MIL-101(Cr) for CO 2 Conversion into Cyclic Carbonates, Under Solvent and Co-Catalyst Free Mild Reaction Conditions

: Mild reaction conditions (nearly room temperature and atmospheric CO 2 pressure) for the cycloaddition of CO 2 with epoxides to produce cyclic carbonates were investigated applying MIL-101(Cr) as a catalyst. The MIL-101 catalyst contains strong acid sites, which promote the ring-opening of the epoxide substrate. Moreover, the high surface area, enabling the adsorption of more CO 2 (substrate), combined with a large pore size of the catalyst is essential for the catalytic performance. Additionally, epoxide substrates bearing electron-withdrawing substituents or having a low boiling point demonstrated an excellent conversion towards the cyclic carbonates. MIL-101(Cr) for the cycloaddition of carbon dioxide with epoxides is demonstrated to be a robust and stable catalyst able to be re-used at least ﬁve times without loss in activity.


Introduction
Nowadays, only 40% of the total amount of carbon dioxide (CO 2 ) generated from human activities is being absorbed by the natural carbon exchange. The remaining amount stays in the atmosphere increasing CO 2 level and is thus the main cause of global warming [1,2]. So far, several technologies have been developed to reduce the CO 2 emission in the atmosphere, namely adsorption or capture. However, CO 2 capture and the carbon control sequestration may cause CO 2 leakage and thus affect the acidification of natural water resources. It would be a great solution, in the context of economic and environmental benefit, to convert CO 2 into useful products instead of CO 2 capture or storage. In this regard, the coupling reaction of CO 2 with epoxides to produce cyclic carbonates that are further useful in the production of polycarbonate is currently perceived as being friendlier than the industrial production of polycarbonates from bisphenol A, which requires the highly toxic reagent phosgene [3][4][5]. A catalyst needed for in the cycloaddition of CO 2 into epoxides due to the inactivity of CO 2 [6,7]. Also, the critical reaction conditions, such as high temperature and pressure of CO 2 , are required to proceed with the reaction. It is thus essential to explore new catalysts which are useful in the catalytic performance under milder reaction conditions (low temperature and CO 2 pressure) [8].

MIL-101(Cr) Characterization
Firstly, the synthesized MIL-101(Cr) was characterized to verify the characteristic properties and compared them with previous reports in the literature. The X-ray diffraction (XRD) analysis was performed to identify the crystalline structure of the synthesized MIL-101 (Cr). The sharp and intense diffraction pattern of MIL-101(Cr), Figure 1a was similar to the patterns of reported MIL-101(Cr) [21,23,28]. The textural morphology and the crystal size of the synthesized material were analyzed using scanning electron microscopy (SEM). The SEM analysis revealed a dense nano-crystal structure of MIL-101(Cr) with an average crystal size of 200 nm, Figure 1b. The porosity property of the synthesized material was evaluated by nitrogen adsorption at 77 K. A large uptake of N 2 in the micropores was observed classifying the materials as an isotherm of Type I, Figure 1c. High surface areas of 2553 m 2 /g and 2785 m 2 /g for BET and Langmuir surface area, respectively, were obtained for the synthesized MIL-101(Cr), although slightly lower than the highest reported surface area of MIL-101(Cr) [21]. A pore volume of 1.089 cm 2 /g and a micropore width of 1.691 nm were indicative of the porous properties of the synthesized material. Next, CO 2 adsorption of the synthesized material was measured, as this is one of the substrate molecules in the CO 2 addition reaction. An excellent CO 2 uptake up to 70 cm 3 /g at 273 K and 1 bar of CO 2 pressure was disclosed for the synthesized MIL-101(Cr), Figure 1d. Besides, the obtained CO2 adsorption capacity is comparable to values reported in the literature, as is the surface area [26].
Catalysts 2020, 10, x FOR PEER REVIEW 3 of 14 500 ηm obtained for the synthesized MIL-101(Cr), although slightly lower than the highest reported surface area of MIL-101(Cr) [21]. A pore volume of 1.089 cm 2 /g and a micropore width of 1.691 nm were indicative of the porous properties of the synthesized material. Next, CO2 adsorption of the synthesized material was measured, as this is one of the substrate molecules in the CO2 addition reaction. An excellent CO2 uptake up to 70 cm 3 /g at 273 K and 1 bar of CO2 pressure was disclosed for the synthesized MIL-101(Cr), Figure 1d. Besides, the obtained CO2 adsorption capacity is comparable to values reported in the literature, as is the surface area [26].

