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3D graphene aerogel anchored tungstophosphoric acid catalysts: Characterization and catalytic performance for levulinic acid esterification with ethanol
Catalysis Communications 85 (2016) 66–69
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
3D graphene aerogel anchored tungstophosphoric acid catalysts: Characterization and catalytic performance for levulinic acid esterification with ethanol Min Wu a, Xiaoli Zhang a, Xiaoli Su a, Xueyang Li a, Xiucheng Zheng a,b,⁎, Xinxin Guan a, Pu Liu a a b
College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China
a r t i c l e
i n f o
Article history: Received 26 April 2016 Received in revised form 23 June 2016 Accepted 24 July 2016 Available online 27 July 2016 Keywords: GA HPW/GA Esterification Levulinic acid Biofuel
a b s t r a c t 3D graphene aerogel (GA) was synthesized from graphene oxide (GO) using a hydrothermal procedure combined with freeze-drying. Then, they were used to assemble the supported H3PW12O40 (HPW) catalysts. The optimization of the reaction parameters for levulinic acid esterification with ethanol was performed and the catalytic performance of the HPW/GA catalysts was comparatively investigated. The results showed that these catalysts had abundant porous structure. In particular, they were found to be efficient in the synthesis of ethyl levulinate, which is promising in reducing the consumption of petroleum-derived fossil fuels. © 2016 Elsevier B.V. All rights reserved.
1. Introduction As an alternative renewable energy resource, biofuel has attracted considerable attention to meet the increasing demands for energy and the growing level of greenhouse gases [1–4]. In particular, ethyl levulinate is an excellent diesel miscible biofuel since it can be used up to 5 wt% directly in regular diesel engines [5]. The conventional catalysts for levulinic acid (LA) esterification are mineral acids such as HCl, H2SO4 and H3PO4. Unfortunately, due to their un-recyclability, harsh reaction conditions and operational problems, replacement of those homogeneous catalysts by the heterogeneous ones is highly desirable [6]. On the other hand, 12-tungstophosphoric acid (HPW) with Keggin structure has been widely used because of its strong acid and good redox properties. However, the high solubility in polar solvents, low thermal stability and low surface area restrict its application as a solid acid catalyst. Supporting or encapsulating HPW species with strong dispersion on or in a suitable carrier with high surface area, which is insoluble in the reaction media, could overcome these limitations [7,8]. The dispersion on or in the carriers may increase the active sites of HPW species and the interaction between HPW and carriers may restrain the leaching of HPW from the resultant heterogeneous catalysts. This could improve the catalytic activity and stability of HPW. ⁎ Corresponding author at: College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China. E-mail address: [email protected] (X. Zheng).
http://dx.doi.org/10.1016/j.catcom.2016.07.023 1566-7367/© 2016 Elsevier B.V. All rights reserved.
As one of the most prevailing 3D macroscopic architectures of graphene, graphene aerogel (GA) exhibits intriguing features such as high porosity, large specific surface area, and high electrical conductivity. Therefore, GA provides exceptional potential in a variety of sustainable applications including catalysis [9,10]. Even so, no work has been reported on the application of GA in esterification reaction. In the present work, GA was hydrothermally assembled and used as supports to prepare HPW-based catalysts. The optimization of the reaction conditions for LA esterification with ethanol was performed and the catalytic performance of the resultant HPW/GA catalysts for the esterification of LA was systematically investigated. 2. Experimental methods GA was prepared from graphene oxide with a hydrothermal method combined with freeze-drying [11] and HPW was synthesized with an improved diethyl ether extraction method [12]. HPW/GA catalysts were prepared by using an impregnation technique similar to that of HSiW/MCM-41 [13] and the resultant catalysts were denoted as x wt% HPW/GA (x = 5, 10, 15, 20, 25, 30, 35, 40, and 45, respectively). The samples were characterized by X-ray diffraction (XRD), nitrogen adsorption-desorption isotherms, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-Transform infrared spectroscopy (FT-IR), and nitrogen adsorption-desorption isotherms. The catalytic performance of the resultant catalysts for the esterification of LA with ethanol was done in a round-bottom flask equipped with
