Towards applications of metal-organic frameworks in catalysis: Friedel-Crafts acylation reaction over IRMOF-8 as an efficient heterogeneous catalyst

Article abstract of DOI:10.1016/j.molcata.2011.08.011

A highly porous metal-organic framework (IRMOF-8) was synthesized by a solvothermal method, and used as an efficient heterogeneous catalyst for the Friedel-Crafts acylation reaction. The solid catalyst was characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), atomic absorption spectrophotometry (AAS), and nitrogen physisorption measurements. High conversions were achieved in the presence of a catalytic amount of the IRMOF-8 (1-5 mol%) without the need for an inert atmosphere. The solid catalyst could be facilely separated from the reaction mixture by simple centrifugation, and could be reused without a significant degradation in catalytic activity. No contribution from homogeneous catalysis of active acid species leaching into the reaction solution was detected.

Full text of DOI:10.1016/j.molcata.2011.08.011

Journal of Molecular Catalysis A: Chemical 349 (2011) 28–35
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Towards applications of metal–organic frameworks in catalysis: Friedel–Crafts
acylation reaction over IRMOF-8 as an efficient heterogeneous catalyst
Lien T.L. Nguyena, Chi V. Nguyenb, Giao H. Danga, Ky K.A. Lea, Nam T.S. Phana,∗
a Department of Chemical Engineering, HCMC University of Technology, VNU-HCM, 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Viet Nam
b Department of Chemical and Food Engineering, Lac Hong University 10 Huynh Van Nghe, Buu Long Ward, Bien Hoa, Viet Nam
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 July 2011
Received in revised form 10 August 2011
Accepted 14 August 2011
Available online 22 August 2011
A highly porous metal–organic framework (IRMOF-8) was synthesized by a solvothermal method, and
used as an efficient heterogeneous catalyst for the Friedel–Crafts acylation reaction. The solid catalyst
was characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission
electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy
(FT-IR), atomic absorption spectrophotometry (AAS), and nitrogen physisorption measurements. High
conversions were achieved in the presence of a catalytic amount of the IRMOF-8 (1–5 mol%) without the
need for an inert atmosphere. The solid catalyst could be facilely separated from the reaction mixture
by simple centrifugation, and could be reused without a significant degradation in catalytic activity. No
contribution from homogeneous catalysis of active acid species leaching into the reaction solution was
detected.
Keywords:
Metal–organic framework
Friedel–Crafts acylation
Heterogeneous catalyst
IRMOF-8
© 2011 Elsevier B.V. All rights reserved.
Leaching
Reuse
1. Introduction
processes in organic synthesis as well as in industrial chemistry
[35]. Traditionally, these reactions require the presence of more
Metal–organic frameworks (MOFs) have been considered as
promising materials for their potential applications in gas separa-
tion and storage, sensors, drug delivery, and catalysis [1–5]. These
materials consist of metal ions or clusters acting as nodes and
polyfunctional organic ligands as linkers [6]. Compared to conven-
tionally used microporous and mesoporous inorganic materials,
these metal–organic structures exhibit advantages such as high
surface areas, tunable pore sizes, and the ease of processability,
flexibility, structural diversity, and geometrical control [7–9]. In
the field of catalysis, several MOFs have been used as solid cata-
lysts or catalyst supports for a variety of organic transformations
such as Knoevenagel condensation [10,11], aldol condensation
[12], oxidation [13–17], epoxidation [18,19], hydrogenation [20],
Suzuki cross-coupling [21], transesterification reaction [22–24],
Friedel–Crafts alkylation [25,26], epoxide ring-opening reaction
[27–29], methylation of aromatic amines [30], activation of alkynes
[31], domino coupling and cyclization reactions [6], and alkene
cyclopropanation [32]. As in the case of zeolites, the application
of MOFs in catalysis is undoubtedly an area that will attract further
research in the near future [28,33,34].
