Formation of Solid Solution and Ternary Phase Diagrams of

Apr 9, 2015 - ABSTRACT: Crash cooling crystallization of different anthracene (ANT) and phenanthrene (PHE) mixtures in toluene, xylene, and N,N- ...
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Formation of Solid Solution and Ternary Phase Diagrams of Anthracene and Phenanthrene in Different Organic Solvents Suping Ding,† Qiuxiang Yin,†,‡ Wei Du,† Xiaowei Sun,† Baohong Hou,†,‡ Meijing Zhang,†,‡ and Zhao Wang*,†,‡ †

School of Chemical Engineering and Technology, State Key Laboratory of Chemical Engineering, Tianjin University, 92 Weijin Road, Nankai District, Tianjin 300072, People’s Republic of China ‡ Collaborative Innovation Center of Chemical Science and Chemical Engineering (Tianjin), Tianjin 300072, People’s Republic of China S Supporting Information *

ABSTRACT: Crash cooling crystallization of different anthracene (ANT) and phenanthrene (PHE) mixtures in toluene, xylene, and N,N-dimethylformamide (DMF) was investigated. The crystals obtained were identified by powder X-ray diffraction, differential scanning calorimetry, and high-performance liquid chromatography. To verify the formation of different solid solutions, ternary phase diagrams of ANT and PHE in toluene, xylene, and DMF at 308.15 K and 0.1 MPa were determined through experiments. Two solid solutions, α and β, respectively, formed at PHE-rich and ANT-rich ends but coexisted in the middle section of the composition. The effect of solvent and separation process optimization of ANT and PHE were further investigated based on the ternary phase diagrams.

1. INTRODUCTION Solid solutions are homogeneous solid-state solutions wherein solute (minor phase) molecules randomly incorporate into the solvent (major phase) crystal lattice.1 In solid solutions, solute molecules generally replace solvent molecules or insert into interstices in the solvent crystal lattice; such replacements or insertions do not destroy the lattice but change the interlayer distances of the solvent crystal lattice.2 The structure and thermodynamic properties of solid solutions are similar to those in the solvent phase and vary with composition, which can be detected by powder X-ray diffraction (PXRD) or differential scanning calorimetry (DSC).3 Solid solution formation is beneficial in alloys, catalysts,4,5 and special materials2,6 but may cause significant problems during separation and purification of sole components from mixtures through recrystallization. Two components cannot be completely separated through recrystallization when they form a solid solution. Anthracene (ANT) and phenanthrene (PHE), as common polycyclic aromatic hydrocarbons (PAHs), are irreplaceable chemical raw materials in the dye and pharmaceutical industries. The molecular structures of these compounds are shown in Figure 1. ANT and PHE have received great attention from industrial manufacturers and scientific researchers because of their special polycyclic structure; the compounds are widely used in dyes,7,8 pesticides,9,10 pharmaceuticals,11 fuels,12 photoelectric materials,13,14 and many other fields. ANT and PHE are known to form solid solutions.15−17 Crude ANT from © 2015 American Chemical Society

Figure 1. Molecular structures of ANT and PHE.

coal tar contains approximately 35 % to 45 % ANT, 15 % to 25 % PHE, and 10 % to 20 % carbazole. During refining of crude ANT, several purification steps, including the separation of ANT and PHE, are required. While many refining methods have been proposed in the literature, the most widely approach is one that combines crystallization and distillation.18−20 As phase diagrams provide very important thermodynamic data during crystallization, they are widely acknowledged to be useful in determining relative phases to improve operating conditions. Most studies have focused on the solvent-free binary solid−liquid equilibrium phase diagrams of organic mixtures of PAHs.21−25 A multiple-phase diagram containing a solvent phase would be of great significance for separating and purifying sole components from PAHs.26−28 In this study, crash cooling crystallization of different ANT and PHE mixtures was performed at 308.15 K, and the Received: December 10, 2014 Accepted: April 3, 2015 Published: April 9, 2015 1401

