Enhancement of catalytic performance in the benzylation of benzene with benzyl alcohol over hierarchical mordenite

Article abstract of DOI:10.1016/j.jcat.2013.06.004

Hierarchical mordenites were prepared by a sequential post-treatment method based on a commercial mordenite with a Si/Al molar ratio of 15. The mordenite obtained by acid treatment showed much higher catalytic activity than the parent mordenite in the benzylation of benzene with benzyl alcohol, even a little better than that obtained by acid-base treatment. The apparent reaction rate constant for acid-leached mordenite is six times higher than that for HM. Further, the mordenite with acid-base-acid treatment exhibited the highest catalytic activity among these samples. The apparent reaction rate constant for acid-base-acid-leached mordenite is 15 times that for HM and twice that for acid-leached mordenite and acid-base-leached mordenite. This remarkably enhanced catalytic performance should be attributed to more accessible acid sites and much better mass transfer ability from rich mesoporosity in acid-base-acid- leached mordenite.

Full text of DOI:10.1016/j.jcat.2013.06.004

Journal of Catalysis 306 (2013) 100–108
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Enhancement of catalytic performance in the benzylation of benzene
with benzyl alcohol over hierarchical mordenite
a
a,c
b
c
c
c
Kunyue Leng , Yi Wang , Changmin Hou , Christine Lancelot , Carole Lamonier , Alain Rives ,
Yinyong Sun
a
a,
⇑
School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, China
State Key Lab of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China
Unite Catalyse & Chim Solide, UMR8181, Université Science & Technologie Lille, F-59655 Villeneuve Dascq, France
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Hierarchical mordenites were prepared by a sequential post-treatment method based on a commercial
mordenite with a Si/Al molar ratio of 15. The mordenite obtained by acid treatment showed much higher
catalytic activity than the parent mordenite in the benzylation of benzene with benzyl alcohol, even a
little better than that obtained by acid–base treatment. The apparent reaction rate constant for acid-lea-
ched mordenite is six times higher than that for HM. Further, the mordenite with acid–base–acid treat-
ment exhibited the highest catalytic activity among these samples. The apparent reaction rate constant
for acid–base–acid-leached mordenite is 15 times that for HM and twice that for acid-leached mordenite
and acid–base-leached mordenite. This remarkably enhanced catalytic performance should be attributed
to more accessible acid sites and much better mass transfer ability from rich mesoporosity in acid–base–
acid-leached mordenite.
Received 30 May 2013
Revised 2 June 2013
Accepted 7 June 2013
Available online 20 July 2013
Keywords:
Hierarchical
Mesoporous
Mordenite
Benzylation
Treatment
Alkylation
Ó 2013 Elsevier Inc. All rights reserved.
1
. Introduction
The liquid-phase benzylation of aromatic compounds by benzyl
methods have been adopted in the synthesis of hierarchical
zeolites with different framework types, such as ZSM-5 [17–22],
Beta [19,22], Y [23–25], and SSZ-13 [26]. Of these, the synthesis
of hierarchical mordenite is of practical importance because mord-
enite, with low preparative cost and strong acidity, has been used
for hydroisomerization, alkylation, and dewaxing processes in
industry. Currently, hierarchical mordenite can be prepared by
dealumination with steam and acid [27–33] or desilication with
base [34–39]. The studies indicate that dealumination reduces
the density of the acidic sites due to the extraction of aluminum
from the framework. As a result, the improvement effect of trans-
port ability on catalytic performance is weakened [36]. For exam-
ple, Boveri et al. showed that the dealuminated catalyst gave a
negligibly improved yield of linear alkylbenzene in the alkylation
of benzene with ethylene [33]. Van Bokhoven et al. reported that
the acid-leached mordenite had a low catalytic activity similar to
that of the parent mordenite in the benzylation of benzene with
benzyl alcohol [40].
