Mechanism of Methanol and Higher Oxygenate Synthesis

The mechanism of methanol synthesis is discussed with account taken of reported isotope labeling and chemical trapping experiments. Recent studies of alkali promotion of methanol synthesis over the Cu/ZnO catalyst revealed the ion specificity Cs>Rb>K>Na, Li. Alkali are also essential for the development of methanol synthesis activity in the recently discovered alkali/MoS2 heterogeneous catalysts and reported Cu+-based homogeneous catalysts. The catalysts are bifunctional and when the synthesis takes place from CO/H2, the basic component activates CO and the Cu/ZnO, MoS2, or Cu+ soluble complex activates hydrogen. Over the ICI Cu/ZnO/Al2O3 catalyst, 14C labeling experiments show that CO2 is hydrogenated preferentially under the low pressure and high H2/(CO+CO2) ratio industrial conditions. Side products are oxygenates, such as higher alcohols, aldehydes, ketones, esters, and ethers, and hydrocarbons in varying amounts. The product distribution is determined by the catalyst used. The synthesis patterns have been successfully modeled over the Cu/ZnO, Cu/ZnO/Cr2O3, Cs/Cu/ZnO, Cs/Cu/ZnO/Cr2O3, CS/MoS2, CS/Co/MoS2 and K/Co/MoS2 catalysts based on a few fundamental mechanistic steps involving Cn → Cn+1 linear growth, Cn → Cn+1 (n≥2) β-addition, and methyl ester forming reactions. Differences in the mechanisms of linear growth and in the ratio of rates of the linear growth to the β-addition result in two substantially different distributions: over the Cu/ZnO-based catalysts, 2-methyl-l-propanol and 1-propanol are the dominant C2+ products, while over the alkali/(Co)/MoS2 catalyst ethanol prevails among the C2+ oxygenates. While methanol synthesis may be operated with selectivity ≥99%, particularly over the (Cs)/Cu/ZnO catalyst, the higher alcohol content may be increased up to 40% of oxygenated products over the alkali-doped copper-based catalysts and to 80% over the alkali/Co/MoS2 catalyst by the choice of reaction conditions. The kinetic models presented here and in the quoted literature permit the prediction of various oxygenated side product compositions at different synthesis conditions from the temperature coefficients of the kinetic constants for linear growth, β-addition, and methyl ester forming reactions.