110th Anniversary: Effects of Alcohol Concentration on the Reactions

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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110th Anniversary: Effects of Alcohol Concentration on the Reactions of Ethyl Acetate and Diethyl Malonate in Hot Compressed Water− Alcohol Mixed Solvents Makoto Akizuki, Kohki Ito, and Yoshito Oshima* Department of Environment Systems, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8563, Japan Downloaded via BUFFALO STATE on July 23, 2019 at 00:53:15 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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ABSTRACT: The effects of the alcohol concentration of a hot compressed water−alcohol (methanol or ethanol) mixture on acid-catalyzed ester reactions were investigated. In the water and water−methanol mixture, ethyl acetate reactions were promoted by the autocatalytic effect of the produced acetic acid, while diethyl malonate reactions were promoted by the catalytic effect of the solvent molecules. The kinetic analyses indicated that the catalytic effect of H+ derived from the dissociation of acetic acid decreased as methanol concentration increased. The catalytic effect of water molecules was highest in a mixed solvent of 10 mol % methanol, and that of methanol molecules increased as methanol concentration increased. In the water−ethanol mixture, the hydroxyl-decarboxylation of diethyl malonate to ethyl acetate and hydrolysis of ethyl acetate to acetic acid proceeded consecutively. The secondary ethyl acetate hydrolysis was suppressed in the water−ethanol mixture because of the differences in the effects of alcohol concentration on each reaction.

1. INTRODUCTION Hot compressed water is a promising solvent for synthetic organic reactions. Because of its small dielectric constant in comparison to that of water under ambient conditions,1 hot compressed water can dissolve many organic materials. Therefore, it could be used as a solvent for organic reactions. Moreover, the solvent properties of water drastically change with temperature and pressure,1−3 and can be utilized for controlling reactions. Acid-catalyzed reactions are particularly suitable reactions as they proceed even without the use of additional acid catalysts. As the ion product of water is particularly large under subcritical conditions (approximately 200−350 °C),2 the H+ formed by the dissociation of water molecules can promote a variety of reactions, such as dehydration4,5 and alkylation.6,7 Furthermore, it has been reported that the hydrogen bonding network of water decreases under supercritical conditions, and the H+ derived from locally dissociated water molecules can act as an acid catalyst.8 In addition to the acid catalytic effects of dissociated water molecules, it has been reported that undissociated water molecules can also catalyze acid-catalyzed reactions.9,10 Moreover, in the case of reactions that produce organic acids, such as the hydrolysis of esters, the H+ produced by the dissociation of organic acids acts as an acid catalyst.11,12 This autocatalytic effect is also considered to be characteristic of hot compressed water. Because of these acid catalytic effects, acidcatalyzed reactions proceed rapidly in hot compressed water. However, in the case of complex reactions, such as consecutive © XXXX American Chemical Society

and branching reactions, it is important to control the acid catalytic effects to obtain high selectivity for the desired reaction products. At temperatures beyond the critical temperature of water (374 °C), the solvent properties can be widely controlled by changing the pressure, even without changing temperature. However, below the critical temperature, the controllable range of the solvent properties by changing pressure is limited as the density of water in the gaseous and liquid phases is not largely affected by pressure, unlike that under supercritical conditions. A promising method of controlling the solvent properties is using a mixture of water and other solvents, such as alcohols. By mixing water and alcohols, the solvent properties can be changed from water-like to alcohol-like.13,14 Because of these changes in the solvent properties and the differences between water and alcohols as chemical species, the characteristic reaction rate and/or reaction selectivity, which cannot be obtained using a single solvent, can be achieved in hot compressed water−alcohol mixed solvents. The conversion of biomass resources, such as woody biomass,15,16 rice straw,17,18 microalgae,19−21 and their model compounds, such as cellulose22,23 and lignin,23,24 in hot compressed water−alcohol mixtures have been widely investigated. In most of these Received: Revised: Accepted: Published: A

April 28, 2019 June 25, 2019 June 27, 2019 June 27, 2019 DOI: 10.1021/acs.iecr.9b02301 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