On the other hand, it has been demonstrated that desilication is
an efficient route for preparing hierarchical mordenite, with good
preservation of the intrinsic acidity [36]. The hierarchical morde-
nite obtained by desilication exhibited much better catalytic per-
formance than the parent and acid-leached mordenite. However,
recent studies suggested that desilication could result in the depo-
sition of part of the Al species on the external surface of the zeolite
framework and thus negatively impact catalytic reactivity. After
chloride or benzyl alcohol is one of the important reactions in or-
ganic chemistry, because it can be used to produce diphenylme-
thane and substituted diphenylmethanes, which are key
industrial compounds as pharmaceutical intermediates or fine
chemicals [1,2]. Generally, such reactions are carried out in the
presence of homogeneous acid catalysts such as AlCl
H
3 3
, FeCl , and
2
SO [3,4]. However, these catalysts meet some problems, such
4
as corrosion and difficulty in separation and recovery. Therefore,
there have been many attempts to replace these homogenous cat-
alysts with heterogeneous solid catalysts [5–8]. Zeolites, as solid
catalysts, have been studied in benzylation reactions [9–13] be-
cause of their strong acidity, large surface area, and regular porous
structure with excellent stability. The results indicate that highly
acidic zeolite catalysts show poor reactivity, mainly because of dif-
fusion limitation caused by their microporous networks [9,10].
Recently, hierarchical zeolites have received much attention be-
cause they cannot only overcome the diffusion limitation of zeo-
lites on the reaction rate but also maintain stability and strong
acidity, like conventional zeolites [14–16]. Several preparative
⇑
Corresponding author. Fax: +86 4006358835 00379.
E-mail address: yysun@hit.edu.cn (Y. Sun).
0
021-9517/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2013.06.004

K. Leng et al. / Journal of Catalysis 306 (2013) 100–108
101
subsequent acid washing, the deposited Al species could be
removed from the external surface. Then, the catalytic perfor-
mance could be further improved due to enhanced accessibility
with an acceleration voltage of 200 kV. Nitrogen sorption
isotherms were obtained at 77 K on a Micromeritics TriStar II
3020 gas sorption and porosimetry system. Samples were normally
prepared for measurement after degassing at 423 K under vacuum
[
39].
In the present work, with the aim of obtaining a highly efficient
À3
27
until a final pressure of 1 Â 10 Torr was reached. Al MAS NMR
experiments were performed at room temperature on a Bruker Ad-
vance AMS-400 spectrometer at a spinning rate of 12 kHz using a
catalyst for Friedel–Crafts alkylation based on mordenite, we pre-
pared hierarchical mordenites by a sequential post-treatment
method based on a commercial mordenite. Considering that benzyl
alcohol is a relatively friendly benzylation reagent due to water as
the byproduct, the benzylation of benzene with benzyl alcohol was
chosen to evaluate the catalytic performance over the catalysts ob-
tained in each step. The relationship between the structural prop-
erties and the catalytic performance will be discussed.
2
7
4-mm probe. In Al MAS NMR, a single pulse length of p/6 and a
2
9
relaxation delay of 1 s were used, while in Si MAS NMR, a high-
powered decoupling pulse sequence and a relaxation delay of 6 s
2
7
were used. The Al chemical shift was referenced to (NH
4
)-
O and the Si MAS NMR chemical shift to octakis-
(trimethylsiloxy)silsesquioxane. X-ray photoelectron spectroscopy
XPS) data were collected on an ESCALAB 250 X-ray photoelectron
spectroscope, using Mg K X-rays as the excitation source. All
binding energies were referenced to C1s of 284.6 eV. Tempera-
ture-programed desorption of NH (NH TPD) was carried out on
2
9
Al(SO
4
)
2
Á12H
2
(
a
2
. Experimental
3
3
2.1. Materials
a FINESORB-3010 equipped with a thermal conductivity detector.
The samples were first outgassed at 823 K for 1 h before the mea-
surement. After being cooled to 303 K, the samples were saturated
Commercial mordenite was purchased from the Catalyst Plant
of Nankai University. HNO , NaOH, NH NO , benzene, and benzyl
3 4 3
alcohol were purchased from Sinopharm Chemical Reagents Co.
and used without further purification.
in an NH
30 ml/min) for 2 h to remove physisorbed NH
profile was determined by increasing temperature from 303 to
73 K at a heating rate of 5 K/min while recording NH desorption
3
stream (5% in Ar) for 1 h and consequently treated in Ar
(
3
. Finally, the TPD
8
3
2.2. Preparation of catalysts
with a thermal conductivity detector. The adsorption of benzene
was performed in an Intelligent Gravimetric Analyzer (IGA-001,
Hiden Isochema). Prior to analysis, the powder samples (60–
The commercial mordenite was ion-exchanged with a solution
4 3
of 1 M NH NO at 353 K for 1 h, under stirring with a speed of
7
0 mg) were outgassed in situ at 573 K for 4 h under vacuum.
1
000 rpm, followed by calcination at 823 K for 3 h. The obtained
When the desired temperature of an experiment was stabilized,
the sample was subjected to a pressure step of benzene vapor
and the weight change was recorded continuously until equilib-
rium was reached.
sample was labeled HM.
