Tautomeric reaction equilibrium of ethyl acetoacetate in CO2–n-pentane and CO2–ethanol mixed solvents in the critical region

Fluid Phase Equilibria 200 (2002) 111–119

Tautomeric reaction equilibrium of ethyl acetoacetate in CO2 –n-pentane and CO2 –ethanol mixed solvents in the critical region Hongping Li, Jun Liu, Xiaogang Zhang, Buxing Han∗ , Zhimin Liu, Jun He, Liang Gao Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China Received 5 July 2001; accepted 14 January 2002

Abstract The tautomeric equilibrium constant KC of ethyl acetoacetate (EAA) in CO2 –n-pentane and CO2 –ethanol was studied by UV–VIS spectroscopy at 308.2 K over the pressure range from 6.5 to 9.0 MPa. This work focuses on how the tautomeric equilibrium changes with pressure and composition of the mixed solvents in the near critical region. The results showed that the effect of pressure on KC was very limited at pressures much higher than the phase separation pressures. However, the KC increased sharply as the pressure approached the critical point or the bubble point of the mixed solvents. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Near critical fluids; Phase behavior; Ethyl acetoacetate; Tautomerism

1. Introduction In recent years, increasing numbers of chemists have begun to study reaction chemistry in supercritical (SC) fluids (SCFs) [1–6]. There are some unique advantages to conduct chemical reactions in SCFs. For example, reaction rates, yields, and selectivity can be adjusted by varying pressure. Mass transfer is improved for heterogeneous chemical reactions, and simultaneous separation and reaction may be accomplished for some reactions. Many papers about chemical reactions in SCFs have been published, and this topic has been reviewed [7–11]. Utilization of SCFs will solve more challenging problems in reaction processes, and after our fundamental understanding of SCFs improves, environmentally more acceptable solvents, such as supercritical (SC) CO2 and H2 O, can be used as solvents to replace many undesirable organic solvents. ∗

Corresponding author. Tel.: +86-10-6256-2821; fax: +86-10-6255-9373. E-mail address: [email protected] (B. Han). 0378-3812/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 1 2 ( 0 2 ) 0 0 0 2 1 - 3

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Study on phase behavior is a long established field [12–14]. However, the effect of pressure and composition of the solvents on chemical reactions in the critical region of mixed solvents is still a very interesting research area. Tautomeric reactions are ideal reversible reactions for probing the effect of different factors on chemical equilibrium. The tautomerism of ethyl acetoacetate (EAA) in liquid solvents [15], SC CF3 H and CF3 Cl [16], and SC CO2 with small amount of co-solvents [17,18] has been studied, which indicate that the equilibrium constant KC is sensitive to the properties of the solvents. Recently, we determined the constant volume heat capacity CV of CO2 + ethanol and CO2 + n-pentane in different phase regions near the critical points of the mixtures [19]. The results show that at fixed composition, CV increases sharply as the pressure approaches the bubble points and the critical points of the mixtures, indicating that the properties of the mixtures are very sensitive to pressure. In this work, the KC of EAA in CO2 + ethanol and CO2 + n-pentane mixed solvents is studied. We focus on the effect of pressure on the KC as the pressure approaches the bubble points and critical point of the mixed solvents.

2. Experimental 2.1. Materials CO2 with a purity of 99.995% was supplied by Beijing Analytical Instrument Factory; n-pentane (>99.7%) and ethanol (>99.5%) were produced by Beijing Chemical Company; EAA was purchased from Aldrich with a purity of >99%. The chemicals were used without further purification. 2.2. Apparatus and procedures for the UV experiments The schematic diagram of the experimental set-up was reported in our previous work [17]. It consisted of a gas cylinder, a high-pressure pump, a pressure gauge, an UV–VIS spectrometer, a temperature-controlled high-pressure UV sample cell, and valves and fittings. UV–VIS spectrophotometer was produced by Beijing General Instrument Company (model TU-1201, resolution: 0.1 nm). The UV sample cell consisted of a stainless steel body and two quartz windows. Outside of the cell body was coiled with electrical heating wire and heat insulation material. The optical length of the UV cell is 1.09 × 10−2 m. To minimize the error originated from temperature and pressure measurements, we used the same pressure gauge and thermometer for phase behavior, density, and UV measurements. The UV experimental method was also similar to that described previously [17]. The main difference was that in this work, mixed solvents were involved, which were prepared in a sample bomb by a gravimetric method. In the experiments, EAA was loaded into the UV sample cell. After thermal equilibrium had been reached, the mixed solvent was charged into the sample cell from the sample bomb until the desired pressure was reached. The UV spectra of EAA were determined. Then, the experiment at a higher pressure was conducted by charging more solvent into the UV sample cell. The volume concentration of EAA was the same at all the pressures because the experiments were carried out from the lowest pressure to highest pressure. To keep the composition of the solvent constant at different pressures, the mixed solvent was always in the single-phase region when charged into the UV sample cell. UV spectra of all the solvents with different compositions were also determined and used as background spectra.

