Construction Of Liulin 3~# Coal Supramolecular Structure Model And Study Of Adsorption Property

The adsorption mechanism of coal to CH₄ and CO₂ has always been a key issue in the development of coalbed methane,which has an important impact on the improvement of CBM recovery.Previous researches on the adsorption of CH₄ and CO₂ by coal have mainly been carried out on the macroscopic and microscopic scales,ie molecule simulating at the molecular level and isothermal adsorption experiments at the macroscopic level.However,coal is a complex molecular assembly composed of many organic substances.The adsorption of CH₄ and CO₂ by coal depends not only on the nature of the molecules that make up the system,but also on its molecular aggregation and intermolecular interactions.Therefore,it is necessary to study the adsorption of CH₄ and CO₂by coal from the supramolecular structure to understand the influence of the intermolecular interaction of coal on the adsorption of gas from the mesoscopic level.It is difficult to study the adsorption of gas by coal at the supramolecular level using traditional experimental methods.The computer molecular simulation technology takes statistical mechanics as the basic principle,and can discuss the interaction process between coal and gas at the molecular and supramolecular levels which providing a new method for the study of coalbed methane.This paper takes Liulin 3^#coal as the research object.Using Soxhlet extraction with pyridine solvent to extract and separate residual coal and asphaltenes,and combining ¹³C nuclear magnetic resonance,industrial analysis and elemental analysis and XPS technologies,macromolecule structure models of residual coal and asphaltenes were constructed.According to the extraction rate and molecular weight,Supramolecular structure models of raw coal and residual coal were constructed.The Monte Carlo（MC）method was used to simulate the adsorption of CH₄ and CO₂ by supramolecular.The following conclusions are drawn:1.The macromolecular structure model of the residual coal and asphaltenes were constructed by means of NMR,industrial analysis and elemental analysis and XPS technology.The main components of residual coal are tricyclic and tetracyclic aromatic structural units and cycloalkanes.The main constituents of asphaltenes are tricyclic aromatic structural units,oxygen-containing functional groups,and aliphatic side chains.The aromaticity of residual coal is greater than asphaltenes,and the alphatic structure and oxygen-containing functional groups are less than asphaltenes.2.Molecular mechanics and molecular dynamics simulation were performed on macromolecular structure models of residual coal and asphaltenes.The optimized structural models have undergone various degrees of bending deformation.The bending of the alphatic chains cause torsion between the aromatic layers.A near-parallel arrangement of aromatic layers appeares in the asphaltenes.This is a stable configuration and do not appear in the residual coal,mainly due to the polycyclic aromatic structural units and cycloalkane of the residual coal.So the structure is more stable and difficult to deform.In both structural models,non-bonding energy,especially van der Waals energy,constitutes the major part of potential energy,which determines the structural stability.There is no hydrogen bond energy in the residual coal,but there is in the asphaltenes,which is related to the alphatic side chains and oxygen functional groups in the coal.3.Using the lowest energy configuration of residual coal and asphaltenes,combined with the extraction rate and molecular weight,a 5-residue coal+1asphaltene and 5 residue coal supramolecular configuration were constructed.Molecular mechanics and molecular dynamics optimization were performed on the supramolecules.The densities of residual coal and asphaltenes are 1.35g/cm3 and 1.40 g/cm3 respectively.A large number of parallel aromatic layers appeare in the supramolecular structures.The alphatic structure also occurs in a parallel orientation.Due to the influence of the alphatic side chains and the surrounding aromatic structure,some aromatic layers bend.Molecular recognition of supramolecule is related to the alphatic side chains.Another recognition mechanism is the key-lock relationship,ie,the host-guest relationship,formed by residual coal and asphaltene molecules.To satisfy this relationship,the aromatic structure and alphatic structure are spatially adjusted to suit the formation of supramolecule.Non-bonding energy,especially van der Waals energy,still constitutes the major part of potential energy in the supramolecular structure.The torsion of the bond and the change of bond length are the basis of the supramolecular ellipsoidal structure.The hydrogen bond in the supermolecular configuration of residual coal is much smaller than that of raw coal,the oxygen-containing functional group and the aliphatic side chain in the structure are the main reasons for this difference.4.The single-component saturated adsorption of CH₄ and CO₂ by raw coal and residual coal supramolecule was simulated using the Giant canonical Monte Carlo method（GCMC）.The saturation adsorption capacity of CH₄ and CO₂ by raw coal supramolecules is 36 moleculars/u.c.and 42 moleculars/u.c.respectively.The saturated adsorption capacity of CH₄ and CO₂ by residual coal supramolecules is30 moleculars/u.c.and 32 moleculars/u.c.respectively.The relationship between the saturation adsorption amount of CH₄ and CO₂ by the two supramolecules is raw coal>residue coal,and CO₂>CH₄.The supramolecular adsorption of CH₄ and CO₂ is physisorption.Only non-bonding energy is involved in the adsorption process,especially van der Waals,followed by electrostatic energy.No hydrogen bonds are involved in CH₄adsorption,and hydrogen bonding occurs in CO₂ adsorption.The hydrogen bond energy of CO₂ adsorption by residual supramolecules is close to zero,which is far less than the hydrogen bond energy of CO₂ adsorption by raw coal supramolecules.This is related to oxygen content,aliphatic side chains and intramolecular hydrogen bonds.The van der Waals energy and electrostatic energy in the adsorption of CH₄ of the same supramolecular configuration are less than that of CO₂.CH₄ and CO₂ molecule exist in the form of tangent triangle and linear respectively in supramolecules.The molecular size and shape have an important influence on adsorption capacity.The smaller the adsorbate molecules are,the more adsorbate molecules enter the smaller pores of the adsorbent,so that the adsorbent surface contains more adsorbate molecules.CH₄ and CO₂ are adsorbed on the edge of the supramolecular structure preferentially and then diffuse into the supramolecular pores.The CH₄ molecules are arranged in pairs of adjacent ethane-like configurations,the CO₂ molecules are adsorbed in the supramolecules in form of intersecting and parallel,and the two adsorbent molecules have a local packing effect.Most of CH₄ and CO₂molecules are distributed in a layered manner along the bending direction of the aromatic layer in the supramolecular molecule,forming an adsorbate molecular layer.In the raw coal supramolecule,the adsorbent molecule tightly surrounds the“key”asphaltene molecule,and some adsorbate molecules enter the elliptical hole,forming a new“key-lock relationship”.This molecular recognition mechanism of supramolecule has an important influence on the adsorption performance of coal.5.The sorption of single component and the binary component of CH₄/CO₂by supramolecule at was simulated 303.15K,0²5MPa pressure.The results show that the relationship between the adsorption capacity is CO₂>CH₄ at the same temperature and the same pressure.For the same adsorbent,the adsorption capacity of raw coal supramolecule is larger than that of residual coal supermolecule,that is,the adsorption of two gases by raw coal supermolecule is stronger than that of residual coal supramolecule.CO₂ has obvious advantages in the adsorption competition of CH₄/CO₂ mixed system.The adsorption of CH₄ in competitive adsorption is less affected by pressure,and CO₂ is more sensitive to changes in pressure.