Tuning Ionic Liquid-Based Catalysts for CO2 Conversion into Quinazoline-2,4(1H,3H)-diones

Carbon capture and storage (CCS) and carbon capture and utilization (CCU) are two kinds of strategies to reduce the CO2 concentration in the atmosphere, which is emitted from the burning of fossil fuels and leads to the greenhouse effect. With the unique properties of ionic liquids (ILs), such as low vapor pressures, tunable structures, high solubilities, and high thermal and chemical stabilities, they could be used as solvents and catalysts for CO2 capture and conversion into value-added chemicals. In this critical review, we mainly focus our attention on the tuning IL-based catalysts for CO2 conversion into quinazoline-2,4(1H,3H)-diones from o-aminobenzonitriles during this decade (2012~2022). Due to the importance of basicity and nucleophilicity of catalysts, kinds of ILs with basic anions such as [OH], carboxylates, aprotic heterocyclic anions, etc., for conversion CO2 and o-aminobenzonitriles into quinazoline-2,4(1H,3H)-diones via different catalytic mechanisms, including amino preferential activation, CO2 preferential activation, and simultaneous amino and CO2 activation, are investigated systematically. Finally, future directions and prospects for CO2 conversion by IL-based catalysts are outlined. This review is benefit for academic researchers to obtain an overall understanding of the synthesis of quinazoline-2,4(1H,3H)-diones from CO2 and o-aminobenzonitriles by IL-based catalysts. This work will also open a door to develop novel IL-based catalysts for the conversion of other acid gases such as SO2 and H2S.


Introduction
Large amounts of CO 2 emissions from the combustion of fossil fuels have caused severe global climate change issues, including the greenhouse effect, global warming, and extreme climate. Carbon capture and storage (CCS) and carbon capture and utilization (CCU) are two kinds of technologies to reduce the CO 2 concentration in the atmosphere [1]. CCS technology means that the captured CO 2 then is pressurized and transported to a storage site, where it is injected into one of a number of types of stable geological features, trapping it for multiple hundreds or thousands of years and preventing its subsequent emission into the atmosphere [2]. Alternatively, CCU technology presents a free, abundant, and non-toxic carbon source with economic benefits to offset high capture costs, wherein CCU systems, atmospheric CO 2 or industrially emitted CO 2 could be recycled and converted into carbon-containing value-added chemicals and fuels [3]. Among these chemicals, quinazoline-2,4(1H,3H)-diones, regarded as pharmaceutical intermediates with a wide range of biological activity, have been successfully synthesized from CO 2 and o-aminobenzonitriles. This reaction is an atom-economical route, and it has been widely investigated using kinds of catalysts, such as metal oxides, [4,5] inorganic/organic supported/grafted catalysts, [6][7][8][9] organic bases, [10][11][12][13][14][15] water, [16] etc. However, most of

CO2 Conversion into Quinazoline-2,4(1H,3H)-diones
The general mechanisms of the reaction of CO2 and o-aminobenzonitriles by catalysts reported in the literature can be classified into three categories: amino preferential activation, CO2 preferential activation, and both amino and CO2 simultaneous activation.

CO 2 Conversion into Quinazoline-2,4(1H,3H)-diones
The general mechanisms of the reaction of CO 2 and o-aminobenzonitriles by catalysts reported in the literature can be classified into three categories: amino preferential activation, CO 2 preferential activation, and both amino and CO 2 simultaneous activation.

