Efficient homogenous catalysis of CO2 to generate cyclic carbonates by heterogenous and recyclable polypyrazoles

Zhen Lu,Jie He,Bogeng Guo,Yulai Zhao,2,Jingyu Cai,2,Longqiang Xiao,2,*,Linxi Hou,2,3,*

1 Department of Materials-Oriented Chemical Engineering,School of Chemical Engineering,Fuzhou University,Fuzhou 350116,China

2 Qingyuan Innovation Laboratory,Quanzhou 362801,China

3 Fujian Key Laboratory of Advanced Manufacturing Technology of Specialty Chemicals,Fuzhou University,Fuzhou 350116,China

Keywords:Carbon dioxide CO2 conversion Cyclic carbonate Heterogeneous catalysis Polypyrazole

ABSTRACT The cycloaddition between CO2 and epoxides to produce cyclic carbonate is an attractive and efficiency pathway for the utilization of CO2 as C1 source.The development of catalyst to mediate cycloaddition between CO2 and epoxides at low temperature and pressure is still a challenge.Herein,a series of polypyrazoles with glass transition temperature (Tg) in the range of 42.3–52.5°C were synthesized and served as catalyst to mediate the cycloaddition of CO2 and epoxides by the assistant of tetrabutylammonium bromide.The catalytic behaviors of polypyrazole on the model cycloaddition of CO2 to epichlorohydrin,including the reaction parameters optimization and versatility were investigated in detail,and excellent yield (99.9%) and selectivity (99%) were obtained under the optimized reaction conditions of 70°C and 1.0 MPa for 6.0 h.Noteworthily,the polypyrazole acts as homogeneous catalyst during reaction(higher than Tg).And under room temperature,polypyrazoles can be easily separated and recovered,which is a promising feature of a heterogeneous catalyst.Furthermore,the reaction mechanism was proposed.The DFT calculation suggested that the formation of hydrogen bond between pyrazole and epoxide greatly reduced the energy barrier,which play an important role in promoting CO2 cycloaddition.

1.Introduction

Global climate change induced by the immoderate carbon dioxide(CO2)emission with the excessive utilization of fossil resources that including petroleum,coal and natural gas as energy resource in our daily life [1,2].The mitigation of excessive CO2emission has become an imminent research topic in both scientific and industrial area [3].Fortunately,CO2is proved as a low-cost,nontoxic,widely available and renewable carbon feedstock [4].One of the promising strategy to reduce CO2emission is to use it as a C1 resource for catalytical conversion into fuel or value-added chemical products,such as cyclic carbonates [5–8].Cyclic carbonates,as one class of important industrial chemicals,can be produced from the cycloaddition of CO2to epoxide oxides(Scheme 1) [9],and served as electrolytes in lithium-ion batteriesm,solvents in chemical synthesis and crucial monomers for preparing polycarbonates [10].

Scheme 1.Illustration of the cycloaddition of CO2 with epoxide.

Nowadays,the efficiency production of various cyclic carbonates via cycloaddition of CO2has been successfully industrialized by homogeneous catalyst systems,such as Lewis acid catalysts by the assistant of tetrabutylammonium bromide (TBAB) [11–13].TBAB and other cocatalysts assist the ring opening of epoxide to form bromoalkoxide in the cycloaddition of epoxides with CO2,making the reaction more efficient [14].Although the homogeneous catalysts play excellent performance especially fast reaction rate and considerable yield [5,15,16],the high temperature(T>200°C) and CO2pressure (P>6.0 MPa) are still required [17].Moreover,the catalysts recovery and purification of final product are cumber some.Taking the advantages of saving cost,and easy separation of catalyst and product,therefore,many studies focused on the heterogenization of homogeneous catalysts have been investigated to recover the catalysts for CO2cycloaddition reactions [18–20].Various heterogeneous catalysts including metal Salen complexes,porphyrin-based complexes,poly(ionic liquids)(PILs),metal organic frameworks(MOFs),and porous organic polymers(POPs)have been extensively investigated to implement catalytic cycles[21–26].However,the requirements of metal and the shortage of catalytic efficiency greatly limits the industrial applications [21,27].

