Application of Br?nsted acid ionic liquids as green catalyst in the synthesis of 2-propanol with reactive distillation☆

Jinbei Yang ,Dongren Cai,Ting Zeng ,Lihua Zhou ,Ling Li,Ruoyu Hong ,Ting Qiu ,*

1 School of Chemical Engineering,Fuzhou University,Fuzhou,Fujian 350108,China

2 School of Ocean Science and Biochemistry Engineering,Fuqing Branch of Fujian Normal University,Fuzhou,Fujian 350300,China

1.Introduction

2-Propanol(IPOH),which is an excellent organic raw material and solvent,is widely used as an intermediate in chemical synthesis,or as an important solvent in the ink,paints,dyes,cosmetics,pharmaceutical industry and food industry[1–6].Ink and paints are the mainly applications which account for about 50%of the total consumption of 2-propanol.Several methods are available for manufacture of 2-propanol.Among them,direct hydration and indirect hydration of propylene are used most widely[7].Propylene and water are used as raw materials of both the processes.Indirect hydration is based on a two step process,in which an ester is formed and then hydrolyzed to 2-propanol.Nevertheless,this route has been gradually phased out in China since the 1980s due to its complex process,low selectivity,high energy requirement and serious corrosion to equipment.Compared to indirect hydration,direct hydration of propylene avoids some corrosion and environment problems and it has been deemed as the preferred process for production of IPOH.However,the process remainssome disadvantages such as high consumption of energy and cost for separation of IPOH from the azeotropic mixture(IPOH and water)[8].Hydrogenation of acetone is another method for producing IPOH.Nevertheless,it is used rarely because of its high demand on raw materials and catalyst.Therefore,it is urgent to develop an efficient,stable,environmentally friendly catalyst and green process for production of IPOH.

The preparation of IPOH via transesterification of is opropyl acetate(IPAc)with methanol(MeOH)as raw materials is a novel process as shown in Eq.(1),which has been investigated by our team using sodium methoxide as catalyst for the first time[3].The high conversion of IPAc(above 99%)was gotten under the optimal conditions.Hunan Zhongchuang Chemical Co.,Ltd.produce IPAc by the addition reaction of acetic acid with propylene on the 20000 t·a?1scale since 2006.This route has high conversion and selectivity,and greatly reduces manufacturing cost.Thus,IPAc is suitable as a feedstock for IPOH production via transesterification.However,sodium methoxide is not the best choice because it is easily inactivated by air or water and cannot be reused.

Ionic liquids(ILs),kinds of environmentally friendly solvent and catalyst,have attracted significant attention of scholars from various fields owing to its adjustable physical and chemical properties[9–16].The main driving force to explore ILs is the fact that they offer distinctive set of attributes,such as non flammable thermally,negligible vapor pressure,excellent thermal stability,high catalytic activity,and excellent recyclability and designability[17–20].Reactions with acid-functionalized ionic liquids as catalysts have gained desired results,such as nitration[21],transesteri fication[22,23],hydrogenation[24],polymerization[25],and Beckmann rearrangement[26].Nevertheless,to the best of our knowledge,their application as catalyst for the synthesis of IPOH via transesterification of IPAc with MeOH hasrarely reported in the literature.

In this work, five Br?nsted acidic ionic liquids were prepared and characterized.Their uses for the synthesis of IPOH via the transesterification of IPAc with MeOH were investigated.Then,the reaction kinetic behavior was studied using[Ps-mim]HSO4as catalyst,which performed best.Finally,a batch reactive distillation was proposed,and the reusability of catalyst was also studied.

2.Experimental Section

2.1.Materials

Isopropyl acetate(＞99.5 wt%analytical grade)was supplied by Hunan Zhongchuang Chemical Co.,Ltd.Other chemicals(AR grade)were commercial products and used without further purification.

2.2.Preparation of Br?nsted acidic ILs

In this paper, five Br?nsted acidic ILs were synthesized according to the procedures previously reported in the literature[27–30],which included 1-propylsulfonate-3-methyliminazole hydrogen sulfate([Ps-mim]HSO4),1-propylsulfonate pyridine hydrogensulfate([Ps-Py]HSO4),propylsulfonate triethylamine hydrogensulfate([Ps-N(Et)3]HSO4),N,N-dimethyl benzylamine-N-propylsulfonate hydrogensulfate([Ps-N-CH2C6H5Me2]HSO4)and N,N-dimethylcyclohexylamine-N-propylsulfonate hydrogensulfate ([Ps-NC6H11Me2]HSO).The structures of ILs were illustrated in Fig.1.

