Efficient Synthesis of Cyclic Carbonates from Atmospheric CO2 Using

Letter pubs.acs.org/journal/ascecg

Efficient Synthesis of Cyclic Carbonates from Atmospheric CO2 Using a Positive Charge Delocalized Ionic Liquid Catalyst Zhiguo Zhang,* Fangjun Fan, Huabin Xing, Qiwei Yang, Zongbi Bao, and Qilong Ren Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Zheda Road 38, Hangzhou 310027, China S Supporting Information *

ABSTRACT: The chemical fixation of CO2 into value-added products has recently received much attention. One of the most well-known methods is the coupling of CO2 with epoxides to form cyclic carbonates. Although this field has progressed considerably, it is still a challenge to perform the reaction with atmospheric CO2 or flue gas. Herein we described the development of 4-(dimethylamino)pyridine hydrobromide ([DMAPH]Br) as a highly efficient and recyclable catalyst for the formation of cyclic carbonates from atmospheric CO2 and epoxides. In the presence of 1 mol % of [DMAPH]Br, excellent conversions and selectivities for a broad range of terminal epoxides were achieved under solvent-free conditions. Furthermore, the catalyst could be recycled over five times without appreciable loss of catalytic activity. The excellent catalytic performance of [DMAPH]Br is attributed to the enhanced synergistic interplay of acidic proton and bromide to epoxides and CO2 through positive charge delocalization on its cation. KEYWORDS: Carbon dioxide, Cyclic carbonates, DMAP, Protic ionic liquids, Hydrogen bonding

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INTRODUCTION Carbon dioxide (CO2) is arguably one of the most well-known greenhouse gases and its continuous emission into the atmosphere has caused increasing environmental concerns. For this reason, the development of efficient chemical processes for capture and utilization of CO2 (CUC) is currently of intensive study.1−3 In this context, the chemical fixation of CO2 into value-added chemicals has seen an explosion of interest, largely due to the potential of CO2 as a renewable and cheap C1 feedstock for the chemical industry.4−8 As a consequence, a number of methodologies have been developed for the chemical fixation of CO2. Of these, the coupling of epoxides with CO2 to form five-membered cyclic carbonates (Scheme 1)

As a result, a number of effective catalysts have been developed for this transformation including transition metal complexes,18−20 alkali metal salts,21−24 metal oxides,25−27 ionic liquids (ILs)13,28−33 as well as organocatalysts.14,34−38 Nevertheless, most of the protocols still operate at high temperatures and pressures with highly purified CO2,11,39 which lead the synthesis of cyclic carbonates a net carbon dioxide emitter rather than consumer. Thus, from a practical point of view, considerable effort has been devoted to the development of efficient catalytic systems for carrying out this transformation at ambient conditions. In this respect, the combination of a welldesigned Lewis acid catalyst with a quaternary ammonium halide cocatalyst has proved to be a viable strategy on promoting this transformation at ambient conditions even with diluted carbon dioxide.40−46 Although CO2 can be efficiently transformed using these catalytic systems, the potential metal contamination sometimes has to be paid attention to. In particular, it is also difficult to recover and reuse these catalysts. Meanwhile, organic molecules based on pyridinium,47,48 imidazolium,3249, ammonium,50,51 and phosphonium salts52,53 as well as N-heterocyclic carbenes54 have been successfully equipped into the arsenal for catalyzing the synthesis of cyclic carbonates. In this context, bifunctional strategies have been often taken for the design of efficient organocatalysts, upon which the incorporation of hydrogen donors into ionic liquids can greatly improve the catalytic activities based on a putative mechanism that epoxides ring

Scheme 1. Synthesis of Cyclic Carbonates from CO2 and Epoxides

has long been considered to be one of the most promising routes for the utilization of CO2,9−14 as cyclic carbonates have a range of applications such as aprotic polar solvents, electrolytes for lithium ion batteries, monomers for polymer synthesis, and intermediates for chemicals and pharmaceuticals.3,9,15−17 Even though the synthesis of cyclic carbonates from epoxides and CO2 is a highly exothermic reaction, however, owing to the inherent thermodynamic stability of CO2, a catalytic system is generally required to activate the epoxides and/or CO2. © 2017 American Chemical Society

Received: February 17, 2017 Revised: March 12, 2017 Published: March 13, 2017 2841

