Reaction kinetics for synthesis of sec-butyl alcohol catalyzed by acid-functionalized ionic liquid☆

Ting Qiu*,Wenli Tang,Chenggang Li,Chengming Wu,Ling Li

College of Chemistry and Chemical Engineering,Fuzhou University,Fuzhou 350108,China

Keywords:Ionic liquids sec-Butyl alcohol Kinetic modeling Transesterification

ABSTRACT The acid-functionalized ionic liquid([HSO3Pmim]HSO4)was synthesized by a two-step method.Nuclear magnetic resonance(NMR)and Fourier transforminfrared spectroscopy(FT-IR)show that the synthesis method is feasible and high purity of ionic liquid can be obtained.Using[HSO3Pmim]HSO4 as the catalyst,we studied the reaction kinetics of synthesizing sec-butylalcohol from sec-butyl acetate and methanol by transesterification in a high-pressure batch reactor.The effects of temperature,initial molar ratio of methanol to ester,and catalyst concentration on the conversion of sec-butyl acetate were studied.Based on its possible reaction mechanism,a homogeneous kinetic model was established.The results show that the reaction heatΔH is 10.94 ×103 J·mol?1,so the reaction is an endothermic reaction.The activation energies E a+and E a? are 60.38 × 103 and 49.44 × 103 J·mol?1,respectively.

1.Introduction

sec-Butyl alcohol(SBA)is widely used in industry.It is used as a methanol co-solvent component for improving the octane number of gasoline and as an important chemical intermediate to produce methyl ethyl ketone[1,2].Several synthetic routes for sec-butyl alcohol have been developed,such as direct hydration of butylene and indirect hydration.These routes suffer from several drawbacks such as high pressure and temperature conditions,high energy consumption,serious corrosion of equipment,low one-way conversion,and high requirement for raw materials[3].The industrial synthesis of sec-butyl alcohol and methylacetate(MeAc)from sec-butylacetate(SBAC)and methanol(MeOH)by transesterification with the catalysis of sodium alkoxide can avoid these problems to some extent,but it involves some difficulties in catalyst separation[4].

The acid-functionalized ionic liquids,which are environmentally friendly solvents and liquid acid catalysts,present high catalytic activity of liquid acid and nonvolatility of solid acid,adjustable molecular structure and acidity,easy separation,high thermal stability,etc.[5-7].Reactions with acid-functionalized ionic liquids as catalysts give desired results,such as nitration[8],transesterification[9,10],esterification[11,12],and Beckmann rearrangement[13].Because of their special properties and functionality,acid-functionalized ionic liquids will become truly designed green solvents and replace conventional catalysts[14-16].Based on the concept“new and efficient green environmental protection,energy conservation and emissions reduction”,a new green process for the synthesis of sec-butyl alcohol via transesterification catalyzed by acid-functionalized ionic liquids has been proposed[17].The transesterification reaction equation is:

CH3COOCH(CH3)CH2CH3+CH3OH?CH3COOCH3+CH3CHOHCH2CH3.In this work,the kinetic behavior of transesterification is investigated using 1-(3-sulfonic acid)propyl-3-methylimidazole hydrogen sulfate[HSO3Pmim]HSO4as catalyst,and the effects of temperature,molar ratio of reactants and catalyst concentration are explored.A homogeneous kinetic model is established based on its possible reaction mechanism.

2.Experimental

2.1.Materials

The initial purities,purification methods, final purities,and sources of the materials are listed in Table 1.Deionized water was prepared in our laboratory.The purities of components were determined by gas chromatography.

2.2.Synthesis of acid-functionalized ionic liquid

The acid-functionalized ionic liquid([HSO3Pmim]HSO4)(Fig.1)was synthesized by the two-step method[18,19],with experimental procedures as follows.Under vigorous stirring,1,3-propanesultone was dissolved in toluene and 1.2 equivalent(eq.)of 1-methyl imidazole was introduced dropwise in an ice bath.The mixture was heated toroom temperature and stirred for 6 h.After filtration,zwitterion formed as white solid.It was washed by ether three times to remove residual material and dried in a vacuum at 353.15 K for 5 h.Then the zwitterion was dissolved in deionized water and equimolar sulfuric acid(98%,by mass)was added slowly at 273.15 K with vigorous stirring.The mixture was slowly heated to 363.15 K and stirred for 6 h.After the water was removed under vacuum,[HSO3Pmim]HSO4appeared as a light yellow transparent viscous liquid.It was dried in a vacuum at 353.15 K for 8 h.[HSO3Pmim]HSO4was obtained with a total yield of 86%.

