Location of extra-framework Co2+, Ni2+, Cu2+ and Zn2+ cations in natural and dealuminated clinoptilolite

In the present work a combination of chemically constrained Rietveld refinement and computer simulation is used for the determination of the positions of the extra-framework cations Co2+, Ni2+, Cu2+ and Zn2+ in the channels of clinoptilolite. The analysis of the metal rich clinoptilolites with two different Si/Al ratio (4.7 and 7.8) demonstrates that most of the cations are located in two extra-framework sites, the first one in the center of channel A and the second one in the center of channel B. In the case of the Cu2+ exchanged zeolite with high Si/Al ratio (4.7), a third site is found. This site is located at 1.65 Å of the center of the A channel and can be considered as a highly disordered first site. All the metal ions are coordinated to water molecules with no coordination to the zeolite framework. The presence of hydronium cations controls the relative occupancy of the transition metals in the observed sites.

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► We study Co2+, Ni2+, Cu2+ and Zn2+ extra-framework cation sites in clinoptilolite.
► The cations are located in two sites, in the centers of channels A and B.
► They do not coordinate to the zeolite framework but only to water molecules.