Synergistic catalysis of isolated Fe3+ and Fe2O 3 on FeOx/HZSM-5 catalysts for Friedel-Crafts benzylation of benzene

Article abstract of DOI:10.1016/j.cclet.2010.12.034

FeO_x/HZSM-5 catalyst with 8 wt.% Fe-loading (8-FeZ) exhibited significantly higher reactivity in the benzylation of benzene with benzyl chloride than FeO_x/HZSM-5 catalyst with 2.5 wt.% Fe-loading (2.5-FeZ) because the synergistic catalysis between isolated Fe³⁺ and superfine Fe₂O₃ occurred on 8-FeZ in the reaction.

Full text of DOI:10.1016/j.cclet.2010.12.034

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 639–642
www.elsevier.com/locate/cclet
Synergistic catalysis of isolated Fe3+ and Fe2O3 on
FeOx/HZSM-5 catalysts for Friedel–Crafts benzylation of benzene
*
Tao Lin, Xin Zhang , Rong Li, Ting Bai, Si Ying Yang
School of Chemical Engineering, Northwest University, Xi’an 710069, China
Received 2 September 2010
Abstract
FeOx/HZSM-5 catalyst with 8 wt.% Fe-loading (8-FeZ) exhibited significantly higher reactivity in the benzylation of benzene
with benzyl chloride than FeOx/HZSM-5 catalyst with 2.5 wt.% Fe-loading (2.5-FeZ) because the synergistic catalysis between
isolated Fe3+ and superfine Fe2O3 occurred on 8-FeZ in the reaction.
# 2010 Xin Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Friedel–Crafts; Synergistic catalysis; Benzene; Diphenylmethane; Fe3+; FeOx/HZSM-5
Friedel–Crafts benzylation of benzene with benzyl chloride (BC) is important for the production of high value
diphenylmethane (DPM) which is used as pharmaceutical intermediates and fine chemicals [1]. Conventionally,
homogeneous acid catalysts such as AlCl3, BF3 and H2SO4 are applied for the reaction [1,2]. However, these catalysts
bring about some disadvantages, such as corrosion, toxicity, difficult separation and recovery. Therefore, it is highly
desirable to develop heterogeneous solid catalysts to replace homogeneous catalysts.
Fe-based solid catalysts, such as Fe-HZSM-5 [3], Fe-SBA-15 [4] and Fe-MCM-41 [5] catalysts have been
considered to be promising catalysts for the benzylation of benzene with BC [2]. Nevertheless, there are some debates
on the active sites for the reaction. Choudhary et al. [3] thought that the reducible Fe species were relative to the
reactivity of Fe-HZSM-5. Sun et al. [4] reported that the highly dispersed iron oxide nanoclusters on Fe-SBA-15 were
active sites for the reaction. Arafat and Alhamed [5] considered that the high redox potential Fe3+ incorporated in the
MCM-41 silica matrix and Fe2O3 were responsible for the high reactivity of Fe-MCM-41. Without deep understanding
structure–reactivity relation, it is difficult to develop an efficient catalyst for the reaction.
Here, based on the results of kinetic studies and the properties of these catalysts characterized by X-ray diffraction
(XRD) and UV–vis spectroscopy, we discuss roles of Fe species on the reactivity of FeOx/HZSM-5 catalysts in
Friedel–Crafts benzylation of benzene and substituted benzene (toluene, p-xylene) with BC.
In this work, FeOx/HZSM-5 catalysts were prepared by incipient wetness method. Under stirring, powder HZSM-5
(SiO2/Al2O3 = 25, Nan Kai Unv.) was impregnated by the calculated amount of ferric nitrate (Fe(NO3)3Á9H2O, Tianjin
Oumi Chem., A.R.) aqueous solution to give 2.5 and 8 wt.% Fe-loading. The resultant mixture was dried at 110 8C for
* Corresponding author.
E-mail address: zhangxinzhangcn@yahoo.com.cn (X. Zhang).
1001-8417/$ – see front matter # 2010 Xin Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.12.034

