Department of Chemistry, Northwestern University, Evanston, IL 60208.
Center for Catalysis and Surface Science, Northwestern University, Evanston, IL 60208.
Proc Natl Acad Sci U S A. 2021 Jun 8;118(23). doi: 10.1073/pnas.2012666118.
The oxidative coupling of methane to ethylene using gaseous disulfur (2CH + S → CH + 2HS) as an oxidant (SOCM) proceeds with promising selectivity. Here, we report detailed experimental and theoretical studies that examine the mechanism for the conversion of CH to CH over an FeO-derived FeS catalyst achieving a promising ethylene selectivity of 33%. We compare and contrast these results with those for the highly exothermic oxidative coupling of methane (OCM) using O (2CH + O → CH + 2HO). SOCM kinetic/mechanistic analysis, along with density functional theory results, indicate that ethylene is produced as a primary product of methane activation, proceeding predominantly via CH coupling over dimeric S-S moieties that bridge Fe surface sites, and to a lesser degree, on heavily sulfided mononuclear sites. In contrast to and unlike OCM, the overoxidized CS by-product forms predominantly via CH oxidation, rather than from C products, through a series of C-H activation and S-addition steps at adsorbed sulfur sites on the FeS surface. The experimental rates for methane conversion are first order in both CH and S, consistent with the involvement of two S sites in the rate-determining methane C-H activation step, with a CD/CH kinetic isotope effect of 1.78. The experimental apparent activation energy for methane conversion is 66 ± 8 kJ/mol, significantly lower than for CH oxidative coupling with O The computed methane activation barrier, rate orders, and kinetic isotope values are consistent with experiment. All evidence indicates that SOCM proceeds via a very different pathway than that of OCM.
使用气态二硫化物（2CH + S → CH + 2HS）作为氧化剂（SOCM）将甲烷氧化偶联为乙烯具有很有前途的选择性。在这里，我们报告了详细的实验和理论研究，这些研究考察了在 FeO 衍生的 FeS 催化剂上将 CH 转化为 CH 的反应机制，该催化剂实现了高达 33%的理想乙烯选择性。我们将这些结果与使用 O（2CH + O → CH + 2HO）的高度放热甲烷氧化偶联（OCM）进行了比较和对比。SOCM 动力学/机理分析以及密度泛函理论结果表明，乙烯是甲烷活化的主要产物，主要通过桥接 Fe 表面位点的二聚 S-S 部分的 CH 偶联来生成，并且在程度上较小，在高度硫化的单核位点上生成。与 OCM 不同，过氧化物 CS 副产物主要通过 CH 氧化而不是通过 C 产物形成，通过吸附在 FeS 表面上的硫吸附位点上的一系列 C-H 活化和 S 添加步骤形成。甲烷转化率的实验速率在 CH 和 S 两者中均为一级，与速率决定的甲烷 C-H 活化步骤中涉及两个 S 位一致，与 CD/CH 动力学同位素效应为 1.78。甲烷转化率的实验表观活化能为 66 ± 8 kJ/mol，明显低于 CH 与 O 的氧化偶联。计算出的甲烷活化能垒、速率顺序和动力学同位素值与实验一致。所有证据都表明，SOCM 与 OCM 的途径非常不同。
