Highly dispersed molybdenum incorporated hollow mesoporous silica spheres as an efficient catalyst on epoxidation of olefins

Molecular Catalysis 433 (2017) 212–223

Contents lists available at ScienceDirect

Molecular Catalysis journal homepage: www.elsevier.com/locate/mcat

Editor’s choice paper

Highly dispersed molybdenum incorporated hollow mesoporous silica spheres as an efficient catalyst on epoxidation of olefins Yirui Shen, Pingping Jiang ∗ , Jian Zhang, Gang Bian, Pingbo Zhang, Yuming Dong, Weijie Zhang The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, PR China

a r t i c l e

i n f o

Article history: Received 25 September 2016 Received in revised form 12 December 2016 Accepted 18 December 2016 Keywords: Hollow mesoporous silica Modified immersion Molybdenum (VI) incorporated catalyst Olefin Epoxidation

a b s t r a c t Highly dispersed Molybdenum (VI) incorporated hollow mesoporous silica catalyst was facilely synthesized with a selective etching strategy and then followed with a modified immersion method. Firstly, hollow mesoporous silica spheres were obtained from solid silica spheres with the interaction of cetyltrimethyl ammonium bromide (CTAB) and Na2 CO3 . Then the catalytic sites were loaded by modified and direct immersion methods as comparison. The catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption/desorption and X-ray photoelectron spectra (XPS). The reactivity of the catalysts was also detected under the same condition in olefin epoxidation. Compared with these two immersion methods, modified method exhibited the best turnover number of 3873 at 6 h with a better reactivity and textural properties than direct one. Highly dispersed molybdenum indeed brought excellent activity and high stability. The conversion could reach 100% with a selectivity above 99% after 12 h. And the catalyst still had a conversion above 86% and a selectivity above 93% after five runs. More importantly, this modified immersion method could also be widely used in the preparation of catalysts as a simple but efficient way. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Epoxides are important intermediates in the production of fine chemicals and polymer synthesis as well as valuable building blocks for the preparation of various bioactive molecules [1]. Among these reactions, epoxidation of olefins is one of the most significant industrial reactions for epoxide products [2,3]. Simultaneously, catalysts display a pivotal position in reaction. During the last decades, great attention has been paid to this field and a variety of catalysts have been developed. Molybdenum-containing complexes are well known of useful epoxidation catalysts for a long time [4,5]. Molybdenum complexes as homogeneous catalysts with high solubility are considered to be very effective catalysts in epoxidation [5–7], such as soluble metal salts, oxodiperoxo molybdenum(VI) complexes [8] and dioxomolybdenum(VI) complexes [6,9–11]. However, hard separation and difficult recycling are still the main drawbacks for homogeneous catalysts. In this regard, active sites are often loaded to multifarious supports to form heterogeneous catalysts. A variety of supports have be used, for example, molecular sieve [12,13], carbon materials [14], metal

∗ Corresponding author. E-mail address: [email protected] (P. Jiang). http://dx.doi.org/10.1016/j.mcat.2016.12.011 1381-1169/© 2017 Elsevier B.V. All rights reserved.

nanoparticles [15] etc. Different approaches have been used in order to obtain heterogeneous molybdenum catalysts in epoxidation. Morey et al. used the anhydrous reaction of metal alkoxides and surface silanol to synthesize MCM-48 mesoporous silica with functionalized pore surface with tungsten and molybdenum metal centers [16]. Jia et al. prepared an organic-inorganic hybrid heterogeneous catalyst system by covalently anchoring oxodiperoxo molybdenum complexes onto the mesoporous silica MCM-41 for the epoxidation of cyclooctene with t-BuOOH as the oxidant [17]. Herrera et al. used the atomic layer deposition (ALD) method for preparing highly dispersed oxides of tungsten, vanadium, titanium, and molybdenum oxide catalyst supported on mesoporous silica [18]. Farahani et al. used aminopropyl modified MCM-41 to incorporate bidentate Schiff base ligands leading to a firmly incorporated catalyst with high reactivity and selectivity of the epoxidation [19]. Abrokwah et al. used one pot hydrothermal procedure to synthesize high surface area M-MCM-41 (M: Cu, Co, Ni, Pd, Zn and Sn) nanocatalysts [20]. Among various approaches, immersion method seems to be a simple and widely used way of loading active sites [21–23], while it will produce aggregation of active sites and severe blocking in channels [23,24]. In order to improve this situation, a modified immersion method was applied in this paper to prepare catalysts with highly dispersed active sites.

Y. Shen et al. / Molecular Catalysis 433 (2017) 212–223

Since the discovery of hollow structures, hollow silica nanomaterials with controlled morphology and structure have attracted a great deal of interest because of their unique properties such as large capacity, low density, excellent thermal and mechanical stability, low toxicity, high biocompatibility and large specific surface areas [25–27]. Based on these excellences, they have tremendous applications including composite materials, controlled drug delivery, catalysis, bioimaging, energy-storage media, electronic devices and so on [28–30]. Recently, more efforts have been committed to the fabrication of hollow nanomaterials due to their superior advantages. Methods of preparing such kind of hollow nanomaterials are varied, many strategies have been developed for constructing hollow nanomaterials. Among them, the template methods including hard templates (e.g. carbon nanoparticles and polystyrene latex particles etc.) and soft templates (e.g. vesicles and emulsion droplets etc.) are one of the most popular methods [31,32]. In these cases, the hollow structure is formed after removing the templates either by calcination at certain temperature or dissolution in relevant solvents. However, both of these two strategies would inevitably destroy the structural integrity and bring out a time-consuming synthetic procedure. Yildirim et al. prepared hollow mesoporous silica nanoparticles with tailored morphology using selective dissolution routine by incubating in phosphate buffered saline (PBS) at 65 ◦ C for one day under mild conditions without the need for corrosive or toxic etchants [33]. Yu et al. used “structural difference-based selective etching” strategy to fabricate hollow/rattle-type mesoporous nanostructures with the interaction of Na2 CO3 and cationic surfactant octadecyltrimethoxysilane (C18 TMS) [34]. Fang et al. developed a straightforward and effective “cationic surfactant assisted selective etching” synthetic strategy for the preparation of high-quality hollow mesoporous silica spheres with either wormhole-like or oriented mesoporous shell [35]. In this work, the catalytic support was synthesized through a cationic surfactant assisted selective etching strategy modified from the reference [35]. After treating solid SiO2 spheres with a solution of Na2 CO3 and cationic surfactant CTAB in a certain amount, hollow mesoporous silica spheres (denoted as HMSS) were easily obtained with wormhole-like shell. As the compounds of molybdenum are soluble in alkaline solution, traditional cocondensation and one-pot synthesis method are not available to incorporate molybdenum in the presence of Na2 CO3 . In order to modify HMSS to be an efficient epoxidation catalyst, active sites were added by modified method. After calcination, incorporated molybdenum was firmly fixed in the shell of hollow mesoporous silica spheres as MoO3 . XRD, TEM, SEM, N2 adsorption/desorption and other characterization means were used to certify the structure and olefin epoxidation were performed to confirm the catalytic activity. After treated with the modified immersion method, the active sites could be located into shell of the material where metal particles could be effectively isolated, and the smooth channels and highly dispersed molybdenum could also be obtained. At the same time, with the hollow interior and the shorter pore channels, reactants and products would be easy to transfer in and out of the supports causing less travelling blockage [36,37]. Both of them will improve the catalytic activity (Scheme 1).

