Carboxylic acid functionalized imidazolium-based ionic liquids efficient catalysts

Reac Kinet Mech Cat

DOI10.1007/s11144-011-0399-8

Carboxylic acid functionalized imidazolium-based ionic liquids:ef?cient catalysts for cycloaddition of CO2

and epoxides

Lina Han?Soo-Jin Choi?Moon-Seok Park?

Seon-Myong Lee?Yu-Jin Kim?Moon-Il Kim?

Binyuan Liu?Dae-Won Park

Received:10July2011/Accepted:31October2011

óAkade′miai Kiado′,Budapest,Hungary2011

Abstract In this study,a series of imidazolium-based ionic liquids(ILs)having carboxylic acid moieties were synthesized and used as new homogeneous catalysts to synthesize cyclic carbonates from CO2and epoxides.Even in the absence of any co-catalyst and organic solvent,carboxylic-acid-functionalized ILs showed better catalytic activity in the coupling reaction of CO2and styrene oxide for the pro-duction of styrene carbonate than did hydroxyl-functionalized ILs and conventional ILs without any functional moieties.A detailed investigation was carried out on a variety of factors that affected the reactivity,such as the alkyl chain length and the molecular composition of IL molecules including the halide ions.The effect of various reaction parameters such as reaction time,temperature,CO2pressure and catalyst amount was also investigated in detail.The mechanism underlying the enhanced rate of the cycloaddition reaction in the presence of carboxylic-acid-functionalized ILs was proposed.

Keywords CatalystáIonic liquidsáCO2áCyclic carbonate

Introduction

In recent decades,the increase in CO2levels has become a burning issue because CO2is the primary greenhouse gas responsible for climate change[1].On the other hand,CO2is a renewable source,which is abundant,nontoxic and inexpensive. L.HanáS.-J.ChoiáM.-S.ParkáS.-M.LeeáY.-J.KimáM.-I.KimáD.-W.Park(&)

Division of Chemical and Biomolecular Engineering,Pusan National University,Jangjeon-dong, Gumjung-gu,Busan609-735,Republic of Korea

e-mail:dwpark@pusan.ac.kr

B.Liu

School of Chemical Engineering,Institute of Polymer Science&Engineering,Hebei University

of Technology,Tianjin300130,China

L.Han et al. Since CO2is an important C1building block,chemical conversion of CO2into useful organic compounds has attracted considerable attention[2–4].One of the most attractive synthetic strategies for utilizing CO2is the coupling reaction between an epoxide and CO2to afford the?ve-member cyclic carbonate that has widespread applications as an aprotic solvent,a precursor for polycarbonate materials,a valuable intermediate in organic synthesis,and an electrolytes in lithium–ion batteries[5–8].For such coupling reactions,many types of catalysts such as alkali metal salts[9–11],metal oxides[12–15],transition metal complexes [16–19],Schiff bases[20,21],ion-exchange resins[22],functional polymers[23–25],and ionic liquids(ILs)have been extensively developed so far.Among these, ILs having quaternary ammonium,phosphonium and imidazolium salts have received the maximum attention owing to their excellent catalytic reactivity and selectivity[26–29].Moreover,ILs possess many outstanding advantages such as environmental friendliness,non-volatility,low melting temperature,and ease of separation,which make them suitable for various applications in the?eld of catalysis.Although signi?cant advances have been made toward the development of IL catalysts for the cycloaddition of CO2with epoxides,ILs have limited application as catalysts because of their low catalytic activity,high water or air sensitivity[30–34],the need for employing a co-solvent,which may be toxic[13, 35,36],and harsh reaction conditions for the catalysis.Therefore,the development of ef?cient,stable,and economical IL-based catalytic systems that facilitate the production of cyclic carbonates under mild conditions is highly desired.

