Template-free method to prepare porous Cu-containing nanotubes with a good catalytic performance for styrene epoxidation

Applied Surface Science 298 (2014) 116–124

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Template-free method to prepare porous Cu-containing nanotubes with a good catalytic performance for styrene epoxidation Chenhui Hu a , Lihong Zhang a , Junfeng Zhang a , Liyuan Cheng a , Zheng Zhai a , Jing Chen b , Wenhua Hou a,∗ a b

Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China Department of Applied Chemistry, College of Science, Nanjing University of Technology, Nanjing 210009, PR China

a r t i c l e

i n f o

Article history: Received 14 October 2013 Received in revised form 18 January 2014 Accepted 22 January 2014 Available online 31 January 2014 Keywords: K4 Nb6 O17 Nanoscrolls Nanotubes CuO Epoxidation

a b s t r a c t Cu-containing nanotubes with a large surface area and pore volume were prepared by using nanoscrolls derived from K4 Nb6 O17 as a support and a subsequent thermal transformation of Cu-containing nanoscrolls into Nb2 O5 nanotubes. The method is facile and template-free. The catalytic performance of the resulted Cu-containing nanotubes was evaluated for styrene epoxidation in the presence of tert-butyl hydroperoxide (TBHP) and H2 O2 , respectively. It was found that Cu-containing nanotubes displayed a relative good catalytic performance with a styrene oxide (SO) selectivity of 46.9% by TBHP and a much higher SO selectivity of 94.6% by H2 O2 . Two possible mechanisms were put forward to explain the different catalytic behaviors in the two types of oxidation systems. Because of the thermal transformation of nanoscrolls into nanotubes, nanoscrolls may be a new kind of promising support for the design and assembly of novel heterogeneous catalysts. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Over the past decade, there has been enormous interest in developing novel types of materials with exceptional properties for the new applications in various fields, particularly in heterogeneous catalysis [1,2]. Among these materials, layered compounds have been extensively investigated as promising candidates due to their considerable ion exchange capacity under mild conditions and other unique physico-chemical properties [3,4]. A great diversity of organic/inorganic species has been incorporated in the interlayer region via intercalation/pillaring and the resulted host-guest materials have exhibited extraordinary chemical, physical and optical properties [5–7]. Recently, the most attractive application of layered compounds is the formation of anisotropic nanosheets with unique morphological features and innovative properties by delamination and the obtained nanosheets are often used as building blocks for preparing advanced nanomaterials [8]. Being quite different from the original orderly stacked layered structure, the disordered reassembly of the resulted nanosheets may lead to the formation of porous structure with a high surface area and more active sites [9,10].

∗ Corresponding author. Tel.: +86 25 83686001; fax: +86 25 83317761. E-mail address: [email protected] (W. Hou). 0169-4332/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2014.01.137

Typically, K4 Nb6 O17 and its acid-exchanged form (noted as Hx K4−x Nb6 O17 ) are two most extensively investigated layered compounds with the distinctive characteristics including semiconducting property, photosensitivity and high loading capability [7]. Since there exist two distinct types of interlayer (interlayers I and II, see Fig. 1) alternately and the top and bottom faces are asymmetrical in each layer, two novel derivatives, nanosheets containing double layers and nanoscrolls consisting of single or double layers folded onto themselves, can be obtained by reaction of K4 Nb6 O17 with aqueous propylammonium hydrochloride and Hx K4−x Nb6 O17 with aqueous tetrabutylammonium hydroxide, respectively [4,11–14]. These two exfoliated derivatives and their modifications (i.e., decorated with RhOH, MgO, etc.) are effective photocatalysts due to their high absorption in the ultraviolet region [3,15]. However, little attention was paid to their applications in heterogeneous catalysis. The styrene oxide (SO) is an important organic intermediate in fine chemicals and pharmaceuticals since it can be further transformed to various useful products [16]. Conventionally, SO was generated via the epoxidation of styrene by molecular oxygen (or air), peracids (3-chloroperoxybenzoic acid) and peroxidecontaining reagents including H2 O2 (or urea-H2 O2 adduct) and tert-butyl hydroperoxide (TBHP) [17–20]. Many desirable catalysts including simple transition metal oxides (NiO, Fe2 O3 , ZnO) and supported nano-silver have been used to enhance the catalytic performance [21–24]. In general, there still exist some disadvantages

