Kinetics of Glycerol Chlorination with Hydrochloric Acid: A New Route

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Ind. Eng. Chem. Res. 2007, 46, 6456-6465

Kinetics of Glycerol Chlorination with Hydrochloric Acid: A New Route to r,γ-Dichlorohydrin R. Tesser,† E. Santacesaria,*,† M. Di Serio,† G. Di Nuzzi,† and V. Fiandra‡ UniVersity of Naples “Federico II”, Department of Chemistry, Complesso UniVersitario Monte S. Angelo, Via Cintia, I-80126 Naples, Italy, and ASER S.r.L., S.S. n.11 Padana Superiore 2/B, 20063 Cernusco sul NaViglio, Milano, Italy

The growing availability of glycerol, as a consequence of the increase in biodiesel production (for which glycerol is a byproduct), is rapidly saturating the market, and consequently, a great interest is now addressed to the development of new process routes for alternative uses of glycerol. Among the various possibilities, our attention has been focused on the glycerol chlorination reaction, with the aim to produce R,γ-dichlorohydrin. This compound can then subsequently be converted into epichlorohydrin, which is an important intermediate in the production of epoxy resins. R,γ-dicholorhydrin, together with R,β-dichlorohydrin, is currently synthesized starting from propylene via allyl chloride. In the present paper, the kinetics of glycerol chlorination with gaseous hydrochloric acid, for the production of R,γ-dichlorohydrin, has been investigated by means of a jacketed glass reactor operated in batch conditions for the substrate (glycerol) and continuously for the hydrochloric acid. Different organic acids have been tested as catalysts with good performances in terms of both activity and, in particular, selectivity toward the desired 1,3-dichlorinated product. A reaction mechanism has been proposed and a consequent kinetic model has been developed in order to quantitatively describe the experimental data collected at various temperatures (80-120 °C), and the kinetic parameters have been evaluated. A generally good agreement between the experimental data and the theoretical model has been found. Introduction The increasing interest in biodiesel production, as an alternative, environmentally friendly, and renewable fuel, has given rise to questions about the possible use of glycerol. In fact, glycerol is the main byproduct in the biodiesel production process (∼0.1 ton/(ton of biodiesel produced)), and its economical exploitation could strongly affect the overall economic competitiveness of the whole process. Nowadays, the glycerol market is becoming rather saturated, and for the near future, it is reasonable to foresee a glycerol surplus due to the steep increase in biodiesel production. These aspects involve the necessity to consider alternative routes, uses, and/or processes for glycerol transformation into useful intermediates or final products. Among the various possibilities for the use of glycerol in the field of fine, specialties, or commodities chemistry, the chlorination reaction for preparing R,γ-dichlorohydrin seems to be particularly promising, as demonstrated by the patents recently published on the subject.1-3 This compound is an important intermediate in the process for synthesizing epichlorohydrin. This compound is highly toxic, harmful if inhaled, and reported as causing cancer. Nearly the same toxicity properties are reported on the material safety data sheet (MSDS) related to dichlorohydrins, while monochlorohydrins are considered moderately toxic but also must be handled with care. The construction of two chemical industrial plants, adopting this new technology, has recently been announced.4 Dichlorohydrins are currently produced industrially starting from propylene through allyl chloride5 and are used to synthesize epoxy resins.6-12 In the process via propylene chlorination, a mixture * Corresponding author. E-mail: [email protected]. † Complesso Universitario Monte S. Angelo. ‡ ASER S.r.L..

of R,γ- (30%) and R,β-dichlorohydrin (70%) is obtained.10,11 This is a drawback of the process, because, the R,β isomer is 10× less reactive than the R,γ-dichlorohydrin in giving epichlorohydrin. On the contrary, the glycerol hydrochlorination is highly selective in the production of R,γ-dichlorohydrin, and this represents an economic advantage with respect to the traditional process. Additionally, even if a detailed comparative economic analysis is outside of the scope of the present work, a further advantage is surely represented by the practically null cost of the starting material (glycerol). In our investigation, the reaction for obtaining R,γ-dichlorohydrin from glycerol has been carried out by means of gaseous hydrochloric acid and a carboxylic acid as catalyst. The overall reaction scheme for preparing R,γ-dichlorohydrin, starting from glycerol and hydrochloric acid, is as follows:

This reaction proceeds by means of a first chlorination of the glycerol, which mostly forms R-monochlorohydrin and water, with small quantities of β-monochlorohydrin, followed by a second chlorination from which the required product is mainly obtained: R,γ-dichlorohydrin and very small amounts of R,βdichlorohydrin. The traditional processes starting from glycerol, described by the old literature, are based on the reaction of glycerol with an aqueous solution of hydrochloric acid, in the presence of acetic acid as a catalyst by adopting a temperature range of ∼80-100 °C.12-19 These old proposed processes are characterized by considerable drawbacks, such as the following: • the loss of catalyst during the reaction due to the relatively low boiling point of acetic acid (117 °C);

10.1021/ie070708n CCC: $37.00 © 2007 American Chemical Society Published on Web 09/05/2007

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Figure 1. Experimental apparatus: (1) jacketed reactor made of Pyrex glass, with a capacity of ∼700 mL; (2) thermostat for the circulation of thermal fluid; (3) peristaltic pump; (4) valve for performing samples withdrawal of the reaction mix; (5) cylinder of hydrochloric acid; (6) vertical condenser for total reflux operation; (7) horizontal condenser for stripping operation; (8) bottle for collecting the condensed reaction product; (9) bubbler provided with a porous disk for introducing gaseous HCl, which ensures effective contact between the gas and liquid phases; and (10) system for suppressing the excess HCl (two or three Drechsel bottles, arranged in series, that contain a solution of sodium hydroxide).