Direct introduction of a certain amount of co-catalysts into the traditional IL catalytic reaction system is a facile approach to effectively enhance the yield of cyclic carbonates.A typical example is as follows:the combination of a Lewis acids and an IL affords a versatile and?exible catalytic system that allows a remarkable enhancement in the catalytic action in the cycloaddition of CO2with an epoxide [37,38].However,owing to the water sensitivity of some Lewis acids such as zinc halides,it is necessary to carry out the catalytic reactions under anhydrous conditions,because of which the synthesis operation becomes complex and the production cost increases.A promising alternative method that can remarkably improve the catalytic performance of cycloaddition reactions involves the inclusion of a suitable amount of a hydroxyl-group-containing solvent into the IL-based catalytic system.For example,the activity of PPh3BuI for the cycloaddition of CO2 to propylene oxide increased by4–5times when the catalyst was used in the presence of water compared to the absence of water[38].In a recent study,we found that the introduction of small amount of water and ethanol into the imidazolium-based IL catalytic system greatly enhanced the activity and selectivity in the cycloaddition reaction between CO2and alkyl glycidyl ether(AGE)[23]. In addition,it has been demonstrated that the incorporation of phenol and urea into IL catalytic systems are favorable for the ring-opening reaction of epoxides to afford cyclic carbonates[39,40].

Another promising alternative that can effectively enhance the catalytic action toward the chemical?xation of CO2to epoxide is functionalization of the alkyl chains of ILs with ether or alcohol moieties[41,42].For instance,Sun et al.[43] synthesized the hydroxyl group-functionalized ILs(HEIMB,HETPPB,HETBAB)

Ef?cient catalysts for cycloaddition of CO2and epoxides

and utilized them to catalyze the cycloaddition reaction of CO2with epoxides;these ILs exhibited higher catalytic activity than did the corresponding ILs without a hydroxyl group.Similarly,a series of quaternary-ammonium-based ILs containing a functional group demonstrated excellent activity and selectivity for the synthesis of cyclic carbonates via cycloaddition reactions[44].

The carboxylic acid group is a stronger Br?nsted acid and hydrogen bond donor than is the hydroxyl group.Therefore,functionalized ILs containing a carboxylic acid group could accelerate the ring opening of epoxides.Thus,an enhanced activity of the catalyst toward the cycloaddition reaction of CO2with epoxides can be expected.To this end,however,the utilization of such functional immidazolium-based IL catalysts still remains little exploration[45,46].Herein,a series of carboxylic acid group-containing imidazolium-based ILs having varied halide ions and alkyl chains have been synthesized and used as homogeneous catalysts for the synthesis of cyclic carbonates via cycloaddition reactions.For comparison,the catalytic performances of imidazolium-based ILs containing hydroxyl groups and those without any functional moieties were also investigated.

Experimental

Materials

1-Methylimidazole,1-ethylimidazole,1-butylimidazole,3-chloropropionic acid, 3-bromopropionic acid,and3-iodopropionic acid were purchased from Sigma-Aldrich and used as received.

Synthesis of functionalized ILs

The carboxylic acid and hydroxyl group functionalized ILs were synthesized according to the following synthetic procedure.In a typical reaction,20mL of anhydrous toluene and1.53g of3-chloropropionic acid(10mmol)were charged into a?ask at room temperature.Thereafter,10mmol of1-methylimidazole was added to the?ask with vigorous stirring.The reaction mixture was re?uxed for5h under nitrogen atmosphere.After removing the solvent under reduced pressure, the remaining oil phase was further subjected to three-circle of rinse with ethyl acetate and a white solid was formed.Finally,1-carboxypropyl-3-methyl imidazo-lium chloride(CMImCl)was obtained after drying in vacuum oven at40°C for 12h.The total yield of CMImCl was95%.Following similar procedures, 1-carboxypropyl-3-methyl imidazolium bromide(CMImBr,white solid,95%yield), 1-carboxypropyl-3-methyl imidazolium iodide(CMImI,slightly yellow solid,93% yield),1-carboxypropyl-3-ethyl imidazolium bromide(CEImBr,white solid,93% yield),1-carboxypropyl-3-butyl imidazolium bromide(CBImBr,white solid,91% yield)and1-hydroxyethyl-3-methyl imidazolium bromide(HMImBr,white solid, 94%yield),were also synthesized(Scheme1).

