Article No : a10_101
Ethylene Glycol SIEGFRIED REBSDAT, Hoechst Aktiengesellschaft, Gendorf, Federal Republic of Germany DIETER MAYER, Hoechst Aktiengesellschaft, Frankfurt, Federal Republic of Germany
1. 2. 3. 4. 4.1. 4.1.1. 4.1.2. 4.2. 4.2.1. 4.2.2. 5. 6.
Introduction. . . . . . . . . . . . . . . . . . . . . . . Physical Properties . . . . . . . . . . . . . . . . . Chemical Properties . . . . . . . . . . . . . . . . Production . . . . . . . . . . . . . . . . . . . . . . . . Ethylene Oxide Hydrolysis. . . . . . . . . . . . Current Production Method . . . . . . . . . . . . Possible Developments . . . . . . . . . . . . . . . Alternative Methods of Ethylene Glycol Production . . . . . . . . . . . . . . . . . . . . . . . . Direct Oxidation of Ethylene . . . . . . . . . . . Synthesis from C1 Units. . . . . . . . . . . . . . . Environmental Protection and Ecology . . Quality Specifications and Analysis . . . . .
. . . . . . .
531 531 532 534 534 534 535
. . . . .
536 536 537 538 538
7. 8. 8.1. 8.2. 9. 10. 11. 11.1. 11.2.
Storage and Transportation. . . . . . . . . . . . Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . Di-, Tri-, Tetra-, and Polyethylene Glycols Ethers and Esters. . . . . . . . . . . . . . . . . . . . Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Economic Aspects . . . . . . . . . . . . . . . . . . . Toxicology and Occupational Health . . . . . Ethylene Glycol . . . . . . . . . . . . . . . . . . . . . Ethylene Glycol Derivatives. . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . .
538 538 539 539 540 541 542 542 543 544
1. Introduction
2. Physical Properties
Ethylene glycol [107-21-1,] 1,2-ethanediol, HOCH2CH2OH, Mr62.07, usually called glycol, is the simplest diol. It was first prepared by WURTZ in 1859 [1]; treatment of 1,2-dibromoethane [106-93-4] with silver acetate yielded ethylene glycol diacetate, which was then hydrolyzed to ethylene glycol. Ethylene glycol was first used industrially in place of glycerol during World War I as an intermediate for explosives (ethylene glycol dinitrate) [2], but has since developed into a major industrial product. The worldwide capacity for the production of ethylene glycol via the hydrolysis of ethylene oxide [75-21-8] (! Ethylene Oxide) is estimated to be ca. 7x106 t/a. Ethylene glycol is used mainly as an antifreeze in automobile radiators (! Antifreezes) and as a raw material for the manufacture of polyester fibers (! Fibers, 4. Synthetic Organic; ! Polyesters).
Ethylene glycol is a clear, colorless, odorless liquid with a sweet taste. It is hygroscopic and completely miscible with many polar solvents, such as water, alcohols, glycol ethers, and acetone. Its solubility is low, however, in nonpolar solvents, such as benzene, toluene, dichloroethane, and chloroform. The UV, IR, NMR, and Raman spectra of ethylene glycol are given in [3]. Following are some of the physical properties of ethylene glycol [4–6]:
2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/14356007.a10_101
bp at 101.3 kPa fp Density at 20 C Refractive index, n20 D Heat of vaporization at 101.3 kPa Heat of combustion Critical data Temperature Pressure Volume
197.60 C 13.00 C 1.1135 g/cm3 1.4318 52.24 kJ/mol 19.07 MJ/kg 372 C 6515.73 kPa 0.186 L/mol
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Ethylene Glycol
Flash point Ignition temperature Lower explosive limit Upper explosive limit Viscosity at 20 C Cubic expansion coefficient at 20 C
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111 C 410 C 3.20 vol % 53 vol % 19.83 mPa s 0.62103 K1
Ethylene glycol is difficult to crystallize; when cooled, it forms a highly viscous, supercooled mass that finally solidifies to produce a glasslike substance. The widespread use of ethylene glycol as an antifreeze is based on its ability to lower the freezing point when mixed with water. The physical properties of ethylene glycol – water mixtures are, therefore, extremely important. The freezing points of mixtures of water with monoethylene glycol and diethylene glycol [111-46-6] are shown in Figure 1. The temperature dependencies of the thermal conductivity, density, and viscosity of ethylene glycol and ethylene glycol – water mixtures are shown in Figures 2–3, and 4 respectively [7]. The Prandtl numbers (the ratio of the viscosity to the thermal conductivity) derived from these values are given in Figure 5 [7]. The vapor pressures of ethylene glycol – water mixtures have been obtained from [8] by interpolation and are listed in Table 1.
