Ind. Eng. Chem. Res. 1988, 27, 215-218
215
KINETICS AND CATALYSIS Stereoselective Hydrogenation of 2-tert-Butylphenol to cis -2-tert -Butylcyclohexanol. 1. Equilibrium Composition of the System 2- tert -Butylphenol/2- tert -Butylcyclohexanone/cis - and trans -2-tert -Butylcyclohexanol and Hydrogen Oemer M. Kut,* Urs R. Datwyler, and Guenther Gutt Department of Chemical Engineering and Industrial Chemistry, Swiss Federal Institute CH-8092 Zurich, Switzerland
of
Technology (ETH),
T h e thermodynamic equilibrium of the system 2- tert-butylphenol/2-tert-butylcyclohexanone/cisand trans-2-tert-butylcyclohexanoland hydrogen is characterized by an unexpectedly low stability of the trans alcohol. Although the incremental method by Van Krevelen and Chermin can be used t o evaluate the thermodynamic properties of 2-tert-butylphenol, 2-tert-butylcyclohexanone, and cis-2-tert-butylcyclohexanolaccurately, the stability of trans-2-tert-butylcyclohexanol is highly overestimated. From incremental calculations, a trans isomer fraction of more than 90% is expected in the equilibrium composition, and the isomerization from the cis to the trans configuration should be slightly exothermic. The equilibrium compositions were experimentally determined over a broad range of temperatures and hydrogen pressures. These experiments have shown that the cis isomer is the dominating product in the real system (>50%) and that the isomerization to the trans alcohol is endothermic (+3 kJ/mol). Alkylated cyclohexanols are easily produced by catalytic hydrogenation of the parent phenols. Some of these cyclohexanols like menthol and 2- and 4-tert-butylcyclohexanol are important intermediates for the fragrance and perfume industry. In the case of 2-tert-butylcyclohexanol, only the cis form is of technical interest. Thus, a main objective of a process design for the hydrogenation of 2tert-butylphenol is to achieve a high stereoselectivity toward the cis isomer. With regard to such a process development, the equilibrium composition of the system 2-tert-butylphenol/2-tert-butylcyclohexanone/2-tert-butylcyclohexanol and hydrogen was studied. A conventional method to evaluate the thermodynamic properties of such compounds is the incremental method by Van Krevelen and Chermin (1951). 2-tert-Butylcyclohexanol is an ortho-disubstituted cyclohexane derivative with a bulky alkyl group causing strong steric effects (Figure 1). Since the next nearest-neighbor interactions are neglected by this method (Benson et al., 1969), the calculated free energies of formation for the individual stereoisomers will not be very accurate. To avoid such uncertainty, the equilibrium compositions of the liquid phase were also determined experimentally. It should be noted that the theoretically possible conformations with axial tert-butyl groups are not existent because of too strong steric hindrance. Since the catalytic hydrogenation of 2-tert-butylphenol to 2-tert-butylcyclohexanol is a consecutive reaction with 2-tert-butylcyclohexanone as a potential intermediate product, a reaction t Deceased October 4,1986.This work was carried out under Professor Gut's direction, and this paper represents a tribute t o
him. 0888-5885/88/2627-0215$01.50/0
scheme according to Figure 2 can be formulated. Further, equatorial substituted cyclohexane derivatives are generally more stable than those with axial substituents. Therefore, the equilibrium fraction of diequatorial trans-2-tert-butylcyclohexanol is expected to exceed that of the cis form.
