Benzylation of benzene with benzyl alcohol on zeolite catalysts

Article abstract of DOI:10.1016/j.apcata.2010.11.044

The catalytic liquid phase benzylation of benzene to diphenylmethane (DPM) with benzyl alcohol (BnOH) was investigated over various zeolite catalysts (HSZ 600 mordenite, CBV 20A mordenite, H-beta zeolites with Si/Al of 10.8 and 35.8, respectively) at 353-423 K and under atmospheric pressure, using three reaction methodologies: (A) by adding all benzyl alcohol at the beginning of the reaction, (B) by dropwise addition of benzyl alcohol (1-2 drops/min for 4 h) and (C) under continuous flow conditions. Nafion-embedded silica and AlCl ₃/MCM-41 were also tested for comparison. The conversion and product distribution largely depended on the experimental conditions and the zeolite nature. Under optimal conditions using the Beta zeolite (Si/Al molar ratio of 10.8) and the (C) methodology, selectivity to DPM of 88.9% for a conversion of BnOH of 99.2% were achieved. Recycling tests showed that the catalyst preserved its activity and selectivity to DPM after several cycles.

Full text of DOI:10.1016/j.apcata.2010.11.044

Applied Catalysis A: General 393 (2011) 206–214
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Benzylation of benzene with benzyl alcohol on zeolite catalysts
N. Candu, M. Florea, S.M. Coman, V.I. Parvulescu∗
Department of Chemical Technology and Catalysis, Faculty of Chemistry, University of Bucharest, Bdul Regina Elisabeta 4-12, Bucharest 030016, Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
The catalytic liquid phase benzylation of benzene to diphenylmethane (DPM) with benzyl alcohol (BnOH)
was investigated over various zeolite catalysts (HSZ 600 mordenite, CBV 20A mordenite, H-beta zeo-
lites with Si/Al of 10.8 and 35.8, respectively) at 353–423 K and under atmospheric pressure, using
three reaction methodologies: (A) by adding all benzyl alcohol at the beginning of the reaction, (B) by
dropwise addition of benzyl alcohol (1–2 drops/min for 4 h) and (C) under continuous flow conditions.
Nafion-embedded silica and AlCl3/MCM-41 were also tested for comparison. The conversion and prod-
uct distribution largely depended on the experimental conditions and the zeolite nature. Under optimal
conditions using the Beta zeolite (Si/Al molar ratio of 10.8) and the (C) methodology, selectivity to DPM of
88.9% for a conversion of BnOH of 99.2% were achieved. Recycling tests showed that the catalyst preserved
its activity and selectivity to DPM after several cycles.
Received 1 September 2010
Received in revised form
29 November 2010
Accepted 30 November 2010
Available online 7 December 2010
Keywords:
Benzylation
Diphenylmethane
Benzyl alcohol
Zeolites
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
saturated linear polyesters improves their thermal stability and its
addition to jet fuels increases the stability and lubricating proper-
Friedel-Crafts alkylation is a very important reaction of organic
chemistry that is used for the production of a large variety of
chemicals for a wide range of industrial segments. Among these
chemicals, benzylated aromatics (an important class of alkylated
aromatics) occupy an important place because they are widely
used in the large-scale synthesis of petrochemicals, cosmetics, dyes,
pharmaceuticals and in many other chemical industries. The benzy-
lation reactions are usually done in liquid phase with homogeneous
acid catalysts such as FeCl3, AlCl3, BF3, ZnCl2, HF, H2SO4 and with
alkyl halides as alkylating agents [1–3].
Among typical criticisms addressed to homogeneous catalysis
(e.g., the difficult separation and recovery of the catalytic species
from the reaction medium) drawbacks like disposal of the spent
catalyst, corrosion or high toxicity are maybe the most impor-
tant ones that should obey the environmental legislation. In the
last years, to overcome these problems, synthetic organic reactions
and especially those for fine and specialty chemicals industries are
carried out with Lewis and Brønsted solid acid catalysts [4].
An important chemical compound obtained through the ben-
zylation of benzene is diphenylmethane which is mostly used in
the fragrance industry and agrochemicals (Scheme 1). Diphenyl-
methane is also recommended as a plasticizer to improve the dying
properties, as solvents for the dyes and as a dyes carrier printing
with disperse dyes. Moreover, the addition of diphenylmethane to
ties [5].
Taking into account the large utilization of this valuable com-
pound, on one hand, and the drawbacks of conditions this is usually
prepared, on the other hand, the optimization of the reaction sys-
tem in the sense of the identification of cheap heterogeneous
active catalysts and best experimental conditions is very impor-
tant. The use of heterogeneous catalysts for this synthesis has
already been reported using benzyl chloride or benzyl alcohol as
alkylating agent. For example, several solid acid catalysts, such as
Fe-modified ZSM-5 and H-beta zeolites, Fe2O3-, FeCl3-, In-, Ga, and
Zn-deposited or included in micro-, meso- and macro-porous sup-
ports, H-beta, H-Y, H-ZSM-5 zeolites, hydrotalcites, FeCl3, MnCl2,
CoCl2, NiCl2, CuCl2, or ZnCl2 supported on acidic alumina and Cu-
, Ga-, Sn-, Zn-, or supported on mesoporous materials (HMS-n)
have been proposed as being active using benzyl chloride as alky-
lating agent [6–17]. However, a disadvantage of this reaction is
the formation of HCl as by-product. Commonly, the use of ben-
zyl alcohol instead of benzyl chloride requires a stronger acid
catalyst, as Amberlyst-15, niobic acid or niobium pentoxide, nio-
bium phosphate, metal triflates supported on MCM-41, SBA-15
mesoporous aluminosilica, metal modified zeolites and Nafion-H
[18–28]. The reaction proceeds via an electrophile intermediate,
which involves the reaction of benzyl alcohol with the acid sites.
