Autocatalytic Kinetic Study of Dimethyl Oxalate Consecutive

Article pubs.acs.org/IECR

Autocatalytic Kinetic Study of Dimethyl Oxalate Consecutive Hydrolysis Yan Xu, Qu Chen, Yu-jun Zhao,* Jing Lv, Zhen-hua Li, and Xin-bin Ma* Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China ABSTRACT: The autocatalytic hydrolysis kinetics of dimethyl oxalate (DMO) was investigated in an isothermal batch reactor at 328.15−358.15 K. It was observed that DMO hydrolysis involved two reactions in series, with monomethyl oxalate (MMO) as an intermediate product. The results showed that water dominates the initial hydrolysis rate of DMO. In later stages, oxalic acid plays a major role in catalyzing DMO hydrolysis and MMO further hydrolysis. Based on these observations, a mechanism involving the ionization of water and oxalic acid was developed that assumes nucleophilic substitution to be the rate-determining step. Concentration-based rate equations were further deduced including the contribution of water startup and the catalytic action of oxalic acid. In addition, the kinetic and equilibrium parameters were estimated from the experimental data through regression analysis.

1. INTRODUCTION Oxalic acid is the simplest organic diacid, and it is well-known for its applications in pharmaceutical synthesis, metallic surface cleaning, rare-earth extraction, textile printing and dyeing, catalyst preparation, and so on.1−5 Traditionally, oxalic acid can be produced by the sodium formate and carbohydrate methods.6,7 The sodium formate process involves the thermal decomposition of sodium formate to sodium oxalate, the conversion of sodium oxalate into insoluble calcium oxalate, and finally the liberation of oxalic acid with sulfuric acid. Although a high yield of oxalic acid is obtained, this process is complicated by the longer reaction path accompanied with high consumption of sulfuric acid and difficult calcium sulfate recovery. The carbohydrate process involves the oxidation of sucrose or cellulose with nitric acid and has the disadvantage of providing a low yield of oxalic acid due to overoxidation. Many articles have mentioned the possibility of oxalate synthesis through carbon monoxide coupling,8−14 and the development of downstream products, such as ethylene glycol,15−18 methyl glycolate, and glycolic acid,19 has begun to attract growing interest. Moreover, oxalic acid can also be produced through the hydrolysis of dimethyl oxalate (DMO). Using DMO from carbon monoxide coupling, the oxalic acid production process includes the steps DMO synthesis 2CO + 2CH3OH +

1 O2 → (COOCH3)2 + H 2O 2

high rate even without additional catalyst. Therefore, it is necessary to investigate the production of highly purified oxalic acid through a green process involving the autocatalytic hydrolysis of DMO. Much attention has been directed toward the study of hydrolysis and its reverse process (esterification),20−23 but only a few studies have considered autocatalytic hydrolysis. Jogunola et al. investigated the hydrolysis of alkyl formates without addition of a catalyst.24−26 They found that there was an induction period (time interval between the commencement of the reaction and the first sign of an appreciable conversion) during methyl formate hydrolysis and that the induction period decreased as temperature increased. They expressed the rate of reaction by introducing a kinetic factor with both noncatalyzed and autocatalyzed contributions. The noncatalyzed contribution was considered to be temperature-dependent, whereas the autocatalyzed contribution was expressed as the product of a rate constant and the concentration of formic acid. This work focuses on the autocatalytic kinetics and reaction pathways of DMO hydrolysis and the effects of water on the initial hydrolysis rate. Additionally, the effects of the initial molar ratio of the reactants and the temperature on the reaction kinetics were also investigated.

2. EXPERIMENTAL SECTION 2.1. Apparatus and Experimental Procedure. All experiments were carried out in a four-necked flask with a capacity of 500 mL. The flask was equipped with a mechanical stirrer, a sampling device, and a thermometer. A reflux condenser was attached to the top of the flask to avoid the loss of volatile liquidphase compounds. The reaction temperature was controlled to within ±0.5 K by locating the flask in a water bath.

