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

The metal-support interaction in gamma-Al2O3 supported WOX catalysts is investigated by a combination of high field quantitative single pulse (SP) Al-27 MAS NMR spectroscopy, 2D MQMAS, H-1(27)-Al CP/MAS, and electronic structure calculations. NMR allows the observation of at least seven different Al sites, including a pentahedral Al site (Al-P), three different tetrahedral Al sites (Al-T), and three octahedral Al sites (Al-O). It is found that the AlP site density decreases monotonically with an increased WOX loading, the Al-O site density increases concurrently, and the density of AlT sites remains constant. This suggests that the Al-P sites are the preferred surface anchoring positions for the WOX species. Importantly, the Al-P site isotropic chemical shift observed for the unsupported gamma-Al2O3 at about 38 ppm migrates to the octahedral region with a new isotropic chemical shift value appearing near 7 ppm when the Al-P site is anchored by WOX species. Density functional theory (DFT) computational modeling of the NMR parameters on proposed WOX/gamma-Al2O3 cluster models is carried out to accurately interpret the dramatic chemical shift changes from which the detailed anchoring mechanisms are obtained. It is found that tungsten dimers and monomers are the preferred supported surface species on gamma-Al2O3, wherein one monomeric and several dimeric structures are identified as the most likely surface anchoring structures.