Efficient Transformation of CO2 to Cyclic Carbonates using Bifunctional Protic Ionic Liquids under Mild Conditions

A series of 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU) based bifunctional protic ionic liquids (DBPILs) was easily prepared by acid-base reaction at room temperature. They were used to catalyze the cycloaddition reaction of CO2 with epoxides under mild conditions. As a metal free catalyst, the best DBPILs showed a 92 % yield of products within 6 hours at 30 C and 1bar CO2 without any solvents and co-catalysts. And it could afford carbonates in good yields with broad epoxide substrate scope and CO2 from simulated flue gas (15% CO2/85% N2). IR spectrum and DFT studies were carried out to invest the mechanism of the cycloaddition


Introduction
Carbon dioxide is one of the main components of flue gas, which is responsible for the global warming. CO2 is also regarded as an abundant, nontoxic and renewable C1 resource. [1][2][3][4] The utilization of CO2 as raw materials for production of energy carriers and chemicals is a promising alternative, which can reduce greenhouse gas thus alleviate the impact of climate change, whilst producing various organic chemical commodities. [5][6][7][8][9][10] However, applications related to CO2 are limited because of its thermodynamic stability and therefore the CO2 activation always requires electrolytic reduction processes or high-energy input. 11 One of the typical feasible routes is the synthesis of five-membered cyclic carbonates by cycloaddition of CO2 with epoxides. 12-14 As one of a few successfully industrial products that efficiently utilize CO2 as a carbon feedstock, cyclic carbonate can be wildly used as solvents in chemical processes, [15][16][17] electrolyte components in lithium batteries, 18 useful monomers for acyclic carbamates 19,20 and carbonates, 21 and intermediates in the production of fine chemicals, etc. 22,23 To date, various catalysts have been developed for the synthesis of useful cyclic carbonates from CO2 and epoxides, such as metal-based catalysts, [24][25][26][27][28][29][30][31][32][33][34] organocatalysts, [35][36][37][38][39][40][41][42][43][44] and ionic liquids etc. [45][46][47][48][49][50][51][52][53] Although most of the catalyst systems show a good yield to produce carbonates, high temperatures (>100 °C) and/or high CO2 pressures are always needed in these studies. Hence, great efforts have been focused on the development of efficient and sustainable catalyst systems that can be carried out chemically converting CO2 at relatively mild reaction conditions. In this respect, many catalysts have been synthesized and exhibited a good performance in the synthesis of cyclic carbonates from CO2. Metal−based catalysts including metal-organic framework (MOF), 54,55 metal (salen) complex 56,57 and metal-porphyrins 58-60 were reported to have a good yield of cyclic carbonates at atmospheric pressure and/or room temperature for the activity sites of metal ions and halide ions. For example, North and coworkers reported an Cr(III) salphen complexes for the cycloaddition reaction under ambient conditions with the presence of TBAB (tetrabutylammonium bromide). 27 The catalyst could catalyze CO2 and epoxides to cyclic carbonates in 57−92% isolated yields after a reaction time of 24 hours. Organocatalyst systems, such as boronic acids, 61 tetraarylphosphonium salts (TAPS) 62 and pyridine-methanol/onium salts, 63 52 Recently studies have been focused on protic ionic liquids, which exhibited not only excellent ability of reversible CO2 capture but also highly efficient CO2 chemical conversion even under ambient conditions because of the powerful H-bonding. 66  Inspired by these works, we reported here the synthesis of DBU-based bifuncitional protic ionic liquids (DBPILs) by acid-base reaction at room temperature based on good acidity of halogenated carboxylic acid and alcohols that caused by inductive effect of the electronwithdrawing group ( Figure 1). DBPILs that composed by alkoxy anion, protic acid and nucleophilic groups were successfully used in cycloaddition reaction of CO2 with epoxides at 1 bar CO2 and 30/50 o C. We found that they were efficient metal-free catalysts for this reaction, which displayed high activity in producing carbonates without the need for long reaction time or co-catalyst. And we explored the influence of the substrate scope on the catalytic behaviour with various epoxides and simulated flue gas. DFT studies, IR spectrum and experiments were conducted to study mechanism of the activation of CO2 and H-bond interaction between DBPILs and epoxide.

