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Mesoporous carbon-supported cobalt catalyst for selective oxidation of toluene and degradation of water contaminants
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Mesoporous carbon-supported cobalt catalyst for selective oxidation of toluene and degradation of water contaminants Yuan Zhuang a , Qisong Lin a , Li Zhang b , Lianshun Luo a , Yuyuan Yao a,∗ , Wangyang Lu a , Wenxing Chen a a b
National Engineering Lab of Textile Fiber Materials & Processing Technology, Zhejiang Sci-Tech University, Hangzhou 310018, China School of Material Science and Chemical Engineering, Ningbo University, Ningbo 315211, China
a r t i c l e
i n f o
Article history: Received 9 March 2015 Received in revised form 8 May 2015 Accepted 20 May 2015 Available online xxx Keywords: Mesoporous carbon Cobalt Bifunctional catalyst Toluene oxidation Dye degradation
a b s t r a c t Mesoporous carbon-supported cobalt (Co-MC) catalysts are widely applied as electrode materials for batteries. Conversely, the development of Co-MC as bifunctional catalysts for application in organic catalytic reactions and degradation of water contaminants is slower. Herein, the catalyst displayed high activity in the selective oxidation of toluene to benzaldehyde under mild conditions, attaining a high selectivity of 92.3%. Factors influencing the catalytic reaction performance were also investigated. Additionally, Co-MC displayed remarkable catalytic activity in degrading dyes relative to the pure metal counterpart. Moreover, the catalyst exhibited excellent reusability, as determined by the cyclic catalytic experiments. The paper demonstrates the potential of Co-MC as a bifunctional catalyst for both toluene selective oxidation and water contaminant degradation. © 2015 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
Introduction Catalytic oxidation is a fundamental reaction occurring in nature as well as a key conversion reaction employed in the synthesis and degradation of organics (Ding et al., 2012; Piera & Bäckvall, 2008). Over the past decades, considerable research efforts have been devoted to design and develop economically feasible and efficient catalysts (Boppana & Jiao, 2011; Huang, Xiang, Li, Jiang, & Guo, 2011; Liang et al., 2011). Transition metals, as a class of promising catalysts, have been applied to catalyze various oxidation reactions based on their multiple oxidation states, particularly in the field of organic catalytic reactions and degradation of water contaminants (Dhakshinamoorthy, Alvaro, & Garcia, 2011; Long, Liu, Wu, Liao, & Li, 2013; bin Saiman et al., 2012; Yan, Li, Zhao, & Chen, 2012; Zhang et al., 2013; Zou et al., 2012). Two groups of transition metal-based catalysts, including noble metal catalysts (Pd, Pt, and Au) and base metal catalysts (Mn, Co, Cu, Fe, and Ni), are used widely. Noble metal catalysts are typically employed because of their high catalytic activity; however, their low abundance and high cost hinder their wide application (Chuang, Liu, Lu, & Wey,
∗ Corresponding author. Tel.: +86 571 86843810; fax: +86 571 86843255. E-mail addresses: [email protected], [email protected] (Y. Yao).
2009). Consequently, base metal catalysts have been increasingly investigated as potential alternatives for oxidation reactions. Among the base metal catalysts studied, cobalt catalysts are promising for use in catalytic oxidation reactions (Jiao & Frei, 2010; Popova et al., 2014). To enhance the stability and dispersion of the active metal, supports, such as SiO2 (Eggenhuisen, Breejen, Verdoes, Jongh, & Jong, 2010; Szegedi, Popova, Mavrodinova, & Minchev, 2008), Al2 O3 (Rane, Borg, Rytter, & Holmen, 2012), TiO2 (Yang, Choi, & Dionysiou, 2007; Shukla, Wang, Sun, Ang, & Tadé, 2010a), MgO (Zhang et al., 2010), and zeolite (Shukla, Wang, Singh, Ang, & Tadé, 2010b), are generally employed. Unfortunately, the interaction between cobalt and SiO2 or Al2 O3 leads to lower catalytic activity because of the formation of mixed compounds during catalyst preparation and reaction (Zhang et al., 2009a). In contrast, the use of TiO2 , MgO, or zeolite supports is limited owing to the unavailability of a suitable approach to recover the nanosized cobalt particles, thereby potentially causing secondary environmental problems (Shukla et al., 2010a; Yang et al., 2007; Zhang et al., 2010). The use of suitable supports would offer excellent catalytic performance (Huang, Bao, Yao, Lu, & Chen, 2014). Therefore, selection of the support is very important toward preparing supported cobalt catalysts with appealing performance and extensive applications. In contrast to the abovementioned supports, mesoporous carbon (MC) has received extensive attention as an excellent support
http://dx.doi.org/10.1016/j.partic.2015.05.010 1674-2001/© 2015 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Zhuang, Y., et al. Mesoporous carbon-supported cobalt catalyst for selective oxidation of toluene and degradation of water contaminants. Particuology (2015), http://dx.doi.org/10.1016/j.partic.2015.05.010
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for catalysts because of its attractive features, i.e., high surface area (up to 2500 m2 /g), large specific pore volume, narrow pore size distribution (2.0–50 nm), and chemical inertness (Almeida, Melo, & Airoldi, 2013; Li, Fu, & Su, 2012; Sun et al., 2012). Furthermore, their open porous network reduces mass transfer limitations when compared with traditional microporous activated carbons (Deng et al., 2010; Wang et al., 2013). Hence, several researchers have investigated mesoporous carbon-supported cobalt for various applications. For instance, Yang et al. (2012) reported the use of ordered mesoporous carbon-supported cobalt-based catalysts for Fischer–Tropsch synthesis, and investigated the influence of the amount of carbon precursor on catalytic performance. Dai & Vogt (2012) reported the synthesis of mesoporous carbon–cobalt composite for the removal of methylene green; the composite catalyst exhibited excellent capacity. Based on these studies, the integration of cobalt into MC holds great potential