A New Approach to CO 2 Capture and Conversion Using Imidazolium Based-Ionic Liquids as Sorbent and Catalyst

Embora tecnologias de captura e armazenamento de carbono (CCS) estejam recebendo grande atenção para a mitigação do efeito estufa, ainda existem muitas desvantagens, tais como o aumento dos custos e gastos de energia associados à sua implementação. No entanto, o uso de CO2 como bloco de construção C1 em síntese orgânica pode ser muito atraente para o desenho de processos ecológicos. Neste trabalho, foram estudados a sorção de CO2 e atividades catalíticas de alguns líquidos iônicos (ILs) base imidazólio para síntese de carbonato cíclico. O trabalho demonstra que a presença de um grupo nucleófilo no sistema catalítico pode melhorar seu desempenho, através da utilização de um IL com ânion halogenado ou por mistura de co-catalisador halogenado com ILs. A última abordagem permitiu a obtenção de um sistema de captura de CO2 eficaz, constituída por IL fluorado mais ZnBr2 que realiza a síntese de carbonato cíclico com 90% de rendimento e 82% de seletividade.


Introduction
The increase of CO 2 concentration in the atmosphere and, consequently, the intensification of global warming are posing major problems to the environment.2][3][4][5] Among the possibilities, are the CO 2 capture and geological storage and CO 2 capture and subsequent conversion in products with higher added value.
The chemical transformation of CO 2 is one of the most interesting options for CO 2 mitigation.The synthesis of cyclic carbonates appears as an interesting and effective solution.][8][9] However, CO 2 is a thermodynamically stable compound and the development of an effective and selective catalytic system is crucial.A catalytic system that could performe cyclic carbonates syntheses under mild conditions still remains as major challenge allied to the difficulties faced for catalyst separation and recycle. 8,90][11][12] Ionic liquids are very versatile compounds.][15][16] Peng and Deng 10 reported the synthesis of propylene carbonate from CO 2 using the ILs [bpy][BF 4 ], [bmim]  [BF 4 ], [bmim][Cl] and [bmim] [PF 6 ] and demonstrated that the catalytic activity of an ionic liquid depends on the nature of the cation and the anion.For the anions, the activity decreased in the order bmim + > bpy + and BF 4 − > Cl − > PF 6 − .The effect of the cation and the anion were also investigated by Yang et al. 9 using IL based on cations C 4 DABCO + , C 8 DABCO + , HDBU + , HTBD + or HHMTA + and anions OH − , Cl − , Br − , BF 4 − , PF 6 − , Tf 2 N − and AcO − .The results showed that the catalytic effect decreases in the order HDBU + > HTBD + ~ OMIM + > C4DABCO + ~ C8DABCO + > BMIM + > HHMTA + .The anion OH − presented the best result and Cl − and Br − showed good performance as well.The acetate anion bound to the cation HDBU also showed catalytic activity comparable to that of Cl -, but its thermal stability was not ideal.The anions Tf 2 N − , PF 6 − , and BF 4 − were not effective.The effect of alkyl chain length was studied by Kawanami et al., 6 in the 1-alkyl-3-methylimidazolium through variations of the alkyl chain length from C 2 to C 8 .The results indicated that the increase of the side alkyl chain in the cation has large effect on the catalyst performance since the yield has been improved with the increasing of the alkyl chain length.This result can be attributed to the increasing solubility of both the epoxide and the CO 2 in the ionic liquid.Further studies showed that the combination of ionic liquids and metallic compounds has proven to be effective, increasing the catalytic activity of ILs in the synthesis of cyclic. 11,15egarding CO 2 capture, the imidazolium cations allied to fluoroalkyl anions stand as very effective systems for CO 2 absorption. 17,18Nevertheless, in relation to the reaction of CO 2 conversion in cyclic carbonates, the results reported in the literature are contradictory in respect to the activity of ionic liquids combined to fluoroalkyl anions.
This study aims to investigate the ionic liquids behavior in both catalytic activity in propylene carbonate syntheses as well as the CO 2 absorption capacity.The reaction conditions as temperature, pressure, reaction time, effect of IL cation and anion as well as the addition of metallic compounds to reaction media and the catalyst recycling were also investigated.

Ionic liquids syntheses
The ionic liquids 1-butyl-3-methylimidazolium chloride ][21] The ionic liquids were characterized by Fourier transform infrared spectroscopy (FTIR), using a Perkin-Elmer spectrophotometer model Spectrum 100 FT-IR with full attenuated reflectance accessory (ATR), as well as by proton nuclear magnetic resonance ( 1 H-NMR) on a Varian Spectrophotometer, model VNMRS 300 MHz, using DMSO-d 6 as solvent and glass tubes of 5 mm in diameter and are in accordance with the literature.

