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Efficient conversion of levulinic acid to ethyl levulinate over silicotungstic acid modified commercially silica-gel sphere catalyst Xiangjin Kong, Shuxiang Wu, Xiaole Li, and Junhai Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01207 • Publication Date (Web): 15 Jul 2016 Downloaded from http://pubs.acs.org on July 19, 2016

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Efficient conversion of levulinic acid to ethyl levulinate over silicotungstic acid modified commercially silica-gel sphere catalyst Xiangjin Kong*, Shuxiang Wu, Xiaole Li, Junhai Liu Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, China. E-mail: [email protected]. Keywords: commercially silica-gel sphere, Levulinic acid, Esterification, Ethyl levulinate ABSTRACT: Silicotungstic acid modified commercially available silica-gel sphere was applied to the catalytic conversion of levulinic acid to give ethyl levulinate. The obtained catalysts were studied by N2 sorptions, XRD, FT-IR and NH3-TPD. The experimental results indicated that catalytic activity of the catalysts were influenced by the amount of acid sites, surface area and porosity. The yield of ethyl levulinate was observed from 70% to 97% with increase of silicotungstic acid load from zero to 40%. The reaction parameters, including catalyst dosage, molar ratio of levulinic acid to ethanol, reaction temperature and time were optimized and a 97.0% yield of ethyl levulinate was obtained at the optimal conditions. The experimental results demonstrated the potential of silicotungstic acid modified commercially available silica-gel sphere catalyst for conversion of levulinic acid to ethyl levulinate under mild process conditions. 1. Introduction With the depletion of nonrenewable resources, especially fossil fuel and coal, causing environmental problems more and more serious. Biomass, potential renewable energy, is a promising alternative for fossil energy due to biomass can be converted into fuels and chemicals [1-2]. Against this background, many research works have been invested in finding biomass

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platform chemicals and varieties of synthesis routes about them. Among of the reported synthetic routes, esterification of levulinic acid with alcohol attracted much more attention of the researchers, since levulinic acid can be obtained from C6 sugar carbohydrates which was derived from renewable lingo-cellulose and, these levulinates or esters are used in the flavor, fragrance and diesel industry with various potential applications [3, 4].For example, ethyl levulinate can be used as the add ingredients (up to 5 wt.%) of diesel miscible biofuel (DMB) to modify the properties of fuel [4-7]. Traditionally, esters were synthesized by using homogeneous inorganic acids such as formic acid, H3PO4 and H2SO4, and often suffered from environmental problems in corrosion, handling and recycling issues [8-10]. Heterogeneous catalysts have gained the attention of the researchers due to the benefits like as the adjustable acidity, easy to be separated and environmental pollution-free. As a new class of solid acid catalyst, heteropoly acids were widely studied because of their very strong Brønsted acidity and special structural properties[11-13]. However, the quite poor stability and small specific surface area have limited their industrial applications. Recently, some methods have been reported to improve its catalytic performance. Among of the methods, loading heteropoly acid on a support has attracted people's much attentions [14-15]. Although some improvement has been achieved, the performance of these catalysts is still poor, and often faced by high cost or complicated preparation procedures [16-18]. Thus it's still a challenge to exploit a fine catalyst to improve the synthetic efficiency of ethyl levulinate. In this contribution, as one of the most studied supports, commercially available silica-gel sphere (SGS) supports were chosen as support. And, several silicotungstic acid modified silicagel sphere catalyst were developed and evaluated for the conversion of levulinic acid to ethyl


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levulinate. Based on the reported literatures, the study may be the first attempt to use commercially available supports for the ethyl levulinate synthesis from levulinic acid. 2. Experimental 2.1. Catalyst preparation and catalytic evaluation Silica-gel sphere supports were provided by Hailang silica-gel drier factory (Qingdao, China). Silicotungstic acid (STA) was provided by Tianjin Kermel Chemical Reagent Co., Ltd (Tianjin, China). Levulinic acid (99%) was provided by Aladdin Reagent Co. (Shanghai, China). The catalysts used in this research were prepared by incipient wetness technique. As an example, 10 wt.% STA/SGS (10 wt.% STA means the weight percentage of silicotungstic acid in the catalyst was 10%.) was prepared as shown below. Silica-gel sphere (20 g) was impregnated in 40.0 mL aqueous solution of 2.0 g silicotungstic acid at room temperature for 4 h, then the catalyst was separated and dried at 393 K for 6 h. The dry catalyst was impregnated with the aqueous solution residue for 4 h again, next dried at 393 K for 6 h. In the end, the catalyst was calcined at 673 K for 2 h. The catalytic performance of the obtained catalysts for the esterification of LA were carried out in a 100 mL three-neck flask attached to a reflux condenser and a magnetic stirrer. For a typical run, the mole ratio of LA to ethanol was 1:8, catalyst to LA ratio is 0.2. After reaction in 355 K for 4 h, the catalyst was separated from the reaction mixture and the liquid product was analyzed by using offline gas chromatograph (SE-54 capillary column: 30 m× 0.32 mm, 0.5 µm film thickness), with ethyl laurate as an internal standard. Furthermore, the composition of products were confirmed by GC-MS (HP-5MS UI capillary column: 30 m × 0.25 mm, 0.25 µm film thickness) equipped with an ion trap MS detector. 2.2. Catalyst evaluation


