MECHANISTIC KINETIC TREATMENT OF THE CHAIN GROWTH-PROCESS IN HIGHER ALCOHOL SYNTHESIS OVER A CS-PROMOTED ZN-CR-O CATALYST

A kinetic analysis has been performed of higher alcohol synthesis over ZnCrO + 15% Cs2O (w/w) using a reaction network strictly based on mechanistic evidence. This includes crossed aldol condensations of aldehydes and ketones and stepwise C, linear additions, both occurring in normal and "oxygen retention reversal" modes, as well as reversible ketonization reactions, and is consistent with chemical equilibrium constraints affecting the following reactions: methanol synthesis, water-gas shift, hydrogenation of carbonyl compounds, ester formation, and ketonization. Kinetic assumptions include reversible equilibrium adsorption of the reacting species on the catalyst sites, equilibrium of the C1 intermediates with CO/H2, and competition of water in adsorbing on the active sites. In line with available chemical data, different reactivities are attributed to the species participating in the chain growth process depending on their molecular structure (aldehydes versus ketones, linear versus branched). Using nine rate parameters, the model successfully describes the concentrations of 20 primary alcohols and ketones (and of the species related by chemical equilibria) as functions of contact time, temperature, pressure, and feed composition. The formation of about 50 additional compounds in trace amounts is also predicted, which are indeed not detected in the synthesis products. The parameter estimates show that aldol condensations are one order of magnitude faster than C1 additions, indicate a greater reactivity of aldehydes than ketones, attribute a significant role to reverse ketonizations in the chain growth process in parallel to aldol condensations, and demonstrate the specific inhibiting action of water on condensations with oxygen retention reversal and ketonization. The apparent activation energies are very small for aldol condensations, ∼63 kJ/mol for C2 linear additions, and ∼155 kJ/mol for ketonizations. © 1992.