Chemoselective and regioselective reduction of citral (3,7-dimethyl-2,6-octadienal) by gas-phase hydrogen transfer over acid–basic catalysts Original Research Article

The selective reduction of citral (3,7-dimethyl-2,6-octadienal) by gas-phase hydrogen transfer over various acid–base catalysts (MgO, ZrO2, MgO/B2O3 and AlPO4/SiO2), using different alcohols as hydrogen donors, was studied. Chemical and surface properties of the solids were characterized by thermal programmed desorption–mass spectrometry (TPD–MS), using pyridine, dimethylpyridine and CO2 to titrate total acidity, Brönsted acidity and total basicity, respectively. Acid (AlPO4/SiO2) and amphoteric catalysts (ZrO2) led to the reduction of the alkene double bonds with 100% selectivity and produced citronellal (ca. 85%) and, to a lesser extent, dihydrocitronellal (15%). On the other hand, basic catalysts such as MgO led to the preferential reduction of the carbonyl group and to geraniol and nerol as end-products. The introduction of B2O3 in the reaction medium (as the MgO/B2O3 catalyst) substantially increased the selectivity for geraniol and nerol, so much so that, under optimal conditions, a selectivity of ca. 31% was readily achieved at a conversion of 15%.