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Acid-Catalyzed Preparation of Biodiesel fromWaste Vegetable Oil: An Experiment for theUndergraduate Organic Chemistry LaboratoryDon Bladt, Steve Murray, Brittany Gitch, Haylee Trout, and Charles Liberko*Department of Chemistry, Cornell College, Mount Vernon, Iowa 52314-1098, United States*cliberko@cornellcollege.edu

Recent interest in the production of biodiesel from vege-table oil (1) has spurred a number of exercises designed for use inthe undergraduate organic laboratory (2-4). These transforma-tions typically involve the conversion of triglycerides into methylesters of the long-chain fatty acids via base-catalyzed transester-ification reactions (Scheme 1). The more polar glycerol that isproduced separates from the biodiesel phase and is easily removed.When the undergraduate students carry out the reaction with newvegetable oil, the procedure goes smoothly and satisfactory yields ofbiodiesel are typically obtained.

The procedures that produce biodiesel from waste cookingoil, however, are complicated by free fatty acids that are present asa result of the hydrolysis of the triglyceride under the conditionsused for cooking (Scheme 2). Under the basic transesterificationconditions (Scheme 3), the free fatty acids become deprotonated.The resulting carboxylates are unreactive toward base-catalyzedesterification. Not only does this decrease the yield of biodiesel,but it also complicates the separation process since the long-chaincarboxylates (soap) create emulsions. This situation is furthercomplicated if water is present in the waste oil. Under the basicreaction conditions, water causes additional saponification.When the free fatty acid content of the waste oil goes higherthan about 10%, the base-catalyzed transesterification reactionbecomes unworkable (5).

Transesterification reactions can also be catalyzed by acids,but generally at a much slower rate. In the acid-catalyzedreactions, higher ratios of alcohol to triglycerides than in thebase-catalyzed reactions are typically used (6). We have foundthat sulfuric acid-catalyzed transesterification of vegetable oilwith methanol remained incomplete even after 6 h at reflux.Since acid-catalyzed esterification of the fatty acids (Fischeresterification) generally occurs at a much faster rate than theacid-catalyzed transesterification of the triglyceride, some proce-dures (7) call for a Fischer esterification of the free fatty acids byacid catalysis followed by a base-catalyzed transesterification toconvert the remaining glycerides into biodiesel. These procedureshave the advantage of not sacrificing the free acids or dealing withthe soapy emulsions, but they are still relatively complicated. Webelieved that a procedure that could convert the free fatty acids inwaste vegetable oil into biodiesel which would avoid the messyemulsions associated with the base-catalyzed reactions and wouldshorten the transesterification reaction time to fit into a typical3-h lab period would be pedagogically valuable for undergraduateorganic students. To that end, we developed a one-pot procedure(Scheme 4) using sulfuric acid to simultaneously catalyze boththe Fischer esterification and the transesterification reactions ofwaste vegetable oil in refluxing 1-propanol.1

Experimental Section

Our procedure involves refluxing a mixture of waste vege-table oil, 1- propanol (5:2 ratio by volume), and concentrated sul-furic acid (0.5% by volume) for 1 h. After cooling, the layers areseparated and the ester layer iswashed anddried.Wehave found thatthe acid-catalyzed reaction with refluxing 1-propanol (boiling point97 �C) occurred at a much faster rate than with methanol (boilingpoint 65 �C). In addition to the higher reaction temperature offeredby the higher boiling solvent, the longer hydrocarbon chain of the1-propanol provides better mixing of the phases. This is consistentwith previous studies showing that the reaction rate is limited by

Scheme 1. Transesterification of Vegetable Oil with Methanol: ARepresentative Triglyceride for Corn Oil Is Shown

Scheme 2. Partial Hydrolysis of Vegetable Oil under Conditions Usedfor Cooking: The R-Groups Are Long-Chain Saturated and UnsaturatedHydrocarbons