• the slowing of the reaction caused by the introduction of water in the reaction mixture, due to the use of aqueous hydrochloric acid, and the failure to remove the water that is formed as a consequence of the reaction itself; and • the difficult separation of R,γ-dichlorohydrin from the reaction medium. These drawbacks, together with the high cost of glycerol, prevented in the past the development of this process. The recent increase in biodiesel production is causing a general increase in glycerol availability and, therefore, a progressive reduction of its costs so that the production route to R,γ-dichlorohydrin, starting from glycerol, would be now more and more attractive. Experimental Section Experimental Apparatus. The experimental runs have been carried out in a laboratory apparatus schematically represented in Figure 1. The reactor is a glass-jacketed reactor equipped with a magnetically driven stirrer and is operated in batch modality for what concerns glycerol and in continuous mode for gaseous hydrochloric acid. The flow of hydrochloric acid is fed, from a cylinder, directly into the liquid glyceric phase in the reactor by using a porous ceramic sparger that, together with the stirrer, ensure a good gas-liquid interphase contact. The temperature of the reaction mixture is kept constant within (0.3 °C by means of a thermostat that continuously circulates thermal fluid into the reactor jacket. The reactor is equipped also with an external recirculation line operated by a peristaltic pump and with a stopping valve for withdrawing samples of the reacting mixture at different times. The peristaltic pump is turned on only when a sample has to be collected and then is stopped. On the head of the reactor, two types of condensers are mounted: the first is vertical and is used for runs at total reflux in which, practically, only hydrochloric acid and small amounts of water are allowed to leave the reactive system; the second condenser is placed horizontally and is used only for runs under stripping conditions, when the flow of hydrochloric acid is used as a stripping agent to remove all the volatile components from the reaction mixture. The choice between the two types of condensers is made by means of two valves mounted on the line at the top of the reactor that allows one to exclude,

alternatively, one of the condensing apparatus. The kinetic runs reported in this paper have all been made in reflux conditions. After the condensers, a reservoir tank is provided for collecting the condensed products eventually present, while the gaseous flow, mainly constituted by unreacted hydrochloric acid, is finally neutralized by bubbling in a series of two or more Drechsel-type bottles containing a solution of sodium hydroxide. The neutralization of the hydrochloric acid excess is monitored by adding an indicator (phenolphthalein) to the sodium hydroxide solution in a way that, when the solution in a bottle is completely neutralized, the indicator changes color and a further neutralization trap is added. All the runs have been conducted at atmospheric pressure of hydrochloric acid because of the limitation of the adopted glass reactor. The increase of the reaction pressure should result in an increase of reaction rate1 as a consequence of the higher concentration of hydrochloric acid in the liquid-phase mixture. Samples Preparation and Analysis. When a sample is collected, the dissolved residue hydrochloric acid, and eventually the catalyst, are neutralized by means of calcium carbonate. About 3 cm3 of sample are treated with 0.5 g of the mentioned salt and kept at 100 °C for 30 min in order to remove also the water formed by the reaction. The vial with the sample is then centrifuged in order to separate the precipitate formed, and the clarified solution is then analyzed by using the following gas chromatographic method. The GC analysis conditions are the following: column, Crompac CP Wax; stationary phase, 100% polyethyleneglycol; length ) 30 m; i.d. ) 0.25 mm; film thickness ) 0.25 µm; FID detector; helium as gas carrier; injector temperature ) 250 °C; detector temperature ) 280 °C; temperature ramp ) 1 min at 40 °C; heating rate ) 20 °C/min to 100 °C, then 40 °C/min up to 200 °C, then hold for 4 min. The sample of the reaction mixture is first diluted with methanol in a volumetric ratio of 1:20. The injected volume of the obtained solution is 1 µL. Experimental Runs. A first set of experimental runs has been performed in the described apparatus with the aim of comparing the activities and the selectivities of different organic acids used as catalysts. This screening has been performed, at 100 °C, by using a constant catalyst concentration of 8% by moles. The

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Table 1. Experimental Runs for Catalyst Screening: Conversion of Glycerol, Selectivity to r,γ-Dichlorohydrin, and Products Distributiona catalyst

amount of catalyst (g)

glycerol conversion at 30 min (%)

glycerol conversion at 3 h (%)

selectivity to R,γ-dichlorohydrin at 3 h (%)

acetic acid malonic acid propionic acid succinic acid citric acid levulinic acid pivalic acid benzoic acid trichloroacetic acid

