3.2 Characterization of Cu-MOR catalysts
In order to identify the copper species present on the Cu-MOR catalysts, we employed a comprehensive array of techniques for sample characterization, including XRF, XRD, H2-TPR, UV-Vis, XPS, and HRTEM. The elemental composition of Si, Al, and Cu in the Cu-MOR catalysts was quantified using XRF, and the results are shown in Table S2. The Si/Al ratio of the purchased commercial MOR zeolite is approximately 17, and the Si/Al ratio of the Cu-MOR samples oscillates around this value. The Cu/Al ratio of the Cu-MOR samples increases slowly with increasing number of ion exchanges. Conversely, for the samples prepared via wetness impregnation, the Cu/Al ratio significantly rises with increasing Cu loading. Correlating these findings with the catalytic performance depicted in Figure 1, a volcano-type trend emerges, highlighting an optimal Cu loading range of ca. 2-4 wt%. The effects of Cu loading on the specific surface area and pore volume of MOR zeolite was investigated using N2-physisorption, and the results are also shown in Table S2. The specific surface area and pore volume of the Cu-MOR samples exhibit a slight decrease with increasing number of ion exchange cycles, primarily attributed to the higher presence of Cu species. Conversely, for the Cu-MOR samples prepared by wetness impregnation, a substantial decrease in specific surface area and pore volume is observed with increasing Cu loading. This drop may be attributed to the presence of CuO particles on the MOR support. The adsorption and desorption curves of MOR and Cu-MOR samples are depicted in Figure S2, illustrating that the MOR support is a typical microporous material with a microporous volume of 0.18 cm3g-1. The grain size and morphology of the MOR support and Cu-MOR IE-3 catalyst were characterized by SEM, as shown in Figure S3. It is evident that the grain size of the MOR zeolite is approximately 250 nm, and both the grain size and morphology of the MOR zeolite remain unchanged after Cu loading.
Figures 2a and 2b present the XRD patterns of the MOR support and Cu-MOR catalysts, synthesized through ion exchange and wetness impregnation methods, respectively. All samples exhibit well-defined diffraction peaks corresponding to MOR zeolite, indicating the preservation of the MOR lattice structure during the preparation process. In Figure 2a, the XRD patterns of Cu-MOR catalysts prepared by ion exchange reveal an absence of characteristic diffraction peaks associated with CuO, Cu2O, or Cu, suggesting that copper species are highly dispersed on MOR.17 In contrast, in Figure 2b, the XRD patterns of Cu-MOR catalysts prepared by wetness impregnation display distinct diffraction peaks of CuO (111) and CuO (-111). The intensities of these peaks increase proportionally with higher Cu loadings, indicating the formation of larger CuO particles at elevated Cu loadings.