Dehydration of ethanol over heteropoly acid catalysts in the gas phase

Abstract

Dehydration of ethanol was studied at a gas–solid interface over a wide range of solid Brønsted acid catalysts based on Keggin-type heteropoly acids (HPAs) in a continuous flow fixed-bed reactor in the temperature range of 90–220°C focussing on the formation of diethyl ether (DEE). The catalysts included H3PW12O40 (HPW) and H4SiW12O40 (HSiW) supported on SiO2, TiO2, Nb2O5 and ZrO2 with sub-monolayer HPA coverage, as well as bulk acidic Cs salts of HPW (Cs2.5H0.5PW12O40 and Cs2.25H0.75PW12O40) and the corresponding core–shell materials with the same total composition (15%HPW/Cs3PW12O40 and 25%HPW/Cs3PW12O40, respectively) comprising HPW supported on the neutral salt Cs3PW12O40. The ethanol-to-DEE reaction was found to be zero order in ethanol in the range of 1.5–10kPa ethanol partial pressure. The acid strength of catalysts was characterised by ammonia adsorption microcalorimetry. A fairly good correlation between the catalyst activity (turnover frequency) and the catalyst acid strength (initial enthalpy of ammonia adsorption) was established, which demonstrates that Brønsted acid sites play important role in ethanol-to-DEE dehydration over HPA catalysts. The acid strength and the catalytic activity of core–shell catalysts HPW/Cs3PW12O40 did not exceed those of the corresponding bulk Cs salts of HPW with the same total composition, which contradicts the literature claims of the superiority of the core–shell HPA catalysts.