Phosphotungstate-Based Ionic Silica Nanoparticles Network for Alkenes Epoxidation

Abstract

An inorganic-organic porous silica network catalyst was prepared by linking silica nanoparticles using ionic liquid and followed by anion-exchange with phosphotungstate. Characterization methods of FT-IR, TG, SEM, TEM, BET, etc., were carried out to have a comprehensive insight into the catalyst. The catalyst was used for catalyzing cyclooctene epoxidation with high surface area, high catalytic activity, and convenient recovery. The conversion and selectivity of epoxy-cyclooctene could both reach over 99% at 70 °C for 8 h using hydrogen peroxide (H₂O₂) as an oxidant, and acetonitrile as a solvent when the catalyst was 10 wt. % of cyclooctene.

1. Introduction

Inorganic-organic hybrid [1,2,3,4,5,6,7,8] materials have attracted considerable interest as the combination of the features of different parts can generate high performance. As the development of nanoscale materials, SiO₂ has indeed stimulated remarkable scientific interest because of its excellent performance and promising applications in scientific and technological fields. In contrast to other inorganic materials, the preparation of SiO₂ nanoparticles has been very mature with a wide source of raw materials, and they possess the merits of high specific surfaces, as well as excellent thermal stabilities. Furthermore, the presence of a large number of silanol (Si–OH) groups on the surface makes it easily for surface organic functionalization [9,10]. Ionic liquids, a class of new type of green environmental protection organic compounds with outstanding properties, were introduced as the organic part, recently. The resulting materials can be applied into numerous fields, like catalysts, anion exchange, selective gas trapping, drug delivery, or electrochemistry [11].

The modification of silica nanoparticles with ionic liquids has been already reported by some researchers [12,13,14]. Ionic liquids are usually just grafted or absorbed onto the surface for next use. However, this is no longer a hot spot and novel method. Meanwhile, it remains separation problems if the nanoparticles are smaller than a certain size. For a few years now, the newly-arising challenge in the field of nanoparticle research is focusing on the development of specific materials based on assemblies of nanoparticles, which approaches to make use of the nanoparticle collective properties [15,16,17,18,19,20]. Thus, a silica network is being prepared by connecting silica nanoparticles with ionic liquid to take the advantage of covalent link of ionic liquid by two different parts.

Epoxides are important raw materials in chemical industrial production [21]. Polyoxometalates (POMs), as is known to all, have been widely used as epoxidation catalysts with H₂O₂ for their high efficient active centers [22,23,24]. However, POMs, themselves, are low efficient and soluble in some epoxidation systems which results in the difficulty in separation of the catalysts. Therefore, ionic liquid-based POM hybrid catalysts come into being.

In this study, we first prepared a silica network by connecting silica nanoparticles with covalent-linked ionic liquid. This material was proved to be porous with high surface area. Phosphotungstic acid anions were then introduced into the material by ion exchange between Keggin-type H₃PW₁₂O₄₀ and ionic liquid as well as protonation of amino group. Ionic liquid had played an important role in two aspects: linking silica nanoparticles and introduction of an active center. The catalysts were used in catalytic epoxidation of olefin for the first time. Structural characteristics and catalytic performance of as-prepared catalysts were carried out to have a comprehensive insight into the catalyst. The study potentially propels the development of nanoparticle networks as promising materials for various fields to take advantage of the collective properties of nanoparticles.

2. Results and Discussion

2.1. Characterization

2.1.1. FT-IR Analysis

The basic modification moieties on the SiO₂ particles were characterized by FT-IR, which were shown in Figure 1. The bands at 3450 and 1630 cm⁻¹ were corresponding to stretching and bending vibrations of surface Si–OH groups on the surface of SiO₂. After grafting with silane coupling agent, the peak intensity decreased and new bands that assigned to the C–H stretching and bending rocking mode respectively appeared in the region of 2970 cm⁻¹ and 1460 cm⁻¹ (Figure 1b–e), which indicated the successful functionalization on the surface of SiO₂ particles. The other bands at 1100 cm⁻¹ and 814 cm⁻¹ were attributed to symmetric and anti-symmetric stretching vibration of Si–O–Si, and band at 960 cm⁻¹ was attributed to the Si–O stretching vibration of Si–OH. For PW (0.058)/SiO₂ Im⁺Cl⁻ SiO₂, a band at 890 cm⁻¹ appeared in Figure 1e, which was assigned to asymmetric stretching of W–O_b–W in the corner-shared octahedral of Keggin-type HPW. Other characteristic bands at 1080, 983, and 805 cm⁻¹ were all overlapped with the bands of SiO₂.