Cycloaddition reaction
The syntheses of propylene carbonate (PC) from CO 2 and propylene oxide (PO) were carried out in the presence of imidazolium cation-based ILs combined to different anions Cl − , BF 4 − and Tf 2 N − .These compounds were tested in the presence of metallic halides (ZnCl 2 or ZnBr 2 ).The use of IL [dmbmim][Tf 2 N] was also evaluated.
All cycloaddition reactions were performed in a stainless steel autoclave of 120 cm 3 equipped with magnetic stirring.For a typical reaction, it was used 100 mmol of propylene oxide, 0.625 mmol of metal salts (ZnCl 2 or ZnBr 2 ) and/or 2.5 mmol of ionic liquid.The syntheses were performed without any additional solvent.The autoclave was pressurized with CO 2 and heated to the desired working temperature.After the reaction completion, the reactor was cooled to room temperature and slowly depressurized.
The separation of the catalyst from propylene carbonate was performed by a simple distillation under inert atmosphere (N 2 ).In the reactions carried out in the presence of metal salts, a filtration step was performed before the distillation process.The resulting liquid mixtures were analyzed using a gas-chromatograph Shimadzu GC-14B equipped with a flame ionization detector (FID) and a DB-5HT column (15 m × 0.32 mm × 0.10 µm) using acetophenone as internal standard and dimethyl ether as solvent.

CO 2 absorption measurements
The CO 2 sorption of the samples were gravimetrically assessed in a magnetic suspension balance (MSB), (Rubotherm Prazisionsmesstechnik GmbH, 35 MPa and 673.15 K) equipped with a single sinker device for absorbate density determination and thermostatized with an oil bath (Julabo F25 ± 273.16 K).The apparatus details are well described in literature. 22,23When compared to other gravimetrical sorption methods, the MSB device has the advantage of allowing high pressure sorption measurements since the sample can be potted into a closed chamber coupled to an external accurate balance (accuracy of ± 10 µg).The samples (0.06 to 0.09 g) were weighed and transferred to the MSB sample container and the system was subjected to a 10 −7 MPa vacuum at the temperature of the sorption measurement, 298.15 K, for 24 h.For all tests, constant weight was achieved in this time.The CO 2 was admitted into the MSB pressure chamber till the desired pressure, 0.1-3 MPa in this study, pressure gauges with an accuracy of 0.01 bar.The solubility of CO 2 in the ILs pressure was measured 3 to 4 h until there was no more weight variation for CO 2 sorption.At this step of solubility of CO 2 in the ILs, the weight reading from the microbalance at pressure P and temperature T is recorded as w t (P,T).After each sorption test, CO 2 desorption were performed and all the samples returned to its original weight at the end.The mass of dissolved CO 2 in the ILs (w g ), was calculated using the equation 1.
where W t (P,T) is the weight of the sample container, ρ(P,T) CO 2 density, directly measured with the MSB coupled single-sinker device, being not necessary the application of any state equation to calculate the CO 2 sorption values.
V sc (T) is the volume of the sample container, determined from a buoyancy experiment when no sample is charged into the sample container, V s (T) is the original volume of the sample, and W s (vac,T) is the weight of the sample under vacuum.The term ρ(P,T)⋅(V sc (T) + V s (T), represents the buoyancy force.The results of CO 2 sorption were expressed in molar fraction.

