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1 Jennifer Stein Department of Chemistry University of Missouri- Columbia 105 Chemistry Building Madison Clark 601 S. College Avenue Columbia, Mo USA April 11, 2017 Dr. Rainer Glaser, Professor of Chemistry Editor, Journal of Organic Chemistry Department of Chemistry, University of Missouri- Columbia Columbia, MO The Cycloaddition of Carbon Dioxide to Epoxides Using Functionalized Ionic Liquid By: Jennifer Stein and Madison Clark Dear Dr. Glaser, We would like to submit the manuscript with the above title for the publication as an article in JOC. The submission is made exclusively to the JOC. The paper comes with supporting information. In this article, we report the results of a study done on the effects of functionalized groups in ionic liquids on various epoxides and their performance affect the overall synthesis performance. With the increasing anxiety of our environmental future, this synthesis offers a means of carbon dioxide utilization, that if found successful, and put on a large scale could aid in fighting the greenhouse effect. As a possible reviewer we would like to suggest Joseph Schell (Mizzou, jrsg24@mail.missouri.edu). With Best Regards, Jennifer and Madison 1

2 The Cycloaddition of Carbon Dioxide to Epoxides Using Functionalized Ionic Liquid Jennifer Stein and Madison Clark Department of Chemistry, University of Missouri-Columbia, Missouri, ; mmcmr5@mail.missouri.edu

3 Abstract A sequence of polydivnylbenzene (P-DVB) polymers inserted with ionic liquids of differential functional groups hydroxyl, carbonyl, and amino were assessed as catalysts for the reaction of carbon dioxide and epoxides to form cyclic carbonates. All catalysts demonstrated good performance across a variety of epoxides. Each catalyst performed well without the presence of a solvent and a co-catalyst. However, the heterogeneous polymer that was grafted with hydroxyl ionic liquid, P-DVB-HElmBr, showed the best catalytic performance. When investigating the catalytic activity of the dependence of PC yield and selectivity on the cycloaddition reaction, P-DVB-HElmBr showed a yield of 97.1%, and a selectivity of 99.6%. The results of functional groups in ionic liquids, initial carbon dioxide pressure, reaction temperature, and reaction time of the cycloaddition reaction were also examined. It is proposed that the joint effect between functional groups, hydrogen bonding, and bromide ions enables the catalytic reaction. It is notable that P-DVB-HElmBr is more crucial for the cycloaddition reaction than the other catalysts due to its ability to hydrogen bond. In addition, it was found from the recycling test that the catalyst could be reused up to six times without any significant loss of catalytic activity. 2

4 Introduction The idea of utilization of carbon dioxide (CO 2 ) is to reuse carbon dioxide by converting it to a new product in order to reduce carbon dioxide emissions. 1 A specific way to utilize carbon dioxide is through organic synthesis where the carbon is taken from a waste product and reused to build useful chemicals. 1 Carbon dioxide is an appealing building block for organic synthesis because it is highly functional, abundant and a renewable chemical reagent that can be used in syntheses with simplicity and at low cost. 2 The paper will focus on the synthesis of cyclic carbonates from carbon dioxide and epoxides, using a cross-linked polymer grafted with functionalized ionic liquid as a reusable and efficient catalyst. The process is favorable due to cyclic carbonates value as polar solvents, electrolytes in batteries, precursors for synthesizing polycarbonates and polyurethanes, and as intermediates in the production of pharmaceuticals. 1 Other advantages include no side product formation from the synthesis, which accounts for the high PC yields. The use of ionic liquids as catalysts is proposed for this synthesis because, unlike common organometallic catalysts, functionalized ionic liquids show good activity and reusability without the use of any organic solvent or co-catalyst. Scheme 1 is an overview of the cycloaddition reaction. Scheme 1. Mechanism proposed for the cycloaddition reaction 3

5 Here we will report on the results of the effects of polymers grafted with various functionalized groups of ionic liquids across a wide range of epoxides to determine which had the best performance correlating with a successful cycloaddition reaction of carbon dioxide and epoxides to yield cyclic carbonates. The hypothesis tested was if the addition of various functionalized ionic liquids will produce high yields. When each functionalized group was compared, the polymer grafted with the hydroxyl group ionic liquid, performed best due to hydrogen bonding. The importance of this investigation is to obtain a low-cost, reusable and effective synthesis with the means to reuse harmful CO 2 emissions in hopes of furthering the methods of carbon dioxide utilization to reduce the amount of carbon dioxide pollution, and, furthermore, to reduce the greenhouse effect. Materials and Methods Reagents Divinylbenzene, 2-bromoethanol, 2-bromethylamine hydrobromide, 3-bromopropionic acid, 2-chloroethanol, and azobis(isobutyronitrile) (AIBN), and CO 2 gas (99.9% purity). Synthesis of Polymers Step A: Step B: AIBN: 2,2 -azobis(isobutyronitrile); DVB: divinylbenzene Scheme 2. Synthesis of (a) HEVImBr and (b) P-DVB-HEImBr 4