[OH]-Based ILs as a Catalyst (1) [Bmim][OH]
According to the results of the study from Xu and Wang et al. [50], the amino group can react with CO 2 when its pKa is above 8.6 ( Figure 3). Therefore, as the pKa of oaminobenzonitrile is relatively small, a basic catalyst is needed, which can improve the reactivity of the amino group on the o-aminobenzonitriles for CO 2 conversion. The first basic IL used as a catalyst for CO 2 conversion with o-aminobenzonitriles for synthesis of quinazoline-2,4(1H,3H)-diones is 1-butyl-3-methyl imidazolium hydroxide ([Bmim][OH]), which is reported by Bhanage et al. [25] They show that a remarkable activity of [Bmim] [OH] can be achieved for the wide variety of substituted o-aminobenzonitriles. The mechanism study indicates that [Bmim][OH] only activates the amino group, and subsequently, the dehydrogenated amino group with high enough basicity to react with CO 2 . Wu et al. [51] verifies the same mechanism using a computational study. The mechanism for the chemical fixation of CO 2 with o-aminobenzonitrile in the presence of [Bmim][OH] is proposed in Figure 4.
indicates that [Bmim][OH] only activates the amino group, and sub drogenated amino group with high enough basicity to react with C ifies the same mechanism using a computational study. The mecha fixation of CO2 with o-aminobenzonitrile in the presence of [Bmim    indicates that [Bmim][OH] only activates the amino group, and subsequ drogenated amino group with high enough basicity to react with CO2. W ifies the same mechanism using a computational study. The mechanism fixation of CO2 with o-aminobenzonitrile in the presence of [Bmim][OH Figure 4.   (2) Supported [OH]-based ILs Bhanage et al. [52] report the synthesis of quinazoline-2,4(1H,3H)-diones through the carboxylative coupling of CO 2 with o-aminobenzonitriles at 120 • C and 30 bar CO 2 using the heterogeneous supported IL phase catalyst 1-hexyl-3-methyl imidazolium hydroxide  Figure 5). They investigate the influence of solvents on the performance of grafted catalysts. The results show that the reaction has proceeded well in DMF or DMSO, while it did not take place in nonpolar solvents (toluene) and aprotic polar solvents (acetonitrile). However, only a low product yield is obtained in a polar protic solvent (methanol), while moderate product yield is obtained in H 2 O. The plausible reaction pathway is also similar to [Bmim][OH].
Bhanage et al. [52] report the synthesis of quinazoline-2,4(1H,3H)-diones carboxylative coupling of CO2 with o-aminobenzonitriles at 120 °C and 30 ba the heterogeneous supported IL phase catalyst 1-hexyl-3-methyl imidazolium supported on silica (  Figure 5). They the influence of solvents on the performance of grafted catalysts. The result the reaction has proceeded well in DMF or DMSO, while it did not take place solvents (toluene) and aprotic polar solvents (acetonitrile). However, only a l yield is obtained in a polar protic solvent (methanol), while moderate prod obtained in H2O. The plausible reaction pathway is also similar to [Bmim][OH

Carboxylate-Based ILs as Catalysts
(1) [OAc]-based ILs Han et al. [54] reports that 1-butyl-3-methylimidazolium acetate ([Bmim act as both solvent and catalyst for the conversion of CO2 and o-aminobenzo quinazoline-2,4(1H,3H)-diones at atmospheric pressure of CO2 with high yi DMF as the solvent, the results show that the yield of product increases with th amount of the IL, indicating that the IL is an active catalyst for the reaction. mechanism is proposed as depicted in Figure 6. After the acetate anion [OAc proton from the amino group, the activated amino group reacts with CO2 rapi to the formation of a carbamate. Subsequently, an intramolecular nucleophilic and the rearrangement occur. The final product will be obtained from the prot

