A zeolite imidazolate framework ZIF-8 catalyst for Friedel-Crafts acylation

Article abstract of DOI:10.1016/S1872-2067(11)60368-9

A zeolite imidazolate framework, ZIF-8, was synthesized and characterized by dynamic laser light scattering, X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, Fourier transform infrared, atomic absorption spectrophotometry, and nitrogen adsorption measurements. The ZIF-8 was highly crystalline and porous with a surface area of over 1600 m²/g. Friedel-Crafts acylation of anisole and benzoyl chloride proceeded well in the presence of ZIF-8 (2-6 mol%) without the need for an inert atmosphere. The reaction afforded a selectivity of 93%-95% to the p-isomer. The solid catalyst can be separated from the reaction mixture by simple centrifugation and reused without significant degradation in catalytic activity. There was no leaching of active acid species into the reaction solution.

Full text of DOI:10.1016/S1872-2067(11)60368-9

CHINESE JOURNAL OF CATALYSIS
Volume 33, Issue 4, 2012
Online English edition of the Chinese language journal
Cite this article as: Chin. J. Catal., 2012, 33: 688–696.
ARTICLE
A Zeolite Imidazolate Framework ZIF-8 Catalyst for
Friedel-Crafts Acylation
Lien T. L. NGUYEN, Ky K. A. LE, Nam T. S. PHAN*
Department of Chemical Engineering, Hochiminh city University of Technology, VNU-HCM, 268 Ly Thuong Kiet, District 10, Ho Chi Minh City
7
0000, Viet Nam
Abstract: A zeolite imidazolate framework, ZIF-8, was synthesized and characterized by dynamic laser light scattering, X-ray powder
diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, Fourier transform infrared,
atomic absorption spectrophotometry, and nitrogen adsorption measurements. The ZIF-8 was highly crystalline and porous with a surface
2
area of over 1600 m /g. Friedel-Crafts acylation of anisole and benzoyl chloride proceeded well in the presence of ZIF-8 (2–6 mol%)
without the need for an inert atmosphere. The reaction afforded a selectivity of 93%–95% to the p-isomer. The solid catalyst can be sepa-
rated from the reaction mixture by simple centrifugation and reused without significant degradation in catalytic activity. There was no
leaching of active acid species into the reaction solution.
Key words: imidazolate framework; zeolite imidazolate framework ZIF-8; Friedel-Crafts acylation; porous material; catalysis
Metal-organic frameworks (MOFs) have emerged as a new
and fragrances [24]. Traditionally, an excess over stoichiomet-
ric amounts of anhydrous strong Lewis acids such as AlCl3,
TiCl , FeCl , or SnCl are required for these reactions [25].
class of porous crystalline materials with numerous potential
applications in gas separation and storage, ion exchange, sen-
sors, drug delivery, catalysis, luminescent and fluorescent
materials, and optoelectronics materials [1–5]. By controlling
the size and functionalization of the organic linkers,
well-defined MOF structures with high surface areas and tun-
able pore sizes can be achieved, thus offering advantages over
traditional microporous and mesoporous inorganic materials
6–8]. In the field of catalysis, several MOFs have already been
used as solid catalysts for a variety of organic transformations
9–18]. Zeolite imidazolate frameworks (ZIFs), a new subclass
of MOFs, have attracted significant attention as they combine
the advantages from both zeolites and conventional MOFs
3
3
4
These catalysts are highly moisture sensitive, and therefore
moisture-free conditions have to be employed to maximize the
atom economy of the reactions [25,26]. Moreover, these cata-
lysts are highly toxic and corrosive, generate a large amount of
waste, and cause difficult purification of the products [27]. For
green chemistry, moisture-insensitive and easily handled solid
acid catalysts should be developed [28]. A variety of solid
catalysts have been employed for the Friedel-Crafts acylation
reactions, including zeolites [29–31], hybrid zeoli-
tic-mesostructured materials [32], mesoporous superacid
catalyst [33], modified clay [34, 35], fly ash-supported cerium
triflate [36], metal triflate loaded SBA-15 [37], Ga/SBA-15
supported on carbon nanofibers composite [38], mesoporous
sulphated zirconia [39], mesoporous sieve AlKIT-5 [40],
nanostructural zinc oxide hollow spheres [41], and silica gel
supported aluminium trichloride [42]. In this work, we report
the utilization of ZIF-8 as a heterogeneous catalyst for
Friedel-Crafts acylation. High conversions were achieved with
a catalytic amount of ZIF-8 (2%–6%) without the need for an
[
[
[
19,20]. During the past decade, research works have focused
on synthesizing new ZIFs for their applications in gas capture
and storage. As compared with conventional MOFs, there are
few applications of ZIFs as catalysts or catalyst supports for
organic transformations in the literature [15, 21–23].
