DISCUSSION ON THE COORDINATION OF NI2-TYPE ZEOLITES FOLLOWED BY DIFFUSE-REFLECTANCE SPECTROSCOPY( IONS TO LATTICE OXYGENS IN CALCINED FAUJASITE)

The coordination of Ni2+ ions to lattice oxygens in calcined X and Y z eolites has been extensively studied. A survey of the literature is pr esented here. From X-ray diffraction, Ni2+ ions are known to occupy pr eferentially the hexagonal prisms (SI) in octahedral symmetry. The mai n d-d absorption bands of Ni2+ ions in calcined NiX and NiY were assig ned on this basis. The corresponding low value of the crystal field (1 0Dq approximate to 6400 cm(-1)) was accounted for by unusual long Ni-O bond lengths at S-I sites. We demonstrate here that this assignment i s not consistent with recently reported extended X-ray absorption fine structure measurements End that the absorption bands should be reassi gned to Ni2+ cations in distorted tetrahedral and trigonal (D-3h) coor dinations. These cations are located in the supercages and/or sodalite cavities.