Assessment of formation channels of cyclohexyl mono- and dicarboxylates in oxidation of cyclohexane

Article abstract of DOI:10.1134/S1070427209020232

Temperature dependences of the rate constants of the esterification reactions of the main carboxylic acids contained in oxidized cyclohexane (adipic, caproic, and formic) by cyclohexanol in a nonpolar medium were determined by solving the inverse kinetic problem.

Full text of DOI:10.1134/S1070427209020232

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2009, Vol. 82, No. 2, pp. 287−294. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © T.S. Kotel’nikova, O.A. Revkov, S.G. Voronina, A.L. Perkel, 2009, published in Zhurnal Prikladnoi Khimii, 2009, Vol. 82, No. 2,
pp. 293−300.
ORGANIC SYNTHESIS
AND INDUSTRIAL ORGANIC CHEMISTRY
Assessment of Formation Channels of Cyclohexyl
Mono- and Dicarboxylates in Oxidation
of Cyclohexane
T. S. Kotel’nikova, O. A. Revkov, S. G. Voronina, and A. L. Perkel
Kuzbass State Technical University, Kemerovo, Russia
Received July 30, 2008
Abstract—Temperature dependences of the rate constants of the esterification reactions of the main carboxylic
acids contained in oxidized cyclohexane (adipic, caproic, and formic) by cyclohexanol in a nonpolar medium
were determined by solving the inverse kinetic problem.
DOI: 10.1134/S1070427209020232
Esters of carboxylic acids and secondary alcohols are
among the main by-products in oxidation of saturated
hydrocarbons and their oxygen derivatives [1–4]. It was
assumed in the early stages of development of the theory
of liquid-phase oxidation of organic compounds by
molecular oxygen that the main source of formation of
these products is the reaction of esterification of carboxylic
acids by alcohols [1, 5]. Later, the significance of the
acid pathway to esters was questioned [6–8] and it was
shown that reactions of alcoholysis of carboxylic acid
anhydrides by alcohols serve as the main channel in which
the esters are formed [4, 9–12]. In [12], the kinetics of
stages of the reaction of alcoholysis of valeric anhydride
by cyclohexanol in the presence of formic acid was
studied, and this enabled estimation of the formation rate
of cyclohexyl esters via the pathway involving carboxylic
acid anhydrides. To evaluate the role of the esterification
reaction in the process of ester formation, it is necessary
to obtain additional data on the kinetics of interaction of
cyclohexanol with carboxylic acids under the conditions
close to those of oxidation.
EXPERIMENTAL
Methods for purification of formic acid, cyclohexanol,
and o-dichlorobenzene were described in [12]. Caproic
acid of pure grade was dried over anhydrous MgSO4
and distilled in a vacuum in a flow of argon. Adipic acid
of chemically pure grade was used without additional
purification.
The esterification of carboxylic acids with cyclohexanol
in an o-dichlorobenzene solution was performed by
the ampule method. Samples of oxidized cyclohexane
(150–155°C, 0.04% cobalt naphthenate, pressure 0.8 MPa)
were obtained from the caprolactam oxidation shop ofAzot
Open Joint-Stock Company, Kemerovo.
Products of the esterification reactions, esters, were
determined by gas-liquid chromatography (GLC). Prior
to determination by GLC, reaction mixtures containing
monocyclohexyl adipate were treated with diazomethane.
The reaction products were analyzed on a 2000×3 mm
column packed with 5% XE-60 silicone on N-AW
0
.20–0.25 mm Chromaton. Hexadecane served as internal
The aim of the present study was to analyze the kinetics
of interaction between cyclohexanol and the main types
of mono- and dicarboxylic acids contained in oxidized
cyclohexane and to estimate the relative contributions of
the above-mentioned main channels to ester formation in
industrial oxidation of cyclohexane to cyclohexanol and
cyclohexanone.
standard. Water in samples of oxidized cyclohexane was
determined by the Fischer method [13]. Methods for
determination of other products in oxidized cyclohexane
were described in [11].
We calculated the sought-for parameters of kinetic
equations by using software implementing the least-squares
method in the Delphi 5.5 environment. The system of
2
87

288
KOTEL’NIKOVA et al.
differential equations was solved in each step by the Euler
or Runge-Kutta method. The calculation was performed
over the entire body of data for the object under study. This
made it possible to improve the accuracy of determination
of the activation energy and pre-exponential factor [14].
fact that the experimental points obtained by varying the
initial concentrations of formic acid or cyclohexanol can
also be described by equations of the type (2), with the
same values of the constants, confirms the validity of the
chosen kinetic scheme. Even the strongest of unsaturated
aliphatic monocarboxylic acids, formic acid, has nearly no
effect of the rate of the esterification reaction.
