Friedel–Crafts benzylation of toluene catalyzed by ZnCl2/SiO2 heterogeneous catalyst to para- and ortho-mono-benzylated toluene

Article abstract of DOI:10.1007/s13738-020-01888-0

A series of catalysts was prepared for the liquid-phase Friedel–Crafts benzylation of toluene with benzyl chloride (BC) by impregnating rice husk ash silica with ZnCl₂ (3?wt%, 6?wt%, 9?wt%, and 12?wt%) via a wet impregnation method. The XRD analysis indicates that the catalysts were amorphous with ill-defined pore systems. The XPS analysis detected the coexistence of ZnO nanoparticles together with ZnCl₂ on the catalyst surface, whereas the ²⁹Si NMR analysis indicates the formation of Si–O–Zn bond. Quantitative conversion of benzyl chloride (100%) was achieved within 3?h at 353?K when a catalyst with 9?wt% ZnCl₂ was used due to its narrow pore size and high surface area (635?m²?g^?1). Para- and ortho-mono-benzylated toluene was obtained as the products. The reaction is proposed to take place via weak attraction between benzyl chloride and the Zn through its chlorine atom. The catalyst was recycled four times with minimum loss (8%) in activity. The benzylation of benzene, toluene, p-xylene, and anisole followed the classical mechanism of Friedel–Craft-type acid-catalyzed benzylation reaction. The BC conversion increased in the order of toluene = p-xylene > anisole > benzene. The catalyst was also screened to be active in the benzoylation of toluene with benzoyl chloride (BOC). The conversion of BOC was 45% with selectivity toward 2-methylbenzophenone (50%) and 3-methylbenzophenone (50%).

Full text of DOI:10.1007/s13738-020-01888-0

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-020-01888-0
ORIGINAL PAPER
Friedel–Crafts benzylation of toluene catalyzed by ZnCl /SiO
2
2
heterogeneous catalyst to para‑ and ortho‑mono‑benzylated toluene
Anwar Iqbal1,3 · Kok‑Hou Tan · U. S. Shaari · N. I. N. B. Ahmad · Farook Adam · Hwei Voon Lee ·
1
1
1
1
2
3
1
4
5
5
Rahimi M. Yusop · N. H. H. Abu Bakar · Hooi Ling Lee · Hariy Pauzi · Muhammad Fadhirul Izwan Abd Malik ·
Ahmad Fadly Jusoh · Lee D. Wilson · M. N. Ahmad · M. Hazwan Hussain · Mohamad Nasir Mohamad Ibrahim
6
7
8
1
1
Received: 28 May 2019 / Accepted: 17 February 2020
©
Iranian Chemical Society 2020
Abstract
A series of catalysts was prepared for the liquid-phase Friedel–Crafts benzylation of toluene with benzyl chloride (BC) by
impregnating rice husk ash silica with ZnCl (3 wt%, 6 wt%, 9 wt%, and 12 wt%) via a wet impregnation method. The XRD
2
analysis indicates that the catalysts were amorphous with ill-deꢀned pore systems. The XPS analysis detected the coexistence
2
9
of ZnO nanoparticles together with ZnCl on the catalyst surface, whereas the Si NMR analysis indicates the formation of
2
Si–O–Zn bond. Quantitative conversion of benzyl chloride (100%) was achieved within 3 h at 353 K when a catalyst with
2
−1
9
wt% ZnCl was used due to its narrow pore size and high surface area (635 m g ). Para- and ortho-mono-benzylated
2
toluene was obtained as the products. The reaction is proposed to take place via weak attraction between benzyl chloride and
the Zn through its chlorine atom. The catalyst was recycled four times with minimum loss (8%) in activity. The benzylation
of benzene, toluene, p-xylene, and anisole followed the classical mechanism of Friedel–Craft-type acid-catalyzed benzylation
reaction. The BC conversion increased in the order of toluene=p-xylene>anisole>benzene. The catalyst was also screened
to be active in the benzoylation of toluene with benzoyl chloride (BOC). The conversion of BOC was 45% with selectivity
toward 2-methylbenzophenone (50%) and 3-methylbenzophenone (50%).
Keywords Rice husk · Rice husk ash silica · Friedel–Crafts benzylation · ZnCl · Para- and ortho-mono-benzylated toluene
2
Introduction
benzyl alcohol (BOH) is among the types of Friedel–Crafts
reactions used to produce important products such as mono-
benzylated toluene (MBT), benzyltoluene, benzoic acid, and
substituted diphenyl methane. These products, especially
MBT, are a valuable commodity in pharmaceuticals and
fragrance industries. Such compounds are also being used
Friedel–Crafts reactions are widely used for the synthesis
of substituted aromatic compounds [1]. In this reaction, the
C–H bond will be converted to the C–C bond in the presence
of a catalyst. Benzylation of toluene, benzene, and xylene
that utilize alkylating agents such as benzyl chloride (BC) or
5
*
Anwar Iqbal
Science and Engineering Research Centre (SERC),
anwariqbal@usm.my
Universiti Sains Malaysia, Engineering Campus,
4
300 Nibong Tebal, Seberang Perai Selatan, Penang,
1
2
School of Chemical Sciences, Universiti Sains Malaysia,
Malaysia
1
1800 Minden, Penang, Malaysia
6
7
Centre for Global Archaeological Research, Universiti Sains
Malaysia, 11800 Minden, Penang, Malaysia
Nanotechnology and Catalysis Research Centre
(
NANOCAT), Institute of Postgraduate Studies, Universiti
Department of Chemistry, University of Saskatchewan,
Malaya, 50603 Kuala Lumpur, Malaysia
1
10 Science Place, Room 156 Thorvaldson Building,
3
4
School of Chemical Sciences and Food Technology, Faculty
of Science and Technology, Universiti Kebangsaan Malaysia,
Saskatoon S7N 5C9, Canada
8
Experimental and Theoretical Research Lab, Department
of Chemistry, Kulliyyah of Science, International
Islamic University Malaysia, Bandar Indera Mahkota,
25200 Kuantan, Pahang, Malaysia
4
3600 UKM Bangi, Malaysia
Nanomaterials Research Group, Universiti Sains Malaysia,
1800 Minden, Penang, Malaysia
1
Vol.:(0
1
123456789)
3

Journal of the Iranian Chemical Society
as monomers for polycarbonate resins, heat-transfer ꢁuids,
aromatic solvents, and ꢀne chemicals [2–6].