3. Chemical Properties Ethylene glycol, like other alcohols, undergoes the reactions typical of its hydroxyl groups, which are described elsewhere ( ! Alcohols, Aliphatic). Thus, only the special chemical
Figure 1. Freezing points of mono- and diethylene glycol – water mixtures a) Monoethylene glycol; b) Diethylene glycol
Figure 2. Temperature dependence of the thermal conductivity of ethylene glycol – water mixtures Ethylene glycol content, mol %: a) 0; b) 25; c) 55; d) 75; e) 100
characteristics and industrially important reactions of ethylene glycol are considered here. The two adjacent hydroxyl groups allow cyclization, and polycondensation; one or both of these functional groups may, of course, also react to give other derivatives. Oxidation. Ethylene glycol is easily oxidized to form a number of aldehydes and carboxylic acids by oxygen, nitric acid, and other
Figure 3. Temperature dependence of the density of ethylene glycol – water mixtures Ethylene glycol content, mol %: a) 0; b) 26.1; c) 50.95; d) 76.9; e) 100
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Table 1. Vapor pressure of ethylene glycol – water mixtures Water content, wt % 0 10 20 30 40 50 60 70 80 90 100
Figure 4. Temperature dependence of the viscosity of ethylene glycol – water mixtures Ethylene glycol content, mol %: a) 0; b) 25; c) 49.90; d) 74.36; e) 100
Vapor pressure, in kPa at 65.1 C
77.7 C
90.3 C
0.30 6.61 11.30 14.70 17.10 18.81 20.16 21.45 22.98 25.08 28.04
0.52 11.65 19.68 25.45 29.68 32.92 35.58 37.92 40.05 41.91 43.34
1.20 19.73 33.01 42.49 49.37 54.60 58.87 62.60 65.98 68.93 71.10
oxidizing agents. The typical products derived from the alcoholic functions are glycolaldehyde (HOCH2CHO) [141-46-8,] glycolic acid (HOCH2COOH) [79-14-1,] glyoxal (CHOCHO) [107-22-2,] glyoxylic acid (HCOCOOH) [298-12-4,] oxalic acid (HOOCCOOH) [14462-7,] formaldehyde (HCHO) [50-00-0,] and formic acid (HCOOH) [64-18-6]. Many of these compounds are described in separate articles. Variation of the reaction conditions can lead to the selective formation of a desired oxidation product. Gas-phase oxidation with air in the presence of copper catalysts is of industrial importance for the production of glyoxal (! Glyoxal and ! Glyoxylic Acid). Glycol cleavage occurs in acidic solution with certain oxidizing agents such as permanganate, periodate, or lead tetraacetate. Cleavage of the C-C bond mainly produces formaldehyde, some of which is further oxidized to formic acid [9]. 1,3-Dioxolane Formation. 1,3-Dioxolanes are formed by reacting ethylene glycol with carbonyl compounds [10]:
Figure 5. Temperature dependence of the Prandtl numbers of ethylene glycol – water mixtures Ethylene glycol content, mol %: a) 0; b) 20; c) 40; d) 60; e) 80; f) 100
Acetalization to the cyclic 1,3-dioxolane proceeds more readily than acetal formation from straight-chain alcohols. If water is removed from the reaction mixture, an excellent yield can be obtained. This reaction is used to protect carbonyl groups in organic syntheses.
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1,3-Dioxolanes can also be formed from ethylene glycol by transacetalization. Examples are the reactions of ethylene glycol with orthoformates [11]:
or with dialkyl carbonates:
1,4-Dioxane Formation. Ethylene glycol can be converted to dioxane [123-91-1 ] by dehydration in the presence of acidic catalysts [12]:
Ether and Ester Formation. Ethylene glycol can be alkylated or acylated by the customary methods to form ethers or esters, respectively. However, the presence of two hydroxyl groups leads to the formation of both monoand diethers and mono- and diesters, depending on the initial concentrations of the individual reactants. The esterification of ethylene glycol with terephthalic acid [100-21-0] to form polyesters is especially important (! Fibers, 4. Synthetic Organic; ! Polyesters).
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industrially since these homologues are formed as byproducts during the production of ethylene glycol (cf. Section 4.1.1).
Decomposition with Alkali Hydroxide. Glycol is a relatively stable compound, but special care is required when ethylene (or diethylene) glycol is heated at a higher temperature in the presence of a base such as sodium hydroxide. Fragmentation of the molecule begins at temperatures above 250 C and is accompanied by the exothermic evolution of hydrogen (D H ¼ 90 to 160 kJ/kg) [13]. This leads to a buildup of pressure in closed vessels.
4. Production Although ethylene glycol has been known since 1859 (WURTZ) [1], it was not produced industrially until World War I. Its synthesis was then based on the hydrolysis of ethylene oxide [7521-8] produced by the chlorohydrin process (! Chlorohydrins, Chap. 4.). Production from formaldehyde [50-00-0] and carbon monoxide was also used commercially from 1940 to 1963 [14]. Neither of these methods is now used, however; the older literature should be consulted for details [2, 12]. Direct oxidation of ethylene [74-85-1] to ethylene glycol was also employed commercially for a short time [15], but was abandoned, probably due to problems caused by corrosion [16].