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oxides (2K2 O/3Nb2 O5 ) was prepared via a conventional impregnation method. 2.2. Catalyst characterization

Fig. 1. The template-free method to prepare porous Cu-containing nanotubes derived from Hx K4−x Nb6 O17 .

for the catalytic epoxidation of styrene, such as expensive and hazardous to handle for peracids, low selectivity for the epoxide formation, incomplete reaction and poor catalyst stability. Therefore, increasing efforts have been made to improve both activity and selectivity for this reaction. Due to its high activity for the decomposition of H2 O2 [25], copper oxide could not be directly used as a catalyst for the epoxidation by H2 O2 . However, supported copper oxide catalysts are considered to be promising catalysts due to the high selectivity of SO, although the conversion of styrene is relatively low [26,27]. Recently, NaHCO3 has been used as an efficient co-catalyst for activating H2 O2 to form HCO4 − which is more effective than H2 O2 in the epoxidation of olefins by Mn-containing catalysts [28]. Unfortunately, to the best of our knowledge, there is still no report about the epoxidation of styrene by H2 O2 , using the supported CuO as a main catalyst and NaHCO3 as a co-catalyst. Here in this paper, in order to develop catalyst with improved activity and selectivity for the epoxidation of styrene and to investigate the catalytic performance of epoxidation by using a new adduct oxidation system (H2 O2 –NaHCO3 ) over CuO supported catalysts, we report the preparation of Cu-containing nanotubes with a large surface area and pore volume by a template-free method. The resulted Cu-containing nanotubes displayed an exceptional catalytic performance for the epoxidation of styrene by TBHP and H2 O2 , respectively. A detailed characterization and discussion about the different catalytic behaviors in different oxidation systems was also provided. 2. Experimental 2.1. Catalyst preparation According to the literature with some changes [11,13,14], Pristine K4 Nb6 O17 was prepared by heating a mixture of K2 CO3 (3.157 g, 22.840 mmol) and Nb2 O5 (8.127 g, 30.570 mmol) at 1100 ◦ C for 24 h. The [Nb6 O17 ]4− nanoscrolls were prepared as follows: 8.000 g K4 Nb6 O17 was exchanged with 1 mol/L HCl solution (1000 mL, changed daily) for 7 days at room temperature. 4.000 g acidexchanged K4 Nb6 O17 (noted as Hx K4−x Nb6 O17 , x < 4) was dispersed in 400 mL deionized water and then treated with 10% wt aqueous TBAOH for 48 h (pH ∼ 10). The resulted slurry was allowed to stand still for 2 h to separate the colloidal nanoscrolls and unexfoliated precipitates. The supernatant [Nb6 O17 ]4− nanoscrolls were carefully collected and then slowly flocculated with 50 mL of 0.25 mol/L Cu(NO3 )2 solution, respectively. After stirring for 12 h, the resulted Cu-containing gels were centrifuged at 9000 rpm for 5 min, thoroughly rinsed with deionized water, and then dried at 80 ◦ C (blue powder, noted as Cu2+ /nanoscrolls). Finally, 0.200 g Cu2+ /nanoscrolls were grinded and calcined in air at 450 ◦ C for 2 h. The resulted bright green catalysts were noted as CuO/nanotubes450. The schematic diagram of the preparation and structure of Cu-containing nanotubes is shown in Fig. 1. For comparison, CuO particles were also obtained by calcinating Cu(NO3 )2 at 450 ◦ C for 2 h. Besides, CuO supported on mixed

