Porous polymer microsphere functionalized with benzimidazolium based ionic liqui

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

Keywords:Supported ionic liquids Porous polymer microsphere Copolymerization Esterification Reactive distillation

ABSTRACT To prepare polymer supported ionic liquids(PSILs)as effective catalysts for esterification,the free radical suspension copolymerization of vinylbenzyl chloride(VBC,monomer),styrene(St,monomer)and divinylbenzene(DVB,crosslinker)with the addition of n-heptane(porogen)was carried out for the fabrication of the porous polymer(PVD)microsphere as support,followed by the immobilization of sulfonic acid-functionalized ionic liquids by the successive treatment of benzimidazole(BIm),1,3-propane sultone and sulfuric acid(H2SO4)or trifluoromethanesulfonic acid(CF3SO3H).The effects of the compositions of DVB and n-heptane on the internal structure of the polymer supports were investigated,and it was found that the support with 40 wt%DVB and 60 wt%n-heptane(with relative to the monomer)could endow the final PSILs with the relatively optimal catalytic performance.The preliminary experiment in the batch reactor indicated that PSILs herein exhibited higher catalytic activities than commercial Amberlyst 46 resin for the esterification of propanoic acid(PROAc)with n-propanol(PROOH).Consequently,the optimal PSILs catalyst,PVD-[Bim-SO3H]HSO4,was selected for further study in the batch reactive distillation column because of low cost and its ease of preparation.The yield of propyl ropionate(PROPRO)could reach up to 97.78%at the optimized conditions of PROOH/PROAc molar ratio(2:1)and catalyst dosage(2.0 wt%).The investigation of the reaction kinetic manifested that the calculated results of second order pseudo-homogeneous kinetic model were in good agreement with experimental values.The pre-exponential factor and activation energy were 4.12×107 L·mol-1·min-1 and 60.57 kJ·mol-1,respectively.It is worth noting that the PSILs catalyst could be simply recovered and reused with relatively satisfactory decrease in the catalytic activity,which made it an environmental friendly and promising catalyst in the industrial application.

1.Introduction

Fischer esterification,one of the most important organic and bioorganic synthesis reactions,plays a significant role in pharmaceuticals,coatings and petrochemical industry[1-4].In recent years,the catalyst for esterification has become the hot research spot because of its significant effect on the production efficiency.As well known,the conventional catalysts for esterification mainly include common inorganic acids(HCl,H2SO4and H3PO4)[5,6]and solid acid(resins and heteropoly acids)[7-11].However,inorganic liquid acids always suffer serious equipment corrosion and environment pollution while solid acids suffer high mass transfer resistance and low active-site density.Therefore,the need of development of environment-friendly catalyst with high catalytic performance is urgent.

