Identification of two types of exchangeable sites for monovalent copper ions exchanged in MFI-type zeolite

Abstract

Three different approaches have been used to characterize the state of exchanged copper ions in copper-ion-exchanged MFI (CuMFI) samples. (1) Two types of an ion-exchangeable site with different adsorption properties for N₂ or CO molecules were identified depending on the pre-treatment temperature (723 or 873 K) of a sample prepared by using an aqueous solution of CuCl₂. (2) The state of the active sites formed by the evacuation of a sample at 873 K that had been prepared using a mixture solution of aqueous NH₄CH₃COO and Cu(CH₃COO)₂ was analysed utilizing both ¹³C¹⁸O and ¹²C¹⁶O to identify the two types of active adsorption sites for CO molecules. (3) CuMFI samples prepared by the ion-exchange method employing anhydrous CuCH₃COO showed a surprising adsorption feature characterized by a single IR band occurring at 2159 cm⁻¹ due to the adsorbed CO molecules, but there was no corresponding IR band due to adsorbed N₂ molecules. A successful preparation of CuMFI, in which the monovalent copper ions exclusively occupied another one of the two types of ion-exchangeable sites, was also carried out utilizing the solid-ion exchange method using Cu(CH₃COO)₂·H₂O. This site exhibits an IR band occurring at 2151 cm⁻¹ for CO molecules and also acts as an active site for N₂ molecules. These experimental data correlate, and clearly indicate that there are at least two types of exchangeable sites for copper ions in MFI-type zeolites.