RETRACTED ARTICLE: Copper-based MOF-74 material as effective acid catalyst in Friedel-Crafts acylation of anisole

Article abstract of DOI:10.1016/j.cattod.2013.11.062

Metal organic frameworks (MOFs) are emerging potential materials for catalytic applications. Among them, the honeycomb M₂(dhtp) family, also known as MOF-74 or CPO-27 materials, are characterized by remarkable textural properties and accessible

Full text of DOI:10.1016/j.cattod.2013.11.062

Catalysis Today 227 (2014) 130–137
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Copper-based MOF-74 material as effective acid catalyst in
Friedel–Crafts acylation of anisole
G. Callejaa, R. Sanza, G. Orcajoa, D. Brionesa, P. Leob, F. Martínezb,∗
a Department of Chemical and Energy Technology, ESCET, Rey Juan Carlos University, c/Tulipán s/n, 28933 Móstoles, Madrid, Spain
b Department of Chemical and Environmental Technology, ESCET, Rey Juan Carlos University, c/Tulipán s/n, 28933 Móstoles, Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Metal organic frameworks (MOFs) are emerging potential materials for catalytic applications. Among
them, the honeycomb M2(dhtp) family, also known as MOF-74 or CPO-27 materials, are characterized by
remarkable textural properties and accessible unsaturated metal sites (M). This work has been focused
on the catalytic properties of novel Cu-MOF-74 material for the Friedel–Crafts acylation of anisole, as
acid-catalyzed reaction test. The influence of reaction variables such as the type of acylating agent and
solvent, and the reaction temperature have been studied. Additionally, the stability of Cu-MOF-74 was
evaluated under different reaction conditions. Cu-MOF-74 achieved a remarkable catalytic performance
in terms of anisole conversion and p-MAP yield. Additionally, the structural MOF-74 phase was preserved
when using diluted concentrations of acetyl chloride as acylating agent. The relative acylating agent con-
centration was demonstrated to be crucial in the collapse of the crystalline MOF-74 phase. The catalytic
activity of Cu-MOF-74, in terms of anisole conversion and selectivity to p-MAP product, was also supe-
rior to other Cu-containing MOF materials (HKUST-1) and conventional inorganic acid catalysts, such
as aluminum-containing microporous zeolites (H-ZSM-5 and BETA) and mesoporous ordered materials
(Al-MCM-41). Cu-MOF-74 also showed high stability and remarkable catalytic performance for seven
consecutive reaction cycles when this MOF material was used in the best reaction conditions.
© 2014 Elsevier B.V. All rights reserved.
Received 17 July 2013
Received in revised form
22 November 2013
Accepted 25 November 2013
Available online 24 January 2014
Keywords:
Copper
MOF-74
Open metal sites
Friedel–Crafts acylation
Anisole
1. Introduction
and the organic ligand can play an important role in the catalytic
reaction mechanism. Regarding the inorganic metallic moiety, they
Metal organic frameworks (MOFs) are a class of materials
constituted by metal or metal clusters bounded to multifunction-
alized organic molecules, such as amines, pyridines, carboxylates,
sulphonates, and phosphonates. The combination of both parts
provides a wide variety of crystalline materials with one-, two-,
or three-dimensional porous structures ranging from microporous
(smaller than 2 nm) to mesoporous (between 2 and 40 nm). Addi-
tionally, the extensive reactivity of metal species and organic
compounds allow the design of a remarkable large number of MOF
structures with a broad diversity of physicochemical properties
[1].
Although MOF materials have been originally used in several
applications such as gas storage, they are especially suitable for
catalysis. MOF materials have been proposed as active and selective
catalysts for a wide range of reactions, from acid–base to redox
catalysis [2–6]. In MOF compounds, both the metallic component
can act as electron-pair acceptor, i.e. Lewis acid, in particular when
uncoordinated metal sites are generated as result of the removal of
coordinated solvent molecules that remain after the MOF synthesis
[7]. In other cases, catalytic groups can be included directly into the
bridging ligands of the organic building blocks of MOFs. The number
of MOFs belonging to this category with demonstrated catalytic
activity is very limited. This is because the reactive groups have to
be accessible to interact with the catalytic substrates and therefore
not being coordinated to the metal ions of the MOF. Therefore, the
difficulty in preparing MOFs containing organic reactive groups lies
in the natural tendency of metals to interact with all the available
functional groups of the ligand.
Several studies on catalytic activity of MOFs with active metal
sites have shown their potential application to reactions like
hydrogenation/isomerization [8–11], cyanosilylation of aldehydes
[12–14], oxidation of organic substrates [15–17], photocatalysis
[18,19], acylation/alkylation of aromatics [20–23], and condensa-
tion reactions [24–26].
The Friedel–Crafts acylation reaction essentially consists of the
production of an aromatic ketone by reacting an aromatic sub-
strate with an acyl component in the presence of a catalyst [27].
∗
Corresponding author. Tel.: +34 914887182; fax: +34 91 488 70 68.
E-mail addresses: guillnan.martnez@gmail.com, guillermo.calleja@urjc.es
(G. Calleja), fernando.castillejo@urjc.es (F. Martínez).
0920-5861/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2013.11.062

G. Calleja et al. / Catalysis Today 227 (2014) 130–137
131
Main reaction products constitute fundamental intermediates in
O
O
O
O
the pharmaceutical, fragrance, flavor, dye, and agrochemical indus-
tries [28]. Conventionally, the electrophilic acylations are catalyzed
by Lewis acids (such as ZnCl2, AlCl3, FeCl3, SnCl4, and TiCl4) or
strong protic acids (such as HF and H2SO4). In particular, the use
of metal halides causes some problems associated with the strong
complex formed between the ketone product and the metal halide
itself which requires the use of more than stoichiometric amounts
of catalyst, and in general a large amount of waste. Therefore, there
is a strong incentive to replace homogeneous processes by green
and efficient heterogeneous processes for economic and environ-
mental reasons. The separation of reaction products also becomes
easier and the catalyst can be reused due to less leaching problems
contrasting with homogeneous catalysts. Solid catalysts used have
been almost exclusively inorganic materials up to now, particularly
micropore zeolites [29,30].
