Temperature dependence of copper(II) migration and formation of new copper(II) species during catalytic propylene oxidation on copper(II)-exchanged Y zeolite and comparison with X zeolite

Electron spin resonance (ESR) and electron spin-echo modulation (ESEM) spectroscopies have been used to characterize the aqueous coordination and location of catalytically active Cu(II) cation-exchanged into K-Y zeolite. The higher Si/Al ratio in CuK-Y compared to CuK-X gives rather different ESR parameters for Cu(II). Under complete hydration, ESR indicates freely rotating Cu(II) ions in the α cage, and the corresponding ESEM indicates six directly bound water molecules. Under partial dehydration, ESR shows that Cu(II) becomes immobilized near the six-ring window of the zeolite, and the corresponding ESEM indicates coordination to three water molecules. Upon dehydration at high temperature, ESR shows that the cupric ions mainly move into the β cage and hexagonal prism. The Cu(II)-catalyzed oxidation of propylene over dehydrated CuK-Y zeolite with low Cu(II) exchange was studied over the temperature range of 160 to 400°C under conditions of excess oxygen in a flow system in order to investigate the temperature dependence of the catalytic activity, the migration of Cu(II) species, the Cu(II) coordination complexes formed in the zeolite framework, and the comparison with previous results on CuK-X zeolite. The catalytic activity of CuK-Y was comparable to that of CuK-X under the same conditions. The propylene conversion increased in the same fashion with the reaction temperature and the extent of exchanged Cu(II) for both X and Y type zeolites. ESR studies also show that Cu(II) migrates during reaction from the initial SI, SI′ sites to SII* and/or SII sites near a six-ring window at which site coordination with propylene occurs prior to reaction. The development of two new Cu(II) species has been observed by ESR during the reaction depending on the reaction temperature. The first new species is developed at relatively low reaction temperature and is suggested to be a Cu(O_z)₃CO₃ complex in site SII*. At higher reaction temperature, another new cupric ion species with reversed g values (g_⊥ > g_∥) is observed and is assigned to a trigonal-bipyramidal cupric ion in site SII coordinating to two hydroxyl groups and three zeolite framework oxygens to give a Cu(O_z)₃(OH)₂ complex. This species is best developed in the reaction temperature range corresponding to 15-50% propylene conversion. At still higher reaction temperature the two new species decompose and disappear, and the cupric ions are redistributed into the same sites as in the dehydrated state. These results are compared with those in X zeolite during cupric ion catalyzed propylene oxidation.