Visible-light-responsive sulfated vanadium-doped TS-1 with hollow structure: Enhanced photocatalytic activity in selective oxidation of cyclohexane

Journal of Catalysis 330 (2015) 208–221

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Visible-light-responsive sulfated vanadium-doped TS-1 with hollow structure: Enhanced photocatalytic activity in selective oxidation of cyclohexane Wenzhou Zhong, Tao Qiao, Jing Dai, Liqiu Mao ⇑, Qiong Xu, Gouqiang Zou, Xianxiang Liu, Dulin Yin, Fiping Zhao National and Local United Engineering Laboratory for New Petrochemical Materials and Fine Utilization of Resources, Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research, Ministry of Education, College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 17 April 2015 Revised 17 June 2015 Accepted 17 June 2015

Keywords: Cyclohexane Selective photocatalytic oxidation Visible light Sulfated V/TS-1 KA-oil

a b s t r a c t A series of visible-light-responsive hollow sulfated V-doped TS-1 photocatalysts with different dopant content has been prepared by impregnation and evaluated in the selective photooxidation of cyclohexane by molecular oxygen. The activity results have been correlated with structural, electronic, and surface examinations of the photocatalysts with the help of X-ray diffraction, N2 physisorption, transmission electron microscopy, and Raman, XPS, infrared, and UV–visible spectroscopy. Irrespective of the reaction, a photocatalytic efficiency enhancement of valuable products (alcohols and ketones) with respect to V-doped titania (P25) references and V-doped and sulfated TS-1 catalysts was observed for hollow sulfated V-doped samples with the TiAO4 species (tetrahedral Ti4+). This is likely linked with an effective narrowing of the band gap and results from a cooperative V, S effect on the structural properties of the TiAO4 network inducing isolated energy levels near the conduction and valence bands. The simultaneous presence of V, S in the TS-1 also promotes the process with higher efficiency (TOF = 2.37 h1, based on the molar amount of V sites) and chemoselectivity (cyclohexanone/cyclohexanol molar ratio = 3.46) than in the homogeneous phase. Extensive computational modeling, together with the O@VAO4ASi and O@VAO4ATi bridging sites on TS-1, was performed to show that O@V and O4ATi (oxygen vacancy) in O@VAO4ATi bridging sites are the main active sites for the selective photooxidation reactions, and this is compatible with experimental observations for varying reaction conditions. Ó 2015 Elsevier Inc. All rights reserved.

1. Introduction The selective oxidation of cyclohexane by molecular oxygen is one of the most challenging and promising subjects from synthetic and industrial points of view [1,2], since this process produces an important KA-oil (a mixture of cyclohexanone and cyclohexanol) intermediate in petroleum industrial chemistry. Modern industrial routes require high temperatures and high pressures and generate undesirable byproducts that lower the product yield and complicate the recovery/separation steps. As a result, the industrial production of KA-oil is energy-intensive. The development of an efficient and highly selective method for cyclohexane oxidation using molecular oxygen as a sole oxidant has been sought in pursuit of the goal of green chemistry.

⇑ Corresponding author. Fax: +86 731 88872531. E-mail address: [email protected] (L. Mao). http://dx.doi.org/10.1016/j.jcat.2015.06.013 0021-9517/Ó 2015 Elsevier Inc. All rights reserved.

Photocatalysis is widely applied in the decomposition of pollutants from water and air. Among various semiconductor-based photocatalysts, bulk TiO2 is the most investigated, and used in commercial water purification. In contrast, relatively fewer studies have been conducted on the development of photocatalysis for organic synthesis using selective oxidation processes [3,4]. Within this field, a particularly interesting and active topic is the selective oxidation of cyclohexane to the corresponding KA-oil, because it completely overcomes the major drawbacks mentioned in the preceding. A main drawback, however, is that bulk TiO2 photocatalyst only absorbs ultraviolet (UV) light because of its large band gap. This has stimulated researchers to develop novel photocatalysts with narrowing band gaps to enhance the response to the more abundant visible light photons [5]. Electronic modification of bulk TiO2 through cationic and/or anionic doping to enhance the optical response to the visible light is a popular strategy [6]. Nevertheless, the resulting photocatalysts often have less efficiency or suffer from a positive impact on charge recombination,

W. Zhong et al. / Journal of Catalysis 330 (2015) 208–221

which is detrimental to the catalytic performance. In particular, maximization of the partial vs. total oxidation yields is very difficult to achieve. Therefore, a number of Ti-substituted porous silica materials (TiAOx; x is the coordination number of the Ti atom with the oxygen atom) as photocatalysts, such as TS-1 and Ti-TUD-1, have been evaluated as alternatives to improve sufficient product selectivity (e.g., no total oxidation products, CO2) [7,8]. In these cases, substrate conversion and product selectivity are correlated with the nature of the TiAOx species. On silica materials with low Ti content, the highly dispersed TiAOx species exist in a tetrahedral coordination environment (TiAO4 species). These isolated TiAO4 species in the silicate frameworks, therefore, lack a bulk semiconducting character and show photocatalytic activity quite different from that of bulk crystalline TiO2, which consists of octahedrally coordinated (aggregated) TiAO6 species. Additionally, TiAO4 species occurring at the solid–solid interfaces of crystalline domains have been hypothesized to be a critical moiety in highly active Ti-based photocatalysts [9]. On TiAO4, some substrates are transformed selectively: for example, CO2 photoreduction [10], propane [11] and benzyl alcohol photooxidation [12], and water splitting [13]. The unique activities of the Ti-substituted molecular sieves are, however, mostly shown under UV-light illumination, and structural advantages in visible-light photocatalytic activity on the catalyst are left unexploited. An opportunity to tune these optical properties lies in the modification of Ti-substituted molecular sieves, yielding an elevated visible light response. Generally, introducing metal ions (such as V, Cr, and Mn) into Ti-substituted materials forms donor states below the conduction band (CB) [14,15]; incorporating nonmetal ions (such as N, S, and C) into Ti-substituted materials modifies the valence band (VB) to form acceptor states above the valence band gap [16,17]. Thus, a low concentration co-doping of metal ions and nonmetal ions should be able to both enhance the visible light absorption efficiency and reduce the recombination of the photogenerated charges. Among the transition metals, V-doping TiO2 or SiO2 in low concentration has been shown to extend the working spectrum to the visible-light region. Klosek et al. reported that V-doped TiO2 photocatalysts resulted in high activity for the oxidation of ethanol in the visible range (396–450 nm) [18]. Mul and his co-workers found that silica-supported vanadium oxide is an effective photocatalyst in selective cyclohexene oxidation under continuous illumination at 458 nm [19]. S is a well-known anion dopant that can occupy O-atom sites in TiO2 to form TiAS bonds by sulfation and that has been reported in the past [20,21]. Particularly, the strong acid sites on sulfated TiO2 increase adsorption strengths and coverage of different organics, which result in enhanced photocatalytic activity [22]. Hence, it can reasonably be assumed that highly visible-light-active photocatalysts may be obtained. In this work, hollow sulfated V-doped TS-1 with the TiAO4 species was prepared by impregnation, using vanadyl sulfate as the source of the dopants in one step. Photocatalytic activity of surface-modified hollow TS-1 with different dopant contents for the oxidation of cyclohexane to KA-oil products in the presence of HCl as an additive was investigated under visible light irradiation. The 5% VOSO4-modified TS-1 was found to exhibit the highest photocatalytic performance (>14% conversion) with selectivity (>94%) to KA-oil (cyclohexanone/cyclohexanol molar ratio = 3.46). From the analysis of the characterization study, the formation of production and density functional theory (DFT) calculations, the contribution of the TiAO4AV@O bridging sites present at the surface of the photocatalyst to the photocatalytic reaction was demonstrated. It was confirmed that a remarkable enhancement of the photocatalytic activity can be attributed to an effective narrowing of the band gap and results from a cooperative V and S effect on structural

