Article pubs.acs.org/IECR
Graphitic C3N4 Photocatalyst for Esterification of Benzaldehyde and Alcohol under Visible Light Radiation Limin Song,*,†,∥ Shujuan Zhang,‡,∥ Xiaoqing Wu,*,§ Haifeng Tian,† and Qingwu Wei† †
College of Environment and Chemical Engineering & State Key Laboratory of Hollow-Fiber Membrane Materials and Membrane Processes and §Institute of Composite Materials & Ministry of Education Key Laboratory of Advanced Textile Composite Materials, Tianjin Polytechnic University, Tianjin 300387, People’s Republic of China ‡ College of Science, Tianjin University of Science & Technology, Tianjin, 300457, People’s Republic of China ABSTRACT: The reaction of alcohols and carboxylic acids catalyzed by acids is the traditional method used to prepare esters in the chemical industry. In this study, we report the synthesis of active graphitic carbon nitride (g-C3N4) that may be used in the one-step reaction between benzaldehyde and alcohol to promote the selective formation of esters under visible light irradiation. Compared with the reaction carried out without illumination, g-C3N4 showed obvious improvements in ester formation. The presence of tin dioxide can also contribute to the formation of esters under visible light irradiation. The use of g-C3N4 for the esterification of various alcohols was explored, and the catalyst showed promising results.
1. INTRODUCTION Novel nonmetal catalysts, such as graphitic carbon nitride (gC3N4), have attracted the attention of many scholars due to their special semiconductor properties (band gap of 2.7 eV), visible light absorption, high stability, nontoxicity, and ease of preparation in aqueous solution.1−4 In particular, the use of gC3N4 in photocatalysis has been extensively studied. As a photocatalyst, the main applications of g-C3N4 include photolysis of water to obtain hydrogen,5,6 light degradation of organic dyes,7,8 and photocatalytic organic reactions.9 Many studies on the use of photocatalytic oxidation to synthesize organic matter have been performed by Wang et al. and Antonietti et al.,10−14 including studies on oxidation of benzene to phenol,10 oxidation of aliphatic C−H bonds,11 oxidative coupling of amines,12,13 and selective oxidation of alcohols.14 However, to the best of our knowledge, the photocatalytic oxidation of benzaldehyde and alcohols to synthesize esters under visible light irradiation has yet to be reported. In several complex synthetic reactions, particularly those of natural products, the direct conversion of benzaldehyde into esters is needed. Thus, synthesis of esters by a one-step route using benzaldehyde as raw material is a promising research endeavor. In this study, benzaldehyde and alcohol were used to produce esters. The esterification of various alcohols was investigated also.
three times with distilled water to remove soluble reactants and impurities, and then washed three times with ethanol to remove some organic impurities. After drying in a vacuum at 60 °C for 4 h, the yellow product was obtained. To prepare SnO2/g-C3N4, 0.86 g of g-C3N4 was mixed with 0.47 g of SnCl4·5H2O and 20 mL of NH3·H2O (25 wt %) under stirring. After 30 min, the resulting mixture was centrifuged at 4000 rpm and washed with distilled water. The product was air-dried at 120 °C for 3 h. The SnO2/g-C3N4 sample was finally obtained as a gray powder after heating at 500 °C for 4 h in a tube furnace. 2.3. Characterization of g-C3N4 Photocatalysts. The asprepared samples were characterized by scanning electron microscopy (SEM, Hitachi S4800), transmission electron microscopy (TEM, Hitachi 7650), and X-ray powder diffraction (XRD, Rigaku D/max 2500). Photoluminescence spectra (PL) were obtained by a fluorescence spectrophotometer (Hitachi F4500). The excitation wavelength was set at 325 nm. UV/vis spectra were recorded on a HP8453 spectrophotometer at room temperature. Binding energies (BEs) were measured by an X-ray photoelectron spectrometer (XPS, Perkin-Elmer PHI5300). In the XPS analysis, the calibration of BE is the standard peak of adventitious carbon (C 1s). The BE has been calibrated according to the standard peak of carbon (C 1s) in the article. Brunauer−Emmett−Teller (BET) surface areas (SBETs) were measured by N2 adsorption at −196 °C using an automatic surface area and pore size analyzer (Autosorb-1-MP 1530VP). Elemental analysis was carried out on an Elemental Vario III elemental analyzer. 2.4. Catalytic Activity Evaluation. In a typical experimental process, the esterifiable reaction was carried out in a 50 mL quartz reactor with a magnetic stirrer under visible light
