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Selective oxidation of glycerol over carbon nanofibers supported Pt catalysts in a base-free aqueous solution
Catalysis Communications 59 (2015) 5–9
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Selective oxidation of glycerol over carbon nanofibers supported Pt catalysts in a base-free aqueous solution Mengyuan Zhang a, Renfeng Nie a, Lina Wang b, Juanjuan Shi a, Weichen Du a, Zhaoyin Hou a,⁎ a b
Institute of Catalysis, Department of Chemistry, Zhejiang University, Hangzhou 310028, China Engineering Research Center for Eco-Dyeing & Finishing of Textiles, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China
a r t i c l e
i n f o
Article history: Received 1 June 2014 Received in revised form 18 September 2014 Accepted 22 September 2014 Available online 28 September 2014 Keywords: Glycerol Oxidation Base-free Carbon nanofibers Platinum
a b s t r a c t Highly dispersed Pt nanoparticles were fabricated on the surface of S-grafted carbon nanofibers (Pt/S-CNFs) and used in selective oxidation of glycerol with molecular oxygen in a base-free aqueous solution. Characterizations disclosed that S-grafted CNFs are more favorable for the dispersion of Pt than that of crude CNFs and HNO3pretreated CNFs. CNFs supported Pt catalysts are more selective toward the formation of glycerin acid, and the initial activity of surface Pt atoms in Pt/S-CNFs reached 750.7 h−1 at 60 °C in a base-free aqueous solution. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Biodiesel is an important renewable biofuel, and it could be produced from animal fats and vegetable oils via transesterification [1], which is helpful to reduce the dependence on fossil energy. Glycerol is a by-product during the production of biodiesel, and the rapidly rising production of biodiesel leads to a serious surplus of glycerol [2]. Catalytic conversion of glycerol to valuable products has become a hot research topic [3–5]. Among these, selective oxidation of glycerol to glycerin acid (GLYA), 1,3-dihydroxyacetone (DIHA), glyceraldehyde (GLYHD), glycolic acid (GLYCA) and tartronic acid (TARAC) has attracted much attention recently [6–10]. In published works, Au-based catalysts were investigated intensively in oxidation of glycerol. It was found that the conversion of glycerol over Au-based catalysts depended strongly on the basicity of reaction medium [11,12]. More recently, Davis et al. disclosed that oxygen atoms incorporated into the GLYA originated exclusively from the water solvent during the oxidation of glycerol over Au-based catalysts in a base solution. The role of oxygen in alcohol oxidation in aqueous media is to remove electrons from the surface of gold which are deposited during oxidation [13,14]. However, selective oxidation of glycerol in a base-free aqueous solution is more interesting because it allows the direct formation of acid products. Villa et al. [9] found that H-mordenite supported AuPt nanoparticles (NPs) are able to oxidize glycerol to GLYA without the use of ⁎ Corresponding author. E-mail address: [email protected] (Z. Hou).
http://dx.doi.org/10.1016/j.catcom.2014.09.036 1566-7367/© 2014 Elsevier B.V. All rights reserved.
base. Brett et al. [15] reported that Au-Pt and Au-Pd NPs supported on Mg(OH)2 are highly active for the selective oxidation of glycerol under base-free conditions. Our previous work found that mono-metal Pt (size 1.2–8.0 nm) on active carbon (AC) is also capable to oxidize glycerol to GLYA in base-free condition [10]. Micropore-free multiwall carbon nanotubes (MWCNTs) supported Pt catalysts are more active for this reaction because of the easier accessibility of Pt on the outer wall of MWCNTs [16–18]. Rodrigues et al. [19–21] also found that MWCNTs supported Au catalysts are more active for glycerol oxidation than that on AC. Carbon nanofibers (CNFs) have drawn a lot of attention for its thermal, mechanical stability, micropore-free graphite structure and low price [22,23]. In this work, CNFs supported Pt catalysts were prepared and tested in the selective oxidation of glycerol in a base-free solution. The excellent performance of Pt/CNFs was discussed with their structure. 2. Experimental 2.1. Catalysis preparation CNFs (N98% carbon basis, from Sigma-Aldrich) were first pretreated with nitric acid (65–68 wt.%) at 75 °C for 10 h, washed with distilled water until the pH of effluent solution reached 6.7, and the solid was dried in vacuum overnight and denoted as HNO3-CNFs. And then, 5 g of HNO3-CNFs was chloridized in 200 mL SOCl2 under refluxing at 70 °C for 12 h, and further grafted with NH2(CH2)2SH in dehydrated toluene for 24 h [24]. The resulting support was denoted as S-CNFs.
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M. Zhang et al. / Catalysis Communications 59 (2015) 5–9
Table 1 Pt dispersion and elemental composition on CNFs. Catalyst
Pt/CNFs Pt/HNO3-CNFs Pt/S-CNFs S-CNFs a b
Particle size of Pt (nm)
Atom concentration (%)a
Pt loading (wt.%) b
XRD
TEM
C
O
Pt
S
N
Pt/ Ptδ+
6.5 5.9 1.5 –
6.8 4.2 1.8 –
91.25 85.62 86.51 79.28
8.42 13.66 11.70 16.13
0.33 0.72 1.45 –
– – b0.01 2.13
– b0.01 b0.01 2.46
5.34 4.26 6.69 –
4.76 4.78 2.46 –
Determined by XPS. Determined by ICP.