M. Wu et al. / Catalysis Communications 85 (2016) 66–69
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a magnetic stirrer and a water-cooled condenser. The products were analyzed by a gas chromatograph (GC2010 II) equipped with an OV-1701 capillary column (30 m × 0.53 mm × 0.33 μm) and a FID detector. 45 wt% HPW/GA was repeatedly used to examine the recyclability of the resultant catalysts. After each catalytic run, the catalysts were recovered from the reaction solution by centrifugation, washed with ethanol and dried at 70 °C. The detailed information about this section was supplied in the “Electronic Supplementary Material”. 3. Results and discussion The XRD patterns shown in Fig. 1 reveal that only the characteristic phase of GA exits in the resultant x wt% HPW/GA catalysts when x is less than 10, suggesting that the HPW species are well dispersed. The characteristic peaks of HPW appear when x is greater than or equal to 10 and the intensity increases with increasing HPW loading. The slight change of the crystalline structure in the HPW/GA catalysts perhaps is caused by the interaction of the oxygen-containing groups in GA and HPW, which may affect the secondary structure of tungstophosphoric acid. One can see from Fig. 2 that the N2 adsorption-desorption isotherms for the HPW/GA catalysts are similar to that of GA, indicating these catalysts retain the textural structure of GA. The loading of HPW significantly affects the textural parameters. As expected, both the specific surface area and pore volume relative to those of GA (SBET = 270.1 m2 g− 1, Vp = 0.2111 cm3 g−1) decrease with increasing HPW loading. This could be attributed to the deposition of HPW species inside the pores or dispersion of them on the surfaces of GA. Even so, The HPW/GA catalysts display much higher textural parameters than that of the pristine HPW (SBET = 5.4 m2 g−1, Vp = 0.0103 cm3 g−1), which is attributed to the contribution of GA (Table 1). For instance, the pore volume and surface area for 45 wt% HPW/GA are 0.0878 cm3 g−1 and 123.1 m2 g− 1, respectively. The unique properties would be helpful for increasing their catalytic activities. Meanwhile, the various loadings of HPW have no much effect on the pore size. This may be explained that part of HPW species block the microspores or are encapsulated in the macropore of GA since these pores could not be detected by the BJH method. As shown in Fig. S1, the FT-IR spectrum of 5 wt% HPW/GA catalysts only exhibits the characteristic IR bands of GA. The characteristic bands of HPW, such as the asymmetry vibrations of P\\Oa (internal oxygen connecting P and W, 1082 cm−1), W\\Od (terminal oxygen bonding to W atom, 983 cm−1), W\\Ob\\W (edge-sharing oxygen connecting W, 890 cm−1) and W\\Oc\\W (corner-sharing oxygen connecting W3O13 units, 804 cm−1) [12], disappear in the catalysts. This is ascribed to the dilution of GA. These bands appear in the case
Fig. 1. XRD patterns of GA, HPW, and the x wt% HPW/GA catalysts.
Fig. 2. N2 adsorption-desorption isotherms of GA, HPW, and the x wt% HPW/GA catalysts.
of 25 wt% HPW/GA catalysts and the absorbance gradually increases with increasing HPW loading, which could be used to identify the existence of Keggin phase in the resultant catalysts. The morphology and structure of the 15 wt% HPW/GA catalysts are characterized by SEM and TEM techniques (Fig. S2). As shown in Fig. S2a, the catalysts retain the well-defined and interconnected 3D porous network of GA, which is caused by the partial overlapping or coalescing of graphene sheets and macroporous architectures [15]. Meanwhile, the HPW particles are anchored uniformly on or wrapped by graphene sheets. This is also confirmed by the TEM images with different magnifications shown in Fig. S2c and d. Ethanol-to-LA molar ratio (nethanol/nLA) and reaction temperature are optimized to maximize LA conversion which is esterified with ethanol over 15 wt% HPW/GA catalysts. The value of nethanol/nLA is varied in the range of 1.0–15.0 and their influences are depicted in Fig. 3a. LA conversion is found to increase from 8.3% to 35.0% with increasing nethanol/ nLA from 1.0 to 10.0 since the esterification reaction is reversible and the excess amount of ethanol used may favor the conversion of LA to some extent. However, the conversion value decreases when nethanol/nLA is further increased. This is may be ascribed to the decrease the contact opportunities for LA species with the catalysts due to the dilution of too much ethanol. Thus, nethanol/nLA is