Catalytic Reaction
MIL-101(Cr) was applied as a heterogeneous catalyst for the CO2 cycloaddition with an epoxide to produce cyclic carbonates. The catalytic performance was studied under different reaction conditions of temperature, pressure, time and amount of the catalyst. Epichlorohydrin was chosen as the model substrate and later on, other epoxide substrates were applied to demonstrate the potential of the catalyst using the same procedure. MIL-101(Cr) as a catalyst for the CO2 addition to cyclic carbonates was previously reported. However, high temperature and CO2 pressure in combination with a co-catalyst next to a few substrates (styrene oxide, propylene oxide) were investigated [26][27][28][29][30]. For example, the results revealed a 97% conversion of styrene oxide at 70 °C under 100 atm of CO2 pressure within 7 h. Unfortunately, a nearly neglectable conversion (9%) was obtained at low pressure (8 atm) and temperature (25 °C) for a reaction time of 24 h. To enhance the catalytic performance of the catalyst, a solution was found by adding a co-catalyst (tetrabutylammonium bromide, TBAB) in the reaction mixture. In this way, a catalytic performance of up to 95% conversion of styrene oxide was obtained (conditions: Temperature: 25 °C; pressure: 8 bars of CO2; reaction time: 48 h). Subsequently, MIL-101(Cr) was investigated for the conversion of propylene oxide under (b)

