ARTICLE pubs.acs.org/IECR
Transesterification of Dimethyl Carbonate with Ethanol To Form Ethyl Methyl Carbonate and Diethyl Carbonate: A Comprehensive Study on Chemical Equilibrium and Reaction Kinetics Tobias Keller,*,† Johannes Holtbruegge,† Alexander Niesbach,† and Andrzej Gorak†,‡ †
TU Dortmund University, Department of Biochemical and Chemical Engineering, Laboratory of Fluid Separations, Emil-Figge-Strasse 70, D-44227 Dortmund, Germany ‡ Lodz Technical University, Department of Environmental and Process Engineering, Lodz, Poland
bS Supporting Information ABSTRACT: This article studies the consecutive transesterification of dimethyl carbonate with ethanol to form ethyl methyl carbonate and diethyl carbonate. The chemical equilibrium and reaction kinetics of this system are investigated experimentally and theoretically. To enhance the reaction rate, the homogeneous catalyst sodium ethoxide was applied. Molar-based and activity-based chemical equilibrium constants were calculated from experimental results, and their temperature dependence was described using the van’t Hoff equation. An activity-based kinetic model that considers the temperature dependence of the reaction rate constants with the Arrhenius equation was derived.
1. INTRODUCTION Reactive distillation (RD), in which the chemical reaction and physical separation are integrated in a single apparatus, represents one of the best-known examples of process intensification1,2 and has been established in process technology. The instantaneous product removal from the reaction zone through distillation leads to an increased yield of chemical equilibriumlimited reactions such as etherification3 and esterification.4 The majority of papers published on RD have investigated chemical systems that are comprised of only one target product (e.g., the ester of an esterification reaction) and have demonstrated the possibility of using RD to increase reactant conversion and the purity of the target product, with selectivity playing a secondary role. In our opinion, the potential of RD has not been fully exploited for reaction systems producing more than one target product. In such multiple-product systems, RD enables the process engineer to vary the selectivity of each target product while still achieving high reactant conversions. The RD column acts as a multipurpose reactor, which substantially extends the current field of applications of RD.5 Only a few reports have been published on the simultaneous control of conversion and selectivity in this type of system. Specifically, Thotla et al.6 investigated the dimerization reaction of acetone to form mesityl oxide and diacetone alcohol as desired products, demonstrating that different operating and design parameters of the RD column resulted in different product ratios. Orjuela et al.7 performed experiments in a pilot-scale RD column to study the parallel esterification of succinic acid and acetic acid with ethanol. The authors verified that both target products, diethyl succinate and ethyl acetate, can be simultaneously produced at high selectivities in a single RD column. Luyben and Yu8 gave an overview of further reaction systems, for which RD can be applied to produce more than one target product. r 2011 American Chemical Society
An industrially relevant example is the reversible, consecutive, second-order transesterification of dimethyl carbonate (DMC) with ethanol to form ethyl methyl carbonate (EMC) and diethyl carbonate (DEC). Because this reaction system consists of two equilibrium-limited transesterification steps, the application of RD is a promising way to intensify this process. For the modeling, simulation and design of an RD column, detailed knowledge about the chemical equilibrium and reaction kinetics of the system is important.9,10 Numerous catalysts are known to enhance the kinetic rate of transesterification reactions. Both acidic and basic catalysts, either homogeneous or heterogeneous, can be used for this purpose.11 Several previous studies have discussed the transesterification reaction of DMC with different alcohols, and an overview of these results is provided online as Supporting Information to the electronic version of this article. Heterogeneous catalysts are usually preferred, because of their reusability and facile separation from the reaction mixture. Dipotassium carbonate on an unspecified phase-transferring agent12 and dipotassium carbonate coated on polyethylene glycol13,14 were found to be effective catalysts for the transesterification reaction of DMC with ethanol. However, the authors reported some difficulties with the mechanical stability and the chemical resistance of the self-coated catalyst. Thus, this catalyst could not be used for an experimental study in an RD column. Zielinska-Nadolska et al.14 examined various heterogeneous catalysts and found that two acidic ion-exchange resins (Lewatit K1221 and Nafion SAC-13) exhibited the highest activities of all of the heterogeneous catalysts investigated, but their reaction rates were still too low Received: July 12, 2011 Accepted: September 1, 2011 Revised: August 29, 2011 Published: September 01, 2011 11073
dx.doi.org/10.1021/ie2014982 | Ind. Eng. Chem. Res. 2011, 50, 11073–11086
Industrial & Engineering Chemistry Research
ARTICLE
Scheme 1. Transesterification of Dimethyl Carbonate (DMC) with Ethanol To Form Ethyl Methyl Carbonate (EMC) and Methanol (Reaction R1)
Scheme 3. Disproportionation of Two Molecules of Ethyl Methyl Carbonate (EMC) To Form Dimethyl Carbonate (DMC) and Diethyl Carbonate (DEC)
Scheme 2. Transesterification of Ethyl Methyl Carbonate (EMC) with Ethanol To Form Diethyl Carbonate (DEC) and Methanol (Reaction R2)
step: the transesterification of EMC with EtOH (see Scheme 2) and the disproportionation of two molecules of EMC, forming DEC and DMC (see Scheme 3). The experiments performed in this study investigated whether the disproportionation reaction was fast enough to be taken into account in the description of the reaction kinetics. In the following text, we refer to the first transesterification (see Scheme 1) as reaction R1 and the second transesterification (see Scheme 2) as reaction R2.
