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

The metal–support interaction in γ - Al 2 O 3 supported WO X catalysts is investigated by a combination of high field quantitative single pulse (SP) 27 Al MAS NMR spectroscopy, 2D MQMAS, 1 H– 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 Al P site density decreases monotonically with an increased WO X loading, the Al O site density increases concurrently, and the density of Al T sites remains constant. This suggests that the Al P sites are the preferred surface anchoring positions for the WO X species. Importantly, the Al P site isotropic chemical shift observed for the unsupported γ-Al 2 O 3 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 WO X species. Density functional theory (DFT) computational modeling of the NMR parameters on proposed WO X /γ-Al 2 O 3 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 γ-Al 2 O 3, wherein one monomeric and several dimeric structures are identified as the most likely surface anchoring structures.