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Processes 2020, 8, 548 8 of 22 Quarternary ammonium halides, one of the organic salts, were among the first active catalysts for the cyclic carbonates synthesis [27,28]. Tetrabutylammonium bromide (TBAB) effectively catalyzed ethylene oxide cycloaddition to ethylene carbonate with 97% yield, but at a harsh condition of 200 ◦C and initial CO2 pressure of 34 bar [27]. Other organic salts such as phosphonium salt and imidazolium salt have been later introduced as effective organocatalysts. Ionic liquid as a catalyst such as imidazolium salt-based ionic liquids (ILs), namely, 1-n-butyl-3-methylimidazolium tetrafluoroborate, exhibited 100% propylene oxide conversion with almost 100% selectivity of propylene carbonate at 110 ◦C and CO2 initial pressure of 40 bar. Good recyclability can be obtained with a slight decrease in conversion and selectivity after 5 cycle runs [29]. The ILs are defined as salts that melt at 100 ◦C or less. Organic salts and ILs can be classified and differentiated by their cation structures. The mechanism of these organocatalysts is intrinsically based on the nucleophilic attack of the anion and the stabilization by interacting with the cation [30]. Because the reaction occurs in the liquid phase, the solubility of the catalyst plays an important role. Many types of ILs have been used to catalyze the reactions, including ILs based on imidazolium, ammonium, phosphonium, and pyridinium salts. For example, Wang et al. [14] studied the dual functional catalytic system by incorporating pyridine alcohol with tetrabutylammonium iodide (TBAI). The results of the binary catalytic showed increase of yield of the different usage of halides ions in the order of Cl− < Br− < I− with yield of 52%, 67%, and 92%, respectively (Table 1). This is probably because chloride ion has stronger hydrogen bond interactions with the alcohol component, when compared to bromide and iodide ions. This study displays conversion of diverse epoxides with high yield, ranging from 85–97%, for which ECH is the highest yield due to electron withdrawing effect of the substituent, which favors the nucleophilic attack at the carbon atom of the epoxide ring. The catalytic system of 2,6-pyridinedimethanol/TBAI was sustainable and economical because the activity was almost constant after recycling at least six times. Deep eutectic solvents (DESs) have recently emerged as captivating multifunctional media. DESs possess analogous properties of ILs; however, it has been proposed as a promising alternative to conventional ILs because of easier preaparation and lower cost. DESs are eutectic mixtures of hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) in a certain stoichiometric ratio. Tak et al. [20] studied the reaction of DES by using choline chloride (ChCl) as HBA and various HBDs, these being urea, ethylene glycol, glycerol, and benzoic acid, with a constant mole ratio of ChCl to HBD of 1:2, finding that urea served as the best HBD of SEO conversion among others because of the greater solubility of CO2 in a ChCl/urea DES mixture (0.301 mol CO2/mol DES). With regard to the activation barrier that hindered the formation of bicyclic spiro-cyclic carbonate, the studies investigated the effect of temperature from 40 to 70 ◦C, significantly influencing the yield of spiro-cyclic carbonate with the increment of 49% to 89% in 5 h. The effect of catalyst loading of ChCl/urea (100–300 mg) was observed, and 300 mg achieved the highest yield (98%) in a shorter reaction period. The effect of substitutions of benzyl (Bn) attached to nitrogen (Figure 3i) with other substrate (i.e., methyl, allyl, and methylbenzyl) provided excellent yields (95–98%); in comparison, the substitutions of aromatic ring-derived substrates (Figure 3i); R = H, F, Cl, benzyl, etc.) required additional duration (6–8 h) in achieving a comparable yield (67–90%), which was somewhat expected due to the steric hindrance of the substituted substrate, and the author suggested that providing higher pressure and temperature may achieve better conversion. The recyclability of ChCl/urea DES displayed consistent yield (≈98%) in four runs. 3.1.2. Metal Salt The most commonly used metal salts are those based on alkali metal salts (also called alkali-metal halides). Alkali metal salts of sodium, potassium, or lithium, which are abundant, low-cost, and non-toxic can be employed to catalyze the CO2/epoxide coupling. For instance, many researchers reported the production of ethylene carbonate under high temperature and pressure. For example, Dow Chemical and Shell patented KI as a catalyst for the ethylene carbonate production from CO2PDF Image | Green Pathway Utilizing CO2 Cycloaddition Reaction Epoxide
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