Beyond Equilibrium: Metal–Organic Frameworks for Molecular Sieving and Kinetic Gas Separation

Abstract

Metal–organic frameworks (MOFs) are a class of crystalline inorganic–organic hybrid materials that have demonstrated huge potential in gas separation due to their ultrahigh porosity, boundless chemical tunability, as well as surface functionality. Most gas separations realized in MOFs are under an equilibrium state and are dependent on the difference in thermodynamic affinities of gases to MOFs, whereas nonequilibrium separation such as kinetic and molecular sieving separation attracting growing attention in the past decade is achieved based on the difference in the size and diffusivity of gas molecules. In this perspective, we first discuss the pore size, temperature, and pressure effect on gas diffusion as well as nonequilibrium gas separation in MOFs. Second, we introduce current techniques reported to measure intracrystalline gas diffusivity. Third, we review recent progress in MOF-based nonequilibrium N2/O2 separation, CO2 capture, and hydrocarbon separation. In addition, we describe the hydrogen isotope separation based on kinetic quantum sieving in MOFs as a special scenario of kinetic gas separation. Lastly, we summarize general design strategies toward MOF-based nonequilibrium gas separation and propose several directions to advance the study in this exciting area.

Proton Conductivity of Composite Polyelectrolyte Membranes with Metal‐Organic Frameworks for Fuel Cell Applications

“…This tunable functionality of MOFs in combination with high porosity and surface area makes the attractive for applications involving interaction with guest species . The chemistry of MOF compounds has received a huge attention over the last decade for their application in many fields of material chemistry such as gas adsorption and separation, biomedical application, catalysis and for electro‐optical devices . Among the several uses, a wide variety of MOFs have been investigated for their potential opportunity in ion and proton conductivity .…”

In Operando Analysis of Diffusion in Porous Metal‐Organic Framework Catalysts

“…Molecular sieving: When a potential sorbate is larger than the available pores, diffusion of the sorbate into the pore does not proceed . Under some conditions, such as low‐temperature, even sorbates that are smaller than the pore dimensions can display prohibitively slow diffusion . Network flexibility or fluxionality can sometimes enable diffusion of guests that are larger than the pore size measured by crystallography …”

Alternatives to Cryogenic Distillation: Advanced Porous Materials in Adsorptive Light Olefin/Paraffin Separations

“…For nonequilibrium separation, the separation performance is determined by the diffusion rate of olefins and paraffins in adsorbents . Kinetic separation occurs when gas adsorption has not reached equilibrium.…”

“…If the pore aperture size of an adsorbent is sufficiently narrow such that one or more components in the gas mixture cannot enter, the adsorption rate(s) of the adsorbate(s) can be regarded as zero, and kinetic separation will evolve to steric separation, which is also called molecular sieving or size‐selective separation. By the rational selection of adsorbents with appropriate pore apertures, namely apertures smaller than the kinetic diameters but greater than the smallest cross‐sectional area of the molecules, it is possible to employ kinetic and molecular sieving separation to perform olefin/paraffin separations 17a,50. However, it should also be bore in mind that crystallographic pore apertures of adsorbents may vary upon gas adsorption due to the prevalent framework flexibility in porous materials such as MOFs.…”

“…Unlike equilibrium separation based on the thermodynamic interactions between gases and adsorbents, nonequilibrium separation is made possible by the differences in diffusion rates of the gases within adsorbents with suitable pore sizes . 4A zeolite, with a pore size around 4 Å, is one of the earliest examples being studied for kinetic olefin/paraffin separations.…”