Proton NMR investigations of surface hydroxyl groups on oxidic Mo-P/gamma-Al2O3 catalysts

Proton MAS NMR spectroscopy has been used to investigate the influence of the impregnation procedure on the surface hydroxyl structure of dehydrated gamma-Al2O3 impregnated with varying amounts of molybdenum or phosphorus. Large differences were observed for catalysts prepared by coimpregnation, sequential impregnation, and sequential impregnation with intermediate calcination. A quantitative analysis of the spectra showed that molybdate ions preferentially reacted with basic OH groups at low molybdate loading, but also with acidic hydroxyl groups at higher. Phosphate ions adsorbed on both types of hydroxyl groups, even at low or moderate coverage, while at high surface coverage a nearly constant density of Al-OH and P-OH groups indicated AlPO4 formation. Phosphorus and molybdenum were well dispersed after coimpregnation, whereas after sequential impregnation (phosphorus first) some pore-plugging occurred and Mo-O compounds adsorbed on underlying phosphates. (C) 1996 Academic Press, Inc.