Structure Evolution of Synthetic Amino Acids-Derived Basic Ionic

Research Article pubs.acs.org/journal/ascecg

Structure Evolution of Synthetic Amino Acids-Derived Basic Ionic Liquids for Catalytic Production of Biodiesel Jingbo Li and Zheng Guo* Department of Engineering, Faculty of Science and Technology, Aarhus University, Gustav Wieds Vej 10, 8000, Aarhus C, Denmark

ACS Sustainable Chem. Eng. 2017.5:1237-1247. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/23/19. For personal use only.

S Supporting Information *

ABSTRACT: A two-step strategy was attempted to develop the best amino acid-based basic ionic liquids for catalytic production of biodiesel via transesterification. Cholinium with various amino acids as paired anions were first synthesized to screen anionic moiety. Arginine and histidine were selected for further structural evolution by varying the substituents of tetraammonium cation. Tetrabutylammonium arginine ([TBA][Arg]) was found to be the most effective catalyst to obtain 98.0%−99.8% yield of biodiesel at 80 °C within 15 min with catalyst loading of 28.84 mmol/100 g high oleic sunflower oil. 13C NMR spectra of reactants and products certified the progress of transesterification structurally. Biodiesel yield of 98.80% was obtained under the optimal conditions: catalyst loading 6% (oil basis, w/w), temperature 90 °C, methanol to oil mole ratio 9:1, and 15 min reaction. The catalytic transesterification by [TBA][Arg] was applicable for different alkyl alcohols, but the activity decreased with increasing alkyl chain length. The catalyst did not show specificity and preference to different glycerides and different fatty acids. The strong protonizability of the guanidine moiety in [Arg]− and stability of [TBA]+, [TMA]+ and [Ch]+ in methanol are suggested to be responsible for the high catalytic activity of the ILs. The developed catalyst significantly reduced the reaction time and might be greener and more sustainable due to the properties of the substrates and the preparation in water. KEYWORDS: Biodiesel, Amino acid-based basic ionic liquids, Basicity, Catalytic mechanism, Transesterification

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hydroxides, as well as sodium or potassium carbonates14 and different types of lipase.15 Ionic liquids (ILs) have attracted tremendous attention in the past decades in almost every aspect of the chemical and physical sciences due to their design and green properties compared to conventional volatile organic solvents.16 The utilization of ILs in biodiesel production, without exception, has also gained momentum. ILs could serve as (1) (co)solvents for chemical and enzymatic preparations of biodiesel, (2) catalysts in biodiesel synthesis, and/or (3) extraction solvents in biodiesel production.16 ILs with catalytic activity for biodiesel production are of particular interest because they could serve as both catalyst and solvent and even benefit subsequent purification processes. Recently, many acidic ILs have been synthesized for esterification reaction to produce biodiesel.17−22 Meanwhile, some basic ILs were synthesized and used as catalysts for biodiesel production via the transesterification process. For example, the amphiphilic alkaline ionic liquid, 1,1,3,3-trimethyl-2-octyl-guanidine hydroxide, was found to be effective for biodiesel production.23 Choline hydroxide was synthesized and used directly as a catalyst for efficient biodiesel production.24 An alkaline ionic liquid 1-butyl-3-methylimida-

INTRODUCTION

Petroleum fuels, in the first place, are depleting as they are from a nonrenewable source, and in the second place, the atmospheric pollution including NOx, SOx, CO, particulate matter, and volatile organic compounds from combustion of them is another major concern.1,2 Biodiesel, as an alternative diesel fuel, is generally made from renewable biological resources like vegetable oils, animal fats, or microalgae lipids.1 The amount of CO2, particulate matter, and greenhouse gas emissions could be reduced by using fuels from renewable biomass.2 Additionally, biodiesel is biodegradable and nontoxic.1−3 Production of biodiesel mainly involves two types of reactions, namely, esterification and transesterification. Esterification generally refers to the reaction between free fatty acids and alcohols catalyzed by either Lewis/Brønsted acids such as solid acids and inorganic acids or biocatalysts such as lipases.4−9 Acid-catalyzed esterification requires longer reaction time and a high methanol/oil mole ratio, resulting in less attention than the alkali-catalyzed transesterification.10 Biodiesel production through transesterification has been intensively studied and reviewed.3,11−13 The typical transesterification for biodiesel production is a reaction between glycerides and alcohols typically driven by basic catalysts in industrial process.11 The most used catalysts include alkaline, metal alkoxides, and © 2016 American Chemical Society

Received: November 11, 2016 Revised: December 1, 2016 Published: December 7, 2016 1237

