Dehydrocyclization of 1-hexene to benzene on Cu3Pt(111)

Abstract

Even though dehydrocyclization is widely practiced in heterogeneous catalysis for the conversion of straight chain hydrocarbons into aromatic compounds, knowledge of the mechanism of this process remains limited, largely because it has not previously been possible to carry out the reaction under conditions amenable to detailed mechanistic study. We report here ultra-high vacuum studies of the dehydrocyclization of submonolayer coverages of 1-hexene to benzene on a Cu3Pt(111) single crystal surface. In these studies reflection-absorption infrared spectroscopy (RAIRS) and temperature-programmed reaction/desorption spectroscopy (TPR/D) have been used to investigate the mechanism of the reaction. The results obtained demonstrate that the mechanism involves two steps: dehydrogenation (T = 270 K) and additional dehydrogenation + cyclization (T = 405 K). The RAIRS and TPR/D results suggest that a planar intermediate with C6H8 stoichiometry (probably 1,3,5-hexatriene) exists on the surface between 270 and 405 K. For surface coverages of up to 12.5% of monolayer saturation the selectivity to benzene formation is 70% with the remaining 30% of the adsorbed 1-hexene dehydrogenating irreversibly to surface carbon and H2. For higher coverages, molecular desorption commences.