Recent Achievements in the Synthesis of Cyclic Carbonates from Oleﬁns and CO 2 : The Rational Design of the Homogeneous and Heterogeneous Catalytic System

: With the consumption of fossil fuels, the level of CO 2 in the atmosphere is growing rapidly, which leads to global warming. Hence, the chemical conversion of CO 2 into high value-added products is one of the most important approaches to reducing CO 2 emissions. Due to being simple, inexpensive and environmentally friendly, the direct synthesis of cyclic carbonates from oleﬁns and CO 2 is a promising project for industrial application. In this review, we discuss the design of the homogeneous and heterogeneous catalytic system for the synthesis of cyclic carbonates from the reaction of oleﬁns and CO 2 . Usually, the catalyst contains the epoxidation active site and the cycloaddition active site, which could achieve the oxidation of oleiﬁns and the CO 2 -insert, respectively. This review will provide a comprehensive overview of the direct synthesis of cyclic carbonates from oleﬁns and CO 2 catalyzed by homogeneous and heterogeneous catalysts. The focus mainly lies on the rational fabrication of multifunctional catalysts, and provides a new perspective for the design of catalysts.


Introduction
The use of CO 2 as a building block for the synthesis of high value-added chemicals has become an important current area of focus in CO 2 conversion [1]. As a cheap, non-toxic and renewable C1 resource, CO 2 has been successfully converted into many high valueadded chemicals such as alcohols [2], carboxylic acids [3], formamides [4] and organic carbonates [5]. Considering environmental awareness and the atom-economic strategy, the direct synthesis of cyclic carbonates from CO 2 and epoxides may be an alternative way of utilizing CO 2 . However, the high price of epoxides prevents the application of cycloaddition reaction of CO 2 . As a result, direct one-pot preparation of cyclic carbonates from olefins and CO 2 has become an important issue for the conversion of CO 2 [6].
Recently, a lot of homogeneous and heterogeneous catalytic systems have been applied in the reaction of olefins and CO 2 [7]. Usually, these catalytic systems have two functionalized active sites: (i) the epoxidation active site for the oxidation of olefins; and (ii) the cycloaddition active site for the coupling reaction between epoxides and CO 2 (Scheme 1) [8,9]. However, how to design catalysts reasonably, and how to promote the synergistic catalysis effect, is a big challenge for the direct oxidative carboxylation between olefins and CO 2 . Herein, we will try to give some suggestions on how to construct the multifunctional catalyst system in the synthesis of cyclic carbonates from olefins and CO 2 . and CO2. Herein, we will try to give some suggestions on how to construct the multifunctional catalyst system in the synthesis of cyclic carbonates from olefins and CO2. This review will be divided into two major parts. The first part will give a summary of the homogeneous catalytic system in recent years, while the second part will focus on the heterogeneous catalytic system and the synergistic effect between the different catalytic moieties. Scheme 1. Synthesis of cyclic carbonate from olefins and CO2.

Ionic Liquids Catalytic System
The straightforward synthesis of cyclic carbonates by direct oxidative carboxylation from olefins and CO2 is a promising method of fixation and utilization of CO2. Ionic liquids, with the advantages of negligible vapor pressure, unique solvation properties and the design-ability of anion and cation, have been widely used in the epoxidation and CO2 cycloaddition [10][11][12]. However, constructing the multifunctional ionic liquids catalytic system with both catalytic ability for olefins epoxidation and CO2 cycloaddition is still a big challenge. The direct formation of styrene carbonate (SC) from styrene and CO2 by employing 1-n-butyl-3-methylimidazolium bromide (BMImBr) as the catalyst and tertbutyl hydroperoxide (TBHP) as an oxidant was achieved (Scheme 2) [13]. The one-pot reaction could give a moderate conversion and selectivity (Conv. 90% and Sel. 40%), while a slightly higher conversion and selectivity (Conv. 99% and Sel. 63%) could be obtained by the two-step reaction. Due to the lack of the epoxidation active sites, the selectivity of styrene oxide is still low, which may significantly decrease the yield of SC. Considering that bicarbonate may react with hyperoxide to form the active oxidant of peroxymonocarbonate ion (HCO4 − ) [14], Zhang et al. reported the synthesis of SC from styrene and CO2 catalyzed by imidazolium hydrogen carbonate ionic liquids ([CnCmIm][HCO3] ILs) [8]. Owing to the epoxidation active sites and cycloaddition active sites in [CnCmIm][HCO3] (Scheme 3), the excellent yield of cyclic carbonates could be obtained by the one-pot reaction. Recently

Ionic Liquids Catalytic System
The straightforward synthesis of cyclic carbonates by direct oxidative carboxylation from olefins and CO 2 is a promising method of fixation and utilization of CO 2 . Ionic liquids, with the advantages of negligible vapor pressure, unique solvation properties and the design-ability of anion and cation, have been widely used in the epoxidation and CO 2 cycloaddition [10][11][12]. However, constructing the multifunctional ionic liquids catalytic system with both catalytic ability for olefins epoxidation and CO 2 cycloaddition is still a big challenge. The direct formation of styrene carbonate (SC) from styrene and CO 2 by employing 1-n-butyl-3-methylimidazolium bromide (BMImBr) as the catalyst and tert-butyl hydroperoxide (TBHP) as an oxidant was achieved (Scheme 2) [13]. The one-pot reaction could give a moderate conversion and selectivity (Conv. 90% and Sel. 40%), while a slightly higher conversion and selectivity (Conv. 99% and Sel. 63%) could be obtained by the two-step reaction. and CO2. Herein, we will try to give some suggestions on how to construct the multifunctional catalyst system in the synthesis of cyclic carbonates from olefins and CO2. This review will be divided into two major parts. The first part will give a summary of the homogeneous catalytic system in recent years, while the second part will focus on the heterogeneous catalytic system and the synergistic effect between the different catalytic moieties.
Scheme 1. Synthesis of cyclic carbonate from olefins and CO2.