As an emerging material,ionic liquids(ILs)have attracted tremendous attentions as green and efficient catalysts in esterification because of their high catalytic activity[12].However,it is difficult to recovery ILs from the liquid mixture of the esterification process,resulting to the limited applications in practical production[13,14].Consequently,researchers began to study the immobilization of ILs onto support materials,such as silica gel[15,16],magnetic nanoparticles[17,18]and polymers[19-21].Among them,polymer supports are relatively promising due to the low cost,large surface area and good compatibility with organic reaction systems.The relevant polymer supported ionic liquids(PSILs)therefore combine the advantages of polymer support with ILs,such as easy separation,high mechanical stability and good catalytic activity.For example,Doherty et al.[22]prepared a number of Cu(II)-bis(oxazoline)-based polymer immobilized ionic liquid phase catalysts(PIILP)with high activities for asymmetric Diels-Alder and Mukaiyama-aldol reactions.As recent literature reported,the PSILs had been applied to a wide range of reactions and technology with excellent performance,e.g.,cycloaddition,H2generation,CO2capture and three-dimensional(3D)printing[23-26].Usually,the PSILs catalysts consist of two parts:(1)Polymer support and(2)ILs active groups.For the polymer support,it was found that most of the existing polymer supports for ILs immobilization are based on polystyrene.Kim and Chi[27]developed a PSILs catalyst using the commercial Merrifield resin(polystyrene-based)as support followed by the immobilization of imidazolium(Im)and such PSILs catalyst could catalyze the nucleophilic substitution reactions efficiently.In addition to commercial resin,some self-synthesized polystyrene-based resin had been used for the fabrication of PSILs.Xu et al.[28]prepared the PSILs catalyst by immobilizing Im-ILs on self-synthesized chloromethylated polystyrene resin and examined its catalytic activity for esterification.The results show that the catalyst possesses high catalytic activity and good selectivity.As heterogeneous catalyst,the internal structure dominates diffusion and mass transfer efficiency,which in turn influence the distribution of acid sites and accessibility of the reactants to the acid sites and hence their overall reactivity[29].Wang et al.[30]developed an efficient polymer supported ionic liquid catalyst to catalyze cyclization of sorbitol to isosorbide,which was synthesized through the suspension polymerization of 4-vinylbenzyl chloride and divinylbenzene,and followed by quaternization reaction.As a result,they found that the porous polymers had high specific surface area and large number of active sites,compared with those of non-porous.Therefore,the catalytic efficiency of PSILs catalyst can be improved by tuning their porosity and internal structure.However,few attentions were paid to the investigation for the optimization of the support structures of PSILs[31-33].For the ILs active groups,Im-type PSILs are the most commonly reported catalyst with good performance for esterification[34-36].Benzimidazolium(BIm),which possesses the similar structure with Im,could also act as cation group for ILs.Theoretically,its improved catalytic properties and stability could be expected because of the presence of the additional benzene ring[37].In previous literature,the BIm-type ILs have been proved to have high catalytic performance for many reactions,demonstrating their great potential in catalytic technologies[38-40].Abbasi[41]applied 1,3-disulfonic acid benzimidazolium based ionic liquid as recyclable catalyst for highly functionalized tetrahydropyridine.High product yield was obtained under mild reaction conditions and shorter reaction time.Kotadia et al.[42]prepared a new Supported Ionic Liquid Catalyst(SILC)by immobilizing BIm based IL on silica gel,and it proved to be an efficient catalyst for solvent less synthesis of 1-amidoalkyl naphthols.However,the studies on BImtype PSILs catalysts are still insufficient.

In this paper,we prepared the PVD support with different internal structures by investigating the effects of the composition of DVB and n-heptane.Two BIm-type PSILs catalysts were synthesized by functionalizing the relatively optimal PVD support with BIm based ionic liquids.The catalytic performance of the PSILs catalysts for esterification was investigated in both batch reactor and batch reactive distillation column.The reaction kinetics for the PROAc-PROOH esterification on PVD-[Bim-SO3H]HSO4was also studied for the further industrialized application.

2.Experimental

2.1.Materials

Vinylbenzyl chloride(VBC,97%)was supplied by Sigma-Aldrich Chemical Co.,USA.Styrene(St,99%),divinylbenzene(DVB,55%),2,2-azo-bis-isobutyronitrile(AIBN,98%),polyvinyl alcohol(PVA),imidazole(Im,99%),benzimidazole(BIm,98%),1,3-propanesulfonate(99%)and trifluoromethanesulfonic acid(98%)were purchased from Aladdin Scientific Co.,Ltd.Shanghai.Sulfuric acid(H2SO4,98%)was provided by Shanghai Lingfeng Chemical Reagent Co.,Ltd.All solvents were of analytic grades and used without any purification.

2.2.Catalyst preparation

2.2.1.Preparation of porous polymer support(PVD)

The PVD support was synthesized by free radical suspension polymerization[43].As shown in Fig.1,4.5 g NaCl,0.75 g AIBN(initiator)and 0.15 g PVA(dispersant)and 150 ml deionized water were added into a three necked round-bottomed flask.The mixture was vigorously stirred for 2 h at 358.15 K until the solution became transparent.Then,the mixture of monomers(mass ratio of VBC:St=2:1),DVB and n-heptane were added gradually into the transparent solution with regular mechanical stirring to create porous polymer microspheres.The weight percentages of DVB and n-heptane with respect to the monomer were varied to optimize the internal structure of the support,respectively.The polymerization reaction was carried out at 348.15 K for 8 h under nitrogen atmosphere.The product was collected by filtration,and sequentially washed with hot water,methanol and ethanol.The Soxhlet extraction was applied to remove the residual reactants and porogen.Finally,the product was dried under vacuum at 333.15 K overnight to obtain the porous PVD support.

Fig.1.Synthesis of PVD support.

