Catalytic Performance of CPM-200-In/Mg in the Cycloaddition of CO2 and Epoxides

Crystalline porous materials (CPM)-200-In and CPM-200-In/Mg metal-organic frameworks (MOFs) were synthesized by a solvothermal method and were characterized by using powder X-ray diffraction (PXRD), FT-IR, Brunauer–Emmett–Teller (BET), temperature programmed desorption (TPD), TGA, XPS, and SEM-EDS. They were used as heterogeneous catalysts for the cycloaddition of CO2 with epoxides and found to be highly efficient toward the cycloaddition reaction at moderate reaction conditions under solvent-free conditions. The catalyst was easily separated by a simple filtration and can be reused up to five consecutive times without any considerable decrease of its initial activity. CPM-200-In/Mg showed excellent catalytic performance in the cycloaddition reaction due to the synergistic role of the acidic sites and basic sites. A plausible reaction mechanism for the CPM-200-In/Mg MOF catalyzed cycloaddition reaction is proposed based on the experimental results and our previously reported DFT (Density Functional Theory) studies.

Recent studies on MOF-based materials for CO 2 mitigation are focused on the development of MOFs having high adsorption capacity and high catalytic performance at the same time [8]. Metal organic frameworks (MOFs), based on transition metals, are predominant over those based on main-group elements. However, it is reported that the MOF-74-Mg exhibited the highest CO 2 uptake capacity over its transition metal analogues such as MOF-74-Co and MOF-74-Ni [54]. The application of MOF-74-Mg as a catalyst for the cycloaddition of CO 2 and epoxides has also been reported previously [55,56]. Recently, bimetallic MOFs (Ni-Co and Co-Zn MOF) have been developed as catalysts in the cycloaddition reaction [57][58][59]. They have shown synergistic effects in promoting the Recent studies on MOF-based materials for CO2 mitigation are focused on the development of MOFs having high adsorption capacity and high catalytic performance at the same time [8]. Metal organic frameworks (MOFs), based on transition metals, are predominant over those based on main-group elements. However, it is reported that the MOF-74-Mg exhibited the highest CO2 uptake capacity over its transition metal analogues such as MOF-74-Co and MOF-74-Ni [54]. The application of MOF-74-Mg as a catalyst for the cycloaddition of CO2 and epoxides has also been reported previously [55,56]. Recently, bimetallic MOFs (Ni-Co and Co-Zn MOF) have been developed as catalysts in the cycloaddition reaction [57][58][59]. They have shown synergistic effects in promoting the catalytic activity by increasing the number of acidic and basic sites. We also reported the synergistic role of Cu-Zr bimetallic MOF, which enhanced the number of basic sites for the cycloaddition reaction [59]. Zhai et al. [60] reported successful preparation of heterometallic MOFs, denoted as crystalline porous materials (CPM-200s) with systematic combinations of trivalent and divalent metals, and their high CO2 adsorption capacity.
In this work, therefore, we selected indium (In) as a second metal to Mg containing MOF and tried to suggest the promotional effect of the bimetallic In-Mg MOF in the cycloaddition reaction. We synthesized CPM-200-In/Mg MOF, and it was used as a catalyst for the first time in the cycloaddition reaction under solvent-free conditions. The bimetallic CPM-200-In/Mg showed better catalytic performance than CPM-200-In since the In-Mg bimetallic MOF exhibited higher CO2 adsorption capacity, higher number of acidic and basic sites, and higher thermal stability. The CPM-200-In/Mg MOF catalyst can be reused up to five consecutive times without a considerable loss of its activity. A reaction mechanism is also proposed based on the experimental results and our previously reported DFT studies on the reaction steps, including reaction intermediates and transition states.

