Catalysis heterogeneous 431 - hydrodesulfurization

The replacement of conventional catalytic processes for purification of mixtures by processes involving extraction with ionic liquids has also been envisioned. For example, the extractive removal of sulfur compounds from fuels has shown promising leads for high selectivity for aromatic sulfur compounds, and removal of such compounds has been a major challenge in conventional heterogeneous hydrodesulfurization catalysis (27,296). 

Sometimes the reacting mixture is in both the liquid and gaseous forms, as in the hydrodesulfurization of heavy petroleum fractions. Of these two types of catalysis, heterogeneous catalysis is the more common type. The simple and complete separation of the fluid produci mixture from the soHd catalyst makes heterogeneous catalysis economically attractive, especially because many catalysts are quite valuable and their reuse is demanded. Only heterogeneous catalysts will be considered in this chapter. 

Research into cluster catalysis has been driven by both intrinsic interest and utilitarian potential. Catalysis involving "very mixed -metal clusters is of particular interest as many established heterogeneously catalyzed processes couple mid and late transition metals (e.g., hydrodesulfurization and petroleum reforming). Attempts to model catalytic transformations arc summarized in Section II.F.I., while the use of "very mixed -metal clusters as homogeneous and heterogeneous catalysis precursors are discussed in Sections I1.F.2. and I1.F.3., respectively. The general area of mixed-metal cluster catalysis has been summarized in excellent reviews by Braunstein and Rose while the tabulated results are intended to be comprehensive in scope, the discussion below focuses on the more recent results. 

OrganometaUic catalysts hydrodesulfurization, 42 413, 417-425, 432 OrganometaUic chemistry, role of phase-transfer catalysis, 35 398-400 OrganometaUic clusters heterogeneous catalysts, 38 283-288 precursors, 38 391-392... 

Recently, however, the implementation of several forms of modern, high resolution spectroscopy has made it possible to characterize, in relatively detailed fashion, the evolving chemistry of heterogeneous catalytic-type systems. In some cases, in fact, it has been possible to map the behavior of actual commercial catalysts, even following use. For example, much of our present understanding of the functionality of hydrodesulfurization catalysis has been a direct result of investigations of commercial systems with modern surface spectroscopies, as is also the case in the development of the basis reference for metals-support interactions (1). 

We focus on heterogeneous catalysis with single and multiple reactant phases, as these are the most common in practice. Examples include environmental catalysis, fat hardening, hydrodesulfurization of oil streams, hydrogenation of fine chemicals, and selective conversions catalyzed by immobilized enzymes or cells in biotechnology. The most popular reactors used in industry for multiphase applications are slurry bubble columns and trickle-bed reactors. They are shovm in Figure 1. 

Hi) To heterogeneous catalysis sulfides of group S(a) often show catalytic activity which may be strongly enhanced in the presence of metal centers of group B elements (c/. the important hydrodesulfurization reactions utilizing Co—Mo—S catalysts). The catalytic activity may be discussed in terms of the different electron densities at the metal centers and the sulfur atoms attached to them. 

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