(49c) Effect of Strength of Soluble and Solid Acid Catalysts on Selectivity in Cross Aldol Reactions

Aldol condensations are important in the synthesis of bulk and fine chemicals, while showing potential for converting biomass-derived feedstocks into fuels. More recently, a side reaction of aldol chemistry, a fission into an olefin and a carboxylic acid has gained attention since it opens up a route from bioethanol to isobutene, via acetone and its aldol reaction [1]. It is debated whether fission occurs at the aldol stage or after dehydration and whether it progresses via carbocations or radicals [2-4]. Very little systematic information exists about the role of the catalyst; although, generally, acidity is needed. Here, we investigate the role of acid strength and the role of homogeneous vs. heterogeneous catalysis for the fission selectivity in cross aldol reactions.

The following trends among soluble organic and inorganic acids with pK_a values from 3.74 to -0.6 were identified: For most mild acids, the reaction was slow and did not progress beyond the aldol intermediate. At intermediate acidity, exclusively aldol condensation was observed. A threshold acidity of pK_a < 2 was a necessary but not a sufficient condition for fission. For example, sulfonic acids exclusively catalyzed condensation while measured rates did not correlate with reported pK_a values. These results suggest selectivity depends on additional factors, such as polarity of acid or solvent.

In contrast, the tested solid acids all catalyzed fission. Zeolites/-types always exhibited high fission selectivity (> 70%), whereas the fission selectivity of sulfonic acid resins was highly variable and depended on the reactants and the conditions. First-order rate constants for fission in zeotypes were a function of the trivalent cation and decreased in the order Al > Ga, Fe ≥ B (Figure 1).

In sum, the results demonstrate that control of the selectivity towards aldol condensation or towards fission is possible by the appropriate choice of catalyst.