optimized to be 10.0 under the reaction conditions. As shown in Fig. 3b, the LA conversion increases with increasing reaction time and changes from 5.8% after 2 h to 18.0% after 9 h. When the reaction time is further increased to 10 h, the value only increases to 18.1%. Thus, 9 h is an optimum reaction time under the present conditions. Fig. 3c shows the catalytic activities of the resultant x wt% HPW/GA catalysts. It is found that the LA conversion increases with increasing HPW loading. The value drastically increases from 11.2% to 89.1% when x increases from 5 to 45, which is consistent with that of the Keggin-anion density (Table 1). On the other hand, in order to reflect actual catalytic performance of HPW species in the resultant catalysts, their activities are also compared with the TOF mode. As shown in the inset of Fig. 3c, although the LA conversion and the Keggin-anion density are the lowest for 5 wt% HPW/GA ascribed to the lowest HPW loading and the highest surface area, it exhibits the highest TOF value (0.031 molLA·h−1·g−1 HPW) among the resultant catalysts. As for 35 wt% HPW/GA catalysts, the LA conversion (70.3%) is higher than the ones with low loadings (x = 5–30) and TOF value (0.028 molLA·h−1·g−1 HPW) is similar or even higher than the ones with high loadings (x = 40, 45). Therefore, the optimized loading for HPW on GA is 35 wt% under the present conditions. In order to investigate the durability of these HPW/GA catalysts, a typical recycle study on the sample with 45 wt% HPW loading is performed, as shown in Fig. 3d. The LA conversion decreases from 89.1% to 76.6% in the second cycle. Comparing with that of the fresh one, the
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M. Wu et al. / Catalysis Communications 85 (2016) 66–69
Table 1 Textural and structural characteristics of GA, HPW and the x wt% HPW/GA catalysts. Samples
SBETa (m2 g−1)
SMb (m2 g−1)
SExc (m2 g−1)
Vpd (cm3 g−1)
VMe (cm3 g−1)
Daf (nm)
Dag (nm)
Dah (nm)
Keggin-anion densityi (HPW nm−2)
GA x=5 x = 10 x = 15 x = 20 x = 25 x = 30 x = 35 x = 40 x = 45 HPW
270.1 244.9 216.3 215.0 202.9 168.9 159.1 156.7 132.8 123.1 5.4
69.6 66.4 64.6 63.4 62.2 49.9 55.9 47.4 40.3 44.0 0.9
200.5 178.5 151.7 151.6 140.7 119.0 103.3 109.3 92.5 79.1 4.5
0.2111 0.1829 0.1610 0.1591 0.1506 0.1265 0.1116 0.1199 0.0922 0.0878 0.0103
0.0322 0.0306 0.0296 0.0291 0.0285 0.0229 0.0255 0.02170 0.0184 0.0200 0.0004
3.13 2.99 2.98 2.96 2.97 2.99 2.80 3.06 2.78 2.85 7.58
3.72 3.56 3.63 3.60 3.65 3.67 3.54 3.83 3.36 3.65 8.65
3.52 3.27 3.29 3.22 3.24 3.22 3.00 3.03 2.99 3.13 7.66
– 0.0427 0.0965 0.1460 0.2060 0.3095 0.3940 0.4669 0.6295 0.7641 38.7062
a
SBET, BET surface area. SM, t-Plot micropore area. SEx, t-Plot external surface area. d Vp, Single point adsorption total pore volume. e VM, t-Plot micropore volume. f Da, Adsorption average pore width (4 V/A by BET). g Da, BJH Adsorption average pore diameter (4 V/A). h Da, BJH Desorption average pore diameter (4 V/A). i The density of Keggin ion was expressed as number of Keggin anion (ionic radius, ~1.2 nm) per square nanometer [14], . ½HPW loading ðwt%Þ 100 6:02 105 Keggin‐anion density ðHPW nm−2 Þ ¼ BET surface area of catalyst ðm2 g−1 Þ 2880:2 b c
absorbance intensity of HPW becomes weak in the IR spectrum for the reused catalysts (Fig. S3a). Meanwhile, the shape of N2 adsorptiondesorption isotherm for the reused catalysts is more similar to that of GA than that of the fresh one shown in Fig. S3b. Additionally, the textural parameters of the reused catalysts (SBET = 194.7 m2 g−1, Vp = 0.1770 cm3 g−1) are larger than those of the fresh one (SBET =
123.1 m2 g− 1, Vp = 0.0878 cm3 g−1). This is mainly due to those HPW species, which have no compact interaction with GA, are partly leached into the reaction media or the washed solvents (water and ethanol). Even so, the SEM images reveal that although there is partial elution during the reaction, most of the HPW species retain in the reused catalysts (Fig. S3c and d). The LA conversion still reaches up to 51.7%
Fig. 3. The influences of ethanol-to-LA molar ratios (with catalyst amount 0.20 g, temperature 80 °C and time 10 h) (a) and reaction time (with catalyst amount 0.20 g, ethanol-to-LA molar ratio 10 and temperature 80 °C) (b) on LA conversion over 15 wt% HPW/GA catalysts; The influences of HPW loading of the x wt% HPW/GA catalysts on LA conversion (with catalyst amount 0.20 g, ethanol-to-LA molar ratio 10, temperature 80 °C and time 9 h) (c); The recyclability of 45 wt% HPW/GA catalysts in the LA conversion (with ethanol-to-LA molar ratio 10, temperature 80 °C and time 9 h) (d).
