Title: New Zeolites Assembled from Layered Building Units
Other Titles: Synthese van nieuwe zeolieten via gelaagde bouweenheden
Authors: Verheyen, Elke
Issue Date: 19-Jun-2014
Abstract: Zeolites are nowadays used in a wide range of applications. Their broad scope of industrial uses includes adsorptive separation and purification, ion exchange and catalysis. The structural diversity of existing zeolites is enormous and the theoretical variability is endless. Up to now 218 framework type codes have been assigned by the International Zeolite Association (IZA). Facing many challenges related to the transition of fossil fuels to renewable chemical feedstock and the need for ever greener, energy- and cost-efficient chemical processes and pollution abatement, there is a compelling need for new generations of zeolites. Zeolites are complex crystalline materials with internal porosity. The scientific community is little by little uncovering the complicated mechanisms governing zeolite formation. Although scientific insight in the role of key parameters in the zeolite crystallization process is growing, synthesizing new zeolite frameworks by design is still in its infancy. The focus of this doctoral research was to investigate the potential of a more rational zeolite synthesis approach via assembly of zeolites layers. The work was inspired by the instability of germanosilicate zeolites. In the last 15 years significant effort has been spend on the synthesis of new germanosilicate zeolites to make use of the structure directing effect of germanium towards extra-large pore zeolites. The high cost of germanium, combined with the instability of most germanium substituted zeolites, due to facile hydrolysis of Ge-O-Si bonds, inspired us to attempt to eliminate the germanate units from zeolite lattices and in this way create new zeolites by cut and paste. The documented instability of germanosilicate IM-12 zeolite (UTL framework type code) was used as an advantage to facilitate the synthesis of new zeolite materials. The reactivity of framework germanium atoms in strong mineral acid was used to selectively remove germanium from the IM-12 zeolite. This resulted in new, nearly all-silica and stable zeolites, named -COK-14 and COK-14 after the Dutch name of our research laboratory (Centrum voor Oppervlaktechemie en Katalyse, COK). IM-12 zeolite forms a unique case in which germanium is found concentrated in a germanate four-ring (Ge-4R), connected to a Si-4R to form a cube, acting as linker of more dense silicate layers. Framework transformation from germanosilicate IM-12 zeolite to the all-silica COK-14 zeolite family through acid leaching occurs via an intermediate phase, Ge-COK-14, in which the germanate four-ring is positioned in the channels of Ge-COK-14. This dislodged germanate unit in Ge-COK-14 is highly important as, most likely, this ensures the proximity of the intact silicate layers and 4Rs, enabling reconnection upon final elimination of germanium. By removing the Ge-4Rs from the IM-12 zeolite, one layer of framework atoms is removed throughout this structure. This corresponds to an inverse sigma transformation of a zeolite framework, a theoretical concept only used to hypothetically simulate new frameworks, and has by this transformation been achieved experimentally for the first time. -COK-14 and COK-14 are new, nearly all-silica zeolites with a two-dimensional intersecting channel system circumscribed with 10- and 12-MR pores. -COK-14 contains periodic interruptions in the framework via systematic silanol groups pairwise present in the 12-MR of -COK-14 zeolite. The International Zeolite Association on July 2013 approved COK-14 as newly synthesized zeolite and COK-14 was assigned the framework type code OKO, which stands for “Oppervlaktechemie en Katalyse One”.The interrupted framework -COK-14 can be transformed to the fully connected COK-14 by a heat treatment in the absence of water. The framework transformation, monitored by high-temperature XRD, occurs gradually between 300 and 650 °C by silanol reorientation and condensation. COK-14 can be retransformed to -COK-14 when exposed to water by suspending COK-14 in warm water. This is the first documented reversible transformation between an interrupted zeolite framework and the fully connected analogue. The high temperature requirements for framework condensation result from the necessary large entropic contribution to the free energy, increasing with temperature, to overcome enthalpy during transformation. The spontaneous reopening occurs due to the lack of sufficient flexibility of the all-silica COK-14 framework. Theoretical analysis revealed flexibility can be restored to the silica-rich COK-14 framework by partial isomorphous substitution of larger tetrahedra such as AlO4 and GeO4. Experimental evidence demonstrating heteroatom incorporation as a strategy for stabilization of COK-14 was established by incorporation of aluminum atoms. Direct aluminum incorporation in the framework during synthesis is not possible as -COK-14 is obtained by degermanation of IM-12 zeolite under strongly acidic conditions where aluminum is highly soluble. Tetrahedral framework aluminum was introduced in all-silica zeolite -COK-14, using Atomic Layer Deposition (Al-ALD) involving alternating exposure to trimethylaluminum and water vapor in an ALD apparatus. Next to tetrahedral aluminum, penta- and octacoordinated aluminum species were present. This is the first example of introducing framework aluminum and generating acid sites in a purely siliceous zeolite by Al-ALD. Part of the extra-framework Al species could be removed by ion exchange with ammonium chloride, followed by calcination at 400 °C. This heat treatment triggered framework condensation to obtain the fully connected OKO zeolite. In contrast to all-silica COK-14, aluminosilicate COK-14 did not hydrolyze back to an interrupted framework, as predicted by the theoretical flexibility analysis. Bifunctional Pt-loaded aluminosilicate COK-14 proved active in the hydroconversion of n-decane. In this way, a new stable aluminosilicate zeolite with intersecting medium and large pores was obtained, which will be a valuable addition to the collection of stable high-silica aluminosilicate zeolites for catalytic application.The zeoliticlayers obtained by removal of sacrificial Ge-4R units from the UTL zeolite bear resemblance to the nanosheets of MFI zeolite synthesized in Ryoo’s research group. The robustness of those layers and their applicability in catalysis was investigated in the last part of this work. MFI nanosheets with thicknesses of 2 and 8 nm were synthesized and converted into bifunctional catalysts. Opposed to the Ge atoms in UTL zeolites, the isomorphous substitution of Al is stable and can be exploited to generate acid catalytic activity. Remarkably, even a 2 nm thin zeolite sheet exhibits molecular shape-selective catalysis.
ISBN: 978-90-8826-364-4
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Centre for Surface Chemistry and Catalysis

Files in This Item:
File Status SizeFormat
PhD thesis_Elke Verheyen.pdf Published 14112KbAdobe PDFView/Open Request a copy

These files are only available to some KU Leuven Association staff members


All items in Lirias are protected by copyright, with all rights reserved.