(555c) A Study on Supported Bimetallic Catalysts Synthesis & Characterization for Direct Conversion of CO2 and Methane into Value Added Chemicals

The growing demand for conversion of natural gas into value-added chemicals has been the focus of a significant number of studies[1, 2]. Specifically, the direct conversion of methane and CO₂ to higher-value oxygenates has substantial industrial value. Due to the thermodynamic stability of CO₂ and methane molecules, direct conversion to high-value product is commercially limited. The following is a two-step process for activation of methane and CO₂: the carboxylation of CH₄ to acetic acid (1), which is then reacted with acetylene to form vinyl acetate (2)

(1) CO₂ + CH₄ → CH₃COOH

(2) CH₃COOH + C₂H₂ → vinyl acetate

This formation of acetic acid (1) is thermodynamically limited, but it can be addressed by coupling this reaction with a thermodynamically favorable reaction, e.g., reacting acetic acid with acetylene (2) to form vinyl acetate monomer (VAM), a high-value industrial intermediate. The overall reaction with acetylene (CH₄ + CO₂ + C₂H₂ → VAM) is thermodynamically more feasible than (1) (ΔG_r = +6.4 kJ/mol). Previously, Spivey et al. demonstrated CH₄ + CO₂ + C₂H₂ → VAM between 200 and 400 ⁰C at 1 atm [3], but VAM formation was not stable at these conditions.

This study focuses on design, synthesis and characterization of bimetallic catalysts for the conversion of methane and CO₂ into high value chemicals in a one-pot reaction. It has been reported[4] that metals like Au, Ag, Pd, Rh can activate methane and help couple it with CO₂ and acetylene. Therefore, TiO₂ supported bimetallic catalysts such as Rh-Au, Pb-Au, Ag-Au have been synthesized in-house followed by characterization via STEM-EDX, L edge XANES, XRD, and XPS analysis.

The bimetallic catalysts were prepared using deposition-precipitation method. 1 wt % of Rh, Ag, Pb, each were added to 1 wt% of Au and the metals were supported on TiO₂ for greater dispersion. After that, the catalysts were characterized to confirm the dispersion of metals and the proximity required to form the bimetallic sites. STEM confirmed that the metals were stationed adjacent to each other to create the bimetallic formation, while EDX confirmed the loading of the active metals on TiO₂. L edge XANES was performed on Au active sites to confirm the transformation of oxidation states as Au react with other metals (Pd, Ag) to form bimetallic sites, similar to the energy shift at white lines (encircled) observed in the literature for Au bimetallics (Figure 1). Successful formation of bimetallic sites was further evaluated via XPS analysis. XRD showed no crystalline structures in the fresh catalysts, which reveals the high dispersion of active metals on TiO₂ surface. The catalysts were later tested for direct activation of methane and CO₂ into oxygenates, which showed successful conversion to C₃ and C₄ products.

Reference:

[1] M.A. Abedin, S. Kanitkar, S. Bhattar, J.J. Spivey, Sulfated hafnia as a support for Mo oxide: A novel catalyst for methane dehydroaromatization, Catalysis Today, 343 (2020) 8-17.

[2] M.A. Abedin, S. Kanitkar, S. Bhattar, J.J. Spivey, Promotional Effect of Cr in Sulfated Zirconia-Based Mo Catalyst for Methane Dehydroaromatization, Energy Technology, 0 (2019) 1900555.

[3] J.J. Spivey, E.M. Wilcox, G.W. Roberts, Direct utilization of carbon dioxide in chemical synthesis: Vinyl acetate via methane carboxylation, Catalysis Communications, 9 (2008) 685-689.

[4] C. Palmer, D.C. Upham, S. Smart, M.J. Gordon, H. Metiu, E.W. McFarland, Dry reforming of methane catalysed by molten metal alloys, Nature Catalysis, 3 (2020) 83-89.