209

properties of the TiAO4 network inducing isolated energy levels near the conduction and valence bands. 2. Experimental 2.1. Catalyst preparation The hollow TS-1 was prepared by a dissolution–recrystalliza tion process in tetrapropylammonium hydroxide (TPAOH), which creates intraparticle voids that facilitate the formation of small and disperse nanoparticles by simple impregnation in recent work [23]. First, TS-1 was synthesized from tetraethyl orthosilicate (TEOS), titanium butoxide (TBOT), and tetrapropylammonium hydroxide (TPAOH) solutions with a composition of SiO2:0.01TiO2:0.4TPAOH:40H2O. After some time, the resulting gel was transferred to a Teflon-lined stainless steel autoclave for 3 days at 170 °C. The solid zeolite was filtered, washed, dried, and finally calcined at 500 °C for 10 h. Second, the hollow TS-1 solid (HTS) was synthesized by treating the previously calcined TS-1 with sulfuric acid solution with a weight ratio of TS-1:sulfuric acid:water = 10:1.0:140, and the acid-treated TS-1 was dispersed in TPAOH solution with a weight ratio of molecular sieve:TPAOH:water = 10:1.5:125. The mixture was then transferred to a Teflon-lined stainless steel autoclave and recrystallized for 3 days at 140 °C. The recrystallized zeolite was filtered, washed, dried, and finally calcined at 500 °C for 10 h. Silicate-1 was prepared by an identical procedure, merely without adding titanium. The hollow sulfated V-doped TS-1 catalysts were prepared by incipient wetness impregnation of hollow TS-1 with an aqueous solution of VOSO4 to give 5 wt.% vanadium in the final product. The impregnated sample was allowed to evaporate slowly at 100 °C till dry, followed by calcination at 500 °C in air for 4 h (with heating rate 5 °C/min). The labels 0 #, 1 #, 2 #, 3 # and 4 # denote samples in which the wt.% of vanadium was 0%, 1%, 5%, 7%, and 10%, respectively. Sulfated V-doped TiO2 (5 #) and sulfated V-doped silicate-1 (6 #) were prepared by an identical procedure. To prepare sulfate-free catalysts, V-doped TS-1 (7 #) was prepared by impregnation of hollow TS-1 with an alcohol solution of VO(acac)2 and the subsequent procedures were the same as those for sulfated V-doped TS-1. 2.2. Characterization techniques The crystalline phase was characterized by a Bruker Advanced D8 diffractometer using Cu Ka radiation (k = 1.542 Å). Size and particle morphology of samples were determined on a Hitachi S-4800 scanning microscope, while the TEM images were taken on a JEOL-JEM-2100 microscope. The textural properties were measured by N2 adsorption at 196 °C on a Tristar 3000 sorptometer. Prior to the tests, degassing was achieved by heating the sample at 200 °C under a reduced pressure of 105 Torr for 3 h. The surface area was determined using the single-point Brunauer–Emmett–T eller (BET) equation, while total pore volume was measured at P/P0 = 0.99. The pore size distributions were determined from the adsorption branch of the isotherms using the Barrett–Joyner–Hal enda (BJH) method. Raman spectra were determined on a Renishaw inVia Raman microscope system with a 514.5 nm laser as the excitation source. A laser output of 30 mW was employed and the maximum incident power at the sample was approximately 6 mW. FT-IR spectra were obtained on an AVATAR 370 Thermo Nicolet spectrophotometer with a resolution of 2 cm1. The diffuse reflectance UV–vis spectra were recorded from 200 to 800 nm on a Varian–Cary 5000 spectrometer using BaSO4 as a reference. X-ray photoelectron spectroscopy (XPS) was performed in

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an imaging photoelectron spectrometer (Axis Ultra, Kratos Analytical Ltd.) using Al Ka radiation (1486.7 eV). Binding energies were calibrated with respect to the signal for adventitious carbon (binding energy = 284.8 eV). XPS software was used for curve fitting. 2.3. DFT calculations The geometrical structure of the smallest models with active sites, Si3TiVO7H11, was optimized using the PBE1PBE method of DFT with 6-311++G(d, p) basis set (DFT PBE1PBE/6-311++G(d, p)). Natural bond orbital (NBO) analyses were performed by the PBE1PBE method on the optimized structure to gain insight into the electronic distribution and bonding patterns of these models. All the calculations were conducted using the Gaussian-09 version C02 package with tight self-consistent field (SCF) convergence and ultrafine integration grids. Models have been considered where one O@VAO4ASi or O@VAO4ATi bridging site on the surface was constructed with a V atom, representing the surface modification of HTS with an isolated V species. In both cases, the unsaturated bonds are saturated by hydrogen atoms. 2.4. Photocatalytic reactions Selective photooxidation of cyclohexane was carried out using a self-made photoreactor equipped with a water-cooled condenser, and a 35 W tungsten–bromine lamp with an UV light filter (Osram brand) was immersed in an acetonitrile solution (5 ml) involving cyclohexane (5 mmol), photocatalyst (50 mg), and concentrated HCl (0.1 ml). We summarized information concerning the reaction set-up (Fig. 1) and lamp characteristics (Figs. S1 and S2 in the Supplementary Data). O2 can be fed into a gas burette to monitor the gas uptake. The reaction mixture was vigorously stirred to eliminate the influence of mass transfer under O2 (1 atm). Under visible light irradiation, the temperature of the reaction solution increased to 36–38 °C because of the heating effect of light irradiation. After the reaction, the catalyst was immediately separated from the suspension by filtration. The components of the liquid phase were quantitatively analyzed on an Agilent 6890N gas chromatograph with a DB-17 polysiloxane

capillary column (30 m  0.32 mm  0.50 lm) and a flame ionization detector using n-hexanol as the internal standard. Both the injector and detector temperature were 250 °C, and the column temperature was 80 °C. The components of the gas phase were analyzed by gas chromatography with a TCD detector. The products were satisfactorily identified by comparing the MS spectra with those of the authentic samples. Mass balances were verified.

3. Results and discussion 3.1. Crystal structure and textural properties of modified HTS catalysts The X-ray diffraction (XRD) patterns of pure HTS (0 #), VOSO4-modified HTS (1 #–4 #), VOSO4-modified TiO2 (5 #), VOSO4-modified silicate-1 (6 #), and VO(acac)2-modified HTS (7 #) samples are shown in Fig. 2. The pure HTS exhibits the characteristic MFI diffraction peaks at 7.95°, 8.94°, 23.2°, and 24.1°, which correspond to the indices of the [1 0 1], [2 0 0], [5 0 1], and [3 0 3] planes [24]. The diffraction peaks due to the crystalline TiO2 phase were not observed in the XRD patterns, suggesting that the Ti species did not condense to form bulk oxides, and possibly were highly dispersed and incorporated into the MFI framework. For the VOSO4-modified HTS samples, with increasing amount of VOSO4, the intensities for the characteristic MFI diffraction peaks decreased and diffraction peaks were slightly broadened due to the coverage of the external surface of HTS with V2O5 nanoparticles or SO2 4 (Fig. 2B). When the amount of V was increased to 10 wt.% (4 #), the peaks at 21.7°, 22.5°, and 32.1°, corresponding to a very small fraction of crystalline V2O5 clusters, were observed, indicating that low content of V in HTS might be well incorporated into the framework of zeolite or homogeneously dispersed in zeolite lattices [25]. In comparison, the VO(acac)2-modified HTS showed an XRD pattern similar to that of VOSO4-modified HTS (2 #), indicating that doped SO2 has no significant effect on the product 4 structure. Additionally, the XRD patterns of VOSO4-modified TiO2 or silicate-1 are shown in Fig. 2A. No diffraction signals assignable to the loaded V2O5 phases were recorded in the supported samples, indicating the uniform distribution of these species on the surfaces of supports.

