Review on Carbon Dioxide Utilization for Cycloaddition of Epoxides by Ionic Liquid-Modiﬁed Hybrid Catalysts: Effect of Inﬂuential Parameters and Mechanisms Insight

: The storage, utilization, and control of the greenhouse (CO 2 ) gas is a topic of interest for researchers in academia and society. The present review article is dedicating to cover the overall role of ionic liquid-modiﬁed hybrid materials in cycloaddition reactions. Special emphasis is on the synthesis of various cyclic carbonate using ionic liquid-based modiﬁed catalysts. Catalytic activity studies have discussed with respect to process conditions and their effects on conversion and product selectivity for the reaction of cycloaddition of CO 2 with styrene oxide. The reaction temperature and the partial pressure of CO 2 have found to play a key role in cyclic carbonate formation. The role of other inﬂuential parameter (solvent effect) is also discussed for the conversion of cyclic/aromatic oxides to polycarbonate production. Our own research work that deals with ionic liquid-based halide-modiﬁed mesoporous catalyst (MCM-41 type) derived from rice husk waste has also been discussed. Finally, the role of carbon dioxide activation and ring-opening mechanisms involved in the cyclic carbonate product formation from CO 2 have been discussed.


Introduction
Greenhouse gas (carbon dioxide-CO 2 ) in the atmosphere helps living things naturally by involving in photosynthesis [1]. About 32% of CO 2 is being produced by hydrocarbon combustion and gasification process that raises concern over environmental pollution [2,3]. The transportation sector contributes nearly 30% to total carbon dioxide emissions [4]. Figure 1 shows the carbon cycle, CO 2 storage, recycle and purification, and utilization mainly of fine chemicals formation by catalysis route. Figure 1 shows the global atmospheric CO 2 concentration for about half a decade from 1958 with respect to continuous research reports carried by the Mauna Loa Observatory in Hawaii [3]. The concentration of atmospheric CO 2 was 399.89 ppm till May 2013, and in 2020, it reached up to 412.78 ppm [5]. The continuous rise in greenhouse gas (CO 2 ) concentration creates global warming issues and continues damage to the green environment. In recent years, smart technologies are developed to store and utilize CO 2 reduction and which makes the pollution free atmosphere [6][7][8]. Carbon dioxide is recognized as a nonflammable [9], nontoxic [10], and inexpensive gas [11]. It is a renewable carbon source [12] and thermodynamically stable compound [13,14]. Interestingly, carbon oxide can replace the following toxic chemical compounds such as carbonyl dichloride known as phosgene (COCl2), carbon monoxide (CO), and isocyanides (R-NCO) for fine chemical applications. Carbon dioxide can be utilized as a mild oxygen source [15]. It can be used as an alternate medium or solvent [16], also work as a supercritical fluid (sc) [17], and act as a carbon source. It can be used based on its unique chemical properties to be incorporated with high "atom efficiency" such as in carboxylation synthesis or in catalyst synthesis [16]. Hutchings [15,17] used supercritical CO2 as an antisolvent for the preparation of Au/scCO2 and sc-VPO (vanadium phosphate) catalysts. Currently, CO2 has been used in various industrial applications such as chemical, pharmaceutical, foodstuff, laboratories and analysis, beverage, and pulp and paper industries [16]. The application of CO2 as C1 raw material in the chemical industry was started in past few decades. It has been reported that approximately 110 million metric tons of CO2 are currently used every year in the chemical industry. In the present decade, carbon dioxide utilization has reached around 110 million MT (metric tons) [18]. Carbon dioxide is also playing major role in the production of urea, [17] methanol [18], salicylic-acid [19], formic-acid [20], cyclic carbonates [21][22][23], copolymers, polymer building blocks, and fine chemicals [24][25][26][27]. Urea is one of the major fertilizers, and CO2 is the source for it [26]. The urea is prepared from ammonia and CO2 in fertilizer [28] and also in fabrication process of various types of polymers, such as melamine and urea-formaldehyde resin [29][30][31][32]. Salicylic acid is produced from phenol and CO2 via the Kolbe-Schmitt reaction [33]. The product is used to produce acetyl salicylic acid which is also known as aspirin, used mostly in healthcare applications [34][35][36]. Everyday monitoring of Carbon dioxide emission in atmosphere have shown in online website (www.co2.earth ) to monitor the Keeling Curve of Atmospheric CO2 concentration emission between 1958 to 16 August 2020 [5,35].
