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Isolation and characterization of Douglas-fir organosolv lignin Cho, Hern J.


Granular water-insoluble lignins were isolated from a series of aqueous organic solvent (organosols) cooks designed for pulping/sac-charification of Douglas-fir sawdust. Among the factors affecting yield and characteristics of the isolated organosolv lignins, only cooking time (5-20 minutes) and concentration of acid catalyst (0-0. IN HC1) were investigated as cooking variables. Cooking temperature (200°C) and solvent composition (acetone/water=60:40) were held constant. It was learned that the acidified organosolv cooking system is far more efficient in delignification and saccharification than a-queous acid hydrolysis under identical conditions. In organosolv cooking, simultaneous dissolution of lignin and sugars occurs in the cooking liquor, allowing continued and total dissolution of the wood constituents. In the present study, only the water-insoluble lignin fraction was isolated and analyzed. An almost quantitative recovery of the precipitable lignin was accomplished by evaporation of the organic solvent from the spent liquor, followed by removal of sugars dissolved in the aqueous solution and reprecipitation of the crude lignin into water. To eliminate the interference from hydrogen bonding and unconjugated carbonyl group in the isolated organosolv lignins, acetylation or reduction was carried out before the lignin samples were characterized. The resulting lignin samples were found to be completely free of cabohydrate contaminants. Both cooking time and acid concentration were found to have a profound effect on the yield of lignin fractions, and chemical and macromolecular properties of the lignin molecules due to two competing reactions, hydrolytic depolymerization and recondensation. These reactions take place simultaneously in the cooking liquor during organosolv cooking. The balance between these two reactions is believed to be responsible for not only the content of functional groups, as revealed by nuclear magnetic resonance, infrared and ultraviolet spectral analyses, but also the size of lignin molecules, as measured by gel permeation chromatographic and scanning electron microscopic analyses of the isolated organosolv lignins. The functional group contents, determined by elemental and spectral analyses, were found to be 0.86-0.97 methoxyl, 0.20-0.49 aromatic hydroxyl and 0.68-0.99 aliphatic hydroxyl groups per C₉-unit of the organosolv lignin molecules. It was also noted that 63-68% of aromatic nuclei have condensed forms with carbon-carbon linkages, having only two hydrogens on each guaiacyl nucleus. The organosolv lignins were found to have much lower molecular weights than those of protoligniri in wood. Typical values of the number average molecular weight of the isolated lignins ranged from 823 to 1,144. The low molecular weight values are due to degradation reactions during the cooking by cleavage of aryl-alkyl linkages of lignin molecules. The particle size of the spherical precipitated lignins ranged from 25 to 500 nm.

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