Suppression of lignin repolymerization via a recyclable formic acid-guaiacol system toward green and sustainable components fractionation

Lignin precursor molecules have received extensive attention in improving the pulp purity and the lignin activity by inhibiting lignin condensation. Herein, a formic acid/guaiacol dual-phase system was established to treat the wheat straw in this work, through a significantly enhanced delignification efficiency. It was attributed to steric hindrance of formyl and methoxy group formed by formic acid, which hindered the π-π stacking and hydrogen bond network reconstruction between lignin fragments. Guaiacol also played an important role in diverting reactive benzyl carbocation toward stable C-O/C-C bond formation through a nucleophilic substitution reaction. The results revealed that the lignin fragment condensation was efficiently suppressed and the resultant cellulose pulp had a high purity. The Mn of lignin could be as low as 588 g/mol and the pulp contained only 7.2 % residual lignin with a 89.7 % α-cellulose content. The resulting paper without bleaching achieved a high brightness of 60.5 %ISO, and exceptional mechanical performance with the tensile index of 39.5 N·m/g and folding cycles of 15 times. Crucially, the formic acid and guaiacol could be recoverable via a simple distillation, minimizing waste generation and decreasing costs. This innovative and green strategy provides an important significance for the efficient lignocellulosic components fractionation.