The NMR characterizations of ILs were shown as follows:

CMImCl 1H NMR (300MHz,DMSO-d6):d 2.95(t,2H),3.90(s,3H),4.38(t,2H),7.79(s,1H),7.87(s,1H),9.43(s,1H),12.80(br,OH)ppm;13C NMR (75MHz,DMSO):d 34.18,35.88,44.89,122.6,123.6137.2,171.7ppm;IR:3148(Imidazole ring C–H stretching vibration),1726(C=O),1568&1450(Imidazole ring stretching),1167(Imidazole ring C–H bending vibration)cm -1.

CMImBr 1H NMR (300MHz,DMSO-d6):d 2.94(t,2H),3.92(s,3H),4.40(t,2H),7.78(s,1H),7.86(s,1H),9.33(s,1H),13.2(br,OH)ppm;13C NMR (75MHz,DMSO)d 34.02,35.96,44.87,122.5,123.6,137.1,171.8ppm;IR:3148(Imidazole ring C–H stretching vibration),1726(C=O),1568&1450(Imidazole ring stretching),1167(Imidazole ring C–H bending vibration)cm -1.

CMImI 1H NMR (300MHz,DMSO-d6):d 2.95(t,2H),3.90(s,3H),4.39(t,2H),

7.74(s,1H),7.82(s,1H),9.21(s,1H)ppm;13C NMR (75MHz,DMSO)d 33.96,36.05,44.88,122.5,123.6,137.0,171.8ppm;IR:3148(Imidazole ring C–H stretching vibration),1726(C=O),1568&1450(Imidazole ring stretching),1167(Imidazole ring C–H bending vibration)cm -1.

CEImBr 1H NMR (300MHz,DMSO-d6):d 1.36(s,3H),2.94(t,2H),4.21(t,2H),4.36(t,2H),7.90(s,1H),7.93(s,1H),9.53(s,1H),11.5(br,OH)ppm;13C NMR (75MHz,DMSO)d 15.2,33.7,44.2,44.8,122.0,122.5,136.2,171.6ppm;IR:3140(Imidazole ring C–H stretching vibration),1726(C=O),1569&1450(Imidazole ring stretching),1167(Imidazole ring C–H bending vibration)cm -1.CBImBr 1H NMR (300MHz,DMSO-d6):d 0.64(s,3H),1.03(t,2H),1.64(t,2H),2.92(t,2H),4.16(t,2H),4.36(t,2H),7.92(s,1H),7.94(s,1H),9.48(s,1H)ppm;13C NMR (75MHz,DMSO)d 13.2,18.7,31.6,33.9,44.8,48.2,122.1,122.4,136.5,171.5ppm.

HMImBr 1H NMR (300MHz,DMSO-d6):d 3.74(q,2H),3.90(s,3H),4.27(t,2H),5.18(s,OH),7.78(s,1H),7.82(s,1H),9.26(s,1H)ppm;13C NMR (75MHz,DMSO)d 35.8,51.6,59.4,122.7,123.4,136.9ppm;IR:3148(Imidazole ring C–H stretching vibration),1728(C=O),1568&1450(Imidazole ring stretching),1167(Imidazole ring C–H bending vibration)cm -1.

Typical cycloaddition reaction

All the cycloaddition reactions were conducted in a 55mL stainless steel autoclave equipped with a magnetic stirrer.Styrene carbonate was synthesized by a

coupling Scheme 1Molecular structures of the functionalized ILs

L.Han et al.

