Additional Brønsted acid sites in [Ga]HZSM-5 formed by the presence of water

The incorporation of Ga(NO3)3 into HZSM-5 zeolite results in additional Brønsted acid sites evidenced by 1H MAS NMR and FTIR. Such acid sites are proposed to arise from the incorporated Ga extra-framework (GaO(OH)) and insertion of such species with defect hydroxyls into the zeolite framework (GaOHSi). These additional Brønsted acid sites are particularly formed under steaming and may well be responsible for the change in the catalytic behaviour of [Ga]HZSM-5 when water is present in the reaction stream. In the reaction using ethylene as a feed, higher hydrocarbons, namely aromatics, can be produced over “reduced Ga species”. In contrast, the conversion of ethanol over [Ga]HZSM-5 showed an increased activity in a manner similar to that over a typical HZSM-5, but with higher acidity. The formation of the additional acid sites is readily reversible at high temperature (>450 °C) and hence the steam treatment can be used for re-dispersing the incorporated Ga species.

GaO(OH) is formed by chemisorbed water on GaO+. This GaO(OH) can be readily inserted into the framework by reactions with defect hydroxyls (>300 °C) forming additional acid sites. Upon heating, the GaO(OH) can either: (i) react with the Brønsted acid sites to form GaO+ back or (ii) become dehydrated to form smaller clusters of Ga2O3 (>450 °C).Figure optionsDownload as PowerPoint slide