Preparation and shaping of solid acid SO4 2?/TiO2 and its application for esterification of propylene glycol monomethyl ether and acetic acid☆

Zhixian Huang,Yixiong Lin,Ling Li,Changshen Ye,Ting Qiu*

School of Chemical Engineering,Fuzhou University,Fuzhou 350116,China

1.Introduction

Propylene glycol methyl ether acetate(PMA)is a kind of multifunctional,colorless and transparent solvent.It has ether bond,alkyl and carbonyl groups.As PMA contains both polar and non-polar groups,it can dissolve polar or non-polar material and the dissolving capacity of PMA is higher than that of other general solvents,so PMA is called as a universal solvent.Due to the kindly environmental performance,excellent thermo-stability and small change of viscosityversustemperature,PMA is gradually replacing ethylene glycol ether and ester solvents and has been extensively applied in ink,coating,printing,dyeing,pesticide and other fields[1–6].

PMA is traditionally manufactured through esterification of propylene glycol monomethyl ether(PM)with acetic acid(HAc).In this reaction,traditional liquid acids such as sulfuric acid and hydro fluoric acid can be used as the catalyst.A detailed kinetic study of esterification of PM and HAc using p-toluenesulfonic as catalyst has been reported in the literature[7].

Traditional liquid acid catalyst belongs to homogeneous catalyst.Although it exhibits high catalyst activity,using traditional liquid acid in this reaction suffers from a number of shortages:(1)the reactant PM is prone to occur side reactions,such as dehydration and oxidation;(2)it is difficult to separate the catalysts from the reactants after the reaction;(3)in order to remove the residual acid catalyst,it is also inevitable to undergo a complex process such as alkali washing;(4)strong acidity has a corrosive effect on equipment so that the use of costly anticorrosive materials for a production apparatus or anti-corrosive treatment of a production apparatus is required.

In recentyears,solid acid catalysts have received sustained attention because oftheirobvious engineering bene fits[8,9].Due to high catalytic activity,low corrosivity,and ease of separation,solid acids become very attractive alternatives to the conventional homogenous catalysts.Therefore,solid acids have been widely used in many kinds of chemical reactions,such as esterification[10–12],alkane isomerization[13,14],aldol condensation[15,16]and ketal reaction[17,18].The solid catalyst can be prepared by impregnating H2SO4solution with transition metal oxides MxOy(such as ZrO2,TiO2,Fe2O3and so on).Some rare earth metal can be introduced into the solid acid to adjust its activity.Therefore,there are many types of solid acid.Among these solid acids,however,SO42?/TiO2solid acid has high catalytic activity and excellent thermal stability.What's more,SO42?/TiO2solid acid is easy to be prepared with low cost and thus has been widely used as an alternative to the conventional homogeneous catalysts.Zhouet al.adopted SO42?/TiO2solid acid to catalyze the esterification reaction of myristic acid with isopropanol[19].The esterifications of fatty acids with methanol and transesterification of vegetable oils with methanol using SO42?/TiO2solid acid as catalyst have been studied[20,21],and it also is found that SO42?/TiO2behaves excellent catalytic activity and stability in the reactions.In this paper,therefore,SO42?/TiO2solid acid was prepared and expected to be substituted for conventional acid catalysts in the esterification ofPMand HAc to produce the desired product PMA.

Because investigation on reaction kinetics in the presence of this emerging heterogeneous catalyst solid acid for such esterification reaction is quite limited,it is necessary to study the effects of operating variables on the esterification reaction,such as temperature,molar ratio of reactants,catalyst concentration and the speed of agitation.Then a heterogeneous kinetic model is established based on its possible reaction mechanism.In addition,the prepared and used solid acid catalyst was powdery,so it still was not easy to be recovered from the reaction system.Therefore,the shaped solid acid catalyst was also studied in the last section.

2.Experimental

2.1.Materials

All chemicals used in this work were of analytical grade,so they were used without further purification.PM,PMA,TiO2,and HAc were supplied by Aladdin Chemical Reagent Co.,Ltd.(Shanghai,China).Sulfuric acid and glycerol were obtained from Sinopharm Chemical Reagent Co.,Ltd.(Shanghai,China).Activated carbon was procured from Wabzhan Carbon Corporation,while pseudo-boehmite and glass fiber from Nuoda Chemical Corporation.

