Highly efficient alkylation of phenol with tert-butyl alcohol using environmentally benign Bronsted acidic ionic liquids

Article abstract of DOI:10.1016/j.apcata.2015.08.031

A series of SO₃-H functionalized environmentally benign Bronsted acidic ionic liquids were prepared using triethylamine, 1-methyl imidazole, pyridine, diethanolamine, 1,4-butane sultone and methane sulfonic acid as the starting materials. These synthesized ionic liquid catalysts were characterized using spectroscopic tools like ¹H NMR, ¹³C NMR. The catalytic activity of the synthesized ionic liquids was investigated in tert-butylation of phenol with tert-butyl alcohol. The effect of kinetic parameters like reaction time, reaction temperature, reactant mole ratio, reactant to catalyst mole ratio, on the conversion of phenol and selectivity to 2-tert-butyl-phenol and 2,4-di-tert-butyl-phenol were investigated. Phenol conversion upto 94.2% was achieved at temperature as low as 80 °C and low alcohol to phenol molar ratio of 2:1, ionic liquid to phenol ratio of 1:1 and reaction time of 2 h. The catalyst was found to retain its activity even after 8 recycles and was completely recoverable. The plausible reaction mechanism was formulated based on product distribution. The kinetic data was found to agree well with classical phenomenological models and the kinetic parameters were estimated. The experimental results were further validated by Density Functional Theory (DFT) optimizations of the ionic liquid structures performed at the B3LYP/6-311G??level using Gaussian 09 package.

