ratio than their bulk equivalents. When the size of a catalyst nanoparticle decreases to a particular nano regime, significant variations in the coordination environment and electronic states of the surface atoms may be possible. As a result, changes in nanoparticle size influence the coordination sphere, adsorption energy, and electronic state of reactant molecules (Hashmi and Hutchings, 2006).

8.3.1.2. Size-Dependent Coordination Environment With decreasing nanoparticle size, the influence of atoms at edges and corners becomes more prominent (Hvolbæk et al., 2007). Cao et al. (2016) analyzed the relationship between the overall size of nanoparticles and surface metal atoms with several CN (coordination number) of cubic geometry and cuboctahedral. They determined that the CNs 4, 7, and 9 of a cuboctahedral nanoparticle and 3, 6, and 8 of cubic nanoparticles highly depend on the nanoparticle’s size. Tao et al. (2010) showed a similar high connection of size-dependent catalytic performance for the room temperature CO oxidation reaction. For example, Pt atoms along the edge of triangular nanoclusters are effective for CO oxidation even at ambient temperature in Pt nanoparticles with a size of roughly 2.2 nm. At ambient temperature, Pt atoms with a CN of 9 on the porch of Pt (111) are not effective for CO oxidation (Che and Bennett, 1989).

8.3.1.3. Size-Dependent Electronic State Metal nanoparticles with an electronic structure of 1–2 nm have a moleculelike electronic structure. As a result, Au nanoparticles with a diameter of < 1 nm are much more molecular than metallic. In contrast to a nanoparticle of larger size, molecule-similar to electronic states of metal nanoparticles of 1–2 nm demonstrates essentially different catalytic activity. Chen and Goodman (2004) achieved this experimentally for the first time in CO oxidation on Au nanoclusters with ternary atomic layers established on TiO2). These researchers discovered that adopted Au nanoparticles of various sizes have variable average CNs after analyzing Au LIII XANES white lines (Valden et al., 1998). As a result, the average CN of a 3 nm Au nanoparticle is 9.5. Similarly, nanoparticles with a diameter of 1 nm have an average CN

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of 6, whereas nanoparticles with a diameter of 0.5–1 nm have an average CN of 3.6. This demonstrates that the electrical environment of smaller Au nanoparticles is size-dependent (Janssens et al., 2006; van Bokhoven and Miller, 2007).

8.3.1.4. Size-Dependent Adsorption Energy In heterogeneous catalysis, adsorption is the first stage. In the literature, sizedependent adsorption energies of reactants on catalyst surfaces with various CN have also been proposed. The adsorption energy of metal nanoparticles is influenced by their coordination environment. For a particular molecule, catalyst atoms with a low CN usually demonstrate stronger adsorption than those with a greater CN (Lu et al., 2015). The adsorption energy of adsorbent surfaces such as OOH, OH, O2, H2O2, H2O, and O on Pt nanocatalyst, for example, decreases linearly as the CN increases from 3 to 9. Additional transition metal catalysts, like Cu, Ni, Co, Ag, Pd, Rh, Au, and Ir, have shown similar linear connections among adsorption energy and CN (Li et al., 2012; Zhang et al., 2014).

8.3.2. Impact of Shape on Catalytic Properties Figure 8.3 depicts typical structures of metal nanoparticles centered on dimensionality. 0D nanoparticles are pseudo-spherical, spherical, octahedral, tetrahedral, dodecahedral, and cubic. Nanotubes, nanorods or nanowires, nanocapsules, and other 1D morphologies of nanoparticles. The 2D shape NPs fit quadrangular plates, triangular, hexagonal or rings, belts, sheets, and other objects. Nanoparticles have complex 3D morphologies, for example, nanostars, nanoflowers, polygonal nano frames, and so on (Lin et al., 2012; Lv et al., 2015).

Figure 8.3. Different kinds of anisotropic nanoparticles. Source: https://www.researchgate.net/figure/Different-types-of-anisotropicnanoparticles_fig1_334820849.