Ionic Liquids Catalytic System
The straightforward synthesis of cyclic carbonates by direct oxidative carboxylation from olefins and CO2 is a promising method of fixation and utilization of CO2. Ionic liquids, with the advantages of negligible vapor pressure, unique solvation properties and the design-ability of anion and cation, have been widely used in the epoxidation and CO2 cycloaddition [10][11][12]. However, constructing the multifunctional ionic liquids catalytic system with both catalytic ability for olefins epoxidation and CO2 cycloaddition is still a big challenge. The direct formation of styrene carbonate (SC) from styrene and CO2 by employing 1-n-butyl-3-methylimidazolium bromide (BMImBr) as the catalyst and tertbutyl hydroperoxide (TBHP) as an oxidant was achieved (Scheme 2) [13]. The one-pot reaction could give a moderate conversion and selectivity (Conv. 90% and Sel. 40%), while a slightly higher conversion and selectivity (Conv. 99% and Sel. 63%) could be obtained by the two-step reaction. Due to the lack of the epoxidation active sites, the selectivity of styrene oxide is still low, which may significantly decrease the yield of SC. Considering that bicarbonate may react with hyperoxide to form the active oxidant of peroxymonocarbonate ion (HCO4 − ) [14], Zhang [15]. In contrast, aromatic and cyclic olefins such as styrene and cyclohexene suffered from easy overoxidation and consequent low selectivity towards the carbonates in the tungstate ionic liquid catalyst. Although the ionic liquids catalyst systems 10 bar CO2 (Scheme 4) [15]. In contrast, aromatic and cyclic olefins such as styrene and cyclohexene suffered from easy overoxidation and consequent low selectivity towards the carbonates in the tungstate ionic liquid catalyst. Although the ionic liquids catalyst systems have good design ability, the development of highly efficient ionic liquids catalysts should be carefully designed.

Metal Complex Catalytic System
It is well-known that some catalysts are inspired by nature and a lot of biological biomimetic catalyst has been applied in the olefin epoxidation reaction [16,17]. Hence, the direct oxidative carboxylation of olefins and CO2 by a metal complex catalytic system seems to be an efficient method. Jing et al. reported the one-pot synthesis of synthesis carbonate from styrene, O2 and CO2 with dioxo(tetraphenylporphyrinato) ruthenium [Ru(TPP)(O)2]/quaternary onium salt [18]. The [Ru(TPP)(O)2] can transform the oxygen atom from the catalyst to form the epoxide which could act as a substrate for the coupling reaction with CO2 (Scheme 5). It is well known that the selectivity of the epoxidation reaction has a great impact on the yield of cyclic carbonate. For example, the yield of styrene carbonate was only 3.6% with benzoic acid (55.3%) and benzaldehyde (37.5%) as major products when using a Fe(TPP)Cl as a comparable catalyst [18]. However, the Ru(Salen)(PPh3)2 catalyst showed a 40.7% yield of styrene carbonate under the same conditions, which meant that the new active site of Ru(Salen)(O) could generate from the Ru(Salen)(PPh3)2 in situ. As a result, the suitable oxidative property and coupling property in the catalyst may play an important role in the catalyst design.

