In the Laboratory
Converting Municipal Waste into Automobile Fuel: Ethanol from Newspaper Mark Mascal* and Richard Scown Department of Chemistry, University of California Davis, Davis, CA 95616; *
[email protected] Dependence on foreign oil is arguably one of the most pressing issues facing the economies of the United States and most other industrialized countries. Of particular concern are both the politically unstable nature of many oil-producing nations and the surge in global demand for oil as a result of the pace of industrialization of emerging market economies such as China and India. Perhaps even more worrisome is the growing carbon dioxide level in the Earth’s atmosphere owing to the burning of fossil fuels, which may presage dramatic changes in global climate. In response to these issues, research into alternative energy sources has gained substantial momentum in recent years (1). One area of marked activity is the production of bioethanol from renewable agricultural feedstocks (as opposed to industrial ethanol generated from petroleum sources). Ethanol produced in this way is commonly used as a fuel additive in “E10 gasohol” (a mixture 10% ethanol and 90% unleaded gasoline). With appropriate modifications to automotive engines, it can also be used in clean burning ethanol-rich fuel blends (E85) or even neat (E100). Emerging applications involve the use of ethanol as a practical “hydrogen carrier” for generation of electricity in fuel cells (2). Bioethanol production requires first the conversion of biomass into fermentable sugars. For the most part, this currently involves the cultivation of sugar crops (sugar cane or beet) or starch crops (corn or other grains). However, the majority of the sugar produced by the photosynthetic reduction of CO2 in plants is in the form of cellulose, the bulk of which either goes unutilized or is discarded as waste. Although the potential
OH HO HO
OH
OH O OH
O
O HO
OH
O HO n
O
OH
OH
H3Oá heat
OH O
HO HO
OH
OH
yeast
OH
CO 2
Scheme I. The hydrolysis of cellulose to glucose and the fermentation of glucose into ethanol.
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of cellulosic biomass to produce ethanol is well understood in the scientific community, the main technological hurdle to its widespread use is the difficult saccharification (i.e., breaking down into simple sugars) of cellulose. The function of cellulose in nature is to serve as a robust structural framework for plant life, and this purpose is not easily defeated. Cellulose is a microcrystalline material that is remarkably resistant to degradation, although it can be hydrolyzed by the action of certain enzymes (microbial cellulases), and the few existing cellulosic ethanol production facilities use this approach (3). Ideally, however, one would apply the simple chemical principle of aqueous acid hydrolysis to break down cellulose ultimately into its glucose monomers. The key to unlocking this reactivity is to disrupt the crystallinity of cellulose (4). This can be achieved hydrothermally (by applying high temperatures and pressures) or by using a solvent to dissolve the cellulose, thus making it vulnerable to hydrolysis. In this experiment, the latter approach is taken whereby waste cellulosic material is first dissolved in 75% H2SO4 and then reprecipitated to give a quasi-amorphous suspension of cellulose that can then be hydrolyzed by heating in dilute acid. Finally, the resulting maple syrup-like product is fermented by yeast into ethanol (Scheme I). Materials Stirrer-hotplate; 55 mm Büchner funnel; 250 mL vacuum filtration flask; 50, 125, and 250 mL Erlenmeyer flasks; 250 mL beaker; thermometer; spatula (ideally PTFE or PTFE-coated metal); magnetic stir bar; filter paper (55 mm); pH paper (0–14 and 5–8); plastic weighing boat; waste newspaper; Celite; Ca(OH)2; KH2PO4; MgSO4; NH4Cl; yeast; stock solutions of 75% w/w H2SO4, 5 M NaOH, and 5 M HCl; TLC plates (silica gel); TLC developing jar; KMnO4 developing solution; heat gun; ethyl acetate; n-propanol; one-hole rubber stopper, oil bubbler; Tygon tubing ; microdistillation kit including 14/10 water condenser, 14/10 