Friedel-Crafts acylation of aromatic compounds with acid
chlorides to form arylketones are fundamental and important
processes in the production of pharmaceuticals, agrochemicals,
Received 20 September 2011. Accepted 16 January 2012.
*
Corresponding author. Tel: +84-8-917416018; Fax: +84-8-38637504; E-mail: ptsnam@hcmut.edu.vn
This work was supported by the Vietnam Department of Science and Technology through contract 40/2010/HD-NDT.
Copyright © 2012, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(11)60368-9

Lien T. L. Nguyen et al. / Chinese Journal of Catalysis, 2012, 33: 688–696
o
o
inert atmosphere.
and heated at a rate of 5 C/min to 140 C in a programmable
oven and held at this temperature for 24 h, then cooled at a rate
o
1
Experimental
of 0.4 C/min to room temperature. After removal of the
mother liquor from the mixture, chloroform (20 ml) was added
to the vials. Colorless polyhedral crystals were collected from
the upper chloroform layer and soaked in DMF (3 ꢀ 15 ml) for
3 d. After that, the DMF was exchanged for dichloromethane
(DCM) (3 ꢀ 15 ml) for 3 d. The residual solvent was removed
1
.1 Materials and instrumentation
All reagents and starting materials were obtained commer-
cially from Sigma-Aldrich and Merck, and were used as re-
ceived without further purification unless otherwise noted.
X-ray powder diffraction (XRD) patterns were recorded using
o
by vacuum at 200 C for 6 h, which yielded 0.30 g of white
polyhedral crystals (25% yield based on 2-methylimidazole).
a Cu K radiation source in a D8 Advance Bruker powder
ꢀ
diffractometer. Scanning electron microscopy (SEM) studies
were conducted on a JSM 740 scanning electron microscope.
Transmission electron microscopy (TEM) studies were per-
formed using a JEOL JEM 1400 transmission electron micro-
scope operated at 100 kV. The ZIF-8 samples were dispersed
on holey carbon grids for TEM observation. Elemental analysis
by atomic absorption spectrophotometry (AAS) was performed
on a Shimadzu AA-6800. The particle size distribution of the
ZIF-8 was determined by the dynamic laser light scattering
1.3 Catalytic test
The Friedel-Crafts acylation of anisole with benzoyl chlo-
ride using the ZIF-8 catalyst was carried out in a magnetically
stirred round bottom flask fitted with a reflux condenser. In a
typical reaction, a mixture of anisole (0.65 ml, 6 mmol), ben-
zoyl chloride (1.4 ml, 12 mmol), and n-dodecane (0.2 ml) as an
internal standard was added into a 50 ml flask containing the
ZIF-8 (69 mg, 6 mol%). The catalyst concentration was cal-
culated with respect to the zinc/anisole molar ratio. The re-
(
(
DLS) method using a LA 920. Fourier transform infrared
FT-IR) spectra were obtained on a Bruker TENSOR37 in-
o
sulting mixture was stirred at 120 C for 6 h. The reaction
strument with the sample dispersed on a potassium bromide
pellet. Nitrogen adsorption measurements were conducted
using a Quantachrome 2200e system. Samples were pretreated
conversion was monitored by withdrawing aliquots from the
reaction mixture at fixed time intervals and quenching with an
aqueous NaOH solution (1%, 0.1 ml). The organic components
were extracted using diethyl ether (3 ꢀ 1 ml), dried over an-
hydrous Na SO , and analyzed by GC. The reaction conversion
o
by heating under vacuum at 150 C for 3 h.
