The application of lignin in the production of advanced bio-based materials has been largely constrained due to its complex structure, which highly depends on its botanical source and isolation method. Typically, lignin requires chemical modification to enhance its utility in material applications. However, conventional procedures often necessitate the use of bulk solvents and excess reagents, resulting in increased production costs and low environmental sustainability. To address this challenge, we explored a solventless modification method using reactive extrusion (REx), a technique that adheres to the principles of green chemistry.
Few studies have reported the use of REx for lignin modification [1–3], although further research is needed to investigate the use of different lignins and modification agents and to assess the sustainability advantage of this method. In our study, we subjected lignin to REx with a cyclic anhydride in a twin-screw extruder. We initially optimized the process parameters using the widely available kraft lignin, subsequently extending the method to various lignin types to assess its broader applicability.
Our results indicated predominant modification of the aliphatic hydroxyls of lignin, with over 80% substitution achieved within minutes. The solventless nature of the process and the elimination of the purification step substantially reduced the E-factor, underscoring the green advances of this technology. Furthermore, the versatility of this method was demonstrated by successfully esterifying various lignin feedstocks. This technique enabled us to produce a thermoplastic lignin derivative that can be further processed and transformed into lignin-based bioplastics through extrusion.
By offering short processing time, efficient material usage, and most importantly, overcoming the challenging solubility of lignin, we envision that REx will play a key role in biorefineries, fundamentally shifting the paradigm for lignin valorization. This significant advancement positions REx as a powerful, green tool to unlock the full potential of lignin as a valuable resource.