1 Introduction

Nowadays, transition metal oxide catalyst such as nickel oxide, [1] cobalt oxide, [2] copper oxide, [3] manganese oxide, [4] vanadium oxide, [5] molybdenum oxide, [6] bimetallic oxide [7] and polymetallic oxide [8] are widely synthesized and used. Among these oxides, nickel oxide is a binary transition metal oxide with a wide range of applications as a p-type semiconductor with a wide band gap. [9] In particular, due to the effect of quantum confinement, the nickel oxide with nanostructure shows excellent optical, magnetic and electronic properties. Owing to its high theoretical specific capacitance, nickel oxide is also a promising material as an electrode of supercapacitor. [10] Furthermore, nickel oxide can also be applied to catalyze the oxidation of CO, methanol and hydrazine, [11] epoxidation of styrene and cyclooctene, [12] decomposition of ethanol, [13] dehydrogenations of ethane, [14] etc.

Scheme 1
scheme 1

The schematic illustration for the synthetic route of xNi-MCM-41.

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However, since the nanoparticle would easily be aggregated into large bulk, quantum-confined nickel oxide nanoparticles, especially the highly-dispersed nickel oxide nanoparticles, tend to be difficult to obtain during synthesis. [15] Therefore, in the process of catalyst synthesis, a support is necessary to disperse the nanoparticles to prevent agglomeration. Mesoporous silica is well-known for its high specific surface area, high porosity and oriented channels, which are the characteristics of ideal catalyst support materials. [16] Particularly, mesoporous MCM-41 is a promising support because of its large surface area, high thermal stability and large pore size. [17]

Numerous studies have revealed that NiO as an active phase showed a superior catalytic performance in olefin epoxidation. [18] Additionally, nickel complexes as active phase, supported over ordered mesoporous silica as catalysts for olefin epoxidation, have also been reported. [19] Styrene oxide is an important intermediate for a large variety of fine chemicals. [20] So catalytic oxidation of styrene to styrene oxide is significant from both academic and commercial point of view. NiO supported on \(\hbox {Al}_{2}\hbox {O}_{3}\) [21] has been used as a catalyst for epoxidation of styrene. However, due to the fact that the specific surface area of \(\hbox {Al}_{2}\hbox {O}_{3}\) is not high enough (336.7 m [2]/g), NiO was not highly dispersed, resulting in the conversion of styrene to only 60.2%. Therefore, using materials with higher specific surface area as support could improve the catalytic activity of the catalyst.

Considering this, we chose MCM-41 with the high specific surface area as support and synthesized supported catalysts with different nickel oxide loadings via the method of combining hydrothermal with calcination. The catalysts were used to catalyze epoxidation of styrene, and to the best of our knowledge, using nickel modified MCM-41 as a catalyst for the selective epoxidation of styrene has not been reported.

Herein, we proposed a novel templating self-assembly route based on the electrostatic assembly mechanism, and the route could illustrate the formation mechanism of the hexagonal channel in MCM-41 adequately. Scheme 1 illustrates the process, formation of micelles by surfactants dispersed in water, and subsequently rod-like micelles due to the aggregation of micelles. Afterwards, the addition of silicon source resulted in the self-assembly of micelles and the formation of the hexagonal outline. Then, electronegative silicate oligomers would match with electropositive ions in the tail of the micelles and an inorganic silicon wall formed on the surface of the organic hexagonal prism, eventually. After that, Ni[2+] anchored on the surface of silicon gel entered the pores that are formed by silicone gel under the effect of surfactant during the hydrothermal reaction and formed highly-dispersed NiO nanoparticles on the surface and pore wall of MCM-41 material during the calcination process.

2 Experimental

2.1 Materials and synthetic procedures

Tetraethyl orthosilicate (TEOS, AR), cetyltrimethyl ammonium bromide (CTAB, AR), ammonia (25%, AR), nickel nitrate hexahydrate (Ni(\(\hbox {NO}_{3})_{2}\cdot \hbox {6H}_{2}\)O, AR), acetonitrile (\(\hbox {CH}_{3}\)CN, AR) and tert-butyl hydroperoxide (TBHP, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. Styrene (\(\hbox {C}_{8}\hbox {H}_{8}\), AR) was purchased from Shanghai Macklin Biochemical Co., Ltd.