Characterization of Catalysts
The powder X-ray diffraction (PXRD) patterns of the prepared CPM-200 catalysts were compared with the simulated single crystal pattern ( Figure 1). The main characteristic peak corresponding to (200) face of CPM-200 catalysts in PXRD matched well with the simulated pattern of the Cambridge Crystallographic Data Center (CCDC) data, confirming the crystalline structure of the prepared catalysts. In this work, therefore, we selected indium (In) as a second metal to Mg containing MOF and tried to suggest the promotional effect of the bimetallic In-Mg MOF in the cycloaddition reaction. We synthesized CPM-200-In/Mg MOF, and it was used as a catalyst for the first time in the cycloaddition reaction under solvent-free conditions. The bimetallic CPM-200-In/Mg showed better catalytic performance than CPM-200-In since the In-Mg bimetallic MOF exhibited higher CO 2 adsorption capacity, higher number of acidic and basic sites, and higher thermal stability. The CPM-200-In/Mg MOF catalyst can be reused up to five consecutive times without a considerable loss of its activity. A reaction mechanism is also proposed based on the experimental results and our previously reported DFT studies on the reaction steps, including reaction intermediates and transition states.

Characterization of Catalysts
The powder X-ray diffraction (PXRD) patterns of the prepared CPM-200 catalysts were compared with the simulated single crystal pattern ( Figure 1). The main characteristic peak corresponding to (200) face of CPM-200 catalysts in PXRD matched well with the simulated pattern of the Cambridge Crystallographic Data Center (CCDC) data, confirming the crystalline structure of the prepared catalysts.         To verify the elemental composition of both catalysts, elemental analysis (EA) and inductively coupled plasma-optical emission spectrometry (ICP-OES) were conducted, and the results in wt% are given in Table 1. X-ray photoelectron spectroscopy (XPS) was also performed (  To verify the elemental composition of both catalysts, elemental analysis (EA) and inductively coupled plasma-optical emission spectrometry (ICP-OES) were conducted, and the results in wt% are given in Table 1. X-ray photoelectron spectroscopy (XPS) was also performed ( Figure 5) to verify the presence of bimetal in CPM-200-In/Mg. In 3d5 and Mg 1s peaks located at binding energies of 445.26 eV and 1304.30 eV show the presence of indium and magnesium metals in the MOF [63][64][65]. CPM-200-In also exhibits In 3d5 peak at the binding energy of 445.12 eV, demonstrating the presence of the indium atom.
The porosity and surface area of both CPMs were studied by the physical adsorption of N 2 at 77 K, and the type I adsorption isotherms revealed the existence of micropores ( Figure 6). Brunauer-Emmett-Teller (BET) surface areas of 1231 m 2 g −1 and 756 m 2 g −1 , and pore volume of 0.50 cm 3 g −1 and 0.44 cm 3 g −1 , were calculated from N 2 adsorption isotherms for CPM-200-In/Mg and CPM-200-In, respectively. The pore size distribution curves confirm the presence of micropores in both catalysts, and the smaller micropore diameter of CPM-200-In/Mg than CPM-200-In. The presence of mesopores is known to be beneficial for promoting mass transfer [51][52][53]; however, no meaningful evidence of mesopore was observed in both CPM-200 catalysts.    The porosity and surface area of both CPMs were studied by the physical ads of N2 at 77 K, and the type I adsorption isotherms revealed the existence of mic ( Figure 6). Brunauer-Emmett-Teller (BET) surface areas of 1231 m 2 g −1 and 756 m 2 pore volume of 0.50 cm 3 g −1 and 0.44 cm 3 g −1 , were calculated from N2 adsorption iso for CPM-200-In/Mg and CPM-200-In, respectively. The pore size distribution curv firm the presence of micropores in both catalysts, and the smaller micropore diam CPM-200-In/Mg than CPM-200-In. The presence of mesopores is known to be be for promoting mass transfer [51][52][53]; however, no meaningful evidence of mesop observed in both CPM-200 catalysts. Temperature-programmed desorption (TPD) of NH3 and CO2 analysis w formed to investigate acid-base nature of the catalysts. As shown in Table 2, CP In/Mg shows a higher number of basic sites, which is necessary for CO2 adsorptio CPM-200-In. NH3 adsorption properties of CPM-200 catalyst was studied to exp presence of acidic sites; these sites are necessary catalytic centers for the activation o ide molecules and also for the coordination with CO2 molecules [8]. The number o sites was almost the same in both catalysts.  Temperature-programmed desorption (TPD) of NH 3 and CO 2 analysis was performed to investigate acid-base nature of the catalysts. As shown in Table 2, CPM-200-In/Mg shows a higher number of basic sites, which is necessary for CO 2 adsorption, than CPM-200-In. NH 3 adsorption properties of CPM-200 catalyst was studied to explain the presence of acidic sites; these sites are necessary catalytic centers for the activation of epoxide molecules and also for the coordination with CO 2 molecules [8]. The number of acidic sites was almost the same in both catalysts. The CO 2 adsorption experiment at 298 K ( Figure 7) showed that CPM-200-In/Mg reached a CO 2 uptake capacity of 89.5 cm 3 g −1 , much higher than CPM-200-In, 49.3 cm 3 g −1 at 298 K, respectively. The increase in the basic sites in CPM-200-In/Mg is responsible to the increase of CO 2 uptake capacity, favoring the attraction between CO 2 and the catalyst. The CO2 adsorption experiment at 298 K ( Figure 7) showed that CPM-200-In/Mg reached a CO2 uptake capacity of 89.5 cm 3 g −1 , much higher than CPM-200-In, 49.3 cm 3 g −1 at 298 K, respectively. The increase in the basic sites in CPM-200-In/Mg is responsible to the increase of CO2 uptake capacity, favoring the attraction between CO2 and the catalyst.

