Recent advances in dialkyl carbonates synthesis and applications.

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Cite this: DOI: 10.1039/c4cs00374h

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Recent advances in dialkyl carbonates synthesis and applications Shouying Huang, Bing Yan, Shengping Wang and Xinbin Ma* Dialkyl carbonates are important organic compounds and chemical intermediates with the label of ‘‘green chemicals’’ due to their moderate toxicity, biodegradability for human health and environment. Indeed, owing to their unique physicochemical properties and versatility as reagents, a variety of phosgene-free processes derived from CO or CO2 have been explored for the synthesis of dialkyl carbonates. In this critical review, we highlight the recent achievements (since 1997) in the synthesis of dialkyl carbonates based on CO and CO2 utilization, particularly focusing on the catalyst design and fabrication, structure–function relationship, catalytic

Received 4th November 2014

mechanisms and process intensification. We also provide an overview regarding the applications of dialkyl

DOI: 10.1039/c4cs00374h

carbonates as fuel additives, solvents and reaction intermediates (i.e. alkylating and carbonylating agents). Additionally, this review puts forward the substantial challenges and opportunities for future research

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associated with dialkyl carbonates.

1. Introduction Dialkyl carbonates have attracted widespread attention during the last decades owing to their extensive applications in most aspects of daily life. Among them, short-chain dialkyl carbonates, particularly dimethyl carbonate (DMC) and diethyl carbonate (DEC), are gaining momentum due to their biodegradability, low toxicity and low bioaccumulation, as well as excellent solubility. As building blocks for the organic synthesis of a variety of chemicals, they have been used as alternatives for toxic and carcinogenic

Key Laboratory for Green Chemical Technology of Ministry of Education, Collaborative Innovation Center of Chemical Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China. E-mail: [email protected]; Fax: +86-22-27403389

Shouying Huang

Shouying Huang obtained her BS (2008) and PhD (2013) degrees in chemical engineering from Tianjin University under the supervision of Professor Xinbin Ma. She is currently a postdoctoral research fellow with Professor Weiping Huang at Nankai University. Her research focuses on catalytic synthesis of organic oxygenated compounds and design of microporous and mesoporous structured zeolite catalysts.

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compounds such as phosgene, dimethyl sulfate and alkyl halide for carbonylation and alkylation reactions, to improve the environmental benefits and process safety.1–3 Additionally, they are widely used as raw materials for the manufacture of agrochemicals, pharmaceuticals, antioxidants etc., and as potential solvents for coating, adhesives and electrolytes in lithium ion batteries.4,5 A report of the Nexant’s ChemSystems in 2012 points out that according to end-use, 51% of DMC is applied for polycarbonate production, 24% for solvent, and the rest for other applications. Recently, oxygen-containing additives have been extensively applied to improve combustion and reduce engine emissions. Due to its high water solubility and persistence in the environment, methyl tert-butyl ether (MTBE) is banned completely or partially in United States and some other countries. Consequently, short-chain carbonates have become strong contenders to supersede MTBE

Bing Yan

Bing Yan obtained her BS (2009) degree in applied chemistry from Shaanxi University of Science Technology and her PhD degree (2014) in chemical engineering from Tianjin University under the supervision of Professor Xinbin Ma. She has been a lecturer of Tianjin University of Science and Technology since 2014. Her research interests in energy and green chemical processes including synthesis of dialkyl carbonates and olefins.

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because of their higher oxygen content, good blending properties, favorable fuel/water partition coefficient as well as negligible effects on the environment. Considering the increased consumption of fuel, there is an intriguing potential market for DMC and DEC.6–8 In view of various synthetic methods of dialkyl carbonates, great efforts have been undertaken under the green chemistry initiatives, which focus on atom-efficiency, waste avoidance, efficient and stable catalyst, nonuse of hazardous toxic substances, reduced investment and increased safety.9 With the global phase-out of the phosgenation processes, the ENIChem process (liquid-phase oxycarbonylation of MeOH),10 the UBE process (methylnitrite (MN) carbonylation)11 and the Texaco process (transesterification of MeOH and ethylene carbonate)12 have been transferred to the industrial scale successively. However, the high price of DMC and DEC caused by the production cost are the main limitation of their utilization as fuel additives throughout the world. In addition, the corrosion, separation and catalyst deactivation remain bottlenecks of these processes despite recent significant improvements. On the other hand, new synthetic routes are put forward under the framework of sustainable developments. Voluminous published literature suggests that the present interest primarily focuses on the rational design of highly efficient and stable catalysts and technological optimization for synthesis of short-chain dialkyl carbonates. There are several excellent reviews focusing on DMC or DEC production, highlighting the limited synthetic approaches.2,4,8,13–16 On the other hand, the description of the development of applications shows lack of systematic and comprehensive methodologies. Oxidative carbonylation and direct or indirect routes based on CO2, have been considered as potential alternatives for the phosgenation process due to their high utilization rate of carbon source and environmental benefits. Nowadays, different synthesis routes are adopted in the available facilities and in those undergoing construction or scheduled. For instance, as one of the biggest PC manufacturers in the world, General Electric Company (GE) applied liquid-phase oxycarbonylation to produce DMC. In China,

more than 90% of DMC is produced by transesterification of MeOH and propylene carbonate (PC). Lately, significant progress in urea alcoholysis has been achieved and applied to an industrial-grade pilot plant. Therefore, although the transesterification route is relatively mature, other routes have gained momentum for development. Even for direct carbonylation of alcohol, which is extremely constrained by thermodynamic limitations, great efforts have been made in the development of novel catalysts and effective dehydration agents. As reported in recent literature, a significant improvement of methanol conversion can be achieved in combination with a dehydration system. When we analyze these routes by focusing on different aspects, such as technological maturity, local resource composition and economical or environmental performances, the optimal route is prone to be different. For example, oxidative carbonylation might be more favorable in respect of thermodynamics, and it is attractive especially for the areas with abundant resource of syngas. Medeiros and co-workers compared six synthesis routes of DMC in terms of economic and environmental performances. They suggested that oxidative carbonylation and direct carbonylation of methanol are greener routes, however, the transesterification and alcoholysis of urea should be eco-technologies when considering CO2 sequestration potential.17 In this review, we have attempted to cover the literature regarding synthetic methodologies and applications of short-chain carbonates from 1997 until now. As shown in Fig. 1, we divided the presented synthetic routes into three parts: conventional phosgene route, oxidative carbonylation, and direct or indirect synthesis from CO2. In comparison with other reviews, we particularly focus on the development of catalyst rational design, catalytic mechanism, and process intensification. As the simplest dialkyl carbonates, the synthesis and applications of DMC and DEC are similar and representative. However, the physicochemical properties such as solubility, polarity and reactivity etc., changes gradually with further increasing the length of the carbon chain, which results in the emergence of differences in synthesis and applications. Here, we present a

Shengping Wang earned her BS (1994) and MS (2000) degrees from Hebei University of Technology and her PhD degree (2003) from Tianjin University, all in Chemical Engineering. She has been a professor of Tianjin University since 2014. She has interests in capture and conversion of carbon dioxide, and synthesis of organic carbonates.

Xinbin Ma received BS and MS degrees in Chemical Engineering from Tianjin University. He obtained his PhD degree in Chemical Engineering from Tianjin University in 1996 under the supervision of Hongfang Chen and Genhui Xu. He continued his academic career as an assistant professor in the same department, and was promoted to full professor in 2004. His main scientific interests are conversion of C1 molecules and Xinbin Ma synthesis of organic carbonates and oxalates. He was named as a NSFC Distinguished Young Scholar in 2014 and Yangtze River Scholar in 2015.

Shengping Wang

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are listed in Tables 1 and 2). The solubility of dialkyl carbonates in different media depends on their length of carbon chain; most of them are slightly soluble in water and easily dissolved in polar organic solvents, such as ethanol. Compared to MTBE, the insolubility in water as well as biodegradability makes them ideal candidates for fuel additives with almost no environmental pollution.8 Based on toxicologic studies, DMC has negligible irritating and mutagenic effects either by contact or inhalation.18 With an increase in the alkyl chain length, the toxicity of dialkyl carbonate increases. For example, DEC is classified as ‘‘an experimental tumorigen and teratogen’’ and ‘‘mildly toxic by subcutaneous route’’, and DPrC is considered as harmful and irritant by inhalation or in contact with skin and eyes. Fig. 1

Synthesis routes of dialkyl carbonates.

3. Synthesis by oxidative carbonylation

detailed discussion about short-chain dialkyl carbonates (DMC and DEC), which is enlightening for the research on synthesis and application of other dialkyl carbonates. Additionally, a brief introduction of dipropyl carbonate (DPrC) as well as ethyl methyl carbonate (EMC) is provided in this paper.

2. Physico-chemical properties The industrially important dialkyl carbonates, especially shortchain symmetrical dialkyl carbonates, are all colorless, transparent and flammable liquids with a pleasant odor and a relatively mild toxicology (detailed physical and toxicologic properties Table 1

Physico-chemical properties of DMC, DEC and DPrC

Molecular mass Melting point (1C) Boiling point (1C) Density (g cm3) Flash point (1C)20 Gasoline/water distribution coefficient8 wt% O Solubility in water (g L1) Vapor pressure (25 1C, mmHg) Enthalpy of vaporization24 (kJ mol1) Molar refractivity (cm3) Molar volume (cm3) Research octane numbera 8 Motor octane numbera 8 a

Up to the 1980s, traditional industrial methods for symmetrical dialkyl carbonate production were carried out via phosgene alcoholysis.27,28 Despite the high yield, employment of toxic phosgene gives rise to equipment corrosion as well as safety and environmental problems. With the global phase-out of the phosgenation processes, the development of phosgene-free routes for dialkyl carbonates synthesis has attracted great interests worldwide from both academic and industrial communities. The direct oxidative carbonylation of alcohols to dialkyl carbonates uses CO, O2 and alcohols as raw materials (eqn (1)). It is the most favorable route in respect of thermodynamics and the operating conditions are relatively moderate. Additionally, the formation of water as the only co-product is considered a green

DMC

DEC

DPrC

90.08 4.61 90.317 1.0698 21.7 (open cup) 18.3 (closed cup) 2.0 53.3 13917 56.021 38.02 0.19 19.4621 87.921 125–131 100–109

118.13 438 1268 0.9758 46 (open cup) 31 (closed cup) B20 40.7 Insoluble4 11.522 44.35 0.22 28.7322 120.922 110–112 95–103

146.19 4119 168.28 0.944 55 (open cup) 64 (closed cup) 32.8 4.119 1.65423 53.22 0.29 37.9923 153.923 110–113 96–104

Gasoline blending values at 3–5 vol%.

Table 2

Comparison of toxicologic properties between DMC, DEC, DPrC, phosgene and dimethyl sulfate

Property

DMC1

DEC25

DPrC26

Oral acute toxicity (rats) Acute toxicity per inhalation or subcutaneous Mutagenic properties Biodegradability Irritability properties

LD50 13.8 g kg1 LC50 140 mg L1 (4 h, inhalation) None 490% (28 days) None

LD50 1570 mg kg1 LD50 8500 mg kg1 (subcutaneous) Equivocal tumorigenic 75% (27 days) None

LD50 980 mg kg1


Phosgene1 LC50 16 mg m3 (1.25 h, inhalation)

Mutagenic Rapid hydrolysis Corrosive

Dimethyl sulfate1 LD50 440 mg kg1 LC50 1.5 mg L1 (4 h, inhalation) Mutagenic Rapid hydrolysis

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benefit of the oxidative carbonylation method compared with the phosgenation approach.29–31 More importantly, the raw materials and CO can be derived from coal, natural gas, shale gas and biomass. Therefore, it is a competitive synthesis route for some countries, such as China, with large reserves of coal and shale gas. For now, the development of industrial catalysts with not only excellent performance but also long service life became the key factor. UBE developed a gas-phase process on the synthesis of DMC from CO and MN over Pd-based catalyst, which was considered as an indirect oxycarbonylation of alcohol. The introduction of alkyl nitrite as an intermediate made the process show higher selectivity and activity, lower operation pressure, and simpler product separation. If alkyl nitrite can be recovered in a closed cycle, the indirect oxycarbonylation can be more easily applied in industry despite the utilization of hazardous materials. In the following subsections, we will describe the current status of the above mentioned oxycarbonylation routes. 2ROH + CO + 1/2O2 - (RO)2CO + H2O R = –CH3, DrG298K = 250.8 kJ mol1, DrH298K = 311.9 kJ mol1; R = –C2H5, DrG298K = 247.5 kJ mol1, DrH298K = 309.5 kJ mol1. (1) 3.1

Direct oxidative carbonylation

According to the state of alcohols, the direct oxidative carbonylation method can be divided into two classes: liquid-phase method and gas-phase method. The gas-phase oxidative carbonylation is directly derived from the liquid-phase method based on the same reaction (eqn (1)). Chlorine-containing catalysts are the most widely used catalysts in both processes, and chlorine plays an important role in the catalytic cycle. Generally speaking, the process in liquid-phase possesses high space time yield (STY) of DMC/DEC, but suffers from several drawbacks, such as product and catalyst separation, equipment corrosion, and high operating pressure. In order to solve these problems, the gas-phase method was developed. In comparison with the liquid-phase method, the gas-phase method operates at relatively low pressure and omits

Table 3

separation and recycling of catalysts. Additionally, the generated H2O can be removed readily in the gas-phase process, which diminishes negative effects on catalysts. 3.1.1 Liquid-phase method 3.1.1.1 Main catalysts. Cuprous chloride (CuCl) was first used for DMC synthesis in the liquid-phase by the Enichem Company in 1983.32 However, this technology suffers from severe corrosive problems due to the presence of the chloride anion in the system. Consequently, attempts were made for heterogenization of the homogenous catalysts by immobilizing CuCl or CuCl2 on polymer supports or N-donor ligands (Table 3).33–43 In these catalysts, the copper salts were stabilized through the coordination between the copper atom and the nitrogen atoms of the polymer supports or N-donor ligands. Polymeric ligands were used as supports to stabilize CuCl or CuCl2 and showed excellent catalytic performance and good heat resistance.33–37 Additionally, a negligible corroding effect and favorable activity in recycling were observed. Among these polymer supported CuCl or CuCl2 catalysts, poly(vinylpyridine) supported CuCl2 catalyst has found commercial applications for the DMC synthesis via oxidative carbonylation of MeOH.33,44 N-donor ligands, including some Schiff bases, were also applied for coordinating with CuXn (X = Cl, Br, I; n = 1, 2) and the corrosion of the reaction system was efficiently inhibited.38–43 Among these N-donor ligands, the addition of N-methylimidazole (NMI) led to the optimal methanol conversion (450%) and selectivity (490%) in DMC production.38 Particularly, 1,10-phenanthroline (phen) was the most effective promoter in terms of both catalytic activity and corrosion inhibition.40 The selectivity of DMC was kept at about 99.5% and the catalytic activity showed no significant change after 16 runs over the CuCl/phen catalyst. Li and co-workers found that phen and NMI presented a synergic effect on the catalytic activity in the synthesis of DEC.39 The ethanol conversion over CuCl/phen/NMI reached 15.2%, which was 3.6-fold that of pure CuCl, and the selectivity of DEC was 99.0%. Additionally, several other metal–Schiff base complexes, such as Co–Schiff base and Au(III)–Schiff-base complexes, were applied for DMC synthesis.41,42 A Y-zeolite-encapsulated Co(salophen) {(N,N0 -bis(salicylidene)-ophenylenediamine)cobalt) catalyst showed excellent activity

Catalytic activities of CuCl or CuCl2 immobilized with polymer or N-donor ligandsa

Reaction conditions Catalyst 1

CuCl2 (0.15 mol L Cu) Pbpy–CuCl2 (0.15 mol L1 Cu) Ppy–CuCl2 (0.15 mol L1 Cu) Bpy–CuCl2 (0.15 mol L1 Cu) PVP–CuCl2 (0.18 mol L1 Cu) (NMI)4CuCl2 (0.3 mmol) CuCl (0.2 mol L1 Cu) CuCl/phen (0.2 mol L1 Cu, nCu/nphen = 1 : 1.5) Co(salophen)-Y (1.0 g) [AuCl2(phen)]Cl/KI (0.15 mmol) a

Pbpy:

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; Pby:

; Bpy:

MeOH/mmol

T/K

P/MPa

PCO/PO2

t/h

MeOH conv. (%)

DMC sel. (%)

Ref.

247.5 247.5 247.5 247.5 247.5 30 1980 1980 40 740

413 413 413 413 433 393 393 393 393 393

2.45 2.45 2.45 2.45 3 5.25 2.4 2.4 3 3

11.5 : 1 11.5 : 1 11.5 : 1 11.5 : 1 11.5 : 1 20 : 1 2:1 2:1 2:1 2:1

2 2 2 2 5 4 2 2 4 4

72 72 76 72 15.5 55 6.4 23.7 25.4 10.8

97 94 94 97 99.6 95 96.4 98.3 99.5 98.5

35 35 35 35 37 38 40 40 41 42

; PVP:

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Catalytic performance of CuCl with or without ILs, CILs and SBA-15 supported CILs


Reaction conditions Catalyst

MeOH/g

T/K

P/MPa

PCO/PO2

t/h

MeOH conv. (%)

DMC sel. (%)

Ref.

