Selective Solubilization of Propane-1,3-diol from Dilute Aqueous

May 14, 2012 - Solution Using Supercritical CO2 Microemulsion. Dan Zhou .... at each condition, and the data of cloud-point pressure at each experimen...
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Selective Solubilization of Propane-1,3-diol from Dilute Aqueous Solution Using Supercritical CO2 Microemulsion Dan Zhou, Wen Yu, and Jian-Zhong Yin* State Key Laboratory of Fine Chemicals, School of Chemical Machinery, Dalian University of Technology, Dalian 116024, P. R. China ABSTRACT: Phase behaviors of scCO2 microemulsions of the quaternary system of 2,6,8-trimethyl-4-nonyl ether (TMN-6) + CO2 + ethyl alcohol (EtOH) + H2O and the quinary system of TMN-6 + CO2 + EtOH + propane-1,3-diol (1,3-PDO) + H2O were investigated in a volume-variable highpressure view cell at temperatures from (302.85 to 317.25) K and at pressures of (6.6 to 12.4) MPa. In these systems, a commercially available nonionic surfactant poly(ethylene glycol) TMN-6 was used as the surfactant and ethanol as the cosurfactant. 1,3-PDO dilute aqueous solutions of 0.025, 0.050, and 0.075 mass fraction were used as model compounds to simulate the real fermentation broth. The results showed that a thermodynamically stable microemulsion can be formed under mild conditions by controlling the operating pressures and temperatures. 1,3-PDO was able to be extracted selectively from the dilute aqueous solution by using scCO2 microemulsion at appropriate temperatures and pressures. This work provides basic thermodynamic data for industrial design and a feasible basis for selective solubilizing 1,3-PDO from fermentation broth.

1. INTRODUCTION As an alternative to organic solvents, supercritical CO2 (scCO2) has drawn much attention due to its attractive properties such as abundance, low cost, nonflammability, nontoxicity, and in particular a mild critical pressure and temperature (TC = 304.25 K, PC = 7.38 MPa).1 However, CO2 is a poor solvent for polar molecules for its nonpolarity, low dielectric constant, and weak van der Waals forces, which have limited its applications in many processes such as separation, reaction, and materials preparation.2,3 scCO2 microemulsion, also called a water-inscCO2 (W/C) microemulsion, is a thermodynamically stable dispersion system that possesses encapsulated aqueous cores. The scCO2 microemulsion could overcome the limitations of the nonpolar scCO2 and thus provide a new approach for the extraction of polar compounds, such as biomacromolecules and polyols. Many studies on water-in-CO2 microemulsions have been carried out in the last few decades.4−6 Smith et al.7−9 found that the high molecular weight protein can be solubilized in supercritical propane with the surfactant sodium bis(2-ethyl1-hexyl) sulfosuccinate (AOT), and they also reported reverse micelles or microemulsions in other supercritical hydrocarbon solvents such as ethane, n-butane, and n-pentane. However, these hydrocarbon solvents are usually inflammable and toxic and have high critical pressures and temperatures. From an environmental perspective, the scCO2 microemulsion is a preferred choice. However, to apply the scCO2 microemulsion, it is a prerequisite to find a surfactant capable of forming thermodynamically stable microemulsions in scCO2. Smith et al.10 © 2012 American Chemical Society

reported that more than 130 kinds of commercial surfactants were not soluble in scCO2. Therefore, new surfactants have be designed and synthesized for improving their solubility in scCO2. Alternatively, the addition of appropriate cosolvents to improve the solubility of commercial surfactants in scCO2 may be another feasible way to apply the W/C microemulsion. Johnston et al.11 successfully employed an ammonium carboxylate perfluoropolyether [CF3O(CF2CFCF3)OCF2COONH4), PFPE] surfactant to form microemulsions in scCO2 and found that the resulting scCO2 microemulsion could solubilize bovine serum albumin (BSA). Following that work, the fluorinated analogues of aerosolOT have been extensively used in the field of microemulsion. Nevertheless, the fluorinate surfactants are high-cost and toxic, which limits its applications in industry. Beckman et al.12 successfully synthesize nonfluorous surfactants of poly(ether carbonate) copolymers in a wide range of compositions from 1 mole fraction polyether to 1 mole fraction polycarbonate. An outstanding advantage of this kind of copolymers is that it can form microemulsions in scCO2 even with a very small amount of addition (0.01 volume fraction polymer) at moderate pressures of (8 to 16) MPa. Eastoe et al.13 reported that the micellization of the hydrocarbon surfactants sodium bis(2,4,4-trimethyl-1pentyl) sulfosuccinate and sodium bis(3,5,5-trimethyl-1-hexyl) sulfosuccinate could be used to form stable microemulsion. Received: February 10, 2012 Accepted: May 8, 2012 Published: May 14, 2012 1787

