STABILIZATION OF THE ETHANE OXIDATION CATALYTIC ACTIVITY OF CU-ZSM-5

The state of isolated copper ions in Cu-ZSM-5 containing additions of La, Ce, and Co was monitored in-situ by ESR under flow conditions. Tre atment by steam at 630 degrees C for 17 h or high-temperature dry calc ination at 850 degrees C induce an irreversible change in coordination for practically all square-planar Cu2+ ions in mono-cationic Cu-ZSM-5 without agglomeration or encapsulation of the isolated ions. All Cu2 ions remain accessible to gas-phase molecules, but the catalytic reac tivity of these altered copper sites decreases drastically. A stabiliz ing effect is noted for samples modified by a relatively large amount, ca. 5.0 wt.-%, of multivalent rare-earth ions La or Ce. Here a part o f the copper ions (20-30%) preserves the parent square-planar Cu2+ Sta te even after calcination at 850 degrees C for 0.5 h. The effect of ca . 1% La or Ce is much less pronounced. The catalytic activity in the c omplete oxidation of ethane correlates well with the number of square- planar cupric cations retained by the samples after different treatmen ts. The introduction of cobalt sharply increases the ethane oxidation activity of samples calcined at 500-650 degrees C.