Ind. Eng. Chem. Res. 2009, 48, 9809–9816
9809
Thermochemistry of Ionic Liquid-Catalyzed Reactions: Theoretical and Experimental Study of the Beckmann RearrangementsKinetic or Thermodynamic Control?† Sergey P. Verevkin,* Vladimir N. Emel’yanenko, and Alexey V. Toktonov Department of Physical Chemistry, UniVersity of Rostock, Hermannstr. 14, 18051 Rostock, Germany
Peter Goodrich and Christopher Hardacre QUILL Research Centre, School of Chemistry and Chemical Engineering, Queen’s UniVersity, Belfast, U.K.
A thermodynamic analysis of the experimental conditions of the Beckmann rearrangement reaction of oximes into amides has been undertaken to examine whether the reaction is under thermodynamic or kinetic control. To answer this question, the thermodynamic properties of the typical Beckmann rearrangement reactions in the ideal gaseous statescyclohexanone oxime to caprolactam and 2-butanone oxime to N-methylpropanamideswere studied by using the quantum mechanical method. Gibbs energy and equilibrium constants of the Beckmann rearrangement have been assessed in the gaseous and the liquid phases. Results of the thermodynamic analysis have shown that Beckmann rearrangements are kinetically controlled. Thus, a search for possible active ionic liquid based catalysts for the mild reaction conditions has been performed. 1. Introduction The Beckmann rearrangement is commonly used in organic chemistry to transform ketoximes into amides often requiring high temperature and a strong acidic catalyst.1 This reaction has been an important subject for improvement, particularly with respect to commercial production of caprolactam in which concentrated sulfuric acid is employed and a large amount of ammonium sulfate is produced as a byproduct. Besides the high commercial utility for the production of caprolactam, the reaction is also of academic interest.2-4 Great efforts have been put into the development of more economical processes. For example, catalytic Beckmann rearrangements were found to be efficiently progressed in the gaseous phase,4-6 but in some cases, low selectivity and catalyst deactivation remained unresolved.6 The use of room temperature ionic liquids as environmentally benign media for catalytic processes is widely recognized7,8 and has been successfully employed at moderate temperatures in the Beckmann rearrangement of cyclohexanone oxime to caprolactam.9-12 In this context, the applicability of Lewis acidic and Brønsted acidic ionic liquids as catalysts to improve the production of caprolactam, especially regarding the conversion of cyclohexanone oxime to caprolactam, has been examined. Previously we studied the thermodynamic properties of caprolactam13 and cyclohexanone oxime14 in the condensed and in the gaseous state. In the present work we explore the thermochemistry of Beckmann rearrangement of cyclohexanone oxime to caprolactam and their catalytic conversion in the presence of a diverse range of ionic liquids (ILs). However, one of the decisive questions for the Beckmann rearrangement is whether the process is controlled kinetically or thermodynamically? In order to answer this question, the thermodynamic properties of the typical Beckmann rearrangements (outlined in reactions R1 and R2) in the ideal gaseous state * To whom correspondence should be addressed. Tel.: +49-381498-6508. Fax: +49-381-498-6524. E-mail: sergey.verevkin@ uni-rostock.de. † This paper is dedicated to Prof. Dr. Gennady J. Kabo (Belorussian State University, Minsk, Belarus) on the occasion of his 70th birthday.
were calculated by using the quantum mechanical method. Gibbs energy and equilibrium constants of the Beckmann rearrangement have been assessed in the gaseous and the liquid phases. Results of the thermodynamic analysis have shown that Beckmann rearrangements are thermodynamically favored. Thus, search for possible active catalysts for the mild reaction conditions has been performed. 2. Experimental Section 2.1. Materials. Cyclohexanone oxime [CAS 100-64-1], ε-caprolactam [CAS 105-60-2], and N-methyl-propanamide [CAS 1187-58-2] (Fluka and Aldrich) had a mass-fraction purity of about 0.99 and were used without further purification. 2-Butanone oxime [CAS 96-29-7] (Aldrich) was purified by repeated distillation at reduced pressure. The degree of purity was determined using a Hewlett-Packard gas chromatograph (GC) 5890 Series II equipped with a flame ionization detector and a Hewlett-Packard 3390A integrator. The carrier gas (nitrogen) flow was 7.2 dm3 · h-1. A capillary column HP-5 (stationary phase cross-linked 5% PH ME silicone) was used with a column length of 30 m, an inside diameter of 0.32 mm, and a film thickness of 0.25 µm. The standard temperature program of the GC was T ) 323 K for 180 s followed by a heating rate of 10 K · min-1 to T ) 523 K. No impurities (e0.01 mass %) could be detected by GC in the combustion sample of 2-butanone oxime. The exact amount of residual water in the sample of 2-butanone oxime was measured by using Karl Fisher
10.1021/ie900633r CCC: $40.75 2009 American Chemical Society Published on Web 09/18/2009
