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3 Dimethyl Carbonate (DMC) as Dual Chemical Reagent

3 Dimethyl Carbonate (DMC) as Dual Chemical Reagent

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12



Application of Dimethyl Carbonate as Solvent and Reagent

methylation

R-NH2 +



O

C

H3CO



367



R NH CH3

O

C



OCH3

carbamoylation



R HN



OCH3



Scheme 12.4 Reactivity of DMC with amines as methylating and carbamoylating agent



Compared to conventional methylating agents such as alkyl halides or dimethyl

sulfate, the use of DMC has two main differences. The first one is that DMC is considered as “green” reagent since it is obtained from CO2 and methanol, and when

performing a methylation reaction, only CO2 and methanol are formed. Therefore,

considering the global life cycle of DMC, the net transformation is methylation of

the nucleophile with methanol in two steps involving the intermediacy of DMC [2].

In contrast, alkyl halides generate as waste a halide anion per mol of product.

Moreover, considering that chloromethane and bromomethane are gases under

ambient conditions, the most typical methylating agent is methyl iodide, and then

the E factor as defined by Sheldon [17] is very large, meaning that the kilograms of

waste produced by kilogram of product is very high on using methyl iodide. The E

factor is considered as an indicator for quantitative measurement of the greenness of

the process that is low when the E factor is high. Similar situations happen with

dimethyl sulfate in which sulfide salts are formed when the methylation occurs.

The second difference of DMC as methylating agent is that, except for very

strong nucleophiles, the reaction requires a catalyst [18]. Typically, methylations

with other reagents as alkyl iodides do not need any catalyst.

Development of catalysts, particularly for heterogeneous catalysis, to promote

methylation of nucleophiles by DMC has been an active topic in green chemistry to

prepare more active and selective catalysts [2]. Several catalytic reactions require

reaction temperatures higher than the boiling point of DMC, and this complicates

considerably the workup of the reactions since close reactors that can stand relatively high pressure are needed. This is particularly important considering that reactions using DMC carried out at 160°C are not uncommon, and the boiling point of

DMC is 90°C. Therefore, more active catalysts should be ideally able to perform the

reaction at ambient pressure, meaning that the maximum temperature should be the

boiling point of DMC.

However, even though catalyst activity is still far from optimum, the key point in

DMC being used as reagent is selectivity because as commented earlier, two different pathways can be promoted in parallel.

In general, the preferential reactivity of DMC is as methylating reagent, methoxycarbonylation (carbamoylation) being much more difficult to achieve in high

selectivity. As a general rule, carbamoylation is always accompanied by some

degree of methylation while, on the other hand, methylation can be obtained almost

free of carbamoylated products.



368



B. Ferrer et al.

Table 12.1 The reaction of DMC with indolyl-3-acetic acid, with 1:1

catalyst:substrate weight ratio, at 180°C, and in the presence of different

zeolites. Isolated products: methyl ester derivative (2b),

N-methoxycarbonyl derivative (3b), decarboxylated derivative (4b)

(Reproduced from Ref. [23]. With kind permission of The Royal Society

of Chemistry)

Isolated product yield (%)

S. No.

Catalyst

t (h)

2b

3b

4b

1

None

3

2

LiY

4

25

43

3

NaY

4

52

21

4

KY

4

50

19

5

LiY

4

87

10

6

NaY

4

97

7

KY

4

98



The selectivity between methylation and carbamoylation depends on a large

number of factors besides the catalyst such as the nature of the nucleophilic species,

reaction temperature, and even the substrate-to-DMC ratio. Generally, hard nucleophiles favor methylation, which is promoted by higher temperatures and high

substrate-to-DMC ratio.