Metal Complex Catalytic System
It is well-known that some catalysts are inspired by nature and a lot of biological biomimetic catalyst has been applied in the olefin epoxidation reaction [16,17]. Hence, the direct oxidative carboxylation of olefins and CO 2 by a metal complex catalytic system seems to be an efficient method. Jing et al. reported the one-pot synthesis of synthesis carbonate from styrene, O 2 and CO 2 with dioxo(tetraphenylporphyrinato) ruthenium [Ru(TPP)(O) 2 ]/quaternary onium salt [18]. The [Ru(TPP)(O) 2 ] can transform the oxygen atom from the catalyst to form the epoxide which could act as a substrate for the coupling reaction with CO 2 (Scheme 5). It is well known that the selectivity of the epoxidation reaction has a great impact on the yield of cyclic carbonate. For example, the yield of styrene carbonate was only 3.6% with benzoic acid (55.3%) and benzaldehyde (37.5%) as major products when using a Fe(TPP)Cl as a comparable catalyst [18]. However, the Ru(Salen)(PPh 3 ) 2 catalyst showed a 40.7% yield of styrene carbonate under the same conditions, which meant that the new active site of Ru(Salen)(O) could generate from the Ru(Salen)(PPh 3 ) 2 in situ. As a result, the suitable oxidative property and coupling property in the catalyst may play an important role in the catalyst design. The functionalization of the ligand of metal-complex could have a great effect on the conversion and selectivity of the reaction, which could be an alternative method to optimize catalyst design. Ghosh et al. developed a manganese (III) complex of an amidoamine ligand/tetrabutylammonium bromide (TBAB) catalytic system, which could efficiently convert the alkene to the epoxide and further react with CO2 to form cyclic carbonate in the presence of CO2 and TBHP [19]. The high selectivity of the epoxide may originate from the transformation of the manganese (III) complex of an amido-amine ligand to a non-heme high-valent manganese-oxo intermediate in situ (Scheme 6). Besides, the Lewis acidity in the manganese (III) complex could also accelerate the CO2 cycloaddition reaction. Based on recent research, the one-pot synthesis of cyclic carbonate catalyzed by the biomimetic transition metals (such as Fe, Mn, Ru) catalytic systems usually includes two parts. Firstly, the metal center is oxidized to form the oxo species, which could act as an intermediate to oxidize the olefin. Secondly, the coupling reaction between the epoxides and CO2 is conducted. Although the homogeneous catalytic system showed excellent activity in the olefins epoxidation and CO2 cycloaddition respectively, the integration of the epoxidation active sites and cycloaddition active sites is not easy work. In addition to the biomimetic transition metals, selective epoxidation of olefins by alkyl hydroperoxide catalyzed by d 0 transition metal complexes (Mo(VI), V(V) and Ti(IV)) has been used in the epoxidation reaction [20]. Hu et al. found that a MoO2(acac)2-quaternary ammonium salt catalytic system could achieve the direct synthesis of cyclic carbonates from olefins and CO2 with TBHP as an oxidant [21]. The one-pot synthesis of cyclic carbonates contained two separate reactions catalyzed by the different active sites. During the epoxidation process, the formation of the alkyl-peroxo M-OOR intermediate could coordinate with the olefin to form a three-membered transition state (Scheme 7). After the transformation of oxygen from M-OOR to olefin, epoxide could be obtained. In the following cycloaddition process, the oxygen atom of the epoxide was activated by the [n-Scheme 5. Proposed mechanism of aerobic oxidative carboxylation of olefins.
The functionalization of the ligand of metal-complex could have a great effect on the conversion and selectivity of the reaction, which could be an alternative method to optimize catalyst design. Ghosh et al. developed a manganese (III) complex of an amido-amine ligand/tetrabutylammonium bromide (TBAB) catalytic system, which could efficiently convert the alkene to the epoxide and further react with CO 2 to form cyclic carbonate in the presence of CO 2 and TBHP [19]. The high selectivity of the epoxide may originate from the transformation of the manganese (III) complex of an amido-amine ligand to a non-heme high-valent manganese-oxo intermediate in situ (Scheme 6). Besides, the Lewis acidity in the manganese (III) complex could also accelerate the CO 2 cycloaddition reaction. Based on recent research, the one-pot synthesis of cyclic carbonate catalyzed by the biomimetic transition metals (such as Fe, Mn, Ru) catalytic systems usually includes two parts. Firstly, the metal center is oxidized to form the oxo species, which could act as an intermediate to oxidize the olefin. Secondly, the coupling reaction between the epoxides and CO 2 is conducted. Although the homogeneous catalytic system showed excellent activity in the olefins epoxidation and CO 2 cycloaddition respectively, the integration of the epoxidation active sites and cycloaddition active sites is not easy work. The functionalization of the ligand of metal-complex could have a great effect on the conversion and selectivity of the reaction, which could be an alternative method to optimize catalyst design. Ghosh et al. developed a manganese (III) complex of an amidoamine ligand/tetrabutylammonium bromide (TBAB) catalytic system, which could efficiently convert the alkene to the epoxide and further react with CO2 to form cyclic carbonate in the presence of CO2 and TBHP [19]. The high selectivity of the epoxide may originate from the transformation of the manganese (III) complex of an amido-amine ligand to a non-heme high-valent manganese-oxo intermediate in situ (Scheme 6). Besides, the Lewis acidity in the manganese (III) complex could also accelerate the CO2 cycloaddition reaction. Based on recent research, the one-pot