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Because of the various numbers of kink sites, edges, and steps appear on the catalyst’s surface in the nanoscale system, innovative anisotropic morphologies offer unique physicochemical advantages compared to conventional isotropic morphologies of nanoparticles. Polyhedral Au NPs with high-indexed features, for example, have been discovered to have outstanding optical and catalytic capabilities (Tran and Lu, 2011; Jing et al., 2014). Various transverse and longitudinal plasmon bands can be seen in Au rods with different length-to-width ratios. A review of numerous forms of anisotropic gold nanoparticles employed in catalysis was recently published by Priecel et al. (2016). For catalysis, surface-improved Raman scattering and branched, sensing Au NPs with many tips assuming shapes like flowers and stars are progressively employed (Zhang and Toshima, 2013).

8.3.3. Composition Effect The influence of composition on catalytic activity is only discussed in this section from the standpoint of bimetallic nanoparticles and alloy. Bimetallic nanoparticles are generally classified into three structure types: alloy, Janus, and core-shell. The synthesis approach used determines the sort of bimetallic or alloy nanostructure generated (Zhang et al., 2011). Bimetallic nanomaterials’ catalytic activity differs from that of their constituent metals. Bimetallic nanoparticles may have synergistic catalytic characteristics rather than being a means of the catalytic activity of their constituents (Yin et al., 2011). Yoo et al. (2011) investigated the catalytic activity of Pt-Y alloy intended for electrocatalytic oxygen reduction, which is an example of the composition effect. When different amounts of Y are added to Pt, the electronic structure of the metal changes, and the binding energy of the oxygen-containing type changes as well. The best catalytic operation was found in a certain Pt-Y alloy mixture. Mazumder et al. (2012) have established that the catalytic activity of monodisperse COPD nanoparticles aimed at formic acid oxidation is composition dependent. In core-shell bimetallic nanoparticles, the composition has an effect. Jiang et al. (2011a, b) discovered that the activity of core-shell Cu@M catalyst nanoparticles intended for hydrolytic dehydrogenation of ammonia borane is composition-dependent. Collaboration of Cu with M in core-shell Cu@M structures can vary the width of the surfaced band, which is helpful for catalytic development. In all three scenarios, the most excellent catalytic activity may be found with an optimal Cu/M ratio (Xia et al., 2009; Han et al., 2014).

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The catalytic oxidation of benzyl alcohol, methanol, CO, dyes, and glucose has all been reported to utilize BNPs extensively. Oxygen reduction, propane dehydrogenation, nitro-aromatic compound hydrogenation, electrocatalytic methanol oxidation, and thiophene desulfurization (Figures 8.4 and 8.5) (Suo et al., 2011; Yamamoto et al., 2011).

Figure 8.4. Various options of bimetallic nanostructures were examined: (a) requested alloy; (b) arbitrary alloy; (c) Janus-like; and (d) core-shell. Source: https://www.researchgate.net/figure/Patterns-exhibited-by-bimetallicnanoalloys-at-various-temperatures-a-Solid-core_fig4_336533626.

Figure 8.5. The shape and size-controlled Pt nanoparticles were set up by the colloid-chemistry-controlled method. Source: https://www.researchgate.net/figure/The-size-and-shape-controlledPt-nanoparticles-prepared-by-the_fig11_283015050.

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8.3.4. Highly Selective Nanocatalysts Highly selective catalysts may lower the amount of energy used in chemical industry product separation and waste disposal processes. Highly selective catalysts are also used in the production of alternative energy resources. A critical step in biomass conversion is the selective conversion of biomassderived carbohydrates to liquid fuels and valuable compounds (Deplanche et al., 2012; Fu et al., 2014). Using surface science investigations, several molecular level parameters impacting catalytic selectivity have been found in our laboratory over the last few decades, allowing nanotechnology to build and engineer very selective catalysts. The rate-determining stages for various products in a multiple-path catalytic reaction normally appear at separate active sites on the catalyst surface. Think about a catalytic reaction with a cyclic hydrocarbon reactant: scission of C-C bonds produces a ring-opening product, whereas dissociation of a C-H link produces a dehydrogenation product. The respective heights of the Gibbs free energy obstacles for the two products control the ratio of product 1 to product 2 made at a specific surface site. The scission of the C-H bond happens more quickly than that of the C-C bond at step sites on platinum surfaces, resulting in a more significant probability of producing the dehydrogenation product. The splitting of the C-C bond becomes more accessible at the kink locations, increasing the ring-opening product. This simplified diagram shows that the discernment of heterogeneous catalytic processes is ultimately dictated by the relative concentrations of active sites for different reaction pathways. The concentration of active sites in nanoparticle catalysts is influenced by the shape and size of the nanoparticles. By creating monodisperse and shape-controlled catalysts, innovations in nanoparticle production enable accurate handling of surface-active sites. Figure 8.6 illustrates how the selectivity of pyrrole hydrogenation on Pt nanoparticles is affected by both shape and size (Perdew et al., 1996). The surface structure modification of nanocatalysts of various shapes and sizes causes the selectivity change.

