Znd. Eng. Chem. Res. 1996,34, 1027-1030
1027
Acid-CatalyzedDehydration of Ethylmethylphenylcarbinol in Phenol-Ethyl Methyl Ketone Mixture Pier Luigi Beltrame,*"Paolo Carnit&**? Aldo Gamba,?Paolo Calaresu,' Oscar Cappellazzo,' and Giuseppe Messinas Dipartimento di Chimica Fisica ed Elettrochimica, Universitci di Milano, I-20133 Milano, Italy, and Enichem ANIC, Centro Ricerche, I-07046Porto Torres (Sassari), Italy
The kinetics of the reaction of ethylmethylphenylcarbinol catalyzed by sulfuric acid in the presence of phenol and ethyl methyl ketone to give phenylbutenes and byproducts were considered. The experimental data were treated within a suitable model by using a n optimization procedure. The kinetic parameters governing the steps of the process and their dependence on water and acid concentrations as well as on temperature were determined. The results were compared with those of the analogous reaction carried out on dimethylphenylcarbinol.
Introduction In the present study, we report the kinetic analysis of the reaction of ethylmethylphenylcarbinol (EMPC), catalyzed by sulfuric acid, in the presence of phenol and ethyl methyl ketone (EMK)to give phenylbutenes (PB), i.e., a mixture of 2-phenyl-1-butene [viz. a-ethylstyrene (aES)],cis-2-phenyl-2-butene [viz. cis-dimethylstyrene (cDMS)], and truns-2-phenyl-2-butene [viz. truns-dimethylstyrene (tDMS)],and byproducts. These byproducts mainly consist of (phenyl-sec-butyllphenylether (PBP), both deriv(PBPE) and (phenyl-sec-buty1)phenol ing from reactions involving phenol. A knowledge of the kinetic parameters governing the different reactions of EMPC in the considered medium as functions of temperature and water and acid concentrations is useful in order to optimize the conditions for the synthesis of phenol and EMK from sec-butylbenzene, EMPC being formed as a side product. This phenol synthesis process is an alternative to the phenol/ acetone process starting from cumene and could become of industrial importance if there should be a drop in demand for the coproduct, acetone, and/or a commercial interest in EMK. Studies on the reactivity of EMPC also appear very interesting as they lead to a greater insight into the whole reaction pattern of the phenol/acetone process. In fact, the starting product in the phenol/acetone process, cumene, is obtained by the benzene alkylation with propylene, a product that may contain amounts of 1-butene, 2-butene, and isobutene. During alkylation butylbenzenes are formed, including sec-butylbenzene which is susceptible to oxidation like cumene. sec-Butylbenzene gives rise to the corresponding peroxyradical, subsequently to the hydroperoxide, and finally in the presence of sulfuric acid to phenol EMK and EMPC. A detailed scheme of the main reactions of the analogous process starting from cumene has already been given (Messina et al., 1983). The present investigation on EMPC follows a study on the carbinol produced from cumene, i.e., dimethylphenylcarbinol(DMPC)(Beltrame et al., 1988).
+
Experimental Section Materials. EMPC was prepared from acetophenone and the Grignard reagent which was obtained by
* To whom all correspondence +
*
should be addressed.
Universita di Milano. Centro Ricerche. 0888-5885/95/2634-1027$09.00/0
Table 1. Reaction Conditions and Numbering of Kinetic Runs
1 2 3 4 5 6
0.289 0.297 0.297 0.299 0.294 0.292
0.305 0.305 0.300
5.211 5.148 5.146 5.385 5.305 5.053
5.318 5.307 5.305 5.495 5.413 5.210
320 320 640 320 320 640
60.0 60.0 60.0 25.0 40.0 80.0
reacting ethyl bromide with magnesium in anhydrous diethyl ether. Phenol, EMK, and 98%sulfuric acid were pure reagents (RPE Carlo Erba). Kinetic Runs. The conditions of the kinetic runs and the numbering adopted throughout the paper can be seen in Table 1. In the set of runs, run 1was considered as the standard, the others being generated from it by varying one condition at a time. The kinetic procedure was as follows: A three-necked cylindrical glass reactor equipped with thermometer, mechanical stirrer, and condenser was loaded with phenol, EMK, and EMPC, and, in three cases, water. The reactor was immersed in a thermostated water bath at the set temperature, and 98%sulfuric acid was added with a 50 p L syringe, under vigorous stirring to prevent high local acid concentrations at the beginning of the reaction. At the employed acid concentrations, the mixture appeared homogeneous. At given intervals, reaction samples were withdrawn and quickly neutralized by contact with ion exchange resins. Some samples were subjected to analyses repeated after hours or days; no variation of the mixture composition was observed. This ensured that the reaction quenching, due to the neutralization associated in most cases with a drop in temperature, was quite effective. Analysis. The analysis of the organic compounds involved in the reaction (Scheme 1) revealed a material balance close to 100, within an experimental error of f 5 % . The reaction samples were analyzed by GC analysis using a TCD apparatus (Carlo Erba, model FTV 2350) with a glass column (2 m long, 2 mm i.d.1, packed with FFAP (15%)on 80-100 mesh Chromosorb WHP. Helium (20 mumin) was used as carrier. The injection temperature was 200 "C; the column temperature was programmed (heating rate of 8 "C/min) from 80 (initial isotherm for 3 min) to 250 "C (final isotherm for 35 mid.