Cycloaddition of CO2 with Epichlorohydrin
The catalytic ability of CPM-200-In/Mg was tested in the cycloaddition of CO2 and epichlorohydrin (ECH). Without catalyst, the reaction did not give any ECH conversion ( Table 3, entry 1). CPM-200-In/Mg and CPM-200-In alone without a co-catalyst showed very low ECH conversion at the standard condition (6 h of reaction, 38.3 mmol of ECH, 1.2 MPa of CO2 pressure, 80 °C). They showed an appreciable conversion at high temperature and long reaction time (110 °C and 12 h). The homogeneous catalyst precursors exhibited much lower ECH conversion (Table 3, entries 2-5). In order to promote the cycloaddition reaction, tetrabutylammonium halides (TBAX, X = Cl − , Br − , I − ) were used as co-catalysts (Table 3, entries 10,13,14). CPM-200-In/Mg catalyst with tetrabutylammonium (TBAB) co-catalyst exhibited very high ECH conversion (90.3%) with >99% (chloromethyl)ethylene carbonate selectivity (Table 3, entry 7). Br − ions showed higher ECH conversion than iodide and chloride ions. Due to the bulky nature of iodide anion, CPM-200-In/Mg with tetrabutylammonium iodide (TBAI) exhibited lower ECH conversion (55%) than with TBAB because of mass transfer limitation. The catalytic performance of the CPM-200-In/Mg/TBAB system increased with the increase in temperature from 60 °C to 80 °C (Table 3, entries 10 and 12).

