Diethyl carbonate: critical review of synthesis routes, catalysts used and engineering aspects

Abstract

Diethyl carbonate (DEC) is a well-known linear organic carbonate that has wide applications. Besides its use as a fuel additive, DEC is an excellent electrolyte for lithium ion batteries and is used for the production of polycarbonates, which are globally used engineering plastics. The synthesis of DEC from CO₂ helps in CO₂ mitigation. It was earlier synthesized by phosgenation of ethanol, which is a toxic and dangerous process. Certain non-phosgene routes have been developed in recent years, which include oxidative carbonylation of ethanol, trans-esterification of carbonate, alcoholysis of urea, ethanolysis of CO₂ and the ethyl nitrite route for DEC synthesis. This review underlines various non-phosgene methods for the synthesis of DEC by critically evaluating the catalysts used, operating conditions and mechanism of synthesis. The performances of various catalysts have been compared graphically along with the identification of problems and potential solutions. Certain engineering aspects, including kinetics and thermodynamics of the various routes, have also been highlighted. The shortcomings and research gaps have been explicitly mentioned and discussed along with required future developments and research work for DEC synthesis.

---

1. Introduction

Organic carbonates are one of the important green chemical raw materials of the 21^st century. Because of their properties such as polarity, low toxicity and higher biodegradability, they have the potential to replace many harmful chemicals for different applications. Synthesis of organic carbonates helps in mitigating and utilizing carbon dioxide (CO₂), which has received much attention in the last few decades due to its indirect role as an environmental pollutant.^1–6 The organic carbonates are broadly classified into linear and nonlinear carbonates.⁷ Important linear carbonates include dimethyl carbonate (DMC) and diethyl carbonate (DEC). Non-linear carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) and glycerol carbonate (GC).

DEC also called carbonic acid diethyl ester is an important organic carbonate.⁸ It is a colorless, transparent liquid with mild toxicity.⁹ Being environment friendly and bio-degradable, its bio-accumulation is also low. It is an important alternative fuel that can be used to replace petroleum-derived fuels, thereby reducing CO₂ and particulate emissions from engines. It is used as a fuel additive because of its high oxygen content. It contains more oxygen (40.6%) as compared to well-known oxygenate methyl tert-butyl ether (MTBE) (18.2%).^10–14 Adding DEC to petroleum derived fuels reduces emission of CO₂ and particulates.¹⁵ The gasoline/water distribution coefficient of DEC is better than that of other carbonates and is preferred in many applications that require low vapour pressure with high amount of energy.^11,14 Table 1 states the physical properties of DEC.¹⁴ Presence of ethyl and carbonyl groups in DEC help in its conversion to important chemicals which include poly-carbonates, carbamates, unsymmetrical alkyl carbonates, ethyl benzene, 3-aryl-2-oxazolidinones, imidazolindin-2-ones.^16–24 It is extensively researched as an electrolyte for lithium ion batteries.^25–27 Being an excellent solvent, it is widely used in pharmaceutical products, fertilizer, pesticide and manufacture of dyes. The applications of DEC are categorised in Fig. 1. DEC decomposes to benign CO₂ and ethanol when released into environment.^11,12,28,29

Table 1 Properties of DEC

Property	Units	DEC
Lower heating value	MBtu per gal	74.3
Density	g cm^−3	0.0975
Melting point	°C	−43
Boiling point	°C	126
Vapour pressure@37.8 °C	mm Hg	0
Blending octane (R + M)/2	—	104–106
Gasoline/water distribution coefficient	—	20
Hydrolysis products	—	Ethanol, CO2

---

Fig. 1 DEC with properties which lead to its various applications.

Various methods used to produce DEC are summarised in Fig. 2. The oldest known among them is the phosgenation of ethanol, however, this process is inherently toxic.³⁰ A number of new non-phosgene routes have been developed in the recent years which include oxidative carbonylation of ethanol, trans-esterification of carbonate, alcoholysis of urea, ethanolysis of CO₂ and de-carbonylation of diethyl oxalate. Fig. 3a shows the number of publications for the synthesis of DEC through various routes and Fig. 3b shows the number of citations of these papers. From the figure, it can inferred that the number of publications on DEC synthesis are rising year wise and the route of DEC synthesis using DMC is being more researched now a days. This may be because of the reactive distillation process in this route which offers high selectivity of DEC. Carbonylation of ethanol using CO₂ is directly related to sequestration of CO₂ but is constrained by its less favourable thermodynamics. Ethanolysis of urea is also widely used because of its low-cost reactants in comparison to other reactions. DEC synthesis using oxy carbonylation by passes the route of urea and CO₂ which are restricted due to their high temperature and pressure. Table 2 summarizes the patents filed related to synthesis of DEC.^31–35

Fig. 2 Various non-phosgene methods for synthesis of DEC.

Fig. 3 Number of citations to publications in synthesis of (DEC) since 2000–2015 through various routes. The data was obtained from Scopus using keyword “diethyl carbonate” with sub keywords such as (“ethanol” OR “ethyl nitrite” OR “CO” OR “CO₂” OR “urea” OR “dimethyl carbonate”).

Table 2 List of important patents filed on DEC synthesis

Year	Inventors	Summary of invention	Reference
1941	Irving E. Muskat and Franklin Strain	This introduced a novel method of preparing carbonate esters of polyhydroxy compounds	31
1972	Ludo K. Frevel and Jo Ann Gilpin	Carbonates are made from alkylene carbonate and non tertiary hydroxyl group in presence of alkali metal	32
1993	UBE INDUSTRIES	Diester of carbonic acid are prepared from carbon monoxide and a nitrite in the presence of a solid catalyst on a carrier, a deactivated catalyst was regenerated by treating with hydrogen and with hydrogen chloride	33
2002	Zaid et al.	The invention generally concerns methods for the synthesis of dialkyl carbonates, like dimethyl carbonate (DMC) and\(DEC). It involves reacting an alcohol or diol, a base and a halogen in the presence of an amine salt catalyst. The first intermediate is reacted with carbon monoxide forming a second intermediate which then reacts with the alcohol or diol in the presence of the amine salt catalyst forming the dialkyl carbonate product	34
2009	Wershofen Stefan; Klein Stephan; Zhou Zhiping; Wang Xingku; Wang Junwei; Kang Maoquing	The present invention relates to preparation of catalyst for the synthesis of organic carbonates, by reacting urea and hydroxyl group containing compounds. The catalyst is a calcinate of hydrous salt containing rare earth element at a moderate calcinating temperature	35

---

Although a number of studies have been reported in the literature, however, a lot of research needs to be done in different areas for each of these routes. This paper presents a critical review of work done for all these routes. It also identifies and highlights the areas where there is research gap and the efforts that need to be placed to fill these gaps.

2. Ethanolysis of urea

2.1 General overview of the reaction

The alcoholysis of urea to produce dialkyl carbonate is a recently developed process. This reaction occurs in two steps. In the first step, ethanol and urea react to form ethyl carbamate (EC) and ammonia. EC further reacts with ethanol to form DEC and ammonia. The alcoholysis of urea can be described by following two reactions:

C₂H₅OH + NH₂CONH₂ ↔ C₂H₅OCONH₂ + NH₃ (1)

C₂H₅OH + C₂H₅OCONH₂ ↔ C₂H₅OCOOC₂H₅ + NH₃ (2)

Process involving urea and ethanolysis is economically favourable due to its cheap and non-toxic raw materials. Since urea itself is formed by reaction of CO₂ and ammonia, therefore, urea can be considered as activated form of CO₂, and hence, this process can be indirectly termed as sink of CO₂. In this reaction, co-product ammonia can be easily recycled back into urea by reacting it with CO₂, hence this process is considered as clean and efficient process. In this process, as no water is produced, and hence, there is no chance of azeotrope formation and thus separation of DEC is easier as compared to other processes.

2.2 Mechanism

A few investigators have reported mechanism of EC and DEC formation by urea alcoholysis reaction using various catalysts.^36–39 A generalized reaction mechanism based on all these studies is shown in Fig. 4.¹⁴ Urea first breaks at temperature >406 K (133 °C) to form ammonia and isocyanic acid (HNCO) via formation of ammonium cyanate salt (NH₄⁺NCO⁻) as an intermediate.^14,40 The highly active isocyanic acid reacts immediately with ethanol to produce EC.^37,38 The reaction of EC with ethanol occurs on a catalyst surface having Lewis acid sites (M^δ+) and basic site (O^δ−) which activates EC and ethanol, respectively. EC reaction with ethanol is a nucleophilic substitution reaction involving addition and elimination reactions as two different steps. Ethanol forms nucleophile CH₃CH₂O⁻ which attacks the carbonyl carbon atom of EC.¹⁴ This attack converts the EC molecule having triangular in a plane structure to form an intermediate molecule with tetrahedral structure which is unstable and forms DEC by releasing ammonia molecule.¹⁴ In the intermediate, the carbon of carbonyl group changes from sp² hybridization to almost sp³ hybridization.^41,42

Fig. 4 Generalized mechanism of DEC synthesis by urea alcoholysis reaction.

2.3 Reaction conditions and thermodynamics

It must be mentioned that various reactions occurring in this process are thermodynamically non-spontaneous. Standard Gibbs free energy change (ΔG°) is large for the formation of DEC (ΔG° = 20.09 kJ mol⁻¹) than that of intermediate compound EC (ΔG° = 0.66 kJ mol⁻¹).¹⁴ Hence, EC is thermodynamically more favourable than DEC. Formation of EC from this route does not even requires catalyst and good yield of EC can be obtained even without catalyst. Moreover, formation of DEC also leads to formation of certain by-products such as biuret, N-ethyl ethyl carbamate, and N-ethyl urea by following reactions:

NH₂CONH₂ ↔ HNCO + NH₃ (3)

HNCO + NH₂CONH₂ ↔ NH₂CONHCONH₂ (4)

(CH₃CH₂O)₂CO + CH₃CH₂OCONH₂ ↔ CH₃CH₂OCONHCH₂CH₃ (5)

This makes this route of DEC synthesis thermodynamically and economically more sensitive. The first step of the decomposition of urea increases rapidly for temperatures >150 °C, therefore, urea alcoholysis reactions are carried out at temperatures >180 °C and pressures are above 25 bar so as to keep the ethanol, EC and DEC in liquid phase.

2.4 Heterogeneous catalysts used for the reaction

Research on the alcoholysis of urea to synthesize DEC has been mainly focused on development of catalysts during the last decade. Among various catalysts, organometallic compounds offer the best yields and selectivity but being homogenous with the system, the problem of separability and deactivation impose problems in process commercialisation.^43–45

Mostly, heterogeneous catalysts have been studied during last decade due to its ease of separation and regeneration. The use of homogeneous catalysts to produce DEC from this route is not yet reported. It may be mentioned that many investigators studied DEC synthesis from urea (via eqn (1) and (2))^14,24,44 whereas a number of investigators directly studied DEC synthesis from EC (via eqn (2)).^10,39,45

Table 3 Comparative analysis of different catalysts used for the synthesis of DEC from ethanolysis of urea

Catalyst	Catalyst preparation method	Reaction conditions			Reference
		Temperature (°C)	Time (h)	DEC yield (%)
a Best performing catalyst or operating condition.
Different metal oxides (ZnO^a)	—	(160–220)190^a	(1–9)5^a	14.2	14
Series of metal oxides (La2O3)^a	Precipitation method	(180–230)210^a	(1–5)3^a	38.6	44
Mg–Zn–Al oxides derived from hydrotalcites	Mg–Zn–Al prepared from co-precipitation method	(180–220)200^a	(1–6)4^a	68	24

---

Table 4 Comparative analysis of different catalysts used for the synthesis of DEC from ethanol and EC

Catalyst	Catalyst preparation method	Reaction conditions			Selectivity (%)	Reference
		Temperature (°C)	Time (h)	DEC yield (%)
a Best performing catalyst or operating condition.
PbO	Thermal decomposition of lead carbonate at 300 °C for 4 h in air	180	7	15.7	43	39
ZnO–Fe2O3	Thermal decomposition of Zn–Fe–CO3 hydrotalcites	180	10	32.3		45
PbO based double mixed oxides, PbO–ZnO^a	Calcinating their corresponding metal compounds	180	7	13.8	40.7	10
Lewis and Brønsted acids (H4P2O7)^a	—	180	6	21.9		58
Slag based catalysts (SN-450)^a and metal oxides	Slags prepared by calcination while metal oxides by co-precipitation method	(190–220)210^a	(1–4)3^a	33.1	74.4	57
Mg–Al mixed oxides derived from hydrotalcites	Catalyst obtained by HTC-Mn possessed max activity	(170–200)190^a	(6–9)8^a	56.7	—	56

