Cu-ZSM-5 catalyst synthesis via ion-exchange with ammonia solution of copper hydroxysalts

Abstract

The properties of dihydroxycarbonate (malachite) and trihydroxynitrate of copper were studied in order to apply their ammonia solutions for the synthesis of copper-substituted ZSM-5 zeolites. Copper concentration and NH4OH/Cu2+ ratio in the solutions varied 1–12 g/l and 6–30, respectively. The parameters of Langmuir, Freundlich, Dubinin-Radushkevich, and Temkin isotherms revealed the favorability of the chosen conditions and the chemical nature of [Cu(NH3)n]2+ ions sorption by zeolite. For both copper salts, the sorption capacity of H-ZSM-5 for Cu2+ ions was higher in the solutions having NH4OH/Cu2+ = 6–10 compared to 30. To identify the synthesis and structure – activity relationship correlations, the ammonia solution and Cu-ZSM-5 samples were characterized by EPR, UV–visible spectroscopy, temperature-programmed reduction by hydrogen (H2-TPR). The calcined Cu-ZSM-5 samples produced at NH4OH/Cu2+ = 6–30 contained two types of the isolated Cu2+ ions (bare Cu2+ and [Cu(OH)]+), differing in their Redox properties. Their reducibility improved with an increase in the copper loading and with a decrease in NH4OH/Cu2+. At NH4OH/Cu2+ = 6–10, the samples additionally comprised the structures of Cu2+ ions with extra-lattice oxygen (ELO), which were formed due to hydrolysis of [Cu(NH3)n(H2O)]2+ to [Cu(NH3)n(OH)]+ and then to [Cu(OH)2Cu]2+. Cu2+–ELO structures possessed a greater ability to complete reduction than both types of isolated Cu2+ ions. Correlations with DeNOx properties in selective catalytic reduction (SCR) of NO by propane and ammonia are discussed.