Fig. 1. Self-assembly photoreactor used in this study.

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[101] [200]

[501]

A

[303]

B

*VO ** 2

0#

*

0#

Intensity (a.u.)

2#

Intensity (a.u.)

5

7# 6#

1# 2# 3#

5# 4#

10

20

30

40

50

60

70

10

80

20

30

40

50

60

70

80

2 Theta (degree)

2 Theta (degree)

Fig. 2. XRD patterns of samples (0 #, 1 #, 2 #, 3 #, 4 #, 5 #, 6 #, and 7 #).

Adsorbed Volume cm3/g

0# 1# 2# 3# 4#

0.0

0.2

0.4

0.6

0.8

1.0

Reletive Pressure (P/P0) Fig. 3. N2 adsorption–desorption isotherms of samples (0 #, 1 #, 2 #, 3 #, and 4 #).

N2 adsorption/desorption isotherms for undoped HTS and modified HTS catalysts are shown in Fig. 3, and the specific surface area, total pore volume, and pore size are listed in Table 1. For each of the samples (0 #–4 #), the presence of intracrystalline voids accessible only via entrances smaller than 4 nm was evident from abrupt closure at p/p0 = 0.42 on the desorption branch [26,27]. These results confirmed that the hollow structures were well retained after VOx doping and SO2 loading. Undoubtedly, the 4 well-defined hollow structure with large surface areas is beneficial

for dispersing nanoparticles and adsorbing small molecules onto the active surfaces of these samples. The mean pore sizes of modified HTS, as shown in Table 1, were slightly increased with increasing amounts of vanadium dopants. This may be attributed to the fact that V species with oxygen had a greater bond length than SiAO, indicating incorporation of V to form SiAOAV or TiAOAV linkages in the zeolite framework sites. The specific surface area and total pore volume of undoped HTS catalyst were 330 m2/g and 0.262 cm3/g, respectively. The increasing amounts of vanadium dopant caused the surface area and pore volume of the modified samples to decrease. Similar decreases in the specific surface area of V-doped TiO2ASiO2 or TiO2AAl2O3 were also observed by Shee et al. [28]. This could be attributed to partial pore blockage by the introduction of VOx and SO2 4 species. For comparison, the VOSO4-modified HTS (2 #) showed a specific surface area higher than that of VO(acac)2-modified HTS (7 #). This is caused by vanadium dopants in the form of interstitial S (TiAOAS or SiAOAS), since the S atom has a covalent radius (0.102 nm) larger than that of O (0.073 nm). 3.2. Morphology and surface characteristics of modified HTS catalysts Fig. 4 shows TEM images of pure HTS and VOSO4-modified HTS (2 #). The TEM image of pure HTS (Fig. 4A1, A2, and A3) display large intraparticle voids with an average particle size of 150– 250 nm, and the large voids are located exclusively in the inner part of the crystals and never communicate directly with the surface. The obvious contrast between the dark edge and the relatively bright center confirms their hollow nature. When compared with

Table 1 The V and S content, BET surface area, pore volume, pore size, and band gap energies of samples.

a

Sample

V (wt.%)a

S (wt.%)a

SBET (m3/g)b

Vp (cm3/g)

Dp (nm)c

Eg (eV)d

HTS (0 #) 1%-VOSO4-HTS (1 #) 5%-VOSO4-HTS (2 #) 7%-VOSO4-HTS (3 #) 10%-VOSO4-HTS (4 #) 5%-VOSO4-TiO2 (5 #) 5%-VOSO4-Silicate (6 #) 5%-VO(acac)2-HTS (7 #)

– 1.1 5.2 7.1 10.1 5.1 5.1 5.2

– 0.6 3.1 4.5 6.4 3.2 3.1 –

330 297 286 263 207 56 283 282

0.262 0.258 0.220 0.217 0.165 0.193 0.251 0.231

8.45 8.76 9.41 9.48 10.8 9.64 9.27 9.36

3.32 2.26 2.14 2.10 2.04 2.63 1.98 2.18

Vanadium and sulfur content of the samples measured by ICP-AES. BET surface area. The pore-size distribution determined by the BJH method. d The band gap energies of these samples were calculated using the equation hm = A(hm  Eg)n/2 where a, m, Eg, and A are the absorption coefficient, light frequency, band gap energy, and a constant, respectively, and n is 4 for the indirect transition. b

c

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Fig. 4. TEM images of samples 0 # (A1, A2, and A3) and 2 # (B1, B2, and B3).

the pure HTS, the VOSO4-modified HTS material has a similar hollow sphere-like structure, which implies that the introduction of VOx or SO2 4 species to HTS did not change the main morphology (Fig. 4B1). It can be seen from Fig. 4B2 and B3 that a large number of dopant species are highly dispersed into a hollow structured zeolite matrix or the inside surface of the catalyst, which is strong evidence for the formation of isolated vanadium species. This kind of hollow structure is also essential to decrease pore diffusion limitations and facilitate bulky molecular transport to catalytic sites in increasing catalyst activity, which was similarly observed in early reports [23,29,30]. Fig. 5 shows the Raman spectra of the 0 # sample, VOSO4-modified HTS (1 #–4 #), the 5 # sample, the 6 # sample and the 7 # sample. From Fig. 5A, it can be seen that the characteristic bands of extraframework Ti appear at 395, 515, and 638 cm1, indicating that the HTS zeolite contained a handful of extraframework Ti species [31]. With increased V content, the Raman frequency of VOSO4-modified HTS (1 #–4 #) is slightly shifted to higher wavenumber (6 cm1), indicating the formation of oxygen vacancies, which could be due to the distortion of the extraframework Ti lattice induced by V modification [28]. For V-modified HTS, oxygen vacancies are prone to form because the lattice vanadium sites act as ‘‘defects’’ by interrupting the ATiAOASiA bridging sites in the crystalline lattice, and the average consecutive length of ATiAOASiA bridging sites will decrease due to the increasing V content [32]. Thus, the shift of the Ti species to higher wavenumber with increasing V content reveals the increasing possibility of Ti atoms meeting V atoms via the formation of TiAOAV in the modified HTS. The reduced intensity of the 638 cm1 characteristic band with vanadium loading further suggests that a VAOATi bond is formed by breaking ATiAOASiA bridging sites. However, the highest concentration of V doping (10%) produces additional

Raman peaks at 283, 303, 487, and 706 cm1 corresponding to the V2O5 phase [33]. In contrast to VOSO4-modified silicate-1, the band at 990 cm1 associated with terminal V@O bands in polymeric tetrahedral metavanadates does not appear (Fig. 5B). This suggests that additional coordination of a monomeric vanadyl species to a surface siliceous site occurs after all exposed surface titanium centers are occupied by vanadyl species and that these overloaded vanadyl species prefer a location near the titanium centers [34]. In contrast to the VO(acac)2, the loaded VOSO4 samples displayed the characteristic band of the S@O bond of isolated SO2 at about 1000 cm1, which might be somewhat hampered 4 by overlap of the assignments in the terminal V@O bands [35]. The IR spectra of catalysts before and after modification in KBr pellets are reported in Fig. 6 in the range 1250–400 cm1, where the skeletal vibrational modes occur. The bands at 1228, 1091, 963, 798, and 470 cm1 perceptible in modified HTS (1 #–4 #) or silicate-1 (6 #) are characteristic of the silica network. The broad features at 1228 and 1091 cm1 are assigned to intertetrahedral and intratetrahedral asymmetric stretching vibrations of TAOAT, respectively. Bands at 798 and 470 cm1 can be assigned to symmetric stretching modes of TAOAT vibration and TAO bending modes, respectively. Finally, the feature at 950 cm1 can be assigned to two overlapping peaks of (SiAOH) and (TiAOASi) vibration [36], but the enhanced intensity of the characteristic band for the HTS-based catalyst further confirms the presence of the TiAOASi bond on the HTS-based catalysts. By comparison with the spectral profile of VOSO4-modified TiO2, a spectral feature of crystalline TiO2 (anatase and/or rutile) with characteristic dominant broad band at 600–650 cm1 [37] was detected in the spectra of KBr pellets of TiO2 supports (Fig. 6A). This is another indication that the titanium species is relatively homogeneously incorporated into the framework for HTS-based catalysts. In addition, the weak

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195 283 395 515 525 638 702

195 283 395

990

638 702

515

990 0#

3# 6# Intensity / c.p.s.

