Effect Of Microscopic Molecular Structure Of Kerogen On The Formation Mechanism Of Non-condensable Gases

Oil shale is a sedimentary rock composed with embedded organic matter and minerals.With its abundant reserves,economics and feasibility,it is listed as an important supplementary energy source for coal and petroleum in the 21st century.Complex chemical changes occur in the pyrolysis of oil shale,it is very difficult to study the specific pyrolysis mechanism of oil shale using traditional experimental techniques in pyrolysis of oil shale.Therefore,a deeper study simulation for the pyrolysis mechanism of oil shale kerogen must be carried out.In this paper,the density functional theory in molecular simulation is used to study the influence of the microscopic molecular structure of kerogen on the formation mechanism of non-condensable gases.Kerogen structural units of containing methyl and oxygen-containing functional groups were selected based on the kerogen model compounds of five regions?American Green River,Estonia,China Huadian,Longkou and Yaojie?.Designing the reaction pathways using selected structural units to find intermediates and products.The bond dissociation energy obtained by the experiment is compared with the calculated value of the simulation to verify the rationality and accuracy of the method.The MS?Materials Studio 2017?software was used to construct the three-dimensional geometry of all the reactants,intermediates and products contained in the 18 reaction pathways of the 11 reactants involved in the pyrolysis of kerogen to form CH₄.Using the density functional theory in quantum chemistry to calculate the bond length and the bond order of the constructed geometry.The effects of different molecular structures,intermediates and heteroatoms on the formation of CH₄ gas were analyzed by appling the density functional theory for transition state search,transition state optimization and transition state confirmation.Based on the kinetic parameters and thermodynamic parameters obtained by density functional theory,the changes of relative total energy,the endothermic and exothermic,spontaneous and non-spontaneous conditions of the demethylation reaction during the reaction process of each reaction path were analyzed.The calculation results show that monocyclic aromatic hydrocarbons and aliphatic hydrocarbon structures preferentially form methane,and polycyclic aromatic hydrocarbon structures should occur at the end.The CO is favorable for demethylation,and CO₂ is not conducive to demethylation reaction in the process of forming CH₄.In the same molecular structure,the further away from the hetero atom,the smaller the effect on the demethylation reaction.Based on the molecular structure study on the effect of CH4 on the pyrolysis of kerogen,the reaction path design and related simulation calculations were carried out for 22 reaction pathways designed by 14 kinds of reactants of kerogen pyrolysis to form another major gaseous product CO.Based on the density functional theory simulation,the optimized product and the corresponding of optimized structure bond length to analyze the structural changes during the chemical reaction.The calculation results show that the ester-containing structure first undergoes decarbonylation reaction to form CO in the process of forming CO,then,the monocyclic aromatic hydrocarbon structure,and finally an aliphatic hydrocarbon structure.In the process of direct decarbonylation,compared with aliphatic hydrocarbons and aromatic hydrocarbons,monocyclic and aliphatic hydrocarbons are prone to decarbonylation reaction to form CO.From the thermodynamic point of view,the decarbonylation reaction of CO formation by intramolecular hydrogen atoms is an endothermic reaction,and the heat absorption decreases with increasing temperature,while the decarbonylation reaction is thermodynamically spontaneous.