Kinetics and mechanism of the catalytic hydration of propylene oxide

Article abstract of DOI:10.1134/S002315841001009X

The kinetics of propylene oxide hydration in the presence of bis(ethane-1,2-diol)molybdate is reported. A mathematical description of PO disappearance and propylene glycol formation is suggested. The most probable scheme for the process is presented. The basic kinetic constants are calculated.

Full text of DOI:10.1134/S002315841001009X

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2010, Vol. 51, No. 1, pp. 50–55. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © R.Z. Shaikhutdinov, L.A. Petukhov, N.V. Sapunov, Kh.E. Kharlampidi, A.A. Petukhov, 2010, published in Kinetika i Kataliz, 2010, Vol. 51, No. 1,
pp. 56 61.
⎯
Kinetics and Mechanism of the Catalytic Hydration
of Propylene Oxide
R. Z. Shaikhutdinova, L. A. Petukhovb, N. V. Sapunovc, Kh. E. Kharlampidib, and A. A. Petukhova
a Nizhnekamsk Institute of Chemical Technology, Nizhnekamsk, 423570 Tatarstan, Russia
b Kazan State Technological University, Kazan, 420015 Tatarstan, Russia
c Mendeleev University of Chemical Technology of Russia, Moscow, 125047 Russia
eꢀmail: sapunovvals@gmail.com
Received July 15, 2008
Abstract—The kinetics of propylene oxide hydration in the presence of bis(ethaneꢀ1,2ꢀdiol)molybdate is
reported. A mathematical description of PO disappearance and propylene glycol formation is suggested. The
most probable scheme for the process is presented. The basic kinetic constants are calculated.
DOI: 10.1134/S002315841001009X
Propylene glycol is a product of largeꢀscale organic was 200 ml. Reaction kinetics was studied under isoꢀ
synthesis. It is used in the production of the environꢀ thermal conditions at 50, 60, and 70°С. The initial PO
mentally safest and least toxic industrial and domestic concentration was varied between 0.42 and 1.45 mol/l;
heat carriers, as well as polyester resins. The synthesis the catalyst concentration, between 0.001 and
of propylene glycol by conventional noncatalytic and 0.007 (gꢀatom Mo)/l. The influence of the reaction
acidꢀcatalyzed methods yields large amounts of diꢀ product MPG, (which was introduced into the reactor
and tripropylene glycols [1].
at concentrations of 1.2 and 2.4 mol/l) on the PO
hydration rate was studied under the same conditions.
Three parallel 1ꢀml samples were taken at certain
time intervals during the reaction. The reaction prodꢀ
ucts were analyzed with a chromatograph. In addition,
the PO content of the reaction mixture was deterꢀ
mined by a standard method. A sample was placed in a
saturated magnesium chloride solution containing
0.02 N hydrochloric acid and was stored for 15 min at
room temperature. The residual hydrochloric acid was
titrated with a KOH solution. The PO conversion in all
entries was 70–95%.
A high selectivity of the epoxide ring opening reacꢀ
tions catalyzed by the molybdenumꢀcontaining comꢀ
pounds has been reported recently [2]. Our prelimiꢀ
nary study demonstrated that the monopropylene glyꢀ
col (MPG) formation selectivity in the presence of
molybdenum glycolates can be as high as 97–99 mol %.
Therefore, investigation of the catalytic activity of the
molybdenumꢀcontaining compounds in the hydration
of olefin oxides is interesting from both theoretical and
practical standpoints.
Here, we report the basic regularities of propylene
oxide (PO) hydration in the presence of a molybdeꢀ
numꢀcontaining catalyst:
RESULTS AND DISCUSSION
Mo6+
H
H
The following investigation strategy was accepted.
(1) At the first stage, the initial PO hydration rates
were measured and a reaction scheme was postulated
based on the mathematical description of the initial
stage of the reaction.
(2) At the second stage, we used nonlinear regresꢀ
sion to obtain a mathematical description of the proꢀ
cess in time (i.e., a soꢀcalled physical model reflecting
the real course of the reaction) and calculated the rate
constant of PO hydration and the constants of some
equilibria observed in the reaction medium.
+ H2O
H2C
C
CH3
H3C
C
CH2
O
OH OH
The occurrence of a single reaction not compliꢀ
cated by other transformations considerably facilitates
the study of hydration details.
EXPERIMENTAL
The starting chemicals were PO (USSR State Stanꢀ
dard GOST 23001ꢀ88), distilled water, and
bis(ethaneꢀ1,2ꢀdiol)molybdate synthesized from
ammonium paramolybdate and ethylene glycol [3].
The hydration of PO was carried out in a batch, magꢀ
netically stirred reactor fitted with a thermometer and
Study of the Reaction by Measuring the Initial Rates
Several series of singleꢀfactor experiments were
a reflux condenser. The volume of the reaction mixture carried out, in which only one process parameter
50