2. EXPERIMENTAL SECTION 2.1. Chemicals. All the reagents were obtained from Tianjin Chemical Reagent Co. All of these reagents were used as received without purification. 2.2. Synthesis of g-C3N4 Photocatalysts. In a typical experiment, 2 g of dicyandiamide (99%) was placed in a ceramic boat and then loaded into the central region of a horizontal tube furnace (Φ 25 mm × 800 mm). The tube furnace was heated to 550 °C in flowing N2 (at a rate of 20 mL min−1) and maintained at 550 °C for 4 h, and then allowed to cool to room temperature naturally. The product was washed © 2012 American Chemical Society
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March 1, 2012 June 11, 2012 June 20, 2012 June 20, 2012 dx.doi.org/10.1021/ie3010226 | Ind. Eng. Chem. Res. 2012, 51, 9510−9514
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radiation. A 300 W Xe lamp with a 420 nm cutoff filter was used as a visible light source. A 100 mg sample of g-C3N4 powder was put into the reactor together with benzaldehyde (2 mL), alcohol (8 mL), perchloric acid (0.1 mL, 70 wt %), and hydrogen peroxide (0.6 mL, 30 wt %). The reactor was heated to 50 °C under visible light radiation and stirring, and was maintained at 50 °C for 4 h. The product was centrifuged and then analyzed by a gas chromatograph (GC) equipped with a flame ionization detector (Agilent Technologies, GC6890). The chemical structures of products were investigated by GC− mass spectrometry (MS) (Agilent Technologies, GC6890N, MS 5975). The conversion was calculated by (n0 − n)/n0, where n is the moles of benzaldehyde after irradiation and n0 is the moles of benzaldehyde before irradiation in the presence of catalyst.
3. RESULTS AND DISCUSSION 3.1. Characterization of g-C3N4 Photocatalysts. Figure 1 shows the typical X-ray diffraction (XRD) pattern of the as-
Figure 2. (a) TEM and (b) field emission SEM images of assynthesized g-C3N4.
394−407 and 532 eV are attributed to N 1s and O 1s electronic transitions, respectively. These results indicate that the g-C3N4 sample is mostly composed of carbon and nitrogen, with trace amounts of oxygen. Adsorbed oxygen molecules and oxides (oxygenated nitrogen, hydroxylic, carboxylic, and so on) on the sample surface are responsible for the appearance of the O 1s peak. The amount of each element present in the sample was calculated by the peak areas of N 1s and C 1s in the spectra obtained. The nitrogen content was determined to be 48.37% (C, 48.64%; O, 2.99%), confirming that a large amount of nitrogen could be obtained on the sample surface. A high-resolution C 1s spectrum in Figure 3B shows two peaks with binding energies of 284.8 and 288.2 eV. Figure 3C shows the high-resolution N 1s XPS spectra of the sample. The Gaussian multipeak fit for each plot shows that the N 1s spectra consist of two strong peaks at 398.5 and 400.4 eV. According to ref 16, these strong peaks could be attributed to N-substituted pyridinic and N-substituted graphitic units, respectively. UV−visible absorption spectra in Figure 4A indicate that the as-prepared g-C3N4 powder has obvious absorption in the visible light range (from 400 to 700 nm), which is an important condition for the photocatalytic activity of g-C3N4 under visible light. The band gap of g-C3N4 was clearly determined to be 2.53 eV. The intrinsic absorption edge of the as-prepared gC3N4 was slightly red-shifted compared with that of the reference4 because a different starting material was used during synthesis. However, the band-to-band excitations of the asprepared g-C3N4 and reference showed no difference. Figure 4B shows the photoluminescence spectra of the as-prepared gC3N4 powder. The apparent shift in the intrinsic fluorescence emission peak to 3.1 eV further confirms the decreased band gap of the as-prepared g-C3N4 compared with the reference.4
Figure 1. X-ray diffraction (XRD) of as-synthesized g-C3N4.
