Characterization and reactivity of Ga + and GaO + cations in zeolite ZSM-5

• Published in: Journal of catalysis 2006-04, Vol.239 (2), p.478-485
• Main Authors: Rane, N., Overweg, A.R., Kazansky, V.B., van Santen, R.A., Hensen, E.J.M.
• Format: Article
• Language: English
• Subjects: Brønsted acid sites; Chemical vapor deposition; Dehydrogenation; HZSM-5; Reduction; Trimethylgallium
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2006.03.004

The reduction of Ga(CH 3) 3/ZSM-5 was closely followed by Fourier transform infrared spectroscopy and Ga K-edge X-ray absorption near-edge spectroscopy. Chemical vapor deposition of trimethylgallium on HZSM-5 (TMG/ZSM-5) resulted in the replacement of nearly all Brønsted acid protons by dimethylgallium species. Removal of the methyl ligands from the cationic Ga clusters gave charge-compensating Ga + and GaH 2 + species. At high temperatures and in the absence of hydrogen, the Ga + species were the most stable, although decomposition of the GaH 2 + species was very slow. Ga + ions can be oxidized by nitrous oxide at low temperature (473 K), resulting in the formation of gallyl (GaO +) cations. A detailed comparison of the reactivity of Brønsted acid protons (HZSM-5) and Ga + ions (reduced TMG/ZSM-5) in propane dehydrogenation showed that the former converted propane via protolytic cracking with methane, ethane, and propene as hydrocarbon products, whereas monovalent Ga + ions produced propene almost exclusively. The reaction data suggest that propane was converted over Ga + cations but not over GaH 2 + cations. The initial rate of propane dehydrogenation was highest for GaO + ions, although rapid deactivation was observed, due to the higher barrier for regeneration of GaO + ions than for formation of less active Ga + ions.