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Highly dispersed iron oxides on mesoporous carbon for selective oxidation of benzyl alcohol with molecular oxygen†
Received 25th September 2013, Accepted 10th December 2013
Longlong Geng, Xiuyan Zhang, Wenxiang Zhang, Mingjun Jia and Gang Liu*
DOI: 10.1039/c3cc47332e www.rsc.org/chemcomm
Highly dispersed iron oxide supported catalysts, prepared using HNO3treated CMK-3 mesoporous carbons as supports, exhibit relatively high catalytic activity in selective oxidation of benzyl alcohol with oxygen.
Iron-based catalysts are particularly attractive because of the ready availability, low cost and low toxicity of this metal.1 Many iron complexes have been reported as efficient homogeneous catalysts for various organic reactions, such as cross-coupling reactions, sulfide oxidations, allylic alkylations and aminations, alcohol oxidations, Michael additions, etc.2 In heterogeneous catalysis, iron oxides are generally considered to be catalytically inactive under mild reaction conditions, which are usually used as catalysts and supports at relatively high temperature (>300 1C) and pressure.1c,3 Recently, iron oxides with nanostructures have proven to be active catalysts in a few heterogeneous catalytic reactions under relatively mild conditions.4 For example, Gao et al. found that layered-carbon-stabilized iron oxide nanostructure composites are active in catalytic oxidation of secondary alcohols with tert-butyl hydroperoxide (TBHP) as oxidants.4b The particle sizes of iron oxides in the composite catalysts are mainly 6–22 nm. Beller and his co-workers reported that nano-iron oxides could be used as active and selective catalysts for alcohol and olefin oxidation with hydrogen peroxide (H2O2) as oxidants.4c They found that iron oxides with particle sizes of 20–50 nm exhibit a 33% conversion of benzyl alcohol with a benzaldehyde selectivity of 97% after 12 h reaction. However, these iron oxide catalysts are not active in the reaction with molecular oxygen (O2) as an oxygen source. Further tuning of particle sizes and the structure of iron oxides is still desirable for designing iron-based heterogeneous catalysts that can directly activate O2 for the selective oxidation of organic substrates. Besides, the dispersion states and the surrounding environment of central metal or metal oxides can significantly influence the reactivity and selectivity of nanostructured supported catalysts.
Key Laboratory of Surface and Interface Chemistry of Jilin Province, College of Chemistry, Jilin University, Changchun, 130021, China. E-mail: [email protected]; Fax: +86 431 85168420; Tel: +86 431 85155390 † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cc47332e
This journal is © The Royal Society of Chemistry 2014
It could even bring some unexpected catalytic behavior or stability in some reactions.5 In this work, highly dispersed iron oxide supported catalysts were prepared using HNO3-treated CMK-3 as a support. CMK-3 is known as a kind of carbonaceous material possessing a well-ordered mesoporous structure with large surface area.6 It is interesting that the resultant catalysts exhibit relatively high activity for the selective oxidation of benzyl alcohol with air as an oxidant source. CMK-3 was synthesized by the nanocasting method using SBA-15 as a hard template and sucrose as the carbon source.6 Before being used as a catalyst support, CMK-3 was chemically treated with HNO3 (4 M, at 60 1C) and the obtained carbon was denoted as H-CMK-3. Supported iron oxide catalysts were prepared by the wet impregnation method with Fe(NO3)39H2O as an iron precursor, and the obtained solid was calcined at 400 1C for 6 h in an argon flow. The loading amount of iron oxides is 5 wt% (calculated with Fe2O3) and the resultant material was denoted as FeOx/H-CMK-3. Low-angle XRD patterns show that H-CMK-3 maintains ordered arrangement of parent CMK-3 (Fig. 1A). It exhibits three peaks in the range 2y = 0.75–31, which can be indexed to the (100), (110) and (200) reflections of the hexagonal space group (P6mm). The patterns of FeOx/H-CMK-3 show a remarkable decrease in the intensity of all reflections compared with that of H-CMK-3. The decrease may be correlated with the partial damage to the structural order of CMK-3 or due to the fact that the introduction of scattering material into the pores leads to an increased phase cancellation between scattering form walls and the pore regions. A similar effect has been discussed in the literature dealing with metal oxide incorporation into the pores of silicate and carbon mesostructures.7 Wide-angle XRD patterns show that all of these samples exhibit two broad diffraction peaks centered at 24.51 and 441 (Fig. 1B), which are generally assigned to diffractions from the (002) and (100) graphite planes of carbon supports.8 No crystalline phase related to iron oxide species could be observed, indicating that the particle size of iron oxides is very small or the degree of crystallization is very low. N2 adsorption–desorption isotherms show that all samples exhibit typical isotherms of ordered mesostructures (type IV) with a welldefined step in the adsorption curve near a P/P0 value of 0.5 (Fig. 1C).
