jp1067523_si_001.pdf (630.04 kB)
Download file

High-Temperature Decomposition of Brønsted Acid Sites in Gallium-Substituted Zeolites

Download (630.04 kB)
journal contribution
posted on 2010-11-18, 00:00 authored by Khalid A. Al-majnouni, Nathan D. Hould, William W. Lonergan, Dionisios G. Vlachos, Raul F. Lobo
The dehydroxylation of Brønsted acid sites (BAS) in Ga-substituted zeolites was investigated at temperatures up to 850 °C using X-ray absorption spectroscopy (XAS), Fourier transform infrared spectroscopy (FTIR), and mass spectrometry-temperature programmed desorption (MS-TPD). X-ray absorption near-edge spectroscopy (XANES) revealed that the majority of gallium has tetrahedral coordination even after complete dehydroxylation. The interatomic gallium−oxygen distance and gallium coordination number determined by extended X-ray absorption fine structure (EXAFS) are consistent with gallium in tetrahedral coordination at low T (<550 °C). Upon heating Ga-Beta and Ga−ZSM5 to 850 °C, analysis of the EXAFS showed that 70 and 80% of the gallium was still in tetrahedral coordination. The remainder of the gallium was found to be in octahedral coordination. No trigonal Ga atoms were observed. FTIR measurements carried out at similar temperatures show that the intensity of the OH vibration due to BAS has been eliminated. MS-TPD revealed that hydrogen in addition to water evolved from the samples during dehydroxylation. This shows that dehydrogenation in addition to dehydration is a mechanism that contributes to BAS decomposition. Dehydrogenation was further confirmed by exposing the sample to hydrogen to regenerate some of the BAS as monitored by FTIR and MS-TPD.