Biodiesel fuels (BDFs) are ecofriendly, biodegradable, and non-toxic renewable fuels that can be readily produced from oils and fats by a transesterification reaction with methanol or ethanol in the presence of an acid or alkaline catalyst , . More than 95% of the feedstocks currently used for the production of BDFs are derived from edible oils, such as rapeseed oil (RO) in Europe, soybean oil in the United States and palm oil in Southeast Asia. However, the cultivation of these feedstocks competes for agricultural land with the growth of crops for human foodstuffs , , , and increases in green-house gas emissions by indirect land-use changes (ILUC) . Therefore, the production of BDFs from edible oils represents an ineffective strategy for mitigating the adverse effects of biodiesel production on climate change. With this in mind, an urgent need exists to find new feedstocks for the production of BDFs that do not have an adverse impact on human foodstuffs or the environment. Vernicia montana Lour. oil (VMO) is a non-edible oil with comparable oil content, free fatty acid (FFA) value, and seed yield characteristics to the Jatropha curcas oil (JCO, another non-edible oil), which has been reported as a high potential feedstock for the production of BDF , . VMO represents around 40%–60% of the weight of the seed kernels obtained from the Vernicia montana plant , , , which has an estimated seed yield of 10 tons/ha/year. In contrast to JCO, VMO has a low FFA value, which is similar to those of many other edible oils. For this reason, VMO does not require additional treatments to obtain high-quality BDF, resulting in a considerable reduction in the costs associated with the production of BDF from this feedstock , , . Vernicia montana can be cultivated under low-temperature/frosty conditions, whereas Jatropha curcas can be damaged by low-temperature conditions (<20°C/frost) , . Furthermore, the area of land devoted to the cultivation of Vernicia montana across Laos and Vietnam has increased considerably during the past decade due to state-funded reforestation programs to prevent land erosion and to the support the generation of oil for BDF production , . Thus, VMO is considered to be a high-potential, non-edible oil feedstock alternative for BDF production in Laos and Vietnam.
However, information pertaining to the production of BDF from VMO is limited. Chen et al.  reported that they obtained a BDF conversion of 98% after a transesterification time of 120 min with 3% (wt/wt) KOH, a rotational speed of 800 rpm, and a methanol/oil molar ratio of 6:1 at 60°C. Given that the transesterification reaction is a reversible process that occurs between two phases (i.e. oil and alcohol phases), the use of large excesses of methanol and catalyst, high temperatures, and/or high speed agitation and long reaction times is required in the mechanical stirring method , ,  and supercritical method , , ,  to enhance the conversion yield for the production of BDF, all of which add to the increasing costs of BDFs. A co-solvent method has been recently developed to overcome the disadvantages associated with conventional methods , , , , and then subsequently applied to the successful production of BDF from JCO and waste cooking oil , , . Patil and Deng  reported the production of BDF from JCO using a mechanical stirring method. The JCO used in their study was subjected to an acid pretreatment step, and a conversion yield was 95% after 120 min using a methanol/oil molar ratio of 9:1, 2% (wt/wt) KOH, and an agitator speed of 1000 rpm at 60°C. Hawash et al.  used a supercritical method to achieve a high conversion yield of 100% from JCO. This process was conducted under 8.4 MPa of pressure at a temperature of 320°C with a methanol/oil molar ratio of 43:1. In contrast, Luu et al.  used the co-solvent method to obtain a conversion yield of 99% after 30 min with methanol/oil molar ratio of 6:1, 1% (wt/wt) KOH, and 20% acetone at 40°C. In this case, the addition of acetone to the reaction mixture as a co-solvent facilitated the transesterification reaction under homogenous conditions, allowing for considerable decreases in the amounts of methanol and catalyst that are added to the reaction. The reaction temperature and the reaction time can also be reduced dramatically under these conditions, thus rendering the use of a high-speed agitator unnecessary. Based on the many benefits associated with this method, we investigated the use of the co-solvent method to enhance the production of BDF from VMO.
