One-Pot Synthesis of Menthol from Citral and Citronellal 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.

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.

Diffusion measurements of hydrocarbons in H-MCM-41 extrudates with pulsed-field gradient nuclear magnetic resonance spectroscopy

Mesoporous materials are promising catalysts for production of biofuels. Herein, H-MCM-41 catalysts with different concentrations of the silica Bindzil binder (10-50 wt%) were prepared and characterized using pulsed-field gradient (PFG) NMR in the powder form and as extrudates. Effective diffusion coefficients (De) are measured in all cases. Diffusivities of n-hexadecane were found smaller for extrudates as compared to the powder catalysts. The estimates of diffusive tortuosity were also determined. PFG NMR data showed one major component that reveals diffusion in interconnected meso- and micropores and one other minor component (1-2%) that may correspond to more isolated pores or may represent complex effects of restricted diffusion. Therefore, several approaches including initial slope analysis of spin-echo attenuation curves, two-component fitting and Laplace inversion were used to discuss different aspects of diffusional transport in the studied H-MCM-41 materials. Correlations between De and the amount of Bindzil, the specific surface area, the micropore volume, the particle size, the total acid sites and the Lewis acid sites are discussed.