Aldol condensation of mixtures of acetaldehyde, acetone and butanone was investigated over a powder TiO₂ catalyst as a strategy for coupling mixtures of light oxygenates generated from biomass pyrolysis into higher-value products. Aldol condensation reactions were performed for single component, binary and ternary mixtures of the three reactants. Pathways for major reaction products generated at industrially relevant temperatures (300-400°C) were identified by comparing the product mixtures for each of the ternary, binary and pure-component reactions. In both binary and ternary mixtures, conversion of acetaldehyde was enhanced while the conversion of either alkenone was suppressed.
To provide further understanding into the mechanisms driving these reactivity trends, kinetics for single component and binary reaction mixtures of acetaldehyde and acetone were measured at relatively low reaction temperatures (150-200°C) and differential conversion to isolate how condensation of mixed streams differed from single-component condensation. Results from kinetic and temperature programmed desorption experiments suggested that acetaldehyde out-competed larger carbonyl compounds for access to active sites. Additionally, trends in product stream selectivity for the acetaldehyde and acetone mixed feed reaction show that acetaldehyde is preferentially activated as an electrophile, interacting with nucleophilic forms of adsorbed acetaldehyde and acetone.
To control rate, selectivity, and stability, the powder TiO₂ was modified in two distinct ways. The first approach was to deposit supported metal nanoparticles to enable the hydrogenation of reactive intermediates and mitigate polymerization and coking reactions. The second approach was to add phosphonic acid self-assembled monolayers to sterically hinder the formation of heavy, non-volatile organics that can block active sites. The inclusion of the supported metal nanoparticles and the hydrogenation functionality resulted in a large increase in conversion and an increase in product selectivity to products in the C₆ to C₁₅ range. The addition of phosphonic acids to the catalyst unexpectedly increased the ratio of C₅ to C₄ products, suggesting a relative enhancement in the activation of acetone as a nucleophile.

Brandon Oliphant

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