Multi SO3H supported on carbon nanotubes: A practical, reusable, and regioselective catalysts for the tert-butylation of p-cresol under solvent-free conditions

Article abstract of DOI:10.1007/s13738-014-0511-x

The present study describes the synthesis, characterization, and catalytic activity of sulfonated multi-walled carbon nanotubes and sulfonated single-walled carbon nanotubes in the tert-butylation of p-cresol. The catalysts were prepared using a chemical and simple process and it characterized by scanning electron microscopy, Fourier transform infrared, and Raman spectroscopy, thermogravimetric analysis, and acid-base titration. The sulfonated multi-walled carbon nanotubes and sulfonated single-walled carbon nanotubes have been used as practical heterogeneous catalytic systems in the tert-butylation of p-cresol under solvent-free conditions. Sulfonated multi-walled carbon nanotubes with the highest total density of SO₃H groups possessed high activity for p-cresol conversion and high selectivity than the sulfonated single-walled carbon nanotubes. This methodology offers some advantages of high selectivity, and high yields, easy work-up, solvent-free conditions, and reusable catalyst. Moreover, the catalytic system was used in scale-up under similar optimized reaction conditions.

Full text of DOI:10.1007/s13738-014-0511-x

J IRAN CHEM SOC (2015) 12:537–549
DOI 10.1007/s13738-014-0511-x
ORIGINAL PAPER
Multi SO H supported on carbon nanotubes: a practical,
3
reusable, and regioselective catalysts for the tert‑butylation
of p‑cresol under solvent‑free conditions
Reza Fareghi‑Alamdari · Mohsen Golestanzadeh ·
Negar Zekri · Zeinab Mavedatpoor
Received: 10 February 2014 / Accepted: 18 July 2014 / Published online: 24 August 2014
©
Iranian Chemical Society 2014
Abstract The present study describes the synthesis, char-
acterization, and catalytic activity of sulfonated multi-walled
carbon nanotubes and sulfonated single-walled carbon nano-
tubes in the tert-butylation of p-cresol. The catalysts were pre-
pared using a chemical and simple process and it characterized
by scanning electron microscopy, Fourier transform infrared,
and Raman spectroscopy, thermogravimetric analysis, and
acid–base titration. The sulfonated multi-walled carbon nano-
tubes and sulfonated single-walled carbon nanotubes have
been used as practical heterogeneous catalytic systems in the
tert-butylation of p-cresol under solvent-free conditions. Sul-
fonated multi-walled carbon nanotubes with the highest total
chemical processes or methodologies has received increas-
ing attention due to their role in safety, energy efficien-
cies, and conservation of the global ecosystem [1]. In this
regard, owing to the simplified recovery and reusability,
the use of heterogeneous catalysts is frequently superior
for many chemical reactions surrounded by both academic
and industrial setting [2]. However, in addition to reus-
ability, one of the most important desiderata in the field
of sustainable chemistry is the replacement of the toxic
and unstable catalysts by safer analogs accompanied with
employing renewable and more accessible energy sources
while demonstrating similar or even higher levels of
activity. Along this line, carbon based catalysts compris-
ing carbon nanotubes (CNTs) have been recognized as
promising tools to attain this goal in organic transforma-
tions [3–9]. Due to the high specific surface area of CNTs,
they are effective supports for immobilization of active
species for the synthesis of solid acid catalysts. Recently,
density of SO H groups possessed high activity for p-cresol
3
conversion and high selectivity than the sulfonated single-
walled carbon nanotubes. This methodology offers some
advantages of high selectivity, and high yields, easy work-up,
solvent-free conditions, and reusable catalyst. Moreover, the
catalytic system was used in scale-up under similar optimized
reaction conditions.
the sulfonated carbon nanotubes (CNTs-SO H) have been
3
achieved by some reported methods [10–15]. The covalent
functionalization of CNTs with sulfonic acid groups pro-
vides stability, considerable solubility and strong surface
Keywords Carbon nanotube · p-Cresol · Solid acid · Tert-
butylation · Solvent-free
acidity. The CNTs-SO H is useful as solid acid catalyst in
3
organic reactions.
Introduction
The synthesis of bulky organic molecules is an impor-
tant area of search for industrial purpose. The tert-butylated
benzene derivatives, typically bulky molecules are com-
mercially important chemical intermediates. They have
been used for the synthesis of antioxidants, antiseptics,
agrochemicals, resins, UV absorber and stabilizer for the
polymers [16–18]. Tert-butylated benzene derivatives, espe-
cially 2-tert-butyl-p-cresol (2-TBC) are widely employed
for the synthesis of antioxidants such as 2,2′-methylene-
bis-(6-tert-butyl-p-cresol) (A.O.2246). The chemical struc-
tures of some antioxidants are shown in Fig. 1.