The Brønsted acid sites polarize the benzylating agent and in turn
produces an electrophile (C6H5CH2+). Then the generated elec-
trophilic species attack the benzene ring, resulting in the formation
of DPM [29,30].
∗
Corresponding author. Tel.: +40 214100241; fax: +40 214100241.
E-mail addresses: vasile.parvulescu@g.unibuc.ro,
Here we report a useful reaction methodology for enhancing the
selectivity to diphenylmethane in the presence of cheap zeolites
v parvulescu@yahoo.com (V.I. Parvulescu).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.11.044

N. Candu et al. / Applied Catalysis A: General 393 (2011) 206–214
207
Diphenylmethane (DPM)
O
Dibenzyl ether (DBE)
catalyst
- H2O
+
OH
Benzyl alcohol (BnOH)
Polyalkylated by-products
Scheme 1. Friedel-Crafts benzylation of benzene with benzyl alcohol.
(e.g., beta zeolites and mordenates) and environmentally friendly
benzyl alcohol as alkylating agent.
furnace and TCD detector. 0.5 g of sample was purged for 2 h with
40 mL/min He at 773 K, after the increase of temperature from
ambient to 773 K using a heating rate of 5 K/min, followed by
lowering temperature to 353 K. Then, 3% NH3/He was flushed for
30 min at 353 K, and then purged with 40 mL/min He at 373 K for
1 h. After that, under the same flow of He, the temperature was
raised from 373 K to 873 K at 10 K/min, analyzing the desorbed
ammonia. The weak, medium, and strong acidities were assigned
to the peaks of NH3-TPD profiles lower than 623 K, between 623
and 773 K, and above 773 K, respectively. FT-IR measurements were
performed at room temperature on a Magna-IR 550 FTIR spectrom-
eter from Nicolet, using a MCT-B liquid nitrogen cooled detector,
and equipped with a heatable cell (up to 773 K) with NaCl win-
dows connected to a vacuum system and a gas manifold. Samples in
the form of self-supporting pellets (around 5 mg/cm2) were placed
into a carousel sample holder for up to 6 pellets. Usually 200 scans
were recorded at a resolution of 2 cm−1 for a single spectrum.
FTIR spectra were normalized to the weight of 10 mg/cm2. Prior to
adsorption of pyridine (Py) the samples were dehydrated by evac-
uation at 673 K, overnight. The FTIR spectra were recorded after
desorption of pyridine at RT, 423, 523, 623 and 723 K, respectively.
All measured spectra were recalculated to a ‘normalized’ wafer of
10 mg. For a quantitative characterization of acid sites the following
bands and absorption coefficients were used: pyridine PyH+ band at
2. Experimental
The commercial zeolites with different structures and chemical
compositions were purchased from different companies. Therefore,
large pore mordenite (HSZ 600) with Si/Al of 3.0, was purchased
from TOSOH Company, mordenite CBV 20A with Si/Al of 10.0,
was purchased from Zeolyst International, while two H-beta zeo-
lites with Si/Al of 10.8 (PQ25) and 35.8 (PQ75), respectively, were
received from PQ-Valfor Company. For comparison they were
also used SAC-13 (Nafion-embedded silica) and AlCl3/MCM-41.
SAC-13 was purchased from Aldrich (Ssp = 400 m2/g; Dp = 10 nm;
Vp = 0.6 mL/g; stable to 473 K, 0.15 mequiv./g, H0 ≥ −12 (96%
H2SO4)). AlCl3-grafted onto MCM-41 was prepared by react-
ing anhydrous AlCl3 with Si-MCM-41 synthesized following a
reported methodology [31], as follows: a suspension of 10 g
Si-MCM-41 in 250 mL dry CCl4 was refluxed for 1.5 h, while
bubbling moisture-free N2 (30 mL/min) through the mixture.
Then 2.6 g (19 mmol) of anhydrous AlCl3 were added, and the
resulting mixture was refluxed under the N2 flow for another
3 h. The obtained AlCl3/MCM-41 suspension was filtered out,
washed with CCl4 and dried under vacuum at 353 K. The resulted
sample was characterized by a series of techniques such as nitro-
gen adsorption–desorption isotherms at 77 K, XRD, XPS, TEM,
and FT-IR. Detailed procedures were given elsewhere [31]. The
commercial zeolites have been characterized as well using nitro-
gen adsorption–desorption isotherms, NH3-TPD, Py-FT-IR and
dynamic light scattering experiments (DLS). The surface areas were
estimated according to the BET model from ex situ BET nitro-
gen physisorption isotherms at 77 K that were recorded on a
Micromeritics ASAP-2010 automated instrument. Calcined sam-
ples were degassed for 15 h at 403 K and 10−6 Torr before analysis.