In this manuscript,we have designed and synthesized cyclic polypyrazoles as efficiency metal-free catalysts for the generation of cyclic carbonates via CO2cycloaddition by the assistance of tetrabutylammonium bromide (TBAB) (Scheme 2).Polypyrazoles own the glass transition temperature (Tg) in the range of 42.3–55.2°C,and are difficult to dissolve in many kinds of solvent,such as methanol,tetrahydrofuran,chloroform,and so on.These features make polypyrazoles act as homogeneous catalysts under high temperature reaction (higher than Tg).And under room temperature,polypyrazoles can be easily separated from the reaction mixture and recovered,which is a promising feature of a heterogeneous catalyst.

Scheme 2.The chemical structure of polypyrazoles.

2.Experimental

2.1.Materials

3-Butyn-1-ol (Macklin,97%),propargyl alcohol (Sinopharm Chemical Reagent,AR),propynol ethoxylate (Macklin,98%),ptoluenesulfonyl hydrazide (Macklin,98%),p-toluenesulfonyl chloride (PTSCl) (Macklin,99%),1,8-diazabicyclo[5.4.0]undec-7-ene(DBU,Macklin,99%),bromoacetyl bromide(Macklin,97%),sodium bicarbonate (Sinopharm Chemical Reagent,AR),sodium chloride(Sinopharm Chemical Reagent,AR),pyridine(Sinopharm Chemical Reagent,AR),and column-layer chromatographic silica gel(Macklin) were purchased and used as received.All solvents,including acetonitrile (MeCN),dichloromethane (DCM),N,Ndimethylformamide (DMF),hexane,petroleum ether (the boiling range of 30–60°C),ethylacetate,tetrahydrofuran (THF),diethyl ether,ethanol,epichlorohydrin (ECH),styrene oxide,cyclohexene oxide,cyclopentene oxide,phenyl glycidyl ether,and tetrabutylammonium bromide (TBAB) were purchased from Sinopharm Chemical Reagent Co.,Ltd and used as received.

2.2.Characterization

The number average molecule weights (Mn) and polymer dispersity index (Mw/Mn,D) of polymers are determined by Gel Permeation Chromatography (Waters GPC 1525) and the eluent are dimethyl formamide (DMF) at a flow rate of 0.8 ml.min-1(40°C).The column system is calibrated with polystyrene (PS) standards.1H NMR spectra are recorded on Bruker Advance III (600 MHz).Gas chromatography (GC) measurements were carried out on a Shimadzu GC-2010 gas chromatograph.The glass transition temperature(Tg)of the obtained polymers was characterized by differential scanning calorimetry(DSC,Thermo Q20)in the range of 25–100°C (heating rate and cooling rate are 5°C.min-1) under nitrogen atmosphere.

2.3.Synthesis of polypyrazoles (P1-P3)

Polypyrazoles P1-P3 was prepared and characterized according to our previously report [28].Monomer (10 mmol) was put into a 25 ml round-bottomed flask and stirred at 100°C.After predetermined time,the obtained slightly yellow solid was dissolved with small amount of DMF and slowly dropped into a large amount of methanol.The white powder was obtained by high-speed centrifugation,washed with methanol and dried under vacuum.

2.4.Cycloaddition of CO2 to epoxide oxides and the recovery of catalyst

In a typical procedure,a mixture of polypyrazole (0.02 mmol),epichlorohydrin(ECH,1 ml,12.8 mmol),and tetrabutylammonium bromide (TBAB,0.1 g,0.3 mmol) were added into a high pressure batch reactor.The reaction kettle was closed and the carbon dioxide gas was led into.Then,the mixture was hearted at predetermined temperature and pressure for a certain period of time.After the mixture was cooled to room temperature,a trace amount of product was collected and characterized by GC to determine the conversion and selectivity.Methanol was added into the product and the precipitation was extracted by centrifugation.The solid was washed with methanol for three times and dried under vacuum.The collected solid can be reused to mediate the cycloaddition of CO2to epoxide oxides.