and RHA and used in the Friedel–Crafts reactions involving
various types of aromatic compounds. For the Friedel–Crafts
benzylation of toluene, Fe- and 4-(methylamino)benzoic acid-
incorporated silica from RH improved the formation of mono-
benzylated toluene to almost 99%. The nitrogen atom from the
benzoic acid formed dative bonds with the carbocation ꢀxing
them at the active sites enabling better interaction with the
toluene to form more mono-benzylated toluene compared to
di-benzylated toluene [13].
Traditionally, strong Lewis and mineral acid catalysts
such as ZnCl , AlCl , FeCl , HF, H SO , TiCl , and BF
2
3
3
2
4
4
3
were employed as homogeneous catalysts in this electro-
philic substitution reaction [1, 3, 7]. Although these catalysts
are very active, they are diꢂcult to recycle. Besides, the
reaction requires an over-stoichiometric amount of catalysts
that generate toxic and corrosive waste [8, 9]. Heterogeni-
zation of the active sites on the surface of a solid can solve
these problems. The heterogeneous catalytic system has
several advantages over the homogeneous catalytic system,
as follows: (1) The solid catalyst can be separated easily
from the reaction mixture through ꢀltration, (2) it produces a
higher yield of products, (3) it requires only very small quan-
tities of the catalyst, and (4) it can be recycled many times.
Rice husk ash-supported indium was synthesized for the
3
+
benzylation of benzene and substituted benzenes. The In
ions were found to be homogeneously distributed on the
surface of the catalyst. The total conversion of benzene was
achieved with ca. 90% selectivity toward diphenylmethane
(DPM). The reactivity trend was shown to be as follows:
benzene>toluene>ethyl benzene>anisole, where this trend
was opposite to the classical acid-catalyzed Friedel–Crafts-
type benzylation reaction [14]. The silica was also used
The surface of silica (SiO ) consists of an abundant
2
amount of silanol (Si–OH) groups which can form bonds
with Lewis acids via a condensation reaction. Lewis acids
supported onto silica have shown remarkable catalytic activ-
ity in various types of Friedel–Crafts reactions. He et al.
3
+
3+
3+
as a support for Al and Ga ions. Similar to the Fe
3
+
3+
ion, the Al and Ga ions were homogeneously distrib-
uted. The selectivity of DPM when the catalysts were used
was ~ 80–86% with 100% BC conversion [15], where the
silica supports used contain mesopores.
[
10] explored the catalytic activity of AlCl immobilized on
3
ordered (SBA-15 and MCM-41) and disordered silica (SiO
2
and SiO -Gel) for the Friedel–Crafts alkylation of benzene
In the present study, we explored the potential of ZnCl
2
2
with a linear chain oleꢀn, with 1-dodecene as the alkylating
agent. Their ꢀndings indicate that the pore channel struc-
ture strongly inꢁuences the catalytic activity of the cata-
lysts. The total conversion of 1-dodecene was achieved when
incorporated on silica from rice husk ash (ZnCl /SiO ) cata-
2
2
lysts for the Friedel–Crafts benzylation of toluene with BC
as the benzylation agent. Zinc chloride (ZnCl ) is widely
2
available and moderately active compared to other Lewis
AlCl immobilized on SBA-15 was used with 50% product
acids. The use of ZnCl as a catalyst for Friedel–Crafts
3
2
selectivity of 2-linear alkylbenzene. A series of thallium-
containing mesoporous silica with diꢃerent Tl contents was
prepared by Baradji et al. [11] for the benzylation of ben-
zene by benzyl chloride. Diphenylmethane was obtained as
the sole product when a stoichiometric ratio of benzene and
benzyl chloride was employed. The catalytic activity was
attributed to the well-dispersed and stabilized thallium oxide
particles on the silica support. Zup et al. [12] utilized silica-
reactions can be traced back to 1989. In that year, ZnCl
2
supported on montmorillonite, K10, was reported and sub-
sequently commercialized under the trade name Clayzinc for
the benzylation of benzene and other aromatic compounds
2
9
[16, 17]. The combined analysis of FTIR, XRD, Si NMR,
and XPS was used to elucidate the surface structure of the
active catalyst. Experimental parameters such as reaction
time, surface texture, reaction temperature, mass of catalyst,
and the molar ratio of toluene:BC were systematically stud-
ied. A detailed mechanism of the reaction which includes
the surface-active sites of the catalyst was proposed as
well. To the best of our knowledge, the physicochemical
and catalytic properties of ZnCl /SiO catalysts have not
gel bound aluminum chloride (Si-AlCl ) for Friedel–Crafts
x
alkylation for a range of aromatics with alkyl halides under
microwave irradiation. The catalysts were found to be much
more reactive compared to a commercial AlCl salt.
