Study of propane oxidation on Cu-zeolite catalysts by in-situ EPR and IR spectroscopies

Three Cu-zeolites with different structure, Cu-TNU-9, Cu-ITQ-2 and Cu–Y have been tested as catalysts for propane oxidation reaction. The activity follows the trend: Cu-TNU-9>Cu-ITQ-2>Cu–Y, and in situ electron paramagnetic resonance (EPR) and infrared (IR) spectroscopies have been used to ascertain the origin of their different behavior. The IR spectra show bands ascribed to the formation of COO− and –CHO adsorbed on the Cu-zeolites as reaction intermediates in the oxidation of propane with an intensity that follows the same tendency that their oxidation activity. The EPR spectra of the three Cu-zeolites show that about 40–50% of total copper is present as isolated Cu2+ species, and heating under propane or propane–oxygen mixture at 350°C provokes the reduction of Cu2+ to Cu+ following the same trend that the oxidation activity, i.e., Cu-TNU-9>Cu-ITQ-2>Cu–Y. Further analysis of the EPR spectra suggests that the reducibility of exchanged Cu2+ cations in the Cu-zeolites is determined by their accessibility to the propane molecules.