Recovery of acetic acid from dilute aqueous solutions using catalytic dehydrative esterification with ethanol

Recovery of acetic acid from dilute aqueous solutions using catalytic dehydrative esterification with ethanol

Chemosphere 91 (2013) 61–67 Contents lists available at SciVerse ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere Re...

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Chemosphere 91 (2013) 61–67

Contents lists available at SciVerse ScienceDirect

Chemosphere journal homepage: www.elsevier.com/locate/chemosphere

Recovery of acetic acid from dilute aqueous solutions using catalytic dehydrative esterification with ethanol Daisuke Yagyu b, Tetsuo Ohishi a, Takeshi Igarashi b, Yoshikuni Okumura b, Tetsuo Nakajo b,  Kobayashi a,⇑ Yuichiro Mori a, Shu a b

Department of Chemistry, School of Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan SHOWA DENKO K.K., 1-13-9, ShibaDaimon, Minato-ku, Tokyo 105-8518, Japan

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

" This work contributes a recovery of

"

"

"

"

acetic acid from dilute aqueous solutions. The esterification yield was significantly increased by the addition of toluene. Low-loading and alkylated polymer catalyst efficiently accelerated the reaction. Addition of inorganic salts, especially CaCl2, was also effective for the reaction. This methodology is expected to be applied to general aqueous reactions.

a r t i c l e

i n f o

Article history: Received 2 August 2012 Received in revised form 26 November 2012 Accepted 26 November 2012 Available online 3 January 2013 Keywords: Direct esterification Acetic acid Polymer-immobilized catalyst Salting-out effect Biphasic medium

SO3H CaCl2

Ethyl acetate + Ethanol

(recovery)

Acetic acid

a b s t r a c t We have developed a direct esterification of aqueous acetic acid with ethanol (molar ratio = 1:1) catalyzed by polystyrene-supported or homogeneous sulfonic acids toward the recovery of acetic acid from wastewater in chemical plants. The equilibrium yield was significantly increased by the addition of toluene, which had a high ability to extract ethyl acetate from the aqueous phase. It was shown that lowloading and alkylated polystyrene-supported sulfonic acid efficiently accelerated the reaction. These results suggest that the construction of hydrophobic reaction environments in water was critical in improving the chemical yield. Addition of inorganic salts was also effective for the reaction under not only biphasic conditions (toluene–water) but also toluene-free conditions, because the mutual solubility of ethyl acetate and water was suppressed by the salting-out effect. Among the tested salts, CaCl2 was found to be the most suitable for this reaction system. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Dilute acetic acid-containing wastewater is one of the common wastes in many important industrial processes, including the production of cellulose acetate, terephthalic acid, poly(vinyl alcohol) and acetaldehyde by the Wacker process, destructive distillation ⇑ Corresponding author. Tel.: +81 358414790; fax: +81 356840634. E-mail address: [email protected] (S. Kobayashi). 0045-6535/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2012.11.078

of wood, and reactions involving acetic anhydride. Although these wastewaters from chemical processes are generally treated with an activated sludge process, the production of a large amount of excess sludge remains as a critical issue in the chemical industry. Therefore, the recovery of acetic acid from wastewater is desirable from an economical and environmental standpoint. However, the recovery of acetic acid is a challenging task because the conventional separation methods such as distillation and extraction are not efficient and are too costly to be applied to industrial processes.

