Solid base catalysts and combined solid base hydrogenation catalysts for the aldol condensation of branched and linear aldehydes Original Research Article

The gas phase condensation reactions of butanal and isobutyraldehyde were studied over a solid base catalyst (4 wt.% Na on SiO2) and combined solid base/hydrogenation catalyst (0.1 wt.% Pd/4 wt.% Na/SiO2). A Pd/Na/SiO2 catalyst can be used to produce a selectively hydrogenated aldol condensation product 2-ethylhexanal from a butanal feed at high selectivity (maximum 94.9% at 42.4% conversion). Selectivity to the hydrogenated product 2-ethylhexanal increases with time on-line and hydrogen pressure. A dual bed system using a Na/SiO2 base catalyst followed by a Cu/Zn hydrogenation catalyst can be used to produce a mixture of butanol and 2-ethylhexanol from a butanal feed. 2-Ethylhexanol production in the dual bed system is increased by contact time and hydrogen pressure. The reaction of isobutyraldehyde over a solid base catalyst produces no products and leads to carbon laydown on the catalyst and in the reactor. The reaction of a 1:1 molar mixture of butanal and isobutyraldehyde produces 2-ethylhexenal and a crossed aldol product 2-ethyl-4-methyl-pent-2-enal. The deactivation of the Pd/Na/SiO2 catalyst with time on-line for the reaction of butanal may be due to the poisoning of base sites by the product 2-ethylhexanal.