Cycloaddition of CO 2 with Epichlorohydrin
The catalytic ability of CPM-200-In/Mg was tested in the cycloaddition of CO 2 and epichlorohydrin (ECH). Without catalyst, the reaction did not give any ECH conversion ( Table 3, entry 1). CPM-200-In/Mg and CPM-200-In alone without a co-catalyst showed very low ECH conversion at the standard condition (6 h of reaction, 38.3 mmol of ECH, 1.2 MPa of CO 2 pressure, 80 • C). They showed an appreciable conversion at high temperature and long reaction time (110 • C and 12 h). The homogeneous catalyst precursors exhibited much lower ECH conversion (Table 3, entries 2-5). In order to promote the cycloaddition reaction, tetrabutylammonium halides (TBAX, X = Cl − , Br − , I − ) were used as co-catalysts (Table 3, entries 10,13,14). CPM-200-In/Mg catalyst with tetrabutylammonium bromide (TBAB) co-catalyst exhibited very high ECH conversion (90.3%) with >99% (chloromethyl)ethylene carbonate selectivity (Table 3, entry 10). Br − ions showed higher ECH conversion than iodide and chloride ions. Due to the bulky nature of iodide anion, CPM-200-In/Mg with tetrabutylammonium iodide (TBAI) exhibited lower ECH conversion (85.8%) than with TBAB because of mass transfer limitation. The catalytic performance of the CPM-200-In/Mg/TBAB system increased with the increase in temperature from 60 • C to 80 • C ( Table 3, entries 10 and 12).
Various reaction parameters such as reaction time, temperature, catalyst amount, and CO 2 pressure are optimized for the catalytic activity of CPM-200-In/Mg/TBAB system. As illustrated in Figure 8, with increasing temperature from 40 to 80 • C, ECH conversion increased steadily; however, a small increase in ECH conversion was shown with increasing reaction time from 8 to 10 h. A maximum in ECH conversion occurred at 1.5 MPa CO 2 pressure, since excessive pressure decreased the concentration of ECH, disturbing the interaction between CPM-200-In/Mg/TBAB and the ECH [25,43]. The optimized reaction parameters were determined at 80 • C, CO 2 pressure of 1.2 MPa, after 6 h, and with 0.6 mol% of CPM-200-In/Mg with TBAB co-catalyst. Various reaction parameters such as reaction time, temperature, catalyst amount, and CO2 pressure are optimized for the catalytic activity of CPM-200-In/Mg/TBAB system. As illustrated in Figure 8, with increasing temperature from 40 to 80 °C, ECH conversion increased steadily; however, a small increase in ECH conversion was shown with increasing reaction time from 8 to 10 h. A maximum in ECH conversion occurred at 1.5 MPa CO2 pressure, since excessive pressure decreased the concentration of ECH, disturbing the interaction between CPM-200-In/Mg/TBAB and the ECH [25,43]. The optimized reaction parameters were determined at 80 °C, CO2 pressure of 1.2 MPa, after 6 h, and with 0.6 mol% of CPM-200-In/Mg with TBAB co-catalyst.

Cycloaddition of CO 2 with Different Epoxides
We tested various aliphatic and aromatic epoxides to investigate the versatility of the CPM-200-In/Mg catalyst. As shown in Table 4, the CPM-200-In/Mg/TBAB system was efficient with regard to various epoxides in the corresponding cyclic carbonates, except for cyclohexene oxide. High conversion of propylene oxide (87.3%), epichlorohydrin (90.3%), and allylglycidyl ether (82.1%) might be ascribed from the effective size of the epoxides, facilitating their access to the active sites. Very low conversion (14.3%) was obtained for cyclohexene oxide since the sterically hindered cyclohexene ring may prevent its approach to the Lewis acidic site [32,37,66]. In addition, it is well known that CO 2 -based polycarbonate is easily formed from the reaction of cyclohexene oxide and CO 2 [67].

Cycloaddition of CO2 with Different Epoxides
We tested various aliphatic and aromatic epoxides to investigate the versatility CPM-200-In/Mg catalyst. As shown in Table 4, the CPM-200-In/Mg/TBAB system w ficient with regard to various epoxides in the corresponding cyclic carbonates, exc cyclohexene oxide. High conversion of propylene oxide (87.3%), epichlorohydrin (9 and allylglycidyl ether (82.1%) might be ascribed from the effective size of the epo facilitating their access to the active sites. Very low conversion (14.3%) was obtain cyclohexene oxide since the sterically hindered cyclohexene ring may prevent proach to the Lewis acidic site [32,37,66]. In addition, it is well known that CO2 polycarbonate is easily formed from the reaction of cyclohexene oxide and CO2 [67

Cycloaddition of CO2 with Different Epoxides
We tested various aliphatic and aromatic epoxides to investigate the versatility CPM-200-In/Mg catalyst. As shown in Table 4, the CPM-200-In/Mg/TBAB system w ficient with regard to various epoxides in the corresponding cyclic carbonates, exce cyclohexene oxide. High conversion of propylene oxide (87.3%), epichlorohydrin (9 and allylglycidyl ether (82.1%) might be ascribed from the effective size of the epo facilitating their access to the active sites. Very low conversion (14.3%) was obtain cyclohexene oxide since the sterically hindered cyclohexene ring may prevent i proach to the Lewis acidic site [32,37,66]. In addition, it is well known that CO2polycarbonate is easily formed from the reaction of cyclohexene oxide and CO2 [67]

