Sulfonated imidazolium ionic liquid-catalyzed transesterification for biodiesel synthesis

Fuel 188 (2017) 483–488

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Sulfonated imidazolium ionic liquid-catalyzed transesterification for biodiesel synthesis Pei Fan a,b,c,d, Shiyou Xing a,b,c,d, Jiayan Wang a,f, Junying Fu a,b,d, Lingmei Yang a,b,d, Gaixiu Yang a,b,d, Changlin Miao a,b,d,⇑, Pengmei Lv a,b,d,e,⇑⇑ a

Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China Key Laboratory of Renewable Energy, Chinese Academy of Sciences, Guangzhou 510640, China University of Chinese Academy of Sciences, Beijing 100049, China d Guangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China e Collaborative Innovation Center of Biomass Energy, Henan Province, Zhengzhou 450002, China f Nano Science and Technology Institute, University of Science & Technology of China, Suzhou 215123, China b c

a r t i c l e

i n f o

Article history: Received 2 September 2016 Received in revised form 9 October 2016 Accepted 12 October 2016 Available online 17 October 2016 Keywords: Ionic liquid Biodiesel Transesterification Sulfonate

a b s t r a c t Four kinds of imidazolium ionic liquids (ILs) were employed to catalyze the transesterification reaction of rapeseed oil. The effects of molar ratio of methanol to rapeseed oil, catalyst dosage, reaction temperature, and reaction time, and the deactivation of water on catalytic activity were explored. The results showed that imidazolium ILs with long alkyl chains and sulfonated groups exhibited the best catalytic activities due to their strong Brønsted acidity. The catalytic activity was depend on the ASO3H group in the cation, not the anion HSO4 . Water molecules competed with the anion to bind with the protons of the imidazolium cation. This results in the disruption of the structure of ILs, leading to deactivation; increasing the reaction temperature could alleviate this negative effect of water. The yield of fatty acid methyl ester (FAME) remained constant (85%) at 130 °C, when the water content increased from 1 wt% to 5 wt%. The highest yield of FAME for the catalyst 1-butylsulfonate-3-methyl imidazolium hydrogen sulfate ([BSO3HMIM][HSO4]) could reach 100% under optimum conditions. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Biodiesel is a form of clean renewable energy and considered an excellent substitute for diesel fuel, derived from vegetable oils and animal fats. In its most common formulation, biodiesel is chemically synthesized by reacting oils such as rapeseed oil, and soybean oil with methanol in the presence of acid or base catalysts. The transesterification process converts triglycerides into the fatty acid methyl ester (FAME) that forms the biodiesel and the by-product glycerol. This transesterification process has so far employed different types of catalysts, including homogeneous and heterogeneous acids or bases like sulfonic acid, potassium hydroxide, sodium hydroxide, or their alkoxides [1]. However, these catalysts have several drawbacks: they are corrosive, cause saponification of fatty acids, and produce high quantities of waste [2], leading to the ⇑ Corresponding author at: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China. ⇑⇑ Corresponding author at: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China. E-mail addresses: [email protected] (C. Miao), [email protected] (P. Lv). http://dx.doi.org/10.1016/j.fuel.2016.10.068 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.

release of environment-unfriendly effluents. Lipases are environment-friendly biocatalysts; however, methanol and the by-product glycerol can partially deactivate the lipase and the enzyme stability is poor [3]. Ionic liquids (ILs) have attracted much interest as relatively clean and promising catalysts and alternative solvents [4–6] due to their wide liquid range, high catalytic activity and thermal stability, design possibilities, tunable physical properties, etc. They have gained tremendous attention as a potential and effective class of catalysts for preparing biodiesel. The role of ILs in preparing biodiesel has been studied extensively [4–7]. ILs have been explored as solvents for the enzymatic methanolysis of triglycerides [8], as hydrophobic additives [9] for the immobilization of enzyme, as catalyst support and co-catalyst (such as modified zeolites) [10], or mixed with heteropolyacids [11] to form new catalytic systems. Further, Brønsted acidic and alkaline IL [12–19] catalysts have been reported for the preparation of biodiesel. While, whether the cation or anion responsible for the catalytic activity was rarely reported. And IL are somewhat hygroscopic [20,21] but acid-catalyzed transesterification is sensitive to water concentration [22], so it is

