Development of heterogeneous catalysts which can achieve both high activity and high selectivity requires establishment of sophisticated and uniform active sites on the surface of solid materials, but formation of uniform active sites is difficult in heterogeneous catalysts because of the nonuniformity of heterogeneous catalysts. Selective hydrogenation of the carbonyl group of unsaturated carbonyl compounds is particularly difficult because hydrogenation of the olefin group is thermodynamically and kinetically preferable to that of the carbonyl groups. Therefore, various heterogeneous catalyst systems have been developed to achieve high selectivity. Recently developed effective heterogeneous catalysts for the selective hydrogenation of unsaturated carbonyl compounds such as unsaturated aldehydes and unsaturated ketones to the corresponding unsaturated alcohols using H2 as a reducing agent are reviewed. Model reactions include liquid-phase selective hydrogenation of crotonaldehyde and liquid-phase selective hydrogenation of benzylideneacetone using a batch reactor, and gas-phase selective hydrogenation of crotonaldehyde with a fixed-bed reactor. Particularly, this review mainly covers metal oxide- or metal cation-modified noble metal catalysts.
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