DOI: 10.1021/acssuschemeng.7b00513 ACS Sustainable Chem. Eng. 2017, 5, 2841−2846

Letter

ACS Sustainable Chemistry & Engineering opening is facilitated via a synergistic effect of hydrogen bonding and nucleophilic anions.55,56 Although significant advances have been achieved in this organocatalytic field, it is still a challenge to perform the reaction at ambient CO2 pressure or with flue gas. 4-(Dimethylamino)pyridine (DMAP) is well-known as an acyl-transfer catalyst for esterification and probably the most frequently used nucleophilic catalyst for various synthetically useful transformations.57−59 In addition, the use of DMAP as a nucleophilic cocatalyst in combination with H-bond donors60,61 or Lewis acids62,63 has also been employed to accelerate the coupling of epoxides with CO2. However, no DMAP-based protic ILs have been explored as a promoter for the title reaction. In view of several recent reports on ammonium ILs catalyzed fixation of CO2 with epoxides50 and the “privileged” structure of DMAP in positive charge delocalization (Scheme 2), we reasoned that DMAP derived protic ILs would serve as

Table 1. Synthesis of Styrene Carbonate (SC) Catalyzed with Different Protic ILs

Scheme 2. Three Mesomeric Forms of Protic DMAP

entrya

catalyst

SO conv. [%]b

SC selec. [%]b

1 2 3 4 5 6 7 8 9 10 11

none DMAP/NaBr 1a [DMAPH]Br 1b [4-NHCH3-PyH]Br 1c [4-NH2-PyH]Br 1d [4-OH-PyH]Br 2a [DBUH]Cl 2b [DBUH]Br 3 [HMIM]Br 4 [HTMG]Br 5 [DABCOH]Br

n.d. 48 96 78 60 47 80 85 84 71 61

n.d. 99 99 99 99 99 99 99 99 99 99

a

Reaction conditions: SO (10 mmol), CO2 (1 atm), catalyst (1 mol %), 120 °C, 4 h. bDetermined by 1H NMR (CDCl3, 400 MHz).

Table 2. Optimization of Reaction Conditions more effective catalysts for the fixation of CO2 into cyclic carbonates than conventional ILs. The enhanced activity was largely ascribed to the weaker electrostatic cation/anion interactions and increased nucleophilicity of anions as well as the simultaneous presence of H-bond interaction with epoxides. Herein, we demonstrated for the first time that [DMAPH]Br was an efficient and recyclable organocatalyst for the synthesis of cyclic carbonates with atmospheric CO2 as well as flue gas. On the basis of these considerations, we commenced our investigations by first synthesis of a series of protonated DMAP derivatives (1a−d) for evaluation. Several known aliphatic protonated onium salts (2a [DBUH]Cl,64 2b [DBUH]Br, 3 [HMIM]Br,65 4 [TMGH]Br, and 5 [DABCOH]Br) have also been prepared for comparison (Scheme 3).

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RESULTS AND DISCUSSION The initial experiment was performed using styrene oxide (SO) as a model substrate in the presence of 1 mol % of protic ILs depicted in Scheme 3 under 1 atm of CO2, and the results are shown in Table 1. Control experiment demonstrated that no reaction occurred in the absence of any catalyst (Table 1, entry

entrya

catalyst loading [mol %]

T [°C]

t [h]

SO conv. [%]b

SC selec. [%]b

1 2 3 4 5 6 7 8 9d

0.1 0.3 0.5 0.7 1.0 1.0 1.0 1.0 1.0

120 120 120 120 120 100 80 80 120

4 4 4 4 4 4 4 12 14

38 (86)c 56 76 85 96 62 36 76 92

99 (99) 99 99 99 99 99 99 99 99

a

Reaction conditions: SO (10 mmol), CO2 (1 atm). bDetermined by H NMR (CDCl3, 400 MHz). cReaction time 24 h, and the conversion is in parathesis. dMixture of 15% CO2 and 85% N2 in volume.

1

1). The use of DMAP/NaBr as the catalyst gave a conversion of 48% under identical condition (Table 1, entry 2). Obviously, the cation structures of the investigated ILs have strong impact on the catalytic activities (entries 3−11) and [DMAPH]Br 1a showed the best activity, giving the desired product in excellent conversion and selectivity (entry 3). The results obtained suggest that a synergetic interplay of protic acid and bromide is crucial for the enhanced catalytic activity. The variation of substituents at 4-position of 1a to less electron-donating groups gave rise to less effective activity, and the yields decreased to 78%, 60%, and 47%, respectively (entries 4−6), presumably due to inefficient positive charge delocalization on these cations. For comparison, several known protic ILs derived from various bases were also tested for the title reaction, which generally showed inferior activity to 1a (entries 7−11). This is in line with previous work using [DBUH]Cl64 and [HMIM]Br65 as

