Specifics of the Synthesis of Some Alkyl tert-Alkyl ...

Petroleum Chemistry, Vol. 45, No. 2, 2005, pp. 92–95. Translated from Neftekhimiya, Vol. 45, No. 2, 2005, pp. 111–114. Original Russian Text Copyright © 2005 by Krasnykh, Levanova, Karaseva, Kirgizova, Varushchenko, Druzhinina, Pashchenko. English Translation Copyright © 2005 by MAIK “Nauka /Interperiodica” (Russia).

Specifics of the Synthesis of Some Alkyl tert-Alkyl Ethers and Their Thermodynamic Properties E. L. Krasnykh*, S. V. Levanova*, S. Ya. Karaseva*, I. N. Kirgizova*, R. M. Varushchenko**, A. I. Druzhinina**, and L. L. Pashchenko** *Samara State Technical University, Samara, Russia **Moscow State University, Moscow, 119899 Russia Received March 19, 2004

Abstract—The conditions of synthesis and isolation of pure (98–99%) and extra pure (>99.9%) alkyl tert-alkyl ethers containing six to eight carbon atoms in their molecule, which are used as high-octane additives to motor fuels, were studied. The key thermodynamic properties, including melting point, density, saturated vapor pressure, enthalpy of vaporization at 298.15 K, normal boiling point, and critical parameters, obtained by experimental and calculation methods, are given.

The protection of atmospheric air from pollutants is one of the most important problems facing modern society. About 80% of the pollutants that get into the atmosphere are discharged by motor transport. Most components of automobile exhaust gases are toxic. Automobile engines release into urban air more than 95% of carbon dioxide, about 65% of hydrocarbons, and 30% of nitrogen oxides. In connection with this, many countries around the world, including Russia, have introduced standards to limit the content of benzene in motor fuel (to at most 1%, since its presence results in the formation of benzo[a]pyrene, the strongest carcinogen) and the requirement that gasolines must necessarily contain oxygen-containing compounds, also known as oxygenates (preferably tertiary ethers), in an amount that ensures the presence of bound oxygen at a level of at least 2–3 wt %. The presence of oxygenates enhances the antiknock resistance of gasoline, increases the completeness of its combustion, and decreases the content of carbon oxides and nitrogen oxides in exhaust.

eight) in the molecule, as the normal boiling points of these ethers are in the temperature range of 363–423 K, which is favorable for the operation of gasoline-fueled internal-combustion engines; and (3) the availability of relatively inexpensive raw materials, namely, ë2–ë5 alcohols and ë4–ë5 isoolefins. The analysis of published data on the synthesis of ATAE shows that they deal mainly with the basic featural regularities and technological details of MTBE production in the liquid phase [3]. However, its production is complicated by the fact that one of the reactants, methanol, is a strong poison. Data on technical solutions for heavier ATAE homologues are scanty and conflicting. There are no national technologies that have been commercially developed. The development of these technologies requires knowledge of the values of saturated vapor pressure, enthalpies of formation and vaporization, and heat capacity data for ATAE, as well as for alcohols and olefins. Creation of a database containing the thermodynamic properties will accelerate the development of the scientific basis for the production of ethers. The important thermodynamic characteristics of the synthesis, including equilibrium constants and enthalpy and entropy of reactions depending of the temperature in the gas and liquid phases, can be calculated from these data. The obtained information will facilitate the choice of the optimal technical parameters (from a thermodynamic point of view) of the process: the temperature, the reactant ratio, and the equilibrium conversion, which will make it possible to obtain ATAE in high yields. To carry out these investigations, it is necessary to develop new approaches to the synthesis and, especially, to the isolation from the reaction mixture of pure

At present, many petrochemical companies have begun to produce methyl tert-butyl (MTBE) and methyl tert-amyl ethers (MTAE) as oxygenates [1, 2]. In Russia, oxygenates have not found proper application yet; therefore, the development and implementation of processes for the industrial manufacture of alkyl tert-alkyl ethers (ATAE) are an important task. The alkylation reaction of isoolefins with lower alcohols (ë1–ë5) is at the heart of ATAE production [3−6]. The choice of the objects is determined by the following factors favoring the use of ATAE as gasoline components: (1) the high octane number (>100), low toxicity of ethers, and the absence of the water separation effect; (2) the number of carbon atoms (six to 92

SPECIFICS OF THE SYNTHESIS OF SOME ALKYL tert-ALKYL ETHERS

(98−99%) specimens for determining the antiknock resistance and extra pure (>99.9%) specimens for precision measurements of thermodynamic characteristics. The complexity of preparing pure specimens is due to several factors: the formation of by-products, mainly dimers and trimers of isoolefins in the presence of proton-donating catalysts, as well as the formation of azeotropic alcohol–ether–olefin mixtures. It is known that the cost of MTBE isolation from the reaction mixture makes up 50–90% of the cost of motor fuels [7]. The aim of this work was to study the conditions of synthesis and isolation of a number of promising tertiary ethers containing six to eight carbon atoms in the molecule: ethyl tert-butyl (ETBE), iso-butyl tert-butyl (IBTBE), n-propyl tert-amyl (NPTAE), and iso-propyl tert-amyl (IPTAE) ethers.

