Catalytic properties of the Cr-HMS materials in the benzylation of benzene with benzyl chloride

Article abstract of DOI:10.1134/S0023158411010022

A series of chromium-containing mesoporous silicas with different Cr contents were prepared and characterized with chemical analysis, N₂ adsorption measurements (BET equation and BJH theory), X-ray diffraction, diffuse reflectance UV-visible and H₂-temperature programmed reduction techniques. Excellent results in benzylation of benzene and substituted benzenes employing benzyl chloride as the alkylating agent were obtained. The mesoporous chromium-containing materials showed both high activity and high selectivity for benzylation of benzene. The activity of these catalysts for the benzylation of different aromatic compounds is in the following order: benzene > toluene > p-xylene > anisole. Kinetics of the benzene benzylation over these catalysts has also been investigated.

Full text of DOI:10.1134/S0023158411010022

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2011, Vol. 52, No. 1, pp. 48–54. © Pleiades Publishing, Ltd., 2011.
Catalytic Properties of the CrꢀHMS Materials
1
in the Benzylation of Benzene with Benzyl Chloride
a,
b
a
b
b
b
b
K. Bachari , A. Touileb , N. Tahir , A. Saadi , D. Halliche , and O. Cherifi
a
Centre de Recherche Scientifique et Technique en Analyses PhysicoꢀChimiques (C.R.AP.C) BP 248,
Alger RP 16004, Algiers, Algeria
Laboratoire de Chimie du Gaz Naturel, Faculté de Chimie, BP 32, 16111 El Alia,
b
U.S.T.H.B., Bab Ezzouar, Algiers, Algeria
eꢀmail: bachari2000@yahoo.fr
Received January 8, 2009
Abstract—A series of chromiumꢀcontaining mesoporous silicas with different Cr contents were prepared and
characterized with chemical analysis, N adsorption measurements (BET equation and BJH theory), Xꢀray
2
diffraction, diffuse reflectance UV–visible and H ꢀtemperature programmed reduction techniques. Excelꢀ
2
lent results in benzylation of benzene and substituted benzenes employing benzyl chloride as the alkylating
agent were obtained. The mesoporous chromiumꢀcontaining materials showed both high activity and high
selectivity for benzylation of benzene. The activity of these catalysts for the benzylation of different aromatic
compounds is in the following order: benzene > toluene >
lation over these catalysts has also been investigated.
pꢀxylene > anisole. Kinetics of the benzene benzyꢀ
DOI: 10.1134/S0023158411010022
1
Chromiumꢀbased catalysts have received much and
Hβ zeolites; Fe O3 or FeCl3 deposited on microꢀ,
2
attention because of their wide use in many catalytic mesoꢀ and macroporous [13]; Feꢀcontaining mesopoꢀ
reactions [1]. Silicaꢀ and aluminaꢀsupported chroꢀ
mium oxides were industrially used for the production
of polyethylene [2, 3] and lower alkenes such as propyꢀ
lene and isobutene through the dehydrogenation of
the corresponding alkanes [4, 5], respectively. The
redox properties and the appropriate dispersion of
chromium species on the support are important in
these catalytic reactions [6]. Crꢀcontaining mesopoꢀ
rous molecular sieves (CrꢀHMS) were reported to
exhibit high photocatalytic reactivity in ethylene polyꢀ
rous molecular sieves materials [14, 15]; Feꢀ, Znꢀ,
Gaꢀ and Inꢀmodified ZSMꢀ5 type zeolite catalysts
16]; Gaꢀ and Mgꢀoxides and chlorides derived from
,
[
GaꢀMgꢀhydrotalicite [17]; GaꢀSBAꢀ15 [18]; Gaꢀ
HMS [19]; InCl , GaCl , FeCl , and ZnCl2 supported
3
3
3
on clays and SiꢀMCMꢀ41 [20]; transition metal chloꢀ
ride supported mesoporous SBAꢀ15 [21]; supported
thallium oxide catalysts [22]; Sb supporting K10 [23];