CuCl (1 mmol) CuCl (1 mmol) [CuCl + 4 Im12] (1.5 mmol Cu) [Cu(Im12)2][CuBr2] (1.5 mmol Cu) CuBr2 (1 mmol Cu) CuBr2/SBA-15 (1 mmol Cu) CuBr2–PyIL/SBA-15 (1 mmol Cu)

4.0 4.0 0.96 0.96 7.7 7.7 7.7

393 393 393 393 393 393 393

2.4 2.4 5.3 5.3 2.4 2.4 2.4

2:1 2:1 50 : 3 50 : 3 2:1 2:1 2:1

4 4 4 4 2 2 2

9.0 17.2 48 62 10.5 8.2 17.0

97.3 97.8 78 89 86 85.3 97.5

46 46 47 47 48 48 48

for DMC synthesis, and could be reused five times without loss of activity.41 Co(salophen) catalyst was evaluated for DEC synthesis and also exhibited good catalytic activity and corrosion inhibition ability.43 The conversion of ethanol and the DEC selectivity reached 15.8% and 99.5%, respectively. Since CuCl is only slightly soluble in methanol, which is unfavorable for methanol to contact with the catalyst efficiently, ionic liquids (ILs) have been used as reaction media to improve the solubility of CuCl and to enhance the activity.45,46 Particularly, N-butylpyridinium tetrafluoroborate ([Bpy]BF4) acted as an effective reaction medium for facile conversion of methanol to DMC via the oxidative carbonylation reaction. The [Bpy]BF4-meditated CuCl catalyst system could be reused at least five cycles with the same selectivity and only slight loss of catalytic activity due to leaching of the catalyst.46 In addition, copper-containing catalytic ionic liquids (CILs) were also found active for the oxidative carbonylation of methanol to DMC.47,48 Sundermeyer and co-workers synthesized and characterized three types of CILs, and the results are listed in Table 4.47 However, the products and unreacted methanol need to be separated from the CILs media by distillation, which is an energy intensive process. CuBr2-PyIL immobilized by SBA-15 (PyIL = 3-trimethoxysilylpropylpyridinium chloride) was found to be more active and selective than CuBr2 and CuBr2/SBA-15 catalysts in DMC synthesis (Table 4).48 Although the CuBr2PyIL/SBA-15 catalyst deactivated rapidly during the initial three runs, it still maintained a comparable activity to the fresh CuBr2/SBA-15 catalyst. Y-zeolite was also employed as an efficient support for heterogenization of CuCl2.49 The activity of the Cu/Y catalyst was lower than the homogeneous CuCl2 catalyst. However, upon the addition of alkali hydroxides, the catalytic activity increased obviously, and the optimal methanol conversion was 42.7% with DMC selectivity of 91.5% at a Na/Cu molar ratio of 1.0.49 Oxides, e.g. SiO2–TiO2, were also used as supports for DMC synthesis by Ren et al.50 The catalyst was synthesized by exchanging Cu+ ions with the Brønsted-acid sites on SiO2–TiO2, showing a promising catalytic property and good corrosion inhibition ability. The incorporation of tetrahedral Ti(IV) species into the silica matrix could enhance the interaction of copper species with the oxide support, and thus improve the catalytic performance.51,52 The catalyst prepared by a sol–gel method showed better catalytic performance and the corrosion problem was reduced, which was attributed to the highly dispersed Cu+ active centers as well as the lower chloride content.52


3.1.1.2 Reaction mechanism. It has been proposed that the oxycarbonylation of methanol followed a two-step redox mechanism, through a copper methoxychloride intermediate (eqn (2) and (3)).53 In the first step, the CuCl was oxidized by oxygen into copper methoxychloride. Secondly, the copper methoxychloride was reduced by CO, and the DMC was formed. In this step, the CuCl phase was regenerated and a new catalytic cycle was initiated. 2CuCl + 2CH3OH + 0.5O2 - 2Cu(OCH3)Cl + H2O 2Cu(OCH3)Cl + CO - (CH3O)2CO + 2CuCl

(2) (3)

A mechanism of DMC synthesis on polymer or N-donor ligand coordinated CuCl2 catalysts was also proposed, as displayed in Scheme 1. One pathway was that two Cu(II) species were revolved in the reaction (Path I). DMC was produced through interaction of the two intermediates CuCl(COOCH3) and CuCl(OCH3). The reduced Cu(I) species were then oxidized to Cu(II) by O2. The other pathway was that one DMC molecule was synthesized through the Cu(II)/Cu(0) redox cycle over one CuCl2 molecule (Path II). In this case, the stability of Cu(0) needs to be considered. Based on the mechanisms proposed above, Sundermeyer and co-workers have taken the activation of oxygen and CO into consideration38 and N-donor ligands were considered to play an important role in the activation of CO.38 Wang’s group has investigated the dissociation of O2 and activation of CO on CuCl(111) surface through Density Functional Theory (DFT) study.54,55 The calculation results suggested that solvents, such

Scheme 1 A proposed reaction mechanism of DMC synthesis on polymer or N-donor ligand coordinated CuCl2 catalysts. Reproduced with permission from ref. 33. Copyright 2000, Elsevier.

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as liquid paraffin and methanol, were in favor of the C–O bond activation, which might improve the reactivity of oxidative carbonylation.54 Their results have also shown that the oxygen species adsorbed on a CuCl(111) surface formed a superoxo species (O2), which contributed to improving the catalytic activity of CuCl for the formation of CH3O.55 In spite of high yield of dialkyl carbonates in liquid-phase catalysis, the problems of catalyst separation and equipment corrosion cannot be solved. Thus, many researchers have focussed on the development of gas-phase methods due to facile separation and mitigated corrosion of devices. 3.1.2 Gas-phase reaction. Gas-phase oxidative carbonylation of alcohols to synthesize carbonates is a promising and competitive method, which was established at the end of the 1980s by the Dow Chemical Company.56,57 This process initially proceeded in the presence of copper chloride compounds (CuCl2 or copper methoxychloride-based pyridine complex) supported on activated carbon (AC) catalysts. Chlorine element plays a crucial role in the reaction. The catalysts can be divided into two categories: chlorine-containing and chlorine-free catalysts. Keller et al. have published an excellent review on the DMC synthesis by 2009 regarding catalysts, mechanisms and industrial processes.15 Up to now, there is a new development for innovative catalysts and reaction mechanism research. Besides introducing the research progress in respects of catalysts, mechanisms and industrial processes for DMC/DEC synthesis, this review pays more attention to the catalyst design and the analysis and comparison of reaction mechanisms. 3.1.2.1 Chlorine-containing catalysts Progress in catalysts. CuCl2/AC catalyst was firstly exploited by the Dow Chemical Company in 1988.57,58 Then the effect of different types of alkali promoters on catalyst activity was investigated. It was found that the addition of the alkali metals influenced the formation and crystal form of copper chloride hydroxide which was in correlation with the catalytic activity. The optimal molar ratio of OH/Cu was 0.5–1.0.59 Afterwards, PdCl2 was introduced into the CuCl2/AC catalyst and the CuCl2– PdCl2 bimetal catalyst (Wacker-type catalyst) showed excellent catalytic property.60,61 Then the Wacker-type catalyst sparked a research upsurge. Ma and co-workers have studied the role of Pd addition through DFT calculation.62 The introduction of Pd caused electron repopulation on the surface and lowered the energy barrier for methanol oxidation. The optimal Pd/Cu ratio was 1 : 17, which was in good agreement with the experimental observation that the PdCl2–CuCl2 catalyst with Pd/Cu = 1 : 20

Table 5

showed the best performance. The next discussion will be presented in the following order: (1) the effect of alkali promoters and the nature of formed active components; (2) the effect of carbon support surface properties; and (3) other supported chlorinecontaining catalysts. For CuCl2 or CuCl2–PdCl2 supported on AC catalyst, the effect of the addition of alkaline promoter was intensively studied (Table 5).60,63–68 Among alkali-metal promoters (Li, Na and K) in the form of respective hydroxides or acetates, the catalyst promoted by CH3COOK exhibited the best catalytic property, because CH3COOK could inhibit the loss of chlorine and improve the charge density of the PdCl2–CuCl2 catalyst.60,63–65 Besides CH3COOK, KOH and KCl also showed an efficient promoting effect on the DMC/DEC synthesis.66–68 By means of varied OH/Cu ratio in the catalyst preparation, different crystal structures of Cu species were successfully observed. Many experiments were designed to clarify the active center. However, the conclusions are still controversial. For NaOH or KOH promoted CuCl2/AC and CuCl2–PdCl2/AC catalysts, Cu2(OH)3Cl was considered the active species.59,65,66,69 Two different crystal habits of Cu2(OH)3Cl were observed on the catalyst surface based on the OH/Cu molar ratio: the phase of the catalyst was mainly a-Cu2(OH)3Cl (atacamite) at the molar ratio of OH/Cu = 0.5; while g-Cu2(OH)3Cl (paratacamite) dominated at the molar ratio of OH/Cu = 1.0.69 The g-type catalyst was found to be more active than the a-type catalyst.66,69 Besides, copper chloride hydroxide with monohydroxy (Cu(OH)Cl) has also shown efficient activity.68,70 The addition of KCl was found to facilitate the formation of Cu(OH)Cl, which was more efficient than g-Cu2(OH)3Cl in regenerating Pd(0) to Pd(II) and reducing Cu(II) to Cu(I), resulting in a higher catalytic activity.68 However, Bell and co-workers have shown that [PdCl2x][CuCl2]x species deposited on the surface of the PdCl2 particles dominated the synthesis of DEC, and the decline of catalytic activity was due to the loss of Cl and the appearance of paratacamite.71,72 Recently, Wang and co-workers reported that the newly formed Cu2O species on PdCl2–CuCl2–KOAc/ AC@Al2O3 during reaction were not only active sites but also inhibitors for the loss of Cl.73 For catalysts supported on carbon-based materials, surface properties of carbon supports have great influences on the activity.72,74 Bell and co-workers have studied the effects of support composition and pretreatment on the catalytic property of PdCunClx catalysts.72 Highly dispersed CuCl2 and PdCl2 could be achieved on acid-pretreated carbon nanofibers, and thus high DEC activity was achieved (9.6 mmolDEC molCu1 s1), which was much higher than that over the AC supported catalyst (5.2 mmolDEC molCu1 s1).72

The effect of alkali-metal promoters in CuCl2/AC or CuCl2–PdCl2/AC catalysts on DMC synthesis

Catalyst CuCl2/AC CuCl2/NaOH/AC CuCl2/NaOH/AC PdCl2–CuCl2/AC PdCl2–CuCl2–NaOH–KCl/AC PdCl2–CuCl2–CH3COOK/AC

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Promoter

OH/Cu

T/K

P/MPa

NaOH NaOH

0 0.5 1.0

403 403 403 418 423 421

1.03 1.03 1.03 0.1 0.64 0.1

NaOH, KCl CH3COOK

1.0

tcontact/s (or SV/h1) 10 10 10 (2100) (2210)

MeOH : CO : O2

MeOH conv. (%)

DMC sel. (%)

Ref.

4/16/1 4/16/1 4/16/1 3.04/2.04/1 4/10/1 2.93/1.93/1

20.4 22.0 23.4 2.1 430 5.3

80.2 84.8 89.3 84.5 B95 76.4

59 59 69 60 68 60


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Catalytic properties of mesoporous silica supported catalysts


Catalyst CuCl2/MCM-41 CuCl2/N-MCM-41a CuCl2/MCM-48 CuCl2/N-MCM-48a CuCl/SBA CuOSi/SBA CuOtBu/SBA PdCl2–CuCl2–TEAB/HMS PdCl2–CuCl2–TEAB/HMS (Cu/Pd = 20, molar ratio) PdCl2/Cu-HMS a

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T/K

P/MPa

403 403 403 403 403 403 403 423 393

1.0 1.0 1.0 1.0

0.64 0.1

423

0.64

GHSV/ mL gcat1 h1 (or tcontact/s) 1200 1200 1200 1200 (3.5) (3.5) (3.5) 700 h1

Reactant (mol%) MeOH/CO/O2 = 16/72/4 MeOH/CO/O2 = 16/72/4 MeOH/CO/O2 = 16/72/4 MeOH/CO/O2 = 16/72/4 MeOH/CO/O2 = 4/9/1 MeOH/CO/O2 = 4/9/1 MeOH/CO/O2 = 4/9/1 MeOH/CO/O = 21/40/5 MeOH/CO/O = 11/8/4

13.6 6.4

EtOH/CO/O2 = 21/40/5

4.8

N-MCM-41 or N-MCM-48 refers to monoamino-functionalized silicas.

Ordered mesoporous carbons (OMCs) were used as the catalyst support and the structural properties of OMCs facilitated the dispersion of active components, which in turn improved the ethanol conversion of B65% over OMC supported Wacker-type catalysts compared with AC supported catalysts.74 Besides carbon materials, several other materials, i.e. mesoporous silica were used as catalyst supports.75–82 Table 6 presents the catalytic properties of these catalysts. Before loading CuCl2, MCM-41 and MCM-48 were modified with 3-aminopropyltrimethoxysilane.75,76 The prepared catalysts showed an enhanced catalytic activity for DMC synthesis, resulting from the acceleration of redox cycle of Cu2+/Cu+ species by the coordination of amino groups with Cu2+. A similar promotion was observed on PdCl2–CuCl2/HMS functionalized with quaternary ammonium salt. The modification by tetraethylammonium bromide (TEAB) showed the best catalytic property (210.9 gDEC Lcat1 h1), owing to facilitated redox processes between Pd(0)/Pd(II) and Cu(II)/ Cu(I).79,80 The precursors also influenced the local structure of Cu dispersed on mesoporous silica. Bell’s group reported that Cu was present primarily as isolated Cu(I) species on the catalysts prepared with CuCl and [CuOSi(OtBu)3]4, whereas [CuOtBu]4 produced Cu(0) particles, which dominated the formation of DMC.77,78 The PdCl2/Cu-HMS catalyst exhibited excellent selectivity for DEC, when Cu was introduced into the framework of HMS during preparation. Also, the Si/Cu molar ratio of mesoporous Cu-HMS showed a remarkable effect on catalytic activity.82 Zhang et al. reported that enhanced hydrophobicity by silylation of Cu-HMS inhibited adsorption of the formed water, resulting in reduction of DEC hydrolysis.83 Deactivation and regeneration. Although chlorine-containing catalysts showed excellent catalytic performance with respect to DMC/DEC yield and selectivity, these catalysts suffered from easy deactivation. The main reason was the loss of chlorine species, which occurred during catalytic reactions.29,58,63,64 Methyl chloride was detected as a by-product in the reaction effluent.29 Besides, sintering of Cu species and reduction of PdCl2 to metallic palladium accounted for the catalyst deactivation.84 The catalyst regeneration can be usually realized through the chlorine supplement. When the deactivated catalyst was treated with 5 vol% HCl–N2 or methanol solution of methyl chloroacetate, the catalytic


Conv. (%)

b

3.9 12.6 2.9 10.2

Sel. (%)

STY or TOFb 1

Ref. 1

97.9 97.5 96.5 96.0 80.1 82.9 80.5 490

1.37 g (gCu h ) 4.36 g (gCu1 h1) 1.02 g (gCu1 h1) 3.60 g (gCu1 h1) 3.5 105 s1 2.1 105 s1 2.0 105 s1 210.9 g (Lcat1 h1) 70 mg (gcat1 h1)

75 75 75 75 78 78 78 78 79

100

110.5 (g gCu1 h1)

82

TOF is reported on the basis of total Cu.

activity can be restored effectively.29,63 The loss of catalyst activity with time on stream could also be overcome by the addition of ppm level of CCl4 to the feed.72 Reaction mechanism. In the process of dialkyl carbonate synthesis over chlorine-containing catalysts, the reactions can be divided into two steps: the alcohol oxidation and the insertion of CO. In the following part, we discuss the similarities and differences of these two reactions over CuCl2, CuCl2–PdCl2 bimetallic chlorides and bimetallic chlorides with Cu2(OH)3Cl from the point of the binding site and coordination of Cu/Pd. For the CuCl2 catalyst, a three-step mechanism (eqn (4)–(6)) was proposed.15 The methanol oxidation occurred on copper(II), along with the production of methoxide coordinated on copper(II) (Cu(OCH3)Cl). Then the CO inserted into Cu(OCH3)Cl with the formation of Cu(COOCH3)Cl, which further reacted with another Cu(OCH3)Cl to produce DMC. In this process, CuCl2 was reduced to CuCl. And the copper(II) state can be recovered through oxidation by O2 (eqn (7)). CuCl2 + 2CH3OH - Cu(OCH3)Cl + CH3Cl + H2O Cu(OCH3)Cl + CO - Cu(COOCH3)Cl Cu(COOCH3)Cl + Cu(OCH3)Cl - CO(OCH3)2 + 2CuCl 2CuCl + 0.5O2 + 2CH3OH - Cu(OCH3)Cl + H2O

(4) (5) (6) (7)

For the CuCl2–PdCl2 bimetallic chlorides catalytic system, there were two mechanisms with different emphasis.15 In one pathway, the palladium was considered the main active center (eqn (8)–(10)). The insertion of CO occurred before the oxidation of methanol and the binding site was Pd(II). In the other pathway, reaction proceeded on Cu (eqn (2), (3) and (11)): the CO reacted with two Cu(OCH3)Cl to produce DMC. Both reactions were redox processes: Pd(II)/Pd(0) and Cu(II)/Cu(0). PdCl2 + CO - Pd(CO)Cl2

(8)

Pd(CO)Cl2 + 2CH3OH - (CH3)2CO + Pd0 + 2HCl

(9)

Pd0 + 2CuCl2 - PdCl2 + 2CuCl

(10)

2CuCl + 2HCl + 0.5O2 - 2CuCl2 + H2O

(11)

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The mechanism of DEC synthesis was proposed on Cu(OH)Cl dominated Wacker-type catalyst by Ma’s group.68 The coordination situation of the active species was assumed to be as follows: (1) Pd(II) interacted weakly with the active copper phase, Cu(OH)Cl, through chlorine and hydroxyl ligands. (2) Both ethanol oxidation and insertion of CO occurred on Pd. (3) The production of DEC induced the reduction of the Pd(II) species to Pd(0). (4) The Pd(0) and Cu(I) was regenerated through oxidization by Cu(OH)Cl and O2, respectively.68 The methanol oxidation over g-Cu2(OH)3Cl was investigated through DFT and DRIFTS studies.85 The Cl atom played a role in the activation of H–O bond of methanol and thus decreased the energy barrier for methoxide formation. Meanwhile, the formed HCl weakly adsorbed on the surface and can easily escape from the surface, resulting in the deactivation of the catalysts during the reaction.

acidic sites in zeolite were exchanged by Cu+ ions, and the formed surface-bound Cu+ was proposed to be the active species for oxidative carbonylation.92–94 This catalytic system was extensively investigated from aspects of structure–function relationship and reaction mechanism. Among microporous zeolites, Cu-Y showed the best activity and selectivity due to its relatively large pore diameter, abundant Brønsted acidic sites and unique threedimensional channel structure. The oxidation state and local coordination environment of Cu+ in Cu-Y zeolites were studied by employing X-ray absorption spectroscopy (XAS). It was found that the form of all the exchanged Cu ions was Cu+, and only Cu+ located at sites II and III 0 was accessible to the reactants (shown in Fig. 2).95 A quantitative relationship between the amount of Brønsted acid sites and catalytic activities was established.96 The effect of Si/Al ratios, which directly influenced the amount of Brønsted acidic sites, was studied via DFT calculations.97

3.1.2.2 Chlorine-free catalysts. Although chlorine-containing catalysts have shown excellent catalytic properties for dialkyl carbonates synthesis through oxidative carbonylation, they showed deactivation and equipment corrosion problems due to the chlorine species. Many efforts have been made to alleviate these issues.86 Notably, King first developed chlorine-free Cu-exchanged Y zeolite (Cu-Y) catalyst.31,87,88 The catalyst displayed good productivity and selectivity for DMC synthesis with a strongly enhanced stability compared with the chloride-based catalysts. The copper species in zeolite supported catalysts existed mainly in the form of isolated Cu(I) coordinated with zeolites. Copper or copper oxide supported on AC was another kind of typical chlorine-free catalysts that has been explored and intensively investigated lately.89–91 Differently from Cu-zeolites, the copper species in carbon supported catalysts existed mainly in the form of CuOx nanoparticles. The activity and selectivity of chlorine-free catalysts still need to be improved, however, they have been considered as potential alternatives to eliminate the negative effects of chlorine. Cu-zeolite catalysts. Table 7 summarizes the catalytic performances of Cu-exchanged zeolites, which can be prepared mainly through solid-state ion exchange (SSIE), as shown in eqn (12) and (13).27,81,82 During the SSIE process, the Brønsted

Table 7

Catalytic properties of Cu–zeolite catalysts for oxycarbonylation of alcohols

Catalyst

T/K

P/MPa

Cu-Y Cu-Y/b-SiC

403 403

1.2 1.2

CuY(Si/Al = 5) CuY-0.3 M Cu-Ya Cu-Yb Cu-b Cu-Y Cu-Y Cu-ZSM-5 Cu-MOR Cu-SAPO-37

413 413 423 443 413 413 403 403 403 403–443

0.7 0.7 0.1 0.8–1.2 0.64 0.64 0.1 0.1 0.1 0.4

a

Fig. 2 Possible locations for Cu+ in faujasite. Reprinted with permission from ref. 95. Copyright 2006, American Chemical Society.