O
Cl
+
Anisole
O
p-MAP
o-MAP
Scheme 1. Simplified reaction scheme for the acylation of anisole with acetyl chlo-
ride.
2.2. Catalyst characterization
According to the large number of publications about MOFs for
catalytic applications during last years, these materials are consid-
ered promising candidates as heterogeneous catalysts [26,31,32].
Properties like well-ordered size and shape of the pores, flexible
activity in response to guest molecules, designable channel sur-
face functionalities, high metal content and high crystallinity make
MOFs unique between porous solids.
Another important point to be evaluated when a given MOF
structure is tested as heterogeneous catalyst is its stability to certain
solvents and reaction conditions. Particularly, MOF materials with
unsaturated metal sites have shown a better performance when
using low polarity solvents, which minimize dissolution of the cat-
alyst [13]. In contrast, donor type solvents, such as tetrahydrofuran
and other ethers, compete against the reaction substrates for the
metal Lewis acid sites, so that they can block the catalytic active
centers or even react with them destroying the MOF structure.
In our previous work [33] a new Cu-based MOF material was
developed, Cu2(dhtp), structurally homologous to the honeycomb-
like MOF-74/CPO-27 series, with unsaturated and accessible metal
sites. Herein we report the catalytic performance of this MOF mate-
rial as a Lewis acid catalyst in the Friedel–Crafts acylation of anisole,
studying its structural stability at different reaction conditions.
Nitrogen adsorption–desorption isotherms at −196 ◦C were
measured using an AutoSorb equipment (Quantachrome Instru-
ments). The micropore surface area was calculated by using the
Brunauer–Emmett–Teller (BET) model [34]. The pore volume and
diameter were estimated by non-local DFT calculations, assuming
a kernel model of N2 at -196 ◦C on carbon (cylindrical pores, NLDFT
equilibrium model) [35]. X-ray powder diffraction (XRD) patterns
were acquired on a PHILIPS X‘PERT diffractometer using Cu K␣ radi-
ation. The data were recorded from 5 to 50◦ (2ꢀ) with a resolution
of 0.01◦. Scanning electron microscopy (SEM) micrographs were
obtained on a PHILIPS XL30 ESEM electronic microscope operating
at 200 kV. Simultaneous thermogravimetry and derivative thermo-
gravimetry analyses (TGA/DTG) were carried out under a nitrogen
atmosphere with an N2 flow of 100 mL min−1 at a heating rate of
5 ◦C/min up to 700 ◦C, using a TA Instruments SDT 2860 apparatus.
2.3. Reaction procedure
In order to evaluate the catalytic performance of Cu-containing
MOF with open metal sites, the acylation of anisole with acetyl
chloride to form methoxyacetophenones (MAPs) was studied as
a reference acid catalyzed reaction (Scheme 1). All the catalytic
experiments were carried out in a round bottom flask placed in a
silicone bath under N2 atmosphere. Reactants and catalyst were
charged at room temperature, and then heated up to the reac-
tion temperature. Stirring was fixed for all runs at 700 rpm in
order to avoid diffusional limitations. The influence of reaction sol-
vent, acylating agent, temperature, acylating agent/anisole molar
ratio and molar catalyst concentration (always referred to the
acylating agent) was studied according to previous operation con-
ditions found in the literature [36]. Samples were withdrawn at
selected reaction times ranging from 0 to 5 h. Anisole and o- and
p-MAPs were identified and quantified by gas chromatography,
using a GC-3900 Varian chromatograph equipped with a CPSIL 8
CB capillary column (30 m × 0.25 mm, film thickness 0.25 ␮m) and
a flame ionization detector (FID). GC temperatures were as fol-
lows: injector 250 ◦C; FID 330 ◦C; program for the oven, 50 ◦C for
1 min; 100 ◦C/min to 175 ◦C and hold for 4.25 min; then 40 ◦C/min
to 270 ◦C. Sulfolane was used as an internal standard and all samples
were analyzed twice.
The relative anisole conversion and p-MAP yield were moni-
tored along the reaction time. Relative anisole conversion (ꢁ) was
calculated taking into account the maximum theoretical anisole
conversion, which depends on the particular anisole/acylating
agent ratio of each catalytic run. Relative p-MAP yield (Y) was esti-
mated as the product of the relative maximum theoretical anisole
conversion and the p-MAP selectivity (S). The p-MAP selectivity
(S) was calculated as mol of p-MAP obtained per mol of anisole
converted.
2. Experimental
2.1. Catalysts preparation
The synthesis procedure of Cu-MOF-74 was slightly modified
from literature [33]. In a typical synthesis, a mixture of 2.2 g
of 2,5-dihydroxyterephthalic acid (H2dhtp, 11.2 mmol, Aldrich)
and trihydrated copper nitrate(II) (5.9 g, 24.6 mmol, Aldrich) were
added over a 20:1 (v/v) solution of N,N-dimethylformamide (DMF)
and 2-propanol (250 mL) in a 500 mL screw cap bottle. The suspen-
sion was stirred until homogeneous solution. Then, the resultant
solution was placed in an oven at 80 ◦C for 18 h. Thereafter, the
sample was cooled down to room temperature and the mother
liquor was separated from reddish needle-shaped crystals by vac-
uum filtration. Thereafter, the crystalline solid sample was washed
with DMF. Afterwards, the crystals were immersed in 100 mL of
methanol for four 4 days, renewing it by fresh methanol every 24 h.
