Mechanism Study Of Low-Temperature Methane Conversion To Methanol

Because of the regional and/or economic limitations,Low-temperature methane conversion to transportable,high-value liquid chemical products has been regarded as one of the most appealing approaches for eventual utilization of the stranded gas and associated petroleum gas that are not effectively used.Despite numerous research and development efforts for past several decades,a cost-effective industry-scale commercialization process has yet to be realized,and the related mechanisms are still lack of recognition.This work aims at improving our mechanistic understandings of catalytic partial oxidative methane conversion and methane chlorination processes so as to provide theoretical guidance and recommendation for the novel engineering process design.Density Functional Theory?DFT?has been applied to investigate the frameworks of the cations in ionic liquids which have good chemical stability in liquid phase catalytic reaction of methane from the perspective of mechanism.The results clearly indicates that the chemical stability of an ionic species cannot be described through analyzing the total energy of compounds,instead,more detailed molecular orbital analysis shows that the aromaticity of the N-substituted heterocyclic compounds strengthens the N-N bond in the ring structure.Consequently,ionic liquids with [pyraz]+ and [1-mpyraz]+ become more stable than the commonly used [1-mim]+ cation which has the lowest energy difference between HOMO and LUMO,that is,most likely produces chemical changes among those N-substituted cationic species.The calculations of ¹HNMR spectra of various cationic species are conducted to assist experimental spectra analyses and verify the experimental conclusions.According to the kinetic study of H/D exchanges of methane in the Shilov's system based on Simplex algorithm,it has been found that the mono-substitution and di-substitution of H-atoms by deuterium isotope on the methane molecule occur nearly simultaneously,and the other two C-H bonds may be further activated to generate CHD3 and CD4.Even without deeper D-substitutions,it is still very difficult to achieve the over +50% mono-substitutive methane compound.According to the theoretical study of mechanisms of methane halogenation and that with SO₂ addition,it has been found that the formation of SO₂Cl· radical significantly reduces the reactivity of Cl· radical towards deeper C-H bond activations.It is initially explored that when the excess SO₂ is introduced into the methane chlorination reaction,deeper chlorination processes for multiple-substituted methane compounds can be prevented.This result offers an important process design guidance to significantly improve the product selectivity of methane chlorination to produce CH₃ Cl.However,this effect could be unique for methane chlorination,because the difference in chemical reactivity for multiple C-H bond activation of SO₂Br·and Br· is much smaller than that of the SO₂Cl·and Cl·.Based on the theoretical predictions from above studies,a novel methane chlorination process involving using methanesurfonly chloride?CH₃SO₂Cl?generated from the overdosed SO₂ assisted chlorination reaction as an intermediate for methanol production is proposed and studied both theoretically and experimentally.The experimental results of catalytic decomposition reactions of CH₃SO₂ Cl indicate that chloride ion is the active material acting as catalyst.The theoretical study of three catalytic systems separately involving Cl-,[1-mim][Cl],and Al2Cl7-based on DFT method proves that the pathway of generating SO₂ and CH₃ Cl by catalytic CH₃SO₂ Cl decomposition performs as SN2 reaction pathway.Both theoretical and experimental results further confirm that an alkalic ionic liquid catalytic system is more favorable over the acidic condition.