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

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.

2. 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.

3. 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.

4. 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.

Acknowledgements

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