Solvent free liquid-phase alkylation of phenol over solid sulfanilic acid catalyst

Article abstract of DOI:10.1016/j.apcata.2011.03.039

Sulfanilic acid was immobilized onto rice husk ash via 3-(chloropropyl) triethoxy-silane to form an acidic solid catalyst denoted as RHAPhSO ₃H. The BET surface area was found to be 308 m² g ^-1. Pyridine adsorption study revealed the presence of Br?nsted acid sites. The EDX analysis showed the presence of S (10.88%) and N (10.37%). The ²⁹Si MAS NMR showed the presence of T ², T³, Q³ and Q⁴ silicon centres. The three carbon atoms of the propyl group were evident from the ¹³C MAS NMR together with a series of chemical shifts consistent with the presence of the benzene ring. In the alkylation of phenol using RHAPhSO₃H as the catalyst resulted in 95% conversion of tert-butyl alcohol at 120 °C with 52% selectivity towards 4-tert-butylphenol. The catalyst was reused several times without significant loss of catalytic activity.

Full text of DOI:10.1016/j.apcata.2011.03.039

Applied Catalysis A: General 399 (2011) 42–49
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Solvent free liquid-phase alkylation of phenol over solid sulfanilic acid catalyst
Farook Adama,∗, Kasim Mohammed Hellob, Tammar Hussein Alia
a School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia
b Chemical Department, Collage of Science, Al-Muthanna University, Iraq
a r t i c l e i n f o
a b s t r a c t
Article history:
Sulfanilic acid was immobilized onto rice husk ash via 3-(chloropropyl)triethoxy-silane to form an acidic
solid catalyst denoted as RHAPhSO3H. The BET surface area was found to be 308 m2 g−1. Pyridine adsorp-
tion study revealed the presence of Brønsted acid sites. The EDX analysis showed the presence of S
(10.88%) and N (10.37%). The 29Si MAS NMR showed the presence of T2, T3, Q3 and Q4 silicon centres.
The three carbon atoms of the propyl group were evident from the 13C MAS NMR together with a series
of chemical shifts consistent with the presence of the benzene ring. In the alkylation of phenol using
RHAPhSO3H as the catalyst resulted in 95% conversion of tert-butyl alcohol at 120 ◦C with 52% selectivity
towards 4-tert-butylphenol. The catalyst was reused several times without significant loss of catalytic
activity.
Received 13 January 2011
Received in revised form 16 March 2011
Accepted 19 March 2011
Available online 26 March 2011
Keywords:
Surface modification
Sol–gel technique
Sulfanilic acid
Alkylation
© 2011 Elsevier B.V. All rights reserved.
4-tert-Butylphenol
1. Introduction
ature review reveals that these alkylation reactions are mostly
carried out in the gas phase with high conversion of phenol [10,11].
In recent years, the study of heterogenation of important organic
ligands including important homogeneous catalysts onto suitable
supports has seen increased interest due to the demand for green
chemistry and environmentally friendly technologies. Rice husk
(RH) is a by-product of the rice milling industry. These husks cause
serious disposal and pollution problems due to the presence of a
high content of silica. The controlled burning of RH in air leads to
the formation of rice husk ash (RHA) which contains ca. 95% silica
[1].
However, gas phase reactions usually involve high temperature
and pressure leading to high cost. Very few studies on the sol-
vent state alkylation of phenol with tertiary butyl alcohol (TBA)
have been published [12,13]. These solvent state reactions how-
ever, usually show very low conversions, i.e., less than 50%. It
will therefore be advantageous to find new environmental friendly
catalysts and milder experimental conditions to increase output
to reduce cost and satisfy the environmental needs at the same
time.
The main method of immobilization of the organic moi-
eties was via the reaction of a particular molecule with the
silanol groups on the silica surface. Typically such a process took
about 12 to 24 h under reflux conditions [2–4]. In this respect,
we had described the surface functionalization of silica with
3-(chloropropyl)triethoxysilane (CPTES) using a simple method
which does not require toxic reagents [5]. The CPTES immobilized
silica was used to heterogenize several organic molecules such as
saccharine and melamine [6,7].
