Catalysts Used in Synthesis of Organic Carbonates - Research Paper Example

Current Applications and Development of Catalyst for the Synthesis of Highly Functional Organic Carbonates no Table of Contents 1. Abstract 3 2. Introduction 3 3. Definitions and Review of Literature 4 3.1. Overview of Organic Carbonate Synthesis 4 3.2. Catalysts Used in Synthesis of Organic Carbonates 6 4. Methodology 9 5. Data and Results 10 5.1. DMC Synthesis and Catalysts Used 10 5.2. Yields of DMC using Different Catalysts 13 6. Interpretation 13 7. Conclusion 13 Nomenclature 14 References 14 Bibliography 16 Appendix 18 1. Abstract Organic carbonates are carbonic acid esters characterized by two aryloxy or alkoxy groups flanking a carbonyl functional group. They are structurally of two kinds, namely – linear and cyclic. Organic carbonates are used in the manufacture of a large number of other industrially important products. This report explores the development of catalysts in the synthesis of organic carbonates. It focuses on the types of catalysts used and the yield of organic carbonates while briefly discussing the various synthesis procedures. The catalysts used in the synthesis of dimethyl carbonate (DMC) and their corresponding yields have been elaborated. 2. Introduction Organic carbonates are industrially important compounds that are being manufactured by companies on a multiton scale (1). These are generally produced from carbonic acid diesterification with hydroxyl compounds (2). The type of carbonate produced, whether it is a dialkyl, dialkyl, etc., depends on the kind of hydroxyl compound used. Organic carbonates are classified into saturated and unsaturated carbonates that are further divided into several other classes (2). Depending on their structure, organic carbonates are either linear or cyclic (1). For the production of these two kinds of organic carbonates, two general pathways have been developed. Unfortunately, although organic carbonates are environment friendly green solvents, their synthesis is not so. The industrial production of linear carbonates involves phosgene and that of cyclic carbonates involves propylene oxide as starting materials. Both of these compounds are highly toxic to the environment and thus, the industrial production of organic carbonates offers quite a challenge to the endeavor of green chemistry (1). Recent research is thus focused on synthesizing organic carbonates directly through non phosgene routes such as from alcohols, epoxides, carbon dioxide, etc. Thus, the production of organic carbonates is proceeding towards greener solutions, and research has been focusing on how the process can be brought in sync with the principles of green chemistry. Organic carbonates are used in the manufacture of a large number of other industrially important products, mainly polyurethanes and polycarbonates. They are employed in the manufacture of pharmaceutical products, herbicides, pesticides, electrolytic fluids, polyimide films, etc., in addition to being used as solvents and fuel additives (3). They are environmentally benign, they are replacing toxic organic solvents, methylating agents and carbonating agents such as methyl halides, dimethyl sulphate and phosgene (3). Other variations of organic carbonates also exist. For instance, aliphatic polycarbonates are biodegradable and hence are medically important (4). They are also used as matrix materials, thermoplastic additives and films in addition to being used for the production of industrially important materials such as specialty polyurethanes (4). Glycerol based carbonate polymers also have a wide range of industrial, pharmaceutical and chemical applications. Polycarbonate esters and other carbonate polymers are used as antibacterial compounds and fluorescent tags in the biomedical field (5). The various routes of synthesis of linear and cyclic organic carbonates are represented schematically in figure 1. Figure 1: Various routes of organic carbonate synthesis and the commonly used building blocks (1). Phosgenation technique, oxidative carbonylation of phenols and alcohols, reaction of urea with phenols and alcohols, reaction of carbon dioxide and oxiranes, and transformation of inorganic metal carbonates are some of the most commonly used methods for synthesis of organic carbonates. The aim of this paper is to study the development of catalysts in the synthesis of organic carbonates. Therefore, this paper will mainly focus on the types of catalysts used and the yield of organic carbonates while briefly discussing the various synthesis procedures. However, it will be difficult to outline the