Catalysis Letters https://doi.org/10.1007/s10562-017-2250-y

Kinetics Study of the Transesterification Reaction of Methyl Acetate with Isooctyl Alcohol Catalyzed by Dicationic Heteropolyanion-Based Ionic Liquids Yong Liu1 · Weihua Liu1 · Xiaonan Shao1 · Jianhong Wang1 · Xiying Li1 · Haiyan Zhang2 Received: 7 September 2017 / Accepted: 10 November 2017 © Springer Science+Business Media, LLC, part of Springer Nature 2017

Abstract In this work, dicationic acidic heteropolyanion-based ionic liquids were synthesized, and used in transesterification reaction of methyl acetate with isooctyl alcohol for producing isooctyl acetate. Compared with ­H2SO4, and ­H3PW12O40, [Bis-BsImB] [HPW12O40] and [Bis-Bs-DABCO][HPW12O40] shown excellent catalytic performance. The reaction kinetics and chemical equilibrium of the transesterification catalyzed by [Bis-BsImB][HPW12O40] and [Bis-Bs-DABCO][HPW12O40] were investigated. The effects of reaction temperature, initial reactant molar ratio, and catalyst concentration on the kinetics were studied in detail. The kinetic data were successfully correlated by a pseudohomogeneous model in the temperature range of 328.15 − 343.15 K. The simulation values obtained by the kinetic model were in good agreement with the experimental data. Moreover, [Bis-BsImB][HPW12O40] and [Bis-Bs-DABCO][HPW12O40] could be easily recovered and reused five times without any obvious decrease in catalytic activity. Graphical Abstract

Keywords  Heteropolyanion-based ionic liquids · Methyl acetate · Transesterification · Kinetics Abbreviations A Methyl acetate B Isooctyl alcohol D Isooctyl acetate E Methanol

* Haiyan Zhang [email protected] 1



Henan Key Laboratory of Polyoxometalate, College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, People’s Republic of China



College of Life Science, Henan University, Kaifeng 475004, People’s Republic of China

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M Molar ratio of isooctyl alcohol to methyl acetate HPAs Heteropoly acids ILs Ionic liquids HPA-ILs Heteropolyanion-based ionic liquids DAHPA-ILs Dicationic acidic heteropolyanion-based ionic liquids PH Pseudo-homogeneous C Concentration (mol L− 1) C0 The initial concentration (mol L− 1) mcat The catalyst concentration (mol L− 1) Ea Activation energy (kJ mol− 1) k Reaction rate constant ­(mol− 1 min− 1) k0 Pre-exponential factor ­(mol− 1 min− 1)

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k+ Forward reaction rate constant ­(mol− 1 min− 1) k− Reverse reaction rate constant ­(mol− 1 min− 1) Ke Equilibrium constant of the reaction T Temperature (K) t Time (min) ΔrH0 The reaction enthalpy (kJ mol− 1) ΔrS Entropy (J mol− 1 K− 1)

1 Introduction Isooctyl acetate is a colorless liquid, with fruit fragrance. It can be used in organic synthesis, coatings, plastics, spices and adhesive agents. Traditional preparation of isooctyl acetate is mainly through esterification reaction of isooctyl alcohol with acetic acid using the acidic catalysts (e.g., ­H2SO4). However, sulfuric acid as a homogeneous catalyst can suffer from many drawbacks such as corrosion of equipment, side reactions, and environmental pollution. Sulfuric acid catalyst has been replaced by other environmentfriendly acidic catalysts. In addition, organic esters can also be synthesized by transesterification of esters with alcohols [1–6]. For example, the liquid-phase transesterification of ethyl acetate with methanol to methyl acetate and ethanol catalyzed by ion-exchange resins [4]. Isopentyl acetate has been synthesized by the transesterification of isoamyl alcohol and glycerol triacetate over acidic ion-exchange resin [5] and immobilized lipase [6]. Methyl acetate is a by-product in polyvinyl alcohol process, and its industrial application is low. Methyl acetate is commonly hydrolyzed to produce methanol and acetic acid, but the high energy consumption limits industrial applications of the technology. Methyl acetate ought to be used for producing other valuable chemicals. Shen et al. [7] reported transesterification of methyl acetate with n-propanol to synthesize n-propyl acetate using Amberlyst-15 resins as catalysts. Cui et al. [8–10] presented transesterification of methyl acetate with n-butanol, ethanol, isoamyl alcohol by ionic liquid (IL), respectively. But as far as we know, the study on the transesterification of methyl acetate and isooctyl alcohol to produce isooctyl acetate has not been reported. Various catalysts have been applied to transesterification reaction, such as ILs [8–10], molecular sieves [11, 12], acidic ion-exchange resins [13, 14], heteropolyacids (HPAs) [15], and lipases [16]. Among them, HPAs with Keggin structures are a kind of multi-core inorganic polymers containing oxygen bridge, which have many excellent performances because of its unique cage structure. HPAs as multifunctional catalysts have attracted the attentions of the researchers. However, there are some disadvantages of HPAs, such as the low surface areas and the solubility,

