Dehydrogenative coupling of methane catalyzed by platinum-added sulfated zirconia and characterization of the catalyst surface

An active and stable catalyst of 3 wt% Pt-added sulfated-zirconia for the dehydrogenative coupling of methane without oxidant was obtained by sulfating zirconia gel with 0.5 M H2SO4, followed by drying, impregnating the sulfated gel with a solution of H2PtCl6, calcining in air at 600 degrees C, and finally reducing in H-2 at 500 degrees C. The catalyst gave conversion of 0.23%, ca 40% of the equilibrium value, at 500 degrees C and was steady during the run of 5 h in a flow reactor (0.5 g catalyst, 10 ml/min methane). The products were ethane and ethylene in a ratio of 9 : 1; CO2 was negligible. A remarkable loss of activity was observed with the catalysts sulfated with ammonium sulfate instead of sulfuric acid. XRD analysis showed that the sulfation of zirconia gel with sulfuric acid led to high dispersion of the Pt matter onto the support; the observations were confirmed by the amount of CO chemisorbed on the surface Pt atom. The adsorption measurement of CO showed the ratio of CO/Pt to be 0.09. Using TG-DTA the sulfur quantity on the catalyst was estimated from a weight loss at 600-1000 degrees C, caused by decomposition of the sulfate species, and the ratio of Pt to S was determined to be 1. XPS showed SO42- for the S state and 10% of the platinate to be in the metallic state. Measurements of TG-DTA and XPS, in addition to the CO chemisorption, showed an active site (in the steady state) to be Pt-0-SO4/ZrO2, surrounded by 10 times platinate-SO4/ZrO2 with various oxidation numbers of Pt except 0, whose species protect the active one from aggregation. The turnover frequencies were estimated to be 0.18 min(-1). (C) 1998 Academic Press