2.2.Preparation of solid acid catalyst SO4 2?/TiO2

2.2.1.Preparation of solid acid catalyst SO4 2?/TiO2

The catalyst used in the experiments was prepared by impregnation method[22].In a typical procedure,a certain quality of TiO2powder was impregnated with 0.5 mol·L?1H2SO4solution,in which the mass ratio of TiO2powder to H2SO4solution was 1:2.Then,the mixture of H2SO4solution and TiO2powder was dried at 393 K for 3 h in a vacuum drying oven.Afterward,the dried mixture was calcined at 673 K for 3 h to obtain solid acid catalyst SO42?/TiO2.The catalyst was used directly in the esterification reaction.

The catalytic activity of the solid acid catalyst was tested for the esterification reaction between PM and HAc in a three-neck flask.The setup was equipped with a heat,a temperature controller(±0.5 K),and a stirrer speed controller.A re flux condenser was installed to avoid any loss of volatile components.Since the reactant HAc was excess,the conversion of PM was chosen as evaluation index.

2.2.2.The optimization of the catalyst preparation condition

Orthogonal design method was used to investigate the effective parameters in the preparation of solid acid catalyst SO42?/TiO2and optimize the preparation condition of impregnation method.According to the preparation method described above,there are five factors that affect the activity of obtained catalyst.They are the concentration of H2SO4solution(A),the mass ratio of TiO2to H2SO4solution(B),calcination temperature(C),drying temperature(D)and calcination time(E).The factors and their levels assigned were presented in Table 1.

Table 1Design experiments in preparation SO4 2?/TiO2

2.3.Characterization of solid acid catalyst SO4 2?/TiO2

The obtained SO42?/TiO2catalyst was characterized for its physicochemical properties such as surface area,crystallinity,morphology and so on.Their specific surface and pore volume were measured by Bruner–Emmett–Teller method(BET;ASAP2010 specific surface area analytical instrument)with N2adsorption.Pyridine adsorbed IR measurements of solid acid catalysts were recorded using a Spectrum 2000 FT-IR absorption spectrometer for KBr pellets in the frequency range 4000–400 cm?1.The thermostability of the solid acid was evaluated by using STA 449C thermal analyzer.The crystal form of the catalyst was obtained on X'Pert Pro MPD X-Ray Powder Diffractometer.The morphology of the catalyst was identified by Scanning Electron Microscope(SEM,XL30 ESEM-TMP).

2.4.Shaping of solid acid catalyst

Shaped solid acid catalyst is easy to separate from the reactants and reuse as opposed to the powder form.However,its catalytic activity may be decreased as compared with a powder catalyst.To obtain a solid acid shaped catalyst that satis fies the characteristics required as a catalyst and has a required mechanical strength,therefore,it is necessary to investigate the molding formulation for the shaping process of solid acid catalyst.

In a typical procedure,a mixture of obtained solid acid catalyst SO42?/TiO2powder 72.5 wt%,binder pseudo-boehmite 5 wt%,reinforcing agent glass fiber 15 wt%,pore forming materials activated carbon 2.5 wt%,lubricant glycerol 5 wt%,and deionized water was put into a kneader and kneaded for 30 min.Then,the mixture was extruded through a circular opening of 1.5 mm diameter to shape the material into cylindrical pellets,which were then dried at room temperature for 24 h and further dried at 393 K for 3 h.By calcining the materials in air at 773 K for 2 h,the precursor shaped catalyst(φ1.5 mm×2 mm)was obtained.

Secondly,the precursor catalyst was sulfated by impregnating in 2 mol·L?1H2SO4solution again.Then,the precursor catalyst was dried at 393 K for 3 h and calcined at 673 K for 3 h to obtain shaped solid acid catalyst SO42?/TiO2.