in developing cobalt catalysts with highly accessible active sites and stable characteristics while retaining the individual functionality. Furthermore, the MC support is expected to provide a good anchoring platform to trap the nanoparticles and prevent catalyst aggregation during catalytic oxidation reactions. To our knowledge, mesoporous carbon-supported cobalt (CoMC) catalysts have been widely applied as negative electrode materials for lithium batteries (Hou, Ndamanisha, Guo, Peng, & Bai, 2009; Liu et al., 2008). In contrast, the use of Co-MC as a bifunctional catalyst for organic catalytic reactions and degradation of water contaminants has been rarely reported. Herein, toluene and various dyes were employed as model compounds to investigate the bifunctional catalytic properties of Co-MC. The results showed that the Co-MC catalyst exhibited high activity toward the selective oxidation of toluene and excellent potential toward the degradation of dyes. Furthermore, compared with free cobalt catalyst, the introduction of MC support into cobalt catalyst system led to a significant enhancement in catalytic activity, and the composite catalyst displayed convenient magnetic separability and excellent reusability. This research offers a new strategy to design catalysts with integrated functions for various advanced applications in organic synthesis, green chemistry, and environmental treatment. Experimental Materials Poly(ethylene oxide)-block-poly(propylene oxide)-blockpoly(ethylene oxide) triblock copolymer Pluronic F127 (PEO106 PPO70 PEO106 ) was purchased from Sigma-Aldrich Co., Ltd. (Shanghai, China). Phenol, formaldehyde, sodium hydroxide, hydrochloric acid, toluene, hydrogen peroxide, acetonitrile, and ethanol were purchased from Gaojing Chem. Co., Ltd. (Hangzhou, China). Cobalt (II, III) oxide (Co3 O4 ) and peroxymonosulfate (PMS; Oxone) were purchased from Aladdin Co., Ltd. (Shanghai, China). Reactive Red M-3BE (RR M-3BE), Acid Red1 (AR 1), Acid Orange 7 (AO 7), Reactive Brilliant Orange K-GN (RBO K-GN), Basic Green (BG), and Weak Acid Pink BS (WAP BS) were commercial compounds (Shinyang Samwoo Fine Chemical Co. Ltd., Hangzhou, China). All chemicals were used as received without any further purification. Doubly distilled water was used in all experiments. Preparation of Co-MC catalyst Soluble resol precursors were prepared using phenol and formaldehyde in a base-catalyzed process according to Sun et al. (2012).The detailed synthesis procedure is available in the Supplementary Material. The Co-MC composite was synthesized through
simple Co3 O4 doping in mesoporous carbon. Typically, 2 g Pluronic F127 was dissolved in 15 ml absolute ethanol at 40 ◦ C. Then, 4.87 g resol precursor solution (51.3 wt% in ethanol) was gradually added, and the mixture was stirred for 10 min, after which 1.47 g Co3 O4 powder was added. After further stirring for 30 min at 40 ◦ C, the mixture was transferred into Petri dishes that were lined with silicon paper to avoid detachment issues. Subsequently, ethanol in the mixture was evaporated for 8 h at 35 ◦ C. The resulting sticky film was subjected to curing at 100 ◦ C. Finally, the obtained solid product was ground into powder. The powder sample was then pyrolyzed in a tube furnace at 800 ◦ C for 4 h under N2 atmosphere (heating rate: 1 ◦ C/min) to decompose the triblock copolymer template and carbonize the resol precursor to finally generate cobalt and its metal oxide. Characterization of catalyst The catalyst samples were characterized by X-ray diffraction (XRD, XTRA, Thermo ARL, Switzerland), transmission electron microscopy (TEM, JEM-2100, JEOL, Japan), Brunner–Emmet–Teller (BET) surface area analysis (3H-2000PS1, Beishide Instrument Technology Co. Ltd., China). The magnetic properties of the CoMC composite catalyst were investigated using a vibrating sample magnetometer (VSM, 7410, Lake Shore, USA). The toluene final reactant and product mixtures were analyzed by gas chromatography mass spectrometry (GC–MS, 6890 N/5973i, Agilent, USA) using bromobenzene as internal standard. The degradation efficiency of the dyes was determined by UV–visible absorption spectroscopy (U-3010, Hitachi, Japan). Catalytic studies The catalytic performance of the Co-MC composite toward the selective oxidation of toluene was examined under atmospheric pressure. Briefly, 3 ml toluene and 15 ml 35% aqueous H2 O2 in acetonitrile (20 ml) were added to an appropriate amount of catalyst in a reaction vessel under continuous vigorous stirring at the desired temperature. After a given reaction time, excess H2 O2 was quenched with sodium thiosulfate (Na2 S2 O3 ), and the reaction mixture was separated by a magnet to remove the catalyst, followed by extraction with ethyl acetate. The final reactant and product mixtures were analyzed by GC–MS using bromobenzene as an internal standard. The yield of benzaldehyde and its selectivity was calculated using the following equations: Conversion (%) = Selectivity (%) =
PhCHO × 100, PhMeall PhCHO × 100, PhMein
(1) (2)
where PhCHO is the molar content of benzaldehyde, PhMein is the molar content of toluene that participated in the reaction, and PhMeall is the initial molar content of toluene that was added to the reaction system. The catalytic degradation of the dyes (RR M-3BE, AR 1, AO 7, RBO K-GN, BG, and WAP BS) was performed in 100 ml glass beakers at a set temperature of 25 ◦ C using a constant temperature shaker water bath. In all experiments, a reaction volume of 50 ml was used. A typical reaction mixture comprised RR M-3BEdye (50 M), unless specified otherwise, Co-MC (0.2 g/L), Co3 O4 (0.2 g/L), and PMS (120 M). pH adjustments were undertaken at the start. At given time intervals, the degradation efficiency of the dyes were determined by UV–vis spectrometer. The residual and removal percentages of the dyes were calculated as follows: Residual percentage dyes (%) = C/C0 × 100 = A/A0 × 100,
(3)
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Fig. 2. Nitrogen sorption isotherm and pore size distribution (inset) of the Co-MC catalyst prepared.
Fig. 1. X-ray diffraction pattern of the Co-MC catalyst prepared.
