Friedel-Crafts alkylations on nanoscopic inorganic fluorides

Article abstract of DOI:10.1016/j.apcata.2010.08.004

The catalytic potential of nanoscopic MF_x (M = Mg, Al; x = 2, 3) has been investigated using batch Friedel-Crafts alkylation of aromatic compounds, including benzene, ethylbenzene, trimethylhydroquinone (TMHQ), and menadiol (MDL), with isophytol and benzyl alcohol. The conversion of isophytol was 100% in the reactions with trimethylhydroquinone (TMHQ), menadiol (MDL) and benzene while the highest selectivity in alkylated compounds was achieved with TMHQ (>99%). The different reaction rates of the alkylation reactions are due to the different nucleophylicities of the substrates, and therefore, due to their ability to delocalize the positive charge in the Wheland intermediate by inductive and resonance effects. The conversions of benzyl alcohol varied between 10 and 84% as a function of the catalyst nature and reaction conditions while the highest selectivity to monobenzyl derived compounds (25%) was achieved with ethylbenzene. The formation of high amounts of dibenzyl ether was also observed, indicating the presence of high amounts of Br?nsted acid sites in this type of catalysts.

Full text of DOI:10.1016/j.apcata.2010.08.004

Applied Catalysis A: General 391 (2011) 169–174
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Friedel–Crafts alkylations on nanoscopic inorganic fluorides
N. Candua, S. Wuttkeb, E. Kemnitzb,∗, S.M. Comana,∗, V.I. Parvulescua,∗
a Department of Chemical Technology and Catalysis, Faculty of Chemistry, University of Bucharest, Bdul Regina Elisabeta, 4-12, Bucharest 030016, Romania
b Humboldt-Universität zu Berlin, Institut für Chemie, Brook-Taylor-Straße 2, D-12489 Berlin, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The catalytic potential of nanoscopic MFx (M = Mg, Al; x = 2, 3) has been investigated using batch
Friedel–Crafts alkylation of aromatic compounds, including benzene, ethylbenzene, trimethylhydro-
quinone (TMHQ), and menadiol (MDL), with isophytol and benzyl alcohol. The conversion of isophytol
was 100% in the reactions with trimethylhydroquinone (TMHQ), menadiol (MDL) and benzene while
the highest selectivity in alkylated compounds was achieved with TMHQ (>99%). The different reaction
rates of the alkylation reactions are due to the different nucleophylicities of the substrates, and therefore,
due to their ability to delocalize the positive charge in the Wheland intermediate by inductive and reso-
nance effects. The conversions of benzyl alcohol varied between 10 and 84% as a function of the catalyst
nature and reaction conditions while the highest selectivity to monobenzyl derived compounds (25%)
was achieved with ethylbenzene. The formation of high amounts of dibenzyl ether was also observed,
indicating the presence of high amounts of Brønsted acid sites in this type of catalysts.
Received 14 January 2010
Received in revised form 18 May 2010
Accepted 4 August 2010
Available online 11 August 2010
Keywords:
Nanoscopic fluorides
Friedel–Crafts alkylation
Vitamins
Benzylation
Diphenylmethane
Benzyl alcohol
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Another example is the synthesis of vitamin K1, an important
compound in the control of blood clotting, which involves as a key-
Nowadays most of the fine chemicals are synthesized under
acid–base catalysis conditions. Traditionally, the syntheses carried
out in liquid-phase follow either a stoichiometric chemistry or use
homogeneous acid and base catalysis. The commonly encountered
drawbacks of such syntheses include salt formation and prob-
lems of waste disposal. Among these syntheses, the Friedel–Crafts
alkylation is widely used in the large-scale production of petro-
chemicals and in a great variety of fine chemicals and intermediates
processes.
step the Friedel–Crafts alkylation of menadiol acetate (MDA) with
isophytol [15].
The industrial-scale synthesis of both products comes up against
corrosion caused by acidic media, contamination of the wastewater
with acids, and the difficult purification of the main product [16].
In line with the need for more environmentally acceptable pro-
cesses, the search for highly efficient, heterogeneous catalysts for
these syntheses, which might replace presently used homogeneous
catalysts, is a challenging task.
