Abstract:
The bulk susceptibilities of cobalt-exchanged zeolites Y and A have been measured over a wide range of temperature using an enclosed Gouy balance.
The magnetic properties indicate that the cobalt ions arc octahedrally coordinated in fully hydrated CoY. On dehydration, the reciprocal susceptibility rises and reaches a maximum when the number of residual water molecules approximately equals the number of cobalt ions. Further dehydration causes the reciprocal susceptibility to fall. It is believed that the maximum in the reciprocal susceptibility corresponds to most cobalt ions being in a tetrahedral or near tetrahedral environment. This coordination is provided by three oxygens from the zeolite framework together with one oxygen from a residual water molecule. Further dehydration removes the remaining water molecules and the final fall in reciprocal susceptibility indicates a migration of the cobalt ions from the tetrahedral coordination to the octahedral coordination associated with the hexagonal prisms of the framework.
The behaviour of CoA is broadly similar to CoY in the initial stages of dehydration. However, there is no fall in the reciprocal susceptibility during the final stages of dehydration. Since there are no hexagonal prisms in the zeolite A structure, the absence of a final decrease of reciprocal susceptibility supports the interpretation of the CoY results.
