Elke Verheyen

Design of zeolite by inverse sigma transformation

Although the search for new zeolites has traditionally been based on trial and error, more rational methods are now available. The theoretical concept of inverse σ transformation of a zeolite framework to generate a new structure by removal of a layer of framework atoms and contraction has for the first time been achieved experimentally. The reactivity of framework germanium atoms in strong mineral acid was exploited to selectively remove germanium-containing four-ring units from an UTL type germanosilicate zeolite. Annealing of the leached framework through calcination led to the new all-silica COK-14 zeolite with intersecting 12- and 10-membered ring channel systems. An intermediate stage of this inverse σ transformation with dislodged germanate four-rings still residing in the pores could be demonstrated. Inverse σ transformation involving elimination of germanium-containing structural units opens perspectives for the synthesis of many more zeolites.

Catalytic activation of OKO zeolite with intersecting pores of 10- and 12-membered rings using atomic layer deposition of aluminium

Tetrahedral framework aluminium was introduced in all-silica zeolite -COK-14 using Atomic Layer Deposition (ALD) involving alternating exposure to trimethylaluminium and water vapour. The modification causes permanent conversion of the originally interrupted framework of -COK-14 to a fully connected OKO type framework, and generates catalytic activity in the acid catalysed hydrocarbon conversion reaction.

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

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, 29Si 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.

Synthesis of an IWW-type germanosilicate zeolite using 5-azonia-spiro[4,4]nonane as a structure directing agent

An IWW-type zeolite with a Si/Ge ratio of 4.9 is obtained using 5-azonia-spiro[4,4]nonane as a template in fluoride-free medium under hydrothermal conditions at 175 °C. In an otherwise identical synthesis, using related 5-azonia-spiro[4,5]decane as a structure directing agent, a mixture of IWW and NON zeolite types was formed. In the absence of GeO2 from the reactant mixture, pure NON formed. The IWW zeolite was characterized by XRD, SEM, and HRTEM. The IWW zeolite displayed a unique morphology and could be calcined at 600 °C without the loss of crystallinity. The Si/Ge ratio of the IWW zeolite was increased by postsynthesis modification. Part of germanium could be eliminated from the as-synthesized IWW zeolite by acid leaching using 6 M HCl solution. Also the calcined material could be degermanated. Here the presence of a silicon source in the acidic leaching solution minimized structural damage. In this way the Si/Ge ratio of the IWW zeolite was increased from 4.9 up to 10.