Being superbasic in nature, NHCs act as nucleophiles to activate CO₂ and form the corresponding imidazolium carboxylates (NHC-CO₂), which subsequently release CO₂ to react with epoxides and afford cyclic carbonates along with the release of NHCs. However, free NHCs are generally unstable as a result of being air- and moisture-sensitive. Zhou et al. ^[40] investigated the thermal stability of 1,3-bis(2,6-diisopropyl- phenyl)imidazolium-2-carboxylate (IPr-CO₂) by in situ monitoring of the v(CO₂) region of the infrared spectrum. They found that most of the IPr-CO₂ decomposed within 1 h at 100 ℃. Interestingly, excess CO₂ was beneficial for the formation of NHC-CO₂ adducts while the presence of propylene oxide (PO) accelerated the decomposition of NHC-CO₂ adducts and simultaneously yielded propylene carbonate (PC). In the case of IPr-CO₂, there was nearly 100% yield of PC at 120 ℃ and 2 MPa for 24 h. However, the solvent CH₂Cl₂ was needed to afford high activity over the NHC-CO₂ adduct. Kayaki et al. ^[35] used ditert-butyl instead of 2,6-diisopropylphenyl, and applied the resulting 1,3-ditert-butylimidazolium-2-carboxylate (IBu-CO₂) for the cycloaddition of CO₂ and epoxides without the need of a solvent at a relatively low temperature (100 ℃). They obtained the corresponding carbonates in yields ranging from 71% to 89% with almost 100% selectivity. The theoretical study by the DFT method suggested that the real catalyst for the cycloaddition was free NHCs rather than NHC-CO₂ ^[41]. N-Heterocyclic olefin (NHO) with high electronegativity at the terminal carbon atom was found to show a strong tendency for CO₂ sequestration, affording a CO₂ adduct (NHO-CO₂) with a C_carboxylate-C_NHO bond longer than the C_carboxylate-C_NHC bond of NHC ^{[42, 43]}. Various CO₂, COS and CS₂ adducts of NHO using N, N’-disubstituted derivatives of 2-methyl imidazolium iodide were also reported. Among these, the NHO-CO₂ adducts were found to be more active (TOF = 3.4 h^-1), while the NHO-CS₂ adducts exhibited the lowest activity for the cycloaddition of CO₂ to epoxides. The poor activity of the NHO-CS₂ adducts was due to difficulty in releasing the highly active NHO as a result of the shorter C_CS2-C_NHO bond. The mechanism over the NHO-CO₂ adducts for the cycloaddition reaction was proposed as: (i) free NHO is released when NHO-CO₂ is heated; (ii) nucleophilic attack of the less hindered carbon atom of epoxide by NHO results in the generation of an alkoxy anion; (iii) nucleophilic attack of the carbon atom in carbonyl group by the alkoxy anion produces cyclic carbonate by intramolecular cyclic elimination, and release of NHO; and (iv) NHO reacts with excess CO₂ to regenerate the NHO-CO₂ adduct (Fig. 1) ^[43].

Fig. 1

Fig. 1. Mechanism for the reaction of CO2 with epoxides catalyzed by NHO-CO2 adduct [43].

Owing to its strong basicity, quaternary ammonium hydroxide is used as CO₂ adsorbent as well as catalyst for the cycloaddition reaction ^{[17, 44]}. With Bu₄N⁺OH^- as catalyst,88% yield of styrene carbonate (SC) was achieved at 120 ℃ and 1 MPa after 24 h. In addition to DFT calculations, a D-labeled epoxide and an optically active epoxide were used to study the reaction mechanism over quaternary ammonium hydroxide. The bicarbonate ion of tetrabutylammonium bicarbonate was recognized as the catalytically active species that attacks the less hindered C atom of the epoxide to generate a ring-opened alkoxide intermediate. The insertion of CO₂ into the alkoxide intermediate forms a carbonate ion which cyclizes to form the cyclic carbonate ^[17].

It was reported that there is a positive synergetic effect between the HBDs and Lewis base in the cycloaddition reaction ^[45-47]. Several typical organic bases such as DBU,N, N-dimethylaminopyridine (DMAP),1,4-diazabicyclo[2.2.2]octane (DABCO), triethanolamine (TEOA), diethanolamine (DEOA), monoethanolamine (MEOA), imidazole (Im), pyridine (Py), triethylamine (TEA), and diethylamine (DEA). and HBDs such as cellulose, chitosan, poly(ethylene glycol)-600 (PEG600), poly(ethylene glycol)-400 (PEG400), glycerine,β-cyclodextrin (β-CD), H₂O and 1,2-propylene glycol (PG) were explored to investigate the synergetic effects of bases and HBDs on the synthesis of cyclic carbonates. The activity order of the above bases was not strictly according to the established pKa order: DBU > DMAP > DABCO ≈ TEOA > DEOA ≈ MIm > MEOA > TEA > DEA ≈ Py > Im, indicating that the basicity of the bases is influential for the cycloaddition reaction. The high activity of DBU may be due to its nucleophilic ability as well as its CO₂ adsorption and activation ability. Moreover, a catalytic activity order of TEOA > DEOA > MEOA > TEA > DEA indicated that the presence of hydroxyl group is also important. Further study showed that the corresponding activity order for the above HBDs is cellulose > chitosan ≈ glycerine ≈ PEG400 > PEG600 ≈ β-CD > H₂O. A plausible mechanism is that nucleophilic attack of DBU on the epoxide coordinated with cellulose through hydrogen bonding results in opening the epoxy ring, and then the reaction with the CO₂ activated by DBU gives the corresponding cyclic carbonate (Fig. 2) ^[46]. Different from the disadvantage of H₂O for the catalytic activity of DBU in the report of Sun et al. ^[46], Roshan et al. found that the presence of a catalytic amount of H₂O was advantageous for the synthesis of PC over common organic bases such as Im, Py and DMAP, giving over 98% product selectivity (120 ℃,1.2 MPa,3 h). According to the DFT investigation, the carboxylate end of the bicarbonate moiety generated in the water-CO₂-base reaction, rather than the carbamate salt or hydroxyl group, was the key active species that gave the higher activity of the base-water systems ^[47].

Fig. 2

Fig. 2. Mechanism for the cycloaddition reaction catalyzed by DBU/ cellulose [46].

With the synergetic effect of HBDs and Lewis base recognized, organic bases functionalized with a hydroxyl group such as 2-pyridinemethanol (2-Py-EtOH) and 2,6-pyridinedimethanol were developed for highefficiency CO₂ and epichlorohydrin (ECH) cycloaddition under mild conditions (T= 25-60 ℃,0.1 MPa of CO₂). It was observed that there was no catalytic activity over benzyl alcohol. When pyridine was used as catalyst, the yield of cyclic carbonate was low (39%,60 ℃). In the case of 2-Py-EtOH, the yield of cyclic carbonate was 75% (60 ℃). The results indicate that both the basic and the hydroxyl group of 2-Py-EtOH are essential for the promotion of reaction. The ¹H NMR spectra of 2-Py-EtOH with and without ECH showed a clear upfield shift of OH proton signal (from δ = 6.50 to 5.49 ppm), indicating the presence of hydrogen bonding between ECH and pyridinemethanol ^[39]. It was recognized that the one-component substrates containing HBDs (-OH and -COOH) and Lewis basic sites (pyridinic N and N-C=N structure) are potential metal- and halide-free catalysts for the cycloaddition reaction.

2.1.2 Halide-containing organic base catalytic systems

2.1.2.1 Binary catalysts containing organic base and halide

Organic bases are excellent adsorbents for CO₂ capture as well as solvents ^[13]. Using DMF as solvent, N-bromosuccinimide (NBS) as catalyst and benzoyl peroxide (BPO) as co-catalyst, Kozak and coworkers ^[48] developed a new method for the continuous synthesis of cyclic carbonates from CO₂ and epoxides. With a residence time of 30 min, the system was operated at steady state for 14 h, and 82% isolated yield of cyclic carbonate was achieved (5 mol% NBS and BPO,2 mol/L 1,2-epoxyoctane in DMF,120 ℃,0.7 MPa). A series of kinetics experiments confirmed the participation of DMF as well as the activation of epoxides by electrophilic bromine (rather than Br^-). A mechanism was proposed: (i) Br₂ is generated in situ by the reaction of NBS with DMF,(ii) Br₂ reacts with epoxides to form bromo-oxonium species while CO₂ is activated over DMF,(iii) the bromo-oxonium species are activated by the activated CO₂ nucleophile, and (iv) after the opening of the epoxide ring, the cleavage of O-Br bond is induced by Br^-, regenerating Br₂ and liberating the alkoxide anion that undergoes cyclization to generate the cyclic carbonate (Fig. 3).

Fig. 3

Fig. 3. Mechanism for CO2 cycloaddition to epoxides catalyzed by NBS/DMF [48].

