Catalytic reactor and organometallic model studies of the mechanism of thiophene hydrodesulfurization
The reactivity of transition metal complexes of thiophene (Th) and 2,3- and 2,5-dihydrothiophenes (DHT) and catalytic reactor studies of thiophene and the dihydrothiophenes have been used to examine the mechanism for the hydrodesulfurization (HDS) of thiophene;Catalytic deuterium exchange of thiophene over HDS catalysts was modeled by the reaction of (pi)-thiophene complexes, CpRu(Th)('+) (where Th = thiophene, 2, or 3 methylthiophene or 2,5-dimethylthiophene) in CD(,3)OD with OH('-). The exchanges follow a rate law Rate = k CpRu(Th)('+) OH('-) which suggests a mechanism involving rate determining proton abstraction from the thiophene by OH('-) followed by deuteration by the solvent. Rates of exchange for the different hydrogens in the complexes follow the trend H(,2,5) >> H(,3,4) > methyl; the same order of reactivity which is observed over HDS catalysts;Reactor studies with thiophene and 2,3- and 2,5-DHT were done at 300(DEGREES) and 400(DEGREES)C over a Re/(gamma)-Al(,2)O(,3) HDS catalyst. At 400(DEGREES)C, the distributions of butane and 1 and 2-butenes were very similar. At 300(DEGREES) however, both 2,3- and 2,5-DHT gave butadiene as the major desulfurized product. Formation of thiophene and tetrahydrothiophene and the interconversion of the dihydrothiophenes was also observed over the catalysts. Deuterodesulfurization studies established that 2,5-DHT directly eliminates butadiene while butadiene formation from 2,3-DHT must involve several surface steps over the catalyst;Several reactions of the dihydrothiophenes over HDS catalysts were modeled using transition metal complexes of 2,3 and 2,5-DHT. The S-bound 2,3-DHT in W(CO)(,5)(2,3-DHT) is converted to tetrohydrothiophene upon treatment with HCl. When Fe(,2)(CO)(,9)2 is reacted in THF with 2,3-DHT though, no isolable Fe(CO)(,4)(2,3-DHT) complex is obtained, but small amounts of 2,5-DHT are observed by ('1)H NMR. Reaction of 2,5-DHT with Fe(,2)(CO)(,9) does result in the formation of Fe(CO)(,4)(2,5-DHT), which readily eliminates butadiene.