Study On The Deoxygenation Mechanism Of Lignin Derivatives Catalyzed By Biochar Based On Oxygen And Nitrogen Sites

The preparation of high-value chemicals from biomass through catalytic pyrolysis is of great significance for sustainable energy development and resource utilization.In current catalytic research,the catalytic production of phenols from lignin by biochar has attracted much attention due to its high activity and selectivity towards phenolic products.However,the catalytic site structure and reaction mechanism of biochar catalysts in phenol production reactions are still unclear.To further explore the mechanism of carbon catalysts in lignin pyrolysis,this study used density functional theory,cluster model methods,machine learning,and catalytic experimental characterization which helps in deeply investigating the mechanism of different structural carbon clusters in catalyzing the production of phenols and other low-oxygen compounds.The main objective of the present study focused on exploring the catalytic mechanism of biochar catalysts in the pyrolysis of lignin（model compound）,elucidating the influence of biochar cluster structure,oxygen-containing groups,and nitrogen element doping on the process,and ultimately proposing a pyrolysis regulation strategy based on the mixed catalyst（biochar+molecular sieve）based on the active sites of biochar.The summary of the present study and important results are as follows.:1.Based on the thermal decomposition products of guaiacol（a lignin-based compound）and the carbon cluster model of the ideal carbon catalyst,the gas-phase free radical control mechanism and the heterogeneous catalytic reaction control mechanism on the carbon surface were systematically studied for the production of phenolic compounds.The results showed that in the gas-phase reaction,the production of phenolic products was promoted by the methyl free radical.In the non-homogeneous catalytic system,methyl and hydroxyphenoxy free radicals were easy to adsorb on the defect positions at the carbon edge and had a competitive adsorption relationship.Compared with the armchair edge,the hydroxyphenoxy free radical adsorbed at the defect position of the zigzag edge to produce hydroxybenzene free radicals and phenol was at a lower dissociative adsorption energy.2.Based on the results of the ideal carbon catalyst phenol production mechanism,a series of biomass carbon catalysts were prepared by pyrolysis of wood chips with different pretreatment methods（CO₂ activation,HNO₃ oxidation）,and catalytic phenol production experiments were conducted using vanillyl alcohol as the raw material to explore the regulatory effect of oxygen-containing groups on the phenol production activity of the carbon catalyst.The catalytic reaction mechanism of the biomass pyrolysis biochar catalyst for phenol production and the promoting effect of oxygen-containing groups（hydroxyl）on the reaction were revealed by using intermediate detection,carbon surface active site analysis,and density functional theory.The results showed that the phenol production activity of the CO₂-activated carbon catalyst was 26%higher than that of the original carbon catalyst.The phenol production activity of the carbon catalyst treated with concentrated HNO₃ was significantly suppressed.Based on the intermediate detection results of the photoionization mass system（PIMS）,the abundance of hydroxyphenoxy free radicals was reduced in the heterogeneous catalytic process.Vibrational and electronic spectral analysis of the changes in surface groups before and after biochar catalysis showed that carbon defect sites with hydroxyl groups were the catalytic sites for the production of phenolic compounds.Subsequent density functional theory calculations confirmed that the catalytic site at the zigzag edge was more active than that at the armchair edge.The synergistic effect of carbon defects and surface hydroxyl groups promoted the transformation of 2-hydroxy-4-methylphenoxy free radicals adsorbed on the surface and the production of phenolic compounds.3.In addition to oxygen-containing groups,nitrogen elements are also important.The edge-doped nitrogen and bulk-doped nitrogen carbon cluster models of the ideal nitrogen-doped carbon catalyst were constructed to discuss the independent mechanism of nitrogen atom doping to enhance the phenol production activity of the carbon catalyst.The thermodynamic barriers for phenol production reactions at the edges of carbon clusters with hydroxyl or amino functional groups were compared,and it was found that the hydrogen supply ability of the amino functional group was lower than that of the hydroxyl functional group.In the nitrogen doped carbon cluster structure,graphite nitrogen improved the hydrogen supply ability of hydroxyl on the catalytic site,thereby promoting the phenol production reaction.The catalytic role of nitrogen atoms in the hydrogen activation reaction and the dissociation reaction of 2-hydroxy-4-methylphenoxy free radicals（bulk-phase adsorption）in the bulk-doped nitrogen carbon cluster model were further analyzed.Based on the density functional theory calculations of the phenol production reaction in the bulk-phase nitrogen-doped cluster model,the activation reaction of hydrogen was found to be the limiting step in the bulk-phase phenol production mechanism.Therefore,the nitrogen-containing groups in the nitrogen-doped carbon catalyst promoted the phenol production reaction by enhancing the hydrogen supply ability of the hydroxyl group.4.To further explore the synergistic effect of oxygen and nitrogen,seaweed-based nitrogen-oxygen co-doped biochar catalysts were prepared from Enteromorpha clathrate（seaweed）and their high catalytic activity towards the conversion of vanillyl alcohol to phenol was confirmed.In order to study the factors that affect the catalytic activity in the microstructure of carbon clusters（hydrogen supply group type,degree of oxidation,cluster size,edge defect degree,etc.）,a series of structurally complex nitrogen-oxygen co-doped carbon catalyst cluster models were constructed using the density functional theory method,and machine learning methods were used to reveal the key indicators（descriptors）that affect the catalytic phenol production activity.The results showed that the gradient boosting regression machine learning model had the best predictive ability for the descriptor-catalytic activity relationship of the cluster structure.The Mulliken charge of the hydrogen supply group was the most important descriptor.In the biochar catalyst,the electron-donating properties of nitrogen atoms improved the catalytic phenol production activity of the carbon skeleton,while the electron-withdrawing properties of basic oxygen atoms inhibited this activity.5.Based on the exploration of the deactivation mechanism of the phenol production active site of the carbon catalyst,it was found that the occupation of the catalytic site by small molecule free radicals such as methyl during the reaction inhibited the heterogeneous catalytic phenol production reaction.Therefore,HZSM-5 catalyst was introduced to promote the polymerization of small molecule free radicals in its channels.A new method of mixed catalyst（biochar+molecular sieve catalyst）for the catalytic production of phenol from vanillin was proposed.The experimental results showed that the selectivity of light aromatic components in the pyrolysis oil produced by the mixed catalyst was increased by six times.TG-MS and density functional theory studies showed that the shape-selective effect of HZSM-5 catalyst promoted the separation of methyl and hydroxyphenoxy free radicals,where the former underwent alkylation and dehydrogenation polymerization through the Bronsted acid site in HZSM-5,and the latter was catalyzed on the carbon surface to produce phenol,realizing effective regulation.