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titration and the appropriate correction to the mass of sample has been performed. Chloroaluminate ionic liquids were prepared according to a reported general procedure.15 The catalyst anhydrous AlCl3 was slowly added to an 1,3-dialkyl imidazolium based ionic liquid. The mixture was stirred at 426 K until the catalyst completely dissolved. Brønsted-acidic ionic liquids were prepared according to a reported general procedure (see the Supporting Information). 2.2. Measurements of the Vapor Pressures Using the Transpiration Method. Vapor pressures of 2-butanone oxime and N-Me-propanamide were determined using the method of transpiration in a saturated nitrogen stream.16,17 Appoximately 0.5 g of the sample was mixed with glass beads and placed in a thermostatted U-shaped tube having a length of 20 cm and a diameter of 0.5 cm. Glass beads with diameter of 1 mm provide a surface which was sufficient for the vapor-liquid equilibration. At constant temperature ((0.1 K), a nitrogen stream was passed through the U-tube and the transported amount of gaseous material was collected in a cooling trap. The flow rate of the nitrogen stream was optimized in order to reach the saturation equilibrium of the transporting gas at each temperature under study. The mass of compound collected after a certain time was determined by dissolving it in a suitable solvent containing an internal standard (hydrocarbon). This solution was then analyzed using gas chromatography. The uncertainty in the sample quantification determined by GC analysis was found to be within 1-3%. The saturation vapor pressure pisat at each temperature T was calculated from the amount of product collected within a definite period of time. Assuming that Dalton’s law of partial pressures applied to the nitrogen stream saturated with the substance i of interest is valid, values of pisat were calculated using (1): psat i ) miRTa /VMi; V ) VN2 + Vi; (VN2 . Vi)
(1)
where R ) 8.314472 J · K-1 · mol-1; mi is the mass of transported compound, Mi is the molar mass of the compound, and Vi its volume contribution to the gaseous phase. VN2 is the volume of transporting gas, and Ta is the temperature of the soap bubble meter. The volume of transporting gas VN2 was determined from the flow rate and time measurements. Data of psat i have been obtained as a function of temperature and were fitted using the following equation:16 R ln psat i
()
T b ) a + + ∆gl Cp,m ln T T0
(2)
where a and b are adjustable parameters and ∆gl Cp,m is the difference of the molar heat capacities of the gaseous and the liquid phase, respectively. T0 appearing in eq 2 is an arbitrarily chosen reference temperature (298.15 K). Consequently, from eq 2, the expression for the vaporization enthalpy at temperature T is derived using eq 3: ∆gl Hm ) -b + ∆gl Cp,mT
(3)
Values of ∆gl Cp,m have been calculated according to a procedure developed by Chickos.18 Experimental results with parameters a and b are listed in Table 1. We have checked experimental and calculation procedures with measurements of vapor pressures of n-alcohols.16 From this data, it was found that the vapor pressures derived from the transpiration method were reliable within 1-3% and their accuracy was governed by the reproducibility of the GC analysis. In order to assess the uncertainty of the vaporization enthalpy, the experimental data were ap-
Table 1. Vapor Pressures p and Enthalpies of Vaporization, ∆lgHm, Obtained by the Transpiration Method T/Ka m/mgb VN2/dm3c
flow/ (dm3 · h-1)
p/Pad
(pexp - pcalc)/Pa
∆lgHm/ (kJ · mol-1)
2-butanone oxime; ∆gl Hm(298.15 K) ) 59.05 ( 0.20 kJ · mol-1 ln(p/Pa) ) (304.28/R) - (78193.23/(R(T/K)) - (64.2/R)ln((T/K)/298.15) 283.4 4.95 285.9 5.49 288.3 5.15 290.7 5.05 293.4 5.26 295.9 5.22 298.3 5.39 300.7 5.10 303.3 6.37 305.8 5.40 308.4 6.06 310.9 5.68 313.4 5.32 315.9 4.46 318.3 7.24 318.6 5.66 321.4 6.84 323.5 8.12 325.4 9.10 327.4 8.96 329.3 10.21
3.18 2.83 2.08 1.68 1.39 1.16 0.946 0.762 0.757 0.553 0.499 0.395 0.302 0.224 0.295 0.231 0.231 0.227 0.231 0.199 0.200
3.47 3.47 3.47 3.05 2.25 3.47 2.27 3.05 2.27 2.01 1.76 1.58 0.91 0.90 0.89 0.92 0.92 0.91 0.92 0.91 0.92
44.51 55.38 70.44 85.23 107.8 127.9 159.2 189.5 234.1 277.2 344.6 406.3 498.9 563.7 691.7 694.3 855.9 1012.4 1140.4 1269.5 1444.2
-0.2 -0.4 1.7 0.8 1.9 -2.2 1.3 -1.3 0.6 -5.2 1.8 -5.4 6.3 -23.8 -2.0 -13.8 -0.1 28.1 25.4 0.7 12.0
60.00 59.84 59.69 59.53 59.36 59.20 59.05 58.89 58.72 58.56 58.40 58.24 58.08 57.92 57.76 57.74 57.56 57.43 57.31 57.18 57.06
N-methyl-propanamide; ∆gl Hm(298.15 K) ) 66.56 ( 0.19 kJ · mol-1 ln(p/Pa) ) (296.10/R) - (83583.26/(R(T/K))) - (57.1/R)ln((T/K)/298.15) 307.2 1.87 309.7 1.78 312.2 2.27 317.3 1.91 322.3 1.79 325.7 1.54 327.2 2.10 329.7 1.86 332.2 1.52 334.8 6.87 337.3 1.70 342.2 3.44 344.7 6.78 347.2 6.40 349.7 6.10 352.2 6.17 357.1 6.19 362.3 6.13 364.7 7.03 367.3 8.28 371.2 10.26
3.69 2.88 2.85 1.65 1.05 0.713 0.876 0.635 0.441 1.66 0.357 0.519 0.810 0.661 0.561 0.472 0.360 0.255 0.255 0.255 0.255
7.64 7.52 7.78 6.61 4.19 2.85 3.50 2.54 1.76 1.72 1.43 2.40 3.04 1.89 2.24 1.89 1.44 1.02 1.02 1.02 1.02
14.37 17.74 23.33 33.57 49.35 61.80 69.27 84.06 99.17 118.9 137.4 191.2 239.1 277.5 311.3 374.0 492.9 680.9 785.9 924.0 1150.7
-0.2 -0.2 1.3 0.5 0.8 -0.8 -0.6 0.3 -0.8 -0.9 -4.9 -6.4 6.5 4.4 -8.4 0.7 -9.5 -0.6 4.1 19.0 28.5
66.04 65.90 65.76 65.47 65.18 64.99 64.90 64.76 64.62 64.47 64.33 64.05 63.90 63.76 63.62 63.48 63.20 62.90 62.76 62.61 62.39
a Temperature of saturation. b Mass of transferred sample, condensed at T ) 243 K. c Volume of nitrogen, used to transfer mass m of sample. d Vapor pressure at temperature T, calculated from m and the residual vapor pressure at the cooling temperature T ) 243 K.