M. Wu et al. / Catalysis Communications 85 (2016) 66–69
in the fifth cycle. The results suggest that the HPW/GA catalysts have allright durability in the reaction. Compared to those heteropoly acids-based catalysts, such as HSiW/ MCM-41 [13], the resultant HPW/GA catalysts, especially those with high HPW loading, exhibit better catalytic activity for the synthesis ethyl levulinate, indicating that the HPW/GA catalysts are efficient in the production of biofuel via esterification of levulinic acid. 4. Conclusions A series of porous solid acid catalysts consisting of 3D graphene aerogel and the supported HPW are synthesized and characterized. The results showed that these catalysts had abundant porous structure and exhibited excellent catalytic performance in the synthesis of ethyl levulinate. The conversion of levulinic acid may reach up to 89.1% under the present conditions. Acknowledgements The authors are grateful for the supports of the National Natural Science Foundation of China (Nos. U1304203 and J1210060), the Foundation of He'nan Educational Committee (No. 16A150046) and the Innovation Foundation of Zhengzhou University (Nos. 201610459062 and 2016xjxm251). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2016.07.023. References [1] P. Sun, G. Gao, Z.L. Zhao, C.G. Xia, F.W. Li, Stabilization of cobalt catalysts by embedment for efficient production of valeric biofuel, ACS Catal 4 (2014) 4136–4142.
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[2] S.M. Abdo, S.Z. Wahba, G.H. Ali, S.A.A. EI-Enin, K.M. EI-Khatib, M.I. EI-Galad, G. EIDiwani, Preliminary economic assessment of biofuel production from microalgae, Renew. Sust. Energ. Rev. 55 (2016) 1147–1153. [3] A. Bohre, S. Dutta, B. Saha, M.M. Abu-Omar, Upgrading furfurals to drop-in biofuels: an overview, ACS Sustain. Chem. Eng 3 (2015) 1263–1277. [4] S. Verma, R.B.N. Baig, M.N. Nadagouda, R.S. Varma, Visible light mediated upgrading of biomass to biofuel, Green Chem 18 (2016) 1327–1331. [5] C.R. Patil, P.S. Niphadkar, V.V. Bokade, P.N. Joshi, Esterification of levulinic acid to ethyl levulinate over bimodal micro-mesoporous H/BEA zeolite derivatives, Catal. Commun 43 (2014) 188–191. [6] Y. Kuwahara, W. Kaburagi, K. Nemotob, T. Fujitani, Esterification of levulinic acid with ethanol over sulfated Si-doped ZrO2 solid acid catalyst: study of the structure-activity relationships, Appl. Catal. A Gen 476 (2014) 186–196. [7] R.M. Ladera, J.L.G. Fierro, M. Ojeda, S. Rojas, TiO2-supported heteropoly acids for low-temperature synthesis of dimethyl ether from methanol, J. Catal 312 (2014) 195–203. [8] A.E.R.S. Khder, Preparation, characterization and catalytic activity of tin oxidesupported 12-tungstophosphoric acid as a solid catalyst, Appl. Catal. A Gen 343 (2008) 109–116. [9] C.H. Sha, B. Lu, H.Y. Mao, J.P. Cheng, X.H. Pan, J.G. Lu, Z.Z. Ye, 3D ternary nanocomposites of molybdenum disulfide/polyaniline/reduced graphene oxide aerogel for high performance supercapacitors, Carbon 99 (2016) 26–34. [10] X.G. Fu, J.Y. Choi, P.Y. Zamani, G.P. Jiang, M.A. Hoque, F.M. Hassan, Z.W. Chen, Co\ \N decorated hierarchically porous graphene aerogel for efficient oxygen reduction reaction in acid, ACS Appl. Mater. Interfaces 8 (2016) 6488–6495. [11] J.K. Meng, Y. Cao, Y. Suo, Y.S. Liu, J.M. Zhang, X.C. Zheng, Facile fabrication of 3D SiO2@graphene aerogel composites as anode material for lithium ion batteries, Electrochim. Acta 176 (2015) 1001–1009. [12] B.B. Dong, B.B. Zhang, H.Y. Wu, S.D. Li, K. Zhang, X.C. Zheng, Direct synthesis, characterization and application in benzaldehyde oxidation