Visible-light-driven oxidation of cyclohexane using Cr-supported mesoporous catalysts prepared via phenyl-functionalized mesoporous silica

Abstract

A series of Cr-supported mesoporous silica (Cr-PFMS) photocatalysts were prepared via phenyl-functionalized mesoporous silica (PFMS) by two steps. Namely Cr-PFMS photocatalysts were firstly assembled by co-condensation of tetraethoxysilane (TEOS) and various amounts of phenyltriethoxysilane (PTES) using PEO–PPO–PEO triblock copolymer (P123) as the template, and then loaded with chromium species with the aid of the phenyl groups. The physical and photophysical properties of the Cr-PFMS photocatalysts were characterized by various techniques including inductive couple plasma optical emission spectrometer (ICP-OES), X-ray photoelectron spectroscopy (XPS) and ultraviolet-visible diffuse reflection spectroscopy (DRS). The visible-light-driven (λ > 420 nm) photocatalytic performance of the Cr-PFMS materials for the cyclohexane oxidation with O₂ at room temperature was investigated. Results showed that the introduction of phenyl groups derived from PTES not only promoted the phase transition of PFMS from high-curvature hexagonal structure to low-curvature cubic structure, but also increased the loading amounts of Cr species of which Cr⁶⁺ chromate species were active sites. Consequently, the hexagonally structured Cr-PFMS photocatalysts possessed higher photocatalytic activity than their counterpart photocatalyst, Cr-SBA-15 prepared without adding PTES. Moreover, the activity of the Cr-PFMS samples with cubic structure was also higher than that of those samples with hexagonal structure. This work may provide more insight into the design of novel supported photocatalysts with high metal loading and catalytic activity for cyclohexane oxidation under mild reaction conditions.

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1. Introduction

The cyclohexane oxidation is crucial for the production of nylon polymers. Moreover, cyclohexanone, one of the two main oxidation products, is an intermediate in the synthesis of various fine chemicals. Unfortunately, there are some shortcomings in cyclohexane oxidation industrial process of conventionally thermal catalysis systems, such as high energy consumption, serious environmental pollution and a large amount of byproducts.^1–3 At present, photocatalysis as an energy-saving and green environmental method is an alternative process to the traditional thermal catalysis. Various photocatalysts for cyclohexane oxidation in heterogeneous systems with O₂ or air have been investigated extensively, such as TiO₂ and modified TiO₂, V₂O₅/Al₂O₃,^4,5 heterogenized decatungstate,⁶ LnVO₄ and LnMo_0.15V_0.85O₄ (Ln = Ce, Pr and Nd),⁷ metal chlorides, Cr/SiO₂,⁸ Cr/MCM-41,⁹ Cr–Si mixed oxides,¹⁰ and Cr–Ti–Si ternary mixed oxides.¹¹ Among them, supported photocatalyst is the preference for liquid reaction because it can help increase the active surface area of the photocatalyst, achieve more efficient separation, and subsequently recycle the photocatalyst.¹² Previous results of the cyclohexane oxidation on MCM-41-supported Cr catalysts,⁹ TS-1-supported Au catalyst and Al₂O₃-supported V₂O₅ (ref. 13) catalyst show the mesoporous structure and active sites loading amount can influence the catalyst performance.

We are interested in developing a new visible-light-driven mesoporous material-supported Cr photocatalyst with spacious mesoporous channels and high chromium loading. For such purpose, we firstly chose the silicon source PTES with phenyl groups coupled with the common silicon source TEOS to assemble PFMS materials with phenyl groups in the channel walls.¹⁴ The presence of phenyl groups is speculated to facilitate the loading of Cr active sites due to their coordination with chromium ions. Secondly, the P123 was used as template since it induced mesoporous materials with larger pore size, thicker pore walls and better structural stability. Thirdly, the mesoporous structure and Cr loading amount were adjusted by varying the PTES dosage in the PFMS synthesis system. A series of PFMS materials were thus prepared and then loaded with Cr species to obtain Cr-PFMS photocatalysts. Their catalytic performance for cyclohexane oxidation to cyclohexanone and cyclohexanol under pure O₂ atmosphere at room temperature was investigated. The effects of the silicon source PTES amount used in the synthesis system on the PFMS structure, loading amount of Cr species and the photocatalytic properties were investigated in detail. This work may provide more insight into the design of novel supported photocatalysts with high metal loading and catalytic activity for cyclohexane oxidation under mild reaction conditions.