solid superacid and silicaꢀsupported polytrifluoroꢀ
merization [7]. In the other hand, Friedel–Crafts methanesulfosiloxane [24]; SiꢀMCMꢀ41ꢀsupported
alkylations comprise a very important class of reacꢀ Ga O3 and In O3 [25]; H SO , HNO , and
2
2
2
4
3
tions which are of common use in organic chemistry. HClO /metakaolinite [26]; alkali metal salts and
4
These reactions are habitually catalyzed by Lewis
acids in liquid phase [8], and the substitution of liquid
acids by solid acid catalysts is a challenging task. The
alkylation of benzene by benzyl chloride is interesting
for the preparation of substituted polychlorobenzenes
used as dielectrics. In homogeneous phase this reacꢀ
tion is catalyzed at the industrial scale by AlCl , FeCl ,
ammonium salts of kegginꢀtype heteropolyacids [27];
ionꢀexchanged clays [28]; clayzic [29]; CuꢀHMS [30];
solid superacids based on sulfated ZrO2 [31]; HY [32];
Fe, Ce, Wꢀmodified
Hβ zeolites [33]; HꢀZSMꢀ5 [34];
and FeCl , MnCl , CoCl , NiCl , CuCl , ZnCl supꢀ
3
2
2
2
2
2
ported on acidic alumina [35] for the benzylation of
benzene and other aromatic compounds.
3
3
BF , ZnCl , and H SO [8–10]. The new environꢀ
3
2
2
4
mental legislation pushes for the replacement of all
liquid acids by solid acid catalysts which are environꢀ
mentally more friendly catalysts and which lead to
minimal pollution and waste [11, 12]. Indeed, several
solid acid catalysts have already been proposed which
are efficient catalysts such as; Feꢀmodified ZSMꢀ5
In the present work, we report the synthesis and
characterization of such materials incorporating chroꢀ
mium and their test as catalysts for the benzylation of
benzene with benzyl chloride. The kinetics of the
reaction over these catalysts has been investigated and
the reaction has been extended to other substrates like
1
The article is published in the original.
toluene, pꢀxylene and anisole.
48

CATALYTIC PROPERTIES OF THE CrꢀHMS MATERIALS IN THE BENZYLATION
49
EXPERIMENTAL
H /Ar as reducing gas with a total flow rate 20 ml/min.
A 50 mg sample was heated from room temperature to
2
Materials
700°C at a heating rate of 10°C/min after preꢀtreatꢀ
Samples were synthesized with hexadecylamine ment at 300°C for 30 min in helium gas flow. The
Aldrich), tetraethyl orthosilicate (TEOS, Aldrich), reduction signal was recorded by a thermal conductivꢀ
(
chromium nitrate (Cr(NO3)3 aqueous solution, ity detector. The reducing gas was cooled by mixture of
Merck) and ethanol (RhoneꢀPoulenc).
nꢀoctane and liquid nitrogen to condense the water
generated from reduction of the catalyst.
Catalysts Preparation
Catalytic Testing
The catalysts CrꢀHMSꢀn (where n is the Si/Cr
The benzylation reactions over a series of chroꢀ
miumꢀcontaining mesoporous silicas catalysts were
carried out in a magnetically stirred glass reactor
25 cm ) fitted with a reflux condenser, having a low
dead volume, mercury thermometer and arrangement
ratio in the precursor gel equal 50, 35, 15, and 5) have
been prepared following the pathway reported by
Tanev et al. [36]. In a representative preparation,
hexadecylamine was added to a solution containing
water and ethanol and the mixture was stirred until
homogeneous. Then tetraethyl orthosilicate was
added under vigorous stirring. The metal precursor (an
aqueous solution of Cr(NO3)3) dissolved in TEOS
itself. This solution was then stirred at room temperaꢀ
ture for 24 h to obtain the products. The solids were
recovered by filtration, washed with distilled water,
and airꢀdried at 393 K. Organic molecules occluded in
the mesopores were removed by solvent extraction.
