Ind. Eng. Chem. Res. 1988,27, 219-225
219
Stereoselective Hydrogenation of 2- tert -Butylphenol to cis -2-tert -Butylcyclohexanol. 2. Kinetics of the Liquid-Phase Hydrogenation of 2- tert -Butylphenol over Nickel, Cobalt, and Noble Metal Catalysts Oemer M. Kut,* U r s R. Datwyler, and G u e n t h e r G u t t Department of Chemical Engineering and Industrial Chemistry, Swiss Federal Institute CH-8092 Zurich, Switzerland
of
Technology (ETH),
T h e kinetics of the stereoselective liquid-phase hydrogenation of 2-tert-butylphenol to cis- and trans-2-tert-butylcyclohexanol were investigated over nickel, cobalt, and ruthenium catalysts in the hydrogen pressure rarige of 10-100 bar and at temperatures up to 200 "C. Time-conversion diagrams show a consecutive competitive reaction behavior with a pressure-dependent selectivity of intermediate 2-tert-butylcyclohexanone. At temperatures below 200 O C , with increasing H2 pressure, an increasing fraction of 2-tert-butylcyclohexanolis formed, apparently direct from the phenol omitting the ketone step (shunt reaction). A modified Langmuir-Hinshelwood-type model is presented based on nonequilibrium adsorption of the ketone. The pressure-dependent ketone selectivity, the extent of the shunt hydrogenations, and the kinetics of establishing the isomeric equilibrium for 2-tert-butylcyclohexanolcan be described quantitatively. Since the model includes the thermodynamical restrictions of the reaction system, it can also be used under reversible conditions. Most of the characteristic features of the catalytic hydrogenation of alkylphenols over a large number of catalysts (nickel, cobalt, and noble metals) are represented by this basic model. The main objective of the process design for the liquid-phase hydrogenation of 2-tert-butylphenol is to achieve a high stereoselectivity toward cis-2-tert-butylcyclohexanol which is used in the perfume and fragrance industry. The effects of different catalysts and operating parameters on the stereoselectivity of the hydrogenation of substituted phenols and cyclohexanones have been studied systematically by various groups since the 1920s. A comprehensive review of the contradictory observations is presented by Bartok (1985)in a recent monograph. Skita (1923)has studied the pH effects on the course of the hydrogenation on colloidal platinum and postulated that under acidic conditions predominantly cis alcohols and under alkaline conditions trans alcohols should be expected. Reviewing a large number of experimental works, Barton (1953)formulated a more precise rule: Axial alcohols are the main hydrogenation products in acidic media (e.g., cis-2-tert-butylcyclohexanol).In neutral media sterically unhindered cyclohexanones are hydrogenated mainly to the corresponding equatorial cyclohexanols. If the catalytic hydrogenation can be performed, highly hindered ketones (e.g., 2-isopropyl- or 2-cyclohexylcyclohexanones (Huckel et al., 1958))are preferably converted to the axial alcohols. The reduction with sodium in ethyl alcohol results in an isomer mixture corresponding to the equilibrium composition (Barton, 1953). Temperature and hydrogen pressure have only feeble effects on the stereoselectivity by a given catalyst. Over various catalysts, higher cis selectivities were reached than were predicted by equilibrium calculations (Datwyler, 1986; Kut et al., 1988). It is not possible to generalize the solvent effects with different catalyst metals: Increasing polarity of the solvent has a positive effect on the cis selectivity when nickel is used as a catalyst (Datwyler, 1986). There is no effect over ruthenium catalysts (Takagi, 1970),and Deceased October 4,1986. This work was carried out under Professor Gut's direction, and this paper represents a tribute to him.
