Ultrasonic-Assisted Preparation of Biodiesel Products from Vegetable Oils

Xisen Wang; Matthew Chrzanowski; Yujuan Liu

doi:10.3791/66689

JoVE Journal > Chemistry

Chimie

Ultrasonic-Assisted Preparation of Biodiesel Products from Vegetable Oils

Published: April 19, 2024

doi:

10.3791/66689

Xisen Wang¹, Matthew Chrzanowski¹, Yujuan Liu¹

¹Department of Chemistry,California State University, Sacramento

Summary

A safe ultrasonic-assisted transesterification method for vegetable oils using an alkaline catalyst is presented here. The method is rapid and efficient for preparing pure biodiesel products.

Abstract

Utilizing vegetable oil as a sustainable feedstock, this study presents an innovative approach to ultrasonic-assisted transesterification for biodiesel synthesis. This alkaline-catalyzed procedure harnesses ultrasound as a potent energy input, facilitating the rapid conversion of extra virgin olive oil into biodiesel. In this demonstration, the reaction is run in an ultrasonic bath under ambient conditions for 15 min, requiring a 1:6 molar ratio of extra virgin olive oil to methanol and a minimum amount of KOH as the catalyst. The physiochemical properties of biodiesel are also reported. Emphasizing the remarkable advantages of ultrasonic-assisted transesterification, this method demonstrates notable reductions in reaction and separation times, achieving near-perfect purity (~100%), high yields, and negligible waste generation. Importantly, these benefits are achieved within a framework that prioritizes safety and environmental sustainability. These compelling findings underscore the effectiveness of this approach in converting vegetable oil into biodiesel, positioning it as a viable option for both research and practical applications.

Introduction

Biodiesel, derived from common, plant-based oils and fats, emerges as a sustainable solution to mitigate reliance on petroleum¹. This renewable substitute showcases reduced greenhouse gas emissions, notably carbon dioxide, while relying on sustainable resources. Furthermore, biodiesel presents distinct advantages over petroleum diesel, characterized by its sulfur-free composition, non-toxic nature, and biodegradability. As an alternative to conventional fossil fuels, biodiesel aligns with the United Nations' (UN's) Net Zero policy by reducing our dependence on non-renewable fossil fuels and mitigating the adverse effects of climate change. Biodiesel offers a promising path to meeting current energy needs, making it a powerful choice for a greener future².

The predominant method used for biodiesel production involves transesterification, a chemical process where triglycerides found in oils and fats react with an alcohol, typically methanol or ethanol, in the presence of a catalyst under elevated temperature conditions¹^,²^,³^,⁴. This reaction yields fatty acid alkyl esters, the principal component of biodiesel. Various types of vegetable oils serve as primary feedstocks for biodiesel production, including both edible⁵ (e.g., extra virgin olive oil and corn oil) and non-edible oils⁶^,⁷^,⁸ (e.g., caper seed oil), as well as waste oils⁹. Methanol is most commonly used for this transesterification process as it is a relatively inexpensive alcohol. Additionally, an array of catalysts such as sulfuric acid, phosphoric acid, potassium hydroxide, sodium hydroxide, or enzymes like lipase can be used to expedite the transesterification process¹^,²^,³^,⁴. Traditionally, the reaction mixture is heated under reflux for prolonged periods, typically 30 min or more. Heating is not as energy efficient as ultrasonication while also posing safety risks⁵. Consequently, there is a need for a safer, faster, and more energy efficient transesterification process.

Ultrasound irradiation emerges as a superior alternative to conventional energy sources such as heat, light, and electricity, primarily due to the phenomenon of acoustic cavitation¹⁰. This phenomenon, characterized by the formation, expansion, and violent collapse of bubbles, generating localized hotspots with temperatures reaching approximately 5000 K and pressures of 1000 atm. Such extreme conditions, coupled with rapid heating and cooling rates (over 10¹⁰ K/s), furnish the requisite energy for a wide array of chemical reactions to occur efficiently at room temperature, including those previously deemed unattainable by conventional means¹⁰. Ultrasonic-assisted synthesis is rapidly gaining ground across diverse research areas. Notably, interest in ultrasonic-assisted synthesis in organic synthesis and solid-state materials is driven by its environmentally friendly nature, energy efficiency, and abbreviated reaction times under ambient conditions⁵^,¹¹^,¹²^,¹³^,¹⁴^,¹⁵^,¹⁶. A prompt and effective technique is introduced here for secure ultrasonic-assisted transesterification of vegetable oils using an alkaline catalyst yielding pure biodiesel products within a short time frame. While extra virgin olive oil serves as the demonstration medium in this study, it is imperative to note that the ultrasonic method holds applicability to a spectrum of vegetables oils⁵^,¹⁷.

