Nanoporous hematite nanoparticles: Synthesis and applications for benzylation of benzene and aromatic compounds

Article abstract of DOI:10.1016/j.jallcom.2013.08.057

The catalytic benzylation of benzene and other aromatic compounds is one of the most important reactions in the synthesis of pharmaceutical compounds. In this study, we report the synthesis of nanoporous α-Fe₂O ₃ nanoparticles via a hydrothermal method and their application in the catalytic benzylation of benzene and benzyl chloride in the fabrication of diphenylmethane. Crystal structure and morphology characterization results demonstrated that the hydrothermal method enabled the fabrication of highly dispersed α-Fe₂O₃ nanoparticles with spherical shape and an average size of 100 nm. The α-Fe₂O₃ nanoparticles have nanopores of less than 10 nm that are randomly distributed inside the nanoparticles. The catalytic benzylation of benzene and benzyl chloride was conducted over the synthesized a-Fe₂O₃ nanoparticles. The results demonstrated that the synthesized a-Fe ₂O₃ nanoparticles are effective catalysts for the benzylation of benzene and benzyl chloride with high activity and selectivity.

Full text of DOI:10.1016/j.jallcom.2013.08.057

Journal of Alloys and Compounds 582 (2014) 83–87
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Nanoporous hematite nanoparticles: Synthesis and applications
for benzylation of benzene and aromatic compounds
c
c
d
Nguyen Duc Cuong a,b,c, Nguyen Duc Hoa a, , Tran Thai Hoa , Dinh Quang Khieu , Duong Tuan Quang ,
⇑
Vu Van Quang a, Nguyen Van Hieu a,
⇑
a International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology, No. 1, Dai Co Viet, Hanoi, Viet Nam
b Faculty of Hospitality and Tourism, Hue University, 22 Lam Hoang, Hue City, Viet Nam
c College of Sciences, Hue University, 77 Nguyen Hue, Hue City, Viet Nam
d College of Education, Hue University, 34 Le Loi, Hue City, Viet Nam
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 July 2013
Received in revised form 7 August 2013
Accepted 7 August 2013
Available online 16 August 2013
The catalytic benzylation of benzene and other aromatic compounds is one of the most important reac-
tions in the synthesis of pharmaceutical compounds. In this study, we report the synthesis of nanoporous
a-Fe2O3 nanoparticles via a hydrothermal method and their application in the catalytic benzylation of
benzene and benzyl chloride in the fabrication of diphenylmethane. Crystal structure and morphology
characterization results demonstrated that the hydrothermal method enabled the fabrication of highly
dispersed
particles have nanopores of less than 10 nm that are randomly distributed inside the nanoparticles. The
catalytic benzylation of benzene and benzyl chloride was conducted over the synthesized -Fe2O3 nano-
particles. The results demonstrated that the synthesized -Fe2O3 nanoparticles are effective catalysts for
the benzylation of benzene and benzyl chloride with high activity and selectivity.
Ó 2013 Elsevier B.V. All rights reserved.
a-Fe2O3 nanoparticles with spherical shape and an average size of 100 nm. The a-Fe2O3 nano-
Keywords:
a-Fe2O3
a
Nanoporous
Benzylation
Diphenylmethane
a
1. Introduction
and NOx in diesel exhausts [9–11], photocatalytic degradation of
salicylic acid [12], thermal decomposition of ammonium perchlo-
rate [13], oxidation of CO [14], decomposition of biomass tars
[15], cracking of catechols and hydroquinones [16], and Fischer–
Tropsch synthesis [17]. An iron-based solid catalyst is also nor-
mally used as a Lewis acid catalyst and/or support in homogeneous
and heterogeneous catalysis [18–20].
Catalytic Friedel–Crafts alkylations are a very important class of
reactions that are commonly used in organic chemistry to synthe-
size various pharmaceutical compounds [1,2]. One of the most
important products synthesized via the Friedel–Crafts reaction be-
tween benzene and benzyl chloride is diphenylmethane (DPM),
which is commonly used in the fragrance industry as (i) both fixa-
tive and scenting agent or (ii) as monomers for the synthesis of
polycarbonate resins [3–5]. Friedel–Crafts alkylations, however, re-
quire a strong Lewis acid catalyst.
