Synthetic Experiment 24 - Biodiesel from Coffee Grounds
Introduction
Coffee grounds are produced at a rate of 16 billion pounds per year. The utilization of this waste product can improve industrial efficiency by recycling for further use and solve waste disposal dilemmas such as land use and water quality brought up by such wide-scale production of coffee beans. The purpose of this experiment is to extract from used coffee grounds, triglycerides which will then be transesterified to fatty acid methyl esters that can be burned as fuel in diesel engines. The application of this diesel biofuel is wide in scope and the potential for coffee grounds, alone, to produce biodiesel is figured at 340 million gallons from 16 billion pounds of coffee beans. While global oil consumption was 86 million barrels per day in 2012 and makes this total coffee biodiesel production enough to provide less than a day of fuel to the world, coffee’s contribution to fuel independence may be more subtle than pure production numbers. Being able to turn our waste into useful energy could be a trend for future consumption. In addition, to using waste, biodiesel has advantages over traditional crude oil for its low greenhouse gas production and can be sourced locally, reducing dependence. Waste produced through the process comes in the form of fuel pellets (for combustion), ethanol, and glycerin, which are all useful products. In this experiment, biodiesel will be synthesized via a base catalyzed transesterification of triglycerides that have been isolated from spent coffee grounds. Triglycerides are a common form of fat found in vegetable and animal oils. Because of this, the process from this experiment could be used to produce biodiesel from any oil that can be isolated from these sources.
An initial amount of spent coffee grounds is dried and then refluxed inside a round-bottom flask to which a reflux condenser is attached in order to isolate the oil from the grounds. This oil is mainly composed of triglycerides and the extraction of these is based on a solid/liquid extraction using hexanes. The extraction works by placing hexanes with the coffee grounds (liquid with solid) and then heating in a reflux condenser in order to speed up extraction of triglyceride oil from the grounds. The extraction works because hexanes is non-polar as is the triglyceride oil. They attract each other because of the similarity in polarity and heating helps this along.The second step of this reaction involves synthesizing the methyl esters from the triglyceride and separating these from the glycerol byproduct. The base used in this reaction was anhydrous potassium carbonate and methanol was the alcohol reactant. Due to the reversibility of this reaction, 6mmol of methanol was used for every 1 of oil, thus driving the reaction toward the product (based on Le Chatelier’s principle). In a transesterification reaction, the deprotonated short chain alcohol attacks the carbonyl carbon, which proceeds to kick off the ester oxygen of the fatty acid components of triglyceride.Since this methyl ester that is formed is relatively nonpolar compared with glycerol, it can be extracted out of reaction mixture with hexanes. What the transesterification does is replaces one alcohol group for another on an ester and is commonly used in production of plastics and biodiesel. The hexanes layer will contain the biodiesel and the hexanes can be evaporated off in order to create an isolated product.
The purpose of this experiment is to demonstrate the viability of biodiesel synthesis from coffee grounds. Biodiesel in this synthesis will come from a transesterification of the triglyceride oil in coffee grounds, isolated by solid/liquid extraction. The waste product of this experiment is glycerol, which is useful by itself, such as a sugar substitute, or as a component in a number of common products such as toothpaste and mouthwash. The “green” quality of the synthesis is established by its formation of useful product from a waste product. To ensure that biodiesel has been isolated after a liquid/liquid extraction from the glycerol/biodiesel mixture after the transesterification, the diesel will be characterized by IR, GC, and GC/MS. As a tool for comparison, the triglyceride oil that comes out of the solid/liquid extraction from the coffee grounds will be characterized by IR.
Fatty Acid Methyl Ester synthesis from triglyceride via transesterification
Experimental4
Triglyceride Oil. Coffee (75.0146g) was brewed and the grounds were dried in a 50oC oven for two days. Spent grounds (49.9832g) were placed in a 500 mL round-bottom flask with 250 mL of hexanes and were refluxed at 80oC for 90 minutes.The reaction mixture was cooled to room temperature and the grounds were removed via vacuum filtration. The hexanes were evaporated with a stream of nitrogen to yield a brown oil (28.5250g, 57.07%). IR (ATR) vmax (cm-1) 2923.11, 1742.00, 1458.87, 1168.58.
