Mechanistic Differences Between Kraft and Soda-AQ Pulping of Hardwoods with Regard to Lignin–Carbohydrate Complexes (LCC)

Abstract

The literature on biomass research contains many references to lignin–carbohydrate complexes (LCC) decreasing the rate of delignification in chemical pulping, decreasing the yield of cellulosic ethanol via fermentation, and decreasing forage digestibility. Regarding wood delignification, there are a few reports on the formation and/or cleavage of lignin–carbohydrate (L–C) bonds during alkaline pulping. The behavior of LCC was investigated to find a potential explanation for the differences between the soda-anthraquinone (soda-AQ or SAQ) and kraft processes with regard to delignification rate in the residual phase of pulping and in the bleaching process. Enzymatically isolated lignin (EL) was prepared from two soda, nine SAQ, and twelve kraft pulps from sugar maple, a hardwood. The range of kappa numbers, after correction for hexenuronic acid (HexA), was 10–60. The bound sugars on each EL were hydrolyzed and converted to monomers by H2SO4 at 121°C. There was evidence in the data suggesting that the bound glucan and xylan on the ELs from soda, SAQ, and kraft pulps were native to the wood. The bound galactan data were somewhat ambiguous, and there was no detection of bound mannan on any EL. The reproducibility and repeatability of bound arabinan attached to ELs (BA) were excellent. Although not conclusive, the totality of the data is suggestive of both L–C bond formation and cleavage involving arabinose units during both kraft and SAQ pulping. There was no decrease in BA when SAQ was used to lower the c-kappa number (HexA-corrected) from ∼60 to ∼25. The case was similar when kraft was used in the range of ∼60 to ∼40. However, there were significant decreases in BA content when c-kappa number was lowered below ∼25 by both SAQ and kraft. A common mechanism was proposed to explain essentially no decrease in BA content at higher kappa numbers, but distinctly different mechanisms were proposed to explain BA cleavage at c-kappa number <25. A mechanism favorable to subsequent bleaching was proposed for kraft, but an unfavorable mechanism was proposed for SAQ.