Effect of Surface Tungstate W5+ Species on the Metathesis Activity of W-Doped Spherical Silica Catalysts

The high surface area W-doped spherical silica (SSP) catalysts were prepared with different sequences of W and Si addition (W–Si (Alt), Si 1 –W 2, and W 1 –Si 2 ) by the sol–gel method with CTAB as a structure directing agent and compared with the impregnated one (W/SSP). All the catalysts exhibited high specific surface area (∼ 1100 m 2 g −1 ) with a closely perfect spherical shape. The presence of surface/sub-surface tungstate W 5+ species, crystalline bulk WO 3 , and tetrahedral tungsten oxide species on the prepared catalysts was investigated by means of X-ray photoelectron spectroscopy depth profile analysis, X-ray diffraction, and Raman spectroscopy. Without in situ reduction by the reactants/products, tungstate W 5+ species was found on the top surface of the as-prepared W–Si (Alt) whereas for the Si 1 –W 2 , W/SSP, and W 1 –Si 2 , the W 5+ appeared only on the sub-surface of the catalysts after 5 and 15 s Ar + etching. The abundance of surface W 5+ species is suggested to facilitate the establishment of the active tungsten carbenes and was correlated well to the catalytic activity in propene metathesis. The surface W 5+ -activity relationship of the WO 3 -based metathesis catalysts is useful especially when the catalyst activity did not depend solely on the amount of active tetrahedral coordinated tungsten oxides.