Previous studies have reported that α-Eleostearic acid, also known as (9Z,11E,13E)-octadeca-9,11,13-trienoic acid, is the main fatty acid component of VMO (75%–80%) , . The high content of unsaturated compounds in VMO is responsible for the low freezing point of the BDF produced from this material (VMO BDF). The low freezing point of VMO BDF is one of its notable properties, and something that we needed to monitor quite closely during this work. In terms of its chemical structure, α-eleostearic acid contains three conjugated double bonds that are prone to oxidation and polymerization reactions, making VMO unstable in air. Therefore, it is very important to evaluate the stability of VMO BDF during storage.
The induction time to oxidative degradation has been widely used to evaluate the stability of BDFs during storage , . However, this parameter is not suitable for determining the stability of BDFs containing 80% α-eleostearic acid methyl ester due to the tendency of this molecule to polymerize, as well as its poor oxidation stability at 110°C, which is the temperature used to determine the induction time . For these reasons, the stabilities of these BDFs are typically determined based on their iodine value and unsaturated ester content. The iodine value provides information pertaining to the degree of unsaturation in the sample, whereas the unsaturated ester content is closely related to the oxidation rate of a specific compound .
The first aim of this study was to improve the production of VMO BDF using the co-solvent method with an alkaline catalyst to achieve a high BDF conversion yield (98%–99%). Several key reaction parameters were evaluated, including the amounts of the co-solvent and reactants as well as the reaction temperature. The BDF properties were determined according the standard methods described by JIS K2390 (Japanese Industrial Standard for biodiesel and testing method) and EN 14214 (European Standard for testing method). The second aim of this study was to investigate the stability of VMO BDF during storage based on variations in its iodine value and unsaturated ester content. For comparison, we also evaluated the stability of the BDF produced from RO (RO BDF). The results of this study can help improve our understanding of the production and storage stability of the VMO BDF.
2 Materials and methods
2.1 Materials and chemicals
Seeds of Vernicia montana were collected directly by farmers working in the Lao Cai Province, Vietnam. RO, methanol (grade 99%), acetone (99.7%), hexane (96%), KOH (85%), H3PO4 (85%), methyl linolenate, methyl stearate, methyl palmitate and methyl heptadecanoate were purchased from Wako Pure Chemical Industries (Osaka, Japan). Methyl linoleate, methyl oleate, monoolein, diolein, and triolein were purchased from Sigma–Aldrich (Tokyo, Japan). Methyl α-eleostearate was purchased from Cayman Chemical (Tokyo, Japan). All other chemicals used in this study were purchased with analytical grade and used without further purification.
2.2.1 VMO extraction:
The seeds of Vernicia montana (3.3 kg) were decorticated, and the kernels were powdered and homogenized for oil extraction. The oil extraction process was conducted in an ultrasonic apparatus, with n-hexane as the solvent. The solvent was used with a sample/solvent ratio of 1:1 (wt/v). After an extraction time of 30 min, the extraction mixture was allowed to settle and separate into an extract solution (upper layer) and a residue (lower layer). The upper layer was transferred into a flask, and the remaining residue was extracted according to the procedure described above. The extraction process was repeated five times in total. The combined extracts were filtered and evaporated to dryness under reduced pressure at 40°C to achieve the desired VMO (1.1 kg).
2.2.2 Optimization of VMO BDF production using the co-solvent method:
Figure 1 shows a flow diagram of the production of VMO BDF using the co-solvent method, along with photographs of the experimental process. Acetone was used as a co-solvent based on the results of a previous report . VMO (20 g) was premixed with an accurate amount of acetone using a magnetic stirrer to give a homogenous mixture. The resulting mixture was placed in a water bath at a pre-determined temperature. A solution of KOH in methanol (MeOH) was then added to the VMO solution. After a reaction time of 60 min, the mixture was transferred to a separating funnel, where it was left to separate into two phases. The upper phase mainly consisted of acetone, fatty acid methyl esters (FAMEs), triglyceride (TG), diglyceride (DG) and monoglyceride (MG), whereas the lower phase mainly consisted of residual MeOH and glycerol. The upper phase was collected and evaporated under reduced pressure at 60°C for 30 min to yield a residue, which was washed three times with water until its pH was approximately 7. The neutral residue was then dried under reduced pressure at 80°C–90°C for 60 min. This washing and drying process removed most of the undesired by-products, reagents, and solvents from the crude BDF. Finally, a 40 μl sample of the dried residue was dissolved in 4 ml of acetone for analysis to determine its FAME, TG, DG and MG contents.