Using benzyl halides and amides (DMF, DMAc, N-formyl- morpholine, N-methylpyrrolidone, and N-formylpiperidine) as catalyst sources, Wang and coworkers ^[49] conducted the cycloaddition of epoxides with ambient CO₂ following an easy and mild procedure to afford five-membered cyclic carbonates in moderate-to-high yields (47%-86%). They reported that DMF was the best (86% yield) among the amides. The BnBr was activated by DMF, and the benzyl cation generated was identified by ¹H NMR analysis. The authors attributed the excellent activity to the electrophilic activation of epoxides by benzyl cations and the nucleophilic activation of CO₂ by DMF (Fig. 4). However, the activity of DMF/BnBr for CO₂ cycloaddition to styrene oxide (SO) at ambient CO₂ pressure was unsatisfactory, even at 120 ℃ for 24 h ^[49]. The organic bases DBU, DABCO, DMAP, Py, TEA, Im and 1,5-diazabicyclo [4.3.0]non-5-ene were also combined with BnBr for the cycloaddition reaction under mild conditions (65 ℃,0.1 MPa). The order of catalytic activity was largely in accordance with the order of the pK_a values of the bases, except for DABCO that showed unsatisfactory activity as a result of high steric hindrance. In addition, the benzyl bromide derivatives with an electron-donating group gave better results than those with an electron-withdrawing group. In the case of the latter, there was decline of leaving of the bromide anion from the aminidinium center. Kinetic studies on the catalytic conversion of CO₂ and ECH to cyclic carbonate over DBU/BnBr indicated the participation of the (Bn-DBU⁺Br^-) complex that was initially formed from DBU and BnBr in the catalytic cycle, similar to that of the DMF/BnBr binary catalytic system. In other words, the opening of the epoxide ring involves the nucleophilic attack from the Br^- of Bn-DBU⁺Br^- rather than from BnBr ^[50].

Fig. 4

Fig. 4. Reaction pathway for the synthesis of cyclic carbonates catalyzed by BrBn/DMF [49].

The above results demonstrated the synergetic effect between the nucleophilic anion and Lewis base as well as that between HBDs and Lewis base for the enhancement of the cycloaddition reaction. Recently, Wang et al. ^[39] developed a Bu₄NX (X = Cl, Br, I) and hydroxyl group functionalized Py (2-Py-EtOH and 2,6-pyridinedimethanol) binary system for the cycloaddition of CO₂ to epoxides at room temperature and atmospheric pressure. The catalysts are efficient and the system is metal-free and solvent-free. In the case of Bu₄NI/2-Py-EtOH, the yields of the corresponding carbonates ranged from 65% to 97% (25 ℃,0.1 MPa,20 h). The excellent performance was attributed to the synergetic effect of the hydroxyl group and halide anion for epoxide ring opening as well as to the basic sites for CO₂ adsorption and activation.

2.1.2.2 Organic base salts

Salts derived from organic bases were used as one-component catalysts for the cycloaddition reaction. From DABCO, DBU, TBD and hexamethylene-tetramine (Hatm), hydrochloride salts were generated and tested ^[51]. Consistent with the report of Yu et al. ^[34], the presence of the “N=C-N” entity in the cation favoredthe cycloaddition. Among the TBD·HX (X = Cl, Br, OAc, C₂F₃O₂) salts, TBD·HBr displayed the best SC yield (88%). The kinetics of the reaction over TBD·HBr and TBABr were monitored by in situ FTIR and Raman spectroscopy, and TBD-HBr was found to be more efficient than TBABr. The DFT calculations on the PO-TBD·HBr complex formed from TBD·HBr and PO indicated that the interaction between the N-H group of TBDH⁺ and the oxygen of PO promotes the ring opening of PO ^[52]. Further DFT study proved that the ring opening of the epoxides before the addition of CO₂ is the rate determining step ^[53]. The carboxyl-functionalized organic base salts were prepared by the reaction of amino acid and HI in water by a microwave-assisted one-pot procedure. The catalysts with an amino group were found to be the most efficient among the various carboxyl-functionalized organic base salts tested. Its performance can be attributed to the role of amine for CO₂ adsorption and activation as well as the synergetic effect of -COOH and the halide anion for epoxide ring opening ^[54].

The results discussed so far indicate that the multi- synergetic effects of HBDs and halide anions for ring opening of epoxide as well as the basic sites for adsorption and activation of CO₂ are essential for CO₂ cycloaddition to epoxides. It is hence desirable to develop one-component catalysts based on the concept of multi-synergetic effects.

2.2 Ionic liquids

ILs have attracted much attention for their unique nature such as being environment-benign, highly stable, non-volatile and non-flammable ^{[55, 56]}. ILs can be employed as a solvent as well as a catalyst in cycloaddition reactions. Their studies were recently reviewed ^{[26, 27, 57, 58]}. In the report of Yang et al. ^[58], task-specific ILs such as amino-functionalized ILs and superbase-derived protic ILs for CO₂ capture and utilization were systematically illustrated. Fiorani et al. ^[26] described ILs ranging from simple “onium” species to supported IL catalysts up to 2014, including those that contained metal for CO₂ cycloaddition to epoxides. For industrialization, Xu et al. ^[27] presented a broad overview of numerous catalysts with the structures and compositions depicted at atomic and molecular levels, the reactor at the unit level, and the process integration at the system level. In this section, we focus mainly on the synergistic effects with the use of ILs from 2014 up to now.

2.2.1 Onium salts

Industrially, quaternary ammonium salts such as TBAB are commonly used for the synthesis of cyclic carbonates from CO₂ and epoxides ^[59]. However, the activity is low even when a massive amount of solvent is used. There were attempts to enhance the activity of the onium salts such as through the combined use of other catalysts or through modification of known catalysts with special functional groups ^[60-64].

2.2.1.1 Quaternary ammonium salts

Bowl-shaped tetraphosphonate cavitands are well recognized as efficient receptors for cationic species. Based on the concept of host-guest chemistry, Mirabaud et al. ^[60] improved catalytic reactivity of quaternary ammonium salts by combining tetraalkyl-ammonium halides (guest) with tetraphosphonate cavitand (host) (Fig. 5). The affinity of the host for the ammonium cation was demonstrated by NMR titration experiments. The strength of the ammonium/halide binding in such host-guest architecture resulted in a dramatic effect on the cleavage of ion pair and consequently the catalytic activity. The Me₄N⁺I^-@2b binary system showed the best performance in accord with the good leaving ability and high nucleophilicity of I^- ions, achieving 92% product yield after 24 h and complete conversion of SO after 48 h.

Fig. 5

Fig. 5. Structure of tetraalkyl-ammonium halides and tetraphosphonate cavitand [60].

After knowing of the synergistic effect of HBDs and nucleophiles, other entities with hydroxyl groups such as silanediols, phenols, pentaerythritol (PETT) and tannic acid (TA) were applied to enhance the activity of onium salts. Silanediols were found to be effective HBDs for CO₂/epoxide coupling at room temperature and atmospheric pressure. In the presence of tetrabutylammonium iodide and SO, there was adownfield shift of the silanediol O-H chemical signal. The phenomenon was attributed to silanediol recognition of both the epoxide and iodide through hydrogen bonding interaction. A mechanism was proposed: (i) the coordination of epoxide with silanediol through hydrogen bonding forms complex Ⅰ; (ii) the activated epoxide undergoes ring opening upon nucleophilic attack of Ⅰ^- and yields alkoxide Ⅱ that is stabilized by hydrogen bonding with silanediol; (iii) the addition of CO₂ to Ⅱ generates silanediol-stabilized intermediate Ⅲ; (iv) cyclic carbonate is formed through an intramolecular cyclic step, together with the release of Ⅰ^- and silanediol (Fig. 6) ^[61]. Alves et al. ^[62] developed a catalytic platform based on an onium salt/phenols binary system efficient for the cycloaddition reaction under mild conditions (60 ℃ and 2 MPa), and found that multi-phenolic compounds were more efficient than phenol. Using online IR spectroscopy, kinetic studies were conducted and the commercially available HBDs (pyrogallol, pyrocatechol, gallic acid, perfluoro-tert-butanol, hexafluoro- (p-tolyl)-isopropanol and 1,3-bis-(2-hydroxyhexafluoroiso- propyl)-benzene) were identified as efficient co-catalysts. A multi-hydroxyl approach was also reported by Wilhelm et al. ^[63]. There was no or little conversion of PO over PETT or nBu₄NI whereas 96% yield of PC was afforded over 5 mol% PETT/Bu₄NI under mild conditions (70 ℃,0.4 MPa,22 h). Sopea et al. ^[64] reported a TA/NBu₄X (X = Br, I) binary catalyst system that was highly efficient for the cycloaddition of CO₂ toward oxiranes. It was reported that the corresponding cyclic carbonates were efficiently generated even at low catalyst loading. In the case of 1,2-epoxyhexane,86% yield of cyclic carbonate was obtained with 0.05 mol% TA and 0.5 mol% Bu₄NI under mild condition (80 ℃,1 MPa,24 h).

Fig. 6

Fig. 6. Reaction pathway for the formation of cyclic carbonates over TBAI/Silanediols [61].

