Current Applications And Development Of Catalyst For The Synthesis Of Highly Functional Organic Carbonates - Essay Example

? Current Applications and Development of Catalyst for the Synthesis of Highly Functional Organic Carbonates no Abstract Organic compounds obtained by carbonic acid and hydroxyl compound diesterification are called organic carbonates. These have diverse industrial, medical and biological applications as intermediates and solvents during lubricant, herbicide, pesticide, plastic synthesis, etc. An exhaustive review on the catalytic production of organic carbonates is lacking. The conventional procedure for synthesis of organic carbonates earlier involved toxic halogen compounds or phosgene, and later on moved to non toxic compounds such as carbon dioxide, alcohols, and epoxides. Recent catalysts include palladium, salen ligands, DBU, transition metal halides such as NiCl2, Lewis bases like Re(CO)5Br, NHC-CO2 adducts, tin, and Cu-exchanged zeolite Y catalyst. Introduction Organic carbonates are organic compounds obtained by carbonic acid and hydroxyl compound diesterification (1). The carbonates are produced in the form of diaryl, dialkyl or substituted dialkyl dialyl products depending on the type of hydroxyl compound used in the diesterification reaction. Organic carbonates have a wide variety of industrial, medical and biological applications (2). More specifically, they are used as intermediates, solvents and protective groups during the synthesis of lubricants, pharmaceuticals, herbicides, pesticides, plastics, etc. (3). Of the different kinds of organic carbonates, dimethyl carbonate is the simplest one, produced industrially from methanol through catalytic oxycarbonylation (1). A review of the literature on the catalytic production of organic carbonates reveals that comprehensive reports on the development of catalysts in the synthesis of organic carbonates is lacking. While earlier, organic carbonate synthesis with the help of catalysts required halogen compounds or phosgene, research on the synthesis of organic compounds later on was more inclined towards substituting halogens with other non toxic compounds. In recent years, research has focused on synthesizing organic carbonates from carbon dioxide, alcohols, and epoxides. Organic carbonate synthesis has thus seen a new dawn, wherein more emphasis was on green chemistry and use of reagents that are least toxic to the environment and to living organisms. The present paper aims at understanding how the use of catalysts in synthesizing organic carbonates progressed over the years. For this purpose, research reports on organic carbonate synthesis will be analyzed and discussed. The types of catalysts employed in organic carbonate synthesis will also be reviewed. The aim is to study the development of catalysts for organic carbonate synthesis, especially in the recent years. Theoretical Background Organic carbonates are formed by the diesterification of hydroxyl compounds with carbonic acid. Carbonic acid does not exist in free state. Its monoester is called hemicarbonic acid and it is formed by the reaction of carbon dioxide with alcohol. It can only be isolated in the form of salts and simple/mixed anhydrides with carboxylic acids because of its unstability. Through the esterification of hemicarbonic acid with hydroxy compounds, organic carbonates can be synthesized. These are classified into two main groups, saturated and unsaturated organic carbonates. Unsaturated carbonates include symmetrical (e.g. diallyl carbonate) and unsymmetrical (e.g. allyl methyl carbonate) organic carbonates. Saturated organic carbonates are further divided into several other categories that include aliphatic, aliphatic aromatic and aromatic. Further classification of organic carbonates is given in figure 1. Dimethyl carbonate is the simplest organic carbonate. Figure 1: Classification of Carbonates (1) Shaikh and Sivaram provided a comprehensive review of the various processes of organic carbonate synthesis. Since then, many more developments have taken place in the field. Before discussing the latest developments in organic carbonate synthesis, the processes enlisted by Shaikh and Sivaram are described here (1): 1. Synthesis of organic carbonates by phosgenation 2. Synthesis of organic carbonates by oxidative