Removal percentage of dyes (%) = ((C0 − C)/C0 ) × 100 = ((A0 − A)/A0 ) × 100,
(4)
where C0 and C are respectively the initial concentration of the dye and actual concentration of the dye after reaction time t; and A0 and A are respectively the initial absorbance and absorbance after reaction time t. Result and discussion Characterization of Co-MC catalyst The XRD pattern of the Co-MC catalyst is shown in Fig. 1. The pattern displayed diffraction peaks at 2 26.4◦ , and 41.5◦ that could be indexed to the turbostratic structure of mesoporous carbon material. The three main diffraction peaks observed at 44.2, 51.4, and 75.7 ◦ could be indexed to face-centered cubic (fcc) Co. In contrast, the diffraction peaks observed at 31.6, 45.4, and 55.8◦ were consistent with those displayed by Co3 O4 (JCPDS No. 42-1467) (Hardjono, Sun, Tian, Buckley, & Wang, 2011; Liu et al., 2008; Wang et al., 2009; Zhang, Chen, Zhou, Wang, & Zhao, 2009b;). Thus, the XRD pattern reveals that the sample consists mainly of Co3 O4 and Co particles on a carbon substrate. The textural properties of Co-MC were investigated by N2 sorption analysis. The nitrogen adsorption–desorption isotherm and associated pore size distribution of Co-MC are shown in Fig. 2. The sorption isotherm was a type IV curve and featured a H2 type hysteresis loop according to IUPAC classification; these features are indicative of the existence of a mesoporous structure (Morris, Horton, & Jaroniec, 2010). The Brunner–Emmet–Teller (BET) surface area and pore volume of the composite were 268 m2 /g and 0.07 ml/g, respectively. Based on the pore size analysis (inset in Fig. 2), the Co-MC composite featured a rather narrow pore size distribution centered at 3.54 nm, as calculated from the desorption branch of the isotherm using Barrett–Joyner–Halenda (BJH) method. The TEM analysis was conducted. As deduced from Fig. 3, cobalt nanoparticles were dispersed on the MC support as evidenced by the dark spots. Furthermore, in general, the Co-MC composite material lacked a well-defined mesostructure, however, a worm-like micelle morphology could be partially observed in the mesoporous carbon support. The magnetic properties of the Co-MC composite catalyst were investigated using a vibrating sample magnetometer, and the results are shown in Fig. 4. The magnetization curve featured a hysteresis loop upon sweeping of the external magnetic field
between −20 and 20 T at room temperature. Furthermore, no obvious remanence or coercivity was observed, thereby suggesting that Co-MC exhibited a soft magnetic character. The saturated mass magnetization was estimated as 16.1 emu/g. Accordingly, it was expected that the Co-MC composite catalyst could be easily separated from the reaction solution upon exposure to an external magnetic field. As shown in the inset in Fig. 4, a residual transparent solution was obtained upon exposure of the suspension to a magnet. A dispersion could be regained by shaking when the magnetic field was removed. This experiment demonstrates the controlled magnetic behavior of Co-MC, which is particularly desirable for practical application in catalysis. Selective oxidation of toluene Toluene can be converted into oxidation products, such as benzyl alcohol, benzaldehyde, and benzoic acid, among which benzaldehyde is the most desirable product (Acharyya et al., 2014; Lu et al., 2004). However, benzaldehyde is susceptible to oxidation to benzoic acid (Rao, Rao, Nagaraju, Prasad, & Lingaiah, 2009).
Fig. 3. TEM image of the Co-MC catalyst prepared.
Please cite this article in press as: Zhuang, Y., et al. Mesoporous carbon-supported cobalt catalyst for selective oxidation of toluene and degradation of water contaminants. Particuology (2015), http://dx.doi.org/10.1016/j.partic.2015.05.010
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Fig. 4. Magnetization curve of the Co-MC catalyst prepared. The inset illustrates the separation process of the catalyst from the reaction mixture upon exposure to an external magnetic field.
Thus, developing advanced catalysts for highly selective oxidation of toluene to benzaldehyde is important. Herein, the Co-MC catalyst was applied to selectively oxidize toluene to benzaldehyde. H2 O2 was chosen as a green oxidant. It is widely used in the liquid oxidation of organic compounds because it is more active than O2 and results in the generation of H2 O as the only byproduct (Wang et al., 2009). The reaction products were analyzed by GC–MS, and the results are shown in Fig. S1. As observed, the products obtained were mostly benzaldehyde and
some benzyl alcohol. This result suggests that the Co-MC composite catalyst exhibited extremely high selectivity toward the oxidation of toluene to benzaldehyde. To optimize the catalytic reaction, various parameters, i.e., reaction temperature, catalyst dosage, and reaction time, which typically influence the selectivity and conversion of toluene oxidation were investigated. First, the effect of reaction temperature on the reaction performance was examined, and the results are shown in Fig. 5. As observed from Fig. 5a, the selectivity to benzaldehyde production varied from 85.7 to 82.6, 92.4, and 94.0% with increasing reaction temperatures from 40 to 60, 80, and 100 ◦ C. The conversion of toluene also increased accordingly with increasing temperatures from 40 to 80 ◦ C, and reached a maximum of 9.0% at 80 ◦ C (Fig. 5b). Further increasing the temperature to 100 ◦ C led to a decrease in the conversion. Thus, by taking into consideration the selectivity and conversion results, a temperature of 80 ◦ C was selected as the most appropriate reaction temperature for the Co-MC-catalyzed oxidation of toluene. Subsequently, the effect of catalyst dosage on the reaction performance was studied. As observed in Fig. 6, when the catalyst dosage ranged from 5 to 15 mg, the selectivity to benzaldehyde production remained mostly unchanged. In contrast, when the amount of catalyst increased to 20 mg, the selectivity to benzaldehyde production decreased considerably to 42%. Additionally, it was noted that conversion decreased from 8.73 to 3.52% with increasing catalyst amounts from 5 to 10 mg, and remained mostly constant with further increase in the catalyst dosage. The decreasing activity trend observed could be due to the slow mass transfer and nonavailability of catalytic sites in the presence of excess catalyst dosage (Karuehanon, Sirathanyarote, & Pattarawarapan, 2012). Thus, a
Fig. 5. Influence of the reaction temperature on (a) selectivity to benzaldehyde production and (b) reaction conversion of 3 ml toluene after 6 h of reaction using 5 mg Co-MC catalyst.
Fig. 6. Influence of the catalyst dosage on (a) selectivity to benzaldehyde production and (b) reaction conversion of 3 ml toluene after 6 h of reaction time at 80 ◦ C.
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Fig. 7. Influence of the reaction time on (a) selectivity to benzaldehyde production and (b) reaction conversion of 3 ml toluene at a reaction temperature of 80 ◦ C using 5 mg Co-MC catalyst.
catalyst dosage of 5 mg was selected as the optimum catalyst dosage for selective oxidation of 3 ml toluene. Finally, the effect of reaction time on the reaction performance was examined and the results are displayed in Fig. 7. As observed, when the reaction time increased from 1 to 6 h, the selectivity to benzaldehyde production remained constant at (92.0 ± 2.0)%. In contrast, the conversion of toluene increased with increasing reaction times. This result indicates that extending the oxidation time is advantageous to the conversion of toluene. Therefore, a reaction time of 6 h was chosen as the optimum oxidation time for the CoMC-catalyzed production of benzaldehyde in terms of selectivity and conversion. Under the optimum conditions established (80 ◦ C, 6 h, and 5 mg catalyst), a high selectivity to benzaldehyde production of 92.3% was achieved. The catalytic performance of the present catalyst was compared with that of some catalysts reported in the literature toward the selective oxidation of toluene to benzaldehyde, and the results are summarized in Table 1. Compared with various catalysts, Co-MC catalyst exhibited higher selectivity to benzaldehyde production obtained under relatively mild conditions.
