Intrazeolite semiconductors: sodium-23 MAS NMR, thallium(1+) luminescence quenching and far-IR studies of acid-base precursor chemistry in zeolite Y

Abstract

Proton-loaded zeolites, prepared from fully dehydrated zeolites and gaseous, anhydrous Bransted acids, represent an important step in the synthesis of intrazeolite semiconductor quantum supralattices. Adsorption-induced 23Na MAS NMR chemical shifts, far-1R Na+ and TI+ translatory mode frequency shifts, and TI+ luminescence quenching effects were chosen as probes of cation-anion interaction in these materials. Samples of zeolite Y with various loadings of TI+ were prepared via aqueous ion-exchange techniques. The samples were characterized by powder X-ray diffraction and far-IR spectroscopy. Luminescence measurements revealed TI+ excitation and emission bands in the UV spectral region. Exposure of thallium TI(1) zeolite Y to anhydrous HBr quenched the luminescence intensity. The intensity quenching followed Stern-Volmer quenching kinetics. Preliminary luminescence lifetime studies of this system supported a static ion pair quenching model. Compelling additional evidence in favor of cation-anion pair formation comes from the observation of a-cage site-specific *)Na MAS NMR chemical shifts in HBr/Na%Y compared to virgin NaFY. The relevance of these observations for proton-loaded zeolite Y to the acid-base precursor chemistry involved in the synthesis of semiconductor nanostructures encapsulated in zeolite Y is critically discussed.