DOI: 10.1021/je501121v J. Chem. Eng. Data 2015, 60, 1401−1407

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Table 1. Weight Composition of the Experiment Points, Mother Liquid, and Solid Products from Cooling Crystallizationa composition of experiment point/% experiment 123456a

M2-DMF M2-toluene M2-xylene M1-DMF M1-toluene M1-xylene

composition of mother liquid/%

composition of solid product/%

xANT

xS

xPHE

xANT

xS

xPHE

xANT

xPHE

9 9 9 3 3 3

73 73 73 54 54 54

18 18 18 43 43 43

2.57 1.99 1.74 2.50 0.79 0.79

78.43 78.35 79.31 56.74 65.48 68.63

19.00 19.66 18.95 40.77 33.73 30.58

87.29 82.97 78.96 20.88 12.34 10.95

12.71 17.03 21.04 79.12 87.66 89.05

The composition is shown by mass fraction x.

products of crystallization were identified through PXRD, DSC, and high-performance liquid chromatography (HPLC). To elucidate the crystallization result, ternary phase diagrams of ANT and PHE in toluene, xylene, and N,N-dimethylformamide (DMF) at 308.15 K and 0.1 MPa were determined through experiments. Solvent effects on crystallization and the resultant ternary phase diagrams were further investigated in detail to optimize the separation process of ANT and PHE.

identified by PXRD and DSC, and its composition was determined by HPLC. The weight composition of the mother liquid was also determined by HPLC. 2.4. Determination of ANT−PHE−Solvent Ternary Phase Diagram. The ternary phase diagrams of ANT and PHE in toluene, xylene, and DMF were experimentally determined. Mixture of ANT, PHE, and solvent at a certain ratio was prepared in a jacketed vessel and then maintained at 308.15 K for 48 h under magnetic stirring. The temperature was controlled by a temperature-controlled water bath (JULABO Circulator, type CF41) and the accuracy was ± 0.1 K. Saturated liquid sample was withdrawn with an injector, filtered through a 0.45 μm syringe filter, and diluted with DMF and eluent, and then the weight composition of the diluted solution was analyzed by HPLC. For each liquid point, three liquid samples were withdrawn and analyzed together to get the average value. The equilibrium solid phase was separated through fast suction filtration of the suspension and dried in an air-dry oven at 323.15 K for 24 h. The obtained solid sample was identified by PXRD and DSC. Also three samples of each solid phase point were prepared and determined together by HPLC. The relative standard uncertainty of the mass fraction (ur(x)) of samples in the experiments is approximately 0.05; it is mainly caused by the sampling procedure. All of the masses in the experiment were determined using a balance (model AL204, Mettler-Toledo, Switzerland) with an accuracy of ± 1· 10−7 kg.

2. EXPERIMENTAL SECTION 2.1. Material. ANT and PHE (supplied by Aladdin with the purities 96% and 95% weight percent, respectively) were further purified from cooling crystallization. Both the purities analyzed by HPLC (Agilent 1100, Agilent Technologies, USA) were above 99%.The purified ANT and PHE were also identified by PXRD and DSC. Toluene, xylene, and DMF (supplied by Tianjin Jiangtian Chemical Reagents Co.) were analytical reagent grade and used without further purification. Acetonitrile used in HPLC was supplied by Tianjin Yuxiang Chemical Reagents Co. with LC purity. 2.2. Analysis Apparatus and Conditions. HPLC. The weight compositions of various samples were determined by Agilent 1100 HPLC, equipped with an Agilent Eclipse Plus C18 column (5 μm, 4.6 mm × 250 mm). The column temperature was 301.15 K, and the flow rate of eluent (100 v = 60 acetonitrile and 100 v = 40 water) was 1.5 mL/min. A UV diode array detector was applied for peak detection at a wavelength of 254 nm. DSC. The thermodynamic properties of various samples were determined by DSC (DSC 1/500, Mettler-Toledo, Switzerland) at a heating rate of 5 K/min under protection of nitrogen atmosphere. The temperature deviations of the measurements were ± 0.3 K. PXRD. The PXRD patterns were obtained by using Cu Kα (1.54) radiation on a D/MAX 2500 X-ray diffractometer. Crystal samples were analyzed over a diffraction-angle (2θ) range of 2° to 40°, at a step size of 0.02°, a dwell time of 1 s, a voltage of 40 kV, and a current of 100 mA. 2.3. Crash Cooling Crystallization of ANT and PHE. Experiments were carried out using a 50 mL jacketed vessel with an agitation rate of 250 rpm by a magnetic stirrer. Solutions were prepared by dissolving the required amounts of ANT−PHE mixture in toluene, xylene, and DMF at about 353.15 K, respectively. After complete dissolution, the solutions were filtered through a preheated 0.45 μm syringe filter, transferred to another jacketed vessel, and then cooled rapidly to 308.15 K by a temperature-controlled water bath (JULABO Circulator, type CF41) with an accuracy of ± 0.1 K. The solid products were collected after nucleation and dried at 323.15 K in an air-dry oven for 24 h. The obtained crystals were

