Glycerol carbonate synthesis from glycerol and dimethyl carbonate using guanidine ionic liquids☆

Xingxing Wang ,Peng Zhang ,Penglei Cui,Weiguo Cheng ,*,Suojiang Zhang ,*

1 Beijing Key Laboratory ofIonic Liquids Clean Process,Key Laboratory ofGreen Process Engineering,State Key Laboratory ofMultiphase Complex Systems,Institute ofProcess Engineering,Chinese Academy of Sciences,Beijing 100190,China

2 University of Chinese Academy of Sciences,Beijing 100049,China

1.Introduction

Glycerol carbonate(GC),originally from biomass,is a kind of high value-added chemicals in industry[1].GC can be used for the synthesis of various chemical intermediates due to its hydroxyl group and 2-oxo-1,3-dioxolane group[2–4].It is also used in the field of cosmetics,coating and paints because of its non- flammable,nonvolatile and biodegradable properties[5].The GC production is regarded as a promising way to deal with the huge excess of glycerol from biodiesel industry[6–9].

GC can be synthesized from glycerol(GL)viaseveral routes including phosgenation,transesterification with dimethyl carbonate(DMC)[6,10–14],carbonation with CO2[15–17],glycerolysis with urea[18,19]and so on.The phosgenation is limited by the toxicity ofphosgene and the carbonation is restricted by the thermodynamic limitation[16].And the glycerolysis has other problems like the byproduct treatment[20].Transesterification of GL with carbonate is one of the most promising and feasible routes to obtain high yield ofGC and the reaction is shown in Fig.1.

Various catalysts were explored to catalyze the transesterification.The homogeneous catalysts such as K2CO3,KOH,1,5,7-triazabicyclo[4.4.0]dec-5-ene(TBD)and 1-n-butyl-3-methylimidazolium-2-carboxylate(Bmin-2-CO2)showed high catalytic activities,but they were not preferable as they cannotbe separated and reused easily[3,21–23].The heterogeneous catalystssuch asmetaloxides,hydrotalcite,and coal fly ash were reported to have good activities[11,14,24–29].However,they stillhad some drawbacks like catalyst deactivation,product contamination or large energy consumption[30].Ochoa-Gómezet al.found that the basic catalysts were much more active compared with the acidic catalysts,which indicated that the performance was closely related to the basic strength of catalysts[21].Under the optimized reaction conditions,the GC yield over the calcined CaO was 95.0%.However,CaO is sensitive to water and prone to deactivation and the catalyst recycling was limited by particle agglomeration.Sandeshet al.used KF impregnated on different supports(carbon,ZrO2,SiO2,H-beta,etc.)to catalyze the synthesis of GC and 95.8%conversion of GL and almost 100%selectivity of GC were obtained[31].Even though the reusability was good,the reaction needed to be carried outin dimethylformamide(DMF),making itdifficultto separate the product from the reaction media.Moreover,it was also emphasized that the low turnover frequencies(TOFs)significantly limited the industrialimplementation.Therefore,itis stilla greatchallenge to explore efficient and environmental friendly basic catalysts for production of GC.

Recently ionic liquids(ILs)have been widely used in organic catalysis,such as Knoevenagel–Michael reaction,direct aldol reactions and aza-Michael reaction due to their unique properties like excellent tunability and good thermal stability[32–35].Importantly,they were also employed as basic catalysts for the synthesis of glycerol carbonate.Although the high yield of GC can be obtained with 1-butyl-3-methylimidazolium imidazolium([Bmin][Im])at 70 °C,the TOF is only 6.6 h?1[36].The basicity of ILs is governed by the strength of cation,anion or their combination[37].Guanidine is a kind of superbase which has been reported to be efficient catalyst in organic catalysis[38,39].The guanidine based ionic liquids possess both easily tunable property of ionic liquids and the required basicity of catalyst[39–41].

Fig.1.The synthesis of GC from GL and DMC.

In this work,a series of guanidine-based ionic liquids were synthesized by neutralizating tetramethylguanidine(TMG)with weak protic acid,and their application on the transesterification of glycerol and dimethylcarbonate undersolvent free conditions was also investigated.The guanidine based ionic liquids were screened and the reaction conditions(temperature,molar ratio of DMC/GL,catalyst amount and reaction time)were sequently optimized.

