HDS mechanism

C0-M0/AI2O3 catalysts have been studied extensively, both for their structure and reaction mechanisms, and many studies have been reported in the literature (1-14). However, the HDS activity Is not completely understood and many conflicting views have been reported. Ho attempt Is made here to explain the HDS mechanism. 

If the partially hydrogenated 2,3-dihydrothiophene intermediate undergoes isomerization to the 2,5-dihydro isomer, the product decomposes rapidly at 120°C to give butadiene in high yield. This resembles the simultaneous pathway discussed for HDS mechanisms catalyzed by MoS2 (see Fig. 23) and has been observed in Re/Al203-catalyzed conversions of 2,3-dihydrothiophene (97). 

This simple mechanistic picture cannot be applied to thiophene derivatives, whose HDS mechanism is still under study. Research on organometallic models... 

DHT does not eliminate 1,3-butadiene under these conditions. The 2,5-DHT also releases 1,3-butadiene when adsorbed on single crystal Mo(llO) surfaces and when passed over a 5% Re/Al203 catalyst at 300 °C under HDS conditions. The more saturated C4 products (equation 3) observed in thiophene HDS are presumably formed by hydrogenation of the 1,3-butadiene. In step (e), the active metal site is regenerated by removal of the sulfide ligand as H2 S upon reaction with H2. The key steps (a-d) in this HDS mechanism (Figure 3) have all been observed either in reactions of thiophene complexes or on catalyst surfaces. 

Weinberg, JM, Harding PG, Humes HD, Mechanisms of gentamicin-induced dysfunction of renal cortical mitochondria effects on mitochondrial monovalent cation transport. Arch Biochem Biophys, 1980,205(1) 232-39. 

Two-electron redox processes are facile in metal sulfide surfaces, and thus M-S bond breaking and re-making in such extended arrays may provide an energetically favorable pathway to modify the electron count at a particular metal center. Although Tl -bonded thiophenes have not been invoked in heterogeneous HDS mechanisms, T T transformations such as the ones defined in organometallic complexes could be envisaged to take place at the active sites of HDS catalysts, as possible routes for... 

If the PMcj and dppe (diphenylphosphino ethane) PtLj analogues are used, complete hydrodesulfurization of DBT and 4-MeDBT to biphenyl and 3-Me-biphenyl, respectively, takes place under 20 atm H, at 100 °C interestingly, the presence of acid-washed alumina promoted the HDS reaction (see e.g. Eq. 4.9). Neither the fate of the sulfur nor the HDS mechanism is clear in this case [42, 43]. 

A further point of great interest that emerges from the work of Curtis and from the mechanism depicted in Fig. 4.14 is the concept of latent vacancies and sulfur mobility . This can be related to some recent important considerations on the existence of real anionic vacancies on Co-Mo-S surfaces, a frequently encountered key feature of HDS mechanisms. Tlie Co atom, which is known to be the primary site of attack of thiols in this case, is electronically saturated, but the empty site required for... 

A general explanation cannot be given, which is valid for different catalysts and reactions, even if these are limited to HDS. One of the characteristic examples is the essential difference in the mechanism of catal3dic effect on sulfided noble metals in comparison with that of molybdenum and tungsten-oxide based catalyst, due to large differences in the energy of their sulfide formation. This difference in HDS mechanism is also well seen from the rather different effect of sulfidation on precious metal promoted MoOx, in comparison with CoMoOx, in the initial period of sulfidation. 

Sulfur uptake and exchange data 1 referred in Sec. 2.3 indicated substantial differences of alumina supported Pd and Pt, in comparison with the molybdena based catalysts. The mobile sulfur amounts were lower [0.25 Smob/Pd (or /(Pt)] than those experienced with the Mo and W based catalysts. Near to all sulfur, accommodated on the noble metal was mobile and — different from the Mo- and W-based catalysts — participated in the reaction the sulfur mobility was substantially higher due to the much lower S-Pd and S-Pt bond strengths. This indicates that the HDS mechanism, detailed above for Mo based catalysts cannot be valid for the noble metals. Again, the number of vacancies for bonding sulfur-organic compounds is not limiting, as in the case with monometallic Co and Ni. 

The comprehensive review on deep HDS published by Whitehurst et provides detailed accounts of the HDS mechanism of a wide range of the model S-containing compounds. This review deserves attention in spite of the fact that it deals predominantly with the y-Al203-supported hydroprocessing catalysts. 

Modes of Adsorption of Sulfur-Containing Compounds. In typical gas oil, more than 70 different sulfur species can be detected by using gas chromatography-mass spectroscopy techniques. Moreover, the major components are alkylbenzothiophenes and alkyldibenzothiophenes. Consequently and to simplify studies already intrinsically complex, an impressive amovmt of studies were devoted to the HDS reactivity of the family of thiophene compovmds ((thiophene, BT, or DBT). The first step of any of these studies concerning the thiophene HDS mechanism is to describe the elemental steps of this process. The most general mechanism accepted in a solid-state point of view is to consider the HDS reaction as a four-electron reduction process releasing H2S and butadiene from thiophene (148). 

Currently, new insight into the HDS mechanism has also been obtained by the combination of STM and DFT combined to obtain detailed information about the structure of unpromoted and promoted M0S2. The conjunction of experimental and theoretical studies has allowed obtaining a detailed picture of the structure of the active phase that prevails under reaction conditions. 

FIGURE 6 Hydrodesulfurization (HDS) mechanisms for thiophene, benzothiophene, and dibenzothiophene in the presence of layered MS2 catalyst. 

Molecular structures of scl PHB, PHBV, PHBHHx and mcl P(HO-co-HD). Mechanical property of the above PHA changes from brittle to flexible to elastic from left to right, while crystallinity decreased from left to right. 

---