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Highly efficient V-Mo-Fe-O catalysts for selective oxidation of toluene to benzaldehyde
Catalysis Communications 86 (2016) 72–76
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Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Highly efficient V-Mo-Fe-O catalysts for selective oxidation of toluene to benzaldehyde Huan Xia a,b, Zili Liu a,b,⁎, Yangyang Xu a,b, Jianliang Zuo a,b, Zuzeng Qin c,⁎ a b c
School of Chemistry & Chemical Engineering, Guangzhou University, Guangzhou 510006, PR China China Key Laboratory of Ministry of Education for Water Quality Security and Protection in Pearl River Delta, Guangzhou 510006, PR China School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, PR China
a r t i c l e
i n f o
Article history: Received 20 May 2016 Received in revised form 2 August 2016 Accepted 4 August 2016 Available online 06 August 2016 Keywords: V-Mo-Fe-O Selective oxidation Toluene Benzaldehyde
a b s t r a c t The V-Mo-Fe-O catalysts were synthesized by a sol-gel method, characterized and used for the selective oxidation of toluene to benzaldehyde with hydrogen peroxide. The results show that V-Mo-Fe-O catalyst is an effective catalyst, exhibiting a toluene conversion of 40.3%, and a benzaldehyde selectivity of 84.5% at 80 °C. Scanning electron microscopy (SEM) shows that the catalyst has nanorod structure with a thin layer compound, which may be constitutive of iron molybdate while the main compound of catalyst is MoV2O8. Furthermore, the catalyst can be easily recycled and reused for four times without significant changes in its activity. © 2016 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Toluene is an aromatic compound which provide three C\\H bonds, and can be oxidized to benzyl alcohol, benzaldehyde, and benzoate [1– 3]. Among them, benzaldehyde is widely used as solvents, perfumes, dyes, and plasticizers. Traditionally, benzaldehyde is produced by chlorinating of –CH3 functionality of toluene and followed by saponifying [4, 5], which may discharge waste with a low benzaldehyde yield. Moreover, the products in this method are limited to utilization in food industries and pharmaceuticals due to the chlorinated contaminants. Therefore, it is important that an effective and environmental friendly way use in the oxidation of toluene to benzaldehyde, and the attempt to resolve this problem was the liquid-phase oxidation of toluene to benzaldehyde [6–9], however, the toluene conversion or the benzaldehyde yield was still very low [10–12]. In our previous studies, we found that V-Mo-O composite oxides can be used as a catalyst for oxidation toluene to benzaldehyde in the hydrogen peroxide at 80 °C under atmospheric pressure for only 30 min, with the toluene conversion of 38.9% and the benzaldehyde selectivity of 69.7% [13]. In the present study, to further increasing the toluene conversion and benzaldehyde selectivity, Fe was introduced to the V-MoO catalysts, and the V-Mo-Fe-O catalysts were used in the liquidphase oxidation of toluene, and the effects of Fe contents on the structure of the V-Mo-Fe-O catalysts were investigated.
2.1. Catalyst preparation
⁎ Corresponding authors. E-mail addresses: [email protected] (Z. Liu), [email protected] (Z. Qin).
http://dx.doi.org/10.1016/j.catcom.2016.08.008 1566-7367/© 2016 Elsevier B.V. All rights reserved.
The V-Mo-Fe-O catalysts were prepared via a sol-gel method according to literature [13]. NH4VO3, (NH4)6Mo7O24·4H2O, and Fe(NO3)3·9H2O were dissolved in 120 mL distilled water at a V:Mo:Fe mole ration of 6:7:X (X = 0, 0.5, 1.0, 1.5, 2.0, and 6.0), and stirred at 80 °C for 10 min. To avoid the rapid precipitation of the macromolecular inorganics and improve the dispersion of the active component, the citric acid [(C6H8O7·H2O), n(C6H8O7·H2O) / n((NH4)6Mo7O24) = 8 / 1] was added to the mixed water solution and stirred for 1 h to obtain a sol. Subsequently, the mixed sol was heated up to 100 °C and stirred for 2 h to form a gel. The gel was dried at 80 °C under vacuum for 8 h, and then calcined at 550 °C for 3 h in air. All the chemical agents were analytical grade without further purification. 2.2. Catalyst characterization Powder X-ray diffractometer (XRD) patterns were employed on a Purkinje MASAL XD-3 diffractometer with Cu Kα radiation (36 kV and 20 mA). The morphology and surface texture of the catalysts were obtained on a JEOLJSM-7001F scanning electron microscope (SEM) using a 10 kV energy source under vacuum. Energy dispersive X-ray spectroscopy (EDS) was used in connection with SEM for the elemental analysis. X-ray photoelectron spectrometer (XPS) was conducted to determine the states of elements on the catalyst surface using a Shimadzu Axis Ultra DLD X-ray photoelectron spectrometer with acrochromatic Al Kα radiation (1486.6 eV) at 10 mA and 12 kV. The H2-temperature
H. Xia et al. / Catalysis Communications 86 (2016) 72–76
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programmed reduction (H2-TPR) was performed in a Micromeritics AutoChem II 2920 in a flow of 30 mL/min 5% H2/Ar mixed gas and with a temperature ramp of 10 °C/min. 2.3. Oxidation of toluene Liquid phase oxidation of toluene was carried out in a three-neck round-bottom flask, and 0.2 g V-Mo-Fe-O catalysts, 1.75 g toluene, and 15 mL acetic acid, acetonitrile, or ethyl acetate using as solvent, were added, and 30 wt% H2O2 was subsequently added dropwise, and reacted at 80 °C for 30 min. After the reaction, the solid catalysts were separated by filtration, and the products were analyzed by using a gas chromatograph (Shimadzu, GC-2014) equipped with a HP5 capillary column and a flame ionization detector (FID), and using the peak-area internal standard method with anisole as the internal standard. 3. Results and discussion
Fig. 1. XRD patterns of V-Mo-Fe-O catalysts (molar ration of V/Mo/Fe = 6.0/7.0/X) with X = 0 (a), 0.5 (b), 1.0 (c), 1.5 (d), 2.0(e), and 6.0 (f).