---

2.4.1. DEC synthesis from urea. Table 3 summarizes recent progress made in synthesis of DEC through ethanolysis of urea.^14,24,44 Wang et al.¹⁴ used a number of metal oxides such as ZnO, CaO, PbO, ZrO, TiO₂, MgO and γ-Al₂O₃ as catalysts for production of DEC from urea and ethanol. Screening of catalysts was done by studying the yield of EC and DEC. Fig. 5a represents the performance of these catalysts. The yield of EC was 84% (via eqn (1)) without any catalyst; however, no yield of DEC (via eqn (2)) was obtained without catalyst. The performance of ZnO and ZrO₂ was best among all the catalysts with reaction parameters being reaction time = 5 h, temperature = 190 °C. Better performance of these catalysts was attributed to the presence of weak acidic and basic sites on both the catalysts.⁴⁶ Maximum DEC yield was 14.2%.¹⁴

Fig. 5 The performance of various metal oxides in terms of yield and selectivity during ethanolysis of urea. (a) Source of data: Wang et al.¹⁴ reaction conditions: ethanol = 150 mL, urea = 15 g, catalyst = 8 g, temperature = 463 K, stirring speed = 600 rpm, reaction time = 5 h, initial pressure = 0.8 MPa, reaction pressure = 2.5 MPa. (b) Source of data: Xin et al.,⁴⁴ reaction conditions: methanol/urea = 10 : 1, catalyst feed = 17% (based on the weight of urea), reaction temperature = 200 °C, reaction time = 3 h, stirring speed = 550 rpm. (c) Mg–Zn–Al hydrotalcites. Source of data: Wang et al.,²⁴ reaction conditions: urea = 20 mmol, ethanol = 200 mmol, catalyst = 0.1 g, temperature = 200 °C, time = 4 h. DEC: EC: ethyl carbamate, NEEC: N-ethyl ethyl carbamate. (d) Yield of DEC and conversion of EC obtained when different catalysts were used. Source of data: An et al.,¹⁰ reaction conditions: 10 wt% PbO in double metal oxides with reaction conditions: catalyst weight percentage = 1%, molar ratio of ethanol : EC = 10 : 1, temperature = 180 °C, and time = 7 h. (e) Slags. Source of data: Wang et al.,⁵⁷ reaction conditions: catalyst = 0.15 g, EC = 3.0 g, ethanol = 20 mL, temperature = 200 °C, time = 3 h. (f) Hydrotalcites derived metal oxides were used. Source of data: Wang et al.⁵⁷ reaction conditions: reaction temperature = 463 K, reaction time = 8 h, catalyst amount = 0.8 g, EC = 0.1 mol, ethanol = 1.0 mol.

PbO was found to be most active catalyst for DEC formation from EC. Lian et al.⁴⁷ extended the study by regenerating and reusing PbO as catalyst. Overall maximum DEC yield of 15.7% was reported. The reason for high yield and selectivity with regenerated catalyst was explored. X-ray diffraction (XRD) analysis showed that the tetragonal PbO in fresh catalyst got converted to cubic metal Pb after reaction. For confirming that mixture of metal Pb and PbO₂ were the main catalytic agent, whose combined action was responsible for high yield and selectivity of DEC. Pure Pb metal, PbO₂, Pb + PbO₂ and fresh PbO were tested as catalyst also. Pb + PbO₂ were found to give best yield and selectivity of DEC. The agent responsible for conversion of tetragonal PbO to metal Pb was also investigated by conducting certain experiments such as reaction of ethanol, EC and DEC with PbO. It was found that DEC is actually responsible for conversion of tetragonal PbO into cubic metal Pb. Before this study, Notario et al.⁴⁸ also reported that DEC can decompose into ethanol, ethylene and CO₂ at 180 °C in presence of PbO.

The problem of leaching of ZnO, as a catalyst, was addressed by Xin et al.⁴⁹ By using lanthanum based oxides and comparing their activities with other metal oxides such as Al₂O₃, MgO, TiO₂, CaO and ZnO as the catalysts for DEC synthesis. La³⁺ cation can behave as Lewis acid site which facilitates the adsorption of groups having dense electron cloud over them such as carbonyl group. It is reported in Kus et al.⁵⁰ that La₂O₃ contains both medium and strong basic sites. Also lanthanum based compounds performed better in the reactions based on urea and alcohols.^51–53 The performance of ZnO and La₂O₃ was investigated by calcinating them at 500 °C. Fig. 5b compares the performances of various catalysts. The yield and selectivity of La₂O₃ was similar to that of ZnO but no leaching was observed as compared to ZnO. Zhao et al.³⁶ studied the effect of ethanol/urea ratio and temperature at supercritical conditions (Fig 6a and b). Maximum yield of DEC was obtained when the ethanol/urea ratio was 10 (Fig. 6a). La₂O₃ performed well, however the problem of low specific surface was encountered. Hydrotalcites are well known for their high surface area and stability against sintering. Wang et al.⁵⁴ later used ternary Mg–Zn–Al hydrotalcites prepared by co-precipitation method, for the reaction between urea and ethanol. Since hydrotalcites performed better in the synthesis of DMC from urea and methanol.⁵⁵ The hydrotalcites containing manganese as transition metal performed best among all (Fig. 5c).

Fig. 6 (a) Effect of ethanol/EC ratio on yield of DEC (reaction temperature = 300 °C and time = 30 min) and (b) EC conversion at time = 30 min. Source of data: Zhao et al.³⁶

2.4.2. DEC synthesis from EC. Table 4 presents comparative analysis of work done for the synthesis of DEC through ethanolysis of EC.^{10,39,45,56–58} Considering better activity of PbO, An et al.¹⁰ investigated direct DEC synthesis from EC using mixed oxide catalysts combining PbO with other oxides to achieve high DEC yield and selectivity. ZnO–PbO performed better than MgO–PbO, CaO–PbO, SrO–PbO, BaO–PbO, ZrO₂–PbO, Al₂O₃–PbO, Fe₂O₃–PbO, CuO–PbO, NiO–PbO, TiO₂–PbO, and La₂O₃–PbO catalysts (Fig. 5d).¹⁰ The effect of PbO content in mixed oxide catalyst was also studied. Maximum yield and selectivity of DEC was obtained when PbO was 15%. Change in lattice structure before and after its usage was also investigated. Hexagonal ZnO, orthorhombic PbO and orthorhombic PbO₂ in fresh catalyst got converted to orthorhombic PbO₂ in the recovered catalyst. When ZnO alone was used as a catalyst, no change in lattice structure was observed. Thus, it was concluded that presence of PbO accelerates the transformation of hexagonal ZnO to some amorphous phase. The yield and selectivity of DEC from recovered ZnO–PbO was found to be better than that of fresh ZnO–PbO. Transformation of ZnO to Zn(NCO)₂(NH₃)₂ was confirmed with the help of infra-red spectra.

Wang and Zhang⁴⁵ studied the yield, selectivity of DEC from ethanol and ethyl carbamate using hydrotalcite derived mixed oxide (ZnO–Fe₂O₃) catalyst. The effect of calcination of hydrotalcite produced mixed phases of ZnO and ZnFe₂O₃. Thermal treatment was considered as one of the factors besides surface area which was responsible for the high yield and selectivity of DEC when ZnO–Fe₂O₃ was obtained after calcination. This catalyst performed best giving 32.3% DEC yield.

Wang et al.⁵⁶ used Mg–Al mixed oxides (synthesized from thermal decomposition of hydrotalcites) as catalyst for the reaction of EC and ethanol to DEC (Fig. 5f). Introduction of transition metals obtained from hydrotalcites lead to much higher performance rather than pure hydrotalcites due to their improved moderate and strong basic sites. Fig. 5f compares the performances of these catalysts.

Wang et al.⁵⁷ used waste slag, whose disposal is an important environmental issue, containing various metal oxides of varying compositions as catalyst for synthesis of DEC from EC. The comparison of the performance of slags (calcined at different temperature) with the metal oxides, prepared with the help of co-precipitation method, was made. Slag containing different proportion (Fig. 7) of metal oxides gave better yield and selectivity then pure metal oxides as shown in Fig. 5e. Slag calcined at 450 °C gave highest yield and selectivity.

Fig. 7 Percentage of different metal oxides in slag after calcination at 350 °C. Source of data: Wang et al.⁵⁷

Qin et al.⁵⁸ used different Lewis and Brønsted acids for synthesis of DEC from EC. Pyrophosphoric acid showed best activity to DEC synthesis from EC and ethanol among various acids. Use of supercritical fluids (SCFs), is a green and catalyst-free technology, which is being researched for its various applications. Reaction temperature and pressure of SCF can be optimized for achieving higher yield and selectivity. Ethanol has critical temperature and pressure of 243.1 °C and 6.38 MPa, respectively. Zhao et al.³⁶ first used this technology for synthesis of DEC using supercritical ethanol without using any catalyst. 22% DEC yield was obtained at 300 °C in supercritical atmosphere. The yield of DEC at different temperatures is shown in the Fig. 6b. Best yield of DEC (23%) was obtained at 563 K after 30 h. Wang et al.⁵⁹ studied the synthesis of DEC using solid basic oxides as catalyst, CaO performed best.

Since transition metal oxides can co-ordinate with ammonia to form complex, they can be termed as Lewis acids.^60,61 They can be used to capture ammonia and shift the equilibrium to the product side. Zhao et al.⁶² studied the performance of transition metal chlorides in synthesis of DEC from EC. Their activity can be attributed to empty d-orbitals of transition metal oxide which activate the N, O atom of EC.⁶³ MnCl₂ > CoCl₂ > ZnCl₂ > CdCl₂ > NiCl₂ was the order of performance of transition metal oxides as the catalyst.

Since the alcoholysis of urea involves two step reactions, in which second step reaction is more difficult. Many authors have investigated second step only to synthesize carbonate from carbamate using zinc based catalysts.⁶⁴ The yield and selectivity was good, however there was difficulty in recovery of catalysts because of the homogenous nature of catalysts. Zhao et al.⁶⁵ investigated formation of DMC from MC using ZnO as catalyst. Transformation of Zn(NCO)₂ into Zn(NCO)₂(NH₃)₂ was suggested as the main reason for low yield of DMC from MC rather than from methanol.

The formation of Zn(NCO)₂(NH₃)₂ was represented by following equations:

ZnO + 2HNCO ↔ Zn(NCO)₂ + H₂O (6)

H₂O + NH₂CONH₂ ↔ NH₄OOCNH₂ (7)

Zn(NCO)₂ + NH₃ ↔ Zn(NCO)₂(NH₃)₂ (8)

ZnO is precursor of homogenous catalyst Zn(NCO)₂(NH₃)₂.

2.5 Summary

DEC synthesis from urea is less favored as compared to synthesis from EC, due to formation of NEEC and N-ethyl urea, in former process, as by-products which suppress the yield of DEC. Removal of DEC and NH₃ as soon as they are formed can drastically improve the yield and selectivity of DEC from urea. This can be done by using catalytic distillation of ethanol and urea. This type of set up and work is missing in the literature. Wang et al.⁵⁴ synthesized DEC with maximum yield of 67.8% using hydrotalcites based catalysts. Mg, Zn and Al based oxides have been found to catalyze the carbonylation of ethanol using urea and hence hydrotalcites were prepared using same metals. Still the need of more active catalyst is required for the process commercialization. Kinetic and thermodynamic parameters also need to be studied in order to design and commercialize the process.