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Nitrogen adsorption-desorption measurement was carried out on an TriStar II3020 equipment and the specific surface areas were calculated by Brunauer-Emmett-Teller (BET) method, while total volume were evaluated from N2 uptake at a relative N2 pressure of 0.99, mesopore volume were calculated by the total volume minus the T-method micro pore volume. Phase identification and degree of crystallinity were observed via using X-ray powder diffraction (XRD, Bruker D8 ADVANCE, Germany). Fourier transform infrared (FT-IR) spectra of the samples pressed in KBr pellets were collected in a Thermo Nicolet 6700 FTIR spectrometer. Temperature-programmed desorption of ammonia (NH3-TPD) was carried out on the automatic multi-purpose adsorption instrument (TP-5080). 3. Results and discussion 3.1. Catalyst characterization The surface area and pore volume of the SGS and STA modified SGS catalysts were measured by nitrogen adsorption-desorption, and the results are listed in Table 1. The surface areas of the supported-type catalysts decreased remarkably from 251.8 m2/g of the SGS with the addition of amounts of silicotungstic acid, and similar downtrends in the catalyst pore volume is also observed. The changes of porosity are probably due to the occupying of some cavities by silicotungstic acid during the catalyst preparation process, which is in accordance with previous studies [19]. The XRD patterns of the silica-gel sphere and the silicotungstic acid modified catalysts were depicted in Fig.1. A broad diffraction peak was observed at 20°-30° for all the samples,which was the diffraction characteristic peak of amorphous silica. The typical peak of silicotungstic acid was observed at 8° [20], with the increased of the silicotungstic acid loading, the typical peak at 8° became more and more intensively. These results demonstrated that the


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silicotungstic acid was successfully impregnated to the catalysts, and basically kept the Keggin structure as reported in previous studies [20-21]. Table 1 N2 sorption characteristics of silica-gel sphere and modified catalysts Area[a], (m2/g)

Sample

Volume[b], cm3/g

Total

External

Micropore

Total

Mesop

SGS

251.8

189.0

62.7

0.92

0.91

10 wt.% STA/SGS

237.0

179.7

57.3

0.85

0.84

20 wt.% STA/SGS

220.4

165.2

55.2

0.80

0.79

30 wt.% STA/SGS

182.6

130.9

51.7

0.65

0.64

40 wt.% STA/SGS 177.1 124.2 53.0 0.64 [a] The calculation of micropore and external area was based on T-plot method,

0.63

[b] The total pore volume was evaluated from N2 uptake at a relative N2 pressure of 0.99. •Silicotungstic acid

•

e

• d

Intensity

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•

c

•

b a

10

20

30

40

50

60

70

2θ£¨°£©

Fig.1 XRD patterns of: (a) SGS, (b) 10 wt.% STA/SGS, (c) 20 wt.% STA/SGS, (d) 30 wt.% STA/SGS, and (e) 40 wt.% STA/SGS To further investigate the structural properties of the catalysts, FT-IR spectra of the silicagel sphere and the silicotungstic acid modified catalysts were shown in Fig.2. The characteristic bands of the SiO2 appeared at 820 cm−1 (Si-O bending) and 1070 cm−1 (Si-O-Si stretching) can


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be observed for all the FT-IR spectra. The characteristic absorption bonds of W=O terminal bonds (970 cm−1) and W-O-W bridges (920 cm−1) were also observed from STA/SGS catalysts, which were in consistent with the pure STA spectra [22]. These results suggested that the silicotungstic acid maintained its Keggin Unit in the catalyst, which was also in agreement with the XRD results.

e

d

c

b a

600

800

1000

1200

1400

-1

Wavenumber(cm )