A Netzsch Thermoanalyzer STA 409 was used for thermo-
2
4
o
gravimetric analysis (TGA) with a heating rate of 10 C/min
was calculated based on anisole. The product identity was
further confirmed by GC-MS. The ZIF-8 catalyst was sepa-
rated from the reaction mixture by simple centrifugation,
washed with copious amounts of anhydrous DCM, dried under
under a nitrogen atmosphere. Gas chromatographic (GC)
analyses were performed using a Shimadzu GC 17-A equipped
with a flame ionization detector (FID) and a DB-5 column
o
(
length = 30 m, inner diameter = 0.25 mm, and film thickness =
vacuum at 100 C for 5 h, and reused. For the leaching test, a
o
0
1
.25 ꢀm). The temperature program for GC analysis was from
catalytic reaction at 120 C was stopped after 1 h, analyzed by
o
o
o
o
00 to 130 C at 15 C/min, from 130 to 200 C at 50 C/min,
GC, and centrifuged to remove the solid catalyst. The reaction
o
o
o
o
from 200 to 215 C at 1.5 C/min, from 215 to 300 C at 50
solution was then stirred with heating to 120 C for a further 5
o
o
C/min and then held at 300 C for 3 min. Inlet and detector
h, and monitored by GC as previously described to determine if
there was any further reaction.
o
temperatures were set at 300 C. n-Dodecane was used as an
internal standard to calculate 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
2 Results and discussion
2.1 Catalyst synthesis and characterization
o
o
GC-MS analysis was from 60 to 280 C at 10 C/min and then
o
held at 280 C for 2 min. The inlet temperature was set at 280
The ZIF-8 was synthesized from the reaction between zinc
o
o
C. MS spectra were compared with the spectra in the NIST
nitrate hexahydrate and 2 methylimidazole in DMF at 140 C
library.
according to a literature procedure [43]. In the zeolite imida-
zolate framework, zinc atoms in the ZIF-8 structure were
connected to nitrogen atoms by 2-methylimidazolate (MeIM)
linkers, which created nanosized pores formed by four, six,
1
.2 Synthesis of ZIF-8
In a typical preparation [43], a solid mixture of zinc nitrate
eight, and twelve membered ring ZnN tetrahedral clusters
4
hexahydrate (Zn(NO
3
)
2
·6H
2
O, 3.76 g, 14.7 mmol) and
[20,44]. It was previously reported that the structure of ZIFs
was similar to that of conventional zeolites in which the T–O–T
bridges (T = Si, Al, P) in zeolites were replaced by M–IM–M
bridges (M = Zn, Co, Cu). More interestingly, the bond angles
2-methylimidazole (H-MeIM, 0.43 g, 11.6 mmol) was dis-
solved in 240 ml of N,N’-dimethylformamide (DMF) that was
then distributed into 20 ml vials. The vial was tightly capped

Lien T. L. Nguyen et al. / Chinese Journal of Catalysis, 2012, 33: 688–696
5
15
25
2
35
45
o
ꢂ/( )
Fig. 1. XRD pattern of the ZIF-8.
in both frameworks are 145° [20]. In the synthesis of porous
MOF-based materials using DMF as solvent, a solvent ex-
change with DCM should be carried out after the solid crystals
were formed to facilitate the evacuation of the material
framework [1,2,45]. Subsequently, weakly interacting DCM
molecules can be easily removed when the ZIF-8 sample is
activated under vacuum [45]. The ZIF-8 was obtained as white
polyhedral crystals with a yield of 25%, which was comparable
with that of previously reported procedures (normally around
Fig. 3. TEM image of the ZIF-8.
microporous and mesoporous inorganic materials. Nitrogen
adsorption measurements (Fig. 4) indicated that the pore
structure of the ZIF-8 was complex and contained both
mesoporous and microporous pores.
FT-IR spectra of the ZIF-8 showed a significant difference
from that of the 2-methylimidazole linker (Fig. 5). In the FT-IR
spectra of 2-methylimidazole, a strong and broad band from
ꢁ
1
ꢁ1
2
0%–25% yields) [3].
3400 to 2200 cm
with the maximum at 2650 cm was as-
The ZIF-8 was then characterized by a variety of different
signed to the NꢁHꢁꢁꢁN hydrogen bond [49]. The resonance
techniques. A zinc loading of 4.17 mmol/g was found from the
elemental analysis with AAS. The particle size distribution of
0.24
the ZIF-8 observed by DLS analysis showed an average size of
0.20
o
159 ꢀm. A very sharp peak below 10 (2ꢁ of 7.20) was ob-
0
.16
.12
served in the XRD pattern of the ZIF-8, indicating that a highly
crystalline material was achieved (Fig. 1). The overall XRD
pattern of the ZIF-8 in this work were in good agreement with
the patterns from single crystal data [21,43,46,47]. It was ap-
parent that the ZIF-8 had better crystallinity as compared with
silica-based materials such as SBA-15, SBA-16, and MCM-41
where broader peaks were observed on their diffractograms
0
0.08
.04
0.00
0
0
.0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
[
48]. The SEM image showed well-shaped and high quality
Pore radium (nm)
cubic crystals for the ZIF-8 (Fig. 2). As expected, the TEM
analysis revealed that the ZIF-8 possessed a porous structure
Fig. 4. Pore size distribution of the ZIF-8.
