Lignin, together with cellulose and hemicellulose, is one of the main components of lignocellulosic biomass and the most abundant source of renewable aromatic molecules on earth. Because of this, lignin is a promising candidate for replacing fossil-based materials such as resins, carbon fibers and plastic materials[1]. Despite the many attempts of using lignin in such materials, industrial applications remain limited. This is mainly due to the degradation of lignin’s structure during isolation, where the harsh pH and temperature pH conditions favour condensation and repolymerization reactions. This yields to isolated lignins with uncontrolled chemical structures and that are thermodynamically difficult to blend with existing materials or to further functionalize[2]. We have developed a process for the fractionation of biomass and isolation of lignin that uses aldehydes to form stable acetals with the diol present within the beta-O-4 bonds of lignin during extraction from lignocellulosic biomass. This functionalization prevents the condensation of the aromatic biopolymer[3]. Here, we developed a method to extract a chemically modified lignin from hardwood by using a multifunctional aldehyde, which allowed us to both isolate an uncondensed lignin and, at the same time, precisely control the degree of functionalization of the biopolymer for further incorporation into new materials.

In this work we used terephthalic aldehyde (TALD) to stabilize lignin during extraction and at the same time functionalize the biopolymer with residual aldehyde groups. Several TALD functionalized lignins were isolated from birch wood by using different concentrations of terephthalic aldehyde in the reaction system. TALD-Lignins were first characterized through HSQC NMR, and quantitative ¹H NMR to determine the molar amount of free aldehydes on the lignin backbone. The quantification showed a linear correlation between the amount of TALD used in the extraction process and the moles of free aldehyde groups bound to the lignin scaffold, demonstrating the possibility of isolating lignins with a controlled quantity of functional groups. Furthermore, to investigate the ability of TALD to efficiently stabilize the aromatic biopolymer, we performed catalytic hydrogenolysis, comparing the results to the reductive catalytic fractionation of birch wood. The high monomer yields obtained in the hydrogenolysis of lignin showed that the terephthalic aldehyde protection strategy is particularly effective at preserving lignin’s native structure. Interestingly, the yield of monomers didn’t decrease rapidly with a reduction in the amount of bound aldehyde, suggesting that TALD stabilized the lignin not only through the formation of acetals, but potentially also through steric hinderance.

Overall, we demonstrated that lignin could be chemically modified with unprecedent control by introducing on its scaffold a free and non-native aldehyde functional group. Given the ease with which aldehydes can be transformed into a variety of other functional groups under mild conditions, this process could help overcome the challenges that limit lignin’s use in new materials such as resins or plastics.

[1] Stefania Bertella,  Jeremy Luterbacher, Trends in Chemistry, 2020, 2, 440–453.
[2] Vivien Romhányi, Dàvid Kun and Béla Pukánszky, ACS Sustainable Chemistry & Engineering, 2018, 6, 14323–14331.
[3] Li Shuai, Masoud Talebi Amiri, Ydna Marie Questell-Santiago, Florent Héroguel, Yandling Li, Hoon Kim, Richard Meilan, Clint Chapple, John Ralph and Jeremy Luterbacher, Science, 2016, 354, 329–333.