Flexibility versus rigidity: what determines the stability of zeolite frameworks? A case study

Abstract

All silica COK-14/-COK-14 with OKO topology is the first case of a zeolite which reversibly transforms from a systematically interrupted to a fully connected state and back. Analysis of the opening/closing behavior allowed the study of entropy and framework flexibility as determinants for the stability of zeolite topologies, which, until now, has been experimentally inaccessible. Interconversion of the all-silica COK-14 zeolite with fully connected OKO topology and its -COK-14 variant with systematic framework interruption was investigated using high-temperature XRD, thermogravimetric analysis, ²⁹ Si MAS NMR, nitrogen adsorption and a range of modelling techniques. Specific framework bonds in the OKO framework can be reversibly hydrolyzed and condensed. Structural silanols of the parent -COK-14, prepared by degermanation of the IM-12 zeolite, were condensed by heating at 923 K, and hydrolyzed again to the initial state by contacting the zeolite with warm water. Molecular modelling revealed an inversion of the relative stabilities for both variants depending on temperature and hydration. Condensation of the structural silanols in -COK-14 to COK-14 is entropy driven, mainly resulting from the release of water molecules. Framework reopening in the presence of water is spontaneous due to the high rigidity of the fully connected OKO framework. Isomorphous substitution was demonstrated as a viable option for stabilization of the fully connected OKO framework as this renders the closed framework flexible

Advances in theory and their application within the field of zeolite chemistry

“…122 Further modeling of the organization phase enabled to predict 20 hypothetical layered zeolites that can be obtained by conversion of existing zeolites like UTL. 112 Static DFT calculations at 0 K can no longer describe this temperature-and time-dependent behavior. 113,123,124 A last point in this section on the interconversion of zeolites is the reversible opening and closing of the (À)COK-14 framework, the bottom part in Fig.…”

“…Silicon cannot form a covalent bond with F À and therefore lacks stability in the D4R. 111,112 Verheyen et al 111 observed that D4Rs often act as a common building unit to expand the framework and to assume the structural role of linking layers. 118 A first computational study of 19 F NMR spectra has been performed by Pulido et al, 202 but these authors used cluster models to model the D4R units.…”

The Si–Ge substitutional series in the chiral STW zeolite structure type

“…The small relative depth of the energy wells served as an indication of accessible metastable phases and of the probability of transitions, which suggests dynamic flexibility behavior, in agreement with previous findings. [26,68,69] This suggests that modelling the structural features of Ge containing zeolites, even with a static view, provides means for predicting the flexibility, if any, of these materials. The addition of a second Ge atom in a D4R cube brings a large asymmetry into the local structure around the Ge atoms, since only one of the two Ge atoms is in close contact with the F atom (at ca.…”

Catalytic activation of all-silica COK-14 zeolite through alumination and particle size reduction using wet ball milling

“…The interruption splits the zeolite into layers. Heating of the obtained allsilica -COK-14 in absence of water or moisture, results in fully connected COK-14 zeolite by reorientation and condensation of silanol groups of the layers [28,37,44]. Upon adsorption of water, even under ambient conditions, the fully condensed framework is locally hydrolyzed again at the same positions.…”