Activation of carbon-supported cobalt-molybdenum catalysts in thiophene hydrodesulfurization

Abstract

Thiophene hydrodesulfurization (HDS) activities and product distributions of impregnated Co/C, CoMo/C and Mo/C catalysts were studied at 673 K and atmospheric pressure. Two kinds of pretreatments (prereduction by hydrogen or presulfidation by H 2/H 2S mixture) were carried out on Co and/or Mo catalysts supported on three kinds of carbon (non-activated, activated by CO 2 for 1 h and 10 h). These catalysts were tested in thiophene HDS. Characteristic synergistic effects were not observed, either in thiophene conversions (CONV) or in hydrogenation selectivities (HYD) of butene products of the HDS reaction. The latter values were similar for Co/C and Mo/C catalysts, in contrast to alumina-supported samples. CONV and HYD values of Co/C catalysts were more stable with increasing reaction time than those of Mo-containing samples, and these values decreased in most cases in the following order: presulfided ‘non-pretreated’ prereduced catalysts. The above order and the gradual decrease in CONV and HYD values with increasing Co content of prereduced samples show that reduction of cobalt is not favourable to high CONV and HYD values, because this reduced Co cannot be sulfided by the hydrogen/thiophene reaction mixture and oxidic Co (in non-pretreated samples) cannot be sulflded efficiently in the HDS reaction. High 1-butene and trans-2-butene excesses over thermodynamic equilibrium ratios of butene isomers were found on non-activated and 1 h-activated carbon-supported catalysts, which indicate the predominance of the reaction route producing 1-butene over these supports. Higher specific surface areas of the activated support seem likely to result in higher surface areas of active phase, too, but it presumably cannot prevent the segregation of metallic Co during prereduction. Widening of micropores and the increasing number of mesopores during activation of the support also contribute to the increasing HDS activities and HYD selectivities.