Thermodynamic Analysis of Isomerization Equilibria of Chlorotoluenes

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Thermodynamic Analysis of Isomerization Equilibria of Chlorotoluenes and Dichlorobenzenes in a Biphasic Reaction Systems Containing Highly Acidic Chloroaluminate Melts Sergey P. Verevkin,*,† Julia Messner,‡ Vladimir N. Emel’yanenko,†,§ Mikhail G. Gantman,‡ Peter S. Schulz,‡ and Peter Wasserscheid‡ †

Department of Physical Chemistry and Department of Science and Technology of Life, Light and Matter, University of Rostock, Dr-Lorenz-Weg 1, D-18059 Rostock, Germany ‡ Lehrstuhl für Chemische Reaktionstechnik, Universität Erlangen Nürnberg, Egerlandstrasse 3, 91058 Erlangen, Germany § Department of Physical Chemistry, Kazan Federal University, Kremlevskaya str. 18, 420008 Kazan, Russia S Supporting Information *

ABSTRACT: Thermodynamics and kinetics of the isomerization of chlorotoluenes and dichlorobenzene to the technically desired metaisomers have been studied in the presence of highly acidic chloroaluminate melts with alkali metal and organic imidazolium cations. Enthalpies of four isomerization processes in reacting systems of chlorotoluenes and dichlorobenzene were obtained from temperature dependencies of the corresponding equilibrium constants in the liquid phase. Experimental reaction enthalpies, enthalpies of vaporization, and absolute vapor pressures of chlorotoluenes and dichlorobenzene have been used for the validation of quantum-chemical methods to predict thermodynamic functions of the four reactions under study successfully. Values of the standard Gibbs energies of formation, standard enthalpies and entropies of formation of chlorotoluenes and dichlorobenzenes in the liquid and in the gas phase have been derived. These values allow optimization of liquid−liquid biphasic manufacturing technologies for halogen-substituted benzenes.

1. INTRODUCTION The application of ionic liquids, both as reaction media and as catalyst, opens new perspectives in chemistry and chemical engineering.1 The use of ionic liquids in industrial processes, namely Lewis acid catalyzed processes, allows to increase the yield of the desired product or to make the separation and the purification of the product easier. Often these results can be achieved due to the fact that the properties of ionic liquids can be varied in a wide range and also can be finely tuned. In particular, the solubility of starting material and product in the ionic liquid and the ionic liquid's acidic properties can be adjusted according to the demands of the process under investigation. In the present manuscript we describe the use of Lewis acidic ionic liquids in the catalysis of two industrially relevant processes: the isomerization of 4-chlorotoluene (4CT) and the isomerization of p-dichlorobenzene (see Figure 1). The main route to produce dichlorobenzene on an industrial scale is the direct chlorination of benzene. This process is kinetically controlled and leads mainly to the formation of oand p-dichlorobenzene due to the directing effect of the first chloro-substituent. The methyl group of toluene shows the same directing effect and the chlorination of toluene also leads to a mixture of ortho- and para-isomers. ortho-Isomers have rather low importance for technology, and the importance of © 2016 American Chemical Society

Figure 1. Isomerization of the chloro-substituted benzenes studied in this work.

para-isomers is relatively higher,2 though namely the metaisomers of dichlorobenzene and chlorotoluene are broadly used as precursors for insecticides, herbicides, pharmaceuticals, and dyes. However, currently for synthesis of these compounds only expensive and atom inefficient routes have been suggested (e.g., the Sandmeyer reaction of m-chloraniline3 or the catalytic dechlorination of 1,2,4-dichlorobenzene4). Received: October 18, 2016 Revised: November 28, 2016 Published: December 2, 2016 13152

DOI: 10.1021/acs.jpcb.6b10529 J. Phys. Chem. B 2016, 120, 13152−13160

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The Journal of Physical Chemistry B

Table 1. System I (p-Chlorotoluene Isomerization) Experimentally Determined Compositions of Equilibrium Mixtures and Kx Values in the Liquid Phasea catalyst

T

xp

xm

x0

Kx(m/p)1

Kx(o/p)2

AlCl3/HCl, 4.25 h [6] AlCl3/AlCl3·H2O, 200 h [11] AlCl3/AlCl3·6H2O, 4 h AlCl3/KCl/NaCl, 4 h AlCl3/LiCl/NaCl, 4 h AlCl3/LiCl/NaCl, 24 h AlCl3/[EMIM]Cl, 4 h AlCl3/[N(CH3)4]Cl, 4 h AlCl3/LiCl/[N(CH3)4]Cl 2:0.6:0.4, 4 h AlCl3/LiCl/[N(CH3)4]Cl, 2:0.5:0.5, 4 h AlCl3/LiCl/[N(CH3)4]Cl, 2:0.4:0.6, 4 h

369 373 413 413 413 413 413 413 413 413 413

17.9 19.0 17.5 17.5 17.5 17.0 85.0 93.0 20.0 22.0 26.0

49.2 38.0 47.0 47.0 47.0 45.0 14.5 6.5 51.0 55.0 59.0

32.9 43.0 35.5 35.5 35.5 38.0 0.5 0.5 29.0 23.0 15.0

2.7 (2.0) 2.7 2.7 2.7 2.6 (0.2) (0.1) 2.6 2.5 2.6

(1.8) 2.3 2.0 2.0 2.0 (2.3) (0.01) (0.01) (1.5) (1.1) (0.6)

a

T is temperature of investigation in K; xi is the mole fraction measured chromatographically. Values in brackets are not at equilibrium either for reaction 1 or for reaction 2.

