Shen, W; Tompsett, GA; Xing, R; Conner, WC, Jr; Huber, GW
Vapor phase butanal condensation was studied over well characterized MgO, MgO/SiO2, SrO/SiO2, MgO-SrO/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, 2ethyl-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 MgO-SrO 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 degrees 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. (C) 2011 Elsevier Inc. All rights reserved.