Literature reported many examples of such syntheses. Thus, an
important example of fine chemicals is the synthesis of (all-rac)-
[␣]-tocopherol (Vitamin E) which displays important antioxidant
properties and shows promising results in the prevention and treat-
ment of heart disease, cancer and Alzheimer’s disease [1–4]. As it is
also used in high amounts in other important industrial markets the
demand of (all-rac)-[␣]-tocopherol is constantly increasing from
year to year [1]. The synthesis of this valuable compound, made
via the condensation of 2,3,6-trimethylhydroquinone (TMHQ)
with isophytol (IP), possibly proceeds through Friedel–Crafts
alkylation–cyclization [5–12] or through an ortho-Claisen rear-
rangement of an intermediary allyl ether [13,14].
Recently, we have developed novel nanoscopic, partly hydrox-
ylated metal fluorides with bi-acidic (Lewis/Brønsted) properties
[17]. We have shown that hydroxylated nanoscopic fluorides (MgF2
and AlF3) can be successfully applied as highly active catalysts in
the synthesis of both vitamins [18,19]. With the aim to extend
the applicability of this promising new class of catalytic materi-
als we applied them in the synthesis of diphenylmethane (DPM)
and its derivatives which are generally prepared via Friedel–Crafts
alkylation (benzylation) reaction. Diphenylmethane and its deriva-
tives are industrially important compounds used as pharmaceutical
intermediates [20] and fine chemicals [21]. In the fragrance indus-
try diphenylmethane has been used as both a fixative and a scenting
soap, as a synergist in some insecticides and as a plastisizer for
dyes [22]. Their synthesis is traditionally performed using H2SO4,
HF, AlCl3, FeCl3, or ZnCl2 [23] as acid catalysts. To overcome the
derivate problems, Lewis acids have been supported by different
solids, such as MCM-41 [24], hydroxyapatite (HAP) [25], and fluo-
rapatite (FAP) and have been used as heterogeneous catalysts for
∗
Corresponding authors. Tel.: +40 214100241; fax: +40 214100241.
E-mail addresses: erhard.kemnitz@chemie.hu-berlin.de (E. Kemnitz),
simona cmn@yahoo.com (S.M. Coman), vasile.parvulescu@g.unibuc.ro,
v parvulescu@yahoo.com (V.I. Parvulescu).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.08.004

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N. Candu et al. / Applied Catalysis A: General 391 (2011) 169–174
the Friedel–Crafts reaction of benzyl chloride with benzene and its
derivatives, respectively. Such heterogeneous catalysts gave rise
to good yields of monoalkylated products and reduced isolation
problems. In spite of the above-mentioned advantages for the use
of supported acids, the preparation of these catalysts is commonly
tedious, furthermore, these catalysts deactivate rapidly due to coke
formation. Moreover, the use of benzyl chloride as the alkylat-
ing agent is undesirable from the green chemistry point of view,
generating hydrogen chloride as by-product. More appropriate is
the use of benzylic alcohol, which generates only benign water as
by-product. Hence, there is still a need to develop an easy-clean
technology more suitable for the synthesis of diphenylmethane and
its derivatives.
Here we will show that, partly hydroxylated, nanoscopic flu-
orides (MgF2 and AlF3) can also be applied in the synthesis of
diphenylmethane and its derivatives, using benzyl alcohol as alky-
lating agent in free solvent conditions. The successful application
of Friedel–Crafts alkylation reactions is strongly influenced by the
main features of the reaction system as the catalyst nature, the sub-
strate and alkylating agent reactivities; an optimal combination of
them being essential.
The crude product was analysed by HPLC (column – EC 125/4.6
NUCLEOSIL 120-5 C18; eluent: acetonitrile; flow rate: 0.8 cm3/min;
wavelength: 280 nm; volume sample: 15 ␮L) chromatography and
1H, 13C NMR (Bruker AV 400 spectrometer, in CDCl3 solvent and
Me4Si as internal standard) spectroscopy.
2.3.2. The synthesis of vitamin K1
In a typical procedure, 174 mg (1 mmol) menadiol (MDL) were
dissolved in 6 cm3 of the solvent (propylene carbonate:heptane
(50:50, vol:vol)) in a glass vial equipped with a magnetic stirrer.