Full text of DOI:10.1016/j.apcata.2015.08.031

Accepted Manuscript
Title: Highly efficient alkylation of phenol with tert-butyl
alcohol using environmentally benign Bronsted acidic ionic
liquids
Author: Tanmoy Patra Salahuddin Ahamad Sreedevi
Upadhyayula
PII:
S0926-860X(15)30126-5
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2015.08.031
APCATA 15525
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
27-3-2015
22-8-2015
25-8-2015
Please cite this article as: Tanmoy Patra, Salahuddin Ahamad, Sreedevi
Upadhyayula, Highly efficient alkylation of phenol with tert-butyl alcohol using
environmentally benign Bronsted acidic ionic liquids, Applied Catalysis A, General
http://dx.doi.org/10.1016/j.apcata.2015.08.031
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1 Highly efficient alkylation of phenol with tert-butyl alcohol using
2 environmentally benign Bronsted acidic ionic liquids
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Tanmoy Patra, Salahuddin Ahamad, Sreedevi Upadhyayula*
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Department of Chemical Engineering, Indian Institute of Technology Delhi, Hauz Khas,
New Delhi–110016, India.
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*Corresponding author. Tel. No.: +91-11-26591083, fax: +91-11-26591120,
E-mail: sreedevi@chemical.iitd.ac.in
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Graphical abstract
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Highlights
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
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Phenol alkylation using Bronsted acidic ionic liquids as catalyst
Kinetics study on phenol alkylation using synthesized ionic liquids
High phenol conversion (94.2%) obtained at 80ꢀ°C within 2ꢀhours of reaction time
DFT studies to correlate acidity and catalytic activity trends of the ionic liquids
Estimation of activation energy (34.8ꢀkJ mol−1) from kinetic modeling
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ABSTRACT
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A series of SO3-H functionalized environmentally benign Bronsted acidic ionic liquids were
prepared using triethylamine, 1-methyl imidazole, pyridine, diethanolamine, 1, 4–butane sultone
and methane sulfonic acid as the starting materials. These synthesized ionic liquid catalysts were
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characterized using spectroscopic tools like H NMR, C NMR. The catalytic activity of the
synthesized ionic liquids was investigated in tert-butylation of phenol with tert-butyl alcohol. The
effect of kinetic parameters like reaction time, reaction temperature, reactant mole ratio, reactant
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to catalyst mole ratio, on the conversion of phenol and selectivity to 2-tert-butyl-phenol and 2,4-
di-tert-butyl-phenol were investigated. Phenol conversion upto 94.2% was achieved at temperature
as low as 80°C and low alcohol to phenol molar ratio of 2:1, ionic liquid to phenol ratio of 1:1 and
reaction time of 2 hours. The catalyst was found to retain its activity even after 8 recycles and was
completely recoverable. The plausible reaction mechanism was formulated based on product
distribution. The kinetic data was found to agree well with classical phenomenological models and
the kinetic parameters were estimated. The experimental results were further validated by Density
Functional Theory (DFT) optimizations of the ionic liquid structures performed at the B3LYP/6-
311G** level using Gaussian 09 package.
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Keywords: Ionic liquids; Alkylation; phenol; tert-butyl alcohol; tert-butylation; 2,4-di-tert-butyl
phenol; Density Functional Theory
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1. Introduction
Alkylation of phenol with tert-butyl alcohol (TBA) is an industrially important Friedel-Crafts
reaction for the production of the fine chemicals, mainly 2-tert-butyl-phenol (2-TBP) and 2, 4-di-
tert-butyl-phenol (2, 4-DTBP) [1]. All the products of this reaction are important lipophilic phenols
used in the production of antioxidants in food industry. These tert-butylated phenols are also used
as additives in jet fuels, petroleum products, cosmetics, pharmaceuticals, rubber manufacturing,
varnishes, printing inks and also as polymerization inhibitors, UV absorbers, antiseptic and
embalming fluids [1-12]. Several investigations on the tert-butylation of phenol using both
homogeneous and heterogeneous catalysts, Lewis acids like AlCl3 and BF3 [2], Bronsted acids like
HF, H3PO4, H2SO4, and HClO3 [3], mesoporous materials [4, 5], near critical as well as super
critical water [6], cation-exchange resins [7], zeolites [7, 8], heteropoly acids [5], solid sulfanilic
acids [9] are reported till date. However, most of the homogeneous liquid acid catalysts are
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environmentally hazardous due to their corrosive nature. The solid acids have the disadvantages
of rapid deactivation by coking and pore blocking as well as solid waste disposal problem. The
cation-exchange resins are not thermally stable to be used in industry [10]. Hence, the need for
greener routes using environmentally benign catalysts is recognized by many researchers. During
the last two decades, ionic liquids, with their tremendous potential both as solvents and catalysts
have evolved immensely as a solution to most of the problems involving catalytic reactions. Room
temperature ionic liquid (RTIL) catalysts, with negligible vapor pressure, ease of separation from
reaction products, excellent thermal stability and almost complete recyclability, have been
considered to be a solution for most of the above mentioned problems [11-20]. However, reports
on alkylation of phenol with TBA using ionic liquid catalysts are scanty [13-20]. Moreover,
Density Functional Theory (DFT) studies on correlating the properties of ionic liquids in catalytic
reactions are almost negligible. Shen et al. reported imidazole based Lewis acidic ionic liquid,
[bmim] PF6 in catalyzing the alkylation of phenol with TBA [13, 14]. However, these Lewis acidic
ionic liquids produce corrosive acids. Gui et al. reported a set of imidazole based Bronsted acidic
ionic liquids with SO3-H as the active functional group. This process is, however, not viable for
large scale alkylated phenols production because of the inherent toxicity of these ionic liquids due
to the presence of the imidazole ring [18]. Elavarasan et al. reported a set of SO3-H functionalized
Bronsted acidic ionic liquids with 86% phenol conversion [20] and Liao et al. made an effort to
correlate the activities of the Bronsted acidic ionic liquids using DFT optimizations of the
structures in Gaussian 09 [21]. In the present study, we report highly energy efficient 94.2%
conversion of phenol using environmentally benign SO3-H functionalized room temperature ionic
liquids in alkylation of phenol with TBA. DFT calculations were used to correlate the experimental
acidity trends of these ionic liquids and hence, to explain their high catalytic activity in this
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reaction. Further those results were used to validate the effect of acidity and functionality variation
on the alkylation mechanism.
The effect of kinetic parameters like reaction time, temperature, reactant mole ratio, catalyst
to reactant mole ratio on phenol conversion were investigated and these parameters were optimized
in a batch reactor. Based on the product distribution, a reaction mechanism was proposed for the
alkylation of phenols using functionalized ionic liquid catalysts. The kinetic data was fitted to
various models reported in literature [20] and the best fit model was used to estimate the kinetic
parameters. The activation energy was finally calculated using the Arrhenius equation.
2. Experimental
2.1. Chemicals
A series of Bronsted acidic ionic liquids with SO3-H functionality were synthesized following
the procedure outlined in literature [20, 22-24]. 1-methyl imidazole was purchased from Sigma
Aldrich Chemicals Pvt. Ltd., UK. Triethylamine, pyridine and diethanolamine were purchased
from Spectrochem Chemicals, India. 1, 4–butane sultone was purchased from Sigma Aldrich
Chemicals Pvt. Ltd, Germany. Phenol and TBA were purchased from Merck Chemicals Pvt. Ltd,
Mumbai, India.
2.2. Synthesis of ionic liquids
As a general methodology for ionic liquid preparation, the individual amine was mixed with
1, 4-butane-sultone and stirred at 60-80°C for 12-18 hours. Resultant zwitterionic solid was
washed three times with diethyl ether and then dried under vacuum (120°C, 0.01 Torr). Finally,
equivalent amount of methane sulfonic acid was added to the solidified zwitterionic mass and
stirred at 80°C for 4-7 hours to obtain the ionic liquid. The structures of the synthesized ionic
liquids are shown in Fig. 1. All the chemicals were research grade and were used without further
purification unless otherwise stated.
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2.3. Characterization of synthesized ionic liquids
The prepared ILs were characterized by 1H NMR and C NMR spectroscopy using Bruker
ARX-300 spectrometer. Thermo gravimetric analysis (TGA) was performed using a TG 209 F3
Tarsus instrument. UV-Visible spectra were recorded on a Shimadzu UV-2450 instrument.