Among the catalysts, transition metal oxides, such as iron oxi-
des, have been immensely popular in a wide range of applications
because of their ease in handling, relatively low cost, nontoxicity,
and environment-friendly property. Iron oxide catalysts are cur-
rently utilized on a large scale basis in laboratory, industrial, and
The catalytic activity of transition metal oxides is strongly
dependent on their morphology and specific surface area. A porous
structure with a relatively large surface area is expected to have a
good catalytic performance. Fe-based solid catalysts, such as
Fe–TUD-1 [21], Fe–ZSM-5 [22], Fe–M–Mor [23], Fe–Al2O3 [24],
Fe–SBA [25], and Fe–MCM-41 [26,27], have been widely used for
the benzylation of benzene with benzyl chloride (BC). However,
findings on the active sites for the reaction [28] and the reaction
induction period are still debatable. Choudhary et al. [29] hypoth-
esized that the reducible Fe species were relative to the reactivity
of Fe–HZSM-5. Sun et al. [30] reported that the highly dispersed
iron oxide nanoclusters on Fe–SBA-15 were active sites for the
reaction. Arafat and Alhamed [31] considered that the high redox
potential Fe3+ incorporated in the MCM-41 silica matrix and
Fe2O3 caused the high reactivity of Fe–CMC-41. With regards to
the reaction induction period, Choudhary et al. [27,29] hypothe-
sized that the reaction induction period was caused by the pres-
ence of moisture in the reaction mixture and that the catalysts
environmental processes [1–8]. Hematite (a-Fe2O3) is one of the
most stable forms of iron oxides, which has been widely used as
a catalyst in numerous reactions, such as the decomposition of soot
⇑
Corresponding authors. Address: International Training Institute for Materials
Science (ITIMS), Hanoi University of Science and Technology (HUST), No. 1, Dai Co
Viet Road, Hanoi, Viet Nam. Tel.: +84 4 38680787; fax: +84 4 38692963.
E-mail addresses: ndhoa@itims.edu.vn (N.D. Hoa), hieu@itims.edu.vn (N.V.
Hieu).
0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.08.057

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N.D. Cuong et al. / Journal of Alloys and Compounds 582 (2014) 83–87
are activated during the initial reaction period. Leng et al. [23] sug-
gested that the reaction induction period seemed to depend on the
catalytic activity. A high activity indicates a short induction period.
However, the function of iron oxides in active catalysts remains
unclear. In addition, the benzylation of benzene with BC over nano-
3. Results and discussion
3.1. Characterization of catalyst
The crystal structure of the as-prepared
were examined via powder XRD measurement, and the data is
a
-Fe2O3 nanoparticles
porous
a-Fe2O3 nanoparticles has not yet been explored. Thereby,
further study is needed to explicitly understand the catalytic per-
shown in Fig. 2. The observed pattern of the collected products
formance of nanoporous
benzene with BC.
a-Fe2O3 nanoparticles for benzylation of
exhibited all the expected peaks from the a-Fe2O3 structure with-
out any detectable peak from impurities and other phases. The dif-
In this article, we report the controlled synthesis of scalable
fraction peaks were well-indexed to the rhombohedral structure of
nanoporous
nanoporous
a
-Fe2O3 nanoparticles for benzene benzylation. The
a
-Fe2O3 [space group: R3c] with structural parameters of
a = b = 0.0356 Å, c = 13.7489 Å, = b = 90°, and = 120°, which
agree with the literature (JCPDS 33-0664). The average crystalline
size of the -Fe2O3 nanoparticles calculated from the XRD data by
using the Scherrer equation is about 35 nm.
The surface state of the synthesized
investigated via FTIR is shown in Fig. 3. The absorption peaks at
a-Fe2O3 nanoparticles were synthesized via a hydro-
a
c
thermal method, and their catalytic performances in the benzyla-
tion of benzene and other aromatics with BC were systematically
investigated. The results showed that the nanoporous hematite
nanoparticles are active and selective catalysts for the benzylation
of benzene with BC to yield DPM.
a
a
-Fe2O3 nanoparticles
474 and 544 cmÀ1 are original peaks from
a-Fe2O3 nanoparticles
[32], whereas the peaks at 1634 cmÀ1 and 3436 cmÀ1 are due to
the OH bending of water and the hydroxyl group (–OH), respec-
tively [33].
