*3.8. Catalytic Activity*

Epoxidation is an important industrial reaction, and epoxides are key intermediates in the manufacture of a wide variety of valuable products [79–81]. The epoxidation of styrene to styrene oxide with aqueous tertbutyl hydroperoxide (TBHP) using compound **1**, **2**, **3**, **4** or **5** as the catalyst was carried out in a batch reactor. In a typical run, the catalyst (compound **1** (2 mg, 0.57 μmol), compound **2** (2 mg, 0.60 μmol), compound **3** (2 mg, 0.58 μmol), compound **4** (2 mg, 0.62 μmol), compound **5** (2 mg, 0.70 μmol), 0.114 mL (1 mmol) of styrene and 2 mL of CH3CN were added to a 10-mL two-neck flask equipped with a stirrer and a reflux condenser. The mixture was heated to 80 ◦C and then 2 mmol of TBHP was injected into the solution to start the reaction. The liquid organic products were quantified using a gas chromatograph (Shimadzu, GC-8A, Beijing, China) equipped with a flame detector and an HP-5 capillary column and identified by comparison with authentic samples and GC-MS coupling. In a blank experiment carried out in the absence of catalyst, no products were observed. Also, the styrene epoxidation reactions in the presence of GeO2 (2 mg, 19.1 μmol) and V2O5 (2 mg, 11.0 μmol) were carried out respectively, and the activities are 24.8% and 71.2%, respectively, after 8 h.

Table 2 shows the catalytic reaction results of TBHP oxidation of styrene over various catalysts. As expected, all the catalysts are active for the oxidation of styrene. Compound **1** as a catalyst shows a performance with 50.1% conversion and 62.8% selectivity to styrene

oxide after 8 h. Compound **2** shows the highest activity among the five with 96.3% conversion and 71.6% selectivity to styrene oxide. Compound **3** shows a catalytic performance with 81.4% conversion and 63.0% selectivity. The performance of compound 4 is similar to that of compound **3** with 84.1% conversion and 55.5% selectivity. The activity and selectivity of compound **5** are 41.7% and 67.1%, respectively. Compounds **3** and **4** are based on Ge6V15, group 12 metals (Cd and Zn) and similar organic ligands (en and enMe), and both exhibit extended framework structures (3-D and 2-D). Therefore, the catalytic activities of the two are similar. The structures of compounds **2** and **5** are more similar to each other. Compounds **2** and **5** are based on similar Cd2Ge8V12 clusters and similar cadmium complexes, and both exhibit similar 1-D extended structures. The significant difference between compounds **2** and **5** is that compound **2** contains aromatic organic ligands but compound **5** dose not; however, the catalytic activities of the two are thoroughly different from each other. To further understand the catalytic mechanism, we still need not only more Ge-V-O crystals but also more catalytic experimental results of the synthesized crystals. Although there have been no investigations on Ge-V-O metal-oxo-clusters as catalysts, there are some similar catalysis studies using catalysts formed by other POMs. The comparisons of the catalytic oxidation of styrene for compounds **1**–**5** and other reported POMs have been summarized in Table S2.


**Table 2.** Catalytic activity and product distribution.

<sup>a</sup> Reaction conditions: catalyst 2 mg, styrene 0.114 mL (1 mmol), CH3CN 2 mL, TBHP (2 mmol), temperature 80 ◦C and time 8 h. <sup>b</sup> So: Styrene oxide, Bza: benzaldehyde; Others: including benzoic acid and phenylacetaldehyde.

The recyclability and reusability of compound **3**, including the conversion and catalyst recovery in three cycles, were studied (Table 3). The same experimental conditions were used. Generally, when using soluble heteropolyacid (e.g., H3[PW12O40]) as the catalyst, the used catalyst was recovered by precipitation and ion exchange [82]. In comparison, it was easy to separate (centrifugation) and recycle compound **3**. The process of recovery possibly resulted in the loss of approximately 40 wt.% after each cycle. The conversion dropped from 81.4% to 44.0% after three cycles.

**Table 3.** Recyclability and reusability of compound **3**.


<sup>a</sup> Reaction conditions: catalyst 2 mg, styrene 0.114 mL (1 mmol), CH3CN 2 mL, TBHP (2 mmol), temperature 80 ◦C and time 8 h. <sup>b</sup> So: Styrene oxide, Bza: benzaldehyde; Others: including benzoic acid and phenylacetaldehyde.

Recovery experiments showed that compound **3** suffered significant activity losses after three cycles. However, the residual catalyst of compound **3** and the as-synthesized crystals used for X-ray analysis can still be considered homogeneous (Figure S5). The FT-IR spectra of compound **3** after the three cycles also remain identical to the one before the reaction (Figure S6).