At last, 2 g S-CNFs (or CNFs, HNO3-CNFs) was dispersed in water, and an aqueous solution containing 0.01 g/mL H2PtCl6 was added dropwise under stirring. The slurry was then sonicated for 2 h and reduced with an aqueous solution of KBH4. After reduction, it was filtered, washed with distilled deionized water until free of chlorine and dried at 60 °C in vacuum. Pt/CNFs and Pt/HNO3-CNFs were used directly for catalytic reaction, and Pt/S-CNFs were further treated in hydrogen at 400 °C for 1 h in order to eliminate the S-containing groups [17]. Actual content of Pt in prepared catalysts was determined through inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Plasma-Spec- II spectrometer) and these data were summarized in Table 1. The dispersion of Pt was detected via pulse CO adsorption as described in literature [10,17]. 2.2. Catalyst characterization Raman spectra of CNFs, HNO3-CNFs and S-CNFs were collected from 800 to 2600 cm− 1 on Renishaw 2000 Confocal Raman Microprobe (Renishaw Instruments, England) using 514.5 nm argon ion laser. X-ray diffraction (XRD) analysis of Pt/CNFs, Pt/HNO3-CNFs and Pt/SCNFs was performed on RIGAKU D/MAX2550/PC diffractometer using Cu Kα radiation at 40 kV and 100 mA. Diffraction data was recorded using continuous scanning at a rate of 0.02°/s and step 0.02°. The morphologies and dimensions of catalysts were observed by transmission
electron microscopy (TEM, JEOL-2010F) using an accelerating voltage of 200 kV. X-ray photoelectron spectra (XPS) were recorded on Perkin-Elmer PHI ESCA System; the X-ray source was Mg standard anode 146 (1253.6 eV) at 12 kV and 300 W.
2.3. Glycerol oxidation Glycerol oxidation was carried out in a 100 mL custom designed stainless autoclave with a glass inner layer, which contains 50 mL glycerol solution and 0.5 g catalyst. The reactor was sealed, filled with a certain amount of O2 and N2 and placed in an oil bath preheated to required temperature and maintained at that temperature for a given time under vigorous stirring with a magnetic stirrer (MAG-NEO, RV-06M, Japan). After reaction, catalyst was removed by filtration and the aqueous solution was analyzed using an Agilent 1100 series high-performance liquid chromatograph (HPLC) equipped with a refractive index detector (RID) and a Zorbax SAX column (4.6 mm × 250 mm, Agilent). The turnover frequency (TOF) on the basis of surface Pt atoms was calculated as: TOF = (number of glycerol molecular converted) / (number of surface Pt atoms) / (reaction time, h). The selectivity of each product was calculated as: (mmol of product in reaction mixture) × (the number of carbon atoms in product) / ((initial mmol of glycerol − mmol of glycerol left) × 3) × 100%.
Fig. 1. TEM images and particle size distributions of (a) Pt/CNFs, (b) Pt/HNO3-CNFs, and (c) Pt/S-CNFs. Image of (d) is the high resolution image of Pt particles in Pt/S-CNFs.
M. Zhang et al. / Catalysis Communications 59 (2015) 5–9
3. Results and discussion 3.1. The structure of catalysts Raman analysis found that that the I(D)/I(G) ratios of CNFs, HNO3CNFs and S-CNFs are 0.78, 0.84 and 0.82, respectively (Fig. S1), which indicated that CNFs were partially oxidized during acid treatment [25,26]. In addition, the position of G peak in HNO3-CNFs and S-CNFs shifts to 1572 cm−1 because of more structural defects formed on the surface of CNFs. XRD patterns show that the mean crystallite size of Pt in Pt/ CNFs and Pt/HNO3-CNFs calculated via Scherrer–Warren equation is 6.5 and 5.9 nm, respectively (Fig. S2). In comparison to Pt/CNFs and Pt/HNO3-CNFs, the diffraction peaks of Pt in Pt/S-CNFs become broad and faint, and the mean crystallite size of Pt decreases to 1.5 nm. These data indicate that Pt disperses highly on S-CNFs, which might
7
Table 2 Oxidation of glycerol using different Pt catalysts a. Catalyst
Conversion (%)
Pt/CNFs Pt/HNO3-CNFs Pt/S-CNFs
30.1 47.3 89.9
Selectivity (%)b GLYA
GLYHD
DIHA
GLYCA
Othersc
84.2 84.9 83.2
6.3 3.7 1.1
9.5 6.5 6.8
0.0 0.0 8.9
0.0 4.9 0.0
a Reaction conditions: catalyst 0.5 g, aqueous solution of glycerol 50 mL (0.1 g/mL), 60 °C, 0.4 MPa O2 + 1.6 MPa N2, 6 h. b Selectivity was calculated as: (mmol of product in reaction mixture) × (the number of carbon atoms in product) / ((initial mmol of glycerol − mmol of glycerol left) × 3) × 100%. GLYA: glyceric acid, DIHA: 1,3-dihydroxyacetone, GLYCA: glycolic acid, GLYHD: glyceraldehyde. c Mono-carbon products (mainly like COx and formaldehyde) were also detected.