DOI: 10.1021/acssuschemeng.6b02732 ACS Sustainable Chem. Eng. 2017, 5, 1237−1247

Research Article

ACS Sustainable Chemistry & Engineering

magnetic agitation at room temperature for 48 h. Water was then removed under vacuum at 80 °C. The solvent mixture, acetonitrile/ methanol (9:1), was employed to precipitate the excessive or unreacted amino acids. Vacuum filtration was used to remove the excessive amino acids. The filtrate was evaporated to remove solvents at 60 °C, and the product was further dried under vacuum for 30 min at 90 °C. The synthesized ILs were stored at 4 °C before use. Characterization of ILs. The synthesized ILs were dissolved individually in methanol at concentrations of 2% or 0.05 mol/L. The pH value was measured thereafter by using a pH meter (InoLab, pH 7110, WTW, Germany). Calibration of the pH meter by the three points program including pH 4.01, pH 7.00, and pH 10.00 was performed just before determining the basicity of the ILs. The catalytic activity of transesterification for each IL was determined by using the method described in the following section. 1 H NMR spectra of all ILs in D2O were recorded by a Bruker Ascend 400 NMR equipment. Residual H2O peak at 4.87 ppm was used as the reference. MestReNova 10 software was used to process the obtained data. General Procedures of Transesterification. Methanol and sunflower oil with high oleic acid content were used as model substrates first to evaluate the effectiveness of the ILs and optimize reaction conditions. A certain amount of sunflower oil and catalyst (ILs) were added in a double-necked flask equipped with a reflux condenser. The temperature of the condenser was controlled at around 22 °C in order to prevent moisture in the air from being condensed and alcohol in the reaction system from being evaporated. The flask was then placed in an oil bath at the desired temperature for 5 min in order to make sure the mixture of sunflower oil and catalyst reached the required temperature as well. The magnetic stirring speed was around 800 rpm to avoid any potential mass and heat transfer problems. Subsequently, a certain amount of methanol was poured in the flask, and a syringe with rubber stopper was used to block another neck. Samples were taken regularly from the equipped syringe. Hexane was used to dissolve and dilute the samples. Glycerol in the system was washed out by water. The hexane phase was analyzed by thin-layer chromatography and flame ionization detection (TLC-FID) afterward. Analytical Methods. Nuclear Magnetic Resonance (NMR) Analyses. In order to verify the transesterification process structurally, 13 C NMR was performed. The hexane phase prepared from transesterification reaction was placed in a vacuum desiccator to remove hexane. The reaction mixture samples were dissolved by 0.7 mL of CDCl3 and subjected to a Bruker Ascend 400 NMR equipment to record 13C NMR spectra. TLC-FID Analyses. An Iatroscan Mark VI TLC-FID (Iatroscan MK6s, Japan) was used to monitor the content of mono-, di-, and triglycerides, free fatty acids, and biodiesel. The sample was spotted on silica coated chromarods (Chromarod-S III, Japan) and developed by the solvent mixture hexane:diethyl ether:formic acid (50:20:0.7, v/v/ v) for 15 min, followed by developing by the solvent mixture benzene:hexane (50:50, v/v) for 30 min. The rods were then dried at 120 °C prior to analysis for 5 min. The results were calculated based on the ratio of peak area of target compound to the total peak area. Determination of Fatty Acid Composition of Nonbiodiesel Fractions. TLC plates coated with silica (TLC Silica gel 60 F254, Sigma-Aldrich) were used to separate the nonbiodiesel compounds. The sample loaded plates were developed by the solvent mixture of hexane:diethyl ether:formic acid (64:16:0.4, v/v/v) until the solvent front reached the top of the plates. The silica gels of sample bands excluding biodiesel band on the developed TLC plate were scraped under UV (254 nm) visualization and then collected and combined for lipid extraction by using chloroform:methanol (2:1, v/v). The resulting fraction was methylated after removal of solvent, and the fatty acid methyl esters were subjected to fatty acid composition analysis. Fatty Acid Composition Analysis. The samples were methylated by using methanolic sodium and BF3-methanol solution. GC-FID (Trace GC Ultra, Thermo Fisher Scientific Inc., Waltham, MA, U.S.A.) was employed to determine the fatty acid composition. Zebron ZB-FFAP, GC Cap. Column 30 m × 0.32 mm × 0.25 μm, column (Phenomenex,

