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

The reduction of Ga(CH3)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 GaH2+ species. At high temperatures and in the absence of hydrogen, the Ga+ species were the most stable, although decomposition of the GaH2+ 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 GaH2+ 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.