3. RESULTS AND DISCUSSION 3.1. Identification of ANT and PHE. The purities of ANT and PHE determined by HPLC are over 99.0%. The DSC curves of ANT and PHE, which were obtained at a heating rate of 5 K·min−1, are given in the Supporting Information. ANT shows a single endothermic peak at 489.47 K and a fusion enthalpy of 24.96 kJ·mol−1, which is very close to the value reported in the literature.29 The DSC thermogram of PHE shows two endothermic peaks at 339.52 and 371.71 K; here, the first peak is attributed to polymorphic transition and the second peak corresponds to the melting point of the stable high-temperature form of PHE.30 The PXRD patterns of pure ANT and PHE are given in the Supporting Information, both ANT and PHE show a strong diffraction peak at around 2θ = 9.4°. This result corresponds to the crystal faces of (0 0 1) and (1 0 0). The similarity of the PXRD patterns of ANT and PHE indicate that the crystal structures of these compounds are similar. 3.2. Formation of Solid Solution of ANT and PHE. The two kinds of ANT-PHE mixtures were designated M1 and M2. The crash cooling crystallization experiments were performed six times at 308.15 K and designated 1-M2-DMF, 2-M21402

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different types of solid solutions may crystallize simultaneously and that solid−solid peritectic transitions occur when the solution is heated to a certain temperature. ANT−PHE− solvent (DMF, toluene, and xylene) ternary phase diagrams were subsequently determined to verify the formation of different solid solutions during crash-cooling. 3.3. ANT−PHE−Solvent Ternary Phase Diagram. The ternary phase diagrams of ANT and PHE in toluene, xylene, and DMF at 308.15 K were experimentally determined and are shown in Figures 4, 5, and 6, respectively; detailed data are

toluene, 3-M2-xylene, 4-M1-DMF, 5-M1-toluene, and 6-M1xylene, respectively. Both the compositions of the experiment points and the corresponding results are listed in Table 1. After crystallization from DMF, the xANT of M1 increased from 6.54% to 20.88% while that of M2 increased significantly from 33.33% to 87.29%; these results indicate that only the composition of ANT can be improved through crystallization because PHE is much more diffluent than ANT. Products from DMF contain more ANT than those from toluene and xylene for both initial compositions of M1 and M2. Thus, DMF favors ANT and PHE separation. The obtained crystals were further identified by PXRD and DSC, and results are shown in Figures 2 and 3. As shown in

Figure 4. ANT−PHE−toluene ternary phase diagram at 308.15 K.

Figure 2. PXRD patterns of the solid products from cooling crystallization.