Catalytic Reaction
MIL-101(Cr) was applied as a heterogeneous catalyst for the CO 2 cycloaddition with an epoxide to produce cyclic carbonates. The catalytic performance was studied under different reaction conditions of temperature, pressure, time and amount of the catalyst. Epichlorohydrin was chosen as the model substrate and later on, other epoxide substrates were applied to demonstrate the potential of the catalyst using the same procedure. MIL-101(Cr) as a catalyst for the CO 2 addition to cyclic carbonates was previously reported. However, high temperature and CO 2 pressure in combination with a co-catalyst next to a few substrates (styrene oxide, propylene oxide) were investigated [26][27][28][29][30]. For example, the results revealed a 97% conversion of styrene oxide at 70 • C under 100 atm of CO 2 pressure within 7 h. Unfortunately, a nearly neglectable conversion (9%) was obtained at low pressure (8 atm) and temperature (25 • C) for a reaction time of 24 h. To enhance the catalytic performance of the catalyst, a solution was found by adding a co-catalyst (tetrabutylammonium bromide, TBAB) in the reaction mixture. In this way, a catalytic performance of up to 95% conversion of styrene oxide was obtained (conditions: Temperature: 25 • C; pressure: 8 bars of CO 2 ; reaction time: 48 h). Subsequently, MIL-101(Cr) was investigated for the conversion of propylene oxide under the same reaction conditions and a comparable conversion of 82% was obtained [29,31]. Herein, mild reaction conditions are the focus of the investigation based on robust, high porosity and high CO 2 uptake of the synthesized MIL-101(Cr) as a heterogeneous catalyst. Therefore, to find the optimum reaction conditions, we fixed the reaction temperature at 35 • C and firstly applied 50 mg catalyst with epichlorohydrin (850 mg, 9.2 mmol) under 4 bar of CO 2 pressure (without co-catalyst and solvent). The obtained result revealed a 100% conversion of epichlorohydrin after 48 h (Table 1, entry 2). Also, a full conversion (100%) was retained when the reaction time was reduced to 24 h (Table 1, entry 3). Next, the CO 2 pressure was investigated, starting from high pressure (8 bar) and reduced till close to ambient pressure (1.5 bar). Also, using these conditions, the catalyst showed an excellent performance (99% conversion), although the reaction time was reduced to 24 h and using a CO 2 pressure of 1.5 bar (Table 1, Entries 1, 2, 6 and 7). Interestingly, the difference between conversion and yield (although there is a high selectivity) could be explained by the fact that the product is adsorbed on the MOF and hence, the adsorbed product is not detectable in H-NMR leading to low values of yield. This is in contrast with the recycling experiments where the obtained yields are acceptable. Here the catalysts is recovered via centrifuge and washing. So, all the products are collected and hence an acceptable yield is obtained. The conversion gradually decreased when shorter reaction times were applied (Table 1, Entries 7 and 10-12). A significant drop in conversion was observed when the catalyst loading was reduced from 50 mg to 25 mg (Table 1, entries 7-9). However, it is worth noting that when using a low catalyst loading, the catalyst showed still an appreciable catalytic activity under these mild conditions (35 • C, 1.5 bar) in the absence of solvent and co-catalyst. Though, higher conversions reached by prolonging the reaction time. the same reaction conditions and a comparable conversion of 82% was obtained [29,31]. Herein, mild reaction conditions are the focus of the investigation based on robust, high porosity and high CO2 uptake of the synthesized MIL-101(Cr) as a heterogeneous catalyst. Therefore, to find the optimum reaction conditions, we fixed the reaction temperature at 35 °C and firstly applied 50 mg catalyst with epichlorohydrin (850 mg, 9.2 mmol) under 4 bar of CO2 pressure (without co-catalyst and solvent). The obtained result revealed a 100% conversion of epichlorohydrin after 48 h (Table 1, entry  2). Also, a full conversion (100%) was retained when the reaction time was reduced to 24 h (Table 1,  entry 3). Next, the CO2 pressure was investigated, starting from high pressure (8 bar) and reduced till close to ambient pressure (1.5 bar). Also, using these conditions, the catalyst showed an excellent performance (99% conversion), although the reaction time was reduced to 24 h and using a CO2 pressure of 1.5 bar (Table 1, Entries 1, 2, 6 and 7). Interestingly, the difference between conversion and yield (although there is a high selectivity) could be explained by the fact that the product is adsorbed on the MOF and hence, the adsorbed product is not detectable in H-NMR leading to low values of yield. This is in contrast with the recycling experiments where the obtained yields are acceptable. Here the catalysts is recovered via centrifuge and washing. So, all the products are collected and hence an acceptable yield is obtained. The conversion gradually decreased when shorter reaction times were applied (Table 1, Entries 7 and 10-12). A significant drop in conversion was observed when the catalyst loading was reduced from 50 mg to 25 mg (Table 1, entries 7-9). However, it is worth noting that when using a low catalyst loading, the catalyst showed still an appreciable catalytic activity under these mild conditions (35 °C, 1.5 bar) in the absence of solvent and co-catalyst. Though, higher conversions reached by prolonging the reaction time. The robustness and stability of this heterogeneous catalyst (MIL-101(Cr)) were proven via catalyst recyclability experiments. The recycled catalyst retained its catalytic properties, both conversion and selectivity, under these optimal mild reaction conditions (Table 1 entries 16-19, Figure S2). It was demonstrated that the catalytic performance remained for at least 5 recycle times. To find out more about the morphology and physical properties of the recycled catalyst, the recycled catalyst (MIL-101(Cr)) was characterized using XRD and compared with the fresh catalyst. The analysis revealed no observable changes in the crystal pattern of the recycled catalyst compared with the fresh catalyst ( Figure S3). However, the surface area of the recycled catalyst was reduced in comparison with the fresh catalyst. This reduction in the surface area could be due to the trapping of reactants or product molecules in the pores of used MIL-101(Cr) ( Figure S4). Although a smaller surface area was observed for the recycled catalyst, this reduced surface area did not change the catalytic performance of the MIL-101(Cr) catalyst under these mild reaction conditions since the conversion was retained.
