Heterogeneous Catalysts for Clean Technology E-Book
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- Herausgeber: Wiley-VCH
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Reactive, but not a reactant. Heterogeneous catalysts play an unseen role in many of today's processes and products. With the increasing emphasis on sustainability in both products and processes, this handbook is the first to combine the hot topics of heterogeneous catalysis and clean technology.
It focuses on the development of heterogeneous catalysts for use in clean chemical synthesis, dealing with how modern spectroscopic techniques can aid the design of catalysts for use in liquid phase reactions, their application in industrially important chemistries - including selective oxidation, hydrogenation, solid acid- and base-catalyzed processes - as well as the role of process intensification and use of renewable resources in improving the sustainability of chemical processes.
With its emphasis on applications, this book is of high interest to those working in the industry.
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Veröffentlichungsjahr: 2013
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Table of Contents
Related Titles
Title Page
Copyright
Dedication
Preface
List of Contributors
Chapter 1: Introduction to Clean Technology and Catalysis
1.1 Green Chemistry and Clean Technology
1.2 Green Chemistry Metrics
1.3 Alternative Solvents
1.4 Heterogeneous or Homogeneous
1.5 Alternative Energy Reactors for Green Chemistry
1.6 Concluding Remarks
References
Chapter 2: Mechanistic Studies of Alcohol Selective Oxidation
2.1 Introduction
2.2 Metal-Catalyzed Alcohol Selox
2.3 Oxide, Sulfide, and Vanadate Catalysts
2.4 Solvent Selection
2.5 In Situ and Operando X-Ray Studies of Selox Catalysts
2.6 Conclusions
References
Chapter 3: Reaction Monitoring in Multiphase Systems: Application of Coupled In Situ Spectroscopic Techniques in Organic Synthesis
3.1 Introduction
3.2 Method Coupling
3.3 Spectroscopic Reactors and Practical Aspects
3.4 Selected Examples of Use
3.5 Conclusion and Outlook
References
Chapter 4: In Situ Studies on Photocatalytic Materials, Surface Intermediates, and Reaction Mechanisms
4.1 Introduction
4.2 In Situ Investigations
4.3 Concluding Remarks
References
Chapter 5: Enantioselective Heterogeneous Catalysis
5.1 Introduction
5.2 Strategies for the Creation of Enantioselective Heterogeneous Catalysts
5.3 Concluding Remarks – A Comparison of the Various Approaches to Heterogeneous Enantioselective Catalysts
References
Chapter 6: Mechanistic Studies of Solid Acids and Base-Catalyzed Clean Technologies
6.1 Introduction
6.2 New Catalytic Systems
6.3 Biomass Conversions
6.4 Summary
References
Chapter 7: Site-Isolated Heterogeneous Catalysts
7.1 Introduction
7.2 Assembled Monolayers of Metal Complexes on Single-Crystal Surfaces
7.3 Reaction-Induced and Photoinduced Formation of Unsaturated Ru Complexes Supported on SiO2 Surfaces
7.4 Manganese Triazacyclononane Catalysts Grafted under Reaction Conditions
7.5 Well-Defined Silica-Supported Mo–Imido Alkylidene Complexes for Metathesis
7.6 Double Catalytic Activation Using a Bifunctional Catalyst with Both Acid and Base on Solid Surfaces
7.7 Summary
References
Chapter 8: Designing Porous Inorganic Architectures
8.1 Introduction
8.2 Templated Methods for the Preparation of Ordered Porous Materials
8.3 Hierarchical Porous Materials
8.4 Concluding Remarks
References
Chapter 9: Tailored Nanoparticles for Clean Technology – Achieving Size and Shape Control
9.1 Introduction
9.2 Size effects – setting the scene
9.3 Size effects illustrated by way of examples of selected industrially important reactions
9.4 Shape effects
9.5 Conclusions
References
Chapter 10: Application of Metal–Organic Frameworks in Fine Chemical Synthesis
10.1 Metal–Organic Frameworks as Heterogeneous Catalysts
10.2 Applications in Carbon–Carbon Bond Formation
10.3 Applications in Oxidation, Carbon–Oxygen, and Carbon–Nitrogen Bond Formation
10.4 Applications in Asymmetric Synthesis
10.5 Concluding Remarks
Acknowledgments
List of Abbreviations
References
Chapter 11: Process Intensification for Clean Catalytic Technology
11.1 Introduction
11.2 Effect of Transport Phenomena on Heterogeneous Catalysis
11.3 Intensification of Transport Phenomena
11.4 Conclusion
List of Symbols
References
Chapter 12: Recent Trends in Operando and In Situ Characterization: Techniques for Rational Design of Catalysts
12.1 Introduction
12.2 Catalyst Nascence
12.3 Synthesis of Silicalite-1 Molecular Sieves
12.4 Preparation of Supported Metal Catalysts
12.5 Catalyst Life
12.6 Elucidating the Reaction Mechanism of Aerobic Oxidation of Benzyl Alcohol
12.7 Determination of the Active Sites in Aerobic Oxidation of Benzyl Alcohol
12.8 Catalyst Death
12.9 Methanol to Hydrocarbons
12.10 Propane Dehydrogenation
12.11 Summary and Conclusions
References
Chapter 13: Application of NMR in Online Monitoring of Catalyst Performance
13.1 Online Monitoring with NMR Spectroscopy
13.2 Quantitative NMR Spectroscopy in Technical Samples
13.3 Flow and High-Pressure NMR Spectroscopy for Reaction Monitoring
13.4 Selected Applications of NMR in Online Monitoring of Catalyst Performance
13.5 Conclusions
Acknowledgments
References
Chapter 14: Ambient-Pressure X-Ray Photoelectron Spectroscopy
14.1 Introduction
14.2 Technical Aspects
14.3 Applications of APXPS
14.4 Outlook
Acknowledgments
References
Index
Related Titles
Serp, P., Philippot, K. (eds.)
Nanomaterials in Catalysis
2013
ISBN: 978-3-527-33124-6
Rodriguez, Jose A., Hanson, Jonathan C., Chupas, Peter J. (eds.)
In-situ Characterization of Heterogeneous Catalysts
2013
ISBN: 978-1-118-00016-8
Zecchina, A., Bordiga, S., Groppo, E. (eds.)
Selective Nanocatalysts and Nanoscience
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2011
ISBN: 978-3-527-32271-8
Crabtree, R.H. (ed.)
Handbook of Green Chemistry - Green Catalysis
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ISBN: 978-3-527-31577-2
Mizuno, N. (ed.)
Modern Heterogeneous Oxidation Catalysis
Design, Reactions and Characterization
2009
ISBN: 978-3-527-31859-9
The Editors
Dr. Karen Wilson
Aston University
European Bioenergy Research Institute
School of Engineering and Applied Science
Birmingham, B4 7ET
United Kingdom
Prof. Adam F. Lee
University of Warwick
Department of Chemistry
Coventry, CV4 7AL
United Kingdom
and
Monash University
School of Chemistry
Victoria 3800
Australia
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany.
All rights reserved. (including those of translation into other languages). No part of this book may be reproduced in any form — by photoprinting, microfilm, or any other means — nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
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We dedicate this book to the memory of Rodney Stewart Lee.