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Although several methods to recover acetic acid have been reported (Golob et al., 1981; Garcia and King, 1989; Cloete and Marais, 1995; Saha et al., 2000; Bianchi et al., 2003; Yu et al., 2003; Gorri et al., 2005; Singh et al., 2006), there is no mature process yet. On the other hand, in terms of the development of environmentally benign and sustainable chemical processes, organic reactions in water have attracted much attention because water is easily available, economical, and harmless to the environment compared with organic solvents. In addition, aqueous reactions make it possible to use chemicals including a large amount of water, such as unrefined bioethanol, as raw materials for chemical processes. Although various efficient catalytic systems in water have been developed (Cornils and Herrmann, 2004; Kobayashi, 2007; Li and Chan, 2007; Lindström, 2007; Li, 2010), dehydrative reactions in water are among the most challenging topics. In the course of our investigations on organic reactions in water, we have developed surfactant-type Brønsted acid-catalyzed dehydrative reactions such as esterification, etherification, thioetherification, dithioacetalization, glycosylation, and carbon–carbon bond-forming reactions in water (Manabe et al., 2001, 2002; Aoyama and Kobayashi, 2006; Shirakawa and Kobayashi, 2007; Shiri and Zolfigol, 2009). In these reactions, a surfactant-type Brønsted acid, pdodecylbenzenesulfonic acid, has been found to be an efficient catalyst for obtaining high chemical yields. Recently, with the aim of recovering acetic acid from wastewater, we have reported dehydrative esterification of dilute acetic acid with a high alcohol content catalyzed by diarylammonium p-dodecylbenzenesulfonates and have applied this esterification system to a flow reaction system (Igarashi et al., 2012). These catalysts showed sufficient hydrophobicity to suppress their leaching into water when dilute acetic acid was employed. However, a problem was the use of an excess amount of a hydrophobic alcohol to construct hydrophobic reaction environments to increase the esterification yield. Furthermore, it is better to convert the esterification product into more valuable products, such as ethyl acetate, by a subsequent process. We then decided to develop alternative processes for the recovery of acetic acid, and focused on the direct esterification of acetic acid with ethanol in water (acetic acid:ethanol = 1:1). Aqueous esterification of hydrophilic substrates that are completely miscible with water is much more difficult than that of hydrophobic substrates. In fact, initial experiments showed that the esterification of dilute acetic acid hardly proceeded. Therefore, we envisioned the use of moderately concentrated wastewater, which was obtained by a preceding concentration process. In addition, considering the application to a practical chemical process, the use of solid catalysts such as polymer-immobilized catalysts is desirable in terms of recovery and reuse of the catalyst. Recovery and reuse of solid catalysts are important in terms of not only environmental issues but also cost-effectiveness. We have already reported that a hydrophobic polystyrene-supported sulfonic acid (PS–SO3H) and an alkylated polystyrene-supported sulfonic acid (ALPS–SO3H) are effective and reusable catalysts for several aqueous reactions of hydrophobic substrates, such as formation of esters, hydrolysis of thioesters, and deprotection of acetals (Manabe and Kobayashi, 2002; Iimura et al., 2003a, 2003b, 2003c). It was expected that these catalysts could be applied to the esterification of hydrophilic substrates in water. Herein, we report an efficient direct esterification of hydrophilic substrates (acetic acid:ethanol = 1:1) in aqueous media toward the recovery of acetic acid from wastewater. We first show polymersupported sulfonic acid-catalyzed esterification in biphasic media (toluene–water) and describe the effect of loading amounts of the acids (sulfonic acid content in the polymer) and the catalyst structure of polymer-supported sulfonic acids on their catalytic activity. Next, to increase the esterification yield further, we have