3
The HM sample was refluxed with a solution of 2 M HNO at
3
73 K for 2 h with a liquid-to-solid ratio of 20 ml/g, under stirring
with a speed of 1000 rpm. Then, the solid sample was filtered,
washed with deionized water, dried at 393 K for 12 h, and ion-ex-
changed thrice with a solution of 1 M NH NO at 353 K for 1 h, fol-
4 3
2.4. Catalytic tests
lowed by calcination at 823 K for 3 h. The final product was labeled
HM-A.
The HM-A sample was treated with a solution of 0.2 M NaOH
with a liquid-to-solid ratio of 20 ml/g at 343 K for 0.5 h under stir-
ring with a speed of 1000 rpm. The product was filtered, washed
with deionized water, dried, ion-exchanged, and then calcined at
The liquid-phase benzylation of benzene with benzyl alcohol
BA) was carried out in a three-necked round-bottom flask
equipped with a reflux condenser and heated in a temperature-
controlled oil bath under atmospheric pressure. In a typical run,
4 ml of benzene was added to the catalyst, which had been acti-
(
1
vated at 773 K in air for 5 h before its use in the reaction. The reac-
tion mixture was maintained for 30 min at the required reaction
temperature and then 0.2 ml of benzyl alcohol was added. This mo-
ment was regarded as the initial reaction time. Liquid samples
were withdrawn at regular intervals and analyzed by gas chroma-
tography on an Agilent 7890A GC with an FID detector using a 30-
m packed HP5 column. The products were also identified by GC–
MS (5975C-7890A) analysis. Since benzene was in excess, conver-
sion was calculated based on the benzylating reagent, i.e., BA. The
selectivity to the product diphenylmethane (DPM) was expressed
as the amount of particular product divided by the amount of total
products and multiplied by 100.
8
23 K for 3 h. The obtained sample was called HM-AB.
The HM-AB sample was treated with a solution of 0.2 M HNO
3
at 323 K for 1.5 h with a liquid-to-solid ratio of 20 ml/g, under stir-
ring with a speed of 1000 rpm. The product was ion-exchanged as
described above, followed by calcination at 823 K for 3 h to obtain
the sample labeled HM-ABA.
The reused catalyst was obtained by filtering the catalyst after
use from the reaction solution, drying, and then calcining at
8
23 K for 5 h.
2.3. Catalyst characterization
Powder X-ray diffraction (XRD) patterns were recorded on a
Rigaku D/Max-2550 diffractometer equipped with a SolX detec-
tor—Cu K radiation with a wavelength of k = 1.5418 Å. Relative
crystallinity was calculated by the summed intensities of the
330), (150), (202), (350), and (511) reflections. The parent
3. Results and discussion
a
3.1. Characterization of catalyst
(
mordenite was assigned a crystallinity of 100%. The crystalline size
was determined from the characteristic peak (2h = 22.1° for the
The powder XRD patterns (Fig. 1) of all the samples exhibited
well-resolved diffraction peaks, which are characteristic of the
mordenite framework structure. Notably, the crystallinity (99%)
of HM-A (Table 1) showed a slight decrease from that of HM,
although the Si/Al molar ratio increased from 15 to 30 (Table 2).
That means that dealumination had a minor influence on the crys-
tallinity of mordenite. A similar result was also observed by an-
other research group [39]. However, after base treatment, the
(
150) reflection) using Scherrer’s equation, D = Kk/b cos h, where
K = 0.9, D represents crystallite size, k represents the wavelength
of Cu K radiation, and b represents the corrected half width of
a
the diffraction peak. SEM images were recorded on a SUPRA 55
operated with an acceleration voltage of 200 kV. Transmission
electron microscopy (TEM) images were recorded on a JEOL 2010

1
02
K. Leng et al. / Journal of Catalysis 306 (2013) 100–108
4
3
3
2
2
1
1
00
50
00
50
00
50
00
HM-ABA
HM-ABA
HM-AB
+
120
HM-AB
HM-A
+ 90
HM-A
HM
+
30
50
HM
35
0.0
0.2
0.4
0.6
0.8
1.0
5
10
15
20
25
30
40
Relative pressure (P/P )
o
2
Theta (degree)
Fig. 2. N
2
adsorption–desorption isotherms of various samples.