XRD patterns were obtained on a Philips X’Pert X-ray diffractometer with monochromatized Cu K␣ radiation. The size distribution and morphology were analyzed by transmission electron microscopy (TEM JEOL JEM-200CX and JEM-1011, operated at an accelerating voltage of 200 and 100 kV, respectively) and scanning electron microscopy (SEM JEOL JEM-6300F). The BET surface area was measured at 77 K by using a Micromeritics ASAP 3020 volumetric adsorption analyzer and the pore-size distributions were obtained using BJH model. The content of metals in the sample was determined by ARL-9800 XP+ (60 kV, 140 mA) X-Ray fluorescence spectrometer. UV–vis reflectance spectroscopic measurements were performed on a Shimadzu UV-3600 spectrophotometer by dispersing sample on BaSO4 . X-ray photoelectron spectroscopic (XPS) analysis was carried out on an X-ray photoelectron spectrometer (THERMO FISHER SCIENTIFIC, K-Alpha) equipped with a hemispherical electron analyzer (pass energy of 20 eV) and an Al K␣ (h = 1486.6 eV) X-ray source. The binding energies (BE) were referenced to the adventitious C 1s peak (284.6 eV) which was used as an internal standard to take into account charging effects. A combination of Gaussian and Lorentzian functions was used to fit the curves. Thermogravimetric analysis (TG) was performed by using an NETZSCH STA 449 C TGA thermal analyzer with a heating rate of 10 ◦ C/min and the temperature range from 26 to 800 ◦ C. 2.3. Catalyst test The epoxidation reactions were carried out as below: 1 mmol styrene, 0.02 mmol catalyst (based on Cu content), 50 ␮L bromobenzene (as the internal standard) and approximately 10 mL mixed solvent (CH3 CN/DMF or CH3 CN/NaHCO3 ) were added into the flask. Then 5 mmol of TBHP (or H2 O2 , as an oxidant) was added to the above mixture. The reaction was maintained at a certain temperature for a desired time and the progress was monitored periodically by gas chromatography. The used catalyst was washed thoroughly with ethanol and deionized water, dried and then reused under the same conditions as above-mentioned. 3. Results and discussion 3.1. XRD analysis As shown in Fig. 2A, pristine K4 Nb6 O17 exhibits all characteristic reflections which are well matched with previous literature and the relative intensity of the (0 4 0) diffraction peak is very strong and sharp, indicating a highly ordered lamellar structure [5,29]. After acid-exchange, the (0 4 0) diffraction peak was obviously shifted to a higher 2 angle and the corresponding d040 value was ˚ revealing that the interlayered K+ decreased from 8.1 A˚ to 7.9 A, ions of K4 Nb6 O17 were partially replaced by H+ ions [30]. In addition, compared with other diffraction peaks (inset of Fig. 2A), the (0 4 0) diffraction peak of Hx K4−x Nb6 O17 is still sharp and strong, suggesting the maintenance of the layered structure. Fig. 2B shows the XRD patterns of Cu2+ /nanoscrolls at different calcination temperature. Though the (0 k 0) reflections are extinct with a rise of calcination temperature (300–400 ◦ C), all patterns have several features in common at a 2 range of 20–50o corresponding to the in-plane diffractions. It indicates that the original orderly stacked layered structure vanished due to exfoliation–flocculation–calcination process while [Nb6 O17 ]4− nanoscrolls still remained in the resulted Cu-containing catalyst and the arrangement of nanoscrolls was rather irregular.

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Fig. 2. XRD patterns of (A) K4 Nb6 O17 and Hx K4−x Nb6 O17 (inset shows their magnified image corresponding 2 = 20–60◦ ) and (B) Cu2+ /nanoscrolls after calcination (300–500 ◦ C) and commercial Nb2 O5 . Table 1 Some textural parameters of Hx K4-x Nb6 O17 and CuO/nanotubes-450. Sample

SBET (m2 /g)

Pore volume (cm3 /g)