Degradation of dyes Reactive Red M-3BE (RR M-3BE) is an azo dye, which presents detrimental effects on the ecosystem owing to its high resistance to microbial and chemical degradation, and can be converted into toxic or carcinogenic compounds (Yao et al., 2013; Zhu, Ai, Ho, & Zhang, 2013). Therefore, in the present work, RR M-3BE was selected as the target dye to evaluate the performance of Co-MC composite toward the degradation/oxidation of dyes. PMS (HSO5 − ) was chosen as an environmentally friendly oxidant, and could be handled easily. The results are shown in Fig. 8. In the sole presence of CoMC composite or MC, the concentration of RR M-3BE in solution remained mostly unchanged. In the presence of PMS only, 59.5% of RR M-3BE was eliminated within 30 min of reaction. In the presence of MC and PMS, the removal rate of RR M-3BE increased to 68.6% within 30 min of reaction. Commercial cobalt (II, III) oxide (Co3 O4 ) was also examined under the same conditions, and 92.9% of RR M-3BE was removed within 30 min of reaction. However, in the presence of Co-MC composite and PMS, more than 96.5% of RR M-3BE was removed within 5 min of reaction. This result suggests that the integration of cobalt into MC support led to a significant enhancement in catalytic activity toward the degradation of RR M3BE dye. The enhanced performance may be attributed to the high porosity and large surface area of the carbon matrix support. Furthermore, GC–MS analysis showed that RR M-3BE dye was oxidized
Fig. 8. Variation in the RR M-3BE dye concentration as a function of time in the presence of Co-MC and PMS, Co3 O4 and PMS, MC and PMS, PMS only, Co-MC only, and MC only. Conditions are [RR M-3BE]: 50 M; T: 25 ◦ C; [PMS]: 120 M; Co-MC dosage: 5 g/L; Co3 O4 dosage: 5 g/L; MC dosage: 5 g/L.
into less toxic and more biodegradable compounds including lactic acid and oxalic acid (Table S1). Furthermore, the effect of the initial pH was investigated by varying the pH value of the aqueous solution in the range of 3–9 at 25 ◦ C. As observed in Fig. 9, the removal rates were 94.6, 97.2, 97.1, and 96.7% at pH values of 3, 5, 7, and 9, respectively. Thus, high removal efficiencies of RR M-3BE using the Co-MC/PMS
Fig. 9. Effect of the initial pH on the removal rate of RR M-3BE in the presence of Co-MC catalyst. Conditions are [RR M-3BE]: 50 M; T: 25 ◦ C; [PMS]: 120 M; Co-MC dosage: 5 g/L.
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Table 1 Comparison analysis of various catalysts employed in the selective oxidation of toluene to benzaldehyde. Catalyst
Optimum conditions
Oxidant
Selectivity (%)
References
Co-MC Au–Pd/TiO2 Mn-HMS MPAV2 /Nb2 O5 Alkali-treated Zeolites CuCr2 O4 spinel nanoparticles
T = 80 ◦ C, [Cat.] = 5 g/L, t = 6 h T = 120 ◦ C, [Cat.] = 20 g/L, t = 48 h T = 170 ◦ C, [Cat.] = 5.8 g/L, t = 3 h Room temperature, [Cat.] = 25 g/L, t = 12 h Low temperature (below 100 ◦ C), [Cat.] = 3.9 g/L, t = 8 h T = 75 ◦ C, [Cat.] = 6.7 g/L, t = 10 h
H2 O2 O2 TBHP* TBHP* H2 O2 H2 O2
92.3 6.0 13.2 76 25.0 84.4
This study Kesavan et al. (2011) Wang et al. (2010) Rao et al. (2009) Du et al. (2012) Acharyya et al. (2014)
*
TBHP: tert-butyl hydroperoxide.
system were achieved under both acidic and alkaline conditions. These results reveal that activation of PMS can proceed over a wide range of pH values. This may be due to the unique characteristics of surface area and pore structure of the carbon support. Loading of the cobalt nanoparticles onto MC could efficiently protect the active sites within a wide pH range and prevent aggregation of the nanoparticles during reaction. Such features make Co-MC more desirable for practical application. Moreover, it should be noted that the abundant framework composition of the catalyst made mass transfer limitations less significant, accordingly leading to accelerated activation of PMS for the removal of RR M-3BE. The reusability of Co-MC composite toward RR M-3BE oxidation was investigated by performing cyclic runs. The catalytic performance of the catalyst was examined. At the end of the reaction, the catalyst was recovered with a magnet. Then, the used catalyst was washed several times with water and dried at 60 ◦ C overnight prior to further use in another catalytic run. These processes were undertaken successively five times. As observed in Fig. 10, the RR M-3BE removal percentage remained constant (96.7–97.3%) across the five consecutive cyclic runs examined. The results demonstrate that the catalyst could be recycled with good catalytic performance, which are highly sought attributes for application in various processes. The catalytic performance of the Co-MC/PMS system was further examined toward the oxidation of other organic dyes, i.e., AR 1, AO 7, RBO K-GN, BG, and WAP BS. Experiments were conducted in the presence of PMS only, or Co-MC composite with or without PMS, and the results are shown in Fig. 11. As observed, the dyes could not be effectively removed in the presence of PMS or Co-MC composite alone. In contrast, in the presence of Co-MC and PMS, the dyes were efficiently removed. In summary, the catalytic studies suggest that Co-MC catalyst can efficiently catalytically oxidize most common dyes, such as acid, reactive, and basic dyes.
Fig. 11. Catalytic degradation of different dyes (BG, RBO K-GN, RR X-3B, AO 7, AR 1, WAP BS). Conditions are [dye]: 50 M; T: 25 ◦ C; [PMS]: 120 M; Co-MC dosage: 5 g/L.
Conclusions The results of this study demonstrated that Co-MC could be used as a promising bifunctional catalyst for both organic catalytic reactions and the degradation of water contaminants. More specifically, the following observations were made: (i) H2 O2 and PMS could be effectively activated to attain high selective oxidation of toluene and degradation of organic dyes, respectively; (ii) the integration of cobalt into MC support led to significantly enhanced catalytic activity of both the selective oxidation of toluene and degradation of dyes that may be attributed to the high porosity and large surface area of the carbon matrix; and (iii) the prepared catalyst demonstrated convenient magnetic separability and excellent reusability without obvious loss in activity. Therefore, the bifunctional Co-MC catalyst developed herein may provide insights in new strategies for designing multifunctional catalysts for various applications. Acknowledgments This work was supported by the State Key Program of National Natural Science of China (No. 51133006), National Natural Science Foundation of China (Nos. 51003096 and 51103133), and Zhejiang Provincial Natural Science Foundation of China (Nos. LY14E030013 and LY14E030015). Appendix A. Supplementary data
Fig. 10. Repeated catalytic degradation of RR M-3BE over recycled Co-MC catalyst with PMS as oxidant. Conditions are [RR M-3BE]: 50 M; T: 25 ◦ C; [PMS]: 120 M; Co-MC dosage: 5 g/L.
