Novel Efficient Procedure for Biodiesel Synthesis from Waste Oils

Apr 12, 2013 - Biodiesel is well-known as a renewable replacement for traditional mineral diesel fuel.(1, 2) Biodiesel has properties similar to those...
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Novel Efficient Procedure for Biodiesel Synthesis from Waste Oils Using Solid Acidic Ionic Liquid Polymer As the Catalyst Xuezheng Liang* Institute of Applied Chemistry, Shaoxing University, Shaoxing 312000, China ABSTRACT: A novel solid acidic ionic liquid polymer (PIL) has been synthesized through the copolymerization of acidic ionic liquid oligomers with divinylbenzene (DVB). An efficient procedure for biodiesel synthesis from waste oils with a high free fatty acid (FFA) content was developed. PIL could efficiently catalyze both the transesterification of triglycerides and the esterification of FFAs with a total yield of >99.0%. The catalytic activities were quite high, with just one step to complete the reactions simultaneously under very mild conditions. The high activities are due to the high hydrophobic surface area, high acidity, and high stability of PIL.

1. INTRODUCTION Biodiesel is well-known as a renewable replacement for traditional mineral diesel fuel.1,2 Biodiesel has properties similar to those of diesel engine fuels and thus can be used in compression-ignition (diesel) engines with little or no modification.3 Its use involves lower emissions of SOx, CO, unburned hydrocarbons, and particulate matter during combustion relative to use of fossil fuels.4,5 Biodiesel is generally produced by the transesterification of vegetable oils with short-chain alcohols. However, the high costs and limited availability of biodiesel feedstocks are critical issues in this industry. The cost of vegetable oils can be up to 75% of the total manufacturing costs, which makes biodiesel production costs approximately 1.5 times higher than those for fossil diesel.6 For this reason, the use of waste vegetable oils can be an effective way of reducing production costs, because waste oils are 2−3 times cheaper than virgin vegetable oils.7 Therefore, waste oils, such as used frying oil, trap grease, and soapstock (a byproduct of vegetable oil refining) that are available cheaply can be considered as feedstocks for biodiesel.8−11 These waste oils often contain significant quantities of free fatty acids (FFAs) and water, which make them unsuitable for homogeneous alkali-catalyzed transesterification processes. Generally, the problem can be circumvented by the esterification pretreatment of the free fatty acids to alkyl esters in the presence of an acid catalyst.12 Homogeneous acid catalysts, such as sulfuric acid and p-toluenesulfonic acid, cannot be reused, and they have other disadvantages such as equipment corrosion, increased byproducts, tedious workup procedures, and environmental problems.13 The use of heterogeneous acid catalysts has been considered to eliminate the problems associated with homogeneous ones. Ionic liquids (ILs) have received extensive attention because of their special properties such as negligible volatility, high conductivity, wide electrochemical window, and strong dissolution ability, and they have been widely used in various areas.14 Many efficient procedures for chemical synthesis using ILs as reaction media or catalysts have been reported.15−18 Ionic liquids functionalized with sulfonic acid groups are one of the most useful types of functional ILs, because they are very © 2013 American Chemical Society

effective for many acid-catalyzed reactions and have activities comparable to those of homogeneous catalysts such as sulfuric acid.19−21 Various acidic ILs have been used for biodiesel synthesis from waste oils. FFAs from soapstock have been used as raw materials with ILs such as [NMP][CH3SO3], [SO3H (CH2)3VPy]HSO4, and dicationic ionic liquids as catalysts in esterification reactions.22−24 These ILs were found to have high activities, with average yields of 96%. However, the byproduct water affected the reaction equilibrium, and harsh reaction conditions were needed to improve the yields.25,26 Base catalysts are not suitable for these waste oils with high FFA contents. Two steps including acid-catalyzed pre-esterification and base-catalyzed transesterification were developed.27,28 However, the processing costs of using both steps would add to the production costs. To simplify the reaction procedure, acid-catalyzed one-step synthetic processes have been studied.29 However, high temperatures (>180 °C) were needed to obtain high activities, resulting in an energy-consuming and expensive process. Furthermore, the total yields were about 96%, which is still not very high. On the other hand, reactions catalyzed by heteropoly acids at low temperature obtained yields of less than 88.6%.30 The ILs also suffered some drawbacks such as limited solubility with organic compounds, especially polar molecules, which not only caused catalyst loss but also added purification difficulties. In addition, the high viscosity of the ILs increased the mass-transfer resistance and limited their industrial application. Many attempts have been made to solve these problems. The immobilization of ILs is a good choice. Various supports and linkages have been employed for this purpose. Porous silica has a high surface area and is rich in hydroxyl groups, thus providing an appropriate support for the anchoring of ionic liquids. Acidic ILs have been successfully immobilized on silica and found to exhibit high activities for many reactions such as the acetalization of carbonyl compounds and the hydrolysis of Received: Revised: Accepted: Published: 6894

