Mechanisms of HDS

H. Topsoe, B. S. Clausen, N. Topsoe, andj. Hyldtoft, Symposium on the Mechanism of HD S / HDN Reactions, Vol. 38, No. 3, Division of Petroleum Chemistry, Preprints, American Chemical Society, Chicago, lU., July 1993. 

As discussed in Section III, when the sulfur content is lowered from 0.20 to 0.05%, the chemistry of HDS of gas oils is essentially the chemistry of alkyl-substituted dibenzothiophenes. Though gas oils initially contain mostly alkyl-substituted benzothiophenes, these are completely removed by the time 0.20% S is achieved. Thus, this review will deal predominantly with the reaction pathways involved in the HDS of alkyl-substituted dibenzothiophenes. There are many excellent reviews on reaction pathways of the more reactive sulfur species such as thiophenes and benzothiophenes (2, 5, 8, 23, 24), and the reader is referred to those reviews for information on the reaction pathways and mechanisms of HDS for the more reactive... 

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. 

Hydrodesulphurization Catalysts Based on Mo or W. - In this Section structural studies of Mo- and W-containing catalysts (unsupported and supported, without and with promoters) and relations between catalyst composition and structure and catalytic properties are reported. Work on sulphides of other metals and on the kinetics and mechanism of hds is reported in the following two Sections. 

Kinetics and Mechanisms of Hds Reactions. - Earlier work and outstanding problems are summarized in the earlier Report. Much of the work reviewed in the present Report concerns thiophen and benzo- and dibenzothiophens that have been of particular interest as model compounds in residual oil and coal hds. Recent reviews include discussions of mechanisms. An outstanding question is whether S elimination from a heterocyclic ring is preceded by ring hydrogenation. The obvious, but important, point has been made that failure to detect a compound in the gas phase, specifically tetrahydrothiophen during thiophen hds, does not exclude the possible importance of a reaction path in which a particular compound is a surface intermediate, since its surface reaction may be faster than its desorption. 

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

For writing the mechanism of HDS (of thiophene) and HYD over the created sites, we took into account data from literature (5,18). It was assumed that the adsorption of hydrogen is rapid, so that the hydrogen atoms ne ed in HYD and HDS reactions are already on the metal sites where thiophene or unsaturated hydrocarbons are sorbed or, alternatively, on the sulfur sites direcdy surrounding these metal sites. In addition, we took into consideration the experimental observations that active sites exist even in the absence of a remote control. This is due to the defect structure of the catalyst surface. These are "original" sites, for which no creation process is needed. 

The mechanism of HDS is a matter of great uncertainty and therefore subject to considerable research effort. According to a popular model, sulfide vacancies are... 

The group of transition metal. sulfide clusters can be viewed as constituting discrete molecular fragments of the metal sulfide phases present in heterogeneous catalysts. However, with a few very notable exceptions, it has so far been difficult to identify sulfide cluster chemistry that directly reflects the mechanisms of HDS. The main exception is the desulfurization of e.g. thiophene by the sulfide vacancy cluster [(Cp )2Mo2Co2(CO)4(//3-S)2( 4-S)], a reaction which closely mimics the mechanism envisaged for the heterogeneous metal sulfide catalysts. 

Scheme 1 General mechanisms of HDS of thiophene, benzothiophene, and dibenzothiophene on solid catalysts.

DMDBT. According to these results, the mechanism of HDS of 4,6-DMDBT (Figure 34) will proceed through the HYD equilibrium between... 

Figure 18. Reaction mechanism of HDS of 4,6-dialky IDBT, assuming the fomation of dihydro intermediate ...

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