Highly Dispersed Copper Species within SBA-15 Introduced by the

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Highly Dispersed Copper Species within SBA-15 Introduced by the Hydrophobic Core of a Surfactant Micelle as a Carrier and Their Enhanced Catalytic Activity for Cyclohexane Oxidation Jinlou Gu,*,† Ying Huang,† S. P. Elangovan,‡ Yongsheng Li,† Wenru Zhao,† Iijima Toshio,‡ Yasuo Yamazaki,‡ and Jianlin Shi* ,†,§ †

Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China ‡ Nippon Chemical Industrial Co. Ltd. 11-1, 9-chome, Kameido, Koto-ku, Tokyo 136-8515 Japan § The State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China ABSTRACT: Highly dispersed copper species were successfully coated onto the inner surface of the rodlike mesoporous SBA-15 silica by a newly developed strategy. The hydrophobic core of the surfactant micelle in the as-synthesized SBA-15 was effectively employed as a carrier for the hydrophobic precursors of copper acetyl acetonate (Cu(acac)2) during hydrothermal treatment (HT) before surfactant removal, and the copper-derivated catalysis centers within SBA-15 were strategically generated on pyrolysis. Characterizations of the synthesized samples by a series of techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer Emmett Teller (BET), electron paramagnetic resonance (EPR), temperature-programmed reduction (TPR), UV vis, X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma atomic emission spectroscopy (ICP AES) manifested that about 0.19 0.85 wt % copper was directly introduced and dispersed into the inner surface of the SBA-15 pore network. While there were few studies, if any, to utilize synthetic copper catalysts for the selective oxidation of cyclohexane oxidation, it was found that currently synthesized Cu-SBA-15 demonstrated very interesting catalytic properties for this industrially important reaction. Under the optimized reaction condition at an oxygen pressure of 1 MPa and a temperature of 120 C for 7 h, the conversion of cyclohexane to the target products was higher than 11%; meanwhile, the total selectivity to cyclohexanol and cyclohexanone satisfactorily reached ca. 80%.

1. INTRODUCTION Supported copper species catalysts have attracted considerable attention due to their recent practical uses in many heterogeneous redox reactions.1 4 Mesoporous materials (MMs), such as SBA15 with intrinsically high surface area and pore volume,5 7 could serve as an excellent platform for their deposition.8 To enhance the catalytic efficiency of the incorporated copper species and take full advantage of the supports, uniform dispersion of the active centers is preferable for free access by the reactants.9 Simultaneously, the size of copper species in the form of nanoparticles, clusters, or isolated copper ions should be controlled at least below the pore size of employed mesoporous supports to prevent the pore blocking and also to ensure enough active surface exposure to the reactants.10 Generally, two preparative routes, i.e., the post modification11,12 and one-pot synthesis,13,14 have been followed for the introduction of active copper catalytic centers. Using post modification methods, such as wet impregnation11 or deposition precipitation,12 the procedure is relatively controllable since the parent support preparation and the grafting process are totally separated; however, it is quite difficult to obtain the uniformly distributed copper species within MMs. Comparatively, the one-pot direct method14,15 r 2011 American Chemical Society

produces better control over the distribution of the introduced copper species since all the reagents for the catalyst preparation are uniformly mixed together but generally at the expense of the mesoscopic ordering of the support. Keeping the pros and cons of the two existent routes in mind, herein we develop a novel and controllable strategy to introduce highly dispersed copper species into rodlike mesoporous SBA-15 silica. It is currently well-known that the hydrophobic core of P123 micelles for SBA-15 templating could provide the compatible environment for the hydrophobic compound, such as Cu(acac)2 in our current case.16,17 However, the SBA-15 is usually synthesized in strong acid media, e.g., 2 M HCl;5,18 such a harsh environment will make the copper precursor unstable if we employ a one-pot synthesis strategy.19 Therefore, we first prepared parent SBA-15 without introducing copper-containing hydrophobic compounds and separated the SBA-15/P123 from its mother solution. Then, during the subsequent hydrothermal procedure, the hydrophobic compound of Cu(acac)2 was added Received: June 29, 2011 Revised: September 23, 2011 Published: September 26, 2011 21211

