Rearrangement of cationic sites in CuH-ZSM-5 and reactivity loss upon high-temperature calcination and steam aging
- Ford Motor Company, Dearborn, MI (United States)
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{degrees}C, 17 h) or dry calcination (850-900{degrees}C, 30 min) induces a change in Cu{sup 2+} coordination without noticeable agglomeration or encapsulation of the isolated ions. All Cu{sup 2+} ions remain accessible to gas-phase molecules. At the same time, the reactivity of these altered Cu{sup 2+} sites and their ability to adsorb different molecules is substantially affected. No stabilization of nitrosyl complexes on specimens calcined at 900{degrees}C was observed, distinct from the case of Cu-ZSM-{sub 500}. The bond strength between an altered Cu{sup 2+} site and the strong ligand-NO{sub 2} is decreased. The sorption of C{sub 3}H{sub 6} on Cu-ZSM-5{sub 900} at 20{degrees}C is not accompanied by noticeable reduction of Cu{sup 2+} sites. Even at 500{degrees}C in a [C{sub 3}H{sub 6} + He] flow a noticeable part of copper ions (20-30%) preserves the Cu{sup 2+} state. A partial stabilizing effect of the Cu{sup 2+} cations on framework Al{sup 3+} in ZSM-5 is demonstrated: the steam aging of pure H-ZSM-5 at 650{degrees}C results in an almost total destruction of the sites where Cu{sup 2+} ions may be exchanged. The introduction of the Cu{sup 2+} before steaming preserves these sites from total destruction and only a transformation in local topography takes place. The surface moieties, containing both Cu{sup 2+} and Al{sup 3+} ions, are a part of the zeolitic framework but must also be {open_quotes}flexible{close_quotes} enough to change symmetry upon additional ligand bonding. The practical implications are discussed. 31 refs., 9 figs., 2 tabs.
- OSTI ID:
- 224030
- Journal Information:
- Journal of Catalysis, Journal Name: Journal of Catalysis Journal Issue: 2 Vol. 157; ISSN 0021-9517; ISSN JCTLA5
- Country of Publication:
- United States
- Language:
- English
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