1、BIOTECHNOLOGY AND BIOENGINEERING Chinese Journal of Chemical Engineering, 18(2) 322 327 (2010) Application of Choline ChloridexZnCl2Ionic Liquids for Preparation of Biodiesel*LONG Tao (龙涛)1,2, DENG Yuefeng (邓岳锋)2, GAN Shucai (甘树才)1and CHEN Ji (陈继)2,*1College of Chemistry, Jilin University, Changchun
2、 130026, China 2State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun 130022, China Abstract The inexpensive and moisture-stable Lewis-acidic ionic liquids were prepared and applied for trans-esterification of soybean
3、 oil to biodiesel. The influences of molar ratio of methanol to soybean oil, reaction tem-perature and amount of ionic liquids were investigated. The transesterification of soybean oil to biodiesel catalyzed by choline chloridexZnCl2ionic liquids showed many advantages such as mild conditions and lo
4、wer cost. On the other hand, the non-ideal yield and complicated separation between biodiesel and soybean oil were also investigated and analyzed. The improvement on the systems of choline chloridexZnCl2was proposed for further investigation. Keywords ionic liquids, biodiesel, transesterification, L
5、ewis acid 1 INTRODUCTION In recent years, room-temperature ionic liquids (ILs) have attracted much attention for synthetic and catalytic application because of their important attrib-utes such as wide liquid range, negligible vapor pres-sure, high catalytic activity, excellent chemical and thermal s
6、tabilities, potential recoverability, design possibilities, and ease of separation of the products from reactants 1. ILs, up to now investigated, can be broadly divided into two types: one based on chloro-metallate anions such as 27Al Cland the other on non-metal-containing anions such as 6PF. The L
7、ewis acidity of the ILs that composed of imidazole cation and chlorometallate anion increased in the order: CuCl25Zn Cl 3ZnClaccording to the slight band shifts from 1449 to 1450 and 1451 cm1. Therefore, the acidity in-creases with increasing the mole ratios of ZnCl2to choline chloride from 1 to 3.
8、This is in agreement with those catalytic activities obtained in transesterification of soybean oil to biodiesel (Fig. 3). The band of 1537cm1in Fig. 2 (d) indicates the presence of Brn-sted acidic sites due to hydrolysis of part anion by the absorbed water mentioned above, which is consistent with
9、C4mimCl/AlCl322. However, the hydrolysis is not reported in all the metal chloride systems because of different experimental conditions such as time, temperature and pH, and thus the explanation couldnt be available in the similar study. Figure 2 FT-IR spectra of (a) pure pyridine, (b) pyridine + ch
10、oline chlorideZnCl2, (c) pyridine + choline chloride2ZnCl2, and (d) pyridine + choline chloride3ZnCl2(pyridine/IL = 1 1 by molar ratios in b-d) (a) Choline chloride3ZnCl2(b) Choline chloride2ZnCl2(c) Choline chlorideZnCl2Figure 3 TG curve of choline chlorideZnCl2, choline chloride2ZnCl2and Choline c
11、hloride3ZnCl23.1.4 Thermal stability and viscosity of choline chloridexZnCl2The thermogravimetric analysis is illustrated in Fig. 3. The decomposition temperatures of choline chlorideZnCl2, choline chloride2ZnCl2and choline chloride3ZnCl2are 320.01 C, 322.82 C and 328.89 C, respectively. There is a
12、mass loss on the TG curve near 100 C which results from the evaporation of water. Therefore, choline chloridexZnCl2exhibit high thermal stabilities. The freezing points of choline chloridexZnCl2vary between 65 C (1 1), 25 C (1 2) and 45 C (1 3) 10. Therefore, choline chloride2ZnCl2is liq-uid at room
13、 temperature while choline chlorideZnCl2and choline chloride3ZnCl2are solid. The viscosity of choline chloride2ZnCl2is determined to be 281 Pas at 298 K, close to the calculated viscosity value about 242 Pas at 298 K 21. 3.2 Preparation of biodiesel and analysis 3.2.1 Effect of different molar ratio
14、s of choline chlo-ride to ZnCl2The reaction was conducted while the molar ratio of methanol to soybean oil is 16 1 at 70 C for 24 h, 48 h, 72 h and 84 h, respectively (see Fig. 4). It is found that choline chloridexZnCl2efficiently pro-moted the transesterification. The transesterification is promot
15、ed by the Lewis acidic species 3ZnCl, 25Zn Cl, Chin. J. Chem. Eng., Vol. 18, No. 2, April 2010 325and 37Zn Clin the catalysts. The yield of biodiesel is slightly enhanced with increasing x from 1 to 3. The conversion of soybean oil increases markedly with time, but there is no significant enhancemen
16、t after 72 h. Hence, 72 h is chosen as the optimized reaction time. Choline chloride2ZnCl2is chosen as a typical exam-ple of catalyst. 3.2.2 Effect of the amount of choline chloride2ZnCl2A series of experiments were carried out using the choline chloride2ZnCl2with different dosages (see Fig. 5). The
17、 amount of choline chloride2ZnCl2is denoted by the IL/oil mass ratio. The results show that with an increase in the relative amount of choline chloride2ZnCl2, the rate of transesterification reaction is obviously enhanced before 10% and then decreased. The highest conversion is achieved at 10% choli