The orthogonal test was applied again to discuss the effects of the quantity of additives which were used in shaping process on the catalytic activity of solid acid catalyst.The conversion of PM and the mechanical strength of catalyst were chosen as the optimizing indexes.Four factors were investigated:the quantity ofbinder pseudo-boehmite(A),reinforcing agent glass fiber(B),pore forming materials activated carbon(C),and lubricant glycerol(D).Three levels of each factor were considered,and the factors and levels of orthogonal test were displayed in Table 2.

2.5.Apparatus and procedure

The esterification reaction kinetic experiments were performed in a batch stainless steel reactor equipped with agitation and temperature control(±0.5 K).The reaction temperatures ranged from 383.15 K to 413.15 K.The schematic diagram of the experimental set-up was presented in Fig.1.

At first,a certain quantity of HAc and catalyst was added into the reactor and then the reactor was sealed.The pressure rose to about 0.4 MPa by filling nitrogen into the reactor,then the reactor was heated and the agitation device started to work.Once the desired reaction temperature was attained,the preheated PM was introduced into the reactor by a tranquil flow pump.This time was considered as the zero reaction time.Samples were withdrawn intermittently from the reactor and immediately cooled to 263 K in order to avoid any further reaction.All samples were analyzed by gas chromatography.The reaction was considered to reach chemical equilibrium when the composition of reaction mixture was nearly constant.

Table 2Design experiments in shaping solid acid catalyst

2.6.Analysis

All samples were analyzed by GC2014C gas chromatography(SHIMADUZ)equipped with a flame ionization detector(FID)and a TM-5 capillary column(0.25 mm×30 m).Nitrogen with a purity of 99.999%was used as the carrier gas.The injection port and detector of the GC were maintained at 523.15 K.The initial oven temperature was 333.15 K and increased with a ramp rate of 30 K·min?1to 373.15 K and hold for 1 min,then increased to 473.15 K at 40 K·min?1and held for 2 min.Here,cyclohexanone was used as the internal standard.Samples were analyzed three times to eliminate the errors.

3.Results and Discussion

3.1.Preparation of solid acid catalyst SO24?/TiO2

3.1.1.Visual and range analysis

In this paper all catalytic activity tests of the samples were performed at initial molar ratio of HAc to PM 4:1,catalyst concentration of 5 wt%,reaction temperature of 388.15 K.Among 25 experiments(as shown in Table 3),the conversion of PM in the 9th experimental run was the highest.So the most effective combination of levels wasA3,B1,C3,D4andE2,that is,CH2SO4was 1.5 mol·L?1,mTiO2:mH2SO4was 1:1,drying temperature 393 K,calcination temperature 673 K,and calcination time 3 h.

Table 3The experimental analysis of orthogonal test

The important order of in fluences of experimental factors on the optimizing index can be obtained through range analysis of orthogonal test.In Table 3,Ki,jis the summation of optimizing index which corresponds withjthfactor andithlevel;ki,jis the average value ofKi,j.The optimal levelofjthfactor can be obtained through comparing the values ofKi,j.The optimal levels of each experimental factor constitute the optimal combination.Kis the summation of total optimizing indexes,andRjis the range ofjthfactor.The lager the value ofRj,the more significant the in fluence ofjthfactor on optimizing index.The optimal levels of each factor were showed in Table 4.

Fig.1.Experimental setup for the synthesis of PMA from PM and HAc.

Table 4Variance analysis

The optimum combination obtained from range analysis wasA2,B3,C2,D4andE3.Through comparing the catalytic activities obtained by visualand range analysis,itcan be concluded thatthe optimalcombination wasA2,B3,C2,D4andE3.

3.1.2.Variance analysis

The significancesofin fluences ofexperimentalfactors on optimizing index can be con firmed by variance analysis.For the orthogonal tableLn(om),the number of test,factor,and level aren,mando,respectively.yi(i=1,2,……,n)is the value of the optimization index in every test.The number of repeat test isw,w=n/r.The procedures of variance analysis are as follow:

(1)Calculation ofsummation ofsquares ofdeviations ofexperimental

factors:

SEis the sum of squares of the errors and is equal to minimum value ofSj.Here,j=1,2,3,4,5 corresponds to factorA,B,C,D,andE,respectively.

(2)Calculation of degree of freedom:

(3)Ftest

In order to judge the significances of in fluences of experimental factors on optimizing index,Ftestofevery factor is carried outaccording Eq.(4).

