Active Sites in Olefin Metathesis over Supported Molybdena Catalysts

Metathesis of propene to ethene and 2-butenes was studied over a series of MoOx/SBA-15 catalysts (molybdenum oxide supported on mesoporous silica SBA-15; Mo loading 2.1-13.3wt%, apparent Mo surface density 0.2-2.5nm(-2)). The catalysts have been prepared by an ion exchange technique. Nitrogen adsorption, (1)HMAS-NMR, Raman, and FTIR spectroscopies were applied to characterize the catalysts. Adsorption of the reactant propene and the probe molecule NH3 was studied by insitu FTIR spectrometry microcalorimetry and temperature-programmed desorption. Irrespective of the loading, only approximate to 1% of the Mo atoms in the MoOx/SiO2 catalysts transform into active carbene (Mo=CHR) sites catalyzing propene metathesis. Isolated, distorted molybdenum di-oxo species in close vicinity to two silanol groups have been shown to be the precursor of the active site. Targeted active site creation by pretreatment with methanol resulted in an increase in initial catalytic activity by a factor of 800.