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Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 3, 1979
Synthesis of Epichlorohydrin by Elimination of Hydrogen Chloride from Chlorohydrins. 1. Kinetic Aspects of the Process S. CarrB,' E. Santacesarla, and M. Morbldelll Isfifutodi Chimica Fisica, Eleftrochimica e Metallurgia, Polifecnico di Milano, Milano, Ita&
P. Schwarz and C. Dlvo Eufeco, Milano, Ita&
The reactions involved in epichlorohydrin industrial production have been studied. The reactions can be divided into two groups: ring closure (dehydrochlorination)and ring opening (epoxide hydrolysis). Two analytical techniques, potentiometry and gas chromatography, have been employed in order to follow the time evolution of the reagents. The kinetic parameters of the reactions have been determined and a kinetic model of the overall system is offered.
Introduction The synthesis of epichlorohydrin (EPY) by elimination of hydrogen chloride from chlorohydrins has become increasingly important because EPY is an intermediate in the production of epoxide resins, glycerine, and several pharmaceutical products, as reported by Lowenheim et al. (1975). In this synthesis, described by Myszkowski and Zielinski (1964) and Huntress (1948), aqueous NaOH or Ca(OH)2acts on a,@ or a,y-dichlorohydrins, according to Scheme I. In such a reaction medium the EPY formed may be converted to glycidol and glycerine, as shown below, resulting in lower yields of EPY (Scheme 11). For this reason EPY must be removed from the reaction environment as quickly as possible. T o accomplish this, EPY is stripped with steam in a distillation column and the contact time must be short to minimize hydrolysis (Pet. Refiner, 1959; Kirk-Othmer, 1965). Thus, the kinetic parameters of the reactions and vapor-liquid equilibrium data are needed to ascertain the optimal operating conditions of the process. Up to now, little information on the kinetics has been available (see, for example, Smith, 1918). The industrial process for EPY production is described in Pet. Refiner (1959), Myszkowski et al. (1965), and Lowenheim et al. (1975). The present paper describes a study of the kinetics of the main reactions involved in EPY synthesis with Ca(OH),. A subsequent paper will discuss the use of these data for simulation of a unit for EPY production. The results of the calculations will be compared with the data obtained from a continuous micropilot unit for EPY synthesis. Experimental Section Equipment and Operating Conditions. Hydrogen chloride is eliminated rapidly, from a,@-and a,y-dichlorohydrins in the presence of hydroxide ions. For this reason the kinetics were studied by continuous potentiometric determination of the amount of hydrogen chloride eliminated. A concentration cell with two Ag/ AgCl electrodes was used. One electrode was immersed in the reaction solution, and the other was placed in a saturated solution of K2S04. The two solutions were separated by a porous disk of fritted Pyrex glass which guaranteed electrical continuity. The electrodes were prepared by anodic attack of a silver rod 0019-7882/79/1118-0424$01.00/0
Scheme I CHz-CH-CH2
I Cl
I
Cl
CH2-CH-CH2
/
OH
I
I
I
Cl
OH
CI
\ CH2-CH-CH2 '0' CI
/ I
+
HCI
in a 0.1 N solution of HC1, a t very low current intensity, as suggested by Ives and Janz (1961). The AgCl coating was renewed daily in order to avoid the effect of electrode aging on kinetic measurements, as described by Ives and Janz (1961). The linearity of the response to the electrodes, under the reaction conditions, was tested before they were used in the actual experiment. Measured quantities of HC1 were added to an aqueous solution containing an excess of Ca(OH)2 at the reaction temperature. Good linearity in the potential difference of the cell (emf) vs. the logarithm of C1- concentration was obtained. The electrodes were calibrated before each determination in order to avoid secondary effects due to aging. The same technique was employed in the study of the reaction of a-monochlorohydrin to glycidol. A stirred batch reactor was used. The equipment is