DEVELOPMENT OF A KINETIC-MODEL FOR ALCOHOL SYNTHESIS OVER A CESIUM-PROMOTED CU/ZNO CATALYST

A kinetic reaction network has been developed for the synthesis of oxygenates from synthesis gave over Cs-promoted Cu/ZnO catalysts in a differential reaction regime. The most important carbon-carbon bond forming reactions of the model are linear growth by addition of an oxygenated C1 intermediate to an alcohol chain that yields linear alcohols and a beta-addition aldol condensation type mechanism between an oxygenated C1 intermediate and an aldehydic C(n) intermediate that yields 2-methyl-branched primary alcohols. The most important carbon-oxygen bond forming reaction is a methoxide addition, alpha-o, that yields methyl esters. Estimates of the kinetic parameters show that the beta-addition is faster than linear growth, which results in high selectivities to branched alcohols, i.e., 2-methyl-1-propanol. Also, the rate of growth of the C2 intermediate is faster than for any other C(n) (n-greater-than-or-equal-to-2) intermediate and this result is specific to the C(s)-promoted catalyst. Comparison of models based on other mechanistic proposals suggests that a type of CO insertion into an aldehyde-catalyst bond that would lead to branched alcohols is not a major carbon-carbon bond forming reaction. In addition, the absence of significant yields of secondary alcohols over the C(s)/Cu/ZnO catalyst shows that an alpha-addition that would yield secondary alcohols is not an important pathway for carbon-carbon bond formation on this catalyst.