| Most of the hemicelluloses are released in the pre-hydrolysis liquor(PHL)during the pre-hydrolysis of fiber materials prior to pulping.The residual fiber materials and the pre-hydrolysis liquor can be used for pulping and separation of hemicelluloses,respectively.The purified hemicelluloses can be used for the production of high value-added products such as hydrogel,package materials and paer chemicals.Based on the concept of biorefinery,aspen chips were used as raw materials,and the effects of the aspen chips storage time on the release of the aspen components during pre-hydrolysis were studied,as well as the negative charge of PHL and the molecular weight of the purified hemicelluloses.The effects of pre-hydrolysis conditions on the components,negative charge and the molecular weight of colloidal lignin in PHL were discussed.The PHL was treated by dialysis,and the effects of dialysis on the components and negative charge of PHL were investigated.The fresh aspen chips and six month storage chips were used for pre-hydrolysis,respectively.The results showed that,the wood chips storage obviously affects the pH and the lignin content of PHL.The pH of PHL obtained from flesh aspen chips was lower that of the PHL obtained from six months storage chips.As the pre-hydrolysis temperature was higher than 150°C,compared the components of the PHL from the six month storage aspen chips,the lignin content in the PHL from flesh chips was lower,and the monosaccharides especially glucose and xylose content was higher.As the pre-hydrolysis temperature was 170°C or more,compared to the aspen chips after six month storage,the xylan content in the PHL of fresh aspen chips was sight lower.As the pre-hydrolysis temperature was 170°C,the molecular weight of the hemicelluloses obtained from the PHL of six months storage aspen chips was sight higher than that of the hemicelluloses from fresh aspen chips,but the molecular weight poly dispersity index of hemicelluloses in fresh wood hydrolysis liquor is lower,which indicated that the hemicelluloses had the narrower molecular weight distribution and more uniform structure.The temperature significantly affected the pre-hydrolysis.With the increase of the pre-hydrolysis temperature,the lignin and saccharides content increased and the pH decreased.As the pre-hydrolysis temperature increased to 180°C,the saccharides content decreased due to the degradation of saccharides and the formation of byproduct such as furfural.The time also affected the pre-hydrolysis,which was a minor factor compared to pre-hydrolysis temperature.With the increase of the pre-hydrolysis time,the lignin and the monosaccharides content increased.When the pre-hydrolysis temperature was 170 °C,the poly(oligo)saccharides content decreased after 90 min pre-hydrolysis,which indicated that poly(oligo)saccharide would degrade to monosaccharides at prolonged time,but the rate of degradation to monosaccharides was much faster that the rate of monosaccharides degradation to furfural.Uronic acids in the pre-hydrolysis liquor existed mainly in the form of oligosaccharides.With the increase of the pre-hydrolysis temperature,the uronic acid especially polygalacturonic acid content decreased,which companied with the decrease of the cationic demand.So the uronic acid especially polygalacturonic acid was an important sources of negative charge of PHL.The PHL was treated by dialysis with different molecular weight cut-off,and the results showed that,the molecular weight of the most lignin in PHL was less than 3500 Da.The molecular weight of a small part of the lignin in PHL was between 3500-14000 Da.The molecular weight of the most poly(oligo)saccharides was less than 3500 Da.After treated by dialysis with molecular weight cut-off of 3500 Da,the uronic acid in PHL content decreased obviously,as well as cationic demand of PHL.31P-NMR was used for the measurement of the groups and their content in the colloidal lignin separated from PHL by centrifugation.The results showed that,with the pre-hydrolysis temperature increase,the aliphatic hydroxyl group of colloidal lignin in PHL decreased,and the phenolic hydroxyl group increased.But carboxyl group content of the colloidal lignin was almost unchanged with the increase of pre-hydrolysis temperature,which showed that the colloidal lignin in the PHL contributed to a small part of negative charge of PHL,especially for the PHL obtained in the lower pre-hydrolysis temperature. |
