DRIFTS study of the nature and chemical reactivity of gallium ions in Ga/ZSM-5

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 Brønsted 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 Brønsted 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.