KINETICS AND MECHANISM OF THE CATALYTIC HYDRATION
51
Table 1. Initial PO hydration rates* at a catalyst concentration of [Mo]0 = 0.0043 gꢀat/l
w0
,
mol l–1 min–1
60
50
°
C
°
C
70°C
[PO]0,
mol/l
MPG added, mol/l
MPG added, mol/l
MPG added, mol/l
0.0
1.2
2.4
0.0
1.2
2.4
0.0
1.2
2.4
0.42
0.78
1.45
0.040
0.013
0.006
(0.006)
[0.006]
0.009
0.083
0.034
0.016
(0.014)
[0.015]
0.023
0.212
(0.22)
[0.209]
0.258
0.092
0.053
(0.039)
[0.040]
0.045
(0.01)
[0.011]
0.020
(0.081)
[0.083]
0.096
(0.023)
[0.024]
0.037
(0.087)
[0.087]
0.138
(0.054)
[0.054]
0.089
(0.042)
[0.044]
0.046
(0.015)
[0.016]
0.026
(0.009)
[0.009]
0.013
(0.097)
[0.104]
0.110
(0.036)
[0.039]
0.061
(0.022)
[0.024]
0.042
(0.25)
[0.265]
0.275
(0.12)
[0.12]
0.158
(0.079)
[0.077]
0.105
(0.045)
[0.047]
(0.021)
[0.023]
(0.014)
[0.014]
(0.108)
[0.12]
(0.052)
[0.055]
(0.034)
[0.036]
(0.27)
[0.283]
(0.15)
[0.15]
(0.106)
[0.097]
Note: Data calculated using formula (8) is given in parentheses, and data calculated using formula (15) are in brackets.
(temperature, catalyst concentration, or the initial PO catalyst concentration; i.e., the reaction order with
or MPG concentration) was varied in each series and respect to the catalyst (Cat) was unity.
the other parameters remained unchanged. The
parameter variation ranges are specified above.
be described by the expression
Thus, the initial reaction rate can mathematically
Since the reaction was carried out in excess water,
the water concentration exerted no effect on the
hydration kinetics. Formally this means that the parꢀ
tial order of the reaction with respect to Н2О can be
taken to be zero.
[
] [
]
(1)
w0 = Cat f1 PO .
0
The initial hydration rates at different initial PO
and MPG concentrations in the temperature range
from 50 to 70°С are listed in Table 1. The dependences
of the initial reaction rates on the initial PO concenꢀ
The partial order with respect to the catalyst was tration [PO]0 (Fig. 1a) are nonlinear and appear as
determined at 60°С and catalyst and PO concentraꢀ curves with a plateau. Such curves are typical of almost
tions of 0.001–0.007 gꢀat Mo/l and 1.24 mol/l, respecꢀ all reactions catalyzed by metal complexes [4] and are
tively. An analysis of PO disappearance curves showed described by a homographic function whose denomiꢀ
that the initial reaction rates depended linearly on the nator is usually a polynomial in substrate concentraꢀ
1
1
w
0, mol l− min−
[PО]0/
40
w0, min
0.3
0.2
0.1
3
2
(a)
(b)
30
20
10
1
2
1
3
0
0.5
1.0 1.5
[PО]0, mol/l
0
0.5
1.0
1.5
[PО]0, mol/l
Fig. 1. (a) Dependences of the initial PO hydration rate on the initial PO concentration and (b) the anamorphoses of these depenꢀ
dences at ( ) 50, ( ) 60, and ( ) 70°С
1
2
3
.
KINETICS AND CATALYSIS Vol. 51
No. 1
2010