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.partic.2015.05. 010.
Please cite this article in press as: Zhuang, Y., et al. Mesoporous carbon-supported cobalt catalyst for selective oxidation of toluene and degradation of water contaminants. Particuology (2015), http://dx.doi.org/10.1016/j.partic.2015.05.010
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Mesoporous carbon-supported cobalt catalyst for selective oxidation of toluene and degradation of water contaminants Yuan Zhuang a , Qisong Lin a , Li Zhang b , Lianshun Luo a , Yuyuan Yao a,∗ , Wangyang Lu a , Wenxing Chen a a b
National Engineering Lab of Textile Fiber Materials & Processing Technology, Zhejiang Sci-Tech University, Hangzhou 310018, China School of Material Science and Chemical Engineering, Ningbo University, Ningbo 315211, China
a r t i c l e
i n f o
Article history: Received 9 March 2015 Received in revised form 8 May 2015 Accepted 20 May 2015 Available online xxx Keywords: Mesoporous carbon Cobalt Bifunctional catalyst Toluene oxidation Dye degradation
a b s t r a c t Mesoporous carbon-supported cobalt (Co-MC) catalysts are widely applied as electrode materials for batteries. Conversely, the development of Co-MC as bifunctional catalysts for application in organic catalytic reactions and degradation of water contaminants is slower. Herein, the catalyst displayed high activity in the selective oxidation of toluene to benzaldehyde under mild conditions, attaining a high selectivity of 92.3%. Factors influencing the catalytic reaction performance were also investigated. Additionally, Co-MC displayed remarkable catalytic activity in degrading dyes relative to the pure metal counterpart. Moreover, the catalyst exhibited excellent reusability, as determined by the cyclic catalytic experiments. The paper demonstrates the potential of Co-MC as a bifunctional catalyst for both toluene selective oxidation and water contaminant degradation. © 2015 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
Introduction Catalytic oxidation is a fundamental reaction occurring in nature as well as a key conversion reaction employed in the synthesis and degradation of organics (Ding et al., 2012; Piera & Bäckvall, 2008). Over the past decades, considerable research efforts have been devoted to design and develop economically feasible and efficient catalysts (Boppana & Jiao, 2011; Huang, Xiang, Li, Jiang, & Guo, 2011; Liang et al., 2011). Transition metals, as a class of promising catalysts, have been applied to catalyze various oxidation reactions based on their multiple oxidation states, particularly in the field of organic catalytic reactions and degradation of water contaminants (Dhakshinamoorthy, Alvaro, & Garcia, 2011; Long, Liu, Wu, Liao, & Li, 2013; bin Saiman et al., 2012; Yan, Li, Zhao, & Chen, 2012; Zhang et al., 2013; Zou et al., 2012). Two groups of transition metal-based catalysts, including noble metal catalysts (Pd, Pt, and Au) and base metal catalysts (Mn, Co, Cu, Fe, and Ni), are used widely. Noble metal catalysts are typically employed because of their high catalytic activity; however, their low abundance and high cost hinder their wide application (Chuang, Liu, Lu, & Wey,
∗ Corresponding author. Tel.: +86 571 86843810; fax: +86 571 86843255. E-mail addresses: [email protected], [email protected] (Y. Yao).
2009). Consequently, base metal catalysts have been increasingly investigated as potential alternatives for oxidation reactions. Among the base metal catalysts studied, cobalt catalysts are promising for use in catalytic oxidation reactions (Jiao & Frei, 2010; Popova et al., 2014). To enhance the stability and dispersion of the active metal, supports, such as SiO2 (Eggenhuisen, Breejen, Verdoes, Jongh, & Jong, 2010; Szegedi, Popova, Mavrodinova, & Minchev, 2008), Al2 O3 (Rane, Borg, Rytter, & Holmen, 2012), TiO2 (Yang, Choi, & Dionysiou, 2007; Shukla, Wang, Sun, Ang, & Tadé, 2010a), MgO (Zhang et al., 2010), and zeolite (Shukla, Wang, Singh, Ang, & Tadé, 2010b), are generally employed. Unfortunately, the interaction between cobalt and SiO2 or Al2 O3 leads to lower catalytic activity because of the formation of mixed compounds during catalyst preparation and reaction (Zhang et al., 2009a). In contrast, the use of TiO2 , MgO, or zeolite supports is limited owing to the unavailability of a suitable approach to recover the nanosized cobalt particles, thereby potentially causing secondary environmental problems (Shukla et al., 2010a; Yang et al., 2007; Zhang et al., 2010). The use of suitable supports would offer excellent catalytic performance (Huang, Bao, Yao, Lu, & Chen, 2014). Therefore, selection of the support is very important toward preparing supported cobalt catalysts with appealing performance and extensive applications. In contrast to the abovementioned supports, mesoporous carbon (MC) has received extensive attention as an excellent support
http://dx.doi.org/10.1016/j.partic.2015.05.010 1674-2001/© 2015 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
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for catalysts because of its attractive features, i.e., high surface area (up to 2500 m2 /g), large specific pore volume, narrow pore size distribution (2.0–50 nm), and chemical inertness (Almeida, Melo, & Airoldi, 2013; Li, Fu, & Su, 2012; Sun et al., 2012). Furthermore, their open porous network reduces mass transfer limitations when compared with traditional microporous activated carbons (Deng et al., 2010; Wang et al., 2013). Hence, several researchers have investigated mesoporous carbon-supported cobalt for various applications. For instance, Yang et al. (2012) reported the use of ordered mesoporous carbon-supported cobalt-based catalysts for Fischer–Tropsch synthesis, and investigated the influence of the amount of carbon precursor on catalytic performance. Dai & Vogt (2012) reported the synthesis of mesoporous carbon–cobalt composite for the removal of methylene green; the composite catalyst exhibited excellent capacity. Based on these studies, the integration of cobalt into MC holds great potential in developing cobalt catalysts