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mainly consists of a high-pressure view cell (net volume is 29.23 mL) capable of operation at pressures up to 30 MPa and temperatures up to 373.15 K. The optical cell was heated by a water bath. In a typical experiment, the designed amounts of surfactant, water, ethanol, and/or 1,3-PDO were introduced to the cell, and the air in the view cell was replaced by CO2. Then, the view cell was sealed. The cell was placed on a magnetic stirrer and heated to the design temperature in the water bath. The temperature was measured using a thermocouple with the accuracy of ± 0.1 K. CO2 was added into the cell very slowly using a high-performance liquid chromatography (HPLC) pump, and the pressure of the system was measured using a pressure transducer (DG1300-BZ-A-2-40) with the accuracy of ± 0.01 MPa. During cloud-point measurement, the pump was stopped upon the formation of an optical transparent single phase solution. Subsequently, the pressure in the cell was gradually decreased by controlling the variable-volume piston (0 to 1.5 mL) until the cloud point was reached. The cloudpoint pressure P was recorded. The pressurization and depressurization process was performed at least three times at each condition, and the data of cloud-point pressure at each experimental condition were averaged with an expanded uncertainty within 6 % (coverage factor k = 2) and shown in Table 1.

Recently, they have made much progress on the study of W/C microemulsions.14−16 Nowadays, scCO2 microemulsions have been used in many fields, such as the extraction of metal ions and macromolecules,17,18 the production of nanoparticles,19−21 and chemical reactions.22,23 All os these works have shown great potentials of W/C microemulsion in applications for industry. As a raw material in chemical industry, propane-1,3-diol (1,3-PDO) plays an important role in the pharmaceutical, cosmetic, coating, and tobacco industries.24 Currently, 1,3-PDO is produced mainly by a biological fermentation process. However, the concentration of 1,3-PDO in aqueous solution produced by this process is only 0.05 to 0.10 mass fraction. Therefore, the separation and purification of 1,3-PDO from the dilute aqueous solution have presently become a challenging problem in industry. Many industrial methods are used to purify 1,3-PDO from its dilute aqueous solution. However, there are some disadvantages for the existing methods, such as high energy consumption for distillation process, and an easy plug for the macroporous resin filtration method. Furthermore, it is difficult to find a highly selective membrane for the separation of 1,3-PDO from excess water. With an attempt to develop an effective way to extracting selectively 1,3-PDO from its dilute aqueous solution, we investigated in this work the possibility of selective solubilization of 1,3-PDO in scCO2 microemulsion from its dilute aqueous solution. The focus of this work is put on the phase behavior of the W/C microemulsion system containing 1,3-PDO. The data obtained in this work are beneficial to large-scale industrial design.

