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Selective oxidation of glycerol over Pt supported on mesoporous carbon nitride in base-free aqueous solution
Accepted Manuscript Selective oxidation of glycerol over Pt supported on mesoporous carbon nitride in base-free aqueous solution Fen-Fen Wang, Shuai Shao, Chun-Ling Liu, Chun-Li Xu, Rong-Zhen Yang, Wen-Sheng Dong PII: DOI: Reference:
S1385-8947(14)01583-6 http://dx.doi.org/10.1016/j.cej.2014.11.115 CEJ 12970
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
27 September 2014 8 November 2014 23 November 2014
Please cite this article as: F-F. Wang, S. Shao, C-L. Liu, C-L. Xu, R-Z. Yang, W-S. Dong, Selective oxidation of glycerol over Pt supported on mesoporous carbon nitride in base-free aqueous solution, Chemical Engineering Journal (2014), doi: http://dx.doi.org/10.1016/j.cej.2014.11.115
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Selective oxidation of glycerol over Pt supported on mesoporous carbon nitride in base-free aqueous solution Fen-Fen Wang, Shuai Shao, Chun-Ling Liu, Chun-Li Xu, Rong-Zhen Yang, Wen-Sheng Dong*
Key Laboratory of Applied Surface and Colloid Chemistry (SNNU), MOE, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an, 710062, China
* To whom correspondence should be addressed: Phone: +86-(0)29-81530806 Fax: +86-(0)29-81530806 E-mail: [email protected]
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Abstract Novel ordered mesoporous carbon nitride (MCN)-supported Pt catalysts were prepared, and then characterized using a combination of powder X-ray diffraction, transmission electron microscopy, N2 adsorption, inductively coupled plasma optical emission spectroscopy, NH3 temperature-programmed desorption, and X-ray photoelectron spectroscopy. These catalysts were evaluated for the selective oxidation of glycerol with molecular oxygen in base-free aqueous solution. The results showed that with increasing N content in the supports, the average Pt particle size in the catalysts decreased, whereas the conversion of glycerol increased. The basicity of the supports is helpful for promoting the oxidation of glycerol to glyceraldehyde and further oxidation to glyceric acid. Glycerol conversion of 63.1% and yields of 58.5% glyceric acid and 24.3% glyceraldehydes were obtained when reacting an aqueous solution of glycerol (0.3 M, 20 mL) with 0.023 g catalyst at 60 °C under 0.3 MPa O2 for 4 h. Upon recycling of the catalyst, the conversion of glycerol gradually decreased. The decrease of catalytic activity during recycling can be attributed to a combination of the growth and oxidation of Pt particles, leaching of N species, and collapse of the ordered mesoporous structure in the carbon nitride support.
Keywords: Glycerol; Oxidation; Pt catalyst; Mesoporous carbon nitride.
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1. Introduction Glycerol is currently produced in large amounts as a byproduct of the manufacture of biodiesel by the transesterification of vegetable oils. One hundred kilograms of glycerol is produced for every 1 ton of biodiesel, and global production of biodiesel has grown from 15,200 barrels per day in 2000 to 335,700 barrels per day in 2010 [1]. Therefore, the conversion of the byproduct glycerol into value-added products would make the biodiesel industry economically more attractive. Glycerol is a highly functionalized molecule and contains three hydroxyl groups. It can be converted into several important chemicals by oxidation, hydrogenolysis, dehydration, esterification, oxidative carbonylation, transesterification, polymerization, and so on [2]. These features make glycerol viable as a versatile biobuilding block. For these reasons, the conversion of glycerol to high-value chemicals has attracted much attention in both academia and industry [3−7]. The selective oxidation of glycerol can produce various valuable oxygenated derivatives, such as glyceric acid, glyceraldehyde, dihydroxyacetone, tartronic acid, glycolic acid, and lactic acid. [8]. However, because of the complex nature of these reaction pathways, it is still a challenge to control the selectivity to give desired products. The aerobic oxidation of glycerol has been extensively studied by many groups using supported Pt [9,10], Pd [10,11], Au [10−22], Rh [23], Ir [24], and Cu-containing catalysts [25−28], and bimetals such as Pt–Bi [29], Au–Pd [30−33], Pd–Ag [34], and Au–Pt [35,36]. The activity and selectivity strongly depend on the reaction conditions (pH, temperature, and substrate to metal ratio) and the structure of the catalyst (metal, particle size, and support). It has been reported that the oxidation of glycerol over gold catalysts is completely inhibited in the absence of the base [10]. Moreover, the sodium salt of glyceric acid is the main product of the reaction, and additional neutralization and acidification is required to obtain the free glyceric acid. From a green chemistry point of view, homogeneous bases should not be used. Recently, advances have been made in base-free 3
oxidation of glycerol. Platinum-containing catalysts have been shown to be effective for this reaction [37−48]. For example, Hou et al. found that Pt/C [39,40], Pt/multiwall carbon nanotubes [40,41], PtCu/C [42], and PtSb/C [43] catalyze the oxidation of glycerol in base-free aqueous solution. Hutchings et al. found that Au–Pt/MgO [44] and Au–Pt–Pd/TiO2 [45] efficiently catalyze base-free oxidation of glycerol. Prati et al. [46] reported that Au−Pt supported on the acidic H-mordenite can also catalyze glycerol oxidation in base-free aqueous solution, although a high reaction temperature is required. Ebitani et al. [47,48] investigated the oxidation of glycerol using Pt/Al2O3, Pt/C, Pt/SiO2, and hydrotalcite-supported platinum catalysts. They found that the basic hydrotalcite-supported platinum catalyst was the most active for this reaction under base-free conditions. Recently, mesoporous carbon nitride (MCN) materials with large surface areas, small particle sizes, tunable pore diameters, and basic character have attracted considerable attention because the incorporation of nitrogen atoms in the carbon nanostructure can enhance the mechanical and conducting properties, field-emission, energy-storage prosperities, and catalytic performance. Vinu et al. found that MCN exhibited excellent performance in the basic catalytic transesterification of β-ketoester and alcohols [49]. Gold-embedded MCN catalysts have also shown good performance in the three-component coupling reaction of benzaldehyde, piperidine, and phenylacetylene for the synthesis of propargylamine [50]. In this study, inspired by the excellent performance of MCN in various reactions, we embedded platinum nanoparticles in MCN, and then used these materials to catalyze the oxidation of glycerol in base-free aqueous solution. 2. Experimental 2.1. Chemicals Pluronic P123 (PEO106PPO70PEO106, Mav = 5800), polyvinyl alcohol (PVA, Mw = 10000, 80%
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hydrolyzed) and glycolic acid (GLYCA) were purchased from Sigma-Aldrich. H2PtCl6·6H2O, NaBH4, tetraethylorthosilicate (TEOS), HCl solution, and hydrofluoric acid were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Glycerol was purchased from Fuchen Chemical Reagent (Tianjin, China). Glyceric acid (GLYA, 40% in water) and glyceraldehyde (GLYDE) were obtained from Alfa Aesar. Dihydroxyacetone (DHA) was obtained from J&K CHEMICA. All chemicals were of analytical grade and used as received without further purification. 2.2. Preparation of catalysts SBA-15 was prepared according to the following procedure. In a typical synthesis, 4 g of P123 was dispersed in 30 g of water and 120 g of a 2 M HCl solution and stirred for 5 h. Thereafter, 9.5 g of TEOS was added to the homogeneous solution under stirring. The resulting gel was aged at 40 °C for 24 h, and then the solution was transferred into a Teflon bottle and heated at 150 °C for 24 h. After being cooled to room temperature, the product was filtered off, washed with distilled water, dried at 338 K, and finally calcined at 600 °C in air for 4 h. MCN materials were prepared by the method developed by Vinu et al. [50] through a polymerization reaction between carbon tetrachloride and ethylenediamine using SBA-15 as a sacrificial template. In a typical preparation, SBA-15 (0.5 g) was added to a mixture of ethylenediamine (0.3 g) and carbon tetrachloride (3 g). The resultant mixture was heated at 90 °C for 6 h under stirring. The obtained dark-brown solid mixture was placed in a drying oven for 12 h and then ground into a fine powder. The template carbon nitride polymer composites were then heat treated in a nitrogen flow of 50 mL min−1 at 600 °C for 5 h using a heating rate of 3 °C min−1. The MCN was obtained after dissolution of the silica framework in 5 wt% hydrofluoric acid followed by filtration, washing with ethanol, and drying at 100 °C. The resultant sample was denoted as MCN-1. The other MCN materials with different N contents were prepared by varying the addition
