CHEMOSELECTIVE AND REGIOSELECTIVE REDUCTION OF CITRAL (3,7-DIMETHYL-2,6-OCTADIENAL) BY GAS-PHASE HYDROGEN-TRANSFER OVER ACID-BASIC CATALYSTS

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 chara cterized by thermal programmed desorption-mass spectrometry (TPD-MS), using pyridine, dimethylpyridine and CO2 to titrate total acidity, Bro nsted 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 c atalysts 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 las the MgO/B2O3 catalyst) substantially i ncreased the selectivity for geraniol and nerol, so much so that, unde r optimal conditions, a selectivity of ca. 31% was readily achieved at a conversion of 15%. (C) 1998 Elsevier Science B.V. All rights reserv ed.