Finally, the sample was dried at 150 ◦C under vacuum (10−3 bar) for
5 h and stored under inert atmosphere.
Other catalyst materials used in this work with the purpose
of comparison like HKUST-1, H-ZSM-5 and BETA were purchased
to Sigma–Aldrich Química S.L., Sud Chemie Iberia S.L. and Zeolyst
International, respectively. Al-MCM-41 was synthesized in our
laboratory following the procedure described in the Supporting
Information.

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G. Calleja et al. / Catalysis Today 227 (2014) 130–137
Table 1
Characteristics of MOFs and other inorganic catalysts used in this study.
Catalyst
Structure
Channel
structure
Channel
diameter (Å)
Surface area
(m2/g)
Cu content
(mmol/g)
Si/Al molar
ratio
Cu-MOF-74
HKUST-1
H-ZSM-5
BETA
MOF
MOF
1D
3D
3D
3D
1D
10–12
10
5.1–5.6
6.6
1126
708
429
710
894
6.2
5.0
–
–
–
–
–
30
19
29
Zeolitic material
Zeolitic material
Mesoporous material
Al-MCM-41
27
In order to assess the activity of copper leached as homogeneous
catalyst, a typical test was performed evaluating the anisole con-
version once the catalyst was removed from the reaction medium
by hot filtration after 1 h.
structure. Note that final thermal treatment of Cu-MOF-74 at 150 ◦C
under vacuum for 5 h assured the activation of the material by evap-
oration of coordinated methanol and generation of free coordinated
metal sites with Lewis acid properties. Fig. 5 of the SI illustrates a
simplified scheme of the activation method for the exchange of the
synthesis solvent and final extraction of methanol over a bidimen-
sional view of the honeycomb structure of Cu-MOF-74. Previous
characterization by single crystal X-ray diffraction analysis con-
firmed this isostructural framework, typical of MOF-74 and CPO-27
family [33]. Table 1 summarizes the most important characteris-
tics of Cu-MOF-74 as well as other catalysts used in this work with
the purpose of comparison: (i) HKUST-1 as another Cu-containing
MOF and (ii) H-ZSM-5 and Al-MCM-41 materials as conventional
inorganic catalysts.
3. Results and discussion
3.1. Characterization of Cu-MOF-74
The most relevant characterization data of the Cu-MOF-74 syn-
thesized by solvothermal method is depicted in the supporting
information (Figs. S1–S5). The porosity of the Cu-based MOF mate-
rial was measured by nitrogen adsorption at −196 ◦C (SI, Fig.
1). The basically type I adsorption/desorption isotherm revealed
a permanent microporosity with a BET specific surface area of
1126 m2/g, a pore volume of 0.57 cm3/g and an average pore diam-
3.1.1. Influence of the reaction solvent
˚
eter of ca. 11 A. Powder X-ray diffraction pattern showed the typical
As mentioned before, the design of MOFs containing unsatu-
rated metal centers is of key importance for catalytic applications,
since they strongly favor the direct interaction between metal and
substrates (reactants or even the solvent molecules present in the
reaction) and the potential activity of metal as Lewis acid sites.
In this sense, the affinity of the solvent molecules for uncoordi-
nated metal sites and their chemisorption on them may affect the
catalytic performance, hindering the access of reactants, and subse-
quently decreasing the catalytic activity. In our particular reaction
of anisole acylation, a series of solvents with different polarity have
been studied in order to assess their effect on the catalytic activity
of Cu-MOF-74 material. Fig. 1 shows the influence of various sol-
vents (nitrobenzene, acetonitrile and dimethylformamide) when
using acetyl chloride as acylating agent. The initial reaction con-
ditions were 120 ◦C, a equimolar anisole/acetyl chloride ratio and
1.5 molar% of catalyst according to operation conditions of a previ-
ous work [36]. Nitrobenzene, being the less polar solvent, achieves
reflections of MOF-74/CPO-27 phase [37], characteristic of a three-
dimensional coordination polymer with a honeycomb topology
that contains 1-D broad channels (SI, Fig. 1). SEM micrographs
(SI, Fig. 3) revealed large needle-shaped crystals, similar to those
shown by crystalline MOF-74/CPO-27 in literature [37,38]. The
thermogravimetric analysis (TGA/DTG) in N2 atmosphere (SI, Fig.
4) indicates the decomposition of the organic moiety of the sample
about 375 ◦C. This fact is a clear evidence of the thermal stability of
the Cu-MOF-74 framework. This thermal stability is very similar to
that shown by the copper terephthalate MOF (Cu-tp) reported by
Tannenbaum and co-workers [39], and slightly lower than the pure
Zn-MOF-74/CPO-27, which is stable up to 400 ◦C [37]. Additionally,
a significant loss of weight was determined at lower temperature
(centered at 85 ◦C) which was attributed to the methanol used for
the removal of original DMF solvent, either physisorbed on the
porous MOF structure or coordinated into the open metal sites of its
70
60
50
40
30
20
10
0
Nitrobenzene
Acetonitrile
Dimethylformamide
0
1
2
3
4
5
Time (h)
Fig. 1. Influence of solvent in the acylation of anisole with acetyl chloride using, Cu-MOF-74.

G. Calleja et al. / Catalysis Today 227 (2014) 130–137
133
70
70
60
50
40
30
20
10
0
70
60
50
40
30
20
10
0
60°C
75°C
90°C
120°C
60
50
40
30
20
10
0
Acetyl Chloride
Benzoyl Chloride
Acetic Anhydride
1 h
5 h
2 h
3 h
0
1
2
3
4
5
Time (h)
Fig. 3. Influence of reaction temperature in the acylation of anisole using Cu-MOF-
74. Normal columns indicate anisole conversion; dense fill columns indicate p-MAP
yield.