10.0 18.0 13.0 20.0 34.0 20.6 18.0 13.9 13.6

80.83 54.30 90.11 65.57 58.02 67.02 8.54 2.04 4.21

99.14 99.45 99.95 98.14 94.85 99.61 11.95 13.66 17.09

29.94 36.05 41.00 30.62 18.57 32.33 9.29 0.00 0.00

products distribution after 3 h of reaction (mol %) catalyst

Gly

R

β

R,γ

R,β

acetic acid malonic acid propionic acid succinic acid citric acid levulinic acid pivalic acid benzoic acid trichloroacetic acid

0.86 0.57 0.00 1.86 5.15 0.39 88.05 86.34 82.91

63.01 55.85 49.88 60.88 70.45 60.04 9.19 12.38 15.08

5.98 7.24 8.77 6.84 6.49 6.98 1.65 1.27 2.01

29.82 35.87 41.00 30.05 17.61 32.20 1.11 0.00 0.00

0.33 0.46 0.35 0.37 0.26 0.37 0.00 0.00 0.00

a Other experimental common conditions: T ) 100 °C, HCl flow rate ) 24 g/min, glycerol loaded ) 200 g, catalyst concentration ) 8% by moles, reaction time ) 3 h, total reflux except for HCl.

catalysts tested are reported in Table 1 together with the other experimental conditions. A second group of runs has the scope of determining the kinetics of the reaction performed in the presence of malonic acid, as catalyst, selected for the good performances obtained in the screening. The explored temperature range for the kinetic runs is 80-120 °C, and at each temperature, the complete evolution with time of the involved species has been evaluated by GC analyses.

to observe that there is not a correlation between the acidity strength of the catalyst and the reaction activity; in fact, trichloroacetic acid, having a pKa equal to 0.66, shows a very low activity. On the other hand, the chemical environment is highly acidic for the presence of anhydrous hydrochloric acid, and the reaction is probably promoted by the ability of the catalyst to give place to a carbocation. Reaction Mechanism and Kinetic Model. The more detailed reaction scheme is as follows:

Results and Discussion Catalytic Screening. A first run (not reported in Table 1) has been performed using HCl and glycerol in the absence of catalyst, and no conversion of glycerol has been observed in this case. The experimental activity has then been focused on the performances of molecules containing carboxylic acid groups with a lower volatility with respect to acetic acid, such as the ones reported in Table 1. In the same table are also reported results in terms of glycerol conversion after 30 min of reaction, conversion after 3 h, and selectivity toward the desired product R,γ-dichlorohydrin. From these results, we can observe that propionic acid can be considered as the best catalyst even if its normal boiling point is only slightly higher than that of the acetic acid. Malonic, levulinic, and succinic acids have also been revealed as interesting both for their performances and also for their very low volatility that, in perspective, can allow one to perform the reaction at higher temperature without appreciable loss of catalyst. For these catalysts, an almost complete conversion of glycerol after 3 h of reaction under total reflux has been observed. Also, the selectivity results of these heavier organic acids can be considered very interesting, with the concentration of the desired product being in the range 3040% by moles, for the best catalysts, in total reflux conditions. From the exposed considerations, the best catalyst for the reaction studied would present the character of high activity and selectivity and, contemporarily, a high volatility in order to ensure the minimization of its losses. In Table 1 is also reported the product distribution obtained after 3 h of reaction for all the catalysts considered for the screening. It is interesting

According to the observed reaction products, the reaction network can be schematized in the following way: In each of the five reactions reported in relation 3, a hydroxyl group initially present on the glycerol is substituted by a chlorine atom and a water molecule is then released. On the basis of the


experimental observations and on the available literature,20,21 a reaction mechanism consisting of several steps can be suggested. The most relevant steps are as follows: (i) a nucleophilic substitution on acylic carbon that consists of an esterification reaction with the formation of a water molecule; (ii) the formation of an oxonium group through alkyl-oxygen bond scission, with the aid of a vicinal group20,21 and the carboxylic acid release; and (iii) the subsequent formation of chlorohydrin by chlorine addition. The described three-step mechanism can be illustrated as in the following scheme, related to glycerol chlorination (reaction 1 in relation 3). The first step is the esterification of glycerol:

Figure 2. Arrhenius plot of the kinetic constants.

Figure 3. Evolution in time of moles of catalyst for the run at T ) 100 °C.

This step is a nucleophilic addition reaction, followed by water elimination, in which glycerol attacks the protonated carbonyl group. This is the classic mechanism, normally accepted for the acid-catalyzed esterification reaction. The second step of the reaction mechanism leads to the formation of a three-membered ring oxonium group and the catalyst in its initial form:20,21

The catalytic cycle is facilitated by the presence of a carboxylic acid, with the carboxylic group as the leaving group being more effective than the hydroxylic group. The last step in the reaction sequence is the nucleophilic substitution SN2 that involves the attack of chlorine anion on the less-substituted carbon atom of the oxonium intermediate (R position):

Also, the β-substitution is possible, even if less probable, giving place in this case to β-monochlorohydrin according to the following:

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Figure 4. Evolution in time of the composition for the experimental run with malonic acid at T ) 80 °C. Comparison between the experimental data and the model behavior.

