One-Pot Synthesis of Menthol from Citral and Citronella! Over Heterogeneous Catalysts

One-pot synthesis of menthol from citronellal or citral was summarized. Both batch and continuous reactors have been recently applied. This reaction is very complex and a bifunctional catalyst exhibiting especially Lewis acid sites for cyclisation of citronellal to isopulegol are needed, while metal particles are required for its hydrogenation to menthols. Typically, too mild acidity of the catalyst and small particles do not catalyze menthol formation. Furthermore, too high acidity causes catalyst deactivation and dehydration of menthol. Very high menthol yields have been obtained in batch reactor over nobel and transition metal supported bifunctional catalysts. Shape selectivity was demonstrated for Ni-supported on Zr-modified beta zeolite, which gave high diastereoselectivity to the desired L-menthol. Recently one-pot synthesis of menthol in a trickle bed reactor has been investigated. Catalyst suffers only minor deactivation in transformation of citronellal to menthol, while more severe catalyst deactivation occurred in transforming citral to menthols. Noteworthy from the industrial point of view is that the product distribution obtained with the same catalyst under kinetic regime or under diffusional limitations differs from each other. The metal location and synthesis method of extrudates can have a major effect on the catalyst performance. Kinetic modelling of the data obtained from the trickle bed reactor considering the effectiveness factor is discussed. [GRAPHICS] .

Automated summary

This study summarized the one-pot synthesis of menthol from citronellal or citral, which can be conducted in both batch and continuous reactors. The reaction requires a bifunctional catalyst with Lewis acid sites for cyclisation of citronellal and metal particles for its hydrogenation. Different catalyst properties and reaction conditions can affect the yield and selectivity of the desired L-menthol. The study also investigated the performance of the catalyst in a trickle bed reactor and discussed the effect of metal location and synthesis method on catalyst performance.