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Fig. 1 Low-angle (A) and wide-angle (B) XRD patterns of CMK-3, H-CMK-3 and supported FeOx catalysts; N2 adsorption–desorption isotherms and BJH pore size distribution (C) and XPS spectra (D) of FeOx/H-CMK-3 and FeOx/CMK-3.
The surface area and pore volume of H-CMK-3 are 918 m2 g 1 and 0.95 cm3 g 1, which are similar to those of CMK-3 (833 m2 g 1 and 1.02 cm3 g 1). FeOx/H-CMK-3 maintains a high surface area (1118 m2 g 1) and large pore volume (1.13 cm3 g 1), indicating that the iron oxides do not block the mesopores of the CMK-3 support. XPS was carried out to detect the surface chemical state of iron species. As shown in Fig. 1D, the Fe (2p) core level is split into Fe2p1/2 and Fe2p3/2 due to spin–orbit coupling, and appears at 724.2 eV and 710.9 eV respectively. These values are close to the range of the Fe2p binding energies of iron in Fe2O3, indicating that most iron on the surface of samples were present at a valence of +3.9 A small amount of Fe2+ should also be present in these samples as suggested by the weak shoulder of the main Fe2p3/2 peak at lower binding energies.9 The morphology of FeOx/H-CMK-3 is investigated with high resolution transmission electron microscopy (Fig. 2a). It can be observed that the FeOx/H-CMK-3 exhibits a linear mesoporous array. Its structural order is a little lower than that of pristine CMK-3 (Fig. S1a, ESI†). Interestingly, no iron oxide particles could be observed in the whole detected region even in high resolution measurement (Fig. 2a and b). STEM and EDX mapping results show that iron species should be present as quite small particles highly dispersed throughout the H-CMK-3 support (Fig. 2c and d and Fig. S3, ESI†). No isolated iron spots should be due to relatively high contents of iron oxides (5 wt%) and the homogeneous dispersion state. In comparison, CMK-3 without being treated with HNO3 was also used for supporting iron oxides (denoted as FeOx/CMK-3). Fig. 2e and f show that iron oxide particles with diameters of 5–10 nm can be observed on the surface of CMK-3. Additionally, a few relatively large iron oxide particles (10–20 nm) were detected in FeOx/CMK-3 (Fig. S1c, ESI†). This sample exhibits a relatively low surface area (503 m2 g 1) and pore volume (0.54 cm3 g 1), which should be due to iron oxide particles blocking the mesopores of the CMK-3 support. Temperature programmed reduction (TPR) in a H2/Ar stream was carried out to detect the reducibility of iron oxides with
2966 | Chem. Commun., 2014, 50, 2965--2967
Fig. 2 HRTEM, HAADF-STEM and EDX mapping images of FeOx/H-CMK-3 (a–d) and HRTEM images of FeOx/CMK-3 (e, f).
different dispersion states. Fig. 3 shows that obvious H2 consumption peaks appear in the profiles of FeOx/H-CMK-3 and FeOx/CMK-3. The intensity of these peaks is much higher than that of CMK-3 and H-CMK-3, suggesting that these peaks can be ascribed to reduction of iron oxides on the surface of carbon supports. The main reduction peak of FeOx/H-CMK-3 (477 1C) is much lower than that of FeOx/CMK-3 (650 1C). This result shows that iron oxides in FeOx/H-CMK-3 are facilely reduced compared with those in FeOx/CMK-3, which should be due to the small particle size of iron oxides in FeOx/H-CMK-3. Selective oxidation of benzyl alcohol with oxygen is an important catalytic reaction in organic synthesis. Previously, it was reported
Fig. 3
H2-TPR profiles of CMK-3, H-CMK-3 and supported FeOx catalysts.