dx.doi.org/10.1021/jp206132a | J. Phys. Chem. C 2011, 115, 21211–21217

The Journal of Physical Chemistry C and spontaneously infiltrated into the hydrophobic core of P123 micelles as the copper source. Followed by pyrolysis, the copperderivated catalytic centers within SBA-15 were strategically prepared. Copper active centers could be highly and uniformly dispersed at the inner surface of the silica framework due to the molecular level dissolution of the precursor compounds into the hydrophobic core of the surfactant template. As expected, the synthesized catalysts demonstrated excellent catalytic properties for the selective oxidation of cyclohexane.20 In comparison with a 4% conversion using traditionally commercial cobalt-based catalysts,21,22 the conversion of cyclohexane to the target products was higher than 11% over the synthesized highly dispersed copper catalysts within SBA-15 under a relatively mild experimental condition. In the present work, we report this newly developed method to fabricate Cu-SBA-15 catalysts and reveal the highly dispersed nature of the incorporated copper species at the inner surface of the mesoporous silica framework. The significantly enhanced catalytic activity for cyclohexane oxidation over the synthesized catalysts is also disclosed.

2. EXPERIMENTAL SECTION 2.1. Preparation of Rodlike SBA-15. An amount of 4.0 g of triblock copolymer Pluronic P123 was added to 150 g of 1.6 M HCl aqueous solution in an autoclave. The mixture was kept under static conditions at 35 C until all the copolymer was completely dissolved. Then, 9.2 mL of TEOS was added under vigorous stirring. After about 5 min stirring, the mixture was aged under static conditions for about 20 h. Thereafter, the solid products were collected by filtration, washed with water several times, and dried at 60 C for 4 h. 2.2. Preparation of Copper-Doped SBA-15 (Cu-SBA-15). The above prepared dried rodlike SBA-15 with P123 trapped (1 g) was treated with 15 mL of distilled water, and the mixture solution was vigorously stirred for about half an hour in an autoclave. Another portion of the mixture solution was prepared by dissolving different amounts of Cu(acac)2 in 5 mL of benzene and 2 mL of trimethylbenzene. Then the prepared copper solution was poured into the above autoclave and then heated at different temperatures, viz. 60, 80, and 100 C, for about 48 h. The resultant Cu(acac)2 infiltrated SBA-15 was collected by filtering the suspension. Then the samples were washed several times with distilled water and ethanol and dried at 100 C for 24 h. The obtained powder was calcined at 550 C for 6 h, and finally, highly dispersed Cu species doped with SBA-15 were prepared. According to the different hydrothermal treatment (HT) temperatures for Cu(acac)2/SBA-15 mixture solutions and the Cu loading levels calculated from the as-synthesized Cu-SBA-15 catalysts, the corresponding samples were labeled as 60-1, 60-2, 80-1, 80-2, 100-1, and 100-2, respectively (the first numbers of 60, 80, and 100 stand for temperatures, and the second numbers of 1 and 2, respectively, represent low and high copper loading levels). 2.3. Catalytic Property Testing. The selective oxidation of cyclohexane was carried out in a 100 mL Teflon-lined stainlesssteel autoclave. Typically, 8 mL of cyclohexane and 50 mg of solid catalyst were added into the reactor. After purging with O2, the reactor was heated to a predetermined temperature, and the O2 pressure was adjusted to 1 MPa with a stirring rate of 100 rpm. During the oxidation process, the O2 pressure was kept between 0.8 and 1 MPa. After a predetermined time of reaction, the