Fig. 1. XRD patterns of various samples.
sis loop in sorption isotherms, like the treated samples, which
suggested the presence of mesopores. For this reason, it may be ex-
plained by mesopores in HM being formed by stacking between
crystal particles. Generally, commercially available mordenite
crystallites have typical dimensions between 100 and 200 nm
and form larger particles [39]. In this work, the commercial mord-
enite used had a crystal size of around 400 nm (Fig. 3). The aggre-
gates of such particles probably resulted in the formation of
intercrystalline mesopores. However, the extent of mesoporosity
crystallinity decreased to 82%. Obviously, the effect of desilication
on the crystallinity was relatively large. Further, the acid-washing
step resulted in a minor decrease in the crystallinity (75%). These
results indicated that the post-treatments could lead to crystallin-
ity loss but the long-range crystal ordering in these samples may
be kept. The effect of disilication on crystallinity is greater than
that of dealumination. Additionally, by Scherrer’s equation, the
crystalline sizes of various samples were determined (Table 1).
The crystalline sizes of HM, HM-A, HM-AB, and HM-ABA are 449,
3
is low. The mesopore volume for HM is 0.1 cm /g, which is lower
than that for the treated samples (Table 1). The acid-leaching step
can improve mesopore volume to some extent, but the enhance-
ment is not significant. Obviously, the mordenite with base treat-
ment had a relatively high mesopore volume. The mesopore
4
24, 401, and 386 nm, respectively. Apparently, the post-treat-
ments resulted in a reduction of crystalline size. Possibly, this is be-
cause aluminum and silicon were extracted during leaching, and
many cracks and faults were formed on the outer surfaces of zeo-
lite grains, which led to partial collapse of large particles into small
particles [40].
3
volume for HM-AB and HM-ABA is 0.21 and 0.32 cm /g, respec-
tively, which is twice and three times that of the parent mordenite.
Additionally, all the treated mordenite had a higher BET special
surface area and external surface area than the parent mordenite,
which should be attributed mainly to the enhancement of porosity
2
The N -sorption isotherms of all the samples are shown in
Fig. 2. Surprisingly, the parent mordenite HM displayed a hystere-
Table 1
2
N sorption data of various samples.
Sample
BET surface area
External surface area
(m /g)
Mesopore surface area
(m /g)
Micropore volume
(cm /g)
Mesopore volume
(cm /g)
Crystalline size
(nm)
Crystallinity
(%)
2
2
a
2
b
3
a
3
b
c
(m /g)
HM
398
479
434
470
36
49
58
49
66
102
151
0.22
0.24
0.23
0.22
0.10
0.14
0.21
0.32
449
424
401
386
100
99
82
HM-A
HM-AB
HM-ABA
119
75
a
t-Plot method.
BJH method (adsorption branch).
XRD.
b
c
Table 2
Textural properties and chemical composition of various catalysts and the catalytic reaction data.
Si/Ala
NH
chemisorbed (mmol/g)b
Conversion of BA (%)c
Apparent reaction rate constant, k
(Â10 min
3
À1
)
TOF (s
À1
)
Sample
3
a
HM
15
30
20
31
1.92
1.32
1.10
1.22
6.1
3.2
0.07
1.42
0.46
3.92
HM-A
HM-AB
HM-ABA
40.2
28.8
99.6
20.9
20.2
45.8
a
b
c
XPS analysis.
NH TPD.
Reaction conditions: amount of catalyst = 0.1 g; benzene = 14 ml; benzyl alcohol = 0.2 ml; reaction temperature = 353 K; reaction time = 0.5 h. The
3
conversion of HM and HM-AB at 0.5 h was estimated according to the curve of conversion with reaction time.

K. Leng et al. / Journal of Catalysis 306 (2013) 100–108
103
Fig. 3. SEM images of HM (a), HM-A (b), HM-AB (c), and HM-ABA (d).
by the additional mesopores (Table 1). HM-ABA had the highest
Most of the particle size is between 200 and 400 nm. Some aggre-
gates can be observed, which may explain the reason for mesopore
formation in the parent mordenite, as mentioned above. In com-
parison, for the treated samples, the crystal shape became irregular
and crystal size turned small. Moreover, the treated samples had a
relatively rough surface. The surface roughness of the samples with
base treatment is more visible than that of the acid-leaching sam-
ple. These results indicated that the process of post-treatment had
an effect on the particle size and morphology of mordenite. TEM
images of various samples are shown in Fig. 4. It can be observed
that there are intercrystalline mesopores and no intracrystalline
mesopores in HM. In contrast, the treated samples displayed the
2
external surface area (119 m /g) among the four samples. The va-
2
lue is more than three times that of HM (36 m /g) and more than
2
2
twice that of HM-A (49 m /g) and HM-AB (58 m /g). These results
indicated that the acid-washing step after base treatment can sig-
nificantly increase the external surface area. The enhancement
should result from the removal of the deposited aluminum species
from the surface of framework. Such high mesopore volume and
external surface area for HM-ABA would be beneficial for the
improvement of mass transfer.
