Kinetics of the synthesis of propyl and butyl acrylates in the presence of some heteropolyacids as catalysts

Article abstract of DOI:10.1002/kin.20358

Esterification reactions of acrylic acid with n-propanol and n-butanol were carried out in the liquid phase in the presence of H₃PW ₁₂O₄₀ or H₃PMo₁₂O₄₀ as a catalyst, at various temperatures, molar ratios of the reactants, and concentrations of the catalyst. The kinetic equations had a nonelementary form.

Full text of DOI:10.1002/kin.20358

Kinetics of the Synthesis of
Propyl and Butyl Acrylates
in the Presence of Some
Heteropolyacids as
Catalysts
JERZY SKRZYPEK,1 TERESA WITCZAK,2 MIROSŁAW GRZESIK,1 MARIUSZ WITCZAK2
1Institute of Chemical Engineering, Polish Academy of Sciences, ul. Bałtycka 5, 44-100 Gliwice, Poland
2Faculty of Food Technology, University of Agriculture, ul. Balicka 122, 30-149 Krako´w, Poland
Received 23 January 2008; revised 4 June 2008; accepted 4 June 2008
DOI 10.1002/kin.20358
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Esterification reactions of acrylic acid with n-propanol and n-butanol were carried
out in the liquid phase in the presence of H3PW12O40 or H3PMo12O40 as a catalyst, at various
temperatures, molar ratios of the reactants, and concentrations of the catalyst. The kinetic
ꢀ
C
equations had a nonelementary form.
12–17, 2009
2008 Wiley Periodicals, Inc. Int J Chem Kinet 41:
tional catalysts of esterification cause to the natural
environment.
INTRODUCTION
While the technological aspects of the esterification
of acrylic acid with propyl and butyl alcohols have
been relatively well understood, the knowledge of the
kinetics and thermodynamics of this process still re-
mains inadequate. There are practically no data on the
kinetics of the synthesis of propyl and butyl acrylate in
the presence of heteropolyacids. Only Chen at al. [1]
suggested that the esterification of acrylic acid with
n-butanol in the presence of heteropolyacid
H3PW12O40 is a second-order reaction. However, their
experimental data are scarce and limited only to the
temperature of 353 K.
The present study aims to determine the ki-
netics of the esterification of acrylic acid with
n-propanol and n-butanol over dodecatungstophos-
phoric acid (H3PW12O40) and dodecamolybdophos-
phoric acid (H3PMo12O40) as catalysts.
Acrylates are important intermediates in the produc-
tion of plastics. Owing to the diverse properties of
polymers and the copolymers obtained on their ba-
sis, they have a wide application in everyday life and
in many industries. Esterification of acrylic acid with
alcohol has commercially been performed by using
liquid catalyst such as sulfuric acid, hydrofluoric acid,
and para-toluenesulfonic acid, but these are toxic and
corrosive. Heteropolyacids are known to be highly ac-
tive acidic catalysts in solution, and so they have been
successfully used in an acid reaction such as esterifica-
tion. Heteropolyacids are less toxic. Replacing mineral
acids with heteropolyacids offers a means for eliminat-
ing or at least reducing the dangers that those conven-
Correspondence to: T. Witczak; e-mail: twitczak@ar.krakow.pl.
c
ꢀ
2008 Wiley Periodicals, Inc.