Hx K4−x Nb6 O17 CuO/nanotubes-450

0.6 50.7

/ 0.22

Element content (mmol/g) Cu

K

Nb

0 1.73

/ 0.17

/ 6.43

450 ◦ C

However, after calcination at (CuO/nanotubes-450), the in-plane (2 2 0) and (0 0 2) diffractions obviously weakened and new diffractions assigned to the (0 0 1) and (1 8 0) facets of Nb2 O5 clearly presented, indicating the formation of Nb2 O5 . As the temperature is further increased to 500 ◦ C, the XRD pattern of calcinated Cu2+ /nanoscrolls is in good agreement with that of Nb2 O5 , suggesting that [Nb6 O17 ]4− nanoscrolls are no longer existed and completely transformed into Nb2 O5 nanotubes (the nanotube morphology was further confirmed by TEM results below). In addition, although no diffraction peaks ascribed to CuO could be observed in Cu-containing catalysts calcined at different temperatures, XRF results in Table 1 show that the contents of Cu and Nb in CuO/nanotubes-450 are respectively 1.73 and 6.43 mmol/g, indicating the successful immobilization and high dispersion of CuO on nanotubes originated from the thermal transformation of nanoscrolls. 3.2. Morphology analysis Fig. 3 displays the morphologies of the pristine K4 Nb6 O17 , nanoscrolls, and CuO/nanotubes-450. Obviously, the original K4 Nb6 O17 has a typical layered structure composed of orderly stacked layers (Fig. 3a), being consistent with the XRD results. However, the compact stacked layers are delaminated into free-standing negatively charged [Nb6 O17 ]4− nanoscrolls after acid-exchange of K4 Nb6 O17 and a subsequent reaction of Hx K4−x Nb6 O17 with TBAOH. TEM images clearly show that nanoscrolls are successfully obtained and the average diameter of nanoscrolls is ∼30 nm while the length is in the range of 200–500 nm (Fig. 3b). After flocculation of nanoscrolls with Cu2+ ions and calcination at 450 ◦ C, the resulted catalyst CuO/nanotubes-450 shows that the original orderly-stacked layered structure completely vanished and a rather disorderly restacked structure mainly composed of nanotubes with an inner diameter of 40 nm was formed (Fig. 3c and d). In addition, the EDS mappings of CuO/nanotubes-450 reveal

that Cu, Nb and O elements are homogenously distributed in the catalysts (Fig. 3e–h). The above results clearly indicate that CuO/nanotubes-450 has a tubular structure with well-distributed Cu species. 3.3. N2 adsorption–desorption analysis Fig. 4 depicts N2 adsorption–desorption isotherm and pore-size distribution curve of CuO/nanotubes-450. Clearly, CuO/nanotubes450 exhibits a characteristic type IV N2 isotherm and a type H3 hysteresis loop, indicating a slit-like pore structure. Besides, the pore-size distribution curve shows a sharp peak at ∼4 nm and a broad peak at ∼40 nm. It is uncertain about the origin of the sharp peak at ∼4 nm for CuO/nanotubes-450, perhaps the well-known artifact of liquid nitrogen instability at 3.8 nm, which only occurs in the desorption branch [31]. Large pores with a diameter of ∼40 nm might be originated from the aggregates of nanotubes and the interior spaces of nanotubes for CuO/nanotubes-450 [32]. Table 1 lists BET surface area and pore volume of Hx K4−x Nb6 O17 and CuO/nanotubes-450. Significantly, CuO/nanotubes-450 has a much higher BET surface area and pore volume than Hx K4−x Nb6 O17 . It can be attributed to the formation of nanotubes and the disorderly restacking of nanotubes, which are in good agreement with the above XRD and SEM results. It indicated that the exfoliation–flocculation–calcination process of Hx K4−x Nb6 O17 is advantageous to the formation of a porous structure without template, resulting in the enhancement of BET surface area and pore volume. 3.4. UV–vis diffuse reflectance spectra analysis UV–vis diffuse reflectance spectra (UV-DRS) can provide useful information related to the copper oxide species and its chemical environment [33,34]. Fig. 5 shows the UV-DRS of the corresponding catalysts. Obviously, K4 Nb6 O17 shows the characteristic absorption at 200–400 nm and no absorption in the infrared region. For CuO, it shows an extensive broad band at 700–900 nm (ca. 733 nm) corresponding to d–d transition of Cu2+ ions in distorted octahedral configuration [35], which is characteristic of bulk CuO species. After immobilization of Cu species, CuO/nanotubes-450 shows not only an enhanced absorption band at 220–350 nm and a red shift of the absorption edge (400–550 nm), but also a new absorption band in the range of 700–950 nm.

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Fig. 3. EM images of (a) K4 Nb6 O17 , (b) nanoscrolls, (c and d) CuO/nanotubes-450, and EDS mappings of CuO/nanotubes-450 (e–h).

