(521cp) Aldol Condensation of Mixed Oxygenates on TiO2 Catalysts

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 heavier, 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 reaction temperatures (300 - 400°C) were identified by comparing the product mixtures for each of the ternary, binary, and pure-component reactions. Selectivity to heavier C₅₊ condensation products was high (>80%) at all temperatures investigated, but undesirable lighter hydrocarbons, formed through cracking of larger condensation products, were also produced during each condensation reaction. The selectivity to both heavier C₅₊ and light hydrocarbon products were found to increase with reaction temperature as well as the average molecular weight of the reactant feed stream. Moderate temperatures (350°C) were found to provide an optimal balance of high conversion and low yield to light hydrocarbon side products.

To provide further understanding into the mechanisms driving these reactivity trends, kinetics for single component and binary reaction mixture reactions 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. Beyond simply lowering the rate via site blocking, the presence of acetaldehyde in the mixture increased the apparent activation barriers and orders for acetone condensation. These changes reflect an unexpected surface interaction between different carbonyl compounds on the catalyst surface.