Hydrogen Bonds in Disulfonic-Functionalized Acid Ionic Liquids for Efficient Biodiesel Synthesis.

Regulating the states of hydrogen bonds in ionic liquids (ILs) is an effective way to improve their catalytic performance. In this paper, disulfonic-functionalized acidic ionic liquids (DSFAILs) were synthesized successfully, including novel SO3H-functionalized binuclear IL (bis[3-(CH2)3SO3H-1-(CH2)2-Im][HSO4]2). For the biodiesel synthesis, compared with the traditional ILs catalysts, DSFAILs bis[(3-(CH2)3SO3H-1-(CH2)2-Im][HSO4]2, [Im(N (CH2)3SO3H)2][HSO4]) had higher catalytic activity even under mild reaction conditions. Using the density functional theory (DFT) method, the role of hydrogen bonds in different SO3H-functionalized acidic ionic liquids (SFAILs) was explored. The forms of hydrogen bonds existing in different ILs directly determine their acidity. It suggested that the forming status of the active sites (hydrogen bonds) were diverse in different SFAILs. Also, deep ionization of the hydrogen atoms from the cation-anion strong interaction could increase the acidity and catalytic performance of SFAILs. From this, the structure-activity relationship between the SFAILs structures and the catalytic activity of methyl oleate synthesis was proposed. Besides, the experimental results also showed that bis[3-(CH2)3SO3H-1-(CH2)2-Im][HSO4]2 catalyst had a high catalytic activity to obtain methyl oleate and the catalyst could be separated easily owing to its larger molecular weight. However, [Im(N(CH2)3SO3H)2][HSO4] had a stronger acidity and a lower steric hindrance and thus a higher catalytic activity and was the optimal catalyst for the methyl oleate synthesis. In the presence of a small amount of catalyst (6 wt %) and at low reaction temperature (353 K), the methyl oleate yield could reach up to 93%. After six recycles of the catalyst, the methyl oleate yield remained at 90%.

Introduction

Biodiesel, a mixture of fatty acid methyl esters (FAMEs), has long been regarded as a clean bioenergy source.[1,2] Compared with traditional petroleum-based fuel, the combustion of pure biodiesel can reduce the CO2 emission by 78%. Moreover, the high cetane number of biodiesel is conducive to enhancing the complete combustion and generating enough motive power. With this in mind, using biodiesel as a jet fuel, the carbon emission can be kept below 50%. Besides, because of its high flash point, biodiesel is more preferable to be stored and easily transported to wherever needed. However, until now, the application of biodiesel is subject to the high cost of feedstock, which accounts for about 75% of the total cost. Moreover, considering the food shortage, the utilization of food sources as a feedstock to produce biodiesel is not practically feasible.[3,4] Thus, exploring the reasons and adopting an efficient synthesis process for biodiesel has attracted more and more interest recently.

The synthesis of biodiesel by esterification of oleic acid with methanol is potentially one of the promising processes that can produce a high-value-added product with oleic acid recycled from waste oil.[5,6] The fatty acid esterification usually proceeds with inorganic acidic catalysts, such as H2SO4, HCl, etc., that are highly corrosive and very hazardous to humans and environment.[7−10] To solve these problems, heteropoly acids,[11,12] acidic ion-exchange resins,[13,14] and solid super acids[15] have been studied as alternative catalysts. Even though these solid acidic catalysts show good catalytic activities in the esterification of fatty acid, their high costs, few active sites per unit volume, and short durability are still obvious drawbacks.[16,17]

