Fixation and conversion of CO2 using ionic liquids

of en , R ng T es, y of epa hi, e 2 f ca their unique properties such as non-volatility, tunable polarity, high stability and so on. In this work, the latest progress on the fixation and conversion of carbon dioxide (CO2) using ILs as absorbents, catalysts or promoters has been summarized. The absorption performance of Catalysis Today 115 (20 conventional ILs and task-specific ILs was systematically investigated, the conversion of CO2 with epoxides, propargyl alcohols and amines using ILs was critically evaluated, and the significant advantages in the fixation and conversion of CO2 using the ILs were demonstrated compared to the conventional absorbents and the catalytic systems without ILs. This research progress may finally lead to building of an in situ fixation–conversion process of CO2 with ILs. If so, we are near an epoch of the fixation and utilization of CO2, although there is obviously a long way to go for us to achieve such a goal. # 2006 Elsevier B.V. All rights reserved. Keywords: CO2; Ionic liquids; Fixation; Conversion; Absorption; Reaction 1. Introduction Carbon dioxide (CO2) produced by combustion of fossil fuels is regarded as the most significant greenhouse gas; the increasingly accumulation of CO2 in the atmosphere has attracted worldwide attention. On the other hand, CO2 is one of the most naturally abundant, inexpensive, non-flammable and non-toxic C1 resources. Recalling the history of chemical industry, we can find many applications of CO2 in the production of valuable products and materials such as carbonated drinks, urea, polycarbonates and so on [1–5]. In order to utilize CO2 as C1 feedstock or sequestrate CO2 for reduction of greenhouse effect, the investigation of efficient methods for capturing CO2 from flue gas, in which CO2 concentration varies from 3 to 14%, is critically important. One of the most commercially applied technologies is the chemical absorption of CO2 by aqueous amines [6]. This technology, however, has shown serious disadvantages, such as the uptake of water into gas stream requires additional drying process and causes serious corrosion. The loss of volatile amines increases the operation cost and other difficulties and the evaporation of water for the release of CO2 upon heating requires excessive cost of energy. The amines used for post-combustion CO2 separation also are known to decompose, causing an environ- mental problem due to waste. Therefore, a novel solvent that could facilitate the separation of CO2 from gas mixtures without concurrent loss of the capture solvent into the gas stream is highly required. In this regard, ionic liquids (ILs) show great potential as an alternative for such applications [7]. Ionic liquids are a kind of novel medium composed entirely of ions. Some typical cation/anion combinations comprising the main types of ILs are listed in Scheme 1. In recent years, significant progress has been made in the application of ILs as alternative solvents and catalysts due to their unique properties such as negligible vapor pressure, a broad range of liquid temperatures, excellent thermal and chemical stabilities, tunable physicochemical characteristics and selective di- ssolution of certain organic and inorganic materials [8–11]. * Corresponding author. Tel.: +86 10 82627080; fax: +86 10 82627080. E-mail address: sjzhang@home.ipe.ac.cn (S. Zhang). 