Ind. Eng. Chem. Res. 1988, 27, 4-1
4
Side Reactions in the Phenol/Acetone Process. A Kinetic Study Pier Luigi Beltrame,*+Paolo Carniti,+Aldo Gamba,+Oscar Cappellazzo,l Loreno Lorenzoni,I and Giuseppe Messina' Dipartimento di Chimica Fisica ed Elettrochimica, Universitd d i Milano, I-20133 Milano, Italy, and E n i c h e m A N I C , Centro Ricerche, I-07046 Porto Torres (Sassari),Italy
The reaction of dimethylphenylcarbinol in the presence of phenol, acetone, and sulfuric acid to give a-methylstyrene and byproducts as well as the reverse reactions was considered. The treatment of the experimental data was performed within a suitable mathematical model by using an optimization procedure. The kinetic parameters governing the different steps of the process and their dependence on temperature and phenol, water, and acid concentrations were determined. Useful indications for the attainment of the best working conditions were obtained. The economy of the process of conversion of cumene into phenol (P) and acetone (A) is heavily affected by the presence of a lot of secondary products, byproducts, and impurities. An overall scheme of the reactions involved was already given (Messina et al., 1983). Dimethylphenylcarbinol (DMFC) and acetophenone are the main byproducts. The latter does not practically undergo further transformations following the process, and it is found almost quantitatively in the pitches. On the other hand, DMFC could be completely recovered as cumene and recycled. In fact, DMFC dehydrates to amethylstyrene (aMS), which can be easily separated and completely reduced to cumene. In the present paper, we report the kinetic analysis of the acid-catalyzed reaction of DMFC in the presence of phenol and acetone to give aMS and byproducts. These are 2,4-diphenyl-4-methyl-l-pentene (aD), cis- and trans-2,4-diphenyl-4-methyl-2-pentene (PD), and 1,1,3trimethyl-3-phenylindane (InD), all derived from aMS dimerization, and o- and p-cumylphenol (CP) and phenyl cumyl ether (PCE), derived from the reactions involving phenol. The knowledge of the kinetic parameters governing the different steps of the process as functions of temperature and phenol, water, and acid concentrations is very important. It allows the optimization of the operational conditions through the reduction of the amount of byproducts.
Table I. Reaction Conditions and Numbering of Kinetic Runs
R = aMS 2 3 4 5 6 7 8 9 10 11
0.391 0.234 0.536 0.388 0.393 0.378 0.397 0.391 0.391 0.395 0.388
0.512 0.494 0.512 0.167 0.775 0.495 0.520 0.515 0.512 0.518 0.508
12 13 14 15 16 17 18 19 20 21 22
0.587 0.357 0.717 0.587 0.588 0.569 0.587 0.587 0.587 0.594 0.582
0.494 0.494 0.494 0.182 0.800 0.478 0.502 0.494 0.494 0.500 0.490
23
0.203
0.544
24
0.198
0.542
1
5.948 5.911 5.828 5.916 5.941 4.912 6.522 5.948 5.948 6.222 5.898
6.101 6.144 6.278 6.135 6.095 7.245 5.400 6.101 6.101 6.382 6.050
620 620 620 620 620 620 620 310 1240 620 620
69.8 69.8 69.8 69.8 69.8 69.8 69.8 69.8 69.8 54.2 80.2
6.122 6.238 5.967 6.151 6.142 7.539 5.354 6.152 6.152 6.222 6.100
620 620 620 620 620 620 620 310 1240 620 620
69.8 69.8 69.8 69.8 69.8 69.8 69.8 69.8 69.8 54.2 80.2
6.101
620
69.8
6.080
620
69.8
R = DMFC 5.906 6.203 6.020 5.907 5.912 4.731 6.550 5.906 5.906 5.973 5.8S6
R = aD 5.948
R = PD
Experimental Section The conditions of the kinetic runs under examination are collected in Table I, together with the numbering adopted throughout the paper. In the two run sets 1-11 and 12-22, runs 1 and 12, respectively, were considered as standard, from which the others were generated by varying one condition at a time. Details on the reactions from aMS and from DMFC were already reported (Messina et al., 1983). For the reactions from a D and from PD, an analogous procedure was followed. Preparation of 2,4-Diphenyl-4-methyI-l-pentene (aD)and 2,4-Diphenyl-4-methyl-2-pentene (PD). The reaction conditions were optimized to minimize the formation of the undesired indanic dimer and other byproducts. The reaction was carried out at 30 "C starting from a mixture of phenol and aMS (molar ratio 2:l) in the presence of 1% wt trichloro- or trifluoroacetic acid in order to enhance the solvating power of the medium. After 3 h, sodium hydroxide (10%) was added to give phenate salt, the organic fraction washed with water, and aMS distilled
5.961
under moderate vacuum. The residue was further rectified under high reflux by using a glass column (1 m long, 25 mm i.d.) packed with Fenske rings. a D was collected a t 13.3 Pa and 118 "C (purity = 99.0% by GLC) and PD at 125 "C (purity = 98.7% by GLC). The time course of the reaction is shown by the following values of molar fractions at various times: t , min aMS aD
5 32.4 0.5
60 20.3 9.2
bD
120 9.7 14.8 0.69
180
5.1 17.8 0.93
The apparent high molar ratio aD/PD is due to the kinetics of the reaction; a thermodynamic value of 0.5 was obtained by standing a M S a t 80 "C in the presence of 0.1% wt H,S04 (96% concentration) as long as a constant ratio was reached (Lorenzoni, 1976).