Preface
Catalytic technologies play a critical role in the economic development of both the chemicals industry and modern society, underpinning 90% of chemical manufacturing processes and contributing to over 20% of all industrial products. Sustainable chemistry is defined as the design and implementation of chemical products and processes that reduce or eliminate the use or generation of hazardous substances, while employing renewable resources in an atom and energy efficient fashion. In accordance with the 12 Principles of Green Chemistry, first advanced by Anastas and Warner, catalysis is a key tool with which to develop sustainable chemistries. New catalytic routes to the manufacture of fine, speciality and pharmaceutical chemicals offer sustainable solutions with minimal environmental impact. In a post-petroleum era, catalysis researchers will need to rise to the challenge of synthesising chemical intermediates and advanced functional materials and fuels from non-petroleum based feedstocks. Success will require an interdisciplinary approach, uniting physical, inorganic, organic and materials chemistry with biotechnology, reaction and process engineering.
To a large extent, the catalytic transformation of individual atoms and molecules into potent drug therapies, advanced fuels, and efficient fertilisers has (to date) depended upon an equal combination of brilliant science and serendipity. This reflects the complex, interdependent interactions between reactants, products, their surrounding environment, and of course the catalyst itself, which in principle should remain unchanged over thousands of reaction cycles. However, recent advances in chemical synthesis, nanotechnology and spectroscopy now offer an unprecedented opportunity to sculpt the atomic structure of solid catalysts and to peer inside their microscopic workings. Our knowledge of the mechanism by which heterogeneous catalysts operate has traditionally been obtained by comparing freshly prepared materials with their spent counterparts isolated post-reaction. While this approach has undoubtedly aided catalyst development, the importance of adsorbate-induced restructuring in modulating surface reactivity, a concept Gabor Somorjai termed the ‘flexible surface’, is now widely accepted. Step-changing discoveries require intelligent catalyst design, informed by quantitative insight into catalyst behaviour under reaction conditions via complementary operando studies of the surface, bulk and atomistic properties of catalysts in action. This book focuses on the development of heterogeneous catalysts for application in clean chemical synthesis, and explores how modern spectroscopic techniques can be employed to aid the design of catalysts (particularly) for use in liquid phase reactions. Examples of catalytic applications to industrially important chemistries including selective oxidation and hydrogenation, solid acid and base catalysed processes, and photocatalytic depollution, while other chapters illustrate the importance of process intensification and use of renewable resources in enhancing the sustainability of chemical processes.
The development of new catalytic processes requires consideration of unconventional reactor technologies which afford improvements in product separation, overall energy efficiency and operational safety. An understanding of the physicochemical properties and behaviour of diverse solid catalysts and associated factors influencing catalyst selection for specific chemical transformations, catalyst compatibility with different reactor designs, and mechanistic insight accessible through time-resolved in-situ spectroscopic tools, will aid industrial and academic researchers in addition to undergraduate students taking courses in sustainable or green chemistry. We hope this text will serve as a central resource for catalytic scientists and engineers across the clean technology community, providing information on next-generation catalyst formulations, process operation, and on-line monitoring. Newcomers to the field of heterogeneous catalysis, particularly undergraduate and postgraduate students, will also be exposed to the fundamental physical principles underpinning an array of spectroscopic methods, and synthetic strategies adopted to prepare high performance nanocrystalline and nanoporous catalysts and to valorise bio-derived, multi-functional feedstocks through atom- and energy economical processes.
This book would not have been possible without the collective work of a number of scientists and engineers spanning catalysis, materials, spectroscopy, process intensification and green chemistry. We would like to express our gratitude to all the contributors, whose time, efforts, and expertise have helped to deliver what we hope will become a valuable scientific resource for beginners and experienced practitioners of clean and sustainable chemistry. We are also grateful to Drs. Elke Maase and Lesley Belfit at Wiley-VCH for their support and useful advice in preparing this book.
List of Contributors
1
Introduction to Clean Technology and Catalysis
James H. Clark
1.1 Green Chemistry and Clean Technology
Traditional chemical manufacturing is resource demanding and wasteful, and often involves the use of hazardous substances. Resources are used throughout the production and including the treatment of waste streams and emissions (Figure 1.1).
Figure 1.1 Resource demands of traditional chemical manufacturing.
Green chemistry focuses on resource efficiency and on the design of chemical products and processes that are more environmentally benign. If green chemistry is used in a process, it should be made simpler, the inputs and outputs should be safer and more sustainable, the energy consumption should be reduced and costs should be reduced as yields increase, and so separations become simpler and less waste is generated [1]. Green chemistry moves the trend toward new, clean technologies such as flow reactors and microwave reactors, as well as clean synthesis. For instance, lower temperature, shorter reaction time, choice of an alternative route, increased yield, or using fewer washings at workup improve the “cleanness” of a reaction by saving energy and process time and reducing waste [2].
At present, there is more emphasis on the use of renewable feedstocks [3] and on the design of safer products including an increasing trend for recovering resources or “closed-loop manufacturing.” Green chemistry research and application now encompass the use of biomass as a source of organic carbon and the design of new greener products, for example, to replace the existing products that are unacceptable in the light of new legislation (e.g., REACH) or consumer perception.
Green chemistry can be seen as a tool by which sustainable development can be achieved: the application of green chemistry is relevant to social, environmental, and economic aspects.
To achieve sustainable development will require action by the international community, national governments, commercial and noncommercial organizations, and individual action by citizens from a wide variety of disciplines. Acknowledgment of sustainable development has been taken forward into policy by many governments including most world powers notably in Europe [4], China [5], and the United States [6].
1.1.1 Ideals of Green Chemistry
In Figure 1.2 and Figure 1.3, the ideals of green chemical synthetic design are shown.
Figure 1.2 Factors for reduction in syntheses.
Figure 1.3 The eight parts of an ideal synthesis.
It is important to note that these green chemistry goals are most effectively dealt with and are easier to apply if they are considered at the design stage rather than retrospectively – green chemistry is not an end-of-pipe solution.
Chemical plants have traditionally concentrated on mechanical safety devices, reducing the probability of accidents. However, mechanical devices are not infallible and safety measures cannot completely prevent the accidents that are happening. The concept of inherently safer design (ISD) was designed with the intention of eliminating rather than preventing the hazards and led to the phrase “What you don't have can't harm you” [7]. ISD means not holding significant inventories of hazardous chemicals or not using them at all.
This approach would have prevented the accident at Bhopal, India in 1984, where many thousands of people were killed or seriously injured. One of the chemicals used in the process at the Union Carbide factory was highly water sensitive, and when a watertight holding tank was breached, the accident occurred, releasing the chemicals into the air, affecting the villages surrounding the factory. The chemical is nonessential and the ISD approach would have been used an alternative, thus eliminating the risk altogether.