development of catalysts for all organic carbonates over time. Therefore, to narrow down the research, the paper focuses on the catalysts used in the synthesis of dimethyl carbonate (DMC). DMC is the simplest organic carbonate and is one of the most industrially useful carbonates (2). It is also used for the synthesis of many other organic carbonates, apart from being used as an organic solvent and fuel additive. It is not ecotoxic and hence, is an environmentally benign compound. The report first provides a brief summary of catalysts used in the synthesis of various organic carbonates and the synthesis procedures. It then goes on to explain the development of catalysts for DMC synthesis as well as the associated production processes. The yields of DMC using various catalysts and reaction conditions will then be reported and analyzed. This paper will thus report a review of the various catalysts used in organic carbonate synthesis over the years, will special emphasis on DMC. 3. Definitions and Review of Literature 3.1. Overview of Organic Carbonate Synthesis Organic carbonates are defined as carbonic acid esters that are characterized by two aryloxy or alkoxy groups flanking a carbonyl functional group (6). The structure of these carbonyl compounds is represented by R1OCOOR2 where R1 and R2 are the alkyl or aryl groups, forming a cyclic architecture (6). As discussed in the earlier section, organic carbonates are structurally of two kinds, namely – linear and cyclic – that are synthesised via the phosgene route, via carbonylative oxidation, carbon dioxide addition to acetals in case of linear and carbon dioxide addition to diols and epoxides in case of cyclic organic carbonates, carbonylative oxidation and carbonate interchange reaction. Carbonates could ideally be synthesized from alcohols by carbon dioxide condensation. However, in this reaction water is produced as part of the equilibrium in this reaction, which is its drawback. Hence, the use of supercritical carbondioxide and acetals in presence of tin catalysts has been suggested, and yields upto 88% were obtained (1). The reaction which synthesizes linear carbonates from phosgene employs two equivalents methanol. ---------- (1) However, the reaction results in the synthesis of hydrochloric acid which is corrosive, requiring the need to recycle or trap it. This method of synthesis also has the disadvantage of using phosgene as a starting material. Phosgene has high ecotoxicity. Other successful methods of organic carbonate production include electrochemical synthesis, oxidative carbonylation of methanol, transesterification of ethylene carbonate in the presence of a base, etc. Synthesis of organic carbonates through electrochemical synthesis is as per the following reaction: ---------- (2) Zhang and Grinstaff reported the synthesis of isotactic and atactic poly 1,2-glycerol carbonates (5). They synthesized these glycerol based organic carbonates by inducing ring opening copolymerization using CO2 in rac-/(R)-benzyl glycidyl ether with the help of [SalcyCoIIIX] complexes. Linear poly(1,3-glycerol carbonate)s are synthesized from cyclic six membered carbonates (3-benzyloxytrimethylene carbonates) or monomers of dimethylacetal dihydroxy acetone carbonate (5). Other glycerol organic carbonates have also been synthesized. Geschwind and Frey synthesized poly(1,2-glycerol carbonate)s from glycidyl ethers and CO2 by combining the monomers of glycidyl ether using two different approaches (7). The synthesis of novel aliphatic poly(butylenes succinate-co-cyclic carbonate)s was reported by Yang et al. (8). They reported the synthesis of these novel biodegradable carbonates from ethyl chloroformate and diols in THF solution at 0 oC. These caboantes were also synthesized from PBS macromers through ring opening polymerization and polycondensation at 210 oC. Polycarbonates have also been synthesized by subjecting bifunctional cyclic carbonates having 5/6 membered rings to anionic ring opening polymerization (ROP) and depolymerization (9). 