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2.3.1. Characterization of ionic liquids by NMR
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IL-1 [N-(4-sulfonic acid) butyl triethylammonium methane sulfonate]: H NMR (D₂O,
∂ (ppm) 0.953 (t,
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300 MHz):
9H), 1.501 (m, 4H), 2.475 (s, 3H), 2.644 (t, 2H), 2.888 (t, 2H), 2.967
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(q, 6H), ∂(ppm)=4.80 [Solvent peak (D₂O)]; C NMR (D2O, 75 MHz): δ(ppm) 4.301, 17.552,
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18.975, 36.151, 47.677, 50.244, 53.600.
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IL-2 [1-(4-sulfonic acid) butylimidazolium methane sulfonate]: H NMR (DMSO, 300
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MHz): δ (ppm) 1.542 (m, 2H), 1.879 (m, 2H), 2.125 (s, 1H), 2.416 (s, 3H), 3.856 (s, 3H), 4.186 (t,
2H), 4.293 (t, 2H), 7.713 (d, 1H), 7.778 (d, 1H), 9.148 (s, 1H), δ (ppm)= 2.508 [Solvent peak
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(DMSO)]; C NMR (DMSO, 75 MHz): δ (ppm) 21.065, 28.065, 30.274, 35.318, 48.044, 49.982,
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121.895, 123.206, 136.209, δ (ppm)= 39.006 [Solvent peak (DMSO)].
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IL-3 [1-(4-sulfonic acid) butylpyridinium methane sulfonate]: H NMR (DMSO, 300
MHz): δ (ppm) 1.615 (m, 2H), 2.022 (m, 2H), 2.281 (s, 1H), 2.580 (s, 3H), 2.603 (t, 2H), 4.677 (t,
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2H), 8.169 (t, 2H), 8.613 (t, 1H), 9.096 (d, 2H), δ (ppm)= 2.511 [Solvent peak (DMSO)]; C
NMR (DMSO, 75 MHz) : δ (ppm) 20.5389, 28.9577, 49.6571, 60.7467, 75.6754, 127.9776,
143.8474, 145.3695, δ (ppm)= 38.2471 [Solvent peak (DMSO)].
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IL-4 [N, N-bis(2-hydroxyethyl)-(4-sulfonic acid)-1-aminium methane sulfonate]: H
NMR (DMSO, 300 MHz): ∂(ppm) 1.640 (m, 2H), 1.733 (m, 2H), 2.410 (s, 3H), 3.020 (m, 1H),
3.160 (m, 1H), 3.221 (t, 2H), 3.301 (t, 2H), 3.658 (t, 2H), 3.742 (t, 4H), 3.933 (broad m, 4H), ∂
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(ppm)= 2.513 [Solvent peak (DMSO)]; 13C NMR (DMSO, 75 MHz): δ (ppm) 21.790, 22.396,
49.280, 53.338, 55.913, 57.993, 63.651, δ (ppm)= 39.510 [Solvent peak (DMSO)].
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2.3.2. Thermo Gravimetric Analysis of ionic liquids
The thermal decomposition temperatures of the ILs were obtained using TG 209 F3 Tarsus at
a heating rate of 10 K/min, under nitrogen atmosphere. Thermal decomposition temperature of
IL-1, IL-2, IL-3 and IL-4 were observed at 312ºC, 324.8ºC, 366.9ºC and 326ºC respectively.
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2.3.3. Acidity of ionic liquids
To compare the Bronsted acidity of different ionic liquids, UV-Visible spectroscopy was used
as mentioned in literature [25-30]. As a general methodology 4-nitro aniline (pKa value of 0.99)
was chosen as a basic indicator to abstract proton from the Bronsted acidic ionic liquids in water.
The variation in acidity was evaluated in terms of the measurable ratio of the unprotonated and
protonated indicator concentration ratio, [B]aq/ [BH+]aq. The Hammett function (H0) was then
calculated using equation 1 as shown below:
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ꢀꢁ = ꢂꢃ(ꢄ)ꢅꢆ + log( ꢄ ꢅꢆ/[ꢄꢀꢇ]ꢅꢆ)
[ ]
(1)
Where, pK(B)aq is the pKa value of the indicator in aqueous solution. The terms [BH+]aq and
[B]aq are protonated and unprotonated indicator molar concentrations in the solvents respectively.
The blank solution of 4-nitro aniline in water showed absorbance maxima at 381 nm. Fig. 2a shows
the absorbance of IL-1 solution at different concentrations. The Hammett function of the other
ionic liquids prepared in the laboratory was also found by similar procedure, and the results are
shown in the Fig. 2b. From the figure, it is clear that the four ionic liquids possess strong acidity
which is very important for this acid-catalyzed reaction.
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2.3.4. Catalytic testing
A series of activity tests using these laboratory synthesized ionic liquids were carried out in a
3 mL polytetrafluoroethylene (PTFE) sealed Borosil glass reactor under autogeneous pressure
placed on a magnetic stirrer. In a typical batch run, 1 mmol each of phenol, TBA and ionic liquid
were taken in the reactor and stirred for 5 hours at 80ºC. The product mixture was analyzed both
using a NUCON GC supplied by AIMIL India Ltd. equipped with a CHROMSORB-WHP (2 m ×
3.175 mm × 2 mm) column and flame ionization detector (FID). The carbon balance was checked
and found to be more than 99.9% while ring balance was obtained to be more than 97.9%.
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2.3.5. Computational methods
MATLAB 8.4 software version was used to perform the optimization in the kinetic modeling.
The DFT studies were performed using Gaussian 09 software [31]. Geometry optimizations of
different ionic liquids were performed at the B3LYP level using the 6–311++G (d, p) basis set. A
vibrational analysis was also performed for each optimized structure to check the absence of
negative frequency which, in turn, confirms the structure to be an energy minimum. The basis set
superposition errors (BSSEs) were confirmed to be negligible by the counterpoise method [32,
33]. Chemcraft software was used to calculate the optimized structural parameters
(http://www.chemcraftprog.com/). The initial structure of all the ionic liquids was started from the
all Trans conformer. The stabilization energy (ΔE) was calculated as the difference between the
energy of the ionic liquid subtracted from the sum of energies of the pure cation and anion as
shown in equation 2:
ꢎꢏ
(
)
[
(
)
(
)(
)]
∆ꢈ ꢉꢊ ꢋꢌꢍ
= ꢈꢐꢑ ꢒꢓꢌꢋꢔꢕ ꢖꢗꢔꢓ − ꢈꢘ + ꢈꢙ ꢒꢓꢌꢋꢔꢕ ꢖꢗꢔꢓ ∗ 2622.9901
(2)
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3. Results & Discussion
3.1. Comparison of activity of the four ionic liquid catalysts
A comparative study of the activity of the prepared ionic liquids with that of the already
available catalysts was performed in this reaction [29]. From Table 1, it is obvious that all the ionic
liquid catalysts gave high phenol conversion at 80°C temperature. The selectivity to 2-TBP and 2,
4-DTBP was higher as compared to the other products compared to literature reports in all cases.
Interestingly, IL-2 and IL-3 showed higher selectivity towards the 2, 6-di-tert-butyl phenol (2, 6-
DTBP) in comparison to IL-1 and IL-4. The reason can be attributed to the sterically hindered
transition state in the formation of the di-ortho substituted phenol, 2, 6-DTBP. Both IL-1 and IL-4
consisting of bulky alkyl chains do not favor the transition state geometry and hence, less selectivity
to 2, 6-DTBP. In case of IL-2 and IL-3 the selectivity towards 2, 6-DTBP was obtained to be as high
as 36.5% and 41.6% respectively. Among the four ionic liquid catalysts, IL-1 gave 94.2% phenol
conversion at 80°C reaction temperature and phenol: TBA: IL mole ratio of 1:2:1 within 2 hours of
reaction time. These reactions were repeated at least five times and the results were found to be
similar. The TOF of IL-1 was observed to be highest at 1.543 min-1. IL-1 was found to be the most
promising catalyst as well as least toxic [30]. Hence, IL-1 was chosen for further detailed kinetic
evaluation. The improved efficiency of this ionic liquid as compared to those with similar
functionality, reported in prior art [20], can be attributed to the solubility factor imparted by the
methane sulfonate anion of IL-1. The presence of this anion resulted in improved solubility of TBA
leading to more reaction with IL-1 to form IL-esters. The product distribution obtained during the
detailed kinetic study helped us to understand the reaction mechanism.
3.2. Effect of reaction time on phenol conversion
To investigate the effect of reaction time on phenol conversion and the selectivity to 2-TBP
and 2, 4-DTBP, a series of reactions were conducted at different temperatures and different
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reactant mole ratios. The results are summarized in Fig. 3a. It is evident from the figure that most
of the phenol conversion was obtained within the first 2 hours of the reaction and, thereafter,
reached steady state. The maximum conversion of phenol was observed to be 94.2%, while the
selectivity to 2-TBP and 2, 4-DTBP reached 24.3% and 52.9% respectively after 2 hours. Beyond
two hours, the conversion of phenol reached steady state. At 30 minutes reaction time, the
selectivity to 2-TBP was 43.9% and beyond that selectivity to 2, 4-DTBP increased. The selectivity
to the other dialkylated product, 2, 6-DTBP also increased slowly up to 13.5% after 5 hours of
reaction time. This significantly lower selectivity towards 2, 6-DTBP can be attributed to the
sterically hindered reaction mechanism of C-alkylation at both the ortho positions of phenol. Thus,
the formation of 2, 6-DTBP is less likely for this type of ionic liquids and hence, selectivity is less
[20]. Also, no significant ether product was observed during the reaction steps. This means that
the mechanism proceeds via alkylation at ortho position followed by tert-butylation at the para
position.
3.3. Effect of reaction temperature