As environment-friendly benign materials, ionic liquids (ILs) were originally used as dielectric materials that dissolve or separate a target substance because of their negligible volatility, significant solubility, excellent thermal stability, and adjustable structure.[18,19] Recently, due to their high catalytic activity, acidic ILs are widely employed in the biodiesel synthesis.[20−25] Among these acidic ILs, SO3H-functional acidic ionic liquids (SFAILs) have drawn increasing attention owing to their typical strong Brônsted acidity, which inherits good features of traditional homogeneous and heterogeneous acidic catalysts. In particular, SFAILs show high acidic catalytic performance with flexible, noncorrosive properties. Therefore, in the esterification reaction, using SFAILs as catalysts for biodiesel synthesis from waste oil feedstock, high catalytic performance can be obtained.[26] Obviously, the strong acidity of these IL catalysts is the main reason for their high acid catalytic activities in the esterification reaction. As novel acidic catalysts, compared with the traditional SFAILs, disulfonic-functionalized acidic ionic liquids (DSFAILs) have been paid more attention because of their stronger acidity. Liu and co-workers found that use of a low dose of DSFAILs as catalysts (only 0.1 mol % based on glycerol) can significantly increase the conversion of glycerol through the esterification reaction.[27] However, the key reasons for the high acid catalytic activity of the DSFAIL catalysts have not been further studied and explained.

In this work, we synthesized methyl oleate using Brônsted acidic ILs as catalysts, which contained two alkyl sulfonic acid groups in the different imidazolium cation structures (Scheme ). Meanwhile, the structure–activity relationship between hydrogen bonds in different cations of disulfonic-functionalized acidic ILs and their catalytic activities were explored and imposed. Besides, using suitable IL as a catalyst, we optimized its reaction conditions, including the dosage of the catalyst, reaction time, reaction temperature, and molar ratio of reactants.

Scheme 1

Acid-Catalyzed Esterification Process of Oleic Acid with Methanol

Results and Discussion

Determination of the SFAILs’ Acidity

Using the Hammett function method,[28] we distinguished the acidities of the SFAILs with different structures (Table ). The results showed that all SFAILs samples showed strong acidity, especially DSFAILs samples, which exhibited significantly stronger acidity than the traditional IL samples with a sulfonic acid group, and the acidity of DSFAILs was even stronger than that of H2SO4. The result indicated that introducing more alkyl sulfonic acid groups in the cationic structure of ILs would be beneficial to improving the ionization intensity of hydrogen atoms in ILs’ cations or anions, which could further increase the ILs’ acidity.

Table 1

Calculations and Comparisons of Hammett Functions of SO3H-Functionalized ILs (32 mmol/L) in Watera

sample	Åmax	[B] (%)	[BH+] (%)	Ho
blank	0.774	100	0	-
[MimN(CH2)4SO3H][HSO4]	0.525	67.83	32.17	1.31
[PyN(CH2)4SO3H][HSO4]	0.515	66.54	33.46	1.29
[(CH3CH2)3N(CH2)4SO3H][HSO4]	0.503	64.99	35.01	1.26
H2SO4	0.425	54.91	45.09	1.08
bis[3-(CH2)3SO3H-1-(CH2)2-Im][HSO4]2	0.369	47.67	52.33	0.91
[Im(N(CH2)3SO3H)2][HSO4]	0.323	41.73	58.27	0.85

4-nitroaniline as the indicator.

Accordingly, in this case, the strong acidity of DSFAILs should be mainly derived from the high ionization intensity of hydrogen atoms, or the hydrogen bonds generated from the strong interaction between cations and anions. Besides, we included that the steric hindrance effect existing in the different molecular sizes of DSFAILs might be the other decisive factor to regulate the acidity of DSFAILs. The Brônsted acidity of these SFAILs follows an order [Im(N(CH2)3SO3H)2][HSO4] > bis[3-(CH2)3SO3H-1-(CH2)3-Mim][2HSO4] > H2SO4 > [(CH3CH2)3 N(CH2)4SO3H][HSO4] > [PyN(CH2)4SO3H][HSO4] > [Mim(CH2)4SO3H][HSO4].

Effect of SFAILs’ Structures on the Esterification Performance

The synthesis of methyl oleate by the esterification of oleic acid with methanol was used as a model reaction for evaluating the catalytic activities of different SFAILs (Table ). The results indicated that all SFAILs had a certain catalytic activity in the synthesis of methyl oleate. Moreover, the order of the catalytic activity of SFAILs was almost identical to that of the acidity determined by the Hammett function method. It was inferred that the high yield of methyl oleate should be related to hydrogen bond in the O–H···O state, which was formed in SFAILs by the strong interaction between cations and anions.[29−33] Also, such SO3H-functionalized acidic ILs always exist in the form of zwitterions and H2SO4.

