Effect of the Kerogen Molecular Structure on the Formation of Methane During Kerogen Pyrolysis

Abstract

In this study, density functional theory (DFT) at the GGA/RPBE level was employed to examine the effects of the kerogen microstructure on the formation mechanism of methane during the pyrolysis of kerogen. The calculations prove that the evolution of CH4 during kerogen pyrolysis corresponds to demethylation, and the process of forming methane involves the interaction of intramolecular hydrogen atom transfer and assistant hydrogen atom transfer. In all reaction paths, the energy barrier of path 5 is the smallest at 260.56 kJ mol−1. The energy barrier of path 6 is the largest at 554.36 kJ mol−1. The results indicate that CO is favourable for demethylation, and CO2 is not conducive to demethylation. Path 1 is the formation of methane by the transfer of assistant hydrogen atoms, and the required energy barrier is 379.45 kJ mol−1. The side chain structure of the aromatic hydrocarbon structure is liable to demethylation to form methane. A comparison of the reaction energy barriers follows the order: path 1 < path 15 < path 14 < path 10, which indicates the that difference in the demethylation reaction is based on the microstructure. In the same reaction process, the benzene ring and the aliphatic hydrocarbon structure are more susceptible to demethylation to form methane. In the heterocyclic bicyclic structures containing O and S, a comparison of the reaction energy barriers follows the order: path 11 ≈ path 12 < path 13, so paths 11 and 12 are close, but path 13 is more difficult to occur, indicating that it is more difficult to demethylate with heteroatoms in the same ring. From a thermodynamic point of view, in the process of assisting the formation of methane by hydrogen atoms, the demethylation reaction is mainly an endothermic reaction. During the transfer of intramolecular hydrogen atoms, the demethylation reaction is mainly an exothermic reaction, and most reactions are spontaneous.