illustrated in Figure 1. The saturated aqueous solution of Ca(OH), (2-4 g of Ca(OHI2in 30 cm3 of water) and the aqueous solution of chlorohydrins (1-2 cm3 in 20 cm3 of water) were heated separately to the reaction temperature. The two solutions were mixed in the reactor and the response of the potentiometer was recorded. The registered emf varied as the reaction progressed and C1- formed. The reaction rate of EPY hydrolysis is much lower than the reaction rate of dehydrochlorination. It was convenient, therefore, to use another technique in order to perform the kinetic runs, taking into account the possibility of electrode aging previously mentioned. For this purpose 1 pL of the reacting solution was withdrawn a t regular intervals and injected in a gas chromatographic column of glass of 5 m length and with an internal diameter of 1 mm, packed with 5% Ucon oil on Chromosorb W (40-60 mesh). The column temperature was 160 "C. Helium (25 cm3/m) has been used as carrier gas. The organic compounds in the injected mixture were detected by the FID detector of an H P 7620 gas chro0 1979 American Chemical Society
Ind. Eng. Ghem. Process Des. Dev., Vol. 18, No. 3, 1979
425
Scheme I1
G ,
O Y
I
53
I
I
150
100
I
200
Figure 3. Trend of conversion vs. time for the dehydrochlorination reaction of a-monochlorohydrin. B--
1
XI
Figure 1. Scheme of the stirred batch reactor: A, heating fluid inlet; B, heating fluid outlet; C, Ag/AgCl electrode; D, Ag/AgCl/K,SO,(sat) reference electrode; E, magnetic stirred rod; F, reagent feeding vessel. X 10
Figure 4. Trend of conversion vs. time for the dehydrochlorination reaction of a,&dichlorohydrin. Table I. Kinetic Parameters of the Kinetic Model i-h = A h e - E k l R T C i 05
reagents
0
0
1
2
t (min)
3
Figure 2. Trend of conversion vs. time for the dehydrochlorination reaction of a,y-dichlorohydrin.
matograph. The prepared gas chromatographic column was able to separate the reaction products and the reagent. Thus, it was possible to evaluate both the decaying in the time of EPY, for effect of hydrolysis, and the evolution of the reaction products glycidol and a-monochlorohydrin. Results The potentiometric curves have been rearranged to show the degree of conversion as a function of time. Calibration of the electrodes before each run made it possible to obtain an accurate dependence of emf on C1- concentration, and thus to derive the degree of conversion of the reagent as a function of time. Conversion as a function of time a t different temperatures for a,y-dichlorohydrin, a,P-di-
Ah,
E h , calimol
S“
m mono
10’ 6.4 X l o 8 3.74 x 1 0 7
11 718 16 984 13 200
EPY
4.92 X
1 8 852
“Y QD
lo8
chlorohydrin, and a-monochlorohydrin is given in Figures 2, 3, and 4. A simple analysis, based on the observation that the half-transformation time is independent of the initial concentration of the reagent, revealed that the experimental data could be described by a pseudo-first-order kinetic model. The kinetic parameters summarized in Table I were obtained by fitting the experimental data reported in Figures 2, 3, and 4. Some experimental runs were performed in the presence of NaOH, instead of Ca(OH)2. A glass electrode was used to follow the reaction, with a saturated calomel electrode as reference. The reaction rate was proportional to the OH- concentration. Actually, if Ca(OH), is employed as a reagent, the OH- concentration does not change significantly as the reaction proceeds, because a buffered solution (Ca(OH)2-CaC12)is formed. In fact, a t a constant temperature, and attributing to the solution an ideal
Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 3, 1979
426
I n 1:
-31 0
I
02
I
I
04
I
OB
06
"I
J
10
tONVERSlON
F i g u r e 5. Decreasing of hydroxyl concentration, in the Ca(OH)2 saturated solution, for the effect of the hydrogen chloride eliminated during the reactions.
F i g u r e 7. Arrhenius plot for the considered reactions. Scheme I11 C H 2 -CH-CHz
I
CI
I
CI
I
OH
I I
O k
0
Figure 6. Plot of In (1 - X)vs. time. The linear trend justifies the assumption of first-order kinetics for the epichlorohydrin hydrolysis.