to be used in an RD column. The same was true for Group IIIB metal oxides such as Sm2O3, which were patented as solid catalysts for the transesterification of DMC with EtOH.15 Because currently available heterogeneous catalysts seem to be instable or not sufficiently active for application in an RD column, we applied a homogeneous catalyst for our study. Mei et al.16 presented a newly developed homogeneous catalyst, lanthanum nitrate hexahydrate, which is far too expensive to use in an RD column. Thus, we screened the following four homogeneous catalysts: potassium hydroxide, sulfuric acid, dibutyl tin acetate, and sodium ethoxide. Based on the screening results, we selected the homogeneous catalyst sodium ethoxide. This paper presents chemical equilibrium data and reaction rate data for the transesterification reaction of DMC with EtOH homogeneously catalyzed by sodium ethoxide. Furthermore, an activity-based reaction rate model was developed, which is an essential requirement for a reliable RD column design. The required activity coefficients were calculated with the UNIQUAC model.
2. REACTION NETWORK This study investigates the consecutive, second-order transesterification of dimethyl carbonate (DMC) with ethanol to form ethyl methyl carbonate (EMC) and diethyl carbonate (DEC). Both EMC and DEC are industrially relevant products. The asymmetric carbonic ester EMC has been found to be a suitable co-solvent for incorporation into nonaqueous electrolytes to enhance the low-temperature performance of rechargeable alkali metal-ion batteries on characteristics such as energy density, discharge, and capacity.1720 DEC represents an attractive replacement for hazardous ethyl halides and phosgene as an ethylization and carbonylation reagent in organic synthesis.21 Similarly to EMC, DEC is widely used as a co-solvent in alkali metal-ion batteries.22 Most importantly, DEC is considered to be the best alternative for methyl tert-butyl ether (MTBE) as an oxygen-containing fuel additive.21,23,24 The reaction converting DMC to DEC is a two-step reaction involving the transesterification of DMC with EtOH to the intermediate EMC and the byproduct MeOH as the first step (see Scheme 1). Two pathways are possible for the second
3. EXPERIMENTAL PROCEDURE 3.1. Experimental Setup. Two experimental series were performed to determine the chemical equilibrium and the reaction kinetics of the transesterification of DMC with ethanol (EtOH). The first series of experiments was performed at atmospheric pressure. However, these experiments were carried out only up to a temperature of only 333 K: above that temperature, the evaporation of the lower-boiling component methanol (MeOH) cannot be controlled, leading to erroneous results. Therefore, a second series of experiments investigating reaction temperatures higher than 333 K was performed under high pressure (approximately 1.5 103 kPa) to prevent the evaporation of the reactants. Different experimental setups were used for the two experimental series. The experiments at atmospheric pressure were performed in a glass three-necked flask with a total volume of 250 mL. Two necks were used to measure the temperature with a thermometer and to withdraw samples with a syringe. A reflux condenser supplied with tap water was connected to the third neck to avoid the loss of volatile compounds. Molecular sieves of 4 Å were attached to the upper end of the reflux condenser to prevent water from entering into the reaction system. A magnetic stirrer with a rotational speed of 300 rpm ensured complete mixing. To reach and maintain the desired reaction temperatures, the threenecked flask was immersed in a thermostat-controlled oil bath with a magnetic stirrer. Figure 1a presents a scheme of the setup for the experiments at atmospheric pressure. The reactants (DMC and EtOH) were placed in the three-necked flask with a total volume of 200 mL and heated to the desired temperature. Based on the preliminary tests, no reaction took place without any catalyst present. To start the reaction, the homogeneous catalyst sodium ethoxide was added to the reaction mixture, and this point was defined as time zero. During the experiment, 1-mL samples were taken from the reaction mixture using a syringe and were added to precooled vials to stop the reaction. Afterward, the vials were placed in dry ice (T = 195 K) and analyzed within 2 h using gas chromatography (see Section 3.2). The experiments at high pressure were carried out in a stainless-steel high-pressure reactor (Parr Instrument Co.) with a total capacity of 300 mL. Figure 1b presents a scheme of this 11074
