Jin Caoa,
Bin Qia,
Jun Liub,
Yuhan Shanga,
Huiwen Liua,
Wenjing Wanga,
Jia Lva,
Zhiyan Chena,
Haibo Zhang*a and
Xiaohai Zhoua
aCollege of Chemistry and Molecular Sciences, Wuhan University, Wuhan, 430072, China. E-mail: haibozhang1980@gmail.com
bHunan University of Arts and Science, Changde, 415000, China
First published on 11th February 2016
A highly efficient and selective method for esterification of formic and acetic acid with alcohols has been achieved at room temperature, with the choline chloride (ChCl)/chromium(III) chloride hexahydrate (CrCl3·6H2O) deep eutectic solvent as a catalyst. High yields and good selectivities of organic esters are obtained using DES [ChCl][CrCl3·6H2O]2 with the molar ratio 5:
1 (carboxylic acids
:
alcohols) at room temperature in 24 h. The ease of recovery and reusability of DES with high catalytic activity makes this method efficient and practical.
Deep eutectic solvents (DESs) are composed of mixing a quaternary ammonium salt with a metal salt or a hydrogen bond donor (HBD).4 DESs can be easily prepared from safe and low cost chemical materials, and through an atom economical approach. In 2001, Abbott5 reported firstly that choline chloride and MCl2 (M = Zn and/or Sn) could form a moisture-stable deep eutectic solvents and the melting points of these liquids were measured. It was found that the decrease in the melting point of the mixture relative to the melting points of the individual components. In recent decades, DESs have been used widely as Lewis acid catalysts and reaction medium for O-acetylation of cellulose and monosaccharides,6 Diels–Alder reactions,7 the synthesis of pillararenes,8 and Kabachnik–Fields reaction.9 In 2007, Prasad and coworkers10 reported that DES [ChCl][ZnCl2]2 as a Lewis acid catalyst as well as reaction medium to prepare long chain wax esters in high yields, but they used high temperature (110 °C) with need of heat supply. Later, Feng11 have used DES [ChCl][SnCl2]2 as a Lewis acid catalyst to prepare phytosteryl esters, however the scope of reaction is limit. Germani's group12 reported a new Brønsted acidic deep eutectic solvent which prepared by mixing new quaternary ammonium methanesulfonate salts with p-toluenesulfonic acid (PTSA) as reaction media for esterification of carboxylic acid with alcohols in good yields, but it needs a large amount of DESs (e.g., 0.3 mol DES per 1 mol carboxylic acid). Recently, Bubalo13 reported the esterification reaction in DESs, which took Novozym 435 (immobilized Candida antarctica lipase B) as catalyst. In this case, however, deep eutectic solvents only used as green solvents instead of the catalysts. Therefore, esterification under mild conditions is of a great interest and importance in application of deep eutectic solvents.
Up to now, chromium based DESs have been widely used in electrochemistry,14 nanoscience15 and leather production processes,16 but there are only few examples about application of organic synthesis. Herein, we initiate a new and distinct method for the esterification of formic and acetic acid with alcohol. The reaction is catalyzed by choline chloride (ChCl)/chromium(III) chloride hexahydrate (CrCl3·6H2O) DES at room temperature, and gives out good efficiency and selectivity. This method meets with the principles of green chemistry and will stimulate and broaden the applications of DESs in different relative fields.
Entry | DES | Yield of n-butyl acetateb | Selectivity to n-butyl acetateb |
---|---|---|---|
a Reaction conditions: n-butanol (0.01 mol), acetic acid (0.05 mol), and DES (0.25 mmol) at room temperature (25 °C) for 24 h.b Yield and selectivity are based on GC. 1,4-Dimethyl-benzene as internal standard.c Isolated yield. | |||
1 | [ChCl][CrCl3·6H2O]2 | 93.0%, 85.7%c | 100% |
2 | [ChCl][FeCl3]2 | 64.5% | 100% |
3 | [ChCl][AlCl3·6H2O]2 | 49.1% | 100% |
4 | [ChCl][MgCl2·6H2O]2 | 18.0% | 100% |
5 | [ChCl][MnCl2·4H2O]2 | 6.3% | 100% |
6 | [ChCl][CoCl2·6H2O]2 | 7.8% | 100% |
7 | [ChCl][NiCl2·6H2O]2 | 10.4% | 100% |
8 | [ChCl][CuCl2·2H2O]2 | 3.7% | 100% |
9 | [ChCl][ZnCl2]2 | 3.0% | 100% |
10 | [ChCl][SnCl2]2 | 3.6% | 100% |
The effect of molar ratio of [ChCl][CrCl3·6H2O]2 to n-butanol on the esterification of acetic acid with n-butanol was investigated. As shown in Table 2, yield of n-butyl acetate increased rapidly with molar ratio of DES/n-butanol below 1/40. When the ratio is over 1/40, however, the yield of n-butyl acetate remained almost unchanged, so the optimal DES amount should be 1/40. The data showed that the reaction could reach high yield of ester at low catalyst amount using [ChCl][CrCl3·6H2O]2, while other DESs, including Brønsted acidic DESs and [ChCl][ZnCl2]2, required a large amount in the esterification to reach the maximum catalytic activity (generally the amount was above 0.3 of carboxylic acids or alcohols).10–12 So, DES [ChCl][CrCl3·6H2O]2 was an economic catalyst with high efficiency for the esterification of acetic acid and n-butanol.