In the recent years, the exploration for new developments
in the chemical industries and environmentally benign
R. Fareghi-Alamdari (*) · N. Zekri · Z. Mavedatpoor
College of Chemistry and Chemical Engineering, Malek-Ashtar
University of Technology, 16765-3454 Tehran, Iran
e-mail: reza_fareghi@yahoo.com
M. Golestanzadeh
Department of Organic Chemistry, Faculty of Chemistry,
University of Kashan, 87317-51167 Kashan, Iran
1
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J IRAN CHEM SOC (2015) 12:537–549
Fig. 1 Selected example of
some important phenolic anti-
oxidants
(H3C)3C
O
O
O
O
O
O
O
O
C(CH3)3
HO
OH
C(CH3)3
C(CH3)3
(
(
H3C)3C
H3C)3C
HO
OH
C(CH3)3
OH
(H3C)3C
A.O.1010
OH
OH
OH
C(CH3)3
(H3C)3C
C(CH3)3 (H3C)3C
C(CH3)3
CH3
CH3
CH3
OH
A.O.2246
Scheme 1 Products distribution in the tert-butylation of p-cresol under different conditions
Owing to the importance of the described compounds,
extensive studies have been carried out on the tert-buty-
lation of phenol derivatives over heterogeneous catalysts
such as [H-Al-MCM-48] [19], [K–Fe-10] [20], Zeolites
bulky alkylated phenols as a green chemical technology. As
shown in Scheme 1, different products of tert-butylation of
p-cresol present depend upon the strength of the acid sites,
temperature and other reaction conditions.
[
21–27], [Fe–Al-MCM-41] [28], Cu Co Fe O [29],X-
Recently, we studied the catalytic activity and selectivity
of nanoCu Co Fe O in the liquid phase tert-butylation of
0
.5
0.5
2
4
SBA-15 [30, 31], [X-MCM-41-SO H] [32–34], Ionic liq-
3
1-x
x
2
4
uids-SO H [35–38], RHAPhSO H [39], TPA-TiO [40],
p-cresol with methyl-tert-butyl ether (MTBE) [52]. In con-
tinuation of our interest toward application of nanocatalysts
in organic reactions [53–55], we decided to investigate the
3
3
2
X-ZrO2 [41–46], HPW-MCM-41 [47], Zn-Al-MCM-41
48],[Al-MCM-41] [49], and homogeneous catalysts such
as brØnsted acids, (H SO , HF, H PO , and HClO ) and
[
possibility of utilization of CNTs-SO H in the tert-butyla-
2
4
3
4
4
3
Lewis acids (AlCl and BF ) [50, 51].
tion of p-cresol.
3
3
Since the catalytic activity and selectivity of tert-but-
ylation of phenol derivatives are highly depended to the
used catalyst and reaction conditions, unfortunately some
of the reported methods have drawbacks. Some of these
most important disadvantages associated with the current
methods are equipment corrosion, environmental pollu-
tion, and use of toxic organic solvents, production of waste,
low selectivity and yield, non-reusable catalyst and rapid
deactivation of the catalysts. Therefore, it is an important
task to find suitable catalysts for the synthesis of these
Experimental
Materials and apparatus
The CNTs used in this study [SWCNTs: (L = 20–30 μm,
D = 1–3 nm)] [MWCNT: (L = 25–35 μm, D = 15–
20 nm)] have been purchased from Research Institute of
Petroleum Industry (RIPI-Iran). The chemicals used in this
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J IRAN CHEM SOC (2015) 12:537–549
539
Scheme 2 Synthetic route
for preparation of the catalyst
(
MWCNTs-SO H & SWCNTs-
3
SO H)
3
work were purchased from Aldrich and Merck chemical
companies. All reactions were carried out under magnetic
stirring in a 25 mL glass reactor. IR spectra were obtained
with KBr pellets in the range of 400–4,000 cm with a
Nicolet-860 spectrometer. NMR spectra were recorded in
described in Scheme 2. The sulfonated single-walled car-
bon nanotubes (SWCNTs-SO H) were prepared by same
3
procedure.
−
1
General procedure for the ***tert-butylation of p-cresol
CDCl solvent on a Brucker 250 MHz spectrometer DPX-
catalyzed by CNTs-SO H under solvent-free conditions
3
3
3
300 using tetramethylsilane (TMS) as an internal stand-
ard. The Raman spectra were recorded with an Almega
Thermo Nicolet Dispersive Raman spectrometer excited
at 532 nm. Thermogravimetric analysis (TGA) was per-
formed on a mettle TA 4000 instrument at a heating rate of
In a 25 mL flask equipped with a condenser and mag-
netic stirring bar, a mixture of p-cresol (5 mmol, 0.541 g),
MTBE (7.5 mmol, 0.661 g) and MWCNTs-SO H (30 mg)
3
was heated at 90 °C under solvent-free conditions for
10 h. The progress of the reaction was monitored by GC.