Figure 5. ANT−PHE−xylene ternary phase diagram at 308.15 K.

listed in the Supporting Information. All of the compositions of the solid and liquid phases were determined by HPLC, and identification of the solid phases was verified by PXRD and DSC. ANT−PHE−Toluene Ternary Phase Diagram. The ANT− PHE−toluene ternary phase diagram at 308.15 K is shown in Figure 4. The red line with symbols represents the saturatedliquid line, while all other lines indicate liquid−solid (L−S) lines connecting the liquid phase composition point with the corresponding equilibrated solid-phase composition point. The L−S lines show that each liquid point is equilibrated with a corresponding solid composition point, which is a typical characteristic of solid-solution phase diagrams.31 However, one liquid point equilibrates with more than one solid point when in the moderate solid composition, which demonstrates that the ternary phase diagram is not a simple solid solution phase diagram because an invariant point marked with a round dot in Figure 4 is determined in the liquid line. The unsaturated or

Figure 3. DSC thermograms of the solid products from cooling crystallization.

Figure 2, the strong diffraction peak at around 2θ = 9.4° presents a slight shift to the left in each curve. Endothermic peaks of the DSC curves in Figure 3 also manifest a shift compared with those from the pure material, which may be attributed to the formation of a solid solution during crashcooling crystallization. The ANT−PHE binary system has been reported to be a discontinuous solid solution system, and a peritectic transition occurs in the system with a moderate solid composition at around 420 K.15−17 PXRD patterns of the crystals obtained from the experiments 1-M2-DMF, 2-M2toluene, and 3-M2-xylene show two peaks at around 2θ = 9.4°, and the thermograms of these crystals in Figure 3 present endothermic peaks at around 420 K, which indicates that 1403

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an invariant liquid point equilibrates with more than one solid phase, which resembles findings in toluene. ANT−PHE−DMF Ternary Phase Diagram. The ANT− PHE−DMF ternary phase diagram at 308.15 K is shown in Figure 6; this phase diagram differs from those obtained with toluene and xylene. Two invariant points may be observed in the ANT−PHE−DMF ternary phase diagram. The first point (point1) is similar to those in toluene and xylene, and the second point (point2) is a eutectic point of PHE and α. The phase diagram can be divided into five regions of L, PHE+L, PHE+α+L, α+β+L, and β+L, as shown in Figure 6. DMF is a highly polar solvent with two negatively charged centers that are provided by O and N; thus, the solvent can form C−H···π hydrogen bonds with ANT and PHE between the N-methyl group of DMF and the benzene ring.32 Lewis et al.33 reported that the amino group of DMF can attach to PHE at C9 or C1 via a short polymethylene chain that forms an exciplex. This phenomenon shows that strong intermolecular interactions exist between PHE and DMF and demonstrates differences among DMF, toluene, and xylene. Differences observed may explain the occurrence of the second invariant point. 3.4. Characteristics of the Solid Solutions. The ternary phase diagrams of ANT and PHE in toluene, xylene, and DMF confirm that different types of solid solutions can form at different regions. To identify the solid solutions obtained during measurement of the ternary phase diagrams further, PXRD patterns and DSC thermograms of the solid solutions in toluene were determined through experiments. PXRD Patterns. The PXRD patterns of different solid solutions from different sections in ANT−PHE−toluene ternary phase diagram are shown in Figure 7; here, the

Figure 6. (a) ANT−PHE−DMF ternary phase diagram at 308.15 K. (b) Enlargement of the part circled in (a).

liquid region in PHE−ANT−toluene ternary phase diagram is narrow because the solubility of ANT in toluene is only 1.73 % (Wt) at 308.15 K, and the invariant point in this diagram cannot be directly determined only from the liquid line because this line is smoothly fitted. Figure 4 further shows that the ternary phase diagram can be divided into four regions as L, α+L, β+L, and α+β+L according to the liquid and solid−liquid lines with different color. The solid phase forms a discontinuous solid solution system consistent with the binary system of PHE and ANT reported in the literature.15−17 α represents a solid solution in which ANT acts as the solute (minor phase) incorporated into the solvent (major phase) crystal lattice of PHE. β represents a solid solution in which PHE acts as the solute (minor phase) incorporated into the solvent (major phase) crystal lattice of ANT. α and β coexist in the α+β+L region, with L corresponding to the invariant point. The clear boundary lines of the three regions of α+L, β+L, and α+β+L could not be determined because it is difficult to obtain solid solutions with continuous compositions near the boundary points in the ANT−PHE axis. ANT−PHE−Xylene Ternary Phase Diagram. The ANT− PHE−xylene ternary phase diagram is shown in Figure 5; here, the unsaturated liquid region is very small compared with the α+L, β+L, and α+β+L regions because of the low ANT solubility in xylene. The ternary phase diagram is divided into four regions according to the liquid and solid−liquid lines, and