Entry | DES amount n(DES)![]() ![]() |
Yield of n-butyl acetateb |
---|---|---|
a Reaction conditions: n-butanol (0.01 mol) and acetic acid (0.05 mol) at room temperature (25 °C) for 24 h.b Yield are based on GC. 1,4-Dimethyl-benzene as internal standard. | ||
1 | 1![]() ![]() |
78.4% |
2 | 1![]() ![]() |
87.5% |
3 | 1![]() ![]() |
93.0% |
4 | 3![]() ![]() |
94.3% |
5 | 1![]() ![]() |
95.8% |
6 | 1![]() ![]() |
96.3% |
Esterification reaction was an equilibrium reaction and generally required the usage of excess amount of the reactants for satisfactory conversion rate. Therefore, the effect of molar ratio of acetic acid to n-butanol was also studied in the presence of [ChCl][CrCl3·6H2O]2. The molar ratio of acetic acid to n-butanol was varied in the range of 1:
4 to 10
:
1 and the results were given in Table 3. When acetic acid and n-butanol was added as stoichiometrically according to the reaction equation, the n-butyl acetate formation only reached 72.6% after 24 h. Although equimolar ratio substrate could appear as ideal in terms of economical aspect of the process, it was observed that such ratio was not advantageous for the formation of n-butyl acetate. When the amount of n-butanol was increased, the yield of ester was increased slightly as well. The highest yield of n-butyl acetate only reached 82.0% using the molar ratio of 1
:
2 (acetic acid to n-butanol). Interestingly, with the increasing the molar ratio of acetic acid to n-butanol from 1
:
1 to 5
:
1, the yield of ester increased greatly. The yield of 93.0% could be obtained using acetic acid/n-butanol ratio of 5
:
1. While the molar ratio of acetic acid/n-butanol was above 5
:
1, the yield of n-butyl acetate became slightly decreasing. These data indicated that the excess of acetic acid was in favor of the formation of n-butyl acetate and the optimal molar ratio of acetic acid/n-butanol was 5
:
1. What is more, the acetic acid price was relatively cheaper and the purification of n-butanol would also be easier, it made the process more industrial application value.
Entry | n(Acid)![]() ![]() |
Yield of n-butyl acetateb |
---|---|---|
a Reaction conditions: n-butanol (0.01 mol) and DES (0.25 mmol) at room temperature (25 °C) for 24 h.b Yield are based on GC. 1,4-Dimethyl-benzene as internal standard. | ||
1 | 1![]() ![]() |
81.4% |
2 | 1![]() ![]() |
81.4% |
3 | 1![]() ![]() |
82.0% |
4 | 1![]() ![]() |
80.0% |
5 | 1![]() ![]() |
72.6% |
6 | 1.5![]() ![]() |
78.1% |
7 | 2![]() ![]() |
80.9% |
8 | 3![]() ![]() |
88.2% |
9 | 4![]() ![]() |
91.2% |
10 | 5![]() ![]() |
93.0% |
11 | 6![]() ![]() |
91.8% |
12 | 8![]() ![]() |
90.5% |
13 | 10![]() ![]() |
87.3% |
The yield of n-butyl acetate was also monitored with the reaction time. The results were shown in Fig. 1. The formation of n-butyl acetate was increased continuously with the reaction time up to 24 h. From 24 h to 48 h, the yield of ester changed a little, only from 93.0% to 98.9%. After 48 h of reaction, the yield of n-butyl acetate almost reached 100% and were almost unchanged. Considering the economic factor, the best time for the esterification of alcohols by carboxylic acids with the DES [ChCl][CrCl3·6H2O]2 was 24 h.