GHSV/h1 1800 1800

3000 6250

12 wt% Cu loading, activated at 1023 K.

Chem. Soc. Rev.

b

Reactant (mol%) MeOH/CO/O2 = 5/15/1 MeOH/CO/O2 = 5/15/1 EtOH/CO/O2 = 4.8/10/1 EtOH/CO/O2 = 4.8/10/1 MeOH/CO/O2 = 18/24/3 MeOH/CO/O2 = 20/40/1.5 EtOH/CO/O2 = 21/10/1 EtOH/CO/O2 = 21/10/1 MeOH/CO/O2 = 12.12/20.2/2.02 MeOH/CO/O2 = 12.12/20.2/2.02 MeOH/CO/O2 = 12.12/20.2/2.02 MeOH/CO/O2 = 8.7/5.8/2.9

DMC/DEC conv. (%)

DMC/ DEC sel. (%)

1

17 3

9.0–11.5 9–11 4.3 4.2 1 1 1 4–6

STY of DMC/DEC

65 54 50–60 72 78.4 23.9 82 35 20 55–45

1

95 g (L-cat h ) 137.5 g (Lcat1 h1), based on Cu-Y 1.5% 2.2% 3.4% 1.0% 0.8% 0.4% 0.2%

Ref. 87 87 96 98 99 100 105 105 104 104 104 30

17.4 wt% Cu loading, activated at 923 K.


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The Cu-Y zeolite with a Si/Al ratio = 6.5 exhibited the highest catalytic activity for the synthesis of DMC, which was in good agreement with the experimental results.96,97 A competitive methanol conversion was achieved by increasing the reaction temperature, but the selectivity of DMC decreased and the formation of DME and CO2 was accelerated. Therefore, taking into account that oxidative carbonylation as an exothermic reaction, thermoconductive b-silicon carbide (b-SiC) was used to support Cu-Y, which offered a better control of the catalytic bed temperature, resulting in limited production of DME and CO2.87 Alkaline treatment, which generated meso- and macropores in zeolite Y and increased the number of exchanged Cu active sites, was also an effective method for increasing DEC yield (Table 7).98 (12)

(13)

Unlike the SSIE process, Richter et al. employed incipient wetness impregnation to synthesize Cu-Y catalyst.99,100 Activation under high temperature is necessary to acquire high catalytic activity, in which auto-reduction of Cu2+ to Cu+ occurred. The Cu2O aggregates were considered to facilitate activation of methanol. Ma and co-workers reported that the activity was in line with the proportions of cuprous species in Cu-Y prepared by ammonia evaporation method, which can be regulated by cupric precursors.101 The DMC decomposition over Cu-Y has been investigated by Root and co-workers, resulting from the residual Brønsted acid sites caused by incomplete exchange during the catalyst preparation.102 Exchanging the acid sites with alkali prior to copper loading was proved to be a promising strategy for decreasing the rate of DMC decomposition. The effects of zeolite structure and chemical composition on the synthesis of DMC were studied over Cu-exchanged Y, ZSM-5 and Mordenite.103,104 Different from Cu-Y, dimethoxymethane (DMM) was the primary product on Cu-ZSM-5 and Cu-MOR. The weaker adsorption of CO on Cu-Y facilitated the insertion of CO into Cu–OCH3, which was the rate-limiting step of the reaction. Ma’s group discovered that b zeolite displayed a shape selective catalysis in the reaction due to its unique three-dimensional interconnected channel system.105 Also it was found that Cu-b catalyst without extra-framework silicon exhibited better catalytic activity because the stereo-hindrance of SiO2 blocked the attack of Cu+ to O atom during the preparation.106 Additionally, Martin’s group successfully synthesized Cu-SAPO-37 using Cu(II) acetyl acetonate powder and the catalytic behavior was comparable with that of Cu-Y zeolite catalyst.30 Reaction mechanism over Cu-zeolites. Based on mainly spectroscopic studies, the reaction mechanism of oxidative carbonylation of methanol over Cu-Y catalyst prepared by SSIE also generally involves two aspects: oxidation of methanol and insertion of CO. Cu(I) is assumed to be the active center. King proposed that the


Scheme 2 Mechanism for the formation of DMC, DMM and MF. Reproduced with permission from ref. 109. Copyright 2008, Elsevier.

first step was oxidation of methanol and Cu(I) to Cu(II)-methoxide in the presence of O2.107 Insertion of CO into Cu(II)-methoxide, leading to the formation of the probable intermediate carbomethoxide, was the rate-limiting step. DMC was formed by reaction of methanol and O2 with carbomethoxide. The results were also supported by the kinetic study of Root and co-workers, which revealed that insertion of gaseous CO into surface methoxide via an Eley– Rideal pathway was the rate-limiting step.31,108 Bell’s group proposed that DMC can be formed by reaction of CO with dimethoxide species (Path I) or interaction of methanol with monomethyl carbonate (MMC) (Path II) (Scheme 2).109 The two major byproducts (DMM and MF) were formed via a hemiacetal intermediate produced by reaction of methanol with formaldehyde. The mechanism was similar to that proposed by King, but different in two aspects: one was that MMC was considered as the reaction intermediate rather than the carbomethoxide species; the other difference was the assumption that DMC can also be formed by insertion of CO to dimethoxide species. The latter was evidenced by the formation of DMC when CO was introduced to the system after the addition of methanol and O2, which cannot be interpreted by the pathway through MMC. Theoretical investigations were in excellent agreement with experimental observations: DMC can be formed via both reaction pathways as mentioned above.110 Methoxide groups bound to Cu+ cations were formed in the presence of gaseous O2, which inhibited the adsorption of CO completely. The reaction mechanism over impregnated Cu-Y catalyst was studied by Bentrup’s group, which was largely consistent with that proposed by Bell’s group, but the role of O2 was controversially discussed.111,112 They claimed that CuOx was essential for oxidation of methanol to methoxide instead of O2. They also found that only monodentate-like MMC(I) species were active in DMC formation because bidentate-like MMC(II) species were strongly adsorbed on the catalyst surface. Moreover, they confirmed via

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isotopic experiments that CO was needed for the formation of DMC, while no additional O2 was apparently required. Ma and co-workers focused on the dissociation of O2 on Cu(I)-zeolite catalyst through DFT calculations. They suggested that the decomposition of O2 occurred readily under reaction conditions and the formed O atom further reacted with methanol to form methoxide.113 Carbon supported copper/copper oxides catalysts. Cu, Cu2O, CuO and their mixture supported on AC as a type of chlorinefree catalyst have shown potential for oxidative carbonylation. Different from zeolite supported catalysts, the active copper species in the carbon supported catalysts were CuOx nanoparticles with mono- or mix-valence state of Cu dispersing on the carbon surface. Wang and co-workers firstly introduced CuO–La2O3/AC catalyst for this reaction.114 The effect of Cu2O/CuO ratio on the catalytic activity and stability was investigated. Subsequently, several studies were reported on AC supported copper catalysts by Li’ group.115–117 Compared with Cu0/AC and CuO/AC, Cu2O/AC catalyst was found to be the most stable one. The impact of surface modification of AC on the catalytic performance was also discussed.118,119 The Cu/AC catalyst treated with ammonia exhibited an excellent activity with methanol conversion and the space time yield of DMC of 7.4% and 152.8 mg gcat1 h1, respectively. The surface oxygen containing groups (OCGs) were found to have played a key role in the oxidative carbonylation of methanol. The optimal Cu loading as well as catalytic activity increased linearly with the amount of OCGs.120 Reaction mechanism over copper/copper oxides supported on AC catalysts. Wang and co-workers investigated the mechanism of oxidative carbonylation over Cu2O/AC catalysts through DFT calculations.89–91 Dissociation of molecularly adsorbed O2 into two O atoms was more favorable both thermodynamically and kinetically on the deficient surface than that on the perfect surface, which improved the catalytic activity of Cu2O obviously.90 Two pathways were proposed for the DMC formation on Cu2O, similar to that on Cu-zeolites.91 The path that CO inserted into methoxide species to produce monomethyl carbonate species was more competitive than CO insertion to dimethoxide species. Ren and his colleagues took the interaction between Cu0 and AC into consideration during the theoretical investigation.121 The results demonstrated that Cu0 tended to adsorb at unsaturated sites on AC and the mechanism of DMC formation was identical to that on Cu2O. 3.2

Methylnitrite carbonylation

UBE developed a process on the synthesis of dimethyl oxalate (DMO) from CO and MN over a Pd/carbon catalyst, and industrialized this technology in 1978.122 The development of DMC synthesis via the MN process can be considered as a derivative of the original plan on DMO synthesis.8 In this route, alkyl nitrite is used as oxidative agent instead of oxygen for reducing the risk of explosion, which is considered as a potential safety problem in direct oxy-carbonylation due to the co-existence of CO and O2. Furthermore, the synthesis of dialkyl carbonates is not accompanied by the formation of H2O, eliminating the negative effect on catalysts. The division of the synthesis process into two parts

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Fig. 3 Schematic view of the UBE process. Reprinted with permission from ref. 15. Copyright 2010, Elsevier.

also avoids the appearance of ternary azeotrope (methanol–water– DMC) in products, which makes products separation much easier. Therefore, synthesis of dialkyl carbonates via alkyl nitrite has gained increasing attention of researchers. However, it is also noteworthy that both NOx and RONO are toxic and corrosive, which go against the principles of ‘‘green chemistry’’. Fig. 3 shows a simplified schematic view of the UBE process, which is a two-step (indirect) method for dialkyl carbonate synthesis.15 In the first reactor, alkyl nitrite is synthesized from NO, O2 and alcohol (eqn (14)). This reaction is non-catalytic and composed of two reactions as follows. NO is first oxidized to N2O3 by oxygen, and then N2O3 reacts with alcohol to produce alkyl nitrite (eqn (14)–(16)). 2NO + 2ROH + 12O2 - 2RONO + H2O 2NO +

1 2O2

- N2O3

2ROH + N2O3 - 2RONO + H2O

(14) (15) (16)

In the second reactor, the synthesis of dialkyl carbonates is catalyzed by Pd-based catalysts from CO and alkyl nitrite based on eqn (17), without the generation of H2O. Therefore, high catalytic activity and selectivity is maintained for a long time as a result of anhydrous condition over the catalyst.123 CO + 2RONO - (RO)2CO + 2NO

(17)

3.3.1 Chlorine containing catalysts. Synthesis of DMC via MN carbonylation can be catalyzed by various kinds of supported catalysts, such as PdCl2–CuCl2, PdCl2–FeCl3, PdCl2–BiCl3, etc.124 Among them, PdCl2–CuCl2/AC catalyst performed best, while CuCl2/AC catalyst was almost inactive.122 This phenomenon suggested that Pd species were the active species of MN carbonylation to DMC, and the Cu species only were only promoters. Moreover, the catalytic performance of the carbonylation of MN reaction was affected by the valence of Pd species: Pd(II) species are the active species of MN carbonylation to DMC; while Pd(0) tends to generate DMO that is evidenced by the treatment of PdCl2/AC catalyst with H2.122 During the carbonylation reaction, Pd(II) species were gradually reduced to Pd(0) species in the presence of reactant CO, leading to the deactivation of catalyst. However, Pd(0) species could also be re-oxidized to Pd(II) species upon treatment with MN and HCl, as shown in eqn (18).


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Therefore, the catalytic performances could be maintained by adding HCl.


Pd0 + 2HCl + 2CH3ONO - PdCl2 + 2CH3OH + 2NO

(18)

Several supports have been employed in bimetallic systems for the synthesis of DMC. AC supported catalyst performed best compared to other supports, the STY of DMC reached 553 g Lcat1 h1.123 Jiang and co-workers found that the acidic property of carriers played an important role in DMC formation.125–127 Generally, acid sites of the catalysts result in decomposition of MN to byproducts DMM and MF (eqn (19) and (20)), which decreases the selectivity of DMC. To overcome the problem, UBE designed a novel catalyst support with spinel type structure.128 The catalyst (PdCl2–CuCl2/Li–Al–O) exhibited outstanding catalytic performance and lifetime. The STY of DMC reached 670 g Lcat1 h1, and the selectivity of DMC was B95% on the basis of consumed CO or MN. Ma’s group deemed that fewer acid sites of the catalyst surface, the structure of spinel and the electronic conductivity of Li ions may be the explanation of its excellent catalytic efficiency.129 4CH3ONO - HCOOCH3 + 2CH3OH + 4NO

(19)

2CH3ONO + CH3OH - H3COCH2OCH3 + 2NO + H2O (20) 3.3.2 Chlorine-free catalysts. Similar to direct oxidative carbonylation, the loss of chlorine is still the main reason of catalyst deactivation. UBE exploited PdNaY catalyst in carbonylation of MN, which exhibited excellent catalytic activity even in the absence of chlorine.130 The initial yield of DMC was 353 g Lcat1 h1 and remained at a rather steady state (B200 g Lcat1 h1) even after 700 h on stream, as illustrated in Fig. 4. It was widely accepted that Brønsted acid sites would accelerate the decomposition of alkyl nitrite, which brought about poor activity and selectivity. Among different microporous zeolites, NaY zeolite was reported to be the optimal carrier due to its certain amount of weak acid sites.131 The interaction between Lewis acid sites and Pd species for the synthesis of DMC from CO and MN were discussed and the results demonstrated that the TOFs increased linearly with the increase of Lewis acid sites in the Pd/FAU catalysts.132 Combined with the results of X-ray photoelectron spectroscopy measurement, a conclusion was inferred that Lewis acid sites could maintain the Pd species in an electron-deficient state, which in turn promoted the synthesis of dialkyl carbonates. 3.3.3 Reaction mechanism. The elementary steps involved in carbonylation of MN over chlorine-containing catalysts and chloring-free catalysts are basically the same, except the difference in interaction of Pd active species with the reactant. Delledonne et al. proposed a catalytic mechanism of chlorinecontaining system, as shown in Scheme 3.133 MN firstly adsorbed on Pd species to form Pd(OCH3)(NO)Cl2 species, followed by insertion of CO to form the reaction intermediate Pd(COOCH3)(NO)Cl2. Then the intermediate reacted with MN to produce DMC, with simultaneous release of NO. A trace amount of methylchloroformate was detected during the formation of DMC catalyzed by the PdCl2 catalyst, which resulted from the reductive elimination of the Pd(COOCH3)(NO)Cl2 intermediate.134 The pathway over Pd/


Fig. 4 Catalytic behavior of 1 wt% PdNaY for DMC synthesis as a function of reaction time. (a) Formation rates of DMC (&), MF (J) and DMM (K) and (b) Selectivities of CO (n) and MN (m) to DMC. Reproduced with permission from ref. 130. Copyright 1997, American Chemical Society.

Scheme 3 Catalytic cycle for Pd catalyzed DMC formation from MN and CO. Reprinted with permission from ref. 133. Copyright 2001, Elsevier.

zeolite was similar to the chlorine-containing catalyst system except that chlorine anions were substituted by zeolite framework oxygen anions.130 However, both reaction mechanisms are rather speculative, and thus systematic investigations are ideally required.