Figure 7. PXRD patterns of solid solutions of α, α+β, and β in the ANT−PHE−toluene phase diagrams and PXRD patterns of pure ANT and PHE.

diffraction peak of solid solution α (xPHE = 81.37 %) at around 2θ = 9.4° shows a slight shift toward the left from 9.354° to 9.059° compared with that of PHE and the diffraction peak of solid solution β (xPHE = 11.51 %) at around 2θ = 9.4° presents a slight shift toward the left from 9.478° to 9.461° compared with that of ANT. Solid samples from region α+β+L in Figure 4 show two diffraction peaks at around 2θ = 9.4°. Each peak slightly shifts toward the left compared with the corresponding peaks of PHE and ANT. This result indicates that solid solutions α and β form simultaneously during measurement of the ternary phase diagram and that the solid solution of ANT and PHE is a discontinuous solid solution system. 1404

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DSC Thermograms. The DSC thermograms of different solid solutions from different sections in ANT−PHE−toluene ternary phase diagram are shown in Figure 8. The DSC curve of

K, are shown in Table 2. The data is the average value of multiple experiments. The order of ANT and PHE solubility in Table 2. Compositions of Invariant Points and Solubility of ANT and PHE at 308.15 Ka,b composition of invariant point/%

solubility/%

phase diagram

xPHE

xS

xANT

SANT

SPHE

ANT−PHE−xylene ANT−PHE− toluene ANT−PHE−DMF

25.51 28.14

72.54 69.64

1.95 2.22

1.72 1.73

34.59 39.89

38.41 49.12

58.71 49.27

2.89 1.61

2.78

48.79

point1 point2

a

The composition and solubility are shown by mass fraction x. bThe relative standard uncertainty of mass fraction x is ur (x) = 0.05, and that of temperature is u (T) = 0.1K

different solvents is Sxylene < Stoluene < SDMF, which indicates that the influence of the similarity between solute and solvent molecules is not as significant as the influence of the polarity of solvent. The invariant point (point1 for DMF) moves toward the point of pure PHE with increasing PHE solubility, thereby increasing xPHE and xANT and decreasing xS. The shift of invariant points in different solvents generates proportion differences in different phase regions and further influences the separation process of ANT and PHE. ANT and PHE cannot be separated completely through recrystallization because they can form solid solutions with different compositions. However, a single solid solution may first be obtained, after which purity could be improved through multistep recrystallization. The separation process for PHE and ANT is shown in Figure 9, point O (O′) is the invariant point,

Figure 8. DSC thermograms of solid solutions of α, α+β, and β in the ANT−PHE−toluene phase diagrams.