The significant levels can be taken α =0.25,α =0.10,α =0.05 or α =0.01.Fα(fi,fE)of every factor is calculated.IfFj＞F0.01(fi,fE),then thejthfactor is highly significant;IfF0.01(fi,fE)≥Fj＞F0.05(fi,fE),then thejthfactor is more significant;IfF0.05(fi,fE)≥Fj＞F0.10(fi,fE),then thejthfactor is significant;IfF0.25(fi,fE)≥Fj＞F0.25(fi,fE),then thejthfactor is slightly significant;IfF0.25(fi,fE)＞Fj,then thejthfactor is no significant.The variance analysis table was showed in Table 4.

Based on variance analysis,the order offactors in fluence on the conversation of PM isA(the concentration of sulfuric acid solution)＞D(drying temperature)＞C(calcination temperature)＞E(calcination time)＞B(mass ratio of sulfuric acid solution and titanium dioxide),which is consistent with range analysis.

3.2.Characterization of solid acid catalyst SO4 2?/TiO2

3.2.1.BET analysis

The surface area of the catalyst was calculated according to BET procedure by using the data of adsorption of N2on the sample(Table 5).It could be seen that after impregnation the surface area decreased from 80.90 m2·g?1to 48.96 m2·g?1.Correspondingly,the pore diameter of the catalyst increased to 16.40 nm from 12.94 nm,which may be the losing of water during drying and calcination processes.

Table 5BET analysis results

Isothermaladsorption and desorption curves of prepared SO42?/TiO2catalyst were showed in Fig.2.It could be found that this isothermal adsorption and desorption curves were IV type,which indicated that SO42?/TiO2belonged to mesoporous material.AtP/P0=0.5–1.0,there was H3 type of hysteresis loop,so the internal pore canals of SO42?/TiO2was mesoporous structure.

Fig.2.Isothermal adsorption desorption curve of SO4 2?/TiO2.

3.2.2.Pyridine adsorbed IR analysis

The pyridine adsorbed IR result was displayed in Fig.3.At wave number of 1450 cm?1,the characteristic absorption peak of Lewis acid was very obvious,so it can be deduced that the prepared catalyst contained a lotofLewis acid.There also existed a smallpeak ofBr?nsted acid at wave number of 1540 cm?1.The summation characteristic absorption peak of this two kinds of acids appeared at wave number of 1490 cm?1.

3.2.3.Thermostability analysis

Thermogravimetry Analysis(TGA)was carried out to understand the thermal stability of the prepared SO42?/TiO2catalyst(Fig.4).It was observed that the change in weight loss was gradual.At 573 K,the mass of sample decreased obviously and there was a sharp peak on DTGcurve,this may be the losing ofphysicaladsorbed wateron catalyst surface.At673 K–773 K,the mass ofsample decreased slightly,this may be the decomposing of free of sulfuric acid and the losing of chemical adsorbed water on the surface of catalyst.At 723 K,the decomposing of SO42?which bonded with titanium dioxide resulted in the losing of sample.Hence,it suggested the stability of catalyst until 723 K.

Fig.3.Pyridine adsorbed IR analysis results of powdery SO4 2?/TiO2 catalyst.

Fig.4.Thermostability analysis result of SO4 2?/TiO2.

3.2.4.XRD analysis

The XRD analysis results of TiO2and SO42?/TiO2were showed in Fig.5.The peak positions of TiO2and SO42?/TiO2catalysts were almost the same.At 25.3°,37.8°,55.1°and 62.7°,the peaks all belonged to anatase type titanium dioxide,which indicated that the titanium dioxide in SO42?/TiO2catalyst was anatase type.

Fig.5.XRD analysis results of powdery catalyst.

3.2.5.SEM analysis

The SEManalysis results ofTiO2and SO42?/TiO2were showed in Fig.6.The surface appearances of TiO2and SO42?/TiO2were similar,and there were both globules on the surface.However,the particle of TiO2was not of uniform size with average diameter of 5 μm,while the particles of SO42?/TiO2were equirotal and its average diameter was about 1 μm.

