World Academy of Science, Engineering and Technology
International Journal of Chemical and Molecular Engineering
Vol:4, No:6, 2010
Comparative Kinetic Study on Alkylation of
p-cresol with tert-butyl Alcohol using Different
SO3-H functionalized Ionic Liquid Catalysts
International Science Index, Chemical and Molecular Engineering Vol:4, No:6, 2010 waset.org/Publication/6235
Pandian Elavarasan, Kishore Kondamudi, Sreedevi Upadhyayula
Abstract—Ionic liquids are well known as green solvents,
reaction media and catalysis. Here, three different sulfonic acid
functional ionic liquids prepared in the laboratory are used as
catalysts in alkylation of p-cresol with tert-butyl alcohol. The
kinetics on each of the catalysts was compared and a kinetic model
was developed based on the product distribution over these catalysts.
The kinetic parameters were estimated using Marquadt’s algorithm to
minimize the error function. The Arrhenius plots show a curvature
which is best interpreted by the extended Arrhenius equation.
Keywords—Alkylation, p-cresol, tert-butyl alcohol, kinetics,
activation parameter, extended Arrhenius equation.
A
I. INTRODUCTION
OF phenols with tert-butyl alcohol is an
industrial important reaction for production of fine
chemicals and anti-oxidants. In particular, alkylation of pcresol with tert-butyl alcohol is an important product called
BHT (butylated hydroxyl toluene), it has many industrial
applications, namely, as antioxidants in food industry as well
as in jet fuels, petroleum products, cosmetics,
pharmaceuticals, rubber, and embalming fluid, antiseptic,
polymerization inhibitor and UV absorber [1-4]. Investigation
of both homogeneous and heterogeneous catalysts for this
typical Friedel-Crafts reaction resulted in different
selectivities and activities based mainly on the acidity of the
catalysts used. Catalysts used for the production of alkylated
p-cresols include Lewis acids (AlCl3, FeCl3 and ZnCl2) [5],
Bronsted acids (H3PO4, H2SO4, HF, HClO4) [6], cationexchanged resins [7], mesoporous materials [8], zeolites [9],
sulfated zirconia [10], heteropolyacids [11] and also ionic
liquids. The liquid acid catalysts cause equipment corrosion
and environmental pollution while solid acids deactivate
rapidly. Although cation-exchanged resins showed promise,
thermal stability and fouling of the resins pose major problems
for their commercialization [7].
LKYLATION
Sreedevi Upadhyayula is with the Indian Institute of Technology Delhi,
New Delhi, 110016 India (corresponding author to provide phone: +91-1126591083; fax: +91-11-26591120; e-mail: sreedevi@chemical.iitd.ac.in).
Pandian Elavarasan is with the Indian Institute of Technology Delhi, New
Delhi, 110016 India. On leave from Department of Chemical Engineering,
Annamalai
University,
Annamalai
nagar,
608002
india(e-mail:
arasu2@gmail.com.).
Kishore Kondamudi is with the Indian Institute of Technology Delhi, New
Delhi, 110016 India (e-mail: withkishore@gmail.com).
International Scholarly and Scientific Research & Innovation 4(6) 2010
Last decade, there has been an increasing interest in
developing catalytic processes with minimum environmental
threats and maximum economic benefits. Room temperature
ionic liquids are finding growing applications as alternative
reaction media for organic transformations and separations.
They possess important attributes, such as negligible vapor
pressure, excellent chemical and thermal stability, potential
recoverability and ease of separation of products from
reactants [12, 13]. Bronsted acidic ionic liquids as novel
benign catalysts have been reported for similar acid catalyzed
reactions [14-16]. Here, we report the comparative alkylation
of p-cresol with tert-butyl alcohol using acidic ionic liquid
catalysts which gave high p-cresol conversion and selectivity
to monoalkylated product. A kinetic model was developed
and the rate parameters estimated and the extended Arrhenius
equation was used to interpret the kinetics of this reaction
well.
