Investigation of the Adsorption and Reactions of Thiophene on Sulfided Cu, Mo, and Rh Catalysts†

The surface chemistry of thiophene (C{sub 4}H{sub 4}S) on sulfided Cu/Al{sub 2} O{sub 3}, Mo/Al{sub 2}O{sub 3}, and Rh/Al{sub 2}O{sub 3} catalysts has been investigated under well-defined conditions using infrared (IR) spectroscopy and temperature programmed desorption (TPD). The results of these experiments have been found to correlate nicely with CO chemisorption measurements of site densities and thiophene hydrodesulfurization (HDS) activities measured in a flow reactor system. In agreement with the literature, the catalysts were found to have dramatically different thiophene HDS activities increasing in the following order: sulfided Cu < sulfided Mo < sulfided Rh. The trend of HDS activities is mirrored in similar trends for the site densities and turnover frequencies for the sulfide catalysts. IR spectra of adsorbed thiophene on the sulfided Cu, Mo, and Rh catalysts at low temperatures show that the amount of thiophene adsorbed on cus metal sites of the sulfide catalysts correlates with the CO chemisorption estimates of site densities. Thiophene reactivity on the catalysts differs significantly, with reaction in thiophene/H{sub 2} mixtures observed on the sulfided Mo and Rh catalysts annealed at 300 and 500 K, respectively, and not at all on a sulfided Cu catalyst annealed up to 700 K. Strongly adsorbedmore » species produced on the sulfided Cu, Mo, and Rh catalysts during the annealing sequence were subjected to TPD experiments that yielded C{sub 4} hydrocarbons and H{sub 2}S, with the predominant C{sub 4} product being butenes. IR and TPD measurements of 1,3-butadiene and 1-butene on sulfided Rh/Al{sub 2}O{sub 3} catalysts, both in the pure gas and in gas/H{sub 2} mixtures, provide evidence for the tentative assignment of strongly adsorbed species produced by cleavage of C-S bonds in thiophene to be a {sigma}-bonded allyl species with C{sub 4}H{sub 7} stoichiometry.« less