3.1. Effects of the modified metal on the selective oxidation Ternary metal oxide V-Mo-O was modified by a variety of transition metal oxides to prepare V-Mo-M-O (M = Co, Cr, Ce, La, Fe) catalysts through a sol-gel method, and these catalysts were used for selective oxidation of toluene to benzaldehyde, the results are shown in Table S1 (in Appendix A. Supplementary data). In our study, the main products were benzaldehyde and benzoic acid, accompanied with a small amount of benzyl alcohol and ester. Table S1 shows that MoO3 show relatively low toluene conversion of 3.2%, and after modified by Vanadium, the toluene conversion increased sharply. From Table S1, the second metal oxides-introduction to V-MoO had a positive influence on the selective oxidation of toluene to benzaldehyde, especially the benzaldehyde selectivity increased obviously. The V-Mo-O catalysts modified with Fe show the greatest benzaldehyde selectivity (84.5%) among the used 8 catalysts. The benzaldehyde selectivity and yield increased 14.8% and 7.0% for V-Mo-Fe-O while compared with V-Mo-O catalyst, respectively, which also much higher than that used for single catalyst, such as V2O5 or MoO3. Therefore, Fe was chosen as the optimal catalyst additive for further investigation. The amount of modifier in the catalyst will affect the catalytic performance of the catalyst, therefore, it is necessary to study the optimal Fe amount in the V-Mo-O catalyst. The V-Mo-Fe-O catalysts with different Fe amounts were prepared at a constant vanadium and molybdenum atom ratio (V/Mo = 6/7) [13], and the effects of Fe amounts, presented by X, on the toluene oxidation to benzaldehyde was shown in Table S1. It can be seen that the toluene conversion, benzaldehyde selectivity and yield were shown in the same change tendency, i.e., an optimal Fe amount was found at X was 1.0, and X = 1.0, the toluene conversion, benzaldehyde selectivity and yield was 40.3%, 84.5%, and 34.1%, respectively. Therefore, the following studies conducted at a Fe amount of Fe:V = 1:6.