carbonylation of phenols/alcohols 3. Synthesis of organic carbonates by reacting phenols/alcohols with urea 4. Synthesis of organic carbonates by reacting carbon dioxide with oxiranes 5. Synthesis of organic carbonates using metal carbonates 6. Synthesis of organic carbonates via carbonate exchange reaction Organic carbonates have also been synthesized from carbon dioxide using ionic liquids as catalysts. Methodology For this review, research papers published by the American Chemical Society were searched and studies on the synthesis of organic carbonates were obtained. The search was performed using various combinations of the keywords “organic carbonates”, “synthesis”, “catalysts”, etc. The search revealed innumerable studies. However, only a few of these studies had a significant mention of the catalysts used. The main focus of the paper is on the development of catalysts in the last ten years. Although an attempt has been made to sum up the history of organic carbonate synthesis, a major part of the discussion will be on the developments in recent years, more specifically in the past ten years. Only papers published by the American Chemical Society have been reviewed. This paper will also attempt at statistically analyzing several aspects of organic carbonate synthesis. Data and Results The first comprehensive review of organic carbonate synthesis was provided by Shaikh and Sivaram in 1996 (1). It described various processes of organic carbonate synthesis. Romano et al. synthesized dimethyl carbonate by subjecting methanol to oxidative carbonylation using copper chloride salt as catalyst (1). This two step reaction is given in the equations 1 and 2. (1) (2) Various other patents report dimethyl carbonate synthesis using various other catalysts and reactions. The use of platinum halide or complexes with alkali/alkali earth metal halides in a continuous gas phase reaction CO and alkyl nitrites has been reported. In this reaction, the yield of dimethyl carbonate was found to be 70-80% (1). In a similar reaction, aromatic hydroxyl compound oxidative carbonylation did not result in high yields. Catalysts such as palladium chloride and palladium carbonyl chloride were experimented with, along with co-catalysts like copper, cobalt, vanadium, and manganese salts. Reports of diaryl carbonate synthesis by the direct oxidative phenol coupling with carbon monoxide/direct condensation with CO2 have also been reported. Palladium compounds, organic/ inorganic bases and alkyl ammonium halide catalysts have been experimented with. A yield of 4-30% diphenyl carbonate has been reported. Other catalysts that have been investigated include zinc acetate/triphenylphosphine, dibutyltin oxide/dimethoxide, triphenyltin chloride, aluminum trioxide etc. are known to give high yields. In the synthesis of organic carbonates, the reaction of aromatic hydroxyl compounds and urea at temperatures of 423.15 – 468.15 K using different catalysts has also been investigated. The synthesis of organic carbonates via the reaction of oxiranes and carbon dioxide is generally catalysed by transition metal complexes, poly-(siloxane)-supported metal halides, organometallic compounds, and Lewis acids. A new catalyst system was developed by Baba et al., who developed it using organotin halides with phosphonium/quaternary ammonium salts to catalyze the reaction between carbon dioxide and oxirane (1). Almost all earlier reactions used in organic carbonate production employed alkyl halides and halogen derivatives as catalysts. Kim et al. reported a coupling reaction of primary and secondary alcohols with several primary alkyl bromides, wherein cesium carbonate was used as a carbon dioxide source and a base for the synthesis of organic carbonates. In 1999, they reported a three way coupling reaction using halides, carbon dioxide and alcohols resulting in the synthesis of mixed alkyl carbonates using cesium alkoxides (2). (3) Verdecchia et al. developed a mild and safe procedure to synthesize mixed organic carbonates. This procedure was developed to avoid the traditionally employed pathway of organic carbonate synthesis which used harmful and toxic chemicals such as carbon monoxide, phosgene and halides (3). They used commercially available tetrabutylammonium ethoxide and methoxide in a reaction with carbon dioxide to