12.3.1



Catalysts for Selective Methylation Using Dimethyl

Carbonate (DMC)



A variety of Lewis metal salts can act as homogeneous catalysts for methylation

using DMC. The list also includes metal triflates, metal halides, and organocatalysts

[19, 20]. The latter field is particularly attractive since, even under homogeneous

conditions, none of the transition metals can be introduced in the system. The general organocatalysts that can be employed are supernucleophilic amines such as

1,4-diazabicyclo[2.2.2]octane (DABCO) and 1,4,7-triazacyclononane (TACN). In

the latter case, protonation of one nitrogen is particularly easy due to charge delocalization, and for this reason, its activity is much higher than the conventional

amines. In one way of the use of organocatalysts that bridges homogeneous and

heterogeneous catalysis, Jacobs and De Vos have covalently attached 1,5,7-triazabicyclo[4.4.0]dec-5-ene to a mesoporous aluminosilicate and used this solid to promote methylation and methoxycarbonylation of amines [21, 22]. Tundo and

coworkers have made significant contributions to the use of DMC as reagent. They

found that sodium zeolites are good heterogeneous catalysts to promote methylation

of a large variety of nucleophiles including aromatic amines [23]. Later, it has been

reported that if sodium is ion exchanged by other metal ions that can act as Lewis

acid sites, then the activity increases with respect to the parent sodium zeolite [21].

Table 12.1 lists some of the results that have been reported using metal exchanged

zeolites as catalysts [23].



12



Application of Dimethyl Carbonate as Solvent and Reagent



12.3.2



369



Catalysts for Methoxycarbonylation



The most important industrial application of DMC that can be envisioned is its use

as phosgene replacement in the synthesis of polyurethane. Polyurethanes are prepared in large quantity by reaction of diisocyanates with polyols. About 85% of the

current polyurethane market corresponds to aromatic diisocyanates derived from

toluene diisocyanate (TDI) or mixtures of isomers of bis(isocyanatophenyl)methane

(Scheme 12.5).



CH3



CH3

N C O

+ HO



N

H



OH



N C O



O

C



O



O C NH

O



CH3

N C O



N C O



TDI



polyurethane



Scheme 12.5 Synthetic route for the preparation of polyurethanes starting from toluene diisocyanate and ethylene glycol



These aromatic diisocyanates are currently produced from the corresponding

aromatic diamines by reaction with phosgene. The current security regulations in

Western countries oblige to build the plant in which phosgene is prepared and used

inside a larger building that can be sealed and the atmosphere evacuated after treatment with bases. From the large capital investment required for phosgene plants, as

well as from the point of view of green chemistry (replacement of toxic chemicals

and development of new processes CO2 neutral), it is desirable to develop alternative processes for the use of phosgene based on CO2. Scheme 12.6 shows a possible

route in which the key process is the methoxycarbonylation of aromatic amines by

organic carbonates.



Cl



CH3



O

C



Cl

CH3



NH2

CH3

NH2

O

C

H3CO



OCH3



N C O

H

NC O

OCH3



HNC O

OCH3



N C O

+ CH3OH



Scheme 12.6 Synthetic routes to toluene diisocyanate (TDI) involving either phosgene or dimethyl carbonate (DMC)



370



B. Ferrer et al.



As we have commented earlier, the problem with the chemical route to prepare

aromatic polyisocyanates using organic carbonates as reagents is the dual reactivity

of these carbonates as alkylating and carbamoylating reagents. Moreover, due to the

lower nucleophilicity of aromatic amines compared to aliphatic amines, the former

exhibit greater tendency to undergo N-alkylation versus carbamoylation. The only

way to circumvent this problem is to develop selective catalysts that promote carbamoylation, minimizing the strongly undesired N-methyl derivatives. While Zn2+

and Pb2+ salts of long alkyl chain carboxylates have been reported as homogeneous

catalysts for carbamoylation of amines, industry prefers heterogeneous catalysts

that can be easily separated from the reaction mixture and also allows the design of

continuous flow processes.

There have been several reports in the literature showing that solid catalysts

such as transition metal–exchanged aluminosilicates can produce carbamates of

aromatic polyamines in variable selectivities depending on the catalyst and reaction conditions [24, 25]. One remarkable example is the report of Sartori, Jacobs,

and coworkers describing that supernucleophilic amines such as TACN covalently

anchored to mesoporous silica can affect carbamoylation of aliphatic amines,

although the selectivity for aromatic amines is very low [21, 22, 26]. It has to be

commented that in order to develop an industrial process, the target for a heterogeneous catalyst should be selectivities over 90% for carbamoylation at conversions

higher than 80% with high catalyst productivity.