4. Synthesis of dialkyl carbonates from CO2 CO2 is the most abundant waste gas especially produced by combustion of fossil fuels (coal, petroleum and natural gas)

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nowadays, which is regarded as the major contributor to the greenhouse effect.135,136 As a potential alternative carbon source to fossil resources, CO2 utilization in chemical synthesis still faces challenges. In the past years, persistent and significant efforts have been made to the synthesis of dialkyl carbonates using CO2 as a raw material. In general, there are three main synthetic routes of dialkyl carbonates from CO2: (1) direct carbonylation of alcohols; (2) urea alcoholysis; and (3) transesterification of cyclic carbonates with alcohols, in which cyclic carbonates are synthesized from epoxides and CO2.13,137–140 Although the former offers the simplest technological processes, the low yield of dialkyl carbonates and rigorous operation conditions prevents its popularization. Unlike the first route, the latter two have been gradually commercialized recently due to high efficiency of carbon resource, inexpensive catalysts and mild conditions.141,142 4.1

Direct synthesis from CO2 and alcohol

The direct synthesis of dialkyl carbonates by reaction between CO2 and alcohol is first proposed in 1980s (eqn (21)). 2CH3OH + CO2 - CO(OCH3)2 + H2O DrG298K = 26.21 kJ mol1, DrH298K = 27.90 kJ mol1

(21)

Differing from other synthetic routes, the carbonyl group is derived from CO2 directly without any other intermediates, which is nontoxic, noncorrosive, nonflammable and abundant. Notably, this route, yielding water as the sole byproduct, has the same atom economy as oxidative carbonylation. Furthermore, the utilization of CO2 avoids the risk of explosion caused by the co-existence of O2 and CO. From the above, it is considered as an attractive and sustainable route for dialkyl carbonates synthesis based on green chemistry principles. From published literature in recent years, most research work focuses on direct carbonylation for the synthesis of dialkyl carbonates. However, there are three main constraints for acquiring high yield of carbonates via this route: (1) activation of CO2 due to its kinetic inertness; (2) thermodynamic limitation and reversibility of the reaction. According to thermodynamic analysis, DMC or DEC synthesis cannot occur spontaneously under usual conditions;143 (3) hydrolysis of the produced carbonates and deactivation of catalyst. Up to now at least, this route is still explored and investigated only at the laboratory-scale. Therefore, rational design of catalysts is expected

Table 8

to facilitate activation of reactant molecules, and appropriate measures of process intensification (e.g. introduction of coupling reaction and dehydrating agent) are taken into account for breaking the unfavorable reaction equilibrium. Herein, we categorize the reported catalyst systems into homogeneous and heterogeneous catalysts, as displayed in Table 8. Since Liu and Ma have given a general review about catalysts in direct synthesis of DMC,139 we place emphasis on the latest advances, particularly catalyst structure and reaction mechanism based on experiment and quantum chemistry calculation. 4.1.1 Homogeneous catalysts 4.1.1.1 Organic metal alkoxy compounds. Metal alkoxides, have long been known to readily adsorb CO2 to form metal alkyl carbonates, which supplies the possibility for their utilization to catalyze CO2 conversion.144 In 1975, DMC formation from CO2 and methanol over organotin was first proposed by Sakai’s group.145 There are two primary requirements for this reaction: activation of CO2 promoted by nucleophilicity of the alkoxy oxygen and strong association of Sn–O bonds. Up to now, organometallics based on Sn, Ti and Nb, including R12Sn(OR2)2, Sn(OR)4, Ti(OR)4, [Nb(OR)5]2, etc., have been intensively studied in literature. Among the metal alkoxide catalysts, R12Sn(OR2)2 is considered as the most promising catalyst.145–147 Various characterizations were applied to provide insight into the structure of the postulated complex and intermediate species under catalytic conditions, and carbonate complexes formed by CO2 insertion into the Sn–O bond were proposed as a key intermediate.148–153 Acidic and basic components are utilized as promoters when using organic metal alkoxide catalysts. Sakakura and co-workers discovered that the addition of a small amount of triflate salt to Bu2SnO or Ti(O-i-Pr)4-based catalysts provided a remarkably enhanced productivity.154 Svec et al. discovered that LCN(n-Bu)2SnOTf (LCN = 2-(N,N-dimethylaminomethyl)phenyl) was the most effective catalyst precursor among different C,N-chelated organotin(IV) trifluoromethanesulfonates, which underwent significant weakening of the Sn–OTf bond in solution.155 Onaka’s group found that the highest increase in the catalytic activity of Sn(Ot-Bu)4 was observed with C6F5OH and pyridine as co-catalysts.156 A generally accepted reaction pathway on conventional R2Sn(OMe)2 catalyst follows as ‘‘double-base activation of methanol’’: after insertion of CO2 into the Sn–OMe bond, intermolecular or intramolecular methyl transfer occurs from an unreacted

Catalysts used for direct synthesis of dialkyl carbonates from CO2 and alcohols

Type of catalyst Homogeneous catalysts Organic metal alkoxy compounds Methyl iodide and potassium base Acetate ILs Heterogeneous catalysts Metal oxides Metal supported catalysts Heteropolyoxometalate Other catalysts

Chem. Soc. Rev.

Compounds R12Sn(OR2)2, Sn(OR)4, Ti(OR)4, [Nb(OR)5]2 etc. CH3I and K2CO3, K3PO4, CH3OK, KOH etc. Mn(OAc)2, Co(OAc)2, Ni(OAc)2, Hg(OAc)2 etc. Basic choline hydroxide, 1-ethyl-3-methylimidazolium hydroxide, 1-butyl-3-methylimidazolium hydroxide etc. ZrO2, CeO2, CexZr1-xO2, ZrO2/SiO2, Nb2O5/ZrO2, acid modified oxides, etc. Cu–Ni/VSiO, Cu–Fe/SiO2, Cu–Ni/C etc. Co1.5PW12O40, Fe1.5PW12O40 etc. Ce-MCM-41, hydrotalcite-like material, zirconium phenylphosphonate phosphite etc.


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carbonyl oxygen on the tin center and methoxy on the carbonyl carbon, required the highest activation energy.146,156,158,159 Aresta et al. proposed three possible pathways for DMC synthesis on [Nb(OMe)5]2, as shown in Scheme 5.160 Compared with the ‘‘double-base activation’’ mechanism (Route C), Route A was more favoured due to its lowest energy barrier, which was considered a ‘‘acid-plus-base activation’’ mechanism. Despite their very high catalytic activity, organic metal alkoxy compounds suffer from inherent disadvantages, such as mammalian toxicity and instability. Therefore, some strategies have been proposed to solve these problems including heterogenization of organotins and development of novel homogeneous catalyst systems.

Scheme 4 The possible catalytic cycle of the Sn-catalyzed DMC synthesis in the presence of dimethyl ketals as a dehydrating agent. Reprinted with permission from ref. 157. Copyright 2008, Elsevier.

methoxo group to the formed Sn[OC(O)OMe].156 Sakakura’s group reported that the transformation of dibutyltin oxide (Bu2SnO)n (Scheme 4, 3) to stannoxane dimer (Scheme 4, 4) was in nearly stoichiometric yield in the presence of dimethyl ketals, which suggested that 4 rather than 1 could be the active intermediate in the catalytic cycle.157 Quantum calculations on the organotin template have afforded more information about the fine-structure of intermediate and reaction mechanism for dialkyl carbonates formation. A concerted reaction via insertion of CO2 to the Sn–OCH3 bond, involving simultaneous attacks of

Scheme 5

4.1.1.2 Methyl iodide and potassium base catalysts. Fujimoto and co-workers first reported that DMC synthesis could be catalyzed using inorganic bases and methyl iodide.161 Subsequently, methyl iodide and potassium solid bases, including K2CO3, KOH, and CH3OK, have been utilized in the synthesis of dialkyl carbonates. The corresponding results are summarized in Table 9. Cheong et al. observed that DMC synthesis could be catalyzed by simple homogeneous iodide catalysts in the presence of trimethylorthoesters or dimethyl acetals.162 Mechanism studies indicated that CH3I acted as a reactant essentially rather than a promoter (eqn (22)–(26)).161,163,164 Via isotope-labelled experiments, Cai et al. confirmed that K2CO3 participated in the synthetic process and the two methyl groups of DMC were derived from methanol and CH3I.165 The activity of different alcohols also has been demonstrated dependent on the length of the carbon chain as well as reaction conditions.163,166 Base + CH3OH - CH3O + H Base

(22)

CH3O + CO2 - [CH3OCOO]

(23)

[CH3OCOO] + CH3I - CH3OCOOH3 + I

(24)

I + H Base+ - HI

(25)

HI + CH3OH - CH3I + H2O

(26)

Three different reaction routes over [Nb(OMe)5]2 catalyst. Reprinted with permission from ref. 160. Copyright 2006, Springer.


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Table 9 Activity of DMC synthesis from CO2 and CH3OH catalyzed by basic catalysts


Catalyst Entry (mmol) 1 2 3 4 5 6 7 8 9 10 11 12

CH3OH/ CH3I/ P/ t/ DMC/ Yield mmol mmol T/K MPa h mmol (%) Ref.

Li2CO3 (3) Na2CO3 (3) K2CO3 (3) Cs2CO3 (3) K3PO4 (3) Me4NOH (3) K2CO3 (3) KHCO3 (3) CH3OK (51) CH3ONa (51) CH3OK (reused) KOH (54)

192 192 192 192 192 192 198 198 850 850 850 850

24 24 24 24 24 24 24 24 80 48 48 48

373 373 373 373 373 373 343 343 353 353 353 353

5 5 5 5 5 5 8 8 7.3 7.3 7.3 2.0

2 2 2 2 2 2 4 4 10 10 10 6

1.3 9.4 11.9 8.6 7.7 7.3 4.0 2.0 67 11 23 36

1.4 9.8 12.4 9.0 8.0 7.6 4.1 2.0 16.2 2.6 5.4 8.5

161 161 161 161 161 161 163 163 164 164 164 143

Besides the two kinds of typical homogeneous catalysts mentioned above, transition-metal acetates (e.g. nickel acetate) also showed a catalytic activity for DMC synthesis.167 Very recently, a series of hydroxyl-functionalized basic ILs were used as catalysts rather than promoters for the direct synthesis of DMC.168 4.1.2 Heterogeneous catalysts. Although homogenous catalysts exhibit high efficiency, complex production separation and recyclability of catalysts give rise to increase of investment in equipment and utilities. Therefore, much work has been devoted to the immobilization of homogenous catalysts as well as the development of new heterogeneous catalyst systems in the past twenty years. 4.1.2.1 Heterogenization of organotins. Heterogenization of organotins by means of encapsulation/immobilization of organotin complexes in/on the host matrixes is desirable to facilitate catalyst separation and reusability.169 Since 2002, organotin compounds (e.g. Sn2(OMe)4, Sn2(OMe)2Cl2, (MeO)2ClSi(CH2)3SnCl3) immobilized on silica, especially mesoporous silicas, have been investigated because of their large surface areas, uniform mesopores and facile surface modification behavior.170–173 Fan et al. attributed the improved activity of organotin-functionalized mesoporous

Table 10

benzene-silica to the enhanced surface hydrophobicity and the presence of a large number of hexa-coordinated Sn species.169 Moreover, different organometallic species behave in different modes when supported on a solid matrix. For example, unlike the grafted organotin species, Nb–silsesquioxane adduct is inactive for DMC production.174 4.1.2.2 Metal oxides catalysts. As mentioned above, there are two prerequisites for direct production of dialkyl carbonates: one is formation of methyl species, and the other is activation of methanol and CO2. The former can proceed via acidic functions, while the latter proceeds on basic functions. Considering the synergetic effect between acidic and basic sites, Tomishige et al. first proposed that the activity and selectivity for DMC formation on amphiprotic oxides profoundly depended on acid– base bifunctional properties on the catalyst surface.175 Since then, ZrO2, CeO2, CexZr1xO2 solid solutions and other composite oxides, as well as acid or hetero-atom modified oxides have been applied to the synthesis of dialkyl carbonates (Table 10).176–178 Up to now, metal oxides are the most intensively investigated catalysts for direct synthesis of dialkyl carbonates, and the present work mainly concentrates on regulation of acidic and basic properties, dispersity of active sites, crystal structure as well as catalytic mechanism. For a single oxide, it is difficult to regulate the innate acidic and basic properties accurately through changing preparation methods, hence, a series of in situ or post acid-modifications were applied to achieve suitable acidic strength and acidity. Doping H3PO4 and H3PW12O40 into oxides, such as ZrO2, V2O5, is a simple and effective approach for improving Brønsted acid sites.179–184 It consequently facilitates the activation of alcohols, which is the rate-determining step of the reaction. On the other hand, ether as an expectable byproduct, could be easily formed via the dehydration of alcohol on strong acid sites. Thus, the appropriate strength of both acidity and basicity plays an important role in product distribution. In addition, single oxides such as ZrO2 and CeO2, suffer from deactivation due to the

Dialkyl carbonates synthesis on oxides catalysts via direct carbonylation of alcohols

Entry

Catalyst

T/K

P/MPa

MeOH/EtOH : CO2

DMC/DEC yield (%)

Ref.

1 2 3 4 5 6 7 8

ZrO2 H3PO4/ZrO2 CeO2/ZrO2 CeO2/ZrO2 H3PW12O40/ZrO2 H3PO4/V2O5 H3PW12O40/CexTi1xO2 CeO2

5 5 21 6 4 0.6 5 5

1:3 0.96 : 1 1 : 5.2 1:1 1 : 1.25 2:1

Al-doped CeO2 Ce0.07Zr0.93O2 CeO2 Ce0.5Zr0.5O2 ZrO2/SiO2 (3.1 Zr wt%) H3PW12O40/Ce0.6Zr0.4O2 Ga2O3/Ce0.6Zr0.4O2 Nb2O5/ZrO2 H3PW12O40/Ce0.1Ti0.9O2 CeO2

373 423 443 443 408 393 413

20 20 6 6 5 2 5

1 (DMC) 0.31 (DMC) 1.67 (DMC) 0.40 (DEC) 4.04 (DMC) 1.80 (DMC) 5 (DMC) 0.73 (DMC) 0.42 (DEC) 1.2 (DMC) 0.23 (DEC) 0.33 (DEC) 0.39 (DMC) 3.2 (DMC, based on m(Zr)) 0.37 (DMC) 0.65 (DMC) 0.30 (DEC) 6.02 (DMC) 0.62 (DMC)

175 179 185 176 182 181 183 178

9 10

433 403 383 383 373 413 443 403 423 408 413

12 13 14 15 16 17 18

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5 8

0.96 : 1 1:1 1.08 : 1 0.38 : 1

193 196 177 195 188 189 186 184 200


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Fig. 5 Correlation of acidity or basicity with catalytic performance of Ce0.6Zr0.4O2 and XGa2O3/Ce0.6Zr0.4O2 (X = 1, 5, 10 and 15). Reprinted with permission from ref. 189. Copyright 2011, Springer.

leaching of basic sites and instability of valence states. Therefore, binary oxides and modified complex oxides were employed to improving catalytic activity and stability. Tomishige et al. reported that the surface structure and acid–base properties of CeO2/ZrO2 catalyst had a significant effect on activity, which could be regulated by the preparation conditions.185 Dibenedetto and co-workers found that 3% Nb dopant increased amounts of acid and base strong sites over CeO2 that contributed to an enhanced DEC yield.186 Song’s group investigated metal oxide/CexZr1xO2 broadly (metal oxide = Ga2O3, La2O3, Ni2O3, etc.), as well as H3PW12O40 modified CexZr1xO2 and CexTi1xO2, and built linear correlations of both basicity-performance and acidity-performance (Fig. 5).187–192 Aresta et al. claimed that inclusion of amorphous alumina enhanced recyclability of CeO2 by repressing reduction of Ce(IV) to Ce(III).193,194 Silica supported ZrO2 exhibited an excellent reusability and an order of magnitude higher yield of DMC than ZrO2, which was due to the high dispersion of active sites and strong interaction between ZrO2 and SiO2 support.195


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Different from acid–base property and dispersity of active sites, the effect of crystal structure and surface morphology are still controversial. ZrO2 or CexZr1xO2 containing predominant tetragonal phase (t-ZrO2) were suggested to possess better activity,175,196,197 while other investigations claimed that monoclinic ZrO2 (m-ZrO2) was the most active species because of stronger Brønsted basic hydroxyl groups and Lewis acid–base pairs.198,199 Recently, four different-shaped CeO2 catalysts were synthesized. Spindle-like CeO2, possessing the most exposed (111) plane, exhibited the highest DMC yield.200 Therefore, a synergism among the plane, defect sites, and acid–basic sites was proposed to be crucial for high reactivity of DMC formation. Many efforts have been made to explore the mechanism on oxides catalysts, aiming at providing guidance of catalyst rational design and controllable fabrication.201 The possible pathways of DMC formation are presented in Scheme 6. In situ Raman and IR studies by Bell’s group revealed that methanol first bounded to Zr4+ Lewis acid sites and then reacted with surface OH group to form H2O. Then, methyl carbonate species could be produced via two pathways: (a) CO2 inserts into the Zr–O bond of the CH3O–Zr species to form (CH3O)COO–Zr (Scheme 6, Path 3), (b) CH3OH reacts with CO2 adsorbed in the form of bicarbonate species (Scheme 6, Path 2). Subsequently, DMC is formed via reaction of methanol with methyl carbonate species. Since the rate of process (b) is much slower than (a), Bell and co-workers presented a complete mechanism as displayed in Path 3.199,202 A similar mechanism has been confirmed for CeO2–ZrO2 and other composite oxides.203 Tomishige’ group elucidated the promoting mechanism by H3PO4 on ZrO2: Brønsted acid sites are more effective than Lewis acid sites for CH3OH activation; CO2 reacts only with terminal –OCH3 (Path 4) rather than bridged –OCH3 (Path 1) to form methyl carbonate species.180 DFT results suggested that the interaction of –OC(O)OCH3 species with gas-phase methanol was easier than the surface-bound OCH3 species.194 Intermolecular attack by two methanol molecules to the hemicarbonate moiety showed the lowest transition state energy. 4.1.2.3 Metal supported catalysts. Transition metals have drawn much attention to accelerate CO2 activation because of their unusual electronic structure and favorable catalytic properties. Also, appropriate supports are beneficial to improve dispersity of metal particles and generate synergetic effects between active species and supports. On the other hand, differing from a batch reactor, a fixed-bed reactor can rapidly remove the formed H2O during the reaction to solve the problems of the catalyst deactivation and carbonates decomposition raised by H2O. Therefore, substantial efforts have been made for application of supported transition-metal catalysts in gas-phase direct carbonylation. Zhong’s group firstly investigated VSiO or ZrSiO supported Cu–Ni bimetallic catalyst in gas-phase direct carbonylation.204–206 Meng and co-workers reported that the crystal type of V2O5 changed during the reduction and a higher crystallinity resulted in a higher yield of DMC.207,208 On Cu–Fe/SiO2 catalyst, a high catalytic performance (CCH3OH = 5.37%, SDMC = 85.9%) was ascribed to the synergism of basic sites functionalised by Cu, Fe and acid sites provided by oxygen vacancies of Fe2O3x.209,210

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Scheme 6 Hypothetic mechanisms of DMC formation over ZrO2 catalyst and promotion of acid modification. Reproduced with permission from ref. 180, 199 and 202. Copyright 2001, American Chemical Society; 2002, Springer; 2000, Springer, respectively.

