(582h) Revealing Specifics of Gas Adsorption in Metal-Organic Frameworks from Compartmentalization of Adsorption Isotherms

Reliable characterization methods are needed to relate the structural specifics of metal-organic framework (MOF) materials to their adsorption properties and to quantify the difference between real samples and ideal crystals. Unique adsorption and transport properties of MOF materials are determined by their complex 3D networks of pore compartments (cages, channels, windows) that differ in size, shape, and chemical functionalities. However, practical MOF samples are rarely the ideal crystals: they contain binders, various defects, and residual solvents. In this work, we propose a novel methodology for assessment from the experimental adsorption isotherms the degree of sample crystallinity, pore type distribution function, adsorption capacity and accessibility of individual pore compartments. Using Monte Carlo simulations, we construct the theoretical adsorption isotherm on the ideal MOF crystal and decompose this isotherm into the fingerprint isotherms corresponding to individual pore compartments. Information about the sample pore structure is obtained from matching the experimental isotherm to the theoretical fingerprint isotherms. This approach is demonstrated on four MOF samples: Cu-BTC, PCN-224, ZIF-412, and UiO-66 using Ar, N₂ and CO₂ at their normal boiling temperatures. The constructed fingerprint isotherms are verified against the experimental data obtained by in-situ adsorption crystallography. The method of pore level compartmentalization of adsorption isotherms provides a better understanding of the adsorption mechanisms and distribution of adsorbate molecule at the pore level that is instrumental for the selection and design of novel adsorbents with improved properties for gas separations, storage, and catalysis.