2.Experimental

2.1.Materials

Glycerol(AR)andN,N-dimethylformamide(99.5%AR)were purchased from Xilong Scientific.Dimethyl carbonate(＞98%GC),1,1,3,3-tetramethylguanidine(99%),2,2,2-tri fluoroethanol(99.5%),and imidazole(99%)were purchased from Aladdin.Benzonic acid(99.5%AR)and Acetic acid(99.5%AR)were from Beijing Chemical Works.Glycerol carbonate(90%),glycidol(96%),and phenol(＞99%AR)were purchased from Macklin,Sigma-Aldrich,and Sinopharm,respectively.All the reagents were used without further purification.

2.2.Preparation of ILs

The ILs were synthesized based on the previous literature and the structures of cation and anions were displayed in Fig.2[34,41–43].In a typical experiment,30 mmol TMG was added into a 100 ml flask,then 30 mmol acid was dropped into the flask slowly.The mixture wasstirred for3 h with good mechanicalstirring.The productsobtained were directly used for characterization and catalytic tests.

2.3.Catalyst characterization

The ionic liquids were con firmed by the NMR.The1H NMR spectra were recorded by a Bruker 700 MHz spectrometer in DMSO-d6or CDCl3with tetramethylsilane as internal standard.

Fig.2.The structures of the cation and anions.

2.4.Catalytic test

The reaction between the glycerol and dimethyl carbonate was carried out in a 50 ml three-necked flask equipped with a re flux condenser and a thermometer.In a typical experiment,0.25 mmol[TMG][TFE]was loaded to the reactor,followed by the addition of 0.025 mol glycerol and 0.05 mol dimethyl carbonate.The mixture was heated to 80°C by oil bath and kept for 90 min.Then the mixture was cooled to room temperature.

2.5.Product analysis

For quantitative analysis,0.5 gn-butanol was added as internal standard to determine the GC and GL.Diluted with 30 ml DMF,the sample was analyzed by the gas chromatograph(Shimadzu GC-2010 plus)with flame ionization detector(FID)and HP-Innowax(30 m×0.25 mm × 0.25 μm).Injector and detector temperatures were 260 °C and 280 °C,respectively.The oven temperature was started at 40 °C,which was held for 2 min,followed by ramping of 25 °C·min?1up to 250°C and lasting for 15 min.

3.Results and Discussion

3.1.Screening of catalysts

The transesterification between the glyceroland dimethylcarbonate was generally catalyzed by the basic catalyst.A series of guanidiniumbased ionic liquids were screened and the results were shown in Table 1.It was demonstrated that the reaction did not occur in the absence of catalyst and no GC was detected.The conversions of GL catalyzed by[TMG][BA]and[TMG][Ac](Entry 2–3)were less than 15%and the selectivities of GC were no more than 53%.On the other hand,the[TMG][TFE],[TMG][Im]and[TMG][Phe](Entry 4–6)showed good performance.The GL conversions were higher than 97%,the GC selectivities were more than 87%and the TOFs were close to 200 h?1catalyzed by the three above.The highest TOF could be 1754.0 h?1under the optimized condition(Entry 7).To the best of our knowledge,the highest TOF of the ionic liquid catalysis for this reaction was 157.4 h?1[36,44–47].It was reported that the GL conversion and GC selectivity could be very high,but they suffered from the long reaction time or the large catalyst amount.

The performance of catalysts was highly dependent on the anion of the ionic liquids[36].The basicities of anions increased in the order of[BA]?＜ [Ac]?＜ [Phe]?≈ [Im]?＜ [TFE]?[40,48,49].The[TFE]?as a Bronsted base has the strongest proton accepting ability and the glycerol is easily activated by the proton exchange to form the glyceroxide.As a result,the[TMG][TFE],[TMG][Im]and[TMG][Phe]showed high activities underthe cooperation ofanion and cation.The basicity of[BA]?and[Ac]?ismuch lowerthan[TFE]?,thusthe activation of glycerolwas notefficientwhich led to lowerconversions and selectivities.This result was consistent with the work reported by Sairiet al.whose catalysts were 2-hydrozyethylammonium formate([HEA][Fmt])and 1-ethyl-3-methylimidazium dimethyl phosphate([Emin][DMP])[45].Totally,[TMG][TFE]exhibited the best performance with 91.0%GC yield and 202.2 h?1TOFs,and it was used for the further investigation.

Table 1Screening of ionic liquids for the glycerol carbonate synthesis

3.2.The optimization of reaction conditions

3.2.1.Effect of temperature

The reaction temperature has a dramatical in fluence on the catalytic activity.Fig.3 showed the changes of GL conversion and GC selectivity at the temperature range of 60–100 °C with the[TMG][TFE]as the catalyst.The GL conversion increased from 81.1%at 60 °C to 97.1%at 80 °C while the GC selectivity decreased from 98.2%to 93.8%.Further increase of temperature did not lead to an increase of GL conversion but the GC selectivity decreased to 84%.Therefore,80°C was the optimal temperature at which the highest GC yield could be obtained.