Green chemistry research has led to the invention of a number of clever processing technologies to save time and energy or reduce waste production, but these technologies mostly exist in academia and, with very few exceptions, industry has been slow to utilize them. Green chemical technologies include heterogeneous catalysis (well established in some sectors but much less used in fine chemicals and pharmaceuticals, see the subsequent text), use of supercritical fluids (as reaction and extraction media), photochemistry, microwave chemistry, sonochemistry, and synthetic electrochemistry. All these replacements for conventional methods and conductive heating can lead to improved yields, reduced reaction times, and reduced by-product formation. Engineered greener technologies also exist, including a number of replacements for the stirred tank batch reactor, such as continuous stirred tanks, fluidized bed reactors, microchannel reactors, and spinning disc reactors as well as microwave reactors, all of which increase the throughput, while decreasing the energy usage and waste. Unfortunately, despite these many new processes, industry is reluctant to use these hardware solutions because of the often massive financial expenditure involved in purchasing these items and the limited number of chemistries that have been demonstrated with them to date. There is also a reluctance to change well-established (and paid for) chemical plant so that newer, cleaner technologies may well have more success in the developing (e.g., the Brazil, Russia, India, and China (BRIC)) nations, where the chemical industry is growing and new plant is required to meet the increasing expectations of local and increasingly affluent markets.
1.2 Green Chemistry Metrics
It is important to be able to quantify the change when changes are made to chemical processes. This enables us to quantify the benefit from the new technology introduced (if there are benefits). This can aid in in-house communication (to demonstrate the value to the workforce) as well as in external communication. For yield improvements and selectivity increases, simple percentages are suitable, but this simplistic approach may not always be appropriate. For example, if a toxic reagent is replaced by a less toxic one, the benefit may not be captured by conventional methods of measuring reaction efficiency. Equally, these do not capture the mass efficiency of the process – a high-yielding process may consume large amounts of auxiliaries such as solvents and reagents, as well as those used in product separation and purification. Ideally, we also need to find a way to include energy and water, both of them have been commonly used in a rather cavalier way but they are now subject to considerable interest that they can vary depending on the location of the manufacturing site.
Numerous metrics have been formulated over time and their suitability discussed at great length [8–12]. The problem observed is that the more accurate and universally applicable the metric devised, the more complex and unemployable it becomes. A good metric must be clearly defined, simple, measurable, objective rather than subjective, and must ultimately drive the desired behavior. Some of the most popular metrics are
Of course, the ultimate metric is life cycle assessment (LCA); however, this is a demanding exercise that requires a lot of input data, making it inappropriate for most decisions made in a process environment. However, some companies do include LCA impacts such as greenhouse gas production in their in-house assessment, for example, to rank solvents in terms of their greenness. It is also essential that we adopt a “life cycle thinking” approach to decision making so that we do not make matters worse when greening one stage in a manufacturing process without appreciating the effects of that change on the full process including further up and down the supply chain.
1.3 Alternative Solvents
Most chemical processes involve solvents – in the reactions and in the workups as well as in the cleaning operations [14, 15]. The environmental impact of a chemical process cannot be properly evaluated without considering the solvent(s). For some time there has been a drive toward replacing or at least reducing the use of traditional volatile organic solvents such as dichloromethane, tetrahydrofuran, and N-methylpyrollidone – commonly used solvents in, for example, catalytic processes.
Ionic liquids, fluorous biphasic systems, and supercritical fluids have all been studied as alternatives to conventional organic solvents. However, because of their nature, some of these novel systems require additional hardware for utilization. For example, some suppliers have designed advanced mixing systems to enable polyphasic systems to be intimately mixed at the laboratory scale. There has also been considerable rethinking of the green credentials of some of these alternative solvents in recent years and many ionic liquids are no longer considered suitable because of their complex syntheses, toxicity, or other unacceptable properties, or difficulty in separation and purification. Fluorous solvents (which are based on heavily fluorinated usually aliphatic compounds) are not considered to be environmentally compatible (as they persist in the environment).
Supercritical solvents are difficult to manipulate because of the high pressures and temperatures often employed. In the case of supercritical water, equipment had to be designed, which could contain the highly corrosive liquid. Vessels for creating supercritical solvents such as supercritical CO2 (scCO2) are now available and are capable of fine adjustments in temperature and pressure to affect the solvents' properties. Very high pressure and temperatures are not required to produce scCO2 and it is becoming an increasingly popular reaction medium as its properties are controllable by varying the temperature and pressure or by the use of a cosolvent [16]. The main environmental benefit of scCO2 lies in the workup, as the product mixture is obtained free from solvent by simply returning to atmospheric conditions. Additionally, carbon dioxide is nontoxic, nonflammable, recyclable, and a by-product of other processes. However, there are energy and safety concerns associated with the elevated temperatures and pressures employed and in particular, there are high capex costs to install a plant. These must be balanced against the benefits of its use.
scCO2 can be a good medium for catalysis, although its low polarity means that either catalysts are heterogeneous or they have to be modified to enable them to dissolve (e.g., by introducing solubilizing substituents on the catalyst ligands).
Ionic liquids are molten salts and are liquid at relatively low temperatures: room-temperature ionic liquids are the most widely studied. Their lack of vapor pressure has been their biggest selling point but the enormous flexibility of choice of ions enables ionic liquids to be designed as catalysts as well as solvent. In particular, they can be powerful combined solvent–acid catalysts. The use of ionic liquids has been reported in various synthetic transformations such as Friedel–Crafts reaction, Diels–Alder reaction, and metal-catalyzed asymmetric synthesis. The problems with their use include toxicity (in some cases), cost of manufacture, and difficulties in separation/purification (they cannot be distilled), and these have hampered their industrial uptake, although they are certainly interesting at least for niche applications [17, 18].
Biphasic systems can be an effective method by which catalyst, substrates, and products can be easily separated into different liquid phases and therefore simplifying and “greening” reaction workup. Fluorous biphasic solvent systems, where the homogeneous catalyst is soluble within the fluorous phase and reactants are soluble within an immiscible conventional solvent, have been extensively studied. Heating leads to the two solvents becoming miscible, enabling the reaction to occur. On completion of the reaction, when cooled, the phases return to being immiscible with the product partitioning into the conventional solvent phase for isolation. However, there have been serious concerns expressed over the “green” credentials of these heavily fluorinated molecules as they persist in the environment and can be hazardous to operators. Phase transfer catalysts (PTCs) have been used for many years in biphasic systems for transferring species into a phase they would not normally be soluble in. They aid the reaction by improving the availability of the substrates [19]. PTCs are commonly quaternary ammonium or phosphonium compounds; they mostly do not present major environmental concerns and continue to be popular for greening organic reactions. Perhaps, the biggest concern is with regard to their recovery from reactions as they are usually very soluble in both phases of the biphasic system, although heterogeneous PTC, involving, for example, silica-supported onium compounds have been reported.
1.4 Heterogeneous or Homogeneous
While homogeneous catalysis generally offers good activity and a homogeneous distribution of active sites, as explained earlier, it is not without problems notably with regard to separation and reuse. Here, heterogeneous catalysis has clear advantages. There are in fact a number of advantages of heterogeneous catalysis compared to homogeneous [20, 21].
The disadvantages of heterogeneous catalysis include added synthesis costs, need for larger amounts of materials, and blocking of catalyst sites. Overall, heterogenization of catalysts (and sometime reagents) is one of the most widely favored green chemical technologies.
1.5 Alternative Energy Reactors for Green Chemistry
There are a number of types of equipment associated with high-energy transfer to the reactants including microreactors, microwave reactors, radio frequency heating, electric pulses, ultrasonication, and spinning disc reactors. Some of these are briefly discussed later.
