Metal Clusters as Models for Hydrodesulfurization Catalysts

Michael Brorson, Jason D. King, Keranio Kiriakidou, Fabio Prestopino and Ebbe Nordlander 

Several reviews have been published on the use of transition metal complexes in HDS studies.t This review aims to summarize some of the important results that 

The types of organosulfur compound present in petroleum feedstocks are alkyl and aryl thiols (RSH), thioethers (RSR ), disulfides (RSSR ), and thiophenic compounds (Fig. 1). The ease with which sulfur is abstracted depends very much on the nature of the sulfur-containing molecule aliphatic compounds (thiols, thioethers) are usually desulfurized much more easily than heteroaromatic (e.g. thiophenes, benzothiophenes, dibenzothiophenes). Among the latter, reactivity decreases in the order thiophene benzothiophene dibenzothiophene. The presence of aliphatic substituent groups can sometimes alter reactivity. The sterically hindered compound 4,6-dimethyldibenzothiophene is, for example, very difficult to desulfurize. 

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The financial and environmental importance of hydrodesulfurization reactions assures continued interest in HDS research. The knowledge of the mechanisms in commercial HDS reactions is still considerably limited determination of the intimate mechanisms of these reactions constitutes a formidable challenge. The use of metal clusters as models for HDS catalysts is continuously developing and we might therefore expect that several new model systems will be investigated in the future. 

In this chemistry, it is natural to focus on models for the catalytic reactions that are most important economically or which are most poorly understood because of difficulties of direct study. One which best fits these criteria is the catalytic hydrotreatment of petroleum feedstocks, which is used to remove sulfur and other heteroatoms, which interfere with subsequent catalytic reactions such as petroleum reforming, from the hydrocarbons. Molybdemun sulfide is the most common metal sulfide used in this catalysis, and hydrogen activation and C-S bond hydrogenolysis are known to be key reactions occurring at the catalyst surface but details are difficult to obtain. Study of model binuclear and cluster complexes has elucidated mechanisms of several of the key reactions and Section 2.6 describes important recent advances in this field, with the focus being on models for hydrodesulfurization catalysts. 

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