However, it is well established that the meta-isomers are the thermodynamically most stable compounds in their respective isomeric mixtures.5,6 As a consequence, a promising route to produce the meta-isomer efficiently is the isomerization of pdichlorobenzene or p-chlorotoluene. This highly selective reaction can be carried out under typical Friedel−Crafts conditions with water promoted aluminum chloride.5,6 However, these classical catalysts have an important drawback: the product isolation procedure demands for the hydrolysis of the reaction system, which leads to the complete destruction of the catalyst. This makes the reuse of the catalyst impossible and the isomerization protocol not sustainable and not economic. A very interesting alternative was presented by Messner et al., who demonstrated that liquid−liquid biphasic reaction systems involving alkali chloroaluminate meltsand in particular lithium chloroaluminate meltsform highly active and recyclable catalyst phases for the dichlorobenzene isomerization.7,8 In focus of this work is the design of novel Lewis acidic catalytic systems for chlorobenzene isomerization processes. The target is to tune the miscibility of the ionic liquid with the isomers in a way that the isomers are sufficiently soluble in the ionic liquid to allow an isomerization reaction. At the same time, a miscibility gap between catalyst phase and isomers is required to allow for a liquid−liquid biphasic process that ensures easy product separation and catalyst recycling. Basic requirement is also a very pronounced Lewis acidity of the applied ionic liquid to promote isomerization to the desired, thermodynamic equilibrium mixture of isomers. It is well established, that catalytic reactions in alkylbenzenes and chlorobenzenes (see Figure S1 in the Supporting Information) are usually limited by the equilibrium of isomerization and disproportionation reactions.5,6,9,10 However, relative rates of these chemical reactions are significantly different and they are crucially dependent on the temperature of experiment. For example, as shown by Olah and Meyer,11 starting from mixtures with different amount of ortho-, meta-, and para-isomers, the distribution of isomers approximates equilibrium only after 50 h at 373 K. Therefore, reactions given in Figure 1, have to be considered both under kinetic and under thermodynamic control. For the development and the optimization of novel biphasic reaction systems, the equilibrium distribution of ortho-, meta, and para-isomers have to be exactly known at any temperature in order to assess and compare activity of catalytic systems. Unfortunately, only few exper-

imental studies dealing with chlorotoluene and dichlorobenzene isomerization in the presence of AlCl3-based catalysts are available.5,6,11,12 Information on isomers distribution available from the literature is not only fragmentary and contradictory, but it also suffers from an ambiguity whether the experimental time at a certain temperature was enough to reach equilibrium or not. The purpose of this work is 3-fold: First, we wish to resolve the contradiction existing between available experimental equilibrium constants for para ↔ meta ↔ ortho interconversions with help of own experimental data. The reliable temperature dependence of equilibrium constants in the liquid phase is able to provide thermodynamic characteristics for isomerization reactions: a standard molar reaction enthalpy, ΔrHm° , and a standard molar reaction entropy, ΔrSm° . The consistency of the latter values derived from equilibrium studies (second law method) can be validated with help of Hess’s law by comparison of the standard molar enthalpies of formation, ΔfHm° , or the standard molar entropies, Sm° , of the reaction participants (second law method). This part of the study aims at providing reliable equilibrium constants and - as a consequencethe quantitative distribution of ortho-, meta-, and para-isomers at any temperature. This knowledge is indispensable for the assessment of the catalytic activity of the novel biphasic reaction systems within this work. However, this knowledge is also of great value to further develop quantum-chemical methods toward their use in optimizing industrial processes. In recent years some of us have intensively applied high-level quantum-chemical methods for the calculation of thermodynamic properties and charcteristics of different, industrially relevant reactions.9,10,13,14 With reliable experimental equilibrium constants for the para ↔ meta ↔ ortho interconversions at hand, it was reasonable to apply quantum chemistry for an “in silico” assessment of these equilibrium constants. Finally, a further aim of the current study is the development of comprehensive tables with thermodynamic data for isomeric chlorotoluenes and dichlorobenzenes. These tables are required for the design and optimization of industrial processes and for implementation of further novel biphasic reaction systems.

2. EXPERIMENTAL PROCEDURES Isomerization of 4-chlorotoluene (4-CT) or 1,4-dichlorobenzene with acidic chloroaluminate melts has been studied in a glass flask equipped with a mechanical stirrer and a reflux 13153