zolium imidazolide effectively catalyzed the conversion of rapeseed, soybean, and sunflower oils into biodiesel.25 Utilization of ILs as cosolvent for transesterification was also studied. For instance, a deep eutectic solvent (DES) consisting of choline chloride and glycerol (1:2 M ratio) was used as cosolvent, and it improved biodiesel yield, reduced saponification, and enabled a straightforward separation when sodium hydroxide was used as catalyst.26 Additionally, mesoporous SBA-15 coated with tetraalkylammonium hydroxides, 4-butyl1,2,4-triazolium hydroxide, or 1,3-dicyclohexyl-2-octylguanidine was proved effective for biodiesel production.27−29 All these mentioned works highlight the effectiveness of basic ILs in biodiesel production via transesterification. Biobased ILs have been recognized as greener and more sustainable media compared with petroleum-derived ILs.30 Amino acids, as bioavailable compounds, can be converted into charged ion species as either anions or cations because each amino acid molecule contains a carboxylic residue and an amine moiety.31,32 Amino acid-based ILs were used in many different aspects such as in adsorption of CO2,33 in metal scavenging and heterogeneous catalysis,34 in chiral separation,35 in pretreatment of lignocellulose,36 and so on. Recently, the acidic ILs derived from amino acids were studied to catalyze the esterification reaction of free fatty acids in our group, and the results showed they are promising (data not published). Additionally, the ILs, cholinium paired with amino acids, have basic properties with pH value ranging from 6.7 to 11.3 at 5 mM concentration in water.36 Therefore, the cholinium amino acids ILs may serve as catalysts for the transesterification process as well. Low toxicity and high biodegradability of cholinium amino acids ILs were proved, making them promising and environmentally friendly catalysts.37 It was thought that the amino acid-based ILs show greener properties than those ILs from synthetic chemicals.38,39 To envisage the technological potential of amino acid-based ionic liquids as a greener catalyst, an evolution strategy was adopted for obtaining the best IL for biodiesel production. Cholinium with varied amino acids as paired anions was in the first place synthesized and tested for its catalytic transesterification efficiency in order to select the most promising anionic amino acid. Subsequently, more ILs derived from the selected promising amino acid (arginine and histidine) by varying the substituents of tetraammonium cation were synthesized and examined. The IL with highest catalytic efficiency was then determined for optimization study of operation variables. Different alcohols and oil feedstocks were applied as well to study the specificity and preference of the amino acid-based IL. Finally, a kinetic study was conducted, and a possible catalytic mechanism was proposed.

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MATERIALS AND METHODS

Materials. Choline hydroxide ([Ch][OH]), tetrabutylammonium hydroxide 30-hydrate ([TBA][OH]·30H2O), tetramethylammonium hydroxide pentahydrate ([TMA][OH]·5H2O), and methanol were purchased from Sigma-Aldrich Co. (St. Louis, MO, U.S.A.). Histidine, arginine, alanine, phenylalanine, and serine were purchased from VWR Bie and Berntsen (Soeborg, Denmark). Deionized Mili-Q water with 18.2 mΩ resistivity was used for synthesis of the ionic liquids. All other mentioned chemicals were purchased from Sigma-Aldrich Co. (St. Louis, MO, U.S.A.) as well. Synthesis of ILs. Either [Ch][OH] or [TBA][OH] or [TMA][OH] aqueous solution was added dropwise to each amino acid aqueous solution. The molar ratio of all amino acids was 1.1 times higher than those bases. The reaction was allowed to proceed with 1238


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ACS Sustainable Chemistry & Engineering Table 1. Catalytic Activity and Basicity of ILs with the Same Cation

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Yield was measured after reaction for 21 h at 50 °C, methanol:triglyceride = 9:1 (mol/mol), catalyst loading 4% triglyceride by TLC-FID as described in Materials and Methods.

Table 2. Catalytic Activity and Basicity of ILs with the Same Anion

*

Yield was measured after reaction for 1 h at 80 °C, methanol:triglyceride = 9:1 (mol/mol), catalyst loading 28.84 mmol/100g triglyceride.

U.S.A.) was used to separate different fatty acid methyl esters and the following detection was performed by the equipped FID. The temperature of injector and detector was 250 °C. The temperature program was as follows: holding at initial temperature of 90 °C for 1 min, followed by increasing up to 150 °C at the speed of 30 °C/min, then increasing further to 225 °C at the speed of 3 °C/min, and finally holding for 8 min.

All synthesized ILs in the present work were in liquid form at room temperature. As shown in Table 1, the pH values of all the ILs dissolved in methanol were higher than 10 when cholinium was employed as the cation. Amino acids are classified into four groups according to their side chains, namely, (1) nonpolar and neutral, (2) polar and neutral, (3) acidic and polar, and (4) basic and polar. Within the investigated amino acids, arginine and histidine are two basic amino acids, whereas all the others are neutral amino acids. The formation of the ILs is a neutralization reaction, during which the carboxyl group in amino acids was neutralized, and therefore, all ILs displayed basic properties due to the protonatable amine groups on either main backbone or/and side chain. [Ch][Arg] showed the highest basicity, while [Ch][Ser] and [Ch][Phe] displayed the lowest basicity among investigated cholinium amino acids ILs, which agrees well with the order of pH values in water.36 The basicity of the ILs shows dependent relations with the basicity of the parental amino acid

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RESULTS AND DISCUSSION Basicity of Synthetic Amino Acid-Based ILs. A two-step strategy was used for structural evolution of synthetic amino acid-based ILs: (1) screening of anionic amino acids with cholinium as a fixed pair, leading to a series of ILs presented in Table 1 and (2) arginine and histidine selected for structural optimization by altering cations, yielding another array of ILs shown in Table 2. All these synthetic ionic liquids have been structurally identified by 1H NMR analysis. The NMR spectra and peak assignments are presented in the Supporting Information. 1239