Figure 1. FT-IR spectra of (a) SiO₂; (b) SiO₂-Im; (c) SiO₂-Cl; (d) SiO₂ Im⁺Cl⁻ SiO₂; (e) PW(0.058)/SiO₂ Im⁺Cl⁻ SiO₂.

Figure 1. FT-IR spectra of (a) SiO₂; (b) SiO₂-Im; (c) SiO₂-Cl; (d) SiO₂ Im⁺Cl⁻ SiO₂; (e) PW(0.058)/SiO₂ Im⁺Cl⁻ SiO₂.

2.1.2. Thermal Stability and Structure

The TG analyses of SiO₂, SiO₂-Cl, SiO₂-Im, SiO₂ Im⁺Cl⁻ SiO₂, and PW (0.058)/SiO₂ Im⁺Cl⁻ SiO₂ were shown in Figure 2. For the pure SiO₂ nanoprticles (Figure 2a), the weight loss of 5.65 wt. % could be only observed before 100 °C, which was assigned to the desorption of water. No weight loss could be seen even with the increasing of temperature to 800 °C. In reality, the formed ionic units of imidazolium presents a higher thermal stability than the aromatic precursors. The precursors started to decompose before 200 °C, while the imidazolium mainly decomposes around 300 °C. From Figure 2b,c, SiO₂-Cl started to decompose at 180 °C, SiO₂-Im started to decompose at 120 °C, and the main weight loss was around 400 °C. From Figure 2d, the first stage ranging from 25 to 120 °C was ascribed to the elimination of adsorbed water. After 120 °C the unreacted organic groups started to decompose and the weight loss around 300 °C was mainly attributed to the decomposition of imidazolium ionic units, which could verify the occurrence of nucleophilic reactions, on one hand. For the PW(0.058)/SiO₂ Im⁺Cl⁻ SiO₂ (Figure 2e), the mass loss after 300 °C also contained the collapse of PW anions in the remainder form of P₂O₅ and WO₃.

Figure 2. Thermogravimetric curves of (a) SiO₂; (b) SiO₂-Cl; (c) SiO₂-Im; (d) SiO₂ Im⁺Cl⁻ SiO₂; and (e) PW(0.058)/SiO₂ Im⁺Cl⁻ SiO₂.

Figure 2. Thermogravimetric curves of (a) SiO₂; (b) SiO₂-Cl; (c) SiO₂-Im; (d) SiO₂ Im⁺Cl⁻ SiO₂; and (e) PW(0.058)/SiO₂ Im⁺Cl⁻ SiO₂.

2.1.3. SEM, EDS, TEM, and DLS Characterization

The morphology changes between SiO₂ nanoparticles and the modified product of SiO₂-Cl, SiO₂-Im, and SiO₂ Im⁺Cl⁻ SiO₂ were provided by the SEM and TEM images presented in Figure 3. Figure 3a,b were the SEM images for SiO₂ nanoparticles before reaction, which presented scattered and small particles size of SiO₂. The particle size could be observed in TEM in Figure 3f,g with an average of 15 nm diameter which was in slight agglomeration. After functionalization, the particle morphology of SiO₂-Cl and SiO₂-Im had almost no change (showed in (c) and (d)) compared to SiO₂ nanoparticles. Figure 3e,h showed the SEM and TEM micrographs after linking nanoparticles through the ionic liquid-like bond. The product, by observation of “islands”, connected into a larger group, like sponge-cake with dispersed holes.

In order to get a profound insight into the microscopic structure, scanning SEM-energy dispersive spectroscopy (EDS) elemental mapping images of catalyst PW(0.058)/SiO₂-Im-SiO₂ was produced, and the results were shown in Figure 3i. Si, O, N, Cl, P, and W were uniformly distributed in this catalyst.

DLS (Dynamic Light Scattering) was carried out after dispersing the samples in ethanol with previous sonication and the results were shown in Figure 4. It also indicated that the particle size of the product SiO₂ Im⁺Cl⁻ SiO₂ was much larger than the one before reaction, which corresponded to the SEM and TEM micrographs. For Figure 4A, a diffraction peak could be observed around 30 nm. The result was larger than the measurement by TEM micrographs, which was due to the slight agglomeration. When reacting with N-(3-triethoxysilylpropyl)-4,5-dihydroimidazol, the particle size was slightly increased in Figure 4B. Nevertheless, for the resulted SiO₂ Im⁺Cl⁻ SiO₂, the most average size showed in Figure 4C was at 700 nm after nucleophilic reaction. The smaller size distribution was due to the incomplete or partial reaction of the functionalized silica particles. Overall, this size was much larger than SiO₂, also indicating the change after the reaction.

