Structure and reactivity of surface vanadia sites in bi-layered supported VO x /AlO x /SiO 2 catalysts via solid-state NMR, first-principles calculations, and catalytic studies

By combining multinuclear 1 H, 29 Si, 27 Al and 51 V solid -state NMR experimental measurements with reactivity and selectivity data, as well as first-principles calculations, for ternary bi-layered supported VO x /Al 2 O 3 /SiO 2 catalysts, the structure of the dehydrated surface vanadia species was determined for each combination of supported VO x and AlO x active components. The specific structure of the surface vanadia sites in the ternary bilayered supported VO x /Al 2 O 3 /SiO 2 catalyst system have been investigated for the first time. The following vanadia sites are proposed based on the current findings: At low vanadia content, surface VO(OH)(OSi) 2 sites form on the alumina -free surface of the SiO 2 support in the bi-layered supported VO x /Al 2 O 3 /SiO 2 catalyst system. In addition, two types of VO(OSi) 3 surface species are likely present in this system. On small alumina clusters, weakly bonded surface VO(OH)(OAl) 2 sites also form, along with the sites containing mixed Si/Al pods VO(OH)(OAl)(OSi). As the size of alumina clusters increases, strongly bonded VO(OAl) 3 centers form, alongside the surface sites containing mixed pods VO(OAl) 2 (OSi) and VO(OAl)(OSi) 2 . The content of these sites increases with the loading of vanadia. Comparing supported VO x /SiO 2 , VO x /Al 2 O 3 , and bi-layered VO x /Al 2 O 3 /SiO 2 catalyst systems revealed differences in catalytic activity due to the presence of new sites with different pods, with OH groups as well as the influence of Al -O -Si bonds on the electronic structure of surface vanadia sites on the alumina clusters. During the formation of dimethyl ether (DME), the turnover frequency (TOF DME ) of the catalysts decreases with increasing domain size of the alumina clusters on SiO 2 . Addition of surface VO x sites, however, slightly decreases the TOF DME with the higher vanadia content weakening the activity of the alumina clusters. When surface VO x exists predominantly as V/Al 1 and V/ Al 2 , TOF redox depend on the vanadia content with lower vanadia content resulting in higher the TOF redox values. The 51 V NMR spectra show that this decrease in redox TOF value correlates with the appearance of vanadia sites associated with silica pods. The activity of the catalysts was tested in methanol conversion reactions.