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

Vapor-phase aldol condensation of n-butyraldehyde to 2-ethyl-2-hexenal was studied at 1 atm and 150 similar to 300 degreesC in a fixed-bed, integral-f low reactor by using NaX, KX, gamma -Al2O3 and Na/NaOH/gamma -Al2O3 catalys ts. 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 gamma -Al2O3 with NaOH and Na causes a large decrease of the surface area but strong enhancement of the c atalyst basicity. The catalytic activity on the basis of unit surface area is in the order Na/NaOH/gamma -Al2O3 > KXU > KXW > NaX >gamma -Al2O3, in ac cordance with the relative catalyst basic strength. The molar ratio of trim eric to dimeric products increases with increasing the reaction temperature and the catalyst basic strength except for Na/NaOH/gamma -Al2O3. Very high selectivity of 2-ethyl-2-hexenal (>98.5%) was observed for reactions over NaX zeolite at 150 degreesC. Based on the FT-IR and the catalytic results, the reaction paths are proposed as follows: self-aldol condensation of n-bu tyraldehyde, 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 calcu lated Arrhenius frequency factor and activation energy are 314 mol/g.h and 32.6 kJ/mol, respectively.