Figure 3. SEM, TEM and EDS images at different magnifications. (a,b) for SEM of SiO₂; (c) for SEM of SiO₂-Cl; (d) for SEM of SiO₂-Im; (e) for SEM of SiO₂ Im⁺Cl⁻ SiO₂; (f,g) for TEM of SiO₂; and (h) for TEM of SiO₂ Im⁺Cl⁻ SiO₂ respectively; (i) for EDS elemental mapping images of the Si, O, N, Cl, P, and W, respectively.

Figure 3. SEM, TEM and EDS images at different magnifications. (a,b) for SEM of SiO₂; (c) for SEM of SiO₂-Cl; (d) for SEM of SiO₂-Im; (e) for SEM of SiO₂ Im⁺Cl⁻ SiO₂; (f,g) for TEM of SiO₂; and (h) for TEM of SiO₂ Im⁺Cl⁻ SiO₂ respectively; (i) for EDS elemental mapping images of the Si, O, N, Cl, P, and W, respectively.

Figure 4. DLS measurement of (A) the starting SiO₂ nanoparticles; (B) SiO₂-Im; and (C) resultant SiO₂ Im⁺Cl⁻ SiO₂.

Figure 4. DLS measurement of (A) the starting SiO₂ nanoparticles; (B) SiO₂-Im; and (C) resultant SiO₂ Im⁺Cl⁻ SiO₂.

2.1.4. Nitrogen Sorption

The porous characteristic of the materials were investigated by Brunauer-Emmet-Teller (BET) method. The N₂ adsorption-desorption isotherms and corresponding pore size distribution curves were shown in Figure 5 (the curves of PW/SiO₂ Im⁺Cl⁻ SiO₂ were particularly similar, so PW(0.058)/SiO₂ Im⁺Cl⁻ SiO₂ was chose as representative). All samples displayed typical type-IV isotherms with a clear adsorption-desorption hysteresis loops at the relative pressure of 0.8 < P/P₀ < 1 (Figure 5, left) as well as broad pore size distribution (Figure 5, right). For the SiO₂ nanoparticles, the pore size was mainly distributed in less than 2 nm, which was due to the micropores of nanoparticles themselves. The size formed above 2 nm was mainly due to the accumulation between the particles. However, when linked by ionic liquid, the SiO₂ network and its catalyst showed more regular in pore size distribution between 20–40 nm. Data of surface area, pore diameter, and pore volume were presented in Table 1. The BET specific surface area of the prepared SiO₂ particles (entry 1) was as high as 381 m²·g⁻¹ and the average pore size was 11.2 nm. When modified by organic reagents and then linked by nucleophilic reaction, specific surface of SiO₂ network (entry 2) was obviously lower than the pure SiO₂ due to the introduction of the organic moieties. After introducing phosphotungstic acid anions into the material by ion exchange, the specific surface of PW(x)/SiO₂ Im⁺Cl⁻ SiO₂ (entries 3–6) also decreased and were lower than the SiO₂ network. Meanwhile, compared to the pure SiO₂, the SiO₂ network and its catalysts exhibited an increase of pore size and pore volume. In particular, the pore size seemed more centralized. This increased the contact area of the substrate and catalysts and led to better catalytic effect. Meanwhile, increased PW anions loading of PW(x)/SiO₂ Im⁺Cl⁻ SiO₂ led to a gradual decrease in pore volume and average pore size (entries 3–6).

Figure 5. Nitrogen adsorption-desorption isotherms (A) and pore size distribution (B) of the samples SiO₂, SiO₂ Im⁺Cl⁻ SiO₂, and PW/SiO₂ Im⁺Cl⁻ SiO₂.

Figure 5. Nitrogen adsorption-desorption isotherms (A) and pore size distribution (B) of the samples SiO₂, SiO₂ Im⁺Cl⁻ SiO₂, and PW/SiO₂ Im⁺Cl⁻ SiO₂.

Table 1. BET parameters of the SiO₂, SiO₂ Im⁺Cl⁻ SiO₂, and PW(x)/SiO₂ Im⁺Cl⁻ SiO₂.

Entry	Compound	SBET (m2·g−1)	Vp (cm3·g−1)	Average Pore Size (nm)
1	SiO2	381	0.93	11.2
2	SiO2 Im+Cl− SiO2	242	1.50	24.1
3	PW(0.035)/SiO2Im+Cl−SiO2	182	1.24	24.0
4	PW(0.058)/SiO2Im+Cl−SiO2	176	1.07	24.0
5	PW(0.074)/SiO2Im+Cl−SiO2	168	1.01	22.4
6	PW(0.17)/SiO2Im+Cl−SiO2	159	0.93	20.3

Table 1. BET parameters of the SiO₂, SiO₂ Im⁺Cl⁻ SiO₂, and PW(x)/SiO₂ Im⁺Cl⁻ SiO₂.