which were assigned to the formation of Fe2(MoO4)3 [14], No any new peak appears in the pattern of V-Mo-Fe-O, which can exclude the cases that Fe species are existed in other new crystal form or loaded on framework in Fe(NO3)3 form. Combined with the data in Table S1 and literature [13], it found that MoV2O8 is the main component in the V-Mo-Fe-O catalysts, and appropriate amount Fe addition would increase the benzaldehyde selectivity, and the excessive amount of doping Fe would form Fe2(MoO4)3, which may cover the active sites of the MoV2O8 and led to a decreasing of toluene conversion and benzaldehyde selectivity, and the optimal V/Mo/Fe atom ratio was 6/7/1. Therefore, the following study would compare the V-Mo-O and V-Mo-Fe-O with a V/Mo/Fe atom ratio of 6/7/1. 3.3. X-ray photoelectron spectroscopy analysis The V 2p, Mo 3d, and Fe 2p XPS spectra of V-Mo-O and V-Mo-Fe-O are shown in Fig. 2 and Fig. S1, and the binding energy of the V, Mo, and Fe in the V-Mo-O and V-Mo-Fe-O were shown in Table S2. In Fig. 2, the binding energy at 232.82 eV (Mo 3d5/2) and 236.02 eV (Mo 3d3/2) in the Mo 3d of V-Mo-O catalyst were assigned to Mo6 + species in an oxidized surrounding of MoV2O8 and MoO3 [15], respectively, which were conformed to the existing of MoV2O8 and MoO3 from the XRD results. Additionally, after modified by Fe, Mo and Fe
3.2. X-ray diffraction analysis The XRD patterns of V-Mo-Fe-O catalysts with Fe content (X) of 0 to 6 were shown in Fig. 1. From Fig. 1, the introduced of Fe did not affect the formation of MoV2O8 due to obvious MoV2O8 diffraction peaks were found at 2θ = 18.5, 21.5, 23.5, 25.0, 27.5, 33.0 and 33.5°. Besides, the molybdenum in the form of MoO3 was observed at the peaks of 2θ = 13.0, 23.0, and 25.9°, which show a disappeared tendency with the increase of the Fe content. With low Fe content, the crystal size of Fe in the sample was too small to be detected by XRD. With the increasing of Fe amount from 0 to 2.0, the MoO3 and MoV2O8 amount decreased while the Fe2(MoO4)3 amount increased, suggesting MoO3 tend to form Fe2(MoO4)3. When the V/Mo/Fe atom ratio further increased to 6/7/6, the peaks at 2θ of 22.9, 24.5 and 27.5° appeared, and the peaks at 2θ of 33.0 and 33.5° reduced and tend to disappear,
Fig. 2. XPS spectra of Mo 3d regions for V-Mo-O and V-Mo-Fe-O catalysts.
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oxides could transform into iron molybdate (Fe2(MoO4)3) at 500 °C [16], and the chemical shifts of Mo 3d5/2 and Mo 3d3/2 were −0.14 eV and −0.18 eV, respectively, which indicated that the introduction of Fe formed an oxidized surrounding of Fe2(MoO4)3 and MoO3, and increased slightly electron cloud density of Mo. From Fig. S1, the binding energies of V 2p3/2 and V 2p1/2 of V-Mo-O catalyst were centered at 517.39 eV and 524.61 eV, respectively. Likewise, the V 2p3/2 and V 2p1/2 in the V-Mo-Fe-O catalyst were 517.38 eV and 524.59 eV showing that V occurred as the form of V5+ in MoV2O8 [19, 20]. As to the V-Mo-Fe-O catalyst at a V/Fe molar ratio of 6/1, the binding energy changes of V 2p3/2 and V 2p1/2 were very slight, indicating that the introduction of Fe did not affect the chemical environment of V element. For Fe 2p XPS spectra in V-Mo-Fe-O catalyst, the Fe 2p3/2 located at approximately 712.0 eV, and Fe 2p1/2 approximately located at 725.0 eV. The Fe 2p3/2 peak is fit into two peaks by the same peak fitting deconvolution technique, and the peaks centered at 711.96 and 714.07 eV can be assigned to Fe2+ and Fe3+ [17,18], respectively. However, the band position in the Fe 2p1/2 region shifts to a higher binding energy upon the presence of Fe-O-Mo, and electron transfer from Mo to Fe [18]. 3.4. SEM analysis V2O5 and MoO3 were easy to form composite oxides with different valence cations replacing each other for not only the ion radius of V5+ (0.59 Å) and Mo6+ (0.62 Å) but structure of their oxides is similar [21, 22]. Fig. 3 shows that both V-Mo-O and V-Mo-Fe-O catalysts are nanorod structure. The V-Mo-O catalyst (Fig. 3a) shows relatively regular nanorods shape, and V-Mo-Fe-O (Fig. 3b) shows agglomerated species covered at the surface of sample and spot (Fig. 3c) at the surface of “rods” which may be caused by external force giving rise to the stripping of catalyst, suggesting the existence of a thin layer compound covering the surface of V-Mo-Fe-O catalyst. The compositions of catalysts surface were estimated by EDS, and it was found the presence of V, Mo and Fe, with the molar ratio of V/Mo/Fe = 5.8/7.1/1. And a roughened surface with defects also found on the surface of V-Mo-Fe-O. It is the defect distributions and the impurities in crystals improved the performance of the catalysts [23,24]. 3.5. H2-TPR studies Fig. 4 was the H2-TPR profile of V-Mo-O and V-Mo-Fe-O. From Fig. 4, in the V-Mo-Fe-O catalyst, vanadium and molybdenum formed to MoV2O8 solid solution, which was confirmed by XRD and SEM, changing the liability of vanadium and molybdenum species. Hence, the H2-TPR peaks of V-Mo-O centered at 575 °C attributed to the reduction of MoO3, and the peak centered at 675 °C was the reduction of MoV2O8 [25,26]. To the V-Mo-Fe-O catalyst, the peaks at 560 °C, 590 °C and 650 °C were attributed to the reduction of MoO3, Fe2(MoO4)3, and MoV2O8 [27,28], respectively. The peak centered at 590 °C of V-MoFe-O catalyst disappeared in the V-Mo-O catalyst, indicated that Fe
Fig. 4. H2-TPR profiles of V-Mo-O and V-Mo-Fe-O catalysts.