Several MOFs were successfully applied to catalyze the addition reaction of CO 2 with epoxide into cyclocarbonates. The activities of these MOFs are related to the nature of the metal center and ligand present in the framework. The presence of acidic sites in some well-known MOFs such as MOF-5, HKUST-1, UiO-66 and so forth disclosed to be the active sites to catalyze this reaction [32][33][34]. However, a co-catalyst and solvent are required to participate in the reaction as basic sites. Consequently, amino-bearing ligands and nitrogen-containing ligands to construct MOFs, for example, UiO-66-NH 2 , Zn-DABCO were reported also exhibiting a catalytic activity comparable to their parent MOFs [7,31,35]. Additionally, binary MOFs such as UiO-66@HKUST-1 and Co@MOF-5 were described as a bimetallic catalyst for the cycloaddition reaction of carbon dioxide with epoxides. Nevertheless, in all reported cases, a co-catalyst, for example, TBAB, was still required to achieve a satisfactory performance [5,36]. On the other hand, zeolitic imidazole frameworks (ZIFs), a subfamily of MOFs, have attracted much attention concerning their properties and applications. For the cycloaddition reaction of CO 2 with epoxides, ZIFs are applied as heterogeneous catalysts exhibiting an excellent catalytic activity in the absence of co-catalyst and solvent-free. The defect structures at the ZIF surface were claimed to bear the active sites for this catalytic reaction [17,31]. Recently, the material obtained after pyrolysis of ZIFs was also reported to be an active heterogeneous catalyst for the cycloaddition of carbon dioxide with epoxides and demonstrated to be advantageous as a robust heterogeneous catalyst [37,38]. Later, the post-synthesis modified ZIF-90 with pyridinium-based ionic liquid (IL) to generate IL-ZIF-90 was developed [39]. The highly porous catalyst (IL-ZIF-90) showed an excellent CO 2 transforming capability. An overview of catalytic activities of reported MOFs with their reaction conditions for the cycloaddition of carbon dioxide with epoxides is summarized in Table 2.  The direct comparison of MIL-101(Cr) with other MOF catalysts reveals that MIL-101(Cr) exhibits the highest catalytic activity under mild reaction conditions. According to previous reports, based on the theoretical calculations (DFT) of the catalytic mechanism, the ring-opening process of the epoxide is the rate-limiting step of this reaction [49]. Also, it was observed that the Lewis acid sites played a main role in the ring-opening process by binding the oxygen atom of the epoxide [50]. A well-known technique to analyze the amount of acid and basic sites on MOFs is temperature program desorption (TPD), which makes use of gas probe molecules (NH 3 and CO 2 to determine the acid and basic amounts, respectively) [48]. Using TPD, Kim et al. determined, based on the integration of NH 3 -TPD signals, the acid sites of several MOFs and reported that MIL-101 contained the highest amount of acid sites in comparison with the other MOFs ( Figure 2) and especially strong acid site (593-673 K) [48]. The presence of this high amount of strong acid sites in MIL-101(Cr) can be applied to elucidate the high catalytic performance of MIL-101 for the cycloaddition of carbon dioxide with epoxides in comparison with other reported MOFs. Recently, it was reported that the defect structure in MOFs plays an active role as active sites in a catalyst [51]. Herein, the defect structures present in MIL-101(Cr) could participate as well and result in an excellent catalytic performance under mild conditions. However, to fully prove the role of the defects, a study of the defect sites in this MIL-101(Cr) based-catalyst compared with the reported MIL-101(Cr) using different characterization and analysis techniques is required. Still, in this report, the focus was mainly directed on the mild reaction conditions. The direct comparison of MIL-101(Cr) with other MOF catalysts reveals that MIL-101(Cr) exhibits the highest catalytic activity under mild reaction conditions. According to previous reports, based on the theoretical calculations (DFT) of the catalytic mechanism, the ring-opening process of the epoxide is the rate-limiting step of this reaction [49]. Also, it was observed that the Lewis acid sites played a main role in the ring-opening process by binding the oxygen atom of the epoxide [50]. A well-known technique to analyze the amount of acid and basic sites on MOFs is temperature program desorption (TPD), which makes use of gas probe molecules (NH3 and CO2 to determine the acid and basic amounts, respectively) [48]. Using TPD, Kim et al. determined, based on the integration of NH3-TPD signals, the acid sites of several MOFs and reported that MIL-101 contained the highest amount of acid sites in comparison with the other MOFs ( Figure 2) and especially strong acid site (593-673 K) [48]. The presence of this high amount of strong acid sites in MIL-101(Cr) can be applied to elucidate the high catalytic performance of MIL-101 for the cycloaddition of carbon dioxide with epoxides in comparison with other reported MOFs. Recently, it was reported that the defect structure in MOFs plays an active role as active sites in a catalyst [51]. Herein, the defect structures present in MIL-101(Cr) could participate as well and result in an excellent catalytic performance under mild conditions. However, to fully prove the role of the defects, a study of the defect sites in this MIL-101(Cr) based-catalyst compared with the reported MIL-101(Cr) using different characterization and analysis techniques is required. Still, in this report, the focus was mainly directed on the mild reaction conditions.

