Post-synthetic modification of nanoporous Cu3(BTC)2 metal-organic framework via immobilization of a molybdenum complex for selective epoxidation

Article abstract of DOI:10.1016/j.molcata.2015.01.014

In the present work, nanoporous Cu₃(BTC)₂ (abbreviated as CuBTC and BTC = benzene-1,3,5-tricarboxylate) metal-organic framework (MOF) was modified in a two-step post-synthetic reaction by covalent attachment of aminopyridine groups followed by the reaction with bis(acetylacetonato) dioxomolybdenum(VI). The prepared molybdenum containing MOF was utilized as a heterogeneous catalyst in the epoxidation of olefins and allylic alcohols. Characterization of the prepared catalyst was performed using Fourier transform infrared, atomic absorption spectroscopies, CHN elemental analysis, powder X-ray diffraction, scanning electron microscopy, thermogravimetric analysis and nitrogen adsorption/desorption techniques. The resulting catalyst showed high selectivity and catalytic activity toward the epoxidation reaction. Moreover, the catalyst demonstrated size-selective properties suggested the reaction occurred inside the pores.

Full text of DOI:10.1016/j.molcata.2015.01.014

Accepted Manuscript
Title: Post-synthetic modification of nanoporous Cu3(BTC)2
metal-organic framework via immobilization of a
molybdenum complex for selective epoxidation
Author: Sara Abednatanzi Alireza Abbasi Majid
Masteri-Farahani
PII:
S1381-1169(15)00018-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.01.014
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25-10-2014
7-1-2015
15-1-2015
Please cite this article as: Sara Abednatanzi, Alireza Abbasi, Majid Masteri-Farahani,
Post-synthetic modification of nanoporous Cu3(BTC)2 metal-organic framework via
immobilization of a molybdenum complex for selective epoxidation, Journal of
Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2015.01.014
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Post-synthetic modification of nanoporousCu3(BTC)2metal-organic framework via
immobilization of a molybdenum complex for selective epoxidation
SaraAbednatanzia, AlirezaAbbasia,*, MajidMasteri-Farahanib
a School of Chemistry, College of Science, University of Tehran, P.O. Box: 14155-6455 Tehran, Iran
b Faculty of Chemistry, Kharazmi University, P.O. Box: 15719-14911Tehran, Iran
*Corresponding author: Alireza Abbasi
Corresponding E-mail address: aabbasi@khayam.ut.ac.ir
Tel.: +98-21-61113644
Fax: +98-21-66495291
1

Graphical abstract
Functionalized nanoporousCu3(BTC)2metal-organic framework (MOF) utilized as
heterogeneous catalyst in epoxidation of olefins and allyl alcohols.
a
Abstract
In the present work, nanoporousCu3(BTC)2 (abbreviated as CuBTC and BTC= benzene- 1,3,5-
tricarboxylate) metal-organic framework (MOF) was modified in a two-step post-synthetic
reaction by covalent attachment of aminopyridine groups followed bythe reaction with
bis(acetylacetonato)dioxomolybdenum(VI). The prepared molybdenum containing MOF was
utilized as a heterogeneous catalyst in the epoxidation of olefins and allylic alcohols.
Characterization of the prepared catalyst was performed using Fourier transform infrared, atomic
absorption spectroscopies,CHN elemental analysis, powder X-ray diffraction, scanning electron
microscopy, thermogravimetricanalysis and nitrogen adsorption/desorptiontechniques. The
2

