REARRANGEMENT OF CATIONIC SITES IN CUH-ZSM-5 AND REACTIVITY LOSS UPONHIGH-TEMPERATURE CALCINATION AND STEAM AGING

The state and reactivity of isolated copper ions in Cu-ZSM-5 treated a t elevated temperatures were monitored in situ by ESR under flowthroug h conditions, Steam aging (620-630 degrees C, 17 h) or dry calcination (850-900 degrees C, 30 min) induces a change in Cu2+ coordination wit hout noticeable agglomeration or encapsulation of the isolated ions, A ll Cu2+ ions remain accessible to gas-phase molecules: O-2 causes dipo le-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 re activity of these altered CU2+ sites and their ability to adsorb diffe rent molecules is substantially affected, No stabilization of nitrosyl complexes on specimens calcined at 900 degrees C was observed, distin ct from the case of Cu-ZSM-5(500). The bond strength between an altere d Cu2+ site and the strong ligand-NO2 is decreased. The sorption of C3 H6 on Cu-ZSM-5(900) at 20 degrees C is not accompanied by noticeable r eduction of CU2+ sites, Even at 500 degrees C in a [C3H6 + He] flow a noticeable part of copper ions (20-30%) preserves the CU2+ state. A pa rtial stabilizing effect of the Cu2+ cations on framework Al3+ in ZSM- 5 is demonstrated: the steam aging of pure H-ZSM-5 at 650 degrees C re sults in an almost total destruction of the sites where Cu2+ ions may be exchanged. The introduction of the Cu2+ before steaming preserves t hese sites from total destruction and only a transformation in local t opography takes place. The surface moieties, containing both CU2+ and Al3+ ions, are a part of the zeolitic framework but must also be ''fle xible'' 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 discuss ed. (C) 1995 Academic Press, Inc.