Zeolite L has a very high thermal stability, maintains a good crystallinity up to 814 °C and can be classified as inflexible . In particular, it is noteworthy that zeolite L undergoes a slight thermal expansion, which is very unusual for an alumino-silicate microporous material. Actually, while most of zeolites show the so-called negative thermal expansion [see 5 for a review], the positive one is observed in a number of neutral siliceous zeolites, like CIT-5, AlPO4-31and the one-dimensional 12MR system ITQ-4. In these materials the expansion is mainly realized along the direction of the main and large 12- or 14-ring channel , as observed in the zeolite L here investigated. Usually, in zeolites, most of the extraframework cations are coordinated by water molecules. Upon dehydration these cations move closer to the framework to form new bonds inducing strains on the framework, resulting in a structural deformation or even in bond breaking. Zeolite L represents an exception, because only one of the three cation sites (KD) is bonded to a water molecule, while the coordination spheres of the other cations are formed by only framework oxygen atoms. Moreover, among the five water sites, only two are at coordination distance from the framework oxygen atoms, while the others are involved only in water-water hydrogen bonds. These structural features explain why the dehydration process does not induce substantial structural modifications in the framework and why zeolite L even undergoes a small unit cell volume increase upon heating.
|Data di pubblicazione:||2013|
|Autori:||Gigli, L.; Arletti, R.; Quartieri, S.; Renzo, F. Di; Vezzalini, G.|
|Titolo:||The high thermal stabilility of the zeolite K-L: dehydration mechanism by in situ SR XRPD experiments|
|Appare nelle tipologie:||Abstract in Atti di Convegno|
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