Heterogeneous catalysis selective catalysts

In heterogeneous catalysis, the catalyst often exists in clusters spread over a porous carrier. Experimentally, it is well established that reactivity and selectivity of heterogeneous reactions change enormously with cluster size. Thus, theoretical studies on clusters are particularly important to establish a basis for the determination of their optimal size and geometry. Cluster models are also important for studying the chemistry and reactivity of perfect crystal faces and the associated adsorption and desorption processes in heterogeneous catalysis (Bauschlicher et al, 1987). 

Key Words Ethylene oxide, Ethylene, Epoxidation, Silver, Cl promotion, Cs promotion. Promotion, Selectivity, Oxametallacycle, Adsorption, Desorption, Chemisorption, Activation energy, Ag-O bond. Reaction mechanism, Oxidation, Cyclisation, Heterogeneous catalysis, Selective oxidation, Eletrophilic oxygen. Nucleophilic oxygen. Subsurface O atoms, Ag/a-A Oj catalyst. 2008 Elsevier B.V. 

Most often, in the heterogeneous catalysis the catalysts are soUds — usually metals, very often precious metals — while the reactants are liquid or gaseous. The catalysts are usually composed of a number of (metal) components in various combinations. The tests are performed in parallel under the otherwise equal temperature and pressure conditicms in a large number of microreactors. fii each microreactor the combination of the tested catalyst components is different and this results in a different yield or selectivity of the test reactirm. This is usually evaluated by composition analysis of the reaction products. 

Heterogeneous Catalysis, Selected Case Histories, Ed. by B. H. Davis and W. P Hettinger, (C. Plank, The Invention of Zeolite Catalytic Cracking Catalysts, p 253), ACS Symposium Series 222 (1983). 

The potential for the use of catalysis in support of sustainability is enormous [102, 103]. New heterogeneous and homogeneous catalysts for improved reaction selectivity, and catalyst activity and stabihty, are needed, for example, new catalytic materials with new carbon modifications for nanotubes, new polymers. 

A very important part of such an undertaking is to be clear about what stages of a chemical process generate the most waste. Often this is found to be the separation stage, after the transformation of reactants to products, where all the various components of the final mixture are separated and purified. Approaches to chemical reactions which help to simplify this step are particularly powerful. Such an approach is exemplified by heterogeneous catalysis. This is an area of chemistry where the catalysts used are typically solids, and the reactants are all in the hquid or gas phase. The catalyst can speed up the reaction, increase the selectivity of the reaction, and then be easily recovered by filtration from the liquid, and reused. 

In catalysis active sites are operative that allow for an alternative reaction path. For a satisfactory catalyst this alternative pathway leads to higher rates and higher selectivity. In heterogeneous catalysis reactant molecules adsorb at active sites on the catalyst surface at the surface sites reactions occur and products are desorbed subsequently. After desorption, active sites are again available for reactant molecules and the cycle is closed. In homogeneous catalysis the situation is essentially identical. Here complexation and decomplexation occur. A complication in heterogeneous catalysis is the need for mass transfer into and out of the catalyst particle, which is usually porous with the major part of the active sites at the interior surface. 

In any catalyst selection procedure the first step will be the search for an active phase, be it a. solid or complexes in a. solution. For heterogeneous catalysis the. second step is also deeisive for the success of process development the choice of the optimal particle morphology. The choice of catalyst morphology (size, shape, porous texture, activity distribution, etc.) depends on intrinsic reaction kinetics as well as on diffusion rates of reactants and products. The catalyst cannot be cho.sen independently of the reactor type, because different reactor types place different demands on the catalyst. For instance, fixed-bed reactors require relatively large particles to minimize the pressure drop, while in fluidized-bed reactors relatively small particles must be used. However, an optimal choice is possible within the limits set by the reactor type. 

Structured catalysts, including monoliths, are very promising as far as pressure drop and high performance for selective reactions are concerned. The perspectives for the use of monolithic catalysts in heterogeneous catalysis have been analysed by Cybulski and Moulijn (1994) and are further discus.sed in Section 5.4.7.7. 

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