with highly accessible active sites and stable characteristics while retaining the individual functionality. Furthermore, the MC support is expected to provide a good anchoring platform to trap the nanoparticles and prevent catalyst aggregation during catalytic oxidation reactions. To our knowledge, mesoporous carbon-supported cobalt (CoMC) catalysts have been widely applied as negative electrode materials for lithium batteries (Hou, Ndamanisha, Guo, Peng, & Bai, 2009; Liu et al., 2008). In contrast, the use of Co-MC as a bifunctional catalyst for organic catalytic reactions and degradation of water contaminants has been rarely reported. Herein, toluene and various dyes were employed as model compounds to investigate the bifunctional catalytic properties of Co-MC. The results showed that the Co-MC catalyst exhibited high activity toward the selective oxidation of toluene and excellent potential toward the degradation of dyes. Furthermore, compared with free cobalt catalyst, the introduction of MC support into cobalt catalyst system led to a significant enhancement in catalytic activity, and the composite catalyst displayed convenient magnetic separability and excellent reusability. This research offers a new strategy to design catalysts with integrated functions for various advanced applications in organic synthesis, green chemistry, and environmental treatment. Experimental Materials Poly(ethylene oxide)-block-poly(propylene oxide)-blockpoly(ethylene oxide) triblock copolymer Pluronic F127 (PEO106 PPO70 PEO106 ) was purchased from Sigma-Aldrich Co., Ltd. (Shanghai, China). Phenol, formaldehyde, sodium hydroxide, hydrochloric acid, toluene, hydrogen peroxide, acetonitrile, and ethanol were purchased from Gaojing Chem. Co., Ltd. (Hangzhou, China). Cobalt (II, III) oxide (Co3 O4 ) and peroxymonosulfate (PMS; Oxone) were purchased from Aladdin Co., Ltd. (Shanghai, China). Reactive Red M-3BE (RR M-3BE), Acid Red1 (AR 1), Acid Orange 7 (AO 7), Reactive Brilliant Orange K-GN (RBO K-GN), Basic Green (BG), and Weak Acid Pink BS (WAP BS) were commercial compounds (Shinyang Samwoo Fine Chemical Co. Ltd., Hangzhou, China). All chemicals were used as received without any further purification. Doubly distilled water was used in all experiments. Preparation of Co-MC catalyst Soluble resol precursors were prepared using phenol and formaldehyde in a base-catalyzed process according to Sun et al. (2012).The detailed synthesis procedure is available in the Supplementary Material. The Co-MC composite was synthesized through
simple Co3 O4 doping in mesoporous carbon. Typically, 2 g Pluronic F127 was dissolved in 15 ml absolute ethanol at 40 ◦ C. Then, 4.87 g resol precursor solution (51.3 wt% in ethanol) was gradually added, and the mixture was stirred for 10 min, after which 1.47 g Co3 O4 powder was added. After further stirring for 30 min at 40 ◦ C, the mixture was transferred into Petri dishes that were lined with silicon paper to avoid detachment issues. Subsequently, ethanol in the mixture was evaporated for 8 h at 35 ◦ C. The resulting sticky film was subjected to curing at 100 ◦ C. Finally, the obtained solid product was ground into powder. The powder sample was then pyrolyzed in a tube furnace at 800 ◦ C for 4 h under N2 atmosphere (heating rate: 1 ◦ C/min) to decompose the triblock copolymer template and carbonize the resol precursor to finally generate cobalt and its metal oxide. Characterization of catalyst The catalyst samples were characterized by X-ray diffraction (XRD, XTRA, Thermo ARL, Switzerland), transmission electron microscopy (TEM, JEM-2100, JEOL, Japan), Brunner–Emmet–Teller (BET) surface area analysis (3H-2000PS1, Beishide Instrument Technology Co. Ltd., China). The magnetic properties of the CoMC composite catalyst were investigated using a vibrating sample magnetometer (VSM, 7410, Lake Shore, USA). The toluene final reactant and product mixtures were analyzed by gas chromatography mass spectrometry (GC–MS, 6890 N/5973i, Agilent, USA) using bromobenzene as internal standard. The degradation efficiency of the dyes was determined by UV–visible absorption spectroscopy (U-3010, Hitachi, Japan). Catalytic studies The catalytic performance of the Co-MC composite toward the selective oxidation of toluene was examined under atmospheric pressure. Briefly, 3 ml toluene and 15 ml 35% aqueous H2 O2 in acetonitrile (20 ml) were added to an appropriate amount of catalyst in a reaction vessel under continuous vigorous stirring at the desired temperature. After a given reaction time, excess H2 O2 was quenched with sodium thiosulfate (Na2 S2 O3 ), and the reaction mixture was separated by a magnet to remove the catalyst, followed by extraction with ethyl acetate. The final reactant and product mixtures were analyzed by GC–MS using bromobenzene as an internal standard. The yield of benzaldehyde and its selectivity was calculated using the following equations: Conversion (%) = Selectivity (%) =
PhCHO × 100, PhMeall PhCHO × 100, PhMein
(1) (2)
where PhCHO is the molar content of benzaldehyde, PhMein is the molar content of toluene that participated in the reaction, and PhMeall is the initial molar content of toluene that was added to the reaction system. The catalytic degradation of the dyes (RR M-3BE, AR 1, AO 7, RBO K-GN, BG, and WAP BS) was performed in 100 ml glass beakers at a set temperature of 25 ◦ C using a constant temperature shaker water bath. In all experiments, a reaction volume of 50 ml was used. A typical reaction mixture comprised RR M-3BEdye (50 M), unless specified otherwise, Co-MC (0.2 g/L), Co3 O4 (0.2 g/L), and PMS (120 M). pH adjustments were undertaken at the start. At given time intervals, the degradation efficiency of the dyes were determined by UV–vis spectrometer. The residual and removal percentages of the dyes were calculated as follows: Residual percentage dyes (%) = C/C0 × 100 = A/A0 × 100,
(3)
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Fig. 2. Nitrogen sorption isotherm and pore size distribution (inset) of the Co-MC catalyst prepared.
Fig. 1. X-ray diffraction pattern of the Co-MC catalyst prepared.
