From waste to resource: advancements in sustainable lignin modification

Abstract

Lignin represents the most abundant natural source of aromatic scaffolds. The majority of lignin is obtained as byproduct from industrial pulping. However, since lignin is often burned after pulping for energy purposes, its real potential as renewable resource for the production of aromatic compounds or polymeric applications is not fully exploited. In this literature review, recent advancements in the field of lignin modification are addressed. In particular, methods for the insertion of different functional groups onto lignin (amines, epoxides, carboxylic acids, esters, multiple bonds and hydroxyl groups) are discussed, focusing on sustainability and safety of the employed procedures and on applications of the respective modifications. The E-factor was chosen as green metric for the evaluation of sustainability, for its clarity and ease of application to different methodologies, together with a careful evaluation of toxicity and hazardousness of the used substances. This review thus provides an overview on lignin modification and applications possibilities and, importantly, quantitatively compares all discussed literature procedures in terms of sustainability.