-1 proximated with the linear equation ln(psat i ) ) f(T ) using the method of least-squares. The uncertainty in the enthalpy of vaporization was assumed to be identical with deviation of experimental ln(pisat) values from this linear correlation. The experimental results are presented in the Table 1. 2.3. Combustion Calorimetry. A homemade isoperibol bomb calorimeter and a commercial combustion calorimeter Parr-1600 (Parr Instrument Deutschland GmbH) were used for the measurement of the energy of combustion of 2-butanone oxime. From a practical perspective, careful encapsulation of a sample is important in combustion calorimetry of liquids. In the present study, commercially available polyethene bulbs (NeoLab, Heidelberg) of 1 cm3 volume were used in order to reduce the capillary effect and make encapsulation easier. Liquid specimens were transferred to polyethene bulbs with a syringe. The narrow neck of the container was compressed with special tweezers and was sealed by heating with hot air. Then, the loaded container was placed in the bomb and burned in oxygen
Ind. Eng. Chem. Res., Vol. 48, No. 22, 2009
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Table 2. Results for Typical Combustion Experiments at T ) 298 K (p° ) 0.1 MPa) of the 2-Butanone Oximea m(substance)/gb m′(cotton)/gb m′′(polyethene)/gb ∆Tc/Kc (εcalor)(-∆Tc)/J (εcont)(-∆Tc)/J ∆Udecomp HNO3/J ∆Ucorr/Jd -m′∆cu′/J -m′′∆cu′′/J ∆cu°(liq)/(J · g-1)e
0.313627 0.003436 0.2823 1.55155 -22966.9 -28.52 61.52 6.52 58.22 13087.71 -31188.2
0.312235 0.003682 0.276796 1.53174 -22673.8 -28.15 59.73 6.43 62.39 12832.54 -31197.1
0.310214 0.003604 0.277683 1.53004 -22648.6 -28.08 59.62 6.45 62.41 12873.66 -31186.3
0.328539 0.003214 0.276697 1.56542 -23172.3 -28.82 65.7 6.55 54.46 12827.95 -31187.9
a For the definition of the symbols, see ref 21. Th ) 298.15 K; V(bomb) ) 0.32 dm3; pi(gas) ) 3.04 MPa; mi(H2O) ) 1.00 g. b Masses obtained from apparent masses. c ∆Tc ) Tf - Ti + ∆Tcorr; (εcont)(-∆Tc) i f ) (εcont )(Ti - 298.15 K) + (εcont )(298.15 K - Tf + ∆Tcorr.). d ∆Ucorr, the correction to standard states, is the sum of items 81-85, 87-90, 93, and 94 in ref 21. e ε ) 14802.6 ( 1.0 J · K-1, water content 1045 ppm.
at a pressure of 3.04 MPa. Results from combustion experiments are given in Table 2. The detailed procedure has been previously described.19 Combustion products were examined for carbon monoxide (Dra¨ger tube) and unburned carbon, but none were detected. The energy equivalent of the calorimeter εcalor was determined with a standard reference sample of benzoic acid (sample SRM 39i, NIST). The correction for nitric acid formation was based on the titration with 0.1 mol · dm-3 NaOH (aq). The atomic weights used were those recommended by the IUPAC Commission.20 The sample masses were reduced to vacuum, taking into consideration their density values (see Table S1 of the Supporting Information). To convert the energy of the actual bomb process to that of the isothermal process and reducing to standard states, the conventional procedure was applied.21 2.4. Study of the Beckmann Rearrangement in the Liquid Phase. Glass vials with screwed caps and small magnetic stirrers were filled with the initial liquid mixture of solvent (0.5 cm3) with cyclohexanone oxime (0.1 mmol) followed by the addition of catalyst (∼10 mass % with respect to oxime). The vial was then heated to the desired temperature and controlled to within Ti ( 0.1 K. After definite time intervals, the vial was cooled rapidly in ice. In all cases, the catalyst was not soluble in the reaction mixture and a clear phase boundary was observed. A sample (∼0.01 cm3) for the GC analysis was taken from the upper liquid phase using a syringe. The reaction mixture was then reheated to the desired reaction temperature waiting further analysis. The composition of the reaction mixtures were determined by gas chromatography. 2.5. Computations. Standard quantum chemistry calculations were performed with the Gaussian 03 rev 04 series of programs.22 Energies were obtained at the G3MP2 and B3LYP/ 6-311G(d,p) level of theory. G3 theory is a procedure for calculating energies of molecules containing atoms of the first and second row of the periodic chart based on quantum chemistry molecular orbital theory. A modification of G3 theory that uses reduced orders of Moller-Plesset perturbation is G3MP2 theory.23 For all the species included in this study, full geometry optimizations were carried out at the HF/6-31G(d) level. The corresponding harmonic vibration frequencies were evaluated at the same level of theory to confirm that the optimized structures found correspond to potential energy minima and to evaluate the corresponding zero-point vibration energies (ZPE) and the thermal corrections at 298 K. ZPE values were scaled by the empirical factor 0.8929.24 All the minima found at the HF/6-31G(d) level were again fully reoptimized at the MP2(FULL)/6-31G(d) level. G3MP2 theory uses geometries from second-order perturbation theory and scaled zero-point
Figure 1. Experimental data of the vapor pressures of N-methyl-propanamide: (O) this work; (•) ref 26; (∆) ref 27; (×) ref 28; (2) ref 29.
