Encapsulation of ultra-small Cu–Fe into ZSM-5 zeolites for NH₃-SCR with broad reaction-temperature ranges

A direct hydrothermal synthesis strategy is developed to prepare the Cu–Fe-ZSM-5 zeolites by using the ethylenediaminetetraacetic acid disodium dihydrate (EDTA-2Na·2H₂O) chelated Cu²⁺ and Fe³⁺ precursors as metal sources. During the crystallization, the metal chelates were spontaneously embedded into the ZSM-5 crystals and subsequently transformed to the zeolites encapsulated copper and iron species after removing the chelating agent via the high-temperature calcination. Rigorous characterizations indicate that the resulted Cu²⁺/Cu⁺ and Fe³⁺/Fe²⁺ species are formed ultra-small nano-composites with an average diameter of ∼2.2 nm and highly dispersed in the ZSM-5 zeolite crystals. The acidity of the ZSM-5 zeolites is subtly modulated by the encapsulated Cu–Fe nano-composite. As expected, the as-prepared Cu–Fe-ZSM-5 exhibits outstanding catalytic performance for the selective catalytic reduction of NO_x with ammonia (NH₃-SCR) in the broad reaction temperature ranges of 220–480 °C, which is much better than the Cu–Fe/ZSM-5 prepared with the conventional wetness impregnation method. Furthermore, the resultant zeolites show high stability and good tolerance to H₂O and SO₂, and it is very competitive for practical application to eliminate NO_x. The developed one-pot hydrothermal synthesis route by using EDTA-2Na·2H₂O as chelating agent is very convenient and easily reproducible, providing a general guideline to design and fabricate the superior transition metal modified zeolite catalysts for NH₃-SCR and the related reactions.