Pressure-induced penetration of guest molecules in high-silica zeolites: the case of mordenite

摘要

A synthetic high-silica mordenite (HS-MOR) has been compressed in both non-penetrating (silicone oil,uds.o.) and penetrating [methanol : ethanol : water (16 : 3 : 1) (m.e.w.), water : ethanol (3 : 1) (w.e.), andudethylene glycol (e.gl.)] pressure transmitting media (PTM). In situ high-pressure (HP) synchrotron X-rayudpowder diffraction (XRPD) experiments allowed the unit cell parameters to be followed up to 1.6, 1.8,ud8.4, and 6.7 GPa in s.o., w.e., m.e.w., and e.gl., respectively. Moreover, e.gl. was also used as a PTM inudin situ HP Raman and ex situ IR experiments. The structural refinement of HS-MOR compressed in e.gl.udat 0.1 GPa – the lowest investigated pressure – revealed the presence of 3.5 ethylene glycol moleculesudper unit cell. The infrared spectrum of the recovered sample, after compression to 1 GPa, is consistentudwith the insertion of ethylene glycol molecules in the pores. XRPD and Raman spectroscopy experimentsudperformed under pressure indicated the insertion of a small number of guest molecules. Ethyleneudglycol is partially retained inside mordenite upon pressure release. A symmetry lowering was observed inuds.o. above 0.8 GPa, while above 1.6 GPa the patterns indicated a rapid loss of long range order. Fromudambient pressure (Pamb) to 1.6 GPa, a high cell volume contraction (DV = 9.5%) was determined. Theudpatterns collected with penetrating PTM suggested the penetration of guest molecules into the porousudhost matrix, starting from a very low P regime. The entrapment of PTM molecules inside microporesudcontributes to the stiffening of the structure and, as a consequence, to the decrease of the compressibilityudwith respect to that measured in s.o. From the structural point of view, HS-MOR reacts to compressionudand to the penetration of different guest species with appropriate framework deformations. Interestingly,udethylene glycol is partially retained inside mordenite upon pressure release, which is of importance forudpotential application of this composite material.