Substrate Scope
The cycloaddition reaction of CO2 applying different substrates catalyzed by MIL-101 (Cr) was also investigated. The obtained results are summarized in Figure 3 and demonstrate that MIL-101

Substrate Scope
The cycloaddition reaction of CO 2 applying different substrates catalyzed by MIL-101 (Cr) was also investigated. The obtained results are summarized in Figure 3 and demonstrate that MIL-101 (Cr) can convert the substrates to their corresponding products under mild reaction conditions. Epibromohydrin and propylene oxide (>99%) were converted fully to their corresponding cyclic carbonates, while phenyl glycidyl ether and styrene oxide display a lower conversion of 25% and 29%, respectively. In this series of epoxides, epoxides having a lower boiling point, for example, propylene oxide (34 • C), epibromohydrin (134 • C) and epichlorohydrin (118 • C) are fully converted to their cyclic carbonates. These results are in line with previous investigation by Taherimehr and co-workers, who also achieved excellent conversion for epoxide substrates with a lower boiling point [52]. In comparison, epoxides having a higher boiling point, for example, phenyl glycidyl ether (245 • C) and styrene oxide (194 • C), are only partially transformed. The allyl glycidyl ether and butyl glycidyl ether with intermediate boiling points (154 • C and 256 • C, respectively), gave an intermediate conversion. Under the investigated conditions, CO 2 mainly remains in the gas phase, while the epoxide substrate having a relatively low boiling point tends to shift into the gas phase. This could be advantageous for good interphase contact between the different components (epoxides, CO 2 and catalytic sites) and thus resulting in achieving a good conversion. Also, substrates containing electron-withdrawing substituents exhibit a higher conversion via enhanced easiness of the ring-opening of the epoxide allowing the nucleophilic attack followed by carbonate formation [31]. The epoxide properties affecting the catalytic performance under mild conditions were illustrated using different MOFs as a catalyst. For instance, using MOF-5 as the catalyst, the following substrate conversion trend was reported-epichlorohydrin > styrene oxide > phenyl glycidyl ether at 50 • C, 0.1 MPa of CO 2 pressure and co-catalyst (n-Bu 4 NBr, 2.5 mol %). Whereas, raising the CO 2 pressure to 6 MPa revealed an increase of the conversion of phenyl glycidyl ether, even higher than the styrene oxide conversion (under similar conditions). A same result was also reported for Zn-DABCO at high pressure (8 bars).
Catalysts 2020, 10, x FOR PEER REVIEW 8 of 14 (Cr) can convert the substrates to their corresponding products under mild reaction conditions. Epibromohydrin and propylene oxide (>99%) were converted fully to their corresponding cyclic carbonates, while phenyl glycidyl ether and styrene oxide display a lower conversion of 25% and 29%, respectively. In this series of epoxides, epoxides having a lower boiling point, for example, propylene oxide (34 °C), epibromohydrin (134 °C) and epichlorohydrin (118 °C) are fully converted to their cyclic carbonates. These results are in line with previous investigation by Taherimehr and co-workers, who also achieved excellent conversion for epoxide substrates with a lower boiling point [52]. In comparison, epoxides having a higher boiling point, for example, phenyl glycidyl ether This could be advantageous for good interphase contact between the different components (epoxides, CO2 and catalytic sites) and thus resulting in achieving a good conversion. Also, substrates containing electron-withdrawing substituents exhibit a higher conversion via enhanced easiness of the ring-opening of the epoxide allowing the nucleophilic attack followed by carbonate formation [31]. The epoxide properties affecting the catalytic performance under mild conditions were illustrated using different MOFs as a catalyst. For instance, using MOF-5 as the catalyst, the following substrate conversion trend was reported-epichlorohydrin > styrene oxide > phenyl glycidyl ether at 50 °C, 0.1 MPa of CO2 pressure and co-catalyst (n-Bu4NBr, 2.5 mol %). Whereas, raising the CO2 pressure to 6 MPa revealed an increase of the conversion of phenyl glycidyl ether, even higher than the styrene oxide conversion (under similar conditions). A same result was also reported for Zn-DABCO at high pressure (8 bars).

