Principles of catalyst design

J. T. Richardson, Principles of Catalyst Design, Plenum Press, New Yrak, 1989. 

After 30 years of effort, we can now safely say that shape selective catalysis has established itself as a new and continually evolving branch of heterogeneous catalysis. I would like to give you a personal view of the major advances made with respect to the general principles of catalyst design and the industrial applications of shape selective catalysis. 

In this chapter we shall iUustrate the above discussion by dealing with the evolution reaction in some detail, and then consider some general principles of catalyst design and other reactions more briefly. 

In this chapter, general principles of catalyst design comprising composition (active components, supports), preparation, conditioning, and testing for screening of different materials are summarized. In this way, the reader is introduced to the challenges associated with catalyst development. 

Recent sol-gel methods have been recognized as promising procedures to prepare catalysts [12-14]. The sol-gel methods allows a unique way of catalyst design, because they represent an ab initio synthesis of the final solid from well defined molecular compounds [13]. By suitable choice of reagents, reaction and drying conditions, such technique allows to predefine pore structure, porosity, composition, surface polarity and crystallinity or amorphicity of metal oxides [12]. In principle, any metal that forms stable oxides can be forced to copolymerise with other metals in sol-gel procedures to provide mixed metal oxides [13]. 

This chapter is devoted entirely to performance models of conventional catalyst layers (type I electrodes), which rely on reactant supply by gas diffusion. It introduces the general modeling framework and employs it to discuss the basic principles of catalyst layer operation. Structure-based models of CCL rationalize distinct regimes of performance, which are discernible in polarization curves. If provided with basic input data on structure and properties, catalyst layer models reproduce PEFC polarization curves. Consistency between model predictions and experimental data will be evaluated. Beyond polarization curves, performance models provide detailed maps or shapes of reaction rate distributions. In this way, the model-based analysis allows vital conclusions about an optimal design of catalyst layers with maximal catalyst utilization and minimal transport losses to be drawn. 

These books in the series will deal with particular techniques used in the study of catalysts and catalysis these will cover the scientific basis of the technique, details of its practical applications, and examples of its usefulness. The volumes concerned with an industrial process or a class of catalysts will provide information on the fundamental science of the topic, the use of the process or catalysts, and engineering aspects. For example, the inaugural volume. Principles of Catalyst Development, looks at the science behind the manufacture of heterogeneous catalysts and provides practical information on their characterization and their industrial uses. Similarly, an upcoming volume on ammonia synthesis will extend from the surface science of single iron crystals to the design of reactors for the special duty of ammonia manufacture. It is hoped that this approach will give a series of books that will be of value to both academic and industrial workers. 

The principles needed to design a polymer of low flammability are reasonably well understood and have been systematized by Van Krevelen (5). A number of methods have been found for modifying the structure of an inherently flammable polymer to make it respond better to conventional flame retardant systems. For example, extensive work by Pearce et al. at Polytechnic (38, 39) has demonstrated that incorporation of certain ring systems such as phthalide or fluorenone structures into a polymer can greatly increase char and thus flame resistance. Pearce, et al. also showed that increased char formation from polystyrene could be achieved by the introduction of chloromethyl groups on the aromatic rings, along with the addition of antimony oxide or zinc oxide to provide a latent Friedel-Crafts catalyst. 

These Mo catalysts with a C2-tether connecting the phosphine and cyclopenta-dienyl ligand provide an example of the use of mechanistic principles in the rational design of improved catalysts, in this case based on information about a decomposition pathway for the prior generation of catalysts. The new catalysts offer improved lifetimes, higher thermal stability, and low catalyst loading. The successful use of a triflate counterion and solvent-free conditions for the hydrogenation are additional features that move these catalysts closer to practical utility. 

A very elegant solution to solve this problem is the introduction of either a permanent or a temporary phase boundary between the molecular catalyst and the product phase. The basic principle of multiphase catalysis has already found implementation on an industrial scale in the Shell higher olefin process (SHOP) and the Ruhrchemie/Rhdne-Poulenc propene hydroformylation process. Over the years, the idea of phase-separable catalysis has inspired many chemists to design new families of ligands and to develop new separation... 

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