Using Molecular Simulations to Unravel the Benefits of Characterizing Mixture Permeation in Microporous Membranes in Terms of the Spreading Pressure

Abstract

The separation performance of microporous crystalline materials in membrane constructs is dictated by a combination of mixture adsorption and intracrystalline diffusion characteristics; the permeation selectivity S perm is a product of the adsorption selectivity S ads and the diffusion selectivity, S diff . The primary objective of this article is to gain fundamental insights into S ads and S diff by use of molecular simulations. We performed configurational-bias Monte Carlo (CBMC) simulations of mixture adsorption equilibrium and molecular dynamics (MD) simulations of guest self-diffusivities of a number of binary mixtures of light gaseous molecules (CO 2 , CH 4 , N 2 , H 2 , and C 2 H 6 ) in a variety of microporous hosts of different pore dimensions and topologies. Irrespective of the bulk gas compositions and bulk gas fugacities, the adsorption selectivity, S ads , is found to be uniquely determined by the adsorption potential, Φ, a convenient and practical proxy for the spreading pressure π that is calculable using the ideal adsorbed solution theory for mixture adsorption equilibrium. The adsorption potential Φ is also a proxy for the pore occupancy and is the thermodynamically appropriate yardstick to determine the loading and composition dependences of intracrystalline diffusivities and diffusion selectivities, S diff . When compared at the same Φ, the component permeabilities, Π i for CO 2 , CH 4 , and N 2 , determinable from CBMC/MD data, are found to be independent of the partners in the various mixtures investigated and have practically the same values as the values for the corresponding unary permeabilities. In all investigated systems, the H 2 permeability in a mixture is significantly lower than the corresponding unary value. These reported results have important practical consequences in process development and are also useful for screening of materials for use as membrane devices.

How Reliable Is the Ideal Adsorbed Solution Theory for the Estimation of Mixture Separation Selectivities in Microporous Crystalline Adsorbents?

“…Since the surface area A is not directly accessible from experimental data, the surface potential, 40 , 43 , with the unit mol kg –1 , serves as a convenient and practical proxy for the spreading pressure π ; the surface potential has also been termed the adsorption potential in several recent publications. 49 − 52 …”

“…In eq , A represents the surface area per kg of framework, and q i 0 ( f ) is the pure component adsorption isotherm; the superscript 0 is used to emphasize that q i 0 ( f ) relates the pure component loading to the bulk fluid fugacity. Since the surface area A is not directly accessible from experimental data, the surface potential, , , with the unit mol kg –1 , serves as a convenient and practical proxy for the spreading pressure π ; the surface potential has also been termed the adsorption potential in several recent publications. − …”

Highlighting the Anti-Synergy between Adsorption and Diffusion in Cation-Exchanged Faujasite Zeolites

“…In eq , q i 0 ( f ) is the pure component adsorption isotherm. Since the surface area A is not directly accessible from experimental data, the surface potential πA / RT ≡ Φ, with the units mol kg –1 , serves as a convenient and practical proxy for the spreading pressure π . − As derived in detail in the Supporting Information, the fractional pore occupancy, θ , is related to the surface potential by where q sat, mix is the saturation capacity for mixture adsorption. Equation implies that Φ may also be interpreted as a proxy for the pore occupancy; it is the fundamentally correct yardstick to compare the adsorption and diffusion characteristics of different host materials. ,− …”