2.2.2.Preparation of polymer supported ionic liquid catalyst

Two different PSILs catalysts,including PVD-[BIm-SO3H]HSO4and PVD-[BIm-SO3H]CF3SO3were prepared based on PVD support using the similar procedure,as shown in Fig.2.Given amounts of PVD and BIm(with the same mole equivalent of the CH2Cl groups of PVD)were firstly added into a 250 ml three necked flask with 100 ml anhydrous N,N-dimethylformamide(DMF).The mixture system was kept at 363.15 K for 24 h under regular stirring,the intermediate product(PVD-[BIm])was obtained after filtration and successively washed with DMF(25 ml),methanol(25 ml)and ethanol(25 ml).Afterwards,PVD-[BIm]and 1,3-propane sultone(with the same mole equivalent of BIm)were added into 100 ml anhydrous toluene in a flask with regular stirring at 363.15 K for 24 h to form PVD-[BIm-SO3],which was then immersed in a flask containing 100 ml dichloromethane and H2SO4/CF3SO3H(with the same mole equivalent of BIm)under mechanical stirring at room temperature for 24 h,followed by filtration and sequentially washed with dichloromethane(25 ml),methanol(25 ml)and ethanol(25 ml)to get the final PVD-[BIm-SO3H]HSO4and PVD-[BIm-SO3H]CF3SO3.

Fig.2.Preparation of PVD-[BIm-SO3H]HSO4and PVD-[BIm-SO3H]CF3SO3catalyst.

2.3.Catalyst characterization

FT-IR spectra were recorded on a Nicolet iS50 spectrometer(Thermo Fisher Scientific).The elemental analysis(EA)was accomplished on the elemental analyzer(Vario EL)(seeing the results in Table S1).X-ray photoelectron spectroscopy(XPS)was carried out on a Thermo Fisher Scientific ESCALAB 250Xi spectrometer with an Al X-ray excitation source(Kα=1487.6 eV).The BET surface area and pore size distribution were measured using N2adsorption-desorption porosimetry(Micromeritics automatic analyzer ASAP2020).Transmission electron microscopy(TEM)was conducted on an electron microscope(FEI,TECNAI G2 F20)with an acceleration of 80 kV.Scanning electron microscopy(SEM)(Hitachi S-4800)was performed to evaluate the morphology and surface structure of catalysts.By thermal gravimetric analysis(TGA)(Netzsch STA449C),the thermal stability of PSILs catalysts was evaluated.Prior to these measurements,all samples were dried for 10 h under vacuum.The compositions of the reactants and products in the esterification application were determined by gas chromatography(GC2014,Shimadzu Corporation,Japan)with a FID and Rtx-1701 capillary column(0.25 mm×30 m)with 0.25 μm film thickness.

2.4.Catalytic procedures of esterification

The esterification experiments were performed in both batch reactor and batch reactive distillation column.The batch reactive distillation devices consisted of a three-necked flask and a glass column section(ϕ22 mm×1750 mm)with θ stainless steel ring packing(2×2 mm),as shown in Fig.S1.The heating mantle and phase segregator are used to provide heat duty and realize the separation of water.The asbestos cloth was applied to reduce heat loss.At the top of the column,the vapor phase is condensed by water and only the oil phase is refluxed.

In the operation of the reactive distillation column,given amounts of PROAc,PROOH and catalyst were introduced into the bottom flask.Then,the heating and stirring apparatus were turned on and the total reflux state was lasted for 0.5 h.Subsequently,aqueous phase was removed from the bottom of the phase segregator and samples were analyzed at regular time intervals by GC and acid-base neutralization titration until the reaction reached equilibrium.In reusability study,the catalyst was subsequently collected by filtration,washed with ethanol and dried at 333.15 K for 6 h under vacuum.

2.5.Kinetic experiments

The kinetic experiments were performed in a 250 ml four necked round-bottomed flask equipped with a mechanical stirrer and condenser.The reaction temperature was controlled by a thermostatic bath.The reaction condition of kinetic experiments consisted of the following:reaction temperature(333.15,343.15,353.15 and 363.15 K),the molar ratio of PROOH to PROAc(1:1,1.2:1,1.4:1),and the dosage of PSILs catalyst(1.0 wt%,1.5 wt%and 2.0 wt%).

At the beginning of the experiment,PROOH and catalyst were added into the flask and heated to the required temperature.Then,pre-heated PROAc with the same temperature was added and the mechanical stirrer was turned on.This moment was considered as the reaction starting point.The sample was periodically withdrawn and cooled hastily in a refrigerator.The conversion of PROAc was calculated by the initial and final concentrations of PROAc,which was analyzed by acid-base neutralization titration.When the conversion of PROAc was constant,it indicated that the state of chemical equilibrium was reached.