resulting catalyst showed high selectivity and catalytic activity toward the epoxidation
reaction.Moreover, the catalyst demonstrated size-selective propertiessuggested thereaction
occurred inside the pores.
Keywords: Metal-organic framework (MOF), modification, nanoporous CuBTC, molybdenum,
epoxidation
1. Introduction
Metal-organic frameworks (MOFs), as a new class of functional materials, are nanoporous
crystalline organic-inorganic hybrid solids constructed from inorganic metal nodes connected to
organic linkers [1-2]. During the last two decades, an increasing number of studiesforthe design
and synthesis of these nanoporous materials have been reported [3-4]. These materials have
potential applications such as gas storage, catalysis, sensing and drug deliverybecause
oftheirmicroporosity, large specific surface area and especially their structural variety [5-7].
In principle, one of the unique properties of MOFs is the possibility to introduce a large variety
of functional groups into the framework by utilizing functionalized ligands directly in aone-step
synthesis. However, it is usually found that the prepared materials still do not contain the desired
chemical functionality needed for more specialized applications. In fact, under solvothermal
conditions most of the modified ligands suffer from chemical or thermal stability [8-9].
Thereforepost-synthetic modification (PSM) is applied to attain functionalized MOFs
[9].FunctionalizedMOFs can be synthesizedthroughchemical modification of either the organic
linkers or unsaturated metal centers created after solvent elimination [9].
CuBTCor HKUST-1,as a good representative for PSM studies, was first discovered in 1999
[10].CuBTCis a 3-D porous MOF possessing square-shaped channels of 0.9nm in diameter with
water molecules coordinated to the copper centers. Removal of water molecules upon thermal
3

activation creates open coordination sites capable ofattaching to organic molecules [11]. Among
different MOFs, CuBTCowing to its facile synthesis and activation, large specific surface area,
relatively large and open pores as well as thermal stability [12] is a good candidate to introduce
desired functionalities.Few reports on the PSM of CuBTClike functionalization using thiol
groups, attachment of a palladium complex and immobilization of amino-functionalized basic
ionic liquid onto CuBTC, have been reported so far [13-15].
Heterogeneous catalysts have several advantages such as easier work-up, recyclability and
stability of active components compared to their homogeneous analogs [16]. In this aspect,
MOFs have been proved to be more suitable supports for heterogenization of active metal
complexes compared to zeolites with pore-limited dimensions[17]. Since Wang and Cohen
introduced PSM of IRMOF-3 in 2007 [18], different oxidation catalysts with good activities and
selectivitieshave been prepared includingsalen-Co(II) complex incorporated into NH2-MIL-101,
manganese(III) porphyrin complex covalently attached to MIL-101, IRMOF-3 modified by
manganese and vanadylcomplexes [19-22].
It has been proved that molybdenum complexes are excellent epoxidation catalysts [23].
Consequently, various solid supports like zeolites, silicates and other mesoporous molecular
sieves have been utilized to immobilize molybdenum complexes [24-26]. We have also recently
demonstrated the catalytic activity of molybdenum Schiff base complexes immobilized on the
surface of carbon nanotubes and activated carbon [27-28].
In spite of many attemptsto immobilizedifferentmolybdenum complexes on solid supports, there
arefew reports on the attachment of these effective complexes using MOFs through PSM. Very
recently PSM of a gallium MOF with MoO2Cl2(THF)2 complex was reportedthatexhibited
goodcatalytic activity and selectivity in epoxidation reactions [29].
4

Considering the significant benefits of CuBTC as solid support and molybdenum complexes as
active components, herein, we report a novel and highly active heterogeneous epoxidation
catalyst by immobilizing bis(acetylacetonato)dioxomolybdenum(VI)complex intonanoporous
CuBTC through aminopyridine linker.The prepared bimetallic catalyst exhibited high activity
and selectivity toward epoxidation reactions compared to previously reported oxidation catalysts
obtained by PSM method [21-22, 29].
2. Experimental
2.1 Materials and instrumentation
All
reagents
were
purchased
from
Merck
chemical
company.
Bis(acetylacetonato)dioxomolybdenum(VI), MoO2(acac)2, was prepared according to the
literature method [30].
Fourier transform infrared (FT-IR) spectra were recorded on Perkin-Elmer Spectrum RXI FT-IR
spectrometer using KBr pellets. Powder X-ray diffraction (XRD) patterns were collected on a
Philips PW1800 diffractometer with Cu Kα radiation in the 2θrange of 4-60° (λ=1.5406 Å).
Scanning electron microscopy (SEM) images were taken with ZEISS-FOS-UT
microscope.Nitrogen adsorption/desorptionstudies were conducted using Quantachrome Nova
2200e, Version 7.11 analyzer at liquid nitrogen temperature. Before the adsorption experiments,
the samples were outgassed under vacuum at 120°C for 24 h.Chemical analyses of the samples
were performed with a Contraa 700 atomic absorption spectrometer. Thermogravimetricanalysis
(TGA) was carried out to determine the stabilities of the samples using a NET-ZSCH STA 409
PC/PG TG Instrument. The samples were heated from 30 to 800°C in air at a constant rate of
20°C min-1. Analyses of the products were examined using a gas chromatograph (HP, Agilent
68909N) equipped with a capillary column (HP-5) and a flame ionization detector (FID). Gas
5