As the supports of catalysts mentioned above suffer from relatively low surface area and high cost, novel supports with the desired properties have been investigated.211 Cu–Ni supported on carbonaceous materials (e.g. AC, graphite oxide and carbon nanotubes) exhibited enhanced interactions between Cu–Ni and supports, good dispersion of metal particles and favorable synergy of Cu, Ni and Cu–Ni alloy, which contributed to an acceleration of catalytic reactivity.211–216 Recently, low-cost and porous natural diatomite was also applied as a support.217 A large number of basic (M) and acid (M+) centers originated from the strong interaction between diatomite and Cu–Ni were suggested to be responsible for the high activity. The catalytic cycle over Cu–Ni supported catalyst was consistent with that on metal oxide, and the electronic transport property of the supporting materials plays an important role.215–217 4.1.2.4 Heteropolyoxometalate catalysts. Heteropolyacid compounds with Keggin structure have been utilized in acidcatalyzed reaction due to their superacidic sites, and the acidity and redox properties of 12-heteropoly compounds depends on both the constituent elements of polyanions and countercations. Aouissi et al. investigated the influence of counteraction (Fe, Co) in heteropolyoxometalate catalysts for DMC synthesis in liquid218–220 and gas phase219,221 respectively. Co1.5PW12O40 was most active, and surprisingly, heteropoly compounds did not undergo deactivation by water either in liquid or gas phase. Table 11

4.1.2.5 Other catalysts. Based on the ‘‘acid–basic catalytic mechanism’’, several novel catalysts have been developed lately (Table 11) and the hydrophilicity of the materials is regarded as an additional favorable factor. Metal ion-exchanged meso- and microporous zeolite catalysts have been investigated in DEC synthesis.222 The local environment of exchanged metal and pore structure of zeolite influence the properties and accessibility of the active sites. The mixed Mg–Al oxides from the calcination of hydrotalcite-like materials supported on silica lyogels (LG-HTO) exhibited high activity and stability owing to its tunable and more accessible acidic–basic sites, as well as the hydrophilicity of the LG.223 Srinivas and co-workers found that the calcined zirconium phenylphosphonate phosphite catalyst was superior to most of solid catalysts because of its hydrophobic nature and appropriate acid–base bifunctionality.224 4.1.3 Utilization of new energy source. Although numerous innovative catalysts have been developed and improved continually, this route is still subject to limited yield of dialkyl carbonates and severe processing conditions. Introduction of new techniques supplies other possibilities for improvement of the process. Compared to conventional thermal treatment, new energy sources including microwave, photocatalysis and electrochemical method, have attracted much attention in recent years. Chun et al. reported that microwave-assistance promoted the DMC yield and reduced the reaction temperature and time dramatically, which was attributed to ‘‘hot-spot effects’’ on point

Catalytic activity of novel catalysts for direct carbonylation of alcohols

Catalyst

T/K

P/MPa

Feed

Yield of DMC/DEC

Reactor

Ref.

Ce–H-MCM-41 (1 g, 16 wt% Ce) Ce–Si-MCM-41 (1 g, 32 wt% Ce) HTO (250 mg) LG-HTO (250 mg) ZrPP-1-C (0.16 g) ZrPP-HF-C (0.16 g)

443 443 403 403 443 443

4.5 4.5

314 mmol EtOH 314 mmol EtOH CO2/MeOH = 25/5 mL min1 CO2/MeOH = 25/5 mL min1 16.02 g 16.02 g

0.09 DEC (mmol)/Ce (mmol) 0.09 DEC (mmol)/Ce (mmol) 1.8% based on DMC 15.9% based on DEC 0.095 mmol DMC 0.808 mmol DMC

Batch Batch Fixed-bed Fixed-bed Batch Batch

222 222 223 223 224 224

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Synthesis of DMC in different catalyst-IL systems

Entry

MeOH/mol

IL (mol)

Sel. (%)

Yield (%)

0.0154 0.0154 0.0154 0.0154 0.0154 0.0154 0.494

BmimBr (0.137) EmimBr (0.137) BmimCl (0.137) BmimOH (0.137) BmimBF4 (0.137) EmimBF4 (0.137) ApmimBr (0.024)

88.4 86.9 86.1 42.5 38.3 17.2 94.3

3.9 3.2 1.7 0.4 0.3 0.2 1.06

a

1 2a 3a 4a 5a 6a 7b


Review Article

a

Bimim = 1-butyl-3-methylimidazolium; Emim = 1-ethyl-3methylimidazolium. Reaction conditions: ambient pressure, reaction time: 48 h, voltage 5.5 V. Cathode Pt, anode Pt.236 b ApmimBr = 1-(3aminopropyl)-3-methylimidazolium. Reaction conditions: ambient pressure, reaction time: 40 h, voltage 5.5 V. Cathode graphite, anode Pt.237

defects or a weak surface bond.225 ILs acted as a reaction medium because of their strong microwave adsorption ability.226 In comparison to thermal catalysis, photocatalysis using ultraviolet or visible light as energy source represents a significant energy saving. A higher conversion of methanol has been observed over Cu/NiO–MoO3/SiO2 and Cu/NiO–V2O5/SiO2.227,228 When using copper modified (Ni, V, O) and (Ni, Mo, O) semiconductor complex, the DMC yield increased to 57% at 1 atm with photo-assistance.208,229 Electrochemical techniques are known to have ability to provide preliminary activation of CO2. Electrocatalytic synthesis of organic carbonates has been investigated from phenols, alcohols and CO2 by several groups.230–235 As shown in Table 12, the reactivity was strongly dependent on the composition of the ILs in electrochemical synthesis.236 Amino-functionalized ILs, especially ApmimBr, exhibited great performance due to its efficient capture and activation of CO2.237 4.2

Synthesis by alcoholysis of urea

Heitz and co-workers first reported a new route of the DMC synthesis by alcoholysis of urea in 1980, as shown in eqn (27).238 This process uses the urea and methanol as raw materials, both of which are low cost and abundant. As urea production is one of the most mature technologies for CO2 capture and utilization at present, this route is an indirect transformation of CO2. If the liberated ammonia (the sole byproduct) is recycled by connecting

Table 13

with the urea production, only CO2 and alcohols are therefore consumed. Moreover, this method has several extra advantages, such as facile separation and purification, mild reaction conditions and safe operations. Hence, the process is sustainable and environmental-friendly. Currently, pilot productions with an annual output of thousands of tons for both one-pot and two-step routes have been finished.141,142 It is anticipated that the commercialization of DMC production via alcoholysis urea will be realized in the coming years. Such a breakthrough in the technique of DMC synthesis can not only lower the operation cost, but also explore a new avenue for the reuse of CO2. NH2CONH2 + 2ROH - (RO)2CO + 2NH3 R = CH3, DrG298K = 12.60 kJ mol1, DrH298K = 47.26 kJ mol1, (27) 4.2.1 Choice of catalysts. Many solid-base catalysts have been used for the synthesis of DMC from urea and methanol, and it was found that the basicity of the catalysts played an important role.239–246 Table 13 summarizes catalytic performances of the solid-base catalysts. Comparing with zinc salts, palladium salts and other oxides (e.g. CaO, MgO, ZrO2, La2O3 and PdO), ZnO showed superior catalytic property with the yield of DMC and DEC at B30% and 14.2% respectively, owing to the formation of Zn(NH3)2(NCO)2 species during the reaction.239–242 Such species could be formed via the coordination of NH3 to Zn(NCO)2, which originated from the reaction of ZnO with HNCO; the latter was the product of urea thermal decomposition. The kinetics of DMC synthesis suggested that the space time and reaction temperature were crucial factors rather than urea concentration in the feed.247 Some composite oxides, including ZnO–CeO2, ZnO–CaO, ZnO– Al2O3 and ZnO–CeO2–MO (MO: La2O3, Y2O3, and Co2O3) were also used for the synthesis of DMC from urea and methanol.243–246 It revealed that catalytic performance had a positive correlation with basicity, while no dependency on the acidity was observed.243,244 ZnO–CaO catalysts showed favourable DMC yield (41.2%) with the Zn/Ca molar ratio of 4 : 1, which was attributed to a synergistic effect between ZnO and CaO and the appropriate acidic/basic sites.246 Zn–Al mixed oxides derived from hydrotalcite-type precursors exhibited high DMC yield (B36.5%), because the

Catalytic performance of solid base catalysts for the DMC/DEC synthesis by alcoholysis of urea

Feed Catalyst

MeOH/EtOH

Urea

T/K

t/h

ZnO CaO MgO ZrO2 PdO ZnO CaO La2O3 MgO ZrO2 ZnO (0.64)–CeO2 (0.26)–La2O3 (0.1) ZnO–CaO Zn/Al mixed oxides

128 g 128 g 128 g 128 g 128 g 150 mL 4 mol 4 mol 4 mol 4 mol 40 mL 64 g 64 g

12 g 12 g 12 g 12 g 12 g 15 g 0.2 mol 0.2 mol 0.2 mol 0.2 mol 3g 0.1 mol 0.1 mol

443 443 443 443 443 463 473 473 473 473 443 453 453

8 8 8 8 8 5 11 11 11 11 4 10 10


Urea conv. (%) 100 100 100 100 100 100 100 100 100 100

DMC/DEC yield

Ref.

29% (DMC) 8% (DMC) 1% (DMC) 1% (DMC) 22% (DMC) 14.2% (DEC) 1.99 (DMC, mmol 2.80 (DMC, mmol 3.07 (DMC, mmol 1.78 (DMC, mmol 50.4 (DMC, %) 41.2 (DMC, %) 36.5 (DMC, %)

239 239 239 239 239 241 242 242 242 242 244 246 245

gcat1 gcat1 gcat1 gcat1

h1) h1) h1) h1)

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formation of ZnAl2O4 spinel significantly modified the weak acidic and basic sites.245 ILs can act as promoters of the reaction through their solvating properties by enhancing the polarity and electrostatic field of the reaction medium (methanol and urea), which resulted in activating the two substrates and thus promoting the reaction process.248 Cai and his co-workers have investigated several ILs, such as Et3NHCl–FeCl3, Et3NHCl–ZnCl2, Et3NHCl–CuCl2, Et3NHCl–SnCl2 and emimBr–ZnCl2, for the synthesis of DMC from methanol and urea.248 Among them, Et3NHCl–ZnCl2 and emimBr–ZnCl2 exhibited higher activity (B26% yield) and surprisingly high selectivity (B100%) to DMC. Metallic compounds, e.g. zinc stearate and organotin, were also proved to be good candidates to catalyze DMC synthesis from urea and methanol.234,235 Due to the gasification of methanol at the desired reaction temperature (about 423 K), polyethylene glycol dimethyl ether, a high boiling electron donor solvent, was used as solvent to enable the reaction temperature to reach the desired value (423–443 K) at atmospheric pressure.249 Too high a reaction temperature would lead to the generation of N-alkyl by-products by consuming DMC, and it was vital to improve DMC yield by continually removing DMC and NH3 from the reaction system.250 Fe2O3 supported on HMCM-49 has been investigated by Cai and co-workers, on which the mutual activation of urea and methanol by Fe [O] and H+ occurred.251 The catalyst exhibited 34.2% conversion of urea and 97.4% selectivity to DMC at 453 K as well as excellent reusability. 4.2.2 Reaction of alkyl carbamates with alcohol in two-step urea alcoholysis. Since urea decomposes easily to ammonia and isocyanic acid even without catalysts, the rate-controlling step of this approach is that MC converts to DMC by consecutive reaction with methanol. Moreover, NH3 accumulated in the first step will restrict the shift of the reaction equilibrium to DMC due to the reversibility of eqn (28). Consequently, it is rational to divide this route into two steps as shown in eqn (28) and (29). Although organotin favors the synthesis of symmetrical dialkyl carbonates from alkyl carbamates and the corresponding alcohols, it is not a desirable candidate because of its strong toxicity and high cost.252

Table 14 Catalytic performance of catalysts in the reaction of alkyl carbamates with alcohol

Entry Catalyst 1a 2a 3a 4a 5a 6a 7a 8a 9b 10b 11b 12b 13b 14b 15b 16c 17c 18c 19c 20c 21d 22d 23d 24d 25d 26d

MC DMC NMMC conv. (%) yield (%) yield (%) Ref.

ZnO 5.6 Zn(OH)2 7.5 ZnSO4 10.2 Zn(NO3)2 23.2 Zn(CH3COO)2 36.1 ZnCl2 50.9 ZnBr2 51.1 Zn(NH3)2Cl2 45.7 La2O3 10.2 LaCl3 73.9 LaF3 20.0 La2(CO3)3 12.4 LaPO4 12.2 La(NO3)3 84.8 LaCl3/Cu(NO3)2 79.1 ZnO–Al2O3 physically mixed 5.4 ZnO–Cr2O3 36.8 ZnO–Fe2O3 47.6 ZnO–Al2O3 56.4 ZnAl2O4 9.3 MgFeO 24.07 NiFeO 23.69 CuFeO 31.81 ZnAlO 24.87 ZnCrO 27.64 ZnFeO 66.58

4.2 3.8 3.7 11.9 178 33.6 28.7 29.5 5.9 28.1 6.1 8.2 8.2 53.7 46.0 3.1 23.5 30.4 34.6 4.2 7.82 3.21 2.66 13.91 17.04 31.48

0 0 0 0 0 9.1 7.8 7.0 0 8.3 0 0 0 14.1 12.2 0 4.2 6.4 7.1 0 0 8 0 3.63 4.01 9.53

253 253 253 253 253 253 253 253 256 256 256 256 256 256 256 152 152 152 152 153 153 153 153 153 153 153

Reaction conditions: MC, 7.5 g; MeOH, 64 g.a Catalyst, 1.0 g; T: 453 K; reaction time, 8 h. b Catalyst, 7.4 mmol; T: 463 K; reaction time, 10 h. c Catalyst, 2.0 g; T: 453 K; reaction time, 10 h. d Catalyst, 1.0 g; T: 453 K; reaction time, 10 h.

(28)

(29)

Sun and his colleagues have made great contributions to the second reaction. Table 14 summarizes the catalytic properties of Zn and La compounds in the reaction of alkyl carbamates with alcohols. MC was activated by Zn2+ through the coordination of the N atom and the essential reaction mechanism was proposed in Scheme 7.253,254 A further DFT study was conducted on the reaction mechanism in the presence of Zn(NH3)2(NCO)2.255 It was noteworthy that on trivalent metal cations, such as La3+, MC was activated via the coordination of the oxygen atom in the

Chem. Soc. Rev.

Scheme 7 Possible reaction mechanism of reaction of MC and methanol over ZnCl2. Reprinted with permission from ref. 254. Copyright 2012, American Chemical Society.

carbonyl group, which was different from Zn2+.256 Furthermore, Zn–Fe and Zn–Al composite oxides have been investigated to overcome the difficulties of recovery and reuse of catalyst. It was demonstrated that preparation methods had a remarkable effect on the activity, and Zn–Fe oxide derived from calcined hydrotalcitelike compounds was more reactive than the samples obtained by


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physical mixing and co-precipitation method.257–259 In order to further improve DMC yield in comparison with batch processes, a new process using a fixed-bed reactor was proposed to facilitate the reaction owing to removal of NH3 from the reaction zone and significant minimization of N-methyl methyl carbamate (NMMC) generation and DMC thermal decomposition. Compared to DMC, DEC synthesis via reaction between ethyl carbamate and ethanol has been rarely reported. Zhao et al. suggested Pd oxide catalyst displayed excellent stability and high activity due to the synergistic action between cubic metal Pb and orthorhombic PbO2.260 In follow-up studies, they found that EC and ethanol are simultaneously activated by Zn(NCO)2 and metal Pb respectively, when using ZnO–PbO as initial catalyst.261 4.3

Synthesis by transesterification reactions

4.3.1 Two-step synthesis from CO2, epoxides and methanol. Dialkyl carbonate synthesis by transesterification of cyclic carbonates and alcohols consists of two steps: the reaction of epoxides with CO2 affords cyclic carbonates first (eqn (30)); then, a cyclic carbonate (e.g., ethylene carbonate (EC) or PC) reacts with an alcohol by ester exchange reaction to produce dialkyl carbonates, such as DMC, DEC, etc. (eqn (31)). Ethylene glycol (EG) or propylene glycol (PG) can be synthesized concomitantly, which enhances the economic efficiency of this route. The process converts CO2 to highvalue chemicals under mild operating conditions. It is by far the most mature alternative for the synthesis of dialkyl carbonates that have been commercialized.16 However, the production of the epoxides represents a constraint and cost driver in production of the carbonates. Moreover, it also involves hazardous compounds and risk of explosion.