focused attention on a salting-out effect by addition of inorganic salts to the aqueous phases, which improves partitioning of organic compounds into the extracting phases. We then describe the effect of various salts on the ester yield in biphasic media (toluene– water) containing inorganic salts. Finally, we show the improvement in the ester yield in water (toluene-free conditions) by the addition of CaCl2, which was found to be the most effective salt for this reaction system. 2. Experimental section 2.1. Material and methods GC analyses were performed using a Shimadzu GC-2010 with a capillary column (Agilent J&W DB-WAXetr). Combustion ion-chromatography was carried out using the following equipment: DIONEX ICS-1500 (ion-chromatography) with an IonPac AS12A (column), Mitsubishi Chemical AQF-100, WS-100, and automatic boat controller. Polystyrene (1% divinylbenzene (DVB) crosslinked, 200–400 mesh (0.037–0.075 mm)) was purchased from Advanced ChemTech. Styrene (Tokyo Kasei Kogyo) and DVB (Sigma– Aldrich, technical grade, 80%) were passed through activated alumina to remove inhibitors before use. Poly(vinyl alcohol) (PVA, degree of polymerization 1500, degree of saponification 86–90 mol%) was purchased from Wako Pure Chemical Industries (Japan). Benzoyl peroxide (wetted with ca. 25% water) was purchased from Tokyo Kasei Kogyo (Japan). Commercial ion-exchange resins, Amberlite IR120B and Amberlite 200CT were used after transformation from the Na form to the H form. Unless otherwise noted, other materials were obtained from commercial suppliers and used without further purification. 2.2. Preparation of polymer-supported sulfonic acid 2.2.1. PS-1%DVB–SO3H A solution of chlorosulfonic acid (3.99 mL, 60.0 mmol) was slowly added to a suspension of polystyrene (30.0 g) in dichloromethane (300 mL) at 0 °C, and the whole was stirred for 6 h. Acetic acid (150 mL) was added, and after being stirred at room temperature for 1 h and left overnight, the resin was collected on a glass filter, rinsed with water, water/tetrahydrofuran (THF), THF, and dichloromethane, and dried in vacuo to give the polymer-supported sulfonic acid (PS-1%DVB–SO3H). The sulfonic acid content was determined as 1.46 mmol g 1 by combustion ion-chromatography. PS-1%DVB–SO3Hs with other loading levels were similarly prepared by changing the ratio of the polymer and chlorosulfonic acid. 2.2.2. C18-ALPS–SO3H Polystyrene (5.21 g) and a solution of stearoyl chloride (22.7 g) were successively added to a mixture of AlCl3 (10.0 g) in carbon disulfide (100 mL) at room temperature. After being stirred for 24 h, 1 N HCl (200 mL) was added to the reaction mixture. After further stirring at room temperature for 12 h, carbon disulfide was evaporated, and the resin was collected on a glass filter, rinsed with 1 N HCl, water, water/THF, THF, and dichloromethane, and dried in vacuo. The resin was reduced in the following step. AlCl3 (3.33 g) was added to a mixture of LiAlH4 (0.95 g) in ether (50 mL) in several portions at 0 °C. After being warmed to room temperature, the resin (4.69 g) was added, and the whole was heated under reflux for 24 h. After being cooled to 0 °C, 1 N HCl (100 mL) was added. The mixture was stirred at room temperature for 24 h and evaporated to remove ether, and the resin was collected on a glass filter, rinsed with 1 N HCl, water, water/THF, THF, and dichloromethane, and

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dried in vacuo. The reduction proceeded quantitatively (based on elemental analysis). Anal. Calcd.: C, 87.6%; H, 12.4%. Found: C, 87.3%; H, 12.4%. The resin was then sulfonated by the procedure given for PS-1%DVB–SO3H. 2.2.3. PS-20%DVB–SO3H Polystyrene (20% cross-linked) was prepared by a typical procedure for suspension polymerization. Styrene (120 mmol), divinylbenzene (24 mmol), and benzoyl peroxide (1.20 mmol) were added to a 300 mL aqueous solution of PVA (2.50 g), and the mixture was stirred at room temperature for 1 h and further stirred at 90 °C for 20 h by a mechanical stirrer under nitrogen atmosphere. After being cooled to room temperature, the resin was collected on a glass filter, rinsed with water and water/THF, THF, and dichloromethane, and dried in vacuo to give PS-20%DVB–SO3H in 90% yield. The particle size of the resin was measured by optical microscopy, and there was not much difference between PS-20%DVB (40– 100 lm) and PS-1%DVB (40–80 lm). The resin was next sulfonated by the procedure given for PS-1%DVB–SO3H. 2.3. Typical procedure for esterification catalyzed by PS–SO3H A mixture of 50 wt% aqueous acetic acid solution (25.0 mmol), ethanol (25.0 mmol), toluene (6.0 g), and 5.0 mol% of PS-1%DVB– SO3H (1.46 mmol g 1, 0.856 g) was stirred at 40 °C for 4 h. Acetone (50 g) was then added to the reaction mixture to form a homogeneous liquid phase, and the ester yield was determined by GC (45% yield). 2.4. Typical procedure for esterification in the presence of CaCl2 A mixture of 50 wt% aqueous acetic acid solution (100 mmol), ethanol (100 mmol), toluene (12.0 g), methanesulfonic acid (MsOH, 5.0 mmol, 5.0 mol%), and CaCl2 (10.0 g, 90.1 mmol, a saturating amount relative to the theoretical water amount in the case that the esterification proceeds quantitatively) was stirred at 40 °C for 72 h. Acetone (100 g) was then added to the reaction mixture to form a homogeneous liquid phase, and the ester yield was determined by GC (94% yield). 2.5. Typical procedure for esterification under toluene-free conditions A mixture of 50 wt% aqueous acetic acid solution (100 mmol), ethanol (100 mmol), MsOH (5.0 mmol, 5.0 mol%), and CaCl2 (10.0 g, 90.1 mmol, a saturating amount relative to the theoretical water amount in the case that the esterification proceeds quantitatively) was stirred at 40 °C for 72 h. The organic and aqueous phases were then separated, and the content of AcOEt, AcOH, and EtOH was determined by GC. Water content was determined by the Karl Fischer method. The ester yields in the organic and aqueous phases were 76 and 7%, respectively.