Fig.6.SEM analysis result:(a)— TiO2;(b)— SO4 2?/TiO2.

3.3.Esteri fication reaction kinetic experiments results

3.3.1.Elimination of external diffusion

In this reaction,there exists mass-transfer resistance across the solid–liquid interface.The effect of the speed of agitation was studied over a range of 100–350 r·min?1to examine the in fluence of an external mass-transfer on the reaction.Fig.7 showed that the external mass-transferresistance could be negligible when the speed ofagitation exceeded 300 r·min?1.Thereby,to avoid abrasion of the catalyst,further experiments were conducted at 300 r·min?1.

3.3.2.Effect of reaction temperature

The effectoftemperature on conversion undersimilarcondition was studied in the range of 383.15 to 413.15 K(Fig.8).As we all know,the increasing temperature is favorable to speed up the reaction.Fig.8 demonstrates thattemperature has significantin fluence on the reaction rate but not on conversion.As the temperature rises,the time required for the reaction to reach equilibrium is gradually reduced.At 383.15 K ittook about260 min to reach the chemicalequilibriumand the equilibrium conversion of PM was 61.58%.Whenp-toluenesulfonic was used as catalyst,the equilibriumtime was about360 min and the equilibrium conversion ofPMwas approximately 60%[7].Hence,using the solid acid SO42?/TiO2as catalyst characterizes faster reaction rate in this reaction.

Fig.7.Effect of the speed of agitation on the conversion of PM(T=413.15 K,catalyst loading 5.0 wt%,and molar ratio of HAc:PM=2:1).

Fig.8.Effect of reaction temperature on reaction rate and conversion of PM:Catalyst loading 5.0 wt%,molar ratio of HAc:PM=2:1 and the speed of agitation 300 r·min?1.

The equilibrium constantKeqcan be calculated from the equilibrium concentration of each component:

The equilibrium constants of different reaction temperatures were shown in Table 6.Itwas obvious thatthe reaction equilibrium constants increased with the increase of reaction temperature,which indicated that the esterification reaction of PM and HAc is an endothermic reaction.

Table 6Values of equilibrium constants at different temperatures

The van't Hoff equation gives:

where ΔHis the reaction enthalpy,J·mol?1.

According to the results presented in Table 6,reaction enthalpy for the reaction was obtained from Eq.(6)by plotting lnKeqagainst 1/T.A satisfactory linear coefficient was obtained.The standard enthalpy of the reaction was estimated,ΔH=7.90 × 103J·mol?1,so this esterification reaction is a weak endothermic reaction.

3.3.3.Effect of initial molar ratio

The initial molar ratio of HAc to PM was varied between 1 and 4 at constanttemperature(413.15 K)and catalyst loading(5.0 wt%).The effect of initial molar ratio of acetic acid to PM on the reaction rate and conversion was shown in Fig.9.It can be found that both the reaction rate and conversion of PM increased with the increase of the molar ratio of HAc to PM.

Fig.9.Effect of initial molar ratio on reaction rate and conversion of PM.

3.3.4.Effect of catalyst loading

The amount of solid acid SO42?/TiO2catalyst varied from 3 wt%to 7 wt%to investigate its in fluence on the reaction rate and conversion of PM at 413.15 K and HAc to PM molar ratio of 2.Fig.10 showed that rate of reaction increases with the increase of catalyst loading because it offers more active catalytic sites for the reaction.The rate of reaction was almost the same for catalyst loading of 5%and 7%and a conversion of 65.5%was obtained in both the cases.Hence,an important point could be obtained thatthe catalystconcentration in fluenced the equilibrium time,but not the equilibrium conversion.

3.3.5.Reaction mechanism and Kinetic model

The reaction of synthesizing PMA from PM and HAc used solid acid SO42?/TiO2as catalyst is esterification reaction and the reaction mechanism of the esterification reaction can be described as follow(the solid acid catalyst was referred as Rs-M):

(1)Acetic acid dissociates out protons(H+)on the catalyst surface acid site:

(2)The nucleophilic addition reaction occurs between proton H+and PM:

Fig.10.Effect of catalyst concentration on reaction rate and conversion of PM.

