Effect of the electronic state and copper localization in ZSM-5 pores on performance in NO selective catalytic reduction by propane

This study was devoted to determination of localization of the individual copper species in the pores of ZSM-5 zeolite after post-synthetic zeolite modification by incipient wetness impregnation with CuCl2 solution followed by hydrolytic polycondensation of the ions in the pores. A series of Cu(n)ZSM-5 samples, where n is the copper concentration varied from 0.5 to 5.0 wt.%, were synthesized using this method. A commercial HZSM-5 zeolite having Si/Al = 17 was used for the synthesis of the catalysts. After modification the Cu(n)ZSM-5 samples were dried and calcined in air at 110 and 450 °C, respectively. The samples were studied by UV–vis–NIR DR spectroscopy, XRD and by measuring and processing the low-temperature (77 K) adsorption of nitrogen and hydrogen. It was shown for the first time that at the concentration of 1.0 wt.% or lower copper was localized in the form of isolated Cu2+Oh cations in the ion-exchange positions in the zeolite micro- and mesopores as well as on oxygen complexes of extraframework aluminum in thin mesopores with D < 3.2 nm in the form of superficial spinel-like structures in octahedral oxygen coordination Cu2+Oh. At the copper concentration above 1.0 wt.% linear associates of weakly bound Cu2+Oh ions with unusual orbital ordering were formed in addition to the two forms discussed above. These associates existing in the forms of one- and two-dimensional nanohydroxocompounds were localized partially in the zeolite micropores but mostly in thin mesopores.The effect of the individual copper species on the catalytic properties of Cu(n)ZSM-5 samples in selective catalytic reduction of NO by propane was also studied. It was found that isolated Cu2+Oh cations in the ion-exchange positions and Cu2+Oh cations localized on the oxygen complexes of extraframework aluminum had the highest catalytic activity in NO SCR by propane. The weakly bound linear associates of Cu2+Oh ions had lower catalytic activity, compared with isolated Cu2+Oh ions. When their amount was high, they caused steric hindrance for contact of the reagents with the more active sites. The Cu2+Oh ion associates were found to transform to nanodispersed Cu2(OH)3Cl during catalytic experiments.

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► Cu(n)ZSM-5 prepared by incipient impregnation with CuCl2 solution followed by hydrolytic polycondensation is active in NO SCR by propane.
► The fresh Cu(n)ZSM-5 contains (1) isolated Cu2+Oh ions on ion-exchange zeolite sites; (2) isolated Cu2+Oh ions on extraframework Al3+Oh; (3) weakly bound linear Cu2+Oh associates. The latter species transform to nanodispersed Cu2(OH)3Cl (4) during catalytic experiments.
► High catalytic activity in NO SCR by propane of Cu(0.5–1%)ZSM-5 is related to the isolated Cu2+Oh ions on ion-exchange zeolite sites and on extraframework Al3+Oh.
► Weakly bound linear Cu2+Oh associates have lower activity. They are active in NO2 SCR by propane, NO2 being formed during NO disproportionation or oxidation on isolated Cu2+.