synthesis of cyclic carbonate catalyzed by the biomimetic transition metals (such as Fe, Mn, Ru) catalytic systems usually includes two parts. Firstly, the metal center is oxidized to form the oxo species, which could act as an intermediate to oxidize the olefin. Secondly, the coupling reaction between the epoxides and CO2 is conducted. Although the homogeneous catalytic system showed excellent activity in the olefins epoxidation and CO2 cycloaddition respectively, the integration of the epoxidation active sites and cycloaddition active sites is not easy work. In addition to the biomimetic transition metals, selective epoxidation of olefins by alkyl hydroperoxide catalyzed by d 0 transition metal complexes (Mo(VI), V(V) and Ti(IV)) has been used in the epoxidation reaction [20]. Hu et al. found that a MoO2(acac)2-quaternary ammonium salt catalytic system could achieve the direct synthesis of cyclic carbonates from olefins and CO2 with TBHP as an oxidant [21]. The one-pot synthesis of cyclic carbonates contained two separate reactions catalyzed by the different active sites. During the epoxidation process, the formation of the alkyl-peroxo M-OOR intermediate could coordinate with the olefin to form a three-membered transition state (Scheme 7). After the transformation of oxygen from M-OOR to olefin, epoxide could be obtained. In the following cycloaddition process, the oxygen atom of the epoxide was activated by the [n-Scheme 6. Structure of manganese (III) complex of an amido-amine ligand and high-valent manganese-oxo intermediate.
In addition to the biomimetic transition metals, selective epoxidation of olefins by alkyl hydroperoxide catalyzed by d 0 transition metal complexes (Mo (VI) , V (V) and Ti (IV) ) has been used in the epoxidation reaction [20]. Hu et al. found that a MoO 2 (acac) 2 -quaternary ammonium salt catalytic system could achieve the direct synthesis of cyclic carbonates from olefins and CO 2 with TBHP as an oxidant [21]. The one-pot synthesis of cyclic carbonates contained two separate reactions catalyzed by the different active sites. During the epoxidation process, the formation of the alkyl-peroxo M-OOR intermediate could coordinate with the olefin to form a three-membered transition state (Scheme 7). After the transformation of oxygen from M-OOR to olefin, epoxide could be obtained. In the following cycloaddition process, the oxygen atom of the epoxide was activated by the [n-Bu 4 N] + cation of the TBAB and then the nucleophilic Br-attacked the less hindered  Table 1 summarizes the homogeneous catalytic system for the conversion of olefins and CO2 to cyclic carbonates. Compared with the different homogeneous catalyst systems, it is found that using the polyoxometalates or metal complexes may be a benefit for the epoxidation reaction and the quaternary ammonium salts may promote the cycloaddition reaction (Table 1, entries 3 and 4). Although ionic liquids have good design ability and can integrate the epoxidation active site and cycloaddition active site into one catalyst, the yield of cyclic carbonates must continue to improve ( Table 1, entries 1 and 2). Interestingly, we found that the reaction temperature had a significant influence on the yield and selectivity of cyclic carbonates during the one-pot reaction [8]. With the increasing temperature, the selectivity of cyclic carbonates is decreasing, which indicates that the high temperature is not beneficial for the epoxidation reaction, and the main byproduct is benzaldehyde. If benzaldehyde is formed, the subsequent CO2 cycloaddition cannot take place. Besides, the pressure of O2 is also an important reaction parameter. The high pressure of O2 may lead to over-oxidation and the byproducts such as benzaldehyde, benzoic acid and acetophenone [19]. As we know, the CO2 cycloaddition is hard to achieve, which means that the CO2-insert reaction is unfavorable under low pressure of CO2. However, further increasing the pressure of CO2 may decrease the yield of cyclic carbonates. Hence, utilization of O2 as an oxidant and without the addition of a co-catalyst is still a huge change for the direct oxidative carboxylation between olefins and CO2.

Entry
Catalyst  Table 1 summarizes the homogeneous catalytic system for the conversion of olefins and CO 2 to cyclic carbonates. Compared with the different homogeneous catalyst systems, it is found that using the polyoxometalates or metal complexes may be a benefit for the epoxidation reaction and the quaternary ammonium salts may promote the cycloaddition reaction (Table 1, entries 3 and 4). Although ionic liquids have good design ability and can integrate the epoxidation active site and cycloaddition active site into one catalyst, the yield of cyclic carbonates must continue to improve ( Table 1, entries 1 and 2). Interestingly, we found that the reaction temperature had a significant influence on the yield and selectivity of cyclic carbonates during the one-pot reaction [8]. With the increasing temperature, the selectivity of cyclic carbonates is decreasing, which indicates that the high temperature is not beneficial for the epoxidation reaction, and the main byproduct is benzaldehyde. If benzaldehyde is formed, the subsequent CO 2 cycloaddition cannot take place. Besides, the pressure of O 2 is also an important reaction parameter. The high pressure of O 2 may lead to over-oxidation and the byproducts such as benzaldehyde, benzoic acid and acetophenone [19]. As we know, the CO 2 cycloaddition is hard to achieve, which means that the CO 2 -insert reaction is unfavorable under low pressure of CO 2 . However, further increasing the pressure of CO 2 may decrease the yield of cyclic carbonates. Hence, utilization of O 2 as an oxidant and without the addition of a co-catalyst is still a huge change for the direct oxidative carboxylation between olefins and CO 2 .