synthesize ethyl and methyl carbonates (TBAEC and TBAMC), which upon reaction with alkyl halides result in high yields of ethyl and methyl carbonates. (4) In the synthesis of five membered organic carbonates like alkylene carbonates, the most commercial method employs the reaction between carbon dioxide and oxirane (4). In this reaction, alkylammonium halide catalysts like tetraethylammonium bromide are used. Subsequently, in recent years, research focused on utilizing carbon dioxide for the synthesis of organic compounds, in sync with the principles of green chemistry for the protection of the environment. Experiments incorporating carbon dioxide for the synthesis of organic substances generally employed transition metal-mediated reactions (5). For instance, cyclic carbonates were synthesized by reacting carbon dioxide with vinyl epoxides in the presence of palladium catalyst, as reported by Fujinami et al. (6). In these reactions, alkoxycarbonyl complexes are key intermediates Yoshida et al. devised a novel process that involved a carbon dioxide recycling reaction that involved 4-methoxycarbonyloxy-2-butyn-1-ols and their reaction with phenols using palladium as catalyst. This reaction results in the production of phenoxy substituted organic carbonates with high efficiencies. At around the same time, Bratt and Taylor published a report describing the synthesis of cyclic carbonates from carbon dioxide with methanesulfonyl carbonates as intermediates (7). These synthetic intermediates formed a new class. In the same year, Okuyama et al. reported a direct synthetic reaction for the production of aromatic polycarbonates from bisphenol A and carbon monoxide using palladium carbene complex as catalyst (8). Earlier reports had demonstrated that oxidative carbonylation can occur in presence of a PdCl2 catalyst. But, polycarbonates resulting from this reaction were found to have low molecular weight (8). In light of this, Okuyama et al. investigated the direct synthetic reaction for aromatic polycarbonate production. Using the palladium complex, they obtained a polycarbonate of 5600 molecular weight using 6,6?-disubstituted-2,2?-bipyridyl ligands. Experiments have investigated suitable alternatives to phosphine ligands, and heterocyclic carbene ligands have been found to be suitable contenders. These prolong the life of the catalyst used in the reaction. Inorganic redox catalysts used in these reactions include Ce(TMHD)4 and Mn(OAc)24H2O. In 2004, Darensbourg et al. attempted to optimize (salen)CrIIIX catalysts to selectively synthesize polycarbonates from carbon dioxide and aliphatic and alicyclic epoxides (9). (5) They employed an iterative catalyst design in which the cocatalyst, salen ligand and initiator as well as the reaction conditions were varied systematically while monitoring the product selectivity and reaction rates through insitu infrared spectroscopy. This method of producing polycarbonates from carbon dioxide and epoxide copolymerization was less expensive and more environment friendly. At higher temperatures of about 80-100?C, aliphatic epoxides are more prone towards the synthesis of cyclic carbonates. Their attempt to use a salen metal catalyst was a novel one. Generally, aromatic polycarbonates were produced through processes that involved bisphenol A and phosgene (10). As already stated, this process posed safety and environmental hazards because of the toxic phosgene reagent that resulted in the production of high amounts of chlorine salts. Some other alternatives to this process include melt transesterification of diphenyl carbonate and bisphenol A. Phosgene free method of diphenyl carbonate is not easily possible due to the equilibrium constraint which makes it important to produce it efficiently if phosgene-free organic carbonate synthesis is desired. Kishimoto and Ogawa further developed a one-pot coproduction process using an amine catalyzed reaction for the synthesis of dimethyl carbonate from methanol, carbondioxides and epoxides (11). For this reaction, 1,8-Diazabicyclo-[5.4.0]undec-7-ene (DBU) was used as a preferred catalyst. The procedure involved the heating of methanol, DBU, carbon dioxide and glycidyl phenyl ether at a temperature and pressure of 423.15 K and 15 MPa respectively, resulting in a high yield of 1,2-diol and dimethyl carbonate. Other