Transformations of Cu/ZSM-5 system upon high-temperature reductive and oxidative treatments

Interaction of O-2 with Cu/ZSM-5 samples (0.5-2.8 wt.% Cu) precalcined in air at 500 or 800 degrees C and then reduced under different conditions was studied. The treatment of the samples precalcined at 500 degrees C by H-2 at the same temperature leads to a quantitative reduction of copper with formation of large (30-60 nm) Cu-0 crystals. A milder treatment of CuH/ZSM-5/(500) (300 degrees C, C3H6) also results in a total reduction of Cu2+, but a noticeable portion of the copper (about 20%) is retained inside the zeolitic channels either as disperse Cu-0 crystals or as Cu+ ions. For the CuH/ZSM-5 samples precalcined above 800 degrees C, the treatment by H-2 or C3H6 at 500 degrees C results in the reduction of only about 3/4 of the CU2+ to Cu-0, and about 1/4 of the cupric ions preserve the valent state Cu2+. The reducibility of isolated cupric cations is related to the coordination state of Cu2+ in ZSM-5, which in turn, depends on the temperature of the oxidative precalcination.