Leveraging Diffusion Kinetics to Reverse Propane/Propylene Adsorption in Zeolitic Imidazolate Framework-8

Abstract

The separation challenge posed by propylene/ propane mixtures arises from their nearly identical molecular sizes and physicochemical properties. Metal−organic frameworks (MOFs) have demonstrated potential in addressing this challenge through the precision tailoring of pore sizes and surface chemistry. However, introducing modifications at the molecular level remains a considerable hurdle. This work presents an approach to reversibly tune the propylene/propane adsorption preference in zeolitic imidazolate framework-8 (ZIF-8) by manipulating the particle size and gas flow rate. Systematically increasing the ZIF-8 crystals from 9 to 224 μm restricts propane diffusion, thereby reversing its preferential adsorption over propylene. Furthermore, raising the gas flow rate of mixed propylene/propane shifts the rate-determining breakthrough step from thermodynamic equilibrium to kinetics, again reversing the adsorption preference in a particular ZIF-8 sample. We propose "dynamic selectivity (S d (t))" as a concept that incorporates both thermodynamic and kinetic factors to elucidate these unexpected findings. Moreover, the driving force equation, grounded on the concept of S d (t), has improved the precision and stability of the computational simulation for fixedbed adsorption processes. This work underscores the potential of diffusion-based modulation, implemented through manageable external changes, as a viable strategy to optimize separation performance in porous adsorbent materials.

Molecular-Sieving Separation of Methanol/Benzene Azeotrope by a Flexible Metal–Organic Framework

“…Interestingly, the inflection point of pore opening moves from P / P 0 ∼ 0.60 (i.e., 7.76 kPa) at 293 K, to P / P 0 ∼ 0.40 (i.e., 6.74 kPa) at 298 K, and to P / P 0 ∼ 0.15 (i.e., 5.29 kPa) at 313 K (Figures a and S14). Compared with conventional examples that increasing temperature will delay the inflection point, , this opposite trend indicated that kinetic factors should play an important role in adsorption and structural transformation (possessing high energy barriers). , Obviously, there was large hysteresis in desorption isotherms (Figure S14), which should be attributed to strong guest–host interactions and framework flexibility . Moreover, even though MeOH molecules could coordinate onto the Ca sites in the structure, the measured adsorption enthalpy by differential scanning calorimetry (DSC) is −52.5 kJ·mol –1 (Figure S16), much higher (more positive) than that of the bond energy of Ca–O (−90 kJ·mol –1 ), which can be attributed to the energy compensation from framework flexibility. ,, …”