The selective hydrogenation of unsaturated ketones (methyl
vinyl
ketone and benzalacetone) and unsaturated aldehydes (crotonaldehyde
and cinnamaldehyde) was carried out with H2 at 2 bar absolute
over Pd/C, Pt/C, Ru/C, Au/C, Au/TiO2, or Au/Fe2O3 catalysts in ethanol or water solvent at 333 K. Comparison
of the turnover frequencies revealed Pd/C to be the most active hydrogenation
catalyst, but the catalyst failed to produce unsaturated alcohols,
indicating hydrogenation of the CC bond was highly preferred
over the CO bond on Pd. The Pt and Ru catalysts were able
to produce unsaturated alcohols from unsaturated aldehydes, but not
from unsaturated ketones. Although Au/Fe2O3 was
able to partially hydrogenate unsaturated ketones to unsaturated alcohols,
the overall hydrogenation rate over gold was the lowest of all of
the metals examined. First-principles density functional theory calculations
were therefore used to explore the reactivity trends of methyl vinyl
ketone (MVK) and benzalacetone (BA) hydrogenation over model Pt(111)
and Ru(0001) surfaces. The observed selectivity over these metals
is likely controlled by the significantly higher activation barriers
to hydrogenate the CO bond compared with those required to
hydrogenate the CC bond. Both the unsaturated alcohol and
the saturated ketone, which are the primary reaction products, are
strongly bound to Ru and can react further to the saturated alcohol.
The lower calculated barriers for the hydrogenation steps over Pt
compared with Ru account for the higher observed turnover frequencies
for the hydrogenation of MVK and BA over Pt. The presence of a phenyl
substituent α to the CC bond in BA increased the barrier
for CC hydrogenation over those associated with the CC
bond in MVK; however, the increase in barriers with phenyl substitution
was not adequate to reverse the selectivity trend.