than stoichiometric amounts of anhydrous strong Lewis acids
such as AlCl3, TiCl3, FeCl3, or SnCl4 [36]. These methods suffer
from high amounts, toxicity and corrosion of the catalysts, gen-
eration of a large amount of waste, and difficult purification of
products [37]. Moreover, these catalysts are highly moisture sen-
sitive and hence moisture-free reaction conditions are required
to achieve the optimal yields of the desired aromatic ketones
[36,38]. With the increasing emphasis on green chemistry, envi-
ronmentally benign processes should be developed to improve the
green credentials of the reaction [39,40]. For the development of
greener processes, moisture-insensitive and easy handling solid
acid catalysts are desired [41]. Furthermore, the use of solid acid
catalysts offers easy catalyst recovery and recycling, as well as
product separation. At the same time, the catalyst recovery also
decreases contamination of the desired products with hazardous
or harmful metals [38,42]. Indeed, several solid acid catalysts have
been investigated for the Friedel–Crafts acylation reactions, such
as metal triflate loaded SBA-15 [43], mesoporous superacid cata-
lyst [40], zeolite [44–46], hybrid zeolitic-mesostructured materials
[47], modified clay [48,49], nafion/silica composite materials [50],
mesoporous sulphated zirconia [51], and mesoporous sieve AlKIT-
5 [52]. Although interesting results have been achieved for the
Friedel–Crafts acylation reaction, they have not led to any very
important industrial application [53]. In this work, we wish to
report the utilization of a highly porous metal–organic framework
(IRMOF-8) as an efficient heterogeneous catalyst for liquid phase
Friedel–Crafts acylation reactions of aromatic compounds with
acid chlorides are considered as fundamental and important
∗
Corresponding author. Tel.: +84 838222447.
E-mail address: ptsnam@hcmut.edu.vn (N.T.S. Phan).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.08.011

L.T.L. Nguyen et al. / Journal of Molecular Catalysis A: Chemical 349 (2011) 28–35
29
Friedel–Crafts acylation reactions. High activity was observed with-
2.3. Catalytic studies
out the need for an anhydrous condition, and the IRMOF-8 catalyst
was easily isolated from the reaction mixture and reused without
significant degradation in activity.
The Friedel–Crafts acylation of toluene with benzoyl chloride
using the IRMOF-8 catalyst was carried out in a magnetically stirred
round bottom flask fitted with a reflux condenser. In a typical reac-
tion, a mixture of toluene (0.60 mL, 5.60 mmol), benzoyl chloride
(0.98 mL, 8.40 mmol) and n-hexadecane (0.15 mL) as an internal
standard was added into a 50 mL flask containing the IRMOF-8
(64 mg, 5 mol%). The catalyst concentration was calculated with
respect to the zinc/toluene molar ratio. The resulting mixture was
stirred at 100 ◦C for 6 h. Reaction conversion was monitored by
withdrawing aliquots from the reaction mixture at different time
intervals, quenching with an aqueous NaOH solution (1%, 0.1 ml).
The organic components were extracted into diethyl ether (3×
1 ml) which was then dried over anhydrous Na2SO4, and analyzed
by GC with reference to n-hexadecane. The product identity was
further confirmed by GC–MS. The IRMOF-8 catalyst was separated
from the reaction mixture by simple centrifugation, washed with
copious amounts of anhydrous toluene and DCM, dried under vac-
uum at 100 ◦C for 5 h, and reused if necessary. For the leaching test,
a catalytic reaction heated to 100 ◦C was stopped after 1 h, analyzed
by GC, and centrifuged to remove the solid catalyst. The reac-
tion solution was then stirred with heating to 100 ◦C for a further
5 h. Reaction progress, if any, was monitored by GC as previously
described.
2. Experimental
2.1. Materials and instrumentation
All reagents and starting materials were obtained commer-
cially from Sigma–Aldrich and Merck, and were used as received
without any further purification unless otherwise noted. Nitrogen
physisorption measurements were conducted using
a Quan-
tachrome 2200e system. Samples were pretreated by heating under
vacuum at 150 ◦C for 3 h. A Netzsch Thermoanalyzer STA 409 was
used for thermogravimetric analysis (TGA) with a heating rate of
10 ◦C/min in air. X-ray powder diffraction (XRD) patterns were
recorded using a Cu K␣ radiation source on a D8 Advance Bruker
powder diffractometer. Scanning electron microscopy studies were
conducted on a JSM 740 Scanning Electron Microscope (SEM).
Transmission electron microscopy studies were performed using a
JEOL JEM 1400 Transmission Electron Microscope (TEM) at 100 kV.