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reactor was cooled to room temperature, and the ethanol was dissolved into the obtained mixture. An excessive amount of triphenyphosphine (Ph3P) was added to the reaction mixture to completely reduce the cyclohexyl hydroperoxide (CHHP), an intermediate in the cyclohexane oxidation to cyclohexanol. The products were analyzed using a gas chromatograph (PerkinElmer, Autosystem XL, USA) with a KB-Wax column. The byproducts of acids and esters were analyzed by GC MS (Agilent, 7890A-5975C). Recycling tests in four consecutive reactions were run at 120 C for 5 h and an isobaric condition of 1 MPa O2. After each cyclohexane oxidation run, the solid catalyst was cooled to room temperature, separated by centrifugation from the product solution, washed with water several times, and then dried at 80 C and reused for the next run under the same conditions. Inductively coupled plasma atomic emission spectroscopy (ICP AES) was employed to analyze the copper contents in the filtered liquid product and the used catalysts. 2.4. Characterizations. UV visible absorption and diffuse reflectance (DR) spectra were recorded on a Shimadzu UV vis 3101 spectroscopy. X-ray diffraction (XRD) patterns were collected with Bruker D8 using Cu Ka radiation (40 kV, 40 mA). N2 adsorption desorption isotherms were obtained on NOVA 4200e at 77 K under a continuous adsorption condition. Prior to the measurements, all samples were pretreated for 12 h at 120 C under nitrogen. The Brunauer Emmett Teller (BET) method was utilized to calculate the specific surface areas using adsorption data in a relative pressure range from 0.2 to 0.4. By using the Barrett Joyner Halenda (BJH) model, the pore volumes and pore size distributions were derived from the adsorption branches of isotherms, and the total pore volumes were estimated from the adsorbed amount at a relative pressure P/P0 of 0.99. The copper contents in the catalysts and filtrated liquid after the catalytic reaction were determined by ICP AES. Transmission electron microscopy (TEM) was conducted on a JEM 2100F electron microscope operated at 200 kV. X-ray photoelectron spectroscopy (XPS) signals were collected on a VG Micro MK II instrument using monochromatic Mg Kα X-rays at 1253.6 eV operated at 150 W. Calibration of the binding energies was made with the C1s binding energy of standard hydrocarbons (284.6 eV). Electron paramagnetic resonance (EPR) was measured by a Bruker X-band EPR spectrometer operated at a frequency of 100 kHz, microwave powder of 9.79 GHz, and a time constant of 500 ms, and the experiments were performed at 100 K. Temperature-programmed reduction of hydrogen (H2-TPR) was performed by using a homemade apparatus loaded with 50 mg of sample. The samples were pretreated at 600 C for 1 h and then cooled to 100 C under a He flow. Pure H2 was injected until adsorption saturation was reached, followed by a flow of He only for 30 min. Then the temperature was raised from 100 K to 600 C with a heating rate of 1 C min 1, and the amount of desorbed hydrogen was detected by a thermal conductivity detector (TCD).

3. RESULTS AND DISCUSSION Figure 1 shows UV vis absorption spectra for the parent SBA-15/P123, pure Cu(acac)2 in ethanol solution, and Cu(acac)2 in the hydrophobic inner core of P123 surfactant micelles after HT for 60-1, 60-2, and 80-2 samples, and it is used to verify the encapsulation process of the hydrophobic compounds. No obvious absorption peak for the parent SBA-15/P123 is present, 21212

dx.doi.org/10.1021/jp206132a |J. Phys. Chem. C 2011, 115, 21211–21217

The Journal of Physical Chemistry C

Figure 1. UV vis absorption spectra for the parent SBA-15/P123 (a), pure Cu(acac)2 dissolved in ethanol solution (b), and Cu(acac)2 dissolved in the hydrophobic inner core of P123 surfactant micelles after hydrothermal treatment for 60-1 (c), 80-2 (d), and 60-2 (e).