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reaction and dried under vacuum at a temperature of no higher than 100°ë. Synthesis of Ethers The synthesis was carried out in glass ampules in which the catalyst and an alcohol were charged and held for several hours to allow the catalyst to swell. Then, the olefin was added. In the case of isoamylenes, the olefin was poured into the ampule; and in the case of isobutylene, the ampule was cooled to –12 to –15°C and then the gaseous olefin was bubbled into the ampule to be condensed there at low temperatures. The alcohol to olefin molar ratio was 3 : 1. After the introduction of olefins, the ampules were tightly closed and thermostated at 40°ë for 48–72 h. Then, the ampules were opened and the resultant mixture was sent to the ether isolation stage.

EXPERIMENTAL The synthesis of ATAE comprises the following stages: preparation of reactants, preparation of a catalyst, synthesis of ethers, and their purification. Preparation of Reactants Alcohols were rectified and then refluxed over potassium metal for 3–4 h to remove water. Then, they were distilled to be used for synthesis. The alcohols were stored over calcined NaX-type zeolites. The purity of each alcohol was at least 99.8%. In this work, we used industrial isobutane–isobutylene and isoamylene fractions. The isoamylene fraction was preliminary rectified to remove tars formed during storage. The composition of the isoamylene fraction (wt %) was as follows: 3-methylbutene-1, 3.7; 2-methylbutene-2, 56.8; 2-methylbutene-1, 35.2; and isopentane, 4.3. The isobutane–isobutylene fraction with an isobutylene content of at least 80% was used in the synthesis without preliminary purification. Catalyst Preparation In the preliminary experiments, we used macroporous KU-23 and KU-2 cation exchangers; however, their application results in the formation of up to 20% of by-products (dimers). It was impossible to purify the obtained ether for the removal of dimers up to the desired purity; therefore, we used less active catalysts, namely, the gel cation exchangers Dowlex and Levatit S-100. The formation of by-products in their presence was minimal. The cation exchanger was prepared according to a conventional procedure as described in [8]: the catalyst was loaded into a separating funnel and treated for 6−8 h with 10% sulfuric acid solution. Then, the cation exchanger was washed with distilled water up to neutral PETROLEUM CHEMISTRY

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Isolation and Purification of Ethers The formation of by-products was negligible with the use of gel cation exchangers. The main problem in the ether isolation was its separation from unreacted alcohol and olefin. At the first step of isolation, we topped the resultant mixture to obtain the fraction with a boiling range below that of the alcohol, which constituted an azeotropic mixture of the alcohol and the ether. This fraction contained about 50% ether. At the second step, the alcohol was washed off with distilled water, as all light alcohols dissolve to different degrees in water much better than the corresponding ethers. The mixture was washed 10–12 times with cold water, 2 or 3 times with a 2% sodium hydrogen carbonate solution, and then with hot water with a temperature 20–30°ë lower than the ether boiling point (until the ether fraction was absolutely clear). The residual alcohol content in the ether was less than 0.02%. After this step, the ether has, on average, 99% purity. At the third step, the ether was purged with an inert gas under vacuum to remove the unreacted olefin. The purity of the ether after removal of the olefin was at least 99.8%. Finally, at the last step, the ether was purified by fractional distillation: middle fractions were collected, which consisted of the desired high-purity product (at least 99.9%). Analysis of Ethers and Reactants Analysis of the reactants and the products was carried out by GLC on a Kristall-2000 instrument with the dedicated Chromatec-Analytica software under the following conditions: a 100-m-long quartz capillary column with the bonded DB-1 phase, an evaporator temperature of 330°ë, a detector temperature of 250°ë, a carrier-gas (helium) flow of 120 ml/min, and a column temperature of 70°ë.

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KRASNYKH et al.