symmetric dialkyl carbonates could also be produced by this reaction. (6) Studies have shown that dicyclohexylcarbodiimide (DCC) enables the facile synthesis of organic carbonates from carbon dioxide and aliphatic alcohols at moderate carbon dioxide pressures (0.1 MPa) and low temperatures upto as much as 310 K (12). This reaction is found to be highly selective towards the formation of carbonates at a temperature of 330 K and the conversion rate is found to increase with an increase in temperature. However, the selectivity is found to decrease at higher temperatures. To understand how the use of catalysts, more specifically Lewis acids such as CuCl2, CuO, CuCl, Cu2I2, Zn(acetate)2 or ZnCl affect the rate of formation of carbonates in this reaction, Aresta et al. (12) compared the selectivity and conversion yield of the alcohol carboxylation using alkyl isoureas and DCC. They performed these reactions in presence and absence of catalysts. It was seen that only the step in which a methanol unit is added to DCC is catalyzed while the other steps are not. As is seen from the earlier reactions, transition metal halides such as MoCl3, NiCl2 and AlCl3, and Lewis bases such as alkalimetal halide are the common catalyst systems that have been used in the coupling reactions of carbon dioxides with epoxides for the synthesis of organic carbonates. Most of these catalyst systems are found to suffer from the need for higher temperatures and pressures, or the use of additional Lewis bases and cosolvents (13). (7) Apart from these, imidazolium-based ionic liquids have also been employed for these coupling reactions. However, their productivity was not practically high. Kim and Verma (13) reported the activities of a few tetrahaloindate(III)-based ionic liquids in microwave reactions. Using ionic liquids of the formula [Q][InX3Y], where Q = imidazolium, ammonium, pyridinium and phosphonium, and X = Chlorine, Iodine and Bromine, and Y = Bromine and Chlorine, they showed that these ionic liquids exhibit high catalytic activity due to their H-bonding interactions. The thermal stability of these catalysts was found to be higher and they seem like promising catalysts for the coupling reactions for organic carbonate synthesis. They are environment friendly and are stable towards moisture and air and their recycling can be done efficiently without the use of volatile organic solvents. Gu et al. reported reactions of propargylic alcohols with carbon dioxide in presence of CuCl/[BMIm][PhSO3] system for the production of organic carbonates in high yields under mild conditions (14). (8) A novel method for synthesizing symmetrical organic carbonates without the use of phosgene was also reported by Jorapur and Chi (15). They synthesized organic carbonates by alkylating a metal carbonate with different sulfonates and halides in 1-n-butyl-3-methylimidazolium hexafluorophosphate. This reaction media is ecofriendly and complies with green chemistry protocols. Alkylation of the metal carbonate such as cesium and potassium carbonates in ionic liquids with 1-bromo-3-phenylpropane was performed. Metal carbonates were chosen because they are highly soluble in ionic liquids. This method resulted in high yields organic carbonates. While this method resulted in high yields of symmetric organic carbonates, the same results could not be obtained for asymmetric organic carbonates. On a different note, another methodology was employed by Jiang et al. to the conventional method of organic carbonate production, i.e. coupling of carbon dioxide to epoxides (16). They performed the reaction with supercritical carbon dioxide in the presence of catalytic amount of the transition-metal complex Re(CO)5Br. The reaction was performed in an autoclave and pressurized carbon dioxide was directly fed to the autoclave and solvent free conditions were maintained. A possible mechanism behind the Re(CO)5Br catalysis of carbon dioxide and epoxide coupling was proposed. This mechanism is shown schematically in figure 2. Figure 2: Proposed mechanism of Re(CO)5Br catalysis for organic carbonate synthesis through carbon dioxide and epoxide coupling (15) The mechanism, as described by the authors involves a 16-electron intermediate formation by Re(CO)5Br decarbonylation. Oxidative addition of the epoxide C-O bond to Re(CO)4Br gives the