of HPWA-SBA-15 mesoporous catalysts, Microporous Mesoporous Mater 176 (2013) 186–193. [13] Y. Chen, X.L. Zhang, M.M. Dong, Y.H. Wu, G.P. Zheng, J. Huang, X.X. Guan, X.C. Zheng, MCM-41 immobilized 12-silicotungstic acid mesoporous materials: structural and catalytic properties for esterification of levulinic acid and oleic acid, J. Taiwan Inst. Chem. Eng 61 (2016) 147–155. [14] S.H. Chai, H.P. Wang, Y. Liang, B.Q. Xu, Sustainable production of acrolein: gas-phase dehydration of glycerol over 12-tungstophosphoric acid supported on ZeO2 and SiO2, Green Chem 10 (2008) 1087–1093. [15] C. Li, G. Shi, Three-dimensional graphene architectures, Nanoscale 4 (2012) 5549–5563.
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
3D graphene aerogel anchored tungstophosphoric acid catalysts: Characterization and catalytic performance for levulinic acid esterification with ethanol Min Wu a, Xiaoli Zhang a, Xiaoli Su a, Xueyang Li a, Xiucheng Zheng a,b,⁎, Xinxin Guan a, Pu Liu a a b
College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China
a r t i c l e
i n f o
Article history: Received 26 April 2016 Received in revised form 23 June 2016 Accepted 24 July 2016 Available online 27 July 2016 Keywords: GA HPW/GA Esterification Levulinic acid Biofuel
a b s t r a c t 3D graphene aerogel (GA) was synthesized from graphene oxide (GO) using a hydrothermal procedure combined with freeze-drying. Then, they were used to assemble the supported H3PW12O40 (HPW) catalysts. The optimization of the reaction parameters for levulinic acid esterification with ethanol was performed and the catalytic performance of the HPW/GA catalysts was comparatively investigated. The results showed that these catalysts had abundant porous structure. In particular, they were found to be efficient in the synthesis of ethyl levulinate, which is promising in reducing the consumption of petroleum-derived fossil fuels. © 2016 Elsevier B.V. All rights reserved.
1. Introduction As an alternative renewable energy resource, biofuel has attracted considerable attention to meet the increasing demands for energy and the growing level of greenhouse gases [1–4]. In particular, ethyl levulinate is an excellent diesel miscible biofuel since it can be used up to 5 wt% directly in regular diesel engines [5]. The conventional catalysts for levulinic acid (LA) esterification are mineral acids such as HCl, H2SO4 and H3PO4. Unfortunately, due to their un-recyclability, harsh reaction conditions and operational problems, replacement of those homogeneous catalysts by the heterogeneous ones is highly desirable [6]. On the other hand, 12-tungstophosphoric acid (HPW) with Keggin structure has been widely used because of its strong acid and good redox properties. However, the high solubility in polar solvents, low thermal stability and low surface area restrict its application as a solid acid catalyst. Supporting or encapsulating HPW species with strong dispersion on or in a suitable carrier with high surface area, which is insoluble in the reaction media, could overcome these limitations [7,8]. The dispersion on or in the carriers may increase the active sites of HPW species and the interaction between HPW and carriers may restrain the leaching of HPW from the resultant heterogeneous catalysts. This could improve the catalytic activity and stability of HPW. ⁎ Corresponding author at: College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China. E-mail address: [email protected] (X. Zheng).
http://dx.doi.org/10.1016/j.catcom.2016.07.023 1566-7367/© 2016 Elsevier B.V. All rights reserved.