The cyclic carbonates are odorless, colorless, and biodegradable [37,38]. The cyclic carbonates are used in industries as aprotic polar solvents [39], as a monomer for polymer synthesis, and as additives [40]. Besides, it is also been used in electrolytic materials such as secondary batteries (lithium batteries) [41], cosmetics, resins, and cleaning utensils [42]. Cyclic carbonates are utilized as an intermediate compound in the biomedical and pharma industries [12]. Cyclic carbonates also play a key role in herbicides and disinfectants synthesis [43] as well as are fuel additives [44].
Traditionally, phosgene with ethane-1,2-diol in dichloromethane solvent was utilized to produce cyclic carbonates, and one of the products is hydrochloric acid obtained Carbon dioxide is recognized as a nonflammable [9], nontoxic [10], and inexpensive gas [11]. It is a renewable carbon source [12] and thermodynamically stable compound [13,14]. Interestingly, carbon oxide can replace the following toxic chemical compounds such as carbonyl dichloride known as phosgene (COCl 2 ), carbon monoxide (CO), and isocyanides (R-NCO) for fine chemical applications. Carbon dioxide can be utilized as a mild oxygen source [15]. It can be used as an alternate medium or solvent [16], also work as a supercritical fluid (sc) [17], and act as a carbon source. It can be used based on its unique chemical properties to be incorporated with high "atom efficiency" such as in carboxylation synthesis or in catalyst synthesis [16]. Hutchings [15,17] used supercritical CO 2 as an antisolvent for the preparation of Au/scCO 2 and sc-VPO (vanadium phosphate) catalysts. Currently, CO 2 has been used in various industrial applications such as chemical, pharmaceutical, foodstuff, laboratories and analysis, beverage, and pulp and paper industries [16]. The application of CO 2 as C1 raw material in the chemical industry was started in past few decades. It has been reported that approximately 110 million metric tons of CO 2 are currently used every year in the chemical industry. In the present decade, carbon dioxide utilization has reached around 110 million MT (metric tons) [18]. Carbon dioxide is also playing major role in the production of urea, [17] methanol [18], salicylic-acid [19], formic-acid [20], cyclic carbonates [21][22][23], copolymers, polymer building blocks, and fine chemicals [24][25][26][27]. Urea is one of the major fertilizers, and CO 2 is the source for it [26]. The urea is prepared from ammonia and CO 2 in fertilizer [28] and also in fabrication process of various types of polymers, such as melamine and urea-formaldehyde resin [29][30][31][32]. Salicylic acid is produced from phenol and CO 2 via the Kolbe-Schmitt reaction [33]. The product is used to produce acetyl salicylic acid which is also known as aspirin, used mostly in healthcare applications [34][35][36]. Everyday monitoring of Carbon dioxide emission in atmosphere have shown in online website (www.co2.earth) to monitor the Keeling Curve of Atmospheric CO 2 concentration emission between 1958 to 16 August 2020 [5,35].
The cyclic carbonates are odorless, colorless, and biodegradable [37,38]. The cyclic carbonates are used in industries as aprotic polar solvents [39], as a monomer for polymer synthesis, and as additives [40]. Besides, it is also been used in electrolytic materials such as secondary batteries (lithium batteries) [41], cosmetics, resins, and cleaning utensils [42]. Cyclic carbonates are utilized as an intermediate compound in the biomedical and pharma industries [12]. Cyclic carbonates also play a key role in herbicides and disinfectants synthesis [43] as well as are fuel additives [44].
Traditionally, phosgene with ethane-1,2-diol in dichloromethane solvent was utilized to produce cyclic carbonates, and one of the products is hydrochloric acid obtained as a by-product, which is harmful to human beings [43]. Scheme 1 shows the conventional synthesis of organic cyclic carbonates. as a by-product, which is harmful to human beings [43]. Scheme 1 shows the conventional synthesis of organic cyclic carbonates. Scheme 1. Synthesis of organic carbonate by the conventional route.
By considering the economic point of view and avoid toxicity generation, the CO2 is a natural choice to phosgene as an alternate and the other advantage is that CO2 can be incorporated into epoxides without side products [45]. However, due to the inert nature of CO2, various catalysts were adopted to activate the epoxide [11]. The cycloaddition of CO2 to epoxide is shown below (Scheme 2).
Ionic liquid (IL) is recently explored as efficient catalysts with growing importance over the past decades [64]. Since 2003, ILs have widely been adopted in the chemical industry as a solvent as well as catalysts for many fine chemical productions [60][61][62][63][64][65]. Hence, a quaternary ammonium ion together with a halide anion, -OH, or a -COOH group with ionic liquid was considered as potential materials for heterogeneous catalysis.

