Differential Penetration of Ethanol and Water in Si-Chabazite: High Pressure Dehydration of Azeotrope Solution

<p>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 (P<sub>amb</sub>) 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 <i>in situ</i> 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 H<sub>2</sub>O and ethanol penetrate Si-CHA porosities even at P<sub>amb</sub>. However, while in these conditions the H<sub>2</sub>O /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 H<sub>2</sub>O 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.</p>