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ACS Sustainable Chemistry & Engineering only to some degree. For example, the most basic amino acid, arginine, resulted in the most basic IL. However, histidine, as a basic amino acid, led to the IL with similar basicity to that of the other neutral amino acid derived ILs (Table 1). The pKa of the imidazole side chain in histidine is approximately 6.0, whereas that of guanidine containing side chain in arginine is approximately 12.5, resulting in the dramatic difference of the two ILs in terms of basicity. In order to study the effect of cation properties on the basicity of the ILs, [TBA][OH] and [TMA][OH] were used to synthesize ILs with histidine and arginine due to their similar chemical structure with [Ch][OH] as shown in Table 2. The basicity of [TBA][His] in both concentrations was higher than that of [TMA][His] and [Ch][His], implying that [TBA]+ enhances the basicity. However, the basicity of arginine-based ILs was not so much different from each other. [TBA]+ has longer side alkane chains compared with [TMA]+ and [Ch]+. The protonated nitrogen atom is therefore more stable because of the bulky substituent surroundings. Catalytic Activity for Transesterification Reaction. Table 1 shows the biodiesel yield after 21 h reaction catalyzed by the ILs with the same cation. The highest yield of 98.97% was catalyzed by [Ch][Arg]. The biodiesel yields catalyzed by [Ch][Ala] and [Ch][His] were 6.34% and 6.79%, respectively. The lowest yield (0.12%) was resulting from the reaction catalyzed by [Ch][Phe]. [Ch][Ser], [Ch][Leu], [Ch][Met], and [Ch][Gly] gave rise to the transesterification efficiency lower than 2.81%. The catalytic efficiency was related to the pH values of their methanol solution. For example, the most basic IL corresponded to the highest catalytic activity, and those less basic ILs resulted in very low biodiesel yield. Since this group of ILs shared the same cation, the catalytic activity should be determined by the anions, which were amino acids in the current case. Furthermore, a close relation between the catalytic activity and isoelectric point of the amino acid could be observed. The isoelectric point of the parental amino acid could be regarded as a balance point between its acidity and basicity. The higher isoelectric point is, the stronger basicity is. In order to find out the effect of cation on the catalytic activity for transesterification, two more similar cations were selected to prepare ILs with histidine and arginine, and the results are shown in Table 2. In line with the results in Table 1, all ILs derived from arginine showed significantly higher catalytic activity than those derived from histidine. Within the histidine derived ILs group, [TBA][His] was the best for transesterifying triglycerides into fatty acid methyl esters, followed by [TMA][His] and then [Ch][His]. However, only 12.22% biodiesel yield was achieved even the best [TBA][His] was used. [Ch][Arg] and [TBA][Arg] catalyzed fully the conversion of triglyceride, whereas [TMA][Arg] displayed slightly lower convertibility. It was difficult to compare [TBA][Arg] and [Ch][Arg] in terms of transesterification efficiency. Therefore, a time course of transesterification reaction was performed using these three ILs, and the results are shown in Figure 1. Apparently, the biodiesel formation rates catalyzed by the ILs were different. The reaction equilibrium was reached within 15, 40, and 60 min for [TBA][Arg], [Ch][Arg], and [TMA][Arg], respectively, suggesting the reaction rate was accelerated when using [TBA][Arg], but the final yield of product was not affected when longer reaction time was offered. It was reported that NaOH and KOH with loading of 2% resulted in biodiesel yields of 93.1% and 92.4%, respectively, for the transesterification reaction at 60 °C for 2.5

Figure 1. Time course of transesterification reaction catalyzed by [TBA][Arg], [Ch][Arg], [TMA][Arg], and arginine. Conditions: 80 °C, methanol:triglyceride = 9:1(mol/mol), catalyst loading 28.84 mmol/100 g triglyceride.

h with methanol to soybean oil ratio of 9:1.24 Under the same reaction condition, the catalyst, [Ch][OH], yielded 95% biodiesel yield with the loading of 4%.24 Compared with these results, the catalytic performance by the ILs developed in the present work is better or comparable to the conventional alkalis. The yield of biodiesel is also higher than the referred data. The transesterification reaction catalyzed by the synthesized [TBA][Arg] using high oleic acid sunflower oil and methanol was also monitored by the 13C NMR technique in order to illustrate the reaction progress structurally. The spectra are shown in Figure 2. Figure 2A was the sunflower oil used in the study. As shown, no peak appeared at 51.52 ppm, which is attributed to the methoxy group in fatty acid methyl ester.40 C2 on the glycerol backbone of triglyceride resonates at 69.06 ppm, while those of 1(3) monoglyceride and 1,2-diglyceride resonated at 70.38 and 72.10 ppm, respectively.41 C1(3) on the glycerol backbone resonates upfield compared with C2 in all glycerides. The sunflower oil used in the present work contained a little 1(3) monoglyceride and 1,2-diglyceride as indicated by Figure 2A. Figure 2B shows the spectrum after reaction for 3 min, and a new peak at 51.52 ppm was observed, indicating the formation of fatty acid methyl esters. Correspondingly, the peak denoting triglyceride decreased. With the increasing reaction time (Figure 2C−E), the peaks of triglyceride, 1(3) monoglyceride, and 1,2-diglyceride became smaller and smaller, clearly suggesting the transesterification reaction proceeded over time. Proposed Reaction Mechanisms. The results in Table 1 clearly suggest that [Arg]− plays a critical role for its catalytic activity of transesterification. Aarginine is the most basic one among the investigated amino acids, which generated the ionic liquid with the highest catalytic activity. Meanwhile, the results in Table 2 and Figure 1 indicate that cation affects the catalytic activity as well. On the basis of these observations, the mechanism of the reaction using the developed catalyst was proposed and is shown in Figure 3. [TBA][Arg] and methanol tend to dissociate in the reaction system. The ions could pair freely with energy favorability. For example, H+ dissociated from methanol tends to associate with the NH group on the 1240