Experimental Section The hydrogenations were performed in a 500-mL stainless steel autoclave without baffles, equipped with a magnetically driven six-bladed Rushton turbine under isothermal and isobaric conditions in the hydrogen pressure range of 10-100 bar and at temperatures between 100 and 280 "C. Higher temperatures led to considerable decomposition reactions, which hindered an accurate determination of the equilibrium compositions. In the runs for equilibrium measurement, 300 mL of 2-tert-butylphenol (STIA, technical grade, >99%, bp 220 "C) was used together with the following commercial powdered catalysts (2-6 g): RCH nickel 55/10 TS (55% Ni on Kieselguhr with activator, Ruhrchemie), Raney nickel B-213 (nonpyrophoric, Degussa), RCH cobalt 45/20 (45% cobalt on Kieselguhr, Ruhrchemie), ruthenium (5% ruthenium on carbon, Engelhard), and copper chromite G-13 (GirdlerSudchemie). The reaction time to establish the chemical equilibrium was at least 24 h. Depending on catalyst and temperature (especially below 140 "C) several days were needed to achieve the equilibrium state. During the course of the reaction 10-13 samdes (0.5 mL) were withdrawn periodically and diluted with 2.5 mL of cyclohexane. The addition of cyclohexane avoided the crystallization of 2tert-butylcyclohexanol and afforded an easier separation of the catalyst by centrifugation. A hydrogen flame gas
0 1988 American Chemical Society
216
Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988
Calculation of the Liquid-Phase Equilibrium
OH
Figure 1. Stereoisomers of 2-tert-butylcyclohexanol(+ enantiomers). cis-2-tert-butylcyclohexanol
H211
K2
Ki
2- ter t-bu tylphenol
#
2-ter t-butylcyc lohexanone
2H2 H 2 1 1 K3
trans-2-tert-butylcyclohexanol
Figure 2. Scheme of the multistep thermodynamic equilibrium system.
chromatograph (PYE UNICAM PU 4500) was used with a 2.1-m column packed with poly(ethy1ene glycol adipate) (PEGA) for quantitative analysis (2' = 140/200 "C). The configuration of the 2-tert-butylcyclohexnol isomers was identified by NMR spectroscopy. Vapor pressures were measured in a conventional VLE apparatus up to 140 "C. At higher temperatures, the boiling points were determined by evaporation under fixed pressure. The VLE experiments were performed with commercial 2-tert-butylcyclohexanol(Aldrich Chemicals, 76% cis, mp 44 "C). 2-tert-Butylcyclohexanone(>99.9% after distillation, mp 2 "C) was produced by catalytic gas-phase dehydrogenation of 2-tert-bu~lcyclohexanol over zinc oxide (Girdler G-72C) at 400 "C (Jaeger and Gut, 1980; Gut and Jaeger, 1982). Published data were used for calculations with 2-tert-butylphenol (McDonald et al., 1959; Handbook of Chemistry and Physics, 1979;Beilstein, 1966, 1980).
Theoretical Section Gas-Phase Equilibria by Incremental Method. Very few data were published about the thermodynamic properties of the system under study. But the AH? and AGfO values of the unsubstituted parent molecules, i.e., phenol, cyclohexanone, and cyclohexanol, are tabulated for a perfect gas state a t 1atm (Stull et al., 1969). Combining these data with incremental corrections for alkylation (Van Krevelen and Chermin, 19611, the enthalpies of the molecules of interest can be calculated. This method is preferable to a simple addition of the increments of all molecular substructures, since the thermodynamics of the ring structure are now based on observable data. Incremental values given by Van Krevelen and Chermin (1961) also refer to an ideal gas state a t 1 atm. It should be noted that 2-tert-butylcyclohexanoneis treated as a single substituted cyclohexane derivative, since the carbonyl group does not perform free rotations. The equilibrium constants according to the modified incremental method are given by eq 1-3 in the temperature range of 300-600 K after conversion of the reference state from 1 atm to 1 bar (Daetwyler, 1986).