As one of the most prevailing 3D macroscopic architectures of graphene, graphene aerogel (GA) exhibits intriguing features such as high porosity, large specific surface area, and high electrical conductivity. Therefore, GA provides exceptional potential in a variety of sustainable applications including catalysis [9,10]. Even so, no work has been reported on the application of GA in esterification reaction. In the present work, GA was hydrothermally assembled and used as supports to prepare HPW-based catalysts. The optimization of the reaction conditions for LA esterification with ethanol was performed and the catalytic performance of the resultant HPW/GA catalysts for the esterification of LA was systematically investigated. 2. Experimental methods GA was prepared from graphene oxide with a hydrothermal method combined with freeze-drying [11] and HPW was synthesized with an improved diethyl ether extraction method [12]. HPW/GA catalysts were prepared by using an impregnation technique similar to that of HSiW/MCM-41 [13] and the resultant catalysts were denoted as x wt% HPW/GA (x = 5, 10, 15, 20, 25, 30, 35, 40, and 45, respectively). The samples were characterized by X-ray diffraction (XRD), nitrogen adsorption-desorption isotherms, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-Transform infrared spectroscopy (FT-IR), and nitrogen adsorption-desorption isotherms. The catalytic performance of the resultant catalysts for the esterification of LA with ethanol was done in a round-bottom flask equipped with
M. Wu et al. / Catalysis Communications 85 (2016) 66–69
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a magnetic stirrer and a water-cooled condenser. The products were analyzed by a gas chromatograph (GC2010 II) equipped with an OV-1701 capillary column (30 m × 0.53 mm × 0.33 μm) and a FID detector. 45 wt% HPW/GA was repeatedly used to examine the recyclability of the resultant catalysts. After each catalytic run, the catalysts were recovered from the reaction solution by centrifugation, washed with ethanol and dried at 70 °C. The detailed information about this section was supplied in the “Electronic Supplementary Material”. 3. Results and discussion The XRD patterns shown in Fig. 1 reveal that only the characteristic phase of GA exits in the resultant x wt% HPW/GA catalysts when x is less than 10, suggesting that the HPW species are well dispersed. The characteristic peaks of HPW appear when x is greater than or equal to 10 and the intensity increases with increasing HPW loading. The slight change of the crystalline structure in the HPW/GA catalysts perhaps is caused by the interaction of the oxygen-containing groups in GA and HPW, which may affect the secondary structure of tungstophosphoric acid. One can see from Fig. 2 that the N2 adsorption-desorption isotherms for the HPW/GA catalysts are similar to that of GA, indicating these catalysts retain the textural structure of GA. The loading of HPW significantly affects the textural parameters. As expected, both the specific surface area and pore volume relative to those of GA (SBET = 270.1 m2 g− 1, Vp = 0.2111 cm3 g−1) decrease with increasing HPW loading. This could be attributed to the deposition of HPW species inside the pores or dispersion of them on the surfaces of GA. Even so, The HPW/GA catalysts display much higher textural parameters than that of the pristine HPW (SBET = 5.4 m2 g−1, Vp = 0.0103 cm3 g−1), which is attributed to the contribution of GA (Table 1). For instance, the pore volume and surface area for 45 wt% HPW/GA are 0.0878 cm3 g−1 and 123.1 m2 g− 1, respectively. The unique properties would be helpful for increasing their catalytic activities. Meanwhile, the various loadings of HPW have no much effect on the pore size. This may be explained that part of HPW species block the microspores or are encapsulated in the macropore of GA since these pores could not be detected by the BJH method. As shown in Fig. S1, the FT-IR spectrum of 5 wt% HPW/GA catalysts only exhibits the characteristic IR bands of GA. The characteristic bands of HPW, such as the asymmetry vibrations of P\\Oa (internal oxygen connecting P and W, 1082 cm−1), W\\Od (terminal oxygen bonding to W atom, 983 cm−1), W\\Ob\\W (edge-sharing oxygen connecting W, 890 cm−1) and W\\Oc\\W (corner-sharing oxygen connecting W3O13 units, 804 cm−1) [12], disappear in the catalysts. This is ascribed to the dilution of GA. These bands appear in the case
Fig. 1. XRD patterns of GA, HPW, and the x wt% HPW/GA catalysts.
