Mechanism of dehydrocyclization of 1-hexene to benzene on Cu3Pt(111):: Identification of 1,3,5-hexatriene as reaction intermediate

We report here ultrahigh vacuum studies of the dehydrocyclization reaction of submonolayer coverages of l-hexene to benzene on a Cu3Pt(lll) single crystal surface, using reflection-absorption infrared spectroscopy (RAIRS), near edge X-ray absorption fine structure (NEXAFS) studies, and temperature-programmed reaction/desorption (TPR/D) spectrometry. As discussed in a previous TPR/D paper, at surface coverages up to 13% of monolayer saturation, l-hexene forms benzene on a Cu3Pt(lll) surface. Selectivity to benzene formation is 70 +/- 10%, with the remaining 30 +/- 10% of the adsorbed 1-hexene dehydrogenating irreversibly to surface carbon and H-2. For higher coverages, molecular desorption commences. Spectroscopic identification of the intermediates of the reaction of l-hexene and other model compounds, such as a 1,3,5-hexatriene, with a Cu3Pt(111) surface suggests that l-hexene and 1,3,5-hexatriene have a common intermediate, and this intermediate has been identified as a rehybridized hexatriene species. Other model compounds, such as trans-3-hexene, have also been used to provide further understanding of the mechanism of the aromatization reaction.