*3.6. Application of Bifunctional (or Multifunctional) Onium Salts*

The functionalization of ILs involves an increase in catalytic activity owing the synergistic effect between the bound functional groups (nucleophilic iodide or bromide anions together with –NH2, –COOH or –OH groups in the role of HBDs). The reached synergism enables a decrease in the quantity of the multifunctional catalyst and simpler separation in an optimal case [93] (Table 22, Figure 12). On the other hand, multifunctional ILs are more difficult to prepare and more expensive due to this reason.

**Table 22.** Carbonation of butylene oxide (BO) using bifunctional phosphonium salts and onium salts [93].

**Figure 12.** Structure of the most active bifunctional phosphonium salt tri-n-butyl-(2-hydroxyethyl) phosphonium iodide [93].

Bifunctional catalysis based on aromatic OH-functionalized onium salt was described by Tsutsumi et al. [94]. This research work demonstrates that it still possible to discover a very active and quite cost-effective bifunctional catalyst such as m-trimethylammonium phenolate, which is much more effective than more structurally complicated ones [94]. This catalyst was only tested, however, for the carbonation of terminal epoxides and still requires higher pressure of CO2 at an elevated temperature [94] (Scheme 10, Table 23).

**Scheme 10.** CO2 activation by ammonium betaine organocatalyst 3-trimethylammonium phenolate [94].

**Table 23.** Comparison of catalytic action of different substituted phenols on carbonation of hexylene oxide [94].


Meng et al. published very promising results obtained by testing a series of OHfunctionalized DBU-based ILs (Figure 13) [95]. They discovered not only a recyclable organocatalyst with excellent activity for carbonation of EPIC at 30 ◦C and an ambient pressure of CO2, but also a compound suitable for the utilization of CO2 from a simulated flue gas (Scheme 11). Its high activity is explained by the cooperative activation of the epoxide ring by protonated DBU in the role of HDB and the activation of CO2 via carbonate formation utilizing alcoholate on a bridge-functionalized bis-DBU anion.

**Figure 13.** Structure of most effective OH-functionalized DBU-based IL [95].

**Scheme 11.** Carbonation of EPIC using simulated flue gas (15% CO2 in nitrogen) catalyzed by OH-functionalized DBU-based IL at ambient pressure and at 30 ◦C [95].

Another described group of catalytically active bifunctional catalysts consists of 1 alkyl-2-hydroxyalkyl pyrazolium salts [96]. The most active one from the broad group of tested derivatives was 1-ethyl-2-(2-hydroxy)ethylpyrazolium bromide (Figure 14). It was demonstrated that using 1 MPa pressure of CO2 at 110 ◦C, even internal epoxide ChO was carbonated to CC with a considerable yield of over 60% after 4 h of action [96].

**Figure 14.** Structure of 1-ethyl-2-(2-hydroxy)ethylpyrazolium bromide, the most effective carbonation catalyst based on 1-alkyl-2-hydroxyalkyl pyrazolium salts [96].

Zhou et al. compared the catalytic activity of quaternized aminoacid glycine (betaine) and quaternized aminoethanol salts (choline salts, Scheme 12) [97].

**Scheme 12.** Structures of tested bifunctional onium salts and scheme of preparation of betaine hydrohalides [97].

The main difference in the structures of these onium salts is the presence of a more acidic carboxylic group (a stronger HBD) in the betaine structure compared with the choline hydroxyl group. In addition, the effects of different anions in the case of protonated betaine were compared, and it was observed that the most active was the corresponding iodide salt. The catalytic activity of different betaine and choline salts decreased in the range: betaine.HI > betaine.HCl > choline.HCl > betaine.HBr > betaine.BF4 −. The authors interpret the low betaine.HBr activity as the effect of its low solubility in the reaction mixture. The tested choline.HCl possesses an activity comparable with Bu4NBr, which supports the positive effect of the hydroxyl group in the activation of the oxirane ring of PO. This positive effect could be both based on HBD action and/or the activation of CO2 on account of carbonate formation. The considerable HDB effect of the carboxylic group of protonated betaines exceeds, however, the effect of the hydroxyl group in choline. The reaction conditions for effective carbonation even of terminal epoxides are, however, harsh (8 MPa CO2, 140 ◦C) [97]. Due to the above-mentioned reasons, the research that focused on ILs functionalized with the carboxylic group(s) provided fruitful results.