be attributed to the S-containing reagents that are more effective for controlling the dimensions and distributions of Pt particles [17,24]. Fig. 1 shows the TEM images and particle size distribution of Pt in Pt/ CNFs, Pt/HNO3-CNFs and Pt/S-CNFs catalysts. It can be found that Pt disperses unevenly on the wall of CNFs, and the detected Pt particle size is 1.5–30.0 nm (with an average diameter of 6.8 nm, Fig. 1a). On HNO3pretreated CNFs, the calculated mean size of Pt decreases slightly to 4.2 nm (Fig. 1b). However, highly dispersed and unique sized Pt particles form on S-CNFs and the calculated mean size reaches 1.8 nm (Fig. 1c–d). These results fit well with the above XRD analysis. XPS analysis confirmed that –S–H (2.46 at.%) and –N–H (2.13 at.%) groups formed on the surface of S-CNFs, and the detected number of –S–H and –N–H groups was quite close (see Fig. S3 and Table 1). These data suggested that NH2CH2CH2SH was successfully grafted on CNFs. High resolution spectra of Pt 4f in Pt/CNFs, Pt/HNO3-CNFs and Pt/S-CNFs catalysts are shown in Fig. 2. Deconvolution of the Pt 4f region shows that the most intense doublets are 71.3 (Pt 4f7/2) and 74.5 eV (Pt 4f5/2), and this doublet could be attributed to metallic Pt [27]. Peaks at 72.2 (Pt 4f7/2) and 75.6 eV (Pt 4f5/2) could be assigned to Ptδ+ that anchored with the C–O group in CNFs [28]. The percentage of Ptδ + on the surface of Pt/HNO3-CNFs is higher than that of Pt/CNFs and Pt/S-CNFs (Table 1), which may be attributed to the higher concentration of oxygen on the surface of HNO3-CNFs. The surface elemental composition that was detected in XPS analysis was summarized in Table 1. It can be found that oxygen increased from 8.42% (in Pt/CNFs) to 13.66% (in Pt/HNO3-CNFs) and 11.70% (in Pt/S-CNFs). These results further indicated that more structural defects formed on the surface of CNFs during HNO3-treatment, and these data fit well with the Raman analysis (see Fig. S1). At the same time, it is quite interesting to note
Fig. 2. XPS spectra of Pt 4f in (a) Pt/CNFs, (b) Pt/HNO3-CNFs, and (c) Pt/S-CNFs.
Fig. 3. Time course of the glycerol oxidation over Pt/S-CNFs catalyst.
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M. Zhang et al. / Catalysis Communications 59 (2015) 5–9
Fig. 4. Glycerol oxidation over Pt/S-CNFs at different temperatures.
that atom percentage of Pt in Pt/S-CNFs reached 1.45%, which might be attributed to the higher dispersion of Pt on the outer surface of S-CNFs. 3.2. Selective oxidation of glycerol The activity and product distribution of selective oxidation of glycerol at 60 °C in base-free aqueous solution over Pt/CNFs, Pt/HNO3-CNFs and Pt/S-CNFs are summarized in Table 2. It was found that GLYA is the main product in all experiments. The conversion of glycerol over Pt/CNFs, Pt/HNO3-CNFs and Pt/S-CNFs are 30.1%, 47.3% and 89.9%, respectively. The calculated TOF of surface Pt in Pt/S-CNFs is 239.3 h−1 (at 6 h). Both the activity and the selectivity of GLYA (at 6 h) are higher than those of MWCNTs (TOF = 145.1 h−1, GLYA selectivity = 68.3%) [17] and AC supported Pt (TOF = 110.4 h− 1, GLYA selectivity = 47.4%) [10]. This enhanced activity might be attributed to that Pt dispersed highly on the outer surface of S-CNFs and the facilitated accessibility of Pt with glycerol. Time-on-stream data during the selective oxidation of glycerol at 60 °C in base-free aqueous solution over Pt/S-CNFs are shown in Fig. 3. It can be found that the conversion of glycerol increased quickly to 47.2% in the first hour, and then increased steadily in the following time. The selectivity of GLYA is higher than 80%. The selectivity rates of DIHA, GLYCA and GLYHD are lower than that of MWCNTs [17] and
Fig. 5. The performance of Pt/S-CNFs at different glycerol/Pt ratios in feed.