3
Since the discovery of MCM-41 in 1992, various types of
silica with pore channel structures ranging from micropores
to macropores have been synthesized and used as a support
for a variety of catalysts. The silica precursor used in the syn-
thesis of most siliceous materials such as tetraethyl orthosili-
cate (TEOS), tetramethyl orthosilicate (TMOS), and sodium
silicate is of synthetic origin and very costly. Rice husk ash
2
2
been reported. The catalyst is appealing on the industrial
viewpoint since the materials used are considered cheap and
widely available.
Materials and methods
(
RHA) offers a sustainable and greener source of silica.
Amorphous silica can be obtained by pyrolyzing rice husk
RH) in a controlled environment at a temperature of around
00–600 °C. In previous studies, Al, Ga, In, and Fe were
immobilized successfully within the matrix of silica from RH
Materials
(
5
The rice husk was obtained from a rice mill in Penang,
Malaysia. Other reagents such as nitric acid (Qrëc 65%),
1
3

Journal of the Iranian Chemical Society
toluene (Qrëc 99.5%), benzyl chloride (SAFC 97%), ben-
zyl alcohol (Acros 99.5%), benzoyl chloride (Fluka 99%),
benzene (Merck 99.7%), anisole (Fluka 99%), p-xylene
were labeled as RHA-x (x=3, 6, 9, and 12) depending on
the wt% of ZnCl .
2
(
(
Sigma 99%), acetone (Sigma-Aldrich ≥ 99.5%), ethanol
Sigma-Aldrich≥99.8%), sodium hydroxide pellets (R&M
Catalyst Characterizations
Chemicals 99%), and CTAB (Riedel–de Haen 98%) were of
analytical grade and were used as received without further
puriꢀcation.
The prepared samples were characterized by FTIR (Perki-
nElmer System 2000) spectroscopy, N adsorption–desorp-
2
tion porosimetry (Micromeritics Instrument Corporation
model ASAP 2000, Norcross), powder X-ray diꢃractom-
etry (Siemens Diꢃractometer D5000, Kristalloꢁex), scan-
ning electron microscopy (SEM, Leica Cambridge S360),
energy-dispersive spectrometry (EDAX FALCON SYS-
TEM), and transmission electron microscopy (TEM, Philips
Methods
Preparation of RHA
29
CM12 Instruments). The solid-state Si MAS NMR spectra
were obtained on a Bruker DSX-3000. Chemical shifts were
referenced to tetramethylsilane (TMS). The XPS wide and
narrow scan spectra were acquired using AXIS Ultra DLD,
Kratos, equipped with an Al Kα X-ray source (1486.6 eV)
at 10 mA, 15 kV, using the multichannel plate and delay line
detector (DLD). The analysis was performed in the hybrid
lens mode with the slot aperture, and the analyzer was oper-
ated in ꢀxed analyzer transmission (FAT) mode with pass
energy of the hemispherical analyzer set at 160 eV for the
survey/wide scan and 20 eV for the high-resolution scans/
narrow scans, while the minimum number of scanning was
ten times. The spectra were analyzed using vision software
The preparation of RHA was carried out based on the
method reported by Adam et al. [18]. Tap water was used to
clean the RH to remove dirt and dried at room temperature
for 24 h. The clean RH (30 g) was stirred in 750 ml of 1.0 M
HNO at room temperature for 24 h. Distilled water was used
3
to wash the RH until the pH of the ꢀltrate remained constant
(
pH 5). The treated RH was dried at 100 °C in an oven for
2
6
4 h followed by pyrolysis in a muꢄe furnace at 600 °C for
h to obtain white RHA.
Preparation of silica support
The silica from RHA was prepared according to the reported
method by Iqbal et al. [19]. In a 500 mL Nalgene bottle,
(
Kratos, UK), which included vision manager and vision
processing. The linear method was used for background sub-
traction and curve ꢀtting. The binding energy was referenced
to an adventitious carbon at 284.6 eV.