Table 2

Comparison of Catalytic Activity over Different SFAILs

sample	yield (%)	ref
blank	13	this work
[MimN(CH2)4SO3H][HSO4]	54	this work
[PyN(CH2)4SO3H][HSO4]	55	this work
[(CH3CH2)3N(CH2)4SO3H][HSO4]	57	this work
H2SO4	88	this work
[Im(N(CH2)3SO3H)2][HSO4]	86	this work
bis[3-(CH2)3SO3H-1-(CH2)2-Im][HSO4]2	72	this work
abis[3-(CH2)3SO3H-1-(CH2)2-Im][HSO4]2	76	this work

Reaction conditions: m(cat.)/m(oleic acid) = 0.06:1.00, n(methanol)/n(oleic acid) = 4:1, t = 5 h, and T = 343 K. The amount of catalyst is 0.26 g.

Besides, the experimental results also showed that with the increase in the amount of alkyl sulfonic acid groups in the cations of SFAILs, the overall catalytic activity of SFAILs would also be enhanced. Thus, DSFAILs [Im(N(CH2)3SO3H)2][HSO4] and (bis[3-(CH2)3SO3H-1-(CH2)2-Im][HSO4]2) displayed higher catalytic activities than the other SFAILs. However, through the comparison of the catalytic activities of these two DSFAILs, using [Im(N(CH2)3SO3H)2][HSO4] as the catalyst, methyl oleate could be obtained in higher yield, irrespective of whether more bis[3-(CH2)3SO3H-1-(CH2)2-Im][HSO4]2 catalysts are added or not. It also suggested that the synergy effect between strong acidity and the small molecular structure of DSFAILs led to their outstanding catalytic performance.

To further explore the origin of acid active sites and their differences in different ILs, the density functional theory (DFT) method was applied to optimize and obtain the most stable geometries of the cations with intramolecular and intermolecular hydrogen bonds of DSFAILs[34,35] and explain the forming process of the strong acid sites. The calculation results indicated that there coexist intramolecular and intermolecular hydrogen bonds in both SFAILs (Figure ). Moreover, compared with the intermolecular hydrogen bonds (O24···H25···O27) existing in [MimN(CH2)4SO3H][HSO4], the total amount of intermolecular hydrogen bonds (O30···H39–O38, O33···H34···O42, and O40···H36–O35) existing in [Im(N(CH2)3SO3H)2][HSO4] was higher. It also indicated that introducing alkyl sulfonic functionalized acidic groups into SFAILs’ cations would favor the formation of hydrogen bonds and ionization of hydrogen atoms caused by the strong cation–anion interactions between adjacent functional groups. Therefore, as the strength and amount of hydrogen bonds increase, the geometry of SFAILs becomes more stable. Also, cations with a small molecular volume were conducive to the formation of hydrogen bonds and contributed to improving the acidity and catalytic efficiency of DSFAILs.

Figure 1

Most stable geometries of the cations with intramolecular hydrogen bonds of SFAILs: (a) [MimN(CH2)4SO3H][HSO4] and (b) [Im(N(CH2)3SO3H)2][HSO4].

From above, it is shown that when the cationic part of SFAILs contains two alkyl sulfonic acid groups, the strong interaction between their anions and cations can achieve deep ionization of hydrogen atoms and further intensify the acidity of DSFAILs (Scheme ). Therefore, the catalytic reaction process of reactants on the acidic active sites (hydrogen bonds) would become easier. Moreover, the DSFAILs ([Im(N(CH2)3SO3H)2][HSO4]) exhibited stronger acidity and catalytic activity than the SFAILs with a single alkyl sulfonic acid group and bis[3-(CH2)3SO3H-1-(CH2)2-Im][HSO4]2, even being stronger than that of H2SO4. [Im(N(CH2)3SO3H)2][HSO4] was identified as the optimal catalyst for oleic acid and methanol esterification, and its catalytic reaction conditions in the oleate reaction were subsequently optimized.

