Thiophenes hydrogenolysis

Progress and prospects in hydrogenation, hydrogenolysis, and desulfurization of thiophenes with soluble metal complexes 98ACR109. 

Olefins and aromatic hydrogenation reaction are undesired in gasoline HDT unfortunately, they cannot be fully inhibited. The high requirement on hydrogenolysis, but low hydrogenation activity, makes CoMo the preferred catalysts. New catalysts are being offered by the manufactures for selective HDS. Speculatively, two concepts have been used to develop new selective catalyst (i) improve thiophene HDS, or (ii) passivate olefin hydrogenation. 

The hydrogenolysis of thiophenes to thiols is a relevant reaction in the HDS process (Equation (16)). 

The combined information gathered from kinetic studies,184 in situ high-pressure NMR experiments,184,185,195 and the isolation of intermediates related to catalysis, leads to a common mechanism for all the hydrogenolysis reactions of (102)-(104) and other thiophenes catalyzed by triphos- or SULPHOS-rhodium complexes in conjuction with strong Bronsted bases. This mechanism (Scheme 41) involves the usual steps of C—S insertion, hydrogenation of the C—S inserted thiophene to the corresponding thiolate, and base-assisted reductive elimination of the thiol to complete the cycle.184 185 195-198... 

The hydrogenolysis of thiophene and its homologs has been investigated in the presence of Ni-Al alloy and aqueous alkali (65) and over vanadium oxide (47). It has been shown that / -(2-thenoyl)-propionic acid (II) on treatment with nickel aluminum alloy in aqueous alkali solution yielded... 

Fig. 29. Correlation between relative hydrogenolysis reactivity and the electron densities on sulfur. ( ) Reaction conditions 300°C, 71 atm, sulfided CoO-Mo03/A1203 (5). (1) DBT (2) benzo[fo]naphtho[2,3-4]thiophene (3) 7,8,9,10-tetrahydrobenzo[b]naphtho[2,3-rf]thio-phene (4) 5b,6,ll,lla-tetrahydrobenzo[fc]naphtho[2,3-d]thiophene. (O) Reaction conditions 450°C, 1 atm, sulfided CoO-Mo03/A1203(6). (5) Thiophene (6) tetrahydrothiophene (7) benzothiophene (8) 2,3-dihydrobenzothiophene. ( + ) Reaction conditions 360°C, 2.9 MPa, sulfided NiO-Mo03/Al203(3). (9) 1-Methyl-DBT (10) 2- or 3-methyl-DBT (11) 4-methyl-DBT (12) 4,6-dimethyl-DBT. (A) Reaction conditions 300°C, 102 atm, sulfided CoO-Mo03/ Al203(8). (13) 2,8-Dimethyl-DBT (14) 3,7-dimethyl-DBT (15) 4-methyl-DBT (16) 4,6-di-methyl-DBT. Reprinted with permission from Ref. 38, Ma et al. (1995). Copyright 1995 American Chemical Society.

By far, the majority of definitive studies which have been made on this catalyst have used thiophene as a model compound for hydrogenolysis. Table IX summarizes some of the more important findings. We will not discuss all the individual papers because few of them attempt any quantia-tive (or even qualitative) correlations with catalyst properties and there is considerable disagreement between workers on the effects obtained. The more significant features to be pointed out are the following ... 

Massoth and Kibby (94) have studied hydrogenolysis activity of thiophene over a series of sulfided Mo/Al catalysts using combined conversions and stoichiometric analyses on catalysts under reaction conditions. They showed that the intrinsic rate constant could be... 

Thiophene hydrogenolysis data over a series of catalysts having different sulfided states correlated well with the model. 

The reactions involving the hydrogenolysis of sulfur compounds encountered in hydroprocessing are exothermic and thermodynamically complete under ordinary operating conditions. The various molecules have very different reactivity, with mercaptan sulfur much easier to eliminate than thiophene sulfur or di-benzothiophene sulfur. 

Hydrogenolysis of adsorbed thiophene (poison) is the rate determining step. Sulfur resistance was Pt > Pt-Ir > Pt-Re.65 66 ... 

Mechanism of hydrogenolysis of thiophene on bimetallic sulfide catalysts 92UK332. 

Thiophene was also hydrogenated to tetrahydrothiophene over rhenium heptasulfide (Re2S7) at 230-260°C246 or rhenium heptaselenide (Re2Se7) at 250°C and at pressures greater than 30 MPa H2 (eq. 12.129),247 without accompanying hydrogenolysis of the carbon-sulfur bond that occurred extensively over molybdenum sulfide.246... 

Temperature Increase Dynamics after the First Cycle. As with the start up of the bed, subsequent temperature cycles resulted in the formation of a mild hot spot. The occurrence of this temperature fluctuation is undesirable since the past history of the catalyst may be altered. The adsorption of thiophene upon the active hydrogenation sites was assumed to be irreversible and therefore unaffected by temperature. However, as will become apparent later, the effect of temperature may have altered the poison coverage/or profile. Lyubarski, et. al [73 determined that, as a result of the hydrogenation of thiophene and subsequent hydrogenolysis to butane, the adsorption capacity of a suported Ni... 

Hydrogenolysis refers to the transformation of a thiophenic substrate into the corresponding thiol. 

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