Heterogeneous catalysis with homogeneous performance

Reactions Catalyzed by Solid-Supported IL Heterogeneous Catalysis with Homogeneous Performance... 

The study of catalytic polymerization of olefins performed up to the present time is certain to hold a particular influence over the progress of the concepts of the coordination mechanism of heterogeneous catalysis. With such an approach the elementary acts of catalytic reaction are considered to proceed in the coordination sphere of one ion of the transition element and, to a first approximation, the collective features of solids are not taken into account. It is not surprising that polymerization by Ziegler-Natta catalysts is often considered together with the processes of homogeneous catalysis. 

We need to develop methods to understand trends for complex reactions with many reaction steps. This should preferentially be done by developing models to understand trends, since it will be extremely difficult to perform experiments or DFT calculations for all systems of interest. Many catalysts are not metallic, and we need to develop the concepts that have allowed us to understand and develop models for trends in reactions on transition metal surfaces to other classes of surfaces oxides, carbides, nitrides, and sulfides. It would also be extremely interesting to develop the concepts that would allow us to understand the relationships between heterogeneous catalysis and homogeneous catalysis or enzyme catalysis. Finally, the theoretical methods need further development. The level of accuracy is now so that we can describe some trends in reactivity for transition metals, but a higher accuracy is needed to describe the finer details including possibly catalyst selectivity. The reliable description of some oxides and other insulators may also not be possible unless the theoretical methods to treat exchange and correlation effects are further improved. 

Pioneering works in this area started in the 1960s by polymer scientists like D. Ballard at ICI and Y. Yermakov at the Novosibirsk Institute of Catalysis [54,55]. The need for polymers with better properties and for better technologies (gas phase processes) has led to the development of various strategies to obtain supported catalytic systems [56-58]. Here, we will concentrate on reactions leading to basic chemicals, and a comparison between homogeneous and heterogeneous catalysts will be performed when possible. 

In heterogeneous catalysis reactants have to be transported to the catalyst and (if the catalyst is a porous, solid particle) also through the pores of the particle to the active material. In this case all kinds of transport resistance s may play a role, which prevent the catalyst from being fully effective in its industrial application. Furthermore, because appreciable heat effects accompany most reactions, heat has to be removed from the particle or supplied to it in order to keep it in the appropriate temperature range (where the catalyst is really fully effective). Furthermore, heterogeneous catalysis is one of the most complex branches of chemical kinetics. Rarely do we know the compositions, properties or concentrations of the reaction intermediates that exist on the surfaces covered with the catalytically effective material. TTie chemical factors that govern reaction rates under these conditions are less well known than in homogeneous catalysis. Yet solid catalysts display specificities for particular reactions, and selectivity s for desired products, that in most practical cases cannot be equaled in other ways. Thus use of solid catalysts and the proper (mathematical) tools to describe their performance are essential. 

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