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mass of ethylenediamine (0.9, 1.35, 2.1, 2.7, and 3.3 g), and the obtained products were denoted as MCN-2, MCN-3, MCN-4, MCN-5 and MCN-6, respectively. Supported Pt catalysts were prepared by immobilizing the colloidal metal particles on the MCN supports. In a typical preparation, the protecting agent was added (Pt/PVA=2:1, weight ratio) to a 50 mL−1 aqueous gold solution (1.12 × 10−3 M) at room temperature (25 °C) under vigorous stirring. Rapid injection of a 0.1 M aqueous solution of NaBH4 (Pt/NaBH4=1:4, mol ratio) led to the formation of a dark solution, indicating the formation of platinum colloids. The immobilization of Pt particles on the MCN materials was accomplished at room temperature (25 °C) by adding the supports to the colloidal Pt solution while stirring, and they were kept in contact until total adsorption (3 wt% Pt on the support) occurred, as indicated by discoloration of the solution. The solids were collected by filtration followed by washing the solid with distilled water to remove all the dissolved species (e.g., Na+ and Cl−). Finally, the solids were dried under vacuum at 110 °C for 12 h. 2.3. Catalyst characterization X-ray diffraction (XRD) patterns of the catalysts were collected on a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation (40 kV, 30 mA). To determine the average sizes of the Pt particles, the samples were finely scanned in the 2θ range from 30 to 50° with a 2θ step size of 0.01° and a step time of 1 s. The Pt particle sizes were estimated using Scherrer’s equation. Transmission electron microscopy (TEM) was conducted on a JEOL JEM 2010 electron microscope operating at 200 kV. X-ray photoelectron spectroscopy (XPS) analysis was performed on an Axis Ultra Kratos (UK) apparatus using an Al Kα source (15 kV, 1486.6 eV). The vacuum in the system was better than 10 −9 Torr. The binding energy was calibrated relative to the C 1s peak (284.8 eV) of the contaminant carbon. The Pt contents in the catalysts were determined with a Varian 720-ES inductively coupled plasma optical emission spectrometer (ICP-OES). Nitrogen adsorption–desorption isotherms were obtained on a Micromeritics ASAP 6
2020M system. Before the measurements, the samples were degassed at 250 °C for 6 h. The Brunauer–Emmett–Teller (BET) method was used to calculate the specific surface areas (SBET). The pore size distributions were derived from the desorption branch of the isotherm based on the Barrett–Joyner–Halenda (BJH) model. The total pore volume (Vp) was estimated from the adsorption branch at a relative pressure (P/P0) of 0.998. The micropore volume (Vmicro) and surface area (Smicro) were determined according to the t-plot method. The volume of the mesopores (Vmeso) was calculated from the difference between Vp and Vmicro. The average pore diameter (Dp) was estimated from the surface area and the total pore volume (Dp = 4Vp/SBET). CO2 temperature-programmed desorption (CO2-TPD) was performed using a Micromeritics AutoChem II 2920 system. A 120 mg sample was loaded into the sample tube and pretreated in He at 500 °C for 1 h, and then the sample was cooled to 50 °C in flowing He. CO2 was introduced and allowed to adsorb at 50 °C for 1 h. The sample was then purged with dry helium at the same temperature to remove weakly adsorbed CO2. Finally, the sample was heated at a linear rate of 10 °C min–1 under dry helium to 500 °C. The effluent was monitored by a thermal conductivity detector. 2.4. Reaction test and product analysis All of the reactions were carried out in a 35 mL stainless steel autoclave equipped with a mechanical stirrer. In a typical experiment, 0.023 g catalyst and 20 mL aqueous solution of glycerol (0.3 M) were charged into the reactor. The autoclave was purged twice with pure O2, and then pressurized to 0.3 MPa with O2 at room temperature. The reaction mixture was heated to 60 °C and held at this temperature for 4 h (unless otherwise stated) under a stirring rate of 600 rpm. After the reaction, the reactor was rapidly cooled to room temperature with an ice-water mixture and depressurized. The post-reaction liquid sample was diluted with mobile phase solution before analysis. The gas mixture was collected in a gasbag for gas chromatograph (GC) analysis. 7
Sample analysis was performed on a Shimadzu HPLC LC-20AT system equipped with both refractive index (RID-10A) and UV (SPD-20A) detectors. An ion-exclusion column (Bio-Rad Aminex HPX-87H) at 40 °C with 0.005 M formic acid (or 0.005 M H2SO4) solution as the mobile phase flowing at 0.30 mL min−1 was used to separate and identify the compounds. The amount of products was determined using calibration curves. Other byproducts, such as CO2, CO, and HCHO, were detected in the gas effluent. The retention times and standard curves of the detected liquid products were determined, and are shown in Table S1 and Fig. S1 (Supplementary Data). 3. Results and discussion 3.1. Structure characterization of Pt/MCN catalysts Low- and wide-angle XRD patterns of the Pt/MCN catalysts are shown in Fig. 1. The Pt/MCN-6 sample had a disordered structure, so the low-angle XRD pattern of this sample is not shown in Fig. 1. For all of the other samples, a sharp diffraction peak at 2θ ≈ 0.9° and a weak peak at 2θ ≈ 1.59° were observed, which can be indexed to the (100) and (110) reflections of the two-dimensional (2D) hexagonal (P6mm) structure, respectively. The pattern is similar to the XRD pattern of the parent mesoporous silica template SBA-15, which consists of a hexagonal arrangement of cylindrical pores interlinked by the micropores present in the walls [51]. Such materials with one-dimensional (1D) mesopores arranged in a hexagonal net are present because the diffraction pattern shows 2D P6mm symmetry. In the wide-angle region, the Pt/MCN catalysts showed two broad diffraction peaks. The peak located at 2θ ≈ 25° corresponds to the (002) diffraction of amorphous carbon. The peak at ~39° was assigned to the (111) reflection of platinum. To determine the average sizes of the Pt particles, the samples were finely scanned in the 2θ range from 30° to 50° with a 2θ step size of 0.01° and a step time of 1 s. The Pt particle sizes were calculated using Scherrer’s equation, and
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the results are shown in Table 1. As shown in Table 1, the Pt particle sizes gradually decreased with increasing N content in the MCN supports. This can be explained as follows. The MCN supports derived from the polymerization between carbon tetrachloride and ethylenediamine have inbuilt –NH2 and –NH groups on the mesoporous walls. These groups can act as a stabilizing agent by providing an anchoring and heterogeneous surface that allows the formation of highly dispersed metal nanoparticles without any agglomeration [50]. The presence of more functional amine groups on MCN helps to reduce the aggregation of Pt nanoparticles and stabilize the formed nanoparticles inside the mesoporous channels. TEM was used to observe the pore structure of the Pt/MCN catalysts. Figure 2 shows TEM images of these catalysts. Pt/MCN-1 to Pt/MCN-5 show highly ordered 1D channels along the [110] direction (Fig. 2a–e). However, no Pt particles were observed owing to the high dispersion of the Pt nanoparticles inside the mesoporous channels. For Pt/MCN-6, ~2 nm Pt particles were homogeneously dispersed on the disordered mesoporous MCN-6 support (Fig. 2f). The results suggest that large amounts of N in the MCN materials adversely affect the ordering of the carbon nitride materials. The physicochemical properties of the Pt/MCN catalysts were characterized by N2 adsorption–desorption. Figure 3 shows the N2 sorption isotherms and the corresponding pore size distributions of the catalysts. All of the samples except for Pt/MCN-6 show type IV isotherms with H1-shaped hysteresis loops in the P/P0 range 0.4–0.8 (Fig. 3a) and narrow pore size distributions (Fig. 3b), which are characteristic of highly ordered mesoporous materials. The Pt/MCN-6 sample also shows a IV isotherm but with a H4-shaped hysteresis loop, which is characteristic of mesoporous materials. The detailed textural properties of the Pt/MCN catalysts are summarized in Table 1. All of the samples had specific surface areas greater than 500 m2 g−1 and mean pore sizes greater than 4 nm. The CO2-TPD curves of the Pt/MCN catalysts are shown in Fig. 4. Pt/MCN-1 shows a 9
desorption peak centered at around 100 °C, indicating that the surface of this sample featured only one type of weak basic site. With increasing N content in the supports, the peak areas corresponding to CO2 desorption increase, indicating that the number of weak basic sites in the Pt/MCN catalysts increased. XPS was used to analyze and identify the surface species and oxidation states on the Pt/MCN catalysts, and the results are shown in Table 2. The binding energy of N 1s in each sample was almost constant (~397.9 ± 0.1 eV), indicating no electronic interaction between N and Pt. There were two Pt species present in all of the catalysts, Pt0 (~70.9 eV) and Ptδ+ (~71.9 eV), because the surface of Pt0 could be immediately oxidized when exposed to air. With increasing N content in the Pt/MCN catalysts, the content of Pt0 species generally decreased. The results are consistent with the XRD analysis. The XRD analysis showed that the Pt particle size decreased with increasing N content in the catalysts. The smaller Pt particles have larger surface area per unit volume than larger particles, and are therefore more easily oxidized. The surface Pt content in all of the Pt/MCN catalysts except Pt/MCN-6 was ~1.9%, which is lower than the bulk content (~3.0%), suggesting that Pt particles mainly entered the pores of the MCN materials.