Compared with pristine K4 Nb6 O17 , the enhanced absorption band of CuO/nanotubes-450 at 220–350 nm indicates the appearance of overlapping absorption in this region. According to the previous literature [36,37], the absorption band at 220–350 nm can be attributed to the ligand-to-metal charge transfer (O2− → Cu2+ ) corresponding to the isolated Cu2+ ions and highly dispersed cluster-like copper species. Besides, the absorption edge of CuO/nanotubes-450 is red shifted towards 400–550 nm. Generally, the absorption edge at 400-550 nm is ascribed to the three-dimensional Cu+ cluster in the CuO matrix [38,39]. However, the existence of Cu+ is less likely in CuO/nanotubes-450 due to the calcination under air atmosphere. Chen prefers to assign this band

Fig. 4. N2 adsorption–desorption isotherm and pore-size distribution curve of CuO/nanotubes-450.

at 400–500 nm to a new Cu-containing phase [40]. According to the XRD results, the possibility of a new Cu-containing phase can be excluded. Besides, as shown in the inset of Fig. 5, CuO/nanotubes450 is colored in transparently green, which is similar to Cu/SiO2 in which Cu2+ ions were dissolved in the matrix of SiO2 [41]. Thus we presume that some Cu2+ ions are incorporated in the NbO6 octahedron matrix at an atomic level, resulting in a red shift of absorption edge, which can be further confirmed by XPS results. Moreover, compared with CuO, the absorption bands of CuO/nanotubes-450 at 700–950 nm become rather broader. The reason may be the high dispersion of CuO species rather than

Fig. 5. UV–vis spectra of (a) K4 Nb6 O17 , (b) CuO and (c) CuO/nanotubes-450.

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Fig. 6. XPS spectra of (a) K4 Nb6 O17 and (b) CuO/nanotubes-450.

the inhomogeneity of CuO particle sizes because the exfoliationflocculation process involves an electrostatic interaction between negatively-charged nanoscrolls and positively-charged Cu2+ ions. In addition, EDS mapping (see Fig. 3) clearly shows that Cu, Nb and O elements are homogenously distributed in the catalyst. It also means that nanoscrolls can be used as a novel support for the dispersion of CuO species. 3.5. XPS analysis As illustrated in Fig. 6A, the spectrum of the survey scan indicates the presence of K, O and Nb in K4 Nb6 O17 and the appearance of Cu in CuO/nanotubes-450. As can be seen in Fig. 6B, the binding energies (BE) of Nb 3d3/2 and Nb 3d5/2 for K4 Nb6 O17 appear at 209.3 and 206.5 eV, respectively. After the immobilization of Cu species, the BEs of Nb 3d3/2 and Nb 3d5/2 are increased to 209.5 and 206.9 eV in CuO/nanotubes450. Besides, the BE of O 1s in K4 Nb6 O17 are located at 531.9 and 529.8 eV, which can be attributed to the hydroxyl oxygen and the lattice oxygen, respectively. However, the BE of lattice oxygen of CuO/nanotubes-450 is shifted to 530.0 eV and two new peaks appear at 532.4 and 531.5 eV due to the hydroxyl oxygen and adsorbed oxygen [42–44]. The BE increment of both Nb 3d and O 1s can be ascribed to the strong interaction between Cu and O.

As shown in Fig. 6D, CuO/nanotubes-450 exhibits Cu 2p region that a Cu 2p3/2 peak appears at ∼933.5 eV and a 2p1/2 peak ∼953.5 eV, along with two shake-up satellite peaks at ∼942.1 and ∼962.1 eV, respectively, indicating a typical characteristic of Cu(II) oxidation state [45–48]. After the fitting of each Cu 2p peaks, two Cu components at BE of 932.7 (1) and 934.6 eV (2) for Cu 2p3/2 and 952.3 (1) and 954.3 eV (2) for Cu 2p1/2 can be resolved. The component (1) at low BE can be ascribed to isolated Cu2+ ions in the NbO6 octahedron matrix while the component (2) at higher BE is probably due to the presence of CuO nanoparticles in the catalysts [46], which is consistent with UV-DRS results. 3.6. Thermal analysis Fig. 7 depicts TG–DTG–DTA curves of the uncalined Cu2+ /nanoscrolls. With an endothermic peak at 95 ◦ C and a maximum rate of weight loss at 155 ◦ C, the weight loss below 200 ◦ C is 7.7 wt% and can be attributed to the evaporation of absorbed water. Besides, the weight loss between 200 and 400 ◦ C is 3.4 wt%, and an obvious exothermic peak appears at 293 ◦ C in DTA curve mainly due to the decomposition of the residual TBA+ ions. However, there is no significant weight loss beyond 400 ◦ C. According to XRD and SEM/TEM results, the [Nb6 O17 ]4− nanoscrolls gradually disappeared above 400 ◦ C and completely