3.2. Catalysts Used in Synthesis of Organic Carbonates A large number of catalysts have been employed in the synthesis of organic carbonates. As the synthesis procedures have evolved over time, so have the different catalysts, depending on the type of starting material, reactants, reaction conditions, and desired product. Some of the earliest catalysts include phosphonium and quarternary ammonium salts used for the synthesis of cyclic carbonates from CO2 and epoxides, a commercial synthetic process since the 1950s (10). Studies have explored the use of tin catalyst in the synthesis dimethyl carbonate from acetals and carbon dioxide. The use of ionic liquids such as BMIMB4 (1-butyl-3-methyl imidazolium tetrafluoroborate) for activating CO2 during DMC production has also been explored. Carbon monoxide and copper (I) chloride have also been used for oxidative carbonylation of methanol for the synthesis of DMC. The advantage of copper catalyst over other metal catalysts such as palladium and mercury is that it undergoes direct reoxidation. When such a catalyst is used in conjunction of ionic liquids, there is a higher yield of DMC. For instance, as reported by Schaffner et al., the yield in presence of ionic liquids is 17%, and is as low as 9% in absence of ionic liquids (1). Other catalysts include Cu-exchanged zeolite Y catalyst and zeolite-encapsulated Co-Schiff base complex for the synthesis of DMC that will be discussed in the forthcoming sections. Bases such as alkoxides, hydroxides, alcoholate amides, and hydrides are also used for organic carbonate synthesis in presence of aromatic and aliphatic alcohols. Most commonly, ethylene carbonate and propylene carbonate are synthesized from CO2 and epoxides . DMC is also synthesized through this transesterification reaction and its advantage is that it does not involve phosgene. Alternatively, DMC is also synthesized from urea using catalysts such as activated dawsonites, homogeneous zirconium, Mg-Al-hydrotalcite materials, MgO, CaO, smectite systems with Ni or Mg, tin, titanium and titanium silicate molecular sieves. Diphenyl carbonates are also synthesized by carbonate interchange reaction that can be reacted with other carbonates and glycerol to yield glycerol based polycarbonates. The catalyst used in such a reaction is immobilized lipase enzyme obtained from Candida antarctica. Tin catalysts having organic moieties are also used for the transesterification reaction for synthesis of glycerol carbonates from diethyl carbonate. By applying this process in a continuous manner quantitative yields are achievable for upto five cycles. For linear and cyclic carbonates, it is not possible to perform direct synthesis from corresponding alcohols because the starting materials are favored by the equilibrium. Therefore, pathways via 1,2-diols are being investigated. Carbonate interchange reactions using tin catalyst (Sn(OCH3)2) and molecular sieves for the synthesis of glycerol carbonate using direct synthesis were found to give low yield. However, the yield was found to be higher when ion exchange resins and zeolites were used in supercritical CO2. The yield was as high as 32%, which is still considerably low. Using zeolites and Bu2SnO as catalyst, an yield of 35% was achieved (1). Zhao, Zhang and Wang proposed a new route of propylene carbonate synthesis from 1,2-propylene glycol and urea in the presence of zinc acetate catalyst (11). Earlier studies on this route were very few. They studied the catalytic properties of zinc acetate on the basis of thermodynamic analysis of propylene carbonate synthesis from 1,2-propylene glycol and urea. The highest yield they obtained was 94%. They also experimented with the immobilization of zinc acetate to enable catalyst recovery and reuse. The support they used was activated carbon and the optimal load of zinc acetate used was 15 wt %. In this case however, the highest yield achieved was 78%, lower than what was achieved through non-immobilized catalyst. The catalysts used before and after the reaction were analyzed through X-ray diffraction, X-ray photoelectron spectroscopy, atomic absorption spectroscopy, and Brunauer-Emmett-Teller surface area analysis. It was found that there were some changes in catalyst during the reaction. Severe loss of zinc acetate was also observed. The synthesis of propylene carbonate from 1,2-proplylene glycol and carbon dioxide instead of urea was also investigated (12). While earlier studies carried out the synthesis of propylene carbonate via this reaction over CeO2-ZrO2 solid solution, the yield obtained was just 2%. However, the selectivity of propylene carbonate was as high as 99.9%. Zhao et al. carried out the same reaction in the presence of zinc acetate as catalyst. They investigated a series of acetate salts and found that zinc acetate has the highest catalytic activity. The yield of propylene carbonate they achieved was 12.3%. The main reaction for this synthesis is as follows: ---------- (3) Another novel catalytic synthesis of organic carbonates is the synthesis of diethyl carbonate using CuCl/phen/NMI [1,10-phenanthroline (phen) and N-methyl imidazole (NMI)] with CuCl catalyst) for the oxidative carbonylation of ethanol. Diethyl