oxorhenium intermediate and subsequent insertion of carbon dioxide into Re-O bond followed by the elimination of C-O bond via reduction resulted in the production of organic carbonate. As discussed earlier, Darensbourg et al. in 2004 experimented with the use of salen derivatives as catalysts for organic carbonate synthesis. In extension to their earlier work, in 2006, Darensbourg and colleagues (17) worked on the applications of metal salen derivatives of aluminium and chromium along with salts of n-Bu4NX, where X = N3 or Cl. They showed that these derivatives are effective catalysts for the selective coupling reaction of trimethylene oxide oxetane with carbon dioxide, resulting in the synthesis of polycarbonates. Organic carbonates are generally intermediates in the formation of polycarbonates (18). Darensbourg and Fitch also reported the copolymerization of carbon dioxide with cyclohexene oxide (19). In their experiment, they observed a high catalytic activity tetramethyltetraazaannulene chromium complex, which results in the discriminate synthesis of poly(cyclohexylene carbonate). Several cocatalysts such as [PPN]X (PPN+) bis(triphenylphosphoranylidene)-ammonium), where X= Br, Cl, N3, CN, and OBzF5. These selectively catalyze the formation of polycarbonates with a turnover frequency of upto 1500 per hour at a temperature of 353.15 K. This catalyst system was active even at a carbon dioxide pressure of 1 bar. They described a copolymerization process which can be modulated to produce either polycarbonates or cyclic carbonates. The selective formation of copolymer vs. organic carbonate was found to be highly dependent on the use of cosolvents. It was seen that without a cosolvent, the rate of production of cyclic carbonates was reduced. Apart from the transition metal complexes, salen complexes, etc., various other compounds have also been employed as catalysts for the production of carbonates. Zhao et al. showed the production of dimethyl carbonate from methanol and methyl carbamate in a batch reactor (20). (9) (10) (11) They found that zinc chloride resulted in the highest yield. In addition, zinc bromide was also found to result in a high yield. The catalytic mechanism was due to the Zn2+ ion. Reaction temperature, reaction time and amount of catalyst were found to strongly influence the performance of the zinc chloride catalyst. As discussed, the coupling reaction of epoxides and carbon dioxide is the popular route for production of organic carbonates. NHC-CO2 adducts are good organic catalysts for this coupling reaction. Zhou et al. in 2008 explored the thermal stability of these adducts in organic solvents (21). In situ FTIR method was employed for this investigated. They showed that imidazolinium carboxylates exhibit a higer thermal stability in their saturated rather than unsaturated form. In the presence of free carbon dioxide, the thermal decomposition of NHC-CO2 adducts is found to be inhibited and their decomposition is accelerated upon the addition of an epoxide, resulting in organic carbonate synthesis. It is seen that soluble adducts of NHC-CO2 are singularly effective in the catalysis of the coupling reaction. Furthermore, it was seen that IPr-CO2 has a higher catalytic activity under the same conditions. However, it is unstable. Catalytic activity of IPr-CO2 was found to increase in the presence of SalenAlEt. The highly active catalytic corrosion inhibitory activity of CuCl, N-methyl imidazole and 1,10-phenanthroline in the synthesis of diethyl carbonate was discovered by Xiong et al. (22). They explored the effects of N-methyl imidazole (NMI) and 1,10-phenanthroline (phen) N-donor ligands on the oxidative carboxylation reaction of ethanol catalyzed by copper chloride. It was seen that these have a synergistic effect on the catalytic activity of cupper chloride. Moreover, the reaction system’s corrosion was also inhibited in the presence of these two ligands. The catalytic activity of copper chloride was found to increase 3.6 fold when used simultaneously with NMI and phen. This mechanism had great implications for the commercial synthesis of diethyl carbonates as the catalytic activity of the conventionally used copper chloride catalyst could be increased greatly. Many other processes have been discussed by Scahffner et al. in their review paper on