Ethanol and Water Adsorption in Conventional and Hierarchical All-Silica MFI Zeolites

Date

2021-11-16

Department

Program

Citation of Original Publication

Swagata Pahari, Matheus Dorneles de Mello, Mansi S. Shah, Tyler R. Josephson, Limin Ren, Huong Giang T. Nguyen, Roger D. Van Zee, Michael Tsapatsis, and J. Ilja Siepmann. ACS Physical Chemistry A DOI: 10.1021/acsphyschemau.1c00026

Rights

This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.

Subjects

Abstract

Hierarchical zeolites containing both micro- (<2 nm) and mesopores (2–50 nm) have gained increasing attention in recent years because they combine the intrinsic properties of conventional zeolites with enhanced mass transport rates due to the presence of mesopores. The structure of the hierarchical self-pillared pentasil (SPP) zeolite is of interest because all-silica SPP consists of orthogonally intergrown single-unit-cell MFI nanosheets and contains hydrophilic surface silanol groups on the mesopore surface while its micropores are nominally hydrophobic. Therefore, the distribution of adsorbed polar molecules, like water and ethanol, in the meso- and micropores is of fundamental interest. Here, molecular simulation and experiment are used to investigate the adsorption of water and ethanol on SPP. Vapor-phase single-component adsorption shows that water occupies preferentially the mesopore corner and surface regions of the SPP material at lower pressures (P/P0 < 0.5) while loading in the mesopore interior dominates adsorption at higher pressures. In contrast, ethanol does not exhibit a marked preference for micro- or mesopores at low pressures. Liquid-phase adsorption from binary water–ethanol mixtures demonstrates a 2 orders of magnitude lower ethanol/water selectivity for the SPP material compared to bulk MFI. For very dilute aqueous solutions of ethanol, the ethanol molecules are mostly adsorbed inside the SPP micropore region due to stronger dispersion interactions and the competition from water for the surface silanols. At high ethanol concentrations (Cᴇₜᴏᵸ > 700 g L–1), the SPP material becomes selective for water over ethanol.