Scheme 2

Possible Prediction of Intramolecular/Intermolecular Hydrogen Bonds Effect/Acidity of SFAILs with Varied Sulfonate Groups

Methyl Oleate Yield from the Esterification of Oleic Acid with Methanol

Effect of Molar Ratio of Methanol to Oleic Acid on the Oleate Reaction

The reaction profile of [Im(N(CH2)3SO3H)2][HSO4] with varying molar ratios was investigated, as shown in Figure .

Figure 2

Effect of n(methanol)/n(oleic acid) on the esterification of oleic acid with methanol. Reaction conditions: m(cat.)/m(oleic acid) = 0.08:1.00, t = 5 h, and T = 353 K.

It could be seen that since this esterification reaction was a reversible reaction process, excess methanol was able to make the equilibrium proceed in the direction of product formation. Methyl oleate yield increased with an increment in oleic acid and then reached an equilibrium when the molar ratio of methanol to oleic acid exceeded 4:1. However, if the molar ratio of methanol to oleic acid (>4:1) continued to increase, the yield of methyl oleate would decrease slightly.

Besides, an excess of methanol probably led to a proportional decrease in the dosage of catalyst per unit volume of the reaction solution and was responsible for the low catalytic activity. That is to say, fewer acidic active sites (hydrogen bonds) would result in less effective esterification of oleic acid with methanol. Taking into account of the cost and the yield of methyl oleate, the optimum molar ratio of methanol to oleic acid should be 4:1.

Effect of Reaction Time on the Oleate Reaction

The oleic acid conversion and the yield of methyl oleate during the oleate reaction also depended on the reaction time. Based on this, we studied the effect of different reaction times on the yield of methyl oleate.

As shown in Figure , the yield of methyl oleate initially improved with the prolonging reaction time. During this esterification reaction process, the catalytic activity was controlled by the dynamics. When the reaction time exceeded 5 h, the esterification reaction reached an equilibrium. If the reaction time was further extended, only the yield of methyl oleate became slightly higher. Therefore, the optimal reaction time was identified as 5 h.

Figure 3

Effect of reaction time on the esterification of methanol with oleic acid catalyzed by different SFAILs. Reaction conditions: m(cat.)/m(oleic acid) = 0.08:1.00, n(methanol)/n(oleic acid) = 4:1, and T = 353 K.

Effect of the IL Dosage on the Oleate Reaction

In general, the amount of DSFAILs used in the esterification reaction is also considered to be one of the key factors affecting the reaction performance. Considering the cost of catalysts having higher catalytic activity, the dosage of catalyst used should match the actual dosage of catalyst required for the actual esterification reaction. As the reference catalyst, [Im(N(CH2)3SO3H)2][HSO4] could be employed to explore the influence of catalyst dosage on the esterification of oleic acid with methanol.

We compared the catalytic performance of different amounts of catalysts, in the range of 1–10 wt % (expressed as a percentage based on the mass of oleic acid), in the oleate reaction, as shown in Figure . As the amount of DSFAILs is increased, the yield of methyl oleate gradually became higher. When the catalyst amount was 6 wt %, the yield of methyl oleate reached 93%. Furthermore, the increase in the amount of catalyst led to a slight increase in catalytic activity. The result suggested that a smaller amount of [Im(N(CH2)3SO3H)2][HSO4] could provide sufficient acidity for the required reaction. On the other hand, moderately strengthening the acidity of SFAILs was an alternative method that could reduce the cost of catalyst while maintaining the high catalytic activity.

Figure 4

Effect of the amount of SFAILs on the esterification of methanol with oleic acid catalyzed by different SFAILs. Reaction conditions: n(methanol)/n(oleic acid) = 4:1, T = 353 K, and t = 5 h.