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which limit their practical application [17, 18]. In order to overcome these drawbacks, immobilization of HPAs could be an effective way to improve their performance [19]. However, due to immobilization of HPAs immobilize hardly, loss active sites easily and has low reaction rate [20, 21]. Therefore, the development of highly active, environmentally friendly, and easily recyclable catalysts for the transesterification of methyl acetate with isooctyl alcohol is necessary. In recent years, ILs, as a cleaning catalyst and an excellent solvent, have attracted intense interest due to their unique properties such as low vapor pressure, remarkable solubility, and high thermal and chemical stability [22, 23]. ILs as the catalysts have been applied in many transesterification reactions, which showed excellent catalytic activity [24]. However, several disadvantage of the ILs still hinder their practical application. For example, a large amount of ILs as reaction medium is needed, recycling ILs from the reactants and products at homogeneous state is not easy. According to the advantages of ILs and HPAs, a novel type of heteropolyanion-based ionic liquids (HPA-ILs) combining Keggin type HPAs anion with ILs cations has gained wide attention. Wang and colleagues [25] first synthesized HPA-ILs and applied them in esterification reaction. After that, several HPA-ILs have been prepared and used as catalysts for oxidation, desulfurization, photodegradation, and condensation reaction [26–29]. In our previous works, a series of HPA-ILs catalysts have been synthesized and used in the esterification of diethylene glycol monobutyl ether with acetic acid [30]. In addition, the research of kinetics is very important for reaction engineering, but only a small number of kinetic studies of reactions have been carried out using ILs as catalysts. The kinetics of the transesterification reaction of methyl acetate with ethanol, n-butanol, and isoamyl alcohol have been studied using ILs as catalysts, respectively [8–10]. Therefore, it is necessary to study the kinetics of transesterification reaction of methyl acetate with isooctyl alcohol using HPA-ILs as catalysts, which will provide optimal operating parameters for the production of isooctyl acetate. In this paper, a series of dicationic acidic HPA-ILs (DAHPA-ILs) were synthesized, and applied in transesterification reaction of methyl acetate with isooctyl alcohol. The kinetics of transesterification reaction using DAHPA-ILs as catalysts were studied in detail. The impact for the reaction of different parameters of catalyst type, reaction temperature, catalyst concentration, and initial reactant molar ratio were also investigated in detail. Then a pseudohomogeneous (PH) kinetic model was adopted to correlate the experimental data, and the corresponding kinetic parameters were estimated.

Kinetics Study of the Transesterification Reaction of Methyl Acetate with Isooctyl Alcohol…

2 Experimental 2.1 Materials Phosphotungstic acid ­(H3PW12O40), imidazole (≥ 99%, AR), and methyl acetate (≥ 98%, AR) were supplied by Sinopharm Chemical Reagent, Shanghai, China. 1,4-Dibromobutane (≥ 98%, AR), 1,4-butyl sultone (≥ 99%, AR), 1,4-diazabicyclo[2.2.2]octane (≥ 99%, AR), isooctyl alcohol (≥ 99%, AR), and 1-methylimidazole (≥ 99%, AR) were purchased by Aladdin Reagent Co. Ltd., Shanghai, China. All reagents were used without further purification.