3. RESULTS AND DISCUSSION 3.1. Effect of the Concentration of TMN-6. The critical micelle concentration (CMC) is defined as the concentration of surfactant to form micelle aggregate in scCO2. The parameter CMC is highly important in predicting the water-microemulsifying power of the surfactant in scCO2. It is therefore important to study the effect of the concentration of surfactant on the phase behavior of the system. Phase behaviors of the system of TMN-6 + CO2 + EtOH + H2O at various concentrations of TMN-6 were investigated, and the obtained P−T phase diagram for the system is shown in Figure 2. A lot of experiments have been done, and the previous results showed that 0.17 mole fraction ethanol and 0.665 mol·L−1 water (0.35 mL)25 were the suitable conditions to form stable microemulsions at moderate pressures. Thus, 0.35 mL of water was used in this work. In addition, different concentrations of TMN-6 ethanol solutions (5.539·10−3 mol·kg−1, 10.527·10−3 mol·kg−1, and 16.291·10−3 mol·kg−1) were prepared before the cloud-point measurement. As can be seen in Figure 2, as the temperature increases, the cloud-point pressure increases at each concentration of TMN-6 studied in this work. On the other hand, at a fixed temperature, the cloud-point pressure becomes the highest at the TMN-6 concentration of 5.539·10−3 mol·kg−1, while it is the lowest at the TMN-6 concentration of 10.527·10−3 mol·kg−1. The possible reason is that a too-low concentration of TMN-6 will result in the formation of only a few numbers of aggregates in ethanol solution, which in turn leads to a decrease in the solubilization capacity of the microemulsion, thus requiring a higher pressure to solubilize all of the water. On the other hand, when the concentration of TMN-6 increases from 10.527·10−3 mol·kg−1 to 16.291·10−3 mol·kg−1, the cloud-point pressure increases, too. This is due to the enhancement of the micelle− micelle interaction, and the weakening of the tail−solvent interaction at a high concentration of TMN-6 requires a corresponding higher pressure to form a stable microemulsion. Therefore, 10.527·10−3 mol·kg−1 is considered as the proper concentration of TMN-6 in ethanol solution for the formation

2. EXPERIMENTAL SECTION 2.1. Materials. Nonionic surfactant Tergitol poly(ethylene glycol) 2,6,8-trimethyl-4-nonyl ether (TMN-6) with a purity higher than 95 % was purchased from Sigma. CO2 (purity ≥ 99 %) was supplied by Guang Ming Gas Corporation of Dalian, China. Absolute ethyl alcohol (EtOH) (analytical reagent grade, purity ≥ 99.5 %) was supplied by Xin Xing Chemical Reagent Company of Shenyang, China. Propane-1,3-diol (1,3-PDO) (analytical reagent grade, purity ≥ 99 %) was supplied by Wei Xin Chemical Company of Dalian, China. In all of the preparations, distilled water was used. 2.2. Phase Equilibrium Measurement. The schematic diagram of a homemade volume-variable optical phase equilibrium apparatus is illustrated in Figure 1. The apparatus

Figure 1. Schematic diagram of phase behavior test apparatus: (1) CO2 cylinder; (2) filter; (3) cooling coil; (4) HPLC pump; (5) surge tank; (6) needle valve; (7) magnetic stirrer; (8) view cell; (9) thermocouple; (10) pressure sensor; (11) vent valve; (12) variablevolumn piston; (13) water bath. 1788

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Table 1. Cloud-Point Pressures of Various Multi-Component Systems concentration of TMN-6 ethanol solution/103 mol·kg−1

