Flexible Aluminum Coordination in Alumino−Silicates. Structure of Zeolite H−USY and Amorphous Silica−Alumina

Abstract

The aluminum coordination in amorphous silica−alumina with different aluminum concentrations as well as in pure γ-Al2O3 has been studied by means of 27Al (MQ) MAS NMR spectroscopy. The effect of ammonia adsorption at 120 °C has been studied and compared with that on zeolite H−USY, as well as with zeolites with different topologies. As for protonic zeolites, part of the octahedral aluminum reversibly changed to tetrahedrally coordinated upon ammonia adsorption on amorphous silica−alumina. The distinction between flexible aluminum coordination in zeolites and silica−alumina was possibly based on the quadrupolar parameters. Ammonia adsorption on pure γ-Al2O3 did not influence the aluminum coordination. A parallel between the structure of zeolite USY and that of amorphous silica−alumina was made on the basis of the results of 27Al MQ MAS NMR spectroscopy. A partial conversion from octahedral to tetrahedral coordination upon ammonia adsorption was observed for zeolite H−USY. Most of the flexible aluminum speci...