Entry	Compound	SBET (m2·g−1)	Vp (cm3·g−1)	Average Pore Size (nm)
1	SiO2	381	0.93	11.2
2	SiO2 Im+Cl− SiO2	242	1.50	24.1
3	PW(0.035)/SiO2Im+Cl−SiO2	182	1.24	24.0
4	PW(0.058)/SiO2Im+Cl−SiO2	176	1.07	24.0
5	PW(0.074)/SiO2Im+Cl−SiO2	168	1.01	22.4
6	PW(0.17)/SiO2Im+Cl−SiO2	159	0.93	20.3

2.1.5. More Evidence of the Synthesis of the Network

In order to further confirm the occurrence of nucleophilic substitution between the imidazoline functional group and chloroalkyl group, anion exchange experiment was carried out as an indirect proof. Once the nucleophilic substitution happened, the chloride ion was easily exchanged by the fluorinated anion to obtain the SiO₂ Im⁺BF₄⁻ SiO₂ and NaCl. First, we detected chloride ions in the solvent and washing phase by the signature of the halide salts in the X-ray diffraction pattern after drying (Figure 6). In the XRD pattern, the obtained salt reflection was marked with a star and was consistent with NaCl. The other Bragg peaks belonged to the fluorinated salt which was in an excess in order to confirm the exchange. Next, EDX analysis of the hybrid material SiO₂ Im⁺BF₄⁻ SiO₂, obtained after the exchange reaction also showed the existence of newly-introduced anions (Figure 7). Typical peaks of fluorine from the newly exchanged anions could be obviously observed while the boron was hidden by the carbon peak. After the exchange reaction, it was noted that chlorine could still be observed in a small amount. The presence of chlorine was due to the residue alkyl chloride groups which did not react with APTES or the residue chlorine ions which were not exchanged.

Figure 6. XRD analyses of (a) the exchange solvent, after separation and washing; (b) NaBF₄. The obtained salt reflection was marked with a star and was consistent with NaCl.

Figure 6. XRD analyses of (a) the exchange solvent, after separation and washing; (b) NaBF₄. The obtained salt reflection was marked with a star and was consistent with NaCl.

Figure 7. EDX spectra of the resulting materials.

Figure 7. EDX spectra of the resulting materials.

2.2. Catalytic Performances

It is known to all that the Keggin-type [PW₁₂O₄₀]³⁻ (PW) can be degraded to peroxo-active species, [PW₄O₈(O₂)₈]³⁻ species, by excess hydrogen peroxide, which is the active species in alkenes epoxidations [25]. The solvent has a great influence on the activity of catalytic reaction and acetonitrile is commonly used in epoxidation reaction as an efficient solvent. Moreover, using H₂O₂ as an oxygen source can do great benefits on the environment and industry [26]. Thus, Table 2 listed the catalytic performance of different catalysts for the epoxidation of cyclooctene in acetonitrile using aqueous 30% H₂O₂ as an oxidant. The results showed that the catalyst PW/SiO₂ Im⁺Cl⁻ SiO₂ had much better catalytic effect than the Keggin-type H₃PW₁₂O₄₀. One reason was the super-hydrophobic of H₃PW₁₂O₄₀, which made it difficult to contact with the oily substrates. Meanwhile, the cations of imidazolium and NH₂⁺ in imidazoline made it easier for the redox of PW. Physical loading of HPW on the pure SiO₂ nanoparticles was carried out and the catalytic activity was very low, which also illustrated the serviceability of SiO₂ Im⁺Cl⁻ SiO₂ for loading the PW anions. In order to further compare the catalytic activity of the network catalysts with traditional silica grafted ionic liquid catalyst, we also performed a catalytic reaction in the presence of the imidazolium and the silica particles labeled as HPW/SiO₂-IL which was prepared by nucleophilic reaction between SiO₂-Cl and methylimidazole, and then reacted with phosphotungstic acid. The catalytic activity showed that the conversion was lower than the network catalysts with similar selectivity (Table 2, entry 4), which demonstrated that the formed network catalysts could lead to high conversion and selectivity.

Table 2. Catalytic performances of cyclooctene with various catalysts.

