580 J ournal of Chemical and Engineering Data, Vol. 49, No. 3, 2004
Ta ble 8. Com p a r ison of th e Sta n d a r d Mola r En th a lp ies
of F or m a tion ∆fH° (l) of ter t-Alk yl Eth er s Der ived fr om
m
En th a lp ies of Rea ction s 1-7 w ith Th ose Ca lcu la ted by
Im p r oved Ben son ’s Sch em e13 a t T ) 298.15 K
∆fH° (l)
m
∆fH° (l)
(Benson)13
/
compound
abbrev (exp)/kJm‚mol-1
kJ ‚mol-1
tert-amyl ethyl ether
tert-amyl propyl ether
tert-amyl butyl ether
tert-amyl iso-propyl ether
tert-amyl sec-butyl ether
TAEE
TAPE
TABE
TAiPE
TAsBE
-372.8 ( 1.6
-398.5 ( 2.3
-423.3 ( 1.5
-400.6 ( 1.5
-426.3 ( 1.5
-430.8 ( 2.7
-373.0
-398.5
-424.1
-400.1
-425.6
-430.6
tert-amyl cyclohexyl ether TAcHE
(-277.0 ( 0.3) kJ ‚mol-1; for 2-propanol, ∆fH° (l) ) (-317.0
m
( 0.3) kJ ‚mol-1; for sec-butanol, ∆fH° (l) ) (-342.7 ( 0.6)
m
kJ ‚mol-1 were taken from Chao et al.17 Data for propanol
∆fH° (l) ) (-302.5 ( 0.2) kJ ‚mol-1 and for butanol ∆fH°
m
m
(l) ) (-327.0 ( 0.2) kJ ‚mol-1 were taken from Mosselman
and Dekker.18 Data for cyclohexanol, ∆fH° (l) ) (-347.4 (
m
2.2) kJ ‚mol-1, were taken from Seller and Sunner.19 These
data together with the enthalpies of reactions 2-7 were
used to calculate standard enthalpies of formation of the
tert-amyl alkyl ethers in the liquid phase (e.g., for tert-amyl
ethyl ether)
F igu r e 5. Equilibrium constants Kx for xROH > 0.5 of reactions
1-7 as a function of temperature. O, methanol; b, ethanol; 4,
propanol; 2, butanol; 0, 2-propanol; *, sec-butanol; +, cyclohexanol.
When using ion-exchange resins as the catalyst, only
tertiary olefins react in the etherification reaction.15 From
the three methylbutenes, 2-methylbutene-1 reacts fastest,
and 2-methylbutene-2 reacts 2-5 times slower depending
on the alcohol, whereas 3-methylbutene-1 gives no reac-
tion.15 In addition to the etherification, isomerization of the
double bond occurs simultaneously
∆f Hm° (liq, TAEE) ) ∆r H°m(liq, 2) + ∆f H°
+
m(liq, EtOH)
∆f H°m(liq, 2MB2) ) - (372.8 ( 1.6) kJ ‚mol-1
The experimental values obtained of enthalpies of forma-
tion derived in this way are given in the Table 8.
We reported recently the improved Benson’s group
additivity increments to estimate the standard enthalpies
of formation alkyl ethers.13 We have applied these incre-
ments for the calculation of the branched ethers studied
in this work (see Table 8). The calculated values of the
2-methylbutene-1 S 2-methylbutene-2
(9)
and the equilibrium favors more stable 2-methylbutene-
2.15 Indeed, in our previous work, we studied the chemical
equilibrium of reaction 9 simultaneously with the methyl
tert-amyl ether synthesis reaction 1.8 In those works, we
established that equilibrium constants Kx of reaction 9 were
independent of the composition of the equilibrium mixtures
and that both reactions 1 and 9 reach equilibrium simul-
taneously. In those works, we used a mixture of 2-meth-
ylbutene-1 and 2-methylbutene-2 (purchased by Merck)
instead of a technical fraction of isoamylenes, which has
been applied in this work. As a consequence, some impuri-
ties of hydrocarbons C4 were present in the technical
fraction and they are eluted together with the peak of
2-methylbutene-1 during the GC analysis following the
proper calculation of the equilibrium constant for the
isomerization reaction 9 was thwarted. That is why in
Tables 1-6 the mole fraction xMB denotes the sum of the
2-methylbutene-1 and residual hydrocarbons C4 in the
equilibrium mixture and why no thermodynamic calcula-
tions for reaction 9 have been performed in the reactive
systems studied. But because of our earlier observation,8
that etherification and isomerization reactions reach equi-
librium simultaneously, insufficient separation of 2-meth-
ylbutene-1 during the GC analysis had no impact on the
interpretation of the etherification reactions 2-7.