Biodiesel. Methanol (8 mL, 194.45 mmol) and potassium carbonate (2.2097g, 17.76 mmol) were added with the triglyceride oil (28.5250g, 32.41 mmol) to a 25 mL round bottom flask and refluxed at 65oC for 30 minutes.The reaction mixture was cooled to room temperature and neutralized by the dropwise addition of 1M acetic acid until neutral pH. The reaction mixture was transferred to a large test tube along with hexanes (10 mL) and 90:10 methanol/water (5 mL). The reaction mixture was agitated and the top organic layer was dried over sodium sulfate and the hexanes were evaporated with a nitrogen stream to yield a yellow oil. (5.3245g, 18.25%), IR (ATR) vmax (cm-1) 2923.15, 2855.02, 1744.33, 1459.14, 1159.90 cm-1; GC RT 10.44 2.906%, 11.76 4.792, 11.96 0.592%, 13.56 0.236%, 15.13 0.043%; GC/MS (dichloromethane, 165-275oC at 10oC/min) RT 11.07 min 270.28 m/z, 12.72 min 294.47 m/z, 12.77 min 297.23 m/z, 12.99 min 298.44 m/z, 14.75 min 326.57 m/z.
Results
Biodiesel was successfully isolated from spent coffee grounds in order to demonstrate how organic chemistry techniques can contribute to more sustainable human activities. The extraction of triglyceride oil from the spent grounds involved a solid/liquid extraction by 90 minute reflux in hexanes and subsequent drying of the hexanes solvent. This solid/liquid extraction works because hexanes is non-polar like the triglyceride oil that is to be extracted. Because of this, the oil dissolves out of the grounds and into the boiling hexanes. From this crude oil, biodiesel was isolated via a base catalyzed transesterifcation reaction as the methoxy group from methanol takes the place of a kicked off the alkoxy group of the triglycerideand a liquid liquid extraction in which the biodiesel is extracted into the organic solution while the glycerol remains in the aqueous solution due to their relative polarities.The evaporation of organic solvent hexanes leaves the pure biodiesel product behind.
The final amount of biodiesel isolated was 5.3245g, from an original amount of 49.9832g of coffee grounds.This gives a 10.65% recovery and from 28.5250g of crude triglyceride oil (57.07% recovered from grounds), an 18.25% yield. The IR spectroscopy was able to suggest the presence of either triglyceride oil of fatty methyl esters but could not indicate purity as the fatty methyl esters were originally part of the triglyceride and altered with a just a methyl group, the IR results are almost identical. In both the IR of the triglyceride and the ester (figures 1 and 2, respectively), the strong2923 cm-1 and 2054 cm-1 peak corresponds to the manyC-C and C-H bonds in the sample. The peak around 1744 cm-1corresponds to the ester carbonyl bond found in both compounds. The peak at 1458 cm-1 corresponds to the methyl groupsat the end of the fatty acids and the peaks at1160 and 1166 cm-1 in figures 2 and 1, respectively, correspond to the ether C-O bond in the glycerol portion of the triglyceride and the ester portion of the fatty acid. The ether would be the distinguishing portion between the triglyceride oil and the biodiesel, but it falls in the same range as the ester and, therefore, the two spectras are indistinguishable. That being said, all components of the crude and purified oils show up in the IR and the presence of both can be confirmed, but the purity cannot as the biodiesel spectrum may contain triglycerides. O-H contaminants were found in neither spectra and cannot be used to determine which compound belongs to which IR analysis. Further analysis of the purified product by GC and GC/MS shows the type of fatty methyl esters present in the biodiesel.