Effect of Reaction Temperature on the Esterification of Oleic Acid with Methanol

The esterification activity of oleic acid with methanol is related to the reaction temperature. Also, we investigated the effect of reaction temperature on the catalytic performance of this esterification reaction (Figure ). The results showed that as the reaction temperature increased from 323 K to 353 K, the yield of methyl oleate increased correspondingly from 82% to 93%.

Figure 5

Effect of reaction temperature on the esterification of methanol with oleic acid catalyzed by different SFAILs. Reaction conditions: m(cat.)/m(oleic acid) = 0.06:1.00, n(methanol)/n(oleic acid) = 4:1, and t = 5 h.

When the reaction temperature was 353 K, it could be confirmed that the dynamic equilibrium of the reaction was reached. The reason was that the yield of methyl oleate did not increase significantly as the reaction temperature continued to increase (>353 K). According to the literature’s report, using SFAILs with a single alkyl sulfonic acid group at a reaction temperature of 403 K could ensure a high yield of methyl oleate (>90%).[25] However, in this esterification reaction, with similar high yield of methyl oleate, the optimal reaction temperature required for the [Im(N(CH2)3SO3H)2][HSO4] catalyst was much lower than the optimal reaction temperature required for other SFAIL catalysts having one alkyl sulfonic acid group. Also, there should be a relationship between the acidity of SFAILs and the reaction temperature. For SFAIL catalysts with one alkyl sulfonic acid group, in a certain solution volume, high catalytic performance could be achieved by increasing the reaction temperature because the reaction rate of acidic active sites (i.e., hydrogen bonds) in these SFAILs was accelerated. Therefore, at a low operating temperature, if the stronger acidity of SFAILs could be ensured, a higher reaction rate and an excellent esterification performance could be also realized. Taking the energy consumption and the product yield into account, 353 K should be the optimal reaction temperature for this esterification reaction.

Stability of Catalyst

For evaluating the [Im(N(CH2)3SO3H)2][HSO4] catalyst’s stability, a series of recycling test experiments were conducted under the optimized reaction conditions, as mentioned above. During the reaction process, DSFAILs were completely miscible with the phase of the reactants. After the reaction, once the reaction solution was cooled to 298 K, the DSFAIL catalysts were easily separated from the reaction mixture at the bottom of the flask. Most of the methyl oleate could be removed by distillation, and then ethyl acetate was added into the reaction mixture. Finally, because DSFAILs was not miscible with ethyl acetate, this sort of IL as a separated phase could remain at the bottom and be reused without further processing.

As depicted in Figure , even after the catalyst had been reused six times, there was still no significant change in the catalytic performance. The catalytic activity of [Im(N(CH2)3SO3H)2][HSO4] sample decreased only from 93 to 90%. From this point of view, the DSFAIL catalysts could have sufficient catalytic stability for the esterification reaction.

Figure 6

Stability of [Im(N(CH2)3SO3H)2][HSO4] catalyst in the esterification of methanol with oleic acid and methanol. Reaction conditions: m(cat.)/m(oleic acid) = 0.06:1.00, n(methanol)/n(oleic acid) = 4:1, t = 5 h, and T = 353 K.

Speculation Mechanism of Synthesizing Methyl Oleate Using DSFAILs

Based on the above results, a possible reaction mechanism for the esterification reaction of oleic acid with methanol using DSFAILs as the catalyst was speculated. Initially, the protons on DSFAILs would rapidly react with the carboxyl group on oleic acid to form a sulfonium salt, causing the carbon atoms to become attached to the more positively charged carboxyl group, thereby attracting nucleophiles and reacting. Among them, the hydroxyl group acted as a nucleophile, attacked the carbonyl carbon, destroyed the acyloxy group on the acid, removed one molecule of water, and finally lost one H+ to form the methyl oleate product (Scheme ).