carbonate was synthesized via homogenous carbonylation (13). It was found that phen and NMI had a synergistic effect on the catalytic activity of CuCl, enabling a much higher efficiency of the catalyst. It was also found that the reaction’s corrosion system was inhibited when both NMI and phen were used simultaneously as ligands. The selectivity of the catalyst to diethyl carbonate was as high as 99% and the yield was 15.2%. When pure CuCl is used as catalyst, the yield is much lower. Using phen and NMI as ligands for CuCl increased the yield by 3.6 fold. Another advantage of this system is that the catalytic activity persisted for a long time during the reaction and corrosion of the catalyst was also avoided. Earlier, PdCl2 catalyst was used because of its efficiency in the reaction. However, its susceptibility to deactivation and its high cost posed major hurdles in the commercialization of this process. Because of the utility of carbon dioxide as feedstock, its catalytic conversion to carbonates and polycarbonates has received much attention. Conventional solid catalysts are generally used for this reaction. These include mesoporous oxides and zeolites. But, there is still a need to develop superior performance catalysts. This requires the development of materials with a different structure, compositional adsorption properties, transport properties etc. that are superior than metal oxides, zeolites, metal phases and other such catalysts. In line with this endeavor, Miralda et al. investigated the catalytic properties of zeolitic imidazolate frameworks (ZIFs) (14). These frameworks are crystalline microporous materials with novel desirable properties such as open porous framework with large pore volume, uniform micropores, high surface area etc., that make them desirable candidates for catalytic processes. The catalytic activity of amine functionalized ZIF-8 (zeolitic imidazole framework-8) catalysts to was applied to synthesize chloropropene carbonate from carbon dioxide and epichlorohydrin (14). No cocatalysts were required in this reaction. Recycling the catalyst during the reaction process led to loss of superior catalytic performance and distinctive crystalline structure of the material. The crystalline instability and catalytic deactivation was attributed to the blocking or poisoning of the active sites of the carbonaceous matter in the catalyst’s pores. Studies have also investigated the synthesis of carbonates such as dipentyl carbonate using basic ionic liquid 1-butyl-3-methylimidazolium hydroxide [bmIm]OH as catalyst in a transesterification reaction. In this reaction, dipentyl carbonate was synthesized by the transesterification of 1-pentanol (1-PeOH) and dimethyl carbonate using alkaline ([bmIm]OH) as catalyst. High catalytic activity was observed and dipentyl carbonate yield of 75.81% was obtained (15). The catalytic activity remained as high as 94% even after using the catalyst repeatedly for five times. Another novel approach towards catalyst development for synthesis of organic carbonates was the use of calcined, rare earth elements incorporated Mg−Al hydrotalcites (HT). The rare earth elements include (Ce3+, La3+, Sm3+, Y3+ and Pr3+). These catalysts were employed for the synthesis of dimethyl carbonate through a transesterification reaction of methanol and propylene carbonate (16). The highest activity was observed for La modified HT, with a propylene carbonate conversion of 65.4 mol % and a dimethyl carbonate selectivity of 88 mol %. The transesterification activity was influenced by the basicity of the catalyst (surface density of basic sites). Only a little loss of activity occurred when the La modified HT catalyst was reused. Park et al. developed a synthetic strategy for the synthesis of aliphatic polycarbonates with high molecular weight (17). For this process, aliphatic diols and dimethyl carbonate were subjected to condensation polymerization. In this reaction, small quantities of sodium alkoxide were used for catalysis. The development of catalysts for such a polymerization reaction for producing polymers with high molecular weight has achieved much attention in the recent past. Earlier, diphenoxytetra-n-butyldistannoxane was used as catalyst for the synthesis of poly(1,4-butylene carbonate). However, an Mn of only 8000 was achieved. 