A series of reactions were conducted in the temperature range of 40°C to 120°C using
different reactant mole ratios for 5 hours. The conversion of phenol decreased significantly beyond
100°C since the dealkylation reaction becomes prominent as reported by earlier researchers [20,
35-36]. Higher temperatures also lead to dehydration of TBA followed by oligomerization, which
in turn, suppresses dialkylated product selectivity [37]. At lower temperature, the extent of
reactions were restricted by mass transfer limitations due to the increased viscosities of the ionic
liquid which was also observed by Gu et al. [34]. Fig. 3b summarizes the results obtained for the
alkylation reaction conducted in the temperature range of 50°C to 100°C up to 2 hours of reaction
time and 1:2:1 phenol: TBA: IL-1 mole ratio. With increase in temperature from 50°C to 80°C,
the conversion of phenol increased from 40.4% to 94.2%. Beyond 80°C, there is a decrease in
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phenol conversion which may be attributed to the vaporization of TBA whose boiling point is
82°C. At 60ºC, IL-1: phenol mole ratio of 1:1 and TBA: phenol mole ratio of 2:1, the selectivity
to 2, 4-DTBP is high (65.5%) and with the increase in temperature, the selectivity to 2, 4-DTBP
decreased up to 52.9% at 80ºC. This decrease in selectivity to 2, 4-DTBP can be attributed to the
decreasing concentration of tert-butyl cation which is more stable at lower reaction temperatures.
The conversion of phenol reached a maximum at reaction temperature of 80°C with moderate
selectivity to 2, 4-DTBP of 52.9%. Hence, considering higher phenol conversion and moderate
selectivity to the desired main product, the reaction temperature of 80°C was taken as optimum
for further kinetic investigations.
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3.4. Effect of TBA to phenol mole ratio
The effect of increase in reactant mole ratio (TBA to phenol) from 0.5 to 4 was investigated
keeping the temperature at 80°C and IL to phenol mole ratio at 1:1. The results are shown in Fig.
3c. The TBA to phenol mole ratio of 2:1 gave the highest phenol conversion beyond which the
conversion of phenol decreased due to dilution of the catalyst by TBA and also due to formation
of oligomers. As the amount of TBA increased, at a constant IL-1 to phenol molar ratio, the
conversion of phenol and selectivity to 2-TBP also increased. While the selectivity to 2, 4-DTBP
decreased gradually with the increase in amount of TBA. This trend in product selectivity indicates
that increased amount of TBA concentration leads to dilution of the acidity of the ionic liquid
catalysts [20, 35]. Thus, the dealkylation becomes predominant and conversion to ortho substituted
phenol, 2-TBP increases in this kinetically controlled reaction regime. The reaction mechanism
proceeds via C-alkylation, favoring the ortho substituted phenol. Hence, considering the phenol
conversion as well as product selectivity, the optimum TBAto phenol mole ratio is 2:1 at a constant
IL-1 to phenol molar ratio of 1:1, temperature of 80°C and a reaction time of 2 hours.
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3.5. Effect of catalyst to phenol mole ratio
The effect of increase in IL-1 to phenol mole ratio from 0.5 to 5 on the phenol conversion and
product selectivity was investigated at 80°C keeping TBA to phenol molar ratio constant at 2:1.
Fig. 3d shows a gradual increase in phenol conversion as the amount of IL to phenol mole ratio to
1:1. Thereafter, phenol conversion remained almost constant. The selectivity to 2-TBP and 2, 4-
DTBP showed opposite trends with increased amount of IL-1 to phenol molar ratio. The selectivity
to 2-TBP increased from 19.4% to 58.6% as IL-1 to phenol molar ratio increased from 0.5 to 5,
while the selectivity to 2, 4-DTBP decreased from 66.4% to 16.4%. The decreased selectivity
towards dialkylated products can be attributed to the increased amount of IL-1 in the reaction
mixture which does not favor the sterically hindered dialkylation reaction step [34]. Since, the
conversion of phenol was highest at IL-1 to phenol molar ratio 1:1, it was considered to be the
optimum catalyst to reactant molar ratio at 80°C within 2 hours of reaction time.
3.6. Recyclability of ionic liquid
The recyclability ofthese prepared ionic liquid catalysts was found to be high when compared
to ionic liquids reported [20] in literature for similar reactions. The residual trace amount of
products of the alkylation reactions were separated from the ionic liquids by extracting with
toluene (5 × 3mL). The resultant layer of the catalyst was washed with diethyl ether (5 × 3mL)
and then dried under vacuum (10 mm of Hg) for 4-5 hours at 125°C. The purity of the recovered
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ionic liquid catalysts was checked by comparing with H NMR, C NMR, etc. as well as thin
layer chromatography of the pure ionic liquid showing negligible amounts of products and
unconverted reactants. No significant amount of oligomerized products were formed at 80ºC. The
other unreacted reactants or products were highly soluble in the washing solvents used. Hence,
the ionic liquids showed high recyclability. The results of the recycle experiments conducted using
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IL-1 are summarized in Table 2. IL-1 was found to maintain significant activity even after 8
recycles.
3.7. Effect of acidity on phenol conversion
Fig. 2b shows the Hammett acidity function (H0) values of the synthesized ionic liquids.
From the figure it is evident that the order of acidity of the ionic liquids is IL-4>IL-1>IL-2>IL-3.
DFT studies on the ionic liquid structures were performed to correlate the acidity trend. The ionic
liquid structures were first geometrically optimized to a minimum using Gaussian 09 program as
shown in Fig. 4. Then, the different structural parameters as well as the thermochemical energy
values were calculated and used to explain the acidity of the ionic liquids. The optimized structural
parameters are summarized in Table 3. The structural parameters for the ionic liquids were
compared with literature [18] reported ionic liquid, 1-methyl-3-(butyl-4-sulfonate) imidazolium
hydrogen sulfate (IL-0), having similar functionality with those reported in this study used in
similar type of reactions. The structural parameters for IL-0 were used to compare the
experimental and theoretical acidity values of the ionic liquids as also reported by Liu et al. [21].
There are two parameters controlling the acidity of the ionic liquids: (i) the interaction between
the cationic species of the ionic liquids with the anion (expressed in terms of stabilization energy)
and (ii) the hydrogen bonding capability between the anionic O atoms and the cationic acidic
proton of O-H bond [21]. Thus, the acidity trend in ionic liquids cannot be directly correlated with
the cationic O-H bond distances. The acidic O-H bond lengths in the cationic part are in the order
of IL-0>IL-3>IL-2>IL-1>IL-4. Again, the stabilization energies of the ionic liquids are in the
order of IL-0>IL-2>IL-3>IL-1>IL-4. Hence, the cation and anion are loosely bound in IL-4 or IL-
1 than in IL-0, IL-2 or IL-3. The distance between anion and the cation in terms of hydrogen
bonding are in the order of IL-4>IL-1>IL-2>IL-3>IL-0, exactly in the opposite order of the acidic
O-H bond distances. Thus, when the hydrogen bonding is weak, the stabilization energy is also
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low and the ion pair is loosely bound. Hence, the acidic proton of SO3-H group will be more active
and more will be the acidity of the corresponding ionic liquid. The relative acidity trend from
these DFT calculations can be summarized as IL-4>IL-1>IL-2>IL-3, which is in agreement with
the experimental results. Surprisingly, IL-4 with the shortest cationic O-H bond distance has the
strongest acidity because of the other two dominating parameters. Hence, it is obvious that both
IL-1 and IL-4, with their strong acidity as compared to IL-2, IL-3 and IL-0, proved to be promising
catalysts in this alkylation reaction. Table 1 shows increase in phenol conversion with acidity for
IL-1, IL-2 and IL-3. However, IL-4 with the highest acidity shows lower phenol conversion which
may be attributed to its increased viscosity due to the presence of -OH groups increasing the H-
bonding capability. IL-1 with its optimum acidity and viscosity showed higher catalytic activity
in the reaction.
3.8. Kinetic modeling
Kinetic runs were conducted at four different temperatures, from 60°C to 100°C at 10°C
interval. At each temperature, the product samples were collected at regular time intervals. In all
these runs, the TBA to phenol mole ratio was kept at 2:1 and the phenol to catalyst mole ratio was
1:1. The tri substituted product (2, 4, 6-tri-tert-butyl phenol) was detected in very small amounts
compared to other isomers. Since, no ether product was obtained, the conversion of tert-butyl
phenol ether (TBPE) to 2-TBP was not considered in the model. In accordance with the product
distribution, a reaction mechanism was proposed as shown in Fig. 5. Based on this mechanism,
the system can be described by the following reaction scheme as shown in between equations 3 to
9:
Conversion of phenol:
298
(3)
13

