One pot synthesis of menthol from (±)-citronellal on nickel sulfated zirconia catalysts

The one-pot synthesis of menthol from (±)-citronellal was carried out using two catalysts: nickel supported on sol–gel sulfated zirconia (Ni/ZrS) and nickel added during the synthesis of sulfated zirconia (NiZrS). The supported catalyst was prepared by impregnation of nickel on the sulfated zirconia support, whereas NiZrS was prepared by the co-gellation of nickel and the sulfated zirconia by the sol–gel method. Two sets of catalytic tests were carried out. In both tests the catalysts were previously treated in He stream. First, the liquid phase cyclization of (±)-citronellal was carried out with both ZrS and NiZrS catalysts. The results showed that both ZrS and NiZrS catalysts are highly active and selective towards isopulegols (100% of selectivity) and highly stereoselective towards (±)-isopulegol (∼70% of selectivity). A second reaction test was the synthesis of menthol. The catalysts were evaluated in a batch reactor at 1.4 MPa of H2 and 100 °C. The results showed that the catalysts allow to obtain a high catalytic activity and selectivity towards stereoisomers of menthol (100% selectivity of menthols) and a high selectivity towards (−)-menthol.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (149 K)Download as PowerPoint slideHighlights► Menthol one pot synthesis from citronellal can be obtained on bifunctional catalysts. ► Cyclization of (±)-citronellal on ZrS catalysts yields isopulegol selectively. ► NiZrS catalysts provide acid and hydrogenating functionalities in menthol synthesis. ► Menthol is selectively obtained from citronellal on NiZrS catalysts. ► Ni supported on ZrS is more active than NiZrS prepared in situ.