Summary
The present review article described the effect of influential parameters such as temperature, pressure, and solvent on the conversion of cyclic epoxide into cyclic carbonate formation in presence of various ionic liquid modified hybrid catalysts. Another section describes the mechanisms insight into the activation of carbon dioxide and the ring-opening process in the process of substrate conversion and acidic/basic characteristics of the catalyst. By considering the economic point of view and avoid toxicity generation, the CO 2 is a natural choice to phosgene as an alternate and the other advantage is that CO 2 can be incorporated into epoxides without side products [45]. However, due to the inert nature of CO 2 , various catalysts were adopted to activate the epoxide [11]. The cycloaddition of CO 2 to epoxide is shown below (Scheme 2). as a by-product, which is harmful to human beings [43]. Scheme 1 shows the conventional synthesis of organic cyclic carbonates. Scheme 1. Synthesis of organic carbonate by the conventional route.
By considering the economic point of view and avoid toxicity generation, the CO2 is a natural choice to phosgene as an alternate and the other advantage is that CO2 can be incorporated into epoxides without side products [45]. However, due to the inert nature of CO2, various catalysts were adopted to activate the epoxide [11]. The cycloaddition of CO2 to epoxide is shown below (Scheme 2).
Ionic liquid (IL) is recently explored as efficient catalysts with growing importance over the past decades [64]. Since 2003, ILs have widely been adopted in the chemical industry as a solvent as well as catalysts for many fine chemical productions [60][61][62][63][64][65]. Hence, a quaternary ammonium ion together with a halide anion, -OH, or a -COOH group with ionic liquid was considered as potential materials for heterogeneous catalysis.

Summary
The present review article described the effect of influential parameters such as temperature, pressure, and solvent on the conversion of cyclic epoxide into cyclic carbonate formation in presence of various ionic liquid modified hybrid catalysts. Another section describes the mechanisms insight into the activation of carbon dioxide and the ring-opening process in the process of substrate conversion and acidic/basic characteristics of the catalyst. To increase cyclic carbonates yield from cycloaddition of CO 2 and epoxides, a different kind of catalyst has been adopted. In the past decades, the wide range of homogeneous and heterogeneous catalysts have been developed to catalyze the so-called CO 2 fixation process. Homogeneous catalysts, such as CoCl 2 /onium salt [44], diimine Ru (II) complex [46], Al-salen-PEA [4], betaine-based quaternary ammonium ion and carboxylic acid [12], N,N-dimethyl formamide (DMF) [47,48], SnCl 4 -organic base [49], Au/Fe(OH) 3 -ZnBr 2 /Bu 4 NBr [50], ionic liquid-highly cross linked polymer [51], BrBu 3 PPEG 600 PBU 3 Br [52], cellulose/KI [53], and Au/R201 [54] have also been studied.
Ionic liquid (IL) is recently explored as efficient catalysts with growing importance over the past decades [64]. Since 2003, ILs have widely been adopted in the chemical industry as a solvent as well as catalysts for many fine chemical productions [60][61][62][63][64][65]. Hence, a quaternary ammonium ion together with a halide anion, -OH, or a -COOH group with ionic liquid was considered as potential materials for heterogeneous catalysis.

Summary
The present review article described the effect of influential parameters such as temperature, pressure, and solvent on the conversion of cyclic epoxide into cyclic carbonate formation in presence of various ionic liquid modified hybrid catalysts. Another section describes the mechanisms insight into the activation of carbon dioxide and the ring-opening process in the process of substrate conversion and acidic/basic characteristics of the catalyst.

Results of Reaction Parameters and Influencing Factors for the Production of Cyclic Carbonates
The production of cyclic carbonates depends on various parameters including catalyst and reaction parameters such as solvents, temperature, and pressure condition. The optimization of all the above parameters could produce maximum product yield. Table 1 summarizes studies that were carried out for cycloaddition of greenhouse gas conversion to styrene oxide (SO) on different types of catalysts . The produce yield was higher due to the presence of both acid and base bifunctional groups present on the catalyst and it synergistically activates the cycloaddition reactions.