The effects of several different parameters on the transesterification of VMO were investigated to determine the optimal process for obtaining the highest possible FAME yield (more than 98%). The molar ratio of MeOH/oil was evaluated using ratios in the range of 3:1–7:1; the amount of acetone was evaluated in the range of 10%–40% (wt/wt based on the mass of VMO); the amount of KOH was evaluated in the range of 0.5–2.0 wt% (based on the mass of VMO); the reaction temperature was evaluated in the range of 30°C–60°C; and reaction time was evaluated in the range of 5–60 min. All these experiments were conducted in triplicate to evaluate the analytical errors (%).
2.2.3 Oxidation stability of VMO BDF and RO BDF:
One set of six glass impingers (50 ml in volume) was divided into two groups. VMO BDF (20 ml) was placed in three of the impingers, whereas RO BDF (20 ml) was placed in the remaining three impingers. The first impinger in each group was connected to an air pump for aeration at an air flow rate of 0.2 l/min. The second impinger in each group was opened and exposed to air. The third impinger in each group was sealed with a cap to avoid exposure to the atmosphere. All six samples were then held in a water bath at 30°C for 1 month. Small aliquots of these samples were taken out every 7 days to determine their iodine values and FAME compositions. The same experiment was conducted at 40°C.
2.3 Analytical methods
2.3.1 Chemical and physical properties of the raw materials:
The iodine values of the samples were determined using the Wijs method . The acid values were determined according to an acid-base titration method using phenolphthalein as an indicator. Water content was determined using a MKC-501 Karl Fisher titration instrument (Kyoto Electric Industry, Kyoto, Japan). Viscosity was determined using a Viscosimeter (Ostwald). All these experiments were conducted in triplicate.
The FAME components were determined using a GC/MS-QP 2010 instrument (Shimadzu, Kyoto, Japan) and a capillary column SP 2380 (30 m×0.25 mm, i.d., 0.20-μm thickness, Supelco, Tokyo, Japan) with He as the carrier gas at a constant flow rate of 1.0 ml/min. These analyses were conducted in the split mode with a 20:1 split ratio and a sample injection volume of 1 μl. The oven temperature was set at 50°C at the beginning of the analytical process and held at this temperature for 1 min, before being increased at a rate of 4°C/min to 250°C, where it was held for 5 min. The injection and ion source temperatures were held at 250°C and 220°C, respectively. Mass spectrometry was conducted in the EI mode over a mass scan range of 35–500 Da with an ionization voltage of 70 eV. The concentration of FAMEs was determined using methyl heptadecanoate as an internal standard.
2.3.2 Estimation of the BDF conversion efficiencies:
The TG, DG, MG and FAME contents of the BDF were determined using a Shimadzu high-performance liquid chromatography (HPLC) system equipped with an Asahipak GF 310 HQ gel permeation column (300×7.5 mm, 5 μm, Shodex, Tokyo, Japan), RID-10A detector (Shimadzu), and an SIL-10AF auto-injector (Shimadzu). HPLC analysis was conducted with a column temperature of 30°C, using acetone as a mobile phase at a flow rate of 0.5 ml/min, and a sample injection volume of 20 μl . The BDF conversion efficiencies are estimated based on the FAME yields (%) which are calculated using the equation 
where WFAMEs and Woil are the weights of the FAMEs and VMO before and after the transesterification reaction; and MFAMEs and Moil the molecule weights of the FAMEs and VMO, respectively.
3 Results and discussion
3.1 Chemical and physical properties of RO, VMO and JCO
The chemical and physical properties of RO, VMO, and JCO are shown in Table 1. The amount of VMO found in the seeds was 33.7%±0.4%, whereas those of RO and JCO were determined to be in the ranges of 37%–50% and 35%–40%, respectively , .
