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

Abstract

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. Graphical Abstract The results from one-pot synthesis of menthol finding applications in pharmaceuticals and fragrances from citral and its hydrogenated product, citronellal over bifunctional catalysts metal–acid are summarized. The relationship between the catalyst properties and the performance is discussed. In the continuous mode catalyst deactivation becomes apparent and in such mode of operation the product distribution might differ from those obtained in a batch reactor.

Essential Oils: Chemistry and Pharmacological Activities—Part II

“…However, L-menthol is presently obtained from Mentha piperita L. essential oil, and the major part is produced by other synthetic procedures on a reasonably large industrial scale ( Figure 8 ). The most important and acclaimed synthesis starts from the pinenes, which was developed by the company Takasago, and is responsible for the major quantities available worldwide (see Figure 3 ) [ 18 , 19 , 20 ]. This is basically due to the greater availability of pinenes.…”

“…Citral can be used for the production of L-menthol [ 18 , 21 ], pseudo-ionone [ 22 ], the ionones, and then on to vitamin A ( Figure 10 ), and also for certain certified cannabinoids.…”