Preparation of Biodiesel PDF

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Synthesis of Biodiesel by Transesterification Introduction: As the idea of replacing fossil fuels, such as gasoline and petroleum, with environmentally-friendly alternatives becomes more of a reality, many people continue to speculate as to how biodiesel can benefit them, their cars, the economy, and the air that they breathe. For starters, studies have shown that the energy output of using biodiesel in place of currently-utilized fossil fuels is 4.56 times as high per lifecycle. Converting to a biodiesel-based fuel will result in greater energy production, while simultaneously producing less waste and pollution than petroleum-based fuels1. The following chart by Baseline Diesel Emissions illustrates the percent reduction of common pollutants formed by fuel containing 20% biodiesel (dark green) versus those formed by 100% biodiesel (yellow):

Figure 1: percent reduction values of 20% biodiesel fuels versus 100% biodiesel fuels 2.

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It is apparent that using 100% biodiesel reduces the impact of pollution significantly by reducing hydrocarbon and carbon monoxide emissions, as well as completely omitting sulfur as a pollutant. Furthermore, soybean oil—whose plants are grown all around the world—serves as the most frequently-used plant oil for biodiesel production. This means that, in contrast to petroleum oil, biodiesel is a renewable energy source that can be synthesized by anyone, anywhere. In addition to these wonderful benefits, biodiesel also benefits the engines of the cars in which they are used in contrast to natural gas and petroleum. Because biodiesel has lubricating properties and lacks any traces of sulfur, converting to it can extend the lifetime of both catalytic convertors and diesel engines as a whole 3. There are few severe disadvantages that exist for the replacement of biodiesel as fuel. For example, biodiesel is currently slightly more expensive than petroleum diesel fuel. One must also take into account the energy required for the actual production of biodiesel, from the farms to the labs. Regardless, it is quite apparent that the pros of biodiesel easily outweighs the cons of using it as a fuel source in place of petroleum. Through the chemical process known as base-catalyzed transesterification, unused plant-based oils can be converted from their typical triglyceride forms into three identical methyl esters (i.e. biodiesel) and a single molecule of glycerol. An alternative method of producing biodiesel fuel is to convert used cooking oil into methyl esters. This method is economical in that it converts waste by-products into useful forms of energy that we may utilize in place of harmful fossil fuels. However, the used cooking oil must be titrated prior to transesterification to neutralize free fatty acids and produce the desired product in good yields.

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Base-catalyzed transesterification is a relatively simple reaction that involves first combining sodium hydroxide with methanol in a flask so they may react to form methoxide anions with water as a by-product. This mixture is then combined with the titrated or unused vegetable oil so that the methoxide anion may perform a nucleophilic attack on the electrophilic carbonyl carbons of the triglyceride. After all three carbonyl groups have reacted, the product will be three methyl esters per triglyceride, as well as the glycerol by-product whose oxyanions will have been protonated by either water or excess methanol. The following diagram illustrates this process:

Figure 2: reaction scheme utilized for the synthesis of biodiesel from soybean oil. “R” serves as the fatty acid chains.

Aside from using vegetable oil as the primary starting material for biodiesel synthesis, animal fats can also be used in this process in much the same way that vegetable oils can. Chicken and swine residues have been a popular choice for this method in the last decade and also proceed through a base-catalyzed transesterification. Many of these procedures utilize slightly different reagents, such as anhydrous ethanol and potassium hydroxide, which react in much the same manner when it comes to the chemical mechanism. As a matter of fact, animal-based biodiesel fuels have been shown to have a higher cetane number, which

corresponds to a better engine ignition quality, than vegetable biodiesels, which have a higher cetane number than petroleum-based fuels4.

Experimental: 100 mL of pure soybean vegetable oil was obtained so that no titration had to be performed prior to biodiesel synthesis. 0.41 grams of sodium hydroxide was mixed in a 250 mL Erlenmeyer flask containing 20.0 mL of methanol. While the sodium hydroxide was being dissolved, 100 mL of vegetable oil was added to a beaker and gently heated to increase the reaction rate of the transesterification process. After all of the sodium hydroxide had dissolved, the vegetable oil was mixed into the flask and stirred constantly for approximately twenty minutes. This solution was then transferred to and stored in a separatory funnel for one week so that the glycerol by-product could properly separate from the biodiesel, which consisted of the newly-synthesized methyl esters. Upon returning to the lab, the stopcock of the separatory funnel was opened in order to drain the glycerol into a beaker. After all of the glycerol had been removed the stopcock was immediately shut to preserve the maximum yield of biodiesel. The mass of the biodiesel obtained was recorded and two tests were performed to verify that the synthesis was successful. First, IR spectra of the starting oil and the biodiesel were taken to confirm the presence of specific functional groups. Afterwards, a viscosity test was performed for the starting oil, the biodiesel, the biodiesel after being cooled in an ice bath, and hexane respectively. 4 Cunha,Ani l do,Vi v i anFedder n,Mar i naC.DePr a,Mar t haM.Hi gar as hi ,Paul oG.DeAbr eu,andAr l eiCol debel l a." Synt hes i sand Char ac t er i z at i onofEt hy l i cBi odi eself r om Ani mal FatWast es . "Sy nt hes i sandChar ac t er i z at i onofEt hy l i cBi odi eself r om Ani malFat Was t es .Sc i enc eDi r ect ,20J une2012.Web.08Apr .2016..