dx.doi.org/10.1021/ie2014982 |Ind. Eng. Chem. Res. 2011, 50, 11073–11086
Industrial & Engineering Chemistry Research
ARTICLE
Figure 1. Setup for the experiments (a) at atmospheric pressure and (b) at a high pressure of ∼1.5 103 kPa.
setup. The reactor had a modular design and several connections on the reactor cover. A pressure gauge, which was used to observe the pressure inside the reactor, and a Model PT-100 thermocouple, which was used to measure the temperature in the liquid phase, were connected to the reactor. The reaction mixture was stirred with a diagonal blade stirrer (300 rpm). The jacked reactor vessel was connected to a thermostat, which controlled the desired temperature of the reaction mixture. At the beginning of each experiment, the two reactants (DMC and EtOH) were placed in the reactor with a total volume of 280 mL. The reactor was inerted and filled with helium until a total pressure of 500 kPa was reached. After the reaction mixture was heated to the desired temperature, the reaction was started by adding the homogeneous catalyst sodium ethoxide dissolved in EtOH, using a stainless-steel cannula with a total volume of 3.8 mL. The cannula filled with the homogeneous catalyst was connected to the helium source to adjust the pressure to 1.5 103 kPa. The valve between the cannula and the reactor was opened for 1 s, to feed the catalyst solution to the reaction mixture. We determined that ∼0.49 ( 0.03 mL of the catalyst solution remained in the cannula. During the experiment, samples were taken from the reaction mixture using a suction line. The liquid from the reaction mixture was collected in a 1-mL precooled stainlesssteel sample bottle. The sample bottle was precooled by placing it in dry ice (T = 195 K). The gas chromatographic analysis was performed within 2 h (see Section 3.2), and each sample was collected in a new sample bottle. Prior to each sample, the suction line was flushed by discarding ∼1 mL of liquid from the line. As a result of the time required for the sampling procedure (screwing a new sample bottle onto the reactor and flushing the suction line), one sample per minute was taken from high-pressure experiments. Systematic errors for both experimental setups were ruled out, because identical results were obtained from kinetic experiments performed in the glass three-necked flask and in the high-pressure reactor. 3.2. Analytical Methods. A gas chromatograph (Shimadzu Model GC-14B) was used to analyze all of the samples offline.
The instrument was equipped with a flame-ionization detector (FID), and helium was used as a carrier gas, with a gas velocity of 27 cm s1. An Innopeg FFAP capillary column was used with a diameter of 0.32 mm, a length of 25 m, and a film thickness of 0.64 μm. The temperature was set to 368 K. Typical retention times were 2.0 min for MeOH, 2.2 min for EtOH, 2.8 min for DMC, 3.2 min for EMC, and 3.9 min for DEC. Mass fractions were obtained directly from the evaluation of the chromatogram based on single-component calibration curves by applying acetonitrile as an internal standard. Acetonitrile was chosen because its retention time was close to those of the target components, and its peak did not overlap with other peaks. The quality of the calibration curves was continuously monitored during the experiments by analyzing test mixtures of known compositions. 3.3. Chemicals. EtOH and MeOH were supplied from VWR International with a guaranteed purity of g99.9 mass %. DMC, EMC, DEC, and the internal standard acetonitrile were purchased from Merck with the same guaranteed quality. The homogeneous catalyst sodium ethoxide was obtained from Merck at 20 mass % in EtOH. In the following text, the mass or molar fraction of the catalyst always refer to the pure amount of catalyst, rather than that of the dilution.
