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.

Abstract:The mechanism of selective production of methyl chloride by a reaction of methane, hydrogen chloride, and oxygen over lanthanum-based catalysts was studied. The results suggest that methane activation proceeds through oxidation-reduction reactions on the surface of catalysts with an irreducible metal-lanthanum, which is significantly different from known mechanisms for oxidative chlorination. Activity and spectroscopic measurements show that lanthanum oxychloride (LaOCl), lanthanum trichloride (LaCl3), and lanthanum phases with an intermediate extent of chlorination are all active for this reaction. The catalyst is stable with no noticeable deactivation after three weeks of testing. Kinetic measurements suggest that methane activation proceeds on the surface of the catalyst. Flow and pulse experiments indicate that the presence of hydrogen chloride is not required for activity, and its role appears to be limited to maintaining the extent of catalyst chlorination. In contrast, the presence of gas-phase oxygen is essential for catalytic activity. Density-functional theory calculations suggest that oxygen can activate surface chlorine species by adsorbing dissociatively and forming OCl surface species, which can serve as an active site for methane activation. The proposed mechanism, thus, involves changing of the formal oxidation state of surface chlorine from -1 to +1 without any changes in the oxidation state of the underlying metal.

The reaction mechanism of water formation from H2 and O2 was studied over a series of silica-supported gold nanoparticles. The metal particle size distributions were estimated with TEM and XRD measurements. Hydrogen and oxygen adsorption calorimetry was used to probe the nature and properties of surface species formed by these molecules. DFT calculations with Au5, Au13, and Au55 clusters and with Au(111) and Au(211) periodic slabs were performed to estimate the thermodynamic stability and reactivity of surface species. Kinetic measurements were performed by varying the reactant partial pressures at 433 K and by varying the temperature from 383 to 483 K at 2.5 kPa of O2 and 5 kPa of H2. The measured apparent power law kinetic parameters were similar for all catalysts in this study: hydrogen order of 0.7-0.8, oxygen order of 0.1-0.2, and activation energy of 37-41 kJ/mol. Catalysts with Si-MFI (Silicalite-1) and Ti-MFI (TS-1 with 1 wt % Ti) exhibited similar activities. The activities of these catalysts with the MFI crystalline supports were 60-70 times higher than that of an analogous catalyst with an amorphous silica support. Water addition in the inlet stream at 3 vol % did not affect the reaction rates. The mechanism of water formation over gold is proposed to proceed through the formation of OOH and H2O2 intermediates. A rate expression derived based on this mechanism accurately describes the experimental kinetic data. The higher activity of the MFI-supported catalysts is attributed to a higher concentration of gold particles comparable in size to Au13, which can fit inside MFI pores. DFT results suggest that such intermediate-size gold particles are most reactive toward water formation. Smaller particles are proposed to be less reactive due to the instability of the OOH intermediate whereas larger particles are less reactive due to the instability of adsorbed oxygen.

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.