Catalytic partial oxidation of propylene to acrolein over copper(II)-exchanged M-X and M-Y zeolites where M = Mg²⁺, Ca²⁺, Li⁺, Na⁺, K⁺, and H⁺: Evidence for separate pathways for partial and complete oxidation

The catalytic oxidation of propylene over copper(II)-exchanged X and Y zeolites in the presence of the different major cocations, Mg²⁺, Ca²⁺, Li⁺, Na⁺, K⁺, and H⁺, was studied as a function of the oxygen/propylene mole ratio in a flow system at 350 °C. The yield of acrolein was optimized for a ratio of unity. The catalytic activities and the changes in cupric ion species were studied by gas chromatography and electron spin resonance. The catalytic activity for this reaction is shown to be due to copper species and is greatly dependent on the type of major cocation in the zeolites. Y zeolites are slightly more effective for the selective oxidation of propylene to acrolein than the corresponding X zeolites. The dependence of the product yields and the Cu(II) concentration on the oxygen/propylene mole ratio indicates two parallel pathways for partial oxidation to acrolein and complete oxidation to carbon dioxide and water. It is suggested that partial oxidation is catalyzed by Cu(I), perhaps in a Cu₂O/CuO phase, and that complete oxidation is catalyzed by Cu(II). An induction period for acrolein formation is observed. Catalyst deactivation is also observed and associated with coke formation which can be monitored by a singlet electron spin resonance signal.