Mechanistic Model A general mechanistic model for the reactivity of DMFC in acid medium in the presence of acetone, phenol, and
Universiti di Milano. Ricerche.
1 Centro
0888-5885/88/2627-0004$01.50/0
0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 1, 1988 5 Scheme 111
Scheme I
DMFC IDMFC)
\
\
jD
CZD
0,p-CP
be always established and applying the steady-state ap-
Scheme I1
DMFC
PPCE aMS
P
aD
0,pCP
small amounts of water is given in Scheme I, where for the sake of simplicity water is omitted (in the phenol-acetone process, both DMFC and aMS give dicumyl peroxide due to the presence of cumyl hydroperoxide). This complex model can be approximated to a simpler one on the basis of the following considerations: (i) the conversion of PCE to aMS appeared to be essentially governed by DMFC concentration (Cappellazzo, 1980); (ii) the formation of the cyclic isomer InD occurs in a medium of stronger acidity than that used in the phenol-acetone process. Accordingly, the model reduces to Scheme 11. In the mathematical treatment, Scheme 11can be further simplified according to the following reasonable assumptions: (i) the equilibrium between C+ and C+dimis fastly reached; (ii) the steady-state approximation holds for the sum of the two carbocations C+ and C+dim,whose concentrations are always very small. Under these conditions, one can prove that Scheme I1 is undistinguishable from Scheme 111, adopted in the kinetic analysis. Hence, the following reactions were considered: ki
DMFC
+P
DMFC
+ H+
k-1
k-2
+ HzO
(1)
C+ + HzO
(2)
PCE
k3
C+CaMS+H+ k L3
C+ + P
2CP + H+
C+ + aMS C+ + aMS
k5
(3) (4)
+ H+
(5)
EOD + H+
(6)
k La
aD
where Kl = kl/k_,, kz = k’fiH+,k-3 = k$aH+, k-, = k’_,aH+, and k4 = k‘,aH+. The set of eq 7-15 allows the calculation of the nine unknowns S, CDmC, CH20,Ccp, Cc+,CpcE,Cam, CaD, and CBD a t various times.
Optimization Procedure Equations 7-15 were considered. The parameters were computed by optimization routines (OPTNOV (Buzzi Fermis, 1968) or, in a few cases, VA04A (Powell, 1965)) associated with an integration routine based on a fourthorder Runge-Kutta method (Carnahan et al., 1969). The objective function to be minimized was Nr Ns Nc
a=
i=lj=lk=l
(Ccalcd
-
Cexptl)~~,kP
NrN,
(16)
where N , is the number of chemical species analyzed, whose calculated and experimental concentrations are Cd and Cqd, respectively;N, is the number of kinetic samples, and N , is the number of kinetic runs.
