- Email: [email protected]
Improvement on the mesostructural ordering and catalytic activity of Co-MCM-41 with ascorbic acid as auxiliary
Materials Letters 100 (2013) 159–162
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Improvement on the mesostructural ordering and catalytic activity of Co-MCM-41 with ascorbic acid as auxiliary Bing Han a,b, Haiqing Wang a, Yan Kong a,n, Jun Wang a a b
State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China School of Material Engineering, Nanjing Institute of Technology, Nanjing 211167, China
ar t ic l e i nf o
a b s t r a c t
Article history: Received 31 July 2012 Accepted 6 March 2013 Available online 15 March 2013
A series of Co-MCM-41 with different cobalt contents were synthesized with ascorbic acid as auxiliary by direct hydrothermal synthesis, in which surprising promotion effect of the ascorbic acid on the longrange ordering of mesoporous structure and its crucial contribution to the catalytic activity were validated. Without the assistance of ascorbic acid, we were unable to synthesize the Co-MCM-41 mesoporous silica with Co/Si molar ratio greater than 0.15, while it is possible for us to obtain sample with Co/Si molar ratio up to 0.30 and more cobalt atoms anchored in the framework of Co-MCM-41 with the assistance of ascorbic acid. Meanwhile, its catalytic activity in the direct hydroxylation of benzene with hydroperoxide as oxidant was highly improved. & 2013 Elsevier B.V. All rights reserved.
Keywords: Co-MCM-41 Ascorbic acid Sol–gel preparation Porous materials Catalytic activity
1. Introduction Since the mesostructural molecular sieves designated as M41S were discovered by researchers at Mobil R&D Corp. [1,2], persistent efforts have been devoted to the study of the synthesis of mesoporous molecular sieves in the last few decades, mainly due to their regularly ordered pore arrangement, very narrow poresize distribution which can be adjusted in the range of 2–10 nm or larger, high surface area and relatively high thermal stability, and therefore could be widely used in the field of catalysis, sorption and separation [3,4]. However, their application to catalytic reaction necessitates the introduction of heteroatom into the framework of them by impregnation or direct synthesis method, due to lack of active sites for pure mesoporous silica. MCM-41 functionalized with metals as aluminum, titanium, iron and cobalt is for long time the object of persistent interests, as they constitute efficient catalysts for various attractive industrial processes [5,6]. Cobalt is an essential ingredient in many catalysts. Recently, the incorporation of cobalt into mesoporous silica framework and the resulting excellent catalytic activity had been widely investigated. It was also found that cobalt-modified mesoporous materials were effective catalysts for Fischer-Tropsch synthesis [7,8], for hydrodesulfurization [9], for CO oxidation [10], and for the oxidation of hydrocarbon, such as cyclohexanol, styrene, and tetralin [11–16], etc. The experimental data suggest the enhanced catalytic activity originates from the Lewis acid sites created by the incorporated
n
Corresponding author. Tel./fax: 86 25 8358 7860. E-mail address: [email protected] (Y. Kong).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.03.015
cobalt species, which are anchoring strongly in the framework of mesoporous silica. Unfortunately, the introduction of heteroatom generally makes the ordering of mesoporous structure become worse, which is usually accompanied by lower catalytic properties, and it is clear that a regular structure of mesoporous materials is important for their applications. In this work, a series of Co-MCM-41 with different cobalt contents were synthesized with ascorbic acid as auxiliary by direct hydrothermal synthesis. The obtained materials were characterized by X-ray diffraction (XRD), N2 physisorption, elemental analysis, transmission electron microscopy (TEM) and hydrogen temperature programmed reduction (H2-TPR). The mesoporous regularity, the valence cobalt atom and the amount of incorporated cobalt atom of prepared Co-MCM-41 material were primarily studied. Besides, the catalytic activity of Co-MCM-41 synthesized with ascorbic acid as auxiliary was preliminary investigated in the hydroxylation of benzene with H2O2 as oxidant. 2. Experimental The preparation of the Co-MCM-41 with different Co/Si atomic ratios (varied between 0.05 and 0.3) was carried out by direct hydrothermal synthesis. The synthesis process is as follows: 5.7 g Na2SiO3 9H2O and 1.8 g of CTAB were dissolved in 50 ml distilled water. The solution was heated up to be homogeneous and then cooled to room temperature (denoted as solution A). The appropriate amount of ascorbic acid (C6H8O6) and cobalt acetate were dissolved in deionized water. Concentrated NH3 H2O was added dropwise to the solution until the solution became transparent
160
B. Han et al. / Materials Letters 100 (2013) 159–162
