Solubility of Terephthalic Acid in Subcritical Water - Journal of

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Solubility of Terephthalic Acid in Subcritical Water Yoshihiro Takebayashi,* Kiwamu Sue, Satoshi Yoda, Yukiya Hakuta, and Takeshi Furuya Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan

ABSTRACT: The solubility of terephthalic acid (TPA) in subcritical water was measured as a function of the temperature from (349 to 547) K at a pressure of 10.0 MPa. A semibatch flow apparatus was developed for the solubility measurement. NMethylpyrrolidone was used as a good solvent for collection of TPA saturated in subcritical water without deposition upon cooling. The solubility of TPA in water showed an exponential increase with increasing temperature from 1.25·10−5 at 349 K to 2.99·10−2 at 547 K. The aqueous solubility of TPA was found to be lower than those of m- and o-phthalic acids by one and three orders of magnitude, respectively. The Gibbs free energy of solution (ΔGsol) as well as the enthalpic and entropic components (ΔHsol and −TΔSsol) were calculated from the temperature dependence of the solubility. The thermodynamic analysis showed that the ΔGsol gradually decreases with increasing temperature, as a result of the competition between a large increase in ΔHsol and a large decrease in −TΔSsol. The ΔGsol value decreased in the order of (p-phthalic acid) > (m-phthalic acid) > (o-phthalic acid) due to a decrease in −TΔSsol, whereas ΔHsol showed a small variation among the three isomers.

1. INTRODUCTION Terephthalic acid (TPA) is of great industrial importance as a raw material for various polyesters.1,2 TPA is commercially synthesized by a partial oxidation of p-xylene in acetic acid, but the product contains colored impurities such as p-carboxybenzaldehyde. The crude TPA is purified by a hydrogenation of the impurities into colorless compounds and by a subsequent recrystallization of TPA to a polymer-grade one. The purification is performed in subcritical water above 523 K, because water at the elevated temperature is a good solvent for TPA. Subcritical water has attracted increasing attention not only as the purification solvent but also as a reaction medium for the TPA synthesis. A novel TPA synthesis using sub- and supercritical water as an environmentally benign solvent in place of acetic acid has been developed in recent years.3,4 For better design and control of these processes, it is essential to understand the solubility of TPA in subcritical water as a function of the temperature. Solubility data of TPA in high-temperature water above 473 K are rarely available in literature, except in a scientific paper by Han et al.5 and in the two encyclopedias, the Kirk−Othmer and Ullmann's ones.1,2 Han et al. measured the solubility of TPA in subcritical water up to 483 K with a static analytical method. Their solubility data, however, disagree markedly with those in the encyclopedias. The encyclopedias afford the solubility data up to 523 K, but no experimental details are presented for the © 2012 American Chemical Society

data. Our aim in this study is to provide solubility data measured up to higher temperatures with a well-defined experimental apparatus and procedure. A semibatch flow method is applied here to the solubility measurement of TPA in subcritical water. In this method, water is passed through a heated extraction column containing an excess amount of solute to prepare saturated aqueous solution under subcritical conditions.6−9 The saturated solution is cooled and collected for the solubility measurement. Since the cooling causes a significant decrease in the solubility of the solute, a mixing with good solvent before the cooling is necessary to avoid a deposition of the solute. For TPA, however, the choice of the good solvent is quite limited, because TPA is insoluble in most ambient solvents, except for concentrated aqueous alkali solutions, pyridine, N,N-dimethylformamide, dimethyl sulfoxide, and N-methylpyrrolidone (NMP).1,2,10 NMP is used here as the collecting solvent, because it is noncorrosive and chemically stable under hydrothermal conditions. The effluent is analyzed by highperformance liquid chromatography (HPLC) to determine the solubility as well as to confirm the thermal stability of TPA in subcritical water. Received: February 29, 2012 Accepted: April 23, 2012 Published: April 30, 2012 1810

