## 2 Results and Discussion
### Catalysts' Characterization
#### 2.1.1 X-ray Fluorescence (XRF) and X-ray Diffraction (XRD)  
The XRF elemental analysis of cerium-incorporated samples is compiled in Table 1. As can be seen, both materials have a similar cerium content, 5.89 and 5.24 wt.% for Ce-MCM-Cl and Ce-MCM-NO\({}_{3}\), respectively. In both cases, the Ce content is lower than the expected value, which led to Si/Ce molar ratios higher than the one used in the synthesis gel (Si/Ce = 25), i.e., Si/Ce = 31 for Ce-MCM-Cl and Si/Ce = 36 for Ce-MCM-NO\({}_{3}\). This result is not uncommon since the incorporation of heteroatom ions in silica matrices is strongly dependent on the synthetic procedure and precursors used [39; 40; 41; 42]. [PERSON] and [PERSON] [39] studied the effects of several synthesis parameters during the preparation of Al-MCM-41 materials with Si/Al = 24, including different aluminum and silicon sources. Some samples showed large deviations from the desired Si/Al ratio, reaching values between 27.48 and 37.61, an error of 15% and 57%, respectively. Also, not all heteroatom ions are incorporated into the framework of silica materials, even when the silicon-to-metal molar ratio obtained in the solid is the same as the one used in the synthesis gel [43; 44]. For example, [PERSON] and co-workers [43] studied Al-MCM-41 with different Si/Al ratios by solid state \({}^{27}\)Al MAS-NMR and observed a signal at 53 ppm, assigned to tetrahedrally coordinated framework aluminum, and another at 0 ppm, associated with octahedrally coordinated non-framework aluminum. The slightly higher Si/Ce ratio obtained for Ce-MCM-NO\({}_{3}\), when compared to Ce-MCM-Cl, suggests that cerium(III) precursors with different anions might influence the outcome in the silica-based molecular sieves' synthesis. Indeed, XRD patterns of the calcined Ce-MCM-41 materials in the low angle region (Figure 1) showed not only differences towards Si-MCM-41, but also among themselves. Si-MCM-41 displayed reflections at 2.30, 3.95, and 4.55\({}^{\circ}\), corresponding to structural planes 100, 110, and 200 of the MCM-41 hexagonal mesophase (planar space group p6m) [45]. For the prepared Ce-MCM-41, the 100 reflection was noticeably shifted towards lower 20 values and the 110 and 200 reflections overlap with each other, forming a broad signal between 3 and 5\({}^{\circ}\). The first indicates an expansion of the unit cell due to cerium incorporation in the MCM-41 framework, which is expected since the ionic radii of the Ce\({}^{4+}\) ions (87 pm for 6-coordination [46]) is higher than Si\({}^{4+}\) (40 pm for 6-coordination [46]). The second observation is characteristic of hexagonal disordered mesoporous materials [47] and has been observed in the literature for Ce-MCM-41 [48; 49; 50; 51]. This loss of mesophase ordering can be attributed to the partial substitution of Si\({}^{4+}\) for Ce\({}^{4+}\) ions due to changes in the Si-O-Si bond angles that lead to the emergence of local structural defects and bond strain [37; 47]. However, metal precursor salts in the synthesis gel could cause changes in the assembly of CTA\({}^{+}\) molecules, leading to less organized materials when compared to Si-MCM-41. As [PERSON] and co-workers [52] demonstrated, even the use of different anions for the same metal precursor can extensively affect the physicochemical properties of particles in solution [52]. Since MCM-41 is a molecular sieve with a bidimensional structure, the calculation of the hexagonal unit cell parameter (\(a\)) could be determined by using Equation (1) (see Materials and Methods). Unfortunately, the 100 reflection cannot be observed in the XRD pattern for the cerium-incorporated materials (Figure 1), as a consequence of the experimental limitations (i.e., the signal of the beam source was only obtainable starting at 20 = 2\({}^{\circ}\)). As mentioned before, the 110 and 200 reflections could be distinguished, but are overlapped.