The carboxyl group is generally considered a stronger HBD than the hydroxyl group. Composed of carboxyl-containing HBDs (EDTA or EDTA-3Na) and quaternary ammonium salts (TBAB or TBAI), the binary catalyst system was efficient for the synthesis of cyclic carbonates from CO₂ and epoxides, giving excellent yield and selectivity without the need of a solvent. When EDTA or TBAB as catalyst, the former gave no product while the latter showed a SC yield of only 58%. In contrast, the combined use of EDTA and TBAB rendered a SC yield of 94% (70 ℃,0.5 MPa,18 h). In addition, the combination of TBAB with EDTA-3Na bearing one carboxyl group gave a SC yield of only 68%. When the amount of EDTA-3Na was raised to four times with equal moles of free carboxyl compared to EDTA, only 78% SC yield was achieved. It was noted that the results were not consistent with that of Zn-EDTA complexes ^[65]. The results demonstrated that having multiple carboxylic groups in one molecule can have the epoxide activated more effectively, and the alkoxide intermediate is stabilized through multi-site hydrogen bonding. Moreover, intramolecular hydrogen bonding is more effective than intermolecular hydrogen bonding in terms of epoxide activation ^[66].

The quaternary ammonium salts functionalized with HBDs are also efficient for the cycloaddition reaction. By alkylation of tertiary amine with haloalcohols, one-component ammonium salts bearing HBDs were synthesized for the coupling of CO₂ with epoxides under mild conditions (45 or 90 ℃). The bifunctional ammonium salts showed activities superior to those of the corresponding ammonium salts. The reaction could be conducted even on a multigram scale ^[67]. The mechanism of CO₂ cycloaddition to epoxides catalyzed by 2-hydroxyl-ethyl-triethylammonium bromide (HETBAB) was investigated using the DFT method. The results suggested that the presence of an -OH functional group is essential for the improvement of catalytic activity ^[68]. Betaine-based salts (QGLY) can be considered as carboxyl functionalized quaternary ammonium salts. Park and coworkerssynthesized QGLY by microwave-assisted quaternization of glycine. DFT was used to simulate the synergistic effect of -COOH group and halide ion of QGLY on the enhancement of catalytic activity ^[69].

2.2.1.2 Quaternary phosphorus salts

The phosphonium-based ILs are efficient for cycloaddition. Carbonate and carboxylate methyltrioctylphosphonium ILs were designed as halide-free and metal-free organocatalysts for CO₂ cycloaddition to SO. The carboxylate and carbonate anions were active nucleophiles for SO conversion. However, as proved bythe ¹H,¹³C, and ³¹P NMR analysis, the above ILs was progressively converted to phosphine oxide ^[70]. Werner et al. ^[71] prepared hydroxyl functional quaternary phosphorus salts (HQPS) by one-step reaction of phosphine and halo derivatives that contained hydroxyl group. In the case of cycloaddition reaction of 1,2-butylene oxide and CO₂ at 90 ℃ and 1 MPa, the yield of cyclic carbonate showed an increasingtrend of Bu₄PI < Bu₄PBr < Bu₄PCl that matched the order of nucleophilicity. However, when [HO(CH₂)₂PBu₃]X was employed, the order changed to [HO(CH₂)₂PBu₃]Cl < [HO(CH₂)₂PBu₃]Br < [HO(CH₂)₂PBu₃]I at the reaction temperature of 90 ℃, and changed to [HO(CH₂)₂PBu₃]I < [HO(CH₂)₂PBu₃]Cl < [HO(CH₂)₂PBu₃]Br when the cycloaddition reaction was conducted at 120 ℃ ^[72]. As previously reported, the presence of hydrogen bond interactions between epoxides and HBDs results in a decrease of the activation energy for epoxide ring opening ^{[67, 68]}. In this case, besides nucleophilicity, the leaving ability of the halide ions is critical as well. In addition, the product yield increased with the change of substituent R of [HO(CH₂)₂PR₃]I in the order of Me < Ph < Cy = Bu = Oct. The phenomenon can be attributed to the increase of nucleophilicity at iodide, since the bulkiness of the cation results in weakening of the ion pair. In the case of [HO(CH₂)₂PBu₃]I,92% yield of 1,2-butylene carbonate was achieved at 90 ℃ in 2 h ^[72].

In the report of Dai et al. ^[73], various functionalized phosphonium-based ILs were prepared and evaluated for the cycloaddition of CO₂ to epoxides in the absence of co-catalyst and solvent. The strong acidity facilitated the ring opening of epoxides, and the weak electrostatic interaction enhanced the nucleophilic attack ability of Br^-. In the presence of alkyl compounds that contained hydroxyl, carboxyl and amino group, there was enhancement of [Ph₃PC₂H₄]Br activity, and [Ph₃PCH₂COOH]Br showed the best performance among these functionalized catalysts. A similar phenomenon was observed in the case of pyridinium-based IL with -COOH,-OH,-SO₃H and -NH₂ functional groups, and only the presence of -COOH resulted in catalytic activity better than that of the traditional pyridinium-based IL ^[74]. In the case of functionalized guanidinium ILs, the order of activity enhancement was amino group > carboxyl > hydroxyl which was similar to the order of the stability of the hydrogen bond between epoxide and HBDs: carbamic acid (generated by the reaction of primary amine with CO₂) > carboxyl > hydroxyl ^[75]. The above results suggested that the selection of an appropriate functional group is essential for the cycloaddition reaction.

Recently, Zhou et al. ^[76] developed a series of phosphorus ylide (P-ylide) CO₂ adducts. The novel P-ylide CO₂ adducts were efficient metal- and halogen-free catalysts that transformed CO₂ into functionalized cyclic carbonates with alkyl, alkenyl, alkynyl, phenyl, halide, ether, amino and ester groups under ambient conditions (25 ℃ and 0.1 MPa of CO₂). The intermolecular synergistic effects of the phosphonium moiety as an onium salt and the carboxylate moiety acting as a nucleophile for activating epoxides was suggested by the kinetic study based on in situ FTIR.

2.2.2 Imidazolium ILs

2.2.2.1 Conventional imidazolium ILs

With tunable structure and function, imidazole ILs are widely used in cycloaddition reactions (Fig. 7) ^[27]. Anthofer et al. ^[77] controlled the activity of imidazolium-based ILs [R¹R²R³Im]Br (R¹ = H, CH₃, benzyl,1-(2,3,4,5,6-pentafluoro) benzyl; R² = H, CH₃, C₂H₅; R³ = n-butyl,n-octyl) for the reaction of CO₂ cycloaddition to PO by changing the substituents of the imidazole cations. The substitution of the acidic ring proton at the 2-position by methyl and ethyl groups resulted in a reduction of PO conversion. The hydrogen bond interaction between the acidic imidazolium proton and the oxygen atom of PO was shown by FT-IR analysis. This suggested that the contact between the imidazolium ring proton at the C2-position and the epoxide oxygen atom facilitated epoxide ring opening by nucleophilic Br^- attack. The bulk steric hindrance on C₁ and C₃ were favorable for high catalytic activity. In the work of Girard et al. ^[78], the effects of cation and anion on the activity were investigated. In the case of 1-n-butyl-3-methylimidazolium (BMIM) derivatives, high conversion and selectivity were achieved with Br^- and I^-, while relatively low selectivity was observed in the cases of BMIM with Cl^-, AcO^-, HCO₃^-, malonate, and proline. It was recognized that good leaving ability of the halide ion was essential for ring closure, which is an SN₂-type reaction. In the case of BMIMI replaced by 1-t-butyl-3-methylimidazolium iodide, the yield (selectivity) declined from 99% (99%) to 77% (87%). This was explained by the stronger acidity of the H-C2 induced by t-Bu at the lateral chain that benefited the generation of phenylacetaldehyde through SO isomerisation. Both the experimental and DFT results testified that the synergetic effect of acidic and basic sites as well as the suitability of hydrogen bond strength are critical for the reaction. Further DFT study of BMIMBr for CO₂ cycloaddition to SO proved that the rate determining step was the ring opening of SO as a result of nucleophilic attack of Br^- on the methylene C atom of SO, involving mainly the cation and anion from the catalyst ^[79].

Fig. 7

Fig. 7. Structure of functionalized imidazolium ILs.

