The efficient use of natural gas will require catalysts that can activate the first C-H bond of methane while suppressing complete dehydrogenation and avoiding overoxidation. We report that single iron sites embedded in a silica matrix enable direct, nonoxidative conversion of methane, exclusively to ethylene and aromatics. The reaction is initiated by catalytic generation of methyl radicals, followed by a series of gas-phase reactions. The absence of adjacent iron sites prevents catalytic C-C coupling, further oligomerization, and hence, coke deposition. At 1363 kelvin, methane conversion reached a maximum at 48.1% and ethylene selectivity peaked at 48.4%, whereas the total hydrocarbon selectivity exceeded 99%, representing an atom-economical transformation process of methane. The lattice-confined single iron sites delivered stable performance, with no deactivation observed during a 60-hour test.

Direct, Nonoxidative Conversion of Methane to Ethylene, Aromatics, and Hydrogen

The conversion of methane to more valuable chemicals is one of the most intensively studied topics in catalysis. The direct conversion of methane is attractive because the process is simple, but unfortunately its products are chemicals that are more reactive than methane. The current status of this research field is discussed with an emphasis on C–H bond activation and future challenges.

Methane activation: the past and future

Recent developments in natural gas production technology have led to lower prices for methane and renewed interest in converting methane to higher value products Processes such as those based on syngas from methane reforming are being investigated Another option is methane aromatization, which produces benzene and hydrogen: 6CH4(g) → C6H6(g) + 9H2(g) ΔGor = +433 kJ mol−1 ΔHor = +531 kJ mol−1 Thermodynamic calculations for this reaction show that benzene formation is insignificant below ∼600 °C, and that the formation of solid carbon [C(s)] is thermodynamically favored at temperatures above ∼300 °C Benzene formation is insignificant at all temperatures up to 1000 °C when C(s) is included in the calculation of equilibrium composition Interestingly, the thermodynamic limitation on benzene formation can be minimized by the addition of alkanes/alkenes to the methane feed By far the most widely studied catalysts for this reaction are Mo/HZSM-5 and Mo/MCM-22 Benzene selectivities are generally between 60 and 80% at methane conversions of ∼10%, corresponding to net benzene yields of less than 10% Major byproducts include lower molecular weight hydrocarbons and higher molecular weight substituted aromatics However, carbon formation is inevitable, but the experimental findings show this can be kinetically limited by the use of H2 or oxidants in the feed, including CO2 or steam A number of reactor configurations involving regeneration of the carbon-containing catalyst have been developed with the goal of minimizing the cost of regeneration of the catalyst once deactivated by carbon deposition In this tutorial review we discuss the thermodynamics of this process, the catalysts used and the potential reactor configurations that can be applied

Catalytic aromatization of methane

Direct conversion of methane under nonoxidative conditions

Progress in the direct catalytic conversion of methane to fuels and chemicals

Mo/H-ZSM5 (1.0−6.3 wt % Mo; Mo/Al = 0.11−0.68) catalysts for CH4 aromatization were prepared from physical mixtures of MoO3 and H-ZSM5 (Si/Al = 14.3). X-ray diffraction and elemental analysis of physical mixtures treated in air indicate that MoOx species migrate onto the external ZSM5 surface at about 623 K. Between 773 and 973 K, MoOx species migrate inside zeolite channels via surface and gas phase transport, exchange at acid sites, and react to form H2O. The amount of H2O evolved during exchange and the amount of residual OH groups detected by isotopic equilibration with D2 showed that each Mo atom replaces one H+ during exchange. This stoichiometry and the requirement for charge compensation suggest that exchanged species consist of (Mo2O5)2+ ditetrahedral structures interacting with two cation exchange sites. The proposed mechanism may provide a general framework to describe the exchange of multivalent cations onto Al sites in zeolites. As the Mo concentration exceeds that required to form a MoOx mon...

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Structure and Density of Mo and Acid Sites in Mo-Exchanged H-ZSM5 Catalysts for Nonoxidative Methane Conversion

Mo/H-ZSM5 (1.0-6.3 wt percent Mo; Mo/Al = 0.11-0.68) catalysts for CH4 aromatization were prepared from physical mixtures of MoO3 and H-ZSM5 (Si/Al= 14.3). X-ray diffraction and elemental analysis of physical mixtures treated in air indicate that MoOx species migrate onto the external ZSM5 surface at about 623 K. Between 773 and 973 K, MoOx species migrate inside zeolite channels via surface and gas phase transport, exchange at acid sites, and react to form H2O. The amount of H2O evolved during exchange and the amount of residual OH groups detected by isotopic equilibration with D2 showed that each Mo atom replaces one H+ during exchange. This stoichiometry and the requirement for charge compensation suggest that exchanged species consist of (Mo2O5)2+ditetrahedral structures interacting with two cation exchange sites. The proposed mechanism may provide a general framework to describe the exchange of multivalent cations onto Al sites in zeolites. As the Mo concentration exceeds that required to form a MoOx monolayer on the external zeolite surface ({approx}4 wt percent Mo for the H-ZSM5 used), Mo species sublime as (MoO3)n oligomers or extract Al from the zeolite framework to form inactive Al2(MoO4)3 domains detectable by 27Al NMR. These (Mo2O5)2+ species reduce to form themore » active MoCx species during the initial stages of CH4 conversion reactions. Optimum CH4 aromatization rates were obtained on catalysts with intermediate Mo contents ({approx}0.4Mo/Al), because both MoCx and acid sites are required to activate CH4 and to convert the initial C2H4 products into C6+ aromatics favored by thermodynamics.« less

Anne Huffsmith

Papers

Structure and Density of Mo and Acid Sites in Mo-Exchanged H-ZSM5 Catalysts for Nonoxidative Methane Conversion

The Structure and Density of Mo and Acid Sites in Mo-ExchangedH-ZSMZ Catalysts