1-n-butyl-3-methylimidazolium-2-carboxylate catalyst also gave a similar result. A higher M­n was achieved with lipase catalyst. With sodium alkoxide catalyst however, an M­n of 100000 – 200000 was achieved. Other aliphatic polycarbonates with high molecular weight were also synthesized using cyclohexane-1,4-dimethanol and 1,6-hexanediol. Most recently, work is in progress on the development of zeolite catalysts, organic catalysts and metal based catalysts. Huang et al. reported an alkaline treatment modification of Y Zeolite for the synthesis of diethyl carbonate. For this purpose, NaY zeolite was treated with NaOH solution and ion exchange with NH4NO3, resulting in the formation of H zeolite (18). CuY was prepared using the treated NaY as support for the catalyst. CuY catalyst with modified NaY supports showed higher conversions than untreated catalyst. The improved catalytic performance was observed because alkaline treatment resulted in zeolite framework defects and dealumination, which in turn caused meso and macropore generation in the Y zeolite. Moreover, there was an increase in the amount of Cu active sites that enhanced the catalytic activity and yield of diethyl carbonate. Aliphatic polycarbonates are synthesized using various catalysts, especially zinc complexes from the copolymerization of carbon dioxide and epoxides. Selective synthesis from epoxides that have electron withdrawing groups such as cyclohexene oxide and propylene oxide is a challenging process. Glycidyl ethers are however rarely used because their copolymerization with CO2 is not possible using common catalysts. Geschwind and Frey investigated the synthesis of poly(1,2-glycerol carbonate) from glycidyl ethers and carbon dioxide using zinc catalyst (7). Sharma et al. investigated a modified zinc based catalyst for the synthesis of ethylene carbonate via the carbonylation of ethylene glycol by urea (19). Zn(NCO)2·2NH was used as catalyst. An yield of 40% was achieved in this process. Figure 2 shows the associative and dissociative reaction mechanisms of this catalyst. Figure 2: Associative and dissociative reaction mechanism of Zn(NCO)2•2NH catalyst. 4. Methodology The aim of this report is to study the current applications and development of catalysts for the synthesis of organic carbonates. Thus, recent literature on the subject and the corresponding data on yields, reaction conditions, catalysts, etc. has been collected, surmised, and analyzed. In order to do this, only publications from American Chemical Society (ACS) have been used. The database of ACS was searched using the search tool offered on its website. Various combinations of the keywords “synthesis”, “organic carbonates”, “yield”, “catalysts”, were used in conjunction with appropriate connectors such as “of”, “using”, etc. The search filters were set to yield only the research papers published from 2003 to 2013 (i.e. the last ten years). Searching the database for “catalysts used in organic carbonate synthesis” yielded more than 3000 research publications. Sorting them in the order of relevance, only those studies were selected that directly dealt with the synthesis of organic carbonates. Other searches were also performed for the literature review without applying the search filter for 2003-2013. Since this paper deals with the development of catalysts for the synthesis of organic carbonates, providing an exhaustive review of all the catalysts for all organic carbonates is impossible owing to the gigantic nature of such data and information. Therefore, to accommodate the information in the present report, the research has been narrowed down to dimethyl carbonate (DMC) synthesis. This is an important organic carbonate that not only has many industrial applications but is also used for the synthesis of several other organic carbonates. For data analysis, the yields of DMC in the presence of different catalysts and reaction conditions have been collected. This was done by first searching for reports on DMC synthesis published in ACS. A search was performed using keywords “DMC” and “synthesis” to find out relevant research on this subject. Data from the research papers was then pooled and used for the analysis. 5. Data and Results This report deals with the development of catalysts in the synthesis of organic carbonates. The various catalysts used for organic carbonate synthesis have already been described in general in the preceding sections. Since an exhaustive review of all the organic carbonates and catalysts used is not possible, only dimethyl carbonate synthesis will be reviewed here. It is the simplest organic carbonate and has a wide array of industrial applications. It is also used for the synthesis of other organic carbonates. This section describes the process of DMC synthesis and catalysts used for the process. The yields of DMC obtained using various catalysts over the years have also been presented. 