Applied Catalysis A: General, Vol.172, No.1, 31-40, 1998
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 characterized by thermal programmed desorption-mass spectrometry (TPD-MS), using pyridine, dimethylpyridine and CO2 to titrate total acidity, Bronsted 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 las 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%.
Keywords:SELECTIVE HYDROGENATION;LIQUID-PHASE;ALPHA;BETA-UNSATURATEDALDEHYDES;MASS-SPECTROMETRY;SOLID CATALYSTS;ALLYL ALCOHOL;KETONES;DEHYDROGENATION;CINNAMALDEHYDE;CROTONALDEHYDE