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The Journal of Physical Chemistry B condenser at ambient pressures, as well as under pressure in a glass autoclave. The detailed description of the experiment has been given elsewhere.7 In short: an acidic mixture of a chloride salt and AlCl3 (n(AlCl3) > n(chloride salt); χAlCl3 = 0.67) was heated above its melting point under gentle stirring within 5 min, and the required amount of the starting isomer was added. The resulting biphasic reaction system was stirred at 500 rpm for 24 h. At certain time intervals, about 1 mL of sample was taken from the flask and dissolved in 5 mL of dichloromethane and extracted with water. Afterward, the organic phase was separated and analyzed by gas chromatography. Sampling was continued until no further change of the compositions was observed indicating that the chemical equilibrium was established. Inorganic as well as organic chloroaluminate melts represent possible acidic catalysts for the isomerization of dichlorobenzenes (DCB) and chlorotoluenes (CT). Organic chloride salts or alkali metal chlorides form with AlCl3 the above-mentioned chloroaluminate melts. The nature of cation in the mixture determines the chemical properties of the resulting chloroaluminate melts. A systematic study of the catalytic activity of different inorganic melts AlCl3−MCl (MCl = LiCl, LiCl−NaCl, KCl−NaCl) and of AlCl3−[EMIM]Cl has been performed. The following chloroaluminate melts were prepared according to a general procedure:7,8 AlCl3/LiCl/NaCl, AlCl3/LiCl/NaCl, AlCl3/LiCl/[N(CH3)4]Cl, 2:0.6:0.4; AlCl3/[C2mim][Cl], AlCl 3 /[N(CH 3 ) 4 ]Cl, 2.0:1.0; AlCl 3 /LiCl/[N(CH 3 ) 4 ]Cl, 2:0.5:0.5, and AlCl3/LiCl/[N(CH3)4]Cl, 2:0.4:0.6. The experimental details are reported in Table 1.

Figure 3. Isomerization of p-chlorotoluene at 413 K in the presence of AlCl3/AlCl3·6H2O as a catalytic system.

concentration of m-chlorotoluene in the reaction mixture even reached a local maximum of 58% before 0.5 h (see Figure 3). However, in the course of the equilibration, the amount of this desired compound slightly decreased to the value predicted by thermodynamics, due to the concurrent formation of ochlorotoluene. After 4 h the reaction mixture reached equilibrium and contained 47 mol % of m-, 17.5 mol % of p-, and 35.5 mol % of o-chlorotoluene. These concentrations were practically constant within the next 8 h of equilibration. Experimental data for the isomerization of p-chlorotoluene by using different catalytic mixtures are presented in Table 1. Both ternary inorganic chloroaluminate melts lead to the formation of mixture with the same content of isomers as AlCl3/AlCl3·6H2O. It is important to note that the extension of reaction time up to 24 h in case of inorganic catalytic systems hardly changed the products distribution. This indicates that, after 4 h at 413 K, the composition of the reaction mixture is close to the equilibrium. In contrast, the use of catalysts with organic chloroaluminate melts leads to a significant decrease of the isomerization rates and interconversions of isomers occurs completely under kinetic control far away from the equilibrium (see Table 1). It has turned out that the catalytic activity of the systems AlCl3− [EMIM]Cl and AlCl3−[N(CH3)4]Cl is much lower compared with the inorganic catalysts. However, the activity of the monophasic system AlCl3−[EMIM]Cl is significantly higher in comparison to the biphasic AlCl3−[N(CH3)4]Cl system (which gave only 6.5 mol % of meta-chlorotoluene, see Table 1). It is likely that diffusion limitation accounts for the decrease in the reaction rate in the AlCl3−[N(CH3)4]Cl system under the applied conditions. According to the results listed in Table 1, catalysts based on AlCl3/LiCl/[N(CH3)4]Cl mixtures show significantly higher activity in the isomerization compared to organic chloroaluminates. Interestingly, when using the system AlCl3−LiCl− [N(CH3)4]Cl (χAlCl3 = 0,67) in catalysis, the activity depends on the amount of LiCl in the mixture. The higher the content of LiCl, the higher the reactivity and the degree of conversion of p-chlorotoluene (see Figure S2). An important characteristic of catalytic systems under study is the amount of o-chlorotoluene produced in the reaction mixture. The latter isomer is concurrently formed according to reaction 2 (see Figure 2). As shown in Table 1, the ternary

3. RESULTS 3.1. Isomerization of 4-Chlorotoluene with LewisAcidic Chloroalumiante Melts (System I). Equilibrium interconversions of ortho-, meta, and para-isomers of chlorotoluenes according to reactions 1 and 2 in Figure 2)

Figure 2. Equilibrium system I for the isomerization of chlorotoluenes.

were studied at 413 K. The disproportion reactions of chlorotoluenes have been out of focus of this work. For the three isomers considered in this study only two equilibrium constants are independent. For the discussion we selected reaction 1 with the equilibrium constant Kx(m/p) and the reaction 2, with the equilibrium constant Kx(o/p) (see Table 1, Figure 3). Different inorganic and organic chloroaluminate melts were used as acidic catalysts. Among these, inorganic melts and AlCl3−[N(CH3)4]Cl formed a biphasic system upon mixing with the reagents, while AlCl3−[EMIM]Cl was completely miscible with the reaction mixture. At first, it was important to find out, how fast the system I was able to reach the equilibrium. The interplay of kinetic and thermodynamic control in the system I is evident from Figure 3 showing the isomerization of p-chlorotoluene with AlCl3/AlCl3·6H2O as catalyst at 413 K. During the first hour of isomerization the system I was completely under kinetic control and the 13154

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The Journal of Physical Chemistry B catalytic compositions based on AlCl3/LiCl/[N(CH3)4][Cl] allows to utilize the kinetic control over the reaction 2 (see Figure 2) and as a consequence significantly higher concentrations (up to 69% at 413 K, see Table 1) of the desired m-chlorotoluene were achieved in the reaction mixture far away from the equilibrium. 3.2. Isomerization of p-Dichlorobenzene with Lewis Acidic Chloroaluminate Melts (System II). The isomerization of p-dichlorobenzene (Figure 4) is an atom-effective

The interplay of kinetic and thermodynamic control on system II with different catalysts is presented in Figure 5. The

