Lignin is an abundant heterogeneous aromatic polymer with a significant potential to serve as a renewable source of aromatic chemicals. However, lignin is highly underutilized as a feedstock for polymeric materials and chemicals as it is commonly burned as a fuel source for pulping plants.
Only an estimated 2% of the 50 million tons of lignin isolated from pulping processes in 2010 was used for specialty products, while the rest was burned as a low-value fuel. This underutilization sparks the desire to develop not only processes to isolate lignin from biomass more efficiently, but also to engineer lignin-based products of increased 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 the synthesis of polymers or either by creating new reactive sites or by structural modification of the functional groups already existing in the lignin. In addition to that, depolymerization of lignin is another option which involves its fragmentation into smaller molecules having higher reactivity that can be utilized for the synthesis of biobased polymers and biofuels.
The chemical products obtained from lignin depolymerization still typically possess some oxygenation, including products such as phenol or vanillin. There are two major thermochemical routes used to isolate useful chemical products from lignin: hydrogenolysis and oxidation.
Hydrogenolysis also referred to as hydrogenation is pyrolysis (the heating of organic substances in the absence of air resulting in smaller fragments while 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 yield the desired products. The first commercial system utilizing the hydrogenation of lignin and a catalyst was the production of approximately 44% mono- phenol products (phenol, o-cresol, p-cresol, p-ethylcresol, and p-propylcresol).
Oxidation is another technique utilized to isolate aromatic products from lignin. The products generated from this process typically possess increased complexity and functionalization, in comparison to products from hydrogenation. While it is clear there are many possible approaches to the oxidative degradation of lignin into useful commercial products, no method has yet to be 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.
As of 2007, only three products are commercially produced from lignin: vanillin, dimethyl sulfide, and dimethyl sulfoxide. Vanillin is commercially isolated through the oxidation of lignin in alkaline conditions. Relevant vanillin derivatives are those that could be reasonably isolated under pulping conditions. In strongly oxidizing conditions, vanillin may be oxidized to vanillic acid or to methoxyhydroquinone via decarboxylation. While in strongly reducing conditions, vanillin can be reduced to vanillic alcohol. With the development of more cost-effective and efficient hydrogenation or oxidation systems, more products may be isolated, and the development of these processes could aid in the reduction of fossil fuel consumption.
The degradation of lignin to phenolic products or monolignols is an energy intensive process, which is one of the reasons lignin currently has more value as a fuel than as a feedstock. Methods that could utilize 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 both phenolic and aliphatic hydroxyl groups can also be used as a macro-monomer for the synthesis of polymers of different nature such as polyurethanes, polyesters, epoxide resins, and phenolic resins.
The increasing demand to develop bio-based polymeric materials and replace those from petroleum-based sources will require significant progress in several different areas. Methods to isolate more useful lignin-based chemical products or methods to chemically functionalize lignin to synthetically useful products without the use of expensive reagents or complicated synthetic routes.
FRACTION aims to develop new downstream conversion technologies of lignin obtained by organosolv extraction, for the synthesis of resins and polyurethanes, but also catalytic depolymerization processes to obtain monophenols with high yield.
Which are the Lignin conversion routes in FRACTION?
O1. Catalytic oxidation
FRACTION project will explore the capacity of new lignin produced by organosolv process to generate high added value products such as vanillin (and its derivatives), veratraldehyde, syringol (and its derivatives, through the catalytic oxidation reaction employing emerging type of catalysts based on supported MOFs.
O2. Lignin depolymerisation (hydrogenolysis/solvolysis)
FRACTION will be focused on the depolymerization of lignin to monophenols with high yields. Depolymerization will be performed in by solvolysis and hydrogenolysis applying advanced solid metal catalysts and low-cost solvents. The isolated monophenols will be used to replace polyols (petrol-based) in polyurethane production.
O3. Synthesis of lignin-based polyurethanes for coatings, adhesives and elastomers
FRACTION project will develop lignin-based polyurethanes by using unmodified lignin from GVL fractionation and depolymerized lignin from solvolysis to replace fossil-based polyols in polyurethane synthesis. Different isocyanates, chain extenders and soft segments will be assessed to obtain bio-based PUs with tailored physicochemical properties for high-value applications such as elastomers, coatings and adhesives. The project will also explore a new approach to synthesize lignin-based non-isocyanate polyurethanes and potentially 100% bio-based polyurethanes.
O4. Polymerization of lignin and lignin-derivates for resin synthesis
FRACTION project aims to develop a Lignin Phenol Formaldehyde (LPF) resin with 20% of phenol substitution which can be turned into an insulation foam and meeting the actual specifications for thermal, mechanical and fire properties.
Dulce Muñoz Ph.D.
Principal Scientist
Advanced Materials Department
FUNDITEC/www.funditec.es
dmunoz@funditec.es
FUNDITEC is leader of FRACTION’s WP4 “GVL lignin conversion into high-value aromatics and polymers”