5.1. DMC Synthesis and Catalysts Used The earliest synthesis of DMC was from carbon monoxide, methanol and oxygen (20). The reaction was catalyzed by cuprous chloride and a slurry reaction system was employed. Maximum DMC yield was obtained when a chlorine/copper ratio of 1 was used. An yield of 15% was achieved. Studies have also investigated the production of DMC from carbon dioxide and methanol using organotin oxides as catalysts (20). However, the catalytic activity of these catalysts is very low because they undergo decomposition in the presence of water, which is a coproduct in the reaction. The use of drying agents to counteract this disadvantage has also been investigated without any significant success. Sakakura et al. thus developed new methodologies to increase the catalytic carbon dioxide conversion efficiency (21). They achieved this through molecular catalysis. Instead of removing water from the reaction (eqn. 4), they dehydrated MeOH and then reacted it with carbon dioxide to produce DMC. ---------- (4) They explored a combination of promoters and metal alkoxides as catalytic system for the conversion of supercritical carbon dioxide with trimethyl orthoacetate. The reaction is dependent on the structure of the catalytic metal alkoxide. DMC yields differed with the catalyst used and were as low as 1% and as high as 70% (21). Dimethyl carbonate synthesis was also carried out by reacting carbon dioxide with acetals using metal catalysts such as tin methoxides. However, the catalytic activity subsides after a few turnovers. Moreover, the synthesis of DMC from ortho esters such as trimethyl orthoacetate, as reported earlier, is not advantageous commercially because of expensive nature of ortho esters to be used as industrial raw materials. In 1999, Sakakura et al. reported another methodology of DMC synthesis from carbon dioxide and acetals catalyzed by metal complexes (22). In this process too, the yields were as low as 1 and as high as 88%. DMC is also synthesized from epoxides, methanol and carbon dioxide. Cui et al. explored the kinetics of one-pot synthesis of DMC from supercritical carbondioxide using methanol and ethylene epoxide as raw materials (23). For this reaction, they used a mixture of K2CO3 and KI. DMC synthesis was generally produced through two step synthesis processes such as phosgene methanolysis, methanol oxidative carbonylation, etc. However, two step processes suffer from high energy consumption, production costs and investment. Thus, one pot synthesis is suggested as an alternative route. Using a mixed catalyst of K2CO3 and KI, one pot synthesis of DMC could be realized. One pot synthesis was also performed using methanol, ethylene oxide and supercritical carbon dioxide using a mixture of KI and K2CO3 catalyst giving yields more than 73% (24). The synthesis of DMC from methanol and urea has also been explored. ZnO is found to have the highest catalytic activity in this synthesis (25). Wang et al. optimized the reaction conditions for this synthesis in a batch process and attained a yield of 30% (25). ZnO was found to have high stability and retained its catalytic activity upon reuse. Other methodologies have also been developed for the synthesis of DMC from urea and methanol. Wang et al. proposed a catalytic distillation technique to increase DMC yield by minimizing the side reactions and preventing an unfavorable equilibrium. Through catalytic distillation over a zinc based catalyst, yields as high as 60-70% were achieved (26). They used a catalytic distillation reactor to carry out chemical reactions and separations over ZnO-Al2O3 (zinc-based heterogeneous catalyst) in one apparatus. This is an extension of their earlier study in which an yield of only 30% was achieved. Wang et al. also carried out the modeling of this catalytic distillation process reported in another paper (27). They developed a non equilibrium model over a solid base catalyst. They illustrated that the catalytic distillation process is superior in that the products could be removed to prevent the reverse reaction. Abimanyu et al. investigated the use of mixed oxide catalysts of MgO and CeO2 in DMC synthesis (28). They prepared mixed oxides of cerium and magnesium through a modified co-precipitation method using ionic liquids at room temperature. These catalysts were used for the co-generation of ethylene glycol and dimethyl carbonate through the transesterification of methanol with ethylene carbonate. The study showed that by adding a template material, i.e. ionic liquid, in the coprecipitation results in an increase in the catalyst’s surface area and reduction in particle size. The surface basicity of the catalyst particles was also found to increase. 