Exchange of Co(II) ions in H-BEA zeolites:: identification of aluminum pairs in the zeolite framework

The Co(II) ion exchange in NH4-, HNa- and H-BEA zeolites (Zeolite beta, BEA) was employed to monitor framework Al pairs in these zeolites balancing the charge of the divalent cations. The character and concentration of unperturbed bridging OH groups or those perturbed via hydrogen bonding, and Lewis sites (defective and tricoordinated framework Al) of NH4-, HNa- and H-BEA zeolites, and their changes induced by Co(II) ion exchange were investigated by quantitative analysis of IR spectra in the region of OH groups and vibrations of C=N groups of adsorbed d(3)-acetonitrile. It is shown that the Co(II) ions in dehydrated BEA zeolites, requiring two AlO2- framework groups for their charge balance, are at cationic sites which were occupied in the parent zeolites by Na ions or by mutually interacting protons or by those exhibiting a high tendency to be dehydroxylated with the formation of Lewis sites. A part of the unperturbed bridging OH groups is not exchanged by divalent Co(II) ions, indicating the far distances of the respective framework aluminum atoms. This indicates that the framework local structures bearing close AlO2- groups in BEA represent an unstable environment with weakened framework Al-O bonds. Metal cations, both Na(I) and Co(II) ions greatly prefer occupation of cationic sites containing two close framework AlO2- groups. Thus metal ion exchange brings about a high stabilization of the BEA framework, and protects the local environment of the aluminum framework against substantial rearrangements leading to the formation of Lewis sites. (C) 2001 Elsevier Science B.V. All rights reserved.