II. EXPERIMENTAL
A. Catalyst Preparation
SO3H-functionalized Bronsted acidic ionic liquids were
prepared in the laboratory following the procedure outlined in
literature [14-16]. N-methyl imidazole, pyridine, triethylamine
and 1,4–butane sultone were purchased from Sigma Aldrich
Chemicals Pvt. Ltd, India. p-cresol and tert-butanol were
purchased from Ranbaxy Ltd., New Delhi, India. In a typical
ionic liquid preparation procedure, N-methyl imidazole was
mixed with 1, 4-butane-sultone stirring at 313-353 K for 1224 hours. After solidification, the zwitterions mass was
washed three times with ethyl ether and then dried under
vacuum (393 K, 0.01 Torr). Stoichiometric amount of sulfuric
acid was then added to the precursor zwitterions. The mixture
was stirred at 353 K for 8h to obtain the ionic liquid. The
structures of the prepared ionic liquid are shown in Fig. 1. All
the chemicals were research grade and were used without
further purification unless otherwise stated.
B. Activity testing
The activity testing of the ionic liquids catalysts in this
reaction was carried out in a 30 ml stainless steel autoclave
lined with a Teflon bomb and equipped with a magnetic stirrer
under autogeneous pressure. A typical batch consisted of 10
384
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World Academy of Science, Engineering and Technology
International Journal of Chemical and Molecular Engineering
Vol:4, No:6, 2010
mmol each of p-cresol, TBA and ionic liquid. Reaction
temperature was maintained at 70°C and reaction mixture was
stirred for 8 hours. A qualitative product analysis was
conducted with a GC-MS and quantitative analyses were
conducted with a NUCON GC supplied by AIMIL India Ltd.
using a CHROMSORB-WHP (2 m x 3.175 mm x 2 mm)
column and flame ionization detector.
International Science Index, Chemical and Molecular Engineering Vol:4, No:6, 2010 waset.org/Publication/6235
III. RESULTS AND DISCUSSION
A. Comparison of activity of the ionic liquids catalysts
The activity of the prepared ionic liquid catalysts was
compared in this reaction and shown in Fig. 2. From the
figure, it is clear that all ionic liquid catalysts gave high pcresol conversion at a temperature as low as 343K. During
reaction phase separation into phases, an IL rich phase and a
product rich phase was observed. Phase separation is because
cause of lipophilic nature of products, water being one of the
byproducts. Table 1 shows the product distribution over the IL
catalysts. The influence of reaction time on conversion of pcresol and the selectivity towards TBC & DTBC was
investigated in the range of 323-363K and different reactant
mole ratios. Fig. 2 shows the conversion of p-cresol at 343 K,
1:1 molar ratio of p-cresol / TBA and 1:1 molar ratio of ILs /
p-cresol. The alkylation of p-cresol with TBA in the ionic
liquid shows high selectively to TBC and 2, 6-di-tert-butyl-3methyl phenol (DTBC). With increase in reaction time, the
conversion of p-cresol and selective yield of TBC increased
rapidly and reached a steady value after 8 h. All three ionic
liquid catalysts, showed 80% of p-cresol conversion and 90%
selectivity to TBC. In all the cases, it was observed that 90%
of p-cresol conversion was achieved within the first hour of
the reaction time. The increase in the selectivity to TBC led to
decrease in selectivity to DTBC with time but the combined
selectivities of TBC & DTBC almost remained constant over
the time period investigated.
B. Reaction mechanism
Based on the experimental results, detailed reaction
mechanism has been proposed in fig. 3. The reactant added to
the system, TBA instantaneously participates in hydrolysis to
form tert-butylium (carbocation) and iso-butylene. The tertbutylium (carbocation)/iso-butylene react with p-cresol which
gives the mono-alkylated (TBC) products. There is no
oligomerization reaction occurring due to solvent activity of
ionic liquids in reaction temperature. The intermediate Oalkylation may be possible, but it was not detected throughout
the reaction. Further, realkylation of mono-alkylated products
to di-alkylated (DTBC) products was observed.
temperatures. Detailed reaction mechanism of tert-butylation
of p-cresol using functional ionic liquid catalyst was studied.
