## 3 Results and Discussion  
### Structure Study  
The crystalline phases and structure of the parent CHA zeolite and Cu-modified CHA zeolites at different conditions were studied using XRD and \({}^{27}\)Al NMR, respectively. The XRD patterns shown in Figure 2 revealed that all the as-synthesized CHA zeolites featured diffractograms identical to the well-known XRD patterns of the CHA framework [26], indicating that the as-synthesized CHA zeolites were successfully fabricated. After the ion exchange with ammonium nitrate (NH\({}_{4}\)-CHA) and potassium (K-CHA), the XRD patterns remained unchanged, as evidenced by the fact that no new diffractogram peaks or changes were observed and, therefore, the fingerprint of the CHA structure with enhanced peak intensities was maintained during the ion exchange. Generally, loading the Cu to the CHA zeolite in both forms enhanced the relative crystallinity with a noticeable enhancement when the loading was carried on the ammonium form (NH\({}_{4}\)-CHA), as presented in Table 1. However, although Cu was loaded onto both the K-CHA and NH\({}_{4}\)-CHA zeolite forms, the XRD diffraction patterns of their Cu-modified forms still corresponded mainly to the single K-CHA and NH\({}_{4}\)-CHA zeolite phases. This is likely related to the lower loadings of Cu onto the zeolite forms.  
For studying the distribution of aluminum in the CHA structure, \({}^{27}\)Al NMR was used. As XRD could not give an account of and indications where Al was located in the structure of all the synthesized zeolite forms, the coordination of aluminum can be determined by the \({}^{27}\)Al NMR spectrum, which has signals at a chemical shift of ca. 50-60 ppm and ca. 0 ppm for tetrahedral and octahedral coordination, respectively [27; 28; 29]. In an attempt to understand the zeolite structure-activity relationship, the location of aluminum atoms in the catalyst framework, the nature or specificity of aluminum bonding in the framework, the closeness of Al framework sites, and the distribution and structure of Al species in non-framework structures are critical factors [30]. In addition, the overall reactivity and activity of the zeolite catalyst may also be influenced by the closeness of framework and non-framework of Al species. Accordingly, in a typical zeolite structural analysis, \({}^{27}\)Al NMR excitation is used to probe the distribution of Al atoms between framework and non-framework sites in zeolite catalysts. Figure 3 shows the \({}^{27}\)Al NMR spectra of all Cu-CHA catalysts in this context. The Cu-CHA catalysts that were ion-exchanged in the ammonium form showed the presence of tetrahedrally coordinated framework Al at bridging acid  
Figure 2: XRD patterns of parent CHA zeolites and CHA zeolite catalysts ion-exchanged with Cu solution at different conditions; K-CHA: CHA as-synthesized, NH\({}_{4}\)-CHA: CHA after ion exchange with ammonium nitrate.  
sites in contrast to non-framework octahedrally coordinated Al at the integrated areas of the \(\sim\)56 and 0 ppm spectral peaks, respectively. A similar result was also observed and reported in the literature [31]. A main spectrum around 56 ppm has been attributed to the tetrahedral coordination of aluminum, while a small peak of \(\sim\)0 ppm in all the samples designating the presence of an extra framework of aluminum assumed to be non-framework octahedrally coordinated Al in the zeolite structural configuration [32]. The presence of an extra framework in NH\({}_{4}\)-form zeolite catalysts resulted from dealumination, which occurred during ion exchange with ammonium ions from the ammonium nitrate solution, as earlier reported in the literature [25]. On the other hand, a different scenario was observed with the K-form of CHA zeolite, as only one peak was observed at ca. 56 ppm, corresponding to the tetrahedral coordination of aluminum in the zeolite catalysts.