∆fH° (l) are in very close agreement with those derived in
m
this work from chemical equilibrium studies. Such close
agreement is a valuable test to establish validity of group
additivty increments suggested earlier.13
In this work, we performed study of the etherification
reactions of linear and branched alcohols with 2-methyl-
butene-2 systematically and obtained a consistent set of
the reaction enthalpies. Experimental results on equilib-
rium mole fractions and temperature dependencies of
equilibrium constants are important for optimization of
yields of tert-alkyl ethers used as gasoline additives.
Liter a tu r e Cited
(1) Izquierdo, J . F.; Cunill, M.; Vila, M.; Iborra, M.; Tejero, J .
Equilibrium Constants for Methyl tert-Butyl Ether and Ethyl tert-
Butyl Ether Liquid-Phase Synthesis Using C4 Olefinic Cut. Ind.
Eng. Chem. Res. 1994, 33, 2830-2835.
(2) Go´mez, C.; Cunill, F.; Iborra, M.; Izquierdo, F.; Tejero, J .
Experimental Study of the Simultaneous Synthesis of Methyl tert-
Butyl Ether and Ethyl tert-Butyl Ether in Liquid Phase. Ind. Eng.
Chem. Res. 1997, 36, 4756-4762.
(3) Rihko, L. K.; Linnekoski, J . A.; Krause, A. O. Reaction Equilibria
in the Synthesis of 2-Methoxy-2-methylbutane and 2-Ethyoxy-2-
methylbutane in the Liquid Phase. J . Chem. Eng. Data 1994, 39,
700-704.
(4) Kitchaiya, P.; Datta, R. Ethers from Ethanol. 2. Reaction Equi-
libria of Simultaneous tert-Amyl Ethyl Ether Synthesis and
Isoamylene Isomerization. Ind. Eng. Chem. Res. 1995, 34, 1092-
1101.
(5) Zhang, T.; J ensen, K.; Kitchaiya, P.; Phillips, C.; Datta, R. Liquid-
Phase Synthesis of Ethanol-Derived Mixed Tertiary Alkyl Ethyl
Ethers in an Isothermal Integral Packed-Bed Reactor. Ind. Eng.
Chem. Res. 1997, 36, 4586-4594.
(6) Linnekoski, J . A.; Kiviranta-Pa¨a¨kko¨nen, P.; Krause, A. O.; Rihko-
Struckmann, L. K. Simultaneous Isomerization and Etherification
of Isoamylenes. Ind. Eng. Chem. Res. 1999, 38, 4563-4570
(7) Linnekoski, J . A.; Krause, A. O.; Rihko, L. K. Kinetics of the
Heterogeneously Catalysed Formation of tert-Amyl Ethyl Ether.
Ind. Eng. Chem. Res. 1997, 36, 310-316.
Enthalpies of reactions 2-7 derived in this work from
the temperature dependencies of equilibrium constants (see
Table 7) were involved in the calculation of the standard
enthalpies of formation of the tert-amyl alkyl ethers. For
this purpose, standard molar enthalpies of formation,
∆fH° (l), of 2-methylbutene-2 and of alkanols are required.
m
Enthalpy of formation in the liquid phase of 2-methyl-
butene-2, ∆fH° (l) ) (-68.1 ( 1.3) kJ ‚mol-1, was taken
m
from combustion experiments.16 Further experimental data
are also available in the literature: for ethanol, ∆fH° (l) )
m