Zeolite Catalytic System
Due to the facile and efficient recovery of the catalyst from the reaction mixture, the heterogeneous method has shown enormous potential in industrial application. Zeolites, the microporous crystalline maters, attract a lot of attention in catalysts due to their tunable catalytic properties and shape selectivities [22]. Recent studies have found that a series of titanosilicate sieves (TS-1 [23], Ti-Beta [24], Ti-MWW, etc. [25]) reveal excellent selective oxidation during the epoxidation reaction. The real active sites of the oxidation reaction could be ascribed to the transformation of the tetrahedrally coordinated titanium atoms to Ti-OOH species [26,27], which may further react with olefins to selectively produce the epoxides (Scheme 8). After adding co-catalyst or functionalized modification of the titanosilicate sieves, the direct oxidative carboxylation of olefins and CO 2 can proceed. Srivastava et al. first reported the synthesis of cyclic carbonates from olefin, oxidant (H 2 O 2 and TBHP, respectively) and CO 2 catalyzed by the microporous titanium-silicalite-1 (TS-1) and mesoporous Ti-MCM-41 [28]. With Ti-MCM-41/TBHP and 4-dimethylaminopyridine (DMAP) as a co-catalyst, a 33% yield of SC can obtain under 6.9 bar CO 2 and 120 • C through two steps reaction. He et al. prepared a quaternary ammonium modified delaminated titanosilicate (Del-Ti-MWW-N + Bu 3 Br − ) which gave a moderate yield (48% for cyclic carbonate) in a one-pot reaction using H 2 O 2 as an oxidant [29]. However, the pore blocking by heavy byproducts may lead to a decrease in catalytic activity and selectivity. Therefore, the development of highly efficient, easily recyclable and hierarchical porous catalysts for the direct synthesis of cyclic carbonates from olefins and CO 2 may overcome these problems.

Zeolite Catalytic System
Due to the facile and efficient recovery of the catalyst from the reaction mixture, the heterogeneous method has shown enormous potential in industrial application. Zeolites, the microporous crystalline maters, attract a lot of attention in catalysts due to their tunable catalytic properties and shape selectivities [22]. Recent studies have found that a series of titanosilicate sieves (TS-1 [23], Ti-Beta [24], Ti-MWW, etc. [25]) reveal excellent selective oxidation during the epoxidation reaction. The real active sites of the oxidation reaction could be ascribed to the transformation of the tetrahedrally coordinated titanium atoms to Ti-OOH species [26,27], which may further react with olefins to selectively produce the epoxides (Scheme 8). After adding co-catalyst or functionalized modification of the titanosilicate sieves, the direct oxidative carboxylation of olefins and CO2 can proceed. Srivastava et al. first reported the synthesis of cyclic carbonates from olefin, oxidant (H2O2 and TBHP, respectively) and CO2 catalyzed by the microporous titanium-silicalite-1 (TS-1) and mesoporous Ti-MCM-41 [28]. With Ti-MCM-41/TBHP and 4-dimethylaminopyridine (DMAP) as a co-catalyst, a 33% yield of SC can obtain under 6.9 bar CO2 and 120 °C through two steps reaction. He et al. prepared a quaternary ammonium modified delaminated titanosilicate (Del-Ti-MWW-N + Bu3Br − ) which gave a moderate yield (48% for cyclic carbonate) in a one-pot reaction using H2O2 as an oxidant [29]. However, the pore blocking by heavy byproducts may lead to a decrease in catalytic activity and selectivity. Therefore, the development of highly efficient, easily recyclable and hierarchical porous catalysts for the direct synthesis of cyclic carbonates from olefins and CO2 may overcome these problems. Because mesoporous materials have a larger pore size than microporous materials, some researchers have designed many mesoporous titanium-silicates applications in oxidative carboxylation of olefins with oxidants and CO2 [20]. For example, Kholdeeva et al. prepared a mesoporous titanium-silicates Ti-MMM-E by evaporation-induced self-assembly (EISA) method [30]. The mesoporous titanium-silicates Ti-MMM-E exhibited superior catalytic performance in the one-step oxidation carboxylation of styrene and 4-methylstyrene with TBHP, TBAB and CO2 under mild conditions (Scheme 9). Moreover, it was found that bromide ions in TBAB may occupy the Lewis acid sites and prevent styrene oxide from further oxidation to benzaldehyde in the presence of TBHP and Ti-MMM-E. To avoid pore blocking, Kholdeeva et al. explored a new method to prepare titaniumgrafted mesoporous silica catalysts by metallic titanium clusters deposition on silica supports [31]. The non-single-site Ti(IV)nOx-like silica-supported catalyst (TixOn-MCM-41 and TixOn-MMM-2) from Ti13 metal clusters deposited onto ordered mesoporous silicas showed a high yield of cyclic carbonate. Compared with the traditional titanium-silicates, Scheme 8. Ti active sites and proposed mechanism for the epoxidation of olefins.
Because mesoporous materials have a larger pore size than microporous materials, some researchers have designed many mesoporous titanium-silicates applications in oxidative carboxylation of olefins with oxidants and CO 2 [20]. For example, Kholdeeva et al. prepared a mesoporous titanium-silicates Ti-MMM-E by evaporation-induced self-assembly (EISA) method [30]. The mesoporous titanium-silicates Ti-MMM-E exhibited superior catalytic performance in the one-step oxidation carboxylation of styrene and 4-methylstyrene with TBHP, TBAB and CO 2 under mild conditions (Scheme 9). Moreover, it was found that bromide ions in TBAB may occupy the Lewis acid sites and prevent styrene oxide from further oxidation to benzaldehyde in the presence of TBHP and Ti-MMM-E. To avoid pore blocking, Kholdeeva et al. explored a new method to prepare titanium-grafted mesoporous silica catalysts by metallic titanium clusters deposition on silica supports [31]. The nonsingle-site Ti(IV) n O x -like silica-supported catalyst (Ti x O n -MCM-41 and Ti x O n -MMM-2) from Ti 13 metal clusters deposited onto ordered mesoporous silicas showed a high yield of cyclic carbonate. Compared with the traditional titanium-silicates, these grafted Ti n O xsilica materials had better accessibility to titanium active sites exposed on the surface of the catalyst (Scheme 10). these grafted TinOx-silica materials had better accessibility to titanium active sites exposed on the surface of the catalyst (Scheme 10). As we know, large pore size molecular sieves are beneficial for the substrate diffusion and the dispersion of the active sites. Since the well-ordered hexagonal mesoporous silica structures (SBA-15) with uniform pore sizes were discovered by Zhao et al. [32], SBA-15 has attracted increasing attention and is considered an important class of nanostructured support materials in heterogeneous catalysis [33]. these grafted TinOx-silica materials had better accessibility to titanium active sites exposed on the surface of the catalyst (Scheme 10). As we know, large pore size molecular sieves are beneficial for the substrate diffusion and the dispersion of the active sites. Since the well-ordered hexagonal mesoporous silica structures (SBA-15) with uniform pore sizes were discovered by Zhao et al. [32], SBA-15 has attracted increasing attention and is considered an important class of nanostructured support materials in heterogeneous catalysis [33].  As we know, large pore size molecular sieves are beneficial for the substrate diffusion and the dispersion of the active sites. Since the well-ordered hexagonal mesoporous silica structures (SBA-15) with uniform pore sizes were discovered by Zhao et al. [32], SBA-15 has attracted increasing attention and is considered an important class of nanostructured support materials in heterogeneous catalysis [33]. Xu et al. reported amino-functionalized ILs@SBA-15 catalysts (N(CH 3 ) 2 -[Im][Br]@SBA-15), which showed impressive catalytic activity for the one-pot synthesis of cyclic carbonates from olefins and CO 2 [34]. The dual function of the N(CH 3 ) 2 -[Im][Br]@SBA-15 could not only catalyze the oxidation of olefins but also promote the CO 2 cycloaddition reaction (Scheme 11). In the oxidation cycle, Br − was first oxidized by TBHP to hypobromite (BrO − ), which could act as an active [O] site to react with styrene to form bromohydrin. After the deprotonation of bromohydrin, styrene oxide was obtained, which was activated by the C2-H on the imidazole ring via the hydrogen bond interaction. Dendritic fibrous nanosilica (DFNS), with the properties of morphology-controlled and intrinsic mesoporous characteristics, have been widely used in catalysis, gas capture, solar-energy harvesting, energy storage, and so on [35]. Sadeghzadeh et al. developed a melamine-Pd(II) Schiff base complex functionalized FeNi3/DFNS core-shell NPs (FeNi3/DFNS/salen/Pd(II) MNPs) (Scheme 12), which showed 94% yield of styrene carbonate even after 10 runs [36]. Recently, Motavalizadehkakhky et al. applied the DFNSsupported platinum (II) complexes (DFNS/Pt(II) NPS) in the synthesis of cyclic carbonate from olefins and CO2 [37]. It was found that the Pt(II) complex in the DFNS played an important role in the direct oxidative carboxylation of CO2 and styrene to styrene carbonate. Dendritic fibrous nanosilica (DFNS), with the properties of morphology-controlled and intrinsic mesoporous characteristics, have been widely used in catalysis, gas capture, solar-energy harvesting, energy storage, and so on [35]. Sadeghzadeh et al. developed a melamine-Pd(II) Schiff base complex functionalized FeNi 3 /DFNS core-shell NPs (FeNi 3 /DFNS/salen/Pd(II) MNPs) (Scheme 12), which showed 94% yield of styrene carbonate even after 10 runs [36]. Recently, Motavalizadehkakhky et al. applied the DFNSsupported platinum (II) complexes (DFNS/Pt(II) NPS) in the synthesis of cyclic carbonate from olefins and CO 2 [37]. It was found that the Pt(II) complex in the DFNS played an important role in the direct oxidative carboxylation of CO 2 and styrene to styrene carbonate.