A typical synthetic process for the preparation of modified MCM-41 follows the reported procedures [22] with minor modification. In a typical synthesis, 2.62 g of CTAB was dissolved in 125 mL of deionized water, and then 36 mL of 25% ammonia was added to the solution under vigorous stirring. Then, 14 mL of TEOS was slowly added to the above solution. After 0.5 h of stirring at \(35\, {^{\circ }}\hbox {C}\) and the hydrolysis of TEOS was complete, the resulting solution turned milk-white, then different amounts of Ni(\(\hbox {NO}_{3})_{2}\cdot \hbox {6H}_{2}\)O (2, 4, 6 mmol) as nickel source were dissolved in deionized water and were added to the above solutions, respectively. After that, the resulting gel was allowed to crystallize at \(110\, {^{\circ }}\hbox {C}\) for 52 h in a Teflon-lined autoclave. The sample was filtered, washed with deionized water up to neutral pH, and dried at \(60 \, {^{\circ }}\hbox {C}\) for 12 h. Finally, the resultant powder was calcined at \(550 \, {^{\circ }}\hbox {C}\) for 6 h. The samples corresponding to different nickel amount were labelled as xNi-MCM-41, in which x stands for the amount of nickel in different samples.

For comparison, pure MCM-41 was synthesized via the above procedure without the addition of nickel source. Pure NiO nanoparticles were also synthesized by following the procedures of preparing Ni-base MCM-41 without adding TEOS.

Fig. 1
figure 1

Small-angle (A) and large-angle (B) XRD patterns of samples: (a) MCM-41, (b) 2Ni-MCM-41, (c) 4Ni-MCM-41, (d) 6Ni-MCM-41.

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2.2 Characterization methods

Crystalline phases of the catalysts were measured by X-ray Diffraction (XRD) with a Cu\(K\alpha \) radiation source (\(\lambda = 0.15406\) nm, Smartlab, Rigaku). The X-ray tubes worked at 40 kV and 30 mA. The small angle XRD was recorded over the 2\(\theta \) range of \(1--10 {^{\circ }}\) with a step size of \(0.02{^{\circ }}\) at an acquisition time of 0.4 s. The wide-angle XRD was collected over the 2\(\theta \) range from \(10{^{\circ }}\) to \(80{^{\circ }}\) with a step size of \(0.1{^{\circ }}\) at a collection time of 0.3 s. Crystal phases were identified by Joint Committee on Powder Diffraction Standards (JCPDS) files.

Textural properties of the samples were derived from \(\hbox {N}_{2}\) adsorption/desorption measurement at 77 K on Quadrasorb SI 2MP. Prior to the measurement, the samples were out-gassed at \(150 \, {^{\circ }}\hbox {C}\) for 12 h. The average pore diameter and pore size distribution curves were established from the adsorption branches of isotherms using the Barrett-Joyner-Halenda (BJH) method.

Surface morphology of the catalysts was examined by transmission electron microscopy (TEM) (Tecnai-G20, FEI) and operated at an accelerating voltage of 200 kV. The samples were dispersed ultrasonically in ethanol and then deposited on a TEM copper grid before the measurement.

The loadings of nickel in catalysts were measured using Inductively Coupling Plasma spectrometer (ICP) (Spectroblue ICP-OES). Fourier transform infrared (FT-IR) spectra were recorded in the range of 400–4000 cm[-1] on Thermo Fisher Nicolet 5700 spectrophotometer in KBr pellets. UV-Vis diffuse reflectance Spectrometer (DRS) (UV-2600, Shimadzu) equipped with a diffuse reflectance accessory was used. The spectra were recorded in the range 200–800 nm at room temperature.

Surface chemical composition of the samples was investigated by X-ray photoelectron spectroscopy (XPS) (K-Alpha, Thermo Fisher) equipped with a standard Al K\(\alpha \) excitation source (1486.6 eV). The C1s peak at 284.6 eV was used as the reference for binding energies.

The reduction behavior of the prepared materials was investigated by hydrogen temperature-programmed reduction (TPR) measurement on a chemisorption analyzer (Micor 2920, Micromeritics) equipped with a thermal conductivity detector (TCD). TPR is a convenient technique for studying the reduction behavior of supported oxide materials reducible at different temperature intervals. Before the TPR analysis, the carbonate and hydrate’ impurities were removed by flowing argon over the catalyst at a flow rate of 30 mL/min at \(300 \, {^{\circ }}\hbox {C}\) for 1 h and the system was then cooled to room temperature. The amount of \(\hbox {H}_{2}\) uptake during the reduction was measured continuously using TCD.

