Homogeneous Esterification Of Sugarcane Bagasse And Its Mechanism

Lignocellulose is one of the most important components of biomass resources,and has many advantages,such as richness,low-cost,renewability,biocompatibility,and biodegradability.However,it also has many disadvantages,e.g.the complicated components,complex linkages between different components,poor dissolubility in common solvents,and poor processability.Therefore,the integrated utilization of lignocellulose is a difficult part for biomass resources utilization with high efficiency.Chemical modification could improve defects and processability of lignocellulose,especially esterification.Here,lignocellulose was originated from sugarcane bagasse?SCB?.The influence of reaction media and esterification reagents on the esterification of SCB was studied,to elucidate the homogenous esterification mechanism of SCB and the potential application of SCB esters.These will provide the theoretical basis and technical support for the high-value conversion and utilization of lignocellulose.In this paper,the homogeneous esterification between SCB and phthalic anhydride?PA?was studied in dimethyl solfoxide/N-methylimidazole?DMSO/NMI?.The SCB and the three isolated components?cellulose,hemicelluloses,and lignin?were comparatively esterified under the same experimental conditions,to study the regulation mechanism of changing PA dosage on the detailed esterification behaviors of SCB.The detailed esterification information of the esterified SCB includes the percentage of substitution?PS?,chemical structure,the PS of each component,active positions in each component,and the PS of acitve positions.Results showed that the PS of SCB phthalates ranged from 23.22%to 30.21%,and the esterification reactitivity of lignin was higher than that of cellulose or hemicellulose.Fourier transform infrared?FT-IR?and ¹H-¹³C heteronuclear single quantum coherence(¹H-¹³C HSQC)confirmed the esterification between SCB and PA.¹H-¹³C HSQC analysis revealed that the esterificatiom reactivity of hydroxyls in anhydrogluceose units?AGU?of cellulose followed the order:C₆-OH>C₂-OH>C₃-OH.The esterificatiom reactivity order of hydroxyls in anhydroxylose units?AXU?of hemicelluloses was as follows:C₂-OH>C₃-OH.The esterificatiom reactivity of lignin aliphatic hydroxyls was higher than that of phenolic hydroxyls.The esterificatiom reactivity of phenolic hydroxyls followed the order:p-hydroxyphenol hydroxyls?H-OH?>guaiacyl hydroxyls?G-OH?>syringyl hydroxyls?S-OH?.Thermogravimetric?TG?analysis revealed that the thermal stability of SCB phthalates was improved.The film from SCB phthalate was prepared by simple solution casting,showing good light transmittance.Besides,the hydrophobicity of the film could be improved by increasing the PS of SCB phthalates.On the basis of studying the hogmogeneous esterification of SCB with PA in DMSO/NMI in detail,the detailed esterification behaviors of SCB in other dual solvents?dimethyl solfoxide/lithium chloride?DMSO/LiCl?,and N,N-dimethylacetamide/lithium chloride?DMAc/LiCl??were further studied.Results showed that the homogeneous esterification of SCB in the three solvents all occurred,and the PS of SCB phthalates obtained in DMSO/NMI was the highest among the three solvents,while the PS of SCB phthalates prepared in DMSO/NMI and DMDSO/LiCl were similar.FT-IR indicated that the three components?cellulose,hemicellulose,and lignin?of SCB were all involved in the esterification reaction,and the esterification reactivity of lignin was higher than that of polysaccharides?cellulose and hemicelluloses?.However,the esterification reactivity of polysaccharides was different in different dual solvents.The esterification reactivity of primary hydroxyls was higher than that of secondary hydroxyls in polysaccharides,the esterfication reactivity of hemicelluloses side-chains was higher than that of hemicelluloses main-chains.In lignin,the esterfication reactivity of lignin aliphtaitc hydroxyls was higher than that of phenolic hydroxyls,however the esterification reactivity order of lignin phenolic hydroxyls was different in different dual solvents.The ionic liquid 1-allyl-3-methylimidazolium chloride?AmimCl?was used as reaction solvent,and the regulation mechanism of chamging PA dosage on the esterification of SCB and its components?cellulose.Hemicellulose,and lignin?was studied.Results showed that the PS range of SCB and cellulose phthalates were 5.66%-22.71%and 11.61%-44.11%within PA dosage 10-50 mmol/g.In the selected conditions,the correlation between PS incraese?PSI?and PA dosage followed the equation:y_PSI=0.004x-0.02.FT-IR and ¹H-¹³C HSQC results suggested that the esterification reactivity of hydroxyls in AGU of cellulose followed the order of C₆-OH>C₂-OH>C₃-OH.The PS of hemicellulose sample was in the range of16.37%-52.15%,and the esterification reactivity of hemicelluloses side-chains was higher than that of main-chains,and abundant hydroxyls in the AXU were esterified.