The NH3-TPD profiles were obtained using a Micromeritics Auto-
ChemII apparatus equipped with a programmable temperature
1545 cm−1, ε = 0.078 cm ␮mol−1, pyridine PyL band at 1454 cm−1
ε = 0.165 cm ␮mol−1 [32].
,
Particle size distribution was determined from DLS measure-
ments using a Mastersize2000 from Malvern Instruments.
The benzylation reactions were carried out in free solvent condi-
tions in magnetically stirred 20 mL glass reactors (methodologies A
and B) and in continuous flow conditions (methodology C). In a typ-
ical procedure, the catalyst (0.12–0.60 g) was mixed with 3.56 mL
(40 mmol) of benzene and 4.16 mL (40 mmol) of benzyl alcohol.
All reactions were carried out at 323–423 K for 5 min-6 h under
vigorous stirring (1400 rpm). Each reaction was carried out follow-
ing three methodologies: (A) by adding all benzyl alcohol at the

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N. Candu et al. / Applied Catalysis A: General 393 (2011) 206–214
Table 1
Textural properties of the investigated catalysts.
Zeolite
Surface area (m2/g)
Pore volume (mL/g)
HSZ 600
CBV 20A
PQ25
438
362
465
534
0.18
0.16
0.19
0.18
PQ75
beginning of the reaction, (B) by dropwise addition of benzyl alco-
hol (1–2 drops/min for 4 h) and (C) flow conditions (with different:
molar ratio of B/BnOH: 1/1, 5/1, 10/1; flow: 0.1, 0.2 and 0.3 mL/min
and temperature: 373 K and 423 K). After the reaction was stopped
(methodologies A and B), the catalyst was filtered off and the reac-
tion product was analyzed through GC (Shimadzu instrument with
FID detection, TR-WAX-TR1MS column of 60 m length and 0.32 mm
inner) and characterized by GC–MS (TERMO Electron Corporation
instrument).
The reactivation of the catalysts was carried out following a pro-
cedure described elsewhere [33,34]. The catalysts were calcined
with 5 K min−1 till 373 K, and then with 1 K min−1 till 773 K, where
they were kept for 4 h.
Fig. 1. NH3-TPD profiles of the investigated zeolites: (a) PQ 25; (b) PQ 75; (c) CBV
20A; (d) HSZ 600.
3. Results and discussion
3.1. Catalysts characterization
The percent of the medium-to-strong acid sites from the total
number of acid sites was about 49% for PQ 25, 43% for PQ 75, 39%
for CBV 20A, and 34% for HSZ 600, respectively.
Table 1 shows the textural properties of the investigated zeo-
lites. While some differences were observed among the surface
areas, depending both on the Si/Al ratios and the type of zeolite,
the pore volume of these materials is very close, that is in a very
good concordance with the typical high external surface area of
beta zeolites.
Py-FTIR spectra revealed the presence of Lewis acid sites as char-
acterized by the 8a adsorption mode of Py (band at 1620 cm−1
)
and the presence of Brønsted acid sites as characterized by the
8a adsorption mode of Py (band at 1638 cm−1). The other bands
in these spectra correspond to the 19b adsorption mode of Py
(1445 and 1545 cm−1), characteristics to Lewis and Brønsted acid
sites, respectively, while those at 1490 and 1595 cm−1 are charac-
teristics to pyridine physically adsorbed. The band at 1490 cm−1
may contain a contribution of PyP (Van der Waals interaction)
and PyH (H-bond interaction) species but also of PyL (interac-
tion with Lewis sites) and PyB (interaction with Brønsted sites)
species [41]. The kinetic diameter of pyridine (0.5 nm) allows
it to react with all acidic hydroxyl groups located in the 12-
ring channels (0.76 nm × 0.64 nm) of beta zeolites but in the
case of mordenites, hydroxyl groups located in 8-ring channels
(0.26 nm × 0.57 nm) are inaccessible for steric reasons. Neverthe-
less, these sites are not accessible for reactants as well. In agreement
From structural point of view, there are some differences among
the two classes of zeolites (e.g., mordenites and beta zeolites):
while beta zeolite has a three-dimensional channel system with
12-membered ring channels (0.76 nm × 0.64 nm), mordenites are
characterized by a bi-dimensional channel system with straight
12-membered ring channels (0.65 nm × 0.70 nm) with crossed 8-
membered ring channels (0.28 nm × 0.57 nm) [35].
The total acidity of zeolites was investigated by NH3-TPD while
the pyridine adsorption gave information about the acidity type
and strength (Brønsted and/or Lewis).
Fig. 1 shows the NH3-TPD profiles of the above zeolites. Mor-
denites showed two large peaks one below 473 K assigned to weak
acid sites and another at 773–873 K assigned to strong acid sites.