December 21, 2012 April 8, 2013 April 12, 2013 April 12, 2013 dx.doi.org/10.1021/ie303564b | Ind. Eng. Chem. Res. 2013, 52, 6894−6900

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cellulose.31,32 Polystyrene (PS) has aromatic rings and double bonds and has also been used as a support. ILs immobilized on PS by certain coupling agents were found to have high activities for the expeditious synthesis of homoallylic alcohols and for esterifications.33,34 However, the expensive and toxic reagents used for immobilization further add to catalyst costs. Furthermore, the stability of these supported ILs should be investigated, and the catalytic activities were found to drop quickly upon recycling. The acid sites on the support surface were easily removed from the surface and reduced the activities. In this work, a novel solid acidic polymeric ionic liquid prepared from a Brønsted acidic ionic liquid and divinylbenzene (DVB) is presented (Scheme 1). DVB was directly

Monomer (3.15g, 10 mmol), ethanol ( 20 mL), and azobisisobutyronitrile (AIBN; 3.2 wt ‰ based on monomer) were mixed together to form a solution. After this solution had been stirred at 70 °C for 4 h, DVB (1.30 g, 10 mmol) and AIBN (3.2 wt ‰ based on DVB) were added to the mixture, and the mixture was stirred for another 4 h. Then, the mixture was left to stand for 12 h at 80 °C to form a white organic gel. A solid monolith was obtained after the evaporation of ethanol solvent at room temperature overnight and ground into a powder. The solid was washed with hot acetone and water until no acidity was detected in the filtrate. The novel solid acidic ionic liquid polymer was obtained after this solid had been dried in an oven at 120 °C overnight with a solid yield of >99%, which indicates that all of the IL was immobilized by crosslinking. 2.2. Procedure for Biodiesel Synthesis. Cooking oil that had been used for frying was employed as the raw material. The oil was obtained directly from a restaurant. Dehydration under reduced temperature and decolorization with active carbon were applied to remove the water and solid residues. The acidity of the waste oil was 45 mg of KOH/g. The fatty acid contents in both FFAs and triglycerides were analyzed through complete methanol esterification. The results are reported in Table 1. The free fatty acid content was about 22 wt %. The

Scheme 1. Synthesis Route of the Novel Acidic Ionic Liquid Polymer

Table 1. Fatty Acid Content in Triglycerides and Free Fatty Acids

copolymerized with the IL monomer, thereby avoiding the use of expensive coupling reagents and reducing the cost. To form ion clusters and improve the ion interactions, the IL was polymerized first to form oligomers. Then, the oligomers were copolymerized with DVB. Poly(DVB) (PDVB) provides a high hydrophobic Brunauer−Emmett−Teller (BET) surface area,35 which enhances the efficiency of mass transfer and prevents the release of acid sites. A novel efficient one-pot synthesis of biodiesel from waste oils using the solid acidic ionic liquid polymer (PIL) was developed. The results showed that the novel PIL was very efficient for the reactions, with yields of greater than 99% under mild conditions.

component

triglycerides

free fatty acids

palmitic acid (C16:0) stearic acid (C18:0) oleic acid (C18:1) linolenic acid (C18:3) linoleic acid (C18:2)

21 29 32 8 10

25 32 28 8 7

molar ratio of methanol to waste oil required was calculated by treating 3 mol of FFAs as 1 mol of triglyceride. The mixture of oil, methanol, and catalyst was stirred at 70 °C in a roundbottom flask for the specified period, and the process was monitored throughout by gas chromatography (GC). The acidity of the reaction mixture was measured by neutralization titration during the reaction process. The catalyst was recycled by filtering. The filtrate formed two phases after the removal of methanol and water through distillation. The top layer was biodiesel, and the lower layer was glycerol with a small amount of glyceride. The biodiesel was collected for chromatographic analysis. Quantitative analysis of the extract solution was carried out on a temperature-programmed Shimadzu (GC-14C) gas chromatograph according to the method provide by Alcantara et al.36 Other analysis methods such as NMR spectroscopy, glycerol titration, and hydroxyl value determination were also used for the analysis, and the results for the other methods fit well with those for the GC analysis, with a difference within 1%. Furthermore, pure biodiesel was obtained under the optimal reaction conditions, and the isolated yield also fit well with the GC analysis, again with a difference within 1%. The conversion of FFAs was calculated from the acidity. The total yield of biodiesel was calculated from the concentration of methyl esters analyzed by GC with the equation