organic carbonates, which is one of the most comprehensive reviews on organic carbonates published in recent years. Published in 2009 in Chemical Reviews, this paper details the use of organic carbonates as solvents in catalysis and synthesis (23). Organic carbonates can ideally be synthesized via the phosgene route, or by addition of carbon dioxide to acetals, carbonylative oxidation of CO, CuCl and MeOH, carbonate interchange reaction, addition of carbon dioxide to epoxides, or by addition of carbon dioxide to diols (23). While the ideal procedure is through condensation of carbon dioxide with alcohols, the limitation of this reaction is the production of water. This limitation could be overcome with the use of acetals. The use of tin catalysts for the reaction was found to result in high yields upto 88%. Cu-exchanged zeolite Y catalyst has also been used in the synthesis of carbonates. Other catalysts include activated dawsonites, CaO, homogeneous zirconium, Mg-Al-hydrotalcites, MgO, smectite with Ni or Mg, tin catalysts, titanium catalysts and titanium silicate molecular sieves (23). These catalysts catalyze the transesteri?cation reaction of ethylene carbonate for the synthesis of organic carbonates. Figure 3: Synthesis of organic carbonates – various routes (23) Another novel methodology was that of Zhang et al. who synthesized dimethyl carbonate using [EMIM]Br/Ce0.5Zr0.5O2 and CexZr1-xO2. This reaction involved methanol and carbon dioxide. In another investigation, Ramidi et al. explored the use of ionic CrV(O) complex for the synthesis of organic carbonates (25). This is a novel instance wherein cyclic amido ligand complex was used for the purpose. Epoxide and carbon dioxide coupling reaction was performed for organic carbonate synthesis. The study showed that in addition to the catalytic activity of CrV(O) complex, the complex’s cation also greatly influenced the overall catalytic activity. This influence was more pronounced in the case of lithium ion. At about the same time, Yan et al. investigated a three-component cyclization reaction for the synthesis of organic carbonates (26). This reaction involved a single operation for synthesis from aldehyde, carbon dioxide and phenacyl bromide and was performed in the presence of LDA (lithium diisopropylamide). (12) Tian et al. also showed the synthesis of organic carbonates from the coupling reaction of epoxides and carbon dioxide using aluminum-salen complexes as catalysts (27). These complexes with quaternary ammonium salts served as efficient catalysts in the reaction. Increase in catalytic activity was owed to the onium salt group attached to the salen ligand. These catalysts are one-component, bifunctional, recyclable, robust, stable to oxygen and moisture, resistant to impurities. The loss in catalytic activity due to recycling is negligible. Catalysts are generally required for the reactions that involve carbon monoxide as the carbonyl carbon source (28). Most reactions that require carbon dioxide starter for the synthesis of organic carbonates also require catalysts. An aluminum complex based catalyst was recently reported by Whiteoak et al. (29). (13) An amino triphenolate ligand sca?old was used for the aluminum complex. Most of the catalytic systems described previously needed high catalyst loadings and were not even at high temperatures. Due to this, their industrial application was limited. Bifunctional catalyst systems combining a nucleophile and Lewis acid had earlier been reported (30). The catalytic protocol developed by Whiteoak et al. had the advantages of having low catalyst loading and the reaction conditions required were mild and could be achieved using abundant, non toxic and cheap metal. As a comparative measure, data for the catalysis of the reactions used in the synthesis of dimethyl carbonate (DMC) were pooled and compared. The data is shown in table 1. Table 1 Year Catalyst Reaction Yield (%) Reference 2004 TMG Catalytic one-pot production from carbon dioxide, glycidyl phenyl ether, and methanol 80 (11) 2004 DBU Catalytic one-pot production from carbon dioxide, glycidyl phenyl ether, and methanol 97 (11) 2004 triethylamine Catalytic one-pot production from carbon dioxide, glycidyl phenyl ether, and methanol 42 (11) 2004 DABCO Catalytic one-pot production from carbon dioxide, glycidyl phenyl