The described reaction mechanism is in agreement with the experimental observation that the amounts of R-monochlorohydrin are always higher than those of β-monochlorohydrin. Moreover, the β-monochlorohydrin is not able to further react, giving place to the formation of R,β-dichlorohydrin with the proposed mechanism. The absence of the vicinal OH group, in this case, prevents the formation of the oxonium ring intermediate and, hence, prevents the second chlorination in the R position. On the contrary, R-monochlorohydrin can undergo a second chlorination with a mechanism similar to the one previously shown. All these findings can also explain the experimental observation that the concentration of β-monochlorohydrin slightly increases during the reaction till glycerol is present in the reaction medium, while at higher reaction times, when glycerol has been almost completely reacted, the concentration of β-monochlorohydrin remains nearly constant. These observations suggest that the conversion of β-monochlorohydrin into dichlorinated product (R,β-dichlorohydrin) by means of reaction 5 in relation 3 can be neglected. On the contrary, R-monochlorohydrin can undergo a second chlorination with a mechanism similar to that expressed by relations 4-7 that leads to the formation of R,γ-dichlorohydrin. On the basis of these considerations, a simplified reaction scheme can be proposed as follows:

Reactions 2 and 4 have been considered irreversible because β-monochlorohydrin accumulates during the reaction and R,β-dichlorohydrin has been obtained always in small

Figure 5. Evolution in time of the composition for the experimental run with malonic acid at T ) 100 °C. Comparison between the experimental data and the model behavior (thin curves, model A; bold curves, model B).

quantities.Relation 8 can be represented by four distinct reactions, which are as follows: K1

glycerol + HCl {\ } R-monochlorohydrin + H2O K -1

K2

glycerol + HCl 98 β-monochlorohydrin + H2O R-monochlorohydrin + K3

} R,γ-dichlorohydrin + H2O (9) HCl {\ K -3

K4

R-monochlorohydrin + HCl 98 R,β-dichlorohydrin + H2O Each of the four reactions is a substitution of a hydroxyl with a chlorine group, occurring according to the mechanism represented by relations 4-7 that consist of a complex sequence of elementary steps. A possible sequence of these steps can be summarized in the following simplified way:

catalyst + H+ f catalyst+

(10.1)

glycerol + catalyst+ f I1+

(10.2)

I1+ f ester + H2O + H+

(10.3)

ester + H+ f ester+

(10.4)

ester+ f I+ + catalyst

(10.5)

I+ + Cl- f R-monochlorohydrin

(10.6)

In the series of elementary steps described by relations 10.110.6, the symbols have the following meaning: catalyst+, protonated catalyst; I1+, first reaction intermediate; ester+, ester in protonated form; and I+, second reaction intermediate in protonated form. For the steps in relations 10.1-10.6, the related reaction rate expressions can be written as

Ind. Eng. Chem. Res., Vol. 46, No. 20, 2007 6461 Table 2. Experimental Run with Malonic Acid at T ) 80 °C; Percent Molar Composition except Water and HCLa t (min)

glycerol (% mol)

R-monochlorohydrin (% mol)

β-monochlorohydrin (% mol)

R,γ-dichlorohydrin (% mol)

R,β-dichlorohydrin (% mol)

0 15 30 45 60 90 120 150 180

100 70.72 59.95 46.76 38.69 26.46 19.33 14.00 11.70

0 26.94 36.51 48.02 55.36 63.63 68.14 71.34 71.07

0 1.81 2.45 3.06 3.69 3.96 4.29 4.57 4.64

0 0.53 1.09 2.16 2.25 5.88 8.16 9.98 12.45

0 0 0 0 0 0.06 0.09 0.11 0.13

a Other experimental conditions: HCl flow rate ) 24 g/min, glycerol loaded ) 200 g, catalyst concentration ) 8% by moles (18 g), total reflux except for HCl.

Table 3. Experimental Run with Malonic Acid at T ) 90 °C; Percent Molar Composition except Water and HCLa t (min)

glycerol (% mol)





0 15 30 45 60 90 120 150 180

100 73.04 40.71 31.49 23.,52 16.17 8.52 5.80 4.19

0 24.61 52.63 59.66 65.57 68.34 70.31 68.79 67.26

0 1.87 3.67 4.20 4.70 5.07 5.39 5.79 5.93

0 0.48 3.00 4.58 6.13 10.29 15.58 19.39 22.36

0 0 0 0.06 0.08 0.12 0.18 0.23 0.26



glycerol (% mol)





0 15 30 45 60 90 120 150 180

100 62.93 45.70 34.69 17.44 5.34 3.80 1.26 0.57

0 33.39 47.62 57.10 66.34 69.64 65.94 63.53 55.85

0 2.85 4.76 5.74 7.42 6.31 7.03 7.27 7.24

0 0.83 1.92 2.46 8.61 18.43 22.87 27.50 35.87

0 0 0 0 0.19 0.27 0.36 0.44 0.46



glycerol (% mol)