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Fig. 4 Catalytic performance of CMK-3, H-CMK-3 and supported FeOx catalysts. Reaction conditions: mcat 0.3 g; toluene 10 mL, benzyl alcohol 1 mmol; temperature 353 K; pressure (air) 1 atm.
that a variety of supported noble metal (e.g. Ru, Pd, Au) catalysts are required for this aerobic oxidation reaction.10 Fig. 4 shows that FeOx/ H-CMK-3 could catalyze this aerobic oxidation reaction. It exhibits a 72% conversion of benzyl alcohol after 8 h reaction, which is much higher than those of FeOx/CMK-3, H-CMK-3 and pristine CMK-3. The selectivity to benzaldehyde is nearly 100%. As for iron oxide, almost no benzyl alcohol conversion can be detected, indicating that iron oxide itself is inactive for the oxidation of benzyl alcohol with air as an oxidant source. Multiple aerobic oxidation cycles were carried out to examine the recoverability of FeOx/H-CMK-3. When the catalyst samples were recovered by simple decanting of the reaction solution, an activity decrease can be observed after two reaction cycles (Fig. S4, ESI†). Then the conversion of benzyl alcohol is maintained at about 33% during the subsequent reaction cycles, which is still much higher than those of other samples. When the used catalysts were thermally treated under an argon atmosphere at 400 1C for 30 min, the samples exhibited nearly consistent activity as the fresh one. This might be due to the complete removal of absorbed species (e.g. aldehydes) after thermal treatment.10d Magnetic measurement shows that FeOx/H-CMK-3 exhibits weak ferromagnetic behaviour (Fig. S6, ESI†). The measured saturation magnetization (Ms) is 1.29 emu g 1. This result suggests that the catalyst can be easily recovered from the reaction system by magnetic separation. Based on the above characterization results, treating CMK-3 with HNO3 is a critical factor for obtaining highly dispersed iron oxide catalysts. It generates a great deal of oxygen-functional groups, including –COOH and CQO, compared with that of pristine CMK-3 (Fig. S5, ESI,† DRIFT spectra). According to the literature and our previous studies, the surface groups should act as nucleation centers for the generation of highly dispersed metal crystallites.11 Besides, the confinement effect of these surface functionalities of CMK-3 mesoporous structure is assumed to stabilize the metal crystallites to minimize metal sintering, contributing to obtaining the highly dispersed iron oxide catalysts. It is widely accepted that decreasing the particle size is related to the higher concentration of low coordinated metal atoms in the small particles. It should be responsible for the facile changing oxidation state of iron oxides and relatively high reduction–oxidation activity over FeOx/H-CMK-3. Additionally, the abundant oxygen-containing functional groups on the surface of carbon supports should provide an environment for these small particles, which is quite similar to the
This journal is © The Royal Society of Chemistry 2014
Communication
ligands surrounding central metals in homogeneous catalysis. The interface iron species may possess similar electronic properties to organometallic compounds, which may function as active sites for the selective oxidation of benzyl alcohol with molecular oxygen. Further work is still needed to clear the nature of active sites. In summary, highly dispersed iron oxide catalysts have been obtained with HNO3 treated mesoporous carbon of CMK-3 as a support. It exhibits a high activity in the selective oxidation of benzyl alcohol with air as an oxidant source. The abundant surface functional groups and the confinement effect of CMK-3 mesoporous structure contribute to obtaining the highly dispersed iron oxide active sites. We believe that the activity of resultant catalysts could be further improved by tuning the surface properties of carbon supports. This work was supported by the National Natural Science Foundation of China (21003059), the Development Project of Science and Technology of Jilin Province (20130101014JC, 201105009) and the Fundamental Research Funds for the Central Universities.