The dried precursor was dispersed in ethanol
5 g/100 ml) containing a small amount of NH Cl
1 g/100 ml) and the mixture was refluxed under vigꢀ
orous stirring for 2 h. The solid was then filtered and
washed with cold ethanol. The extraction procedure
was repeated twice before drying the samples at 393 K
in an oven. Finally the samples were calcined at 873 K
in air for 4 h.
3
(
for continuously bubbling moistureꢀfree nitrogen N2
3
–1
(
flow rate 30 cm min through the liquid reaction
mixture, at the following reaction conditions: reaction
mixture, containing 15 ml of moistureꢀfree liquid aroꢀ
matic compound (or 2.5 ml of moistureꢀfree aromatic
compound mixed with 12.5 ml of moistureꢀfree solꢀ
vent), 1.0 ml of benzyl chloride, 0.1 g of catalyst and
reaction temperature 353 K. The reaction was started
by injecting benzyl chloride in the reaction mixture,
containing catalyst and aromatic compound with or
without solvent. Measuring quantitatively the HCl
evolved in the reaction by acid–base titration (by
absorbing the HCl carried by N2 in a 0.1 M NaOH
solution, containing phenolphthalein indicator) folꢀ
lowed the course of the reaction. The polybenzyl chloꢀ
rides (which are formed by the condensation of benzyl
chloride) were isolated from the reaction mixture by
the procedure given by Choudhary et al. [37]. In all the
cases, the major product formed was mainly
monobenzylated compound along with polybenzyl
chlorides as side products depending upon the condiꢀ
tion used. Samples were analyzed periodically on a gas
chromatograph HPꢀ6890 equipped with a flame ionꢀ
ization detector and a capillary column RTXꢀ1
(
(
4
Characterization of the Samples
The chemical compositions of the samples were
determined by a combination of wet chemical methꢀ
ods and atomic absorption spectrometry (Hitachi Z
00). Adsorption–desorption experiments using N2
were carried out at 77 K on a Nova 2000 porosimeter
Quantachrome) instrument. Before each measureꢀ
ment the samples were first outgassed at 423 K for 12 h
8
(
30 m × 0.32 nm i.d.). The products were also identiꢀ
fied by GCꢀMS (HPꢀ5973) analysis.
(
–3
at
at 3.9
5 × 10 Torr and then at room temperature for 2 h
–9
×
10 Torr. The N2 isotherms were used to
RESULTS AND DISCUSSION
determine the specific surface areas and pores diameꢀ
ter using the BET (Brunauer–Emmett–Teller) equaꢀ
tion and BJH (Barrett–Joyner–Halenda) theory.
Powder Xꢀray diffraction (XRD) patterns were
Characterization
The results of the chemical composition and charꢀ
acteristics of the catalysts are given in the Table 1. The
chromium compositions of the solids corresponded
relatively well to those fixed for the synthesis except at
low chromium content (CrꢀHMSꢀ50) where a loss of
chromium was observed. Most of the values of the speꢀ
cific surface areas of the solids were larger than
recorded on Siemens D500 diffractometer with Cu
K
α
radiation. They were recorded with 0.02° (2
and 1 s counting time per step over two angular
domains form 1 to 10° (2 and from 10 to 80° (2
θ
) steps
°
θ)
°
θ
).