AC [10] supported Pt catalysts. At the same time, the calculated TOF on the basis of surface Pt reached 750.7 h−1 at the beginning of the reaction (10% conversion of glycerol). The performances of Pt/S-CNFs catalyst for selective oxidation of glycerol at different temperatures and different glycerol/Pt ratios are shown in Figs. 4 and 5, respectively. It was found that the conversion of glycerol increased quickly from 13.8% (at 25 °C) to 78.1% (at 80 °C) with rising temperature, while the selectivity of GLYA and DIHA decreased slightly due to the C–C cleavage as the selectivity of GLYCA rose (Fig. 4). In addition, the excellent performance of Pt/S-CNFs was also confirmed at different glycerol/metal ratios (440–3520) in feed. The conversion of glycerol decreased from 64.7% to 15.6% with the increasing amount of glycerol, and the selectivity of GLYA remains higher than 80%. The catalyst could be recycled for 5 times (see Table S2). According to the catalytic performance of Pt/S-CNFs in this work and those achievements in published papers [6–12,16–21], we think that DIHA and GLYHD should be the initial products in glycerol oxidation and the formed GLYHD are very reactive to form GLYA. But C–C cleavage would happen at higher temperature and/or higher conversion of glycerol. This proposed mechanism is shown in Fig. S4. 4. Conclusion It was found that unique sized and highly dispersed Pt can be prepared on the surface of S-CNFs. Pt/S-CNFs are more active and selective for the oxidation of glycerol to GLYA in a base-free aqueous solution. Characterizations indicated that the dispersion of Pt depends strongly on the surface properties of the support. Pt deposited mainly on the outer wall of S-CNFs, which can contact with glycerol and/or oxygen easily. Acknowledgments This research work was supported by the National Natural Science Foundation of China (21473155, 21273198, 21073159) and the Zhejiang Provincial Natural Science Foundation (LZ12B03001). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2014.09.036. References [1] C.-H. Zhou, J.N. Beltramini, Y.-X. Fan, G.Q. Lu, Chem. Soc. Rev. 37 (2008) 527–549. [2] B. Katryniok, S. Paul, F. Dumeignil, ACS Catal. 3 (2013) 1819–1834. [3] M. Pagliaro, R. Ciriminna, H. Kimura, M. Rossi, C. Della Pina, Angew. Chem. Int. Ed. 46 (2007) 4434–4440. [4] B. Katryniok, H. Kimura, E. Skrzynska, J.-S. Girardon, P. Fongarland, M. Capron, R. Ducoulombier, N. Mimura, S. Paul, F. Dumeignil, Green Chem. 13 (2011) 1960–1979. [5] J.J. Bozell, G.R. Petersen, Green Chem. 12 (2010) 539–554. [6] W. Hu, D. Knight, B. Lowry, A. Varma, Ind. Eng. Chem. Res. 49 (2010) 10876–10882. [7] E.G. Rodrigues, S.A. Carabineiro, X. Chen, J.J. Delgado, J.L. Figueiredo, M.F. Pereira, J.J. Órfão, Catal. Lett. 141 (2011) 420–431. [8] S. Carrettin, P. McMorn, P. Johnston, K. Griffin, G.J. Hutchings, Chem. Commun. (2002) 696–697. [9] A. Villa, G.M. Veith, L. Prati, Angew. Chem. 122 (2010) 4601–4604. [10] D. Liang, J. Gao, J. Wang, P. Chen, Z. Hou, X. Zheng, Catal. Commun. 10 (2009) 1586–1590. [11] R. Garcia, M. Besson, P. Gallezot, Appl. Catal. A Gen 127 (1995) 165–176. [12] S. Carrettin, P. McMorn, P. Johnston, K. Griffin, C.J. Kiely, G.J. Hutchings, Phys. Chem. Chem. Phys. 5 (2003) 1329–1336. [13] B.N. Zope, D.D. Hibbitts, M. Neurock, R.J. Davis, Science 330 (2010) 74–78. [14] M.S. Ide, R.J. Davis, Acc. Chem. Res. 47 (2014) 825–833. [15] G.L. Brett, Q. He, C. Hammond, P.J. Miedziak, N. Dimitratos, M. Sankar, A.A. Herzing, M. Conte, J.A. Lopez-Sanchez, C.J. Kiely, D.W. Knight, S.H. Taylor, G.J. Hutchings, Angew. Chem. 123 (2011) 10318–10321. [16] Z. Lin, H. Chu, Y. Shen, L. Wei, H. Liu, Y. Li, Chem. Commun. (2009) 7167–7169. [17] D. Liang, J. Gao, H. Sun, P. Chen, Z. Hou, X. Zheng, Appl. Catal. B Environ. 106 (2011) 423–432. [18] R. Nie, D. Liang, L. Shen, J. Gao, P. Chen, Z. Hou, Appl. Catal. B Environ. 127 (2012) 212–220.
M. Zhang et al. / Catalysis Communications 59 (2015) 5–9 [19] E.G. Rodrigues, S.A. Carabineiro, J.J. Delgado, X. Chen, M.F. Pereira, J.J. Órfão, J. Catal. 285 (2012) 83–91. [20] E.G. Rodrigues, M.F. Pereira, X. Chen, J.J. Delgado, J.J. Órfão, J. Catal. 281 (2011) 119–127. [21] E.G. Rodrigues, M.F.R. Pereira, J.J. Delgado, X. Chen, J.J.M. Órfão, Catal. Commun. 16 (2011) 64–69. [22] K.P. De Jong, J.W. Geus, Catal. Rev. Sci. Eng. 42 (2000) 481–510. [23] V. Castranova, P.A. Schulte, R.D. Zumwalde, Acc. Chem. Res. 46 (2013) 642–649.