dx.doi.org/10.1021/je300263z | J. Chem. Eng. Data 2012, 57, 1810−1816

Journal of Chemical & Engineering Data

Article

Aqueous TPA solution exiting from the column was mixed with NMP in a union tee (Swagelok SS-200-3) before cooling to prevent a deposition of TPA due to the solubility decrease upon cooling. The mixing was performed by injecting the aqueous solution into the stream of NMP to avoid a contact of the aqueous solution with the tube wall. NMP was supplied by an HPLC pump (Shodex DS-4) at a constant flow rate and was heated in a preheating coil (1/16 in. o.d., 0.5 mm i.d., 5.0 m length) before the mixing. The mixture was cooled to room temperature in a cooling coil (1/16 in. o.d., 1.0 mm i.d., 0.5 m length) and was collected after depressurization by a backpressure regulator (JASCO 880-81). The pressures of water, NMP, and the mixture were monitored by strain gauges P1 and P2 (Kyowa Electronics Instruments, PG-500KU) and a pressure gauge P3 attached to the back-pressure regulator, respectively. The pressures and the temperatures were recorded by a data logger (Agilent 34970A). 2.3. Procedure. Sampling conditions (the flow rate of water vwater, the flow rate of NMP vNMP, and a sampling period tsample) optimized at each target temperature studied are summarized in Table 1. The flow rate of water was fixed at 0.10 cm3·min−1,

In this paper, we report details of the experimental apparatus and procedure developed for the solubility measurement of TPA in subcritical water. The solubility is measured over a wide range of temperature from (349 to 547) K at a constant pressure of 10.0 MPa. The solubility of TPA is compared with those in literature and with those of o- and m-phthalic acids. The temperature dependence of the solubility is systematically interpreted in terms of the thermodynamic properties of solution. The Gibbs free energy of solution and its enthalpic and entropic components are calculated as functions of temperature to elucidate how the temperature dependence of the solubility and its difference among the isomers are determined by the thermodynamic properties.

2. EXPERIMENTAL METHODS 2.1. Reagents. Water was purified to the specific resistance of 18 MΩ·cm by a Millipore Milli-Q system. Terephthalic acid (TPA; Tokyo Chemical Industry, 99 %), N-methylpyrrolidone (NMP; Wako, 99 %), phosphoric acid (Wako, 85 % aqueous solution), acetonitrile (Wako, 99.5 %), and benzoic acid (Wako, 99.5 %) were used as purchased. 2.2. Apparatus. A schematic diagram of the experimental apparatus is shown in Figure 1. Water was provided by a

Table 1. Sampling Conditions at Each Target Temperature Studied: vwater, Flow Rate of Water; vNMP, Flow Rate of NMP; tsample, Sampling Period

Figure 1. Schematic diagram of experimental apparatus: SV1 to SV3, stop valves; P1 to P3, pressure gauges; TP and TC, thermocouples.

target temperature

vwater

vNMP

tsample

K

cm3·min−1

cm3·min−1

min

348 373 398 423 448 473 498 523 548

0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10

0.40 0.40 0.90 0.90 0.90 0.90 0.90 0.90 1.40

30.0 30.0 20.0 20.0 10.0 5.0 5.0 5.0 2.5

while the flow rate of NMP was varied from (0.40 to 1.40) cm3·min−1 for the complete collection of TPA saturated in water at each temperature. After the extraction column loaded with TPA was installed in the oven, the syringe pump was started at the constant flow rate of vwater with a low-deadvolume stop valve SV2 (SSI 6020-14220) closed and stop values SV1 and SV3 open. The back-pressure regulator was set at 10.0 MPa. After the pressures P1 and P3 reached the target pressure, the HPLC pump was started at the flow rate of vNMP. When the pressure P2 reached 9.0 MPa, the valve SV2 was opened to mix the aqueous solution with NMP. After the pressures P1 to P3 were equilibrated, heating with the oven was started. It took ca. 40 min until the column temperature TC reached a plateau around the target temperature. After a 10 min equilibrium period, the effluent was collected in 25 cm3 volumetric flasks. The collection was performed three times at each target temperature for the given sampling period tsample. During the samplings, the fluctuation in the column temperature TC was within ± 0.1 K, and those in the pressures P1 to P3 were within ± 0.1 MPa. No pressure increase due to plugging was observed during the three samplings and the subsequent 60 min run. This indicates that the extracted TPA was completely collected in the effluent without deposition in the tubing. The amount of TPA in the effluent was determined by HPLC analysis after a dilution of the effluent with NMP to the volume

