Valorisation of organosolv lignin in dynamic boronate ester gels by ethanol/water tandem batch depolymerization and exploiting glycerol carbonate functionalization

Abstract

In this study, a tandem process combining depolymerization and oxyalkylation was applied to beech organosolv lignin (MW ≈ 8300 g mol−1) to synthesize an optimal 1,2-diol rich bio-based substrate for boronate ester gels. A 70/30 vol% ethanol/water solvent system was selected as depolymerization medium due to its optimal lignin solubility and depolymerization degree. The depolymerization was conducted at 250 °C for 8 h and yielded lignin products with tuneable molecular weights (MW ≈ 2100–5900 g mol−1), influenced by the presence of Pd-catalyst and/or formic acid. The phenolic OH content increased across all samples through cleavage of β-O-4 linkages to similar values of ≈ 4.0–4.5 mmol g−1, particularly in condensed phenolic OHs due to the high S/G ratio. The aliphatic OH content decreased by acid-catalysed depolymerization while the addition of the Pd-catalyst mitigated this effect via in-situ H2 generation. Subsequent oxyalkylation with glycerol carbonate at 175 °C for 30 min, catalysed by K₂CO₃, converted phenolic OHs into 1,2-diols. The increase in alOH content is affected by the initial ratio of different OH functionalities and by the unwanted internal transesterification that led to discrepancies between theoretical values and experimental results. Finally, boronate ester gels with stoichiometric diol/boronic acid ratio were made and 1,2-diol quantification via 13C NMR proved more accurate than 31P NMR, which overestimated the 1,2-diol content, leading to excessive crosslinker addition and consequently diol capping. Despite a lower 1,2-diol content, gels from depolymerized lignin exhibited higher storage and loss moduli, attributed to the increased effective crosslink density.