Fig.3.Effect of temperature on GC synthesis.(Reaction conditions:GL:0.025 mol;n(DMC)/n(GL)=2;catalyst:0.3 mol%of[TMG][TFE]based on GL;reaction time:90 min).

3.2.2.Effect of catalyst amount

The effect of catalyst amount ranging from 0.1 mol%–5 mol%(based on the glycerol amount)was investigated while other parameters were kept constant.It is observed in Fig.4 that the GL conversion was almost around 96%with different catalyst amount.However,the selectivity of GC decreased from 94.8%to 68.7%at 80°C for 90 min with DMC/GL molar ratio of 2.This could probably be explained by the enhancement ofside reaction.The use of0.1 mol%ofthe[TMG][TFE]was good enough for the transesterification between GL and DMC.

3.2.3.Effect of DMC/glycerol molar ratio

The effect of molar ratio of DMC and GL was studied in Fig.5.Theoretically,one mole GL reacted with an equimolar amount of DMC to produce one mole GC and two mole byproduct of methanol.However,the conversion of GL and yield of GC were 76.5%and 89.9%respectively when the molar ratio of DMC and GL was 1.Since the DMC and methanol can be separated from the reaction system by distillation easily[30],excess DMC was given in the reaction,resulting in 95.2%GL conversion and 94.8%GC yield at the ratio of 2.Further increase of the amount of DMC did not lead to the significant increase of the GL conversion but decreased the GC selectivity.The optimal molar ratio of DMC and GL was 2 at 80°C.

Fig.4.Effect of catalyst amount on GC synthesis.(Reaction conditions:GL:0.025 mol;n(DMC)/n(GL)=2;temperature:80°C;reaction time:90 min).

Fig.5.Effect of molar ratio between DMC and GL on GC synthesis.(Reaction conditions:GL:0.025 mol;catalyst:0.1 mol%of[TMG][TFE]based on GL;temperature:80°C;reaction time:90 min).

3.2.4.Effect of reaction time

The effect of reaction time was also discussed in Fig.6.It can be found thata GL conversion of47.7%and GC selectivity of92.2%were obtained at 15 min.The increase of reaction time to 30 min gave the 91.8%conversion ofGL and 95.5%selectivity of GC.Further prolonging of reaction time had little in fluence on both the conversion and selectivity.The reaction time was studied in the range of 15–90 min and 30 min was found to be the best.

3.3.The possible reaction mechanism

Apossible reaction mechanism forglycerolcarbonate synthesis from glycerol and dimethyl carbonate was proposed shown in Fig.7.This is an anion–cation cooperative catalysis.The nucleophilic activation was triggered by the proton exchange between the[TFE]?and the glycerol to form the glyceroxide.The electrophilic activation of carboxylic oxygen was done by the interaction with hydrogen in[TMG]+.Then the glycerol methyl carbonate and methanol were obtained.With further activation of glycerol methyl carbonate,the glycerol carbonate formed by fast intramolecular cyclisation.The similar activation of carbonyl by[TMG]+and hydroxy by conjugate base was also observed in other's works[42,45,50].The reaction mechanism was speculated and further verification is under way.

Fig.6.Effect of reaction time on GC synthesis.(Reaction conditions:GL:0.025 mol;n(DMC)/n(GL)=2;catalyst:0.1 mol%of[TMG][TFE]based on GL;temperature:80°C).

3.4.The comparison of catalysts

The comparison of TOFs with other catalysts was listed in Table 2.It can be observed that the[TMG][TFE]had the highest TOF value of 1754.0 h?1compared with the catalysts from the literatures.DBU and CH3OK also showed high TOFs,1254.4 h?1and 1146.0 h?1,respectively.Other TOFs were much lower than that of[TMG][TFE].The[TMG][TFE]was an efficient catalyst for the transesterification of GL and DMC among the reported catalysts.

Table 2The comparison of TOFs between different catalysts

4.Conclusions

A series of guanidine-based ionic liquids showed good performance for the transesterification of glycerol carbonate synthesis.[TMG][TFE]was found to be the most effective catalyst for its high basic property and anion–cation cooperation.The conversion of glycerol was 91.8%and selectivity of glycerol carbonate was 95.5%under the optimized reaction conditions.Remarkably,TOF as high as 1754.0 h?1during the transesterification was obtained.Furthermore,a reaction mechanism was proposed.This work provides an example for the synthesis of carbonateviatransesterification by ionic liquids.

Fig.7.The possible reaction mechanism.

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