Yuxin Wu,Zhuo Chen*,Xiaohui Zhang,Jian Chen,Yundong Wang,Jianhong Xu*

The State Key Laboratory of Chemical Engineering,Department of Chemical Engineering,Tsinghua University,Beijing 100084,China

Keywords:Carbon dioxide Microreactor Kinetics Ionic liquids

ABSTRACT Carbon dioxide (CO2) utilization and fixation have become one of the most important research areas nowadays due to the increase of global greenhouse effect.Cyclic carbonate,which is widely used in various fields,can be synthesized by fixation of CO2 with epoxide in industry.Moreover,the synthesis of cyclic carbonate is a 100%atom economical reaction,which makes it eco-friendly and promising.To enhance the reaction efficiency and safety,a microreaction system was used as the platform for cycloaddition reaction.In this work,tetrabutylammonium bromide(TBAB)was chosen as catalyst,and propylene oxide(PO) as a mode substrate.Interestingly,the addition of water can increase the propylene carbonate (PC)yield and decrease the activation energy considerably,proving water as catalyst promoter for PC synthesis.PC yield and selectivity could reach 91.6% and 99.8%,respectively.The Influence factors and kinetic equation for CO2 cycloaddition were obtained as well.

1.Introduction

Global warming,generated by increasing greenhouse gases levels in the atmosphere,has aroused great attention in a worldwide level [1].Carbon dioxide is the mainly kind of greenhouse gases,which is a potential C1 building block to synthesize highvalued organic compounds [2],has the advantages of abundant,cheap and nontoxic[3].In order to reduce global warming,the fixation of CO2into useful compounds has received considerable concern and is beneficial to reduce CO2emission [4],which will not introduce acid or solvents and perform as a 100% atom economy reaction.Cyclic carbonates have been widely used as polar solvents,electrolytic elements,monomers and raw materials for fine chemicals synthesis[5],and are usually synthesized by cycloaddition of CO2and epoxides in industry [6].Moreover,the catalytic formation of cyclic carbonates is a 100%atom economical reaction(Fig.1),which makes it eco-friendly and receive increasing attention in recent years.

Among all kinds of cyclic carbonate,propylene carbonate is a‘‘green”sustainable alternative solvent with low toxicity,high boiling point and low vapor pressure,and is widely used in lithium-ion batteries and removal of CO2[7].In order to convert propylene oxide into propylene carbonate,numerous catalysts including metal oxide[8],transitionmetal complexes[9],alkaline metal salts[10],ionic liquids [11] and supported catalysts [12] have been reported for this process.However,most of these catalysts are not commercial because of low conversion [13],extreme conditions requirement [14] and low stability [15].Among them,ionic liquids (ILs) have received considerable concern due to their advantages of thermal stability,uninflammability and low volatility[16,17].Previous literatures have reported that hydroxyl group in conjunction with ionic liquids would efficiently activate the epoxide and catalyze the reaction [3,18].Sunet al.[19] reported that both propylene oxide (PO) conversion and propylene carbonate (PC) yield would dramatically increase with the addition of water catalyzed by almost all the Lewis base catalysts (e.g.KI,[bmim]Br,TBAB,ZnBr2,etc.),which indicated that water was also a good hydroxyl group-containing solvent.Since water is green and cheap,herein,TBAB/H2O was decided to be catalysts for cycloaddition of CO2with PO served as a model substrate.

Fig.1.Reaction diagram for cyclic carbonate synthesis.

However,batch reactors,which are commonly served as equipment for CO2cycloaddition,have the disadvantages of long residence time (R.T.),multi-phase heterogeneous mixing and potential risks at high temperature and pressure.Normally it takes 1-20 hours for cycloaddition of CO2in batch reactor [20-22].Moreover,the real application of CO2cycloaddition suffers from danger caused by increasing CO2pressure and long residence time,which are limiting factors of energy saving and safety production.Kumaret al.[23]synthesized styrene carbonate(SC)using TBAB as catalyst in a batch reactor,and it took 8.5 hours to achieve SC yield of 35%.Ryuet al.[24] reported that it took at least 6 hours to produce propylene carbonate in batch reactor catalyzed by TBAB.Even in the presence of water,which can decrease the activation energy to shorten the reaction time,the cycloaddition of CO2still needs long enough time due to the structural stability of CO2and the shortcoming of batch reactors.Liuet al.[18] reported that it took 1 hour to achieve 78% PC yield in batch reactor,while only 23%with TBAB alone.Similarly,Sunet al.[3] also reported that it took 1 hour to reach 86% PC yield with water as hydroxyl groupcontaining solvent,and it would decrease to 56% without water.Moreover,the kinetic laws of gas-liquid multiphase reaction process are greatly affected by two-phase mass and heat transfer process,which can be improved by high contact area and intensive mixing.

