Highly selective synthesis of menthols from citral in a one-step process

We report for the first time the selective synthesis of menthols from citral in a one-step process. Bifunctional metal/acid catalysts active and selective for menthol synthesis were developed by studying the individual steps involved in the reaction pathway leading to menthols from citral. The metallic component was selected by testing silica-supported metals for citral hydrogenation to citronellal. Acid site requirements to efficiently isomerize citronellal to isopulegols were investigated on different solid acids. Potential bifunctional metal/acid catalysts were then prepared and tested for citral conversion to menthols. The best catalyst was Ni/Al-MCM-41, which yielded about 90% menthols and gave 70–75% of racemic (±)-menthol in the menthol mixture.