Revealing Active Sites and Reaction Pathways in Methane Non‐Oxidative Coupling over Iron‐Containing Zeolites

Abstract

Non‐oxidative coupling of methane is a promising route to obtain ethylene directly from natural gas. We synthesized siliceous [Fe]zeolites with MFI and CHA topologies and found that they display high selectivity (>90 % for MFI and >99 % for CHA) to ethylene and ethane among gas‐phase products. Deactivated [Fe]zeolites can be regenerated by burning coke in air. In situ X‐ray absorption spectroscopy demonstrates that the isolated Fe3+ centers in zeolite framework of fresh catalysts are reduced during the reaction to the active sites, including Fe2+ species and Fe (oxy)carbides dispersed in zeolite pores. Photoelectron photoion coincidence spectroscopy results show that methyl radicals are the reaction intermediates formed upon methane activation. Ethane is formed by methyl radical coupling, followed by its dehydrogenation to ethylene. Based on the observation of intermediates including allene, vinylacetylene, 1,3‐butadiene, 2‐butyne, and cyclopentadiene over [Fe]MFI, a reaction network is proposed leading to polyaromatic species. Such reaction intermediates are not observed over the small‐pore [Fe]CHA, where ethylene and ethane are the only gas‐phase products.

Non-Oxidative Coupling of Methane Catalyzed by Heterogeneous Catalysts Containing Singly Dispersed Metal Sites

“…As shown in Table 1, catalysts containing singly dispersed metal sites have been discovered to be efficiently active for non-oxidative coupling of methane. Fe©CRS 1080 Conversion of CH4 = 6.9-5.8% Selectivity of C2 hydrocarbons = 86.2% [72] [Fe]CHA 700-800 Conversion of CH4 = ~2% Selectivity of C2 hydrocarbons = >90% [73] Pt1@CeO2 975 Conversion of CH4 = 14.4% Selectivity of C2 hydrocarbons = 74.6% [74] [Ta]-SiO2 250-475 conversion of CH4 = 0.5% Selectivity of C2H6 = 50% (>98% among hydrocarbons) Cumulative TON = 40 [75] [W]-H@𝛾-Al2O3 1 350…”

“…In 2023, another catalyst of singly dispersed iron sites on zeolite [Fe]MFI and [Fe]CHA was reported to present high selectivity to ethylene and ethane from the non-oxidative activation of methane. Such Fe-based catalysts were synthesized via specifically developed direct hydrothermal synthesis techniques [73]. Compared to the MFI-or CHA-supported Fe 2 O 3 nanoclusters formed using the impregnation method, single-Fe catalysts obtained 4-5-fold more selectivity to C 2 hydrocarbons (>90% for [Fe]MFI and >99% for [Fe]CHA).…”

Materials Enabling Methane and Toluene Gas Treatment

“…Moreover, burning coke in the air can replenish deactivated [Fe]zeolites. Oxide semiconductors coated with metals are significant photocatalysts for the nonoxidative coupling of methane [44]. Because they greatly reduce the CH4 dissociation energy barrier and enhance C-C interaction, precious metals predominate in the NOCM.…”