0 1995 American Chemical Society
1028 Ind. Eng. Chem. Res., Vol. 34,No. 4,1995
concentration of phenol was considered constant at its initial value, due to its great excess. A numerical integration of the system of eqs 5-11 allows the calculation of the seven unknowns, S, CEMPC,CPBPE, CH,O, CPBP,Cc+,and CPB, at various times.
Scheme 1 Et I Ph-C-O-Ph I
Et P h 4 *CH,
Et
(aES1
*
I
I
Ph-:-OH
Optimization Procedure I
4.
Me,C=C/H Ph'
'Me
(t DMSI
Equations 5- 11 were considered. The parameters were obtained minimizing the objective function:
Et I Ph-C-C,H,OH
I
In symbols
Me
where N , is the number of chemical species analyzed, their calculated and experimental concentrations being Ccalcd and Cexptl,respectively,N , is the number of kinetic samples, and N , is the number of kinetic runs. The computer program was based on the optimization routine OPTNOV (Buzzi Ferraris, 19681, associated with an integration routine based on a fourth-order RungeKutta method (Carnahan et al., 1969).
PBP
Mechanistic Model The mechanistic model for the reactions of EMPC in acidic medium in the presence of phenol, EMPC, and small amounts of water is given in Scheme 1,where P indicates phenol and, for the sake of simplicity, water is omitted. Hence, the following reactions were considered: EMPC
+ P A PBPE + HzO
(1)
+ H+ 2C+ + HzO
(2)
+
(3)
k-i
EMPC
k-2
C +k-3 ~ P BH+
C+
+ P 5 PBP + H+
(4) On the basis of reactions 1-4, assuming that equilibrium 1is rapidly established and applying the steadystate approximation for C+, the following set of equations can be written in the absence of initial amounts of PB and PBP:
S = CEMPC + CPBPE
IZZcEMPC + k-3cPB e,+= IZ-ZCH,O + + k4CP
--- -k2CEMPC dt
+ k-2CC+CH,0
d:y
-- - k3Cc+ - k - , C p ~ where K1 = kl/k-l, k2 = k;uH+, and k-3 = k:,u,.
(5)
(9) (10)
Results and Discussion On the basis of the findings obtained for the analogous reaction involving DMPC (Beltrame et al., 1988, coefficients kz and k-3, which embody the acid activity, can be considered the only ones markedly affected by a variation in water concentration. This dependence is well described by the equations:
(13)
(14) where kp and w are positive constants. Assigning to w the value previously obtained for DMPC (14.9 M-l), and considering kinetic runs 1and 2 carried out at the same acidity and temperature, values of the parameters KI, ki, k-2, k3, k13, and k4 were obtained according to the optimization procedure described above. Kinetic run 3, in which acidity was varied with respect t o runs 1 and 2 while temperature remained unchanged, was employed to determine the extent of the consequent modification of parameters k i and k:, assuming for both of them the same effect. It was found that doubling the concentration of sulfuric acid resulted in their increase by a factor of 1.75. This value is somewhat higher than that previously estimated in the case of DMPC (1.53). By using as initial values the parameter values obtained in the two above computations, the influence of the temperature on the reaction was examined considering the complete set of kinetic runs including runs 4-6. This effect was evaluated adopting in the optimization procedure an Arrhenius-type dependence according to the equation:
(11)
The
where k333 refers to the standard temperature condition
Ind. Eng. Chem. Res., Vol. 34,NO. 4,1995 1029 Table 2. Optimized Coefficients"of the Reactions Indicated in Scheme 1 aa Functions of Temperature and Their Values at 60 "C (C& = 0.29-0.30 M C i = 5.1-5.4
M;
= 5.2-5.5 M)
at 60 "C
coefficient Kl= exp(5.19 - 3190/T) k, =-exp(38.05 - 9610/T) for CH2so= 320 ppm = ex&38.61 - 9610/T) for C, = 640 ppm k-2 ='exp(21.19 - 16lO/n k 3 = exp(19.73 - 4340/T) k:, p exp(4.74 - 2550/T) for cH2S0, = 320 ppm = exp(5.30 - 2550/T) for cR*so4= 640 PPm k4 = exp(12.30 - 3330/T) a
K1 = k l h - 1 ; ki = u&(l
1.25 x 9.95 x
io3 min-'
1.74 x
lo4min-'
1.27 x 8.18 x 5.48 x
lo7 M-l
11
I
aES
9.59 x 10-2 min-l
,
I
I
1.01 x 10 M-l min-'
+ WCH'O)with w = 14.9 M-l.