Fig.2 FT-IR spectra of (a) SGS, (b) 10 wt.% STA/SGS, (c) 20 wt.% STA/SGS, (d) 30 wt.% STA/SGS, and (e) 40 wt.% STA/SGS The NH3-TPD curves of the silica-gel sphere and the silicotungstic acid modified catalysts were shown in Fig.3. The two desorption peaks at the lower and the higher temperature correspond to the weak and strong acid sites on the catalyst, respectively. Table 2 listed the total acidity values of the catalysts together with the temperature of the NH3 desorption peak. With the increase of STA loadings, the acidity value of the weak acid sites was increased, but the stronger acidity was reduced. It demonstrated that the strong acid sites of the supports were neutralized by the doped silicotungstic acid. Table 2 also displayed that, with the increase of STA loadings, the total acidity value increased to 1.321 and then declined to 1.115. The acidity value of 30 wt.%


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Intensity

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e d

c b a 523

573

673

623

723

773

823

873

923

973

Desorption temperature £¨K£©

Fig.3 NH3-TPD curves of (a) silica-gel sphere, (b) 10 wt.% STA/SGS, (c) 20 wt.% STA/SGS, (d) 30 wt.% STA/SGS, and (e) 40 wt.% STA/SGS Table 2 NH3-TPD analysis of the STA /SGS catalysts with different loading Weak

Strong

Sample

Total acidity Temp.

acidity

Temp.

acidity

SGS

621.65

0.121

841.24

0.526

0.647

10 wt.% STA/SGS

607.73

0.247

851.81

0.870

1.117

20 wt.% STA/SGS

609.33

0.619

841.94

0.702

1.321

30 wt.% STA/SGS

613.97

0.783

837.83

0.376

1.159

40 wt.% STA/SGS 615.26 1.023 622.27 0.092 1.115 Note: Temp. means the peak temperature(K), acidity means the ammonia acidity(mmol/g). STA/SGS and 40 wt.% STA/SGS were lower than that of 20 wt.% STA/SGS catalyst. High silicotungstic acid loadings result in uneven low acidity capacity of the catalysts, it makes this results sound to be unbelievable. As can be found in Table 1, when the loading of silicotungstic acid is more 20%, a sharply decreased of the surface area was observed. Thus, we inferred that the drop of acidity capacity of 30 wt.% STA/SGS and 40 wt.% STA/SGS is caused by the poor


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dispersion or aggregation of silicotungstic acid, which may block up the channels of silica-gel sphere, and similar results were also reported by Kexin Li et al. [23] and Yihang Guo et al. [24]. 3.2 Catalytic Activity The catalytic performance of silica-gel sphere and silicotungstic acid modified catalysts was evaluated by the synthesis of EL under the experimental conditions (mole ratio LA to ethanol 1:8, catalyst to LA ratio: 0.2, 355 As can be found, all of the silicotungstic acid modified catalysts exhibited better catalytic activity than the silica-gel sphere. With the loading of silicotungstic acid on silica-gel sphere was increased to 20 wt.%, the yield of EL increased sharply from 70% to 96.5%. It is noteworthy that when silicotungstic acid loading was more than 20 wt.%, the EL yield only increased to 97.0% for the 40 wt.% STA/SGS catalyst slightly, this may due to the poor porous structure of 30 wt.% STA/SGS and 40 wt.% STA/SGS catalyst as can be found in Table 1. In view of the above results, 20 wt.% STA/SGS can be selected as the optimal catalyst for the current reaction. Table 3 Effect of different STA loading on SGS Sample

SGS

10 wt.% STA/SGS

20wt.%STA/SGS

30wt.%STA/SGS

40wt.%STA/SGS

Yield(%)

70.0

85.3

96.5

96.9

97.0

3.3. Optimization of process parameters Since 20wt.%STA/SGS was elected as the optimal catalyst for the current reaction, the reaction conditions including catalyst dose, molar ratio of levulinic acid to ethanol, reaction temperature and reaction time were optimized to further reveal the catalytic performance of 20wt.%STA/SGS. 3.3.1. catalyst dosage


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The effect of catalyst dosage was studied to optimize catalytic performance of 20 wt.%STA/SGS, the results are shown in Fig.4. It can be observed that the yield of EL increased from 91.9% to 96.4% with the increased dosage of catalyst to LA ratio from 0.1 to 0.2. When further increased the dosage of catalyst, the almost identical EL yield is observed. Thus, the optimum catalyst to LA ratio is 0.2 in the present case.