Ef?cient catalysts for cycloaddition of CO2and epoxides

reaction between styrene oxide(SO)and CO2in the presence of ILs,as illustrated in Scheme2.In a typical reaction,0.6mmol of the IL was introduced into the reactor containing40mmol SO.The reaction was carried out at a preset CO2pressure and different temperatures.After the reaction was completed,the reactor was quickly cooled to0°C in an iced bath.Then,the catalyst was separated from the reaction mixture by centrifugation.The product was analyzed by Agilent6890GC and GC–MS systems equipped with a FID(?ame ionization detector)using dichloromethane as the internal standard.The SO conversion,the selectivity and yield of styrene carbonate were calculated from GC results.Each catalytic reaction and catalysis was repeated for three times to secure reproducibility.

Results and discussion

Synthesis of styrene carbonate from styrene oxide and CO2

Styrene carbonate was produced from cycloaddition reaction of CO2with SO using the carboxylic-acid-functionalized ILs as homogeneous catalysts(Scheme2).The corresponding performances of these ILs are summarized in Table1.As shown in Run1,in the absence of an IL catalyst,the desired product was not detected. However,using these ILs,cyclic carbonates were obtained in good yield and high selectivity toward styrene carbonate(Run2–9).

When the carboxylic-acid-functionalized ILs(CMImCl,CMImBr and CMImI) bearing various halide ions were used to catalyze the cycloaddition reaction,the yields of styrene carbonate varied in the following order for different halide ions: Cl-\Br-\I-(Run2–4).Particularly,CMImI afforded the highest SO conver-sion,up to94%;this could be attributed to the fact that the nucleophilicity and leaving ability of the anion increase in the order Cl-\Br-\I-.Generally,a highly nucleophilic anion tends to attract the epoxide ring more ef?ciently to form a reactive intermediate(Scheme3),while the anion with higher leaving ability is favorable for the production of cyclic carbonates in the catalytic reaction process [44].Therefore,I-plays a key role in improving the catalytic reactivity of CMImI. SO yields similar to that reported previously for carboxylic acid-functionalized ammonium-based IL(HBetI,Ref.in Table1)[44]were achieved with CMImI,

(shorter

although the catalytic reaction was conducted under milder conditions Array Scheme2Synthesis of styrene carbonate via cycloaddition reaction between CO2and styrene oxide

reaction time,lower CO 2pressure,and smaller catalyst amount).Consequently,from the viewpoint of the industrial production,the carboxylic-acid-functionalized imidazolium-based IL is more competitive than the carboxylic acid-functionalized ammonium-based IL.

The effect of the alkyl chain length of the carboxylic-acid-functionalized ILs on the catalytic reactivity was also investigated.CEImBr and CBImBr exhibited SO conversion of 93and 96%(Run 5and 6),respectively;those SO conversion were higher than that of CMImBr (Run 3).This fact indicated that a longer alkyl chain length helped in achieving a high yield of styrene carbonate [47].It is postulated that an increase in the bulkiness of the alkyl chain,results in a decrease in the electrostatic interaction between anion and cation as both experience a mutual repulsion;consequently,the availability of the anion increases.

In order to compare the catalytic activities of the carboxylic-acid-functionalized ILs,the catalytic performance of ILs having a hydroxyl functional group (HMImBr)and that of the conventional IL without any functional moieties (EMImBr)were examined.The corresponding results showed that the catalytic activities of the ILs varied in the order CMImBr (92%)[HMImBr (88%)[EMImBr (79%)(Run 3,7,and 8).On the basis of this result,it could be suggested that the involvement of carboxylic acid group in the IL molecules is bene?cial for activating the ring opening of epoxide through a hydrogen bond resulting from the coordination between the H atom of the IL and the O atom of the epoxide;and thus,the cyclic carbonate is obtained in high yield.Encouraged by the excellent catalytic performance achieved when using carboxyl-based ILs,we attempted to introduce CH 3COOH (as co-catalyst)into the traditional EMImBr catalysis system for the synthesis of styrene carbonate.Interestingly,with the use of CH 3COOH,the SO conversion of EMImBr was signi?cantly enhanced from 79to 90%(Run 8and 9).Molecules containing carboxylic acid moieties are more suitable for enhancing the Table 1Effects of ILs bearing varied functional groups on the synthesis of styrene carbonate Run