Table 1 Purities of chemicals

Fig.1.Structure of[HSO3Pmim]HSO4.

2.3.Characterization of acid-functionalized ionic liquid

The structure of[HSO3Pmim]HSO4was analyzed by nuclear magnetic resonance(NMR)and fouriertransforminfrared spectroscopy(FT-IR).1H NMR and13C NMR spectra in DMSO-d6 were obtained using an AVANCE III 500 Bruker instrument,with tetramethylsilane(TMS)as internal standard.FT-IR spectra were recorded using a Perkin-Elmer Spectrum 2000 FT-IR spectrophotometer for KBr pellets.

1HNMR(500 MHz,DMSO-d6,TMS)δ(ppm):9.11(s,1H,N-CH2-N),7.78(m,1H,N-CH2-CH2),7.69(m,1H,CH2-CH2-N),4.30(t,2H,N-CH2-CH2),3.85(s,3H,-CH3),2.41(t,2H,CH2-CH2-S),and 2.09(p,2H,CH2-CH2-CH2);13C NMR(126 MHz,DMSO-d6)δ(ppm):137.24,124.09,122.79,48.19,47.82,36.20,and 26.59.

FT-IR(KBr,cm?1):3400,3116,3158,2875,2961,1510,1172,1068,and 850.

The NMR spectral and FT-IR spectral data of the[HSO3Pmim]HSO4agree with its designed structures(Fig.1).No impurity peak appears in the1H NMR spectra.This demonstrates that the purity of the[HSO3Pmim]HSO4is more than 98%.Therefore,the synthesis and purification methods for the[HSO3Pmim]HSO4are feasible.

2.4.Apparatus and procedure

In this study,the reaction system has a lower boiling point(about 338.15 K)at atmospheric pressure.In order to increase the reaction temperature to enhance the reaction rate,the reaction was carried out at pressure of 0.608 MPa.The reaction temperatures were 343.15-373.15 K.

The kinetic experiments were carried out in a stainless steel reactor(material:0Cr18Ni9,reactor volume:500 ml)equipped with an agitation and temperature control device(±0.05 K).Its schematic diagram is shown in Fig.2.In each run,the reactants were proportionately added to the reactor.The pressure was raised to 0.608 MPa by filling nitrogen,with some fluctuations and the maximum deviation of±0.1 kPa.In order to ensure uniform temperature in the reactor,a stirrer was set at a desired level.Once the desired temperature was attained,the preheated catalyst(dissolved in methanol)was charged into the reactor by a tranquil flow pump and the time was regarded as the initial time.Samples were taken at a fixed time interval,cooled rapidly,and then analyzed by gas chromatography.The reaction was considered to reach chemical equilibrium when the composition of the reaction mixture was nearly constant.Then the mixture was removed from the reactor and kept for recycling catalyst.

Fig.2.Apparatus of kinetic experiment.1—heating mantle;2—stainless steel reactor;3—thermocouple;4—mechanical stirrer;5—pressure tap;6—tranquil flow pump;7—control cabinet;8—nitrogen cylinder;9—condenser.

2.5.Sample analysis

All samples were analyzed by gas chromatography(GC,Shanghai Branch Chong GC9800),with 1,4-dioxane as the internal standard and parameters as follows: flame ionization detector(FID);column:ATFFTP,50 m × 0.32 mm × 0.5 μm;column temperature:started at 333.15 K for 2 min,increased to 341.15 K at 1.5 K·min?1and held for 1 min,and then increased to 453.15 K at 15 K·min?1and held for 1 min;injector temperature:453.15 K;and detector temperature:493.15 K.All samples were analyzed at least three times to eliminate the error.The mole fraction was within an uncertainty of±0.002.