CH2
1
-
5
-C H -CH2 \o/
CH2-CH-CH2
I
Lo/
OH
CI
Table 11. Solubility C o n s t a n t s of Ca( OH),
t, " C 10 30 40 50 71.7 90 95.3 99
g of CaO/lOO g of sol. 0.125 0.109 0.100 0.0917 0.0657 0.0591 0.0561 0.0523
K, 4.45 x 10-5 2.95 x 10-5
2.28 x 10-5 1.76 x 10-5 0.647 X 10.'
0.470 X lo-' 0.402 X lo-' 0.326 X 10.'
behavior, the following relationship can be obtained combining the solubility product of Ca(OH), with the electroneutrality relationship, both expressed as a function of the advancement degree X of the reaction [OH-I3 + XC0[OH-l2- 2K, = 0 (1) Co is the initial concentration of the reagent and K , is the solubility product. Figure 5 shows the behavior of [OH-] as a function of X a t different values of Co and temperature, as derived from the previous equation. The values of K , were evaluated from the solubility data for calcium hydroxide reported by Stephen (1964) and given in Table 11. As can be seen from Figure 5 , the concentration of hydroxyl ions decreases very slowly after the initial stage, justifying the employment of first-order kinetics as an effective model in describing our experimental data. It should be noted from the data reported in Table I1 that the solubility of Ca(OH), decreases with increasing temperature. The influence of this fact on the values of the effective pseudo-first-order reaction rate constants will be brought out in the discussion. Hydrolysis of EPY was followed by gas chromatography. The main reaction product was glycidol. Nevertheless, a small amount of a-monochlorohydrin was detected, and
CI
OH
Cl
Table I11 1
12 1 2 5
aP
ay
-1 0 0
-1
0 0
EPY
GLY
1
0 0
1 -1
1
H,O 1 1
1
this went through a maximum as a function of time. It can thus be concluded that a-monochlorohydrin is an intermediate species. The hydrolysis of EPY can also be described by firstorder kinetics, as shown by the plot in Figure 6. The kinetic parameters are summarized in Table I. Finally the Arrhenius plots of all the reactions studied are summarized in Figure 7. These plots allow a direct comparison of the reactivities of the different species investigated. Discussion The experiments performed provided the kinetic parameters for a complete description of the epichlorohydrin production process. The proposed reaction path is shown in Scheme 111. a-Monochlorohydrin is an intermediate in the formation of glycidol. Actually, since reaction 3 is slow and reaction 4 is fast, in a pseudo-first-order model, such as the one adopted, the formation of glycidol can be described by only one effective reaction rate constant. The use of a single constant can be justified by the simple application of pseudo-stationary approximation. As reported by Myszkowski et al. (19651, glycidol is very slowly converted
Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 3, 1979 Scheme
IV
r o
OH
I -C-CI
427
I I ci
+
1
OH-
to glycerol. Therefore the rate of variation (moles/time volume) of the ith species can be written as Ri = rk'aik where ark is the stoichiometric matrix given in Table 111, where rk is the row vector of the reaction rates (k is the index of the kth reaction) rk = Ir1, r2, r 5 l Each term is expressed as Tk = Ake-Ek/RTCL The values of Ak and Ek are summarized in Table I. The different reactions studied can be divided into two groups: (1)ring closure or dehydrochlorination, according to Myszkowski and Zielinski (1964); Myszkowski et al. (19651, and Lowenheim et al. (1975),and ( 2 ) ring opening or epoxide hydrolysis (see Huntress 1948). The reactions of group 1 can be regarded as an internal nucleophilic substitution (SN2), preceded by a base-catalyzed dissociation equilibrium, as shown in Scheme IV. See, for example, Patai (1967) and Weissberger (1964). The reaction rate can be written as r = kK[R][OH-] (2) where [R] is the reagent concentration, K is the equilibrium constant for the formation of the intermediates ion, and k is the reaction rate constant. The experimentally determined pseudo-first-order kinetics can be attributed to the small variation in hydroxyl ion concentration, owing to the formation of CaC1,. According to eq 2 the reactivity of chlorohydrins is strongly affected by the equilibrium constant for the formation of the intermediate ion. This constant, of course, depends on the position of substituent groups and on their electron withdrawing character. The reactivities decrease in the following order: a,y-dichlorohydrin > a-monochlorohydrin > a,@-dichlorohydrinwith reactivity ratios of 300:lOO:l. These figures are easily explained in the case of a,y-dichlorohydrin and a-monochlorohydrin. The first compound, in fact, can react in either of two ways as the intermediate ion is formed. ,.**'
p-.