4.1. Ethylene Oxide Hydrolysis 4.1.1. Current Production Method Ethoxylation. Ethylene glycol reacts with ethylene oxide to form di-, tri-, tetra-, and polyethylene glycols. The proportions of these glycols found in the reaction product are determined by the catalyst system that is used and the glycol excess. A considerable excess of glycol is required to obtain the lower homologues in a satisfactory yield. This reaction is rarely used
Only one method is currently used for the industrial production of ethylene glycol. This method is based on the hydrolysis of ethylene oxide obtained by direct oxidation of ethylene with air or oxygen (! Ethylene Oxide). The ethylene oxide is thermally hydrolyzed to ethylene glycol without a catalyst. Figure 6 shows a simplified scheme of a plant producing ethylene glycol by this method. The ethylene oxide – water
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Figure 6. Flow diagram for a glycol plant a) Reactor; b) Drying column; c) Monoethylene glycol column; d) Diethylene glycol column; e) Triethylene glycol column; f) Heat exchanger
mixture is preheated to ca. 200 C, whereby the ethylene oxide is converted to ethylene glycol. Di-, tri-, tetra-, and polyethylene glycols are also produced, but with respectively decreasing yields (see also Chap. 3). The formation of these higher homologues is inevitable because ethlyene oxide reacts with ethylene glycols more quickly than with water; their yields can, however, be minimized if an excess of water is used – a 20-fold molar excess is usually employed. Figure 7 shows the composition of the resulting product mixture as a function of the ratio of water to ethylene oxide. Although the values were determined by using sulfuric acid as a catalyst [17], they also apply as a good
approximation for the reaction without a catalyst. Thus, in practice almost 90 % of the ethylene oxide can be converted to monoethylene glycol, the remaining 10 % reacts to form higher homologues:
Figure 7. Composition of the product obtained on hydrolysis of ethylene oxide (EO) as a function of the water to ethylene oxide ratio a) Monoethylene glycol; b) Diethylene glycol; c) Triethylene glycol; d) Higher poly(ethylene glycols)
4.1.2. Possible Developments
After leaving the reactor, the product mixture is purified by passing it through successive distillation columns with decreasing pressures. Water is first removed and returned to the reactor, the mono-, di-, and triethylene glycols are then separated by vacuum distillation. The yield of tetraethylene glycol is too low to warrant separate isolation. The heat liberated in the reactor is used to heat the distillation columns. A side stream must be provided to prevent the accumulation of secondary products, especially small amounts of aldehydes, which are produced during hydrolysis. The shape of the reactor affects the selectivity of the reaction. Plug-flow reactors are superior to both agitator-stirred tanks and column reactors [18].
The glycol production method described in Section 4.1.1 is the only one of current industrial
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Ethylene Glycol
importance. It is simple, but has some major drawbacks: 1. The selectivity of the first step – the production of ethylene oxide – is low (ca. 80 %). 2. The selectivity of ethylene oxide hydrolysis is low – ca. 10 % is converted to di- and triethylene glycol. 3. Energy consumption for the distillation of the large amount of excess water is high. Therefore, much research has been carried out to improve this process. The search for better silver catalysts is an objective for point 1 (! Ethylene Oxide). Points 2 and 3 must be considered together, since higher selectivity for ethylene oxide hydrolysis automatically reduces the excess of water required. Many catalysts have been described in the literature that are able to optimize selectivity or lower the reaction temperature and the required excess of water. Acids and bases are known to accelerate the reaction rate. The kinetics of the acid [19] and base [20] catalysis of ethylene oxide hydrolysis have been thoroughly investigated; mechanisms are discussed in [21]. The industrial feasability of catalysis with ion-exchange columns in the liquid phase [22, 23] and the gas phase [24] has been tested. Although the use of catalysts allowed the reaction temperature to be lowered, selectivity was not significantly enhanced. Furthermore, the catalyst needed to be separated and either fed back into the reaction mixture or replaced. As a result of these disadvantages, these types of catalysis have not proved to be of commercial use. However, catalysts that improve selectivity have been described in patents; they include molybdates [25], vanadates [26], ion exchangers [27], and organic antimony compounds [28]. However, their advantages do not yet seem to justify their use on an industrial scale. The selective synthesis of ethylene glycol via the intermediate ethylene carbonate (1,3-dioxolan-2-one) [96-49-1] seems to be a promising alternative. This compound is obtained in high yield (98 %) by reacting ethylene oxide with carbon dioxide and can be selectively hydrolyzed to give a high yield of ethylene glycol. Only
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double the molar quantity of water is required for this reaction.
According to a Halcon patent, ethylene oxide can be extracted from the aqueous solution, formed during its production, with supercritical carbon dioxide [29]. An ethylene oxide – carbon dioxide solution is obtained, which reacts to form ethylene carbonate. Hydrolysis of the ethylene carbonate then yields ethylene glycol. Possible catalysts for this reaction are quaternary ammonium and phosphonium salts, such as R4NHal, R4PHal, or Ph3PCH3I. Problems such as product separation and catalyst feedback still need to be resolved, but this method for the selective synthesis of ethylene glycol from ethylene oxide seems to be the most promising for industrial-scale application.
4.2. Alternative Methods of Ethylene Glycol Production The low selectivity of ethylene oxide production and increasing ethylene prices warrant the search for alternative ways of producing ethylene glycol. 4.2.1. Direct Oxidation of Ethylene As mentioned earlier, catalytic oxidation of ethylene [74-85-1] with oxygen in acetic acid has already been used on an industrial scale, but this method was soon abandoned due to problems caused by corrosion. The yield of ethylene glycol (>90 %) was much higher than that obtained in the more indirect route via ethylene oxide [30].