3.Results and Discussion

3.1.Effect of reaction temperature

Fig.3 shows the effect of temperature(from 343.15 to 373.15 K)on the reaction rate and conversion of sec-butyl acetate.The reaction rate and the conversion increase with temperature,while the equilibrium conversion at different temperatures changes slightly.For a weakly endothermic reaction,the effect of temperature on the equilibrium conversion is slight,but is significant on the reaction rate.At 343.15 K,the reaction reached equilibrium at 480 min and the conversion of sec-butyl acetate was 87.91%.Xiao used sodium alkoxide as catalyst and performed experiments with an equilibrium time of 12 min and conversion of 57.5%at323 K[20].Here we use[HSO3Pmim]HSO4as catalyst,acquiring a lower reaction rate but higher equilibrium conversion,and the catalyst can be reused.

Fig.3.Effect of temperature on reaction rate and conversion of sec-butyl acetate.(molar ratio of methanol to sec-butyl acetate=3.5,catalyst concentration=1.5%).

The equilibrium constant of the reaction can be calculated by

where K is the equilibrium constant and c is the concentration,mol?L?1.

Eq.(1)can be rewritten as

The van't Hoff equation gives

where ΔH is the activation energy,J·mol?1.

In a small temperature interval,ΔH can be considered to be constant and Eq.(3)becomes

A plot of ln K versus 1/T is shown in Fig.4.The temperature dependence of the equilibrium constant can be expressed as

The activation energy of 1.094 × 104J·mol?1means a weakly endothermic reaction.The mean squared error between experimental and calculated equilibrium constants at different temperatures is 1.03×10?3.

3.2.Effect of initial molar ratio

Fig.4.Equilibrium constant K versus reaction temperature T.

Fig.5.Effect of initial molar ratio of methanol to sec-butyl acetate on the conversion of secbutyl acetate(catalyst concentration=1.5%,by mass;T=363.15 K).

Fig.5 shows the effect of the initial molar ratio of methanol to secbutyl acetate(Rn)on the reaction rate and conversion.As the ratio increases from 2.5 to 4.5,the conversion of sec-butyl acetate and the reaction rate increase,so the effect of the molar ratio on the equilibrium conversion of sec-butyl acetate and the reaction rate is significant.Since the transesterification reaction is a reversible reaction,increasing the molar ratio is favorable for the forward reaction,improving the equilibrium conversion of sec-butyl acetate.Theoretically,the equilibrium constant should be the same at the same temperature.Equilibrium constants calculated by Eq.(2)are listed in Table 2.They are similar with the ratio above 3.0,but that at the ratio of 2.5 is different.The possible reason is that the reaction system becomes heterogeneous when the reaction reaches equilibrium,because the methanol concentration decreases and the ionic liquid is insoluble in other constituents of this system.

Table 2Equilibrium constants K at different molar ratios R n

3.3.Effect of catalyst concentration

The transesterification reaction is catalyzed by a homogeneous catalyst,[HSO3Pmim]HSO4,which is soluble in methanol.The effect of catalyst concentration(from 1.0%to 2.0%,by mass)on the reaction is shown in Fig.6.The increment of catalyst concentration enhances the reaction rate.The catalyst concentration influences the time at which the reaction reaches equilibrium,but not the equilibrium conversion.However,catalyst concentration should not be too high,because the concentration of methanol decreases with reaction time and the ionic liquid shows extraction effect,reducing the solubility of methanol and sec-butyl acetate,methyl acetate,sec-butyl alcohol,then the reaction system becomes heterogeneous.

Fig.6.Effect of catalyst concentration on the reaction rate and conversion of sec-butylacetate.(R n=3.5;T=363.15 K).

3.4.Reaction mechanism

The reaction mechanism of acid-functionalized ionic liquids catalyzed transesterification is similar to that of proton acid.They all provide protons,then a series of reactions proceed.The process can be divided into four steps as follows.

(1)Acid-functionalized ionic liquid([HSO3Pmim]HSO4)dissociates into protons(H+)and anions.

(2)The carbocation resulted from the combination of proton and carbonyl group has high electron affinity.It is beneficial to the attack of hydroxyl group to form a new carbon-oxygen bond.

(3)Tetrahedron structure is formed by a nucleophilic substitution reaction with the hydroxyl group of methanol attacking the carbocation.

(4)The tetrahedron structure dissociates into H+,sec-butyl alcohol and methyl acetate.