Effect of Influence of Reaction Temperature for Cycloaddition of Epoxides with CO 2
The reaction temperature is an important parameter in a catalytic reaction for an effective collision between molecules to enhance the bond-breaking step. Hence, the more molecular collision is the reason for the more yield of final products. Aresta et al. [26,41] reported the temperature effect on the production of styrene carbonate (SC) using Nb 2 O 5 as a catalyst from styrene oxide by CO 2 addition. Below 100 • C, the reaction did not yield any product. However, it provides 80% yield at temperature of 135 • C. Jutz et al. [40] studied the effect of reaction temperature on the performance of Mn (salen) Br. The highest yield was obtained at 160 • C, and with a further increase in temperature, the yield was dramatically reduced. This was attributed to changes in the phase distribution observed at higher reaction temperatures. Zhou et al. [11] reported that the reaction carried out at 140 • C results in the formation of propylene carbonate (PC) with the highest yield of 98%. Increase in the temperature from 140 to 150 • C dropped the yield up to 78% due to problems of side product generation at high-temperature conditions. Bai et al. [83,84] reported that in some instances, high-temperature conditions are causing the catalyst to decompose resulting in a decrease in propylene carbonate (PC) yield [84]. Qiao et al. [91] explained that styrene oxide (SO) is difficult to convert to styrene carbonate (SC) compared to all other epoxides due to the lower reactivity of β-carbon atom. They found that the temperature of 120 • C (98%) with suitable catalyst was the best-optimized condition than the high-temperature condition (130 • C (~80%) and 140 • C (~95%)).
Recently, Lee et al. [98] studied the cycloaddition reaction between ally glycidyl ether (AGE) and carbon dioxide using PS-hexyl-Methyl iodide at 12 bar of total pressure and different temperature conditions. They reported that the yield of allyl glycidyl carbonate (AGC) increases from 80 to 140 • C, and it decreased with a further increase in the temperature to 160 • C. The yield decreased was due to the generation of oligomers and other side products like 3-allyl oxy-1,2-propanediol. Zhong (2014) et al. [100] studied and reported the effect of temperature in the range between 120 and 160 • C for propylene carbonate formation. The yield of PC in the presence of 0.78 and 13.7 m/mol of DMF solvents for comparative purpose was studied. They found that the usage of a large amount of DMF was favorable to provide a higher yield at the lower reaction temperature.

Effect of Influence of Reaction Pressure Condition for Cycloaddition of Epoxides with CO 2
The reaction pressure of the carbon dioxide insertion has been established as one of the most crucial and critical conditions for affecting the epoxide cycloaddition reaction [91][92][93][94][95][96][97][98]. The inserted CO 2 acts as an important reactant for all catalytic transformations [42]. Two phases are established in the reaction system; the bottom phase is rich with epoxide and the top phase is enriched with CO 2 . According to Xie et al. (2007), the reactant CO 2 favors the reaction when the bottom phase is under high pressure. However, the above condition is not favorable for the high-pressure reactions (120 bar) as concentration of epoxide for example propylene oxide [49].
Ghosh et al. [36] reported that at lower pressure (7 bar), the catalyst retains moderate reaction activity turn over frequency (TOF) of 312 h −1 and with increasing pressure up to 20 bar, increased TOF value of 351 h −1 was observed. However, the pressure more than 20 bar results in diminishes overall reactivity due to polarity and solubility problem of the catalysts. Qiao et al. [91] reported different pressure conditions, such as mild pressure (15 bar), medium pressure (80 bar), and supercritical pressure (140 bar), for the styrene oxide with CO 2 cycloaddition reactions. This was due to changes in the phase from gas to supercritical fluid, where a part of styrene oxide (SO) dissolves in the supercritical condition. On the other hand, Wang, et. al (2017) [65] approach was for the same reaction but different perspective, i.e., introduction of high concentration of CO 2 dissolves within a substrate or "liquefies" the formation of intermediate complex. Xiang et al. (2009) [52] reported that the many oligomers were produced as the side products such as propylene oxide and styrene oxide due to insertion of CO 2 at high pressure in a solvent-less condition. Jutz et al. [40,53] reported that a ratio of 1:4 (epoxide: CO 2 ) was the best reactant ratio condition instead of 1:16 for conversion of both epoxides.