In this context, we have reported recently that CeO2-supported gold nanoparticles

meet the requirements to become an industrial catalyst for the carbamoylation of

aromatic amines [27]. The key point in the catalyst is the unique behavior of CeO2

nanoparticles to promote carbamoylation against methylation. Thus, for instance,

TiO2, Fe2O3, Al2O3, ZrO2 and other metal oxides in the absence of gold promote

N-methylation under the same conditions in which CeO2 promotes carbamoylation

[27]. However, even though the selectivity of CeO2 toward carbamoylation is high,

its activity is low, and therefore longer reaction times are needed to achieve the

required amine conversions. The effect of gold, at very low loadings (below 1 wt%),

is precisely to boost the activity without altering the selectivity toward carbamoylation. Table 12.2 summarizes some of the results that have been reported illustrating

the role of CeO2 and the effect of gold as well as other noble metals.

Concerning the reaction mechanism, absorption of DMC on CeO2 shows that

this molecule splits into two fragments. One is a methyl group anchored on surface

OH (methylation of the surface), and the other part is methoxycarbonyloxy bonded

to Ce metal ion. When the material is heated, it is observed that the methyl group

remains strongly bonded to the surface, but the methoxycarbonyloxy group is lost.

This thermal behavior that can be easily monitored by in situ infrared (IR) spectroscopy justifies the observed catalytic behavior since it demonstrates the tendency of

CeO2 to retain the methyl group and, therefore, to minimize methylation. In fact, an

analogous in situ IR study of the behavior of DMC on TiO2 shows exactly the same

bond cleavage and splitting of DMC, but a contrasting behavior upon heating is

noticed since the methyl group is more easily desorbed. Figure 12.1 illustrates the

changes occurring in the IR spectra of the CeO2 and TiO2 metal oxides after exposing the surface to DMC and evacuation at 100°C.



12



Application of Dimethyl Carbonate as Solvent and Reagent



371



Table 12.2 Results for the reaction of toluene diamine (DAT) with DMC in the presence of a

series of catalysts (Reproduced from Ref. [27]. With kind permission of John Wiley & Sons)

o-1 + p-1NN,NMass

DAT

carbamoylated Carbamoylated N-Methylated

Catalyst

balance (%) conv. (%) products (%)

product (%)

product (%)

95 ± 3

99

36

25

39

Zn(OAc)2

ZnO (40 nm)

97 ± 3

16

1



99

Au/CeO2 (0.44%) 99 ± 2

99

4

96



fresh

99



100



Au/CeO2 (0.44%) 98 ± 2

3rd reuse

65

73



27

Au/CeO2 (0.44%; 99 ± 3

40 nm)

97 ± 3

92

48

52



CeO2 (5 nm)

Au/TiO2 (0.44%) 99 ± 3

58

10



90

Au/Fe2O3 (0.44%) 95 ± 3

27





100

Pd/CeO2 (0.44%) 96 ± 3

87

34

65

1

Pd/TiO2 (0.44%)

95 ± 3

10

4



96

Pt/TiO2 (1.5%)

98 ± 2

8





100

Reaction conditions: DAT (0.98 mmol), DMC (26.69 mmol); catalyst: Au, Pd, Pt, Zn (0.5% mol

respect to DAT), CeO2 and ZnO (100 mg), 7 h, 140°C



1104



Absorbance (a.u.)