Figure 2. Small-angle XRD patterns of parent SBA-15 (a), 60-1 (b), and 60-2 (c). The inset shows the wide-angle XRD pattern of sample 60-2.

while all the other samples demonstrate the typical strong absorption bands at about 240 300 nm assigned from Cu(acac)2.19 Due to the slight difference in microenvironment of the hydrophobic core of surfactant micelles from ethanol solution, the absorption peaks for the encapsulated Cu(acac)2 shift very slightly. These results manifest that the Cu(acac)2 has been really infiltrated into the hydrophobic core of the surfactant micelles and also verify the synthetic mechanism we proposed. We also found that the peak intensity increased when a larger amount of Cu(acac)2 was added into the SBA-15/P123 suspension under the same HT temperature of 60 C. However, when we increased the temperature to 80 C (Figure 1d) or 100 C (data not shown), it was found that the loaded Cu(acac)2 in the P123 hydrophobic core decreased corresponding to lower peak intensity of the UV vis adsorption. These observations imply that the treatment at overhigh temperature is unfavorable for the assembly of the hydrophobic copper compounds into the hydrophobic core of the P123 micelles, assuming that the harsh condition of higher temperature will partially change its inherent hydrophobicity to hydrophilicity23 and make the metal organic compounds no longer dissolvable into the hydrophobic cores of the surfactant micelle. Since 60 C was the most optimum temperature of HT based on UV vis measurements, our further work was centered on 60-1 and 60-2 samples. The small-angle XRD was applied to monitor the mesostructure evolvement of the synthesized samples. As manifested in Figure 2a, the well-defined and ordered pore structure is present for parent SBA-15. Three well-resolved reflections of (100), (110), and (200) could be clearly indexed in the hexagonal space group of

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p6mm. All the diffraction peaks for samples 60-1 and 60-2 shift to lower angle direction, evidencing the enlarged structure units. This provides further evidence that Cu(acac)2 is really effectively incorporated into the hydrophobic region of SBA-15/P123 and expands the pore diameter. Three peaks are perfectly maintained after HT at 60 C for 48 h, indicating that the structure ordering of the SBA-15 support is not destroyed. In the wide-angle region for 60-2, the sample with the highest Cu content, no diffraction peak corresponding to crystalline CuO clusters could be detected as shown in the inset of Figure 2. It is thus deduced that the particle size of the doped Cu species is probably too small to be detected by X-ray diffraction. The pore structure parameters of the synthesized samples with different copper contents were also characterized by nitrogen sorption isotherms. Consistent with the above XRD analysis, all the samples possess N2 sorption isothermal curves similar to that of parent SBA-15. Their irreversible type IV adsorption desorption isotherms with a H1 hysteresis are indicative of the presence of a mesoporous structure with narrow pore size distributions. The capillary condensation steps shift to a relatively higher pressure, and pore sizes are clearly expanded for all the copper-doped samples as depicted in Figure 3. This pore size expansion and corresponding pore volume enhancement should be attributed to the introduction of hydrophobic solvent and Cu(acac)2 in the hydrophobic core of P123 micelles. The specific pore structure parameters as well as precise amounts of copper loading level in different samples measured by ICP AES are listed in Table 1. TEM images were taken to analyze the distribution of copper species in the meso-channels and to directly observe the morphology of the synthesized samples. As shown in Figure 4, SBA-15 rods with meso-channels are clearly present for all the samples including parent and Cu-doped SBA-15, which indicates that the introduction of copper oxide did not destroy the ordered mesostructure and the discrete rodlike morphology of the parent SBA-15. The preservation of the mesostructure ordering could be more clearly visible from the enlarged image of Figure 4d, and parallel channels cross through the whole SBA-15 rods. More importantly, no CuO particles or aggregations are observable outside or inside the channels, even when the Cu content is as high as 0.85 wt % as shown in Figure 4c for sample 60-2. Therefore, it is concluded that uniform copper species with very small sizes are highly dispersed into the channels, which is believed to benefit from the inherent merits of the current strategy in which the hydrophobic precursors of Cu(acac)2 were dissolved in hydrophobic cores of surfactant micelles at the molecular level. All the above results also verify that the introduced copper active centers are highly accessible to guest molecules as the meso-channels are unblocked by introduced Cu species. The incorporation of copper species into the SBA-15 and the elemental state in the composite materials were further explored by XPS. The results show that the binding energy of Cu2p3/2 is centered at 934.5 eV for both samples of 60-1 and 60-2 (Figure 5), which is a little higher than that of bulk CuO as reported.24 Such a shift of Cu 2p3/2 peak toward higher energy is previously ascribed to the better dispersion of introduced copper species in the MMs25 and is consistent with the above TEM analysis. The signal intensity for sample 60-2 is relatively stronger than that for 60-1 and also illustrates that the Cu loading level is higher for 60-2 in agreement with ICP and UV vis spectroscopy measurements. 21213



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Figure 3. N2 adsorption desorption isotherms (A) and the corresponding pore size distribution curves (B) of the samples for parent SBA-15 (a), 60-1 (b), and 60-2 (c).

