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SCOPEThe chemistry of catalytic hydrodesulfurization is not well understood although the hydrodesulfurization of petroleum distillates has been practiced industrially for years. The subject remains a neglected area of research, although there has been a recent surge in process development motivated by air pollution control standards requiring low sulfur fuel oils. The developments focus on processing of high-molecular-weight feeds, including residual oils. The results include large scale fixed-and fluidizedbed residuum hydrodesulfurization processes.This review emphasizes the recent literature in summarizing the catalytic chemistry of hydrodesulfurization and identifying the process engineering problems.The subject of reaction intermediates in hydrodesulfurization is reviewed thoroughly; kinetics for pure compounds and petroleum feedstocks are summarized; and structures of alumina-based catalysts (especially Ni/W and Co/Mo) are given in detail, leading to consideration of the catalytic sites.The engineering literature of the new processes is sparse; consequently, the review is qualitative and speculative. The unique problems are related to plugging of catalyst pores by inorganic reaction products and to the possibility of unstable operation with exothermic reactions of explosive mixtures. CONCLUSIONS AND SIGNIFICANCEReaction networks prevailing in catalytic hydrodesulfurization are known for only a few reactants, and results are not easily generalized; thiophene is desulfurized to butadiene before it is hydrogenated, but benzothiophenes are more rapidly hydrogenated than desulfurized. Rates of hydrodesulfurization vary greatly among feeds, being lowest for high-molecular-weight aromatic compounds like benzothiophene. Hydrodesulfurization of many reactants over a wide range of conditions is first order in reactant concentration and in hydrogen partial pressure; reaction is inhibited by product H2S and by strongly adsorbed hydrocarbons.Catalysts have been described in terms of two models, the intercalated sulfide model in which the catalytic sites for hydrogenation and possibly hydrodesulfurization involve W " + ions adjacent to vacancies at crystal edges and the monolayer model with catalytic sites involving M o~+ ions octahedrally coordinated in a surface layer which is stabilized by the presence of promoter Co*+ ions located in tetrahedral sites of the underlying support.The processes for hydrodesulfurization of petroleum distillates are well established, and until recently there had been little incentive for improvement of catalysts. The more challenging problems of hydrodesulfurizing residual feeds stem from the lack of reactivity of the heavier sulfur compounds and the presence of organometallic feed components; these react to form inorganic products, which can permanently plug the pores of catalysts and contribute to plugging of the interstices of fixedbed reactors. The catalyst aging phenomena have been countered by catalyst-and reactor-designs maximizing the capacity for unwanted products and optimizing th...

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