even a decrease of cis selectivity is observed over platinum (Rylander, 1979). On most of the conventional hydrogenation catalysts, the conversion of the alkylphenols to the alkylcyclohexanols is a normal consecutive reaction, with the corresponding alkylcyclohexanone as an intermediate product (Coussemant and Jungers, 1950;Takagi, 1970;Kut and Gut, 1980). During the hydrogenation of phenols with longer alkyl substituents like 2-tert-butylphenol, high ketone selectivities can be achieved (xOn>50%, in some cases even >go%) depending on the catalyst metal (Kut et al., 1983;Bartok, 1985). For a successful scale-up and reactor design, reliable kinetic models are necessary which sufficiently represent the main features of the reaction. Zwicky and Gut (1978) described the consecutive competitive hydrogenation of phenolic compounds with a Langmuir-Hinshelwood-type model assuming noncompetitive adsorption of organic species and hydrogen. But in several cases it was observed that the cyclohexanol is additionally formed by a parallel reaction directly from the phenol, omitting the ketone step (Rylander, 1979;Kut and Gut, 1980). This pressure-dependent "shunt reactionn was also modeled by assuming that certain adsorption equilibria are not established under specific reaction conditions (Kut and Gut, 1980;Kut et al., 1983). A detailed analysis of internal and external masstransfer restrictions on hydrogenation reactions showing Langmuir-Hinshelwood-typekinetic behavior was recently presented (Gut et al., 1986). In this paper, the hydrogenation kinetics of 2-tert-b~tylphenol over nickel, cobalt, and ruthenium catalysts are modeled in the hydrogen pressure range of 10-100 bar and at temperatures up to 220 OC (beginning of cracking processes). The solvent and pH effects are discussed elsewhere (Datwyler, 1986). Equilibrium calculations (Kut et al., 1988)have shown that 2-tert-butylphenol is not completely convertible to 2-tert-butylcyclohexanol at temperatures above 160 "C in the pressure range studied. Additionally, some isomerization of the cis- or trans-cyclohexanols is expected, depending on the reaction conditions. The simple Langmuir-Hinshelwood model for
OSSS-5885/88/ 2627-O219$O1.5O/O 0 1988 American Chemical Society
220 Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 0i. 3%
&?J
--
2-tert,butvlphenal (:h)
IJ
Z-tert,butyli i c 1 o11exano:e
trans-i-rert,butyl crcloiexarc ( t r
(an1
Figure 1. Basic reaction scheme for kinetic modeling.
D
irreversible competitive consecutive hydrogenations (Zwicky and Gut, 1978) has to be expanded in order to describe these phenomena. The hydrogenation of 2-tert-butylphenol to cis- and trans-2-tert-butylcyclohexanol can be represented by the reaction scheme in Figure 1. As was shown in part 1 (Kut et al., 1988), the dehydrogenation of 2-tert-butylcyclohexanoneto 2-tert-b~tylphenol can be neglected under the experimental conditions studied. Experimental Section The kinetic experiments were performed with technical grade 2-tert-butylphenol (STIA, Switzerland, >99%, bp 220 "C) and 2-tert-butylcyclohexone (>99.5%, mp 2 "C, bp 204 "C) without solvents. The intermediate product 2-tert-butylcyclohexanonewas produced by the gas-phase dehydrogenation of 2-tert-butylcyclohexanolover zinc oxide (Girdler G-72C, T = 400 "C) and purified by distillation. The following commercial powdered catalysts were used (dp < 25 pm): RCH-Ni 55/10 TS (55 wt % Ni on Kieselguhr with activator, Ruhrchemie),RCH-Co 45/20 TS (45 wt % Co on Kieselguhr, Ruhrchemie), and 5% ruthenium on carbon (Engelhard). When necessary, the catalyst was prereduced overnight using 50 bar of hydrogen (>99.999%) at 150 "C. The hydrogenations were performed in a 500-mL stainless steel autoclave without baffles, equipped with a magnetically driven six-bladed Rushton turbine (Autoclave Engineers) under isothermal and isobaric dead-end conditions. The hydrogen pressure was varied between 10 and 100 bar and the temperature between 100 and 220 "C. With an external heating-cooling system, the reador temperature was kept within the range of *1 "C of the fixed values. To prevent external masstransfer effects, the speed of agitation was maintained over 1500 rpm (Buhlmann et al., 1982). Heating up occurred under nitrogen atmosphere to prevent hydrogenation. During the course of the reaction, 10-13 samples were withdrawn at appropriate intervals and diluted with cyclohexane. A FID gas chromatograph (PYE UNICAM PU 4500) was used with a 2.1-m glass column packed with poly(ethy1ene glycol adipate) (PEGA) for quantitative analysis (T= 140/200 "C). Kinetic Modeling A typical time-conversion diagram for the hydrogenation of 2-tert-butylphenol over ruthenium/carbon is presented in Figure 2. The time-conversion curves of different catalysts show that a useful kinetic model for the hydrogenation of 2tert-butylphenol should be able to describe the following features of this reaction: The cyclohexanolisomers are mainly formed by a competitive consecutive reaction: phenol cyclohexanone cis- or trans-cyclohexanol.
