Zinc-based deep eutectic solvent-mediated hydroxylation and demethoxylation of lignin for the production of wood adhesive

Abstract

Choline chloride–ZnCl₂ deep-eutectic solvent (ChCl–ZnCl₂ DES), mole ratio 1 : 2, was used to improve the chemical reactivity of wheat straw alkali lignin under different temperatures and times of pretreatment. The chemical structure of the resulting modified lignin was studied by UV, FT-IR, ¹H, ¹³C and ³¹P-NMR spectroscopies, TGA, NALDI-TOF MS and ICP. Interestingly up to 10 wt% of lignin can be readily dissolved in ChCl–ZnCl₂ DES under the optimized pretreatment conditions (80 °C, 1 h) yielding ca. 65% of modified lignin upon precipitation using water as an antisolvent. As a result of the chemical modification of lignin occurring during its dissolution in DES, the total phenolic hydroxyl of the fraction precipitated increased ca. 1.9-fold while methoxyl moieties were reduced between 1.6 and 2.2-fold when compared with untreated lignin. Thus, in the fraction of lignin precipitated, phenolic hydroxyl formation took place at the expense of selective methoxyl cleavage from the aromatic ring, judging by the decrease of S units, whereas β-O-4′ linkages and molecular weight remain unchanged. Finally modified lignin was used as a phenol replacement in the synthesis of phenol-formaldehyde (PF) adhesives. Remarkably, the strength of the modified resin (1.3 MPa) compared with PF resin was practically the same when 40 wt% of the phenol was replaced by the modified lignin. This work shows that lignin can be readily modified in a DES – improving its reactivity – further advancing its prospective use in the wood industry.