¹³C-NMR and¹H-¹³C HSQC suggested that the esterification reactivity of C₂-OH and C₃-OH in AXU was similar.The PS of lignin phthalate ranged from 41.13%to 68.81%.³¹P-NMR analysis indicated that lignin aliphatic hydroxyls were more reactive than phenolic hydroxyls,and the esterification reactivity order of phenolic hydroxyls was as follows:H-OH>G-OH>S-OH.The homogeneous synthsize of SCB phalate with catalyst 4-dimethylaminopyridine?DMAP?was studied in the ionic liquid AmimCl.The SCB and the three isolated components were comparatively esterified with catalyst under the same experimental conditions in AmimCl,to study the detailed esterification behaviors of SCB.Results showed that the esterification reactivity of lignin was higher than that of polysaccharides?cellulose or hemicelluloses?,and the skeleton of SCB three fractions were stable during the dissolution and regeneration processes.The esterification reactivity of AGU hydroxyls in cellulose followed the order of C₆-OH>C₂-OH>C₃-OH.The esterification reactivity order of hydroxyls in AXU of hemicelluloses was as follows:C₂-OH>C₃-OH.Lignin aliphatic and phenolic hydroxyls were both involved in the homogeneous esterification,and lignin aliphatic hydroxyls were more reactive than phenolic hydroxyls during the esterification.The esterification reactivity of phenolic hydroxyls followed the order of S-OH>G-OH>H-OH.Compared with the prepared SCB esters under the similar experimental conditions in the ionic liquid without catalyst,the PS obviously decreased.Among this,the PS of cellulose component had no significant change;however,the PS of hemicellulose and lignin components significantly decreased,and the mass loss of weight of the esterified lignin occurred.Compared with the esterification without catalyst,the PS of C₆-OH,C₂-OH,and C₃-OH in AGU of cellulose were all increased;the esterification reactivity of C₂-OH in AXU of hemicellulose was increased;the esterification reactivity of lignin phenolic hydroxyls changed as:S-OH>G-OH>H-OH.The homogeneous esterification of SCB and the three isolated components with maleic anhydride?MA?were studied in AmimCl.Results showed that the PS of the maleated SCB and its components?cellulose,hemicelluloses,and lignin?were 12.52%,13.50%,10.89%,and 14.30%,respectively.FT-IR and NMR analysis confirmed that the three components were all involved in the homogeneous maleation.¹H-¹³C HSQC NMR analysis revealed that the primary mono-esterification of cellulose,diesterification of hemicellulose and lignin,and the degradation of the three fractions occurred during the homogeneous maleation.The esterification reactivity of hydroxyls of AGU in cellulose followed the order of C₆-OH>C₂-OH>C₃-OH.The esterification reactivity of hydroxyls in AXU of hemicelluloses followed the order of C₂-OH>C₃-OH.Lignin aliphatic and phenolic hydroxyls were both esterified,and the esterification reactivity of aliphatic hydroxyls was higher than that of phenolic hydroxyls.The esterification reactivity order of phenolic hydroxyls was as follows:H-OH>G-OH>S-OH.The influence of changing GA dosage on the homogeneous esterification of SCB was studied.Resutls showed that the three components of SCB?cellulose,hemicelluloses,and lignin?were all involved in the homogneous esterification.The PS of SCB glutarylate was gradually increased with the GA dosage rising from 10 to 40 mmol/g,and decreased when the GA dosage further increased to 50 mmol/g.During the homogneous esteriifcation,the esterification reactivity of the three components followed the order of lignin>hemicellulose>cellulose.The esterification reactivity of AGU hydroxyls in cellulose followed the order:C₆-OH>C₂-OH>C₃-OH.The esterification reactivity of C₂-OH and C₃-OH in AXU of hemicelluloses was similar,and the esterification reactivity of secondary hydroxyls on the hemicelluloses side-chains was higher than that on hemicelluloses main-chains.Lignin aliphatic hydroxyls were more reactive than phenolic hydroxyls.Gel permeation chromatography?GPC?analysis suggested the occurrence of degadation of SCB during the homogeneous the esterificaiton.