3.Results and Discussion

3.1.The characterization of polypyrazoles

The preparation of three kinds of polypyrazoles were taken out,which has been reported in our previously work [28].The characteristics data of the obtained polypyrazoles (P1-P3) examined by GPC and DSC is shown in Table 1.The number average molecule weight and dispersity index of P1-P3 are in the range of 3800–4400 g.mol-1and 1.63–1.71,respectively.Importantly,the glass transition temperature of P1-P3 is in the range of 42.3–55.2°C,which means the polymer will turn to high elastic state when the reaction temperature higher than 60°C.

Table 1 Characteristics data of the obtained polypyrazoles①

3.2.Polypyrazoles catalyzed the cycloaddition of carbon dioxide to epichlorohydrin (ECH)

The cycloaddition of carbon dioxide to epichlorohydrin (ECH)mediated by polypyrazoles (P1,P2 and P3) was carried out and the results were presented in Table 2.As shown in the entries 1–3,without the assistant of co-catalyst TBAB,the cycloaddition of carbon dioxide to epichlorohydrin do not occur.In the present of TBAB (0.3 mmol),(chloromethyl)ethylene carbonate (CMEC) was produced with high selectivity (99%),and the yields in the range of 57.8%–68.7% after a 4 h reaction at 60°C and 1.0 MPa pressure.

3.3.The cycloaddition of carbon dioxide with ECH mediated by P1 in different reaction condition

In order to address the most suitable reaction condition of the catalyst dosage,pressure,temperature and TBAB dosage,ECH was selected as the model starting material to react with CO2.As shown in Table 3(Entries 1–3),the dosage of P1 and the TBAB were kept at 0.02 mmol and 0.3 mmol respectively,the conversion increases gradually along with the increase in the pressure every 0.5 MPa at 60°C,and the highest yield could reach 86.1% with the pressure rising to 1.5 MPa.However,slightly increasement of yield (7.6%) was detected when the pressure was increased from 1.0 to 1.5 MPa.Thus,1.0 MPa was chosen as reaction pressure in the following research.Generally,higher temperature offers higher reaction rate and yield more products.As shown in entry 1,4 and 5 in Table 3,when the reaction temperature was improved from 50 to 70°C,the yield of target carbonate was greatly increased.The same experiment result was achieved when the dosage of catalyst was increased from 0.01 to 0.02 mmol,which due to the increasement of catalytic active site.As mentioned in Table 2 (Entry 2),no carbonate was produced in the absence of TBAB.Then,the dosage of TBAB was increased from 0.1 to 0.3 mmol.As depicted in Table 3(Entries 1,8 and 9),the yield of carbonate was sharply increased from 28.6%to 78.5%,which suggests the important and indispensable of TBAB.Finally,the cycloaddition of carbon dioxide with ECH mediated by P2 was taken out in the optical reaction condition(Table 3,entry 10),which generated target carbonate with 99.9%yield and 99% selectivity,respectively.