The GC in figure 5 shows the five most common oils in the biodiesel sample. At 10.44 minutes, palmitic acid has a peak corresponding to 2.906% of the total sample. At 11.76 minutes, Linoleic acid has a peak corresponding to 4.792% of the total sample. At 11.96 minutes, Oleic acid has a peak that corresponds to 0.592% of the total sample. At 13.56 minutes, Stearic acid has a peak corresponding to 0.236% of the total sample. At 15.13 minutes, eicosanoic acid has a peak corresponding to 0.043% of the total sample. GC/MS reveals the biodiesel’s five major fatty methyl esters as hexadecanoic acid (palmitic acid), linoleic acid, oleic acid, stearic acid, and eicosanoic acid. The percent composition of these compounds was 36.52% palmitic acid (Figure 6) with a retention time of 11.07 minutes, 40.57% linoleic acid (Figure 7) with a retention time of 12.72 minutes, 10.17% oleic acid (Figure 8) with a retention time of 12.77 minutes, 9.27% stearic acid (Figure 9) with a retention time of 12.99 minutes, and 3.47% eicosanoic acid (Figure 10) with a retention time of 14.75 minutes. Palmitic acid showed a molecular ion peak at 270.28 m/z with a most abundant fragment at 73.92 m/z which is a CH2CH3COOH molecule. Linoleic acid had a molecular ion peak at 294.47 m/z and a most abundant fragment (CHCHCH2CHCH) at 66.92 m/z. Oleic acid had a molecular ion peak at 297.23 m/z and a most abundant fragment (CH2CHCHCH2CH2CH2) at 55.01 m/z. Stearic acid had a molecular ion peak at 298.44 m/z with a most abundant fragment (CH2CH2COOH) at 73.90 m/z. Eicosanoic acid had a molecular ion peak at 326.57 m/z and a most abundant fragment (CH2CH2COOH) at 74.02 m/z.Because these compounds are all fatty acid methyl esters, it indicates that the biodiesel was correctly isolated from the glycerol and consists of the expected type of components. According to results obtained by Kondamudi et al., lineoleic acid was the most abundant free fatty acid, followed by palmitic acid and with stearic and oleic acid grouped at low relative abundance, trailed by eicosanic acid at a very low abundance. This trend follows the results obtained in this experiment, further securing the results as valid.
The number that does not match the literature values is the percent yield result. An 18.25% yield is extremely low when compared with the (questionable) 100% yield obtained by Knondamudi1, 98.5% yield from Burton2, and 96% from Vardon3. The reason behind this is suspected to be a misreported crude oil weight that was in fact not dry from hexanes. This would make the denominator for the % yield calculation higher than it would have been had all hexanes been evaporated and pushed the percentage lower. For percent recovery from the spent grounds, the number comes closer to matching reported values. Compared with 15%1, 16.3%2, and 11-20%4 recoveries, the 10.65% recovery in this experiment is expected. Places for loss of product include transfer between lab glass, reflux, and incomplete transesterification.
Conclusion
Triglyceride oils were isolated from spent coffee grounds by solid/liquid extraction with hexanes. The triglyceride oils then underwent a base catalyzed transesterifiaction reaction with potassium carbonate and methanol to yield biodiesel. The synthesis of biodiesel from spent coffee grounds was successful in terms of recovery from the grounds. The expected percentage of oil in coffee grounds is between 11 and 20% and this experiment yielded 10.65% recovery. In terms of yield, the amount of diesel separated from the crude oil was disappointing. With expected results above 90%, the calculated yield was only 18.25%. Despite this, the product was pure biodiesel made up of 5 types of fatty acid methyl esters whose percentages mirror those found in earlier experiments. While IR data was inconclusive, it did show that the correct structures were present and that the experiment was on the correct path. For improvement, complete drying of the triglyceride oil would have improved the percent yield and achieved the results that are present in the literature.
References:
1. Kondamudi, Narasimharao; Mohapatra, Susanta K; Misra, Mano,Spent coffee grounds as a versatile source of green energy. J. Agric. Food Chem., 2008, 56 (24), pp 11757–11760
2. Rachel Burton; Xiaohu Fan; Greg Austic, Evaluation of Two-Step Reactionand Enzyme Catalysis Approaches for Biodiesel Production from Spent Coffee Grounds,Int. J. Green Energy,2010, 7:5, 530-536
3.Derek R. Vardon; Bryan R. Moser; Wei Zheng; Katie Witkin; Roque L. Evangelista; Timothy J. Strathmann; Kishore Rajagopalan; and Brajendra K. Sharma,Complete Utilization of Spent Coffee Grounds To Produce Biodiesel, Bio-Oil, and Biochar. ACS Sustainable Chem. & Eng.20131 (10), 1286-1294.
4. Rummel, Sheryl. Experiment 24: Biodiesel from Coffee Grounds. Pennsylvania State University Chemistry 213B Synthetics2013.
5. Minard; Masters; Halmi; and Williamson. Lab Guide for Chemistry: Introduction to Organic Chemistry. 2012-2013 Edition by Haydn McNeil. P.39-69.
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