Could pinane, a derivative of the terpene pinene, replace common solvents like n-hexane?
Solvents like n-hexane and other hydrocarbons are commonly used to extract botanical materials. These solvents from fossil resources are generally considered hazardous and toxic. They pose dangers to both humans and to the environment. Green solvents are more environmentally friendly and less dangerous. Discovering new green solvents that are economic and as effective as hydrocarbons is an important area of research.
In a recent paper published in Green Chemistry, researchers compare pinane to n-hexane for the extraction of carotenoids, lipids, aromas, and antioxidants. [1]
Some physical and chemical properties of terpenes like α-pinene, D-limonene, and p-cymene are similar to those of hexane, so they have real potential to replace petroleum-based solvents. They are also safer, renewable, economic, and widely available. They meet many of the criteria for a green solvent.
What’s a Green Solvent?
- Biodegradable
- Non-toxic or low toxicity
- Recyclable
- Stable price
- High purity grade
- Synthesis is an energy saving process
- Low flammability
- Stable storage
- Renewable
- No green solvents meet all criteria
- Examples: ethanol, glycerol, and D-limonene
A key limitation of terpenes as solvents is their unsaturated nature, which makes them unstable. Researchers explain that they “are easily oxidized on air exposure,” causing irritation and allergies. The hydrogenation of terpenes could make them more stable and superior solvents.
To avoid the toxicity of residual metal solvents, the team used palladium on carbon (Pd/C), a reusable and heterogeneous catalyst to synthesize pinane from the pinene molecules in turpentine oil (naturally produced from pine trees). Specifically, hydrogenation was performed with 10% Pd/C at room temperature and 400 Psi with an autoclave for 3 hours. This created a “cis-rich pinane (cis/trans 7/3),” which was filtered through Celite pad.
They then performed solid-liquid extraction of carrots, rapeseed, and caraway seeds using the ULTRA-TURRAX® Tube Drive System.
Pinane was found to be comparable or superior to n-hexane in the extractions. Pinane extracted about 95% of the carotenoids from carrots while n-hexane extracted only 78%. The researchers noted that “no differences were observed in the fatty acid profiles” of rapeseed oil, and the yields were similar. The terpene profiles from caraway seeds were also comparable.
The team found that hydrogenation of terpenes α-pinene and β-pinene to get stable saturated pinanes overcomes the drawback of oxidation that is an issue with terpenes in their unsaturated chemical form. With pinane performing as good as or better than n-hexane, it is possible that this green solvent could make botanical extraction safer for workers and the environment without adding burdens of high cost or scarce supplies. [1]
Image: by Kayla Warner on Unsplash
References
- Yara-Varón E, Selka A, Fabiano-Tixier AS, et al. Solvent from forestry biomass. Pinane a stable terpene derived from pine tree byproducts to substitute n-hexane for the extraction of bioactive compounds. Green Chemistry. 2016;18(24):6596-6608. doi:10.1039/c6gc02191c. [Impact Factor: 9.405; Times Cited: 15 (Semantic Scholar)]