After completion of the reaction, acetone (2 × 10 mL) was
added. The catalyst was filtered by simple filtration under
reduced pressure. The obtained liquid was heated by evap-
oration on a rotary evaporator and the remained yellow
compound was purified on a silica gel column (n-hexane:
EtOAc/5:1) for the separation of 2-TBC, 2,6-di-tert-butyl-
p-cresol (2,6-DTBC) and other byproducts. The identity of
the desired product was performed by FT-IR and FT-NMR
spectral data. The same experiment was performed by use
−1
1
0 °C min under nitrogen atmosphere. SEM images of
CNTs were taken on a Philips XL30 SEM instrument. The
identity of the products was confirmed by gas chromatogra-
phy (GC, HP-5890) equipped with a capillary column.
Preparation of the catalysts
In a typical experiment, multi-walled carbon nanotubes
(MWCNTs) (1 g) and 100 mL deionized water were added
in a becher and it sonicated for 30 min. After the sonica-
tion, the solvent was removed under reduced pressure and
the obtained MWCNTs were transferred to another flask
containing [hydrochloric acid (37 %), 25 mL; nitric acid
of SWCNTs-SO H (35 mg).
3
Spectral data of 2-TBC
(63 %), 25 mL] and stirred at 80 °C for 4 h under a nitrogen
atmosphere. Then the solution was filtered under reduced
pressure and the obtained materials were first washed thor-
oughly with deionized water and then dried at 100 °C for
2-TBC: white crystal, mp 50–52 °C, FT-IR (KBr):υ
−1
(cm ) = 3,620, 3,415, 2,960, 2,875, 1,480, 1,440, 1,370,
1
1,235, 870. H-NMR (250 MHz, CDCl ):δ (ppm): 6.5–7.24
3
H
1
0 h. In this step (I), the MWCNTs-COOH was obtained.
(m, 3H, Ar), 4.58 (s, 1H, OH), 2.26 (s, 3H, CH ), 1.39 (s, 9H,
3
ter-butyl). C-NMR, (62.5 MHz, CDCl ): δ (ppm): 151.9,
3 C
135.8, 129.6, 127.8, 127.2, 116.4, 34.4, 29.6, and 20.8.
Recycling of the catalyst
13
After the step (I), the MWCNTs-COOH (1 g) and 50 mL
deionized water were sonicated for 15 min. Then, the water
was filtered and 40 mL H SO (98 %) was added to the set-
2
4
up at 250–270 °C (H SO boils at 335 °C) for 20 h under
2
4
a nitrogen atmosphere. After cooling the solution to room
temperature, the solution was filtered and washed com-
pletely with deionized water for several times. The solid
obtained materials dried at 100 °C for 15 h. These steps are
The reusability of the catalyst was investigated in the reac-
tion of p-cresol and MTBE. After completion of the reac-
tion, the catalyst was separated by simple filtration, and
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J IRAN CHEM SOC (2015) 12:537–549
then washed exhaustively with chloroform, deionized water
and ethanol, and it dried at 80 °C for 10 h before being used
with fresh p-cresol and MTBE. The catalyst can be reused
for 5 runs without efficient loss of its catalytic activity.
which are commercially available were converted to CNTs-
COOH with HCl (37 %) and HNO (63 %). This step was
3
carried out in order to increase the reactivity and purity of
the CNTs and to remove amorphous carbon, fullerene and
catalyst particles such as iron, cobalt and nickel. This step
also introduced oxygen containing groups, mainly carboxyl
groups on the CNTs [58, 59].After this step, H SO (98 %)
The acid–base titration of the catalyst
2
4
The acidic capacity of the CNTs-SO H catalysts was inves-
was reacted with CNTs-COOH to afford the CNTs-SO H.
3
3
tigated by an acid–base titration method [56, 57]. First
1
00 mg of MWCNTs-SO H dispersed in water and soni-
Characterization of the MWCNTs-SO H
3
3
cated in a water bath for 10 min under nitrogen atmosphere
and SWCNTs-SO H
3
to degas CO . Next 10 mL of NaOH (0.098 N) was added
2
and the mixture stirred for 2 h at room temperature. Sub-
sequently, the mixture was filtered through sintered glass
The prepared catalysts were characterized by scanning
electron microscopy (SEM), Fourier transform infrared
(FT-IR) and Raman spectroscopies, thermogravimetric
analysis (TGA) and acid–base titration.
(G-4) and washed several times with deionized water. The
filtrate was then titrated with HCl (0.1 N) until reaching the
neutral point as monitored by phenolphthalein as indicator.