3.2. Catalytic performance of Pt/MCN catalysts Table 3 summarizes the activity of the Pt/MCN catalysts for the oxidation of glycerol in base-free aqueous solution. The main products on the catalysts were glyceric acid and glyceraldehyde. For all of the Pt/MCN catalysts, the conversion of glycerol increased with increasing N content in the catalysts, indicating that more basic sites promote the oxidation of glycerol. The oxidation of an alcohol to an aldehyde in aqueous solution over Pt catalysts likely occurs in three steps [52]. First, the alcohol adsorbs on the metal surface, producing an adsorbed
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metal alkoxide. Second, β-hydride elimination occurs to produce a carbonyl species and a metal hydride. Finally, the metal–hydride is oxidized by associatively adsorbed oxygen to regenerate the metal surface. Among these steps, β-hydride elimination has been proposed as the rate-determining step. The basic sites contained in the supports may promote β-hydride abstraction during the reaction, and thus accelerate the oxidation of alcohols [53,54]. Ebitani et al. [47] also found that basicity of the hydrotalcite support is advantageous for promoting glycerol oxidation. On the other hand, with increasing N content in the Pt/MCN catalysts, the Pt particle sizes slightly decreased, which may also contribute to the conversion of glycerol, but we believe this may not be the main reason. From Table 3, with increasing N content in the Pt/MCN catalysts, the selectivity of glyceraldehyde decreased, whereas the selectivity of glyceric acid increased (except for Pt/MCN-6). The results suggest that more basic sites promoted the sequential oxidation of glyceraldehyde to glyceric acid. This is inconsistent with the results reported by Ebitani et al. [47] in the selective oxidation of glycerol over hydrotalcite-supported platinum catalysts and by Villa et al. [53] in the selective oxidation of alcohols. Table 3 also shows that the selectivity of dihydroxyacetone decreased, whereas the selectivity of glycolic acid increased with increasing N content in the Pt/MCN catalysts except for Pt/MCN-6. Among the catalysts, Pt/MCN-6 showed the lowest selectivities of glyceraldehyde and glyceric acid but the highest selectivities of C1 products and glycolic acid, suggesting that more basic sites promoted C–C bond cleavage reactions. Pt/MCN-6 also exhibited the highest selectivity of dihydroxyacetone, although the reason for this is unclear and further investigation is required. Figure 5 shows the time course of glycerol oxidation on Pt/MCN-5 in base-free aqueous solution. At the initial stage of the oxidation reaction (0.5 h), glycerol conversion of 29.5% was
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obtained. The corresponding selectivities of glyceric acid, glyceraldehyde, glycolic acid, and dihydroxyacetone were 76.2%, 10.8%, 2.8%, and 2.6%, respectively. With prolonged reaction time, glycerol conversion increased and the selectivity of glyceric acid gradually decreased, whereas the selectivities of glyceraldehyde, glycolic acid, and dihydroxyacetone gradually increased. Glycerol oxidation is a sequential reaction that follows multiple paths [52]. The oxidation of a primary or secondary alcohol of glycerol produces glyceraldehyde or dihydroxyacetone, respectively. Glyceraldehyde can be sequentially oxidized to glyceric acid, and both glyceraldehyde and dihydroxyacetone can be oxidized to hydroxypyruvic acid. The sequential oxidation of glyceric acid typically produces tartronic acid. Moreover, glycerol oxidation products can undergo carbon cleavage to form C2 acids, such as glycolic and oxalic acid, as well as C1 products. In the present study, glyceric acid, glyceraldehyde, dihydroxyacetone, glycolic acid, and small amounts of C1 products were detected. The yield of dihydroxyacetone was rather low, whereas the yields of glyceraldehyde and glyceric acid were much higher during the reaction, suggesting that the formation rate of dihydroxyacetone over Pt/MCN-5 was much lower than that of glyceraldehyde. Therefore, a plausible reaction pathway for the oxidation of glycerol over Pt/MCN-5 in base-free aqueous solution is proposed in Scheme 1. The reusability of Pt/MCN-5 for the oxidation of glycerol was investigated. After the first run, the catalyst was separated from the reaction solution by filtration, washed with distilled water, dried, and then reused in another four catalytic runs. The results in Fig. 6 show that both the conversion of glycerol and the selectivity of glyceric acid decreased, whereas the selectivities of glyceraldehyde, dihydroxyacetone and glycolic acid gradually increased. As previously reported, a major disadvantage of Pt catalysts is that they can rapidly deactivate under certain reaction conditions. Deactivation of the Pt catalysts could be caused by over-oxidation of the metal to form oxides on the surface and poisoning of the surface with acid
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products, byproducts, or cleavage products [52]. To determine the real reasons for catalyst deactivation, the used catalyst was characterized by TEM, XPS, and subjected to BET analysis, and the reaction solutions after each cycle were analyzed by ICP-OES. ICP analysis confirmed that no detectable leaching of Pt into the reaction mixture occurred during recycling. XRD analysis (Fig. S2a) confirmed that the average Pt particle size in the used Pt/MCN-5 catalyst increased to ~5.8 nm. The TEM image (Fig. S2b) showed that the ordered mesoporous structure in Pt/MCN-5 was destroyed after recycling. XPS analysis (Fig. S3) showed that the binding energy of Pt4f7/2 increased (>0.7 eV), while the concentration of O species on the surface of the used catalyst increased, suggesting that the Pt species were more oxidized. In addition, the N concentration decreased in the used Pt/MCN-5 catalyst, indicating that leaching of N-containing species occurred. Hence, the decrease of catalytic activity during recycling can be attributed to a combination of growth and oxidation of Pt particles, leaching of N species, and collapse of the ordered mesoporous structure.
4. Conclusions We prepared novel ordered mesoporous carbon nitride (MCN)-supported Pt catalysts. The Pt nanoparticles were homogenously embedded in the ordered mesoporous channels. These synthesized materials were evaluated for the selective oxidation of glycerol with molecular oxygen in base-free aqueous solution. The results showed that with increasing N content in the supports, the average Pt particle sizes in the catalysts decreased. The basicity of the supports could promote the sequential oxidation of glycerol to glyceraldehyde and then to glyceric acid. The Pt/MCN catalysts showed high conversion of glycerol and selectivities of glyceric acid and glyceraldehydes. Upon recycling of the catalyst, the conversion of glycerol gradually decreased. The decrease of catalytic activity during recycling can be attributed to a combination of the growth and oxidation of Pt particles, leaching of N species, and collapse of the ordered
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mesoporous structure in the carbon nitride support.
Acknowledgments The authors gratefully acknowledge financial support from the Program for Key Science and Technology Innovation Team of Shaanxi Province (2012KCT-21) and the Fundamental Research Funds for the Central Universities (GK201305011).