1588



0.25

1044



0.1



1054

1121

1157



a



d



1566

a

d

1750



1500



1250



Wavenumber (cm-1)



1000



1750



1500



1250



1000



Wavenumber (cm-1)



Fig. 12.1 FTIR spectra of nanocrystalline CeO2 (left) and Au/TiO2 (right) after DMC adsorption

(a, the spectrum on the top) and desorption at increasing temperatures 303 K (b, first intermedium

spectrum), 343 K (c, second intermedium spectrum), and 393 K (d, the spectrum on the bottom)

(Reproduced from Ref. [27]. With kind permission of John Wiley & Sons)



When similar Fourier-transformed infrared (FTIR) studies are carried out with

Au/CeO2, the same behavior as for CeO2 is observed, with the important difference

that desorption of methoxycarbonyloxy group takes place at even lower temperature. This justifies very nicely the effect of gold enhancing the activity by favoring

desorption of methoxycarbonyloxy group.



372



B. Ferrer et al.



In addition, since aromatic amines are industrially obtained from aromatic nitro

compounds and gold exhibits a remarkable activity for the selective hydrogenation

of nitro groups, we have developed a tandem reaction (hydrogenation + carbamoylation) as one-step process in which nitro aromatic compounds are converted to the

corresponding methoxycarbonyl diamines with high selectivity [27] as shown in

Scheme 12.7.

CH3



CH3

NO2

+

NO2



H2



O

C

H3CO



OCH3



Au/CeO2



H

NC O

OCH3



HNC O

OCH3



Scheme 12.7 Tandem reaction (hydrogenation + carbamoylation) of 2,4-dinitrotoluene with DMC

to obtain the corresponding methoxycarbonyl diamine



Finally, the aromatic methyl carbamate can be transformed into the corresponding

aromatic isocyanate by thermolysis in the presence or absence of catalyst (see

Scheme 12.6). This renders a synthetic route for commercial diisocyanates starting

from nitro aromatics or aromatic amines using CO2 and methanol as reagents and

going through DMC as intermediate. In the last step of isocyanates formation, methanol is recovered (see Scheme 12.6) and can be recycled for the formation of fresh

DMC. In this way, only CO2 and nitro compounds are consumed in the overall cycle.



12.4



Concluding Remarks and Future Prospects



Taking into account the wide range of potential DMC uses as solvent and reagent

and the expected growth of these novel uses, it can be easily anticipated that the

production of DMC and other organic carbonates (particularly diethyl carbonate

and propylene carbonate) will grow dramatically in the years to come. In addition,

as the production of DMC rises, there will be a large incentive to replace the current

industrial preparation of DMC based on CO to a new process based on CO2 fixation.

Among the novel important uses of DMC, application in Li batteries, supercapacitors, solar cells and other uses related to new energy resources will increase at a very

high pace.

Concerning the use of DMC as reagent, methylation is not very important since

it can be performed directly with methanol or suitable derivatives. However, polyurethanes will be a new market for increasing demand of DMC.

Acknowledgment Financial support by Spanish Ministry of Science and Innovation

(CTQ2009/11586), Consolider MULTICAT and Polytechnic University of Valencia (20101196) is

gratefully acknowledged.



12



Application of Dimethyl Carbonate as Solvent and Reagent



373



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2. Tundo P, Selva M (2002) The chemistry of dimethyl carbonate. Acc Chem Res 35:706–716

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16. Yang H, Huamg M, Wu J, Lan Z, Hao S, Lin J (2008) The polymer gel electrolyte based on

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Mater Chem Phys 110:38–42

17. Sheldon RA (1992) Chem Ind 23:903–906

18. Chankeshwara SV (2008) Dimethyl carbonate (DMC): a versatile and environmentally benign

building block. Synlett 4:624–625

19. Mei F, Chen E, Li GX (2009) Lanthanum nitrate as an efficient and recoverable homogeneous

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96(1):27–33

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MCM-41–TBD in the selective preparation of carbamates and unsymmetrical alkyl carbonates

