Partial oxidation of ethylbenzene by H₂O₂ on VO_x/HZSM-22 catalyst

Abstract

A series of VO_x/HZSM-22 catalysts for the partial oxidation of ethylbenzene was prepared with 0.09–2.86 wt% V loading by an incipient wetness impregnation method, and the catalysts were characterized by inductively coupled plasma-atomic emission spectrometer, X-ray diffraction, Fourier transform infrared spectroscopy, N₂ physisorption analysis, X-ray photoelectron spectroscopy, diffuse reflectance ultraviolet-visible spectroscopy, temperature-programmed calcination-mass spectroscopy, and temperature-programmed hydrogen reduction. The results showed that the chemical nature of vanadium species on HZSM-22 depended on V loading. The amount of vanadium species incorporated into the framework of HZSM-22 increased with V loading firstly and reached a relatively stable value of about 0.2 wt%. The decomposed ammonia might reduce V⁵⁺ to V⁴⁺ in the calcination process. Highly dispersed extra-framework vanadium species, especially surface extra-framework V⁴⁺, promised a good effect on the oxidation of ethylbenzene, under optimized conditions, a high yield of 17.5% with 72.5% selectivity to acetophenone was obtained.

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1. Introduction

Traditionally, the production of aromatic ketones was primarily achieved by the reaction of Friedel–Crafts acylation utilizing acid halides or acid anhydrides as reactants employing a stoichiometric amount of AlCl₃ as catalyst, which led to a large volume of environmentally hazardous and corrosive waste.^1–3 Consequently, great efforts have been devoted to producing the phenyl ketones from the oxidation of alkylbenzenes through a more efficient and environmentally friendly method in the past few decades.^4–9 It is of profound significance to find more sustainable methods using friendly oxidants such as H₂O₂ and recyclable catalysts to obtain the aromatic ketones because conventional procedures bear some drawbacks, such as multiple-step reactions, corrosion of equipment, harsh process conditions, unfriendly oxidants etc.^10,11 Acetophenone is regarded as an important intermediate in the production of agrochemicals, perfumes, esters, pharmaceuticals, drugs, tear gas and resins.^12–15 A great deal of work associated with this one-pot reaction has been done to obtain good activity and selectivity, including homogeneous catalysts and heterogeneous catalysts.^12,16–20

Heterogeneous catalysts for the oxidation of ethylbenzene were highly expected. Typically, Chuan-De Wu and his coworkers incorporated metal 5,10,15,20-tetrakis(3,5-biscarboxylphenyl)porphyrin (M-H₈OCPP) into porous metal–organic framework, and this catalyst gave out more than 99% conversion of ethylbenzene and selectivity to the acetophenone.²¹ M. Ghiaci and his co-workers used SiO₂/Al₂O₃-supported manganese catalyst and obtained a conversion of 91% and a selectivity of 98% in the presence of supercritical carbon dioxide with good catalyst stability.²² V. Murugesan et al. synthesized bimetallic Mn–Ti-SBA-15 and got a ethylbenzene conversion of 92% with 87% selectivity to acetophenone.²³ However, these studies used tert-butyl hydroperoxide as the oxidant, which is expensive, poisonous and explosive at high temperature.

Therefore, using H₂O₂ or O₂ as oxidant is the present focus in the oxidation of ethylbenzene. In this aspect, homogeneous catalysts were mostly used. Bernhard Gutmann et al. reported that the oxidation of ethylbenzene catalyzed by CoBr₂ in acetic acid obtained 87% conversion of ethylbenzene and 36% selectivity to the acetophenone at 80 °C.²⁰ Zaihui Fu et al. employed 8-quinolinolato manganese(III) complexes to get 26.1% ethylbenzene conversion and 65% oxidation yield.¹² These processes generally suffered from the separation of reaction mixture and the difficulty of the reuse of the catalyst.

Thus, developing heterogeneous catalyst using H₂O₂ as oxidant under mild reaction conditions to realize the target partial oxidation is highly desired. The pioneer work of Mona Hosseini-Sarvari et al.²⁴ and Rajaram Bal et al.²⁵ stimulated us to exploit broader kinds of heterogeneous catalysts using H₂O₂ as oxidant.

Zeolites is the most abundant nanoporous materials with great pore channels and surface area, the mostly famous being ZSM-5, silicalite-1, zeolite beta, zeolites A, X and Y.²⁶ In order to modulate the performance on different kinds of reactions of the zeolites, the incorporation of transition metals to the extra-framework and framework is crucial for the catalytic activity for the targeted product.