Notes and references 1 (a) D. H. Deng, L. Yu, X. Q. Chen, G. X. Wang, L. Jin, X. L. Pan, J. Deng, G. Q. Sun and X. H. Bao, Angew. Chem., Int. Ed., 2013, 52, 371; (b) B. Plietker, Iron Catalysis in Organic Chemistry, Wiley-VCH, Weinheim, 2008; (c) A.-H. Lu, J.-J. Nitz, M. Comotti, C. Weidenthaler, K. Schlichte, C. W. Lehmann, ¨th, J. Am. Chem. Soc., 2010, 132, 14152; (d) T. R. O. Terasaki and F. Schu Simmons and V. Artero, Angew. Chem., Int. Ed., 2013, 52, 6143. 2 (a) C. Bolm, J. Legros, J. L. Paih and L. Zani, Chem. Rev., 2004, 104, 6217; (b) S. Enthaler, K. Junge and M. Belle, Angew. Chem., Int. Ed., 2008, ¨der, B. Join, A. J. Amali, K. Junge, X. Ribas, M. Costas 47, 3317; (c) K. Schro and M. Beller, Angew. Chem., Int. Ed., 2011, 50, 1425; (d) S. Fleischer, S. L. Zhou, K. Junge and M. Beller, Angew. Chem., Int. Ed., 2013, 52, 5120. ¨ckner, S. M. Zhang and 3 (a) F. Shi, M. K. Tse, M. M. Pohl, J. Radnik, A. Bru M. Beller, J. Mol. Catal. A: Chem., 2008, 292, 28; (b) B. Moens, H. D. Winne, S. Corthals, H. Poelman, R. D. Gryse, V. Meynen, P. Cool, B. F. Sels and P. A. Jacobs, J. Catal., 2007, 247, 86; (c) M. Qing, Y. Yang, B. S. Wu, J. Xu, C. H. Zhang, P. Gao and Y. W. Li, J. Catal., 2011, 279, 111. 4 (a) D. Verma, S. Verma, A. K. Sinha and S. L. Jain, ChemPlusChem, 2013, 78, 860; (b) Y. J. Gao, D. Ma, G. Hu, P. Zhai, X. H. Bao, B. Zhu, B. S. Zhang and D. S. Su, Angew. Chem., Int. Ed., 2011, 50, 10236; ¨ckner, S. M. Zhang and (c) F. Shi, M. K. Tse, M.-M. Pohl, A. Bru M. Beller, Angew. Chem., Int. Ed., 2007, 46, 8866; (d) D. Obermayer, A. M. Balu, A. A. Romero, W. Goessler, R. Luque and C. O. Kappe, Green Chem., 2013, 15, 1530. 5 (a) D. Tanaka, Y. Oaki and H. Imai, Chem. Commun., 2010, 46, 5286; (b) G. Liu, X. L. Wang, X. Wang, H. X. Han and C. Li, J. Catal., 2012, 293, 61. 6 S. Jun, S. H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna and O. Terasaki, J. Am. Chem. Soc., 2000, 122, 10712. 7 H. Huwe and M. Froba, Carbon, 2007, 45, 304. ¨th, 8 (a) A.-H. Lu, W.-C. Li, E.-L. Salabas, B. Spliethoff and F. Schu Chem. Mater., 2006, 18, 2086; (b) G. Liu, Y. Liu, Z. L. Wang, X. Z. Liao, S. J. Wu, W. X. Zhang and M. J. Jia, Microporous Mesoporous Mater., 2008, 116, 439. 9 (a) C. L. Corkhill and D. J. Vaughan, Appl. Geochem., 2009, 24, 2342; (b) M. Mullet, V. Khare and C. Ruby, Surf. Interface Anal., 2008, 40, 323; (c) S. Guo, G. K. Zhang, Y. D. Guo and J. C. Yu, Carbon, 2013, 60, 437. 10 (a) F. F. Wang, Z. S. Lu, L. Yang, Y. X. Zhang, Q. H. Tang, Y. M. Guo, X. M. Ma and Z. X. Yang, Chem. Commun., 2013, 49, 6626; (b) S. Wang, W.-C. Li, G.-P. Hao, Y. Hao, Q. Sun, X.-Q. Zhang and A.-H. Lu, J. Am. Chem. Soc., 2011, 133, 15304; (c) X. M. Huang, X. G. Wang, X. S. Wang, X. X. Wang, M. W. Tan, W. Z. Ding and X. G. Lu, J. Catal., 2013, 301, 217; (d) J. J. Zhu, J. L. Faria, J. L. Figueiredo and A. Thomas, Chem.–Eur. J., 2011, 17, 7112. 11 (a) J. M. G. Salazar, M. I. Brrena, C. Merino and N. Merino, Mater. Lett., 2008, 62, 494; (b) G. M. Zhao, J. H. Shi, G. Liu, Y. Liu, Z. L. Wang, W. X. Zhang and M. J. Jia, J. Mol. Catal. A: Chem., 2010, 327, 32; (c) Y. H. Zu, J. Y. Tang, W. C. Zhu, M. Zhang, G. Liu, Y. Liu, W. X. Zhang and M. J. Jia, Bioresour. Technol., 2011, 102, 8939.
