Vapor-phase aldolization of n-butyraldehyde to 2-ethyl-2-hexenal over solid-base catalysts

Article abstract of DOI:10.1002/jccs.200000169

Vapor-phase aldol condensation of n-butyraldehyde to 2-ethyl-2-hexenal was studied at 1 atm and 150～ 300°C in a fixed-bed, integral-flow reactor by using NaX, KX, γ-Al₂O₃ and Na/NaOH/γ-Al₂O₃ catalysts. Ion exchange of NaX zeolite with potassium acetate solution results in a decrease of crystallinity and apparent lowering of surface area, whereas the basic strength is enhanced. Treatment of γ-Al₂O₃ with NaOH and Na causes a large decrease of the surface area but strong enhancement of the catalyst basicity. The catalytic activity on the basis of unit surface area is in the order Na/NaOH/γ-Al₂O₃ > KXU > KXW > NaX>γ-Al₂O₃, in accordance with the relative catalyst basic strength. The molar ratio of trimeric to dimeric products increases with increasing the reaction temperature and the catalyst basic strength except for Na/NaOH/γ-Al₂O₃. Very high selectivity of 2-ethyl-2-hexenal (>98.5%) was observed for reactions over NaX zeolite at 150°C. Based on the FT-IR and the catalytic results, the reaction paths are proposed as follows: self-aldol condensation of n-butyraldehyde, followed by dehydration produces 2-ethyl-2-hexenal, which then reacts with n-butyraldehyde and successively dehydrates to 2,4-diethyl-2,4-octadienal and 1,3,5-triethylbenzene. For the reaction over NaX, the calculated Arrhenius frequency factor and activation energy are 314 mol/g-h and 32.6 kJ/mol, respectively.