Research On The Structure Of Different Rank Coals Based On The Yield Distribution Of Benzene Carboxylic Acids

Our country's energy structure of "rich coal,poor oil,and less gas"determines that coal will occupy a dominant position in Chinese energy consumption for a long time,and the study of coal structure plays a guiding role in the clean and efficient use of coal.Over the past few decades,the scholars of coal chemical industry have used various technical means to build more than a hundred chemical structural models,and can explain some phenomena in the process of coal utilization,but they all have their own limitations.For example,these structural models can't explain the yield distribution of twelve kinds of benzene carboxylic acids obtained by alkali-oxygen oxidation of coal.This article has constructed the macromolecular structural models of these three coals including Xiaolongtan lignite,Kailuan bituminous coal,and Hebi anthracite based on the yield distribution of benzene carboxylic acids obtained by alkali-oxygen oxidation,combined with the contents of several main elements obtained by elemental analysis,various carbon structural parameters obtained by 13C NMR,and the contents of functional groups obtained by FTIR and XPS.On the basis of these constructed structural model,the concentrations of the main covalent bonds in coal were counted,and the relationships between the types and contents of elements contents,aromatic cluster structure,functional groups and chemical bonds with the carbon content in coal were discussed,and an attempt was made to explain how the coalification process works.The main conclusions are summarized as follows.(1)The structural model of Xiaolongtan lignite is dominated by benzene and naphthalene,the structural model of Kailuan bituminous coal is dominated by four-ring and five-ring aromatics,and the structural model of Hebi anthracite is dominated by eight-ring and nine-ring aromatics.With the increase of coal rank,the aromatic cluster structure gradually condensed into polycyclic aromatic hydrocarbons,and bridgehead carbons accounted for an increasing proportion of aromatic carbons,and at the same time,fewer and fewer aromatic carbons were substituted.The length of the alkyl side chain on the periphery of the aromatic cluster structure and the length of the aliphatic bridge connecting the two adjacent aromatic cluster structures have been shortened to a certain extent.(2)From the point of view of functional groups,oxygen-containing functional groups have been reduced to varying degrees.First,methoxy and alcoholic hydroxyl groups only exist in peat and low-rank lignite.Secondly,the cleavage of fatty ether bonds and the removal of-OH from carboxyl groups.Finally,the structures of phenyl ether bonds,phenolic hydroxyl groups and carbonyl groups are also reduced.For nitrogen-containing functional groups,there is an obvious trend that the five-membered pyrrole is converted into a sixmembered pyridine structure.It may be that two five-membered rings react with each other through resonance stable free radicals to form a six-membered ring,and the nitrogen atom may play a catalytic role in it.(3)From the perspective of chemical bonds,the concentration of Car-Car bonds increases with the increase of coal rank and the slope increases;the concentration of Car-Cal bonds does not show a decreasing trend from lignite to bituminous coal,and reduces until the anthracite stage.The concentration of the Cal-Cal bond decreases with the increase of coal rank and it decreases faster and faster;the concentration of the biphenyl bond has a greater increase in the anthracite stage.For C-O bonds,the fastest decrease in concentration is the CalO bond,which means the breaking and elimination of oxygen-containing functional groups.The concentration of O-H bonds first decreased and then increased,which may be due to the cleavage and removal of alcoholic hydroxyl groups and the rearrangement of phenolic hydroxyl groups.(4)In the process of coalification,the cleavage of functional groups may form activated free radicals on alkyl side chains and bridge bonds,triggering the cyclization and aromatization of aliphatic carbons,and realize the polycondensation of aromatic cluster structures.