Liquid-Phase Methane Peroxidation in the Presence of Cu-ZSM-5: Effect of Modification with Palladium

The properties of Pd-modified Cu-ZSM-5 catalysts in liquid-phase methane peroxidation have been studied. The catalysts were prepared by polycondensation of Cu2+ and Pd2+ ions in the zeolite pores. Solutions of ammonia or ammonium complexes of one of the introduced metals were used as an alkaline agent promoting the polycondensation of hydrated metal ions. The electronic states of Cu2+ and Pd2+ ions and the acidic and redox properties of the catalysts, as well as their textural characteristics, were studied. Associated Cu2+ ions and polynuclear hydroxo/oxo complexes of Cu2+/Pd2+ ions were found to form in the pores during polycondensation, which are located in the channels and on the surface of zeolite crystallites. The addition of Pd affects the redox, acidic, and catalytic properties of Cu-ZSM-5, but this effect does not always lead to increased selectivity of the process with respect to liquid oxygenates (methanol and formic acid). Among the bimetallic catalysts, Cu/Pd-ZSM-5 was the most active catalyst, which was most selective for liquid oxidation products; it was synthesized by introducing [Cu(NH3)4]2+ ions after Pd2+ ions. It was assumed that its catalytic characteristics are associated with the lower strength of Cu2+–LAS and higher stability of isolated Cu2+ ions and polynuclear PdO-like clusters against reduction compared with the same characteristics of the sample obtained by simultaneous introduction of Cu2+ and Pd2+.