The IRMOF-8 samples were dispersed on holey carbon grids for
TEM observation. Elemental analysis with atomic absorption spec-
trophotometry (AAS) was performed on an AA-6800 Shimadzu.
Fourier transform infrared (FT-IR) spectra were obtained on a
Bruker TENSOR37 instrument, with samples being dispersed on
potassium bromide pallets.
3. Results and discussion
3.1. Catalyst synthesis and characterization
Gas chromatographic (GC) analyses were performed using a Shi-
madzu GC 17-A equipped with a flame ionization detector (FID)
and a DB-5 column (length = 30 m, inner diameter = 0.25 mm, and
film thickness = 0.25 ␮m). The temperature program for GC analy-
sis heated samples from 100 to 130 ◦C at 15 ◦C/min; then heated
them from 130 to 200 ◦C at 50 ◦C/min; then heated them from 200
to 215 ◦C at 1.5 ◦C/min; then heated them from 215 to 300 ◦C at
50 ◦C/min and held them at 300 ◦C for 3 min. Inlet and detector
temperatures were set constant at 300 ◦C. n-Hexadecane was used
as an internal standard to calculate reaction conversions. GC–MS
analyses were performed using a Hewlett Packard GC–MS 5972
with a RTX-5MS column (length = 30 m, inner diameter = 0.25 mm,
and film thickness = 0. 5 ␮m). The temperature program for GC–MS
analysis heated samples from 60 to 280 ◦C at 10 ◦C/min and held
them at 280 ◦C for 2 min. Inlet temperature was set constant at
280 ◦C. MS spectra were compared with the spectra gathered in
the NIST library.
In this work, the IRMOF-8 was synthesized using zinc nitrate
tetrahydrate and 2,6-napthalenedicarboxylic acid by a solvother-
mal method in DMF, according to a literature procedure developed
by Yaghi and co-workers [54–58]. As a member of isoreticular
metal–organic framework family, IRMOF-8 is constructed by 2,6-
napthalenedicarboxylate (NDC) linkers joining Zn4O clusters to
produce an extended three-dimensional cubic porous network,
which has the formula unit Zn4O(NDC)3 [55,57,59,60]. In the syn-
thesis procedure of porous MOF-based materials using formamide
solvents, the solvent exchange should be carried out as this step
will facilitate the evacuation of the material frameworks [1,2,61]. It
was therefore decided to immerse the IRMOF-8 samples in excess
DCM at room temperature without stirring for 2 days. During this
solvent exchange step, strongly interacting DMF molecules were
replaced by more weakly interacting DCM molecules that would
be facilely removed under vacuum at high temperature in the sub-
sequent activation step [61]. This activation step was also crucial to
obtain a fully porous structure [62]. It was found that the IRMOF-
8 was achieved as pale-yellow cubic-shaped crystals in a yield of
71% (based on 2,6-napthalenedicarboxylic acid). From experimen-
tal point of view, it should be noted that decreasing the zinc:NDC
molar ratio resulted in a significant drop in the yield of the IRMOF-8.
The IRMOF-8 was then characterized using a variety of different
techniques. Elemental analysis with AAS indicated a zinc loading of
4.38 mmol/g. The crystalline phase purity of the IRMOF-8 after the
activation step was confirmed by powder XRD analysis [62]. The
overall XRD patterns of the IRMOF-8 were in good agreement with
the theoretical patterns from the single crystal data, and with those
previously reported in the literature [54,62–64]. A very sharp peak
below 10◦ (with 2ꢀ of 6.0) was observed on the XRD diffractogram
of the IRMOF-8, indicating that a highly crystalline material was
achieved (Fig. 1). As with other MOF-based materials, the XRD pat-
terns of the IRMOF-8 exhibited a better crystallinity as compared to
those of other crystalline materials such as SBA-15, SBA-16, MCM-
41, and MCM-48 where broader peaks were normally observed on
2.2. Synthesis of IRMOF-8
In
a typical preparation [54,55], a solid mixture of zinc
nitrate tetrahydrate (Zn(NO3)2 4H2O) (0.37 g, 1.40 mmol) and
2,6-naphthalenedicarboxylic acid (0.06 g, 0.27 mmol) (NDCH) was
dissolved in 15 mL of N,Nꢀ-dimethylformamide (DMF) in a 20 mL
vial. The vial was tightly capped and heated at 100 ◦C in an isother-
mal oven, and held at this temperature for 24 h. After unassisted
cooling of the vial to room temperature, crystals formed in the
vial were removed by decanting with mother liquor. The solid
product was washed with DMF (3× 10 mL) by immersing the
sample in the solvent without stirring for 3 days. After that, the
DMF was exchanged by dichloromethane (DCM) (2× 10 mL) for
2 days. The residual solvents were removed under vacuum at
140 ◦C for 5 h, yielding 0.061 g pale-yellow cubic-shaped crystals
(71% based on 2,6-napthalenedicarboxylic acid). Found C: 46.00,
H: 2.47, Zn: 28.43%, Zn4O(NDC)3 requires C: 46.99, H: 1.97, Zn:
28.32%.