2.3.Characterization of Br?nsted acidic ILs

NMR spectra were obtained on a Bruker AV500 spectrometer(Bruker Co.,Ltd.,Switzerland)in DMSO-d6 and calibrated with TMS as the internal reference.IR measurements of ILs were recorded using a Spectrum 2000 FT-IR absorption spectrometer(Perkin Elmer Inc.,American)for KBr pellets in the frequency range of 4000–400 cm?1.

The acidity of ILs was determined by using the theoretical model and UV–visible spectroscopic methods.UV–visible spectra were performed on an Elmer Lambda 900 spectrometer(Perkin Elmer Inc.,American)with a basic indicator(4-nitroanline)by following the literature reported previously[31,32],and the solvent was methanol.

2.3.1.NMR analysis

The spectraldata of1HNMRand13C NMRfor the five Br?nsted acidic ILs were shown as follows:

2.3.2.FT-IR analysis

The spectral data of FT-IR for the five Br?nsted acidic ILs were listed as follows:

Fig.1.Structures of five Br?nsted acidic ILs.

The structures of the five Br?nsted acidic ILs were investigated using1HNMR,13CNMR and FT-IR spectroscopy,which were coincided with the theoretical structure(Fig.1).Moreover,1HNMR spectra showed no impurities,which demonstrate that high-purity five ILs were synthesized.

2.3.3.Hammett acidity evaluation

The acidity strength of Br?nsted acidic ILs can be efficiently expressed by the Hammett acidity function,H0,which can be calculated by the following equation:

According to the Lambert–Beer's Law,the[I]/[IH+](I represents indicator)could be determined and calculated from the changes of UV–visible absorbance of 4-nitroanline after the addition of Br?nsted acidic ILs.In this paper,the maximal absorbance of the unprotonated form of the indicator was observed at 370 nm under the same concentration of 4-nitroanline(54 μmol·L?1,p Ka=0.99)and ILs(80 mmol·L?1)in methanol.The absorption curves and the calculated of Hammett acidity values were shown in Fig.2 and Table 1,respectively.

Fig.2.UV visible spectra of the five Br?nsted acidic ILs.(a)blank;(b)[Ps-NCH2C6H5Me2]HSO4;(c)[Ps-N(Et)3]HSO4;(d)[Ps-N-C6H11Me2]HSO4;(e)[Ps-Py]HSO4;(f)[Ps-mim]HSO4.

Table 1 Calculation of Hammettacidity function(H0)values of different ILs in methanol(293.15 K)

As shown in Table 1 and Fig.2,the results showed that the order of the H0values of the five ILs is as follows:[Ps-N-CH2C6H5Me2]HSO4＞[Ps-N(Et)3]HSO4＞[Ps-N-C6H11Me2]HSO4＞[Ps-Py]HSO4＞[Ps-mim]HSO4.On the contrary,the acidity of[Ps-mim]HSO4is strongest.It is clearly shown that the acidity of the ILs depended on the types of cations,although the influence was insignificant.

2.4.Experimental apparatus and procedure for 2-propanol synthesis

In this study,the boiling point of the reaction system was relatively low at atmospheric pressure(about 338.15 K).Therefore,the experiment of the transesterification was carried out at high pressure(higher than 0.2 MPa)in order to increase the temperature to enhance the reaction rate.

2.4.1.Kinetic experiments

The kinetic experiments of the transesterification were conducted in a stainless steel reactor(reaction volume:500 ml,material:316 L)equipped with an agitation and temperature-controlling device(±0.1 K)(Fig.3).A HPLC pump was used for injection of the raw material into the reactor.

The desired amount mixture ofIPAc and MeOHwas charged into the reactor firstly.After the reactor was sealed up,nitrogen was introduced into the reactor to ensure the reactants keep liquid under the desired reaction temperature.Then,the reactants were heated to specified temperature with moderate agitation.Once the reaction temperature was reached,the preheated catalyst(dissolved in methanol)was fed into the reactor quickly by the HPLC pump,at the same time,the stirrer was set at an appointed rate and the time was regarded as the initial time.Samples were withdrawn at a fixed time interval,cooled down quickly and then analyzed by gas chromatography(GC2014,Shimadzu Corporation,Japan).

The experiments at the reaction temperature range of 338.15–393.15 K,the initial molar ratio of MeOH to IPAc of 2.0–5.0 and the catalyst dosage of 1.0 wt%,1.5 wt%and 2.0 wt%were studied.The reaction could be considered as reaching chemical equilibrium when the composition of the reaction mixture was nearly constant.All samples were performed in triplicate and mean values were quoted as results.

