Selective one step synthesis of (−)menthol from (+)citronellal on Ru supported on modified SiO2 Original Research Article

The isomerisation of (+)citronellal has been performed under mild conditions on ZnX2 (X=Cl, Br, NO3) 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 ZnBr2/SiO2 for a Zn content ≥4.0 mmol/g SiO2 ZnBr2/SiO2 is more active than ZnCl2/SiO2, while the selectivity to (−)isopulegol is similar. Zn(NO3)2/SiO2 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–ZnBr2/SiO2, on which the one step hydrogenation of (+)citronellal to (−)menthol occurs with a (−)menthol yield >85%.