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2+

Fig. 7. TG–DTG–DTA curves of uncalcined Cu /nanoscrolls.

transformed into Nb2 O5 nanotubes at 500 ◦ C. This fact indicates that the resulted Cu-containing nanotubes is rather stable. 3.7. Catalytic activity 3.7.1. Epoxidation with TBHP The catalytic behaviors of CuO/nanoscrolls-450 were investigated in the epoxidation of styrene by TBHP and H2 O2 . For comparison, CuO supported on mixed oxides (2K2 O/3Nb2 O5 ) was prepared via conventional impregnation method (noted as CuO2 K2 O/3Nb2 O5 -450) and evaluated under the same conditions. The results are listed in Table 2. For the epoxidation of styrene by TBHP, both K4 Nb6 O17 and Hx K4−x Nb6 O17 showed a rather poor catalytic performance with a very low conversion and TOF (entries 1 and 2). Except trace amount of benzaldehyde and other products (benzoic acid and phenyl acetic acid), no styrene oxide could be obtained. It indicates that the epoxidation of styrene did not occur in the presence of only support. Nevertheless, for CuO particles, the epoxidation of styrene did occur and a conversion of 66.9% and a SO selectivity of 35.4% were obtained (entry 3). In addition, it was found that CuO-2 K2 O/3Nb2 O5 -450 had a conversion of 62.7% and a SO selectivity of 46.0% (entry 4). The results show that CuO supported on mixed oxides has a relatively better catalytic behavior than bare CuO particles. Interestingly, CuO/nanotubes-450 had a conversion of 94.5% and a SO selectivity of 46.9% (entry 5), which is much superior to bare CuO particles and CuO-2 K2 O/3Nb2 O5 -450. Even for the reused CuO/nanotubes-450, the catalytic performance is still better than that of bare CuO particles (entry 6). These results suggest that the nanotubular morphology is greatly beneficial to the improvement of catalytic performance, especially for the enhancement of styrene conversion. The reason may be attributed to the large surface area and pore volume of nanotubes. In addition, it was also found that the SO selectivity was decreased noticeable from 46.9% to 35.4% while the conversion was increased from 94.50% to 99.0% if DMF not used in the solvent (entry 7), revealing that the SO selectivity is dependent upon DMF. The reason may be the strong interaction of DMF molecules with the active Cu2+ sites, which would not only impair the coordination of styrene with Cu2+ species but also promote the separation of SO from the active sites and prevent the deep oxidation of SO by TBHP [49]. Fig. 8 shows the influence of calcination temperature on the catalytic activity of Cu-containing nanotubes. The best conversion and SO yield were achieved when the calcination temperature was 450 ◦ C. The SO selectivity was almost the same below 450 ◦ C

121

Fig. 8. The influence of calcination temperature on styrene epoxidation by TBHP. Reaction conditions: catalyst (2 mmol%), styrene (1 mmol), TBHP (5 mmol), solvent (9 mL CH3 CN + 1 mL DMF), reaction time = 8 h, reaction temperature = 83 ◦ C.

but decreased dramatically at 500 ◦ C. The reason should be the completely transformation of [Nb6 O17 ]4− nanoscrolls into Nb2 O5 nanotubes at 500 ◦ C. Fig. 9 displays the influence of the substrate/oxidant ratio on the performance of CuO/nanotubes-450. Obviously, the substrate/oxidant ratio plays a significant influence on styrene conversion but has little effects on SO selectivity. It was found that the desirable substrate/oxidant ratio was 1:5. 3.7.2. Epoxidation with H2 O2 As shown in Table 2, a conversion of 75.5% and a SO selectivity of 64.0% were obtained by bare CuO (entry 8). However, CuO/nanotubes-450 showed a much enhanced selectivity (up to 94.6%) although the conversion was reduced to some extent, indicating that CuO species on nanotubes is also propitious to the enhancement of SO selectivity by H2 O2 . As shown in entries 10–12, the reaction time and substrate/oxidant ratio did not have a significantly effect on SO yield. However, when the amount of catalyst was doubled, both conversion and selectivity were obviously decreased, revealing that the catalytic behavior of CuO/nanotubes-450 in the H2 O2 –NaHCO3 oxidation system is sensitive to active Cu2+ sites. Fig. 10 shows the influence of CH3 CN/NaHCO3 volume ratio on SO yield in the presence of H2 O2 . Clearly, the epoxidation of styrene did not occur without NaHCO3 and SO yield has a linear relationship with the volume of NaHCO3 within a range of 0-5 mL, suggesting that H2 O2 –NaHCO3 adduct is superior to sole H2 O2 for styrene

