Investigating the Surface Structure of γ-Al2O3 Supported WOX Catalysts by High Field 27Al MAS NMR and Electronic Structure Calculations

Abstract

The metal–support interaction in γ-Al2O3 supported WOX catalysts is investigated by a combination of high field quantitative single pulse (SP) 27Al MAS NMR spectroscopy, 2D MQMAS, 1H–27Al CP/MAS, and electronic structure calculations. NMR allows the observation of at least seven different Al sites, including a pentahedral Al site (AlP), three different tetrahedral Al sites (AlT), and three octahedral Al sites (AlO). It is found that the AlP site density decreases monotonically with an increased WOX loading, the AlO site density increases concurrently, and the density of AlT sites remains constant. This suggests that the AlP sites are the preferred surface anchoring positions for the WOX species. Importantly, the AlP site isotropic chemical shift observed for the unsupported γ-Al2O3 at about 38 ppm migrates to the octahedral region with a new isotropic chemical shift value appearing near 7 ppm when the AlP site is anchored by WOX species. Density functional theory (DFT) computational modeling of the NMR pa...