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

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.