The system can be described by the reaction scheme given in
figure 8. Based on the product distribution, the reaction
mechanism was formulated and the kinetic model was
developed based on the following assumptions:
1. Formation of tert-butylium/iso-butene gas is negligible due
to fast hydrolysis of TBA.
2. The amount of TBA and water calculated from mass
balance of the major products.
3. tert-butylation of p-cresol is considered as irreversible.
4. TBC to DTBC reaction is considered as reversible.
5. There is no consideration of intermediate formations (Oalkylation).
B. Batch reaction kinetic model
The mechanism of the reaction is detailed in fig. 3. From
this reaction mechanism, a second-order rate equation is
formulated. The intrinsic kinetics of phase separation reaction
in the liquid phase is extremely difficult to explain, hence,
here the activity co-efficients of the reactants and products
were assumed to be unity and the rate constant estimated. The
rate of formation of different components can be expressed as
follows:
Alkylation of p-cresol
dCC
= −k1 * CC * CTBA
dt
(1)
Rate of conversion of tert-butyl alcohol
dCTBA
= −k1 * CC * CTBA − k2 * CTBC * CTBA + k3 * CDTBC * CW
dt
dCTBC
= k1 * CC * CTBA − k2 * CTBC * CTBA + k3 * CDTBC
dt
A. Kinetic modeling
The kinetic runs were carried out at five different
temperatures, 333, 343, 353, 363 and 363 K respectively in a
batch reactor at autogeneous pressure. Table 3 shows the final
concentration of reactants and products at various
International Scholarly and Scientific Research & Innovation 4(6) 2010
(3)
Rate of formation of 2, 6-DTBC
dCDTBC
= k2 * CTBC * CTBA − k3 * CDTBC * CW
dt
(4)
Rate of formation of water
dCW
= k1 * CC * CTBA + k2 * CTBC * CTBA − k3 * CDTBC * CW
dt
(5)
Where, C is the concentration of respective components
mol / L, t is the batch reaction time in sec, k is the rate
constant of respective reaction in L / mol-1 sec-1. A nonlinear
regression algorithm was used for parameter estimation for the
above batch model eq. (1-5). A software for parameter
estimation in dynamic model algorithm was followed as given
in fig. 4 [17]. The optimum values of the parameters were
estimated by minimizing the objective function given by
n
IV. KINETIC MODELING
(2)
Rate of formation of 2-TBC
(
)
f = ∑ ⎡⎢ x pred − ( xexp ) ⎤⎥
⎣
i
i⎦
2
(6)
Batch reaction rate constants were optimized shown in table
4. The standard error of estimates for the rate reaction from
concentration of reactant and products was of the order of 10-5
given by equation (6). The experimental and the predicted
yield of reactants and products were plotted in fig. 5 at 343 K
i =1
385
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World Academy of Science, Engineering and Technology
International Journal of Chemical and Molecular Engineering
Vol:4, No:6, 2010
for IL-2. It shows that the proposed reaction rate expression
predicts the alkylation values comparable with the
experimental ones. Similarly, all rate constant were estimated
and reported in table 2.
Evaluated kinetic rate constants at various temperatures
were used to determine the activation energy and frequency
factor using Arrhenius relationships as shown in fig. 10 & 11.