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Figure 2. 13C NMR of the glyceride fraction from the reaction mixture catalyzed by [TBA][Arg] as reaction progress. (A) Sunflower oil (0 min). (B) Reaction after 3 min. (C) Reaction after 6 min. (D) Reaction after 9 min. (E) Reaction after 12 min.

guanidine of [Arg]− because of its high pKa value. This kind of dissociation and reassociation generates methoxyl ions which serve as nucleophiles. The transesterification reaction to produce biodiesel from glycerides is a typical nucleophilic substitution reaction. The negatively charged nucleophiles attack the positively charged carbonyl carbon of glycerides, and then, the reaction occurs.24 Therefore, [Arg]− is the key to initiate the reaction as compared to the anions derived from other amino acids. Meanwhile, cations like [TBA]+, [TMA]+, and [Ch]+ also influenced the catalytic velocity as shown in Figure 1. The catalytic activity order was [TBA][Arg]> [Ch][Arg]> [TMA][Arg]. As discussed above, the generated methoxyl ions also tend to pair with cations like [TBA]+, [TMA]+, and [Ch]+. When a stronger association exists between methoxyl and the cations from ILs, the catalytic activity is lowered. The experimental data also support this hypothesis. [TBA]+ contains more alkyl groups, and therefore, the paired methoxyl is freer than that paired with the other two. According to this theory, [Ch][Arg] should be more active than [TMA][Arg], and this fits the experimental data. The generated methoxyl anions then attacks the carbonyl group of triglyceride to form an intermediate. After rearrangement and splitting, with the involvement of methanol, diglyceride, biodiesel, and the methoxyl anions are released again. These steps are well described by many reports.1,23,24 OH− is the main active species for activating methoxyl anions and further stimulates the reaction in these reports.1,23,24 However, in the present work, [Arg]+ was found to be the active species for generating methoxyl anions. It should be

noted that pure arginine was unable to catalyze the reaction as shown in Figure 1, suggesting the arginine has to be in the form of ILs in order to endow its catalytic activity for transesterification. Optimization of reaction conditions Effect of Catalyst Dosage. Mechanism of base-catalyzed transesterification suggests that methoxide acts as a nucleophile for the transesterification reaction.3 The methoxide is formed from the reaction between base catalyst and methanol, and the concentration of methoxide determines the transesterification reaction rate. Therefore, the effect of catalyst loading on biodiesel yield against time was investigated, and the results are shown in Figure 4. At the initial stage, biodiesel yield increased with increasing reaction time regardless of catalyst loading. However, higher catalyst loadings gave rise to shorter reaction time to reach the reaction equilibrium. For example, only 15 min was required for catalyst loading of 6% compared to 25 min for the loading of 4%. Biodiesel yield was around 99% for both catalyst loadings of 4% and 6% after 40 min reaction. As reported, biodiesel yield of 95% was achieved within 2.5 h at 60 °C using 4% [Ch][OH] as catalyst.24 Compared with [Ch][OH], the catalyst developed in the present work is more effective by achieving higher yield in a shorter reaction time. Biodiesel yield of 74.4% was obtained using KOH as catalyst at a loading of 6% within 4 h.23 It was pointed out that soap will appear when NaOH or KOH loading is over 2.5% when the reaction temperature is 55 or 60 °C.23,24 No soap formation was observed in the current study even though the catalyst loading added up to 7.5% (data not shown). [TBA][Arg] at the loading of 6% was selected as the optimal 1241


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Figure 3. Proposed reaction mechanisms of [TBA][Arg] ionic liquid catalyst.

final biodiesel yield.18 The effects of different temperatures on the biodiesel yield at atmospheric pressure was studied, and the results are shown in Figure 5. As expected, higher temperature

Figure 4. Effect of catalyst concentration (wt % based on triglycerides) on biodiesel yield. Reaction conditions: 80 °C, methanol:triglyceride (n/n) = 9:1. Mean value ± standard deviation from two independent determinations is shown.