syringe pump (ISCO 100DX) at a constant flow rate and was heated in a SUS316 preheating coil (1/16 in. o.d., 0.5 mm i.d., 2.0 m length) before entering into an extraction column from the bottom. The preheating coil and the extraction column were heated in a gas chromatograph oven (Shimadzu GC-8A). The temperature of the preheated water was monitored by a 1/ 16-in. sheathed thermocouple TP (Chino) inserted into an union tee (Swagelok SS-200-3) located between the preheating coil and the extraction column to confirm that preheating to the target temperature was attained. The extraction column was composed of two pieces of SUS316 tube (1/2 in. o.d., 8.3 mm i.d., 75 mm length), a pair of column-end fittings (Swagelok SS-810-6-1ZV), and a reducing union tee (Swagelok SS-810-3-8-4). The extraction column had an internal volume of 10.6 cm3. The column was filled with 7.0 g of TPA mixed with 30 g of SUS304 stainless steel beads (1/8 in. diameter). The steel beads were used to facilitate a contact of flowing water with TPA without channeling. The temperature in the column was measured by a 1/8-in. sheathed thermocouple TC inserted into the center of the column via the reducing union tee. In both the column-end fittings, sintered stainless steel filters with a nominal pore size of 0.5 μm were installed to retain undissolved TPA in the column. 1811



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consistent with the experimental result by Dunn et al.12 They reported that TPA is thermally stable in subcritical water up to 573 K for 60 min. Mole fraction solubility x2 of TPA in subcritical water determined from the HPLC signal area is summarized in Table 2 as a function of the temperature T. In the following discussions, the temperature T denotes the column temperature measured by the thermocouple TC and averaged over the sampling period. At each temperature, three solubility values obtained from the first, the second, and the third fractions of the effluent are listed in the table, together with their average value and standard deviation. No significant change in the solubility was observed among the three fractions, showing that equilibrium was attained during the measurement. The solubility is plotted in Figure 3 against the temperature. Note that the vertical axis is on the logarithmic scale. The results of the double-column experiments are compared with those of the single-column ones. Negligible difference was found in the solubility-temperature profile between the singleand double-column experiments. This indicates that the single extraction column could provide a sufficient residence time of water and a sufficient amount of TPA for the saturation of water with TPA. The solubility x2 of TPA in subcritical water shows an exponential increase with increasing temperature T over the wide range from 1.25·10−5 at 349 K to 2.99·10−2 at 547 K. Such an exponential increase in the solubility with the temperature is commonly observed, though empirically, for aromatic solids in subcritical water.9,13−15 Here let us express the temperature dependence of ln x2 by the following function:

of 25 cm3. At target temperatures higher than 473 K, the solution was further diluted with NMP by a factor of 10, because the concentration of TPA was too high for the HPLC analysis. The amount of water in the effluent was calculated from the flow rate of water vwater = 0.10 cm3·min−1, the sampling period tsample = (2.5 to 30.0) min, and the density of pure water ρwater = 1.002 g·cm−3 at the pumping condition (298 K and 10.0 MPa).11 In addition to the single-column experiments described above, double-column experiments were carried out to confirm a saturation of water with TPA in the extraction column. In the double-column experiment, an additional extraction column was inserted between the union tee for the thermocouple TP and the main extraction column. The additional column was also filled with 7.0 g of TPA and 30 g of steel beads. The double-column experiments were performed at the target temperatures of (423, 473, and 523) K. The results were compared with those of the corresponding single-column experiments. 2.4. HPLC Analysis. The amount of TPA in the effluent was determined with an HPLC system (Agilent LC1100). The mobile phase was a mixture of 20 vol % acetonitrile and 80 vol % aqueous acidic solution (pH ∼ 2.0) containing 0.1 vol % phosphoric acid. The phosphoric acid was added to prevent a dissociation of the carboxyl groups of TPA (pKa1 = 3.54)2 into the ionic form (−COOH → −COO− + H+). The mobile phase was supplied at a constant flow rate of 1.0 cm3·min−1. Chromatographic separation was performed with a ZORBAX SB-C3 column (4.6 mm i.d., 15 cm length, 5 μm particle size) thermostatted at 313 K. The injection volume of the sample was 2.0 mm3. Detection was made by a diode-array detector (Agilent G1315A) at the wavelength of 250 nm with the slit width of 4 nm. A calibration curve was prepared in the concentration range of (0.01 to 1.0) g·dm−3 to confirm a linearity of the HPLC signal area to the TPA concentration.

ln x 2 = a + b·(T /K)

(1)

The coefficients a and b were determined by a least-squares fit to the experimental data. The values of a and b for the best fit are listed in Table 3, together with the root-mean-square deviation (rmsd) of the regression in ln x2 defined by

3. RESULTS AND DISCUSSION 3.1. Solubility Data. Before discussing the solubility of TPA in subcritical water, let us confirm that TPA is chemically stable under the high-temperature conditions without any hydrothermal decompositions, such as a decarboxylation into benzoic acid. The HPLC chromatogram of the effluent at 547 K, which is the highest temperature examined in this study, is shown in Figure 2, in comparison with that of benzoic acid. A large peak at 4.3 min is assigned to TPA, and a signal around 2.3 min is attributed to NMP. Benzoic acid (6.7 min) was not detected in the chromatogram of the effluent, indicating that no decarboxylation occurred at thermodynamic conditions. This is

rmsd =

1 N

x2)exp i

N

∑ [(ln x2)iexp − (ln x2)icalc ]2 (2)

i=1

x2)calc i

where (ln and (ln are the ith experimental and calculated data of ln x2, respectively, and N is the number of the data. The value of b = 0.03951 thus determined means that the solubility of TPA increases by an order of magnitude every temperature increase of 58 K. The corresponding regression line is shown in Figure 3. The regression line describes well the temperature dependence of the solubility experimentally obtained. The solubility data obtained here are compared with those in literature. As shown in Figure 3, our solubility data are in good agreement with those reported in the two encyclopedias, Kirk− Othmer and Ullmann,1,2 in the temperature range of (393 to 523) K. The agreement supports the validity of our measurement. Good agreement is found also at ambient temperature. An extrapolation of eq 1 to 298 K gives a solubility value of x2 = 1.7·10−6. This value is close to those in literature at that temperature (x2 = 1.8·10−6 2 and 2.1·10−6 1). In contrast, solubility data measured by Han et al.5 exhibit marked deviations from the above three sets of data. The reason for the discrepancy is unclear, but it should be specific to TPA, because solubility data for o- and m-phthalic acids measured by Han et al.5 agree well with the literature data in the encyclopedias, as discussed in the next section.

Figure 2. (a) HPLC chromatogram of the effluent at 547 K in comparison with (b) that of benzoic acid. 1812



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Table 2. Mole Fraction Solubility x2 of TPA in Water as a Function of the Temperature T at a Constant Pressure of 10.0 MPaa 103·x2 T/K

first

second

third

average

SD

extraction column

348.8 373.4 398.8 422.8 447.1 473.2 497.9 523.7 547.4 423.8 472.9 523.2

0.0125 0.0310 0.0904 0.240 0.609 1.77 4.69 12.6 29.9 0.252 1.72 12.0

0.0125 0.0307 0.0888 0.237 0.611 1.77 4.68 12.5 29.8 0.251 1.72 12.1

0.0124 0.0309 0.0899 0.237 0.609 1.78 4.72 12.5 30.1 0.252 1.72 12.0

0.0125 0.0309 0.0897 0.238 0.610 1.77 4.70 12.5 29.9 0.252 1.72 12.0

0.0001 0.0002 0.0008 0.002 0.001 0.01 0.02 0.1 0.1 0.001 0.01 0.1

single single single single single single single single single double double double

a At each temperature, three solubility values obtained from the first, the second, and the third fractions of the effluent are listed with their average value and standard deviation (SD). The results of the double-column experiments are compared with those of the single-column ones.