-
-
L
2
3
4
5
6
7
9
TIME [HI Figure 2. Time-conversion diagram for the hydrogenation of 2tert-butylphenol over ruthenium/carbon: T = 180 "C, pH= 100 bar, m K = 1 w t %. Curves simulated by using the model eq 1 and 29-38; parameters from Table IV.
A considerable fraction of the alcohols are produced from the phenol by omitting the intermediate ketone step, especially at higher H2 pressures. This shunt reaction decreases the maximum concentration of 2-tert-butylcyclohexanone. The primary distribution of the stereoisomeric alcohols changes by the subsequent isomerization until the equilibrium composition at the given temperature is reached. A complete conversion of 2-tert-butylcyclohexanone cannot be performed above 160-200 "C depending on the hydrogen pressure. Therefore, the dehydrogenation reactions forming 2-tert-butylcyclohexanonefrom the isomeric 2-tert-butylcyclohexanolsare not negligible under such conditions. Development of the Nonequilibrium Adsorption Model The following characteristic observations were made with all the catalysts studied. At temperatures below 140-160 OC, the maximum yield of the intermediate 2-tert-butylcyclohexanonedecreases with increasing hydrogen pressure. Additionally, some 2-tert-butylcyclohexanolis formed by an apparently parallel reaction from 2-tert-butylphenol (Figure 1). With increasing temperature, this pressure effect diminishes and at about 180-220 "C (depending on the catalyst), the maximum yield of ketone is almost independent of hydrogen pressure. Consequently, there is no observable shunt reaction under such conditions. The pressure dependence of the maximum ketone concentration also disappears under e,xternal hydrogen mass-transfer conditions. In this absorption-controlled regime, no shunt reaction can be detected. In the reaction range studied with fine powdered catalysts, effects of pore diffusion on the intermediate product selectivity can be excluded. These observations are rationalized by a nonequilibrium adsorption of the intermediate cyclohexanone at the catalyst surface according to Rylander (1979) and Kut et al. (1983). In hydrogen-poor media (slow chemical reaction) or at high temperatures (fast desorption), the desorption rate is fast enough relative to the chemical reaction rate to establish the adsorption/desorption equilibrium of 2tert-butylcyclohexanone (Figure 3). The intermediate selectivity is then not affected by the hydrogen pressure (Kut et al., 1983). The dehydrogenation of 2-tert-butylcyclohexanoneto 2-tert-butylphenol is neglected by formulating the kinetic model in the whole range of interest as it was shown by
Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 221 The fractional occupancies0, in eq 7 may be expressed by OL (eq 8-11). Solving this equation for OL gives
cs
11 (CS)
-2
k-z(l - OH) (k2 + k 3 ) o ~ + kd
on
tl k3‘ H2
I/
k-3
(tr)
11 tr
Figure 3. Reaction scheme for the sorption model. The adsorption/desorption equilibrium for 2-tert-butylcyclohexanoneis not established. Adsorbed species are given in parentheses.