Supported vanadium oxides catalysts have been widely studied for catalyzing different reactions.^27–31 These vanadium oxides catalysts favored to the side-chain oxidation of ethylbenzene.^32,33 The vanadium species incorporated into the framework of SBA-16 was mainly in the form of highly dispersed tetrahedral coordination.³² The highly dispersed tetrahedral V=O species with lower vanadium loading are associated with the conversion of oxidation activity and the formation of Ce–O–Ce and the V–O–V structure are related to the production of acetophenone.³³ Centi et al. reported a polynuclear vanadium species with three different states (V³⁺, V⁴⁺ and V⁵⁺) and an octahedral VO²⁺ prefers to interact with the OH groups, also a symmetrical framework tetrahedral V⁵⁺ species on VS-1 via direct hydrothermal method.³⁴

Some papers reported that HZSM-22 zeolite has TON structure type and three different kinds of framework topology rings, including 5, 6 and 10-membered-ring structures, which are smaller than ZSM-5, ZSM-11, and ZSM-35, and they also have acidic properties and shape selectivity on the surface of zeolites.^35–37 Besides, impregnation method is useful to gain a high percentage of metal loading.³⁸

In the present work, a series of VO_x/HZSM-22 catalysts was synthesized and used to catalyze the ethylbenzene to acetophenone using hydrogen peroxide as oxidant. To gain a further insight into the nature of the vanadium species as well as the connection between vanadium species and the catalytic performance on ethylbenzene, we studied the correlation with conversion of ethylbenzene, yield of products as well as selectivity of products and the structure of vanadium species in the HZSM-22.

2. Experimental

2.1. Catalyst preparation

VO_x/HZSM-22 catalyst was prepared by incipient wetness impregnation method using commercially available HZSM-22 as support, which was calcined at 600 °C for 4 h before impregnation. Firstly, the HZSM-22 support was evacuated to 0.8–0.9 Torr for 0.5 h in a 100 mL test tube with side neck. Then aqueous solution of NH₄VO₃ was dropped onto the HZSM-22 support, and the slurry was stood for 24 h at room temperature. The excess H₂O was removed by heating the tube in a water bath at 80 °C, and the resultant solid was dried overnight in electric oven with forced convection at 100 °C, and then calcined at 550 °C for 6 h in air to obtain the catalysts, denoted as x wt% support (x was the controlled V content).

Temperature-programmed calcination of the impregnated sample was also performed using multichannel Hiden HP R-20QIC mass spectrometer to monitor the gaseous products formed. Firstly, 100 mg sample was added into a stainless steel U-tube. Then the sample was swept using helium at room temperature for nearly 2 h until the baseline of MS signals stabilized. After that, the temperature was raised to 550 °C with a ramp of 5 °C min⁻¹ in flow air (20 mL min⁻¹). The signals of exhausted gas were monitored and a quantity of ammonia, a small amount of nitrous oxide and nitric oxide with MS signals of m/z = 17, 44 and 30 were observed. The evolution of the NO_x demonstrated the oxidation of ammonia, probably by V⁵⁺ species, which was reduced to V⁴⁺.

2.2. Catalyst characterization

The actual V content loaded on the catalysts was determined by Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES) (IRIS Advantage ER/S).

X-ray diffraction (XRD) measurement of the catalysts were performed on a SHIMAZU XRD-6100 diffractometer using Cu-kα radiation (λ = 0.15406 Å) with a scanning angle (2θ) from 5 to 80°. The tube voltage was 40 kV and the current was 30 mA.

Fourier transform infrared (FTIR) spectra were collected at 2 cm⁻¹ resolution using a Nicolet Nexus 670 FTIR spectrometer equipped with a mercury cadmium telluride detector. The powder samples were mixed with KBr (1 wt%) and pressed into translucent disks at room temperature.

The surface area, pore volume, and average pore size distribution were measured by N₂ physisorption analysis at liquid nitrogen temperature (77 K) using a Quantachrome Autosorb-iQ instrument. The samples were degassed at 573 K for 4 h before adsorption measurement. The surface area was calculated by BET equation with a Micropore BET Assistant, and the micropore surface area and micropore volume were determined by V–t method. Moreover, the pore size distribution was calculated by Saito–Foley method.

X-ray Photoelectron spectroscopy (XPS) was used to determine the surface species and their composition on the samples. XPS experiments were carried on an AXIS Ultra DLD (KRATOS) high performance electron spectrometer with Al Kα radiation (1486.6 eV) and the source being operated at 12 kV and 12 mA. The obtained binding energy (BE) was calibrated using the C 1s peak at 284.6 eV as the reference, and the linear background was subtracted from all the spectra and peak fitting was performed using an 80/20 Lorentz–Gauss function.

Diffuse reflectance ultraviolet-visible spectroscopy (DR UV-vis spectroscopy) spectra was obtained on a SHIMADZU UV-3600 in the range from 190 to 850 nm using BaSO₄ as the reference. The spectra were deconvoluted into sub-bands by Peakfit v4 software.