3. Carbonylation of ethanol using carbon-di-oxide

3.1 General overview of the reaction

Among different approaches, direct synthesis of organic carbonates from alcohols and CO₂ is gaining huge interest in recent times due to its high commercial significance.⁷ Various attempts have been made to convert CO₂ to stable carbonates.^66–80 Leino et al.⁷⁰ reviewed conventional synthesis methods of dialkyl carbonates from alcohols and CO₂. Ethanol as compared to methanol is highly abundant, non toxic substance, however, being less acidic, it is less reactive than methanol.⁷¹ Main reaction by this route is as follows^72,73

2C₂H₅OH + CO₂ ↔ (C₂H₅O)₂CO + H₂O (9)

3.2 Thermodynamics of reaction

CO₂ being linear and centro symmetric in nature is a very stable compound. Hence, the reactions of CO₂ are not thermodynamically favorable. Table 5 shows the thermodynamic data of various substances involved in this reaction.^79,80 The standard enthalpy change (ΔH°), standard entropy change (ΔS°) and ΔG° for the reaction are −16.60 kJ mol⁻¹, −175.99 J mol⁻¹ K⁻¹ and 35.84 kJ mol⁻¹, respectively. Thus, this reaction is exothermic and favors backward reaction if temperature is increased. Since ΔG° > 0, therefore, this reaction does not occurs spontaneously. At fixed temperature of 100 °C, this reaction becomes spontaneous only when pressure exceeds 7.25 × 10⁵ MPa which is technically not feasible. Hence, the reaction can be shifted to DEC side using effective catalysts and co-reactants or by using dehydrating agents such as butylene oxide and molecular sieves.⁷⁰

Table 5 Various thermodynamic data for substances^79,80

Substance	ΔH^of	S^of	Cp
DEC	−637.9	412.2	152.1
H2O	−241.8	188.8	33.6
CO2	−393.5	213.8	37.1
C2H5OH	−234.8	281.6	65.6

---

3.3 Mechanism

A number of investigators used one-pot synthesis of DEC using this method whereas many other investigators studied two-pot synthesis.^70,74

Fig. 8 illustrates the direct synthesis of DEC from CO₂ as reported by Arbelaez et al.⁷⁵ First, the catalyst gives C₂H₅O⁻ group from ethanol. Cu–Ni bimetallic catalyst was used for the purpose. The CO₂ gets activated giving CO₂–M species. Transformation between the two yields to M–O(C₂H₅)–C(O)–M type complex. Finally another activated ethanol molecule reacts to give DEC and regenerated M sites.

Fig. 8 Mechanism of synthesis of DEC from ethanol and CO₂.

Tomishige et al.⁷⁶ synthesized DMC from CO₂ and methanol using H₃PO₄/ZrO₂ as heterogeneous catalyst. They inferred that bi-functional catalyst with both acidic and basic sites perform better giving high selectivity for this type of reactions.

3.4 Comprehensive study of process

3.4.1. Direct synthesis. Yoshida et al.⁷⁷ used ceria as the catalyst for synthesis of DMC from methanol and DEC, EMC from ethanol. Selectivity and yield of DMC were directly proportional to the active sites of CeO₂. Ceria based catalyst performed better for the synthesis of organic carbonates from CO₂.⁷⁸ EMC and DEC were also synthesized from the reaction containing ethanol, methanol in presence of CO₂. The trans-esterification rate was found to be on the lower side. This was attributed to the adsorbed ethyl carbonate species and methanol.

Leino et al.⁷⁹ investigated the reaction of ethanol and CO₂ over CeO₂ as catalyst and synthesized DEC via one-pot synthesis. As the water gets produced continuously during the reaction, butylene oxide was used as dehydrating agent due to its inherent water absorbing property and low toxicity. Introduction of butylene oxide as water trap increased the DEC yield by 9 folds. The effect of reaction temperature, applied pressure and reaction time on DEC yield was also studied.⁸⁰ Leino et al.⁸¹ further used CeO₂ (synthesized with precipitation method) to study the effects of various catalyst synthesis parameters and synthesis time on DEC yield. It was found that the catalyst CeO₂ synthesized at pH 11 gave the highest DEC yield. Kumar et al.⁸² synthesized Ce–H-MCM-41, Ce–Si-MCM-41, K-MCM based mesoporous catalyst, and Cs-ZSM-12 and Na-ZSM-12 microporous catalysts. Maximum DEC formation was observed with 16 wt% Ce–H-MCM-41 and 32 wt% Ce–Si-MCM-41 mesoporous catalysts. This was attributed to presence of weak and strong basic sites on the catalysts.

Zhang et al.⁸³ observed 2.5 fold increase in the DEC yield when 3A molecular sieves was used as co-catalyst (as a dehydrating agent) along with ZrO₂ as a main catalyst. Pore size of molecular sieve and acidic–basic properties of ZrO₂ was main factor responsible for the activity of catalyst. Arbelaez et al.⁷⁵ used Cu–Ni supported on activated carbon as the catalyst for direct synthesis of DEC from ethanol and CO₂. Copper and its oxide doping itself enhanced the activity in similar reactions.^84,85 Cu–Ni showed good activity as catalyst for other reactions also.⁸⁶

Since mixed (ceria and zirconium) oxide Ce_xZr_1−xO₂ performed better as catalyst for carbonylation of alcohols,⁷⁶ hence, Wang et al.⁸⁷ used the same catalyst for DEC synthesis. It was reported that an increase in the value of x from 0.18 to 0.8 decreased the activity of catalyst due to formation of strong acid–base sites. Prymak et al.^88–90 prepared Ce_xZr_1−xO₂ by citrate method and evaluated its performance in a plug flow reactor for carbonylation of ethanol. It was found that the introduction of more zirconia in CeO₂ increased the acid–base sites. Performance of various catalysts varied due to the difference in stoichiometric ratio of Ce and Zr; and also due to difference in amount of acidic and basic sites present.

Some ionic liquids also have been tried as the catalyst for CO₂ fixation to organic carbonates. Ionic liquids combined with K₂CO₃ form effective catalytic system and an easy method for the synthesis of organic carbonates.⁹¹

3.4.2. Two-step synthesis. Use of organic iodide such as ethyl iodide (C₂H₅I) as co-reactant along with some basic catalyst (K₂CO₃) is reported to give better alcohol conversion.⁹² One-pot synthesis of DEC using this procedure can be represented by following reaction.⁷⁴

C₂H₅OH + CO₂ + K₂CO₃ + C₂H₅I ↔ (C₂H₅O)₂CO + KI + KHCO₃ (10)

Above reaction can be carried out in two steps/pots using K₂CO₃ in the first pot and C₂H₅I in the second pot. In the first step, formation of potassium ethyl carbonate (PEC, C₃H₅O₃⁻K⁺) and potassium bicarbonate (KHCO₃) takes place. This step acts as a trap for CO₂. In the second step, PEC reacts with ethyl iodide (C₂H₅I) to form DEC. These reactions steps can be represented as:

C₂H₅OH + CO₂ + K₂CO₃ ↔ C₃H₅O₃⁻K⁺ + KHCO₃ (11)

C₃H₅O₃⁻K⁺ + C₂H₅I ↔ (C₂H₅O)₂CO + KI (12)

For the second step, ethanol can be used in place of ethyl iodide to synthesize DEC by following reaction:

C₃H₅O₃⁻K⁺ + C₂H₅OH ↔ (C₂H₅O)₂CO + H₂O (13)

Various co-reactants have been tested which gave different yield of DEC from PEC. Fig. 9a shows the performance of these co-reactants. For one-pot synthesis maximum 46% yield of DEC was obtained. This method didn't give good yield though it was very benign in nature.

Fig. 9 (a) Effect of various co-reactants on yield of DEC at temperature = 110 °C and time = 4 h. Source of data: Gasc et al.⁷⁴ (b) Performance of various catalysts used at CO₂ initial pressure = 3 MPa, reaction temperature = 420 K, reaction time = 3 h, magnetic stirring rate = 500 rpm. Source of data: Wang et al.⁹¹

Since two-pot synthesis involves high energy consumption and hence high cost is involved. Wang et al.⁹³ extended the work by carrying out synthesis of DEC in one-pot using ethanol, CO₂ and ethylene oxide. This reaction is shown below

(CH₂)₂O + CO₂ + 2C₂H₅OH ↔ (C₂H₅O)₂CO + (CH₂OH)₂ (14)

Using this method, co-product glycol was produced which itself is an important raw material in manufacture of polyester fibers and fabric industry. The reaction was studied using different heterogeneous and homogenous catalysts and was also investigated. The effect of various reaction parameters on the yield and selectivity of DEC was also studied. The reaction occurs in two steps:

• Cyclo-addition of CO₂ to ethylene oxide

• Trans-esterification of ethanol and ethylene carbonate to give DEC.

Different binary catalysts (mixture of ionic salts and basic catalyst) for trans-esterification of ethylene carbonate were used.⁶⁸ Fig. 9b compares the performances of various catalysts. KI/EtONa was observed to the be best catalyst combination that gave 30.8% DEC yield. The reaction mechanism for one-pot synthesis of DEC is shown in Fig. 10.

Fig. 10 Reaction mechanism for one-pot synthesis of DEC.

The synthesis of organic carbonates from CO₂ suffers from deactivation of homogenous catalysts and heterogeneous catalysts. Table 6 summarizes the work progress in transformation of CO₂ to DEC in recent years.^{74,75,77,81,82,87,93}

Table 6 Comparative analysis of different catalysts used for the synthesis of DEC from CO₂

Co-reactants or catalyst used	Preparation method	Reaction conditions			DEC mmol	Reference
		Rxn temp. (°C)	Time (h)	Pressure (MPa)
a Best performing catalyst or operating condition.
CH3OH, C2H5OH, CeO2^a	Thermal decomposition	(110–170)170^a	2	—	0.42 mmol	77
(1) Two pot synthesis, (i) EtOH + CO2 + K2CO3, (ii) EtCO3 + EtI	—	110	4	8	22.8 mmol	74
(2) One-pot synthesis, EtOH + CO2 + K2CO3. Supercritical conditions, (EtI/EtOH/PTC)
CexZr1−xO2 (Ce0.07Zr0.93O2)^a	Co-precipitation	(110–140)140^a	2	—	0.28 mmol	87
Cu–Ni/AC	Impregnation			—	—	75
Ce–H-MCM-41, Ce–Si-MCM-41, Cs-MCM-41, K-MCM-41 and Na-ZSM-12, Cs-ZSM-12	Evaporation impregnation	170	23	—	0.4 mmol	82
Cerium oxide	Precipitation method^a, hydrothermal method, CeO2-SBA-15 composite material	180	25	4.5	0.34 mmol	81
Solid basic oxides (KI + EtONa)^a	Precipitation method	(130–180)170^a	(1–5)2^a	(1–4)2^a	14.34 mmol	93

---

3.5 Summary

Due to poor thermodynamics, this reaction needs better catalyst and operating conditions for commercialization. This route of DEC synthesis has very high significance as it can directly serve as a way for sequestration of CO₂. Ceria–zirconia based catalyst have performed better under supercritical conditions. However breaking of C=O bond impedes the reaction and hamper its conversion. Therefore, typical phase equilibrium should be studied for the process, and behavior of system at high temperature and pressure should be modeled and predicted using different equation of states.

4. Oxidative carbonylation of ethanol

4.1 The nature and facts about reaction

Oxidative carbonylation of ethanol, in comparison to CO₂, is quiet favorable and promising method of producing DEC on industrial scale because of the high selectivity of DEC obtained.¹¹ The reaction is given below:

2CH₃CH₂OH + CO + 1/2O₂ ↔ (C₂H₅)₂CO + H₂O (15)

This reaction may occur in both liquid and gas phases. In liquid phase, difficulties such as separation of products from catalysts, corrosion of equipments, usage of CO in excess and deactivation of catalysts are encountered. In gas phase, DEC is obtained at stoichiometric amount of CO at low temperature and pressure.

4.2 Catalysts used in the study

A number of catalysts have been used for DEC synthesis by oxidative carbonylation of ethanol.^94–120 Table 7 summarizes the performance of different catalyst used in the process.^{11,98,99,102,103,105,109,117,120} Palladium has been proven best catalyst for the oxy carbonylation of ethanol, however high cost and easy deactivation of catalyst are the major issues.⁹⁵ Considering this, a number of other investigators have tested various other types of catalysts. Dunn et al.¹¹ synthesized DEC from oxidative carbonylation of ethanol over heterogeneous catalysts. CuCl₂ in place of copper acetate was found to enhance the chances of corrosion, however the yield obtained with CuCl₂ was much better than the other catalysts. The use of CuCl₂ along with PdCl₂ gave highest yield. The effect of support was also explored. Activated carbon was found to be the best performing support as compared to silica and alumina. Since MCM-41 is known to be good support with its weak acidity and high surface area. Chen et al.⁹⁶ dispersed CuCl on MCM-41, and used it as the catalyst for oxy carbonylation of ethanol to synthesize DEC. The incorporation of Al by impregnation method resulted in an increase in acid sites without structural changes.