0.10
14
t
1
min-' lo2min-' min-'
i
0.20
0.10
0.10
RUN 1
1
RUN 6
RUN 1
3 0
20
40
60
80
t (mid
Figure 2. Distribution of phenylbutenes aES, cDMS, and tDMS during the course of kinetic runs 1 and 6.
5 0.301
I
8 1
0.10
0 0
40
60
80
t (mid
Figure 1. Time courses of kinetic runs 1and 5 described in Table 1.
(60 "C). The final overall computation gave the results collected in Table 2. In every optimization step, determination indices higher than 0.99 were obtained. Experimental points of kinetic runs 1and 5 and the corresponding calculated curves are shown in Figure 1. This figure reveals that the proposed mechanism and the parameters obtained by the optimization procedure satisfactorily account for the observed reaction time courses. The high values of constants 12; and k-2 indicate that the equilibrium between EMPC and C+ is established fast: the term k>k-2, when compared with the corresponding one evaluated at the same temperature in the case of DMPC, appears lower by a factor of ca. 40, suggesting a more unstable nature of intermediate
species C+ in the present case. In fact, it forms more slowly by factors 10 and 5 from reactions 2 and -3 respectively, and reacts more rapidly by factors 4,20, and 8 in reactions -2,3,and 4,respectively. However, it must be noticed that in the case of DMPC the corresponding intermediate species (C'9 is also involved in a further reaction with the product of reaction 3, a-methylstyrene (aMS),giving rise to dimers 2,4diphenyl-4-methyl-1-pentene (aD)and cis- and trans2,4-diphenyl-4-methyl-2-pentene (PD). The model employed here assumes phenylbutenes (PB) as a whole and, consequently, considers a single parameter for reaction -3. A detailed analysis of these products formed with time has evidenced that their isomerization occurs during the course of the process, making the mathematical treatment more complex. Such an isomerization is evident in Figure 2,where the distribution of aES, cDMS, and tDMS, within the PB fraction, is shown as a function of time in the case of runs 1and 6. It can be observed that aES undergoes a marked isomerization: its concentration reaches a maximum and then decreases, while those of cDMS and tDMS are still increasing. To assess valid values to all the kinetic coefficients involved in this isomerization, it would be useful to perform an independent study. Thus, the values given in Table 2 have t o be mainly regarded as effective parameters able to fairly reproduce the experimentally observed trends of concentrations CEMPC, CPBPE,CPB,and CPBP. An interesting comparison between the present case and that of DMPC in terms of concentration trends vs time can be made by grouping the products formed from the intermediate species C'+ as depicted in Scheme 2, where MSD comprises aMS, aD,and PD. PCE is phenyl cumyl ether, and CP represents 0 - and p-cumylphenols.
1030 Ind. Eng. Chem. Res., Vol. 34, No. 4,1995 0.30I
- DMPC
.........
1
1
PCE
____________------
_ _ _ _ _ _ _ _ _ - - - - - - - '
. *
0
Y
0.2oc
I'
40
20
60
0
- - - - - - - - - - - --
4
0
4
8
12
16
20
Figure 3. Calculated time courses of the reactions of DMPC carried out under the conditions described in Table 1 for runs 1 (a and a') and 5 (b and b').
Scheme 2
The time courses reported in Figure 3 for the reaction of DMPC carried out under the conditions corresponding to those described in Table 1 for runs 1 and 5 appear quite similar to those relative to EMPC shown in Figure 1, when using time scales of 0-8 and 0-20 min, respectively. Thus, the reaction of DMPC requires shorter times by a factor of 10 a t 60 "C with respect to EMPC, while a t 40 "C such a factor becomes somewhat lower, close to 4, due to the different dependence on temperature of the process in the two cases.
Literature Cited Beltrame, P. L.; Carniti, P.; Gamba, A.; Cappellazzo, 0.;Lorenzoni, L.; Messina, G. Side Reactions in the PhenoYAcetone Process. A Kinetic Study Znd. Eng. Chem. Res. 1988,27,4-7. Buzzi Ferraris, G. Metodo Automatic0 per "rovare l'Ottimo di una Funzione. (An Automatic Method of Finding the Optimum of a Function.) Quad. Zng. Chim. Ztal. 1968,4 , 171-192. Carnaham, B.; Luther, H. A.; Wilkes, J. 0. The Approximation of the Solution of Ordinary Differential Equations. Applied Numerical Methods; Wiley: New York, 1969; pp 361-366. Messina, G.; Lorenzoni, L.; Cappellazzo, 0.; Gamba, A. Side Reactions and Related by-Products in the PhenoVAcetone Process. Chim. Znd. 1983,65, 10-17.
Received for review December 8 , 1994 Accepted December 29, 1994@ IE9305531 Abstract published in Advance ACS Abstracts, March 1, 1995. @