The most abundant fatty acid component of VMO was determined to be α-eleostearic acid (C18:3) with a content of 80.3%±0.5%, whereas those of RO and JCO were oleic acid and linoleic acid, respectively (Table 1). For feedstocks containing a high acid value, such as JCO and rubber seed oil (more than 2.5 wt% or 5 mg KOH/g oil), the BDF production process consisted of two stages, including (i) the esterification of the FFAs in the oil using an acid catalyst (e.g. sulfuric acid) or metallic catalyst (e.g. zinc or zinc acetate) to reduce the acid value of the oil; and (ii) the transesterification of TG, DG, and MG with an alkaline catalyst, such as KOH , , . Given that the acid value of the VMO used in the current study was 1.5 mg KOH/g oil, this was used directly for BDF production without any pretreatment process.
3.2 Optimization of the process for VMO BDF production using the co-solvent method
The effects of several different parameters on the transesterification reaction of VMO using the co-solvent method were evaluated, including the molar ratio of MeOH/oil, amount of catalyst, amount of co-solvent, reaction temperature, and reaction time. The results of these experiments are shown in Figure 2 together with the analytical error bars for each parameter.
3.2.1 Effect of the molar ratio of MeOH/oil
The molar ratio of MeOH/oil was evaluated using ratios in the range of 3:1–7:1. Figure 2A shows the relationship between the FAME yield and the molar ratio of MeOH/oil. The results revealed the possibility of achieving a FAME yield greater than 90% with a MeOH/oil ratio of more than 3:1, representing the stoichiometric molar ratio of the transesterification reaction. This result was identical to those previously reported for the transesterification reaction , , . Notably, the FAME yield reaches 99% when the MeOH/oil molar ratio was 6:1 or 7:1. Based on this result, the molar ratio was set to 6:1 for the transesterification process.
3.2.2 Effect of the amount of KOH catalyst
The amount of KOH was evaluated in the range of 0.5–2.0 wt% (based on the mass of VMO). The relationship between the FAME yield and the amount of KOH added to the reaction is shown in Figure 2B. The results showed that the FAME yield increased considerably from 90% to 99% when the amount of KOH added to the reaction increased from 0.5% to 1%. However, further increases in the amount of KOH did not lead to further increases in the FAME yield. Similar results have been reported previously for the co-solvent method , , , , . The optimum amount of KOH was therefore determined to be 1%.
3.2.3 Effect of the amount of solvent
The amount of acetone was evaluated in the range of 10%–40% (wt/wt based on the mass of VMO). The effect of the amount of acetone added to the reaction on the FAME yield is shown in Figure 2C. As can be seen, when the amount of acetone was increased from 10% to 20%, the FAME yield increased from 96% to 99%, which was the highest value observed in this study. Further increases in the amount of acetone to values in the range of 30%–40% led to a reduction in the FAME yield to 94%–95%. This phenomenon was also observed during the production of BDF from JCO (JCO BDF) , and can be explained in terms of the dilution of the reactants with acetone, leading to a reduction in the rate of the reaction. Taken together, these data indicated that the amount of solvent was critical to achieving a high FAME yield. The optimum amount of acetone co-solvent for VMO BDF production was thus set at 20%.
3.2.4 Effect of the reaction temperature
The reaction temperature was evaluated for temperature in the range of 30°C–60°C. The effect of the reaction temperature on the FAME yield is shown in Figure 2D. As can be seen, the FAME yield increased from 95% to 99% when the temperature increased from 30°C to 40°C. However, further increases in the temperature led to a slight decrease in the FAME yield to 97%, which can be attributed to the evaporation of acetone under the high-temperature conditions, compromising the homogenous nature of reaction mixture. It is noteworthy that high reaction temperatures are typically used in the mechanical stirring method to produce BDFs, such as 60°C for VMO  and canola oil , and 80°C for corn oil . Based on these results, the optimum temperature for this transesterification reaction was set at 40°C.
3.2.5 Effect of the reaction time
The reaction time was evaluated in the range of 5–60 min. Figure 2E shows the relationship between the reaction time and the FAME yield. Small aliquots (2 ml) of the transesterification reaction mixture were collected at specific time intervals and neutralized by the addition of 5% phosphoric acid to stop the reaction. A FAME yield of 87% was achieved after 5 min, whereas a BDF conversion of greater than 98% was obtained after 30 min. It is noteworthy that mechanical stirring methods for BDF production usually require 60 min to achieve a high BDF conversion yield , .
