Water/Alcohol Mixture Adsorption in Hydrophobic Materials: Enhanced Water Ingress Caused by Hydrogen Bonding

Abstract

Microporous crystalline porous materials such as zeolites, metal−organic frameworks, and zeolitic imidazolate frameworks (ZIFs) have potential use for separating water/alcohol mixtures in fixed bed adsorbers and membrane permeation devices. For recovery of alcohols present in dilute aqueous solutions, the adsorbent materials need to be hydrophobic in order to prevent the ingress of water. The primary objective of this article is to investigate the accuracy of ideal adsorbed solution theory (IAST) for prediction of water/alcohol mixture adsorption in hydrophobic adsorbents. For this purpose, configurational bias Monte Carlo (CBMC) simulations are used to determine the component loadings for adsorption equilibrium of water/methanol and water/ethanol mixtures in all-silica zeolites (CHA, DDR, and FAU) and ZIF-8. Due to the occurrence of strong hydrogen bonding between water and alcohol molecules and attendant clustering, IAST fails to provide quantitative estimates of the component loadings and the adsorption selectivity. For a range of operating conditions, the water loading in the adsorbed phase may exceed that of pure water by one to two orders of magnitude. Furthermore, the occurrence of water−alcohol clusters moderates size entropy effects that prevail under pore saturation conditions. For quantitative modeling of the CBMC, simulated data requires the application of real adsorbed solution theory by incorporation of activity coefficients, suitably parameterized by the Margules model for the excess Gibbs free energy of adsorption.

Structure and dynamics of a water/methanol mixture confined in zeolitic imidazolate framework ZIF-8 from atomistic simulations

“…The hydrogen bonding between water and methanol molecules is believed to lead to mutual slowing-down 68,69 of each component. Recently, the formation of hydrogen bonds between water and alcohol molecules has been found to enhance the water ingress, 70,71 which cause failure of an ideal adsorbed solution theory (IAST) to provide a quantitative description of adsorption equilibrium. It is common to define the hydrogen bond between a donor and an acceptor on the basis of energetic and/or geometric criteria.…”

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 …”

“…For quantitative modeling of the data on activity coefficients, a variety of models such as regular solution, 43 Wilson, 50 , 51 , 55 NRTL, 56 SPD, 48 and Margules 52 , 57 have been used. For example, the Margules model takes the following form …”

“…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. − …”

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

“…Because the surface area A is not directly accessible from experimental data, the adsorption potential π A / RT ≡ Φ, − with the units mol kg –1 , serves as a convenient and practical proxy for the spreading pressure π. For binary mixture adsorption, each of the equalities on the right hand side of eq must be satisfied.…”