energies from Hartree-Fock theory followed by a series of single-point energy calculations at the MP2(Full), QCISD(T), and MP2/GTMP2Large levels of theory. No corrections for internal rotors have been taken into account. The enthalpy values of at T ) 298 K were evaluated according to standard thermodynamic procedures.25 3. Results and Discussion The enthalpy of formation in the gas phase of any compound is made up of two contributions shown in eq 4: ∆fH0m(g) ) ∆gl Hm + ∆fH0m(1)
(4)
0 (l) is the where ∆gl Hm is the enthalpy of vaporization and ∆fHm enthalpy of formation in the liquid state. The experimental 0 (l) of the compounds under determination of ∆gl Hm and ∆fHm study is discussed below. 3.1. Vapor Pressures and Enthalpies of Vaporization. A summary of vapor pressures measured in this work are presented in Table 1. Comparison of the vapor pressures measured in the present work for N-Me-propanamide with the literature data is given in Figure 1. As can be seen, the results are in good agreement with those from the ebulliometric technique,26,28 although it should be noted that the direct comparison is difficult because the literature data were measured at elevated temperatures close to the boiling point. In contrast, the data from static manometry29 readings are in substantial disagreement with the vapor pressure measured in the present work; however, it is not clear as to the reason for this discrepancy. Available data28,30 on vaporization enthalpies of N-Me-propanamide are given in Table. 3. The vaporization enthalpies reported in the literature were not adjusted to the reference temperature 298 K. Therefore, the primary experimental results from refs 26-30 were treated using eqs 2 and 3 in the same way as the experimental results and the derived ∆gl Hm (298 K) are given in Table 3. The vaporization enthalpy ∆lgHm (298 K) for N-Me-propanamide measured in this work is again in agreement with those from ebulliometry and in disagreement with results from the static method, following the same pattern as for the vapor pressure. Only two earlier experimental data points for the vapor pressure
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Table 3. Compilation of Data on Vaporization Enthalpies, ∆lgHm, of N-Methyl-propanamide and 2-Butanone Oxime compounds
techniquea
temperature range/K
∆lgHm(T)/kJ · mol-1
-∆Cpb(Clp)/J · mol-1 · K-1
∆lgHm(298 K)/kJ · mol-1c
ref
N-methyl-propanamide
S E C S E T S T
303-363 380.4-479.9 298.15 362.2-414.1 368.1-473.5 307.2-371.2 313-333 283.4-329.3
49.0 ( 1.7 56.6 ( 0.2
57.1 (179.0)49
50.9 ( 1.7 63.9 ( 0.2 64.9 ( 0.3 59.0 ( 0.2 64.7 ( 0.4 66.56 ( 0.19 57.7 59.05 ( 0.20
27 28 30 29 26 this work 31 this work
2-butanone oxime
53.9 ( 0.2 57.9 ( 0.4 64.0 ( 0.2 57.2 58.55 ( 0.2
64.2 (206.3)
a Techniques: E ) ebulliometry; C ) calorimetry; T ) transpiration; S ) static method. b Values of ∆Cp have been derived from the isobaric molar heat capacity of the liquid, Clp compounds (they are in brackets) are according to the procedure developed by Chickos and Acree.18 c Vapor pressure available in the literature were treated using eqs 2 and 3 in order to evaluate the enthalpy of vaporization at 298 K in the same way as our own results in Table 3. -1
Table 4. Thermochemical Data at T ) 298 K (p° ) 0.1 MPa) for Compounds Studied in this Work (kJ · mol ) compounds 2-butanone oxime N-methyl-propanamide cyclohexanone oxime ε-caprolactam
0 ∆fHm (cr)
0 ∆fHm (l)
-153.2 ( 2.547 -329.4 ( 1.748
-139.9 ( 0.9 -320.1 ( 0.836 -143.5 ( 2.5a -316.5 ( 1.7a
g ∆cr Hm
∆lgHm
0 ∆fHm (g)
78.8 ( 0.114 87.5 ( 0.613
59.1 ( 0.2 66.6 ( 0.2 69.1 ( 0.1b 74.6 ( 0.6b
-80.8 ( 1.1 -253.5 ( 0.8 -74.4 (2.514 -241.9 ( 1.813
a g l l Hm - ∆cr Hm, using the values ∆cr Hm(298 Derived as explained in Table S2 of the Supporting Information. b Calculated as the difference ∆lgHm ) ∆cr K) derived in Table S2.
