Article pubs.acs.org/IECR
Solubilities of p‑Toluenesulfonic Acid Monohydrate and Sodium p‑Toluenesulfonate in Aqueous Sulfuric Acid Solutions and Its Application for Preparing Sodium p‑Toluenesulfonate Wen Zhao, Wei Zou, Tong Liu, Feng-Bao Zhang, Guo-Liang Zhang, and Qing Xia* School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, PR China S Supporting Information *
ABSTRACT: The solubilities of p-toluenesulfonic acid monohydrate (PTSA·H2O) and sodium p-toluenesulfonate (NaPTS) in aqueous sulfuric acid solutions were investigated at a temperature range from 278.45 to 342.95 K. The experimental results showed that the solubilities of PTSA·H2O and NaPTS in aqueous sulfuric acid solutions increased with temperature from 278.45 to 342.95 K and decreased with sulfuric acid concentrations from 0 to 0.7. The experimental data were correlated with the modified Apelblat equation. The calculated results showed good agreement with the experimental data. A new strategy which sodium sulfate was used as a starting material in the preparation of NaPTS was profiled according to the solubility difference between PTSA·H2O and NaPTS. In the continuous preparation of NaPTS, the favorable mole ratio (PTSA·H2O to sodium sulfate) is chosen as 4.0, and the optimum operation conditions of the recycle process were determined, which improved the present process of 4-methylphenol production.
1. INTRODUCTION PTSA·H2O (CAS No. 6192-52-5) is an intermediate in producing 4-methylphenol which is widely used in the synthesis such as dyes, medicines, pesticides, spices, cosmetics, polymer material, and other fields.1,2 4-Methylphenol is in short supply for a long time all over the world, especially in the developing countries. Currently, the sulfonation-alkali fusion process is adopted for the traditional industrial production of 4methylphenol.3,4 The process is shown in Figure 1, and it includes four steps: (1) sulfonating toluene with concentrated sulfuric acid to produce p-toluenesulfonic acid (PTSA) which precipitates as PTSA·H2O; (2) neutralizing PTSA·H2O with sodium sulfite to obtain NaPTS (CAS No. 657-84-1) and byproduct sulfur dioxide; (3) alkali fusion of NaPTS with sodium hydrate at temperatures of 330−360 °C to get 4methylphenol sodium salt;5 (4) acidifying 4-methylphenol sodium with sulfur dioxide to produce 4-methylphenol and byproduct sodium sulfite. In the neutralization and acidification steps, byproducts sulfur dioxide and sodium sulfite can be recycled as raw material in acidification and neutralization steps, respectively. The purity of 4-methylphenol produced in the traditional sulfonation-alkali fusion process is only 0.92−0.95 mass fraction, which limits the application of 4-methylphenol. The impurities are mainly produced in sulfonation and neutralization steps. The sulfonation reaction-crystallization method has replaced the traditional sulfonation method to increase the purity of PTSA·H2O from 0.90 to 0.98 mass fraction.6,7 In the neutralization step, the purity of raw material sodium sulfite which is recovered from organic wastewater of the acidification step is only about 0.92 mass fraction.8 Using this sodium sulfite will bring new impurities to subsequent steps and increase the difficulty of 4-methylphenol purification. Li et al.9,10 have proposed a new strategy to produce sodium 4-nitrotoluene-2sulfonate (NTSNa) by neutralizing 4-nitrotoluene-2-sulfonic © 2013 American Chemical Society
acid (NTS) with sodium sulfate. Because the solubility of NTSNa in a certain concentration of aqueous sulfuric acid solution is much lower than that of NTS, the NTSNa will precipitate out and the aqueous sulfuric acid solutions will be obtained after the reaction. Both NTSNa and NaPTS are obtained through sulfonation and neutralization reaction, so we want to use the idea of Li to improve the present sulfonationalkali fusion process. The feasibility of the new strategy depends on the solubility difference of PTSA·H2O and NaPTS in aqueous sulfuric acid solutions. Only solubility data of PTSA· H2O in aqueous sulfuric acid solutions at T = 298.15 K were reported by Kozlov et al.,11 and solubility data of NaPTS in water were reported by Renich and Taft12 and Shcherbakova et al.13 The information about solubility is not enough. Therefore, in this work, the solubilities of PTSA·H2O and NaPTS in aqueous sulfuric acid solutions have been measured by a dynamic method over the temperature range from 278.45 to 342.95 K and the mass fractions of aqueous sulfuric acid solutions range from 0 to 0.8. The modified Apelblat equation14,15 was used to correlate the experimental data. According to the determined data, a new strategy for preparing NaPTS is brought up and discussed.
