SURFACE SCIENCE AND HIGH PRESSURE REACTION STUDIES OF THIOPHENE HYDRODESULFURIZATION OVER MOLYBDENUM SINGLE CRYSTAL CATALYSTS

Surface science experiments combined with high pressure reaction studies have been used to investigate the hydrodesulfurization (HDS) of thiophene over Mo single crystal catalysts. Characterization of the sulfided Mo(100) surface and the surface chemistry of thiophene on Mo have been performed under UHV conditions while HDS reaction were performed at pressures of 1 atm.; At low coverages ((theta)(,S) (LESSTHEQ) 0.05) sulfur atoms are adsorbed on the Mo(100) surface in the fourfold hollow site. As the sulfur coverage is increased past (theta)(,S) = 0.67 a second binding site becomes occupied. The simultaneous occupation of two binding sites is induced by repulsive interactions between adsorbed atoms. The heat of adsorption at low coverages is 110 kcal/mole and drops to 75-90 kcal/mole at high coverages.; At high coverages thiophene adsorbs on the Mo(100) surface in both a reversibly bound state that desorbs upon heating and an irreversibly bound state that decomposes. The irreversibly bound molecule decomposes via a low temperature mechanism (175K - 350K) and a second, high temperature process in the range 600K - 680K. The adsorbed molecule is (pi)-bonded to the surface with its sulfur atom bound in a chemical environment similar to that in gas phase thiophene. The (alpha)-CH bonds are weakened to a greater extent by their interaction with the surface than the (beta)-CH bonds. During the low temperature decomposition process (alpha)-CH bond scission occurs prior to (beta)-CH bond scission. This adsorbed species is stable on the surface to temperatures (GREATERTHEQ)600K (the temperature at which high pressure HDS reactions are performed).; The high pressure HDS reaction mechanism proceeds via initial desulfurization to yield butadiene. The subsequent hydrogenation reactions producing butenes and butane proceed via a hydrocarbon intermediate that saturates its available binding sites on the surface. Sulfiding of the surface selectively blocks these binding sites and thus selectively blocks butene and butane production.; The initial desulfurization step of the thiophene HDS reaction occurs via a mechanism that does not deposit sulfur onto the metal surface i.e. not via the Lipsch-Schuit mechanism. A mechanism involving direct hydrogenation of CS bonds to produce H(,2)S is consistent with these results. An intramolecular dehydrodesulfurization mechanism, as proposed by Kolboe, cannot be ruled out.