(434g) Ring Opening of 1,3-Dimethylcyclohexane: Effects of Ni and K on Supported Iridium Catalysts

In this work, the ring opening (RO) reactions of 1,3-dimethylcyclohexane (DMCH) on different modified Ir catalysts have been studied. 1,3-DMCH is used as model feed to choose the most selective metal catalyst, which could be implemented in the treatment of gasoline for improving octane numbers (ONs). Ring opening of 1,3-DMCH over iridium catalysts proceeds via three mechanisms: dicarbene, adsorbed olefin or metallocyclobutane. The dicarbene reaction path, which occurs on Ir/SiO₂ catalyst, results in unsubstituted C-C bond cleavages. Therefore, the corresponding products with high degree of branching have higher ONs. In contrast, the other mechanisms prefer to break C-C bond at substituted positions, which yields products with lower ONs than the original feed. Ir/Al₂O₃ appears as the most active and selective toward this type of ring opening. Since the activity of Ir/Al₂O₃ is high, the primary RO products undergo excessively secondary hydrogenolysis to lighter hydrocarbons as conversion increases. To avoid the cracking problem and make Ir/Al₂O₃ more selective toward dicarbene mode, second metals (Ni or K) were added to Ir catalysts. It has been found that the addition of K and Ni alters both activity and selectivity of Ir catalysts. The activities of alumina-supported Ir-Ni and Ir-K catalysts, together with the dispersions, are lower than that of alumina-supported Ir catalyst. The incorporations of Ni and K to Ir/Al₂O₃ catalyst at a certain ratios can suppress the cleavage of C-C bond at substituted positions and excessive secondary hydrogenolysis, which in turn results in the increasing ONs of product mixtures. This might be due to the fact that the introductions of Ni and K block the strong interaction of iridium with alumina. Therefore, both dispersion and number of active sites responsible for formation of adsorbed olefin or metallocyclobutane intermediates decrease. On the other hand, the dispersion of Ir-K/SiO₂ is higher than that of silica-supported Ir catalyst. Potassium probably helps to create small particle sizes of Ir. Consequently, the addition of K into silica catalyst facilitates the rupture of more sterically hindered C-C bonds, which causes a decrease in ON.