2.4.2.Batch reactive distillation experiments

The batch reactive distillation experiment of the transesterification was performed in a stainless steel column(internal diameter:20 mm,height:3 m,bottom volume:500 ml)equipped with an electronic reflux splitter to control reflux ratio and a heater in the bottom to heat the reaction mixture.The column was packed with θ stainless steel packing(φ3×3).There was a condenser located at the top of column for cooling down the vapor.The equipment diagram was shown in Fig.4.

Firstly,the reactants including catalyst ILs were proportionately added to the reboiler.The column pressure was increased to the specified value by introducing nitrogen after all correlative equipment was sealed up.Secondly,the reactants were heated and then total reflux operation was lasted for 10 min after the distillate appeared at the top.Subsequently,the distillates were continuously removed out.Samples were withdrawn at different time intervals from the reboiler,cooled down quickly and then analyzed in triplicate by gas chromatography until the pointed reaction time was reached.The used catalyst ILs could be easily separated from mixture of the bottom by removing the sample mixture with the rotary evaporator(353 K,2000 Pa),and reused for the next time.

2.4.3.Sample analysis method

All samples were quantitatively analyzed by gas chromatography(GC 2014,Shimadzu Corporation,Japan)equipped with a FID detector and AT-FFAP capillary column(50 m×0.32 mm×0.5μm),using 1,4-dioxane as internal standard and N,N-dimethyl for mamide as solvent.The temperature program of column was as follows:started at 328.15 K for 1 min,increased to 338.15 K at 2.5 K·min?1and maintained for 1 min,and then increased to 353.15 K at 30 K·min?1,the temperature was increased to 453.15 Kat40 K·min?1finally.Injector and detector temperatures were both maintained at 533.15 K.

3.Results and Discussion

3.1.Performance of five IL catalysts

Fig.3.Equipment diagram of kinetic experimental.1—heating mantle,2—high pressure reactor,3—thermocouple,4—mechanical stirrer,5—pressure tap,6—HPLC pump,7—control cabinet,8—exhaust valve,9—condenser,10—nitrogen cylinder.

Fig.4.Equipment diagram of high-pressure reactive distillation.1—reboiler,2—stainless steel packing,3—column,4—condenser,5—reflux splitter,6—re flux splitter controller,7—stainless steel receiving tanks,8—exhaust valve,9—nitrogen cylinder.

The five IL catalysts were tested for their activity under the initial MeOH/IPAc molar ratio of 3.0,catalyst dosage of 1.5%(mass fraction,based on the total reactant mass),reaction temperature of 353.15 K,and reaction time of 240 min.The results were depicted in Table 2.As seen in Table 2,all ILs are active for the transesterification,and the catalytic activity(conversion of the IPAc)of these ILs was listed in the following order:[Ps-mim]HSO4＞[Ps-Py]HSO4＞[Ps-NC6H11Me2]HSO4＞[Ps-N(Et)3]HSO4＞[Ps-N-CH2C6H5Me2]HSO4,which is consistent with the order of acidity of these ILs(Table 1).

Based on the Hammett acidity evaluation and the performance testing of five acidic IL catalysts,it can be concluded that the catalyticactivity of the ILs is influenced by the type of cation,although the effect was insignificant.The effect of cation on the catalytic activity was as follows:[Ps-mim]+＞[Ps-Py]+＞[Ps-N-C6H11Me2]+＞[Ps-N(Et)3]+＞[Ps-N-CH2C6H5Me2]+.Obviously,[Ps-mim]HSO4shows highest catalytic activity and the yield of[Ps-mim]HSO4is highest among the five acidic ILs.

Table 2 Catalytic activity of different ILs on the conversion of IPAc①

As reported in earlier literature[28,33],the thermal decomposition temperature of[Ps-mim]HSO4is above 320°C,which shows that[Ps-mim]HSO4has high thermal stability,wide liquid ranges and suitable to be used as catalyst under high temperature compared to the resin,solid acid catalyst etc.Therefore,[Ps-mim]HSO4was chosen as catalyst for reaction kinetics and batch reactive distillation experiment in follow-up experiments.

