INFLUENCE OF LA3-ZSM-5 ZEOLITE AND REACTIVITY LOSS UPON HIGH-TEMPERATURE TREATMENTS(, CE4+, GA3+, AND BA2+ IONS ON REARRANGEMENT OF CATIONIC SITES IN CUH)

The state and reactivity of isolated copper ions in pure and La-, Ce-, Ga-, or Ba-promoted CuH-ZSM-5 zeolites treated at elevated temperatur es were monitored by in-situ ESR under flow conditions. Steam-aging (6 30 degrees C, 17 h) or dry calcination (850-900 degrees C, 30 min) of monocationic CuH-ZSM-5 induce an irreversible change in the Cu2+ coord ination without noticeable agglomeration or encapsulation of the isola ted ions, with disappearance of practically all the most reactive squa re-planar Cu2+ ions. All Cu2+ ions remain accessible to adsorption of O-2, H2O C3H6, and NO2 molecules from the gas phase, with a substantia l loss of adsorption strength on the altered Cu2+ sites. The catalytic activity of these altered Cu2+ sites in total ethane oxidation also d rastically decreases. A substantial retardation of high-temperature re arrangement of the local structure of Cu2+ sites is noted for the CuH- ZSM-5 promoted by a relatively large amount (5 wt %) of rare-earth ele ments, Like La or Ce ions, in this case, a noticeable part of Cu2+ ion s (20-30%) preserves the parent square-planar Cu2+ state even after th e sample calcination at 850 degrees C for 30 min. No stabilizing effec t of Ga and Ba additives is found. The catalytic activity of the sampl es in complete ethane oxidation correlates very well with the number o f square-planar Cu2+ cations retained by the samples after different t reatments.