2. Experimental
2.1. Materials
The morphologies of the nanoporous
which were characterized from SEM and TEM images, are shown
in Fig. 4(A and B), respectively. The -Fe2O3 nanoparticles fabri-
a-Fe2O3 nanoparticles,
Urea, Fe(NO3)3Á9H2O, benzene, benzyl chloride, toluene, and p-xylene were pur-
chased from Merck. Cetyltrimethylammonium bromide (CTAB) was purchased from
Aldrich. All chemicals were used as received without further purification.
a
cated via hydrothermal synthesis have very homogenous morphol-
ogies with nearly spherical shapes and regular dispersion
(Fig. 4(A)). The average particle size of the nanoparticles is about
100 nm, which is larger than the average crystalline size estimated
from the Scherrer equation (35 nm). This result suggests that the
nanoparticle is not a single crystal but is a poly-crystal. The TEM
image also indicates that nanopores of about less than 10 nm are
2.2. Synthesis of nanoporous
a-Fe2O3 nanoparticles
Nanoporous -Fe2O3 nanoparticles were synthesized via a hydrothermal meth-
a
od by using urea and iron(III) nitrate nonahydrate as precursors and CTAB as a sur-
factant. Ferric nitrate (2 mmol), CTAB (1 g), and urea (15 mmol) were dissolved in
35 mL of distilled water via magnetic stirring at room temperature for 2 h to obtain
a slurry solution. The slurry solution was then transferred into a 100 mL Teflon-
lined autoclave for hydrothermal processing. The hydrothermal processing was car-
ried out at 80 °C for 36 h, and the solution was then cooled down to room temper-
ature. The products were collected by several cycles of centrifugation and washed
with distilled water and ethanol. The collected products were dried at 60 °C for
24 h and then heat-treated at 500 °C to remove organic residues.
randomly distributed inside the
a-Fe2O3 nanoparticles (Fig. 4(B)).
These nanopores are believed to be generated because of the pres-
ence of bubbles during the hydrothermal process. The water expul-
sion of iron oxyhydroxide via heat treatment at 500 °C may also
contribute to the formation of the nanopores. However, the XRD
pattern of as-hydrothermal
cal diffraction peaks of an
a
-Fe2O3 samples (Fig. S1) shows typi-
a
-Fe2O3 phase but not of iron oxyhy-
2.3. Material characterization
droxide. Therefore, the possibility of the formation of the
nanopores as a result of water expulsion could be eliminated.
Fig. 5 shows the N2 adsorption/desorption isotherms and pore
The crystal structure of the nanoporous
ized using an X-ray diffractometer (XRD, D8 Advance, Brucker, Germany) with Cu
(k = 1.54 nm) radiation. Infrared spectroscopy was performed to evaluate the
a-Fe2O3 nanoparticles was character-
Ka
size distributions of the a-Fe2O3 nanoparticles. The sample exhibits
surface state of the synthesized materials. The infrared spectra were obtained using
a Nicolet 6700 FTIR Spectrometer in the range of 400–4700 cmÀ1 with a spectral
resolution of 4 cmÀ1 in transmittance mode. The KBr pellet technique was used
for sampling with 1 wt.% of materials for infrared spectroscopy measurement.
The morphology and the average particle size of hematite nanoparticles were inves-
tigated via scanning electron microscopy (SEM, Model JSM-5300LV) and transmis-
sion electron microscopy (TEM, Model JEOLE-3432, Japan). The nitrogen adsorption/
desorption isotherms of the heat-treated samples were obtained using a Micromer-
itics at 77 K. The specific surface area of the sample was calculated using the Bru-
nauer–Emmett–Teller (BET) method, whereas the pore-size distribution was
determined from the desorption isotherm data by using the Barrett–Joyner–Ha-
lende (BJH) model.
a type IV N2 adsorption/desorption isotherm with the H1 hystere-
sis loop (Fig. 5), which indicates the presence of mesoporosity
within the nanoparticles [34]. The pore diameters (inset of Fig. 5)
calculated using the BJH method are about 81 nm, and the pores
are caused by the interparticle space that is formed by the aggrega-
tion of Fe2O3 nanocrystals upon calcination. The BET specific sur-
face area of the nanoporous
a
-Fe2O3 nanoparticles is 15 m2/g.
This value is relatively high considering the size of the nanoparti-
cles. The catalytic activity greatly depends on the morphologies
and specific surface area of the catalysts because they have differ-
ent fractions of atoms located at different corners and edges as well
as different defects that are caused by the loss of atoms at these
locations.
2.4. Friedel–Crafts type benzylation of benzene
A scheme that represents the proposed mechanism for the benzylation of ben-
zene over the nanoporous
a-Fe2O3 nanoparticles catalysts is shown in Fig. 1 [22,26].
The liquid phase Friedel–Crafts benzylation reactions over
a-Fe2O3 nanoparticle
catalysts were carried out in a mechanically stirred mode in a two-neck round-bot-
tomed flask (capacity: 100 mL), which was fitted with a reflux condenser and
heated in a precisely controlled oil bath. All the reactions were performed in air
atmosphere. Benzene (30.55 g, 391 mmol), benzyl chloride (1.1 g, 8.7 mmol), and
hematite catalyst (50 mg) were mixed and stirred at 358 K at a moderate speed
for at least 3 min. The liquid samples were extracted, filtered periodically, and ana-
lyzed via gas chromatography (GC-2010 Shimadzu series) equipped with a thermal
conductivity detector (TCD) to evaluate the catalytic activity. The catalytic conver-
sion was calculated from the amount of benzyl chloride that was consumed during
the reaction, and the products were confirmed via GC–MS. Details about the GC–MS
results can be found in the electronic supporting information (ESI).