1.5.1 Microchannel Reactors
The principle of the microreactor is based on the simple fact that having very small volumes of reactants coming together at any given time, the risk of a potentially hazardous thermal runaway is minimized. This is an example of “process intensification,” which has many benefits including uniform treatment of all molecules and transport rates that match the reaction rates. These can lead to improved selectivity and yield and reduced processing time as well as reduced risk through limited exposure. Typically, submillimeter channels are etched into quartz or plastic units [22]. These units have been successfully demonstrated for liquid/liquid reactions, for example, fixing palladium cross-coupling catalysts to fine glass pipes.
Using a “scaling out” rather than “scaling up” approach, a more flexible production capacity is available with the opportunity to rapidly switch product output as market demands change, and very importantly (in the light of such disasters as Bhopal), the storage of hazardous product should become redundant.
1.5.2 Microwave Reactors
Microwave irradiation is a high-frequency electric field, with wavelength in the centimeter range, which places it between radio waves and infrared in the electromagnetic spectrum (Figure 1.4). Microwave energy is very low, around 1 J mol−1, which means that microwaves cannot directly break the bonds. Microwaves interact with dipoles or ions, and create “molecular heating” by causing dipole rotations (or ionic conduction). Both of these mechanisms of receiving energy are caused by the molecules attempting to align with the rapidly oscillating microwave field. Thus, microwave reactors are capable of enhancing reaction rates as they allow more molecules to have sufficient energy to overcome the activation barrier of the reaction. These high-energy molecules are created by preventing the molecules from relaxing from the excited state: kinetic relaxation occurs in 10−5 s, whereas microwaves apply energy in 10−9 s, which creates a nonequilibrium state.
Figure 1.4 Microwaves in the electromagnetic spectrum.
There has been an exponential growth in microwave-related publications from the first articles involving organic synthesis in 1986 [23]. Early studies used domestic ovens and gave erratic results that are often caused by hot spots, which in some cases led to explosions. A number of companies have now manufactured systems designed for chemistry, using mono-mode microwave generators and laboratory-scale apparatus usually on a scale of 1–100 ml. With these more reliable systems, chemists have reported microwave benefits including decreased reaction times, reduced overall energy consumption, and improved yield and selectivity. Microwave technology is “enabling a wide range of reactions to be performed easily and quickly” [24].
Microwave processing has now been shown to be effective at large continuous processing scale, for example, in waste treatment including food waste gasification. Microwave-assisted organic chemical reaction can be considerably more energy efficient than that using conventional heating [25] and as such is another example of process intensification, especially when combined with flow systems that can help overcome limitations of microwave penetration and allow optimum continuous operation. Furthermore, because of the instantaneous nature of the heating, microwaves offer a major advantage in controllability over conventional heating. Microwave activation of the active center of heterogeneous catalysts has also been proposed [26], but it must be noted however that this is a little understood reaction and no detailed explanation or theories have been proposed.
Microwave reactions have been successfully demonstrated for many different organic reactions including metal-mediated catalysis, cyclo-additions, heterocyclic chemistry, rearrangements, electrophilic and nucleophilic substitutions, and reduction. Many reactions work well in water, adding to the techniques green credentials [27].
1.6 Concluding Remarks
Green chemistry shows many great challenges for the future and many opportunities where technologies such as catalysis can play an important role. It also teaches us that we must only introduce changes in full recognition of the effects across the life cycle including resources and all aspects of the process.
The development of new heterogeneous catalysts, which can be integrated into emerging intensive processes or can be operated with alternative solvents, is critical to establish viable clean technologies for industry. Catalyst design requires improved understanding of the mechanism of catalyzed processes and ability to probe catalyst active sites under operation as is discussed in the subsequent chapters of this book.
References
1. Clark, J.H. (2009) Nat. Chem., 1, 12.
2. RSC Royal Society of Chemistry Green Chemistry Book Series, RSC Publishing http://www.rsc.org/shop/books/series/81.asp (accessed 6 April 2013).
3. Clark, J.H., Deswarte, F.E.I., and Farmer, T.J. (2009) Biofuels, Bioprod. Biorefin., 3, 72.
4. El-Agraa, A.M. (2004) The European Union: Economics and Policies, 7th edn, Prentice Hall, Harlow.
5. Jintao, H. (2007) in his keynote speech at the 17th National Congress of the Communist Party of China (CPC), October 2007.
6. United States of America Policy (2010) President Obama's Development Policy and the Global Climate Change Initiative, 22nd September 2010.
7. Kletz, T. (1998) Process Plants: A Handbook for Inherently Safer Design, Taylor & Francis, London.
8. Bennett, M. and James, P. (eds) (1999) Sustainable Measures, Greenleaf Publishing Ltd., Sheffield.
9. Committee on Industrial Environmental Performance Metrics, National Academy of Engineering, National Research Council (1999) Industrial Environmental Performance Metrics, Challenges and Opportunities, The National Academy Press, Washington, DC.
10. Corporate Environmental Performance 2000 (1999) Strategic Analysis, Vol. 1, Haymarket Business Publications, London.
11. Curzons, A.D., Constable, D.J.C., Mortimer, D.N., and Cunningham, V.L. (2001) Green Chem., 3, 1.
12. Lapkin, A. and Constable, D. (2008) Green Chemistry Metrics, Wiley-VCH Verlag GmbH, Weinheim.
13. Sheldon, R. (2007) Green Chem., 9, 1273.
14. Adams, D.J., Dyson, P.J., and Tavener, S.J. (2004) Chemistry in Alternative Reaction Media, John Wiley & Sons, Ltd, Chichester.
15. Kerton, F. (2009) Alternative Solvents for Green Chemistry, RSC Publishing, Cambridge.
16. Hunt, A.J., Sin, E.H.K., Marriott, R., and Clark, J.H. (2010) ChemSusChem, 3, 306.
17. Petkovic, M., Seddon, K., Rebelo, Z.P., and Pereira, C.S. (2011) Chem. Soc. Rev., 40, 1383–1403.
18. Piechkova, N.V. and Seddon, K. (2008) Chem. Soc. Rev., 37, 123–150.
19. Starks, C.M., Liotta, C.L., and Halpern, M. (1994) Phase-Transfer Catalysis, Chapman & Hall, London.
20. Fadhel, A.Z., Pollet, P., Liotta, C., and Eckert, C.A. (2010) Molecules, 15(11), 8400–8424.
21. Hagen, J. (2006) Industrial Catalysis; À Practical Approach, John Wiley & Sons, Ltd, Chichester.
22. Greenway, G.M., Haswell, S.J., Morgan, D.O., Skelton, V., and Styring, P. (2000) Sens. Actuators, B Chem., 63, 153.
23. Hayes, B.L. (2002) Microwave Synthesis – Chemistry at the Speed of Light, CEM Publishing, Matthews, NC.
24. Leadbetter, N. (2004) Chem. World, 1, 38.
25. Gronnow, M.J., White, R.J., Clark, J.H., and Macquarrie, D.J. (2005) Org. Process Res. Dev., 9, 516.
26. Budarin, V.L., Clark, J.H., Taverner, S.J., and Wilson, K. (2004) Chem. Commun., 23, 2736–2737.
27. Polshettiwar, V. and Varma, R.S. (eds) (2010) Aqueous Microwave Assisted Chemistry, RSC Publishing, Cambridge.
2
Mechanistic Studies of Alcohol Selective Oxidation
Adam F. Lee
2.1 Introduction
Catalytic selective oxidation (selox) is an important class of clean chemical transformations employed in the synthesis of valuable chemical intermediates, and a test bed for many fundamental concepts within heterogeneous catalysis and surface science. The selox of alcohols, carbohydrates, and aromatics is especially challenging in terms of understanding the dynamics of chemical reactions at the liquid–solid–gas interface, and requires new spectroscopic tools and analytical protocols to provide quantitative spatiotemporal information on structure–function relationships in order to optimize reaction conditions and design next-generation selox catalysts. Advances in inorganic methodologies to synthesize tunable nanostructures, and synchrotron science and the parallel development of multidimensional spectroscopies, afford new possibilities for understanding the operation of catalysts under working conditions (operando), and thereby nanoengineering the active site for improved activity, selective, and lifetime in selox chemistry.