2. Experimental

2.1. Materials

PEO–PPO–PEO (P123, 99.9%), triethoxyphenylsilane (PTES, 98%), tetraethyl orthosilicate (TEOS, A.R.), chromic nitrate nonahydrate (Cr(NO₃)₃·9H₂O, A.R.), hydrochloric acid (HCl, A.R.), N,N′-dimethylformamide (DMF, A.R.) were obtained commercially and used without treatment. Deionized water was self-made. Cyclohexane was purchased from Sinopharm and purified prior to use by passing through a column filled with neutral alumina to remove traces of possible oxidation products. All other reactants were used without further purification or treatment.

2.2. Preparation of the photocatalysts

2.2.1 Synthesis of the PFMS-X% and SBA-15 support. 1.5 g P123 used as the template was dissolved in 40 mL HCl (1.5 mol L⁻¹) in a beaker and stirred for 1 h at room temperature. A total amount of 15.2 mmol of PTES and TEOS was added slowly to the above mixture under intensive stirring for 15 min at 35 °C and then the final mixture was stirred for 20 h at 35 °C. After that, the reaction mixture was transferred to an autoclave at 130 °C for 24 h. The solid product was filtered, washed several times with deionized water, dried at 60 °C and calcined at 250 °C for 6 h to yield the PFMS samples. The PFMS samples were named as PFMS-X%, wherein X = 3, 5, 7, 10, X% represents the molar percent of PTES in the total silicon source. For comparison, SBA-15 was synthesized following the same procedure but only using TEOS without PTES as silicon source.

2.2.2 Chromium supporting of the PFMS-X% and SBA-15 support. 0.12 g Cr(NO₃)₃·9H₂O was dissolved thoroughly in 25 mL DMF to form blue solution. 0.50 g PFMS-X% or SBA-15 sample was added to the solution and stirred for 30 min at room temperature, then transferred to a stainless steel autoclave and heated at 130 °C for 48 h. The product was filtrated, washed thoroughly with deionized water and dried to obtain the powder samples denoted as Cr-supported PFMS-X%. And then these samples were calcined at 550 °C for 6 h under air to obtain yellow powder denoted as Cr-PFMS-X% or Cr-SBA-15.

2.3. Characterization

The low angle XRD patterns were conducted using a Bruker Axs/D8 focus with CuKα radiation at a scan rate of 2° min⁻¹. The FT-IR spectra were carried out on Magna-IR 560 E.S.P spectrometer with a 0.35 cm⁻¹ resolution value. The BET specific surface areas were measured by nitrogen adsorption at −196 °C with an Micromeritics ASAP 2020M+C static volumetric gas adsorption instrument. Prior to the sorption analysis, samples (0.1–0.2 g) in the analysis chamber were subject to a vacuum of 10⁻⁵ Torr at 250 °C for 10 h. The ICP analysis was performed on a Perkin Elmer optima 7300 V spectrometer. The morphologies of the samples were obtained on a FEI Quanta 200F SEM with an accelerating voltage of 10 kV. The TEM images were performed on a JEOL JEM-2100 TEM microscope. The XPS measurements were recorded on the Thermo VG Scientific Escalab 250 spectrometer using monochromatized AlKα excitation. The binding energy scale was calibrated with respect to C 1s peak of hydrocarbon contamination of 284.6 eV. The UV-vis spectra of photocatalysts were detected by a Shimadzu UV-4100 spectrophotometer using BaSO₄ as the reference material.

2.4. Photocatalytic activity

The performances of the as-prepared samples for the cyclohexane oxidation were investigated in a customized reactor. For each experiment, 50 mg catalyst was added into the reactor. Then the reactor was sealed using a quartz septum cap, and evacuated for 2–3 min. After that, 10 mL cyclohexane was injected through the injection port. Prior to test, pure oxygen with the 15 mL min⁻¹ flow was bubbled through the solution for 5 min at 0 °C to prevent the evaporation of cyclohexane. A 300 W xenon lamp with a 420 nm cutoff filter was used as the light source, positioned vertically with respect to and in very close proximity to the reactor. The reaction was carried out at 25 °C for 5 h. After photoirradiation, the resulting solution was recovered by centrifugation, and the catalyst was washed thoroughly with 10 mL MeCN. The concentrations of cyclohexanol and cyclohexanone were analyzed by a gas chromatograph (GC, SP-3420) equipped with an FID detector. The activity is expressed by turnover number (TON), which is defined as the number of moles of cyclohexanol and cyclohexanone that a mole of catalyst can produce before becoming inactivated.