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Chapter 13



Application of Supercritical Fluids

for Biodiesel Production

Ikumei Setsu, Ching-Hung Chen, Chao-Rui Chen, Wei-Heng Chen,

Chien-Hsiun Tu, Shih-Ming Lai, and Chieh-Ming J. Chang



Abstract This chapter elucidates supercritical carbon dioxide (SC-CO2) extraction

of triglycerides from powdered Jatropha curcas kernels and seeds, followed by a

series of subcritical hydrolysis and supercritical methylation of the extracted SC-CO2

oil to obtain a 98.5% purity level of biodiesel. Effects of the reaction temperature,

the reaction time, and the solvent-to-solid ratio on free fatty acid (FFA) in the hydrolyzed oil and fatty acid esters in the methylated oil via two experimental designs

were also examined. Supercritical methylation of the hydrolyzed oil following subcritical hydrolysis of the SC-CO2 extract yielded a methylation reaction conversion

of 99%, and activation energies of hydrolysis and transesterified reactions were

found, respectively. This study demonstrates that supercritical methylation preceded

by subcritical hydrolysis of the SC-CO2-extracted oil is a feasible two-step process

in producing biodiesel from powdered Jatropha kernels better than that from

Jatropha seeds. The economical estimation of the process was also examined.

I. Setsu • C.-H. Chen • W.-H. Chen • C.-M.J. Chang (*)

Department of Chemical Engineering, National Chung Hsing University,

250 Kuo-Kuang Road, Taichung 402, Taiwan, ROC

e-mail: ymsetsu@gmail.com; pilicity@hotmail.com; lemonice0806@yahoo.com.tw;

cmchang@dragon.nchu.edu.tw

C.-R. Chen

Department of Chemical Engineering, National Chung Hsing University,

250 Kuo-Kuang Road, Taichung 402, Taiwan, ROC

Chemical Engineering Division, Institute of Nuclear Energy Research,

1000 Wen-Hua Road, Lungtan, Taoyuan 325, Taiwan, ROC

e-mail: g8965128@mail.nchu.edu.tw

C.-H. Tu

Department of Applied Chemistry, Providence University, 200 Chungchi Road,

Taichung 43301, Taiwan, ROC

S.-M. Lai

Department of Chemical and Materials Engineering, National Yunlin University

of Science and Technology, 123 University Road, Touliu, Yunlin 640, Taiwan, ROC

A. Mohammad and Inamuddin (eds.), Green Solvents I: Properties

and Applications in Chemistry, DOI 10.1007/978-94-007-1712-1_13,

© Springer Science+Business Media Dordrecht 2012



375



376



13.1

13.1.1



I. Setsu et al.



Introduction

Supercritical Fluids



Among the various fields that supercritical fluids have been applied include extraction, chromatography, and recrystallization, due to their high diffusivity, high permeability, and low surface tension. Supercritical fluid extraction (SFE) is extensively

performed for concentrating bioactive compounds. The SFE parameters, for example,

pressure, temperature, and cosolvent ratio are easily controlled, and desired compounds can be obtained in high purity and high total yield (TY) [1]. Some valuable

bioactive compounds, such as flavones and colorants, can be extracted using SFE with

suitable cosolvents. On the other hand, some undesirable compounds, such as caffeine

and pesticides, can be removed utilizing SFE. For example, supercritical fluids technology can be used to remove wax to enhance the concentration of flavonoids in the

propolis extract [2]; extract the antioxidant components from propolis by SC-CO2

antisolvent fractionation [3]; and extract 3,5-diprenyl-4-hydroxycinnamic acid

(DHCA) from Brazilian propolis by SC-CO2 modified with cosolvent, which process

is followed by column chromatography to yield 95% pure DHCA [4]. Biodiesel preparation by using supercritical technique was introduced in 2001 [5, 6]. Recent publication indicated that biodiesel production from Jatropha oil via noncatalytic supercritical

methanol transesterification resulted in high yield of fatty acid methyl esters [7].



13.1.2



Biodiesel



Fatty acid methyl ester (FAME), commonly known as biodiesel, has been considered a potential source of future renewable and environmentally friendly energy,

replacing exhaustible petroleum-derived diesel.

The daily depletion of fossil fuels necessitates the development of an alternative

fuel source that satisfies energy demands worldwide. Producing biodiesel from natural resources is one of the most promising methods. However, because biodiesel fuels

are more expensive than petroleum fuels as well as highly viscous, diesel engines

cannot efficiently atomize biodiesel fuels [8], and vegetable oils have not been widely

accepted as a diesel engine fuel. Conversely, biodiesel is better than diesel fuel in

terms of sulfur content, flash point, aromatic content, and biodegradability [9, 10].

