Toward the aluminum coordination in dealuminated mordenite and amorphous silica–alumina: A high resolution 27Al MAS and MQ MAS NMR study

Abstract

27Al MAS and MQ MAS NMR spectroscopy were used to study the aluminum coordination in dealuminated mordenite. A 27Al MAS NMR signal ascribed to distorted tetrahedrally coordinated Al (DTetrAl) was detected. By comparison with amorphous silica–alumina and alumina materials, in which no such signal was seen, the presence of DTetrAl species could be a typical feature in crystalline zeolites after activation and dealumination. An ammonium–water solution treatment under pressure in an autoclave was introduced; besides realumination of the activated zeolite, this treatment can transform six- and five- coordinate Al species to a tetrahedral coordination state, both in dealuminated zeolites and in amorphous silica–alumina, but not in alumina. From these experimental results we suggest that: (i) amorphous silica–alumina is a better model for the extra-framework material in dealuminated zeolites than alumina; (ii) DTetrAl species, which are absent in thermally treated amorphous silica–alumina, might be regarded as transitional from the crystalline framework lattice state to the amorphous silica–alumina state during the process of dealumination.