The physical assignment of the low temperature peak received in
the literature different assignments: chemisorption of NH3 to zeo-
lite cations, to weakly acid silanol groups or to weak Lewis acid sites
[36–39]. Contrarily to mordenites the NH3-TPD profiles of beta zeo-
lites corresponded to only one peak centered very close to 700 K
and assigned to a medium-to-strong acidity. However, the peaks
of both mordenite and beta show a quite large non-symmetry that
can be interpreted in the sense they correspond to the contribution
of several surface species with similar acid strength.
According to Camilot et al. [40], in the case of beta zeolites,
the non-symmetry is explained by two maxima: a first one, more
intense, at about 683 K and another at 702 K. As the aluminum con-
tent decreased the intensity of the peak at 683 K diminishes faster
than the second one and this latter is shifted about 17 K to higher
temperatures. The shift of the second peak indicates an increase in
acid strength of the sites that according to these authors [40] can
be attributed to a higher proton activity as a consequence of the
lower negative charge density of the zeolite framework. A similar
effect can be expected to mordenites as well.
ˇ
with Cejka et al. [42] the acidity measured in this manner is
more adequate to the actual catalytic capability of these zeo-
lites.
In Fig. 2 are exemplified Py-FTIR spectra at different Py-
desorption temperatures on PQ25 beta zeolite sample. During
desorption, the band of PyH+ species (1545 cm−1) systematically
decreases while a new band located at 1462 cm−1 start to appear
[43]. The latter band (assigned to iminium ions, formed by the
attack of protons on the pyridine complex bonded to Lewis acid
sites: C5H5NL + H+ = C5H5NL+ [42]) increases with desorption tem-
perature, which can be due to the combined effect of pyridine
migration and “in situ” formation of stronger Lewis sites. The inten-
sity of the iminium ion band relative to that of PyL (1462 and
1450 cm−1) increased distinctly with the increase of the desorption
temperature. The increase of the desorption temperature generates
the removal of Py from both Brønsted and Lewis sites and causes
an increased distance between these sites, and also of the aver-
age acid strength of the remaining hydroxyl groups (and hence
the mobility of the protons). This is most likely the reason for

N. Candu et al. / Applied Catalysis A: General 393 (2011) 206–214
209
3.2. Benzylation of benzene following the methodology A
Table 3 depicts the results of benzylation of benzene with benzyl
alcohol (BnOH) as alkylating agent over the investigated zeolites
H-beta (PQ25 and PQ75) and mordenite (CBV 20A and HSZ 600)
(Scheme 1). Obviously, the benzyl alcohol is a less reactive alkylat-
ing agent than benzyl chloride, and as a result, it is expectable a
reaction rate to diphenylmethane (DPM) inferior to those reported
with benzyl chloride [11]. Under the investigated conditions H-beta
zeolites were more active compared to mordenites (entries 1–2
versus entries 3–4). In terms of selectivity, H-beta zeolites were
also found to be more selective than mordenites in this reaction
although the selectivity is not satisfactory. Both the activity and
selectivity were affected by the Si/Al molar ratio for both zeolites
(Beta and Mordenite). A higher Si/Al molar ratio was found to allow
an increased BnOH conversion. For example, after 2 h, the conver-
sion of BnOH on PQ75 sample (H-beta zeolite; Si/Al = 35.8) was
almost 66%. The results are in contradiction with early reports [11]
showing that a high Si/Al molar ratio of H-beta lowers the alkylating
agent (benzyl chloride) conversion.
It is well known that a higher Si/Al ratio decreases the density
of acid sites and increases their strength. To generate the elec-
trophile (C6H5CH2+) needed for the Friedel-Crafts alkylation, the
benzyl alcohol requires stronger acid sites. Furthermore, it was
observed for both zeolites that the selectivity to DPM is signifi-
cantly affected by the Si/Al ratio, especially in the case of mordenite
zeolites. Thus, apart the high DBE formed amounts, high molecu-
lar weight polybenzylated by-products were also observed, their
concentration depending on the type of catalyst. The lower selec-
tivity to DPM at higher Si/Al ratios can be attributed to the residual
Brønsted acid sites, which seems to be also the main responsible for
catalyzing the consecutive reactions in the benzylation of benzene.
However, the variation of the BnOH conversion and the selectivity
to DPM do not parallel the Si/Al molar ratio (Table 3, entries 2 and
3) showing that the zeolites behavior is dictated not only by their
acidity but also by their porosity. Accordingly, although the differ-
ence in the Si/Al molar ratio for PQ25 Beta zeolites (Si/Al = 10.8)
and CBV 20A mordenite (Si/Al = 10.0) is insignificantly large differ-
ences were observed in the case of selectivity to DPM (it decreased
from 30.9% for PQ25 Beta to 3.1% for CBV 20A mordenite), while
the observed difference in the conversion of BnOH is quite low
(48.4% in the case of PQ25 Beta versus 44.5% in the case of CBV 20A
mordenite). A somehow surprising result was obtained on HSZ 600
mordenite (Table 3, entry 4), in the presence of which the amount
Fig. 2. Py-FTIR spectra at different Py-desorption temperatures on PQ25 beta zeolite
sample.
the increase of the number of iminium ions formed per PyL [44].