3.2.Reaction kinetics

3.2.1.Effect of reaction temperature

The effect of reaction temperature is important to the kinetic experiments,from which the activation energy of the transesterification of IPAc with MeOHcan be obtained.During the course of the experiments,the reaction temperature was varied in the range of 338.15–393.15 K,and the results were illustrated in Fig.5.As can be seen in Fig.5,the increment of temperature is apparently favorable to accelerate the reaction rate of transesterification,while the equilibrium conversion decreases slightly with the increasing of reaction temperature.Hence,it can be concluded the transesterification reaction is a marginal exothermal reaction.

Fig.5.Effect of temperature on the conversion of IPAc.Conditions:catalyst,[Psmim]HSO4;initial mole ratio of MeOH to IPAc,3:1;catalyst dosage,1.5 wt%.The dots represent experiment results;the lines represent model results.

3.2.2.Effect of initial molar ratio

The effect of initial molar ratio of MeOH to IPAc was studied experimentally.The mole ratio varied from 2:1 to 5:1,and the conversions of IPAc were obtained and shown in Fig.6.As shown in Fig.6,the reaction rate and equilibrium conversion are sensitive to the initial reactant molar ratio.The more the MeOH was added,the higher the conversion of IPAc was obtained in the same reaction time.Among them,the equilibrium conversion is 90.7%with MeOH to IPAc mole rate of 4:1 in 180 min.Further increase of this mole ratio makes weak influence to the conversion and reaction rate.Theoretically,the equilibrium constant should be the same at the same temperature.Equilibriumconstants calculated by Eq.(4)were listed in Table 3,from which we can see that they are all similar to each other.This is in a good agreement with the theory.

Fig.6.Effect of molar ratio of MeOH to IPAc on the conversion of IPAc.Conditions:catalyst,[Ps-mim]HSO4;reaction temperature,363.15 K;catalyst dosage,1.5 wt%.The dots represent experiment results;the lines represent model results.

Table 3 Equilibrium constants K eq at different molar ratio

3.2.3.Effect of catalyst dosage

A homogeneous catalyst,[Ps-mim]HSO4(soluble in methanol),was used as catalyst for the transesterification reaction.The catalyst dosage,defined as the mass percentage of the catalyst to the total feed mass,was varied over a range of 1.0 wt%–2.0 wt%at the temperature of 363.15 K and the initial molar ratio of MeOH to IPAc of 3.0.The results were exhibited in Fig.7.As illustrated in the figure,the reaction rate of IPAc increases with the increase of catalyst dosage from 1.0 wt%to 1.5 wt%,yetthe equilibrium conversion is constant.Furthermore,the reaction rate is improved slightly using 2.0 wt%catalyst.Therefore,a suitable catalyst dosage should be determined.

3.2.4.Chemical equilibrium

The equilibrium constants of the transesterification reaction catalyzed by[Ps-mim]HSO4in this work were defined by Eq.(3).The equilibrium constant Keqcan be deduced from the equilibrium composition obtained from the experiment:

Fig.7.Effect of catalyst dosage on the conversion of IPAc.Conditions:catalyst,[Psmim]HSO4;reaction temperature,363.15 K;initial mole ratio of MeOH to IPAc,3:1.The dots represent experiment results;the lines represent model results.

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

In a small temperature interval,ΔH can be considered to be constant and therefore,

where C is a constant.

A plot of ln Keqversus 1/T was shown in Fig.8.The temperature dependence of the equilibrium constant can be expressed by Eq.(7).

According to Eq.(7),the activation energy is?5.26 kJ·mol?1,which means that the reaction is weakly exothermal.The mean squared error between experimental and calculated equilibrium constants at different temperatures is 2.36×10?4.

3.2.5.Reaction mechanism

Fig.8.Temperature dependence of the chemical equilibrium constants:ln K eq versus 1/T(■)and the best fit of our data(—).

The reaction mechanism of the transesterification catalyzed by acid-functionalized ionic liquids is similar to that of proton acid.Therefore,the possible reaction mechanism can be deduced as shown in Fig.9.Steps 1 and 3 are relatively fast reactions and they are always assumed to be in chemical equilibrium.Step 2 is considered as a rate-controlling step for the slowest reaction rate.All in all,the reaction mechanism can be predigested into two steps,as shown in Fig.10.

Fig.9.Reaction mechanism of the transesterification of IPAc and MeOH.

Fig.10.Simplified reaction mechanism of the transesterification of IPAc and MeOH.

3.2.6.Kinetic modeling

Based on the mechanism of the reaction as mentioned above(Fig.10),a kinetic model can be deduced:

whereare the forward and backward reaction rate constants of Step 2,respectively.