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To determine the absorbance of the EAA of a sample, the background absorbance of the corresponding solvent was subtracted from the spectrum.

3. Results and discussion 3.1. Equilibrium constant KC of tautomerization In this work, we determined the KC of EAA tautomeric reaction in CO2 (1) + n-pentane (2) and CO2 + ethanol systems at 308.2 K. The concentration of EAA in the solution was 1.05 × 10−1 mol m−3 for all of the experiments. The critical point pressures (PC ) and bubble point pressures (Pb ) of CO2 (1) + n-pentane (2) and CO2 +ethanol systems at 308.2 K were reported in our previous work [19], which are also shown in Table 1. The data in Table 1 allowed us to select typical compositions and pressures of the mixed solvents. For CO2 + n-pentane system, the mixtures with X2 = 0.010, 0.021, 0.050, and 0.100 were chosen, and for CO2 + ethanol system X2 = 0.010, 0.020, 0.050, 0.084 were selected. All the experiments are carried out at homogeneous conditions. The tautomerism reaction of EAA can be expressed as follows:

(1) The equilibrium of the tautomerism reaction changes with the properties of solvent. The properties of a fluid in the critical region depend strongly on the pressure. Thus, the concentration of enol tautomer changes with pressure because the total concentration of EAA is fixed (1.05 × 10−1 mol m−3 ). UV–VIS was used to determine the concentration of the enol tautomer in different solvents. For example, Fig. 1 shows the UV absorbance of EAA in a CO2 (1) + ethanol (2) mixture (X2 = 0.020) at 308.2 K and different pressures. There is an absorption maximum at about 241 ± 1 nm, which is assigned to the π → π ∗ transition of the C=C group for the enol isomer [17]. The intensities of the spectra were expressed by the apparent molar absorption coefficient (εapp ), which was calculated by the Lambert–Beer law Table 1 Bubble point (Pb ) of CO2 (1) + n-pentane (2) and CO2 (1) + ethanol (2) systems at 308.2 K CO2 (1) + n-pentane (2)

CO2 (1) + ethanol (2)

X2

Pb (Mpa)

X2

Pb (MPa)

0.010 0.021b 0.050 0.100

Single phasea 7.47b 7.17 6.52

0.010 0.020b 0.050 0.084

Single phasea 7.70b 7.42 7.16

a b

Single phase in the whole pressure range. Critical composition and critical pressure.

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Fig. 1. Typical UV spectra of the absorbance of EAA in CO2 (1)–ethanol (2) mixture (X2 = 0.020) at 308.2 K and different pressures.

εapp =

A bC0

(2)

where A is the absorbance at the absorption maximum, C0 the total concentration of EAA in the cell and b the path length of the cell. The enol content (% enol) and equilibrium constant (KC ) was calculated from εapp % enol = (3) εmax KC =

% enol [enol] = [keto] 1 − % enol

(4)

where [enol] and [keto] denote the equilibrium concentrations of enol and keto tautomers, respectively. The εmax is the maximum molar absorption coefficient of the pure enol tautomer and εmax = 16000 l mol−1 cm−1 is adopted [20]. In our previous work [17], we determined the percentage of enol tautomer in liquid solvents of different polarities by UV method using the value of εmax = 16000 l mol−1 cm−1 . The results agreed well with that determined by 1 H NMR method determined by other authors [21]. This indicated that the effect of solvent properties on εmax was not considerable. Thus, the KC could be calculated on the basis of the εapp determined and Equations (3) and (4). The enol contents (% enol), apparent molar absorption coefficients (εapp ) and equilibrium constants (KC ) of EAA tautomerization in different solvents at various pressures are listed in Table 2. 3.2. Effect of pressure and composition of the solvents on KC At pressures lower than PC or Pb the solvents should separate into a liquid and a vapor phase. The KC become meaningless at these conditions; thus, the lowest pressures examined were PC or Pb at these compositions. The equilibrium constants are graphically shown in Figs. 2 and 3. The figures show that

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Table 2 Enol contents (% enol), apparent molar absorption coefficients (εapp ) and equilibrium constants (KC ) of EAA tautomerization at 308.2 K and various pressures CO2 (1) + n-C5 H12 (2)

CO2 (1) + ethanol (2)

P (MPa)

εapp (l mol−1 cm−1 )

