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

Abstract

Microporous crystalline adsorbents such as zeolites and metal–organic frameworks (MOFs) have potential use in a wide variety of separation applications. The adsorption selectivity S ads is a key metric that quantifies the efficacy of any microporous adsorbent in mixture separations. The Ideal Adsorbed Solution Theory (IAST) is commonly used for estimating the value of S ads , with unary isotherms of the constituent guests as data inputs. There are two basic tenets underlying the development of the IAST. The first tenet mandates a homogeneous distribution of adsorbates within the pore landscape. The second tenet requires the surface area occupied by a guest molecule in the mixture to be the same as that for the corresponding pure component. Configurational-bias Monte Carlo (CBMC) simulations are employed in this article to highlight several scenarios in which the IAST fails to provide a quantitatively correct description of mixture adsorption equilibrium due to a failure to conform to either of the two tenets underpinning the IAST. For CO 2 capture with cation-exchanged zeolites and MOFs with open metal sites, there is congregation of CO 2 around the cations and unsaturated metal atoms, resulting in failure of the IAST due to an inhomogeneous distribution of adsorbates in the pore space. Thermodynamic non-idealities also arise due to the preferential location of CO 2 molecules at the window regions of 8-ring zeolites such as DDR and CHA or within pockets of MOR and AFX zeolites. Thermodynamic non-idealities are evidenced for water/alcohol mixtures due to molecular clustering engendered by hydrogen bonding. It is also demonstrated that thermodynamic non-idealities can be strong enough to cause selectivity reversals, which are not anticipated by the IAST.

Evaluating solvothermal and mechanochemical routes towards the metal–organic framework Mg2(m-dobdc)

“…Consistently, the N 2 capacity of Mg 2 ( m -dobdc)-MC at 750 mbar and 40 °C (0.89 mmol g −1 ) is higher than that reported for Mg 2 (dobdc) as well. Given the unclear suitability of calculating selectivities in open metal site MOFs using ideal adsorbed solution theory (IAST), 31 we elected to calculate non-competitive CO 2 /N 2 selectivities under conditions relevant to coal flue gas capture for Mg 2 ( m -dobdc)-ST, Mg 2 ( m -dobdc)-MC, and Mg 2 (dobdc) instead (Fig. 6d).…”

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.…”

A critical review on the practical use of lithium cycle as an upfront nitrogen removal technology from natural gas

“…For example, the ideal adsorbed solution theory is often used, [53][54][55][56] which is a theoretical method commonly used to calculate said selectivities using only adsorption results for individual pure components, but such a method assumes homogenous gas molecules distribution on the adsorbent surface and similarity in surface area occupation for that of the 63 mixture molecule and the pure gas molecule, which was proven not to be necessarily the case in many systems. 65 Membranes have also been investigated in the separation of NG components as summarized in Table 5 of the last two years in literature. Membrane technology is based on the rejection of certain components and permeation of others by semi-permeable "barriers" due to differences in molecular sizes of approaching gas components.…”

“…The existing literature on using adsorption for N 2 /CH 4 separation is also compromised by two additional crucial pieces of information: first, most of such research if not all conducts experiments on equimolar feeds, which is unrepresentative of NG in LNG before the cold section, and second, many rely on numerical and theoretical calculations to determine mixture selectivities instead of experimental results. For example, the ideal adsorbed solution theory is often used, 53–56 which is a theoretical method commonly used to calculate said selectivities using only adsorption results for individual pure components, but such a method assumes homogenous gas molecules distribution on the adsorbent surface and similarity in surface area occupation for that of the mixture molecule and the pure gas molecule, which was proven not to be necessarily the case in many systems 65 …”