3.Results and Discussion

3.1.The effects of DVB and n-heptane on the performance of PVD support

Seeing that the internal structure is an important factor for the catalytic efficiency of PSILs catalyst,the effects of cross-linker and porogen were investigated by changing the amount of DVB and n-heptane in the process of free radical suspension polymerization to obtain the PVD support with optimal internal structure.The structural parameters such as specific surface area(SBET),pore volume(VP)and average pore diameter(DP)were measured by a N2adsorption-desorption instrument.As can be seen from Fig.3a,SBETand VPof PVD support increase with the increase of DVB concentration while DPpresents the decrease trend[44].According to the theory of polymerization process,many nuclei form as the smallest composition units of polymer beads and their surface area determines the SBETof the polymer support.Therefore,the above-mentioned phenomenon could be interpreted as the fact that the increasing amount of DVB could enhance the intermolecular crosslinking degree of nuclei,which leads to the PVD support with larger SBET.In other words,high DVB concentration could also increase the number of micropore and decrease the average pore diameter.The similar results were obtained by Moustafa et al.[45].With n-heptane content increasing from 20 wt%to 100 wt%,SBETand VPof the samples increase obviously while DPdecreases first from 19.31 to 11.79 nm and then increases to 14.17 nm as shown in Fig.3c.The similar results were also obtained by Liu[46,47].The phenomenon mentioned above was account for that the increasing porogen concentration induced crosslinked polymer beads with more loose structure which would form more pores in the polymer network.Then,in the case where large amount of pores is generating,some micropores are gradually connected which results in the increase of average pore diameter during the further increase of n-heptane amount from 60 wt%.The results in Fig.3 reveal that the internal structure of PVD support strongly depends on the concentration of DVB and n-heptane.Therefore,the study provided a simple methodology to prepare the polymer support with tunable internal structure.

In order to evaluate the performance of the PVD supports with different internal structures,the corresponding PVD-[BIm-SO3H]HSO4catalysts were synthesized using the PVD as support and the catalytic performance was evaluated by catalyzing the esterification of PROAc with PROOH.As presented in Fig.3(b)and(d),the reaction rate varied correspondingly with the PSILs catalysts possessing different internal structures,manifesting that large surface area and pore volume of PSILs catalyst have positive effects on the catalytic efficiency.The phenomenon is attributed to the significant influence of the internal structure of PSILs catalyst on the processes of thermal conductivity,mass transfer and diffusion[29].However,with the amount of cross-linker continuously increasing from 40 wt%,the positive influence on the reaction weakened.The phenomenon can be explained by the finding that the polymer with excessive cross-linking degree would bring too dense polymer network and reduce reaction contact area.Considering the above factors,the preparation scheme was finally established with the amount of DVB and n-heptane accounting for 40 wt%and 60 wt%(by mass of monomer mixtures),respectively.

3.2.Characterization of catalysts

3.2.1.Fourier transform infrared spectroscopy(FT-IR)analysis

Fig.3.(a)and(b)The effects of cross linker amount on structure parameters and catalytic efficiency;(c)and(d)the effects of porogen amount on structure parameters and catalytic efficiency.

The FT-IR spectra of the support and PSILs catalysts are shown in Fig.4.For PVD,the peaks at 1603 and 1491 cm-1verify the existence of CC skeleton vibrations of aromatic ring in the support[17].The peaks at 674 and 1265 cm-1are attributed to the bending frequency of functional group CH2Cl in the PVD support[20].For PSILs,the aforementioned peaks corresponding to aromatic ring were also observed,however,the peaks at 674 and 1265 cm-1disappeared in the synthesized PSILs catalysts with the appearance of new peaks at 745 and 1561 cm-1,which is responsible for CN and CN stretching vibration of benzimidazole ring[16].Moreover,two significant peaks near 1031 and 1151 cm-1,are originated from the asymmetric and symmetric stretching frequencies of SO[17],suggesting the existence of SO3H groups on PSILs catalyst.The peak at 1223 cm-1is associated with the presence of CF bond which is the characteristic peak of functional group for PVD-[BIm-SO3H]CF3SO3[19].The FT-IR analysis reveals that the BIm based ILs were successfully coated on PVD support by covalent bonding.

Fig.4.FT-IR spectra of PVD support,PSILs catalyst and reused PVD-[BIm-SO3H]HSO4catalyst.