Intensity / c.p.s.

2# 1# 0#

2# 7# 5#

4#

A 100

200

300

400

500

600

700

800

900

B 1000 1100

100

200

300

400

500

600

700

800

900

1000 1100

Wavenumber/cm-1

Wavenumber/cm-1

Fig. 5. Visible Raman spectra (A) and (B) of samples (0 #, 1 #, 2 #, 3 #, 4 #, 5 #, 6 #, and 7 #).

1128

Absorbance (a.u.)

Absorbance (a.u.)

950

2# 5# 6# 7# 0#

A 400

0# 1# 2# 3# 4#

B 600

800

1000

1200

Wavenumbers (cm-1)

400

600

800

1000

1200

Wavenumbers (cm-1)

Fig. 6. FT-IR spectra of (A) and (B) samples (0 #, 1 #, 2 #, 3 #, 4 #, 5 #, 6 #, and 7 #).

peak at 1128 cm1 is typical for the SO2 4 mode of vibration of a chelating bidentate sulfate ion coordinated to a metal cation [38]. When SO2 4 is bound to the catalyst surface, the symmetry can be lowered to either C3V or C2V. The bands obtained in the range 1100–1225 cm1 are typical of sulfate complexes in bidentate configuration with C2V symmetry. However, the characteristic feature might be somewhat hampered by overlap of the assignments of the silica network on the HTS-based catalysts. 3.3. Optical properties and band gap energy of modified HTS catalysts Fig. 7 shows UV–vis diffuse reflectance spectra of pure HTS, VOSO4-modified HTS (1 #–4 #), VOSO4-modified silicate-1, VOSO4-modified TiO2, and VO(acac)2-modified HTS. The corresponding UV–vis spectrum of pure HTS showed two absorption bands (see inset in Fig. 7A): the first band, 215 nm, would be assigned to a charge transfer from O2 to tetrahedral Ti4+ in the zeolite framework; the second band, 300 nm, indicated the formation of extraframework Ti in the HTS [39]. The corresponding values of energy edge are 3.82 and 3.32 eV, which were exhibited only in the UV region. After HTS is doped with VO(acac)2, the resulting catalyst (7 #) exhibits visible-light absorption at a wavelength above 400 nm; nevertheless it shows no absorption for

VOSO4-modified TiO2. This might be related to charge transfer from the 3d orbitals of V ions to the conduction band of crystalline TiAOASi by formation of isolated impurity energy levels below the bottom of the TiAOASi crystalline conduction band [15]. This means that these impurity energy levels are beneficial for extending the absorption spectrum wavelength toward the visible light region. In contrast to 7 #, VOSO4-modified HTS exhibits a two-step absorption edge. The first edge is only slightly shifted to the visible region from 568 to 576 nm, while the second edge extends to the visible region from 600 to 800 nm (Fig. 7A). The S doping causes a slight decrease in the band gap to 2.14 eV in connection with the formation of localized S2p states just above the valence band maximum of crystalline TiAOASi due to substitutional and interstitial S species [19]. In this case, some authors have assigned the absorption band in the visible range to the occurrence of oxygen vacancies, which causes a charge imbalance between S6+ and O2 ions [40]. Furthermore, the extending of the second edge could indicate the existence in this sample of a certain number of intermediate states within the band gap that contribute to the absorption process in the visible region [35]. It is clearly visible from the spectra that the tail of the absorption edge is significantly shifted to the visible region for the samples with increasing V concentration in HTS powder (Fig. 7B). The highest V-doping (10%)

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1.4

A

1.4

1.2

B

1.0

1.0

0.4 0.2 0.0 300

0.6

1.0

0.4 1# 7# 6# 5# 0#

0.2 0.0

1# 7# 6# 5# 0#

Absorbance

Absorbance

0.8

0.8

200

300

F (R)

F (R)

1.0

0.6

0.8

1.2

1.2

400

500

600

700

0.8

0.4

0.2

1# 2# 3# 4# 0#

0.0

0.6

800

200

300

Wavelength/nm

600

700

800

Wavelength/nm

500

600

700

800

1# 2# 3# 4# 0#

0.2

500

400

Wavelength/nm

0.4

0.0 400

0.6

300

400

500

600

700

800

Wavelength/nm

Fig. 7. UV–vis spectra of (A) and (B) samples (0 #, 1 #, 2 #, 3 #, 4 #, 5 #, 6 # and 7 #).

causes the band gap to decrease to 2.04 eV, mainly attributed to the presence of crystalline V2O5 species, having a low Fermi level, on the surface of the HTS.

suggests that sulfur (S6+) is present in the form of sulfate, has coordination with the surface of Ti4+, and has a ATiAOASA linkage [22]. 3.5. Photooxidation of cyclohexane

3.4. Chemical state of modified HTS catalysts The chemical states of the dopants incorporated into HTS were investigated by XPS. The core level of Ti2p, V2p, O1s, and S2p in VOSO4-modified HTS (2 #) and the pure HTS sample is reported in Fig. 8. Fig. 8A shows the XPS spectra of Ti2p photoelectrons for the pure HTS and 2 # samples. The Ti2p3/2 peak for the pure sample was found to be symmetric at the binding energy of 460.1 eV (which is typically assigned to the tetrahedral Ti in the HTS [41]), while the peak for the VOSO4 modified sample showed a shoulder (with a fractional amount of 18%) at lower binding energy. This can be attributed to the fact that V and S ions are incorporated into the HTS lattice and influence the local chemical state of Ti4+ ions. It is obvious that the Ti2p region of VOSO4-modified HTS can be resolved into three parts, one at 460.1 eV originating from Ti4+ species, the second at 459.2 eV resulting from VOSO4-modified HTS combined with the surface sulfate species [42], and the third at 457.2 eV corresponding to Ti3+ species. As a result, the reduction of these Ti4+ to Ti3+ in the framework with tetrahedral coordination indicates that oxygen vacancies can be created by V and S doping. In the V2p XPS spectrum of the 2 # sample (Fig. 8B), two peaks at 517.2 eV and 524.9 eV are assigned to V2p3/2 and V2p1/2, which are higher than 516.1 and 523.6 eV for referenced values of V2O5 [43], respectively. This analysis indicates that V atoms were incorporated into the HTS crystal lattice under our experimental conditions. Due to similar radii, V4+ ions (0.78 Å) may be incorporated into the HTS lattice by substitutionally replacing Ti4+ ions (0.745 Å) and forming TiAOAV bonds. The XPS spectrum of the V2p3/2 level was deconvoluted into two peaks having binding energy values of 517.3 and 516.2 eV, which have been attributed to V5+ and V4+ states, respectively [44]. This indicates that V exists in the HTS in the form of V5+ and V4+, with a larger quantity of V5+ ions, as indicated by the area under the peak in XPS. The XPS spectra of the O1s region are given in Fig. 8C. With the introduction of VOSO4, the O1s region of the 2 # sample was significantly changed, and curve fitting suggests that lattice TiAO, VAO, oxygen in surface sulfate species (SO2 4 ), and AOH exist [42]. Fig. 8D shows the XPS spectra of the S2p band for the VOSO4-modified HTS sample. The binding energy of 168.5 eV

