Lignin is an abundant heterogeneous aromatic polymer with significant potential to serve as a renewable source of aromatic chemicals. However, lignin is largely underutilized as a feedstock for polymeric materials and chemicals, as it is typically burned as a fuel source for pulp mills. It is estimated that only 2% of the 50 million tons of lignin isolated from pulp manufacturing processes in 2010 was used for specialty products, while the rest was burned as a low-value fuel. This underutilization raises the desire to develop not only processes to isolate lignin from biomass more efficiently, but also to design lignin-based products of higher commercial value.
Different chemical modifications can be applied to convert lignin into useful (chemical) products, either by polymerization, which uses lignin as a monomer in polymer synthesis, or by creating new reactive sites or structurally modifying existing functional groups in lignin. In addition, depolymerization of lignin is another option involving its fragmentation into smaller molecules with higher reactivity that can be used for the synthesis of bio-based polymers and biofuels.
The chemicals obtained from lignin depolymerization usually still have some oxygenation, including products such as phenol or vanillin. There are two main thermochemical routes used to isolate useful chemicals from lignin: hydrogenolysis and oxidation.
Hydrogenolysis, also called hydrogenation, is the pyrolysis (the heating of organic substances in the absence of air that results in smaller fragments, minimizing combustion to carbon dioxide) in the presence of hydrogen. By adding solvents and/or catalysts to these reactions, the hydrogenation process can be accelerated to obtain the desired products. The first commercial system using lignin hydrogenation and a catalyst produced approximately 44% monophenolic products (phenol, o-cresol, p-cresol, p-ethylcresol and p-propylcresol).
Oxidation is another technique used to isolate aromatic products from lignin. The products generated from this process tend to have greater complexity and functionalization compared to products from hydrogenation. Although it is clear that there are many possible approaches for the oxidative degradation of lignin into useful commercial products, no method has yet been high yielding, industrially relevant or low cost. With the development of better catalysts in the future, it is reasonable to believe that lignin could be used as a feedstock for oxidized aromatic products.
Vanillin is commercially isolated by oxidation of lignin under alkaline conditions. The relevant vanillin derivatives are those that could reasonably be isolated under pulping conditions. Under strongly oxidizing conditions, vanillin can be oxidized to vanillic acid or methoxyhydroquinone by decarboxylation. Whereas, under strongly reducing conditions, vanillin can be reduced to vanillyl alcohol. With the development of more cost-effective and efficient hydrogenation or oxidation systems, more products can be isolated, and the development of these processes could help reduce fossil fuel consumption.
The degradation of lignin to phenolic products or monolignols is an energy-intensive process, which is one of the reasons why lignin currently has more value as a fuel than as a feedstock. Methods that could use lignin as a source to synthesize new materials without any additional degradation would not only be useful, but also energetically and environmentally favorable.
Lignin, which contains phenolic and aliphatic hydroxyl groups, can also be used as a macromonomer for the synthesis of polymers of different nature, such as polyurethanes, polyesters, epoxy resins and phenolic resins.
The increasing demand for the development of bio-based polymeric materials and replacement of petroleum-based ones will require significant advances in several areas Application of methods to isolate more useful lignin-based chemicals or methods to chemically functionalize lignin to obtain useful synthetic products without the use of expensive reagents or complicated synthetic routes.
FRACTION’s goal is to develop new downstream conversion technologies of lignin obtained by organosolvent extraction, for the synthesis of resins and polyurethanes, but also catalytic depolymerization processes to obtain monophenols with high yields.
What are the lignin conversion routes in FRACTION by FUNDITEC?
O1. Catalytic oxidation. The FRACTION project will explore the capacity of the new lignin produced by the organosolv process to generate high value-added products such as vanillin (and its derivatives), veratraldehyde, syringol (and its derivatives) by catalytic oxidation reaction using a type of emerging catalysts based on supported MOFs.
O2. Synthesis of lignin-based polyurethanes for coatings, adhesives and elastomers. The FRACTION project will develop lignin-based polyurethanes using unmodified lignin from GVL fractionation and depolymerized lignin from solvolysis to replace fossil-derived polyols in the synthesis of polyurethanes.
Author: Dr. Dulce Muñoz, Principal Investigator, Department of Advanced Materials, FUNDITEC.