2. Experimental 2.1. Chemicals Analytical reagent grade olefins were purchased from Aladdin, China. Tetraethyl orthosilicate (TEOS), ammonia solution (25–28 wt%), cetyltrimethyl ammonium bromide (C16 TAB), anhydrous ethanol, sodium carbonate, ammonium molybdate

213

Scheme 1. Synthesis of catalyst Mo/HMSS.

and other solvents were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). All the reagents were used without further purification. Deionized water was used all around the experimental process. 2.2. Synthesis of hollow mesoporous SiO2 sphere catalyst 2.2.1. Synthesis of SiO2 spheres Solid SiO2 spheres (denoted as SiO2 ) were synthesized following a modified Stöber method referred to Zhang et al. [38]. In a typical process, 9 mL of ammonium aqueous solution, 16.25 mL of anhydrous ethanol and 24.75 mL of deionized water were added in a flask with a magnetic stirring of 1100 rpm. 9 mL of TEOS and 45.5 mL of anhydrous ethanol were fully mixed in a beaker before adding. Then the mixed solution of TEOS and ethanol was poured into the flask quickly under fiercely stirring. After stirring for 1 min, the stirring speed was turned down to 400 rpm. The mixture was then sequentially stirred at room temperature for 2 h, resulting in the formation of a white silica colloidal suspension. Finally, the obtained silica particles were centrifugally separated from the suspension and fully washed with adequate deionized water and ethanol. 2.2.2. Synthesis of hollow mesoporous SiO2 spheres Hollow mesoporous SiO2 spheres was synthesized in a simple method referred to Fang et al. [35]. 0.5 g of as-prepared SiO2 were homogeneously dispersed in 90 mL of deionized water by ultrasonication for 10 min. Then 10 mL of CTAB aqueous solution (12.5 mg·mL−1 ) was added, the mixture was further stirred at room temperature for 30 min before the introduction of Na2 CO3 (2.12 g). Afterwards, the reaction was constantly stirred at 35 ◦ C for 10 h and 50 ◦ C for another 10 h. Finally, the hollow mesoporous SiO2 spheres were collected by centrifugation and fully washed with deionized water and ethanol (denoted as HMSS). 2.2.3. Incorporated with molybdenum by direct immersion method Hollow mesoporous SiO2 spheres incorporated by molybdenum was prepared by excessive immersion method taking ammonium molybdate as the source of molybdenum. Firstly, we used the direct immersion method to incorporate active sites. HMSS was fully impregnated in a certain amount of ammonium molybdate solution, treated with ultrasonication, evaporated at 50 ◦ C and ultimately calcined at 823 K for 6 h [21,24]. The mass of molybdenum in hollow mesoporous SiO2 spheres were 0.4, 1, 2, 5, 10 wt%

214

Y. Shen et al. / Molecular Catalysis 433 (2017) 212–223

(denoted as Mo/HMSS-0.4, Mo/HMSS-1, Mo/HMSS-2, Mo/HMSS-5 and Mo/HMSS-10). 2.2.4. Incorporated with molybdenum by modified immersion method The pre-synthesized hollow mesoporous silica spheres was soaked by a given amount of ammonium molybdate solution (the mass of molybdenum in hollow mesoporous SiO2 spheres was 5 wt%), then treated by ultrasonication for 30 min, after which the mixture was stirred at 60 ◦ C for 8 h in order to be absolutely impregnated, finally washed with deionized water for 3 times to remove the residual metal, dried, calcined at 823 K for 6 h. (denoted as Mo/HMSS-X) 2.3. Olefin epoxidation As our previous experiments [39], the catalytic activity tests were carried out with epoxidation of cyclohexene as a model reaction under neat conditions. In a typical procedure, cyclohexene (2.5 mmol), 65 wt% tert-butyl hydroperoxide aqueous solution (TBHP, 5.0 mmol), n-octane (internal standard, 1.25 mmol) and catalyst (25 mg) were added to 1, 2-dichloroethane (1, 2-DCE, 10 mL) in a 25 mL two-neck round bottom flask equipped with a reflux condenser at 80 ◦ C. Samples were taken at intervals and stored in the refrigerator. After centrifugation, the supernatant liquid was analyzed by a gas chromatograph (GC, SP-6890A) equipped with a FID detector and a capillary column (SE-54 30m × 0.32mm × 0.25 ␮m). The catalytic activity for the epoxidation of cyclohexene was evaluated by the transformation of cyclohexene to cyclohexene epoxide. Conversion, selectivity, turnover number (TON) and turnover frequency (TOF) were criterions to assess the reactivity. The calculation method was as follow. [Conversion (%)] = ×

100 A(olefin) A(n-octane)

  A(olefin) A (n-octane)

− t=oh



 A  (olefin) A (n-octane)

3. Results and discussion 3.1. SEM and TEM analysis As shown in Fig. 1a, solid SiO2 spheres with an average diameter of 426 nm was obtained from the Stöber method with monodispersity. After treating with Na2 CO3 and CTAB (Fig. 1b), the spherical structure remained intact without a break. This demonstrated that exterior feature of Mo/HMSS-X kept the same as former. Meanwhile, compared with Fig. 1c and d, it was obvious that solid SiO2 spheres were successfully converted into the hollow structure. Besides, according to Fig. 1e, the shell of Mo/HMSS-X was a wormhole-like structure [40,41] ascribing to the contribution of CTAB. To get a detailed view of the incorporated Mo species, a TEM image of Mo/HMSS-X (with Mo content 0.23 wt%) was shown in Fig. 1f, where many small dark dots (MoO3 ) were evenly dispersed in the shell. It could be seen that MoO3 were highly dispersed and the size of MoO3 was smaller than 5 nm. In order to demonstrate the high dispersion of Mo species in Mo/HMSS-X, a TEM image of Mo/HMSS-0.4 (under the similar Mo content of 0.27 wt%) was also shown in Fig. 1g by comparison, where gathered dark dots were appeared. This indicated the highly dispersed active sites in Mo/HMSS-X. 3.2. EDS analysis

t=xh



(1) t=oh

[Selectivity (%)] = A (product) ×

Rigaku D/MAX-2500PC diffractometer (Rigaku Co., Japan) equipped with Cu-K␣ radiation (␭ = 1.5418 Å) and operated at 40 kV and 100 mA. Molybdenum content in each sample was analyzed with inductively coupled plasma-atom emission spectroscopy (ICP-AES) on Perkin-Elmer AA-300 spectrophotometer. X-ray photoelectron spectroscopy (XPS) measurements were conducted using an ESCALAB 250Xi (Thermo, USA) spectroscopy with Al K␣ line as the excitation source (h␯ = 1484.6 eV).

