Selective hydrodesulfurization of FCC naphtha with supported MoS2 catalysts: The role of cobalt

The catalytic activity and selectivity for hydrodesulfurization (HDS) and o lefin hydrogenation of FCC naphtha have been determined for MoS2 (no Co) ca talysts on different supports and for a commercial CoMo/alumina HDS catalys t both with and without the addition of alkali. For MoS2 catalysts, the spe cific HDS activity is higher on silica than alumina, while addition of Cs r esulted in no change in the activity. The differences in activity, however, are relatively small, a factor of less than two. EXAFS and XRD structural analysis indicate that small MoS2 particles are present on all catalysts. T he differences in rate are not due to differences in particle size, dispers ion, or support physical properties, but are likely due to the modification of catalytic properties by an interaction with the support. While there is a small influence on the rate, the composition of the support, or modifica tion by Cs, has no effect on the HDS/olefin hydrogenation selectivity. The olefin hydrogenation conversion increases linearly with HDS conversion, and at high HDS conversion, few olefins remain in the FCC naphtha, Similar to the effect for Cs promotion of MoS2 on alumina, the addition of K to sulfid ed CoMo/alumina had little affect on the activity or selectivity for HDS an d olefin hydrogenation. Unlike MoS2 catalysts, however, with sulfided CoMo at less than about 85% HDS conversion, the rate of olefin hydrogenation is low, but it increases rapidly as the sulfur in the naphtha drops below abou t 300 ppm. Selective HDS of FCC naphtha appears to correlate primarily to t he formation of the CoMoS phase, rather than to the basic nature of the sup port. It is proposed that the enhanced olefin hydrogenation selectivity of CoMo catalysts is due to the competitive adsorption of sulfur compounds, wh ich inhibit adsorption and saturation of olefins in the naphtha. (C) 2000 A cademic Press.