3-methylmalonate was prepared following a previously published method.27 2.2. Experimental Procedures. The reactions were conducted using a flow reactor composed of SUS316 (Figure 1) at 250 °C and 30 MPa. The molar ratio of alcohol in the

studies, the use of mixed solvents achieved larger product yields than the use of single solvents. For example, during the conversion of microalgae in a hot compressed water−ethanol mixed solvent at 300 °C, Ji et al. obtained the maximum bio-oil yield at an ethanol content of 75 vol % and the minimum solid residue yield at an ethanol content of 50 vol %.21 It was suggested that water promotes the hydrolysis of microalgae as it is more acidic than ethanol, while ethanol promotes the decomposition of high-molecular-weight products as its dielectric constant is smaller than that of water. Furthermore, ethanol also acts as a hydrogen donor and suppresses repolymerization reactions. Ji et al. reported that these roles of water and ethanol synergistically affect the product yields in mixed solvents. Aida examined the solvolysis of polyethylene terephthalate in hot compressed water−methanol mixed solvent at 350 °C and 23−30 MPa, and 400 °C and 28 MPa.25 From the results of their experiment and molecular dynamics simulation, they suggested that the local concentration of methanol near the ester group was larger than the bulk concentration, and the local concentration affected the reaction rate. Ermakova et al. investigated the isomerization of α-pinene in a hot compressed water−ethanol mixed solvent at 384 °C and 230 atm,26 and found that the selectivity of limonene was higher with a higher water content. They suggested that the speed of α-pinene isomerization by the ionic mechanism was higher at a higher water content compared than that by the radical mechanism, and the change in the reaction mechanism contributed to the higher selectivity of limonene. On the basis of these results, they empirically estimated the concentration of H+-derived from the dissociation of water molecules in each mixed solvent composition. Although the characteristics of hot compressed water− alcohol mixed solvents are becoming clear, fundamental knowledge about the roles of solvents in reactions is insufficient and must be elucidated. In this study, we investigated the effects of the alcohol concentration of hot compressed water−alcohol (methanol or ethanol) mixtures on typical acid-catalyzed reactions. The solvolysis (hydrolysis and alcoholysis) of ethyl acetate (a simple ester of monocarboxylic acid) and diethyl malonate (an ester of dicarboxylic acid) was selected as a model reaction. The reactions of these esters are also important as models of consecutive reactions as the hydroxy-decarboxylation of diethyl malonates to corresponding acetates, known as Krapcho decarboxylation, is a useful method of synthesizing acetates. The purpose of this study is to elucidate the effects of alcohol addition on the acid catalytic effects in hot compressed water.

Figure 1. Schematic diagram of the flow reactor.

water−alcohol mixtures ranged from 0 to 30 mol %. The distilled water, alcohol, and reactants were pumped separately using three pumps (PU-2080; Jasco Corp., Japan and 260D; Teledyne ISCO, Inc., USA). The distilled water and alcohols were mixed and preheated by line preheating so that the temperature at the reactor entrance reached 250 °C. After mixing with the reactants, the solution was fed into the coil reactor with an internal diameter of 1.0 mm. The reactant concentrations of the water−methanol mixtures were 0.83− 0.90 and 1.3−1.4 mol/dm3 for the diethyl malonate and ethyl acetate reactions, respectively. For the reaction with the addition of acetic acid, equimolecular acetic acid was added to the esters. The stream emitted from the reactor was immediately cooled using a water-cooled heat exchanger and then depressurized using a back-pressure regulator (SCF-Bpg; Jasco Corp., Japan). The residence time, defined as the reactor volume divided by the volumetric flow rate of the hot compressed solvents,13 was controlled by changing the length of the reactor and the volumetric flow rate. 2.3. Analysis. The effluent was mixed with known amounts of alcohols to prevent phase separation at an ambient temperature, and then analyzed using gas chromatographs (GCs) equipped with a capillary column (TC-1701; GL Sciences, Inc., Japan). A GC-flame ionization detector (GC2014; Shimadzu Corp., Japan) was used for quantitative analysis, and a GC-mass spectrometer (GC-2010; Shimadzu Corp., Japan) was used for qualitative analysis.