2.2.2.2 Functionalized imidazolium ILs

Compared with conventional Im ILs, functionalized Im ILs generally showed better catalytic activity owing to the synergistic effect between the functional groups (hydroxyl, carboxyl and amino) and nucleophiles such as halide anions (Fig. 7). As described by Wang et al. ^[80], the process for the cycloaddition reaction between CO₂ and epoxides catalyzed by 1-(2-hydroxyl-ethyl)-3-methylimidazolium (HEMIM) chloride changed from single-step to multipath compared to the non-catalyst process, and the nucleophilic attack of anion and hydrogen bonding were two critical factors for the promotion of reaction. In the report of Liu et al. ^[81], the combined use of HBDs and 1-(2-hydroxyl-ethyl)-3-butylimidazolium bromide (HEBimBr) afforded significant enhancement of activity compared to the use of HEBimBr alone, and with 94% PC yield the EG/HEBimBr system showed the best performance (140 ℃,2.0 MPa,2.5 h). The DFT results suggested that the ring closing step was rate determining in the case of HEBimBr while the ring opening step was rate determining in the case of EG/HEBimBr. Anthofer et al. ^[82] synthesized hydroxy-functionalized mono- and bis-imidazolium bromides and found that the activity of the latter was higher than that of the former even with the equal molarity of midazolium cations. The conversion of PO over hydroxy-functionalized bis-imidazolium bromides at 70 ℃ and 0.4 MPa for 16 h was 95%. The above functional groups were located at the nitrogen of the imidazolium rings that are commonly not stable especially in the presence of a base owing to the reactive C(2)-H of imidazolium. Zhang’s group ^[83] designed a series of base-stable 2-hydroxymethyl-functionalized ILs by introducing a hydroxyl methyl group to the reactive C(2)-H of imidazolium. Different from the traditional ILs, the alkyl length of the ILs cation had little influence on the catalytic activity due to the absence of C(2)-H. The performance of the 2-hydroxymethyl- functionalized ILs was much better than that of traditional ILs, and afforded a PC yield of 83%-92% (110 ℃,1 MPa,1 h). In addition, the DFT results again confirmed that hydrogen bonding is beneficial for stabilizing the intermediates and transition states in the cycloaddition process.

It was reported that the synergetic effect of carboxyl or hydroxyl group and halides promoted the cycloaddition reaction. Xiao et al. ^[84] investigated the influence of acidic strength on the catalytic activity of Brnsted acidic ILs. Weak acidity favored the increase of catalytic activity owing to the fact that strong acidic ILs, such as sulfonic acid, result in strong hydrogen bonding with the epoxide that hinders the insertion of CO₂. In the case of 1-(2-carboxyethyl)-3-methylimidazolium bromine (Ⅲ) as catalyst, the yield of PC was 96.3% (100 ℃,1.5 MPa,2 h). As suggested by the DFT study by Wang et al. ^[85], the elongation of the alkyl chain in the cation improvedthe product yield, and the use of imidazole group as cation was better than that of pyridine group.

Amino-functionalized Im ILs was prepared and used as catalysts for thecycloaddition of epoxides with CO₂ ^[86]. Compared to the carboxyl-functionalized Im ILs, the amino-functionalized Im ILs showed better catalytic activity. This was attributed to the enhancement of CO₂ adsorption and activation by amine through the formation of ammonium carbamate as well as to the hydrogen bonding between the ammonium-group and epoxide. Consistent with the report of Anthofer et al. ^[77], a long N-alkyl chain in amino- functionalized Im ILs was beneficial for the enhancement of catalytic activity. Recently, Liu et al. ^[87] developed ILs catalysts based on urea derivatives (denoted as UDILs) with multi-secondary amine groups. The UDILs showed double molar CO₂ adsorption and reversible CO₂ capture ability, as well as outstanding catalytic activity for the cycloaddition of CO₂ to epoxides. Based on the kinetic studies, the authors proposed a mechanism that used the activation of epoxide and CO₂ by the urea-functionalized cations and the ring-opening of epoxide induced by the nucleophilic anion.

The above results indicated that the high activity of HBDs/Ils binary catalysts and functionalized ILs can be attributed to the synergistic effect of the functional group (hydroxyl, carboxyl and amino) and nucleophile such as halide anions. In addition, the multi-hydroxyl and multi-cation concepts are suitable for the design of efficient metal-free catalysts. However, it is noted that the high price of ILs and difficulty in product separation inhibit the industrialization of ILs for the production of PC.

2.3 Other homogeneous metal-free catalysts

With chemically reactive acidic and basic sites,α-amino acids (AAs) are efficient for the cycloaddition of CO₂ to PO when a proper amount of H₂O is added to the reaction system. Basic Aas, such as L-arginine, L-histidine, L-lysine showed better performance than acidic AAs such as L-aspartic acid and L-glutamic acid, and L-histidine performed the best. In the case of L-histidine, increasing the amount of H₂O from 1.3 mol% to 13 mol% resulted in a conversion increase from 64% to 90% while the selectivity declined from 96% to 91%. A reaction mechanism involving the amine moiety of the L-his imidazole ring and the synergistic interplay of the -OH groups in H₂O and the COO^- end of the zwitterionic L-his was proposed ^[88].

As acidic counterparts of the well-known proaza- phosphatrane superbases, azaphosphatranes are used as efficient catalyst (0.1 mol% loading) for CO₂ cycloaddition to epoxides at 80 ℃ and atmospheric pressure. Being structurally tunable, the catalytic activity of azaphosphatranes can be modulated by changing the substituents attached to the peripheral N-atoms. The azaphosphatrane bearing bulkier substituents such as p-methoxybenzyl and neo-pentyl exhibited relatively higher stability than those bearing methyl substituents, and the azaphosphatrane bearing p-methoxybenzyl groups showed the highest catalytic activity. The kinetic studies suggested that epoxide was activated through hydrogen bonding with P-H, and with the insertion of CO₂ into the P-N bond, there was the generation of an unusual tricyclic phosphoryl-carbamate intermediate ^[89]. DFT investigation revealed that intermolecular proton transfer was essential for the reaction, and the insertion of CO₂ into the P-N bond of the catalyst led to catalytic deactivation ^[90]. Subsequently, Chatelet et al. ^[91] developed an encapsulation method by self-assembly giving a nano-cage to improve the catalytic activity and stability of azaphosphatranes. It was reported that the encapsulated azaphosphatrane showed catalytic activity and stability much better than those of the non-caged counterpart.

3 Heterogeneous catalysts

3.1 Immobilized catalysts

Although homogeneous catalysts are generally efficient for cycloaddition, the complicated separation process is unwelcome in industrialization. The immobilization of homogeneous catalysts on suitable supports (e.g., mesoporous materials and polymers) is an efficient and simple method to afford heterogeneous catalysts. The catalysts based on carbon materials are not presented here. They are reviewed in the later section of “Carbon materials”.

3.1.1 Immobilizedorganic base catalysts

As described previously, the presence of a conjugated N=C-N structure is beneficial for the cycloaddition reaction. Having a large specific surface area and being porous, together with having an abundance of hydroxyl groups, mesoporous silica materials (SiO₂, SBA-15, MCM-41) are excellent supports for organic bases that contain the N=C-N structure such as TBD, guanidine and NHC. The most common method to immobilize organic bases on silica is the silylanization of the silica materials with chlorine-terminal silanes followed by the nucleophilic substitution reaction of unsaturated amines ^{[36, 38]}.

With a high specific surface and well-ordered architecture, the hydroxyl-rich MCM-41 is a good solid support for loading homogeneous catalysts ^[38]. Zhou and coworkers ^[92] prepared NHC-functionalized MCM-41 through the silylanization of MCM-41 with 3-mercaptopropyltrimethoxy- silane followed by reaction with 1,3-bis-(4-allyl-2,6-diisopropylphenyl) imidazolium chloride. In situ diffuse reflectance infrared fourier transform spectroscopy was applied to investigate the reversible CO₂ capture-release ability of MCM-41-NHC. It was reported that MCM-41-NHC can effectively trap CO₂ at a relatively low temperature and release the adsorbed CO₂ at a higher temperature. Furthermore, MCM-41-MHC-CO₂ was proved to be an efficient heterogeneous catalyst for the cycloaddition of CO₂ to epoxides with excellent regioselectivity under mild conditions. In the case of PO,100% yield of PC was obtained at 120 ℃ after 48 h. Abundant with silanols, silica (SiO₂) is also used as support for heterogenizing an organic base catalyst. Adam et al. ^[93] grafted 3-chloropropyl silica with tetramethylguanidine and obtained spherical nanoparticles with an average size of 19.62 nm and a narrow pore size distribution (3.5-4.2 nm). A conversion of 92% was achieved under the optimized reaction conditions (130 ℃,5.0 MPa,8 h), and the PC selectivity was 98%. CO₂ was adsorbed and activated at Lewis base sites to generate the carbonate species.

In another case, both mesoporous organosilica and silica were applied as supports. Prasetyanto et al. ^[94] incorporated T-type melamine-based triorganosilsesquioxane of large size as an organic linker in the periodic mesoporous organosilica. The resulting melamine-based organosilica possessed a well-ordered p6mm hexagonal mesostructure and performed effectively in the cycloaddition of CO₂ to PO. Based on the calculated geometric parameters and adsorption energy, the authors proposed that compared to the single-type amine species, the combined use of a secondary amine and tertiary amine at a particular geometric position would result in better CO₂ activation.

As discussed above, the process for cycloaddition over bases is generally by the adsorption and activation of CO₂ to afford a carbamate or carbamic acid species as the initial step, followed by the subsequent nucleophilic attack to epoxides. However, the nucleophilicity of the carbamate and carbamic acid species is not enough to cause epoxide activation, and the activity of the immobilized organic base catalysts is limited. It is hence necessary to develop immobilized catalysts that have high nucleophilicity.

3.1.2 Immobilized ILs catalysts

In recent years, various ILs were designed as catalysts or co-catalysts for cycloaddition ^{[26,27,60-63]}. Despite their efficiency, the immobilization of them is essential to synthesize cyclic carbonates by a continuous technology.

