SITE STRUCTURE SENSITIVITY OF DIENE HYDROGENATION AND ISOMERIZATION-REACTIONS ON MOS2-GAMMA-AL2O3 CATALYSTS

The hydrogenation and isomerization of two diene molecules, cis-1,3-pentadiene and 2-methyl1,3-butadiene (isoprene), have been performed at 50°C on a 13.8 wt% MoO3/γ-Al2O3 sulfided hydrotreating catalyst. The alumina support was not active for hydrogenation and its activity for isomerization was found poisoned by the presence of the MoS2 phase. The observed diene hydrogenation and isomerization activities are therefore attributed to the sulfide phase. Both the activity and the product distribution have been found strongly dependent on the sulfur content of the (1010) edge plane of the MoS2 slabs. In that edge plane all the Mo ions are similar. Therefore the active sites responsible for the various catalytic functions must be distinguished on the basis of the environment of the Mo ions. More precisely hydrogenation has been found sensitive to Mo ions tricoordinatively unsaturated (cus) and isomerization to 2-cus and 4-cus Mo ions. However, as each vacancy or sulfur ion is shared by two Mo ions in the (1010) edge plane, a full description of the sites leads us to consider an "elementary ensemble" of two Mo ions and their six associated sulfur ions and/or vacancies. The active sites responsible for the different catalytic functions observed correspond primarily to different configurations of sulfur ions and vacancies in the "elementary ensemble" or, in other words, to different site structures. It is shown that due to its strongly preferred 3-4 hydrogenation, the reaction scheme of isoprene hydrogenation is simple enough to correspond to a catalytic function for each product. It is then shown that different site structures could be assigned to each of these functions and that simple statistical calculations can satisfactorily explain the experimental product distribution. The selectivity and activity of the catalyst for the diene hydrogenation and isomerization reactions studied are therefore dependent on the structure of the sites present in the (1010) edge plane of the MoS2 slabs. © 1990.