(
Fig. 3) which was different from that of conventionally used
(
(
1)
2)
3
800 3300 2800 2300 1800ꢁ 1300
800
1
Wavenumber (cm )
Fig. 2. SEM image of the ZIF-8.
Fig. 5. FT-IR spectra of the ZIF-8 (1) and 2-methylimidazole (2).

Lien T. L. Nguyen et al. / Chinese Journal of Catalysis, 2012, 33: 688–696
1
00
1
00
o
(
a)
100 C
1
1
0
.0
-0.5
1.0
1.5
o
10 C
9
9
8
8
7
7
5
0
5
0
5
0
80
o
20 C
6
0
0
-
4
-
20
0
-2.0
0
1
2
3
4
5
6
1
00 200 300 400 500 600 700 800 900
Time (h)
o
Temperature ( C)
100
(b)
o
1
00 C
Fig. 6. TGA analysis of the ZIF-8.
o
9
9
9
9
9
8
6
4
2
0
110 C
o
1
20 C
between the NꢁHꢁꢁꢁN out-of-plane bending and the NꢁH
ꢁ
1
stretching vibrations was found at 1846 cm [49]. These ab-
sorption bands were not seen in the spectra of the ZIF-8, con-
firming that the 2-methylimidazole linkers were fully depro-
tonated during the formation of the ZIF structure. In the TGA
result for the ZIF-8, there was little weight loss in the tem-
perature range of 200–400 C, indicating that the ZIF-8 was
stable up to 400 C (Fig. 6). The thermal stability of the ZIF-8
was therefore comparable with that reported in the literature
43]. The specific surface area was up to 1632 m /g for the
o
o
88
1
2
3
4
5
6
Time (h)
Fig. 7. Effect of temperature on reaction conversion (a) and selectivity
b).
2
[
ZIF-8, which was in good agreement with that in the literature.
Several ZIF-8 samples with surface areas ranging from 1300 to
810 m /g were previously reported [19,43,50–8].
(
2
1
conversion using 4 mol% ZIF-8 catalyst and an ani-
sole:benzoyl chloride molar ratio of 1:4 and 100, 110, and 120
C. Aliquots were withdrawn from the reaction mixture at
2
.2 Catalytic test
o
The ZIF-8 was assessed for its activity as a solid catalyst in
different times and analyzed by GC to give kinetic data for the
course of the reaction. A conversion of 58% was obtained after
6 h reaction at 120 C. Decreasing the temperature resulted in a
the Friedel-Crafts acylation of anisole with benzoyl chloride to
form p-benzoylanisole as the major product and
o-benzoylanisole as the minor product (Scheme 1). In the
ZIF-8 structure, the six-membered ring pore windows are as
narrow as 0.34 nm [20,51], and large molecules cannot enter
the pores. Chizallet and coworkers [22] recently investigated
the transesterification reaction using ZIF-8 as catalyst, and
pointed out that all active sites are located on the external
surface, and not in the micropores. It can be inferred that the
Friedel-Crafts acylation of anisole with benzoyl chloride would
also occur on the external surface of the ZIF-8 particles. Initial
studies addressed the effect of temperature on the reaction
o
significant drop in the reaction rate, with 44% and 36% con-
o
versions after 6 h at 110 and 100 C, respectively (Fig. 7(a)).
The reaction selectivity remained almost unchanged at
93%ꢁ95% p-benzoylanisole (Fig. 7(b)).
For an organic transformation using a solid catalyst, the re-
agent ratio is an important factor. In this work, it was found that
the reagent molar ratio had a profound effect on the reaction
o
conversion. The data were from the reaction at 120 C for 6 h,
using 4 mol% ZIF-8 catalyst with anisole:benzoyl chloride
molar ratios of 1:2, 1:3, and 1:4. The reaction carried out with
the reagent ratio of 1:4 afforded 58% conversion after 6 h.
