(537e) CO2 and H2o in the Calf-20 MOF: Molecular Insights into the Commercialized MOF for Wet CO2 Capture Processes

Metal-organic frameworks (“MOFs”) are a class of porous solid sorbents that have great potential for adsorption-based gas separations, particularly for CO₂ capture processes.¹ The broad spectrum of unique MOF structures has highlighted serious challenges for commercializing MOFs to industrial scale CO₂ capture that stereotypically include scalability and their interactions with water in its various forms.

CALF-20, a MOF developed by the Shimizu group at the University of Calgary² captures CO₂ with high capacity and selectivity over water vapor despite a physisorptive capture mechanism. The mixed pillared-layered zinc₂ triazolate₂ oxalate structure is stable in boiling water, wet acid gases, and resists oxidative loss of CO₂ capacity.

The surface area of the pure MOF can be fully regenerated after exposure to 150 °C steam for 7 days, while further tests show that the material retains it’s CO₂ capacity after > 450,000 steam exposure cycles. Competitive separations on structured CALF-20 show preferential CO₂ physisorption below 40% relative humidity (RH), introducing a critical new variable (dehydration level) in process optimization of CO₂ capture using solid sorbents. Additionally, a scalable synthesis under mild conditions has enabled commercial deployment using this MOF for CO₂ capture from cement plants.^2,3

This work will delve further into CALF-20's unique ability to selectively adsorb CO₂ in the presence of water vapor as a quantitative structure-property relationship. The structural moiety of overlapping aromatic rings resulting from the pillared zinc-triazolate bilayer creates ~ 7 Å distance across the pore window, as seen in Figure 1, that introduces a slight hydrophobic shielding effect. Furthermore, the lack of any open metal sites or functional groups like amine binding sites creates a unique pore electronic environment characterized by strong but diffuse electrostatic charge throughout the pore that can still induce a weak quadrupole moment in CO₂ streams with up to 60% RH. A similar pore shape was recently recognized elsewhere ⁴ as the ideal MOF pore for selective CO₂ adsorption from wet gas streams.

Variable temperature diffuse reflectance infrared transmission spectroscopy (DRIFTS) will be used to probe the unique host-guest relationship of CO₂ + H₂O in CALF-20 at above ambient temperatures. Initial results suggest that elevated temperatures enhance the hydrophobicity of the MOF, as the saturation pressure of water vapor increases significantly within the range of relevant operating conditions. Process optimization and valuation of this new variable of dehydration level for adsorption-based CO₂ capture will be discussed in the context of post-combustion CO₂ capture and other opportunities for capture from other carbon sources and compositions will be highlighted.

Works Cited

[1] Trickett, C. A., et al., The chemistry of metal-organic frameworks for CO2 capture, regeneration and conversion. Nature Reviews Materials 2, 17045 (2017).

[2] Lin, J.B., et al., A scalable metal-organic framework as a durable physisorbent for carbon dioxide capture. Science, 374, 1464-1469 (2021).

[3] Hovington, P., et al, Rapid Cycle Temperature Swing Adsorption Process Using Solid Structured Sorbent for CO2 capture from Cement Flue Gas. Proceedings of the 15th Greenhouse Gas Control Technologies Conference 15-18 March 2021. https://dx.doi.org/10.2139/ssrn.3814414

[4] Boyd, P.G., et al. Data-driven design of metal–organic frameworks for wet flue gas CO2 capture. Nature 576, 253–256 (2019).