Fig. 9. The influence of substrate/oxidant ratio on styrene epoxidation by TBHP. Reaction conditions: catalyst (2 mmol %), styrene (1 mmol), solvent (9 mL CH3 CN + 1 mL DMF), reaction time = 8 h, reaction temperature = 83 ◦ C.

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Table 2 Performance of the corresponding catalysts in the epoxidation of styrene. Entry

Catalyst

Epoxidation with TBHPa K4 Nb6 O17 1 Hx K4−x Nb6 O17 2 3 CuOb 4 CuO-2K2 O/3Nb2 O5 -450 5 CuO/nanotubes-450 CuO/nanotubes-450c 6 CuO/nanotubes-450d 7 Epoxidation with H2 O2 e 8 CuOb 9 CuO/nanotubes-450 CuO/nanotubes-450f 10 CuO/nanotubes-45g 11 CuO/nanotubes-450i 12

Styrene conv. (%)

Selectivity (%)

SO yield (%)

TOF (h−1 )

SO

BA

OP

9.8 8.2 66.9 62.7 94.5 99.0 99.0

Trace Trace 35.4 46.0 46.9 37.8 35.4

24.3 16.4 7.7 8.5 2.7 2.2 2.5

75.7 83.6 56.9 45.5 50.4 60.0 62.1

Trace Trace 23.7 28.8 44.3 37.4 35.1

0.6 0.5 4.2 3.9 5.9 6.2 6.2

75.5 65.7 65.3 68.6 58.6

64.0 94.6 92.3 87.9 55.6

Trace 2.4 2.0 2.3 Trace

36.0 3.0 5.7 9.8 44.4

48.4 61.1 60.2 60.8 32.6

18.9 16.4 8.2 17.2 14.7

a Reaction conditions: Styrene (1 mmol), solvent (∼10 mL, CH3 CN:DMF = 9:1), reaction time = 8 h, reaction temperature = 83 ◦ C, n(styrene):n(oxidant) = 1:5. Amount of copper oxide containing catalyst (based on Cu content, 0.02 mmol). For K4 Nb6 O17 and Hx K4−x Nb6 O17 , 10 mg. SO = Styrene oxide, BA = benzaldehyde and OP = other products. TOF = [product]/([catalyst] × time). b CuO particles obtained by decomposition of Cu(NO3 )2 at 450 ◦ C for 2 h. c Reused catalyst. d 10 mL CH3 CN. e Reaction conditions: Styrene (1 mmol), Solvent (∼10 mL, CH3 CN:NaHCO3 = 3:7), Reaction time = 2 h, Reaction temperature = 40 ◦ C. Amount of copper oxide containing catalyst (based on Cu content, 0.02 mmol). n(styrene): n(oxidant) = 1:5. f Reaction time = 4 h. g n(styrene):n(oxidant) = 1:10. i Amount of catalyst ∼ 0.04 mmol.

epoxidation over CuO/nanotubes-450 and the catalytic behavior depends on the amount of NaHCO3 . The best SO yield was obtained in CH3 CN/NaHCO3 volume ratio 3:7 while seriously decreased in 1:9. It might be attributed to a high pH value of the reaction system in the presence of an excess NaHCO3 and relatively weak solubility of styrene and epoxide in a small amount of CH3 CN, both could impair styrene epoxidation. Fig. 11 depicts the influence of reaction temperature on styrene epoxidation by H2 O2 –NaHCO3 adduct. It was found that the proper temperature range was 40–60 ◦ C and the catalytic performance significantly decreased outside the range. A lower temperature is unfavorable to the adsorption of substrate and H2 O2 –NaHCO3 adduct on CuO/nanotubes-450 while a higher temperature can accelerate both decomposition of oxidant and deep oxidation of epoxides. In conclusion, compared with the earlier reports in which the conventional ion-exchange methods were used [24,25],

CuO/nanotubes-450 in this work shows a higher SO selectivity and SO yield. The present work implies that the exfoliation-flocculation process with nanoscrolls is not only helpful to immobilize CuO species, but also generally leads to a high surface area and a large pore volume originated from the random and disorderly rearrangement of nanotubes, which are both advantageous to the catalytic performance.

