Insights into the nuclearity and local environment of transition metal species in ion-exchanged CHA zeolites and effects on the oxidation of methane

Ion-exchange method is a popular and facile way to prepare metal-containing zeolites, and transition-metal-exchanged zeolites have been widely used for the activation of small molecules such as CO₂, CH₄, and NH ₃ , focusing on their transformation into valuable products or environmentally friendly chemicals. Understanding the exchange way between metal ions and zeolite and inferring the local environment of metal species is crucial to comprehending and predicting the activity of metal-exchanged zeolite catalysts. Herein, the frequently employed early transition metals (Fe ³⁺ , Co ²⁺ , and Cu ²⁺ ) were adopted to exchange with CHA-type aluminosilicate zeolite (SSZ-13). The acidity of transition-metal-exchanged CHA zeolites was measured by NH ₃ -TPD, and the metal speciation was identified by HAADF-STEM images, UV-vis, and XAS spectra. The linear trend line of the strong acid content and the metal/Al ratio of exchanged catalysts was used to calculate the ion-exchanged degree between metal cations and acidic protons of zeolite. The exchanged ratios of n :1 ( n ˃2), 1:2, and 1:1 for Fe, Co, and Cu/CHA zeolites specified the possible nuclearity and local environment of metal species. Therefore, these zeolite catalysts exhibited completely different reaction characteristics toward direct and continuous oxidation of methane using N ₂ O as the oxidant. Taking full advantage of these features, high performance in N ₂ O decomposition, direct conversion of methane to hydrocarbon via methanol as the intermediate, and direct conversion of methane to methanol with high methanol selectivity were realized by Fe, Co, and Cu-exchanged CHA zeolites, respectively. These findings provide guidance and reference for preparing metal-ion-exchanged zeolite catalysts and their suitable applications.