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mass transfer between the phases (8). The use of vigorous mixing ofthe phases and the use of longer-chain alcohols to improvesolubility (9) would certainly improve the mass transport as wouldthe addition of a co-solvent (10) or the use of ultrasound irradia-tion (11). The reaction reported here comes to completion in aboutan hour. The separation of the glycerin proceeds smoothly and using1MNaCl(aq) towash keeps any emulsions in check. The procedureis simple and produces satisfactory yields of biodiesel even fromheavily degraded vegetable oil.2 The resulting biodiesel may beanalyzed by simple flammability and viscosity tests as well as by1H NMR spectrometry.

Hazards

Full splash goggles should be worn. Concentrated sulfuricacid is corrosive and contact with skin should be avoided.1-Propanol is flammable. Flames are a hazard in any lab andshould be used with adequate precautions. Using biodiesel, wewere not able to maintain the flame without the presence of awick so the dangers from performing the flammability testsshould be minimal.

Results and Discussion

One initial concern with this method was the possible sidereactions that might occur with the unsaturated sites on the fattyacids in the presence of concentrated sulfuric acid because manyacid-catalyzed reactions of alkenes are possible. Indeed, if con-centrated sulfuric acid is combined with an unsaturated oil, themixture quickly darkens and becomes more viscous. By firstcombining the sulfuric acid with the alcohol and then adding theoil, this can largely be avoided. When the reaction was carriedout in this way, 1H NMR analysis of the product showed nosignificant change in the alkene region of the spectrum betweenthe oil and the biodiesel. We have found that the production ofbiodiesel using acid catalysis proceeds faster with the waste oilthan with the new oil, presumably because of the ability of the

degraded oil to provide for better transfer between the phases andbecause of the faster rate of the esterification versus the transes-terification reaction.

It is difficult to visually distinguish a typical sample ofbiodiesel from new vegetable oil. Infrared spectra of the triglyc-erides and the monoesters are nearly identical. Two simple testshave been developed that allow students to easily distinguishbiodiesel from vegetable oil. To assess flammability, students pre-paredminiature oil-lamps from small vials, a cotton string wick, andaluminum foil to hold the tip of the wick above the vial (Figure 1).The vegetable oil wick is noticeably more difficult to light than thebiodiesel wick. Once lit, the biodiesel will maintain a flame from thewick similar to that of a candlewhereas the vegetable oil will smolderand go out after the wick is consumed. A basic test for viscosityinvolves filling separate Pasteur pipets with vegetable oil and bio-diesel and observing the length of time it takes for the pipets to drainby gravity. Typically, students will discover a 5:1 drip ratio betweenthe biodiesel and vegetable oil. More sophisticated techniques usingGC analysis of the methyl esters of fatty acids (12), HPLC analysisof triglycerides (13), and determination of viscosity (14) have beenreported, but our simple qualitative methods have been satisfactoryfor this particular exercise.

If time and facilities permit, 1H NMR is quite useful forfollowing the progress of the transesterification reaction. The 1HNMR (CDCl3) spectrum of new vegetable oil and biodiesel aresurprisingly well resolved. The vegetable oil shows a multiplet atδ = 4.0-4.4 ppm for the glyceryl hydrogens, whereas the newlyprepared biodiesel lacks this multiplet and shows a triplet at δ =4.1 ppm for the methylene (-O-CH2-) protons of the propylester. These are readily distinguishable from the analogous

Scheme 3. Base-Catalyzed Transesterification of Partially DegradedVegetable Oil

Scheme 4. Acid-Catalyzed Esterification and Transesterification ofDegraded Vegetable Oil in 1-Propanol

Figure 1. Time sequence showing the comparison in appearance andflammability of vegetable oil versus biodiesel.

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methylene protons from any 1-propanol (δ = 3.6 ppm) con-tamination.3 Analysis by NMR spectroscopy is valuable to thestudents because it demonstrates the ability of the technique toidentify samples, track reactions, and assess purity.