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[24] Y.-T. Kim, K. Ohshima, K. Higashimine, T. Uruga, M. Takata, H. Suematsu, T. Mitani, Angew. Chem. 118 (2006) 421–425. [25] V. Chandra, J. Park, Y. Chun, J.W. Lee, I.-C. Hwang, K.S. Kim, ACS Nano 4 (2010) 3979–3986. [26] H. Wang, X. Xiang, F. Li, J. Mater. Chem. 20 (2010) 3944–3952. [27] Y. Liang, H. Zhang, H. Zhong, X. Zhu, Z. Tian, D. Xu, B. Yi, J. Catal. 238 (2006) 468–476. [28] J. Zhang, K. Sasaki, E. Sutter, R. Adzic, Science 315 (2007) 220–222.
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Selective oxidation of glycerol over carbon nanofibers supported Pt catalysts in a base-free aqueous solution Mengyuan Zhang a, Renfeng Nie a, Lina Wang b, Juanjuan Shi a, Weichen Du a, Zhaoyin Hou a,⁎ a b
Institute of Catalysis, Department of Chemistry, Zhejiang University, Hangzhou 310028, China Engineering Research Center for Eco-Dyeing & Finishing of Textiles, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China
a r t i c l e
i n f o
Article history: Received 1 June 2014 Received in revised form 18 September 2014 Accepted 22 September 2014 Available online 28 September 2014 Keywords: Glycerol Oxidation Base-free Carbon nanofibers Platinum
a b s t r a c t Highly dispersed Pt nanoparticles were fabricated on the surface of S-grafted carbon nanofibers (Pt/S-CNFs) and used in selective oxidation of glycerol with molecular oxygen in a base-free aqueous solution. Characterizations disclosed that S-grafted CNFs are more favorable for the dispersion of Pt than that of crude CNFs and HNO3pretreated CNFs. CNFs supported Pt catalysts are more selective toward the formation of glycerin acid, and the initial activity of surface Pt atoms in Pt/S-CNFs reached 750.7 h−1 at 60 °C in a base-free aqueous solution. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Biodiesel is an important renewable biofuel, and it could be produced from animal fats and vegetable oils via transesterification [1], which is helpful to reduce the dependence on fossil energy. Glycerol is a by-product during the production of biodiesel, and the rapidly rising production of biodiesel leads to a serious surplus of glycerol [2]. Catalytic conversion of glycerol to valuable products has become a hot research topic [3–5]. Among these, selective oxidation of glycerol to glycerin acid (GLYA), 1,3-dihydroxyacetone (DIHA), glyceraldehyde (GLYHD), glycolic acid (GLYCA) and tartronic acid (TARAC) has attracted much attention recently [6–10]. In published works, Au-based catalysts were investigated intensively in oxidation of glycerol. It was found that the conversion of glycerol over Au-based catalysts depended strongly on the basicity of reaction medium [11,12]. More recently, Davis et al. disclosed that oxygen atoms incorporated into the GLYA originated exclusively from the water solvent during the oxidation of glycerol over Au-based catalysts in a base solution. The role of oxygen in alcohol oxidation in aqueous media is to remove electrons from the surface of gold which are deposited during oxidation [13,14]. However, selective oxidation of glycerol in a base-free aqueous solution is more interesting because it allows the direct formation of acid products. Villa et al. [9] found that H-mordenite supported AuPt nanoparticles (NPs) are able to oxidize glycerol to GLYA without the use of ⁎ Corresponding author. E-mail address: [email protected] (Z. Hou).
http://dx.doi.org/10.1016/j.catcom.2014.09.036 1566-7367/© 2014 Elsevier B.V. All rights reserved.
base. Brett et al. [15] reported that Au-Pt and Au-Pd NPs supported on Mg(OH)2 are highly active for the selective oxidation of glycerol under base-free conditions. Our previous work found that mono-metal Pt (size 1.2–8.0 nm) on active carbon (AC) is also capable to oxidize glycerol to GLYA in base-free condition [10]. Micropore-free multiwall carbon nanotubes (MWCNTs) supported Pt catalysts are more active for this reaction because of the easier accessibility of Pt on the outer wall of MWCNTs [16–18]. Rodrigues et al. [19–21] also found that MWCNTs supported Au catalysts are more active for glycerol oxidation than that on AC. Carbon nanofibers (CNFs) have drawn a lot of attention for its thermal, mechanical stability, micropore-free graphite structure and low price [22,23]. In this work, CNFs supported Pt catalysts were prepared and tested in the selective oxidation of glycerol in a base-free solution. The excellent performance of Pt/CNFs was discussed with their structure. 2. Experimental 2.1. Catalysis preparation CNFs (N98% carbon basis, from Sigma-Aldrich) were first pretreated with nitric acid (65–68 wt.%) at 75 °C for 10 h, washed with distilled water until the pH of effluent solution reached 6.7, and the solid was dried in vacuum overnight and denoted as HNO3-CNFs. And then, 5 g of HNO3-CNFs was chloridized in 200 mL SOCl2 under refluxing at 70 °C for 12 h, and further grafted with NH2(CH2)2SH in dehydrated toluene for 24 h [24]. The resulting support was denoted as S-CNFs.
