Striving for a greener world with lignin waste
15 April 2023
The world needs to find ways to reduce its use of fossil fuels because of their adverse effects on the environment. Lignocellulosic biomass is an excellent sustainable alternative that can be used to make green products. Lignin, being one of the major components in lignocellulosic biomass, is the second most abundant natural polymer, but it's used sparingly.
Lignin waste is typically burned to produce energy, which contributes to little added value. In 2017, 70 million tonnes of lignin waste was produced, but only 1 to 2 per cent of lignin was used to make non-fuel products like fertilisers and adhesives. This means we're not fully using lignin's potential, making it harder for companies to produce sustainable products via biorefinery.
In plants, lignin acts as a protective barrier, making them strong and keeping them safe from biological or chemical damage. However, lignin is difficult to be converted into other products because it's strong and doesn't break down easily. Lignin depolymerisation processes that exist today are expensive and use harsh chemicals. This makes it hard for companies to make sustainable products from lignin at a low cost with green approaches.
Recently, researchers have looked into the use of ionic liquids and deep eutectic solvents (DESs) to make it easier to turn lignin into valuable products. While most ionic liquids are expensive and possess potential hazards, DESs are better alternatives because they are more environmentally friendly and less costly. DESs have been used in biomass pretreatment and transformation to make various bio-based products such as furfural.
"Researchers found that combining water and DES could make converting lignin into other products easier. This research adds an alkaline solution to lignin to make it into aromatic monomers. Using DESs could make it easier and less expensive for companies to produce sustainable products at one atmospheric pressure," stated Associate Professor Wu Ta Yeong, Associate Head of School (Graduate Research), School of Engineering, Monash University Malaysia.
Associate Professor Wu and his team, including his former PhD student Dr Victor Ong Zhenquan, and current PhD students Yong Khai Jie and Tnah Shen Khang, tested a method using sodium hydroxide (NaOH) solution with aqueous DES in biomass pretreatment and transformation. They tried different NaOH concentrations, temperatures and time periods to see what worked the best for such a process. They also added different metal chloride catalysts to see if they could improve the depolymerisation process. The goal was to find a way to break down and transform lignin from biomass more efficiently at one atmospheric pressure.
The research concludes that using NaOH-aqueous DES as a depolymerisation medium could be promising for promoting milder and inherently safer lignin depolymerisation at one atmospheric pressure. The addition of metal chloride catalysts also showed to improve the process by increasing aromatic monomer yields while reducing reaction temperatures. Furthermore, physical and chemical characterisation studies were performed to provide further evidence supporting this proposed approach.