299
Isomerization of monoalkylated phenols:
300
301
(4)
Alkylation of 2-TBP:
302
(5)
(6)
303
304
Alkylation of 4-TBP:
305
306
(7)
Isomerization of dialkylated phenols:
307
308
(8)
(9)
Conversion of ether to 2-TBP:
309
310
311
312
313
Model parameters were estimated by minimizing the objective function, as shown in equation
10, which is the sum of squares of errors between the predicted and measured values for all the
state variables for a dynamic run:
ꢡ
ꢟ
ꢞ
∑
∑
ꢚ = ꢜꢠꢏ ꢝꢠꢏ(ꢛꢜꢝ − ꢛꢁ,ꢜꢝ
)
(10)
314
315
Where, x is computed value, x0 is observed value, n is total number of state variables and m
is total number of observations.
14

316
317
318
319
320
321
322
323
324
325
326
327
The program requires the input data in six main steps described as follows:
 Model equations (Given in Sec. 3.8.1).
 Number of model parameters (k1 to k11).
 Initial, lower and upper bounds of the parameters.
 Starting, final and sampling times for experimental data.
 Number of state variables (There are 7 state variables starting from CP (conc. of phenol)
to CW (conc. of water).
 Experimental measurements of state variables.
3.8.1 Batch reactor kinetic model
The rate equations considered in the kinetic model for all the individual reactions involved in
the alkylation are as follows (equations 11 to 17):
Alkylation of phenol:
ꢢꢘ
ꢣ
328
329
330
= −ꢉꢏ ∗ ꢥꢦ ∗ ꢥꢧꢨꢙ
(11)
ꢢꢤ
Rate of conversion of TBA:
ꢢꢘ
ꢩꢪꢫ = −ꢉꢏ ∗ ꢥꢦ ∗ ꢥꢧꢨꢙ − ꢉꢬ ∗ ꢥꢞꢎꢧꢨꢦ ∗ ꢥꢧꢨꢙ + ꢉꢭ ∗ ꢥꢞ,ꢬꢎꢮꢧꢨꢦ ∗ ꢥꢯ − ꢉꢰ ∗ ꢥꢞꢎꢧꢨꢦ ∗ ꢥꢧꢨꢙ
+
ꢢꢤ
331
332
ꢉꢱ ∗ ꢥꢞ,ꢰꢎꢮꢧꢨꢦ ∗ ꢥꢯ − ꢉꢲ ∗ ꢥꢬꢎꢧꢨꢦ ∗ ꢥꢧꢨꢙ + ꢉꢳ ∗ ꢥꢞ,ꢬꢎꢮꢧꢨꢦ ∗ ꢥꢯ
Rate of formation of 2-TBP:
(12)
ꢢꢘ
ꢴꢵꢩꢪꢣ
ꢢꢤ
333
= −ꢉꢏ ∗ ꢥꢦ ∗ ꢥꢧꢨꢙ − ꢉꢞ ∗ ꢥꢞꢎꢧꢨꢦ ∗ ꢥꢞꢎꢧꢨꢦ + ꢉꢶ ∗ ꢥꢬꢎꢧꢨꢦ ∗ ꢥꢬꢎꢧꢨꢦ − ꢉꢬ ∗ ꢥꢞꢎꢧꢨꢦ
∗
334
335
ꢥꢧꢨꢙ + ꢉꢭ ∗ ꢥꢞ,ꢬꢎꢮꢧꢨꢦ ∗ ꢥꢯ − ꢉꢰ ∗ ꢥꢞꢎꢧꢨꢦ ∗ ꢥꢧꢨꢙ + ꢉꢱ ∗ ꢥꢞ,ꢰꢎꢮꢧꢨꢦ ∗ ꢥꢯ
(13)
Rate of formation of 4-TBP:
ꢢꢘ
ꢷꢵꢩꢪꢣ
336
337
= ꢉꢏ ∗ ꢥꢦ ∗ ꢥꢧꢨꢙ + ꢉꢞ ∗ ꢥꢞꢎꢧꢨꢦ ∗ ꢥꢞꢎꢧꢨꢦ − ꢉꢶ ∗ ꢥꢬꢎꢧꢨꢦ ∗ ꢥꢬꢎꢧꢨꢦ − ꢉꢲ ∗ ꢥꢬꢎꢧꢨꢦ
∗
ꢢꢤ
ꢥꢧꢨꢙ + ꢉꢳ ∗ ꢥꢞ,ꢬꢎꢮꢧꢨꢦ ∗ ꢥꢯ
(14)
15