MOFs Catalytic System
Metal-organic frameworks (MOFs), as a new kind of porous materials with high crystalline, high metal content, large surface area and tunable pore structure, have shown great potential applications in gas storage [38], energy storage [39] and catalysis [40]. Some

MOFs Catalytic System
Metal-organic frameworks (MOFs), as a new kind of porous materials with high crystalline, high metal content, large surface area and tunable pore structure, have shown great potential applications in gas storage [38], energy storage [39] and catalysis [40]. Some pioneering research has been reported in the olefins epoxidation reaction and CO 2 cycloaddition reaction respectively [41][42][43][44][45]. The direct synthesis of cyclic carbonates from olefins and CO 2 catalyzed by MOFs usually contained two parts, including the epoxidation reaction and cycloaddition reaction [46]. As a result, combining these two reactions into one functionalized MOFs catalyst may not only avoid the co-catalyst but also promote the conversion and selectivity. Duan et al. used MOF as support and incorporated a pyrrolidine moiety as a chiral organocatalyst and a polyoxometalate as an oxidation catalyst (ZnW-PYIs, ZnW: ZnW 12 O 40 6− ) for the direct synthesis of cyclic carbonates from olefins and CO 2 (Scheme 13) [47]. The oxygen-enriched surface of ZnW 12 O 40 6− could accelerate the transformation of the catalytic precursors to the active intermediate of the epoxidation [48]. Meanwhile, pyrrolidine-2-yl-imidazole (PYI) could act as a cooperative catalytic site, which may enhance the activities of the oxidants and drive the catalysis asymmetrically [49]. Based on the synergistic effect, 72-92% yields of cyclic carbonates and 55-80% enantiomeric excesses were obtained with styrene and its derivatives in the presence of TBHP and TBAB. Compared with the polyoxometalate, Mn-porphyrin is also a suitable candidate for epoxidation reaction [50]. Nagaraja et al. , which exhibited catalytic activity for the synthesis of cyclic carbonates through the twostep catalytic process [52]. The spatial matching between the Mo(VI) and alkaline µ 3 -OH tricopper (II) cores could provide the compatibility with the Mo=Ot-activated epoxidation intermediate during the epoxidation reaction. In the CO 2 cycloaddition reaction, the µ 3 -OH tricopper (II) core could increase the adsorption of CO 2 with the formation of a bidentate bridge or tridentate linear adsorption state [53], which could promote the CO 2insertion reaction. Hence, improving the adsorption or desorption property of a catalyst can significantly influence the catalytic activity [54]. the compatibility with the Mo=Ot-activated epoxidation intermediate during the epoxidation reaction. In the CO2 cycloaddition reaction, the μ3-OH tricopper (II) core could increase the adsorption of CO2 with the formation of a bidentate bridge or tridentate linear adsorption state [53], which could promote the CO2-insertion reaction. Hence, improving the adsorption or desorption property of a catalyst can significantly influence the catalytic activity [54].
Scheme 13. The schematic representation of tandem catalysis for the asymmetric cyclic carbonate transformation fromolefins and carbon dioxide.
The rational design of multifunctional MOFs can achieve selective capture and conversion of CO2 under mild conditions [55], but combining CO2 capture with its conversion is still a problem. Recently, Nguyen et al. prepared lanthanide-based metal-organic frameworks (MOF-591 and MOF-591), which showed good adsorption of CO2 [56]. Additionally, MOF-590 revealed excellent catalytic activity for the one-pot oxidative carboxylation of olefins with CO2. Usually, the oxidative carboxylation of styrene and CO2 catalyzed by Scheme 13. The schematic representation of tandem catalysis for the asymmetric cyclic carbonate transformation fromolefins and carbon dioxide.
The rational design of multifunctional MOFs can achieve selective capture and conversion of CO 2 under mild conditions [55], but combining CO 2 capture with its conversion is still a problem. Recently, Nguyen et al. prepared lanthanide-based metal-organic frameworks (MOF-591 and MOF-591), which showed good adsorption of CO 2 [56]. Additionally, MOF-590 revealed excellent catalytic activity for the one-pot oxidative carboxylation of olefins with CO 2 . Usually, the oxidative carboxylation of styrene and CO 2 catalyzed by the MOF-590 consisted of two steps, including the epoxidation catalyzed by MOF-590/nBu 4 NBr-TBHP and the cycloaddition catalyzed by MOF-590/nBu 4 NBr (Scheme 14). The Nd 3+ clusters of MOF-590 were first oxidized to Nd 4+ -peroxy species and Nd 4+ -peroxy species released a t-butoxy radical to oxidize styrene to obtain the styrene oxide through the butoxperoxy intermediate [42]. Secondly, the styrene oxide was first coordinated by the Nd unit in the MOF-90 and the C-O bond in the styrene oxide was polarization and elongated. Then Br − attacked the less steric hindrance C atom in the styrene oxide forming the metal-coordinated bromoalkoxide intermediate, which was stabilized by the Nd unit. The metal-carbonate intermediates were formed by the insertion of CO 2 into the metalcoordinated bromoalkoxide intermediate. After the ring-closing reaction, styrene carbonate was formed and the MOF-590 and nBu 4 NBr were regenerated. Scheme 13. The schematic representation of tandem catalysis for the asymmetric cyclic carbonate transformation fromolefins and carbon dioxide.
The rational design of multifunctional MOFs can achieve selective capture and conversion of CO2 under mild conditions [55], but combining CO2 capture with its conversion is still a problem. Recently, Nguyen et al. prepared lanthanide-based metal-organic frameworks (MOF-591 and MOF-591), which showed good adsorption of CO2 [56]. Additionally, MOF-590 revealed excellent catalytic activity for the one-pot oxidative carboxylation of olefins with CO2. Usually, the oxidative carboxylation of styrene and CO2 catalyzed by the MOF-590 consisted of two steps, including the epoxidation catalyzed by MOF-590/nBu4NBr-TBHP and the cycloaddition catalyzed by MOF-590/nBu4NBr (Scheme 14). The Nd 3+ clusters of MOF-590 were first oxidized to Nd 4+ -peroxy species and Nd 4+ -peroxy species released a t-butoxy radical to oxidize styrene to obtain the styrene oxide through the butoxperoxy intermediate [42]. Secondly, the styrene oxide was first coordinated by the Nd unit in the MOF-90 and the C-O bond in the styrene oxide was polarization and elongated. Then Br − attacked the less steric hindrance C atom in the styrene oxide forming the metal-coordinated bromoalkoxide intermediate, which was stabilized by the Nd unit. The metal-carbonate intermediates were formed by the insertion of CO2 into the metalcoordinated bromoalkoxide intermediate. After the ring-closing reaction, styrene carbonate was formed and the MOF-590 and nBu4NBr were regenerated. To avoid the addition of a co-catalyst, Ahn et al. constructed a zirconium-based porphyrinic MOF-545 (Zr 6 O 8 (H 2 O) 8 (H 2 -TCPP) 2 ) using a tetrakis(4-carboxyopheny) porphyrin (H 6 -TCPP) ligand and Zr 6 clusters, followed by Mn(III) metalation to the porphyrin units for the epoxidation reaction (Scheme 15) [57]. Then, 1-methyl-3-(2-carboxyethyl)imidazolium bromide (ImBr) ionic liquid was introduced to the Zr 6 clusters forming ImBr-functionalized MOF-545(Mn) (ImBr-MOF-545(Mn)) for the CO 2 cycloaddition reaction. The MOF-545 could act as a support platform that integrated the Lewis acidic Mn(III)/Zr(IV) ions and Br − in the adjacent position and exhibited a high yield of cyclic carbonates in the absence of any co-catalyst under mild reaction conditions.

Metal Nanoparticles Catalytic System
Metal nanoparticles (NPs) or single-atom catalytic sites (SACs) have been wildly used in a variety of catalytic reactions due to their excellent activity [58,59]. For example, Sun and co-workers prepared a series of Au-supported catalysts which could synthesize the cyclic carbonates from olefins and CO 2 [60,61]. They applied the Au/SiO 2 -ZnBr 2 /Bu 4 NBr catalyst system for the preparation of cyclic styrene carbonate from styrene and CO 2 [60], which could give a 42% selectivity towards cyclic styrene carbonate under mild reaction conditions (80 • C, 1 MPa CO 2 ). Moreover, it was found that cumene hydroperoxide was a better oxidant than Bu 4 NBr, indicating that choosing the suitable oxidation may have a significant effect on the yield of styrene carbonate. Using a strong basic resin R201 as support to replace the SiO 2 , a 51% yield of styrene carbonate was obtained in the Au/R201 catalyst with anhydrous TBHP as an oxidant. The different catalytic activity between the Au/SiO 2 -ZnBr 2 /Bu 4 NBr and Au/R201 catalyst may originate from the different particle sizes of Au nanoparticles, which may play important role in the epoxidation reaction. The work function of Au species with different atomicity is strongly dependent on the particle size [58]. The appropriate size of Au nanoparticles could enhance the selectivity of epoxides during the epoxidation reaction [62]. Recently, Zhang et al. fabricated a doubleshell microencapsulated nanoreactor as a bifunctional catalyst for the one-pot synthesis of cyclic carbonates from olefins [63]. Au nanoparticels (NPs) were confined between an inter shell of nitrogen-doped porous carbon rich in Zn single-atom catalytic sites (SACs) and an outer shell of mesoporous SiO 2 (noted as Zn-N-C/Au@mSiO 2 , Scheme 16). The synergistic catalysis of the ultrafine Au NPs and Zn SACs showed high catalytic efficiency and tandem catalytic function for the direct oxidative carboxylation from olefins and CO 2 . To avoid the addition of a co-catalyst, Ahn et al. constructed a zirconium-based porphyrinic MOF-545 (Zr6O8(H2O)8(H2-TCPP)2) using a tetrakis(4-carboxyopheny) porphyrin (H6-TCPP) ligand and Zr6 clusters, followed by Mn(III) metalation to the porphyrin units for the epoxidation reaction (Scheme 15) [57]. Then, 1-methyl-3-(2-carboxyethyl)imidazolium bromide (ImBr) ionic liquid was introduced to the Zr6 clusters forming ImBr-functionalized MOF-545(Mn) (ImBr-MOF-545(Mn)) for the CO2 cycloaddition reaction. The MOF-545 could act as a support platform that integrated the Lewis acidic Mn(III)/Zr(IV) ions and Br − in the adjacent position and exhibited a high yield of cyclic carbonates in the absence of any co-catalyst under mild reaction conditions. Scheme 15. The synthetic procedure of ImBr-MOF-545(Mn) by post-synthetic Mn metalation followed by ImBr ionic liquid functionalization.

