Lignin is the only renewable resource that offers the potential for extracting large quantities of aromatic molecules of the kind that are essential for producing many platform chemicals and materials, for example. When lignin is produced using the Kraft process for making paper pulp, the heterogeneity, high molar mass and low functionality essentially prove a major hindrance for many applications. Using base-catalyzed (BCD) or oxidative depolymerization to cleave the lignin molecule is one way of overcoming these restrictions. Depending on how the process is controlled, this enables the molar mass and heterogeneity to be reduced while also generating new functionalities.
Base-catalyzed depolymerization reduces heterogeneity, oxidation increases functionality
In the BCD process, lignin is depolymerized into phenolic monomers, oligomers and various by-products. Using the kind of continuous BCD technique that is available at Fraunhofer CBP, the weight-average and number-average molecular weights of commercial Kraft lignin can, for example, be reduced by approximately 70 and 50 percent respectively. By contrast, oxidation only enables a slight decrease in the weight-average and number-average molecular weights of Kraft lignin to be achieved because it is often the case that only the side chains of the lignin get oxidized. However, the upside of oxidation is that it allows the introduction of highly reactive carboxyl groups.
Combining the advantages of both processes
Thus, KoBaOx aims to sequentially combine base-catalyzed lignin depolymerization (BCD) with alkaline oxidation involving hydrogen peroxide. In this way, innovative carboxylated lignin derivatives can be produced for use in various applications – such as thermoplastics. Two synthesis routes are being explored within this context: i) oxidation of the lignin with subsequent BCD, ii) implementation of the processes the other way round as an alternative. Both possibilities lead to advantages and disadvantages from a process technology perspective and these are to be compared in the course of the study.
For instance, linking the two processes (oxidative and base-catalyzed depolymerization of lignin) eliminates processing steps that would otherwise have to be performed on the cleaving solutions obtained. In addition, membrane technology is likewise to be used as a means of reducing the number of processing steps and chemicals involved. The innovative carboxylated lignin derivatives with a low molar mass could potentially offer improved water solubility, thereby facilitating film applications. If these aims are achieved, it will open up a new source of high-quality products for the bioeconomy.