Continuous dehydration of cavansite under dynamic conditions: an in situ synchrotron powder-diffraction study

Abstract

The dehydration dynamics of cavansite, Ca(VO)(Si4O10)*4H2O was studied by timeresolved in situ synchrotron powder diffraction between 298 and 900 K. The crystal structure evolution was continuously monitored through 20 Rietveld structure refinements (Pnma space group) in the 298-810 K range whereupon cavansite turned amorphous without any precursor to a polymorphic phase transition to pentagonite. The results achieved from the series of time-resolved Rietveld refinements allowed to highlight the out-of-equilibrium effects which govern dehydration of cavansite powders in dynamic conditions. While confirming the general picture of cavansite dehydration, as previously reported by static single crystal work, our dynamic study discovered the occurrence of an important transient phenomenon which is the cell volume expansion caused by framework relaxation resulting from breakdown of the hydrogen bonding network in the initial heating stages. We also documented that the channels formed by elliptical eight-membered tetrahedral rings of cavansite, when heated under the typical dynamic conditions of an industrial process, undergo a series of short-lived "breathing" pulses which could be exploited for fine tuning of gas diffusion paths in possible applications of synthetic analogues of these vanadosilicates.

Detailed Investigation of Thermal Regeneration of High-Silica ZSM-5 Zeolite through in Situ Synchrotron X-ray Powder Diffraction and Adsorption Studies

“…The weakening of hydrogen bonding network is well-known in the zeolite frameworks during the initial heating stages (e.g., in hydro-sodalite, analcime, wairakite, silica sodalite, cavansite, ZSM-5-toluene, and DCE-loaded, and in the same as-synthesized-high silica-ZSM-5 used in this work); recently, it was described by Martucci et al as a “pore-mouth-breathing motion”. This breathing is associated with a transient slight opening and regularization of the pore apertures, which is followed by a contraction.…”

“…At the same time, no changes are observed in extraframework site occupancies reported in Figure 3a,b, indicating that this transient framework expansion is mainly related to the release of host−guest interactions. The weakening of hydrogen bonding network is well-known in the zeolite frameworks during the initial heating stages (e.g., in hydro-sodalite, 44 analcime, 45 wairakite, 46 silica sodalite, 47 cavansite, 48 ZSM-5-toluene, 18 and DCE-loaded, 20 and in the same as-synthesized-high silica-ZSM-5 49 used in this work); recently, it was described by Martucci et al 48 as a "pore-mouthbreathing motion". This breathing is associated with a transient slight opening and regularization of the pore apertures, which is followed by a contraction.…”

Elastic behavior and pressure-induced structural modifications of the microporous Ca(VO)Si4O10·4H2O dimorphs cavansite and pentagonite

“…Moreover, the Ca coordination number coupled with the different flexibility of the frameworks seems to be responsible for either phase transitions or structural collapse in pentagonite and cavansite, respectively. The dehydration dynamics of cavansite were probed also by in situ synchrotron powder diffraction [22]. The knowledge of the pressure-induced structural modifications in cavansite is limited to a Raman spectroscopic study by Ravindran et al [23].…”

Organic Guests within a Ferroelastic Host: The Case of High Silica Zeolite ZSM-5

“…On the contrary, the desorption of guest molecules embedded in the ZSM-5 channels occurs at higher temperature, and their mobility inside the framework is controlled by configurational effects . As a matter of fact, refined bond distances indicate that the extraframework species interact with the coadsorbed water molecules, thus forming organic–water oligomers (clusters or short chains) interacting with the framework oxygens. − , According to the literature, ,,− this result suggests that the monoclinic ferroelastic phase shows an initial breathing to a “pore-mouth-breathing motion” because of the weakening of the hydrogen bonding network (host–guest interactions). After the ferroelastic-to-paraelastic phase transition, a sudden slope change in the unit-cell parameters occurs, and the release/decomposition of guest organic molecules takes place.…”