0 0 Table 5. Results of Calculations of the Enthalpies of Reaction, ∆rHm (kJ · mol-1), Gibbs Energies of Reaction ∆rGm (kJ · mol-1) and Entropies of 0 Reaction ∆rSm (J · mol-1 · K-1) in the Gaseous Phase at 298 K
G3MP2
B3LYP/6-311G(d,p)
reaction
0 ∆rGm
0 ∆rHm
0 ∆rSm
0 ∆rGm
0 ∆rHm
0 ∆rSm
R1 R2
-164.3 -175.9
-164.5 -170.4
-0.6 18.6
-172.1 -187.2
-172.4 -179.7
-1.0 25.1
of 2-butanone oxime have been measured using an isoteniscope,31 and this data, in the form of the vaporization enthalpy (see Table 4), is in good agreement with our new results (Figure S1 of the Supporting Information). 3.2. Experimental Enthalpies of Formation in the Liquid, 0 0 ∆fHm (l), and gaseous state, ∆fHm (g). The results from combustion experiments for 2-butanone oxime are summarized in Table 2. The mean of the standard specific energy of combustion ∆cu° ) -31189.9 ( 2.4 J · g-1 was derived from two experiments with the homemade calorimeter and four 0 (l) ) -139.9 experiments with the Parr-6100. To derive ∆fHm ( 0.9 kJ · mol-1 from the measured molar enthalpy of combus0 ) -2720.4 ( 0.7 kJ · mol-1, molar enthalpies of tion ∆cHm formation of H2O(l) ) -285.830 ( 0.042 kJ · mol-1 and CO2(g) ) -393.51 ( 0.13 kJ · mol-1 have been used as assigned by CODATA.32 The total uncertainty was calculated according to the guidelines presented by Olofsson.33 The uncertainty assigned 0 is twice the overall standard deviation and includes to ∆fHm the uncertainties from calibration, the combustion energies of the auxiliary materials, and the uncertainties of the enthalpies of formation of the reaction products H2O and CO2. Only one previous experimental value of enthalpy of formation for 2-butanone oxime has been determined by Landrieu34 using combustion calorimetry, and it is in disagreement by 10 kJ · mol-1 with the result obtained in this work. Surprisingly their 0 (cr) ) -151 kJ · mol-1, referred to the solid state; value, ∆fHm however, the melting temperature for 2-butanone oxime is reported to be Tm ) 243.65 K.35 We do not have any reasonable explanation for such a disagreement, but a validation of our value using ab initio calculations is given below. The enthalpy 0 (l) ) -320.1 ( 0.8 of formation of N-Me-propanamide, ∆fHm kJ · mol-1, measured using combustion calorimetry by Vasil’eva and Zhil’tsova36 was used in this work. Values of the vaporization enthalpies of 2-butanone oxime and N-Me-propanamide, derived in this work (Table 1), have
been used together with the results from combustion experiments (Table 4) to obtain the gaseous standard enthalpies of formation, 0 (g), at 298 K calculated according to eq 4. The resulting ∆fHm 0 values of ∆fHm (g) are given in Table 4. 3.3. Quantum Chemical Calculations. Recently, we showed the remarkable ability of the G3MP2 method to predict accurately equilibrium constants and reaction enthalpies of different types of chemical reactions such as isomerization or transalkylation.37 In this case, the chemical reactions were known to be thermodynamically controlled, thus the yields of desired products could be predicted with the help of quantum chemistry calculations. In the current work, we have applied the quantum chemistry procedure in order to understand whether the Beckmann rearrangement is governed by kinetic or thermodynamic control. 3.3.1. Thermodynamic Analysis of the Beckmann Rearrangement: Kinetic or Thermodynamic Control? In general, 0 knowledge of the Gibbs energy ∆fGm of any chemical reaction allows an assessment of the thermodynamic equilibrium constant KP for this reaction in the gaseous phase according to the general equation: ∆fG0m ) -RT ln KP
(5)
In the case that the equilibrium constant KP g 1 estimated according to eq 5, the yields of desired products could be higher than 50%. In this work, the thermodynamic functions for 0 , reaction enthalpy, reactions R1 and R2, Gibbs energy ∆fGm 0 0 ∆rHm, and reaction entropy, ∆rSm, have been calculated using the B3LYP and G3MP2 method and the calculated values are given in Tables 5-7. However, before a calculation of the equilibrium constant KP for the Beckmann rearrangement may be undertaken, the test of validity of the quantum chemistry methods toward compounds involved in the chemical reactions R1 and R2 must be established. Such a test provides a
Ind. Eng. Chem. Res., Vol. 48, No. 22, 2009 Table 6. Enthalpies of reactions (R1 and R2)
a
0 ∆rHm
9813
-1
(kJ · mol ) in the Gaseous and the Liquid Phase at T ) 298 K
reaction
0 ∆rHm (g)DFTa
0 ∆rHm (g)G3MP2
0 ∆rHm (g)exp
0 ∆rHm (liq)DFTa
0 ∆rHm (liq)G3MP2
0 ∆rHm (liq)exp
R1 R2
-172.4 -179.7
-164.5 -170.4
-167.5 ( 3.1 -172.7 ( 1.4
-177.9 -187.2
-174.9 -177.9
-173.0 ( 3.0 -180.2 ( 1.4
B3LYP/6-311G(d,p).