(30)

(31)

Cyclic carbonates can be synthesized by the insertion of CO2 into epoxides with a high conversion and selectivity under moderate conditions, both of which could be close to 100% in the first step. The recent book of Dibenedetto and Angelini on the synthesis of organic carbonates offers a good general review on the subject.262 There are also some excellent reviews by different research groups focusing on the transformation of CO2 into cyclic carbonates.137,140,263,264 Hence, the key point of this route is how to improve the conversion of cyclic carbonates and the selectivity of dialkyl carbonates in the second step. A number of acid and base catalyst systems, including tertiary amine and quaternary ammonium functionalized resins, group VB and VIB bases, ammonium-exchanged Y zeolites, zirconium, and titanium and tin soluble catalysts, have been evaluated by Knifton et al.265 In general, the base catalysts appeared to be more effective because of their comparable pK values and the


activity increased with increasing operating temperatures and base strength. 4.3.1.1 Homogeneous catalysts. Since transesterification of cyclic carbonates and alcohols to generate dialkyl carbonates follows a nucleophilic substitution mechanism, basic compounds are considered as promising catalysts and the strength of nucleophilicity plays a crucial role in their activity. The catalytic activities of different homogenous catalysts are listed in Table 15. For soluble inorganic bases, alkali-metal bases including KOH, LiOH, NaOH provided a great catalytic activity.266 Differing from inorganic bases, the diversity of organic bases offered more possibility to modulate the basicity and basic strength. Alkalimetal alkoxides are applied due to their stronger alkalinity than corresponding inorganic bases. Among them, sodium alkoxide was considered as the most promising catalyst for industrial application because of its low price and excellent performance.267–270 However, considering corrosivity of sodium alkoxide, nonionic strong nitrogen bases were investigated by Williams et al.271 As shown in Table 15, entries 16–23, these strong nitrogen bases performed approximately as well as alkoxides, such as CH3ONa, CH3OLi. In consideration of the recycling of catalysts, Jagtap et al. applied poly(4-vinylpyridine) as a novel, homogeneous base catalyst and it could be separated from reaction system by distillation or phase separation and showed little loss of activity after three recycles.272 Recently, ILs have been also utilized as catalysts for dialkyl carbonates synthesis via transesterification, due to their unique advantages we have mentioned above. It was noteworthy that the activity of the IL catalysts depended on the bulkiness of the cation as well as the nucleophilicity of the anion (Table 15, entries 5–11 and 24–35).273–275 Some controversial experimental results about the influence of the length of alkyl chain on catalytic activity have been reported in the literature. Park’s group suggested that those with longer alkyl chain substituent on the cation and more nucleophilic anions showed better catalytic performance.274 However, Yang et al. ascribed the decrease of activity and selectivity with the increase of alkyl length and the reduction of hydrophilicity as well as solubility for DABCO-derived basic ILs.276 Afterwards, Zhang and co-workers found that carboxylic functionalized imidazolium salts were more reactive compared with typical ILs, with the larger steric hindrance giving more monoester products.277 Interestingly, microwave assistance revealed less significance of the alkyl chain length of the cation on the catalytic activity.275 In order to simplify the catalyst separation, immobilization of IL has also attached researchers’ interests, and is discussed below. 4.3.1.2 Heterogeneous catalysts. Immobilization of ILs onto supports can simplify the separation process while retaining catalytic properties. Commercial silica, MCM-41 and polystyrene resin (PS) were utilized to immobilize ILs by impregnation or onepot synthesis, and the heterogenized catalysts were easily recovered and reused or allowed to continuously perform in a fixed bed reactor without any considerable loss of activity.277–279 However, rigid cross-linking of PS and narrow mesopores (3–5 nm) of MCM-41 may restrain the immobilization of ILs as well as the diffusion of reactants and products. Therefore, Xu et al. used

Chem. Soc. Rev.

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Review Article


Table 15

Chem Soc Rev

The comparison of the activity of various soluble base catalysts

Entry

Catalyst

T/K

P/MPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

KOH (0.1 g) K2CO3 (0.1 g) LiOH (0.1 g) NaOH (0.1 g) TBACa (2 mmol) TPACla (2 mmol) THACla (2 mmol) TOACla (2 mmol) TDodACla (2 mmol) TBABra (2 mmol) TBAIa (2 mmol) Polyvinylpyrrolidone (0.44 g) PVPb (0.44 g) LiOMe (8.4 mmol) NaOMe (8.4 mmol) DABCOc (8.4 mmol) DBNc (8.4 mmol) DBUc (8.4 mmol) MTBDc (8.4 mmol) TBDc (8.4 mmol) 1ac (8.4 mmol) 1bc (8.4 mmol) 1cc (8.4 mmol) EMImCld (2 mmol) BMImCld (2 mmol) HMImCld (2 mmol) EMImBF4d (2 mmol) EMImPF4d (2 mmol) [C4DABCO]OH (0.1 mmol) [C4DABCO]Cl (0.1 mmol) [C4DABCO]Br (0.1 mmol) [C8DABCO]Br (0.1 mmol) [C8DABCO]PF6 (0.1 mmol) [C8DABCO]NTf2 (0.1 mmol) Me4NBr (0.1 mmol) DMICe (0.1 mmol) Choline chloride EtONa IMesHCl, tBuOKf (5 mol%) ICyBF4, tBuOKf (5 mol%) SIPrHCl, tBuOKf (5 mol%) SIMesHCl, tBuOKf (5 mol%)

298 298 298 298 413 413 413 413 413 413 413 413 413 393 393 393 353 353 353 353 353 353 353 413 413 413 413 413 343 343 343 343 343 343 343 383 383 303 313 298 298 298

0.34 0.34 0.34 0.34 2.07 2.07 2.07 2.07 2.07 2.07 2.07 0.1 0.1

1 1 1 1 6 6 6 6 6 6 6 4 4

1.38 1.38 1.38 1.38 1.38

6 6 6 6 6 4 4 4 4 4 4 4 1.33 1.33

a

t/h

10 10 10 10

Feed (mmol/mmol)

Conv. (%)

Yield (%)

Ref.

EC/MeOH = 1/4 EC/MeOH = 1/4 EC/MeOH = 1/4 EC/MeOH = 1/4 PC/MeOH = 25/200 PC/MeOH = 25/200 PC/MeOH = 25/200 PC/MeOH = 25/200 PC/MeOH = 25/200 PC/MeOH = 25/200 PC/MeOH = 25/200 PC/MeOH = 25/200 PC/MeOH = 25/200 PC/MeOH = 420/1730 PC/MeOH = 420/1730 PC/MeOH = 420/1730 PC/MeOH = 420/1730 PC/MeOH = 420/1730 PC/MeOH = 420/1730 PC/MeOH = 420/1730 PC/MeOH = 420/1730 PC/MeOH = 420/1730 PC/MeOH = 420/1730 EC/MeOH = 25/200 EC/MeOH = 25/200 EC/MeOH = 25/200 EC/MeOH = 25/200 EC/MeOH = 25/200 EC/MeOH = 10/100 EC/MeOH = 10/100 EC/MeOH = 10/100 EC/MeOH = 10/100 EC/MeOH = 10/100 EC/MeOH = 10/100 EC/MeOH = 10/100 EC/MeOH = 20/200 EC/MeOH = 20/200 PC/EtOH = 1/8 EC/MeOH = 1/3 EC/MeOH = 1/3 EC/MeOH = 1/3 EC/MeOH = 1/3

42.66 55.26 61.94 56.70 58.8 48.9 59.0 60.2 61.6 43.2 37.6 56 96

40.79 55.17 61.52 55.12 47.6 33.4 39.5 41.2 41.6 33.3 28.0 50 82 78 77 74 74 75 75 75 70 71 71 76.2 68.5 60.4 41.2 53.7 50 25 26 22 30 34 41 81 45

266 266 266 266 273 273 273 273 273 273 273 272 272 271 271 271 271 271 271 271 271 271 271 274 274 274 274 274 276 276 276 276 276 276 276 277 277 268 433 433 433 433

76.3 74.1 67.0 60.8 56.3 72 57 46 55 58 55 76 82 68 66

88 79 81 84

Tetrabutylammonium (TBA+), tetrapropylammonium (TPA+), tetrahexylammonium (THA+), tetraoctylammonium (TOA+), tetradodecyl-

ammonium (TdodA+). b Poly(4-vinylpyridine) (PVP). c

. d 1-Ethyl-3-methylimidazolium

(EMIm), 1-n-butyl-3-methylimidazolium (BMIm), 1-n-hexyl-3-methylimidazolium (HMIm). e 1,3-Dimethylimidazolium-2-carboxylate (DMIC). f

.

mesocellular silica as supports to promote the catalytic activity due to its 3D mesoporosity with a ultra-large pore size (20–50 nm), which was beneficial for enhanced amount of immobilized ILs as well as mass transfer in the reaction.280 To date, a variety of heterogeneous catalysts, such as modified molecular sieves,281–285 single or mixed metal oxides,261,286–301 basic anion-exchange resin,302,303 smectite,304 hydrotalcite and derivative,258,305,306 dawsonite,307 and graphitic carbon nitride308

Chem. Soc. Rev.

have been developed in DMC and DEC synthesis via transesterification. The activity and selectivity of typical catalysts are summarized in Table 16. Sun and co-workers have performed intensive and systemic research on this reaction.2 Here, we review the published work focusing on the development of catalyst design, consisting of three parts: (1) basicity and base strength, (2) stability and recycling, and (3) dependence on the length of carbon chain.


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Activity, selectivity and reaction conditions of transesterification over heterogeneous catalysts

EC/PC Conversion (%)

Dialkyl carbonate Yield (%)

Reactant (mmol/mmol)

Reaction conditions

Reactor

Ref.

K-TS-1 (0.089 g) Hydrotalcite (Mg/Al = 2.5) (89 mg) Smectite (Ni–Na–Li) (0.45g) MgO (1.80 wt%) CaO (1.82 wt%) CaO/Carbon (1.80 wt%) n-Bu2N-MCM-41 (3.5 g) CaO–ZrO2 (15 g) Mixed-CaO–ZrO2 (0.5 g) IL-MgO–CeO2 (1 g) MgO–CeO2 (Ce = 24.4 mol%) Na-mesoporous CeO2 (0.08 g) Na-ZrO2 (0.4 g) Amberlyst A-21 (2.25 g) Na-dawsonite (10 g) KF/g-Al2O3 (0.5 g) THA-MS41 (0.2 g) ZnO2-Y2O3 (0.07 g) NaZSM-5 (0.25 g) Eggshell (0.013 g) Pr3+-modified HT (0.21 g) MgO@meso-SiO2 (0.03 g) Mesocellular SiO2-[SmIm]OH (0.5 g) Mesostructured graphitic carbon nitride (0.15 g) Fe–Zn double-metal cyanide complexes (0.25 g)

68 70 71.2 92 57 43 42 97 52 65.3 67 N/A 53 96 N/A N/A 78.4 55 79.3 81 34.1 90 85.0 81.8

57 58 66 36 48 41 N/A N/A N/A 56.6 64 64.6 N/A 32 65 45.2 76.3 54 77 79 31.3 N/A 82.9 81.3

EC/MeOH = 31/124 EC/MeOH = 31/124 EC/MeOH = 25/200 PC/MeOH = 1/4 PC/MeOH = 1/4 PC/MeOH = 1/4 EC/MeOH = 1/8 PC/MeOH PC/MeOH = 1/6 EC/MeOH = 1/4 EC/MeOH = 1/8 EC/EtOH = 0.1/0.5 PC/MeOH = 0.1/0.5 EC/MeOH = 1/8 EC/MeOH = 1/4 EC/EtOH = 40/400 EC/MeOH = 25/200 EC/MeOH = 30/240 EC/MeOH = 30/240 PC/MeOH = 1/10 PC/MeOH = 10/100 EC/MeOH = 1/8 EC/MeOH = 1/10 EC/MeOH = 25/250

Reflux condition Reflux condition 423 K, 1 MPa 433 K 283 K 323 K 423 K, 0.2 MPa LHSV, 3 h1 423 K, 0.5 MPa LHSV, 0.03 h1 413 K 423 K, 0.2 MPa, LHSV, 3 h1 423 K, 0.2 MPa, LHSV, 3 h1 403 K, 0.1 MPa 433 K 393 K 343 K, 0.1 MPa 323 K, 0.1 MPa 453 K, 1.17 MPa 338 K, 0.1 MPa 343 K 298 K, 0.1 MPa 423 K 413 K, 0.1 MPa 338 K, 0.1 MPa 433 K, 0.6 MPa

Autoclave Autoclave Autoclave Autoclave Autoclave Autoclave Tube reactor Tube reactor Autoclave Tube reactor Tube reactor Flask Autoclave Autoclave Autoclave Autoclave Autoclave Autoclave Autoclave Flask Autoclave Flask Flask Autoclave

281 305 304 286 286 287 283 288 289 291 292 298 293 302 307 295 279 297 284 300 306 299 280 308

N/A

Autoclave

310

62 65 60 50 59.8

PC/MeOH = 10/100 PC/EtOH = 10/100 PC/Propanol = 10/100 PC/Butanol = 10/100 PC/ Hexanol = 10/100 PC/MeOH = 1/8 PC/MeOH = 1/4 PC/MeOH = 1/8 PC/MeOH = 1/8 PC/MeOH = 1/10 EC/MeOH = 1/5

443 K

Amberlyst 26 OH (1 g)

86.6 79.4 77.5 69.3 62.5 61 60 57 55 55.9 32.4

313 263 313 263 423 338

Flask Tube reactor Flask Tube reactor Autoclave Flask

303

Catalyst


Review Article

PA312L/OH (1 g) Mg/La oxides (Mg/La = 1/3, 80 mg) Li/C–SiO2 (0.5 wt% of methanol)

For transesterification of alcohols and cyclic carbonates, the main function of the solid base is to abstract H+ from alcohol molecules to form alkoxy species. Therefore, the strength of base dominates the rates of reaction, while the amount of basic sites determines the reaction rate and product contribution. Additionally, well-developed porosity and high surface area usually offer more exposed basic sites and enhance their accessibility to reactants. Therefore, many efforts have been made with respect to these issues. Basic metal oxides are considered as a promising catalyst system due to their strong and large amount of basic sites, low cost and preparation process. Compared to MgO, CaO apparently accelerates the reaction of MeOH and PC at low temperature due to its higher base strength and larger numbers of basic sites.286,309 Incorporation of Ca2+ into ZrO2 by coprecipitation was proved favorable for DMC synthesis,289,290 because neighboring Ca2+ and Zr4+ in homogeneous CaO–ZrO2 solid solution increased the basicity of lattice oxygen on the surface. Deng et al. found that the generation of large amount of medium basic sites (7.2 o H o 9.8, as determined by Hammett indicator method) in ZnO–Y2O3 was responsible for the superior catalytic activity, which depended on the elemental composition and calcination temperature.297 A similar conclusion on smectite


K, 0.1 MPa K WHSV = 0.9 h1 K, 0.1 MPa K WHSV = 0.9 h1 K K, 0.1 MPa

303 301 285

catalyst was proposed by Bhanage et al.304 Abimanyu et al. demonstrated that the dopant of Ce into MgO facilitated reaction because of the enhancement of basicity and base strength. However, a high proportion of strong basic sites, which resulted from the high concentration of cerium content, had a negative effect on DMC generation by producing by-products.292 In order to modify the basicity, they utilized ILs as template material to prepare Ce–Mg mixed oxides.291 As a typical solid-base catalyst, hydrotalcite-type materials were also investigated for DMC synthesis. It was claimed that the intercalated anions affected the amount and strength of basic sites, which dominated the conversion of EC and the yield of DMC.305 Rare-earth element modification of Mg–Al hydrotalcites resulted in the enhancement of DMC yield, and a linear correlation between PC conversion or TON and surface density of basic sites (Fig. 6) was reported by Srinivas’s group.306 Despite their appropriate basicity, low specific area and restricted porosity of metal oxides limit their catalytic efficiency. Mesostructured materials were used to provide more accessible basic sites. Improvements of the catalytic performance was so achieved through mesoporous metal oxides prepared by NaOH post-treatment and one-pot synthesis, such as Na–ZrO2 and CaO–ZrO2, Na–CeO2.298 The hierarchical flower-like MgO coating on mesoporous silica also exhibited good performance for DMC production.299

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Chem Soc Rev


This approach is considered as one of the promising reactions in the development of environmentally benign processes based on the utilization of CO2. The introduction of epoxides can promote the activation of CO2 and play a key role in the process of the direct synthesis of DMC.

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Fig. 6 Correlation between catalytic activity and density of basic sites on La-hydrotalcites calcined catalysts. Reprinted with permission from ref. 306. Copyright 2012, American Chemical Society.

Introduction of a second component (e.g. Na+ and Ca2+ into the ZrO2 lattice) can successfully provide a binding effect to inhibit the leaching of basic sites in a continuous reaction system or during catalyst separation.288–290,293,294,296 Besides, particle size and mechanical strength also influence the stability with respect to industrial applications. Utilization of supports with large surface area and excellent mechanical properties, such as carbon and mesoporous silica were reported to stabilize the active phase during separation and recycling.287,299 The molecule size of alcohols and cyclic carbonates also have a profound effect on the yield of dialkyl carbonates due to the spatial effects and pore diffusion limitation. With increasing alkyl chain length of alcohols and steric hindrance of alkylene carbonates, both the yield and selectivity of dialkyl carbonates decreased obviously over different catalysts, including microporous zeolite, Amberlyst A-21 and Fe–Zn double-metal cyanide complexes.302,310 4.3.2 One-pot synthesis from CO2, epoxides and methanol. Unlike two-step transesterification, one-pot synthesis of DMC from CO2, methanol and epoxides (e.g. ethylene oxide (EO) or propylene oxide (PO)) have some advantages, such as low energy consumption, low investment and production costs owing to avoidance of the intermediate cyclic carbonate separation (eqn (32)).

Table 17

Basicity is advantageous to the catalyzed reaction of one-pot synthesis of dialkyl carbonates from CO2, alcohol and epoxides. Up to now, the catalysts used for this reaction system include metal oxides, smectite, alkaline metal salts (halides, hydroxides and carbonates), amines, quaternary ammonium salts, ILs, and combinations of the above compounds. The catalytic activities of metal oxides and alkaline metal salts supported on oxides catalysts which showed excellent catalytic performance in DMC synthesis are listed in Table 17. Arai and co-workers have investigated several metal oxides (MgO, CaO, ZnO, ZrO2, La2O3, CeO2 and Al2O3) used for this reaction, and found that MgO was the best catalyst for both two-step and onepot synthesis.311 The influence of the alcohol chain length on the catalytic activity was studied, which indicated that the activity decreased as the carbon number of the alcohol increases (Table 17, rows 1–3). Characterization results suggested that both strong and moderate basic sites on the catalysts were active for the reaction of the epoxides and CO2, while only the latter was responsible for the reaction of cyclic carbonates and methanol. Although the DMC production can be realized in one-pot synthesis, the selectivity is not satisfactory because of the alcoholysis of the epoxide. Further they used a Mg containing smectite catalyst (S-Mg) which could avoid the undesirable methanolysis reaction of PO for the one-pot synthesis of DMC, and enhanced the selectivity of DMC effectively.312 Since the basicity was beneficial to the reaction, some basic components were introduced into the catalysts to improve the catalytic activity.313,314 Fujita and co-workers added alkali-metal hydroxides, such as NaOH, KOH and LiOH, into Mg-containing

Catalytic activities of basic catalysts in one-pot synthesis

Feed

Selectivity (%)

Catalyst

mol (epoxide)

CO2/MPa

MeOH/mol

T/K

t/h

Epoxide conv. (%)

EC/PC

DMC

EG/PG

Ref.

