Development of Highly Active Heterogeneous Oxidation Catalysts Based on the Properties of Metal Hydroxides

Abstract

This manuscript summarizes our recent research results on heterogeneously catalyzed aerobic oxidation reactions using supported metal hydroxide catalysts. Metal hydroxide species possess both Lewis acid and Bronsted base sites on the same metal sites, and various kinds of substrates including alcohols, primary amines, nitriles, and terminal alkynes can be activated by the concerted action of the Lewis acid and Bronsted base pair sites. In the presence of a supported ruthenium hydroxide catalyst (Ru(OH) x /Al2O3), oxidative dehydrogenation of alcohols and primary amines efficiently proceeds to afford the corresponding carbonyl compounds (aldehydes and ketones) and nitriles, respectively. In the presence of ammonia (THF solution), Ru(OH) x /Al2O3 can catalyze ammoxidation of primary alcohols to the corresponding nitriles. This ammoxidation proceeds through the sequential reactions of oxidative dehydrogenation of primary alcohols to aldehydes, dehydrative condensation of the aldehydes with ammonia to aldimines, and oxidative dehydrogenation of the aldimines. In addition, formal α-oxygenation of primary amines to the corresponding primary amides can be realized through the Ru(OH) x /Al2O3-catalyzed oxidative dehydrogenation and nitrile hydration. Moreover, a supported copper hydroxide species on a manganese oxide-based octahedral molecular sieve (Cu(OH) x /OMS-2) can act as an efficient and highly durable heterogeneous catalyst for oxidative homocoupling of terminal alkynes to the corresponding diynes. The catalyses for the above-mentioned oxidation reactions are truly heterogeneous, and the catalysts can be reused several times with keeping their high catalytic performance.