Figure 4. Equilibrium system II for the isomerization of dichlorobenzenes. Figure 5. p-Dichlorobenzene isomerization−formation of m-dichlorobenzene over time applying different, acidic chloroaluminate melts at 443 K.

way to synthesize m-dichlorobenzene. Classical Friedel−Crafts catalysts are active in this reaction, but hydrolysis for product isolation leads to complete catalyst loss and large amounts of aluminum oxide and hydroxide waste (contaminated with aromatic compounds) that is difficult to dispose of. Compared to system I, the isomerization of dichlorobenzenes (system II) requires harsher conditions. We used liquid− liquid biphasic systems with (X(AlCl3) = 0.67 with 20 mol % AlCl3 in respect to the organic compound and experimental studies have been performed at 443 K. The latter was limited from one side by the boiling point of p-dichlorobenzene (447 K) and from another side by the melting point of the binary AlCl3−LiCl system (442 K), which is important for the comparison of the activity of chloroaluminate melts. Ternary systems containing chloroaluminates possess lower melting points compared to binary mixtures, which also enables to perform the isomerization process at temperatures lower than 443 K with these systems. Results of the isomerization of p-dichlorobenzene in the presence of both inorganic chloroaluminates AlCl3−LiCl (2:1), AlCl3−LiCl−NaCl (2:0.4:0.6), AlCl3−KCl−NaCl (2:0.4:0.6) and organic chloroaluminates AlCl3−[EMIM]Cl (2:1) are given in Table 2. For comparison we also collected experimental data on equilibrium compositions of dichlorobenzenes available from the literature.5,12,15

highest activity was observed for the reference catalyst (AlCl3/ AlCl3·6H2O). In this case the concentration of m-dichlorobenzene reached 21% after 2 h reaction time and further extension of the reaction time to 24 h showed no further change. Note that the organic chloroluminate system AlCl3− [EMIM]Cl is not active in this very demanding isomerization reaction (see Figure 5) which is in full agreement with earlier reports.7 The activity of inorganic chloroaluminates was dependent on the nature of the cation. The highest yield of mdichlorobenzene was reached in the presence of the AlCl3− LiCl melt. In this case, after 24 h the yield of 24% of mdichlorobenzene, was even higher, than the yield obtained with the reference system (AlCl3/AlCl3·6H2O). Other ternary systems showed lower catalytic activity as compared to the binary AlCl3−LiCl. The system AlCl3−LiCl-NaCl reached 12 mol % of m-dichlorobenzene, and the melt AlCl3−KCl-NaCl reached only 5% after 24 h reaction time (see Figure 5). It is interesting to note that despite the fact that the reference system (AlCl3/AlCl3·6H2O) showed the highest activity in the first 2 h, the reaction stopped significantly far away from the expected equilibrium ratios (see Table 2). This could be due to partial sublimation of aluminum chloride under the harsh

Table 2. System II (p-Dichlorobenzene Isomerization) Experimentally Determined Composition of Equilibrium Mixtures and Kx Values in the Liquid Phasea catalyst

T

xp

xm

x0

Kx(m/p)

Kx(o/p)

AlCl3 [12] AlCl3 [12] AlCl3 [12] AlCl3 [12] AlCl3·6H2O [5] AlCl3·H2O [15] AlCl3/LiCl/HCl, 4 h AlCl3/LiCl/NaCl, 4 h AlCl3/LiCl/NaCl/HCl, 4 h AlCl3/[EMIM]Cl, 24 h

393 433 433 453 473 513 443 443 443 443

33.6 25.7 30.0 30.6 32.6 37.0 34.0 49.0 37.0 90.0

55.1 45.2 54.0 57.2 59.5 42.0 60.0 49.0 58.0 5.0

11.3 29.1 16.0 12.2 7.9 21.0 6.0 2.0 4.0 5.0

1.6 1.8 1.8 1.9 1.8 (1.1) 1.8 (1.0) (1.6) −

0.28 − − 0.39 (0.24) 0.56 (0.18) (0.04) (0.11) −

a

T is temperature of investigation in K; xi is the mole fraction measured chromatographically. Values in brackets are not at equilibrium either for reaction 3 or for reaction 4. 13155

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Table 3. Thermodynamic Functions ΔrHm° and ΔrSm° of Reactions 1−4 in the Liquid Phase and Temperature Dependences ln Kx = a + b × (T/K)−1 reaction

Ta (K)

R1

a

b

373−413

0.6

144

R2

373−413

−0.6

538

R3

393−453

1.8

−535

R4

393−513

1.6

−1156

ΔrHm° b (kJ mol−1) −1.2 −1.4 −1.7 −4.5 −3.9 −4.0 4.5 4.3 −0.1 9.6 7.5 4.1

± ± ± ± ± ± ± ± ± ± ± ±

1.0 2.0c 3.5d 1.0 2.0c 3.5d 0.4 1.8c 3.5d 1.0 1.8c 3.5d

ΔrSm° b (J mol−1 K−1)

Kx/298 K

Kx/443 K

5.0 ± 3.5

3.0

2.5

−5.0 ± 3.5

3.3

1.8

15.2 ± 1.0

1.0

1.8

13.7 ± 2.2

0.1

0.4

Temperature range of the equilibrium study. bThe values of the enthalpies ΔrH°m and entropies ΔrS°m of reactions 1−4 were derived for the average temperatures of the experimental intervals given in the column 2. It was assumed that the enthalpies of the reaction hardly change on passing from the average temperature of the experimental range to T = 298 K. cCalculated from experimental enthalpies of formation (see Table S1) of reaction participants; dCalculated with eq 2.

