Effect of Surface Tungstate W 5+ 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), Si1–W2, and W1–Si2) 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 m2 g−1) with a closely perfect spherical shape. The presence of surface/sub-surface tungstate W5+ species, crystalline bulk WO3, 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 W5+ species was found on the top surface of the as-prepared W–Si(Alt) whereas for the Si1–W2, W/SSP, and W1–Si2, the W5+ appeared only on the sub-surface of the catalysts after 5 and 15 s Ar+ etching. The abundance of surface W5+ 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 W5+-activity relationship of the WO3-based metathesis catalysts is useful especially when the catalyst activity did not depend solely on the amount of active tetrahedral coordinated tungsten oxides.