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L.T.L. Nguyen et al. / Journal of Molecular Catalysis A: Chemical 349 (2011) 28–35
Fig. 1. X-ray powder diffractogram of the IRMOF-8.
Fig. 4. TGA analysis of the IRMOF-8.
Fig. 5. FT-IR spectra of the IRMOF-8 (a), and 2,6-napthalenedicarboxylic acid (b).
Fig. 2. SEM micrograph of the IRMOF-8.
from that of conventionally used microporous and mesoporous
inorganic materials.
BET surface areas of up to 2110 m2/g were observed for the
IRMOF-8, which is significantly higher than those of convention-
ally used microporous and mesoporous inorganic materials [1]. As
with other solid materials, it is necessary to investigate the thermal
degradation of the IRMOF-8 since many applications would depend
on its thermal stability [66]. The most significant feature in the TGA
result for the IRMOF-8 was that little weight loss was observed
between the temperature range of 200–450 ◦C, indicating that the
IRMOF-8 was stable up to over 450 ◦C (Fig. 4). The thermal stability
of the IRMOF-8 ensured that it could be used across a wide tem-
perature range. FT-IR spectra of the IRMOF-8 exhibited a significant
difference as compared to that of 2,6-napthalenedicarboxylic acid
(Fig. 5). There were strong peaks at 1574–1600 cm−1 in the spectra
of the IRMOF-8, which was lower than the C O stretching vibra-
tion observed in free 2,6-napthalenedicarboxylic acid (1692 cm−1).
These strong peaks were due to the stretching vibration of carboxy-
late anions, which were formed from the reaction of the linkers with
the metal ions [67,68]. Furthermore, broad bands near 3400 cm−1
were assigned to the O–H stretching vibration of the physisorbed
water. This could be rationalized based on the fact that the mois-
ture was absorbed into the sample when it was being dispersed
on potassium bromide pallets in air for FT-IR analysis. Indeed, TGA
analyses were carried out for a series of IRMOF-8 samples, indi-
cating that the material was highly hygroscopic, and it absorbed
moisture readily during the handling or the storage of the sam-
ple. If the analysis was performed right after the activation step, no
moisture was found in the material (Fig. 4). However, the presence
of water was clearly observed for samples stored or handled in air.
Fig. 3. TEM micrograph of the IRMOF-8.
their diffractograms [65]. The SEM micrograph showed that well-
shaped, high quality cubic crystals were obtained for the IRMOF-8
(Fig. 2). The XRD patterns, together with the SEM image, indicated
that the IRMOF-8 sample was highly crystalline. Images from TEM
analysis showed that the IRMOF-8 possessed a highly porous struc-
ture (Fig. 3). Indeed, the TEM image of the IRMOF-8 was different

L.T.L. Nguyen et al. / Journal of Molecular Catalysis A: Chemical 349 (2011) 28–35
31
Scheme 1. Friedel–Crafts acylation of toluene with benzoyl chloride using IRMOF-8
catalyst.