Scheme 3

Possible Reaction Mechanism of Synthesizing Methyl Oleate Using [Im(N(CH2)3SO3H)2][HSO4] Catalyst

From a comparison of various previously reported acid catalysts for the esterification reaction of oleic acid with methanol (Table ), it could be seen that these catalysts exhibited different catalytic performances for synthesizing methyl oleate. Among them, some catalysts had lower yields of methyl oleate, while others required more severe reaction conditions (reaction temperature, catalyst amount, etc.). However, using DSFAILs as a catalyst, the outstanding acidic catalytic performance could still be obtained in mild reaction conditions. To further confirm this, using DSFAILs as the catalyst, the catalytic activity of this DSFAILs in other esterification reactions with different acids was explored.

Table 3

Comparison of Catalytic Activity Using Different Catalystsa

sample	T	t	molar ratio	catalyst (wt %)	yield (%)	refs
MPEG-350-[SO3H-(CH2)4-HIM][HSO4]	341	4	10	4.84	84.5	(36)
[BMIm][TS]	413	5	2	7	93	(37)
6.0% WO3/USY	473	2	6	10	80	(38)
HPW@MIL-100	384	5	11	15	40	(39)
[HMIM]HSO4	383	8	15	14	90	(40)
F-SO42–/MWCNTs	338	6	12	0.9	90	(41)
MMFP-IL	363	3	12	4	95	(42)
ZrFe-SA-SO3H	363	5	12	9	96.6	(43)
[(CH2COOH)2IM]@H-UiO-66	353	5	10.39	6.28	93.71	(44)
MPEG-350-[SO3H-(CH2)4-HIM][HSO4]	341	4	10	4.84	84.5	(36)
[Im(N(CH2)3SO3H)2][HSO4]	353	5	4	6	93	this work

T = temperature (K); t = reaction time (h); SA, solid acid; MMFP, mesoporous melamine–formaldehyde polymer; molar ratio = n(methanol)/n(oleic acid).

As shown in Table , [Im(N(CH2)3SO3H)2][HSO4] also exhibited high catalytic performance in other esterification reactions under mild reaction conditions. It means that as long as SFAILs have sufficiently strong Brônsted acidity, they can effectively improve the catalytic activity and the harsh experimental conditions for esterification reactions. Therefore, DSFAILs can be used to efficiently catalyze the esterification reaction with different feedstocks. Two key reasons for the catalyst are having a strong Brônsted acidity and a low molecular weight to avoid the steric hindrance. Thus, modifying the structure of SFAILs by regulating the functional groups is still an effective method to improve their catalytic performance.

Table 4

Esterification Performance of Other Acids with Methanola

feedstock	the yield of lipid (%)
apalmitic acid + methanol	93
alauric acid + methanol	92
bn-caprylic acid + methanol	85

Reaction conditions: n(acid)/n(methanol)/n(cat.) = 1:1.5:0.1, room temperature, t = 5 h.

n(acid)/n(methanol)/n(cat.) = 1:1:0.1, room temperature, t = 4 h, without water-carrying agent.

Conclusions

This research work reveals that under mild reaction conditions, DSFAILs with strong acidity can be used as a new suitable acidic catalyst for biodiesel synthesis with oleic acid and methanol. The strong acidity of DSFAIL catalysts is mainly due to their higher number of hydrogen bonds and deeply ionized hydrogen atoms per unit volume. That also leads it to their high activity for the oleate reaction. A comparison with different SFAIL catalysts showed that the new DSFAILs (bis[3-(CH2)3SO3H-1-(CH2)2-Im][HSO4]2) also had high catalytic activity and easy recovery for methyl oleate synthesis. However, the methyl oleate yield using bis[3-(CH2)3SO3H-1-(CH2)2-Im][HSO4]2 as the catalyst was lower than that using [Im(N(CH2)3SO3H)2][HSO4] as a catalyst (93%) because of the larger molecular weight of bis[3-(CH2)3SO3H-1-(CH2)2-Im][HSO4]2. Besides, the steric hindrance effect of the catalyst is the other key reason influencing the catalytic activity. After six recycles, [Im(N(CH2)3 SO3H)2][HSO4] can still maintain high catalytic activity. Disulfonic-functionalized acidic ILs can be a potential catalyst for a low-cost operating process to synthesize methyl oleate under mild reaction conditions.