2.2 Preparation and Characterization of HPA‑ILs In this work, two DAHPA-ILs composed of different cations and the HPW12 O2− 40 anion were prepared (Fig. 1). The detailed synthesis procedure of these DAHPA-ILs were as following. DAHPA-IL1 1,4-bis(imidazol-1-(4-sulfonicacid)butyl) butane phosphotungstate ([Bis-BsImB][HPW12O40]) was prepared as follows [31]: imidazole (0.1  mol) was dissolved in 20 mL DMSO mixtures (1:1) in a 100 mL flask, and NaOH (0.1 mol) was added. Then the mixtures were placed in a water bath to 60 °C, and refluxed and agitated for 5  h. The mixtures were cooled down to 40  °C, and 1,4-dibromobutane (0.05 mol) was added dropwisely, producing yellow solid mixture of 1,4-bis(imidazol-1-yl)butane and NaBr. Then mixtures were dissolved by chloroform to remove NaBr, and purified 1,4-bis(imidazol-1-yl)butane was obtained by precipitation in water after evaporating chloroform. After that, 1,4-bis(imidazol-1-yl)butane (0.1 mol) reacted with 1,4-butane sultone (0.2 mol) were placed in a round-bottom flask and stirred at room temperature for 72 h. The solids were obtained, washed with ether for three times, and then vacuum dried under reduced pressure at 60 °C. Next, a certain amount of an aqueous solution of ­H3PW12O40

was dropped. Then, the mixtures were placed in a water bath to 80 °C, and refluxed and agitated for 6 h. After the reaction, the solid samples [Bis-BsImB][HPW12O40] were obtained by using the rotary evaporator to remove water. 1 H NMR (400 MHz, ­D2O, DMSO) δ: 9.01–9.19 (m, 2H), 7.62–7.75 (m, 4H), 4.16–4.28 (m, 8H), 2.56–2.58 (m, 4H), 1.87–1.92 (m, 4H), 1.73–1.83 (m, 4H), 1.49–1.54 (m, 4H). DAHPA-IL2 1,4-bis(4-sulfobutyl)-1,4-diazabicyclo[2.2.2]octane-1,4-diium phosphotungstate ([Bis-BsDABCO][HPW12O40]) was synthetized as follows: 1,4-diazabicyclo[2.2.2]octane (0.1  mol) and 1,4-butane sultone (0.1 mol) were placed in a round-bottom flask and stirred at room temperature for 72 h. In the process of the reaction, the white solid precipitation was obtained. Then the mixtures were washed with ethyl acetate until the upper liquid became transparent. The white solid was dried at 80 °C for 12 h in vacuum. Next, a certain amount of an aqueous solution of ­H3PW12O40 was dropped. The mixtures were placed in a water bath to 80 °C, and refluxed and agitated for 6 h. After the reaction, [Bis-Bs-DABCO][HPW12O40] was obtained by using the rotary evaporator to remove water. 1H NMR (400 MHz, DMSO) δ: 4.00–4.34 (m, 18H), 3.61–3.69 (m, 4H), 3.33–3.8 (m, 8H), 1.80–1.90 (m, 2H), 1.64–1.72 (m, 2H).

2.3 Apparatus and Procedure A round-bottom flask (50 mL) was placed in a constant temperature water bath of 70 °C. 0.1 mol methyl acetate and 0.1 mol isooctyl alcohol were added to the flask. After the reactants reached the reaction temperature, a certain amount of DAHPA-ILs catalysts were added to the flask. The agitator and time measurement were started immediately. The reaction was carried out for 8 h with stirring. In each experiment, samples were taken out from the reactor in specific intervals, and then rapidly put it to 273.15 K ice bath to avoid its further reaction. Then these samples were analyzed by gas chromatography.

Fig. 1  Structures of DAHPAILs

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2.4 Analysis The samples were analyzed by a gas chromatograph (Fuli, 9790) equipped with a DB-1 capillary column (30 m × 0.539 mm × 1.50 μm) and a hydrogen flame ionization detector (FID). The temperatures of injector and FID were set at 523.15 and 573.15 K.