Catalysts	Conversion (%)				Selectivity (%)
	2 h	4 h	6 h	8 h	2 h	4 h	6 h	8 h
SiO2	-	-	-	-	-	-	-	-
HPW	4.9	8.6	11.2	14.2	70.3	67.9	61.8	43.2
HPW/SiO2	0.8	1.6	4.4	6.6	36.8	34.5	35.8	32.4
HPW/SiO2-IL	30.5	52.7	72.4	79.8	98.7	98.1	97.4	97.0
PW(0.035)/SiO2Im+Cl−SiO2	54.3	68.4	76.5	84.1	98.5	98.2	97.9	96.9
PW(0.058)/SiO2Im+Cl−SiO2	77.2	88.8	94.4	99.4	99.4	99.4	99.3	99.2
PW(0.074)/SiO2Im+Cl−SiO2	68.5	78.2	91.1	93.5	99.3	98.4	98.9	98.9
PW(0.17)/SiO2Im+Cl−SiO2	45.3	63.6	70.2	77.6	98.7	98.1	98.7	98.2

Reaction conditions: cyclooctene: 2 mmol, acetonitrile: 5 mL, H₂O₂: 6 mmol, catalysts: 0.02 g, temperature 70 °C.

Table 2. Catalytic performances of cyclooctene with various catalysts.

Catalysts	Conversion (%)				Selectivity (%)
	2 h	4 h	6 h	8 h	2 h	4 h	6 h	8 h
SiO2	-	-	-	-	-	-	-	-
HPW	4.9	8.6	11.2	14.2	70.3	67.9	61.8	43.2
HPW/SiO2	0.8	1.6	4.4	6.6	36.8	34.5	35.8	32.4
HPW/SiO2-IL	30.5	52.7	72.4	79.8	98.7	98.1	97.4	97.0
PW(0.035)/SiO2Im+Cl−SiO2	54.3	68.4	76.5	84.1	98.5	98.2	97.9	96.9
PW(0.058)/SiO2Im+Cl−SiO2	77.2	88.8	94.4	99.4	99.4	99.4	99.3	99.2
PW(0.074)/SiO2Im+Cl−SiO2	68.5	78.2	91.1	93.5	99.3	98.4	98.9	98.9
PW(0.17)/SiO2Im+Cl−SiO2	45.3	63.6	70.2	77.6	98.7	98.1	98.7	98.2

Reaction conditions: cyclooctene: 2 mmol, acetonitrile: 5 mL, H₂O₂: 6 mmol, catalysts: 0.02 g, temperature 70 °C.

Furthermore, Table 2 also showed that the loading of PW anions of PW/SiO₂Im⁺Cl⁻SiO₂ affected the catalytic activity of the catalysts. This was correlation with the amount of active sites in the catalyst and the inside structure of material. Obviously, PW(0.058)/SiO₂Im⁺Cl⁻SiO₂ showed the best activity. When the loading of PW anions was less than 0.058, the conversion decreased accordingly. However, excessive PW loading resulted in the agglomeration of excessive HPW in the channel, which might change the specific surface area and pore volume, therefore, led to the decrease of catalytic activity.

To investigate the importance of the IL moiety, the imidazoline based PW/SiO₂-Im was prepared by stirring 0.6 g HPW with 1 g SiO₂-Im, and the loading of PW were ca. 0.082 mmol/g. The catalytic activity and reusability were compared in Table 3. Imidazoline-modified SiO₂ can be protonated with HPW. The results showed that the catalyst of PW/SiO₂-Im offered even higher conversion than PW(0.058)/SiO₂Im⁺Cl⁻SiO₂ in the first run. This is because of the increased protonating of SiO₂-Im per unit mass leading to more PW loading, and the PW(0.058)/SiO₂Im⁺Cl⁻SiO₂ also contains the part of SiO₂-Cl. However, the reusability of PW/SiO₂-Im was far less active than the catalyst of PW(0.058)/SiO₂ Im⁺Cl⁻SiO₂. There were two possible reasons: one was that the stability of NH₂⁺ in imidazoline was low and the other was that the recovery efficiency of individual SiO₂ nanoparticles was much lower than the collective SiO₂ network on account of the catalysts size.

Table 3. Catalytic reusability of PW(0.058)/SiO₂Im⁺Cl⁻SiO₂ and PW(0.082)/SiO₂-Im for the epoxidation of cyclooctene.

