High interfacial resistances of CH4 and CO2 transport through Metal-Organic framework 5 (MOF-5)

Abstract

Metal-organic framework 5 (MOF-5) has wide applications, such as gas capture, energy storage, drug delivery, compound separation and purification. Equilibrium molecular dynamics (EMD) simulations were performed to investigate effects of gas species (including CH4 and CO2) and gas loadings on the transport diffusion in MOF-5 with different dimensions, at different temperatures. The results reveal that the corrected diffusivities of CH4 and CO2 increase with thickness of MOF-5, and eventually converge to constants when thickness prolongs to an infinity. Further, the corrected diffusivities are found to increase with temperature. Based on the temperature dependency of corrected diffusivities, the energy barriers for CH4 and CO2 are determined, revealing CO2 normally has stronger energy barrier than CH4. It is found that stronger sorbate-solid interactions and larger inertia of sorbate both lead to higher energy barrier, associated with enhanced interfacial resistance and slower diffusion rate. The critical membrane thickness of CO2 is in tens of nanometer scale, which is nearly twice as much as that of CH4, emphasizing the essential role of the interfacial resistance on determining the transport diffusion procedure. While the corrected diffusivities, D0, increase with sorbate loading both for CH4 and CO2, and the corresponding ratios of the interfacial resistance to the total resistance, Rf/Rt, exhibit slight decrease and unobvious changes accordingly. Our results further reveal that the enhanced interactions among gas molecules at higher loadings actually facilitate molecules to overcome the diffusion energy barrier and subsequently enhance the diffusion rate. Our simulations essentially prompt the understanding of gases transport through nano-porous media, and benefit the optimal design of nano-porous membrane.