Interestingly, decreasing the reagent ratio to 1:4 to 1:3 resulted
in an enhancement in reaction rate, with 66% conversion after
6 h. The value improved to 76% for the reaction using the
anisole:benzoyl chloride molar ratio of 1:2 (Fig. 8(a)). From
the experimental point of view, using a molar ratio of less than
1:2 would cause difficulty in stirring the reaction mixture
containing the solid catalyst. In several cases, Friedel-Crafts
acylation reactions are carried out under solvent-free condition
OMe
OMe
COCl
OMe O
ZIF-8
+
+
O
o-isomer
p-isomer
Scheme 1. Friedel-Crafts acylation of anisole with benzoyl chloride
using ZIF-8 catalyst.

Lien T. L. Nguyen et al. / Chinese Journal of Catalysis, 2012, 33: 688–696
00
1
100
(a)
80
60
40
20
0
8
6
4
2
0
0
0
0
0
8
4 mol%
Leaching test
1
:2
:3
1
1:4
0
1
2
3
4
5
6
0
1
2
3
4
5
6
Time (h)
Time (h)
9
9
9
9
9
Fig. 9. Leaching test to indicate no active acid species leaching into the
(
b)
1:2
reaction solution.
1
:3
6
4
2
0
1:4
with aliquots being sampled at different times and analyzed by
GC. Within experimental error, no further reaction was ob-
served after the solid catalyst was removed from the reaction
mixture. Therefore, it can be inferred that there was no leached
active species in the liquid phase, and that the reaction could
only proceed in the presence of the solid ZIF-8 catalyst (Fig. 9).
It should also be noted that no conversion was observed for the
reaction in the absence of the ZIF-8 catalyst.
1
2
3
4
5
6
Time (h)
We also investigated the effect of the catalyst concentration
on the reaction conversion. It was previously reported that for
the Friedel-Crafts acylation using traditional catalysts, an ex-
cess over stoichiometric amounts of the Lewis acid can be
required, as a complex between the metal and the oxygen atom
of the aroyl products can be formed in significant amounts
[27,56]. The catalyst concentration can be reduced dramati-
cally with several solid catalysts for this reaction, such as
mesoporous superacid catalyst [33,36], metal triflate loaded
SBA-15 [37], beta zeolite [29,30], and hybrid zeoli-
tic-mesostructured materials [32]. Indeed, the Friedel-Crafts
acylation reaction can require from less than 1 mol% to more
than 10 mol% catalyst, depending on the nature of the catalyst
as well as the substrate. In this work, it was found that the
benzoylation reaction of anisole can proceed in the presence of
a catalytic amount of the ZIF-8. The reaction has been carried
Fig. 8. Effect of anisole:benzoyl chloride molar ratio on reaction con-
version (a) and selectivity (b).
with a large excess of one reagent as the reaction medium
[
25,27,52–54]. In this work, the Friedel-Crafts acylation reac-
tion using the ZIF-8 catalyst was carried out under solvent-free
condition with excess benzoyl chloride also acting as the sol-
vent. In a heterogeneous reaction, mass transfer limitation in
the system can have a significant effect on the reaction rate.
Therefore, increasing the amount of solvent can lead to a drop
in reaction rate. The reaction selectivity remained almost un-
changed, with 93%ꢁ95% p-benzoylanisole being observed
(
Fig. 8(b)).
For a liquid phase organic transformation using a solid
catalyst, there is the possibility that some active sites are dis-
solved into the liquid phase, and these leached species can
contribute significantly to the reaction conversion [55]. In
order to determine if leaching occurred in the Friedel-Crafts
acylation reaction using the ZIF-8 catalyst, an experiment was
carried out to estimate the contribution, if any, of leached active
species. A simple centrifugation was performed during the
course of the reaction to remove the solid ZIF-8. If the catalytic
reaction continued, this would indicate that active species had
been leached. The solid ZIF-8 catalyst was removed from the
reaction mixture after 1 h reaction time by simple centrifuga-
tion (the reaction was started with 4 mol% of fresh ZIF-8
o
out at 120 C for 6 h with a reagent molar ratio of 1:2 in the
presence of 2 mol%, 4 mol%, and 6 mol% ZIF-8 catalyst. On
increasing the catalyst concentration from 4 mol% to 6 mol%, a
conversion of 82% was achieved after 6 h. As expected, the
reaction proceeded more slowly for the case of 2 mol% cata-
lyst, although 67% conversion was still obtained after 6 h (Fig.
