Vapor phase butanal self-condensation over unsupported and supported alkaline earth metal oxides

Vapor phase butanal condensation was studied over well-characterized MgO, MgO/SiO2, SrO/SiO2, MgOsingle bondSrO/SiO2 and MgO/HY in a fixed-bed flow reactor. CO2-TPD showed that the base strength and the number of basic sites decreased when the alkaline earth metal oxides were supported on silica. NH3-TPD illustrated that new acid sites were generated when the alkaline metal oxides were supported on silica. The primary product for all catalysts was 2-ethyl-2-hexenal (EHEA) produced by aldol condensation of butanal. Side reactions produced several other products including butanoic acid, heptanone, 2-ethyl-2-hexenol, butanoic acid 2-ethyl-2-hexenol ester and 2,4,-diethyl-2-,4-octandienel by reactions including the Tishchenko cross esterification reactions, double aldol addition, ester hydrolysis, ketonization, cyclization and dehydration. As compared to the unsupported MgO, the silica supported MgO, SrO and MgOsingle bondSrO and MgO/HY exhibited an almost five times higher activity, a higher stability and an improved selectivity. This indicates that both acid and base sites are needed to achieve good catalytic performance for gas phase aldol condensation reactions. Catalyst activation and deactivation mechanisms were studied by butanal-TPD-DRIFTS and butanoic acid TPD-TGA. At temperatures below 300 °C, all catalysts deactivated due to poisoning by the butanoic acid, which was produced as a by-product. The weak bonding of the adsorbed species on the MgO/SiO2 surface contributed to the improved stability of the catalyst for vapor phase butanal condensation.