In situ Ga K-edge XANES study of Ga-exchanged zeolites at high temperatures under different atmospheres including vacuum, CO, and pressurized H2

Abstract

Ga-exchanged zeolites are promising catalysts for dehydrogenative alkane transformations. Reduced Ga species, such as Ga hydrides and Ga⁺ cations, are possible active sites for alkane dehydrogenation based on in situ spectroscopic studies. However, most of the reported studies were conducted at lower temperatures (<300 °C) compared with those encountered in actual operating conditions (>600 °C). In this study, the in situ Ga K-edge XANES analysis of Ga-exchanged MFI zeolite was performed under actual operating temperatures (>700 °C) and different atmospheres (H₂, He, CO, vacuum, and C₂H₆). The absorption edge appeared at similarly lower energy values than that of Ga₂O₃ in all atmospheres, whereas the main absorption peak intensity differed depending on the presence or absence of a hydride source in the gas phase. Higher intensities were observed under H₂ and C₂H₆ atmospheres than under CO and vacuum atmospheres. It was also noted that the main absorption peak intensity increased with increasing H₂ pressure, indicating that the higher intensity was responsible for the presence of Ga-hydride species. Similar in situ XANES measurements were conducted for other Ga-exchanged zeolites. The obtained results were compared with our previously reported data on the relative formation amounts and steady-state activity for ethane dehydrogenation.