“Soft” oxidative coupling of methane to ethylene: Mechanistic insights from combined experiment and theory

Abstract

The oxidative coupling of methane to ethylene using gaseous disulfur (2CH4 + S2 → C2H4 + 2H2S) as an oxidant (SOCM) proceeds with promising selectivity. Here, we report detailed experimental and theoretical studies that examine the mechanism for the conversion of CH4 to C2H4 over an Fe3O4-derived FeS2 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 O2 (2CH4 + O2 → C2H4 + 2H2O). 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 CH2 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 CS2 by-product forms predominantly via CH4 oxidation, rather than from C2 products, through a series of C–H activation and S-addition steps at adsorbed sulfur sites on the FeS2 surface. The experimental rates for methane conversion are first order in both CH4 and S2, consistent with the involvement of two S sites in the rate-determining methane C–H activation step, with a CD4/CH4 kinetic isotope effect of 1.78. The experimental apparent activation energy for methane conversion is 66 ± 8 kJ/mol, significantly lower than for CH4 oxidative coupling with O2. 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.

Origin of Rate and Selectivity Trends and Exceptional Yield in Sulfur-Oxidative Propane Dehydrogenation Over Supported Vanadium Catalysts

“…One possible explanation for this effect, proposed in previous work on S 2 as an oxidant, is the favorable reaction thermodynamics (Table ). Due to the weaker nature of C–S bonds compared to C–O bonds, the thermodynamic driving force toward total oxidation is far less severe for the S 2 than the O 2 system. − Nevertheless, it is also plausible that additional effects such as kinetics play a role in SODHP versus ODHP chemistries since kinetics play an important role in traditional ODHP product distributions . This will be discussed further in the DFT analysis below.…”

“…Sulfur-containing gases represent another class of alternative oxidants. Previously, this laboratory showed that sulfur vapor, S 2 , can be utilized as an oxidant for various light alkane partial oxidations, including methane coupling, − ethane ODH, and ODHP. , Unlike CO 2 –ODHP, SODHP remains exergonic at 500 °C. Furthermore, as shown in Table , the thermodynamic driving force toward total oxidation is also significantly reduced with S 2 compared to O 2 .…”

Sustainable Synthesis of Drop-In Chemicals from Biomass via Chemical Catalysis: Scopes, Challenges, and the Way Forward

“…reported gaseous S 2 as an alternative oxidant to O 2 for the OCM into ethylene over Fe 3 O 4 -derived FeS 2 catalyst. The authors obtained a 33% selectivity toward ethylene at around 10% conversion of CH 4 . Electrocatalysis has also shown promise, where a mixed metal oxide electrocatalyst grown in situ on the electrode surface promoted OCM into ethylene.…”

“…The authors obtained a 33% selectivity toward ethylene at around 10% conversion of CH 4 . 78 Electrocatalysis has also shown promise, where a mixed metal oxide electrocatalyst grown in situ on the electrode surface promoted OCM into ethylene. A 12.1% yield to ethylene was obtained at 41% conversion of CH 4 .…”

Impacts of framework Al distribution and acidic properties of Cu-exchanged CHA-type zeolite on catalytic conversion of methane into methanol followed by lower hydrocarbons

“…The development of a catalytic process that can directly convert CH 4 into useful chemical feedstocks such as CH 3 OH and C2-C4 hydrocarbons has extensively been pursued. [1][2][3][4][5][6] Among various types of heterogeneous catalysts, Cu-containing zeolites have extensively been investigated as a catalyst for the methane to methanol (MTM) reaction; small pore-zeolites such as CHA, AEI, and AFX frameworks exhibit a better CH 4 reactivity and CH 3 OH selectivity than medium-(MFI) and large-(*BEA and MOR) ones. [7][8][9][10][11] Regarding the active Cu species, the density functional theory (DFT) studies by Yoshizawa and co-workers have confirmed that the activation energy required for the cleavage of the C-H bond of CH 4 , which is the rate-limiting step, is governed by the Cu-O-Cu angle and also depends on the crystallographic position in the zeolite cavity.…”