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A more recently developed catalyst system is based on the use of Pd(II) complexes [31]. A mixture of PdCl2, LiCl, and NaNO3 in acetic acid and acetic anhydride has been shown to give a 95 % selectivity for glycol monoacetate and glycol diacetate formation (60 – 100 C, 3.04 MPa) [32]. During this process, Pd(II) is reduced to Pd(0). The precipitation of Pd(0) is prevented because it is reoxidized to Pd(II) by the nitrate ions. The available oxygen finally regenerates the nitrate, thus providing a complete catalytic system. The formation of ethylene glycol monoacetate [542-59-6] (50 % yield) and ethylene glycol diacetate [111-55-7] (7 % yield) has also been investigated using the catalyst system PdCl – NO2 – CH3CN dissolved in acetic acid. Studies with radioactive isotopes showed that the NO2 functions as an oxidizing agent [33]. Vinyl acetate is formed as a byproduct (20 % yield). However, the catalytic action of this system is quickly exhausted due to the precipitation of palladium compounds. If a PdCl2 – CuCl2 – CuOCOCH3 system is used, the reaction proceeds under mild conditions (65 C, 0.5 MPa) without the formation of a precipitate; a yield of over 90 % is obtained [34].
Ethylene Glycol
537
In recent years, increasing attention has been paid to Pd(II) systems as catalysts for the direct oxidation of ethylene to ethylene glycol. In spite of the widespread interest in this alternative, industrial applications have yet to be realized. 4.2.2. Synthesis from C1 Units The long-term shortage and increased price of crude oil have led to an intensive search for methods of producing organic intermediates from C1 units (i.e., methods based on coal). Many publications have appeared on the synthesis of ethylene glycol by this approach. Only the most important methods that rely on synthesis gas or carbon monoxide [630-08-0] are discussed here; they are summarized in Figure 8. At a high pressure, carbon monoxide and hydrogen react directly to produce ethylene glycol [35, 36]. However, the reaction is slow and the catalyst is both sensitive and expensive [37]. Other methods involve the formation of formaldehyde [50-00-0], methanol [67-56-1] [38], or esters of oxalic acid [144-62-7] as intermediates [37, 39]. The only method to attain industrial importance was that employed by Du Pont from 1940 to 1963, which used formaldehyde and
Figure 8. Production of ethylene glycol from carbon monoxide (percentages indicate approximate yields)
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glycolic acid [79-14-1] as intermediates. High operating pressure and temperature were required, however (48 MPa, 220 C). This process was significantly improved by the introduction of hydrogen fluoride [7664-39-3] as a catalyst (1 – 2 MPa, 60 C) [40]. At the present time, none of the described methods based on C1 units can compete with the ethylene ! ethylene oxide ! glycol pathway. However, if crude oil prices increase, the synthesis of ethylene glycol from C1 units will become more economically attractive [41].
[45]. The UV absorption of fiber-grade glycol is often used as an additional parameter for quality control. Gas chromatography is commonly used for the quantitative determination of ethylene glycol. Monoethylene glycol can be detected by oxidation with periodic acid even if di- and triethylene glycols are also present; however, aldehydes, glycerol, and monopropylene glycol falsify the results [4].
7. Storage and Transportation
5. Environmental Protection and Ecology
Pure anhydrous ethylene glycol is not aggressive toward most metals and plastics. Since ethylene glycol also has a low vapor pressure and is noncaustic, it can be handled without any problems; it is transported in railroad tank cars, tank trucks, and tank ships. Tanks are usually made of steel; high-grade materials are only required for special quality requirements. Nitrogen blanketing can protect ethylene glycol against oxidation. At ambient temperatures, aluminum is resistant to pure glycol. Corrosion occurs, however, above 100 C and hydrogen is evolved. Water, air, and acid-producing impurities (aldehydes) accelerate this reaction. Great care should be taken when phenolic resins are involved, since they are not resistant to ethylene glycol.
Ethylene glycol is readily biodegradable [42]; thus, disposal of wastewater containing this compound can proceed without major problems. The high LC50 values of over 10 000 mg/L [43, 44] account for its low water toxicity: LC50 crayfish (Procambarus) 91 000 mg/L, LC50 fish (Lepomis macrochirus) 27 540 mg/L.
6. Quality Specifications and Analysis Since ethylene glycol is produced in relatively high purity, differences in quality are not expected. The directly synthesized product meets high quality demands (‘‘fiber grade’’). The ethylene glycol produced in the wash water that is used during ethylene oxide production is normally of a somewhat inferior quality (‘‘antifreeze grade’’). The quality specifications for mono-, di-, and triethylene glycols are compiled in Table 2
8. Derivatives Only the most important of the many derivatives of ethylene glycol will be discussed in this sec-
Table 2. Quality specifications of mono-, di-, and triethylene glycols [45] Property
Purity, % Diethylene glycol content, wt % Boiling range (at 101.3 kPa), C Density (20 C), g/cm3 Refractive index, n20 D Water content, wt % Acid number, mg of KOH/g
Method
Monoethylene glycol Antifreeze grade
Fiber grade
Diethylene glycol
Triethylene glycol
Gas chromatography Gas chromatography
>98.00 <0.50.
