The state of excessively ion-exchanged copper in mordenite: Formation of tetragonal hydroxy-bridged copper ion

Copper ions are exchanged in mordenite in amounts in excess of the value expected from stoichiometric considerations in the process of repeated ion-exchange of sodium ions in mordenite with copper ions. This is not the case for nickel and calcium ions used as exchanger ions. A series of experiments was designed to elucidate the state of excessively ion-exchanged copper in mordenite. DRS and EPR analyses together with XANES spectra revealed that the oxidation state of copper ion in mordenite is divalent in each exchange stage. IR, EPR, XANES and EXAFS studies provided evidence for the existence of a tetragonal hydroxy-bridged polymer of copper ions in excessively ion-exchanged mordenite; the Cu-O bond length is 1.97 Å and the Cu-Cu distance is 3.05 Å. This polymer species exhibited IR absorption bands near 3350 and 930 cm^-1 for OH stretching and MOH bending vibrations of bridged species, respectively. The difference in exchange behaviour of divalent metal ions is interpreted in terms of the difference in the magnitude of their hydrolysis constants.