Fig. 2. Influence of acylating agent in the acylation of anisole using Cu-MOF-74.
was considered the best acylating agent for Cu-MOF-74 in the
Friedel–Crafts acylation reaction of anisole.
64% anisole conversion after 5 h of reaction, whereas DMF, with
the highest polarity, shows a negligible anisole conversion. These
results indicate that the lower solvent polarity, the better perfor-
mance of Cu-MOF-74 catalysts. Thus, the affinity of polar solvent
molecules for uncoordinated metal sites seems to have a significant
influence in the Friedel–Crafts acylation reaction, being detrimen-
tal to access substrate molecules to the active acid centers. These
results prove that the selection of solvent is crucial, not only in
terms of favoring the diffusion of reactants and products in the
catalytic system, but also to increase the availability of unsatu-
rated metal sites of MOFs for catalytic applications. These results
are in agreement with other studies reported in the literature with
other catalysts like MCM-41 and borate zirconia type-acid catalyst
[40,41].
3.1.3. Influence of reaction temperature
The influence of the reaction temperature in the catalytic per-
formance of the Cu-MOF-74 was also evaluated in the range of
60–120 ◦C for the acylation of anisole with acetyl chloride and
nitrobenzene as solvent. In this part of the study, both relative
anisole conversion and p-methoxyacetophenone (p-MAP) yield
were monitored along the reaction time. The results shown in Fig. 3
revealed the increase of the anisole conversion with the temper-
ature. A slight increase of anisole conversion was observed from
60 to 90 ◦C. However, the increase of temperature until 120 ◦C
yields a remarkable enhancement of the anisole conversion. More-
over, this increase of activity in terms of anisole conversion was
also accompanied by an increase of p-MAP yield. Concerning the
reaction kinetics, high initial reaction rates can be estimated for
the first hour following a remarkable decrease for the next four
hours, regardless the reaction temperature. Similar results have
been also found in the literature for the acylation of anisole using
zeolitic materials as Lewis acid catalysts [42] or perfluorosulphonic
mesoporous materials as Brønsted acid catalysts [43]. The main
reason of this loss of activity is attributed to chemisorbed reac-
tants and acylated products on the acidic sites of the materials
[44]. Cu-MOF-74 achieved a 65% of anisole conversion with a rel-
ative p-MAP yield of ca. 35%. Ruling out the selectivity to o-MAP,
which was negligible according to quantitative analysis in the liquid
phase of reaction, the formation of other polyacylated byproducts
or the partial chemisorption of the target p-MAP into the microp-
orous structure of Cu-MOF-74 cannot be discarded. Several studies
reported in literature for microporous zeolitic and mesoporous
materials have confirmed these hypotheses, although the p-MAP
yields were higher for that materials [36,44,45].
3.1.2. Influence of the acylating agent
The effect of different acylation reagents on the Friedel–Crafts
acylation of anisole with Cu-MOF-74 has been also studied using
acetyl chloride, benzoyl chloride and acetic anhydride. Fig. 2 illus-
trates the anisole conversion for the three acylation reagents at
120 ◦C, equimolar anisole/acylating agent ratio and 1.5 molar% of
catalyst. As it is well-known, Lewis acid catalysts are able to form
electrophilic acylium ions (R C+ O) from acyl halides or acid anhy-
drides, which can be attacked by ␲-electrons of the C C aromatic
rings following a typical nucleophilic addition to the acylium ions.
In general, the more electrophilic is the acylium ion, the more
favored is the nucleophilic attack of the aromatic ring. In the case
of acetyl chloride and benzoyl chloride, the acylium ions gener-
ated from the benzoyl chloride are less electrophilic due to the
resonance stabilization of the aromatic ring. The anisole conver-
sion using acetyl chloride was remarkably much higher than that
obtained with benzoyl chloride. These results seem to confirm the
higher catalytic performance by the more electrophilic character
of the acylium ions. Nevertheless, it must be noted that benzoyl
chloride as acylating agent produces more bulky acylated prod-
ucts which could be also affecting the activity of Cu-MOF-74 due to
diffusion constraints.
3.2. Stability of Cu-MOF-74 for the acylation of anisole
3.2.1. Catalytic stability of Cu-MOF-74
For liquid-phase reactions where solid metal-based catalysts
are used, the dissolution of some active metal species of the solid
phase into the solution during the reaction is considered a serious
problem. In the case of partial solubility of metal species, the reac-
tion could proceed under homogeneous or partially homogeneous
catalysis conditions [31]. Moreover, in the case of MOF materials,
leaching of metal species can also produce a partial or total collapse
of the MOF structure, since the metal is forming an essential part
of the material structure.
In the case of acetyl chloride and acetic anhydride, both of them
generate the same acylium ion (CH3 C+ O), but the catalytic per-
formance of Cu-MOF-74 was also significantly different. This fact
can be explained by the weakness of the C Cl bond in acetyl chlo-
ride, which contributes to the higher activity of this acylating agent
as compared to the acetic anhydride. This behavior of both acy-
lating agents have been also reported in the literature for zeolitic
H-ZSM-5 catalyst [29,36]. Regarding these results, acetyl chloride

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G. Calleja et al. / Catalysis Today 227 (2014) 130–137
70
60
50
40
30
20
10
0
Fresh catalyst
Used catalyst
Cu-MOF-74
Leaching test
0
1
2
3
4
5
Time (h)
Fig. 4. Influence of the homogeneous contribution from the copper leaching of Cu-
MOF-74.
