(668c) Equilibrium and Non-Equilibrium Insights on the Direct Air Capture Performance of K-MER and 13X in Humid Environments

Direct air capture (DAC) of CO₂ has been identified as a critical component of Net-Zero strategies to mitigate global carbon emissions.¹ It is estimated that 10 GT/year of CO2 capture by DAC technologies will be required to achieve Net-Zero emissions by 2050, but current DAC plants can only achieve <0.1% of that target.² Realizing more widespread implementation of DAC technologies hinges on the development of more efficient adsorbent materials that can satisfy multiple criteria pertaining to uptake capacity, kinetics, selectivity, stability, and cost. Of the various adsorbent candidates, zeolites have demonstrated promising performance for DAC applications owing to their high thermal stability, competitive uptake capacity, low energy requirements for regeneration, and cheap cost. Unfortunately, the presence of humidity in DAC environments significantly hinders their performance under realistic operating conditions since water outcompetes CO₂ for binding sites and facilitates the formation of carbonates that require prohibitively high regeneration temperatures. As a result, the economic and energy penalties of water remain a large barrier for DAC technologies based on zeolite-based adsorbents.

However, a recent breakthrough⁴ has demonstrated that certain zeolites, including K-MER (potassium-exchanged merlinoite), contain double 8-member ring (D8MR) subunits that have privileged sites for CO₂ capture even in the presence of water vapor at high CO₂ concentrations under equilibrium conditions. This finding highlights a new opportunity for zeolite design for humid CO₂ capture and opens questions pertaining to (1) the concentration-dependence of this phenomenon, and (2) the performance of zeolites such as K-MER under the dynamic (i.e., non-equilibrium) operation representative of a DAC process. The work we present in this talk will address these two questions. Herein, we characterize the performance of K-MER under humid DAC concentrations and benchmark its performance to 13X (Na-FAU), a widely studied zeolite for CO₂ capture. Through dynamic column breakthrough (DCB) adsorption and breakthrough studies, we validate the results of this previous work under 100% CO₂, 5% RH conditions for K-MER at higher Si/Al ratio. However, we demonstrate that the equilibrium behavior observed at 100% CO₂ does not hold at 400 ppm at 5% RH, 30 ºC (CO₂:H₂O: ~0.2 mol:mol).

Despite this finding, we will demonstrate that K-MER is still able to retain >90% of its dry CO₂ uptake capacity throughout 5 cycles, in stark contrast to 13X (~50%), even under only moderate temperature swing regeneration of 100 ºC. Further, we will show how K-MER regenerates 90% of the adsorbed CO2 species using only a 30 °C purge with dry air over multiple cycles, in contrast to the 50% regeneration achieved under the same conditions for 13X. Desorption measurements from thermogravimetric analysis paired with insights from diffuse reflectance infrared spectroscopy (DRIFTS) studies show that the improved humid capture performance in K-MER can be attributed to weaker CO₂ site strengths that are agnostic to water presence as well as greater water regeneration at moderate temperatures (100 °C). Finally, we also introduce the concept of an “nth cycle” test where humid CO₂ is allowed to flow until water breakthrough, after which the cycling regeneration protocol is applied, and humid CO₂ is again flowed until only CO₂ breakthrough. This experiment seeks to estimate the minimum capacity drop that may be expected from a sorbent due to full water accumulation in the bed without requiring the ~100s if not ~1000s of cycles that would be needed to achieve water saturation of the entire bed. Collectively, these results highlight the importance of characterizing both equilibrium and non-equilibrium phenomena in binary adsorption in zeolites and set the stage for future studies on MER zeolites to elucidate the CO₂ and H₂O concentration-dependent performance of K-MER in capture applications.

(1) Intergovernmental Panel on Climate Change, I. Climate Change 2023 Synthesis Report, 2023.

(2) Direct Air Capture - Energy System. IEA. https://www.iea.org/energy-system/carbon-capture-utilisation-and-storag….

(3) Fu, D.; Davis, M. E. Toward the Feasible Direct Air Capture of Carbon Dioxide with Molecular Sieves by Water Management. Cell Rep. Phys. Sci. 2023, 4 (5), 101389. https://doi.org/10.1016/j.xcrp.2023.101389.

(4) Lee, H.; Xie, D.; Zones, S. I.; Katz, A. CO2 Desorbs Water from K-MER Zeolite under Equilibrium Control. J. Am. Chem. Soc. 2024, 146 (1), 68–72. https://doi.org/10.1021/jacs.3c10834.