2. EXPERIMENTAL METHODS 2.1. Reagents. Methanol, ethanol, diethyl malonate, ethyl acetate, and acetic acid were purchased from FUJIFILM Wako Pure Chemical Co., Japan., and methyl diethyl malonate and nbutyl diethyl malonate were purchased from Tokyo Chemical Industries Co., Ltd., Japan. Distilled water was prepared using distillation equipment (RFD240HA; Advantec Toyo Kaisha Ltd., Japan). Prior to use, the distilled water, methanol, and ethanol were degassed by bubbling with N2 gas. Dimethylmalonate, methyl acetate, malonic acid, hexanoic acid (FUJIFILM Wako Pure Chemical Co., Japan), monoethyl malonate, ethyl propionate, ethyl hexanoate (Tokyo Chemical Industries Co., Ltd., Japan), and propionic acid (Kanto Chemical Co., Inc., Japan) were used to analyze the reaction products, and 1-ethyl

3. RESULTS AND DISCUSSION 3.1. Reactions of Ethyl Acetate in the Water− Methanol Mixture. The time-dependence of ethyl acetate concentrations during the reactions of ethyl acetate in water and water−methanol mixtures is shown in Figure 2a. The reactions proceeded without the addition of a catalyst at each methanol content, and the reaction rates decreased as the methanol content increased. In addition, the reaction rate of ethyl acetate increased with increasing residence time, regardless of the methanol content. The major reaction products were acetic acid and ethanol. Methyl acetate was also produced in the water−methanol mixtures. Figure 3 shows the time dependence of acetic acid (a) and methyl acetate (b). B

DOI: 10.1021/acs.iecr.9b02301 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. Time dependence of the ethyl acetate concentration during the reaction of ethyl acetate at 250 °C and 30 MPa. (a) Reactions without the addition of acetic acid, (b) reactions with the addition of acetic acid. Molar ratios of methanol: (blue open and closed circles) 0 mol %, (green open and closed squares) 10 mol %, (orange open and closed triangles) 20 mol %, (red open and closed diamonds) 30 mol %.

Figure 3. Time dependence of the acetic acid (a, c) and methyl acetate (b, d) concentrations in the reaction of ethyl acetate at 250 °C and 30 MPa. (a, b) Reactions without the addition of acetic acid, (c, d) reactions with the addition of acetic acid. Molar ratios of methanol: (blue open and closed circles) 0 mol %, (green open and closed squares) 10 mol %, (orange open and closed triangles) 20 mol %, (red open and closed diamonds) 30 mol %.

According to the time dependence of the reaction rate, the conversion of ethyl acetate was promoted by the autocatalytic effect of the acetic acid produced in the reactions. To elucidate the catalytic effect, ethyl acetate reactions were conducted with the addition of additional acetic acid (1:1 molar ratio). The time dependence of the ethyl acetate concentrations is shown in Figure 2b. Compared with the reaction without the addition of further acetic acid (Figure 2a), the reactions were faster, regardless of the methanol content, suggesting that acetic acid served as a catalyst. Similar to the reactions without the addition of acetic acid, the reaction rates decreased with an increase in the methanol content. Figure 3 presents the time dependence of acetic acid (c) and methyl acetate (d). In the reactions in water and the 10 mol % methanol mixture, the acetic acid concentrations continuously increased as the

residence time increased, whereas they did not increase in the 20 and 30 mol % methanol mixtures. This suggests that the hydrolysis and methanolysis of ethyl acetate and the methyl esterification of acetic acid occurred. The acetic acid reactions were conducted in water−methanol mixtures (Figure 4), and the conversion of acetic acid to methyl acetate was confirmed. 3.2. Reactions of Diethyl Malonate in the Water− Methanol Mixture. Figure 5a shows the time dependence of the diethyl malonate concentrations in the diethyl malonate reactions without the addition of acetic acid. The reactions proceeded faster than the ethyl acetate reactions, regardless of the methanol concentration. When the methanol concentration increased, the reaction rates slightly decreased. The major reaction products were ethyl acetate, acetic acid, and ethanol, and monoethyl malonate was produced as a C


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Scheme 1. Reaction Path of Diethyl Malonate in the Hot Compressed Water−Methanol Mixture

Figure 4. Time dependence of the acetic acid concentration in its reaction at 250 °C and 30 MPa. Molar ratios of methanol: (green squares) 10 mol %, (orange triangles) 20 mol %, (red diamonds) 30 mol %.