3.1.2.1 Silicon-based materials supported ILs catalysts

Silicon-based materials are also suitable supports for the immobilization of ILs and organic bases. Dai et al. immobilized 3-(2-hydroxyl-ethyl)-1-propylimidazolium bromide (HEPIMBr) onto SiO₂, SBA-15 and Al-SBA-15 mesoporous molecular sieves by the chemical grafting method. SBA-15 and unsupported HEMIMBr displayed poor activity for CO₂ cycloaddition to PO while the activity remarkably increased when HEMIMBr was loaded onto the silicon materials. The activity order was SBA-15-HEPIMBr > Al-SBA-15-HEPIMBr > SiO₂-HEPIMBr > HEMIMBr. The excellent performance of SBA-15-HEPIMBr was attributed to the synergetic effects between the active component and support as well as -OH and halide anions ^[95]. Cheng et al. ^[96] found that the loading of ILs with -OH or -COOH functional groups was less than that of ILs without a functional group, but the former was more efficient than the latter. Based on experimental and DFT studies, a mechanism was proposed that involves the activation of epoxide under the influence of hydrogen bonding and nucleophilic Br^- attack as well as the adsorption and activation of CO₂ by the tertiary nitrogen atoms of the catalyst (Fig. 8).

Fig. 8

Fig. 8. Mechanism for the fixation of CO2 in the form of cyclic carbonates [96].

In the studies of Adam and coworkers ^[97], imidazole was immobilized on MCM-41 using 3-chloropropyltriethoxysilane as anchoring agent, and then subjected to alkylation with 1,2-dibromoethane (Fig. 9). The material obtained was designated as MCM-41-Imi/Br. According to the ion chromatography analysis, the concentration of bromide in MCM-41-Imi/Br was 0.44 mmol/g. At 100 ℃ and 3 MPa for 4 h,100% conversion and 98.8% selectivity to SC over MCM-41-Imi/Br was achieved under solvent-free condition, while there was an obvious decline of activity with the addition of a solvent such as 1,2-dichloroethane, acetonitrile, DMF and toluene. The decline of activity wasattributed to the adsorption of solvent molecules on the active sites that inhabited the activation of substrates. When SC was heated at 100℃ together with MCM-41-Imi/Br in the absence of CO₂ for 4 h,2-bromo-1-phenylethanol was detected as major product by GC-MS. The result confirmed that the ring opening of epoxide took place by a nucleophilic attack of Br^- at the methylene carbon of epoxide ^[98].

Fig. 9

Fig. 9. Reaction sequence and the structure of MCM-41-Imi/Br [97].

The “multilayered covalently supported” approach was applied to increase the loading of ILs. Agrigento et al. ^[99] prepared multilayered covalently supported ILs by grafting different bis-vinylimidazolium salts on thiol-functionalized silica. The SiO₂ supported bis-imidazolium iodide salt with xylene or octane as linker was identified as the most active IL-based catalyst. The superior productivity was attributed to the high IL loadings (53 wt.% and 61 wt.%). In addition, the “multi-hydroxyl functionalized” approach was used to generate efficient heterogeneous as well as homogeneous catalysts. Kohrt et al. ^[100] designed and prepared SiO₂-supported triethanolammonium iodide by reacting supported amines with iodoethanol. This catalyst gave the conversion of several terminal epoxides to the corresponding cyclic carbonates in 67%-99% yield under mild conditions (90 ℃,1 MPa,6 h). Compared to the triethanolammonium iodide supported on polystyrene (PS), the SiO₂-supported triethanolammonium iodide gave slightly lower yields but showed much better stability in the reusability test. In the case of 1,2-butene oxide as substrate,99% yield of the corresponding cyclic carbonate was achieved and the catalyst could be reused 13 times with the high activity retained.

Polyhedral oligomeric silsesquioxane functionalized with imidazolium chloride peripheries (POSS-Imi) was successfully synthesized and used for the cycloaddition of CO₂ to epoxides with excellent performance. In the presence of PrOH, POSS-Imi (TON = 533) showed enhanced catalytic activity when compared to unsupported 1-butyl-3-methylimidazolium chloride (TON = 326). The excellent performance of POSS-Imi was ascribed to the synergistic effect of POSS and ILs as well as to the high concentration of imidazolium species on POSS ^[101].

3.1.2.2 Polymer-immobilized ILs catalysts

Being cheap, separable and easily surface-modified, organic polymers are promising candidates to support ILs and have wide applications in the synthesis of cyclic carbonates. Zhang et al. ^[102] developed quaternary ammonium salts functionalized polystyrene (PS) by the direct quaternarization of chloromethylated PS with N,N-dimethylethylamine (PS-QNS) for cycloaddition of CO₂ to PO. There was no PC produced over PS, while over QNS and PS-QNS, the PC yield was 50.7% and 7.9%, respectively (150 ℃,2 MPa,5 h). The results indicated that there is a synergistic effect between the support and catalytically active species. In the work of Deng et al. ^[103], the “multi-hydroxyl functionalized” approach was applied to prepare PS-supported bis-ammonium ILs based on DABCO with -CH₂COOH,-CH₂CH₂OH, and -CH₂CH₂NH₂ as end groups. With the -COOH,-OH, and -NH₂ functional groups, there was enhanced catalytic efficiency. Saptal et al. ^[104] designed and developed dicationic and di-hydroxyl ILs based on DABCO for CO₂ cycloaddition to PO (Fig. 10). It was reported that the [PS-DABCO-PDO][Br-Cl] catalyst performed well, giving 98% PC yield, and canbe recycled up to seven times without loss of catalytic activity (100 ℃,2 MPa,3 h).

Fig. 10

Fig. 10. Structures of functionalized and PS-supported DABCO based Ils [104].

1,2,4-Triazolium- and Imidazole-based ILs were also covalently anchored onto PS for the cycloaddition reaction. Whiteoak et al. ^[105] developed PS-supported 1,2,4-triazolium-based ILs containing 1,2,3-trihydroxy- benzene as catalyst for cycloaddition under mild conditions (45 ℃ and 1 MPa). The catalyst can be easily recycled and reactivated through reaction with methyl iodide. Jadhav et al. ^[106] found that NTf₂^- was more suitable than halide anions such as Br^-and Cl^- in the PS-Im catalyst system.

A cross-linked divinylbenzene polymer (PDVB) was employed as support for the formation of heterogeneous catalysts. Dai et al. ^[107] developed a series of PDVB-supported ILs by reacting brominated alkanes with PDVB that was grafted with 1-(3-amino-propyl) imidazole. The catalytic activity of PDVB-ILs was in the order PDVB-HEIMBr > PDVB-AEIMBr > EtOH/PDVB-EIMBr > PDVB-EIMBr, and such an order indicates the importance of hydroxyl for the cycloaddition reaction. Subsequently, a carboxyl functional group was tested and found to be good for HBDs in PDVB-Im ^[108]. Recently, PDVB grafted with carboxyl-, hydroxyl-, or amino-functionalized di-cation (P-FDⅡLs) were developed (Fig. 11). The catalyst based on the di-cation showed better performance than that based on mono-cation. The activity of PDVB-supported di-cation with different functional groups was in the order: -COOH > -NH₂ > -OH, and the PC yield was up to 97.8% for the -COOH functionalized catalyst at 130 ℃ and 2.5 MPa in 4 h. The results suggested that the synergetic effect between the carboxyl group and halide anion facilitate the cycloaddition reaction ^[109].

Fig. 11

Fig. 11. Preparation of polymers grafted with functionalized DⅡLs [109].

Furthermore, FDU-type periodic polymer with a mesoporous organic framework and abundant phenolic hydroxyl was applied as support to immobilize imidazolium-based ILs (Fig. 12). In the cycloaddition of CO₂ to PO, the activity of FDU-EIMBr without a hydroxyl group in the ILs was similar to those with a hydroxyl group while the homogeneous catalyst HEIMBr with a hydroxyl group showed much higher activity than EIMBr. In the case of using FDU-EIMBr as catalyst,91% yield of PC was obtained (110 ℃,1 MPa,3 h). Combined with the DFT study, it is recognized that the presence of halide ions and the strong hydrogen bonding interaction between the phenolic hydroxyl group and PO are important factors that facilitate the cycloaddition reaction. In addition, the DFT study indicated that the activation of CO₂ can be attributed to the tertiary ammonium of the imidazolium ring moiety ^[110].

Fig. 12

Fig. 12. Schematic for preparing a series of FDU-15 mesoporous polymer supported imidazolium-based ILs [110]. (a) FDU-HEIMBr; (b) FDU-CMIMBr; (c) FDU-DHPIMBr; (d) FDU-EIMBr.