ˇ
In accordance with Cejka et al. [45] two processes could operate
simultaneously at 723 K: pyridine desorption and dehydroxyla-
tion of liberated OH groups. As some part of the hydroxyl groups
is still kept by pyridine, dehydroxylation is somehow restricted,
as not enough OH groups are available for further condensa-
tion.
It is worth to note that for most of the zeolites, Lewis acid sites
are much stronger than the Brønsted acid sites. The Py-desorption
spectra (Fig. 2) confirm this fact, at 723 K the amount of strength
Lewis acid sites being by far higher than the strength Brønsted acid
sites. Table 2 gives the relative population of these sites and their
concentration.
Table 2
The relative population of the Brønsted and Lewis acid sites and the concentration of Brønsted and Lewis sites.
Zeolite
Si/Al molar ratio
Brønsted acid sitea
PyH+ mmol/gb
Lewis acid sitea
PyL mmol/gb
PQ 75
PQ25
CBV 20A
HSZ 600
35.8
10.8
10.0
3.0
0.21
0.9
0.415
0.605
0.016
0.07
0.03
0.051
0.7
1.56
2.64
0.008
0.11
0.26
0.43
0.047
a
Calculated by the ratio of the integrated area of IR adsorption peak to sample weight (Py desorption – 423 K).
Calculated from the absorption coefficients [32].
b
Table 3
Benzylation of benzene with benzyl alcohol catalyzed by zeolites following methodology A.
Entry
Catalyst
Si/Al molar ratio
CBnOH (%)
SDPM (%)
By-products (%)
DBE
Polyalkylated benzenes
1
2
PQ75
PQ25
35.8
10.8
65.8
48.4
21.7
30.9
71.6
63.5
6.7
5.6
3
4
CBV 20A
HSZ 600
10.0
3.0
44.5
38.9
3.1
9.1
94.0
73.3
2.9
17.6
Reaction conditions: 3.56 mL (40 mmol) of benzene; 4.16 mL (40 mmol) of BnOH; 600 mg of catalyst; reaction temperature = 353 K; reaction time = 2 h.

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N. Candu et al. / Applied Catalysis A: General 393 (2011) 206–214
80
of polyalkylated benzene by-product was considerably higher than
on the other catalysts. Such a behavior proves once more the effect
of both the chemical and textural zeolite characteristics upon the
catalytic performances.
70
60
50
40
30
20
10
0
Increasing the reaction temperature to 373 K, as expected, was
leading to an increase of the conversion of BnOH, irrespective of the
zeolite nature, but the selectivity was still far on those of practical
interest (Table 4). However, although the change in the temper-
ature was small a different change in the selectivity has been
observed as a function of the zeolite nature. Thus, in the case of
Beta zeolites (Table 4, entries 1 and 2), the increase of the temper-
ature lowers the selectivity to DPM while for mordenites, the same
increase of temperature led to a higher selectivity in DPM (Table 4,
entries 3 and 4).
At higher temperatures and in the presence of Beta zeolites, the
decrease in the selectivity to DPM may be attributed to the forma-
tion of higher amounts of by-products (e.g., DBE and polyalkylated
benzene). In the case of mordenites, the different porosities con-
nected with a certain surface chemical composition changes again
the product distribution.
C BnOH (%)
S DPM (%)
S DBE (%)
323
343
353
363
373
Reaction temperature (K)
Fig. 3. The variation of the conversion of BnOH and selectivities to DPM and DBE
as a function of the reaction temperature following methodology B (reaction condi-
tions: reaction mixture: substrate = 3.56 mL (40 mmol); BnOH = 4.16 mL (40 mmol);
amount of PQ25 catalyst = 600 mg; reaction time = 2 h).
To the same temperature (373 K) and a longer reaction time (4 h)
the selectivity to DPM is slightly increased (Table 5). Moreover, the
amount of the catalyst used in the reaction influence both the con-
version of BnOH and the selectivity to DPM. In Table 5 are presented
the results obtained using different amounts of PQ25 Beta zeolite
(0.03–0.15 g). Nevertheless, for the same amount of catalyst (0.15 g)
the selectivity to DPM is slightly lower at 373 K after 4 h (Table 5,
entry 6) than at 353 K after 2 h (Table 3, entry 2) but slightly higher
than at 373 K after 2 h (Table 4, entry 2).In conclusion, following the
methodology A the etherification of benzyl alcohol to DBE is faster
than the benzylation of benzene DPM [45]. On the other hand, the
variation of the selectivity to DPM as a function of the reaction tem-
perature and time suggest that the generated dibenzyl ether (DBE)
acts as an alkylating agent.
as it is that characteristic to beta zeolites is sufficient to generate
electrophile (C6H5CH2+) species necessary for Friedel-Craft alkyla-
tion. Moreover, higher acidity strength (e.g., SAC-13 sample) favors
the etherification thus decreasing the selectivity to the alkylated
product, while the low activity –OAlCl2 and –O2AlCl units formed
during the immobilization of AlCl3 on the surface of MCM-41 sam-
ple [31] led to a decrease of both the conversion of benzyl alcohol
and the selectivity to DPM. Therefore, the nature of the catalyst has
a considerable influence on the reactivity and selectivity of the alky-
lating agent, the activated electrophile species existing as a more
or less tight ion pair with a considerable degree of covalent bond-
ing between the carbocation and the catalyst macroanion. In these
conditions, its relative stability is very important for determining
the rate of alkylation.
