The cell walls of woody plants predominantly consist of cellulose and lignin, which are long-chain compounds that give plants rigidity. While quite a few methods are available to process cellulose, no industrial process is currently available for processing lignin, which is the “glue" that binds the cellulose fibres together. This is because lignin is a very stable molecule that is extremely resistant to being broken down into its building blocks.

At the same time, lignin is the only natural macromolecule whose basic structure consists of aromatic rings, and is therefore suitable for use as a resource for producing what are known as bio-aromatics. Aromatic compounds are important chemical building blocks that have many applications, e.g. in solvents and as components for many everyday items such as packaging and medications. Moreover, lignin is a virtually inexhaustible natural resource that cannot be used as food or feed.

Up to 50 million tonnes of lignin accumulate every year as a by-product of the paper industry alone. However, the vast majority of it is combusted. As is the case with all renewable materials, the way of producing biomass-derived platform chemicals is slightly different from that involving fossil resources such as petroleum or coal. This is down to the fact that plants already possess higher-valent molecules.

Quite some time ago, the Baden-Württemberg government established the “Lignocellulose” research consortium as part of its Bioeconomy Research Programme. Dr. Ursel Hornung and her research group from the Institute of Catalysis Research and Technology (IKFT) at the Karlsruhe Institute of Technology (IKT) are part of the large-scale project, which inter alia aims to promote the use of lignin. The interdisciplinary institute has many years of experience in studying processes to convert biomass into chemical building blocks, also called basic chemicals or platform chemicals. “We think that it is possible to make use of some of the lignin that accumulates as a by-product of the pulping process without having to change the process too much,” said Hornung. “Since paper consumption is already declining considerably in the digital era, the pulping industry is already thinking about alternatives. However, high-value products have yet to be produced. If lignin could be converted into its building blocks, these compounds could be used as platform chemicals for producing many different products, which would save large quantities of petroleum.”

Around six years ago, Hornung and her team therefore began to develop methods for breaking down lignin into its basic units. In principle, two different methods can be used and are selected in relation to the function of the products that will be made from the lignin building blocks. “Platform chemicals usually have additional functionalities,” said Hornung. “This is why we need to use methods that do not involve removing certain functional groups from the molecules when we want to produce chemical building blocks. However, if the plan is to make solvents such as benzene and toluene, then we have to use another lignin splitting method.” We are interested in both methods.”

For producing platform chemicals, the KIT’s chemical engineers use hydrothermal processes for converting lignin into the required building blocks. These methods involve splitting lignin in water at raised temperatures and pressures (known as near-critical conditions), which stop the water evaporating. “This process severely limits the range of products that can be synthesised. Although hydrothermal splitting leads to elevated amounts of catechol and guaiacol, the quantities are still too low for the intended purposes. The compounds are not pure enough either.” Catechol is used as a chemical building block for paints, fragrances and medications. In future, catechol monomers might also be used to replace resorcinol in formaldehyde resins.

Producing the second type of chemical from lignin, i.e. solvents, involves splitting lignin in such a way that only the sought-after compounds are formed. “These reactions are temperature sensitive, and different temperatures are chosen in relation to the side chain that is to be maintained,” said Hornung. “This method enables us to precisely control the type of compound we want to obtain. The process is called hydrogenolysis and is also used in the liquefaction of coal.” The researchers have already been able to produce BTEX solvents consisting of benzene, toluene, ehtylbenzene and xylene. These are also used as raw materials in the chemical industry.

Another important goal of the project is to elucidate the reaction pathway of lignin decomposition in order to be able to optimise the processes used. “We have built a loop reactor for this purpose,” says the chemist. We hope to obtain an in-depth understanding of the splitting reaction and find out the effect of subsequent reactions. Polymerisation reactions tend to lead to unwanted products, and are difficult to control. A loop reactor controls the backmixing and thus influences the formation of larger cleavage products, i.e. oligomers. In addition to monocylic products, certain oligomers might have quite interesting properties.

The researchers from Karlsruhe not only focus on lignin that accumulates as a by-product of the pulping industry. They are also studying the potential of waste products such as bark as raw material. “Bark accumulates in large quantities in the wood processing industry and is usually burnt. It would therefore be extremely interesting to use bark waste as lignin source,” said Hornung. The researchers have discovered that this is possible, and that the reaction with bark is quite similar to that involving wood-derived lignin. “We have already made contact with an SME in Baden-Württemberg that has bark available. Bark is a waste product with huge potential. We are therefore carrying out tests with kraft lignin produced by the pulping industry and also with bark waste,” said Hornung.

In the near future, the IKFT researchers are planning to work with lignin oligomers, i.e. molecules consisting of several units. At present, they are focussing on monocyclic products. “The production of oligomers is difficult to understand; the splitting methods are even more complicated and characterisation is far more complex than that of monomers,” concluded Hornung.