Fig. 2. N2 adsorption-desorption isotherms of GA, HPW, and the x wt% HPW/GA catalysts.
of 25 wt% HPW/GA catalysts and the absorbance gradually increases with increasing HPW loading, which could be used to identify the existence of Keggin phase in the resultant catalysts. The morphology and structure of the 15 wt% HPW/GA catalysts are characterized by SEM and TEM techniques (Fig. S2). As shown in Fig. S2a, the catalysts retain the well-defined and interconnected 3D porous network of GA, which is caused by the partial overlapping or coalescing of graphene sheets and macroporous architectures [15]. Meanwhile, the HPW particles are anchored uniformly on or wrapped by graphene sheets. This is also confirmed by the TEM images with different magnifications shown in Fig. S2c and d. Ethanol-to-LA molar ratio (nethanol/nLA) and reaction temperature are optimized to maximize LA conversion which is esterified with ethanol over 15 wt% HPW/GA catalysts. The value of nethanol/nLA is varied in the range of 1.0–15.0 and their influences are depicted in Fig. 3a. LA conversion is found to increase from 8.3% to 35.0% with increasing nethanol/ nLA from 1.0 to 10.0 since the esterification reaction is reversible and the excess amount of ethanol used may favor the conversion of LA to some extent. However, the conversion value decreases when nethanol/nLA is further increased. This is may be ascribed to the decrease the contact opportunities for LA species with the catalysts due to the dilution of too much ethanol. Thus, nethanol/nLA is optimized to be 10.0 under the reaction conditions. As shown in Fig. 3b, the LA conversion increases with increasing reaction time and changes from 5.8% after 2 h to 18.0% after 9 h. When the reaction time is further increased to 10 h, the value only increases to 18.1%. Thus, 9 h is an optimum reaction time under the present conditions. Fig. 3c shows the catalytic activities of the resultant x wt% HPW/GA catalysts. It is found that the LA conversion increases with increasing HPW loading. The value drastically increases from 11.2% to 89.1% when x increases from 5 to 45, which is consistent with that of the Keggin-anion density (Table 1). On the other hand, in order to reflect actual catalytic performance of HPW species in the resultant catalysts, their activities are also compared with the TOF mode. As shown in the inset of Fig. 3c, although the LA conversion and the Keggin-anion density are the lowest for 5 wt% HPW/GA ascribed to the lowest HPW loading and the highest surface area, it exhibits the highest TOF value (0.031 molLA·h−1·g−1 HPW) among the resultant catalysts. As for 35 wt% HPW/GA catalysts, the LA conversion (70.3%) is higher than the ones with low loadings (x = 5–30) and TOF value (0.028 molLA·h−1·g−1 HPW) is similar or even higher than the ones with high loadings (x = 40, 45). Therefore, the optimized loading for HPW on GA is 35 wt% under the present conditions. In order to investigate the durability of these HPW/GA catalysts, a typical recycle study on the sample with 45 wt% HPW loading is performed, as shown in Fig. 3d. The LA conversion decreases from 89.1% to 76.6% in the second cycle. Comparing with that of the fresh one, the
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M. Wu et al. / Catalysis Communications 85 (2016) 66–69
Table 1 Textural and structural characteristics of GA, HPW and the x wt% HPW/GA catalysts. Samples
SBETa (m2 g−1)
SMb (m2 g−1)
SExc (m2 g−1)
Vpd (cm3 g−1)
VMe (cm3 g−1)
Daf (nm)
Dag (nm)
Dah (nm)
Keggin-anion densityi (HPW nm−2)
GA x=5 x = 10 x = 15 x = 20 x = 25 x = 30 x = 35 x = 40 x = 45 HPW
270.1 244.9 216.3 215.0 202.9 168.9 159.1 156.7 132.8 123.1 5.4
69.6 66.4 64.6 63.4 62.2 49.9 55.9 47.4 40.3 44.0 0.9
200.5 178.5 151.7 151.6 140.7 119.0 103.3 109.3 92.5 79.1 4.5