affects the redox properties after modified the V-Mo-O catalyst. In addition, it can be assigned to the V\\Mo composite oxide being reduced at low temperature due to the modification of Fe. After adding Fe, the dispersions of Mo oxide were changed, which was advantageous to the reduction process [18]. Furthermore, the reactive Mo_O existed in the surface deformity of MoO3 would led to the production of oxygen vacancies around Mo6+, and accelerate the high valence Mo to become low valence Mo. For this reason, the reduction of the oxide of the active component was promoted by formation of low valence Mon+ [29]. The shift in the reduction peaks of the V-Mo-Fe-O catalyst demonstrates that the MoO3 in V-Mo-Fe-O was more easily reduced to a lower valence state, the lattice oxygen in the catalyst had a higher moving performance, and was easier reduced than V-Mo-O catalyst.
3.6. Effect of reaction conditions and the reusability of the catalysts The effect of reaction time on the oxidation of toluene was examined and results shown in Fig. 5, revealed that the conversion was rapidly increased with reaction time, achieving a toluene conversion of 40.3% and a benzaldehyde selectivity of 84.5% at 30 min. From 30 to 120 min, the toluene conversion increased from 40.3% to 46.9% while the benzaldehyde selectivity decreased from 84.5% to 54.3%. The amount of H2O2 in the system is an important factor affecting the oxidation of toluene, and the results were shown in Fig. S2(A). The increment in H2O2 molar ratio allowed oxidation of the side chain and consequently the toluene conversion increased. But the excess H2O2 amount caused the decrease of benzaldehyde selectivity. When the H2O2 molar ratio was above 8, both the toluene conversion and benzaldehyde selectivity were almost unchanged. The percentage of H2O2 consumed is determined by iodometric titration after the end of experiment and found to be in the range of 89% to 91%. This suggests that more than a stoichiometric amount of H2O2 is utilized in the reaction,
Fig. 3. SEM images of (a) V-Mo-O and (b, c) V-Mo-Fe-O catalysts.
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Acknowledgment The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (No. 21276054), and the Key Laboratory of Applied Surface and Colloid Chemistry (Shaanxi Normal University) (No. 2016017).
Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2016.08.008.
References
Fig. 5. The effects of reaction time on the toluene oxidation.
which could be due to the thermal decomposition of H2O2. Therefore, the optimal molar ratio of H2O2 to toluene was 6. The nature of the solvent has a strong influence on the oxidation of toluene. Therefore, three solvents with different acidity have been investigated. The results from Table S3 indicate that the effect of solvent on the catalytic activity is mainly related to the acidity of solvent. The acidity of solvent was found to be in the following order: ice acetic acid N acetonitrile N ethyl acetate, and both the toluene conversion and benzaldehyde selectivity increased with increasing acidity of the solvent, suggesting that proton in the solvent may be involved in the catalytic reaction, whereas the influence of the solvent acidity on catalytic reaction is complex. In addition, acetic acid has proper polarity which can turn the reactants and hydrogen peroxide to form homogeneous reaction system and favor the release of lattice oxygen of catalysts [30,31]. However, with the data in Table S3, the acetic acid was used as the optimal solvent. 3.7. Reusability of the catalyst Reuse of the recovery catalyst had also been studied to further evaluate performance of the catalyst. The catalyst was separated from the system and washed with acetone and dried for the next run under the same reaction conditions. From Fig. S2(B), the catalyst V-Mo-Fe-O show a good leaching resistance capability and the reaction was carried out four times in consecutive run with only a slight decrease in activity of the reused catalyst, and the XRD patterns of fresh and reused catalyst (not list here) were not changed, which demonstrates that no structural change was observed, and the amount measurement of the reused catalyst demonstrates that no significant loss in catalyst was observed. This finding implied that the catalyst can be effectively recovered and easily recycled. 4. Conclusions The V-Mo-Fe-O composite catalyst was synthesized by a sol-gel method and used in toluene selective oxidation to benzaldehyde. A toluene conversion of 40.3% with 84.5% selectivity towards benzaldehyde was obtained with hydrogen peroxide as an oxidant under atmospheric pressure over the V-Mo-Fe-O rod-like catalyst. The high catalytic performance depends on the catalyst composed by MoV2O8 (nanorod structure), on which a thin layer compound of MoO3 and (Fe2(MoO4)3) was covered. The introduction of Fe increased the electron cloud density of molybdenum and provided richer electronic center for catalytic oxidation reaction. Additionally, the tightly covered multilayers of MoO3 and MoV2O8 play an important role in the oxidation of toluene to benzaldehyde, and the catalyst is low-cost, easily recycled and shows high potential application in industry.
