New Insights into the Oxidative Coupling of Methane from Spatially Resolved Concentration and Temperature Profiles

Multi-Scale Studies of 3D Printed Mn–Na–W/SiO2 Catalyst for Oxidative Coupling of Methane

This work presents multi-scale approaches to investigate 3D printed structured Mn–Na–W/SiO2 catalysts used for the oxidative coupling of methane (OCM) reaction. The performance of the 3D printed catalysts has been compared to their conventional analogues, packed beds of pellets and powder. The physicochemical properties of the 3D printed catalysts were investigated using scanning electron microscopy, nitrogen adsorption and X-ray diffraction (XRD). Performance and durability tests of the 3D printed catalysts were conducted in the laboratory and in a miniplant under real reaction conditions. In addition, synchrotron-based X-ray diffraction computed tomography technique (XRD-CT) was employed to obtain cross sectional maps at three different positions selected within the 3D printed catalyst body during the OCM reaction. The maps revealed the evolution of catalyst active phases and silica support on spatial and temporal scales within the interiors of the 3D printed catalyst under operating conditions. These results were accompanied with SEM-EDS analysis that indicated a homogeneous distribution of the active catalyst particles across the silica support.

Reactor engineering calculations with a detailed reaction mechanism for the oxidative coupling of methane

The oxidative coupling of methane (OCM) is a potential option for conversion of excess natural gas to higher value products or useful feedstocks. The preferred or ideal OCM stoichiometry is: 2CH4 + O2 → C2H4 + 2H2O, but real OCM produces a variety of species. Using a detailed mechanism from the literature for OCM over a La2O3/CeO2 catalyst that combines coupled elementary gas phase and surface reactions, a reactor engineering study has been done. Adiabatic packed bed reactor (PBR, modeled as plug flow) and continuous stirred tank reactor (CSTR, perfect mixing) simulations using this mechanism are presented. Each reactor simulation used the same total number of catalyst sites. Process variables included CH4/O2 feed ratio (7, 11), feed temperature (843–1243 K), and feed rate. All runs were conducted at 1.01E5 Pa pressure. The results show the CSTR produces high conversions at much lower feed temperatures than those required by the PBR. Once full PBR “light off” occurs, however, its CH4 conversions exceed CSTR. The simulations reveal OCM over this catalyst at these conditions gives a mixture of synthesis gas (CO, H2) and C2Hx (primarily C2H4 plus small quantities of C2H6 and C2H2). The CSTR favors the production of synthesis gas, while the PBR favors C2Hx. Within the suite of CSTR cases, C2Hx is favored at the lowest feed temperature and highest CH4/O2 feed ratio.

Synthesis of Value-Added Chemicals via Oxidative Coupling of Methanes over Na2WO4-TiO2-MnO x /SiO2 Catalysts with Alkali or Alkali Earth Oxide Additives

Na₂WO₄-TiO₂-MnO _x /SiO₂ (SM) catalysts with alkali (Li, K, Rb, Cs) or alkali earth (Mg, Ca, Sr, Ba) oxide additives, which were prepared using incipient wetness impregnation, were investigated for oxidative coupling of methane (OCM) to value-added hydrocarbons (C₂₊). A screening test that was performed on the catalysts revealed that SM with Sr (SM-Sr) had the highest yield of C₂₊. X-ray photoelectron spectroscopy analyses indicated that the catalysts with a relatively low binding energy of W 4f_7/2 facilitated a high CH₄ conversion. A combination of crystalline MnTiO₃, Mn₂O₃, α-cristobalite, Na₂WO₄, and TiO₂ phases was identified as an essential component for a remarkable improvement in the activity of the catalysts in the OCM reaction. In attempts to optimize the C₂₊ yield, 0.25 wt % Sr onto SM-Sr achieved the highest C₂₊ yield at 22.9% with a 62.5% C₂₊ selectivity and a 36.6% CH₄ conversion. A stability test of the optimal catalyst showed that after 24 h of testing, its activity decreased by 18.7%.

Catalyst screening for the oxidative coupling of methane: from isothermal to adiabatic operation via microkinetic simulations

OCM catalysts underperforming in typical isothermal conditions could result in above average performances in adiabatically-relevant operating conditions.

First principles studies of CO2 and O2 chemisorption on La2O3 surfaces

Periodic density functional theory calculations were performed to study the surface structures and stabilities of the La₂O₃ catalyst in CO₂ and O₂ environments, relevant to the conditions of the oxidative coupling of methane (OCM) reaction. Thermodynamic stabilities of the clean surfaces were predicted to follow the order of (001) ≥ (011) ≫ (110) > (111) > (101) > (100), with their direct band gaps at the Γ point following the similar order of (001) > (011) > (110) > (111) > (100) > (101). Hubbard U corrections to the La 4f and 5d orbitals do not qualitatively change the predictions of surface energies and band gaps. For the most stable (001) surface, CO₂ chemisorption to form carbonate species is exothermic by -0.60 eV with a negligible energy barrier of 0.07 eV, whereas O₂ chemisorption to form peroxide species is endothermic by 0.64 eV with a considerable energy barrier of 1.29 eV. For the slightly less stable (011) surface, both CO₂ and O₂ chemisorption can occur at different surface sites, and the same applies to the other studied surfaces. Dissociation temperatures of surface carbonate species range from 300 to 1000 K at p_CO2 of 1 bar, which follow the order of (101) ≈ (110) > (111) ≈ (100) ≈ (011) ≫ (001), showing their strong sensitivity to the surface structure. Dissociation temperatures of surface peroxide species are mostly lower than the room temperature except for those of the (011) and (111) surfaces, although the significant kinetic barriers predicted should prevent their facile dissociation. Insights into the temperature-programmed desorption experiments and the methane reactivity of La₂O₃ in the OCM reaction were also given based on the results of our calculations.

Effects of Ir-doping on the transition from oxidative coupling to partial oxidation of methane in La2O3–CeO2 nanofiber catalysts: spatial concentration and temperature profiles

The spatial profiles suggest this direct CH₄ partial oxidation to syngas may ensue by a hybrid of established kinetic mechanisms.

Thermal Methane Activation by [Si2 O5 ].+ and [Si2 O5 H2 ].+ : Reactivity Enhancement by Hydrogenation

The thermal reactions of methane with the oxygen-rich cluster cations [Si₂ O₅ ]^⋅+ and [Si₂ O₅ H₂ ]^⋅+ have been examined using Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry in conjunction with state-of-the-art quantum chemical calculations. In contrast to the inertness of [Si₂ O₅ ]^.+ towards methane, the hydrogenated cluster [Si₂ O₅ H₂ ]^.+ brings about hydrogen-atom transfer (HAT) from methane with an efficiency of 28 % relative to the collision rate. The mechanisms of this process have been investigated in detail and the reasons for the striking reactivity difference of the two cluster ions have been revealed.