Continuous flow synthesis of menthol via tandem cyclisation–hydrogenation of citronellal catalysed by scrap catalytic converters

Abstract

A continuous flow synthesis of menthol in a tandem system using catalysts derived from the ceramic core of scrap catalytic converters is reported.

Cascade transformations of (±)-citronellal to menthol over extruded Ru-MCM-41 catalysts in a continuous reactor

“…Menthol is a fine chemical, one of the most common aroma compounds with a worldwide demand of more than 25 000-30 000 metric tons per year. [1][2][3][4][5] Three asymmetric carbon atoms in the cyclohexane ring lead to a chiral compound exhibiting four diastereoisomeric pairs, namely (±)-menthol, (±)-neomenthol, (±)-isomenthol and (±)-neoisomenthol. Only (−)-menthol has a strong physiological cooling effect and the characteristic odour worthy to be commercially used in i.e.…”

“…The other seven isomers are typically considered less valuable. [1][2][3][5][6][7][8][9] Synthetic (−)-menthol is mainly obtained by the industrial Takasago five step process with myrcene as the main reactant. 7,[10][11][12] The last two steps in this process are the 'ene' cyclization of (+)-citronellal to (−)-isopulegol in the presence of Lewis acid aqueous ZnBr 2 catalyst.…”

“…Recently, a continuous flow tandem system for the (+)-citronellal transformation over a low cost waste-derived powder catalyst with 77% of (−)-menthol yield was reported. 1 Furthermore work of Cova et al 23 demonstrated applicability of a wastederived powder material, scrap ceramic-cores of automotive catalytic converters (SCATs) in the continuous-flow hydrogenation of isopulegol to menthol. Under 30 bar, 100°C , 0.1 mL min −1 of 20 mM (−)-isopulegol in toluene, SCATs showed almost comparable (−)-menthol yield of 91.3% as a commercial 1% Pd/C catalyst (94% of yield).…”

From Waste to Green Applications: The Use of Recovered Gold and Palladium in Catalysis

“…In addition, scanning transmission electron microscope high-angle annular dark-field (STEM-HAADF) imaging of the PW-TWC indicated that neither leaching nor aggregation of palladium had occurred after 8 h of reaction. Additional research by the Luque group has expanded on their work on PW-TWC, applying material derived from scrap to the continuous in-flow synthesis of menthol from citronellal [111]. The PW-TWC sample was employed both as a solid support for iron oxide nanoparticles (Fe-PW-TWC) to catalyse the cyclisation of citronellal and as a catalyst for the hydrogenation of (−)-isopulegol (Figure 16).…”

Recent Progress in Continuous‐Flow Hydrogenation

“…Another example is the tandem cyclization–hydrogenation that was exploited to synthesize menthol from citronellal by Luque and co‐workers . The developed Lewis acid catalyst consisted of supporting iron oxide nanoparticles on the ceramic core of SCATs.…”