Photooxidation of cyclohexane over the as-synthesized samples in the presence of concentrated HCl as an additive was evaluated under visible light irradiation and compared with the commercial TiO2 Degussa P25, pure HTS and VOSO4, as shown in Table 2. It is clearly revealed that P25 and HTS show no photocatalytic activity for this photooxidation reaction when illuminated with visible light (entries 1 and 2). This corresponds well with the lack adsorption ability of the single TiO2 or HTS for visible light, as supported in the UV–vis spectral characterization. However, similar visible light stimulation of homogeneous VOSO4 as a possible reference results in considerable oxidation of cyclohexane in the CH3CN solution (entry 3), which indicates that fast electron–hole recombination is the dominant process under these experimental conditions. For VOSO4-modified HTS samples, remarkably high visible light photocatalytic activity could be observed for the system under study, affording cyclohexanol, cyclohexanone, and chlorocyclohexane as major reaction products (entry 5). HTS is composed of TiAOx species existing in tetrahedral coordination (TiAO4 species). Since the TiAO4 species does not absorb any visible light, the VOx and SO2 4 species in the HTS are responsible for the visible light absorption and the associated photocatalytic activity. Therefore, VOSO4-modified silicate-1 was investigated as a possible reference (entry 12). Low photocatalytic activity, however, was obtained under similar conditions. This could be a result of the change in surface speciation of such silicate-1 in the absence of TiAO4 species. Another traditional photocatalyst containing the same titanium oxide, but with a different coordination environment, is bulk TiO2. In bulk TiO2, the TiAOx species clusters are in octahedral coordination (TiAO6 species). VOSO4-modified TiO2 also exhibits visible light photocatalytic activity; however, the photocatalytic activity is lower than that of VOSO4-modified HTS (entry 11). CO2 production was also detected in VOSO4-modified TiO2 samples, which indicates that this TiAO6 species lacks sufficient product selectivity [45,46]. As stated in the Introduction, incorporating nonmetal ions S into Ti-substituted materials is advantageous for recombination. Furthermore, a VO(acac)2-modified HTS sample was measured (entry 13). Although this photocatalyst has a similar VOx species in the HTS, differences in the absence of SO2 could result in 4

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470

465

460

455

Intensity (a.u.)

Intensity (a.u.)

B V 2p

HTS 5% VOSO4-HTS

Ti 2p

Intensity (a.u.)

A Ti 2p

450

Binding Energy (ev)

460.1

459.2

V5+ 2P3/2

V5+ 2P1/2

V4+ 2P3/2

457.2

526

472 470 468 466 464 462 460 458 456 454 452 450 448

524

522

Binding Energy (ev)

520

518

516

514

Binding Energy (eV)

D

C

O 1s

S 2p

HTS 5% VOSO4-HTS

O-Ti SO42542

540

538

536

534

532

530

528

540

538

S 2p

O-V

Binding Energy (ev)

542

Intensity (a.u.)

O-H

Intensity (a.u.)

Intensity (a.u.)

O 1s

536

534

532

530

528

174

172

170

168

Binding Energy (ev)

E

Survey

164

162

160

158

156

1

HTS VOSO4-HTS

O 2p Si 2p

166

Binding Energy (ev)

Intensity (a.u.)

C 1s Ti 2p

V 2p

0

100

200

300

400

500

600

700

800

900 1000 1100

Binding Energy (ev) Fig. 8. XPS spectra of (A) Ti2p, (B) V2p, (C) O1s, and (D) S2p of samples (0 # and 2 #), and (E) survey spectrum of samples (0 # and 2 #).

differences in photocatalytic performance. As expected, sulfated modification has been proven beneficial for visible light (photo-) catalytic activity. In particular, the sulfated samples demonstrated enhanced cyclohexanone selectivity, which might be attributed to the formation of strong acid sites by sulfation and the increase in adsorption strengths. In order to improve the photocatalysis cycling of VOSO4-modified HTS (2 #), we tried to use concentrated HCl as an additive for cyclohexane oxidation, and the results are listed

in Table 2. Entries 5 and 9 show that adding 0.1 or 0.2 ml of concentrated HCl to the reaction system significantly promoted this photocatalytic reaction, providing much higher conversion than without the additive HCl (entry 2). This is likely due to the formation of Cl atom radical species from HCl, which is highly effective in capturing H atoms of cyclohexane in the photocatalytic cycling of the oxidation reaction [47]. When the additive concentrated HBr rather than concentrated HCl was introduced into this photocatalysis reaction system, the photocatalytic efficiency was not

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Table 2 Visible-light-driven oxidation of cyclohexane with molecular oxygen catalyzed by V-doped catalysts.a Entry

Catalyst

Conv. (%)b

1 2 3 4 5 6 7 8e 9f 10g 11 12 13 14h 15i 16j 17k 18l

HTS TiO2 VOSO4 1%-VOSO4-HTS (1 #) 5%-VOSO4-HTS (2 #) 7%-VOSO4-HTS (3 #) 10%-VOSO4-HTS (4 #) 5%-VOSO4-HTS (2 #) 5%-VOSO4-HTS (2 #) 5%-VOSO4-HTS (2 #) 5%-VOSO4-TiO2 (5 #) 5%-VOSO4-Silicate (6 #) 5%-VO(acac)2-HTS (7 #) 5%-VOSO4-HTS (2 #) 5%-VOSO4-HTS (2 #) 5%-VOSO4-HTS (2 #) 5%-VOSO4-HTS (2 #) 5%-VOSO4-HTS (2 #)

– – 0.4 4.3 14.5 12.1 6.7 0.6 14.9 – 5.8 2.6 5.1 16.2 6.9 0.4 2.3 3.4

Selectivity (%)c A

B

C

D

– – 21.3 4.6 6.0 11.2 21.4 8.3 – 10.9 22.7 2.3 5.3 5.2 11.6 _ –

– – 45.0 13.7 21.4 19.5 15.4 10.3 23.3 – 15.0 10.3 27.9 16.4 15.7 20.9 – –

– – 33.7 81.7 72.6 69.3 63.2 89.7 68.4 – 63.1 67.0 69.8 78.3 79.1 67.5 100 100

– – – – – –– – – – 11 – – – – – –

Ketone/alcohol molar ratio

TOF (h1)d

– – 0.76 6.08 3.46 3.62 4.19 8.88 2.95 – 4.29 6.63 2.55 4.87 5.14 3.30 _ –

– – 0.01 3.32 2.37 1.45 0.07 0.10 2.38 – 0.97 0.43 0.83 2.65 1.28 0.07 0.38 0.56

a All reactions were done with 0.050 g catalyst, 4 mmol cyclohexane, O2 (1 atm), 5 ml of acetonitrile, at 36–38 °C, time 6 h, concentrated HCl (0.1 ml) as an additive, and a tungsten–bromine lamp (35 W) as visible light source. b Conversion (%) based on substrate = {1  [(concentration of substrate left after reaction)  (initial concentration of substrate)  1]}  100. c Product selectivity = content of this product/(adding cyclohexane amount (mmol)  the amount of cyclohexane recovered (mmol))  100%; A = chlorocyclohexane; B = cyclohexanol; C = cyclohexanone; D = small amounts of unidentified products and CO2. d Turnover frequency (TOF): number of moles of cyclohexane converted per mole of catalyst. Number in parentheses is based on number of moles of V sites only (from ICPAES data). e Without adding concentrated HCl to the photooxidation system. f Using concentrated HCl (0.2 ml) as an additive in the photooxidation system. g The lamp was packaged in silver paper. h Adding water (0.1 ml) to the photooxidation system. i Adding water (0.4 ml) to the photooxidation system. j Using acetone (5 ml) instead of acetonitrile. k Using cyclohexanol (4 mmol) instead of cyclohexane. l Using cyclohexanol (4 mmol) instead of cyclohexane with water (0.1 ml) in the photo-oxidation system.