3.2.2.X-ray photoelectron spectroscopy analysis

The surface compositions and chemical states of PVD support and the PSILs catalysts were analyzed by XPS and the results were depicted in Fig.5.The peaks for elements C,O and Cl are found in the survey spectrum(Fig.5a)confirming the structure of PVD support.The peaks around 400.0 and 169.0 eV corresponding to N and S are observed in Fig.5b verifying the presence of benzimidazolium groups and SO3H groups on synthesized PSILs catalysts.The characteristic peak of F at 688.1 eV(Fig.5b)further certifies the successful introduction of trifluoromethanesulfonic anion[48].Fig.5(a)and(b)shows that PSILs catalysts presented a significant decrease at the peak of 200.1 eV in comparison with the support,demonstrating that the Cl groups are substituted by N-containing species.In the N(1s)XPS spectra(Fig.5c),the peaks near 401.2 and 402.1 eV are assigned to the quaternary ammonium atoms of benzimidazole ring in PSILs catalysts[17].The S(2p)XPS spectra(Fig.5d)show a main peak belonging to the S(2p3/2),which confirms the SO3H group on catalysts[49].The XPS analysis also suggests the PVD support was well functionalized with BIm based ILs.

3.2.3.Nitrogen adsorption-desorption analysis

Fig.6 depicts the N2adsorption-desorption isotherms and the Barrett-Joyner-Halenda(BJH)pore size distribution curves for the PVD support and PSILs catalysts.All samples present type IV isotherm with apparent H1 hysteresis loops at the relative pressure P/P0ranging from 0.8 to 0.9,which indicates the mesoporous structure of the PSILs catalysts.The structural parameters of the PVD and PSILs are summarized in Table 1.Comparing with the structure parameters of blank PVD support,SBETand VPof PSILs catalysts decreased,while DPhad a certain increase.The phenomenon could be attributed to the reason that some small pore channels of support are occupied with the immobilized ILs.

Fig.5.(a)XPS spectra of PVD;(b)XPS spectra of PSILs catalysts;(c)N 1s of PSILs catalysts;(d)S 2p of PSILs catalysts.

Fig.6.(a)N2adsorption-desorption isotherms of samples;(b)pore size distributions of samples.

Table 1 Structural properties of the PVD support and PSILs catalyst

3.2.4.SEM analysis

The morphology and surface structure of PVD support and PSILs catalysts were characterized by SEM.As exhibited in Fig.7(a-1),the PVD support presents microsphere morphology with uniform size.The PSILs catalysts still retain an intact spherical structure and the size had no obvious change under the same magnification,as compared with the support(see Fig.7(b-1)and(c-1)).By increasing the magnification,it can be found that the surface of PSILs catalysts becomes much plainer than that of PVD,as shown in Fig.7(a-2),(b-2)and(c-2).In order to observe the internal morphology,the samples were fractured in liquid nitrogen to get a cross section.It's clear to observe the high crosslinking degree in the core structure and large amount of mesopores in Fig.7(a-3),(b-3)and(c-3),which can provide good access of reactants towards catalytic sites for catalyzing process.Moreover,the results are consistent with the results of N2adsorption-desorption.

Fig.7.(a-1),(a-2)and(a-3)The SEM micrographs of the surface and cross section of PVD support,(b-1),(b-2)and(b-3)PVD-[BIm-SO3H]HSO4and(c-1),(c-2)and(c-3)PVD-[BIm-SO3H]CF3SO3.

3.2.5.TEM analysis

The sliced specimens of PVD support and two prepared PSILs catalysts were observed by TEM to further analyze the porosity,and the TEM images with scale bar 100 nm and 25 nm are displayed in Fig.8.The images show the pore structure inside the PVD support and PSILs catalysts are dominated by mesopores.The results are consistent with the results of SEM observations and BET analysis.

3.2.6.Thermogravimetric analysis

The thermal stability curves obtained by TGA are exhibited in Fig.9.Only one drastic weight loss stage(350-450°C)was observed from the TG curve of PVD(Fig.9a),which was attributed to the pyrolysis of the backbone chain of polymer.The synthesized PSILs presented two step degradation patterns as shown in Fig.9(b)and(c),which refer to the decomposition of grafted BIm-ILs(220-350 °C)and the backbone chain of polymer(400-450°C)[30],respectively.The reason for the slight weight loss that occurred near 100°C was due to the evaporation of physical adsorbed moisture.By contrast,the commercial ionexchange resin Amberlyst 46™(A46)decomposed remarkably from 130°C,as exhibited in Fig.8d.It can be concluded that the PSILs catalysts possess high thermal stability.

3.3.Catalytic activity of PSILs catalysts

Fig.8.The TEM images of PVD support(a),PVD-[BIm-SO3H]HSO4(b)and PVD-[BIm-SO3H]CF3SO3(c).