Figure 4. Comparison of the solubility x2 among o-, m-, and p-phthalic acids in water as a function of the temperature T: ◇, Han et al. (o-, m);5 +, Kirk−Othmer Encyclopedia of Chemical Technology (m-, p-);1 ×, Ullmann's Encyclopedia of Industrial Chemistry (o-, m-, p-);2 ○, this work (p-). The lines are least-squares fits to the data with eq 1.

Figure 3. Mole fraction solubility x2 of TPA in water as a function of the temperature T in comparison with literature data: ○, this work with single-column experiments; □, this work with double-column experiments; , least-squares fit to our experimental data with eq 1; +, Kirk−Othmer Encyclopedia of Chemical Technology;1 ×, Ullmann's Encyclopedia of Industrial Chemistry;2 ◆, Han et al.5

Table 3. Coefficients of Equation 1 Determined by LeastSquares Fit to the Solubility Data for o-, m-, and p-Phthalic Acids in Water and the Root-Mean-Square Deviation (rmsd) of the Regression in ln x2 solute

a

103·b

rmsd

solubility data

o-phthalic acid m-phthalic acid p-phthalic acid (TPA)

−20.44 ± 0.39

44.03 ± 1.10

0.16

literature2,5

−23.86 ± 0.25

42.41 ± 0.67

0.18

literature1,2,5

−25.07 ± 0.09

39.51 ± 0.19

0.04

this work

each other. In other words, the value of b differs slightly among the three isomers. The solubility of TPA is lower than those of m- and ophthalic acids by one and three orders of magnitude, respectively. The isomeric effect on the solubility can be explained by a difference in the dipole moment among the isomers. The dipole moment of TPA (ca. 0 D) is much smaller than those of m-phthalic acid (2.3 D) and o-phthalic acid (2.6 D),16 because of a cancellation of partial dipoles in a TPA molecule between the two carboxyl groups at the opposite directions. The nonpolar molecular structure of TPA leads to the low solubility in water compared with those of the o- and m-isomers. 3.3. Thermodynamic Analysis. To gain deeper insight into the aqueous solubility of TPA, the temperature dependence is interpreted in terms of the thermodynamic properties of solution. Figure 5 shows the van't Hoff plot of the solubility: that is, ln x2 is plotted against 1/T. In addition to our experimental data, the literature value at ambient conditions (ln x2 = −13.2 at 1/T = 3.35·10−3 K−1)2 is also plotted in the figure. The van't Hoff plot is not linear but has a positive curvature. The positive curvature indicates that the enthalpy of solution ΔHsol is an increasing function of the temperature, as discussed later. We thus regressed the temperature dependence of ln x2 to the modified Apelblat-type equation:17

3.2. Comparison with the Isomers. In Figure 4, the solubility of TPA (p-phthalic acid) in subcritical water is compared with those of o- and m-phthalic acids. Solubility data measured by Han et al.5 as well as the literature data in the two encyclopedias1,2 are plotted against the temperature. For o- and m-phthalic acids, these sets of data are consistent with each other, in contrast to the discrepancy for TPA shown in Figure 3. The solubilities of o- and m-phthalic acids increase exponentially with increasing temperature, in a similar way to that of TPA, and thus can be expressed by eq 1. The values of a and b for the best fit and the rmsd of the regression are summarized in Table 3. The corresponding regression lines are shown in Figure 4. The regression lines are almost parallel with 1813



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Figure 6. Thermodynamic properties of the solution for TPA in water as functions of the temperature T: ●, the Gibbs free energy of solution ΔGsol; ◇, the enthalpic component ΔHsol; □, the entropic component −TΔSsol; +, the entropy of solution ΔSsol. The lines are polynomial fits to the data.