6 The desired product is P-DVB-HEImBr and was produced through a two-step process as shown in Scheme 2. Step A synthesized the cross-linked 1-(2-hydroxyl-ethyl)-3-vinyl imidazolium bromide (HEVImBr). The product of step A could have been grafted with a hydroxyl-, carboxyl-, and amino-functionalized imidazolium ionic liquid (IL). After completion of step A, the synthesized imidazolium (C 3 H 5 N + 2 ) IL polymer was prepared by radical copolymerization to produce the final product of P-DVB-HEImBr. Step A occurred through a 2-bromoethanol and 1-vinylimidazole addition. The mixture was heated for at 70 C for 24 hours, at which point the product of the step A reaction was then cooled to room temperature. Two byproducts were then produced, a top and a bottom layer. The top layer was discarded and the remaining residue was washed twice with acetonitrile followed by a single wash with diethyl ether. The product was then dried at 60 C for 4 hours under vacuum. Step B proceeded with the synthesized product from step A. The polymer linked with imidazolium ILs was prepared by radial copolymerization. The DVB and AIBN were added to a 160mL CH 3 Cl solution that contained dissolved HEVImBr. As in step A, this reaction was run under atmospheric nitrogen, however, it was refluxed for 48 hours instead of 24 hours. The solidified mixture that was produced, was filtered and washed with single rinses of THF, acetone and methanol before drying for 6 hours in a vacuum of 60 C. Finally, the catalyst P-DVB- HEImBr is produced. Analogs Furthermore, analogs of the P-DVB-HEImBr product were synthesized. The reaction conditions were held the same: Step A was run at 70 C for 24 hours and Step B was dried for 6 hours in a vacuum of 60 C. The only difference in the reaction was the reagent used. Instead of 5

7 grafting the P-DVB with HEVImBr, P-DVB was grafted with CEVImBr, AEVImBr, EVImBr, and HEVImCl, which produced P-DVB-CEImBr, P-DVB-AEImBr, P-DVB-EImBr, and P- DVB-HEImCl, respectively. For the preparation of P-DVB-AEImBr, once the solid was obtained and the copolymerization was completely filtered out, a 5mL solution of CH 3 CN/H 2 O and NaOH was added and left to stir overnight to neutralize the hydrogen bromide. The rinsing of all the analog products proceeded the same as the P-DVB-HEImBr: washed with single rinses of THF, acetone and methanol. Cycloaddition Reaction P-DVB-HEImBr proved to be the most efficient catalyst. The synthesized product was then used as a catalyst in a series of cycloaddition reactions containing the epoxides shown in Table 1. The reactions were run under a reactor charged with epoxide and the catalyst. The reactor was then induced with CO 2, heated and stirred. The reactor was cooled to 0 C in an icewater bath, and the remaining CO 2 was released and absorbed in a solution saturated with K 2 CO 3. Table 1. Epoxides combined with the P-DVB-HEImBr produced in Step B to give their respective products 6

8 Results and Discussion Catalyst characterization The grafted polymers of P-DVB-EImBr, P-DVB-HEImBr, P-DVB-CEImBr and P-DVB- AEImBr were characterized by FT-IR analysis shown in Figure S1. All the previously stated polymers exhibited characteristic bands of the imidazolium ring functional group. Furthermore, all the molecules exhibited bands corresponding to their functional groups. P-DVB-EImBr exhibited O-H and C-O stretching vibrations; P-DVB-CEImBr exhibited O-H, C=O, and C-O stretching vibrations; and P-DVB-AEImBr exhibited N-H and C-N stretching vibrations as shown in Figure S1a-d, respectively. Analysis of Catalysts Propylene oxide (PO) was used as a substrate to fix CO 2. Data was evaluated for the synthesis of propylene carbonate (PC) through cycloaddition of CO 2 to PO and recorded in Table 2. (Note that the structure of the substrate propylene oxide (PO) along with its product propylene carbonate (PC) is entry 2 in Table 1.) Table 2 shows the catalytic performance of P-DVB grafted with various IL polymers. P- DVB alone showed no catalytic activity (entry 1). However, P-DVB grafted with hydroxyl, carboxyl, and amine groups exhibited much higher catalytic activity (entries 3-5). In contrast, the P-DVB-EImBr complex was less efficient for the cycloaddition reaction (entry 6). 7

