| Coal pyrolysis is an important approach in realizing the coal multiple conversion to generate clean gas, liquid and solid production with high quality. And it requires a deep understanding of the coal structure characteristics and the essential mechanism of conversion as theoretical basis. All of the existing coal structure models and knowledge of coal pyrolysis process are based on the macro speculation through the spectrum analysis and sample analysis of final products. It is difficult to propose the mechanism of coal pyrolysis because of lacking sufficient knowledge of numerous intermediates in the conversion process. Quantum chemistry becomes a new and efficient method to understand the molecular structure and reactivity of coal.In this work, the bond dissociation enthalpies (BDE) and radical stabilization energies (RSE) of various typical coal model compounds were investigated to understand the characteristics and dissociation behavior of the chemical bonds in coal. Vacuum ultraviolet single-photon ionization coupled with molecular beam mass spectrometry was employed to identify the reactants, radicals, and products, which provide useful experimental information for studying the generation mechanism of pyrolysis products. The most plausible position for hydroxyl group in coal structure was studied by theoretical calculations from energy point of view. The major contents and results in this work are summarized as follows:(1) The performance of several theoretical methods including DFT (Density Functional Theory), double-hybrid DFT, and high-level composite methods to evaluate the BDE of different types of compounds were examined to look for alternative methods that can balance the computational cost and higher precision to the best for large systems. The tested sets contain monocyclic and polycyclic aromatic molecules, branched hydrocarbons, small inorganic molecules, etc. The results show that the mPW2PLYP and G4MP2 methods achieve reasonable agreement to the benchmark values for most tested molecules. We recommend the G4MP2 as the most appropriate method for small systems (atoms number? 20); the double-hybrid DFT methods are advised for large aromatic molecules in medium size (20? atoms number?50), and the double-hybrid DFT methods with empirical dispersion correction are recommended for long-chain and branched hydrocarbons in the same size scope; the DFT methods are advised to apply for large systems (atoms number^ 50), and the M06-2X and B3P86 methods are more favorable.(2) The homolytic BDE of various bonds (C-H, C-C, C-O, O-H, and C-N) for coal based model compounds that are representative of the functionalities present in coal were computed by using mPW2PLYP method based on the previous research. The BDE for C-H, C-C, C-O,O-H, and C-N cover a range from 111.4 to 81.2 kcal/mol,114.1 to 62.8 kcal/mol, 107.6 to 52.6 kcal/mol,111.2 to 86.6 kcal/mol, and 104.0 to 59.0 kcal/mol, respectively. The so-called "weak bond" is associated with the BDE and highly depends on the stabilization of the fragmented radicals. The higher stability of produced radicals leads to the lower BDE, and the bond is easier to be cracked. Our work suggests that the initial steps of the coal pyrolysis are most probably the cleavage of bonds to form the phenoxy radical or benzyl radicals. With the increase of temperature, the loss of alkyl and hydrogen groups becomes feasible, whereas the phenyl-like radicals are the most difficult to form.(3) A new method to apply on the BDE calculations based on the radical stabilization energies (RSE) values was proposed, and a set of corrections for heteroatom system (NH2, OH, OCH3, C=O, SH, F, Cl and I) was established for evaluating the BDE through RSE value. The mean absolute deviation (MAD) between the results from the method and theoretical calculation ranges from 0.57 to 2.20 kcal/mol by testing BDE of plentiful typical chemical bonds in coal.(4) The initial reaction in pyrolysis of phenyl ethers is the homolysis of PhO-C, which was supported by theoretical calculations and experimental observation. The methyl substituent and its substituted positions on the benzene ring will affect the subsequent reactions of phenoxyl radicals. The length of C-C bridge bond plays an influential role in cracking of corresponding bonds in three a, ?-diarylalkanes pyrolysis process. The order of BDE of the bridge bond is Ph-Ph> PhCH-Ph> PhCH2-CH2Ph, which was confirmed by experimental observation.(5) Coal structure is very complicated, and the proportion of various functional groups in coal may be studied by existing analysis techniques, but the interactions of these functional groups are still mostly unknown because of lack of very effective way to get this information yet. In this work we assumed that the most stable position for a certain group could be the most plausible position for that group in the coal structure, then with the help of DFT calculations we have studied the most plausible position for hydroxyl group in coal structure. We found the order of preferential position for hydroxyl group is:pyridine ring> polycyclic aromatic hydrocarbons> monocyclic aromatic hydrocarbons> alicyclic ring. Besides, double substituted hydroxyl groups on pyridine and aromatic ring are possible. Hydroxyl groups tend to be in the positions where they can form hydrogen bond with each other. Electron donating and withdrawing groups do not obviously change the preference of hydroxyl group unless there is hydrogen bond formed. |
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