Reversible Tetrahedral−Octahedral Framework Aluminum Transformation in Zeolite Y

Abstract

The hydration of a mildly calcined zeolite Y sample leads to the local destruction of the zeolite lattice. This way, besides the dealumination of the framework, a partial hydrolysis of framework Al−O bonds occurs generating framework connected Al−OH species. This hydrolysis is evidenced by a peak at 3.1 ppm in the 1H MAS NMR spectrum, which can only be resolved by applying the 1H spin−echo MAS NMR technique with different echo delay times. These framework Al−OH sites can host water molecules giving rise to octahedrally coordinated Al species seen in the 27Al MAS NMR spectrum at 0 ppm. The subsequent adsorption of ammonia on this sample after hydration converts this octahedral coordinated Al back to tetrahedral Al sites. In the 27Al MAS NMR spectrum these species are now seen by the increased line width of the 60 ppm line.