In accordance with the task formulated, we chose
caproic, adipic, and formic acids as objects of study for
examining the esterification reaction. The first two acids
are the most important representatives of mono- and
dicarboxylic acids of oxidized cyclohexane [1], and the
last of these imparts specific properties to the esterification
process in oxidation of cyclohexane [11, 12].
In a similar way, we obtained the rate constants of the
stages, activation energies, and pre-exponential factors
for the reaction of esterification of caproic acid (0.14 M)
by cyclohexanol (1.77 M), carried out in the temperature
range 160–190°C (see figure, Table 1).
In the absence of acid catalysts, the esterification of
caproic and formic acid by cyclohexanol occurs by the
reversible reaction
In the reaction of adipic acid with cyclohexanol,
dicyclohexyl adipate is formed in two successive reversible
stages:
k
R1C OH
+
1
HO C(CH ) C OH
R C OC H + H O ,
6
11 2
2
4
+
C H OH
6
11
k_
O
O
O
O
(1)
+
ROH; k+1
1
R –H, C
5
H
11
HO C(CH ) C OR
_
2 4
H O; k_
1
2
and the accumulation kinetics of the corresponding ester
is described by the differential equation
O
O
+
ROH; k+2
1
RO
OR
(4)
C(CH ) C
.
_
2
4
d[R COOR]
1
H O; k_
2
=
k [R COOH][ROH]
2
i+
O
d
t
O
1
−
k [R COOR][H O] ,
(2)
In this case, the kinetics of accumulation of mono- and
dicyclohexyl adipate at ith temperature can be described
by a system of differential equations
i−
2
1
1
where [ROH], [H O], [R COOH], and [R COOR] are the
2
running concentrations of cyclohexanol, water, and the
d[HOOC(CH ) COOR]
2
4
=
k [HOOC(CH ) COOH]
+1 2 4
corresponding carboxylic acid and ester; k and k are the
i+
i−
dt
rate constants of reaction (1) at ith temperature.
×
[ROH]− k [HOOC(CH ) COOR][H O]
−1
2
4
2
On passing to experiments at jth temperature, values
of the corresponding rate constants can be expressed by
the reaction
−
k [HOOC(CH ) COOR][ROH]+ k
+2 2 4
−2
×
[ROOC(CH ) COOR][H O] ,
(5)
2
4
2
E
R
⎛
⎜
⎜
⎝
⎞
⎟
⎟
⎠
1
Ti
1
Tj
ln k j= ln k i
+
−
(3)
d[ROOC(CH ) COOR]
2
4
=
k [HOOC(CH ) COOR]
+2 2 4
dt
×
[ROH]− k [ROOC(CH ) COOR][H O] , (6)
−2 2 4 2
Experiments on interaction of formic acid (0.14 M) with
cyclohexanol (1.77 M) were carried out in the temperature
range 150–180°C in a solution of o-dichlorobenzene. The
values obtained for the cyclohexyl formate concentrations
are presented in the figure. The rate constants of the stages
of reaction (1), activation energies, and pre-exponential
factor were calculated by solving the inverse kinetic
reaction with the use of Eqs. (2) and (3) (Table 1). The
where [HOOC(CH ) COOH], [HOOC(CH ) COOR],
and [ROOC(CH ) COOR] are the running concentrations
of adipic acid and mono- and dicyclohexyl adipates,
respectively.
2 4
2 4
2 4
The reaction of esterification of adipic acid (0.075 M)
by cyclohexanol (1.86 M) was studied in the temperature
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ASSESSMENT OF FORMATION CHANNELS OF CYCLOHEXYL MONO- AND DICARBOXYLATES 289
Table 1. Results of calculation of rate constants, activation energy, and pre-exponential factor for esterification of formic, ca-
proic, and adipic acids by cyclohexanol
5
−1
−1
k × 10 , s , Μ ,
range 160–190°C. The experimental data were used
to calculate by Eqs. (3), (5), and (6) the corresponding
rate constants of the stages, pre-exponential factors, and
activation energies of this reaction (Table 1). The resulting
calculated curves of ester accumulation rather well describe
the experimental values of the concentrations of mono- and
dicyclohexyl adipates (see figure).