Fig. 10. The influence of CH3 CN/NaHCO3 volume ratio on SO yield catalyzed by CuO/nanotubes-450 in the presence of H2 O2 . Reaction conditions: catalyst (2 mmol %), styrene (1 mmol), n(styrene):n(oxidant) = 1:5, reaction time = 2 h, reaction temperature = 40 ◦ C.

Fig. 11. The influence of reaction temperature on styrene epoxidation by H2 O2 –NaHCO3 adduct. Reaction conditions: catalyst (2 mmol%), styrene solvent (∼10 mL, CH3 CN:NaHCO3 = 3:7), reaction time = 2 h, (1 mmol), n(styrene):n(oxidant) = 1:5.

3.8. The possible catalytic mechanism of epoxidation To explain the improved selectivities and the different effects of DMF and NaHCO3 in the epoxidation mediated by CuO/nanotubes450, according to the literature [28,50], two possible mechanisms were put forward and illustrated in Fig. 12. In the epoxidation by TBHP, the terminal oxygen of TBHP is activated by Cu2+ sites and then could be reacted with the styrene. The possible transition-state is shown in Fig. 12a. The reaction is

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Fig. 12. Proposed mechanisms for the epoxidation of styrene by (a) TBHP and DMF, and (b) H2 O2 –NaHCO3 adduct.

impaired because of the strong interaction of DMF molecules with the active Cu2+ sites, and thus a long time and a high temperature are required. However, the strong interaction can also promote the separation of SO from the active sites and prevent the deep oxidation of SO by TBHP, resulting in an enhancement of SO selectivity. On the other hand, the epoxidation by H2 O2 –NaHCO3 adduct might be another mechanism. H2 O2 and HCO3 − react in an equilibrium process to produce HCO4 − [28] which is more active than sole H2 O2 , leading to the formation of five-membered ring by HCO4 − and Cu2+ sites (Fig. 12b). Being different from the epoxidation by TBHP, more styrene molecules are adsorbed on the Cu2+ sites due to the absence of DMF. The styrene and the active O in the fivemembered ring are more approachable due to the curved surface of nanotubes, thus giving a higher SO selectivity on Cu-containing nanotubes than on bare CuO. 4. Conclusion In summary, Cu-containing nanotubes were prepared by using [Nb6 O17 ]4− nanoscrolls as a support and a subsequent thermal treatment. The method is facile and template-free. The resulted Cu-containing nanotubes displayed a relative good catalytic performance with a styrene oxide (SO) selectivity of 46.9% by TBHP and a much higher SO selectivity of 94.6% by H2 O2 , which are both higher than that of bare CuO. Two possible mechanisms were put forward to explain the different catalytic behaviors in the two types of oxidation systems. Because of the transformation of nanoscrolls into nanotubes, nanoscrolls may be a new kind of promising support for the design and assembly of novel heterogeneous catalysts. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 21073084 and 20773065), Natural Science Foundation of Jiangsu Province (Grant No. BK2011438), 973 Project (Grant No. 2009CB623504, Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20130091110010) and Modern Analysis Center of Nanjing University. References [1] S. Shylesh, V. Schünemann, W.R. Thiel, Magnetically separable nanocatalysts: bridges between homogeneous and heterogeneous catalysis, Angew. Chem. Int. Ed. 49 (2010) 3428–3459. [2] X.H. Liu, R. Yi, N. Zhang, R.R. Shi, X.G. Li, G.Z. Qiu, Cobalt Hydroxide nanosheets and their thermal decomposition to cobalt oxide nanorings, Chem. Asian J. 3 (2008) 732–738. [3] R.Z. Ma, Y. Kobayashi, W.J. Youngblood, T.E. Mallouk, Potassium niobate nanoscrolls incorporating rhodium hydroxide nanoparticles for photocatalytic hydrogen evolution, J. Mater. Chem. 18 (2008) 5982–5985.

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