SEM images of all the samples are shown in Fig. 3. Obviously,
the parent mordenite had a smooth surface with a prismatic shape.
Fig. 4. TEM images of HM (a), HM-A (b), HM-AB (c), and HM-ABA (d).

1
04
K. Leng et al. / Journal of Catalysis 306 (2013) 100–108
existence of intracrystalline mesopores. These results are in accor-
dance with the data from N sorption.
To detect the coordination state of aluminum species in the
2
2
7
27
samples, Al MAS NMR was done. Fig. 5 shows Al MAS NMR
spectra of all the samples. Apparently, they exhibited a strong peak
centered at around 54 ppm, corresponding to tetrahedrally coordi-
nated aluminum, and a small peak centered at around 0 ppm, cor-
responding to octahedrally coordinated aluminum, indicating that
Al coordination in all the samples was mainly tetrahedral with a
small amount octahedral. Additionally, it can be noted that for
the treated samples, the peak corresponding to tetrahedrally coor-
dinated aluminum became wide and the total peak area turned
small compared with that for the parent mordenite. This suggested
that the Al content in the treated samples decreased and some Al
sites in the mordenite framework might be slightly changed. Obvi-
ously, the resonance peak area of HM-AB is larger than that of HM-
A and HM-ABA, indicating that the Al content in HM-AB is higher
than that in HM-A and HM-ABA. These results are in agreement
with the measurement results of the Si/Al molar ratio (Table 2).
To further uncover the structural changes, the experiments of
2
9
Si MAS NMR for various samples have been done. As can be seen
2
9
in Fig. 6, the Si MAS NMR of all the samples is mainly composed
29
of different tetrahedral sites. The deconvolution of Si spectrum
led to six peaks centered at À97 to À103 ppm (SiOH), À106.25,
and À110.08 ppm (Si(3Si, 1Al)), and À112.82 and À115.30 ppm
(
Si(4Si)), respectively. When the proportions of different Si sites
are compared, it can be found that acid leaching obviously re-
moved the Al species in the framework because the proportion of
Si(3Si, 1Al) decreased from 31.39 to 23.43%. Further, the desilica-
tion almost did not change the proportion of Si(4Si) and Si(3Si,
1
Al). Generally, it is thought that during desilication, the Si species
such as Si(4Si) can easily be leached out and it is difficult to remove
the Si species such as Si(3Si, 1Al) because the negatively charged
À
AlO4 tetrahedra prevent the hydrolysis of the Si–O–Al bond in
À
the presence of OH [41]. In this case, if the desilication process
only removed the Si species such as Si(4Si) and SiOH in HM-A,
the proportion of Si(3Si, 1Al) sites should increase. However, the
fact is that the proportion of Si(4Si) and Si(3Si, 1Al) in HM-A and
HM-AB is almost the same, suggesting that the base treatment
should result in partial loss of framework Al species. Possibly, this
part of Al species existed in HM-AB as extra-framework Al. After
the final acid-washing step, they could be removed. The fact that
the proportion of SiOH, Si(3Si, 1Al), and Si(4Si) sites in HM-ABA
is very close to that in HM-AB and the Si/Al molar ratio increased
from 20 to 31 could support this opinion.
The acidity of the samples was characterized by NH
can be seen in Fig. 7, all the samples showed two desorption peaks
of NH in the range 303–773 K. These desorption peaks corre-
3
TPD. As
3
sponded to two different type of acid sites. The low-temperature
desorption peak (303–523 K) was attributed to weak acid sites,
and the high-temperature desorption peak (523–773 K) was due
Fig. 6. 29Si NMR spectra of various samples.
to NH
3
bound to strong acid sites. The total number of acid sites
3
listed in Table 2 was calculated by peak area of desorbed NH .
The number of acid sites was 1.92 mmol/g for HM, 1.32 mmol/g
for HM-A, 1.1 mmol/g for HM-AB, and 1.22 mmol/g for HM-ABA,
respectively. Notably, HM possessed more acid sites than the trea-
ted samples, because HM had a lower Si/Al molar ratio (Si/Al = 15).