Results and Discussion
Effect of IL and co-catalyst Aiming to evaluate the influence of the anion in the syntheses of propylene carbonate (PC) from CO 2 and propylene oxide (PO), cycloaddition reactions were carried out in the presence of ionic liquids (ILs) based on imidazolium cation combined to Cl − , BF 4 − and Tf 2 N − anions.The first choice for assessing the ILs catalytic activity for CO 2 conversion reactions were [bmim][Tf 2 N] and [bmim][BF 4 ], since these compounds are reported in the literature as good solvents for carbon dioxide. 17,24The results depicted in Table 1 show that these ILs exhibited extremely low activities in the cyclic carbonates syntheses.Despite the good ability for CO 2 absorption presented by Tf 2 N − and BF 4 − anions their catalytic activities were not satisfactory (entry 1 and 7).
On the other hand, the IL [bmim][Cl] presented an interesting catalytic performance presenting an yield of 84% and 90% of selectivity in PC and a turnover frequency (TOF) of 5 h −1 (entry 6) as seen in Table 1.This behavior is probably due to its good nucleophilic character. 9These results demonstrate that the activity of ILs is strongly influenced by the nature of the anion and are in accordance with the results described by Yang et al. 9 in relation to the anions behavior.The low catalytic activity for Tf 2 N − and BF 4 − anions can be directly linked to the structural volume of the anions and their low nucleophilicity indicating that the best ILs for CO 2 capture are not necessarily the best catalysts for CO 2 transformation.However, the combination of these ILs with metallic compounds contributed to an increase on the catalytic activity.
The results of the combination of metal compounds to [bmim][Tf 2 N] and [bmim][BF 4 ] revealed that the addition of ZnBr 2 contributed to an increase in catalytic activity for both ILs (entry 3-4; 9, Table1), indicating that the presence of Lewis acidic compounds as cocatalysts greatly enhances the activity of ionic liquid for the cyclic carbonate syntheses. 25With the exception of [bmim][Cl], the catalytic activities of ionic liquids were quite influenced by the nature of halide bonded to zinc atoms.The reactivity was found to be in the order ZnBr 2 > ZnCl 2 as seen in Table 1, suggesting the importance of the nucleophilicity of halide counterions.
For example, when [bmim][Tf 2 N] was used as catalyst, the addition of ZnCl 2 the yield of the carbonation reaction was 19% and the selectivity 60%.When ZnBr 2 was added, the yield increased to 90% and the selectivity to 82%.
This behavior is probably due to the high reactivity of Zn(II) combined with the nucleophilicity of bromide, 7,26 which has favored the propylene oxide ring-opening.Although the activity of ZnBr 2 containing systems is higher than those with ZnCl 2 , the latter is commonly chosen as the cocatalyst for the coupling of carbon dioxide with epoxides because it is the cheapest Zn(II) salt and has satisfactory activity. 7However, when bmimCl was used as catalyst, the metallic halides contribution were not significant (entry 8).In this case the yield, selectivity and activity remained almost constant.
We also performed cycloaddition reactions only with the metals compounds without IL.No activity was observed with ZnCl 2 (entry 11) or ZnBr 2 (entry 12) according to previous studies. 7,11,25he effect of the branching in the alkyl chain of imidazolium cation was also evaluated.The IL [dmbmim][Tf 2 N] alone, as well as its combination with ZnBr 2 , presented low activities in the PC syntheses (entry 2 and 5, Table1).It was expected that the branching on the alkyl lateral chain would increase the CO 2 solubility resulting in a higher activity.The alkyl side branching increases the free volume and consequently improves the CO 2 solubility in the ionic liquid. 17Probably, the steric hindering caused by the branching in the alkyl side chain interfered with the approach of the substrate/catalyst, difficulting the reaction.
Aiming to increase the catalytic activity, an equimolar mixture of the catalytic ILs [bmim][Tf 2 N] + [bmim][Cl] and the metallic compound ZnBr 2 has been evaluated (entry 10).However, the yield of this system was lower than that attained separately for each IL in the presence of ZnBr 2 (entry 4-8).
One of the acceptable mechanisms for the cycloaddition of CO 2 to epoxide catalysed by IL involves a nucleophilic attack of the IL anion to the less hindered carbon atom of the epoxide ring.In the next step, an oxy anion species is formed.The carbon atom of the CO 2 interacts with the anion species producing an anion alkylcarbonate converted to the cyclic carbonate by intermolecular cyclic elimination. 15n our work, we concluded that the Tf 2 N -and BF 4

−
containing ILs need a metal halide, a Lewis acid to interact with the oxygen of the epoxide atom allowing the IL basic anion to perform the attack to the less hindered atom of the epoxide ring, see Scheme 1a.This could be assigned to the non-nucleophilic natures of the Tf 2 N -and BF 4 − anions. 27When [bmim][Cl] is used as catalyst, probably the attack of the epoxide is performed by the Cl − in the less hindered carbon atom, in the first step of the reaction, see Scheme 1b.
As we can see in Table 1, the IL [bmim][Tf 2 N] combined with ZnBr 2 showed the best yield (90%) (entry 4).Thus, it has been selected for the evaluation of temperature; pressure, reaction time as well as the behavior of this system in recycle.

Effect of temperature
The selectivity, yield and activities values obtained at different temperatures are shown in Table 2.It is evidenced that the increase in temperature from 373.15 K to 383.15 K resulted in a significant increase in the yield (entry 13-4), and after 383.15K the increasing of the temperature decreases the yield (entry 4-16).It is possible that the yield decreasing is associated with the reduction in the PC selectivity in higher temperature, resulting in side reactions such as PO isomerization and PC polymerization. 28

Influence of CO 2 pressure
Pressure affects cyclic carbonate synthesis, and the optimal pressure occurs at around the CO 2 critical pressure. 12From the results shown in Table 3, it can be seen that our results showed a different behavior.The results shown in Table 3 are in accordance to the literature, evidencing that an insufficient or excessive amount of CO 2 results in relatively low conversions, i.e., there is an ideal molar ratio of propylene oxide and CO 2 to obtain a high conversion.A higher CO 2 pressure can retard the interaction between epoxide and the catalyst.On the other side, it can be highlighted a significantly improve in the carbonate selectivity with the pressure augmentation. 9,10fluence of the reaction time The influence of the reaction time on the propylene carbonate synthesis is presented in the Table 4.It was observed that the reaction time reduction from 6 up to 2 h (entry 4-19), provided a decreases in yield, curiously the increase in reaction time from 6 to 8 h (entry 4-21) decreases the yield.The reaction time of 6 h was ideal for this the system.