chromatography-mass spectrometry (GC-MS) was recorded using a Shimadzu-14A fitted with a
capillary column (CBP5-M25).
2.2 Catalyst preparation
2.2.1 Synthesis of aminopyridine functionalized CuBTC, (AMP-CuBTC)
Preparation of CuBTC.nH2OMOF was carried out using the previously reported hydrothermal
method [31]. Typically, 0.875g (3.6 mmol) of Cu(NO3)2.3H2O in 12 ml deionized water was
added to 0.42 g (2.0 mmol) of H3BTC in 12 ml of ethanol. The mixture was transferred to a
Teflon-lined steel autoclave and heated to 120°C for 12h. The obtained blue crystals were
filtered and washed with deionized water and ethanol.Before functionalization, 0.5g of
CuBTC.nH2O wasfirst activated at 150°C for 24 h to remove water molecules. Afterwards, a
solution of 4-aminopyridine((AMP), 50 mg, 0.53 mmol) in dichloromethane was added to the
activated material suspended in 15 ml toluene. The mixture was refluxed for 16h to yield
aminopyridine modified samples. After filtration, the product waswashed with dichloromethane,
and thendried at 100°C for 3h.
2.2.2 Synthesis of supported molybdenum catalyst, (Mo-AMP-CuBTC)
A solution of MoO2(acac)2 (16 mg, 0.05 mmol) in 5 ml dichloromethane was added to a solution
of AMP-CuBTC (200 mg) in dichloromethane (10 ml). The mixture was then refluxed for 4 h,
filtered and soxhlet extracted with dichloromethane for 24 h to remove the unreacted
molybdenum complex. The obtained molybdenum catalyst was dried at 100°C for 3h.
2.2.3 Catalytic epoxidation of olefins using Mo-AMP-CuBTC catalyst
As a typical procedure, the epoxidation reaction was carried out as follows: To a mixture of
catalyst (100 mg) and olefin (8 mmol) in solvent (10 ml) was added oxidant (14.4 mmol).Tert-
butyl hydroperoxide (TBHP, 80% in di-tertiary butyl peroxide), cumenehydroperoxide (CHP,
6

80% in cumene) and hydrogen peroxide (H2O2, 30% in water) were used as oxidants. The
mixture was refluxed forvarious times and the final products were quantified with isooctane
(8.75 mmol) as internal standard.
3.
Results and discussion
3.1 Synthesis and characterization of the supported molybdenum catalyst,(Mo-AMP-CuBTC)
To activate CuBTC.nH2Oand eliminate its water molecules, the sample was heated at 150 °C for
24 h. After activation, CuBTC modification was conducted by the covalent attachment of 4-
aminopyridine to the free metal sites followed by the addition of molybdenum complex. The
synthetic route used to prepare the catalyst is illustrated in Fig. 1.
Molybdenum complex with dimensions of about 8.5 × 6.2 × 6.2 has the appropriate size to
diffuse through the accessible pores present in CuBTC MOF (~ 0.9 nm). Therefore, AMP-
CuBTC metal binding can be done inside the pores.
N
NH2
150 °C
Cu
Cu
Cu
N
NH2
Cu
Cu
Cu
- H2O
Toluene
H2O
CuBTC.nH2O
CuBTC
AMP-CuBTC
MoO2(acac)2
CH2Cl2
O
N
Cu
N
Cu
O
Mo
O
O
O
Mo-AMP-CuBTC
Fig. 1.The schematic illustration for modification of CuBTC with molybdenum complex.
7