Table 7 Comparative analysis of different catalysts used for the synthesis of DEC from CO

Catalyst	Catalyst preparation method	Reaction conditions		DEC yield (%)	Selectivity (%)	Reference
		Temperature (°C)	Pressure (Psig)
a Best performing catalyst or operating condition.
CuCl2/PdCl2/AC	Impregnation method	170		10	—	11
CuCl2/PdCl2/KOH/AC	Impregnation method	150	100	—	—	98
CuCl2/PdCl2/AC	Impregnation method	120	—	—	100	102
CuCl2–PdCl2/AC	Impregnation method	(120–180)150^a	(25–125)100^a	12.5	—	103
CuCl2–PdCl2–KCl–NaOH/AC	Impregnation method	140	92.8	32	90	99
Co (salophen)		(100–180)140^a	(145–725)725	15.7	99.5	105
PdCl2/Cu-HMS	—	150	92.82	—	100	109
CuCl2/PdCl2 on amorphous carbon KCl and NaOH	Impregnation method	150	45	—	—	120
PdCl2/mCuO	—	150	92.82	—	91.4	117

---

It was also found that the addition of hydroxides to silica and alumina hampered the selectivity. On the other hand, it enhanced the selectivity when activated carbon was used. KOH was found to be the most effective hydroxide which gave best yield with activated carbon. The DEC yield along with the effect of catalyst, support and addition of hydroxyl group is shown in Fig. 11.

Punnoose et al.⁹⁷ attributed formation of copper atacamite increased the activity of catalyst. Roh et al.⁹⁸ conducted the same experiment in pulse quench flow reactor and studied the effect of reaction parameters such as temperature and pressure on yield and selectivity. Excess of CO caused kinetic saturation while the excess of ethanol gave high by-products in the reactor.

Punnoose et al.⁹⁷ reported paratachamite (Cu(OH)₃Cl) as more active catalysts than CuCl₂. Zhang et al.⁹⁹ extended the work by using potassium salts as promoter over CuCl₂–PdCl₂–NaOH/AC. KCl performed best among all different types of salts giving 32% DEC yield. Conversion of Cu₂(OH)₃Cl to Cu(OH)Cl was responsible for the enhanced activity of catalyst as Cu(OH)Cl was found to be more efficient than Cu₂(OH)₃Cl in regenerating Pd⁰ to Pd²⁺ and reducing Cu²⁺ to Cu⁰. The formation of Cu(OH)Cl was attributed to addition of KCl as promoter.

The pretreatment of catalyst significantly affects the crystal structure which in turn affects the catalytic activity of the catalyst.¹⁰⁰ Introduction of quaternary ammonium salts as surfactant significantly influenced textural properties on silica support. Later Zhang et al.¹⁰¹ showed PdCl₂/Cu–Cu₂O was the most active specie for the synthesis of DEC using Pd–Cu bimetallic nanoparticle catalysts.

Liu and Chang¹⁰² reported CuCl–PdCl₂/C as catalyst for the reaction. Insertion of carbon monoxide to ethoxy was observed to be the rate limiting step. The catalyst was easily deactivated due to sintering of CuCl and decomposition of PdCl₂. Fig. 12 shows the mechanism of the method. Since batch reactor cannot predict the behavior of chemical reaction, hence Roh et al.¹⁰³ conducted the reaction using same catalyst but in continuous flow reactor and reported the deactivation of catalyst at higher temperature. This was consistent with the results previously reported by Tomishige et al.⁹⁴ for the synthesis of DMC.

Fig. 11 Comparison of ethanol conversion, DEC yield and selectivity obtained using various catalysts at reaction conditions: EtOH = 25 mL, concentration of catalyst = 0.12 mol L⁻¹, initial pressure = 3.0 MPa, P(CO) : P(O₂) = 2 : 1, time = 2.5 h, 140 °C. Source of data: Zhu et al.¹⁰⁵

Fig. 12 Mechanism of catalyst being deactivated due to sintering of CuCl and decomposition of PdCl₂.

Xiong et al.¹⁰⁴ studied the process using CuCl/Schiff base as catalyst and defined inhibition efficiency to be:

Xiong et al.¹⁰⁴ suggested that different N-ligand promoters can enhance the carbonylation to DEC. The performances of different promoters are summarized in Fig. 13. The combination of 1,10-phenanthroline (phen) and N-methyl imidazole (NMI), phen/NMI performed best among all with the high catalytic activity and excellent corrosion inhibition. The reaction approached kinetic saturation when CO was present in excess while by-product diethoxy methane was favoured when ethanol was in excess. The selectivity of DEC was over 99% but conversion of ethanol was not up to the mark.

Palladium is very expensive and CuCl₂ is corrosive in nature. Hence, Zhu et al.¹⁰⁵ used Schiffs base complexes as catalyst for the oxidative carbonylation of ethanol to produce DEC. The performances are shown in Fig. 14. Salophen type ligand showed phenyl ring conjugated to salicyl moieties. Conjugation improved the stability of Co (salophen) complex. Activated carbon based catalyst deactivate with time due to loss of chlorine. It can be regenerated by offline treatment with Cl₂ containing gas such as HCl.^106,108

Zhang et al.¹⁰⁷ used different promoters such as tetra butyl ammonium bromide (TBAB), cetyl trimethyl ammonium bromide (CTAB), tetra ethyl ammonium butyl (TEAB), cetyl trimethyl ammonium chloride (CTAC) to find out the effect of PdCl₂–CuCl₂/HMS catalyst on DEC selectivity and conversion of ethanol. Fig. 15 shows the mechanism of the process. TEAB promoted catalyst showed best catalytic properties by giving highest selectivity and ethanol conversion.

Fig. 13 The effect of PdCl₂–CuCl₂/HMS catalyst on DEC selectivity and conversion of ethanol.

Zhang et al.¹⁰⁰ used HMS as support to synthesize DEC by impregnating Cu-HMS with Pd containing solution. The activity of resulting catalyst PdCl₂/Cu-HMS was better than Cu-HMS, PdCl₂/HMS. PdCl₂/HMS exhibited good STY but not good selectivity. Zhang and Ma¹¹¹ compared the performances of Cu exchanged β and γ zeolite catalyst. The selectivity of Cu_β was much more than Cu_γ. Due to special architecture of β zeolite, it didn't allow DEC to diffuse through its pores. Huang et al.¹¹⁰ studied the effects of modification with NaOH treatment on properties of γ zeolite and its catalytic activity during oxidative carbonylation of ethanol. The increase in conversion of ethanol along with almost constant selectivity was observed.

Zhang and Ma,¹¹¹ investigated PdCl₂/Cu-HMS catalyst for gas phase oxidative carbonylation of ethanol. At optimized Si/Cu molar ratio of about 50, preferred degree of mesoporous structure was obtained. Although CuCl₂–PdCl₂ supported on activated carbon or mesoporous silica catalysts exhibited excellent activity, however, the catalyst used was costly and easily deactivated. Since King¹¹² reported that Cu exchanged zeolite posses good activity for the synthesis of DMC through oxidative carbonylation of methanol. Hence, Huang et al.¹¹³ prepared CuY catalysts with three different methods and compared the performances of all three catalyst for the preparation of DEC from oxidative carbonylation of ethanol. Ammonia evaporation method was found to be the most promising for Cu loading. The precursors also affected the yield of DEC. Later Huang et al.^113,116 modified CuY zeolite with different NaOH solutions and studied its activity for DEC. Conversion of ethanol and yield of DEC increased considerably.

Zhang et al.¹¹⁴ used a novel method where hydrolysis of diethyl ether was used to consume the water continuously obtained on the product side, and hence enhance the activity of PdCl₂/Cu-HMS.¹¹⁴ The hydrolysis of diethyl ether is an endothermic reaction and oxidative carbonylation is exothermic. The free energy of the coupling reaction is less than zero.

Zhang et al.¹¹⁵ synthesized methyl and ethyl modified PdCl₂/HMS catalyst and studied the effect of silylation. The silylation improved the PdCl₂/HMS but the catalytic performance of PdCl₂/Si–Cu-HMS-Ben didn't increase as PdCl₂/HMS due to collapse of framework. Therefore, silylation agents needs to be chosen carefully during modification, and catalytic performance was closely related to both hydrophobicity and structure of molecular sieves, however, latter was main factor in the synthesis of DEC by gas-phase oxidative carbonylation of ethanol. Zhang et al.¹⁰⁹ combined TMCS and Cu-HMS to get a hybrid form of PdCl₂/Si–Cu-HMS-x which exhibited higher conversion and activity as compared to PdCl₂/Cu-HMS. As removal of water is must to obtain higher conversion, silylation of modified Cu-HMS enhanced its hydrophobic property thereby improving conversion and selectivity. Zhang et al.¹¹⁷ synthesized PdCl₂ supported on mesoporous copper oxide (CuO) prepared from hard template (HMS). The performances were also compared among mesoporous copper oxide (CuO) prepared with different copper precursors. PdCl₂/CuO catalyst prepared by Cu₂(OH)₂CO₃ gave best conversion and selectivity.

DEC was synthesized from oxy carbonylation of ethanol using CuCl₂–PdCl₂/AC as the catalyst.^118,119 The effect of support was latter studied by dispersing CuCl₂ and PdCl₂ on activated carbon and nano fibers. For same surface loading of CuCl₂ and PdCl₂, carbon nano fibers had higher dispersion of active components than activated carbon. Pd[CuCl₂]₂ species which were active sites for DEC synthesis were stabilized by bonding with oxygen-containing species.¹²⁰ The study was further continued to explore the effect of support composition and pretreatment on activity and selectivity of DEC using carbon-supported PdCu_nCl_x catalysts for the synthesis of DEC. With same CuCl₂ and PdCl₂ loading on partially oxidized carbon nano fibers resulted in a higher dispersion of the active components and a higher DEC activity than could be achieved on activated carbon. Over oxidation of edges was found to be the reason for the loss in activity of activated carbon.¹²¹ Chen et al.¹²² used Pd(PPh₃)₂Cl₂ as a promoter on Cu and Pd loading on activated carbon to enhance the yield and selectivity of DEC.

Huang et al.¹¹⁰ established a quantitative relationship between amount of Brønsted acid sites on Cu based catalysts and their activities during oxidative carbonylation. It was found that Brønsted acid sites on internal and external surface of zeolite form active sites of catalyst.

Zhang et al.¹²³ studied the effect of pretreatment of catalyst with NH₃·H₂O on activity of Cu β catalysts for gas-phase oxidative carbonylation of ethanol. After the treatment of catalyst with NH₃·H₂O H β zeolite (prepared by mixing H-zeolite and 50% CuCl) facilitated selective extraction of extra framework silicon. Fig. 16 shows the mechanism.

Fig. 14 Mechanism showing selective extraction of extra framework silicon.

4.3 Summary

The DEC synthesis route has been studied vigorously and a number of catalyst and their activities have been studied. Easy operation of reaction due to its favorable thermodynamics gives an edge over all other routes. PdCl₂–CuCl₂ based catalyst have shown good activity but still the problem of corrosion and safety parameters attached with CO need to be removed. Also, no kinetic and thermodynamics study is reported for the reaction which is essential for commercialization.

5. Synthesis of DEC from DMC

5.1 A two step method to synthesize DEC

The synthesis of DEC can be done by trans-esterification reaction between DMC and ethanol. The process can be termed as ‘green’ as DMC can be synthesized from CO₂. The reaction is limited by chemical equilibrium, an intermediate and formation of three azeotropes.