The alkylation of phenol is of great industrial importance.
Some 450,000 tonnes of alkylated products like tertiary butylphe-
nols are used in the industry per year. Mono-alkylphenols and
di-alkylphenols are used in the manufacture of antioxidants, UV
absorbers and for the production of phenolic resins [8,9]. Liter-
In this work, sulfanilic acid was immobilized onto silica from
rice husk ash via CPTES to form an acidic solid catalyst. The catalyst
was used successfully for the solvent free liquid-phase alkylation
of phenol. Herein we report the preparation, characterization and
the catalytic activity of this solid sulfanilic acid catalyst.
2. Experimental
2.1. Chemicals
The chemicals used in this study were sodium hydrox-
ide (Systerm, 99%), CPTES (Sigma–Aldrich, 95%), sulfanilic acid
(Sigma–Aldrich, 99%), nitric acid (Systerm, 65%), toluene (J.T. Baker,
99.8%), acetic acid (Systerm, 99.5%), TBA (Merck, 99%), phenol
(Scharlau, 99.5%), acetonitrile (Qrec, 99.9%) and acetophenone
(Nacalai tesque, 99.9%). The rice husk (RH) was collected from a
rice mill in Penang, Malaysia. All chemicals were AR grade or of
high purity and used directly without further purification.
∗
Corresponding author. Tel.: +60 1 64069717; fax: +60 4 6574854.
E-mail addresses: farook@usm.my, farook dr@yahoo.com (F. Adam).
0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.03.039

F. Adam et al. / Applied Catalysis A: General 399 (2011) 42–49
43
2.2. Extraction and modification of silica from RHA
of TBA. Phenol (3.7 g, 0.04 mol) was transferred to the round bottom
flask containing 0.15 g of the catalyst (pre-dried at 110 ◦C for 24 h
and cooled in a desiccator to minimize moisture content). After
the reaction temperature reached 120 ◦C, TBA (7.6 mL, 0.08 mol)
was added. The reaction mixture was refluxed for 9 h. Samples for
analysis (∼0.50 mL) were withdrawn at regular intervals from the
reaction mixture and 20 ␮L of acetonitrile (as internal standard for
TBA) and or acetophenone (as internal standard for phenol) was
added to the mixture. This mixture was then analyzed by GC and
the products were confirmed by GC–MS.
2.2.1. Sources of silica
The rice husk ash (RHA) was chosen as the source of amorphous
silica [14] as it was available in abundance. The silica was extracted
from rice husk according to a reported method [15,16].
2.2.2. Synthesis of RHAPhSO3H
RHA silica was functionalized with CPTES according to the
method reported elsewhere [5]. The resulting, RHACCl was used
to immobilize sulfanilic acid onto the silica surface. A 1.0 g sample
of RHACCl was dispersed in 30 mL dry toluene. To this suspen-
sion, about 1.5 g (8.6 mmol) of sulfanilic acid was added followed
by 1.2 mL (8.6 mmol) of triethylamine acting as a proton scav-
enger. The mixture was refluxed for 48 h at 110 ◦C to produce a
brown solid. The product was filtered off and washed with toluene,
dichloromethane and finally with acidified ethanol. The sample was
dried at 110 ◦C for 24 h and ground to powder. The product yield
was 1.85 g and was labelled as RHAPhSO3H.
3. Results and discussion
The synthesis of RHAPhSO3H is shown in Scheme 1. The immo-
bilization of sulfanilic acid onto RHACCl was done under reflux
condition in dry toluene for 48 h. The following chemical and phys-
ical analyses were used to support the successful immobilization
of sulfanilic acid.
3.1. The elemental analyses
2.3. Sample characterization
The elemental analysis showed the presence of 0.59% of N
(10.37% by EDX). Sulphur was determined by EDX and showed
10.88% in RHAPhSO3H. These elements were not present in RHACCl
and RHA [5]. From the amount of Cl present in the EDX of RHACCl
(4.08%) and RHAPhSO3H (0.6%), the percentage loading of sulfanilic
acid on the silica was calculated to be 87.5%. The results also show
an increase in the percentage of C at 16.72% for RHAPhSO3H com-
pared to RHACCl.