Simulated CuCl
In order to confirm the influence of Cu-leaching from Cu-MOF-
74 catalyst in acylation reaction of anisole, a typical experiment
was performed at 120 ◦C, equimolar anisole/acetyl chloride molar
ratio, 1.5 molar% of catalyst and nitrobenzene as solvent, but after
1 h of reaction the solid Cu-MOF-74 was removed from the reac-
tion medium by hot filtration. The liquid mixture after filtration
was maintained at 120 ◦C for another 4 h in the same reaction
system. Fig. 4 shows anisole conversion profiles for the typical
experiment using the solid catalyst and the new experiment being
the solid catalyst removed after 1 h of reaction. As observed, the
anisole conversion did not continue after the solid catalyst separa-
tion, which evidenced a low contribution of homogeneous catalysis
to the anisole acylation. Thus, the active role of the solid Cu-MOF-
74 in the acylation of anisole is clearly proven when comparing
the anisole conversion after the first hour of reaction for both
experiments. Moreover, it must be noted that similar activity was
achieved by trihydrated copper nitrate(II) when this homogeneous
catalytic system was used with the same copper content (data not
shown). Although the catalytic performance of the homogeneous
catalyst was also remarkable, this reaction has the environmen-
tal drawback of the metal presence in the liquid phase. However,
novel heterogeneous materials as the described here avoid these
environmental impacts.
Although the contribution of active leached species of Cu-MOF-
74 was ruled out, additional characterization of fresh and used
catalyst after reaction by XRD was performed. These difractograms
evidenced the complete disappearance of the typical Cu-MOF-
74 phase (Fig. 5). A secondary crystalline phase was detected by
intense diffraction peaks located at 29, 47.5 and 56 degrees, which
clearly match up with typical XRD pattern of copper(I) chloride.
These facts indicated the collapse of microporous Cu-MOF-74 crys-
talline phase accompanied to the formation of a new copper(I)
chloride phase, possibly due to the acidic conditions of the media. A
similar collapse phenomenon of Cu-based MOF has been described
recently for Opanasenko et al. [24] In order to know the possi-
ble catalytic activity of Cu(I)Cl in the acylation of anisole, a new
experiment using Cu(I)Cl with the same copper content was per-
formed. This experiment achieved a 28% of conversion after 1 h
of reaction, which was significantly lower than the 55% obtained
by Cu-MOF-74 when its structure was partially degraded (Fig. 6).
So, these results suggest that the catalytic activity, cannot only
come from the Cu(I)Cl formed, but also from the MOF structure,
evidenced by an important extra contribution in the anisole con-
version.
10 20 30 40 50 60 70 80 90
2 (°)
Fig. 5. X-ray diffraction patterns of fresh and used solid catalyst.
3.2.2. Influence of catalyst concentration and anisole/acetyl
chloride ratio
Previous stability tests of Cu-MOF-74 in nitrobenzene at 120 ◦C
and presence of the reagents used for the acylation reaction evi-
denced that acetyl chloride affected dramatically the stability of
the Cu-MOF-74 phase. According to the typical mechanism of acyla-
tion reactions [46], the preliminary formation of acylium ions over
uncoordinated Lewis acid centers, in this case uncoordinated cop-
per sites, can alter the coordination sphere of copper producing the
collapse of Cu-MOF-74 phase.
Taking into account the stability tests, the catalyst concen-
tration was increased from 1.5 to 15 molar% in order to get a
higher catalyst/acetyl chloride ratio and after that, the influence
of the acylating agent concentration was explored by increasing
of anisole/acetyl chloride molar ratio (MR) from 1/1 to 5/1. Fig. 6
shows the catalytic performance using 15 molar% of catalyst and
different anisole/acetyl chloride molar ratios. The catalysts were
recovered after methanol washing and vacuum drying, being
MR = 1:1
MR = 2.5:1
MR = 5:1
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
1 h
2 h
3 h
5 h
Fig. 6. Influence of the anisole/acetyl chloride ratio for the acylation of anisole using,
a Cu-MOF-74 catalyst concentration of 15 molar%. Normal columns indicate anisole,
conversion; dense fill columns indicate p-MAP yield.

G. Calleja et al. / Catalysis Today 227 (2014) 130–137
135
120
120
100
80
60
40
20
0
Cu-MOF-74
H-ZSM-5
HKUST-1
BEA
Al-MCM-41
100
80
60
40
20
0
1 h
2 h
5 h
3 h
Fig. 8. Comparison of different types of acid catalysts for the acylation of anisole.,
Normal columns indicate anisole conversion; dense fill columns indicate p-MAP
yield.
giving rise a 3-D framework with a unique 1-D honeycomb like
porous channel system with a diameter of about 1.1–1.2 nm.
The higher interconnectivity of the HKUST-1 porous network
did not enhance its catalytic performance. The higher activity of
Cu-MOF-74 as compared to this material was attributed to its
higher surface area and metal content (see Table 1).
Fig. 7. X-ray diffraction patterns of fresh and used Cu-MOF-74 catalyst for different,
Regarding the inorganic catalysts, both materials are consid-
ered potential acid Lewis catalysts. Zeolitic H-ZSM-5 material is an
aluminosilicate with crystalline MFI-type microporous structure,
whereas Al-MCM-41 is an amorphous aluminosilicate mesoporous
material with hexagonal arrangement. Both inorganic materials
led to lower anisole conversions as well as lower p-MAP yields.
