Sustainable menthol from a by-product of the paper industry

Turpentine oil is produced during paper production when wood fibres are boiled down. At the headquarters of our partner UPM Kymmene alone, several tens of thousands of tonnes are produced annually, which are then thermally recycled or converted into biodiesel. In our considerations of alternative uses, we focused on the chemical compound 3-carene, which makes up about 30 percent of the oil and can be used to produce menthol. A major challenge here is separating the oil into its components," explains project manager Prof. Dr. Matthias Eisenacher from the Faculty of Applied Natural Sciences at TH Köln.

Separation column and nanofiltration

In order to extract 3-carene from turpentine oil, the project partners erected an approximately eight-metre-high separation column on a pilot plant scale at the Deutz campus. All components were custom-made in the workshop at TH Köln. ‘Our plant carries out what is known as rectification, a thermal separation process that separates the components of the liquid according to their boiling points. This is particularly difficult with turpentine oil because the boiling points of the individual substances are very close together,’ explains research assistant Katharina Göbel from the Institute for Plant and Process Engineering at TH Köln.

The institute also investigated whether the oil could be broken down into its components using organic nanofiltration and tested various commercial plastic membranes for this purpose. "This process has not yet been extensively researched for organic solutions and is characterised by a relatively complex interaction between the starting material, the recyclable material and the membrane. We were able to achieve good results and demonstrate that this process is a useful addition to rectification for extracting other substances besides 3-carene," says Göbel. For example, alpha- and beta-pinene were dissolved from the turpentine oil – organic hydrocarbon compounds that are used in medical applications, for example.

Synthesis in a few steps

At the same time, the chemical synthesis of 3-carene to menthol was developed. ‘It was particularly important to us that our process could also be realistically implemented in industry. That's why we wanted to use as few synthesis steps as possible and only use commercially available methods and equipment,’ says Eisenacher. Ultimately, the researchers need four steps, i.e. individual chemical conversions, to produce menthol. The first three steps are already practical with a yield of 90 percent and more. ‘Despite all the optimisation, the last step only achieves a yield of 65 percent. Further research is needed here, for example on the use of enzymes,’ says Eisenacher.

The project ‘Waste2Menthol – Synthesis of menthol from waste from the paper industry’ was funded by the Federal Ministry of Education and Research through the FHprofUnt programme with around 660,000 euros. Partners were Symrise AG, a manufacturer of fragrances and flavours, and paper manufacturer UPM Kymmene.