The activation energy and frequency factor from fig. 10 and
11 are calculated to be 14.46 kcal/mol and 5.19 x 106 for IL-2
and 13.65 kcal/mol and 1.87 x 106 for IL-3 in the temperature
range 323 – 363 K respectively. The activation energy values
for various reactions compare well with the values for same
reactions in IL-1. The activation energy and frequency factor
from previous data 15.63 kcal/mol and 2.65*107 in the
temperature range 323 – 363 respectively [14]. As compared
to the batch alkylation of p-cresol using IL-1 catalysts, the
activation energy using IL-2 and IL-3 are low showing that
the reaction is intrinsically kinetic controlled. The activation
energy of the system decreases from IL-1 < IL-2 < IL-3 which
may be attributed to the properties of the IL due to change in
organic cations which vary the physical properties of ILs [12,
13]. The plots for the rate constants k2 and k3 show a
curvature, this deviation from the Arrhenius relation is
attributed to heat and mass transfer resistances at low
temperature and also the solvolysis effect in the reaction [18].
In order to incorporate these effects, a modified empirical
form of Arrhenius equation is applied as given below
ln k = A+B (1/T-1/T0 ) + ε
TABLE I PERCENTAGE YIELD OF REACTANTS AND PRODUCTS IN
ALKYLATION OF P-CRESOL WITH TBA AT VARIOUS TEMPERATURES USING IL-2
AND IL-3 AS CATALYSTS
Temp.
K
IL-2
323
333
343
353
363
IL-3
323
333
343
353
363
p-cresol
TBA
TBC
DTBC
water
65.7
77.6
85.1
87.1
88.1
34
22
14
12
11
28
15
5
1
2
54
71
78
81
83
11
7
9
9
7
77
85
95
99
98
65.7
77.6
85.1
87.1
88.1
36
24
20
18
18
29
21
11
10
9
50
66
71
72
74
10
6
8
9
7
71
79
90
90
91
ALKYLATION OF P-CRESOL WITH TERT-BUTYL ALCOHOL IN IL-1 AND IL-2
CATALYSTS
(9a)
ΔH= -R ( B+T0 )
(9b)
ΔS=R ( A-ln ( k B T0 / h ) + ΔH/RT )
(9c)
The empirical Arrhenius equation for the temperature
dependent rate constant and its interpretation by the transition
state theory using the parameter A and B was reported in
literature for similar reactions [41]. The empirical Arrhenius
equation given by the linear relationship with the error
function ‘ε’ in eq. (7) and (8) interpret the reaction rate
constant in terms of transition state theory. Linear regression
analysis was used to estimate the parameters A, B, activation
energy, enthalpy of activation and entropy of activation
calculated from the eq. (9) and valid only in an interval around
the temperature T0 K. The parameters, enthalpy of activation
and entropy change during activation are valid only in the
transition state theory and the values are given in table 3.
Extended Arrhenius equation
ln k = A'+B' (1/T-1/T0 ) + C ' (1/T-1/T0 ) + ε
2
)
ΔC p = R C'/T02 − 1
ΔH= -R ( B'+T0 )
ΔS=R ( A'-ln ( k B T0 / h ) + ΔH/RT )
(10)
(11a)
(11b)
(11c)
Most kinetic data can be adequately described by the
International Scholarly and Scientific Research & Innovation 4(6) 2010
yield
APPARENT ACTIVATION ENERGIES AND PRE-EXPONENTIAL FACTORS FOR
(8)
E a = − RB
conversion of
p-cresol
TABLE II OPTIMIZED PARAMETERS OF SECOND-ORDER RATE CONSTANTS,
(7)
ln k = ln ( kT / h ) - ΔH/RT + ΔS/R
(
empirical Arrhenius equation. In this case, the enthalpy and
entropy of activation given in table 5 suggest that the extended
Arrhenius equation needs to be used to interpret the kinetics
better. The extended Arrhenius equation is in the form of
second-order quadratic expression (10). Non-linear regression
analysis was used to solve for parameters A', B' and C' with
the error estimate in the range of 10-5. The activation
parameter, enthalpy change and the entropy change were
calculated from eq. (11c) and the values are given in table 4.