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Figure 8.6. (Upper panel) The kinked Pt surface of the model. At the kink and step sites, the C-C and C-H bonds are detached, respectively. (Panel on the lower left) The two-pathway reaction’s schematic free energy potential surface. Breaking the C-C bond produces Product 1, and disrupting the C-H bond produces Product 2. Source: https://www.intechopen.com/chapters/67023.

Bimetallic alloys, which give surface-active areas with specific atomic configurations of metal components, are a significant class of selective catalysts. The advent of coherent catalyst design at the nanoscale has recently changed this sector. Computer-based high-throughput screening identifies the atomic and elements structure of alloy candidates with the best catalytic characteristics. After that, nanoalloys are created and tested. The fact that alloy particle size reduction usually results in a decreased immiscible gap allows for the suitable method of alloy catalysts with the desired composition. A NiZn3 alloy catalyst with improved selectivity for limited hydrogenation of acetylene has been discovered using this method (Figure 8.7) (Linkov and Satterstrom, 2008).

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Figure 8.7. (a) The selectivity of pyrrole hydrogenation as a function of nanoparticle size at the specific reaction conditions: 4 Torr pyrrole, 400 Torr H2, 413 K. Small nanoparticles have a strong pyrrolidine selectivity. (b) Pyrrole hydrogenation selectivity is affected by nanoparticle shape underneath the following conditions: 400 Torr H2, 4 Torr pyrrole nanopolyhedra particles exhibit a more excellent selectivity for pyrrolidine than nanocubes at lower temperatures. Source: https://www.intechopen.com/chapters/67023.

8.4. SOLAR-DRIVEN WATER SPLITTING Solar-driven water splitting necessitates the use of a synergist of light absorbers, electrocatalysts, and product separation procedures. These roles can be accomplished as independent process stages, such as using a solar cell in conjunction with an electrolyze or as part of an incorporated system that produces distinct streams of O2 (g) and H2 (g) using just water and sunshine as inputs. Although there are some similarities between the discrete component and integrated systems approaches, there are significant variances in functionality, materials, and design strategy. This status report focuses on fully integrated systems (Vollath, 2008). Status Stand-alone solar-driven water split systems are uncommon, with no available for commercial use or large-scale display. However, in recent laboratory displays, similar systems’ efficiency has grown dramatically. Solar-to-hydrogen conversion competencies of more than 10% have been shown in systems that generate reliable, distinct streams of O2 and H2, with effectiveness of up to 19% described for systems that coevolve stoichiometric combinations of O2(g) and H2(g). Four system-level conditions should be

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met concurrently to be evaluated for ultimate commercialization: safety, stability, high efficiency, and the ability for low-cost production. At this time, no system has been demonstrated that meets all these characteristics at the same time. The long-term strength of effective solar light absorbers make contact with a liquid electrolyte, as well as active, earth-abundant electrocatalysts for the redox reactions required to create fuel sustainably, is a long-standing difficulty. The most efficient systems are unable to scale and remain stable. Chemical techniques and physical mechanisms that maintain adequate product separation are now not scalable, cost-efficient, or stable. When all the components in a comprehensive system should operate below the same set of reaction circumstances, additional obstacles arise. Nonetheless, compared to discrete photovoltaic/electrolyzer assemblies, combined systems have significant potential benefits in terms of performance, cost, and simplicity, making unified systems beautiful targets for component, device, and engineering design development, proof-of-concept, and exploration (Somorjai et al., 2008).