2.1.1 Applications of Selective Oxidation
The oxidative dehydrogenation of alcohols represents key steps in the synthesis of aldehyde, ketone, ester, and acid intermediates employed within the fine chemical, pharmaceutical, and agrochemical sectors, with allylic aldehydes in particular high-value components used in the perfume and flavoring industries [1]. For example, crotonaldehyde is an important agrochemical and a valuable precursor for the food preservative sorbic acid, while citronellyl acetate and cinnamaldehyde confer rose/fruity and cinnamon flavors and aromas, respectively. There is also considerable interest in the exploitation of biomass-derived feedstocks such as glycerol (a by-product of biodiesel synthesis from plant or algal triacyl glycerides) for the production of high-value fine chemicals such as dihydroxyacetone, tartronic acid, and mesoxalic acid [2, 3]. Likewise, the selox of hydroxymethyl furfural (HMF), derived from acid-/base-catalyzed cellulose hydrolysis and dehydration, to 2,5-furandicarboxylic acid (FDCA), offers a potential sustainable replacement for terephthalic acid in clothing and plastics [3]. The synthesis of methyl lactate and methyl pyruvate has also been demonstrated from oxidation of 1,2-propanediol [4].
2.1.2 Oxidant Considerations
Molecular oxygen (and air) is overwhelmingly the oxidant of choice for vapor-phase catalytic combustion or partial oxidation of hydrocarbons; although N2O can offer improved selectivity in the oxidative coupling of methane to ethene/ethane [5, 6] and alkene epoxidation [7–9], the associated cost and risk of nitrogen oxide greenhouse gas emissions have prohibited commercialization. In contrast, liquid-phase alcohol oxidations may be driven by a range of oxidants including metal salts [10, 11], t-butyl hydroperoxide [12, 13], and H2O2. Over the past two decades, the popularization of green chemistry ideals has led to a shift away from the use of toxic reagents, notably Cr(VI), as an alcohol oxidant [14, 15]. In addition to safety concerns, such oxidants are also atom inefficient because of the formation of large quantities of metal salt by-products that necessitate expensive separation steps to isolate the desired organic product and associated waste treatment. Explosion hazards also render the large-scale implementation of peroxo-oxidants problematic, while current manufacture is also atom inefficient [16], although in situ H2O2 synthesis [17, 18] and simultaneous utilization for alcohol oxidations [19] may offer a future solution to these issues. A majority of heterogeneously catalyzed selox processes thus employ O2 or air to afford safe, economic, and environmentally benign alcohol selox [20], although this presents new challenges in terms of activating the O=O bond at temperatures typically below 160 °C in a three-phase system, while maintaining high selectivity to aldehydes, ketones, and carboxylic acids against competing combustion and C–C cleavage. Catalyst development is no longer looked upon simply in terms of optimizing atom and energy efficiencies, but as a clean technology, where aspects of the overall process design, such as choice of solvent-free/green solvent operation [21] and methods of catalyst separation and waste disposal, must be considered [22]. Sheldon and coworkers [23, 24] were the first to successfully demonstrate the catalytic aerobic oxidation of diverse alcohols to carbonyl compounds using water-soluble Pd complexes. Several excellent reviews address the early development of platinum group metal (PGM) alcohol selox catalysts [25–28] and broader aspects of hydrocarbon partial oxidation [29]; hence, this chapter focuses more on recent breakthroughs in understanding the underpinning adsorbate–surface interactions, oxidation mechanisms, and parallel tailoring of catalyst structure to optimize the performance.
2.2 Metal-Catalyzed Alcohol Selox
2.2.1 Monometallic Catalysts
Late transition metals, notably gold, palladium, and platinum, have proven to be the most successful heterogeneous catalysts employed for alcohol selox [30]. Size-controlled Pd nanoclusters supported on TiO2, comprising Pd0, Pd+, and Pd2+ centers, are active for alcohol selox using molecular oxygen [31], with cinnamyl alcohol oxidation to cinnamaldehyde favored over 2060-atom Pd clusters with predominantly Pd+ character. Low-loading Pd/MgO catalysts also showed high activity toward a variety of alcohols under mild conditions in the absence of additional acid or base [32]. Particle-size-dependent selox of benzyl alcohol to benzaldehyde has also been reported for Pd clusters dispersed on SiO2–Al2O3 and NaX zeolite supports [33, 34]. The optimum particle size of around 3 nm reported for such benzyl alcohol oxidation implies a structure-sensitive reaction, with edge and corner Pd atoms believed to be more active than terrace sites. Interestingly, geraniol and 2-octanol selox did not show such size-dependent reactivity [34]. However, the impact of the size of Pd nanoparticle on the reactivity of crotyl and cinnamyl alcohols was the subject of a recent systematic investigation [35], wherein no such optimum size was noted, with selox turnover frequencies (TOFs) increasing monotonically with decreasing loading into the sub-1 nm regime. This observation is supported by earlier high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) studies, which demonstrated that even atomically dispersed palladium is extremely active toward aerobic oxidation of crotyl, cinnamyl, and benzyl alcohols with selectivity to their respective aldehydes exceeding 70 % [36].
Although a range of other transition metal nanoparticles, including Ag [37–39], Ru [40], Pt [41], Cu [42, 43], and Mo [44], have been shown to be effective for alcohol selox, a majority of recent studies have focused on gold. While bulk gold is well known for its chemical inertness [45], pioneering work by Haruta and Hutchings has highlighted the unique catalytic properties of nanoparticulate gold, particularly in conjunction with reducible supports such as ceria, iron oxide, and titania [46, 47]. Prati and coworkers [48, 49] were the first to demonstrate the use of supported gold for alcohol selox, in particular for converting diols and sugars into monoacids. Selox of primary alcohols to aldehydes and esters, primarily for the flavorings and fragrance sectors, has also been catalyzed by gold on supports including carbon [50]. Supported gold clusters can exhibit greater selectivity and stability during alcohol and carbohydrate selox than their PGM counterparts [51, 52], although their TOFs are generally lower [53], for example, benzyl alcohol oxidation over 2.5 wt% Au/HAP = 12 400 h−1 versus 2.5 wt% Pd/HAP = 24 800 h−1 at 433 K. Gold on amino-modified fumed silica can efficiently catalyze primary alcohol selox to yield their esters in single step [54]. Using nanocrystalline (5 nm) ceria, Corma and coworkers [55, 56] synthesized gold catalysts that gave superior selectivity for cinnamyl alcohol oxidation to cinnamaldehyde over comparative mono- and bimetallic transition metal catalysts. It is worth mentioning that the mode of preparation of gold catalysts is crucial for determining their subsequent activity, with the deposition-precipitation method producing superior monometallic-supported gold catalysts [46]. Aberration-corrected HAADF-STEM has shed some insight into the nature of the catalytically active Au species when supported over iron oxide [57], with rapid CO oxidation attributed to bilayer clusters of diameter ∼0.5 nm containing around 10 gold atoms. This observation conforms with earlier predictions from gold clusters deposited on ultrathin TiOx films [58].