Table 1 Examination of addition of various organic solvents.

Entry

Organic solventa

AcOH aq.: solventb

Yield (%)

1 2 3 4 5 6 7 8 9 10

None n-Hexane (14.9) Toluene (18.2) Toluene (18.2) Benzene (18.6) p-Xylene (18.0) n-Octylbenzene (17.4) Cyclohexane (16.8) Anisole (19.5) Dibenzyl ether (19.3)

– 1:1 1:1 1:2 1:2 1:2 1:2 1:2 1:2 1:2

37 49 65 72 71 70 63 62 67 62

Other substances’ values are as follows: ethyl acetate (18.1), acetic acid (21.4), ethanol (26.5). a Data in brackets refer to Hansen’s solubility parameter (MPa1/2) cited in (Barton, 1983). b Initial mass ratio of 50 wt% AcOH aq. and an organic solvent.

yield. We envisioned that after the esterification reaction, the organic phase containing ethyl acetate might be easily separated from the aqueous phase and that ethyl acetate might be purified by distillation and then the organic solvent might be recovered and used repeatedly. Table 1 shows the results of esterification along with the values of Hansen’s solubility parameter (Barton, 1983). The solubility parameter is a numerical estimate of the degree of interaction between materials and is a good indication of the relative solvency behavior of a specific solvent. That is, materials with similar values are likely to be miscible. The highest yield was achieved by adding benzene, toluene, or p-xylene, whose values for the solubility parameters are close to the value for ethyl acetate (18.1). These results demonstrate that it is important to employ an organic solvent that has a high extraction ability for esters from an aqueous phase to improve the esterification yield. We then prepared several types of polystyrene-supported sulfonic acids according to the literature, to investigate the effect of the structure of the supported acids on ester yields. We initially carried out the esterification catalyzed by PS–SO3H (1.46 mmol g 1), which had a moderate loading level (sulfonic acid content in the polymer) and low DVB cross-linkage (Fig. 1). This result shows that the esterification yield increased approximately

100

3. Results and discussion Initially, we carried out the esterification of 50 wt% aqueous acetic acid with ethanol (the molar ratio of acetic acid to ethanol = 1:1) in the presence of 5 mol% MsOH at 40 °C for 72 h, and the desired ethyl acetate was obtained in 37% yield (Table 1, entry 1). The presence of a large excess amount of water has a detrimental effect on the equilibrium yield of the esterification. Although a large excess of one of the reactants is often employed to improve the yield, it is important to react at a 1:1 M ratio to minimize the amount of the residual reactants in water. To avoid the use of the excess reactant, we next attempted to add organic solvents as extraction agents for ethyl acetate to improve the esterification

Yield (%)

80 60 40 20 0 0

20

40

60

80

100

Reaction time (h) Fig. 1. Reaction profile for esterification catalyzed by PS-1%DVB–SO3H (1.46 mmol g 1). Conditions: 50 wt% AcOH aq., 1.0 equiv. of EtOH, 5 mol% of PS-1%DVB–SO3H (1.46 mmol g–1), initial mass ratio of AcOH aq. and toluene = 1:2, 40 °C.