To this mixture 0.4 cm3 (1 mmol) of IP and 50 mg of the catalyst
were added. After that, the vial was closed, immersed in an oil
bath with a temperature of 100 ◦C, and the charged mixture was
stirred (1.250 rpm) for 1–5 h. The resulting reaction mixture was
cooled down to room temperature and the catalyst was removed
by filtration from the two phase solvent mixture, the heptane
phase (containing the reaction product) was separated from the
propylene carbonate phase and the solvent was removed under
vacuum to give a crude product. The crude product was analysed
by UPLC chromatography (Ultrahigh Pressure Liquid Chromatogra-
phy) Thermo Scientific (column – EC 125/4.6 NUCLEOSIL 120-5 C18;
eluent: acetonitrile; flow rate: 0.5 ␮L/min; wavelength: 280 nm;
volume of sample: 1 ␮L) and 1H, 13C NMR spectroscopy (Bruker
AV 400 spectrometer, in DMSO-d6 or CDCl3 solvent and Me4Si as
internal standard).
2. Experimental
2.1. Catalyst preparation
2.3.3. The synthesis of biphenylmethane
All the synthesis reagents were analytically pure, and were
used as received from Aldrich. The catalysts were synthesized
as reported elsewhere [17,26]. Nanoscopic partly hydroxylated
aluminum fluoride was prepared from aluminum iso-propoxide
(Aldrich, 98%) using a one-pot fluorolytic sol–gel synthesis [18].
The obtained sample was named as AlF3-50, where 50 indicates the
concentration of the aqueous HF solution used in the synthesis. In
a similar way, other five MgF2-based samples were obtained from
dissolving metallic Mg (Aldrich, 99.98%) [17] in methanol followed
by the sol–gel-fluorination. The prepared catalysts were referred
to hereafter as MgF2-40, MgF2-57, MgF2-71 and MgF2-87, where
the numbers indicate the different concentrations of aqueous HF
solutions used.
In a typical procedure, to a solution of 0.40–1.56 mg of ben-
zene and 0.54–1.08 mg of benzyl alcohol (corresponding to a molar
ratio of 1/2 and 4/1), 100 mg of catalyst were added. The obtained
mixture was kept to a reaction temperature of 100–120 ◦C, for
4–24 h. The same experiments were performed using ethylbenzene
instead of benzene. After the reaction, the catalyst was sepa-
rated by filtration and the reaction products were analyzed by
gas-chromatography GC (Shimadzu instrument with FID detec-
tion, TR-WAX-TR1MS column of 60 m length and 0.32 mm inner)
and identified by GC–MS (TERMO Electron Corporation instrument,
Treace GC Ultra and DSQ).
3. Results and discussion
2.2. Catalyst characterization
The synthesis and characteristic properties of the nanoscopic
metal fluoride catalysts were presented in more detail recently
[17,28]. In brief, when a metal alkoxide is reacted with stoichio-
metric amounts of aqueous HF, a competition between fluorolysis
(reaction with HF) and hydrolysis (reaction with water) occurs
resulting in the formation of hydroxofluorides. The F/OH ratio in
the products formed this way depends on the free Gibbs enthalpy
of each reaction and on the reaction rate. In fact both, the thermo-
dynamic data as well as the reaction rate (the fluorolysis rate higher
than the hydrolysis rate) let expect the dominant formation of the
metal fluoride [28]. However, by varying the H2O to HF-ratio in the
hydrofluoric acid differently hydroxylated metal fluoride phases
can be obtained. This competitive reaction system is schematically
illustrated in Eq. (1). Thus, the simultaneous fluorolysis and hydrol-
ysis reaction leads to partly hydroxylated aluminium or magnesium
fluorides, respectively, as has been already demonstrated [17,18].
XRD, CP-MAS-NMR, TEM, thermal analysis, and elemental anal-
ysis have been applied to study the structure, composition, and
thermal behavior of the bulk materials. XPS measurements, FTIR
with probe molecules, NH3-TPD, and the determination of N2/Ar
adsorption–desorption isotherms have been carried out to inves-
tigate the surface properties. All characterization procedures were
described elsewhere [17,18,27,28].