SEM images of the catalysts
Results and discussions
The SEM images of MWCNTs before and after the H SO
2
4
treatment are shown in Fig. 2. The SEM images of MWC-
Preparation of the catalyst
NTs (Fig. 2a) and MWCNTs-SO H (Fig. 2b) indicate that
3
the functional groups such as hydroxyl, sulfonic and car-
boxylic acid have been anchored to the MWCNTs. Com-
The MWCNTs-SO H and SWCNTs-SO H were synthe-
3
3
sized in two steps as described in Scheme 2. First, the CNTs
pared with the pristine MWCNTs, the MWCNTs-SO H
3
Fig. 2 SEM images of a
MWCNTs (normal magnifica-
tion), b MWCNTs-SO H (high
3
magnification), c SWCNTs
(normal magnification), d
SWCNTs-SO H (high magni-
3
fication)
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J IRAN CHEM SOC (2015) 12:537–549
541
−
1
was covered by a layer of foreign matter, resulting in thick-
ened MWCNTs bundles and a denser network of nano-
tubes. In addition, the SEM images of SWCNTs (Fig. 2c)
around 1,700 cm [58], which could be assigned to C=O
stretching mode of the COOH group. In Fig. 3e, another
band that becomes more prominent is the one at about
−1
and SWCNTs-SO H (Fig. 2d) are shown in Fig. 2. The
1,720 cm , which can be assigned to the carbonyl group of
the carboxylic acid group attached to the end and side wall
3
observed morphologies, by SEM images, show significant
difference between SWCNTs and SWCNTs-SO H. It can
of the SWCNTs. The HNO : HCl oxidation process intro-
3
3
be attributed to the presence of –OH; –COOH and –SO H
groups on the side wall of the SWCNTs which can form
hydrogen-bond-type interactions.
duces not only carboxylic acid, but also alcohol or ketone
species [60]. In the Fig. 3b, c weak band at approximately
3
−1
1,150 cm is due to the C–O stretching mode in alcoholic
group [61]. Also, Fig. 3c, f shows IR spectra of MWCNTs-
SO H and SWCNTs-SO H respectively. In Fig. 3c (MWC-
FT‑IR studies of the catalysts
3
3
−
1
NTs-SO H), the broadband centered at about 3,297 cm is
3
FT-IR spectra of the catalysts confirmed that functionaliza-
tion of the CNTs is performed with hydroxyl, carboxylic
and sulfonic acids. Figure 3 shows the FT-IR spectra of
resulted from the OH stretching mode of COOH and SO H
3
groups [61]. In the spectrum of SWCNTs-SO H (Fig. 3f)
3
−1
this band presented at approximately 3,300 cm . H SO
2
4
pristine CNTs, CNTs-COOH and CNTs-SO H of two types
treatment with MWCNTs-COOH also results in the appear-
ance of peaks at about 1,300 and 1,120 cm (Fig. 3c),
3
−1
of the catalyst. The high symmetry presented on pristine
MWCNTs and SWCNTs generates very weak infrared sig-
nals, due to the weak difference of charge states and very
small induced electric dipole. The peaks related to C=C
which corresponds to the SO asymmetric and symmetric
2
stretching modes respectively. In addition, in the low fre-
−1
quency part of spectrum the peak at 550 cm was assigned
to the C–S stretching mode, suggesting the existence of
covalent sulfonic acid groups (Fig. 3c). The presence of the
−
1
double bond at approximately 1,500 cm for MWCNTs
−
1
(
Fig. 3a) and 1,550 cm for SWCNTs (Fig. 3d) are not
seen in the spectra [58].The functionalization of the CNTs
with sulfonic and carboxylic acids breaks the symmetry of
the CNTs, which enhances the generation of induced elec-
tric dipoles and signs as detected. Fig. 3b, e depicts MWC-
NTs-COOH and SWCNTs-COOH respectively. FT-IR
spectrum of the MWCNTs-COOH (Fig. 3b) shows peaks at
sulfonic acid group in the side wall of the SWCNTs-SO H
3
(Fig. 3f) is also demonstrated by the bands at 1,120, 1,300
−1
and 590 cm , which correspond to the symmetric, asym-
metric SO and C–S stretching modes respectively.
2
Raman spectroscopy studies of the catalysts
Raman spectroscopy is a technique frequently used to
characterize CNTs [62]. In the region spectrum (1,300–
−1
1
,600 cm ), two bands are observed showing the charac-
teristic of the CNTs. These bands point the graphite band
(
(
G-band) and disorder and defects of the structure, named
D-band). The ratio between the intensity of the D-band
and the G-band, noted I
value, corresponds to a higher
D/G
3
proportion of ***sp carbon [63]. These features are very
useful for characterizing functionalized CNTs. Fig. 4a–f
presents the Raman spectra of the two types of the catalysts
and Fig. 4a–c illustrates the Raman spectra of MWCNTs,
MWCNTs-COOH and MWCNTs-SO H respectively. As
3
shown in Fig. 4a, the spectrum exhibits two peaks near to
−1
−1
1
,320 and 1,571 cm , of which the peak in 1,571 cm is
identified with the tangential mode (G-band). Also, the ori-
−1
gin of the peak at 1,320 cm is the D-band and its appear-
ance has been tentatively assigned to a symmetry lower-
ing effect, such as the presence of iron, cobalt and nickel
(during synthesis of CNTs) and functional groups such as
hydroxyl, sulfonic and carboxylic acid on the side walls
of MWCNTs. The calculated ID/IG values of MWCNTs,
MWCNTs-COOH and MWCNTs-SO H are 0.24, 0.37
3
and 0.84, respectively. This increasing value implies that
a strong damage to the side walls of the MWCNTs is due
Fig. 3 FT-IR spectra of a MWCNTs, b MWCNTs-COOH, c MWC-
NTs-SO H, d SWCNTs, e SWCNTs-COOH, f SWCNTs-SO H
3
3
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J IRAN CHEM SOC (2015) 12:537–549
Fig. 4 Raman spectra of a MWCNTs, b MWCNTs-COOH, c MWCNTs-SO H, d SWCNTs, e SWCNTs-COOH, f SWCNTs-SO H
3
3
to the functionalized groups. These values for SWCNTs,
as if the tube was breathing. The RBM features are very
useful for characterizing nanotube diameter through the Eq.