Catalytic Mechanism
The carbon dioxide coupling mechanism with epoxides to generate cyclic carbonates is depending on the type of the catalyst used. For example, in case the catalyst contains both Lewis acid and basic sites, it is accepted that the acidic site activates the epoxide. In contrast, the basic site

Catalytic Mechanism
The carbon dioxide coupling mechanism with epoxides to generate cyclic carbonates is depending on the type of the catalyst used. For example, in case the catalyst contains both Lewis acid and basic sites, it is accepted that the acidic site activates the epoxide. In contrast, the basic site activates the carbon dioxide [53]. On the other hand, the co-catalyst is the source of the basic site for the reaction when the catalyst provides the Lewis acidic site only. A mechanism for the reaction of epoxide and CO 2 catalyzed by catalysts with Lewis acidic sites only in the absence of co-catalyst was also proposed [48]. Carbon dioxide can act as nucleophile or electrophile in the reaction. Here, the first step of the mechanism is the electrophilic activation of the epoxide ring by the Lewis acidic site of the MIL-101(Cr) catalyst resulting in the ring-opening of the epoxide. Next, a nucleophilic attack from the oxygen atom of CO 2 occurs and binds with the carbon of the epoxide to form intermediate metal species. Finally, the ring-closing occurs where the oxygen anion of the open ring interacts with the carbon atom of the carbon dioxide and extrudes the cyclic carbonate, as shown in Figure 4. Also, high surface features and a large pore volume of the catalyst for example, MOFs, promote the coordination sites required for the epoxide binding, which plays a crucial role in the overall catalytic performance of the cycloaddition of CO 2 to generate the cyclic carbonate product.
Catalysts 2020, 10, x FOR PEER REVIEW 9 of 14 activates the carbon dioxide [53]. On the other hand, the co-catalyst is the source of the basic site for the reaction when the catalyst provides the Lewis acidic site only. A mechanism for the reaction of epoxide and CO2 catalyzed by catalysts with Lewis acidic sites only in the absence of co-catalyst was also proposed [48]. Carbon dioxide can act as nucleophile or electrophile in the reaction. Here, the first step of the mechanism is the electrophilic activation of the epoxide ring by the Lewis acidic site of the MIL-101(Cr) catalyst resulting in the ring-opening of the epoxide. Next, a nucleophilic attack from the oxygen atom of CO2 occurs and binds with the carbon of the epoxide to form intermediate metal species. Finally, the ring-closing occurs where the oxygen anion of the open ring interacts with the carbon atom of the carbon dioxide and extrudes the cyclic carbonate, as shown in Figure 4. Also, high surface features and a large pore volume of the catalyst for example, MOFs, promote the coordination sites required for the epoxide binding, which plays a crucial role in the overall catalytic performance of the cycloaddition of CO2 to generate the cyclic carbonate product.