solid solution α (xPHE = 81.37 %) shows two endothermic peaks: the peak at 359.05 K corresponds to polymorphic transitions,26 and the peak at 379.04 K corresponds to the melting point of solid solution α. The transition temperature is higher than that of pure PHE, which indicates that incorporating ANT into the lattice cannot prevent polymorphic transitions but improves the transition temperature via molecular interactions between ANT and PHE. The melting point of solid solution α is close to that of PHE because PHE serves as the solvent in solid solution α. The DSC curve of solid solution β (xPHE = 11.51 %) manifests one endothermic peak at 483.24 K, the melting point of solid solution β, and is close to the melting point of pure ANT; such a finding can confirm that ANT serves as the solvent in solid solution β. The DSC thermogram of a mixture comprising solid solutions α and β (xPHE = 48.10 %) shows three endothermic peaks at 362.88, 389.40, and 425.14 K. The first peak corresponds to polymorphic transitions, while the two other peaks are broader and overlapped each other. The temperatures of these peaks are between the melting peaks of pure ANT and pure PHE, which indicates that solid solutions α and β coexist in the sample. The peak at 389.40 K is the melting point of solid solution α, which shifts to right from that of PHE. The melting peaks of solid solutions α and β may mask the transition peak because peritectic transition (α+L → β) occurs at around 420 K.15−17 Thus, the peak at 425.14 K cannot be determined to be one thermal event. The melting points of the same types solid solutions (α or β) differ, and this difference may be attributed to differences in composition. 3.5. Effect of Solvent and Optimization of Separation Process. Toluene, xylene, and DMF were used as the solvent phase in the determination of phase diagram to investigate the solvent effect on the ternary phase diagram of ANT and PHE. Toluene and xylene are common aromatic solvents with weak polarities. The solubility of ANT is lower than that of PHE in both toluene and xylene, thus the ternary phase diagrams of ANT and PHE in the two solvents have the same characteristics as mentioned above, except the invariant point varies with different solvents. The compositions of invariant points in the three phase diagrams, as well as the solubilities of ANT and PHE at 308.15

Figure 9. Scheme of separation process of PHE and ANT by recrystallization.

point A is the boundary point between solid solution α and the mixture of solid solution α and β, point B is the boundary point between solid solution β and the mixture of solid solution α and β, and the other points mentioned in the latter correspond different phases and compositions in the ternary system. Point M represents the raw material composition of ANT and PHE. Points S and M are connected and intersect with line OB at point G. A certain amount of solvent is added to move the system toward the composition of point G, and a product of solid solution β is obtained; here, the composition of point B corresponds to the composition of the mother liquid located at point O. The shift of invariant points when using different 1405

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solvents changes the slope of OB (O′B). Therefore, the amount of solvent and recrystallization time are varied. For example, if DMF is used as the solvent, the invariant point (point 1 for DMF) moves toward the point of pure PHE and the slope of OB (O′B) is larger. Then the higher purity product will be obtained with the same amount of solvent. So DMF is the best solvent for separation of ANT and PHE, which is according to the conclusion of cooling crystallization experiment. Repeating the process above, points S and B are connected and intersect with S−L line CD at H. The solvent is added for reaching the composition point H. The higher purity product are obtained at composition point D. This procedure is repeated several times to significantly improve the purity of the raw material after multistep recrystallization.

4. CONCLUSIONS Crash-cooling crystallization of different ANT and PHE mixtures in toluene, xylene, and DMF was investigated at 308.15 K, respectively. Different solid solutions were obtained during crystallization and elucidated by the ternary phase diagrams of ANT and PHE in the three solvents. Both the ANT−PHE−toluene and ANT−PHE−xylene ternary phase diagrams show an invariant liquid point that equilibrates with the liquid phase, solid solution α and β. Therefore, the phase diagram can be divided into four regions as L, α+L, β+L, and α+β+L. For ANT−PHE−DMF ternary phase diagram, one more invariant point was found which is the eutectic point of PHE and α, therefore the phase diagram can be divided into five regions as L, PHE+L, PHE+α+L, α+β+L, and β+L. All of the diffraction peaks at around 2θ = 9.4° in the PXRD patterns of the solid solutions showed a slight shift to the left, and the melting points of solid solutions α and β were close to those of PHE and ANT, respectively. Different solvents resulted in differences in the separation process of ANT and PHE because of shifting of invariant points. The shift of invariant points changes the slope of S−L lines, therefore, the amount of solvent and recrystallization time are varied.

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ASSOCIATED CONTENT

S Supporting Information *

DSC thermograms, PXRD patterns, and phase equilibrium data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-22-27405754. Fax: 86-22-27314971. E-mail: zhao_ [email protected]. Funding

The support from National Natural Science Foundation of China (no. 21206109) and Tianjin Municipal Natural Science Foundation (no. 12JCQNJC04500) are greatly appreciated. Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/je501121v J. Chem. Eng. Data 2015, 60, 1401−1407