(1)

DMO hydrolysis (COOCH3)2 + 2H 2O ⇌ (COOH)2 + 2CH3OH

(2)

If the methanol hydrolyzate were recovered and completely returned to the process of DMO synthesis, oxalic acid could theoretically be synthesized from carbon monoxide, oxygen, and water. This means that the production of oxalic acid from DMO is environmentally benign and atomically economical. Preliminary investigations showed that DMO hydrolysis proceeds at a © 2014 American Chemical Society

Received: Revised: Accepted: Published: 4207

December 16, 2013 February 23, 2014 February 24, 2014 February 24, 2014 dx.doi.org/10.1021/ie404253x | Ind. Eng. Chem. Res. 2014, 53, 4207−4214

Industrial & Engineering Chemistry Research

Article

The flask was filled with the desired amount of distilled water and heated to the reaction temperature. The required amount of DMO (J&K Chemical Co., 99.0%), preheated to the desired reaction temperature, was discharged into the flask through a thermostatic funnel to start the hydrolysis reaction. All experiments in this work were conducted at a stirring speed of 350 rpm to ensure that the reactants mixed thoroughly. The extent of reaction was monitored by determining the compositions of small samples withdrawn from the reaction mixture at regular intervals. The corresponding sample was diluted with acetonitrile to prevent the crystallization of oxalic acid or DMO and further hydrolysis before examination by highperformance liquid chromatography (HPLC). 2.2. Analytical Methods. All samples were analyzed with an Agilent 1100 high-performance liquid chromatograph with a ZORBAX 125 SB-C18 column (4.6 mm × 250 mm, 5 μm). The eluent was 10% acetonitrile and 90% phosphoric acid (pH 2.38, adjusted with phosphoric acid) flowing at 0.6 mL/min. UV detection was at 210 nm. Peak identification was established by comparison of the retention times of the sample and standard solutions. The concentrations of oxalic acid and DMO were determined by calibration curves obtained by analyzing standard solutions with known concentrations, whereas the concentrations of the other compounds were calculated based on stoichiometric relationships. Gas chromatography/mass spectrometry (GC/MS) analysis of the reactants was performed using an Agilent 6890GC5975MSD system equipped with an HP-5MS capillary column (30 m × 0.25 mm i.d. × 0.25 μm). Helium was used as both the carrier (1 mL/min) and makeup (45 mL/min) gases. The injector was operated at 220 °C in split mode with a split ratio of 100:1.

Figure 1. Mole fractions of the main components versus reaction time (T = 348.15 K, RW/DMO = 19.7). The continuous lines represent the results of the proposed model.

3. RESULTS AND DISCUSSION 3.1. Reaction Pathway. Based on the results of product analysis by HPLC, another compound was found in addition to oxalic acid and DMO. The peak of this unknown compound appeared between those of DMO and oxalic acid in the HPLC chromatograms. The compound was determined to be monomethyl oxalate (MMO) with a molecular weight of 104 upon further analysis of the mass spectrum. The products of the reaction indicate that multiple reactions occur during the process of DMO hydrolysis. Figure 1 shows typical concentration profiles with time. The concentration of MMO increased significantly in the first 10 min, but the oxalic acid generation rate was rather low at this stage. From 10 to 30 min, the formation of oxalic acid accelerated noticeably, and the concentration of MMO increased at the same time. After about 30 min, the concentration of MMO peaked and decreased gradually, whereas oxalic acid was generated continuously, until the concentrations of the reactants tended to remain constant. The concentration variations revealed that DMO was first hydrolyzed to generate MMO and then MMO was further hydrolyzed to generate oxalic acid. Moreover, other evidence also indicated that MMO, not oxalic acid, was produced directly from DMO. Figure 2 shows a plot of the overall selectivities of MMO and oxalic acid versus DMO conversion. MMO was observed to have a large intercept with the y axis, whereas no intercept of oxalic acid on the y axis was found. According to the Delplot method developed by Klein and coworkers,27,28 MMO should be formed directly from DMO, and oxalic acid should be produced from MMO. In addition, the concentrations of all of the reactants remained almost constant