Chem. Commun., 2014, 50, 2965--2967 | 2967
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Cite this: Chem. Commun., 2014, 50, 2965
Highly dispersed iron oxides on mesoporous carbon for selective oxidation of benzyl alcohol with molecular oxygen†
Received 25th September 2013, Accepted 10th December 2013
Longlong Geng, Xiuyan Zhang, Wenxiang Zhang, Mingjun Jia and Gang Liu*
DOI: 10.1039/c3cc47332e www.rsc.org/chemcomm
Highly dispersed iron oxide supported catalysts, prepared using HNO3treated CMK-3 mesoporous carbons as supports, exhibit relatively high catalytic activity in selective oxidation of benzyl alcohol with oxygen.
Iron-based catalysts are particularly attractive because of the ready availability, low cost and low toxicity of this metal.1 Many iron complexes have been reported as efficient homogeneous catalysts for various organic reactions, such as cross-coupling reactions, sulfide oxidations, allylic alkylations and aminations, alcohol oxidations, Michael additions, etc.2 In heterogeneous catalysis, iron oxides are generally considered to be catalytically inactive under mild reaction conditions, which are usually used as catalysts and supports at relatively high temperature (>300 1C) and pressure.1c,3 Recently, iron oxides with nanostructures have proven to be active catalysts in a few heterogeneous catalytic reactions under relatively mild conditions.4 For example, Gao et al. found that layered-carbon-stabilized iron oxide nanostructure composites are active in catalytic oxidation of secondary alcohols with tert-butyl hydroperoxide (TBHP) as oxidants.4b The particle sizes of iron oxides in the composite catalysts are mainly 6–22 nm. Beller and his co-workers reported that nano-iron oxides could be used as active and selective catalysts for alcohol and olefin oxidation with hydrogen peroxide (H2O2) as oxidants.4c They found that iron oxides with particle sizes of 20–50 nm exhibit a 33% conversion of benzyl alcohol with a benzaldehyde selectivity of 97% after 12 h reaction. However, these iron oxide catalysts are not active in the reaction with molecular oxygen (O2) as an oxygen source. Further tuning of particle sizes and the structure of iron oxides is still desirable for designing iron-based heterogeneous catalysts that can directly activate O2 for the selective oxidation of organic substrates. Besides, the dispersion states and the surrounding environment of central metal or metal oxides can significantly influence the reactivity and selectivity of nanostructured supported catalysts.
Key Laboratory of Surface and Interface Chemistry of Jilin Province, College of Chemistry, Jilin University, Changchun, 130021, China. E-mail: [email protected]; Fax: +86 431 85168420; Tel: +86 431 85155390 † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cc47332e
This journal is © The Royal Society of Chemistry 2014
It could even bring some unexpected catalytic behavior or stability in some reactions.5 In this work, highly dispersed iron oxide supported catalysts were prepared using HNO3-treated CMK-3 as a support. CMK-3 is known as a kind of carbonaceous material possessing a well-ordered mesoporous structure with large surface area.6 It is interesting that the resultant catalysts exhibit relatively high activity for the selective oxidation of benzyl alcohol with air as an oxidant source. CMK-3 was synthesized by the nanocasting method using SBA-15 as a hard template and sucrose as the carbon source.6 Before being used as a catalyst support, CMK-3 was chemically treated with HNO3 (4 M, at 60 1C) and the obtained carbon was denoted as H-CMK-3. Supported iron oxide catalysts were prepared by the wet impregnation method with Fe(NO3)39H2O as an iron precursor, and the obtained solid was calcined at 400 1C for 6 h in an argon flow. The loading amount of iron oxides is 5 wt% (calculated with Fe2O3) and the resultant material was denoted as FeOx/H-CMK-3. Low-angle XRD patterns show that H-CMK-3 maintains ordered arrangement of parent CMK-3 (Fig. 1A). It exhibits three peaks in the range 2y = 0.75–31, which can be indexed to the (100), (110) and (200) reflections of the hexagonal space group (P6mm). The patterns of FeOx/H-CMK-3 show a remarkable decrease in the intensity of all reflections compared with that of H-CMK-3. The decrease may be correlated with the partial damage to the structural order of CMK-3 or due to the fact that the introduction of scattering material into the pores leads to an increased phase cancellation between scattering form walls and the pore regions. A similar effect has been discussed in the literature dealing with metal oxide incorporation into the pores of silicate and carbon mesostructures.7 Wide-angle XRD patterns show that all of these samples exhibit two broad diffraction peaks centered at 24.51 and 441 (Fig. 1B), which are generally assigned to diffractions from the (002) and (100) graphite planes of carbon supports.8 No crystalline phase related to iron oxide species could be observed, indicating that the particle size of iron oxides is very small or the degree of crystallization is very low. N2 adsorption–desorption isotherms show that all samples exhibit typical isotherms of ordered mesostructures (type IV) with a welldefined step in the adsorption curve near a P/P0 value of 0.5 (Fig. 1C).