byproduct. In the reactions in the water−methanol mixtures, methyl acetate was also obtained as a major product, and 1ethyl 3-methylmalonate and dimethylmalonate were formed as byproducts. Figure 5b shows the time dependence of the diethyl malonate concentrations in the diethyl malonate reactions with the addition of further acetic acid. Unlike the reaction without the addition of acetic acid (Figure 5a), the time dependence did not greatly differ between the different methanol contents. This indicates that the diethyl malonate reactions were not promoted by the autocatalytic effect of the produced acetic acid. The reaction products could be separated into two groups based on the diethyl malonate reaction pathways (Scheme 1). One group consists of hydrolysis products, including the primary products of the hydrolysis of diethyl malonate and the secondary products of one or more hydrolysis reactions. The other consists of methanolysis products alone. Figure 6 shows the concentrations of the hydrolysis (a) and methanolysis (b) products during the diethyl malonate reactions without the addition of acetic acid. With an increase in the methanol content, fewer hydrolysis products were produced and more methanolysis products formed. Regardless of the methanol content, the concentrations of the hydrolysis products continuously increased, while those of methanolysis products initially increased then decreased with an increase in the residence time.

3.3. Kinetic Analyses of the Ester Reactions in the Water−Methanol Mixture. As described in the previous sections, the reaction of ethyl acetate and that of diethyl acetate proceeded in different manners, and each reaction can be regarded as a model of acid catalyzed reaction proceeds in a different mechanism. To quantitatively discuss the effects of methanol concentration on each type of reaction, we conducted kinetic analyses. The results in Section 3.1 suggest that the ethyl acetate reactions are mainly promoted by the catalytic effect of acetic acid. In acid catalyzed hydrolysis and alcoholysis of an ester, the carbonyl group of ester is protonated, and water or alcohol is added to the protonated carbonyl carbon. Also, in an acidcatalyzed esterification, the carbonyl group of carboxylic acid is protonated, and an alcohol is added to the protonated carbonyl carbon. By assuming that each reaction rate in Scheme 2 (R1− R3, R−3) is proportional to the concentrations of the reactant, solvent molecule, and H+ formed by the dissociation of acetic acid, the reaction rates (r1−r3, r−3) can be described by eqs 1 and 2). ri = k iC H+CreactantCwater = ki(KACacetic acid)0.5 CreactantCwater = k i′Cacetic acid 0.5Creactant(i = 1, −3)

(1)

Figure 5. Time dependence of the diethyl malonate concentration in its reaction at 250 °C and 30 MPa. (a) Reactions without the addition of acetic acid, (b) reactions with the addition of acetic acid. Molar ratios of methanol: (blue open and closed circles) 0 mol %, (green open and closed squares) 10 mol %, (orange open and closed triangles) 20 mol %, (red open and closed diamonds) 30 mol %. D


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Figure 6. Time dependence of the product concentrations in the reaction of diethyl malonate without the addition of acetic acid at 250 °C and 30 MPa. (a) Concentration of hydrolysis products, (b) concentration of methanolysis products. Molar ratios of methanol: (blue circles) 0 mol %, (green squares) 10 mol %, (orange triangles) 20 mol %, (red diamonds) 30 mol %.

data were explained well by using the model described above. The evaluated kinetic rate coefficients are summarized in Table 1. At all methanol contents examined in this study, methyl acetate was mainly produced by the methyl esterification of acetic acid (R3), and the values of k2′ (corresponding to the methanolysis of ethyl acetate (R2)) were negligible. However, the reactions of diethyl malonate were not promoted by the acid catalytic effects of acetic acid (Section 3.2). A potential reaction mechanism is the SN2 reaction catalyzed by solvent molecules. In SN2 hydrolysis and methanolysis of dimethyl acetate, water or methanol is added to carbonyl carbon of dimethyl acetate as nucleophile, and then, elimination of EtO group occurs. By assuming that each reaction in Scheme 1 (R1−R3) is proportional to the concentration of reactants and that Cwater and Cmethanol are constant, the reaction rates (r1−r3) can be described according to eqs 3 and 4.