1
00 mL of 0.8 M NaOH solution was mixed with 3.2 g of
RHA. The mixture was stirred at 80 °C under constant stir-
ring for 1 h. The resulted sodium silicate was then ꢀltered to
remove undissolved RHA. A CTAB solution was prepared
by dissolving 2.9 g of CTAB in 50 mL of distilled water at
room temperature. This CTAB solution was added to the
clear sodium silicate while constantly stirring at 80 °C. The
pH of the mixture was reduced to pH 10 by adding 2.0 M
The acidity of the catalysts was studied using temper-
ature-programmed desorption (TPD-NH ) with NH as a
3
3
probe molecule. The analysis was carried out using TPD/
R/O 1100 (Thermo Finnigan) instrument equipped with a
thermal conductivity detector (TCD). The catalyst (~0.05 g)
was pretreated by N gas ꢁow for 30 min at 250 °C, fol-
2
HNO . A white gel started to form at pH 10. The gel was
3
lowed with NH gas for an hour at ambient temperature
3
aged in the mother liquor under constant stirring at 80 °C for
to allow adsorption of NH onto the surface. The excess
3
5
7 h, followed by centrifugation and ꢀltration. The gel was
NH was subsequently ꢁushed with the N gas ꢁow at a
3
2
ꢀ
nally rinsed with distilled water and acetone. The recovered
3
rate of 20 cm /min. The desorption of NH from the acid
3
xerogel was dried in the oven for 24 h at 100 °C. The xero-
gel was ground into a ꢀne powder and calcined in a muꢄe
furnace for 6 h at 600 °C to remove the CTAB. The xerogel
sites of the catalyst was detected by TCD under helium gas
ꢁ
ow (30 mL/min) from 50 to 900 °C and held for 30 min
[
20]. The bulk Zn content was determined using inductively
coupled plasma-optical emission spectrometry (ICP-OES)
PerkinElmer, ELAN 6100).
was labeled as RHA-SiO .
2
(
Impregnation of ZnCl on the surface of silica
2
Catalytic activity
In a typical procedure, the ZnCl salt (based on silica
2
weight) was dissolved in a beaker containing 20 mL of etha-
In a typical liquid-phase Friedel–Craft benzylation reaction,
a mixture of toluene (15.0 mL, 140 mmol) and benzyl chlo-
ride (1.0 mL, 9 mmol) was taken in a 50.0-mL round-bottom
ꢁask ꢀtted with a reꢁux condenser. The reaction mixture was
stirred well in a preheated oil bath at 353 K before adding
nol. Into this solution, 1.0 g of RHA-SiO was added and
2
vigorously stirred at 50 °C in a water bath until all the sol-
vent evaporated. The resulting powder was dried in an oven
at 110 °C before calcination at 200 °C for 2 h. The catalysts
1
3

Journal of the Iranian Chemical Society
the catalyst. The reaction is considered to begin when the
catalyst was added to the ꢁask. The reaction medium was
stirred for 6 h. Samples (0.5 mL) were withdrawn at regu-
lar intervals and analyzed with gas chromatography (GC),
model Clarus 500 PerkinElmer setup with an Elite 5 capil-
lary column (PerkinElmer), and a ꢁame ionization detector
(
FID). About 0.2 μL of the sample was injected into the
GC–FID for analysis. The temperature program for the GC
analysis was as follows: initial temperature: 90 °C; initial
°
time: 0 min; Ramp 1:4 C/min; Temp 2: 175 °C/min; ꢀnal
temperature: 250 °C. Gas chromatography–mass spectrom-
etry (GC–MS), model Clarus 600 PerkinElmer, was used to
conꢀrm the products. Conditions used for GC–MS analysis
were similar to the GC. For the hot ꢀltration test, the catalyst
was removed at 10 min and the reaction was continued using
the ꢀltrate. For the reusability test, the catalyst was separated
by hot ꢀltration followed by washing with hot solvent and
then dried at 373 K before reusing.
Fig. 1 The FTIR spectra of (a) RHA-SiO , (b) RHA-3, (c) RHA-6,
2
(
d) RHA-9, and (e) RHA-12. The disappearance of IR band attributed
to ν (Si–O–Si) at 1224 cm− is indicated by the red arrows
1
as
Results and discussion
of Si–O–Zn bonds. A group of bands at 961, 804, and
−
1
4
57 cm are assigned to ν (Si–OH), ν (Si–O–Si), and
as
s
Catalyst characterization
Si–O–Si, respectively. The bending mode of adsorbed
water molecules is indicated by the appearance of the band
−
1
The ICP-OES analysis indicates that the total concentra-
tion of Zn increased as the wt% was increased. From the
XPS analysis, the maximum concentration of Zn that can
be supported on the support is~1.0 wt%. The concentration
of elements detected on the surface of the catalysts by XPS
is given in Table 1 together with the total Zn concentration
detected by ICP-OES.
at 1635 cm [21, 22].
Figure 2a represents the low-angle X-ray diffraction
(LAXD) diꢃractograms of the catalysts. From Fig. 2a, three
signatures are noted in the diꢃractogram of RHA-SiO at
2
2θ=2.5°, 4.32°, and 4.98°. These reꢁections correspond to
the (1 0 0), (1 1 0), and (2 0 0) planes, respectively. The
weak intensity of this peak indicates that RHA-SiO has
2
The FTIR spectra of the catalysts are shown in Fig. 1.
poor crystallinity and ill-deꢀned hexagonal pore geometry
−
1
The broad band at ~ 3400 cm (shown by the red dot-
ted line) is associated with the –OH bond stretching
vibration of the surface silanol groups and hydrogen-
bonded adsorbed water molecules. The bands at 1224 and
[23]. In comparison with RHA-SiO , the reꢁection line of
2
(1 0 0) plane shifted to 2θ = 2.7° when 3 wt% and 6 wt%
of ZnCl was impregnated. Increasing the ZnCl content
2
2
to 9 wt% and 12 wt% diminished all the reꢁection lines.