The RHAPhSO3H was characterized by Powder X-ray diffrac-
tion (Siemens diffractometer, D5000, Kristalloflex). The nitrogen
adsorption porosimetry was carried out on an automatic physisorp-
tion porosimeter (Autosorb-1 CLP, Quantachrom, USA). The FT-IR
spectra were recorded on a PerkinElmer spectrometer (System
2000). The 29Si and 13C MAS NMR was obtained using a Bruker
(DSX-300) machine at the NMR research centre in IIS Bangalore.
The scanning electron microscopy (SEM) (Leica Cambridge S360)
and energy dispersive spectrometry (EDX) (Edax Falcon System)
were used to study the surface morphology of the catalysts. The
TEM micrographs were obtained using Philips CM12 equipment.
Thermogravimetric analysis (TGA) was performed using a TGA
SDTA851e instrument.
3.2. Fourier transformed infrared spectroscopy analysis
The FT-IR spectra of RHACCl and RHAPhSO3H are shown in Fig. 1.
The FT-IR spectra of RHA and RHACCl had been described previ-
ously by Adam et al. [5]. The typical broad band around 3458 cm−1
is usually assigned to O–H vibration of SiO–H and HO–H of adsorbed
water [18]. The stretching absorption band of NH appeared at
3198 cm−1. The stretching vibration of C–H aromatic ring was
observed at 3064 cm−1. The stretching aliphatic C–H vibration was
observed at 2929 cm−1. The band at 1633 cm−1 was assigned to
the bending vibration of trapped water molecules within the sil-
2.4. The surface acidity of RHAPhSO3H
The sample together with a beaker of pyridine was placed in a
desiccator equipped with a valve connection to a membrane vac-
uum pump (AMB Greiffenberger Antriebstechnik, model MZ2C, CE
2002/06). The system was evacuated for 1 h at a rate of 1.7 m3 h−1
.
ica matrix. The peaks at 1602–1547 cm−1 could be due to the C
bond. The asymmetric and symmetric stretching of the O
C
O
The system was then closed and the desiccator was kept under vac-
uum for 48 h to equilibrate. The atmosphere in the desiccator was
evacuated again for 1 h at the same pumping rate. The sample was
then removed and analyzed by FT-IR using KBr disc. The spectrum
was recorded in absorption mode.
S
fragment was observed at 1319, 1157 and 1114 cm−1 respectively.
The Si–C stretching vibration was observed at 1244 cm−1. The band
at 1075 cm−1 was attributed to the Si–O–Si stretching vibrations in
RHACCl. This peak had shifted to 1035 cm−1 in RHAPhSO3H. The
FT-IR spectrum gives a good indication of the successful immobi-
lization of the sulfanlic acid onto RHACCl.
2.5. Cation exchange capacity (CEC)
The cation exchange capacity (CEC) was done according to
reported method [17]. Sodium chloride, 1.0 g was dissolved in
25 mL of distilled water in a conical flask with a magnetic stirrer.
A 1.0 g ( 10 mg) sample of RHAPhSO3H was added and left to stir
for 30 min. Phenolphthalein (2–3 drops) was added and the sample
was titrated with standard NaOH solution. An average of 3 separate
titrations were performed to obtain an average value for the cation
exchange capacity of RHAPhSO3H.
3.3. Powder X-ray diffraction pattern
The X-ray diffraction pattern (not shown) shows a broad peak
at ca. 22.36◦, which indicated the amorphous nature of the sample.
This was similar to the observed diffraction pattern for amorphous
silica in RHA and RHACCl as reported earlier [5].
3.4. N2 adsorption–desorption analysis
2.6. tert-Butylation of phenol
Fig. 2 shows the nitrogen adsorption isotherm obtained for
RHAPhSO3H with the inset showing the pore size distribution
graph. The hysteresis loop observed in the range of 0.4 < P/P0 < 1.0,
is associated with capillary condensation according to IUPAC clas-
sification. The isotherm exhibited by RHAPhSO3H is of type IV and
exhibited an H3 hysteresis loop [19].