Figure 5. van't Hoff plot of the solubility x2 of TPA in water. The line is a least-squares fit to the data with eq 3.

ln x 2 = A +

B + C ln(T /K) (T /K)

(3)

where the third term C ln T arises from an assumption that ΔHsol is an increasing function of the temperature with a constant isobaric heat capacity ΔCP,sol of solution. The coefficients A, B, and C for the best fit and the rmsd of the regression are listed in Table 4. The regression line shown in Figure 5 has a good correlation with the data. The temperature dependence of the mole fraction solubility x2 under isobaric conditions is related to the Gibbs free energy change ΔGsol upon the dissolution of TPA into water as

ΔGsol = −RT ln(γ2x 2)

the dissolution of TPA into water is endothermic. The ΔHsol shows a marked increase with increasing temperature from 26 kJ·mol−1 at 298 K to 93 kJ·mol−1 at 547 K, where the isobaric heat capacity of solution ΔCP,sol defined by ⎛ ∂ΔHsol ⎞ ⎟ = RC ΔCP ,sol = ⎜ ⎝ ∂T ⎠ P −1

is 271 ± 3 J·mol ·K . The large positive ΔHsol causes the significant increase in the solubility of TPA in subcritical water with increasing temperature. In contrast to the increase in ΔHsol, the entropic term −TΔSsol is a decreasing function of the temperature. The −TΔSsol value is positive at ambient temperature. In other words, the entropy change ΔSsol is negative upon the dissolution of TPA into ambient water. The negative ΔSsol is often observed for nonpolar solutes in ambient water and is possibly due to an iceberg-like structure formation of water molecules around TPA.18 At temperatures higher than 348 K, however, −TΔSsol turns out to be negative and shows a marked decrease down to −77 kJ·mol−1 with increasing temperature up to 547 K. The large decrease in −TΔSsol cancels the increase in ΔHsol, resulting in the gradual decrease in ΔGsol with increasing temperature. Finally, the thermodynamic properties of solution for TPA are compared with those for o- and m-phthalic acids. The thermodynamic properties for o- and m-phthalic acids were calculated according to eqs 3 to 7 from the literature data shown in Figure 4.1,2,5 The coefficients A, B, and C in eq 3 for the best fit are listed in Table 4. The ΔGsol values for the three isomers are plotted in Figure 7a as functions of the temperature, and the decompositions into ΔHsol and −TΔSsol are shown in Figure 7b. For all three isomers, the ΔGsol shows a gradual decrease with increasing temperature, as a result of the competition between a large increase in ΔHsol and a large decrease in −TΔSsol.

(4)

where R is the gas constant and γ2 is the activity coefficient of TPA in the aqueous solution. In the following analysis, the activity coefficient γ 2 is assumed to be unity as an approximation, because it is unknown how the γ2 value varies with the temperature and the concentration of TPA, although a deviation from the ideal solution should be noted especially at high temperatures due to the high solubility of TPA in water. The Gibbs free energy of solution ΔGsol consist of the two terms, the enthalpic component ΔHsol and the entropic one −TΔSsol. These terms are calculated using the Gibbs− Helmholtz equation as ⎛ ∂(ΔGsol /T ) ⎞ ΔHsol = −T 2·⎜ ⎟ = R(CT − B) ⎝ ⎠P ∂T

(5)

where eq 3 was used to evaluate the temperature derivative, and −T ΔSsol = ΔGsol − ΔHsol

(7)

−1

(6)

The thermodynamic properties of solution, ΔGsol, ΔHsol, −TΔSsol, and ΔSsol, for TPA in water are plotted in Figure 6 as functions of the temperature. The ΔGsol shows a gradual decrease with increasing temperature from 33 kJ·mol−1 at 298 K to 16 kJ·mol−1 at 547 K. The ΔGsol is decomposed into the enthalpic and entropic components. The enthalpic component ΔHsol has a positive value at every temperature studied, that is,

