Purification And Refinement Of Xylo-oligosaccharides From Pre-hydrolysis Liquor Of Eucalyptus Wood

In recent years,the market demand for dissolving pulp is increasing year by year.Containing a large amount of hemicellulose,the pre-hydrolysis liquor could be produced during the preparation of dissolving pulp.With the concept of biorefinery being put forward,the efficient utilization of hemicellulose of pre-hydrolysis liquor has become a research hotspot for researchers work in science and technology.However,the presence of lignin in pre-hydrolysis liquor would hinder the preparation of hemicelloluse-based high value-added products and reduce the purity of downstream products.In this paper,calcium hydroxide?Ca?OH?₂?,activated carbon?AC?and laccase were employed to purify the pre-hydrolysis liquor for removing impurities such as lignin with the minimum loss of xylan.Then,microwave-assisted acid hydrolysis and enzymatic hydrolysis were applied toprepare xylo-oligosaccharides with degree of polymerization in the range of 2 to 4.Besides,the structure and properties of the prepared xylo-oligosaccharides were analyzed.The purpose was to provide theoretical basis and technical support for the high value-added utilization of pre-hydrolysis liquor.Firstly,calcium hydroxide was used to purify the pre-hydrolysis liquor,the effect of calcium hydroxide dosage on the contents of lignin,furfural,acetic acid and xylose in pre-hydrolysis liquor was explored,then the purification mechanism of calcium hydroxide was studied.The results showed that when the dosage of calcium hydroxide was 1.5%,the removal of lignin and furfural were 41.2%and 47.4%,respectively,the loss of xylose and xylan were unconspicuous,while the concentration of acetic acid increased significantly from 6.2 to 11.7 g/L.The purification of calcium hydroxide can remove the lignin and furfural effectively without an obvious effect on the contents of xylose and xylan.In addition,calcium hydroxide would lead the removal of acetyl groups from xylan and form acetic acid during the purification process.Then,the AC adsorption was employed to purify the pre-hydrolysis liquor treated with calcium hydroxide?PHL-1?and the effect of AC dosage on the contents of lignin,furfural,acetic acid and xylose in pre-hydrolysis liquor was conducted,then the purification mechanism of AC was studied.The results indicated that when 1.0%of AC was added into pre-hydrolysis liquor treated with calcium hydroxide,the removal of lignin and furfural were 54.4%and 98.0%,respectively,and the loss of xylose was only 6.0%.According to the analysis of AC adsorption mechanism,AC would selectively adsorb lignin first,and its adsorption capacity of xylan is better than xylose,nevertheless,the small molecule of lignin is not easy to be adsorbed by AC.It is concluded that the order of the four factors is as follows.The pre-hydrolysis liquor treated with calcium hydroxide and AC?PHL-2?was further purified via laccase-assisted AC treatment,the key parameters affecting the efficiency of laccase treatment was optimized by Design-Expert software,the results showed that the order of the above parameters from strong to weak was as follows:pH value>laccase dosage>treatment temperature>treatment time.The functional model between the lignin removal and parameters,such as pH value,treatment temperature,laccase dosage and treatment time,was established.The results indicated that the removal of lignin was up to 65.4%when 5 U/g of laccase was added and the purification process was conducted at 45?for 3 h with the pH value of 5.After purification with laccase and AC,the total removal of lignin and furfural were 90.7%and 100%,respectively.However,the total loss of xylose and xylan were 5.7%and12.0%,respectively.Microwave-assisted acid treatment were used to produce xylo-oligosaccharides?XOS,the degree of polymerization in the range of 2 to 4?from the pre-hydrolysis liquor purified with calcium hydroxide and AC.The effects of the type of acid,acid dosage,temperature and time on the contents of xylose and XOS were investigated and the structure of generated XOS was characterized.The results showed that when hydrolysis reaction was conducted at 120?for 20 min with 1.4%sulfuric acid,the content of XOS increased up to 38.8%with the assisted pretreatment of microwave.The produced XOS contains part of side-chains of arabinose and uronic acid.Xylanase treatment was empolyed to prepare xylo-oligosaccharides from the pre-hydrolysis liquor purified with calcium hydroxide and AC.The effects of xylanase dosage,time,temperature and pH value on the contents of XOS were investigated,the structure of produced XOS were analyzed and characterized.The results illustrated that when enzymatic hydrolysis reaction was processed at 55?for20 min with 2 U/g xylanase,the content of XOS was up to 12.3 g/L,increased by 68.5%compared to the PHL untreated with xylanase,and the content of XOS was 54.0%based on the Eucalyptus original PHL.The XOS contains part of side-chains of arabinose and uronic acid.