equilibrium calculations in part 1of these papers (Kut et al., 1988). Model Assumptions. The rates for the individual components can be described by Langmuir-Hinshelwood-type equations: (a)
=
-rph
(b)
~o~ -6 ,) +r, = k 2 m ~ o -~ k_2m~8,,(1
(2)
(C)
+rtr = k 3 m ~ 8 -~k-,m~8,(1 ~o~ - 0,)
(3)
(
(12)
Diverging from the classical Langmuir-Hinshelwood-type model, the fractional occupancies of the adsorbing organic compounds depend on the hydrogen pressure, since the hydrogen affects the amount of adsorbed intermediate ketone. Kinetics of the Formation of cis- and trans-2tert -Butylcyclohexanol. The combination of eq 2 with the steady-state expression for the catalyst coverage by 2-tert-butylcyclohexanone (eq 4) gives after some rearrangements
kdk-20c8(1- 0,)
(1)
klmKOphOH
)
- OH) + Ktrctr 1 + (kz -I- k 3 ) o ~+ kd
kak20LCon0H
)
- k42OCs(1- OH) (13)
The use of the analytical expressions 5,6, and 8-10 for the fractional occupancies of the reacting compounds leads to
Since the adsorbed 2-tert-butylcyclohexanonemust be a rather unstable species at the catalyst surface, a steadystate assumption for its fractional occupancy of active sites can be made: dOonldt = 0. Oon
= [k,oph@H+ (k-@cs + k-30tr) X (1- 0,) + kacono~l/[(k2+ k 3 ) o ~+ kdl (4)
It was assumed that the adsorption of hydrogen at the hydrogen-specificactive sites is not influenced by the adsorption of organic species (Zwicky and Gut, 1978; Kut and Gut, 1980; Kut et al., 1983, 1984). It is further assumed that one hydrogen site adsorbs two atoms of hydrogen. Other hydrogen pressure functions would not change the principles of the model derivation, since they only affect the mathematical expression for OH: OH
The model equation of the formation of trans-2-tert-b~tylcyclohexanol can be derived similarly.
= K H C H / (+ ~ KHCH) = (KH/H)PH/[~+ (KH/H)pHI (5) 1 - OH = 1/(1 + KHCH) = 1/[1 (KH/H)PHI (6)
Calculation of the Fraction of Unoccupied Active Sites for Organic Compounds. A balance of the coverage of the active sites for organic species gives Oph don 6- 6, 6L = 1 (7)
+
+ + +
The fractional occupancies of the individual components under conditions of equilibrium adsorption are Oph
= KphCphOL
(8)
dm
= KcaCca~L
(9)
otr = KtrCtroL Substituting eq 8-10 into eq 4 gives
Oon/OL
= IklKphCph0H + (IZ-2Kc8cm+ k-3Ktrctr) (1 - OH) + k,cOJ/[(b + OH
-
k-2k30cs(1- 0,) k2kaCon0L
(
1--
k2k-3@tr) + 1 k-Zk3°cs (17)
-1eq
=
+k-2k38,,(1 - OH) (1--)+1 kak3cOne~
eq
(10)
+ kdl
(11)
Combining eq 18 with eq 13 for the equilibrium case gives now
222 Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 CphoL'
(30)
'CS
= QoicCse~'
(31)
otr
=
(32)
oph
Combining the definitions of the thermodynamic equilibrium constants with eq 18 and 19 provides kak2KH/
kaOL
-(21) k2k-3Ktr K2 Substituting eq 20 and 21 together with eq 14 and 15 into eq 13, we get the kinetic model expression for the formation of cis-2-tert-butylcyclohexanol. --
K3
QolCtreL'
eon is given by eq 4, taking into account
= K,
kdk-2Kcs k-2k3KC8
=
=
(33)
k,/OL'
Finally, we get the kinetic equations of the hydrogenation of 2-tert-butylphenol to cis- and trans-2-tert-butylcyclohexanol, which can be used for parameter estimation using an appropriate expression for OH (eq 5 or 43): rph
= -klmKephoH
(1)
The model equation of the formation of trans-2-tert-butylcyclohexanol can be derived similarly.
