Extraction And Separation Of Hemicelluloses From Poplar And Its Preliminary Application

In traditional pulping process,cellulose in wood fibers is utilized effectively,while a large amount of hemicellulose and lignin dissolved in the black liquor are sent to the alkali recovery section for combustion.It is a great waste of biomass resources.In recent years,the high-valued utilization of various components of lignocellulosic materials has become a research hotspot after the birth of biomass refining concept.The lignocellulosic materials were treated by pre-hydrolysis before cooking,and the pre-hydrolysis liquor(PHL)could be used to produce the high value-added products.The lignocellulosic materials after pre-hydrolysis can also be used for producing dissolved pulp with high(?)-cellulose contents,which will bring additional economic benefits for pulp and paper industry.In this dissertation,the hemicellulose in poplar was extracted by hydrolysis of hot water.The dissolution of hemicellulose as well as the dissolution of lignin,furfural and acetic acid in the different pre-hydrolysis conditions were studied.The results showed that the highest total sugars extraction was obtained at the condition of solid-liquid ratio of 1:6,pretreatment temperature of 170℃ and time at maximum temperature of 60 min,which reached at 18.43 g/L.The highest lignin content was obtained at 180℃/60 min,while the highest furfural and acetic acid contents were obtained at 180℃/80 min.The prehydrolysis conditions of the highest total sugars content and other degradation products were not consistent,which was beneficial to the separation and purification of the carbohydrate components in the following experiments.The optimum pre-hydrolysis conditions for the recovery of hemicellulose were obtained by the ethanol precipitation combined with evaporation concentration.The composition,structure and the thermal stability of recycled hemicellulose were studied.The results showed that the best condition to acquire the largest amount of precipitation was 160℃/80 min.and 98.1%of hemicellulose of PHL was precipitated.The purity of the recovered hemicellulose was 77.6%.The increase of sugar content in the PHL does not mean that the amount of ethanol precipitation will increase.When the ethanol precipitation was carried out after adjusting the pH of the concentrated PHL to 3,80.3%of hemicellulose of PHL was precipitated,while the purity of the recovered hemicellulose was increased to 87.9%.The main composition of the hemicelluloses were xylose,accounting for 54.7%in mass,and 2.25%of arabinose,7.25%of galactose,9.25%of glucose and 14.65%of mannose were alsofound.The recovered hemicellulosesobtained from different pre-hydrolysis conditions showed a same composition and structure with small differences in the amount of precipitation and the content of sugars by the analysis of ion chromatography and FTIR.The thermos-gravimetric analysis indicated that thermal stability of recovered hemicelluloses decreased along with the improvement of the pretreatment temperature and extension of the pretreatment time;the thermal stability of recovered hemicellulose obtained after adjusting the pH of the concentrated PHL to 3 was higher than the hemicellulose obtained without adjusting the pH.The FTIR and NMR analysis indicated that the backbone of hemicellulose was(?)-D-pyranoidxylose,linkage was(?)-glucosidic bond and branches of hemicelluloses were L-arabino and 4-O-methy lgoucuronic-acid.The separation and purification of the PHL(170℃/60min)by activated carbon and Ion-exchange resin was also studied.Results showed that using a small amount of activated carbon with many times to treat the PHL can effectively obtain the high removal of lignin and low loss of sugars.The three stages of activated carbon(1:120,1:120,1:240)and one stage of ion exchange resins(1:20)achieved 96.6%,100%and 57.4%of lignin,furfural and acetic acid removal rate respectively,and 88.0%of sugars recovery.The hemicelluloses obtained by ethanol precipitation and the PHL after purification were all used to produce the hydrogel,which were used to adsorb the Zn2+and Cd2+.The chemical structure and surface porous of two types of hydrogel was observed and analyzed by FTIR.The optimal preparation conditions of N,N-methylene diacrylamide(MBA)/hemicellulose and acrylic acid(AA)/hemicellulose of two kinds of hydrogel were all 0.1 and 9,respectively.The optimal adsorption conditions of two types of hydrogel were all the pH of 6.5 for Zn2+ and Cd2+,the time of 120 min and the metal ion concentration of 800 mg/g.Under these conditions,the maximum adsorption capacities of hydrogel made from hemicelluloses obtained by ethanol precipitation for Zn2+ and Cd2+ were 307 and 508 mg/g,respectively.The maximum adsorption capacities of hydrogel made by the PHL after purification for Zn2+ and Cd2+ were 300 and 462 mg/g,respectively.In addition,two types of hydrogel can be reused and there was slight difference in the amount of adsorption and recovery efficiency.