3.1.2.3 Biopolymer-supported ILs catalysts

Biopolymers have the advantages of being naturally abundant, non-toxic and biocompatible as a support for homogeneous catalysts. Zhao et al. ^[111] were the first to develop quaternary ammonium salt functionalized chitosan (CS) for the cycloaddition of CO₂ to PO without the need of co-catalyst and solvent. 1-Butyl-triphenylphosphonium- bromide was grafted onto CS by reacting the amine on CS with the alkyl bromide of phosphorus salt (denoted as CS-[BuPh₃P]Br). Using CS-[BuPh₃P]Br as catalyst, the yield of PC was 98.3% at 120 ^oC and 2.5 MPa in 4 h. The ring opening of the epoxides was a combined result of polarization (caused by hydrogen bonding with the hydroxyl groups), electronic interaction (by [BuPh₃P]⁺), and nucleophilic attack (by bromide anion). In addition, the CS-[BuPh₃P]Br performed well in five consecutive runs without any loss of PC yield and selectivity ^[112]. The CS-supported 1-ethyl-3-methyl imidazolium halides were also demonstrated to be recyclable and efficient under mild conditions ^[113]. It is noted that different from the above method for the immobilization of quaternary ammonium salt, Tharun et al. ^[114] prepared quaternized chitosan in situ by the reaction of CH₃I with -NH₂ on CS.

As discussed above, the concept of multi-synergetic effects is applicable for the design of an efficient immobilized catalyst. It is meaningful to develop environment-benign, architecture well-ordered porous materials that are efficient for the adsorption of CO₂ and epoxides. In addition, methods of chemical immobilization of active components should be developed to avoid catalyst deactivation due to leaching.

3.2 Organic copolymer catalysts

The use of organic copolymers as heterogeneous catalysts is highly desirable because they are cheap to prepare, easy to separate, and the catalytically active sites are tunable and high in density.

3.2.1 Poly(ionic liquids) catalysts synthesized by olefin polymerization

Imidazole ILs are efficient homogeneous catalysts for the cycloaddition of CO₂ to epoxides. The development of poly(ionic liquids) (PILs) as a heterogeneous catalyst is beneficial in terms of catalyst separation.

Xie et al. ^[115] first synthesized highly crosslinked PILs by copolymerization of 3-butyl-1-vinylimidazolium chloride ([VBIM]Cl) with cross-linker divinylbenzene (DVB) (Fig. 13). The amount of [VBIM]Cl was 1 mmol per 1 g PDVB-[VBIM]Cl. The catalytic activity of PDVB-[VBIM]Cl was comparable to or even better than those of the liquid catalysts [BMIM]Cl and [VBIM]Cl which are PSIL monomers. In the case of PDVB-[VBIM]Cl, the yield of PC was 97.4% at 110 ℃ and 6 MPa in 6 h. In addition, PDVB-[VBIM]Cl showed much better performance than PVBIMCl that was synthesized by direct polymerization of [VBIM]Cl without the use of the cross-linker DVB. The above phenomenon was also seen in hydroxyl-, carboxyl- and amino-functionalized PILs with a similar framework ^[116]. This was ascribed to the much higher density of active sites in PVBIMCl compared to PDVB-[VBIM]Cl ^{[115, 116]}. In addition, hydroxyl-, carboxyl- and amino- functionalized PILs showed much higher catalytic activity than non-functionalized PILs. Using carboxyl-functionalized PILs as catalyst,95% PC yield and 100% selectivity was obtained at 120 ℃ and 2 MPa in 4 h ^[116]. By means of suspension polymerization, Han et al. ^[117] prepared a series of PDVB-based PILs by reacting a variety of alkyl halides with highly cross-linked porous poly(N-vinylimidazole-co- divinylbenzene) (PVIm) (Fig. 14). The results demonstrated that the improved catalytic activity of PVIm was related to factors such as more nucleophilic anions, bulkier alkyl chains, and presence of hydroxyl groups. Dani et al. ^[118] developed porous PILs that were high in surface area by an innovative method by the synthesis of a non-ionic co-polymer by precipitation polymerization, and the co-polymer was then quaternized by an alkylation step using microwave heating. The amount of ILs was 1.6 mmol/g PILs when the DVB : VIm volume ratio was 3:7. The yield of EC depended on the loading of VIm.

Fig. 13

Fig. 13. Synthesis of the cross-linked PILs [115]. AIBN: azobis(isobutyronitrile).

Fig. 14

Fig. 14. Schematic for the synthesis of PVIm-RX [117].

Besides the PDVB-based ILs, various cross-linked ionic polymers based on styrene-functionalized imidazolium salts were prepared by the direct polymerization of 1,3-bis(4- vinylbenzyl)imidazolium salts ([bvbim]X, X = Cl, PF₆, or BF₄). Poly[bvbim]Cl and [bvbim]Cl exhibited almost identical catalytic activity while catalytic activity over poly[bvbim]-[PF₆] and poly[bvbim][BF₄] was negligible. The results testified to the importance of nucleophilic anions ^[119]. In the work of Wang et al. ^[120], bis-vinylimidazolium salts (Fig. 15) were used as monomers, and in the free radical self-polymerization process, meso-macroporous hierarchical PILs (MPILs) with tunable structure and extremely high ionic site density were formed. The MPILs were highly efficient for the cycloaddition reaction at atmospheric pressure and low temperature, in particular for epoxides with a long carbon chain that are known to be extremely inert. The excellent performance was ascribed to the porous structure being suitable for CO₂ adsorption as well as to the enrichment of Br^- anions needed for epoxide ring opening.

Fig. 15

Fig. 15. Structure of synthesized vinylimidazolium salt monomers [120].

3.2.2 Poly(ionic liquids) catalysts prepared by non-olefin polymerization

The above PILs were all synthesized by olefin polymerization with the ILs located at the side chain which limited the loading of ILs and restricted the stability as well as the recyclability of PILs. By the reaction of 1,2,4,5-tetrakis(bromomethyl)benzene (TBB) and 4,4’-bipyridine, a new highly cross-linked cationic polymer (TBB-BPy) was synthesized. The material was generated as uniform microspheres with a smooth surface and size of 2-3 μm. The activity of TBB-BPy was higher than that of TBB-Py and TBB-Py functionalized with hydroxyl, carboxyl or amino. In the case of TBB-BPy, the yield of PC reached 99% in 4 h (120 ℃ and 1 MPa). In addition, with a cross-linked covalent cationic structure, the catalyst can be reused six times without loss of activity ^[121]. Yang et al. ^[122] adopted a two-step alkylation approach for the synthesis of fluoro-functionalized polymeric ILs (F-PIL-Br) with the ILs located at the main chain (Fig. 16). The resulting PILs were stable up to 300 ℃. The catalytic activity increased with fluorine content in the cations, following the order: PIL-Br < F_0.5-PIL-Br < F-PIL-Br. In addition, F-PIL-Br showed activity three times higher than that of non-fluorous PIL-Br in the cycloaddition of CO₂ to SO. This may be due to the CO₂-philic property of the fluorine-containing materials that enhanced CO₂ adsorption around the catalytic sites ^[77]. Wang et al. prepared main chain poly-imidazolium salts by the condensation reaction of bisimidazoles and silicon tetrachloride (Fig. 17). The existence of Si-OH in the poly-imidazolium salt was confirmed by the FT-IR stretching vibration signal at 3360 cm^-1 and a ¹H NMR peak at 15.03 ppm. The experimental and DFT results evidenced that the presence of Si-OH in the polymer chain was the key factor for achieving high activity ^[123].

Fig. 16

Fig. 16. Synthetic route and structures of fluoro-functionalized polymeric ILs [122].

Fig. 17

Fig. 17. Synthesis of main chain poly-imidazolium salts by condensation reaction of bisimidazoles and silicon tetrachloride [123].

Various novel hypercrosslinked porous polymers embedded with a phosphonium salt were prepared by the Friedel-Crafts reaction using phosphonium salts, benzene, and formaldehyde dimethyl acetal as monomer (Fig. 18). The surface area decreased with increasing anion diameter as well as alkyl chain length of cations. The pore size increased with increasing anion diameter of Cl^-, Br^- and I^-. It was observed that the catalytic performance showed the order of Br^- > Cl^- > I^- while CO₂ capacity had the order of Br^- > Cl^- > I^-. Hence, the high activity of the porous polymers with Br^- was attributed to the combined effects of high CO₂ capacity, suitable nucleophilicity and leaving ability of Br^- as well as the large surface area and pore size ^[124].

Fig. 18

Fig. 18. Synthesis of hypercrosslinked phosphonium polymers [124].

3.2.3 Halogen-free polymers catalysts

Amorphous triazine-based covalent organic framework (CTF) was prepared by thermal polymerization of 1,4-dicyanobenzene or 2,6-dicyanopyridine (Fig. 19). With high thermal and chemical stability, the CTF with abundant basic nitrogen sites was an efficient catalyst for CO₂ cycloaddition to epoxides. The mesopores, large surface area and high nitrogen content were beneficial for the conversion of CO₂ and epoxides ^[125]. With abundant amino and hydroxyl groups, dicyandiamide-formaldehyde polymer (DFP) was apt for the cycloaddition reaction. The mechanism involves the ring-opening of epoxides assisted by the hydroxyl, and the activation of CO₂ induced by amine. In addition, the introduction of ammonium salts markedly enhanced the activity of DFP ^[126]. A nanoporous polymer-based NHC (NP-NHC) with an excellent 3D microporous structure, relatively high specific surface area, and large pore volume was prepared (Fig. 20). The super basic nature of NP-NHC facilitated its reaction with CO₂ to afford the corresponding carbonate adducts with 97% CO₂ fixation efficiency at room temperature. In the cycloaddition of CO₂ and epoxides, NP-NHC showed excellent performance comparable to NHC, and showed unique substrate selectivity due to its microporous structure (0.4 nm). NP-NHC effectively catalyzed CO₂ cycloaddition to monoaliphatic substituted terminal epoxides, affording the corresponding cyclic carbonates in excellent yields (92%-98%) and 100% selectivity. However, the product yield was bad with phenyl and benzyl substituted epoxides. The product yield was in trace amount (1%) for the former and zero for the latter ^[23].

