Abstract
Abstract The influence of the intrapore cation on the fluorination of zeolite Y from dilute fluoride solutions has been studied, revealing fluoride reacts with the zeolite framework in the presence of a Bronsted acid to form [SiO3F] and [AlO3F] moieties. 29Si{1H} Cross-polarised MAS NMR indicates the reaction proceeds by the substitution of surface hydroxide moieties for fluoride. The fluorination reaction is strongly influenced by the nature of the intrapore cation. Intrapore Bronsted acids facilitate fluorination of the framework by in situ ion-exchange, releasing the acidic ions to the zeolite surface. The fluorination reaction may be further promoted by the presence of intrapore alkaline earth cations (viz. Mg2+, Ca2+, Sr2+ and Ba2+). The conclusions of this work are significant to the preparation of fluorinated zeolite catalysts, the application of zeolites in defluoridation and the labelling of zeolite-based tracers with 18F for application in positron imaging techniques.
Highlights
Fluoride may be used as a mineraliser to catalyse condensation re actions in sol-gel syntheses of zeolites and related materials
We have investigated the interaction between dilute fluoride solutions and zeolites, determining the influence the intrapore cation has on the affinity for fluoride, the fluoride containing moieties present in the products and the likely mechanism by which defluoridation occurs
The 29Si magic-angle spinning (MAS) NMR spectrum recorded on H–Y along with peak positions, assignments and integrals may be found in the Supporting Information (Fig. S2 and Table S2)
Summary
Fluoride may be used as a mineraliser to catalyse condensation re actions in sol-gel syntheses of zeolites and related materials. Fits to the Dubinin-Radushkevitch isotherm reveal fluoride interacts by chemi sorption with Al3+-modified zeolites A, X and Y [12,14] In these modified zeolites, it is believed fluoride substitutes for a hydroxide in surface-sorbed M3+-complexes [12,14]. The mechanism by which the fluorination of zeolites occurs is ascribed either to the addition of H+F− ion pairs across T-O-T bonds, or alternately, the substitution of fluoride for hydroxide at surface silanol (Si–OH) or aluminol (Al–OH) moieties. A greater understanding of how defluoridation by zeolites occurs from low concentration fluoride solutions could inform strategies to enhance fluoride loadings. We have investigated the interaction between dilute fluoride solutions and zeolites, determining the influence the intrapore cation has on the affinity for fluoride, the fluoride containing moieties present in the products and the likely mechanism by which defluoridation occurs
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