Particularly, HM-AB (Si/Al = 20) exhibited fewer acid sites than
HM-A and HM-ABA, although it had a lower Si/Al molar ratio than
HM-A (Si/Al = 30) and HM-ABA (Si/Al = 31). Possibly, the deposi-
tion of partial Al species on the mordenite framework after the
desilication made some acid sites inaccessible. This fact that the
Fig. 5. 27Al NMR spectra of various samples.

K. Leng et al. / Journal of Catalysis 306 (2013) 100–108
105
100
HM-A
HM
HM-ABA
HM-A
HM-AB
HM-ABA
80
HM-AB
HM
6
4
2
0
0
0
0
0
50
100
150
200
Reaction time (min)
100
200
300
400
500
600
o
Fig. 8. Catalytic activities of various catalysts in the benzylation reaction of
benzene with benzyl alcohol at 353 K.
Temperature ( C)
3
Fig. 7. NH TPD profiles of various samples.
acid-washing step increased the number of acid sites could be a
proof of this supposition.
ably enhanced catalytic activity, which almost ended this reaction
after a reaction time of 0.5 h. The apparent reaction rate constant
for HM-ABA is 15 times that for HM and twice that for HM-A
and HM-AB (Table 2). Additionally, the turnover frequency (TOF)
has been estimated based on the mole number of diphenylme-
thane produced per mole Al per second. The value of TOF was
3.2. Catalytic activity
À1
À1
À1
À1
To evaluate the catalytic performance of these catalysts, the
0.07 s
for HM, 1.42 s
for HM-A, 0.46 s
for HM-AB, and
benzylation of benzene with benzyl alcohol was first chosen at a
reaction temperature of 353 K. The reaction pathway in the benzy-
lation is displayed in Scheme 1. The products in the reaction are
mainly diphenylmethane (DPM) and dibenzyl ether (DBE). The
product DBE can be further converted into DPM. Meanwhile, the
polyalkylated products may be formed by the benzylation of DPM.
The conversion of benzyl alcohol with reaction time over vari-
ous catalysts is shown in Fig. 8. Obviously, HM gave a low catalytic
activity, with a BA conversion of 6.1% after a reaction time of 0.5 h
3.93 s for HM-ABA, respectively (Table 2). Based on the compar-
ison of TOF data, it can be seen that the enhancement of catalytic
performance is much greater. For example, the TOF of HM-ABA is
50 times higher than that of HM, twice that of HM-A, and eight
times that of HM-AB. However, as far as the selectivity is con-
cerned, all the catalysts are similar. The obtained products con-
tained mainly DPM (above 95%) with a small amount of DBE
(below 5%). No polyalkylated products were detected.
In comparison with previous reports on mesoporous zeolites in
the benzylation of benzene alcohol with benzene, HM-ABA is a
highly efficient catalyst. For example, Prins et al. showed that mes-
oporous ZSM-5 prepared by a soft-template method took 10 h to
end this reaction at the same reaction temperature [42]. Van
Bokhoven et al. displayed that mesoporous mordenite obtained
by an acid–base-leaching method completed the reaction after
3 h [40]. Ryoo et al. reported that an ordered mesoporous zeolite
MMS gave a BA conversion of 44% with a DMP selectivity of 77%
and a disordered mesoporous Beta showed a BA conversion of
33% with a DMP selectivity of 68% after 5 h at the same reaction
temperature [22]. Although in the above works the amounts of cat-
alyst and benzyl alcohol are different from those used in this work,
it can be estimated that HM-ABA is still a very active catalyst in
(Table 2). Interestingly, HM-A gave much higher catalytic activity
than HM. The BA conversion reached 40.2% after a reaction time
of 0.5 h, even a little better than that of HM-AB (28.8%). This result
is different from the previous report from van Bokhoven’s group
[
40]. They showed that acid-leaching mordenite gave a very low
catalytic activity, similarly to the parent mordenite, and a much
lower catalytic activity than the acid–base-leaching mordenite.
This difference might be due to the use of different parent morde-
nite. In that work, the used mordenite had a plate-like morphology
and a large crystal size (around 5 lm). In comparison, in this work,
the commercial mordenite had a prismatic morphology and a small
crystal size (around 400 nm). However, further study was required
to make the reason clear. Importantly, HM-ABA exhibited remark-
OH
OH
Catalyst
H O
-
Catalyst
2
OH
Catalyst
Catalyst
O
Scheme 1. Benzylation reaction pathway of benzene with benzyl alcohol.

