Catalytic transformations of citral in a continuous flow over bifunctional Ru-MCM-41 extrudates

The three-step reaction of citral to menthol was investigated in an autoclave and a trickle-bed reactor over Ru-MCM-41 catalysts in powder and shaped forms, respectively, with the same composition and controlled metal location. All catalysts were characterized in detail and the results were correlated with catalytic tests. Activity and selectivity were strongly affected by controlling the location of Ru in the powder catalyst applied in the batch experiments. The catalyst with the largest distance between the metal and acid sites, and at the same time, with the highest total acidity,i.e.with Ru deposited exclusively on a binder Bindzil, displayed the highest yield of menthols. In contrast, in the trickle-bed reactor with extrudates the Ru location was of almost no importance which is related to mass transfer. Comparison between batch and continuous experiments also revealed significant differences in the product distribution. The highest yield of the desired menthol of 6% with stereoselectivity of 66% was obtained at a residence time of 12.5 min after 3 h of time-on-stream over egg-shell extrudates with Ru distribution at the outermost layer, deposition of Ru on both H-MCM-41 and the binder Bindzil, and the smallest Ru particle size.