Temperature-programmed hydrogen reduction (H₂-TPR) was utilized to characterize the samples with different vanadium loading. The sample (100 mg) was placed in a fixed-bed reactor and pretreated at 423 K for 30 min in N₂ with a flow rate of 30 mL min⁻¹. After the temperature was decreased to 353 K, the sample was heated from 353 K to 1103 K with a ramp of 5 K min⁻¹ in 5% H₂/Ar flow. The consumption of H₂ was recorded by a GC (FULI-9790) equipped with a thermal conductivity detector (TCD).

2.3. Measurement of catalytic activity

Partial oxidation of ethylbenzene (EB) was carried out in a 50 mL two-necked round flask equipped with a reflux condenser and a stirrer in a water bath. 8.2 mmol of substrate was added into acetonitrile solvent with the designed amount of catalyst. When the water bath temperature reached 70 °C and kept constant, hydrogen peroxide (HP) aqueous solution (30 wt%) was added to the reaction flask. The resulting mixture was refluxed for designed time, and then cooled to room temperature. After filtration of the reaction mixture, the liquid products were identified by GC-MS analyses (Agilent, 5973 NETWORK 6890 N), and analyzed quantitatively by GC (FULI, GC-9790 equipped with hydrogen flame detector and a capillary column AT, SE-54) using nitrobenzene as internal standard. The products included acetophenone (AP), benzaldehyde (BD), 1-phenylethanol (1-PE), 2-phenylethanol, ethylphenols and 2-ethylcyclohexa-2,5-diene-1,4-dione (EQ). The yields were calculated by the amount of product (mmol) over the amount of initial ethylbenzene (mmol) added. The selectivity was calculated by the amount of product (mmol) over the amount of all the products (mmol), ignoring the ring-opening products, tar and polycondensed aromatic hydrocarbons because no such products were observed, avoiding the interference of volatility of ethylbenzene.³⁹ The reaction scheme of ethylbenzene is in Scheme S1 (see ESI†). The conversion (conv.) of EB is the sum of all yields of the products. The yield (Y_i, i = AP, BD, 1-PE, 2-PE, ethylphenols and EQ) of products, the selectivity (S_i, i = AP, BD, 1-PE, 2-PE, ethylphenols, EQ, side-chain and ring) are calculated as follows:

3. Results and discussion

3.1. Catalyst characterization

3.1.1. ICP-AES. The results of ICP-AES are displayed in Table 1. Ammonium acetate can dissolve the extra-framework vanadium species while aqua regia can dissolve both the extra-framework and framework vanadium species.^40,41 Thus, vanadium content obtained from aqua regia dissolution might represent the actual total vanadium content, while that from ammonium acetate dissolution may represent the content of the extra-framework vanadium species. With the controlled vanadium contents increasing from 0.10 to 3.50 wt%, the total actual content of vanadium on the support rose monotonically from 0.09 to 2.86 wt%. Similarly, the content of extra-framework vanadium increased from 0.07 to 2.08 wt%. At low controlled vanadium loading of 0.10 and 0.50 wt%, only small amount of vanadium (0.02 and 0.04 wt% respectively) could be incorporated into the framework of HZSM-22. With the controlled vanadium varied from 1.00 to 2.50 wt%, the amount of framework vanadium kept almost the same (about 0.20 wt%). While further increase of controlled vanadium content made a significant increase of both extra-framework and framework vanadium species. It was indicated that the distribution of vanadium species varies with the controlled amount of vanadium in the preparation process, which dominated the amount of vanadium incorporated in the framework of HZSM-22 and that of extra-framework. In the preparation of vanadium silicate-1 catalyst, a threshold value of about 1.0 wt% vanadium was found to be favored to incorporate into the framework of silicate-1.⁴⁰ The 0.20 wt% vanadium incorporation over the range of 1.00 to 2.50 wt% controlled vanadium content is similar to the previous work.⁴⁰ However, with higher controlled amount of vanadium, the amount of incorporated vanadium greatly increased. This might be caused by the nature of the support.

Table 1 The distribution of vanadium species on VO_x/HZSM-22 depending on the controlled loading

Entry	Vanadium content (wt%)
	Controlled	Total	Extra-framework	Framework
1	0.10	0.09	0.07	0.02
2	0.50	0.36	0.32	0.04
3	1.00	0.71	0.52	0.19
4	1.50	0.96	0.75	0.21
5	2.50	1.81	1.59	0.22
6	3.50	2.86	2.08	0.78

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3.1.2. XRD. Fig. 1 shows the XRD patterns of the fresh catalysts with different V loading. The diffraction patterns of the samples with actual vanadium loading of 0.09–0.96 wt% exhibited no significant difference from those of HZSM-22, indicating that the vanadium species were highly dispersed because of the low loading. However, crystalline V₂O₅ with diffraction peaks at around 15.4°, 21.6° based on the literature^42–44 was observed when the vanadium content increased from 1.81 to 2.86 wt%. It is indicated that higher loading led to the aggregation of vanadium species on HZSM-22. The formation of crystalline V₂O₅ with increasing vanadium content was also observed on mesoporous SBA-15, and the critical content was found to be about 2.8 wt%, which was higher than the value observed in the present work (1.81 wt%).⁴⁵ The different properties of the supports might be the reason for this variation.