Cycloaddition of CO2 with Different Epoxides
We tested various aliphatic and aromatic epoxides to investigate the versatility CPM-200-In/Mg catalyst. As shown in Table 4, the CPM-200-In/Mg/TBAB system w ficient with regard to various epoxides in the corresponding cyclic carbonates, exce cyclohexene oxide. High conversion of propylene oxide (87.3%), epichlorohydrin (9 and allylglycidyl ether (82.1%) might be ascribed from the effective size of the epo facilitating their access to the active sites. Very low conversion (14.3%) was obtain cyclohexene oxide since the sterically hindered cyclohexene ring may prevent i proach to the Lewis acidic site [32,37,66]. In addition, it is well known that CO2polycarbonate is easily formed from the reaction of cyclohexene oxide and CO2 [67]

Cycloaddition of CO2 with Different Epoxides
We tested various aliphatic and aromatic epoxides to investigate the versatility CPM-200-In/Mg catalyst. As shown in Table 4, the CPM-200-In/Mg/TBAB system w ficient with regard to various epoxides in the corresponding cyclic carbonates, exce cyclohexene oxide. High conversion of propylene oxide (87.3%), epichlorohydrin (90 and allylglycidyl ether (82.1%) might be ascribed from the effective size of the epox facilitating their access to the active sites. Very low conversion (14.3%) was obtaine cyclohexene oxide since the sterically hindered cyclohexene ring may prevent it proach to the Lewis acidic site [32,37,66]. In addition, it is well known that CO2-b polycarbonate is easily formed from the reaction of cyclohexene oxide and CO2 [67]

Cycloaddition of CO2 with Different Epoxides
We tested various aliphatic and aromatic epoxides to investigate the versatility CPM-200-In/Mg catalyst. As shown in Table 4, the CPM-200-In/Mg/TBAB system w ficient with regard to various epoxides in the corresponding cyclic carbonates, exce cyclohexene oxide. High conversion of propylene oxide (87.3%), epichlorohydrin (9 and allylglycidyl ether (82.1%) might be ascribed from the effective size of the epo facilitating their access to the active sites. Very low conversion (14.3%) was obtain cyclohexene oxide since the sterically hindered cyclohexene ring may prevent proach to the Lewis acidic site [32,37,66]. In addition, it is well known that CO2 polycarbonate is easily formed from the reaction of cyclohexene oxide and CO2 [67

Cycloaddition of CO2 with Different Epoxides
We tested various aliphatic and aromatic epoxides to investigate the versatility CPM-200-In/Mg catalyst. As shown in Table 4, the CPM-200-In/Mg/TBAB system ficient with regard to various epoxides in the corresponding cyclic carbonates, exc cyclohexene oxide. High conversion of propylene oxide (87.3%), epichlorohydrin ( and allylglycidyl ether (82.1%) might be ascribed from the effective size of the ep facilitating their access to the active sites. Very low conversion (14.3%) was obtain cyclohexene oxide since the sterically hindered cyclohexene ring may prevent proach to the Lewis acidic site [32,37,66]. In addition, it is well known that CO polycarbonate is easily formed from the reaction of cyclohexene oxide and CO2 [67

Cycloaddition of CO2 with Different Epoxides
We tested various aliphatic and aromatic epoxides to investigate the versatility CPM-200-In/Mg catalyst. As shown in Table 4, the CPM-200-In/Mg/TBAB system w ficient with regard to various epoxides in the corresponding cyclic carbonates, exce cyclohexene oxide. High conversion of propylene oxide (87.3%), epichlorohydrin (9 and allylglycidyl ether (82.1%) might be ascribed from the effective size of the epo facilitating their access to the active sites. Very low conversion (14.3%) was obtain cyclohexene oxide since the sterically hindered cyclohexene ring may prevent i proach to the Lewis acidic site [32,37,66]. In addition, it is well known that CO2polycarbonate is easily formed from the reaction of cyclohexene oxide and CO2 [67]

