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Processes 2020, 8, 548 9 of 22 and ethylene oxide under harsh conditions, around T = 190 ◦C and P = 13 bar [16]. For propylene carbonate production, 99% of propylene oxide conversion can be achieved at 120 ◦C and 30 bar within 5 h when the reaction is catalyzed by KI, whereas only 27% and 3% conversion occurs in the cases of KBr and KCl catalysts, respectively [31]. However, the activity of alkali metal salts is relatively low, and therefore a co-catalyst is commonly necessary [32,33]. 3.1.3. Metallic Salen Complex Monometallic Salen Complex Homogeneous metal–salen and affine complexes (salphen, salophen, salalen, etc.) have been well developed over the last two decades as salen ligands that can coordinate with many transition metals, and have been widely studied for cycloaddition reaction [5]. Salen complexes are readily available, and can be finely tuned and prepared in enantiomerically pure form, thus enabling the creation of an asymmetric environment around the active metal site [21]. These metal complex catalysts have been used in many organic reactions, targeting cycloaddition reaction of CO2. Castro-Osma et al. [21] found an interesting finding of Cr(III) salen complexes with the use of the salophen ligand and tetrabutylammonium bromide (TBAB) that could catalyze CO2 coupling under ambient condition (Figure 6). Regarding the monometallic salen complex (Figure 6a; R = tert-butyl, X = Cl) with TBAB as a co-catalyst, the cycloaddition of CO2 with styrene oxide gave 100% conversion with selectivity higher than 99% under ambient condition for 24 h. Modifying the salen complex (Figure 6a; R = tert-butyl, X = Cl) by replacing the tert-butyl substituents with methoxy groups (Figure 6a; R = MeO, X = Br), results in higher rate of reaction, verifying the fact that replacing substituent with electron-donating groups (MeO) could produce more active catalysts. Bimetallic Salen Complexes The development of bimetallic salen complex with TBAB as co-catalyst was the earliest catalytic system capable of catalyzing the insertion of CO2 into terminal epoxides at ambient pressure and temperature [34]. On the basis of Wang et al. [22], the calculation using density functional theory (DFT) proved bimetallic (salen-Co) complexes were one of the most efficient catalysts, with an activation energy of merely 9.94 kcal/mol, indicating that the reaction could occur under ambient conditions. The experimentally produced yield of 75.8% with salen-Co catalyst was the highest yield attained when compared with salen-Al and salen-Zn at ambient conditions (Figure 6b) [17]. 3.1.4. Metallic Non-Salen-Based Complexes Monometallic Non-Salen-Based Complexes Due to a unique framework of porphyrin, coordination with metal referred to as “metalloporphyrin” is highly versatile and allows tunable electronic properties of the metal center, which can enhance catalytic activity and exhibit good thermal stability [23]. It is one of the most studied organometallic complexes besides salen-based complexes. For example, Maeda et al. [23] studied functionalized Zn(II) TPP (tetraphenylporphyrin) with eight quaternary ammonium bromides at the ortho, meta, or para positions of the meso-phenyl groups (Figure 7a; R = O(CH2)6N+Bu3Br−). The meta-substituted Zn(II) complexes showed very high activity. At 20 ◦C, they reported a cyclic carbonate yield of 82% with a TON (turnover number) of 1640 in 48 h under atmospheric pressure of CO2. A very high TON of 240,000 can be achieved by increasing reaction temperature to 120 ◦C and initial CO2 pressure to 17 bar. In a mechanistic approach, DFT (density functional theory) calculation was employed to reveal the origin of merit of the meta-substituted catalyst. De et al. [24] reported the synthesis of nonsymmetric aza-oxa cryptand derivatized with L-proline. The trinuclear Co(II) complex {[Co3(L)2(NCS)6]·(15CH3CN) (5acetone)(6H2O)} can be formed by reacting the cryptand and Co(II) perchlorate in the presence of KSCN. With TBAB as a co-catalyst,PDF Image | Green Pathway Utilizing CO2 Cycloaddition Reaction Epoxide
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