Study On The Structure Evolution Of Coal Particles And Depolymerized Products In The Process Of Lignite Depolymerization

High-value-added chemicals such as benzene polycarboxylic acids（BPCAs）could be produced during the depolymerization of lignite,therefore the depolymerization of lignite is a potential pathway for the high-value utilization of lignite.The process of lignite depolymerization involves a complex process of converting heterogeneous coal particles into homogeneous soluble products.However,up to now,the structure evolution of lignite during depolymerization is still not clear,therefore,it is hard to precisely regulate and control the reaction process,and thus the selective regulation of target products still faces challenges.Disclosing the structure evolution process from lignite particles to soluble products is of great significance to understand the depolymerization process of lignite and control the depolymerizaton process.In this thesis,the structure evolution of lignite during the depolymerization process of ruthenium ions catalyzed oxidation（RICO）was studied.The changes of the size of lignite particles and molecular weight of the soluble products were emphatically analyzed.The specific research contents and conclusions of this paper are as follows:（1）By using RICO to depolymerize lignite,the particle size and distribution of depolymerization residues under different conditions were investigated,and the effects of Na IO₄/lignite mass ratios,Ru Cl₃/lignite mass ratios,reaction temperature and time on particle size changes were studied.After the relationship between the degree of lignite depolymerization and particle size evolution would be further analyzed.The results showed that the amount of Na IO₄and Ru Cl₃has significant effects on the depolymerization degree and particle size changes of lignite.When the mass ratio of Na IO₄/lignite increases from 0.0 to 20.0,the depolymerization degree of lignite increases significantly from nearly 0%to 92.5%,and the average diameter of lignite particles decreased from ca.965 nm to ca.149 nm.These results demonstrate that the depolymerization of lignite involves the conversion of lignite from microparticles to nanoparticles.As the depolymerization proceeds,the zeta potential becomes more negative and the content of oxygen-containing functional groups such as hydroxyl groups increase.Combined with the gradual reduction of particle size of lignite particles,it can be reasonably inferred that lignite nanoparticles gradually depolymerized and released the soluble products into the solution,transforming lignite itself from heterogeneous particles into homogeneous water-soluble products.（2）In order to deeply study the evolution process of lignite conversion into soluble products,the relationship between BPCAs and the particle size evolution of lignite was studied through quantitative analysis of the yield of BPCAs.After that,the changes in the molecular weight of soluble products were studied by investigating the effects of different reaction conditions（Na IO₄/lignite mass ratios,Ru Cl₃/lignite mass ratios,reaction temperature and time）on the molecular weight and molecular weight distribution of soluble products,and the structure evolution process of lignite during the RICO depolymerization process was further proposed.The results showed that increasing the Na IO₄/lignite mass ratios,the total yield of BPCAs increases significantly,the maximum yield of BPCAs can reach 10.0%and the average particle size of lignite significantly decrease.Indicating that BPCAs were initially produced during the conversion of microparticles into nanoparticles.Molecular weight analysis showed that,the molecular weight of the soluble products had a wide distribution from dozens of thousand Dalton to several hundred Dalton,indicating that the macromolecules released from lignite particles were gradually decomposed and formed a series of intermediate products.Based on these results,the microstructural evolution process of lignite during depolymerization was speculated.Lignite particles mainly experienced the microparticles,nanoparticles,soluble macromolecules,humic acids,and finally small molecule products such as BPCAs.The formation of macromolecules proceeded fast and the subsequent formation of small molecules（such as BPCAs）from macromolecules was the rate control step.（3）By studying the RICO reaction process of 3-benzyloxybenzyl alcohol,naphthalene,anthracene,graphene,graphite and other different lignite model compounds,the fracture behavior of chemical bond in the RICO reaction process was further analyzed,and the structural evolution process of lignite in the RICO reaction process was revealed.The results showed that during the RICO reaction,the fat bridge bonds（such as-CH₂-,-CH₂CH₂-,etc.）and the ether bond C-O-C will be broken,and the aromatic rings in 2～3fused ring structures will be selectively oxidized by Ru O₄,ultimately generating substances such as BPCAs,and the depolymerization rate of the fused ring structure in graphene during RICO was only 12.5%,and the depolymerization rate of graphite oxide was 57.5%,but the depolymerization rate of graphite oxide after pyrolysis at 600°C was only 11.9%,Indicating that the degree of aromatic core polymerization and the content of oxygen-containing functional groups or bridge bonds are key factors affecting the depolymerization effect,determining the depolymerization rate and organic acid yield of lignite.The research results of this article deepen the understanding of the microstructure evolution process of lignite oxidation and depolymerization,providing theoretical guidance and basic data for the preparation of high-value chemicals from lignite oxidation and depolymerization.