0 15 30 45 60 90 120 150 180

100 62.64 35.74 16.84 9.37 4.12 2.15 1.80 1.00

0 33.22 55.91 67.79 70.17 67.53 62.94 57.67 54.23

0 3.29 4.87 5.90 6.07 6.59 6.59 6.85 6.94

0 0.85 3.41 9.32 14.19 21.47 27.94 33.21 37.32

0 0 0.06 0.14 0.20 0.29 0.38 0.47 0.50


r1 ) k1[Cat][H+] r2 ) k2[Gly][Cat+] r3 ) k3[I1+]

r4 ) k4[ester][H+] r5 ) k5[ester+]

(11)

r6 ) k6[I+][Cl-]

According to this step sequence, each of the four reactions in relation 9 could be described by a complex kinetic model characterized by six parameters k1-k6 and by the concentration of intermediate species that cannot be measured.

Following the approach of the rate-determining step (RDS), a first hypothesis has been made in which the oxonium ion formation (reaction 10.5), that is, the formation of a threemembered ring, would be much slower with respect to the other steps, so we can consider this as rate-determining step. As a consequence, the other reaction steps can be considered at equilibrium. With these assumptions and by mathematical manipulation, the resulting reaction rate for a generic reaction in which a hydroxyl group is substituted by chlorine will result as follows:

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glycerol (% mol)





0 15 30 45 60 90 120 150 180

100 41.29 24.07 13.87 9.25 4.49 2.46 0.95 0.58

0 51.77 64.47 68.71 69.92 68.13 64.01 59.69 55.23

0 4.41 6.19 6.37 6.75 6.94 7.34 6.95 7.60

0 2.53 5.15 10.86 13.84 20.14 25.81 31.95 36.07

0 0 0.11 0.17 0.22 0.30 0.38 0.44 0.52


r ) (k5keq4keq3keq2keq1)[Cat][Gly][H+]/[H2O]

(12)

If the constants appearing in relation 12 are lumped into a single pseudo-constant defined as

K ) (k5keq4keq3keq2keq1)

(13)

the final rate expression becomes

r ) K[Cat][Gly][H+]/[H2O]

(14)

According to this RDS hypothesis, the following expression for the set of reaction rates (model A) would be valid:

[

]

CHCGly K1 C r1 ) CC K1 CW KE1 R r2 ) K2CC

[

r 3 ) C C K3

(15)

CHCGly CW

CHCR K3 C γ CW KE3 R

(16)

]

(17)

C HC R r4 ) K4CC CW

(18)

An alternative assumption of the RDS in relations 10.1-10.6 can be represented by step 10.3, which is the ester formation. According to this assumption, a different kinetic model can be derived and the reaction rate related to the generic hydroxyl substitution with chlorine becomes

r ) (k3keq2keq1)[Cat][Gly][H+]

(19)

As in the previous case (see relations 13 and 14), a pseudoconstant K can be introduced and the resulting expression for the reaction rate is the following:

r ) K[Cat][Gly][H+]

(20)

Therefore, by assuming the ester formation as the RDS, the kinetic model becomes (model B)

[

r1 ) CC K1CHCGly -

K1 C C KE1 R W

r2 ) K2CCCHCGly

]

(21) (22)

[

r3 ) CC K3CHCR -

K3 C C KE3 Rγ W

]

r4 ) K4CCCHCR

(23) (24)

The equations representing model A (eqs 15-19) or model B (eqs 21-24) have been used to solve the material balance equations that, for the liquid phase in a batch reactor, can be written as follows:

dnG ) (-r1 - r2)VR dt dnβ ) (r2)VR dt

dnR ) (r1 - r3 - r4)VR dt dnRγ ) (r3)VR dt

dnW ) (r1 + r2 + r3 + r4)VR dt

(25)

dnRβ ) (r4)VR dt

In the set of ordinary differential equations (ODEs) (eq 25), ni is the moles of component i in the liquid phase and VR is the volume of the reacting liquid mixture. In order to solve the system of ODEs (eq 25), we have introduced other assumptions in calculating the reaction volume and the catalyst concentration: (i) The only component that was not condensed is HCl. (ii) The reaction volume increases by the effect of HCl addition and decreases as a consequence of samples withdrawing. Experimentally, an almost stationary liquid volume has been observed for all the runs performed, so the volume has been considered constant. (iii) A certain amount of catalyst was subtracted from the reaction mixture with the samples, so its concentration decreases along the reaction time. This effect has been taken into account by calculating the actual catalyst number of moles by subtracting the catalyst removed with samples and correlating these values with an empirical polynomial of the following type:

molCatalyst ) At2 + Bt + C 0 min < t (time) < 180 min (26) As an example, in Figure 3 is reported the decreasing trend of catalyst moles number related to the run at T ) 100 °C with malonic acid. In this plot is reported both the estimated values (open circles) and the empirical correlation represented by eq 26 (continuous line). (iv) The amount of HCl in the liquid phase has been evaluated by means of a standard NaOH titration on the samples collected. The evaluated acidity has been attributed totally to HCl, and that related to the catalyst has been neglected in this context. Also in this case, an empirical correlation has been employed to describe the evolution in time of HCl number of moles.


