Study of copper(2+) location in zeolites Na-A and K-A by electron spin resonance and electron spin echo spectroscopies

Abstract

Electron spin resonance (ESR) and electron spin echo (ESE) spectroscopies are used to determine the cation site location of Cu/sup 2 +/ in hydrated and dehydrated forms of Cu/sup 2 +/-doped zeolites Na-A and K-A. This is achieved by partial exchange of the Na-A and K-A zeolites by Cs/sup +/ cations which reside in well-defined crystallographic sites. Monitoring the weak hyperfine interaction between the Cu/sup 2 +/ and Cs/sup +/ cations using ESE makes it possible to determine the cation siting of Cu/sup 2 +/ relative to the matrix of Cs/sup +/ cations. At low Cu/sup 2 +/ loadings the Cu/sup 2 +/ favor sites on the threefold axis close to a zeolite six-ring. The larger the number of water molecules coordinated to the Cu/sup 2 +/ the more the displacement of the Cu/sup 2 +/ from the center of the six-ring face - except in the case of a trigonal-bipyramidal Cu/sup 2 +/ complex coordinated to one water molecule in the ..cap alpha..-case, one in ..beta..-cage, and three equatorial lattice oxygens which is located in site S2. An octahedrally coordinated Cu/sup 2 +/ complex in both zeolites, Na-A and K-A, coordinated to three H/sub 2/O molecules is displaced 0.09more » nm into the ..beta..-cage while a tetrahedrally coordinated Cu/sup 2 +/ complex bound to only one water molecule in K-A is displaced 0.02 nm into the ..beta..-cage. After complete dehydration the Cu/sup 2/ moves almost into the plane of the six-ring to give the strongest possible cation-zeolite interaction with a Cu/sup 2 +/-O/sub z/ bond length of 0.23 nm (O/sub z/ = zeolite oxygen).« less