Results and Discussion The ratio R56 = k5/k6 was obtained from the concentrations of aD and OD in the early stage of their formation (R56 = 17.5) and was kept constant throughout the computations. Coefficients k2 and k,, which embody the acid activity, appeared to be the only ones markedly affected by the variation of water concentration. This dependence was found to be well described by ko,
k6
The relevant set of equations was written on the basis of reactions 1-6, considering that equilibrium 1appeared to
where koi and w are constants PO). These empirical
6 Ind. Eng. Chem. Res., Vol. 27, No. 1, 1988 Table 11. Optimized Coefficients of Reactions 1-6a Carried Out at 69.8 OC, with CHZSO1 = 620 ppm, Cop = 5.97 M, and CA = 6.14 M K , = 9.33 x 10-3 k , = 31.9 M-' min-' k " , = 4.48 X lo5 min-' k 0 - , = 2.50 X min-' k - , = 3.87 X 10, M-l m i d k , = 1.82 M-' min-' k , = 73.3 min-' k o _ , = 8.35 X min-' u = 14.9 M-I k"_, = 1.27 min-' k, = 1 9 7 k1-I min-' " K , = k,/k I
1; K O ,
= aH+k:(1
+ WCH~O). 7 -
I
bl
I
ri 11 03-1
Y
t
CI
1
'
0
I
1
I
310
620
930
CHso 2
4
I
1
1240
(PPm)
Figure 2. Values of GH+ as a function of sulfuric acid concentration (with Cop = 5.97 M) or of phenol concentration (with CH2SOa= 620 PPm).
Figure 1. Time courses of the kinetic runs 1 (a), 14 (b), and 15 (c) a M S , (m) DMFC, ( 0 )CP,(A)aD, (A)PD, described in Table I: (0) (0) PCE.
equations account for the decrease of k2 and k-3 when the increase of water concentration causes a lowering of acid activity. It was reasonably assumed that equations analogous to (17) and (18) hold for k-, and k-6 too. The values of k-, and k, at CHzO= 0.54 M were derived from the initial rate measurements of a D and OD disappearance by considering runs 23 and 24, respectively. Taking into account the above relationships, all the parameters were obtained by optimizing coefficients K,, ko2,k-2, k,, k0-3, kl, kg,and w by using runs 1-5 and 12-16 all together. The results are collected in Table 11. The high values of constants ko2and k-, indicate that the equilibrium between DMFC and C+is established fast: the value of the equilibrium constant K2 = ko2/k-, (11.6 M, in the conditions listed in Table 11) is likely more significant than the single values of the related kinetic coefficients. Experimental points of kinetic runs 1,14, and 15 and the corresponding calculated curves are shown in Figure 1, as examples. The influence of acidity on the overall process was studied by examining kinetic runs 8 and 19 with CH2SO4= 320 ppm and runs 9 and 20 with CHZSo4 = 1240 ppm, the other conditions remaining close to the standard ones, i.e., to those of runs 1 and 12. The constants involved to the greatest extent are k2, k-3, k+,, and k-6. These constants were expressed, at the two different levels of acidity, through the relationship k, = k,,,t8'H+ (i = 2, -3, -5, -6) (19) where 8/H+ is the acid activity ratio between the actual and the standard (st) conditions, for which 9',+ = 1. The values of 8',,+at the two levels of acidity exhibit, as expected, the same trend of acid concentration (Figure 2). The following operative equation describing the dependence of 8',+ on CHzSOl(ppm) was derived: 8 ' H + = 0.308 + 1.115 X 10-3CH,so, (20) The solvent effect was investigated by considering kinetic runs 6 and 17 with = 4.82 M and C, = 7.39 M and = 6.54 M and = 5.38 M, the other runs 7 and 18 with
cp
cp
c.k
conditions remaining close to the standard ones. By assuming that the solvent changes influence essentially the acid activity of the medium, equations analogous to (19) were applied:
k, = ki,stt?''H+
(i = 2, -3, -5, -6)
(21)
where 8",+ is the acid activity ratio between the actual and the standard (st) conditions. The dependence of 9'1,+ on the phenol concentration (M) is shown in Figure 2 and is described by the operative equation 9"H+
= 0.309
+ 4.29 X 10%p8
(22)
Of course, any change of phenol concentration modifies the product distribution since phenol is involved in the reaction pattern. Thus, in the reaction from DMFC, higher yields of CP and PCE were obtained by increasing phenol concentration. However, the catalytic role of phenol is particularly significant. For instance, in runs 17 and 18, similar conversions (-60%) were reached in 2 and 0.5 min, respectively. The same consequences on reaction time were found for runs 6 and 7 from a M S . From Figure 2, it can be noted that the change of Cp from 5 to 6.5 M affects the OH+ value nearly to the same extent as the change of CH$O, from 310 to 1240 ppm. For concurrent changes of acid and phenol concentrations with respect to standard conditions, QH+ = 9 'H+8 'rH+ should apply. The effect of temperature was examined through kinetic runs 10 and 21 and 11 and 22. In the optimization procedure, an Arrhenius-type dependence was adopted according to
where k343refers to the standard temperature condition (69.8 "C). The resulting activation energies of the most significant reactions are collected in Table 111, together with the reaction enthalpy of reaction 1. Inspection of Figures 1 and 3 reveals that the proposed mechanism and the parameters obtained by the optimization procedure satisfactorily account for the reaction time courses. In every case, calculated curves were obtained in fair agreement with experimental trends. The reliability of parameters is supported by the high ratio (- 30) of experimental measurements to the number of variables.
