Abstract
The desilication of zeolite Y (of Si/Al = 31) that was previously dealuminated by steaming and acid treatment was studied. Desilication of zeolites of high Si/Al module in alkali solutions extracts both Si and Al from zeolite crystals, but while Si remains in solution, Al is reinserted into the zeolite grain. The main goal of our study was to follow the status of Al reinserted into zeolite during the desilication procedure, and its role in the formation of acid sites of the Brønsted and Lewis types. The properties of Al were followed by 27Al MAS NMR spectroscopy (for parent samples and zeolites treated either with NaOH or NaOH/tetrabutylammonium hydroxide), whereas the acid sites generated in the final stages were studied by IR spectroscopy with NH3 and CO as probe molecules. In non-desilicated zeolite, most of the Al was in a typically zeolitic tetrahedral coordination, while both NMR and quantitative IR studies of NH3 sorption evidenced that Al that was extracted by desilication and was subsequently reinserted had a tetrahedral coordination similar to amorphous aluminosilicates and showed an ion exchange ability. After the exchange of Na+ to NH4+ and decomposition of NH4+ ions, reinserted Al forms generated protonic sites from which some condensed at higher temperatures producing Lewis acid sites (with stoichiometry typical for zeolites i.e., the condensation of two protonic sites produces one Lewis site) but some other kept their character.
Highlights
Zeolites are very important catalysts in the chemical industry, especially in oil refineries
One method of improvement of the catalytic efficiency of zeolites is the preparation of hierarchical zeolites with developed pore system comprising mesopores
A lot of data on the synthesis of mesoporous zeolites, their properties, and catalytic applications were presented in a monograph edited by J
Summary
Zeolites are very important catalysts in the chemical industry, especially in oil refineries. The most important advantages of zeolites as catalysts are related to the fact, that active sites are situated inside the micropores and the carbocations produced by the addition of protons to reactants are stabilized by the negative charge of the framework. Another advantage of zeolites is the presence of very strong Brønsted acid sites. One method of improvement of the catalytic efficiency of zeolites is the preparation of hierarchical zeolites with developed pore system comprising mesopores. The desilication of zeolites in alkaline solutions turned out to be the most effective way of producing mesoporous zeolites. A lot of data on the synthesis of mesoporous zeolites, their properties, and catalytic applications were presented in a monograph edited by J. Discussion concerning the mechanism of desilication has been presented in [2,3,4,5,6]
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