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) ionic liquid was found to be the most effective in the preparation of MgO-CeO2 catalyst, which was nanoporous and had high catalytic activity. A DMC yield of 56.6% was achieved. A popular mode of DMC synthesis is the reaction of methanol and carbon dioxide. Eta et al. reported DMC synthesis from methanol and carbon dioxide using KCl doped ZrO2 (29). The reaction was carried out using chemical traps to restrict water in order to overcome the thermodynamic limitations of the reaction. Magnesium played a major role in catalysis. Carbonated magnesium methoxide (CMM) was formed in the process and was adsorbed on ZrO2. The oxygen atom of ZrO2 migrated to the surface methoxy groups of the carbonated magnesium methoxide to form DMC. This also resulted in the formation of MgO which then underwent a reaction with methanol to regenerate magnesium methoxide and produce water. The water then reacted with a dehydrating agent and the equilibrium shift resulted in a higher DMC yield. Through method, a yield of 7.2% was achieved. A similar reaction of methanol with carbon dioxide for DMC synthesis was carried out by Zhang et al. (30). The reaction was carried out over CexZr1-xO2 and [EMIM]Br/Ce0.5Zr0.5O2. This solid solution had a bimodal pore structure. A 1:1 Ce/Zr molar ratio was found to result in highest catalytic activity and a yield of 7.9% was achieved. By combining this catalyst with a dehydrating agent, a higher yield of 10.4% was achieved. Loading 1-ethyl-3-methylimidazolium bromide ionic liquid Ce0.5Zr0.5O2 doubled the catalytic activity. Bustamante et al. carried out modeling of the gas phase behavior and chemical equilibrium of the popular reaction between methanol and carbon dioxide for the synthesis of dimethyl carbonate (31). They showed that the conversion of MeOH is increased at higher pressure and lower temperature. In the one-step process of DMC synthesis from carbon dioxide and methanol, many catalysts are used successfully. These include organometallic compounds, n-butyl(alkoxy) stannanes, carbodiimides, molecular sieves, metal oxides, etc. Apart from the synthesis from methanol and carbon dioxide, other reactions for the synthesis of DMC have also been investigated. Zhu et al. carried out the synthesis of DMC by oxidative carbonylation using a Co-Schiff Base/Zeolite recyclable catalyst. Zeolite-encapsulated Co complexes were highly stable and were efficient catalysts. A yield of 25.4% was achieved (32). As described earlier, another novel approach towards catalyst development for synthesis of organic carbonates was the use of calcined, rare earth elements incorporated Mg−Al hydrotalcites (HT) (16). These catalysts were employed for the synthesis of dimethyl carbonate through a transesterification reaction of methanol and propylene carbonate and the rare earth elements used include (Ce3+, La3+, Sm3+, Y3+ and Pr3+). The highest activity was observed for La modified HT, with a yield of 65.4%. The catalyst was stable and reusable. Other reactions for DMC synthesis include that of methanol with carbon monoxide through electrochemical carbonylation using CuCl2−2,2′-bipyridyl catalyst (33). DMC has also been synthesized from methyl carbamate and methanol using zinc compounds as catalysts in a batch reactor (34). ZnCl2 was found to have the highest catalytic activity and a yield of 33.6% was obtained. 5.2. Yields of DMC using Different Catalysts The DMC yields for the reactions discussed in the previous section have been tabulated in table 1 in appendix. The yields are represented chronologically and the reaction and reactants have also been presented. 6. Interpretation From table 1 it is inferred that the highest yield achieved was 88% using Bu2Sn(OMe)2 as catalyst in the transformation of carbon dioxide using acetals. The lowest yield was achieved in the catalytic conversion of carbon dioxide using trimethyl orthoacetate and Bu3Sn(OMe) as catalyst. The average yield taking all the yields into consideration was found to be 40.08% with a standard deviation of 26.3. Only a change in the kind of catalyst used, keeping the other reactants same resulted in a huge variation in the yield. For instance, the yields achieved in catalytic conversion of carbon dioxide using trimethyl orthoacetate was as low as 1% with Bu2Sn(OMe)2 as catalyst and as high as 70% using Bu2Sn(OMe)2 with Bu4PI as additive. Moreover, urea methanolysis using ZnO catalyst gave a low yield of 30% while a high yield of 70% was achieved through catalytic distillation