Continuous flow reactors have distinguished advantages over conventional batch reactor such as high rate of mass and heat transfer [25],considerable safety under extreme conditions [26]and great controllability in strong exothermic reaction [27].With high contact area and intensive mixing of gas and liquid,the residence time can be reduced into minutes or even seconds.Huanget al.[28] produced cyclopropylamine in a continuous-flow microreaction system,and the CPA yield could reach as much as 96% in only 4 min,while it took several hours in batch reactor.Moreover,the formation of by-products could be prevented due to the precise temperature control of microreaction system.Another advantage of microreactor is that it is convenient to study the kinetic rules for many kinds of reactions because changing reaction conditions is quick and precise.de Bellefonet al.[29]studied the kinetic models of almost 214 reactions in a microreactor,which could be obtained in short time period because it could avoid the problem of time consuming using traditional batch reactor,and in the meantime keep the reaction safe.Moreover,in traditional batch reactor,pressure should be high enough to initiate CO2fixation process,and we can relieve pressure and open up the reactor only when the temperature drops to a moderate degree.As a result,kinetic study is difficult to realize,but it is convenient to achieve using microreaction system because operation conditions can be varied easily in the process of reaction.Werhanet al.[30] studied the kinetic parameters of gas-liquid reaction under high pressure in a microreactor.Since microreactor can achieve precise control,kinetic study is highly reliable,and can be obtained much faster than batch reactor.

Therefore,microreaction system is suitable for improving reaction process efficiency of CO2cycloaddition reaction with the precondition of high conversion and selectivity.Several literatures have studied the cycloaddition of CO2in microreaction system.Zhaoet al.[31] first reported CO2fixation reaction in microreactor catalyzed by ionic liquids with hydroxyl modified,the reaction time could be reduced dramatically to seconds,but the catalyst system was not commercial.In addition,expensive commercially purchased microreactor would result in limitations for applications in industry.Rehmanet al.[32]studied styrene carbonate synthesis catalyzed by TBAB and ZnBr2.In their study,long residence time(45 min)was needed in a tube-in-tube reactor.Therefore,both fast reaction process and low cost are needed in order to realize scaleup process in the future.Moreover,TBAB is a cheap and abundant catalyst which is often used in industry,and the thermolysis of quaternary ammonium salts,commonly known as‘‘Hofmann Elimination”,happens at 300-600 °C [33],indicating the stability of TBAB.To our knowledge,there is no relevant report on cycloaddition of CO2in microreactor catalyzed by cheap and abundant catalyst system.

Herein,CO2fixation process to synthesize propylene carbonate catalyzed by TBAB/water in a continuous-flow microreaction system was reported.In this reactor,the influence of temperature,pressure,catalyst and water concentration were studied to find the appropriate reaction conditions.In order to explain the function of water,PC yield,product selectivity and the activation energy for fixation process with and without water were studied.Moreover,the kinetic equation could be derived via experimental results and kinetic models,which will be helpful to deduce the mechanism of reactions and instruct the real process in industry.

2.Experimental

2.1.Reagents

Propylene oxide(PO,99%)was obtained from Shanghai Aladdin Technology Co.,LTD,Tetrabutylammonium bromide (TBAB,＞99%)was obtained from Shanghai Dibai Biotechnology Co.,Ltd,diphenyl(GC,＞99.5%) was used as internal standard substance and was obtained from Shanghai Aladdin Bio-Chem Technology Co.,LTD.Carbon dioxide (99.99%) was commercially available.

2.2.Apparatus

The microreactor system consists of a T-shaped micromixer(internal diameter 2.0 mm) and a delay reactor (1/8-inch stainless,internal diameter 2.0 mm,long 30 m) as shown in Fig.2.Tetrabutylammonium bromide (TBAB) and water were dissolved in propylene oxide (PO) at ice bath (to prevent it from evaporating)as solution A.A constant flow pump (purchased from Xingda Science and Technology Development Co.,Ltd)was used to deliver solution A with a volume flow rate of 1.0 ml·min-1,and a back pressure regulating valve was used to control the liquid pressure to prevent gas flow backward.On the other hand,a mass flowmeter for gases was served as controller for the mass flow rate of CO2.Solution A was served as continuous phase,while CO2as dispersed phase.After combing two phases at the T-shaped micromixer,the delay loop was used to keep the reaction going.Another loop(1/8-inch stainless tube,inner diameter 2.0 mm,8 m long)and a water bath were used to quench the reaction.Pressure regulating function was achieved by another back pressure regulating valve.Gas-liquid mixture was separated in the separator.After waiting for about 20 min,the product was collected to continue product analysis.

