Differential penetration of ethanol and water in Si-chabazite: High pressure dehydration of azeotrope solution

Giorgia Confalonieri,Gloria Tabacchi,Giovanna Vezzalini,Ettore Fois,Rossella Arletti,Simona Quartieri,T Jean Daou

doi:10.1016/j.micromeso.2019.04.032

Abstract

Abstract This study is aimed to shed light on the mechanisms at the basis of the differential penetration of alcohol and water in hydrophobic zeolites at ambient (Pamb) and non-ambient pressure. Here we report the effects of the penetration of water and alcohol in an all-silica chabazite (Si-CHA) compressed with an ethanol/water azeotrope solution (ethanol: water = 95.63 : 4.37 by mass %). We collected in situ synchrotron X-ray Powder Diffraction (XRPD) data in order to monitor the structural modifications induced by the fluid penetration and to investigate the guest-guest and host-guest interactions. First principles molecular dynamics simulations allowed to complete the structural description at high pressure, providing an atomistic level description of the guest-guest hydrogen bond network. For a comprehensive understanding of the processes involving the Si-CHA + azeotrope interactions, both the zeolite and the alcohol/water solution were firstly investigated separately under pressure. The results obtained prove that both H2O and ethanol penetrate Si-CHA porosities even at Pamb. However, while in these conditions the H2O/ethanol ratio adsorbed inside Si-CHA is similar to that of the external azeotrope solution, under pressure the zeolite extra-framework content corresponds to a composition much richer in H2O than the azeotrope one. Hence, our results suggest that a dehydration effect occurred on the azeotrope solution, promoted by pressure. In addition, the experiment performed to test the elastic behavior of Si-CHA with a non-penetrating pressure transmitting medium interestingly indicates that Si-pure chabazite is the most compressible zeolite among those up to now studied in silicone oil.

Highlights

The shape selective properties of zeolites are at the basis of their success in adsorption processes and catalytic activity
In this paper we report the intrusion of an azeotrope solution in an all silica chabazite (Si-CHA) under pressure
The results of this work show that, when Si-CHA is immersed in the ethanol/water azeotrope solution, it immediately absorbs both H2O and ethanol molecules, even at ambient conditions

Summary

Introduction

The shape selective properties of zeolites are at the basis of their success in adsorption processes and catalytic activity. One of the challenges in renewable energy fuel production is the purification of ethanol from water. While the water/ethanol separation in ethanol rich solutions by hydrophilic zeolites is extremely effective (e.g. LTA membranes [5]), the ethanol/water separation performances in H2O rich solutions operated by hydrophobic zeolites are definitely worse. This is due to the presence of silanol defects, or Al hydrophilic sites - accidentally present in “nominally silicatic“ zeolites which favor the adsorption of water molecules during the purification process

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