Computer simulation of coal organic mass structure and its sorption properties
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Structural model of C100H79O7NS coal organic mass was obtained within density functional theory in the localized orbital basis set using the B3LYP hybrid functional. The model was compared with the known experimental data for coal of different grades and its sorption properties were studied with respect to CH4, CO2 and H2O. It has been shown that macromolecule of coal organic mass has bulk structure with a pore inside it. Interaction between coal and CH4 molecules consists of typical physical adsorption with oligomer formation on the pore border, physical adsorption with elements of chemical adsorption was also observed between coal and H2O molecules. Interaction between coal and H2O molecules included both physical and chemical adsorbion.
KeywordsPolycyclic compounds Coal organic mass Computer simulation IR-spectra Pore structure Methane
Coal is one of the basic energy materials of organic origin, it is also the source of raw materials for chemical industry. Coal is a complex heterogeneous rock, consisting both of organic and inorganic impurities, it is characterized by developed porosity and has both chemical and physical structure. As for its chemical composition, coal is a mixture of high-molecular-weight polycyclic aromatic compounds, such as benzene C6H6, toluene C6H5CH3, xylene C6H4(CH3)2, naphthalene C10H8, anthracene C14H10, pyrene C16H10 and their derivatives with high mass fraction of carbon, as well as of water and volatile substances with small amount of mineral impurities. There is a great number of works where structures of these elementary objects are studied in detail (Zhuravlev et al. 2012). Physical structure of coal is defined by its supramolecular structure, size and pore distribution. According to the X-ray structural analysis, the pore size in coal varies from 5 to 1000 Å, the maximum being 80–100 Å.
Elemental composition of coal organic mass, structure of macromolecules and nature of supramolecular structure determine the basic physical, chemical and technological properties of coal (Gyulmaliev et al. 2003). It is impossible to determine if coal is suitable for specific technological processes without taking into account physical and chemical characteristics of coal structure. One of the main problems of coal chemistry is prediction of physical and chemical properties of coal organic mass in accordance with the structural and chemical data, that is elemental, functional and fragment. Theoretical research methods play a certain role in resolving this task (Kairbekov et al. 2015).
Nowadays there is a great number (~ 150) of molecular models of coal organic mass, their review can be found in (Castro-Marcano et al. 2012; Roberts et al. 2015; Zhang et al. 2017). Along with the growth of computation capacity, computer simulation has recently made it possible to use 3D structures of coal and predict their physical and chemical properties. Equilibrium structures and infrared spectra for four typical molecular models of coal have been determined in the works (Gagarin and Gyulmaliev 2007; Arenillas et al. 2003). Due to combination of theoretical calculations with the experimental IR-spectra it is possible to suggest probable molecular structure for wet coal. It is shown in (Xiang et al. 2014) that it is possible to successfully describe interaction of C222H185N3O17S5 coal molecule with CH4, CO2 and H2O molecules using molecular dynamics method and taking into account the contribution of van der Waals forces, electrostatic interactions and hydrogen bonds to the reduction of non-bonding interactions.
Methane plays a significant role in physical processes occurring in coal that is why the study of sorption properties of coal with regards to methane is a challenging task (Cheng et al. 2017). It is shown in (Yua et al. 2017) that methane is within coal substance, it is released on its surface in a molecular form, i.e. during the desorption process methane molecules do not dissociate when they enter the solid solution in coal. The bonding of methane molecules with coal is determined by van der Waals forces, while the incorporation energy of a methane molecule into solid solution is not a constant quantity. It depends on a specific localization of a molecule: whether it is in aliphatic compound or inside a crystallite. The methane sorbed by coal is distributed among the solid solution (absorption) and surface of cracks and pores (adsorption). As coal has a very developed internal surface, the amount of adsorbed methane can equal that of the absorbed.
The study of interaction process of CO2 and H2O with coal is also of great interest (Meng et al. 2018a, b). Chemical composition of coal is complex that makes coal both hydrophilic and hydrophobic. Compared to non-polar gases CH4 and CO2, coal-water interactions are more complicated, and mainly consist of dispersion force, hydrogen bond and chemical adsorption between water and minerals. A large number of literary sources state that as H2O content in coal increases, its adsorption properties relative to CH4 and CO2 decrease. Moisture content is one of the main parameters influencing gas adsorption in coal seams (Jinxuan et al. 2018; Meng et al. 2018a, b). CO2—coal interaction is of interest because due to it we can increase the volume of methane recovered from coal seams. The injection of CO2 into coal seams does not only reduce carbon dioxide emissions, but also improves the desorption capacity of methane displacement (Wu et al. 2019).
The purpose of this research is to establish molecular 3D-structure of coal organic mass and determine possible ways of CH4, CO2 and H2O sorption using computer-based simulation methods.
2 Computational details
The exchange part of the B3LYP functional includes 20% of the Hartree–Fock exchange, 8% of the Slater-Dirac exchange and 72% of the Becke’s exchange, while the correlation part includes 19% of the VWN local functional (Vosko, Wilk and Nusair) and 81% of the LYP functional, taking into account gradient corrections.