The absence of notable scattering in the diꢃractograms of
RHA-9 and RHA-12 indicates the presence of a highly dis-
−
1
1
085 cm are attributed to ν (Si–O–Si). The intensity
as
−
1
of the band at 1224 cm (shown by red arrows) pro-
gressively reduced as the ZnCl wt% was increased. The
ordered pore system. The d-spacing (d ) values of RHA-
2
100
reduction in the band intensity suggests that the surface
SiO , RHA-9, and RHA-12 were 1.77, 1.63, and 1.63 nm,
2
Si–OH undergone structural change leads to the formation
respectively, whereas the unit cell parameters (a ) were 2.04,
0
1
.88, and 1.88 nm, respectively. The decrease in the d and
100
a indicates the loss of structural regularity and pore-size
Table 1 The elements detected on the surface of the catalysts by XPS
0
analysis
narrowing [24]. The d was calculated from the XRD pat-
100
tern by Bragg’s equation (2dsin θ=λ, where λ=1.5406 Å
Catalyst
XPS analysis (wt%)
ICP-
OES
for the Cu Kα line), whereas the a was determined using
0
(
ppm)
the formula a =2d /√3. The wide-angle X-ray diꢃraction
0
100
Si
Zn
Cl
O
Zn
(WAXD) pattern of the catalysts is shown in Fig. 2b. The
broad band centered at 2θ = 23° is due to the presence of
amorphous silica [25]. The absence of reꢁection lines related
to the ZnO indicates that the concentration of ZnO particles
might be too low for detection [26] or the ZnO nanoparticles
exist in a highly amorphous state [27].
RHA-3
RHA-6
RHA-9
RHA-12
20.66
26.94
28.03
27.28
0.38
1.05
1.06
1.08
0.17
0.23
0.26
0.34
58.66
63.42
66.43
63.96
147
347
607
885
1
3

Journal of the Iranian Chemical Society
RHA-SiO2
RHA-3
(
a)
(b)
RHA-6
RHA-9
RHA-12
2
3
4
5
6
7
8
9
10
10
20
30
40
50
60
70
80
2
Theta (degree)
2 thetha (deg.)
Fig. 2 The a LAXD and b WAXD diꢃractograms of RHA-3, RHA-6, RHA-9, and RHA-12
The TEM micrographs of the catalysts are shown in
micropores, whereas the Type IV isotherm is evidenced by
the existence of a hysteresis loop near the relative pressure
Fig. 3. The pores in RHA-SiO can be seen to be arranged
2
in parallel and spirally order with stacked layers of silica
0.8–1.0. The loop is associated with the condensation of N
2
sheets (Fig. 3a). The pore arrangement progressively deteri-
in the interparticle pore domain space of the powder grains.
The RHA-9 and RHA-12 show a Type IV isotherm which is
typical of mesoporous materials. These catalysts also show
a H4 hysteresis loop. According to the IUPAC, the H4 hys-
teresis loop is associated with materials having narrow slit
pores [29].
orated as the wt% of ZnCl was increased. The pore structure
2
can still be clearly seen in the TEM micrographs of RHA-3
(
Fig. 3a) and RHA-6 (Fig. 3b). The spiral architecture cannot
be seen in the TEM micrographs of RHA-9 and RHA-12.
The TEM results are in agreement with the XRD results. The
pore deterioration is indicated by decreased d and a val-
The pore-size distribution of the catalysts was determined
1
00
0
ues and the disappearance of the XRD signature at 2θ=2.7°.
using the DFT model. For RHA-SiO , pores in the range of
2
The extent of silica condensation in the catalysts can be
supermicropores (11–20 Å) were detected. The impregnation
2
9
observed from the Si MAS NMR spectra in Fig. 4. The
of ZnCl resulted in larger pore diameter. The pore-size dis-
2
3
4
two strong signals related to the Q (− 102 ppm) and Q
tribution of RHA-3 was 10–16 Å and 20–45 Å, whereas the
2
(
− 112 ppm) and a weak signal related to Q (− 93 ppm)
pore-size distribution of RHA-6 was 10–19 Å and 43–55 Å.
2
3
are observable in the spectrum of RHA-SiO . The weak Q
Increasing the ZnCl content to 9 wt% and 12 wt% yielded
2
2
signal corresponds to the geminal silanols, whereas the Q
a catalyst with a single narrow pore size, while the pore size
of RHA-9 and RHA-12 was 31 Å. During the impregna-
tion process, the Zn ions may have blocked the micropores
and leaving the larger mesopores open. The incorporation
4
and Q signals are associated with the isolated silanols and
siloxanes, respectively [28]. In the spectra of ZnCl contain-
2
3
4
ing catalysts, only Q and Q signals can be observed. The
2
disappearance of the Q signal suggests the substitution of
of 3 wt% and 6 wt% of ZnCl resulted in a decrease in the
2
2
−1
H atoms of the silanols by Zn atom to form Si–O–Zn bonds.
surface area (SA, 20–100 m g ) compared to the BET SA
2
−1
The TPD-NH and acidity strength distribution are given
of RHA-SiO (1016 m g ). A further increase to 9 wt% and
3
2
in Fig. 5. The TPD pattern of the catalysts showed NH des-
12 wt% is accompanied by a more signiꢀcant SA decrease.
