Monte Carlo Simulations of Nanopore Compartmentalization Yield Fingerprint Adsorption Isotherms as a Rationale for Advanced Structure Characterization of Metal-Organic Frameworks

Unique adsorption and transport properties of metal-organic framework (MOF) materials are determined by their complex nanostructures composed of three-dimensional (3D) networks of pore compartments (cages, channels, and windows) that differ in size, shape, and chemical functionalities. Practical MOF samples are rarely ideal crystals: they contain binders, various defects, and residual solvents. Reliable nanopore structure characterization methods are needed to quantify the difference between real samples and ideal MOF crystals. Here, we construct theoretical isotherms in the individual pore compartments of MOF crystals using Monte Carlo simulations and use them as reference fingerprint isotherms. The comparison of the experimental isotherms with the theoretical fingerprint isotherms allows one to calculate the pore type distribution function, degree of sample crystallinity, adsorption capacity, and accessibility of individual pore compartments. This information cannot be obtained with the currently available methods of adsorption characterization. The proposed methodology is demonstrated drawing on the examples of Ar, N2, and CO2 adsorption on porous coordination network-224 (PCN-224) and zeolitic imidazolate framework-412 (ZIF-412) MOF crystals. The constructed fingerprint isotherms are verified against the literature experimental data obtained by in situ adsorption crystallography. The pore-level compartmentalization of adsorption isotherms provides a better understanding of the specifics of the adsorption mechanisms and distribution of adsorbed molecules between the individual pore compartments, which is instrumental for the selection and design of adsorbents with improved properties for gas separation, storage, and catalysis.