One pot transformation of citronellal to menthol over Ni/γ-al₂o₃

Citronella oil from Cymbopogon winterianus is widely produced in Indonesia (Java type). A way to increase the value of this product is to transform citronellal contained in the oil into menthol, one of the most sought after fragrance products. The one pot citronellal to menthol transformation is also a war horse of tandem reactions over heterogeneous catalysts. Here we wish to report our results in the one pot transformation of citronellal into menthol over a Ni/Al₂O₃ catalyst. While alumina showed to be a poor catalyst for the transformation of citronellal into isopulegol, ZnBr₂ modified alumina allowed to obtain very fast formation of isopulegol. However, when Ni was supported on the modified alumina and used under hydrogenation conditions only isopulegol was obtained even after long reaction times. On the contrary a 10% Ni/ Al₂O₃ catalyst showed fairly good activity. These results can be ascribed to very big differences in Ni particle size and to reduced surface area of the Zn containing catalyst.