+
ron= -(rph + rCs rtr)
Simplification of the Model Equations. The kinetic model eq 1,22, and 23 implicitly consist of 11parameters. The following reasonable assumptions were made to reduce the number of model parameters by two. (a) The stereoisomers of 2-tert-butylcyclohexanolare adsorbed equally well. K, = K,, = Kol
(24)
(mass balance)
(36)
This set of differential equations has nine parameters to be determined. For further calculations, optional sets of seven parameters can be chosen. The remaining two parameters are fixed by thermodynamic restrictions (eq 20 and 21). The following set of parameters was taken for the nonlinear regression: kl, k2, k3, k-z, k,', Qol,and KH/H. The depending parameters were fixed by the thermodynamic relations 20 and 21, using the modifications 24-28:
(b) In the liquid phase the catalyst is almost entirely covered by organic species. OL --* 0, thus l/6L >> 1 (25)
(37)
It is useful to define the following new parameters to simplify the calculations: (a) BL' = OLKph (26)
(38)
k,' =
(b) (c)
Qol
= &/+,
ka/Kph
(27)
(so-called "relative adsorptivity") (28)
Equation 12 is therefore modified by the new definitions (26)-(28) to give
The thermodynamical equilibrium of the system 2-tertbutylphenol/ 2-tert-butylcyclohexanone/cis-and trans-2tert-butylcyclohexanol was treated in part 1 (Kut et al., 1988; Datwyler, 1986). It can be represented in the liquid phase by three equilibrium constants (pH= 10-100 bar; T = 100-300 "C): Kl = X , , / ( X ~ ~ H ~=) exp(15481/T - 32.034) (39)
Kz = X,,/(X,,,~H) = exp(8108/T - 17.498)
(40)
exp(7717/T - 16.973)
(41)
K3
The value of the fractional occupancy of Z-tert-butylcyclohexanone is influenced by the chemical kinetics. Thus, not only organic compounds but also the noncompetitive adsorbing hydrogen determines the extent of catalyst coverage by 2-tert-butylcyclohexanone.The remaining fractional occupancies are calculated by
=
X~~/(X,JJH= )
A multivariable, nonlinear regression routine was used to estimate the seven unknown parameters of the system. The algorithm was based on the method of Marquart (1963),whereas the integration of the differential equations was carried out by a conventional integration subroutine. The regression program simultaneously minimized the residual sum of squares of all the measurable mole fractions. Thus, the residual function is given by A
F = kC(xij(exp) - xij(fitted))2 i=l I
where i = ph, on, cs, tr and j = number of the sample.
Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 223 Table 1. Nonequilibrium Adsorption Model for the Hydrogenation of 2-tert-Butylphenol over RCH-Co 45/20 TS: T = 140 "C model 2, eq 43 model 1, eq 5 Darameter D std dev s, parameter p std dev s, 6.97 X 2.20 X 1.24 k, 3.88 X lo-' 1.64 1.52 X lo-' 4.50 X 4.57 X lo-' k2 2.34 X lo-* 7.29 X 2.18 X lo-' k3 6.28 X 6.33 X 1.98 X IO-' 4.42 X 1.31 X
k-2
k,' Qoi
KH/H
1.08 X 3.06 X loT2 7.08 X 4.74 X
Calculated from Equations k d = 2.87 X lo-' k-3 = 1.22 X lo-' Kz = 1.484 K3 = 1.058
7.41 X 1.87 X IO-' 4.92 X lo-' 5.40 X
1.01 X 2.29 X 5.95 X 2.99 X
i\
4
0
Modeling of the Hydrogen Pressure Dependence. In the case of nickel and cobalt, two different model functions of hydrogen pressure dependence were tested (Table I). (a) Two adjacent metallic atoms form a catalytic active site; e.g., one hydrogen molecule can be adsorbed at one site (Gut, 1982). Whether the molecule is dissociating or not does not affect the mathematical treatment for this case. model 1: OH = ( K H / H ) ~ H /+[ ~(KH/H)PH] (5)
\ / \"./
4
i
I
\ d
4
4
37, 38, 40, and 41 kd = 3.06 X lo-' k-3 = 1.38 X K2 = 1.484 K3 = 1.058
1
I
P 1
3
2
4
5
6
TIME [HI Figure 4. Time-conversion diagram for the hydrogenation of 2tert-butylphenol over RCH-Co 45/20 TS: T = 180 OC, pH= 10 bar, mK = 2 wt %. Curves simulated by using the model eq 1 and 29-38; parameters from Table I; eq 43 used.