This process involves a two step reaction, first ethyl methyl carbonate is formed and in the second step, DEC is formed. The intermediate ethyl methyl carbonate itself finds many applications because of its low viscosity and freezing point.¹²⁴ Table 8 summarizes the progress in the synthesis of DEC from trans-esterification of DMC with ethanol.^125–129 The equations are represented below:

(CH₃O)₂CO + C₂H₅OH ↔ C₂H₅OCOOCH₃ + CH₃OH (16)

C₂H₅OCOOCH₃ + C₂H₅OH ↔ (C₂H₅O)₂CO + CH₃OH (17)

Table 8 Comparative analysis of different catalysts used for the synthesis of DEC from DMC

Catalyst	Reactants	Reaction conditions			Reference
		Reaction temperature (°C)	Time (h)	Selectivity DEC (%)
a Best performing catalyst or operating condition.
NKF/Al2O3, 30KF/Al2O3^a	DMC = 20 mmol, ethanol = 80 mmol, catalyst 20KF/Al2O3 = 2 wt%	80	4	57	127
Mg–Al–O–t-Bu hydrotalcite	n(EtOH) : n(DMC) = 5 : 1, catalyst 1 wt%	80	7	70.8	126
Metal triflates (yttrium triflate)^a	DMC (18.0 g, 0.2 mol), ethanol (55.2 g, 1.2 mol), 1.4 mol% of catalyst based on DMC	(76–80)	7	80.9	125
MgO	DMC, 0.05 mol; DEC, 0.05 mol	103	1	—	128
Sodium ethoxide	The ratio changes from 4 : 1 to 10 : 1	40	2	—	129

---

ΔH° for the first trans-esterification reaction is 1.595 kJ mol⁻¹ whereas it is 0.809 kJ mol⁻¹ for the second step. The E_a of forward and reverse directions for first and second reaction were calculated to be 45.633 kJ mol⁻¹ and 47.228 kJ mol⁻¹; and 55.058 kJ mol⁻¹ and 54.24 kJ mol⁻¹, respectively.¹³⁰

5.2 Homogenous catalysts

Since metal triflates were active for organic transformations.^131–133 Mei et al.¹²⁵ tested metal triflates, well known Lewis acids, as homogenous catalyst for the trans-esterification of DMC with ethanol. Yttrium triflate was found to be the best performing catalyst with easy separation and high repeatability. Mei et al.¹²⁶ used lanthanum nitrate and other lanthanum based compounds as homogenous catalyst. Its reusability was also found to be on a positive side.

5.3 Heterogeneous catalysts

Nadolska et al.¹³⁴ studied the trans-esterification reaction of ethanol and DMC using different catalyst which included zeolite, Lewatit K, etc. The screening of catalysts was done on the basis of yield and selectivity of DEC obtained. Fig. 17 shows the performances of various catalysts. Lewatit K1221 resin, Nafion SAC-13 and modified potassium carbonate gave good yield and selectivity of DEC. Kinetics of the reaction was also studied and the kinetic constants based on Arrhenius equation were estimated. The reactions were elementary in nature and second step was the rate controlling step.

Fig. 15 The effect of catalyst, support and addition of hydroxy groups to catalysts at conditions: catalyst = 0.50 g, ethanol = 3.6 g, P_CO = 2.76 bar, P_air = 4.14 bar, P_N2 = 6.89 bar, temperature = 170 °C, time = 4 h. Source of data: Dunn et al.¹¹

Fig. 16 Conversion and selectivity obtained using different promoters at conditions: ethanol = 80 mL, P_CO/P_O2 = 2 : 1, P = 2.4 MPa, T = 393 K, time 3 h and stirring speed 1000 rpm. Source of data: Xiong et al.¹⁰⁴

Fig. 17 Screening of various catalysts performance for synthesis of DEC from DMC at time = 1600 min and temperature = 348 K. Source of data: Nadolska et al.¹³⁴

Since Ando et al.¹³⁵ reported the importance of fluoride group in basicity of catalyst, Murugan and Bajaj¹²⁷ studied the yield and selectivity of DEC using ionic fluorides as catalyst supported on alumina. Fig. 18 compares the performances of different catalysts. KF/Al₂O₃ was then used as the standard catalyst to evaluate dependence of various reaction parameters on yield and selectivity of DEC and EMC.

Fig. 18 Performances of various catalysts. Reaction conditions: DMC = 20 mmol, ethanol = 80 mmol, catalyst 20KF/Al₂O₃ = 2 wt%, time = 4 h, and temperature = 80 °C. Source of data: Murugan and Bajaj.¹²⁷

Palani et al.¹³⁶ synthesized Al-MCM-41 (50), Al-MCM-41 (100), Al–Zn-MCM-41 (50) and Al–Zn-MCM-41 (100) molecular sieves with Si/Al ratio of 50 and 100 and used them for trans-esterification of DMC and ethanol in vapour phase. High temperature favored conversion of DMC. Al–Zn-MCM-41 (50) and Al–Zn-MCM-41 (100) performed well. Decrease in conversion with increase in time was observed and attributed to coke formation.

Mei et al.¹²⁶ synthesized Mg–Al–O–t-Bu hydrotalcite and reported 86.4% DMC conversion and 61.2% DEC selectivity. The activity after regeneration of catalyst was also tested till 5 runs and found the catalyst to be active. Zhao et al.¹²⁸ used carbon supported MgO catalyst synthesized by wet impregnation method and studied its performance. As for DEC synthesis compared to other porous carbon based support, NC-2 performed best as well as its resistance to leaching also added to its activity. Shi et al.¹³⁷ compared the performance of amorphous mesoporous aluminophosphate as the heterogeneous catalyst with other solid acid or base catalyst. The high activity, stability and recoverability of catalyst was observed and its high activity was attributed to weak acid–base pairs on the surface of amorphous mesoporous aluminophosphate.

5.4 Reactive distillation process

Reactive distillation process integrates reaction and distillation in single assembly. Some of the advantages of such type of reactor include enhanced conversion, improved selectivity by-passing azeotropic limitations. Compared to conventional reaction separation processes, reactive distillation involves high reactant conversion, high selectivity and better energy utilization.¹³⁸ In contrast to its advantages, several constraints such as complexity of design, difficult to scale-up limits its usage.^139,140 Keller et al.¹⁴¹ used a validated reactive distillation model and studied the effect of reflux ratio, molar feed ratio and catalyst molar fraction in the feed. These operational parameters were optimized using evolutionary algorithm. The results showed that not only DEC and EMC can be synthesized with 95% selectivity each but also the problem of low conversion of ethanol can be addressed. Optimization study was also performed to determine optimal combinations of EMC selectivity and DMC conversion that can be obtained in reactive distillation column.

Luo and Xiao¹⁴² studied the modeling and simulation of reactive distillation column. Very high conversion with DEC selectivity of 99.5% was observed. Problem of separation of alcohols was accomplished. Analysis of Murphree tray efficiency confirmed reliability of model. Lukacs et al.¹⁴³ analyzed the feasibility of batch reactive distillation and detailed study of the reaction in two reversible cascade reactions in batch reactive distillation process. Qiu et al.¹⁴⁴ integrated the process of reaction and distillation to enhance the reaction using sodium ethoxide as homogenous catalyst. Results indicated that DEC can be easily removed by distillation during the reaction itself therefore it overcomes reaction equilibrium limitations. Keller et al.¹⁴⁵ investigated the kinetics of reaction experimentally and theoretically. The molar and activity based equilibrium constants were also calculated. The temperature dependency on equilibrium constants was also described using van't Hoff equation. Continuing the work Keller et al.¹⁴⁶ performed homogenous phase trans-esterification reaction between DMC and ethanol in a pilot scale reactive distillation to produce EMC and DEC with a high selectivity of 79.2% and 80.2%, respectively. Wei et al.¹⁴⁷ studied the design and control of reactive distillation process and optimized the feed location to suppress the intermediate EMC and increased the conversion of DMC. The heavier DMC should be located at bottom and lighter ethanol should be at the top. The proposed design was compared with conventionally used reactive distillation column. Keller et al.¹⁴¹ used experimental data's and compared them to the simulation results. The non equilibrium stage model using effective diffusion coefficient and equilibrium stage model assuming chemical equilibrium was used. The non equilibrium stage model was not sufficient for proper description of experiments.

DEC from DMC is one of the most researched processes for DEC synthesis, therefore commercialization of this process is most likely in near future. Although the design of reactive distillation is widely done and high purity of DEC is being obtained, still more models with better correlations are needed.

6. Synthesis of DEC by ethyl nitrite route

The vapor phase synthesis of DEC via ethyl nitrite was developed using Pd catalysts, however few studies have been reported on DEC synthesis route.^148,149 DEC synthesis by this route occurs in two steps. First ethanol gets oxidized in presence of nitric oxide and oxygen to form ethyl nitrite which reacts with carbon monoxide to form DEC. Intermediate ethyl nitride acts as oxidant and maintains the activity of catalyst such as palladium.¹⁵⁰ These steps are shown below:

2C₂H₅OH + 2NO + 1/2O₂ ↔ (C₂H₅O)₂NO (18)

(C₂H₅O)₂NO + CO ↔ (C₂H₅O)₂CO (19)

6.1 Catalysts used and other parameters

MCM-41 type mesoporous sieves are well known as a catalyst due to their high surface area and high thermal stability.¹⁵¹ The reaction used palladium supported on Si-MCM-41 and Ti-MCM-41 for synthesis of DEC.¹⁵² The effect of copper and titanium, which were used as additives, on activity was observed. The effect of support and additives is shown in Fig. 19 (at temperature of 105 °C).

Fig. 19 The effect of support and additives on TOF of DEC. Source of data: Zhen et al.¹⁵²

Pd based catalyst performed better for the synthesis of similar type of carbonates via ethyl nitrite route.^153–156 Hence, Ma et al.²⁹ investigated the reaction using various binary catalysts supported on activated carbon. The performances are compared in Fig. 20a. The space time yield (STY) of DEC was good with PdCl₂–CuCl₂/AC. Different additives were added to observe the change in yield and selectivity of DEC. The effect of various additives such as LaCl₃, CeCl₃, PrCl₃, CH₃COOK, BiCl₃, (NH₄)₆Mo₇O₂₄ on STY of DEC was studied. Various additives increased the STY yield of DEC as well as the by-products but some catalyst like CeCl₃ gave best STY as well as they suppressed the production of by-products. The mechanism of redox cycles and deactivation of catalysts was also explored. It is shown in Fig. 21, and reactions are given as follows:

PdCl₂ + C₂H₅ONO ↔ PdClx(OC₂H₅)NO (20)

PdClx(OC₂H₅)NO + CO ↔ PdClx(COOC₂H₅)NO (21)

PdClx(COOC₂H₅)NO ↔ Pd(0) + ClCOOC₂H₅ + NO (22)

Fig. 20 (a) The effect of various binary catalysts on STY of DEC at reaction conditions: T = 383 K, GHSV = 2200 h⁻¹, inlet gas composition CO = 20%, C₂H₅ONO = 20%. Source of data: Ma et al.²⁹ (b) Conversion of DEO and selectivity of DEC obtained with different catalysts at reaction condition: T = 513 K, GHSV = 1000 h⁻¹, N₂ = 25 mL min⁻¹, reaction time = 1 h. Source of data: Hao et al.¹⁴⁹

Fig. 21 Mechanism of redox cycles of catalyst.

6.2 Synthesis of DEC from diethyl oxalate (DEO)

Hao et al.¹⁴⁹ analysed thermodynamics of preparation of dialkyl carbonates from dialkyl oxalates and found synthesis of DEC from DEO to be feasible. Although synthesis of diaryl carbonates from diaryl oxalates have been reported earlier, however, there is still need of good catalyst. The reaction below summarizes the process:

(C₂H₅OCO)₂ ↔ (C₂H₅O)CO + CO (23)

Li et al.¹⁵⁷ used Pd as the catalyst in the fixed bed reactor and used Runge–Kutta method to estimate parameters of kinetic models. Fan et al.¹⁴⁸ synthesized DEC by using Wacker type catalysts on different types of support. The performance on different supports and the effect of solvent on performance was observed.

Hao et al.¹⁴⁹ investigated the decarbonylation of DEO to DEC using various alkali catalysts supported on silica, alumina, zeolite, zirconia, activated carbon and MgO. The performance of catalyst and their support is summarized in Fig. 20b. The problem of leaching of potassium element from the catalyst was reflected in decrease in selectivity of DEC. Other parameters such as effect of temperature, catalyst loading were also addressed.