9 Table 2. Catalytic Performance of Various P-DVB-grafted IL polymers using PO as a substrate Entry Catalysts Yield(%) Sel. (%) 1 P-DVB Trace -- 2 HEVlmBr P-DVB-HElmBr P-DVB-CElmBr P-DVB-AElmBr P-DVB-ElmBr P-DVB-ElmBr/ethanol P-DVB-ElmBr/H2O P-DVB-ElmBr/phenol Ethanol Trace H2O Trace Phenol P-DVB-HElmCl P-HElmBr Reports show hydrogen bonding between hydroxyl (or carboxyl) groups and epoxides. This effect has a positive effect on epoxy-ring opening which facilitated the cycloaddition reaction. 5 However, strong hydrogen bonds might weaken the nucleophilicity of anions, thus strengthening the ring-opening capability of cations. 5 From the data compiled in this experiment, P-DVB-HEImBr, with moderate H-bonding, exhibited the best activity. In the case when HEVImBr was used as a homogeneous catalyst, 64.2% yield and 99.6% selectivity were obtained (entry 2). Under the same reaction conditions, P-DVB-HEImBr obtained 97.1% yield and 99.6% selectivity (entry 3). The results concluded that the heterogeneous P-DVB-HEImBr was superior to the homogeneous HEVImBr catalyst. To further investigate the role of hydroxyl groups, a compound containing three O-H functional groups ethanol, H 2 O, and phenol were each added to a P-DVB-EImBr complex. Ethanol, H 2 O and phenol were extremely low in catalytic activity by themselves (entries 10-12), 8

10 however, once bound to the P-DVB-EImBr complex, they rose significantly in catalytic activity (entries 7-9). The percent selectivity detected in the case of P-DVB-EImBr/H 2 O (95.7%) was lower than P-DVB-EImBr/ethanol and P-DVB-EImBr/phenol (both > 99% selectivity), likely a result of hydrolysis within the substrate (PO). Further investigations in the presence of hydroxyl groups resulted in increased catalytic activity when P-DVB was bound with anions. It was found the P- DVB-HEImBr (entry 3) exhibited a higher catalytic activity than P-DVB-HEImCl (entry 13) due to difference in nucleophilic anions. It is apparent that the presence of hydroxyl groups resulted in increased catalytic activities. Based on the results of the presented study, a mechanism has been proposed for the cycloaddition of CO 2 to PO with P-DVB-HEImBr (Scheme 1). P-DVB-HEImBr Catalytic Performance Table 2 demonstrates that P-DVB-HEImBr was the most effective catalyst for the cycloaddition of CO 2 to epoxides. Typically, temperature, time and pressure have significant effects on product yield. Therefore, the three parameters were investigated and yield and selectivity were recorded. A reaction with the parameters of 5 hours and 140 C was experimentally found to produce the best results. The graphs of % yield and selectivity at 140 C versus CO 2 pressure are shown in Figure 1. Increasing pressure from 4.5 to 5 MPa resulted in an increase of yield. However, increasing from 5 to 5.5 MPa, resulted in a decrease of yield. Therefore, 5 MPa of CO 2 was determined to be the most efficient. 9

11 Yield (%) yield sel Selectivity (%) Pressure (MPa) 75 Figure 1. Effect of CO 2 pressure on catalytic activity of P-DVB-HEImBr Compared to previous epoxide-catalyzed studies conducted, a cross-linked polymer grafted with IL, as presented in this study, was proven to be a better catalyst than other methods of epoxide linkage such as quaternary phosphonium salts. 6 As shown in Figure 2, phosphonium salts produced high selectivity rates, however the % yield decreased drastically as more MPa CO 2 was induced into the system. The reaction conditions were run similarly: 150 C for 6 hours. P-DVB-HEImBr was found to withhold 2.5 times more CO 2 than quaternary salts Yield (%) Yield Sel Selectivity (%) Initial CO2 Pressure (MPa) Figure 2. Effect of CO 2 pressure on catalytic activity of quaternary phosphonium salts. 10