MgO MgO MgO S-Mg Na2CO3–KCl/ZnO K2CO3–KCl/MgO K2CO3–KCl/MgOa K2CO3–KCl/Al2O3 K2CO3–KCl/Al2O3a

0.062 (EO) 0.025 (PO) 0.055 (styrene oxide) 0.021 (PO) 1 (PO) 1 (PO) 1 (PO) 1 (PO) 1 (PO)

8 8 8 8 2.5 2.5 2.5 2.5 2.5

0.2 0.2 0.2 0.2 4 4 4 4 4

423 423 423 423 433 433 433 433 433

15 15 15 15 5 5 5 5 5

96.1 99.2 98.4 95 85.8 84.9 100 85.7 100

31.4 14.4 13.8 24 34.0 32.3 1.2 35.4 18.6

28.0 13.6 12.5 34 21.6 21.4 46.2 20.1 35.4

26.3 14.8 0.0 36 36.1 40.0 50.1 36.7 40.1

311 311 311 312 314 314 314 314 314

a

Calcined under air atmosphere at 873 K for 3 h.

Chem. Soc. Rev.


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mesoporous smectites, and found that increasing the content of alkali-metal caused an increase of DMC yield and a decrease of propylene epoxide (PO) methanolysis.313 Jiang and co-workers demonstrated that supported double basic component (Na2CO3– KCl) on Al2O3, ZnO, MgO promoted the catalytic one-pot synthesis of DMC.314 Calcination could improve the basicity of the catalysts, which in turn enhanced the catalytic activities (Table 17, rows 6–9). A variety of amines with different structures and basicities are attractive and readily controlled as catalysts for the one-pot coproduction of dialkyl carbonates and 1,2-diols. Kishimoto et al. investigated the influence of the reaction conditions on the yield of dialkyl carbonates (DMC, DEC, dibutyl carbonate) in detail.315 The reactivity measurements showed that 1,8diazabicyclo[5.4.0]undec-7-ene served as a suitable catalyst, and the yields decreased with the increase of carbon chain length of alcohols under similar reaction conditions. High initial CO2 pressure was found to favor dialkyl carbonates formation and supercritical CO2 can increase the mass transfer efficiency of the reactants during the reaction. Therefore, one-pot synthesis of DMC under reaction conditions at which CO2 was in the supercritical state was studied.316–318 KI and K2CO3 were efficient catalysts in this reaction process.316,318 Wang and co-workers have demonstrated that the selectivity of DMC and the conversion of EO could reach 73.0% and 100.0%, respectively, over a mixture of KI and K2CO3 (weight ratio of KI/ K2CO3 = 1 : 1).316 Han and co-workers have supported KI or KI combined with K2CO3 on ZnO, and the catalysts were very active and selective for the reaction after calcination.318 At 423 K, the yields of DMC and glycol exceeded 57% when EO was used. It has been proved that the quaternary ammonium salts are active for the cycloaddition of epoxide and CO2, while Lewis bases or acids can successfully promote the transesterification reaction. Based on this point, a homogeneous binary catalyst system of nBu4NBr/n-Bu3N was applied for one-pot synthesis of DMC from methanol, styrene oxide and supercritical CO2, and the DMC yield could reach 84% at styrene oxide conversion of 98%. A possible mechanism (Scheme 8) was also proposed by He’s group, which involved two cycles: (1) the n-Bu4NBr-catalyzed cycloaddition of CO2; (2) the n-Bu3N-promoted transesterification reaction of cyclic carbonate with methanol.318 Inspired by this result, Deng and co-workers synthesized novel ILs with a tertiary amino moiety and a quaternary ammonium group for DMC synthesis from EO, CO2 and methanol. Among the evaluated bifunctional ILs catalysts, 99% EO conversion and 74% DMC selectivity were achieved by using 6-(N 0 ,N 0 -dimethylamino)-1-(N,N,N-trimethylammonium)hexane iodide as catalyst under relatively mild reaction conditions, which could be attributed to its stronger basicity by altering anions. Moreover, these ILs catalysts can be recovered for several times.319 Cai and his co-workers found that basic IL (choline hydroxide) supported on MgO showed a good catalytic performance with 98% conversion of PO and above 90% selectivity to DMC and PC.320 4.3.3 DEC and DPrC synthesis from transesterification of DMC and ethanol or propanol. For the synthesis routes mentioned above, an increase in the number of carbon atoms of the alcohol results in a decrease in both yield and selectivity of the dialkyl carbonates, because of increasing steric hindrance and inactivity of long-chain


Review Article

Scheme 8 A possible mechanism for the present n-Bu4NBr/n-Bu3Ncatalyzed one-pot synthesis of DMC. Reprinted with permission from ref. 318. Copyright 2006, Elsevier.

alcohols.284,302 Thus, dialkyl carbonates with more than one carbon atom in alkyl group, such as DEC and DPrC, are also considered to be produced indirectly by the transesterification of DMC with alcohols. The reaction is a two-stage process (eqn (33) and (34)): (CH3O)2CO + ROH 2 CH3OCOOR + CH3OH R = C2H5, DrG333K = 8.41 103 kJ mol1, DrH333K = 5.94 kJ mol1 R = C3H7, DrG300K = 1.34 kJ mol1, DrH300K = 0.27 kJ mol1 (33) CH3OCOOR + ROH 2 (RO)2CO + CH3OH R = C2H5, DrG333K = 7.37 105 kJ mol1, DrH333K = 1.13 kJ mol1 R = C3H7, DrG300K = 2.10 kJ mol1, DrH300K = 0.27 kJ mol1 (34) This system consists of two parallel reactions with respect to ethanol/propanol, while two consecutive reactions with respect to ethyl/propyl methyl carbonate (EMC/PMC).321 The process can be catalyzed by homogeneous base catalysts, such as alkali-metal methoxides, however, these catalysts are toxic,

Chem. Soc. Rev.

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flammable, corrosive and sensitive to air and water.322 Li and co-workers found that lanthanide triflates and lanthanum nitrate were efficient homogeneous catalysts.323,324 In addition, these two types of catalyst are stable to air, water and the reaction mixture, and are reusable and easy to handle. The results of the heterogeneous catalysts screening, including zeolite ion-exchange resins, hydrotalcites, supported K2CO3 and ZrO2, indicated that base catalyst appeared more competitive.321 Additionally, diffusion limitation also influenced the catalytic performance. Compared to AC and Hb, MCM-48 supported K2CO3 showed better catalytic performance due to its large surface area and large pore volume.325–327 Murugan et al. reported that supported alkali halides also exhibit excellent activity due to their high strong basic nature, and 20 wt% KF/Al2O3 gave 61.6% DEC selectivity and 96% DMC conversion.328 Over calcined hydrotalcite-like compounds containing La, the maximum conversion of DMC and selectivity of DPrC reached above 98% and 95.4%, respectively. Ma et al. discovered that La content played an important role in tuning the structural and morphological properties as well as moderate basic sites of the catalysts, which are crucial factors of catalytic performance.329 4.3.4 EMC synthesis from transesterification of DMC and DEC. EMC, as an ideal cosolvent in a nonaqueous electrolyte, has a great application potential by significantly improving discharge characteristics of the cells such as the energy density, discharge capacity, etc.330 However, its production and fairly high price have limited its widespread application. Conventional esterification of methyl chloroformate with ethanol using basic catalysts has environmental and safety problems. In comparison, the synthesis of EMC from DEC and DMC avoids the utilization of highly toxic reactant, chloroformate (eqn (35)). Table 18 summarizes the catalytic activity of various catalysts reported in the literature. CH3OCOOCH3 + C2H5OCOOC2H5 - 2CH3OCOOC2H5 DrG300K = 3.46 kJ mol1, DrH300K = 0.03 kJ mol1

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Ti(OBu)4 and Bu2SnO are applied as homogenous catalysts for the synthesis of EMC with high activity as well as remarkable selectivity. However, the disadvantage of separation and high cost of the catalyst restricts their further application.331 Therefore, both base and acid heterogeneous catalyst are applied

Table 18

as alternatives in the synthesis of EMC. MgO, as a typical base catalyst, showed similar catalytic properties to Bu2SnO either in liquid or vapor phase.330 However, high operating temperature gave rise to coke formation, resulting in a decrease of conversion. Thus, the development of efficient catalysts under mild conditions deserves to be paid attention. Jia and his colleagues attempted to fabricate carbon-supported MgO nanocomposites using porous carbon as support.332 The presence of rich oxygencontaining surface groups was in favor of the high dispersion of MgO particles, so increasing the activity and stability of the catalyst. Study on Al–Zn-MCM-41 and Al-MCM-41 catalyst showed that the activity of the catalysts followed the order of the acidity of the catalysts.333 Han and co-workers proved that MOFs could catalyze the reaction smoothly at 373 K, acting as a Lewis acid catalyst.334 Additionally, materials with large surface and acid-basic bifunctional groups, including amorphous mesoporous aluminophosphate, zeolitic imidazolate framework, and mesoporous MgAl2O4 spinel, were reported as good candidates for EMC synthesis.335–337

5. Process intensification 5.1

Elimination of water

In consideration of a severe thermodynamic and equilibrium limitation, hydrolysis of the produced carbonate and catalyst deactivation caused by H2O, many efforts have been made in recent years to search for effective dehydrating agents under reaction conditions in direct synthesis of dialkyl carbonates from CO2 and alcohols. Since conventional inorganic dehydrating agents exhibit limited performance due to their reversibility under the relatively high reaction temperature, organic dehydrating agents such as orthoesters, acetal, nitriles and epoxides have been applied to make the reaction thermodynamically favorable and shift the original equilibrium towards dialkyl carbonates production. Sakakura and Kohno have given a detailed review in 2009 and summarized the published results of catalytic measurements in the presence of dehydrating agents.138 Tomishige and co-workers have paid great efforts on the investigation of the catalytic direct synthesis of organic carbonates from CO2 and alcohols combined with dehydration systems. Recently they summarized recent progress

Catalytic performance of EMC synthesis over different catalysts

Entry

Catalyst

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Ti(OBu)4 Ti(OPh)4 Bu2SnO Bu2SnCl3 MgO La2O3 ZnO CeO2 Al–Zn-MCM-41(50) MgO/porous carbon Mesoporous AlPO Zn4(O)(BDC)3 Hydrotalcite ZIF-8 MgAl2O4 spinel

Chem. Soc. Rev.

Reactant conv. (%)

EMC yield (%)

DMC/DEC (mmol/mmol)

Reaction conditions

Ref.

43.1 (DEC) 41.7 (DEC) 45.6 (DEC) 43.3 (DEC) 44.2 (DEC) 12.6 (DEC) 26.5 (DEC) 7.3 (DEC) 85 (DMC) 49 (DEC) 49 (DEC) 50.1 25.9 34.0

42.8 41.5 45.6 43.3 44.2 12.6 26.5 7.3 89 N/A N/A 50.1 25.9 34.0 49.0

1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1

376 376 376 376 376 376 376 376 448 376 366 373 373 373 373

331 331 331 331 330 330 330 330 333 332 337 334 334 335 336

K, K, K, K, K, K, K, K, K, K, K, K, K, K, K,

3h 3h 3h 3h 4h 4h 4h 4h LHSV: 3 h1 10 min 60 min 3h 3h 3h 0.5 h


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on this subject. They categorized the dehydration systems into nonreactive dehydration systems and reactive dehydration systems to discuss the features and effects of the dehydration systems.338 In this review, we only focus on the recent advances on utilization of dehydrating agents from 2009 until now. Nitriles including acetonitrile, benzonitrile, 2-cyanopyridine etc., are proposed as the most efficient dehydrating agent for organic carbonate synthesis.338–341 Recently, almost complete conversion of methanol in the direct synthesis route was firstly reported when comprising a heterogeneous CeO2 catalyst and 2-cyanopyridine, and the regeneration of nitrile by dehydration of 2-picolinamide can be realized over a Na2O/SiO2 catalyst.342 Not only that, various organic carbonates with longer alkyl or phenyl groups as well as cyclic carbonates can be obtained in high yields mediated by 2-cyanopyridine or acetonitrile.340,343,344 Investigation of various nitriles revealed that having a CN group at the 2-position relative to N atom of 6-membered ring heteroaromatics led to better dehydrating efficiency.343 Subsequently, Bansode and Urakawa employed 2-cyanopyridine as a recyclable dehydrating agent to mediate direct synthesis of DMC from CO2 and methanol in a fixed bed continuous flow reactor, which showed an excellent (495%) methanol conversion and high DMC selectivity (499%). This study presents new opportunities to achieve high DMC yield (up to 1 gDMC gcat1 h1) with a much shorter reaction time than in a batch operation.345 In addition, methoxide ILs can also function as a water trapping medium to shift the reaction equilibrium towards DMC formation and limit byproduct formation.346 As mentioned above, introduction of dehydrating agents indeed facilitates the production of dialkyl carbonates. However, both inorganic and organic dehydrating agents have superiorities and inevitable limitations. Therefore, some other strategies accompanied with application of dehydrating agents, particularly on equipment remodeling, were developed for H2O removal. Sakakura’s group designed a separated dehydrater from the reaction system, in which 3A molecular sieve were kept at room temperature, meanwhile a part of the reaction mixture is circulated through the dehydrating tube by a high-pressure circulation pump.347 The yield of DMC improves from 2.1% to 29.2% in this system. Li et al. have investigated a membrane catalytic reactor applied for DMC synthesis, and found that polyimide–silica hybrid membrane promoted catalytic activity even at lower pressure due to its excellent thermal and hydrophilic properties.348 For the oxidative carbonylation process, H2O slightly decreased the conversion of alcohols and the selectivity of dialkyl carbonates from CO.29 A coupling reaction, hydrolysis of diethyl ether was introduced to the reaction system of DEC synthesis to reduce the negative influence of H2O.349 The STY of DEC has been improved with a decrease of the water content in the system and the coupling reaction exhibits sensitive dependence on the temperature. 5.2

Reactive distillation (RD)

For alcoholysis of urea, the reaction equilibrium was thermodynamically unfavorable. In addition, it conformed to a consecutive reaction. Major side reactions, including the thermal decomposition of DMC and the reaction between MC and DMC,


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reduced the DMC yield in the batch process. Therefore, researchers have focussed on improving the yield of dialkyl carbonates by removing NH3 and carbonates effectively and quickly from the reaction system.241,247,249 Sun and co-workers have developed a catalytic distillation technique to minimize the side reactions and break the unfavorable equilibrium. A nonequilibrium model of the catalytic distillation at the bench scale was proposed.350 In the catalytic distillation reactor, the DMC yield reached 60–70% over a Zn-based catalyst.351 The reactant conversions of transesterification of cyclical carbonates and alcohols are also strongly limited by their unfavorable chemical equilibrium.268,352 Furthermore, the formation of binary azeotropes between MeOH and DMC, EC and EG, makes the purification of DMC not only challenging but also energy and cost intensive.353 Homogeneous processes, using a cheap alkali as catalyst, particularly CH3ONa, have great potential for commercialization. Thus, more detailed studies about process intensification using CH3ONa as a catalyst are imperative. In comparison with conventional distillation, RD column is one of the most promising possibilities to overcome the chemical equilibrium limitation and reduce apparatus investment.267 Fang et al. have designed a laboratory RD column for acquiring a complete conversion of EC and built a rigorous mathematical model based upon kinetic and thermodynamic studies.269 Subsequently, a plant-wide design and control of DMC synthesis process was reported by Wang ´rak and co-workers presented the et al.270 In further work, Go first experimental investigation of transesterification of PC with methanol in a pilot-scale RD column.352 The experimental and simulated results defined the operating range for the economic optimization for an industrial-scale RD process. Recently, Wang’s group designed a RD process with a separation column optimized by minimizing the total annual cost for synthesizing DEC.354 In addition, an alternative thermally coupled RD process was proved to reduce energy consumption.