Table 4. Coefficients of Equation 3 Determined by Least-Squares Fit to the Solubility Data for o-, m-, and p-Phthalic Acids in Water and the Root-Mean-Square Deviation (rmsd) of the Regression in ln x2

a

solute

A

B

C

rmsd

solubility data


−250.41 ± 48.75 −214.61 ± 24.86 −220.93 ± 2.76

7902 ± 2507 6004 ± 1304 6597 ± 159

38.026 ± 7.093 32.166 ± 3.601 32.575 ± 0.393

0.15 0.19 0.02

literature2,5 literature1,2,5 this worka

Including literature data at 298 K.2 1814



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thermodynamic properties of solution were calculated from the temperature dependence of the solubility. The enthalpy of solution ΔHsol was positive and showed a monotonic increase with increasing temperature, whereas the entropic component −TΔSsol was a decreasing function of the temperature. The competition between ΔHsol and −TΔSsol resulted in a gradual decrease in the Gibbs free energy of solution ΔGsol for all three isomers. Among the three isomers, the value of ΔGsol decreased in the order of (p-phthalic acid) > (m-phthalic acid) > (ophthalic acid). The difference is mainly attributed to −TΔSsol, because the enthalpic component ΔHsol was weakly affected by the isomeric difference.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ Figure 7. (a) Comparison of the Gibbs free energy of solution ΔGsol among o-, m-, and p-phthalic acids in water as a function of the temperature T, and (b) those of the enthalpic component ΔHsol and the entropic one −TΔSsol: △, o-phthalic acid; □, m-phthalic acid; ○, p-phthalic acid (TPA). The lines are polynomial fits to the data.

The ΔGsol value, however, differs much among the isomers. The ΔGsol value decreased in the order of (p-phthalic acid) > (m-phthalic acid) > (o-phthalic acid). The decrease in ΔGsol is attributed to −TΔSsol rather than to ΔHsol. The ΔHsol value is weakly variant among the isomers, leading to the similar temperature dependence of the solubility shown in Figure 4. The isobaric heat capacity of solution ΔCP,sol summarized in Table 5 is also negligibly affected by the isomeric difference. In Table 5. Isobaric Heat Capacity ΔCP,sol of Solution for o-, m-, and p-Phthalic Acids in Water Calculated Using Equation 7 and the Value of C Listed in Table 4 ΔCP,sol solute