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Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Highly efficient V-Mo-Fe-O catalysts for selective oxidation of toluene to benzaldehyde Huan Xia a,b, Zili Liu a,b,⁎, Yangyang Xu a,b, Jianliang Zuo a,b, Zuzeng Qin c,⁎ a b c
School of Chemistry & Chemical Engineering, Guangzhou University, Guangzhou 510006, PR China China Key Laboratory of Ministry of Education for Water Quality Security and Protection in Pearl River Delta, Guangzhou 510006, PR China School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, PR China
a r t i c l e
i n f o
Article history: Received 20 May 2016 Received in revised form 2 August 2016 Accepted 4 August 2016 Available online 06 August 2016 Keywords: V-Mo-Fe-O Selective oxidation Toluene Benzaldehyde
a b s t r a c t The V-Mo-Fe-O catalysts were synthesized by a sol-gel method, characterized and used for the selective oxidation of toluene to benzaldehyde with hydrogen peroxide. The results show that V-Mo-Fe-O catalyst is an effective catalyst, exhibiting a toluene conversion of 40.3%, and a benzaldehyde selectivity of 84.5% at 80 °C. Scanning electron microscopy (SEM) shows that the catalyst has nanorod structure with a thin layer compound, which may be constitutive of iron molybdate while the main compound of catalyst is MoV2O8. Furthermore, the catalyst can be easily recycled and reused for four times without significant changes in its activity. © 2016 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Toluene is an aromatic compound which provide three C\\H bonds, and can be oxidized to benzyl alcohol, benzaldehyde, and benzoate [1– 3]. Among them, benzaldehyde is widely used as solvents, perfumes, dyes, and plasticizers. Traditionally, benzaldehyde is produced by chlorinating of –CH3 functionality of toluene and followed by saponifying [4, 5], which may discharge waste with a low benzaldehyde yield. Moreover, the products in this method are limited to utilization in food industries and pharmaceuticals due to the chlorinated contaminants. Therefore, it is important that an effective and environmental friendly way use in the oxidation of toluene to benzaldehyde, and the attempt to resolve this problem was the liquid-phase oxidation of toluene to benzaldehyde [6–9], however, the toluene conversion or the benzaldehyde yield was still very low [10–12]. In our previous studies, we found that V-Mo-O composite oxides can be used as a catalyst for oxidation toluene to benzaldehyde in the hydrogen peroxide at 80 °C under atmospheric pressure for only 30 min, with the toluene conversion of 38.9% and the benzaldehyde selectivity of 69.7% [13]. In the present study, to further increasing the toluene conversion and benzaldehyde selectivity, Fe was introduced to the V-MoO catalysts, and the V-Mo-Fe-O catalysts were used in the liquidphase oxidation of toluene, and the effects of Fe contents on the structure of the V-Mo-Fe-O catalysts were investigated.
2.1. Catalyst preparation
⁎ Corresponding authors. E-mail addresses: [email protected] (Z. Liu), [email protected] (Z. Qin).
http://dx.doi.org/10.1016/j.catcom.2016.08.008 1566-7367/© 2016 Elsevier B.V. All rights reserved.
The V-Mo-Fe-O catalysts were prepared via a sol-gel method according to literature [13]. NH4VO3, (NH4)6Mo7O24·4H2O, and Fe(NO3)3·9H2O were dissolved in 120 mL distilled water at a V:Mo:Fe mole ration of 6:7:X (X = 0, 0.5, 1.0, 1.5, 2.0, and 6.0), and stirred at 80 °C for 10 min. To avoid the rapid precipitation of the macromolecular inorganics and improve the dispersion of the active component, the citric acid [(C6H8O7·H2O), n(C6H8O7·H2O) / n((NH4)6Mo7O24) = 8 / 1] was added to the mixed water solution and stirred for 1 h to obtain a sol. Subsequently, the mixed sol was heated up to 100 °C and stirred for 2 h to form a gel. The gel was dried at 80 °C under vacuum for 8 h, and then calcined at 550 °C for 3 h in air. All the chemical agents were analytical grade without further purification. 2.2. Catalyst characterization Powder X-ray diffractometer (XRD) patterns were employed on a Purkinje MASAL XD-3 diffractometer with Cu Kα radiation (36 kV and 20 mA). The morphology and surface texture of the catalysts were obtained on a JEOLJSM-7001F scanning electron microscope (SEM) using a 10 kV energy source under vacuum. Energy dispersive X-ray spectroscopy (EDS) was used in connection with SEM for the elemental analysis. X-ray photoelectron spectrometer (XPS) was conducted to determine the states of elements on the catalyst surface using a Shimadzu Axis Ultra DLD X-ray photoelectron spectrometer with acrochromatic Al Kα radiation (1486.6 eV) at 10 mA and 12 kV. The H2-temperature
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programmed reduction (H2-TPR) was performed in a Micromeritics AutoChem II 2920 in a flow of 30 mL/min 5% H2/Ar mixed gas and with a temperature ramp of 10 °C/min. 2.3. Oxidation of toluene Liquid phase oxidation of toluene was carried out in a three-neck round-bottom flask, and 0.2 g V-Mo-Fe-O catalysts, 1.75 g toluene, and 15 mL acetic acid, acetonitrile, or ethyl acetate using as solvent, were added, and 30 wt% H2O2 was subsequently added dropwise, and reacted at 80 °C for 30 min. After the reaction, the solid catalysts were separated by filtration, and the products were analyzed by using a gas chromatograph (Shimadzu, GC-2014) equipped with a HP5 capillary column and a flame ionization detector (FID), and using the peak-area internal standard method with anisole as the internal standard. 3. Results and discussion
Fig. 1. XRD patterns of V-Mo-Fe-O catalysts (molar ration of V/Mo/Fe = 6.0/7.0/X) with X = 0 (a), 0.5 (b), 1.0 (c), 1.5 (d), 2.0(e), and 6.0 (f).