Metal Nanoparticles Catalytic System
Metal nanoparticles (NPs) or single-atom catalytic sites (SACs) have been wildly used in a variety of catalytic reactions due to their excellent activity [58,59]. For example, Sun and co-workers prepared a series of Au-supported catalysts which could synthesize the cyclic carbonates from olefins and CO2 [60,61]. They applied the Au/SiO2-ZnBr2/Bu4NBr catalyst system for the preparation of cyclic styrene carbonate from styrene and CO2 [60], which could give a 42% selectivity towards cyclic styrene carbonate under mild reaction conditions (80 °C, 1 MPa CO2). Moreover, it was found that cumene hydroperoxide was a better oxidant than Bu4NBr, indicating that choosing the suitable oxidation may have a significant effect on the yield of styrene carbonate. Using a strong basic resin R201 as support to replace the SiO2, a 51% yield of styrene carbonate was obtained in the Au/R201 catalyst with anhydrous TBHP as an oxidant. The different catalytic activity between the Au/SiO2-ZnBr2/Bu4NBr and Au/R201 catalyst may originate from the different particle sizes of Au nanoparticles, which may play important role in the epoxidation reaction. The work function of Au species with different atomicity is strongly dependent on the particle size [58]. The appropriate size of Au nanoparticles could enhance the selectivity of epoxides during the epoxidation reaction [62]. Recently, Zhang et al. fabricated a double-shell microencapsulated nanoreactor as a bifunctional catalyst for the one-pot synthesis of cyclic carbonates from olefins [63]. Au nanoparticels (NPs) were confined between an inter shell of nitrogen-doped porous carbon rich in Zn single-atom catalytic sites (SACs) and an outer shell of mesoporous SiO2 (noted as Zn-N-C/Au@mSiO2, Scheme 16). The synergistic catalysis of the ultrafine Au NPs and Zn SACs showed high catalytic efficiency and tandem catalytic function for the direct oxidative carboxylation from olefins and CO2.

Others
Frustrated Lewis pairs (FLPs) are combinations of Lewis acids and Lewis bases that are prevented from forming a stable adduct by steric or electronic hindrance [64].

Others
Frustrated Lewis pairs (FLPs) are combinations of Lewis acids and Lewis bases that are prevented from forming a stable adduct by steric or electronic hindrance [64].
Due to the sterically hindered Lewis acid and Lewis base, the FLPs have shown the ability in the activation and transformation of inert small molecules such as H 2 and CO 2 [65][66][67]. Qu et al. reported a defect-enriched CeO 2 with constructed interfacial FLPs (two adjacent Ce 3+ ···O 2− ), which could activate CO 2 via the interactions between C/Lewis basic lattice O 2− and the two O atoms in CO 2 /two adjacent Lewis acidic Ce 3+ ions [68]. When increasing the surface of CeO 2 , an 88% yield of styrene carbonate can be obtained catalyzed by porous nanorods of CeO 2 (PN-CeO 2 ) under 80 • C and 24 h. The excellent performance of the selective tandem conversion of olefins and CO 2 to cyclic carbonates may derive from the efficient CO 2 activation for cycloaddition and weakening of the adsorption of in situ-produced epoxides to suppress their hydrolysis (Scheme 17). Scheme 16. Synthesis procedure of Zn-N-C/Au@mSiO2.