In order to confirm the modification of the CuBTCframework, the FT-IR spectra of all samples
were recorded and shown in Fig. 2.In the AMP-CuBTC spectrum (Fig. 2b), the characteristic
band at 1617 cm-1wasascribed to the stretching vibration of the C=N within the pyridine ring.
Furthermore, the band at 3364 cm-1 can be attributed to the N-H symmetric stretching of 4-
aminopyridine. In the Mo-AMP-CuBTC spectrum (Fig. 2c), the presence of two adjacent bands
at 907 and 939 cm-1was assigned to Mo=O vibrations. In addition,the absorption band at 1646
cm-1 was attributed to C=N vibration of the Schiff base ligand upon complexation with
MoO2(acac)2. These results revealed successful modification of the MOF with the molybdenum
complex. The observation of the N-H vibrations at 3362 and 3361 cm-1 in fresh and recycled
Mo-AMP-CuBTC catalyst (Fig. 2c and Fig. 2d) wasdue to the presence of free aminopyridine
groups which were not reacted with the molybdenum complex (see the obtained results from
elemental analysis).
8

Fig. 2.FT-IR spectra of (a) CuBTC, (b) AMP-CuBTC, (c) Mo-AMP-CuBTC, (d) recycled Mo-AMP-
CuBTC.
Further evidence for the modification of the MOF with the molybdenum Schiff base complex
was confirmed by atomic absorption spectroscopy (Mo: 0.73%, 0.076 mmol g-1) and CHN
analysis for the modified samples. (AMP-CuBTC: C 30.68, H 2.56, N 2.63%, Mo-AMP-CuBTC:
C 32.63, H 2.75, N 2.99%). CHN analysis revealed the presence of 2.99% nitrogen in the Mo-
AMP-CuBTC catalyst suggesting the attachment of aminopyridine groups to the metal sites.The
obtained results from CHN and atomic absorption spectroscopy showed that about half of
aminopyridine groups were functionalized withthe molybdenum complex.
The phase purity and crystallinity of the bulk samples were examined by powder X-ray
diffraction (Fig.3). The patterns of the synthesized CuBTC before modification were similar to
that of simulated ones,indicatingthe phase purity of the preparedMOF. Also, the framework
structure remained intact after PSM showing the stability of the framework during
themodification process.As expected, the modification of the CuBTCcaused a decrease in the
intensity of the peaks in the XRD patterns.
9

Fig. 3.XRDpatterns of (a) Simulated CuBTC, (b)as-synthesized CuBTC, (c) Mo-AMP-CuBTC,
(d) recycled Mo-AMP-CuBTC.
The surface morphology of the obtained catalysts was investigated by SEM (Fig. 4). The SEM
imagesof the as-synthesized CuBTC showswell-defined octahedral crystals with particlesizes
between 4 and 10 µm(Fig. 4a). After modification of CuBTC with the molybdenum complex, the
size of the particles is about 6 µm, however some of the particles are agglomerated (Fig 4b).
(a)
(b)
10

Fig. 4.SEMimages of (a) as-synthesized CuBTC, (b) Mo-AMP-CuBTC.
Thermal behavior of the samples was investigated by thermogravimetric analysis. As can be seen
in Fig. 5a, the as-synthesized CuBTCshowedthe first weight loss of about 34% up to290 °C
which was ascribed to the loss of surface and inner water moleculesof the framework. The
second weight loss above 290 °C was due to the MOF collapse. The final residue of about
31%was assigned to CuO. In the TGA curve of theMo-AMP-CuBTC catalyst (Fig. 5b),a weight
loss of only 7% was observedup to 110°Cas the result ofwater molecules replacement in CuBTC
by aminopyridine groups in the modified catalyst. In the second step, a weight loss of about48%
between110-440°C might be attributed to the loss of the organic components attached to the
coppercenters through PSM together with the BTC linkers of the framework.
11