4. CALCULATION OF LIQUID-PHASE ACTIVITIES To describe a real, nonideal, liquid mixture, the interactions between the different components must be considered in the kinetic approach by using activities ai for each component. The activity is related to the molar fraction xi through the following equation: ai ¼ γi xi The activity coefficients γi can be calculated using any applicable activity coefficient model (for example, the Wilson, NRTL, UNIQUAC or UNIFAC models).25 In this study, the UNIQUAC model26 was applied, following the recommendations presented by Carlson.27 In the UNIQUAC equations, the size 11075
dx.doi.org/10.1021/ie2014982 |Ind. Eng. Chem. Res. 2011, 50, 11073–11086
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. Pure-Component Volume Parameter ri and Area Parameter qi Used in the UNIQUAC Model component
ri
qi
ref
MeOH
1.431
1.432
28
EtOH DMC
2.105 3.048
1.972 2.816
28 29
EMC
3.781
3.388
30
DEC
4.397
3.896
29
Table 2. UNIQUAC Binary Interaction Parameters aij and bij (see eq 1) (the Corresponding Values of the Coefficient of Determination (R2), and the Publication Sources for the Experimental VLE Data Are Also Included) component 1 component 2 MeOH
MeOH
and volume of the molecules are described with the volume parameter ri and the area parameter qi. Both pure component parameters were taken from the literature and are listed in Table 1. The UNIQUAC model used in this work considers the temperature dependence of the activity coefficient γi by eq 1, in which τij is the binary interaction parameter between component i and j: bij τij ¼ exp aij þ ð1Þ T
DMC
j
R2
aij
bij [K]
1 2
0.000
48.990 0.9999
2 1
0.000
1 2 0.201 2 1
ref 28
38.517 14.870 0.9999
31
0.273 306.550
MeOH
EMC
1 2 0.086 38.822 0.9995 2 1 0.026 278.229
32
MeOH
DEC
1 2 0.322
33
EtOH
The coefficients aij and bij were obtained by regressing them to experimental vaporliquid equilibrium data available in the literature for all binary mixtures (see references in Table 2). The resulting values are listed in Table 2. In addition, the corresponding values of R2, the coefficient of determination, are given to demonstrate the excellent description of the binary vaporliquid equilibria by the UNIQUAC equations. A graphical comparison between the experimental and calculated vaporliquid equilibria of all of the binary systems is available online as Supporting Information.
EtOH
DMC
EMC
71.678 0.9987
2 1
0.369 376.829
1 2
0.000
93.595 0.9999
2 1
0.000
81.760
1 2
0.156
21.509 0.9960
31
32
2 1 0.091 197.632 EtOH
DEC
1 2 0.298 2 1
DMC
DMC
5. RELEVANCE OF THE DISPROPORTIONATION REACTION As described in Section 2, there are two different pathways for the reaction of EMC to DEC: the transesterification of EMC with EtOH and the disproportionation of two molecules of EMC to form DEC and DMC. To investigate whether the disproportionation reaction proceeds sufficiently fast, additional experiments were performed without another reactive component besides EMC. The reactants DMC and EtOH were replaced with the inert solvent acetone. In the absence of EtOH, EMC can only be converted to DEC via the disproportionation reaction and cannot react via the transesterification reaction. If DEC is detected within the usual time frame of an experiment, then the disproportionation reaction proceeds at a considerable rate and must be considered in the future analysis of the reaction kinetic data. To verify that acetone was inert with regard to the reaction, an additional experiment was performed with an initial molar ratio between EtOH and DMC of χEtOH/DMC = 3 in the presence of acetone with a molar fraction of 0.5 mol mol1. The reaction temperature was set to 323 K, and the catalyst molar fraction was 5.0 104 mol mol1. The same chemical equilibrium constants and the same reaction rate were obtained as in the experiment without added acetone, which demonstrated that acetone does not influence the reaction rate and can be regarded as inert. The experiment with EMC dissolved in acetone showed that neither DMC nor DEC was formed within the usual time frame of 3 h. Even after 36 h, no DMC or DEC were detected in the
EtOH
i
EMC
DEC
1 2 0.230
30.426 0.9997
2 1 0.030