3. Results and discussion

3.1. Characterization of the photocatalysts

The low-angle XRD patterns of the Cr-PFMS-X% (X = 3, 5, 7, 10) and Cr-SBA-15 photocatalysts are shown in Fig. 1. Cr-SBA-15 exhibits three diffraction peaks at 0.8, 1.5 and 1.7° corresponding to (1 0 0), (1 1 0) and (2 0 0) crystal facets. The strongest peak at 0.8° shifts to higher angle for the Cr-PFMS-X% samples due to introduction of phenyl content. Cr-PFMS-3% also shows three diffraction peaks similar to Cr-SBA-15. This shows that they both possess ordered 2d-hexagonal p6mm SBA-15 structure. But the characteristic diffraction peaks (1 1 0) and (2 0 0) of 2d-hexagonal structure disappear when adding content of PTES increases to 5%. Instead, a well-defined shoulder peak and a broad diffraction peak in the range of 2θ = 1.3–2.0° are observed, which suggests the transition from the 2d-hexagonal SBA-15-like symmetry to the cubic Ia3̄d structure. Taking the Cr-PFMS-7% sample as example, it exhibits an intense peak at 2θ = 1.02° indexed as (2 1 1) and a shoulder peak at 2θ = 1.18° indexed as (2 2 0) in the XRD pattern (Fig. 3d).¹⁵ The other two weak peaks, indexed as (4 0 0) and (4 2 0) reflections, of cubic Ia3̄d mesoporous structure in the range of 2θ = 1.3–2.0° are also observed.¹⁶ However, adding higher amounts of PTES, e.g. 10%, only less defined cubic Ia3̄d materials are obtained, suggesting too much phenyl groups may disturb the formation of cubic mesoporous structures. The possible explanation for the formation of Ia3̄d mesoporous structure is due to the local effective surfactant packing parameter, g = V/a₀l, where V is the total volume of the surfactant chains plus any cosolvent organic molecules between the chains, a₀ is the effective head group area at the micelle surface, and l is the kinetic surfactant tail length or the curvature elastic energy. The g parameter is a useful molecular structure-directing index to characterize the geometry of the mesophase products, and phase transitions may be viewed as a variation of g in the liquid-crystal-like solid phase.¹⁷ When g is 1/2, hexagonal mesophase (p6mm) is expected. The larger values of g between 1/2 and 2/3 favor the formation of the cubic (Ia3̄d) mesophase. When PTES is added to the synthetic system, the phenyl groups of PTES with hydrophobic property will associate with the hydrophobic PPO blocks and enlarge the hydrophobic volume V. Since g = V/a₀l, the increase of V leads to increased g values. This induces the phase transition from high-curvature hexagonal p6mm mesostructure to low-curvature cubic Ia3d̄ mesophase. Based on the XRD results, it is inferred that the formation of 2d or 3d mesostructure depends on the dosage of PTES acted as a second silicon source in the synthetic system. The results show that the content of phenyl groups has important effects on the porous structure of the as-synthesized samples.

Fig. 1 Powder XRD patterns of (a) Cr-SBA-15; (b) Cr-PFMS-3%; (c) Cr-PFMS-5%; (d) Cr-PFMS-7%; (e) Cr-PFMS-10%.

Fig. 2 shows the SEM images of Cr-SBA-15 sample and Cr-PFMS-5% sample. It is clear that there are no obvious difference in morphology observed for the two samples. The TEM images of the Cr-PFMS-5% sample are shown in Fig. 3. The black dots on mesoporous materials are inferred to be chromium species, which indicates that the chromium species with size of 20–30 nm are supported successfully and isolated from each other (Fig. 3a). The HRTEM images are taken along [1 0 0], [1 1 1], [1 1 0] directions, respectively. It further proves the well-defined Ia3̄d structure in agreement with XRD results.

Fig. 2 SEM images of (a) Cr-SBA-15 and (b) Cr-PFMS-5%.

Fig. 3 TEM images of the Cr-PFMS-5% sample.

