Along with cellulose, lignin is one of the most common organic compounds on earth. Researchers from the Fraunhofer Institute for Chemical Technology ICT are working on optimising the yield of aromatic platform chemicals using innovative sustainable processes for the extraction and fractionation of lignin. The ultimate goal is to provide an alternative to petroleum in the pharmaceutical, plastics and food industries.
Paper is made from wood, as everyone knows from their schooldays. However, the ability to produce plastics, medications and adhesives from wood is not quite so easy to understand at first. A natural material called lignocellulose is the basis for paper as well as for plastics. Lignocellulose forms the structural framework in the cell wall of trees. It consists of cellulose, hemicelluloses and lignin. Cellulose is currently used to produce paper and pulp, while lignin is already being used as feedstock for plastics and adhesives.
“Due to its structure and composition, lignin has very interesting functionalities,” says Dr. Detlef Schmiedl, project manager at the Fraunhofer Institute for Chemical Technology ICT in the city of Pfinztal. “Lignin is the most important renewable source of aromatic compounds.” Aromatic compounds such as benzene, phenol, toluene and vanillin are currently extracted from petroleum and used for producing plastics, or as platform chemicals in the pharmaceutical industry.
Many pulp mills separate lignin and cellulose from each other using so-called sulphate or sulphite processes involving sodium hydroxide, sodium sulphite and sulphate. In addition to the main product, i.e. cellulose, the pulping process leads to a lignin stream in the form of so-called black liquor. The black liquor is incinerated to produce electrical power and water vapour, as well as to recover chemicals for reuse in the pulping process (with the aim of making pulp mills self-sufficient in terms of energy). According to Schmiedl, up to 20% of the black liquor can also be used for other processes, as lignin can be removed from black liquor using different processes (LignoboostTM and LignoforceTM). “These processes lead to a mixture of low-, medium- and high-molecular weight lignins,” explains the chemist. Industry and science are looking for ways to fractionate and subsequently modify these different types of lignin. In some areas, this is already being done; a company called Tecnaro GmbH produces plastics granules from so-called kraft lignins. These granules can be moulded into different shapes using a thermoplastic process. Lignin can also be used as raw material for vanillin, apocynin and syringaldehyde. However, the disadvantage of lignin that is removed from black liquor is that it contains sulphur as a result of the process used. “Some sulphur compounds are harmful to the environment and often also a problem in catalysed reactions as they can inhibit catalytical processes,” explains Schmiedl.
The investigations being carried out by the Fraunhofer ICT focus, among other things, on the sulphur-free isolation of lignin and cellulose using an acid-catalysed process called Organosolv. In a project called “Optimisation of an Organosolv pulping process – grass and hardwood/fractionation of lignin and generation of polyfunctionated intermediates”, which is funded by the Baden-Württemberg Bioeconomy Research programme, Viktoria Rohde’s doctoral research is focused on finding out how lignin-based polyfunctional building blocks can be sustainably produced in high yields and used for developing chemical products such as varnishes and adhesives. The researcher from the Fraunhofer ICT uses poplar wood with bark (hardwood) or miscanthus (grass), which can be grown in short-rotation plantations (SRP). “Both plants are suitable for the project as they have a lignin content of as high as 25 to 30 percent,” says Schmiedl. “Another advantage is that they are available relatively quickly, i.e. between three to five years, and the carbon dioxide is quickly removed from the atmosphere.”
The Organosolv process breaks down wood into lignin, cellulose and hemicelluloses using water, ethanol and high pressure. “Sulphuric acid is only added in catalytic amounts of around 0.5%,” explains Schmiedl, “so that the recovered lignin is highly pure and sulphur-free.” The method can be specifically adapted to the type of biomass used. This means that there are specific processes for poplars and miscanthus, aimed at achieving the highest possible lignin yield and best possible lignin quality. All three fractions can be used. Cellulose remains as a macromolecule and can be used for industrial applications (pulp, viscose). “Cellulose can also be used to extract second-generation sugars,” says the project manager. “These can then be used for biotechnological processes.” A lignocellulose biorefinery pilot plant is already in operation at the Fraunhofer Centre for Chemical-Biotechnological Processes CBP in Leuna.
The Fraunhofer researchers are particularly interested in sulphur-free lignin. Viktoria Rohde analyses lignin types and separates them into low- and high-molecular weight fractions using thermal processes. The resulting lignin-based synthetic building blocks are then further functionalised by inserting different functional chemical groups into the molecule, thus modifying the properties. The aim is to synthesise alkyl aryl ethers with different functionalities that are then a good basis for various material applications.
“One objective is to be able to use the compounds in polyurethanes or epoxy resins such as the ones used in foams and adhesives, to name a couple of examples,” says Schmiedl. Polymer additives can also be developed. According to Schmiedl, the production of aromatic platform chemicals is particularly interesting. These include 2-methoxyphenols, 2,6-dimethoxyphenols and 1,2-dihydroxybenzenes, which are the basis for vanillin, apocynin, syringaldehyde and aromatic diamines. Syringaldehyde is a drop-in chemical for use in photostabilisers and polymer additives. Guaiacol is a raw material used for producing vanillin, the feedstock for many natural substance syntheses. It can also be used for producing medical compounds, e.g. L-DOPA, which is a precursor of dopamine and used for managing the symptoms of Parkinson’s disease.
Miscanthus and poplar lignin are structurally different. Schmiedl explains that the lignin of grass is significantly less cross-linked than that of poplar and composed of three building blocks. This has several advantages: the selective, catalysed depolymerisation of so-called H-G-S lignin (hydroxyphenyl, guaiacyl and syringyl units) is simpler, and higher monomer yields can be expected. Rohde’s doctoral thesis deals, amongst other things, with the different monomer contents of poplar lignin (S-G lignin, syringyl and guaiacyl units) and miscanthus lignin. The aim is to make the subsequent functionalisation sustainable (The 12 Principles of Green Chemistry) and scalable. “The best result would be getting the catalyser to remain in the molecule so that it could be used for the final formulation,” says Schmiedl. This would result in a continuous process, no side products and no time-consuming purification process. However, Schmiedl already knows that lignocellulose is a resource with a great future. “Selective processes help increase the yield of platform chemicals and produce novel structures,” says the expert. The former energy carrier lignin therefore has the potential to become a basic resource for a large number of new products in the future.