Carboxylate-Based ILs as Catalysts (1) [OAc]-based ILs
Han et al. [54] reports that 1-butyl-3-methylimidazolium acetate ([Bmim][OAc]) can act as both solvent and catalyst for the conversion of CO 2 and o-aminobenzonitriles into quinazoline-2,4(1H,3H)-diones at atmospheric pressure of CO 2 with high yields. Using DMF as the solvent, the results show that the yield of product increases with the increased amount of the IL, indicating that the IL is an active catalyst for the reaction. A plausible mechanism is proposed as depicted in Figure 6. After the acetate anion [OAc] captures a proton from the amino group, the activated amino group reacts with CO 2 rapidly, leading to the formation of a carbamate. Subsequently, an intramolecular nucleophilic cyclization and the rearrangement occur. The final product will be obtained from the proton transfer. (2) Atypical carboxylate-containing ReILs The reversible ionic liquids (ReILs) have first been reported by Jessop et al. [55], where the mixture of superbase and alcohol could capture equimolar CO2 to form carboxylate-containing ReILs. Zheng et al. [56] have synthesized a series of ReILs as both the solvents and catalysts for the conversion of CO2 or CS2 and o-aminobenzonitriles into quinazoline-2,4(1H,3H)-diones or quinazoline-2,4(1H,3H)-dithiones. A plausible mechanism is proposed as depicted in Figure 7. As can be seen, the reaction mechanism using this atypical carboxylate-containing ReILs as the catalyst is similar to other mechanisms using typical carboxylate anion-containing ILs as the catalysts. The CO2 conversion can be performed at 40 °C and 1 bar CO2 with a high yield of quinazoline-2,4(1H,3H)-diones.  (2) Atypical carboxylate-containing ReILs The reversible ionic liquids (ReILs) have first been reported by Jessop et al. [55], where the mixture of superbase and alcohol could capture equimolar CO 2 to form carboxylatecontaining ReILs. Zheng et al. [56] have synthesized a series of ReILs as both the solvents and catalysts for the conversion of CO 2 or CS 2 and o-aminobenzonitriles into quinazoline-2,4(1H,3H)-diones or quinazoline-2,4(1H,3H)-dithiones. A plausible mechanism is proposed as depicted in Figure 7. As can be seen, the reaction mechanism using this atypical carboxylate-containing ReILs as the catalyst is similar to other mechanisms using typical carboxylate anion-containing ILs as the catalysts. The CO 2 conversion can be performed at 40 • C and 1 bar CO 2 with a high yield of quinazoline-2,4(1H,3H)-diones. Compared with the reaction conditions using different ILs based on anions [OH] (at 120 • C and 30 bar), [WO 4 ] (at 140 • C and 1 bar), and [OAc] (at 90 • C and 1 bar), the advantages using ReILs as the catalyst can be mild conditions, high efficiency, easy separation of products, and the reusability of catalysts. (2) [Im−CO2] complexes It is known that anions play a key role in CO2 capture or CO2 utilization. However, Wang et al. [57] report that the basicity of cations affect the catalytic activity of ILs dramatically, and the hydrogen bond from cations could promote this reaction at 80 °C and 1 bar CO2. In their research, 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene imidazolate (2) [Im−CO 2 ] complexes It is known that anions play a key role in CO 2 capture or CO 2 utilization. However, Wang et al. [57] report that the basicity of cations affect the catalytic activity of ILs dramatically, and the hydrogen bond from cations could promote this reaction at 80 • C and 1 bar CO 2 . In their research, 7-methyl-1,5,7-triazabicyclo [4.4.0]dec-5-ene imidazolate ([HMTBD][Im]) with a higher basicity (pKa = 13.0) could improve the yield to 87%, while 1-methylimidazolium imidazolate (HMIm][Im]) with a lower basicity (pKa = 7.1) exhibits poor catalytic activity with a yield of 16%, indicating that the cation can impact this reaction dramatically. In addition, the results of the quantum-chemical calculations, NMR spectroscopic investigations, and controlled experiments indicate that in-situ generated [Ch][Im−CO 2 ] is the real catalyst for the conversion of CO 2 and o-aminobenzonitriles into quinazoline-2,4(1H,3H)-diones. Thereby, the possible reaction mechanism can be illustrated in Figure 8, which is similar to that of using [OAc]-based ILs as catalysts.

Aprotic Heterocyclic Anion ILs as Catalysts
(1) Azolate-based anion IL Dyson et al. [58] investigate the relationship between the pKa of the IL anion (co gate acid) and the yield of quinazoline-2,4(1H,3H)-dione. The results show that a li relationship is found in the pKa range of 9.2~14.4. They have determined the acidity o quinazoline-2,4(1H,3H)-dione product using UV/Vis spectroscopy, and the measured is 14.7. Thus, all catalysts with pKa values above 14.7 only act as pre-catalysts toward formation of the quinazolide anion IL catalysts, explaining the uniform reaction yield served for all ILs with a pKa above this value ( Figure 9). The results indicate that neu ization of the original catalyst and formation of the alternative quinazolide anion cat leads to the efficient CO2 conversion.

Aprotic Heterocyclic Anion ILs as Catalysts
(1) Azolate-based anion IL Dyson et al. [58] investigate the relationship between the pKa of the IL anion (conjugate acid) and the yield of quinazoline-2,4(1H,3H)-dione. The results show that a linear relationship is found in the pKa range of 9.2~14.4. They have determined the acidity of the quinazoline-2,4(1H,3H)-dione product using UV/Vis spectroscopy, and the measured pKa is 14.7. Thus, all catalysts with pKa values above 14.7 only act as pre-catalysts towards the formation of the quinazolide anion IL catalysts, explaining the uniform reaction yield observed for all ILs with a pKa above this value ( Figure 9). The results indicate that neutralization of the original catalyst and formation of the alternative quinazolide anion catalyst leads to the efficient CO 2 conversion. is 14.7. Thus, all catalysts with pKa values above 14.7 only act as pre-catalysts towards the formation of the quinazolide anion IL catalysts, explaining the uniform reaction yield observed for all ILs with a pKa above this value ( Figure 9). The results indicate that neutralization of the original catalyst and formation of the alternative quinazolide anion catalyst leads to the efficient CO2 conversion. Figure 9. Proposed catalytic cycle for the preparation of quinazoline-2,4-diones by catalysts with a pKa < 14.7 (cycle 1) and the cycle for catalysts with a pKa > 14.7 (cycle 2). The IL cation was omitted for clarity. Reprinted with permission from Ref. [58]. Copyright 2017 Wiley-VCH. [Suc] is −0.793 e. As more negative charge is favorable for activating the substrate, the actual catalytic species should be [Suc] rather than [Suc−CO 2 ]. Therefore, the possible reaction mechanism can be illustrated in Figure 10, including amino activation, cyclization, ring-opening, and ring-closure and catalyst regeneration. [Suc] is −0.793 e. As more negative charge is favorable for activati the substrate, the actual catalytic species should be [Suc] rather than [Suc−CO2]. Therefo the possible reaction mechanism can be illustrated in Figure 10, including amino activ tion, cyclization, ring-opening, and ring-closure and catalyst regeneration.