0920-5861/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2006.02.021 Fixation and conversion Suojiang Zhang a,*, Yuhuan Ch Wenbin Dai c a Research Laboratory of Chemical Engineeri Chinese Academy of Scienc b Graduate School of Chinese Academ c SCF Solution Group, Business Incubation D 1002-14, Mukohyama, Naka-s Available onlin Abstract Ionic liquids (ILs), a kind of novel green media composed entirely o CO2 using ionic liquids a,b, Fuwei Li a, Xingmei Lu a, yohei Mori c echnology, Institute of Process Engineering, Beijing 100080, PR China Sciences, Beijing 100039, PR China rtment, Mitsubishi Materials Corporation, Ibaraki-ken 311-0102, Japan 9 March 2006 tions and anions, have recently attracted considerable attention due to www.elsevier.com/locate/cattod 06) 61–69 is Today 115 (2006) 61–69 een conventionally used ionic liquids. These unique features of macroscopic properties are essentially determined by the specific microstructures and interactions of S. Zhang et al. / Catalys62 Scheme 2. Hydrogen bond formed betw Scheme 1. The constituents of ILs. Both spectroscopic investigation and quantum computa- tions discovered the existence of hydrogen bonds between ionic pairs [12–16], and more interesting is the recent work discovering the hydrogen bond network in ILs as shown in Schemes 2 and 3 [17]. Obviously, these studies are helpful for us to understand the structure–property relationship of ILs, and will essentially lead to the rational design of functional ILs. If so, it can be expected that the exponential increase of publications on the fundamental and application studies of ILs will be continued as shown in Fig. 1. The neglectable volatility of ILs results in a non- contaminated target gas and makes it especially fascinating in absorption of CO2. More interesting is that CO2 can significantly dissolve in the ILs as compared to conventional organic solvents even in the case of physical absorption [18,19]. Such higher solubilities show the great potential of ILs as not only good absorbents for CO2 capture but also good solvents or catalysts for CO2 reacting with other compounds such as epoxides, because the higher concentration of CO2 in the ILs phase is a substantially positive factor for promoting the reaction of CO2. Based on the above-mentioned merits of ILs as both absorbents and reaction media, we can further draw a fascinating picture like Fig. 2, which draws an in situ fixation– conversion coupling process of CO2 in ILs powered by sunlight. Although there is obviously a long way to go for us to achieve such a dream or goal, the rapid progress on the fixation and conversion of CO2 using ILs seems to be continuously validating the truth of our dream. In the limited pages of this paper we are trying to summarize these latest research results, especially focusing on the fixation/absorption of CO2 in two chloride anion and imidazolium cation. classes of ILs, namely conventional ILs and task-specific ILs, and the conversion of CO2 to valuable carbonyl-containing compounds via the reaction between CO2 and epoxides, Scheme 3. The hydrogen bond network of [emim]Cl ((a, c and e) Cl� anions; (b, d and f) [emim]+ cations). alcohols and amines using ILs as reaction solvents and/or cations, and the anions such as BF4 �, PF6 �, Tf2N �, NO3 � and EtSO4 �. Blanchard et al. [20] determined the solubility of CO2 in a series of imidazolium-type ILs including 1-n- butyl-3-methylimidazolium hexafluorophosphate ([bmim]- S. Zhang et al. / Catalysis Today 115 (2006) 61–69 63 Fig. 1. Publications on the ionic liquids from 1994 to 2003. Fig. 3. Relationship between the solubility of CO2 and pressure in six kinds of ionic liquids [20]. catalysts or promoters. 2. Fixation of CO2 using ionic liquids A number of investigations have shown that CO2 is remarkably soluble in ILs. According to the structural features and fixation/absorption mechanisms, the ILs can be classified into two categories, conventional ILs and task-specific ILs. The conventional ILs could absorb/fix less amount of CO2 because of the physical interactions between CO2 and ILs. The task-specific ILs with alkaline groups could sequester larger amount of CO2 than that of conventional ILs because of the chemical interactions or reactivities between CO2 and alkaline groups of ILs. 2.1. Fixation of CO2 using conventional ionic liquids There are several reported works on the fixation/absorption of CO2 in conventional