Effect of Strength of Soluble and Solid Acid Catalysts on Selectivity in Aldol Reactions

Aldol reactions can follow the classical condensation path and produce an α,β-unsaturated carbonyl compound and water, but in some cases, aldol chemistry may be steered toward a fission pathway that gives an olefin and a carboxylic acid. This investigation examines how the strength of Bro̷nsted acid sites─both in homogeneous and heterogeneous catalysis─controls these two pathways and their kinetics. The cross-aldol reaction between benzaldehyde and 3-pentanone served as a test case. Batch reactions, conducted in toluene as solvent at a temperature of 140 °C under autogenous pressure, were analyzed by GC and in situ ATR-FTIR spectroscopy to determine product distributions and rate constants for condensation and fission pathways. A series of soluble acids, including a family of sulfonic acids, mostly favored the condensation pathway, with formic acid as the weakest in the series, due to its pKa being inactive for aldol chemistry. Significant amounts of fission products were rare except for higher concentrations of benzenesulfonic acid and the known selectivity of phosphoric acid. For the sulfonic acid family, the logarithm of the first-order condensation rate coefficients scaled only roughly with pKa (water) values, whereas a good correlation was obtained with calculated deprotonation Gibbs energies in toluene. A series of H-forms of isomorphously substituted beta zeolites, HESiBEA with E = Al, Ga, Fe, or B, favored the fission pathway. Site density and site strength were characterized by calorimetric measurements of the heats of adsorption of isopropylamine, which decreased in the order Al > Ga, Fe > B. The logarithm of the site-normalized first-order fission rate coefficients scaled roughly with the heat of adsorption and correlated linearly with reported deprotonation energies. In conclusion, acid strength mainly affects activity and can be seen as a prerequisite for either aldol condensation or fission chemistry, whereas additional, yet to be fully clarified, catalyst properties and reaction conditions are required to steer aldol chemistry toward fission selectivity.