HDS catalysis

Aksenov D.G., Klimov O.V., Startsev A.N. (1997) Alumina supported sulfide catalysts. VI. Hydrogenolysis of tetrahydrothiophen. Kinet. Katal 38, 903-7. Yukelson 1.1., (1969) Methods of the Basic Organic Synthesis, Nauka, Moscow. Startsev A.N. (1995) The Mechanism of HDS Catalysis. Catai. Rev.-Sci. Eng., 37, 353-423. 

B-G. Johnson, F.E. Massoth and Bartholdy, Diffusion and catalyt.lt activity studies on res id-deactivated HDS catalysis, AIChE J, 32 (1986) 1930-1937. 

There is disagreement about the role (if any) of Mo in hds catalysis. Mo concentration correlated with hds activity for sulphided Mo03 and for various Ni-Mo/Al203 catalysts (diesel oil, thiophen ). The concentration of Mo (e.s.r.) was, however, much less in M0/AI2O3 catalysts promoted by Fe, Co, Ni than in the unpromoted catalysts. Since the promoted catalysts were more active in gas oil hds, Mo was not considered to be a hds site. 

Startsev, A. The mechanism of HDS catalysis. Catalysis Reviews—Science and Engineering, 1995, 37, 352. 

Many other metals have been shown to be active in HDS catalysis, and a number of papers have been published on the study of periodic trends in activities for transition metal sulfides [15, 37-43]. Both pure metal sulfides and supported metal sulfides have been considered and experimental studies indicate that the HDS activities for the desulfurization of dibenzothiophene [37] or of thiophene [38, 39] are related to the position of the metal in the periodic table, as exemplified in Fig. 1.2 (a), 1.2 (b), and 1.2 (c). Although minor differences can be observed from one study to another, all of them agree in that second and third row metals display a characteristic volcano-type dependence of the activity on the periodic position, and they are considerably more active than their first row counterparts. Maximum activities were invariably found around Ru, Os, Rh, Ir, and this will be important when considering organometallic chemistry related to HDS, since a good proportion of that work has been concerned with Ru, Rh, and Ir complexes, which are therefore reasonable models in this sense however, Pt and Ni complexes have also been recently shown to promote the very mild stoichiometric activation and desulfurization of substituted dibenzothiophenes (See Chapter 4). 

A more elaborate and perhaps more appealing proposal advanced by several authors [54] involves the heterolytic activation of H2 on e.g. a Mo=S or Mo-S-Mo bond to form Mo-H plus Mo-SH species. The metal hydride is sometimes though to take part in HDS catalysis, but also its oxidation by molybdenum to form a second -SH group can be envisaged as a facile process for a highly mobile H atom in such an sulfur rich surface (Eq. 1.4) ... 

This extensive [(triphos)Ir] model chemistry demonstrates that all the necessary steps for the complete HDS of thiophene or for the hydrogenolysis of benzothiophene can be achieved without any need to previously saturate the heterocyclic ring, in agreement with one of the commonly invoked mechanisms in HDS catalysis - the desulfurization route - as discussed in Chapter 1. 

Prior to about 1985, less than a handful of metal complexes containing coordinated thiophenes were known, and virtually no mention of them could be found in the HDS literature of that time. Today, not only the number and the variety of such compounds have grown to be very large, but also they have become of obliged reference for workers in the heterogeneous HDS field. In turn, organometallic chemists have become aware of the wealth of information available on the chemistry of thiophenes on solid catalyst surfaces, and of the main standing issues that need to be understood and solved in HDS catalysis. 

In relation to HDS catalysis, C-S bond cleavage of thiols has also been extensively investigated. Co/Mo/S cluster shown in Scheme 3.79 reacts with benzenethiol to give a new /r-sulhdo cluster by releasing benzene. Such reactions are also regarded as one of the model reactions of heterogeneous HDS catalyst... 

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