Analysis of phenolation potential of spruce kraft lignin and construction of its molecular structure model

Abstract

The structure of kraft lignin is heterogeneous and the reaction activity is low due to the complicated and somewhat uncertain chemical reactions under the strong alkali and high temperature conditions of kraft pulping chemistry. At present, people still lack understanding of its structure, so it is difficult to be used as high quality industrial raw material for many years despite its huge output. In this paper, the phenolation potential of industrial Spruce Kraft Lignin (SKL) in phase separation system was explored. Compared with previous studies on phase separation reaction, due to the direct use of lignin as feedstock, sulfuric acid would only act as a phenolation catalyst, and is no longer used for swelling cell walls and hydrolyzing carbohydrates, so a modest reduction of acid concentration is feasible. In the phase separation system, the benzyl position of lignin was activated to selectively graft the p-cresol groups. Due to the protection of organic phase on lignin, the breakage of the β linkages and the condensation reaction of lignin were effectively prevented, so the original structure of lignin was well preserved. In order to capture the detailed structural changes of lignin after phenolation in phase separation system, FT-IR, GPC and quantitative NMR techniques (1H-NMR, 31P-NMR and 2D-NMR) were used to analyze the changes of main functional groups, linkages and substructures of industrial lignin samples before and after phenolation. The results showed that the industrial spruce kraft lignin still have many active sites which can be modified by phenolation. And through p-cresol treatment, a kind of high activity industrial lignin with more phenolic hydroxyl groups, more uniform molecular size and higher purity was obtained. Then the molecular structure models of spruce kraft lignin before and after phenolation is proposed respectively based on the molecular weights, quantitative analysis of functional groups, linkages, substructures and introduced p-cresols. This will be helpful to understand the structural changes of industrial spruce kraft lignin during phenolation in phase separation system, and provide guidance for the high value utilization of this high activity industrial lignin in the future.