18、ne chloride2ZnCl2. It is very likely that the amount of choline chloride2ZnCl2higher than 10% would result in a decrease in the catalytic activity and the reason needs to be further investigated. Figure 5 Effect of choline chloride2ZnCl2amount on the transesterification reaction IL, n(methanol) n(oi
19、l) = 16 1, 70 C 24 h; 48 h; 72 h 3.2.3 Effect of the molar ratio of methanol to soy-bean oil It is shown that the biodiesel content rapidly in-creases with increasing molar ratio of methanol to oil (see Fig. 6). Since biodiesel production by transesteri-fication is a reversible reaction, the product
20、ion yield could be elevated by introducing excess amount of the reactant methanol to change the equilibrium. When the ratio is less than 16, the molar ratio of methanol to oil has a significant effect on the catalytic activity. When methanol is further increased, the concentration of catalyst is dil
21、uted at a fixed amount of choline chloride2ZnCl2and soybean oil, and the amount of methanol has a slight effect on the catalytic perform-ance after 16. Moreover, a higher molar ratio of methanol to oil will cause the separation problem during recycling. Therefore, the optimal molar ratio of methanol
22、 to soybean oil of 16 is preferable. 3.2.4 Effect of the reaction temperature In general, the reaction temperature can influence the reaction rate and biodiesel yield. In present work, the reaction temperature varies within a range from 50 to 90 C. The experimental results are shown in Fig. 7. The b
23、iodiesel yield increases with increasing tem-perature every 10 C from 50 to 90 C. The increasing rates are 13.21%, 7.16%, 2.84% and 4.38%, respec-tively. The temperature intervals of the increase are equal, but the correspondingly increased biodiesel content is reduced gradually. These results show
24、that the influence of reaction temperature on the trans-esterification reaction becomes smaller with an in-crease in temperature. Furthermore, the reaction tem-perature consumedly exceeds the boiling point of methanol such as 80 and 90C, and the methanol will quickly vaporize and form a large number
25、 of bubbles, which inhibits the reaction on the two-phase interface. Moreover, in order to save energy, it is necessary to choose the relative low temperature. Therefore, the Figure 4 Effect of different molar ratios of choline chlo-ride to ZnCl2catalyst 10%, n(methanol) n(oil) = 16 1, 70 C 1 1; 1 2
26、; 1 3 Figure 6 Effect of methanol/soybean oil molar ratio on thetransesterification reaction (IL 10%, 70 C) 24 h; 48 h; 72 h Figure 7 Effect of reaction temperature on the transesteri-fication reaction n(methanol) n(oil) =16 1, IL 10% 24 h; 48 h; 72 h Chin. J. Chem. Eng., Vol. 18, No. 2, April 2010
27、326optimum reaction temperature for the transesterifica-tion of soybean oil to biodiesel is considered to be around 70C. 3.2.5 Effect of moisture The parallel experiment was carried out with and without sealed nitrogen with 16 1 molar ratio, 70 C and 72 h. The conversations are 54.52% and 56.14% wit
28、hout and with nitrogen protection, respectively. Therefore, choline chloride2ZnCl2is insensitive to moisture. 3.3 Problems and analyses The conversion rate of transesterification using choline chloride2ZnCl2as catalyst under molar ratio of methanol to oil of 16 1 with addition of 10% catalyst at 70
29、C for 72 h is 54.52%. The reaction mechanism of transesterification is shown in Scheme 2. 25Zn Clis the predominant species in IL so that the transesterification is mainly catalyzed by 25Zn Cl. The non-ideal yield of biodiesel predominately attributes to the weak acidity of 25Zn Cl. Therefore, the m
30、ain method increasing the conversion is to improve the acidity of catalyst. The recycling utilization of choline chloride2ZnCl2plays an important role in preparation procedure. Choline chloride2ZnCl2exists in methanol phase with glycerol. Glycerol was separated from ILs by vacuum distillation in the
31、 reported literatures 23-27. However, the high boiling point and high vis-cosity of glycerol make the separation between ILs and glycerol difficult. Furthermore, the requirement for equipment is high in vacuum distillation, which has not been realized in industry. Therefore, the sepa-ration between
32、glycerol and ILs may become the bot-tleneck problem. Thus, the ILs immobilized on some solid materials such as resin to catalyze transesterifi-cation to biodiesel or purification of ILs by solvent extraction are proposed, which will be further studied. 4 CONCLUSIONS Choline chloridexZnCl2were applie
33、d as Lewis acidic catalysts for transesterification of soybean oil, which extended their application. FT-IR investigation demonstrated that Lewis acid strength of ILs increased with the increase of ZnCl2, which was in agreement with the activities observed in the preparation of bio-diesel. Because o
34、f the weak acidity of catalyst, the yield of biodiesel was lower than other ILs. Choline chloridexZnCl2is a new system with many excellent advantages such as simple preparation and low prices. The preparation process of biodiesel using choline chloridexZnCl2as Lewis acidic cata-lysts is effective. R
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