100

Yield (%)

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95

90 0.05

0.10

0.15

0.20

0.25

0.30

0.35

Cataly to LA Ratio

Fig.4 Effect of catalyst dosage 3.3.2. Effect of LA to ethanol molar ratio The parameter plays an important role in the present reaction due to the reversible reaction nature. The effect of the parameter (LA: ethanol molar ratio) has been studied, the experimental results are presented in Fig.5. As shown in Fig.5, when the amount of substance of ethanol is four times that of LA increase to eight times, the yield of EL also increased simultaneously. Clearly this results was inconsistent with the nature of reversible reaction, since adding excess of ethanol to the reaction could enhance the equilibrium conversion of LA. But with the further addition of ethanol (up to 12 times), the yield of EL dropped from 96% to 89%. This situation


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may be caused by too much ethanol diluted levulinic acid, this does not favor the reaction, so the yield of EL has been lower. Therefore, 1:8 is an appropriate molar ratio of LA to ethanol.

110

Yield (%)

100

90

80

70 1:4

1:6

1:8

1:10

1:12

LA to Ethanol Molar Ratio

Fig.5 Effect of LA to ethanol molar ratio 3.3.3. Effect of reaction temperature

100

95

Yield (%)

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90

85

80 353

354

355

356

Reaction Temperature (K)

Fig.6 Effect of reaction temperature


357

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The temperature has influence on both reaction rate and yield. The esterification is an endothermic reaction. In order to ensure that the reaction is under the reflux condition, the selected temperature is slightly higher than the boiling point of ethanol. The reactions were carried out at temperature range from 353 K to 357 K. As shown in Fig.6, the yield of EL increases with increase of temperature. But as the temperature continues to rise, the yield of EL has been leveling off. Hence, 357 K was the optimal reaction temperature. 3.3.4. Effect of reaction time The reaction time was varied from 2 h to 6 h in the same experimental conditions over 20 wt.% STA/ SiO2 catalyst. As the experiments were performed at 357 K, the reaction time range (2-6 h) was chosen. As shown in Fig.7, the catalyst of the reaction was used for 6 h with reaction temperature of 357 K. The yield of 96.9% is the highest when the reaction has ran for 4 h, and then leveling off. So, 4 h was chosen as the best reaction time.

100

Yield (%)

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90

2

3

4

5

Reaction Time (h)

Fig.7 Effect of reaction time 4. Conclusions


6

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The synthesis of ethyl levulinate from levulinic acid with ethanol was systematically studied using solid acid catalyst, and EL yield of 97% was obtained using 20% silicotungstic acid modified commercially available silica-gel sphere as catalyst. Silicotungstic acid supported on commercially silica-gel sphere supports as catalyst may be used for the first time for the esterification of levulinic acid. This modified catalyst can be considered an efficient catalyst for the esterification of levulinic acid, providing an alternative of inorganic acid to synthesis ethyl levulinate. Acknowledgements Supported by National Natural Science Foundation of China (Grant No. 21406103), and Natural Science Foundation of Shandong Province (Grant No. ZR2015BM014). References [1] Huber, G.W.; Iborra, S.; Corma, A. Synthesis of transportation fuels from biomass: chemistry, catalysts, and engineering. Chem. Rev. 2006, 106, 4044-4098. [2] Roman-Leshkov, Y.; Barrett, C.J.; Liu, Z.Y.; Dumesic, J.A. Production of dimethylfuran for liquid fuels from biomass-derived carbohydrates. Nature. 2007, 447, 982-986. [3] Rackemann, D.W.; Doherty, W.O.S. The conversion of lignocellulosics to levulinic acid. Biofuel. Bioprod. Bior. 2011, 5, 198-214. [4] Bozell. J.J. Chemistry. Connecting biomass and petroleum processing with a chemical bridge. Science. 2010, 329 (5991), 522-523. [5] Fernandes, D.R.; Rocha, A.S.; Mai, E.F.; Mota, C.J.A.; Silva, V.T.D. Levulinic acid esterification with ethanol to ethyl levulinate production over solid acid catalysts. Appl. Catal. A: Gen. 2012, 425-426, 199-204.