Fig. 1 The XRD patterns of the fresh catalysts with different V loading and pure vanadium pentoxide. (a) HZSM-22, (b) 0.09 wt% VO_x/HZSM-22, (c) 0.36 wt% VO_x/HZSM-22, (d) 0.71 wt% VO_x/HZSM-22, (e) 0.96 wt% VO_x/HZSM-22, (f) 1.81 wt% VO_x/HZSM-22, (g) 2.86 wt% VO_x/HZSM-22, (h) V₂O₅.

3.1.3. FTIR. The FTIR spectra of the sample are shown in the Fig. 2 and S1.† The bands at about 550 cm⁻¹ and 640 cm⁻¹ were attributed to the bending vibration of the framework Si–O–Si bond of HZSM-22, while the bands at around 1090 cm⁻¹ were assigned to asymmetric stretching vibration of Si–O–Si.^46,47 The bands at around 1200 cm⁻¹ were ascribed to Al–O bond, and the shifts of these bands to higher wavenumbers were indicative of vanadium species incorporated into framework of HZSM-22.^48,49 There were no significant changes in FTIR with low actual vanadium loading of 0.09–0.71 wt% compared to HZSM-22. Over the vanadium loading range of 0.96–1.81 wt%, the intensity of the bands at 550, 640 cm⁻¹ increased with increasing vanadium loading, while with the vanadium loading increasing to 2.86 wt%, the intensities of these bands had a further increase. Moreover, the band at around 1090 cm⁻¹ had a blue shift. It might be deduced that the vanadium species that is incorporated into framework vanadium species affected significantly the Si–O–Si bond of HZSM-22.

Fig. 2 FTIR spectra of samples with different vanadium content: (a) HZSM-22, (b) 0.09 wt% VO_x/HZSM-22, (c) 0.36 wt% VO_x/HZSM-22, (d) 0.71 wt% VO_x/HZSM-22, (e) 0.96 wt% VO_x/HZSM-22, (f) 1.81 wt% VO_x/HZSM-22, (g) 2.86 wt% VO_x/HZSM-22.

3.1.4. N₂ physisorption analysis. The N₂ adsorption–desorption isotherms and the textural properties of VO_x/HZSM-22 are displayed in Table 2, Fig. S2 and S3.† As the vanadium content increased from 0.09 to 2.81 wt%, the BET surface area, micropore surface area and micropore volume decreased significantly from 255.7 to 63.6 m² g⁻¹, 196.7 to 16.2 m² g⁻¹ and 0.079 to 0.008 cm³ g⁻¹, respectively, because of the presence of framework and extra-framework vanadium species. Nevertheless, the external surface area only changed in the range of 41.3–67.9 m² g⁻¹. At low vanadium content of 0.09 wt%, the changes of BET surface area, micropore surface area and micropore volume was not obvious. However, with increasing vanadium content, the surface area and the pore volume decreased significantly. The slight decrease of BET surface area and pore volume with 0.09 wt% vanadium content might be caused by the incorporation of V into the framework and/or the anchor of vanadium on the wall of the pores of HZSM-22. With increase of vanadium content, a certain amount of vanadium species blocked the micropores of the catalysts, causing dramatic drop of surface area and pore volume. The formation of crystalline V₂O₅ species made further decrease of these properties, which was similar to the results reported in literature.⁵⁰ The shift of largest pore size to bigger value with increasing vanadium from 0.09 to 2.86 wt% confirmed the above deduction.

Table 2 Physicochemical properties of the catalysts

Samples	SBET (m^2 g^−1)	Smic (m^2 g^−1)	Sext (m^2 g^−1)	Vmic (cm^3 g^−1)
HZSM-22	255.7	196.7	59.0	0.079
0.09	238.4	180.3	58.1	0.074
0.36	178.3	137.0	41.3	0.055
0.71	167.0	99.1	67.9	0.041
0.96	165.4	105.5	59.9	0.043
1.81	149.2	90.0	59.3	0.037
2.86	63.6	16.2	47.3	0.008

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3.1.5. XPS. The XPS results are given in the Table 3 and Fig. S4.† Clearly, as the content of actual vanadium increased from 0.09 to 2.86 wt%, the surface vanadium content increased monotonically. All the peaks at around 517.0 eV were fitted into two peaks. The former one around 517.5 eV was ascribed to the V⁵⁺, while the latter one around 516.5 eV was assigned to V⁴⁺.^40,51 The presence of V⁴⁺ suggested that small quantities of framework and extra-framework V⁵⁺ were reduced by the ammonia in the calcination process, which was confirmed by the results of mass spectroscopy. The V⁵⁺/V⁴⁺ decreased monotonically with increasing vanadium content. The co-existence of V⁴⁺ and V⁵⁺ agreed with citations,^52–54 and the balance between the reduction of V⁵⁺ to V⁴⁺ by NH₃ released and the oxidation of V⁴⁺ to V⁵⁺ by air deserved further investigation.