ether, and methanol 70 (11) 2004 potassium tert-butoxide Catalytic one-pot production from carbon dioxide, glycidyl phenyl ether, and methanol 50 (11) 2004 TBAB Catalytic one-pot production from carbon dioxide, glycidyl phenyl ether, and methanol 17 (11) 2007 MgO Synthesis from methanol, epoxides and CO2 14 (18) 2008 ZnO Methyl carbamate conversion 4.2 (20) 2008 Zn(OH)2 Methyl carbamate conversion 3.8 (20) 2008 ZnSO4 Methyl carbamate conversion 3.7 (20) 2008 NaCl Methyl carbamate conversion 3.4 (20) 2008 Zn(NH3)2Cl2 Methyl carbamate conversion 29.5 (20) 1996 ClCH2CO2Na Carbon interchange reaction of alcohols and ethylene carbonate 60 (1) 1996 Tl2O3 Carbon interchange reaction of alcohols and ethylene carbonate 25 (1) 1996 CH3OH Carbon interchange reaction of alcohols and ethylene carbonate 48 (1) Interpretation, Conclusion and Limitations Data in table 1 was used to draw conclusions on the performance of the catalysts in the synthesis of DMC. It is seen that the yields of DMC in presence of catalyst range from as low as 3.2 to as high as 97%. The mean of the yields is 36.50 with a standard deviation of 30.38. Highest yields are seen for the catalytic one-pot production from carbon dioxide, glycidyl phenyl ether, and methanol in presence of DBU catalyst. This data however cannot be used for an accurate comparison of the catalysts because it does not take into account the reaction conditions and the types of reactions are different for all catalysts listed. This paper set out to provide an outline of the various catalysts used for the synthesis of organic carbonates. The aim was to show the sequential development of catalysts for the process. The discussion is based on a chronological review of the papers published on the subject. As seen from the review, most of the catalytic procedures for organic carbonate synthesis are based on halo compounds and phosgene. In order to avoid the use of these toxic compounds, symmetrical organic compounds like DMC have been synthesized by the coupling reaction of epoxides, carbon dioxide and alcohols. Many catalysts and reaction mechanisms have used for this coupling reaction. For instance, palladium catalyst, salen ligands, amine, DBU, transition metal halides such as MoCl3, NiCl2 and AlCl3, and Lewis bases such as alkalimetal halide, Re(CO)5Br, NHC-CO2 adducts, tin catalysts, Cu-exchanged zeolite Y catalyst, activated dawsonites, CaO, homogeneous zirconium, Mg-Al-hydrotalcites, MgO, smectite with Ni or Mg, tin catalysts, titanium catalysts and titanium silicate molecular sieves, CrV(O) complex, aluminum-salen complexes, etc. Other methods for organic carbonate synthesis include the reaction between oxiranes and carbon dioxide and metal complexes and onium salts are used in this reaction as homogenous catalysts. Salen complexes of aluminum, zinc, cobalt, chromium, tin, etc. are found to exhibit high catalytic activities. Ionic liquids have also been experimented with for the production of organic carbonates. Ionic liquids such as imidazolium salts and tetrahaloindate(III)-based ionic liquids are used as catalysts for the synthesis. A combination of metal salts and ionic liquids has also been used in catalyzing these reactions. Lithium bromide and other heterogenous catalysts such as magnesium oxide and ZnO-SiO2 have also been used as catalysts. While earlier the synthesis of organic carbonates was more primarily focused on the use of halogen compounds or phosgene, research on the synthesis of organic compounds shifted towards substituting halogens with other non toxic compounds such as the use of carbon dioxide, alcohols, and epoxides. This review provides a brief outline of the catalytic mechanisms applied in the synthesis of organic carbonates. One of its limitations is that it is based on a year-wise review of catalytic mechanisms. More extensive review of the catalytic mechanisms could not be performed. Moreover, the catalysts could not be compared keeping in view the reaction conditions. Nomenclature AlCl3 = aluminum chloride CO = carbon monoxide CO2 = carbon dioxide Cu2I2 = copper iodide CuCl = copper chloride CuCl2 = copper chloride CuO = copper oxide DBU = 1,8-Diazabicyclo-[5.4.0]undec-7-ene DCC = dicyclohexylcarbodiimide DMC = dimethyl carbonate LDA = lithium diisopropylamide MoCl3 = molybdenum chloride NiCl2 = nickel chloride