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meaningful to research the effect of water on the catalytic activity during the reaction. In this paper, ILs including 1-propyl-3-methyl imidazolium hydrogen sulfate ([PrMIM][HSO4]) (a), 1-propylsulfonate-3methyl imidazolium hydrogen sulfate ([PrSO3HMIM][HSO4]) (b), 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIM][HSO4]) (c), and 1-butylsulfonate-3-methyl imidazolium hydrogen sulfate ([BSO3HMIM][HSO4]) (d) (Fig. 1), were studied as catalysts for preparing biodiesel from rapeseed oil and methanol; the effect of water on these catalysts was also explored. The FAMEs were analyzed by gas chromatography with an internal standard. Moreover, different reaction conditions that might influence the yield of FAMEs, including reaction temperature, reaction time, molar ratio of substrates, and catalyst dosage, were explored in this experiment. 2. Experimental detail 2.1. Materials and reagents The ILs a, b, c, and d were purchased from the Shanghai Chengjie chemical company and rapeseed oil was obtained from a supermarket in Guangzhou. The reference standards—all of GC grade (>99.0%)—were obtained from Sigma-Aldrich. The methyl ester standards included methyl myristate, methyl palmitate, methyl oleate, methyl linoleate, methyl linolenate, methyl cis-11-eicosenoate, methyl cis-13-docosenoate. Methanol (P98%) was purchased from Tianjin Fuyu Fine Co. All chemicals were used as received without pre-treatment. 2.2. Experiments The reaction was performed in a thick-walled pressure vessel equipped with a magnetic stirrer in an oil bath. Agitation sufficient to overcome mass transfer limitations during biodiesel production was applied for all experiments. The operating parameters designated for the IL-catalyzed process include a temperature range of 90–140 °C, six levels of catalyst concentrations 0.5–3.0 wt% (relative to the weight of rapeseed oil), molar ratios of methanol to rapeseed oil in the range 3:1–18:1, and reaction time in the range of 2–5 h. For each experiment, 5 g of rapeseed oil was used as the starting material. After the reaction mixture was cooled to room temperature, the upper layer was separated by decantation, washed with water to remove the residuals, and then distilled under vacuum to eliminate excess methanol. The FAME was analyzed by gas chromatography. 2.3. Analysis of biodiesel The FAME was analyzed using a Shimadzu Gas Chromatograph (GC-2010) equipped with the AOC-20i automatic injection port and a flame ionization detector (FID). The capillary column was a DB-WAX (30 m  0.25 mm  0.25 lm); methyl heptadecanoate

a

c

b

d

Fig. 1. The structures of the ILs used in this study.

was used as the internal standard. Thermogravimetric analysis (TGA) was performed using a TGA Q50 V20.13 Build 39. The samples (10 mg) were placed in aluminum pans and run at a rate of 10 K min 1 to 800 °C under nitrogen gas at a flow rate of 20 mL min 1. FT-IR transmission spectra in the frequency range of 350–4000 cm 1 were recorded on a TENSOR27 FT-IR spectrometer. IL samples were prepared as KBr disks. Acidities of the ILs were analyzed by METTLER TOLEDO pH instruments. 3. Results and discussion 3.1. TG analyses of ILs The decomposition temperatures of the ionic liquids were determined by TGA. According to the weight loss curve of the ionic liquid in Fig. 2, it can be seen that the thermal decomposition temperatures of the four catalysts were much higher than temperatures in this experiment. The onset temperatures for ILs a, b, c, and d were 224 °C, 355 °C, 211 °C, and 355 °C, respectively; the respective final temperatures were 324 °C, 441 °C, 350 °C, and 441 °C. The weight loss curves also showed the ionic liquids possessed high thermal stabilities and wide liquid ranges. From Fig. 2 we can see that the ILs are stable at the temperatures of this experiment. 3.2. Acidity analyses of ILs In this study, the Brønsted acidities of the ILs were compared and rationalized by electron induction theory. For a and c, the positive charge on the electron-withdrawing imidazole ring is stabilized by the electron donating capability of the alkyl chain that increases with increasing length of the carbon chain. Thus, the anion HSO4 in c is difficult to dissociate H+, leading to weak acidity. For b and d, which have sulfonate-bearing alkyl chains, the spatial effect is dominant. The Coulomb force between the cation and the anion becomes weak due to the larger volume of the cation when the carbon chain is longer; therefore, it is easier for the ASO3H moiety to lose H+. Based on the above analysis, the Brønsted acidities of the ILs are in the sequence: d > b > a > c. This is verified by the pH data in Table 1. 3.3. Effects on the yield of FAME 3.3.1. Effect of reaction temperature Reaction temperature is one of the major factors affecting the yield of FAME. The effect of temperature on the yield of FAME is depicted in Fig. 3. The reaction temperature was varied from 90 to 140 °C in six experiments, keeping other parameters constant (catalyst concentration = 10 wt%, molar ratio of methanol to rapeseed oil = 10:1, and reaction time = 5 h), the catalysts showed different catalytic activities. For ILs a and c, their maximum conversion during the transesterification process were not more than >20% (19.74% and 8.89%, respectively); ILs b and d showed good catalytic activity and displayed the same trends in the conversion of FAME, which increased as the temperature. This is similar to previous reports [23], which stated that ILs with sulfonated longer carbon chains tend to deprotonate more easily, giving rise to stronger Brønsted acidity. Obviously, the catalytic activity was connected to the ASO3H moiety on the cation and not the anion HSO4 , although a and c have strong acidities similar to HCl (in Table 1). Possibly, the H+ in the HSO4 anion is mainly used to maintain the charge neutrality of the IL; therefore, a and c showed poor catalytic activities during transesterification. Fig. 3 shows that the FAME yield increased from <40% to >90% when the temperature was increased from 90 °C to 120 °C, and up to a maximum of