Others
Frustrated Lewis pairs (FLPs) are combinations of Lewis acids and Lewis bases that are prevented from forming a stable adduct by steric or electronic hindrance [64]. Due to the sterically hindered Lewis acid and Lewis base, the FLPs have shown the ability in the activation and transformation of inert small molecules such as H2 and CO2 [65][66][67]. Qu et al. reported a defect-enriched CeO2 with constructed interfacial FLPs (two adjacent Ce 3+ •••O 2− ), which could activate CO2 via the interactions between C/Lewis basic lattice O 2− and the two O atoms in CO2/two adjacent Lewis acidic Ce 3+ ions [68]. When increasing the surface of CeO2, an 88% yield of styrene carbonate can be obtained catalyzed by porous nanorods of CeO2 (PN-CeO2) under 80 °C and 24 h. The excellent performance of the selective tandem conversion of olefins and CO2 to cyclic carbonates may derive from the efficient CO2 activation for cycloaddition and weakening of the adsorption of in situ-produced epoxides to suppress their hydrolysis (Scheme 17). Considering the utilization of "carbon-neutral" resources, using a biomass-based catalyst for the synthesis of styrene carbonate from styrene and CO2 may be a green and sustainable method. Due to the abundant hydroxyl groups in the biomass-based catalyst Scheme 18. The reaction procedure of TBA-TBD-P with CO 2 and calculated two different ways of activating CO 2 on TBA-TBD-P.
Considering the utilization of "carbon-neutral" resources, using a biomass-based catalyst for the synthesis of styrene carbonate from styrene and CO 2 may be a green and sustainable method. Due to the abundant hydroxyl groups in the biomass-based catalyst (such as cellulose, chitosan and lignin), these biomass-based catalysts have been applied in the CO 2 cycloaddition reaction [5,[70][71][72]. Although the biomass-based catalysts exhibit excellent catalytic activity for the CO 2 cycloaddition reaction, how to introduce the epoxidation active sites in the biomass-based catalysts and achieve the direct synthesis of cyclic carbonates remains a big challenge. Jain et al. Fabricated magnetically separable chitosan (MCS) modified with Co(II) acetylacetonate and triphenylphosphonium bromide catalyst (Co(acac) 2 -QPB@MCS) for the direct synthesis of cyclic carbonates from olefins and CO 2 with O 2 as an oxidant (Scheme 19) [73]. They used chitosan as a support platform to combine the epoxidation active site (cobalt(II) acetylacetonate complex, Co(acac) 2 ) and the cycloaddition active site (quaternary triphenylphosphonium bromide, P + Ph 3 Br), which could achieve 73.1% yield of styrene carbonate. However, the yield of styrene carbonate was lower when using QPB@MCS alone or a physical mixture of Co(acac) 2 @MCS and QPB@MCS (1:1) as a catalyst, which indicated that the covalent grafting of Co(acac) 2 and P + Ph 3 Br moieties to MCS showed obvious synergistic catalytic effect during the one-pot reaction. Scheme 18. The reaction procedure of TBA-TBD-P with CO2 and calculated two different ways of activating CO2 on TBA-TBD-P.
Considering the utilization of "carbon-neutral" resources, using a biomass-based catalyst for the synthesis of styrene carbonate from styrene and CO2 may be a green and sustainable method. Due to the abundant hydroxyl groups in the biomass-based catalyst (such as cellulose, chitosan and lignin), these biomass-based catalysts have been applied in the CO2 cycloaddition reaction [5,[70][71][72]. Although the biomass-based catalysts exhibit excellent catalytic activity for the CO2 cycloaddition reaction, how to introduce the epoxidation active sites in the biomass-based catalysts and achieve the direct synthesis of cyclic carbonates remains a big challenge. Jain et al. Fabricated magnetically separable chitosan (MCS) modified with Co(II) acetylacetonate and triphenylphosphonium bromide catalyst (Co(acac)2-QPB@MCS) for the direct synthesis of cyclic carbonates from olefins and CO2 with O2 as an oxidant (Scheme 19) [73]. They used chitosan as a support platform to combine the epoxidation active site (cobalt(II) acetylacetonate complex, Co(acac)2) and the cycloaddition active site (quaternary triphenylphosphonium bromide, P + Ph3Br), which could achieve 73.1% yield of styrene carbonate. However, the yield of styrene carbonate was lower when using QPB@MCS alone or a physical mixture of Co(acac)2@MCS and QPB@MCS (1:1) as a catalyst, which indicated that the covalent grafting of Co(acac)2 and P + Ph3Br moieties to MCS showed obvious synergistic catalytic effect during the one-pot reaction.  Table 2 summarizes the heterogeneous catalytic system for the conversion of olefins and CO 2 to cyclic carbonates. It is found that most of the heterogeneous catalyst with good yields and selectivity towards the cyclic carbonates usually includes two important parts: (i) a high selectivity epoxidation active site for the oxidation of olefins, (b) the Lewis acid and base sites for the CO 2 cycloaddition reaction ( Table 2, entries 10-17). In addition, the suitable catalyst support with high surface area, controllable pore size, and the easy diffusion of substrate molecules can accelerate the direct oxidative carboxylation of olefins with CO 2 ( Table 2, entries 4-9). Interestingly, some of the heterogeneous catalysts integrating the different catalytic moieties show a higher catalytic activity than the physical mixture of the different catalytic moieties, which indicates that the stereoconfinment effect and the distribution of the different active sites can efficiently stabilize and transform the intermediates and promote the reaction ( Table 2, entries 10-17).  The reaction parameters such as oxidant, temperature and CO 2 pressure can significantly affect the activity and selectivity for both the epoxidation and cycloaddition reaction during the oxidative carboxylation of styrene. Usually, the organic oxidants (such as TBHP, CHP and PhIO) exhibit higher activity than H 2 O 2 or O 2 . For example, the oxidation of TBHP can enhance the interaction with the substrate and react with the catalyst forming the main driver of epoxidation [52]. Besides, the high temperature is a disadvantage in improving the selectivity of the epoxidation process. Hence, the low temperature is usually used for the epoxidation reaction ( Table 2, entries 1-3, 13 and 19). However, the high temperature benefits the activation of CO 2 during the CO 2 cycloaddition reaction [29]. The high temperature of the direct oxidative carboxylation of olefins may lead to the over-oxidation of styrene oxide and polymerization of styrene [57]. It is found that the higher cyclic carbonates can be obtained with a higher initial CO 2 pressure ( Table 2, entries 8 and 9). The higher CO 2 pressure can enhance the contact between CO 2 molecules and substrate, so it is necessary to develop the catalyst with a higher CO 2 capture capacity.

Conclusions and Perspectives
The direct oxidative carboxylation of olefins with CO 2 is an economic and sustainable strategy for the synthesis of cyclic carbonates. However, the one-pot reaction usually contains two different reactions: (i) the epoxidation reaction; (ii) the CO 2 cycloaddition reaction and the optimal reaction conditions of these two reactions are obvious different. For example, the optimal reaction conditions of the epoxidation reaction are 50 • C with TBHP as an oxidant (Table 2, entry 19). However, the optimal reaction conditions of the CO 2 cycloaddition reaction can be obtained at 150 • C with 40 bar CO 2 . Hence, how to combine these two different reactions into a one-pot reaction may be a huge challenge. Although utilization of two different catalysts can achieve the one-pot reaction, the low conversion and selectivity may prevent the large-scale reaction ( Table 2, entries 1, 2 and 4-5). As a result, the integration of two different catalytic sites into one catalyst may be an alternative method to avoid the problems mentioned above ( Table 2, entries 17 and 23).
In this review, we carefully investigate the method in the design of the homogeneous and heterogeneous catalytic system for the direct oxidative carboxylation of olefins with CO 2 . Considering the oxidation reaction is the first step, the effectively transferring active [O] atom from oxidation to olefins via the epoxidation active site may play a significant role in the selectivity of the epoxides (Schemes 3 and 6). Compared with the TBHP, the use of H 2 O 2 or O 2 as oxidation is safe and environmentally sustainable. However, using H 2 O 2 or O 2 as oxidation needs well-designed epoxidation active sites such as POMs or Mn-porphyrin (  (Table 2, entry 21). However, CO 2 is hard to convert due to its thermodynamically stable and kinetically inert. As a result, a high temperature or CO 2 pressure is used in the CO 2 cycloaddition reaction. However, a high temperature may affect the oxidation reaction in the first step because some by-products (acid or aldehyde) can be found at a higher temperature ( Table 2, entries 1-3, 13 and 19). Therefore, the integration of the suitable epoxidation active site and cycloaddition active site into a porous material could be a benefit for the direct oxidative carboxylation of olefins with CO 2 .