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M. Zhang et al. / Catalysis Communications 59 (2015) 5–9
Table 1 Pt dispersion and elemental composition on CNFs. Catalyst
Pt/CNFs Pt/HNO3-CNFs Pt/S-CNFs S-CNFs a b
Particle size of Pt (nm)
Atom concentration (%)a
Pt loading (wt.%) b
XRD
TEM
C
O
Pt
S
N
Pt/ Ptδ+
6.5 5.9 1.5 –
6.8 4.2 1.8 –
91.25 85.62 86.51 79.28
8.42 13.66 11.70 16.13
0.33 0.72 1.45 –
– – b0.01 2.13
– b0.01 b0.01 2.46
5.34 4.26 6.69 –
4.76 4.78 2.46 –
Determined by XPS. Determined by ICP.
At last, 2 g S-CNFs (or CNFs, HNO3-CNFs) was dispersed in water, and an aqueous solution containing 0.01 g/mL H2PtCl6 was added dropwise under stirring. The slurry was then sonicated for 2 h and reduced with an aqueous solution of KBH4. After reduction, it was filtered, washed with distilled deionized water until free of chlorine and dried at 60 °C in vacuum. Pt/CNFs and Pt/HNO3-CNFs were used directly for catalytic reaction, and Pt/S-CNFs were further treated in hydrogen at 400 °C for 1 h in order to eliminate the S-containing groups [17]. Actual content of Pt in prepared catalysts was determined through inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Plasma-Spec- II spectrometer) and these data were summarized in Table 1. The dispersion of Pt was detected via pulse CO adsorption as described in literature [10,17]. 2.2. Catalyst characterization Raman spectra of CNFs, HNO3-CNFs and S-CNFs were collected from 800 to 2600 cm− 1 on Renishaw 2000 Confocal Raman Microprobe (Renishaw Instruments, England) using 514.5 nm argon ion laser. X-ray diffraction (XRD) analysis of Pt/CNFs, Pt/HNO3-CNFs and Pt/SCNFs was performed on RIGAKU D/MAX2550/PC diffractometer using Cu Kα radiation at 40 kV and 100 mA. Diffraction data was recorded using continuous scanning at a rate of 0.02°/s and step 0.02°. The morphologies and dimensions of catalysts were observed by transmission
electron microscopy (TEM, JEOL-2010F) using an accelerating voltage of 200 kV. X-ray photoelectron spectra (XPS) were recorded on Perkin-Elmer PHI ESCA System; the X-ray source was Mg standard anode 146 (1253.6 eV) at 12 kV and 300 W.
2.3. Glycerol oxidation Glycerol oxidation was carried out in a 100 mL custom designed stainless autoclave with a glass inner layer, which contains 50 mL glycerol solution and 0.5 g catalyst. The reactor was sealed, filled with a certain amount of O2 and N2 and placed in an oil bath preheated to required temperature and maintained at that temperature for a given time under vigorous stirring with a magnetic stirrer (MAG-NEO, RV-06M, Japan). After reaction, catalyst was removed by filtration and the aqueous solution was analyzed using an Agilent 1100 series high-performance liquid chromatograph (HPLC) equipped with a refractive index detector (RID) and a Zorbax SAX column (4.6 mm × 250 mm, Agilent). The turnover frequency (TOF) on the basis of surface Pt atoms was calculated as: TOF = (number of glycerol molecular converted) / (number of surface Pt atoms) / (reaction time, h). The selectivity of each product was calculated as: (mmol of product in reaction mixture) × (the number of carbon atoms in product) / ((initial mmol of glycerol − mmol of glycerol left) × 3) × 100%.
Fig. 1. TEM images and particle size distributions of (a) Pt/CNFs, (b) Pt/HNO3-CNFs, and (c) Pt/S-CNFs. Image of (d) is the high resolution image of Pt particles in Pt/S-CNFs.
M. Zhang et al. / Catalysis Communications 59 (2015) 5–9
3. Results and discussion 3.1. The structure of catalysts Raman analysis found that that the I(D)/I(G) ratios of CNFs, HNO3CNFs and S-CNFs are 0.78, 0.84 and 0.82, respectively (Fig. S1), which indicated that CNFs were partially oxidized during acid treatment [25,26]. In addition, the position of G peak in HNO3-CNFs and S-CNFs shifts to 1572 cm−1 because of more structural defects formed on the surface of CNFs. XRD patterns show that the mean crystallite size of Pt in Pt/ CNFs and Pt/HNO3-CNFs calculated via Scherrer–Warren equation is 6.5 and 5.9 nm, respectively (Fig. S2). In comparison to Pt/CNFs and Pt/HNO3-CNFs, the diffraction peaks of Pt in Pt/S-CNFs become broad and faint, and the mean crystallite size of Pt decreases to 1.5 nm. These data indicate that Pt disperses highly on S-CNFs, which might
7
Table 2 Oxidation of glycerol using different Pt catalysts a. Catalyst
Conversion (%)
Pt/CNFs Pt/HNO3-CNFs Pt/S-CNFs
30.1 47.3 89.9
Selectivity (%)b GLYA
GLYHD
DIHA
GLYCA
Othersc
84.2 84.9 83.2
6.3 3.7 1.1
9.5 6.5 6.8
0.0 0.0 8.9
0.0 4.9 0.0
a Reaction conditions: catalyst 0.5 g, aqueous solution of glycerol 50 mL (0.1 g/mL), 60 °C, 0.4 MPa O2 + 1.6 MPa N2, 6 h. b Selectivity was calculated as: (mmol of product in reaction mixture) × (the number of carbon atoms in product) / ((initial mmol of glycerol − mmol of glycerol left) × 3) × 100%. GLYA: glyceric acid, DIHA: 1,3-dihydroxyacetone, GLYCA: glycolic acid, GLYHD: glyceraldehyde. c Mono-carbon products (mainly like COx and formaldehyde) were also detected.