Diffuse reflectance UV–visible (DR UV–vis) specꢀ
2
–1
troscopic measurements were recorded on a UV2100 1000 m g , which was typical for mesoporous mateꢀ
spectrometer. The spectra were collected at 230– rials. When the chromium content increased, they
7
00 nm referenced to BaSO . H2ꢀTemperature proꢀ decreased slightly. The N adsorption isotherms of the
4
2
grammed reduction (H ꢀTPR) of catalysts was perꢀ samples revealed a uniform pore size. Diffuse reflecꢀ
2
formed on Chembet3000 chemical adsorption appaꢀ tance UV–visible spectra of CrꢀHMSꢀ
n with different
ratus (Quantachrome) by using a mixture of 5 vol % Si/Cr ratio are shown in Fig. 1. It is generally well
KINETICS AND CATALYSIS Vol. 52
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2011

50
BACHARI et al.
Table 1. Chemical composition and characteristics of the catalysts
SiO /Cr O (molar ratio)
2
2
3
Surface area,
Sample
φp, Å*
2
–1
m g
gel
chemical analysis
Cr, wt %
HMS
–
–
50
35
15
5
–
1170.0
1143.0
1114.0
1085.0
989.3
38
35
36
35
34
CrꢀHMSꢀ50
CrꢀHMSꢀ35
CrꢀHMSꢀ15
CrꢀHMSꢀ5
78.0
39.0
17.2
6.4
1.8
4.2
6.9
9.1
*
φ is the mean pore diameter obtained from N adsorption isotherms.
p 2
known that two bands at 260–280 and 370–380 nm in tetrahedral coordination at low chromium loading and
the charge transfer region are characteristic of the the increase of chromium loading causes formation of
presence of Cr (VI) in tetrahedral coordination, Cr(III) in octahedral coordination on CrꢀHMSꢀ
whereas bands at 450–465 and 580–600 nm relate to alysts. Furthermore, the pure siliceous HMS mesopoꢀ
the A2g T1g and A2g
T2g transitions of Cr(III) in rous molecular sieve gives one broad peak at low angle
octahedral symmetry respectively [38, 39]. From region corresponding to 100 reflection in XRD patꢀ
n catꢀ
d
Fig. 1, two bands at 260–270 and 360–370 nm appear tern, indicating the presence of uniform mesoporous
apparently on CrꢀHMSꢀ50 and CrꢀHMSꢀ35, indicatꢀ channels. Chromiumꢀcontaining mesoporous silicas
ing the presence of Cr(VI) in tetrahedral coordination. CrꢀHMSꢀ
n show one similar XRD peak as pure siliꢀ
But absorption band at 600 nm characteristic of ceous HMS mesoporous molecular sieve at about
2 =
θ
Cr(III) in octahedral coordination is not detectable on 2°. Moreover, the intensity of the peak decreased when
both samples with lower chromium loading. A weak the chromium content increased showing that the
shoulder at 450–470 nm ascribes to Cr(VI) in chroꢀ addition of chromium has a negative effect on the
mate ions on these two samples. The appearance of crystallinity. At the same time, two weak peaks at
band at 600 nm and weakening of bands at 260 and 44.5 and 54.8° corresponding to diffraction of crystal
70 nm demonstrate that part of Cr species form to Cr are observed on XRD pattern of CrꢀHMSꢀ35.
Cr(III) in octahedral coordination on CrꢀHMSꢀ15 When the chromium content grows (CrꢀHMSꢀ15 and
and CrꢀHMSꢀ5 samples. The O
Cr(III) charge CrꢀHMSꢀ5), clear diffraction peaks of rhombohedral
transfer also contributes to absorption bands at 450– Cr crystal appear (Fig. 2). In fact, the CrꢀHMSꢀ50
65 nm overshadowed that of Cr(VI) chromate. These does not show any XRD peaks of metal oxide. Indeed,
results reveal that most Cr species form to Cr(VI) in the CrꢀHMSꢀ5 shows more intense reflection than
2θ
=
3
O
2 3
O
3
2
4
Absorbance
Intensity
4
3
4
3
2
2
1
1
20
40
60
80
, deg
300
400
500
600 700
Wavelength, nm
2θ
Fig. 1. DR UV–vis spectra of the CrꢀHMSꢀ
CrꢀHMSꢀ50,
HMSꢀ5.