6. Applications 6.1

Fuel additives

Since the 1980s, DMC and DEC were patented as options for meeting oxygenate specifications on gasoline, due to their excellent blending property, their outstanding oxygen content, the more appropriate gasoline/water distribution than C1–C3 light alcohols, and most importantly, their environmental friendliness. Pacheco and Marshall have reviewed the manufacture of DMC and its characteristics as a fuel additive in 1997, in which they summarized the patents concerning the use of DMC in fuels and compared the physicochemical properties of several common oxygenates.8 Among the listed oxygenates in ref. 8, DMC have the least detrimental effect on reformulated gasoline. Moreover, the mixture of several light alkyl carbonates have been regarded as a better quality gasoline stock, because of a higher heating value, a lower freeze point and volatility, and slightly less mutual solubility in water. Nowadays, diesel engines are widely used for both on-road and non-road applications. In comparison with gasoline engines, diesel

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engines possess higher thermal efficiency, reliability, durability and have improved torque characteristics.6,355 However, they suffer from inherent higher NOx and particulate matter (PM) emissions.356 The fine particles in PM, especially with an aerodynamic diameter smaller than 10 mm and 2.5 mm (PM10 and PM2.5), are the most likely to cause problems of the human respiratory tract.357 We recommend the Nicola and co-workers’ review about emissions from ethers and organic carbonate fuel additives, in which they retrospectively discuss the studies on emissions of DMC and DEC blended diesel fuel.6 Herein, we only focus on the latest advances in research on diesel with dialkyl carbonates additives. Tsolakis’s group reported that low level addition of DMC (4 vol%) resulted in a reduction of total hydrocarbons, CO, and PM up to 50%.7 By using DMC blended diesel fuel, Zhu et al. confirmed that the smoke opacity, the particulate mass concentration as well as the total number of particulates were reduced, and the proportion of soluble organic fraction in particles could be significantly increased.355 Despite of the above advantages of diesel–DMC blends, a negative aspect is that the high value solubility temperature. Considering this reason, the mixture cannot be used in cold regions unless a blend pre-heater is added to the engine system.6 Pandian et al. claimed that the addition of DMC along with exhaust gas recirculation made the engine emit less smoke and lower NOx, CO, and hydrocarbon emissions.358 6.2

Solvents

As the simplest dialkyl carbonates, DMC and DEC have also been labeled as ‘‘green solvents’’, aiming at the replacement of conventional organic solvents, such as halogenated organic solvents as well as ketones, ethers and ester acetates. Besides the environmental friendly characteristics, they offer a low vapor pressure, suitable liquid temperature range, and unique solubility property. In recent years, it is remarkable that DMC and DEC are known as solvents not only for organic synthesis but also electrochemical and extractive applications. Schaffner et al. gave a compendium of organic carbonates as alternative solvents in synthesis, in which DMC and DEC are two of the representatives they focused on.5 Furthermore, they can also be used as non-reactive solvents in paints, adhesives and so on. With fast-paced development of lithium ion batteries, the corresponding solvents consisting of organic carbonates have been attracted considerable industrial interest. Although PC and EC allow a good dissolution of lithium salts, their strong viscosity limits the efficiency of the lithium electrochemical cycle. In view of this point, linear alkyl carbonates, such as DMC, DEC and EMC are usually introduced as co-solvents to increase the conductivity of the electrolyte because of their low viscosity as well as great solvation force towards lithium ions, which leads to diminution of the electrolyte resistance.359 On the other hand, it has been suggested that the dialkyl carbonatecontaining solvent system is one of the best electrolyte systems for the purpose of the formation of a passive film on the surface of the anode, which results in a good electrochemical stability and chemical stability to maintain the total performance of the lithium ion battery.360,361 Therefore, their binary and multicomponent mixtures, including cyclic carbonates and linear

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alkyl carbonates, are expected to play an important role for energy storage for domestic portable devices. Moreover, the purity of DMC has been confirmed to strongly influence the electrochemical performance of lithium ion battery.362 Recently, Li–O2 or Li–air batteries are receiving a great deal of interest because of their higher energy density and larger energy storage capacity. Organic carbonate-based electrolytes have been the most widely used in Li–O2 cells to date.363,364 However, because they are susceptible to the attack by superoxide radical anion and oxidation of alkyl carbonates during discharge and charging respectively, they are not suitable to serve as electrolytes for the Li–O2 system at present, and substantial challenges need to be addressed.365 In addition, dialkyl carbonate-containing electrolyte are also considered as a competitive candidate used for high power density electrical double layer capacitors (supercapacitors) that have been proved helpful in the automotive industry to save energy consumption.366 The region of ideal polarisability of the system can be obtained mainly by the electrochemical stability of the LiPF6 EC–DMC electrolyte.367 As mentioned above, because of the excellent solubility of ions and the high ionic diffusion in DMC, the carbonate-based electrolyte results in convenience for the preparation of dyesensitized solar cells with a long-term stability and high conversion efficiency, which is a third possible application of DMC related to energy.366 To sum up, in the view of the wide range of potential DMC applications as solvent, particularly in energy needs, the massive production of short-chain dialkyl carbonates (e.g. DMC, DEC and EMC) is anticipated in the coming years. 6.3

Organic synthesis

Dialkyl carbonates, can be handled safely without the special precautions due to their moderate toxicity, in comparison with the conventional carbonylating (i.e. phosgene) and alkylating agents (i.e. alkyl halides and dimethyl sulphate).368 Besides, the formation of inorganic salts as byproducts and the related disposal problems are avoided.369 The third advantage of dialkyl carbonates is their appreciable selectivity following the hard–soft acid–base (HSAB) principle. According to the HSAB theory, the carbonyl of the molecules can be considered as the harder electrophile, while the two alkyl groups represent softer electrophiles. With changes in reaction temperature, the nucleophilic substitution on DMC follows two different mechanisms (shown in Scheme 9), therefore, low temperatures allow carboxymethylation reaction, whereas high temperatures give methylation derivatives.1,370,371 Tundo et al. have overviewed the reactivity of some soft/hard mono and bidentate nucleophiles at N, O, and S with DMC and analyzed the effect of their nature on discrimination between the C atoms of DMC.371 In the published literature, they have shown good reactivity and selectivity with a number of nucleophilic substrates, such as phenols,372–378 alcohols,379–381 carboxylic acids,378,381–383 primary amines,384 indole,385 alkenes,386 etc. Taking account of the large amount of literature about DMC application as an intermediate in organic synthesis, we present a brief introduction from the two aspects of carboxymethylation and methylation reaction.


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bimetallic oxides (with the heterometal Ni, Fe, Pd, Al) and found that the catalytic activity was affected by the type and concentration of the heterometal.397

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Scheme 9 Nucleophilic substitution on DMC by BAC2 and BAL2 mechanisms. Reprinted with permission from ref. 1. Copyright 2002, American Chemical Society.

6.3.1 Carboxymethylation and carbonylation agent. With raising awareness of environmental problems, clean and ecofriendly synthesis processes, which avoid or minimize utilization of poisonous or harmful substances, are more attractive and competitive. DMC, as a safe carboxymethylation or carbonylation agent for substituting phosgene, has attracted immense interest in industry and academic research. Tundo et al. discussed the reaction of DMC with oximes, ketones and amines respectively in their review.1 Here, we only gave some representative examples and their latest advances. 6.3.1.1 Diphenyl carbonate (DPC) synthesis. Polycarbonate synthesis via a phosgene-free route involves in the transesterification of DPC with bisphenol A (eqn (36)). In this route, diphenylcarbonate as an intermediate, can be produced by a transesterification–disproportionation process (eqn (37)–(39)). Gong et al. published a review about diphenylcarbonate synthesis in 2007, in which the reaction of DMC and phenol was introduced in detail.387 The commonly used homogeneous catalysts in this process are Lewis acids, Al, Ti, Sn, V halides, alkoxides, organometallic compounds and ILs. Park and his co-worker suggested that the highest methylphenyl carbonate can be obtained by using (NH4)8Mo10O34 as a catalyst precursor, based on comparison work for the transesterification between DMC and phenol over various homogeneous catalysts.388 Bhanage’s group reported that Brønsted and Lewis acidic ILs significantly enhanced the yield of DPC by using dibutyltin oxide catalyst.389 In consideration of separation and regeneration, single and mixed oxides, including V2O5,390 MoO3/SiO2,391,392 TiO2/SiO2,393 PbO/ MgO394 etc., were investigated in heterogeneous catalytic process, some of which showed good activity. However, reusability of these catalysts were not satisfactory. Recently, Chen and co-workers prepared core–shell TiO2@SiO2 (ref. 395) and multi-walled carbon nanotube supported TiO2 (ref. 396) catalysts. The core–shell structure and the strong interaction between TiO2 and carbon nanotube effectively prevented leaching of TiO2, so that only slight loss of activity was observed. Dibenedetto et al. screened Ce-based


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(39) 6.3.1.2 Glycerol carbonate (GC) synthesis. GC, an important added-value products derived from glycerol, has high versatility in applications, such as a ‘‘green’’ solvent for cosmetics, personal care items and medicines, a component of solid laundry detergents, electrolyte for Li-ion batteries, and intermediate for manufacturing polycarbonates and polyurethanes etc.398,399 The transesterification of glycerol with a dialkyl carbonate over basic catalysts is one of the preferred methods because of its mild and green process conditions. Scheme 10 gives a schematic representation of GC synthesis.400 According to the calculated equilibrium constant, transesterification of glycerol with alkyl carbonates is thermodynamically favourable for producing GC, and increasing temperature can increase the chemical equilibrium constant.401 A coupling reaction and azeotropic distillation for the synthesis of GC from glycerol and DMC was considered.402 GC yield reached as high as 98% by using benzene as azeotropic agent in order to remove methanol from the reaction system continuously. A catalyst screening has been performed by Gomez et al. for clarifying the influence of different basic and acid homogeneous and heterogeneous catalysts, and the results showed that basic catalysts are proved more favorable for GC generation.403 Up to now, various catalysts, including alkaline oxide (e.g. CaO),,402,404,405 amphoteric oxide (e.g. CeO2),406 binary or ternary oxides (e.g. Mg–Ca,407 Mg–La,408 Mg–Al,409 Ca–Al,410 Mg–Al–Zr,411 Mg–Zr–Sr412), oxide supported alkali (e.g. K2CO3/MgO,413 NaOH/Al2O3414), ILs (e.g. tetramethylammonium hydroxide,415 DBU416), layered hydroxides,417 lipase,418,419 etc. have been extensively studied. The catalytic activity was strongly dependent on the strength of basicity, especially for heterogeneous catalysts. 6.3.1.3 Carbamoylation of aniline. Carbamoylation of aniline with DMC for the synthesis of carbamates, precursors of isocyanates, is probably one of the most promising synthetic

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Scheme 10 Schematic representation of GC synthesis: (1) glycerol; (2) DMC; (3) unstable intermediate; (4) GC; (5) glyceroldicarbonate and (6) diglycerol tricarbonate.400 Copyright 2014, Elsevier.

approaches via substitution of phosgene.420 As shown in eqn (40) and (41), these reactions can proceed under mild conditions, with methanol as the sole by-product. Tundo’s group investigated the synthesis of N-phenylcarbamate in the presence of homogeneous, supported and heterogeneous catalysts, and concluded that basic zinc carbonate was the best catalyst giving almost quantitative conversion and selectivity for the carbamate.420 The continuousflow process was also carried out with the remarkable reactivity and selectivity.421,422 The presence of water was suggested to greatly facilitate the methoxycarbonylation of aliphatic aniline, because the generated OH- was the active species and played a pivotal role in hydrogen abstraction from the amino group (rate-determining step).422

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RNH2 + DMC - RNHCO2CH3 + CH3OH

Table 19

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6.3.2 Alkylation agent. In the past two decades, a great interest has been fuelled towards dialkyl carbonates as innovative alkylating agents, especially for DMC. The environmentally benign features of DMC have been readily recognized: non-toxic, safety, considerable selectivity, and avoidance or minimization of solvent and derivatization sequences, as shown in Table 19.423 Additionally, its versatile reactivity imparts to it a great potential as a methylation reagent of several O-, S-, N- and C-nucleophiles (eqn (42)).423 The formed aryl alkyl ethers and monomethylaniline have found immense applications as intermediates in the manufacture of fine chemicals, such as pharmaceuticals, herbicides, pesticides, stabilizers for polymers, dyes and so on. Differing from classical reagents, the reactions of DMC with primary aromatic amines and with CH2-active compounds exclusively generate the corresponding mono-N- and mono-C-methyl derivatives. Selva et al. have made a comparative evaluation of DMC, MeI, dimethyl sulfate and MeOH as methylating agents through examining three model transformations: O-methylation of phenol, the mono-C-methylation and the mono-N-methylation of aniline.423 They claimed that the favourable atom economy and

General properties of several alkylation agents, including DMC, DMS, MeI and methanol423

DMC

DMS

MeI

MeOH

Oral acute toxicity (LD50 rats/mg kg ) Acute toxicity per contact (LD50 cavy/mg kg1) Acute toxicity per inhalation (LC50 rats/mg L1, 4 h) Mutagenic properties Irritating properties (rabbits, eyes, skin) Hazard identification

13 800 42500 140 None None Highly flammable

76 110 na Possible teratogen Irritant to skin Very toxic

Use of solvents Waste water treatment NaOH consumption By-productsb Thermodynamic Cost/h L1

No No No MeOH, CO2 Not or slightly exothermic B30

440 naa 1.5 Mutagenic Causes burns Very toxic and corrosive Yes Yes Yes NaSO4Me Exothermic B30

5700 na 64 na Irritant to skin, eyes Toxic and highly flammable No Yes/no No H2O

1

a

na: not available.

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b

Yes Yes Yes NaI Exothermic Z120

15–20

By-products defined for methylation processes.


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mass index of methylation processes with DMC and MeOH inherently reflect the greenness of these reactions. The use of a base in stoichiometric amounts was required by using dimethyl sulfate or alkyl halides because the leaving group (SO42 or Hal) can not be removed from the reaction medium.13 Moreover, DMC has been proved as a more efficient methylating agent than MeOH at low temperature.424 At the present time, the main catalysis systems are gas–liquid phase-transfer catalysts,425 organotin,374 ILs,375,376,426,427 basic zeolites,428–430 mixed oxides, hydrotalcite378,424 and MOFs.384 In order to realize a highly selective transformation, different continuous processes have been proposed by several groups, and the target product (e.g. anisole) exhibited an excellent selectivity at a substantially quantitative conversion of the substrate.375,376,431

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7. Summary and perspective It has been witnessed by voluminous publications and patents for the past decades that remarkable progress has been achieved both in the synthetic approaches and applications of dialkyl carbonates. The wide applications of dialkyl carbonates in chemical synthesis, electronic consumption industry, energy, transport as well as daily life lead to continuously increasing interests around the world. Now, DMC is apparently already being applied as a diesel additive to some extent in some countries. Despite of the uncertainties with respect to very large scale applications for fuel additives in certain areas, there is still a considerable expansion of the market for dialkyl carbonates due to their manifold established and semi-established applications, especially for polycarbonates intermediate production, clean solvents, etc. Therefore, improvement and optimization of existing commercial processes as well as development of novel and environmentally benign processes, deserve great and persistent attention in the future. Meanwhile, the relevant research on application fields such as production of high-value chemicals need more and special input. In recent years, significant efforts have been made towards increasing energy efficiency, promoting the catalytic performance, improving the security of production, as well as reducing the investment and environmental pollution. Many researches have sought the rational design of catalysts, clarification of structure–performance relationship, process optimization and exploration of reaction mechanisms. Recent advances in isolated or combined characterization methods under operando conditions and relevant apparatuses, such as in situ IR, nuclear magnetic resonance, UV-visible spectroscopy, Raman and XAS etc., contribute to analyzing the strength, concentration and location of active sites, elucidating solid catalyst structures, identification of surface species and reaction intermediates, and construction of the complete mechanism network for dialkyl carbonates synthesis. The rapid development of computational chemistry, particularly DFT, also provides a better understanding of chemical structure


and reactivity. However, there are still many substantial challenges to be faced and addressed at the present stage. Oxy-carbonylation appears the most favorable reaction from a thermodynamics standpoint. In addition, the abundant and inexpensive raw materials, as well as high atom economy and non-toxic byproduct provide a promising prospect to the scientific and industrial community. More importantly, the raw materials (e.g. methanol, CO) can be derived from syngas, so that it is an attractive route for the countries and areas with large reserves of coal and shale gas. The reaction is catalyzed over transition-metal chlorides, such as CuCl, CuCl2 and PdCl2 etc. Despite an excellent initial activity and selectivity, the inherent problems of such catalysts are rapid deactivation and equipment corrosion caused by the irreversible loss of chlorine. Although some strategies have been explored for immobilization of chlorine by different groups, it is significant that catalyst deactivation cannot be avoided and only delayed. By using the negative framework of aluminosilicate zeolites to provide a similar local environment, Cu-exchanged zeolites have been intensively investigated as an alternative for chlorine-containing catalysts. The effects of zeolite structure, composition and acidity as well as local environment of Cu+ on dialkyl carbonates synthesis are elucidated by combining different characterizations with activity measurements. Although the Cu–zeolites system exhibits very good on-steam stability, unfortunately, the activity and selectivity are still far from viable for industrial application. In addition, the steric hindrance and diffusion limitation of microporous structure play an important role in the apparent reaction rate and product distribution with increasing the length of the alkyl group. In the future, the rational design and controllable fabrication of chloride-free catalysts are anticipated as the main direction. Recently, the synergy between multi-active sites on Cu–zeolites and copper oxides supported on carbon materials has aroused researchers’ interest. Furthermore, the exploration of new materials, such as hierarchically porous materials, is another promising strategy for enhancing activity, eliminating diffusion limitation and modulating products distribution. CO2 routes, including urea alcoholysis, epoxide route via transesterification and straightforward synthesis, utilize waste greenhouse gas CO2 as raw material, which allow for possibility and opportunity to alleviating global climate changes and to reduce the dependence of chemical industry on fossil-based carbon resources, in particular petroleum. Due to the thermodynamical stability and the kinetic inertness of the CO2 molecule, direct synthesis from CO2 and alcohol remains at the laboratory level, although many attempts has been made to solve the thermodynamic restrictions and acquire high yield of dialkyl carbonates by developing and regulating catalysts, introducing dehydrating agent and using supercritical CO2. In contrast, urea alcoholysis and epoxide route via transesterification have been realized industrialization or semiindustrialization. In terms of technical maturity, transesterification is the most competitive candidate, however, other two routes still present promising prospects due to their own features. Compared with homogeneous catalysts, heterogeneous catalysts are gaining momentum because of easier post-reaction separation and

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recyclability, which lead to the substantial reduction of investment in equipment and utilities. In order to combine these advantages with the great efficiency of homogeneous catalysts, significant progress has been made in various directions, including the immobilization of homogeneous catalysts, exploitation of novel heterogeneous catalysts, modulation of the properties of catalysts and the use of novel synthesis technologies, such as electrochemical and photocatalysis processes. Given the diversity of approaches in dialkyl carbonates synthesis, the nature of different catalyst systems and their corresponding catalytic microprocesses are not well understood. The rapid development of modern analytical techniques and quantum chemistry provide valuable and pertinent information for probing and elucidating the role of catalysts and reaction mechanisms. In the next few years, despite that the forecast growth rates for polycarbonate consumption vary widely by region, the global consumption will maintain a fast growth rate, particularly in Asia (including 4.9% for China and 8.0% for India).432 As a raw material of non-phosgene processes of polycarbonate production, the demand of dialkyl carbonates will continue to increase. Furthermore, the rapid development of lithium batteries as well as environmentally friendly solvents and high-valued chemical manufacture will also stimulate the production of dialkyl carbonates. In conclusion, the vast expanse of possibilities that organic carbonates chemistry offers will undoubtedly invoke more attention and input around the world for the foreseeable future.

Acknowledgements Financial support from the National Science Foundation of China (20936003, 21325626, 21406120) and the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP) (Grant 20090032110021) is gratefully acknowledged.

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