2.3 Catalytic activity measurements

The epoxidation of styrene reaction was carried out in a 50 mL two-necked flask equipped with a condenser. Generally, 1041.5 mg styrene, 7.9 mg acetonitrile and 50 mg catalyst were added to a 50 mL two-necked flask, then 10 mmol tert-butyl hydroperoxide as oxidant was added. Finally, the reaction began after immersing the flask into oil bath, and kept at 80 \(^{\circ }\)C. The reaction was carried out under magnetic stirring. The products of epoxidation were analyzed by a gas chromatograph (INESA GC-126) equipped with OV-1701 capillary column and a FID detector with toluene as internal standard. The characterization of the main and by-products were performed by comparison of their retention times with standard samples. The alkene conversion and selectivity of products were calculated from peak area by standard addition method using Eqs. (1) and (2), respectively:

$$\begin{aligned}&\hbox {Conversion}\,(\%)=\frac{\hbox {mole of reactant converted}}{\hbox {mole of the reactant in feed}}\times {100} \end{aligned}$$
(1)
$$\begin{aligned}&\hbox {Product selectivity}\,(\%)\nonumber \\&\qquad \qquad \qquad \qquad =\frac{\hbox {mole of the product formed}}{\hbox {mole of the reactant converted}}\times {100}\nonumber \\ \end{aligned}$$
(2)

In a typical experiment, after recovering the catalyst from the reaction mixture at optimum conditions, it was washed several times with acetonitrile and dried, and then reused under the same conditions for catalyst reusability test.

Table 1 The textural properties of samples.
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Fig. 2
figure 2

\(\hbox {N}_{2}\) adsorption-desorption isotherms (A), and pore size distribution curve derived from the adsorption branch (V: pore volume, D: pore diameter) (B) for samples with different nickel oxide loadings: (a) MCM-41, (b) 2Ni-MCM-41, (c) 4Ni-MCM-41 and (d) 6Ni-MCM-41.

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Fig. 3
figure 3

TEM images of MCM-41 (\(\hbox {a}_{1}\) and \(\hbox {a}_{2})\), 2Ni-MCM-41 (\(\hbox {b}_{1}\) and \(\hbox {b}_{2})\) and 4Ni-MCM-41 (\(\hbox {c}_{1}\) and \(\hbox {c}_{2})\).

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Fig. 4
figure 4

TEM images of 6Ni-MCM-41 (A and B) and SAED pattern (C), particle-size distribution of NiO nanoparticles (D).

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3 Results and Discussion

3.1 Structure and morphology

The X-ray powder diffraction measurement was used to characterize the crystallographic structure of the synthesized materials. The small-angle XRD patterns of samples are shown in Figure 1A. An intense main diffraction peak and two small peaks were well-resolved at \(2.1{^{\circ }}\), \(3.6{^{\circ }}\) and \(4.2{^{\circ }}\), corresponding to the (111), (200) and (220) planes, which are the characteristics of the hexagonal pore mesostructure of the synthesized materials (JCPDS 49-1712). [23] Compared with pure MCM-41, the diffraction peak of Ni-based materials was shifted toward higher angles, indicating the decrease of \(\hbox {d}_{100}\) interplanar spacing, which can be seen in the data in Table 1. The \(\hbox {d}_{100}\) values were obtained from Jade 5.0 and the hexagonal unit cell parameter \(\hbox {a}_{0}\) was calculated byh the equation, \(\hbox {a}_{0}= \hbox {2d}_{100}\surd 3\). The increase of the d-values and \(\hbox {a}_{0}\) can be attributed to the higher ionic radii of \(\hbox {Ni}^{2+}\) (69 pm) in comparison to \(\hbox {Si}^{4+}\) (40 pm) and due to the longer bond lengths of Ni-O in comparison to Si-O bonds, [24] suggesting the successful incorporation of NiO.

Furthermore, when the load of nickel was less than or equal to 4 mmol, there was no obvious change in the intensity of the peaks, indicating that the structural integrity was retained after modification. However, when the load of nickel reached 6 mmol, the intensity of the peaks weakened, especially for the intensity of (100) peak, indicating that the structures were retained but with a slight distortion of the channels. [25]