2.3.Product analysis

After reaction,PC yield was determined gravimetrically because of the low boiling point of propylene oxide,which means that there is no remaining PO in the output liquid.The test method for determining the selectivity of product was realized by gas chromatography Agilent 7890B using internal standard method,OV-101 column (25 m × 0.53 mm × 3 μm) was used as chromatographic column.The initial temperature is 70 °C,hold initial temperature for 2 min,then increase the temperature to 260 °C with a rate of 20 °C·min-1and hold for 10 min.The standard curve is shown in Fig.S4 (Supplementary Material).

Fig.2.Scheme of the microreaction system for propylene synthesis.

Fig.3.Effect of reaction temperature on PC yield TBAB/H2O/PO (mass ratio)=8/12.4/100,3.0 MPa,CO2/PO (mol)=2,R.T.=166 s.

3.Results and Discussion

3.1.Effect of reaction conditions on cycloaddition reaction

3.1.1.Effect of reaction temperature

As shown in Fig.3,PC yield increased when the temperature ranged from 110 °C to 140 °C,and the growth rate gradually slowed with the increase of temperature.In detail,the PC yield increased from 36.1%to 74.3%with temperature increasing.When the molar ratio of TBAB was settled,the suitable reaction temperature was determined as well.The rise in temperature enhanced the kinetics of CO2fixation and the catalyzing activity of TBAB/H2O,consequently leading to the improvement on the cycloaddition reaction performance and PC yield.And dynamic equilibrium of the reaction process is reached above 140 °C with this catalyst amount.

3.1.2.Effect of reaction pressure

Fig.4 showed that pressure affected PC yield obviously.The PC yield increased with pressure increasing from 1.5 MPa to 3.5 MPa,and the increase rate gradually reduced as pressure increased.In detail,the PC yield increased from 59.3%to 73.0%as reaction pressure increased from 1.5 MPa to 3.5 MPa.Therefore,reaction pressure was favorable to PC yield,because it resulted in improvement in CO2solubility and mass transfer rate.The concentration of CO2in the liquid phase increased as pressure increased,resulting in the improvement in PC yield.

Fig.4.Effect of reaction pressure on PC yield TBAB/H2O/PO (mass ratio)=8/12.4/100,140 °C,CO2/PO (mol)=2,R.T.=166 s.

Fig.5.Effect of TBAB concentration on PC yield H2O/PO (mass ratio)=12.4/100,140 °C,3.0 MPa,CO2/PO (mol)=2,R.T.=166 s.

3.1.3.Effect of TBAB concentration

Next,the effect of TBAB concentration on the yield of PC was investigated.As shown in Fig.5,with the increase of the TBAB molar concentration,the reaction rate was accelerated,and PC yield increased accordingly.Specifically,PC yield increased from 44.2% to 72.5% with TBAB molar ratio ranged from 0.425% to 1.44%.The increase of TBAB concentration was beneficial to the reaction kinetics,but overly-abundant TBAB would negatively lower the economic value of the microreaction system,so we chose proper molar ratio of TBAB with 1.44%to study the influence laws and reaction kinetics.

3.1.4.Effect of water concentration

Water is the key factor in the catalyst system,the influence of water content on PC yield and product selectivity was studied as shown in Fig.6.In the presence of water,both PC yield and product selectivity were considerably increased,which indicated that TBAB/H2O performed high activities.Little amount of PC was obtained without water at lower temperature,because-OH environment is not formed without water,resulting in polymerization reaction and other complex reaction as shown in Fig.S3.With water,side reaction,which is only hydrolysis of propylene carbonate,happens,and GCMS data shown in Fig.S1 confirms this as well.Moreover,with the increase of water content from 5% (mass) to 10% (mass),PC yield and selectivity increased clearly,indicating the activation energy of hydrolysis of propylene carbonate is lower than that of cycloaddition of CO2.However,when water content was conducted above 10% (mass),the product selectivity decreased.Over-abundant water would cause side effect in which PO would be converted to propylene glycol.The activity of Brcould be improved by water,because the activity of anions varied in the order of Br-＞Cl-＞I-in the presence of water,while it was I-＞Br-＞Cl-without water existence [19].

3.1.5.Optimization of PC yield

As shown above,PC yield could be achieved as high as 72.9%when the mass fractions of TBAB,H2O and PO were fixed.If higher yield was needed,we could increase the reaction temperature,water content and TBAB content in the same time.Finally,91.6%of PC yield could be obtained with the TBAB molar ratio of 4.3%and water of 24.8%at 160°C.The results showed that the reaction performances could be improved with microreaction system(Table 1).