Hickman still head, 14/10 air condenser packed with stainless steel sponge or an equivalent material, and 14/10 50 ml round-bottom flask. Glassware and reagents were sourced from a combination of vendors including of Sigma-Aldrich, Corning, and Chemglass. The yeast was purchased in a supermarket (Fleischmann’s active dry yeast). The heat gun used in the TLC experiment may be obtained from a hardware store. General Procedure Saccharification of Newspaper Students cut 4 g of newspaper into small pieces and add it portionwise into a 75% w/w solution of H2SO4. After each addition the mixture is thoroughly agitated until the paper imbibes the acid and gradually degrades into a dark, homogeneous paste. Hot water is then carefully added, leading to a fine, gray–brown suspension. The mixture is transferred into an Erlenmeyer flask
Journal of Chemical Education • Vol. 85 No. 4 April 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
and heated near the boiling point for 90 minutes. The reaction is then cooled to room temperature and may be stoppered and left at this point until the following laboratory period. A vacuum filtration flask is set up with a Büchner funnel and filter paper. A layer of Celite is spread over the filter paper and wetted with water. The above reaction mixture is filtered through the bed of moist Celite. The dark filter residue is washed twice with water and the combined filtrate is then transferred into a flask. To this is added solid Ca(OH)2 to neutralize the acid, and the resulting CaSO4 precipitate is collected by suction filtration. The CaSO4 filter cake is washed with water, and the pH of the filtrate is checked at this point and adjusted to neutral with either aqueous NaOH or HCl as needed. The volume of the mixture is then reduced to about 25 mL by heating on a hotplate and then transferred into a large plastic weighing boat and allowed to evaporate in a fume hood until the following lab period. The product is a heavy, sweet smelling, pale-brown syrup or semi-solid mass. Product Analysis by TLC If desired, the product can be analyzed by thin-layer chromatography (TLC) on silica gel against a glucose standard using a 7:2:1 n-propanol:ethyl acetate:water mobile phase. Visualization is achieved by dipping in a KMnO4 solution (5) and gently heating with a heat gun. Fermentation The above product is dissolved in 15 mL of a standard fermentation medium containing KH2PO4, NH4Cl, and MgSO4 plus 0.2 g of dried yeast. The fermentation apparatus consists of an Erlenmeyer flask with a rubber one-hole stopper, into which a simple bubbler is inserted. The progress of the fermentation can be monitored by observing the quantity of CO2 evolved. Depending on the ambient temperature, fermentation is complete within 2–5 days. The resulting, murky brown mixture is decanted from the yeast residue and is ready for distillation. Distillation A micro-distillation apparatus is assembled, consisting of a 14/10 water condenser atop a 14/10 Hickman still head. The still assembly is inserted into the top of a fractional distillation column made by filling a 14/10 air condenser with stainless steel sponge or an equivalent packing material. Finally, this column is inserted into a 14/10, 50 mL round-bottom flask containing the decanted fermentation mixture and a magnetic stir bar. The distillation temperature is monitored by suspending a thermometer through the top of the apparatus (which must be left open for venting). The flask is gradually heated with good stirring until the distillate rises through the column. The fraction containing the ethanol will have a different appearance (less surface tension) than the predominantly aqueous fraction below it. Distillate is collected until the temperature just reaches 100 °C. The overall yield is ca. 0.5 mL of the 95% ethanol–5% water azeotrope. Hazards 75% H2SO4, 5 M NaOH, and 5 M HCl are highly corrosive. Calcium hydroxide is a severe irritant and care must be taken to avoid raising dust. The TLC experiment involves the use of small quantities of flammable organic solvents and basic permanganate dip that is caustic and a strong oxidizer.
