Selective one step synthesis of (-)menthol from (+)citronellal on Ru supported on modified SiO2

The isomerisation of (+)citronellal has been performed under mild condition s on ZnX2 (X=Cl, Br, NO3) supported on high surface area silica. The influe nce of the Zn loading, counter anion and temperature of pretreatment of the catalyst on the activity and selectivity to (-)isopulegol was investigated . The catalytic activity increases with the Zn loading. The selectivity to (-)isopulegol also increases reaching the maximum value (86%) on ZnBr2/SiO2 for a Zn content greater than or equal to 4.0 mmol/g SiO2 ZnBr2/SiO2 is mo re active than ZnCl2/SiO2, while the selectivity to (-)isopulegol is simila r. Zn(NO3)(2)/SiO2 is far less active and selective. An increase of the tem perature of pretreatment increases the selectivity to (-)isopulegol. A bifu nctional catalyst was formulated, namely Ru-ZnBr2/SiO2 on which the one ste p hydrogenation of (+)citronellal to (-)menthol occurs with a (-)menthol yi eld >85%. (C) 2000 Elsevier Science B.V. All rights reserved.