Synthesis of menthol from citronellal over supported Ru- and Pt-catalysts in continuous flow†
One-pot menthol synthesis from citronellal in a trickle bed reactor was investigated using Ru/H-beta-300 zeolite extrudates without any binder and Pt- and Ru-extrudates containing 70 wt% H-beta-25 zeolites and 30 wt% bentonite binder using different methods of metal loading on the extrudates. The catalytic results were correlated with the physico-chemical properties of bifunctional zeolite-based extrudates. The conversion of citronellal to menthol was nearly the same for Pt extrudates being independent of catalyst acidity or the metal particle size. Ru extrudates also exhibited similar conversion levels as a function of time-on-stream, except for very mildly acidic Ru/beta-300 extrudates, which gave much lower conversion. Pt extrudates showed better stability with time on stream in comparison with Ru extrudates. The best catalyst giving the highest menthol yield and a low amount of acyclic hydrogenation products was Ru-beta-bentonite extrudates where Ru was located randomly in the mixture of 70% H-beta-25 and 30% bentonite binder. Hydrogenation was more prominent for Pt than for Ru catalysts, most probably due to their higher hydrogenation ability. Stereoselectivity to menthol varied in the trickle bed reactor in the range of 67 to 73%. In a batch reactor for Ru- and Pt-catalysts independent of support acidity, it was in the range of 70–71%. The reuse of Ru/H-beta-300 extrudates was successfully demonstrated.