Abstract
The high-pressure behavior of a synthetic siliceous ferrierite has been studied by in situ single-crystal and powder synchrotron X-ray diffraction with a diamond anvil cell, using four different P-transmitting fluids: the non-penetrating silicone oil and the potentially pore-penetrating methanol:ethanol:H2O = 16:3:1 mixture, ethylene glycol and 2methyl-2propen-1ol. The high-pressure experiment in silicone oil shows a remarkable flexibility of the FER framework. Two displacive phase transitions, following the path Pmnn-to-P121/n1-to-P21/n11 with pressure, were observed. The three polymorphs were found to share a virtually identical bulk compressibility, though showing a different anisotropic pattern. The experiments with potentially penetrating media enhanced the occurrence of a complex scenario, from which the P-induced intrusion of fluid molecules into the FER structural voids can be assumed by the different phase-transition paths and compressibility patterns, by the calculated residual electron density and by the different deformation mechanisms at the atomic scale, observed as a function of the used medium. The starting orthorhombic polymorph was always restored upon decompression in all the experiments. The roles of the different surface area in single crystal and polycrystalline samples, and of the process kinetics on the compressibility and crystal–fluid interactions, are discussed.
Highlights
In the last years, a significant number of studies has been devoted to the behavior of natural and synthetic zeolites under pressure (HP), e.g. [1,2] and references therein
The following phase-transitions path may be proposed for siliceous ferrierite at varying T and P: Immm ← (↑Temp.) Pmnn (↑Pres.) → P121/n1 → (P-1) → P21/n11 where Pmnn is the stable polymorph at ambient conditions and P-1 is a necessary transient state between the P121/n1-to-P21/n11 phase transition
This HP study demonstrates the remarkable flexibility of Si-FER, an industrially important porous material
Summary
A significant number of studies has been devoted to the behavior of natural and synthetic zeolites under pressure (HP), e.g. [1,2] and references therein. To determine the exploitability and the efficiency of a porous material in HP processes, it is essential to study its elastic behavior and the P-induced structural deformations under a series of experimental conditions, e.g. different P regimes and different P-transmitting fluids (PTF). It is well known that HP studies on porous materials can be performed using penetrating or non-penetrating PTF [3]. The former are usually aqueous/organic mixtures, with molecular sizes small enough to penetrate the zeolite pores, e.g. While non-penetrating PTF are mainly used to study the zeolites compressibility, along with P-induced phase transitions and amorphization, the penetrating ones can be used for investigating the P-induced intrusion of extra-guest molecules in the framework pores, e.g. Molecular ‘spring’, ‘damper’ or ‘shock absorber behavior can be observed, e.g. [11,12,13,14]
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