(b) Complete dissociation of the hydrogen molecule occurs at the catalyst surface. An active site consists of one metallic atom where one hydrogen atom can adsorb. dH,st
= [(KH/H)pHl'/'/{1
+ [(KH/H)pH1'/2)
(42)
In most of the experiments, the hydrogen pressure dependence of the kinetics was higher than 0.5. Thus, it can be assumed that the addition of the second hydrogen atom proceeds slowly: model 2: OH = (OH,at)2 ([(KH/H)pH]'/2/ {I+ [(KH/H)pH]1/2))2= (eq 42)2 (43)
Kinetics for Various Catalysts Kinetics for RCH-Co 45/20 TS. A characteristic feature of the hydrogenation of 2-tert-butylphenol on the cobalt catalyst is considerable variation of the maximum ketone concentration in the hydrogen pressure range of 10-80 bar. 10 bar: x,,,,, = 0.35 80 bar:
x,,,,,
= 0.27
Simulations based on an equilibrium adsorption of the intermediate product give a steeper increase of 2-tert-b~tylcyclohexanoneand a slower production rate of the final products compared with the experimental observations (see Figure 5, dashed lines). An important fraction of the alcohol seems to be produced directly from the adsorbed intermediate ketone before leaving the catalyst surface. The 12,' and k d values from both models are almost identical. The quotient Q,, = k,'/kd = 0.61 (model 2) indicates a similar adsorption strength of 2-tert-butylphenol and 2-tert-butylcyclohexanone. The percentage of nondesorbing Z-tert-butylcyclohexanone is given by (44)
0
1
3
2
TIME
4
5
6
[HI
Figure 5. Time-conversion diagram for the hydrogenation of 2tert-butylphenol over RCH-Co 45/20 T S T = 180 "C, pH = 40 bar, m~ = 0.5 wt %. Curves simulated by using the model eq 1 and 29-38; parameters from Table I; eq 43 used. Dashed lines: simulations based on an equilibrium adsorption of 2-tert-butylcyclohexanone (ordinary model of a consecutive competitive reaction). Table 11. Percentage of Nondesorbing 2-tert-Butylcyclohexanoneon RCH-Co 45/20 TS: Calculated from Equation 44; k2,kJ,kd,and KH/Hfrom Table I Y (eq 44), % PH,bar OH (eq 43) 10 40 100
0.036 0.101 0.179
17.7 37.9 52.1
The percentages in Table I1 indicate that a t higher hydrogen pressures a considerable fraction of the alcohol is formed by a shunt reaction. These model predictions were confirmed by simulations (Figures 4 and 5) and comparison with experimental results. This kinetic model may also be used for rhodium, palladium, or platinum catalysts (Datwyler, 1986). Kinetics for RCH-Ni 55/ 10 TS. The results from the nonequilibrium adsorption model for nickel are less satisfying than those for cobalt, since the thermodynamical restrictions 37 and 38 cause isomerization rates which are much too small compared with the experimental values (Table 111, Figure 6). Further, a hydrogen pressure dependence of the isomer selectivity is observed on nickel catalysts. This fact cannot be described by a single adsorption term for the intermediate ketone. The model provides an average selectivity which agrees well with the experiment at p H = 40 bar.
224 Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988
1
on
--
1-
i
L
--1
0
z
0
1
2
4
3
TIME
5
6
10
5
[HI
Figure 6. Time-conversion diagram for the hydrogenation of 2tert-butylphenol over RCH-Ni 55/10 T S T = 140 "C, PH = 40 bar, m K = 1w t %. Curves simulated by using the model eq 1and 29-38; parameters from Table 111; eq 43 used. Table 111. Nonequilibrium Adsorption Model for the Hydrogenation of 2-tert -Butylphenol over RCH-Ni 55/10 TS: T = 140 OC model 2. ea 43 model 1, eq 5 std dev sp std dev sp parameter p parameter p kl 2.01 x 10-1 6.87 x 10-3 7.20 X lo-' 3.63 X lo-'3.38 X 10-1 2.06 k2 5.22 X lo-' 8.07 X k3 1.86 X lo-' 2.88 X 7.34 x 10-1 1.20 x 10-1 k-2 1.24 x 10-3 2.35 x 10-4 1.65 x 10-3 2.82 X lo-" 1.26 X lo-' 3.63 X 4.06 X k,' 1.30 X lo-' 4.87 X 2.53 X 4.52 x 10-3 Qoi 2.56 X 3.38 X 3.80 x 10-3 3.29 X KHIH 9.04 X Calculated from Equations kd = 2.30 k-3 = 6.72 X Kz = 8.39 K3 = 5.50
C
37, 38, 40, and 41 kd = 2.50 kT3 = 8.94 X K2 = 8.39 K3 = 5.50
Table IV. Nonequilibrium Adsorption Model for the Hydrogenation of 2-tert -Butylphenol over Ruthenium/Carbon (Eauations 45 and 46) parameter p std dev sp parameter p sM dev sp k-2 7.63 X 8.22 X kl* 1.35 X lo" 1.68 X lo6 kz* 2.25 X 1.04 X lo4 k,' 1.90 X 1.35 X Qo, 2.98 X 2.15 X k3* 4.62 X lo-' 3.81 X Calculated from Equations 37, 38, 40, and 41 Kz = 1.484 kd = 1.26 K3 = 1.058 k-, = 2.20 X