KINETICS OF THE SYNTHESIS OF PROPYL AND BUTYL ACRYLATES
13
EXPERIMENTAL
RESULTS
Kinetic experiments were carried out in a typical
batch reactor of 100-mL capacity, which was me-
chanically agitated with an impeller and fitted with
a reflux condenser. The temperature was maintained
within 0.5◦C. The mixing speed of about 1000 rpm
was determined to be sufficient to eliminate any mass
transfer limitation. No change in the reaction rate
was detected when the stirrer speed was varied from
550 to 1500 rpm. To initiate an experimental run, a
known amount of alcohol was charged to the reactor
and heated to the desired temperature. Acrylic acid
was added at the desired mole ratio. Next, the cata-
lyst H3PW12O40 or H3PMo12O40 was introduced to
the reaction medium. To avoid side reactions of poly-
merization, hydroquinone was added to the reactor.
Chromatography revealed that the reaction mixture
contained mostly the products of esterification (ester
and water) and only trace amounts of the products
of side reactions. Therefore, it was possible to check
the progress of the reaction by determining the acidity
of the reaction system. The acid number was estab-
lished through the titration of KOH samples. Samples
of the reaction mixture were immediately diluted in
cold 2-propanol, and reaction stopped because of cool-
ing and dilution. The duration of the process and the
frequency of sampling were selected for individual ex-
periments, depending on the conditions of the reaction.
The experiments were conducted till the moment when
minimal changes in the concentrations of the reactants
were noticed.
The esterification of acrylic acid with aliphatic alco-
hol is a typical reversible reaction described by the
following equation:
H2C CH COOH + RꢁOH ⇔ H2C CHCOORꢁ
+ H2O
(1)
Based on the stoichiometric equation, an extended
kinetic equation, in which acrylic acid is a reference
reactant, can be written in the following form:
ꢀ
ꢁ
cEc cWd
Ka
rK = k1 cKa cAb −
(2)
where K and A refer to acrylic acid and alcohol, re-
spectively, and W and E refer to water and one of the
possible esters, respectively.
Then, using the mass balance of the reference con-
stituent in a batch reactor (assuming a constant den-
sity), the following relation is obtained:
ꢀ
ꢁ
dcK
dt
cEc cWd
Ka
−
= k1 cKa cAb −
(3)
If the actual concentration of acrylic acid is represented
by its initial concentration, CK , and degree of conver-
0
sion, αK , Eq. (3) can be written as follows:
ꢀ
ꢀ
ꢁ
b
dαK
dt
nA
0
= cK(a+b−1)k1 (1 − αK )a
− αK
0
The molar ratio of alcohol to acid at the beginning
of the reaction was 3:1, 5:1, and 10:1. The catalyst
concentrations were 1.23, 2.95, 4.92, and 9.84 wt%
for dodecatungstophosphoric acid and 3.12, 6.24, and
12.48 wt% for dodecamolybdophosphoric acid. The
experiments were carried out at several temperatures
from 318 to 368 K for n-propanol and from 343 to
373 K for n-butanol.
nK
0
ꢂ
ꢃ
ꢁ
αK(c+d)
− cKc+d−a−b
(4)
0
Ka
where nA and nK are the initial numbers of moles.
0
0
By integrating Eq. (4) for the different values of
exponents assumed, we tried to find a solution that
Table I The Estimated Activity Coefficients for n-Butanol at a Temperature of 363 K and the Initial Molar Ratio of
Acrylic Acid to Alcohol of 1:3
xK
xA
xE
xW
γK
γA
γE
γW
0.225
0.200
0.175
0.150
0.125
0.100
0.075
0.050
0.025
0.725
0.700
0.675
0.650
0.625
0.600
0.575
0.550
0.525
0.025
0.050
0.075
0.100
0.125
0.150
0.175
0.200
0.225
0.025
0.050
0.075
0.100
0.125
0.150
0.175
0.200
0.225
0.932
0.899
0.866
0.835
0.805
0.776
0.748
0.720
0.694
1.006
1.018
1.030
1.042
1.055
1.067
1.080
1.092
1.105
1.967
1.922
1.879
1.836
1.793
1.751
1.710
1.670
1.630
2.823
2.833
2.841
2.848
2.853
2.858
2.861
2.864
2.866
International Journal of Chemical Kinetics DOI 10.1002/kin