2.3. Catalytic tests
2.3.1. The synthesis of vitamin E
Trimethylhydroquinone (TMHQ) (97 wt.%) and isophytol (IP)
(95 wt.%) were purchased from Across Organics. The other reagents
(analytical grade) were obtained from Merck. In a typical proce-
dure, 152–304 mg (1–2 mmol) TMHQ was dissolved in 6 cm3 of
solvent (heptane:propylene carbonate (50:50, vol:vol)) in a glass
vial (standard capacity of 8 cm3) equipped with a magnetic stir-
rer. To this mixture 0.4 cm3 (1 mmol) of IP and 50 mg of catalyst
were added. After that the vial was closed, immersed in an oil bath
with a temperature of 100 ◦C, and the charged mixture was stirred
(1250 rpm) for 5–60 min. After reaction the catalyst was separated
from the two-phase solvent mixture, the heptane phase (containing
the tocopherol) was separated from the propylencarbonate phase
and the solvent removed under vacuum to give a crude product.
M(OR)n + nHF + xH2Ofluo−r→olysisMFn−x(OH) + nROH + n − xHF (1)
x
hydrolysis
Based on TEM, MAS NMR, and XPS results, we showed that adjust-
ing the HF-stoichiometry HF: Mg to 2 but varying the water content
results in shell-like nanoparticles of MgF2−x(OH)x phases, the inner
cores (bulk) of which mainly consist of essentially pure MgF2. In
their outer shells (surface), they become increasingly hydroxylated
(with x still <0.1), as evidenced by XPS and FTIR results. This is

N. Candu et al. / Applied Catalysis A: General 391 (2011) 169–174
171
because the competing hydrolysis reaction becomes increasingly
relevant after the fluorolysis reaction has consumed the HF in
the reaction system [17]. These materials possess medium-strong
Lewis acid sites and, by increasing the amount of water, Brønsted
acid sites as well [28]. Therefore, using different amounts of water
during the sol–gel synthesis the Lewis/Brønsted acid site ratio can
be tuned very precisely.
Since a long time it has been established that acidic halides,
which are typically Lewis acids, have little or no activity for alky-
lation when used in a pure state. They can be activated by the
addition of low concentrations of co-catalysts which interact with
the Lewis acids to generate actual or potential Brønsted sites
[29]. Thus, against any expectation, partly hydroxylated MgF2 was
found to carry Brønsted acidic instead of basic OH groups. The
unexpected results obtained in the synthesis of vitamins E and
K1 clearly indicate that the high activity in combination with an
exciting high selectivity to the main product is due to the syner-
gistic effect of the presence of both Brønsted and Lewis sites on
the surface of the nanoscopic hydroxylated fluorides [17]. There-
fore, the best catalyst for the target product can be obtained
through a very simple tuning of the acidic properties of the material
[19].
density of Brønsted sites in disfavour of the Lewis sites slowers even
more the rate of the alkylation step down (Fig. 1C and D).
At the moment, it is not entirely clear as to whether the total
number of acidic sites has an influence upon the synthesis, but
it is obvious that the presence of Brønsted acid sites is essen-
tial to promote the first alkylation synthesis step, while for the
general synthesis the Lewis/Brønsted acid site ratio is a critical
feature. It seems that the optimal combination of Lewis/Brønsted
acid sites was generated in the sample MgF2-71, while an opti-
mal acidic strength was characteristic for AlF3-50. Moreover, the
decreasing alkylation rate is accompanied by an increasing of the
selectivity to phytadienes, formed through the dehydration of the
isophytol.
By replacing the TMHQ with MDL (Scheme 1) the reaction rate of
the Friedel–Crafts alkylation became slower, conversions of 100%
of MDL being obtained only after 5 h. Such behaviour is due to the
lower nucleophilicity of MDL as compared with TMHQ and, as a
consequence, due to its lower ability to interact with the activated
electrophile.
The overall yields to the alkylated intermediates in the syn-
thesis of Vitamin E and Vitamin K1, as a function of the catalyst’s
characteristics, are presented in Table 1.