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polymer particles. The inner area of highly cross-linked particles such as PS-20%DVB is difficult to sulfonate compared with low cross-linked particles (Pepper, 1951). Therefore, the more highly cross-linked PS–SO3H have higher sulfonic acid group density in the outer area of the polymer particles, which reduces the catalytic activity (Jerabek, 1980). In addition, commercially available ion exchange resins, Amberlite IR120B and Amberlite 200CT, were ineffective, presumably because of their high loading levels. These studies on both the effects of loading levels of the sulfonic acids and the catalyst structures revealed that a low-loading and alkylated polystyrene-supported sulfonic acid is one of the best catalysts for the esterification in aqueous media. Under these biphasic media (toluene–water) conditions, although the equilibrium yield was ca 70%, this was an acceptable value at this stage because we planned to recover ca 50% of total acetic acid from wastewater in the concentration process and this esterification process. However, we next tried to increase the equilibrium yield further and thus focused on the salting-out effect by adding inorganic salts to the aqueous phase (Herbert, 1972; Roman-Leshkov et al., 2007; Roman-Leshkov and Dumesic, 2009). In general, the salting-out effect is regarded as the result of hydration of ions and the interactions between hydrated ions and nonelectrolyte components (Gorgenyi et al., 2006). Hydration of cations and anions results from dipole interactions of ion species and water molecules, and it makes water molecules partially structured and reduces the degree of freedom. These water molecules are unavailable as a solvent; therefore, organic components tend to be less soluble in water containing salts. As indicators for comparison of the hydration ability of different ions, hydrated radius, hydration number, hydration enthalpy, and the Jones–Dole viscosity coefficient are widely used (Tansel et al., 2006). In addition, the ratio of charge squared to ionic radius, z2/r, correlates strongly with hydrated radius and hydration enthalpy. Basically, it is recognized that the higher the charge density of the ion, the larger the effect of salting out, although there are exceptions caused by interaction with coexisting substances (Kasikov et al., 2006). Investigation of the salting-out effect on esterification yield was carried out by addition of saturated or constant amounts of various inorganic salts to the aqueous phases, which improves partitioning of organic compounds into the extracting phases and leads to an increase in the ester yield. As shown in Fig. 3, the esterification yield was improved by addition of salts except for the case of sodium sulfate and magnesium sulfate. Although sulfates have higher salting-out ability than halides (Gorgenyi et al., 2006), the use of sulfates in acid-catalyzed reactions is undesirable, because formation of hydrogensulfate ion decreases reaction rates (Roman-Leshkov and Dumesic, 2009). Compared with ester yields when constant amounts of salts were added (initial molar ratio of salt to water = 1:10), the order of alkali metal chlorides was LiCl > NaCl > KCl > RbCl > CsCl, which shows that the ester yield de-

Table 2 Catalytic activity of polymer-supported catalysts and homogeneous ones.

Entry

Catalyst

Loading (mmol g

1 2 3 4 5 6a 7 8

PS-1%DVB–SO3H PS-1%DVB–SO3H PS-1%DVB–SO3H PS-1%DVB–SO3H PS-1%DVB–SO3H PS-1%DVB–SO3H MsOH HCl

4.09 2.77 1.46 0.74 0.22 4.09 – –

1

)

Yield (%) 34 39 45 49 52 36 37 37

a Addition of polystyrene (1% DVB cross-linked), the weight ratio of polystyrene/ catalyst = 4.5.

linearly within the first few hours and came to an equilibrium position in ca 20 h. Next, we examined the effect of the loading levels of the sulfonic acids on the catalytic activity by conducting the esterification for 4 h using PS-1%DVB–SO3H. As shown in Table 2, it was found that the lower the loading, the higher the yields (entries 1–5). These results indicate that hydrophobicity, which depends on the loading amounts of the sulfonic acids, is very important for the catalytic activity, as in the case employing hydrophobic substrates (Manabe and Kobayashi, 2002; Iimura et al., 2003c). Interestingly, low-loading PS-1%DVB–SO3H showed higher activity than homogeneous acids such as MsOH and HCl, although highly hydrophilic substrates are likely to encounter homogeneous catalysts in the aqueous phase. To elucidate the influence of the polymer volume on the esterification yield, 1% cross-linked polystyrene was added; however, little effect on the yield was observed (entry 6). Furthermore, we examined the effect of the loading amounts of the sulfonic acids and catalyst structure of polymer-supported sulfonic acids on their activity by comparison of the esterification yields after 4 h (Fig. 2). As a result, in all cases, the yields were better when the lower-loading polymer-supported sulfonic acids were employed (Erdem and Kara, 2012). C18-ALPS–SO3H showed the highest activity. Introduction of long alkyl chains increased the hydrophobicity of the catalysts, and the construction of hydrophobic reaction environments in the catalyst structure might be effective in removing water molecules generated in the reaction and could improve the reaction yield. In the cases of PS-20%DVB– SO3H and PS-1%DVB–SO3H, the difference in the yields increased for the higher loading levels. Presumably, this was caused by the difference in the sulfonic acid group distribution within the