Temperature
ºC
Rate constant (L / mol s)
k1 ( x 10-4)
k2 ( x 10-4)
k3 ( x 10-3)
IL-2
363
353
343
333
323
Activation energy
(Ea), kcal/mol
Pre-exponential factor
IL-3
363
353
343
333
323
Activation energy
(Ea), kcal/mol
Pre-exponential factor
386
0.86
1.60
3.29
6.67
9.29
14.41
5.19 x 10
6
0.98
2.02
4.17
7.49
9.36
13.65
1.87 x 106
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93.00
5.76
1.24
5.23
11.24
16.05
2.04
0.18
0.29
0.98
-
-
31.85
4.08
0.89
4.18
6.64
53.84
10.97
0.82
1.13
3.37
-
-
World Academy of Science, Engineering and Technology
International Journal of Chemical and Molecular Engineering
Vol:4, No:6, 2010
TABLE III ESTIMATED PARAMETERS IN EMPIRICAL ARRHENIUS EQUATION
AT T0 = 70°C
reaction rate constants
k1
k2
k3
A
-5.44
-5.05
-4.94
B (103)
-7.28
14.46
13.8
-29.5
5.41
-10.7
-11.4
-102.2
9.18
-18.2
-18.9
-123.8
-5.31
-6.87
13.64
13.0
-31.6
-5.48
3.93
-7.81
-8.50
-94.5
-5.85
9.45
-18.82
-19.51
-127.1
IL-2
Ea, kal/mol
ΔH, kcal
ΔS, cal
International Science Index, Chemical and Molecular Engineering Vol:4, No:6, 2010 waset.org/Publication/6235
IL-3
A
B (103)
Ea, kcal/mol
ΔH, kcal
ΔS, cal
Fig. 2: Effect of reaction time on ionic liquids. Reaction
conditions: p-cresol (10 mmol): TBA: IL molar ratio of (1:1:1), 343 k
and autogeneous pressure at 800 rpm.
TABLE IV ESTIMATED PARAMETERS IN EXTENDED ARRHENIUS EQUATION
AT T0 = 70°C
Reaction rate constant
k2
k3
-6.21
-4.82
9.83
-5.9
-0.11
8.93
1.6
1.45
8.84
-45.5
-0.48
-71.85
-6.41
-4.33
7.94
-6.75
1.44
7.70
1.30
1.263
7.89
-48.67
-3.54
-82.18
IL-1
A'
B' (103)
C' (107)
ΔC p , kcal/mol K
ΔH , kcal
ΔS , cal
IL-3
A'
B' (103)
C' (107)
ΔC p , kcal/mol K
ΔH , kcal
ΔS , cal
Fig. 3: Possible reaction mechanism for alkylation of p-cresol with
tert-butyl alcohol in ionic liquids.
Initial guess of parameters (x)
Integration of dynamic model
Model solver
Calculation of objective function (f(x))
Experimental data
Optimum
objective
Yes
Routine complete
No
Fig. 1: Structure of ionic liquids.
Return to algorithm to calculate new (x)
Fig. 4: Iteration algorithm of parameter estimation.
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387
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International Journal of Chemical and Molecular Engineering
Vol:4, No:6, 2010
V. CONCLUSION
The kinetics of alkylation of p-cresol with tert-butyl alcohol
using different ionic liquids in a batch reactor was studied. All
the three ionic liquids gave more than 80 % p-cresol
conversion and 90% selectivity to TBC. Although, the pcresol conversion was almost similar in all ionic liquids, the
activation energy varied due to change in the physicochemical
properties of ILs. The kinetics of the reaction are well
interpreted using the extended Arrhenius equation. The
activation parameter, enthalpy of activation and entropy
changes were estimated for the reaction in IL-2 and IL-3. The
experimental yields of the products match well with model
predicted yield suggesting that the reaction rate model is
appropriate.
Fig. 5: Comparison of experimental and predicted product yields for
alkylation of p-cresol with tert-butyl alcohol using IL-2 at 343 K.