The alkylation was carried out in liquid phase under argon in
a 50 mL round bottom flask, equipped with a magnetic stirrer and
water condenser. The water for the condenser was chilled to 2–3 ◦C
in ice. A pump (Astro 500 liquid filter) was used to circulate this
chilled water through the condenser to minimize the evaporation

44
F. Adam et al. / Applied Catalysis A: General 399 (2011) 42–49
RT
Sodium silicate
NaOH
RHA
+
C1
C3
O
O
O
Si
Si
SiO 2
Cl
(a)
C2
HNO 3 / RT
Sodium silicate
+
(EtO) 3SiCH 2CH2CH2Cl
+
pH = 3 / (45 min.)
O
O
SiO 2
Cl
(b)
HO
C5
C1
C3
C6
C
O
C4
7SO 3
H
O
Si
SiO 2
+
NH
C2
O
C5
*
C6
*
(a1)
sulfanilic acid /Toluene
/ Et 3
N
Ref. 110 oC / (48 h)
O
O
Si
NH
SO 3H
SiO 2
HO
(b2)
Scheme 1. The reaction sequence for the synthesis of RHAPhSO3H. Two possible structures of the catalyst are postulated based on 13C and 29Si MAS NMR spectral studies.
The approximate times taken for the completion of the experimental process are also shown.
spectrum of RHACCl showed chemical shifts attributed to Q4 and
Q3 silicon atoms at −109.92 and −100.65 ppm respectively [5].
These chemical shifts were shifted to −110.9 and −100.9 ppm
respectively in RHAPhSO3H as shown in Fig. 3a. RHAPhSO3H
showed a chemical shift at −65.8 ppm which was assigned to
the T3 silicon centre. A similar chemical shift was observed at
−65.2 ppm for RHACCl. The T2 silicon centre in RHACCl was
present at −57.4 ppm which was however shifted to −56.3 ppm in
RHAPhSO3H.
The specific surface area of RHAPhSO3H was found to be
308 m2 g−1 with an average pore volume of 0.6 cm3 g−1. The pore
size distribution of RHAPhSO3H reveals a narrow pore size range
of ca. 4 - 7 nm. This can be advantageous as it can accommodate
only specific size range of molecules in its pores, leading to catalytic
selectivity. The specific surface area of RHACCl was 633 m2 g−1 with
an average pore volume of 0.7 cm3 g−1 [5]. The decrease in the sur-
face area of RHAPhSO3H, however, could be due to the reduction of
the surface sites due to the immobilization of sulfanilic acid caus-
ing the surface to be over crowded with the ligand network on the
surface and thus blocking the smaller pores.
3.6. 13C MAS NMR
The 13C MAS NMR spectrum of RHAPhSO3H is shown in Fig. 3b.
It clearly shows three chemical shifts at 8.5, 24.7 and 45.5 ppm
attributed to the C1, C2 and C3 carbon atoms of the propyl chain
(Scheme 1). Similar chemical shifts for RHACCl appeared at 10.3,
26.7 and 47.6 ppm, respectively [5].
3.5. 29Si MAS NMR
The 29Si MAS NMR spectra of RHA [16] showed only the pres-
ence of Q4 (Si(OSi)4) and Q3 (Si(OSi)3(OH)) silicon centres’, having
chemical shifts at −110 and −100 ppm, respectively. The 29Si NMR
101
1157 1037
1245
1933
1113
RHA-SO3H
1547
1602
1633 1499
1423
829796
2647
%T
30642929
3198
571
560
428
(b)
RHAPhSO3H
1319
3441
686
690
1244
1010
1157 1035
1141
(a)
RHACCl
562
2960
798
1638
955
467
1214
3458
1075
4000.0 3600
3200
2800
2400
2000
1800
1600
1400
1200
1000
800
600
400.0
cm-1
Fig. 1. The FT-IR spectra of (a) RHACCl and (b) RHAPhSO3H.

F. Adam et al. / Applied Catalysis A: General 399 (2011) 42–49
45
499–892 ◦C was attributed to the decomposition of the remaining
organic groups. The FT-IR for this mass loss showed the presence
of –CO2–, C4H6O2 and H2O moiety. The loss of water, specifically
resulting from the condensation of silanol groups to form siloxane
structure at the higher temperature.
