Ag0 and Co0 nanocolloids as recyclable quasihomogeneous metal catalysts for the hydrogenation of α,β-unsaturated aldehydes to allylic alcohol fragrances Original Research Article

The hydrogenation of the α,β-unsaturated aldehydes cinnamaldehyde (trans-3-phenyl-2-propenal), citral (3,7-dimethyl-2,6-octadienal) and leaf aldehyde (trans-2-hexenal) was investigated with metal colloids as quasihomogeneous catalysts in organic solvents. Using polyvinylpyrrolidone-stabilized Ag0 and Co0 nanoclusters, the carbonyl compounds were chemoselectively hydrogenated to the desired allylic alcohol fragrances. Particularly the Ag0 nanosols emerged as a worthwhile alternative for Pt or Ru based catalysts. Amides proved to be excellent solvents for the synthesis as well as for the application of the metal nanocolloids in the hydrogenation of α,β-unsaturated aldehydes. In the challenging hydrogenation of citral, a selectivity exceeding 70% towards the doubly unsaturated alcohol isomers (geraniol and nerol) was attained at 90% conversion using Ag0 nanocolloids dispersed in N,N-dimethylacetamide. Reactions were performed under mild conditions (2.0 MPa H2, 323 K) at a molar citral/silver ratio of 200/1. The addition of Lewis acid cations, such as Fe3+ or Zn2+ resulted in a further improvement of the catalytic activity and chemoselectivity. Finally, by application of membrane filtrations, an efficient separation of the metallic nanocolloids from the reaction mixtures was realized. In the recycling of the metal nanosols, the metal catalysts’ nanodispersion and performance were satisfactorily preserved.