Continuous production of higher alcohols from synthesis gas and ethanol using Cs-modified CuO/ZnO/Al2O3 catalysts

The sustainable synthesis of higher alcohols is important for future energy scenarios, mobility applications and the production of chemicals. This study presents the synthesis of higher alcohols from synthesis gas and ethanol using Cs-doped Cu/ZnO catalysts supported on Al2O3 with focus on continuous operation. Ethanol was added to overcome the rate-limiting step of the reaction, which corresponds to the C-1 to C-2 chain growth step. Catalysts prepared by wet impregnation showed the highest selectivity and yield to higher alcohols during screening in batch reactors with 1-butanol as the main alcohol product. The highest yield and selectivity to higher alcohols in a continuous process was found for a space velocity of 19,400 L(STP)/(kg(cat).h), a temperature of 593 K and an ethanol to CO ratio of 0.3:1. In general, alcoholic products were received from aldol-type couplings of C1 or C2 intermediates with adsorbed alcohol derivates. Two types of coupling paths were observed: (i) retention of the oxygen of the C-n intermediate (1-propanol, branched alcohols with a methyl side chain and 2-butanol) and (ii) retention of the oxygen of the adsorbed alcohol (1-propanol and 1-butanol). Thus, products derived from the homocoupling of ethanol (1-butanol/2-butanol) allowed the identification of the reaction path. The corresponding reaction path could be influenced by changing the ethanol to CO ratio resulting in a shift of the alcohol products mainly from reaction path (i), to a mixture of products derived from both paths (i) and (ii) via coupling of C-1 and C-2 intermediates and, finally, to products mostly derived from the homocoupling of ethanol via path (ii). Aldehydes and ketones were identified as reaction intermediates. Ester production competed with the formation of alcohols. Lower ethanol to CO ratios reduced the formation of esters via side reactions.