Interpretation of the Experimental Runs and Discussion. As a first approach in the interpretation of the experimental data, we have focused our attention in the discrimination between models A and B to ascertain the RDS that effectively affects the overall observed reaction rate. The catalytic cycle for the chlorination of glycerol can be expressed according to the simplified scheme below:

For this cycle, the two following reaction rate expressions can be written:

rest ) KestCC*CGly overall esterification reaction

(27)

rchlor ) KchlorCE*CH overall chlorination reaction

(28)

By applying a steady-state approximation to the ester concentration, and taking into account for the catalyst the material balance CC ) CC* + CE*, an overall reaction rate equation for the product formation can be derived:

rchlor ) KchlorCCCGCH

[

1 Kchlor CG + C Kest H

]

(29)

The application of relation 29 to the four reactions of set 9 results in an eight-parameter model in which the relative contributions to the reaction rate of esterification and chlorination can be evaluated separately. In order to estimate these contributions, eq 29 has been applied to the run performed at 100 °C with malonic acid as the catalyst, and the related parameters have been calculated by nonlinear regression.22 From the values obtained for these parameters, a ratio of 15-90 has been found between Kchlor and Kest, and this suggests that the esterification step is much slower than the chlorination one. Moreover, on the basis of this result, the two models A and B have also been applied to the run with malonic acid at 100 °C, and the results are compared in Figure 5. From the plot, it is evident that the quality in the description of the experimental data with model A (step 10.5 as RDS) is lower than that of model B (step 10.3 as RDS). This finding represents a further confirmation that the assumption of the ester formation as the rate-determining step (model B) should be more correct, and the correspondent model has, therefore, been selected by us for the full kinetic analysis. Model B, coupled with eq 25, has been applied to the description of all the experimental runs reported in Tables 2-6. The experimental data set, related to each temperature, has been submitted to mathematical nonlinear regression for evaluation of kinetic parameters K1, K2, K3, and K4 by using the package Berkeley Madonna.22 The kinetic parameters are reported in Table 7 together with their confidence limits at 95% level of probability, and in the same table is reported also the activation energy and the pre-exponential factor as obtained from the Arrhenius plot of Figure 2. From this diagram, it is evident that, with all the lines being almost parallel, the obtained values for

Figure 6. Evolution in time of the composition for the experimental run with malonic acid at T ) 120 °C. Comparison between the experimental data and the model behavior. Table 7. Kinetic Constants and Arrhenius Parameters for the Runs on Malonic Acid T (°C)

K1a

K2a

K3a

K4a

80 90 100 110 120

7 667 ( 940 11 704 ( 1 272 13 274 ( 1 692 19 433 ( 2 216 27 411 ( 2 861

450 ( 41 764 ( 60 1 089 ( 87 1 465 ( 123 2 215 ( 170

714 ( 227 1 109 ( 307 1 784 ( 407 2 383 ( 532 2 179 ( 685

8(3 13 ( 5 26 ( 7 32 ( 9 31 ( 13

Ea (kJ mol-1) ln A a

reaction 1

reaction 2

reaction 3

reaction 4

35.2 ( 0.3 20.9 ( 9

44.3 ( 0.2 21.3 ( 0.7

34.9 ( 0.8 18.6 ( 2.2

42.1 ( 1.0 16.5 ( 2.8

Kinetic constants are expressed in cm6/(mol2 min).

Table 8. Equilibrium Constants for Reactions 1 and 3 Evaluated from Standard Gibbs Energy of Formation for the Runs with Malonic Acid T (°C)

KE1

KE3

80 90 100 110 120

3846 3064 2470 2015 1660

194 167 146 128 113

the activation energies related to the four reactions are quite similar. The values of the equilibrium constants KE1 and KE3 have been estimated from standard thermodynamic calculations and are reported in Table 8. As can be seen, these values are rather high, and this means that reactions 1 and 3 are almost completely shifted toward the products in the adopted operative conditions. By using the parameters reported in Tables 7 and 8, the kinetic model has been used for the description of the experimental data obtained by using malonic acid as catalyst, and some results are reported, as examples of the accuracy of fitting, in Figures 4-6. As can be appreciated from these plots, the agreement between the experimental data and the proposed model is good.

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Ind. Eng. Chem. Res., Vol. 46, No. 20, 2007 Table 10. Selectivities in Terms of Ratios between Kinetic Constants for Various Catalysts at T ) 100 °C

acetic acid malonic acid citric acid levulinic acid succinic acid propionic acid

Figure 7. Evolution in time of the composition for the experimental run with levulinic acid at T ) 100 °C. Comparison between the experimental data and the model behavior. Table 9. Kinetic Constants at T ) 100 °C for Various Catalysts acetic acid malonic acid citric acid levulinic acid succinic acid propionic acid a

K1a

K2a

34 619 ( 4 012 13 274 ( 1 692 3 307 ( 387 16 905 ( 1 801 13 549 ( 1 290 25 545 ( 2 871

2 342 ( 198 1 089 ( 87 247 ( 21 1 315 ( 119 1 028 ( 95 2 265 ( 201

Kinetic constants are expressed in

K3a

K4a

1 576 ( 302 17 ( 4 1 784 ( 407 26 ( 7 269 ( 41 0.5 ( 0.2 1 421 ( 85 17 ( 6 1 354 ( 78 14 ( 5 1 197 ( 91 12 ( 4

cm6/(mol2

min).