Unsupported Au nanoclusters (or those contacting an inert support material such as BN) exhibit strong size-dependent reactivity, with optimal oxidation performance typically reached < 5 nm diameter [59]. For example, colloidal gold stabilized by polyvinylpyrrolidone (PVP) shows pronounced size effects in the aerobic oxidation of benzylic alcohols in water under ambient conditions [60]. Figure 2.1 illustrates this phenomenon for p-hydroxybenzyl alcohol oxidation, wherein 1.3 nm Au clusters achieve 80 % conversion, whereas 9.5 nm clusters are catalytically dead. Differential oxygen adsorption onto these gold clusters is believed to play a crucial role in regulating reactivity.
Figure 2.1 Size-dependent selox activity of PVP-stabilized Au clusters in p-hydroxybenzaldehyde production.
(Reprinted with permission from Ref. [60]. Copyright 2005, American Chemical Society.)
For unsupported Au nanoparticles, the choice of stabilizer also influences the resulting catalytic performance [61], with tetrakis(hydroxypropyl)phosphonium chloride (THPC) dramatically enhancing liquid-phase glycerol oxidation compared with poly(vinyl alcohol) (PVA) and citrate ligands (TOF = 2478 h−1 vs 715 and 160 h−1, respectively). This is attributed to both particle-size effects, and more restricted reactant accessibility to the active site when using PVA. Monolithic, nanoporous gold, prepared by selective leaching of Ag from AuAg alloys and visualized in Figure 2.2, also yields an efficient selox catalyst for the oxidative coupling of methanol to methyl formate, reaching 97 % selectivity below 80 °C [62].
Figure 2.2 (a) Scanning electron microscopic (SEM) image and mechanistic cartoon of nanoporous Au catalyst for CH3OH selox to HCO2CH3. (b) Excellent low-temperature performance characteristics.
(From Ref. [62]. Reprinted with permission from AAAS.)
2.2.2 Bimetallic Selox Catalysts
Platinum and palladium are effective catalysts for alcohol oxidation when used alone; however, significant stability and selectivity improvements have been observed on incorporation of a second (usually less active) metal promoter such as Bi, Pb, and Sn [63–65]. These observations are common to aerobic selox of allylic and benzylic alcohols, as well as polyols such as propylene glycol [64] and glycerol [66]. In the case of Bi, in situ X-ray absorption spectroscopy (XAS) and attenuated total reflection infrared spectroscopy (ATR-IR) indicate that the promoter protects Pt against deactivation by overoxidation and prevents site blocking by, for example, aromatic solvents [67].
Recent interest in gold-catalyzed selox has led to the investigation of bimetallic variants, most notably involving gold and palladium [47, 68–70] or platinum [71–73] mixtures. Prati and coworkers [73] first reported significant rate enhancements in the liquid-phase selox of d-sorbitol to gluconic/gulonic acids following Au addition to Pd/C and Pt/C catalysts. This was accompanied by improved resistance to onstream deactivation. Hutchings and coworkers [53] subsequently observed similar rate enhancements for selox of diverse allylic, benzylic, and primary alkyl alcohols on alloying Au with Pd over titania. Their alloy catalysts also retained very high selectivities, characteristic of pure gold clusters. Figure 2.3 shows an HAADF-STEM image of a Au–Pd cluster supported on titania, which outperforms its individual metallic components in benzyl alcohol oxidation. Sol-immobilization was subsequently employed to synthesize more well-defined Au–Pd catalysts on titania [74, 75] for solvent-free oxidation of alcohols. Catalysts prepared by the latter technique showed superior activity at lower metal loadings compared with those prepared by conventional impregnation methods.
Figure 2.3 (a) An HAADF-STEM image of an Au–Pd bimetallic cluster on TiO2. (b) Comparative activity and selectivity of Au/TiO2, Pd/TiO2, and Au–Pd/TiO2 in solvent-free benzyl alcohol oxidation.
(From Ref. [53]. Reprinted with permission from AAAS.)
The influence of varying Au–Pd compositions on catalytic selox has been explored for PVP-stabilized mono- and bimetallic nanoparticles [76]. An optimum Au:Pd composition of 1 : 3 was found for 3 nm particles in the aqueous-phase aerobic oxidations of benzyl alcohol, 1-butanol, 2-butanol, 2-buten-1-ol, and 1,4-butanediol, with the bimetal system possessing better activity than Au or Pd nanoparticles alone. Mertens and coworkers [77] also studied benzyl alcohol oxidation over PVP-stabilized Au–Pd clusters. In this case, the best catalysts comprised clusters of ∼1.9 nm diameter containing 80 % Au. PVP-stabilized bimetallic Au–Pd nanoparticles prepared by coreduction or sequential reduction strategies have also been investigated for the room-temperature aerobic oxidation of crotyl alcohol to crotonaldehyde by Scott and coworkers [78]. The chemical, structural, and electronic properties of these nanoparticles were investigated by XAS, which for the sequentially reduced bimetallic system indicated an Au core/Pd shell structure in which the palladium was heavily electronically perturbed relative to monometallic Pd or coreduced Au–Pd nanoparticles. This Pd surface-enriched core–shell nanostructure was extremely active toward (base-free) oxidation of crotyl alcohol to crotonaldehyde with ∼90 % selectivity, and a surface redox mechanism postulated involving an active Pd(II) species (Figure 2.4).
Figure 2.4 Proposed mechanism for the base-free, aqueous selective aerobic oxidation of crotyl alcohol to crotonaldehyde over PVP-stabilized Au core/Pd shell nanoparticles highlighting the importance of Pd2+ centers.
(Reprinted with permission from Ref. [78]. Copyright 2011, American Chemical Society.)
Similar synergic Au–Pd interactions were reported for bimetallic Au–Pd catalysts supported on polyaniline (PANI) [79] for benzyl alcohol oxidation. Here, the colloidal preparative route provided a narrow particle size distribution (∼3 nm) with a Pd-rich shell encapsulating an Au-rich core. In this instance, the optimal composition was Au:Pd = 1 : 9. Benzyl alcohol oxidation has likewise been studied over an Au–Pd/TiO2 catalyst in which the deposition-precipitation method was used to improve the particle size distribution and activity versus wet impregnation [80]. In contrast, liquid-phase oxidation of cinnamyl, benzylic, octenol, and octenal alcohols using Au–Pd-activated carbon indicated that sequential metal deposition was an effective means to generate highly active catalysts with wide-ranging Au:Pd compositions [72, 81, 82]. Benzyl alcohol oxidation has also been tested over ceria-supported Au–Pd, in which the ceria was prepared by an antisolvent precipitation technique using supercritical CO2 (scCO2). This scCO2 method offers greener catalyst synthesis and gives more active materials compared to conventional ceria because of increased metal dispersion [83].