The FT-IR spectra of SBA-15, PFMS-X%, Cr-supported PFMS-5% and Cr-PFMS-5% samples are shown in Fig. 4. Contrast to SBA-15, two additional absorption peaks attributed to phenyl groups, appear at 1433 and 741 cm⁻¹ in the spectra of the PFMS-X% samples. The results show that the phenyl groups are successfully introduced into the PFMS-X% samples by using a second silicon source PTES during the synthetic system. By comparing the FT-IR spectra, the intensities of characteristic peaks of phenyl group increase with the increase of adding content of PTES. It is indicated that the introducing content of phenyl groups could be adjusted by PTES dosage. The absorption peaks at 1080 and 795 cm⁻¹ are attributed to the asymmetric stretching vibration and the symmetric stretching vibration of Si–O–Si bond of the PFMS-X% and SBA-15 samples respectively. The relatively strong absorption peaks at 3440 and 1650 cm⁻¹ are assigned to the adsorption of water and the vibration of Si–O–H bond. After chromium supporting of the PFMS-5% sample, the intensity of the band at 1650 cm⁻¹ in the spectrum of Cr-supported PFMS-5% increases obviously, which is attributed to the surface hydroxyl groups formed due to the supported chromium species. Moreover, in comparison with PFMS-5%, the intensities of absorption peaks at 1080 cm⁻¹ decrease obviously for the Cr-supported PFMS-5% and Cr-PFMS-5% samples (see inset in Fig. 4). This intensity change may be caused by the loading of chromium species via coordination of chromium ions with phenyl groups which can be bonded to a transition-metal moiety, as in the chromium benzene complexes.^18,19 And observing the peaks at 1433 and 741 cm⁻¹ attributed to phenyl groups finds that the phenyl groups are kept in Cr-supported PFMS-5% after calcination at 250 °C, but lost in the Cr-PFMS-5% samples after calcination at 550 °C. This indicates the calcination step at 250 °C in the preparation of PFMS-5% does not destroy the phenyl groups, which later facilitate the supporting of Cr species. However, for the Cr-PFMS-5% sample the calcination step at 550 °C after chromium loading in order to oxidize the chromium species leads to the removal of phenyl groups.

Fig. 4 The FT-IR spectra of: (a) SBA-15; (b) PFMS-3%; (c) PFMS-5%; (d) PFMS-7%; (e) PFMS-10%. (f) Cr-supported PFMS-5%; (g) Cr-PFMS-5%.

Nitrogen adsorption/desorption isotherms were further measured to characterize the porous structure of the samples. It is found from Fig. 5 that all the samples exhibit typical type IV nitrogen adsorption/desorption isotherms with a pronounced uptake in the relative pressure range (p/p₀) of 0.6–0.8, indicating the presence of the uniform mesoporous structure. This shows that for the PFMS-X% samples, the low calcination temperature at 250 °C is enough to remove the template P123 forming porous channels and the Cr-PFMS-X% samples maintain porous structure after the high temperature calcination at 550 °C. It can be seen from Table 1 that the BET surface area of PFMS-3% sample is similar to that of SBA-15 since they have the same hexagonal structure. However, the PFMS-X% (X = 5, 7 and 10) samples with cubic structure have much higher BET surface area and the surface areas first increases and then decreases with the increase of the phenyl group content introduced in the PFMS-X% (X = 5, 7 and 10) samples. It is observed that PFMS-5% sample possesses the highest BET surface area of 1053 m² g⁻¹ and corresponding pore volume of 1.49 cm³ g⁻¹. The BET surface area and pore volume of Cr-PFMS-X% and Cr-SBA-15 samples decreases a little than those values of PFMS-X% and SBA-15, indicating that the supported chromium does not plug the channel. The average pore diameter decreases with increasing the content of phenyl groups introduced, which proves that the phenyl groups were successfully introduced into the pore walls.

Fig. 5 N₂ adsorption–desorption isotherms of as-synthesized samples: (a) SBA-15; (b) PFMS-5%; (c) PFMS-10%; (d) Cr-SBA-15; (e) Cr-PFMS-5%; (f) Cr-PFMS-10%.

Table 1 The pore parameters of the as-synthesized samples

Sample	SBET (m^2 g^−1)	Pore volume (cm^3 g^−1)	Average pore diameter (nm)
SBA-15	648	1.24	7.9
PFMS-3%	639	1.14	7.0
PFMS-5%	1054	1.49	5.6
PFMS-7%	920	1.17	5.1
PFMS-10%	919	1.07	4.5
Cr-SBA-15	520	1.04	7.8
Cr-PFMS-5%	731	1.02	5.7
Cr-PFMS-10%	507	1.00	5.6

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The ICP analysis (Table 2) gives the chromium weight percent of the photocatalysts. Generally, the chromium supported amount of the Cr-PFMS-X% sample is higher than that of Cr-SBA-15 sample without phenyl groups, indicating that the introduction of phenyl groups could help chromium species to be supported on the pore wall. As for the Cr-PFMS-X% samples, the chromium content first increases and then decreases with increasing the content of phenyl groups introduced. This trend is the same to the changes of BET surface area and pore volume with the phenyl group content. The maximum amount of supported chromium species is 0.18 wt% for the Cr-PFMS-5% sample with 5% PTES silicon source. However, when the dosage of PTES silicon source is more than 5%, although more phenyl groups are introduced, the chromium supported amount decreases because of the decrease of BET surface area and pore volume. Therefore, the loaded amount of chromium species can be controlled by changing the introducing amount of phenyl groups.