Figure 2. Plot of overall product selectivity versus DMO conversion (T = 348.15 K, RW/DMO = 19.7). The continuous line represents the results of the proposed model.

after about 60 min, indicating that the reactions could reach equilibrium within a relatively short time. Therefore, two overall reactions should be considered in the kinetic model: hydrolysis of DMO to MMO and further hydrolysis of MMO to oxalic acid. That is, the two reactions occur in series and can be written as follows

3.2. Effects of the Reaction Parameters on Kinetics. 3.2.1. Effects of the Initial Molar Ratio of Water to DMO. The effects of the initial molar ratio of water to DMO (RW/DMO) on the hydrolysis were investigated. As shown in Figure 3, higher RW/DMO had a positive effect on the DMO hydrolysis process. Excess water shifts the chemical equilibrium toward the products and thus increases the equilibrium conversion of DMO and the yield of oxalic acid. Moreover, the rates both of DMO conversion and oxalic acid generation increased with increasing RW/DMO, 4208

dx.doi.org/10.1021/ie404253x | Ind. Eng. Chem. Res. 2014, 53, 4207−4214


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3.2.2. Effect of the Reaction Temperature. Hydrolysis kinetic experiments were carried out at different temperatures from 328.15 to 358.15 K. As shown in Figure 4, the hydrolysis rate

Figure 3. Effects of the initial reactant molar ratio on the hydrolysis reaction (T = 348.15 K): (a) conversion of DMO and (b) yields of oxalic acid and MMO versus reaction time. The continuous and dotted lines represent the results of the proposed model.

Figure 4. Effects of the temperature on the hydrolysis of DMO (RW/DMO = 19.7): (a) conversion of DMO and (b) yields of oxalic acid and MMO versus reaction time. The continuous and dotted lines represent the results of the proposed model.

which is different from the results of ion-resin-catalyzed hydrolysis.19 In ion-resin-catalyzed hydrolysis, the initial reaction rate is proportional to the product of the concentration of reactants at the same temperature. At the beginning of the ionresin-catalyzed hydrolysis reaction, increasing the water/ester ratio under conditions of excess water supply decreases the product of the concentrations of reactants and, thereby, the reaction rate. Unlike the ion-resin-catalyzed system, the amount of substances, acting as catalyst in the autocatalytic system, varied with water decreasing and acid producing. At the start of hydrolysis, the dissociation of water is the only way to provide hydrogen ions to initiate the hydrolysis of DMO; the limited hydrogen ions result in slow reactions. Then the hydrolysis will be accelerated by increasing amounts of H+ ions provided by oxalic acid produced from the reactions. Consequently, the overall autocatalytic hydrolysis is influenced by the initial rates of the reactions. DMO hydrolysis takes place in a liquid−liquid partly miscible system, and it proceeds faster in the aqueous phase because of its relative abundance of H+ compared to the organic phase. The more water added, the larger the amount of aqueous phase that can be obtained. As a result, DMO hydrolysis is promoted. With increase of RW/DMO, the presence of more H+ resulted in higher reaction rate. Because the initial rate of DMO hydrolysis is more sensitive to the amount of water, the effects of water on the initial hydrolysis should be taken into account for the hydrolysis kinetics.

increased significantly with increasing temperature. It can be observed that the equilibrium conversions of DMO were nearly the same in the experimental temperature range and the yields of oxalic acid increased from 52% to 61% with increasing temperature. The variations in equilibrium conversion and yield with temperature exhibit the typical features of an endothermic reaction. However, the yield of oxalic acid was still low even when hydrolysis was carried out at high temperature because of chemical equilibrium inhibition. Accordingly, techniques such as reactive distillation would be available for surpassing equilibrium, thereby improving the yield of oxalic acid. Thus, a reliable knowledge of the reaction kinetics is required for the simulation and design of a reactive distillation process. 3.3. Mechanism and Kinetic Model for DMO Autocatalytic Hydrolysis. 3.3.1. Mechanism and Kinetics. Based on the acid-catalyzed hydrolysis mechanism,29 ester hydrolysis involves the attack of water on the protonated ester, proton transfer, and elimination of alcohol and protons.20,30,31 Combining the predicted hydrolysis pathway, the following mechanism of DMO hydrolysis is proposed: Oxalic acid is generated from MMO following the carbonyl substitution of MMO, and MMO forms upon the substitution of one carbonyl of DMO. The attacks of water on the protonated diester and 4209