In addition, pores in the domain of mesopores, as in the case
of SAC-13 and AlCl3/MCM-41, allow the formation of by-products
such as polyalkylated benzenes and DBE, thus decreasing the selec-
tivity to the main reaction product – DPM.
The dependence of the conversion and the selectivity to DPM on
the reaction temperature was investigated using PQ25 Beta zeolite,
in a range of 323–373 K (Fig. 3).
3.3. Benzylation of benzene following the methodology B
A common procedure to favor the increase of the conversion is to
work in an excess of the substrate. Nevertheless, such an approach
comes in contradiction with the principles of green chemistry, any
excess of the reactants assuming not only higher raw materials con-
sumption but also the generation of wastes. Having these in mind,
experiments in which BnOH was dropwise added finally reaching
a molar ratio of the reactants of 1/1 have also been carried out.
In this way it is generated an apparently high excess of one of the
substrates into the reaction medium and the etherification reaction
may be confined.
Table 6 presents the reaction results following the methodology
B, at 353 K. Tested zeolites were compared with two other cat-
alytic samples with strong pure Brønsted acidity (SAC-13; Nafion
(13 wt%) embedded silica; amount of acid sites: 0.15 mequiv./g,
H0 ≥ −12; Dp = 10 nm) or pure Lewis acidity (AlCl3/MCM-41,
amount of active phase = 1.9 mmol/g; Dp = 2.4 nm)). As Table 4
shows, the major improvement is that in the presence of zeolites,
the dropwise addition of BnOH improves the selectivity to DPM
in high degree, thus approaching values of practical interest. Using
PQ75 zeolite, following this procedure the selectivity in DPM was of
65% for a conversion of 62% in 4 h, and with PQ25 the selectivity in
DPM was 77% for a conversion of 58% under the same experimental
conditions. An improvement was observed in fact for all zeolites.
Keeping the system for another 2 h under the same experimental
conditions was not accompanied by significant changes. Notewor-
thy, the behavior of zeolites beta was superior to that of SAC-13 or
of AlCl3/MCM-41 catalysts.
As we expected the conversion of BnOH was found to increase
with the increase of the reaction temperature. However, like in the
experiments carried out following the methodology A, the selectiv-
ity to DPM registered a maximum value (72.2%) at 353 K. At higher
temperatures (373 K), the decrease in DPM selectivity is attributed
to the formation of bulkier polybenzylated as consecutive products
and to the etherification to DBE.
3.4. Benzylation of benzene following the methodology C
The use of flow methodology brings several technical advan-
tages as, for example, the improved heat and mass transfer and the
precise control over reaction conditions. Such technical advantages
bring in turn important improvements of the chemical reaction in
terms of yield improvement, better selectivity and reproducibility.
An optimization of the benzylation can be made by varying
different reaction parameters as: temperature (373–423 K), flow
rate of the reactants (0.1–0.3 mL/min), and benzene/BnOH molar
ratios (1/1–1/10). The most significant results are presented in
Tables 7 and 8. As Table 7 shows, in the presence of PQ25 beta zeolite
and flow rate of 0.2 ml/min, the conversion of BnOH increases from
These data suggest that in fact the benzyl alcohol activation
does not require a very strong acidity, and a middle-strong acidity

N. Candu et al. / Applied Catalysis A: General 393 (2011) 206–214
211
Table 4
Benzylation of benzene with benzyl alcohol at 373 K following methodology A.
Entry
Catalyst
Si/Al molar ratio
CBnOH (%)
SDPM (%)
By-products (%)
DBE
Polyalkylated benzenes
1
2
PQ75
PQ25
35.8
10.8
72.1
72.6
15.6
18.7
77.8
77.9
6.6
3.4
3
4
CBV 20A
HSZ 600
10.0
3.0
45.9
51.8
16.3
30.9
80.7
57.8
3.0
11.3
Reaction conditions: 3.56 mL (40 mmol) of benzene; 4.16 mL (40 mmol) of BnOH; 600 mg of catalyst; reaction temperature = 373 K; reaction time = 2 h.
Table 5
Benzylation of benzene with benzyl alcohol with different amounts of PQ25 Beta zeolite following methodology A.
Entry
PQ25 beta catalyst (mg)
CBnOH (%)
SDPM (%)
By-products (%)
DBE
Polyalkylated benzene
1
2
3
4
5
6
120
200
300
400
500
600
63.2
67.8
76.2
72.3
80.3
83.5
17.7
19.1
19.9
21.9
22.5
25.6
75.9
74.5
74.0
73.2
72.7
69.8
6.4
6.4
6.1
4.9
4.8
4.6
Reaction conditions: 3.56 mL (40 mmol) of benzene; 4.16 mL (40 mmol) of BnOH; temperature = 373 K; reaction time = 4 h.
Table 6
The benzylation of benzene following methodology B.