338
339
Rate of formation of 2, 4-DTBP:
ꢢꢘ
ꢴ,ꢷꢵꢸꢩꢪꢣ = ꢉꢬ ∗ ꢥꢞꢎꢧꢨꢦ ∗ ꢥꢧꢨꢙ − ꢉꢭ ∗ ꢥꢞ,ꢬꢎꢮꢧꢨꢦ ∗ ꢥꢯ + ꢉꢲ ∗ ꢥꢬꢎꢧꢨꢦ ∗ ꢥꢧꢨꢙ − ꢉꢳ ∗
ꢢꢤ
340
341
ꢥꢞ,ꢬꢎꢮꢧꢨꢦ ∗ ꢥꢯ + ꢉꢏꢁ ∗ ꢥꢞ,ꢰꢎꢮꢧꢨꢦ ∗ ꢥꢞ,ꢰꢎꢮꢧꢨꢦ − ꢉꢏꢏ ∗ ꢥꢞ,ꢬꢎꢮꢧꢨꢦ ∗ ꢥꢞ,ꢬꢎꢮꢧꢨꢦ
(15)
Rate of formation of 2, 6-DTBP:
ꢢꢘ
342
ꢴ,ꢹꢵꢸꢩꢪꢣ = ꢉꢰ ∗ ꢥꢞꢎꢧꢨꢦ ∗ ꢥꢧꢨꢙ − ꢉꢱ ∗ ꢥꢞ,ꢰꢎꢮꢧꢨꢦ ∗ ꢥꢯ − ꢉꢏꢁ ∗ ꢥꢞ,ꢰꢎꢮꢧꢨꢦ ∗ ꢥꢞ,ꢰꢎꢮꢧꢨꢦ + ꢉꢏꢏ
∗
ꢢꢤ
343
344
ꢥꢞ,ꢬꢎꢮꢧꢨꢦ ∗ ꢥꢞ,ꢬꢎꢮꢧꢨꢦ
(16)
Rate of formation of Water:
ꢢꢘ
ꢺ
345
= ꢉꢏ ∗ ꢥꢦ ∗ ꢥꢧꢨꢙ + ꢉꢬ ∗ ꢥꢞꢎꢧꢨꢦ ∗ ꢥꢧꢨꢙ − ꢉꢭ ∗ ꢥꢞ,ꢬꢎꢮꢧꢨꢦ ∗ ꢥꢯ + ꢉꢰ ∗ ꢥꢞꢎꢧꢨꢦ ∗ ꢥꢧꢨꢙ −
ꢢꢤ
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
ꢉꢱ ∗ ꢥꢞ,ꢰꢎꢮꢧꢨꢦ ∗ ꢥꢯ + ꢉꢲ ∗ ꢥꢬꢎꢧꢨꢦ ∗ ꢥꢧꢨꢙ − ꢉꢳ ∗ ꢥꢞ,ꢬꢎꢮꢧꢨꢦ ∗ ꢥꢯ
(17)
Where, CP, CTBA, C2-TBP, C4-TBP, C2,4-DTBP, C2,6-DTBP and CW are the concentrations of
respective components, and k1 to k11 are the second order rate constants. A comparison of
experimental and predicted phenol conversion with time at 80ºC is shown in Fig. 6a. The predicted
data fits well with the experimental measurements at five different temperatures 60ºC, 70ºC, 80ºC,
90ºC and 100ºC. The optimized parameters are summarized in the Table 4. The value of the
activation energy and pre-exponential coefficient were obtained for each of the rate constants after
plotting ln (k) vs 1/T as shown in Fig. 6b (shown for k1 only).
4. Conclusions
SO3-H functionalized room temperature Bronsted acidic ionic liquids were prepared in the
laboratory using a greener approach and characterized using spectroscopic techniques. The acidity
was optimized to suite the alkylation reaction chosen. A highly efficient and greener route for
selective liquid phase tert-butylation of phenol with TBA, in room temperature ionic liquids, was
systematically conducted in a batch autoclave under autogeneous pressure. The kinetic parameters
like temperature, reaction time, reactant ratio, catalyst to reactant ratio were optimized. DFT
16

