Study On Lignin Removal And Hydrodeoxygenation Transformation Of Corn Straw

Promoting the coordinated development of energy supply and ecological environment under the context of carbon is an important issue of concern in various fields and disciplines.The efficient conversion of biomass is an effective pathway for promoting the transition of energy structure towards clean and low-carbon.Lignin is the only compound with an aromatic ring structure among the three major components of lignocellulosic biomass,and it has the potential to produce high value-added chemicals,high-performance fuels,and new environmentally friendly materials.Therefore,conducting comprehensive research on lignin removal,depolymerization,and derivative conversion to enhance the overall utilization value of lignin components is essential to promote the development of biomass resource utilization technologies.The chemical structure of natural lignin is complex and prone to alteration during the pretreatment process,which affects the subsequent catalytic depolymerization conversion.Current pretreatment research is more focused on the development of efficient separation systems,and there is insufficient exploration of the removal laws of lignin at the microscopic scale.Moreover,there is a lack of profound understanding of the pretreatment mechanisms,resulting in a failure to establish a systematic theory to guide the subsequent high-value utilization.In addition,lignin is a three-dimensional polymer composed of phenylpropane units connected by C-O bonds and C-C bonds,with abundant oxygen-containing functional groups.The catalytic hydrodeoxygenation refining of lignin phenolic derivatives is usually carried out under high temperature,high pressure,and strong reducing agents,which is not conducive to industrial largescale applications and environmental protection requirements.Therefore,designing an economically and environmentally friendly catalytic system to achieve the depolymerization of lignin macromolecules and the low-temperature and highefficiency conversion of lignin phenolic derivatives is a research topic of great value and significance.To address the aforementioned issues,this paper focuses on the common biomass resource of corn stover and conducts research on the migration patterns of lignin in the cell wall during organic solvent pretreatment,in order to deepen our understanding of the pretreatment mechanism.Additionally,an efficient catalytic system is developed to depolymerize the lignin separated by the organic solvent into lignin-derived phenols,which are further hydrogenated and deoxygenated into high value-added products.The main content and conclusions are presented as follows.Firstly,using corn stalks as raw material,the removal of lignin in an acidic dioxane/water solvent system was studied,and the correlation mechanism between the migration pattern of lignin in the cell wall microstructure and chemical structure changes during pretreatment was obtained.Results showed that at a pretreatment temperature of 70℃,the removal of lignin firstly occurred in the secondary cell wall and became significant after 75 min,with a 10.8%increase in removal rate in the first 5 minutes.The removal of lignin progressed towards the compound middle lamella.Through experimentation,changes in lignin chemical structure characteristics were observed at 75 min,including monomer composition（S/G）,linkage numbers（β-O-4,β-β,β-5）,and molecular weight.It was further explained that the differential distribution of lignin in the cell wall led to different degrees of removal difficulty,resulting in differences in the chemical structure of recovered lignin at different stages of pretreatment.To verify the trends in lignin structural unit changes and depolymerization properties,this study designed Pt/C-heteropolyacid synergistic system to catalyze the depolymerization of lignin recovered by organic solvents pretreatment（DOLs）.The typical C-O structures（4-O-5,α-O-4,and β-O-4）can be cleaved under the catalytic of Pt/C and phosphotungstic acid.Under the reaction conditions of 240℃ and 2 h,DOLs60 and DOLs-120 obtained by recovering lignin after pretreating with acidic dioxane for 60 min and 120 min were depolymerized.The monomer yields were 15.9%and 17.4%,respectively,indicating that DOLs have high reaction activity and can be depolymerized into a large number of monomers.In addition,the higher relative content of G unit in the depolymerization product of DOLs-60 proved that the types of monomers in the depolymerization product are related to the structure of DOLs.Building on the completion of lignin depolymerization,this paper further investigates the hydrodeoxygenation（HDO）of lignin phenolic derivatives.An organic group-modified TiO₂（B）nanosheet was developed and used to prepare the 5%Pt/TiO₂（B）catalyst,which achieved complete conversion of propylphenol at 180℃ and 1 MPa 10%H₂/N₂,with a selectivity to propylbenzene of 73%.Comparison with Ptbased catalysts on different supporters confirmed that the supporter can affect the size of Pt particle,thus influencing the catalytic activity for HDO.The Pt nanoparticles on the surface of Pt/TiO₂（B）had an average particle size of 1.49 nm and isolated single atoms,demonstrating that the organic groups on TiO₂（B）favored the high dispersion of Pt particles and improved the activity of Pt/TiO₂（B）.It was also found that the strong metal-supporter interaction promoted the formation of Ti³⁺ defects,which facilitated the efficient catalytic deoxygenation of propylphenol.This study provides a new avenue for enhancing the catalytic activity of catalysts by modifying the supporter.Based on the previous chapter,we further optimized the catalytic deoxygenation system with Pt/TiO₂（B）catalyst as the core and established a catalytic HDO system for phenolic compounds under near-room temperature conditions.Complete conversion of phenol to cyclohexane and cyclohexanol was achieved under 50℃ and 1 atm H₂.Preparation of Pt/TiO₂（B）catalyst with different states of surface Pt species demonstrated that coexistence of nanoparticles and single atoms was most favorable for the Pt species involved in phenol HDO.In-situ monitoring of the reaction confirmed that stable organic groups on TiO₂（B）support were favorable for the generation of key intermediates.Possible reaction mechanisms were inferred through DFT calculations of elementary reaction barriers and validation experiments of intermediates.Specifically,phenol hydrogenation deoxygenation under mild condiction was achieved by breaking the C-O bond of low-energy enol intermediates.This catalytic system provides an energy-saving and environmentally friendly solution for efficient HDO of lignin-derived phenolic.