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Supported Catalysts and their Applications
By D.C. Sherrington, A.P. Kybett The Royal Society of Chemistry
Copyright © 2001 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-880-9
CHAPTER 1
SELECTIVITY IN OXIDATION CATALYSIS
B. K. Hodnett
Department of Chemical and Environmental Sciences and The Materials and Surface Science Institute University of Limerick, Limerick, Ireland
ABSTRACT
Selectivity in oxidation catalysis has been reviewed for conventional catalysts used for the production of bulk chemicals and epoxidations. The point of activation of the substrate is identified as a key factor identifying three mechanistic features. These are (i) activation of the weakest C-H bond in a substrate, (ii) activation of the strongest C-H bond and (iii) electrophilic attack in olefins. Key features of each type of reaction are identified and new catalyst types needed to break through existing selectivity barriers are discussed.
1 INTRODUCTION
It has been established for some time that the chemical structure of substrates (reactants) is important in determining reactivity over heterogeneous catalysts. Yao' established the following order of reactivity for alkane total oxidation over supported platinum catalysts n - C4,H10 > C3 H8 > C2H6 > CH4 (> more reactive) and there is a body of work which indicates that C-H bond strength is an important factor in determining reactivity; molecules with weak C-H bonds tend to be more reactive.
The situation with respect to selectivity is less clear. Some examples of selective oxidation catalysts used in commercial practice are listed in Table 1. A consistent feature is that many oxidation catalysts are not highly dispersed when viewed on the atomic scale. Hence particle sizes tend to be large, even for supported precious metal catalysts. This feature, in turn, has led to descriptions of active site structures on these catalysts that are extensions of bulk structures.
2 SUBSTRATE ACTIVATION BY C-H BOND RUPTURE
The term activation is often used in relation to hydrocarbon reactivity over heterogeneous catalysts. Here, it is defined as identifying the primary point of attack on a reacting molecule. Literature evidence relating to kinetic isotope effect (KIE) studies of selective oxidation and ammoxidation reactions are listed in Table 2. In each case, a KIE is observed only in relation to a specific C-H bond in the substrate. For example, in n-butane oxidation to maleic anhydride, a KIE is observed only when the methylene (-CH2-) hydrogens are replaced by deuterium, consistent with these C-H bonds being the point of activation in n-butane and their rupture being the slow step in the overall reaction. Further analysis of these results indicate that the point of activation is the weakest C-H bond available in substrate. Individual C-H bond strengths are annotated in Column 2 of Table 2.
This activation feature identifies one class of selective oxidation catalyst namely, those that activate the substrate through rupture of the weakest C-H bond. The performance of these selective oxidation catalysts is best presented in terms of selectivity-conversion plots. Using this approach, multiple selectivity-conversion plots can be generated, such as that shown in Figure 1 for isobutene and isobutane oxidation to methacrolein. These plots are intended to illustrate that there exists in relation to each selective oxidation reaction an upper performance limit beyond which experimental studies have not yet progressed.
Consideration of this mechanism leads logically to the conclusion that if activation in the direction of selective oxidation results from rupture of the weakest C-H bond in the substrate, then the selective oxidation product so formed must be subject to attack (destruction) via the same process. In general, rupture of any bond in the selective oxidation product would lead to its destruction. Hence, the function DοHC-H REACTANT - Dο HC-H or C-C PRODUCT, namely the difference in bond strengths between the weakest C-H bond in the reactant and the weakest bond in the product has been evaluated for 24 oxidation reactions. Figure 2 presents a plot of the selectivity at 30% conversion for a wide range of oxidation reactions against the function DοHC-H REACTANT - Dο HC-H or C-C PRODUCT. The point zero on this scale represents the situation where the weakest C-H bond in a given substrate has the same bond strength as the weakest bond in the selective oxidation product. The data in Figure 2 clearly shows that active sites in conventional oxidation catalysts are capable of selectively activating a C-H bond in a substrate in the presence of similar bonds in the selective oxidation product provided that there is no bond in the product with a bond strength less than 30-40 kJ mol-1 of the value for the weakest C-H bond in the substrate.
In recent years, a new class of commercial oxidation catalyst has emerged, namely the Fe-ZSM5 catalsts used for phenol production from benzene using nitrous oxide as oxidising agent. This system is said to generate the so-called a-oxygen species. Since this is a zeolite based catalyst in which diffusion limitations can normally be expected, kinetic isotope effect studies are not useful. However, the a-oxygen does appear to have a different reactivity pattern to conventional oxidation catalysts. In a study of the reactivity of a-oxygen towards isopropylbenzene the product distribution shown in Figure 3 was observed, namely that the preferred point of activation of the hydrocarbon is the strongest available C-H bond. This feature identifies a second class of selective oxidation reaction, much less common, namely where the preferred point of activation of the hydrocarbon is the strongest C-H bond in the structure.