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monoester are separately rate-limiting steps of the DMO hydrolysis reaction (reaction 3) and MMO further hydrolysis (reaction 4), whereas the proton donation steps, as well as the subsequent steps, reach equilibrium rapidly. Then, the mechanism of hydrolysis was simplified, as shown in reactions 5−8

⎛ c H O+ ⎞⎡ c MMOc M ⎤ r1′ = k1′⎜⎜ 3 ⎟⎟⎢c DMOc H2O − ⎥ K 2(k1′/k −′ 1) ⎦ ⎝ c H 2 O ⎠⎣

(16)

⎛ c H O+ ⎞⎡ cOAc M ⎤ ⎥ r3′ = k 3′⎜⎜ 3 ⎟⎟⎢c MMOc H2O − K4(k 3′/k −′ 3) ⎦ ⎝ c H 2 O ⎠⎣

(17)

where the products K2(k′1/k′−1) and K4(k′3/k′−3) are equal to the equilibrium constants of the overall reactions 3 (KE1) and 4 (KE2). Thus, r1′ and r3′ can be written as ⎛ c H O+ ⎞⎛ c c ⎞ r1′ = k1′⎜⎜ 3 ⎟⎟⎜c DMOc H2O − MMO M ⎟ KE1 ⎠ ⎝ c H 2 O ⎠⎝

(18)

⎛ c H O+ ⎞⎛ c c ⎞ r3′ = k 3′⎜⎜ 3 ⎟⎟⎜c MMOc H2O − OA M ⎟ KE2 ⎠ ⎝ c H 2 O ⎠⎝

(19)

The concentration of hydroxonium ion can be obtained according to the charge balance and ionization equilibrium and is expressed as c H3O+ = c HOOCCOO− + cCH3OOCCOO− + cOH− (20) K a,OA =

Hydroxonium ions, which act as the catalytic agent in DMO hydrolysis, come from the ionization of water, MMO, and oxalic acid. The ionization equilibrium can be reached rapidly. Additionally, oxalic acid is a medium strong acid. Its first ionization equilibrium constant is 1.271 (in aqueous solution at 298 K), which is considerably larger than its secondary ionization equilibrium constant (4.272).32 Therefore, as shown in reactions 9−11, only the first ionization of oxalic acid is considered

c HOOCCOO−c H3O+

K a,MMO = K a,H2O =

cOAc H2O

(21)

cCH3OOCCOO−c H3O+ c MMOc H2O

(22)

cOH−c H3O+ c H 2O 2

(23)

After inserting the ionization equilibrium into the charge balance, the concentration of hydroxonium ion can be expressed as (c H3O+)2 = (K a,OAcOA + K a,MMOc MMO + K a,H2Oc H2O)c H2O (24)

from which the ratio of hydroxonium ion to water is solved as c H3O+ c H2O

H 2O + H 2O ⇌ OH− + H3O

r3′ = k 3′c MMOc H3O+ − k −′ 3c Bc M

(13)

⎛ c c ⎞ ⎜c DMOc H2O − MMO M ⎟ KE1 ⎠ ⎝

r3′ =

(26)

K a,OAcOA + K a,MMOc MMO + K a,H2Oc H2O dnOA = k 3′ c H 2O V dt

⎛ c c ⎞ ⎜c MMOc H2O − OA M ⎟ KE2 ⎠ ⎝

cOAc H3O+ c Bc H2O

(25)

dnDMO V dt K a,OAcOA + K a,MMOc MMO + K a,H2Oc H2O = k1′ c H 2O

where A represents CH3OOCC(OH)2+, B represents HOOCC(OH)2+, and M represents CH3OH. The concentrations of intermediates A and B can be obtained by applying the quasiequilibrium hypothesis to rapid steps 6 and 8 c MMOc H3O+ K2 = cAc H2O (14)