Taken together, these results indicated that the optimal conditions required to obtain a FAME yield of 99%±0.3% for the production of VMO BDF using the co-solvent method were as follows: a MeOH/oil molar ratio of 6:1, 20% (wt/wt) acetone co-solvent, and 1% (wt/wt) KOH at a reaction temperature of 40°C. Compared with the conventional mechanical stirring method , the co-solvent method used in this study led to considerable reductions in the reaction time (30 min for the co-solvent method vs. 120 min for the conventional method), the consumption of KOH (1% vs. 3%), and the reaction temperature (40°C vs. 60°C) required to achieve a FAME yield of 99%. Therefore, these changes also led to a considerable decrease in energy consumption for BDF production.
3.2.6 Properties of VMO BDF
The properties of the VMO BDF produced in this study are shown in Table 2, together with the quality criteria defined by EN 14214 and JIS K2390. The results revealed that the BDF had a FAME content of 99%, density of 897 kg/m3 (15°C), water content of 275 mg/kg, acid value of 0.3 mg KOH/g oil, MG content of 0.5%, DG content of 0.2%, TG content of 0.1%, contained no free glycerol, and had a total glycerol content of 0.18%, which were all well within the required quality criteria. The freezing point of the VMO BDF produced in the current study (−12°C) was much lower than those of JCO BDF and RO BDF (1.8°C and −10.5°C, respectively), thereby suggesting that this material could be used under cold weather conditions. However, this material had an iodine value of 158.5 g I2/100 g and a kinetic viscosity of 7.7 mm2/s (40°C), which are both outside of the required criteria. Therefore, this VMO BDF material must be blended with other BDFs, such as canola oil BDF or palm oil BDF, to enhance its iodine value and viscosity characteristics . Furthermore, it would be important to evaluate the stability of this VMO BDF during storage because it comprises 80.3% α-eleostearic acid methyl ester, which consists of three conjugated double bonds, making VMO BDF unstable in air.
3.3 Oxidation stability of VMO BDF
3.3.1 Iodine value (IV)
As shown in Figures 3A and B, the IV of RO BDF was determined to be 118 g I2/100 g at the beginning of the investigation and remained largely unchanged when it was held in storage at 30°C and 40°C, except after 4 weeks of aeration with air at 40°C, when it decreased from 118 to 90 g I2/100 g. In contrast, the IV of VMO BDF decreased from an initial value of 159 to 91 g I2/100 g at 30°C and 69 g I2/100 g at 40°C after 1 week of aeration with air (Figures 3C and D). However, when VMO BDF was stored in a closed vessel, it remained stable for more than 1 month, even when the cap was opened and the surface of the BDF was exposed to the atmosphere at 30°C (Figure 3C). To develop a deeper understanding of the processes responsible for the stability of VMO BDF during storage, we also monitored for changes in its unsaturated ester content.
3.3.2 Unsaturated ester content of VMO BDF
The variations observed in the unsaturated FAME content of VMO BDF over a 4-week period are shown in Table 3. The unsaturated FAME content of VMO BDF gradually decreased when the material was aerated with air. Most notably, the amount of α-eleostearic acid methyl ester decreased from an initial value of 80% to 11% when it was held at 30°C for 7 days. Notably, this material degraded completely when it was held at 40°C for 7 days under the same conditions. VMO BDF was oxidized much more readily than RO BDF, because it contained a much higher amount of unsaturated FAMEs containing three conjugated double bonds than RO BDF (80% vs. 8%). Based on the data in Table 3, the oxidative degradation of the unsaturated esters was determined to be of the order eleostearates (C18:3)>linoleates (C18:2)>oleates (C18:1), consistent with the results of a previous report . Otherwise, the compositions of the VMO BDF samples that were sealed with a cap at temperatures in the range of 30°C–40°C remained unchanged during the 4-week storage period. Therefore, these results demonstrate that the amount of oxygen exposure had a significant impact on the stability of the unsaturated FAMEs.