be attributed to the S-containing reagents that are more effective for controlling the dimensions and distributions of Pt particles [17,24]. Fig. 1 shows the TEM images and particle size distribution of Pt in Pt/ CNFs, Pt/HNO3-CNFs and Pt/S-CNFs catalysts. It can be found that Pt disperses unevenly on the wall of CNFs, and the detected Pt particle size is 1.5–30.0 nm (with an average diameter of 6.8 nm, Fig. 1a). On HNO3pretreated CNFs, the calculated mean size of Pt decreases slightly to 4.2 nm (Fig. 1b). However, highly dispersed and unique sized Pt particles form on S-CNFs and the calculated mean size reaches 1.8 nm (Fig. 1c–d). These results fit well with the above XRD analysis. XPS analysis confirmed that –S–H (2.46 at.%) and –N–H (2.13 at.%) groups formed on the surface of S-CNFs, and the detected number of –S–H and –N–H groups was quite close (see Fig. S3 and Table 1). These data suggested that NH2CH2CH2SH was successfully grafted on CNFs. High resolution spectra of Pt 4f in Pt/CNFs, Pt/HNO3-CNFs and Pt/S-CNFs catalysts are shown in Fig. 2. Deconvolution of the Pt 4f region shows that the most intense doublets are 71.3 (Pt 4f7/2) and 74.5 eV (Pt 4f5/2), and this doublet could be attributed to metallic Pt [27]. Peaks at 72.2 (Pt 4f7/2) and 75.6 eV (Pt 4f5/2) could be assigned to Ptδ+ that anchored with the C–O group in CNFs [28]. The percentage of Ptδ + on the surface of Pt/HNO3-CNFs is higher than that of Pt/CNFs and Pt/S-CNFs (Table 1), which may be attributed to the higher concentration of oxygen on the surface of HNO3-CNFs. The surface elemental composition that was detected in XPS analysis was summarized in Table 1. It can be found that oxygen increased from 8.42% (in Pt/CNFs) to 13.66% (in Pt/HNO3-CNFs) and 11.70% (in Pt/S-CNFs). These results further indicated that more structural defects formed on the surface of CNFs during HNO3-treatment, and these data fit well with the Raman analysis (see Fig. S1). At the same time, it is quite interesting to note
Fig. 2. XPS spectra of Pt 4f in (a) Pt/CNFs, (b) Pt/HNO3-CNFs, and (c) Pt/S-CNFs.
Fig. 3. Time course of the glycerol oxidation over Pt/S-CNFs catalyst.
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M. Zhang et al. / Catalysis Communications 59 (2015) 5–9
Fig. 4. Glycerol oxidation over Pt/S-CNFs at different temperatures.
that atom percentage of Pt in Pt/S-CNFs reached 1.45%, which might be attributed to the higher dispersion of Pt on the outer surface of S-CNFs. 3.2. Selective oxidation of glycerol The activity and product distribution of selective oxidation of glycerol at 60 °C in base-free aqueous solution over Pt/CNFs, Pt/HNO3-CNFs and Pt/S-CNFs are summarized in Table 2. It was found that GLYA is the main product in all experiments. The conversion of glycerol over Pt/CNFs, Pt/HNO3-CNFs and Pt/S-CNFs are 30.1%, 47.3% and 89.9%, respectively. The calculated TOF of surface Pt in Pt/S-CNFs is 239.3 h−1 (at 6 h). Both the activity and the selectivity of GLYA (at 6 h) are higher than those of MWCNTs (TOF = 145.1 h−1, GLYA selectivity = 68.3%) [17] and AC supported Pt (TOF = 110.4 h− 1, GLYA selectivity = 47.4%) [10]. This enhanced activity might be attributed to that Pt dispersed highly on the outer surface of S-CNFs and the facilitated accessibility of Pt with glycerol. Time-on-stream data during the selective oxidation of glycerol at 60 °C in base-free aqueous solution over Pt/S-CNFs are shown in Fig. 3. It can be found that the conversion of glycerol increased quickly to 47.2% in the first hour, and then increased steadily in the following time. The selectivity of GLYA is higher than 80%. The selectivity rates of DIHA, GLYCA and GLYHD are lower than that of MWCNTs [17] and
Fig. 5. The performance of Pt/S-CNFs at different glycerol/Pt ratios in feed.