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[34] S. Hirasawa, Y. Nakagawa, K. Tomishige, Selective oxidation of glycerol to dihydroxyacetone over a Pd-Ag catalyst, Catal. Sci. Technol. 2 (2012) 1150–1152. [35] Y. Shen, S. Zhang, H. Li, Y. Ren, H. Liu, Efficient synthesis of lactic acid by aerobic oxidation of glycerol on Au–Pt/TiO2 catalysts, Chem. Eur. J. 16 (2010) 7368–7371. [36] R.K.P. Purushothaman, J. van Haveren, D.S. van Es, I. Melián-Cabrera, J.D. Meeldijk, H.J. Heeres, An efficient one pot conversion of glycerol to lactic acid using bimetallic gold-platinum catalysts on a nanocrystalline CeO2 support, Appl. Catal. B: Environ. 147 (2014) 92–100. [37] S.-S. Liu, K.-Q. Sun, B.-Q. Xu, Specific selectivity of Au-catalyzed oxidation of glycerol and other C3‑ polyols in water without the presence of a base, ACS Catal. 4 (2014) 2226–2230. [38] E.G. Rodrigues, M.F.R. Pereira, X. Chen, J.J. Delgado, J.J. M. Órfão, Selective oxidation of glycerol over platinum-based catalysts supported on carbon nanotubes, Ind. Eng. Chem. Res. 52 (2013) 17390−17398. [39] D. Liang, J. Gao, J. Wang, P. Chen, Z. Hou, X. Zheng, Selective oxidation of glycerol in a base-free aqueous solution over different sized Pt catalysts, Catal. Commun. 10 (2009) 1586–1590. [40] J. Gao, D. Liang, P. Chen, Z. Hou, X. Zheng, Oxidation of glycerol with oxygen in a base-free aqueous solution over Pt/AC and Pt/MWNTs catalysts, Catal. Lett. 130 (2009) 185–191. [41] D. Liang, J. Gao, H. Sun, P. Chen, Z. Hou, X. Zheng, Selective oxidation of glycerol with oxygen in a base-free aqueous solution over MWNTs supported Pt catalysts, Appl. Catal. B: Environ. 106 (2011) 423–432. [42] D. Liang, J. Gao, J. Wang, P. Chen, Y. Wei, Z. Hou, Bimetallic Pt-Cu catalysts for glycerol oxidation with oxygen in a base-free aqueous solution, Catal. Commun. 12 (2011) 1059–1062. [43] R. Nie, D. Liang, L. Shen, J. Gao, P. Chen, Z. Hou, Selective oxidation of glycerol with oxygen in base-free solution over MWCNTs supported PtSb alloy nanoparticles, Appl. Catal. B: Environ. 127 (2012) 212–220. [44] 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, Selective oxidation of glycerol by highly active, bimetallic catalysts at ambient temperature under base-free conditions, Angew. Chem. Int. Ed. 50 (2011) 10136–10139. [45] S.A. Kondrat, P.J. Miedziak, M. Douthwaite, G.L. Brett, T.E. Davies, D.J. Morgan, J.K. Edwards, D.W. 17
Knight, C.J. Kiely, S.H. Taylor, G.J. Hutchings, Base-free oxidation of glycerol using titania-supported trimetallic Au–Pd–Pt nanoparticles, ChemSusChem 7 (2014) 1326–1334. [46] A. Villa, G. M. Veith, L. Prati, Selective oxidation of glycerol under acidic conditions using gold catalysts, Angew. Chem. Int. Ed. 49 (2010) 4499–4502. [47] A. Tsuji, K.T.V. Rao, S. Nishimura, A. Takagaki, K. Ebitani, Selective oxidation of glycerol by using a hydrotalcite-supported platinum catalyst under atmospheric oxygen pressure in water, ChemSusChem 4 (2011) 542–548. [48] A. Takagaki, A. Tsuji, S. Nishimura, K. Ebitani, Genesis of catalytically active gold nanoparticles supported on hydrotalcite for base-free selective oxidation of glycerol in water with molecular oxygen, Chem. Lett. 40 (2011) 150–152. [49] X. Jin, V.V. Balasubramanian, S.T. Selvan, D.P. Sawant, M.A. Chari, G.Q. Lu, A. Vinu, Highly ordered mesoporous carbon nitride nanoparticles with high nitrogen content: a metal-free basic catalyst, Angew. Chem. Int. Ed. 48 (2009) 7884–7887. [50] K.K. Datta, B.V.S. Reddy, K. Ariga, A. Vinu, Gold nanoparticles embedded in a mesoporous carbon nitride stablizer for highly efficient three-component coupling reaction, Angew. Chem. Int. Ed. 49 (2010) 5961–5965. [51] A. Vinu, K. Ariga, T. Mori, T. Nakanishi, S. Hishita, D. Golberg, Y. Bando, Preparation and characterization of well-ordered hexagonal mesoporous carbon nitride, Adv. Mater. 17 (2005) 1678–1652. [52] S.E. Davis, M.S. Ide, R.J. Davis, Selective oxidation of alcohols and aldehydes over supported metal nanoparticles, Green Chem. 15 (2013) 17–45. [53] A. Villa, C.E. Chan-Thaw, G.M. Veith, K.L. More, D. Ferri, L. Prati, Au on nanosized NiO: a cooperative effect between Au and nanosized NiO in the base-free alcohol oxidation, ChemCatChem 3 (2011) 1612–1618. [54] G. Wu, X. Wang, Y. Huang, X. Liu, F. Zhang, K. Ding, X. Yang, Selective oxidation of glycerol with O2 catalyzed by low-cost CuNiAl hydrotalcites, J. Mole. Catal. A: Chem. 379 (2013) 185–191.
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Table 1 Compositions and textural properties of supported Pt catalysts Pt content
Pt particle
SBET
Smeso
Vp
Vmeso
Dp
(wt%)a
size (nm)b
(m2 g−1)
（m2 g−1)
(cm3 g−1 )
(cm3 g−1 )
(nm)
Pt/MCN-1
2.9
3.4
509
459
0.66
0.63
4.0
Pt/MCN-2
2.9
2.9
710
628
0.72
0.68
4.4
Pt/MCN-3
3.1
2.8
506
471
0.61
0.60
4.1
Pt/MCN-4
3.0
2.6
502
433
0.68
0.65
4.4
Pt/MCN-5
3.1
2.3
608
498
0.73
0.66
4.5
Pt/MCN-6
3.2
2.0
806
705
1.5
1.44
6.6
Sample
a
Measured by ICP-OES.
b
Measured by XRD.
19
Table 2 XPS analysis of Pt catalysts supported on mesoporous carbon nitrides Binding Energy (eV) Catalyst
O 1s
N 1s
(C–O)
(C–N)
Pt/MCN-1
532.1
398.0
Pt/MCN-2
531.9
397.8
Pt/MCN-3
531.9
397.8
Pt/MCN-4
531.9
397.9
Pt/MCN-5
531.9
397.8
Pt/MCN-6
531.9
397.9
Surface concentration (wt%)
Pt species Pt4f7/2
(%)
71.0
Pt0 : 56.2
71.9
Ptδ+: 43.7
70.9
Pt0 : 56.0
71.9
Ptδ+: 44.0
70.8
Pt0 : 55.5
71.8
Ptδ+: 44.5
70.9
Pt0 : 54.5
71.7
Ptδ+: 45.5
70.6
Pt0 : 54.9
71.5
Ptδ+: 45.1
70.7
Pt0 : 53.7
72.1
Ptδ+: 46.3
20
O
C
N
Pt
20.2
75.2
2.6
1.9
18.6
74.4
4.5
1.9
17.1
74.3
6.6
1.8
16.2
74.8
7.1
1.7
13.5
75.2
9.4
1.8
10.1
71.8
13.5
3.8
Table 3 Oxidation of glycerol using Pt catalysts supported on mesoporous carbon nitrides Selectivity (%)
Conversion (%)
GLYA
GLYDE
GLYCA
DHA
C1 products
Pt/MCN-1
37.7
35.8
46.2
2.2
13.9
1.9
Pt/MCN-2
40.5
35.9
45.9
2.6
13.6
2.0
Pt/MCN-3
47.5
45.1
38.9
4.2
9.5
2.3
Pt/MCN-4
56.4
54.5
30.4
4.6
6.9
3.6
Pt/MCN-5
63.1
58.5
24.3
6.3
6.2
4.7
Pt/MCN-6
68.5
35.9
4.9
12.1
18.2
28.9
Catalyst
Reaction conditions: 0.023 g catalyst, 20 mL aqueous solution of glycerol (0.3 M), glycerol/Pt molar ratio 750, 60 °C, 0.3 MPa O2, 4 h.
