Comparison of the structural characterization of lignophenols and alkaline lignins before and after methylolation

Abstract

This study reports on the isolation of bamboo lignophenols (LPs) and alkaline lignins (ALs) followed by their subsequent modification by methylolation. Methylolated lignins are quite useful and in high demand in wood–plastics composites or phenolic resin. The structural characterizations of the two lignin samples both before and after methylolation were compared by gel permeation chromatography, Fourier transform infrared spectroscopy, and ¹H–¹³C heteronuclear single quantum coherence nuclear magnetic resonance spectroscopy (2D HSQC NMR). It was found that after methylolation, the weight average molecular weight (Mw) and polydispersity (D = Mw/Mn) of the LPs varied to a greater extent than those of the ALs (c.f., Mw: from 1167 (D = 1.96) to 8267 (D = 5.47) for the LPs and Mw: from 917 (D = 1.45) to 1580 (D = 1.62) for the ALs). These results indicated that the LPs have more functional groups prone to methylolation compared to ALs. Quantitative ¹³C and 2D-HSQC NMR analysis demonstrated that both LPs and ALs were hydroxyl phenol, guaiacyl, syringyl-type (HGS-type), and that the esterified p-coumarate groups in the native bamboo lignins were cleaved under alkaline cooking conditions but were not cleaved in the phase separation system. In addition, the 1,1-bis(aryl)propane-2-O-aryl ether unit was confirmed to be the basic unit of the LPs, and both the introduced and unreacted p-cresol moieties were observed by 2D HSQC NMR spectroscopy. Finally, the higher ratio of syringyl unit to guaiacyl unit (S/G ratio) and methoxy group content of the ALs compared to the LPs explained the poorer reactivity of the ALs toward methylolation.