Jatropha curcas (JC), a plant that grows naturally in the wild, can grow without

irrigation under a broad rainfall range (250–3,000 mm/year) [11]. Additionally, as a

pressed cake, JC can be used as fertilizer, and its organic waste products can be

digested to produce biogas methane [12, 13]. The JC kernel forms a large proportion of the seed, accounting for 61.3 ± 3.1%. Experimental kernels exhibited large

variations in crude protein (19–31%), lipids (43–59%), neutral detergent fibers

(3.5–6.1%), and ash (3.4–5.0%) contents. The gross energy of kernels was generally

similar (28.5–31.2 MJ/kg) [14, 15].



13



Application of Supercritical Fluids for Biodiesel Production



377



Most studies have reported that increased amounts of biodiesel were produced

when a catalyst was employed to accelerate methylation because the solubility of

oil in methanol is poor. The alkali-catalyzed reaction is the most commonly used

process for practical biodiesel production. Cvengro and Povaz used 4% NaOH as

a catalyst to produce biodiesel using two-stage low-temperature transesterification

of cold-pressed rapeseed oil with methanol at temperatures up to 343 K [16].

Kaieda et al. synthesized methyl esters from plant oil and methanol in a solventfree reaction system using lipase from Rhizopus oryzae [17]. Shimada et al. examined enzymatic alcoholysis for biodiesel fuel production by employing Candida

antarctica lipase as the catalyst [18]. Pizarro and Park analyzed the production of

biodiesel fuel with Rhizopus oryzae lipase as the catalyst from vegetable oils contained in waste activated bleaching earth [19].

However, the alkali-catalytic process has some shortcomings in that it is unsuitable for oil feed containing FFAs as the alkali and side-products are difficult to

remove. Additionally, enzymatic methylations do not yield consistent conversions.

Zhang et al. demonstrated that the acid-catalytic process employing used cooking

oil proved technically feasible and was less complex than the alkali-catalytic process using the same oil [20]. Ghadge and Raheman utilized sulfuric acid to produce

biodiesel from Madhuca indica oil, which has high amount of FFAs [21]. Tashtoush

et al. used sulfuric acid in an experimental study for evaluating and optimizing the

conversion of waste animal fat into biodiesel [22]. Nonetheless, the acid-catalytic

process takes much time in the esterification process. No catalyst is required for

transesterification of supercritical methanol or ethanol, and nearly complete conversions can be achieved in very short periods [23]. This is primarily because supercritical methanol and oil exist in a single phase [24, 25]. Cao et al., who carried out

transesterified soybean oil in supercritical methanol without a catalyst [26], demonstrated that in addition to the alkali-catalytic and acid-catalytic processes, supercritical methylation is also an effective method. The transesterification reaction of

rapeseed oil in supercritical methanol without any catalyst was investigated by Saka

and Kusdiana [27]. They further discussed the effects of water on biodiesel fuel

production via treatment with supercritical methanol [28].

After the development of the single-step supercritical methylation process, a few

researchers developed a two-step process. Ramadhas et al. investigated biodiesel

production from rubber seed oils with high FFA using a two-step transesterification

process that converts the oils into their monoesters [29]. Babcock et al. investigated

the conversion of chicken fat and tall oil into biodiesel via a two-step process involving hydrolysis of triglyceride-containing feeds followed by supercritical esterification of the resulting/existing FFAs [30]. Hydrolysis and the subsequent supercritical

methanol treatment process developed by Minami and Saka is a promising alternative to the single-step supercritical methanol method. It concluded that hydrolysis of

triglycerides liberates FFAs that can then be methylated easily [31].

An international standard that describes the minimum requirements for biodiesel

indicates that a minimum of 96.5% FAMEs is required for vehicles as well as only

trace amounts of mono-, di-, and triglyceride, and total glycerine can exist in the

fuel [30]. Our study concludes that high concentrations of triglycerides in the extract



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