Cycloaddition of CO2 with Different Epoxides
We tested various aliphatic and aromatic epoxides to investigate the versatility CPM-200-In/Mg catalyst. As shown in Table 4, the CPM-200-In/Mg/TBAB system w ficient with regard to various epoxides in the corresponding cyclic carbonates, exce cyclohexene oxide. High conversion of propylene oxide (87.3%), epichlorohydrin (90 and allylglycidyl ether (82.1%) might be ascribed from the effective size of the epox facilitating their access to the active sites. Very low conversion (14.3%) was obtaine cyclohexene oxide since the sterically hindered cyclohexene ring may prevent it proach to the Lewis acidic site [32,37,66]. In addition, it is well known that CO2-b polycarbonate is easily formed from the reaction of cyclohexene oxide and CO2 [67]

Cycloaddition of CO2 with Different Epoxides
We tested various aliphatic and aromatic epoxides to investigate the versatility CPM-200-In/Mg catalyst. As shown in Table 4, the CPM-200-In/Mg/TBAB system w ficient with regard to various epoxides in the corresponding cyclic carbonates, exce cyclohexene oxide. High conversion of propylene oxide (87.3%), epichlorohydrin (9 and allylglycidyl ether (82.1%) might be ascribed from the effective size of the epo facilitating their access to the active sites. Very low conversion (14.3%) was obtain cyclohexene oxide since the sterically hindered cyclohexene ring may prevent i proach to the Lewis acidic site [32,37,66]. In addition, it is well known that CO2polycarbonate is easily formed from the reaction of cyclohexene oxide and CO2 [67]

Cycloaddition of CO2 with Different Epoxides
We tested various aliphatic and aromatic epoxides to investigate the versatility CPM-200-In/Mg catalyst. As shown in Table 4, the CPM-200-In/Mg/TBAB system w ficient with regard to various epoxides in the corresponding cyclic carbonates, exce cyclohexene oxide. High conversion of propylene oxide (87.3%), epichlorohydrin (9 and allylglycidyl ether (82.1%) might be ascribed from the effective size of the epo facilitating their access to the active sites. Very low conversion (14.3%) was obtain cyclohexene oxide since the sterically hindered cyclohexene ring may prevent i proach to the Lewis acidic site [32,37,66]. In addition, it is well known that CO2polycarbonate is easily formed from the reaction of cyclohexene oxide and CO2 [67]

Reusability of Catalyst
After easy separation using a centrifuge, the recyclability studies of CPM-200-In/Mg/ TBAB catalyst were carried out under optimal reaction conditions (80 • C, 6 h, 0.6 mol% catalyst, and 1.2 MPa of CO 2 pressure). As shown in Figure 9, CPM-200-In/Mg/TBAB system can be recycled five times, without considerable loss in its initial activity. The reused CPM-200-In/Mg catalyst was analyzed by FT-IR, PXRD, and TGA to investigate its chemical and thermal stability. PXRD patterns and FT-IR spectra of the fresh and reused CPM-200-In/Mg are identical (Figures S1 and S2). The TGA of the recycled CPM-200-In/Mg catalyst showed no destruction of the original structure of the fresh one ( Figure S3).
After easy separation using a centrifuge, the recyclability studies of CPM-200-In/Mg/TBAB catalyst were carried out under optimal reaction conditions (80 °C, 6 h, 0.6 mol% catalyst, and 1.2 MPa of CO2 pressure). As shown in Figure 9, CPM-200-In/Mg/TBAB system can be recycled five times, without considerable loss in its initial activity. The reused CPM-200-In/Mg catalyst was analyzed by FT-IR, PXRD, and TGA to investigate its chemical and thermal stability. PXRD patterns and FT-IR spectra of the fresh and reused CPM-200-In/Mg are identical (Figures S1 and S2). The TGA of the recycled CPM-200-In/Mg catalyst showed no destruction of the original structure of the fresh one ( Figure S3).   [2][3][4][5] in the cycloaddition of ECH and CO2 are also compared with CPM-200-In/Mg (Table 5, entry 6). It also confirms superior performance of the present work.    [2][3][4][5] in the cycloaddition of ECH and CO 2 are also compared with CPM-200-In/Mg (Table 5, entry 6). It also confirms superior performance of the present work.