500
400
300
200
100
0
3.0
2.5
2.0
1.5
1.0
0.5
0.0
3.9. The surface acidity
0
10
20
30
40
50
60
Pore Diameter [nm]
The concentrations of exchangeable proton on the RHAPhSO3H
was determined in water by exchanging the ionisable proton with
excess Na+ (from NaCl) followed by titrating the H+ liberated with
standard NaOH solution. The cation exchange capacity (CEC) was
found to be 23.0 mmol per 100 g RHAPhSO3H.
0.0
0.2
0.4
0.6
0.8
1.0
The nature of the acid sites on the surface of RHAPhSO3H was
also studied qualitatively by pyridine adsorption and desorption
experiment monitored by FT-IR spectroscopy. The FT-IR spectra
(Supplementary 1) of RHAPhSO3H after pyridine adsorption show
strong bands at 1484 and 1434 cm−1 which are due to the inter-
action between adsorbed pyridinium ion and the surface Brønsted
acid site [21]. No Lewis acid sites were revealed on the catalyst
surface by this study.
P/Po
Fig. 2. The N2 adsorption–desorption isotherm of RHAPhSO3H. Inset shows the pore
size distribution.
The broad chemical shifts at 120, 120.8, 121.2 and 124.8 ppm
were assigned to the benzene ring (C4–C7). These results show that
the sulfanilic acid was immobilized onto silica via the formation of
the C–N bond as shown in Scheme 1.
3.10. Alkylation study
3.7. Transmission electron microscopy
Tert-butylation of phenol using TBA was carried out over
RHAPhSO3H by varying the phenol:TBA ratio. The products
obtained were 2-(tert-butyl)phenol (2-TBP), 4-TBP and 2,4-di(tert-
butyl)phenol (2,4-DTBP). Scheme 2 shows the reaction and the
products detected over RHAPhSO3H. Small amounts of tert-
butylphenol ether (TBPE) and isobutene (IBE) and its dimmers were
also observed.
The scanning electron micrographs (SEM) are shown in Fig. 4a
and b. The SEM shows that the sample is granular and porous. The
granular particles of different sizes are arranged randomly on the
surface building up the porous structure of the material.
Transmission electron micrographs (TEM) of RHAPhSO3H are
also shown in Fig. 4c and d. The particles are irregular in shape and
arranged in a non-orderly manner. These irregular particles may
result from the non directional arrangement of pores due to the
amorphous nature of the resulting material [20].
3.10.1. The effect of time on TBA conversion
The conversion of TBA as a function of time is shown in Fig. 5.
Table 1 shows the product distribution and selectivity at selected
time intervals. After 1 h, the conversion of TBA was ca. 19%. The
TBPE and IBE were the main products. During the second hour,
the formation of 2-TBP and 4-TBP was observed. The formation of
2,4-DTBP was observed after 6 h. As the reaction time increased
further, the conversion of TBA and the selectivity of 2-TBP, 2,4-
DTBP and 4-TBP increased, while the product selectivity towards
TBPE and IBE decreased. The decrease in the selectivity of TBPE and
IBE could be due to further reaction of these products in the pres-
ence of RHAPhSO3H. The conversion of TBA approached 95% at 9 h
(Fig. 5). At this instant calculation showed a 57% conversion of phe-
nol. No change in the conversion of TBA after 9 h was observed. The
optimized reaction time was thus found to be 9 h.
3.8. Thermal analysis by TGA FT-IR
The FT-IR spectrum (not showing) of the evolved gasses from
TGA analysis (TGA-FTIR) was used to characterize each mass loss.
The TGA of RHAPhSO3H showed four characteristic decomposition
stages: the first starting at 34–142 ◦C, assigned to water loss (ca.
2.45%) which was confirmed by the FT-IR. The second mass loss (ca.
