Multinuclear gallium-oxide cations in high-silica zeolites

Periodic DFT calculations of the stability of mononuclear and oligonuclear Ga-oxo cations in mordenite (MOR) have been carried out. Independent of the aluminium distribution in the zeolite framework the stability of cyclic Ga2O22+ ions is much higher than that of the isolated GaO+ (gallyl) ions in a high-silica mordenite (Si/Al = 23) model. As to the location of such dimers, favorable tetrahedral coordination environment of Ga dominates over the necessity to compensate the positive extraframework charges directly with proximate negative framework charges. Charge alternation can occur in Ga2O2/MOR models in which positive charges of the cationic complex are separated from the framework anionic sites. Oligomerization of four isolated gallyl ions in a MOR model with Si/Al = 11 results in the formation of cubic Ga4O44+ ions. Also in this case direct interaction of the cluster is limited to two anionic sites, while two other framework [AlO2](-) units are significantly remote. Binuclear sites are argued to account for the enhanced activity of oxygenated gallium-exchanged high-silica zeolites in alkane dehydrogenation. These sites, however, tend to decompose via water desorption upon the catalytic reaction resulting in less reactive reduced Ga+ ions. As per predictions from the quantum-chemical calculations, the experimental results show that the high alkane dehydrogenation activity can be maintained by in situ hydrolysis of the reduced extraframework Ga species.