concentration of TMN-6 ethanol solution/103 mol·kg−1

concentration of TMN-6 ethanol solution/103 mol·kg−1

concentration of TMN-6 ethanol solution/103 mol·kg−1

concentration of TMN-6 ethanol solution/103 mol·kg−1

TMN-6 + CO2 + EtOH + H2O Quaternary System, 0.35 mL H2O 302.98 305.92 308.56 311.39 314.32 317.06 10.22 10.12 10.23 10.89 11.57 12.40 10.527 303.08 305.82 308.65 311.59 314.23 317.06 7.95 8.17 8.65 9.52 9.93 10.35 16.291 302.79 305.92 308.95 311.49 314.13 317.06 9.95 9.83 10.06 10.42 10.91 11.66 TMN-6 + CO2 + EtOH + 1,3-PDO Quaternary System, 0.35 mL 1,3-PDO 0 T/K 305.62 308.85 311.68 314.62 316.96 P/MPa 7.43 8.24 9.19 9.92 10.50 5.539 T/K 308.56 311.20 314.23 317.26 P/MPa 7.65 8.37 9.26 10.12 10.527 T/K 309.14 311.98 314.72 317.75 P/MPa 7.36 8.01 8.80 9.52 16.291 T/K 302.88 305.82 308.65 311.78 315.11 317.16 P/MPa 6.77 7.66 8.39 9.12 10.01 10.68 TMN-6 + CO2 + EtOH + 1,3-PDO + H2O Quinary System, 0.35 mL, 0.025 mass fraction, 1,3-PDO Aqueous Solution 5.539 T/K 302.49 305.43 308.07 311.00 313.93 316.96 P/MPa 9.62 9.55 9.93 10.32 10.81 11.60 10.527 T/K 303.37 306.21 309.14 311.98 314.52 317.75 P/MPa 6.61 7.30 7.81 8.44 9.06 9.68 16.291 T/K 302.79 305.92 308.95 311.49 314.13 317.06 P/MPa 10.20 10.12 10.08 10.24 10.67 11.12 TMN-6 + CO2 + EtOH + 1,3-PDO + H2O Quinary System, 0.35 mL, 0.050 mass fraction, 1,3-PDO Aqueous Solution 5.539 T/K 302.98 305.92 308.75 311.29 314.72 317.36 P/MPa 9.03 8.74 9.15 9.89 10.43 11.06 10.527 T/K 303.18 305.72 308.75 311.59 314.52 317.26 P/MPa 8.70 8.88 9.08 9.36 9.85 10.32 16.291 T/K 302.88 305.92 308.75 311.29 314.62 317.26 P/MPa 8.53 8.78 9.22 9.69 10.69 11.22 TMN-6 + CO2 + EtOH + 1,3-PDO + H2O Quinary System, 0.35 mL, 0.075 mass fraction, 1,3-PDO Aqueous Solution 5.539 T/K 302.49 305.72 308.46 311.39 314.32 317.26 P/MPa 8.39 8.65 9.12 9.63 10.42 11.15 10.527 T/K 302.88 305.92 308.85 311.49 314.03 317.36 P/MPa 7.84 8.18 8.74 9.21 9.68 10.57 16.291 T/K 302.98 305.72 308.85 311.29 314.23 317.06 P/MPa 7.92 8.30 8.68 9.19 9.86 10.5 5.539

T/K P/MPa T/K P/MPa T/K P/MPa

TMN-6 + CO2 + EtOH + 1,3-PDO and the quinary system TMN-6 + CO2 + EtOH + 1,3-PDO + H2O were investigated to show the possibility of selective solubilization of 1,3-PDO in scCO2 microemulsion and to find the proper operating conditions for selective solubilization such as the phase composition, temperature, and cloud-point pressure. 3.2.1. Phase Behavior of the TMN-6 + CO2 + EtOH + 1,3PDO System. Cloud-point pressures for the quaternary system of TMN-6 + CO2 + EtOH + 1,3-PDO were measured at temperatures from (302.85 to 317.25) K when the volume of pure 1,3-PDO added into the system was 0.35 mL. The P−T phase diagram is shown in Figure 3. A clear single-phase microemulsion was observed above the solid lines for each TMN-6 concentration. As can be seen in Figure 3, the cloudpoint pressure increased with increasing temperature at constant concentrations of TMN-6. This is due to the decrease of scCO2 density with increasing the temperature. In addition, as observed in this figure, 1,3-PDO can be dissolved in the system of CO2 + EtOH even without TMN-6, but at a high cloud-point pressure. When 5.539·10 −3 mol·kg −1 and 10.527·10−3 mol·kg−1 TMN-6 ethanol solution were added, the cloud-point pressures became lower than that of without TMN-6, which implies that 1,3-PDO is dissolved more facilely

Figure 2. P−T phase diagram for the system TMN-6 + CO2 + EtOH + H2O at different concentrations of TMN-6: ●, 16.291·10−3 mol·kg−1; ■, 10.527·10−3 mol·kg−1; ▲, 5.539·10−3 mol·kg−1.

of a stable microemulsion for the system of TMN-6 + CO2 + EtOH + H2O. 3.2. Phase Behavior of TMN-6 + CO2 + EtOH + 1,3-PDO and TMN-6 + CO2 + EtOH + 1,3-PDO + H2O Systems. The formation of microemulsion, namely, the existence of a core of bulk water in scCO2, can be used to solubilize polar substance. Since both water and 1,3-PDO are soluble in scCO2 microemulsion, the phase behaviors of the quaternary system of 1789