Starting from the free energy of formation in the ideal gas state, the corresponding value of a compound in the liquid phase at constant hydrogen pressure and temperature is calculated by splitting the phase change into three steps (p, < po): isothermal expansion of the gaseous compound from the reference state (p, = 1bar) to its vapor pressure; vapor-liquid equilibrium (AG = 0); isothermal recompression of the liquid compound from the vapor pressure to the reference state ( p , = 1 bar). It can be shown that only the isothermal gas compression provides a significant contribution to the change of free energy due to condensation a t fixed pressure: G ( l ) = AGf(g) + RT In (P,/P,) (4) The free energy of a chemical reaction in the liquid phase is now given by
AG.0)
= CvjAGfj(g)+ RT J
Cvj In
( ~ v j / ~ o )
I
(5)
Finally, the equilibrium constant in the liquid phase is derived from eq 5 as In KO) = In K(g) - Cvj In J
-
bv,j/~,)
(6)
For the reaction studied (A(1) + nH2(g) B(l)J,the general 6 is simplified to In K(1) = In K(g) - In p V , B + In pV,A (7) The measured vapor pressure data are fitted by the equations @, < 1 bar) 2- tert-butylphenol: P v , p h = exp(-6355/T + 12.853) (8) 2-tert-butylcyclohexanone: pv,on = exp(-5672/T
+ 11.898)
pv,ol= exp(-6436/T
+ 13.470)
(9) 2-tert-butylcyclohexanol(mixture of isomers, 76% cis): (10) By use of eq 7 together with the appropriate numerical values for the gas-phase equilibrium (eq 1-3) and the vapor pressures (eq 8-10), the liquid system is described by KIi(1) = r o n / ( ~ p h p=~exp(15481/T 2) - 32.034) (11) Kzi(1) = X,,/(X~JJH) = exp(8495/T - 18.030) (12) &i(l) = x ~ ~ / ( x , J I H=) exp(9261/T - 17.354) (13)
Results and Discussion According to the modified incremental method, trans2-tert-butylcyclohexanolshould be the main product in the temperature range of interest (Figure 3). Depending on hydrogen pressure, some 2-tert-butylcyclohexanoneis formed between 150 and 400 "C; above 400 "C 2-tert-butylphenol is the only component expected (PH< 100 bar). The isomerization of cis- to trans-2-tert-butylcyclohexanol should be slightly exothermic (AHR,is= -6.4 kJ/mol). Observed Equilibrium Compositions. Regressions of the experimental data from Table I result in the following temperature functions of the equilibrium constants for the liquid phase: Kz = x ~ ~ / ( x ~ J I=H exp(8108/T ) - 17.498) (14) K 3 / K 2= xtr/r,, = exp(-391/T + 0.525) (15)
Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 217 L O
1.0
-
TR
1
Y
\
\ I
/
i
V I
w
i
.a .6
c ii 4
E
G
.4
w
-1 0
=
.2 0.
0
200
100
TEMPERATURE
300
400
t°C1
PH,bar 10 10 10 10 20 40 40 40 40 40 40 40 40 40 40 60 80
xph
Ton
0.016 0.040 0.080 0.148 0.021
0.001
0.002 0.005 0.009 0.018 0.034 0.068 0.121 0.228 0.012 0.017
x, 0.582 0.561 0.532 0.485 0.576 0.625 0.613 0.602 0.594 0.580 0.566 0.548 0.519 0.485 0.415 0.568 0.557
17.
Xtr
0.401 0.399 0.388 0.366 0.403 0.375 0.387 0.396 0.401 0.411 0.416 0.418 0.413 0.393 0.356 0.420 0.426
The equilibrium composition of the isomeric 2 - t e r t - b ~ tylcyclohexanols in cyclohexane was studied formerly by Pasto and Rao (1970). Their results are fitted by eq 16, which differs only slightly from (15). K , / K 2 = xtr/xcs = exp(-405/T + 0.438) (16) The isomerization of cis- to trans-2-tert-butylcyclohexanol in the liquid phase is represented by the following parameters (derived from eq 15): cis -trans AGR = +1.19 kJ/mol(200 "C); A H R = +3.25 kJ/mol; ASR = +4.36 J/ (mol K). The equilibrium constants for the liquid system were recalculated using the observed composition data for the ketone reduction: The hydrogenation of 2-tert-butylphenol to 2-tert-b~tylcyclohexanone is still described by the modified incremental method (in the experimental range studied, no significant amount of 2-tert-butylphenol remained in the equilibrium state). The formation of the 2-tert-butylcyclohexanolsfrom 2-tert-butylcyclohexanoneis calculated from eq 14 and 15. The equilibrium constants of the real system are now given by (100 OC < T < 300 "C; 10 bar < p H < 100 bar; Ki = KJ.1)) K , = x , , / ( x ~ $ ~ ~ ) = exp(15481/T - 32.034) (11)
400
300 LOCI
Figure 4. Equilibrium composition at pH = 10 bar; eq 11, 14, and
Table I. Observed Average Equilibrium Compositions (Deviation. f0.002) 160 180 200 220 180 100 120 140 160 180 200 220 240 260 280 200 220
200
TEMPERATURE
Figure 3. Equilibrium composition calculated by the modified incremental method, pH = 10 bar, eq 11-13.