Scheme 2. Reaction Path of Ethyl Acetate in the Hot Compressed Water−Methanol Mixture

rj = k jC H+CreactantCmethanol = k j(KACacetic acid)0.5 Creactant Cmethanol = k j′Cacetic acid 0.5Creactant(j = 2, 3)

(2)

where k is the kinetic rate coefficient, C is the concentration, and KA is the dissociation coefficient of acetic acid. As Cwater and Cmethanol were large and remained almost unchanged during the reactions, they can be regarded as constants. The kinetic rate constants were evaluated by fitting these equations to the experimental data of the ethyl acetate and acetic acid reactions. The fitting was conducted using the solver function of Microsoft Excel, and the equations were numerically integrated following the Runge−Kutta method. The parity plot is shown in Figure 7a and the fitting results of each component are shown in Figures S1−S3. The experimental

ri = k iCreactantCwater = k i′Creactant(i = 1, 3)

(3)

rj = k jCreactantCmethanol = k j′Creactant (j = 2)

(4)

These equations were analytically integrated, and the kinetic rate coefficients were evaluated by fitting them to the experimental data. Figure 7b presents the parity plot, Figure S4 presents the fitting results, and Table 2 presents the values of the kinetic rate coefficients. The data calculated based on the model were in good accordance with the experimental data.

Figure 7. Parity plots of the (a) reaction system of ethyl acetate, (b) reaction system of diethyl malonate. Symbols: (blue closed diamonds) ethyl acetate, (orange closed squares) ethanol, (red closed triangles) acetic acid, (green closed circles) methyl acetate, (blue open diamonds) diethyl malonate, (red open triangles) hydrolysis products, and (green open circles) methanolysis products. E


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Industrial & Engineering Chemistry Research Table 1. Kinetic Rate Coefficients of the Ethyl Acetate Reaction System k′ ((dm3)0.5 mol−0.5 s−1) k1′

molar ratio of MeOH 0 0.1 0.2 0.3

7.70 4.18 3.04 2.11

× × × ×

k2′ 10−3 10−3 10−3 10−3

0 0 0

k′/Csolvent ((dm3)1.5 mol−1.5 s−1)

k3′ 7.04 × 10−3 5.98 × 10−3 4.85 × 10−3

k−3′

k1′/Cwater

2.77 × 10−2 9.15 × 10−3 5.17 × 10−3

1.68 1.15 1.07 9.48

× × × ×

k2′/CMeOH

k3′/CMeOH

k−3′/Cwater

0 0 0

1.75 × 10−3 8.40 × 10−4 5.09 × 10−4

7.65 × 10−4 3.22 × 10−4 2.33 × 10−4

10−4 10−4 10−4 10−5

Table 2. Kinetic Rate Coefficients of the Diethyl Malonate Reaction System k′ (s−1) k1′

molar ratio of MeOH 0 0.1 0.2 0.3

6.83 6.05 4.24 2.41

× × × ×

k′/Csolvent (dm3 mol−1 s−1)

k2′ 10−2 10−2 10−2 10−2

7.05 × 10−3 1.60 × 10−2 2.35 × 10−2

k3′ 9.11 × 10−3 3.17 × 10−2 4.89 × 10−2

k1′/Cwater 1.49 1.67 1.49 1.08

× × × ×

10−3 10−3 10−3 10−3

k2′/CMeOH

k3′/Cwater

1.75 × 10−3 2.25 × 10−3 2.46 × 10−3

2.51 × 10−4 1.11 × 10−3 2.20 × 10−3

bulk concentrations decreased with increasing methanol concentration. Although the increase in the local concentration is the general effect of the addition of methanol on hot compressed water, the dependence of methanol concentration differs depending on the solvent properties, which can be changed by changing the temperature and pressure. For example, the values of the excess molar volume,13 which relate to the repulsive and attractive interactions between solvent molecules, were negative and decreased with increasing methanol concentration at 250 °C and 30 MPa (same as our study). However, under similar conditions to those of Aida’s study, the values at 345 °C and 20 MPa were largely positive and increased an increase in methanol concentration, while those at 345 °C and 30 MPa changed slightly from negative to positive. Such differences in the characteristics of a mixed solvent are considered to affect the dependence of the methanol concentration. 3.4. Application of the Water−Alcohol Mixture for Controlling Reaction Selectivity: Reactions of Diethyl Malonate Derivatives in the Water−Ethanol Mixture. In this section, we discuss how the characteristics of the acid catalytic reactions in the water−alcohol mixtures investigated above can be utilized for controlling reaction selectivity, taking the hydroxy-decarboxylation of diethyl malonates to the corresponding acetates as an example. This reaction is useful for synthesizing acetates through Krapcho decarboxylation. The reactions were conducted in a water−ethanol mixture to allow us to focus on controlling two consecutive hydrolysis reactions. (Scheme 3) The reactions of diethyl malonate in the water−ethanol mixture were conducted at 250 °C and 30 MPa. The ethanol concentrations were 0−20 mol %, and the initial concentrations of diethyl malonate were 0.82−0.90 mol/dm3. The major product was ethyl acetate, while acetic acid was