Protocol

1. Oil source and preparation

Add 2.0 mL of HPLC-grade methanol into a 15 mL centrifuge tube.
CAUTION: Methanol is a highly flammable liquid. It is toxic if swallowed, in contact with skin, or if inhaled, and it causes damage to the eyes. Ensure to wear personal protective equipment (PPE) when working with methanol and use it in the fume hood.
Add one pellet of KOH (~0.10 g) to the centrifuge tube and dissolve the KOH solid using the ultrasonic cleaner (40 kHz) by just turning on the ultrasonic.
CAUTION: KOH is harmful if swallowed. It causes severe skin burns, eye damage, and serious eye damage. Ensure to wear PPE when working with this substance.
NOTE: For optimal results, place the centrifuge tube inside a beaker filled with water and then position the beaker within the ultrasonic bath. This immersed configuration guarantees thorough exposure of the reaction mixture to the ultrasonic irradiation, maximizing its effectiveness.

2. Transesterification process

Add 8.0 mL of extra virgin olive oil to the centrifuge tube.
Cap and close the centrifuge tube tight and shake the centrifuge tube vigorously to mix the oil and the potassium methoxide solution.
NOTE: Keep the centrifuge cap tight when shaking the centrifuge tube.
Loosen the cap and put the centrifuge tube into the ultrasonic bath. Turn on the ultrasonic bath for 1 min.
After the first 1 min, close the centrifuge cap tight and shake the centrifuge tube vigorously again.
Loosen the cap and put the reaction mixture into the ultrasonic bath for another 14 min.
Transfer the reaction mixture to a separatory funnel and drain the bottom glycerin layer.
Wash the top layer with 15 mL of saturated NaCl solution 3x, to wash the excess methanol and residual catalyst out of the ester. Ensure the pH of the final washing is neutral by testing with a pH paper.
Transfer the top biodiesel layer into a dry, clean vial, add anhydrous Na₂SO₄ to the vial, swirl the mixture, and let the mixture stand for about 15 min till the biodiesel is clear. Use the clear biodiesel product for characterization.

3. Characterization of biodiesel

Fourier-transform infrared (FT-IR) analysis
1. Record the FT-IR spectra across a wide range of 4000-400 cm^-1. Measure each sample by co-adding 16 scans at a resolution of 4 cm^-1. Perform background subtraction by acquiring a fresh air spectrum before each sample scan. This is to ensure accurate baseline correction and minimized sample contamination. Before each new sample, clean the ATR plate using methanol then dry with lint-free wipes.
Proton nuclear magnetic resonance (¹H NMR) analysis
1. To analyze the chemical composition of the biodiesel product, record the nuclear magnetic resonance (NMR) spectra of biodiesel and extra virgin olive oil on a 500 MHz NMR spectrometer at room temperature. Utilizing a high-resolution 5 mm double gradient probe, prepare each sample by dissolving 50 mg of the sample in 0.7 mL of deuterated chloroform (CDCl₃) containing 0.05% tetramethylsilane (TMS) as an internal standard. Acquire ¹H NMR spectra using the TOPSPIN program with 16 scans and referenced to the TMS standard at 0.0 ppm.
  CAUTION: CDCl₃ is harmful if swallowed and toxic if inhaled. It causes skin irritation and serious eye irritation. Wear PPE when working with this substance.
Viscosity analysis
1. Prepare two 5.75 inch glass Pasteur pipettes and one pipette pump.
2. Make two marks on each pipette with a pen. The top mark is on the body of the pipette, and the second mark is on the narrow stem, approximately 2 cm up from the tip.
3. Use a pipette pump to fill the pipette with extra virgin olive oil with the meniscus at the top mark.
4. Remove the pipette pump and start the stopwatch. Stop the stopwatch as extra virgin olive oil reaches the lower mark.
5. Repeat steps 3.3.3 and 3.3.4 with biodiesel product instead of extra virgin olive oil.
6. Determine the relative viscosities of biodiesel versus the extra virgin olive oil by timing their passage through a glass pipette.
  Relative Viscosity = (oil time)/(biodiesel time).
Flammability tests
1. Immerse a cotton string of approximately 2 cm in length into biodiesel and another cotton string into extra virgin olive oil. Ensure complete saturation of the string with the respective liquid. Place the coated cotton strings on aluminum foil.
2. In a designated laboratory area, away from flammable solvents, assess the ease of igniting each cotton string and observe the quality of the flame produced. Determine if one cotton string ignites more readily than the other. Evaluate which liquid exhibits superior wicking capabilities and which sustains a stronger burn.