6.60%, 0.67 mg) occurred between 142 and 357 ◦C, assigned to the
decomposition of sulfanilic acid on the silica surface. In accordance
to this mass loss, it was calculated that 0.0039 mmol of sulfanilic
acid was loaded on 10.25 mg of RHAPhSO3H. Similar calculation
had been reported for saccharine [6]. The third continuous mass
loss (ca. 11.68%) occurred between 357 and 499 ◦C, which was
attributed to the decomposition of unreacted chloropropyl group
anchored on the silica. The fourth mass loss (ca. 10.60%) in the range
The reaction catalysed by 3.2 mg (0.019 mmol) of sulfanilic acid
(homogenous) also showed 82.8% conversion of TBA. The yield con-
tained 48% of 2-TBP, 22% of 4-TBP, and 25% of TBPE in the same
reaction time. We believe this is the first report of sulfanilic acid
Fig. 3. The solid sate NMR spectra of RHAPhSO3H. (a) The 29Si MAS NMR spectrum (b) The 13C MAS NMR spectrum.

46
F. Adam et al. / Applied Catalysis A: General 399 (2011) 42–49
Fig. 4. The SEM and TEM images of RHAPhSO3H: (a) SEM at 900k magnification, (b) SEM at 1500k magnification. (c and d) TEM at 260k magnification.
being used as a catalyst in a chemical reaction, be it homogeneous
or heterogeneous. However, it is very clear that the heterogeneous
RHAPhSO3H is a much more efficient catalyst than the homogenous
sulfanilic acid.
3.10.2. The effect of catalyst mass on the catalytic activity
The influence of catalyst mass on the activity and selectivity of
TBA was studied in the range of 25–200 mg of RHAPhSO3H at 120 ◦C
using 1:1 (phenol:TBA) mole ratio for 9 h. The product distribu-
tion and selectivity during each reaction time are shown in Table 2.
O
+
IBE
TBPE
O H
O H
C H 3
R H A P hS O 3
H
H O
C H 3
+
120 oC / 9 h
2-TBP
C H 3
O H
O H
O H
+
+
4-TBP
2,4-TTBP
2,4,6-TTBP
Scheme 2. Tert-butylation of phenol over RHAPhSO3H and the structures of the products as determined by GC and confirm by GC–MS.

F. Adam et al. / Applied Catalysis A: General 399 (2011) 42–49
47
Table 1
Catalytic activity and product distribution as a function of reaction time. The reaction conditions: molar ration of phenol:TBA = 1:1, 50 mg mass of catalyst at 120 ◦C.
Reaction time (h)
Conversion of TBA (mol%)
Selectivity of product (mol%)
2-TBP
4-TBP
2,4-DTBP
TBPE
IBE
0
1
2
3
4
5
6
7
8
9
0
10
26
32
44
66
82
86
90
95
0
2
3
4
8
16
26
40
49
52
0
0
1
2
3
0
0
0
0
0
0
1
4
4
5
0
87
89
90
82
70
56
32
23
16
0
10
7
4
7
7
7
6
3
7
10
17
21
23
3
Table 2
Catalytic activity and product distribution as a function of catalyst mass. The reaction conditions: molar ratio of phenol:TBA = 1:1, time 9 h, and temperature 120 ◦C.
Catalyst amount (mg)
Conversion of TBA (mol%)
Selectivity of product (mol%)
2-TBP
4-TBP
2,4-DTBP
TBPE
IBE
25
50
100
200
85
96
95
81
44
53
52
43
19
23
23
18
3
5
5
3
27
17
16
30
6
3
3
5
100
favours the formation of alkyl substitution on the ring. No product
was observed at room temperature. However, in a control reaction
where no catalyst was used, only TBPE was formed and the conver-
sion of TBA was found to be ca. 8.63% after 9 h. This shows that the
catalyst and the correct temperature are essential for the alkylation
to take place.
Variable
Homogeneous Sulfanilic acid
RHAPhSO3H
80
60
40
20
0
3.10.4. The effect of reactant molar ratio
The effect of the molar ratio of phenol:TBA on the conversion
of TBA and the product selectivity is shown in Table 4. The result
shows that the conversion of TBA increased progressively when
the mole ratio was varied from 1:4 to 1:1. However, when the mole
ratio was varied from 2:1 to 4:1, the TBA conversion decreased. The
maximum conversion of 97% was obtained when the mole ratio of
phenol:TBA was 2:1. However, the selectivity of 2-TBP was only
42% which was lower than the selectivity afforded by 1:1 mole ratio
which was ca. 53% 2-TBP. Under the same condition the conversion
of phenol was ca. 57%.