57.864
1 2 1.523 2 1
EMC
DEC
131.676 0.9987
0.410
1 2 0.333 2 1
33
0.381 365.987
57.293 0.9984
32
30
232.946 195.435 0.9999
32
0.378 219.275
reaction mixture, which proves that the disproportion reaction of EMC does not need to be considered in the analysis of the chemical equilibrium and reaction kinetic data for the transesterification of DMC with EtOH using the homogeneous catalyst sodium ethoxide. In contrast, Shen et al.34 found that the homogeneous catalyst tetrabutoxytitanium favors the disproportionation reaction of EMC. These different results agree with the observations made by Williams et al.35 that the course of the disproportionation reaction is dependent on the catalyst selected and that especially tetrabutoxytitanium promotes this kind of reaction. Apparently, the disproportionation reaction is enhanced by organometallic catalysts, which was also found by Haubrock et al.36 The authors investigated the transesterification reaction of DMC with phenol using the homogeneous catalyst tetrabutoxytitanium and demonstrated that the disproportionation reaction—in this case, the disproportionation of methyl phenyl carbonate—occurred at a significant rate and could not be neglected in the analysis of the kinetic reaction data. Furthermore, Haubrock et al.36 showed that the disproportionation reaction is intrinsically faster than the two transesterification reactions. As we have shown, this is not the case for the transesterification of DMC with EtOH using the homogeneous catalyst sodium ethoxide. 11076
dx.doi.org/10.1021/ie2014982 |Ind. Eng. Chem. Res. 2011, 50, 11073–11086
Industrial & Engineering Chemistry Research
ARTICLE
Table 3. Operating Conditions for All of the Experiments Performed To Determine the Chemical Equilibrium and Reaction Kinetics (the Pressure P, Temperature T, Initial Molar Ratio between EtOH and DMC (χEtOH/DMC), and Catalyst Molar Fraction xcat Were Varied) experiment
T [K]
χEtOH/DMC
xcat [mol mol1]
Atmospheric Pressure (P = 101.3 kPa) E1 E2
323 323
3.0 3.0
4 104 5 104
E3
323
2.0
5 104
E4
323
2.0
5 104
E5
323
3.0
2 103
E6
333
3.0
4 104
E7
333
3.0
5 104
E8
333
2.0
5 104
should be used for RD column design, we selected the temperature range for the kinetic experiments to cover the expected temperature range of the RD column. Two different experimental setups were used for the experimental investigation, which were previously described in Section 3.1. Note that in experiments E18E21, which were performed at the highest temperatures (373403 K), only the chemical equilibrium and no reaction kinetics could be determined (see Section 8 for discussion).
7. CHEMICAL EQUILIBRIUM 7.1. Thermodynamics. In the special case of ideal liquidphase behavior, all of the activity coefficients are set to 1, yielding the following mathematical formulations for the molar-based equilibrium constants Kx,R1 and Kx,R2:25 eq
High Pressure (P = 1.5 103 kPa) E9 E10 E11
343 343 343
3.0 3.0 2.0
4 104 5 104 5 10
4 4
E12
353
3.0
4 10
E13
353
3.0
5 104 4
E14
353
2.0
5 10
E15
363
3.0
4 104
E16
363
3.0
5 104
E17 E18
363 373
2.0 3.0
5 104 4 104
E19
383
3.0
5 104
E20
393
3.0
5 104
E21
403
3.0
5 104
6. OVERVIEW OF EXPERIMENTS To provide reliable experimental data on the chemical equilibrium and on the reaction kinetics, 21 experiments (E1E21) on the transesterification of DMC with EtOH were successfully performed in a batch reactor. The homogeneous catalyst sodium ethoxide was applied to enhance the reaction rate. Because the reaction kinetics experiments were continued until the chemical equilibrium was reached, experimental data regarding the chemical equilibrium and the reaction kinetics were obtained in a single experiment. In all of the experiments, the chemical equilibrium was assumed to be reached when the concentration profiles of each component reached constant levels. The time to achieve chemical equilibrium was typically in the range of 230 min. To guarantee that chemical equilibrium was reached, each experiment was performed for at least 90 min. Moreover, the concentration profiles were judged to be flat when the absolute difference between the component mass fractions of two sampling points was