361
362
363
364
365
366
367
368
369
studies on the ionic liquid structures were performed and from the calculated structural parameters
the experimental acidity trends were correlated. The synthesized ionic liquids proved to be very
efficient catalysts for this alkylation reaction. They are inexpensive, completely recoverable and
recyclable. Catalytic amount of the laboratory prepared IL-1 showed very high phenol conversion,
complete alcohol conversion and high dialkylated product selectivity at low temperatures. Thus,
these Bronsted acidic ionic liquid catalysts show promise for the development of a continuous,
environmentally benign, energy efficient and economical process route for the production of
alkylated phenols. The activation energy for the ionic liquid catalyzed alkylation process was
estimated to be 34.8 kJ mol-1 with a pre-exponential factor value of 28.7 lit. mol-1 min-1.
370
371
372
Acknowledgement
Tanmoy Patra thanks Council of Scientific and Industrial Research, India, for a research
fellowship.
17

373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
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Fig. 1. Structure of ionic liquids.
Fig. 2. Relative acidity measurements using UV-Visible spectroscopy: a) Absorption spectrum of
4-nitroaniline for different concentration of IL-1 in H2O, b) Hammett Function (acidity) of ILs in
water with 4-nitroaniline as indicator (pKa=0.99), Amax=381 nm.
Fig. 3. Kinetics study for alkylation of phenol with tert-butyl alcohol: a) Effect of reaction time on
phenol conversion and product selectivity, b) Effect of reaction temperature on phenol conversion
and product selectivity, c) Effect of TBA to phenol mole ratio on phenol conversion and product
selectivity, d) Effect of catalyst (IL-1) to phenol mole ratio on phenol conversion and product
selectivity.
Fig. 4. Optimized structure of different ionic liquids: a) Structure of IL-0, b) Structure of IL-1, c)
Structure of IL-2, d) Structure of IL-3, e) Structure of IL-4.
Fig. 5. Proposed reaction mechanism involving IL-1.
Fig. 6. Kinetic parameter estimation from kinetic modeling: a) Comparison of experimental and
predicted concentrations in alkylation of phenol with tert-butyl alcohol using IL-1 at 80ºC, b)
Arrhenius plot for rate constant, k1.
20