Results:

Soybean Oil Biodiesel

Amount Consumed 100 mL / 91.93 g ---

Density5 0.9193 g/mL 0.880 g/mL

Molecular Weight 920 g/mol 292.2 g/mol

Amount Synthesized --86.48 g

Table 1: compares amount of soybean oil used with amount of biodiesel synthesized; values used in the percent yield calculation.

Percent Yield=

86.48 g Actual Yield x 100 %= 98.73 % yield = Theoretical Yield 87.59 g

Substance Biodiesel Cold Biodiesel Vegetable Oil Hexane (Control)

Viscosity Time 15.60 s 22.54 s 128.36 s 4.53 s

Table 2: displays results from viscosity test described in the procedure.

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Figure 3: IR spectra obtained for the soybean oil used for biodiesel synthesis. It is hypothesized that ethanol, the agent used to clean the IR plate, was still remaining during IR analysis. Therefore, this IR spectra represents ethanol more than vegetable oil, rendering it inconclusive.

Figure 4: IR spectra obtained for synthesized biodiesel. Saturated carbon-hydrogen bonds, a few unsaturated carbon-hydrogen double bonds from the fatty acid chain, and strong absorptions corresponding to esters and C-O bonds confirm the identity of the methyl ester (biodiesel).

Discussion:

As stated earlier, this transesterification occurs through a base-catalyzed procedure in which methoxide—produced from adding sodium hydroxide to a solution of methanol— performs a nucleophilic attack on one of the carbonyl groups of the triglyceride. This single mechanism produces a methyl ester and a diglyceride whose oxyanion is protonated by the protic methanol solvent. This mechanism occurs two more times since there are two remaining carbonyl groups on the diglyceride. The final product is three methyl esters, the main component of biodiesel, as well as a single molecule of glycerol. The mechanism for a single transesterification that converts a triglyceride to a diglyceride is as follows:

Figure 5: the mechanism for base-catalyzed transesterification of a triglyceride to a diglyceride. The steps are: 1) nucleophilic attack by methoxide on carbonyl carbon, 2) carbonyl double bond reforms to kick out the rest of the diglyceride as a leaving group, and 3) the negatively-charged oxygen atom is protonated by methanol. This mechanism occurs twice more to obtain three methyl esters and a single molecule of glycerol (figure 2).

Using pure, unused vegetable oil prevents one from having to titrate the starting material to remove free fatty acids. Free fatty acids in cooking oil are formed via the hydrolysis of oil and will contaminate the final product if not removed properly. However, it should be noted that the use of waste (used) cooking oil for the synthesis of biodiesel is environmentally favored as it convert waste by-products into extremely useful materials that can replace

harmful petroleum fuels that we utilize on a daily basis. This method is often preferred over using pure vegetable oil that could be used for cooking and then subsequently recycled to promote biodiesel production. The big question that remains after biodiesel synthesis is, “can this be used efficiently in a diesel engine?” Based on the IR spectrum obtained for the synthesized biodiesel ( figure 4), the synthesized compound contains carbonyl groups, as well as carbon-oxygen single bonds that are observed in ester functional groups. Additionally, the compound contains mostly saturated carbon-hydrogen bonds along with a small amount of unsaturated carbon-hydrogen double bonds that are found in the fatty acid chain. Furthermore, the percent yield for this procedure was very high, containing only a small fraction of contamination. Following further analysis and confirmation, it would be safe to say that this biodiesel can safely be used in a standard diesel engine. Although biodiesel may serve as a minimal pollutant in comparison to burning petroleum, we must also take into account the “greenness” of this reaction with regard to the environment. One efficient value for determining the impact of waste produced from a reaction is called the E-factor (Environmental factor). This value is easily obtained by determining the ratio between the wastes produced over the amount of useful product obtained through a synthesis. The E-factor for this specific reaction is 0.2509, which is relatively low in comparison to other industrial processes around the world. Roger Sheldon, renowned founder of the Efactor value, has constructed a table correlating industrial processes with their amount of product versus the E-factor calculation obtained through their reactions. As one can deduce from table 3, this reaction has a low E-factor compared to typical bulk production of chemicals, and just slightly higher than those calculated through the oil refining industry:

Table 3: Roger Sheldon’s calculations for E-factors concerning various industry segments around the world6.

In conclusion, it is fair to say that biodiesel may soon hold an important role in our society as we continue to consume large amounts of fuel on a daily basis. Considering this, and the myriad of problems that come along with burning fossil fuels, it is important that we find methods of creating energy that are minimally impactful towards the environment in which we thrive. The benefits of biodiesel range from relatively mild synthesis processes to improved fuel efficiency in diesel engines to minimal environmental pollutants and so on. With luck, this exciting and relatively new idea will radiate across the world and result in a cleaner, healthier planet.

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