The general mechanism of alkylation involves first the inter-
action of the alkylating agent with the acid catalyst to form an
activated electrophile, E+, which adds to the aromatic ring acting
as a nucleophile, followed by proton elimination (Eq. (2)):
As Table 1 shows, in terms of overall yield to the alkylated inter-
mediates, the differences from one catalyst to another one are,
apparently, insignificantly, the values varying in a narrow domain.
Generally speaking, in both cases, the MgF2-40 sample, with a
high amount of Brønsted acidity is less selective to alkylated inter-
mediates than the other samples due to the increasing amount
of phytadienes in the reaction products. Therefore, although the
catalyst is able to generate the activated electrophile through its
interaction with the alkylating agent, the Friedel–Crafts alkylation
takes place to a lower degree. Comparing the results obtained in
the presence of MgF2 samples, this behaviour seems to be more a
consequence of the Lewis/Brønsted acid sites ratio and not of the
acidic sites density (Table 1).
In the case of benzene (Scheme 1) the only detected reaction
products were phytadienes formed by the dehydration of isophytol,
because of the very low nucleophilicity of the substrate.
Furthermore, the catalysts were re-used in the benzylation of
benzene and ethyl benzene. Obviously, the benzylic alcohol is a less
reactive alkylating agent than isophytol (IP), taking into account
the higher stability of the former. Even alkylation with benzylic
alcohol (instead of isophytol) as alkylation agent was not expected
as long as the alkylation conditions were the same. Therefore, the
Friedel–Crafts alkylation of benzene (1a) and ethylbenzene (1b)
with benzyl alcohol (2) (Scheme 2) were made in solvent-free con-
ditions under autogenic pressure. The best results obtained in the
alkylation of benzene are summarized in Table 2. TOF values are
given as the number of benzylic alcohol molecules transformed in
time at the acidic sites, calculated from NH3-TPD and N2-sorption
isotherms measurements (Table 1).
E
+ + ArH → [E − Ar − H+] → E − Ar + H+
(2)
1
where 1 is known as a Wheland intermediate [29]. The nature of
the catalyst has a considerable influence on the reactivity and selec-
tivity of the alkylating agent, the activated specie (E+) existing as a
more or less tight ion pair with a considerable degree of covalent
bonding between the carbocation and the catalyst macroanion. In
these conditions, its relative stability is very important for deter-
mining the rate of alkylation.
On the other hand the substrate selectivity is governed by its
nucleophilicity and its ability to delocalize the positive charge in
the Wheland intermediate 1 by inductive and resonance effects
[29].
Therefore, the successful application of Friedel–Crafts alkylation
reactions is strongly influenced by several features of the reaction
system as the catalyst nature, the substrate and alkylating agent
reactivities, and the reaction parameters, an optimal combination
of these features being essential.
As we have already shown the nature of the catalyst (i.e., the
Brønsted/Lewis acid site ratio) is crucial in the synthesis of two
important vitamins: vitamin E and K1, both involving as first step
the Friedel–Crafts alkylation of an aromatic ring (e.g., trimethyl-
hydroquinone (TMHQ) and menadiol (MDL), respectively) with
isophytol (IP) as alkylating agent (Scheme 1) [19]. Therefore, taking
this into account for the synthesis of vitamin E, the overall rate of
the synthesis seems to be dictated by the rate of the chromane ring
closure step (Fig. 1A–D). The first alkylation step of TMHQ to phytil-
hydroquinone (PHQ) is much faster than the chromane ring closure
of the phytil-hydroquinone (PHQ) step and is dictated both by the
acid strength and the Brønsted/Lewis acid site ratio of the cata-
lysts. Moreover, the same catalytic features influence the overall
selectivity to the main product (vitamin E).
The time dependent analysis of the reaction products (Fig. 1A–D)
shows that the acid strength has a positive influence both on the
reaction rate and the selectivity to vitamin E (VE). The alkylation
intermediate (PHQ) is formed in a high degree with a maximum of
69.6% in only 5 min (Fig. 1A). A decrease in the acid strength slowed
down the rate of the alkylation step, the maximum of alkylated
intermediate being obtained after 10 min (Fig. 1B). Increasing the
As Table 2 shows, the highest activity in the benzylation of
benzene with benzylic alcohol is obtained by the MgF2-71 sam-
ple (Table 2, entries 4–6). A higher amount of benzylic alcohol
led to a faster transformation but, unfortunately, predominantly
results into the benzylic ether by-product. In this case, nor the
Brønsted/Lewis acid sites ratio neither the acidic strength increases
the selectivity to DPM, the highest values being below 15%, irre-
spective of the catalyst nature or of the reaction conditions.