21
Figure Captions Fig. 1. Low- (A) and wide-angle (B) XRD patterns of the Pt catalysts supported on mesoporous carbon nitrides: (a) Pt/MCN-1, (b) Pt/MCN-2, (c) Pt/MCN-3, (d) Pt/MCN-4, (e) Pt/MCN-5, and (f) Pt/MCN-6. Fig. 2. TEM images of the Pt catalysts supported on mesoporous carbon nitrides: (a) Pt/MCN-1, (b) Pt/MCN-2, (c) Pt/MCN-3, (d) Pt/MCN-4, (e) Pt/MCN-5, and (f) Pt/MCN-6. Fig. 3. N2 sorption isotherms (A) and corresponding pore size distributions (B) for the Pt catalysts supported on mesoporous carbon nitrides: (a) Pt/MCN-1, (b) Pt/MCN-2, (c) Pt/MCN-3, (d) Pt/MCN-4, (e) Pt/MCN-5, and (f) Pt/MCN-6.
Fig. 4. CO2-TPD profiles for the Pt catalysts supported on mesoporous carbon nitrides: (a) Pt/MCN-1, (b) Pt/MCN-2, (c) Pt/MCN-3, (d) Pt/MCN-4, (e) Pt/MCN-5, and (f) Pt/MCN-6. Fig. 5. Time course of glycerol oxidation over Pt/MCN-5 in water. Fig. 6. Recycling of Pt/MCN-5 during the oxidation of glycerol (reaction conditions: 0.023 g catalyst, 20 mL glycerol aqueous solution (0.3 M), 60 °C, 0.3 MPa O2, 6 h).
Scheme 1. Proposed reaction route for the oxidation of glycerol over Pt/MCN-5.
22
Intensity (a.u.)
(A)
e d c b a 1
2
3
4
5
2 theta (degree)
(B) Pt (111)
Intensity (a.u.)
a b c d e
f 10
20
30
40
50
2theta (deg.)
Figure 1
23
60
70
80
Figure 2
24
2000
(A)
f
1200
e d
3
-1
Vads (cm g ), STP
1600
800
c b a
400
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative presure( P/Po)
(B)
0.4
f 0.3
e
3
-1
-1
dV/dD (cm g nm )
0.5
d
0.2
c b
0.1
a 0.0 0
5
10
15
20
Pore Diameter (nm)
Figure 3
25
25
30
35
Intensity (a.u.)
a b c d e f 0
100
200
300 o
Temp. ( C)
Figure 4
26
400
500
Conversion GLYA GLYHYDE GLYCA DIHA
Conversion & Selectivity (%)
100
80
60
40
20
0 1
2
3
Time (h)
Figure 5
27
4
5
Conversion or Selectivity (%)
80
Conversion GLYA GLYHYDE GLYCA DIHA
70 60 50 40 30 20 10 0 1
2
3
Recycling times
Figure 6
28
4
5
OH O
HO
O
2
O
HO
OH
OH O2
HO
OH
fa st
OH
GLYDE
GLYA
OH O
2
w slo
O HO
OH
Scheme 1
29
O2
O OH GLYCA
O2
C1 products
Graphical abstract
30
Highlights •
Ordered mesoporous carbon nitride-supported Pt catalysts were prepared.
•
Pt nanoparticles were homogenously embedded in the ordered mesoporous channels.
•
With increasing N content, the average Pt particle size in the catalysts decreased.
•
The catalyst can catalyze the oxidation of glycerol in base-free aqueous solution.
31
S1385-8947(14)01583-6 http://dx.doi.org/10.1016/j.cej.2014.11.115 CEJ 12970
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
27 September 2014 8 November 2014 23 November 2014
Please cite this article as: F-F. Wang, S. Shao, C-L. Liu, C-L. Xu, R-Z. Yang, W-S. Dong, Selective oxidation of glycerol over Pt supported on mesoporous carbon nitride in base-free aqueous solution, Chemical Engineering Journal (2014), doi: http://dx.doi.org/10.1016/j.cej.2014.11.115
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Selective oxidation of glycerol over Pt supported on mesoporous carbon nitride in base-free aqueous solution Fen-Fen Wang, Shuai Shao, Chun-Ling Liu, Chun-Li Xu, Rong-Zhen Yang, Wen-Sheng Dong*
Key Laboratory of Applied Surface and Colloid Chemistry (SNNU), MOE, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an, 710062, China
* To whom correspondence should be addressed: Phone: +86-(0)29-81530806 Fax: +86-(0)29-81530806 E-mail: [email protected]
1
Abstract Novel ordered mesoporous carbon nitride (MCN)-supported Pt catalysts were prepared, and then characterized using a combination of powder X-ray diffraction, transmission electron microscopy, N2 adsorption, inductively coupled plasma optical emission spectroscopy, NH3 temperature-programmed desorption, and X-ray photoelectron spectroscopy. These catalysts were evaluated for the selective oxidation of glycerol with molecular oxygen in base-free aqueous solution. The results showed that with increasing N content in the supports, the average Pt particle size in the catalysts decreased, whereas the conversion of glycerol increased. The basicity of the supports is helpful for promoting the oxidation of glycerol to glyceraldehyde and further oxidation to glyceric acid. Glycerol conversion of 63.1% and yields of 58.5% glyceric acid and 24.3% glyceraldehydes were obtained when reacting an aqueous solution of glycerol (0.3 M, 20 mL) with 0.023 g catalyst at 60 °C under 0.3 MPa O2 for 4 h. Upon recycling of the catalyst, the conversion of glycerol gradually decreased. The decrease of catalytic activity during recycling can be attributed to a combination of the growth and oxidation of Pt particles, leaching of N species, and collapse of the ordered mesoporous structure in the carbon nitride support.
Keywords: Glycerol; Oxidation; Pt catalyst; Mesoporous carbon nitride.
2
1. Introduction Glycerol is currently produced in large amounts as a byproduct of the manufacture of biodiesel by the transesterification of vegetable oils. One hundred kilograms of glycerol is produced for every 1 ton of biodiesel, and global production of biodiesel has grown from 15,200 barrels per day in 2000 to 335,700 barrels per day in 2010 [1]. Therefore, the conversion of the byproduct glycerol into value-added products would make the biodiesel industry economically more attractive. Glycerol is a highly functionalized molecule and contains three hydroxyl groups. It can be converted into several important chemicals by oxidation, hydrogenolysis, dehydration, esterification, oxidative carbonylation, transesterification, polymerization, and so on [2]. These features make glycerol viable as a versatile biobuilding block. For these reasons, the conversion of glycerol to high-value chemicals has attracted much attention in both academia and industry [3−7]. The selective oxidation of glycerol can produce various valuable oxygenated derivatives, such as glyceric acid, glyceraldehyde, dihydroxyacetone, tartronic acid, glycolic acid, and lactic acid. [8]. However, because of the complex nature of these reaction pathways, it is still a challenge to control the selectivity to give desired products. The aerobic oxidation of glycerol has been extensively studied by many groups using supported Pt [9,10], Pd [10,11], Au [10−22], Rh [23], Ir [24], and Cu-containing catalysts [25−28], and bimetals such as Pt–Bi [29], Au–Pd [30−33], Pd–Ag [34], and Au–Pt [35,36]. The activity and selectivity strongly depend on the reaction conditions (pH, temperature, and substrate to metal ratio) and the structure of the catalyst (metal, particle size, and support). It has been reported that the oxidation of glycerol over gold catalysts is completely inhibited in the absence of the base [10]. Moreover, the sodium salt of glyceric acid is the main product of the reaction, and additional neutralization and acidification is required to obtain the free glyceric acid. From a green chemistry point of view, homogeneous bases should not be used. Recently, advances have been made in base-free 3
oxidation of glycerol. Platinum-containing catalysts have been shown to be effective for this reaction [37−48]. For example, Hou et al. found that Pt/C [39,40], Pt/multiwall carbon nanotubes [40,41], PtCu/C [42], and PtSb/C [43] catalyze the oxidation of glycerol in base-free aqueous solution. Hutchings et al. found that Au–Pt/MgO [44] and Au–Pt–Pd/TiO2 [45] efficiently catalyze base-free oxidation of glycerol. Prati et al. [46] reported that Au−Pt supported on the acidic H-mordenite can also catalyze glycerol oxidation in base-free aqueous solution, although a high reaction temperature is required. Ebitani et al. [47,48] investigated the oxidation of glycerol using Pt/Al2O3, Pt/C, Pt/SiO2, and hydrotalcite-supported platinum catalysts. They found that the basic hydrotalcite-supported platinum catalyst was the most active for this reaction under base-free conditions. Recently, mesoporous carbon nitride (MCN) materials with large surface areas, small particle sizes, tunable pore diameters, and basic character have attracted considerable attention because the incorporation of nitrogen atoms in the carbon nanostructure can enhance the mechanical and conducting properties, field-emission, energy-storage prosperities, and catalytic performance. Vinu et al. found that MCN exhibited excellent performance in the basic catalytic transesterification of β-ketoester and alcohols [49]. Gold-embedded MCN catalysts have also shown good performance in the three-component coupling reaction of benzaldehyde, piperidine, and phenylacetylene for the synthesis of propargylamine [50]. In this study, inspired by the excellent performance of MCN in various reactions, we embedded platinum nanoparticles in MCN, and then used these materials to catalyze the oxidation of glycerol in base-free aqueous solution. 2. Experimental 2.1. Chemicals Pluronic P123 (PEO106PPO70PEO106, Mav = 5800), polyvinyl alcohol (PVA, Mw = 10000, 80%
4
