Impact of OH Radical Generator Involvement in the Gas‐Phase Radical Reaction Network on the Oxidative Coupling of Methane—A Simulation Study

Abstract

The impact of OH• generation during the oxidative coupling of methane (OCM) was simulated using state-of-the-art gas-phase chemistry and a comprehensive chemical kinetic model. The inclusion of the quasi-equilibrated formation of OH• from a H2O-O2 mixture into the combustion chemistry network enhanced the CH4 conversion rate and C2 selectivity, consistent with the previously proposed mechanism involving catalytically generated OH•. The OH-pathway increased the CH3• concentration resulting in an enhanced transformation rate from CH3• to C2H6 (second-order in CH3•) more than to CO (first-order in CH3•). Relative to other H-abstracting radical species, the OH• weakened the sensitivity of the H abstraction rate constant to C-H bond energy, or lower 2 6 4 C H CH / kk , which comparatively slows the C2H6 conversion rate relative to CH4, thus enhancing C2 selectivity. Concurrent dehydrogenation of C2H6 to C2H4 may maximize the C2H4 selectivity even after O2 depletion. With the involvement of the OH•-mediated pathway, this study addresses the effects of Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff))

Oxidative-Coupling-Assisted Methane Aromatization: A Simulation Study

“…Such OCM processes were investigated in more detail in our previous study with a particular focus on the OH • effect on the OCM process (see also the product profiles without OH • chemistry [reaction ] in the Supporting Information, Figure S1 and Table S1). The resulting conversion and selectivity profiles with quasi-equilibrated OH • formation as a function of residence time are shown in Figure a,a′, with a′ showing the magnified profiles of the region before the O 2 depletion point. The reactant conversion and product distribution at the maximum C 2 H 4 yield and the maximum C 6 H 6 yield points are summarized in Table .…”

“…37,44 To represent the quasi-equilibrated nature of OH• produced from the catalytic reaction in the OCM simulation (reaction 3), the contribution was explicitly included in the ChemKin simulation with the forward rate constant of kf = 10 18 . The detailed discussion on the kinetic contribution of OH• for OCM rates and selectivities can be found in our previous report, 26 and this study includes only the sufficiently-large contribution of this OH• contribution (see also Supporting Information).…”

“…This study, therefore, investigates single plug-flow reactor (PFR) modeling that achieves generation of aromatics and H 2 from CH 4 via combining the OCM and C 2 H 4 transformation using the state-of-the-art detailed gas-phase chemical kinetic model. This study is based on the catalytic contribution via OH • generation in reaction (i.e., adding catalytic contribution as a gas-phase reaction) that is used to improve OCM selectivity, which was extensively studied in our previous study. − , Then, the OCM products are transformed only in the homogeneous gas-phase reactions in the latter part of the PFR. Therefore, this approach quantitatively identifies gas-phase contributions to the product distribution in this CH 4 -O 2 scheme, which makes this study unique compared to the previously reported studies on heterogeneous catalytic aromatization.…”

Oxidative Coupling of Methane for Ethylene Production: Reviewing Kinetic Modelling Approaches, Thermodynamics and Catalysts

“…Based on the evaluated scientific papers, seven different main subject streams have been identified: (1) modelling of the OCM process, embracing phenomenological and empirical mathematical descriptions of reactors and respective chemical kinetics [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22], but also involving theoretical analyses of mechanistic models [23][24][25][26][27][28][29]; (2) economic evaluation of actual OCM commercial implementations, involving the analysis of capital and operational costs [30][31][32][33][34][35]; (3) assessment of environmental impacts, mainly involving the analysis of CO 2 emissions [31]; (4) development of new and/or improved catalysts for the production of ethylene from OCM reactions [27,; (5) development of alternative processes for CH 4 conversion into ethylene and/or other products, including, for instance, the non-oxidative coupling of methane [74][75][76]; (6) investigation of downstream purification strategies [77][78][79], concerning mainly the separation of products from the OCM reactor output stream; and finally, (7) the design, optimisation and development of OCM reactors for ethylene production, including the analysis of distinct reactor concepts (such as chemical looping…”

Noncatalytic Oxidative Coupling of Methane (OCM): Gas-Phase Reactions in a Jet Stirred Reactor (JSR)

“…To support this theory, the formation of hydroxyl radicals has been directly observed using LIF spectroscopy 14 and the contribution of the hydroxyl radical generation rate in the gas-phase network on OCM has also been investigated for simulation study, which could theoretically reach the maximum C 2 H 4 yield of 32%. 15 …”