3.2. Benzylation of benzene and other aromatic compounds
To evaluate the catalytic performance of the nanoporous
a-
Fe2O3 nanoparticles, the benzylation of benzene with BC was
investigated. The reaction was carried out at 358 K and 50 mg of
the catalyst was used. Fig. 6 shows the conversion of BC and prod-
uct distribution as a function of reaction time. The BC conversion
reached 100% in a very fast reaction time of about 3 min. The reac-
tion induction period depends on the catalytic activity [23], but the

N.D. Cuong et al. / Journal of Alloys and Compounds 582 (2014) 83–87
85
Fig. 1. Scheme depicting the proposed mechanism for the benzylation of benzene over the nanoporous a-Fe2O3 nanoparticles as catalyst.
20
30
40
50
60
70
2θ (deg.)
Fig. 2. XRD pattern of the
a-Fe2O3 nanoparticles fabricated via the hydrothermal
method.
Fig. 4. (A) SEM and (B) TEM images of the nanoporous Fe2O3 nanoparticles.
with the highest iron content showed the highest catalytic activity
[5,21,23,35]. In addition, the high catalyst activity of the Fe-solid
catalyst is usually caused by its porous structure and the presence
of iron oxide. For instance, Anand et al. [5] studied the benzylation
of benzene over FeKIT-5 (12), which contains a lot of iron species.
The higher catalytic activity of FeKIT-5 (12) was attributed to its
three-dimensional cage-type porous structure and high iron con-
tent. Hamdy et al. [21] demonstrated that the nanoparticles of iron
oxide were less coordinately saturated (because of lattice strain
and defects) than isolated iron species or the iron centers present
in larger particles, thereby giving rise to more active sites. Leng
et al. reported that the presence of more iron oxide nanoclusters
could contribute to the improvement of catalytic activity [23]. In
Fig. 3. FTIR spectra of the a-Fe2O3 nanoparticles.
a
-Fe2O3 nanoparticles in the current study had a very high activity
with almost no reaction induction period. A high activity indicates
a short induction period. Comparing the catalytic activity of the
a-Fe2O3 nanoparticles with those of other Fe-solid
based catalysts, such as Fe–SBA-15 [1], FeKIT-5 [5], Fe–M–Mor
[23], Fe–Al2O3 [24], Fe/Si–MCM-41 [27], and Fe2O3/HZM-5 [35],
the nanoporous a-Fe2O3 nanoparticles are more active. In general,
nanoporous
the catalytic activity depends on the iron content in the catalyst,
the present study, the higher catalytic activity of
a-Fe2O3 nanopar-
which comes into contact with the benzylation agents. The catalyst
ticles was possibly due to its high iron content and porous

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N.D. Cuong et al. / Journal of Alloys and Compounds 582 (2014) 83–87
Table 1
160
120
80
40
0
Benzylation of benzene with benzyl chloride at different catalyst mass.
absorption
desorption
Catalyst (mg)
Time (min)
BC conversion (%)
DPM selectivity
10
50
100
500
3
3
3
3
5.23
100
100
100
100
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
81 nm
97.76
95.33
89.93
Conditions: benzene = 391.1 mmol, benzyl chloride = 8.7 mmol, time = 3 min,
temperature = 358 K.
0
20 40 60 80 100 120 140
Table 2
Pore diameter (nm)
Catalytic activities of the nanoporous
temperatures.
a-Fe2O3 nanoparticles at different
Temperature (K)
Time (min)
BC conversion (%)
DPM selectivity
0.0
0.2
0.4
0.6
0.8
1.0
P/Po
343
358
373
393
60
3
3
3.5
100.00
97.76
95.24
85.86
100
100
100
Fig. 5. Nitrogen adsorption/desorption isotherms and BJH pore size distribution
3
(inset) of the nanoporous
a
-Fe2O3 nanoparticles.
Condition: benzene = 391.1 mmol, benzyl chloride = 8.7 mmol, catalyst = 50 mg.
100
80
60
40
20
0
100
80
60
40
20
0
Table 3
Influence of different electron donating substitution groups for the benzene benzy-
lation reaction over
a-Fe2O3 nanoparticles.