a

The First Law Method (Combustion Enthalpies Measurements). The standard molar enthalpies of formation Δf Hm° of organic compounds in the liquid or crystalline state are usually derived from combustion or solution calorimetry. Enthalpies of formation of chloro-substituted benzenes (relevant to the present work) have been evaluated just recently16−18 and they are collected in Table S1 (see Supporting Information). Thus, having the evaluated enthalpies of formation Δf H°m(liq) of all participants of reactions 1−4 at hand, the reaction enthalpies, ΔrHm° , can be independently derived according to Hess’s law and used for an additional prove of the ΔrHm° , obtained from the temperature dependences of equilibrium constants of the reactions 1−4 according to the the second law method. 4.2. Liquid Phase: Equilibrium Constants and Reaction Enthalpies. The experimental results of the chemical equilibria studies of systems I and II are listed in Tables 1 and 2. The experimental values of Kx for all reactions considered in this work are only slightly changing with the temperature. Experimental values of Kx for reactions 1−4 were approximated as a linear function of temperature by the equation ln Kx = a + b × (T/K)−1 using the method of least-squares. The slopes of these lines when multiplied by the gas constant afford the standard enthalpies of these reactions, ΔrH°m. The intercept of these lines when multiplied by the gas constant afford standard entropies of these reactions ΔrSm° . Numerical results are presented in Table 3. The errors in the thermodynamic functions from equilibrium studies are given by the standard deviations for the meaningful level 0.05. The reaction enthalpies of positional isomerization on the benzene ring usually reflect the relative intensity of substituents interactions. As can be seen from Table 3 reaction enthalpies in the system I are slightly exothermic. The enthalpy of reaction 2 is responsible for the sterically demanding para ↔ortho interconversion, that is why the enthalpy of this reaction is obviously larger than those for the reaction 1, where para ↔ meta isomerization do not exhibit any noticeable interactions of CH3 and Cl-substituents at the benzene ring. The reaction enthalpies in system II are slightly endothermic. Also, the enthalpy of reaction 4 accounting for the para ↔ ortho interconversion is noticeably larger than that for reaction 3, the para ↔ meta isomerization, where the less intense interaction

reaction conditions. Moreover, the formation of gaseous HCl during the equilibration was observed. Apparently both factors reduce the catalytic activity of the reference system already after 2 h. Compositions of isomers obtained in the presence of different binary and ternary melts are collected in Table 2. As can be seen from this table after 4 h of equilibration the catalytic systems under study have been able to reach distribution of isomers close to the equilibrium with the good yield of the desired m-dichlorobenzene at the level of 49−60 mol %.

4. DISCUSSION 4.1. General Aspects. Standard molar enthalpies of chemical reactions, ΔrH°m, can be determined experimentally from the temperature dependence of chemical equilibrium constants (in accordance with the second law of thermodynamics) or by means of calorimetric measurements of the standard molar enthalpies of formation of all reaction participants (with the help of Hess’s law or in accordance with the first law of thermodynamics). The Second Law Method (Equilibrium Constant Measurements). For a general chemical reaction in the liquid phase, the true thermodynamic equilibrium constant, Ka, is defined as the ratio of the activities ai of the products and the reagents under equilibrium conditions. From our previous experiences, activity coefficients of isomers are very close to unity and for this reason we can use for the current equilibrium study an assumption that Ka = Kx, where the equilibrium constant, Kx, is calculated from the experimental mole fractions of reaction 1−4 participants as they are given in Tables 1 and 2. From the Van’t Hoff relation d[ln Kx]/dT = Δr Hm°/RT 2

(1)

the slope of a plot (ln Kx) vs 1/T gives the standard enthalpy of reaction Δ rH m° if the temperature dependence of the equilibrium constant Kx is known. This procedure, involving study of the changes of equilibrium constants with temperature, is referred to as second law method. Hence, reaction enthalpies, ΔrH°m, for reactions 1−4 have been calculated from the temperature dependence of the equilibrium ratios Kx (see Tables 1 and 2). 13156

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Table 4. G3MP2 Calculated Thermodynamic Functions, ΔrGm° , ΔrHm° , and ΔrSm° and Thermodynamic Equilibrium Constants of Reactions 1−4 in the Gas Phase at 298 K

a

reaction

ΔrGm° (kJ mol−1)

ΔrHm° (kJ mol−1)

ΔrSm° (J mol−1 K−1)

KP

Kx(theor)a

Kx(exp)b

R1 R2 R3 R4

−0.67 −1.72 −0.36 5.69

−1.07 −4.07 −0.38 5.26

−1.36 −7.99 −0.09 −1.42

1.31 1.99 1.15 0.10

1.33 1.86 1.07 0.13

3.0 3.3 1.0 0.1

Calculated by eq 3. bCalculated by equation ln Kx = a + b × (T/K)−1 with coefficients given in Table 3.