100
90
80
70
60
50
40
30
20
10
0
93%
×
66%
×
45%
(b)
(a)
×
×
31%
0
100
200
300
400
500
600
700
800
Temperature/°C
Fig. 5.TGAcurves of (a) CuBTC, (b) Mo-AMP-CuBTC.
Textural properties of the pristine CuBTC and Mo-AMP-CuBTC catalyst were investigated with
low temperature N2 adsorption/desorption analyses. The adsorption isotherms of both samples
are shown in Fig.6. In the adsorption isotherm of pristine CuBTC (Fig. 6a), the uptake of N2
increases at high relative pressure and exhibits type I/IV mixed isotherm with a hysteresis loop
which indicates the microporous and mesoporous nature of the sample. The observed
mesoporosity might be attributed to the stacking of nanoparticles.
The adsorption isotherm of Mo-AMP-CuBTC catalyst (Fig. 6b) is mainly type II, as deduced
from the shape of the isotherm at relative pressures below 0.2 and at higher relative pressures,
which is the characteristic of nonporous materials. This is mainly due to a pore blocking effect,
especially micropores, which is also evident from the analysis of the uptake at low relative
pressures in the adsorption/desorption isotherm. The pore blocking is probably resulted from the
12

reaction of acac ligands of molybdenum precursor with aminopyridine groups located in the
pores of the framework. The isotherm also shows a small hysteresis loop due to the presence of
few mesopores in the Mo-AMP-CuBTC catalyst. These observations are in good agreement with
our previous researches [27-28].
(a)
(b)
Fig. 6.Nitrogen sorption isotherms of (a) CuBTC, (b) Mo-AMP-CuBTC (insert view shows the
magnified view).
The textural parameters of the unmodified and modified CuBTC are collected in Table 1. A clear
point when comparing these data is a significant decrease in the Brunauer-Emmett-Teller surface
area (SBET) and total pore volume upon tethering the molybdenum complex within the pores of
the MOF which confirms the previously mentioned pore blocking effect.
13

Table 1. Texture parameters of the samples obtained from nitrogen sorption studies.
b
Sample
CuBTC
SBET(m2g-1) Smicroa(m2g-1)
VT (mlg-1)
Vmicroa(mlg-1)
Vmesoc(mlg-1)
0.149
861
14
765
0
0.538
0.389
0
Mo-AMP-CuBTC
0.070
0.070
a Calculated by the t-method.
b Total pore volume at p/p0=0.98.
c Obtained by subtracting the micropore volume from the total pore volume at p/p0=0.98.
3.2.Catalytic epoxidation in the presence of the supported molybdenum catalyst,(Mo-AMP-
CuBTC)
Selective epoxidation of some olefins and allylic alcohols was conducted using Mo-AMP-
CuBTCas catalyst.In an attempt to assess the optimal reaction conditions, different solvents were
applied to the epoxidation of 3-Methyl-2-buten-1-ol in the presence of H2O2, CHP and TBHP as
oxidants. As observed in Fig. 7,in all three solvents Mo-AMP-CuBTC can act as active catalyst
with TBHP as oxidant. A clear point when comparing the results is the higher reactivity of the
substrate in CHCl3 and CH2Cl2compared with CH3CN in the presence of TBHP and CHP as
oxidants. This observation can be explained with the coordination ability of CH3CN and its
competition with the oxygen molecule to occupy the coordination sites of the catalyst. In
addition, Mo-AMP-CuBTC also exhibited more catalytic activity with TBHP rather than CHP as
oxidant. This behavior can be attributed to more electrophilic character of the peroxidic oxygen
in TBHP. Furthermore,the molybdenum catalyst showedpoor catalytic activity in the epoxidation
of substrates in the presence of H2O2 as oxidant. However, with H2O2as oxidant higher
conversion was obtained for the epoxidation of 3-Methyl-2-buten-1-ol in CH3CN as solvent.
This may be due to the better miscibility of the reactants in this solvent. These observations are
consistent with previous reports [28, 32]. Regarding these results the epoxidation reaction was
14