As nucleophilic substitution reaction are relatively fast,and considered to be in equilibrium,the rates of Step 1 can be expressed as:

whereare the forward and backward reaction rate constants of step 1,respectively.Eqs.(8)and(9)lead to

whereis the initial concentration ofIPAc.cH+is the concentration of catalyst.

3.2.7.Estimation of the reaction rate constants

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

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

According to the nonlinear Eq.(11),a fourth-order Runge–Kutta method is used to calculate the conversion of IPAc under different time.An objective function(OF)is to minimize the squared differences between the calculated value Xcaland the experimental value Xexp,as shown in Eq.(13).The optimal parameters for the kinetics are estimated by using the Nelder–Mead simplex method,as shown in Table 4.

The fitted parameters were selected to calculate the conversion of IPAc by the homogeneous reaction kinetic model,and the results were shown in Figs.5,6 and 7.There is close agreement between the calculated and experimental values.Therefore,the kinetic model can describe the kinetic behavior of the system reliably.

3.3.Batch reactive distillation experiments

According to the previous work,the transesterification of IPAc with MeOH is a reversible reaction.High conversion of IPAc can be obtained by removing one product out of the system in time.The transesterification was carried out in a stainless steel column as shown in Fig.4.During the experimental period,several operation conditions were studied:reaction pressure,catalyst dosage,initial molar ratio and reaction time.Moreover,the stability of the catalyst was also investigated.

3.3.1.Effect of reaction pressure

The reaction temperature and evaporation are both associated with operating pressure.The effect of reaction pressure on the conversion of IPAc was examined by fixing the catalyst dosage of 1.0 wt%,reaction molar ratio of MeOH to IPAc of 3:1 and reaction time of 3.0 h.As can be seen in Fig.12(a),the conversion of IPAc significantly increasesfrom 94.5%to the maximum value of 98.1%with the increase of the reaction pressure from 0.1 to 0.2 MPa.However,with further increase of the reaction pressure,the conversion of IPAc decreases.A possible reason is that the increase of reaction pressure leads to high reaction temperature,which accelerates the reaction rate and then the high conversion is obtained.Nevertheless,the amount of vaporization is decreasing with the increase of reaction pressure,which is not conducive to removing products out.Then,the forward reaction is inhibited.Therefore,the optimum reaction pressure is between 0.2 and 0.3 MPa,and the conversion of IPAc is above 97.5%.

Table 4 Chemical reaction rate constant of the transesterification at various temperatures

Fig.11.Arrhenius plotfor the reaction rate constants of IPAc with MeOH.Forward reaction k+(■)and backward reaction k? (▲).The lines represent the results of the linear regression.

3.3.2.Effect of catalyst dosage

The effect of catalyst dosage on the conversion of IPAc was studied and illustrated in Fig.12(b).The reaction conditions are:catalyst,[Psmim]HSO4;reaction pressure,0.3 MPa;initial mole ratio of MeOH to IPAc,3:1;and reaction time,3.0 h.As the catalyst dosage increased from 0.25 to 1.0 wt%,the conversion of IPAc was found to increase from 92.4%to 97.6%due to the increase of catalytic active sites.Nevertheless,with further increase of the catalyst dosage,the conversion of IPAc changes insignificantly.As a result,in consequence of the cost and activity of catalyst,the suitable catalyst dosage is around 1.0 wt%.

Fig.12.Conversion of IPAc by transesterification over[Ps-mim]HSO4 as a function of(a)reaction pressure,(b)catalyst dosage,(c)reaction molar ratio,and(d)reaction time.

3.3.3.Effect of reaction molar ratio

Fig.12(c)shows the effect of molar ratio of reactants(MeOH/IPAc)on the conversion of IPAc.The reaction conditions are:catalyst,[Psmim]HSO4;catalyst dosage,1.0 wt%;reaction pressure,0.3 MPa;and reaction time,3.0 h.Overall,the reaction molar ratio has great effect on the conversion of IPAc as depicted in Fig.12(c).As the reaction molar ratio increased from 2.0 to 4.0,the conversion of IPAc raises obviously from 92.0%to 99.4%.However,when the reaction molar ratio is larger than 4,the effect on conversion is weak.Actually,the increase of initial molar ratio can lead to the increase of energy consumption of subsequent separation.Therefore,in the consideration of the conversion of IPAc and energy consumption of subsequent separation,a molar ratio(MeOH/IPAc)of 4.0 is the most suitable.