J·mol−1·K−1


316 ± 59 267 ± 30 271 ± 3

REFERENCES

(1) Bemis, A. G.; Dindorf, J. A.; Horwood, B.; Samans, C. Phthalic Acids and Other Benzenepolycarboxylic Acids. In Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; Mark, H. F., Othmer, D. F., Overberger, C. G., Seaborg, G. T., Eds.; John Wiley & Sons: New York, 1982; Vol. 17, pp 732−777. (2) Sheehan, R. J., Terephthalic Acid, Dimethyl Terephthalate, and Isophthalic Acid. In Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; Elvers, B., Hawkins, S., Russey, W., Eds.; Wiley-VCH: Weinheim, 1995; Vol. A26, pp 193−204. (3) Dunn, J. B.; Savage, P. E. Terephthalic Acid Synthesis in HighTemperature Liquid Water. Ind. Eng. Chem. Res. 2002, 41, 4460−4465. (4) Dunn, J. B.; Savage, P. E. Economic and Environmental Assessment of High-Temperature Water as a Medium for Terephthalic Acid Synthesis. Green Chem. 2003, 5, 649−655. (5) Han, N.; Zhu, L.; Wang, L.; Fu, R. Aqueous Solubility of mPhthalic Acid, o-Phthalic Acid and p-Phthalic Acid from 298 to 483 K. Sep. Purif. Technol. 1999, 16, 175−180. (6) Miller, D. J.; Hawthorne, S. B. Method for Determining the Solubilities of Hydrophobic Organics in Subcritical Water. Anal. Chem. 1998, 70, 1618−1621. (7) Srinivas, K.; King, J. W.; Howard, L. R.; Monrad, J. K. Solubility of Gallic Acid, Catechin, and Protocatechuic Acid in Subcritical Water from (298.75 to 415.85) K. J. Chem. Eng. Data 2010, 55, 3101−3108. (8) Zhang, D. F.; Montanes, F.; Srinivas, K.; Fornari, T.; Ibanez, E.; King, J. W. Measurement and Correlation of the Solubility of Carbohydrates in Subcritical Water. Ind. Eng. Chem. Res. 2010, 49, 6691−6698. (9) Karasek, P.; Planeta, J.; Roth, M. Solubility of Solid Polycyclic Aromatic Hydrocarbons in Pressurized Hot Water: Correlation with Pure Component Properties. Ind. Eng. Chem. Res. 2006, 45, 4454− 4460. (10) Wang, Q.; Xu, H.; Li, X. Solubilities of 4-Carboxybenzaldehyde and 1,4-Benzenedicarboxylic Acid in N-Methyl-2-Pyrrolidone in the Temperature Range from (343.2 to 468.2) K. J. Chem. Eng. Data 2005, 50, 243−245. (11) Thermophysical Properties of Fluid Systems; NIST Chemistry WebBook. http://webbook.nist.gov./chemistry/fluid/ (accessed April 28, 2012). (12) Dunn, J. B.; Burns, M. L.; Hunter, S. E.; Savage, P. E. Hydrothermal Stability of Aromatic Carboxylic Acids. J. Supercrit. Fluids 2003, 27, 263−274. (13) Carr, A. G.; Mammucari, R.; Foster, N. R. A Review of Subcritical Water as a Solvent and Its Utilisation for the Processing of Hydrophobic Organic Compounds. Chem. Eng. J. 2011, 172, 1−17. (14) Karasek, P.; Planeta, J.; Roth, M. Aqueous Solubility Data for Pressurized Hot Water Extraction for Solid Heterocyclic Analogs of

contrast, −TΔSsol shows a marked decrease in the order of (pphthalic acid) > (m-phthalic acid) > (o-phthalic acid). For oand m-phthalic acids, −TΔSsol is negative at 298 K, whereas it is positive for TPA, probably reflecting the difference in the polarity among the solute molecules. The difference in −TΔSsol is the thermodynamic origin of the low solubility of TPA compared with those of o- and m-phthalic acids.

4. CONCLUSIONS The solubility of TPA in subcritical water was measured by a semibatch flow method using NMP as a collecting solvent. The mole fraction solubility increased exponentially with increasing temperature from 1.25·10−5 at 349 K to 2.99·10−2 at 547 K under the isobaric condition of 10.0 MPa. The aqueous solubility of TPA was lower than those of m- and o-phthalic acids by one and three orders of magnitude, respectively. The 1815



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Anthracene, Phenanthrene and Fluorene. J. Chromatogr., A 2007, 1140, 195−204. (15) Karasek, P.; Planeta, J.; Roth, M. Solubilities of Oxygenated Aromatic Solids in Pressurized Hot Water. J. Chem. Eng. Data 2009, 54, 1457−1461. (16) Yaws, C. L.; Narasimhan, P. K. Dipole moment - Organic Compounds. In Thermophysical Properties of Chemicals and Hydrocarbons; Yaws, C. L., Ed.; William Andrew: Norwich, 2008; pp 672− 682. (17) Apelblat, A.; Manzurola, E. Solubilities of L-Aspartic, DLAspartic, DL-Glutamic, p-Hydroxybenzoic, o-Anistic, p-Anisic, and Itaconic Acids in Water from T = 278 K to T = 345 K. J. Chem. Thermodyn. 1997, 29, 1527−1533. (18) Franks, F.; Reid, D. S. Thermodynamic Properties. In Water, A Comprehensive Treatise; Franks, F., Ed.; Plenum: New York, 1973; Vol. 2, pp 323−380.

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