3
2
−1
orption peak has T lower than 200 °C. This implies that
The SA of RHA-9 and RHA-12 was 623 and 494 m g ,
respectively. Aside from clogging/blocking the micropores,
max
the catalysts contain weak acid sites. The RHA-3 has the
highest acid density (1089 µmol/g), whereas RHA-12 has the
lowest density (229 µmol/g). The acid density of RHA-6 and
RHA-9 was 495 and 635 µmol/g, respectively.
the deposition of ZnCl or ZnO nanoparticles in the pores
2
may result in SA reduction, as evidenced by the reduction
in the BET surface area. The micropores SA of RHA-SiO ,
2
2
−1
The N adsorption–desorption isotherms of the catalysts
RHA-3, and RHA-6 was 899, 842, and 814 m g , respec-
tively, where the textural parameters are summarized in
Table 2.
2
are shown in Fig. 6. The uptake proꢀles of RHA-SiO , RHA-
2
3
, and RHA-6 resemble Type I and Type IV isotherms. The
Type I isotherm is indicated by a high N uptake observed
The XPS spectra of all the catalysts were similar, and
the representative spectra are shown in Fig. 7. The XPS
2
at a very low relative pressure (P/P ) due to the ꢀlling of
o
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3

Journal of the Iranian Chemical Society
Fig. 3 The TEM micrographs of a RHA-SiO , b RHA-3, c RHA-6, d RHA-9, and e RHA-12
2
spectrum of Zn 2p (Fig. 7a) suggests the presence of two dif-
Signals related to the surface silanol groups (Si–OH)
which generally appear at 534 eV were not observed. The
absence of this signal strongly suggests the complete con-
densation of silanol groups to siloxane and the formation
of Si–O–Zn species. The O 1s result is in agreement with
ferent types of Zn species. The presence of ZnCl species is
2
indicated by a doublet at 1023.2 and 1044.0 eV correspond-
ing to the Zn 2p and Zn 2p lines, respectively [30, 31].
3
/2
1/2
Another band at 1017.0 eV is a characteristic of ZnO [32].
2
9
The existence of this band indicates that a small amount
the Si NMR results which did not reveal signal related
of the ZnCl was converted to ZnO during heat treatment.
to surface Si–OH for the ZnCl containing catalysts as
2
2
Compared to the XRD, XPS is more sensitive and able to
detect ꢀner particles of ZnO at lower concentrations.
The chlorine spectrum indicates the presence of Cl 2p
well. However, silica itself is rich in oxygen. Thus, it can
be diꢂcult to resolve the types of oxygen bound to silica
and zinc. Schmidt et al. [33] and Pillai et al. [34] reported
the diꢂculty in resolving the signals of oxygen bound to
zinc due to the excessive oxygen content in zeolite and alu-
mina. They concluded that the surface hydroxyl groups of
1
/2
and Cl 2p at 199.0 and 201.7 eV, respectively [33]. Only
3
/2
the signal related to the Si–O–Si was detected at 532.8 eV
when the O 1s peak of the catalysts was deconvoluted.
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Journal of the Iranian Chemical Society
Fig. 6 The nitrogen adsorption–desorption isotherms of (a) RHA-
SiO , (b) RHA-3, (c) RHA-6, (d) RHA-9, and (e) RHA-12
2
Table 2 Textural properties of RHA-SiO and its modiꢀed forms
2
2
9
Fig. 4 The solid-state Si NMR spectra of (a) RHA-SiO , (b) RHA-
(RHA-3, 6, 9, and 12) according to nitrogen adsorption isotherm
2
3
, (c) RHA-6, (d) RHA-9, and (e) RHA-12
results
Catalyst
BET surface Micropore
Micropore
volume
DFT (Å)
2
−1
area (m g
)
surface area
2
−1
3
−1
(
m g
)
(cm g
)
RHA-SiO2 1000
899
842
0.40
0.41
11–20
10–16
RHA-3
965
2
6–45
RHA-6
919
814
0.43
10–19
4
3–50
RHA-9
RHA-12
623
494
–
–
–
–
31
31
Friedel–Crafts Benzylation of Toluene
The inꢁuence of reaction time and types of benzylation
agent in the Friedel–Crafts benzylation of toluene is shown
in Fig. 9. The benzylation agents used were benzyl chloride
(
BC) and benzyl alcohol (BOH). Complete BC conversion
Fig. 5 The TPD-NH analysis for catalysts: (a) RHA-3, (b) RHA-6,
3
was achieved within 3 h with 100% selectivity toward mono-
benzylated toluene (para- and ortho-mono-benzylated tolu-
ene). The selectivity of para- and ortho-mono-benzylated
toluene was 50% for each isomer, respectively. The con-
version of styrene was~ 30% when BOH was used as the
benzylating agent. The water molecules produced from the
benzylation reaction between toluene and BOH may have
strongly adsorbed on the active sites of the catalyst surface
thus preventing the reaction from taking place. The consecu-
tive investigation was carried out using BC due to the higher
conversion percentage. For comparison, the reaction was
carried out using RHA-SiO and ZnCl . The reaction did
(
c) RHA-9, and (d) RHA-12
the supports were eliminated during the modiꢀcation with
ZnCl forming Al–O–Zn bonds. The existence of XPS sig-
2
nals related to Si 2p at 103.7 eV is due to the existence of
4
+
Si in the catalyst’s framework [35].
2
9
Based on the results of Si NMR and XPS analyses,
the proposed surface structure of the catalyst is shown in
Fig. 8. The ZnO nanoparticles are proposed to be depos-
ited inside the pores, where the particles are held by the
oxygen atoms from the silica framework.