The products distribution obtained for other catalysts is qualitatively similar to that of malonic acid and, for comparison purposes, in Figure 7 is reported the evolution in time of the products mole fractions for a run performed at 100 °C where levulinic acid was used as the it catalyst. Another interesting consideration can be made by examining the ratio of the kinetic constants obtained. As example, by considering the run with malonic acid at T ) 100 °C, the ratio K1/K2 indicates that the chlorine substitution in the R position is ∼12× faster with respect to the same substitution in the β position. In analogy, the ratio K3/K4 shows that the formation of R,γ-dichlorohydrin is 68× faster if compared to that of R,βdichlorohydrin. According to the proposed mechanism, represented by relations 4-6, the reactions steps for the first and second chlorination are substantially the same, so the ratios K1/ K2 and K3/K4 should result in being quite similar, but actually, the Ki are defined as pseudo-kinetic constants and their values, ultimately, depend also on the esterification equilibrium constants of the single elementary step (see eq 13). On the other hand, the ratios K1/K3 and K1/K4, indicate that, starting from R-monochlorohydrin, the substitution reaction in the γ position is 7.4× slower than the first chlorination, while the attack of chlorine in the β position is 510× slower than the first chlorination. All these considerations seem to support the presence of a certain induction effect of the first chlorine atom that favors the attack of the second chlorine in the γ position

K1/K2 ) R/β

K3/K4 ) Rγ/Rβ

K1/K3 ) R/Rγ

K1/K4 ) R/Rβ

14.8 12.2 13.4 12.9 13.2 11.3

92.7 68.6 538.0 83.6 96.7 99.8

22.0 7.4 12.3 11.9 10.0 21.3

2 036.4 510.5 6 614.0 994.4 967.8 2 128.8

rather than in the β position, in agreement to what was expressed by the proposed reaction mechanism. In Table 9 are reported for comparison the kinetic constants, evaluated for the different tested organic acids, at T ) 100 °C, and their related confidence intervals. From these values, it is possible to observe that acetic acid is the most active in the experimental conditions of total reflux adopted. Other catalysts, such as propionic and malonic acids, have shown lower values of the hydrochlorination kinetic constant K1 (R-monochlorohydrin formation) and comparable values of K3 (R,γ-dichlorohydrin formation).The use of the mentioned alternative catalysts is, therefore, of great potential interest by considering their low volatility that could result in a more constant concentration, avoiding losses by evaporation. Finally, in Table 10, the selectivities toward the different products, expressed in terms of ratios between the kinetic constants, are reported for the tested catalysts. In all the examined cases, the first chlorination in the R position is an order of magnitude faster with respect to that in the β position, in analogy to what was observed for malonic acid. Although citric acid resulted in being less active, it has shown the highest selectivity toward R,γ-dichlorohydrin with respect to R,βdichlorohydrin. Conclusions In the present paper, the glycerol chlorination reaction by gaseous hydrochloric acid has been investigated. Different organic acids have been tested in this reaction as valid alternatives to the traditional catalyst, which is acetic acid. The use of less volatile catalysts, in fact, represents an improvement for what concerns the minimization of catalyst losses by evaporation. Moreover, the use of pure gaseous HCl, instead of its aqueous solution, involves further advantages for this reaction, such as more efficient water removal and the possibility of a selective recovery of the desired product 1,3-dichlorohydrin. This compound has shown a relatively low volatility, in comparison with that of the others chlorinated products observed. This finding involves the possibility to separate the 1,3dichlorohydrin directly by stripping with gaseous HCl, and this aspect will be the object of a future investigation. The interpretation of the collected experimental data has been based on a detailed three-step reaction mechanism in which the presence of a vicinal hydroxyl group plays a fundamental role in the catalyst reintegration reaction through alkyl-oxygen bond scission. In this mechanism, the most probable rate-determining step resulted in the esterification reaction between glycerol and the carboxylic acid being used as catalyst. Starting from the proposed reaction mechanism, kinetic expressions have been derived for the description of the evolution with time of the observed products distribution. The mathematical model has been applied to the description of isothermal batch runs, and the kinetic parameters have been evaluated. The results of the mentioned model are in good agreement with the experimental observations.