3.3.4.Effect of reaction time

The suitable reaction time was explored in a range of 1.0–3.5 h,as shown in Fig.12(d).The reaction conditions are:catalyst,[Psmim]HSO4;catalyst dosage,1.0 wt%;reaction pressure,0.3 MPa;and initial mole ratio of MeOH to IPAc,4:1.It can be seen from Fig.12(d)that the conversion of IPAc increases with the increase of reaction time.When the reaction time was longer than 3.0 h,the conversion remains invariant.This phenomenon can be explained by the fact that the transesterification of IPAc with MeOH is nearly completed with a time of around 3.0 h.Meanwhile,the temperature and mass fraction of MeAc and IPAc at the bottom were recorded and analyzed,respectively.The results were listed in Fig.13(a)and(b).It was observed that the temperature at the bottom of column is gradually increased with reaction time due to the continuous change of composition at the bottom of column(Fig.13(a)).As shown in Fig.13(b),the top temperature was close to the binary azeo tropic point(MeOH/MeAc).Hence,the preferred reactant time is 3.0 h.

3.3.5.Repeatability experiment

From the experimental study done in this work,the optimum batch reactive distillation operation conditions can be concluded as follows:reaction pressure,0.3 MPa;catalyst dosage,1.0 wt%;initial mole ratio of MeOH to IPAc,4:1;and reaction time,3.0 h.Under the optimum conditions,the conversion of IPAc is above 99%.

For the purpose of verifying the reliability of the experimental data,a repeatability experiment was performed.The results were listed in Table 5.As depicted in Table 5,there is no significant difference in the three parallel experiments,which indicates that the experimental data are reliable and accurate.

3.3.6.Reusability of catalyst

In order to evaluate the potential reusability of catalyst in the transesterification of IPAc with MeOH,a series of recycle experimentswere conducted in stainless steel column under the optimum conditions.The reuse performances of the ILs[Ps-mim]HSO4were shown in Table 6.It can be seen thatthe conversion ofIPAc only slightly decreases from 99.4%to 99.0%after the catalyst was repeatedly used for four times.Furthermore,the FT-IR,1H NMR and13C NMR spectroscopy analyses of recycled ILs were carried out to verify that there are no new functional groups formed for the used IL catalyst.The results of recycle experiments and characterization both indicate that the ILs[Psmim]HSO4as the catalyst for the transesterification of IPAc with MeOH is recyclable and stable.

Table 5 Repeatability experiment results

Table 6 Effect of recycle times on the conversion of IPAc①

3.4.Comparison of[Ps-mim]HSO4 and traditional catalyst

According to the previous study of our team[3],sodium alkoxide was used as catalyst in the transesterification of IPAc with MeOH.The reaction reached equilibrium in 120 min at 333 K and the conversion of IPAc is about 88.0%with catalyst dosage of 0.3 wt%and initial mole ratio of MeOH to IPAc of 3:1.However,sodium methoxide is sensitive to water and air,and it cannot be reusable,which would produce large amounts of solid waste and cause environmental pollution.In this paper,[Ps-mim]HSO4was used as catalyst.The reaction reached equilibrium in 480 min at 353 K and the conversion of IPAc is 87.4%with catalyst dosage of 1.5 wt%and initial mole ratio of MeOH to IPAc of 3:1.Compared to sodium alkoxide,[Ps-mim]HSO4as catalyst can acquire almost the same equilibrium conversion,the reaction rate is relatively low.However,ionic liquids offer a new possibility for developing environmentally-friendly acidic catalysts owing its advantages,such as stability in air and water,excellent thermal stability,high catalytic activity,and easy reusability.In this work,the high conversion of IPAc was obtained under optimum conditions using[Ps-mim]HSO4as catalyst,and the catalyst was recycled easily by removing the reaction mixture with the rotary evaporator and reused without any further procedure.Ionic liquids are promising to replace conventional catalysts and therefore have a great potential for industrial application.

Fig.13.Effect of reaction time on(a)the purity of MeAc and IPAc in bottom and(b)the bottom and top temperature.

4.Conclusions

In this work,we have prepared and characterized five Br?nsted acidic ionic liquids.The experimental results show that[Ps-mim]HSO4is more active than the other ILs in the synthesis of 2-propanol(IPOH)via the transesterification of isopropyl acetate(IPAc)with methanol(MeOH),which was chosen as catalyst for further study.The reaction kinetics of IPOH synthesis was studied.A homogeneous second order kinetic model for the transesterification was established,and it has been found that the kinetic model established can be well used in describing the transesterification.The high-pressure batch reactive distillation experiment of the transesterification was performed in a stainless steel column.As a result,a high conversion of IPAc of 99.4%was obtained under the optimal reaction conditions.Moreover,the catalyst[Psmim]HSO4can be recycled easily by the rotary evaporator and reused without any further treatment.The catalyst had been repeatedly used for four times and no obvious changes in the structure of catalyst could be observed.

