Isomorphous solid-state substitution of Si for framework Al in Y zeolite using crystalline (NH4)2[SiF6] as reactant

Abstract

Solid-state reactions in ground mixtures of NH₄-Y zeolite and crystalline (NH₄)₂[SiF₆] at temperatures below the decomposition point of the complex compound were investigated. The reaction products were characterized by chemical analysis, IR and MAS NMR spectroscopy, XRD and thermal analysis. Isomorphous substitution of silicon for framework aluminium proceeded when the escape of gaseous products (NH₃) from the reaction system was prevented or retarded by use of appropriate reaction vessels. Thus-performed dealumination of Y zeolite resulted in a considerable increase of both acid strength and thermal stability of bridging hydroxyls (Brønsted acid sites). Product inhibition manifested itself at application of more than about 25 (NH₄)₂[SiF₆] per unit cell. Secondary processes (desilication and non-substitutive dealumination of the zeolitic framework), induced by the reaction product (NH₄)₃[AlF₆] and adsorbed water, were found to proceed to a minor degree. They result in the formation of non-acidic silanol groups. Evidently, these undesired reactions can be effectively suppressed by (partial) dehydration of the zeolitic component of the reaction mixture. Another irreversible reaction route involving the release of ammonia and formation of NH₄[AlF₄] and NH₄HF₂ was found to be dominant in open systems. Thus-obtained products exhibited low crystallinity, less acidic and thermally highly unstable bridging hydroxyl groups, Lewis acid sites (cationic aluminium species) and acidic silanol groups.