Conclusions

This laboratory exercise produces usable biodiesel in satis-factory yield and the laboratory exercise including viscosity andflammability tests can be carried out in about 2.5 h. The exerciseis popular with students owing to the current interest in produc-ing fuel from biomass. This procedure serves to demonstrate theconcept of transesterification as well as Fischer esterification.The viscosity and flammability tests are simple ways to distin-guish biodiesel from vegetable oil and serve to reinforce theimportance of physical properties of motor fuels. While the costof 1-propanol versus ethanol or methanol as well as the energyneeded to heat the mixture under reflux may make this particularprocedure unsuitable for large-scale production of biodiesel, theprocedure does serve to demonstrate important concepts in theorganic laboratory.

Acknowledgment

This work was supported by Cornell College. The authorsare grateful to the reviewers for their many helpful suggestions forimproving the manuscript. Sodexho Food Services provided uswith waste vegetable oil.

Notes

1. Heating could be avoided if conserving energy was the mostimportant factor under consideration such as for a large-scaleproduction. The room-temperature reaction takes about oneweek with active stirring to complete. We have opted toincrease the reaction rate, to conserve lab time.

2. Used cooking oil was obtained from the college food service andtitration with KOH showed it to contain∼10% free fatty acids.Even with the students' rudimentary technique, typical studentisolated yields of biodiesel are ∼70% from the acid-catalyzedreaction. Losses in the yield may occur during the separation

and washing. It is also possible that some of the waste oil mayhave suffered degradation beyond simple hydrolysis.

3. It is difficult to completely remove excess 1-propanol bywashing since it is quite soluble in the biodiesel layer. We havenot found the 1-propanol impurity to be a problem in theperformance of the biodiesel in our laboratory tests; it likelyadds to the decreased viscosity and the increased flammabilityof the sample.

Literature Cited

1. For a review see: Kulkarni, M. G.; Dalai, A. K. Ind. Eng. Chem. Res.2006, 45, 2901–2913.

2. Clarke, N. R.; Casey, J. P.; Brown, E. D.; Oneyma, E.; Donaghy, K. J.J. Chem. Educ. 2006, 83, 257.

3. Akers, S. M.; Conkle, J. L.; Thomas, S. N.; Rider, K. B. J. Chem.Educ. 2006, 83, 260–262.

4. Bucholtz, E. C. J. Chem. Educ. 2007, 84, 296–298.5. Canakci, M.; Van Gerpen, J. Trans. ASAE 2001, 44, 1429–1436.6. Zheng, Z.; Kates, M.; Dube, M. A.; McLean, D. D. Biomass

Bioenergy 2006, 30, 267–272.7. Bucholtz, E. C. J. Chem. Educ. 2007, 84, 296–298.8. Ataya, F.; Dube, M. A.; Ternan, M. Energy Fuels 2008, 22,

697–685.9. Ma, F.; Clements, D.; Hanna, M. Ind. Eng. Chem. Res. 1998, 37,

3768–3771.10. Zhou, W.; Konar, S. K.; Boocock, D. G. B. J. Am. Oil Chem. Soc.

2003, 80, 367–371.11. Stavarache, C.; Vinatoru, M.; Nishimura, R.; Maeda, Y.Chem. Lett.

2003, 32, 716–717.12. Cover, R. E. J. Chem. Educ. 1968, 45, 120.13. Farines, M.; Soulier, R; Soulier, J. J. Chem. Educ. 1988, 65, 464.14. Fountain, C. W.; Jennings, J.; McKie, C. K.; Oakman, P.; Fetterolf,

M. L. J. Chem. Educ. 1997, 74, 224.

Supporting Information Available

A student handout; instructor notes; base-catalyzed preparation ofbiodiesel; the acid-catalyzed preparation of biodiesel. This material isavailable via the Internet at http://pubs.acs.org.