The effects of the methanol concentration will be discussed based on the evaluated kinetic rate coefficients. Each k′ includes the water or methanol concentration. To discuss the catalytic effects of the water−methanol mixtures as solvents, the values of k′ divided by the solvent concentration as a reaction product are also presented in Tables 1 and 2. In the subsequent discussions, k1′ and k2′ were used to discuss the reaction rate coefficients of the hydrolysis and methanolysis of diethyl malonate, respectively. In the ethyl acetate reaction system (Table 1), the k′/Csolvent values decreased with increasing methanol concentration in all reactions, indicating that the catalytic effect of H+ derived from the dissociation of acetic acid is smaller with a higher methanol concentration. The dielectric constants at 250 °C and 30 MPa are 28.3 for pure water1 and 7.6 for pure methanol,29 and the values of the hot compressed water−methanol mixtures have been reported to decrease with increasing methanol concentrations.14 Under a smaller dielectric constant, ionic species become less stable. As the transition state of the acid-catalyzed mechanism is ionic, the activation energy increases, thereby decreasing the reaction rate. Furthermore, as H+ is an ionic species, the dissociation of acetic acid is suppressed and its concentration would decrease. In the diethyl malonate reaction system (Table 2), the k1′/ Cwater value of the 10 mol % methanol mixture was larger than that of pure water, while the value decreased as the methanol concentration increased above 10 mol %. These results suggest that the catalytic effect of water molecules reached highest at a methanol concentration of 10 mol %. In contrast, the k2′/ Cmethanol value increased as the methanol concentration increased from 10 to 30 mol %. Although water and methanol have good miscibility, the microscopic structure of the mixture is not uniform.28 A plausible explanation for the decrease in the k1′/Cwater value and increase in the k2′/Cmethanol value as the methanol concentration increased above 10 mol % is that the local concentration of methanol near the active site of diethyl malonate is higher than that of the bulk solvent, and the difference between the local concentration and bulks is larger with a higher methanol concentration. Aida also reported a higher local concentration of methanol than that of the bulk solvent in the water−methanol mixture in his research on the solvolysis of polyethylene terephthalate in a hot compressed water−methanol mixture at 350 °C and 23−30 MPa.25 However, in his study, the difference between the local and

Scheme 3. Reaction Path of the Diethyl Malonate Derivatives in the Hot Compressed Water−Ethanol Mixture

F


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Figure 8. Time dependence of the ester yields in the reaction of diethyl malonate derivatives in water and the water−ethanol mixture. (a) Ethyl acetate yield of the diethyl malonate reaction at 250 °C and 30 MPa, (b) ethyl propionate yield of the methyl diethyl malonate reaction at 300 °C and 30 MPa, (c) ethyl hexanoate yield of the buthyl diethyl malonate reaction at 300 °C and 30 MPa. Molar ratios of ethanol: (blue circles) 0 mol %, (green squares) 10 mol %, (orange triangles) 20 mol %.

malonate were 0.67−0.75 mol/dm3, and those of n-buthyl diethyl malonates were 0.52−0.56 mol/dm3. Figure 8b, c show the time dependence of the ester yield. As was observed for the reactions of diethyl malonate, the secondary hydrolysis of esters was suppressed in the water−ethanol mixture. Although the maximum ester yield was not largely different between the water and water−ethanol mixtures, high ester yields could be obtained under a wide range of residence times in the water− ethanol mixture. These results indicate that using a water− alcohol mixture is an effective method of controlling the reactions of diethyl malonate derivatives.

