HYDROGENOLYSIS OF N-HEPTANE OVER THE NI-TH SYSTEM - KINETIC AND MECHANISTIC STUDIES

The reaction rates of n-heptane hydrogenolysis in the presence of Ni-T h catalysts are studied as a function of the partial pressure of hydro gen in a dynamic reaction system. Three different types of reactions a re observed: a) demethylation; b) deethylation; and c) internal fissio n reaction. The reaction order with respect to hydrogen partial pressu re is negative and varies from -1.8 to -3. Such a highly negative orde r is very common for hydrogenolysis reactions, and can be justified by the presence of an alpha alpha gamma-triadsorbed intermediate. When t he carbon-metal double bond is located at the terminal carbon atom, th e carbon-carbon bond scission occurs with an apparent activation energ y of about 160 kJ mol(-1) It occurs with a higher apparent activation energy, in the 215-240 kJ mol(-1) range, when the C=M double bond is s hifted along the interior of the hydrocarbon chain. it seems that the presence of thorium in these catalysts modifies the electronic propert ies of nickel. We also attempt to propose a suitable mechanism for the hydrogenolysis reaction. We believe that the reaction between the che misorbed hydrogen and hydrocarbon takes place on the same type of site s as in the Langmuir-Hinshelwood mechanism. Another mechanism consider ing the dissociative adsorption of hydrocarbon is also discussed.