8.4.1. Electrocatalysts Precious metals like Ru and Ir are found in the most active electrocatalysts for water oxidation to O2 (g) in acidic media. Due to the rarity of these components, scaling up these systems in their present form to fulfill existing world fuel consumption would be impossible. Pt is the highly active electrocatalyst for proton reduction to H2 in acidic environments (g). Considering different active electrocatalysts that are also steady under operational circumstances is a challenge. The hunt for a non-precious noble metal for water oxidation in acidic conditions is arguably an oxymoron. For the extremely kinetically challenging four-electron oxidations of two water molecules to produce O2 (g), a material must have enough covalency that the sometimes ionic electrocatalyst constituent of the material is reduced insoluble in acid and is not leaked from the structure, whereas at low overpotentials also showing a high degree of activity. For the evolution of H2 (g) in acidic circumstances, phosphide-based sulfide-based and earthabundant transition-metal electrocatalysts have been created. Although these materials have proven long-term stability in the laboratory, with day and night cycling of the electrocatalyst beneath operational potentials, longterm stability in real-world settings has yet to be shown (Terrones, 2003).

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In alkaline channels, stable earth-copious electrocatalysts created on Mo–Ni and similar alloys are utilized commercially in alkaline electrolyzes for H2 (g) evolution. Mo stabilizes the active Ni electrocatalyst site. Water oxidation to O2 (g) in alkaline environments is catalyzed by moderately low overpotential earth-plentiful electrocatalysts centered on FeNi oxyhydroxides and similar compounds, which are stable for long periods. Unlike in acidic media, where the inadequate suite of stable earth-abundant O2 (g)-developing electrocatalysts has extremely high overpotentials, a slight performance increase in an integrated system can be achieved in alkaline media by lowering electrocatalyst overpotentials compared to now accessible materials. As a result, the biggest issue in alkaline media is integrating all the presented electrocatalysts into a completely working system. Electrocatalysts that advance porosity throughout operation can offer pathways for the electrolyte to corrode the fundamental bubble formation; light absorber wants to be measured and usefully exploited to improve local mass transport deprived of deleterious electrochemical and visual effects on process, and long-term constancy below light and dark cycling situations needs to be minimized and managed. Sufficient electrocatalyst stability and activity at near-neutral pH have yet to be established. Operating a method at near-neutral pH necessitates either conceding safety by coevolving stoichiometric mixes of O2 (g) and H2 (g) or requiring a membrane that takes the lead to electrodialysis of the electrolyte, preventing effective system operation. Using an electromotive force in synthetic photosynthetic systems instead of the protonmotive force employed in natural photosynthesis poses a significant problem. In the first case, the pH gradient that is an unavoidable result of trans-membrane electron transport must be neutralized, resulting in the loss of the chemical potential associated with it. On the other hand, natural photosynthesis takes advantage of the protonmotive force’s pH gradient to enable proton cross-membrane movement actively. It supplies energy in the reductive components of the system by creating and splitting chemical bonds (Zhao et al., 1998). Regardless matter whether active, balanced electrocatalyst materials and elements for water-splitting are created, energy-efficient, scalable, and passive means of solving this critical obstacle stay a hurdle at the system level below near-neutral pH operating circumstances (Huang et al., 2008).

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8.4.2. Light Absorbers Optimal light absorbers mix proper solar spectrum absorption with operatingcondition stability. Because no photoanode in water is thermodynamically fixed under lighting, kinetic stability should be used. Even though oxides are usually stable in the lab, these materials all feature an extremely lowlevel valence band such as O2– in nature. It is unclear whether stability is yet achievable in principle for an oxide-based material with a reduced bandgap achieved by significantly elevating the energy of the valence band more toward the proper capacity for water oxidation. Protection layers, for instance, amorphous TiO2, have been used to stabilize photoelectrodes via conducting holes and prevent corrosive contact with the electrolyte. The long-term disaster process of these systems, along with techniques to reduce pitting-induced corrosion because of coating flaws or local passivation procedures to expand operating life cycles from weeks to decades, is still unknown (Murray et al., 2000). In comparison to photoanode stabilization, photocathode stabilization provides extra difficulties due to the differences in failure mechanisms and, as a result, the techniques necessary for developing appropriate safety layers. Furthermore, cathodes are more susceptible to poisoning due to metal impurity plating than anodes’ oxidative self-cleaning characteristic. They so have more strict constraints on the cleanliness of the input water stream (Zhang et al., 2007). Imperfections or pinholes in the defending coating can also cause performance deterioration, resulting in diversions when plated metal layers from ohmic connections or local Schottky obstacles along with the fundamental semiconductor (Xia et al., 2003).