NMI = N-methyl imidazole TBAEC = tetrabutylammonium ethoxide carbonate TBAMC = tetrabutylammonium methoxide carbonate ZnCl = zinc chloride References 1. (Shaikh and Sivaram, 1996) 2. (Kim et al., 1999) 3. (Verdecchia et al., 2002) 4. (Clements, 2003) 5. (Yoshida et al., 2003) 6. (Giannoccaro et al., 2006) 7. (Bratt and Taylor, 2003) 8. (Okuyama et al., 2003) 9. (Darensbourg et al., 2004) 10. (Kim et al., 2004) 11. (Kishimoto and Ogawa, 2004) 12. (Aresta et al., 2005) 13. (Kim and Verma, 2005) 14. (Gu et al., 2005) 15. (Jorapur and Chi, 2005) 16. (Jiang et al., 2005) 17. (Darensbourg, Ganguly and Choi, 2006) 18. (Sakakura, Choi and Yasuda, 2007) 19. (Darensbourg and Fitch, 2008) 20. (Zhao et al., 2008) 21. (Zhou et al., 2008) 22. (Xiong et al., 2009) 23. (Schaffner et al., 2010) 24. (Zhang et al., 2011) 25. (Ramidi et al., 2011) 26. (Yan et al., 2011) 27. (Tian et al., 2012) 28. (Zhang, Liu and Yue, 2012) 29. (Whiteoak et al., 2013) 30. (Cohen, Chu and Coates, 2005) Bibliography (1) Shaikh, A.G. and S. Sivaram, Chemical Reviews, 96:3 951?976 (1996) (2) Kim, S., Chu, F., Dueno, E.E. and K.W. Jung, The Journal of Organic Chemistry, 64:13 4578-4579 (1999) (3) Verdecchia, M., Feroci, M., Palombi, L. and L. Rossi, The Journal of Organic Chemistry, 67:23 8287-8289 (2002) (4) Clements, J.H., Industrial & Engineering Chemistry Research, 42:4 663-674 (2003) (5) Yoshida, M., Fujita, M., Ishii, T. and M. Ihara, Journal of the American Chemical Society, 125:16 4874-4881 (2003) (6) Giannoccaro, P., Cornacchia, D., Doronzo, S., Mesto, E., Quaranta, E. and M. Aresta, Organometallics, 25:11 2872-2879 (2006) (7) Bratt, M.O. and P.C. Taylor, The Journal of Organic Chemistry, 68:14 5439-44 (2003) (8) Okuyama,K., Sugiyama, J., Nagahata, R., Asai, M., Ueda, M. and K. Takeuchi, Macromolecules, 36:19 6953-6955 (2003) (9) Darensbourg, D.J., Mackiewicz, R.M., Phelps, A.L and D.R. Billodeaux, Accounts of Chemical Research, 37:11 836-844 (2004) (10) Kim, W.B., Joshi, U.A. and J.S. Lee, Industrial & Engineering Chemistry Research, 43:9 1897-1914 (2004) (11) Kishimoto, Y. and I. Ogawa, Industrial & Engineering Chemistry Research, 43:26 8155-8162 (2004) (12) Aresta,M., Dibenedetto, A., Fracchiolla, E., Giannoccaro, P., Pastore, C., Pa pai, I. and G. Schubert, The Journal of Organic Chemistry, 70:16 6177-6186 (2005) (13) Kim, Y.J. and R.S. Varma, The Journal of Organic Chemistry, 70:20 7882-7891 (2005) (14) Gu,Y., Zhang, Q., Duan, Z., Zhang, J., Zhang, S. and Y. Deng, The Journal of Organic Chemistry, 70:18 7376-7380 (2005) (15) Jorapur, Y.R. and D.Y. Chi, The Journal of Organic Chemistry, 70:26 10774-10777 (2005) (16) Jiang, J., Gao, F., Hua, R. and X. Qiu, The Journal of Organic Chemistry, 70:1 381-383 (2005) (17) Darensbourg, D.J., Ganguly, P. and W. Choi, Inorganic Chemistry, 45:10 3831?3833 (2006) (18) Sakakura, T., Choi, J. and H. Yasuda, Chemical Reviews, 107:6 2365?2387 (2007) (19) Darensbourg, D.J. and S.B. Fitch, Inorganic Chemistry, 47:24 11868-11878 (2008) (20) Zhao, W., Wang, F., Peng, W., Zhao,N., Li, J., Xiao, F., Wei,W. and Y. Sun, Industrial & Engineering Chemistry Research, 47:16 5913–5917 (2008) (21) Zhou, H., Zhang, W., Liu, C., Qu, J. and X. Lu, The Journal of Organic Chemistry, 73:20 8039–8044 (2008) (22) Xiong, H., Mo, W., Hu, J., Bai, R. and G. Li, Industrial & Engineering Chemistry Research, 48:24 10845–10849 (2009) (23) Schaffner, B., Schaffner, F., Verevkin, S.P. and A. Borner, Chemical Reviews, 110:8 4554–4581 (2010) (24) Zhang, Z., Liu, Z., Lu, J. and Z. Liu, Industrial & Engineering Chemistry Research, 50 1981–1988 (2011) (25) Ramidi, P., Sullivan, S.Z., Gartia, Y., Munshi, P., Griffin, W.O., Darsey, J.A., Biswas, A., Shaikh, A.U. and A. Ghosh, Industrial & Engineering Chemistry Research, 50 7800–7807 (2011) (26) Yan, P., Tan, X., Jing, H., Duan, S., Wang, X. and Z. Liu, The Journal of Organic Chemistry, 76 2459–2464 (2011) (27) Tian, D., Liu, B., Gan, Q., Li, H. and D.J. Darensbourg, ACS Catalysis, 2 2029?2035 (2012) (28) Zhang, H., Liu, H. and J. Yue, Chemical Reviews, (yet to be published). (29) Whiteoak, C.J., Kielland, N., Laserna, V., Escudero-Ada?n, E.C., Martin, E. and A.W. Kleij, Journal of the American Chemical Society, 135 1228?1231 (2013) (30) Cohen, C.T., Chu, T. and G.W. Coates, Journal of the American Chemical Society, 127:31 10869-10878 (2005) Read More