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10

5.5

A

d

)

b

Deriv.Weight Change(%/

Weight/mg

8

C

6

B

5.0

a 4

2

4.5

d

4.0 3.5 3.0

c

2.5 2.0

b

1.5 1.0 0.5

a

0.0

0 0

100

200

300

400

500

600

700

800

0

temperature/

100

200

300

400

500

600

700

800

temperature/ Fig. 2. (A) TGA (B) DTA curve for ILs.

Table 1 The pH of ILs used.

pH

HCl

H2SO4

a

b

c

d

1.70

1.48

1.71

1.62

1.86

1.52

Note: the concentration of all measured solutions was 0.02 mol/L in water.

the efficiency of transesterification was investigated in detail. As shown in Fig. 4, the FAME content increased with increasing catalyst dosage from 0.5 wt% to 3 wt%. Both the catalysts b and d showed high catalytic activities to afford FAME in yields up to 89.48% and 93.4% at a catalyst loading of 0.5 wt%. They showed obvious advantages over the other ILs [24–26] due to their ASO3H functionalization. The catalytic activities of ILs with an alkane sulfonic acid group were close to that obtained with H2SO4 [23]; the acidities of these ILs (Table 1) are also similar to that of H2SO4. Concentrated H2SO4 is corrosive. Further, ILs as catalysts exhibit many advantages such as high catalytic activity, excellent stability and environmental benefits.

110 100 90

yield of FAME %

80 70 60

b d a c

50 40 30 20 10 0 90

100

110

120

130

140

110

reaction temperature/

100 Fig. 3. Effect of reaction temperature on the yield of FAME; reaction conditions: n (methanol):n(oil) = 10:1, catalyst dosage 10 wt%, 5 h.

90

130 °C. It is clear that higher temperatures allowed the system to exceed the activation energy required for the reaction, leading to increased conversion rate. At 140 °C, the FAME yield decreased because the methanol in the liquid-phase reaction system was rare. Therefore, 130 °C was chosen as the optimal reaction temperature for future experiments; this is lower than that in other reports [23,12,24]. Under the optimum reaction temperature, catalyst d showed the maximum conversion of 100%, followed by 94.91% conversion afforded by catalyst b. It could be concluded that the sulfonate-bearing alkyl chain showed improved catalytic performance. 3.3.2. Effect of catalyst dosage In general, the amount of catalyst has a significant effect on the catalytic performance. Therefore, the effect of catalyst dosage on

yield of FAME %

80 70 60 50

b d

40 30 20 10 0 0.5

1.0

1.5

2.0

2.5

3.0

catalyst dosage wt% Fig. 4. Effect of catalyst dosage on yield of FAME; reaction conditions: n(methanol): n(oil) = 10:1, 5 h, 130 °C.

