The molecular design and experimental study on the catalytic cleavage of linkages in lignin with binuclear ionic liquid

Abstract

Abstract The β-O-4 linkages are dominant in lignin with the high activity of cleavage under the acidic conditions and difficult to reunite. Therefore, it is significant to study the cleavage of them. As for the amounts of ILs are huge, the theory of Frontier Molecular Orbital was proposed to predict the “fracture precision” of linkages especially β-O-4 linkages in lignin. Finally, a binuclear ionic was designed as the liquid catalyst to improve the catalytic activity. Based on the result of molecular design, [PrSO3HMIM]+ and [HSO4]− were more efficient to break the β-O-4 linkages. Then, [PrSO3HMIM]2[2HSO4] was designed through increasing the number of [PrSO3HMIM]+ and [HSO4]−. Subsequently, the yield of guaiacol was used as the experimental indicator to evaluate the cleavage selectivity of β-O-4 linkages. The experimental results indicated that the optimal yield of guaiacol was 27.23% with [PrSO3HMIM]2[2HSO4] as catalyst, which was higher than that with [PrSO3HMIM][HSO4] as catalyst, and the reaction time to the optimal yield was shortened. At last, the reaction mechanism was also studied that the cleavage of linkages was attributed to the hydrogen bonding interaction between cation and lignin (dominant) as well as anion and lignin (synergistic) by the NBO analysis. Through simulation design and experimental verification, [PrSO3HMIM]2[2HSO4] enhanced the hydrogen bonds interaction between IL and lignin,which showed an excellent effect on lignin degradation.