Superior behavior of MOF materials over zeolites has been also
shown in the Pechmann reaction of naphthol and Prins conden-
sation reaction of ␤-pinene [24,26]. Taking into account the similar
Al/Si ratio for both materials, the slight improved activity of micro-
porous H-ZSM-5 is associated to better accessibility of Al centers
in the crystalline microporous structure of H-ZSM-5 material as
compared to the amorphous pore walls of Al-MCM-41. In addi-
tion, microporous BETA zeolitic material with a larger pore size
anisole/acetyl chloride ratios and a Cu-MOF-74 catalyst concentration of 15 molar%.
characterized by XRD (Fig. 7). The catalytic activity of Cu-MOF-74
was slightly improved with the increase of the catalyst loading
as compared to previous results (anisole conversion and p-MAP
yield obtained for equimolar anisole/acetyl chloride ratio and 1.5
molar% of catalyst). The increase of the anisole/acetyl chloride
ratio allowed enhancing the relative anisole conversion above
90% with respect to the maximum conversion achievable with the
initial acetyl chloride concentration. Moreover, the p-MAP yield
also increased as the relative acetyl chloride concentration was
reduced. In terms of Cu-MOF-74 stability, it was observed that the
increase of the catalyst concentration, which meant an increase of
the catalyst/acetyl chloride ratio, was not effective to preserve the
crystalline MOF-74 phase. However the combination of decreasing
the acylating agent concentration by increased the anisole/acetyl
chloride ratio and the increase of catalyst concentration allowed
keeping the crystalline MOF-74 phase, as observed in the recovered
catalyst after reaction. These results confirm the negative effect of
the acylating agent concentration on the stability of Cu-MOF-74.
˚
than ZSM-5 (6.6 vs 5.5 A) showed a better catalytic performance
than its aluminosilicate homologues, but still below Cu-MOF-74.
These results could be explained not only due to the large pore size
of BETA, but also because the higher acidity (Si/Al = 19) respect to
the other aluminosilicate materials used (Si/Al ∼ 30). So, the Cu-
MOF-74 exhibited the best anisole conversion and higher p-MAP
yield. Regarding other zeolitic materials reported in literature for
the same reaction, such as hierarchical ZSM-5 zeolites and hybrid
zeolitic-ordered mesoporous materials [30], the anisole conver-
sion of Cu-MOF-74 was still higher than theses inorganic materials.
Therefore, the acid capacity of copper atoms located into the
hybrid MOF-74 phase as well as its remarkable surface area makes
Cu-MOF-74 a promising material for acid catalyzed reactions, in
particular for the acylation of anisole.
3.3. Catalytic performance of hybrid Cu-MOF-74 versus other
inorganic catalysts
The catalytic performance of Cu-MOF-74 was assessed in com-
parison to other different acid catalysts: (i) HKUST-1 as another
Cu-containing MOF and (ii) other conventional inorganic catalysts
typically used in acid catalyzed reaction such as H-ZSM-5 and BETA
zeolitic materials and Al-MCM-41 mesoporous materials. Fig. 8 dis-
plays the catalytic performance of all the catalysts studied in terms
of relative anisole conversion and p-MAP yield. All the experiments
were carried out at 120 ◦C, anisole/acetyl chloride molar ratio of 5/1
and 15 molar% of catalyst, using nitrobenzene as solvent.
The copper-based MOF material, HKUST-1, is characterized by
a 3-D porous channel system, with a pore diameter ca. 1 nm, as a
result of the linkage between well-known paddle-wheel metallic
clusters and tritopic linkers (trimesic acid). In contrast, Cu-MOF-74
material is built up from infinitive rod-like metallic clusters joined
together by tetratopic linkers (2,5-dihydroxiterephthalic acid)
3.4. Reusability of Cu-MOF-74
The reusability of the catalyst was also studied in the acylation
of anisole. In this particular case, since most of the anisole conver-
sion was normally achieved after 1 h of reaction, the reutilization of
the catalyst was carried out for reactions performed at 1 h of time.
Likewise, in order to assure enough amount of catalyst for several
consecutive cycles, the catalyst concentration was increased from
15% to 37.5% respect to the acylating agent. The anisole/acylating
agent ratio was also set to 2.5:1 with the purpose of increasing the
overall production of p-MAP product. The results displayed in Fig. 9

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G. Calleja et al. / Catalysis Today 227 (2014) 130–137
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60
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0
100
Acknowledgments
The authors wish to thank the Spanish Ministry of Economy
and Competitiveness for their financial support of CICYT (CTQ2012-
38015 Project).
80
60
40
20
0
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.cattod.
2013.11.062.
References
1
2
3
4
5
6
7
Reaction cycles
[1] H. Zhou, J. Long, O. Yaghi, Introduction to metal–organic frameworks, Chem.
Rev. 112 (2012) 673.
[2] S. Neogi, M.K. Sharma, P.K. Bharadwaj, Knoevenagel condensation and cyanosi-
lylation reactions catalyzed by an MOF containing coordinatively unsaturated
Zn(II) centers, J. Mol. Catal. A: Chem. 299 (2009) 1.
Fig. 9. Cyclability of Cu-MOF-74 in the acylation reaction of anisole. Normal columns
indicate anisole conversion; dense fill columns indicate p-MAP yield.
[3] R.Q. Zou, H. Sakurai, Q. Xu, Preparation, Adsorption properties, and catalytic
activity of 3D porous metal–organic frameworks composed of cubic building
blocks and alkali-metal ions, Angew. Chem. Int. Ed. Engl. 45 (2006) 2542.
[4] F. Gándara, B. Gómez-Lor, E. Gutiérrez-Puebla, M. Iglesias, M.A. Monge, D.M.
Proserpio, N. Snejkoal, An indium layered MOF as recyclable Lewis acid catalyst,
Chem. Mater. 20 (2008) 72.
[5] M.H. Alkordi, Y.L. Liu, R.W. Larsen, J.F. Eubank, M. Eddaoudi, Zeolite-
like metal–organic frameworks (ZMOFs) as platforms for applications: on
metalloporphyrin-based catalysts, J. Am. Chem. Soc. 130 (2008) 12639.
[6] F. Schröder, D. Esken, M. Cokoja, M.W.E. van den Berg, O.I. Lebedev, G. van Ten-
deloo, B. Walaszek, et al., Ruthenium nanoparticles inside porous [Zn4O(bdc)3]
by hydrogenolysis of adsorbed [Ru(cod)(cot)]: a solid-state reference system
for surfactant-stabilized ruthenium colloids, J. Am. Chem. Soc. 130 (2008) 6119.