established and they are available in the literature16−18 (see Table S1 in the Supporting Information). Enthalpies of reactions ΔrH°m(g) required for eq 2 were calculated (see Tables 4) directly from H298 enthalpies of the reaction participants using the G3MP2. The resulting enthalpies of reactions 1 −4, ΔrH°m(liq) in the liquid phase are given for comparison with the experimental values in Table 3. The calculated values of the ΔrHm° (liq) for reactions 1 and 2 are in good agreement with those derived from the chemical equilibrium studies. Agreement for reactions 3 and 4 is acceptable within the combined boundaries of uncertainties. As a consequence, the energetics of isomerization reactions in the liquid phase can be reliably assessed from quantumchemical calculations with help of empirical data for the vaporization enthalpies. The thermodynamic equilibrium constant, kp, calculated for the gas phase reactions 1−4 is related to the thermodynamic constant Kx of these reactions in the liquid phase by eq 3:

of two Cl-substituents on the benzene ring are similar to metainteractions of CH3 and Cl-substituents. Analysis of equilibrium constants for reactions 1−4 reported in the literature (see Table 1 and 2) have revealed that the position of equilibrium was not always easy to approach starting from the individual para-, meta-, or ortho-isomer. In many cases the true position of equilibrium was not reached even after hundreds hours of equilibration. This fact has heavily aggravated the analysis of the available experimental distribution of isomers. For this reason the independent validation of reaction enthalpies, ΔrH°m, obtained from temperature dependences of equilibrium constants of reactions 1−4 according to the second law method was virtually required. Enthalpies of reactions 1−4 independently derived according to Hess’s law using the liquid-phase enthalpies of formation, Δf H°m(liq), of chlorotoluenes and dichlorobenzenes from Table S1 are in agreement to those from the second law method within the boundaries of experimental uncertainties (see Table 3). This validates the selection of equilibrium constants taken for the thermodynamic analysis of the interconversions of para-, meta-, or ortho-isomers. Coefficients of temperature dependences ln Kx = a + b × (T/K)−1 given in Table 3 can be used for calculation of the benchmark equilibrium constants Kx at any temperature, which are important as indicators of the catalytic activity of the liquid−liquid biphasic reaction system. 4.3. Gas Phase: Reaction Enthalpies and Equilibrium Constants from Quantum Chemical Calculations. In the recent work by some of us,9,10,13,14 we established a remarkable ability of high-level quantum-chemical methods to predict gaseous thermodynamic functions of organic compounds accurately. Admittedly, the chemical reactions of isomerization and transalkylation are thermodynamically controlled, thus, the desired yields of goal products can be predicted by the quantum-chemical calculations. Having established in this work the reliable experimental equilibrium constants for the para ↔ meta ↔ ortho interconversions of chloro-substituted benzenes, it was reasonable to test the ability of quantum chemistry for the “in silico” assessment of these equilibrium constants. In order to validate this procedure, the following thermodynamic functions were calculated for the gas phase reactions 1−4 using the G3MP2 method at 298 K: the Gibbs energy ΔrGm° , the reaction enthalpy, ΔrH°m, and the reaction entropy, ΔrS°m. Calculated values are compiled in Table 4. Results of the quantum-chemical calculations are referred to the gas phase. The standard molar reaction enthalpies ΔrHm° (g) in the gas phase are related to reaction enthalpies, ΔrH°m(liq), in the liquid state by the equation: Δr Hm◦(liq) = Δr Hm◦(g) −

∑ νiΔlg Hm◦ ,i i

Kx = KP ×

Ppara ‐ isomer,0 Pmeta ‐ isomer,0

or Kx = KP ×

Ppara ‐ isomer,0 Portho ‐ isomer,0

(3)

where Pi,0 are the saturated vapor pressures of the pure components. Saturated vapor pressures for chlorobenzenes were available from the literature15−17 (see Table 5 and Tables S2 and S3 in the Supporting Information). Using the values of KP for the gaseous reactions 1−4, calculated with help of G3MP2 and presented in Table 4, the theoretical thermodynamic constants Kx(theor) in the liquid phase were calculated (see Table 4). Theoretical ratios Kx for the reactions 1−4) (calculated using eq 3 given in Table 4) are compared there with the experimental values Kx at 298 K. Theoretical values of Kx calculated at the G3MP2 level are in acceptable agreement with the experimental values. Thus, the procedure developed in this work could be practically applied for the assessment of yields of isomerization reactions similar to reactions 1−4) studied in this work. Following such a procedure could be recommended in general for a broad range of reactions, such as alkylation and interconversion of substituted aromatic compounds. 4.4. Standard Molar Thermodynamic Functions of Halogen-Substituted Benzenes. The basic thermodynamic equation for the Gibbs free energy Δf Gm◦ = Δf Hm◦ − T × Δf Sm◦

(4)

where ΔfH°m and ΔfS°m are the enthalpy and the entropy of formation, is responsible for the assessment of feasibility of any chemical process. Recently, some of us have shown19 that the gas-phase standard molar entropies, Sm° (g, 298 K), of different organic molecules can be calculated with an acceptable accuracy. In this work, we calculated S°m(g, 298 K)-values for isomeric chlorotoluenes and dichlorobenzenes by the G3MP2 method, optimizations and frequency calculations were

(2)

Δg1H°m,i

where are the molar enthalpies of vaporization of the pure compounds i at the reference temperature 298 K. The latter values for compounds involved in reactions 1−4 are well 13157

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From treatment of vapor pressures temperature dependencies (see Supporting Information). bFrom Table 6. cCalculated from Sm° (g) and entropy of vaporization Δg1Sm° (298 K) given in this table.

