Methanol dehydration to dimethylether over Al2O3 catalysts

Abstract

The vapor-phase dehydration of methanol to dimethylether (DME) has been investigated over a set of eight commercial and self-prepared Al2O3 catalysts with different physicochemical characteristics. Materials were characterized with respect to their textural properties (B.E.T. and B.J.H. methods), acidity (TPD of ammonia), crystallinity, phase composition and morphology (XRD, TEM). Catalytic activity and selectivity have been evaluated in the temperature range of 150–400°C in the absence and in the presence of water in the feed. The adsorption/desorption properties of catalysts toward methanol and water as well as some aspects of the reaction mechanism have been investigated with the use of transient-MS and in situ DRIFTS techniques. Results show that catalytic activity per gram of catalyst varies by two orders of magnitude from one sample to another in a manner that depends on the physicochemical characteristics of Al2O3. All samples investigated are highly selective to DME at temperature lower than ca. 325°C, whereas small amounts of CO and CH4 are formed at higher reaction temperatures. As a general trend, activity per gram of catalyst increases with increase of SSA (amount of acidic surface sites) and is enhanced over samples characterized by high crystallinity and relatively high porosity. The presence of water vapor in the feed suppresses conversion of methanol at a given temperature due to competition for active sites and thermodynamic limitations. Methanol adsorbs on the Al2O3 surface both molecularly and in the form of methoxy species. Evidence is provided that formation of DME involves weakly adsorbed methoxy species whereas methoxy species that are bonded more strongly on the Al2O3 surface are converted to surface formates, which decompose to yield CO, H2 and CH4 in the gas-phase.