Ind. Eng. Chem. Res., Vol. 27, No. 1, 1988 7 60
Table 111. Optimized Values of Activation Energy and Reaction Enthalpy of the Reactions and Equilibrium Related to the Listed Constants constant E., kcal/mol-' AH, kcal mol-' k02 23.8 k-2 8.5 k3 14.7 kO-3 22.6 k4 10.6 K1 -1.7 I
-
a1
I
I
I
\
I
C L F c = 0.6 M
r.
CJ
i
1
R
k'
I
I
OH+
Figure 5. Values of the reaction time 7 corresponding to the maxas a function of QH+. imum yield of CYMS 20
40
Mi
0
20
40
Mi
Figure 3. Time courses of the kinetic runs 8 (a), 18 (b), and 22 (c) described in Table I (symbols as in Figure 1).
As shown in Figures 4 and 5, at constant temperature the highest yields of aMS are associated with the lowest concentrations of DMFC, P, and HzO, while the corresponding reaction times are inversely correlated to the values of OH+. The curves drawn in Figure 5 can be described by 1 7=(24) kH+$H+
40 02
06
04
C:plM'
08
4
5
6
CF(M'
7
02
04
06
08
CLFC(M)
Figure 4. Maximum yield of aMS (7") in various reaction conditions computed by using the optimized parameters.
The proposed model and the optimized coefficients allow one to calculate the dependence of the maximum yield of aMS from DMFC (v-) on the various process parameters. This is useful in choosing the best operational conditions under different circumstances. Examples of the calculated dependence of vmaxon the initial concentrations of water, phenol, and DMFC at various temperatures are shown in Figure 4. The trends of the curves are not influenced by the solvent acidity which, on the other hand, strongly affects the reaction time corresponding to v,,(T). This is shown in Figure 5 where the acidity is expressed as 29H, regardless of its origin, either from sulfuric acid or phenol.
The value of kH+ is 0.27 min-l at 70 OC,and its dependence on temperature corresponds to an activation energy of 25.1 kcal mol-'. Equation 24 reflects a direct relation between the reaction rate of the process and the acidity of the medium. From this point of view, a high phenol concentration is a favorable condition since it allows fast formation of aMS. However, as already pointed out, the same result can be obtained by increasing the concentration of sulfuric acid that enhances the rate without lowering vmax. Registry No. P, 108-95-2; A, 67-64-1; DMFC, 617-94-7; CYMS, 98-83-9; PCE, 24318-48-7; CP, 27576-86-9; CYD,6362-80-7; PD, 6258-73-7; HzS04, 7664-93-9; CBH,CH(CHJz, 98-82-8.
Literature Cited Buzzi Ferraris, G. Ing. Chim.I t . 1968, 4, 171, 180. Cappellazzo, 0. Thesis, University of Sassari, Italy, 1980. Carnahan, B.; Luther, H. A.; Wilkes, J. 0. Applied Numerical Methods; Wiley: New York, 1969; p 361. Lorenzoni, L. Thesis, University of Sassari, Italy, 1976. Messina, G.; Lorenzoni, L.; Cappellazzo, 0; Gamba, A. Chim. Ind. 1983, 65, 10. Powell, M. J. D. Comput. J . 1965, 7, 303.
Received for review May 12, 1986 Accepted August 26, 1987
