Acetylene cyclotrimerization over Ni/SiO2 catalysts in hydrogen atmosphere

Non-conventional Ni catalysts supported on low surface area SiO2 were prepared by reduction of nickel acetate by aqueous hydrazine, characterized by XRD and H-2 chemisorption properties or TPSR and tested in acetylene cyclotrimerization in the presence of hydrogen. Their performances were compared with those of classical catalysts supported on silica of low or high surface area. Hydrogen TPD profiles exhibited the presence of active sites involving nickel or nickel in interaction with the support. Hydrogen storage increased for non-classical catalysts or low surface area support. Under the reaction conditions used, no cyclotrimerization of acetylene was observed in the absence of hydrogen, probably as a result of strong adsorption of benzene precursor species. In the presence of hydrogen, benzene and ethylene were the main products in the low reaction temperature regime (<80 degreesC) whereas ethane predominated in the higher temperature domain. Butane and higher hydrocarbons were also formed but in small amounts or as traces. Decreasing the nickel loading increased acetylene conversion and benzene selectivity to some extent but diminished selectivity of ethylene and ethane in the same proportions. A non-classical catalyst and a low surface area support favoured acetylene trimerization whereas a classical catalyst and a high surface area silica preferentially formed ethane. Increasing the hydrogen partial pressure increased conversion and C-2 and C-4 hydrogenation products at the expense of cyclotrimerization to benzene. Increasing the partial pressure of acetylene strongly deactivated the catalyst but favoured selectivity of both benzene and ethylene. The presence of both water vapour and hydrogen improved cylotrimerization to benzene at the expense of hydrogenation products, notably at low reaction temperatures. The effect of the different parameters on the sorptive and catalytic properties of the nickel catalysts prepared are discussed, notably the role of hydrogen and water in the determination of activity and reaction paths. (C) 2003 Elsevier B.V. All rights reserved.