Rearrangement of Cationic Sites in Cuh-ZSM-5 and Reactivity Loss upon High-Temperature Calcination and Steam Aging

• Published in: Journal of catalysis 1995-12, Vol.157 (2), p.603-610
• Main Authors: Kucherov, A.V., Hubbard, C.P., Shelef, M.
• Format: Article
• Language: English
• Subjects: Catalysis; Catalysts: preparations and properties; Chemistry; Exact sciences and technology; General and physical chemistry; Ion-exchange; Surface physical chemistry; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Zeolites: preparations and properties
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1006/jcat.1995.1325

The state and reactivity of isolated copper ions in Cu-ZSM-5 treated at elevated temperatures were monitored in situ by ESR under flowthrough conditions. Steam aging (620-630°C, 17 h) or dry calcination (850-900°C, 30 min) induces a change in Cu2+ coordination without noticeable agglomeration or encapsulation of the isolated ions. All Cu2+ ions remain accessible to gas-phase molecules: O2 causes dipole-dipole signal broadening; H2O sorption leads to the increase of the local crystal field symmetry; and the admission of CCl4 results in a noticeable change in CU2+ local coordination. At the same time, the reactivity of these altered CU2+ sites and their ability to adsorb different molecules is substantially affected. No stabilization of nitrosyl complexes on specimens calcined at 900°C was observed, distinct from the case of Cu-ZSM-5500. The bond strength between an altered Cu2+ site and the strong ligand-NO2 is decreased. The sorption of C3H6 on Cu-ZSM-5900 at 20°C is not accompanied by noticeable reduction of CU2+ sites. Even at 500°C in a [C3H6 + He] flow a noticeable part of copper ions (20-30%) preserves the CU2+ state. A partial stabilizing effect of the Cu2+ cations on framework Al3+ in ZSM-5 is demonstrated: the steam aging of pure H-ZSM-5 at 650°C results in an almost total destruction of the sites where Cu2+ ions may be exchanged. The introduction of the Cu2+ before steaming preserves these sites from total destruction and only a transformation in local topography takes place. The surface moieties, containing both CU2+ and Al3+ ions, are a part of the zeolitic framework but must also be "flexible" enough to change symmetry upon additional ligand bonding. The high-temperature calcination or steaming at intermediate temperatures appears to affect this ability. The practical implications are discussed.