prepared g-C3N4 powder calcined at 550 °C for 4 h. All peaks correspond to the graphitic phase of C3N4. The most intense XRD peak at 27.4° corresponds to the (002) peak and is attributed to stacking of the conjugated aromatic system.4 The morphology and nanostructure of the as-prepared gC3N4 obtained at 550 °C were further characterized by transmission electron microscopy (TEM) (Figure 2a). The particles showed a slicelike morphology, which is consistent with the reference sample.15 The brief period of ultrasonification during the preparation of TEM samples may have brought about irregularities in their morphology. The particles tended to aggregate because they had a slicelike morphology so that we were unable to obtain a detailed TEM image of g-C3N4. Scanning electron microscopy (SEM) images of the g-C3N4 sample obtained at 550 °C are shown in Figure 2b. The microstructures were uniformly formed on the holder surface. Although the particles appeared to aggregate, each particle was composed of many small pieces on the sample surface upon further inspection. Thus, the SEM and TEM results are highly similar. Figure 3A shows the full-range X-ray photoelectron spectrum (XPS) of the g-C3N4 sample obtained at 550 °C. The full-range XPS spectrum of the sample shows a main peak about 280−290 eV, which is attributed to the C 1s transition. XPS peaks around 9511
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Figure 4. (A) UV−vis absorption and (B) PL spectra of as-synthesized g-C3N4.
different reaction conditions used to synthesize esters are listed in Table 1. Only a small amount of ethyl benzoate was produced in the esterification of benzaldehyde and ethanol (24%, Table 1, entry 1). Esterification could be improved to 32% (Table 1, entry 2) when the reaction was carried out under visible light irradiation (λ > 420 nm). Thus, g-C3N4 could clearly be used as a photocatalyst in the direct conversion of benzaldehyde and ethanol to ethyl benzoate although the photocatalyst had low activity. Direct esterification using the nonmetal photocatalyst under visible light irradiation proved to be a promising finding. Ion doping is an effective method of further enhancing the activity of photocatalysts.8,10,13 According to ref 17, SnO2 is a solid superacid. It is a very good catalyst for the esterification reaction. SnO2 also shows very good activity in the esterification of aldehydes.18 In addition, SnO2 is also a very mild oxidant. Thus, in order to improve the activity of gC3N4 in the esterification reaction, an experiment using SnO2/ g-C3N4 (SnO2 19 wt %; the BET specific surface area of the SnO2/g-C3N4 sample is 6.1 m2/g) as a photocatalyst was carried out with all other reaction conditions kept the same. Conversion reached 34% (Table 1, entry 15) in the dark and 41% (Table 1, entry 16) under visible light irradiation, proving that the activity could be further improved in the presence of SnO2/g-C3N4. In a comparison of pure g-C3N4 (Table 1, entries 1 and 2) and SnO2/g-C3N4 (Table 1, entries 15 and 16), the improvement of activity under irradiation that resulted
Figure 3. X-ray photoelectron spectra (XPS) for as-synthesized gC3N4: (A) full range, (B) C 1s, and (C) N 1s.
