Role of surface silanols and confinements of siliceous MFI supports on stability of active Ga species for ethane dehydrogenation

Abstract

Effect of surface silanol and confinement of siliceous MFI supports on the anchoring stability of active Ga species was demonstrated for ethane dehydrogenation. The catalysts were prepared by impregnation of Ga(NO3)3 solution on siliceous MFI (Si/Al >500) and amorphous SiO2, and characterized by XRD, XRF, SEM-EDX, H2-TPR, NH3-TPD, in situ XANES, and EXAFS. Extra-framework Ga3+ species were present with different dispersions and reducibility, depending on the surface silanols. Proximity of the silanols within the surface confinement played an essential role on anchoring stability of the extra-framework Ga3+ species. All Ga catalysts provided > 93% ethylene selectivity with appreciable TOF ~60 h−1 for ethane dehydrogenation at 650°C. In situ XANES, EXAFS, and H2-TPR suggested that the highly dispersed extra-framework Ga3+ species could exist as dimeric Ga oxide [Ga2O2]2+ species. This active site could be reversibly interconverted with the less active [HGaOH]+ species under the H2 flow.