K4 =

c H2O

r1′ = −

Steps 5 and 7 are rate-limiting, whereas steps 6 and 8 are rapid. The rate-determining steps are expressed as (12)

K a,OAcOA + K a,MMOc MMO + K a,H2Oc H2O

Thus, the preliminary kinetic equations can be obtained as

(11)

r1′ = k1′c DMOc H3O+ − k −′ 1cAc M

=

(27)

Further kinetics regression requires accurate calculation of the molar concentration, which depends entirely on the density evaluation of the reaction mixture. However, the reaction mixture’s density is closely related to temperature and

(15)

Then, the rate equations can be rewritten as 4210



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composition. With the limited solubility of DMO and the lack of physical properties of MMO, the accurate density evaluation of the DMO hydrolysis system becomes considerably difficult. The volume of the liquid-phase reaction mixture is generally regarded as approximately constant, and the total number of moles is constant during DMO hydrolysis. As shown below, the rate equations are expressed in terms of mole fractions rather than concentrations nt0 dt V K a,OAyOA + K a,MMOyMMO

rDMO = −

dyDMO

ln KE2 = −

1726.5 + 2.9484 T

(34)

= k1′

yH O

+ K a,H2O

2

⎛ y y ⎞ ⎜yDMO yH O − MMO M ⎟ 2 KE1 ⎠ ⎝ rOA =

dyOA dt

= k 3′

nt0 V

(28)

K a,OAyOA + K a,MMOyMMO yH O

Figure 5. Temperature dependencies of the equilibrium constant.

+ K a,H2O

The standard reaction enthalpies ΔrH01 and ΔrH02 in these experiments were found to be 8.97 and 14.35 kJ/mol, respectively, which are close to the values calculated from the standard enthalpies listed in Table 1.

2

⎛ y y ⎞ ⎜yMMO yH O − OA M ⎟ 2 KE2 ⎠ ⎝

(29)

n0t

where V is the volume of the reaction mixture and is the initial total number of moles of the reaction mixture, which can be represented as 0 nt0 = (1 + R W/DMO)nDMO

Table 1. Standard Formation and Reaction Enthalpies in the DMO Hydrolysis System

(30)

n0DMO

where is the initial number of moles of DMO. As can be seen, the kinetic terms are dependent on the temperature and RW/DMO in the mechanism models. Therefore, the kinetic models were further simplified by fitting the experimental data with temperature and RW/DMO for practical use. 3.3.2. Parameter Estimation. The basic parameters appearing in the rate expression include the rate constants, equilibrium constants, and ionization equilibrium constants. The equilibrium constants based on liquid concentrations were calculated as ⎛ y y ⎞ KE1 = ⎜⎜ MMO M ⎟⎟ ⎝ yDMO yH2O ⎠eq

KE2

⎛ y y ⎞ = ⎜⎜ OA M ⎟⎟ ⎝ yMMO yH2O ⎠eq

a

1078.5 + 2.8381 T

ΔfH°(kJ/mol)

dimethyl oxalate methanol oxalic acid water monomethyl oxalate

−756.3 −239.1 −825.1 −285.8 −793.5c

ΔrH°calc(kJ/mol)

ΔrH°exp(kJ/mol)

a

9.5 15.1b

8.97 14.35

Overall reaction 1. bOverall reaction 2. cBenson method.33

The differential terms dyDMO/dt and dyOA/dt were obtained by differentiating the polynomial relating to the mole fraction−time data. It was found that the relationships of both (dyDMO/dt)/ [yDMOyH2O − (yMMOyM/KE1)] and [dyOA/dt)/(yMMOyH2O − (yOAyM/KE2)] with (yOA/yH2O)1/2 were linear. The kinetic equations can thus be simplified as follows according to the experimental observations