The stability of VMO BDF at 20°C was also evaluated and compared with that of RO BDF. The results for these experiments were similar to those obtained at 30°C. Taken together, these data suggested that VMO BDF was as stable as RO BDF to 4 weeks of storage at temperatures in the range of 20°C–40°C in closed vessels, despite the fact that VMO BDF contained a large amount of α-eleostearic acid methyl ester. For good stability under high-temperature storage conditions, the results of the current study suggest that VMO BDF should be stored in a sealed container to avoid exposure to the air (oxygen). In terms of its practical application, the results of the current study suggest that VMO BDF should be blended with petroleum diesel or saturated esters to reduce its degree of unsaturation. Given that VMO represents a potential feedstock for the production of BDF, we are currently evaluating the stability of VMO BDF for longer storage times (2, 3, and 6 months) to achieve a better understanding of its stability.
The application of the co-solvent method to the production of BDF from VMO led to a reduction in the amount of energy consumed by this process as a result of considerable reductions in the reaction time, consumption of KOH, and reaction temperature compared with other conventional methods. The BDF produced using our newly optimized procedure satisfied the EN 14214/JIS K2390 quality criteria regarding its FAME, water, MG, DG, TG, free glycerol, and total glycerol contents, as well as its acid value and density. VMO BDF contained in closed vessels showed good stability equal to RO BDF following 4 weeks of storage at temperatures in the range of 20°C–40°C. However, the stability of VMO BDF needs to be investigated for longer storage periods to develop a better understanding of its long-term stability.
The authors acknowledge the financial support from the Science and Technology Research Partnership for Sustainable Development (SATREPS, Project: Multi-beneficial Measure for the Mitigation of Climate Change by the Integrated Utilization of Biomass Energy in Vietnam and Indochina countries), JST-JICA.
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About the article
Hanh Ngoc Thi Le
Hanh Ngoc Thi Le achieved her Master of Science in Analytical Chemistry in 2011 from the University of Natural Sciences, Vietnam National University, Ho Chi Minh City. Since 2006, she has been an employee of An Giang University, Vietnam. She is currently a doctoral student at Osaka Prefecture University, Japan. Her research activities concern the production of biodiesel fuels via the co-solvent method and the utilization and chemical compositions of feedstocks used for biodiesel fuel production.
Kiyoshi Imamura received his PhD in Chemical Engineering from Osaka Prefecture University in 1995. He has been a quest researcher of the University upon his retirement from the Environment Institute of Osaka Prefecture. His current research is BDF production, particularly the analysis of biologically active chemicals in BDF plants.
Masakazu Furuta received his PhD from Kyoto University, Japan, in 1993. He is now a professor at the Radiation Research Center in Research Organization for University-Community Collaborations, and Department of Quantum and Radiation Engineering, Graduate School of Engineering, Osaka Prefecture University. His research focuses on novel applications of gamma radiation and electron beams in the field of food and medical-material science and technology, identification of irradiated foods, and analysis of inactivation mechanisms of microorganisms in food and medical devices.
Luu Van Boi
Luu Van Boi received his Doctor of Science degree in Chemistry from the N.D. Zelinskiy Institute of Organic Chemistry, Russian Academy of Sciences in 1999 after obtaining his PhD diploma in Chemistry in 1991 at the Kiev National University. His scientific interests are focused on chemistry of bioactive sulfur- and nitrogen-containing heterocyclic compounds, technology for production of biomass energy and pour point depressants for crude oils and biodiesel fuels. At present, he is the Director of the Vietnam National University Key Laboratory for Bioenergy Development.
Yasuaki Maeda received his PhD in chemical engineering from Tokyo Institute of Technology in 1970. He currently works as a research professor at the Research Organization for University-Community Collaborations, Osaka Prefecture University, Japan. He is also a member of the editorial board of Ultrasonic Sonochemistry, has an honorary doctorate at Vietnam National University, Hanoi, and is a distinguished Professor in Vietnam National University Ho Chi Minh. His research activities focus on environmental analysis, sonolytical chemistry, and biomass energy production.
Published Online: 2017-06-13
Published in Print: 2018-04-25