3.2. Photocatalytic Activity of g-C3N4 Photocatalysts. The result of elemental analysis shows that the as-prepared gC3N4 treated at 550 °C for 4 h are composed of nitrogen (60.54 wt %) and carbon (34.61 wt %). The atomic ratio of N to C is near 1.5. In general, the atomic ratio of N to C is 1.33 in the molecular formula of C3N4. This result further illustrates that the as-synthesized g-C3N4 has a high amount of nitrogen in the bulk phase of the sample. In addition, the BET specific surface area of the g-C3N4 sample is 11 m2/g, and the porous structure was not found in the experiment of N2 adsorption. The photocatalytic reaction of g-C3N4 was carried out in an H2O2/HClO4 medium in a stirred quartz reactor at 50 °C for 4 h under visible light irradiation. The experimental results of 9512
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Table 1. Photocatalytic Activities for Ester Production from Benzaldehyde and Alcohol by g-C3N4 Photocatalysts with (+) or without (−) Visible Light (λ > 420 nm) alcohol
hν
catalyst
oxidant
conversion (%)
− + −
g-C3N4 g-C3N4 g-C3N4
H2O2 H2O2 H2O2
24 32 21
+
g-C3N4
H2O2
28
5 6 7
ethanol ethanol n-propyl alcohol n-propyl alcohol 1-butanol 1-butanol 1-pentanol
− + −
g-C3N4 g-C3N4 g-C3N4
H2O2 H2O2 H2O2
20 28 20
8
1-pentanol
+
g-C3N4
H2O2
27
9 10 11
− + −
g-C3N4 g-C3N4 g-C3N4
H2O2 H2O2 H2O2
18 24 16
+
g-C3N4
H2O2
21
−
g-C3N4
H2O2
0
+
g-C3N4
H2O2
0
15
1-octanol 1-octanol isopropyl alcohol isopropyl alcohol 2-methyl-2propyl alcohol 2-methyl-2propyl alcohol ethanol
−
H2O2
34
16
ethanol
+
SnO2/gC3N4 SnO2/gC3N4
ethyl benzoate ethyl benzoate propyl benzoate propyl benzoate butyl benzoate butyl benzoate pentyl benzoate pentyl benzoate octyl benzoate octyl benzoate isopropyl benzoate isopropyl benzoate 2-methyl-2propyl benzoate 2-methyl-2propyl benzoate ethyl benzoate
H2O2
41
ethyl benzoate
entry 1 2 3 4
12 13
14
Article
AUTHOR INFORMATION
Corresponding Author
*Tel./fax: +86-22-83955458. E-mail:
[email protected] (L.S.);
[email protected] (X.W.).
product
Author Contributions ∥
These authors contributed equally to this work.
Notes
The authors declare the following competing financial interest(s): photocalysis.
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REFERENCES
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was 25% (pure g-C3N4), whereas with modified catalyst (SnO2/ g-C3N4) only 17% improvement resulted. In the modified gC3N4 with SnO2, SnO2 mainly showed an acidic effect. The gC3N4 exhibited photocatalytic oxidation. On the SnO2/g-C3N4 surface, the load of SnO2 reduced the active sites of the g-C3N4 exposed on its surface, which made the activity under light radiation activity less improved than that of pure g-C3N4. The reactions of various alcohols were carried out under the same reaction conditions, and the experimental results are listed in Table 1. When ethanol was substituted with n-propyl alcohol, 1-butanol, 1-pentanol, 1-octanol, isopropyl alcohol, and 2-methyl-2-propyl alcohol, the corresponding esters except 2methyl-2-propyl benzoate were obtained both in the dark (Table 1, entries 3−14) and under visible light (Table 1, entries 3−14). From entries 3 to 12, higher activities were obtained in all reactions carried out under visible light compared with those in the dark.
4. CONCLUSION In the study, g-C3N4 was prepared by a simple route. Tin ions can also be successfully loaded into a g-C3N4 by a simple softchemical method. The structures and properties of those catalysts were analyzed by a variety of techniques. The above experimental results clearly showed that g-C3N4 is a good photocatalyst to activate the esterifiable reaction of benzaldehyde and alcohol under visible light radiation. The synergistic effect of SnO2 and g-C3N4 facilitated these transformations of benzaldehyde and alcohol. 9513
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(16) Bian, S. W.; Ma, Z.; Song, W. G. Preparation and characterization of carbon nitride nanotubes and their applications as catalyst supporter. J. Phys. Chem. C 2009, 113, 8668. (17) Chavan, S. P.; Zubaidha, P. K.; Dantale, S. W. Use of solid superacid (sulphated SnO2) as efficient catalyst in facile transesterification of ketoesters. Tetrahedron Lett. 1996, 37, 233. (18) Qian, G.; Zhao, R.; Ji, D. Facile oxidation of benzaldehyde to esters using S·SnO2/SBA-I-H2O2. Chem. Lett. 2004, 33, 834.
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