7.88 7.93 8.00 8.27 8.40 8.61 8.83 9.00

5677.6 5593.7 5528.3 5497.0 5473.8 5455.2 5442.6 5438.3

7.49 7.60 7.72 7.83 7.96 8.06 8.30 8.57 8.77 9.04

5681.6 5618.0 5584.4 5562.1 5542.6 5521.9 5510.3 5494.5 5480.5 5477.6

7.11 7.17 7.26 7.40 7.69 8.23 8.60 9.00

5710.7 5658.1 5614.4 5596.8 5583.4 5562.9 5550.0 5528.7

6.52 6.64 6.75 7.10 7.57 7.92 8.50 9.01

5720.5 5684.1 5664.5 5633.7 5628.9 5610.7 5593.6 5583.4

Enol (%)

KC

P (MPa)

εapp (l mol−1 cm−1 )

35.5 35.0 34.6 34.4 34.2 34.1 34.0 34.0

0.550 0.538 0.528 0.523 0.520 0.517 0.516 0.515

7.86 7.92 7.96 8.01 8.06 8.21 8.43 9.02

4783.8 4604.3 4487.6 4391.3 4313.7 4249.6 4160.1 4080.9

35.5 35.1 34.9 34.8 34.6 34.5 34.4 34.3 34.3 34.2

0.551 0.541 0.536 0.533 0.530 0.527 0.525 0.523 0.521 0.521

7.71 7.79 7.84 8.00 8.32 8.80 9.02

4578.7 4411.3 4283.8 4175.0 4079.2 4008.8 3971.7

35.7 35.4 35.1 35.0 34.9 34.8 34.7 34.6

0.555 0.547 0.541 0.538 0.536 0.533 0.531 0.528

7.46 7.60 7.65 7.75 8.03 8.49 8.72 9.01

4276.0 4147.9 4057.6 3997.7 3928.9 3878.5 3827.3 3793.8

35.8 35.5 35.4 35.2 35.2 35.1 35.0 34.9

0.557 0.551 0.548 0.543 0.543 0.540 0.538 0.536

7.16 7.26 7.30 7.34 7.40 7.71 7.87 8.31 9.00

3795.8 3702.1 3638.0 3615.4 3592.0 3573.8 3550.5 3527.5 3501.0

X2 = 0.010

Enol (%)

KC

29.9 28.8 28.0 27.4 27.0 26.6 26.0 25.5

0.427 0.404 0.390 0.378 0.369 0.362 0.351 0.342

28.6 27.6 26.8 26.1 25.5 25.1 24.8

0.401 0.381 0.366 0.353 0.342 0.334 0.330

26.7 25.9 25.4 25.0 24.6 24.2 23.9 23.7

0.365 0.350 0.340 0.333 0.325 0.320 0.314 0.311

23.7 23.1 22.7 22.6 22.5 22.3 22.2 22.0 21.9

0.311 0.301 0.294 0.292 0.289 0.288 0.285 0.283 0.280

X2 = 0.010

X2 = 0.021

X2 = 0.020

X2 = 0.050

X2 = 0.050

X2 = 0.100

X2 = 0.084

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Fig. 2. Effect of pressure on the equilibrium constant KC in pure CO2 and CO2 (1)–n-pentane (2) mixtures at 308.2 K.

at fixed solvent composition, the KC at subcritical conditions has a similar trend to that at supercritical conditions. The KC increases sharply as the pressure approaches Pb or PC . At higher pressures, however, the effect of pressure on the KC is not significant. The KC in the SCFs is very sensitive to the pressure near the critical point. From the results in Figs. 2 and 3 we can deduce that properties of subcritical fluids are also very sensitive to pressure as pressure approaches the bubble points. However, the effect of pressure becomes less pronounced as the composition of the solvent is further away from the critical composition. The effect of the pressure on KC can be discussed qualitatively in the following paragraphs. Lu et al. [17] determined the KC of EAA in alcohols and ethanol–cyclohexane mixtures. The UV results indicated that KC was related to the dielectric constant and density of the solvent, and the KC decreased with increasing dielectric constant and the density of the solvents. The densities of solvents determined by gravimetric method [22] in this work are listed in Table 3, and graphically shown in Figs. 4 and 5. It is shown from Figs. 4 and 5 that the density of the solvents decreases dramatically as pressure approaches PC or Pb. The dielectric constant of the solvents should decrease with decreasing density.

Fig. 3. Effect of pressure on the equilibrium constant KC in pure CO2 and CO2 (1)–ethanol (2) mixtures at 308.2 K.

