(419b) Evidence of Combined Heat Transfer and Surface Barrier Resistances for Propane in Commercial ZIF-8 Crystals Probed By Pressure-Swing Frequency Response

Shedding light on the controlling transport step in nanoporous materials, pressure swing frequency response has been developed to investigate the mass transfer mechanisms and rates for propane in commercial ZIF-8 crystals with submicron size, at multiple pressures from 0.079 to 1 bar and temperatures of 10, 30, and 50 °C. The system shows a bimodal response that cannot be described by a single diffusion or surface barrier model. The concept of introducing inert metal beads is shown to be adequate to differentiate between additional heat and mass transfer steps - the resulting change of the response curves demonstrates that heat transfer dominates the response and leads to the bimodal behavior with a faster mass transfer step than the heat transfer step. Furthermore, mathematical treatment has been developed to eliminate non-adsorption dynamics at high frequency. As a result, the corrected amplitude ratio curve depends on the adsorption kinetics only, and the technique allows the upper-frequency range to extend at least one order of magnitude, from the previous 0.1 Hz to 1 Hz for the current system. Therefore, it allows the determination of the mass transfer mechanism of propane in the submicron ZIF-8 crystals, controlled by the combined effects of heat transfer and surface barrier, in contrast to most accounts of micropore diffusion in literature. The surface barrier resistance strongly depends on the concentration and temperature with an activation energy of 28 kJ/mol extracted for this system. This example suggests extreme caution should be taken with respect to assumed mechanisms in play during the evaluation of kinetic data. The contribution of heat and surface barriers can be significant for kinetic studies on the adsorption system with appreciable adsorption heat in small crystals. Accordingly, the interference of both heat and surface barriers could hinder the separation performance, different from expected by kinetic selectivity from diffusivities alone.