3.1. Effects of the modified metal on the selective oxidation Ternary metal oxide V-Mo-O was modified by a variety of transition metal oxides to prepare V-Mo-M-O (M = Co, Cr, Ce, La, Fe) catalysts through a sol-gel method, and these catalysts were used for selective oxidation of toluene to benzaldehyde, the results are shown in Table S1 (in Appendix A. Supplementary data). In our study, the main products were benzaldehyde and benzoic acid, accompanied with a small amount of benzyl alcohol and ester. Table S1 shows that MoO3 show relatively low toluene conversion of 3.2%, and after modified by Vanadium, the toluene conversion increased sharply. From Table S1, the second metal oxides-introduction to V-MoO had a positive influence on the selective oxidation of toluene to benzaldehyde, especially the benzaldehyde selectivity increased obviously. The V-Mo-O catalysts modified with Fe show the greatest benzaldehyde selectivity (84.5%) among the used 8 catalysts. The benzaldehyde selectivity and yield increased 14.8% and 7.0% for V-Mo-Fe-O while compared with V-Mo-O catalyst, respectively, which also much higher than that used for single catalyst, such as V2O5 or MoO3. Therefore, Fe was chosen as the optimal catalyst additive for further investigation. The amount of modifier in the catalyst will affect the catalytic performance of the catalyst, therefore, it is necessary to study the optimal Fe amount in the V-Mo-O catalyst. The V-Mo-Fe-O catalysts with different Fe amounts were prepared at a constant vanadium and molybdenum atom ratio (V/Mo = 6/7) [13], and the effects of Fe amounts, presented by X, on the toluene oxidation to benzaldehyde was shown in Table S1. It can be seen that the toluene conversion, benzaldehyde selectivity and yield were shown in the same change tendency, i.e., an optimal Fe amount was found at X was 1.0, and X = 1.0, the toluene conversion, benzaldehyde selectivity and yield was 40.3%, 84.5%, and 34.1%, respectively. Therefore, the following studies conducted at a Fe amount of Fe:V = 1:6.
which were assigned to the formation of Fe2(MoO4)3 [14], No any new peak appears in the pattern of V-Mo-Fe-O, which can exclude the cases that Fe species are existed in other new crystal form or loaded on framework in Fe(NO3)3 form. Combined with the data in Table S1 and literature [13], it found that MoV2O8 is the main component in the V-Mo-Fe-O catalysts, and appropriate amount Fe addition would increase the benzaldehyde selectivity, and the excessive amount of doping Fe would form Fe2(MoO4)3, which may cover the active sites of the MoV2O8 and led to a decreasing of toluene conversion and benzaldehyde selectivity, and the optimal V/Mo/Fe atom ratio was 6/7/1. Therefore, the following study would compare the V-Mo-O and V-Mo-Fe-O with a V/Mo/Fe atom ratio of 6/7/1. 3.3. X-ray photoelectron spectroscopy analysis The V 2p, Mo 3d, and Fe 2p XPS spectra of V-Mo-O and V-Mo-Fe-O are shown in Fig. 2 and Fig. S1, and the binding energy of the V, Mo, and Fe in the V-Mo-O and V-Mo-Fe-O were shown in Table S2. In Fig. 2, the binding energy at 232.82 eV (Mo 3d5/2) and 236.02 eV (Mo 3d3/2) in the Mo 3d of V-Mo-O catalyst were assigned to Mo6 + species in an oxidized surrounding of MoV2O8 and MoO3 [15], respectively, which were conformed to the existing of MoV2O8 and MoO3 from the XRD results. Additionally, after modified by Fe, Mo and Fe
3.2. X-ray diffraction analysis The XRD patterns of V-Mo-Fe-O catalysts with Fe content (X) of 0 to 6 were shown in Fig. 1. From Fig. 1, the introduced of Fe did not affect the formation of MoV2O8 due to obvious MoV2O8 diffraction peaks were found at 2θ = 18.5, 21.5, 23.5, 25.0, 27.5, 33.0 and 33.5°. Besides, the molybdenum in the form of MoO3 was observed at the peaks of 2θ = 13.0, 23.0, and 25.9°, which show a disappeared tendency with the increase of the Fe content. With low Fe content, the crystal size of Fe in the sample was too small to be detected by XRD. With the increasing of Fe amount from 0 to 2.0, the MoO3 and MoV2O8 amount decreased while the Fe2(MoO4)3 amount increased, suggesting MoO3 tend to form Fe2(MoO4)3. When the V/Mo/Fe atom ratio further increased to 6/7/6, the peaks at 2θ of 22.9, 24.5 and 27.5° appeared, and the peaks at 2θ of 33.0 and 33.5° reduced and tend to disappear,
Fig. 2. XPS spectra of Mo 3d regions for V-Mo-O and V-Mo-Fe-O catalysts.