n
sample:
1
—
Fig. 2. XRD patterns of the CrꢀHMSꢀ
domain of 10 –80° (2 ): —CrꢀHMSꢀ50,
35, —CrꢀHMSꢀ15, —CrꢀHMSꢀ5.
n
catalysts in the
2
—CrꢀHMSꢀ35,
3
—CrꢀHMSꢀ15, —Crꢀ
4
°
θ
1
2—CrꢀHMSꢀ
3
4
KINETICS AND CATALYSIS Vol. 52
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2011

CATALYTIC PROPERTIES OF THE CrꢀHMS MATERIALS IN THE BENZYLATION
51
logKa
1
2
3
4
100
200
300
400
500
600 700
Temperature, °C
2.74
2.76
2.78
2.80
2.82
1/
2.84
−1
3
T
, 10 K
Fig. 3. H ꢀTPR patterns of the CrꢀHMSꢀ
CrꢀHMSꢀ50,
HMSꢀ5.
n
samples:
1—
2
2
—CrꢀHMSꢀ35,
3
—CrꢀHMSꢀ15, —Crꢀ
4
Fig. 4. Arrhenius plot of log a
K
as a function of 1/
T
for the
benzene benzylation reaction over CrꢀHMSꢀ15 catalyst.
CrꢀHMSꢀ15. These results correspond to the observaꢀ Bz/BzCl = 15) over the CrꢀHMSꢀ15 catalyst could be
tion from DR UV–vis that higher content of chroꢀ fitted well to a pseudoꢀfirstꢀorder rate law:
mium leads to formation of Cr(III) species on Crꢀ
log(1/1 – x) = (Ka /2.303)(t – t0),
HMS.
where Ka is the apparent rate constant,
x is the fracꢀ
H2ꢀTPR patterns of CrꢀHMSꢀ
n
with different
tional conversion of benzyl chloride, is the reaction
t
chromium content are shown in Fig. 3. One strong
peak at about 500°C with a shoulder at 400–450°C is
observed for each sample. These H2 consumption
peaks are assigned to the reduction of Cr(VI)
Cr(III) [39]. Based on the results of Cavani et al. [40],
two kinds of Cr(VI) species, the grafted and the soluble
Cr(VI), are expected to form on CrꢀHMS. The grafted
Cr(VI) offers greater interaction with silica support
and is harder to be reduced than the soluble Cr(VI),
which presents as isolated chromates on the surface of
catalyst. Thus, the strong peak at higher temperature
corresponds to the reduction of grafted Cr(VI) species
and the weak shoulder at lower temperature may result
from reduction of soluble Cr (VI).
time, and t0 is the induction period corresponding to
the time required for reaching equilibrium temperaꢀ
ture. As a function of the time a plot of log (1/1 – x)
gives a linear plot over a large range of benzyl chloride
conversions. The effect of temperature on the rate was
studied by conducting the reaction at 343, 348, and
3
53 K under the standard reaction conditions (stoichiꢀ
ometric ratio Bz/BzCl = 15 and 0.1 g of catalyst). The
results showed that the catalytic performances of the
catalyst strongly increased with a growth of the reacꢀ
tion temperature (Table 2). Indeed, the time for 50%
conversion of benzyl chloride and the apparent rate
constant–1Ka changed from 337.4 min and 50.4
×
×
⎯
4
1
0 min at 343 K to, respectively, 75 min and 132.8
–
4
–1
10
min at 353 K. By contrast, the selectivity to
diphenylmethane decreased from 100 to 70.4%.
