(263f) Spatial Species Concentration and Temperature Profiles in La2O3-CeO2 Nanofiber Catalytic Beds for the Oxidative Coupling of Methane Reaction

Oxidative coupling of methane (OCM) is a high-temperature process to transform methane into ethylene, which is a valuable intermediate for the chemical industry ^[1]. In spite of decades long research that resulted in thousands of papers and hundreds of patents, these efforts have failed so far to render OCM as a competitive process. Although many OCM catalysts were reported ^[2], there appears to be an upper limit of C₂₊ yield of about 25% per reactor pass for which the kinetic reasons are largely unknown ^[2]. A likely explanation for this limit is that at above a certain temperature, homogeneous gas phase reactions start to dominate over catalysis, so that product selectivities and yields are no longer determined by the catalyst. In order to make progress it is crucial that an improved quantitative understanding of the underlying detailed chemical kinetic mechanisms (DCKM) of the coupled surface and gas phase reactions in OCM be developed and validated over a very broad range of conditions encountered in the process. With the availability of DCKM, we will be in a better position to identify improved OCM conditions, superior reactor configurations and new leads for catalytic materials that are needed to exceed the 25% limit for C₂₊ product yields. However, the validation of DCKM require experimental data of high information content since detailed mechanisms contain a large number of species participating in hundreds of elementary reactions. Spatially resolved concentration and temperature profiles over a broad range of conditions represent the necessary and critical data sets for mechanism validation and also represent a new direction in OCM research.

In this presentation, we report on the spatial species concentration and temperature profiles in a fixed bed OCM reactor packed by novel La₂O₃-CeO₂ nanofiber fabric catalysts^[3] at different CH₄/O₂ feed ratios at their respected ignition temperatures. The reactor was 6 mm diameter quartz tube and was packed with 20 mg catalyst, which was sandwiched between quartz wool plugs. The concentration profiles were determined by capillary sampling followed by GC analysis. Temperature profiles were determined by thermocouples. The measured mole fraction profiles of CH₄, O₂, C₂H₆, C₂H₆, C₃H₆, CO, CO₂, H₂ and H₂O reveal important quantitative insights on the sequence of reactions of CH₄ and O₂ that are essential for the initial development and validation of detailed chemical kinetic mechanisms for the OCM process.   

References

1.      M. C. Alvarez-Galvan, N. Mota, M. Ojeda, S. Rojas, R. M. Navarro, J. L. G. Fierro, Catal. Today 2011, 171, 15-23.

2.      U. Zavyalova, M. Holena, R. Schlögl, M. Baerns, ChemCatChem 2011, 3, 1935-1947.

3.      Noon, D., Seubsai, A., Senkan, S., "Oxidative Coupling of Methane by Nanofiber  Catalysis",

      ChemCatChem, 5, 146-149, 2013.