Pd-catalyzed completely selective hydrogenation of conjugated and isolated CC of citral (3,7-dimethyl-2, 6-octadienal) in supercritical carbon dioxide

Abstract

Mesoporous Pd-MCM-48 as well as Pd/SiO₂, Pd/Al₂O₃ catalysts containing 1% of Pd were successfully used in the selective hydrogenation of citral in scCO₂. The reported results show that it is possible to hydrogenate the conjugated and isolated CC double bond forming a fully saturated aldehyde, dihydrocitronellal (3,7-dimethyloctanal) under mild conditions. The ascendancy of the scCO₂ medium is established in comparison with the conventional organic solvent and solventless conditions. Changes in different reaction parameters such as H₂ pressure, temperature and nature of the support do not affect the selectivity. Moreover, the catalyst can be recycled several times without any further treatment. Easy separation of the liquid product from the catalyst and the use of environmentally benign solvent make this procedure a viable and an attractive ideal green chemical process for large-scale application. Experimental findings are supported by the theoretical calculations to propose a plausible explanation for the reduction of the conjugated and isolated CC double bonds in scCO₂.