The conversion and selectivity for 2-TBP, 4-TBP and 4,2-DTBP
decreased gradually when the mole ratio of TBA increased from
1:2 to 1:3. The selectivity of TBPE was 100% when the mole
ratio was 1:4 at the percentage conversion of 15%. On the other
hand, when the molar ratio was 4:1, the conversion of TBA was
81 mol%. This indicates that an over population of TBA will pref-
erentially adsorb and block the active sites on the catalyst. This
will reduce the possibility of reaction with the phenol. This is
also a good indication that both reactants must be present side-
by-side on the catalyst for reaction to take place. The catalytic
reaction should thus be a second order SN2 reaction, eliminating
the possibility of free carbocation species leading to purely SN1
reaction. Under the experimental condition, the kinetic data sat-
isfies the pseudo-first order reaction kinetics in the presence of a
large excess of one reactant. Given these experimental data, one
could also essentially select etherification or alkylation products
by simply selecting the appropriate mole ratio between phenol and
TBA.
0
2
4
6
8
10
12
Time (h)
Fig. 5. Conversion of TBA in the alkylation of phenol with TBA over RHAPhSO3H
and homogeneous sulfanilic acid as a function of reaction time. Reaction conditions:
molar ration of phenol:TBA = 1:1, mass of catalyst = 50 mg (3.2 mg homogenous cat-
alyst) reaction temperature 120 ◦C.
When the catalyst mass was increased from 25 mg to 50 mg the
conversion of TBA increased from 85 to 96% and the selectivity of
2-TBP reached ca. 53%.
When the catalyst mass was increased to 100 and 200 mg, the
conversion of TBA decreased slightly. A two fold increase in TBPE
was observed when the catalyst mass was 200 mg resulting in a
decrease in the selectivity of 2-TBP (43%).
3.10.3. The effect of reaction temperature on the conversion of
TBA
The effect of temperature on the conversion of TBA and the cat-
alytic performance was studied in the range of 100–120 ◦C and
the results are summarized in Table 3. When the temperature
increased the conversion and selectivity for all products increased.
The selectivity for TBPE was observed to be very high (91%) at the
lower temperature. This could be due to the molecular associa-
tion that reduces adsorption and dissociation on the active sites
[22]. This could also be due to the lower activation energy needed
for etherification resulting in the formation of TBPE. Increasing the
temperature to 120 ◦C results in a six fold decrease in TBPE and
3.10.5. Reusability studies of RHAPhSO3H
The catalyst was regenerated by washing with ethanol and then
dried at 170 ◦C for 24 h before each reuse. The elemental analysis
(the value of EDX in bracket) for the regenerated catalyst showed

48
F. Adam et al. / Applied Catalysis A: General 399 (2011) 42–49
Table 3
The catalytic activity and the product distribution as a function of reaction temperature. The reaction conditions: molar ratio of phenol:TBA = 1:1, reaction time 9 h and mass
of catalyst 50 mg.
Temperature (◦C)
Conversion of TBA (mol%)
Selectivity of product (mol%)
2-TBP
4-TBP
2,4-DTBP
TBPE
IBE
100
110
120
20
31
95
2
3
52
0.5
1
23
1
0.3
5
91
4
16
5
2
3
Table 4
The catalytic activity and the product distribution as a function of mole ratio. The reaction conditions: temperature 120 ◦C, time 9 h and 50 mg mass of catalyst.
Molar ratio phenol:TBA
Conversion of TBA (mol%)
Selectivity of product (mol%)
2-TBP
4-TBP
2,4-DTBP
TBPE
100
93
91
17
30
48
12
IBE
1:4
1:3
1:2
1:1
2:1
3:1
4:1
15
27
50
95
97
93
81
0
1
6
53
42
29
53
0
3
2
23
22
13
24
0
1
0
2
1
3
4
1
6
5
2
2
4
Fig. 6. The SEM images of used RHAPhSO3H at 1.4k magnification.
the presence of N (10.17%), Cl (0.7%) and S (7.0%). The results did not
show significant changes in these values. However, some change
in the morphology of the surface was observed as in Fig. 6. The
catalyst had agglomerated into rod shaped material. The result of
the reusability study is shown in Table 5. The catalyst was found
to be stable under the regeneration conditions and could be used
several times without loss of catalytic efficiency.
law according to Eq. (1).