Results and discussion
To synthesize the protic ionic liquids, electron withdrawing groups are selected to increase acidity of alcohols and carboxylic acid through inductive effect. Halogens are well known as electron-withdrawing groups. It has been proven that Bris an excellent nucleophilic ion for the cycloaddition reaction in our previous work. 47 Table 1.  The effects of reaction time and temperatures were presented in Figure 2. The yields of CPC increased rapidly to 82% within the first 4 h at 30 o C, then slowly reached to 96% in the next 4 hours. The results indicated an obviously decrease of reaction rates. As the reaction temperature increased to 50 o C, the yield of CPC increased to 98% sharply in 4 h without an obvious slowgrowth stage. This may be caused by the increasing viscosity of the reaction system with the producing of CPC, which have a negative effect on the CO2 transferring in the liquid phase. The mechanism mentioned above was confirmed by the same behavior of the catalyst loading study shown in Figure 3, which also had a slow-growth stage with the increasing of 1f. Scheme 1. Cycloaddition of epichlorohydrin with simulated flue gas catalyzed by 1f.
CO2 is known as the main component of dry flue gas. More attentions has been paid on the conversion of CO2 from the flue gas. 1,26,32 In order to investigate whether DBPILs could be used in the flue gas, a 15% CO2/85% N2 system was chosen to simulate flue gas for the cycloaddition reaction with epoxides (Scheme 1). However, only 24% of CPC was obtained within 6 hours catalyzed by 1f, which was much lower than pure CO2 at the same conditions (24% vs 92%). The yield of CPC can reach to 90% after 36 hours reaction. The results indicated the feasibility for conversion of CO2 in the flue gas catalyzed by DBPILs, but the comparably low activity was still the drawback need to be overcome. A range of different substituted terminal epoxides were examined under a balloon of CO2 condition in the presence of 1f DBPILs. The results were summarized in Table 2.
Epibromohydrin (2a) could afford the product 3a in a good yield of 88% at the optimal condition.
Taking into account of the industrial application, the loading of 1f was reduced, and the reaction temperature increased to 50 o C. However, the carbonate 3b showed much lower yield than 3c at the same condition, which probably due to the high stereo-hindrance effect. Furthermore, styrene oxide 2d and glycidol derivatived 2e-2h were examined. All of these epoxides generated the corresponding cyclic carbonates in good yields (3d-3h).  2f was subsequently chosen as an optimal terminal epoxide to study the cycling performance of DBPILs 1f. When we tried to recycle 1f after the reaction, we found that the yield of 3f decreased obviously from 95% to 77% after four runs (Figure 4). Based on the report, the reduction of catalytic activity after used might be caused by the partial loss of the catalyst for the sublimation property 66    It is reported that H-bond interaction between catalyst and epoxide can reduce the activation energy of ring-opening step. 47 Hence, comparison of 1f and 1h catalyzed ring-opening step was examined by DFT study to identify whether hydrogen proton from 1f had a more powerful Hbond interaction with epoxide than -OH group from 1h. All calculations were carried out with B3LYP-D3/6-31+G** level implemented in Gaussian 09 package. As shown in Figure 6, 1hcatalyzed ring-opening step has an energy barrier of 44.2 kcal/mol, which is a little higher than Based on all the results above, a possible mechanism of the 1f-catalyzed process is proposed, and the DFT study was used to study the mechanism. As shown in Scheme 2 and Figure 7 transition state TS1 with a barrier of 39.6 kcal/mol. As CO2 was added into the reaction system, it could be activated by alkoxy anion, and led to the formation of complex C (step 3).
Subsequently, the alkyl carbonate D generated by the nucleophilic attack of the intermediate through TS2 through a low energy barrier of 2.9 kcal/mol (step 4). Finally, the cyclic carbonate was obtained by ring-closure step with an energy barrier of 7.9 kcal/mol (TS3). The study illustrated that the epoxy ring-opening step was a rate-limited step with the highest energy barrier of 39.6 kcal/mol (TS1).

Conclusions
In summary, this work exhibits a simple way to prepare DBPILs by introducing electron

Conflicts of interest
The authors declare no conflicts of interest.