Table 2 Cycloaddition of carbon dioxide to ECH①

Table 3 Cycloaddition of carbon dioxide to ECH mediated by P1

3.4.Kinetic study of the cycloaddition of carbon dioxide to ECH mediated by P1

To explore the catalytic activity of catalysts,kinetic study of the cycloaddition of carbon dioxide to ECH was carried out.Therefore,the effect of temperature on catalytic activity of catalyst was investigated.The reactions were performed at 40,50,and 60°C,respectively.As the result shown in Fig.1a,the conversion of cyclooctene increased gradually from 24.8% to 78.5% along with the reaction temperature increase from 40 to 60°C,which is well agreement with the previous results.Since,the initial k of the reaction carried out at different temperature was obtained by the linear fitting of the time versus conversion curve (Fig.S1,Supplementary Material).Then,according to the Arrhenius equation of ln k=ln A-Ea/RT (where A is pre-exponential factors,Eais the activation energy,and T is the temperature),the relationship between ln k and 1/T was plotted,which indicates good linear relationship (Fig.1b).Eaand A calculated by the Arrhenius equation should be 84.69 kJ.mol-1and 8.41×1012mol-1.dm3.s-1,respectively.Theresult indicated that the cycloaddition of carbon dioxide to ECH has high conversion speed [29].

Fig.1.(a) The conversion profiles of ECH obtained at different temperatures.(b) Arrhenius curve and equation obtained from the reaction of carbon dioxide and epichlorohydrin catalyzed by P1.

3.5.Mechanism study elucidated by theoretical calculation of the cycloaddition of carbon dioxide to epoxide oxides

It has been well known that the CO2cycloaddition with epoxide is initiated by the interaction of the hydrogen with oxygen atom in the epoxides [30].In this step,the hydrogen bond was formed by the interaction of N-H with oxygen,which could active the epoxy ring.After that,the Br-ion provided by TBAB attacks the carbon atom which has higher steric hindrance of the coordinated epoxide,followed by the ring opening step to generate oxygen anion.Then,the oxygen anion interacts with CO2to produce alkyl carbonate anion,and then the corresponding carbonate was synthesized by the ring closing step.Meanwhile,the catalyst was regenerated(Fig.2a).

To quantitatively calculate the activation energy of each step(Fig.2b),the DFT calculation was carried out(the detailed information was shown in the supporting information,Table S1).As for the high molecular weight polypyrazole (P2) and high steric hinder of ECH,dimer of pyrazole (DP) and propylene oxide (PO) are used as the simulation model.The reaction of DP with PO is exothermic with an energy barrier (ΔE) value of -26.6 kcal.mol-1(1 kcal=4.186 kJ),which means the pyrazole could absorb PO by the hydrogen bond formed between pyrazole and epoxide (IM1),and subsequent generate a relatively stable transition state (IM2).Then,the attack of Br-anion on PO is endothermic with ΔE value of 19.1 kcal.mol-1(TS1),then producing a stable anion intermediate(IM4)with ΔE value of-14.0 kcal.mol-1(exothermic process).The insertion of CO2to produce alkyl carbonate anion is an intramolecular process with the ΔE value of -16.0 kcal.mol-1(IM5).Finally,the desorption of the nucleophile Br-to produce the target cyclic carbonate is endothermic with ΔE value of 6.2 kcal.mol-1.What is not doubt is that the attack of nucleophile Br-anion on PO is the rate-determining process [29].This calculation suggested that the formation of hydrogen bond between pyrazole and epoxide play an important role in promoting the cycloaddition of CO2.

3.6.Evaluation of the reusability and stability of P1

Considering to reduce the production cost,the reusability of catalyst is very important[31].Due to the low glass transition temperature,polyprazoles can dissolve in the reaction mixture and act as homogeneous catalysts at the reaction temperature.When the reaction is finished and cooled to room temperature,the catalysts can be precipitated in methanol and further separated by centrifugation.Therefore,the catalyst was recovered and recycled,and the recyclable catalytic performance was evaluated.The cycloaddition reaction of CO2with ECH was taken out under conditions of 0.02 mmol of P1,0.03 mmol of TBAB,and 1.0 MPa CO2at 60°C for 6 h.After each test,P1 was simply separated by centrifugal,then washed,dried,and reused in the next operation.After been reused for 5 times,P1 still showed good catalytic performance(Fig.3a).The yield of corresponding carbonate higher than 90%in each running.Furthermore,as shown in Fig.3b,the1H NMR characteristic peaks for the recycled sample are nearly in accordance with the newly prepared P1,confirming that recovered P1 could be reused.