P. Fan et al. / Fuel 188 (2017) 483–488

3.3.3. Effect of the molar ratio of methanol to oil The stoichiometry of the transesterification reaction is such that 3 mol of methanol reacts with 1 mol of the triglycerides present in rapeseed oil to give 3 mol of FAME. The stoichiometric ratio of methanol to rapeseed oil of 3:1 provided the lowest conversions of <70% (Fig. 5). The yield of FAME steeply increases when the ratio was increased from 3:1 to 12:1. This is due to the excess methanol shifting the reaction equilibrium towards the products’ side and producing more FAME. The yield of FAME both achieved the highest when ratio was 12:1 and it kept constant as ratio increased from 12:1 to 18:1 which could be explained that the transesterification reaction was at equilibrium state. 3.3.4. Effect of reaction time Fig. 6 shows the distinct increasing trend in biodiesel yield with increasing reaction time. Over 60% of FAME was obtained within 2 h while the highest yield (100%) was achieved after 3 h. When the reaction time was increased from 4 h to 5 h, the yield of FAME remained almost unchanged. Hence, 3 h could be the optimal reaction time for the reaction. In this set of experiments, catalysts with long alkyl chains showed significant advantage. FAME conversion was 88.73% for catalyst d (59.32% for catalyst b) after 2 h reaction and achieved 100% conversion 2 h earlier than catalyst b. This shows that reaction time is an important feature in the transesterification process. 3.4. Effect of water on catalytic activity at different temperatures It is known that the water in feedstock may easily cause deactivation. Acid-catalyzed transesterification is sensitive to water concentration [27] because water favors the formation of free fatty acids (FFAs) by hydrolysis of triglycerides [28]. The side reaction decreased the activity of the catalyst and consequently the yield of FAME. On the other hand, water molecules can form waterrich clusters around H+ [27], decreasing the activity of the catalysts; therefore, the yield of FAME significantly decreased when water was added [22,27] in the acid-catalyzed method. Water content should be kept under 0.5 wt% to achieve ester yield higher than 90% [29]. The effect of water on catalytic activities of ILs is unknown even though they were reported to be hygroscopic [20,21]. Fig. 7 shows that the yield of FAME decreased when 1% water was added to the reaction system. At 90 °C and 110 °C, IL d showed poor catalytic activity when the water content increased

110 100 90 80

yield of FAME%

486

70 60 50 40 30

b d

20 10 0 1

2

3

4

5

6

reaction time/h Fig. 6. Effect of reaction time on biodiesel yield; reaction conditions: catalysts dosage 2 wt%, n(methanol):n(oil) = 12:1, 130 °C.

to 5%. In Fig. 8, the bands at 1004 cm 1and 1170 cm 1 are assigned to the ASO3H group, and the band at 1575 cm 1 to C@N ring stretching vibrations of imidazole; these bands, along with other special bands for IL d disappeared after adding water. It might be explained that the hydrogen bonding between the [BSO3HMIM]+ cation and the HSO4 anion [30] was destroyed by water molecules that compete with the anion to bind with hydrogen atoms of imidazolium cation [31] (Fig. 9). This results in the disruption of the structure of the ILs leading to deactivation of IL d. At 130 °C, the yield of FAME is nearly unchanged (about 85%) when the water content is increased from 1 wt% to 5 wt%, probably due to the water evaporated from the reaction system at 130 °C. Hence, increasing the reaction temperature could alleviate the negative effect of water. 3.5. Mechanism of acid-catalyzed transesterification reaction The Brønsted acid ILs catalyzed the conversion of triglyceride and methanol to FAME and glycerol. H+ from the IL catalyst attacks the triglyceride; the protonated triglyceride reacts with methanol

110 100 90

100

70

80

60

yield of FAME%

yield of FAME%

80

50 40 30

b d

20 10

60

90 110 130

40

20

0

0 3

6

9

12

15

18

molar ratio of methanol to oil

0

1

2

3

4

5

water content wt% Fig. 5. Effect of molar ratio of methanol to oil on the yield of FAME; reaction conditions: catalysts dosage 2 wt%, 5 h, 130 °C.

Fig. 7. Influence of water content on transesterification performance of IL d.

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2.0

1.5

d -1

1568cm C=N str.

Absorbance

-1

1004cm S=O str.

-1

1170cm SO3H str.

1.0

OH str.

watered d

0.5

0.0

500

1000

1500

2000

2500

wavenumber/cm

3000

3500

4000

-1

Fig. 8. IR spectra of IL d. d with water and then dried to constant weight at 80 °C.

4. Conclusion

Fig. 9. Water destroys the structure of IL d.

to form a tetrahedral intermediate that decomposes to form a diglyceride [32]. The protonated fatty acid methyl ester deprotonates to the product FAME and H+ [33], which is captured by the diglyceride and begins the second catalytic cycle, generating FAME and glycerol (Fig. 10).

