## 1 Introduction
Although the SAPO-34 and SSZ-13 materials are examples of small pore-sized zeolites with the same structure (CHA topology), the composition difference results in different catalytic performances in the SCR reaction. For instance, Bronsted acid sites in SSZ-13 are related to the aluminum content in the framework. In contrast, the Bronsted site in SAPO-34 is controlled by introducing Si to the AlPO\({}_{\rm 4}\)-34 structure [15], as the introduction of Si at a higher concentration causes the formation of isolated pure Si structures that hold no acidity. Accordingly, controlling the acidity of SAPO-34 is more difficult than its counterpart, SSZ-13. Moreover, the form, location, and concentration of the copper (Cu) in the CHA structure generally plays a crucial role in the catalytic performance in NO\({}_{\rm x}\) SCR catalysis. The Cu might exist as isolated Cu\({}^{\rm 2+}\) in the CHA structure, which is the active form in most cases, or as copper oxides (Cu\({}_{\rm x}\)O\({}_{\rm x-1}\)) [16]. Even though SSZ-13 and SAPO-34 are structurally similar, the role of each form of Cu\({}^{\rm 2+}\) and CuO is different. [PERSON] et al. confirmed, by using electron paramagnetic resonance (EPR), that Cu\({}^{\rm 2+}\) was the active form of copper in SAPO-34 [17]. Conversely, in a similar study, it was found that both isolated Cu\({}^{\rm 2+}\) and CuO were the significant actors in contributing to the catalytic activity of Cu/SAPO-34 [18]. However, it was further found that CuO species were more active at lower temperatures while the isolated Cu\({}^{\rm 2+}\) species were preferentially active at higher temperatures [19]. This indicates that it is not only the material's intrinsic composition that controls which form of Cu is present in SAPO-34 but also the condition under which the material works. Additionally, apart from the Cu distribution and its chemical forms, inherent zeolite composition, such as the Si/Al ratio, affects the hydrothermal stability of the materials. For example, the low Si/Al ratio of CHA improved the material's stability and the widespread distribution of Cu\({}^{2+}\) over the material, making ammonia more reactive, enhancing the NOx SCR activity [13]. Therefore, the Si/Al ratio in the material influences the distribution and concentration of Cu species, their forms, and their locations in the CHA structure. At a lower concentration of Cu, isolated Cu\({}^{2+}\) might be in the six-member ring and function as an active site, while at a higher concentration, the Cu\({}^{2+}\) ions agglomerate to form CuO and exist in the eight-member ring. However, in SAPO-34, it has been found that both Cu\({}^{2+}\) and CuO coexisted even at lower concentrations of Cu loading. A higher Si/Al ratio gave rise to a higher concentration of Al sites in the six-member ring (6 MR). In comparison, with a lower Si/Al ratio, it is unlikely for the material to acquire two Al sites ready to be compensated by the Cu\({}^{2+}\)[20]. Consequently, Cu\({}^{2+}\) species are located in the 6 MR at a lower Si/Al ratio and in the 8 MR at a higher Si/Al ratio. Additionally, the chemical form in which Cu exists in the CHA framework also depends on the technique followed during Cu loading. Several approaches can be used to incorporate Cu into the structure of a zeolite, such as ion exchange, solid-state ion exchange, vapor deposition, wet mixing, impregnation, and in situ incorporation within the organic structure-directing agents. Among all of the methods mentioned, the ion exchange was the most effective approach for forming isolated Cu\({}^{2+}\) in CHA zeolite's structural framework. Cu-CHA zeolites have been used as an efficient catalyst for NOx reduction at temperatures higher than 200 \({}^{\circ}\)C. However, the major bottleneck in using CHA zeolite is the cost required to synthesize the CHA structures, which mainly revolves around using expensive organic structure-directing agents (OSDAs) [21; 22].