100 [A (product) + A (by − products)] (2)

TON = n (convertal substrate) /n (metal atoms in the catalyst)

(3)

TOF = TON/Reaction duration

(4)

2.4. Characterization Morphologies of the samples were observed on a Hitachi S-4800 scanning electron microscopy (SEM) equipped with a fieldemission gun with an accelerating voltage at 2 kV and a JEOL JEM-2100F transmission electron microscopy (TEM) with an acceleration voltage of 200 kV. Energy-dispersive X-ray spectroscopy (EDS) mappings and line scanning analysis were recorded using a Hitachi S-4800 scanning electron microscopy equipped with an Oxford Instruments 80 mm2 silicon drifted detector with an accelerating voltage of 15 kV and current of 15 mA. FT-IR spectra were recorded on a NICOLET 6700 FT-IR Thermoscientific spectrometer using a KBr pellet technique in the 500–4000 cm−1 region. Nitrogen adsorption/desorption isotherms were measured at −196 ◦ C with nitrogen as the analysis gas using a Micromeritics ASAP 2010 instrument. All samples were degassed for 4 h at 180 ◦ C before measurements. X-ray diffraction (XRD) patterns of the materials were carried out at room temperature using a

EDS elemental mapping images of Mo/HMSS-X (Fig. 2b-d) revealed the distribution of elements O, Si and Mo, which constituted of the catalysts. The presence of Si, O and Mo elements and their uniform distribution were clearly illustrated in the images, further confirming the successful incorporation of molybdenum into the shell of hollow mesoporous silica spheres. Moreover, line-scanning analysis was also employed in order to compare the distribution of molybdenum in samples with different metal ratios. The EDS line-scanning analysis of Mo/HMSSX, Mo/HMSS-0.4, Mo/HMSS-1, Mo/HMSS-2, Mo/HMSS-5 and Mo/HMSS-10 were showed in Fig. 3. Element line scanning analysis also exhibited the concentrations of O, Si and Mo [42]. Compared with the concentration of Mo, Mo/HMSS-X had more uniform distribution than Mo/HMSS-0.4, Mo/HMSS-1, Mo/HMSS2, Mo/HMSS-5 and Mo/HMSS-10. These results firmly certified the high dispersion of Mo incorporated by modified method [43]. 3.3. FT-IR analysis The FT-IR spectra of HMSS and Mo-containing catalysts were shown in Fig. 4. All samples displayed absorption bands around 805, 963, 1100, 1381, 1627 and 3453 cm−1 . The band at 805 and 1100 cm−1 corresponded to symmetric and asymmetric stretching of Si-O-Si, respectively [44]. The 1381 cm−1 band could be indexed to the absorption of CO2 [44]. The 1627 cm−1 band observed in all spectra was assigned to water [44]. Moreover, the band at 3453 cm−1 was attributed to the vibration of OH including Si O H [45]. A signal at 963 cm−1 was assigned to Si-OH [46]. It also should be noted that with the adding of Mo, a new band at around 910 cm−1 appeared, which could be ascribed to Mo O groups in Mo O Si vibration [9–11,47]. Furthermore, with the increase of

Y. Shen et al. / Molecular Catalysis 433 (2017) 212–223

Fig. 1. SEM images of SiO2 spheres (a) and Mo/HMSS-X (b); TEM images of SiO2 spheres (c), Mo/HMSS-X (d, e, f) and Mo/HMSS-0.4 (g).

Fig. 2. EDS elemental mapping images of Mo/HMSS-X for elements O, Si and Mo (b–d), respectively.

215

216

Y. Shen et al. / Molecular Catalysis 433 (2017) 212–223

Fig. 3. EDS line-scanning analysis of Mo/HMSS-X(a), Mo/HMSS-0.4(b), Mo/HMSS-1(c), Mo/HMSS-2(d), Mo/HMSS-5(e) and Mo/HMSS-10(f), respectively.

Mo content, the band at 963 cm−1 became weaker revealing that Mo was fixed on the surface of the materials and Si-OH group decreased.

(A)

(B)

a b c

a

910

d e f g

c d e

3453

1627 1381

f g

805

1100

4000

b

3500

3000 2500 2000 1500 -1 Wavenumber (cm )

963

1000

500

1000 800 600 -1 Wavenumber (cm )

Fig. 4. FT-IR spectra of HMSS(a), Mo/HMSS-X(b), Mo/HMSS-0.4(c), Mo/HMSS-1(d), Mo/HMSS-2(e), Mo/HMSS-5(f) and Mo/HMSS-10(g).

3.4. N2 adsorption-desorption isotherms and ICP-AES analysis Molybdenum content loaded on materials was determined by ICP-AES analysis, with a content of 0.23 wt% (Mo/HMSSX), 0.27 wt% (Mo/HMSS-0.4), 0.59 wt% (Mo/HMSS-1), 1.21 wt% (Mo/HMSS-2), 3.52 wt% (Mo/HMSS-5) and 8.34 wt% (Mo/HMSS10). These results were consistent with the theoretical value accompanying with an inevitable loss. Specific surface area and porosity of the catalysts were investigated by N2 adsorption-desorption measurements. Fig. 5A showed N2 adsorption-desorption curves of HMSS, Mo/HMSS-X, Mo/HMSS0.4, Mo/HMSS-1, Mo/HMSS-2, Mo/HMSS-5 and Mo/HMSS-10 with a type IV isotherm and a type H4 hysteresis loop, indicating the formation of mesoporous hollow structure [48]. Fig. 5B showed the corresponding pore size distributions, It was clear that all the samples tested were mesoporous structure with pore size ranged between 2 and 4 nm. As shown in Table 1, Brunauer-EmmettTeller (BET) surface area and pore volume of original HMSS without impregnated molybdenum species was 990 m2 /g and 1.06 cm3 /g, respectively. After modified with molybdenum through modified

Y. Shen et al. / Molecular Catalysis 433 (2017) 212–223

217

(B)

Volume adsorbed (cm /g)

(A) 3

a

a b c

b c d e f g 0.0

d e f g

0.2

0.4

0.6

0.8

1.0

2

4

Relative pressure (P/Po)

6

8

10

12

14

16

18

20

Pore diameter (nm)

Fig. 5. N2 adsorption-desorption isotherms (A) and the corresponding pore size distributions (B) of HMSS(a), Mo/HMSS-X(b), Mo/HMSS-0.4(c), Mo/HMSS-1(d), Mo/HMSS-2(e), Mo/HMSS-5(f) and Mo/HMSS-10(g).

immersion method, BET surface area and pore volume decreased to 484 m2 /g and 0.72 cm3 /g, respectively. Whereas, at the same content of treated molybdenum, direct immersion strategy made BET surface area and pore volume dramatically decline to 58 cm3 /g and 0.27 cm3 /g, respectively. Besides, lower contents of incorporated molybdenum with 0.4 wt%, 1 wt% and 2 wt% also possessed a low BET surface area of 133 m2 /g (Mo/HMSS-0.4), 132 m2 /g (Mo/HMSS-1) and 102 m2 /g (Mo/HMSS-2) as well as pore volume of 0.23 cm3 /g (Mo/HMSS-0.4), 0.25 cm3 /g (Mo/HMSS-1) and 0.21 cm3 /g (Mo/HMSS-2), respectively. Apparently, the addition of molybdenum species had a pivotal influence on the decrease of specific surface area and pore volume. The immersed molybdenum was deemed to be sintered onto the surface of the materials as MoO3 after calcination, which would occupy the channels in the materials leading to the distinct depression of specific surface area and pore volume. With the increasing ratio of molybdenum, specific surface area decreased and reached the minimum at 5 wt% (Mo/HMSS-5) whose pores were considered to be mostly occupied by incorporated Mo. When the immersed ratio reached 10 wt%, aggregate MoO3 became larger than the pore diameters, MoO3 would exist outside the pores instead of inside the pores leading to a rise in specific surface area. Compared with these two immersion methods, BET surface area and pore volume of the materials decreased more by direct immersion method than modified immersion method. Severe pore-filling effects were supposed to generate in the channels by direct immersion, while washing away the residual metal after immersion would improve this situation and result in highly dispersed active sites, which explained the great difference between modified and direct methods on BET surface area and pore volume.