0 Table 7. Thermodynamic Function ∆rGm (kJ · mol-1) and Equilibrium Constants KP and Ka of the Reactions R1 and R2 at T ) 298 K
reaction
0 ∆rGm (G3MP2)
0 ∆rGm (DFT)a
KP(G3MP2)
KP(DFT)a
Ka(theor)b(G3MP2)
Ka(theor)b(DFT)a
R1 R2
-164.3 -175.9
-172.1 -187.2
6.2 × 1028 6.7 × 1030
1.5 × 1030 6.3 × 1032
(9.6 × 1029)c (1.6 × 1032)d
(2.2 × 1031)c (1.5 × 1034)d
a B3LYP/6-311G(d,p). b Calculated according to eq 7. c Ratio of saturation vapor pressures of reaction participants P1/P2 ) 15.5 (see text). d Ratio of saturation vapor pressures of reaction participants P1/P2 ) 23.6 (see text).
0 0 Table 8. Results of Calculations of the Enthalpies of Reaction, ∆rHm (kJ · mol-1), Entropies of Reaction ∆rSm (J · mol-1 · K-1), Gibbs Energies of 0 Reaction, ∆rGm (kJ · mol-1), and Equilibrium Constants KP in the Gaseous Phase at 298 K and at 373 K and at Total Pressure P ) 1 atm, for the Beckmann Rearrangement of Cyclohexanone Oxime into ε-Caprolactam (R1)
HF/3-21G*
B3LYP/6-311G(d,p)
T
0 ∆rGm
0 ∆rHm
0 ∆rSm
KP
0 ∆rGm
0 ∆rHm
0 ∆rSm
KP
298 K 373 K
-180.7 -174.2
-182.6 -176.2
-6.2 -5.5
4.6×1031 2.4×1024
-172.2 -166.2
-172.5 -165.9
-1.0 0.74
1.5 × 1030 1.8 × 1023
comparison of the reaction enthalpies of reactions (R1 and R2) obtained from experimental enthalpies of formation (∆fH0m) and from quantum chemistry calculations. In general, the enthalpy 0 , is defined as the stoichiometric of a chemical reaction, ∆rHm difference of the enthalpies of the products and educts in the pure states. The enthalpies of reactions R1 and R2 calculated directly from the enthalpies of reaction participants using B3LYP and G3MP2 are given in Table 6. It is clear that the 0 (g)G3MP2, are values calculated using the G3MP2 method, ∆rHm in a good agreement with the experimental values (see columns 3 and 4 in Table 6) whereas the values calculated using the density functional theory (DFT) method, ∆rH0m(g)DFT, are slightly overestimated (see columns 2 and 4 in Table 6). 0 (g) are related to the standard reaction The values of ∆rHm 0 (l), in the liquid state by enthalpies, ∆rHm ∆rH0m(1) ) ∆rH0m(g) -
∑ν∆ H i
g l
mi
(6)
i
where ∆gl Hmi 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 R1 and R2 are available in the literature13,14 and have also been measured in this work (see Table 4). The enthalpies of reactions R1 and 0 0 (l)DFT and ∆rHm (l)G3MP2, in the liquid phase calculated R2, ∆rHm according to eq 6 are listed for comparison with experimental 0 (l)DFT for values in Table 6. The calculated values of the ∆rHm reactions R1 and R2 are again slightly more negative, but the 0 (l)G3MP2 are in good agreement (within the values of ∆rHm boundaries of experimental uncertainties) with those derived from the experimental enthalpies of formation (see Table 6, columns 5, 6, and 7). It should be stressed that the values of 0 0 (l)DFT and ∆rHm (l)G3MP2 for reactions R1 and R2 have the ∆rHm been calculated using enthalpies H298 (see Table S3 of the Supporting Information) of the reaction participants directly available from the ab initio protocol. Using this procedure, any ambiguity due to calculations leading to the enthalpies of 0 (g), are avoided, as described below. Such a formation, ∆fHm straightforward procedure allows calculating a required enthalpy of reaction by quantum chemistry methods more reliable. Having established the ability of quantum chemistry methods to predict thermodynamic properties of oximes and amides, the thermodynamic equilibrium constants in the gaseous phase, KP,
of the Beckmann rearrangement (reactions R1 and R2) at 298 K have been calculated using eq 5 (see Table 7). The very high values of KP for both reactions provide evidence, that, at 298 K, the equilibrium of both Beckmann rearrangements is completely shifted to the desired direction of the amide formation. Taking into account that the ε-caprolactam synthesis under industrial conditions is performed at elevated temperatures (373-403 K), the calculations of KP at 373 K have been repeated (see Table 8). The values of KP of the Beckmann rearrangements are substantially dependent on the temperature, but even at 373 K, the position of equilibrium remains completely shifted to that of amide formation. However, the industrial ε-caprolactam synthesis is performed in the liquid phase; therefore, the results of the quantum chemistry calculations need to be transformed to the liquid state. The thermodynamic equilibrium constant KP calculated for the gas phase reaction (R1 or R2) is related to the thermodynamic constant Ka of this reaction in the liquid phase by eq 7: Ka ) KP(P1 /P2)
(7)
