Catalytic and electron spin resonance studies of propylene oxidation on copper(II)-exchanged X zeolite: Temperature dependence of copper(II) migration

The copper(II)-catalyzed oxidation of propylene on CuK-X zeolite was studied under conditions of excess oxygen over the temperature range of 100 to 400°C in a flow system to investigate the temperature dependence of the catalytic activity and the migration of copper(II) species and Cu(II) coordination complexes in the zeolite framework. It is shown that the percent of propylene conversion depends upon the reaction temperature and upon the extent of Cu(II) exchanged into the zeolite. Temperature causes the cupric ions to migrate near the six-ring window between the α and β cages of the zeolite structure where they complex with propylene and induce the oxidation reaction. Arrhenius activation energies decrease with increasing copper(II) content. The decreasing activation energy with decreasing Cu(II) content is associated with different site occupancies of Cu(II) within the zeolite structure. Electron spin resonance studies show that cupric ions are initially distributed in SI hexagonal prism sites and SI′ sites in the β cage adjacent to the hexagonal prisms in dehydrated CuK-X zeolite. During reaction, Cu(II) migrates from these sites to sites SII′ and SII near the six-ring window between the α and β cages at which site coordination with propylene, constrained to the α cage, occurs prior to reaction. Cu(II) is reduced to Cu(I) by propylene to form a Cu(I) complex, which is subsequently oxidized by oxygen to regenerate Cu(II) and to yield CO₂ and water as reaction products. A new cupric ion species with g_∥ = 2.267 and A_∥ = 183 × 10^-4 cm^-1 developed during the reaction. This is proposed to be a cupric ion in site SII′ in the β cage coordinating to CO₂ located in the α cage of the zeolite structure based on CO₂ adsorption experiments and on electron spin-echo modulation analysis.