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1,3-PDO can be dissolved in scCO2 microemulsion with TMN-6 as the surfactant and ethanol as the cosurfactant. To study whether the TMN-6 + CO2 + EtOH system can extract selectively 1,3-PDO from dilute aqueous solution, the phase behavior of the quinary system TMN-6 + CO2 + EtOH + 1,3-PDO + H2O was investigated at temperatures ranging from (302.85 to 317.25) K when the volume of water and 1,3-PDO aqueous solution were both 0.35 mL. The P−T phase diagrams for the quinary system TMN-6 + CO2 + EtOH + 1,3-PDO + H2O at different TMN-6 concentrations and 1,3-PDO concentrations are shown in Figures 4 to 6. As illustrated in Figure 4b,c, Figure 5a,c, and Figure 6a,c, respectively, a transparent phase solution was observed above the solid line for each concentration of TMN-6 ethanol solution. The cloud-point pressures of the quinary system are lower than that of the quaternary system of TMN-6 + CO2 + EtOH + H2O at the same temperatures, which implies that the system of TMN-6 + CO2 + EtOH + H2O can selectively dissolve 1,3-PDO rather than water at appropriate operating pressures. However, a crossover temperature (TCO) appeared at 309.35 K when the concentration of TMN-6 was 16.291·10−3 mol·kg−1 (Figure 4a) and appeared at 310.85 K when the concentration of TMN-6 was 10.527·10−3 mol·kg−1 (Figure 5b), respectively. As we all know, the temperature affects the stability of the microemulsion in two opposite ways. The stability of the water core is weakened with increasing temperature. The interaction between the CO2 and the surfactant decreases, which results in a decrease of the water domains or hydrodynamic radius in the microemulsion aggregates. The decrease of water domain indicates that the polarity of the environment in the water core is weakened, which leads to the solubilization of more 1,3-PDO aqueous solution than water. On the other hand, the volatility of the solute increases with temperature, which is favorable to the increase of the solubility of TMN-6. More

Figure 3. Cloud-point pressures of the system TMN-6 + CO2 + EtOH + 1,3-PDO at different TMN-6 concentrations. Concentrations of TMN-6: ■, 16.291·10−3 mol·kg−1; ●, 10.527·10−3 mol·kg−1; ▲, 5.539·10−3 mol·kg−1; ▼, without TMN-6.

in the microemulsion of the ternary system of TMN-6 + CO2 + EtOH than in the binary system of CO2 + EtOH. Moreover, at a fixed temperature, the pressure decreases with the increase of the concentration of TMN-6. However, when the concentration of TMN-6 ethanol solution is 16.291·10−3 mol·kg−1, a higher pressure is needed, suggesting that an optimal concentration of TMN-6 ethanol solution exists between 10.527·10−3 mol·kg−1 and 16.291·10−3 mol·kg−1. Thus, more experiments are required to find a proper TMN-6 concentration to get the lowest cloud-point pressure as far as a largescale industrial process is concerned. 3.2.2. Phase Behavior of the TMN-6 + CO2 + EtOH + 1,3PDO + H2O System. To simulate the real fermentation broth and identify the influence of the concentration of 1,3-PDO on the phase behavior of the system of TMN-6 + CO2 + EtOH + 1,3-PDO + H2O, various concentrations of 1,3-PDO dilute aqueous solution (0.025, 0.050, and 0.075 mass fractions) were prepared as a model compound. As mentioned above, pure

Figure 4. Cloud-point pressures of the quinary system TMN-6 + CO2 + EtOH + 1,3-PDO + H2O (0.025 mass fraction 1,3-PDO aqueous solution) and quaternary system of TMN-6 + CO2 + EtOH + H2O at different concentrations of TMN-6: a, 16.291·10−3 mol·kg−1; b, 10.527·10−3 mol·kg−1; c, 5.539·10−3 mol·kg−1. ■, TMN-6 + CO2 + EtOH + H2O system; ●, TMN-6 + CO2 + EtOH + 1,3-PDO + H2O system. 1790

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Figure 5. Cloud-point pressures of quinary system of TMN-6 + CO2 + EtOH + 1,3-PDO + H2O (0.050 mass fraction 1,3-PDO aqueous solution) and quaternary system of TMN-6 + CO2 + EtOH + H2O at different concentrations of TMN-6. Concentrations of TMN-6: a, 16.291·10−3 mol·kg−1; b, 10.527·10−3 mol·kg−1; c, 5.539·10−3 mol·kg−1. ■, TMN-6 + CO2 + EtOH + H2O system; ●, TMN-6 + CO2 + EtOH + 1,3-PDO + H2O system.