Table 3 Surface compositions of VO_x/HZSM-22 determined by XPS

Catalyst (wt%)	V 2p3/2 (V)			V 2p3/2 (IV)			V 2p3/2 (V)/V 2p3/2 (IV)
	BE (eV)	Area (%)	FWHM	BE (eV)	Area (%)	FWHM
0.09	517.5	88.49	1.4	516.5	11.51	0.8	7.7
0.36	517.5	87.30	1.4	516.5	12.70	0.8	6.9
0.71	517.5	86.94	1.5	516.5	13.06	1.0	6.7
0.96	517.4	73.65	1.6	516.4	26.35	1.2	2.8
1.81	517.5	70.45	1.6	516.2	29.55	1.2	2.4
2.86	517.6	69.23	1.6	516.5	30.77	1.2	2.2

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3.1.6. DR UV-vis spectroscopy. Fig. S5† displays the DR UV-vis spectroscopy spectra of all the catalysts. According to the literature,^3,55,56 the bands centered at around 220 nm were assigned to V⁴⁺ characteristic of tetrahedral coordination, those at around 250–260 nm to framework V⁵⁺ with tetrahedral coordination, while those at around 380 nm to V⁵⁺ with either pentagonal or pseudo-octahedral coordination, due to interaction with water molecules.⁵⁷ As shown in Fig. S5,† all the catalyst samples give DR UV-vis reflection spectra over the range of 190–850 nm and exhibited strong low charge-transfer (CT) bands below 500 nm. To study them in detail, the spectra were fitted into various sub-bands, as shown in Fig. S6,† and the quantitative results obtained from the Peakfit v4 are shown in Table 4 and the peak area of the three types of peaks at around 220, 250 and 380 nm were denoted as S₁, S₂ and S₃, respectively. The intensity of all the peaks increase consistently as the vanadium content increasing, but the relative intensity of different sub band varied in different ways. With the increase of vanadium loading, the peak area at around 220 nm increased monotonically, which might represent the increase of V⁴⁺ incorporated into the framework of the HZSM-22,⁵⁵ that is, with low vanadium loading of 0.09–0.96 wt%, the V⁴⁺ with tetrahedral coordination increased monotonically. With further increase of vanadium loading from 1.81 to 2.86 wt%, the V⁴⁺ with tetrahedral coordination kept almost the same. A slight red shift from vanadium content of 0.09–2.86 wt% was also observed, which may be assigned to the longer bond length of V–O than that of Si–O.⁵⁸ The bands at around 250 nm may represent the framework V⁵⁺.⁵⁹ Over the vanadium content tested, the amount of framework V⁵⁺ increased monotonically. Moreover, the ratio of S₁/S₂ decreased monotonically, indicating that the V⁴⁺ may decrease while the framework V⁵⁺ may increase monotonically. With the vanadium content of 0.09–2.86 wt%, the bands at around 380 nm representing V⁵⁺ with either pentagonal or pseudo-octahedral coordination increased gradually. There was a further dramatic increase as the vanadium loading increased from 1.81 to 2.86 wt%,⁵⁷ and the color of all the calcined VO_x/HZSM-22 transformed from white to yellowish when they were exposed to the atmosphere at room temperature, intensifying with the increase of vanadium content.⁶⁰ This could be resulted from the additional coordination of water molecules to form octahedral coordinated vanadium ions in the microporous defect sites.⁶¹ The bands at approximately from 600 to 800 nm were corresponded to the d–d transition of isolated vanadyl VO²⁺ ions with tetrahedral coordination, but their intensity was very low compared with the major bands.^56,62–64

Table 4 Quantitative results of the DR UV-vis sub-bands

Catalyst (wt%)	S1	S2	S3	S1/S2
0.09	4.6	4.5	0.0	1.0
0.36	7.7	9.9	8.3	0.8
0.71	17.9	29.7	39.1	0.6
0.96	25.5	39.3	40.6	0.6
1.81	28.1	67.5	84.8	0.4
2.86	28.4	84.3	126.4	0.3