[7] L. Ma, C. Abney, W. Lin, Enantioselective catalysis with homochiral
metal–organic frameworks, Chem. Soc. Rev. 38 (2009) 1248.
[8] B. Gomez-Lor, E. Gutierrez-Puebla, M. Iglesias, M.A. Monge, C. Ruiz-Valero,
N. Snejko, In(2)(OH)(3)(BDC)(1.5) (BDC = 1,4-benzendicarboxylate): an In(III)
supramolecular 3D framework with catalytic activity, Inorg. Chem. 41 (2002)
2429.
[9] J.A.R. Navarro, E. Barea, J.M. Salas, N. Masciocchi, S. Galli, A. Sironi, C.O. Ania, J.B.
Parra, H2, N2, CO, and CO2 sorption properties on a series of robust isoreticular
sodalite type microporous coordination polymers, Inorg. Chem. 45 (2006) 2397.
[10] F.X. Llabrés i Xamena, A. Abad, A. Corma, H. Garcia, MOFs as catalysts: activity,
reusability and shape-selectivity of a Pd-containing MOF, J. Catal. 250 (2007)
294.
Fig. 10. X-ray diffraction patterns of fresh and used Cu-MOF-74 catalyst after several
[11] X. Zhang, F.X. Llabrés i Xamena, A. Corma, Gold(III)–metal organic framework
bridges the gap between homogeneous and heterogeneous gold catalysts, J.
Catal. 265 (2009) 155.
reaction cycles.
[12] M. Fujita, Y.J. Kwon, S. Washizu, K. Ogura, Preparation, clathration ability, and
catalysis of a two-dimensional square network material composed of cad-
mium(II) and 4,4ꢀ-bipyridine, J. Am. Chem. Soc. 116 (1994) 1151.
[13] K. Schlichte, T. Kratzke, S. Kaskel, Improved synthesis, thermal stability and
catalytic properties of the metal–organic framework compound Cu3(BTC)2,
Microporous Mesoporous Mater. 73 (2004) 81.
[14] S. Horike, M. Dinca, K. Tamaki, J.R. Long, Size-selective Lewis-acid catalysis in a
microporous metal–organic framework with exposed Mn2+ coordination sites,
J. Am. Chem. Soc. 130 (2008) 5854.
[15] L.C. Tabares, J.A.R. Navarro, J.M. Salas, Cooperative guest inclusion by a zeolite
analogue coordination polymer. sorption behavior with gases and amine and
group 1 metal salts, J. Am. Chem. Soc. 123 (2001) 383.
[16] K.S. Suslick, P. Bhyrappa, J.H. Chou, M.E. Kosal, S. Nakagaki, D.W. Smithenry,
S.R. Wilson, Microporous porphyrin solids, Acc. Chem. Res. 38 (2005) 283.
[17] A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Metal organic frameworks as effi-
cient heterogeneous catalysts for the oxidation of benzylic compounds with
t-butylhydroperoxide, J. Catal. 267 (2009) 1.
evidence a catalytic activity practically constant in terms of relative
anisole conversion and p-MAP yield after seven cycles. Addition-
ally, XRD patterns of the catalyst after each reaction cycle (Fig. 10)
confirm the stability of Cu-MOF-74 phase, suggesting this material
arises as an interesting catalyst for acylation reactions.
4. Conclusions
The activity of Cu-MOF-74 for acid catalyzed reactions, in partic-
ular the Friedel–Craft acylation of anisole using acetyl chloride, has
been proven. The polarity of the solvent molecule was considered a
crucial factor for the use of Cu-MOF-74 as a catalyst, due to the affin-
ity of polar molecules to uncoordinated metal sites, which decrease
the accessibility of reactants and subsequently the loss of activity.
High concentrations of the acylating agent in the catalytic system
promoted the degradation of the crystalline Cu-MOF-74 phase. The
structural stability of the material was maintained by increasing of
catalyst concentration and the anisole/acylating agent ratio. Under
these reaction conditions, Cu-MOF-74 exhibited a better catalytic
performance than HKUST-1 as representative of other Cu-MOF,
and conventional zeolitic and mesoporous inorganic materials (H-
ZSM-5 BETA and Al-MCM-41). Moreover, the catalytic performance
of Cu-MOF-74 was maintained quite constant for seven catalytic
cycles, showing very promising results for its application in acid
catalyzed reactions.
[18] Z.T. Yu, Z.L. Liao, Y.S. Jiang, G.H. Li, G.D. Li, J.S. Chen, Construction of a microp-
orous inorganic–organic hybrid compound with uranyl units, Chem. Commun.
(2004) 2004.
[19] P. Mahata, G. Madras, S. Natarajan, Novel photocatalysts for the decomposition
of organic dyes based on metal–organic framework compounds, J. Phys. Chem.
B 110 (2006) 13759.
[20] L. Nguyen, K. Le, N. Phan, A zeolite imidazolate framework ZIF-8 catalyst for
Friedel–Crafts acylation, Chin. J. Chem. 33 (2012) 688.
[21] N. Phan, K. Le, T. Phan, MOF-5 as an efficient heterogeneous catalyst for
Friedel–Crafts alkylation reactions, Appl. Catal. A: Gen. 382 (2010) 246.
[22] P. Horcajada, S. Surble, C. Serre, D.Y. Hong, Y.K. Seo, J.S. Chang, J.M. Greneche,
I. Margiolaki, G. Ferey, Synthesis and catalytic properties of MIL-100(Fe), an
iron(III) carboxylate with large pores, Chem. Commun. (2007) 2820.