×[HOOC(CH ) COOR][ROH] +k−2
2 4
×
[ROOC(CH ) COOR][H O] +k−2
2 4 2
(8)
West = k [HOOC(CH ) COOR][ROH]−
+2
2 4
−
k [ROOH(CH ) COOR][H O].
(9)
−2
2
4
2
It can be seen from the data in Table 1 that the formation
rate constant of cyclohexyl formate substantially exceeds
that of other esters. These results are in good agreement
with the data of [15], according to which the reactivity of
formic acid in the reaction with cyclohexanol is 30 times
that of other aliphatic carboxylic acids.
According to Table 1, the rate constants of the forward
and reverse reactions at 150°C are as follows (s M ):
−1
−1
−5
−5
k = 3.8 ×10 and k = 1.7 × 10 for formic acid; k =
+
−
+
−6
−5
1
4
.85 × 10 and k = 2.65 ×10 for caproic acid; and k =
− +1
−6
−5
−6
.87 × 10 , k = 4.76 × 10 and k = 2.03 ×10 , k =
−1
+2
−2
The kinetic dependences obtained and the data on the
composition of products in industrial samples of oxidized
cyclohexane (Table 2) enabled us to calculate the formation
rates of cyclohexyl formate and cyclohexyl caproate in the
esterification reaction by Eq. (7) and those of mono- and
dicyclohexyl adipates, by Eqs. (8) and (9), respectively:
3.51 × 10−5 for adipic acid. The calculated formation rates
of cyclohexyl formate, caproate, and adipate in samples of
oxidized cyclohexane are listed in Table 3.
As already noted, an alternative to the esterification
reaction pathway to cyclohexyl esters is the alcoholysis
of anhydrides by cyclohexanol. To evaluate the relative
importance of these two channels, it is necessary to
calculate the rate of the alcoholysis reactions. The difficulty
is in that oxidized cyclohexane contains more than ten
products of acid nature [1, 11]. Naturally, mixed and
1
1
West = k [R COOH][ROH]− k [R COOR][H O] ,
(7)
+
−
2
West
=
k [HOOC(CH ) COOH][ROH]
−
k−1
+1
2 4
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 82 No. 2 2009

290
KOTEL’NIKOVA et al.
symmetric anhydrides will be formed in considerably
(a)
greater amounts and there is no way of determining their
individual composition by the existing methods. It is
possible, however, to use the circumstance that, according
to the results of [10, 11], the ratios between the yields of
carboxylic acids and their cyclohexyl esters are close for
all mono- and dicarboxylic acids (except formic acid). If
we assume that mixed anhydrides containing acyls of all
carboxylic acids found in oxidized cyclohexane (except
formic acid) must have reactivities close to those of the
corresponding anhydrides containing the acyl of valeric
acid, then the rate of the alcoholysis reaction can be
estimated using temperature dependences reported in [12]
for the rate constants of the stages of the reaction between
valeric anhydride and cyclohexanol in the presence of
formic acid.
t, min
b)
(
The analytically determined sum of carboxylic
acid anhydrides (Σ[R C(O)OC(O)R ]) in oxidized
i
j
cyclohexane includes anhydrides with different reactivities,
with formic acid {Σ[HC(O)OC(O)R']} and without it
{Σ[R'C(O)OC(O)R']}:
i
j
∑
[R C(O)OC(O)R ] = ∑[HC(O)OC(O)R']
+
∑[R'C(O)OC(O)R'].
(10)
t, min
c)
The ratio between these groups of anhydrides is
determined by the equation for the equilibrium constant
12]:
(
[
∑
[HC(O)OC(O)R']∑[R'COOH]i
K =
,
∑
[R'C(O)OC(O)R'][HCOOH]
(11)
where K is the equilibrium constant (K = k /k = 26.94)
1
−1
[
12]; [HCOOH], concentration of formic acid; and
Σ[R'COOH]I, total content of carboxylic groups without
formic acid.