imidazolium-type ILs [19–24], which are composed of 1-alkyl-3-methylimidazolium ([rmim]+) Fig. 2. Proposed integrative fixation–conversion process of CO2 in ionic liquids. PF6), 1-n-octyl-3-methylimidazolium hexafluorophosphate ([omim]PF6), 1-n-octyl-3-methylimidazolium tetrafluorobo- rate ([omim]BF4), 1-n-butyl-3-methyl imidazolium nitrate ([bmim]NO3), 1-ethyl-3-methylimidazolium ethyl-sulfate ([emim]EtSO4) and n-butylpyridinium tetrafluoroborate ([N-bupy]BF4) in the pressure region from 0.1 to 10 MPa. The solubility data at 313 K are presented in Fig. 3 [20]; it can be seen that the solubility of CO2 follows the sequence of [bmim]PF6/[omim]PF6 > [omim]BF4 > [N-bupy]BF4 > [bmim]NO3 > [emim]EtSO4. Cadena et al. [22] studied the mechanism of CO2 dissolution in imidazolium-type ILs by experimental and molecular modeling, and found that the anions have larger impact on the solubility of CO2. Kazarian Fig. 4. Image of the equilibrium state of CO2 in [bmim]PF6 by molecular dynamic simulation. 2.2. Fixation of CO2 using task-specific ionic liquids Considering the very limited capability of the conventional ILs in the absorption/fixation of CO2, it is essentially necessary to explore novel ILs with the specific function for absorption/ fixation of CO2. Due to the unique ‘‘self-designable’’ characteristics of ILs, alkaline group such as –NH2 can be is Today 115 (2006) 61–69 Fig. 5. Phosphonium-amino acids ionic liquids: (I) [P(C4)4]-L-Gln; (II) [P(C4)4]-L-Asn; (III) [P(C4)4]-L-b-Ala; (IV) [P(C4)4]-Gly; (V) [P(C4)4]-L-Ser; (VI) [P(C4)4]-L-Ala; (VII) [P(C4)4]-L-Lys; (VIII) [P(C4)4]-L-Trp; (IX) [P(C4)4]- L-Tyr; (X) [P(C4)4]-L-Thr; (XI) [P(C4)4]-L-Val; (XII) [P(C4)4]-L-Pro; (XIII) [P(C4)4]4-L-Arg; (XIV) [P(C4)4]-L-His; (XV) [P(C4)4]-L-Glu; (XVI) [P(C4)4]- L-Ile; (XVII) [P(C4)4]-L-Met; (XVIII) [P(C4)4]-L-Cys; (XIX) [P(C4)4]-L-Leu; Scheme 4. Proposed reaction mechanism between [pabim]BF4 and CO2. et al. [25] found that there was evidence of a weak Lewis acid– base interaction between CO2 and PF6 � or BF4 � anions using ATR-IR spectroscopy. The image of the equilibrium state of CO2 in [bmim]PF6 is shown in Fig. 4 by molecular dynamic simulation carried out in our laboratory. The simulations were performed at 298 K and 1 atm for a system composed of 192 molecules of [bmim]PF6 and 10 molecules of CO2 with the standard periodical boundary conditions. In the obtained equilibrium state it can be seen that the molecules of CO2 disperse well in the [bmim]PF6 ILs. These microscopic studies provide valuable information for understanding the solubility behavior of CO2 in the conventional imidazolium-type ILs. The Henry’s constants show higher solubilities of CO2 in the ILs compared to conventional organic solvents, for example, the Henry’s constant at 298.15 K is 5.34 MPa in [bmim]PF6 [19], while it is 8.43 MPa in heptane, 13.33 MPa in cyclohexane, 10.41 MPa in benzene and 15.92 MPa in ethanol, respectively [26]. The relatively higher solubility of CO2 in imidazolium-type ILs is due to the activity of 2-H in imidazolium ring [22,27]. More interesting are the very small excess volumes for CO2 dissolution in the ILs compared to that in conventional organic solvents, although there is a relatively larger solubility of CO2 in the ILs than that in conventional organic solvents. For example, the liquid phase composition of 0.69 mole fraction CO2 in [bmim]PF6 produces a mere 18% volume increase over the pure IL, whereas a liquid phase composition of 0.740 mole fraction CO2 in toluene gives a 134% volume increase over the pure solvent. Brennecke and co-workers looked for insight into the solubility of CO2 in a series of imidazolium-type ionic liquids [19,21,28]. According to the different solubilities of CO2 and methane, ILs have potential to be utilized in separation of CO2 from natural gas. Anthony et al. [20] found that CO2 solubility in [rmim]PF6 is much higher than other gases such as CO, CH4, H2, N2 