(denoted as solution B). Solution B was then added to solution A, the reaction mixture was stirred at room temperature for 10 min, its pH value was afterwards adjusted to 11 with H2SO4 (1 mol L−1). The obtained mixture was subsequently transferred into a Teflonlined autoclave to age at 120 1C for 72 h. After the hydrothermal treatment, the resulting solid products were filtered, washed repeatedly with deionized water and dried under ambient condition. The pre-dried solid was calcined for 5 h at 550 1C with heating rate of 1 1C/min under flowing air (80 ml/min) to completely remove the organic templating agent. The obtained samples are denoted as xCo, where x is 100 times of the calculated amount of cobalt acetate for different Co/Si atomic ratios, while the samples prepared by similar procedure but without coreagent are denoted as xCo-1. XRD patterns were recorded on a Rigaku D/Max-RAX instrument with Cu-Kα radiation. N2 physisorption was measured with the Micromeritics ASAP-2020 instrument. Elemental analysis was performed on Jarrell-Ash 1100 ICP emission spectrometer. TEM images were examined on a JEM 2010 high resolution transmission electron microscope. Hydrogen temperature programmed reduction (H2-TPR) was performed on a JAPAN BELCAT-Analyzer. The catalytic activity assessment of the Co-MCM-41was performed as follows: 0.1 g catalyst and 2 ml benzene were added to 15 ml acetic acid in a 50 ml flask placed in a water bath. The temperature was adjusted to 65 1C and maintained throughout the reaction. Then a certain amount of H2O2 (30 vol%, the molar ratio of benzene/H2O2 ¼1/3) was added dropwise. The reaction mixture was stirred for 12 h, and then the resulting solution was centrifuged and analyzed with gas chromatography (SP-6890) equipped with a capillary column (0.32 mm 30 m SE-54). The benzene conversion and selectivity of phenol were calculated by the normalization method.
3. Results and discussion Characterizations of Co-MCM-41 catalyst: The XRD patterns of Co-MCM-41samples are shown in Fig. 1. It presented three characteristic peaks corresponding to reflections on the (100), (110) and (200) planes, which indicated the formation of wellordered two-dimensional hexagonal structure [17]. The intensity of the peaks decreased gradually with the increase of cobalt content, which meant that the ordered mesostructure was disturbed by the incorporation of cobalt species. As it can be seen from the XRD patterns, the xCo-1 samples only had a weaker and broader (100) characteristic peak, while the xCo samples had three
stronger (100), (110) and (200) characteristic peaks, which indicated the adding of ascorbic acid made the mesoporous structure become more regular in the same cobalt content. It was also found that the peaks of the xCo samples had an obvious shift towards small angle compared with the xCo-1 samples, by which we can infer there may be a larger number of cobalt atoms incorporated inside the framework of MCM-41 [18]. The physical properties of all samples are summarized in Table 1. It can be noted that the BET surface area decreased as the content of cobalt increased, which may be due to the blocking of pores of MCM-41 by cobalt oxides scattered on the outer surface of mesoporous channels, and the samples 15Co and 10Co had the maximum pore size and total pore volume respectively. Fig. 2 shows that typical hexagonal arrangement of the pores of the MCM-41 mesoporous materials [1] was quite clear in the two samples. An additional proof of the hexagonal symmetry structure is given by the FT pattern (inset Fig. 2(a)). By contrast the TEM images of samples 5Co and 5Co-1, the pore structure of the latter transforms to be wormhole-like and the regularity becomes poorer obviously, which is consistent with the result of XRD. So, we can draw an inference that the regularity of samples can be improved greatly by the adding of ascorbic acid. On the other hand, compared with the samples xCo-1, the samples xCo with high cobalt content up to 0.3 and highly ordered mesopores can be synthesized with ascorbic acid as auxiliary, which can be explained preliminarily that there exists electrostatic interaction between the two templates, as a result, promoting the regularity of the C16TMABr surfactant micelles and/or that ascorbic acid can make the rate of hydrolysis of cobalt ions and silicate synchronize by the electrostatic interaction between cobalt ion and ascorbic acid. The specific reasons are still for further study. As can be seen from Fig. 3, the TPR profiles of the Co-MCM-41 samples generally exhibit two peaks. The first reduction peak appeared at around 500 1C would correspond to the reduction of large particles of cobalt oxide scattered on the outer surface of
Table 1 The physical properties of Co-MCM-41 samples. Sample Co/Si (mol%)a SBET (m2 g−1) a0 (nm) DP (nm) ts (nm) VP (cm3 g−1) 5Co 10Co 15Co 20Co 25Co a
4.7 11.1 14.7 18.7 20.5
648 608 603 586 559
ICP results.