440
Table 1. Activity of the various catalysts in alkylation of phenol with TBA
% Selectivity
Temperature
Mole ratio
Conversion
(%)
Catalysts
(0C)
80
70
80
70
80
80
70
70
(TBA: phenol)
2-TBP 4-TBP 2, 4-DTBP 2, 6-DTBP
IL-1a
IL-2a
IL-2a
IL-3a
IL-3a
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
94.2
24.3
19.9
20.2
23.8
14.1
26.5
13.1
25.8
10.9
21.8
7.2
52.9
36.4
36.1
48.9
39.2
45.6
74.9
60.2
11.9
21.9
36.5
14.4
41.6
15.8
2.6
72.6
77.4
70.1
12.9
5.1
78.2
IL-4a
82.5
12.1
9.4
[bmim]PF6 [13]
[hbim-SO3H] HSO4
[18]
91.5
80.3
9.0
5.1
[TEA-(CH2)4-SO3H]
HSO4 [21]
TPA/TiO2 [5]b
Mordenite [7]c
USY [7]d
RHAPhSO3H [22]e
TPA/MCM-41[14]f
70
2:1
84.5
23.8
10.2
57.6
7.4
80
60
6 :10
6 :10
6 :10
1 :2
60.0
44.8
52.7
57.0
44.8
3.6
96.4
49.1
23.0
22.0
49.0
-
-
-
-
48.1
63.0
42.0
29.0
2.8
70
13.1
2.0
120
70
2 :1
21.2
-
441
442
443
444
445
446
447
448
a Reaction Conditions: phenol (1 mmol), TBA (1 to 2 mmol), ionic liquid (1 mmol); stirring
speed= 800 rpm; reaction time: 2 h.
b Reaction Conditions: 250mg catalyst; reaction time: 1 h.
c Reaction Conditions: 200mg catalyst; reaction time: 3 h.
d Reaction Conditions: 200mg catalyst; reaction time: 8 h.
e Reaction Conditions: 50mg mass of catalyst; reaction time: 9 h.
f Reaction Conditions: 282mg catalyst; reaction time: 4 h.
449
450
451
452
21

453
Table 2. Recyclability of ionic liquids
Experimental
Run
% Conversion with
respect to Phenol
% Selectivity
2-TBP
4-TBP
2, 4-DTBP
Fresh Catalyst
Recycle 1
Recycle 2
Recycle 3
Recycle 4
Recycle 5
Recycle 6
94.2
93.9
93.5
93.9
93.0
92.4
91.1
24.3
10.9
52.9
23.9
23.6
24.4
23.7
22.3
21.6
9.2
9.6
54.2
53.4
52.1
53.0
53.9
54.9
11.1
10.1
9.6
9.1
Recycle 7
Recycle 8
90.4
90.1
20.3
20.2
8.6
8.1
55.9
56.9
454
455
456
457
458
459
Reaction conditions: phenol (1 mmol): TBA: IL-1 (1:2:1) molar ratio; 80ºC; 2 h.
460
461
462
463
464
465
466
22

467
Bond type
IL-1
IL-2
IL-3
IL-4
IL-0
Cationic O-H bond
distance (Å)
1.022
1.049
1.421
1.021
1.491
O-H bond distance
between anionic O
and cationic H (Å)
1.545
1.446
1.059
1.546
1.036
ΔE (kcal/mole)
-1447.215
-1620.129
-1541.095
-1391.998
-1728.352
468
469
470
471
472
473
474
475
476
477
478
479
480
481
Table 3. DFT Study - Structural parameters and stabilization energy values for the ionic liquids
23

482
483
484
485
486
Table 4: Kinetic parameters for alkylation of the phenol with tert-butyl alcohol using ionic liquid (IL-
1) as a catalyst.
Rate Constant
Temperature (0C)
80
Activation
energy
(Ea) (kJ
mol-1)
Pre-
exponential
factor
(lit. mol-1 min-1)
60
70
90
100
(lit. mol-1 min-1)
28.74
k1
0.93× 10- 1.31× 10- 1.91× 10-4 2.75× 10- 3.50× 10-4
34.8
4
4
4
k2
0.55× 10- 0.73× 10- 0.92× 10-4 1.06× 10- 2.16× 10-4
31.5
99.4
20.0
54.6
62.7
158.4
55.8
115.4
67.1
98.0
4.8
4
4
4
k3
0.27× 10- 0.75× 10- 1.12× 10-4 7.44× 10- 10.79× 10-
1.2× 1011
31.7
4
4
4
4
k4
2.10× 10- 2.93× 10- 3.32× 10-5 3.80× 10- 5.20× 10-5
5
5
5
k5
0.1× 10-5 0.4× 10-5 0.5× 10-5 1.00× 10- 1.20× 10-5
6.3× 102
1.6× 106
3.4× 1020
6.6× 104
6.6× 1013
4.7× 105
8.4× 109
5
k6
0.19× 10- 0.34× 10- 1.03× 10-3 1.41× 10- 1.97× 10-3
3
3
3
k7
0.01× 10- 0.02× 10-
0.025×
10-2
0.5× 10-2 3.06× 10-2
2
2
k8
0.92× 10- 1.70× 10- 5.84× 10-3 6.07× 10- 7.1× 10-3
3
3
3
k9
k10
0.75× 10- 1.21× 10- 2.23× 10-4 8.22× 10- 84.34× 10-
4
4
4
4
1.20× 10- 2.90× 10- 4.90× 10-5 8.30× 10- 17.8× 10-5
5
5
5
k11
0.3× 10-5 0.8× 10-5 2.5× 10-5 8.0× 10-5 10.1× 10-5
487
24

488
489
Fig 1
490
491
Fig 2
25

492
493
Fig 3
26

494
495
Fig 4
27

496
497
Fig 5
28

498
499
Fig 6
29