Experimental Section

Materials

All organic reagents were commercial products of the high purity (>98% purity) from Sinopharm Chemical Reagent and used for the reaction without further purification. Oleic acid (>99%), methyl oleate (>99%), and methyl heptadecanoate (>97%) were purchased commercially (Sigma-Aldrich and Aladdin) without further purification.

Synthesis of SFAILs

The synthesis of SFAILs was according to previous literature.[45−49] The specific steps are given in Scheme .

Scheme 4

Synthetic Routes of Different SO3H-Functionalized Acidic ILs

N-Methylimidazole (0.05 mol), pyridine, or triethylamine was mixed with 0.05 mol of 1,4-butane sultone, and the solution was stirred for 24–48 h. The product was then repeatedly washed with ethyl acetate and dried in a vacuum (<133 Pa, 373 K) to obtain white solid IL precursors. Finally, mixing 0.05 mol of H2SO4, 20 mL of cyclohexane, and 0.05 mol of precursors while stirring for 8 h at 353 K could result in the formation of the SFAIL samples [MimN(CH2)4SO3H][HSO4], [PyN(CH2)4SO3H][HSO4], [(CH3CH2)3N(CH2)4SO3H][HSO4].

Under the ice-bath condition, 0.1 mol of 1,3-propane sultone and 0.05 mol of trimethylsilyl imidazole were mixed and stirred for 4 h. Then, the reaction was continued at 353 K for 10 h. After adding an appropriate amount of deionized water and reacting for 2 h, white solid IL precursors could be obtained. Finally, mixing 0.1 mol of H2SO4, 20 mL of cyclohexane, and 0.1 mol of precursors while stirring for 10 h at 353 K could result in the formation of the DSFAILs [Im(N(CH2)3SO3H)2][HSO4].

The synthesis of SO3H-functionalized binuclear IL is shown in Scheme .

Scheme 5

Synthetic Route of SO3H-Functionalized Binuclear Acidic IL

Trimethylsilyl imidazole (0.1 mol) and 0.05 mol of 1,4-dibromobutane were mixed at 353 K for 24 h, then washed with ethyl acetate and methanol, and dried in a vacuum to get the IL precursor. The 0.5 mol of precursor was dissolved with water, 0.1 mol of 1,3-propane sultone was added slowly under an ice-bath and continuously reacted for 4 h. And then, the liquids were magnetically stirred for 12 h at room temperature, washed with ethyl acetate and methanol three times, and vacuum-dried (vacuum <133 Pa, 373 K, 4 h) to get a yellow viscous liquid. Next, it was mixed with sulfuric acid and silver oxide at 1:2:1, stirred magnetically for 8 h to get a colorless transparent liquid and yellow precipitate AgBr, which could be removed by filtration, and vacuum-dried (vacuum <133 Pa, 373 K, 8 h) to obtain bis[3-(CH2)3SO3H-1-(CH2)2-Im][HSO4]2.

Esterification of Oleic Acid with Methanol

Oleic acid (2.82 g, 0.01 mol) was added to a 50 ml round bottom flask, methanol (0.04 mol) and SO3H functionalized acidic ILs catalyst (0.169 g) were added respectively, and the reaction was performed with magnetic stirring. The mixture was heated in an oil bath (reflux condensation). After the completion of the reaction, two phases were formed and the desired product remained mainly in the upper phase. Samples (1 mL) were withdrawn from the upper phase and centrifuged, and the supernatant liquid (5 μL) was mixed with 1 μL of methyl heptadecanoate (internal standard) before gas chromatography (GC) analysis.

A quantitative analysis was carried out on a GC (Agilent 7890) equipped with an HP-5 capillary column (30 m × 0.25 mm Agilent Technologies, Inc., Santa Clara) and a flame ionization detector. The column temperature was kept constant at 483 K and nitrogen was chosen as the carrier gas. The injector and detector temperature were both set at 523 K. The yield of methyl oleate was calculated as yield = m(MO) × 100%/m, where m(MO) is the mass of methyl oleate obtained and m is the mass of initial oils.