Terpenes have been explored as green solvents with the potential to replace petroleum-based solvents such as n-hexane. Limonene is a terpene solvent of great interest. It is a common waste product (e.g., citrus peels) and has illustrated success when extracting oil from rapeseed and microalgae as well as carotenoids from tomatoes, among others.  That said, Madji et al  note that limonene “oxidizes easily under exposure to air,” and “oxidation products are labelled as allergens.” Given these limitations, the researchers determined to hydrogenate limonene to synthesize a more stable yet still effective terpene solvent: para-menthane.
Initially, they conducted a theoretical study using specialized software to predict solubility of certain target compounds in n-hexane or p-menthane. The commonly extracted foods and targets were:
- Carotenoids from carrots
- Triglycerides from rapeseeds
- Terpenes D-limonene and S-carvone from caraway
The researchers noted that “p-menthane [had] probability of solubility higher than, or similar to, n-hexane.” The experimental part of the study then tested each solvent’s ability to extract the listed compounds from their respective food sources.
Reprinted from: Madji S, Hilali S, Fabiano-Tixier AS, et al. para-Menthane as a stable terpene derived from orange by-products as a novel solvent for green extraction and solubilization of natural substances. Molecules. 2019;24(11):2170. doi:10.3390/molecules24112170. License: Attribution 4.0 International (CC By 4.0)
The limonene came from essential oil of citrus peel waste. A relatively low temperature (40-50° C) and ruthenium catalyst on activated charcoal were used to hydrogenate limonene to p-menthane. The main extractions relied on the ULTRA-TURRAX® Tube Drive System with 1 g ground plant material (carrot, rapeseeds, or caraway) and 10 ml solvent (n-hexane or p-menthane) for solid-liquid extraction.
The solvents yielded similar quantities of α- and β-carotene from carrots although the mass yield from p-menthane was slightly higher (not significantly). The triglyceride profiles of the rapeseed oil extractions were nearly identical. Results for caraway’s key terpenes, D-limonene and S-carvone, were similar although n-hexane produced slightly higher (not significantly) yields. Thus, the solvents were comparable with no significant differences.
To further investigate p-menthane, the researchers conducted two additional “common analytical extraction procedures”: Dean-Stark Distillation (separating water from plant matrices) for carotenoids from carrots and Soxhlet extraction for oil from rapeseeds. These experiments compared p-menthane to toluene rather than n-hexane. Again, results between solvents were very similar. The only significant difference regarded Dean-Stark Distillation, where complete water recovery with p-menthane required 55% less time compared to water recovery with toluene. This improvement came after a brief starting delay due to p-menthane’s higher boiling point.
Ultimately, the authors declare, p-menthane could be a “green replacer for petroleum-based solvents such as n-hexane or toluene.” 
- Madji S, et al. para-Menthane as a stable terpene derived from orange by-products as a novel solvent for green extraction and solubilization of natural sMolecules. 2019;24(11):2170. doi:10.3390/molecules24112170. [Impact Factor: 3.267; Times Cited: 1 (Research Gate)]