14
SKRZYPEK ET AL.
Table II Calculated Values of the Equilibrium
Constants and Equilibrium Degree of Conversions of
Acrylic Acid at Various Temperatures
ꢀ) was difficult to reach, and the model obtained gave
major errors. Since the chemical reactions under study
took place in the liquid state and the reactants formed
a multicomponent mixture of polar substances, we de-
termined their activity coefficients. The determination
was made by the UNIFAC method, and the necessary
data were derived from [2]. Table I shows examples
of activity coefficients for the reaction of acrylic acid
with butanol at a constant temperature. Because the
order of the values obtained for the other reactions
at various temperatures was similar, we inferred that
the reaction rate was proportional to the activity rather
than the concentration of the reactants. Therefore, the
kinetic model was expressed in terms of both the ac-
tivities and molar concentrations. Because the reaction
system was better described when, was assumed to be
nonideal of the liquid mixture, we replace the concen-
tration of the reactants in Eq. (2) with their activities.
Then, using the mass balance we obtained the relation
T
α∗
n
A0/nB0
Kx
Kγ
Ka
n-Propanol
318
328
338
348
0.775
0.811
0.831
0.849
0.866
1:3
1:3
1:3
1:3
1:3
1.20
1.59
1.88
2.22
2.61
5.70
5.66
5.57
5.48
5.39
6.87
9.00
10.48
12.17
14.07
358
n-Butanol
343
353
0.905
0.925
0.955
1:3
1:3
1:3
2.10
2.80
4.00
7.50
7.36
7.23
15.74
20.62
28.93
363
would ensure the best fit between the reaction rate
equation and the experimental data. To do so, we used
a numerical integration method.
ꢀ
ꢁ
dcK
dt
γEc cEc γWd cWd
Ka
k1t = c(a+b−1)
= k1 γKa cKa γAbcAb −
(6)
K0
−
αK
ꢄ
dαK
ꢅ
ꢆ
ꢀ
ꢁ
where γ is the activity coefficient of the component.
The differential equations (3) and (6) after transfor-
mation were integrated numerically by the Simson’s
method.
Using the equilibrium degrees of conversion, deter-
mined on the basis of the acrylic acid concentration
established experimentally in the state of equilibrium,
αK(c+d)
ꢅ
ꢆ
b
nA0
nK0
(1 − αK )a
− αK − cKc+d−a−b
0
Ka
0
= ꢀ (αK )
(5)
Our numerous calculations showed that a linear re-
lationship ꢀ(αK ) = k1t in the coordinate system (t,
14
12
10
8
6
4
2
0
0
50
100
150
200
250
300
350
Time (min)
ꢀ
Figure 1 Model fitting for the esterification of acrylic acid with n-propanol in the presence of H3PW12O40 ( : T = 348 K
1:3 catalyst 4.92 wt%, ꢁ: T = 358 K 1:3 catalyst 4.92 wt%, + : T = 358 K 1:5 catalyst 4.92 wt%, ×: T = 358 K 1:10 catalyst
4.92 wt%. : T = 358 K 1:3 catalyst 2.95 wt%, ꢂ: T = 368 K 1:3 catalyst 4.92 wt%, ∗: T = 358 K 1:3 catalyst 9.84 wt%).
•
International Journal of Chemical Kinetics DOI 10.1002/kin

KINETICS OF THE SYNTHESIS OF PROPYL AND BUTYL ACRYLATES
15
25
20
15
10
5
0
0
50
100
150
200
250
300
350
400
Time (min)
ꢀ
Figure 2 Model fitting for the esterification of acrylic acid with n-propanol in the presence of H3PMo12O40 ( : T = 348 K
ꢄ
1:3 catalyst 3.12 wt%, : T = 358 K 1:3 catalyst 3.12 wt%, : T = 368 K 1:3 catalyst 3.12 wt%, +: T = 368 K 1:5 catalyst
ꢃ
3.12 wt%, : T = 368 K 1:10 catalyst 3.12 wt%, : T = 368 K 1:5 catalyst 6.24 wt%, ∗: T = 368 K 1:5 catalyst 12.48 wt%).
◦
•
we determined the thermodynamic equilibrium con-
stants Ka from the following equation:
Table II shows the values of the equilibrium de-
grees of conversion for propanol and butanol and the
values of the constants at various temperatures. The
equilibrium constant was expressed as a function of
temperature as follows:
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
ꢁ
aEaW
aK aA
xExW
xK xA
γEγW
γK γA
Ka =
=
(7)
ꢀ
ꢁ
eq
eq
eq
−16700
n-propanol Ka = 3.630 × 103 exp
RT
where a is the activity of the component and x is the
molar fraction of the component.
(8)
16
14
12
10
8
6
4
2
0
0
100
200
300
400
500
600
700
Time (min)
ꢀ
Figure 3 Model fitting for the esterification of acrylic acid with n-butanol in the presence of H3PW12O40 ( : T = 353 K 1:3
catalyst 2.95 wt%, : T = 363 K 1:3 catalyst 2.95 wt%, ꢁ: T = 363 K 1:5 catalyst 2.95 wt%, +: T = 363 K 1:10 catalyst
◦
2.95 wt%, : T = 363 K 1:3 catalyst 1.23 wt%, ꢂ: T = 373 K 1:3 catalyst 2.95 wt%, ∗: T = 363 K 1:3 catalyst 4.92 wt%).
•
International Journal of Chemical Kinetics DOI 10.1002/kin