Reaction Kinetics
Two Bz/BzCl ratios have been investigated. The reꢀ
The kinetic data for the benzene benzylation reacꢀ sults obtained are reported on Table 3. It appears that
tion in excess of benzene (stoichiometric ratio the stoichiometric ratio between benzene and benzyl
Table 2. Catalytic activities of CrꢀHMSꢀ15 at 343, 348, and 353 K (Bz/BzCl = 15 and mcat = 0.1 g)
Selectivity, %
Apparent rate constant
Ka, 10 min
Temperature, K
Time*, min
4
–1
diphenylmethane
polybenzyl chlorides
343
348
353
337.4
150.0
75.0
100.0
94.0
70.4
–
50.4
64.5
6.0
29.6
132.8
*
Time required for 50% conversion of benzyl chloride.
KINETICS AND CATALYSIS Vol. 52 No. 1
2011

52
BACHARI et al.
Table 3. Influence of the stoichiometric ratio Bz/BzCl on mechanism is different from that for the classical acid
selectivity to diphenylmethane at 348 K over CrꢀHMSꢀ15
catalyst
catalyzed benzylation reactions. On the other hand,
the activation energy estimated from the Arrhenius
–1
plot is 96 kJ mol (Fig. 4). This value is lower than the
values recorded for the catalysts In O /HꢀZSMꢀ5 [16]
Selectivity, %
2
3
Benzene/benzyl
cloride ratio
Time*, min
and Ga O /H [41]. Indeed, it was interesting to comꢀ
β
diphenylꢀ polybenzyl
chlorides
2
3
methane
pare the solids with Crꢀexchanged clays investigated
earlier under similar conditions (Bz/BzCl = 15,
5
437.5
306.2
70.2
29.8
–
3
53 K) [28]. Both systems reached a final conversion
of 100% with complete selectivity to monoalkyl; the
half reaction time was about 480 min for Cr/K10 and
was here about 75 min, so that CrꢀHMSꢀ15 is more
active than clays.
1
5
100.0
*
Time required for the complete conversion of benzyl chloride.
chloride has a strong influence on the selectivity to
diphenylmethane. With a low ratio, the secondary reꢀ
actions to dibenzylbenzenes and tribenzylbenzene
were favored. Results showing the influence of differꢀ
ent substituent groups attached to aromatic benzene
Catalytic Performances of CrꢀHMS Materials
in the Benzylation of Benzene
A comparison of the catalytic properties of the solꢀ
nucleus on the conversion of benzyl chloride in the ids tested is presented on Table 4.
benzylation of corresponding substituted benzenes at
The pure mesoporous silica (HMS) and the comꢀ
pounds containing less chromium (CrꢀHMSꢀ50 and
CrꢀHMSꢀ35) were totally inactive. The other comꢀ
3
low.
53 K over the CrꢀHMSꢀ15 catalyst are presented beꢀ
Reaction rates for substituted benzenes at 353 K over pounds showed an activity increasing with their chroꢀ
CrꢀHMSꢀ15 catalyst (mcat = 0.1 g and substituted benꢀ
zenes/BzCl = 15)
mium content. However, the selectivity to diphenylꢀ
methane at complete conversion of benzyl chloride
decreased while the Cr content increased.
