Structural Transformation Of Lignin During Pretreatment And Its Correlation To Enzymatic Saccharification

Lignocelluloses are the only sustainable and renewable resources that can provide alternative of crude oil,fossil fuels,etc.The conversion of abundant lignocelluloses to biofuels presents a viable option for enhancing energy security and reducing greenhouse gas emissions.Unfortunately,although the biochemical conversion of lignocelluloses into biofuels has significant economical and technical potential,lignocelluloses naturally resist enzymatic attack due to many physicochemical factors,such as the existence of lignin and hemicelluloses,cellulose crystallinity,and accessible surface.Thus,an efficient pretreatment system must be applied prior to the enzymatic saccharification to overcome recalcitrance of lignocelluloses and improve the accessibility of cellulases to the pretreated substrates.In this paper,the effect of steam explosion,dilute sulfuric acid,and hydrothermal pretreatments on the chemical composition,structural features and the enzymatic digestibility of different lignocelluloses were comparatively studied.However,although above-mentioned pretreatments are promising technology for lignocelluloses,some disadvantages,such as only partial degradation and removal of hemicelluloses and lignin,as well as incomplete disruption of lignin-hemicelluloses matrix will impede the enzymatic digestibility of pretreated substrate.Among them,lignin typically plays a negative role in enzymatic saccharification and can significantly inhibit enzyme efficiency.Therefore,a further post-treatment is required to remove lignin and degraded hemicellulosic products from the pretreated substrates so as to improve enzymatic digestibility.Alkaline treatment is a promising technology for the effective removal of lignin and hemicelluloses,which can dramatically enhance enzymatic digestibility of the substrate.As a biorefinery process,lignin attracted close attentions for its potential value in developing bio-based materials and chemicals.In short,all the results will provide useful information in the utilization of lignocellulosic materials for large-scale enzymatic saccharification in the prospective biorefinery industry.The results were summarized as follows:Three combined pretreatment systems based on steam explosion,dilute acid,and hydrothermal pretreatments and mild alkaline post-treatment have been developed to remove lignins and obtain digestible substrates from different lignocelluloses(bamboo,rice straw,and sweet sorghum stems)for bioethanol production.The yield,purity,dissociation mechanisms,structural features,and structural transformations of alkali lignins obtained from the combined systems were investigated.It was found that pretreatment facilitated the subsequent alkaline delignification,releasing lignin with the highest yield and purity from the pretreated residue.All the results indicated that the depolymerization and condensation reactions of lignins simultaneously occurred under the harsh conditions.In general,these lignins had low molecular weights,narrow polydispersities,and high content of phenolic hydroxyl groups.In addition,removal of hemicelluloses by the pretreatment and release of lignin by the subsequent alkaline post-treatment resulted in higher cellulose saccharification ratios than the only pretreated or alkaline treated substrates.Thus,the combination of pretreatment and alkaline treatment is an environmentally friendly and economical feasible method for the production of glucose and high-purity lignin,which will be further converted into high value-added products based on biorefinery.To investigate the complexity and inhomogeneity of the lignin from sweet sorghum stem,successive alkali treatments were applied to extract lignin fragments.The successive treatments released 80.3%of the original lignin.The chemically structural inhomogeneity of the isolated lignins was comparatively and comprehensively investigated by UV,FT-IR,and NMR spectra.The lignins were found to be predominantly composed of ?-O-4 aryl ether linkages,together with minor amounts of ?-?,?-5,?-1,and ?,?-diaryl ethers linkages.In addition,hydroxycinnamic acid(mainly p-coumaric acid),which was found to be attached to lignin,was released and coprecipitated in the lignin fractions isolated in the initial extracting steps,whereas hydroxycinnamic acid were not detected in the subsequently alkaline extracted lignin fractions.Moreover,the high proportion of carbon-carbon structures was potentially related to the high amounts of guaiacyl units in the lignin investigated.Thermogravimetric analysis revealed that the higher molecular weights of lignins resulted in relatively higher thermal stability,and the higher content of carbon-carbon structures in the lignin probably led to a higher "char residue".These findings suggested that the lignin fractions extracted from sweet sorghum stem by successive alkali extractions had inhomogeneous features both in chemical composition and structure.Residual lignin plays an inhibitory role in the enzymatic saccharification of cellulosic biomass.The structural changes in the isolated lignins from the hot water hydrothermal pretreatment of aspen and their inhibitory effects on the enzymatic saccharification of Avicel were examined.The functional groups of the isolated lignins were determined using quantitative 13C,2D HSQC and 31P NMR.The increase in pretreatment severity significantly increased the condensed and non-condensed syringyl and guaiacyl OH group content in the isolated lignins,but decreased the aliphatic OH,p-hydroxybenzoate OH and carboxylic OH group content.A compelling adverse association(r2=0.998)was observed between the condensed syringyl and guaiacyl phenolic OH group content in lignins and their inhibitory effects on enzymatic saccharification.Langmuir adsorption isotherms showed that a higher pretreatment severity resulted in a higher binding ability between the isolated lignins and the cellulase enzymes,which led to more non-productive binding.It is hypothesized that condensed syringyl and guaiacyl phenolic units are mainly responsible for the inhibitory effect of lignin on enzymatic saccharification,in which the condensed aromatic rings enhance the hydrophobic interactions and the phenolic OH group boosts the hydrogen bonding.The combination of hydrophobic interactions and hydrogen bonding can further intensify undesirable non-productive binding.