It is possible to derive from Eq. (11), using Eq. (10), an
expression for calculating the total concentration of mixed
anhydrides including the formic acid residue:
t, min
i
j
∑[R C(O)OC(O)R ]
Experimental values of the concentrations c and the calcu-
lated curves of accumulation of esters in the interaction of (a)
formic, (b) caproic, and (c) adipic acids with cyclohexanol at
various temperatures. (t) Time. T (°C): (a) (1) 150, (2) 160, (3)
∑
[HC(O)OC(O)R'] =
(12)
1
+ ∑[R'COOH] /([HCOOH])
1
0
70, (4) 180, and (5) 180 at c0
= 0.14 M and c0
=
The formation rate of cyclohexyl formate is determined
by the equation
HCOOH
ROH
.88 M, (6) 180 at c0
= 0.067 M and c0 = 1.77 M; (b)
HCOH
ROH
(
1
(
1) 160, (2) 170, (3) 180, and (4) 190; (c) (1, 1') 160, (2, 2')
70, (3, 3') 180, and (4, 4') 190; (1–4) cyclohexyl adipate and
1'–4') dicyclohexyl adipate.
West = k∑[HC(O)OC(O)R'][ROH] ,
(13)
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ASSESSMENT OF FORMATION CHANNELS OF CYCLOHEXYL MONO- AND DICARBOXYLATES 291
Table 2. Effect of the degree of hydrocarbon conversion on the content of some products in samples of cyclohexane oxidized
at 150°C
c
c
where k is the rate constant of alcoholysis of a mixed
anhydride, including the acyl of formic acid, into cyclohexyl
formate, and [ROH] is the cyclohexanol concentration.
expression (12) on the assumption that the concentration
of mixed anhydrides of a carboxylic acid is proportional
to the concentration of its carboxylic groups:
According to the results of [12], the temperature
dependence of the formation rate constant of cyclohexyl
formate by the reaction of alcoholysis of a mixed anhydride,
including the acyl of formic acid, by cyclohexanol has the
form
[
HC(O)OC(O)C H ]
5
11
i
j
∑[R C(O)OC(O)R ]
=
,
2
1
+ ∑[R'COOH] /(K[HCOOH][C H COOH])
(16)
i
5
11
8
9.30 ± 0.02 kJ mol−
1
and Σ[R'C(O)OC(O)C H ] can be calculated by the
equation
ln k = (24.27 ± 0.07 −
.
5
11
(14)
RT
Hence we find the formation rate constant of cyclohexyl
−2
−1
−1
formate, k = 32.2 × 10 s M at 150°C. The calculated
formation rates of cyclohexyl formate in samples of
oxidized cyclohexane are listed in Table 4.
In contrast to that of cyclohexyl formate, the formation
rate W of cyclohexyl caproate is equal to the sum of
alcoholysis rates of formic-caproic anhydride, W ', and
of other mixed anhydrides including the acyl of caproic
acid, W'':
(17)
According to [12], the temperature dependence of the
rate constant of cyclohexyl caproate formation by the
reaction of alcoholysis of a mixed anhydride, including the
acyl of formic acid, by cyclohexanol has the form
W = W '+W '' = k '[HC(O)OC(O)C H ][ROH]
5
11
+
0.5k ''∑[R'C(O)OC(O)C H ][ROH] ,
(15)
67.44 ± 0.06 kJ mol−1
5
11
ln k ' = (14.04 ± 0.18) −
.
RT
(18)
where k' is the rate constant of alcoholysis of formic-
caproic anhydride into cyclohexyl caproate; k'', rate
constant of alcoholysis of a mixed anhydride, including
the acyls of caproic and another carboxylic acid, into
cyclohexyl caproate; [HC(O)OC(O)C H ], concentration
Table 3. Effect of the degree of cyclohexane conversion on
the calculated rates of formation of cyclohexyl esters of car-
boxylic acids by the reaction of esterification in samples of
cyclohexane oxidized at 150°C
5
11
of formic-caproic anhydride; Σ[R'C(O)OC(O)C H ], total
5
11
concentration of other mixed anhydrides including the acyl
of caproic acid; and 0.5, coefficient that accounts for the
fact that, in alcoholysis of a mixed anhydride of caproic
and another carboxylic acid, the formation of cyclohexyl
ester of the latter acid is equiprobable.
cyclohexyl cyclohexyl monocyclo- dicyclohex-
formate
caproate hexyl adipate yl adipate
Theequationnecessaryforcalculatingtheconcentrations
of formic-caproic anhydride can be derived from
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 82 No. 2 2009

292
KOTEL’NIKOVA et al.