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Fig. 1 Low-angle (A) and wide-angle (B) XRD patterns of CMK-3, H-CMK-3 and supported FeOx catalysts; N2 adsorption–desorption isotherms and BJH pore size distribution (C) and XPS spectra (D) of FeOx/H-CMK-3 and FeOx/CMK-3.
The surface area and pore volume of H-CMK-3 are 918 m2 g 1 and 0.95 cm3 g 1, which are similar to those of CMK-3 (833 m2 g 1 and 1.02 cm3 g 1). FeOx/H-CMK-3 maintains a high surface area (1118 m2 g 1) and large pore volume (1.13 cm3 g 1), indicating that the iron oxides do not block the mesopores of the CMK-3 support. XPS was carried out to detect the surface chemical state of iron species. As shown in Fig. 1D, the Fe (2p) core level is split into Fe2p1/2 and Fe2p3/2 due to spin–orbit coupling, and appears at 724.2 eV and 710.9 eV respectively. These values are close to the range of the Fe2p binding energies of iron in Fe2O3, indicating that most iron on the surface of samples were present at a valence of +3.9 A small amount of Fe2+ should also be present in these samples as suggested by the weak shoulder of the main Fe2p3/2 peak at lower binding energies.9 The morphology of FeOx/H-CMK-3 is investigated with high resolution transmission electron microscopy (Fig. 2a). It can be observed that the FeOx/H-CMK-3 exhibits a linear mesoporous array. Its structural order is a little lower than that of pristine CMK-3 (Fig. S1a, ESI†). Interestingly, no iron oxide particles could be observed in the whole detected region even in high resolution measurement (Fig. 2a and b). STEM and EDX mapping results show that iron species should be present as quite small particles highly dispersed throughout the H-CMK-3 support (Fig. 2c and d and Fig. S3, ESI†). No isolated iron spots should be due to relatively high contents of iron oxides (5 wt%) and the homogeneous dispersion state. In comparison, CMK-3 without being treated with HNO3 was also used for supporting iron oxides (denoted as FeOx/CMK-3). Fig. 2e and f show that iron oxide particles with diameters of 5–10 nm can be observed on the surface of CMK-3. Additionally, a few relatively large iron oxide particles (10–20 nm) were detected in FeOx/CMK-3 (Fig. S1c, ESI†). This sample exhibits a relatively low surface area (503 m2 g 1) and pore volume (0.54 cm3 g 1), which should be due to iron oxide particles blocking the mesopores of the CMK-3 support. Temperature programmed reduction (TPR) in a H2/Ar stream was carried out to detect the reducibility of iron oxides with
2966 | Chem. Commun., 2014, 50, 2965--2967
Fig. 2 HRTEM, HAADF-STEM and EDX mapping images of FeOx/H-CMK-3 (a–d) and HRTEM images of FeOx/CMK-3 (e, f).
different dispersion states. Fig. 3 shows that obvious H2 consumption peaks appear in the profiles of FeOx/H-CMK-3 and FeOx/CMK-3. The intensity of these peaks is much higher than that of CMK-3 and H-CMK-3, suggesting that these peaks can be ascribed to reduction of iron oxides on the surface of carbon supports. The main reduction peak of FeOx/H-CMK-3 (477 1C) is much lower than that of FeOx/CMK-3 (650 1C). This result shows that iron oxides in FeOx/H-CMK-3 are facilely reduced compared with those in FeOx/CMK-3, which should be due to the small particle size of iron oxides in FeOx/H-CMK-3. Selective oxidation of benzyl alcohol with oxygen is an important catalytic reaction in organic synthesis. Previously, it was reported
Fig. 3
H2-TPR profiles of CMK-3, H-CMK-3 and supported FeOx catalysts.