Fig. 19

Fig. 19. Structures of covalent triazine frameworks: CTF-1 (left) and CTF-P (right) [125].

Fig. 20

Fig. 20. Structure of nanoporous polymer based NHC [23].

The above polymer catalysts are mainly prepared by olefin polymerization, and showed poor chemical and thermal stability. In addition, the problem of swelling typical of olefin polymers is inevitable. Therefore, the design of an innovative monomer and development of a method for the preparation of a well-ordered architecture and nanoporous polymer with high stability and low swelling tendency is desirable. It is envisaged that the fabrication of polymer-containing composite materials is a way to tackle the problems.

3.3 Carbon materials

Recently, carbon materials have received increasing attention as metal-free heterogeneous catalysts owing to easy access from natural sources, ease of preparation and regeneration, excellent thermal and chemical stability, and high specific surface area. With the easy recovery feature, carbon materials exhibit advantages in terms of activity and regenerability over traditional metal-containing catalysts due to their high flexibility forpore structure modification and surface functionalization ^[127]. There are many carbon materials such as graphene oxide (GO) and graphitic carbon nitride (g-C₃N₄) used in cycloaddition reactions. Most of these are listed in Table 2.

表 2 (Table 2)

Table 2 Catalytic performance of various carbocatalysts for the cycloaddition of CO2 to PO. Catalyst Temperature (℃) Pressure (MPa) Time (h) m(cat.) (mg) n(PO) (mmol) Yield (%) TOF a (h-1) MS-MCN 140 0.6 10 20 21.5 30.6 3.3 [128] u-g-C3N4-480 130 2 4 100 21.5 58.4 3.2 [129] Zn2+/g-C3N4/SBA-15 150 3.5 1.5 100 42.9 96.1 28.1 [130] n-butBr/mp-C3N4 140 2.5 6 200 143 87.7 10.7 [132] g-C3N4 b 130 3.5 2 50 64 5.6 3.3 [133] g-C3N4-S b 130 3.5 2 50 64 99.1 58.1 [133] Bu4NBr/P-C3N4-2 c 100 2 4 50 28.6 93.1 13.3 [131] Bu4NBr/Si/SiC@C d 45 1 10 40 28.5 96.0 7.0 [139] Bu4NBr/GO e 100 2.25 1 50 28.6 81.3 47.4 [25] Bu4NI/AP-GO f 70 1 12 30 10 91 2.6 [135] [BMIM]Br/GO g 80 1 6 20 6 99 3.1 [134] GO-[SmIm]I 140 2 4 600 214 96 8.7 [137] GO-HEIMBr 140 2 4 400 214 99 13.5 [138] GO-DMEDA-I 120 2 3 100 28.6 89.4 8.7 [136] GO-DMEDA-I 140 2 0.5 125 28.6 99.4 46.4 [136] CNT-(CH2)3(Et)3N+Cl- 110 2 6 660 214 66 3.6 [140] AA-950b 150 4 16 5 20 84.3 28.7 [142] CNFWs 150 2 12 10 20 86 14.6 [143] a Mass of synthesized cyclic carbonate over per gram carbocatalyst per hour. b Using ECH as substrate. c Using 2.4 mol% Bu4NBr as co-catalyst. dUsing 3 mol% Bu4NBr as co-catalyst. e Using 2.5 mol% Bu4NBr as co-catalyst. f Using 1 mol% Bu4NI as co-catalyst. g Using 2.5 mol% [BMIM]Br as co-catalyst.

Table 2 Catalytic performance of various carbocatalysts for the cycloaddition of CO2 to PO.

3.3.1 Catalysts based on carbon nitride

g-C₃N₄ is a polymeric carbon material mainly made up of C and N atoms connected in tris-triazine-based patterns, with H being the major impurity. Being rich in nitrogen species of different kinds, g-C₃N₄ is CO₂-phillic and can be used for CO₂ adsorption and activation. Ansari et al. ^[128] first applied mesoporous carbon nitrides (MCN) prepared by a nano-casting method using mesoporous silica as template and melamine as precursor for CO₂ cycloaddition to PO. However, a low activity was recorded despite the use of a massive amount of DMF as solvent.

Many methods were explored to enhance the activity ofg-C₃N₄, including the enhancement of defect sites ^[129], increase of surface area ^[130], introduction of Lewis acidic sites and immobilization of functional groups ^[131-133]. Urea-derived g-C₃N₄ with a higher stability and more active centers was prepared at different temperatures (450 -550 ℃). With a decrease of preparation temperature from 550 ℃ to 480 ℃, g-C₃N₄ with lower crystallinity and smaller polymerization degree was generated. Due to the enhanced amount of edge defects, there was higher catalytic activity. Using u-g-C₃N₄-480 as catalyst, complete conversion and high yield (98.9 %) of PC was attained at 130 ℃,2 MPa and 24 h ^[129]. In order to enlarge surface area, g-C₃N₄ nanophases were well dispersed in SBA-15 mesochannels by two-step vapor condensation of dicyandiamide. It was demonstrated that the defect-rich g-C₃N₄ nanophases were active for the cycloaddition reaction. Moreover, the introduction of Lewis acid sites was an efficient way to improve activity ^[130]. However, the leaching of the active ions was an inevitable problem. Therefore, Lan et al. ^[131] designed a simpl e and efficient method to prepare P-modified g-C₃N₄ (P-C₃N₄) with acid sites by thermolysis of a mixture of melamine and hexachlorotriphosphazene under metal-free conditions. For the first time, acid-base bifunctional P-C₃N₄ combined with Bu₄NBr was used as catalyst for the synthesis of cyclic carbonates by CO₂ cycloaddition to epoxides under mild conditions. It was observed that the catalytic activity of P-C₃N₄ increased with P content as a result of the formation of acid sites. P-C₃N₄-2 exhibited the best activity with a PC yield of 94.6% at 90 ℃ and 2 MPa in 4 h. The excellent performance of Bu₄NBr/P-C₃N₄-2 was attributed to the synergetic effect of the acid sites and halide anions for ring opening of epoxide as well as to the basic sites for adsorption and activation of CO₂.

New mesoporous-C₃N₄ (mp-C₃N₄) based quaternary ammonium salts were prepared by the reaction of tertiary amine on mp-C₃N₄ with alkyl halides. For the alkyl halides with long alkyl chains, the corresponding mp-C₃N₄ showed low activity due to steric hindrance during the reaction with the N-containing heterocycles of mp-C₃N₄. The halide anions and the amine species located at the edges of mp-C₃N₄ sheets were suggested to be the catalytically active species. The former activate the PO molecules while the latter promote the adsorption and activation of CO₂ ^[132]. Recently, Huang et al. ^[133] prepared amino- and hydroxyl-rich g-C₃N₄ by the hydrolysis of g-C₃N₄ in 60% H₂SO₄. The catalytic activity of g-C₃N₄ increased with the increase of H₂SO₄ treatment temperature from 40 to 80 ℃. Further rise of temperature to 100 ℃ resulted in the dissolution of g-C₃N₄ in H₂SO₄ to form a clear solution. It was considered that the enhanced catalytic activity was a result of enlarged specific surface area as well as the generation of amino and hydroxyl groups.

3.3.2 Catalysts based on graphene oxide

Being rich in oxygen functional groups (e.g., hydroxyl, epoxide, carbonyl, and carboxyl), GO has received increasing attention as a metal-free heterogeneous catalyst owing to outstanding features such as water-tolerance, high adsorption capacity, easy modification,2D morphology and sheet structure. Lan et al. ^[25] reported that GO exhibited excellent catalytic performance in the cycloaddition of CO₂ with epoxides under mild conditions in the presence of Bu₄NBr. With PO as substrate,96% yield of PC was afforded in 1 h (100 ℃ and 2MPa), and at room temperature and atmospheric pressure, the PC yield was 79% in 48 h. The synergetic action of the hydroxyl group and nucleophilic anion was proved by FT-IR. In addition, the presence of a proper amount of H₂O enhanced the conversion of epoxide remarkably. Subsequently, Luo et al. ^[134] conducted a kinetics study on the cycloaddition of CO₂ to SO over imidazolium IL/GO binary catalyst. Recently, Saptal et al. ^[135] designed (3-aminopropyl)-trimethoxysilane functionalized GO (APGO) with acid-base cooperative catalytic functions. The presence of HBDs (acid) and amines (base), and the porous nature of AP-GO were essential factors for the promotion of the cycloaddition reaction. Combined with TBAI, AP-GO was efficient for the synthesis of cyclic carbonates at 100 ℃ and atmospheric pressure, and 94% yield of SC was obtained in 27 h. Nonetheless, the recycle of homogeneous nucleophiles is complex.

