Characterization of Se-Loaded Molecular Sieves A, X, Y, AIPO-5, and Mordenite

Selenium has been successfully loaded into molecular sieves A, X, Y, AlPO-5, and mordenite, and the products were characterized by using EXAFS, solid-state NMR, and diffuse-reflectance techniques. This study reveals selenium is predominantly of the trigonal (helical chains) form in all but the A sample, where only the Se₈-crown ring form is found. A mixture of allotropes and helical chains occupy the large 3D-pore and channel systems of molecular sieves X and Y; however, a single, probably fixed-pitch helical-chain allotrope occupies the more constrained 12-membered-ring channels found in mordenite and A1PO-5. The high degree of order in these last two sieves is reflected in a strong second-shell feature in the EXAFS spectra.