2. EXPERIMENTAL SECTION All organic reagents were commercial products of the highest purity available (>98%) and were used for the reactions without further purification. 2.1. Synthesis of the Novel Acidic Ionic Liquid Polymer. Triethylene amine (9.5 g, 0.1 mol), 1,3-propanesulfonate (12.2 g, 0.1 mol), and tetrahydrofuran (20 mL) were mixed and stirred magnetically for 72 h at room temperature. Then, white solid zwitterions formed. The white solid zwitterions were filtered and washed repeatedly with ether. After being dried in a vacuum (110 °C, 1.33 Pa), the white solid zwitterions were obtained in good yield (91%). An equimolar amount of concentrated sulfuric acid was added to the obtained zwitterions, and the mixture was stirred for 4 h at 60 °C to form the ionic liquid monomer. 1H NMR for the zwitterions [400 MHz, D2O, tetramethylsilane (TMS)]: δ 2.15 (m, 2H), 2.75 (t, J = 7.6 Hz, 1H), 2.12 (t, JH−H = 7.2 Hz, 2H), 5.37 (d, 3H), 5.51 (d, 3H), 6.21 (m, 3H). IR (KBr): 1046 and 924 cm−1 (−SO3H), 1166 cm−1 (C−N). 6895

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yield (%) =

(

weight of biodiesel produced MW of biodiesel

) × biodiesel concentration

(weight of oil/MW of oil) × 3 × 100

(1)

3. RESULTS AND DISCUSSION 3.1. Characterization of the Solid Acidic Ionic Liquid Polymer. The acidity of the novel solid acidic PIL was 4.4 mmol/g, as determined by neutralization titration. This acidity is much higher than the values for common heterogeneous acids. This acidity is also in accordance with the use of an equimolar mixture of IL monomer and DVB, which could be adjusted easily by changing the molar ratio. The more IL monomer used, the higher the acidity obtained. On the other hand, the hydrophobic BET surface area decreased with increasing DVB content. PIL obtained using an equimolar mixture of IL monomer and DVB had a BET surface area of 323 m2/g and a large pore volume of 0.83 cm3/g. The pore size of PIL was distributed around 35.0 nm. Because the average pore size of PIL is large (35.0 nm), bulky reactants can easily diffuse into the interior of this catalyst. This will allow reactants to come into contact with more acid sites, resulting in a better activity for the catalyst. According to elemental analysis, PIL contained 54.3% C, 6.8% H, and 13.1% S. These results correspond well with the polymer structure shown in Scheme 1 and also confirm that all of the IL monomer and DVB participated in cross-linking and entered into the solid structure. The IR spectrum (Figure 1) of

Figure 2. SEM image of PIL.

monomer contained three terminal double bonds and could participate in the cross-linking of DVB, resulting in a highly porous structure. The small particles were interconnected together, lacking an obvious boundary. The PIL sample exhibited abundant disordered nanopores with uniform pore sizes ranging from 20 to 200 nm, in agreement with the BET surface analysis. This highly cross-linked structure made the recovery of PIL quite simple without specific operations other than filtration. 3.2. Catalytic Activities for Biodiesel Synthesis. The waste oil used in this work contained both free fatty acids and triglycerides, and PIL should efficiently catalyze both the transesterification and esterification. First, the catalytic activity for the esterification of pure oleic acid was investigated (Figure 3). It can be seen from Figure 3 that the catalyst was very

Figure 1. IR spectrum of PIL. Figure 3. Catalytic activity for the esterification of pure oleic acid. Reaction conditions: oleic acid, 5 g; methanol, 1.70 g; catalyst, 1 wt % based on oleic acid; 70 °C. Yield was calculated based on acidity.

PIL shows the absorptions of sulfonic acid groups at 1038 and 880 cm−1, thus confirming the presence of these acid groups. The Fourier transform IR spectrum also shows that PIL contained resident functionalities including C−C (1162 cm−1), Ar−H (2941 cm−1), and OH (3400 cm−1). Scanning electron microscopy (SEM) was used to determine the morphology of the PIL material, observed as comprising irregular spheres with a particle size of about 0.1−0.2 μm, as shown in Figure 2. This structure is quite similar to that of PDVB obtained from solvothermal polymerization.37 The IL

efficient for this reaction, with a yield of >99% after 6 h, much higher than that of the acidic ionic liquid, which was 99.4% after 8 h, which was also much higher than the yields obtained with acidic ILs.24,25 The catalytic activities for biodiesel synthesis from used cooking oil were investigated. The effect of reaction time on the yield was investigated first (Figure 5). Figure 5 shows that PIL was very efficient for both the transesterification and esterification reactions. A high total yield of 99.1% was obtained after 12 h; thus, this one-step biodiesel synthesis from waste oil at low temperature provided a much higher yield than another reported catalyst (99.0% 0.01%

confirming that the novel catalyst would be one of the best choices for the synthesis of biodiesel.

4. CONCLUSIONS The novel solid acidic catalyst PIL has been synthesized through the copolymerization of acidic ionic liquid oligomers and DVB. PIL showed high activities for one-pot biodiesel synthesis from waste oil with a high FFA content at low temperature with a total yield greater than 99%. High acidity, high stability, and high activities are the key features of PIL. Because of its operational simplicity, high yield, and mild reaction conditions, the biodiesel synthesis process using this catalyst holds great potential for industrial application.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-575-88345681. Fax: +86-575-88345681. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 21103111). REFERENCES

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