INTRODUCTION
The growing global demand for energy, driven by increasing population and industrialization, has intensified the reliance on non-renewable fossil fuels. This dependency not only accelerates the depletion of natural resources but also contributes significantly to environmental degradation, including rising greenhouse gas emissions and global warming. As a result, there is an urgent need to identify alternative energy sources that are renewable, sustainable, and environmentally benign.
Biodiesel has emerged as a promising substitute for conventional diesel due to its renewability, non-toxic nature, biodegradability, and reduced emissions of carbon oxides (COx) and sulfur oxides (SOx). Produced primarily through the transesterification of triglycerides—derived from plant oils or animal fats—with short-chain alcohols (such as methanol or ethanol), biodiesel mimics the combustion characteristics of petroleum diesel while offering superior environmental performance.
Initially, edible oils like rapeseed, soybean, and coconut were utilized as biodiesel feedstocks. However, concerns over the food-versus-fuel debate have prompted a shift toward second-generation biofeedstocks. Non-edible oils from sources such as algae, jatropha, rubber seeds, and candlenut (Aleurites moluccana) are now being explored due to their high oil content and limited impact on food supply chains.
Despite its advantages, biodiesel quality and economic feasibility are often constrained by the presence of impurities—such as excess alcohol, unreacted glycerides, water, and residual catalysts—that remain after the transesterification process. These impurities can lead to decreased engine performance, corrosive effects, and the formation of harmful combustion byproducts. Therefore, purification is a critical step in biodiesel production to ensure compliance with international fuel quality standards (ASTM D6751 and EN 14214).
Conventional purification methods, such as wet washing with water, are commonly employed but suffer from several drawbacks. Water-based purification not only contributes significantly to the overall production cost but may also promote the hydrolysis of esters into free fatty acids, leading to soap formation and fuel degradation.
In recent years, alternative solvent systems like ionic liquids (ILs) and deep eutectic solvents (DESs) have gained attention for biodiesel purification. DESs, in particular, have shown promising potential due to their low toxicity, biodegradability, ease of preparation, and tunable physicochemical properties. Composed of hydrogen bond donors (HBDs) and hydrogen bond acceptors (HBAs), DESs exhibit lower melting points than their individual components due to strong intermolecular interactions. These features enable DESs to interact effectively with both polar and non-polar biodiesel impurities.
Previous studies have demonstrated the capability of choline chloride-based DESs in extracting glycerol and other contaminants from biodiesel. However, limited research has been conducted on their application in purifying biodiesel derived from non-edible sources such as lumbang oil.
This study aims to assess the efficiency of three choline chloride-based DES systems—ChCl:Glycerol, ChCl:Ethylene Glycol, and a ternary ChCl:Glycerol:Ethylene Glycol mixture—in purifying biodiesel produced from lumbang oil through direct acid-base catalyzed transesterification. The purification performance of these DESs is evaluated through physicochemical characterization, including FAME content, viscosity, and residual impurity levels, in accordance with recognized biodiesel quality standards.
MATERIALS AND METHODS
Materials
Lumbang seeds were sourced from the University of the Philippines Los Baños. Analytical grade choline chloride, glycerol, and ethylene glycol (Sigma Aldrich, Singapore) were desiccated prior to use. Methanol, hexane, sulfuric acid, and sodium hydroxide (Merck) were used as received.
Oil Extraction
Lumbang seeds were cleaned, air-dried (48 h), and cracked. Kernels were oven-dried (100°C, 3 h), ground, and subjected to Soxhlet extraction with 150 mL of hexane for 3 hours. The oil-hexane mixture was filtered and solvent removed via rotary evaporation at 40°C.
Physicochemical Properties of Lumbang Oil
- Acid and Free Fatty Acid Value: 5 g of oil was titrated with 0.1 M KOH in ethanol (25 mL) at 65°C.
- Saponification Number: Reflux of oil with 0.5 M alcoholic KOH; titrated with HCl.
- Iodine Number: Oil reacted with Wijs solution, followed by titration with Na₂S₂O₃.
- Density and Viscosity: Measured using a pycnometer and Oswald viscometer respectively, at room temperature.
Biodiesel Synthesis via Acid-Base Catalysis
100 g of lumbang oil was pre-esterified with 25 mL methanol and 0.7 g H₂SO₄ (65°C, 2 h), followed by neutralization and base transesterification with 25 mL methanol containing 0.5 g NaOH (2 h, 65°C). Crude biodiesel was separated from glycerol overnight.
