Ethanol Dehydration over Silica-Supported H₃PW₁₂O₄₀ and the Role of Water

Catalytic dehydration of (bioderived) ethanol to ethylene or diethyl ether (DEE) offers an atom-efficient route to commodity chemicals and renewable aviation fuels. Here, the impact of silica support morphology and dispersion of H3PW12O40 (HPW) on the vapor-phase dehydration of ethanol to ethylene and DEE was investigated. Ethanol conversion at 175 °C and ambient pressure was inversely proportional to HPW dispersion over a fumed silica and mesoporous SBA-15 support, with specific activity directly proportional to the crystalline water content, highlighting the importance of catalysis within the pseudo-liquid phase. A common turnover frequency of ∼2500 h–1 was determined for HPW/SBA-15, with all acid sites participating. Catalyst deactivation at 175 °C could be suppressed by co-feeding 10 wt % water, likely by mitigating the loss of crystalline (acidic) water; higher reaction temperatures induce decomposition of the heteropolyanion to WO3 and could also be partially suppressed by co-fed water. In the presence of co-fed water, the optimum 50 wt % HPW/SBA-15 catalyst could be used for three consecutive reactions at 175 °C with minimal loss of activity or selectivity without any reactivation protocol. Ethanol dehydration was selective to DEE (∼80%) for reaction <225 °C, with higher temperatures inducing a switchover to ethylene (87% ≥ 300 °C) in accordance with thermodynamic predictions. Maximum steady-state DEE productivity was 600 mmol·gcat–1·h–1 at 175 °C, and maximum steady-state ethylene productivity was 1800 mmol·gcat–1·h–1 at 225 °C. In situ DRIFTS identified the protonated ethanol dimer (C2H5OH)2H+ as the reactive intermediate to DEE formation, with higher temperatures favoring the formation of protonated ethanol (C2H5OH)H+ and ethoxy intermediates to ethylene.