(100) 30Co 15Co
Intensity (a.u.)
(110) (200)
15Co-1
10Co 10Co-1
5Co 5Co-1
1
2
3
4
5
6
7
2θ (Degree) Fig. 1. XRD patterns of Co-MCM-41samples.
8
4.32 4.44 4.60 4.47 4.47
2.50 2.63 2.80 2.58 2.60
1.82 1.80 1.79 1.89 1.87
0.60 0.66 0.59 0.57 0.55
B. Han et al. / Materials Letters 100 (2013) 159–162
161
Fig. 2. (a) and (b) TEM images of Co-MCM-41 samples 5Co and 5Co-1, and corresponding FT pattern (right upper corner in Fig. 2(a)).
benzene conversion under the same reaction conditions. The result of catalytic activity may be due to the increase of the content of the divalent cobalt or more ordered mesostructure for the samples xCo synthesized with the adding of ascorbic acid.
4. Conclusions With ascorbic acid as auxiliary in the synthesis process, the samples Co-MCM-41(xCo) with high cobalt content, highly ordered mesopores and more cobalt incorporated in the framework of Co-MCM-41 were successfully synthesized. The application of the Co-MCM-41 catalyst to the one-step hydroxylation of benzene by using hydrogen peroxide as oxidant gave a satisfactory phenol yield, and the resulting long-range ordered mesoporous structure was proved to be crucial for the catalytic activity of the Co-MCM-41 catalyst.
Acknowledgments Fig. 3. H2-TPR profiles of Co-MCM-41 samples 10Co, 10Co-1 and 25Co.
mesoporous channels [18]. The second reduction peak at higher temperature around 700 1C could be explained by the strong cobalt-silica interaction, in particular for the reduction of cobalt silicates [19]. For 10Co and 25Co samples, the reduction peaks at around 500 1C are much weaker than that of 10Co-1, while the reduction peaks appear at around 700 1C are stronger. Combined with the patterns of X-ray diffraction, we can conclude that more cobalt species should be located in the framework of MCM-41 [20] with the assistance of ascorbic acid. Moreover, by chemical titration analysis method, we got the molar ratio of Co3+/Co2+ in framework of 10Co (0.071) was smaller than that of 10Co-1(1.052), which may be due to the reducing property of ascorbic acid which can significantly increased the content of divalent cobalt. 3.1. Catalytic activity By studying the catalysts Co-MCM-41 for benzene hydroxylation, we found that the phenol selectivity for all samples was 100% and the sample 10Co with Co/Si ¼0.1, which had the largest pore volume and highly ordered mesopores as characterized by nitrogen physisorption and X-ray diffraction, gave the maximum benzene conversion 35.4%. While the sample 10Co-1, despite its identical Co/Si molar ratio with the sample 10Co, only gave 18.3%
