Study On Enzymatic Hydrolysis Of Sugarcane Bagasse Pretreated By Green Liquor And Its Lignin's Catalyzed Depolymerization Pathways

Sugarcane bagasse,the main by-product of the sugarcane industry,is one of the major agro-industrial residues produced in large quantities.It could be a potential lignocellulosic feedstock for production of bio-ethanol or bio-chemicals.However,its recalcitrant structure makes its enzymatic hydrolysis difficult and results in low ethanol yield.Nowadays,most studies still focus on the pretreatment methods to improve the enzymatic hydrolysis of lignocellulosic biomass.Lignin remains poorly utilized though it is a promising raw material for the fuel and chemical industries.In this study,different green liquor pretreatment methods for sugarcane bagasse were investigated in order to improve the efficiency of enzymatic hydrolysis.At the meantime,the structure of black liquor lignin was characterized.And then the catalyzed hydroprocessing of sugarcane bagasse black liquor lignin and different lignin model compounds were studied.Finally,the possible routes for lignin depolymerization were proposed.Green liquor combined with sulfite or ethanol was effectively utilized to improve the enzymatic hydrolysis of sugarcane bagasse by selectively removing lignin.The superior performance of Kraft GL allows conversion of cellulose to glucose more significantly than Soda GL.The highest glucose yield of 96.8%could be reached after Kraft GL-sulfite pretreatment at 140?.Kraft GL combined with ethanol(Kraft GL-ethanol)could not only reduce the lignin content of sugarcane bagasse,but also remove some non-crystalline hemicellulose.The increase in glucose yield after pretreatment at low temperatures(e.g.80? and 100?)was significant.The maximum glucose yield(98.26%)of sugarcane bagasse after 72 h of enzymatic hydrolysis was achieved at a pretreatment temperature of 160?.Compared with Soda GL-H2O2 pretreatment,more lignin was removed during Soda GL-Ethanol pretreatment.The chemical structure of lignin was considerably more degraded by GL-Ethanol pretreatment than that by GL-H2O2 pretreatment.According to the structure of black liquor lignins from Kraft GL-ethanol pretreatment,it could be concluded that there was less re-condensation of lignin fragments during Kraft GL-ethanol pretreatment which was beneficial for delignification.?-O-4 linkage is the most prevalent linkage in real lignin.Firstly,4-methoxy-?-[(2-methoxyphenoxy)methyl]-benzenemethanol was chose as substrate and investigated its hydroprocessing reactions using different catalysts.It could be found that about 45.66 wt%(86.59 mol%)monomers were obtained when using Ni/USY as catalyst with H2 at 280? in ethanol/water.When using pure methanol as solvent,the model compound was converted into 68.30 wt%(124.47 mol%)by Ni/?-25.In the study of 4-methoxy-a-[(4-methoxyphenoxy)methyl]-benzenemethanol,it could be found that the position of methoxyl group in B ring may affect the possible pathways of hydroprocessing.When using Ni/Mg-Al LDH as catalyst,this dimer owned a low monomer yield but high yield of an ether compound.This ether compound was produced by the reaction between dimer and ethanol.Compared to Ni/p-25,the dimer yielded more monomers about 35.38 wt%(67.74 mol%)when using Ni/USY.The mechanism of y-OH dimer hydroprocessing was more complicated than that of ?-OH dimer.During the hydroprocessing of ?-OH dimer,many reactions occurred such as cleavage of ?-O-4,reaction between dimer or intermediate and ethanol,cleavage of C?-C?,dehydration of ?-OH.The cleavage of side chains would generate more gaseous product and result in a low yield.In all the reactions catalyzed by Ni/USY and Ni/?-25,it could be found that the intermediate could react with ethanol to produce an ether compound in accordance with published literatures.However,for the other model compounds with?-1,?-5 and 4-0-5 linkage,these catalysts did not work well.Based on the studies of model compounds,the screened catalysts were used for hydroprocessing of real lignin and the liquid products were analyzed.In this part,two kinds of sugarcane bagasse lignin were prepared:alkaline ethanol lignin and alkaline hydroperoxide lignin.The chemical structure of lignins showed that there are a few differences between these two ligins which may affect the distribution of liquid products.In absence of catalyst,the monomer yield of alkaline ethanol lignin was higher than that of alkaline hydroperoxide lignin.Lignin could be partial depolymerized just with hydrogen and solvent.For alkaline ethanol lignin,more monomers could be produced with addition of catalysts.The highest monomer yield(34.68 wt%)was obtained when using Ni/Mg-AlLDH as catalyst and with a high selectivity for syringol.However,this catalyst did not work well for alkaline hydroperoxide lignin.Its highest yield(33.27 wt%)was obtained when catalyzed by Ni/?-25.Moreover,the solid acid catalyzed depolymerization of lignins produced more compounds with carbonyl group than hydroprocessing.The yield of alkylpehnol increased in the presence of catalysts but still in a low level.