Table 1. Pore Structure Parameters for the Synthesized Samples and the Cu Content Calculated from ICP Measurements S

pore width

pore volume

Cu

sample

(m2/g)

(nm)

(cm3/g)

(wt %)

SBA-15

614

4.6

0.65

0

60-1 60-2

687 690

5.9 6.4

1.00 1.05

0.19 0.85

To further probe the coordination environment and dispersion status of the introduced copper species, EPR analysis was carried out, and the recorded spectra were shown in Figure 6. Four splitting features (mI = 3/2, 1/2, +1/2, +3/2) are clearly observed in the low-field region for the parallel component due to the hyperfine interaction between the unpaired electron and the nuclear spin of copper (I = 3/2), whereas the signal for the perpendicular component (g^= 2.08) is not resolved. These features are typical of Cu2+ cations in axial symmetry,26 and the anisotropic parameters (gII = 2.39 2.40) are in agreement with Cu2+ in octahedral coordination.27 These EPR lines of paramagnetic Cu2+ ions will otherwise disappear for bulk CuO due to a strong dipole dipole interaction. Therefore, highly dispersed Cu2+ species are prevailed in both samples of 60-1 and 60-2, while small clusters cannot be totally excluded.28 Actually, for copper-containing SBA-15 prepared by a conventional method, even the copper content was as low as 0.4 wt %,3 and the EPR signals have become weakened due to the dipole interaction among the aggregations between the neighboring paramagnetic species. For the current synthesized 60-2 sample with copper content of 0.85 wt %, the gII signals are more resolved, and the intensity of the g^ signal is much stronger as compared with 60-1, corresponding to the highly dispersed state even at relatively higher CuO content. This confirms that the current strategy really benefits from the homogeneous dispersion of the introduced copper species. We found that UV vis diffuse reflectance (UV vis DR) spectra could be used for further characterization of the state of the incorporated Cu species as displayed in Figure 7. Both samples reveal two bands at 200 300 and 600 800 nm, while no absorption band is present for parent SBA-15. The absorption at 200 300 nm is assignable to the ligand to the metal charge-transfer transitions between the surface oxygen and the isolated Cu2+ ions.29 The broad but weak band at 600 800 nm,

Figure 4. Typical TEM images for the parent SBA-15 (a), 60-1 (b), and 60-2 (c) samples. The enlarged image of Cu-SBA-15 rods in sample 60-2 (d).

Figure 5. XPS spectra of Cu2p electrons for samples 60-1 (a) and 60-2 (b).

especially for sample 60-2, can be assigned to d d transitions of Cu2+ in a tetragonally distorted octahedral environment,30 probably arising 21214



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Scheme 1. Chemical Equation for the Cyclohexane Oxidation over the Synthesized Cu-SBA-15 Catalysts

Figure 6. EPR spectra at 100 K for copper species within the SBA-15 samples of 60-1 (a) and 60-2 (b).

Figure 7. UV vis DR spectra for parent SBA-15 (a), 60-1 (b), and 60-2 (c) samples.

