(43b) Understanding Isotherm Shape and Competitive CO2 /H2o Breakthrough Measurements on Novel Adsorbents for Carbon Capture.

The application of metal organic frameworks (MOFs) for carbon capture remains a growing field
due to the adaptability of their chemistry. [1] Changes in organic linkers and metal centers can
significantly alter how a microporous solid interacts with different adsorbates. The success of
the recently commercialized MOF CALF-20 is due, in part, to its ability to maintain CO₂ capacity
in the presence of water even at a relative humidity as high as 50%. This behaviour is due to the
nature of the binary competition in the pores of CALF-20 and reveals itself in the unique shapes
of the associated CO₂/H₂O competitive isotherms. [2] Some MOFs reported in the literature exhibit
H₂O and CO₂ isotherms that are comparable to CALF-20 but not all of these adsorbents
manifest a pronounced binary competition between the two adsorbates. [3] Understanding how
the isotherm shapes and the competitive behaviour of the adsorbates are connected should
provide insight into what MOF structures could be well-suited for adsorptive CO₂ capture
processes. The lack of experimental data and the uncertainty associated with binary mixing
rules for MOFs places process development and optimization at a disadvantage.

To address this scarcity, we have started a survey of MOFs with a range of chemistries and
adsorptive characteristics using a microscale Dynamic Column Breakthrough (DCB) instrument
we have constructed. [4] The instrument provides independent control of CO₂ and water mole
fractions and uses less than 200 mg of sample for analysis. Both adsorption and desorption
experiments were carried out to solve the mass balance for both component loadings. MIL-120
was studied as it displays a highly rectangular Type 1 isotherms for both water and CO₂. [5] Al-
Fumarate and CAU-10-H were studied for their Type 1 CO₂ isotherm and Type 5 water
isotherms. The experimental results suggest that the position of the inflection in the water
isotherm and the shape is an important factor in the binary CO₂ equilibrium loading. The data
also demonstrates in certain, should a shift in the water isotherm inflection point in the presence of CO₂ be
observed, the equilibrium capacity for CO₂ is enhanced at higher relative humidity. The total
loadings calculated from the DCB experiments were corroborated using a dynamic vapour
sorption (DVS) apparatus to confirm the reliability of the measurements.

[1] S. L. James, “Metal-organic frameworks,” Chem. Soc. Rev., vol. 32, no. 5, pp. 276–288,
2003, doi: 10.1039/B200393G.

[2] J.-B. Lin et al., “A scalable metal-organic framework as a durable physisorbent for
carbon dioxide capture,” Science, vol. 374, no. 6574, pp. 1464–1469, Dec. 2021, doi:
10.1126/science.abi7281.

[3] B. Zhang, Z. Zhu, X. Wang, X. Liu, and F. Kapteijn, “Water Adsorption in MOFs:
Structures and Applications,” Adv. Funct. Mater., vol. n/a, no. n/a, p. 2304788, doi:
10.1002/adfm.202304788.

[4] N. S. Wilkins, J. A. Sawada, and A. Rajendran, “Quantitative Microscale Dynamic
Column Breakthrough Apparatus for Measurement of Unary and Binary Adsorption Equilibria on
Milligram Quantities of Adsorbents,” Ind. Eng. Chem. Res., vol. 61, no. 20, pp. 7032–7051, May
2022, doi: 10.1021/acs.iecr.2c00115.

[5] R. P. Loughran et al., “CO₂ capture from wet flue gas using a water-stable and cost-
effective metal-organic framework,” Cell Rep. Phys. Sci., vol. 4, no. 7, Jul. 2023, doi:
10.1016/j.xcrp.2023.101470.