12 Catalyst Recycling Under 140 C, CO 2 pressure 5.0 MPa for 5 hours and PO as a substrate, the reusability of P-DVB-HEImBr was measured. P-DVB-HEImBr was recovered by filtration and then rinsed and vacuumed. The catalyst was reused for the next run and was found to have no significant loss to the catalytic activity through the completion of 6 trials (Figure 3). It was found that the FT-IR spectrum of the recovered catalyst is similar to that of the first one (Figure 4), concluding that P- DVB-HEImBr demonstrates great reusability. Figure 3. Catalytic activity of recycled P-DVB-HEImBr Figure 4. FT-IR spectra of P-DVB-HEImBr: (a) fresh and (b) recovered 11

13 The above results indicate that P-DVB-HEImBr is the most effective, stable and reusable catalyst of the synthesized options for the cycloaddition of CO 2 to PO. Conclusion The cross-linked polydivinylbenzene grafted with various (hydroxyl, carboxyl, and amino) functional ionic liquid, was found to be an effective heterogeneous catalyst for the synthesis of cyclic carbonates from CO 2 and epoxides. The various functional ionic liquids, through hydrogen bonding with the bromide anion, greatly promoted the cycloaddition reaction. The polymer grafted with the hydroxyl group, P-DVB-HEImBr, showed the greatest catalytic performance and thermal stability compared to the other polymers grafted with various functionalized ionic liquids. When investigating the dependence of PC yield and selectivity on the cycloaddition reaction, P-DVB-HEImBr showed a yield of 97.1% and a selectivity of 99.6%. The results indicated relative hydrogen bonding was advantageous to supporting the cycloaddition reaction. The polymer grafted hydroxyl catalyst, P-DVB-HEImBr, showed a large efficiency for reusability. It could be reused up to 6 runs without a loss of 90% catalytic activity. The catalyst was overall very appealing because of its stability and reusability. Overall P-DVB- HEImBr, can be used as an effective catalyst for the utilization synthesis of cyclic carbonates, via cycloaddition, from CO 2 and epoxides. Having an effective utilization reaction such as this opens a small window into the large problem of carbon dioxide emissions. This reaction provides a means to successfully utilizing excess emissions of CO 2, which is a small step to combating global warming. 12

14 Supplementary Material Available The appendix explains in more details of the processes and preparations of the desired products. References (1) Liu, Q.; Wu, L.; Jackstell, R.; Beller, M. Using Carbon Dioxide as a Building Block in Organic Synthesis. Nature Communications. 2015, (2) Holscher, M.; Gurtler, C.; Keim, W.; Muller, E. T.; Peters, M.; Leitner, W. Carbon Dioxide as a Carbon Resource- Recent Trends and Perspectives. JOC. 2012, 67b, (3) Sun, J.; Zhang, S.J.; Cheng, W.G.; Hydro, R. Hydroxyl-Functionalized Ionic Liquid. Tetrahedron Letters. 2008, 49, (4) Sun, J.; Han, L.J.; Cheng, W.G.; Wang, J.Q.; Zhang, X.P.; Zhang S.J. Efficient Acid-Base Bifunctional Catalysis for the Fixaton of CO 2 with Epoxides. ChemSusChem. 2011, 4, (5) Xiong, Y.; Bai, F.; Cui, Z.; Guo, N.; Wang, R. Cycloaddition Reaction of Carbon Dioxide to Epoxides Catalyzed by Polymer-Supported Quaternary Phosphonium Salts. JOC

15 Supporting Information Jennifer Stein and Madison Clark Department of Chemistry, University of Missouri-Columbia, Columbia, Missouri,

16 Table of Contents Reagents of Functionalized Ionic Liquids.....S2 Reaction Conditions for Step A.S2 Reaction Conditions for Step B.S2 Cycloaddition Reaction S2 Catalyst Characterization.....S2-S4 Analysis of Catalysts S4-S5 P-DVB-HEImBr Catalyst Performance.S5 Bibliography..S6 S1