The ethanol product is flammable. All parts of the experiment must be carried out wearing gloves, labcoats, and safety glasses. The sugars and fermentation products of this experiment should never be tasted. Discussion This lab exercise was introduced into the third (and final) quarter of the second-year organic chemistry course for majors, which typically has enrollments between 10–40 students. The experiment was developed in the context of updating an aging lab manual with procedures that (i) involved modern, relevant chemistry; (ii) gave students a good workout in lab methodology; (iii) demonstrated applications of chemistry to real-world problems; and (iv) were fun to do. We found an article published in this Journal (6) that also dealt with ethanol fermentation (starting from frozen sweet corn), which seems likely to have also drawn its inspiration from the energy crisis of that era. We however wished to highlight the potential use of materials that would otherwise be discarded as waste as the starting point for fuel synthesis. The educational objectives of this experiment are the following: (i) to give students practical experience in carbohydrate chemistry; (ii) to introduce students to the use of biotechnology (fermentation) in organic synthesis, which is also a central feature of the pharmaceutical chemical industry; (iii) to give students practical experience in microdistillation, vacuum filtration, and thin-layer chromatography; and (iv) to inspire students with the potential of chemistry to take on problems of global significance (alternative fuels, environmental issues, political issues). When following the above procedure, students typically report a mass of crude product from the newspaper hydrolysis on the order of 2 g, but the precise yield of glucose is not determined since the product is not easily dried or crystallized. Newspaper is about 50% cellulose so the maximum theoretical yield in this experiment would be around 2 g of pure glucose. In practice, we estimate a glucose yield of 50% based on the cellulose present, that is, about 1 g, which is consistent with the reported efficiency of related hydrolysis procedures (7), and is also supported by the quantity of ethanol students produce in the fermentation. Completion of the full experiment requires about 9 laboratory hours that, due to necessary breaks in the procedure for the evaporation of the sugar solution and the fermentation, are distributed over four 3-hour laboratory sessions. The format of this experiment is further discussed in the online supplement. Student feedback to this experiment has been positive, with students particularly commenting on its “applied” nature in terms of biofuel production. Although the method as described above is not commercially viable, the experiment highlights the role of chemistry in alternative energy research. The main issue with commercializing processes such as described above is the formation of large quantities of calcium sulfate waste. For example, in this experiment 21 g of CaSO4 are generated in the production of about 0.5 g of ethanol, which would equate to 5600 lbs of waste to fuel up a typical automobile! Although hydrates of calcium sulfate are used in plaster of Paris (CaSO4⋅0.5H2O) and gypsum (CaSO4⋅2H2O) as drywall or finishing materials in the building industry, they are low-value commodities that are not in short supply. However, it has been shown that instead of neutralizing with base, the acidic sugar solution can be passed through a bed
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 4 April 2008 • Journal of Chemical Education
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In the Laboratory
of ion exchange resin that is able to effect separation of the sugar from the acid, the latter of which can then be recycled (8). This suggests that, in principle at least, a hydrolytic process could be developed that avoids the inorganic waste problem. Indeed, research into the promise of cellulosic biomass conversion continues apace (9–11). Acknowledgments Michael Jarosh and Nema Hafezi are thanked for help analyzing the products of this experiment. Literature Cited 1. Ragauskas, A. J.; Williams, C. K.; Davison, B. H.; Britovsek, G.; Cairney, J.; Eckert, C. A.; Frederick, W. J., Jr.; Hallett, J. P.; Leak, D. J.; Liotta, C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski, T. Science 2006, 311, 484. 2. Song, S.; Tsiakaras, P. Applied Catalysis B: Environmental 2006, 63, 187–193. 3. McCoy, M. Chem. Eng. News 2006, 24–25. 4. Xiang, Q.; Lee, Y. Y.; Pettersson, P. O.; Torget, R. W. Applied Biochem. Biotech. 2003, 105–108, 505–514. 5. Levine, S. G. J. Chem. Educ. 1996, 73, A4–A6.
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6. Maslowsky, E., Jr. J. Chem. Educ. 1983, 60, 752. 7. Iranmahboob, J.; Nadim, F.; Monemi, S. Biomass Bioenergy 2002, 22, 401. 8. Neuman, R. P.; Rudge, S. R.; Ladisch, M. R. Reactive Polymers 1987, 5, 55–61. 9. Ragauskas, A. J.; Williams, C. K.; Davison; B. H.; Britovsek, G.; Cairney, J.; Eckert, C. A.; Frederick, W. J., Jr.; Hallett, J. P.; Leak, D. J.; Liotta, C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski, T. Science 2006, 311, 484–489. 10. Sun, Y.; Cheng, J. Bioresource Technology 2002, 83, 1–11. 11. U.S. Department of Energy’s Energy Efficiency and Renewable Energy Biomass Program. http://www1.eere.energy.gov/biomass (accessed Dec 2007).
Supporting JCE Online Material
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Journal of Chemical Education • Vol. 85 No. 4 April 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