Entry
Catalyst
CBnOH (%)
SDPM (%)
By-products (%)
DBE
Polyalkylated
benzenes
4 h
6 h
4 h
6 h
4 h
6 h
4 h
6 h
5.5
7.2
0
1.0
6.1
12.8
1
2
3
4
5
6
PQ75
PQ25
CBV 20A
HSZ 600
SAC-13
62.4
57.9
41.2
43.5
59.9
37.0
64.2
64.8
42.1
44.7
61.4
41.9
65.5
77.0
29.2
42.9
21.0
3.1
63.2
72.2
29.1
42.3
16.2
3.3
27.8
17.2
70.8
56.3
71.6
91.9
31.3
20.6
70.9
56.7
77.7
83.9
6.7
5.8
0
0.8
7.4
5.0
AlCl3/MCM-41
Reaction conditions: reaction mixture: 3.56 mL (40 mmol) of benzene; 4.16 mL (40 mmol) of BnOH; 600 mg of catalyst; reaction temperature = 353 K; reaction time = 4 h and
6 h, respectively (including the dropwise adding time of BnOH).
Table 7
Benzylation of benzene with benzyl alcohol at 373 K following methodology C.
Entry
Molar ratio-B/BnOH
CBnOH (%)
SDPM (%)
By-products (%)
DBE
Polyalkylated benzene
1
2
3
1/1
5/1
10/1
10/1
20
24.8
50.1
88.9
56.7
75.2
49.9
9.4
1.7
1.8
1.7
0.2
89.8
99.2
96.8
4*
43.3
Reaction conditions: 3.56–35.6 mL of benzene; 4.16 mL of BnOH; 600 mg of catalyst PQ25; temperature = 373 K; and *methodology A: 35.6 mL of benzene; 4.16 mL of BnOH;
600 mg of catalyst; temperature = 373 K, reaction time = 2 h.
Table 8
Benzylation of benzene with benzyl alcohol at 423 K following methodology C.
Entry
Molar ratio-B/BnOH
CBnOH (%)
SDPM (%)
By-products (%)
DBE
Polyalkylated benzene
1
2
3
1/1
5/1
10/1
42.7
97.1
96.4
3.3
34.7
76.7
96
64.5
23
0.7
0.8
0.3
Reaction conditions: 3.56–35.6 mL of benzene; 4.16 mL of BnOH; 600 mg of catalyst PQ25; reaction temperature = 423 K.
20% to 99.2% with the increases of the benzene/BnOH molar ratio
from 1/1 to 10/1 (entries 1–3). The selectivity to DPM paralleled the
conversion of BnOH increasing from 24.8 to 88.9%.
continuously through the reactor until the efficiency of the pro-
cess decreased. The reaction products were analyzed every 10 min
(Fig. 4).
For a more realistic evaluation of the reactor-catalyst efficiency,
both reactants (e.g., benzene and benzyl alcohol) were flowing
As Fig. 4 shows, after approximately 2 h the conversion of
BnOH started to decrease, indicating that the active sites become

212
N. Candu et al. / Applied Catalysis A: General 393 (2011) 206–214
100
90
80
70
60
50
40
30
20
10
0
C BnOH (%)
S DPM (%)
S DBE (%)
Reaction time (min)
Fig. 5. Particle size distribution of zeolite catalysts: (a) PQ 25 (d = 20. A 8.9 (sh), and
34.3 ␮m); (b) PQ 75 (d = 8.7 (sh), and 28.7 ␮m); (c) HSZ 600 (d = 6.4 ␮m); (d) CBV
20A (d = 3.8 ␮m).
Fig. 4. Benzylation of benzene with benzyl alcohol at 373 K following route C
(reaction conditions: B/BnOH molar ratio = 10/1: 35.6 mL of benzene and 4.16 mL
of BnOH; 600 mg of catalyst PQ25; temperature = 373 K; flow rate of the reac-
tants = 0.2 mL/min; time = 10 min for each reaction).
Table 9
Thiele modulus modified by Weisz for the investigated catalysts.
progressively poisoned by the polyalkylated benzenes formed as
by-products (<2%; not shown in Fig. 4). After cca. 7 h the catalyst
was almost completely deactivated and the conversion of BnOH
was 2.2%.
Zeolite
PQ 75
PQ25
CBV 20A
HSZ 600
ϕ
9.13 × 10−2
9.56 × 10−2
0.001 × 10−2
0.003 × 10−2
Interestingly, after the first 10 min, the selectivity to DPB was
only 3.9% while the selectivity to DBE was of 95.8%, after 10 min
the selectivity of DBE decreased at 40.5% while the selectivity of
DPB increased at 58.8%. The increase of the conversion of BnOH
from 15.6% after the first 10 min to 25.3% after 20 min does not jus-
tify the high difference in the DPM selectivity (from 3.9% to 58.8% in
this reaction period). To explain these results we have to take into
consideration that the benzylation takes place through a mecha-
nism which involves as benzylation agents both BnOH and DBE, in
agreement with data reported previously [46] (Scheme 2).
Keeping the same reaction conditions but changing the reaction
methodology (from C to A) the selectivity to DPM was drastically
reduced while the conversion of BnOH remained at the same level
(Table 7, entries 3 and 4).