[Ch][OH] + Water
Ma and Han et al. [60] report that the transformation of CO2 and o-aminobenz nitriles to quinazoline-2,4(1H,3H)-diones in water can be promoted by choline hydroxi ([Ch] [OH]. The results of yield vs. CO2 pressure show that the yield becomes independe of the pressure above 2 MPa. Additionally, the results of yield vs. temperature show th the yield can be reached 92% at 90 °C, and then remain unchanged with further-increasi temperature. The reaction mechanism is discussed and proposed as Figure 11. It is we known that CO2 can form carbonic acid (H2CO3) in water.

[Ch][OH] + Water
Ma and Han et al. [60] report that the transformation of CO 2 and o-aminobenzonitriles to quinazoline-2,4(1H,3H)-diones in water can be promoted by choline hydroxide ([Ch] [OH]. The results of yield vs. CO 2 pressure show that the yield becomes independent of the pressure above 2 MPa. Additionally, the results of yield vs. temperature show that the yield can be reached 92% at 90 • C, and then remain unchanged with further-increasing temperature. The reaction mechanism is discussed and proposed as Figure 11. It is well-known that CO 2  nucleophilically attacks the C atom of the nitrile group. After a series of rearrangement and catalyst regeneration steps, the quinazoline-2,4(1H,3H)-dione is formed.
Molecules 2023, 28, x FOR PEER REVIEW 10 of 23 Figure 11. Plausible mechanism for the reaction of CO2 and 1a catalyzed by choline hydroxide aqueous solution. Reprinted with permission from Ref. [60]. Copyright 2014 Royal Society of Chemistry.

NHC as a Catalyst
Shi et al. [61] reports that the quinazoline-2,4(1H,3H)-diones can be obtained from CO2 and o-aminobenzonitriles catalyzed by N-heterocyclic carbenes (NHCs) at 120 °C and 1 bar CO2. The NHCs are generated from a series of imidazolium chloride in the presence of the base K2CO3 in DMSO. The plausible reaction mechanism is reported and illustrated

NHC as a Catalyst
Shi et al. [61] reports that the quinazoline-2,4(1H,3H)-diones can be obtained from CO 2 and o-aminobenzonitriles catalyzed by N-heterocyclic carbenes (NHCs) at 120 • C and 1 bar CO 2 . The NHCs are generated from a series of imidazolium chloride in the presence of the base K 2 CO 3 in DMSO. The plausible reaction mechanism is reported and illustrated in Figure 12. The authors suggest that NHCs are the catalytic active species, and NHCs transfer CO 2 to quinazoline-2,4(1H,3H)-diones via the formation of NHC−CO 2 adducts. After the nucleophilic addition of zwitterionic adducts to the nitrile group and the intramolecular cyclization, the NHCs are regenerated. Finally, quinazoline-2,4(1H,3H)diones are produced via a series of proton transfer, ring opening, and intramolecular nucleophilic addition. ous solution. Reprinted with permission from Ref. [60]. Copyright 2014 Royal Society of Chemistry.

NHC as a Catalyst
Shi et al. [61] reports that the quinazoline-2,4(1H,3H)-diones can be obtained from CO2 and o-aminobenzonitriles catalyzed by N-heterocyclic carbenes (NHCs) at 120 °C and 1 bar CO2. The NHCs are generated from a series of imidazolium chloride in the presence of the base K2CO3 in DMSO. The plausible reaction mechanism is reported and illustrated in Figure 12. The authors suggest that NHCs are the catalytic active species, and NHCs transfer CO2 to quinazoline-2,4(1H,3H)-diones via the formation of NHC−CO2 adducts. After the nucleophilic addition of zwitterionic adducts to the nitrile group and the intramolecular cyclization, the NHCs are regenerated. Finally, quinazoline-2,4(1H,3H)diones are produced via a series of proton transfer, ring opening, and intramolecular nucleophilic addition.