Removal percentage of dyes (%) = ((C0 − C)/C0 ) × 100 = ((A0 − A)/A0 ) × 100,
(4)
where C0 and C are respectively the initial concentration of the dye and actual concentration of the dye after reaction time t; and A0 and A are respectively the initial absorbance and absorbance after reaction time t. Result and discussion Characterization of Co-MC catalyst The XRD pattern of the Co-MC catalyst is shown in Fig. 1. The pattern displayed diffraction peaks at 2 26.4◦ , and 41.5◦ that could be indexed to the turbostratic structure of mesoporous carbon material. The three main diffraction peaks observed at 44.2, 51.4, and 75.7 ◦ could be indexed to face-centered cubic (fcc) Co. In contrast, the diffraction peaks observed at 31.6, 45.4, and 55.8◦ were consistent with those displayed by Co3 O4 (JCPDS No. 42-1467) (Hardjono, Sun, Tian, Buckley, & Wang, 2011; Liu et al., 2008; Wang et al., 2009; Zhang, Chen, Zhou, Wang, & Zhao, 2009b;). Thus, the XRD pattern reveals that the sample consists mainly of Co3 O4 and Co particles on a carbon substrate. The textural properties of Co-MC were investigated by N2 sorption analysis. The nitrogen adsorption–desorption isotherm and associated pore size distribution of Co-MC are shown in Fig. 2. The sorption isotherm was a type IV curve and featured a H2 type hysteresis loop according to IUPAC classification; these features are indicative of the existence of a mesoporous structure (Morris, Horton, & Jaroniec, 2010). The Brunner–Emmet–Teller (BET) surface area and pore volume of the composite were 268 m2 /g and 0.07 ml/g, respectively. Based on the pore size analysis (inset in Fig. 2), the Co-MC composite featured a rather narrow pore size distribution centered at 3.54 nm, as calculated from the desorption branch of the isotherm using Barrett–Joyner–Halenda (BJH) method. The TEM analysis was conducted. As deduced from Fig. 3, cobalt nanoparticles were dispersed on the MC support as evidenced by the dark spots. Furthermore, in general, the Co-MC composite material lacked a well-defined mesostructure, however, a worm-like micelle morphology could be partially observed in the mesoporous carbon support. The magnetic properties of the Co-MC composite catalyst were investigated using a vibrating sample magnetometer, and the results are shown in Fig. 4. The magnetization curve featured a hysteresis loop upon sweeping of the external magnetic field
between −20 and 20 T at room temperature. Furthermore, no obvious remanence or coercivity was observed, thereby suggesting that Co-MC exhibited a soft magnetic character. The saturated mass magnetization was estimated as 16.1 emu/g. Accordingly, it was expected that the Co-MC composite catalyst could be easily separated from the reaction solution upon exposure to an external magnetic field. As shown in the inset in Fig. 4, a residual transparent solution was obtained upon exposure of the suspension to a magnet. A dispersion could be regained by shaking when the magnetic field was removed. This experiment demonstrates the controlled magnetic behavior of Co-MC, which is particularly desirable for practical application in catalysis. Selective oxidation of toluene Toluene can be converted into oxidation products, such as benzyl alcohol, benzaldehyde, and benzoic acid, among which benzaldehyde is the most desirable product (Acharyya et al., 2014; Lu et al., 2004). However, benzaldehyde is susceptible to oxidation to benzoic acid (Rao, Rao, Nagaraju, Prasad, & Lingaiah, 2009).
Fig. 3. TEM image of the Co-MC catalyst prepared.
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Fig. 4. Magnetization curve of the Co-MC catalyst prepared. The inset illustrates the separation process of the catalyst from the reaction mixture upon exposure to an external magnetic field.
Thus, developing advanced catalysts for highly selective oxidation of toluene to benzaldehyde is important. Herein, the Co-MC catalyst was applied to selectively oxidize toluene to benzaldehyde. H2 O2 was chosen as a green oxidant. It is widely used in the liquid oxidation of organic compounds because it is more active than O2 and results in the generation of H2 O as the only byproduct (Wang et al., 2009). The reaction products were analyzed by GC–MS, and the results are shown in Fig. S1. As observed, the products obtained were mostly benzaldehyde and
some benzyl alcohol. This result suggests that the Co-MC composite catalyst exhibited extremely high selectivity toward the oxidation of toluene to benzaldehyde. To optimize the catalytic reaction, various parameters, i.e., reaction temperature, catalyst dosage, and reaction time, which typically influence the selectivity and conversion of toluene oxidation were investigated. First, the effect of reaction temperature on the reaction performance was examined, and the results are shown in Fig. 5. As observed from Fig. 5a, the selectivity to benzaldehyde production varied from 85.7 to 82.6, 92.4, and 94.0% with increasing reaction temperatures from 40 to 60, 80, and 100 ◦ C. The conversion of toluene also increased accordingly with increasing temperatures from 40 to 80 ◦ C, and reached a maximum of 9.0% at 80 ◦ C (Fig. 5b). Further increasing the temperature to 100 ◦ C led to a decrease in the conversion. Thus, by taking into consideration the selectivity and conversion results, a temperature of 80 ◦ C was selected as the most appropriate reaction temperature for the Co-MC-catalyzed oxidation of toluene. Subsequently, the effect of catalyst dosage on the reaction performance was studied. As observed in Fig. 6, when the catalyst dosage ranged from 5 to 15 mg, the selectivity to benzaldehyde production remained mostly unchanged. In contrast, when the amount of catalyst increased to 20 mg, the selectivity to benzaldehyde production decreased considerably to 42%. Additionally, it was noted that conversion decreased from 8.73 to 3.52% with increasing catalyst amounts from 5 to 10 mg, and remained mostly constant with further increase in the catalyst dosage. The decreasing activity trend observed could be due to the slow mass transfer and nonavailability of catalytic sites in the presence of excess catalyst dosage (Karuehanon, Sirathanyarote, & Pattarawarapan, 2012). Thus, a
Fig. 5. Influence of the reaction temperature on (a) selectivity to benzaldehyde production and (b) reaction conversion of 3 ml toluene after 6 h of reaction using 5 mg Co-MC catalyst.
Fig. 6. Influence of the catalyst dosage on (a) selectivity to benzaldehyde production and (b) reaction conversion of 3 ml toluene after 6 h of reaction time at 80 ◦ C.
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Fig. 7. Influence of the reaction time on (a) selectivity to benzaldehyde production and (b) reaction conversion of 3 ml toluene at a reaction temperature of 80 ◦ C using 5 mg Co-MC catalyst.
catalyst dosage of 5 mg was selected as the optimum catalyst dosage for selective oxidation of 3 ml toluene. Finally, the effect of reaction time on the reaction performance was examined and the results are displayed in Fig. 7. As observed, when the reaction time increased from 1 to 6 h, the selectivity to benzaldehyde production remained constant at (92.0 ± 2.0)%. In contrast, the conversion of toluene increased with increasing reaction times. This result indicates that extending the oxidation time is advantageous to the conversion of toluene. Therefore, a reaction time of 6 h was chosen as the optimum oxidation time for the CoMC-catalyzed production of benzaldehyde in terms of selectivity and conversion. Under the optimum conditions established (80 ◦ C, 6 h, and 5 mg catalyst), a high selectivity to benzaldehyde production of 92.3% was achieved. The catalytic performance of the present catalyst was compared with that of some catalysts reported in the literature toward the selective oxidation of toluene to benzaldehyde, and the results are summarized in Table 1. Compared with various catalysts, Co-MC catalyst exhibited higher selectivity to benzaldehyde production obtained under relatively mild conditions.