conductedin the presence of TBHPas oxidant and CHCl3 as solvent and the results are
summarized in Tables2, 3 and Fig.8.
100
90
80
70
60
50
40
30
20
10
0
Chloroform
Dichloromethane
Acetonitrile
TBHP
H O
2 32
CHP
1
2
Fig. 7.3-Methyl-2-buten-1-ol epoxidation in the presence of Mo-AMP-CuBTC catalyst.
To clarify the catalytic role of the prepared Mo-AMP-CuBTC catalyst, the epoxidation reaction
was also done in the presence of unmodified CuBTC as catalyst. Although, CuBTC can act as a
heterogeneous catalyst in oxidation reactions [33-34], PSM route enhances catalytic efficiency of
the CuBTC framework. As can be seen from Table 2, entry 1 and Fig. 8awhen unmodified
CuBTC was applied as catalyst the catalytic activity toward the formation of epoxideis clearly
lower than Mo-AMP-CuBTC.
15

100
80
60
40
20
0
(b)
(a)
0
5
10
15
20
25
Time (h)
Fig. 8.Effect of time on 3-Methyl-2-buten-1-ol conversion in the presence of (a) CuBTC, (b) Mo-AMP-
CuBTC.
In order to evaluate the effect of substrate structure on catalyst activity different olefins and
allylic alcohols were tested and the results are shown in Tables 2 and 3. The high reactivity of
the allylic alcohols is probably due to the participation of hydroxyl group in approaching the
double bond to the electrophilic oxygen center. The order of increasing reactivities of the olefins
based on either conversions or TOFs are as styrene ~ cyclooctene>norbornene> cyclohexene >
1-hexene > 1-octene >trans-stilbene >cis-stilbene. To explain this trend, two determining factors
should be taken into consideration. It is expected that olefins with higher electron density of the
double bond show more epoxidation reactivity. Hence, the reaction rates for cyclooctene and
cyclohexene with double bond driven from secondary carbons are higher than 1-octene and 1-
hexene. Cyclooctene and norbornene are more reactive than cyclohexene owing to the existence
of more electron donating cyclic bridge connected to the double bond. That 1-octene is
epoxidized slower than 1-hexene can be inferred since larger hexyl group connected to double
16

bond sterically hinders it in approaching to the catalyst metal center with respect to 1-hexene
which its double bond carries a smaller butyl group. However, in the case of larger olefins such
as trans-stilbene and cis-stilbene, Mo-AMP-CuBTC showed no significant catalytic activity
toward epoxidation reaction(Table 2, entry 8 and 10). This difference in reactivity is because of
the different dimensions of the substrates and confirms that the reaction occurs inside the pores.
To ensure that the epoxidation reactions take place within the pores of the catalyst, a size
selectivity study was done by utilizing MoO2(acac)2 as catalyst. Although the reactions catalyzed
by the homogeneous catalyst were completed within 2h, only 7 or 8% conversion of the
substrates was obtained in the presence of Mo-AMP-CuBTC after the same period of time (Table
2, entry 8to11). A similar size selectivity behavior of MOFs as heterogeneous catalystsis also
observed in previous reports [35-38].As an example,a size selective heterogeneous catalyst for
oxidation of olefins has been synthesized through metalloporphyrin encapsulation in a MOF with
CuBTC topology. The order of reactivities based on either conversions or TOFs were as styrene
>trans-stilbene>triphenylethylene. Conversion of styrene reached 85% after 10 h, compared with
conversions of 40% and < 5% for trans-stilbene and triphenylethylenerespectively. This
observationconfirmed that the oxidation reaction occurs in the cages of the framework [38].
Table 2.Catalytic epoxidation of olefins using different catalysts.
Entry
Catalyst
Substrate
Cyclooctene
Cyclooctene
Cyclohexene
Styrene
Time (h) Conversiona (%) Selectivityb (%)
TOFc(h-1)
-
1
2
3
4
5
CuBTC
6
2
2
2
2
46
99
90
99
93
50
Mo-AMP-CuBTC
Mo-AMP-CuBTC
Mo-AMP-CuBTC
Mo-AMP-CuBTC
521
>99
>99
>99
>99
474
521
489
Norbornene
17

