Influence of Pore Size on Hydrocarbon Transport in Isostructural Metal-Organic Framework Crystallites

Hydrocarbon separations using porous materials such as metal-organic frameworks (MOFs) have been proposed to reduce the energy demands associated with current distillation-based methods. Despite the potential of these materials to distinguish hydrocarbons through thermodynamic or kinetic mechanisms, experimental data quantifying hydrocarbon transport in MOFs is lacking. Such mass transfer measurements are vital to elucidate structure-property relationships and design future high-performing separation materials. In this work, we aim to isolate the influence of pore size on hydrocarbon diffusion by studying a pair of isoreticular MOFs, Co₂Cl₂BBTA and Co₂Cl₂BTDD. We use a volumetric method to extract mass transport coefficients for six hydrocarbon probe molecules of varying size and chemical functionality. From these nonequilibrium mass transport measurements, we determine the rate-limiting diffusion mechanism and identify trends in hydrocarbon surface permeabilities in the MOFs based on pore size, hydrocarbon chain length, and temperature.