Physicochemical properties of some alkyl tert-alkyl ethers ρ, g cm–3 PVO, kPa 298.15 K 298.15 K

Ether

Bp (K)

Ethyl tert-butyl ether (ETBE)

179.3 ± 0.01

0.7351

16.76

n-Propyl tert-butyl ether (NPTBE)

179.6 ± 0.1

0.7455

6.16

184.78 ± 0.05

0.7354

9.71

–

0.7582

161.0 ± 0.1 157.7 ± 0.1

iso-Propyl tert-butyl ether (IPTBE) n-Butyl tert-butyl ether (NBTBE) iso-Butyl tert-butyl ether (IBTBE) Ethyl tert-amyl ether (ETAE)

∆νH, kJ mol–1 298.15 K

TIBP, K

Tc, K

Pc, MPa

345.96 505 345.86 [21] 370.89 533

1.98

33.06 ± 0.4 32.97 ± 0.16 [20] 36.57 ± 0.2 36.76 ± 0.2 [20] 34.48 ± 0.2 34.67 ± 0.18 [20] 41.58 ± 1.7

0.7478

3.25

39.17 ± 0.27

386.12

0.7614

5.26

38.19 ± 0.16

374.824 542 2.65 374.70 [21] 552 [21]

360.67 520 2.50 360.43 [21] 528 [21] 2.80 [21] 396.58 561 2.35 547

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tures with ethers and will deteriorate the quality of motor fuels in the future. The ethers were obtained by the following reversible reaction: CH3 CH 2 =C–CH 2 –CH 3 + ROH

CH3 CH 3 –C–CH 3 , OR – –

–

CH 2 =C–CH 3 + ROH CH3

CH3 CH 3 –C–CH 2 –CH 3 OR – –

CH3 CH 3 –C=CH–CH 3 + ROH –

RESULTS AND DISCUSSION The choice of ethers for the investigation is determined by the fact that, in addition to complying with the aforementioned requirements, they have one significant advantage over their methylated homologues, namely, the enhanced reactivity of ë2–ë5 alcohols and their higher equilibrium degree of conversion in reactions with isoalkenes [16]. However, the yield of byproducts is also higher in this case, thus determining the necessity of searching for new, milder conditions of synthesis, since the dimers [17] form azeotropic mix-

2.56

–

Determination of Thermodynamic Properties A set of promising tertiary ethers was investigated by experimental and calculation methods in order to obtain the key thermodynamic properties that determine the choice of ATAE as a high-octane additive to motor fuel. The set of experiments carried out to create the database included the determination of the temperature dependence for the saturated vapor pressure of liquid ATAE by comparative ebulliometry in the moderate pressure range 2–101 kPa with an accuracy of σP = ± 20 Pa and σT = ±0.01 K; the enthalpies of vaporization by a calorimetric technique with an accuracy of σç = ±(0.002–0.005)∆v H; and the densities of liquids in the temperature range 293–343 K with an accuracy of σρ = ±0.0005 g/cm3 [9–11], as well as heat capacity by vacuum adiabatic calorimetry in the temperature range 5–353 K with an average accuracy of σ C p = ±0.002Cp [12–14]. The investigations were carried out for six ethers: ETBE, IBTBE synthesized in this work with a purity >99.9%, n-butyl tert-butyl (NBTBE) [14], n-propyl tert-butyl (NPTBE) [15], isopropyl tert-butyl (IPTBE) [15] and ethyl tert-amyl (ETAE) ethers [11].

2.81

where ROH represents ë2–ë4 alcohols. In the case of isoamylenes, the reaction proceeds with the olefins having the double bond at a tertiary carbon atom. It is known that the reaction of ë1−ë5 alcohols with olefins yields not only the corresponding ethers but also tertiary alcohols and dimers and trimers of the olefins [15]. For ë1–ë2 alcohols, equilibrium is difficult to establish: there are considerable losses due to high volatility of alcohols, as the alcohols are well soluble in water and form azeotropic mixtures with it; the equilibrium is displaced towards the reactant side. The equilibrium for ë3–ë5 alcohols is reached considerably more quickly and is shifted to the product side. The alcohol conversion to ether at 323 K increases from 60 to 87% on passing from methanol to n-amyl alcohol, according to [3, 18]. The yield of pure products (95–98%) in the synthesis is less than 20%. In all cases, the catalyst was a macroporous sulfonated cation exchanger. It is known that the reaction in the presence of sulfonated cation exchangers [19] is greatly limited by internal diffusion at T > 333 K and conversions close to equilibrium ones. Isoolefins accelerate the reaction and PETROLEUM CHEMISTRY

