A new strategy for the improvement of the N₂ adsorption properties of copper-ion-exchanged MFI-type zeolites at room temperature

A copper-ion-exchanged MFI-type zeolite, CuMFI, which exhibits an extremely efficient adsorption of the dinitrogen (N₂) molecule at room temperature, has been successfully prepared by ion exchange in an aqueous solution of Cu(CH₃COO)₂ containing a component of NH ₄CH₃COO. This ion-exchange method has unique characteristics as follows; first, the copper ion exchange takes place exclusively on the ion-exchangeable site that acts as the active site for N ₂ adsorption, and second, the extent of the reduction of the divalent copper ion exchanged attains a high value of 88%. In addition, the CuMFI sample thus prepared exhibits a high efficiency for N₂ adsorption at 298 K where the monovalent copper ions act as adsorption sites; approximately 86% of them are effective for N₂ adsorption. In this case, the heats of adsorption of N₂ were large, 87-60 kJ mol^-1. These observations were rationalized as showing that the site-selective ion exchange with copper ions occurs, accompanying acetate ligands by which the effect of the size is brought, and that the divalent copper ions exchanged in this way can be reduced easily and efficiently to the monovalent ions; on that occasion the organic ligands incorporated in the ion-exchange process are decomposed, promoting the reduction of the copper ions. From the observations of photoluminescence spectra and X-ray absorption near-edge structure spectra in the processes both before and after N₂ adsorption, it was deduced that the three-coordinated monovalent copper ions are active sites for N ₂adsorption at room temperature, which was also supported by the results of both diffuse reflectance spectra and density functional theory calculations. The results obtained are expected to provide significant information for the development of materials that function efficiently as N ₂-fixation or N₂-activation catalysts as well as NO _x decomposition catalysts.