Surface Chemistry of Ketones and Diketones on Lewis Acidic γ-Al2O3 Probed by Infrared Spectroscopy

Abstract

To advance the understanding of surface reactions of biomass-derived oxygenates, adsorption and conversion of ketones and diketones on Lewis acidic γ-Al2O3 are studied. The model compounds include ketones with hydroxyl groups (di/hydroxyacetone) as well as diketones with different distances between the two carbonyl groups (2,3-butanedione and 3,4-hexanedione, 2,4-pentanedione, and 2,5-hexanedione as α-, β-, γ-diketones, respectively). In situ infrared (IR) spectroscopy is utilized to experimentally observe the surface species. The deconvoluted IR spectra acquired between 50 and 250 °C suggest that intermolecular aldol condensation is the most common reaction path for the studied di/ketone reagents. This reaction path consists of sequential enolization, dimerization, and dehydration to form conjugated products with lower ν(C═O) and ν(C═C) frequencies. Exceptions included intramolecular aldol condensation of 2,5-hexanedione and isomerization and dehydration of dihydroxyacetone. Density functional theory calculations suggest that diketones bind as monodentate surface species provided their stability on γ-Al2O3. This study provides insight on di/ketone chemistry on Lewis acidic γ-Al2O3 that will be relevant for many fields including prebiotic and industrial chemistry.