Among above three steps,the reaction rates of steps 1 and 2 are very fast while step 3 is the slowest,so step 3 can be considered as the ratecontrolling step.The rate equation could be written as follows according to Eq.(10),

Whereris the reaction rate,mol·L?1·min?1,k3+andk3?represent the forward and reverse reaction rate constant in Step 3,respectively,L·min?1·mol?1;Cis the concentration of components in the reaction system,mol·L?1.

Through the above 3 reaction steps,its kinetic equation can be deduced,

whereKeqis the total equilibrium constant.SinceCδcan be regarded as constant during the reaction process,supposedk3?Cδ=k,then Eq.(11)can be rewritten as:

wherexis the conversion of PM.

Since Eq.(12)is a nonlinear equation,a fourth-order Runge–Kutta method is used to calculate the conversion of PM under different reaction time.The sum of squared differences of the experimental valuesxand calculation values~xwas regarded as the optimization objective,as shown in Eq.(13).The optimal parameters for the kinetics are estimated by using the Nelder–Mead simplex method,as shown in Tables 7.

Table 7k+and k?at different temperatures

The relationship between rate constantkand temperatureTcan be described by the Arrhenius equation:

whereEais the activation energy,J·mol?1andk0is the pre-exponential factor,L·min?1·mol?1.

Arrhenius plot of lnkagainst 1/T(K)was made(Figs.12 and 13)to getEaandk0.

3.3.6.Validation of kinetic model

In order to verify the reliability of the proposed kinetic model,the calculated data were compared with the experimental data(as shown in Fig.11).All the experimental data were found to be in excellent agreement with calculated data,so the proposed kinetic model was reliable.

Fig.11.Comparison of the calculated data with experimental data.

3.4.Shaping of solid acid catalyst SO4 2?/TiO2

3.4.1.Visual and range analysis

The experimental conditions of catalytic activity test of the shaped solid acid catalyst were as follow:initial molar ratio of HAc to PM 2:1,catalyst concentration 5 wt%,reaction temperature 413.15 K.Results of orthogonal experimental design(L9(34))were displayed in Table 8.Because the conversions of PM in all cases were almost the same,the mechanical strength of catalyst was regarded as optimizing index.The mechanical strength of the catalyst obtained in the 9th experimental run was the highest and its molding formulation was as follows:binder 7 wt%,reinforcing agent 20 wt%,pore forming material 2.5 wt%,lubricant 4 wt.%.

Table 8The experimental analysis of orthogonal test

The important order of in fluences of experimental factors on the optimizing index could be obtained through range analysis.Based on the values ofRj,the important order of factors in fluences on optimizing indexes was obtained:A(binder)＞C(pore forming material)＞B(reinforcing agent)＞D(lubricant).The optimal levels of experimental factors were showed in Table 8.

The optimum formula obtained from range analysis wasA3,B1,C1,andD1.Through compared results obtained by visual and range analysis,it can be concluded that the optimum level of each factor wasA3,B3,C2,andD1.

3.4.2.Variance analysis

The significances of factor in fluences on optimizing index can be con firmed though variance analysis.Table 9 gives the results ofvariance analysis.Based on variance analysis,the important order of factor is binder,pore forming material,reinforcing agent and lubricant,which is consistent with range analysis.

Table 9Variance analysis

3.5.Characterization of shaped solid acid catalyst SO4 2?/TiO2

3.5.1.BET analysis results

Shaped solid acid catalystwas characterized by surface area analyzer and its isothermaladsorption desorption curve was showed in Fig.12.It could be found that after shaped the pore diameter increased to 17.16 nm from 16.40 nm.Correspondingly,the surface area decreased to 30.56 m2·g?1from 48.96 m2·g?1,it was due to the adding of pore forming material.The enlargement of pore diameter is advantageous for diffusion of reactants in catalyst.The isothermal adsorption desorption curve ofshaped catalystis IVtype and there is H3 type ofhysteresis loop atP/P0=0.5–1.0,which indicate that the internal pore canal of shaped catalyst also is mesoporous structure.

Fig.12.Isothermal adsorption desorption curve of shaped catalyst.