3 Results and Discussion 3.1 Catalyst Characterization FTIR spectra of catalysts are shown in Fig. 2. From Fig. 2c, some peaks at wave number of 1080 cm− 1(P–O stretching vibration), 977 cm− 1 (W=O stretching vibration), 895 cm− 1 (W–Ob–W stretching vibration), and 807 cm− 1 (W–Oc–W

Fig. 2  FTIR spectra for (a) [Bis-BsImB][HPW12O40], (b) [Bis-BsDABCO][HPW12O40], and (c) ­H3PW12O40

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stretching vibration) were observed. The FTIR spectra of [Bis-BsImB][HPW12O40] and [Bis-Bs-DABCO][HPW12O40] also show that they all have peaks at wave number of 1080, 977, 895 and 807 cm− 1, showing they all retain the Keggin structure of ­H3PW12O40. As shown in Fig. 2a, the bands at 3450 and 3112 cm− 1 attribute to the stretching vibration of N–H and C–H respectively to imidazole rings. The bands at 2934 and 2873 cm− 1 are attributed to the stretching vibration of –CH2–. It shows some peaks at 1631 and 1563 cm− 1 (C–C skeletal vibration), 1466 cm− 1 (–CH2 stretching vibration). The peak at 1159 and 1042 cm− 1 are assigned to the asymmetrical and symmetric stretching vibration of the –S=O to –SO3H, respectively. As seen in Fig. 2b, the bands at 3450 and 3024 cm− 1 are attributed to the stretching vibration of N–H and C–H, respectively. It shows some peaks at 1624 and 1202 cm− 1 (C–C skeletal vibration), 1472 cm− 1 (–CH2 bending vibration), 1398 cm− 1 (C–N stretching vibration). The peaks at 1140 and 1043 cm− 1 are assigned to the asymmetrical and symmetric stretching vibration of the –S=O to –SO3H, respectively. The FTIR spectra show that both DAHPA-ILs of Fig.  2a, b not only preserve the Keggin structure of ­H3PW12O40, while also retain the organic cation structure. TG–DSC curves of catalysts are shown in Fig. 3. As shown in Fig.  3a, [Bis-BsImB][HPW 12O40] is gradually decomposed. The endothermic peak at 170  °C in DSC curve could be attributed to the evaporation of water in the samples, and the samples completely dehydrated when the temperature reached 250 °C. Three endothermic peaks are appeared in DSC curve between 400, 500 and 600 °C, while TG curve associated with weightlessness, which described [Bis-BsImB] was gradually decomposed. As seen in Fig. 3b, a endothermic peak appears at 200 °C which is caused by the evaporation of water in the sample. A significant weight

Fig. 3  TG–DSC curves of a [Bis-BsImB][HPW12O40], and b [Bis-Bs-DABCO][HPW12O40]

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Kinetics Study of the Transesterification Reaction of Methyl Acetate with Isooctyl Alcohol…

loss can be seen in TG curve between 25 and 100 °C. And with severe weight loss after 200 °C while three small endothermic peak appear between 300 and 600 °C, which show that [Bis-Bs-DABCO] is gradually decomposed. The weight loss is only about 12% when the temperature reaches 700 °C. The TG–DSC curves of [Bis-BsImB][HPW12O40] (a) and [Bis-Bs-DABCO][HPW12O40] (b) show that they have high thermal stability.

3.2 Catalyst Performance The transesterification reaction of methyl acetate with isooctyl alcohol was investigated to test their catalytic activities under the same reaction conditions using different DAHPAILs and other acidic catalysts. The results of the conversion of methyl acetate were shown in Table 1. As can be seen in Table 1, [Bis-BsImB][HPW12O40] and [Bis-Bs-DABCO] [HPW12O40] also show excellent catalytic performance. The conversion of methyl acetate reached 53.36 and 48.29% at 8 h using [Bis-BsImB][HPW12O40] and [Bis-Bs-DABCO] [HPW 12O 40] as catalysts, respectively. In comparison, the conversion of methyl acetate at 8  h were only 2.05 and 2.32% in the presence of [­ DABCO][HPW12O40] and ­[MIM]3PW12O40 as catalysts, respectively. As shown in Table 1, the conventional acidic catalyst sulfuric acid, and ­H3PW12O40 showed the best catalytic performance. However, sulfuric acid and ­H3PW12O40 as homogeneous catalysts have some disadvantages such as difficult separation from the reaction mixtures, environmental pollution. DAHPA-ILs as heterogeneous catalysts are insoluble in reaction system, and can be easily separated from the reaction mixtures by simple filtration. DAHPA-ILs have no environmental pollution and corrosion of equipment. Therefore, [Bis-BsImB]

Table 1  Comparison of the conversions of methyl acetate Entries

Catalysts

Time (h)

X (%)

1

[Bis-BsImB][HPW12O40]