10(a)). The selectivity for the p-isomer was almost the same for
all cases, with 93%ꢁ95% of p-benzoylanisole being obtained
(Fig. 10(b)). Firouzabadi and co-workers previously used
aluminum dodecatungstophosphate as a heterogeneous catalyst
for the Friedel-Crafts acylation reaction between anisole and
benzoyl chloride, and obtained a selectivity of 80% to the
p-isomer [57]. In the same reaction using a zeolite-based
o
catalyst at 120 C). The liquid phase was then transferred to a
o
new reactor vessel and stirred for an additional 5 h at 120 C

Lien T. L. Nguyen et al. / Chinese Journal of Catalysis, 2012, 33: 688–696
100
100
(a)
80
60
40
20
0
80
60
2
4
mol%
mol%
40
9
2 ꢃm
159 ꢃm
10 ꢃm
6 mol%
20
2
0
1
2
3
4
5
6
0
0
1
2
3
4
5
6
Time (h)
9
9
9
9
9
8
6
4
2
0
Time (h)
(
b)
2 mol%
4
6
mol%
mol%
Fig. 11. Effect of catalyst particle size on reaction conversion.
groups in the acyl chloride. In this work, it was observed that
the presence of the methoxy group (electron-donating) in the
benzoyl chloride structure accelerated the reaction. The acyla-
tion reaction of anisole with 4-methoxybenzoyl chloride af-
forded 87% conversion after 6 h, while 82% conversion was
obtained for the case of benzoyl chloride under identical con-
ditions. The experimental result showed that the Friedel-Crafts
acylation of anisole with 4-chlorobenzoyl chloride proceeded
with more difficulty than in the case of benzoyl chloride, al-
though 73% conversion was still observed after 6 h (Fig.
1
2
3
4
5
6
Time (h)
Fig. 10. Effect of catalyst concentration on reaction conversion (a) and
selectivity (b).
1
2(a)). Although the reaction using 4-methoxybenzoyl chloride
catalyst, Choudhary and coworkers improved the p-isomer
selectivity to 90% [58]. Singh and coworkers [59] also
achieved a selectivity of 90% for the same reaction using
100
(a)
80
60
40
20
0
Cu(OTf) as a catalyst. Kemnitz and coworkers [60] improved
2
the selectivity to 93%ꢁ95% by using a borate zirconia solid
acid catalyst. However, the reaction required the presence of
nitrobenzene as solvent [60]. The selectivity of ZIF-8 in the
Friedel-Crafts benzoylation of anisole was therefore compa-
rable with those previously reported in the literature. Moreover,
it was found that increasing the particle size of the ZIF-8
catalyst resulted in a significant drop in reaction rate. Conver-
sions of 92%, 76%, and 69% were obtained after 6 h reaction
using 4 mol% ZIF-8 with particle sizes of 92, 159, and 210 ꢀm,
respectively (Fig. 11). Since the reaction occurred on the ex-
ternal surface of the ZIF-8 particles, smaller size crystals
should exhibit higher catalytic activity due to the increased
external surface of the smaller crystals [61].
4
-chlorobenzoylchloride
benzoylchloride
4-methoxybenzoylchloride
0
1
2
3
4
5
6
Time (h)
1
110
00
20
(b)
4
-chlorobenzoylchloride
benzoylchloride
4-methoxybenzoylchloride
1
The study was then extended to the Friedel-Crafts acylation
of anisole with different acylating reagents, including benzoyl
chloride, 4-methoxybenzoyl chloride, and 4-chlorobenzoyl
9
0
80
o
chloride. The reaction was carried out for 6 h at 120 C using 6
7
0
0
mol% ZIF-8 catalyst with the reagent molar ratio of 1:2. Pos-
ternak and coworkers [56] previously reported a positive effect
of electron-withdrawing groups present in the acyl chloride for
Friedel-Crafts acylation. However, Fernandes and coworkers
6
1
2
3
4
5
6
Time (h)
[
27] found that the yield of Friedel-Crafts acylation was dra-
Fig. 12. Effect of different acylation reagents on reaction conversion (a)
and reaction selectivity (b).
matically decreased in the presence of electron-withdrawing

Lien T. L. Nguyen et al. / Chinese Journal of Catalysis, 2012, 33: 688–696
100
(a)
9
8
7
6
0
0
0
0
Conversion
(1)
p-isomer
(2)
1
2
3
Run
4
5
5
10 15 20 25 30 35 40 45 50
1
00
o
2
ꢂ/( )
(b)
Fig. 15. XRD patterns of the fresh (1) and reused (2) ZIF-8.