produced as a byproduct. In addition, a small amount of monoethyl malonate was detected in the initial stage of the reaction. These product behaviors suggest that the major reaction path included the following consecutive reactions; 1, hydrolysis and subsequent decarboxylation of diethyl malonate to ethyl acetate; and 2, hydrolysis of ethyl acetate to acetic acid. Figure 8a shows the time dependence of the ethyl acetate yield of the water and water−ethanol mixtures. In water, the yield was highest at approximately 40 s, and then decreases with increasing residence time owing to the secondary hydrolysis of ethyl acetate. However, the secondary reaction was suppressed by adding ethanol to the solvent, and larger ethyl acetate yields were obtained in the water−ethanol mixture. As reported in the previous section, the hydrolysis of ethyl acetate was inhibited as the methanol concentration increased due to the lower catalytic effect of H+ in the water− methanol mixture. In addition, as the hydrolysis of ethyl acetate is autocatalytic, the reaction rate increased once acetic acid was produced as a reaction product. Therefore, the smaller catalytic effect of H+ in a solvent with a higher alcohol concentration contributed to the inhibition of the hydrolysis of ethyl acetate to acetic acid. In addition, the ethyl esterification of acetic acid, which is the reverse reaction of the hydrolysis of ethyl acetate, was faster with a higher ethanol content, and this reaction also reduced the apparent reaction rates of the hydrolysis of ethyl acetate. The reactions of other diethyl malonate derivatives (methyl diethyl malonate and n-butyl diethyl malonate) were also investigated. As the reactions did not sufficiently proceed at 250 °C and 30 MPa, the reactions were conducted at 300 °C and 30 MPa. The initial concentrations of methyl diethyl

4. CONCLUSIONS The reactions of ethyl acetate and diethyl malonate derivatives in hot compressed water−alcohol with concentrations ranging from 0 to 30 mol % were investigated as typical models of acidcatalyzed reactions. From the experiments and kinetic analyses, the following conclusions regarding the features of hot compressed water−alcohol mixed solvents for acid-catalyzed reactions were drawn. 1 The hydrolysis reaction of ethyl acetate in the water− methanol mixture mainly proceeded in an autocatalytic manner. As the methanol content of the solvent increased, the catalytic effect of the produced acetic acid decreased. The smaller dielectric constant of the water−methanol mixture contributed to the larger activation energy of the ionic reaction and/or the lower dissociation of acetic acid. 2 The hydrolysis and methanolysis of diethyl malonate in the water−methanol mixture did not occur by the catalytic effect of acetic acid, but by the catalytic effect of the solvent molecules. The reaction rate coefficient of G


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hydrolysis in the 10 mol % methanol mixture was higher than that in pure water, suggesting that the catalytic effect of water molecules is higher in the 10 mol % methanol mixture. As the methanol concentration increased from 10 to 30 mol %, the reaction rate coefficients of hydrolysis decreased while those of methanolysis increased. This could be due to the different local concentrations of water and methanol near the active site of diethyl malonate to those in the bulk solvent. 3 The major path of the reactions of the diethyl malonate in the water−ethanol mixture was the consecutive reaction, which involved two key hydrolysis reactions. Unlike the reaction in water, the secondary hydrolysis reaction was suppressed in the water−ethanol mixture, contributing to the large yield of ethyl acetate. The suppression of secondary hydrolysis was also observed in the reactions of other diethyl malonate derivatives. In this research, the effects of the alcohol concentration of hot compressed water−alcohol mixtures on certain acidcatalyzed reaction rates are elucidated. The findings will aid in controlling reactions by changing the composition of water− alcohol mixed solvents.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b02301.

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Fitting results of the ethyl acetate reactions, acetic acid reactions, and diethyl malonate reactions (PDF)

AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. ORCID

Makoto Akizuki: 0000-0001-9350-0036 Yoshito Oshima: 0000-0002-6222-3774 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Part of this work was financially supported by the Kurita Water and Environment Foundation Grant (Grant 16D018). The GC-MS analyses were conducted using facilities at Kashiwa Branch, Environmental Science Center, The University of Tokyo. We highly appreciate this support. We also thank Editage for English language editing.

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REFERENCES

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