2. EXPERIMENTAL SECTION 2.1. Materials Preparation. The information of the materials used in the experiment is listed in Table 1. Prior to the measurement, PTSA·H2O was dried in a vacuum oven to constant weight at 333.15 K (PTSA·H2O will melt when the temperature is higher than 373.15 K) and NaPTS was dried in Received: Revised: Accepted: Published: 18466
September 29, 2013 November 19, 2013 November 26, 2013 November 26, 2013 dx.doi.org/10.1021/ie403228w | Ind. Eng. Chem. Res. 2013, 52, 18466−18471
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Figure 1. The sulfonation and alkali fusion process in the production of 4-methylphenol.
flask equipped with a mechanical stirrer, thermometer, dropping funnel, and Dean−Stark trap. The solution was heated to 60 °C with stirring while the certain amount of saturated solution of sodium sulfate (the quantity is calculated on the basis of the mole ratio of PTSA·H2O and sodium sulfate) was added dropwise over a period of 20 min. The reaction mixture was heated under agitation up to 90−95 °C and refluxed until the calculated amount of water (controlling the concentration of sulfuric acid was 0.2 solute-free mass fraction) was collected in a Dean−Stark trap. After the completion of reaction, the solution was cooled to room temperature, and the NaPTS was separated by filtration. The cake and the filtrate were weighed to determine the yield of NaPTS. Evaporation-Crystallization. The above filtrate was put into the four-neck round-bottom flask and heated. By controlling the amount of water evaporated, the concentration of sulfuric acid in filtrate is concentrated from 0.20 to 0.70 (solute-free mass fraction). After the solution was cooled to room temperature, the remaining unreacted PTSA·H2O was crystallized out and filtrated. Continuous operation was achieved by the reuse of the cake (almost PTSA·H2O) in neutralization reaction and filtrate (mainly sulfuric acid and water) in sulfonation reaction.
Table 1. Provenance and Mass Fraction Purity of the Materials Used in This Study materials
mass fraction purity
PTSA·H2O
0.995
NaPTS
0.990
sodium sulfate
>0.990
sodium carbonate
0.998
concentrated sulfuric acid deionized water
0.965
sources Meryer Chemical Co., Ltd., Shanghai, China Heowns Biochemical Technology Co., Ltd., Tianjin, China Guangfu Chemical Reagents Co., Tianjin, China Meryer Chemical Co., Ltd., Shanghai, China Tianjin Kewei Reagent Co., Tianjin, China Nankai Chemical Reagents Co., Tianjin, China
an oven to constant weight at 473.15 K. Differential scanning calorimetry (Netzsch, DSC 204 F1, Germany) was used to confirm that solid PTSA·H2O did not lose the water of hydration. In the thermogram of DSC, there was no peak before the melting point. Therefore, PTSA·H2O as a monohydrate was taken as the solute during the determination. The exact mass fraction of concentrated sulfuric acid was calibrated with anhydrous sodium carbonate and methyl red indicator before it was used for preparing a certain concentration aqueous sulfuric acid solution. 2.2. Apparatus and Procedure of Solubility Measurement. The solubility was determined using the dynamic method. The laser monitoring system was used to observe the dissolving processes and determine the solid−liquid equilibrium temperature of a certain composition mixture. The apparatus and procedure of the solubility experiments was already described in detail elsewhere.16,17 Briefly, the solute and solvents were weighed by an analytical balance (Gibertini, Crystal 200, Italy, with an accuracy of 0.0001 g) and placed into a jacketed glass vessel. The mixtures were stirred continuously with a magnetic stir bar and heated slowly with a rate less than 0.2 K·h−1. A refrigerated/heating circulator (Julabo FP45-HE, Germany, temperature stability ±0.01 K) was used to control the temperature of the solution, and the temperature was measured by a platinum resistance thermometer (PT-100, calibrated with an accuracy of 0.01 K). The temperature at which the solid disappeared and the intensity of the laser beam reached a maximum was taken as the solid−liquid equilibrium temperature. The standard uncertainties of the measured temperature of PTSA·H2O and NaPTS were ±0.2 and ±0.3 K, respectively, which was calculated from the repeated experimental measurements. 