Figure 6. Cloud-point pressures of the quinary system TMN-6 + CO2 + EtOH + 1,3-PDO + H2O (0.075 mass fraction 1,3-PDO aqueous solution) and quaternary system of TMN-6 + CO2 + EtOH + H2O with different concentrations of TMN-6. Concentrations of TMN-6: a, 16.291·10−3 mol·kg−1; b, 10.527·10−3 mol·kg−1; c, 5.539·10−3 mol·kg−1. ■, TMN-6 + CO2 + EtOH + H2O system; ●, TMN-6 + CO2 + EtOH + 1,3-PDO + H2O system.

in selective solubilization of 1,3-PDO at the corresponding pressures. However, at the temperatures higher than TCO, as seen in Figures 4a and 5b, the effect of temperature on the stability of the structure of water core becomes dominant, which

aggregates can be formed which is beneficial for solubilizing more water than 1,3-PDO. Thus, the cloud point pressure of TMN-6 + CO2 + EtOH + 1,3-PDO + H2O system is lower than that of the TMN-6 + CO2 + EtOH + H2O system, which results 1791

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is favorable to the increasing of the solubility of 1,3-PDO aqueous solution and demonstrates the selective solubilization of 1,3-PDO in the microemulsion at the range of pressures studied. On the contrary, when the temperature is lower than TCO, the effect of temperature on the volatility of the solute becomes dominant, and an opposite result is then obtained. In addition, Figure 6b shows little difference in the cloud-point pressures between the two systems, which is difficult to extract selectively 1,3-PDO from dilute aqueous solution. Therefore, 5.539·10−3 mol·kg−1 and 16.291·10−3 mol·kg−1 TMN-6 ethanol solutions were chosen to extract 0.075 mass fraction 1,3-PDO aqueous solution. These suggest that besides temperature, both the concentrations of TMN-6 and 1,3-PDO have important influences on the selective solubilization of 1,3-PDO. The feasibility of selectively solubilizing 1,3-PDO from dilute aqueous solution at different concentrations of TMN-6 is shown in Table 2. The results of this work provide basic thermodynamics

*Tel./fax: +86-411-84986274. E-mail: [email protected]. Funding

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (20976026, 20976028). Notes

The authors declare no competing financial interest.

■

103 mol·kg−1

0.025

302.85 < T < 309.35

5.539 10.527 16.291 5.539 10.527 16.291 5.539 10.527 16.291

a a b a a a a a a

309.35 ≤ T < 310.85

310.85 ≤ T < 317.25

0.050 0.075 a b a a a a a a a

REFERENCES

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concentration of 1,3-PDO aqueous solution/mass fraction

temperature/K

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Table 2. Feasibility of Selectively Solubilizing 1,3-PDO from Dilute Solution at Different Concentrations of TMN-6 concentration of TMN-6 ethanol solution

Article

a b a a b a a b a

a

Represent the conditions that solubilization is feasible. bRepresent the conditions that could not realize solubilization.

data for industrial design and are of instructive significance for the industrial process.

4. CONCLUSION Phase behaviors of scCO2 microemulsion of the quaternary system of TMN-6 + CO2 + EtOH + H2O and the quinary system of TMN-6 + CO2 + EtOH + 1,3-PDO + H2O were investigated in a volume-variable high-pressure view cell at temperatures ranging from (302.85 to 317.25) K and pressures of (6.6 to 12.4) MPa. The W/CO2 microemulsion is stabilized by the commercially available surfactant TMN-6 and cosurfactant ethanol. At each experimental condition, a transparent phase solution was observed, which confirmed that both water and 1,3-PDO could be dissolved in the microemulsion. Moreover, at appropriate temperatures and pressures, 1,3-PDO could be selectively solubilized in the polar core of the scCO2 microemulsion, showing the potential of the scCO2 microemulsion for the selective solubilization of 1,3-PDO from dilute aqueous solution. When 0.025 and 0.050 mass fraction 1,3-PDO dilute aqueous solution were added, a crossover temperature (TCO) appeared at 309.35 K when the concentration of TMN-6 ethanol solution was 16.291·10−3 mol·kg−1 and appeared at 310.85 K when the concentration of TMN-6 ethanol solution was 10.527·10−3 mol·kg−1, respectively. The results obtained in this work may be useful for the separation of 1,3-PDO in bioprocess. 1792

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