and CH3CH3, which suggested that [rmim]PF6 might be potentially applied as absorbent from separation of CO2 from a coal steam gas mixture. Recently there are some reported works on the solubility of CO2 in sulfonate ILs. Zhang et al. [29] determined the solubility of CO2 in trihexyl (tetradecyl) phosphonium dodecylbenzene- sulfonate ([P666,14]C12H25PhSO3) and trihexyl (tetradecyl) phosphonium methylsulfonate ([P666,14]MeSO3) at tempera- tures ranging from 305 to 325 K and the pressures ranging from 2 to 9 MPa. At a given temperature, the magnitude of Henry’s constants for CO2 follows the sequence of [P666,14]MePh- SO3 > [P666,14]MeSO3 > [bmim]BF4, which indicates that the solubility of CO2 in sulfonate ILs is generally lower than that in imidazolium-type ILs. In general, the absorption/fixation of CO2 in the conven- tional ILs such as imidazolium-type ILs and sulfonate ILs is very limited because of its physical nature of interactions, although it is relatively higher than the solubility of CO2 in the conventional organic solvents such as heptane, ethanol, benzene and so on. The equilibrium solubility of CO2 in these conventional ILs is about 0.10–0.15 wt% at room temperature and atmospheric pressure, which is obviously too low for S. Zhang et al. / Catalys64 industrial application for CO2 capture. attached to the structure of cations or anions of ILs while still keeping the merits of the ILs [30]. The designed task-specific ILs obviously can break the limitation of the conventional ILs and tackle the disadvantages of the commercially applied absorbents such as aqueous amines. Bates et al. [30] reported a task-specific IL, 1-n-propyla- mine-3-butylimidazolium tetrafluoroborate ([pabim]BF4), for CO2 capture, the saturated concentration of CO2 in [pabim]BF4 reaches a level of 7.4 wt%. The proposed reaction mechanism is shown in Scheme 4 [30], which is basically the same as that for the amines currently used as CO2 absorbents. CO2 molecule attacks the free electron-pair of N atom and forms a new COO� group, simultaneously the NH2 group of another [pabim] + accepts one H+ and becomes –NH3 + group, which accounts for the saturation molar ratio of 1:2 between CO2 and [pabim]BF4. Recently, Zhang et al. [31] reported a new kind of task- specific ILs, tetrabutylphosphonium amino acids ([P(C4)4]AA). Tetrabutylphosphonium bromide [P(C4)4]Br was transformed into tetrabutylphosphonium hydroxide [P(C4)4]OH by anion exchange resin and neutralized by amino acids such as glycine, L-alatine, L-b-alatine, L-serine and L-lysine to produce [P(C4)4]AA. The photographic image of the synthesized 20 kinds of [P(C4)4]AA ILs is presented in Fig. 5. To increase the (XX) [P(C4)4]-L-Phe. absorption/fixation rate of CO2 in these highly viscous ILs, the [P(C4)4]AA ILs were coated on porous silica gel to form a thin However, in the presence of small amount of water, the reaction between CO2 and H2O is very complex. For example, the –NH2 group can catalyze the formation of bicarbonate. CO2 and H2O react to form H2CO3 and HCO3 � and the H+ proton resourced from the deprotonation of H2CO3 to HCO3 � attacks the free electron-pair of NH2 group and forms –NH3 + group as shown in Scheme 5. Therefore, one molecule of [P(C4)4]AA can absorb oneCO2molecule, which accounts for the absorptionmolar ratio of 1:1 between CO2 and [P(C4)4]AA in the presence of water. It is worthwhile to notice that not all the ILs containing – NH2 group can absorb/fix CO2 effectively. Our experimental studies [32] showed the guanidine ILs, e.g., 1,1,3,3-tetra- methylguanidium lactate (TMGL), can only absorb/fix 0.25 wt% CO2, which is much lower than the expected amount according to the absorption molar ratio of 1:2 between CO2 and –NH2 group if it follows the same mechanism as [pabim]BF4 and [P(C4)4]AA. The underlying reason is the large FMO energy gap (9.53 eV) between HOMO-5 of TMGL and LUMO of CO2, which is much larger than the