Cycloaddition Reaction Mechanism
A plausible reaction mechanism based on the experimental results and our previous DFT studies [31,33,37] is proposed, for the cycloaddition of epoxide and CO 2 catalyzed by CPM-200-In/Mg and co-catalyst (Scheme 2). In the first step, Lewis acidic metal centers interact with the O atom of the epoxide ring. Then, the Br − ion of the co-catalyst attacks the least hindered C atoms of epoxide, resulting in the ring opening of epoxide. Thereafter, the generated partial positive charge on the carbon atom of epoxide polarizes the CO 2 molecule. Finally, the cyclic carbonate is generated by the ring closure with the elimination of the bromide ion. The synergistic role of acidic sites (Lewis acidic In and Mg metal sites) and basic sites (Lewis basic motif inside MOF pores) seems essential for the improved catalytic performance of CPM-200-In/Mg.

Preparation of CPM-200-In
CPM-200-In was prepared according to the reported method [60]. In a 20 mL glass vial, 22.0 mg of In(NO3)3·xH2O and 16.0 mg of H4ABTC were dissolved in a mixture of 2.0 g of DMA and 1.0 g of ACN. After addition of 120.0 mg HNO3, the vial was sealed and placed in a 90 °C oven for 3 days. After cooling to room temperature, pure yellow cubic crystals were obtained.

Preparation of CPM-200-In
CPM-200-In was prepared according to the reported method [60]. In a 20 mL glass vial, 22.0 mg of In(NO 3 ) 3 ·xH 2 O and 16.0 mg of H 4 ABTC were dissolved in a mixture of 2.0 g of DMA and 1.0 g of ACN. After addition of 120.0 mg HNO 3 , the vial was sealed and placed in a 90 • C oven for 3 days. After cooling to room temperature, pure yellow cubic crystals were obtained.

Preparation of CPM-200-In/Mg
For the preparation of CPM-200-In/Mg in a 20 mL glass vial, 22.1 mg of InCl 3 , 86.0 mg of Mg(OAc) 2 , 35.6 mg of H 4 ABTC were dissolved in a mixture of 4.0 g of DMA and 0.8 g of H 2 O. After addition of 120.0 mg HCl, the vial was sealed and placed in a 90 • C oven for 2 days. Pure yellow cubic crystals were obtained.

Cycloaddition of CO 2 and Epoxide
The cycloaddition reactions were carried out in a semi-batch reactor according to our previously reported method [43]. The detailed procedure is described in the Supplementary Materials. The products were analyzed by using a gas chromatograph (GC, Agilent technologies, HP 7890 A) with a flame ionization detector. Dichloromethane was used as an internal standard.

Conclusions
In this work, CPM-200-In and CPM-200-In/Mg MOF were successfully prepared by a solvothermal method and were characterized by using PXRD, SEM-EDS, FT-IR, TGA, XPS, BET, CO 2 , and NH 3 TPD analysis. Both MOFs were used as catalysts for the CO 2 fixation with various epoxides, and they were shown to be highly active (TOF of 25.0/h for ECH conversion) for the cycloaddition reaction under moderate operating conditions and solvent-less conditions. Especially, CPM-200-In/Mg revealed superior catalytic performance to CPM-200-In in the cycloaddition process due to higher surface area, higher CO 2 adsorption capacity, and higher number of basic sites. CPM-200-In/Mg catalyst could be recycled five times, without considerable loss in its initial activity. Based on the experimental data and our previous DFT studies, a plausible reaction mechanism, including the synergistic role of the acidic sites and basic sites for the bimetallic catalyzed epoxide-CO 2 cycloaddition reaction, was also suggested.