I
-0
~H~-cH-*;H~
CI~~-CH--CH~
I (CI
I CI
I
CI
In addition, the reactivity of a-monochlorohydrin is reduced by internal hydrogen bonding between the adjacent hydroxyl groups
I
C H 2 --CH--CH2
I
I
CI
a,@-Dichlorohydrin is less reactive than a,y-dichlorohydrin because its intermediate ion is less stable. In fact, the inductive effects of both the chlorine atoms of a,ydichlorohydrin favor the hydrogen mobility of the hydroxyl
F i g u r e 8. Compensation effect observed in the dichlorohydrin parameters according t o t h e Brphsted catalysis law. Scheme
V
I / \O CH ' I
I I CHOH I
I =I
CHOH
C HOH
CH
+ OH-
CHO-
I
+
OH-
group. On the contrary, in a,@-dichlorohydrin,the adjacent chlorine atoms interact with each other, reducing, as a consequence, the overall effect on the hydrogen mobility. This finding seems to be substantiated by the lower activation energy. In fact, in a pseudo-first-order model, r = k,[R], the effective rate constant k,, can be written as follows with good approximation
k,
ik
K m
(3)
Therefore the apparent activation energy is correlated with the changes in enthalpy both of the reaction ( S H R )and of calcium hydroxide dissolution (AH,). It follows that 1 EappE E,,. + AHR jAH, (4)
+
The differences observed in the activation energies may thus be attributed to the differences in the values of AHR. Such values can also be affected by steric factors since the OH- group attacks a primary carbon atom in the case of a,&dichlorohydrin and a-monochlorohydrin and a secondary carbon atom in the case of a,y-dichlorohydrin. The assumption that A H R is mainly responsible for the observed order of reactivity seems to be substantiated by the fact that our reagents follow the Br6nsted catalysis law, as seen in Figure 8. This finding implies the existence of a correlation between AH, and the activation energies. According to Patai (1967), the ring-opening reaction takes place according to Scheme V. Because the reaction is base catalyzed, pseudo-first-order kinetics in the presence of calcium hydroxide is justified in this case as well. This reaction could lead to the formation of amonochlorohydrin, which quickly reacts to form glycidol. Therefore the kinetics of epichlorohydrin degradation tend to correspond to the kinetics of glycidol formation. Literature Cited Huntress, E. ti.. "Organic Chlorine Compounds", Wiley, New York, N.Y., 1948. Ives, D. J. G., Janz, G. J., "Reference Electrodes",Academic Press, New York, N.Y., 1961. "Kirk-Othmer Encyclopedia of Chemical Technology", Wiley, New York, N.Y., 1965. Lowenheim, F. A., Moran, M. K., Wiley-Interscience, New York, N.Y., 1975. Myszkowski, J., Zielinski, A. Z.,Chim. Ind , 91, 654 (1964). Myszkowski, J., Zielinski, A. Z., Lashowska, E., Przem. Chem., 44, 565 (1965). Patai, S., "The Chemistry of Functional Groups", Interscience, New York, N.Y., 1967. Pet. Refiner, 38(11), 253 (1959). Smith, L., 2. Phys. Chem., 92, 717 (1918). Smith, L., 2. Phys. Chern., 93, 59 (1918). Stephen, H., "Sdubilitiisof Inorganic and Organic Compounds", Pergamon Ress, Oxford, 1964. Weissberger, A,, "The Chemistry of Heterocyclic Compounds", Interscience, New York, N.Y., 1964. R e c e i v e d for r e v i e w F e b r u a r y 21, 1978 A c c e p t e d D e c e m b e r 27, 1978