>99.80 <0.10
99.60
99.50
DIN 53171, D 1078 DIN 51757, D 1122 DIN 53491, DIN 51777, DIN 53402,
ASTM
195 – 200
196 – 199
242 – 247
285 – 295
ASTM
1.113 – 1.115
1.1135 – 1.1140
1.1160 – 1.1175
1.1235 – 1.1245
ASTM 1747 ASTM E 203 ASTM D 1613
1.431 – 1.433 <0.50 <0.01
1.4315 – 1.4320 <0.100 <0.005
1.4460 – 1.4475 <0.20 <0.05
1.4555 – 1.4565 <0.10 <0.05
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Table 3. Physical properties of mono-, di-, tri-, and tetraethylene glycols [4, 5, 46] Property
Monoethylene glycol
Diethylene glycol
Triethylene glycol
Tetraethylene glycol
Molecular formula CAS registry number Mr bp(at 101.3 kpa), o C fp, C Vapor pressure (20 C), Pa Density (20 C), g/cm3 Refractive index, n 20 d Heat of combustion, MJ/kg Heat of vaporization (101.3 kPa), kJ/mol Viscosity (20 C) mPa*s Surface tension, N/m2 Flash point, C Ignition temperature, C Lower explosive limit, vol %
HOCH2CH2OH [107-21-1] c62.7 197.6 –13 5.3 1.1130 1.4318 19.07 52.24 19.83 4.84 (20 C) 119 410 3.2
H(OCH2CH2)2OH [111-46-6] 106.12 244.8 –8 2.7 1.1160 1.4470 22.32 52.26 36.0 4.85 (20 C) 141 390 0.7
H(OCH2CH2)3OH [112-27-6] 150.17 287.4 –7 0.5 1.1230 1.4560 23.68 61.04 49.0 4.22 (25 C) 177 370 0.9
H(OCH2CH2)4OH [112-60-7] 194.23 decomp. –4.1 <1.3 1.1247 1.4598
tion, namely di-, tri-, and polyethylene glycols, the methyl, ethyl, and butyl glycol ethers and the acetate esters.
62.63 61.9 191
with the appropriate alcohol. A mixture of homologues is obtained, all displaying the following general structure: RO(CH2CH2O)n–H
8.1. Di-, Tri-, Tetra-, and Polyethylene Glycols The higher homologues of ethylene glycol are formed as byproducts during the synthesis of ethylene oxide and monoethylene glycol or are prepared directly by reacting monoethylene glycol with ethylene oxide. Di-, tri-, and possibly tetraethylene glycol can be purified by distillation. For quality specifications of these compounds, see Table 2; Table 3 shows some of their physical properties. The temperature dependence of the thermal conductivity, density, viscosity, and Prandtl numbers of di- and triethylene glycol – water mixtures is described in [46]. Poly(ethylene glycols) (PEG) [25322-68-3] are produced as mixtures of higher molecular mass homologues (see also ! Polyoxyalkylenes) and are characterized by their mean molecular masses (e.g., PEG 400). Physical properties of some poly(ethylene glycol) mixtures are listed in Table 4.
8.2. Ethers and Esters Ethers. Ethylene glycol monoethers are usually produced by reaction of ethylene oxide
The glycol monoethers can be converted to diethers by alkylation with common alkylating agents, such as dimethyl sulfate or alkyl halides (Williamson synthesis). Glycol dimethyl ethers are formed by treatment of dimethyl ether with ethylene oxide [48]. The properties of some ethylene glycol mono- and diethers are shown in Tables 5 and 6, respectively; they are mainly used as solvents. Esters. Ethylene glycol esters are produced from reaction of ethylene oxide and an appropriate acid and are used also as solvents. Use of a suitable catalyst can influence the outcome of the Table 4. Physical properties of poly(ethylene glycols) [49] Poly(ethylene glycol), mean Mr
fp, C
Refractive index, n70 D
Viscosity, mPa s*
200 600 1000 2000 10 000 20 000 35 000
ca. 50 17 – 22 35 – 40 48 – 52 55 – 60 ca. 60 ca. 60
1.458 (25 C) 1.452 1.453 1.454 1.456 1.456 1.456
60 – 70 (100 %) 16 – 19 24 – 29 50 – 60 530 – 1000 2700 – 3500 11 000 – 14 000
*
Values refer to a 50 wt % aqueous poly(ethylene glycol) solution except where otherwise stated.
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Table 5. Physical properties of some ethylene glycol monoalkyl ethers, RO(CH2CH2O)n–H [4, 5] R
n CAS registry number
Mr
fp, C
bp Density Refractive Specific heat Vapor Flash point, Ignition Explosive (101.3 kPa), (20 C), index, n20 pressure C temperature, limits, D (20 C), C kg/m3 J g1 K1 (20 C), Pa C vol %
CH3 C2H5 C4H9 CH3 C2H5 C4H9 CH3 C2H5
1 1 1 2 2 2 3 3
76.094 90.120 118.172 120.146 134.172 162.224 164.198 178.224
85.1 100.0 70.4 50.0 54.0 68.1 38.2 18.7
124.50 135.10 171.20 194.10 201.90 230.60 249.00 255.50
[109-86-4] [110-80-5] [111-76-2] [111-77-3] [111-90-0] [112-34-5] [112-35-6] [112-50-5]
964.6 929.7 900.8 1021.0 989.0 953.0 1047.5 1020.0
1.4024 1.4079 1.4194 1.4265 1.4275 1.4316 1.4381 1.4380
reaction so as to produce monoesters (sodium acetate) or diesters (sulfuric acid). Glycol monoethers can be converted to the corresponding ether esters by the usual methods. Some important properties of ethylene glycol esters and ether esters are listed in Table 7. Trade Names. Some common trade names of ethylene glycol ethers and esters are as follows: Monoethylene Glycol Derivatives. Monomethyl ether: Methyl Cellosolve Solvent, Dowanol EM Glycol Ether, Methyl Oxitol Glycol. Monoethyl ether: Cellosolve Solvent, Dowanol EE Glycol Ether. Monobutyl ether: Butyl Cellosolve Solvent, Dowanol EB Glycol Ether, Butyl Oxitol Glycol. Dimethyl ether (also known as glyme): Dimethyl Cellosolve Solvent. Diethyl ether: Diethyl Cellosolve Solvent. Monomethyl ether acetate: Methyl Cellosolve Acetate. Monoethyl ether acetate: Cellosolve Acetate, Poly-solv EE Acetate. Diethylene Glycol Derivatives. Monomethyl ether: Methyl Carbitol Solvent, Dowanol DM Glycol Ether. Monoethyl ether: Carbitol Solvent, Dowanol DE Glycol Ether. Monomethyl ether
2.24 930 2.32 530 2.44 80 2.15 24 2.31 17 2.29 4 2.18 2.17 (15 C) 0.4
38 40 66 90 93 99 118 195
285 215 225 215 190 195
2.4 1.8 1.1 1.6 1.8 0.7
– – – – – –
20.6 15.7 10.6 16.1 12.2 5.9
acetate: Methyl Carbitol Acetate. Monoethyl ether acetate: Carbitol Acetate. Monobutyl ether acetate: Butyl Carbitol Acetate. Diethylene glycol dimethyl ether is also known as diglyme. Triethylene Glycol Derivatives. Monomethyl ether: Dowanol TM Glycol Ether.
