## 3 Results and Discussion  
### Impact of Hydrothermal Synthesis Under Different Conditions  
MIL-100(Fe) was prepared by the hydrothermal method at different reaction conditions. A comparison of the powder X-ray diffraction patterns of the as-prepared MIL-100(Fe)is shown in Figure 1. Furthermore, for comparison, experimental patterns of MIL-100(Fe) reported in the literature [9; 29; 30] (CIF file No. 7102029) and those of \(\alpha\)-F\({}_{2}\)O\({}_{3}\) were also added. The XRD spectrum of MF-S0, which used a smaller trimesic acid-to-iron precursor ratio compared to MF-S1, did not exhibit clear, prominent peaks typically associated with the MIL-100(Fe) structure. However, upon closer inspection of the acquired diffraction pattern, subtle peaks can be identified around 4\({}^{\circ}\) and 6.5\({}^{\circ}\) and the peaks at approximately 10.2\({}^{\circ}\) and 11\({}^{\circ}\), respectively, which can be attributed to the crystallographic features of MIL-100(Fe). These emerging peaks suggest the presence of this material, albeit not in a dominant form within the sample. In addition, some peaks at 24.1\({}^{\circ}\), 33.1\({}^{\circ}\), and 35.7\({}^{\circ}\) are clearly identifiable, related to the iron oxide phase (\(\alpha\)-Fe\({}_{2}\)O\({}_{3}\)) [31; 32]. This is consistent with the interpretation that the small amount of trimesic acid relative to the iron precursor led to the formation of MIL-100(Fe) with a high concentration of defects, impurities, and secondary reaction products. The sample prepared using MF-S1 exhibited an XRD pattern consistent with the MIL-100(Fe) pattern. This suggests that the obtained structure is compatible with MIL-100(Fe). However, some purity concerns could be addressed, as the peaks at small angles (at 3.5\({}^{\circ}\) and 4.0\({}^{\circ}\)) are less prominent than in the experimental pattern, which suggests that there may be some residual impurities from the synthesis process in the sample. The increase in the hydrothermal synthesis time up to 16 h in MF-S2 resulted in the formation of a mixture of MIL-100(Fe) and another crystalline phase. This phase was characterized by X-ray diffraction peaks at 20 angles of 24.1\({}^{\circ}\), 33.1\({}^{\circ}\), and 35.7\({}^{\circ}\), which, as previously discussed, correspond to the characteristic peaks of iron (III) oxide, specifically \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\). This suggests that the extended synthesis time promoted the formation of \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\), along with MIL-100(Fe). Analogously, increasing the amount of water in the preparation recipe, a step employed for the MF-S3 batch, also resulted in the production of defective MIL-100(Fe) with \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) iron oxide particles. This was confirmed by its X-ray diffraction analysis, which showed that the material had weak peaks at 10\({}^{\circ}\) that were characteristic of MIL-100(Fe), but it also had strong peaks that corresponded to an iron oxide phase. This suggests that the excess water in the reaction mixture led to the precipitation of iron oxide nanoparticles, which were then incorporated into the MIL-100(Fe) structure. Figure 1: XRD of MIL-100(Fe) samples synthesized hydrothermally under different synthesis conditions. Triangle marker: \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\). Circle marker: MIL-100(Fe). Overall, the results of this study suggest that the formation of MIL-100(Fe) with iron oxide defects is sensitive to the amount of water used in the synthesis and the trimesic acid-to-iron precursor ratio. This is important to consider when optimizing the synthesis of MIL-100(Fe) for specific applications. The MF-S1 protocol is distinguished from other protocols by its use of a low Fe\({}^{3+}\)/H3 BTC ratio and a water solvent concentration in the synthesis mixture. The lower water solvent concentration is a crucial factor in achieving the desired purity of MIL-100(Fe).