Catalyst and Process Effects in the Solvent-Free Hydrogenation of p-Cymene to p-Menthane

The hydrogenation of p-cymene to p-menthane, a bio-based solvent, over four platinum-group catalysts, was thoroughly investigated in this study. The effect of the support material, pressure, and temperature were explored. Rhodium was the most effective metal, even under normal pressure conditions. Charcoal was a better metal support compared to alumina, offering better selectivity at lower pressure and outstanding recyclability. Hydrogen pressure had no effect on the selectivity; however, the conversion rate was maximal at higher pressure. At higher temperatures, the thermodynamically stable trans-isomer was favored, whereas at lower temperatures the cis-isomer became predominant. Remarkably, Rh/C achieved >99% conversion of p-cymene and maintained stable activity and selectivity over 66 recycling cycles, whereas the same metal-based catalyst on alumina was only recycled twice. These findings demonstrate that the solvent-free hydrogenation of p-cymene can be efficiently achieved using commercially available catalysts, with Rh/C emerging as a promising benchmark for sustainable and green catalytic processes.