70

Yield (%)

50

SO3H

40

PS-DVB-SO3H

30 20 10 0

C18H37

SO3H

60

0

1

2

3

4

5

6

C18-ALPS-SO3H

C18-ALPS-SO3H PS-1%DVB-SO3H PS-20%DVB-SO3H Amberlite IR120B Amberlite 200CT

SO3H loading (mmol g-1) Fig. 2. Effects of the SO3H loadings and the catalyst structure. Conditions: 50 wt% AcOH aq., 1.0 equiv. of EtOH, 5 mol% of catalyst, initial mass ratio of AcOH aq. and toluene = 1:2, 40 °C, 4 h.

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AcOH aq.

MsOH (5 mol%) inorganic salt

EtOH

+

(1.0 equiv)

50 wt%

AcOEt

H2O/toluene 40 ˚C AcOH aq. : toluene = 1 : 1

100 salt : water = 1 : 10 (mol/mol) saturated with salt

Yield (%)

90 80 70 60

gC

M

Cl 2 rCl 2 Br a S N

Ca

4

l2

Cl

Cs

2

Cl

Rb

Cu Cl 2 Zn Cl 2 Sn Cl Na 2 NO Na 3 2 SO 4 Mg SO

l

KC

2

Cl

Na

Cl

Cl

Li

Ni

ne

no

KB r Ca Br

40

N aI

50

Fig. 3. Effect of inorganic salts as additives on esterification yields. Violet bars correspond to experiments at initial molar ratio of salt to water = 1:10, for 48 h, and purplish red bars correspond to experiments in the presence of a saturating amount of salt relative to the theoretical water amount in the case that esterification proceeds quantitatively, for 72 h. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

creases with increase in ionic radius of the alkali metal cations. Higher yields were obtained when divalent salts such as CaCl2, MgCl2, SrCl2, NiCl2, and CuCl2 were added. Although Mg ion has a higher charge density than Ca ion, the yield using MgCl2 was lower than that using CaCl2. Because the order of salting-out ability between MgCl2 and CaCl2 differs depending on the literature (Kasikov et al., 2006; Tansel et al., 2006; Roman-Leshkov and Dumesic, 2009; Wu et al., 2010), it is considered that not only the charge density of ions but also interactions of ions with other components are important factors. In the case of adding saturated amounts of salts, the highest yield (94%) was obtained using CaCl2, which has a high solubility in water. It is noted that CaCl2 is economical and harmless to the environment. In contrast, in the case of using PS-1%DVB–SO3H instead of MsOH, similar yields were obtained in the presence of CaCl2. Recovery and reuse of PS-1%DVB–SO3H were successfully conducted after reactivation with hydrochloric acid. During the reactivation, Ca salts were converted into free acids. Next, the effect of CaCl2 on the esterification was examined for a shorter reaction time (4 h, Table 3). The yield was remarkably increased from 37% to 79% by addition of saturating amounts of CaCl2, and reached 90% on raising the temperature from 40 to 60 °C. Although the esterification was catalyzed slightly by CaCl2 on its own, the combination of CaCl2 with MsOH was important for significant improvement in the esterification yield. Although only the esterification using 50 wt% AcOH aq has been examined so far, the enhancement of the equilibrium yield by means of the salting-out effect was also applicable to low concentration acetic acid solutions. As shown in Fig. 4a, the difference in the equilibrium yields between those obtained in the presence of CaCl2 and those obtained in the absence of CaCl2 increased with

Table 3 Esterification of acetic acid with ethanol in the presence or absence of CaCl2 for 4 h.a Entry

Catalyst

CaCl2

Yield (%)

1b 2b,c 3c 4

MsOH MsOH None None

None Saturated Saturated None

37 79 [90]d 13 Trace

a Conditions: 50 wt% AcOH aq., 1.0 equiv. of EtOH, initial mass ratio of AcOH aq. and toluene = 1:2, 40 °C, 4 h. b In the presence of 5 mol% of MsOH. c Saturated with CaCl2. d At 60 °C.