value due to the shorter reaction time required to reach the maximal biodiesel yield. Effect of Reaction Temperature. For a reversible reaction, the increase in transesterification temperature results in increase of the initial reaction rates and further increases the

Figure 5. Effect of reaction temperature on biodiesel yield. Reaction conditions: methanol:triglyceride = 9:1, catalyst loading 6%. Mean value ± standard deviation from two independent determinations is shown. 1242


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ACS Sustainable Chemistry & Engineering up to 90 °C accelerated the reaction rate and improved the biodiesel yield after 15 min reaction. When the reaction temperature was 50, 80, and 90 °C, biodiesel yield reached equilibrium after reaction for 6 or 9 min. On the contrary, the biodiesel yield increased continuously with reaction time when the temperature was 65 °C. The possible reason might be that 50 °C is too low to initiate the reaction effectively, while 80 and 90 °C are high enough to complete the reaction in a short time. The optimal temperature was found to be 60 °C when catalyst [Ch][OH] was used, and the increase in temperature to 65 °C resulted in a decrease in biodiesel yield.24 The decrease was attributed to quick evaporation of methanol in the reaction mixture at 65 °C.24 However, it was not observed in the present study. In another report, 60 °C was the best for biodiesel production using an alkaline ionic liquid 1-butyl-3-methylimidazolium imidazolide as catalyst.25 Biodiesel yield decreased if higher reaction temperature than 70 °C was employed.25 It should be noted that the reaction time in the previous reports24,25 was 2.5 and 1 h, respectively, while in the present work it was only 15 min. Shorter reaction time might be the reason that no decrease in biodiesel yield at higher temperature was observed. The optimal reaction temperature was 90 °C for catalytic biodiesel production through transesterification by the catalyst [TBA][Arg]. Effect of Methanol/Triglyceride Ratio. Increase in reactants could shift the equilibrium of a reversible reaction toward the formation of products. Therefore, it was expected that the increase in methanol to triglyceride ratio could enhance triglyceride conversion into biodiesel. The effect of mole ratio of methanol/triglyceride on biodiesel yield was investigated, and the results are shown in Figure 6. Theoretically, 3 mol

played role as reactant but also some others. The biodiesel yield from the ratio of 12 before 9 min was lower than that from the ratios of 6 and 9, and it was comparable after 12 min. It could be due to the high loading of the methanol-diluted catalyst and triglyceride concentration. Thus, except for the reactant role, methanol also plays the role as solvent with effects on relative dosage of catalyst and triglyceride. This phenomenon was also observed by some other researchers.23,24,26 On the basis of the results obtained in the present work, an n(methanol)/ n(triglyceride) ratio of 9 is recommended due to its higher biodiesel yield. Transesterification with Different Alcohol Donors. In order to explore the specificity and activity of the new catalyst on transesterifying triglyceride by different alcohols, experiments were performed, and the results are shown in Figure 7.

Figure 7. Transesterification efficiency with different alcohols of the catalyst. Reaction conditions: 90 °C, catalyst loading 6%, alcohol/ triglyceride ratio (9:1, n/n). Mean value ± standard deviation from two independent runs is shown.

Alcohols with longer chain lengths resulted in lower fatty acid alkyl ester formation. Only 68% biodiesel was obtained after 15 min reaction when ethanol was employed. Additionally, extension of the reaction time to 30 min increased the final biodiesel yield by only 22% in the case of ethanol. When 1propanol and 1-butanol was used, much lower (around 27%) biodiesel was obtained after 30 min reaction. 1-Hexanol gave rise to biodiesel yield of 22%. 2-Propanol resulted in the lowest biodiesel yield of 5.5%, indicating the catalyst [TBA][Arg] is not effective for secondary alcohols. The higher steric hindrance of 2-propanol greatly inhibits the transesterification processes with triglyceride. Within the primary alcohols group, the more alkyl groups they have, the weaker dissociation degree the alcohol possesses, meaning the acidity of the alcohols becomes weaker with longer chain length. When the acidity gets weaker, they are more difficult to be activated to form alkoxides.24,42 The specificity of [TBA][Arg] for the transesterification reaction might be used for separating different alcohols. Transesterification Efficiency for Different Sources of Plant Oils. In order to study the specificity and activity of the new catalyst further on different oil types, rapeseed oil, soybean oil, and palm oil were subjected to the reaction, and the results are shown in Figure 8. Very similar patterns for biodiesel yield were observed, although different types of oil were used.