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Table 3 Densities of CO2 (1)–n-pentane (2) and CO2 (1)–ethanol (2) mixtures at 308.2 K and various pressures ρ mix (103 mol m−3 )

P (Mpa)

X2 = 0.010 9.79 16.32 9.40 15.92 9.04 15.55 8.70 14.95 8.41 14.43 8.23 13.89 8.04 12.97 7.97 12.24 7.93 11.85 7.87 11.03 7.83 10.42 7.76 9.69 7.71 9.14 7.60 8.18 7.52 7.45 7.33 6.69 7.13 6.01 6.88 5.39

X2 = 0.021 9.90 9.65 9.27 8.91 8.75 8.54 8.36 8.16 8.04 7.95 7.86 7.81 7.75 7.68 7.63 7.58 7.55 7.52 7.49 7.38

(X2 = 0.0102) 9.92 16.62 9.07 15.81 8.56 14.99 8.21 13.97 8.05 13.14 7.93 11.74 7.87 10.86 7.81 10.07 7.76 9.16 7.71 8.83 7.62 7.95 7.52 7.33 7.30 6.28 7.11 5.71 6.83 5.05 6.51 4.49

X2 = 0.020 9.97 9.60 9.37 9.00 8.66 8.30 8.11 7.96 7.88 7.81 7.75 7.70 7.67

ρ mix (103 mol m−3 )

16.31 16.18 15.98 15.70 15.54 15.31 15.11 14.81 14.62 14.46 14.23 14.10 13.93 13.67 13.48 13.13 12.88 12.59 12.25 11.23

17.04 16.77 16.60 16.21 15.85 15.32 14.91 14.49 14.16 13.79 13.27 12.60 12.14

P (Mpa)

ρ mix (103 mol m−3 )

CO2 (1)–n-pentane (2) X2 = 0.050 10.02 9.65 9.25 8.81 8.61 8.37 8.16 8.01 7.97 7.92 7.85 7.75 7.63 7.52 7.42 7.31 7.19 7.09

16.37 16.20 16.01 15.75 15.62 15.45 15.28 15.15 15.11 15.06 14.99 14.88 14.77 14.63 14.49 14.30 14.09 13.51

CO2 (1)–ethanol X2 = 0.050 10.13 9.70 9.27 8.90 8.60 8.21 7.92 7.75 7.59 7.51 7.48 7.44 7.41 7.37

17.67 17.45 17.22 16.96 16.76 16.41 16.09 15.86 15.55 15.04 14.81 13.87 12.81 11.84

P (MPa)

ρ mix (103 mol m−3 )

X2 = 0.100 9.88 9.30 9.01 8.68 8.46 8.28 8.01 7.88 7.80 7.70 7.53 7.36 7.12 6.98 6.79 6.56 6.45

16.35 16.19 16.11 16.02 15.95 15.89 15.80 15.75 15.72 15.68 15.62 15.56 15.46 15.40 15.30 15.18 15.12

X2 = 0.084 9.92 9.33 8.75 8.16 7.72 7.45 7.37 7.28 7.23 7.13

17.51 17.41 17.31 17.17 16.90 15.95 15.76 14.34 12.70 10.36

Thus, both changes in density and dielectric constant of the solvents are favorable for increasing the KC as the pressure approaches PC or Pb , and KC increases significantly as pressure approaches the phase border.Fig. 3 illustrates that KC in CO2 –ethanol decreases with increasing concentration of ethanol at the higher pressures where the density of solvents is not sensitive to pressure. The main reason is probably

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Fig. 4. Densities of CO2 (1)–n-pentane (2) mixtures in different phase regions at 308.2 K.

Fig. 5. Densities of CO2 (1)–ethanol (2) mixtures in different phase regions at 308.2 K.

that since the dielectric constant of ethanol is much larger than CO2 , the dielectric constant of the solvent increases with increasing ethanol concentration. In CO2 –n-pentane mixtures, however, the KC increases with increasing concentration of n-pentane, which is difficult to explain. List of symbols A absorbance at the absorption maximum of EAA b optical path length of the UV cell (m) C0 total concentration of EAA in the mixture (mol m−3 ) Enol content of enol tautomer KC equilibrium constant of tautomerism reaction Pb bubble point pressure (MPa) PC critical point pressure (MPa) X2 mole fraction of component 2

H. Li et al. / Fluid Phase Equilibria 200 (2002) 111–119

εapp εmax ρ max

119

apparent molar absorption coefficient of EAA tautomer (l mol−1 cm−1 ) maximum molar absorption coefficient of enol tautomer (l mol−1 cm−1 ) density of mixture (103 mol m−3 )

Acknowledgements This work was supported by Ministry of Science and Technology of China (G2000048010) and National Natural Science Foundation of China (20133030). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

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