Effect of Influence of Solvent for Cycloaddition of Epoxides with CO 2
A variety of solvents are adopted to synthesis cyclic carbonate from cycloaddition reaction. A solvent plays a key role in minimizing carbonaceous deposits on the catalyst surface. [55]. Aresta et al. [26,41] exploited that the N,N-dimethyl formamide (DMF) alone yields 34.7% of styrene carbonate at 50 bar pressure of CO 2 and predict that amide group was a good promoter. Di-methyl acetamide (DMA) produced SC about 28% yield without the catalyst at 50 bar pressure and at temperature of 135 • C for 12 h. The role of DMF in the cycloaddition of epoxides mechanism and transformation is as follows (Scheme 3). bar), medium pressure (80 bar), and supercritical pressure (140 bar), for the styrene oxide with CO2 cycloaddition reactions. This was due to changes in the phase from gas to supercritical fluid, where a part of styrene oxide (SO) dissolves in the supercritical condition. On the other hand, Wang, et. al (2017) [65] approach was for the same reaction but different perspective, i.e., introduction of high concentration of CO2 dissolves within a substrate or "liquefies" the formation of intermediate complex. Xiang et al. (2009) [52] reported that the many oligomers were produced as the side products such as propylene oxide and styrene oxide due to insertion of CO2 at high pressure in a solvent-less condition. Jutz et al. [40,53] reported that a ratio of 1:4 (epoxide: CO2) was the best reactant ratio condition instead of 1:16 for conversion of both epoxides.

Effect of Influence of Solvent for Cycloaddition of Epoxides with CO2
A variety of solvents are adopted to synthesis cyclic carbonate from cycloaddition reaction. A solvent plays a key role in minimizing carbonaceous deposits on the catalyst surface. [55]. Aresta et al. [26,41] exploited that the N,N-dimethyl formamide (DMF) alone yields 34.7% of styrene carbonate at 50 bar pressure of CO2 and predict that amide group was a good promoter. Di-methyl acetamide (DMA) produced SC about 28% yield without the catalyst at 50 bar pressure and at temperature of 135 °C for 12 h. The role of DMF in the cycloaddition of epoxides mechanism and transformation is as follows (Scheme 3). According to the mechanism, role of amide is to facilitate the nucleophilicity of the respective oxygen atoms in CO2 or epoxides. From the results, it is evident that the methylene chloride could contribute to stabilize the polar or ionic intermediates through the dipole effect, i.e., C δ--H δ+ and C δ+ -Cl δ-. The cooperative solvation effect occurred in the presence of tetrachloro methane and ethanol-like additive used along with DMF for cycloaddition reactions. Kawanami et al. (2000) [45] used supercritical condition (sc) to study the effect of DMF as a catalyst and solvent on epoxide formation, which is dissolved in DMF-scCO2. Recently, Zhong (2000) et al. [100] utilized DMF as cocatalyst with ZnBr2 as a catalyst for cycloaddition reaction of propylene oxide with carbon dioxide. In their study, they have observed that DMF acted as a solvent as well as carbon dioxide activator. Both high conversion and 100% selectivity was obtained at 150 °C and pressure condition of 30 bar with very quick reaction time of 10 min [100]. Alvaro (2004) et al [12] used 0.4 According to the mechanism, role of amide is to facilitate the nucleophilicity of the respective oxygen atoms in CO 2 or epoxides. From the results, it is evident that the methylene chloride could contribute to stabilize the polar or ionic intermediates through the dipole effect, i.e., C δ− -H δ+ and C δ+ -Cl δ− . The cooperative solvation effect occurred in the presence of tetrachloro methane and ethanol-like additive used along with DMF for cycloaddition reactions. Kawanami et al. (2000) [45] used supercritical condition (sc) to study the effect of DMF as a catalyst and solvent on epoxide formation, which is dissolved in DMF-scCO 2 . Recently, Zhong (2000) et al. [100] utilized DMF as cocatalyst with ZnBr 2 as a catalyst for cycloaddition reaction of propylene oxide with carbon dioxide. In their study, they have observed that DMF acted as a solvent as well as carbon dioxide activator. Both high conversion and 100% selectivity was obtained at 150 • C and pressure condition of 30 bar with very quick reaction time of 10 min [100]. Alvaro (2004) et al. [12] used 0.4 mL dichloromethane (DMC) or dimethyl carbonate as a cosolvent to enhance product solubility in the supercritical medium and it serves as a cocatalyst as well. The results showed 70% conversion and 100% selectivity in presence of Cr-salen base catalyst under supercritical condition (100 bar, 80 • C, reactor volume = 50 m, 6 h reaction time).
A new approach was taken by Jiang et al. [46] in order to study the solvent effect for the synthesis of chloropropene carbonate from epichlorohydrin. Protic alcohols (methanol and ethanol) acted as good solvent for the formation of chloropropene carbonate with 90% and 82% yield. However, higher molecular weight alcohol such as benzyl alcohol was found to give less productivity yield (12%), whereas the same reaction carried out at 110 • C for 20 h with DMF as the solvent resulted in achieving for highest yield (f > 99%).