obviously improved, providing bromocyclohexane (content 83.4%) as main product with 1.2% of conversion. This is likely because the Br atoms formed cannot easily capture H atoms of cyclohexane due to their low activity, and the formation of cyclohexyl hydroperoxide is also easily hampered in the presence of high Br free radicals [48]. Entry 14 shows that the phot-catalytic oxygenation was improved by introducing water (0.1 ml) into the above acids-mediated photocatalysis systems, providing higher cyclohexanone selectivity (78.3%) than in entry 5. This indicates that water (or water from concentrated HCl solution) can accelerate the hydrogen abstraction and promote the oxidation of alcohol to ketone as a proton carrier. An attempt to further increase the amount of water led to a negative effect on cyclohexane conversion (entry 15), which might be due to the deactivation of the ⁄ charge-transfer excited state [Ti3+AO L ] [49]. Further, when the lamp was packaged in silver paper, the catalytic activity of VOSO4-modified HTS was nearly negligible, even if the solution temperature went up from 15 to 36–38 °C due to the heating effect of the lamp (entry 10). These findings undoubtedly support the conclusion that the oxidation reaction is triggered by light irradiation rather than heating. In addition, cyclohexane conversion increased with increasing vanadium loading up to 5 wt.% V (entries 4 and 5). At higher loadings, cyclohexane conversion decreased (entries 6 and 7). To elucidate whether the dispersion degree of the vanadium species is a key factor in determining the catalytic performances, we have calculated the turnover frequency (TOF) values for cyclohexane conversion based on V sites on catalyst. Table 2 shows a comparison of the cyclohexane TOFs of V sites on HTS. The TOF values for cyclohexane conversion are markedly high for low V content. The trend is typical of supported Lewis acid

catalysts, with monotonically decreasing reactivity with increasing loading, and would lead one to conclude that the rate of reaction is greater on lower-nuclearity sites. As already discussed, the monomeric vanadyl species became progressively oligomerized with increasing surface coverage. Therefore, the catalytic activity was strongly influenced by the oligomerization degree of the vanadyl species. A similar dependency on surface coverage and number of vanadyl species was reported by Wachs et al. for V oxides on different supports in propene oxidation [50]. The influence of solvents on the photocatalytic oxidation was also investigated by using the catalyst VOSO4-modified HTS (2 #). The photocatalytic reaction hardly occurred in acetone (see entry 16), whereas it could proceed efficiently in MeCN (entries 5 and 9), probably being due to the better electron transfer ability of the catalyst in MeCN than in acetone [4]. In order to understand the product distribution in cyclohexane photooxidation, we studied the effect of the reaction parameters. First, the effects of the amount of VOSO4-modified HTS (2 #) on the photooxidation of cyclohexane were studied and the obtained results are shown in Fig. 9A. The cyclohexane conversion and chlorocyclohexane selectivity increased continuously with the catalyst weight in the range 0.01–0.05 g, with a concomitant decrease in the selectivity of oxygenated cycohexanone products. However, further addition of catalyst led to a negative effect on conversion, likely because of a screening effect of excess particles that masks part of the photosensitive surface [51]. The effects of reaction time on the photooxidation reaction is shown in Fig. 9B. The low conversion of cyclohexane at first indicates a short induction period, which most likely relates to initiation of the radical reaction. This is followed by a constant rate of cyclohexane conversion for up

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70

60

50

50

40

40 30

cyclohexanol

20

chlorocyclohexane

10

0.02

0.04

0.06

0.08

50

50 cyclohexanol

40 30

30

20

20 chlorocyclohexane

0

0 0.10

5

80

cyclohexanone

60 50

40

40 conversion cyclohexanol

20

30 20

chlorocyclohexane

10 1.0

0.8

0.6

10

0.4

0.2 -1

Cyclohexane concentration / mol L

Product selectivity / %

70

50

30

90

80

C

60

10

15

20

25

Iirradiation time / h

Conversion / %

Product selectivity / %

70

10 0

0

Catalyst amount / g 80

40 conversion

10

10 conversion

60

cyclohexanone

D

90 chlorocyclohexane cyclohexanol cyclohexanone

conversion/%

80

70

70

60

60

50

50

40

40

30

30

20

20

10

10

0

Conversion / %

20

60

Conversion / %

60

0 0.00

70

70

cyclohexanone

30

80

B

80

Product selectivity / %

70

Product selectivity / %

80

A

Conversion / %

80

0 1 atm

2 atm

3 atm

O2 pressure / atm

Fig. 9. Effect of the catalyst amount (A), irradiation time (B), cyclohexane concentration (C), and O2 pressure (D) on the conversion of cyclohexanane and KA oil and chlorocyclohexane selectivity over the synthesized 5% VO(SO4) HTS catalysts.

to about 18 h of reaction time, after which a significant leveling off of the conversion rate occurs. A possible explanation for this phenomenon is a radical-type autoxidation process. In order to find out the reaction intermediates, we took aliquots of the sample at regular intervals and analyzed them by iodometric titration. No hydroperoxide was detected, which indicates that these possible reaction intermediates are quickly transformed into more stable cyclohexanone or cyclohexanol. From changed selectivity achieved in reaction times less than 12 h with decreased cyclohexanol selectivity and increased cyclohexanone selectivity observed at prolonged reaction times, it is logical to conclude that cyclohexanol formed initially can be transformed to cyclohexanone via a sequential oxidation process as the reaction proceeds. Fig. 9C shows the effect of cyclohexane concentration on photooxidation. The conversion increased from 14.5% to 24.8% with the decrease of the concentration of cyclohexane from 1.0 to 0.20 mol/l, while the selectivity for the oxygenated and chlorinated products hardly varied. Finally, the oxygen pressure was detrimental to the photooxidation reaction, as shown in Fig. 9D. A further promotion effect of increasing O2 pressure on cyclohexane conversion and cyclohexanone selectivity was observed, with a concomitant decrease in the selectivity of chlorocyclohexane products. This finding undoubtedly supports the conclusion that the increase in O2 pressure can play a role in improving combination of the cyclohexyl radicals with O2 molecules and reducing reaction probability of the Cl atoms with cyclohexyl radicals [47]. In order to further explore the reason that simultaneous presence of V and S in the hollow TS-1 is capable of promoting a process with a higher cyclohexanone/cyclohexanol molar ratio (3.46)

than in the homogeneous phase, we measured the photocatalytic activity of VOSO4-modified HTS (2 #) in the oxidation of cyclohexanol with or without water under the same conditions, and the results are shown in Table 2 (entries 17 and 18). It is found that 100% cyclohexanone selectivity with cyclohexanol conversion (2.3%) was obtained in the photooxidation cyclohexanol experiment (entry 17), indicating that a consecutive reaction of formed cyclohexanol to cyclohexanone may occur simultaneously during the photocatalyzed cyclohexane oxidation. Notably, when a suitable amount of water was introduced into these photocatalysis cyclohexanol systems, the cyclohexanol conversion was further improved to some extent (entry 18), indicating that water can promote the oxidation of cyclohexanol to cyclohexanone as proton carrier. With going into too much detail on the route to cyclohexanone from cyclohexane, we note that cyclohexanone was the primary product, with 86.7% selectivity at 100% conversion when the decomposition of cyclohexyl hydroperoxide intermediate on VOSO4-modified HTS (2 #) was tested under the same photoirradiation conditions. This supports the conclusion that the cyclohexanone formation rate is dominated by the reactions depending strongly on the type of the zeolite host, and can be explained as follows. First, the presence of OH groups and Ti(III) centers on the zeolite surface may be responsible for the formation of a peroxide intermediate that subsequently changes to cyclohexanone in zeolite-induced selective (photo-) oxidation, as proposed for a previously reported reaction mechanism [45,52]. Second, we may capitalize here on product shape-selectivity. The hollow nature of the catalyst with large intraparticle voids facilitates the transport of cyclohexanone out of the crystal’s channels. In the present

