Deciphering the enzymatic grafting of vanillin onto lignosulfonate for the production of versatile aldehydes-bearing biomaterials

Abstract

Modification of lignin plays a crucial role in extending its applications. While chemical functionalization has been extensively applied, exploring the enzyme-catalyzed approach for grafting phenolic molecules presents a promising avenue. Herein, we investigate the controlled laccase-mediated grafting of vanillin onto lignosulfonates (LS) as a sustainable approach to introduce aldehydes into LS, paving the way for further (bio)chemical functionalizations (e.g., reductive amination and Knoevenagel-Doebner condensations). The resulting vanillin-grafted LS is comprehensively characterized (HPLC, SEC, Pyrolysis-GC/MS, FTIR). The study reveals four key steps in the grafting process: (i) vanillin acts as a mediator, generating the phenoxyl radical that initiates LS oxidation, (ii) the oxidation leads to depolymerization of LS, resulting in a decrease in molecular weight, (iii) rearrangement in the vanillin-grafted LS, evidenced by the replacement of labile bonds by stronger 5-5 bonds that resist to pyrolysis, and (iv) if the reaction is prolonged after complete consumption of vanillin, condensation of the vanillin-grafted LS occurs, leading to a significant increase in molecular weight. This study provides valuable insights on the behavior of vanillin and LS throughout the process and allows to identify the optimal reaction conditions, thereby enhancing the production of vanillin-grafted LS for its subsequent functionalization.