6
7
Mo-AMP-CuBTC
Mo-AMP-CuBTC
Mo-AMP-CuBTC
MoO2(acac)2
2
2
2
2
2
2
62
72
8
>99
>99
>99
>99
>99
>99
326
379
42
-
1-Octene
1-Hexene
8
trans-Stilbene
trans-Stilbene
cis-Stilbene
cis-Stilbene
9
89
7
10
11
Mo-AMP-CuBTC
MoO2(acac)2
37
-
82
Reaction conditions: catalyst (100mg), olefins (8mmol), TBHP (14.4mmol), refluxing chloroform (10ml).
aGC yield based on starting substrate.
bSelectivity toward the formation of epoxide determined by GC-Mass or injection of reference.
cCalculated as mmol of product formed per mmol of molybdenum in the catalyst per time.
Table 3.Catalytic epoxidation of allylic alcohols using Mo-AMP-CuBTC as catalyst.
Entry
Catalyst
Substrate
3-Methyl-2-buten-1-ol
trans-2-Hexen-1-ol
1-Octen-3-ol
Time (h) Conversiona (%) Selectivityb (%)
TOFc(h-1)
516
1
2
3
4
Mo-AMP-CuBTC
Mo-AMP-CuBTC
Mo-AMP-CuBTC
Mo-AMP-CuBTC
2
2
2
2
98
89
79
99
>99
>99
>99
>99
468
416
Allyl alcohol
521
Reaction conditions: catalyst (100 mg), allylic alcohols (8 mmol), TBHP (14.4 mmol), refluxing
chloroform (10 ml).
aGC yield based on starting substrate.
bSelectivity toward the formation of epoxide determined by GC-Mass or injection of reference.
cCalculated as mmol of product formed per mmol of molybdenum in the catalyst per time.
The effect of catalyst amount on the epoxidation of 3-Methyl-2-buten-1-ol has been summarized
in Table 4. It is clear that substrate conversion increases with increasing the amount of catalyst
up to 100 mg, and then remains constant with larger amount of the catalyst.
Table 4.Catalytic epoxidation of 3-Methyl-2-buten-1-ol using different amounts of catalyst.
18

Entry
Catalyst
Catalyst amount (mg) Time (h) Conversiona (%) Selectivityb (%)
TOFc(h-1)
1663
832
1
2
3
4
5
Mo-AMP-CuBTC
Mo-AMP-CuBTC
Mo-AMP-CuBTC
Mo-AMP-CuBTC
Mo-AMP-CuBTC
25
50
2
2
2
2
2
75
79
94
98
98
>99
>99
>99
>99
>99
75
660
100
150
516
344
Reaction conditions:3-Methyl-2-buten-1-ol (8mmol), TBHP (14.4mmol), refluxing chloroform (10ml).
aGC yield based on starting substrate.
bSelectivity toward the formation of epoxide determined by GC-Mass.
cCalculated as mmol of product formed per mmol of molybdenum in the catalyst per time.
The reusability of the Mo-AMP-CuBTC catalyst was tested in the epoxidation of 3-Methyl-2-
buten-1-ol after 2 h of reaction for five times and the results are shown in Fig. 9. It can be
concluded that the recovered catalyst maintained similar activity as the atomic absorption
spectroscopy analysis showed no significant loss of the molybdenum after five cycles of the
epoxidation reaction (Molybdenum: 0.074 mmol g-1). In addition, the similarity of the FT-IR
spectrum and XRD of the recycled catalyst to the fresh one clearly indicates the catalyst stability
under the reaction conditions (Fig. 1d and 3d).
To measure the possible catalytic contribution of the solubilized molybdenum species, in a
separate test with cyclohexene, the reaction mixturewas filtered off after 2 h reaction and then
the filtrate left to react for a further 24 h. The cyclohexene conversion reached 92% after 24 h,
compared with conversion of 90% before the catalyst filtration. From these data it can be
concluded that the catalytic reaction is truly heterogeneous in nature and there is no significant
molybdenum species in the solution.
19