[Bu 4 P][2-MIm] as a Catalyst
Liu et al. [62] reports that quinazoline-2,4(1H,3H)-diones can be obtained in excellent yields from atmospheric CO 2 and o-aminobenzonitriles using tetrabutylphosphonium 2-methylimidazolate ([Bu 4 P][2-MIm]) as a catalyst. A possible reaction mechanism is proposed and illustrated in Figure 13. It can be seen that the carbamate intermediate [2-MIm−CO 2 ] is first generated from the reaction of CO 2 and [2-MIm] anion. After IL nucleophilically attacks the CN group and is followed by intramolecular nucleophilic cyclization and hydrogen transfer, the corresponding quinazoline-2,4(1H,3H)-dione is obtained and the catalyst is regenerated.
yields from atmospheric CO2 and o-aminobenzonitriles using tetrabutylphosphonium 2methylimidazolate ([Bu4P] ) as a catalyst. A possible reaction mechanism is proposed and illustrated in Figure 13. It can be seen that the carbamate intermediate [2-MIm−CO2] is first generated from the reaction of CO2 and  anion. After IL nucleophilically attacks the CN group and is followed by intramolecular nucleophilic cyclization and hydrogen transfer, the corresponding quinazoline-2,4(1H,3H)-dione is obtained and the catalyst is regenerated.    yields from atmospheric CO2 and o-aminobenzonitriles using tetrabutylphosphonium 2methylimidazolate ([Bu4P] ) as a catalyst. A possible reaction mechanism is proposed and illustrated in Figure 13. It can be seen that the carbamate intermediate [2-MIm−CO2] is first generated from the reaction of CO2 and  anion. After IL nucleophilically attacks the CN group and is followed by intramolecular nucleophilic cyclization and hydrogen transfer, the corresponding quinazoline-2,4(1H,3H)-dione is obtained and the catalyst is regenerated.     can be used as both the catalyst and solvent for the reaction of CO 2 with various oaminobenzonitriles at atmospheric pressure and room temperature, producing corresponding quinazoline-2,4(1H,3H)-diones in excellent yields. The reported possible reaction pathway can be found in Figure 15. It can be seen that [HDBU][TFE] activates both CO 2 and the substrates simultaneously, resulting in dehydrogenated amino and [TFE−CO 2 ] active species. After the nucleophilic attack, intramolecular nucleophilic cyclization, rearrangement, and hydrogen transfer, quinazoline-2,4(1H,3H)-diones are obtained and [HDBU][TFE] is regenerated. However, Mu and Liu et al. [66] investigate using systematic DFT calculations that one o-aminobenzonitrile molecule requires two CO 2 molecules to yield one quinazoline-2,4-(1H,3H)-dione. One CO 2 acts as a reactant, while another transferred to [TFE-CO 2 ] acts as the catalyst. The conversion mechanism begins with nitrile activation, which is different from Figure 15 [TFE] can be used as both the catalyst and solvent for the reaction of CO2 with various o-aminobenzonitriles at atmospheric pressure and room temperature, producing corresponding quinazoline-2,4(1H,3H)-diones in excellent yields. The reported possible reaction pathway can be found in Figure 15. It can be seen that [HDBU][TFE] activates both CO2 and the substrates simultaneously, resulting in dehydrogenated amino and [TFE−CO2] active species. After the nucleophilic attack, intramolecular nucleophilic cyclization, rearrangement, and hydrogen transfer, quinazoline-2,4(1H,3H)-diones are obtained and [HDBU][TFE] is regenerated. However, Mu and Liu et al. [66] investigate using systematic DFT calculations that one o-aminobenzonitrile molecule requires two CO2 molecules to yield one quinazoline-2,4-(1H,3H)-dione. One CO2 acts as a reactant, while another transferred to [TFE-CO2] acts as the catalyst. The conversion mechanism begins with nitrile activation, which is different from Figure 15. Recently, [HDBU][TFE] grafted on Fe3O4 for fixation of CO2 into quinazoline-2,4(1H,3H)-dione has been reported by Vishwakarm and Mahto et al. [67].   (3) Phenolate ILs as catalysts Zhu and Wang et al. [69] have synthesized a series of aprotic phenolate ILs with cholinium cation and substituent phenolate anions. Among these ILs, cholinium 2,4dichlorophenolate ([Ch] [2,) can efficiently promote the conversion of CO 2 to quinazoline-2,4(1H,3H)-diones at 40 • C and 1 bar CO 2 . The reported plausible catalytic reaction mechanism can be found in Figure 17.  Figure 16. Plausible reaction mechanism for the synthesis of quinazoline-2,4(1H,3H)-di printed with permission from Ref. [68]. Copyright 2022 Elsevier Ltd.