52
SHAIKHUTDINOV et al.
1
1
w
0, mol l− min−
0.3
1
1
(a)
(b)
0.04
0.02
0.2
2
3
2
0.1
3
0
0.5
1.0
1.5
0
0.5
1.0 1.5
[PО]0, mol/l
Fig. 2. Effect of MPG additions on the initial rate of PO hydration at (a) 50 and (b) 70 °С: (
1) no MPG, (2) [MPG] = 1.2 mol/l,
0
and ( ) [MPG] = 2.4 mol/l.
3
0
tions. In the case considered, this function is f2 [PO]0, [PO]0 by the current PO concentration [PO] using the
where f2 is the soꢀcalled catalyst “complexation” funcꢀ above constants
α'
and
β':
tion. This kind of relationship is due to the equilibrium
formation of complexes of the catalyst with reactants
d PO
PO
[
]
[
]
(5)
−
=
.
and reaction products [4]. Thus, Eq. (1) takes a more
definite form:
dt
α'+β' PO
[
]
However, this attempt was unsuccessful. The calcuꢀ
lated curves coincided with experimental curves only
in the initial region. Beyond this region, they ran far
below the experimental curves. Therefore, substances
inhibiting the hydration of PO appear during the reacꢀ
tion. It is reasonable to assume that MPG is one of
these substances. The addition of MPG to the reaction
medium indeed sharply decreases the initial PO disapꢀ
pearance rate at various [PO]0 values (Table 1). The
inhibiting effect of MPG is illustrated in Fig. 2.
Cat PO
[
][
]
0
(2)
w0 =
.
f PO
[
]
2
0
Indeed, the dependences of [PO]0/
constant catalyst concentration (Fig. 1) between 50
and 70°С are linear and are described by the equation
w
0 on [PO]0 at a
PO
[
]
0
(3)
−
= α'+ β' PO .
0
[
]
w0
This effect of MPG can be explained by the equiꢀ
librium formation of the catalyst complexes with
MPG or mixed complexes including the catalyst, PO,
and MPG. Similar complexes were described earlier
[5–7].
At [Cat] = const, Eq. (2) will appear as
PO
[
]
0
(4)
w0 =
.
α'+ β' PO
[
]
0
The numerical values of the constants
follows: at 50°С = 2 and = 21; at 60°С
and = 8; at 70°С = 0.5 and = 3.4.
α
'
and
β
'
are as
The dependence of the initial PO hydration rate on
[PO]0 somewhat changes upon the addition of MPG.
At comparable [PO]0 and [MPG]0 values, the “curvaꢀ
ture” of the dependence decreases, indicating that the
formation constants of the catalyst complexes with PO
and MPG are approximately equal. Thus, the equaꢀ
tion for the initial rate of PO hydration should have the
following form:
α'
β'
α
' = 1.8
β'
α'
β'
Equation (4) describes the initial rates of the proꢀ
cess and corresponds to the reaction scheme that
assumes the rapid primary formation of a catalyst–
PO complex followed by attack by water on this comꢀ
plex [4]:
Cat PO
[
][
]
K
k(+H O)
2
p
0
(6)
w0 =
.
[Cat] + [PO]
[Cat · PO]
+ [MPG].
[Cat]
f2([PO]0,[MPG]0)
(I)
In order to determine the type of the catalyst complexꢀ
ation function f2, we constructed the plots shown in
Fig. 3. These plots can be fitted to linear functions.
It is likely that the high epoxide ring opening selecꢀ
tivity under the action of water is due to PO coordinaꢀ
tion in the ligand sphere of molybdenum, which is
accessible to attack only by the small molecules of
water.
This means that
tial MPG concentration; i.e.,
[MPG]0:
α
'
and
β
' in Eq. (3) depend on the iniꢀ
α
' = f[MPG]0 and ' =
β
f
We attempted to describe PO disappearance during
the reaction in terms of the differential form of Eq. (4)
obtained by replacing the initial PO concentration
at 50°С:
α
' = 2 + 25[MPG]0,
β' = 21 + 3[MPG]0;
KINETICS AND CATALYSIS Vol. 51
No. 1
2010