7. Other reactions for synthesis of DEC

DEC can be synthesized from trans-esterification of ethylene carbonate and ethanol. This method posses advantages as both ethylene carbonate and ethanol are low toxic and as ethylene carbonate is industrially produced from CO₂ and oxirane, hence this method can be considered as sink to CO₂. Xianping and Wende¹⁵⁸ studied the trans-esterification of ethylene carbonate with ethanol using sodium ethylate as homogenous catalyst also kinetic models were proposed. The parameters of the model were estimated by Runge–Kutta method. Qiu et al.¹⁵⁹ synthesized DEC from ethylene carbonate using KF/γ-Al₂O₃ as a catalyst. KOH and K₂CO₃ were the most active species for the activity. 72% yield of DEC was obtained at 298 K. To shift the equilibrium to the product side, reactive distillation was used by Qiu et al.¹⁶⁰ with sodium ethoxide as the catalyst. Effect of different parameters like reactant ratio, reflux ratio, catalyst concentration on yield of DEC was studied. DEC yield of 91% with 97% purity was obtained.

Bae et al.¹⁶¹ synthesized DEC through liquid phase hydrodechlorination of CCl₄ in a medium containing ethanol with co-production of acetaldehyde. The method can also termed as green one as it involves sink for CCl₄, a ozone depletion compound.^162,163 Although exothermic reaction impedes the implementation of reaction, the use of reaction in liquid phase is inevitable for synthesis. Pd and Pt supported on activated carbon were used as the catalyst in presence of protic solvent ethanol. The reaction without ethanol yielded low conversion of CCl₄ but conducting the reaction in presence of ethanol enhances the conversion of CCl₄ and DEC yield.

Wang et al.¹⁶⁴ synthesized DEC by trans-esterification of propylene carbonate and ethanol by semi-continuous process by using potassium carbonate. The reaction was found to be endothermic and the reaction was favored at high temperature. Yield and selectivity of DEC obtained were 92.5% and 80.8%, respectively. Reaction mechanism along with reaction parameters was also explored. Guo et al.¹⁶⁵ synthesized through this route using tetramethyl-guanidine in a semi-continuous mode. Under optimal conditions the yield and selectivity of DEC was 95.8% and 90.7%.

More studies are required on these routes for the understanding their reaction mechanism and other engineering parameters.

8. Thermodynamics and kinetics study in DEC

From the text, it can be inferred that synthesis of DEC from CO is the most suitable as compared to other routes. DEC synthesis from CO₂ is the most green method as it involves direct conversion of CO₂ to valuable and environment friendly compound but thermodynamically least favorable among all. Synthesis of DEC from DMC is also favorable, however it suffers from azeotropic problem.

Very few phase equilibrium studies have been reported in the literature with respect to DEC.^166,167 Ding¹⁶⁸ evaluated excess Gibbs free energy of mixing of five organic carbonates with ten binary combinations by fitting their binary phase diagrams. The activity coefficient was calculated from these plots. Molecular cyclicity plays an important role in determining the intermolecular forces as the evaluated excess Gibbs energies deviate from those of an ideal solution, in the positive direction for the opposite-cyclicity combinations and in the negative direction for the same-cyclicity combinations. The linear–linear ones showed small values implying nearly ideal solution while cyclic–cyclic had considerable negative values, thus demonstrating a strong attractive force between molecules.

Zhu et al.¹⁶⁹ studied vapor liquid equilibrium system for CO + DEC and CO + ethyl acetate (EA) at 293.2 K, 313.2 K and 333.2 K at the elevated pressures up to 12 MPa. The experiments were conducted in cylindrical autoclave with a moveable piston and an observation window. Peng–Robinson (PR) and Peng–Robinson–Stryjek–Vera (PRSV) equation of state with the two-parameter van der Waals II or Panagiotopoulos–Reid mixing rule were used to correlate to the experimental data. A good correlation of model with the experimental values was found at higher temperature and pressure. The experimental and calculated results showed good correlation.

Till date no study is reported on ternary system containing DEC, CO₂, ethanol and water, however, some studies on binary systems are reported. The vapor–liquid equilibria for mixtures containing organic carbonates + n-alkanes using several versions of the UNIFAC model have been predicted.¹⁷⁰ The UNIFAC and UNIQUAC, predicts excess molar enthalpies whereas later model can be used to predict excess molar enthalpies (h^E) and Gibbs energy (g^E), and infinite dilution coefficients.^171,172 The later versions of UNIFAC were used for calculation of interaction parameters between carbonate and methylene groups.^173–175 The h^E and g^E values of systems containing a linear carbonate and an n-alkane are fairly well represented by all the tested versions of the UNIFAC model.

Behavior of phase equilibrium at high pressure for the binary systems of [carbon dioxide (1) + DMC (2)] and [carbon dioxide (1) + DEC (2)] at temperatures of 273 K, 283 K, and 293 K have been reported.^176,177 The experimental data were correlated with the Peng–Robinson (PR) equation of state and the Peng–Robinson–Stryjek–Vera (PRSV) equation of state with van der Waals-1 or Panagiotopoulos–Reid mixing rules. The correlations produced reasonable values for the interaction parameters. The comparisons between calculation results and experimental data indicate that the PRSV equation of state coupled with the Panagiotopoulos–Reid mixing rule produced better correlated results.

Keller et al.¹⁷⁸ studied the chemical equilibrium and reaction kinetics of system containing, DMC and ethanol, experimentally and theoretically. Homogeneous catalyst sodium ethoxide was applied to enhance the reaction rate. Molar-based and activity-based chemical equilibrium constants were calculated from experimental results, and their temperature dependence was described using the van't Hoff equation. An activity-based kinetic model that considers the temperature dependence of the reaction rate constants with the Arrhenius equation was derived. The activity coefficients γ_i was calculated by UNIQUAC or UNIFAC model.

Zhang et al.¹²⁹ synthesized DEC from propylene carbonate and ethanol using sodium ethoxide as homogenous catalyst. The reaction was found to be reversible with a propylene carbonate equilibrium conversion of about 64% at an ethanol to propylene carbonate mole ratio of 8.0 and a reaction temperature of 303 K. The thermodynamic data of reactants and products was obtained from literature.^129,175 Since equilibrium conversion changed very little with the temperature, equilibrium constant based on partial pressure (K_p) was approximately equal to equilibrium constant on molar basis (K_c). Change in enthalpy of reaction determined experimentally (ΔH^o_exp) was found with the slope of the plot between K_c and the result was very much similar to change in enthalpy derived from modeling (ΔH^o_m). Model parameters were estimated by least square method. The E_a was calculated to be 31.29 kJ mol⁻¹. The trans-esterification of propylene carbonate and ethanol was postulated to be relatively fast and weakly exothermic reversible process.

Meng et al.¹⁷⁹ studied the kinetics of CO coupling reaction in presence of ethyl nitrite over supported palladium catalyst in a continuous flow integral-type fixed-bed reactor to give diethyl oxalate (DEO) and DEC. An integral-type fixed-bed reactor was employed, in which resistances of the intra particle diffusion and external mass transfer were ruled out. Power law kinetic rate equations were used to represent the experimental data. Parameters in the model were determined by means of the damped least-square method. At diameter of catalyst particles (d_p) < 3 mm, rate of reactions were found to be within kinetic regime, and both intra particle and external mass-transfer resistance were negligible.

Nadolska et al.¹³⁴ screened heterogeneous catalysts for the two-stage reaction of trans esterification of DMC to DEC. For the most active catalysts kinetic parameters were determined. The process was analyzed in reactive distillation column, with the best catalyst incorporated as an element of the structured packing. Modified potassium carbonate, Lewatit K1221 and Nafion SAC-13 were proved to be the most active heterogeneous catalysts.

Overall, very few kinetics and thermodynamics studies are reported for various synthesis routes of DEC and a research focus on these aspects is immediately required so that these routes may be commercially developed.

9. Summary and future research perspectives

DEC being and environment friendly, low bio-accumulative and easily biodegradable compound, can be used as such in various applications and as a reactant for the synthesis of important compounds like ethyl benzene. Since phosgene route of organic carbonates has been abandoned, therefore, non-phosgene environment friendly routes to synthesize DEC are becoming more popular. The synthesis of DEC was reviewed in this paper with focus on different catalysts used and mechanisms through synthesis from various routes.

Process involving urea and ethanol is not only economically favourable due to its cheap and non-toxic material present, but it also possesses extra edge because of no azeotrope formation.^180–187 Although different types of catalysts have enhanced the yield and selectivity of DEC, but it still needs to be improved by some folds for its commercialisation. Yield can be improved further by removing the DEC formed in the reactor as soon as it is formed. This can be done by implying catalytic distillation, reactive distillation, fixed bed reactor type setup has been successful in similar type of reactions.^180–182 Ionic liquids are widely used as the catalyst for different reactions but are not used for DEC synthesis.¹⁸⁵ Ceria based catalysts should be used further for DEC synthesis that have shown excellent activity for trans esterification reaction.^183,184 The homogenous catalysts performed better than heterogenous catalysts but the problem of separation of homogenous catalysts can be met by substituting some groups like sulfonic acid on it which ease to its separation.¹⁸⁶ However, no kinetic study is reported yet and hence no insight view of the activity of catalyst which include calculation of E_a or frequency factor.¹⁸⁷

The sequestration of CO₂ into mineral carbonates is not only environment friendly and benign process, it also a green method which acts as a sink for CO₂. But the process is limited by unfavorable thermodynamics. Low acidity of ethanol makes it passive for the reaction. Some catalysts, co-catalysts must be used to activate ethanol. Certain dehydrating agents may be introduced in order to shift the equilibrium to the product. This reaction proceeds at very high pressure and hence study of vapour liquid equilibrium is very important. This will help in prediction of the conversion and yield at higher temperature and pressure that will help to minimize the need of experiments at high pressure.¹⁸⁸ Moreover, no kinetic study has been performed for this method so far, which is urgently required.

Oxidative carbonylation of ethanol occurs at low temperature and pressure; this makes the process more favorable as compared to DEC synthesis from CO₂. Moreover high selectivity of DEC obtained favors the commercialization of process. However, the use of poisonous CO as the reactant and absence of highly active noble catalyst restricts the process on large scale. Oxy carbonylation of ethanol is best suited method for commercialization as it involves low temperature, pressure and gives selectivity of nearly 100%. However, the quick deactivation of catalysts and corrosive nature of gas phase reaction needs some attention.

Synthesis of DEC by trans-esterification of DMC with ethanol can be termed as a green one but it suffers from formation of azeotropes. Reactive distillation is the new method which bypasses conventionally used reaction followed by distillation to get final product with improved yield and selectivity.

DEC is a compound which can be used for the synthesis of many lucrative products and has different applications in various fields; therefore these research gaps need to be filled.