Run a	PW(0.058)/SiO2Im+Cl−SiO2		PW(0.082)/SiO2-Im
	Conversion (%)	Selectivity (%)	Conversion (%)	Selectivity (%)
1	94.4	99.3	96.0	98.2
2	88.7	97.5	27.1	87.3
3	82.1	95.1	25.8	81.6
4	75.2	94.6	18.6	78.3

The reusability experiment was carried out by adding the residual catalyst in the next reaction after centrifugation and drying without any change of other components. ^a Reaction conditions: cyclooctene: 2 mmol, acetonitrile: 5 mL, H₂O₂: 6 mmol, catalysts: 0.02 g, temperature: 70 °C, reaction time: 6 h.

Table 3. Catalytic reusability of PW(0.058)/SiO₂Im⁺Cl⁻SiO₂ and PW(0.082)/SiO₂-Im for the epoxidation of cyclooctene.

Run a	PW(0.058)/SiO2Im+Cl−SiO2		PW(0.082)/SiO2-Im
	Conversion (%)	Selectivity (%)	Conversion (%)	Selectivity (%)
1	94.4	99.3	96.0	98.2
2	88.7	97.5	27.1	87.3
3	82.1	95.1	25.8	81.6
4	75.2	94.6	18.6	78.3

The reusability experiment was carried out by adding the residual catalyst in the next reaction after centrifugation and drying without any change of other components. ^a Reaction conditions: cyclooctene: 2 mmol, acetonitrile: 5 mL, H₂O₂: 6 mmol, catalysts: 0.02 g, temperature: 70 °C, reaction time: 6 h.

2.2.1. Effects of Temperature and Time

Each chemical reaction was accompanied by thermal effects. The same reaction carried out at different temperatures would result in quite different results. The reaction time was an important basis to judge it effective or not. The effects of temperature and time on the epoxidation of cyclooctene using catalyst PW(0.058)/SiO₂Im⁺Cl⁻SiO₂ were shown in Figure 8. It was observed that with the increase of temperature, the molecular energy, and relative content of activated molecules increased, which increased the effective collision between the reactant molecules, thereby improving the conversion. However, the increase of temperature also increased the possibility of ring rupture, resulting in the decrease of selectivity. When temperature reached 70 °C, the reaction had tended to balance. However, the conversion decreased at 80 °C and the selectivity was in a sharp decline which might be due to the excessive activation of the catalyst. And with the increase of reaction time, the conversion increased while selectivity decreasing. As a whole, the conversion and selectivity were both higher than 90% at 70 °C in 6 h; specifically, 94.4% and 99.3%, respectively.

Figure 8. Effect of temperature and time on (A) conversion; and (B) selectivity. (a): 60 °C; (b): 70 °C; and (c): 80 °C.

Figure 8. Effect of temperature and time on (A) conversion; and (B) selectivity. (a): 60 °C; (b): 70 °C; and (c): 80 °C.

2.2.2. Effect of Catalyst Dosage

The influences of different catalyst amounts were investigated and the results were shown in Figure 9. It was indicated that the conversion increased with the increase of catalyst amount, but further improved slightly when the amount was over 300 mg. The selectivity had almost no change when the amount was over 200 mg. Taking economics and green chemistry into consideration, a low catalyst amount (200 mg) was used in further experiments.

Figure 9. Effect of temperature and time on (A) conversion and (B) selectivity. (a): 0.01 g; (b):0.02 g; (c): 0.03 g; (d): 0.04 g.

Figure 9. Effect of temperature and time on (A) conversion and (B) selectivity. (a): 0.01 g; (b):0.02 g; (c): 0.03 g; (d): 0.04 g.

2.2.3. Effect of Solvent

In order to investigate the general application of the synthetic catalyst in different solvents, the influences of solvents on the oxidation of cyclooctene were studied and the results were summarized in Table 4. Acetonitrile, methanol, ethanol, chloroform, 1,2-dichloroethane, and ethyl acetate were used as solvents. The results demonstrated that the solvents were general relevant to alkenes epoxidation, especially for acetonitrile, methanol, and chloroform. In particular, the catalytic reaction was more suitable in the polar solvent. Due to the high boiling point and stable selectivity of the product, acetonitrile was, thus, chosen as the reaction medium in our experiment.

Table 4. Effect of solvents on epoxidation of cyclooctene.

Entry	Solvent	Boiling Point (°C)	Conversion		Selectivity
			4 h	8 h	4 h	8 h
1	Acetonitrile	81.6	88.8	99.4	99.4	99.2
2	Methanol	64.5	92.9	96.8	99.1	96.6
3	Ethanol	78.2	52.0	77.1	98.8	98.8
4	Chloroform	61.2	98.9	100.0	98.6	96.6
5	1,2-Dichloroethane	83.5	8.8	15.0	94.0	93.6
6	Ethyl acetate	78.3	4.5	5.5	85.8	88.2

Reaction conditions: cyclooctene: 2 mmol, solvent: 5 mL, H₂O₂: 6 mmol, catalysts: 0.02 g, temperature: 70 °C.