Amino Acid Anion IL as a Catalyst
He et al. [73] reports that tetra-butylphosphonium argininate ([TPB][Arg]) could promote cyclization of o-aminobenzonitrile with CO2. The authors have investigated the influences of the reaction parameters, including CO2 pressure, reaction temperature, and time. The results showed that the yield decreases from 95% to 16% with CO2 pressure decreasing from 8.5 to 0.1 MPa. In addition, the comparative yields are obtained at 120 °C and 100 °C after 12 h reaction. The possible pathway for the reaction of o-aminobenzonitrile with CO2 catalyzed by [TBP][Arg] is illustrated in Figure 21. It can be seen that both the amino group and CO2 are initially activated by the bifunctional anion (carboxyl group and guanidine group, respectively) in [TBP] [Arg]. After the nucleophilic attack, intramolecular nucleophilic cyclization, rearrangement, and hydrogen transfer, quinazoline-2,4(1H,3H)-dione can be obtained while [TPB][Arg] can be regenerated.

Amino Acid Anion IL as a Catalyst
He et al. [73] reports that tetra-butylphosphonium argininate ([TPB][Arg]) could promote cyclization of o-aminobenzonitrile with CO 2 . The authors have investigated the influences of the reaction parameters, including CO 2 pressure, reaction temperature, and time. The results showed that the yield decreases from 95% to 16% with CO 2 pressure decreasing from 8.5 to 0.1 MPa. In addition, the comparative yields are obtained at 120 • C and 100 • C after 12 h reaction. The possible pathway for the reaction of o-aminobenzonitrile with CO 2 catalyzed by [TBP][Arg] is illustrated in Figure 21. It can be seen that both the amino group and CO 2 are initially activated by the bifunctional anion (carboxyl group and guanidine group, respectively) in [TBP] [Arg]. After the nucleophilic attack, intramolecular nucleophilic cyclization, rearrangement, and hydrogen transfer, quinazoline-2,4(1H,3H)dione can be obtained while [TPB][Arg] can be regenerated. [Im] is chosen as a catalyst, and the plausible mechanism could be illustrated in Figure 22. With simultaneous activation of both amino and CO2, following Reprinted with permission from Ref. [73]. Copyright 2015 Royal Society of Chemistry.

Aprotic Heterocyclic Anion ILs as Catalysts
(1) Azolate anion ILs as catalysts He et al. [74] reports a series of azolate anion ILs for the carboxylative cyclization of o-aminobenzonitriles with CO 2 at 120 • C and 1 bar CO 2  [Im] is chosen as a catalyst, and the plausible mechanism could be illustrated in Figure 22. With simultaneous activation of both amino and CO 2 , following the dehydrogenation, nucleophilic attack, intramolecular nucleophilic cyclization, rearrangement, and hydrogen transfer, quinazoline-2,4(1H,3H)-dione is obtained and [HTMG][Im] is regenerated.  [Im] is chosen as a catalyst, and the plausible mechanism could be illustrated in Figure 22. With simultaneous activation of both amino and CO2, following the dehydrogenation, nucleophilic attack, intramolecular nucleophilic cyclization, rearrangement, and hydrogen transfer, quinazoline-2,4(1H,3H)-dione is obtained and [HTMG][Im] is regenerated.  Liu et al. [75] have reported a series of triazolate anion ([Triz]) ILs for CO 2 conversion into quinazoline-2,4(1H,3H)-dione, and [HTMG][Triz] has exhibited a high activity at 50 • C and 1 bar CO 2 without any organic solvents. The plausible reaction mechanism can be found in Figure 23, which is similar to the reaction using [HTMG][Im] as the catalyst. Liu et al. [75] have reported a series of triazolate anion ([Triz]) ILs for CO2 conversion into quinazoline-2,4(1H,3H)-dione, and [HTMG][Triz] has exhibited a high activity at 50 °C and 1 bar CO2 without any organic solvents. The plausible reaction mechanism can be found in Figure 23, which is similar to the reaction using [HTMG][Im] as the catalyst.  (2) Imide anion IL as a catalyst Liu et al. [76] reports the 1,1,3,3-tetramethylguanidinium succinimide ([HTMG][Suc]) as the solvent and catalyst for the efficient transformation of CO 2 and o-aminobenzonitriles into quinazoline-2,4(1H,3H)-diones at 60 • C and 20 bar CO 2 . The reported possible pathways of CO 2 and o-aminobenzonitrile catalyzed by [HTMG][Suc] are illustrated in Figure 24, where both the amino group and CO 2 are simultaneously activated. Figure 23. Plausible reaction mechanism. Reprinted with permission from Ref. [75]. Copyright 2020 American Chemical Society.
(2) Imide anion IL as a catalyst Liu et al. [76] reports the 1,1,3,3-tetramethylguanidinium succinimide ([HTMG][Suc]) as the solvent and catalyst for the efficient transformation of CO2 and o-aminobenzonitriles into quinazoline-2,4(1H,3H)-diones at 60 °C and 20 bar CO2. The reported possible pathways of CO2 and o-aminobenzonitrile catalyzed by [HTMG][Suc] are illustrated in Figure 24, where both the amino group and CO2 are simultaneously activated.  (3) Hydantoin anion IL as a catalyst Xu et al. [48] have synthesized a hydantoin anion-functional IL, tri-n-butylethylphosphonium 1-methylhydantoin ([P 4442 ][1-MHy]), for efficient CO 2 capture and catalytic conversion of CO 2 to quinazoline-2,4(1H,3H)-dione. The results of CO 2 capture show that the capacity of IL at 30 • C and 1 bar is up to 1.58 mol CO 2 per mol IL, which is attributed to the multiple-site cooperative interactions. Moreover, the results of CO 2 conversion show that the yield of quinazoline-2,4(1H,3H)-dione is 97% at 60 • C and 2 MPa CO 2 . Possible reaction mechanisms of CO 2 with o-aminobenzonitrile catalyzed by [P 4442 ][1-MHy] are shown in Figure 25, similar to the mechanisms of this reaction using other aprotic heterocyclic anion ILs as catalysts.  Figure 25, similar to the mechanisms of this reaction using other aprotic heterocyclic anion ILs as catalysts.