KINETICS AND MECHANISM OF THE CATALYTIC HYDRATION
0, min
(a)
53
[PО]0/
15
w
(b)
3
100
3
10
5
2
60
2
1
1
20
0
0
0.5
1.0
1.5
0.5
1.0
1.5
[PО]0, mol/l
Fig. 3. Effect of MPG additions on the dependence of the [PO] /
w ratio on [PO] at (a) 50 and (b) 70°С: (1) no MPG, (2)
0
0 0
[MPG] = 1.2 mol/l, and (3) [MPG] = 2.4 mol/l.
0
0
ence of the [MPG]0[PO]0 term indicates that a mixed
complex can form:
at 60°С:
' = 7 + 1.5[MPG]0;
at 70°С: 0.5 + 2[MPG]0,
' = 3.4 + 1.0[MPG]0.
α
' = 1.8 + 10[MPG]0,
(7)
β
K
pi
[Cat
The rateꢀdetermining step of the process is the
reaction of the complex [Cat PO] with water (reacꢀ
⋅
PO] + [МPG]
[Cat
⋅
PО
⋅
МPG]. (III)
α' =
β
⋅
tion (I)). The formation of complexes via reactions
(II) and (III) and the resulting inhibition of the main
process was called inhibition by reaction products and
“cross” inhibition [4].
After the
α
'
and values were substituted into
β
'
Eq. (4), we obtained a mathematical model for the iniꢀ
tial rates for PO hydration under the conditions examꢀ
ined:
In view of the catalyst balance equation
at 50°С
[Cat]0 = [Cat] +
Кр[Cat][PО]
PO
[
]
+
Ki[Cat][МPG] + KpKpi[PО][МPG],
0
,
w0 =
(9)
2 + 25 MPG + (21 + 3[MPG] ) PO
[
]
[
]
0
0
0
the initial rate of the reaction can be expressed as
at 60°C
w0
PO
[
]
(8)
0
kK [Cat] PO
[
]
(10)
w0 =
,
p
0
0
=
.
1.8 + 10 MPG + (8 + 1.5[MPG] ) PO
[
]
[
]
0
0
0
1 + K MPG + K PO + K K MPG PO
]0 [
0
[
]
[
]
[
]
i
p
i
pi
0
0
at 70°С
At the next stage of the study, we confirmed the forꢀ
mation of all of the above complexes in the reaction
mixture and refined the numerical values of the
parameters in Eqs. (8).
PO
[
]
0
.
w0 =
0.5 + 2 MPG + (3.4 + 1.0[MPG] ) PO
[
]
[
]
0
0
0
The initial rates calculated using Eq. (8) coincide
well with the observed values in all series of experiꢀ
Study of the Reaction by Kinetic Curve Analysis
ments (Table 1). The correlation coefficient is
R =
For this purpose, it was necessary to analyze all
kinetic curves describing PO concentration variation
during the reaction. We attempted to describe the PO
disappearance kinetics by Eq. (8) in differential form.
The replacement of w0 by the corresponding derivative
and the replacement of the initial concentrations
[PO]0 and [MPG]0 by the current concentrations led
to the following equations:
0.99, which indicates that Eq. (8) provides a good fit to
the initial rates of PO hydration on the molybdenum
catalyst under various conditions.
However, the influence of the reaction products on
the PO hydration rate still remains unclear. Evidently,
MPG can exert an inhibiting effect due to the formaꢀ
tion of an equilibrium complex with the catalyst:
K
at 50°С
i
[Cat] + [MPG]
[Cat
⋅
MPG].
(II)
d PO
PO
[
]
[
]
This is indicated by the appearance of a term containꢀ
ing [MPG]0 in the denominator of Eqs. (8). The presꢀ
,
−
=
dt
2 + 25 MPG + (21 + 3[MPG]) PO
[
]
[
]
KINETICS AND CATALYSIS Vol. 51
No. 1
2010