performed at the HF/6-31G(d) level of theory. These values for dichlorobenzenes are in good agreement with values derived from statistical thermodynamics (ST).20 Having such a good agreement the liquid-phase entropies S°m(liq, 298 K) for chloro-substituted benzenes were calculated by subtracting the molar entropy of vaporization Δg1S°m (see Table 5) from the entropy in the gas state Sm° (g, 298 K) given in Table 6, column 2. The molar entropy of vaporization Δg1Sm° Table 6. Comparison of the S°m(g, 298 K) Values from Quantum Chemical Calculations (G3MP2) and from Statistical Thermodynamics (ST) in J·K−1·mol−1 compounds

S°m(g)(G3MP2)

S°m(g)(ST)

2-chlorotoluene 3-chlorotoluene 4-chlorotoluene 1,2-dichlorobenzene 1,3-dichlorobenzene 1,4-dichlorobenzene

361.5a 368.1 369.5 344.3 345.7 345.8

341.5 343.5 340.4b

The value is calculated as the sum S°m(g, 298 K)G3MP2 = 346.6 J·K−1· mol−1 and the contribution R × ln σ for the symmetry number (σ = 6); bThe value is calculated from the experimental S°m(cr, 298 K) = 174.0 J·K−1·mol−1 22 and the sublimation entropy ΔgcrSm° = 166.4 J·K−1· mol−1 derived from the vapor pressure measurements24 between 273.5 and 303.5 K (see Table S3). a

values were derived (see Table 7) from the experimental vapor pressures available from the literature (see Tables S2 and S3). The standard molar entropies of formation, ΔfS°m, in the gas and in the liquid phase were calculated on the basis of eq 5: aCgraphite + (b/2)H 2(g) + (c /2)Cl 2(g) = Ca HbClc (g or liq or cr)

(5)

using the S°m values given in Table 7 and the values of entropy of formation for Cgraphite (5.74 ± 0.13) J·K−1·mol−1, for H2(g) (130.52 ± 0.02) J·K−1·mol−1, and for Cl2(g) (223.081 ± 0.010) J·K−1·mol−1 recommended by Chase.21 Values of the standard Gibbs energies of formation, ΔfG°m, were estimated by eq 4 from the values of ΔfH°m and ΔfS°m (see Table 7). The standard molar thermodynamic functions in the liquid and in the gas phase collected in Table 7 can be used for optimizing technologies for the manufacturing of halogensubstituted benzenes as well as for design and implementation of novel biphasic reaction systems.

5. CONCLUSIONS In conclusion, we have studied thermodynamics and kinetics of the isomerization of chlorotoluenes and dichlorobenzenes. Our experiments using highly acidic ionic liquid and molten salt catalyst systems provided temperature-dependent equilibrium constants in the liquid phase from which thre reaction enthalphies of four isomerization processes were derived. We could demonstrate that thermodynamic functions derived from quantum-chemical calculation matched the experimental values within the boundaries of uncertainty. These will allow one in the future to optimize isomerization processes of aromatic compounds, for example in the liquid−liquid biphasic reaction mode, by the help of quantum chemical calculations.

a

250.6 256.3 259.0 232.3 236.6 − 361.5 368.1 369.5 344.3 345.7 340.4 1.3 1.7 1.1 0.2 0.3 0.6 ± ± ± ± ± ± 110.9 111.8 110.5 112.0 109.1 166.4 0.4[15] 0.5[15] 0.5[15] 0.5[16] 0.5[16] 0.3[16] ± ± ± ± ± ± 46.4 46.9 46.5 48.8 47.3 66.0 451 412 421 188 264 245 274.2−306.2 274.2−309.2 274.3−309.2 263.2−373.2 278.3−311.3 273.5−303.5 2-chloro-methylbenzene (liq) 3-chloro-methylbenzene (liq) 4-chloro-methylbenzene (liq) 1,2-dichlorobenzene (liq) 1,3-dichlorobenzene (liq) 1,4-dichlorobenzene(cr)

Δg1H°m(exp) (kJ·mol−1) p0(exp) (Pa) T-range(exp) (K) compound

Table 5. Thermodynamic Properties of Halogen-Substituted Benzenes at T = 298 K

Δg1S°m(exp)a (J·K−1·mol−1)

S°m(g)(G3MP2)b (J·K−1·mol−1)

S°m(liq)(est)c (J·K−1·mol−1)

The Journal of Physical Chemistry B

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The Journal of Physical Chemistry B Table 7. Experimental Standard Molar Thermodynamic Properties of Chloro-Substituted Benzenes at T = 298 K compound

state

ΔfHm° (kJ·mol−1)

ΔfSm° a (J·K−1·mol−1)

ΔfGm° b (kJ·mol−1)

Sm° c (J·K−1·mol−1)

Cp,m ° d (J·K−1·mol−1)

2-chloromethylbenzene

liq gas liq gas liq gas liq gas liq gas cr liq gas

−33.0 [15] 13.1 [15] −30.5 [15] 16.3 [15] −29.1[15] 17.1[15] −17.4 ± 1.3[16] 31.4 ± 1.4[16] −20.5 ± 1.0[16] 26.8 ± 1.1[16] −42.3 ± 1.0[16] −25.1 ± 1.1[16] 22.5 ± 1.2[16]

−357.9 −247.0 −352.2 −240.4 −349.5 −239.0 −286.3 −174.3 −282.0 −172.9 −344.6 −288.8 −178.2