Discussion of the Mechanism Insight of Cycloaddition of Epoxides with CO 2 4.1. Activation of CO 2 for Cycloaddition of Epoxides
Lu et al. [59] reported that aluminum pthalocyanine complex formation on MCM-41 support for the cycloaddition reaction of epoxides They observed that CO 2 activated through nucleophilic attack at the carbon atom of CO 2 by the alcoholate group (-OCH 2 CH 2 Br). The weak interaction between the central metal ion of pthalocyanine complex and the lone pairs oxygen in CO 2 makes synergistic mechanism. The halide intramolecular substitution facilitate the epoxide into cyclic carbonates. In another related study, Barbarini et al. [10] argued that the mechanism of CO 2 activation through the formation of the zwitterion compound. Scheme 4 shows that the CO 2 adds to the epoxide via nucleophilic attack. the synthesis of chloropropene carbonate from epichlorohydrin. Protic alcohols (methanol and ethanol) acted as good solvent for the formation of chloropropene carbonate with 90% and 82% yield. However, higher molecular weight alcohol such as benzyl alcohol was found to give less productivity yield (12%), whereas the same reaction carried out at 110 °C for 20 h with DMF as the solvent resulted in achieving for highest yield (f > 99%).

Activation of CO2 for Cycloaddition of Epoxides
Lu et al. [59] reported that aluminum pthalocyanine complex formation on MCM-41 support for the cycloaddition reaction of epoxides They observed that CO2 activated through nucleophilic attack at the carbon atom of CO2 by the alcoholate group (-OCH2CH2Br). The weak interaction between the central metal ion of pthalocyanine complex and the lone pairs oxygen in CO2 makes synergistic mechanism. The halide intramolecular substitution facilitate the epoxide into cyclic carbonates. In another related study, Barbarini et al. [10] argued that the mechanism of CO2 activation through the formation of the zwitterion compound. Scheme 4 shows that the CO2 adds to the epoxide via nucleophilic attack. The efficient Epichlorohydrin conversion was obtained (62.3%) after functionalization of SBA-15 with adenine group compared to bulk SBA-15 (1.5%). This increased in conversion was related to the intensity of the band at 1609 cm −1 ascribed due to CO2 bonded with amine sites, which recognize the importance of such sites for CO2 activation. In a related study, Srivastava, et al, [77,86] exploited and compared the importance of the basic sites present in the catalysts such as alkyl amines (-NH2), adenine (Ade), imidazole (Im) and guanine (Gua) to activation process of carbon dioxide. Different types of coordination modes of CO2 was discussed in detail in the past and its well known in the field of carbon dioxide chemistry [75]. Scheme 6 shows the stability of the activated CO2 complex formation occurs on the basic amine sites at the catalyst surface decreased as follows: primary > secondary > tertiary amines [75,77 and 86]. The formation of carbamate anions from different type of amine groups are demonstrated. The metal-electron deficient part of the catalyst facilitates the reaction rate for the formation of cyclic carbamate ions. The efficient Epichlorohydrin conversion was obtained (62.3%) after functionalization of SBA-15 with adenine group compared to bulk SBA-15 (1.5%). This increased in conversion was related to the intensity of the band at 1609 cm −1 ascribed due to CO 2 bonded with amine sites, which recognize the importance of such sites for CO 2 activation. In a related study, Srivastava et al. [77,86] exploited and compared the importance of the basic sites present in the catalysts such as alkyl amines (-NH 2 ), adenine (Ade), imidazole (Im) and guanine (Gua) to activation process of carbon dioxide. Different types of coordination modes of CO 2 was discussed in detail in the past and its well known in the field of carbon dioxide chemistry [75]. Scheme 6 shows the stability of the activated CO 2 complex formation occurs on the basic amine sites at the catalyst surface decreased as follows: primary > secondary > tertiary amines [75,77,86]. The formation of carbamate anions from different type of amine groups are demonstrated. The metal-electron deficient part of the catalyst facilitates the reaction rate for the formation of cyclic carbamate ions.
In Scheme 5 it is more clearly explained the role activation of CO 2 for their efficient conversion towards carbon dioxide activation process.
From their point of view, Scheme 5 shows the stability of the activated CO 2 complex formation that occurring on the basic amine sites at the surfaces of catalyst decreased as follow: primary > secondary > tertiary amines [75,77,86]. The formation of carbamate anions from different types of amine groups are demonstrated. The metal-electron deficient part of the catalyst facilitates the reaction rate for the formation of cyclic carbamate ions.

