DRIFTS study of the nature and chemical reactivity of gallium ions in Ga/ZSM-5 II. Oxidation of reduced Ga species in ZSM-5 by nitrous oxide or water

The oxidation of reduced Ga species in ZSM-5 zeolite modified via anchoring of trimethyl gallium or by incipient wetness impregnation with a Solution of gallium nitrate was studied. Reduction Of Such materials in hydrogen and evacuation at high temperature resulted in quantitative replacement of the Bronsted acidic protons by Ga+ ions. However, the extent of dealumination of the material prepared by incipient wetness impregnation was higher. Oxidation of univalent gallium species by nitrous oxide at 673 K by either preparation method led to the quantitative formation of charge-compensating [Ga3+(O2-)](+) fragments. Ga+ ions can also be oxidized at 573 K by water vapor, whereupon approximately 25% of the Ga+ ions are transformed into [GaO](+) with concomitant release of molecular hydrogen. Dissociative chemisorption of molecular hydrogen via oxidative addition of hydrogen atoms to Ga+ cations at 773 K resulted in the formation of gallium dihydride species, which are very stable and are decomposed only in vacuum above 673 K. Oxidation of such [Ga3+(H-)(2)](+) cations by N2O at 573 K gave positively charged [Ga3+(H-)(OH)(-)](+) species. At higher temperature (673 K), the latter are further oxidized by nitrous oxide, giving neutral GaOOH species and Bronsted acidic protons. The similarity of infrared spectra of ethane adsorbed on Ga/ZSM-5 and HZSM-5 at 298 K indicates weak polarization of ethane molecules by Ga+ cations. However, oxidative addition of ethane at 523 K led to the formation of [Ga3+(H-)(C2H5-)](+) species. Above 573 K the grafted ethyl fragments decomposed to gallium hydride species and ethylene. Indications were also found for Subsequent oligomerization and aromatization of the resulting olefin. (c) 2005 Elsevier Inc. All rights reserved.