2.3. Preparation of NaPTS from PTSA·H2O with Sodium Sulfate. Neutralization. A saturated solution of PTSA·H2O was put into a 500 mL four-neck round-bottom
3. RESULTS AND DISCUSSION 3.1. Solubility Data and Correlation. The experimentally determined solubility data of PTSA·H2O and NaPTS in aqueous sulfuric acid solutions are presented in Tables S1 and S2 (Supporting Information), where Texp is the measured absolute temperature, m1 and m2 are molality solubilities of PTSA·H2O and NaPTS in aqueous sulfuric acid solutions, respectively, and w03 is the solute-free mass fraction of sulfuric acid in the solvent mixtures. Solid−liquid equilibrium of aqueous multicomponent electrolyte systems is characterized by the common ion effect and the electrolyte−electrolyte interactions. In aqueous sulfuric acid solutions, ionic reaction among various species becomes complicated because sulfuric acid has the first and second thermodynamic dissociation. The application of electrolyte thermodynamic models such as the electrolyte-NRTL model will lead to unsatisfactory results.18 At even higher sulfuric acid concentrations, these models are no longer valid and even “salting in” may occur.19,20 Since the modified Apelblat equation21,22 has been used in many previously published studies of solid−liquid equilibrium data in sulfuric acid system as an empirical equation,14,15 the experimental data in this work was correlated by the modified Apelblat equation 18467
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Table 2. Parameters of the Modified Apelblat Model and σ for PTSA·H2O and NaPTS in Binary Sulfuric Acid (w03) + Water (1 − w03) Solvent Mixtures (Defined by eqs 1 and 2) solute PTSA·H2O
NaPTS
ln m = A +
A
B
C
σ/K
−117.57 −157.65 164.75 −186.42 213.27 −551.26 −871.81 −8.7403 −94.855 143.39 −1216.8 −296.15 −127.12
4685.5 6466.8 −9534.4 1377.3 −15803 19318 33490 −432.60 3230.3 −8282.4 49527 9195.5 1686.8
18.395 24.360 −23.048 31.271 −28.887 84.917 133.56 2.0108 14.889 −20.229 183.84 45.878 20.871
0.53 0.43 1.46 1.57 1.22 1.61 0.23 0.21 0.87 1.07 1.35 0.43 0.26
solvent w03 w03 w03 w03 w03 w03 w03 w03 w03 w03 w03 w03 w03
= = = = = = = = = = = = =
0.00 0.20 0.40 0.60 0.70 0.80 0.00 (281.55 K < T ≤ 294.1 K) 0.00 (294.1 K < T < 339.65 K) 0.20 0.40 0.60 0.70 0.80
B + C ln T T
(1)
where m is the molality solubility of the solute, T is the calculated absolute temperature, and A, B, and C are constants. The root-mean-square deviation σ between Texp and T is taken as the object function which is defined as N
σ = [∑ (T exp − T )2 /(N − 1)]0.5 i=1
(2)
where N is the number of experimental data points. A nonlinear minimization function in Matlab is used for minimizing σ and estimating parameters A, B, and C. The values of the model parameters and σ for determined systems are presented in Table 2. The root-mean-square deviations range from 0.21 to 1.61 K. Figures 2 and 3 show the curves of m1 and m2 versus temperature at w03 ≤ 0.40 and 0.60 ≤ w03 ≤ 0.80, respectively. It
Figure 3. Molality solubility m1 (m2) of PTSA·H2O (NaPTS) in binary sulfuric acid + water solvent mixtures (0.60 ≤ w03 ≤ 0.80). Mass fractions of sulfuric acid on the solute-free basis are as follows: (★, ☆) w03 = 0.60; (◆, ◇) w03 = 0.70; (▼, ▽) w03 = 0.80; solid symbols (★, ◆, ▼) express molality solubilities of PTSA·H2O; hollow symbols (☆, ◇, ▽) express molality solubilities of NaPTS; () the calculated results of the modified Apelblat equation.
