Sustainable production of acrolein: catalytic performance of hydrated tantalum oxides for gas-phase dehydration of glycerol

Abstract

A series of tantalum oxide samples (Ta₂O₅-T) were prepared by varying the calcination temperature T in the range of 110–700 °C of a highly hydrated precursor (Ta₂O₅·2.1H₂O), which was obtained from a hetero-phase reaction between tantalum pentachloride and aqueous ammonia. These Ta₂O₅-T samples were characterized by TG/DTA, XPS, nitrogen adsorption, XRD, and UV-Raman, and were employed to catalyze the gas-phase dehydration of glycerol (GL) to produce acrolein (AC) at around 315 °C. n-Butylamine titration using Hammett indicators, NH₃- and CO₂-TPD, and IR of adsorbed pyridine were measured to assess the surface acid–base properties of the catalysts. The Ta₂O₅-350 catalyst, which was in a hydrated amorphous state and showed no basicity but a maximum strong acidity at −8.2 < H₀ ≤ −3.0 (H₀ being the Hammett acidity function) prior to the reaction, was identified among all Ta₂O₅-T samples as the best performing catalyst in terms of AC selectivity and GL consumption rate. The AC selectivity increased with increasing the fractional strong acidity at −8.2 < H₀ ≤ −3.0 but the catalytic GL consumption rate increased with increasing the number of strong acid sites on the catalyst prior to the reaction. The partly (Ta₂O₅-600) and fully (Ta₂O₅-700) crystallized samples showed almost no strong acidity but they exhibited some basicity in the CO₂-TPD experiments; their much lower AC selectivity (<40 mol%) as compared with that on Ta₂O₅-350 (75 mol%) indicated that surface basicity was detrimental to the AC selectivity. The best performing Ta₂O₅-350 catalyst also showed after activation in the initial hours of the reaction a good catalytic stability in long term reactions, regardless of the reaction temperature (305–340 °C) and GHSV_GL (80–400 h⁻¹).