Benzylation of benzene by benzyl chloride over silica-supported iron sulfate catalysts

Article abstract of DOI:10.1016/j.mencom.2014.06.015

The silica-supported Fe-containing catalysts prepared using FeSO ₄ as a precursor exhibit high activity toward the reaction of benzene with benzyl chloride.

Full text of DOI:10.1016/j.mencom.2014.06.015

Mendeleev
Communications
Mendeleev Commun., 2014, 24, 231–232
Benzylation of benzene by benzyl chloride
over silica-supported iron sulfate catalysts
Maria A. Shuvaeva,a Alexey L. Nuzhdin,*a Oleg N. Martyanova,b and Galina A. Bukhtiyarovaa
a G. K. Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk,
Russian Federation. Fax: +7 383 330 8056; e-mail: anuzhdin@catalysis.ru
b Novosibirsk State University, 630090 Novosibirsk, Russian Federation
DOI: 10.1016/j.mencom.2014.06.015
The silica-supported Fe-containing catalysts prepared using FeSO4 as a precursor exhibit high activity toward the reaction of benzene
with benzyl chloride.
The Friedel–Crafts alkylation comprises an important class of
reactions commonly used in organic chemistry.1 Among these reac-
tions, the liquid-phase benzylation of benzene and other aromatic
compounds by benzyl chloride is used to produce diphenyl-
methane and substituted diphenylmethanes, which are key inter-
mediates for pharmaceutical and fine chemicals.2 Generally, such
reactions are carried out using strong homogeneous acid, such as
AlCl3, FeCl3, TiCl4, H2SO4 and GaCl3,1,3,4 which induce problems
associated with high corrosion, toxicity and separating of homo-
geneous catalysts.
Efforts to develop more eco benign heterogeneous catalysts
instead of the traditionally used homogeneous acids have revealed
the promising features of iron oxide or sulfate.2,5–14 Nanosized
Fe2O3 clusters with a high degree of coordinative unsaturation are
responsible for high catalytic activity.9,10 Arata and co-workers
found that bulk iron-containing materials, such as iron sulfate
and iron oxide, are also effective catalysts for the Friedel–Crafts
benzylation of toluene with benzyl chlorides.13,14 Previously, we
found that the impregnation of silica with iron(ii) sulfate led to
the formation of sulfate-containing iron compounds after thermal
treatment at 110–500°C, which transformed into nanosized
e-Fe2O3 particles after calcination at higher temperatures.15–17
The aim of this work was to study iron-containing silica-
supported catalysts prepared using iron(ii) sulfate as a precursor
in the Friedel–Crafts benzylation of benzene.†
The calcination temperature strongly affects the catalytic pro-
perties of the catalysts containing 4 wt% iron in the benzylation
reaction at 60°C: the sample 4FS400 provides the reaction, whereas
neither the samples FS110, FS300, FS500, FS600, FS700 and
FS900 nor the silica support induce the benzylation of benzene
even after 24 h. The effect of calcination temperature on the
transformation of iron species in a FeSO4/SiO2 catalyst was
studied using Mössbauer spectroscopy, XRD, TGA, TEM and
XPS.15–17 The complex compounds containing randomly distri-
buted oxide, hydroxide and sulfate groups as anions surrounding
iron atoms are formed on the catalyst surface after calcination at
400°C. An increase in calcination temperature is accompanied
by a gradual loss of sulfate groups and the formation of e-iron
oxide nanoparticles at 900°C. We can propose that complex
iron(iii) compounds containing hydroxide and sulfate anions
exhibit high catalytic activities toward benzene benzylation, while
e-Fe2O3 particles are not active despite their nanosized character.
Figure 1 shows the conversion of benzyl chloride as a func-
tion of reaction time in the presence of the 4FS400 catalyst at
60°C
100
80
60
40
20
0
50°C
25°C
Three sets of samples containing approximately 2, 4 and 6 wt%
iron were prepared by the impregnation of silica with aqueous
FeSO4 solution and subsequent drying and calcination. The
samples were denoted as nFS300, nFS400, etc., where 300, 400,
etc. denotes the calcination temperature and n corresponds roughly
to the iron concentration (wt%).
0
100
200
300
400
Reaction time/min
Figure 1 Effect of reaction temperature on the conversion of benzyl chloride
in the benzylation of benzene over the 4FS400 catalyst.
†
The catalysts were prepared using KSKG silica (KhromAnalit; specific
surface area, 287 m2 g–1; average pore diameter, 141 Å; pore volume,
0.81 cm3 g–1). Silica powder with a grain size of 0.25–0.50 mm was
impregnated with an aqueous solution of iron(ii) sulfate heptahydrate
(Acros Organic) and dried at room temperature for 24 h and then at
110°C for 4 h. Portions of the dried samples were calcined at 300, 400,
500, 600, 700 and 900°C for 4 h. The chemical composition of the catalysts
was determined by inductively coupled plasma atomic emission spectro-
metry using an OPTIMA 4300DV spectrometer from Perkin Elmer.
The liquid-phase benzylation of benzene with benzyl chloride was
carried out at 25–60°C in a magnetically stirred round-bottomed flask
fitted with a reflux condenser and heated in a precisely controlled water
bath. In a typical run, 5 ml of benzene and 0.45 ml of benzyl chloride were
added to the catalyst (0.05 g, activated at 110°C for 30 min). To follow
the reaction, the liquid was sampled at regular intervals and analysed by
gas-liquid chromatography (Agilent 6890N instrument equipped with a
19091S-916 HP-1MS capillary column, 60.0 m×320 mm×0.24 mm, and