T,"c
100
0
l '. Oa
---,
.6
.4 .2 0. 200
100
0
400
300
TEMPERATURE
LOCI
Figure 5. Equilibrium composition at pH = 40 bar; eq 11, 14, and 17. F r a c t i o n o f cis-Isomer
(%)
B 80 -
40 20-
Trans
c:s
j
Figure 6. Cis isomer fraction as a function of temperature calculated by the incremental method (A) and by regression from experimental data (B). (- - -) Equilibrium composition in cyclohexane (Pasto and Rao, 1970).
trans-2-tert-butylcyclohexanol are shown in Figures 4 and 5 as a function of temperature for hydrogen pressures of 10 and 40 bar. The observed compositions are also indicated. In Figure 6, the results of the calculations using the incremental method (eq 12 and 13) and the experimental K2 = ~,,/(x,,,,pH) = exp(8108/T - 17.498) (14) data (eq 15) are compared. The equilibrium measurements by Pasto and Rao (1970) are also given. K3 = xtr/(x,,,p~)= exp(7717/T - 16.973) (17) It can be seen from Figures 4-6 that cis-2-tert-butylcyclohexanol is the dominating product in the temperature Calculated equilibrium compositions for the liquid system 2-tert-butylphenol/2-tert-butylcyclohexanone/cis-, range of 100-200 O C . Against the prediction by the in-
218 Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 Table 11. Comparison of Equilibrium Constants Calculated by the Modified Incremental Method (Ki(1)) and from Exoerimental Comoositions ( K ) 160 200 240 280
3.39 0.70 0.18 0.059
4.87 0.93 0.23 0.069
2.32 0.52 0.15 0.049
56.07 9.20 2.00 0.54
cremental method, the isomerization of cis- to trans-2tert-butylcyclohexanol is endothermic (AHR,is= +3.3 kJ/mol). 2-tert-Butylcyclohexanolis an exceptional ortho-disubstituted cyclohexane derivative. The cis isomer has a lower enthalpy (AHR,,> 0) and is thermodynamically more stable (AGR,is > 0) than the trans form. In addition, the intermediate product, 2-tert-butylcyclohexanone, is thermodynamically more favored than expected from incremental calculations (xonC 0.2, Figure 3). Between 150 and 200 "C, hydrogen pressures above 10 bar are needed to achieve a practically complete conversion to 2-tert-butylcyclohexanol. As shown in Table 11, the formation of cis-2-tert-b~tylcyclohexanol is predicted with sufficient precision by the modified incremental method (K2compared to KJI)). The formation of the trans isomer, however, is not represented with acceptable accuracy by this method (K3 compared to K3i(l)). The modified incremental method does not take into account the destabilization of the trans configuration due to the bulky tert-butyl group. Studies with di-tert-butylcyclohexanolsby Pasto and Rao (1970) have shown that in such systems chair conformations may be so strongly deformed that even the usually unstable twist conformations can represent the most preferred form. The strong repulsion between the hydroxyl and the alkyl groups leads t o this torsion of the cyclohexane skeleton. The atomic distances and the angles of torsion of 2tert-butylcyclohexane were calculated by Altona and Sundaralingam (1970). The most stable conformation is reached when the bulky tert-butyl group forms an angle of 17O to the cyclohexane ring, which