Recent studies of titania-supported Au shell (five-layer)–Pd core (20 nm) bimetallic nanoparticles utilizing in situ X-ray photoelectron spectroscopy (XPS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), extended X-ray absorption fine structure (EXAFS), and X-ray diffraction (XRD) and ex situ high-resolution transmission electron microscopy (HRTEM) showed limited Au–Pd intermixing, and particle growth occurs below 300 °C [84]. Higher temperatures promoted alloying, accompanied by significant sintering and surface roughening. These structural changes were mirrored by the nanoparticle-catalyzed liquid-phase-selective aerobic oxidation of crotyl alcohol to crotonaldehyde. Figure 2.5 shows how Pd surface segregation enhanced both activity and selectivity, with the most active (and selective) surface alloy attainable containing ∼40 atom% Au.
Figure 2.5 (a) HRTEM images of Au–Pd/TiO2 nanoparticles as a function of annealing (inset shows fcc Au lattice fringes). (b) Influence of Au/Pd surface alloying on crotyl alcohol-selective oxidation to crotonaldehyde.
(Reprinted from Ref. [84]. Copyright 2010, with permission from Elsevier.)
Additional fundamental insight into the synergy between Au and Pd has been derived from temperature-programmed reaction mass spectrometry studies of crotyl alcohol [85, 86], and its reaction products, crotonaldehyde and propene [87], over Au/Pd(1 1 1) model single-crystal catalysts. These demonstrated that Au incorporation suppressed the decomposition of alcohol reactant and aldehyde product, favoring desorption of the desired crotonaldehyde product. Surprisingly, preadsorbed oxygen actually promoted crotonaldehyde desorption, preventing its subsequent decarbonylation (and combustion) to propene (Figure 2.6).
Figure 2.6 Impact of coadsorbed oxygen and surface-alloyed Au on the decomposition of crotonaldehyde and propene over a Pd(1 1 1) model catalyst determined by temperature-programmed reaction spectrometry.
(Adapted from Ref. [87] by permission of the PCCP Owner Societies.)
2.2.3 Support Effects
Gold nanoclusters on gallia polymorphs have also been proven to be versatile green catalysts for the oxidative esterification of several alcohols [88, 89]. Early work by Hutchings [90] demonstrated the potential of dispersed gold to selectively oxidize alcohols under solvent-free conditions, an important step in achieving a green process. Gold on Cs2CO3 also exhibits excellent selectivity for the room-temperature oxidation of primary alcohols to their corresponding aldehyde, reducing the net energy input [91]. Ethanol selox to acetaldehyde can also be undertaken using Au on oxide supports, with high initial rates attained over TiO2, ZnO, and Al2O3, and higher selectivity over TiO2 and ZnO [92]. A comparison of the catalytic activity of Au/TiO2 and Au/MgAl2O4 for ethanol oxidation to acetic acid suggested that titania outperformed the spinel support [93]. Au/TiO2 systems can affect direct transformation of primary alcohols to their corresponding methyl esters under aerobic conditions [94]. More exotic supports, for example, U3O8, have also been investigated for Au nanoparticles; deposition–precipitation results in an excellent catalyst for the solvent-free oxidation of benzyl alcohol to benzaldehyde (although in this instance the oxidant was tert-butyl hydroperoxide) [95].
Mesoporous aluminas are hypothesized as efficient supports in terms of catalysis for anchoring metal nanoparticles. They offer higher stability and dispersion of the nanoparticles apart from facilitating easy diffusion of reactants and product molecules. Hackett and coworkers [36] recently showed that low Pd concentrations dispersed across high-area (350 m2 g−1) mesoporous alumina show exceptional activity toward allylic alcohol selox. Specifically, production of crotonaldehyde and cinnamaldehyde is enhanced 10-fold over that achievable using a conventional γ-alumina support (Figure 2.7), which is attributed to the greater dispersion achievable over the higher area alumina [96], and its ability to stabilize single-site Pd2+ catalytic centers. Amorphous silica and mesoporous SBA-15, SBA-16, and KIT-6 have also been employed to support Pd nanoparticles for alcohol selox [35, 97, 98]. These catalysts show high thermal stability and recyclability, and careful comparison of initial rates and evaluation of surface metal and oxide contents by XPS and CO chemisorption provided conclusive proof that surface PdO was the active site, independent of silica support architecture [35].
Figure 2.7 Crotyl alcohol selox over Pd/Al2O3 is a strong function of support morphology and Pd oxidation state, with atomically dispersed Pd2+ centers obtained over mesoporous alumina offering maximum crotonaldehyde production.
(Adapted with permission from Ref. [36]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA.)
Hydroxyapatite (Ca10(PO4)6(OH)2) has also attracted considerable interest as a catalyst support. In these materials, wherein Ca2+ sites are surrounded by PO43− tetrahedra, the introduction of transition metal cations such as Pd into the apatite framework can generate stable monomeric phosphate complexes that are efficient for aerobic selox catalysis [99]. Carbon-derived supports have also been utilized for this chemistry, and are particularly interesting because of the ease of precious metal recovery from spent catalysts simply by combustion of the support. Carbon nanotubes (CNTs) have received considerable attention in this latter regard because of their superior gas adsorption capacity. Palladium nanoparticles anchored on multiwalled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs) show better selectivity and activity for aerobic selox of benzyl and cinnamyl alcohols [100, 101] compared to activated carbon. Interestingly, Pd supported on MWCNTs showed higher selectivity toward benzaldehyde, whereas activated carbon was found to be a better support in cinnamyl alcohol oxidation. Functionalized polyethylene glycol (PEG) has also been employed successfully as a water-soluble, low-cost, recoverable, non-toxic, and non-volatile support with which to anchor nanoparticulate Pd for selox catalysis of benzyl/cinnamyl alcohols and 2-octanol [102–104].
Micro- and mesoporous oxide architectures possessing regular channels and high surface areas have been widely used as scaffolds for metal nanoparticles, limiting metal agglomeration and associated deactivation. Gold nanoparticles confined within SBA-15 and GMS mesoporous silicates are very selective for the selective aerobic oxidation of benzyl alcohol to benzaldehyde [105, 106]. Kim et al. [107] adopted a different approach using aluminum tri-sec-butoxide and pluronic surfactants in a one-pot synthesis of 10 nm gold clusters embedded within mesostructured aluminum oxyhydroxide fibers. The latter catalysts were efficient for aerobic oxidation of diverse primary and secondary alcohols under ambient conditions, and demonstrated good reusability without significant activity loss. Most recently, 3D tomography was used to show that bottom-up synthetic approaches confer both great thermal stability on 1–2 nm sized Au nanoparticles embedded within silicalite-1 and substrate size selectivity in the aerobic oxidation of benzaldehyde derivatives [108]; it remains to be seen whether restricted substrate access through these zeolite micropores promotes similar enhanced size-/shape-selective catalysis in alcohol oxidations. Microgels have recently been used as an alternative matrix for growing and supporting noble metal nanoparticles with the resulting tailored catalysts using much promise in aerobic oxidations [109, 110]. Gold on a methylthio-ethyl methacrylate (MTEMA)-based microgel exhibits comparable activities to more common oxide-supported systems. These Au microgels are quasi-homogeneous, enabling reactions to be carried out in water, while facilitating efficient catalyst separation.