9. Uses Ethylene glycol lowers the freezing point of water (see Fig. 1). Its ease of handling makes it a perfect antifreeze [51], which accounts for over 50 % of its commercial uses (! Antifreezes). Commercial antifreezes based on glycol also contain corrosion inhibitors and are used, for example, in motor vehicles, solar energy units, heat pumps, water heating systems, and industrial cooling systems. Protection against freezing is directly related to the glycol concentration; 60 % glycol prevents freezing down to a temperature of 55 C. There is little point using higher concentrations because the freezing point then starts to increase (Fig. 1).
Table 6. Physical properties of some ethylene glycol dialkyl ethers RO(CH2CH2O)n-R [2, 4, 50] R
n CAS registry number
Mr
fp, C bp Density Refractive Specific heat Vapor Flash point, Ignition Lower (20 C), pressure C temperature, explosive (101.3 kPa), (20 C), index, n20 D C kg/m3 J g1 K1 (20 C), Pa C limit, vol %
CH3 C2H5 C4H9 CH3 C2H5 C4H9 CH3
1 1 1 2 2 2 3
90.120 118.172 174.276 134.172 162.224 218.328 178.224
58.0 74.0 69.1 64.0 44.3 60.2 45.0
[110-71-4] [629-14-1] [112-48-1] [111-96-6] [112-36-7] [112-73-2] [112-35-6]
85.2 121.4 203.6 163.0 188.4 254.6 216.0
867.0 841.0 836.0 944.0 906.6 883.8 987.0
1.3796 1.3920 1.4131 1.4078 1.4115 1.4233 1.4233
2.01 2.04 2.10 1.80
8100.0 1250.0 27.0 267.0 (15 C) 51.0 (13 C) 1.3 120.0
6 35 85 51 82 118 113
200 205
1.6
1.4
195
0.7
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Ethylene Glycol
541
Table 7. Physical properties of some ethylene glycol acetates and ether acetates [4] Compound
CAS registry Mr number
fp, C bp Density Refractive Ignition Flash Explosive (101.3 kPa), (20 C), index, n20 tempera- point, limits, D C kg/m3 ture, C C vol %
Ethylene glycol monoacetate Ethylene glycol diacetate Ethylene glycol methyl ether acetate Ethylene glycol ethyl ether acetate Ethylene glycol butyl ether acetate Diethylene glycol methyl ether acetate Diethylene glycol ethyl ether acetate Diethylene glycol butyl ether acetate
[542-59-6] [111-55-7] [110-49-6] [111-15-9] [112-07-2] [629-38-9] [112-15-2] [124-17-4]
80.0 31.0 65.1 61.7 63.5 99.3 25.0 32.2
104.10 146.10 118.13 132.16 160.21 162.19 176.21 204.26
Ethylene glycol is also a commercially important raw material for the manufacture of polyester fibers, chiefly poly(ethylene terephthalate) (! Fibers, 4. Synthetic Organic). This application consumes ca. 40 % of the total ethylene glycol production. Polyesters are, however, used for other purposes, e.g., for producing recyclable bottles (! Polyesters). Other minor uses of ethylene glycol are as a humectant (moisture-retaining agent), plasticizer, softener, hydraulic fluid, and solvent [52]. Ethylene Glycol Derivatives, mainly the ethers and esters, are frequently employed as reaction media, as absorption fluids, and as solvents for dyes, lacquers, and various cellulose products (cellulose esters, ethers, and nitrocellulose) [53]. Diethylene Glycol serves as a solvent, softener (cork, adhesives, paper, etc.), dye additive (printing and stamping inks), deicing agent for runways and aircraft [54], and a drying agent for gases (e.g., natural gas) [55]. Triethylene Glycol is used for the same purposes as diethylene glycol (e.g., as a solvent, plasticizer, and drying agent for gases) [56]. Poly(ethylene Glycols) with varying molecular masses find numerous uses in the pharmaceutical industry (e.g., in ointments, liquids, and tabletting) and the cosmetic industry (e.g., creams, lotions, pastes, cosmetic sticks, and soaps). They are also used in the textile industry (e.g., cleaning and dyeing aids), in the rubber industry (e.g., lubricants and mold parting
182.0 190.5 145.0 156.4 192.3 209.1 217.4 246.7
1110 1106 1006 974 942 1041 1011 981
1.4209 1.4159 1.4019 1.4058 1.4138 1.4209 1.4213 1.4262
410 390 355
290
102 105 49 54 74 110 112 116
7.8 – 27.7 1.7 – 8.2 3.18 – 10.1 1.66 – 8.37
agents), and in ceramics (e.g., bonding agents and plasticizers) [57].