Thanks to the financial support of the National Natural Science Foundations of China (21276125, 20876077), the Major Projects of the Natural Science Fund for High Education of Jiangsu Province (10KJA530015 and 11KJB430008) and the Jiangsu Province Natural Science Fund (BK2011372). References [1] Beck JS, Vartuli JC, Roth WJ, Leonowicz ME, Kresge CT, Schmitt KD, et al. J Am Chem Soc 1992;114 10834–34. [2] Kresge CT, Leonowicz ME, Roth WJ, Vartuli JC, Beck JS. Nature 1992;359:710–2. [3] Niwa S, Eswaramoorthy M, Nair J, Raj A, Itoh N, Shoji H, et al. Science 2002;295:105–7. [4] Notte PP. Topics Catal 2000;13:387–94. [5] Corma A. Chem Rev 1997;97:2373–419. [6] Taguchi A, Schuth F. Microporous Mesoporous Mater 2005;77:1–45. [7] Jung JS, Kim SW, Moon DJ. Catal Today 2012;185:168–74. [8] Eggenhuisen TM, Den Breejen Johan P, Verdoes D. J Am Chem Soc 2010;132:18318–25. [9] Alonso-Nunez G, Bocarando J, Huirache-Acuna R, Alvarez-Contrerasc L, Huang ZD. Appl Catal A Gen 2012;419:95–101. [10] Wang WX, Li YW, Zhang RJ, He DH, Liu HL, Liao SJ. Catal Commun 2011;12:875–9. [11] Taghavimoghaddam J, Knowles GP, Chaffee AL. J Mol Catal A Chem 2012;358:79–88. [12] Cui HT, Zhang Y, Qiu ZG, Zhao LF, Zhu YL. Appl Catal B Environ 2010;101:45–53. [13] Ma YP, Zeng ML, He J, Duan LN, Wang JF, Li JJ. Appl Catal A Gen 2011;396:123–8.
162
[14] [15] [16] [17] [18]
B. Han et al. / Materials Letters 100 (2013) 159–162
Masoud SNJ. Mol Catal A Chem 2008;283:120–8. Masoud SNJ. Mol Catal A Chem 2009;310:51–8. Masoud SN, Elahe E, Hamideh S, Mehdi B. Inorg Chim Acta 2011;375:11–9. Yuan YZ, Cao W, Weng WZ. J Catal 2004;228:311–20. Lin HY, Chen YW. Mater Chem Phys 2004;85:171–5.
[19] Ataloglou T, Vakros J, Bourikas K, Fountzoula C, Kordulis C, Lycourghiotis A. Appl Catal B 2005;57:299–312. [20] Todorova S, Parvulescu V, Kadinov G, Tenchev K, Somacescu S, Su BL. Microporous Mesoporous Mater 2008;113:22–30.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Improvement on the mesostructural ordering and catalytic activity of Co-MCM-41 with ascorbic acid as auxiliary Bing Han a,b, Haiqing Wang a, Yan Kong a,n, Jun Wang a a b
State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China School of Material Engineering, Nanjing Institute of Technology, Nanjing 211167, China
ar t ic l e i nf o
a b s t r a c t
Article history: Received 31 July 2012 Accepted 6 March 2013 Available online 15 March 2013
A series of Co-MCM-41 with different cobalt contents were synthesized with ascorbic acid as auxiliary by direct hydrothermal synthesis, in which surprising promotion effect of the ascorbic acid on the longrange ordering of mesoporous structure and its crucial contribution to the catalytic activity were validated. Without the assistance of ascorbic acid, we were unable to synthesize the Co-MCM-41 mesoporous silica with Co/Si molar ratio greater than 0.15, while it is possible for us to obtain sample with Co/Si molar ratio up to 0.30 and more cobalt atoms anchored in the framework of Co-MCM-41 with the assistance of ascorbic acid. Meanwhile, its catalytic activity in the direct hydroxylation of benzene with hydroperoxide as oxidant was highly improved. & 2013 Elsevier B.V. All rights reserved.