1
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K. Leng et al. / Journal of Catalysis 306 (2013) 100–108
this reaction after the effect factor of amounts of catalyst and ben-
zyl alcohol is deducted.
1.0
To find why HM-ABA exhibited remarkably enhanced catalytic
activity in the reaction, the relationship between catalytic perfor-
mance and structural properties needs to be considered. In general,
it is thought that the benzylation reaction proceeds via an electro-
philic intermediate involving the interaction of benzyl alcohol with
the acid sites. The acid sites polarize the benzylating agent and in
turn produce an electrophile. Then, the generated electrophilic
species attack the benzene ring, resulting in the formation of
DPM [9,43]. Therefore, the number of acid sites is an important fac-
tor that affects catalytic activity. However, it can be noted that HM
had the largest total number of acid sites but exhibited the lowest
catalytic activity among the four samples and HM-ABA possessed a
similar total number of acid sites to HM-A but showed much high-
er catalytic activity than HM-A (Table 2). These results indicated
that the total number of acid sites had no good relationship with
catalytic activity. Possibly, the accessibility of acid sites played a vi-
tal role in this reaction. Nevertheless, the characterization of acces-
sible acid sites is not easy. Although an accessibility index derived
from infrared spectroscopy of substituted alkylpyridines with dif-
ferent sizes has been proposed to characterize the accessibility of
acid sites in hierarchical zeolites, and this method has been dem-
onstrated to be efficient over hierarchical ZSM-5 [44], its applica-
bility to other types of zeolites needs to be further investigated.
Recently, Park et al. measured the number of external acid sites
in mesoporous ZSM-5 by the chemisorption of 2,6-di-tert-butyl-
pyridine and found that mesoporous ZSM-5 with more external
acid sites exhibited superior catalytic performance in the benzyla-
tion of aromatics with benzyl alcohol [45].
On the other hand, mass transfer ability is another important
factor that influences catalytic performance. It is well known that
the micropore system of mordenite consists of a 12-membered
ring (MR) pore channel of 0.67 Â 0.70 nm and an 8-MR side pocket
of 0.34 Â 0.48 nm. Generally, mordenite is considered a one-
dimensional pore system due to inaccessibility of the 8-MR pores
for most molecules [39]. In the benzylation reaction, the molecular
size of reactants and products is around 0.5 nm, which is close to
the size of the pore channel with 12-MR and larger than that of
the pore channel with 8-MR [45]. Obviously, such a pore system
is not beneficial for mass transfer. Additional mesoporosity and
external surface area can improve the ability for mass transfer.
As listed in Tables 1 and 2, the treated samples with higher meso-
pore volume and external surface area had a much higher catalytic
activity than the parent mordenite. HM-ABA, with the highest
external surface area and mesopore volume, exhibited the best cat-
alytic activity. These results seemed to reveal that the catalytic
performance had a close relationship with the mass transfer abil-
ity. However, HM-A especially had a lower mesopore volume and
external surface area than HM-AB but showed slightly better cata-
lytic activity, probably because the deposited Al species after desi-
lication blocked partial pore channels and covered some acid sites,
and as a result, partial acid sites in HM-AB are inaccessible.
To demonstrate that the sample with large external surface area
and mesopore volume had better mass transfer ability, the diffu-
sion experiments were carried out by the adsorption of benzene
0.8
0
0
.6
.4
HM
0.2
HM-A
HM-AB
HM-ABA
0.0
0
200
400
Time (min)
600
800
1000
Fig. 9. Diffusion curves of benzene in various samples.
at 298 K and 0.1 bar. The adsorbed amount of benzene at adsorp-
tion equilibrium is 27.4 mg/g for HM, 38.6 mg/g for HM-A,
53.5 mg/g for HM-AB, and 62.6 mg/g for HM-ABA (Table 3). The
normalized uptake curves of various samples are shown in Fig. 9.
The effective diffusivity was calculated according to the reported
method [46]. The values for HM, HM-A, HM-AB, and HM-ABA are
À20
À20
À20
À20
2
1.93 Â 10 , 2.76 Â 10 , 5.98 Â 10 , and 6.93 Â 10
m /s,
respectively (Table 3), which increased with the mesopore volume
and external surface area. Among these samples, HM-ABA had the
highest effective diffusivity, which is three times larger than HM.
These results are in good agreement with our supposition. Based
on the above analysis, it can be concluded that a highly efficient
catalyst in this reaction should have a good combination of the
number of accessible acid sites, mesopore volume, and external
surface area. The catalyst with more accessible acid sites, larger
mesopore volume, and greater external surface area can be catalyt-
ically more active in this reaction. Therefore, such outstanding cat-
alytic performance over HM-ABA catalyst should be attributed to
more accessible acid sites and much better mass transfer ability
from rich mesoporosity in HM-ABA.