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[6] Pasquale, G.; Vazquez, P.; Romanelli, G.; Baronetti, G. Catalytic upgrading of levulinic acid to ethyl levulinate using reusable silica-included Wells-Dawson heteropolyacid as catalyst. Catal. Commun. 2012, 18, 115-120. [7] Joshi, H.; Moser, B.R.; Toler, J.; Smith, W.F.; Walker, T. Ethyl levulinate: A potential biobased diluent for biodiesel which improves cold flow properties. Biomass Bioenergy. 2011, 35 (7), 3262-3266. [8] Liu, Y.J.; Lotero, E.; Goodwin Jr., J.G. A comparison of the esterification of acetic acid with methanol using heterogeneous versus homogeneous acid catalysis. J. Catal. 2006, 242(2), 278286. [9] Ronnback, R.; Salmi, T.; Vuori, A.; Haario, H.; Lehtonen, J.; Sundqvist, A.; Tirronen, E. Development of a kinetic model for the esterification of acetic acid with methanol in the presence of a homogeneous acid catalyst. Chem. Eng. Sci. 1997, 52(19), 3369-3381. [10] Lilja, J.; Murzin, D.Y.; Salmi, T.; Aumo, J.; Mäki-Arvela, P.; Sundell, M. Esterification of different acids over heterogeneous and homogeneous catalysts and correlation with the Taft equation. J. Mol. Catal. A: Chem. 2002, 182-183, 555-563. [11] Sambeth, J.E.; Romanelli, G.; Autino, J.C.; Thomas, H.J.; Baronetti, G.T. A theoreticalexperimental study of Wells-Dawson phospho-tungstic heteropolyacid: An explanation of the pseudoliquid or surface-type behavior. Appl. Catal. A: Gen. 2010, 378(1), 114-118. [12] Engin, A.; Haluk, H.; Gurkan, K. Production of lactic acid esters catalyzed by heteropoly acid supported over ion-exchange resins. Green Chem. 2003, 5, 460-466. [13] Fernandesa, D.R.; Rochaa, A.S.; Maia, E.F.; Motab, C.J.A.; Silva, V.T.D. Levulinic acid esterification with ethanol to ethyl levulinate production over solid acid catalysts. Appl. Catal. A: Gen. 2012, 425-426, 199-204.


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[14] Dharne, S.; Bokade, V.V. Esterification of levulinic acid to n-butyl levulinate over heteropolyacid supported on acid-treated clay. J. Nat. Gas Chem. 2011, 20, 18-24. [15] Pasquale, G.; Vázquez, P.; Romanelli, G.; Baronetti, G. Catalytic upgrading of levulinic acid to ethyl levulinate using reusable silica-included Wells-Dawson heteropolyacid as catalyst. Catal. Commun. 2012, 18, 115-120. [16] Song, D.Y.; An, S.; Sun, Y.N.; Guo, Y.H. Efficient conversion of levulinic acid or furfuryl alcohol into alkyl levulinates catalyzed by heteropoly acid and ZrO2 bifunctionalized organosilica nanotubes. J. Catal. 2016, 333, 184-199. [17] Song, D.Y.; An, S.; Lu, B. Guo, Y.H.; Leng, J.Y. Arylsulfonic acid functionalized hollow mesoporous carbon spheres for efficient conversion of levulinic acid or furfuryl alcohol to ethyl levulinate. Appl. Catal. B: Environ. 2015, 179, 445-457. [18] Nandiwale, K.Y.; Sonar, S.K.; Niphaskar, P.S. Catalytic upgrading of renewable levulinic acid to ethyl levulinate biodiesel using dodecatungstophosphoric acid supported on desilicated H-ZSM-5 as catalyst. Appl. Catal. A: Gen. 2013, 460-461, 90-98. [19] Richard, F.; Célérier, S.; Vilette, M.; Comparot, J.D.; Montouillout, V. Alkylation of thiophenic compounds over heteropoly acid H3PW12O40 supported on MgF2. Appl. Catal. B: Environ. 2014, 152-153, 241-249. [20] Yan, K.; Wu, G.S.; Wen, J.L.; Chen, A.C. One-step synthesis of mesoporous H4SiW12O40SiO2 catalysts for the production of methyl and ethyl levulinate biodiesel. Catal. Commun. 2013, 34, 58-63. [21] Jadhav, A.H.; Kim, H.; Hwang, I.T. An efficient and heterogeneous recyclable silicotungstic acid with modified acid sites as a catalyst for conversion of fructose and sucrose into 5hydroxymethylfurfural in superheated water. Bioresource Technol. 2013, 132, 342-35.


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[22] Talebian-Kiakalaieh, A.; Amin, N.A.S.; Zakaria, Z.Y. Gas phase selective conversion of glycerol to acrolein over supported silicotungstic acid catalyst. J. Ind. Eng. Chem. 2016, 34, 300312. [23] Li, K.X.; Hu, J.L.; Li, W.; Ma, F.Y.; Xu, L.L.; Guo, Y.H. Design of mesostructured H3PW12O40-silica materials with controllable ordered and disordered pore geometries and their application for the synthesis of diphenolic acid. J. Mater. Chem. 2009, 19, 8628-8638. [24] Guo, Y.H.; Li, K.X.; Yu, X.D.; Clark, J.H. Mesoporous H3PW12O40-silica composite: Efficient and reusable solid acid catalyst for the synthesis of diphenolic acid from levulinic acid. Appl. Catal. B: Environ. 2008, 81, 182-191.


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