(1), where the A and B parameters are determined experi-
SWCNTs-COOH and SWCNTs-SO H (Fig. 4d–f) are 0.22,
3
0
.34 and 0.80 respectively. In according to the two features
in the Raman spectra (D-band and G-band) we make sure
that two types of the catalysts are functionalized.
Third important region in the Raman spectrum of the
CNTs is radial breathing mode (RBM). The RBM region is
in the low frequency of the spectrum. The Fig. 5a shows the
atomic vibration of the carbon atoms in the radial direction,
mentally and d is diameter of the CNTs [62]. For SWC-
t
−1
NTs bundles have been found that A = 234 cm nm and
−1
B = 10 cm (where B is an upshift coming from tube–tube
interaction). For more information, for isolated SWCNTs
−1
−1
these parameters are A = 248 cm nm and B = 0 cm .
The diameters of SWCNTs used in this research were
between 1.04 and 2.66 nm (Fig. 5b). For large diameters,
tubes such as MWCNTs the intensity of the RBM features
is weak or very weak and is hardly observable (Fig. 5c).
A
ωRBM
=
+ B
(1)
dt
TGA studies of the catalysts
TGA thermograms were used to determine the amount of
formed sulfonic and carboxylic acid groups on the side
wall of the CNTs. Figure 6a–d shows thermograms of
CNTs-COOH, and CNT-SO H for two types of the cata-
lysts. The weight loss between 100 and 200 °C is entirely
due to the presence of amorphous carbon, water, metals and
3
Fig. 5 a Atomic vibration of RBM feature in CNTs, b RBM feature
of SWCNTs (overlapping of three sample), c RBM feature of MWC-
NTs
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J IRAN CHEM SOC (2015) 12:537–549
543
Fig. 6 TGA thermograms of a MWCNTs-COOH, b MWCNTs-SO H, c SWCNTs-COOH, d SWCNTs-SO H
3
3
so on [64]. Also, the weight loss between 200 and 600 °C
is completely due to functional groups on the end and side
wall of the CNTs. It is possible that these functional groups
600 °C, and M is the molecular weight of the desorbed
functional groups.
w
ꢀ g
ꢁ
in this research are OH, COOH, and SO H groups. As
R(%) × Mw
L(%) × 12
3
mol
X =
ꢀ
ꢁ
(2)
g
mol
shown in Fig. 6a, the attached COOH groups decomposed
at about ~250 °C and 18 % weight was decreased. After
treatment of the H SO with MWCNTs-COOH, TGA of
Acidity measurement of the catalysts
2
4
the MWCNTs-SO H showed that 49 % weight was deleted.
3
Accordingly these results indicate that 67 % weight can be
ascribed to the covalently of sulfonic and carboxylic acid
groups to the side wall of the MWCNTs. The weight losses
Acid–base titration was employed to determine the den-
sity of sulfonic acid groups on the side wall of the MWC-
NTs-SO H and SWCNTs-SO H. The volume required to
3
3
in the SWCNTs-COOH and SWCNTs-SO H were 10 and
reach the neutral point was subtracted from the initial vol-
ume of NaOH used to obtain the volume of NaOH which
3
3
0.3 % respectively. Taking into account that the covalently
SO H measurements indicated one functionality every ~5
has reacted with SO H group on the side wall MWCNTs
3
3
C atoms in the MWCNTs-SO H. Also, in the SWCNT the
(Eq. 3). The measurement was repeated three times for each
sample and the average calculated. The acid–base titration
3
SO H functional groups measurements were shown one
3
SO H every ~13 C atoms. Results obtained by TGA ther-
mograms showed good agreement with results of FT-IR
spectroscopy and Raman spectroscopy.
