## 2 Results and Discussion  
### Physicochemical Properties of Nb/HMS and Nb/MCS Catalysts  
The thermogravimetric (TG) and differential thermal analysis (DTA) curves of HMS and MCS are compared in the Supplementary Material (SM); Figure S1. Both pure silica HMS and MCS nanocomposite exhibited the weight loss between room temperature and 100 \({}^{\circ}\)C, which corresponded to physisorbed moisture (3-4 wt.%). The weight loss observed for MCS at 300-650 \({}^{\circ}\)C was ascribed to the decomposition of carbon in the silicate framework (13.3 wt.%). Moreover, MCS showed a small loss of weight (2.4 wt.%) in the range of 200-300 \({}^{\circ}\)C, which was related to the oxygen-functional groups on the carbon surface [24]. The X-ray photoelectron spectroscopy (XPS) was used to identify the type of surface functional groups of MCS as shown in Figure S2 (SM). The C1s XPS spectrum can be decomposed into four peaks, comprised of carbon groups (C=C, CH\({}_{\text{x}}\), and C-C), hydroxyl groups or ether linkages (C-O, C-O-C), carbonyl groups (C=O), and carboxyl or ester groups (O-C=O) at 283.6, 285.7, 287.3, and 289.0 eV, respectively [25; 26]. These results confirmed the presence of the carbon moieties and oxygen functional groups, such as hydroxyl group (-OH) and carboxyl group (-COOH) [24], in the MCS nanocomposite. The X-ray diffraction (XRD) patterns and nitrogen (N\({}_{2}\)) physisorption isotherms of HMS, NR/HMS and MCS are shown in Figure 1. All samples exhibited the reflection peak at 2\(\theta\) in the range of 20-25\({}^{\circ}\) (Figure 1A), which corresponded to amorphous silica [27; 28]. When compared to HMS, NR/HMS showed an increased intensity of amorphous phase since the incorporated NR hindered the condensation of silicate framework of HMS. The carbonization used in the MCS preparation promoted the formation of siloxane bond (Si-O-Si), decreasing the amorphous nature of the silica matrix. The N\({}_{2}\) physisorption isotherms revealed the mesoporous character of these materials (Figure 1B), according to the IUPAC classification. Compared to the pristine HMS, the mesoporous of NR/HMS and MCS was relatively lower (Table 1) due to the presence of organic moieties dispersed in the mesostructure framework [23]. The XRD patterns of the Nb/HMS and Nb/MCS with different Nb loading amounts are shown in Figure 2A,B, respectively. The characteristic peaks of Nb\({}_{2}\)O\({}_{5}\) were not observed for both 10-Nb/HMS and 10-Nb/MCS, indicating the small crystallites of Nb\({}_{2}\)O\({}_{5}\) well dispersed on the HMS and MCS surface. An increase in the amount of Nb loaded on the HMS and MCS surface to 20 and 30 wt.% enhanced the characteristic peaks of Nb\({}_{2}\)O\({}_{5}\) at 2\(\theta\) = 22.6\({}^{\circ}\), 28.5\({}^{\circ}\), 36.7\({}^{\circ}\), 46.2\({}^{\circ}\), 50.5\({}^{\circ}\), 55.2\({}^{\circ}\), 64.2\({}^{\circ}\) and 70.8\({}^{\circ}\)[29], which indicate the enlarged  
Figure 1: (**A**) XRD patterns and (**B**) N\({}_{2}\) physisorption isotherms of HMS, NR/HMS, and MCS. Nb\({}_{2}\)O\({}_{5}\) particles [30]. The crystallite size of Nb\({}_{2}\)O\({}_{5}\) on HMS (39.7-47.4 nm) was smaller than that on MCS (45.8-49.0 nm). It was explained by a higher surface area for HMS than MCS (Table 1) [31]. As shown in Figure 2C, the images obtained from transmission microscopy (TEM) confirmed a high dispersion of Nb\({}_{2}\)O\({}_{5}\) crystallites on both the HMS and MCS surface but formed agglomerates at an increased Nb loading level. The N\({}_{2}\) adsorption-desorption isotherms indicated that the mesoporous nature of HMS and MCS was retained after the Nb loading (Figure 3). The textural parameters of these catalysts are summarized in Table 1. The impregnation of Nb onto both HMS and MCS decreased \(S_{\text{BET}}\), \(V_{\text{t}}\), and \(D_{\text{p}}\), which confirmed the presence of Nb\({}_{2}\)O\({}_{5}\) species dispersed on the surface of mesostructured HMS and MCS [32].