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(CO)
and ν(CC) 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.