[Doi 10.1002%2F14356007.a10_101] Rebsdat, Siegfried -- Ullmann's Encyclopedia of Industrial Chemistry Ethylene Glycol

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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Ethylene Glycol

533

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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Ethylene Glycol

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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Ethylene Glycol

535

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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Ethylene Glycol

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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 99.80