[1]E.Grousseau,J.Lu,N.Gorret,S.E.Guillouet,A.J.Sinskey,Isopropanolproduction with engineered Cupriavidus necator as bioproduction platform,Appl.Microbiol.Biotechnol.98(9)(2014)4277–4290.

[2]K.Inokuma,J.C.Liao,M.Okamoto,T.Hanai,Improvement of isopropanol production by metabolically engineered Escherichia coli using gas stripping,J.Biosci.Bioeng.110(6)(2010)696–701.

[3]T.Qiu,P.Zhang,J.B.Yang,L.Xiao,C.S.Ye,Novel procedure for production of isopropanol by transesterification of isopropyl acetate with reactive distillation,Ind.Eng.Chem.Res.53(36)(2014)13881–13891.

[4]L.Xiao,Q.L.Wang,H.X.Wang,T.Qiu,Isobaric vapor-liquid equilibrium data for the binary system methyl acetate+isopropylacetate and the quaternary system methyl acetate+methanol+isopropanol+isopropyl acetate at 101.3 kPa,Fluid Phase Equilib.344(2013)79–83.

[5]S.J.Wang,D.S.Wong,Control of reactive distillation production of high-purity isopropanol,J.Process Control 16(4)(2006)385–394.

[6]A.Rahman,S.Salem,A review on reduction of acetone to isopropanol with Ni nano superactive,heterogeneous catalysts as an environmentally benevolent approach,Appl.Catal.A Gen.469(2014)517–523.

[7]Y.Xu,K.T.Chuang,A.R.Sanger,Design of a process for production of isopropyl alcohol by hydration of propylene in a catalytic distillation column,Chem.Eng.Res.Des.80(6)(2002)686–694.

[8]S.M.Mahajani,M.M.Sharma,T.Sridhar,Direct hydration of propylene in liquid phase and under supercritical conditions in the presence of solid acid catalysts,Chem.Eng.Sci.57(22)(2002)4877–4882.

[9]M.I.Kim,S.J.Choi,D.W.Kim,D.W.Park,Catalytic performance of zinc containing ionic liquids immobilized on silica for the synthesis of cyclic carbonates,J.Ind.Eng.Chem.20(2014)3102–3107.

[10]L.Zhang,M.Xian,Y.He,L.Li,J.Yang,S.Yu,X.Xu,A Br?nsted acidic ionic liquid as an efficient and environmentally benign catalyst for biodiesel synthesis from free fatty acids and alcohols,Bioresour.Technol.100(19)(2009)4368–4373.

[11]N.Muhammad,Y.A.Elsheikh,M.I.A.Mutalib,A.A.Bazmi,R.A.Khan,H.Khan,Z.J.Man,An overview of the role of ionic liquids in biodiesel reactions,J.Ind.Eng.Chem.21(2015)1–10.

[12]T.Qiu,W.L.Tang,C.G.Li,C.M.Wu,L.Li,Reaction kinetics for synthesis of sec-butyl alcohol catalyzed by acid-functionalized ionic liquid,Chin.J.Chem.Eng.23(1)(2015)106–111.

[13]H.X.Wang,C.M.Wu,X.W.Bu,W.L.Tang,L.Li,T.Qiu,A benign preparation of secbutanol via transesterification from sec-butyl acetate using the acidic imidazolium ionic liquids as catalysts,Chem.Eng.J.246(2014)366–372.

[14]A.Ghorbani-Choghamarani,M.Norouzi,Synthesis and characterization of ionic liquid immobilized on magnetic nanoparticles:A recyclable heterogeneous organocatalyst for the acetylation of alcohols,J.Magn.Magn.Mater.401(2016)832–840.

[15]M.Olkiewicz,N.V.Plechkova,M.J.Earle,A.Fabregat,F.Stüber,A.Fortuny,J.Font,C.Bengoa,Biodieselproduction from sewage sludge lipids catalysed by Br?nsted acidic ionic liquids,Appl.Catal.B Environ.181(2016)738–746.

[16]A.Chinnappan,A.H.Tamboli,W.J.Chung,H.Kim,Green synthesis,characterization and catalytic efficiency of hypercross-linked porous polymeric ionic liquid networks towards 4-nitrophenol reduction,Chem.Eng.J.285(2016)554–561.

