n-Butyraldehyde Self-Condensation Catalyzed by Sulfonic Acid Functionalized Ionic Liquids

Self-condensation of n-butyraldehyde to 2-ethyl-2-hexenal is one of the important processes for the industrial production of 2-ethylhexanol. In the present work, several sulfonic acid functionalized ionic liquids (SFILs) were synthesized. Their acid strengths were determined by the Hammett method combined with UVvis spectroscopy, and their catalytic performances in n-butyraldehyde self-condensation were investigated. The results show that the conversion of n-butyraldehyde correlated well with the acid strength of the SFILs with the same cation. The SFILs with triethylammonium cations showed a better catalytic performance than those with imidazolium cations or pyridinium cations, and [HSO3-b-N(Et)3]p-TSA (b, butyl) exhibited the highest selectivity. Under the optimal reaction conditions of the mass ratio of [HSO3-b-N(Et)(3)]p-TSA to n-butyraldehyde = 0.1, reaction temperature = 393 K, and reaction time = 6 h, the conversion of n-butyraldehyde was 89.7% and the selectivity to 2-ethyl-2-hexenal was 87.8%. [HSO3-b-N(Et)(3)]p-TSA could be reused four times without a significant loss in its catalytic performance. A kinetic analysis result showed that this is a reversible second-order reaction. Compared with the kinetic parameters from the reaction catalyzed by an aqueous base or acid catalyst, the pre-exponential factor is lower due to the restriction of the high viscosity of [HSO3-b-N(Et)(3)]p-TSA. Finally, a possible reaction mechanism for n-butyraldehyde self-condensation catalyzed by [HSO3-b-N(Et)(3)]p-TSA was proposed.