CHARACTERIZATION AND CATALYTIC STUDIES ON DEFECT SITES FORMED UPON THE THERMAL-DECOMPOSITION OF SODIUM IONIC CLUSTERS IN NAX ZEOLITE

Sodium ionic clusters have been prepared by chemical vapor deposition of sodium metal at 225 degrees C and 1 x 10(-4) Torr in NaX zeolites. A maximum loading of approximately 1.2 wt % sodium was vaporized onto dehydrated NaX zeolite. Ionic clusters prepared in this study have bee n determined to have the formula Na-6(5+). In this complex the sodium ions are arranged in octahedral geometries in the sodalite cages of Na X zeolite. Identification of these species was accomplished by electro n paramagnetic resonance, magic angle spinning nuclear magnetic resona nce. Fourier transform infrared spectroscopy, luminescence spectroscop y, and X-ray diffraction experiments. These ionic clusters have a loca lized electron trapped in the center of the octahedra, giving rise to a color center. Infrared analysis of pyridine adsorption onto NaX and NaX exposed to sodium vapor (Na(v)/NaX) revealed that initial Bronsted acidities of untreated NaX zeolites are low and that a complete loss of Bronsted acidity is observed upon treatment with Na vapor. The cata lytic properties of materials exposed to sodium vapor have been studie d in isomerization reactions. Defect sites or holes on framework oxyge n sites are proposed as the source of catalytic activity at elevated t emperatures (>340 degrees C). A catalytic mechanism for isomerization of cyclopropane to propylene over positively charged defect sites asso ciated with Na-6(5+) clusters in Na(v)/NaX zeolites is discussed. Acti vation energies, based on Arrhenius plots for first-order reactions, a nd rates of propylene formation are determined for cyclopropane isomer ization reactions over defect sites in Na(v)/NaX catalysts. Two reacti on pathways are proposed for the treatment of sodium vapor on these ma terials. The dominating reaction produces Na-6(5+) clusters in the sod alite cages of NaX zeolites. In a second reaction, Na vapor reduces Br onsted acid sites on terminal hydroxyl groups. The latter reaction lea ds to the formation of hydridic species on framework silicon atoms at high temperatures and the generation of-electron holes on framework ox ygen. Activation of defect sites has been observed only at high temper atures. Electron paramagnetic resonance studies show that naphthalene, sublimed on defect sites, results in the formation of naphthalene rad icals.