2

[Bis-Bs-DABCO][HPW12O40]

3

[DABCO][HPW12O40]

4

[MIM]3PW12O40

5

H2SO4

6

H3PW12O40

4 8 4 8 4 8 4 8 4 8 4 8

44.56 53.36 37.39 48.29 1.58 2.05 1.96 2.32 50.21 55.47 44.16 55.04

Reaction conditions temperature of 343.15  K, catalyst concentration of 0.0181 mol L− 1, and initial molar ratio of 1:1. No byproducts were found by GC

[HPW12O40] and [Bis-Bs-DABCO][HPW12O40] were found to have promising application in transesterification of methyl acetate with isooctyl alcohol and were used as catalysts for further kinetic experiments.

3.3 Effect of Reaction Temperature As shown in Fig. 4, the conversion of methyl acetate at different reaction temperatures in the range of 328.15–343.15 K was studied using [Bis-BsImB][HPW12O40] and [Bis-BsDABCO][HPW12O40] under the same reaction conditions. The reaction rate increased with increasing reaction temperature. For [Bis-BsImB][HPW12O40] and [Bis-Bs-DABCO] [HPW12O40], when the reaction temperatures was increased from 328.15 to 343.15 K, the conversion of methyl acetate at 8 h increased obviously from 47, 34–53, 48%, respectively. These results show that the increase of temperature is helpful to the improvement of the reaction rate, which is due to that an increase in temperature brings about more successful collisions between reactants that have sufficient energy to break the bonds and form products and thus lead to a higher conversion of methyl acetate.

3.4 Effect of Catalyst Concentration The effect of different catalyst concentrations on the conversion of methyl acetate with in the range of 0.00776–0.0233  mol  L − 1 was investigated using [BisBsImB][HPW12O40] and [Bis-Bs-DABCO][HPW12O40] as catalysts under the same reaction conditions, respectively. The results were shown in Fig. 5. It can be observed that the conversion of methyl acetate increased with increasing catalyst concentration, and the time to reach equilibrium was shorter. This is due to the increased catalyst concentration improves the active ingredient concentration, and thus lead to speed up the reaction rate. The conversion of methyl acetate increased from 42 to 55% at 8 h when the catalyst concentration increased from 0.00776 to 0.0233 mol L− 1 using [Bis-BsImB][HPW12O40] as catalyst. And in the same reaction conditions, the conversion of methyl acetate increased from 25 to 49% using [Bis-Bs-DABCO][HPW12O40] as catalyst.

3.5 Effect of Initial Reactant Molar Ratio The effect of initial reactant molar ratio on the conversion of methyl acetate was studied using [Bis-BsImB] [HPW12O40] as catalysts at temperature of 343.15 K, and with catalyst concentrations of 0.0181 mol L− 1. As can be seen in Fig. 6, the conversion of methyl acetate increased obviously as the molar ratio was changed from 1:2 to 2:1. This is because an increase in the concentration of isooctyl alcohol brings the reaction toward positive reaction side for

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Fig. 4  Effect of reaction temperature on the conversion of methyl acetate using [Bis-BsImB][HPW12O40] (a) and [Bis-Bs-DABCO] [HPW12O40] (b) as catalysts at temperature of (filled triangles)

328.15 K, (filled diamonds) 333.15 K, (filled circles) 338.15 K, (filled squares) 343.15  K, with catalyst concentration of 0.0181  mol  L− 1, and methyl acetate to isooctyl alcohol molar ratio of 1:1

Fig. 5  Effect of catalyst concentration on the conversion of methyl acetate using [Bis-BsImB][HPW12O40] (a) and [Bis-Bs-DABCO] [HPW12O40] (b) as catalysts at temperature of 343.15 K, with methyl acetate to isooctyl alcohol molar ratio of 1:1, and catalyst con-

centration of (filled triangles) 0.00776  mol  L− 1, (filled diamonds) 0.0129 mol L− 1, (filled circles) 0.0181 mol L− 1, and (filled squares) 0.0233 mol L− 1

this reversible reaction. However, when the molar ratio was further increased to 3:1, the conversion of methyl acetate decreased. This can be attributed to the fact that the concentration of methyl acetate has been diluted after continuing increase of molar ratio. Therefore, an excess of isooctyl alcohol is not conducive to the transesterification of methyl acetate, and the optimal molar ratio is 2:1.