Mitsui et al. (1973) and Bartok (1985) reported that in the case of nickel catalysts the adsorption of a cyclohexanone derivative determines the stereochemistry of the subsequent hydrogenations, whereas in the case of noble metals only the half-hydrogenated states are critical for the form of the final product. Thus, nickel requires a kinetic model including two distinguishable adsorption states of the ketone. One of these states would lead to cis-2-tert-butylcyclohexanol;the other would form the trans alcohol. Regarding possible isomerization steps at the catalyst surface, the model consists of 14 parameters which are hardly determinable by regression calculations because of the statistical intercorrelations. Kinetics for Ruthenium on Carbon. The characteristic feature of the hydrogenation on a ruthenium catalyst is the linear time-concentration relationship for the organic substances (Table IV, Figures 2, 7, and 8). The hydrogenation of 2-tert-butylcyclohexanonestarts only when 2-tert-butylphenol has completely disappeared. Both hydrogenation steps are not affected by the products formed. The remarkable bend of the concentration profiles
TIME
15
[HI
Figure 7. Time-conversion diagram for the hydrogenation of 2tert-butylphenol over ruthenium/carbon: T = 180 O C , p~ = 10 bar, mK = 0.9 wt %. Curves simulated by using the model eq 1 and 29-38; parameters from Table IV; OH, 1- OH according to eq 45 and 46.
i'
6-:
\
/j
\ e
- /
:
I
\ /
2
k-
3
4
5
6
7
E
TIME [HI Figure 8. Time-conversion diagram for the hydrogenation of 2tert-butylphenol over ruthenium/carbon: T = 180 "C, PH = 40 bar, m K = 1wt %. Curves simulated by using the model eq 1and 29-38; parameters from Table IV; OH, 1 - OH according to eq 45 and 46.
of 2-tert-butylcyclohexanola t the starting point of the ketone reduction (Figures 2 and 8) separates two phases of cyclohexanol formation: alcohol which was formed before this point originates directly from 2-tert-butylphenol via nondesorbing intermediate, whereas all the cyclohexanols formed afterwards are due to the normal ketone hydrogenation. In contrast to nickel and cobalt, the reaction rate is first order in hydrogen pressure. Therefore, a numerical value of KH/H cannot be determined. The hydrogen term and the rate constant are modified as I
Conclusions Compared with a pure consecutive reaction scheme, the nonequilibrium adsorption model describes additionally the following observations: decrease of the maximum ketone concentration with increasing hydrogen pressure; shunt hydrogenation of 2-tert-butylphenol to 2-tert-b~tylcyclohexanol showing a higher reaction order (>1)toward hydrogen pressure; isomerization of the cyclohexanol isomers by a dehydrogenation to the ketone which is immediately rehydrogenated.
Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 225
10
2
35
T = temperature, "C t = reaction time, min, h x = mole fraction y = percentage of nondesorbing ketone, defined by eq 44, %
1
I3
Greek Symbols 8, = fractional occupancy of active sites by compound j OL' = modified fractional occupancy, eq 26, L/mol
1
-
T 0
~/ , c
~ 20
, 40
~
~ 60
HYDROGEN PRESSURE
,
I
j
14OoC
, 80
~ 100
,
, 120
[BAR]
Figure 9. Reaction rate per amount of catalyst as a function of the hydrogen pressure; hydrogenation of 2-tert-butylphenol over RCHNi 55/10 TS: (- - -) calculated from model 1, eq 5; (-1 calculated from model 2, eq 43.
For the description of the hydrogen pressure dependence, two model functions were tested. The discrimination of one of these models was not possible with any statistical significance. The residual sum of squares of model 2 is about 25% smaller than for model 1, both for nickel and cobalt. But the residuals are within the experimental error range at least during the phenol reduction as shown in Figure 9 (RCH-Ni 55/10 TS). Figure 9 gives a plot of the hydrogen pressure dependence of the observed initial rates and of the corresponding model functions. Model 1 is more favorable because of the simpler structure. A discrimination between the models and further conclusions about mechanisms cannot only be based on kinetic measurements.