3.5. XRD analysis The mesostructure of catalysts was further confirmed by small X-ray diffraction (SXRD). Obviously, materials of HMSS and Mo/HMSS-X exhibited a distinct (100) reflection diffraction peak at 2␪ 0.7–1.2◦ , which was solid evidence for mesostructure (Fig. 6). Nevertheless, the samples prepared by direct impregnation gave rise to a strong decrease in the intensity of (100) reflection peak, together with a shift of the position toward lower 2␪ values. This result could be ascribed to the pore-filling effects which would reduce the scattering contrast between the pores and the silica walls [49,50]. It was also consistent with that of N2 adsorptiondesorption analysis which explained the change of specific surface area, total pore volume and BJH average pore diameters. From the results of wide angle X-ray diffraction (Fig. 7), a broad peak appeared at around 2␪ 22.0◦ , a typical XRD diffraction peak for SiO2 , which corroborated the existence of SiO2 [51]. Except the peak of SiO2 XRD diffraction, no obvious diffraction peak of MoO3 could be observed in the XRD spectra of HMSS and Mo/HMSS-X. The reason might be that MoO3 was highly dispersed on the support and the size of MoO3 crystallites was less than 5 nm, which was the detection limit of XRD [52], and this was also consistent with the result of TEM (Fig. 1f). When the sample was prepared by direct immersion method, small diffraction peaks of crystalline MoO3 appeared regularly. The obvious diffraction peaks at 2␪ 11.87, 23.51, 27.38, 39.01, 45.61 and 54.53 were observed, which could be ascribed to the crystalline phase of MoO3 (JCPDS file: 05-0508) [24,53]. Meanwhile, the intensity of the diffraction peaks for MoO3 strengthened with the enhanced content of molybdenum; that is,

Table 1 Textural characteristics of the catalysts. Sample

Mo contenta

SBET b

VBJH c

DBJH d

HMSS Mo/HMSS-0.4 Mo/HMSS-1 Mo/HMSS-2 Mo/HMSS-5 Mo/HMSS-10 Mo/HMSS-X

– 0.27 0.59 1.21 3.52 8.34 0.23

990 133 132 102 58 155 484

1.06 0.23 0.25 0.21 0.27 0.34 0.72

4.26 7.01 7.66 8.20 18.49 8.80 5.98

a From ICP-AES analysis, mass fractions of molybdenum in hollow mesoporous SiO2 spheres (wt%). b BET surface area, SBET (m2 /g). c Pore volume, VBJH (cm3 /g). d Pore diameter, DBJH (nm).

Intensity (a.u.)

(100)

a b c d e f g 1

2

3

4

5

2 Theta (deg) Fig. 6. Small XRD patterns of HMSS (a), Mo/HMSS-X (b), Mo/HMSS-0.4 (c), Mo/HMSS-1 (d), Mo/HMSS-2 (e), Mo/HMSS-5 (f) and Mo/HMSS-10 (g).

218

Y. Shen et al. / Molecular Catalysis 433 (2017) 212–223

Intensity (a.u.)

a b c d e f g JCPDS:05-0508 10

20

30

40

50

60

h

70

80

2 Theta (deg) Fig. 7. Wide angle XRD patterns of HMSS(a), Mo/HMSS-X(b), Mo/HMSS-0.4(c), Mo/HMSS-1(d), Mo/HMSS-2(e), Mo/HMSS-5(f), Mo/HMSS-10(g) and MoO3 (h).

the particle size of MoO3 increased. The conclusion certified the existence of highly dispersed MoO3 prepared by modified immersion method. 3.6. XPS analysis X-ray photoelectron spectroscopy was utilized to investigate the nature and surface exposure of molybdenum species for Mo/HMSS-X catalysts [54]. All the binding energies were referenced to the C 1s hydrocarbon peak at 284.80 eV. XPS spectra of Mo/HMSS-X were shown in Fig. 8A, peak of Mo certified the successful incorporation. Meanwhile, Fig. 8B exhibited the typical spectrum of Mo 3d with two peaks at 232.78 eV and 235.68 eV corresponding to Mo 3d5/2 and Mo 3d3/2 as Mo6+ in MoO3 , respectively [37,52,55,56]. From the result of XPS, it was certified that molybdenum was fixed on the surface of the materials as MoO3 .

10 wt% owned a weaker reactivity than Mo/HMSS-5 with the ratio of 5 wt%. It was acknowledged that with the growth of molybdenum, conversion of the catalysts was improved. However, high loading of molybdenum species would also cause aggregation [57] and the blockage in channels, which led to the reduction of activity for Mo/HMSS-10. Hence, the optimal ratio was fixed at 5 wt%. In addition, all the catalysts had an impressive selectivity above 80% indicating the fine reactivity (Fig. 9C). Mo/HMSS-X was prepared through modified immersion under the same ratio as Mo/HMSS-5 (5 wt%). Although Mo/HMSS-X was provided with low content of active sites according to the results of ICP-AES analysis (0.23 wt%), its reactivity could match with the best catalyst prepared by direct immersion method Mo/HMSS5 (Fig. 9A). However, compared with the results of TON values, Mo/HMSS-X prepared by modified immersion exhibited superior catalytic activity than other catalysts prepared by direct immersion (Fig. 9B). From this perspective, Mo/HMSS-X was proved to possess excellent reactivity, whose TON value reached 3873 at 6 h, while TON values of Mo/HMSS-0.4, Mo/HMSS-1, Mo/HMSS2, Mo/HMSS-5 and Mo/HMSS-10 were 1915, 1318, 514, 268 and 110 at 6 h, respectively. Meanwhile, the selectivity of Mo/HMSS-X was higher as well. Relevant to the results of BET surface area and pore volume, catalysts synthesized through modified immersion had better textural properties than direct immersion method. It was suggested that the incorporated metal through direct immersion would cause serious blockage in the mesopores, thence, washing after immersion would remove the residual substances trapped in channels resulting in smooth channels and good transportation/diffusion. Through this method, active sites in materials would be isolated in the shell with high dispersion and provided maximum contact between substrate and oxidant. No molybdenum was detected from the supernatant of epoxidation through ICP-AES (the detection limit of the method is 0.0001 wt%) which indicated that the active sites were firmly immobilized on the supports with no leaching during the reaction.

3.7. Catalytic epoxidation activity 3.7.1. Catalytic activity with two immersion methods In order to compare the catalytic activity of different immersion methods, epoxidation of cyclohexene was carried out to assess the reactivity of Mo/HMSS-0.4, Mo/HMSS-1, Mo/HMSS-2, Mo/HMSS-5, Mo/HMSS-10 and Mo/HMSS-X. Compared with different mass of molybdenum in hollow mesoporous SiO2 spheres prepared by direct immersion, reactivity of the catalysts increased with the rising of mass fraction and reached the maximum with the ratio of 5 wt% with a conversion of 96.89% and a selectivity of 92.71% at 6 h (Fig. 9A, 9C). Afterwards, activity declined with higher loading. Mo/HMSS-10 with a ratio of

(A)

3.7.2. Effects of different oxidants Different oxidants were investigated using the model reaction of cyclohexene in the same conditions. As shown in Fig. 10, the active order of conversion was TBHP > 50% H2 O2 > 30% H2 O2 > 70% H2 O2 , and the order of selectivity was TBHP > 50% H2 O2 > 70% H2 O2 > 30% H2 O2 . As an organic oxidant, TBHP revealed the best catalytic activity, good compatibility with reactants, which made the optimal effect. Compared with different concentration of H2 O2 , 50% H2 O2 with the proper concentration reacted in the best results with 81.89% conversion in the 6 h. 30% H2 O2 with a low effective concentration provided less oxidants and followed with a small amount of water which would make negative influence on the reaction,

(B)

Si 2p Mo 3d C 1s

1200

1000

800

600

400

Binding Energy (eV)

Mo 3d

Mo6+

Intensity (a.u.)