![]() | ||
Fig. 1 The effect of reaction time on the esterification reaction: n-butanol (0.01 mol), acetic acid (0.05 mol), and DES (0.25 mmol) at room temperature (25 °C). Error bars represent the SD. |
In order to investigate the scope and limitation of the DES [ChCl][CrCl3·6H2O]2 as a catalyst for esterification, varies of alcohols and acids as the substrates were also tested, and the results were outlined in Table 4. As can be seen, the length of the alkyl chains of the alcohols did not affect the yield of ester much. In the reaction of n-octanol and acetic acid, the yield of n-octyl acetate was still above 76% (Table 4, entry 1–9). While with the increase of chain length of the carboxylic acids, the rate of esterification decelerated dramatically and the yield of n-butyl butyrate was poor (Table 4, entry 10–12). In addition, the alcohols with a branched chain, which had a huge steric hindrance, reacted with acetic acid and had slower reaction rate. Among the alcohols mentioned in Table 4, cyclohexanol had the biggest strict hindrance, so it had the lowest yield.
Entry | Alcohols | Carboxylic acids | Yield of estersb |
---|---|---|---|
a Reaction conditions: alcohols (0.01 mol), carboxylic acids (0.05 mol), and DES (0.25 mmol) at room temperature (25 °C) for 24 h.b Yield are based on GC-MS.c Isolated yield. | |||
1 | Methanol | Acetic acid | 95.1% |
2 | Ethanol | Acetic acid | 93.2% |
3 | Propanol | Acetic acid | 92.8% |
4 | Isopropanol | Acetic acid | 88.7% |
5 | n-Butanol | Acetic acid | 93.0%, 85.7%c |
6 | Isoamyl alcohol | Acetic acid | 85.1% |
7 | n-Hexanol | Acetic acid | 82.5% |
8 | n-Octanol | Acetic acid | 76.4% |
9 | Iso-octyl alcohol | Acetic acid | 64.9% |
10 | n-Butanol | Formic acid | 97.1% |
11 | n-Butanol | Propanoic acid | 59.8% |
12 | n-Butanol | Butyric acid | 29.4% |
13 | Cyclohexanol | Acetic acid | 31.1% |
14 | Benzyl alcohol | Acetic acid | 74.7%, 68.2%c |
15 | 2-Chloro-1-ethanol | Acetic acid | 79.2% |
16 | Propargyl alcohol | Acetic acid | 65.4% |
17 | Isoamyl alcohol | Formic acid | 92.2% |
18 | n-Hexanol | Formic acid | 90.9% |
19 | n-Octanol | Formic acid | 87.5% |
20 | Benzyl alcohol | Formic acid | 85.6%, 77.9%c |
Benzyl alcohol's hydroxide radical formed the conjugated system with the benzene ring decreasing the strict hindrance, and its methylene was mobilizable, so the yield of methyl phenylacetate was higher than cyclohexyl acetate (Table 4, entry 13–14). Also, the alcohols with electron-withdrawing groups had poorer reactivity (Table 4, entry 15–16). Moreover, esterification of alcohols and formic acid (Table 4, entry 17–20), catalyzed by DES also had high efficiency. These results indicated that [ChCl][CrCl3·6H2O]2 DES could be a very potential catalyst for the synthesis of formate and acetate.
Once the reactions were complete, more attention was paid to separate esters from the reaction system and recycle the DES. In fact, the product can be very easily isolated, DES and water formed the lighter phase while the ester product and reactants remained in the upper liquid phase and it was simply and conveniently separated by decanting. After that, the reusability test was conducted by using the former model reaction. As shown in Table 5, although the yield of n-butyl acetate slightly decreased, the reused DES [ChCl][CrCl3·6H2O]2 still had higher activity for the yield of n-butyl acetate after reusing 4 times. The slight decrease of activity might be ascribed to the slight deactivation and loss of DES during the process of reuse.
Deep eutectic solvents are formed by mixing metal halides and choline chloride. So, it shows both Lewis acidity and polar solubility in an esterification reaction. During the reaction, the reactants alcohols and carboxylic acids are dissolved in the DES phase, and the product ester are separated to a non-polar phase. As a result, the chemical equilibrium of the esterification was pulled forward, and the reaction occurred simultaneously without removing the by-product water.
As a conclusion, the DES [ChCl][CrCl3·6H2O]2 showed good catalytic ability in the reaction between organic acids and alcohols, and the unique solubility of the DES was beneficial to generate more products. Further experiments, aiming at the specific reaction mechanism, are now ongoing in our labs.
Footnote |
† Electronic supplementary information (ESI) available: Experimental details, preparation of deep eutectic solvents, the synthesis process of organic esters, representative examples of GC chromatograms and the GC-MS spectra of esters. See DOI: 10.1039/c6ra01029f |
This journal is © The Royal Society of Chemistry 2016 |