3.2. Catalytic studies
The IRMOF-8 was assessed for its activity as a solid acid catalyst
in the Friedel–Crafts acylation of toluene with benzoyl chloride to
form p-benzoyltoluene as the major product and o-benzoyltoluene
as the minor one (Scheme 1). The initial reaction was carried out
using 5 mol% IRMOF-8 catalyst relative to toluene at 80 ◦C for 6 h. It
was observed that the reaction could afford over 50% conversion
under this condition without the need for an inert atmosphere,
with a selectivity to p-benzoyltoluene of 80% being obtained. No
trace amount of m-benzoyltoluene was detected from the product
mixture. Initial studies addressed the effect of temperature on the
reaction conversion, having carried out the reaction using 5 mol%
IRMOF-8 catalyst and benzoyl chloride:toluene molar ratio of 5:1
at 80 ◦C, 90 ◦C, and 100 ◦C, respectively. Aliquots were withdrawn
from the reaction mixture at different time intervals and analyzed
by GC, giving kinetic data during the course of the reaction. It was
found that increasing the reaction temperature from 80 ◦C to 90 ◦C
led to a significant enhancement in reaction rate. A conversion
of 72% was achieved after 6 h at 90 ◦C, while the reaction selec-
tivity remained almost unchanged with 80% of p-benzoyltoluene
being observed. As expected, a higher reaction conversion was
obtained for the reaction carried out at 100 ◦C (Fig. 6). It was
previously reported that the liquid-phase Friedel–Crafts acyla-
tion of toluene could be carried out in the temperature range of
60–120 ◦C, depending on the nature of the catalyst as well as the
nature of the substrates [36,37,69–72]. Longer reaction time and/or
higher catalyst concentration were needed to achieve reasonable
conversion values at lower reaction temperature. Indeed, several
Friedel–Crafts acylation processes have been effectively operated
at 100 ◦C [36,72].
The reagent ratio is also an important factor that should be taken
into consideration. Friedel–Crafts acylation reactions are normally
carried out under solvent-free condition, and a large excess of one
reagent is required to act as the reaction medium [36,37,70–72].
However, organic solvents were previously employed for the
reaction in several cases, including both homogeneous and het-
erogeneous processes [37,69,73–75]. In this research, it was found
that the Friedel–Crafts acylation reaction of toluene with benzoyl
chloride using the IRMOF-8 catalyst could occur under solventless
condition, and that the reagent molar ratio had a profound effect
on the reaction conversion. Interestingly, decreasing the benzoyl
chloride:toluene molar ratio from 5:1 to 4:1 resulted in a signif-
icant enhancement in reaction rate, with 82% conversion being
obtained after 6 h at 100 ◦C for the latter case. Moreover, the reac-
tion using 5 mol% IRMOF-8 catalyst at 100 ◦C afforded a conversion
of up to 95% after 6 h at the benzoyl chloride:toluene molar ratio
of 1.5:1. It should be noted that the reaction was carried out under
solvent-free condition, and the excess benzoyl chloride could also
act as the solvent for the reaction. In a heterogeneous reaction, mass
transfer limitation in the system might have a significant effect on
the reaction rate. Therefore, increasing the amount of the solvent
could lead to a drop in reaction rate. From experimental points
of view, it should be noted that using a benzoyl chloride:toluene
molar ratio of less than 1.5:1 could cause difficulty in stirring the
Fig. 6. Effect of temperature on reaction conversion (a) and reaction selectivity (b).
reaction mixture containing the solid catalyst. It was also observed
that the selectivity to the p-isomer remained almost unchanged,
being approximately 80% upon changing the reagent molar ratio
from 5:1 to 1.5:1 (Fig. 7). The reaction selectivity observed for
the acylation reaction using the IRMOF-8 catalyst was comparable
to that previously reported in the literature, where the p-isomer
ratio in the product mixture could range from 66% to 99%, depend-
ing on the nature of the catalyst as well as the reaction condition
[36,69,70,72,76].
With such results in hand, we then decided to investigate the
effect of catalyst concentration on reaction conversion. It was pre-
viously reported that in many cases, more than stoichiometric
amounts of the Lewis acid such as anhydrous AlCl3, TiCl4, and FeCl3
were needed for the Friedel–Crafts acylation reaction due to com-
plex formation with the oxygen atom of the aroyl products [37,76].
Replacement of environmentally unacceptable anhydrous Lewis
acids with solid acids has been a desired approach to effectively
reduce the amount of catalyst for Friedel–Crafts acylation reactions.