Table 2 ICP results of synthesized catalysts using different contents of phenyl groups

Run	Sample	Amount of chromium supported (wt%)
1	Cr-SBA-15	0.11
2	Cr-PFMS-3%	0.16
3	Cr-PFMS-5%	0.18
4	Cr-PFMS-7%	0.14
5	Cr-PFMS-10%	0.12

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3.2. Photocatalytic performance

The as-synthesized photocatalysts are used to catalyze the cyclohexane oxidation with molecular oxygen in a solvent free condition. The results of cyclohexane oxidation under visible light are shown in Table 3. It is shown that the cyclohexanol and cyclohexanone yields for the Cr-PFMS-X% (X = 3, 5, 7) samples are higher than those for the Cr-SBA-15 sample. This is attributed to two reasons: on the one hand, contrast to the Cr-SBA-15 sample, the Cr-PFMS-X% (X = 3, 5, 7) samples all possess more Cr species. On the other hand, the structures of two types of photocatalysts are different. As is known, MCM-48 with cubic structure can improve molecule diffusion compared with MCM-41 with hexagonal structure.²⁰ Similarly, the Cr-SBA-15 sample possesses two-dimensionally hexagonal channels, whereas the Cr-PFMS-5% and Cr-PFMS-7% samples have three-dimensionally cubic channels in favor of molecule diffusion.

Table 3 Results of visible-light-driven photocatalytic oxidation of cyclohexane

Run	Samples	Cyclohexanol/μmol	Cyclohexanone/μmol	TON^c
a CH2Cl2 used as solvent.b After the reaction (run 3), the catalyst was recovered by filtration and rinse, and used for reaction.c The yields of [(cyclohexanol + cyclohexanone)]/(Cr amount on the catalyst).
1	Cr-SBA-15	3.33	5.16	8.03
2	Cr-PFMS-3%	4.87	7.66	8.14
3	Cr-PFMS-5%	9.97	11.82	12.59
4	Cr-PFMS-7%	4.37	6.93	8.39
5	Cr-PFMS-10%	2.24	2.42	4.04
6	Cr-PFMS-5%^a	57.33	36.76	54.39
7	Cr2O3	0	0	0
8	Cr-PFMS-5%^b	10.37	11.28	12.51

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However, the Cr-PFMS-10% sample produces less amount of cyclohexanol and cyclohexanone than Cr-SBA-15. This is because the former has lower BET specific area and pore size although they have almost the same chromium loading. The lower BET specific area and pore size may be caused by the partial collapse or blocking of porous material. This can increase the flow resistance of liquid reactants and suppress the access of cyclohexane to the chromate species. Moreover, TON firstly increases and then decreases with increasing phenyl groups in the Cr-PFMS-X% (X = 3, 5, 7) samples. The TON value is far higher than previously reported.^8,9,11 Cr₂O₃ has not catalytic activity in the visible light region, indicating that Cr⁶⁺ from chromate species acted as the main active sites of the as-synthesized samples. The solvent has great influence on the photocatalytic results. It is shown in run 6 that when dichloromethane is used as solvent the total yield of cyclohexanol and cyclohexanone is fourfold and the molar ratio of cyclohexanol/cyclohexanone is much higher. As far as we know, the TON is the best result reported at present for photocatalytic cyclohexane oxidation with solvent using Cr-supported porous catalyst.

To verify the heterogeneity of the photocatalytic process, a leaching test is performed with Cr-PFMS-5% sample according to ref. 21 (Fig. 6). We filter the Cr-PFMS-5% photocatalyst after 5 h. At this time, half the volume is filtered and the resulting clear solution is allowed to react. The percentage of leaching is estimated by comparing the time-conversion plot of the twin reactions with and without solid. It is found that there is no cyclohexanol or cyclohexanone generated in filtered liquid, which proves that the photocatalytic process was heterogeneous reaction.

Fig. 6 The total yield of cyclohexanol and cyclohexanone formed during the controlled experiments.