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oxides could transform into iron molybdate (Fe2(MoO4)3) at 500 °C [16], and the chemical shifts of Mo 3d5/2 and Mo 3d3/2 were −0.14 eV and −0.18 eV, respectively, which indicated that the introduction of Fe formed an oxidized surrounding of Fe2(MoO4)3 and MoO3, and increased slightly electron cloud density of Mo. From Fig. S1, the binding energies of V 2p3/2 and V 2p1/2 of V-Mo-O catalyst were centered at 517.39 eV and 524.61 eV, respectively. Likewise, the V 2p3/2 and V 2p1/2 in the V-Mo-Fe-O catalyst were 517.38 eV and 524.59 eV showing that V occurred as the form of V5+ in MoV2O8 [19, 20]. As to the V-Mo-Fe-O catalyst at a V/Fe molar ratio of 6/1, the binding energy changes of V 2p3/2 and V 2p1/2 were very slight, indicating that the introduction of Fe did not affect the chemical environment of V element. For Fe 2p XPS spectra in V-Mo-Fe-O catalyst, the Fe 2p3/2 located at approximately 712.0 eV, and Fe 2p1/2 approximately located at 725.0 eV. The Fe 2p3/2 peak is fit into two peaks by the same peak fitting deconvolution technique, and the peaks centered at 711.96 and 714.07 eV can be assigned to Fe2+ and Fe3+ [17,18], respectively. However, the band position in the Fe 2p1/2 region shifts to a higher binding energy upon the presence of Fe-O-Mo, and electron transfer from Mo to Fe [18]. 3.4. SEM analysis V2O5 and MoO3 were easy to form composite oxides with different valence cations replacing each other for not only the ion radius of V5+ (0.59 Å) and Mo6+ (0.62 Å) but structure of their oxides is similar [21, 22]. Fig. 3 shows that both V-Mo-O and V-Mo-Fe-O catalysts are nanorod structure. The V-Mo-O catalyst (Fig. 3a) shows relatively regular nanorods shape, and V-Mo-Fe-O (Fig. 3b) shows agglomerated species covered at the surface of sample and spot (Fig. 3c) at the surface of “rods” which may be caused by external force giving rise to the stripping of catalyst, suggesting the existence of a thin layer compound covering the surface of V-Mo-Fe-O catalyst. The compositions of catalysts surface were estimated by EDS, and it was found the presence of V, Mo and Fe, with the molar ratio of V/Mo/Fe = 5.8/7.1/1. And a roughened surface with defects also found on the surface of V-Mo-Fe-O. It is the defect distributions and the impurities in crystals improved the performance of the catalysts [23,24]. 3.5. H2-TPR studies Fig. 4 was the H2-TPR profile of V-Mo-O and V-Mo-Fe-O. From Fig. 4, in the V-Mo-Fe-O catalyst, vanadium and molybdenum formed to MoV2O8 solid solution, which was confirmed by XRD and SEM, changing the liability of vanadium and molybdenum species. Hence, the H2-TPR peaks of V-Mo-O centered at 575 °C attributed to the reduction of MoO3, and the peak centered at 675 °C was the reduction of MoV2O8 [25,26]. To the V-Mo-Fe-O catalyst, the peaks at 560 °C, 590 °C and 650 °C were attributed to the reduction of MoO3, Fe2(MoO4)3, and MoV2O8 [27,28], respectively. The peak centered at 590 °C of V-MoFe-O catalyst disappeared in the V-Mo-O catalyst, indicated that Fe
Fig. 4. H2-TPR profiles of V-Mo-O and V-Mo-Fe-O catalysts.