[23] U. Ravon, M.E. Domine, C. Gaudillere, A. Desmartin-Chomel, D. Farrusseng,
MOFs as acid catalysts with shape selectivity properties, New J. Chem. 32 (2008)
937.

G. Calleja et al. / Catalysis Today 227 (2014) 130–137
137
ˇ
[24] M. Opanasenko, M. Shamzhy, J. Cejka, Solid acid catalysts for coumarin synthe-
[36] D. Serrano, R. García, D. Otero, Friedel–Crafts acylation of anisole over
hybrid zeolitic-mesostructures materials, Appl. Catal. A: Gen. 359 (2009)
69.
[37] N. Rosi, J. Kim, M. Eddaoudi, B. Chen, M. O’Keeffe, O. Yaghi, Rod packings and
metal–organic frameworks constructed from rod-shaped secondary building
units, J. Am. Chem. Soc. 127 (2005) 1504.
[38] P. Dietzel, R. Johnsen, R. Blom, H. Fjellvåg, Structural changes and
coordinatively unsaturated metal atoms on dehydration of honeycomb anal-
ogous microporous metal–organic frameworks, Chem. Eur. J. 14 (2008)
2389.
sis by the Pechmann reaction-MOFs versus zeolites, ChemCatChem 5 (2013)
1024.
[25] M. Opanasenko, A. Dhakshinamoorthy, J. Cejka, H. Garcia, Deactivation path-
ways of the catalytic activity of metal–organic frameworks in condensation
reactions, ChemCatChem 5 (2013) 1553.
[26] M. Opanasenko, A. Dhakshinamoorthy, Y. Hwang, J. Chang, H. Garcia, J. Cejka,
ˇ
ˇ
Superior performance of metal–organic frameworks over zeolites as solid acid
catalysts in the Prins reaction: green synthesis of nopol, ChemSusChem6 (2013)
865.
[27] P.H. Gore, in: G.A. Olah (Ed.), Aromatic Ketone Synthesis in Friedel–Crafts and
Related Reactions, vol. III, Part 1, John Wiley & Sons Inc., London, 1964, p. 1.
[28] A. Corma, M.E. Davis, Issues in the synthesis of crystalline molecular sieves:
towards the crystallization of low framework-density structures, Chem. Phys.
Chem. 5 (2004) 304.
[39] C. Carson, K. Hardcastle, J. Schwartz, X. Liu, C. Hoffmann, R. Gerhardt, R. Tan-
nenbaum, Synthesis and structure characterization of copper terephthalate
metal–organic frameworks, Eur. J. Inorg. Chem. (2009) 2338.
[40] E.A. Gunnewegh, S. Gopie, H. van Bekkum, MCM-41 type molecular sieves
as catalysts for the Friedel–Crafts acylation of 2-methoxynaphthalene, J. Mol.
Catal. A: Chem. 106 (1996) 151.
[41] P. Patil, K. Malshe, P. Kumar, M. Dongare, E. Kemnitz, Benzoylation of
anisole over borate zirconia solid acid catalyst, Catal. Commun. 3 (2002)
411.
[42] D. Bhattacharya, S. Sharma, A.P. Singh, Selective benzoylation of naphthalene to
2-benzoylnaphthalene using zeolite H-beta catalysts, Appl. Catal. A: Gen. 150
(1997) 53.
[43] F. Martínez, G. Morales, A. Martín, R. van Grieken, Perfluorinated Nafion-
modified SBA-15 materials for catalytic acylation of anisole, Appl. Catal. A: Gen.
347 (2008) 169.
[44] U. Freese, F. Heinrich, F. Roessner, Acylation of aromatic compounds on H-Beta
zeolites, Catal. Today 49 (1999) 237.
[45] M. Selvaraj, K. Lee, K. Yoo, T. Lee, Synthesis of 2-acetylmethoxynaphtalene using
mesoporous SO4-2/Al-MCM-41 molecular sieves, Microporous Mesoporous
Mater. 81 (2005) 343.
[46] I. Lezcano-González, J. Vidal-Moya, M. Boronat, T. Blasco, A. Corma, Iden-
tification of active surface species for Friedel–Crafts acylation and Koch
carbonylation reactions by in situ solid-state NMR spectroscopy, Angew. Chem.
Int. Ed. 52 (2013) 5138.
[29] G. Sartori, R. Maggi, Use of solid catalysts in Friedel–Crafts acylation reactions,
Chem. Rev. 106 (2006) 1077.
ˇ
[30] D.P. Serrano, R.A. García, G. Vicente, M. Linares, D. Procházková, J. Cejka, Acidic
and catalytic properties of hierarchical zeolites and hybrid ordered meso-
porous materials assembled from MFI protozeolitic units, J. Catal. 279 (2011)
366.
[31] A. Corma, H. García, F.X. Llabrés i Xamena, Engineering metal organic frame-
works for heterogeneous catalysis, Chem. Rev. 110 (2010) 4606.
[32] T. Shi, Z. Guo, H. Yu, J. Xie, Y. Zhong, W. Zhu, Atom-economic synthesis of
optically active Warfarin anticoagulant over a Chiral MOF organocatalyst, Adv.
Synth. Catal. 355 (2013) 2538.
[33] R. Sanz, F. Martínez, G. Orcajo, L. Wojtas, D. Briones, Synthesis of a honeycomb-
like Cu-based metal–organic framework and its carbon dioxide adsorption
behaviour, Dalton Trans. 42 (2013) 2392.
[34] S. Brunauer, P. Emmett, E. Teller, Adsorption of gases in multimolecular layers,
J. Am. Chem. Soc. 60 (1938) 309.
[35] J. Jagiello, M. Thommes, Comparison of DFT characterization methods based
on N2, Ar, CO2 and H2 adsorption applied to carbons with various pore size
distributions, Carbon 42 (2004) 1227.