New insight into N₂ adsorption and ion-exchange features of CuMFI with different Si/Al ratios

The effects of the Si/Al ratio of the mother MFI on the N₂ adsorption phenomena at 301 K and on the ion-exchange features of copper-ion-exchanged MFI zeolites (CuMFI) were investigated. The CuMFI samples were prepared by ion exchange in an aqueous solution of Cu(C₂H ₅COO)₂, Cu(CH₃COO)₂, CuCl ₂ or Cu(NO₃)₂. The sample with an Si/Al ratio of 19.8, which had been prepared by using a Cu(C₂H ₅COO)₂ solution, exhibited extremely efficient N ₂ adsorption in terms of both the number of adsorbed molecules and its energetic behaviour, as compared to samples with other Si/Al ratios and/or with other ion-exchange solutions. The coordination environment of the copper ion adsorbing N₂ on this sample was clarified for the first time by analysis of the EXAFS spectra. The IR and DR-UV-Vis spectra revealed that the state of exchanged Cu²⁺ in CuMFI differs remarkably, depending on the value of the Si/Al ratio of the MFI and the types of counter ion in the exchange solution. The Cu²⁺ species coordinated with propionate or acetate ion were preferentially ion-exchanged in samples with higher Si/Al ratios (i.e., 19.8 and above), in comparison with aqua complexes of Cu ²⁺ ions. From the measurements of heats-of-adsorption of CO and photoemission spectra, it was elucidated that the relative proportion of the number of the sites located in the neighbourhood of the three lattice oxygen atoms (i.e., three-coordinate sites) to the total number of exchangeable sites in MFI including the two-coordinate sites increased with an increase in the Si/Al ratio; Cu²⁺ with propionate or acetate ion were exchanged selectively on the three-coordinate sites. The DR-UV-Vis and EPR spectroscopic signatures of two different Cu²⁺ species located in the two- and the three-coordinate sites were determined.