Menthol is an important taste and fragrance for medication, food and cosmetics. Now chemists have developed a new synthesis path through which this mint aroma can be obtained from turpentine oil – a waste product of the paper industry. This environmentally friendly menthol production could replace menthol synthesis on a petroleum basis in the long term. However, there are still a few hurdles to be mastered.
If wood fibers are boiled when making paper, enormous amounts of turpentine oil are incurred as a waste, about 190,000 tons annually. So far, this by -product has mainly been burned to gain energy from it, or to continue to use biodiesel. Now researchers around Dominik Dylong from the Technical University (TH) Cologne have found a way together with industrial partners to recycle the turpentine oil more profitable and environmentally friendly. They focused on the chemical compound 3-carn, which is about 30 percent of the oil. Menthol can be made from it – provided the connection is in pure form.
Raw material extraction from waste
“A big challenge is to separate the oil into its components,” explains senior author Matthias Eisenacher from TH Cologne. In order to separate the 3-carn from the turpentine oil, the researchers therefore built a test system with a rectification column around eight meters high. “The components of the liquid are separated according to their boiling points. This is particularly difficult for turpentine oil because the boiling points of the individual substances are close together,” explains Katharina Göbel from TH Cologne. In addition to this distillation separation, the team examined whether the 3-carn and other ingredients can be filtered out of the turpentine oil via plastic membranes. “So far, this complex procedure has been little researched for organic solutions,” says Göbel.
The result: With the combination of both techniques-rectification and subsequent membrane filtration-turpentine oil can be reliably broken down into its components. Not only 3-cars were isolated from turpentine oil, but also similar organic hydrocarbon compounds such as alpha and beta pins. Menthol and many other fragrances and flavors can also be made from these.
Menthol synthesis in four steps
At the same time, Dylong and his colleagues worked on a sustainable synthesis process to turn 3-carn menthol. Their goal was to get along with as few steps as possible with industrial devices and environmentally friendly reaction conditions. They succeeded and they only needed four steps to produce menthol, as they report. In contrast, previous 3-care-based procedures needed seven or eight steps. The team around Dylong also used common sustainable reagents such as hydrogen and hydrogen peroxide, various catalysts and no solvents.
The first three steps of the new method are very efficient and are already practical with a yield of at least 90 percent. However, the application has so far failed due to the fourth reaction: “Despite all the optimization, the last step only comes to a yield of 65 percent. Further research is necessary here, for example on the use of enzymes,” says Eisenacher. If this hurdle is also mastered, the procedure could in future deliver “green” menthol from waste and replace the previously common, petroleized synthesis method for menthol. “Overall, the feasibility of the new route has been proven. Further research is required to raise this process to a commercially competitive level,” the chemists write.
Source: Cologne University of Applied Sciences; Specialist articles: chemsuschem, DOI: 10.1002/CSSC.202500515