(31)

rDMO = −

−

(32)

The equilibrium compositions of the reaction mixture were measured after the kinetic experiments had been run until reaction equilibrium was reached. As the equilibrium constant is a function of only temperature, the value of Keq at a particular temperature was calculated from the equilibrium compositions with different molar ratios. The equilibrium constants were determined from plots of ln Keq versus 1/T, as illustrated in Figure 5. Then, the following expressions were obtained for the equilibrium constants as functions of temperature ln KE1 = −

component

dyDMO dt

⎞⎛ ⎛ y = ⎜k1 OA + k 0⎟⎜yDMO yH O ⎟⎝ ⎜ 2 yH O ⎠ ⎝ 2

yMMO yM ⎞ ⎟ KE1 ⎠

rOA =

dyOA dt

(35)

⎞⎛ ⎛ y y ⎞ y = ⎜k 2 OA + k 0⎟⎜yMMO yH O − OA M ⎟ ⎟⎝ ⎜ 2 KE2 ⎠ yH O ⎠ ⎝ 2 (36) 1/2

In rate eqs 35 and 36, k1(yOA/yH2O) and k2(yOA/yH2O)1/2 represent the catalytic effect of oxalic acid, in which (yOA/yH2O)1/2 represents the concentration effect of oxalic acid on the reaction rate, and k0 represents the startup effect of the hydroxonium ions from water dissociation. The experiments show that k1 and k2 are temperature-dependent. Furthermore, the ionization of MMO

(33) 4211



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⎛ YOA,calc − YOA,exp 1⎜ MRD2 = ⎜ ∑ n ⎝ all samples YOA,exp

can be ignored during the hydrolysis because, according to the structures of MMO and oxalic acid, the ionization capability of MMO should be much weaker than that of oxalic acid. A similar case was reported by Ashworth et al.,20 who assumed that monomethyl maleate did not catalyze the esterification of maleic acid because monomethyl maleate (pKa = 8.3 in methanol) was observed to have a considerably lower acidity than maleic acid (pKa = 5.5 in methanol). As can be seen from eqs 35 and 36, k0 is related to the initial rate of DMO hydrolysis and is calculated as −(dyDMO/dt) |t=0/

⎞ ⎟ × 100 ⎟ ⎠

(42)

where XDMO = 1 − (yDMO/yDMO ° ) and YOA = yOA/yDMO ° . The values of yDMO,calc and yOA,calc were obtained by the fourth-order Runge−Kutta method. The MRD results are reported in Table 2. Table 2. Parameters of the Autocatalytic Hydrolysis Model reaction rate

A (min−1)

rDMO rOA

9.7875 × 10 4.9503 × 103 7

Ea (kJ/mol)

MRD (%)

54.19 26.94

3.95 5.67

The kinetic equations obtained from the proposed model are as follows ⎛ rDMO = ⎜9.7875 × 107e−6517.77/ T ⎜ ⎝

yOA yH O 2

⎞ + k 0⎟ ⎟ ⎠

⎛ y y ⎞ ⎜yDMO yH O − MMO M ⎟ 2 KE1 ⎠ ⎝ ⎛ rOA = ⎜4.9503 × 103e−3240.87/ T ⎜ ⎝ ⎛ y y ⎞ ⎜yMMO yH O − OA M ⎟ 2 KE2 ⎠ ⎝

Figure 6. Analysis of the relationship between k0 and RW/DMO as a function of T.