3 OLEFIN EPOXIDATION
Olefin epoxidation may be viewed as a third class of selective oxidation reaction in that it does not involve C-H bond rupture in the substrate. A feature of commercial operation of this type of chemistry has been the use of silver catalysts for ethylene epoxidation by oxygen. Another consistent feature is the inability of this same catalyst system to epoxidize propene. Indeed further analysis of a range of substrates over the silver-oxygen system, some of which are presented in Table 3, indicates that substrate structure is important in determining selectivity in epoxidation. Good selectivities in the silver-oxygen system are possible only for substrates without allylic C-H bonds. Hence, in Table 3, 1-3 butadiene and styrene can be selectively epoxidized in the silver-oxygen system but propene and 1-butene cannot. This data is further analysed in Figure 4 which plots selectivity to epoxide for a range of olefin substrates against the bond dissociation enthalpy of the weakest C-H bond in the olefin. For the silver-oxygen system, the presence of a C-H bond in the olefin with a bond energy below 400 kJ mol-1 leads to a very low selectivity, presumably because of activation of a weak C-H bond rather than by electrophilic attack at the double bond.
The situation when a TS-1 peroxide catalyst system is used is entirely different. The temperatures involved are lower and the oxidizing species involved here appears to be much more electrophilic and capable of epoxidizing those substrates in Table 3 (propene and 1-butene) where the silver-oxygen system failed. When the selectivity to epoxide is plotted against the weakest C-H bond in the olefin (Figure 4), there is a clear increase in the range of application of this system over the silver-oxygen system with the oxidizing species on TS-1 being capable of electrophilic attack even when very weak bonds (340 kJmol-1) are present in the olefin.
Clearly, the level of sophistication involved in the TS-1 catalyst is greater than that involved with the other catalyst systems listed in Table 1. The generation of 2.5 mol% titanium in solid solution in silicalite makes for a very dilute system with a limited number of active sites per unit volume. However, this approach seems to be necessary to expand the range and applicability of selective oxidation catalysis.
4 CONCLUSIONS
Selective oxidation has been reviewed and points to a mature technology associated with conventional selective oxidation catalysts where substrate activation occurs via the weakest C-H bond. Discriminating capacity and selectivity of active sites on these catalysts is limited to being able to activate a C-H bond in a substrate that is 30-40 kJmol-1 weaker than a similar bond in the selective oxidation product. There are a number of emerging iron-based systems where the strongest C-H bond in a given substrate is activated. Selectivity in olefin epoxidation is related to competition between electrophilic attack and C-H bond rupture. The more electrophilic nature of the oxidizing species in the TS-1 peroxide system gives it a much greater range of applicability by comparison with the silver-oxygen system.
CHAPTER 2
THE DEVELOPMENT AND APPLICATION OF SUPPORTED REAGENTS FOR MULTI-STEP ORGANIC SYNTHESIS
Steven V. Ley and Ian R. Baxendale
Department of Chemistry University of Cambridge Lensfield Road Cambridge, CB2 1EW, UK
1. Introduction
Synthetic organic chemistry is a continuously evolving subject with new techniques, reactions and methods being developed at an ever increasing rate. In an era when the world is becoming increasingly aware of the limits of its natural resources and the environmental impact of disposing of waste materials, the chemical industries are under considerable pressure to discover, develop and utilise more efficient manufacturing protocols. The areas which have seen the most change in recent years have been the pharmaceutical and agrochemical sectors. These communities are constantly seeking new ways to meet the demand for new, diverse and structurally interesting molecules for biological evaluation. Their traditional approaches to lead compound discovery and optimisation have been both expensive and time consuming. The challenge is therefore to find more efficient and cost-effective methods to produce an ever-increasing number of chemical entities as quickly and as cleanly as possible. This has led to the emergence of combinatorial chemistry and related automation technologies as essential components of the discovery process.' Owing to the development of high throughput screening techniques, the speed of biological evaluation of potential drug candidates has increased dramatically. In order to match these advances it is necessary to develop suitable protocols for the fast and efficient generation of chemical libraries. These libraries of small molecules have normally been prepared either in solution or assembled on solid support. The greater flexibility offered by solution phase chemistry is outweighed by the need for time consuming work-up and purification of the individual library components. As a consequence, solid supported reagents have been developed and are becoming increasingly popular since they combine the advantages of polymer-supported chemistry with the versatility of solution phase reactions, allowing clean reactions and removal of contaminating by-products by simple filtration.
2. Polymer-supported reagents
The concept of immobilising reagents on a support material is not new; catalytic hydrogenation and numerous other processes that occur on a solid surface can be classified as examples of supported-reagent systems. It is conceivable that with the appropriate choice of support a diverse variety of reagents could be tethered. Indeed, not only have supported variants of many commonly used reagents been prepared, but also a growing number of scavenging agents capable of sequestering unwanted by-products and excess reactants from solution have also been described. A typical example of how these concepts work in practice to give clean products is shown in Scheme 1. Although the idea of using solid-supported reagents has been known for a long time their specific application in the generation of large chemical arrays via organised multi-step syntheses has to date been little explored. Studies such as these are required to demonstrate the full range of advantages that these reagents offer such as ease of handling, low toxicity and simple reaction monitoring. Furthermore, the increased speed of purification gained by their use means that this application of chemistry is of special significance in multi-parallel syntheses. We describe below some of our efforts in this area and illustrate how these methods have a broad ranging potential for organic synthesis in the future.
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Excerpted from Supported Catalysts and their Applications by D.C. Sherrington, A.P. Kybett. Copyright © 2001 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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