References

1. A. Behr, Angew. Chem., Int. Ed. Engl., 1988, 27, 661–678.
2. W. Leitner, Coord. Chem. Rev., 1996, 153, 257–284.
3. M. Aresta and A. Dibenedetto, J. Mol. Catal. A: Chem., 2002, 182, 399–409.
4. S. Therdthianwong, C. Siangchin and A. Therdthianwong, Fuel Process. Technol., 2008, 89, 160–168.
5. N. A. S. Amin and T. C. Yaw, Int. J. Hydrogen Energy, 2007, 32, 1789–1798.
6. R. Raudaskoski, M. V. Niemela and R. L. Keiski, Top. Catal., 2007, 45, 57–60.
7. B. Schaffner, F. Schaffner, S. P. Verevkin and A. Borner, Chem. Rev., 2010, 110, 4554–4581.
8. E. M. Eyring, H. L. C. Meuzelaar and R. J. Pugmire, Annual Report for Cooperative Research in C1 Chemistry (DF-FC26-99FT40540), 2001, pp. 32–36.
9. M. A. Pacheco and C. L. Marshall, Energy Fuels, 1997, 11, 2–29.
10. H. An, X. Zhao, L. Guo, C. Jia, B. Yuan and Y. Wang, Appl. Catal., A, 2012, 433–434, 229–235.
11. B. C. Dunn, C. Guenneau, S. A. Hilton, J. Pahnke, E. M. Eyring, J. Dworzanski and H. L. Meuzelaar, Energy Fuels, 2002, 16, 177–181.
12. M. Honda, S. Kuno, N. Begum, K. I. Fujimoto, K. Suzuki, Y. Nakagawa and K. Tomishige, Appl. Catal., A, 2010, 384, 165–170.
13. N. S. Roh, B. C. Dunn, E. M. Eyring, R. J. Pugmire and H. L. Meuzelaar, Fuel Process. Technol., 2003, 83, 27–38.
14. D. Wang, B. Yang, X. Zhai and L. Zhou, Fuel Process. Technol., 2007, 88, 807–812.
15. M. Kozak, J. Merkisz, P. Bielaczyc and A. Szczotka, SAE, 2009, 2009, 2696.
16. Y. Li, B. Xue and Y. Yang, Fuel Process. Technol., 2009, 90, 1220–1225.
17. B. Veldurthy, J. M. Clacens and F. Figueras, Eur. J. Org. Chem., 2005, 10, 1972–1976.
18. E. H. Elageed, B. Wang, Y. Zhang, S. Wu and G. Gao, J. Mol. Catal. A: Chem., 2015, 408, 271–277.
19. Y. Li, B. Xue and X. He, J. Mol. Catal. A: Chem., 2009, 301, 106–113.
20. B. Xue, Y. Li, S. Wang, H. Liu and C. Xu, React. Kinet., Mech. Catal., 2010, 100, 417–425.
21. K. U. Nandhini, B. Arabindoo, M. Palanichamy and V. Murugesan, Catal. Commun., 2006, 7, 351–356.
22. R. Badru and B. Singh, RSC Adv., 2014, 4, 38978–38985.
23. M. Sawicki and L. L. Shaw, RSC Adv., 2015, 5, 53129–53154.
24. P. Wang, S. Liu, X. Ma, Y. He, A. S. Alshammari and Y. Deng, RSC Adv., 2015, 5, 25849–25856.
25. M. Selva, A. Caretto, M. Noè and A. Perosa, Org. Biomol. Chem., 2014, 12, 4143–4155.
26. J. Wang, Y. Wu, X. Xuan and H. Wang, Spectrochim. Acta, Part A, 2002, 58, 2097–2104.
27. A. M. Haregewoin, E. G. Leggesse, J. C. Jiang, F. M. Wang, B. J. Hwang and S. D. Lin, Electrochim. Acta, 2014, 136, 274–285.
28. A. A. Shaikh and S. Sivaram, Chem. Rev., 1996, 96, 951–976.
29. X. Ma, M. Fan and P. Zhang, Catal. Commun., 2004, 5, 765–770.
30. I. E. Muskat and F. Strain, US Pat., 2 379 250, 1941.
31. S. Franklin and I. E. Muskat, US Pat., 2370571, 1941.
32. L. K. Frevel and J. A. Gilpin, US Pat., 3642858, 1972.
33. T. Matsuzaki, T. Shimamura, Y. Toriyahara and Y. Yamasaki, Eur Pat., 0565076, 1993.
34. G. H. Zaid and B. A. Wolf, US Pat., 6452036, 2002.
35. S. Wershofen, S. Klein, Z. Zhou, X. Wang, J. Wang and M. Kang, US Pat., 8445713, 2009.
36. L. C. Zhao, Z. Q. Hou, C. Z. Liu, Y. Y. Wang and L. Y. Dai, Chin. Chem. Lett., 2014, 25, 1395–1398.
37. M. H. Wang, N. Zhao and W. Wei, Ind. Eng. Chem. Res., 2005, 44, 7596–7599.
38. M. H. Wang, H. Wang and N. Zhao, Catal. Commun., 2006, 7, 6–10.
39. G. Lian, Z. Xinqiang, A. Hualiang and W. Yanji, Chin. J. Catal., 2012, 33, 595–600.
40. P. M. Schaber, J. Colson and S. Higgins, Thermochim. Acta, 2004, 424, 131–142.
41. Y. Y. Gao, W. C. Peng, N. Zhao, W. Wei and Y. Sun, J. Mol. Catal. A: Chem., 2011, 351, 29–40.
42. W. Zhao, W. Peng, D. Wang, N. Zhao, J. Li, F. Xiao, W. Wei and Y. Sun, Catal. Commun., 2009, 10, 655–658.
43. J. Y. Ryu, US Pat., 0203307 A1, 2005.
44. S. Xin, L. Wang, H. Li, K. Huang and F. Li, Fuel Process. Technol., 2014, 126, 453–459.
45. D. Wang and X. Zhang, Adv. Mater. Res., 2012, 479–481, 1768–1771.
46. B. M. Bhanage, S. Fujita, Y. Ikushima and M. Arai, Green Chem., 2003, 5, 429–432.
47. G. Lian, X. Zhao, H. An and Y. Wang, Chin. J. Catal., 2012, 33(4), 595–600.
48. R. Notario, J. Quijano, C. Sanchez and E. Velez, J. Phys. Org. Chem., 2005, 18, 134–141.
49. S. Xin, L. Wang, H. Li, K. Huang and F. li, Fuel Process. Technol., 2014, 126, 453–459.
50. S. Kus, M. Otremba and M. Taniewski, Fuel, 2003, 8, 1331–1338.
51. D. Wang, X. Zhang, Y. Gao, F. Xiao, W. Wei and Y. Sun, Fuel Process. Technol., 2010, 91, 1081–1086.
52. D. Wang, X. Zhang, C. Liu and T. Cheng, React. Kinet., Mech. Catal., 2015, 115, 597–609.
53. X. Zhang, D. Wang, C. Liu, L. Shi, Q. Fang and Z. Song, Petrochem. Technol., 2015, 44, 1182–1187.
54. P. Wang, S. Liu, F. Zhou, B. Yang, A. S. Alshammaric and Y. Deng, RSC Adv., 2015, 5, 19534–19540.
55. D. Wang, X. Zhang, W. Zhao, W. Peng, N. Zhao, F. Xiao, W. Wei and Y. Sun, J. Phys. Chem. Solids, 2010, 71, 427–430.
56. D. Wang, X. Zhang, C. Liu, T. Cheng, W. Wei and Y. Sun, Appl. Catal., A, 2015, 505, 478–486.
57. L. Wang, H. Li, S. Xin and F. Li, Catal. Commun., 2014, 50, 49–53.
58. X. Qin, B. Liu, B. Han, W. Zhao, S. Wu and P. Lian, Adv. Mater. Res., 2013, 821–822, 1081–1084.
59. D. Wang, X. Zhang, W. Wei and Y. Sun, Petrochem. Technol., 2012, 41, 396–400.
60. K. S. Weil, J. Solid State Chem., 2008, 18, 199–210.
61. H. Zhu, X. Gu, K. Yao, L. Gao and J. Chen, Ind. Eng. Chem. Res., 2009, 48, 5317–5320.
62. X. Qin, B. Liu, B. Han, W. Zhao, S. Wu and P. Lian, Adv. Mat. Res., 2013, 821, 1081–1084.
63. J. A. R. Navarro and B. Lippert, Coord. Chem. Rev., 1999, 185, 653–667.
64. W. Zhao, F. Wang, W. Peng, N. Zhao, J. Li, F. Xiao, W. Wei and Y. Sun, Ind. Eng. Chem. Res., 2008, 47, 5913–5917.
65. H. Zhao, X. Zhao, H. An and R. Wang, Petrochem. Technol., 2009, 2, 139–144.
66. J. Liu, Y. Li, J. Zhang and D. He, Appl. Catal., A, 2016, 513, 9–18.
67. E. Privalova, S. Rasi, P. Arvela, K. Eranen, J. Rintala, D. Murzin and J. Mikkola, RSC Adv., 2013, 3, 2979–2994.
68. T. Sakakura, J. C. Choi and H. Yasuda, Chem. Rev., 2007, 107, 2365–2387.
69. M. Kharoune, J. Lavoie and Y. Comeau, CA Patent, 469, 974, 2005.
70. E. Leino, M. P. Arvela, V. Etaa, Y. D. Murzin, T. Salmi and J. P. Mikkola, Appl. Catal., A, 2010, 383, 1–13.
71. J. Murto, Acta Chem. Scand., 1964, 18, 1043–1053.
72. U. Romano, N. Dimuzio and F. Rivetti, BE. Pat., 80-203043 886495, 1981.
73. A. Heyden, DE. Pat., 109933, 1900.
74. F. Gasc, S. T. Roux and Z. Mouloungui, J. Supercrit. Fluids, 2009, 50, 46–53.
75. O. Arbelaez, A. Orrego, F. Bustamante and A. L. Villa, Top. Catal., 2012, 55, 668–672.
76. K. Tomishige, T. Sakaihori, Y. Ikeda and K. Fujimoto, Catal. Lett., 1999, 58, 225–229.
77. Y. Yoshida, Y. Arai, S. Kado, K. Kunimori and K. Tomishige, Catal. Lett., 2006, 115, 95–101.
78. P. Kumar, P. With, V. C. Srivastava, R. Glaser and I. M. Mishra, J. Environ. Chem. Eng., 2015, 3, 2943–2947.
79. E. Leino, P. Mäki-Arvela, K. Eränen, M. Tenho, D. Y. Murzin, T. Salmi and J. P. Mikkola, Chem. Eng. J., 2011, 176, 124–133.
80. E. Leino, N. Kumar, P. Mäki-Arvela, A. Aho, K. Kordás, A. R. Leino, A. Shchukarev, D. Y. Murzin and J. P. Mikkola, Mater. Chem. Phys., 2013, 143, 65–75.
81. E. Leino, M. P. Arvela, V. Eta, N. Kumar, F. Demoisson, A. Samikannu and J. P. Mikkola, Catal. Today, 2013, 210, 47–54.
82. N. Kumar, E. Leino, P. Arvela, A. Aho, M. Kaldstrom, M. Tuominen, P. Laukkanen, K. Eranen, J. P. Mikkola, T. Salmi and D. Y. Murzin, Microporous Mesoporous Mater., 2012, 152, 71–77.
83. X. Zhang, D. Jia, J. Zhang and Y. Sun, Catal. Lett., 2014, 144, 2144–2150.
84. M. S. Seehra, S. Suri and V. Singh, J. Appl. Phys., 2012, 111(7), 07B516.
85. P. Dutta, M. S Seehra, Y. Zhang and I. Wender, J. Appl. Phys., 2008, 103(7), 07D104.
86. O. Arbalaez, T. Reina, S. Ivanova, F. Bustamante, L. IvVilla, A. IvCenteno and A. Odriozola, Appl. Catal., A, 2015, 497, 1–9.
87. W. Wang, S. Wang, X. Ma and J. Gong, Catal. Today, 2009, 148, 323–328.
88. I. Prymak, V. N. Kalevaru, S. Wohlrab and A. Martin, Catal. Sci. Technol., 2015, 5, 2322–2331.
89. I. Prymak, V. Kalevaru, P. Kollmorgen, S. Wohlrab and A. Martin, DGMK Tagungsber., 2013, 2013, 249–256.
90. I. Prymak, V. Kalevaru, S. Wohlrab, O. Prymak and A. Martin, DGMK Tagungsber., 2014, 2014, 211–220.
91. J. Q. Wang, W. G. Cheng, J. Sun, T. Y. Shi, X. P. Zhang and S. J. Zhang, RSC Adv., 2014, 4, 2360–2367.
92. T. Zhao, Y. Han and Y. Sun, Fuel Process. Technol., 2000, 62, 187–194.
93. L. Wang, H. Li, S. Xin, P. He, Y. Cao, F. Li and X. Hou, Appl. Catal., A, 2014, 471, 19–27.
94. K. Tomishige, S. Tomohiro, I. Yoshiki and F. Kaoru, Catal. Lett., 1999, 58, 225–229.
