Characterization of Conversion Behavior of Major Constituents in Lignocellulosic Biomass During Acid-Catalyzed Organosolv Pretreatment

The objective of this study is to understand conversion characteristics of glucose and xylose as major monosaccharide standards of lignocellulosic biomass, and furthermore, to investigate hydrolysis and degradation characteristics of cellulose and hemicellulose as polysaccharide standards, and finally, to evaluate those influences on the recalcitrance of Liriodendron tulipifera during organosolv pretreatment with 50% (v/v) ethanol at various concentrations of sulfuric acid as a catalyst.\n\tIn respect of glucose and xylose standards, the acid-catalyzed organosolv pretreatment with ethanol as an organic solvent was obviously different from acid-catalyzed process under an aqueous medium. Among three stages of conversions, glucose and xylose were decomposed very slowly no more than 1.7-3.0% and 1.9-4.4% at the second stage of 150-170˚C and 140-160˚C, respectively. In particular, due to the influence of combined conversion reactions, which occurred under both ethanol-rich and water-rich characteristics, ethyl levulinate and methyl levulinate were formed from glucose standard by esterification reaction. However, whole conversion reactions of xylose were progressed at lower temperature than those of glucose.\n\tIn case of Avicel PH-101 and ashless filter paper as cellulose standards and xylan as hemicellulose standard, xylan was completely hydrolyzed at much lower temperature than the celluloses. The hydrolysis and conversion of xylan was almost unaffected by the increase of degree of polymerization (DP), but the difference of DP between glucose and cellulose strongly influenced those hydrolysis and conversion. Moreover, hydrolysis rate and maximum yields of the major products were different between Avicel PH-101 and ashless filter paper. In particular, at higher temperatures, hydrolysis and conversion profiles of the cellulose and xylan showed the same pattern as that of glucose and xylose, respectively.\n\tThe major decomposition products of glucose and celluloses were 5-HMF, furfural, levulinic acid and formic acid, and that of xylose and xylan was only furfural. The minor decomposition products were very different from each other. Therefore, overall reactions related to the formation of the decomposition products from xylose were much simpler than those of glucose.\n\tOn the basis of the results mentioned above, conversion characteristics of major constituents in whole lignocellulosic biomass, L. tulipifera were investigated. During organosolv pretreatment of L. tulipifera, glucan was slowly hydrolyzed no more than 15.9% on the basis of initial glucan content in the unpretreated L tulipifera. At lower temperatures, the hydrolysis and degradation of xylan and lignin were rapidly progressed simultaneously, and at 180˚C, xylan was completely hydrolyzed and converted. At 180˚C, only 10.4% Klason lignin and 1.0% acid soluble lignin remained in pretreated solid residue along with 88.6% glucan.\n\tDue to the lignin-carbohydrate complex in L. tulipifera, during the simultaneous hydrolysis and degradation of xylan and lignin, breakdown of internal lignin and xylan bonds and lignin solubilization occurred simultaneously, but after the complete hydrolysis of xylan, the degradation of lignin and its solubilization into liquid hydrolysate almost did not progress. Therefore, remained lignin accelerated the formation of lignin droplets, which coalesced, migrated and was redeposited, and finally, the number and content of the lignin droplets rapidly increased.\n\tCrystallinity index of pretreated solid residue continuously increased with increasing temperature. Through FT-IR analysis of pretreated solid residue, the rapid hydrolysis and degradation of xylan and lignin after organosolv pretreatment, and the slow hydrolysis of glucan were confirmed.\n\tAs a result of enzymatic hydrolysis, it was confirmed that the critical temperature condition (140-150˚C), which was revealed by the investigation on the hydrolysis and conversion of major components and those related physicochemical properties of L. tulipifera, led to the significant changes of the biomass recalcitrance in L. tulipifera during organosolv pretreatment, resulting in the drastic improvement of cellulose to glucose conversion yield and enzymatic digestibility. In that temperature range, the simultaneous decrease of xylan and lignin contents followed by the destruction of lignin-carbohydrate complex and the distribution of lignin, as well as the deacetylation of hemicellulose occurred.\n\tIn conclusion, the investigation on conversion characteristics of major components in lignocellulosic biomass and the evaluation of those influences on physicochemical properties of lignocellulose biomass during organosolv pretreatment provided us the significant keys to effectively overcome the lignocellulosic biomass recalcitrance.