can be observed that the calculated solubility curves show good agreement with the experimental data. The solubility data of NaPTS in pure water presented here behave differently, having two distinct regions, below and above T = 294 K. In order to confirm its validity, the heating rate was set as 0.02 K·h−1 in the reproducibility experiments and the same results were obtained. The solubility curve is described by two equations, and the parameters of the equations are shown in Table 2. 3.2. Comparison with Literature Data. The solubility of PTSA·H2O in different concentration aqueous sulfuric acid solutions at T = 298.15 K is calculated by eq 1 and compared with the results of Kozlov et al.11 in Figure 4. It can be seen that the present results show good agreement with Kozlov’s results in w30 ≤ 0.40 aqueous sulfuric acid solutions but are considerably higher than Kozlov’s results in low concentration aqueous sulfuric acid solutions. Also, the experimental solubility data of NaPTS in pure water are compared with literature data in Figure 5. It can be observed that the present data is higher than the data reported
Figure 2. Molality solubility m1 (m2) of PTSA·H2O (NaPTS) in binary sulfuric acid + water solvent mixtures (0.0 ≤ w03 ≤ 0.40). Mass fractions of sulfuric acid on the solute-free basis are as follows: (■, □) w03 = 0.00; (●, ○) w03 = 0.20; (▲, △) w03 = 0.40; solid symbols (■, ●, ▲) express molality solubilities of PTSA·H2O; hollow symbols (□, ○, △) express molality solubilities of NaPTS; () the calculated results of the modified Apelblat equation. 18468
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Shcherbakova et al.13 is 67 g·L−1, which is shown as a square in Figure 5. The value does not agree with the present result. However, if Shcherbakova’s data is multiplied by 10, the result would coincide closely with the present work. 3.3. Discussion of Solubility Data. It can be seen from Figures 2 and 3 that the solubilities of PTSA·H2O and NaPTS both increase with temperature and decrease first, increase afterward with w03. The highest and the lowest solubility are obtained in pure water and w03 = 0.70 sulfuric acid solution for both systems within the studied solvent compositions. Therefore, adjusting the concentration of aqueous sulfuric acid solutions to w03 = 0.70 is a benefit for the precipitation of PTSA·H2O and NaPTS. The solubility difference of PTSA·H2O and NaPTS in the same concentration of aqueous sulfuric acid solutions is also shown in Figures 2 and 3. The different scales of the vertical axis are used in Figures 2 and 3. It is observed that the solubility difference of PTSA·H2O and NaPTS decreases with w03. An obvious solubility difference exists in 0 ≤ w03 ≤ 0.30. Especially in 0 ≤ w03 ≤ 0.20, the solubility of PTSA·H2O is significantly higher than that of NaPTS. However, the solubility difference is small and irregular in 0.60 ≤ w03 ≤ 0.80, which is shown in Figure 3. This irregularity can be attributed to the complicated electrolyte−electrolyte interactions in high concentration sulfuric acid solution. 3.4. Application of the Solubility Difference in the Preparation of NaPTS. 3.4.1. New Strategy for Preparation of NaPTS. The above determined solubility data are used to design a new strategy which is shown in Figure 6. Sodium sulfite is replaced by sodium sulfate, and the neutralization reaction is carried out by adding sodium sulfate to the aqueous solutions of PTSA·H2O. PTSA is converted to NaPTS, and aqueous sulfuric acid solution is obtained at the same time. The amount of water in the neutralization reaction is controlled to ensure forming 0.20 solute-free mass fraction aqueous sulfuric acid solutions. Then, the solution is cooled to room temperature and NaPTS will precipitate out. After filtration, the filter cake NaPTS is sent to the alkali fusion step, and the filtrate which contains the remaining PTSA, byproduct sulfuric acid, and water is concentrated in an evaporation-crystallization step to recover PTSA·H2O. In the evaporation-crystallization step, by controlling the amount of water evaporated, the filtrate is concentrated from w03 = 0.20 to w03 = 0.70. Because PTSA· H2O has the lowest solubility in 0.70 solute-free mass fraction aqueous sulfuric acid solution, almost all of the remaining PTSA·H2O can be separated by filtration. The separated PTSA·
Figure 4. Molality solubility m1 of PTSA·H2O at a temperature of 298.15 K in binary sulfuric acid (w03) + water (1 − w03) solvent mixtures: (●) the data calculated from eq 1 in this work; (△) Kozlov et al.11
Figure 5. Molality solubility m2 of NaPTS in pure water: (●) this work; (△) Renich and Taft12; (□) Shcherbakova et al.13
by Renich and Taft12 when the temperature is higher than 300 K. The solubility curve in the present work has an obvious turning point which also occurred for the data of Renich and Taft. The solubility of NaPTS measured at T = 293.15 K by
Figure 6. The flowsheet of the new strategy. 18469
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H2O will be recycled to the neutralization step, and the filtrate of w03 = 0.70 aqueous sulfuric acid solutions can be reused in the sulfonation reaction, as shown in Figure 6. 3.4.2. Determination of Mole Ratio of PTSA·H2O to Sodium Sulfate. In order to determine the suitable mole ratio of PTSA·H2O to sodium sulfate which is used in the neutralization reaction, the experiment described in the Neutralization subsection of section 2.3 is carried out. Because PTSA·H2O can be recovered through the evaporationcrystallization step in the new strategy, excessive PTSA·H2O is used to increase the conversion of sodium sulfate to NaPTS. Different mole ratios of PTSA·H2O to sodium sulfate versus the yield of NaPTS are given in Figure 7, which shows the yield of
fraction) as the starting material in the neutralization step. The impurities that were introduced by the starting material could be avoided, and the purity of the final product 4-methylphenol could be improved. This decreases the difficulty in purification of the final product. Byproduct 0.70 solute-free mass fraction aqueous sulfuric acid solution in the neutralization step would be recycled as a starting material in the sulfonation reaction. It can be calculated that the amount of sulfuric acid consumed in the sulfonation step would reduce by half. Byproduct sodium sulfite in the acidification step can be sold as an industrial product. The profits described above can offset the energy consumption in evaporation. Therefore, this new strategy has advantages of economy in materials and an environmentally friendly effect.