To further investigate the catalytic reactivity of HM-ABA, the ef-
fect of reaction temperature was tested. As shown in Fig. 10, reac-
tion temperature had a strong effect on BA conversion. For
example, at 343 K, the BA conversion was 40% after a reaction time
of 0.5 h, while at 353 K, the BA conversion reached nearly 100%.
The rate data for the benzylation of benzene in excess of benzene
1
00
3
53 K
348 K
3
43 K
80
60
40
20
0
Table 3
Adsorbed amount at adsorption equilibrium and diffusivity of benzene at 298 K and
0.1 bar on various samples.
À20
2
Sample
Adsorption amount (mg/g)
Diffusivity (10
m /s)
0
20
40
60
80
100
HM
27.4
38.6
53.5
62.6
1.93
2.76
5.98
6.93
Reaction time (min)
HM-A
HM-AB
HM-ABA
Fig. 10. Effect of reaction temperature on the benzylation reaction of benzene with
benzyl alcohol over HM-ABA catalyst.

K. Leng et al. / Journal of Catalysis 306 (2013) 100–108
107
over HM-ABA catalyst could be well fitted to a pseudo-first-order
rate law, ln(1/1 À x) = k (t À t ), where k is the apparent rate con-
stant, x is the fractional conversion of benzyl alcohol, t is the reac-
tion time, and t is the induction period corresponding to the time
required to reach equilibrium temperature. A linear plot of log(1/
À x) versus t À t can be obtained. The reaction rate became high
with increasing temperature. At 343 K, the apparent rate constant
benzyl alcohol. The fresh catalyst gave an activity of 15.3 mmol/g
at a reaction time of 0.5 h. Correspondingly, the first and second re-
used catalysts presented an activity of 15.2 and 15.4 mmol/g,
respectively. These results indicate that the catalyst could be re-
used and maintain the initial activity even after three cycles. Such
catalytic performance is of great importance for potential indus-
trial application.
a
0
a
0
1
0
À4
À1
À4
k
a
was 2.23 Â 10 min , while at 348 K it reached 3.54 Â 10
-
À1
min , which is nearly 1.6 times that at 343 K. Further increase
in temperature to 353 K increased k
as high as at 343 K. These first-order rate constants gave a linear
Arrhenius plot (Fig. 11) with an estimated activation energy of
À4
À1
4. Conclusions
a
to 4.58 Â 10 min , twice
Hierarchical mordenites were prepared by a sequential post-
treatment method based on a commercial mordenite. The hierar-
chical mordenite obtained by acid–base–acid treatment exhibited
remarkably enhanced catalytic activity in the benzylation of ben-
zene with benzyl alcohol. The apparent reaction rate constant is
15 times that for the parent mordenite and twice that for the
acid-leaching mordenite and base-leaching mordenite. Such out-
standing catalytic performance should be attributed to more acces-
sible acid sites and much better mass transfer ability from rich
mesoporosity in HM-ABA. The activation energy for HM-ABA in
this reaction is calculated with a low value of 74 kJ/mol. Addition-
ally, the catalyst HM-ABA could be reused and almost maintain the
initial activity even after several cycles. These results indicate that
hierarchical mordenites have potential applications in Friedel–
Crafts alkylation.
7
4 kJ/mol for the HM-ABA catalyst. This value is much lower than
that of the reported mesoporous ZSM-5 [42].
3.3. Reusability of the catalyst
The reusability of heterogeneous catalysts is one of their main
advantages. To test its reusability, a reaction using the HM-ABA
catalyst was carried out at 353 K. Fig. 12 shows the catalytic activ-
ities of fresh and reused HM-ABA in the benzylation of benzene by
-
-
-
-
-
-
-
1.2
1.3
1.4
1.5
1.6
1.7
1.8
Acknowledgments
a
The project was sponsored by Scientific Research Starting Fund-
ing from Harbin Institute of Technology, Open Funding from the
State Key Lab of Inorganic Synthesis and Preparative Chemistry, Ji-
lin University, and the Scientific Research Foundation for Returned
Overseas Chinese Scholars, State Education Ministry. We thank
Professor Shuxia Liu and Xingquan Wang from Northeast Normal
University for the diffusion measurements.
K
0.00284
0.00286
0.00288
0.00290
0.00292
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