Author:

Keywords:

mordenite, aluminum coordination, mq mas nmr, dealumination, angle-spinning nmr, zeolite-y, cross-polarization, double-rotation, spectroscopy, catalysts, state, sites, aluminosilicates, faujasites, Science & Technology, Physical Sciences, Technology, Chemistry, Applied, Chemistry, Physical, Nanoscience & Nanotechnology, Materials Science, Multidisciplinary, Chemistry, Science & Technology - Other Topics, Materials Science, MQ MAS NMR, CROSS-POLARIZATION, DOUBLE-ROTATION, ZEOLITE-Y, STATE, SITES, SPECTROSCOPY, USY, 03 Chemical Sciences, 09 Engineering, Materials, 34 Chemical sciences, 40 Engineering

Abstract:

Al-27 MAS and MQ MAS NMR spectroscopy were used to study the aluminum coordination in dealuminated mordenite. A Al-27 MAS NMR signal ascribed to distorted tetrahedrally coordinated Al (DTetrA]) 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. (c) 2005 Elsevier Inc. All rights reserved.