where P1 and P2 are appropriate saturation vapor pressures of an appropriate oxime (P1) and amide (P2). The latter vapor pressures have been measured in the present work using the transpiration method (see Table 4) or, in the case of cyclohexanone oxime and caprolactam, obtained from the the literature.13,14 Using the values of KP for the gaseous reactions R1 and R2 calculated with help of B3LYP and G3MP2 and presented in Table 7, the thermodynamic equilibrium constants Ka in the liquid phase were calculated. From the value of Ka ) 9.6 × 1029 for the Beckmann rearrangement of cyclohexanone oxime in the liquid phase, the equilibrium of the reaction is also completely shifted to the formation of ε-caprolactam. According to these computational studies, it is possible to conclude that the Beckmann rearrangement is a kinetically controlled process demonstrating strong temperature dependence and which becomes thermodynamically more favorable at lower temperatures. Following, elevated temperatures are not necessarily required to reach high yields, provided that an active catalytic system is applied. According to our calculations using the G3MP2 method, the side reaction of the Beckmann rearrangement of cyclohexanone oxime to cyclohexanone at 298 K (reaction R3):
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Ind. Eng. Chem. Res., Vol. 48, No. 22, 2009 Table 9. Results of G3MP2 Calculations of the Standard Enthalpy 0 of Formation ∆fHm (g) of 2-Butanone Oxime at 298 K (kJ · mol-1) 0 ∆fHm calc
0 has also very large negative free energy (∆fGm ) -122.9 -1 kJ · mol ; see Table S5 of the Supporting Information) which 0 is comparable with the main reaction R1 (∆fGm ) -164.3 -1 kJ · mol ) given in Table 7. These calculations provide evidence that the both main and side reactions are thermodynamically favored and only strong and selective catalysis is responsible for the high yield of the goal product. 3.3.2. G3MP2 Enthalpies of Formation of Amides and Oximes. In a series of our recent works,38-43 we have established a remarkable ability of the G3MP2 ab initio method to predict enthalpies of formation of different compounds accurately. We continue to test this composite method for amides and oximes with the help of experimental data obtained in the current work. The gaseous enthalpies of formation of amides and oximes were calculated with help of the standard atomization reactions44 as well as bond separation reactions.45 We have calculated the ∆fH0m(g) of cyclohexanone oxime and caprolactam, 2-butanone oxime and N-Me-propanamide, with help the following bond separation reactions:
C6H11ON + 12CH4)NH3 + H2O + 9C2H6
(R4)
C6H11ON + 6CH4)C2H5NO + 5C2H6
(R5)
C6H11ON + 10CH4)NH2-OH + 8C2H6
(R6)
C4H9ON + 8CH4)NH3 + H2O + 6C2H6
(R7)
C4H9ON + 2CH4)C2H5NO + 2C2H6
(R8)
C4H9ON + 6CH4)NH2-OH + 5C2H6
(R9)
Using enthalpies of these reactions calculated by the G3MP2 0 (g) for simple molmethod and enthalpies of formation ∆fHm ecules as recommended by Pedley et al.,46 the enthalpies of formation of the compounds under study have been calculated (see Tables 9-12). Using the atomization procedure and bond separation reactions, the calculated enthalpies of formation of oximes and amides, are found to be in good agreement with the experimental results (see Tables 9-12). 3.4. Experimental Study of the Beckmann Rearrangement in the Liquid Phase. Having established the dominance of kinetic control for the Beckmann rearrangement, a search for an active catalytic system which is able to perform the reaction under mild conditions was undertaken. Strong Lewis acids such as AlCl3 are able to catalyze the production of ε-caprolactam in high yields even in a solid state reaction at moderate temperatures 323-353 K.50 It is well-known that that chloroaluminate ionic liquids exhibit a wide range of Lewis acidity depending on the molar ratio of reactants37 and tuning this acidity has the potential to develop more selective conditions in comparison to AlCl3. In this work, the Beckmann rearrangement of cyclohexanone oxime in toluene or mesitylene in the presence of different chloroaluminate-1,3-dialkylimidazolium based catalysts (10 mol %) at 378-413 K (see Table S4 of the Supporting Information) was investigated. Under slightly Lewis acidic conditions using 1-butyl,3-methyl imidazolium chloride ([BMIM]Cl)/AlCl3 (1/1.1), the main product observed was cyclohexanone with less than 10% of the desired ε-caprolactam. Increasing the Lewis acidity of the chloroaluminate ionic liquid
atomization
R7
R8
R9
0 ∆fHm (g)exp
-79.82
-81.89
-83.33 average
-88.53 -83.4 ( 1.9a
-80.8 ( 1.1
a Calculated as the average from bond separation and isomerization reactions.
Table 10. Results of G3MP2 Calculations of the Standard Enthalpy 0 of Formation ∆fHm (g) of N-Methyl-propanamide at 298 K (kJ · mol-1) 0 ∆fHm calc
atomization
R7
R8
R9
0 ∆fHm (g) exp
-250.24
-252.30
-253.75 average
-258.95 -253.8 ( 1.9a
-253.5 ( 0.8
a Calculated as the average from bond separation and isomerization reactions.
Table 11. Results of G3MP2 Calculations of the Standard Enthalpy 0 of Formation ∆fHm (g) of Cyclohexanone Oxime at 298 K (kJ · mol-1) 0 ∆fHm calc
atomization
R4
R5
R6
0 ∆fHm (g) exp
-73.94
-74.18
-75.63 average
-80.83 76.2 ( 1.6a
-74.4 ( 2.5
a Calculated as the average from bond separation and isomerization reactions.