Representative Results

In this demonstration, the transesterification reaction of extra virgin olive oil and methanol, catalyzed by KOH, produces biodiesel at room temperature in an ultrasonic bath (Figure 1)⁵. The starting materials in the centrifuge tube show the reactants are immiscible and divided into two layers as seen in Figure 2A. The upper layer is a mixture of methanol and KOH while the lower layer is composed of extra virgin olive oil. To promote homogenization, a brief pre-mixing of the reactants is recommended before subjecting the centrifuge tube to ultrasonic agitation.

After 1 min in the ultrasonic bath, the reaction mixture undergoes noticeable homogenization as seen in Figure 2B. After another 14 min in the ultrasonic bath, product separation occurs with the upper layer comprising biodiesel products and the lower layer containing glycerol esters as shown in Figure 2C. Allowing the mixture to settle for a few minutes further improves separation, as shown in Figure 2D. Hence, the ultrasonic-assisted transesterification reaction efficiently yields biodiesel at room temperature and under atmospheric pressure within a short time. Moreover, the reaction facilitates accelerated separation of biodiesel from the glycerin layer for facile work-up. The biodiesel products can be further purified by separation in a separatory funnel and subsequent washing with saturated NaCl solutions. Characterization of the biodiesel product can be accomplished through ¹H-NMR analysis to confirm the successful synthesis of biodiesel.

Figure 1: Synthesis of biodiesel from vegetable oils via transesterification reaction. The image shows the synthesis steps described here.))) represents ultrasonic treatment; r.t. represents room temperature. Please click here to view a larger version of this figure.

Figure 2: Reaction mixture. The images show the reaction mixture (A) at time 0 min, (B) after 1 min of ultrasonic treatment, (C) just after finishing (after 15 min of ultrasonic treatment), and (D) at 5 min after the reaction finished. Please click here to view a larger version of this figure.

The FT-IR spectra of the extra virgin olive oil (Figure 3A) and biodiesel (Figure 3B) are analyzed to confirm the production of biodiesel. The peak at 1435 cm^-1 is present in the biodiesel showing CH₃ asymmetric bending, while this peak is absent in the extra virgin olive oil. The peak at 1195 cm^-1 shows the O-CH₃ stretching is present in biodiesel, but this peak is absent in extra virgin olive oil. Three peaks are present in both extra virgin olive oil and biodiesel: the peaks between 2800-3000 cm^-1 show the CH₂ stretching and the asymmetric CH₃ and CH₂ stretching, and the peak between 1700-1800 cm^-1 shows the stretch of C=O. A band at 721-723 cm^-1 indicates a Z (cis) double bond in the hydrocarbon chain of the biodiesel. (Z)-Unsaturation lowers the melting point of the product. The results are consistent to previous literature¹⁸.

Figure 3: FT-IR spectrum. The FT-IR spectrum measured from 4000-400 cm^-1 for (A) extra virgin olive oil and (B) biodiesel. Please click here to view a larger version of this figure.

For ¹H NMR results, the extra virgin olive oil contains a multiplet at δ = 4.1-4.3 ppm and a multiplet at δ = 5.2-5.3 ppm for its glyceryl protons (Figure 4A). The biodiesel product exhibits an absence of glyceryl protons, displaying a singlet at δ = 3.67 ppm for the methyl protons within the methyl ester (Figure 4B). The peaks are consistent to previous literature¹⁹^,²⁰. This singlet differs from the CH₃ singlet (δ = 3.49 ppm) observed in MeOH. Based on the ¹H NMR findings, it is evident that the ultrasonic-assisted method can produce biodiesel with a purity of close to 100%.

Figure 4: ¹H NMR spectrum. The ¹H NMR (500 MHz) spectrum in CDCl₃ of (A) extra virgin olive oil with the corresponding assignment of the major peaks of the glycerol unit and the fatty acid chains and (B) biodiesel with corresponding assignment of the major peaks of methyl ester and the fatty acid chains. Spectra demonstrate the different functional groups in the vegetable oil and biodiesel and the purity of the biodiesel product. Please click here to view a larger version of this figure.

Biodiesel which has a viscosity closer to that of petroleum diesel is about 8x less viscous than extra virgin olive oil. Extra virgin olive oil with its 10x viscosity disadvantage, makes it impractical for fuel systems.

When tested for flammability, cotton string soaked in biodiesel ignited faster and burned more intensely than the one soaked in extra virgin olive oil. This suggests biodiesel's potential as a diesel fuel alternative.

Discussion

In this demonstration, an ultrasonic assisted method of base-catalyzed production of biodiesel is elucidated for optimal efficacy. For optimal results, the centrifuge tube should be placed inside a beaker filled with water and then the beaker should be placed within the ultrasonic bath. This immersed configuration guarantees thorough exposure of the reaction mixture to the ultrasonic treatment, maximizing its effectiveness. If desired, a centrifuge rack can also be used to replace the beaker inside the ultrasonic bath, which can further reduce the noise of ultrasonication.