Scheme 3. Compounds Prepared as Potential Catalysts for the Title Reaction

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ACS Sustainable Chemistry & Engineering Scheme 4. Proposed Catalytic Mechanism

Figure 1. Catalyst recycling for cycloaddition reaction (reaction conditions similar to those of Table 1, entry 3).

the catalysts, where the reaction had to undergo at high pressure of carbon dioxide. Table 3. Reactions of Various Epoxides with CO2 using [DMAPH]Br as the Catalyst

a

Reaction conditions: 1 (10 mmol), CO2 (1 atm), 120 °C, 1a (1 mol %), 4 h. bDetermined by 1H NMR (CDCl3, 400 MHz). c12 h. 2843


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ACS Sustainable Chemistry & Engineering

times without considerable loss in activties. Further exploitation of [DMAPH]Br to other transformations is currently under investigation in our laboratory.

After [DMAPH]Br 1a was established as the optimal catalyst, we next investigated the effects of other reaction parameters on the reaction including catalyst loadings, reaction temperatures, and concentrations of CO2 (Table 2). In general, the conversions of SO were proportional to the catalyst loadings and 1 mol % of catalyst loading was optimal for this reaction (entries 1−5). Of note, decreasing the catalyst loading to 0.1 mol %, a decent conversion of SO can also be obtained albeit with a prolonged reaction time (24 h) (Table 2, entry 1). Furthermore, the SO conversions were strongly affected by the reaction temperature, which grew steadily from 38% to 96% when the reaction temperature was increased from 80 to 120 °C (entries 5−8). The results in Table 2 display that 120 °C was a suitable temperature. It is particularly worth mentioning that under optimal conditions and using a mixture of 15% CO2 and 85% N2 to mimic flue gas in industry, catalyst 1a was able to convert SO into SC with excellent conversion (entry 9), which, to our knowledge, represents the first example of protic ILs catalyzed synthesis of cyclic carbonate with diluted carbon dioxide. To test catalyst reusability, the reaction was carried out in the presence of a catalytic amount of [DMAPH]Br under the optimal condition. After the reaction was completed, 5.0 mL of water was added into the mixture, which was then subjected to centrifugal separation. The aqueous phase was collected and the catalyst was recovered after removal of water by rotary evaporation. The recovered catalyst was used for next run without further purification. The results shown in Figure 1 demonstrate that the catalyst could be recycled over five times without appreciable loss of catalytic activity. Under the optimal reaction conditions, the substrate scope of this protocol was also examined and the results are summarized in Table 3. Generally, terminal epoxides bearing with both electron-withdrawing and electron-donating groups could be transformed to the corresponding cyclic carbonates with excellent conversions and selectivities (entries 1−6), but for internal cyclohexene oxide, a relatively low conversion and selectivity was obtained even when the reaction time was prolonged to 12 h (entry 7), presumably due to the steric hindrance of cyclohexene oxide. On the basis of previous reports31,33,34,56,65 and our experimental results, a plausible mechanism was proposed for [DMAPH]Br-catalyzed cyclization of epoxides with atmospheric CO2 (Scheme 4). First, epoxide was activated via the hydrogen bonding between the proton of [DMAPH]Br and the oxygen of the epoxide, which resulted in the polarization of the C−O bond and therefore facilitated its ring opening by nucleophilic attack of the bromide. The formed bromo-alkoxide can be stabilized by coordination to the cation part through hydrogen bonding, and then reacts with CO2 and cyclizes to give the cyclic carbonate with the regeneration of [DMAPH]Br.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00513. Preparations of ILs, NMR and FT-IR of ILs, NMR of products, procedure of the cycloaddition reaction (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Z. Zhang. E-mail: [email protected]. ORCID

Zhiguo Zhang: 0000-0003-1681-4853 Qiwei Yang: 0000-0002-6469-5126 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the MOST (2016YFA0202900), the National Natural Science Foundation of China (21376212, 21436010), the Fundamental Research Funds for the Central Universities (2016FZA4019) and the Natural Science Foundation of Zhejiang Province, China (LY13B060001).

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REFERENCES

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CONCLUSION In summary, [DMAPH]Br 1a has been developed as a highly efficient and recyclable catalyst for the synthesis of cyclic carbonates from epoxides and atmospheric carbon dioxide. Various terminal epoxides bearing with both alkyl and aryl substituents are tolerated with this protocol, giving the corresponding product in excellent conversions and selectivities. It is particularly worth mentioning that using a mixture of 15% CO2 and 85% N2 to mimic flue gas in industry, catalyst 1a was also able to convert SO into the product with excellent conversion. Moreover, the catalyst can be recyled over five 2844


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