Formation of new copper(II) species during propylene oxidation on copper(II)-exchanged X zeolite

The migration of copper(II) species and their coordination complexes in the zeolite framework was investigated during propylene oxidation on CuK-X zeolite in a flow system under conditions of excess oxygen and correlated with the reactivity over the temperature range of 100-400°C. Electron spin resonance (ESR) and electron spin echo modulation (ESEM) are used to monitor the different Cu(II) species generated, their stereochemistry, their site locations, their migration, and the coordination during the reaction in K-X zeolite. The development of three new Cu(II) species has been observed by ESR during the reaction depending upon the reaction temperature. One new cupric ion species developed at relatively low reaction temperature is assigned to a copper(II)-CO₂ complex in site SII′ in the β-cage. At higher reaction temperature, the development of another new cupric ion species with g_⊥ > g_∥ is assigned to a trigonal bipyramidal cupric ion in site SII between the α- and β-cages. This cupric ion coordinates 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 20-40% propylene conversion. A third Cu(II) species developed at still higher reaction temperature is suggested to be a copper(II)-CO₃ complex in site SII* in the α-cage on the basis of CO₂ adsorption experiments and ESEM analysis.