The same model has also been used for comparing activities and selectivities of the different tested catalysts. From the kinetic approach, it resulted that acetic acid is the best catalyst, but the activities shown by the other catalysts are comparable, and considering that the volatility of these catalysts are much lower than that of acetic acid, their use is more convenient in a continuous plant. Acknowledgment Thank are due to ASER srl for the financial support. List of Symbols r, ri ) reaction rate of ith reaction (mol/(cm3 min)) rest ) reaction rate for esterification reaction (mol/(cm3 min)) rchlor ) reaction rate for esterification reaction (mol/(cm3 min)) Ki ) kinetic constant of the ith reaction (cm6/(mol2min)) K ) pseudo-kinetic constant (cm6/(mol2min)) ki ) kinetic constant of the ith elementary step (note: the units are different according to the stoichiometry of each elementary step) Kest ) kinetic constant for esterification reaction (cm3/(mol min)) Kchlor ) kinetic constant for chlorination (cm3/(mol min)) KE1 ) equilibrium constant of reaction 1 KE3 ) equilibrium constant of reaction 3 keqi ) equilibrium constant for the ith elementary step CC ) catalyst concentration (mol/cm3) CC* ) actual catalyst concentration (mol/cm3) CE* ) actual ester concentration (mol/cm3) CH ) HCl concentration (mol/cm3) CGLY ) glycerol concentration (mol/cm3) CR ) R-monochlorohydrin concentration (mol/cm3) Cβ ) β-monochlorohydrin concentration (mol/cm3) CRγ ) R,γ-dichlorohydrin concentration (mol/cm3) CRβ ) R,β-dichlorohydrin concentration (mol/cm3) CW ) water concentration (mol/cm3) A, B, C ) empirical constants in relation 26 t ) time (min) VR ) reaction volume (cm3) ni ) number of moles of ith component (mol) EAi ) activation energy for the ith reaction (kJ/mol)

(2) Krafft, P.; Gilbeau, P.; Gosselin, B. Claessens, S. Process for producing dichloropropanol from glycerol, the glycerol coming eventually from the conversion of animal fats in the manufacture of biodiesel. WO 054167 A1, 2005. (3) Siano, D.; Santacesaria, E.; Fiandra, V.; Tesser, R.; Di Nuzzi, G.; Di Serio, M.; Nastasi, M. Process for the production of alpha, gammadichlorohydrin from glycerin and hydrochloric acid. WO 111810 A2, 2006. (4) Ullmann’s Encylopedia of Industrial Chemistry, 7th ed.; WileyVCH: Weinheim, Germany, 2006. (5) Reisch, M. S.; Tullo, A. H. Chem. Eng. News 2006, Dec, 34. (6) Solvay Inc. Process for the production of alpha-epichlorhydrin. GB 799,567, 1958. (7) Bruin, P. Epoxy ethers, their polymers and derivatives. GB 814,695, 1959. (8) Brojer, Z.; Penczec, P. A.; Sas, J. H. Process of manufacturing epoxide resins. GB 1,001,364, 1965. (9) Kawabe, T.; Masaaki, M. Process for continuous production of epichlorohydrin. U.S. Patent 4,113,746, 1978. (10) Carra`, S.; Santacesaria, E.; Morbidelli, M. Synthesis of epichlorohydrin by elimination of hydrogen chloride from chlorohydrins. 1. Kinetic aspects of the process. Ind. Eng. Chem. Process Des. DeV. 1979, 18 (3), 424-427. (11) Carra`, S.; Santacesaria, E.; Morbidelli, M. Synthesis of epichlorohydrin by elimination of hydrogen chloride from chlorohydrins. 2. Simulation of the reaction unit. Ind. Eng. Chem. Process Des. DeV. 1979, 18 (3), 428-433. (12) Clarke, H. T.; Hartman, W. W. Epichlorohydrin. In Organic Syntheses, Coll.; 1941; Vol. 1, p 233. (13) Conant, J. B.; Quayle, O. R. Glycerol R-monochlorohydrin. In Organic Syntheses, Coll.; 1941; Vol. 1, p 294. (14) Conant, J. B.; Quayle, O. R. Glycerol R,γ-dichlorohydrin. In Organic Syntheses, Coll.; 1941; Vol. 1, p 292. (15) Fauconnier, S. Bull. Soc. Chim. 1887, 2 (48), 236-240. (16) Hill, A. J.; Fischer, E. J. A synthesis of beta-chloro-allyl chloride. J. Am. Chem. Soc 1922, 44, 2582-2595. (17) Britton, E. G.; Heindel, R. X. Preparation of glycerol dichlorohydrin. U.S. Patent 2,144,612, 1939. (18) Britton, E. C.; Slagh, H. R. Glycerol dichlorohydrin. U.S. Patent 2,198,600, 1940. (19) Thompson, W. P. Recovery of aliphatic chlorohydrins having from 3 to 5 carbon atoms from aqueous solution. GB 931,211, 1963. (20) Graham Solomons, T. W. Organic Chemistry, 7th ed.; John Wiley & Sons: New York, 1999; pp 741-744. (21) March, J. AdVanced Organic Chemistry, 5th ed.; Wiley-Interscience: New York, 2001; pp 308-312. (22) Macey, R.; Oster, G. Berkeley MadonnaTMsModeling and Analysis of Dynamic Systems, release 8.3; University of California Berkeley: Berkeley, CA, 2001.

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ReceiVed for reView May 17, 2007 ReVised manuscript receiVed July 16, 2007 Accepted July 21, 2007 IE070708N

Kinetics of Glycerol Chlorination with Hydrochloric Acid: A New Route

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