Catalyst Preparation
MIL-101(Cr) was synthesized using the hydrothermal method previously reported with a slight modification in the synthesis procedure [25]. In a typical synthesis, the Cr(NO3)3.9H2O (0.4 g, 1 mmol) and BDC (0.249 g, 1.5 mmol) were dissolved in deionized water (DI, 10 mL) and followed with the addition of HF (10 μL). The mixed suspension was vigorously stirred for 20 min in a Teflon liner (40 mL) before transferred into the stainless steel autoclave. The autoclave was kept under autogenic pressure at 180 °C for 8 h and then cooled to room temperature. The resulting green solid was separated from the mixture via centrifuge (8500 rpm, 5 min) and washed with DI water and

Catalyst Preparation
MIL-101(Cr) was synthesized using the hydrothermal method previously reported with a slight modification in the synthesis procedure [25]. In a typical synthesis, the Cr(NO 3 ) 3 .9H 2 O (0.4 g, 1 mmol) and BDC (0.249 g, 1.5 mmol) were dissolved in deionized water (DI, 10 mL) and followed with the addition of HF (10 µL). The mixed suspension was vigorously stirred for 20 min in a Teflon liner (40 mL) before transferred into the stainless steel autoclave. The autoclave was kept under autogenic pressure at 180 • C for 8 h and then cooled to room temperature. The resulting green solid was separated from the mixture via centrifuge (8500 rpm, 5 min) and washed with DI water and DMF. The solid product was post-treated in water (20 mL) at 80 • C for 1 h and ethanol (20 mL) at 70 • C for 3 h, respectively. After centrifugation, the obtained solid (as-synthesized MIL-101(Cr)) was dried at 120 • C under vacuum overnight before further use and analysis (activated MIL-101(Cr)).

Catalytic Reaction
The synthesized MIL-101(Cr) catalyst (50 mg) and substrates (9.2 mmol) were charged in a high-pressure reactor (15 mL stainless steel, XINGDA company, Beijing, China) with a magnetic stirrer. The atmosphere in the reactor was exchanged with CO 2 before pressurized to 1.5 bar of CO 2 pressure. The reactor was immersed in a pre-heated oil bath at 35 • C under stirring conditions (300 rpm) for the required period of reaction time. The reactor was cooled to room temperature and 1,3,5-trioxane, as the internal standard (3 mmol) and CDCl 3 (1 mL) were added. The reaction mixture was analyzed via 1 H NMR using CDCl 3 as a solvent. The catalyst was recovered by the centrifugation and then washed three times with dichloromethane and then dried at 80 • C under vacuum for 12 h for the next reuse. The same reaction conditions were applied to other substrates.

Catalyst Characterization
The powder X-ray diffraction (XRD) D8 Advanced Diffractometer, Bruker (Germany) (Bragg-Brentano geometry) at 40 kV and 45 Ma with Cu Kα radiation source (λ = 1.54056 Å at 40 kV and 45 Ma) and scanning rate of 2 • min -1 was used to characterize the crystalline structure of the material. The morphology was observed by SEM (JSM-IT300/JSM-IT300, JEOL, Japan). The gas adsorption-desorption measurement was carried out by Micrometrics instrument (ASAP 2020 Analyzer) using N 2 and CO 2 gases of 99.999% purity. The pore distribution and surface area were analyzed by Brunauer-Emmett-Teller (BET) and Langmuir methods. The linearized BET and Langmuir equations were fitted in the range of 0.003 < P/P 0 < 0.05.

Conclusions
In this study, MIL-101(Cr) was synthesized via the hydrothermal method and applied as a heterogeneous catalyst for the cycloaddition of CO 2 into epoxide. The catalytic activity of the MIL-101(Cr) catalyst was investigated, aiming at mild reaction conditions (reaction temperature, pressure, time, catalyst loading and epoxide substrate). The MIL-101(Cr) demonstrates excellent conversion and selectivity under mild reaction conditions (near room temperature (35 • C) and an atmospheric pressure of CO 2 (1.5 bar)). The epoxides containing an electron-withdrawing group could easily be converted into the cyclic carbonate product under these mild conditions, even in the absence of co-catalyst and solvent. The presence of strong Lewis acids and high porosity in the MIL-101(Cr) is beneficial for CO 2 adsorption and improves the catalytic performance. Besides, MIL-101(Cr) exhibits a non-noticeable loss in the catalytic conversion during at least five cycles. The results demonstrate a highly active, greener and economically viable heterogeneous catalyst for the cyclic carbonate production from epoxide and CO 2 compared to any reported MOFs.