Intensity (a.u.)

O 1s

200

0

232.78 eV 235.68 eV

245

240

235

230

Binding Energy (eV)

Fig. 8. XPS survey spectra of Mo/HMSS-X (A) and Mo 3d present in Mo/HMSS-X (B).

225

220

Y. Shen et al. / Molecular Catalysis 433 (2017) 212–223

(B)

(A) 100

Mo/HMSS-X Mo/HMSS-0.4 Mo/HMSS-1

5000

80 4000

Mo/HMSS-x Mo/HMSS-0.4 Mo/HMSS-1 Mo/HMSS-2 Mo/HMSS-5 Mo/HMSS-10

40 20

3000 2000 1000

0 0

2

4

6

8

10

0

12

0

2

4

Time (h)

(C)

Mo/HMSS-2 Mo/HMSS-5 Mo/HMSS-10

60

TON

Conversion (%)

219

6

8

10

12

Time (h)

100

Selectivity (%)

95 90 85 Mo/HMSS-x Mo/HMSS-0.4 Mo/HMSS-1 Mo/HMSS-2 Mo/HMSS-5 Mo/HMSS-10

80 75 70 0

2

4

6

8

10

12

Time (h) Fig. 9. Epoxidation of different catalysts (A) conversion, (B) TON, (C) selectivity. Reaction conditions: catalyst (25 mg), cyclohexene (2.5 mmol), TBHP (5.0 mmol), 1, 2-DCE (10 mL), n-octane (1.25 mmol), temperature 80 ◦ C.

(B)

(A) 100

a Selectivity (%)

a

80

Conversion (%)

100

b c

60 40

d

20

90

b d c

80

70

0 0

2

4

6

8

10

12

Time (h)

0

2

4

6

8

10

12

Time (h)

Fig. 10. Epoxidation of different oxidants (A) conversion (B) selectivity. (a) TBHP, (b) 50 wt% H2 O2 , (c) 30 wt% H2 O2 , (d) 70 wt% H2 O2 . Reaction conditions: catalyst (25 mg, Mo/HMSS-X), cyclohexene (2.5 mmol), oxidant (5.0 mmol), 1, 2-DCE (10 mL), n-octane (1.25 mmol), temperature (80 ◦ C).

resulting in the low reactivity with 74.99% conversion in the 6 h. Meanwhile, the high content of water covered the surface of catalysts and inhibited the contact between catalyst and oxidant [58]. Moreover, 70% H2 O2 with a high concentration reacted violently accompanying with local heat which caused a lot of byproducts and led to a low selectivity of 80–85%.

3.7.3. Effects of different solvents Solvents played a crucial role in epoxidation reaction, several organic reagents were used to inquiry their impacts here. The reaction was performed in cyclohexene epoxidation with different solvents and corresponding reaction temperature. The results

of epoxidation with CHCl3 , 1, 2-DCE, EtOAc, and CH3 CN were summarized in Fig. 11, in which catalytic activity of different solvents varied a lot. CHCl3 and 1, 2-DCE as solvents provided better conversion and selectivity in the reason of free incorporation with active metal sites as well as fine compatibility of oxidants and olefins. As a polar solvent, CH3 CN would enhance the acidity in the channels causing the ring-opening reaction. As shown in Fig. 11B, the selectivity was only around 20–25%, which concluded a poor catalytic performance. With the presence of carbonyl group, EtOAc could provide electronic pair to incorporate with molybdenum leading to the declination of activity. In summary, EtOAc and CH3 CN which contained donor atoms (O or N) were easy to incorporate strongly with molybdenum and reduced the catalytic activity.

220

Y. Shen et al. / Molecular Catalysis 433 (2017) 212–223

(A)

(B)

100

80

Selectivity (%)

Conversion (%)

60

b

40

c

20

a b

a

80

100

d

60

40

d 20

c

0 0

2

4

6

8

10

12

Time (h)

0

2

4

6

8

10

12

Time (h)

Fig. 11. Epoxidation in different solvents (A) conversion (B) selectivity. (a) 1, 2-DCE, (b) CHCl3 , (c) EtOAc, (d) CH3 CN. Reaction conditions: catalyst (25 mg, Mo/HMSS-X), cyclohexene (2.5 mmol), TBHP (5.0 mmol), solvent (10 mL), n-octane(1.25 mmol), temperature: 1, 2-DCE (80 ◦ C), CHCl3 (55 ◦ C), CH3 CN (75 ◦ C), EtOAc (75 ◦ C).

3.7.4. Epoxidation of different olefins In order to research the general applicability of catalysts, catalytic epoxidation of different kinds of olefins was further measured with the best catalyst Mo/HMSS-X (Table 2). After reacting for 4 h, cyclic olefin showed excellent catalytic performance, the conversion of cyclopentene, cyclohexene and cyclooctene were 80.21, 85.02 and 91.07%, selectivity were 90.31, 98.81 and 97.68%, respectively. Catalytic activity improved with the increase of the number of carbon atoms in cyclic olefin molecules with the reason of a more electron donating (CH2 )n cyclic bridge connected to the double bond [59]. As for linear olefins after reacting for 4 h, the conversion of 1- pentene, 1- hexene and 1- octene were 73.68, 67.59 and 30.96%, selectivity were 91.78, 84.70 and 84.28%, respectively. Conversely, activity of linear olefins decreased with the rise of alkyl chain. The length of alkyl chain would increase the steric hindrance, which made the substrates hard to get through the pore canal and

contact with the metal active sites. The pivotal factor of epoxidation may be the electronic effect, higher electronic density of the double bond was believed to show better efficiency. Therefore, cyclopentene, cyclohexene and cyclooctene with double bonds driven from secondary carbons would exhibit better activities in comparison with 1- pentene, 1-hexene and 1-octene which contained double bonds between secondary and primary carbons. In order to further detect this hypothesis, epoxidation of 1-phenyl-1-cyclohexene and 4-methyl-1-cyclohexene were carried out. Conversion of 1phenyl-1-cyclohexene and 4-methyl-1-cyclohexene were 34.78 and 84.58%, selectivity were 57.89 and 92.15%, respectively. The difference was obvious, 1-phenyl-1-cyclohexene with an electron-withdrawing group showed inferior reactivity, while 4methyl-1-cyclohexene with an electron-donating group exhibited superior activity. The conclusion confirmed the assumption that

Table 2 Epoxidation of different olefins. Entry

Substrate

Product

Conversion (%)

Selectivity (%)

1

80.21

90.31

2

85.02

>99

3

91.07

>98

4

34.78

57.89

5

84.58

92.15

6

73.68

91.78

7

67.59

84.70

8

30.96

84.28

Reaction conditions: catalyst (25 mg, Mo/HMSS-X), olefin (2.5 mmol), TBHP (5.0 mmol), 1,2-DCE (10 mL), n-octane(1.25 mmol), temperature (80 ◦ C), reaction time(4 h).