95. S. T. Gadge and B. M. Bhanage, RSC Adv., 2014, 4, 10367–10389.
96. P. Chen, S. Huang, J. Zhang, S. Wang and X. Ma, Front. Chem. Sci. Eng., 2014, 9(2), 224–231.
97. A. Punnoose, M. S. Seehra, B. C. Dunn and E. M. Eyring, Energy Fuels, 2002, 16, 182–188.
98. N. S. Roh and B. C. Dunn, Am. Chem. Soc., Div. Fuel Chem., Prepr. Symp., 2002, 47, 142–143.
99. Z. Zhang, X. Ma, J. Zhang, F. He and S. Wang, J. Mol. Catal. A: Chem., 2005, 227, 141–146.
100. Z. Zhang, X. Ma, P. Zhang, Y. Li and S. Wang, J. Mol. Catal. A: Chem., 2007, 266, 202–206.
101. P. Zhang, Y. Zhou, M. Fan and P. Jiang, Chin. J. Catal., 2015, 36, 2036–2043.
102. T. C. Liu and C. S. Chang, Appl. Catal., A, 2006, 304, 72–77.
103. N. Roh, B. Dunn, E. Erying, R. Pugmire and H. Meuzelaar, Fuel Process. Technol., 2003, 83, 27–38.
104. H. Xiong, W. Mo, J. Hu, R. Bai and G. Li, Ind. Eng. Chem. Res., 2009, 48, 10845–10849.
105. D. Zhu, F. Mei, L. Chen, W. Moa, T. Li and G. Li, Fuel, 2011, 90, 2098–2102.
106. X. Z. Jiang, Y. H. Sua, B. J. Lee and S. H. Chien, Appl. Catal., A, 2001, 211, 47–51.
107. P. Zhang, S. Wang, S. Chen, Z. Zhang and X. Ma, Chem. Eng. J., 2008, 143, 220–224.
108. J. H. Ko, R. S. Park, J. K. Jeon, D. H. Kim, S. C. Jung, S. C. Kim and Y. K. Park, J. Ind. Eng. Chem., 2015, 32, 109–112.
109. P. Zhang, Y. Zhou, M. Fan, P. Jiang, X. Huang and J. Lou, Catal. Lett., 2014, 144, 320–324.
110. S. Huang, Y. Wang, Z. Wang, B. Yan, S. Wang, J. Gong and X. Ma, Appl. Catal., A, 2012, 417–418, 236–242.
111. P. Zhang and X. Ma, Chem. Eng. J., 2010, 163, 93–97.
112. S. T. King, J. Catal., 1996, 161, 530–538.
113. S. Huang, P. Chen, B. Yan, S. Wang, Y. Shen and X. Ma, Ind. Eng. Chem. Res., 2013, 52, 6349–6356.
114. P. Zhang, M. Fan and P. Jiang, RSC Adv., 2012, 2, 4593–4595.
115. P. Zhang, Y. Zhou, M. Fan and P. Jiang, Appl. Surf. Sci., 2014, 295, 50–53.
116. S. Huang, J. Zhang, Y. Wang, P. Chen, S. Wang and X. Ma, Ind. Eng. Chem. Res., 2014, 53, 5838–5845.
117. P. Zhang, Y. Zhou, M. Fan and P. Jiang, Appl. Surf. Sci., 2015, 332, 379–383.
118. Y. Zhang, I. J. Drake, D. N. Briggs and A. T. Bell, J. Catal., 2006, 244, 219–229.
119. D. N. Briggs, Y. Zhang and A. T. Bell, J. Catal., 2007, 251, 443–452.
120. D. N. Briggs, K. H. Lawrence and A. T. Bell, Appl. Catal., A, 2009, 366, 71–83.
121. D. N. Briggs, G. Bong, E. Leong, K. Oei, G. Lestari and A. T. Bell, J. Catal., 2010, 276, 215–228.
122. X. Q. Chen, F. Gao and C. F. Xu, J. Fuel Chem. Technol., 2006, 34, 607–610.
123. P. Zhang, S. Huang, S. Wang and X. Ma, Chem. Eng. J., 2011, 172, 526–530.
124. E. J. Plichta and W. K. Behl, J. Power Sources, 2000, 88, 192–196.
125. F. M. Mei, E. X. Chen and G. X. Li, Kinet. Catal., 2009, 50, 666–670.
126. F. Mei, E. C. G. Li and A. Zhang, React. Kinet. Catal. Lett., 2008, 93, 101–108.
127. C. Murugan and H. C. Bajaj, Fuel Process. Technol., 2011, 92, 77–82.
128. G. Zhao, J. Shia, G. Liua, Y. Liua, Z. Wang, W. Zhang and M. Jia, J. Mol. Catal. A: Chem., 2010, 327, 32–37.
129. Y. Zhang, Z. Ji, R. Xu and Y. Fang, Chem. Eng. Technol., 2011, 35, 693–699.
130. H. P. Luo and W. D. Xiao, Chem. Eng. Sci., 2001, 56, 403–410.
131. Y. Arai, T. Masuda, Y. Masaki and M. Shiro, J. Chem. Soc., Perkin Trans. 1, 1995, 91, 2913–2917.
132. G. A. Olah, O. Farooq, S. M. F. Farnia and J. A. Olah, J. Am. Chem. Soc., 1988, 110, 2560–2564.
133. E. Fillion, D. Fishlock, A. Wilsily and J. M. Goll, J. Org. Chem., 2005, 70, 1316–1327.
134. I. Z. Nadolska, K. Warmuzinski and J. Richter, Catal. Today, 2006, 114, 226–230.
135. T. Ando, S. J. Brown, J. H. Cark, P. G. Cork, T. Hanfusa, J. Ichihara, J. M. Miller and M. S. Robertson, J. Chem. Soc., Perkin Trans., 1986, 2, 1133–1139.
136. A. Palani, N. Gokulakrishnan, M. Palanichamy and A. Pandurangan, Appl. Catal., A, 2006, 304, 152–158.
137. J. Shi, G. Liu, Z. Fan, L. Nie, Z. Zhang, W. Zhang, Q. Huo, W. Yan and M. Jia, Catal. Commun., 2011, 12, 721–725.
138. G. T. Van and A. Stankiewicz, Ind. Eng. Chem. Res., 2009, 48, 2465–2474.
139. H. Schoenmakers and B. Besslin, Chem. Eng. Process., 2003, 42, 145–155.
140. K. Sundmacher and U. Hoffmann, Chem. Eng. Sci., 1996, 51, 2359–2368.
141. T. Keller, B. Dreisewerd and A. Górak, Chem. Process Eng., 2013, 34, 17–38.
142. H. Luo and W. Xiao, Chem. Eng. Sci., 2001, 56, 403–410.
143. T. Lukacs, C. Steger, E. Rev, M. Meyer and Z. Lelkes, Int. J. Chem. Eng., 2011, 2011, 231828.
144. P. Qiu, L. Wang, X. Jiang and B. Yang, Energy Fuels, 2012, 26, 1254–1258.
145. T. Keller, J. Holtbruegge, A. Niesbach and A. Gorak, Ind. Eng. Chem. Res., 2011, 50, 11073–11086.
146. T. Keller, J. Holtbruegge and A. Gorak, Chem. Eng. J., 2012, 180, 309–322.
147. H. Y. Wei, A. Rokhmah, R. Handogo and I. L. Chien, J. Process Control, 2011, 21, 1193–1207.
148. M. Fan, P. Zhang and X. Ma, Fuel, 2007, 86, 902–905.
149. C. Y. Hao, S. Wang and X. Ma, J. Mol. Catal. A: Chem., 2009, 306, 130–135.
150. T. Matsuzaki and A. Nakamura, Catal. Surv. Asia, 1997, 1, 77–88.
151. J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmin, T. U. Chu, D. H. Olsen, E. W. Sheppard, S. B. McCullen and J. B. Higgins, J. Am. Chem. Soc., 1992, 114, 10834–10843.
152. J. X. Zhen, S. Y. Hua and C. S. Hua, Catal. Lett., 2000, 69, 153–156.
153. N. Manada, M. Murakami, K. Abe, Y. Yamamoto and T. Kurafuji, US Pat., 5403949, 1995.
154. N. Manada, T. Kurafuji, Y. Yamamoto and M. Murakami, US Pat., 5426209, 1995.
155. K. Nishihira, S. Tanaka, Y. Nishida, N. Manada, T. Kurafuji and M. Murakami, US Pat., 5869729, 1999.
156. K. Nishihira, K. Mizutare and S. Tanaka, US Pat., 5162563, 1992.
157. Z. Li, C. He, D. Yin, B. Wang, X. Ma and G. Xu, J. Chem. Ind. Eng., 2005, 56, 2315–2319.
158. Z. Xianping and X. Wende, Chem. React. Eng. Technol., 2006, 22, 418–419.
159. P. Qiu, B. Yang, C. Yi and S. Qi, Catal. Lett., 2010, 137, 232–238.
160. P. Qiu, L. Wang, X. Jiang and B. Yang, Energy Fuels, 2012, 26, 1254–1258.
161. J. W. Bae, E. J. Jang, D. H. Jo, J. S. Lee and K. H. Lee, J. Mol. Catal. A: Chem., 2003, 206, 225–238.
162. H. C. Choi, S. H. Choi, J. S. Lee, K. H. Lee and Y. G. Kim, J. Catal., 1996, 161, 790–797.
163. J. W. Bae, E. D. Park, J. S. Lee, K. H. Lee, Y. G. Kim, S. H. Yeon and B. H. Sung, Appl. Catal., A, 2001, 217, 79–89.
164. M. Wang, X. Q. Zhao, H. L. An and Y. J. Wang, J. Chem. Eng. Chin. Univ., 2010, 24(4), 663–669.
165. L. Guo, M. Wang, X. Zhao, H. An and Y. Wang, Petrochem., Tech., 2011, 40(6), 590–593.
166. F. Bustamante, F. Orrego, S. Villegas and L. Villa, Ind. Eng. Chem. Res., 2012, 51, 8945–8956.
167. V. Eta, P. Arvela, R. Leino, K. Kordas, T. Salmi, D. Murzin and J. Mikkola, Ind. Eng. Chem. Res., 2010, 49, 9609–9617.
168. M. S. Ding, J. Solution Chem., 2005, 34(3), 343–359.
169. R. Zhu, J. Zhou, S. Liu, J. Ji and Y. Tian, Fluid Phase Equilib., 2010, 291(1), 1–7.
170. J. Garcia, E. R. Lopez, J. Fernandez and J. L. legido, Thermochim. Acta, 1996, 286, 321–332.
171. A. Fredenslund, R. L. Jones and J. M. Prausnitz, AIChE J., 1975, 21, 1086–1099.
172. D. S. Abrams and J. M. Prausnitz, AIChE J., 1975, 21, 116–128.
173. L. Gmehling, J. Li and M. Schiller, Ind. Eng. Chem. Res., 1993, 26, 178–193.
174. B. L. Larsen, P. Rasmussen and A. Fredenslund, Ind. Eng. Chem. Res., 1987, 26, 2274–2286.
175. D. R. Lide, CRC Handbook of Chemical and Physics, CRC Press, Boca Raton, FL, 80th edn, 2000.
176. C. Lu, Y. Tian, W. Xu, D. Li and R. Zhu, J. Chem. Thermodyn., 2008, 40, 321–329.
177. D. Y. Peng and D. B. Robinson, Ind. Eng. Chem. Fundam., 1976, 15, 59–64.
178. T. Keller, J. Holtbruegge, A. Niesbach and A. Gorak, Ind. Eng. Chem. Res., 2011, 50, 11073–11086.
179. F. Meng, G. Xu, R. Guo, H. Yan and M. Chen, Chem. Eng. Process., 2004, 43, 785–790.
180. M. Wang, H. Wang, N. Zhao, W. Wei and Y. Sun, Ind. Eng. Chem. Res., 2007, 46, 2683–2687.
181. D. Wang, X. Zhang, W. Wei and Y. Sun, Chem. Eng. Technol., 2012, 35, 2183–2188.
182. A. Swapnesh, V. C. Srivastava and I. D. Mall, Chem. Eng. Technol., 2014, 37, 1765–1777.
183. W. Joe, H. J. Lee, U. G. Hong, Y. S. Ahn, C. J. Song, B. J. Kwon and I. K. Song, J. Ind. Eng. Chem., 2012, 18, 1730–1735.
184. W. Joe, H. J. Lee, U. G. Hong, Y. S. Ahn, C. J. Song, B. J. Kwon and I. K. Song, J. Ind. Eng. Chem., 2012, 18, 1018–1022.
185. H. Wang, B. Lu, X. Wang, J. Zhang and Q. Cai, Fuel Process. Technol., 2009, 90, 1198–1201.
186. X. Qin, W. Zhao, B. Han, B. Liu, P. Lian and S. Wu, Korean J. Chem. Eng., 2015, 32, 1064–1068.
187. W. B. Zhao, B. Han, N. Zhao, F. K. Xiao and W. Wei, J. Cent. South Univ., 2012, 19, 85–92.
188. R. Piñero, J. García, M. Sokolova and M. J. Cocero, J. Chem. Thermodyn., 2007, 39, 536–549.

---