hydrolyzed) and glycolic acid (GLYCA) were purchased from Sigma-Aldrich. H2PtCl6·6H2O, NaBH4, tetraethylorthosilicate (TEOS), HCl solution, and hydrofluoric acid were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Glycerol was purchased from Fuchen Chemical Reagent (Tianjin, China). Glyceric acid (GLYA, 40% in water) and glyceraldehyde (GLYDE) were obtained from Alfa Aesar. Dihydroxyacetone (DHA) was obtained from J&K CHEMICA. All chemicals were of analytical grade and used as received without further purification. 2.2. Preparation of catalysts SBA-15 was prepared according to the following procedure. In a typical synthesis, 4 g of P123 was dispersed in 30 g of water and 120 g of a 2 M HCl solution and stirred for 5 h. Thereafter, 9.5 g of TEOS was added to the homogeneous solution under stirring. The resulting gel was aged at 40 °C for 24 h, and then the solution was transferred into a Teflon bottle and heated at 150 °C for 24 h. After being cooled to room temperature, the product was filtered off, washed with distilled water, dried at 338 K, and finally calcined at 600 °C in air for 4 h. MCN materials were prepared by the method developed by Vinu et al. [50] through a polymerization reaction between carbon tetrachloride and ethylenediamine using SBA-15 as a sacrificial template. In a typical preparation, SBA-15 (0.5 g) was added to a mixture of ethylenediamine (0.3 g) and carbon tetrachloride (3 g). The resultant mixture was heated at 90 °C for 6 h under stirring. The obtained dark-brown solid mixture was placed in a drying oven for 12 h and then ground into a fine powder. The template carbon nitride polymer composites were then heat treated in a nitrogen flow of 50 mL min−1 at 600 °C for 5 h using a heating rate of 3 °C min−1. The MCN was obtained after dissolution of the silica framework in 5 wt% hydrofluoric acid followed by filtration, washing with ethanol, and drying at 100 °C. The resultant sample was denoted as MCN-1. The other MCN materials with different N contents were prepared by varying the addition
5
mass of ethylenediamine (0.9, 1.35, 2.1, 2.7, and 3.3 g), and the obtained products were denoted as MCN-2, MCN-3, MCN-4, MCN-5 and MCN-6, respectively. Supported Pt catalysts were prepared by immobilizing the colloidal metal particles on the MCN supports. In a typical preparation, the protecting agent was added (Pt/PVA=2:1, weight ratio) to a 50 mL−1 aqueous gold solution (1.12 × 10−3 M) at room temperature (25 °C) under vigorous stirring. Rapid injection of a 0.1 M aqueous solution of NaBH4 (Pt/NaBH4=1:4, mol ratio) led to the formation of a dark solution, indicating the formation of platinum colloids. The immobilization of Pt particles on the MCN materials was accomplished at room temperature (25 °C) by adding the supports to the colloidal Pt solution while stirring, and they were kept in contact until total adsorption (3 wt% Pt on the support) occurred, as indicated by discoloration of the solution. The solids were collected by filtration followed by washing the solid with distilled water to remove all the dissolved species (e.g., Na+ and Cl−). Finally, the solids were dried under vacuum at 110 °C for 12 h. 2.3. Catalyst characterization X-ray diffraction (XRD) patterns of the catalysts were collected on a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation (40 kV, 30 mA). To determine the average sizes of the Pt particles, the samples were finely scanned in the 2θ range from 30 to 50° with a 2θ step size of 0.01° and a step time of 1 s. The Pt particle sizes were estimated using Scherrer’s equation. Transmission electron microscopy (TEM) was conducted on a JEOL JEM 2010 electron microscope operating at 200 kV. X-ray photoelectron spectroscopy (XPS) analysis was performed on an Axis Ultra Kratos (UK) apparatus using an Al Kα source (15 kV, 1486.6 eV). The vacuum in the system was better than 10 −9 Torr. The binding energy was calibrated relative to the C 1s peak (284.8 eV) of the contaminant carbon. The Pt contents in the catalysts were determined with a Varian 720-ES inductively coupled plasma optical emission spectrometer (ICP-OES). Nitrogen adsorption–desorption isotherms were obtained on a Micromeritics ASAP 6
2020M system. Before the measurements, the samples were degassed at 250 °C for 6 h. The Brunauer–Emmett–Teller (BET) method was used to calculate the specific surface areas (SBET). The pore size distributions were derived from the desorption branch of the isotherm based on the Barrett–Joyner–Halenda (BJH) model. The total pore volume (Vp) was estimated from the adsorption branch at a relative pressure (P/P0) of 0.998. The micropore volume (Vmicro) and surface area (Smicro) were determined according to the t-plot method. The volume of the mesopores (Vmeso) was calculated from the difference between Vp and Vmicro. The average pore diameter (Dp) was estimated from the surface area and the total pore volume (Dp = 4Vp/SBET). CO2 temperature-programmed desorption (CO2-TPD) was performed using a Micromeritics AutoChem II 2920 system. A 120 mg sample was loaded into the sample tube and pretreated in He at 500 °C for 1 h, and then the sample was cooled to 50 °C in flowing He. CO2 was introduced and allowed to adsorb at 50 °C for 1 h. The sample was then purged with dry helium at the same temperature to remove weakly adsorbed CO2. Finally, the sample was heated at a linear rate of 10 °C min–1 under dry helium to 500 °C. The effluent was monitored by a thermal conductivity detector. 2.4. Reaction test and product analysis All of the reactions were carried out in a 35 mL stainless steel autoclave equipped with a mechanical stirrer. In a typical experiment, 0.023 g catalyst and 20 mL aqueous solution of glycerol (0.3 M) were charged into the reactor. The autoclave was purged twice with pure O2, and then pressurized to 0.3 MPa with O2 at room temperature. The reaction mixture was heated to 60 °C and held at this temperature for 4 h (unless otherwise stated) under a stirring rate of 600 rpm. After the reaction, the reactor was rapidly cooled to room temperature with an ice-water mixture and depressurized. The post-reaction liquid sample was diluted with mobile phase solution before analysis. The gas mixture was collected in a gasbag for gas chromatograph (GC) analysis. 7
Sample analysis was performed on a Shimadzu HPLC LC-20AT system equipped with both refractive index (RID-10A) and UV (SPD-20A) detectors. An ion-exclusion column (Bio-Rad Aminex HPX-87H) at 40 °C with 0.005 M formic acid (or 0.005 M H2SO4) solution as the mobile phase flowing at 0.30 mL min−1 was used to separate and identify the compounds. The amount of products was determined using calibration curves. Other byproducts, such as CO2, CO, and HCHO, were detected in the gas effluent. The retention times and standard curves of the detected liquid products were determined, and are shown in Table S1 and Fig. S1 (Supplementary Data). 3. Results and discussion 3.1. Structure characterization of Pt/MCN catalysts Low- and wide-angle XRD patterns of the Pt/MCN catalysts are shown in Fig. 1. The Pt/MCN-6 sample had a disordered structure, so the low-angle XRD pattern of this sample is not shown in Fig. 1. For all of the other samples, a sharp diffraction peak at 2θ ≈ 0.9° and a weak peak at 2θ ≈ 1.59° were observed, which can be indexed to the (100) and (110) reflections of the two-dimensional (2D) hexagonal (P6mm) structure, respectively. The pattern is similar to the XRD pattern of the parent mesoporous silica template SBA-15, which consists of a hexagonal arrangement of cylindrical pores interlinked by the micropores present in the walls [51]. Such materials with one-dimensional (1D) mesopores arranged in a hexagonal net are present because the diffraction pattern shows 2D P6mm symmetry. In the wide-angle region, the Pt/MCN catalysts showed two broad diffraction peaks. The peak located at 2θ ≈ 25° corresponds to the (002) diffraction of amorphous carbon. The peak at ~39° was assigned to the (111) reflection of platinum. To determine the average sizes of the Pt particles, the samples were finely scanned in the 2θ range from 30° to 50° with a 2θ step size of 0.01° and a step time of 1 s. The Pt particle sizes were calculated using Scherrer’s equation, and
8