In large-angle XRD patterns (Figure 1B), all samples showed a broad diffraction peak at \(23{^{\circ }}\), which was ascribed to the amorphous framework of MCM-41. [26] And, 6Ni-MCM-41 revealed three peaks corresponding to (111), (200) and (220) reflections assigned to the crystalline NiO (JCPDS 47-1049). While for 4Ni-MCM-41, several weak peaks which are specific to nickel oxide were detected. As for 2 Ni-MCM-41, the peak that corresponds to nickel oxide was hardly found, suggesting that no NiO nanoparticle was present on the external surface of MCM-41 materials or the micro-morphology grain formed was below the detection limit of XRD. [27] These results indicated that the emerging crystal diffraction peaks were accompanied by an enhanced trend with the increase of nickel loading in the samples, owing to the increment of crystallinity and enlargement of grain size for nickel oxide nanoparticles according to Scherrer’s Equation. [28]

To further investigate the textural information of all samples, the \(\hbox {N}_{2}\) adsorption/desorption isotherms and pore size distributions were obtained and depicted in Figure 2A and B, respectively. The surface areas were measured with the BET method, pore sizes were calculated from the adsorption branch by BJH method. The surface area, pore volume, pore size and wall thickness (\(\hbox {W}_{d})\) are also summarized in Table 1. As observed in Figure 2A, all the samples exhibited typical IUPAC type IV adsorption isotherm with a \(\hbox {H}_{1}\) type hysteresis loop, indicating that the mesostructure of the synthesized samples [29] and the incorporation of nickel oxide did not obviously destroy the mesophase of the samples. And along with the increase of nickel loading in the samples, the size of the hysteresis loop varied with an increasing trend, indicating a declining trend of pore size of samples (Table 1).

For all the samples, another common feature is that the sharp step over the narrow range of relative pressure at P/\(\hbox {P}_{0}\) of 0.25–0.4 corresponded to the characteristic of capillary condensation inside the conventional mesoporous present in MCM-41 structure. [30] Upon nickel incorporation, the location of the inflection step remained the same, suggesting that all the samples kept ordered mesoporous structure (Figure 2B), while the sharpness turned flat and the adsorption amount decreased significantly, owing to the fact that the declining specific surface area (Table 1) restrained monolayer adsorption of nitrogen towards the walls of the mesopores. [31] Furthermore, as shown in Figure 2B, all the prepared materials were mesoporous with a narrow pore size distribution at the range of 2.5–3.5 nm. And the vertices of the curves all appeared at 2.75 nm, indicating that the number of pores with a diameter of 2.75 nm is the most.

The TEM images of the samples are shown in Figures 3 and 4. For MCM-41, the well-ordered mesopores and straight channel are clearly depicted. The diameter of mesopores was about 2.73 nm and the thickness of mesoporous wall was about 1.69 nm (inset Figure 3a\(_{2})\), so the distance between two cylindrical mesopores was about 4.42 nm. The data is consistent with the data in Table 1. Compared to the pure MCM-41, Ni-based materials also showed well-ordered mesoporous channel with minor defects in mesoporous walls as a consequence of incorporation of nickel oxide nanoparticles in the walls. Moreover, NiO nanoparticles were not observed over 2Ni-MCM-41 and 4Ni-MCM-41, but the data from Table 1 confirms the presence of nickel and the molar ratio of nickel to silicon is very close to that of the initial mixture. NiO nanoparticles could be observed over 6Ni-MCM-41 (Figure 4B), and the results are consistent with XRD analysis.

The selected-area electron diffraction (SAED) pattern and the particle-size distribution of NiO nanoparticles are also shown in Figure 4. Diffraction rings assigned to the (111), (200), (220) and (311) planes of NiO nanocrystals are clearly seen in Figure 4C. As shown in Figure 4D, the particle size of the NiO nanoparticles varied from 1 to 10 nm, and the vast majority was in the 5–7 nm range.

The TEM image and XRD pattern of as-prepared pure NiO nanoparticles are presented in Figure 5. The sharp diffraction peaks (Figure 5B) are assigned to NiO nanocrystals (JCPDS 47-1049) which proved that the NiO with high crystallinity had been synthesized successfully. As shown in Figure 5A, the length and width of rod-like NiO nanoparticles are about 42 nm and 24 nm, respectively. While the maximum size of NiO nanoparticles supported on MCM-41 is only about 10 nm, which is much smaller than that of pure NiO nanoparticles, indicating that the silicon source is beneficial to the formation of small size NiO nanoparticles.

Fig. 5
figure 5

(A) TEM image and (B) XRD pattern of prepared NiO nanoparticles.

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Fig. 6
figure 6

FT-IR spectra of samples with different nickel oxide loadings and NiO: (a) MCM-41, (b) 2Ni-MCM-41, (c) 4Ni-MCM-41, (d) 6Ni-MCM-41 and (e) NiO.