(37)

where f1 and f 2 were both found to be temperature-dependent. After the data for f1 and f 2 had been separately correlated with temperature, the following expression was then obtained for k0 10 −1.0947 × 104 / T

k 0 = 5.2417 × 10 e

(38)

− 4.5322 × 10 e

Plots of −(dyDMO/dt)/[yDMOyH2O − (yMMOyM/KE1)] − k0 and −(dyOA/dt)/[yMMOyH2O − (yMMOyM/KE2)] − k0 versus (yOA/ yH2O)1/2 gave straight lines, allowing k1 and k2 to be obtained from their slopes. Regression results for k1 and k2 by the Arrhenius equation are given by ln k1 = −

6517.77 + 18.3992 T

(39)

ln k 2 = −

3240.87 + 8.5072 T

(40)

The qualities of the fits were evaluated in terms of the mean relative deviation (MRDs) ⎛ XDMO,calc − XDMO,exp 1⎜ MRD1 = ⎜ ∑ n ⎝ all samples XDMO,exp

yH O 2

⎞ + k 0⎟ ⎟ ⎠ (44)

4. CONCLUSIONS The kinetics of DMO hydrolysis without the addition of a catalyst was investigated in a batch reactor by varying the initial molar ratio of reactants and the temperature. It was found that consecutive reactions occur in the process of DMO hydrolysis, with MMO acting as an intermediate product that is further hydrolyzed to oxalic acid. The standard reaction enthalpies of the two reactions were estimated to be 8.97 and 14.35 kJ/mol, respectively, indicating that high operating temperatures favor the formation of oxalic acid. A mechanistic kinetic model involving the effects of water startup and acid catalysis was developed to describe the autocatalytic hydrolysis of DMO. The ionization of MMO can be ignored in the mechanism, because of the weaker ionization capability of MMO compared to oxalic acid. The kinetic parameters including the rate constants for water startup and the catalytic action of oxalic acid were determined in this work. The catalytic action of oxalic acid was found to be proportional to the 1/2 power of its concentration, and the effect of water startup was found to be dependent on temperature and RW/DMO. Higher RW/DMO not only increases the equilibrium conversion of DMO and the yield of oxalic acid, but also accelerates the initial reaction. The model proposed was found to be in a good agreement with the experimental data and

R W/DMO

14 −1.3387 × 104 / T

yOA

The parameters in the model equations have the following ranges: RW/DMO = 15−60, T = 328.15−358.15 K. The fits of the kinetic model to the experimental results are shown in Figures 1, 3, 4, and 7. Good agreement between the calculated curves and experimental points can be observed. From the kinetic model obtained, it is noticeable that both water startup and acid catalysis should be taken into account to describe the rates of DMO hydrolysis and oxalic acid formation.

(y°DMOy°H2O). Figure 6 shows that, at constant temperature, k0 is in a linear relationship with RW/DMO; thus, k0 can be expressed as k 0 = f1 + f2 R W/DMO

(43)

⎞ ⎟ × 100 ⎟ ⎠ (41) 4212



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Figure 7. Comparison of the experimental results with the kinetic model for DMO hydrolysis.

(2012D-5006-0503), and the Tianjin Natural Science Foundation (13JCZDJC33000).

would be applicable to the design of hydrolysis reactive distillation processes.

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■

AUTHOR INFORMATION

Corresponding Authors

*Fax: +86-22-87401818. E-mail: [email protected]. Tel.: +86-2227409880. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by the National High Technology Research and Development Program of China (2011AA051002), the National Natural Science Foundation of China (21276186), the PetroChina Innovation Foundation 4213

NOMENCLATURE A = pre-exponential Arrhenius factor, min−1 Ea = apparent activation energy, kJ/mol Keq = thermodynamic equilibrium constant n = number of experimental data points rDMO = reaction rate of DMO, min−1 rOA = reaction rate of OA, min−1 RW/DMO = initial molar ratio of water to DMO T = absolute temperature, K t = time, min X = conversion (%) Y = yield (%) y = mole fraction dx.doi.org/10.1021/ie404253x | Ind. Eng. Chem. Res. 2014, 53, 4207−4214


Article

ΔrH° = standard reaction enthalpy, kJ/mol

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Subscriptsand Superscripts

calc = calculated value exp = experimental value Abbreviations

DMO = dimethyl oxalate M = methanol MMO = monomethyl oxalate MRD = mean relative deviation OA = oxalic acid W = water

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REFERENCES

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Autocatalytic Kinetic Study of Dimethyl Oxalate Consecutive

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