2
2
not occur when RHA-SiO was used, whereas only ~60% of
2
1
3

Journal of the Iranian Chemical Society
(
a)
(b)
1060
1050
1040
1030
1020
1010
210
208
206
204
202
200
198
196
194
192
Binding Energy (eV)
Binding Energy (eV)
(c)
(d)
5
40
538
536
534
532
530
528
526
524
1
10
108
106
104
102
100
98
96
Binding Energy (eV)
Binding Energy (eV)
Fig. 7 The XPS spectra of a Zn 2p, b Cl 2p, c O 1s, and d Si 2p regions from the catalysts
BC
BOH
1
00
RHA-SiO2
ZnCl2
8
6
4
2
0
0
0
0
0
0
1
2
3
4
5
6
Fig. 8 The proposed surface structure of the catalyst
Time (h)
toluene was converted in the presence of ZnCl . Even though
2
Fig. 9 The conversion of BC and BOH with respect to time. The
ZnCl is active, the metal salt cannot be easily separated and
2
reaction carried out using ZnCl and RHA-SiO was also included for
2
2
reused. The inactivity of RHA-SiO is attributed to the very
2
the comparison. The reaction was carried out using 75 mg of catalyst
at 353 K. The molar ratio of toluene:BC/BOH used was 15:1
mildly acidic nature of silica.
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Journal of the Iranian Chemical Society
The benzylation of toluene with BC was proposed to take
place via electrophilic aromatic substitution. The reaction
was initiated when the BC interacts on the surface of the
catalyst to form a charged complex, A (Fig. 10). The interac-
tion between Zn and the Cl atom of the BC is hypothesized
to be weak. The interaction will also weaken the strength of
the C–Cl bond of the BC [14].
the conversion of BC compared to the types of pores. Thus,
RHA-9 was used as the best catalyst for the optimization of
other parameters.
The inꢁuence of the mass of RHA-9 on the conversion
of BC is shown in Fig. 13. The catalyst weight was varied
between 25 and 100 mg at 353 K with the reaction time set
at 3 h, with 20:1 toluene:BC molar ratio. The BC conver-
sion increased from 80 to 100% when the catalyst mass was
increased from 25 to 50 mg due to the increase in the num-
ber of active sites. The further mass increase did not result
in a signiꢀcant change in BC conversion. The product dis-
tribution was not aꢃected by the change in the catalyst mass.
Based on the result, the optimum catalyst weight was 50 mg.
The inꢁuence of the molar ratio of toluene:BC on the con-
version was investigated using 50 mg of RHA-9 for 3 h at the
reꢁux temperature of 353 K. The mole ratio of toluene:BC
studied was 10:1, 15:1, and 20:1, where the results are
shown in Fig. 14. Increasing the molar ratio from 10:1 to
15:1 increased the conversion of BC from 60 to 85%. At
the molar ratio of 20:1, the conversion reached 100%. The
dilution of the reaction medium enabled BC to approach the
active sites (Zn) for adsorption. The reaction conditions also
prevented the reactants and products from being strongly
adsorbed on the surface of the catalyst [38]. The optimum
molar ratio of toluene:BC for this reaction was determined
as 20:1.
The reaction between A and toluene, which is a weak
nucleophile, formed a mixture of two intermediates: B and
B’. The reorganization of electrons in the intermediates will
form para- and ortho-mono-benzylated toluene and hydro-
chloric acid (HCl) as a by-product. The formation of para-
and ortho-mono-benzylated toluene is shown in Fig. 11a, b,
respectively. Based on the highest BC conversion rate when
the reaction was carried out for 3 h, it was chosen as the
optimum reaction time.
From the reaction proꢀle shown in Fig. 12, the conversion
of BC in the presence of RHA-3 (25%) and RHA-6 (43%)
was lowest despite their larger BET SA. In the presence of
micropores, the reactants had limited accessibility to the
active sites, thus causing lower BC conversion. Besides that,
a high concentration of product(s) may delay the desorption
process of the products and possibly accumulate with the
pore system [36]. These limitations may eventually lead to
catalyst deactivation [37]. The conversion of BC increased
to 100% when RHA-9 was used due to the presence of
mesopores. It dropped to 62% when RHA-12 was used. This
was due to its lower surface area compared to RHA-9.
The Friedel–Crafts benzylation reaction depends predom-
inantly on the Lewis acidity of the catalysts. However, in
this study, the trend of BC conversion was not in agreement
with the acid density values obtained herein. Even though
the acid density of the RHA-3 and RHA-6 was higher, the
BC conversion was lower compared to RHA-9. Therefore, it
can be concluded that acid density has moderate inꢁuence on
In conclusion, to convert 100% BC to para- and
ortho-mono-benzylated toluene, 50 mg of RHA-9 was
required, where the reaction was carried out at 353 K with a
molar ratio of toluene:BC, 20:1, for 3 h. The results for the
recyclability of RHA-9 are shown in Fig. 15. The catalyst
was used for four cycles (fresh and three reuses) without
any signiꢀcant loss. The BC conversion for the fresh and
ꢀrst reuse was 100%. The conversion of BC dropped by ca.