[17]K.Scott,N.Basov,R.J.J.Jachuck,N.Winterton,A.Cooper,C.Davies,Reactor studies of supported ionic liquids:Rhodium-catalysed hydrogenation of propene,Chem.Eng.Res.Des.83(10)(2005)1179–1185.

[18]J.P.Hallett,T.Welton,Room-temperature ionic liquids:Solvents for synthesis and catalysis.2,Chem.Rev.111(5)(2011)3508–3576.

[19]J.B.Yang,L.H.Zhou,X.T.Guo,L.Li,P.Zhang,R.Y.Hong,T.Qiu,Study on the esterification for ethylene glycol diacetate using supported ionic liquids as catalyst:Catalysts preparation,characterization,and reaction kinetics,process,Chem.Eng.J.280(2015)147–157.

[20]M.Deetlefs,K.R.Seddon,M.Shara,Predicting physical properties of ionic liquids,Phys.Chem.Chem.Phys.8(5)(2006)642–649.

[21]G.Cheng,X.Duan,X.Qi,C.Lu,Nitration of aromatic compounds with NO2/air catalyzed by sulfonic acid-functionalized ionic liquids,Catal.Commun.10(2)(2008)201–204.

[22]K.X.Li,L.Chen,Z.C.Yan,H.L.Wang,Application of pyridinium ionic liquid as a recyclable catalyst for acid-catalyzed transesterification of Jatropha oil,Catal.Lett.139(3–4)(2010)151–156.

[23]T.Qiu,X.T.Guo,J.B.Yang,L.H.Zhou,L.Li,H.X.Wang,Y.Niu,The synthesis of biodiesel from coconut oil using novel Br?nsted acidic ionic liquid as green catalyst,Chem.Eng.J.296(2016)71–78.

[24]I.Cerna,P.Kluson,M.Bendova,T.Floris,H.Pelantova,T.Pekarek,Intensification of the use of ionic liquids as efficient reaction co-solvents in asymmetric hydrogenations,Chem.Eng.Process.50(3)(2011)264–272.

[25]W.Och?dzan-Siod?ak,K.Dziubek,D.Siod?ak,Biphasic ethylene polymerisation using 1-n-alkyl-3-methylimidazolium tetrachloroaluminate ionic liquid as a medium of the Cp2TiCl2titanocene catalyst,Eur.Polym.J.44(11)(2008)3608–3614.

[26]X.Liu,L.Xiao,H.Wu,Z.Li,J.Chen,C.Xia,Novel acidic ionic liquids mediated zinc chloride:Highly effective catalysts for the Beckmann rearrangement,Catal.Commun.10(5)(2009)424–427.

[27]A.C.Cole,J.L.Jensen,I.Ntai,K.L.T.Tran,K.J.Weaver,D.C.Forbes,J.H.Davis,Novel Br?nsted acidic ionic liquids and their use as dual solvent-catalysts,J.Am.Chem.Soc.124(21)(2002)5962–5963.

[28]H.Li,S.Yu,F.Liu,C.Xie,L.Li,Synthesis of dioctyl phthalate using acid functionalized ionic liquid as catalyst,Catal.Commun.8(11)(2007)1759–1762.

[29]G.Schmidt-Naake,A.Schmalfu?,I.Woecht,Free radical polymerization in ionic liquids—In fluence of the IL-concentration and temperature,Chem.Eng.Res.Des.86(7)(2007)765–774.

[30]J.Fraga-Dubreuil,K.Bourahla,M.Rahmouni,J.P.Bazureau,J.Hamelin,Catalysed esterifications in room temperature ionic liquids with acidic counteranion as recyclable reaction media,Catal.Commun.3(5)(2002)185–190.

[31]C.Thomazeau,H.Olivier-Bourbigou,L.Magna,S.Luts,Determination of an acidic scale in room temperature ionic liquids,J.Am.Chem.Soc.125(18)(2003)5264–5265.

[32]Y.Wang,D.Jiang,L.Dai,Novel Br?nsted acidic ionic liquids based on benzimidazolium cation:Synthesis and catalyzed acetalization of aromatic aldehydes with diols,Catal.Commun.9(15)(2008)2475–2480.

[33]Q.Wu,H.Chen,M.Han,D.Wang,J.Wang,transesterification of cottonseed oil catalyzed by Br?nsted acidic ionic liquids,Ind.Eng.Chem.Res.46(24)(2007)7955–7960.