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3.1.7. H₂-TPR. The TPR results are presented in Fig. 3. Obviously, we can find three peaks at approximately 658–677 K, 760–780 K and 890 K, which might be assigned to isolated vanadium species, polymeric vanadium species and bulk-like V₂O₅, respectively.^65–67 As the content of vanadium increased from 0.09 to 2.86 wt%, the first peak shifted slightly to higher temperature with slight increase of intensity, because of the increase of isolated vanadium species. Additionally, the second peak changed in a similar way, but the intensity for the samples with vanadium content of 2.86 wt% increased sharply compared with the other catalysts with vanadium loading 0.09–1.81 wt%, meaning that a large quantity of polymerized vanadium species formed. The third peak only occurred on the sample containing 2.86 wt%, indicating that the bulk-like V₂O₅ was produced, which is in accordance with the XRD results.

Fig. 3 H₂-TPR of all the catalysts: (a) 0.09 wt% VO_x/HZSM-22, (b) 0.36 wt% VO_x/HZSM-22, (c) 0.71 wt% VO_x/HZSM-22, (d) 0.96 wt% VO_x/HZSM-22, (e) 1.81 wt% VO_x/HZSM-22, (f) 2.86 wt% VO_x/HZSM-22.

3.2. Catalytic oxidation of ethylbenzene

The results of activity test in terms of ethylbenzene conversion, acetophenone yield, and selectivity to different products are shown in Tables 5–9.

Table 5 Effect of V loading on the oxidation of EB^a

Catalyst (wt%)	Conv. (%)	SAP (%)	Sside-chain (%)	Sring (%)	YAP (%)	YBD (%)	Y1-PE (%)	YEQ (%)	Yethylphenols (%)
a Reaction condition: 200 mg of VOx/HZSM-22, 70 °C, 4 h, 49.9 mmol 30% H2O2, 8.2 mmol EB, 10 mL acetonitrile.
HZSM-22	0.8	63.3	100.0	0.0	0.5	0.2	0.1	0.0	0.0
0.09	0.6	50.0	83.3	16.7	0.3	0.1	0.1	—	0.1
0.36	5.5	52.7	83.6	16.4	2.9	0.6	1.1	0.1	0.8
0.71	15.0	64.7	90.0	10.0	9.7	1.8	1.9	0.6	0.9
0.96	19.3	72.5	94.3	5.7	14.0	2.1	2.0	0.3	0.8
1.81	24.4	71.7	92.6	7.4	17.5	2.3	2.7	0.1	1.7
2.86	17.8	71.3	94.4	5.6	12.7	2.0	2.0	0.1	0.9

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Table 6 Effect of amount of catalysts on the oxidation of EB^a

Catalyst (mg)	Conv. (%)	SAP (%)	Sside-chain (%)	Sring (%)	YAP (%)	YBD (%)	Y1-PE (%)	YEQ (%)	Yethylphenols (%)
a Reaction condition: 1.81 wt% VOx/HZSM-22, 70 °C, 4 h, 49.9 mmol 30% H2O2, 8.2 mmol EB, 10 mL acetonitrile.
25	4.0	57.5	85.0	15.0	2.3	0.4	0.7	0.1	0.5
50	12.0	63.3	91.7	8.3	7.6	1.5	1.8	0.3	0.7
100	18.1	68.5	91.7	8.3	12.4	2.0	2.2	0.2	1.3
200	24.4	71.7	92.6	7.4	17.5	2.3	2.7	0.1	1.7
300	18.6	72.0	93.5	6.5	13.4	1.9	2.1	0.1	1.1
400	16.0	67.5	90.6	9.4	10.8	1.8	1.8	0.4	1.1

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Table 7 Effect of reaction time on the oxidation of EB^a

Reaction time (h)	Conv. (%)	SAP (%)	Sside-chain (%)	Sring (%)	YAP (%)	YBD (%)	Y1-PE (%)	YEQ (%)	Yethylphenols (%)
a Reaction condition: 1.81 wt% VOx/HZSM-22, 200 mg catalyst, 70 °C, 49.9 mmol 30% H2O2, 8.2 mmol EB, 10 mL acetonitrile.
1	16.9	72.2	93.5	6.5	12.2	1.8	1.8	0.8	0.3
2	18.6	72.6	94.1	5.9	13.5	1.9	2.1	0.2	0.9
3	21.0	74.3	94.8	5.2	15.6	2.0	2.2	0.1	1.0
4	24.4	71.7	92.6	7.4	17.5	2.3	2.7	0.1	1.7
5	19.8	71.7	93.9	6.1	14.2	2.0	2.2	0.0	1.2
6	20.4	71.1	92.6	7.4	14.5	2.1	2.2	0.1	1.4

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Table 8 Effect of reaction temperature on the oxidation of EB^a

