Extraction And Characterization Of Hemicelluloses From Corn Stalks And Preparation Of Activated Carbon From Corncob Hydrolysis Residue

More and more attention has been focused on corn stalk for biotransformation into bioethanol,chemicals and materials due to its renewability,abundance and widely distribution in China.As well known,biomass is primarily made up of cellulose?40⁴5%?,hemicelluloses?25³5%?and lignin?20³0%?,where in hemicelluloses are a group of complex glycan including xylan,glucan et al.In this paper,hemicelluloses of corn stalk were separated and its structure was analysed by ion chromatography,FTIR,NMR and TGA.The results lay the theoretical foundations for subsequent conversion and utilization of hemicelluloses.Firstly,the chemical composition of corn stalk was analysed and hemicelluloses of corn stalk were extracted with hot-compressed water.The results showed that the main compositions of corn stalk were celluloses?39.1%?,hemicelluloses?29.5%?and lignin?18.9%?.The optimal conditions of hemicelluloses extraction were at temperature of 170? with extraction time of 50 min,where xylan content in pre-extracted liquid reached the maximum?24.7%?with low content of degradation products of hemicellulose and dissolved.Three different methods of hemicellulose extraction were adopted for corn stalk.Hemicelluloses of corn stalk were extracted by 10% KOH solution then followed by graded ethanol precipitation.The results showed that the hemicelluloses low branching density were obtained in low ethanol concentration,while hemicelluloses with high branching density were precipitated in high concentration ethanol.The FTIR and NMR resultsshowed that back bone of hemicellulose was D-xylpyranosyl and the side-chain units of hemicellulose were L-arabinan and 4-Omethylglucuronic-acid.Hemicelluloses with yield of 74% were obtained in the continuous extracting process of water,KOH aqueous solution with different concentrations,and mixture solution of KOH and ethanol.The hemicelluloses with more branches could be obtained by hot water extraction,while hemicelluloses with low branch density could be obtained by alkali extraction.According to the spectral analysis,the alkali-soluble hemicelluloses in sample H1.5 were L-arabino-4-O-methyl-glucurono-D-xylan.TG analysis showed that the thermal stability of water soluble hemicelluloses was superior to that of alkali soluble hemicellulose.Hemicelluloses with yield of 88.5% from corn stalk could be obtained in the continuous extracting process with dimethyl sulfoxide,dioxane triethylamine,saturated barium hydroxide,1M KOH,1M NaOH and 3M KOH aqueous solutions.The hemicelluloses extracted by organic solvent contained more branched structure,while alkali-soluble hemicelluloses have a straight-chain structure.The hemicelluloses extracted by DMSO mainly consisted of 4-O-methyl-glucurono-arabinoxylans.The thermal analysis results showed that the thermal stability of alkali-soluble hemicelluloses was superior to that of the hemicelluloses extracted by organic solvent.Upgrading corncob residues?CCR?to a high quality energy resource is an effective utilization of an underutilized industrial lignocellulosic waste.The hydrothermal carbonization technique was therefore employed to generate a high heating value?HHV?hydrochar.Results showed that HHV increased 47% after treatment at 230 °C for 1.5 h.However,prolonging the residence time beyond 0.5 h had a negligible effect on the HHV.Decreases in H/C and O/C ratio verified that reductions in C and O reactions occurred following hydrothermal carbonization.The chemical and thermal properties of the final hydrochar by FT-IR,TG/DTG and XRD analyses indicated that dehydration and decarboxylation were the predominant pathways for C and O reduction.The present hydrothermal carbonization process offered a promising approach to upgrade CCR into hydrochar with high heating value under mild conditions.Finally,preparation and characterization of activated carbon with high surface area from hydrochars were investigated.The optimal conditions for activated carbon preparation were temperature at 400? reaction time of 1 h with impregnation ratio of 3,in which surface area and pore volume of activated carbon reached the maximum 2192 m²/g and 1.269 cm³/g,respectively.The N₂ adsorption isotherms showed that the activated carbon from hydrochars had better micropore structure.The FTIR analysis indicated that the chemical modification occurred in activation process of hydrochars.SEM analysis confirmed that amorphous and heterogeneous structures with cavities and pores formed on the surface of the activated carbons in products.In addition,the prepared activated carbon exhibited a high adsorption capacity on lignin and acetic acid in prehydrolysis liquor of dissolving pulp production.