1
ln
= ka(t − t0)
(1)
1 − x
where ka is the apparent rate constant, x is the fractional conversion
of TBA, t is the reaction time, and t0 is the induction period.
The kinetic parameters for the catalyst are tabulated in
Supplementary 3. The pseudo-first order rate constants were cal-
culated from the slope of the experimental plots which gave the
Arrhenius plot as shown in Supplementary 4. The apparent activa-
tion energy was deduced from the Arrhenius plot and was found to
3.10.6. Reaction kinetics
Kinetic parameters were obtained from experiments conducted
at three different temperatures, i.e., 100, 110 and 120 ◦C. The reac-
tion conditions for these experiments were 1:1 molar ratio of
phenol:TBA, 50 mg catalyst mass and 9 h of reaction time. The
data obtained from these experiments were used to determine
the reaction kinetics of the catalyst (RHAPhSO3H) as shown in
Supplementary 2. It was observed that the experimental results
obtained were in good agreement with the pseudo-first order rate
be 10.4 kcal mol−1
.
Krishnan et al. [8] evaluated the apparent activation energy (Ea)
for the alkylation of phenol with tert-butyl alcohol under pressure
in batch mode over various zeolite catalysts. The values obtained
were greater than 6.5 kcal mol−1. They concluded that the reaction
was controlled by surface reaction. The value of Ea obtained in this
Table 5
The conversion of TBA and the product selectivity in the alkylation of phenol as a function of reusability of catalyst. Reaction condition: molar ratio of phenol:TBA = 1:1,
temperature = 120 ◦C, time 9 h and 50 mg catalyst.
Catalyst
Conversion of TPA
Selectivity
2-TBP
4-TBP
2,4-DTBP
5
11
5
3
TBPE
IBE
Fresh
95
96
94
94
53
50
54
51
23
24
25
22
17
4
10
17
3
4
6
6
1 reuse
2 reuse
3 reuse

F. Adam et al. / Applied Catalysis A: General 399 (2011) 42–49
49
CH3
CH3
H3C
CH3
H2O
CH3
H3C
CH3
C+
H3C
H3C
H
O
S
O+
+ H+
OH
H
H3C
H
O
H
CH3
H
O
O
O
O
NH
Si
SiO2
(a)
OH
H3C
CH3
CH3
O
C+
O-
S
O
O
O
H
C
O
Si
N
SiO2
H3C
H3C
HO
CH3
H
(b)
a) Carbocation attack
H+
HO
CH-2
O
H
O-
S
C+
CH3
O
O
O
C
O
Si
H3C
N
SiO2
H
HO
0
b) Alkene attack
Scheme 3. The proposed mechanism for the tert-butylation of phenol over RHAPhSO3H showing the formation of carbocation and the alkene intermediate. (a) The carbocation
route and (b) the alkene route.
study is comparable in magnitude to that of the work of Krishnan
et al. [8].
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.apcata.2011.03.039.
3.10.7. The proposed mechanism
The strong Brønsted acid sites in RHAPhSO3H could protonate
the hydroxyl group in TBA to form the oxonium ion which can easily
liberate H2O molecule to form the carbocation (CH3)3C⊕ [23]. The
formed carbocation could either directly attack the ortho- or para-
position of phenol via formation of transition state (a) as shown in
Scheme 3 to give the ortho- or para-alkyl phenol in a seemingly
pseudo-first order reaction. The carbocation on the catalyst can
also undergo elimination of proton to form the alkene which could
also attack the ortho or para position of phenol via formation of
transition state (b).
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We would like to thank the Malaysian Government for
the USM-RU-PRGS grant (1001/PKIMIA/842020) and RU grant
(10001/PKIMIA/814019) which partly supported this work.
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