Direct methane conversion into aromatic hydrocarbons over catalysts with molybdenum (Mo) nanostructures supported on shape-selective zeolites is a promising technology for natural gas liquefaction. We determined the identity and anchoring sites of the initial Mo structures in such catalysts as isolated oxide species with a single Mo atom on aluminum sites in the zeolite framework and on silicon sites on the zeolite external surface. During the reaction, the initial isolated Mo oxide species agglomerate and convert into carbided Mo nanoparticles. This process is reversible, and the initial isolated Mo oxide species can be restored by a treatment with gas-phase oxygen. Furthermore, the distribution of the Mo nanostructures can be controlled and catalytic performance can be fully restored, even enhanced, by adjusting the oxygen treatment.

Mo carbide nanoparticles supported on ZSM-5 zeolites are promising catalysts for methane dehydroaromatization. For this and other applications, it is important to identify the structure and anchoring sites of Mo carbide nanoparticles. In this work, structures of Mo2C x (x = 1, 2, 3, 4, and 6) and Mo4C x (x = 2, 4, 6, and 8) nanoparticles are identified using a genetic algorithm with density functional theory (DFT) calculations. The ZSM-5 anchoring sites are determined by evaluating infrared vibrational spectra for surface OH groups before and after Mo deposition. The spectroscopic results demonstrate that initial Mo oxide species preferentially anchors on framework Al sites and partially on Si sites on the external surface of the zeolite. In addition, Mo oxide deposition causes some dealumination, and a small fraction of Mo oxide species anchor on extraframework Al sites. Anchoring modes of Mo carbide nanoparticles are evaluated with DFT cluster calculations and with hybrid quantum mechanical and molecular mechanical (QM/MM) periodic structure calculations. Calculation results suggest that binding through two Mo atoms is energetically preferable for all Mo carbide nanoparticles on double Al-atom framework sites and external Si sites. On single Al-atom framework sites, the preferential binding mode depends on the particle composition. The calculations also suggest that Mo carbide nanoparticles with a C/Mo ratio greater than 1.5 are more stable on external Si sites and, thus, likely to migrate from zeolite pores onto the external surface of the zeolite. Therefore, in order to minimize such migration, the C/Mo ratio for zeolite-supported Mo carbide nanoparticles under hydrocarbon reaction conditions should be maintained below 1.5.

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The literature on methane dehydroaromatization (MDA) to benzene using ZSM-5 supported, group V–VIII transition metal-based catalysts (MOx/ZSM-5) is critically reviewed with a focus on in situ and operando molecular insights.

Cr/ZSM-5 catalysts with 0.5−2.6 wt % Cr loadings and zeolites with 15−140 Si/Al ratios were characterized with in situ UV−vis, IR, and Raman spectroscopies, including operando Raman measurements under reaction conditions in methane conversion at 773−1123 K with a simultaneous online analysis of reaction products. DFT calculations with cluster and periodic models were performed with DMol 3 , Gaussian 09, and VASP software packages. Isolated Cr(VI) dioxo and Cr(III) mono-oxo structures on framework Al anchoring sites were identified as the dominant species under most conditions. In the absence of gas-phase O 2 (under Ar flow) at 773 K, the Cr(VI) dioxo species on framework Al anchoring sites autoreduce, and the Cr(III) mono-oxo species remain the only observable surface Cr oxide structures. For ZSM-5 zeolites with a relatively low concentration of framework Al atoms (Si/Al ≥ 25), exposure to gas-phase O 2 at 773 K forces surface Cr oxide species to migrate from framework Al anchoring sites to Si sites on the external surface of the zeolite and form dioxo (Si− O−) 2 Cr(O) 2 and mono-oxo (Si−O−) 4 Cr(O) structures. The activity of Cr/ZSM-5 catalysts in methane conversion with the production of benzene and hydrogen as the main products is lower than that of Mo/ZSM-5 catalysts. The rate of benzene formation over Cr/ZSM-5 catalysts, however, is relatively stable with time on stream, in comparison to a rapidly declining rate over Mo/ZSM-5 catalysts. The zeolite-supported Cr species are highly mobile under the reaction conditions and can migrate between zeolite particles.