NMR study on residual lignins isolated from chemical pulps of beech wood by enzymatic hydrolysis

Two residual lignins isolated from kraft and ASAM pulps of beech wood (Fagus sylvatica L.) were analyzed by 1H and 13C NMR spectra to confirm the structural features previously obtained from wet chemical degradation methods. 1H NMR spectra revealed that the most distinct features of ASAM lignin was the signal at δH 5.9 ppm and δH 6.6 ppm for Hα in β-O-4 linkage and protons for syringyl units. The abundance of syringyl units in ASAM lignin was also evidenced by the signals at δC 154 ppm and δC 104 ppm in the 13C NMR spectra, responsible for aromatic C3/C5 and C2/C6 of syringyl units. The important functional groups, such as phenolic OH, aliphatic OH and methoxyl groups, were indirectly evaluated by 13C NMR spectra of acetylated residual lignin. In particularly, the increment of phenolic OH, one of the parameter for cleavage of β-O-4 linkages, was determined to relative low in of β-O-4 linkages in ASAM residual lignin as compared to kraft residual lignin, indicating that ASAM procedures may run by quite different pathway for delignification unlike the fragmentation of β-O-4 linkages and cleavage of side chain in the general alkaline pulpings.