A facile sol-gel synthesis of highly active nano α-aluminum fluoride catalyst for dehydrofluorination of hydrofluorocarbons

Nano α-aluminum fluorides with high surface area were synthesized by a simple ethylene glycol mediated sol–gel technique using dilute aq. HF as the F source, and their performances were investigated for gas-phase dehydrofluorination of 1,1,1,2-tetrafluoroethane at relatively low reaction temperature (400 °C). Aluminum precursor was crucial for the phase composition, textural and acidic properties of the resulting fluoride. Commercially low-cost and available inorganic Al was firstly used as precursor for synthesis of aluminum fluorides via the sol–gel route, and the as-prepared α-AlF3 possessed high surface area (up to 170 m2 g−1) and strong surface acidities even after thermal treatment of 400 °C. The presence of sulfate ions suppressed the crystallization of aluminum fluoride during calcination when Al2(SO4)3 was used as precursor, leading to an amorphous phase for as-prepared AlF3. Aluminum fluorides with β and pyrochlore phase were prepared using aluminum isopropoxide and aluminum diacetate hydroxide, respectively, as precursors. Based on the results of IR, XRD and element analysis, it is concluded that the formation of Al-glycolates during the sol–gel process can effectively reduce the hydrolysis of precursor and suppress particle agglomeration, thus leading to dominant formation of nano aluminum fluorides. The residual O derived from minor hydrolysis can resist sintering under high-temperature treatment, which is a key factor to improve the thermostability of as-prepared aluminum fluoride. Due to their large Lewis acidic amounts, the α-AlF3 derived from AlOOH and Al2O3 exhibited remarkably high catalytic activity with CF3CH2F conversion of 24%–28% and CF2=CHF selectivity above 98%, which were far higher than that of AlF3 prepared by traditional methods.