Indeed, by using solid acid catalysts such as mesoporous superacid
catalyst [40], beta zeolite [44,45], hybrid zeolitic-mesostructured
materials [47], and modified clay [48,49], the catalyst concentra-
tions could vary from less than 1 mol% to more than 10 mol%,
depending on the nature of the catalyst as well as that of the sub-
strate. The catalyst concentration, with respect to the zinc content
in the IRMOF-8, was studied in the range of 1–5 mol% relative
to toluene. The reaction was carried out using a benzoyl chlo-
ride:toluene molar ratio of 1.5:1 at a reaction temperature of 100 ◦C
for 6 h. It was found that the Friedel–Crafts acylation of toluene and

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L.T.L. Nguyen et al. / Journal of Molecular Catalysis A: Chemical 349 (2011) 28–35
Fig. 8. Effect of catalyst concentration on reaction conversion (a) and reaction selec-
tivity (b).
Fig. 7. Effect of benzoyl chloride:toluene molar ratio on reaction conversion (a) and
reaction selectivity (b).
benzoyl chloride proceeded readily in the presence of a catalytic
amount of the IRMOF-8. Conversions of 95%, 90%, and 88% were
achieved after 6 h at the catalyst concentration of 5 mol%, 3 mol%,
and 1 mol%, respectively. The catalyst concentrations used for the
Friedel–Crafts acylation reaction in this study were therefore com-
parable to those in the literature. As expected, it was found that
the selectivity to the p-isomer remained almost unchanged, being
approximately 80% in the catalyst concentration range of 1–5 mol%
(Fig. 8).
The activity of the IRMOF-8 in the Friedel–Crafts acylation
of toluene was also compared to that of MOF-5 (also called
IRMOF-1), a well-known member of isoreticular metal–organic
frameworks. The MOF-5 was synthesized and characterized as pre-
viously reported [25]. The reaction using the MOF-5 catalyst was
carried out using a benzoyl chloride:toluene molar ratio of 1.5:1 at
a reaction temperature of 100 ◦C for 6 h. Kinetic studies indicated
that the IRMOF-8 exhibited slightly higher activity than the MOF-5.
Reaction conversions of 90%, 88%, and 82% were observed after 6 h
at the MOF-5 concentration of 5 mol%, 3 mol%, and 1 mol%, respec-
tively. Furthermore, the selectivity to the p-isomer of the reaction
using the IRMOF-8 (approximately 80%) was also slightly higher
than that for the case of the MOF-5 (approximately 77%). However,
it was apparent that the MOF-5 could be used as an alternative
catalyst for the Friedel–Crafts acylation reaction. Indeed, Farrus-
seng and co-workers previously reported that the IRMOF-8 and
MOF-5 exhibited almost similar activity in the Friedel–Crafts alky-
lation reaction of toluene and tert-butyl chloride, though slightly
higher conversions and selectivity were observed for the case of
the IRMOF-8 [77]. It should be noted that MOF-5 (composition of
Zn4O(BDC)3 in which BDC = 1,4-benzenedicarboxylate) is topologi-
cally similar to IRMOF-8 (composition of Zn4O(NDC)3 in which NDC
is 2,6-naphthalenedicarboxylate), and the MOF-5 possessed signifi-
cantly higher surface areas than those of the IRMOF-8. However, the
median pore diameter of the IRMOF-8 was larger than that of the
MOF-5 [56], thus indicating that the pore size of the IRMOFs might
have a more pronounced effect on the reaction than the surface
areas.
For a liquid-phase reaction using solid catalysts, an important
problem that should be taken into account is the possibility that
some of active sites could migrate from the solid phase to the solu-
tion and that these leached species could become responsible for a
significant part of the catalytic activity [78]. In order to determine
if leaching was a problem for the Friedel–Crafts acylation reaction
using the IRMOF-8 catalyst, an experiment was performed to esti-
mate the contribution of leached active species to the total reaction
conversion by performing a simple centrifugation during the course
of the reaction to remove the solid catalyst. If the reaction conver-
sion continued increasing after the solid catalyst was removed, this
would indicate that the real active species was leached acid rather
than the solid IRMOF-8 catalyst. The organic phase was separated
from the solid catalyst after 1 h reaction time by simple centrifu-
gation, having used 5 mol% of fresh IRMOF-8 catalyst at 100 ◦C. The
reaction solution was then transferred to a new reactor vessel, and
stirred for an additional 5 h at 100 ◦C with aliquots being sampled

L.T.L. Nguyen et al. / Journal of Molecular Catalysis A: Chemical 349 (2011) 28–35
33
Fig. 9. Leaching test indicated no contribution from homogeneous catalysis of active
acid species leaching into reaction solution.
at different time intervals, and analyzed by GC. The data from GC
analysis results gave quantitative information about residual, cat-
alytically active acid in the reaction solution. Within experimental
error, no further reaction was observed after the solid IRMOF-8
was removed, proving that there was no contribution from leached
active species and that conversion was only being possible in the
presence of the solid IRMOF-8 catalyst (Fig. 9).