3 Results and discussion
Atoms charges have been obtained for all the reported structure fragments. Thus, the standard for the benzene ring is the Mulliken positive charge on hydrogen about + 0.1 e (e—is an electron charge) and negative charge on carbon about − 0.1 e. The sulphur atom acquires +0.18 e positive charge in C16H10S (b) molecule, while the closest carbons acquire − 0.31 e charge. As for C12H9N (a) molecule, the nitrogen atom acquires − 0.4 e negative charge, however, directly attached to it carbon atoms have a weak positive charge (0.04 e). The oxygen atom in C18H12O molecule is negative − 0.3 e, while the two attached to it carbon atoms are positive (+ 0.06 e and + 0.01 e, respectively).
The volume (104.79 Å3) of the pore indicated in (Fig. 2b) is 1.64% from the volume of the molecular model of coal organic mass. Due to this, there is enough space for up 5 CH4 methane molecules, as the volume of one such molecule is about 12 Å3. The study of methane adsorption on coal organic mass was conducted in the following way. Methane molecules were alternately placed into a pore in such a way that the distance between the methane atoms and coal organic mass atoms was larger than that needed for chemical bond formation (~ 2 Å). The “freezing” of atoms was applied for geometry optimization for those atoms that were at some distance from the atoms’ pore of coal organic mass. The simulated tests indicated that these atoms of coal organic mass do not interact with methane molecules and do not bring significant changes to the geometry. The error of bond length is 0.4%, and the error of bond angles is 1.2%. At the same time the total energy for the “frozen” and free models differs by less than 0.01%.
Physical methane adsorption is shown in (Fig. 2a). Since the distance between any methane atom and the atom of coal organic mass molecule in the optimized structure is more than 2 Å, chemical bond is not formed. The methane concentration in the obtained model is 3.24% from the total molar weight of the molecule.
Analyzing the geometry of the coal organic mass structure and different methane concentrations in the pore, it is possible to draw a conclusion that as the methane concentration increases, the distance from the coal organic mass structure to the nearest molecule does not increase. Thus, it proves that the pore is able to maintain the obtained structure.
It turned out that one methane molecule, placed in the center of the pore, after optimization tends to the outside and is located near the surface of the molecule. In this case the bond energy is calculated according to formula (5). At n = 1, the bond energy is − 117.26 kcal/mol. The second molecule, initially placed in the centre of the pore, also comes to the surface and together with the first methane molecule forms a dimer with a typical distance between carbon atoms of 3.7 Å. The bond energy was calculated according to formula (6), at n = 2, it is − 128.39 kcal/mol. The third methane molecule comes to the surface, too, and transforms methane dimer to trimer with carbon atoms distances of 3.7 Å and 3.8 Å. The bond energy for n = 3 is − 131.98 kcal/mol. As the methane concentration continues to increase, the tendency for polymerization is preserved. With the forth added methane atom, a tetramer is formed where the distances between carbons are 3.7 Å, 3.7 Å and 3.9 Å, the energy bond is − 147.42 kcal/mol. It should be noted that in all the cases the distance between carbons in dimer, trimer or tetramer, located close to the surface of the coal organic mass macromolecule, were less than the corresponding distances in free dimer, trimer or tetramer.
Similarly, the sorption properties of the obtained model with respect to CO2 and H2O were studied. When up to two molecules of carbon dioxide (CO2) were placed in the pore, physical adsorption takes place. With concentration increase, both physical and chemical adsorption occur, which becomes apparent when one of the CO2 molecules is destroyed and oxygen atom joins coal organic mass molecule. The remaining part of the molecule joins the other CO2 molecule, C2O3 structure is formed, which is located close to the surface of the pore. The distances between atoms in the formed molecule are 1,5 Å for single chemical bond C-O and 1,2 Å for double chemical bond. Wherein the distance between any atom of C2O3 and the closest atoms of the coal organic mass model is above 2,3 Å, which indicates the absence of chemical bond between these atoms. During the study of the sorption interaction with a molecule of water, physical adsorption with elements of chemical adsorption is observed. As the concentration of H2O molecules increases, dimers are formed.
Due to the conducted calculation within DFT theory using B3LYP hybrid functional, it is possible to develop the optimal scheme to calculate the structures of polycyclic compounds and further the structure of coal organic mass with composition similar to coal. The obtained C100H79O7NS 3D molecular model is in good agreement with the known experimental data on coal. The model is volumetric (V = 3848 Å3) and porous, so it is possible to study sorption properties on its basis. The model of physical adsorption of methane in coal is predicted, with formation of such structures as dimer, trimer or oligomer takes place. The models of sorption interaction of the obtained coal organic mass with the molecules of carbon dioxide and water are determined. All the obtained oligomers are located near the surface of the macromolecule of coal organic mass and retained by it.
The work was supported by the Ministry of Education and Science of the Russian Federation within the scope of contract 15.3487.2017/PP.
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