8.4.3. Developments in Science and Technology to Solve Problems The crucial developments in science and technology to address the concerns are all linked to materials. The preferred features and features of catalysts, separators, and light absorbers are well understood and easily quantifiable. Still, the discovery and identification of materials with the desired mix of chemical and physical properties is currently lacking. To make rapid progress, a combination of theory, experiment, and high-quantity data-driven methodologies will be required for every one of the crucial components of an incorporated solar-driven water-splitting system. Furthermore, the required attributes must be demonstrated synergistically in an incorporated

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system that runs under a set of mutually agreed-upon circumstances for each component in the system. Since the first proven integrated system is probably the last deployment system, maintaining flexibility is critical. A range of options should be investigated in parallel to maximize the possibilities of success (Law et al., 2004; Yin et al., 2005).

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plasmonic peaks from visible to near-IR range. Chemistry of Materials, 26(5), 1794–1798. 56. Zhang, Y., Grass, M. E., Habas, S. E., Tao, F., Zhang, T., Yang, P., & Somorjai, G. A., (2007). One-step polyol synthesis and Langmuir− Blodgett monolayer formation of size-tunable monodisperse rhodium nanocrystals with catalytically active (111) surface structures. The Journal of Physical Chemistry C, 111(33), 12243–12253. 57. Zhao, D., Peidong, Y., & Qisheng, H., (1998). Topological construction of mesoporous materials. Current Opinion in Solid State and Materials Science, 3(1), 111–121.

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INDEX

A

B

abandoned chemical plants 186 absorption spectroscopy 38, 41, 56 acetaldehyde 7 acetaldehyde degradation 7 Acetaldehyde pyrolysis 31 alcohol 7, 12 Alkali metals 127 alkyl group 64 Alkynes 70 alumina/silica hydrocracking catalysts 130 ammonia 10, 14, 120, 126, 127, 128, 130, 132, 156, 161 amorphous solids 125, 133 aquatic cultures 175 Aquifer pyrophilus 187 Asymmetric catalysts 200 asymmetric reactions 203 atmosphere 5, 9 atmospheric chemistry 5 atomic force microscopy 186 atoms 31, 42 automotive exhaust converter 2

bacteria 186 bamboo 175 beer 186 Bennett Chandler process 32 biocatalysis 66, 67 Biocatalysts 186, 187, 190, 194 Bio-Derived wastes 175 biological transformations 189 biomass 175, 176 biotechnology 187, 188, 191, 192, 198 borohydrides 12 bread 186 Brufen 200 bulk chemicals 2, 16

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C calcined 132, 133, 136 carbohydrates 7 carbonaceous materials 175, 176 carbon-based materials 220 carbon monoxide 8, 20 catalyst 2, 3, 4, 5, 7, 8, 9, 11, 17, 18, 19, 21, 24 catalytic collision 26

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catalytic processing 121 catalytic reaction 2, 3, 4 catalytic reforming 126 Cefixime 200 cell 189, 191, 196, 197 Ceprozil 200 chemical bond 19 chemical kinetics 26, 32 chemical reaction 2 chemical synthesis 187, 190 chemical vapor deposition 126 chemistry 30, 31, 45, 47, 51, 60 chiral catalysts 205 chiral compounds 205 clusters 126, 131, 148, 162 commercial biocatalysts 188 complex catalysis 67 composite materials 175, 176 compound 3 computational chemistry 17 construction 18 covalent bond 69 crop production 120 crystalline materials 125 D De-insertion 75 dendrimers 218 Divided Saddle Theory 32 DNA synthesis 6 dust particles 173 E ecological development 202 electrocatalytic methanol oxidation 225 electron counting 66 energy 3, 4, 7, 8, 9, 12, 23 engineering 188, 191, 192