Figure 6. Effect of methanol to triglyceride ratio (n/n) on biodiesel yield. Reaction conditions: 90 °C, catalyst loading 6%. Mean value ± standard deviation from two independent runs is shown.

methanol is required to obtain a complete conversion of 1 mol triglyceride. However, in practice, the biodiesel yield was only around 62% when the methanol/triglyceride ratio was 3. Increase in the ratio to 4.5 improved the biodiesel yield to 80% within 15 min. Further increase of the ratio to 6, 9, and 12 resulted in biodiesel yields of 96%, 99%, and 97%, respectively. This clearly indicates that higher methanol loading increases the final biodiesel yield within 15 min. However, biodiesel yield within the first 3 min did not positively correlate to the methanol/triglyceride mole ratio, suggesting methanol not only 1243


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contents may be the most important factor that affects the activity and the absolute dosage of the basic catalyst because of the neutralization reaction. The final biodiesel percentage showed direct negative correlation with the free fatty acid content in each oil. Therefore, although the functional group of the developed catalyst (Table 1 and 2, Figure 3) is not the same as sodium hydroxide, the influence of free fatty acid seems similar. The predominant fatty acids in the investigated oils are different. For example, rapeseed oil is rich in C18:1, and soybean oil contains more C18:2, while palm oil contains equal amount of C16:0 and C18:1. These differences would benefit for finding out the fatty acid preference of the catalyst. Therefore, the fatty acid composition of each type of oil and their corresponding nonbiodiesel components were analyzed in order to study the preference of the catalyst to different types of fatty acid (Table 4). Consequently, the fatty acid composition of substrate and its corresponding nonbiodiesel components after reaction was quite similar, indicating no preference of fatty acid was possessed by the catalyst. Kinetic Study of [TBA][Arg]-Catalyzed Transesterification. Biodiesel production from transesterification reaction consists of three steps as shown below.1

Figure 8. Transesterification of different oil feedstocks. Reaction conditions: 90 °C, catalyst loading 6%, methanol/triglyceride ratio (9:1, n/n). Mean value ± standard deviation from two independent runs is shown.

However, the final biodiesel yield was different, following the order of soybean oil > rapeseed oil > palm oil. The chemical composition of the starting materials was analyzed, and the results are shown in Table 3. Compared with

Triglyceride (A) + MeOH (B) k1

⇌ Diglyceride (C) + Biodiesel (D) rapeseed oil triglyceride % free fatty acid % diglycerides % monoglyceride %

99.49 0.25 0.18 0.07

± ± ± ±

0.09 0.01 0.03 0.05

soybean oil 99.72 0.04 0.15 0.08

± ± ± ±

0.28 0.07 0.09 0.12

(1)

k4

Table 3. Chemical Composition of Different Oils* palm oil 98.02 0.62 0.95 0.42

± ± ± ±

Diglyceride (C) + MeOH (B)

0.55 0.35 0.54 0.13

k2

⇌ Monoglyceride (E) + Biodiesel (D)

(2)

k5

*

The analyses were carried out by the TLC-FID method. An Iatroscan Mark VI TLC-FID (Iatroscan MK-6s, Japan) was used to monitor the content of mono-, di-, and tri-glycerides, free fatty acids, and biodiesel. The sample was spotted on silica coated chromarods (Chromarod-S III, Japan) and developed by the solvent mixture hexane:diethyl ether:formic acid (50:20:0.7) for 15 min, followed by developing by the solvent mixture benzene:hexane (50:50, v/v) for 30 min. The rods were then dried at 120 °C prior to analysis for 5 min. The results were calculated based on the ratio of peak area of target compound to total peak area.

Monoglyceride (E) + MeOH (B) k3

⇌ Glycerol (F) + Biodiesel (D)

(3)

k6

where k1, k2, and k3 indicate the forward reaction rate constant, while k4, k5, and k6 mean the backward reaction rate constant of each reversible reaction. The intermediates including diglyceride and monoglyceride were very little during the reaction process when high oleic sunflower oil was used. Therefore, the whole process could be simplified as follows.

rapeseed and soybean oils, palm oil contains slightly less triglyceride and more of the other components. Free fatty acid

Table 4. Fatty Acid Composition of Different Oils and Their Corresponding Unconverted Glyceridesa

C14 C16 C18 C18:1 C18:2 C18:3 C20 C20:1 unknown a

rapeseed oil (%)

nonbiodiesel components from rapeseed oil (%)

soybean oil (%)

nonbiodiesel components from soybean oil (%)

palm oil (%)

nonbiodiesel components from palm oil (%)

ND 4.55 1.62 61.98 18.24 7.55 1.50 0.79 4.68

ND 4.78 2.10 60.96 18.76 7.79 1.35 0.41 3.83

ND 10.76 4.58 22.81 53.49 6.94 ND ND 1.42

ND 11.36 4.06 24.82 53.04 5.11 ND ND 1.60

0.96 42.27 4.18 39.81 9.36 ND ND ND 3.43

0.92 39.59 4.41 41.61 9.03 ND ND ND 4.34

ND: not detected. Unknown indicates the peak area percentage of unknown compounds. 1244


Research Article


Figure 9. Kinetic study of the transesterification reaction. (A) Plot of −ln|1 − X| versus time for determination of reaction rate constants; X is conversion of triglyceride. (B) Arrhenius plot for oleic acid conversion to biodiesel.

apparent with high R2 values. Meanwhile, the reaction constant increased with the increased temperature. The activation energy could be calculated according to the Arrhenius eq 10.

