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- Natural zeolite clinoptilolite (1) (entfernen)
Clinoptilolite is a natural, medium pore size, zeolite. It contains a 2-deminsional pore system containing of oxygen-8- membered and oxygen-10-membered rings. It is one of the most abundant natural zeolites. Clinoptilolite has a crystalline structure showing unique ion exchange and sorption properties. It is mainly used as ion exchanger and adsorbent for removal of toxic elements and compounds from the environment. The use of natural zeolite as spent catalysts may open new perspectives in the chemical use of heavy feed feedstock by chemical conversion. Natural zeolites are not expensive, widely available and environment friendly. Clinoptilolite is the most abundant natural zeolite. Although clinoptilolite is a potential acid catalyst, reports on its application in catalysis are limited so far.
This contribution deals with the modification of the acidity of the natural zeolite clinoptilolite, the characterization of the acid sites and the catalytic activity in the Brønsted acid catalyzed acetalization of benzaldehyde with butandiol-1,3. The samples were acidified by via ammonium-ion exchange followed by calcination and acid treatment. The obtained catalysts were characterized by XRD regarding crystallinity and phase composition. The morphology was studied by TEM images The acidity was studied by ammonia-TPD measurements and 1H solid state MAS NMR as well as REDOR experiments. Structure and structural changes caused by applied modifications were studied by 27Al- and 29Si MAS NMR measurements and thermal analysis.
The ion experiments show that a part of the cations of clinoptilolite readily exchange with ammonium ions and protons from HCl treatment. The NMR results give first hints for a relation of the catalytic conversion with the presence of 5-fold coordinated aluminum. The results will be discussed in terms of a collaborative action of Brønsted acid sites and 5-fold coordinated aluminum.