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Fig. 7
figure 7

UV-Visible spectra of the samples with different nickel oxide loadings: (a) MCM-41, (b) 2Ni-MCM-41, (c) 4Ni-MCM-41 and (d) 6Ni-MCM-41 and (e) NiO.

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3.2 State of nickel species

Figure 6 shows the FT-IR spectra of all samples in the region from 4000 to 400 \(\hbox {cm}^{-1}\). The band at 3445 \(\hbox {cm}^{-1}\) observed in all samples are ascribed to the symmetrical stretching vibration of -OH groups on the surface of MCM-41 and the band at 1627 \(\hbox {cm}^{-1 }\)is assigned to the bending vibration of adsorbed water. [32] The bands at 1083 and 808 \(\hbox {cm}^{-1}\) are related to the asymmetric stretching vibration and symmetric stretching vibration of Si-O-Si in the framework of silica, [33] respectively. Additionally, the absorption bands at 976 \(\hbox {cm}^{-1 }\)confirmed the presence of silanol group Si-OH and its symmetric stretching vibration; [34] moreover, the intensity of the absorption peaks showed a decreasing trend with increasing loading of nickel in these samples, suggesting the decreasing of surface Si-OH in the samples. That could be attributed to the fact that the nickel oxide nanoparticle incorporated in the silicate walls and interacted with the walls. [35] And the FT-IR bands at 457 \(\hbox {cm}^{-1}\) are related to the bending vibration of the rocking mode of Si-O-Si. [36]

Fig. 8
figure 8

XPS spectra of 6Ni-MCM-41.

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The presence of Ni=O stretching vibration was confirmed by the absorption band at 669 \(\hbox {cm}^{-1}\), demonstrating the presence of NiO in MCM-41. [37] For comparison, the FT-IR of NiO is also depicted in Figure 6, and the band at 669 \(\hbox {cm}^{-1}\) was also observed in NiO, further confirming that NiO has been loaded into MCM-41. In addition, the absorption band at 426 \(\hbox {cm}^{-1}\) in NiO is assigned to the Ni-O vibration mode [38] which could be covered by the band at 457 \(\hbox {cm}^{-1}\) related to the bending vibration of the rocking mode of Si-O-Si in Ni-base MCM-41.

UV-Vis spectra of the samples are shown in Figure 7. A strong absorption in the UV region was observed at 378 nm in 6Ni-MCM-41, which approached the absorption at 360 nm and 364 nm, attributed to the band gap absorption in NiO that is reported in literature. [38] What is more, the as-synthesized NiO also showed the absorption at the same wavelength, confirming the presence of NiO in MCM-41. While the band was not found in other Ni-based materials, which could be due to the fact that the loading of NiO was too little to be detected. Deservedly, the parent MCM-41 showed no absorption assigned to NiO, either.

Figure 8 shows the XPS spectra of 6Ni-MCM-41, and the survey spectrum (Figure 8A) identified the presence of Ni, O and Si clearly. The high resolution Ni 2p XPS spectrum (Figure 8B) showed two prominent peaks at 874 and 856 eV, assigned to Ni \(\hbox {2p}_{1/2}\) and Ni \(\hbox {2p}_{3/2}\) states of \(\hbox {Ni}^{2+ }\)in NiO, [39] respectively. And the peak at 880 eV corresponded to the satellite peak of Ni \(\hbox {2p}_{1/2}\), while the peak at 862 eV is assigned to the satellite peak of Ni \(\hbox {2p}_{3/2}\). [40] Besides, as shown in Figure 8C, the peak centered at 532 eV could be attributed to the binding energy of O 1s, which is ascribed the \(\hbox {O}^{2-}\) in the NiO. [41] All these results confirmed the presence of NiO.