3.3. Photocatalytic mechanisms

Fig. 7 displays the Cr 2p high-resolution XPS spectra of Cr-PFMS-5% samples. The significant bands at binding energies of 577.0–578.0 eV and 586.0–588.0 eV, respectively corresponding to Cr 2p_3/2 orbital and Cr 2p_1/2 orbital, which could be attributed to Cr³⁺ species. The bands of higher binding energies 580.0–580.5 eV and 589.0–590.0 eV corresponding to Cr⁶⁺ species are also observed in the sample.²² The results indicate that chromium exists in a form of Cr⁶⁺ chromate species and Cr³⁺ chromium oxide. The above catalytic results show the Cr³⁺ chromium oxide has no catalytic activity for the visible light driven cyclohexane oxidation with molecular oxygen.

Fig. 7 Cr 2p high-resolution XPS spectra of Cr-PFMS-5% sample.

The diffuse reflectance UV-vis spectra show that chromium supported catalysts all have visible-light response. It is shown in Fig. 8 that the Cr-PFMS-X% samples all possess two distinctive absorption bands at 350 and 450 nm, assigned to a ligand-to-metal charge transfer (LMCT) band from O²⁻ to Cr⁶⁺ transitions of the chromate species, which are dispersed in silica matrices and isolated from each other.²² In addition, the slight absorption bands at 600 nm are assigned to a d–d transition of the octahedral Cr³⁺ species in a Cr₂O₃ cluster.^8,9,11 Contrast to Cr-SBA-15, the Cr-PFMS-X% samples show the similar absorption edge and obviously enhance light absorption in the visible region. It further indicates the existence of Cr⁶⁺ and Cr³⁺, which is in agreement with XPS results.

Fig. 8 Diffuse reflectance UV-vis spectra of: (a) Cr-SBA-15; (b) Cr-PFMS-3%; (c) Cr-PFMS-5%; (d) Cr-PFMS-7%; (e) Cr-PFMS-10%.

Based on the results presented here, as well as those analyses in previous studies,^8–11 it is obvious that the photocatalytic performance could be explained as follows (shown in Scheme 1). Calcination of the last step in synthetic process lead to the removal of adsorbed water, and the dehydrated chromium oxide species anchor on the catalyst surface by reaction with the surface Si–OH groups. This is an acid–base reaction: the weaker acid H₂O is replaced by the stronger acid H₂CrO₄. It is possible that on PFMS materials not all chromium are anchored due to the presence of Cr³⁺ species in the Cr-PFMS-X% photocatalysts. Chromium species are impregnated on a “rough” surface of the Cr-PFMS-X% samples due to calcination leads to the removal of phenyl fragments, thus, resulting in the formation of “distorted” chromium species (Scheme 1a). The reduction of Cr⁶⁺ to Cr⁵⁺* occurs at vacuum through the terminal oxygen (O_T)–Si bond formation associated with the removal of H₂O from adjacent Si species (Scheme 1b). After irradiation, the transfer of photo-generated electrons from O_T to Cr results in the formation of the excited state (Cr⁴⁺*) (Scheme 1c), which is inferred to the active site for cyclohexane oxidation. And its amount could be promoted due to the more readily charge transfer between O_T and Cr. The O_T, formed by photo-generated charge transfer, has an electrophilic character and thus acts as a positive hole to oxidize cyclohexane with molecular oxygen to cyclohexanol and cyclohexanone in the end.

Scheme 1 The proposed mechanism of Cr-PFMS-X% photocatalysts.^8,9,11

4. Conclusions

This report presents a series of novel visible-light-driven Cr-supported catalysts possessing ordered mesoporous structure with large pore size (>5 nm) prepared via PFMS materials. The introduction of phenyl groups promotes the formation of large-pore cubic structure, and facilitates chromium loading. The Cr-PFMS-X% samples have high photocatalytic activity for the cyclohexane oxidation to cyclohexanone and cyclohexanol with molecular oxygen under visible light irradiation. Among them, the Cr-PFMS-5% sample shows the highest TON 12.59 for cyclohexane oxidation without solvent. And its TON increases to 54.39 when using dichloromethane as solvent. The work may provide more insight into the structure design for novel supported photocatalyst with high metal loading and catalytic activity.

Acknowledgements

We acknowledge the support of this work by the National Natural Science Foundation of China (Grants 51072230 and U1162118), Specialized Research Fund for the Doctoral Program of Higher Education of China, and the Beijing Young Talents Plan.

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