Catalyst Conversion (%)Selectivity (%)Yield (%)1

None 0––2

CMImCl 6494603

CMImBr 9299914

CMImI 9499945

CEImBr 9399926

CBImBr 9698957

HMImBr 8898868

EMImBr 7998779

EMImBr/CH 3COOH 909989Ref.

b HBetI ––96a Reaction condition:Catalyst =0.6mmol;Styrene oxide =40mmol;CO 2Pressure =1.3MPa;Temperature =120°C;Time =2h

b

According to Ref.[44]:Catalyst/epoxide =2.5mmol %;CO 2Pressure =8.0MPa;Tempera-ture =140°C;Time =8h L.Han et al.

catalytic activity of conventional ILs towards the chemical ?xation of CO 2to epoxides than are molecules containing hydroxyl groups.Although the use of those carboxylic acid or hydroxyl group contained molecules as co-catalyts also provide an ef?cient alternative to improve the catalytic performance of traditional IL catalysts,the extra puri?cation procedures are necessary to remove them from the ?nal products.

To get further insights into the cycloaddition reaction of CO 2with SO,the effects of various reaction parameters such as reaction temperature,time,CO 2pressure,and the amount of catalysts on the catalytic performance of ILs have been studied.In these reactions,CMImBr was selected as the standard catalyst.Fig.1a shows the effect of reaction temperature on the yield of styrene carbonate.As the temperature was increased from 90to 120°C,the yield of styrene carbonate was increased markedly from 68to 91%.However,with a further increase in the temperature

to

Scheme 3The proposed mechanism for the synthesis of cyclic carbonate catalyzed by the carboxylic-acid-functionalized imidazolium-based ionic liquids

Ef?cient catalysts for cycloaddition of CO 2and epoxides

L.Han et al. 130°C,a decrease in the styrene carbonate yield was observed.This could have been caused by the side reactions that tend to occur at high reaction temperatures. Fig.1b demonstrates that the yield of styrene carbonate increased with an increase in the CO2pressure to1.62MPa.A further increase in the CO2pressure led to a decrease in the styrene carbonate yield.This could be attributed to the high concentration of CO2in the reaction mixture;this high CO2concentration caused a decrease in the interaction between the epoxide and the catalyst,and a consequent decrease in the epoxide concentration in the vicinity of the catalyst.In addition,the amount of catalyst employed had a signi?cant effect on the yield of styrene carbonate(Fig.1c).When the amount of CMImBr was increased from0.4to 0.6mmol,the corresponding yield of styrene carbonate increased from79to91.1%. However,a slight reduction in the yield was observed when the using amount of catalyst was further increased up to0.8and1.0mmol.The in?uence of the reaction time on the yield was also investigated(Fig.1d).The results indicated that a reaction time of2h was necessary for the complete conversion of SO under the investigated reaction conditions.A longer reaction time may result in the occurrence of side reactions;this may lead to a slight decrease in the yield of carbonate.On the basis of the results discussed above,the optimized conditions for the cycloaddition reaction of CO2and SO were con?rmed to be as follows:temperature,120°C;CO2 pressure,1.62MPa;amount of CMImBr,0.6mmol;reaction time,2h. Synthesis of cyclic carbonate from CO2and other epoxides

The above results indicate that CMImBr is an ef?cient catalyst for the cycloaddition reaction of SO and CO2under solvent-free conditions.In order to study the feasibility of using the catalyst for cycloaddition reaction involving other epoxides, the reactions of other epoxides with CO2were examined and the results are shown in Table2.CMImBr could be used with a variety of epoxides for producing the corresponding cyclic carbonates in high yield with high selectivity.When cyclohexane oxide was allowed to react with CO2,cyclic carbonates were obtained in82%yield within12h.In contrast,for other cyclic carbonates,high yields in the range of96–98%were achieved within shorter reaction periods.The lowering of the reaction rate for cycloaddition of CO2and cyclohexane oxide was mainly caused by the high steric hindrance of reactant resulting from the bulky cyclohexane ring in the reactant.