Figure 8. H2-TPR profiles for the bulk CuO (a), 60-1 (b), and 60-2 (c) samples.

from the small copper cluster. These results suggest that copper species are indeed highly dispersed in SBA-15 in isolated ions or small clusters in consistence with the EPR analysis. TPR measurements were carried out to investigate the reducibility of the copper species. Figure 8 presents the hydrogen reduction profiles of samples 60-1, 60-2, as well as bulk CuO. Three hydrogen consumption peaks exhibit for the sample 60-2 catalyst. The peak at 225 C could be assigned to highly dispersed copper species31 such as oxocations (Cu O Cu)2+, while the reduction peak at 306 C could be ascribed to the small CuO clusters at the inner surface of SBA-15.32 It should be emphasized that the cylindrical mesopores of SBA-15 are generally surrounded by a corona, which may represent either a microporous layer or some surface corrugations of the pore wall.33 The introduction of the copper species might smooth the corrugated inner surface of the synthesized SBA-15.34 These results are well

correlated with EPR and UV vis DR measurements and demonstrate the high dispersion state of the introduced copper species. A very weak signal at 411 C with a small peak area (the ratio to the total peak area is less than 5%), which is shifted to higher temperature as compared to bulk CuO with a reduction temperature of 360 C (Figure 8a), is most probably attributed to the isolated CuO in the SBA-15 framework as reported by Chu et al.35 Very weak hydrogen consumption signals for sample 60-1 are corroborating to the very low copper content in the sample but are still distinguishable at almost the same positions as sample 60-2. Currently, while copper could serve as the active center for many heterogeneous redox reactions,1,2 there are few studies, if any, to utilize synthetic copper catalysts for the selective oxidation of cyclohexane oxidation. Herein we tested the catalytic properties of the Cu-SBA-15 for the selective oxidation of cyclohexane. The reaction is of increasing importance because it is one of the methods36,37 to produce cyclohexanone (K) and cyclohexanol (A) (ketone/alcohol or K/A oil), which are starting materials in the manufacture of nylon-6 and nylon66 polymers.20,35 The traditional commercial process for cyclohexane oxidation using molecular oxygen or air is generally carried out under relatively harsh conditions at a temperature of 150 160 C and an oxygen pressure of 1 2 MPa, affording about 4% conversion and 70 85% selectivity to cyclohexanone and cyclohexanol.21,22 The specific reaction process is shown in Scheme 1. First, we compared the catalytic performances for cyclohexane oxidation under a temperature of 120 C and oxygen pressure of 1 MPa over the synthesized Cu-SBA-15 with different copper content, and the results were listed in Table 2. A low conversion of 3.6 mol % was obtained over the 60-1 catalyst. For the 60-2 catalyst with 0.85 wt % Cu, the conversion of cyclohexane reached 9.5 mol %. As a control experiment, we did not detect any K/A oil converted from cyclohexane oxidation without using the copper catalysts. These suggest that the contents of Cu in the synthesized catalysts have a crucial influence on their activity in cyclohexane oxidation.38 Figure 9 presents the effect of the reaction temperature on the cyclohexane oxidation over the 60-2 catalyst with a time duration of 5 h and isobaric condition of 1 MPa oxygen. It can be found that the conversion of cyclohexane increases quickly while increasing the reaction temperature from 100 to 120 C and reaches a maximum of 9.5 mol % at 120 C. Subsequently, the conversion gradually drops to a lower value of 7.0 mol % at 140 C, which is most probably due to the decrease of oxygen amount at increased temperature under isobaric conditions.39 Meanwhile, at the initial stage, the selectivity of K/A oil decreases with the increase of temperature. The distribution of cyclohexanone also reaches its maximum of 59.4% at 100 C, and the change trend is similar to that of selectivity to K/A oil. However, as reaction temperature increases, the distribution of cyclohexanol remains nearly unchanged, while the molar ratio of K to A goes down when the temperature is increased, which suggests 21215



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Table 2. Properties of the Different Catalysts for the Cyclohexane Oxidation Reactiona cyclohexane catalysts

conversion (mol %)

product selectivity (mol %) cyclohexanol (A)

byproducts

—

—

60-1

3.6

48.7

47.9

3.4

60-2

9.5

31.8

49.4

18.8

blank

—

cyclohexanone (K)

—

Reaction conditions: cyclohexane 8 mL, catalyst 50 mg, oxygen 1 MPa, temperature 120 C, reaction time 5 h. Byproducts mainly include adipic acid, valeric acid, formic acid cyclohexyl ester, and pentanoic acid cyclohexyl esters.