a JAS atomic emission detector). The reaction products were identified
by GC-MS. The primary reaction product is diphenylmethane, and the
by-products are o-, p- and m-dibenzylbenzenes. Because benzene was in
an excess, the catalytic activity of the iron-containing samples was charac-
terised by the conversion of benzyl chloride. The selectivity was expressed as
the ratio of the moles of diphenylmethane formed to the moles of benzyl
chloride consumed in the reaction. The heterogeneity of the catalytic action
was confirmed by the observations that the filtrate obtained after the catalytic
reaction was inactive, and it did not contain paramagnetic species (detected
by ESR). Moreover, the elemental analysis data for the catalysts indicated
that the Si/Fe ratio is identical for a sample before and after reaction.
© 2014 Mendeleev Communications. All rights reserved.
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Mendeleev Commun., 2014, 24, 231–232
100
80
60
40
20
0
decreased. After the first regeneration, the complete conversion
of benzyl chloride at 60°C was achieved in 180 min instead of
60 min for the as-synthesized 4FS400 catalyst. After the second
regeneration, the complete conversion of benzyl chloride required
300 min. Despite the decreased activity in the second experiment
relatively to the first one, the proposed catalytic system can be
practically applicable due to its simple, reproducible and green
preparation, which is performed without organic solvent and with
cheap and readily available materials. It is well known that, in
the production of fine and specialty chemicals, where the high
added-value of the final compounds may account for the use of
valuable and costly reactants, the solid catalyst is often considered
as a consumable reagent.18
25°C
50°C
60°C
0
20
40
60
80
100
Conversion (%)
Figure 2 Selectivity for diphenylmethane formation as a function of benzyl
chloride conversion over the 4FS400 catalyst.
In summary, the benzylation of benzene with benzyl chloride
was investigated in a temperature range of 25–60°C over silica-
supported Fe-containing catalysts prepared by the impregnation
of silica with aqueous FeSO4 and the subsequent drying and
calcination. The calcination temperature is the primary factor
determining the catalytic activity: the catalysts show high activity
toward the reaction of benzene with benzyl chloride after calcina-
tions at 400°C, whereas the catalysts calcined at other tempe-
ratures did not exhibit any activity. The selectivity of diphenyl-
methane formation reaches 86% at a 100% conversion of benzyl
chloride and does not depend on the reaction temperature and
iron content. The proposed catalytic system seems promising for
practical use due to its facile and environmentally safe preparation
with the use of cheap and readily available materials.
different reaction temperatures. The reaction usually started after
an induction period and then proceeded rapidly, especially at
temperature of 60°C. The 4FS400 sample ensured 100% con-
version of benzyl chloride in 60 min at 60°C; decreasing the
reaction temperature beyond this point reduces the catalytic
activity. For example, at 50°C, the reaction was completed after
180 min; the induction period increased by a factor of 2, but
approximately the same time was needed for the conversion of
benzyl chloride. The sample storage and pretreatment conditions
and the water content of the reagent (benzene) also influenced
the induction period. A thorough drying of the catalytic sample
and benzene shortened the induction period, and the reaction
did not proceed at all if the water content of benzene exceeded
100 ppmw. The dependence of the induction period duration
on catalyst humidity and reaction temperature was observed
earlier.2,6,9 The selectivity of diphenylmethane formation decreased
with the conversion of benzyl chloride. At 100% conversion, the
selectivity was approximately 86%. The difference in the reac-
tion temperature and the catalyst pretreatment conditions did not
have a significant effect on the selectivity for a fixed molar ratio
of reagents (Figure 2).
The influence of iron content on the catalytic activity of the
iron-containing silica-supported samples in the benzene benzyla-
tion reaction was assessed using catalysts containing 2.1, 4.4 and
6.0 wt% Fe. The growth of the iron content from 2.1 to 4.4 wt%
increased the catalytic activity, but a further increase in iron
loading up to 6 wt% did not improve the catalytic properties
(Figure 3). The selectivity of diphenylmethane formation was not
affected by the iron content of the catalysts.
The iron-containing catalysts lost their activity after the first
run, and they should be regenerated by burning out the carbon-
containing organic deposition formed on the surface.8,12 The
regeneration of the 4FS400 catalyst by calcination at 400°C for
4 h in an air flow preserved the selectivity for diphenylmethane
formation (86%) but the activity of the regenerated catalyst
This work was supported by the Presidium of the Russian
Academy of Sciences (programme no. 27, project no. 46).
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Figure 3 Conversion of benzyl chloride in the benzylation of benzene at
60°C over the FS400 catalysts with different iron contents.
Received: 1st November 2013; Com. 13/4240
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