Selective hydrogenation of citral over amorphous NiB and CoB nano-catalysts

Alloy catalysts of P-1NiB and P-1CoB were prepared by reduction of their acetate salts with NaBH4 in aqueous solution. Alloy catalysts of P-2WNiB and P-2WCoB were similarly prepared in ethanolic solution. The NiB and CoB catalysts prepared by this method were characterized as nano-scale (10–20 nm) particles with amorphous structure. These nano-scale alloy catalysts were used for the selective hydrogenation of citral to citronellal, nerol/geraniol and citronellol, which are important perfumery products. These catalysts are compared with the popular commercial catalysts: Raney nickel and Raney cobalt. The NiB and CoB catalysts were generally much more active and selective than Raney nickel or Raney cobalt. The distribution of products depended on the catalysts, the solvent used to prepare the catalysts and the solvent employed as the reaction medium. Over P-1NiB, the conjugated CC bond was preferentially reduced and citronellal was the main product in a reaction medium of cyclohexane, while citronellol was the major one in an ethanol medium. P-2WNiB was much more active than P-1NiB, but citronellol was further reduced to form the completely saturated product 3,7-dimethyloctanol. Over CoB catalysts, the conjugated CO bond was preferentially reduced to form nerol/geraniol and then consecutively reduced to citronellol; no observable 3,7-dimethyloctanol was formed. A yield of citronellal over 85% was obtained over P-1 and P-2WNiB in cyclohexane, and a 100% yield of citronellol was obtained over P-1 and P-2WCoB in ethanol. Only a 65% yield of nerol/geraniol was obtained over CoB catalysts.