Abnormal CO2 and H2O Diffusion in CALF-20(Zn) Metal-Organic Framework: Fundamental Understanding of CO2 Capture

Y Magnin and E Dirand and G Maurin and PL Llewellyn, ACS APPLIED NANO MATERIALS, 6, 19963-19971 (2023).

DOI: 10.1021/acsanm.3c03752

Carbon mitigation is one challenging issue that the world is facing. To tackle the deleterious impacts of CO2, processes emerged, including chemisorption from amine-based solvents and, more recently, physisorption in nanoporous solids. Physisorption in metal-organic frameworks (MOFs) is currently attracting considerable attention; however, the selection of the optimum sorbent is still challenging. While CO2 adsorption by MOFs has been widely explored from a thermodynamics standpoint, dynamical aspects remain less explored. CALF-20(Zn) MOF was recently proposed as a promising alternative to the commercially used CO2 13X zeolite sorbents; however, an in-depth understanding of the nanoscopic mechanisms originating its good performance still has to be achieved. To do so, we deliver some insights into the adsorption and diffusion of CO2, H2O, and mixtures in CALF-20 through atomistic simulations. CALF-20(Zn) was revealed to exhibit unconventional guest-host behaviors that give rise to abnormal guest thermodynamics and dynamics. The hydrophobic nature of the nanoporous solid leads to a low water adsorption enthalpy at low loading, followed by a continuous increase, driven by strong water hydrogen bonds, found to arrange as quasi 1D molecular wires in MOF nanoporosity, recalling water behavior in small-diameter carbon nanotubes. While no superdiffusion was found in the CALF-20(Zn) as compared to carbon nanotubes, this behavior was shown to impact the guest-loading diffusion coefficient profile, with the presence of a minimum that correlates with the inflection point in the adsorption isotherm corresponding to the H2O wires formation. Interestingly, the diffusion coefficients of CO2 and H2O were also found to be of the same order of magnitude, with similar nonlinear profiles as a function of the guest loading. We further demonstrated that the diffusion coefficient for CO2 in the presence of water decreases with increasing water loading.

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