Keywords: Co-MCM-41 Ascorbic acid Sol–gel preparation Porous materials Catalytic activity
1. Introduction Since the mesostructural molecular sieves designated as M41S were discovered by researchers at Mobil R&D Corp. [1,2], persistent efforts have been devoted to the study of the synthesis of mesoporous molecular sieves in the last few decades, mainly due to their regularly ordered pore arrangement, very narrow poresize distribution which can be adjusted in the range of 2–10 nm or larger, high surface area and relatively high thermal stability, and therefore could be widely used in the field of catalysis, sorption and separation [3,4]. However, their application to catalytic reaction necessitates the introduction of heteroatom into the framework of them by impregnation or direct synthesis method, due to lack of active sites for pure mesoporous silica. MCM-41 functionalized with metals as aluminum, titanium, iron and cobalt is for long time the object of persistent interests, as they constitute efficient catalysts for various attractive industrial processes [5,6]. Cobalt is an essential ingredient in many catalysts. Recently, the incorporation of cobalt into mesoporous silica framework and the resulting excellent catalytic activity had been widely investigated. It was also found that cobalt-modified mesoporous materials were effective catalysts for Fischer-Tropsch synthesis [7,8], for hydrodesulfurization [9], for CO oxidation [10], and for the oxidation of hydrocarbon, such as cyclohexanol, styrene, and tetralin [11–16], etc. The experimental data suggest the enhanced catalytic activity originates from the Lewis acid sites created by the incorporated
n
Corresponding author. Tel./fax: 86 25 8358 7860. E-mail address: [email protected] (Y. Kong).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.03.015
cobalt species, which are anchoring strongly in the framework of mesoporous silica. Unfortunately, the introduction of heteroatom generally makes the ordering of mesoporous structure become worse, which is usually accompanied by lower catalytic properties, and it is clear that a regular structure of mesoporous materials is important for their applications. In this work, a series of Co-MCM-41 with different cobalt contents were synthesized with ascorbic acid as auxiliary by direct hydrothermal synthesis. The obtained materials were characterized by X-ray diffraction (XRD), N2 physisorption, elemental analysis, transmission electron microscopy (TEM) and hydrogen temperature programmed reduction (H2-TPR). The mesoporous regularity, the valence cobalt atom and the amount of incorporated cobalt atom of prepared Co-MCM-41 material were primarily studied. Besides, the catalytic activity of Co-MCM-41 synthesized with ascorbic acid as auxiliary was preliminary investigated in the hydroxylation of benzene with H2O2 as oxidant. 2. Experimental The preparation of the Co-MCM-41 with different Co/Si atomic ratios (varied between 0.05 and 0.3) was carried out by direct hydrothermal synthesis. The synthesis process is as follows: 5.7 g Na2SiO3 9H2O and 1.8 g of CTAB were dissolved in 50 ml distilled water. The solution was heated up to be homogeneous and then cooled to room temperature (denoted as solution A). The appropriate amount of ascorbic acid (C6H8O6) and cobalt acetate were dissolved in deionized water. Concentrated NH3 H2O was added dropwise to the solution until the solution became transparent
160
B. Han et al. / Materials Letters 100 (2013) 159–162
(denoted as solution B). Solution B was then added to solution A, the reaction mixture was stirred at room temperature for 10 min, its pH value was afterwards adjusted to 11 with H2SO4 (1 mol L−1). The obtained mixture was subsequently transferred into a Teflonlined autoclave to age at 120 1C for 72 h. After the hydrothermal treatment, the resulting solid products were filtered, washed repeatedly with deionized water and dried under ambient condition. The pre-dried solid was calcined for 5 h at 550 1C with heating rate of 1 1C/min under flowing air (80 ml/min) to completely remove the organic templating agent. The obtained samples are denoted as xCo, where x is 100 times of the calculated amount of cobalt acetate for different Co/Si atomic ratios, while the samples prepared by similar procedure but without coreagent are denoted as xCo-1. XRD patterns were recorded on a Rigaku D/Max-RAX instrument with Cu-Kα radiation. N2 physisorption was measured with the Micromeritics ASAP-2020 instrument. Elemental analysis was performed on Jarrell-Ash 1100 ICP emission spectrometer. TEM images were examined on a JEM 2010 high resolution transmission electron microscope. Hydrogen temperature programmed reduction (H2-TPR) was performed on a JAPAN BELCAT-Analyzer. The catalytic activity assessment of the Co-MCM-41was performed as follows: 0.1 g catalyst and 2 ml benzene were added to 15 ml acetic acid in a 50 ml flask placed in a water bath. The temperature was adjusted to 65 1C and maintained throughout the reaction. Then a certain amount of H2O2 (30 vol%, the molar ratio of benzene/H2O2 ¼1/3) was added dropwise. The reaction mixture was stirred for 12 h, and then the resulting solution was centrifuged and analyzed with gas chromatography (SP-6890) equipped with a capillary column (0.32 mm 30 m SE-54). The benzene conversion and selectivity of phenol were calculated by the normalization method.