is no longer frequent changes, it means that equilibrium is reached. The chemical equilibrium constant could be get from the sample’s concentration when equilibrium is reached by the equation:

3.6 Chemical Equilibrium The kinetic experiments were carried out at different temperatures while other conditions were same. Samples were getting out from the reactor after certain time intervals, and were analyzed. When the concentration of the sample

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Ke =

CD CE , CA CB

(1)

where Ke is the equilibrium constant, C is the equilibrium concentration of component (mol L− 1), and A, B, D, E is methyl acetate, isooctyl alcohol, isooctyl acetate, methanol, respectively. The reaction enthalpy (ΔrH0) and entropy (ΔrS0) were calculated by the van’t Hoff equation, where the van’t

Kinetics Study of the Transesterification Reaction of Methyl Acetate with Isooctyl Alcohol…

Hoff equation provides information about the temperature dependence the equilibrium constant.

ln Ke =

−Δr H 0 Δr S0 + . RT R

(2)

The reaction enthalpy (ΔrH0) and entropy (ΔrS0) were obtained from Eq. (2) by plotting lnKe versus 1/T, as shown in Fig. 7. It can be seen that the equilibrium constant increased when the temperature increased. From Fig. 7, the reaction enthalpy (ΔrH0) and entropy (ΔrS0) of this reaction were found to be 25.92 kJ mol− 1 and 88.79 J mol− 1 K− 1, respectively.

3.7 Reaction Mechanism and Kinetic Modeling

Fig. 6  Effect of initial reactant molar ratio on the conversion of methyl acetate using [Bis-BsImB][HPW12O40] as catalysts at 343.15 K, with catalyst concentration of 0.0181 mol L− 1, and methyl acetate to isooctyl alcohol molar ratio of (filled triangles) 1:2, (filled diamonds) 1:1, (filled circles) 2:1, and (filled squares) 3:1

Fig. 7  Effect of reaction temperature on the equilibrium constant

The transesterification reaction equation of methyl acetate with isooctyl alcohol was written as k+

A + B ⟷ D + E. k−

(3)

The mechanism of the transesterification of methyl acetate and isoamyl alcohol catalyzed by ­[HSO3bmim][HSO4] has been studied [9]. In this paper, methyl acetate reacts with isooctyl alcohol to synthesize isooctyl acetate and methanol using [Bis-BsImB][HPW12O40] and [Bis-Bs-DABCO] [HPW12O40] as catalysts. This reaction is similar with the other transesterification reaction of methyl acetate reported in the literatures [9]. Therefore, the reaction mechanism of methyl acetate and isooctyl alcohol was shown in Fig. 8. A PH model was usually applied for the liquid-phase reaction system [30, 32–35]. In this paper, the transesterification kinetic of methyl acetate and isoamyl alcohol using [BisBsImB][HPW12O40] and [Bis-Bs-DABCO][HPW12O40] as catalysts can be established on the basis of the PH model. Assuming this transesterification reaction is reversible secondorder reaction, the equation of reaction rate is as follows:

( ) dcA = mcat k+ cA cB − k− cD cE dt ( ) = mcat k+ cA cB − cD cE ∕Ke ,

r=−

(4)

Fig. 8  Reaction mechanism for transesterification

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where k+ is the forward reaction rate constant, k− is the reverse reaction rate constant, C is molar concentrations, mcat is the catalyst loading per unit volume, Ke is the equilibrium constant, Ke = k+/k−. Since the initial concentration of isooctyl acetate and methanol are zero and the conversion of methyl acetate is X at the time t, the Eq. (4) can be written as

r=

[ ] dX = mcat cA0 k+ (1 − X)(M − X) − X 2 ∕Ke , dt

(5)

where M is the molar ratio of isooctyl alcohol to methyl acetate. The integration of the Eq. (5) can be written as [( )] )( n3 + n2 − 2n1 X n2 − n3 ln = n3 cA0 k1 t = Y, (6) n2 − n3 − 2n1 X n2 + n3 where n 1  = 1  −  1/Ke , n 2  = M + 1, n3 = [n22 − 4n1 M]1∕2 , k1 = mcatk+. The Y values can be determined according the formula (6). Figure 9 shows the relationship between Y and time t at different temperatures. It is observed that Y is linearly related with time through the origin, irrespective of the reaction temperature. From the slopes of the lines given in Fig. 9, the positive reaction rate constant k+ was obtained. It was found that the rate constant increase with the increase with the increase in the reaction temperature. The reverse reaction rate constant k− was calculated using the equilibrium constant Ke and k+. The relationship between temperature and the reaction rate can be written as Arrhenius equation

ln k = ln k0 −

Ea . RT

(7)

According to Eq. (7), the relationship between ln k and 1/T is plotted in Fig. 10. The forward and reverse reaction activation energy and the pre-exponential factor can be obtained from the slope and intercept respectively, which can be seen in Table 2.

