2022 Annual Meeting

(264h) Improving Low-Temperature CH4 Oxidation Performance with High-Silica Pd/CHA Zeolite Catalysts

Authors

Jingzhi Liu - Presenter, Syracuse University
Tala Mon, University at Buffalo
Viktor Cybulskis, Syracuse University
Eleni Kyriakidou, SUNY at Buffalo
Methane (CH4) is a vital energy carrier and chemical feedstock but has more than 80 times the global warming potential of CO2 over a 20‑year span [1]. Thus, complete catalytic oxidation of CH4 to CO2 is often desirable to reduce the environmental impact of CH4­ emissions by more than 96% while generating far less NO­x compared to thermal combustion. Pd/Al2O3 catalysts are state-of-the-art for CH4 oxidation around 500 ºC under CH4-rich conditions but require high precious metal loadings (> 5%) and cannot maintain sufficient CH4 conversion (> 90%) at lower temperatures (< 400 ºC) or under lean/ultra‑lean conditions [2]. Additionally, H2O and sulfur poison Pd sites and metal dispersion is lost through sintering upon regeneration at high-temperatures, resulting in decreased CH4 oxidation performance and shortened catalyst life. Recently, Pd-containing high-silica zeolites, such as Pd/SSZ-13 (CHA topology), have emerged as promising catalysts for low‑temperature CH4 oxidation owing to their hydrophobicity and superior hydrothermal durability [3].

High-silica CHA zeolites (Si/Al molar ratios = 15-137) were prepared in both hydroxide (OH) and fluoride (F) media, ion-exchanged to 1 wt.% Pd, and then evaluated for CH4 oxidation activity before and after simulated aging for 1 h at 650 ºC under wet-lean conditions (0.15% CH4, 5% O2, 5% H2O, bal. Ar). A comparison of temperatures required to achieve 50% and 90% CH4 conversion (T50, T90), revealed that Pd/CHA (Si/Al > 33) outperforms Pd/Al2O3 (Figure 1) with steady-state CH4 oxidation rates on Pd/CHA‑F (80) were ~4× higher than Pd/Al2O3 at 250 ºC (0.4 % CH4, 5% O2, N2 bal.). Decreasing T50 and T90 temperatures for Pd/CHA with increasing Si/Al molar ratios and results for Pd/CHA‑OH (137) and Pd/CHA-F (80) of similar Si/Al ratio suggest that more hydrophobic zeolites result in improved lower temperature performance and durability in the presence of H2O.

References

  1. Solomon, S., Manning, M., Marquis, M., Qin, D. Cambridge University Press (2007).
  2. Raj, A., Matthey Technol. Rev. 2016, 60, 228.
  3. Dusselier, M.; Davis, M. E., Rev. 2018, 118, 5265.