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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Crabtree, R.H. (ed.)
Handbook of Green Chemistry - Green Catalysis
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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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We dedicate this book to the memory of Rodney Stewart Lee.
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
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].
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.
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.
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.
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.
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.
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.
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].
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.
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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.
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].
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.
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.)
−1−1Figure 2.2