Reaction temperature (°C)	Conv. (%)	SAP (%)	Sside-chain (%)	Sring (%)	YAP (%)	YBD (%)	Y1-PE (%)	YEQ (%)	Yethylphenols (%)
a Reaction condition: 1.81 wt% VOx/HZSM-22, 200 mg catalyst, 4 h, 49.9 mmol 30% H2O2, 8.2 mmol EB, 10 mL acetonitrile.
30	2.3	43.5	60.9	39.1	1.0	0.1	0.3	0.3	0.5
40	6.0	51.7	73.3	26.7	3.1	0.5	0.8	0.8	0.8
50	11.5	62.6	83.5	16.5	7.2	1.0	1.4	1.0	0.9
60	15.7	70.7	89.8	10.2	11.1	1.3	1.7	0.4	1.2
70	24.4	71.7	92.6	7.4	17.5	2.3	2.7	0.1	1.7
80	19.7	73.6	97.0	3.0	14.5	2.3	2.2	0.2	0.4

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Table 9 Testifying whether the catalyst is heterogeneous^a

Catalyst	Conv. (%)	SAP (%)	Sside-chain (%)	Sring (%)	YAP (%)	YBD (%)	Y1-PE (%)	YEQ (%)	Yethylphenols (%)
a Reaction condition: 1.81 wt% VOx/HZSM-22, 200 mg catalyst, 70 °C, 4 h, 49.9 mmol 30% H2O2, 8.2 mmol EB, 10 mL acetonitrile.
Used solvent	1.0	20.0	40.0	60.0	0.2	0.1	0.1	—	0.6
Used catalyst	17.0	74.1	93.5	6.5	12.6	1.6	1.6	0.2	0.9
Fresh	24.4	71.7	92.6	7.4	17.5	2.3	2.7	0.1	1.7

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3.2.1. The influence of vanadium loading on the activity. The catalytic performance of the samples and the support for the partial oxidation of ethylbenzene is shown in Table 5. A 0.8% conversion of ethylbenzene with 63.3% selectivity was obtained on HZSM-22. This is negligible compared to the results obtained on the catalysts. Thus, the support might just have effect on the vanadium species to form active sites, and then affect the reaction. The partial oxidation of EB in the present system could be classified into two categories, the side-chain oxidation producing BD, 1-PE, etc.; and benzene ring oxidation producing phenols. It was obvious that over the vanadium loading range tested, the selectivity of side-chain C–H bond oxidation was higher than that of the ring C–H bond oxidation. As the vanadium loading increased, the selectivity of side-chain increased but that of the ring decreased. The conversion of EB increased with vanadium content firstly, and reached the maximum value of 24.4% at 1.81 wt% vanadium content, whereas further increase of vanadium content to 2.86 wt% decreased it to 17.8%. This variation trend is in accordance with that of the amount of extra-framework vanadium species from 0.07 to 1.59 wt%. It implied that highly dispersed extra-framework vanadium species might be responsible for the partial oxidation of EB as well as the yield of AP, BD and 1-PE. With further increase of the vanadium loading to 2.86 wt%, although the extra-framework vanadium species increased dramatically, the formation of crystalline V₂O₅ might not be beneficial for the conversion of EB. XPS results showed that with the vanadium loading increasing, the surface V⁴⁺ increased monotonically. It might imply that the surface V⁴⁺ extra-framework vanadium species was beneficial for the conversion of EB and other side-chain products. TPR results also confirmed that those species were isolated vanadium species. With the vanadium content increasing from 0.09 to 0.96 wt%, the selectivity of AP increased from 50.0 to 72.5%, and with further increase of vanadium content to 1.81 wt%, it decreased slightly to 71.7% while on 2.86 wt% catalyst, the selectivity decreased further to 71.3%. These data proved further that highly dispersed extra-framework vanadium species were beneficial for the selectivity of AP while oligomerized vanadium species might result in deeper oxidation. However, the presence of oligomer of aggregated V species and crystalline V₂O₅ would decrease the selectivity of AP and side-chain of C–H bond oxidation.

