Synthesis ofp-menthane-3,8-diol from citronellal over lignin-derived carbon acid catalysts

p-Menthane-3,8-diol (PMD) is receiving growing attention as a natural mosquito repellent with lower toxicity compared to the widely-usedN,N-diethylm-toluamide (DEET). In this study, sustainable carbon acid catalysts derived from alkaline lignin (AL) were prepared for synthesizing PMD from a popular chemical (+/-)-citronellal in an environmentally friendly solvent of water. The catalytic performances of the AL-derived carbon acid catalysts prepared at different pyrolysis temperatures were better than those of other catalysts such as carbon black and H-USY. In particular, when the AL pyrolyzed at 500 degrees C was used as the carbon acid catalyst, the conversion of (+/-)-citronellal was as high as 97% with a high PMD yield of 86%, indicating that waste alkaline lignin from pulp and paper industries can be used as a source of acid catalysts. It is found that the formation of PMD is preferred over catalysts with weaker acid sites, whereas isopulegol was more easily formed over stronger acid sites. Moreover, the reaction route of the citronellal cyclization-hydration reaction was more dominantviathe carbocation-hydration pathway rather than the isopulegol hydration route on the weaker carbon acid catalyst.