DES Synthesis and Application
DESs were synthesized by mixing choline chloride with either glycerol, ethylene glycol, or both in specific molar ratios and heated at 80°C until clear. DESs were mixed with biodiesel at 1:1 and 2:1 molar ratios and stirred at 30°C for 4 h. Separation occurred after 2 h of settling.
Characterization Methods
- 1H NMR Spectroscopy: Bruker AVANCE II (300 MHz) in D₂O for DES confirmation.
- FTIR Spectroscopy: Nicolet 6700 FTIR to compare purified and unpurified biodiesel.
- DOE Certification Tests: FAME (EN 14214), glycerides (EN 14105), and kinematic viscosity (ASTM D445).
RESULTS AND DISCUSSION
Lumbang Oil Properties
Table 1 summarizes the properties of crude oil. Oil yield was 56%, with high saponification (103.82 mg KOH/g) and 6.52% free fatty acids—indicative of high soap-forming potential if base-catalyzed synthesis is used alone.
Table 1. Physicochemical Properties of Crude Lumbang (Aleurites moluccana) Oil
| Property | Experimental Value |
|---|---|
| Density (g/mL) | 0.9271 |
| pH | 5.13 |
| Iodine Number (g I₂/100 g oil) | 120.50 |
| Moisture Content (% w/w) | 12.22 |
| Acid Value (mg KOH/g oil) | 11.97 |
| Saponification Number (mg KOH/g oil) | 103.82 |
| Free Fatty Acid Content (%) | 6.52 |
| Kinematic Viscosity (mm²/s) | 27.34 |
Biodiesel Production and Purification
Direct acid-base transesterification yielded 92.31% biodiesel. Impurities such as methanol and glycerides were prevalent in crude biodiesel and were removed using DESs.
DES Confirmation and Purification Process
DES formation was confirmed via 1H NMR, showing downfield peak shifts due to hydrogen bonding. DESs effectively clarified after 15 min stirring at 80°C.
Table 2. Composition and Molar Ratios of Deep Eutectic Solvents (DESs) Used for Biodiesel Purification
| DES Composition | DES Label | DES Molar Ratio | DES:Biodiesel Molar Ratio |
|---|---|---|---|
| Choline Chloride : Glycerol | DES1-11 | 1 : 2 | 1 : 1 |
| Choline Chloride : Glycerol | DES1-21 | 1 : 2 | 2 : 1 |
| Choline Chloride : Ethylene Glycol | DES2-11 | 1 : 2 | 1 : 1 |
| Choline Chloride : Ethylene Glycol | DES2-21 | 1 : 2 | 2 : 1 |
| Choline Chloride : Glycerol : Ethylene Glycol | DES3-11 | 1 : 2 : 2 | 1 : 1 |
| Choline Chloride : Glycerol : Ethylene Glycol | DES3-21 | 1 : 2 : 2 | 2 : 1 |
Abbreviations: ChCl = Choline Chloride; Gly = Glycerol; Eg = Ethylene Glycol
Physicochemical Changes Post-Purification
- Methanol Removal: DES-purified samples showed >98% reduction (Table 3).
- FTIR Profiles: Indicated reduced hydroxyl-containing compounds post-extraction
- FAME Content: DES2 (ChCl:Eg, 1:1) achieved 98.13%, exceeding EN 14214 standards (96.5%)
- Free and Total Glycerin: All DESs lowered free glycerin to ≤0.01%; total glycerin dropped below 0.24% in all cases except DES3-21
- Glyceride Removal: High extraction efficiencies for mono-, di-, and triglycerides
- Viscosity: Reduced by up to 18% in purified samples improving combustion compatibility.
Table 3. Volume of Methanol Removed Under Vacuum from Unpurified and DES-Purified Lumbang Biodiesel
| Sample Type | DES:Biodiesel Molar Ratio | Methanol Removed (mL) |
|---|---|---|
| Unpurified Biodiesel | – | 6.10 |
| DES1 (ChCl:Gly) Purified | 1:1 | 0.10 |
| DES1 (ChCl:Gly) Purified | 2:1 | 0.10 |
| DES2 (ChCl:Eg) Purified | 1:1 | 0.11 |
| DES2 (ChCl:Eg) Purified | 2:1 | 0.12 |
| DES3 (ChCl:Gly:Eg) Purified | 1:1 | 0.11 |
| DES3 (ChCl:Gly:Eg) Purified | 2:1 | 0.10 |
CONCLUSION
Choline chloride-based DESs—particularly the ChCl:Ethylene Glycol system at a 1:1 molar ratio—significantly improved biodiesel purity and quality by reducing contaminants and enhancing key fuel properties. These solvents provide a promising, green approach to biodiesel purification, making lumbang-derived biodiesel more suitable as an alternative renewable fuel.
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