affects the redox properties after modified the V-Mo-O catalyst. In addition, it can be assigned to the V\\Mo composite oxide being reduced at low temperature due to the modification of Fe. After adding Fe, the dispersions of Mo oxide were changed, which was advantageous to the reduction process [18]. Furthermore, the reactive Mo_O existed in the surface deformity of MoO3 would led to the production of oxygen vacancies around Mo6+, and accelerate the high valence Mo to become low valence Mo. For this reason, the reduction of the oxide of the active component was promoted by formation of low valence Mon+ [29]. The shift in the reduction peaks of the V-Mo-Fe-O catalyst demonstrates that the MoO3 in V-Mo-Fe-O was more easily reduced to a lower valence state, the lattice oxygen in the catalyst had a higher moving performance, and was easier reduced than V-Mo-O catalyst.
3.6. Effect of reaction conditions and the reusability of the catalysts The effect of reaction time on the oxidation of toluene was examined and results shown in Fig. 5, revealed that the conversion was rapidly increased with reaction time, achieving a toluene conversion of 40.3% and a benzaldehyde selectivity of 84.5% at 30 min. From 30 to 120 min, the toluene conversion increased from 40.3% to 46.9% while the benzaldehyde selectivity decreased from 84.5% to 54.3%. The amount of H2O2 in the system is an important factor affecting the oxidation of toluene, and the results were shown in Fig. S2(A). The increment in H2O2 molar ratio allowed oxidation of the side chain and consequently the toluene conversion increased. But the excess H2O2 amount caused the decrease of benzaldehyde selectivity. When the H2O2 molar ratio was above 8, both the toluene conversion and benzaldehyde selectivity were almost unchanged. The percentage of H2O2 consumed is determined by iodometric titration after the end of experiment and found to be in the range of 89% to 91%. This suggests that more than a stoichiometric amount of H2O2 is utilized in the reaction,
Fig. 3. SEM images of (a) V-Mo-O and (b, c) V-Mo-Fe-O catalysts.
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Acknowledgment The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (No. 21276054), and the Key Laboratory of Applied Surface and Colloid Chemistry (Shaanxi Normal University) (No. 2016017).
Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2016.08.008.
References
Fig. 5. The effects of reaction time on the toluene oxidation.
which could be due to the thermal decomposition of H2O2. Therefore, the optimal molar ratio of H2O2 to toluene was 6. The nature of the solvent has a strong influence on the oxidation of toluene. Therefore, three solvents with different acidity have been investigated. The results from Table S3 indicate that the effect of solvent on the catalytic activity is mainly related to the acidity of solvent. The acidity of solvent was found to be in the following order: ice acetic acid N acetonitrile N ethyl acetate, and both the toluene conversion and benzaldehyde selectivity increased with increasing acidity of the solvent, suggesting that proton in the solvent may be involved in the catalytic reaction, whereas the influence of the solvent acidity on catalytic reaction is complex. In addition, acetic acid has proper polarity which can turn the reactants and hydrogen peroxide to form homogeneous reaction system and favor the release of lattice oxygen of catalysts [30,31]. However, with the data in Table S3, the acetic acid was used as the optimal solvent. 3.7. Reusability of the catalyst Reuse of the recovery catalyst had also been studied to further evaluate performance of the catalyst. The catalyst was separated from the system and washed with acetone and dried for the next run under the same reaction conditions. From Fig. S2(B), the catalyst V-Mo-Fe-O show a good leaching resistance capability and the reaction was carried out four times in consecutive run with only a slight decrease in activity of the reused catalyst, and the XRD patterns of fresh and reused catalyst (not list here) were not changed, which demonstrates that no structural change was observed, and the amount measurement of the reused catalyst demonstrates that no significant loss in catalyst was observed. This finding implied that the catalyst can be effectively recovered and easily recycled. 4. Conclusions The V-Mo-Fe-O composite catalyst was synthesized by a sol-gel method and used in toluene selective oxidation to benzaldehyde. A toluene conversion of 40.3% with 84.5% selectivity towards benzaldehyde was obtained with hydrogen peroxide as an oxidant under atmospheric pressure over the V-Mo-Fe-O rod-like catalyst. The high catalytic performance depends on the catalyst composed by MoV2O8 (nanorod structure), on which a thin layer compound of MoO3 and (Fe2(MoO4)3) was covered. The introduction of Fe increased the electron cloud density of molybdenum and provided richer electronic center for catalytic oxidation reaction. Additionally, the tightly covered multilayers of MoO3 and MoV2O8 play an important role in the oxidation of toluene to benzaldehyde, and the catalyst is low-cost, easily recycled and shows high potential application in industry.
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