*3.1. Characterization of Solids*

The FTIR spectrum of Nb-MCM-41 materials is shown in Figure 1, where bands are observed between 1400 and 400 cm<sup>í</sup><sup>1</sup> due to the fundamental vibrations of the mesoporous structure. A large band around 3450 cm<sup>í</sup><sup>1</sup> and a band at ~1630 cm<sup>í</sup><sup>1</sup> correspond to O–H stretching and surface water [24]. The bands at ~1230, ~1080, and ~810 cm<sup>í</sup>1 are assigned to Si–O symmetric and asymmetric stretch vibrations; the band at ~460 cm–1 is characteristic of silica compounds and corresponds to O–Si–O group stretching [25].This infrared spectrum was the same independent of the procurement method and percent incorporation of niobium.

Raman spectroscopy characterization permits to determine, to a certain degree, the incorporation of niobium pentoxide onto the MCM-41 mesoporous structure. The lack of a neat Raman band in the spectrum at ~680 cm<sup>í</sup><sup>1</sup> —corresponding to niobium oxide polyhedra symmetric stretching modes—or bands between 200 and 300 cm<sup>í</sup><sup>1</sup> —assigned to flexion modes of Nb–O–Nb bonds [26]—would indicate a lack of Nb2O5 crystalline nanoparticles on the silica structure. Bearing this in mind, it can be observed in Figure 2 that the Raman band at ~680 cm<sup>í</sup><sup>1</sup> is present in most of the solids analyzed, except for the mesoporous solids with 10% niobium synthesized through the sol-gel method, Figure 2d, and with 20% niobium obtained through the incipient impregnation method, Figure 2b, which indicates that most of the niobium added was incorporated into the silica structure [26]. However, in the other solids the presence of this band indicates that not all the niobium added was incorporated into the silica, but that a certain percentage of it was leached. Though the increase occurring in the background of the Raman spectra, Figure 2b,d, can be associated with the fluorescence of organics present in the sample [27], due to the presence of carbon in the sample as it was found in the images of SEM-EDS (Figure 3), however the carbon content is small.

**Figure 1.** Infrared spectrum of Nb-MCM-41 sample.

**Figure 2.** Raman spectrum of Nb-MCM-41 through the incipient impregnation method (**a**), (**b**), and (**c**), and through the sol-gel method (**d**), (**e**), and (**f**).

In order to verify the niobium concentration on the surface of the MCM-41 mesoporous structure, EM-EDS analysis was conducted, shown in Figure 3.

The Nb-MCM-41 powders obtained through the sol-gel method showed a greater distribution of niobium on the surface. In those obtained by the incipient impregnation method, however, the niobium distribution was not homogeneous.

The Nb-MCM-41 materials obtained using the incipient impregnation method (Figure 4) produces an X-ray diffractogram characteristic of MCM-41, which contains four peaks at low angles, as noted in the figure; the first of these is the most intense peak, appearing around 2ș = 2°, corresponding to Miller index (100). Other lower-intensity reflections appear between 3° < 2ș < 10°, corresponding to Miller indexes (110) and (200), which verifies a hexagonal symmetry of the structure [24,25,27]; the X-ray diffractogram is the same independent of the percentage of niobium incorporated. This result is similar to that obtained by Ziolek and Nowak [16].

Furthermore, observing the complete diffractogram between 10° and 90°, Figure 4b, similar for all samples impregnated with Nb, two ridges are observed in the regions where, normally, the peaks characteristic of Nb2O5 are located, so that these samples may contain crystals of this oxide forming. Regarding the solids obtained using the sol-gel method, these produce a diffractogram (Figure 5) that differs from the typical MCM-41structure, given that it does not show this structure's characteristic peaks. However, these solids show peaks at low angles, which could indicate that a mesoporous structure is being obtained, but not an MCM-41 type structure.

**Figure 4.** X-ray diffractogram of Nb-MCM-41 obtained using the incipient impregnation method to (**a**) in the region of low angles, and (**b**) in the high-angle region.

These diffraction patterns are different from those obtained by Ziolek and Nowak [16] in that the mesoporous Nb-MCM-41 synthesized by the sol-gel method should show a different pore structure. Given the similarity of the XRD patterns of Figure 5 to the small-angle XRD patterns of the mesoporous Nb samples synthesized by Chen *et al.* [19] that showed obvious peaks between 0.58 and 1.58 and no other peak at higher degrees, it can be concluded that these came from a less ordered mesoporous structure and that the d value of each mesoporous sample would lie between 7.7 and 14.9 nm.

Moreover, observing the full diffractogram between 10° and 90°, Figure 5b, similar for all the samples synthesized using different concentrations of Nb precursor, this is similar to that of Figure 4b, so that it is possible these samples also contain in their structure some forming Nb2O5 crystals.

To corroborate the mesoporous structure, the N2 adsorption and desorption isotherms are shown in Figure 6. This analysis allowed to determine of the Nb-containing mesoporous MCM-41 solid along with large surface areas after the incorporation of niobium to the structure of the silica. As the load increases niobium, surface area decreases as expected (Table 1).

**Figure 5.** X-ray diffractogram of Nb-MCM-41 obtained through the sol-gel method with 10%, 20% and 30% of niobium.

**Figure 6.** N2 adsorption and desorption isotherms of 20% Nb-MCM-41 obtained through (**a**) sol-gel and (**b**) incipient impregnation methods, respectively.

However, although the samples exhibited a mesoporous phase, which was corroborated using DRX at low angles, their hysteresis loop is different compared to that of MCM-41, possibly through the formation of species on the surface of the pores from the niobium, which leads to a "plugging" of the channels, and which was reflected in the narrowness of the hysteresis loop area [28], as well as decreasing both the surface area and pore size. This plugging would be generated by the synthesis methods used to obtain the samples of interest. In the case of impregnation of the MCM-41, it is very likely that niobium species are deposited, during the impregnation process, in the pores of the substrate (MCM-41), plugging them and causing a reduction in the hysteresis loop (see Figure 6b). In the case of the samples synthesized by sol-gel, after forming the micelles and adding the organic Nb precursor (ammonium oxalate), it is possible that due to its nature, some of it is distributed within the micelles (lyophobic area) and remains there throughout the process of synthesis, leading finally to plugging of the pores and thus a reduction of the hysteresis loop (Figure 6a). Clearly, on increasing the concentration of Nb precursor, a greater plugging of the pores can be expected, and a reduction in their size, as shown in Table 1.


**Table 1.** Textural properties of mesoporous solids synthesized in this work.

The TEM micrographs for the Nb-MCM-41 solids obtained through the incipient impregnation method (Figure 7) should not present optimal pore distribution, mainly because these solids were calcined again following impregnation with the niobium precursor, which would generate some loss of organized structure.

**Figure 7.** TEM micrographs corresponding to Nb-MCM-41 mesoporous solids obtained using the incipient impregnation method with 30% niobium.

The TEM micrographs corresponding to Nb-MCM-41 mesoporous solids synthesized using the sol-gel method are also shown in Figure 8, illustrating that pore distribution is uniform, which indicates that the pore size for these solids ranges between 2 and 10 nm. These transmission electron micrographs are typical of mesoporous structures, according to several reports [29].

**Figure 8.** TEM micrographs corresponding to Nb-MCM-41 mesoporous solids obtained using the sol-gel method with niobium percentages of (**a**) 10%, (**b**) 20%, and (**c**) 30%.