For calculation the extent of functionalization per C
atoms, weight loss values were employed together with
the molecular weight of the different groups and the Eq.
showed that the amount of SO H attached to MWCNTs is
3
3
−1
1.80 mmol g . In addition, after preparation of MWCNTs-
COOH, we calculate amount of –OH and –COOH by acid–
base titration as described above. Acid–base titration for
−1
MWCNTs-COOH was 0.15 mmol g . The acid–base titra-
tion results for the SWCNTs-SO H and SWCNTs-COOH
3
−1
(2) was applied [65]. Where X stands for the number of
were 0.94 and 0.10 mmol g respectively.
carbon atoms in the CNTs sample per each covalent func-
tional group, R (%) is the residual mass at 600 °C in the
TGA plot, L (%) is the weight loss in the range of 200 and
NaOH (mmol reacted with SO H)
3
= NaOH (primary mmol) − NaOH (mmol unreacted with SO H) (3)
3
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Catalytic activity
In this research, we studied the influence of catalytic activ-
ity of MWCNTs-SO H and SWCNTs-SO H in the tert-but-
3
3
ylation of p-cresol under different solvents, temperatures,
molar ratios, amounts of the catalyst, and tert-butylation
agent. In addition, we investigated recovery and reusability
of the catalyst in this reaction.
The influence of the solvent in the tert‑butylation
of p‑cresol
In order to choose the reaction media, different solvents
were investigated in tert-butylation of p-cresol. Differ-
ent solvents such as n-hexane, chloroform, acetonitrile,
and nitromethane were chosen as the reaction medium
(Table 1). Also the reaction was carried out under sol-
vent-free conditions. We observed that from n-hexane to
nitromethane the conversion of p-cresol was improved in
tert-butylation of p-cresol catalyzed by MWCNTs-SO H
3
and SWCNTs-SO H. The yield of 2-TBC was increased
3
with increasing polarity of the solvents and the amounts
of the other products were decreased to 1.1 and 1.6 %
for MWCNTs-SO H and SWCNTs-SO H respectively
3
3
(Table 1, entry 4). In addition, we checked the reaction
under solvent-free conditions (Table 1, entry 5). In this
condition, the yield of 2-TBC and conversion of p-cresol
were improved. Hence, we choose the solvent-free con-
ditions, because higher catalytic activity and selectivity
were observed. It should be mentioned that the absence of
organic solvent is in good agreement with green chemistry
principles.
The influence of temperature in the tert‑butylation
of p‑cresol under solvent‑free conditions
The influence of the reaction temperature on the conver-
sion of p-cresol and the selectivity to 2-TBC is shown
in Table 2. The catalytic activity and selectivity of the
MWCNTs-SO H and SWCNTs-SO H can be improved
3
3
by increasing of temperature until 90 °C (Table 2).The
reaction was carried out under different conditions.
Depending on the reaction conditions and the nature cat-
alytic system tert-butylation of p-cresol can be take place
at the oxygen (O-alkylation) (TBCE) and/or at the ring
carbon atoms (C-alkylation). The previous papers indi-
cate that the selectivity is highly depending to the acid
strength of the catalyst, the amount of the tert-butylating
agent, and temperature too, which agrees with the results
reported elsewhere [19, 20, 46]. In general, the C-alkyla-
tion requires stronger acid sites and higher temperature
than for the O-alkylation. The conversion of p-cresol
and the selectivity of 2-TBC reached maxima at 90 °C.
1
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J IRAN CHEM SOC (2015) 12:537–549
545
Table 2 The influence of temperature in the tert-butylation of p-cresol under solvent-free conditions
Entry Temperature MWCNTs-SO H
SWCNTs-SO H
3
3
(°C)
a
a
Conv. p-cresol 2-TBC (%) 2,6-DTBC Other
Conv. p-cresol 2-TBC (%) 2,6-DTBC
Other
products (%)
a
a
b
a
a
a
b
a
(%)
(%)
products (%) (%)
(%)
1
2
3
4
5
50
70
90
54.3
75.8
82.9
85.1
84.3
33.3
60.9
70.7
69.3
64.1
2.9
4.1
8.8
7.1
9.2
18.1
10.8
3.4
47.5
73.9
77.1
83.4
82.8
29.8
60.1
68.7
68.1
60.0
7.8
2.8
9.9
11.0
2.3
6.1
100
120
8.7
9.2
6.1
11.0
12.1
10.7
General reaction conditions: catalyst (25 mg), p-cresol (5 mmol): MTBE (5 mmol) (1:1), solvent-free, Time: 10 h
a
GC yields
b
The other products are TBCE and oligomers
Table 3 The influence of molar ratio in the tert-butylation of p-cresol
Entry Molar ratio
p-cresol:
MWCNTs-SO H
SWCNTs-SO H
3
3
(
a
b
a
Conv. p-cresol 2-TBC (%) 2,6-DTBC
Other products Conv. p-cresol 2-TBC (%) 2,6-DTBC
Other prod-
ucts (%)
MTBE)
a
a
a
a
a
b
a
(%)
(%)
(%)
(%)
(%)
1
2
3
4
5
1:1
82.9
84.8
89.8
90.9
90.5
70.7
74.5
80.1
81.5
76.1
8.8
7.9
3.4
2.4
2.7
0.6
77.1
80.2
87.7
89.3
88.6
68.7
71.9
75.6
77.2
70.1
6.1
6.8
2.3
1.5
4.1
3.0
2.0
1:1.25
1:1.5
1:1.75
1:2
7.0
8.0
8.8
9.1
14.4
Trace
16.5
General reaction conditions: catalyst (25 mg), solvent-free, T = 90 °C, Time: 10 h
a
GC yields
b
The other products are TBCE and oligomers
The TBCE turn into 2-TBC at high temperature and a
slightly further C-alkylation of 2-TBC was happened
with MTBE. It also indicates that lower temperature
favors the formation of TBCE, while higher temperature
favors the formation of 2-TBC. We observed that at tem-
peratures higher than 90 °C the conversion of p-cresol
was improved, while the selectivity of the desired prod-
uct (2-TBC) was decreased for two types of catalysts
ratio up to 1:2, which has been ascribed to the preferen-
tial adsorption of MTBE on the BrØnsted acid sites as
compared with iso-butene (from tert-butyl carbocation).
