Oxidation States of Copper Ions in ZSM-5 Zeolites. A Multitechnique Investigation

The redox behavior of Cu/ZSM-5 zeolites prepared by ion exchange from Cu²⁺ aqueous solutions has been followed by a variety of spectroscopic techniques to provide a thorough picture of the so-called "self-reduction" of cupric ions, which occurs upon dehydration of the hydrated system at various temperatures, and of the reverse process of reoxidation as well. Conflicting hypotheses on both these processes are, in fact, present in the literature. The experimental techniques employed in this work are electron paramagnetic resonance (EPR), IR, and optical spectroscopies, extended X-ray absorption fine structure, and X-ray absorption near-edge structure. The early stages of dehydration (from room temperature to about 470 K) involve cupric ion migration and formation of EPR silent moieties but no reduction to Cu⁺. The onset of this latter phenomenon starts at 470 K and, in the range 470-670 K, involves the majority of copper ions present in the system. Rehydration of Cu⁺-containing samples does not cause direct Cu⁺ oxidation to Cu²⁺ but favor this latter process when O₂ is used as oxidant. Oxidation of Cu⁺ to Cu²⁺ by molecular oxygen, in fact, does not take place at room temperature if O₂ is contacted with the dehydrated material but easily occurs when oxygen is adsorbed on rehydrated samples at the same temperature.