over ZnO. There hasn’t been a large amount of change in the DMC yield over the years as is evident from the yield of 15-20 in 1980-1998 and 25.4 in 2009. The data shows that there is no direct increase in yield with time, although research on catalysts has been full-fledged. This can be explained on the basis of the motivation behind the development of these catalysts. While earlier studies focused on phosgene based routes and transition metal based catalysts, these were subsequently abandoned because of their high eco toxicity. Research later on directed towards more environment friendly catalysts and synthesis routes in order to develop eco-friendly synthetic routes for organic compounds. 7. Conclusion This report explored the development of catalysts for the synthesis of organic carbonates. Hundreds of catalysts have been investigated over the past years and synthesis of organic carbonates has become more inclined towards eco-friendly routes. Most of the catalysts used for organic carbonate synthesis are metal based. Nanoporous compounds such as zeolite have also been a main area of focus. Earlier, transition metal, palladium and copper metal catalysts were used more frequently. However, their disadvantages such as cost, instability, susceptibility to deactivation and corrosion etc., in addition to their environmental toxicity led to the focus on development of more stable, reusable, non-corrosive and ecofriendly catalysts. Nomenclature [bmIm]OH – 1-butyl-3-methylimidazolium hydroxide 1-PeOH – 1-pentanol ACS – American Chemical Society BMIMB4 – 1-butyl-3-methyl imidazolium tetrafluoroborate CMM – Carbonated magnesium methoxide CO2 – Carbon dioxide DMC – Dimethyl carbonate HT – Hydrotalcites NMI – N-methyl imidazole Phen – 1,10-phenanthroline ROP – Ring opening polymerization ZIFs – Zeolitic imidazolate frameworks References 1. (Schaffner et al. 2010) 2. (Shaikh and Sivaram 1996) 3. (Verevkin et al. 2008) 4. (Jiang et al. 2007) 5. (Zhang and Grinstaff 2013) 6. (Zhang, Liu and Yue, 2012) 7. (Geschwind and Frey 2013) 8. (Yang et al. 2004) 9. (Endo et al. 2005) 10. (Beattie et al. 2012) 11. (Zhao et al. 2004) 12. (Zhao et al. 2008) 13. (Xiong et al. 2009) 14. 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No. Year Reaction Reactants Catalyst Used Maximum Yield (%) Reference 1 1980 Oxidation of CO with O2 in methanol Carbon monoxide, oxygen, methanol Cuprous chloride 15 20 2 1998 Catalytic conversion of carbon dioxide using trimethyl orthoacetate Carbon dioxide, trimethyl orthoacetate Bu2Sn(OMe)2 20 21 3 1998 Catalytic conversion of carbon dioxide using trimethyl orthoacetate Carbon dioxide, trimethyl orthoacetate Bu3Sn(OMe) 1 21 4 1998 Catalytic conversion of carbon dioxide using trimethyl orthoacetate Carbon dioxide, trimethyl orthoacetate Me2Sn(OMe)2 7 21 5 1998 Catalytic conversion of carbon dioxide using trimethyl orthoacetate Carbon dioxide, trimethyl orthoacetate Bu2Sn(OMe)2 with Bu4NOTs additive 48 21 6 1998 Catalytic conversion of carbon dioxide using trimethyl orthoacetate Carbon dioxide, trimethyl orthoacetate Bu2Sn(OMe)2 with Bu4PI additive 70 21 7 1998 Catalytic conversion of carbon dioxide using trimethyl orthoacetate Carbon dioxide, trimethyl orthoacetate Cp2Ti(OMe)2 with Bu4PI additive 37 21 8 1998 Catalytic conversion of carbon dioxide using trimethyl orthoacetate Carbon dioxide, trimethyl orthoacetate Cp*2Ti(OMe)2 with Bu4PI additive 47 21 9 1999 Transformation of carbon dioxide using acetals Carbon dioxide, acetals Bu2Sn(OMe)2 57 22 10 1999 Transformation of carbon dioxide using acetals Carbon dioxide, acetals Bu2Sn(OMe)2 88 22 11 2003 One pot synthesis from supercritical carbon dioxide Ethylene oxide, methanol, carbon dioxide Mixture of KI and K­2CO3 73 24 12 2005 Urea methanolysis Urea, methanol ZnO 30 25 13 2007 Urea methanolysis Urea, methanol catalytic distillation over ZnO-Al2O3 catalyst 70 26 14 2007 Transesterification of methanol with ethylene carbonate Ethylene carbonate, methanol MgO-CeO2 mixed oxide catalyst modified with ionic liquid 56.6 28 15 2010 Direct synthesis from methanol and carbon dioxide Methanol, carbon dioxide KCl doped ZrO2 7.2 29 16 2011 Direct synthesis from methanol and carbon dioxide Methanol, carbon dioxide CexZr1-xO2 with a dehydrating agent 10.4 30 17 2009 Direct synthesis from methanol and carbon dioxide Methanol, carbon dioxide Co-Schiff Base/Zeolite 25.4 32 18 2012 Transesterification of propylene carbonate with methanol Propylene carbonate, methanol La modified Hydrotalcite 65.4 16 19 2008 DMC synthesis from methyl carbamate and methanol in batch reactor Methanol, methyl carbamate ZnCl2 33.6 34 Read More