Degradation of dyes Reactive Red M-3BE (RR M-3BE) is an azo dye, which presents detrimental effects on the ecosystem owing to its high resistance to microbial and chemical degradation, and can be converted into toxic or carcinogenic compounds (Yao et al., 2013; Zhu, Ai, Ho, & Zhang, 2013). Therefore, in the present work, RR M-3BE was selected as the target dye to evaluate the performance of Co-MC composite toward the degradation/oxidation of dyes. PMS (HSO5 − ) was chosen as an environmentally friendly oxidant, and could be handled easily. The results are shown in Fig. 8. In the sole presence of CoMC composite or MC, the concentration of RR M-3BE in solution remained mostly unchanged. In the presence of PMS only, 59.5% of RR M-3BE was eliminated within 30 min of reaction. In the presence of MC and PMS, the removal rate of RR M-3BE increased to 68.6% within 30 min of reaction. Commercial cobalt (II, III) oxide (Co3 O4 ) was also examined under the same conditions, and 92.9% of RR M-3BE was removed within 30 min of reaction. However, in the presence of Co-MC composite and PMS, more than 96.5% of RR M-3BE was removed within 5 min of reaction. This result suggests that the integration of cobalt into MC support led to a significant enhancement in catalytic activity toward the degradation of RR M3BE dye. The enhanced performance may be attributed to the high porosity and large surface area of the carbon matrix support. Furthermore, GC–MS analysis showed that RR M-3BE dye was oxidized
Fig. 8. Variation in the RR M-3BE dye concentration as a function of time in the presence of Co-MC and PMS, Co3 O4 and PMS, MC and PMS, PMS only, Co-MC only, and MC only. Conditions are [RR M-3BE]: 50 M; T: 25 ◦ C; [PMS]: 120 M; Co-MC dosage: 5 g/L; Co3 O4 dosage: 5 g/L; MC dosage: 5 g/L.
into less toxic and more biodegradable compounds including lactic acid and oxalic acid (Table S1). Furthermore, the effect of the initial pH was investigated by varying the pH value of the aqueous solution in the range of 3–9 at 25 ◦ C. As observed in Fig. 9, the removal rates were 94.6, 97.2, 97.1, and 96.7% at pH values of 3, 5, 7, and 9, respectively. Thus, high removal efficiencies of RR M-3BE using the Co-MC/PMS
Fig. 9. Effect of the initial pH on the removal rate of RR M-3BE in the presence of Co-MC catalyst. Conditions are [RR M-3BE]: 50 M; T: 25 ◦ C; [PMS]: 120 M; Co-MC dosage: 5 g/L.
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Table 1 Comparison analysis of various catalysts employed in the selective oxidation of toluene to benzaldehyde. Catalyst
Optimum conditions
Oxidant
Selectivity (%)
References
Co-MC Au–Pd/TiO2 Mn-HMS MPAV2 /Nb2 O5 Alkali-treated Zeolites CuCr2 O4 spinel nanoparticles
T = 80 ◦ C, [Cat.] = 5 g/L, t = 6 h T = 120 ◦ C, [Cat.] = 20 g/L, t = 48 h T = 170 ◦ C, [Cat.] = 5.8 g/L, t = 3 h Room temperature, [Cat.] = 25 g/L, t = 12 h Low temperature (below 100 ◦ C), [Cat.] = 3.9 g/L, t = 8 h T = 75 ◦ C, [Cat.] = 6.7 g/L, t = 10 h
H2 O2 O2 TBHP* TBHP* H2 O2 H2 O2
92.3 6.0 13.2 76 25.0 84.4
This study Kesavan et al. (2011) Wang et al. (2010) Rao et al. (2009) Du et al. (2012) Acharyya et al. (2014)
*
TBHP: tert-butyl hydroperoxide.
system were achieved under both acidic and alkaline conditions. These results reveal that activation of PMS can proceed over a wide range of pH values. This may be due to the unique characteristics of surface area and pore structure of the carbon support. Loading of the cobalt nanoparticles onto MC could efficiently protect the active sites within a wide pH range and prevent aggregation of the nanoparticles during reaction. Such features make Co-MC more desirable for practical application. Moreover, it should be noted that the abundant framework composition of the catalyst made mass transfer limitations less significant, accordingly leading to accelerated activation of PMS for the removal of RR M-3BE. The reusability of Co-MC composite toward RR M-3BE oxidation was investigated by performing cyclic runs. The catalytic performance of the catalyst was examined. At the end of the reaction, the catalyst was recovered with a magnet. Then, the used catalyst was washed several times with water and dried at 60 ◦ C overnight prior to further use in another catalytic run. These processes were undertaken successively five times. As observed in Fig. 10, the RR M-3BE removal percentage remained constant (96.7–97.3%) across the five consecutive cyclic runs examined. The results demonstrate that the catalyst could be recycled with good catalytic performance, which are highly sought attributes for application in various processes. The catalytic performance of the Co-MC/PMS system was further examined toward the oxidation of other organic dyes, i.e., AR 1, AO 7, RBO K-GN, BG, and WAP BS. Experiments were conducted in the presence of PMS only, or Co-MC composite with or without PMS, and the results are shown in Fig. 11. As observed, the dyes could not be effectively removed in the presence of PMS or Co-MC composite alone. In contrast, in the presence of Co-MC and PMS, the dyes were efficiently removed. In summary, the catalytic studies suggest that Co-MC catalyst can efficiently catalytically oxidize most common dyes, such as acid, reactive, and basic dyes.
Fig. 11. Catalytic degradation of different dyes (BG, RBO K-GN, RR X-3B, AO 7, AR 1, WAP BS). Conditions are [dye]: 50 M; T: 25 ◦ C; [PMS]: 120 M; Co-MC dosage: 5 g/L.
Conclusions The results of this study demonstrated that Co-MC could be used as a promising bifunctional catalyst for both organic catalytic reactions and the degradation of water contaminants. More specifically, the following observations were made: (i) H2 O2 and PMS could be effectively activated to attain high selective oxidation of toluene and degradation of organic dyes, respectively; (ii) the integration of cobalt into MC support led to significantly enhanced catalytic activity of both the selective oxidation of toluene and degradation of dyes that may be attributed to the high porosity and large surface area of the carbon matrix; and (iii) the prepared catalyst demonstrated convenient magnetic separability and excellent reusability without obvious loss in activity. Therefore, the bifunctional Co-MC catalyst developed herein may provide insights in new strategies for designing multifunctional catalysts for various applications. Acknowledgments This work was supported by the State Key Program of National Natural Science of China (No. 51133006), National Natural Science Foundation of China (Nos. 51003096 and 51103133), and Zhejiang Provincial Natural Science Foundation of China (Nos. LY14E030013 and LY14E030015). Appendix A. Supplementary data
Fig. 10. Repeated catalytic degradation of RR M-3BE over recycled Co-MC catalyst with PMS as oxidant. Conditions are [RR M-3BE]: 50 M; T: 25 ◦ C; [PMS]: 120 M; Co-MC dosage: 5 g/L.
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.partic.2015.05. 010.
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Please cite this article in press as: Zhuang, Y., et al. Mesoporous carbon-supported cobalt catalyst for selective oxidation of toluene and degradation of water contaminants. Particuology (2015), http://dx.doi.org/10.1016/j.partic.2015.05.010