Symmetries of the oxygen ligands of Ni(II) and Co(II) ions in synthetic A, X, and Y zeolites

Abstract

The magnetic susceptibility of Ni 2+ and Co 2+ ions in fully and partly dehydrated X, Y, and A zeolites has been measured over the temperature range from 4.2 to 300 K. Zeolites of various exchange levels (11–69%) have been investigated. A comparison of ionic radii of Ni 2+, Co 2+, and of O 2− shows that Ni 2+ and Co 2+ ions in dehydrated zeolites can assume C 3 v and C 4 v symmetries as well as the previously proposed (11, 12) O h and T d coordinations. The temperature dependences of the magnetic susceptibility of Ni 2+ and Co 2+ in C 3 v and C 4 v O 2− ligand symmetries were calculated from crystal field theory, using Van Vleck's formula. By comparison of the experimentally found χ M ( T) dependences with those theoretically calculated for C 3 v and C 4 v as well as with those already known for O h and T d symmetries, we have tried to determine a sequence of occupation and distribution of Ni 2+, Co 2+ ions in zeolites as depending on temperature, dehydration time and dehydration level. It was established that in fully dehydrated Y zeolite there is a preference for Ni 2+ ions to occupy C 4 v sites and for Co 2+ to occupy C 3 v sites, whereas in X and A zeolites no such preference to occupy specific sites is found. No relation between the quantity of exchanged ions and distribution or sequence of occupation of exchange positions seems to exist for A and X zeolites. In partly dehydrated zeolites the change of the χ M −1( T) slope with the degree of dehydration is similar to that described in other papers. It seems that the main factors controlling the process of occupation and distribution of the ion exchange sites are the parameters of the dehydration process and ion migration in the zeolites. In fully dehydrated zeolites the exchange sites of low coordination number and C 3 v or C 4 v symmetries are most probable on geometrical, electrostatic and chemical grounds.