Recycling
To investigate the reuse effect in the catalyst system ([bmim][Tf 2 N] and ZnBr 2 ) the recycling experiment was conducted at the best conditions achieved (4.0 MPa, 383.15K and 6 h).The procedure adopted for the separation of the catalytic system and propylene carbonate was the same used for all syntheses and described in experimental section.This procedure was repeated four times.The catalytic activity was not maintained even for only four recycle reactions.This behavior is probably due to the loss of the metallic compound (ZnBr 2 ) in the PC separation operation, decreasing the catalytic activity. 7iming to study the ZnBr 2 loss in PC recycle, reactions were carried out in the same conditions of the recycle reactions presented before.After the PC separation step the ZnBr 2 was re-added (0.625 mmol) and the reaction performed.With the addition of the ZnBr 2 after each reaction the yield increased 17% when compared to the reaction without the addition of the metal halide and remained almost constant.These results evidenced that the loss of the ZnBr 2 is probably occurring as described in literature. 7n order to improve the separation process and also study the influence of the increase of IL concentration in the reaction medium, a reaction with 100 mmol of PO, 28 mmol of [bmim][Tf 2 N], 0.625 mmol de ZnBr 2 , in the same reaction conditions (383.15 K, 6 h and 4 MPa of CO 2 ) was performed.The yield, selectivity and activity were the same obtained with 2.5 mmol of IL.The increment of IL content in the reaction medium, in this case, does not facilitate the PC separation.These results evidenced that the catalytic conversion of propylene oxide and CO 2 in PC works with catalytic quantities of IL but for integration of CO 2 capture and transformations process it is possible to use a higher IL volume and maintain the same yield.The results shown for PC syntheses using ILs as catalysts evidenced the importance of the anion.For CO 2 capture also the anion plays a major role.In order to combine the CO 2 capture and conversion, it is important to find a system that can act both as a solvent for CO 2 separation and as a catalyst for CO 2 transformation.In Figure 1 it can be seen the different CO 2 absorption capacity for the ILs that proved to perform also the role as a catalyst for propylene carbonate syntheses.
As it can be seen from Figure 1, the use of [bmim][Cl] − for CO 2 capture is not a good option, despite of this IL act as a good catalyst for PC synthesis.The CO 2 sorption at 0.5 MPa is near 0 mol% stead for the combination of [bmim][Tf 2 N] and ZnBr 2 it is approximately 14 mol%.The fluoroalkyl anions allied to imidazolium cations present a relatively high CO 2 solubility and have been described in the literature for CO 2 absorption process. 17The use of these compounds for PC syntheses needs the addition of a metallic compound in order to achieve an acceptable yield.It must be mentioned that the best conditions for CO 2 capture and chemical conversion are distinct.Nevertheless, the results presented in Figure 1 evidenced that the CO 2 solubility in a solvent combining [bmim][Tf 2 N] and ZnBr 2 can be a good option for CO 2 capture and conversion of this molecule in a PC adding value to this chemical.

Conclusions
The results showed that the best ILs for capturing CO 2 are not necessarily the best catalysts for CO 2 conversion.The activities of the ILs in both processes are strongly influenced by the nature of the anion.While the solubility of CO 2 is improved by fluorinated anions in catalysis for CO 2 conversion, the activity is favored by a higher nucleophilicity of the anion.Our results evidenced that the combination of imidazolium-based ILs/fluoroalkyl anions and a metal halide compound is a good approach for combining the CO 2 capture and conversion.In catalytic conversion of CO 2 to carbonates, the temperature, pressure and reaction time showed to play an important role in the conversion and selectivity.The best result was obtained with the catalytic system [bmim][Tf 2 N]/ZnBr 2 at 4.0 MPa of pressure, 383.15K of temperature and 6 h of reaction time.

Scheme 1 .
Scheme 1. Proposed mechanism for the synthesis of propylene carbonate from propylene oxide and CO 2 using different ionic liquids: (a) IL combined to the anions Tf 2 N − and BF 4 − and (b) [bmim][Cl].

Table 2 .
Temperature effect in the PC synthesis using [bmim][Tf 2 N] and ZnBr 2 as catalyst

Table 3 .
CO 2 effect pressure in the PC synthesis using bmimTf 2 N +ZnBr 2