# Mesoporous Niobium Oxyhydroxide Catalysts for Cyclohexene Epoxidation Reactions
## 3 Results and Discussion
### Epoxidation Reactions  
Figure 4a shows the preliminary catalytic results for the catalysts before and after calcination. A first analysis identifies that the materials that did not undergo calcination presented a greater cyclohexene conversion capacity.  
Figure 3: IR spectra for acidity analysis by pyridine adsorption on CINbS-600 and CINbS-AC-600 catalysts.  
The catalysts CINbS-AC and CINbS are niobium oxyhydroxides (NbO\({}_{2}\)OH) and as proposed by [PERSON] et al., these materials are capable of forming peroxo groups in the presence of H\({}_{2}\)O\({}_{2}\). These groups are highly oxidizing, which would explain the better catalytic efficiency of the CINbS-AC (80%) and CINbS (50%) catalysts in the oxidation of cyclohexene [27]. The calcined catalysts do not present these surface hydroxyls for this reason they are not able to form peroxo groups, so their catalytic activity in the oxidation of cyclohexene are lower. The CINbS-AC catalyst showed the highest cyclohexene conversion (84%) under the conditions studied. [PERSON] et al., also analyzed the conversion of cyclohexene using niobium compounds as a catalyst under similar reaction conditions and obtained a maximum conversion of 58% [28].  
The results of the catalyst reuse tests are presented in Figure 4b. A small conversion decrease in the cycles of reaction was observed, which may be related to loss of catalyst mass during the washes and the deactivation of the catalytic sites after some reactions, however the catalyst showed a certain stability, with considerable cyclohexene conversions (~60%) after two reuse tests. However, in the third reuse of the catalyst there is a marked drop in the conversion capacity. The catalyst was recovered after this test and it was observed that the hydrophobizing group had been leached from the surface after the third use. The presence of the group that makes the catalyst have greater affinity for the solvent where the substrate is dissolved [23]. Other forms of synthesis are being studied in order to maintain a more stable structure and increase its reuse capacity.  
\({}^{1}\)H and \({}^{13}\)C spectra were obtained to prove epoxide formation by comparing the \({}^{13}\)C chemical shifts of the experimental and simulated. The values of the chemical displacements are presented in Table 2.  
\begin{table}
\begin{tabular}{c c c} \hline \hline C1 & 51.22 & 52.07 \\ C2 & 25.36 & 24.50 \\ C3 & 20.53 & 19.47 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Experimental and simulated chemical displacement values for epoxide carbon.  
Figure 4: (**a**) Epoxidation reactions using niobium catalysts. Conditions: 25 \({}^{\circ}\)C; 60 min; molar ratio (2:1) H\({}_{2}\)O\({}_{2}\)/cyclohexene; 50 mg of catalyst. (**b**) CINbS-AC catalyst reuse test at 25 \({}^{\circ}\)C, 60 min, 50 mg catalyst and H\({}_{2}\)O\({}_{2}\)/cyclohexene molar ratio 2:1.  
The \({}^{13}\)C spectrum is shown in Figure 5. The presence of the acetonitrile solvent, the cyclohexene substrate and the major product, epoxide, are observed. Other cyclohexene oxidation products, such as cyclohexanone, were not formed as no ketone carbonyl signal was observed.