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

The isomerisation of (+)citronellal has been performed under mild conditions on ZnX(2) (X=Cl, Br, NO(3)) supported on high surface area silica. The influence 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 ZnBr(2)/SiO(2) for a Zn content greater than or equal to 4.0 mmol/g SiO(2) ZnBr(2)/SiO(2) is more active than ZnCl(2)/SiO(2), while the selectivity to (-)isopulegol is similar. Zn(NO(3))(2)/SiO(2) is far less active and selective. An increase of the temperature of pretreatment increases the selectivity to (-)isopulegol. A bifunctional catalyst was formulated, namely Ru-ZnBr(2)/SiO(2) on which the one step hydrogenation of (+)citronellal to (-)menthol occurs with a (-)menthol yield >85%. (C) 2000 Elsevier Science B.V. All rights reserved.