Xiao et al. suggested that the influence of suitable acidity, even with the flexibility of the bound -(CH2)n-COOH chain in the IL structure, is crucial for the carbonation of epoxides due to the cooperation function of the ring-opening of oxiranes [98]. When 1-(2 carboxyethyl)-3-methylimidazolium bromide was used as bifunctional IL, the PC from PO was obtained with ca. 96% yield using pressurized CO2 (1.5 MPa) at 110 ◦C after 2 h. The IL showed high thermal stability and could be recycled with a slight loss in activity, while the selectivity of the cyclic carbonates remained at over 98%. The catalytic activity of the described functionalized IL-based carboxylic acids is still not unique enough, however, and these types of catalysts still require elevated pressure of CO2 to maintain a high conversion of epoxides to cyclic carbonates. In addition, the carbonation of internal epoxide is still quite slow even at the above-mentioned high pressure and elevated temperature [98].

The construction of bridge-functionalized bisimidazolium-based ILs improves the catalytic activity of acidic ILs, as was discovered by Kuhn et al. [99]. The most active catalyst is the most branched one, bis(imidazoyl)isobutyric acid derivative, *N*-arylated with hydrophobic mesitylene (Figure 15). It is well recyclable and active even using 0.4 MPa CO2 at 70 ◦C. It is ineffective, however, in the case of internal epoxides' carbonation [99].

**Figure 15.** Multifunctional bisimidazolium bromides tested for carbonation of epoxides [99].

An additional direction of research related to the significant increasing of catalytic activity was discovered by Han et al. [100] and Wang et al. [101]. The Han and Wang research groups recognized the crucial role of ion pairs produced by a combination of acidic ILs with guanidinium cations. Using the same acidic ILs, neutralized by alkylated guanidines, enables an increase in activity, probably due to the distinctive activation of reacting CO2. This catalytic system is active at 0.1 MPa CO2 and 30 ◦C for the carbonation of EPIC, but fails even in the case of PO (Table 24). Additionally, the used ion pairs are simply separable from the produced cyclic carbonates by means of extraction with ethyl acetate, enabling simple recycling without a significant drop in conversion.


**Table 24.** Catalytic activity of multifunctional IM-based ILs on the synthesis of CPC by carbonation of EPIC [100,101].

Bridged methylene(bis)imidazolium salts substituted on both *N* -nitrogens by carboxymethyl groups are more active after neutralization with tetramethylguanidine [101] (Figure 16, Table 24). Even this catalytic system is active at 0.1 MPa CO2 and 50 ◦C but the carbonation of internal ChO seems to be sluggish using the above-mentioned reaction conditions. Additionally, the used catalyst is simply separable from the produced cyclic carbonates and enables simple recycling without a significant drop in conversion.

**Figure 16.** Structures of multifunctional IM-based ILs tested for the carbonation of EPIC [100,101].

The reverse activation of dual amino-functionalized ILs neutralized with acidic aminoacids, such as glutamic or aspartic acids, is possible, as was documented by Yue et al. [102]. This ion-pair-based catalytic system produces, however, high yields at 0.5 MPa and a temperature of 105 ◦C after 13 h of CO2 action. It is recyclable without loss of activity and works well in the case of terminal epoxides [102] (Figure 17).

**Figure 17.** Structures of dual amino-functionalized IM-based ILs [102].

A very attractive alternative approach was published by Kumar et al. [103]. Their research was focused on the utilization of CO2 from model flue gas (5–15% CO2 in N2 stream at atmospheric pressure) at 80 ◦C using task specific AA-based ILs (Scheme 13). The authors verified that tetrabutylammonium salt with histidine dissolved in dialkyl carbonate enables the capture and usage of CO2 for the carbonation of terminal epoxides at the above-mentioned reaction conditions, with a high yield. This research work is one of the very infrequent examples of the direct capture and subsequent utilization of CO2 from (model) flue gas. The authors documented that this catalyst is recyclable with no drop in efficiency after six recycling steps. This catalytic system was proved, however, only on terminal epoxides [103].

**Scheme 13.** Carbonation of terminal epoxides using model flue gas (5–15% CO2 in nitrogen) [103].

A different approach was used for the preparation of highly catalytically active bifunctional ILs using allylation by Hui et al. [104] (Figure 18). They discovered tetramethylguanidinebased allylated IL with superior activity for the capture and utilization of CO2 from simulated flue gas at 120 ◦C, ambient pressure and solventless conditions (Table 25). This catalyst is effective even for carbonation of ChO and reusable with low loss of activity after five recycling steps [104]. This type of catalyst seems to be very attractive even for the carbonation of other internal epoxides including eventually epoxidized fatty acid esters.

**Figure 18.** Structures of functionalized ILs tested as highly active CO2 cycloaddition catalysts [104].


**Table 25.** Effect of different PILs and ILs on the carbonation of SO [71].
