Investigation of the adsorption and reactions of thiophene on sulfided Cu,Mo, and Rh catalysts

The surface chemistry of thiophene (C4H4S) on sulfided Cu/Al2O3, Mo/Al2O3, and Rb/Al2O3 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 wit h CO chemisorption measurements of site densities and thiophene hydrodesulf urization (HDS) activities measured in a flow reactor system. In agreement with the literature, the catalysts were found to have dramatically differen t thiophene HDS activities increasing in the following order: sulfided Cu < sulfided Mo < sulfided Rh. The trend of HDS activities is mirrored in simi lar 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 chemisorpt ion estimates of site densities. Thiophene reactivity on the catalysts diff ers significantly, with reaction in thiophene/H-2 mixtures observed on the sulfided Mo and Rh catalysts annealed at 300 and 500 K, respectively, and n ot at all on a sulfided Cu catalyst annealed up to 700 K. Strongly adsorbed species produced on the sulfided Cu, Mo, and Rh catalysts during the annea ling sequence were subjected to TPD experiments that yielded C-4 hydrocarbo ns and H2S, with the predominant C-4 product being butenes. IR and TPD meas urements of 1,3-butadiene and 1-butene on sulfided Rh/Al2O3 catalysts, both in the pure gas and in gas/H-2 mixtures, provide evidence for the tentativ e assignment of strongly adsorbed species produced by cleavage of C-S bonds in thiophene to be a sigma-bonded allyl species with C4H7 stoichiometry.