3. Results and discussion Characterizations of Co-MCM-41 catalyst: The XRD patterns of Co-MCM-41samples are shown in Fig. 1. It presented three characteristic peaks corresponding to reflections on the (100), (110) and (200) planes, which indicated the formation of wellordered two-dimensional hexagonal structure [17]. The intensity of the peaks decreased gradually with the increase of cobalt content, which meant that the ordered mesostructure was disturbed by the incorporation of cobalt species. As it can be seen from the XRD patterns, the xCo-1 samples only had a weaker and broader (100) characteristic peak, while the xCo samples had three
stronger (100), (110) and (200) characteristic peaks, which indicated the adding of ascorbic acid made the mesoporous structure become more regular in the same cobalt content. It was also found that the peaks of the xCo samples had an obvious shift towards small angle compared with the xCo-1 samples, by which we can infer there may be a larger number of cobalt atoms incorporated inside the framework of MCM-41 [18]. The physical properties of all samples are summarized in Table 1. It can be noted that the BET surface area decreased as the content of cobalt increased, which may be due to the blocking of pores of MCM-41 by cobalt oxides scattered on the outer surface of mesoporous channels, and the samples 15Co and 10Co had the maximum pore size and total pore volume respectively. Fig. 2 shows that typical hexagonal arrangement of the pores of the MCM-41 mesoporous materials [1] was quite clear in the two samples. An additional proof of the hexagonal symmetry structure is given by the FT pattern (inset Fig. 2(a)). By contrast the TEM images of samples 5Co and 5Co-1, the pore structure of the latter transforms to be wormhole-like and the regularity becomes poorer obviously, which is consistent with the result of XRD. So, we can draw an inference that the regularity of samples can be improved greatly by the adding of ascorbic acid. On the other hand, compared with the samples xCo-1, the samples xCo with high cobalt content up to 0.3 and highly ordered mesopores can be synthesized with ascorbic acid as auxiliary, which can be explained preliminarily that there exists electrostatic interaction between the two templates, as a result, promoting the regularity of the C16TMABr surfactant micelles and/or that ascorbic acid can make the rate of hydrolysis of cobalt ions and silicate synchronize by the electrostatic interaction between cobalt ion and ascorbic acid. The specific reasons are still for further study. As can be seen from Fig. 3, the TPR profiles of the Co-MCM-41 samples generally exhibit two peaks. The first reduction peak appeared at around 500 1C would correspond to the reduction of large particles of cobalt oxide scattered on the outer surface of
Table 1 The physical properties of Co-MCM-41 samples. Sample Co/Si (mol%)a SBET (m2 g−1) a0 (nm) DP (nm) ts (nm) VP (cm3 g−1) 5Co 10Co 15Co 20Co 25Co a
4.7 11.1 14.7 18.7 20.5
648 608 603 586 559
ICP results.
(100) 30Co 15Co
Intensity (a.u.)
(110) (200)
15Co-1
10Co 10Co-1
5Co 5Co-1
1
2
3
4
5
6
7
2θ (Degree) Fig. 1. XRD patterns of Co-MCM-41samples.
8
4.32 4.44 4.60 4.47 4.47
2.50 2.63 2.80 2.58 2.60
1.82 1.80 1.79 1.89 1.87
0.60 0.66 0.59 0.57 0.55
B. Han et al. / Materials Letters 100 (2013) 159–162
161
Fig. 2. (a) and (b) TEM images of Co-MCM-41 samples 5Co and 5Co-1, and corresponding FT pattern (right upper corner in Fig. 2(a)).
benzene conversion under the same reaction conditions. The result of catalytic activity may be due to the increase of the content of the divalent cobalt or more ordered mesostructure for the samples xCo synthesized with the adding of ascorbic acid.
4. Conclusions With ascorbic acid as auxiliary in the synthesis process, the samples Co-MCM-41(xCo) with high cobalt content, highly ordered mesopores and more cobalt incorporated in the framework of Co-MCM-41 were successfully synthesized. The application of the Co-MCM-41 catalyst to the one-step hydroxylation of benzene by using hydrogen peroxide as oxidant gave a satisfactory phenol yield, and the resulting long-range ordered mesoporous structure was proved to be crucial for the catalytic activity of the Co-MCM-41 catalyst.
