Synthesis of p-Menthane-3,8-diol from Citronellal over Lignin-Derived Carbon Acid Catalysts
used N,N-diethyl m-toluamide (DEET). In this study, sustainable carbon acid catalysts derived from alkaline lignin (AL) were prepared for synthesizing PMD from a popular chemical of (±)-citronellal in an environmentally friendly solvent of water. The catalytic performances of the AL-derived carbon acid catalysts prepared at different pyrolysis temperatures were more excellent than other catalysts such as carbon black and H-USY. Especially, when the AL pyrolyzed at 500 °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 the wasted alkaline lignin can be used as a source of acid catalysts. It is found that the formation of PMD preferred over the catalysts with weaker acid sites whereas isopulegol was more easily formed over stronger acid sites. Moreover, the reaction route of citronellal cyclization-hydration reaction was more dominant via carbocation-hydration pathway rather than isopulegol hydration route on the weaker carbon acid catalyst.