Symbol
CC
CDTBC
CTBA
CTBC
CW
Ea
k1, k2 and k3
-1
1/T (k )
0.0027 0.0028 0.0028 0.0029 0.0029
0.003
0.003 0.0031 0.0031 0.0032
-1.5
k1
-2.5
A', B' & C'
k2
ln (k )
k3
Quantity
yield of p-cresol
yield of 2,6-di-tert-butyl-p-cresol
yield of tert-butyl alcohol
yield of 2-tert-butyl-p-cresol
yield of water
activation energy.
second-order rate constant.
parameters in expansion of Arrhenius
equation
-3.5
ΔCp
activation parameter,
-4.5
ΔH
ΔS
enthalpy of activation,
entropy of activation,
parameters in empirical Arrhenius
equation
error function
Boltzmann’s constant
Planck’s constant
butylated hydroxytoluene
2,6-di-tert-butyl-p-cresol
Ionic liquids
N-(4-sulfonic acid)butyl
triethylammonium hydrogen sulfate
1-(4-sulfonic acid) butyl pyridinium
hydrogen sulfate
1-(4-sulfonic acid) butyl-3-methyl
imidazolium hydrogen sulfate
tert-butyl alcohol
2-tert-butyl-p-cresol
experimental yield
predicted yield
A&B
ε
-5.5
y = -7277.3x + 15.463
2
R = 0.9917
kB
h
BHT
DTBC
ILs
-6.5
-7.5
IL-1
Fig. 6: Arrhenius plot for alkylation of p-cresol with tert-butyl
alcohol using IL-2 catalyst.
0.0027
0.00275
0.0028
0.00285
1/T (K-1)
0.0029 0.00295
IL-2
IL-3
0.003
0.00305
0.0031
0.00315
TBA
TBC
xexp
xpred
-2
-3
k1
Unit
kcal/mol
L/mol s
kcal/mol
K
kcal
cal
k2
-4
ln (k )
International Science Index, Chemical and Molecular Engineering Vol:4, No:6, 2010 waset.org/Publication/6235
NOMENCLATURE
k3
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y = -6873.8x + 14.429
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Fig. 7: Arrhenius plot for alkylation of p-cresol with tert-butyl
alcohol using IL-3 catalyst.
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Pandian Elavarasan received his master’s degree in Chemical engineering,
Annamalai University, Annamalai nagar, India in 2005, and is presently
pursuing Ph. D. degree at Department of Chemical Engineering, Indian
Institute of Technology Delhi, New Delhi, India. He is lecturer in the
Department of Chemical Engineering, Annamalai Unversity, Annamalai
nagar, India. He has worked on mass transfer on waste water treatment, ionic
liquids, catalysis and reaction engineering.
Kishore Kondamudi received his master’s degree in Chemical
engineering with Process Engineering and Design, Indian Institute of
Technology Delhi, New Delhi, India in 2008, and is presently pursuing Ph. D.
degree at Department of Chemical Engineering, Indian Institute of
Technology Delhi, New Delhi, India. He has worked on Catalysis and reaction
enginnering, ect.
Dr. Sreedevi Upadhyayula received her master’s degree in Chemical
engineering with Petroleum Refinery Engineering and Petrochemical
Technology as specialization, Indian Institute of Technology, Kharagpur,
India in 1993, and received Ph. D. degree NCL, Pune & Indian Institute of
Technology, Kharagpur, India in 2003. presently she is is an Assistant
Professor in the Department of Chemical Engineering, Indian Institute of
Technology Delhi, New Delhi, India. She was Assistant Professor,
Department of Chemical Engineering, IIT Kharagpur, West Bengal, India in
2004-2006. She has worked on rational design of novel catalytic materials and
catalytic conversion technologies, solid acid catalysis, ionic liquids and
reaction engineering, ect.
International Scholarly and Scientific Research & Innovation 4(6) 2010
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