As mentioned earlier, the replacement of environmentally
unacceptable homogeneous Lewis acids with solid acids offers sev-
eral advantages including easy catalyst recovery and recycling.
Indeed, solid acid catalysts were previously investigated for the
Friedel–Crafts acylation reactions, and it was reported that several
of them could be recycled and reused [44,45,48,79]. The IRMOF-8
catalyst was therefore investigated for recoverability and reusabil-
ity in the Friedel–Crafts acylation reaction over five successive runs.
The reaction was carried out at 100 ◦C using the benzoyl chlo-
ride:toluene molar ratio of 1.5:1 at 5 mol% catalyst for 6 h. After each
run, the catalyst was separated from the reaction mixture by sim-
ple centrifugation, then washed with copious amounts of toluene
to remove any physisorbed reagents. The recovered IRMOF-8 was
dried under vacuum at 100 ◦C overnight, and then reused in fur-
ther reactions under identical conditions to those of the first run.
Aliquots were withdrawn from the reaction mixture at different
time intervals and analyzed by GC, giving kinetic data during the
course of the reaction using the fresh and recycled catalyst, respec-
tively. It was found that the IRMOF-8 catalyst could be recovered
and reused in further reactions without a significant degradation in
activity. Conversions of 95%, 95%, 96%, 90%, and 82% were achieved
after 6 h for the 1st, 2nd, 3rd, 4th, and 5th run, respectively. The
selectivity to the p-isomer remained almost unchanged over five
successive runs, being approximately 80% for all cases (Fig. 10).
Although it was previously reported that no loss of activity was
observed for reused solid catalysts in the Friedel–Crafts acylation
reaction, no kinetic data was provided [35,40]. Indeed, only conver-
sions at the end of the experiment were mentioned. Unfortunately,
stable activity cannot be proven by reporting only similar reaction
yields at long times. Kinetic studies are the true test of catalyst
deactivation [78]. Further investigations would be needed to clarify
the reason of the catalyst deactivation in the Friedel–Crafts acyla-
tion reaction using the IRMOF-8. However, it was found that the
IRMOF-8 crystals were broken into smaller particles after three cat-
alytic runs. Particle agglomeration was then observed in the 4th
and 5th runs, which should be one of the reasons of the catalyst
deactivation.
Fig. 10. Catalyst recycling studies: conversion (a) and selectivity (b).
The study was then extended to the Friedel–Crafts acylation
reaction of benzoyl chloride with several aromatic hydrocarbon
having different substituents, including anisole, toluene, p-xylene,
and ethylbenzene, respectively. Reactions were carried out at
100 ◦C with 5 mol% catalyst loading and at the benzoyl chlo-
ride:aromatic hydrocarbon molar ratio of 1.5:1. However, it was
difficult to observe the difference of the reaction rate as the ben-
zolylation proceeded with high conversions in all cases. It was
therefore decided to decrease the catalyst concentration to 3 mol%
when investigating the effect of different substituents on the
reaction conversion and selectivity. As expected, the anisole ben-
zoylation using the IRMOF-8 catalyst proceeded with a higher
reaction rate than that of toluene. This result is consistent with the
classical Friedel–Crafts acylation mechanism, in which the reac-
tion is favored by the presence of more electron-donating groups
in the aromatic substrate. Indeed, Choudhary, Jang and co-workers
employed several solid acid catalysts for the Friedel–Crafts ben-
zoylation and found that the reactivity of the aromatic nucleus
increased with the number of electron-donating groups [36,80].