Fig.9.(a)TG of PVD support,(b)PVD-[BIm-SO3H]HSO4,(c)PVD-[BIm-SO3H]CF3SO3and(d)A46.

A set of experiments were performed in a batch reactor under the same reaction conditions to examine the catalytic activities of various catalysts with the esterification of PROAc with PROOH as model reaction.The results presented in Table 2 indicate that the reaction rate and the yield of PROPRO were enhanced significantly with the presence of PSILs catalyst compared with that of the blank experiment.It was found that the catalytic activity followed the order of PVD-[BIm-SO3H]HSO4≈PVD-[BIm-SO3H]CF3SO3＞A46＞PVD-[Im-SO3H]HSO4＞PVD,which demonstrates that the PSILs catalysts are more active than A46 ion-exchange resin for esterification.It also proved that the BIm-type PSILs actually possess better catalytic performance than Im-type PSILs,which could be attributed to the additional benzene ring of its organic structure[16,30].Then,PVD-[BIm-SO3H]HSO4was selected for subsequent study because of the less toxicity and lower cost of raw material.

Table 2 Catalytic activities of various catalysts for the esterification of PROAc and PROOH①

3.4.Batch reactive distillation experiments

Reactive distillation(RD)as a coupling process of chemical reaction and separation can remove product timely and facilitate the chemical equilibrium to move forward,which possesses the advantages of high conversion rate,less energy consumption and low investment[50].Yang et al.[51]investigated the synthesis process of ethylene glycol diacetate by batch reactive distillation using silica supported ILs as catalyst and the product was obtained in high yield.In this section,the effects of reactants molar ratio and catalyst dosage on the synthesis of PROPRO in a batch reactive distillation column were investigated.

3.4.1.Effect of the molar ratio of PROOH to PROAc

As shown in Fig.10,with the molar ratio of PROOH to PROAc increasing from 1.0 to 2.0,the yield of PROPRO increases from 88.69%to 97.78%.Then the continuing increase of molar ratio has a very slight effect on the yield of PROPRO.In contrast with the batch reactor(Table 2),the batch reactive distillation process notably enhanced the equilibrium yield at the same reaction time,which was attributed to the prompt removal of the product water from reaction system to break the limitation of chemical equilibrium.However,the increase of reactant molar ratio would lead to an increase of energy consumption in subsequent separation.Thus,the suitable molar ratio of PROOH to PROAc 2.0 was determined.

Fig.10.The effect of reactants molar ratio on the yield of PROPRO.Reaction conditions:catalyst,PVD-[BIm-SO3H]HSO4;catalyst dosage of 2.0 wt%.

3.4.2.Effect of catalyst dosage

The results depicted in Fig.11 demonstrate that the catalyst dosage had a negligible effect on the yield of PROPRO.Moreover,the yield all reached above 97%under the conditions of different catalyst dosages and the same reactant molar ratio(2:1).However,the equilibrium time deduced obviously with the catalyst dosage increased from 1.0 wt%to 2.0 wt%,as shown in Fig.11.Then the impact weakened gradually with the further increase of catalyst dosage.Based on the economics of the process,the preferred catalyst dosage was 2.0 wt%.

Fig.11.The effect of catalyst dosage on the yield of PROPRO.Reaction conditions:catalyst,PVD-[BIm-SO3H]HSO4;PROOH:PROAc(molar ratio)of 2:1.

3.4.3.Applications of PSILs in various esterification systems

To broaden the scope of application systems,various esterification reactions were catalyzed by PVD-[BIm-SO3H]HSO4under the same condition in the batch reactive distillation column.The results summarized in Table 3 show that the yields of various esters were all higher than 95%,even up to 99.19%.It was sufficient to demonstrate that the PSILs catalyst is a superior heterogeneous catalyst applicable to a variety of esterification reactions.By comparing the catalytic activities of PSILs catalysts with other supported ILs catalysts,it can be found that the BImtype PSILs we synthesized show better catalytic performance for esterification.

