Pretreating Lignocellulose Via PHP Pretreatment: Lignin Deconstruction Mechanism And Isolated Products Utilization

Traditional fossil energy consumption has caused climate change and environmental pollution,which promotes the development of low-carbon and sustainable energy.As a renewable carbon neutral biomass,lignocellulose has unique advantage to be the alternative of traditional fossile resources by converting into bulk chemicals and polymer materials.However,the complex structure and intrinsic recalcitrance of lignocellulose make great challenge to efficient bioconversion.Traditional tretreatment techniques possess single products that will waste resource and pollute environment.It is a good strategy to improve lignocellulose availability by fractionating lignocellulose into cellulose,hemicellulose and lignin.As a major factor of biomass recalcitrance,lignin valorization is the key to improve resource utilization and build a high efficiency biorefining.In this study,the phosphoric acid plus hydrogen peroxide（PHP）was used to pretreat lignocellulose for improving cellulose availability and recovering lignin and hemicellulose compositions.To solve the problem of lignin valorization effectively,the physical and chemical properties and lignin deconstruction mechanism were analysed.The resultant lignin was used for adsorbent and porous carbon according to its physical and chemical properties.The main studies and results of this dissertation are summarized as follows:（1）A cellulose-first fractionation process with the co-production of water-soluble polysaccharides and lignin（PHPL）was built based on the PHP pretreatment.Detailed research about the effect of pretreatment conditions on lignocellulose deconstruction and components recovery.Typical agricultural and forestry residues were used to demonstrate the feedstock compatibility of PHP refinery.H₃PO₄-H₂O₂ ratio and pretreatment temperature had great impact on lignocellulose deconstruction and product recovery.A high H₃PO₄-H₂O₂ratio cooperated with elevated temperature achieved 96.7%of hemicellulose removal,but decreased the recovery of water-soluble polysaccharides.The low H₃PO₄-H₂O₂ ratio achieved a high delignification of 76.6%,while lignin was degraded to decrease lignin recovery.As for cellulose-rich fraction,the accessibility of cellulose hydrolytic enzymes was enhanced,achieving 100%of cellulose-glucose conversion.Moreover,100 g of poplar（hardwood）,pine（softwood）,and corn stalk（grassy）can be efficient deconstruted by PHP pretreatment,producing 18.4 g of bioethanol with the co-production of water-soluble polysaccharides（6.6 g）and lignin（6.8 g）.（2）The physical and chemical properties of PHPL preteated with different time,PHPL and the residual lignin in cellulose-rich fraction（PHPRL）from different raw materials were comprehensive researched to clarify deconstruction mechanism of PHP pretreatment.PHPL-1h,PHPL-3h and PHPL-5h were characterized with high purity（≤1.0%residual carbohydrates）and low molecular weight（1383-1436 g/mol）.Compared to the cellulolytic enzyme lignin,PHPL-1h exhibited abundant Ar-OH（1.90 mmol/g）,-COOH（0.54 mmol/g）and carbony（0.28 per Ar）.However,the content of Ar-OH and-COOH of PHPL-5h decreased to 0.28 mmol/g and 0.54 mmol/g,respectively,with the disappearing of the aromatic signals and the increasing of carbony（0.36 per Ar）.PHPRL and PHPL underwentβ-O-4′cleavage and aromatic ring oxidization in PHP pretreatment,thus having similar structure.However,PHPL had lower molecular weight（1182-1682 g/mol）and Ar-OH content（0.68-1.42 mmol/g）,which exhibited higher depolymerization degree,benzene ring oxidization and ring-opening than PHPRL.The oxidation system produced by the synergy of H₃PO₄ and H₂O₂ achieved efficient lignin deconstruction by cleaving side chain and oxidizing benzene ring.（3）The oxidation severity of PHP pretreatment was tuned to research the effect on simultaneous lignocellulose deconstruction and lignin modification.PHPLs were used as adsorbent to verify the relationship between oxygen-containing functional groups and adsorbent capacity of lignin.Single H₃PO₄ only achieved 94.5%of hemicellulose and 32.7%of lignin removal.PL had high molecular weight（4190 g/mol）and low content of hydroxyl groups,suggesting the limited fractionation and deconstruction ability of single H₃PO₄pretreatment.H₂O₂ input improved the removal of hemicellulose（～100%）and lignin（73.3%）,and the cellulose-glucose conversion（～100%）.With the increasing of H₂O₂ input,the ability ofβ-O-4’cleavage,Ar-OH and benzene ring oxidation were improved,thus resulting in PHPL with low molecular weight and abundant oxygen-containing groups.High oxidation severity of PHP pretreatment improved lignin deconstruction and recovery,but yielding low carbohydrate recovery.The adsorbent capacity of these lignins that fractionating by H₃PO₄ and H₂O₂ pretreatment with different H₂O₂ input followed the order of PHPL-3.53%>PHPL-1.77%>PHPL-0.88%>PL.The enriched oxygen-containing functional groups of PHPL facilitated adsorbent capacity.（4）PHPLs with low molecular weight and abundant oxygen-containing functional groups were selected as carbon precursor to prepare porous carbon with high specific surface area（SSA）by one-step KOH-activation.KOH facilitated the generation of micropores and mesopores of PHPLCs,producting high SSA（3094 m²/g）,high pore volume（1.72 cm³/g）and wide pore size distribution（0.5-6 nm）.With increasing of activation degree,carbon defects increased,graphitization and oxygen content decreased.In the three-electrode system,PHPLC exhibited a specific capacitance of 352.9 F/g.In the two-electrode system,PHPLC had excellent energy density of 9.5 Wh/kg at power density of 25.0 W/kg.Moreover,5000 cycles of charge/discharge reached 88.46%retention at 5 A/g,implying a good cycle stability.Under the same activation parameters,the oxygen content（12.0-5.5 at.%）of PHPLCs decreased,while SSA（3383-3923 m²/g）and specific capacitance（225.7-350.9 F/g）of PHPLCs increased with the improving of oxidation degree of PHPLs.With the increase of oxidation severity of PHPLs,the content of oxygen-containing functional groups that can be easy consumed by KOH was increased,thus tuning the development of pore and electrochemical performance of porous carbon.This study established a new process to efficient deconstruct and fractionate agricultural and forestry lignocellulose residues by PHP pretreatment.Lignin structure characteristics and deconstruction mechanism have been comprehensively researched,and the relationship of lignin structure and performance as adsorbent and porous carbon were enriched,which provided efficient biorefining approach for lignocellulose utilization,facilitating the development of biomass utilization and carbon neutrality of China.