The promoting role of Cr and K in catalysts for high-pressure and high-temperature methanol and higher-alcohol synthesis

Abstract

The role of Cr and K promoters in the activity and selectivity of methanol and higher alcohol Zn-based catalysts has not been completely clarified. The CO + H₂ reaction on catalysts with different Zn/Cr atomic ratios together with results from X-ray diffraction have shown that the active species is a non-stoichiometric spinel-like phase; potassium increased the ZnO crystal size and decreased the non-stoichiometry of this spinel-like phase. Following methanol adsorption, as well as during the CO + H₂ reaction, surface methoxy and formate groups are formed; the formate/methoxy ratio is much higher on K-doped catalysts. These species decompose when heated, and K-bonded carbonates are formed. The formation of methyl formate at very low temperature occurs only on K-doped catalysts. Chemical enrichment experiments led to the hypothesis that chain growth in the synthesis of higher alcohols takes place through insertion of a C₁ unit in the β-position of a C_n unit. Adsorbed methyl formate could play a fundamental role in this mechanism.