54
SHAIKHUTDINOV et al.
ing values:
at 60°C
PO
at 50°С
β* = –4 + 3([MPG]0 + [PО]0),
α
* = 2 + 25([MPG]0 + [PО]0),
γ
d PO
(11)
[
]
[
]
−
=
,
= –3;
dt
1.8 + 10 MPG + (8 + 1.5[MPG]) PO
[
]
[
]
at 60°С * = 1.8 + 10([МPG]0 + [PО]0),
* = –2 + 1.5([МPG]0 + [PО]0), = –1.5;
at 70°С * = 0.5 + 2([MPG]0 + [PО]0),
* = 1.4 + 1.0([MPG]0 + [PО]0), = –1.0.
α
at 70°С
β
γ
(13)
α
d PO
PO
[
[
]
]
,
]
−
=
β
γ
dt
0.5 + 2 MPG + (3.4 + 1.0[MPG]) PO
[
]
[
Integrating Eq. (12) yields
where [PO] and [MPG] are the current concentraꢀ
tions and [MPG] = [MPG]0 + [PO]0 – [PO].
⎛[PO]0 ⎞
F(α,β,γ) = α*ln
+ β*([PO]0 −[PO])
⎟
⎜
[PO]
(14)
⎝
⎠
Use of standard integration programs resulted in
small and nonsystematic deviations of the calculated
PO concentrations from the experimental values. To
refine the numerical values of the parameters in
Eq. (11), we performed a statistical analysis of its inteꢀ
gral form. Since the current MPG and PO concentraꢀ
tions are linearly interrelated (due to the high selectivꢀ
ity of the process), Eq. (11) can be transformed into
+ γ([PO]20 −[PO]2) − t = 0,
where
t
is the reaction time.
The parameters
α*,
β*, and were determined by
γ
processing experimental data using the leastꢀsquares
method for nonlinear functions. The values correꢀ
sponding to the initial rates were chosen as the initial
approximations. Next, the square deviations of the
[
β
F
*
(
α
*,
β
*, and
γ
)]2 function were minimized and
*,
α
d PO
PO
[
]
[
]
, and
γ
values somewhat differing from those correꢀ
.
(12)
−
=
2
sponding to the initial rates were obtained.
As a result of this optimization, we arrived at the
, and are functions of the iniꢀ following mathematical description of the kinetics of
dt
α* + β* PO + γ PO
[
]
[
]
The parameters
α
*,
β
*
γ
tial PO and MPG concentrations and take the followꢀ the process:
d[PO]
dt
[PO]
at 50°С
at 60°С
at 70°С
,
−
−
−
=
=
=
3.0 + 23[МPG] + (20.9 + 2.3[МPG])[PO]
d[PО]
dt
[PО]
,
(15)
2.2 + 9.2[МPG] + (6.8 + 1.6[МPG])[PО]
d[PО]
dt
[PО]
.
0.96 + 1.5[МPG] + (2.5 + 2.0[МPG])[PО]
8800
The initial rates calculated using Eq. (15) also coinꢀ
cide with the observed values (Table 1). By comparing
Eq. (15) with Eq. (12), we calculated the rate constant
and equilibrium constants for complex formation
(Table 2). The dependences of these constants on the
Ki = 1.1×10−11e l/mol
T
,
Kpi = 1.1×1018e−14200l/mol
T
.
The adequacy of the model was estimated in terms
of the Fischer criterion . The following results were
obtained for a significance level of 0.05:
reaction temperature
sions
T
are described by the expresꢀ
F
kapp = 1.6 ×1014e−11500l2 mol−1 min−1
T
<
,
Fcalcd = 2.95 Ftabl = 19.3
,
5500
which proves that the model is adequate to the experiꢀ
mental results.
Kp = 2.6 ×10−7e l/mol,
T
The analysis of the kinetics of PO hydration cataꢀ
lyzed by molybdenum glycolate indicates a rather
complicated behavior of the process. The main reacꢀ
tion is complicated by the formation of a number of
complexes of the catalyst with PO and MPG and the
mixed complex. In the latter case, both PO and MPG
are present in the ligand sphere of the central Mo
atom. The catalyst–PO complex interacts with water
to form the reaction products via reaction (I). The
high epoxide ring opening selectivity is due to the
coordination of PO in the ligand sphere of molybdeꢀ
Table 2. Constants in Eq. (10)
kapp
,
Kp
l/mol
,
Ki
l/mol
,
Kpi,
l/mol
T, °C
l2 mol–2 min–1
50
60
70
0.05
0.15
0.40
6.97
3.09
2.60
7.70
4.18
1.56
0.10
0.18
1.33
KINETICS AND CATALYSIS Vol. 51
No. 1
2010

KINETICS AND MECHANISM OF THE CATALYTIC HYDRATION
55
Problems of Solvation and Complex Formation in Soluꢀ
tions, Ivanovo, Russia, 1998, abstract H411.
num, which is accessible to attack only by the small
molecules of water. The formation of complexes via
reactions (II) and (III) withdraws part of the catalyst
from the process (inhibition by the reaction products
and “cross” inhibition [4]), thus inhibiting the entire
hydration process.
4. Schmid, R. and Sapunov, V., NonꢀFormal Kinetics: In
Search for Chemical Reaction Pathways, Weinheim:
Chemie, 1982.
5. Losada, M., Nguyen, Ph., and Xu, Yu., J. Phys. Chem.
A
, 2008, vol. 112, p. 5621.
REFERENCES
6. Su, Zh., Wen, Q., and Xu, Yu., J. Mol. Spectrosc., 2006,
vol. 128, p. 6755.
1. US Patent 2006025 637 A1, 2006.
2. Gil’manov, Kh.Kh., Khim. Tekhnol., 2006, no. 9, p. 24.
3. Galiev, R.G., Rzhevskaya, N.N., Petukhov, A.A.,
7. Faller, T.H., Csanady, Gy.A., Kreuzer, P.E., Baur, C.M.,
and Filser, J.G., Toxicol. Appl. Pharmacol., 2001,
vol. 172, p. 62.
Belyaev, S.P., and Yablonskiy, O.P., VII Int. Conf. on the
KINETICS AND CATALYSIS Vol. 51
No. 1
2010