It is clear that, under these conditions, the etherification reaction
is much faster than the benzylation due to the low reactivity of the
substrate.
Replacing benzene by ethyl benzene, that exhibits a stronger
ability to delocalize the positive charge in the Wheland intermedi-
ate 1 by inductive and resonance effects, the results are somehow
better (Table 3).

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N. Candu et al. / Applied Catalysis A: General 391 (2011) 169–174
Scheme 1. The Friedel–Crafts alkylation of various aromatic substrates with different nucleophilicities.
Comparing the results obtained in the alkylation of ethyl
main product (PDPM versus DPM) is a result of the substrate
nucleophilicity.
These results show that the electron donating –C2H5 group
enhances the benzylation reaction because the transition state is
benzene (Table 3) with those obtained in the alkylation of ben-
zene (Table 2) with benzylic alcohol, it is clear that the increase
of the TOF’s and, more important, of the selectivities to the
Table 1
Textural and chemical characterization of the catalysts.
Entry
Catalyst
BET surfacea (m2 g−1
)
Pore sizea (nm)
Amount of acid sitesb (mmol g−1
)
Overall yield
(5 + 8)c (wt.%)
(7 + 10 + 11)d (wt.%)
1
2
3
4
MgF2-40
MgF2-57
MgF2-71
AlF3-50
180
235
276
187
2.3
2.5
2.4
6.5
0.168
0.262
0.163
0.624
85.9
87.8
91.1
99.9
85.9
89.1
91.2
90.8
a
Determined from N2-sorption isotherms measurements.
Determined from NH3-TPD measurements.
b
c
Reaction conditions: conversion: 100%; 50 mg catalyst, 304 mg TMHQ + 0.4 mL IP (2/1 molar ratio), 6 mL solvent (propylencarbonate/heptane = 50/50, vol/vol), 100 ◦C,
reaction time = 0–1 h.
d
Reaction conditions: conversion: 100%; catalyst amount: 50 mg; reaction temperature: 100 ◦C, MDL/IP molar ratio: 1/1, solvent: propylene carbonate/heptane (50/50,
vol/vol), reaction time 5 h. The differences to 100% represent the phytadiene by-products.

N. Candu et al. / Applied Catalysis A: General 391 (2011) 169–174
173
Fig. 1. The distribution of the reaction products in the synthesis of Vitamin E as a function of the catalyst’s nature (reaction conditions: 50 mg catalyst, 304 mg TMHQ + 0.4 cm3
IP (2/1 molar ratio), 6 cm3 solvent (propylencarbonate/heptane = 50/50, vol/vol), 100 ◦C, reaction time = 0–60 min). IP, isophytol; VE, vitamin E; PHQ, phytil-hydroquinone.
Table 2
The conversion of benzylic alcohol and selectivity to diphenylmethane under different reaction conditions as a function of the catalyst nature and reaction conditions.
Entry
Catalyst
T (◦C)
1a/2 molar ratio
C (wt.%)
TOF (h−1
)
S (wt.%)
3a, DPM
4
1
2
3
4
5
6
7
8
9
MgF2-57
MgF2-57
MgF2-57
MgF2-71
MgF2-71
MgF2-71
AlF3-50
AlF3-50
AlF3-50
100
100
120
100
100
120
100
100
120
1/2
4/1
4/1
1/2
4/1
4/1
1/2
4/1
4/1
11.1
12.6
38.2
17.5
18.4
42.0
14.1
15.4
52.3
1.8
1.0
3.1
4.5
2.35
5.4
0.9
0.5
1.7
3.5
11.7
14.5
2.7
10.1
13.2
7.1
96.5
88.3
85.5
97.3
89.9
86.8
92.9
89.7
87.3
10.3
12.7
Reaction conditions: 100 mg catalyst, 24 h.
stabilized by the electron donating groups both on benzene and
benzyl alcohol. The order in the activity of the samples was the
same, the most active being the MgF2-71 sample, followed by
MgF2-57 and AlF3-50 samples. Although not in a spectacular way,
the selectivity to PDPM increases, the highest value reaching almost
25% at 120 ◦C and a 1b/2 molar ratio of 4/1.