3.8 Reusability of Catalyst For the application in industry, the stability and reusability of catalysts are important factors. The reusability of [BisBsImB][HPW12O40] and [Bis-Bs-DABCO][HPW12O40] in the transesterification reaction of methyl acetate and isooctyl alcohol were studied. These reactions are liquid–solid biphasic heterogeneous reactions by using [Bis-BsImB] [HPW12O40] and [Bis-Bs-DABCO][HPW12O40] as catalysts, so the solid catalyst could be easily recycled. After being filtered, washed three times, and dried at 60 °C for 12 h under a vacuum, the catalysts were used for the next run. As shown in Fig. 11, the conversion of methyl acetate is not significantly reduced after reused the catalysts of [Bis-BsImB] [HPW12O40] and [Bis-Bs-DABCO][HPW12O40] five times, which showed that [Bis-BsImB][HPW12O40] and [Bis-BsDABCO][HPW12O40] have good stability and reusability.

4 Conclusions The reaction kinetics for transesterification of methyl acetate with isooctyl alcohol have been studied using DAHPA-ILs as catalyst. The kinetic experimental data indicated that [Bis-BsImB][HPW 12 O 40 ] and [Bis-BsDABCO][HPW 12O 40] have displayed comparable catalytic activities with ­H2SO4, and ­H3PW12O40, and could be easily recycled and can be reused five times without any

Fig. 9  Relationship between Y in formula (6) and time (t) at different temperature of (filled triangles) 328.15  K, (filled diamonds) 333.15  K, (filled circles) 338.15 K, and (filled squares) 343.15 K using [Bis-BsImB][HPW12O40] (a) and [Bis-Bs-DABCO][HPW12O40] (b) as catalysts

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Kinetics Study of the Transesterification Reaction of Methyl Acetate with Isooctyl Alcohol…

Fig. 10  Relationship between k and T. Filled squares positive, filled circles negative catalyst: [Bis-BsImB][HPW12O40] (a) and [Bis-Bs-DABCO] [HPW12O40] (b)

Table 2  Parameters of the PH model

Catalysts

[Bis-BsImB][HPW12O40] [Bis-Bs-DABCO][HPW12O40]

k0 ­(L2 mol− 2 min− 1)

Ea (kJ mol− 1)

Positive

Negative

Positive

Negative

4.72 × 105±1.07 9.84 × 104±1.64

1.31 × 103±3.12 1.33 × 102±1.41

42.98 ± 1.82 40.57 ± 1.34

29.18 ± 1.97 23.28 ± 0.71

are suitable catalyst for the transesterification of methyl acetate with isooctyl alcohol. Acknowledgements  This work was supported by the National Natural Science Foundations of China (Nos. 21206031, 21676072), the Science and Technology Development Program of Henan Province of China (No. 172102210196), the Foundation of Education Department of Henan Province of China (No. 16A530001).

Compliance with Ethical Standards  Conflict of interest  There are no conflict of interest for each contributing author.

Fig. 11  Reusability of [Bis-BsImB][HPW12O40] (red column) and [Bis-Bs-DABCO][HPW12O40] (green column) at 343.15 K, with catalyst concentration of 0.0181 mol L− 1, methyl acetate to isooctyl alcohol molar ratio of 1:1, and the reaction time of 8 h

obvious decrease in catalytic activity. The effect of various parameters such as reaction temperature, catalyst concentration, and initial reactants molar ratio on the conversion of methyl acetate was studied in detail. The PH model was used to correlate the kinetic experimental data. The calculated data by the estimated kinetics parameters was consistent with the experimental data. As a result, the [BisBsImB][HPW 12O 40] and [Bis-Bs-DABCO][HPW 12O 40]

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