the results are shown in Table 1. As shown in Table 1, the Pt particle sizes gradually decreased with increasing N content in the MCN supports. This can be explained as follows. The MCN supports derived from the polymerization between carbon tetrachloride and ethylenediamine have inbuilt –NH2 and –NH groups on the mesoporous walls. These groups can act as a stabilizing agent by providing an anchoring and heterogeneous surface that allows the formation of highly dispersed metal nanoparticles without any agglomeration [50]. The presence of more functional amine groups on MCN helps to reduce the aggregation of Pt nanoparticles and stabilize the formed nanoparticles inside the mesoporous channels. TEM was used to observe the pore structure of the Pt/MCN catalysts. Figure 2 shows TEM images of these catalysts. Pt/MCN-1 to Pt/MCN-5 show highly ordered 1D channels along the [110] direction (Fig. 2a–e). However, no Pt particles were observed owing to the high dispersion of the Pt nanoparticles inside the mesoporous channels. For Pt/MCN-6, ~2 nm Pt particles were homogeneously dispersed on the disordered mesoporous MCN-6 support (Fig. 2f). The results suggest that large amounts of N in the MCN materials adversely affect the ordering of the carbon nitride materials. The physicochemical properties of the Pt/MCN catalysts were characterized by N2 adsorption–desorption. Figure 3 shows the N2 sorption isotherms and the corresponding pore size distributions of the catalysts. All of the samples except for Pt/MCN-6 show type IV isotherms with H1-shaped hysteresis loops in the P/P0 range 0.4–0.8 (Fig. 3a) and narrow pore size distributions (Fig. 3b), which are characteristic of highly ordered mesoporous materials. The Pt/MCN-6 sample also shows a IV isotherm but with a H4-shaped hysteresis loop, which is characteristic of mesoporous materials. The detailed textural properties of the Pt/MCN catalysts are summarized in Table 1. All of the samples had specific surface areas greater than 500 m2 g−1 and mean pore sizes greater than 4 nm. The CO2-TPD curves of the Pt/MCN catalysts are shown in Fig. 4. Pt/MCN-1 shows a 9
desorption peak centered at around 100 °C, indicating that the surface of this sample featured only one type of weak basic site. With increasing N content in the supports, the peak areas corresponding to CO2 desorption increase, indicating that the number of weak basic sites in the Pt/MCN catalysts increased. XPS was used to analyze and identify the surface species and oxidation states on the Pt/MCN catalysts, and the results are shown in Table 2. The binding energy of N 1s in each sample was almost constant (~397.9 ± 0.1 eV), indicating no electronic interaction between N and Pt. There were two Pt species present in all of the catalysts, Pt0 (~70.9 eV) and Ptδ+ (~71.9 eV), because the surface of Pt0 could be immediately oxidized when exposed to air. With increasing N content in the Pt/MCN catalysts, the content of Pt0 species generally decreased. The results are consistent with the XRD analysis. The XRD analysis showed that the Pt particle size decreased with increasing N content in the catalysts. The smaller Pt particles have larger surface area per unit volume than larger particles, and are therefore more easily oxidized. The surface Pt content in all of the Pt/MCN catalysts except Pt/MCN-6 was ~1.9%, which is lower than the bulk content (~3.0%), suggesting that Pt particles mainly entered the pores of the MCN materials.
3.2. Catalytic performance of Pt/MCN catalysts Table 3 summarizes the activity of the Pt/MCN catalysts for the oxidation of glycerol in base-free aqueous solution. The main products on the catalysts were glyceric acid and glyceraldehyde. For all of the Pt/MCN catalysts, the conversion of glycerol increased with increasing N content in the catalysts, indicating that more basic sites promote the oxidation of glycerol. The oxidation of an alcohol to an aldehyde in aqueous solution over Pt catalysts likely occurs in three steps [52]. First, the alcohol adsorbs on the metal surface, producing an adsorbed
10
metal alkoxide. Second, β-hydride elimination occurs to produce a carbonyl species and a metal hydride. Finally, the metal–hydride is oxidized by associatively adsorbed oxygen to regenerate the metal surface. Among these steps, β-hydride elimination has been proposed as the rate-determining step. The basic sites contained in the supports may promote β-hydride abstraction during the reaction, and thus accelerate the oxidation of alcohols [53,54]. Ebitani et al. [47] also found that basicity of the hydrotalcite support is advantageous for promoting glycerol oxidation. On the other hand, with increasing N content in the Pt/MCN catalysts, the Pt particle sizes slightly decreased, which may also contribute to the conversion of glycerol, but we believe this may not be the main reason. From Table 3, with increasing N content in the Pt/MCN catalysts, the selectivity of glyceraldehyde decreased, whereas the selectivity of glyceric acid increased (except for Pt/MCN-6). The results suggest that more basic sites promoted the sequential oxidation of glyceraldehyde to glyceric acid. This is inconsistent with the results reported by Ebitani et al. [47] in the selective oxidation of glycerol over hydrotalcite-supported platinum catalysts and by Villa et al. [53] in the selective oxidation of alcohols. Table 3 also shows that the selectivity of dihydroxyacetone decreased, whereas the selectivity of glycolic acid increased with increasing N content in the Pt/MCN catalysts except for Pt/MCN-6. Among the catalysts, Pt/MCN-6 showed the lowest selectivities of glyceraldehyde and glyceric acid but the highest selectivities of C1 products and glycolic acid, suggesting that more basic sites promoted C–C bond cleavage reactions. Pt/MCN-6 also exhibited the highest selectivity of dihydroxyacetone, although the reason for this is unclear and further investigation is required. Figure 5 shows the time course of glycerol oxidation on Pt/MCN-5 in base-free aqueous solution. At the initial stage of the oxidation reaction (0.5 h), glycerol conversion of 29.5% was
11
obtained. The corresponding selectivities of glyceric acid, glyceraldehyde, glycolic acid, and dihydroxyacetone were 76.2%, 10.8%, 2.8%, and 2.6%, respectively. With prolonged reaction time, glycerol conversion increased and the selectivity of glyceric acid gradually decreased, whereas the selectivities of glyceraldehyde, glycolic acid, and dihydroxyacetone gradually increased. Glycerol oxidation is a sequential reaction that follows multiple paths [52]. The oxidation of a primary or secondary alcohol of glycerol produces glyceraldehyde or dihydroxyacetone, respectively. Glyceraldehyde can be sequentially oxidized to glyceric acid, and both glyceraldehyde and dihydroxyacetone can be oxidized to hydroxypyruvic acid. The sequential oxidation of glyceric acid typically produces tartronic acid. Moreover, glycerol oxidation products can undergo carbon cleavage to form C2 acids, such as glycolic and oxalic acid, as well as C1 products. In the present study, glyceric acid, glyceraldehyde, dihydroxyacetone, glycolic acid, and small amounts of C1 products were detected. The yield of dihydroxyacetone was rather low, whereas the yields of glyceraldehyde and glyceric acid were much higher during the reaction, suggesting that the formation rate of dihydroxyacetone over Pt/MCN-5 was much lower than that of glyceraldehyde. Therefore, a plausible reaction pathway for the oxidation of glycerol over Pt/MCN-5 in base-free aqueous solution is proposed in Scheme 1. The reusability of Pt/MCN-5 for the oxidation of glycerol was investigated. After the first run, the catalyst was separated from the reaction solution by filtration, washed with distilled water, dried, and then reused in another four catalytic runs. The results in Fig. 6 show that both the conversion of glycerol and the selectivity of glyceric acid decreased, whereas the selectivities of glyceraldehyde, dihydroxyacetone and glycolic acid gradually increased. As previously reported, a major disadvantage of Pt catalysts is that they can rapidly deactivate under certain reaction conditions. Deactivation of the Pt catalysts could be caused by over-oxidation of the metal to form oxides on the surface and poisoning of the surface with acid
12
products, byproducts, or cleavage products [52]. To determine the real reasons for catalyst deactivation, the used catalyst was characterized by TEM, XPS, and subjected to BET analysis, and the reaction solutions after each cycle were analyzed by ICP-OES. ICP analysis confirmed that no detectable leaching of Pt into the reaction mixture occurred during recycling. XRD analysis (Fig. S2a) confirmed that the average Pt particle size in the used Pt/MCN-5 catalyst increased to ~5.8 nm. The TEM image (Fig. S2b) showed that the ordered mesoporous structure in Pt/MCN-5 was destroyed after recycling. XPS analysis (Fig. S3) showed that the binding energy of Pt4f7/2 increased (>0.7 eV), while the concentration of O species on the surface of the used catalyst increased, suggesting that the Pt species were more oxidized. In addition, the N concentration decreased in the used Pt/MCN-5 catalyst, indicating that leaching of N-containing species occurred. Hence, the decrease of catalytic activity during recycling can be attributed to a combination of growth and oxidation of Pt particles, leaching of N species, and collapse of the ordered mesoporous structure.