Interestingly, the rate of the toluene benzylation using the IRMOF-
8 catalyst was found to be significantly higher than that of p-xylene,
which is different from the classical Friedel–Crafts acylation mech-
anism. Experimental results also indicated that the reaction of
ethylbenzene proceeded slower than the case of toluene, with 70%
conversion being observed after 6 h (Fig. 11). It was also found
that the selectivity to the p-isomer in the reaction of anisole was
slightly higher than the case of toluene and ethylbenzene. The fact
that p-xylene and ethylbenzene were less reactive than toluene in

34
L.T.L. Nguyen et al. / Journal of Molecular Catalysis A: Chemical 349 (2011) 28–35
Fig. 12. Effect of different acylation reagents on reaction conversion (a) and reaction
selectivity (b).
Fig. 11. Effect of substitutents on reaction conversion (a) and reaction selectivity
(b).
tion of the moisture with the Lewis acid sites of the catalyst could
enhance the reaction rate appreciably [80]. The exact catalytic cen-
ters still remain to be clarified, though it could be proposed that
both Lewis acid and Bronsted acid sites might be responsible for the
catalytic activity of the IRMOF-8. However, further studies would
be necessary to elucidate the acid centers present in the catalyst,
as well as the mechanism of the Friedel–Crafts acylation reaction
using the IRMOF-8.
the benzoylation reaction still remains to be explained in further
investigation.
The effect of different acylation reagents on the Friedel–Crafts
acylation reaction of toluene was also studied, having carried out
the reaction using benzoyl chloride, 4-methoxybenzoyl chloride,
and 4-chlorobenzoyl chloride, respectively, at 100 ◦C with 3 mol%
catalyst loading and at the reagent molar ratio of 1.5:1. It was
found that the reaction of 4-methoxybenzoyl chloride afforded 93%
conversion after 6 h, while a conversion of 83% was observed for
the case of 4-chlorobenzoyl chloride (Fig. 12). Indeed, Fernandes
and co-workers previously reported that the presence of electron-
withdrawing groups in the acyl chloride significantly reduced the
yield of the Friedel–Crafts acylation reaction [37]. In contrast,
Posternak and co-workers reported a positive effect of electron-
withdrawing groups in the acyl chloride for the Friedel–Crafts
acylation reaction [76]. Although the zinc center is of medium acid-
ity, the acylation reaction could proceed to high conversions in the
presence of a catalytic amount (1–5 mol%) of the IRMOF-8 without
the need for an inert atmosphere. Farrusseng and co-workers pre-
viously employed IRMOFs as catalysts for Friedel–Crafts alkylation,
and proposed that Zn–OH species, which were formed as structural
defects in the synthesis step or upon water adsorption, could con-
tribute to the catalytic activity of the IRMOFs [26,77]. Choudhary
and co-workers previously carried out the Friedel–Crafts acyla-
tion using zeolite-based catalysts in the presence of moisture, and
pointed out that new Bronsted acid sites created by the interac-
4. Conclusions
In conclusion, highly crystalline porous IRMOF-8 was achieved
by a solvothermal method using zinc nitrate tetrahydrate and
2,6-napthalenedicarboxylic acid. The IRMOF-8 was characterized
using a variety of different techniques, including FT-IR, TEM, SEM,
XRD, TGA, AAS, and nitrogen physisorption measurements. The
Friedel–Crafts acylation of toluene and benzoyl chloride proceeded
readily in the presence of a catalytic amount of the IRMOF-8
(1–5 mol%), without the need for an inert atmosphere. The solid
catalyst could be facilely separated from the reaction mixture by
simple centrifugation or filtration, and could be reused without a
significant degradation in catalytic activity. No contribution from
homogeneous catalysis of active acid species leaching into the reac-
tion solution during the course of the reaction was detected. The
IRMOF-8 catalyst clearly exhibited advantages over conventional
Lewis acid catalysts such as anhydrous AlCl3, TiCl4, and FeCl3, and
could be used as an alternative to other solid acid catalysts for the

L.T.L. Nguyen et al. / Journal of Molecular Catalysis A: Chemical 349 (2011) 28–35
35
Friedel–Crafts acylation reaction. The fact that the IRMOF-8 was
readily synthesized using a simple procedure would be interested
to the chemical industry. Current research in our laboratory has
been directed to the design and synthesis of several MOF-based
catalysts for a wide range of organic transformations.
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