Table 4. Effect of solvents on epoxidation of cyclooctene.

Entry	Solvent	Boiling Point (°C)	Conversion		Selectivity
			4 h	8 h	4 h	8 h
1	Acetonitrile	81.6	88.8	99.4	99.4	99.2
2	Methanol	64.5	92.9	96.8	99.1	96.6
3	Ethanol	78.2	52.0	77.1	98.8	98.8
4	Chloroform	61.2	98.9	100.0	98.6	96.6
5	1,2-Dichloroethane	83.5	8.8	15.0	94.0	93.6
6	Ethyl acetate	78.3	4.5	5.5	85.8	88.2

Reaction conditions: cyclooctene: 2 mmol, solvent: 5 mL, H₂O₂: 6 mmol, catalysts: 0.02 g, temperature: 70 °C.

3. Experimental Section

3.1. Materials and Methods

N-(3-triethoxysilylpropyl)-4,5-dihydroimidazol and 3-aminopropyl-trimethoxysilane (APTES) were purchased from Meryer (Meryer, Shanghai, China). Other commercially-available chemicals were bought from local suppliers (Sinopharm Chemical Reagent, Beijing, China). All reagents were purified by standard procedures before use. FT-IR spectra were obtained as potassium bromide pellets in a Nicolet 360 FT-IR thermoscientific spectrometer in the 4000–400 cm⁻¹ region (Thermo Fisher Scientific, Waltham, MA, USA). The elemental analyses were performed on a CHN elemental analyzer (Elementar, Hanau, HE, Germany). TG analysis was carried out with a STA409 instrument in dry air at a heating rate of 10 °C·min⁻¹ (Mettler Toledo, Zurich, Switzerland). SEM image was performed on a HITACHI S-4800 field-emission scanning electron microscope (Hitachi, Tokyo, Japan). Transmission electron microscopic (TEM) photographs of the prepared samples were taken in JEOL JEM 2100 electron microscope under an accelerating voltage of 200 kV (JEOL, Tokyo, Japan). The metal loading of the host materials of Tungsten were determined by inductively-coupled plasma atomic emission spectroscopy (ICP-AES) on a Perkin-Elmer AA-300 spectrophotometer (Shimadzu, Kyoto, Japan). Nitrogen adsorption/desorption isotherms were measured at −196 °C using a Quantachrome Quadrasorb SI automated gas sorption system (Micromeritics instrument corp, Atlanta, GA, USA). Samples were degassed under vacuum for 5 h at 120 °C. The micropore volume was obtained with the t-plot method, while the Brunauer-Emmet-Teller (BET) method was applied to calculate the specific surface area. Pore size distributions were evaluated from desorption branches of nitrogen isotherms using the BJH model. The total pore volume was determined at P/P₀ 0.95. The X-ray diffraction (XRD) pattern of the material was recorded on a Bruker D8 advanced powder X-ray diffractometer using Cu Ka (k = 1.5406 Å) as the radiation source in 2θ range of 4°–70° with a step size of 4° and a step time of 1 s (Bruker Axs Gmbh, Karlsruhe, BW, Germany). DLS experiments were carried out with previous sonication of the samples. The run time of the measurements was 10 s. Every size distribution curve was obtained by averaging six measurements. The apparatus was an ALV/DLS/SLS-5022F light scattering electronic spectrometer (ALV-GmbH, Langen, Germany).

3.2. Catalyst Preparation

Silica nanoparticles, as well as the surface functionalization of the silica nanoparticles with N-(3-triethoxysilylpropyl)-4,5-dihydroimidazol and APTES, were prepared according to literature with minor modification [27]. The typical preparation procedure of the catalyst was in Scheme 1.

Scheme 1. Typical preparation procedure of the catalysts PW(x)/SiO₂ Im⁺Cl⁻ SiO₂. (A) Synthesis of SiO₂-Im; (B) Synthesis of SiO₂-Cl; (C) Synthesis of PW(x)/SiO₂ Im⁺Cl⁻ SiO₂.

Scheme 1. Typical preparation procedure of the catalysts PW(x)/SiO₂ Im⁺Cl⁻ SiO₂. (A) Synthesis of SiO₂-Im; (B) Synthesis of SiO₂-Cl; (C) Synthesis of PW(x)/SiO₂ Im⁺Cl⁻ SiO₂.

