(539g) Calculation and Analysis of RAST Activity Coefficients for Ethanol/Water Adsorption in All-Silica Zeolites

Understanding mixture adsorption in nanoporous materials such as zeolites is a key piece of information for designing efficient adsorption- or membrane-based separation processes. In practice, the Ideal/Real Adsorbed Solution Theory (IAST/RAST) of Myers and Prausnitiz¹ is often used to predict mixture isotherms from single-component data. However, significant deviations from IAST have been reported for nonideal mixtures², while measuring or predicting non-ideal mixture adsorption isotherms remains a challenging task both experimentally and computationally. In this work, we extend our previous study on the adsorption of binary water/ethanol mixtures and apply the RAST formalism to extract the pore-phase activity coefficients in all zeolites catalogued by the Structure Commission of the International Zeolite Association. Grand-canonical Monte Carlo simulations are used to calculate the binary isotherms and the unary isotherms of ethanol. To obtain the unary isotherms of water that typically require long equilibration periods, we adopt an alternative, computationally less expensive method by fitting a Langmuir-Freundlich isotherm for each zeolite to its Henry’s constant (computed from Widom insertion), saturation loading (estimated from pore volume), and pressure at half loading (computed from an iterative GCMC procedure). With the three isotherms, RAST activity coefficients are extracted and expressed in terms of the Margules activity model. Comparing ethanol-selective zeolites of different pore dimensions, we observe that activity coefficients are framework and topology dependent, and generally increase with pore sizes. The activity coefficients are higher than those in bulk solutions for medium- and large-pore zeolites (e.g., MFI, BEA, FAU zeolites) and lower for small-pore zeolites (e.g., LTA zeolite). We examine how these observations generalize to the adsorption of other aqueous mixtures and anticipate that these results will provide new insights into how confinement modifies bulk nonidealities.

[1] Myers & Prausnitz. AIChE J. 11, 121 (1965).

[2] Bai, Tsapatsis & Siepmann. Langmuir 28, 15566 (2012).