10. Economic Aspects Ethylene glycol is one of the major products of the chemical industry. Its economic importance is founded on its two major commercial uses – as an antifreeze and for fiber production. Since ethylene glycol is currently produced exclusively from ethylene oxide (! Ethylene Oxide), production plants are always located close to plants that produce ethylene oxide. The proportion of ethylene oxide that is converted to glycol depends on the local conditions, such as transport facilities and the market situation. Almost all of the ethylene oxide produced in recently installed plants in Saudi Arabia is, for example, converted to ethylene glycol because the domestic market for ethylene oxide is negligible and because ethylene glycol is easier to transport than ethylene oxide. Detailed information about the glycol capacities of individual plants is difficult to obtain, but an estimated 60 % of the total world production of ethylene oxide is converted to ethylene glycol. Therefore, the worldwide ethylene oxide production capacity of 8 106 t corresponds to an annual ethylene glycol production of 6.7 106 t. About 50 % of the ethylene glycol that is produced is used in antifreeze. Another 40 % is channeled into the fiber industry. Consequently, the ethylene glycol demand is closely connected to the development of these two sectors. Surplus capacities and growing competition for market shares are expected on account of increasing
542
Ethylene Glycol
glycol capacities – not only in Saudi Arabia, but also in the Eastern Bloc. Japan has already reduced its glycol capacities considerably [58]. In view of the increasing price of crude oil, alternative production methods based on synthesis gas – hitherto still in the experimental phase – are likely to become more important and increasingly competitive.
11. Toxicology and Occupational Health 11.1. Ethylene Glycol Oral Toxicity. The oral LD50 values of ethylene glycol are 14 – 15 g/kg [59, 60] for mice and ca. 8 g/kg [61] for rats. Oral toxicity in humans is higher, 1.4 g/kg can be lethal [59]. In rats, long-term administration of ethylene glycol in food at concentrations of 10.0 and 20.0 g/kg led to formation of bladder stones (calcium oxalate), damage to the renal tubules, and centrolobular fatty degeneration of the liver [62]. In other investigations, comparable changes were already apparent at a concentration of 5.0 g/ kg, no changes were observed at 2.0 g/kg [63]. No changes were observed in the kidneys of monkeys fed a diet providing a maximal daily dose of 0.17 g/kg of body mass over a period of three years [64]. However, addition of ethylene glycol to the drinking water given to monkeys at a concentration corresponding to a daily dose of 0.24 g/kg of body mass led to the formation of oxalate crystals in the kidneys after about five months. Oxalate crystals were also formed in the brains of monkeys given very high doses (10 wt %) in drinking water [65]. Eye and Skin Irritation. Introduction of a single dose of ethylene glycol into the conjunctival sac does not affect the rabbit eye [66], but repeated administration leads to mild conjunctivitis [67]. Ethylene glycol vapor at a concentration of 17 mg/m3 produces ocular damage in humans [68]. No effects were observed in the eyes of monkeys exposed to a vapor concentration of 265 mg/m3 [68]. Ethylene glycol has not been shown to irritate mucous membranes. Repeated exposure of the skin to ethylene glycol can cause mild irritation, similar to that produced by glycerol.
Vol. 13
Inhalation. The inhalative toxicity of an atmosphere saturated with ethylene glycol (0.5 mg/L) is low. Rats tolerated this concentration for four weeks without any permanent injuries, only mild narcosis was observed [69]. No adverse effects were detected in rats, rabbits, guinea pigs, dogs, and monkeys exposed to an ethylene glycol concentration of 57 mg/L for six weeks [70]. Monkeys survived after inhaling concentrations of 600 mg/m3 administered as an aerosol over a period of several months. Human volunteers exposed to maximal concentrations of 67 mg/m3 for one month reported some irritation of the nasal mucous membranes and occasionally headache. The presence of ethylene glycol in the inhaled atmosphere was perceived at concentrations above 140 mg/m3 [71]. A concentration of 200 mg/m3 was found to be intolerable [72]. Rats, rabbits, dogs, and monkeys that inhaled pure oxygen containing 100 mL/m3 of ethylene glycol at a pressure of 34.5 kPa for three weeks did not show any adverse effects apart from local irritation [73]. Mutagenicity and Carcinogenicity. Ethylene glycol gives a negative result in the Ames test [74, 75]. Many studies have been performed on its carcinogenicity [64, 76, 77]. Rats were fed with diets containing ethylene glycol concentrations corresponding to 1000, 200, and 40 mg/kg of body mass; no carcinogenic effects were observed. However, typical formation of oxalate crystals in the kidneys and other organs was found [78]. Similar experiments did not increase the incidence of tumor formation in mice [79]. Reproduction. In a study extending over three generations, ethylene glycol was added to the diet fed to rats at a concentration of 0.04, 0.2, and 1.0 g kg1 d1. The fertility and viability of the offspring were not affected by this treatment [80]. Teratogenic effects were not detected in rats receiving maximal feed concentrations of 1000 mg/kg, only a mild fetal toxicity was observed [81]. Teratogenic and fetotoxic effects were produced in mice and rats after administration of very high doses (1.25, 2.50, and 5.00 g/kg of body mass) in the form of a bolus with the aid of an esophageal probe [82].
