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.