Ring Opening of Epoxide
The ring-opening mechanism of the epoxide is described in two ways such as (i) Lewisacid catalyzed cleavage and (ii) Lewis-base catalyzed cleavage. Bu et al. (2007) [101] found that the ruthenium complex ((2,2 -bipy)RuCl 3 (CH 3 OH)) supported by cetyl-trimethyl ammonium chloride (CTAC) catalyzes propylene oxide (PO) in the presence of CO 2 and achieved 100% yield for propylene carbonate (PC). In the above catalyst system, Ru acts as a Lewis-acid to activate the PO to form adduct of Ru-PO. CTAC addition enhances reaction rate and strikes the less sterically hindered carbon to break the epoxide ring while forming the oxy-anion species.
Bai et al. [83,96] reported that the bi-functional metal porphyrins M(TTMAPP)I 4 (X) (M = Co, Mn, Fe, and Cr; X = OAc, CCl 3 COO, CF 3 COO, OTs, I, Cl, and Br) were highly efficient catalysts for the respective cycloaddition (formation of propylene carbonate). In the above catalyst system, metal ion incorporation acts as a Lewis acid center to facilitate the catalytic reaction rate. The order of activity of the catalysts was Co > Mn > Fe > Cr. The catalytic activity of cobalt porphyrin decreased with different counter ions as follows: Barbarini et al. [10] reported mesoporous silica (MCM-41) with hexagonal morphology in which Si-OH (hydroxyl and silanol functionalized)-supported guanidine catalysts are studied for cycloaddition reactions. The enhanced reactivity was obtained due to mechanisms involved in hydrogen bonding. Zhou et al. [11] studied the mechanism of cyclic carbonate formation in the presence of betaine (HBetX) and choline cation (ChoX) catalyst. They compared the anion effect and hydroxyl and carboxyclic acid group activation towards catalyst function. The order of reactivity for PC conversion and yield decreased as follows: The role of leaving group ability has also been studied, and the activity follows in this order: I − > Br − > Cl − . Adopting better nucleophilic anions could improve the epoxide ring opening/breaking efficiency of the catalyst.
The carboxylic acid group is found to be best for ring-opening mechanism with respect to suitable halide anions. The reason behind the halide anions to activate the ring opening is due to presence of stronger BrøØnsted acid and thereby involved in hydrogen bonding. Scheme 6 shows the reaction mechanism for the cycloaddition reaction and its halide anion interaction.