a

Figure 9. Effect of reaction temperature on conversion and selectivity in cyclohexane oxidation over the 60-2 catalyst for the conversion of cyclohexane (a); selectivity of cyclohexanol (b); selectivity of cyclohexanone (c); selectivity of cyclohexanol and cyclohexanone (d). Reaction conditions: cyclohexane 8 mL, catalyst 50 mg, oxygen pressure 1 MPa, reaction time 5 h.

that cyclohexanone is continuously oxidized to byproduct.40 Generally, the acid and ester are the products of deep oxidation of K/A oil.41 The distributions of these byproduct increase with increasing temperature, indicating that, at higher temperature, the oxidization rate of the cyclohexane to the desired products is slower than that of K/A oil oxidization to byproduct.42 Thus, temperature higher than 120 C has a negative effect on both conversion and selectivity. To maintain a high yield to the desired products, a relatively mild reaction temperature of 120 C seems most suitable for the current synthesized Cu-SBA-15 catalysts. To further probe the optimum reaction conditions for the synthesized 60-2 sample, the effect of reaction time on cyclohexane conversion and product distributions at a reaction temperature of 120 C and oxygen pressure of 1 MPa was explored and shown in Figure 10. It can be found that the cyclohexane conversion steadily increased with increasing reaction time and reached as high as 11.4 mol % in 7 h, and by prolonging the reaction time beyond 7 h, the increasing trend flattened. In addition, the selectivity of K/A oil rose to its maximum of 92.3% in 1 h and decreased gradually with a further increase in reaction time, which was most possibly due to the deep oxidation of cyclohexanol and cycloheanone by molecular oxygen.40 Obviously, the over long reaction time was unfavorable for the selective production of K/A oil. Catalyst recycling is one of the important aspects for practical applications. The reusability of Cu-SBA-15 catalyst in the oxidation of cyclohexane has been examined under the optimum conditions of 120 C and 1 MPa O2. The results in Table 3 show that the conversion did not change so much with a value around 9%, and the selectivity slightly decreased from 81.2% to 76.0% even after three runs. ICP AES analyses of the filtered liquid product and the used catalysts did not detect the obvious leaching

Figure 10. Effect of reaction time on conversion and selectivity in cyclohexane oxidation over sample 60-2 for conversion of cyclohexane (a); selectivity to cyclohexanol (b); selectivity to cyclohexanone (c); and selectivity to cyclohexanol and cyclohexanone (d). Reaction conditions: cyclohexane 8 mL, Cu-SBA-15 catalyst 50 mg, oxygen pressure 1 MPa, temperature 120 C.

Table 3. Effect of Recycling of Sample 60-2 on the Catalytic Properties in the Cyclohexane Oxidation Reactiona product cyclohexane cycle of

conversion

catalyst

(mol %)

selectivity (mol %)

cyclohexanol

cyclohexanone

byproducts

first (fresh)

9.5

31.8

49.4

18.8

second third

8.8 8.5

30.8 29.9

46.7 46.0

22.5 24.1

fourth

4.4

43.0

33.1

23.9

a

Reaction conditions: cyclohexane 8 mL, catalyst 50 mg, oxygen 1 MPa, temperature 120 C, reaction time 5h. Byproducts mainly include adipic acid, valeric acid, formic acid cyclohexyl ester, and pentanoic acid cyclohexyl esters.

of the copper ions from the synthesized 60-2 sample. These further confirm the stability and recyclablility of the synthesized catalysts for the oxidation of the cyclohexane with oxygen in a solvent-free system.

4. CONCLUSIONS In summary, a facile approach has been developed to incorporate highly dispersed copper species into the pore channels of mesoporous SBA-15 by employing a molecular assembly template as a carrier for hydrophobic compounds. The key to the success in synthesizing the catalysts is to encapsulate the hydrophobic Cu(acac)2 into purified SBA-15/P123 during post hydrothermal treatment, in which the copper precursor is unaffected by the harsh reaction environment such as strong acid media 21216


The Journal of Physical Chemistry C employed for SBA-15 synthesis. The synthesized catalysts demonstrated remarkably enhanced catalytic activity for cyclohexane oxidation. In optimized reaction conditions, the conversion of cyclohexane to the target products reached above 11 mol %. It is clear that this newly developed route is general to introduce different highly dispersed metallic active centers at the inner surface of SBA-15 by choosing the proper hydrophobic metallic precursor compounds dissolvable in hydrophobic solvents. Further work will be carried out on the other metal doping in SBA-15 and their catalytic applications.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; [email protected].