AC [10] supported Pt catalysts. At the same time, the calculated TOF on the basis of surface Pt reached 750.7 h−1 at the beginning of the reaction (10% conversion of glycerol). The performances of Pt/S-CNFs catalyst for selective oxidation of glycerol at different temperatures and different glycerol/Pt ratios are shown in Figs. 4 and 5, respectively. It was found that the conversion of glycerol increased quickly from 13.8% (at 25 °C) to 78.1% (at 80 °C) with rising temperature, while the selectivity of GLYA and DIHA decreased slightly due to the C–C cleavage as the selectivity of GLYCA rose (Fig. 4). In addition, the excellent performance of Pt/S-CNFs was also confirmed at different glycerol/metal ratios (440–3520) in feed. The conversion of glycerol decreased from 64.7% to 15.6% with the increasing amount of glycerol, and the selectivity of GLYA remains higher than 80%. The catalyst could be recycled for 5 times (see Table S2). According to the catalytic performance of Pt/S-CNFs in this work and those achievements in published papers [6–12,16–21], we think that DIHA and GLYHD should be the initial products in glycerol oxidation and the formed GLYHD are very reactive to form GLYA. But C–C cleavage would happen at higher temperature and/or higher conversion of glycerol. This proposed mechanism is shown in Fig. S4. 4. Conclusion It was found that unique sized and highly dispersed Pt can be prepared on the surface of S-CNFs. Pt/S-CNFs are more active and selective for the oxidation of glycerol to GLYA in a base-free aqueous solution. Characterizations indicated that the dispersion of Pt depends strongly on the surface properties of the support. Pt deposited mainly on the outer wall of S-CNFs, which can contact with glycerol and/or oxygen easily. Acknowledgments This research work was supported by the National Natural Science Foundation of China (21473155, 21273198, 21073159) and the Zhejiang Provincial Natural Science Foundation (LZ12B03001). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2014.09.036. References [1] C.-H. Zhou, J.N. Beltramini, Y.-X. Fan, G.Q. Lu, Chem. Soc. Rev. 37 (2008) 527–549. [2] B. Katryniok, S. Paul, F. Dumeignil, ACS Catal. 3 (2013) 1819–1834. [3] M. Pagliaro, R. Ciriminna, H. Kimura, M. Rossi, C. Della Pina, Angew. Chem. Int. Ed. 46 (2007) 4434–4440. [4] B. Katryniok, H. Kimura, E. Skrzynska, J.-S. Girardon, P. Fongarland, M. Capron, R. Ducoulombier, N. Mimura, S. Paul, F. Dumeignil, Green Chem. 13 (2011) 1960–1979. [5] J.J. Bozell, G.R. Petersen, Green Chem. 12 (2010) 539–554. [6] W. Hu, D. Knight, B. Lowry, A. Varma, Ind. Eng. Chem. Res. 49 (2010) 10876–10882. [7] E.G. Rodrigues, S.A. Carabineiro, X. Chen, J.J. Delgado, J.L. Figueiredo, M.F. Pereira, J.J. Órfão, Catal. Lett. 141 (2011) 420–431. [8] S. Carrettin, P. McMorn, P. Johnston, K. Griffin, G.J. Hutchings, Chem. Commun. (2002) 696–697. [9] A. Villa, G.M. Veith, L. Prati, Angew. Chem. 122 (2010) 4601–4604. [10] D. Liang, J. Gao, J. Wang, P. Chen, Z. Hou, X. Zheng, Catal. Commun. 10 (2009) 1586–1590. [11] R. Garcia, M. Besson, P. Gallezot, Appl. Catal. A Gen 127 (1995) 165–176. [12] S. Carrettin, P. McMorn, P. Johnston, K. Griffin, C.J. Kiely, G.J. Hutchings, Phys. Chem. Chem. Phys. 5 (2003) 1329–1336. [13] B.N. Zope, D.D. Hibbitts, M. Neurock, R.J. Davis, Science 330 (2010) 74–78. [14] M.S. Ide, R.J. Davis, Acc. Chem. Res. 47 (2014) 825–833. [15] G.L. Brett, Q. He, C. Hammond, P.J. Miedziak, N. Dimitratos, M. Sankar, A.A. Herzing, M. Conte, J.A. Lopez-Sanchez, C.J. Kiely, D.W. Knight, S.H. Taylor, G.J. Hutchings, Angew. Chem. 123 (2011) 10318–10321. [16] Z. Lin, H. Chu, Y. Shen, L. Wei, H. Liu, Y. Li, Chem. Commun. (2009) 7167–7169. [17] D. Liang, J. Gao, H. Sun, P. Chen, Z. Hou, X. Zheng, Appl. Catal. B Environ. 106 (2011) 423–432. [18] R. Nie, D. Liang, L. Shen, J. Gao, P. Chen, Z. Hou, Appl. Catal. B Environ. 127 (2012) 212–220.
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