Y. Shen et al. / Molecular Catalysis 433 (2017) 212–223

221

Table 3 Catalytic performance of Mo/HMSS-X compared with other catalysts in epoxidation of cyclohexene with TBHP as oxidant. Entry

Catalyst

Conditions ◦

Time (h)

TOF (h−1 )

Conversion (%)

Selectivity (%)

Ref.

893 255 179 546 (10 min) 25 72.3 118

87 88 99 100

99 98 94 100

This work [19] [62] [7]

30 90 100

85 100 91

[57] [53] [63]

1 2 3 4

Mo/HMSS-X MoO2 acpyAmpMCM-41 Mo-MCM-41 MoO3 nanoparticles

1, 2-DCE (80 C) Chloroform (Reflux) Decane (80 ◦ C) Decane (120 ◦ C)

4 4 4 1

5 6 7

Mo-TUD-1 MoO3 /SiO2 Mo-VPO

Dichloroethane (40 ◦ C) 1,2-DCE (80 ◦ C) Acetonitrile (90 ◦ C)

6 2 10

Conversion

100

Selectivity

80

80

Conversion (%)

Conversion (%)

100

60 40 20 0 1st

2nd

3rd

4th

No catalyst separation Catalyst filtered

40 20 0

5th

0

Run number Fig. 12. Recycle tests of optimal catalysts via epoxidation of cyclohexene in five consecutive runs. Reaction conditions: catalyst (25 mg, Mo/HMSS-X), cyclohexene (2.5 mmol), TBHP (5.0 mmol), 1, 2-DCE (10 mL), n-octane (1.25 mmol), temperature 80 ◦ C, reaction time (8 h).

60

2

4 6 Time (h)

8

Fig. 13. Conversion of cyclohexene in epoxidation to screen catalyst leaching. Reaction conditions: catalyst (25 mg, Mo/HMSS-X), cyclohexene (2.5 mmol), TBHP (5.0 mmol), 1, 2-DCE (10 mL), n-octane (1.25 mmol), temperature 80 ◦ C.

4. Conclusions electronic density displayed a significant role in epoxidation [60,61]. 3.7.5. Recycle tests Stability of catalysts was a crucial property for heterogeneous catalysts. Recovery of the catalyst was carried out after the first run according to the literature [60]. The catalyst was firstly separated by centrifugation and decantation of the mixture. Then washed with 1, 2-DCE as safe solvent and dried under vacuum. Afterwards the retreated catalyst was used directly for the next round in the same condition. The recycling results were shown in Fig. 12, the conversions were 95.23, 93.51, 92.90, 90.61 and 86.60% after reacting for 8 h in the first, second, third, fourth and fifth recycle, respectively. The selectivity remained at a high level above 93%. The above outcome indicated the outstanding reusability of the catalyst. In order to further confirm the stability of Mo/HMSS-X, leaching experiments for the epoxidation of cyclohexene were carried out in 1, 2-DCE at 80 ◦ C. Two comparative epoxidation reactions were carried out in the same conditions. After reacting for 2 h, catalyst in one of the reactions was separated by hot filtration to evaluate the possible leaching of the metal active sites into the reaction medium. Then both of the reactions were monitored again till 8 h. According to Fig. 13, the reaction treated with hot filtration did not proceed further which demonstrated the firm incorporation of active sites [7]. 3.7.6. Comparison with literature data Table 3 exhibited different catalysts’ performance in olefin epoxidation of cyclohexene as a model substrate with TBHP as oxidant based on a literature survey. The catalysts shown in Table 3 covered nanoparticles, organic and inorganic mesoporous materials. Compared with these Mo-containing catalysts, our samples prepared by modified immersion method had higher TOF value.

An efficient catalyst for epoxidation was successfully synthesized through a selective etching strategy and then followed with modified immersion. As a result, this etching strategy ensured a perfectly spherical shape with hollow interior which would improve the transportation of reactants and products; modified immersion dramatically declined the pore-filling effect in channels with a specific surface area of 484 m2 /g. Furthermore, active sites were also isolated in the shell with high dispersion, resulting in impressive conversion (94.75%), selectivity (98.44%) and TON value (3873) at 6 h. Moreover, the catalyst still had a continuous stability with a conversion above 85% and selectivity above 93% for the fifth round at 8 h. In conclusion, our work highlights that a modified immersion method reported here will become an efficient method to fabricate highly dispersed active site incorporated hollow mesoporous silica spheres as an efficient catalyst. Acknowledgments Thanks for the financial support from the Fundamental Research Funds for the Central Universities (JUSRP51507), the Innovation Foundation in Jiangsu Province of China (BY2014023-08), the Postgraduate Innovation Project of Jiangsu Province (KYLX16 0787) and the financial support from MOE & SAFEA, 111 Project (B13025). References [1] D. Banerjee, R.V. Jagadeesh, K. Junge, M.M. Pohl, J. Radnik, A. Bruckner, M. Beller, 1; Convenient and mild epoxidation of alkenes using heterogeneous cobalt oxide catalysts, Angew. Chem. Int. Ed. 53 (2014) 4359–4363. [2] J. Silvestre-Alberó, M.E. Domine, J.L. Jordá, M.T. Navarro, F. Rey, F. Rodríguez-Reinoso, A. Corma, Spectroscopic calorimetric, and catalytic evidences of hydrophobicity on Ti-MCM-41 silylated materials for olefin epoxidations, Appl. Catal. A: Gen. 507 (2015) 14–25.

