Structural Changes Of Lignin In Pre-hydrolysis Of Aspen Wood And Effect On Following Pulp

Extraction of hemicelluloses from raw material as much as possible prior to pulping is the fundamental to realize the biorefinery of the biomass, which will be important for the application of hydrolysis to lignocellulosic materials. The hot-water hydrolysis of the aspen chips was investigated in this paper. The components of the chips, the hydrolysate, the wood residual lignin and the later pulping were analyzed under different hydrolysis conditions. The degradation rule of the carbohydrates and the lignin were discussed emphatically.Firstly, the chemical compositions of the aspen chips and the hydrolysates were analyzed during the different hydrolysis conditions. The results indiated that the degradation of hemicelluloses and lignin increased along with the strengthen temperature and reaction time. The pre-hydrolysis could extract about 37% of carbohydrates and 20% of lignin from the aspen chips. However, higher temperature could not cause more lignin to become extracted. Alternatively, more lignin was found to exist in the wood residue, probably due to the redeposition of the dissolved lignin on the wood fiber surface under severe extraction conditions. Also, the redeposition of the lignin might occur in the exothecium rather than the endothecium of the wood chips.A minimum reaction temperature of 160℃ was necessary for a satisfactory hemicelluloses removal. When the reaction temperature was changeless, prolonging the reaction time at the maximum reaction temperature (> 30 min) could further extract hemicelluloses from wood residues. At the same time, the dissolution of lignin and celluloses also occured. The sugars level in wood residues was almost at a stable state with increasing the severe extraction conditions, indicating that the dissolution and readsorption of sugars reached a balance and that no more sugars in hydrolysate were adsorbed on the wood surface. However, the more severe conditions could indeed reduce the concentration of sugars dissolved. The decrease of sugars dissolved in hydrolysate under severe extraction conditions could result from the formation of furfural or its derivatives. Therefore, under the hydrolysis condition at 170℃ in 60 min, we can obtain more sugars and lignin in hydrolysate.The milled wood lignins of residue wood chips were analyzed from using the modern instrument analytical technique, including the laser scattering instrument, FTIR and 13P-NMR. The laser scattering instrument results showed that the average molecular weight of the lignin increased along with the increasing hydrolysis temperature. However, the lignin polydispersity decreased under higher temperature. The FTIR results incidated that the contents of the-OH units, the benzene ring C-H units, the phenol-ether lignin and the C=O guaiacyl lignin increased during the pre-hydrolysis process. However, the contents of the syringyl lignin, the condensation-type guaiacyl lignin, the aliphatic hydroxyl and the C-H units were lower than control group. In addition, the detection of 31P-NMR discovered that the contents of the aliphatic hydroxyl decreased after hydrolysis, but the contents of the syringyl hydroxyl, the guaiacyl hydroxyl, the condensed phenolic hydroxyl and the non-condensed phenolic hydroxyl increased along with the increasing hydrolysis conditions. Compared these results with the analyzed of lignin thermal detection, we found that the lignin degradation and condensation reactions occured during the pre-hydrolysis processs.The effects of the hot water hydrolysis on the subsequent kraft pulping of the aspen chips were analyzed. The Kappa value and the brightness increased along with increased hydrolysis conditions. However, the yield, the viscosity burst index and the tensile index decreased along with increased hydrolysis conditions. In addition, the fiber length and tearing index increased but then decreased. Therefore, the hydrolysis of the aspen chips was disadvantageous for the following pulping. However, when the alkali content was increased from 18% to 20%, the strength of the pulp was improved.