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reaction, the diameter of cyclohexane is 0.49 nm (the distance between oxygen and the most distant hydrogen is 0.47 nm; the van der Waals radius of hydrogen is 0.10 nm) and that of the products cyclohexanone, cyclohexanol, and cyclohexyl hydroperoxide should be 0.51, 0.58, and 0.69 nm, respectively; the average pore diameter of TS-1 is 0.55 nm. We therefore envisage that the product cyclohexanone can easily come out of the intracrystalline voids of the VOSO4-modified HTS catalyst via the pores. However, the product cyclohexanol and reaction intermediates cyclohexyl hydroperoxide formed within the intracrystalline voids of the VOSO4-modified HTS catalyst will be held in the vicinity of the active centers until the reaction proceeds further to yield the more mobile product cyclohexanone. 3.6. Photocatalytic mechanism The above results clearly indicate the enhanced visible-light-responsive activity of VOSO4-modified HTS as compared with HTS. This enhanced activity may result from the creation of new active sites in the catalyst and the presence of O@VAO4ATi bridging sites as a result of the occupation of the basic sites on the surface of HTS by vanadia. The increase in the loading of vanadia increases the formation of O@VAO4ATi bridging sites, which create impurity energy levels between VD and CD with formation of photogenerated charges [53], thereby increasing conversion. However, as the vanadia doping increases relative to available surface sites, polymeric vanadates begin to form due to the lack of available basic sites on the catalyst surface. The polymeric vanadates creating VAOAV bridging bonds are formed by breaking VAO4ATi bonds of the monomeric vanadyl species, as already identified by Raman spectroscopy (Fig. 5). This leads to reduced visible-light activity for oxidation of cyclohexane to KA oils. The O@VAO4ATi bridging sites in VOSO4-modified HTS, therefore, play a role in the photocatalytic oxidation of cyclohexane. On the basis of examination of surface species by Raman, XPS, and FT-IR methods, in this work we imagine two kinds of smallest structural models containing the different active bridging sites (O@VAO4ASi and O@VAO4ATi). This is a highly simplified system, and DFT calculations were carried out only to elucidate the

electronic distributions and band patterns of the different active bridging sites. Fig. 10 illustrates the electronic structure of Si3TiVO7H11 models including the calculated atomic NBO charge of selective V, Ti, and O (O@V) atoms taken from the same sites in the (a) and (b) models. The charge (1.025 e) of the V atom at O@VAO4ATi bridging sites is smaller than that (1.035 e) at O@VAO4ASi bridging sites, implying that the V@O bond at O@VAO4ATi bridging sites possesses a stronger mixed covalent characteristic [54]. The NBO charge of the V impurity has an obvious change at O@VAO4ATi bridging sites. The different charges of Ti atoms located at the definite sites in the different models were observed, indicating a mixed valence state of Ti ions, suggesting that the existence of an oxygen vacancy significantly affects Ti3d states. In the (b) models with O@VAO4ASi bridging sites, the NBO charge (1.025 e) of the V atom is smaller than that (1.564 e) of the Ti atom, showing that the covalent bond feature between V and O4 atoms is much stronger than that between Ti and adjacent O4 atoms. It can also be seen clearly that the charge (0.671 e) of the adjacent O4 atom at O@VAO4ATi bridging sites is smaller than that (0.932 e) at O@VAO4ASi bridging sites, due to the obvious change of the p-state electron by the Ti atom. These results indicate that more electrons should transfer from Ti to adjacent O4 atoms rather than being shared between V and O4 atoms at O@VAO4ASi bridging sites, facilitating the charge separation of photogenerated carriers and thus benefiting the photocatalytic performance [55]. Also, the charge (0.299 e) of the O atom (V@O) at O@VAO4ATi bridging sites is higher than that (0.278 e) at O@VAO4ASi bridging sites, indicating that the V@O bond at O@VAO4ATi bridging sites possesses a stronger adsorption capacity for the reactant. The high photocatalytic performance of the as-synthesized VOSO4–HTS composites may be related to their unique energy bands, specific surface structure, and recombination rate of photogenerated hole–electron pairs. The band gap energy is highly dependent on the number of vanadium 3d electrons [53], which shows a low value for V (3d34s2), with three 3d electrons. Under irradiation by visible light, electrons can be excited from the valence band to the unoccupied V5+(3d0) 3d energy level below the conduction band of HTS [15], from the S3p energy level to

Fig. 10. Electronic structure of optimized models for the different active (A) and (B).

W. Zhong et al. / Journal of Catalysis 330 (2015) 208–221

the conduction band, or from the S3p energy level to the unoccupied V5+3d energy level. The difference of gap band energy in the singly doped HTS and silicate-1 and codoped HTS may result in different photocatalytic activity, which is connected with their electron configurations. At the same time, V5+ ions present on the surfaces of HTS particles in the form of monomeric V2O5 species are responsible for e and h+ separation. Due to the lower Fermi level of V2O5 species, the photogenerated electrons may immediately transfer to V5+ ions, leaving holes on the HTS, resulting in the effective separation of e and h+ [56]. The V4+ species, created from V5+ by electron trapping, easily release and transfer the electron to oxygen molecules absorbed onto the surfaces of HTS to produce superoxide radicals O 2 . The holes in the valence band react with H2O to produce hydroxyl radicals, which can oxidize Cl to form Cl atoms. At the same time, visible-light-driven electron-transfer processes between metal and Cl ions can also lead to oxidation of Cl ions to Cl atoms and the reduction of V(V) to V(IV) ions. The obtained Cl atom radical species is highly

219

effective in capturing H atoms of cyclohexane with the formation of cyclohexyl radicals, resulting in improved photocatalytic activity in the presence of concentrated HCl. Similarly, in S doping, the sulfation procedure via calcination led to electron transfer from TiAO4 species to the oxygen atoms of sulfate ions, causing delocalization and a new electron distribution on the surface of HTS. This attributed electron transfer to sulfate ions, leading to an electron deficiency on Ti atoms and to the formation of Lewis acid sites. We thought that this irreversible charge transfer acted as a charge trap [57], increasing the space charge separation between the photogenerated electrons and holes, prolonging their lifetime and hindering their recombination in photocatalysis. An optimum S and V content can help to separate the photogenerated charges efficiently by acting as trapping sites for the electrons and holes, respectively. Thus, the probability of photogenerated charges reaching the catalyst surface before recombination is far better for codoped HTS than for singly doped HTS.

Fig. 11. Mechanism for the photooxidation of cyclohexane with O2 vapor over the VO(SO4)-modified HTS photocatalyst.