Table 12. Results of G3MP2 Calculations of the Standard Enthalpy 0 of Formation ∆fHm (g) of ε-Caprolactam at 298 K (kJ · mol-1) 0 ∆fHm calc
atomization
R4
R5
R6
0 ∆fHm (g)exp
-238.44
-238.69
-240.14 average
-245.34 -240.7 ( 1.6a
-241.9 ( 1.8
a Calculated as the average from bond separation and isomerization reactions.
Figure 2. Structures of cations [PPyr]+ and [BPyr]+.
BMIM to 1/2 only altered the rate of formation of cyclohexanone. Brønsted-acidic catalysts are traditionally used for the Beckmann rearrangement, and we focused our attention on the catalytic activity of two hydrogensulfate Brønsted-acidic ionic liquids namely 1-ethyl,3-methyl imidazolium hydrogensulfate ([EMIM][HSO4]) and 1-butyl,3-methyl imidazolium hydrogensulfate ([BMIM][HSO4]). As found with the chloroaluminate ionic liquids, the main product of the Beckmann rearrangement of cyclohexanone oxime using the hydrogensulfate based ionic liquids was cyclohexanone (see Table S4 of the Supporting Information). This study was extended to sulfonic acid (-SO3H) functionalized ionic liquids, Figure 2. These task-specific ionic liquids containing the [HSO4] anion have shown good catalytic activity in esterification reactions.15 Two task-specific ionic liquids, [PPyr][HSO4] and [BPyr][HSO4] (Figure 2), were prepared in house and tested for catalytic activity in the Beckmann rearrangement of cyclohexanone oxime (Table 13). Under the optimized conditions, complete conversion of cyclohexanone oxime with 85% selec-
Ind. Eng. Chem. Res., Vol. 48, No. 22, 2009 Table 13. Beckmann Rearrangement of Cyclohexanone Oxime into ε-Caprolactama Tb
catalyst [PSPy][HSO4]
378 378 378 [PSPy][HSO4] 413 413 413 [BSPy][HSO4] 413 413 413 413
time (h) xcyclohexanone xcyclohexanone_oxime xcaprolactam 0.5 1.0 15 0.3 0.5 1.0 0.3 0.5 1.0 2.0
0.8102 0.8659 0.8737 0.3288 0.1084 0.1437 0.2249 0.3597 0.4699 0.5104
0.1288 0.0653 0.0011 0.2251 0.1153 0.0097 0.5222 0.2985 0.1525 0.1127
0.0610 0.0687 0.1252 0.4461 0.7763 0.8466 0.2529 0.3418 0.3775 0.3769
a
Experimentally determined composition of mixtures in the liquid phase (T is the temperature of investigation in kelvin; xi is the mole fraction measured chromatographically). b Studies at 378 K were performed in toluene and at 413 K in mesitylene. Table 14. Beckmann Rearrangement of Cyclohexanone Oxime catalyst Cl3C-CH(OH)2 2,4,6-trichloro-triazine 2,4,6-trichloro-triazine/ZnCl2 H2NSO3H ClSO3H (HBO2)n [1-Me-3-(CH2)3-SO2ClIm][CF3SO3] [1-Et-3-(CH2)3-SO2ClIm][PF6] [1-Et-2-F-3-MePyr][FSO3] P2O5/CF3SO3H [1-Bu-3-MeIm][PF6]/P2O5 [1-Bu-3-MeIm][CF3COO]/PCl5 ([1-Bu-3-MeIm][PF6]/(HBO2)n
solvent
deg C/h yield, % ref
90/7.5 25/3 90/2 90/6 90/0.5 140/42 25/0.25 80/2 dimethoxyethane 25/6 diMe-formamide 120/0.5 75/16 80/2 90/3 diMe-formamide acetonitrile acetonitrile toluene
67 100 30 40 71 62 99 98 94 63 99 99 97
50 51 52 53 54 55 56 57 58 59 9 10 11
tivity to ε-caprolactam was achieved within 1 h at 413 K using [PPyr][HSO4] ionic liquid as the catalyst. In spite of small amounts of the side product (cyclohexanone), this catalyst has a promising potential for further improvement and optimization of the reaction conditions. Interestingly, under similar reaction conditions at 378 K, complete conversion was achieved with high selectivity toward cyclohexanone showing that higher temperatures favor the kinetic product. These results may be compared with other catalytic systems for Beckmann rearrangements under mild conditions which have been collected from the literature in Table 14. In accordance with the theoretic investigation, herein, very high yields (94-100%) of ε-caprolactam could be already achieved even at room temperatures using 2,4,6-trichloro-triazine,51 Lewis acidic ionic liquids,56,57 and 2-halogene-pyridinium salts.58 However, a search for active and technologically suitable catalysts for the mild conditions of the Beckmann rearrangements still remains a challenging task. Nevertheless, thermodymanic analysis of the Beckmann rearrangements performed in this study strongly support efforts to develop an environmentally friendly route to nylon-6 in mild conditions. Acknowledgment This work has been supported by Research Training Group 1213 “New Methods for Sustainability in Catalysis and Technique” (DFG) and a Portfolio Partnership from the EPSRC. Supporting Information Available: Auxiliary data for compounds used in this work (Table S1); calculation of enthalpies of formation of cyclohexanone oxime and caprolactame in the liquid phase (Table S2); total energies at 0 K and enthalpies at 298 K (in Hartree) of the molecules studied in this work (Table S3); results of experimental studies of the Beckmann rearrangements (Table S4); comparison for experi-
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ReceiVed for reView April 23, 2009 ReVised manuscript receiVed August 30, 2009 Accepted September 1, 2009 IE900633R