causes a slight torsion of the ring structure. Consequence of the torsion is a decrease of the atomic distances between the tert-butyl group and equatorial hydrogen atoms. This means in the case of 2-tert-butylcyclohexanolthat there must exist strong repulsive interactions between the equatorial hydroxyl group in the trans isomer and the methyl groups of the alkyl chain. Therefore, the trans configuration is strongly destabilized. The axial hydroxyl group of cis-2tert-butylcyclohexanol is less affected by repulsive forces. The kinetics of the hydrogenation of 2-tert-butylphenol considering these thermodynamic equilibria are discussed in the following paper in this issue. Nomenclature AG = change of Gibbs free energy, kJ/mol A H = change of enthalpy, kJ/mol K1 = thermodynamic equilibrium constant, ph
on, bar-2
K z = thermodynamic equilibrium constant (experimental), on s cs, bar-' K3 = thermodynamic equilibrium constant, on e tr, bar-' K,, = thermodynamic equilibrium constant (calculated by modified incremental method), ph e on, bar-2 K,,, K3i = thermodynamic equilibrium constants, (calculated by modified incremental method), on e cs and on F! tr, respectively, bar-' p = pressure, bar R = gas constant = 8.314 J/(mol K) A S = change of entropy, J/(mol K) T = temperature, K, O C v = molar volume, m3/mol x = mole fraction Greek Symbol Y = stoichiometric coefficient Subscripts cs = cis-2-tert-butylcyclohexanol f = formation (for thermodynamic parameters) g = gas phase H = molecular hydrogen i = from incremental method is = isomerization j = unspecified component 1 = liquid phase o = reference state 01 = 2-tert-butylcyclohexanol(mixture cis/trans) on = 2-tert-butylcyclohexanone ph = 2-tert-butylphenol R = reaction (for thermodynamic parameters) tr = trans-2-tert-butylcyclohe~anol v = vapor, vapor phase Registry No. 2-HOC6H4C(CH3)3, 88-18-6; tert-butylcyclo5448-22-6; hexanone, 1728-46-7; trans-2-tert-butylcyclohexanol, '
cis-2-tert-butylcyclohexanol, 7214-18-8.
L i t e r a t u r e Cited Altona, C.; Sundaralingam, M.Tetrahedron 1970, 26, 925. Beilstein 1966, 6(E 111), 1861. Beilstein 1980, 6(E IV), 3292. Benson, S. W.; Cruickshank, F. R.; Golden, D. M.; Haugen, G. R.; O'Neal, H. E.; Rodpers, - A. S.;Shaw, R.; Walsh, R. Chem. Reu. 1969, 69, 279. Datwyler, U. R. Ph.D. Thesis 8131, ETH Zurich, 1986. Gut. G.: Jaeger. R. Chem. Eng. Sci. 1982.37, 319. Handbook Chemistry and physics, 60th ed.; CRC: Boca Raton, FL, 1979. Jaeger, R.; Gut, G. Chimia 1980,34, 283. McDonald, R. A.; Shrader, S. A.; Stull, D. R. J. Chem. Eng. Data 1959, 4 , 311. Pasto, D. J.; Rao, D. R. J. Am. Chem. Soc. 1970, 92, 5151. Stull, D. R.; Westrum, E. F.; Sinke, G. C. The Chemical Thermodynamics of Organic Compounds; Wiley: New York, 1969. Van Krevelen, D. W.; Chermin, H. A. G. Chem. Eng. Sci. 1951,1,66. Van Krevelen, D. W.; Chermin, H. A. G. In Landolt-Bornstein Zahlenwerte und Funktionen aus Physik, Chemie, Astronomie, Geophysik, Technik, 6th ed.; Springer Verlag: Berlin, 1961; Vol. 11/4, p 18.
07
Received for review December 3, 1986 Revised manuscript received August 3, 1987 Accepted September 14, 1987