Triglyceride (A) + 3MeOH (B) k

⇌ Glycerol (F) + 3Biodiesel (D) k′

(4)

The reaction rate for reaction 4 can be expressed as the following eq 5. dCA = k′C FC D3 − kCAC B3 dt

ln k = ln A +

Theoretically, 3 mol methanol could convert 1 mol triglyceride completely into biodiesel. However, in order to prompt the conversion of triglyceride, 9 mol of methanol was loaded to react with 1 mol of triglyceride. Methanol is more excessive than triglyceride, biodiesel, and glycerol. Therefore, CB can be considered as a constant value. Meanwhile, k′ is much lower than k according to very little detectable monoglycerides. It is also believed that the equilibrium lies toward the production of biodiesel and glycerol.1 Therefore, the eq 5 could be simplified into eq 6. (6)

CA = CA0(1 − X )

(7)

where X is triglyceride conversion. Combining eqs 6 and 7, the following eq 8 could be obtained. dX = k(1 − X ) dt

(8)

■

After integration, eq 9 is obtained. −ln|1 − X | = kt + a

(10)

where Ea is the activation energy, R is the gas constant (J/mol K), T is the absolute temperature (K), and A is the preexponential factor for the reaction. With known k, R, and T values, a linear function between ln k and 1/T could be plotted, and Ea could be further calculated. The Arrhenius plot is shown in Figure 9B. According to the regressed equation, the activation energy (Ea) and the preexponential factor are 84.38 kJ/mol and 64.95 × 1010 min−1, respectively. The higher Ea indicates the reaction is more temperature dependent and more sensitive to temperature change. It was found that the Ea was 61.51, 59.41, and 26.78 kJ/ mol for hydrolysis reaction of triglyceride, diglyceride, and monoglyceride from palm oil, respectively, when KOH was used as catalyst.43 The Ea was 79.1 kJ/mol when KOH was used to catalyze biodiesel production from palm oil and dimethyl carbonate.44 Comparably, the Ea was 71.25 kJ/mol for biodiesel production catalyzed by calcium methoxide.45 Additionally, the Ea was 51.69 and 72.49 kJ/mol when dolomite (activated at 800 °C) and calcium oxide (activated at 800 °C) were employed, respectively, as catalysts for biodiesel production.46 Compared with all the mentioned reports, the Ea of the currently studied catalyst is higher, meaning more energy is needed to activate the reactants to form the products. However, the total reaction time is dramatically shortened.

(5)

dCA = −kCA dt

Ea −RT

(9)

CONCLUSION [TBA][Arg] was selected as the best catalyst for transesterification from two series of ILs. 13C NMR results clearly proved the formation and evolution of biodiesel against the reaction time. It was found that the basicity of the ILs showed positive correlation with the catalytic activity. Reaction conditions including catalyst loading, reaction temperature, and methanol to oil mole ratio were investigated and optimized. The resultant optimal conditions were catalyst loading 6% (based on oil, w/w), methanol to oil ratio (9:1, mol/mol), and

where t is time and a is a constant value. On the basis of eq 9, the linear regression could be performed between−ln|1 − X| and t at each specific temperature (Figure 9A), from which the reaction rate constant at specific temperature could be obtained. The transesterification reaction catalyzed by [TBA][Arg] was very fast. Therefore, conversion within 9 min at lower temperature and conversion within 6 min at 90 °C were included in the regression. As shown in Figure 9A, the linear relationship is 1245


Research Article

ACS Sustainable Chemistry & Engineering 90 °C for 15 min, resulting in near-complete conversion of triglyceride into biodiesel. Alcohols with different chain lengths were used, and the biodiesel yield decreased with the chain length. Specifically, very low biodiesel yield was obtained when the alcohols with more than three carbons were used, implying potential application in alcohols separation. No specificity and preference for different glycerides or fatty acids species were found. The activation energy is 84.38 kJ/mol, which is slightly higher than that of KOH. Both the guanidine moiety in [Arg]− and cations like [TBA]+, [TMA]+, and [Ch]+ were thought to be responsible for the high catalytic ability. The easy preparation of the ILs in aqueous solution and the biorenewable properties of amino acids allow the whole process to be greener and more sustainable.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02732. 1 H NMR spectra and peak assignments. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Fax: (+45) 76123178. Tel: (+45) 89425285. ORCID

Jingbo Li: 0000-0001-5960-806X Zheng Guo: 0000-0002-8436-7082 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Graduate School of Science and Technology (GSST), Aarhus University, DLG (Dansk Landbrugs Grovvareselskab) Food Oil, Denmark, and the NOVO NORDISK Foundation (NNF16OC0021740) is gratefully acknowledged. Technical support from Anette Andersen is acknowledged as well.

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Structure Evolution of Synthetic Amino Acids-Derived Basic Ionic

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