Abnormal CO2 and H2O Diffusion in CALF-20(Zn) Metal-Organic Framework Angstropores

Abstract: Carbon mitigation is one challenging issue that the world is facing. To tackle deleterious impacts of CO2, processes emerged, including chemisorption from amine based solvents, and more recently physisorption in porous solids. While CO2 capture from amine is more mature, this process is corrosive and detrimental for environment. Physisorption in Metal-Organic Frameworks (MOFs) is currently attracting a considerable attention, however the selection of the optimum sorbent is still challenging. While CO2 adsorption by MOFs have 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, in-depth understanding of microscopic mechanisms originating its good performance still have to be achieved. In this report, we deliver a microscopic insight of CO2 and H2O in CALF-20(Zn) by atomistic simulations. CALF-20(Zn) revealed to exhibit unconventional guest-host behaviors that give rise to abnormal thermodynamic and diffusion. The hydrophobic nature of the solid leads to a low water adsorption enthalpy at low loading followed by a gradual increase, driven by strong water hydrogen bonds, found to arrange as quasi 1D water wires in MOF porosity, recalling water behavior in carbon nanotubes and aquaporins. While no super-diffusion found, this behavior was shown to impact diffusion along with guests loading, with a minimum correlated with inflection point of adsorption isotherm corresponding to wires formation. Interestingly, diffusion of both CO2 and H2O were also found to be of the same order of magnitude with similar non-linear behaviors.