Effects of support and reaction pressure for the synthesis of dimethyl ether over heteropolyacid catalysts.

Dimethyl ether (DME) is an advanced second-generation biofuel produced via methanol dehydration over acid catalysts such as γ-AlO, at temperatures above 240 °C and pressures above 10 bar. Heteropolyacids such as tungstosilicic acid (HSiW) are Brønsted acid catalysts with higher DME production rates than γ-AlO, especially at low temperatures (140-180 °C). In this work, we show that the performance of supported HSiW for the production of DME is strongly affected by the nature of the support. TiO and SiO supported HSiW display the highest DME production rates of ca. 50 mmol/h/g. Characterization of acid sites via H-NMR, NH-isotherms and NH-adsrobed DRIFT reveal that HSiW/X have Brønsted acid sites, HSiW/TiO showing more and stronger sites, being the most active catalyst. Methanol production increases with T until 200 °C where a rapid decay in methanol conversion is observed. This effect is not irreversible, and methanol conversion increases to ca. 90% by increasing reaction pressure to 10 bar, with DME being the only product detected at all reaction conditions studied in this work. The loss of catalytic activity with the increasing temperature and its increasing with reaction pressure accounts to the degree of contribution of the pseudo-liquid catalysis under the reaction conditions studied.