3.4.4.Reusability study

The yield of PROPRO decreased from 97.78%to 78.95%after reusing for six times,as presented in Fig.12.The relatively satisfactory results indicate PSILs catalyst possesses high stability of catalytic activity.Inorder to further investigate the change on the structure of reused catalyst,it was characterized by SEM,FT-IR and elemental analysis.The SEM image of the catalyst reused 6 times(see Fig.S2 in supporting information)presents almost no difference with the fresh catalyst in the morphology and particle size.The fact that the synthesized PSILs catalyst wasn't crushed in recycling experiment demonstrated the high mechanical strength.The FTIR spectra in Fig.4 show that all the characteristic peaks for the fresh PVD-[BIm-SO3H]HSO4catalyst still existed in that for the reused catalyst,suggesting that the bonding between ILs and PVD support is stable.The new peaks at 960 and 1730 cm-1should be contributed to the CO and CO stretching vibrations of the n-propyl propionate adhering on catalyst surface.The attachment of n-propyl propionate covered its original acid sites of catalyst,which could be identified as one reason for the decline of catalytic activity in recycling experiments.In addition,the element analysis results(Table S1)show that the S content of reused PVD-[BIm-SO3H]HSO4catalyst decreased from 5.47 wt%to 4.54 wt%after reusing for six times.The decrease of S content is another reason for the decrement of catalytic activity,which was attributed to the partial loss of the anion(HSO4-).In other words,part of the anion(HSO4-)was lost in the form of H2SO4during recycling process[29,56].

Table 3 Catalytic activities of various SILs catalysts for esterifications

Fig.12.Recycling of PVD-[BIm-SO3H]HSO4catalyst for the synthesis of PROPRO.Reaction conditions:PROOH:PROAc(molar ratio)of 2:1,catalyst dosage of 2.0 wt%.

3.5.Mechanism of esterification

As the previous literature reported,heterogeneous esterification follows the Eley-Rideal mechanism[28,57].The esterification process has two key steps,protonation of acid and nucleophilic attack of the pronated acid by the alcohol.The catalytic mechanism of the prepared BIm-type PSILs catalyst for the esterification process illustrated in Fig.13 is consistent with aforementioned mechanism.From Fig.13,acid(ROOH)first accepts a proton from the catalyst PVD-[BIm-SO3H]HSO4and carbocation I is formed.In the second step,the O atom of alcohol(R′OH)attacks the carbocation(I)to give the tetrahedral intermediate II.Then,the H+of intermediate(II)transfers and loses one molecule water to get protonated ester III.Finally,with the breaking of the hydrogen-oxygen bond IV,final product carboxylic acid ester V is obtained and the catalyst is also recovered.Moreover,the hydrophobic property of the PVD support of PSILs could promote the chemical equilibrium to move forward by efficiently keeping the by-product(water)far away from the active sites.

3.6.Kinetic experiments

According to the excellent performances described above,the PSILs catalyst could be considered as a promising catalyst in industrial applications.In order to design a reactive distillation column for the production of PROPRO with PVD-[BIm-SO3H]HSO4as catalyst,the reaction kinetic for the esterification of PROAc with PROOH was further studied.

3.6.1.Elimination of internal diffusion and external diffusion

To determine suitable catalyst particle size and stirring speed,the same experiments were performed under different particle sizes(0.18,0.25 and 0.38 mm)and agitation speeds(150,350 and 450 r·min-1).The corresponding results are shown in Fig.14.It is noticeable that the particle size almost has no influence on the conversion of PROAc,which means that the internal diffusion could be neglected in the particle size range of 0.18-0.38 mm.The reaction rate had an obvious increase with the stirring speed increased from 150 to 350 r·min-1.The conversion of PROAc was almost constant at the same reaction time with the stirring speed increased from 350 to 450 r·min-1,which indicates the effect of external diffusion could be ignored at stirring speed higher than 350 r·min-1.Consequently,all further experiments were conducted with the catalyst particle size of 0.25-0.38 mm and the stirring speed of 350 rpm.

3.6.2.Effect of catalyst dosage

A series of experiments with different catalyst dosages(1.0 wt%,1.5 wt%and 2.0 wt%)were studied in this section.The reaction temperature was 353.15 K and initial molar ratio of PROOH and PROAc was 1.2.As shown in Fig.15a,with the catalyst dosage increasing from 1.0 wt%to 1.5 wt%(by weight of reactant mixture),the reaction rate was accelerated.Nevertheless,by comparing the two sets of experiments with catalyst dosages of 1.5 wt%and 2.0 wt%,it could be found that the reaction rate was nearly identical under the same reaction time.The results demonstrated that the optimum catalyst dosage was 1.5 wt%.

Fig.13.Mechanism of esterification catalyzed over PVD-[BIm-SO3H]HSO4catalyst.