17 Supporting Information Reagents of Functionalized Ionic Liquids All reagents were purchased from Aldrich Chemical Co. and were used as received no further treatment was performed on them once received. Reaction Conditions For Step A: 4.374g 2-bromoethanol and 3.294g 1-vinylimidazole were poured, respectively, into a 50 ml flask under atmospheric Nitrogen. The mixture was heated at 70 C for 24 hours, then stored at room temperature. Reaction Conditions for Step B: Product from step a (P-DVB-HEImBr), 3.321g DVB, AIBN (used as an initiator) added to 160mL CH 3 Cl solution containing 1.0g HEVImBr dissolved. Cycloaddition Reaction The cycloaddition reactions were carried out in a 30mL high-pressure stainless-steel autoclave, equipped with a magnetic stirring bar. The typical run includes 28.6 mmol epoxide,.65% mol (which was calculated according to the amount of IL present) and an appropriate amound of biphenyl (used as the internal standard). The CO 2 was released using an aspirator and absorbed by the aqueous K 2 CO 3 solution. A GC-mass spectrometer and 1 H NMR were conducted on the resulting mixtures. Catalyst Characterization The synthesized catalysts exhibited bands of an imidazolium ring functional group (1606, 1157, 1041 cm -1 ). The P-DVB-EImBr complex exhibits stretching vibration bands of O-H (3434 cm -1 ) and C-O (1221 cm -1 ). The P-DVB-CEImBr exhibits stretching vibration bands of O-H S2

18 (3396 cm -1 ), C=O (1710 cm -1 ), and C-O (1238 cm -1 ). The P-DVB-AEImBr complex exhibited stretching vibration bands of N-H (3413 cm -1 ) and C-N (1256 cm -1 ). Figure S1. FT-IR Spectra of (a) P-DVB-EImBr, (b) P-DVB-HEImBr, (c) P-DVB-CEImBr and (d) P-DVB-AEImBr The results of elemental analysis and BET measurements are summarized in Table S1. The initial amount of IL was calculated according to the result of N-element analysis. Based off the cross-linked polymers, P-DVB-CEImBr is the largest in amount of functional IL (1.20 mmol/g) but smallest in specific surface area (303.2 m 2 /g). The P-HEImBr is extremely low in specific surface area (0.4 m 2 /g), due to the absence of DVB (cross-linker). S3

19 Catalyst Elemental results HIL S BET N (wt.%) C (wt.%) H (wt.%) Br (wt.%) (mmol/g) (m 2 /g) P-DVB- HEImBr P-DVB- CEImBr P-DVB- AEImBr P-DVB- EImBr P-HEImBr 0.4 Table S1. Physicochemical properties of catalysts: % element, amount of HIL, and specific surface area. Analysis of Catalysts The main reason the heterogeneous catalyst is superior to the homogeneous catalyst is due to the poor HEVImBr affinity for propylene oxide (PO). The IL does not disperse well in the reaction system; however, under stirring, the P-DVB-HEImBr complex disperses well. Based on the results of the present study, a mechanism has been proposed for the cycloaddition of CO 2 to PO with P-DVB-HEImBr (Scheme 2). First, the O atom of the epoxide and the hydroxyl group of the catalyst interact through hydrogen bonding, resulting in the formation of intermediate I. Then, the B-carbon atom of the epoxide undergoes nucleophilic attack by the bromide ion; simultaneously, the ring of the epoxide opens and generates an oxyanion, II. CO 2 is then inserted into the reaction, resulting in the formation of a linear halocarbonate, III. Through intramolecular substitution, a cyclic carbonate forms through substitution of the bromide ion. P-DVB-HEImBr Performance Temperature and time have significant effects on product yield. Therefore, we investigated the dependence of yield and selectivity on reaction conditions. Figure S2 shows the S4

20 relationship between % yield and reaction time, while temperature was set at 140 C. 94.1% yield was obtained after 4 hours and 97.1% after 5 hours with selectivity remaining above 99.5% throughout. Depicted in Figure S3 shows the relationship between PC yield and reaction temperature; reaction time is 4 hours. The reaction is very temperature-sensitive. There is hardly any change in selectivity, however, there is a rapid rise in PC yield from 35.4% to 99.5% which the temperature is raised from 110 to 150 C. Figure S2. Effect of reaction time on catalytic activity of P-DVB-HEImBr Figure S3. Effect of reaction temperature on catalytic activity of P-DVB-HEImBr S5

21 Bibliography Wei-Li D.; Sheng-Lian L.; Shuang-Feng Y.; Xu-Bizo L.; Chak-Tong A. Cross-Linked Polymer Grafted with Functionalized Ionic Liquid as Reusable and Efficient Catalyst for the Cycloaddition of Carbon Dioxide to Epoxides. Journal of CO 2 Utilization. 2013, 3, S6