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SPECIFICS OF THE SYNTHESIS OF SOME ALKYL tert-ALKYL ETHERS

alcohols inhibit it. The alkyl tert-alkyl ether produced also accelerates the reaction at low concentrations but inhibits it when the equilibrium state is approached. Since the reaction is a highly exothermic equilibrium process and is accompanied by a decrease in volume, the conversion of reactants is increased by continuous withdrawal of the heat and the products from the reaction zone. The yield of the ethers increases with an increase in the alcohol to isoolefin ratio; in this case, the conversion of isoolefins reaches 97–99%. It was found that the resultant mixture always contains isoolefin, whose amount corresponds to its solubility in the alcohol at a given temperature, and this has to be taken into account in the manufacturing of extra-purity products. The determined physicochemical properties of the ethers are given in the table and compared with reliable literature data. The values of Tc and Pc were calculated from vapor pressure and density data by the law of corresponding states (LCS). The critical parameters are used to calculate, in terms of the LCS, some properties that are not measured experimentally: viscosity, surface tension, and heat capacity with an accuracy suitable for technological calculations. Due to lower energies of dispersion interactions, the values of IBP, ∆v H and Tc for iso-ATAE are lower than for isomeric n-ATAE. The estimate of increments for of ëç2 group in the series of n-ATAE showed of selfconsistence of IBP and ∆v H data within ±0.3 K and ±0.5 kJ mol–1, respectively. Three increments for Tc had an identical value of 28 K. ACKNOWLEDMENTS This work was supported by the Russian Foundation for Basic Research, project no. 02-02-17009, and the Samara oblast administration. REFERENCES 1. V. Yukhnev, V. Zvyagin, and Yu. Moroshkin, Neft Ross., No. 10 (2000). 2. V. E. Emel’yanov, Khim. Tekhnol. Topl. Masel, No. 2, 5 (1995).

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3. M. N. Stryakhileva, G. N. Krylova, et al., in Thematic Review of TSNIITNeftekhim, Ser. Neftekhim. Neftepererab. (Moscow, 1988), No. 8, p. 15. 4. E. M. Rivin and L. Ya. Gorozhankina, in Thematic Review of TsNIITNeftekhim, Ser. Prom-st. Sint. Kauch. (Moscow, 1995), No. 1, p. 11. 5. V. G. Ryzhikov, V. F. Vasil’ev, L. V. Shalimova, et al., RU Patent No. 2117030, Byull. Izobret., No. 34 (1998). 6. N. A. Baryshnikova, N. S. Gorbik, O. F. Skresanova, V. F. Vasil’ev, et al., RU Patent No. 2176634, Byull. Izobret., No. 34 (2001). 7. C. Streicher, L. Asselineau, and A. Forestair, in Proceedings of 13 IUPAC Conference on Chemical Thermodynamics, Clermont-Ferrand, 1994; Pure Appl. Chem. 67, 985 (1995). 8. G. V. Odabashyan, Laboratory Manual on Chemistry and Technology of Organic and Petrochemical Synthesis (Khimiya, Moscow, 1982) [in Russian]. 9. R. M. Varushchenko, L. L. Pashchenko, A. I. Druzhinina, et al., Zh. Fiz. Khim. 76, 632 (2002). 10. R. M. Varushchenko, Ch. A. Aitkeeva, A. I. Druzhinina, et al., Zh. Fiz. Khim. 76, 824 (2002). 11. R. M. Varushchenko, L. L. Pashchenko, A. Yu. Churkina, and A. V. Shabanova, Zh. Fiz. Khim. 76, 1027 (2002). 12. R. M. Varushchenko, A. I. Druzhinina, A. Yu. Churkina, and Zi-Cheng-Tan, Zh. Fiz. Khim. 75, 1351 (2001). 13. O. V. Dorofeeva, V. S. Yungman, R. M. Varushchenko, and A. I. Druzhinina, Int. J. Thermophys. (in press). 14. A. I. Druzhinina, R. M. Varushchenko, O. V. Dorofeeva, et al., Zh. Fiz. Khim. 78, 2141 (2004). 15. V. Macho, V. Kavala, M. Orkesa, and M. Polievka, Ropa Uhlie, No. 24, 397 (1982). 16. K. G. Sharonov, A. M. Rozhnov, V. I. Barkov, and R. I. Cherkasova, Zh. Prikl. Khim. (Leningrad) 60, 359 (1987). 17. N. A. Baryshnikova, S. V. Levanova, N. S. Gorbik, and O. F. Skresanova, Ekol. Prom-st. Ross., 12 (2003). 18. V. V. Safronov, K. G. Sharonov, and A. M. Rozhnov, Zh. Prikl. Khim. (Leningrad) 62, 824 (1989). 19. T. Zhang and R. Datta, Ind. Eng. Chem. Res. 34, 730 (1995). 20. V. Majer and V. Svoboda, Enthalpies of Vaporization of Organic Compounds: A Critical Review & Data Compilation (Blackwell, 1985). 21. M. A. Krahenbuhl and J. Gmehling, J. Chem. Eng. Data 39, 759 (1994).