Substituent
Benzene Toluene
121.2
p
ꢀXylene Anisole
Apparent rate conꢀ 132.8
stant Ka, 10 min
114.5 92.5
4
–1
Recycling of the Catalysts
According to the classical mechanism of the
The stability of the catalysts has been studied by
Friedel–Crafts type acid catalyzed benzylation reacꢀ running the reaction successively with the same cataꢀ
tion, the benzylation of an aromatic compound is easꢀ lyst (CrꢀHMSꢀ15) under the same conditions without
ier if one or more electron donating groups are present any regeneration between two runs. The reaction was
in the aromatic ring [8]. Hence, the order for the rate first run under the standard conditions (benzene/benꢀ
of benzylation for the aromatic compound is expected zyl chloride ratio is 15, 353 K) to the complete converꢀ
as follows: anisole > pꢀxylene > toluene ӷ benzene. sion of benzyl chloride. Then after a period of 8 min
But, what is observed in the present case is totally another quantity of benzyl chloride was introduced in
opposite to that expected according to the classical the reaction mixture leading to the same benꢀ
mechanism. The apparent rate constant for the benzyꢀ zene/benzyl chloride ratio. After the achievement of
lation of benzene and substituted benzenes is in the the second run, the same protocol was repeated a secꢀ
following order: benzene > toluene > pꢀxylene > aniꢀ ond time. The results, presented in Table 5, showed
sole. This indicates that, for this catalyst, the reaction that the catalyst could be used several times in the benꢀ
Table 4. Catalytic properties of the catalysts in the benzylation of benzene with benzyl chloride at 348 K (Bz/BzCl = 15 and
mcat = 0.1 g)
Selectivity, %
Apparent rate constant
Ka, 10 min
Catalyst
HMS
Time*, min
4
–1
diphenylmethane
polybenzyl chlorides
–
–
–
–
–
–
–
–
CrꢀHMSꢀ50
CrꢀHMSꢀ35
CrꢀHMSꢀ15
CrꢀHMSꢀ5
–
–
–
–
306.2
216.4
100.0
90.5
–
64.5
137.6
9.5
*
Time required for the complete conversion of benzyl chloride.
KINETICS AND CATALYSIS Vol. 52
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CATALYTIC PROPERTIES OF THE CrꢀHMS MATERIALS IN THE BENZYLATION
53
Table 5. Effect of recycling of the catalysts in the benzylation of benzene with benzyl chloride at 353 K over CrꢀHMSꢀ15
catalyst (Bz/BzCl = 15 and mcat = 0.1 g)
Selectivity, %
Apparent rate constant
Ka, 10 min
Running
Fresh
Time*, min
4
–1
diphenylmethane
polybenzyl chlorides
427.1
447.3
459.8
80.6
81.5
79.3
19.4
18.5
20.7
132.8
129.5
130.7
First run
Second run
*
Time required for the complete conversion of benzyl chloride.
zene benzylation process without a significant change 12. Cao, J., He, N., Li, C., Dong, J., and Xu, Q., Mesopoꢀ
rous Mol. Sieves, 1998, vol. 117, p. 461.
of its catalytic activity.
1
3. Choudhary, V.R., Jana, S.K., and Mamman, A.S.,
In conclusion, mesoporous catalysts CrꢀHMSꢀ
were synthesized with different Cr content and exhibꢀ
n
Microporous Mesoporous Mater., 2002, vol. 56, p. 65.
ited excellent activities in the benzylation of benzene 14. He, N., Bao, S., and Xu, Q., Appl. Catal., A, 1998,
and substituted benzenes reaction. XRD, DR UV–vis
and H ꢀTPR characterizations demonstrate that
chromium content in CrꢀHMSꢀn has significant effect
vol. 169, p. 29.
2
15. Bachari, K., Millet, J.M.M., Benaichouba, B., Cherꢀ
ifi, O., and Figueras, F., J. Catal., 2004, vol. 221, p. 55.
on oxidation state and coordination of chromium speꢀ
cies. Cr (VI) species corresponding to chromate form
the main local structure of chromium at low Cr conꢀ
tent. The percentage of Cr (VI) decrease and crystal
Cr O appear with an increase of Cr loading. Two
kinds of Cr (VI) with different reduced abilities are difꢀ
ferentiated by H2ꢀTPR method. Furthermore, the
mechanism involves probably a redox step at the reacꢀ
tion initiation. This gives a greater independence to
1
1
1
6. Choudhary, V.R. and Jana, S.K., Appl. Catal., A, 2002,
vol. 224, p. 51.
7. Choudhary, V.R., Jana, S.K., and Narkhede, V.S., Appl.
Catal., A, 2002, vol. 235, p. 207.
2
3
8. El Berrichi, Z., Cherif, L., Orsen, O., Fraissard, J., Tesꢀ
sonnier, J.P., Vanhaecke, E., Louis, B., Ledoux, M.J.,
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