3.2.2. The influence of reaction parameters.
The influence of the amount of catalysts. The effect of the amount of catalysts on the activity is demonstrated in Table 6. With the amount of catalysts increasing from 25 mg to 200 mg, the conversion of EB increased from 0.6 to 24.4%, and then decreased to 16.0% with the amount of catalyst of 400 mg. Besides, the selectivity of AP increased from 50.0 to 71.7%, and kept almost the same with 300 mg catalyst (72.0%) then decreased to 67.5%. Obviously, this reaction relies heavily on the amount of catalyst. However, more amounts of catalysts may result in the decomposition of a large volume of hydrogen peroxide, being disadvantageous for the selective reaction of EB to AP, to 200 mg of catalyst is suitable for the present system.
The influence of reaction time. The results obtained with different reaction time on the catalytic oxidation of EB over VO_x/HZSM-22 are listed in Table 7. The yield of AP increased gradually from 16.9 to 24.4% with the reaction time of 1–4 h. The yield of by-products of BD, 1-PE and ethylphenols changed in the same way, but the selectivity of AP did not vary significantly. Nevertheless, when the reaction time exceeded 4 h, the yield of all products decreased continuously. The apparent decrease of the conversion of EB might be caused by the fact that the sum of the yield of the products decreased while the formation of other products that could not be detected by our present techniques did not contributed to the conversion calculation.
The influence of temperature. As shown in Table 8, as the temperature rose from 30 to 70 °C, the yield of AP increased monotonically, and then decreased, the optimal reaction temperature was 70 °C under the reaction conditions of hydrogen peroxide/EB molar ratio of 6.1 : 1, 10 mL acetonitrile as solvent, and 4 h of reaction time, where the highest yield of AP was 17.5%. By contrast, the selectivity of AP increased monotonically until the reaction temperature of 80 °C, reaching 73.6%. Moreover, with the temperature increasing to above 60 °C, the self-decomposition of hydrogen peroxide was extremely serious, resulting in the decrease of the conversion of EB.⁶⁸

The effect of several solvents (ethanol, acetone, isopropyl, ethyl acetate, n-butanol, glycol, methanol, acetic acid and acetonitrile), the amount of H₂O₂ used and the amount of CH₃CN was investigated. However, the best yield of ethylbenzene and selectivity remained 17.5% and 72.5%, respectively. Further work is needed to get better results.

Heterogeneous nature of the catalytic reaction. To study whether the selective oxidation of EB is heterogeneous, the VO_x/HZSM-22 was added to a 50 mL two-necked round-bottom flask, and then adding 10 mL acetonitrile, and let them keep 4 h in a water bath with a reflux condenser at 70 °C. After four hours, the mixture was filtered to obtain the used solvent and used catalyst, 8.2 mmol EB, 10 mL acetonitrile and 49.9 mmol 30% H₂O₂ were added to the round-bottom flask with used catalyst and kept for 4 h at 70 °C. The same procedure was employed to the used solvent. All the results combing with the results of fresh VO_x/HZSM-22 are shown in Table 9. The used solvent showed a slight conversion of EB but the used catalyst showed 17.0% conversion of EB. Both the conversion of EB and the selectivity of AP were lower than that of the fresh VO_x/HZSM-22. It demonstrated that the reaction is heterogeneous and a small amount of vanadium species was leached in the solvent. The stability of the catalyst deserved further study.
The recyclability of the catalyst. The 1.81 wt% VO_x/HZSM-22 was reused for two times by conducting the same experiments with the used catalyst. The results are shown in the Table S1 (ESI†). After second times' usage, the catalyst almost lost the catalytic activity because the catalyst had a 55.6% leaching rate from the results of the ICP-AES. At first, to explore if carbon deposition deactivated the catalyst, the used catalyst was calcined at 550 °C for 6 h and then reused to test the catalytic activity. The results are shown in Table S1 (ESI†). The conversion of ethylbenzene using the regenerated catalyst was a little higher than that of the second circle (without calcination). The above data indicated that the origin of the deactivation of the catalyst was not carbon deposition. In order to figure out further the reason of activity decrease, we performed another contrast experiment. The designed catalyst, acetonitrile and hydrogen peroxide were added into the round-bottom flask under reaction conditions and kept for 4 h. Then filtrating the mixture to obtain solution and used solid catalyst, the solution and used solid catalyst were used to do the same catalytic experiment, respectively. The results are shown in Table S1.† The conversion of ethylbenzene was only around 2.1% by using the solution obtained. When the treated solid catalyst was used, the conversion of ethylbenzene decreased significantly, only 2.2% ethylbenzene conversion was obtained. Although the catalytic reaction is heterogeneous, the hydrogen peroxide can dissolve a large amount of extra-framework vanadium species. Thus, enhancing the stability of the catalyst remained one challenge.

4. Conclusions

The results showed that the vanadium species could be incorporated into the framework and/or existed extra-framework of VO_x/HZSM-22 depending on the loading. The highly dispersed extra-framework vanadium species was advantageous for the selective oxidation of ethylbenzene with the EB conversion of 24.4% and good AP selectivity of 72.5%. Specifically, the surface extra-framework V⁴⁺ was responsible for the conversion of EB as well as yield of AP, BD and 1-PE. Furthermore, the formation of crystalline V₂O₅ was disadvantageous for both the conversion of ethylbenzene and side-chain oxidation.

Acknowledgements

This work is financially supported by National Natural Science Foundation of China (No. 21372167), and the technical support from the Analytical and Testing Centre of Sichuan University are cordially acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra05906f

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