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Fig. 4 Catalytic performance of CMK-3, H-CMK-3 and supported FeOx catalysts. Reaction conditions: mcat 0.3 g; toluene 10 mL, benzyl alcohol 1 mmol; temperature 353 K; pressure (air) 1 atm.
that a variety of supported noble metal (e.g. Ru, Pd, Au) catalysts are required for this aerobic oxidation reaction.10 Fig. 4 shows that FeOx/ H-CMK-3 could catalyze this aerobic oxidation reaction. It exhibits a 72% conversion of benzyl alcohol after 8 h reaction, which is much higher than those of FeOx/CMK-3, H-CMK-3 and pristine CMK-3. The selectivity to benzaldehyde is nearly 100%. As for iron oxide, almost no benzyl alcohol conversion can be detected, indicating that iron oxide itself is inactive for the oxidation of benzyl alcohol with air as an oxidant source. Multiple aerobic oxidation cycles were carried out to examine the recoverability of FeOx/H-CMK-3. When the catalyst samples were recovered by simple decanting of the reaction solution, an activity decrease can be observed after two reaction cycles (Fig. S4, ESI†). Then the conversion of benzyl alcohol is maintained at about 33% during the subsequent reaction cycles, which is still much higher than those of other samples. When the used catalysts were thermally treated under an argon atmosphere at 400 1C for 30 min, the samples exhibited nearly consistent activity as the fresh one. This might be due to the complete removal of absorbed species (e.g. aldehydes) after thermal treatment.10d Magnetic measurement shows that FeOx/H-CMK-3 exhibits weak ferromagnetic behaviour (Fig. S6, ESI†). The measured saturation magnetization (Ms) is 1.29 emu g 1. This result suggests that the catalyst can be easily recovered from the reaction system by magnetic separation. Based on the above characterization results, treating CMK-3 with HNO3 is a critical factor for obtaining highly dispersed iron oxide catalysts. It generates a great deal of oxygen-functional groups, including –COOH and CQO, compared with that of pristine CMK-3 (Fig. S5, ESI,† DRIFT spectra). According to the literature and our previous studies, the surface groups should act as nucleation centers for the generation of highly dispersed metal crystallites.11 Besides, the confinement effect of these surface functionalities of CMK-3 mesoporous structure is assumed to stabilize the metal crystallites to minimize metal sintering, contributing to obtaining the highly dispersed iron oxide catalysts. It is widely accepted that decreasing the particle size is related to the higher concentration of low coordinated metal atoms in the small particles. It should be responsible for the facile changing oxidation state of iron oxides and relatively high reduction–oxidation activity over FeOx/H-CMK-3. Additionally, the abundant oxygen-containing functional groups on the surface of carbon supports should provide an environment for these small particles, which is quite similar to the
This journal is © The Royal Society of Chemistry 2014
Communication
ligands surrounding central metals in homogeneous catalysis. The interface iron species may possess similar electronic properties to organometallic compounds, which may function as active sites for the selective oxidation of benzyl alcohol with molecular oxygen. Further work is still needed to clear the nature of active sites. In summary, highly dispersed iron oxide catalysts have been obtained with HNO3 treated mesoporous carbon of CMK-3 as a support. It exhibits a high activity in the selective oxidation of benzyl alcohol with air as an oxidant source. The abundant surface functional groups and the confinement effect of CMK-3 mesoporous structure contribute to obtaining the highly dispersed iron oxide active sites. We believe that the activity of resultant catalysts could be further improved by tuning the surface properties of carbon supports. This work was supported by the National Natural Science Foundation of China (21003059), the Development Project of Science and Technology of Jilin Province (20130101014JC, 201105009) and the Fundamental Research Funds for the Central Universities.
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