Table 1 Optimization of PC yield

3.2.The kinetics of CO2 cycloaddition reaction

In order to study the mechanism of PC synthesis catalyzed by TBAB/H2O,a detailed study of reaction kinetics in the microreaction system was needed,and the equations of reaction rate were listed in Eqs.(1)-(5) based on reaction rate theory.Assuming that the reaction was according with pseudo-first order reaction kinetic model for propylene oxide [34],and the CO2concentration was within 30%.Moreover,the concentration of TBAB and H2O were nearly unchanged during the reaction process.Therefore,the obvious reaction rate was related to the concentration of CO2,TBAB and H2O.The activation energy could be fitted by Arrhenius equation as shown in Eq.(5).

[PO],[CO2],[TBAB]and [H2O] were the concentrations of each feed components,anda,b,cwere the reaction orders for propylene oxide,CO2,TBAB and water,kobswas the observed rate constant.

3.2.1.Activation energy with and without water

Fig.6.Effect of water concentration on (a) PC yield and (b) product selectivity TBAB/PO (mass ratio)=8/100,3.0 MPa,CO2/PO (mol)=2,R.T.=166 s.

Fig.7.Arrhenius fitting for the fixation of CO2 with and without water existence.

As shown in Fig.7,the activation energy was determined by the experiment data at the temperature ranged from 110150 °C using Arrhenius Eq.(5).The activation energy for PC synthesis with TBAB only was fitted to be 51.12 kJ·mol-1as shown in Fig.7 and Table 2,while Rehmanet al.[32] determined activation energy for SC synthesis with TBAB only to be 55 kJ·mol-1,demonstrating the rationality of this result.Then,the activation energy was reduced to 27.1 kJ·mol-1in the presence of water.Therefore,water was served as efficient medium for CO2fixation process,because the activation energy decreased in the presence of water,making the reaction easier to perform.

Table 2 Activation energy for the fixation of CO2 with and without water existence

3.2.2.Reaction order for PCO2

In order to determine the reaction order of CO2,experimental data performed at different CO2pressure was used with other reaction conditions remained unchanged.The reaction rate increased with pressure increasing,and the experimental data was observed to be first order reaction for CO2as shown in Fig.8,suggesting that the value of CO2reaction order is 1.And the results were reported the same by other literatures [32,35].

3.2.3.Reaction order for TBAB concentration

The reaction order of TBAB was obtained by the reaction rate at different TBAB concentrations.The experiment data shown in Fig.5 was used to perform a first-order fitting,where [cat] means the concentration of catalyst,and the result was shown in Fig.9.The result indicated that reaction order of TBAB was determined to be 1,and the results were constant with the previous report [32].In detail,one anion (Br-) attacked on the least hindered C atom,and then the ring of one molecular PO opened.

3.2.4.Reaction order for water concentration

The reaction order with respect to water was studied using experimental data performed at various water concentrations.And the fitting result was shown in Fig.10,where [H2O] means the concentration of water.The concentration of TBAB was fixed,and it could be observed that first-order system was good with a slope of 0.51,suggesting a value of 0.5 for this reaction order.The role of water in this catalyst system was to provide hydrogen bonds to accelerate the ring-opening process.

Fig.8.First-order fitting model of different CO2 pressure.

Fig.9.First-order fitting model of different TBAB concentrations.

Fig.10.Linear fitting for observed rate constant in different water concentration.

As a result,the kinetic equation for CO2cycloaddition catalyzed by TBAB/H2O can be determined as below:

3.2.5.Possible reaction mechanism

According to the kinetic Eq.(6),reaction mechanism for propylene carbonate synthesis catalyzed by TBAB/H2O was proposed in Fig.S5.Totally,there are four steps in the process: (1) H atom of water and O atom of propylene oxide coordinateviaa hydrogen bond,and the C-O bond was polarized.(2)Br-attacks on the least hindered C atom,and then the ring of one molecular PO opens.(3)The intermedia reacts with CO2to form an alkylcarbonate anion and (4) propylene carbonate is formed and the catalyst could be recycled again.The mechanism proposed has also been proved by the previous reports [3,19,36,37].

4.Conclusions

Highly efficient process of CO2cycloaddition was realized in a microreaction system,and the kinetic parameters were obtained using experimental data,proving the positive effect of water.The experimental results showed that temperature,pressure,TBAB and water concentration provided a positive influence on the PC yield.The variation of activation energy with and without water also proved it as efficient medium for CO2cycloaddition catalyzed by TBAB.The kinetic study suggested a first-order dependence of the reaction rate on TBAB and CO2concentrations,and the kinetic order value of water concentration was proved to be 0.5.Moreover,the possible reaction mechanism for CO2cycloaddition catalyzed by TBAB/H2O was proposed,which could be verified by experimental data and previous reports.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge the supports of the National Natural Science Foundation of China (22025801,22108147),Shui Mu Xue Zhe of Tsinghua University(2020SM056),China Postdoctoral Science Foundation(2021M691761) for this work.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2022.05.019.