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  • Author: Dimitris S. Argyropoulos x
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Abstract

During this investigation, softwood (picea mariana) milled wood lignin was subjected to kraft pulping conditions at the temperature range 100-160 oC, for variable lengths of time. After quantitative isolation of the treated lignins, they were subjected to 31PNMR analyses which allowed the absolute rates of scission of the erythro and thewo diastereomers of the arylglycerol-B-aryl ether structures present in lignin to be derived.The plots of the kinetic data revealed that two kinetic regimes operate i.e. an initial fast phase that is followed by another slower phase. In agreement with previous accounts, the rate constants for these scission reactions were found to follow a pseudo-first-order rate law, during both phases. Rate constant data obtained during this effort, invariably indicated that the erythro isomers of the R-0-4 units of softwood milled wood lignin cleave faster than their thr-eo counterparts during both phases. At elevated temperatures (160 "C) the difference was found to be augmented. The general stability of the threo diastereomers toward kraft pulping seems to be the manifestation of a considerably slower reaction of the pulping reagents with the threo diastereomers of the B-aryl ethers. Arrhenius and Eyring plots o i these data supplied the complete profile of thermodynamic parameters governing this commercially important ether scission reaction.

Abstract

Despite the growing importance of Eucalyptus wood as raw material for pulp and paper, there is a lack of knowledge on the chemistry of their macromolecular components. The present paper addresses this issue by applying the recently developed protocol for isolating enzymatic mild acidolysis lignins (EMAL) from Eucalyptus grandis, Eucalyptus globulus and the softwood species Douglas fir and white fir, which were used for comparative purposes. The structures of EMALs were investigated by quantitative 31P NMR, DFRC/31P NMR (derivatization followed by reductive cleavage followed by quantitative 31P NMR) and size exclusion chromatography (SEC). Overall, the yields of EMALs isolated from Eucalyptus were higher than those from the softwoods examined. Lignin from E. globulus was found to contain higher contents of arylglycerol-β-aryl ether structures, free phenolic hydroxyl groups and syringyl-type units than lignin from E. grandis. New insights provided by the DFRC/31P NMR revealed that up to 62.2% of arylglycerol-β-aryl ether structures in E. globulus are uncondensed, while in E. grandis the amount of such uncondensed structures was found to be lower than 48%. SEC analyses showed that lignins from E. grandis and softwoods associate in greater extension than lignin from E. globulus.