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

Cascade transformations of (+/-)-citronellal in a continuous mode were investigated over a bifunctional shaped ruthenium catalyst bearing metal clusters of the size 7-13 nm. Four types of Ru/H-MCM-41 extrudates (1.5 x 10 mm) containing 30% of Bindzil-50/80 colloidal silica binder were prepared varying in metal location and metal-to-acid ratio, while the concentration of Bronsted and Lewis acid sites and textural properties of the final extrudates were comparable. Catalytic tests were performed in the trickle-bed reactor under 70 degrees C, 10 bar of H-2, and the initial reactant concentration in cyclohexane 0.086 mol L-1 for the liquid residence time of 12.5 min. As a reactant, isopulegol, citronellol or (+/-)-citronellal was used. Metal location in extrudates has a significant effect on the catalytic activity and selectivity especially in terms of isopulegol isomers which content correlated with the metal-to-acid site ratio. Stereoselectivity to the desired (+/-)-menthol was 68-70%. The highest amount of the desired menthol, 32% yield, was obtained over extrudates where Ru was deposited on the catalytic support, i.e. with the shortest distance between the acid and metal sites, the lowest Bronsted acidity, the lowest Bronsted-Lewis acid sites ratio, the highest specific surface area and the narrowest range of the Ru particle size distribution.