Brønsted-acid ionic liquid catalysts were evaluated in terms of yield of biodiesel from rapeseed oil. From various studies (reaction temperature, dosage of the ILs, molar ratio of methanol to rapeseed oil, reaction time, and the influence of water content on catalyst), the optimal parameters were established. The ILs a, b, c, and d had strong acidities, similar to that of concentrated H2SO4. The ASO3H-functionalized ILs showed improved catalytic performance compared to ILs without any sulfonate group; thus, the good catalytic activity could be attributed to ASO3H moiety in the cation. The highest yield of FAME was achieved under mild reaction conditions: reaction temperature = 130 °C, 2 wt% [BSO3HMIM][HSO4], methanol-to-rapeseed oil molar ratio = 12:1, and reaction time = 3 h. Water in the feedstock could destroy the structure of ILs leading to deactivation; increasing reaction temperature could alleviate this negative effect. The yield of FAME is constant at 85% even when the water content increased from 1 wt% to 5 wt % at 130 °C. This work showed that ILs have great potential for preparing biodiesel.

Fig. 10. Mechanism of acid-catalyzed transesterification process.

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Acknowledgement The work was supported by National Natural Science Foundation of China (No. 21506217), National Natural Science Foundation of China (No. 21576260), Natural Science Foundation of Guangdong Province (No. 2016A030308004), Natural Science Foundation of Guangdong Province (No. 2015A030313720) and Special Funds of Applied Science and Technology Research of Guangdong Province (No. 2015B020241002). References [1] Wilson K, Lee AF. Rational design of heterogeneous catalysts for biodiesel synthesis. Catal Sci Technol 2012;2:884–97. [2] Wilson K. The application of calcined natural dolomitic rock as a solid base catalyst in triglyceride transesterification for biodiesel synthesis. Green Chem 2008;10:654–9. [3] De Paola MG, Ricca E, Calabro V, Curcio S, Iorio G. Factor analysis of transesterification reaction of waste oil for biodiesel production. Bioresour Technol 2009;100:5126–31. [4] Rocha LAAJD. Use of ionic liquids in biodiesel production: a review. Braz J Chem Eng 2012;29:1–13. [5] Qureshi ZS, Deshmukh KM, Bhanage BM. Applications of ionic liquids in organic synthesis and catalysis. Clean Technol Environ Policy 2013;16:1487–513. [6] Zhao H, Baker GA. Ionic liquids and deep eutectic solvents for biodiesel synthesis: a review. J Chem Technol Biotechnol 2013;88:3–12. [7] Xie H, Zhao ZK. Biofuel production with ionic liquids. Biofuels Biorefineries 2014;1:171–93. [8] Zhao H, Song Z, Olubajo O, Cowins JV. New ether-functionalized ionic liquids for lipase-catalyzed synthesis of biodiesel. Appl Biochem Biotechnol 2010;162:13–23. [9] Cabrera-Padilla RY, Lisboa MC, Pereira MM, Figueiredo RT, Franceschi E, Fricks AT, et al. Immobilization of Candida rugosa lipase onto an eco-friendly support in the presence of ionic liquid. Bioprocess Biosyst Eng 2015;38:805–14. [10] Salminen E, Kumar N, Virtanen P, Tenho M, Maki-Arvela P, Mikkola J-P. Etherification of 5-hydroxymethylfurfural to a biodiesel component over ionic liquid modified zeolites. Top Catal 2013;56:765–9. [11] Wu J, Gao Y, Zhang W, Tan Y, Tang A, Men Y, et al. Esterification of cooking oil for biodiesel production using composites Cs2.5H0.5PW12O40/ionic liquids catalysts. Appl Petrochem Res 2014;4:305–12. [12] Han M, Yi W, Wu Q, Liu Y, Hong Y, Wang D. Preparation of biodiesel from waste oils catalyzed by a Bronsted acidic ionic liquid. Bioresour Technol 2009;100:2308–10. [13] Tong-mei Z. Synthesis of biodiesel through catalyzed by a novel transesterification alkaline ionic liquid. J Fuel Chem Technol 2014;42:200–6. [14] Jinhua L. Preparation of biodiesel by transesterification from cottonseed oil using the basic dication ionic liquid as catalysts. J Fuel Chem Technol 2010;38:275–80. [15] Zhang A. Synthesis of biodiesel using new basic ionic liquid as catalyst. Appl Chem Ind 2009;38:167–77.

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