4. Conclusions We prepared novel ordered mesoporous carbon nitride (MCN)-supported Pt catalysts. The Pt nanoparticles were homogenously embedded in the ordered mesoporous channels. These synthesized materials were evaluated for the selective oxidation of glycerol with molecular oxygen in base-free aqueous solution. The results showed that with increasing N content in the supports, the average Pt particle sizes in the catalysts decreased. The basicity of the supports could promote the sequential oxidation of glycerol to glyceraldehyde and then to glyceric acid. The Pt/MCN catalysts showed high conversion of glycerol and selectivities of glyceric acid and glyceraldehydes. Upon recycling of the catalyst, the conversion of glycerol gradually decreased. The decrease of catalytic activity during recycling can be attributed to a combination of the growth and oxidation of Pt particles, leaching of N species, and collapse of the ordered
13
mesoporous structure in the carbon nitride support.
Acknowledgments The authors gratefully acknowledge financial support from the Program for Key Science and Technology Innovation Team of Shaanxi Province (2012KCT-21) and the Fundamental Research Funds for the Central Universities (GK201305011).
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18
Table 1 Compositions and textural properties of supported Pt catalysts Pt content
Pt particle
SBET
Smeso
Vp
Vmeso
Dp
(wt%)a
size (nm)b
(m2 g−1)
（m2 g−1)
(cm3 g−1 )
(cm3 g−1 )
(nm)
Pt/MCN-1
2.9
3.4
509
459
0.66
0.63
4.0
Pt/MCN-2
2.9
2.9
710
628
0.72
0.68
4.4
Pt/MCN-3
3.1
2.8
506
471
0.61
0.60
4.1
Pt/MCN-4
3.0
2.6
502
433
0.68
0.65
4.4
Pt/MCN-5
3.1
2.3
608
498
0.73
0.66
4.5
Pt/MCN-6
3.2
2.0
806
705
1.5
1.44
6.6
Sample
a
Measured by ICP-OES.
b
Measured by XRD.
19
Table 2 XPS analysis of Pt catalysts supported on mesoporous carbon nitrides Binding Energy (eV) Catalyst
O 1s
N 1s
(C–O)
(C–N)
Pt/MCN-1
532.1
398.0
Pt/MCN-2
531.9
397.8
Pt/MCN-3
531.9
397.8
Pt/MCN-4
531.9
397.9
Pt/MCN-5
531.9
397.8
Pt/MCN-6
531.9
397.9
Surface concentration (wt%)
Pt species Pt4f7/2
(%)
71.0
Pt0 : 56.2
71.9
Ptδ+: 43.7
70.9
Pt0 : 56.0
71.9
Ptδ+: 44.0
70.8
Pt0 : 55.5
71.8
Ptδ+: 44.5
70.9
Pt0 : 54.5
71.7
Ptδ+: 45.5
70.6
Pt0 : 54.9
71.5
Ptδ+: 45.1
70.7
Pt0 : 53.7
72.1
Ptδ+: 46.3
20
O
C
N
Pt
20.2
75.2
2.6
1.9
18.6
74.4
4.5
1.9
17.1
74.3
6.6
1.8
16.2
74.8
7.1
1.7
13.5
75.2
9.4
1.8
10.1
71.8
13.5
3.8
Table 3 Oxidation of glycerol using Pt catalysts supported on mesoporous carbon nitrides Selectivity (%)
Conversion (%)
GLYA
GLYDE
GLYCA
DHA
C1 products
Pt/MCN-1
37.7
35.8
46.2
2.2
13.9
1.9
Pt/MCN-2
40.5
35.9
45.9
2.6
13.6
2.0
Pt/MCN-3
47.5
45.1
38.9
4.2
9.5
2.3
Pt/MCN-4
56.4
54.5
30.4
4.6
6.9
3.6
Pt/MCN-5
63.1
58.5
24.3
6.3
6.2
4.7
Pt/MCN-6
68.5
35.9
4.9
12.1
18.2
28.9
Catalyst
Reaction conditions: 0.023 g catalyst, 20 mL aqueous solution of glycerol (0.3 M), glycerol/Pt molar ratio 750, 60 °C, 0.3 MPa O2, 4 h.
21
Figure Captions Fig. 1. Low- (A) and wide-angle (B) XRD patterns of the Pt catalysts supported on mesoporous carbon nitrides: (a) Pt/MCN-1, (b) Pt/MCN-2, (c) Pt/MCN-3, (d) Pt/MCN-4, (e) Pt/MCN-5, and (f) Pt/MCN-6. Fig. 2. TEM images of the Pt catalysts supported on mesoporous carbon nitrides: (a) Pt/MCN-1, (b) Pt/MCN-2, (c) Pt/MCN-3, (d) Pt/MCN-4, (e) Pt/MCN-5, and (f) Pt/MCN-6. Fig. 3. N2 sorption isotherms (A) and corresponding pore size distributions (B) for the Pt catalysts supported on mesoporous carbon nitrides: (a) Pt/MCN-1, (b) Pt/MCN-2, (c) Pt/MCN-3, (d) Pt/MCN-4, (e) Pt/MCN-5, and (f) Pt/MCN-6.
Fig. 4. CO2-TPD profiles for the Pt catalysts supported on mesoporous carbon nitrides: (a) Pt/MCN-1, (b) Pt/MCN-2, (c) Pt/MCN-3, (d) Pt/MCN-4, (e) Pt/MCN-5, and (f) Pt/MCN-6. Fig. 5. Time course of glycerol oxidation over Pt/MCN-5 in water. Fig. 6. Recycling of Pt/MCN-5 during the oxidation of glycerol (reaction conditions: 0.023 g catalyst, 20 mL glycerol aqueous solution (0.3 M), 60 °C, 0.3 MPa O2, 6 h).
Scheme 1. Proposed reaction route for the oxidation of glycerol over Pt/MCN-5.
22
Intensity (a.u.)
(A)
e d c b a 1
2
3
4
5
2 theta (degree)
(B) Pt (111)
Intensity (a.u.)
a b c d e
f 10
20
30
40
50
2theta (deg.)
Figure 1
23
60
70
80
Figure 2
24
2000
(A)
f
1200
e d
3
-1
Vads (cm g ), STP
1600
800
c b a
400
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative presure( P/Po)
(B)
0.4
f 0.3
e
3
-1
-1
dV/dD (cm g nm )
0.5
d
0.2
c b
0.1
a 0.0 0
5
10
15
20
Pore Diameter (nm)
Figure 3
25
25
30
35
Intensity (a.u.)
a b c d e f 0
100
200
300 o
Temp. ( C)
Figure 4
26
400
500
Conversion GLYA GLYHYDE GLYCA DIHA
Conversion & Selectivity (%)
100
80
60
40
20
0 1
2
3
Time (h)
Figure 5
27
4
5
Conversion or Selectivity (%)
80
Conversion GLYA GLYHYDE GLYCA DIHA
70 60 50 40 30 20 10 0 1
2
3
Recycling times
Figure 6
28
4
5
OH O
HO
O
2
O
HO
OH
OH O2
HO
OH
fa st
OH
GLYDE
GLYA
OH O
2
w slo
O HO
OH
Scheme 1
29
O2
O OH GLYCA
O2
C1 products
Graphical abstract
30
Highlights •
Ordered mesoporous carbon nitride-supported Pt catalysts were prepared.
•
Pt nanoparticles were homogenously embedded in the ordered mesoporous channels.
•
With increasing N content, the average Pt particle size in the catalysts decreased.
•
The catalyst can catalyze the oxidation of glycerol in base-free aqueous solution.
31