decreasing concentrations of AcOH aq. This result shows that the new recovery method using direct esterification in water may have applicability to wastewater containing low concentrations of acetic acid (5–10 wt%). To develop a more efficient recovery system for acetic acid, we next examined the esterification under toluene-free conditions. Without using toluene as an extracting phase, the recycling process of toluene is not needed, and the concentration of ethyl acetate in the organic phase is dramatically increased to reduce the purification cost. As shown in Fig. 4b, the organic phase composed of concentrated ethyl acetate formed gradually during the reaction because the mutual solubility of the aqueous and organic phases was decreased by the salting-out effect; as a result, the equilibrium shifted to the product. Actually, the aqueous esterification saturated with CaCl2 under toluene-free conditions proceeded remarkably in high yield (83% total yield, 76% in organic phase, 7% in aqueous phase) in comparison with that under salt-free conditions (Table 1, entry 1). Concentration of ethyl acetate in the organic phase was as high as 92 wt%, with acetic acid (4.6 wt%), ethanol (2.7 wt%), and water (0.8 wt%). Although the partitioning of ethyl acetate into the aqueous phase was increased as compared with the esterification under AcOH aq/toluene (1/1) conditions (94% total yield, with only 0.3% in aqueous phase), the toluene-free esterification has a significant advantage in cost economy because the purification process can be simplified by using the concentrated ethyl acetate as the organic phase. Finally, the effect of the molar ratios of acetic acid and ethanol on toluene-free esterification was investigated by increasing the amount of ethanol (Fig. 4c). Interestingly, it was found that the maximum value of the ester yield was reached when 1.1 equiv. of ethanol was used. The use of an excess amount of ethanol brought about an increase in the mutual solubility of the aqueous and organic phases, and led to an increased partitioning of ethyl acetate into the aqueous phase. From the viewpoint of recovery of organic compounds from wastewater, although the highest yield (90%) is obtained by using 1.1 equiv. of ethanol, there is little advantage compared with the 1:1 M ratio conditions, because an excess amount of ethanol remains in the aqueous phase.

4. Conclusion We have developed an esterification of aqueous acetic acid with ethanol to synthesize ethyl acetate using polystyrene-supported

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to dilute aqueous acetic acid and to treatment of wastewater are now in progress.

(a) 100

Yield (%)

80

Acknowledgment

60

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan.

40

References 20

0

0

10

20

30

40

50

Concentration of acetic acid (wt%) EtOH CaCl2

(b)

AcOH EtOH CaCl2 H2O

AcOH H2O AcOH aq. reaction

AcOEt

phase separation

CaCl2 H2O

(c)

AcOEt CaCl2 H2O

100

Yield (%)

95

90

85

80 1.0

1.2

1.4

1.6

1.8

2.0

EtOH (equiv.) Fig. 4. Esterification of acetic acid with ethanol saturated with CaCl2 under aqueous conditions. (a) Esterification of low-concentration acetic acid in the presence or absence of CaCl2. The solid line corresponds to the ester yield saturated with CaCl2, other conditions as follows: 1–50 wt% AcOH aq. (6.0 g), EtOH (1.0 equiv.), MsOH (0.49 g), toluene (6.0 g), 40 °C, 72 h. The dotted line corresponds to the ester yield in the absence of CaCl2, other conditions as follows: 1–50 wt% AcOH aq. (6.0 g), EtOH (1.0 equiv.), MsOH (20 mol%), toluene (6.0 g), 40 °C, 168 h. (b) Schematic diagram for the process of recovery of concentrated ethyl acetate under toluene-free conditions. (c) Effect of ethanol amounts on the toluene-free esterification. Conditions: 50 wt% AcOH aq., 1.0–2.0 equiv. of EtOH, 5 mol% of MsOH, in the presence of a saturating amount of CaCl2, at 40 °C for 16 h.

sulfonic acid catalysts. Addition of an organic solvent significantly improved the reaction yield. Furthermore, hydrophobic lowerloading sulfonic acid catalysts significantly accelerated the reaction. This methodology is expected to be applied to general aqueous reactions using hydrophilic substrates. The effect of the addition of inorganic salts on the ester yields was investigated, and CaCl2 was found to be the most effective salt in improving the equilibrium yield. Further investigations to apply this system

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