Fig.2.Reaction process and theoretical calculation.(a)Scheme of reaction process for cycloaddition of CO2 with propylene oxide(PO)catalyzed by single pyrazole molecule.(b)Relative energy for each step of CO2 cycloaddition with propylene oxide(PO)catalyzed by pyrazole.Color scheme:blue for nitrogen,red for oxygen,gray for carbon,and white for hydrogen.（1kcal=4.186 kJ）

Fig.3.(a)Characterization of the reusability of P1 in the cycloaddition reaction of CO2 with ECH.(b)The 1H NMR spectra of newly prepared P1(blue line)and recovered P1(pink line).

3.7.Cycloaddition reaction of CO2 and various epoxides mediated by P1

In order to evaluate the catalytic universality for the cycloaddition of epoxides potential and CO2,the optimized conditions of 0.02 mmol of P1,0.03 mmol of TBAB,and 1.0 MPa CO2at 60°C for 6 h was applied to other epoxides,including styrene oxide,cyclohexene oxide,cyclopentene oxide,and phenyl glycidyl ether,with which the effects from the substituent group and steric hindrance could be expounded.As listed in Table 4,P1 exhibits good catalytic characteristic on the cycloaddition transformation of epoxides into related cyclic carbonates with satisfying high yields.According to the test results,strong electron absorbing groups could enhance the yield (Entry 1>4>2>3) due probably to the stronger hydrogen bond formation by polypyrazole with epoxide.However,the adverse effect from steric hindrance is obvious as confirmed.The yield of related carbonate declines (Entry 4>2>3)along with the increase in the molecular size of epoxides,which could be attributed to the fact that the attacking of Br-anion to epoxide is more difficult and subsequently influence the ultimate yield.

Table 4 Cycloaddition of carbon dioxide to different epoxides①

4.Conclusions

In summary,a series of cyclic polypyrazoles with low glass transition temperature were synthesized and acted as catalyst to mediate the cycloaddition of CO2and epoxides in the presence of tetrabutylammonium bromide.polypyrazoles act as homogeneous catalysts under high temperature reaction(higher than Tg),and can be easily separated from the reaction mixture and recovered.With the regulation of the catalyst dosage,pressure,temperature and TBAB dosage,the optimized conditions of 0.02 mmol of P1,0.03 mmol of TBAB,and 1.0 MPa CO2at 60°C for 6 h was gained and applied to generate target cyclic carbonate with 99.9% yield and 99% selectivity.Kinetic study of the cycloaddition of carbon dioxide to ECH was studied,indicating that the cycloaddition reaction has high conversion speed.Moreover,the DFT calculation was carried out to quantitatively calculate the activation energy of each step,depicting that the attack of nucleophile Br-anion is the rate-determining process and the formation of hydrogen bond between pyrazole and epoxide play important role.Furthermore,the catalyst can be recovered due to its high stability without losing the catalytic ability after been reused for 5 times.This work paves a facile way to design polymer-based catalyst with great potential for practical applications in conversion of CO2,and provides useful insights into the reaction mechanism of CO2with epoxide.

Author Contribution

Zhen Lu synthesized the polyoyrazoles,carried out the additional reaction of CO2and epoxides,and wrote the manuscript.Jie He accomplished the characterization of all materials,reactions,and wrote the manuscript.Bogeng Guo and Yulai Zhao analyzed and organized the data.Jingyu Cai finished the DFT calculation.Xiao Longqiang and Linxi Hou provided the idea,completed research plan,and revised the manuscript.

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

This work was financially supported by the National Natural Science Foundation of China(21504025),the Natural Science Foundation of Fujian Province (2019J05040),Fujian Provincial Department of Education (JT180038),Key Program of Qingyuan Innovation Laboratory (00221003),Fuzhou University Testing Fund of precious apparatus（2021T022),Talent Program (GXRC-18041) and Higher Education Disciplinary Innovation Program（‘111’ Program）of Fuzhou University.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2022.01.009.