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Ethylene Glycol
Metabolism. In the body, ethylene glycol is degraded by alcohol dehydrogenase to form glycolaldehyde, which is further degraded by an aldehyde oxidase to produce glyoxal. Glyoxal is subsequently metabolized to glycolic and glyoxylic acids, from which oxalic acid, formic acid, glycine, 2-oxo-4-hydroxygluconate, 2-hydroxy-3-oxoadipate and oxomalate are formed [83, p. 3827].
543
stones is attributed to contamination of the diethylene glycol with monoethylene glycol (see Section 11.1) [87, 88]. The low cumulative toxicity of diethylene glycol has been confirmed [89–91]. Diethylene glycol does not irritate the eyes or skin [92]; absorption by the skin is extremely low [93]. Diethylene glycol does not affect the fertility of rats [94]. It does not display mutagenicity in the Ames test [95]; other studies also suggest that the compound is not carcinogenic [87, 88].
11.2. Ethylene Glycol Derivatives Toxicity data and exposure limits for ethylene glycols and some monoethylene glycol derivatives are listed in Table 8. Diethylene Glycol has a low acute oral toxicity (see Table 8). A LDLo value of 1.0 mL/kg is often erroneously cited but this value was determined using a preparation that contained sulfanilimide [85]. In long-term studies performed over a period of two years, dietary levels of 1, 2, and 4 % diethylene glycol administered to rats produced calcium oxalate stones with associated kidney and liver damage [86]. Chronic mechanical irritation of the bladder epithelium by the resulting bladder stones led to the formation of benign tumors. Production of calcium oxalate
Triethylene Glycol has an extremely low oral toxicity when given either in a single dose (see Table 8) or in repeated doses [86, 96]. This compound causes only slight irritation of the skin and mucous membranes, it is not teratogenic [97]. Tetraethylene Glycol is, like triethylene glycol, nontoxic (see Table 8). Poly(Ethylene Glycols) have an extremely low acute oral toxicity (LD50 in rats > 30 g/kg) which decreases as their molecular mass increases [98]. Studies on rats [98] and dogs [99] have shown that their cumulative toxicity is also very low. Irritation of the skin and mucous membranes in humans and animals is slight. When applied to
Table 8. Toxicity data and exposure limits for ethylene glycols and ethylene glycol derivatives Compound
Ethylene glycols Monoethylene glycol Diethylene glycol Triethylene glycol Tetraethylene glycol Monoethylene glycol derivatives Monomethyl ether (methoxyethanol) Monomethyl ether acetate (methoxyethyl acetate) Monoethyl ether (ethoxyethanol) Monoethyl ether acetate (ethoxyethyl acetate) Monobutyl ether (butoxyethanol) Monobutyl ether acetate (butoxyethyl acetate) *
mL/kg
Single oral LD50 in rats, g/kg
Reference
MAK, ppm
TLV-TWA, ppm
8 20.8 22.1 30.8*
[61] [61] [61] [83, p. 3843]
10 10
50
3.4
[84]
5
5
3.93
[61]
5
5
5.5
[84]
5
5
5.1
[61]
5
5
2.5
[84]
20
25
20
544
Ethylene Glycol
the skin, pure poly(ethylene glycols) do not cause sensitization [98, 100, 101] and are nontoxic [98]. Poly(ethylene glycols) with a molecular mass < 1000 are absorbed in the intestine and excreted in the urine and feces. Intestinal absorption decreases markedly as the molecular mass increases [102–104].
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Further Reading G. P. Chiusoli, P. M. Maitlis (eds.): Metal-catalysis in Industrial Organic Processes, Royal Society of Chemistry, Cambridge, UK 2006. R. J. Farn (ed.): Chemistry and Technology of Surfactants, Blackwell, Oxford, UK 2006. M. W. Forkner, J. H. Robson, W. M. Snellings, A. E. Martin, F. H. Murphy, T. E. Parsons: ‘‘Glycols’’, Kirk Othmer
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Encyclopedia of Chemical Technology, John Wiley & Sons, Hoboken, NJ, online DOI: 10.1002/ 0471238961.0520082506151811.a01.pub2. J. Hagen: Industrial Catalysis, 2nd ed., Wiley-VCH, Weinheim 2006. M. Kjellin, I. Johansson (eds.): Surfactants from Renewable Resources, Wiley, Chichester 2010.
Vol. 13 H. K. Phlegm: The Role of the Chemist in Automotive Design, CRC Taylor & Francis, Boca Raton, FL 2009. J. Tulla-Puche, F. Albericio: The Power of Functional Resins in Organic Synthesis, Wiley-VCH, Weinheim 2008. U. Zoller, P. Sosis: Handbook of Detergents, CRC Press, Boca Raton, FL 2009.
