(39b) Oxidative Coupling of Methane over Supported Mn-Na-W Catalyst: Influence of Preparation Method and SiC Crystal Phase | AIChE

(39b) Oxidative Coupling of Methane over Supported Mn-Na-W Catalyst: Influence of Preparation Method and SiC Crystal Phase


Kulkarni, S. - Presenter, Ghent University
Velisoju, V. K., University of Alicante
Lezcano, G., King Abdullah University of Science and Technology
Castano, P., King Abdullah University of Science & Technology
  1. Introduction

The oxidative coupling of methane is regarded as a potential one-step route for C2 chemicals from natural gas. However, its industrial application relies on high C2 yields (>30%) with prolonged times on stream and thus, the development of catalysts that can fulfill such requirements is key. Over the years, the high selectivity of Mn-Na-W/SiO2 has been systematically reported. These 3 metals create a system that converts amorphous SiO2 into cristobalite which selectively allows the coupling of methyl radicals to form C2 compounds. Nonetheless, the high exothermicity of the reaction typically leads to deactivation and the inclusion of SiC in the support could help dissipate the heat. Although there are some works available wherein SiC is the support of choice for the OCM, there is no study on partially replacing SiO2 for SiC and the effect type of SiC crystal phase. The present work outlines a novel SiC-SiO2 support synthesized by spray drying to extend the lifetime compared to the benchmark SiO2-supported catalyst prepared by wetness impregnation. Through the inclusion of SiC, reaction heat is effectively dissipated while attaining high C2 selectivity. The effect of SiC crystal phase is also evaluated, with α+β SiC fostering the formation of a convenient SiC-cristobalite core-shell structure.

  1. Experimental

Mn-Na-W catalysts with a SiO2-SiC supports were prepared by spray-drying, a novel, cost-effective and scalable process with perfect control of the properties of the final solid. Prior to spray-drying, a slurry of precursors containing the metals, SiO2 and SiC was prepared and ball-milled. The slurry was then spray-dried with compressed air acting as carrier gas through a two-fold nozzle and droplets were dried in the chamber at 200 ºC. Finally, catalysts were calcined at 800 ºC for 6 h with a ramp of 10 ºC min-1. Catalysts with various SiC crystal phases (α, β, α+β) were prepared. A pure SiO2-supported Mn-Na-W catalyst prepared by impregnation was considered as benchmark. Catalysts were thoroughly characterized and tested in a high-throughput 16-fold parallel reactor.

  1. Results and discussion

The evolution of CH4 conversion with time on stream for standard Mn-Na-W/SiO2 catalyst prepared by impregnation (IMP SiO2) and spray-dried Mn-Na-W/SiO2-SiC (SD SiO2-SiC) at 800 ºC. IMP SiO2 presents a clear descending trend, with 66% residual conversion at the end of the run (70 h). This observation antagonizes with the remarkably stable behavior observed in the case of SD SiO2-SiC, showing a little drop in CH4 conversion over time on stream even after 70 h. Based on H2-TPR and SEM characterization, a more uniform distribution of active phases and SiC across the particles was observed for SD SiO2-SiC compared to IMP SiO2. From the Raman spectra of fresh and used catalysts, the presence of surface α-cristobalite could be confirmed for both fresh catalysts, which was transformed into quartz after reaction, in line with PXRD observations. Characteristic Na2WO4 (identified as a key species for OCM) peaks in both fresh and used IMP SiO2 samples could also be observed, while for the SD SiO2-SiC, only a meagre Na2WO4 peak could be identified in the fresh sample, due to the transition of these species into MnWO4. In fact, the inclusion of SiC into the support led to the presence of Mn7SiO12 and MnWO4 in almost absence of Na2WO4, suggesting that the improved thermal conductivity of SD SiO2-SiC accelerates the dynamics of active phase transfer between Na2WO4-Mn7SiO12 to MnWO4-Na2O which results in an enhanced stability.

When studying the effect of SiC crystal phase, the catalyst supported onto α+βSiC (SD SiO2-a+bSiC) performed reasonably better than its more porous analogue (SD SiO2-βSiC) in terms of CH4 conversion. The bulk nature of the SiC was confirmed by TG analysis, where βSiC was converted to SiO2 in greater proportions. Therefore, a bulk oxidation of SiC and more uniform distribution of SiC and SiO2 was anticipated for the catalyst supported onto SiO2-βSiC compared to SD SiO2-α+βSiC, where a SiC-enriched core a SiO2-rich shell is hypothesized based on XPS experiments and is also further confirmed from SEM analysis. As a result, the increased thermal conductivity explains the observed enhanced OCM activity of SD SiO2-α+βSiC.

  1. Conclusions

In the present work, the positive effect of incorporating SiC as a support ingredient was corroborated compared to the benchmark Mn-Na-W/SiO2 catalyst. SiC inclusion endows the catalysts with more phase uniformity and increased thermal conductivity besides presumably altering the dynamics of the active sites. At the same time, the importance of SiC crystal phase in the formulation of the support was also confirmed, with an α-SiC component preventing the major oxidation of SiC to SiO2, as observed in the β counterpart.


The authors acknowledge financial support, resources and facilities provided by the King Abdullah University of Science and Technology (KAUST).