energy gap (6.07 eV) S. Zhang et al. / Catalysis Today 115 (2006) 61–69 65 Scheme 6. Proposed adsorption mechanism between [P(C4)4]b-Ala and CO2 without water. Scheme 5. Proposed adsorption mechanism between [P(C4)4]Gly and CO2 in the presence of water. film, four cycles of sorption–desorption proved their stable, fast and reversible behavior comparing to bubbling CO2 through bulk ILs which usually takes more than 3 h. The saturated molar ratio between CO2 and [P(C4)4]AA reached a level of 1:2 at room temperature and atmospheric pressure. Interestingly, in the presence of small amount of water, the [P(C4)4]AA ILs could adsorb equal molar amounts of CO2, i.e., the absorption/ fixation capability of these ILs was double that in the case of no water. Spectroscopic investigations suggested differences in the absorption mechanism with or without water as shown in Schemes 5 and 6, respectively. As shown in Scheme 6, in the case of no water, CO2 molecule attacks the free electron-pair of N atom and forms a new COOH group which constructs a hydrogen bond O���H���N with the NH2 group of another AA�. The hydrogen bond partly occupies the free electron-pair of the N atom and makes it inert to reaction with CO2; therefore, the saturated molar ratio is 1:2 between CO2 and –NH2 groups. Fig. 6. Comparison of the HOMO and LUMO energies for [pabi between HOMO of [pabim]BF4 and LUMO of CO2 as shown in Fig. 6. It is the carbocation that lowers the HOMO-5 energy of TMGL and weakens its nucleophilicity; as a result, TMGL cannot effectively interact with CO2. 3. Conversion of CO2 using ILs 3.1. Conversion of CO2 with epoxides using ILs as catalysts One of the most promising technologies in the utilization of CO2 is the cycloaddition between epoxides and CO2 to produce five-membered cyclic carbonates as shown in Scheme 7, which are excellent aprotic polar solvents and intermediates extensively applied in the production of a variety of indispensable products such as pharmaceuticals, fine chemicals and so on (Scheme 8). m]BF4, GTML and CO2 at B3LYP/6-31G** theory of level. advantages over the conventional catalysts such as high catalytic efficiency (TOF), mild reaction conditions, non-toxic reagents and recycling of the ILs catalysts, although the cylcoaddition between propylene oxide and CO2 could not be effectively catalyzed by using [bmim]BF4 solely [45]. The catalytic system comprised of zinc chloride (ZnCl2) and 1-butyl-3-methylimidazolium bromide ([bmim]Br) achieved 95% yield, >98% selectivity and 5410 h�1 TOF under mild reaction conditions without any cosolvents [46], and it could be reused for five times with a little loss of catalytic activity. Also, the ZnCl2/[bmim]Br catalyst showed excellent activity and selectivity for a variety of other epoxides listed in Table 2 [46]. Interestingly, the cis-stereochemistry cyclic carbonate was S. Zhang et al. / Catalysis Today 115 (2006) 61–6966 Table 2 The yield and efficiency of the reaction between CO2 and various epoxides reactions catalyzed by ZnCl2/[bmim]Br Substrate Product Yield (%) TOF (h�1) 95 4887 95 3332 Scheme 7. A variety of catalysts such as alkali metal halides [33–36], metal oxides [37–39] and metal complexes [40–44] have been intensively studied for this kind of reaction; there are however a number of disadvantages such as low catalytic activity, severe reaction conditions, difficult recycling of the catalysts and so on. As one of the alternative approaches for tackling these problems, ILs have been investigated as the catalysts or promoters for this kind of reaction. Herein we present some typical examples to demonstrate the performance of ILs in the cycloaddition reactions. In Table 1, a comparison of the ILs catalytic systems and the conventional catalyst systems for synthesis of propylene carbonate are presented. It can be seen that the ILs cata- lytic systems such as ZnCl2/[bmim]Br, SalenAl/TBAI and Ni(PPh3)2Cl2/Zn/TAB