The schematic diagram is shown in Fig.7.Steps 1 and 3 are very fast and they are assumed to be in equilibrium.Step 2 is the slowest process and it is the rate-controlling step.As a result,the reaction mechanism can be simplified to two steps,as shown in Fig.8.

3.5.Kinetic model

Based on the mechanism of reaction in Fig.8,the kinetic model can be expressed as

where k2and k?2are the forward and reverse reaction rate constants of Step 2,respectively.

Step 1 can be considered to be in equilibrium,so that

where k1and k?1are the forward and reverse reaction rate constants,respectively,of Step 1.Eqs.(6)and(7)lead to

where k+and k?are the reaction rate constants of forward and reverse reactions,respectively.

The total reaction rate equation of the transesterification is

With Eqs.(9)and(10),we have

Fig.6 shows that the effect of catalyst concentration on the reaction rate is significant.We assume that[HSO3Pmim]HSO4can quickly provide protons and the dissociation process is in equilibrium state.The initial reaction rate r0is a function[21]of catalyst concentration ccat,

where E and F are constants.

Because the activity of H+is related to the catalyst concentration and r0is proportional to,is used to replace cH+in Eq.(11).The reaction rate equation can be expressed as

Since we assume that the reaction is in equilibrium state,and the values of r0at different catalyst concentrations are obtained by Eq.(11)and experimental data in Fig.6,Eq.(12)becomes

The mean squared error between experimental and calculated initial reaction rates at different catalyst concentrations is 1.09×10?7.

The values of XSBACcan be calculated by Eq.(13).As a nonlinear equation,Eq.(13)is solved by using the Runge-Kutta method with Matlab.The optimal parameters for the kinetics are estimated by the least squares method.The correlated results are listed in Table 3.

The relationship between rate constant k and temperature T can be expressed by the Arrhenius equation in the form of

where Eais the activation energy,J·mol?1,and A is the pre-exponential factor or apparent frequency factor,L·min?1·mol?1.

Fig.8.Simplified reaction mechanism.

Table 3 Reaction rate constant k+and k?at different temperatures

According to Eq.(15),ln k changes linearly with 1/T.The plot of the experimental data is shown in Figs.9 and 10.Eaand A can be calculated from the slope and the intercept of the line.The equation of k+and k?can be expressed as

Fig.9.Forward reaction rate constant k+versus temperature T.(R n=3.5,catalyst concentration=1.5%).

Fig.10.Reverse reaction rate constant k?versus temperature T.(R n=3.5,catalyst concentration=1.5%).

The results calculated by the homogeneous reaction kinetic model are shown in Figs.3,5 and 6.The calculated values are in good agreement with the experimental data.Therefore,the kinetic model can describe the kinetic behavior of the system reliably.However,the deviation of calculated and experimental values is relatively large at Rn=2.5,which may illustrate that the reaction system is a heterogeneous phase.

4.Conclusions

High purity ionic liquid([HSO3Pmim]HSO4)was prepared and used as catalyst.Based on the reaction mechanism,the homogeneous kinetic model for the transesterification of sec-butyl acetate and methanol to sec-butyl alcohol was established.Reaction rate constants were determined,and the activation energies of the forward and reverse reactions were obtained based on the Arrhenius law.The result shows that the calculation fits to the experimental data well,indicating that the homogeneous reaction kinetic model gives a good representation for the system.

Nomenclature

A pre-exponential factor,L·min?1·mol?1

c concentration,mol?L?1

ccatcatalyst concentration,mol?L?1

cA0initial concentration of sec-butyl acetate,mol?L?1

E,F constants in Eq.(12)

Eaactivation energy,J·mol?1

ΔH activation energy,J·mol?1

K equilibrium constant

k+forward reaction rate constant,mol?1.5·L1.5·min?1

k?reverse reaction rate constant,mol?1.5·L1.5·min?1

N number of data points

R gas constant,J·mol?1·K?1

Rninitial molar ratio of methanol to sec-butyl acetate

r reaction rate,mol·L?1·min?1

r0initial reaction rate,mol·L?1·min?1

T absolute temperature,K

XSBACconversion of sec-butyl acetate

Subscripts

calc calculated value

cat catalyst

MeAc methyl acetate

MeOH methanol

1,2 forward reaction of Steps 1 and 2 in Fig.8

?1,?2 reverse reaction of Steps 1 and 2 in Fig.8