Recently, Lan et al. ^[136] functionalized GO with silanol group, amine and quaternary ammonium salts by silylanization of GO with chlorine-terminal silanes followed by subsequent nucleophilic substitution reaction with N,N'-dimethylethylene- diamine through a one-pot approach (Fig. 21). It was found that the multi-functionalized GO was a water-tolerant and recyclable catalyst effective for the cycloaddition reaction at 120 ℃ and 2 MPa with a yield of 97.8% in 4 h. The non-toxic, metal-free, water-tolerant, and high-efficiency catalyst showed a TOF of 46.4 h^-1 under mild condition, which was the highest among the reported heterogeneous carbocatalysts. The excellent performance was attributed to the synergetic effect of the silanol group and halide anion for epoxide ring opening as well as the ability of amine for the adsorption and activation of CO₂. Li and coworkers ^{[137, 138]} immobilized imidazole ILs and hydroxyl-functionalized ILs on GO using the method reported by Lan et al. ^[136]. The presence of hydroxyl groups was responsible for the high catalytic performance of the GO that was grafted with hydroxyl-functionalized ILs. The hydroxyl groups provided efficient synergistic sites to accelerate the ring opening of epoxide. The supported catalyst can be easily separated and reused for at least seven times without significant loss of activity.

Fig. 21

Fig. 21. Preparation of GO grafted with quaternary ammonium salts bearing terminal amino groups [136].

3.4 Other carbon materials

Liu et al. ^[139] synthesized hollow-structured Si/SiC@C nanospheres (HSSC) with high specific surface areas (BET 239 m²/g) by magnesiothermic reduction of resin-coated SiO₂ spheres. A PC yield of 96% was obtained over TBAB/HSSC at 45 ℃ and 1.0 MPa in 10 h. Different from the TBAB/GO catalyst system, the excellent performance was attributed to the synergetic effect between Brnsted acid (HSSC) and Lewis base (TBAB) as well as to the high surface area and porosity of HSSC.

Other carbon materials were used to graft ILs. Different from the process for the generation of GO-based ILs ^[136-138], the carbon nanotube-supported quaternary ammonium salts (QCNT) were synthesized by acylation (reaction of carboxyl groups on oxidized carbon nanotube with thionyl chloride) and esterification (reaction of acyl chloride with hydroxyl on functionalized quaternary salts). The cycloaddition reaction of CO₂ with epoxides over QCNT followed first order kinetics, at least for the first 3 h. In addition, a synergy was observed between the carboxyl and the ammonium moiety grafted on the carbon nanotubes, and the distance between carboxyl and quaternary N had a strong impact on the cycloaddition reaction ^[140]. Coconut shell activated carbon (CSAC) grafted with ILs for the continuous cycloaddition of CO₂ to ECH in a packed bed reactor. The conversion of ECH over the CSAC tethered Bmim/Br, Bmim/BF₄, Bmim-OH/Br and Bmim-COOH/Br were 63.4%,74.5%,83.4% and 85.9% at 140 ℃ and 1.4 MPa, respectively. Hydroxyl or carboxyl groups on the CSAC grafted with Bmim-OH/Br and Bmim-COOH/Br formed hydrogen bondswith the oxygen of ECH, playing a synergistic role with Br^- to enhance the cycloaddition reaction ^[141].

N-doped carbon materials were efficient catalysts for the cycloadition reaction. In the work of Ma et al. ^[142], an N-doped porous carbon monolith with a large specific surface area was prepared from alginic acid at different pyrolysis temperatures (defined as AA-750, AA-850, AA-950, AA-1000). The rise of the pyrolysis temperature resulted in the decline of N content but enhancement of CO₂ adsorption capacity and surface area (20 mmol/g and 2486 m²/g for AA-1000). AA-950 showed the best catalytic activity in the cycloaddition of CO₂ to ECH at 150 ℃ and 4 MPa with a product yield of 84.3% in 16 h. The high activity of AA-950 wasascribed to the synergic effect of surface area and pyridinic-N. Li et al. ^[143] also efficiently synthesized cyclic carbonates by the cycloaddition of CO₂ to epoxides over nitrogen-doped porous carbon nanofiber webs (CNFWs) prepared by a one-step carbonization-activation treatment of pre-synthesized polypyrrole nanofiber webs. The CNFWs exhibited outstanding adsorption capacity (4.42 mmol/g) at 25 ℃ and 1 MPa. The cycloaddition reaction mechanism was related to the adsorption and activation of CO₂ by Lewis base N sites as well as to the nucleophilic attack of pyridine N at epoxide activated by the hydroxyl group on CNFWs through hydrogen bonding.

However, the limited amounts of functional groups on carbon materials hinder the use of carbocatalysts. It is envisaged that the multi-hydroxyl and multi-cation concept together with the “multilayered covalently supported” approach can bring about activity improvement forcarbocatalysts. Moreover, the development of the controlled synthesis of novel carbon materials as efficient catalysts is meaningful.

4 Conclusions and perspectives

Recent advances in the design of metal-free catalysts including organic bases, ILs, supported catalysts, organic copolymers and carbon materials as well as progress in the cycloaddition mechanism were discussed. The synergistic effects were described in detail at the molecular level. The cooperative effects of HBDs and organic bases, organic bases and nucleophilic anions, HBDs and nucleophilic anions as well as that of the active component and support for the enhancement of catalytic activity were emphasized. The review affords insights into the design and development of efficient metal-free catalytic systems. The ultimate challenge is to design efficient metal-free catalytic systems for flow reactors where the cycloaddition reaction is conducted at ambient temperature and atmospheric pressure using CO₂ of low concentration such as “waste” CO₂. For CO₂ adsorption as well as the activation of CO₂ and epoxides, the use of the multi-synergetic strategy is advisable for the development of a suitable catalyst system for the cycloaddition reaction. The multi-functional approach was highlighted. In addition, DFT calculations are recommended for the advanced design of catalysts and better understanding of the catalytic mechanism.

Abbreviations

HBDs  Hydrogen bond donors

DFT  Density functional theory

ILs  Ionic liquids

NHCs  N-heterocyclic carbenes

TBD  1,5,7-Triaza-bicyclo[4.4.0]dec-5-enium

DBU  1,8-Diazabicyclo-[5.4.0]undec-7-ene

NHC-CO₂  Imidazolium carboxylates

IPr-CO₂  1,3-Bis(2,6-diisopropylphenyl) imidazolium-2-carboxylate

PO  Propylene oxide

PC  Propylene carbonate

IBu-CO₂  1,3-Ditert- butylimidazolium-2-carboxylate

NHO  N-Heterocyclic olefin

SC  Styrene carbonate

DMAP  N, N-dimethylaminopyridine

DABCO  1,4-Diazabicyclo[2.2.2]octane

TEOA  Triethanolamine

DEOA  Diethanolamine

MEOA   Monoethanolamine

Im  Imidazole

Py  Pyridine

TEA  Triethylamine

DEA  Diethylamine

PEG600  Poly(ethylene glycol)-600

PEG400  Poly(ethylene glycol)-400

β-CD  β-cyclodextrin

PG  1,2-Propylene glycol

2-Py-EtOH  2-Pyridinemethanol

ECH  Epichlorohydrin

NBS  N-bromosuccinimide

BPO  Benzoyl peroxide

SO  Styrene oxide

PETT  Pentaerythritol

TA  Tannic acid

HEMIM  1-(2-Hydroxyl-ethyl)-3-methylimidazolium

HEBimBr  1-(2-Hydroxyl-ethyl)-3-butylimidazolium bromide

P-ylide  Phosphorus ylide

UDILs  Urea derivatives ILs

AAs  α-Amino acids

HEPIMBr  3-(2-Hydroxyl-ethyl)-1-propylimidazolium bromide

POSS-Imi  Polyhedral oligomeric silsesquioxane functional-ized with imidazolium chloride peripheries

PS  Polystyrene

CS  Chitosan

[VBIM]Cl  3-Butyl-1-vinylimidazolium chloride

DVB  Divinylbenzene

PVIm  Poly(N-vinylimidazole-co-divinylbenzene)

AIBN   Azobis(isobutyronitrile)

[bvbim]X  1,3-bis(4-vinylbenzyl)imidazolium salts

MPILs  Meso-macroporous hierarchical PILs

TBB  1,2,4,5-Tetrakis(bromomethyl)benzene

TBB-BPy  Cross-linked cationic polymer

F-PIL-Br  Fluoro-functionalized polymeric ILs

CTF  Triazine-based covalent organic framework

DFP  Dicyandiamide-formaldehyde polymer

g-C₃N₄  Graphitic carbon nitride

P-C₃N₄  P-modified g-C₃N₄

GO  Graphene oxide

mp-C₃N₄  Mesoporous-C₃N₄

HSSC  Hollow-structured Si/SiC@C nanospheres

QCNT  Carbon nanotube-supported quaternary ammonium salts

CSAC  Coconut shell activated carbon

CNFWs  Nitrogen- doped porous carbon nanofiber webs

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