4. CONCLUSIONS Using a laser monitoring observation technique, the solubilities of PTSA·H2O and NaPTS in aqueous sulfuric acid solutions have been investigated in a temperature range from 278.45 to 342.95 K. Both of them increase with temperature. The lowest solubilities of PTSA·H2O and NaPTS are both exhibited at w03 = 0.70, and the highest solubilities are both observed in pure water. The solubility data have been described with the modified Apelblat equation. The results calculated by the model show good agreement with the experimental data. A new strategy, based on the solubility difference between PTSA·H2O and NaPTS in aqueous sulfuric acid solutions for the production of NaPTS, is carried out in laboratory scale, and the favorable mole ratio of the reactants should be 4.0 (PTSA· H2O to sodium sulfate). The amount of raw material and the yield of NaPTS tend to be stable after six-times-recycled experiments. The new strategy has potential in the industrial application of 4-methylphenol production, given that the laboratory scale has been shown to be feasible.
Figure 7. Plots of yield of NaPTS versus mole ratio of PTSA·H2O to sodium sulfate.
NaPTS by 0.967 if the mole ratio of PTSA·H2O to sodium sulfate is greater than or equal to 4.0. The optimum condition with a mole ratio of 4.0 is adopted in the latter process. 3.4.3. The Cycle Experiments and Brief Economic Analysis of the Strategy. Under optimum conditions, the experiments which are described in the Evaporation-Crystallization subsection of section 2.3 are carried out cyclically. Each time, PTSA·H2O recovered in the evaporation-crystallization step is recycled to the neutralization reactor as a part of the raw material and the amount of sodium sulfate used in the neutralization reaction is calculated to keep a mole ratio of 4.0. The number of experimental time, amount of raw materials, and products in the cycle experiments are shown in Table 3. It can be observed that the amount of material and the yield of product tend to be stable after six-times-recycled experiments. In this new strategy, sodium sulfate which is in high purity, cheap, and easily available replaces sodium sulfite (recovered from wastewater of acidification step, only about 0.92 mass
■
ASSOCIATED CONTENT
S Supporting Information *
Tables of the experimental solubilities of PTSA·H2O and NaPTS at temperature Texp and solute-free mass fraction of sulfuric acid in binary sulfuric acid + water solvent mixtures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86-22-27400292. Fax: +86-22-27408778. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
Table 3. The Cycle Experiments of the Neutralization Step in the New Strategy number of experiments
raw material: PTSA·H2O/g
raw material: sodium sulfate/g
product: NaPTS/g
recycling: PTSA·H2O/g
1 2 3 4 5 6
152.19 221.82 256.44 289.91 305.22 304.95
28.41 41.46 47.87 54.13 56.98 56.93
76.47 105.70 125.30 142.38 154.82 154.12
69.63 104.25 137.72 153.03 152.76 152.98
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itaconic acids in water from T=278 K to T=345 K. J. Chem. Thermodyn. 1997, 29, 1527−1533. (22) Apelblat, A.; Manzurola, E. Solubilities of o-acetylsalicylic, 4aminosalicylic, 3,5-dinitrosalicylic, and p-toluic acid, and magnesiumDL-aspartate in water from T= (278 to 348) K. J. Chem. Thermodyn. 1999, 31, 85−91.
ACKNOWLEDGMENTS The authors thank the Program of Introducing Talents of Discipline to Universities, China (No. B06006), for support.
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REFERENCES
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