3.5.2.Thermostability analysis

TG curves of the shaped catalyst(Fig.13)showed weight loss gradually below 523 K due to the removalofadsorbed water.At 523–773 K,the mass of sample decreased slightly because of the decomposing of free sulfuric acid and the losing of chemical absorbed water on the surface of catalyst.While there was a remarkable weight loss at 873 K owe to the decomposing of SO42?which is boned with titanium dioxide.Thus,thermogravimetry studies reveal that after shaping the thermostability of the catalyst is improved.

Fig.13.Thermostability analysis result of shaped catalyst.

3.5.3.XRD analysis results

The X-ray diffraction patterns of the shaped catalyst were shown in Fig.14.There were six characteristic peaks of anatase type titanium dioxide at 25.3°,37.8°,48.1°,53.9°,55.1°,and 62.7°,which indicated that the titanium dioxide particles in the shaped catalystbelonged to anatase type.Therefore,it can be concluded that after shaping the catalyst still remained anatase type titanium dioxide which is very useful for esterification of PM and HAc.

Fig.14.XRD analysis results of shaped catalyst.

3.6.Performance test of shaped solid acid catalyst

The performance of shaped solid acid catalyst was evaluated by repeated usage in the batch stainless steel reactor.Catalyst cycles were performed under the conditions of 413.15 K,2:1 initial molar ratio of HAc to PMand 5 wt%catalystdosage.For each ofthe repeated reactions,the catalyst was recovered and washed thoroughly with diethyl ether and then dried overnight in air before reused,the experimental results were showed in Fig.15.The catalyst was consecutively reused five times without a slightly loss of its activity,which indicated that the shaped solid acid catalyst exhibited high activity and stability.

Fig.15.Performance for repeated used of shaped solid acid catalyst.

4.Conclusions

The solid acid catalyst SO42?/TiO2was prepared using impregnation technique and orthogonal test was used to explore the in fluences of experimental factors on catalytic activity of solid acid catalyst SO42?/TiO2.The results showed that the optimal synthesis method of solid acid catalyst were as follow:the concentration of sulfuric acid solution was 1 mol·L?1,the mass ratio of sulfuric acid solution and titanium dioxide powder 1:2,drying temperature 393 K,calcination temperature 723 K,and calcination time 3 h.

The kinetic for esterification of HAc with PM in the presence of solid acid catalyst SO42?/TiO2was investigated.The effects of various operating variables on the conversion of PM were studied.A heterogeneous kinetic model was established and the kinetic parameters were determined.The conversion of PM calculated by the proposed kinetic model was found to be in excellent agreement with the experimental results.

The shaping method of impregnation–shaping–impregnation was applied.The optimal molding formulation of catalyst was determined based on the results of orthogonal test.The performance of the shaped solid acid catalyst was evaluated by repeated usage in the batch stainless steel reactor.It was found that the shaped solid acid catalyst exhibited high stability,it was consecutively reused five times without a slightly loss of its activity.

Nomenclature

Cithe concentrationsiin the system,mol·L?1

CP0Mthe initial concentration of PM,mol·L?1

Eathe activation energy,J·mol?1

F(x) the objective function

Fjthe value ofFtest

fjdegree of freedom

ΔHthe reaction enthalpy,J·mol?1

Kthe summation of total optimizing indexes

Keqthe equilibrium constant

Ki,jthe summation of optimizing index

kforward reaction rate constant,L·min?1·mol?1

ki,jthe average value ofKi,j

mthe number of factor

nthe number of data points

othe number of repeat test

Rthe gas constant,8.31 J·mol?1·K?1

Rnthe initial molar ratio of HAc to PM

rthe reaction rate,mol·L?1·min?1

SEthe sum of squares of the errors

Sjthe um of squares of deviations

Tthe reaction temperature,K

tthe reaction time,min

xthe experimental conversion of PM

xthe calculated conversion of PM

yithe value of the optimization index

Subscripts

eq equilibrium state

HAc acetic acid

PM propylene glycol monomethyl ether

PMA propylene glycol monomethyl ether acetate

rthe number of level

Γ intermediate product

δ catalyst

1,2,3 reaction of step 1,2 and 3

+,? forward and reverse reaction

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