An La-Sr/CaO catalyst has been chemically imaged during activation and under operando conditions during the oxidative coupling of methane reaction (OCM) at high temperature using X-ray diffraction computed tomography (XRD-CT) in combination with full pattern Rietveld refinement. At room temperature the main components of the catalyst were present as carbonates and hydroxides. During the activation stage (temperature ramp) they decomposed, 2 forming La2O3, SrO and mixed CaO-SrO oxides. Under the OCM reaction conditions, the predominant phases present were (~20 % wt.) La2O3 and CaO-SrO (~45 % wt.) and these remained stable throughout the entire reaction, whereas SrO, formed during activation, reacted with produced CO2 leading to formation of SrCO3 (~ 35 % wt). Two polymorphs of SrCO3, orthorhombic and rhombohedral, were found to be stable under reaction conditions although the extent to which these phases were observed varied spatially and temporally with reactant gas composition. The presence of the high temperature rhombohedral polymorph can be associated with higher combustion activity and since the Rietveld analysis is performed on a pixel-by-pixel basis it is possible to observe, for the first time, domains of differing activity within the reactor.
Experimental data for adsorption of pure carbon dioxide, methane, and nitrogen on zeolite
13X granules at different temperatures (288.15–318.15 K) and
pressure up to 20 bar are reported. The cation of adsorbent is exchanged
with H+, Li+, and Cu2+, and the adsorption
of pure gases is measured. The equilibrium adsorption isotherms of
gases are performed with a static volumetric adsorption instrument,
which was designed and built. The results show that the adsorption
capacity of carbon dioxide is higher than that of methane and nitrogen
and that the cation exchange improved the adsorption capacity of pure
gases. The LiX adsorbent has the highest adsorption capacity among
the studied adsorbents. However, the relative selectivity of carbon
dioxide over methane for zeolite 13X has the highest value of 30.48.
The CuX adsorbent shows the highest selectivity for carbon dioxide
over nitrogen. The adsorption isotherms for all of the pure gases
are fitted successfully with the Toth model.
This work presents a detailed description of the synthesis of a 5A zeolite and its further granulation into molecular sieve beads. The XRD spectrum of synthesized crystals showed the characteristic peaks of a type A sodium zeolite, and XRF results of the ion-exchanged material with a calcium chloride solution allowed us to verify the formation of a 5A zeolite structure. The analysis of SEM images indicated that the granulated process generated a core−shell structure with suitable mechanical properties for industrial use. The granulated material exhibited similar properties (roundness, 96%; crush strength, 57 N; packing density, 607 kg/m 3 ; and Langmuir surface area, 410.5 m 2 /g) to most commercial molecular sieves. However, its lower water sorption capacity (0.142 kg/kg) reveals that zeolite adsorption is affected by the cementing material. Adsorption equilibrium studies at different temperatures with oxidative coupling of methane (OCM) effluent gases indicated that the material showed a higher affinity for CO 2 , followed by ethylene, ethane, and methane, whereas nitrogen was barely retained. The affinity trend was in agreement with the observed heats of adsorption, which were in the range of 13−30 kJ/mol for tested gases. Isotherms were fitted with a Langmuir model, and adsorption parameters were obtained. Regressed equilibrium equations can be used for further modeling of a swing adsorption separation for OCM effluent gases.
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