73.7 86.8 74.5 88.0 75.1 88.4 67.9 83.4 63.6 78.3 60.4 61.0 75.6

250.6 361.5 256.3 368.1 259.0 369.5 232.3 344.3 236.6 345.7 174.0 [21] 229.8e 340.4

178.1 121.2 171.7 116.5 176.6 120.1 171.3 111.9 170.6 112.3 143.8 [21] 179.4 112.3

3-chloromethylbenzene 4-chloromethylbenzene 1,2-dichlorobenzene 1,3-dichlorobenzene 1,4-dichlorobenzene

Calculated by eq 5. bCalculated by eq 4. cFrom Tables 5 and 6. dFrom Table S4. eThe value is calculated from the experimental S°m(cr, 298 K) = 174.0 J·K−1·mol−1 22 and the entropy of fusion Δ1crSm° = 55.8 J·K−1·mol−1.23 a

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(6) Norris, J. F.; Turner, H. S. The Rearrangement of Certain Derivatives of Toluene by the Action of Aluminum Chloride. J. Am. Chem. Soc. 1939, 61, 2128−2131. (7) Messner, J.; Schulz, P. S.; Taccardi, N.; Kuhlmann, S.; Wasserscheid, P. Isomerisation of 1,4-Dichlorobenzene using Highly Acidic Alkali Chloroaluminate Melts. Chem. Commun. 2014, 50, 11705−11708. (8) Kuhlmann, S.; Böger, U.; Wasserscheid, P.; Schlenk, S.; Messner, J.; Weber, H.-M. WO 2013/189848, Lanxess Deutschland GmbH (Chem. Abstr., 2013, 993608). (9) Verevkin, S. P.; Emel'yanenko, V. N.; Toktonov, A. V.; Goodrich, P. C.; Hardacre, C. Thermochemistry of Ionic Liquid Catalyzed Reactions. Experimental and Theoretical Study of Chemical Equilibria of Isomerization and Transalkylation of tert-Amylbenzenes. J. Phys. Chem. B 2009, 113, 12704−12710. (10) Verevkin, S. P.; Emel’yanenko, V. N.; Hopmann, E.; Arlt, W. Thermochemistry of Ionic Liquid-Catalysed Reactions. Isomerisation and Transalkylation of tert-Alkyl-Benzenes. Are These Systems Ideal? J. Chem. Thermodyn. 2010, 42, 719−725. (11) Olah, G. A.; Meyer, M. W. Friedel-Crafts Isomerization. IV. Aluminum Halide-Catalyzed Isomerization of Halotoluenes. J. Org. Chem. 1962, 27, 3464−3469. (12) Spryskov, A. A.; Erykalov, Yu. G. Orientation by Substitution in Aromatic Row. V. Isomerisation of Dichlorobenzenes. Zh. Org. Khim. 2009, 29, 2795−2803. (13) Verevkin, S. P.; Emel'yanenko, V. N.; Kozlova, S. A.; Smirnova, I.; Arlt, W. Experimental and Theoretical Study of Chemical Equilibria in the Reacting System of the di-Alkyl Carbonate Synthesis. Ind. Eng. Chem. Res. 2011, 50, 9774−9780. (14) Verevkin, S. P.; Emel'yanenko, V. N.; Toktonov, A. V.; Goodrich, P.; Hardacre, C. Thermochemistry of Ionic LiquidCatalysed Reactions. Theoretical and Experimental Study of the Beckmann Rearrangement − Kinetic or Thermodynamic Control? Ind. Eng. Chem. Res. 2009, 48, 9809−9816. (15) Olah, G. A.; Tolgyesi, W. S.; Dear, R. E. A. Friedel-Crafts Isomerization. I. Effect of Promoted Aluminum Halides on Halobenzenes. J. Org. Chem. 1962, 27, 3441−3449. (16) Verevkin, S. P.; Sazonova, A. Yu.; Emel'yanenko, V. N.; Zaitsau, D. H.; Varfolomeev, M. A.; Solomonov, B. N.; Zherikova, K. V. Thermochemistry of Halogen-Substituted Methylbenzenes. J. Chem. Eng. Data 2015, 60, 89−103. (17) Verevkin, S. P.; Melkhanova, S. V.; Emel'yanenko, V. N.; Varfolomeev, M. A.; Solomonov, B. N.; Zherikova, K. V. Thermochemistry of Dihalogen-Substituted Benzenes: Data Evaluation Using Experimental and Quantum Chemical Methods. J. Phys. Chem. B 2014, 118, 14479−14492. (18) Verevkin, S. P.; Emel'yanenko, V. N.; Varfolomeev, M. A.; Solomonov, B. N.; Zherikova, K. V. Enthalpies of Vaporization of a

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b10529. Isomerization of p-chlorotoluene, comparison of conversion of p-chlorotoluene, thermochemical data for halogenobenzenes, methods of calculations, vapor pressures, and compilation of data on molar heat capacities and heat capacity differences (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(S.P.V.) E-mail: [email protected]. Telephone: +49-381-498-6508. Fax: +49-381-498-6502. ORCID

Sergey P. Verevkin: 0000-0002-0957-5594 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the German Science Foundation (DFG) in the framework of Priority Program SPP 1708 “Material Synthesis Near Room Temperature”. This work has been partly supported by the Russian Government Program of Competitive Growth of Kazan Federal University and Russian Foundation for Basic Research No. 15-03-07475.

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REFERENCES

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