Sustainable lignin modifications and processing methods - green chemistry as the way forward

Abstract

The anthropologic impact on climate has reached severe heights since humanity is still holding onto non-renewable fossil sources. Lignins can play a key role in the transition from petroleum based chemistry towards a more sustainable and green approach. Commercially available lignins are in a large extent technical lignins obtained from the soda, Kraft or sulfite process and in a lesser extent lignins from biorefineries (i.e., organosolv and hydrolysis lignin…). However, it is far from enough to solely incorporate lignins at application level in order to improve sustainability and increase the greenness. The path from lignin biomass to the final end product should be taken into consideration as well. Therefore, this review starts of with an adaptation of the CHEM21 metrics toolkit to assess the greenness of valorization strategies based on strategically chosen criteria. A detailed solvent evaluation helps the reader to inform the greenness of a wide variety of solvents and in addition, it can be used as an instrument to design green ionic liquids which can be applied during the valorization of lignins. Green lignin modification and depolymerization strategies (to increase the solubility, reactivity and processability and to improve the structural uniformity and alter functionalities) were evaluated bearing the green chemistry principles and the adapted CHEM21 metrics toolkit in mind. Finally, the processing methods of lignins (i.e., blending and polymer synthesis) were assessed on their sustainable approach.