222

Y. Shen et al. / Molecular Catalysis 433 (2017) 212–223

[3] Y. Zhu, Q. Wang, R.G. Cornwall, Y. Shi, Organocatalytic asymmetric epoxidation and aziridination of olefins and their synthetic applications, Chem. Rev. 114 (2014) 8199–8256. [4] F. Zhang, H. Hu, H. Zhong, N. Yan, Q. Chen, Preparation of ␥-Fe2 O3 @C@MoO3 core/shell nanocomposites as magnetically recyclable catalysts for efficient and selective epoxidation of olefins, Dalton Trans. 43 (2014) 6041–6049. [5] T.M.K. Stefan, V. Kotov, Mariana G. Georgieva, Preparation and use of novel molybdenum-containing organic complexes as catalysts in the epoxidation of cyclohexene, J. Mol. Catal. A: Chem. 195 (2003) 83–94. [6] Y. Sui, X. Zeng, X. Fang, X. Fu, Y. Xiao, L. Chen, M. Li, S. Cheng, Syntheses structure, redox and catalytic epoxidation properties of dioxomolybdenum(VI) complexes with Schiff base ligands derived from tris(hydroxymethyl)amino methane, J. Mol. Catal. A: Chem. 270 (2007) 61–67. [7] C.I. Fernandes, S.C. Capelli, P.D. Vaz, C.D. Nunes, Highly selective and recyclable MoO3 nanoparticles in epoxidation catalysis, Appl. Catal. A: Gen. 504 (2015) 344–350. [8] N. Gharah, S. Chakraborty, A.K. Mukherjee, R. Bhattacharyya, Highly efficient epoxidation method of olefins with hydrogen peroxide as terminal oxidant, bicarbonate as a co-catalyst and oxodiperoxo molybdenum(VI) complex as catalyst, Chem. Commun. 263 (2004) 2630–2632. [9] A. Günyar, D. Betz, M. Drees, E. Herdtweck, F.E. Kühn, Highly soluble dichloro, dibromo and dimethyl dioxomolybdenum(VI)-bipyridine complexes as catalysts for the epoxidation of olefins, J. Mol. Catal. A: Chem. 331 (2010) 117–124. [10] S.M. Bruno, C.C.L. Pereira, M.S. Balula, M. Nolasco, A.A. Valente, A. Hazell, M. Pillinger, P. Ribeiro-Claro, I.S. Gonc¸alves, New chloro and triphenylsiloxy derivatives of dioxomolybdenum(VI) chelated with pyrazolylpyridine ligands: catalytic applications in olefin epoxidation, J. Mol. Catal. A: Chem. 261 (2007) 79–87. [11] J. Zhao, X. Zhou, Ana M. Santos, Eberhardt Herdtweck, Carlos C. Romão, Fritz E. Kühn, Molybdenum(VI) cis-dioxo complexes bearing sugar derived chiral Schiff-base ligands: synthesis, characterization, and catalytic applications, Dalton Trans. (2003) 3736–3742. [12] A.L. Cánepa, C.M. Chanquía, G.A. Eimer, S.G. Casuscelli, Oxidation of olefins employing mesoporous molecular sieves modified with copper, Appl. Catal. A: Gen. 462–463 (2013) 8–14. [13] S.M. Jeong, A. Burri, N. Jiang, S.E. Park, Microwave synthesis of hydrophobic Ti-TUD-1 mesoporous silica for catalytic oxidation of cycloolefins, Appl. Catal. A: Gen. 476 (2014) 39–44. [14] Y. Lin, X. Pan, W. Qi, B. Zhang, D.S. Su, Nitrogen-doped onion-like carbon: a novel and efficient metal-free catalyst for epoxidation reaction, J. Mater. Chem. A 2 (2014) 12475. [15] A. Bento, A. Sanches, E. Medina, C.D. Nunes, P.D. Vaz, MoO2 nanoparticles as highly efficient olefin epoxidation catalysts, Appl. Catal. A: Gen. 504 (2015) 399–407. [16] M.S. Morey, J.D. Bryan, S. Schwarz, G.D. Stucky, Pore surface functionalization of MCM-48 mesoporous silica with Tungsten and Molybdenum metal centers: perspectives on catalytic peroxide activation, Chem. Mater. 12 (2000) 3435–3444. [17] M. Jia, A. Seifert, W.R. Thiel, Mesoporous MCM-41 materials modified with oxodiperoxo Molybdenum complexes: efficient catalysts for the epoxidation of cyclooctene, Chem. Mater. 15 (2003) 2174–2180. [18] J.E. Herrera, J.H. Kwak, J.Z. Hu, Y. Wang, C.H.F. Peden, Synthesis of nanodispersed oxides of vanadium titanium, molybdenum, and tungsten on mesoporous silica using atomic layer deposition, Top. Catal. 39 (2006) 245–255. [19] M. Masteri-Farahani, F. Farzaneh, M. Ghandi, Synthesis and characterization of molybdenum complexes with bidentate Schiff base ligands within nanoreactors of MCM-41 as epoxidation catalysts, J. Mol. Catal. A: Chem. 248 (2006) 53–60. [20] R.Y. Abrokwah, V.G. Deshmane, D. Kuila, Comparative performance of M-MCM-41 (M: Cu Co, Ni, Pd, Zn and Sn) catalysts for steam reforming of methanol, J. Mol. Catal. A: Chem. 425 (2016) 10–20. [21] S. Hashimoto, H. Takeda, S. Honda, Y. Iwamoto, Novel coloring of geopolymer products using a copper solution immersion method, Constr. Build. Mater. 88 (2015) 143–148. [22] L. Lizama, T. Klimova, Highly active deep HDS catalysts prepared using Mo and W heteropolyacids supported on SBA-15, Appl. Catal. B: Environ. 82 (2008) 139–150. [23] T. Huang, W. Huang, J. Huang, P. Ji, Methane reforming reaction with carbon dioxide over SBA-15 supported Ni-Mo bimetallic catalysts, Fuel Process. Technol. 92 (2011) 1868–1875. [24] C. Lin, K. Tao, H. Yu, D. Hua, S. Zhou, Enhanced catalytic performance of molybdenum-doped mesoporous SBA-15 for metathesis of 1-butene and ethene to propene, Catal. Sci. Technol. 4 (2014) 4010–4019. [25] N. Hao, K.W. Jayawardana, X. Chen, M. Yan, One-step synthesis of amine-functionalized hollow mesoporous silica nanoparticles as efficient antibacterial and anticancer materials, ACS Appl. Mater. Interfaces 7 (2015) 1040–1045. [26] J.C. Song, F.F. Xue, Z.Y. Lu, Z.Y. Sun, Controllable synthesis of hollow mesoporous silica particles by a facile one-pot sol-gel method, Chem. Commun. 51 (2015) 10517–10520. [27] X. Li, F. Luo, G. He, Activation of the solid silica layer of aerosol-based C/SiO2 particles for preparation of various functional multishelled hollow microspheres, Langmuir 31 (2015) 5164–5173.