Acknowledgments Fig. 3. H2-TPR profiles of Co-MCM-41 samples 10Co, 10Co-1 and 25Co.
mesoporous channels [18]. The second reduction peak at higher temperature around 700 1C could be explained by the strong cobalt-silica interaction, in particular for the reduction of cobalt silicates [19]. For 10Co and 25Co samples, the reduction peaks at around 500 1C are much weaker than that of 10Co-1, while the reduction peaks appear at around 700 1C are stronger. Combined with the patterns of X-ray diffraction, we can conclude that more cobalt species should be located in the framework of MCM-41 [20] with the assistance of ascorbic acid. Moreover, by chemical titration analysis method, we got the molar ratio of Co3+/Co2+ in framework of 10Co (0.071) was smaller than that of 10Co-1(1.052), which may be due to the reducing property of ascorbic acid which can significantly increased the content of divalent cobalt. 3.1. Catalytic activity By studying the catalysts Co-MCM-41 for benzene hydroxylation, we found that the phenol selectivity for all samples was 100% and the sample 10Co with Co/Si ¼0.1, which had the largest pore volume and highly ordered mesopores as characterized by nitrogen physisorption and X-ray diffraction, gave the maximum benzene conversion 35.4%. While the sample 10Co-1, despite its identical Co/Si molar ratio with the sample 10Co, only gave 18.3%
Thanks to the financial support of the National Natural Science Foundations of China (21276125, 20876077), the Major Projects of the Natural Science Fund for High Education of Jiangsu Province (10KJA530015 and 11KJB430008) and the Jiangsu Province Natural Science Fund (BK2011372). References [1] Beck JS, Vartuli JC, Roth WJ, Leonowicz ME, Kresge CT, Schmitt KD, et al. J Am Chem Soc 1992;114 10834–34. [2] Kresge CT, Leonowicz ME, Roth WJ, Vartuli JC, Beck JS. Nature 1992;359:710–2. [3] Niwa S, Eswaramoorthy M, Nair J, Raj A, Itoh N, Shoji H, et al. Science 2002;295:105–7. [4] Notte PP. Topics Catal 2000;13:387–94. [5] Corma A. Chem Rev 1997;97:2373–419. [6] Taguchi A, Schuth F. Microporous Mesoporous Mater 2005;77:1–45. [7] Jung JS, Kim SW, Moon DJ. Catal Today 2012;185:168–74. [8] Eggenhuisen TM, Den Breejen Johan P, Verdoes D. J Am Chem Soc 2010;132:18318–25. [9] Alonso-Nunez G, Bocarando J, Huirache-Acuna R, Alvarez-Contrerasc L, Huang ZD. Appl Catal A Gen 2012;419:95–101. [10] Wang WX, Li YW, Zhang RJ, He DH, Liu HL, Liao SJ. Catal Commun 2011;12:875–9. [11] Taghavimoghaddam J, Knowles GP, Chaffee AL. J Mol Catal A Chem 2012;358:79–88. [12] Cui HT, Zhang Y, Qiu ZG, Zhao LF, Zhu YL. Appl Catal B Environ 2010;101:45–53. [13] Ma YP, Zeng ML, He J, Duan LN, Wang JF, Li JJ. Appl Catal A Gen 2011;396:123–8.
162
[14] [15] [16] [17] [18]
B. Han et al. / Materials Letters 100 (2013) 159–162
Masoud SNJ. Mol Catal A Chem 2008;283:120–8. Masoud SNJ. Mol Catal A Chem 2009;310:51–8. Masoud SN, Elahe E, Hamideh S, Mehdi B. Inorg Chim Acta 2011;375:11–9. Yuan YZ, Cao W, Weng WZ. J Catal 2004;228:311–20. Lin HY, Chen YW. Mater Chem Phys 2004;85:171–5.
[19] Ataloglou T, Vakros J, Bourikas K, Fountzoula C, Kordulis C, Lycourghiotis A. Appl Catal B 2005;57:299–312. [20] Todorova S, Parvulescu V, Kadinov G, Tenchev K, Somacescu S, Su BL. Microporous Mesoporous Mater 2008;113:22–30.