2.3 Oxide, Sulfide, and Vanadate Catalysts
A range of inorganic compounds have also been explored as alternatives to transition metal nanoparticle catalysts for alcohol selox. Mixed metal oxides of Mo and V have shown much promise in this regard, but remain less active than their PGM counterparts. Wang and coworkers [111–114] recently demonstrated that crystalline Mo–V–O with an orthorhombic structure can efficiently catalyze benzyl alcohol oxidation to benzaldehyde using either dioxygen or air. Such mixed metal oxides can be reused without appreciable selectivity loss. In an interesting study, Bowker and coworkers [115, 116] examined the oxidative dehydrogenation of methanol to formaldehyde over MoO2, MoO3, and iron molybdates. They identified Mo6+ as a crucial factor for high formaldehyde selectivity, with Mo4+ favoring CO production and selox proceeding via a Mars–van Krevelen mechanism.
Supported oxides and hydroxide nanoparticles have also been employed for selox [107, 117, 118]. Zeolite-confined nano-RuO2 exhibits excellent activity and selectivity toward aldehydes and ketones without employing sacrificial reductants or cocatalysts [117]. Titania-supported nano-PdO is another promising candidate for benzyl alcohol selox, which demonstrates excellent activity [118]. Vanadyl sulfates have recently been shown as efficient catalysts for benzyl alcohol oxidation in the presence of trace water [119].
2.4 Solvent Selection
One of the biggest challenges in oxidation catalysis is the development of clean technologies that can operate without standard laboratory organic solvents [120]. Here, we describe some of the solutions to this problem.
2.4.1 Supercritical Fluids
scCO2 has found widespread appeal as a green solvent because of its low cost and toxicity, ready availability, excellent heat capacity and miscibility with a wide range of polar and apolar molecules, including dioxygen, and simple and efficient recyclability. The use of scCO2 has been recently reviewed by Seki and Baiker [121]; hence, we only highlight selectively from this field. There are several reports related to Pd-nanoparticle-catalyzed alcohol selox in scCO2 [122–124]. Biphasic aerobic oxidation of 3-methyl-2-butene-1-ol (and a range of allylic and benzylic alcohols) over Pd/PEG was recently found to be very efficient under continuous-flow operation [125] (Figure 2.8). It is proposed that the PEG support prevents aggregation and deactivation of the most active “giant” 561-atom Pd clusters, while scCO2 provides a safe working environment for using high molecular O2 pressures.
Figure 2.8 Continuous-flow, aerobic biphasic selox of 3-methyl-2-butene-1-ol catalyzed by PEG-supported Pd561 clusters under scCO2. Induction period during the first run may be associated with Pd redispersion.
(Reproduced with permission from Ref. [125]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA.)
Chapman and coworkers also recently designed a miniature scCO2 reactor for continuous selox of secondary alkyl and benzyl alcohols, and 1-octanol, using a Pt–Bi/Al2O3 catalyst. This system has achieved good yields and mass balances, indicating limited combustion [126].
2.4.2 Ionic Liquids and Water
The application of ionic liquids in catalysis has been reviewed by Hardacre and Parvulescu [127]; hence, only selected examples are discussed herein. Molten salts have been used as an “immobilization” medium for homogeneous selox catalysis [128], for example, RuCl3 [129], Cu(ClO4)2 [130], or CuCl [131] (the latter in conjunction with molecular sieve 3A as a heterogeneous solid acid promoter). Van Doorslaer et al. [132] also reported the use of imidazolium ionic liquids for Pd(II)-acetate-catalyzed oxidation of alcohols to ketones, wherein, for example, 1-butyl-3-methylimidazolium tetrafluoroborate helped to expel products from the catalyst mixture. Ionic liquids have also been used to modify catalyst supports. For example, molybdovanadophosphoric acid immobilized on imidazolium-modified SBA-15 exhibited high activity in the aerobic oxidation of benzylic, allylic, and aliphatic alcohols with limited overoxidation [133]. The presence of ionic liquids also improved the photocatalytic activity of 12-tungstophosphoric acid on MCM-41 for alcohol selox [134]. Kantam and coworkers [135] found that basic choline hydroxide helped to stabilize Ru on nanocrystalline MgO, improving the catalyst reuse in primary and secondary alcohol seloxes over several cycles.
Water is usually considered the most environmentally benign solvent; however, the poor solubility of bulkier alcohols and molecular oxygen under ambient conditions has limited its application (in the absence of surfactants [25]). Pt nanoparticles supported on a water-soluble anion exchange resin exhibit excellent E-factors (12.8 kilo waste per kilo product) in addition to very good activity and selectivity for alcohol oxidation [136]. Similarly, water has been used as solvent for, for example, benzylic and primary alcohol oxidations over supported or stabilized Pd [137], Pt [138], and Au [139] clusters.
2.5 In Situ and Operando X-Ray Studies of Selox Catalysts
2.5.1 X-Ray Absorption Spectroscopy
Fundamental understanding of structure–function relationships is central for the design of improved selox catalysts, and has been greatly assisted by the development of new analytical tools with which to probe active sites at subnanometer spatial resolution [36] and subsecond time resolution. X-ray-based methods in particular can provide detailed insight into chemical composition and environment of active components and reacting adsorbates [140–142]. Quick and dispersive XAS have the capability to monitor dynamic changes in catalyst structure under reaction conditions (so-called operando spectroscopy) and have been applied to alcohol selox over Pd [96, 143–146], Pt [67, 147], and Ru [147] nanoparticles.
A combined in situ XAS/Fourier transform infrared (FT-IR) study by Grunwaldt and coworkers [148] examined the nature of the active phase of Pd during benzyl alcohol oxidation. X-ray absorption near-edge spectra suggested that palladium remained in metallic form throughout the catalytic cycle. In contrast, operando XAS and ex situ XPS of a series of Pd/γ-alumina catalysts demonstrated that the oxidation rates of crotyl and cinnamyl alcohols were proportional to the palladium dispersion and concentration of surface palladium oxide (Figure 2.9) [96]. On-stream reduction of supported Pd nanoparticles has been shown to strongly correlate with deactivation possibly because of irreversible adsorption of strongly adsorbed by-products over metallic palladium sites [144]. Selox of benzyl alcohol oxidation over Pd/Al2O3 under scCO2 conditions also suggests that Pd is partially oxidized under reaction conditions [149]. Interestingly, no deactivation was observed on increasing the O2 concentration under these scCO2 conditions, suggesting palladium over-oxidation is not detrimental to selox. A parallel operando study of cinnamyl alcohol selox under scCO2 confirmed that reactant-induced reduction was an important process, and that oxygen supply to the catalyst was mass-transport limited [150].
Figure 2.9 Evidence that surface PdOx catalyzes the aerobic selective oxidation of allylic alcohols over Pd/Al2O3: (a) Strong dependence of TOF on surface oxide concentration and (b) in situ reduction of active PdO phase accompanying onstream deactivation.
(Adapted from Refs [96] and [144] by permission of the Royal Society of Chemistry.)
A related XAS investigation of untreated and Bi-promoted Pd/Al2O3