However, further increases of p-cresol:MTBE molar ratio
to 1:1.5, result in an increasing yield of 2-TBC. But the
selectivity for 2-TBC decreases with increasing p-cresol:
MTBE molar ratio, while the selectivity 2,6-DTBC
increases. This is due to a higher concentration of the
alkylating agent in the reaction medium, which facilitates
the formation of the dialkylated product. The best molar
ratio was 1:1.5.
(
Table 2, entries 4, 5). Hence, we chose 90 °C as opti-
mum temperature.
The influence of molar ratio in the tert‑butylation
of p‑cresol under solvent‑free conditions
The influence of the amount of catalyst in the
tert‑butylation of p‑cresol under solvent‑free conditions
Table 3 shows the effect of the molar ratio p-cresol:
MTBE on the catalytic activity and selectivity of MWC-
NTs-SO H and SWCNTs-SO H in tert-butylation of
The catalytic activity and selectivity of MWCNTs-SO H
3
and SWCNTs-SO H at different amounts of the catalyst are
3
3
3
p-cresol at 90 °C under solvent-free conditions. All of
the results are summarized in Table 3. Conversion of
the p-cresol increases with the p-cresol:MTBE molar
shown in Table 4. As the amount of the catalyst increase,
both the conversion of p-cresol and the selectivity to 2-TBC
first increase and then reach a steady state. The selectivity
1
3

5
46
J IRAN CHEM SOC (2015) 12:537–549
Table 4 The influence amount of the catalyst in the tert-butylation of p-cresol
Entry Catalyst (mg) MWCNTs-SO H
SWCNTs-SO H
3
3
a
a
Conv. p-cresol 2-TBC (%) 2,6-DTBC
Other
products (%) (%)
Conv. p-cresol 2-TBC (%) 2,6-DTBC
Other
products (%)
a
a
b
a
a
a
b
a
(%)
(%)
(%)
1c
2
3
4
5
6
7
–
–
–
–
–
–
–
–
–
10
15
20
25
30
35
25.2
33.3
55.1
89.8
94.1
94.5
6.0
5.1
10.1
9.7
14.1
5.4
1.7
2.7
0.7
0.7
20.1
29.3
54.9
87.7
87.3
92.2
4.1
6.5
8.8
8.9
8.0
4.3
3.1
9.5
2.2
4.2
4.1
3.9
0.9
17.8
43.7
80.1
91.1
91.3
18.3
41.8
75.6
79.1
88.2
7.0
2.3
2.5
General reaction conditions: p-cresol (5 mmol): MTBE (7.5 mmol) (1:1.5), solvent-free, T = 90 °C, Time: 10 h
a
GC yields
b
The other products are TBCE and oligomers
c
Time: 48 h
Table 5 The influence of alkylating agent in the tert-butylation of p-cresol
a
b
Entry tert-butylation agent MWCNTs-SO H
SWCNTs-SO H
3
3
Conv. p-cresol 2-TBC
2,6-DTBC Other
Conv. p-cresol 2-TBC
2,6-DTBC Other
(%)
c
c
c
d
c
c
c
c
products (%)c
d
(%)
(%)
(%)
products (%) (%)
(%)
1
2
MTBE
94.1
94.5
91.1
85.6
2.3
6.7
0.7
2.2
92.2
91.8
88.2
81.4
3.1
7.7
0.9
2.7
tert-BuOH
a
b
c
d
General reaction conditions: catalyst (30 mg), p-cresol (5 mmol): MTBE (7.5 mmol) (1:1.5), solvent-free, T = 90 °C, Time: 10 h
General reaction conditions: catalyst (35 mg), p-cresol (5 mmol): MTBE (7.5 mmol) (1:1.5), solvent-free, T = 90 °C, Time: 10 h
GC yields
The other products are TBCE and oligomers
Fig. 7 The reusability of the
MWCNTs-SO H
3
to 2,6-DTBC decreases. However, a strong acid system
does not lead to further alkylation of 2-TBC with MTBE.