Table 4. Effect of the degree of conversion on the calculated rates of formation of cyclohexyl esters of carboxylic acids by the
reactions of alcoholysis of anhydrides by cyclohexanol in samples of cyclohexane oxidized at 150°C
W
Table 5. Equilibrium concentrations ceq and the ratio of the observed concentrations cobs to the equilibrium concentrations of
cyclohexyl formate, caproate, and adipate in samples of cyclohexane oxidized at 150°C
Hence we find the rate constant of cyclohexyl caproate
formation, k' = 5.89 × 10 s M at 150°C.
that at which cyclohexyl caproate and mono-and
dicyclohexyl adipates are produced at all degrees of
cyclohexane conversion. The main contribution to the
formation of these three esters is made by alcoholysis
of mixed anhydrides that do not contain the acyl of
formic acid (Table 4). Comparison of the formation rates
of cyclohexyl esters of these acids by the reactions of
esterification and alcoholysis of anhydrides (Tables 3 and
−3
−1
−1
The temperature dependence of the rate constant
of cyclohexyl caproate formation by the reaction of
alcoholysis of a mixed anhydride, containing the acyl of
another carboxylic acid, by cyclohexanol has the form
[
12]
−1
4
) shows that formation rate of all the cyclohexyl esters
6
8.65 ± 0.22 kJ mol
ln k '' = (14.94 ± 0.62) −
.
(19)
in the industrial process of cyclohexane oxidation by the
esterification reactions is negligible as compared with the
alcoholysis rate of mixed anhydrides. For example, for
cyclohexyl formate, this rate is more than 25 thousand
times lower than the rate of its formation by the reaction
of alcoholysis of mixed anhydrides including a formic
acid residue. For mono- and dicyclohexyl adipates,
the importance of the esterification reaction somewhat
exceeds that for the formate, but in this case too, its
contribution does not exceed 0.2%. It can also be seen
from the data in Tables 3 and 4 that, for these esters, the
ratio between the rates of esterification and alcoholysis
of the anhydrides by cyclohexanol is almost independent
of the degree of cyclohexane conversion. For cyclohexyl
caproate, this ratio varies. This possibly occurs because
RT
Hence we find the rate constant of cyclohexyl caproate
−2
−1
−1
formation, k'' = 1.02 × 10 s M at 150°C.
Using the values obtained for the rate constants and
taking into account the assumptions of their applicability
to other mixed anhydrides of oxidized cyclohexane, we
calculated W ' and W '' by Eqs. (16) and (17) and then, using
Eq. (16), also W for cyclohexyl caproate and mono- and
dicyclohexyl adipates. The results of the calculation are
listed in Table 4.
The data in Table 4 indicate that the rate at which
cyclohexyl formate is formed by the reactions of
alcoholysis of mixed anhydrides substantially exceeds
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ASSESSMENT OF FORMATION CHANNELS OF CYCLOHEXYL MONO- AND DICARBOXYLATES 293
the esterification reaction is close to equilibrium in this
case and, as a result, the calculation accuracy of the
rate becomes poorer. In particular, this is indicated by
the negative value of the formation rate of cyclohexyl
caproate (occurrence of hydrolysis) at a cyclohexane
conversion of 3.2% (Table 3).
(2) The contribution of the reaction of esterification
of mono- and dicarboxylic acids by cyclohexanol to
the formation of cyclohexyl esters in the oxidation
of cyclohexane is less than 0.2% relative to that
from the main channel of their accumulation:
alcoholysis of mixed carboxylic acid anhydrides by
cyclohexanol.
In contrast to the reaction with an anhydride, the reaction
of esterification of an alcohol by an acid is reversible and,
under certain conditions, can make lower the content of
cyclohexyl esters formed by the reaction of alcoholysis of
carboxylic acid anhydrides by alcohols.
ACKNOWLEDGMENTS
The authors are grateful to O.V. Borodina, head of the
quality control department Tokem ProductionAssociation
Limited-Liability Company (Kemerovo) for assistance
in determination of water in oxidized cyclohexane
samples.
Using the running concentrations of cyclohexanol,
water, and the corresponding carboxylic acids in oxidized
cyclohexane (Table 2), we calculated by the formula
k+[ROH][R'COOH]
[
R'COOR] =
i
(20)
k [H O]
−
2
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1
4. Kotel’nikova, T.S., Revkov, O.A., Voronina, S.G., and
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 82 No. 2 2009