Proper solvent selection is another critical component for the synthesis of biodiesel by ultrasonic-assisted synthesis. Although both methanol and ethanol are green solvents²¹ and can be used to produce biodiesel, compared to ethanol, methanol offers several advantages, including cost-effectiveness, faster reaction times, efficient glycerin separation, and friendly purity assessment using ¹H NMR²². Therefore, methanol is used in this demonstration. Moreover, the biodiesel product resulted from methanol is easier to characterize using H¹ NMR⁵.

Water poses a significant challenge in the transesterification of vegetable oil, hindering both catalyst efficiency and biodiesel purity. Excess water triggers hydrolysis, converting valuable biodiesel into problematic free fatty acids that react with the alkaline catalyst, forming troublesome soap. This soap impedes separation and purification, reduces catalyst activity, and ultimately lowers biodiesel yield. However, the reaction mixture in a closed system will cause a lower yield of biodiesel. Therefore, the cap of the centrifuge tube needs to be loosened under ultrasonic treatment.

The ultrasonic-assisted method is advantageous over the conventional heating methods because biodiesel can be synthesized accurately, precisely, and quickly at room temperature under atmospheric pressure. In this method, biodiesel is synthesized under ambient temperature and atmospheric pressure, which is more energy efficient and safer than conventional heating methods. Therefore, the ultrasonic-assisted method is a greener method in comparison to conventional heating method. Due to its environmental friendliness, energy efficiency, and short reaction time under ambient conditions, biodiesel can be prepared within 15 min using the ultrasonic-assisted method. This method is also applicable for other vegetable oils, such as soybean oil, corn oil, canola oil, peanut oil, and olive oil. The results have been reported in our previous publication⁵. These vegetable oils are commercially available and easily accessible, allowing for wider adoption of the ultrasonic-assisted method.

Different ultrasonic baths (40 kHz) can be used, including low-cost ultrasonic cleaners or ultrasonic jewelry cleaners, although the reaction time may vary. Compared with an ultrasonic probe, an ultrasonic bath has one frequency, which is the limitation of this method. Because the reaction time is short and multiple reactions can be run simultaneously in the ultrasonic bath, the ultrasonic-assisted method can also be used in teaching laboratory courses as a good example to introduce to students the application of 12 principles of green chemistry such as the use of renewable feedstocks, design for energy efficiency, inherently safer chemistry for accident prevention, and catalysis.

In conclusion, an ultrasonic-assisted transesterification method was introduced for the preparation of pure biodiesel from commercially available vegetable oils at room temperature and atmospheric pressure. Compared to traditional heating methods, ultrasonic irradiation offers significant advantages in oil transesterification: shorter reaction times, decreased molar ratios of alcohol to oil, up to 50% lower energy consumption, and increased reaction rates. This efficient process leverages ultrasound irradiation to achieve effective energy distribution within the reaction mixture, leading to superior performance¹⁷^,²³. Moreover, it is advantageous to run small-scale reactions using low-cost ultrasonic baths; therefore, ultrasonic baths as the ultrasound irradiation source can be easily and widely adopted in research and teaching laboratories.

Divulgations

The authors have nothing to disclose.

Acknowledgements

The work was supported by Author YL's start-up fund and Pedagogy Enhancement Award (PEA) at California State University, Sacramento.

Materials

Chloroform-d	Fisher Scientific	865-49-6	• Harmful if swallowed. • Causes skin irritation. • Causes serious eye irritation. • Toxic if inhaled. • Suspected of causing cancer. • Suspected of damaging fertility or the unborn child. • Causes damage to organs through prolonged or repeated exposure
Heated Ultrasonic Baths, Digital, Branson Ultrasonic	Branson	89375-492
Methanol	Fisher Scientific Company	67-56-1	Highly flammable liquid and vapor. Toxic if swallowed, in contact with skin or if inhaled. Causes damage to organs (Eyes).
Potassium hydroxide	Fisher Scientific Company	1310-58-3	May be corrosive to metals. Harmful if swallowed. Causes severe skin burns and eye damage. Causes serious eye damage
Sodium chloride	Sigma-Aldrich	7647-14-5	Not hazardous
Vegetable oils			A commonly consumed food with a long history of safe use in pesticides.

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Wang, X., Chrzanowski, M., Liu, Y. Ultrasonic-Assisted Preparation of Biodiesel Products from Vegetable Oils. J. Vis. Exp. (206), e66689, doi:10.3791/66689 (2024).