Synthesis of C 2+ oxygenates from methanol at atmospheric pressure over alkali-promoted zinc-chromium oxide catalysts

Abstract

Zinc-chromium oxide based catalysts have been tested for the synthesis of higher oxygenates from methanol and hydrogen at atmospheric pressure and in the temperature range 300–405dgC. It has been found that potassium-promoted zinc-chromium oxide, while decomposing a large part of methanol to carbon monoxide and hydrogen, also produces significant amounts of C 2+ oxygenates. It is proposed that formation of higher oxygenates occurs by a slow C 1→C 2 step, which involves methanol-related C 1 species, followed by rapid aldol-type condensation reactions and by fast hydrogenation to alcohols. Hydrogen is not necessary for the formation of higher oxygenates, but appears to prevent catalyst deactivation. Marked changes in the product selectivity have been observed when using unpromoted zinc-chromium oxide: higher oxygenates are no longer produced and the fraction of methanol not decomposed to carbon monoxide and hydrogen is converted to dimethyl ether and hydrocarbons. It is concluded that alkali addition plays a crucial role in the formation of C 2+ oxygenates over zinc-chromium oxide catalysts.