’ ACKNOWLEDGMENT This work was financially supported by Shanghai Pujiang Program (No. 09PJ1403000), Research Fund for the Doctoral Program of Higher Education (No. 20090074120009), Natural Science Foundation of China (No. 51072053, 21001043), the New Century Excellent Talents in University (No. NCET10-0379), the Fundamental Research Funds for the Central Universities (No. WD0911012 and No. WK1013001), and the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT0825). ’ REFERENCES (1) Chen, C.; Xu, J.; Zhang, Q.; Ma, H.; Miao, H.; Zhou, L. J. Phys. Chem. C 2009, 113, 2855–2860. (2) Chanquía, C. M.; Andrini, L.; Fernandez, J. D.; Crivello, M. E.; Requejo, F. G.; Herrero, E. R.; Eimer, G. A. J. Phys. Chem. C 2010, 114, 12221–12229. (3) Li, Y.; An, D. L.; Zhang, Q. H.; Wang, Y. J. Phys. Chem. C 2008, 112, 13700–13708. (4) Chen, L. F.; Guo, P. J.; Zhu, L. J.; Qiao, M. H.; Shen, W.; Xu, H. L.; Fan, K. N. Appl. Catal. A: Gen. 2009, 356, 129–136. (5) Sayari, A.; Han, B. H.; Yang, Y. J. Am. Chem. Soc. 2004, 126, 14348–14349. (6) Martın-Aranda, R. M.; Cejka, J. Top Catal. 2010, 53, 141–153. (7) Taguchi, A; Ferdi Schuth, F. Microporous Mesoporous Mater. 2005, 77, 1–45. (8) Drake, I. J.; Fujdala, K. L.; Baxamusa, S.; Bell, A. T.; Tilley, T. D. J. Phys. Chem. B 2004, 108, 18421–18434. (9) Chen, C. K.; Chen, Y. W.; Lin, C. H.; Lin, H. P.; Lee, C. F. Chem. Commun. 2010, 46, 282–284. (10) Zhang, G. Y.; Long, J. L.; Wang, X. X.; Zhang, Z. Z.; Dai, W. X.; Liu, P.; Li, Z. H.; Wu, L.; Fu, X. Z. Langmuir 2010, 26, 1362–1371. (11) Patel, A.; Rufford, T. E.; Rudolph, V.; Zhu, Z. H. Catal. Today 2011, 166, 188–193. (12) Yin, A. Y.; Guo, X. Y.; Dai, W. L.; Fan, K. N. J. Phys. Chem. C 2010, 114, 8523–8532. (13) Chanquía, C. M.; Sapag, K.; Castellon, E. R.; Herrero, E. R.; Eimer, G. A. J. Phys. Chem. C 2010, 114, 1481–1490. (14) Bachari, K.; Cherifi, O. Catal. Commun. 2006, 7, 926–30. (15) Balsamo, N. F.; Chanqua, C. M.; Herrero, E. R.; Casuscelli, S. G.; Crivello, M. E.; Eimer, G. A. Ind. Eng. Chem. Res. 2010, 49, 12365–12370. (16) Gu, J. L.; Shi, J. L.; Xiong, L. M.; Chen, H. R.; Ruan, M. L. Microporous Mesoporous Mater. 2004, 74, 199–204. (17) Minoofar, P. N.; Hernandez, R.; Chia, S.; Dunn, B.; Zink, J. I.; Franville, A. C. J. Am. Chem. Soc. 2002, 124, 14388–14389. (18) Zhao, D. Y.; Feng, J. L.; Huo, Q. S.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548–552.

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