## 2 Results and Discussion
### Physicochemical Properties of Nb/HMS and Nb/MCS Catalysts
Furthermore, with increasing the amount of Nb loading, the Nb/HMS series exhibited a relatively large reduction of textural  
Figure 2: **(A,B)** XRD patterns of **(A)**\(X\)-Nb/HMS and **(B)**\(X\) -Nb/MCS catalysts, and **(C)** TEM images of representative \(X\)-Nb/HMS and \(X\)-Nb/MCS catalysts at magnification of 400,000\(\times\). properties when compared to the Nb/MCS samples. To rationalize the advantage of MCS in this aspect, the field-emission scanning electron microscopy (FE-SEM) was carried out (Figure 4). Both of the pristine HMS and MCS exhibited tiny spherical particles (Figure 4A,B, respectively), which were conformed to the previous work [24]. The Nb loading resulted in a significant agglomeration of 10-Nb/HMS particles (Figure 4C), while the morphology of 10-Nb/MCS remained unchanged (Figure 4D). These results implied that the carbon moieties in the MCS nanocomposite created a hydrophobic environment, which could reduce the interaction, and so the agglomeration, between silica particles. Figure 4: FE-SEM images of (**A**) HMS, (**B**) MCS, (**C**) 10-Nb/HMS, and (**D**) 10-Nb/MCS at magnification of 50,000\(\times\). The temperature-programmed desorption of NH\({}_{3}\) (NH\({}_{3}\)-TPD) profiles of representative HMS and MCS catalysts are compared in Figure S3 (SM). The acid sites of Nb/HMS and Nb/MCS catalysts were classified into three types, corresponding to three ranges of desorption temperatures. The desorption occurred at 50-200 \({}^{\circ}\)C and was assigned to the acid sites weakly adsorbed NH\({}_{3}\), such as silanol groups (Si-OH) on the HMS and MCS surface, and hydroxide groups of Nb (Nb-OH) [10]. The medium acid sites were related to the desorption of NH\({}_{3}\) at 200-300 \({}^{\circ}\)C, which were attributed to Lewis acid character of Nb\({}^{5+}\) in the oxide crystals (Nb-O) [12]. The desorption above 300 \({}^{\circ}\)C was designated to strong Lewis acid sites, as the Nb\({}^{5+}\) species on the edges and corners of the oxide crystals. As shown in Table 1, MCS had a lower amount of weak acid sites than HMS due to a smaller number of Si-OH groups on the nanocomposite surface. The Nb impregnation not only increased the content of weak acidity but also generated the medium acid sites. The smaller Nb\({}_{2}\)O\({}_{5}\) crystallites on the HMS surface should contribute to the higher total acidity of 10-Nb/HMS than 10-Nb/MCS [33]. With increasing the Nb loading amount on MCS, the weak acidity was decreased, while the content of medium acid sites increased. It was explained by an enlarged crystallite size of Nb\({}_{2}\)O\({}_{5}\) as evidenced by the XRD and TEM analysis. The amount of Bronsted and Lewis acid sites in the catalysts was determined by in situ Fourier transform infrared spectroscopy using pyridine as probe molecule (Py-FTIR). As revealed in Figure 5, the bands at 1445 and 1595 cm\({}^{-1}\) were assigned to the pyridine molecules adsorbed on weakly acidic groups (Si-OH and oxygen functional groups on the carbon surface). The bands at 1450, 1607 and 1623 cm\({}^{-1}\) were attributed to the pyridine molecules coordinated with Lewis acid sites, while the bands at 1540 and 1640 cm\({}^{-1}\) were corresponded to the pyridinium ions (PyH\({}^{+}\)) adsorbed on Bronsted acid sites [34]. The superposition of bands related to the adsorbed pyridine on both Bronsted and Lewis acid sites was present at 1490 cm\({}^{-1}\). The parent HMS and MCS materials exhibited the absence of Bronsted acidity [35]. After the Nb impregnation, the bands related to weakly acidic sites were decreased, which indicated the deposition of Nb\({}_{2}\)O\({}_{5}\) on the surface of HMS and MCS. The amount of Bronsted and Lewis acid sites as calculated from the Py-FTIR spectra is reported in Table 1. With increasing the Nb loading, the total acidity was decreased. It is explained by an enlarged crystallite size as indicated by the XRD result (Figure 2). [PERSON] et al.