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environmental catalysis 166, 167, 168, 169, 171, 172, 174, 180, 181, 182, 183, 184 enzyme activity 37 enzyme-catalyzed reaction 27 enzyme development 188 Enzymes 186, 187, 189, 190 equation 26, 32, 33, 35, 39, 43, 45 equilibrium 64, 69, 75, 80, 82, 96, 98 Erythromycin 200 Ethylene glycol 10 ethylene oxide 11, 14 F fermentation 186, 190 first-order reaction 29, 30, 31, 58 flames 26, 54 flash 37, 38, 39, 58, 59, 60 Flash photolysis 37 fuels 2 Fusion 131 G Garbage cleanup 13 gasoline 171 Gibbs free energy 3 glassy metals 126 green chemistry 202 greenhouse gas abatement 166 H halocarbon wax 37 hazardous chemicals 10 heat transfer 123 Heavy metals 205 Heterogeneous catalysis 120, 156 Homogeneous catalysis 6, 12, 21 homogeneous metal 67

Index

241

homometallic oxides 126 HTC (Hydrothermal carbonization) 175 hydrocarbon molecule 76 Hydrogenation 205, 212, 213 hydrogen peroxide 6 hydrothermal synthesis 129, 134, 155

microporous substrates 216 microscopic reversibility 72 Migration 73 Mixed-oxide catalysts 131 Mobile emission control 166 molecules 26, 31, 37, 43, 44, 47, 52, 56 multidisciplinary field 2

I

N

Inhomogeneous catalytic reaction 64 inorganic molecules polymers 218 ionic liquids 218 ions 31, 41, 53 iridium (Ir) 69 iron-based complex 120

Nanocomposites 219 nanomaterials 216, 221, 224 nanomedicine 217 nanoparticles 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 228, 233, 234, 235, 236, 237 nanoscience 217, 236 nanotechnology 216, 217, 226 Naproxin 200 natural gas 171, 173 natural systems 26 nitrogenase 187 nitrogen dioxide 10 Nitrogen fixation 120 Noble metal nanoparticles 220 NTP (Non-thermal plasma) 173

K Kinetic chemical reactions 30 kinetics 4, 11, 16, 17 L L-Dopa 200 ligand field theory 66 lignocellulosic materials 175 liquid waste treatment 166 living organisms 2 M Mass transmission 123 mechanism analysis 17 Metal crystal surfaces 124 metallic glasses 131 metal nanoparticles 175, 177 metal-organic frameworks 175, 177 metal oxidation state 76, 83 metal oxide nanoparticles 175, 178 methanol 171

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O optics 217 Organic Chemistry 187 organic compounds 12 organic syntheses 12 organism 186, 189 organometallic complexes 6 organometallic compounds 64, 65, 85, 98 oxidation states 66, 72

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oxidative addition reaction 70 Oxidative coupling 70 oxide coatings 126 oxygen 6, 8, 9, 22, 23 oxygen atoms 8, 9 Oxygen reduction 225 P palladium (Pd) 8, 205 Paracetamol 200 pharmaceutical business 200, 201, 202, 205 Pharmaceutical compounds 200 pharmaceutical research 186 photocatalytic cycle 216 platinum 7, 8 Poisons 127, 128 pollution control 2 polyethers 10 polyurethanes 10 potential energy 26, 47 propane dehydrogenation 225 protein 6 R Raney metals 130, 132 rate equation 28, 29, 31, 46 rate law 29, 32, 33, 34, 35, 44, 56, 59, 60 raw materials 10 reaction mixture 28, 36, 37, 40, 43 Reductive elimination 71, 72 rhodium (Rh) 8

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S s-bond metathesis 77 selective ethylene oxidation (SEO) 10 selective hydrogenation 10 skeletal catalysts 131, 161 Solid catalysts 121 solids catalyze reactions 7 solvents 10, 12, 218 spectroscopy 17 starch, 175 stoichiometric oxidations 12 stoichiometric reactions 9 T thermodynamics 4, 20 thermophilic bacteria 186 thiophene desulfurization 225 TMHQ (Trimethylhydroquinone) 205 Transition metals 220 V Vitamin K 200 W waste 10, 11, 12, 13 Z zeolites 5, 20, 22, 123, 125, 130, 134, 139, 155, 219, 220 zinc 12

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