Ring Opening of Epoxide
The ring-opening mechanism of the epoxide is described in two ways such as (i) Lewis-acid catalyzed cleavage and (ii) Lewis-base catalyzed cleavage. Bu et al ( 2007) [101] found that the ruthenium complex ((2,2'-bipy)RuCl3(CH3OH)) supported by cetyl-trimethyl ammonium chloride (CTAC) catalyzes propylene oxide (PO) in the presence of CO2 and achieved 100% yield for propylene carbonate (PC). In the above catalyst system, Ru acts as a Lewis-acid to activate the PO to form adduct of Ru-PO. CTAC addition enhances reaction rate and strikes the less sterically hindered carbon to break the epoxide ring while forming the oxy-anion species.
Bai et al. [83,96] reported that the bi-functional metal porphyrins M(TTMAPP)I4(X) (M = Co, Mn, Fe, and Cr; X = OAc, CCl3COO, CF3COO, OTs, I, Cl, and Br) were highly efficient catalysts for the respective cycloaddition (formation of propylene carbonate). In the above catalyst system, metal ion incorporation acts as a Lewis acid center to facilitate the catalytic reaction rate. The order of activity of the catalysts was Co > Mn > Fe > Cr. The catalytic activity of cobalt porphyrin decreased with different counter ions as follows: Barbarini et al. [10] reported mesoporous silica (MCM-41) with hexagonal morphology in which Si-OH (hydroxyl and silanol functionalized)-supported guanidine catalysts are studied for cycloaddition reactions. The enhanced reactivity was obtained due to mechanisms involved in hydrogen bonding. Zhou et al. [11] studied the mechanism of cyclic carbonate formation in the presence of betaine (HBetX) and choline cation (ChoX) catalyst. They compared the anion effect and hydroxyl and carboxyclic acid group activation towards catalyst function. The order of reactivity for PC conversion and yield decreased as follows: Cl − > BF4 − > PF6 − . The role of leaving group ability has also been studied, and the activity follows in this order: I − > Br − > Cl − . Adopting better nucleophilic anions could improve the epoxide ring opening/breaking efficiency of the catalyst.
The carboxylic acid group is found to be best for ring-opening mechanism with respect to suitable halide anions. The reason behind the halide anions to activate the ring opening is due to presence of stronger BrøØnsted acid and thereby involved in hydrogen bonding. Scheme 6 shows the reaction mechanism for the cycloaddition reaction and its halide anion interaction. Scheme 6. The plausible cycloaddition reaction mechanism for epoxide ring opening with CO2 by betaine (HBetX) catalysts [11].
Ramalingam et al. [102] and our group recently reported halide ion-modified mesoporous silica catalysts for solvent-free cycloaddition of styrene oxide with CO 2 . For above reaction, imidazole was first immobilized on MCM-41 (derived from biomass materials) using 3-chloropropyltriethoxysilane (CPTES) as the anchoring agent followed by alkylation with 1,2-dibromoethane at 110 • C. The prepared catalyst was mentioned as MCM-41-Imi/Br. The catalyst was used in the cycloaddition of styrene oxide, glycidol, epichlorohydrin and phenyl glycidyl ether, and allyl glycidyl ether.
The halide ion (Br) and the tertiary amine from imidazole anchored over mesoporous support (MCM-41-Imi/Br) involved in the ring opening and activation of CO 2 . In Scheme 7, the mechanism of ring opening of the epoxide carried out by a nucleophilic attack by the bromide ion at the less sterically hindered β-carbon resulted to the formation of haloalkoxy species. carbonates was achieved under mild conditions without solvent and in the absence of a co-catalyst. Biopolymer chitosan-grafted quaternary phosphonium ionic liquid (CS-(BuPh3P)Br) was reported as an excellent catalyst [98]. The authors proposed that the bromide anion of the catalyst played a major role in epoxy ring opening activated by the hydroxyl groups and phosphonium cation interaction. The same type of catalysts was developed by reported hydroxyl, carboxyl, and amino-functionalized phosphonium-based ionic liquid catalyst. They observed that a similar mechanism as mentioned above for the opening of the epoxide ring via polarization of epoxide C-O bond [90][91][92][93][94][95][96][97][98]. Excellent selectivity and good yield were obtained for cyclic carbonates under suitable or optimizable reaction conditions [90][91][92][93][94][95][96][97][98][99][100][101]. The following yields were obtained for the each cyclic carbonates such as epichlorohydrine (97.0%),) glycidol (98.3%), styrene oxide (98.8%), phenyl glycidyl ether (96.7%) for allyl glycidyl ether (97.5%,) and 1,2-epoxyhexane (100% for) at the reaction time of three hours (3 h) [101,102].
Ramalingam et al. [102] and our group recently reported halide ion-modified mesoporous silica catalysts for solvent-free cycloaddition of styrene oxide with CO2. For above reaction, imidazole was first immobilized on MCM-41 (derived from biomass materials) using 3-chloropropyltriethoxysilane (CPTES) as the anchoring agent followed by alkylation with 1,2-dibromoethane at 110 °C. The prepared catalyst was mentioned as MCM-41-Imi/Br. The catalyst was used in the cycloaddition of styrene oxide, glycidol, epichlorohydrin and phenyl glycidyl ether, and allyl glycidyl ether.
The halide ion (Br) and the tertiary amine from imidazole anchored over mesoporous support (MCM-41-Imi/Br) involved in the ring opening and activation of CO2. In Scheme 7, the mechanism of ring opening of the epoxide carried out by a nucleophilic attack by the bromide ion at the less sterically hindered β-carbon resulted to the formation of haloalkoxy species. Scheme 7. Mechanism of the halide ion (Br) and the tertiary amine from imidazole anchored over mesoporous support (MCM-41-Imi/Br) catalysts on cycloaddition of epoxide with CO 2.

Conclusions
The present review explained the different types of porous and mesoporous solid acid-base and ionic liquid-modified mesoporous catalysts for the effective conversion for cycloaddition reaction of various epoxide with CO 2 . In addition, the influence of various parameters, such as reaction temperature, pressure, and usage of solvents or solvent-free conditions, is discussed.
Both excellent selectivity and good yield were obtained for cyclic carbonates under tuned reaction conditions by ionic liquid immobilized MCM-41 catalyst. The higher yields were obtained for the conversion of cyclic epoxides. The above higher conversion proves that the value of the development of ionic liquid-based mesoporous catalytic materials and their future applications. The mechanism insight of ring opening of epoxide at various catalyst systems has also been discussed. The Ft-IR spectroscopy is very useful to exploit the activation mechanism of CO 2 for cycloaddition reaction using the various aminefunctionalized solid catalyst. Hence, the development of a hybrid composite catalyst based on ionic liquid could be the potential material for direct usage of emerging greenhouse gas for various chemical processes.