6[()TD$FIG] 40
T. Lin et al. / Chinese Chemical Letters 22 (2011) 639–642
Fig. 1. Pseudo 1st order plots of benzylation of benzene with benzyl chloride on 2.5-FeZ and 8-FeZ catalysts at different temperature.
4 h and then calcined at 500 8C for 5 h in air. FeOx/HZSM-5 catalysts with 2.5 wt.% and 8 wt.% Fe-loading are
remarked as 2.5-FeZ and 8-FeZ, respectively.
Liquid phase benzylation of benzene and substituted benzene (toluene, p-xylene) with BC on these catalysts was
carried out in a three necked round-bottomed flask (250 mL) equipped with a reflux condenser and magnetic stirring.
The flask was heated in a precisely controlled water bath under atmospheric pressure. In a typical run, 30 mL benzene
(toluene, or p-xylene) was added to 0.2 g catalyst placed in the flask and the mixture was heated to required reaction
temperature. The mixture was maintained for 5 min at the temperature and then 2.7 mL BC was added. The moment
was regarded as initial reaction time. The catalyst was separated from liquid mixture by centrifuge. Reactants and
products were analyzed by gas chromatography (Schimadu GC-14C) with FID and packed column polyethene glycol
(3 m). Since benzene was in excess, conversion was calculated based on BC.
Fe2O3 and HZSM-5 showed no activity in the benzylation of benzene with BC. It can be seen in Fig. 1, distinctive
conversion of benzyl chloride with 100% selectivity of DPM (not shown in Fig. 1) was obtained on 8-FeZ and 2.5-FeZ
under the investigated reactionconditions. Hence, the interaction of Fewith HZSM-5 occurred in these catalysts. In order
to further evaluate the catalytic reactivity of 8-FeZ and 2.5-FeZ, kinetics of the benzylation of benzene with benzyl
chloride on these catalysts were studied. It is hypothesized that the reaction rate of BC follows a pseudo-first-order rate
equation, ln[1/(1 À x)] = k(t À t0), where k is the first-order rate constant (minÀ1), x is the conversion of BC (mol%), t is
the reaction time (min) and t0 is the induction period (min). In addition, activation energy (Ea, kJ/mol) of the benzylation
of benzene with BC on these catalysts is estimated from Arrhenius equation, k = A exp(ÀEa/RT), in which A is the
frequency factor (minÀ1), R is the universal gas constant (8.314 J/(mol K)) and T is the reaction temperature (K).
The linear plots of ln[1/(1 À x)] versus t was obtained by linear regression method and the rate constant k was
calculated by these plots (Fig. 1). It is found that the reaction rate of BC could be fitted well to the pseudo-first-order
rate law. Rate constant k8-FeZ (on 8-FeZ) and k2.5-FeZ (on 2.5-FeZ) gradually increased with the increase of reaction
temperature. Hence, the rise of reaction temperature favored the conversion of BC on 8-FeZ and 2.5-FeZ. k8-FeZ is
much higher than k2.5-FeZ under the same reaction conditions. Fig. 2 shows Arrhenius plots of the reaction on 8-FeZ
and 2.5-FeZ catalysts. Ea on 8-FeZ is ca. 132.6 kJ/mol and Ea on 2.5-FeZ is ca. 140.2 kJ/mol. These results confirm
that 8-FeZ has higher catalytic reactivity than 2.5-FeZ in the reaction.
The reusability of 8-FeZ in the benzylation of benzene with BC was tested. The reused catalyst was obtained by
separating from reaction solution and then calcined at 550 8C for 5 h. It can be seen in Table 1, 8-FeZ could keep high
conversion of BC (>90%) with 100% selectivity of DPM even after three reactions ran. Such catalytic performance is
important for the potential industrial application. On the other hand, the catalytic reactivity of 8-FeZ in the benzylation
of substituted benzene (toluene or p-xylene) with BC was also investigated. 8-FeZ showed high catalytic reactivity in
the benzylation of substituted benzene (Table 1).
XRD patterns of these catalysts, Fe2O3 and HZSM-5 were collected by Rogaku Rotflex D/Max-C powder X-ray
diffractometer with Cu Ka radiation (l = 0.15046 nm) operated at 40 kV and 30 mA (Fig. 3). XRD characteristic
peaks of HZSM-5 were observed on 8-FeZ and 2.5-FeZ, indicating that the Fe loading could not significantly change
the framework of HZSM-5. However, XRD peaks of HZSM-5 detected in these catalysts slightly shifted toward the
high 2u (8) with respective to XRD peaks of zeolite HZSM-5. These results further indicate that the interaction of Fe

