Reduction of Menthone with Isopropanol in the Presence of Palladium on Sibunit (ICT-3-31)

Catalytic properties of the industrial «palladium on Sibunit» catalyst (ICT-3-31, 0.5 wt % Pd) was studied in the reaction of menthone reduction to menthol. Menthone was reduced at 250–300 °C using isopropanol as the H-donor under conditions of the reaction of hydrogen transfer (RHT). For comparison, the catalyst free reaction was conducted under identical conditions. Practically identical menthone conversions (61–62 %) were observed with and without catalyst at 350 °C but the selectivity to menthol – the target product – decreased considerably (from 98 % to 23 %) and the proportion of side products increased (from 2 to 77 % expressed as the quantity of reacted menthone) in the presence of the catalyst. When the temperature of the catalytic reaction reduced to 250 °C, the reduction selectivity increased up to 42 % but menthone conversion decreased down to 10 %. All the products of menthone conversion were identified and pathways of their formation suggested. It was established that the carbon support Sibunit catalyzed menthone dehydration at 250–350 °C thus impeding the achievement of appropriate yields of the target alcohol. The other side reactions catalyzed by ICT-3-31 were dehydrogenation followed by aromatization of p-menthenes to produce substituted benzenes – p-cymene and thymol. The application of ICT-3-31 for RHT is only advisable provided that more reactive organic substrates are involved in the reaction at below 200–250 °C or stronger H-donors are used.