As for the other cases, the increase of the reaction temperature
led to an increase of the conversion of BnOH while the selectivity
to DPM showed a major decrease (Table 8) in the favor of the DBE
and polyalkylated benzene.
Irrespective of the experimental methodology (Tables 3–8)
the zeolite PQ25 was the most effective catalyst. Actu-
ally, the performances of the catalysts followed the order
PQ25 > PQ75 > HSZ600 > CBV20A. This order paralleled per-
fectly the order of the particle size 34.3 ␮m (PQ25) > 28.7 ␮m
(PQ75) > 6.4 ␮m (HSZ600) > 3.8 ␮m (CBV20A) (Fig. 5).
different catalysts. These values are much smaller than 1, i.e. the
value for which is accepted the diffusion limits the overall process
(Table 9). In addition the differences between the values measured
for the two supports are in concordance with the larger pore sizes
of the beta-zeolites. However, there are strong arguments the reac-
tion occurs on the external surface of the catalysts that is quite
contradictory with these results.
Subtle differences between zeolites of similar pore sizes and
dimensionality can be usually explained based on the differences in
the acidity of the individual zeolites [42]. In the studied benzylation
reaction the zeolites behavior may account for the fact the reaction
occurs mainly on the external surface of the above zeolites. How-
ever an analysis of the selectivities in this case is speculative and
has little sense since most of the acid sites are distributed within
the grain and only those situated at the surface work.
These data also show that the behavior is different working with
benzyl alcohol instead of benzyl chloride. Using benzyl chloride
medium-to-strong acid sites are not required maybe because of
the participation of the released HCl [50]. This behavior is different
using benzyl alcohol and is also confirmed by the data presented
in Fig. 3 for the first 2 h reactions. The strong acid sites are nec-
essary to generate DBE that becomes the alkylating agent. Once,
these sites are blocked further reaction of DBE is limited and both
the conversion and the selectivity to DPM dramatically decreased.
The performances of the catalysts also follows fairly good the
percent of the medium-to-strong acid sites from the total number
of acid sites PQ 25 (49%) > PQ 75 (43%) > CBV 20A (39%) > HSZ 600
(34%). Exception makes only CBV 20A and HSZ 600 but for the last
one the particle size is larger.
3.5. Recycle experiments
PQ75 Beta zeolite used in the benzylation of benzene was
recycled four times in order to check the activity, stability and
reusability of the catalyst following the methodology A. After com-
pletion the reaction on fresh catalyst, the catalyst was separated
from the reaction mixture by filtration, washed with acetone and
calcined at 773 K for 4 h in the presence of air. The PQ75 cata-
lyst practically kept the activity and selectivity to DPM after each
cycle (Table 10) irrespective of the reaction methodology has been
used.
However, no alteration of the kinetics as an effect of the internal
diffusion linitation has been evidenced. The calculation of the Thiele
modulus modified by Weisz [47–49]
vR2
ϕ =
DeffC0
where v is the rate (mol s g−1) calculated for conversions smaller
than 15%, R is the radius of the catalyst grains, C0 is the concen-
tration (mol ml−1), and Deff is the diffusion coefficient (cm2 s−1
)
This is another important advantage of using BnOH since stud-
ies using benzyl chloride as alkylating agent reported a loss of the
activity after recycling, although in a low extent, due to a minor
indicated that indeed the diffusion resistance is absent, and the
reaction rate is representative of activity for the comparison of

N. Candu et al. / Applied Catalysis A: General 393 (2011) 206–214
213
catalyst
+
- H2O
OH
+
2
+
OH
O
Scheme 2. Friedel-Crafts benzylation of benzene involving both BnOH and DBE as benzylation agents.
Table 10
It has to be pointed out that the catalytic system has also
some practical advantages such as: (i) availability and low costs
of the catalysts; (ii) simple and practical experimental set-up; (iii)
easier purification of the final product due to the lower content
of by-products. The catalytic efficiency combined with practical
advantages of the catalytic experimental set-up and the “green”
elements of the reaction itself make this process appealing even in
the commercial stage.
Recycling of PQ75 zeolite in four catalytic runs.
Entry
Run
CBnOH (%)
SDPM (%)
1
2
3
4
Fresh catalyst (1st run)
2nd run
3th run
65.8
65.6
65.7
65.6
21.7
21.3
21.4
21.5
4th run
Reaction conditions: 3.56 mL (40 mmol) of benzene; 4.16 mL (40 mmol) of BnOH;
600 mg of catalyst; reaction temperature = 353 K; reaction time = 2 h.
Acknowledgement
dealumination of the zeolite by HCl, which is produced during the
reaction as a byproduct [11,51,52].
The authors thank the CNCSIS for PNCDI II 37/2007 financial
support.
Using benzyl alcohol as alkylating agent the only inorganic by-
product formed during the reaction is water. However, it seems that
even the formed water blocks the acid sites and the prolongation
of the reaction time is leading to a plateau in the benzyl alcohol
conversion (Table 6). A simple azeotropic removal of water during
the reaction may solve this inconvenient.
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