[28] X.W. Lou, C. Yuan, Q. Zhang, L.A. Archer, Platinum-functionalized octahedral silica nanocages: synthesis and characterization, Angew. Chem. Int. Ed. 45 (2006) 3825–3829. [29] Y. Li, J. Shi, Hollow-structured mesoporous materials: chemical synthesis, functionalization and applications, Adv. Mater. 26 (2014) 3176–3205. [30] J. Hu, X. Wang, L. Liu, L. Wu, A facile and general fabrication method for organic silica hollow spheres and their excellent adsorption properties for heavy metal ions, J. Mater. Chem. A 2 (2014) 19771–19777. [31] K. Wenelska, K. Kierzek, R.J. Kalenczuk, X. Chen, E. Mijowska, Nanoconfinement induced formation of core/shell structured mesoporous carbon spheres coated with solid carbon shell, ACS Appl. Mater. Interfaces 5 (2013) 3042–3047. [32] X. Wu, C.M. Crudden, Chiral hybrid mesoporous silicas: assembly of uniform hollow nanospheres and helical nanotubes with tunable diameters, Chem. Mater. 24 (2012) 3839–3846. [33] A. Yildirim, M. Bayindir, A porosity difference based selective dissolution strategy to prepare shape-tailored hollow mesoporous silica nanoparticles, J. Mater. Chem. A 3 (2015) 3839–3846. [34] Y. Chen, L. Guo, Q. He, F. Chen, J. Zhou, J. Feng, J. Shi, Hollow/rattle-type mesoporous nanostructures by a structural difference-based selective etching strategy, ACS Nano 4 (2009) 529–539. [35] X. Fang, C. Chen, Z. Liu, P. Liu, N. Zheng, A cationic surfactant assisted selective etching strategy to hollow mesoporous silica spheres, Nanoscale 3 (2011) 1632–1639. [36] Y. Chen, Q. Wang, T. Wang, Fabrication of thermally stable and active bimetallic Au-Ag nanoparticles stabilized on inner wall of mesoporous silica shell, Dalton Trans. 42 (2013) 13940–13947. [37] J. Wang, H. Liu, The effect of the support structure on catalytic activity: a case study on hollow and solid MoO3 /SiO2 , RSC Adv. 6 (2016) 2374–2378. [38] T. Zhang, Q. Zhang, J. Ge, J. Goebl, M. Sun, Y. Yan, Y. -s. Liu, C. Chang, J. Guo, Y. Yin, A self-templated route to hollow silica microspheres, J. Phys. Chem. C 113 (2009) 3168–3175. [39] J. Zhang, P. Jiang, Y. Shen, W. Zhang, X. Li, Molybdenum(VI) complex with a tridentate Schiff base ligand immobilized on SBA-15 as effective catalysts in epoxidation of alkenes, Microporous Mesoporous Mater. 206 (2015) 161–169. [40] M. Tan, X. Wang, X. Wang, X. Zou, W. Ding, X. Lu, Influence of calcination temperature on textural and structural properties reducibility, and catalytic behavior of mesoporous ␥-alumina-supported Ni-Mg oxides by one-pot template-free route, J. Catal. 329 (2015) 151–166. [41] X. Yang, X. Li, Z. Li, G. Zhang, D. Wu, Mesoporous wormholelike carbon with controllable nanostructure for lithium ion batteries application, Mater. Res. Bull. 66 (2015) 83–87. [42] Q. Fu, H. Li, X. Shi, K. Li, G. Sun, Silicon carbide coating to protect carbon/carbon composites against oxidation, Scr. Mater. 52 (2005) 923–927. [43] P. Qiao, S. Zou, S. Xu, J. Liu, Y. Li, G. Ma, L. Xiao, H. Lou, J. Fan, A general synthesis strategy of multi-metallic nanoparticles within mesoporous titania via in situ photo-deposition, J. Mater. Chem. A 2 (2014) 17321–17328. [44] K. Tao, Q. Ma, N. Tsubaki, S. Zhou, L. Han, Molybdenum containing cage like mesoporous KIT-5 for enhanced catalytic conversion of 1-butene and ethylene to propene, J. Mol. Catal. A: Chem. 416 (2016) 39–46. [45] Maria D. Alba, Z. Luan, Jacek Klinowski, Titanosilicate mesoporous molecular sieve MCM-41: synthesis and characterization, J. Phys. Chem. 100 (1996) 2178–2182. [46] L. Zhang, J. Shi, J. Yu, Z. Hua, X. Zhao, M. Ruan, A new In-Situ reduction route for the synthesis of Pt nanoclusters in the channels of mesoporous silica SBA-15, Adv. Mater. 14 (2002) 1510–1513. [47] E. Zamanifar, F. Farzaneh, J. Simpson, M. Maghami, Synthesis, crystal structure and catalytic activity of a new Mo Schiff base complex with Mo histidine immobilized on Al-MCM-41 for oxidation of sulfides, Inorg. Chim. Acta 414 (2014) 63–70. [48] T. Yang, J. Liu, R. Zhou, Z. Chen, H. Xu, S.Z. Qiao, M.J. Monteiro, N-doped mesoporous carbon spheres as the oxygen reduction reaction catalysts, J. Mater. Chem. A 2 (2014) 18139–18146. [49] H. Yang, X. Lu, Y. Yuan, Superior performance of gold supported on titanium-containing hexagonal mesoporous molecular sieves for gas-phase epoxidation of propylene with use of H2 and O2 , J. Phys. Chem. C 113 (2009) 8186–8193. [50] N. Gargiulo, A. Peluso, P. Aprea, F. Pepe, D. Caputo, CO2 adsorption on polyethylenimine-functionalized SBA-15 mesoporous silica: isotherms and modeling, J. Chem. Eng. Data 59 (2014) 896–902. [51] H. Wang, P. Wu, H. Shi, W. Tang, Y. Tang, Y. Zhou, P. She, T. Lu, Hollow porous silicon oxide nanobelts for high-performance lithium storage, J. Power Sources 274 (2015) 951–956. [52] Y. Lei, X. Chen, C. Xu, Z. Dai, K. Wei, Enhanced catalytic performance in the gas-phase epoxidation of propylene over Ti-modified MoO3 -Bi2 SiO5 /SiO2 catalysts, J. Catal. 321 (2015) 100–112. [53] P. Chandra, D.S. Doke, S.B. Umbarkar, A.V. Biradar, One-pot synthesis of ultrasmall MoO3 nanoparticles supported on SiO2 , TiO2 , and ZrO2 nanospheres: an efficient epoxidation catalyst, J. Mater. Chem. A 2 (2014) 19060–19066. [54] S.S. Bhoware, S. Shylesh, K.R. Kamble, A.P. Singh, Cobalt-containing hexagonal mesoporous molecular sieves (Co-HMS): Synthesis characterization and catalytic activity in the oxidation reaction of ethylbenzene, J. Mol. Catal. A: Chem. 255 (2006) 123–130. [55] L. Li, X. Liu, L. Lyu, R. Wu, P. Liu, Y. Zhang, Y. Zhao, H. Wang, D. Niu, J. Yang, Y. Gao, Modification of ultra-thin NPB interlayer on the electronic structures of

Y. Shen et al. / Molecular Catalysis 433 (2017) 212–223

[56] [57]

[58] [59]

the CH3 NH3 PbI3 /NPB/MoO3 interface, J. Phys. Chem. C 120 (2016) 17863–17871. N.V. Alov, XPS study of MoO3 and WO3 oxide surface modification by low-energy Ar+ ion bombardment, Phys. Status Solidi C 12 (2015) 263–266. M.S. Hamdy, G. Mul, Synthesis, characterization and catalytic performance of Mo-TUD-1 catalysts in epoxidation of cyclohexene, Catal. Sci. Technol. 2 (2012) 1894. I.W.C.E. Arends, R.A. Sheldon, Recent developments in selective catalytic epoxidations with H2 O2 , Top. Catal. 19 (2002) 133–141. B. Dutta, S. Jana, A. Bhattacharjee, P. Gütlich, S.I. Iijima, S. Koner, ␥-Fe2 O3 nanoparticle in NaY-zeolite matrix: preparation, characterization, and heterogeneous catalytic epoxidation of olefins, Inorg. Chim. Acta 363 (2010) 696–704.

223

[60] M. Jafarpour, A. Rezaeifard, M. Ghahramaninezhad, F. Feizpour, Dioxomolybdenum (VI) complex immobilized on ascorbic acid coated TiO2 nanoparticles catalyzed heterogeneous oxidation of olefins and sulfides, Green Chem. 17 (2015) 442–452. [61] E.P. Talsi, T.V. Rybalova, K.P. Bryliakov, Ti-salalen mediated asymmetric epoxidation of olefins with H2 O2 : effect of ligand on the catalytic performance and insight into the oxidation mechanism, J. Mol. Catal. A: Chem. 421 (2016) 131–137. [62] F. Bigi, C.G. Piscopo, G. Predieri, G. Sartori, R. Scotti, R. Zanoni, R. Maggi, Molybdenum-MCM-41 silica as heterogeneous catalyst for olefin epoxidation, J. Mol. Catal. A: Chem. 386 (2014) 108–113. [63] G.C. Behera, K.M. Parida, A comparative study of molybdenum promoted vanadium phosphate catalysts towards epoxidation of cyclohexene, Appl. Catal. A: Gen. 464–465 (2013) 364–373.