\(\hbox {H}_{2}\)-TPR technique was adopted to study the reduction behavior of supported NiO materials. The \(\hbox {H}_{2}\)-TPR curve of 6Ni-MCM-41 is shown in Figure 9. 6Ni-MCM-41 materials exhibited two reduction peaks at about \(360 \, {^{\circ }}\hbox {C}\) and \(600 \, {^{\circ }}\hbox {C}\). The peak at \(360 \, {^{\circ }}\hbox {C}\) could be assigned to the reduction of the NiO on the external surface of MCM-41materials, and the other peak at \(600 \, {^{\circ }}\hbox {C}\) could be ascribed to the reduction of the NiO in the mesoporous walls of MCM-41materials. According to some literature, the reduction peak at \(600 \, {^{\circ }}\hbox {C}\) could be related to nickel oxide layer that interacted strongly with silicate surface. [42] Additionally, the peak at \(600 \, {^{\circ }}\hbox {C}\) was much higher than that of \(360 \, {^{\circ }}\hbox {C}\), indicating that the content of NiO on the external surface of MCM-41 was much less than the NiO in the mesoporous walls of MCM-41 material. That is because of the fact that \(\hbox {Ni}^{2+}\) prefers to enter the pores formed by silicone gel under the effect of capillarity during the hydrothermal reaction and incorporated into the pore wall which then formed NiO during the calcination process. That could also explain above analysis results. As for 2Ni-MCM-41 and 4Ni-MCM-41, the content of \(\hbox {Ni}^{+}\) was so little that all \(\hbox {Ni}^{2+}\) were incorporated into pore wall and highly dispersed NiO nanoparticles were formed eventually, so there was no NiO nanoparticle on the external surface of MCM-41material, and hence NiO could not be detected. While for 6Ni-MCM-41, there was enough NiO present on the external surface of MCM-41 to be detected.

3.3 Catalytic results

The catalytic properties of all samples were evaluated by the liquid-phase epoxidation of styrene using tert-butyl hydroperoxide as oxidant and the corresponding catalytic results are presented in Table 2. For comparison, the catalytic properties of pure MCM-41 and NiO NPs were also investigated. As we can see, pure MCM-41 showed no catalytic activity towards the epoxidation of styrene, while NiO nanoparticles exhibited good catalytic activity, suggesting that nickel oxide should be the active centers for oxidation of styrene. But, as for 2Ni-MCM-41 and 4Ni-MCM-41, the conversion of styrene was poor as compared to NiO NPs although the selectivity of styrene oxide increased to a certain extent. That may be due to the fact that the loading of NiO was too little to achieve excellent catalytic effect, and the catalytic performance of 6Ni-MCM-41 mentioned below confirmed that. Owing to high dispersion of nickel oxide on the support and the synergistic effect between NiO nanoparticles and the supports, 6Ni-MCM-41 gave higher conversion and selectivity compared to pure NiO nanoparticles. What is more, a conclusion that NiO nanoparticles with smaller size possessed better catalytic performance, which can be understood by considering the size of NiO nanoparticles supported on MCM-41 was much smaller than that of pure NiO nanoparticles. The catalytic performance of 6Ni-MCM-41 was also better than that of other supported nickel oxide catalysts (Table 2).

For heterogeneous catalysts, catalyst reusability is an important parameter. So, the reusability of 6Ni-MCM-41 catalyst was investigated considering it exhibited the best catalytic performance among all the catalysts. The results are shown in Figure 10A. For comparison, the reusability of pure NiO nanoparticles was also investigated and the results are shown in Figure 10B. As displayed in Figure 10A, a sustainable catalytic activity was obtained even after five recycling experiments, indicating that 6Ni-MCM-41 had good stability attributed to the special interaction between nickel oxide and support. While pure NiO nanoparticles showed obvious reduction of the conversion efficiency, the selectivity of styrene oxide stayed at a high level. That could be due to the agglomeration of NiO nanoparticles without the support and dispersion in MCM-41 material.

Fig. 9
figure 9

\(\hbox {H}_{2}\)-TPR pattern of 6Ni-MCM-41.

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Table 2 The catalytic performance of all the samples in the epoxidation of styrene \(^{\mathrm{a}}\)
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Fig. 10
figure 10

Reusability of 6Ni-MCM-41 (A) and pure NiO nanoparticles (B) catalysts in the epoxidation of styrene with TBHP.

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4 Conclusions

In this paper, well-ordered MCM-41 containing highly-dispersed NiO nanoparticles in mesoporous walls and on the surface was synthesized successfully. A novel templating self-assembly route that could illustrate the formation mechanism of hexagonal channel in MCM-41 adequately is proposed. The characterization results revealed that \(\hbox {Ni}^{2+}\) prefers to enter the pores that are formed by silicone gel under the effect of capillarity during the hydrothermal reaction and smaller size NiO nanoparticles are easier to be formed on the MCM-41 materials. Additionally, catalytic results indicated that NiO is the catalytic active center for the oxidation of styrene and the efficient catalytic property of 6Ni-MCM-41 is attributed to high dispersion of nickel oxide on the support.