Aldol condensation is an important C-C bond formation reaction in chemical synthesis that finds versatile applications in bulk and fine chemical industries and has potential for upgrading of biomass to fuels. However, aldol reactions pose a challenge for controlling the product selectivity by forming a mixture of desired and undesired self, cross, and poly-condensation products. Furthermore, a side reaction, the fission of aldol products resulting in an olefin and a carboxylic acid, has garnered attention recently as a route to isobutene from acetone. While a high level of selectivity control is achievable by homogeneous catalysis, selective solid catalysts, which would be more robust, facilitate separation and make aldol processes more sustainable, are lacking. This thesis presents the factors that govern chemo- and regioselectivity in heterogeneously catalyzed cross aldol reactions.
Several families of catalysts were applied in liquid phase batch reactions to explore the effect of materials properties and process conditions. The nature of the active site (strong/weak, Brønsted/Lewis, acid/base), porosity (micro/meso), and hydrophobicity were systematically varied. Soluble analogs were included for comparison.
Regioselectivity was investigated using benzaldehyde and 2-butanone, which may form branched (reaction at -CH2- of butanone) or linear (reaction at -CH3) addition and condensation products. Product distributions analyzed at different conversions revealed that kinetics and equilibria of the addition and dehydration steps govern regioselectivity. Solid acid (-SO3H functionalized) catalysts favored the branched condensation product like their soluble analog. This result is explained with fast dehydration catalyzed by the strong acid and regioselectivity control through preferred formation of the branched aldol. In the absence of strong acid sites (BEA zeotypes), the linear condensation product predominated since dehydration of the respective aldol is facile, consistent with literature. Solid amine catalysts preferentially formed linear condensation products as a consequence of the higher stability of the intermediate formed at the less substituted methyl carbon.
A kinetic model was developed for the aldol condensation and fission of 3-pentanone and benzaldehyde that accurately describes the trends in the experimental data and estimates reaction orders and rate constants. Arrhenius analysis indicated a higher activation energy for fission (to β-methylstyrene and acetic acid) than for condensation explaining the increased fission selectivity at higher reaction temperatures with a sulfonic acid functionalized MCM-41 catalyst. The high fission selectivity observed with zeolites of different frameworks and proton densities can be rationalized by hindered dehydration of the aldol owing to the antiperiplanar orientation required for the E2 elimination mechanism.
These findings demonstrate that both regio- and chemoselectivity in solid-catalyzed aldol reactions can be directed by tuning surface functionality and texture of catalyst materials and by optimizing the reaction conditions.