Mechanisms Analysis
It is known that the pKa of o-aminobenzonitrile at 25 °C is only 0.77, [77] and insufficient nucleophilicity cannot induce nucleophilic attacks on CO2. Thus, it is necessary to add catalysts with basicity and nucleophilicity to improve the conversion of o-aminobenzonitrile and CO2. However, the different basicity and different nucleophilicity of catalysts result in different mechanisms. As aforementioned, there are three kinds of mecha- Reprinted with permission from Ref. [48]. Copyright 2022 American Chemical Society.

Mechanisms Analysis
It is known that the pKa of o-aminobenzonitrile at 25 • C is only 0.77, [77] and insufficient nucleophilicity cannot induce nucleophilic attacks on CO 2 . Thus, it is necessary to add catalysts with basicity and nucleophilicity to improve the conversion of o-aminobenzonitrile and CO 2 . However, the different basicity and different nucleophilicity of catalysts result in different mechanisms. As aforementioned, there are three kinds of mechanisms of the reaction of CO 2 and o-aminobenzonitriles catalyzed by ILs reported in the literature, including amino preferential activation, CO 2 preferential activation, and both amino and CO 2 simultaneous activation. According to the literature and our critical analysis, although the mechanisms in each group are different in detail, the steps are basically similar in general. Thus, the general reaction mechanisms for CO 2 conversion into quinazoline-2,4(1H,3H)diones can be summarized as follows.
For "amino preferential activation", the anion ([X]) of IL with proper basicity first dehydrogenates the hydrogen of the amino group in the o-aminobenzonitrile, resulting in the active amino group with a negative charge (-HN -). Second, the negative-charged amino nucleophilically attacks the C atom of CO 2 , resulting in the carbamate anion (-HNCOO -). Third, the carbamate anion nucleophilically attacks the C atom of the nitrile group in the o-aminobenzonitrile, leading to an intramolecular nucleophilic cyclization. Last, the ring is rearranged along with proton transfer, resulting from the presence of tautomeric forms (-NHC( = O)--N = C(OH)-) or the assistance from the anion (-NH + [X] -N -+ [HX]). After these four steps, product can be synthesized.
For "CO 2 preferential activation", the carbamate or carbonate intermediate [X−CO 2 ] anion is generated first from the reaction of CO 2 and the anion [X]. Second, the [X−CO 2 ] anion nucleophilically attacks the C atom of the nitrile group in the o-aminobenzonitrile, resulting in a negative-charged N atom (-C = N -). Third, proton transfer from amino group to the negative-charged N atom results in -C = NH and a negative-charged amino group. Four, the negative-charged amino nucleophilically attacks the C atom of -COO-, leading to an intramolecular nucleophilic cyclization, accompanied by the removal of the anion [X]. Last, the ring is rearranged along with proton transfer, resulting in the target product.
For "both amino and CO 2 simultaneous activation", CO 2 is first activated by the anion [X], forming a carbamate or carbonate intermediate [X−CO 2 ] anion, and the amino group is simultaneously activated by [X], forming a negative-charged amino group -HNvia dehydrogenation. Second, the negative-charged amino nucleophilically attacks the C atom of CO 2 in [X−CO 2 ] accompanied by the removal of anion [X], resulting in the carbamate anion (-HNCOO -). After intramolecular nucleophilic cyclization, ring rearrangement, and proton transfer, the target product can be obtained.
The "both amino and CO 2 simultaneous activation" mechanism is similar to the mechanism of "amino preferential activation". The difference between these mechanisms is the first two steps. With both amino and CO 2 simultaneous activation, the energy consumption of the nucleophilic attack in the second step of the "both amino and CO 2 simultaneous activation" mechanism is lower than the other one.