[()TD$FIG]
T. Lin et al. / Chinese Chemical Letters 22 (2011) 639–642
641
Fig. 2. Arrhenius plots of benzylation of benzene with benzyl chloride on these catalysts.
Table 1
Benzylation of different aromatic compound with BC on 8-FeZ catalyst.
Acromatic substrate
Benzene
Runs
Conv. BC (mol%)
Sele. Product (mol%)
T>90 (min)
Fresh
98.8
99.6
98.5
Diphenylmethane (100%)
Diphenylmethane (100%)
Diphenylmethane (100%)
5
5
7
First reuse
Second reuse
Toluene
Fresh
Fresh
100
100
para(ortho, meta)-Benzylated (100%)
5
p-Xylene
2,5-Di methyl diphenylmethane (100%)
10
Reaction condition: 30 mL aromatic compound, 2.7 mL benzyl chloride, 0.2 g catalyst, 80 8C, reaction time = 3 h. T>90: time required for more than
90% conversion of BC.
with HZSM-5 occurs on these catalysts. XRD peaks belonging to Fe oxides could not be found on 2.5-FeZ, suggesting
that Fe species might be highly dispersed and isolated on the catalyst. 8-FeZ showed low intensity of the XRD peaks
relative to 2.5-FeZ, and 8-FeZ presented weak broad XRD peaks belonging to Fe2O3. Hence, the dispersion of Fe
species decreased and a little superfine Fe2O3 formed on 8-FeZ.
UV–vis spectra of these catalysts dispersed well in deionized water were recorded by Schimadu UV–vis 256
instrument (Fig. 4). Fe2O3 showed two broad bands at ca. 300 and 600 nm, belonging to Fe3+–O2À in bulk Fe2O3 [5].
Both 2.5-FeZ and 8-FeZ exhibited a shoulder peak at ca. 235 nm, tentatively assigned to the charge transfer between
isolated Fe3+ and O2À attached the framework of HZSM-5 [5,6]. 8-FeZ appeared a peak at ca. 305 nm, possibly due to
[()TD$FIG]
Fig. 3. XRD patterns of these catalysts.

6[()TD$FIG] 42
T. Lin et al. / Chinese Chemical Letters 22 (2011) 639–642
Fig. 4. UV–vis spectra of these catalysts.
the presence of Fe2O3 particles in the catalyst. Therefore, isolated Fe3+ species and Fe2O3 located in 8-FeZ. The result
is agreement with that of XRD.
Benzylation of benzene with benzyl chloride is electrophilic substitute reaction [1–5,7]. The possible reaction
mechanism has been proposed. Active metal ions (Men+) firstly attack benzyl chloride to form carboncation C6H5–
CH2+ and Me(nÀ1)+ through oxidation process; subsequently, benzene without activation directly reacts with C6H5–
CH2+ to produce diphenylmethane, which is rate determining step [1,2,7]. In case of this work, the interaction of Fe
with HZSM-5 induced the formation of isolated Fe3+ species on 8-FeZ and 2.5-FeZ catalysts. 8-FeZ and 2.5-FeZ
exhibited high catalytic reactivity in the benzylation of benzene with benzyl chloride in comparison with Fe2O3 and
HZSM-5. Therefore, isolated Fe3+ species are close relative to the catalytic reactivity of 8-FeZ and 2.5-FeZ. Fe2O3 had
no significant activity in the reaction, possibly due to the shortage of isolated Fe3+ species.
8-FeZ had lower activation energy and higher rate constant than 2.5-FeZ, indicating that 8-FeZ possessed more
active sites than 2.5-FeZ. On the other hand, the previous works revealed that the nano-particle Fe oxide could activate
benzene to form C6H5–H (C6H5dÀÁ Á ÁÁ Á ÁHd+) species which more easily reacted with C6H5–CH2+ than benzene [4,5,7].
Hence, the presence of a little superfine Fe2O3 on 8-FeZ enhanced the activation of benzene and favored the rate
determining step in the benzylation of benzene with benzyl chloride. The isolated Fe3+ species and superfine Fe2O3
occurred synergistic catalysis to improve the catalytic reactivity of 8-FeZ. In summary, 8-FeZ exhibited higher
catalytic reactivity than 2.5-FeZ in the benzylation of benzene with BC, due to the occurrence of synergistic catalysis
between isolated Fe3+ species and Fe2O3 on 8-FeZ. Under the reaction conditions of 30 mL benzene, benzene/
BC = 14.36 (mol ratio), 80 8C, 0.2 catalyst and reaction time = 5–7 min, the high conversion of BC with 100%
selectivity of DPM was achieved on 8-FeZ catalyst even after three reactions ran. In addition, 8-FeZ showed good
catalytic reactivity in the benzylation of substituted benzene (toluene, p-xylene) with BC.
Acknowledgments
This work was financially supported by Ministry of Education (No. NCET-10-878, 20096101120018, 2009-37th of
SRFROCS), Shaanxi Province (No. 2009ZDKG-70, 09JK793), Northwest University (No. PR200905) and State Key
Lab for SSPC (2009).
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