Spectroscopic investigation into the nature of the active sites for epoxidation reactions using vanadium-based aluminophosphate catalysts

VAlPO-5 and VAlPO-11 catalysts were synthesized and their structural features characterized by a range of spectroscopic techniques (UV-Vis, Raman and IR spectroscopies) revealing valuable information on the nature of the incorporated vanadium species. A preliminary investigation into the catalytic potential of these catalysts was carried out by studying the epoxidation of cyclopentene to its corresponding epoxide using a solid source of "active" oxygen, acetylperoxyborate (APB) as the oxidant. UV-Vis spectra showed strong bands in the 40,000-35,000 cm^-1 range, that can be assigned to isolated Td-like oxovanadium(V) (V⁵⁺ = O vanadyls), along with a component around 35,000 cm^-1 due to a small fraction of Td V⁵⁺ = O in di- or oligomeric V_xO_y clusters. The V⁵⁺ = O ions were reversibly transformed to V⁴⁺ = O species by reduction, and could also change their coordination (to octahedral geometry) in the presence of water molecules. The presence of isolated V ⁵⁺ = O was confirmed by resonant Raman studies. IR spectroscopy of adsorbed probes (CO and NO) revealed the presence of two families of framework V⁴⁺ = O ions with different Lewis acidity VA4+andVB4+ forming monocarbonyls (bands at 2192 cm^-1 and 2184 cm^-1) and di-nitrosyls (bands at 1905-1755 cm^-1 and 1835-1680 cm^-1). Small differences in the local environment of the vanadyl ions (symmetry, defects), that can have an influence over their Lewis acidity, did not play a major role in influencing the overall product selectivity, whilst the differences in rate and turnover frequency could be explained on the basis of the differing pore diameters of the two catalysts.