8% when the catalyst was reused for the second and third
cycles. From these experimental results, RHA-9 is consid-
ered stable, where it must be noted that the selectivity toward
the mono-benzylated toluene remained constant. Hence, the
use of RHA-9 for the benzylation of toluene with BC has
excellent potential for use in industrial applications. The hot
ꢀ
ltration test (HFT) was conducted to determine the hetero-
geneity of the catalyst. After the removal of the catalyst, the
conversion of toluene remained at 20%. The lower conver-
sion of BC compared to RHA-9 further conꢀrmed that the
active sites did not leach from the support. If leaching had
happened, the conversion rate will increase as the reaction
progress. The reaction proꢀle is shown in Fig. 16. The ICP-
OES analysis did not detect any presence of Zn ion in the
reaction mixture of the third cycle as well.
The benzylation reaction of diꢃerent types of arenes was
also investigated under the optimum conditions. The results
are shown in Table 3. According to the classical mechanism
of Friedel–Crafts-type acid-catalyzed benzylation reaction,
Fig. 10 The interaction of BC on the catalyst surface of the catalyst.
The adsorption will form a weak interaction between the Cl and Zn
atoms and weakens the C–Cl bond
1
3

Journal of the Iranian Chemical Society
(
a)
(
b)
Fig. 11 The proposed mechanism for the Friedel–Crafts benzylation of toluene with benzyl chloride. The formation of para- and ortho-mono-
benzylated toluene is shown in a and b, respectively
the reaction tends to proceed faster (easier) if one or more
electron-donating groups are present in the aromatic ring
the oxygen atoms can also form hydrogen bonds with the
surface silanol group of the catalyst which may poison the
catalyst [14]. Despite variation in the BC conversion, only
mono-benzylated product was obtained.
[
39]. From the table, the BC conversion increased in the
order of toluene=p-xylene>anisole>benzene which is in
accordance with the classical mechanism. Toluene contains
The catalyst (RHA-9) was also screened for benzoylation
of toluene with benzoyl chloride (BOC). The reaction was
carried out under the optimum conditions determined for
the benzylation of toluene. The conversion of BOC was 45%
with selectivity toward 2-methylbenzophenone (50%) and
3-methylbenzophenone (50%). However, the conditions used
for the screening may not be optimum. Thus, more in-depth
investigations of various parameters are required.
a single electron-donating group (–CH ), whereas p-xylene
3
contains two. The presence of greater number of electron-
donating groups increased the electrophilic substitution
reaction. The methoxy (–OCH ) group of anisole is elec-
3
tron withdrawing by the inductive eꢃect of the oxygen atom
which can reduce its reactivity toward electrophilic substi-
tution reaction compared to toluene and p-xylene. Besides,
1
3

Journal of the Iranian Chemical Society
1
00
RHA-3
RHA-6
RHA-9
RHA-12
100
90
80
70
60
50
40
30
20
10
0
10:1
15:1
20:1
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
0
1
2
3
0
1
2
3
Time (h)
Time (h)
Fig. 12 The reaction proꢀle expressed as conversion of BC versus
Fig. 14 The conversion of BC with respect to time for diꢃerent molar
ratios of toluene:BC. The reaction was carried out at 353 K using
50 mg of RHA-9
time of the respective catalysts. The reaction was performed using
7
2
5 mg of catalyst at 353 K. The molar ratio of toluene:BC used was
0:1
100
100
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
80
60
40
20
0
2
5 mg
50 mg
7
5 mg
1
00 mg
Fresh
1st reuse
2nd reuse
3rd reuse
0
1
2
3
Cycle
Time (h)
Fig. 15 The eꢃect of recycling on the BC conversion at optimum
conditions (RHA-9 mass: 50 mg, the molar ratio of toluene:BC: 20:1,
reaction time: 3 h, and reaction temperature: 353 K)
Fig. 13 The eꢃect of diꢃerent masses of RHA-9 on BC conversion.
The reaction was carried out at 353 K. The molar ratio of toluene:BC
used was 20:1
The catalytic activity of the catalyst was mainly influ-
enced by the textural properties (pore system and sur-
face area) rather than surface acidity. The electrophilic
aromatic substitution reaction proceeded through the
formation of a series of charged complexes. It was noted
that the catalyst revealed minor leaching in the overall
process. The reactivity trend for benzylation of diꢃerent
arenes was shown to adhere to the classical mechanism of
Friedel–Crafts-type acid-catalyzed benzylation reaction;
toluene=p-xylene>anisole>benzene.
Conclusion
The most catalytically active catalysts in the liquid-phase
Friedel–Crafts benzylation of toluene with benzyl chloride
(
(
BC) were found to be RHA-9. The highest conversion
100%) of BC was achieved when 50 mg of catalyst was
used together with the molar ratio of toluene:BC at 20:1.
Under the optimum conditions, para- and ortho-mono-
benzylated toluene was formed with 50:50 selectivity.
1
3

Journal of the Iranian Chemical Society
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Acknowledgements We would like to thank the Malaysian
Government,Universiti Sains Malaysia and Universiti Kebangsaan
Malaysia for supporting this work through the Research University
Grant (RUI) (1001/PKIMIA/8011083), Short Term Grant (304/
PKIMIA/6313215) and Geran Universiti Penyelidikan Universiti
Kebangsaan Malaysia (GUP-2019-045). We would like to extend our
gratitude to the Ministry of Education, Malaysia (Higher Education)
and Universiti Sains Malaysia for the Postdoctoral Training Fellowship
awarded to Anwar Iqbal.
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