Desired Synthetic Method under Mild Reaction Conditions
Although some reported ILs as the catalysts have drawbacks (high CO 2 pressures, high temperatures, difficulties in recycling the catalysts, etc.) as conventional catalysts (metal oxides, inorganic/organic bases, etc.), ILs still have great advantages as solvents and catalysts due to their unique tunable structures and properties. According to the aforementioned discussion, the "both amino and CO 2 simultaneous activation" mechanism could provide an alternative opportunity to obtain the product under mild reaction conditions with low energy consumption. Through tuning the structures of ILs, CO 2 -philic task-specific ILs or functional ILs could act as the catalysts following the "both amino and CO 2 simultaneous activation" mechanism. These CO 2 -philic task-specific ILs or functional ILs with desired basicity could be used in CO 2 capture. Typical functional anions and corresponding absorption mechanisms can be found in Part 2. Generally, the catalytic performance of ILs (especially anions) is affected by both cations and anions as well as the cation-anion interaction. A typical example anion is imidazolate anion ([Im]). The results of the quantum-chemical calculations, NMR spectroscopic investigations, and controlled experiments from Wang et al. [57] indicate that insitu-generated [Ch][Im−CO 2 ] is the real catalyst for the conversion of CO 2 and oaminobenzonitriles following the "amino preferential activation" mechanism. Liu et al. [62] agrees that 2-methyl-substituted imidazolate ([2-MIm]) could generate [2-MIm−CO 2 ] when [Bu 4 P][2-MIm] is used as the catalyst; however, the carbamate nucleophilically attacks the CN group and becomes a part of product. One possible reason is that the methyl group changes the interaction between the imidazolate anion and CO 2 . He et al. [74] show that [HTMG][Im] could simultaneously activate both amino and CO 2 , probably due to the strong interaction between the cation and anion as well as the proton on the cation. Thus, the effects of structures and interactions of anions and cations cannot be ignored. Additionally, features can be used to develop the innovative synthetic routes for the synthesis of quinazoline-2,4(1H,3H)-diones from CO 2 and o-aminobenzonitriles.

Conclusions and Outlook
Because of their unique properties, ILs could be used as solvents and catalysts for CO 2 capture and conversion into value-added chemicals. After the first IL has been reported in 2009 for the conversion of CO 2 and o-aminobenzonitriles into quinazoline-2,4(1H,3H)-diones, kinds of ILs with basic anions such as [OH], carboxylates, aprotic heterocyclic anions, etc., have been developed for converting CO 2 and o-aminobenzonitriles into quinazoline-2,4(1H,3H)-diones. The different catalytic mechanisms, including amino preferential activation, CO 2 preferential activation, and simultaneous amino and CO 2 activation, have been investigated systematically. This review is benefited for understanding the synthesis of quinazoline-2,4(1H,3H)-diones from CO 2 and o-aminobenzonitriles using IL-based catalysts. However, it is clear that this field is still in its infancy. Several issues should be paid more attention to and need to be investigated further as follows.
(1) Developing IL-based heterogeneous catalysts. There are only a few examples of ILbased heterogeneous catalysts used in the synthesis of quinazoline-2,4(1H,3H)-diones from CO 2 and o-aminobenzonitriles. Because of the advantages in the separation of catalysts and products from reaction system, heterogeneous catalysts based on ILs, especially functional ILs, should be developed.
(2) Conversion CO 2 under flue gas conditions. "In situ" strategies for CO 2 capture and subsequent conversion into value-added chemicals have been developed as potential methods for directly transforming waste CO 2 to value-added CO 2 -based chemicals without purification. There are only a few examples of the catalysts that could be used under flue gas conditions [57]. However, more IL-based catalysts with high efficiency should be developed for CO 2 conversion at low temperature (40~60 • C) and low CO 2 partial pressure (0.1~0.15 bar CO 2 ).
(3) Conversion mechanisms should be investigated deeply. It can be seen from the above discussion in Part 3 that reported mechanisms of ILs with anions such as carboxylate anions, aprotic heterocyclic anions, etc., in different literatures followed different routes. The main concern is whether the mechanism begins with a preferential activation or a simultaneous activation.
Generally, functional ILs with tunable structures and properties contribute an opportunity to achieve efficient CO 2 capture and conversion into the value-added chemicals under flue gas conditions. This review article opens a door to develop novel IL-based systems for CCUS and other gases utilization.