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In addition, the photocatalytic reaction takes place by absorption of HCl, O2, and H2O on the surface of the photocatalyst to generate active radical species by electron and holes, respectively. In the present photocatalyst, the presence of intracrystalline voids accessible only via entrances smaller than 4 nm produced by recrystallization decreases pore diffusion limitations and increases the contact area between the active sites and the reactants, thus resulting in improved quantum efficiency of HTS [58]. In addition, the good dispersion of tetrahedrally coordinated Ti in hollow crystals facilitates e and h+ transport to the surface for faster reaction [6]. The presence of surface V5+ ions in the form of monomeric V2O5 species produces a spatial charge layer at the hollow interface with the TiAO4 band in HTS due to difference in electrochemical potential [59]. The stationary electric field at the hollow interface provides the driving force for the photogenerated e on TiAO4 species to be instantaneously injected into the V5+ species [15]. Thus intracrystalline voids with surface V5+ ions provide the ideal scenario for fast charge transfer on the surface for photocatalytic reaction. Based on the above experimental and computational results, the photocatalytic mechanism of VOSO4-modified HTS is presented in Fig. 11 as a series of numbered chemical equations. The absorption of a photon excites an electron to the CB (eCB) under visible light irradiation, generating a positive hole in the VB (mVB+) (Eq. (1)). The photogenerated electrons in the CB are transferred to adsorbed O2 on the surface of the photocatalyst, which act as electron scavengers to produce superoxide anion radicals (O 2 ) (Eq. (2)). The resulting O2 radicals then react with H+ to generate hydroperoxyl radicals (HO2) and subsequently H2O2, as shown in Eqs. (3) and (4) [60]. H2O2 may further react with TiOH to yield  OH radicals; the OH radicals react with water to yield the hydroxyl radical–water complex (H2OAO), which is responsible for the production of Cl (Eqs. (5)–(7)). In addition, a visible-light-driven electron-transfer process between metal and Cl ions can also lead to oxidation of Cl ions to Cl atoms and reduction of V(V) to V(IV) ions (Eq. (8)). Thus, the obtained radical species Cl may abstract one H from cyclohexane to form cyclohexyl radicals (Cy, Eq. (9)), followed by addition of O2 to the Cy to form a cyclohexyl peroxy radical (CyOO). The latter is further converted to a cyclohexyl hydroperoxide (CyOOH) with the production of another cyclohexyl radical via CyOO abstracting one H atom of cyclohexane or subsequent couples by two cyclohexyl peroxy radicals to product KA-oil and O2 (Eq. (11)). The bulky Ti(OOCh) species was possibly formed by the reaction of TiOH with CHHP, and the homolytic fragmentation of Ti(OOCh) can yield Ti-centered radical species such as Ti(IV)AO or Ti(IV)AOO [31], which are generally regarded as active centers for cyclohexane oxidation. Of course, the formed holes can react with H2O molecules to produce OH radicals, which are involved in the catalytic cycling pathway. Another minor pathway via OH radicals abstracting one H atom of cyclohexane to yield CyOOH may also exist (Eq. (10)). Finally, CyOOH mostly rearranges to cyclohexanone and water in the zeolite cage environment (Eq. (12)), as described by Heinz et al. [52]. In addition, another minor pathway via OH radicals coupling with cyclohexyl radicals to product a cyclohexanol may also exist and the formed cyclohexanol may be oxidized further to cyclohexanone by a consecutive reaction (Eq. (13)). Undoubtedly, the formation of chlorocyclohexane should be due to the combination of Cl with cyclohexyl radicals (Eq. (14)) and the product, as supported by the above experiments.

4. Conclusions Novel V, S-codoped TS-1 with intracrystalline voids was synthesized via a simple recrystallization route and tested for

photocatalytic selective oxidation of cyclohexane under visible light irradiation. The results showed that the vanadium was mostly dispersed on the surfaces of intracrystalline voids in the form of monomeric V2O5 species, and a handful of V atoms modified the surface of HTS in the form of VAO4ATi. The introduction of V onto the HTS also promotes the formation of oxygen vacancies and isolated Ti3+ on the surface of HTS. The introduction of S onto HTS via calcination leads to an electron transfer from TiAO4 species to the oxygen atoms of sulfate ions. Consequently, V, S-modified HTS samples demonstrate enhanced cyclohexane conversion with a yield of KA oils 3–7 times that on the reference samples (TiO2, silicate-1, and HTS). The significantly enhanced KA oils selectivity on surface V, S-modified hollow HTS samples is mainly attributed to the synergistic effects of the highly dispersed VAO4ATi bridging sites and the formation of isolated Ti3+ and oxygen vacancies on the adsorption of the cyclohexane and oxygen, as well as the separation of photogenerated electrons and holes. The present work may provide new insight into the fabrication of novel visible-light photocatalysts with excellent performance for photooxidation of cyclohexane to KA oils. Acknowledgments This work was supported by the Natural Science Foundation of Hunan Province (Grant 14JJ2056) and the Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province and Scientific Research Fund of Hunan Provincial Education Department (Grant 13C562). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2015.06.013. References [1] J.M. Thomas, R. Raja, G. Sankar, G.G. Bell, Nature 398 (1999) 227–230. [2] U. Schuchardt, D. Cardoso, R. Sercheli, R. Pereira, R.S. da Cruz, M.C. Guerreiro, D. Mandelli, E.V. Spinacé, E.L. Pires, Appl. Catal. A: Gen. 211 (2001) 1–17. [3] A. Maldotti, A. Molinari, R. Amadelli, Chem. Rev. 102 (2002) 3811–3836. [4] Y. Shiraishi, T. Hirai, J. Photochem. Photobiol. C 9 (2008) 157–170. [5] A. Kubacka, G. Colón, M. Fernández-García, Chem. Rev. 112 (2012) 1555–1614. [6] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269–271. [7] J. Matosa, A. García, S.-E. Park, Appl. Catal. A: Gen. 393 (2011) 359–366. [8] M. Anpo, Photofunctional Zeolites, Nova Science Publishers Inc., New York, 2000. [9] G. Li, N. Dimitrijevic, L. Chen, J. Nichols, M. Graham, T. Rajh, K.A. Gray, J. Am. Chem. Soc. 130 (2008) 5402–5403. [10] M. Anpo, J.M. Thomas, Chem. Commun. 31 (2006) 3273–3278. [11] F. Amano, T. Tanaka, Catal. Lett. 35 (2006) 468–473. [12] T.R. Eaton, M.P. Campos, K.A. Gray, J.M. Notestein, J. Catal. 309 (2014) 156–165. [13] X. Chen, S. Shen, L. Guo, S.S. Mao, Chem. Rev. 110 (2010) 6503–6570. [14] J. Zhu, Z. Deng, F. Chen, J. Zhang, H. Chen, M. Anpo, Appl. Catal. B: Environ. 62 (2006) 329–335. [15] R. Dholam, N. Patel, A. Miotello, Int. J. Hydrogen Energy 36 (2011) 6519–6528. [16] S.U.M. Khan, M. Al-Shahry, W.B. Ingler Jr., Science 297 (2002) 2243–2245. [17] X. Chen, C. Burda, J. Am. Chem. Soc. 130 (2008) 5018–5019. [18] S. Klosek, D. Raftery, J. Phys. Chem. B 105 (2001) 2815–2819. [19] G. Mul, W. Wasylenko, M.S. Hamdy, H. Frei, Phys. Chem. Chem. Phys. 10 (2008) 3131–3137. [20] T. Umebayashi, T. Yamaki, H. Itoh, K. Asai, Appl. Phys. Lett. 81 (2002) 454–456. [21] F. Jiang, Z. Zheng, Z.Y. Xu, S.R. Zheng, Z.B. Guo, Q. Li, L. Chen, J. Hazard. Mater. 134 (2006) 94–103. [22] D.S. Muggli, L.F. Ding, Appl. Catal. B: Environ. 32 (2001) 181–194. [23] G. Zou, W. Zhong, L. Mao, Q. Xu, J. Xiao, D. Yin, Z.g. Xiao, S.R. Kirk, T. Shu, Green Chem. 17 (2015) 1884–1892. [24] J. Wang, L. Xu, K. Zhang, H. Peng, H. Wu, J. Jiang, Y. Liu, P. Wu, J. Catal. 288 (2012) 16–23. [25] J. Scholz, A. Walter, T. Ressler, J. Catal. 309 (2014) 105–114. [26] Y. Wang, M. Lin, A. Tuel, Micropor. Mesopor. Mater. 102 (2007) 80–85. [27] G. Zou, W. Zhong, Q. Xu, J. Xiao, C. Liu, Y. Li, L. Mao, S.K. Kirk, D. Yin, Catal. Commun. 58 (2015) 46–52. [28] D. Shee, G. Deo, A.M. Hirt, J. Catal. 273 (2010) 221–228. [29] J.C. Groen, T. Bach, U. Ziese, A.M.P. Donk, K.P.D. Jong, J.A. Moulijn, J.P. Pe0 rezRamirez, J. Am. Chem. Soc. 127 (2005) 10792–10793.

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