3.6.3.Effect of the initial molar ratio

The similar experiments were conducted with reactant molar ratios of 1.0:1,1.2:1 and 1.4:1(PROOH/PROAc)at 353.15 K and the dosage of catalyst 1.5 wt%to determine the optimal condition.The influence of reactant molar ratio on conversion of PROAc was displayed in Fig.15b.It's easy to find that the conversion of PROAc was conspicuously enhanced in the primary stage of reaction with the initial reactant molar ratio varying from 1.0:1 to 1.2:1.However,with the molar ratio increasing from 1.2:1 to 1.4:1,the influence of reactant molar ratio on conversion of PROAc weakened.Given the above results,the reactant molar ratio of 1.2:1 was selected for the optimum condition for the esterification of PROAc with PROOH.

3.6.4.Effect of the reaction temperature

The same experiments were conducted at the reaction temperatures of 333.15,343.15,353.15 and 363.15 K with catalyst dosage of 1.5 wt%and reactant molar ratio of 1.2:1 to investigate the effect of reaction temperature.The conversion of PROAc significantly increased with the increase of reaction temperature at the same reaction time,as shown in Fig.15c.It suggests that high temperature could obviously facilitate the forward reaction.Moreover,Fig.15c also depicts the comparison between the experimental conversion of PROAc with model values and the results show that model predictions were consistent with the experimental data.

3.7.Kinetic model measurement

3.7.1.Chemical equilibrium study

The reaction of PROAc to PROPRO can be expressed by the following equation:

As shown in Eq.(2),the equilibrium constant(Keq)of this reaction was obtained from equilibrium concentration(CPROAc,CPROOH,CPROPRO,CH2O)which can be deduced by equilibrium conversion rate(XPROAce)and reactant molar ratio(Rm).

Fig.14.(a)The effect of particle size and(b)agitation speed.Reaction conditions:catalyst,PVD-[BIm-SO3H]HSO4;catalyst dosage of 1.5 wt%;reaction temperature of(353.15±0.5)K;PROOH:PROAc(molar ratio)=1.2:1.

Fig.15.(a)The effect of catalyst dosage,(b)reactant molar ratio and(c)reaction temperature on the conversion of PROAc.

According to the van't Hoff isobaric equation(Eq.(3)),Keqwas associated with the standard enthalpy of reaction(kJ ⋅mol-1).

Due to the narrow temperature range,could be assumed as constant and Eq.(4)could be obtained.

By calculating Keqof different temperatures on the basis of Eq.(2)and making the relationship plot of lnKeqand 1/T(Fig.16a),the enthalpy of reaction could be estimated.As a result,the valuewas 4.61 kJ·mol-1,indicating the esterification of PROAc with PROOH was a slightly endothermic reaction.

3.7.2.Kinetic modeling

Considering that the esterification of PROAc with PROOH could be regarded as a second-order reaction,the reaction rate rPROAccould be written as the following equation[58].

After replacing the molar concentration of each component with PROAC conversion XPROAcand reactant molar ratio Rm,as Eq.(5)can be written as:

According to the experimental data,kinetic parameters k+and kwere correlated by the integral form of Eq.(6).The Arrhenius equation expounded the relationship between reaction rate constant and reaction temperature.

where Aiand Ea,irefer to the pre-exponential factor and activation energy of this reaction,respectively.Based on Eq.(8),the linear relationship plot of lnkiand 1/T was presented in Fig.16b.Hence,the corresponding values of Aiand Ea,iwere estimated by linear fitting and listed in Table 4.

4.Conclusions

Two PSILs catalysts were prepared based on the presynthesized copolymer support with adjustable internal structure.They exhibited higher catalytic activities than the commercial A46 resin with the esterification of PROAc and PROOH as model reaction under the same condition.Then,the effects of catalyst dosage and reactants molar ratio on the yield of PROPRO production were investigated in the batch reactive distillation process:under the optimal conditions,PROPRO yield could reach 97.78% within 4 h.The catalytic activities were investigated via a series of esterification reactions as well.PSILs catalysts could be easily recovered by simple washing-drying and reused for six times with a slight decrease in yield,which was caused by the residual ester covering on the catalyst surface and the loss ofIn addition,the reaction kinetics parameters of the synthesis of PROPRO could be fitly described in the second order pseudo-homogeneous kinetic model,which could provide the basic data for the design of reactive distillation column in continuous application.The PSILs catalyst prepared herein is an environmental friendly and promising catalyst in the industrial application.

Fig.16.(a)Temperature dependence of the chemical equilibrium constants and(b)Arrhenius plot for forward reaction rates and backward reaction rates.

Table 4 Kinetic parameters for the kinetic model

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2019.01.039.