The highest conversion of p-cresol and selectivity of 2-TBC
were observed using 30 and 35 mg catalyst for MWCNTs-
SO H and SWCNTs-SO H respectively (Table 4, entries 6,
The influence of alkylating agent in the tert‑butylation
of p‑cresol under solvent‑free conditions
In this part, we investigated the influence of alkylating agent
in the catalytic activity and selectivity of the catalyst (Table 5).
We observed that MTBE for two types of the catalyst are suit-
able while tert-BuOH decreased the selectivity of the 2-TBC.
3
3
7). In addition, it is well known that in the absence of the
catalyst, the reaction was not carried out (Table 4, entry 1).
1
3

J IRAN CHEM SOC (2015) 12:537–549
547
Scheme 3 The proposed reac-
tion mechanism
Table 6 Comparison of
the results obtained for the
tert-butylation of p-cresol
catalyzed by CNTs-SO H with
3
those obtained by the recently
reported catalyst
Entry
Catalyst
T (°C)
Conversion of p-cresol (%)
Yield (%)
-TBC 2,6-
Ref.
2
DTBC
1
2
3
4
5
6
7
8
9
1
1
1
HPW@MCM-41
K10-FeAl20
TPA@ZrO2
110
80
85
78
6.7
[47]
[20]
[43]
[48]
[40]
[44]
[49]
[36]
[52]
[46]
–
86.7
81.4
98.6
89.5
92.4
90.4
90
1.7
18.1
–
100
120
130
130
90
61
Zn-Al-MCM-41
TPA@TiO2
93.2
82
7.5
6.3
–
WO -ZrO
69.8
88.2
80
x
2
Al-MCM-41
Ionic Liquid-SO H
70
9.5
0
3
Cu1-xCo Fe O
4
100
80
70
100
97
x
2
0
1
2
12-TPA-ZrO2
83
–
MWCNTs-SO H
90
94.1
92.2
91.1
88.2
2.3
3.1
This work
This work
3
SWCNTs-SO H
90
3
The reusability and stability of the catalyst
acidic catalyst do not recovered even one time, in contrast
the CNTs-SO H catalyst can be filtered and reuse several
3
For practical applications of a heterogeneous catalyst, level
of reusability is a very important feature. The homogeneous
times without significant loss of its catalytic activity. We
examined the reusability of the MWCNTs-SO H in the
3
1
3

5
48
J IRAN CHEM SOC (2015) 12:537–549
multiple sequential reaction of p-cresol and MTBE (Fig. 7).
After each run, the catalyst was isolated by filtration,
washed exhaustively with chloroform, deionized water and
ethanol, and it dried at 80 °C for 10 h before being used
with fresh p-cresol and MTBE.
we conclude that the excellent catalytic activity and selec-
tivity of the MWCNTs-SO H and SWCNTs-SO H can be
correlated to solvent, temperature, molar ratio, amount of
the catalyst, and alkylating agent. We observed that the
best conditions for the catalytic activity and selectivity of
3
3
MWCNTs-SO H are at 90 °C, molar ratio (1:1.5), catalyst
3
The proposed reaction mechanism
30 mg, 10 h, MTBE as alkylating agent, and solvent-free
conditions. Also, the best conditions for SWCNTs-SO H
3
The suggested mechanism in Scheme 3 seems to be rea-
are as the same conditions for the MWCNTs-SO H. But
3
sonable for the tert-butylation of p-cresol with MTBE cata-
the amount of the SWCNTs-SO H is 35 mg. The results
3
lyzed by CNTs-SO H under solvent-free conditions. The
showed that in this reaction the MWCNTs-SO H is a lit-
3
3
methoxy group in MTBE is first activated by protonation
tle better than SWCNTs-SO H, because the acid strength
3
with CNTs-SO H to give I. The oxonium ion can easily
of the MWCNTs-SO H was higher than SWCNTs-SO H.
3
3
3
liberate CH OH molecule to form II. The carbocation II
We also investigated the recycling of the MWCNTs-SO H
3
3
on the catalyst can also undergo elimination of proton to
form the alkene III. The formed carbocation could directly
attack the ortho position of p-cresol via formation of transi-
tion state (a). The alkene (III) could also attack the ortho
position of p-cresol via formation of transition state (b).
The attack of (II) and (III) on the p-cresol gives the species
and it reused in five consecutive runs and showed slight
decrease of the catalytic activity, selectivity and conver-
sion of p-cresol after each recycling. In addition we car-
ried out tert-butylation of p-cresol in the scale-up.
Acknowledgments We gratefully acknowledge the financial sup-
port by Malek-Ashtar University of Technology (MUT). We thank Dr.
Rashidi in Research Institute of Petroleum Industry (RIPI) for useful
help.
(IV) and (V), respectively, which is subsequently converted
to the 2-TBC (VI) and release CNTs-SO H for the next
3
catalytic cycle.
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