0.2111 0.1829 0.1610 0.1591 0.1506 0.1265 0.1116 0.1199 0.0922 0.0878 0.0103
0.0322 0.0306 0.0296 0.0291 0.0285 0.0229 0.0255 0.02170 0.0184 0.0200 0.0004
3.13 2.99 2.98 2.96 2.97 2.99 2.80 3.06 2.78 2.85 7.58
3.72 3.56 3.63 3.60 3.65 3.67 3.54 3.83 3.36 3.65 8.65
3.52 3.27 3.29 3.22 3.24 3.22 3.00 3.03 2.99 3.13 7.66
– 0.0427 0.0965 0.1460 0.2060 0.3095 0.3940 0.4669 0.6295 0.7641 38.7062
a
SBET, BET surface area. SM, t-Plot micropore area. SEx, t-Plot external surface area. d Vp, Single point adsorption total pore volume. e VM, t-Plot micropore volume. f Da, Adsorption average pore width (4 V/A by BET). g Da, BJH Adsorption average pore diameter (4 V/A). h Da, BJH Desorption average pore diameter (4 V/A). i The density of Keggin ion was expressed as number of Keggin anion (ionic radius, ~1.2 nm) per square nanometer [14], . ½HPW loading ðwt%Þ 100 6:02 105 Keggin‐anion density ðHPW nm−2 Þ ¼ BET surface area of catalyst ðm2 g−1 Þ 2880:2 b c
absorbance intensity of HPW becomes weak in the IR spectrum for the reused catalysts (Fig. S3a). Meanwhile, the shape of N2 adsorptiondesorption isotherm for the reused catalysts is more similar to that of GA than that of the fresh one shown in Fig. S3b. Additionally, the textural parameters of the reused catalysts (SBET = 194.7 m2 g−1, Vp = 0.1770 cm3 g−1) are larger than those of the fresh one (SBET =
123.1 m2 g− 1, Vp = 0.0878 cm3 g−1). This is mainly due to those HPW species, which have no compact interaction with GA, are partly leached into the reaction media or the washed solvents (water and ethanol). Even so, the SEM images reveal that although there is partial elution during the reaction, most of the HPW species retain in the reused catalysts (Fig. S3c and d). The LA conversion still reaches up to 51.7%
Fig. 3. The influences of ethanol-to-LA molar ratios (with catalyst amount 0.20 g, temperature 80 °C and time 10 h) (a) and reaction time (with catalyst amount 0.20 g, ethanol-to-LA molar ratio 10 and temperature 80 °C) (b) on LA conversion over 15 wt% HPW/GA catalysts; The influences of HPW loading of the x wt% HPW/GA catalysts on LA conversion (with catalyst amount 0.20 g, ethanol-to-LA molar ratio 10, temperature 80 °C and time 9 h) (c); The recyclability of 45 wt% HPW/GA catalysts in the LA conversion (with ethanol-to-LA molar ratio 10, temperature 80 °C and time 9 h) (d).
M. Wu et al. / Catalysis Communications 85 (2016) 66–69
in the fifth cycle. The results suggest that the HPW/GA catalysts have allright durability in the reaction. Compared to those heteropoly acids-based catalysts, such as HSiW/ MCM-41 [13], the resultant HPW/GA catalysts, especially those with high HPW loading, exhibit better catalytic activity for the synthesis ethyl levulinate, indicating that the HPW/GA catalysts are efficient in the production of biofuel via esterification of levulinic acid. 4. Conclusions A series of porous solid acid catalysts consisting of 3D graphene aerogel and the supported HPW are synthesized and characterized. The results showed that these catalysts had abundant porous structure and exhibited excellent catalytic performance in the synthesis of ethyl levulinate. The conversion of levulinic acid may reach up to 89.1% under the present conditions. Acknowledgements The authors are grateful for the supports of the National Natural Science Foundation of China (Nos. U1304203 and J1210060), the Foundation of He'nan Educational Committee (No. 16A150046) and the Innovation Foundation of Zhengzhou University (Nos. 201610459062 and 2016xjxm251). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2016.07.023. References [1] P. Sun, G. Gao, Z.L. Zhao, C.G. Xia, F.W. Li, Stabilization of cobalt catalysts by embedment for efficient production of valeric biofuel, ACS Catal 4 (2014) 4136–4142.
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