Production Of Hemicellulose-derived Sugars From Corncob By Pretreatment

As one of typical agricultural wastes,corncob is considered to be a promising candidate for replacing fossil resources to produce biofuels and high value-added chemicals because of its low price,renewable and abundant reserves.The main chemical components of corncob are cellulose,hemicellulose and lignin.The hemicellulose content in corncob is higher than other straws,which is the promising raw material for the preparation of hemicellulose-derived products such as functional sugars.However,due to the complex structures such as the strong hydrogen bonding,high degree of polymerization and high crystallinity of cellulose,the three-dimensional structure of lignin,and the lignin-hemicellulose complex,it is difficult to hydrolyze corncob.Therefore,it is necessary to break through its natural barrier by pretreatment techniques for achieving the efficient utilization of corncob.In this study,different high-efficiency pretreatment approaches were developed to prepare high value-added hemicellulose-derived sugars from corncob,which achieved the selective depolylmerization of hemicellulose during pretreatment.（1）Hemicellulose-derived sugars were prepared by microwave-induced hydrothermal pretreatment of corncob when a trace of NaOH was added for suppressing side reactions.The effects of NaOH concentration and hydrothermal pretreatment severity factor（SF）were investigated on the product distribution of the hydrolysate and solid residue composition.The physicochemical properties of the residue were characterized by SEM and XRD.The results indicated that NaOH and SF had a significant effect on the yield of hemicellulose-derived sugars.With increasing SF,the total dissolution rate of hemicellulose increased,and the pH value controlled by NaOH could change the distribution of the hemicellulose-derived sugars and suppressed the formation of by-products.When SF was 4.00 and NaOH concentration was 0.005%,the total hemicellulose derived sugar yield of 88.71%could be obtained.In the meantime,the furfural yield was only 1.72%and the unknown loss of hemicellulose was1.51%.It was also found that more than 90%cellulose still remained in treated corncob,which could be applied for the production of biofuels.（2）A feasible approach was proposed to prepare xylose from corncob using metal chlorides as catalysts in a biphasic system.The catalytic performances of metal chlorides from different groups,valence states and concentrations were investigated,and the effect of pretreatment was discussed on the enzymatic hydrolysis of the residue.The structures,crystallinity and surface morphology of the residue after pretreatment were analyzed by SEM,XRD and solid ¹³C NMR techniques.The results showed that the highest xylose yield of 87%was achieved when the pretreatment was conducted at 140 ^oC for 20 min with 25 mmol/L FeCl₃ as the catalyst under microwave heating in the biphasic system（2-MTHF/water,1:3）.The enzymatic hydrolysis of the residue was carried out by cellulase（25 FPU/g residue）,and the maximum glucose yield was 89%.（3）Xylooligosaccharides were produced from corncob using organic acids under the control of NaOH.The influences of the types and concentration of organic acids,the amount and time of NaOH addition were investigated on the yield of xylo-oligosaccharides.The corncob residue after pretreatment was treated with p-toluenesulfonic acid at 80°C to release lignin from treated corncob.The residue after the second treatment was catalyzed by TEMPO to make cellulose to be nanofibers.The separated lignin and cellulose were analyzed by AFM,¹³C NMR and SEM.Results showed that 38.30%of xylo-oligosaccharides were achieved when corncob was pretreated with 0.2 mol/L formic acid at 160 ^oC for 60 min,and then 0.4%NaOH was added,the reaction was continued at the original temperature for 60 min.In addition,the analysis of AFM and SEM showed that nano-scale lignin and cellulose were obtained by the subsequent treatment.