The formation of ether, observed for these reactions
(Tables 2 and 3), was also reported for other catalysts [30–32],
Scheme 2. Friedel–Crafts reaction of benzene and ethylbenzene with benzyl alcohol.

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N. Candu et al. / Applied Catalysis A: General 391 (2011) 169–174
Table 3
The conversion of benzylic alcohol and selectivity to p-ethyldiphenylmethane (PDPM) under different reaction conditions as a function of the catalyst nature and reaction
conditions.
Entry
Catalyst
T (◦C)
1b/2 molar ratio
C (wt.%)
TOF (h−1
)
S (wt.%)
3b, PDPM (ortho + para)
4
1
2
3
4
5
6
7
8
9
MgF2-57
MgF2-57
MgF2-57
MgF2-71
MgF2-71
MgF2-71
AlF3-50
AlF3-50
AlF3-50
100
100
120
100
100
120
100
100
120
1/2
4/1
4/1
1/2
4/1
4/1
1/2
4/1
4/1
12.8
45.6
64.0
20.7
51.0
70.3
16.0
64.2
84.6
2.0
3.6
5.1
5.3
6.5
9.0
0.5
2.1
2.8
4.0
9.6
22.5
5.2
11.4
22.7
7.8
21.8
24.3
96.0
90.4
77.5
94.8
88.6
77.3
92.2
78.2
75.7
Reaction conditions: 100 mg catalyst, 24 h.
the authors associating its formation to both, Brønsted and Lewis
acid sites. On the other hand, Deshpande et al. [31] reported that
in the benzylation with benzyl alcohol the etherification is faster
than the alkylation reaction, and the generated dibenzyl ether can
act as alkylating agent. Moreover, Lachter and co-workers [32]
showed that the rate of dibenzyl ether formation is the same for
the toluene, ethylbenzene and propylbenzene.
be improved above 25% irrespective of the catalyst’s nature (e.g.,
the Brønsted/Lewis acid sites ratio and acidity strength) suggest-
ing a limitation of the catalyst’s ability for this kind of reactions.
Nevertheless, the MgF2-57 catalyst’s porosity and the density of
the acid sites enabled selectivity to the para-isomer as the almost
quantitatively formed reaction product of the benzylation.
The interesting feature of the benzylation in the presence of
nanoscopic fluorides is the isomer composition of the reaction
products: In accordance with a typical electrophilic aromatic sub-
stitution pathway, benzylation products obtained from aromatic
alkylation should predominantly consist of ortho–para substi-
tuted compounds. While in the presence of MgF2-71 and AlF3-50
samples, the percentage of ortho-isomer represent 33–40% from
the total ortho–para DPM, in the presence of MgF2-57 fluoride
(Ssp = 235 m2/g; Dp = 2.5 nm; amount of acid sites = 0.262 mmol/g),
the main benzylation product is the para-isomer, ortho-isomer
being formed by less than <1%. Such behavior may by an effect both
of the catalyst’s porosity (shape-selectivity) and of the density of
the acid sites onto the surface.
The recovered catalysts showed consistent activity. The catalytic
activity and selectivity were maintained even after the third re-use.
Hydroxilated AlF3 and MgF2 materials exhibit very low solubility
because of their high lattice energy. Moreover, these fluorides are
hydrolysis resistant as is commonly known [18]. Consequently, the
leaching test carried out showed that, indeed, the nanoscopic metal
fluorides correspond to the stable catalysts. After the separation of
the catalyst from the liquid solution, neither the level of the IP con-
version nor the products distribution was changed for another 1 h.
It can be concluded that these catalysts are stable under the reac-
tion conditions, and the reaction takes place under heterogeneous
conditions.
Acknowledgement
The authors thank the CNCSIS for PNCDI II 40/2007 financial
support.
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