3.2.1. Synthesis of Silica Nanoparticles

74 μL ammonia solution (25%–28%) and 1.98 g (110 mmol) water were added to 100 mL absolute methanol in a 250 mL round bottom flask. The solution was stirred for 5 min before adding dropwise 10.41 g (500 mmol) TEOS. The final solution was stirred for three days at ambient temperature. The resulting solid was centrifuged and washed with methanol and water several times, and dried under vacuum.

3.2.2. Synthesis of Modified Silica Nanoparticles

0.6 g (0.01 mol) previously prepared silica nanoparticle was dispersed in 50 mL anhydrous toluene by sonication for 60 min. Then 0.005 mol N-(3-triethoxysilylpropyl)-4,5-dihydroimidazol (or APTES) was added dropwise. The solution was stirred under 110 °C for 24 h. The product was filtered, washed in a Soxhlet apparatus with diethyl ether and dichloromethane for 24 h, and then dried at 50 °C under vacuum. The obtained powder was named as SiO₂-Im and SiO₂-Cl, respectively.

3.2.3. Synthesis of Silica Nanoparticle Network

The synthesis was driven by a nucleophilic substitution occurring between an imidazoline functional group and a chloroalkyl group present on the surface of the silica nanoparticles. 0.5 g silica nanoparticles modified with N-(3-triethoxysilylpropyl)-4,5-dihydroimidazol and 0.5 g silica nanoparticles modified with APTES were introduced into a 100 mL round bottom flask with 50 mL anhydrous toluene. The solution was stirred over 2 days at 70 °C and filtered, washed, finally dried under vacuum. A pale yellow powder was obtained and labelled as SiO₂ Im⁺Cl⁻ SiO₂. Elemental analysis: found C: 7.25 wt. %, H: 1.45 wt. %, N: 1.96 wt. %.

3.2.4. Synthesis of Phosphotungstate-Loaded Silica Nanoparticle Network

Catalysts of PW(x)/SiO₂ Im⁺Cl⁻ SiO₂ with different H₃PW₁₂O₄₀ loadings on SiO₂ Im⁺Cl⁻ SiO₂ were prepared by following strategy. 1.0 g silica nanoparticle network was dispersed in 20 mL deionized water and then added dropwise into an aqueous solution (20 mL) with various amount of H₃PW₁₂O₄₀ (0.2 g, 0.6 g, 1.0 g, 1.4 g). The solution was stirred for 24 h at ambient temperature. Upon centrifugation, the precipitate was washed with deionized water several times, and then dried in vacuum overnight at room temperature. The obtained catalysts were labelled as PW(x)/SiO₂ Im⁺Cl⁻ SiO₂, where x was the loading amount of PW. The PW loading could be calculated by ICP-AES. The results showed that the loading of PW were ca. 0.035, 0.058, 0.074, or 0.17 mmol/g, respectively.

3.2.5. Anion Exchange Experiment

The starting compound SiO₂ Im⁺Cl⁻ SiO₂ was dispersed in 25 mL acetone. The salt NaBF₄ for the exchange was added in mass in a ratio of 1:1, compared to the starting compound. The dispersion was stirred for 24 h at room temperature. Then, the resultant product SiO₂ Im⁺BF₄⁻ SiO₂ was centrifuged and washed with acetone and deionized water. The solvent and washing phases were combined and evaporated under vacuum. The salt obtained after evaporation and was dried in vacuum over P₂O₅. White powders are obtained as NaBF₄ and NaCl.

3.3. Catalytic Performance

Cyclooctene (2 mmol), CH₃CN (5 mL), and catalyst (0.02 g) were added to a 25 mL flask. The reaction started after the addition of aqueous H₂O₂ (30 wt. %, 6 mmol) at a definite temperature within 10 min under vigorous stirring. After reaction, the product mixture was analyzed by gas chromatography mass spectrometry (GC-MS). The catalyst was recovered by centrifugation, dried and used for the next run.

4. Conclusions

In this research, a silica network catalyst was prepared by connecting silica nanoparticles with ionic liquid and followed reacting with phosphotungstate. A series of characterization methods were carried out to confirm the successful synthesis of the material. The synthetic catalyst was effective heterogeneous catalyst for the epoxidation of cyclooctene with H₂O₂. Conversion and selectivity of epoxy-cyclooctene could both reach over 99% at 70 °C for 8 h using hydrogen peroxide as an oxidant in acetonitrile. Compared with silica nanoparticles, the reported work demonstrated that the inorganic-organic hybrid silica network performed as a more promising material in various fields for its collective properties: better pore structure, much easier to separate and, thus, to recycle.