8
6
4
2
0
0
0
0
0
catalyst. The combination of Lewis acid and Brönsted acid
sites could contribute to the catalytic activity of the ZIF-8 in the
Friedel-Crafts acylation reaction, but further studies are nec-
essary to elucidate the acid centers present in the catalyst, and
the mechanism of the Friedel-Crafts acylation using the ZIF-8
catalyst.
When one uses a solid catalyst, a crucial issue that should be
taken into consideration is the ease of separation as well as the
deactivation and reusability of the catalyst. The ZIF-8 catalyst
was therefore investigated for recoverability and reusability
over five successive runs. The reaction was carried out at 120
Reused catalyst
Fresh catalyst
0
1
2
3
4
5
6
Time (h)
Fig. 13. Catalyst recycling studies.
o
gave
a
higher conversion than benzoyl chloride or
C using the reagent molar ratio of 1:2 and 6 mol% catalyst for
4
-chlorobenzoyl chloride, a lower selectivity for the p-isomer
6 h. After each run, the catalyst was separated from the reaction
mixture by simple centrifugation, then washed with copious
amounts of toluene and DCM to remove any physisorbed
reagents. The recovered ZIF-8 was dried under vacuum at 100
was obtained with this acylating reagent (Fig. 12(b)). Chizallet
and coworkers [22] previously investigated the presence of
acid-basic sites in the ZIF-8 by combining CO adsorption
monitored by FT-IR and DFT calculations, and confirmed that
some strong Lewis acid sites (in particular ZnII species) and
strong Brönsted acid sites (NH groups), together with basic
sites, existed on the surface of the catalyst [22]. Choudhary and
coworkers [58] reported that the rate of the Friedel-Crafts
acylation using a zeolite-based catalyst was significantly im-
proved in the presence of new Brönsted acid sites formed by
the interaction of moisture with the Lewis acid sites of the
o
C overnight, and then reused under identical conditions to
those of the first run. It was found that the activity of the ZIF-8
catalyst decreased gradually after each run. However, the
catalyst could be reused with conversions of 82%, 80%, 78%,
66%, and 66% after 6 h for the 1st, 2nd, 3rd, 4th, and 5th run,
respectively (Fig. 13(a)). Kinetic studies also indicated that the
catalytic activity of the ZIF-8 decreased slightly after each use
(Fig. 13(b)). The reaction selectivity remained almost un-
changed with 93%–95% of p-benzoylanisole being observed in
all cases. As compared with the FT-IR spectra of the fresh
catalyst, a slight difference was observed with the ZIF-8 after
the first run (Fig. 14). XRD result of the reused catalyst re-
vealed that the ZIF-8 maintained its crystallinity during the
course of the reaction. However, a slight difference in the
overall structure was observed for the reused ZIF-8 (Fig. 15).
Further investigations are needed to clarify the reason of
catalyst deactivation in the Friedel-Crafts acylation reaction
using the ZIF-8 catalyst.
(1)
(2)
Fig 14
3
500 2900 230020001800 ꢁ 1400
600
1
000
1
Wavenumber (cm )
3 Conclusions
Fig. 14. FT-IR spectra of the reused (1) and fresh (2) ZIF-8.
Highly crystalline and porous ZIF-8 was synthesized from

Lien T. L. Nguyen et al. / Chinese Journal of Catalysis, 2012, 33: 688–696
the reaction of zinc nitrate hexahydrate and 2-methylimidazole
by a solvothermal method. The ZIF-8 was characterized by
FT-IR, TEM, SEM, XRD, TGA, AAS, DLS and nitrogen ad-
sorption, and used as catalyst for Friedel-Crafts acylation be-
tween anisole and benzoyl chloride. The catalyst afforded high
conversions with a catalytic amount (4%–6%) without the need
for an inert atmosphere. The solid catalyst can be easily sepa-
rated from the reaction mixture by simple centrifugation or
filtration, and can be reused without significant degradation in
catalytic activity. There was no active acid leaching into the
reaction solution.
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