The selective adsorption of tellurium in the aluminosilicate regions of AFI- and MOR-type microporous crystals

Abstract

Attempts have been made to load tellurium (Te) atoms into the one-dimensional nano-channels of microporous crystals of aluminophosphate AlPO₄-5 and of aluminosilicate mordenites of the Na⁺ form (Na-MOR) and the H⁺-form (H-MOR) at 673 K. The density of the atoms adsorbed was in the sequence 0 ∼ AlPO₄-5 ≪ H-MOR < Na-MOR. AlPO₄-5 provides a shallow potential of periodical charge fluctuation for Te atoms, from the alternate ordering of Al and P atoms through O atoms. Mordenite offers a sufficiently strong potential for Te adsorption, but the magnitude varies with the type of cation. Dipoles between framework AlO₂⁻ anion sites and their Na⁺ counter-ions in Na-MOR provide a stronger potential than the Brønsted acid points in H-MOR. The adsorption of Te atoms in the silico-aluminophosphate SAPO-5 was between that of AlPO₄-5 and H-MOR, leading us to suspect that Te atoms are selectively adsorbed in the aluminosilicate regions accompanying the Brønsted acid points distributed in the major aluminophosphate network. The aluminosilicate regions in SAPO-5 are below 500 nm in size and are distributed throughout a single crystal.