16
SKRZYPEK ET AL.
35
30
25
20
15
10
5
0
0
50
100
150
200
250
300
350
400
Time (min)
ꢀ
Figure 4 Model fitting for the esterification of acrylic acid with n-butanol in the presence of H3PMo12O40 ( : T = 353 K
1:3 catalyst 3.12 wt%, : T = 363 K 1:3 catalyst 3.12 wt%, ꢁ: T = 363 K 1:5 catalyst 3.12 wt%, +: T = 363 K 1:10 catalyst
◦
3.12 wt%, : T = 373 K 1:3 catalyst 3.12 wt%, : T = 363 K 1:5 catalyst 6.24 wt%, ∗: T = 363 K 1:5 catalyst 12.48 wt%).
ꢃ
•
ꢀ
ꢁ
Table III Kinetic Parameters
−31400
n-butanol Ka = 9.603 × 105 exp
E(J mol−1
)
k0 (m4.5 mol−1.5 min−1
)
RT
Catalyst
(9)
n-Propanol
H3PW12O40
H3PMo12O40 69,400 1000
n-Butanol
H3PW12O40
H3PMo12O40 72,300 800
65,200 600
6.08 × 103
3.26 × 104
We observed that temperature had a stronger effect
on chemical equilibrium in the case of the butyl acry-
late synthesis.
Finally, using Eq. (6), we obtained (in all cases) a
nonelementary form of the kinetic equation:
66,000 400
1.12 × 104
9.82 × 104
ꢀ
ꢁ
Table IV Time Needed to Reach the Degree of
Conversion of 0.6 at a Temperature of 363 K (the Initial
Molar Ratio of Acrylic Acid to Alcohol of 1:3 and the
aK aA
aE0.5
aE0.5aW
Ka
rK = k1 × ccat
−
(10)
Catalyst Concentration of 15 mol m−3
)
Figures 1– 4 show the graphs of the function
ꢀ(αK ) = k1t in the coordinate system (t, ꢀ). As the ex-
perimental points in the graphs were scattered around
a straight line, we assumed that the adopted form of
the kinetic equation was correct. The determining cri-
terion for the form of the equation was the graph of
the function ꢀ(αK ) = k1t for experiments conducted
at the same temperature but at different initial molar
ratios of alcohol to acrylic acid.
Time (min)
H3PW12O40
H3PMo12O40
n-Propanol
n-Butanol
166
128
130
120
CONCLUSIONS
The reaction rate constant was related to the tem-
perature using the Arrhenius equation:
The results of our extensive research provided the basis
for working out kinetic equations and determining the
values of constants appearing in the equations.
As indicated by the catalytic activities of the
heteropolyacids under study, dodecamolybdophos-
phoric acid was more active than dodecatungstophos-
phoric acid both for n-propanol and n-butanol
(Table IV). The time needed to reach the degree of
ꢀ
ꢁ
−E
k1 = k0 exp
(11)
RT
Table III shows the values of the frequency factor k0
and the energy of activation E established by a linear
regression.
International Journal of Chemical Kinetics DOI 10.1002/kin

KINETICS OF THE SYNTHESIS OF PROPYL AND BUTYL ACRYLATES
17
conversion 0.6 for the reactions carried out in the
presence of both catalysts was shorter for n-butanol
(Table IV), which suggests that this reaction proceeded
faster.
ysis). In contrast, for heteropolyacids in this study the
kinetic equation in all cases had a nonelementary form.
BIBLIOGRAPHY
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the kinetics was elementary, whereas for n-propanol
[3] and lower alcohols [5] the reaction was fourth
order (second order in acid and alcohol for esterifi-
cation, and second order in ester and water for hydrol-
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International Journal of Chemical Kinetics DOI 10.1002/kin