## 3 Results and Discussion
However, the size of the microspheres increased after oxidation, e.g., from 148 nm to 198 nm with DVB:FVPDSe = 1:1, 149 nm to 186 nm with DVB:FVPDSe = 3:1 and 167 nm to 188 nm with DVB:FVPDSe = 4:1, while the Se3d XPS spectra in Figure 3d show the binding energy of diselenide at 56.5 eV, and the binding energy of Se3d shifted to 60.2 eV which corresponds to selenium acid after oxidation [31]. We speculated that the diselenide bond in DSe-PS is oxidized to form selenium acid after being oxidized by H\({}_{2}\)O\({}_{2}\), and becomes porous and loose, which leads to an increase in the measured particle size in methanol. At the same time, the morphology of the sphere did not change after oxidation, and the microspheres could be recycled and reused, which presents the possibility of DSe-PS as a heterogeneous catalyst. To evaluate the catalytic activity of DSe-PS, the oxidation of acrolein was used as a model reaction (Scheme 5). The oxidation of acrolein under the catalysis of selenium-containing compounds has been covered well in the literature [28; 32]. Two main products, e.g., acrylic acid (AA) and methacrylate (MA), were obtained after oxidation. It was found that the molar fraction of MA in the products varied from 0.3-0.5 to 0.9-0.95 when using different selenium-based catalysts. A molar fraction of MA in the products is 0.3-0.5 in the oxidation of acrolein when H\({}_{2}\)SeO\({}_{3}\) is used as a catalyst [32], while by introducing diselenide moiety into microgels and using such selenium-modified microgels as the catalyst, the molar fraction of MA in the products could be increased to 0.9-0.95 [28]. Following such results, DSe-PS was used as the catalyst in the same oxidative reaction. The result showed that the molar fraction of MA in the products was 1.0, i.e., the reaction offered high selective activity for methyl acrylate (Table 2). Figure 2: SEM images of DSe-PS. (**a**) Se-1-D-2-20, (**b**) Se-1-D-2-40, (**c**) Se-1-D-2-60. (**d**) Particle size distribution curve of DSe-PS microspheres, polydispersity indexes (PDI) were a = 0.1, b = 0.155, c = 0.013. With these initial results, the reaction was investigated in detail. Firstly, the kinetic of reaction was investigated by using oxidized Se-1-D-5-40 as the catalyst at 50 \({}^{\circ}\)C. The results are summarized in Figure 4a. The reaction was carried out smoothly and reached over 80% \begin{table}
\begin{tabular}{c c} \hline \hline
**Catalyst** & **Molar Fraction of MA in the Products** \\ \hline H\({}_{2}\)SeO\({}_{3}\)[32] & 0.3–0.5 \\ Selenium-Modified Microgels [28] & 0.9–0.95 \\ DSe-PS & 1.0 1  \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparation of the product selectivity of selenium-containing catalysts for the oxidation of acrolein in the literature. Figure 3: Particle size distribution curve of DSe-PS before and after oxidation by H\({}_{2}\)O\({}_{2}\) (black-before oxidation; red-after oxidation). (**a**) Se-1-D-1-40, (**b**) Se-1-D-3-40, (**c**) Se-1-D-4-40. (**d**) XPS spectra of Se in DSe-PS and Se-PS which was oxidized by 5 wt% H\({}_{2}\)O\({}_{2}\) overnight. SEM images of Se-1-D-3-40 (**e**) before and (**f**) after oxidation. yield of MA after 24 h. Secondly, the effect of reaction temperature on the yields of MA was investigated. The results are summarized in Figure 4b. They showed that the yield of MA was increased with the increasing of temperature. The highest yield of MA was obtained as 94% at 60 \({}^{\circ}\)C. Thirdly, the effect of selenium concentration in PS particles on the reaction was investigated. The results are shown in Figure 4c. The maximum yield of MA was 73% in the case of catalyzing by DSe-PS prepared with a weight ratio of FVPDSe and DVB of 1:1. Decreasing the selenium content in DSe-PS resulted in decreasing yields of MA from 73% to 45%. It should be noted here that only MA was obtained in the oxidation of acrolein catalyzed by DSe-PS.