# Mo(VI) Complexes Immobilized on SBA-15 as an Efficient Catalyst for 1-Octene Epoxidation
## 2 Results and Discussion
### Catalyst Reusability  
To study the reusability of the Mo(VI) heterogeneous catalyst, the solid was properly recovered by filtration after epoxidation tests and washed with the solvent reaction at room temperature. Runs 1 and 2 were performed under the same experimental conditions: molybdenum/1-octene molar = 1:1000, decane as solvent and TBHP as oxidant. Figure 9 shows the reaction results (product yield and oxidant efficiency) obtained with both the fresh and the used catalyst, indicating that the achieved 1,2-epoxioctane yield decreases in 10% in the second use, whereas the oxidant efficiency remains almost unaltered. According to previous works [3; 25] molybdenum leaching is the most probable cause to justify this catalytic activity reduction. Indeed, the metal content of the used catalyst (3.20 wt %, determined by ICP-OES) is 12% lower than the value corresponding to the fresh catalyst (3.76 wt %, see Table 1), confirming that molybdenum leaching occurs under our experimental conditions. On the other hand, high oxidant efficiency obtained for both fresh and used sample indicates that side reactions are boosted poorly by using this Mo(VI) heterogeneous catalyst.  
Finally, X-ray photoelectron spectroscopy (XPS) was employed to study the molecular environment of the active sites in the fresh and used catalyst. Figure 10 shows XPS spectra of both samples in the Mo3d (A) and O1s (B) energy regions. The Mo3d region shows well-resolved spectral lines at 231.3 and 234.4 eV, typical of the Mo3d5/2 and Mo3d3/2 spin orbit, respectively [29; 30]. As observed, very similar Mo3d XPS spectra are obtained for fresh and used catalyst, indicating that molybdenum oxidation state and its coordination sphere remains unaltered during the epoxidation reaction. On the other hand, results coming from the O1s spectra show slight differences between both samples. Thus, the spectrum of the used catalyst shows three different signals: a small one located at 533.5, which can be ascribed to the presence of Mo-OH species, a very intense one located at 532.5 eV which can be attributed to the oxygen of the silica (Si-O-Si bonds existing on SBA-15 material) and a small shoulder at 530.4 eV, corresponding to oxygen atoms bonded to molybdenum sites (species Mo=O, see Scheme 1). Fresh catalyst shows these three peaks, and an additional one located at 527 eV, that is probably related to the oxygen of acac ligands coming from the molybdenum source, and which remain attached to molybdenum sites after catalyst preparation. Taking into account that the used catalyst does not present this signal and the coordination sphere of molybdenum is similar for fresh and used catalyst (as shown Mo3d XPS results), acac groups should be replaced by other oxygenated  
Figure 9: 1,2-epoxioctane yield and oxidant efficiency achieved with fresh (run 1) and used catalyst (run 2) (using decane as solvent, TBHP as oxidant and Mo/olefin molar ratio = 1:1000).  
species during the first epoxidation run. These oxygenated species should come from the oxidant, which must interact with molybdenum centers to achieve olefin epoxidation.  
According to the previous descriptions, Scheme 2 shows the proposed cyclic mechanism for describing the changes occurring in the environment of molybdenum sites during epoxidation runs. As observed, during the first run, the acac group is replaced by an oxidant molecule (TBHP, in this case) that interacts with the olefin substrate at the same time, thus leading to the epoxide and the corresponding alcohol (tert-butanol, in this case), which is still linked to the metal site. Then, a new oxidant molecule approaches the molybdenum, releasing the alcohol and starting the cycle again.  
Figure 10: XPS spectra of fresh and used catalyst in the Mo3d (**A**) and O1s (**B**) energy regions.