Proposed mechanism of coupling reaction

A previous research suggested that hydroxyl-groups-containing ILs can accelerate the ring-opening reaction of epoxides by hydrogen bonds formation[21].In the current study,ILs bearing a carboxylic acid moiety were found to have better catalytic activity than did ILs with a hydroxyl functional group and ILs without any functional group(Table1,Run3,7,8).It was also found that the catalytic activity of a traditional IL can be signi?cantly enhanced by adding certain amount of acetic acid to the catalytic reaction system.Therefore,it can be deduced that the carboxylic acid group promotes the cycloaddition reaction more ef?ciently than

does the hydroxyl group.The proposed mechanism is shown in Scheme 3;the X anion (Lewis base)of the catalyst opens the epoxy ring activated by the carboxylic acid group and the electronic interaction with the imidazolium cation affords an intermediate.Then,the intermediate reacts with CO 2to form the corresponding cyclic carbonate,and the catalyst is regenerated.Thus,in our current catalysis systems,the coexistence of hydrogen-bond donors (–COOH),the imidazolium cation,and a halide anion showed a synergistic effect in promoting the coupling reaction.This effect might be the main reason for the high catalytic activity and selectivity of carboxylic-acid-based

ILs.

Fig.1Effect of reaction parameters on the cycloaddition reaction between CO 2and styrene oxide:a The in?uence of temperature.Other conditions:CMImBr (0.6mmol),CO 21.3MPa,2h.b The in?uence of CO 2pressure.Other conditions:CMImBr (0.6mmol),120°C,2h.c The in?uence of catalyst amount.Other conditions:CO 21.3MPa,120°C,2h.d The in?uence of reaction time.Other conditions:CMImBr (0.6mmol),CO 21.3MPa,120°C

Ef?cient catalysts for cycloaddition of CO 2and epoxides

Conclusions

A series of carboxylic-acid-functionalized imidazolium-based ILs bearing various halide ions and alkyl chains have been synthesized and used to catalyze the cycloaddition reaction of CO 2with various epoxides.The corresponding cyclic carbonates were obtained in high yields.Moreover,long alkyl chains and anions with high nucleophilicity are favorable for epoxide conversion.IL molecules with carboxylic acid moieties in their backbones can accelerate the cycloaddition reaction more effectively than do hydroxyl groups functionalized ILs and ILs without any functional groups.Alternatively,the addition of a certain amount of acetic acid into the catalytic system can also enhance the catalytic performance of the ILs to a considerable extent.On the basis of the effect of the reaction parameters on the catalytic activity of the ILs,we concluded that cyclic carbonates can be obtained in high yield and with excellent selectivity under the optimized conditions.The proposed mechanism indicated that the carboxylic acid group,which is a stronger Br?nsted acid and hydrogen-bond donor than is the hydroxyl groups,showed a synergistic effect with halide ions and thus accelerated the cycloaddition reaction more ef?ciently than did the hydroxyl group.

Table 2Varied cyclic carbonates synthesized from different epoxides

Entry Epoxides Product Temperature

(o C)Time (h)

Yield (%)1Cl

O Cl O O O 1102982O O O O O O 115296

3Ph O

O

Ph O O O O 1152984O O O

O 115

1282a Reaction condition:epoxide,40mmol;catalyst CMImBr,0.6mmol;CO 2pressure,1.62MPa L.Han et al.

Ef?cient catalysts for cycloaddition of CO2and epoxides

Acknowledgments This work was supported by the National Research Foundation of Korea(R01-2009-0070580)and by the National Foundation of China(No.50973026).The authors are also thankful for the?nancial support of2010NRF-NSFC cooperative program(D00020and No.5101140349)and Pohang Accelerator Laboratory.

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