Nomenclature c = concentration, mol/L c ho = initial concentration of 2-tert-butylphenol, mol/L 2 = particle diameter, mm, m lf = Henry's constant for hydrogen, bar L/mol K , = thermodynamic equilibrium constant, ph G on, bar-2 K z = thermodynamic equilibrium constant, on F-? cs, bar-l K3 = thermodynamic equilibrium constant, on F! tr, bar-' KH = adsorption constant for hydrogen, L/mol Kph = adsorption constant for 2-tert-butylphenol, L/mol k = chemical rate constant, mol/(L min w t %) k, = adsorption rate constant, l/(min w t %) k,' = k,/KPh,.mol/& min w t %) k d = desorption rate constant, mol/(L min w t %) mK = relative amount of catalyst based on 2-tert-butylphenol, wt%
p = pressure, bar Qj = relative adsorptivity of compound j = Kj/Kph r = reaction rate, mol/(L min) rpho= initial hydrogenation rate of 2-tert-butylphenol,mol/(L
min)
Subscripts a = adsorption at = atomic hydrogen cs cis-2-tert-butylcyclohexanol ~ d =~= desorption ~ ~ ~ 1 ~ , ~ eq = equilibrium H = hydrogen (usually molecular) j = unspecified component L = unoccupied catalytic site max = maximum on = 2-tert-butylcyclohexanone ph = 2-tert-butylphenol tr = trans-2-tert-butylcyclohexanol (ph), (on), (cs), (tr) = adsorbed species
~
,
,
,
Registry No. 2-HOCBH4C(CH3&,, 88-18-6; Ru, 7440-18-8; Co, 7440-48-4; Ni, 7440-02-0; 2-tert-butylcyclohexanone,1728-46-7; cis-2-tert-butylcyclohexanol, 7214-18-8; trans-2-tert-butylcyclohexanol, 5448-22-6.
Literature Cited Bartok, M. Stereochemistry of Heterogenous Metal Catalysis; Wiley: Chichester, 1985. Barton, D. H. R. J. Chem. SOC. 1953, 1027. Buhlmann, Th.; Gut, G.; Kut, 0. M. Chimia 1982, 36, 469. Chim. Belg. 1950,59, 295. Coussemant, F.; Jungers, J. C. Bull. SOC. Datwyler, U. R. Ph.D. Thesis 8131, ETH Zurich, 1986. Gut, G. Swiss Chem. 1982,4/3a, 17. Gut, G.; Kut, 0. M.; Yuecelen, F.; Wagner, D. In Catalytic Hydrogenation; Cerveny, L., Ed.; Stud. Surf. Sci. Catal. 27; Elsevier: Amsterdam, 1986; p 517. Huckel, W.; Mayer, M.; Jordan, E.; Seeger, W. Justus Liebigs Ann. Chem. 1958,616,46. Kut, 0. M.; Buhlmann, Th.; Mayer, F.; Gut, G. Ind. Eng. Chem. Process Des. Dev. 1984,23, 335. Kut, 0. M.; Datwyler, U. R.; Gut, G. Ind. Eng. Chem. Res. 1988, preceding paper in this issue. Kut, 0. M.; Gut, G. Chimia 1980,34, 250. Kut, 0. M.; Gut, G.; Biihlmann, Th.; Lussy, A. In Mass Transfer with Chemical Reaction in Multiphase Systems; Alper, E., Ed.; NATO A S I Series E 73; Martinus Nijhoff: The Hague, 1983; Vol. 11, p 225. Marquart, D. W. J. SOC. Ind. Appl. Math. 1963,11, 431. Mitsui, S.; Saito, H.; Yamashita, Y.; Kaminaga, M.; Senda, Y. Tetrahedron 1973,29,1531. Rylander, P. N. Catalytic Hydrogenation in Organic Synthesis; Academic: New York, 1979; p 92. Skita, A. Chem. Ber. 1923, 56, 1014. Takagi, Y. Sci. Pap. Inst. Phys. Chem. Res. (Jpn.) 1970, 64, 39. Zwicky, J. J.; Gut, G. Chem. Eng. Sci. 1978, 33, 1363. Received for review December 3, 1986 Revised manuscript received August 3, 1987 Accepted September 14, 1987