BC Conversion
Selectivity to DPM
Aromatic Reaction
substances time
(min)
Reaction
temperature
(K)
BC
Reaction product(s)
conversation (selectivity %)
Benzene
Toluene
3
3
358
368
100
100
Diphenylmethane
(97.76%)
0-Benzyltoluene
(52.36%) p-
benzyltoluene
(47.64%)
p-Xylene
3
378
100
2-Benzyl-1,3-
dimethylbenzene
(99.93%)
0
2
4
6
8
10
12
Reaction time (min)
Fig. 6. Percentage conversion and selectivity versus time for the reaction between
benzene and BC at 358 K (molar ratio benzene/BC of 45:1, 50 mg of
catalyst).
a-Fe2O3 as a
The influence of the stoichiometric ratio between benzene and
BC for the benzylation reaction was also studied, and the data is
shown in Fig. 7. The benzylation of benzene was carried out at
358 K for 3 min with various molar ratios of benzene/BC by using
50 mg of the catalyst. The benzene/BC ratio was changed by keep-
ing the amount of BC constant while varying the amount of ben-
zene. The result shows that the ratio between benzene and BC
appears to have a strong influence on the selectivity to DPM
(Fig. 7). The conversion of BC reached to 100% in all the ratios.
The selectivity towards DPM is increased with increasing benzene
content. At a ratio benzene/BC of 45, a selectivity of approximately
100% can be achieved. The increase in DPM selectivity with
increasing benzene/BC ratio can be attributed to enhancement of
benzene over the catalyst surface and a higher dilution of BC.
structure, which provided larger active sites. Moreover, the difference
in the catalytic activity of the -Fe2O3 nanoparticles could possibly be
a
attributed to the existence of mesopores that greatly overcame the
diffusion limitation compared with microporous catalysts.
120
BC conversion;
DPM;
Others
100
80
60
40
20
0
The reaction profile when different amounts of nanoporous
Fe2O3 nanoparticle catalysts were used reveals that selectivity to
DPM is higher at a lower concentration (Table 1). At a higher
a-
a-
Fe2O3 concentration, the conversion of BC increases to an optimum
value and the formation rate of other products also increases.
Hence, an optimum amount of 50 mg of catalyst in the reaction
mixture is ideal for achieving better conversion (100%) and selec-
tivity for DPM (97.76%).
The effect of temperature on the benzylation activity of a-Fe2O3
0
15
30
45
60
nanoparticles was also investigated, and the results are shown in
Table 2. The performance of the catalysts strongly increases with
increasing reaction temperature. At a reaction temperature of
343 K and a reaction time of 1 h, the conversion of BC and the
Benzene/BC (molar ratio)
Fig. 7. Influence of benzene/BC molar ratio on the conversion of BC and product
distribution (Reaction condition: 358 K, 50 mg of -Fe2O3).
a

N.D. Cuong et al. / Journal of Alloys and Compounds 582 (2014) 83–87
87
selectivity to DPM are 3.5% and 100%, respectively. When the reac-
Appendix A. Supplementary material
tion temperature was increased to 358 K, the BC conversation in-
creases from 3.5% to 100%, but the selectivity to DPM decreases
from 100% to 97.76% for a reaction time of 3 min. The selectivity
to DPM is only 85.86% at 393 K. These results reveal that the con-
version of BC and the selectivity towards DPM can be easily tuned
by simply adjusting the reaction temperature.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jallcom.2013
.08.057.
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The benzylation reaction over
a-Fe2O3 nanoparticles was also
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We introduced the synthesis and application of nanoporous a-
Fe2O3 nanoparticles and investigated their catalytic performances
for the benzylation of benzene and several aromatic compounds.
Well-dispersed nanoporous
size of 100 nm, which had nanopores of less than 10 nm, were pre-
pared via a hydrothermal method. The -Fe2O3 nanoparticles
a-Fe2O3 nanoparticles with an average
a
showed excellent catalytic performance in the benzylation of ben-
zene with benzyl chloride, which leads to the formation of DPM
with high selectivity. The BC conversion reached 100% in a reaction
time of 3 min with 97.76% selectivity to DPM. The conversion and
product distribution are largely dependent on the experimental
conditions. The selectivity of DPM increases with increasing ben-
zene/BC molar ratio but decreases with increasing catalyst amount
and reaction temperature. The nanoporous
a-Fe2O3 nanoparticles
also had potential applications in other Friedel–Crafts alkylations,
especially in large molecular reactions.
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Acknowledgment
This work was financially supported by the Vietnam’s National
Foundation for Science and Technology Development (Nafosted,
Code: 104.03-2012.54)