100
90
80
70
60
50
40
30
20
10
0
1
2
3
4
5
Number of runs
Fig. 9.Recyclability ofthe Mo-AMP-CuBTC catalyst for the epoxidation of 3-methyl-2-buten-1-ol.
Comparison the results of the prepared catalyst with the data presented in our previous works on
the epoxidation of cyclooctene with other molybdenum catalysts derived from different solid
supports are given in Table 5 [23, 27-28]. As it is clear from their TOFs, Mo-AMP-CuBTC
catalyst shows higher efficiency and catalyzes the epoxidation reaction in shorter times. This
behavior may be related to the catalytic activity of the CuBTC framework as solid support while
activated carbon (AC), multi-wall carbon nanotube (MWCNT) and mesoporous material (MCM-
41) are inert supports and are not involved in the epoxidation reaction. Moreover, the
molybdenum content of the current prepared catalyst is higher than the previously reported ones
(Table 5).
20

Table 5.Comparison of the results obtained for epoxidation of cyclooctene catalyzed by different
molybdenum catalysts immobilized on various solid supports.
Conversiona (%)
TOFb(h-1)
Catalyst
Mo content Time (h)
(mmol g-1)
Reference
Mo-AMP-CuBTC
MoO2acacAmp@MCM-41
MoO2(acac)sal-MWCNT
Mo-APTS-AC
0.07
0.06
0.05
0.06
2
7
8
8
99
99
99
99
521
138
198
165
This work
23
27
28
Reaction conditions: catalyst (100mg), cyclooctene (8mmol), TBHP (14.4mmol), refluxing chloroform
(10ml).
aGC yield based on starting substrate.
bCalculated as mmol of product formed per mmol of molybdenum in the catalyst per time.
The catalytic activity of the prepared catalyst in the epoxidation of cyclohexene was also
compared to the other oxidation catalysts obtained by attachment of different active components
to MOFs as solid support and the results are presented in Table 6.In the case of IRMOF3-Vsal
(Table 6, entry 1)[21] the poor catalytic activity with loss of framework crystallinity during the
reaction limited its application as heterogeneous catalyst. IRMOF3 [Mn] and Mo@COMOC-4
catalysts showed good catalytic activity and the substrate conversion increased with increasing
temperature (Table 6, entry 2to 5). The high catalytic efficiency of the prepared system can be
concluded from the data presented in Tables5 and 6.
21

Table 6.Comparison of the results obtained for epoxidation of cyclohexene catalyzed by different
catalysts immobilized on MOFs.
Conversion (%)
40
Entry
1
Catalyst
Reaction conditions
Time
(h)
Reference
21
IRMOF3-Vsal
30µl cyclohexene, 1.5 eq TBHP
(5.5M solution in decane), 5 ml
THF, 10 mg catalyst, 60 °C
72
2
3
IRMOF3 [Mn]
IRMOF3 [Mn]
1 mmol cyclohexene, 2 mmol
trimethylacetaldehyde, O2, 5 ml
toluene, 15 mg catalyst, 40 °C
1 mmol cyclohexene, 2 mmol
trimethylacetaldehyde, O2, 5 ml
toluene, 15 mg catalyst, 55 °C
25 mmol cyclohexene, 9 ml
TBHP, 30 ml chloroform, 50 °C
25 mmol cyclohexene, 9 ml
TBHP, 30 ml chloroform, 50 °C
6
6
68
78
22
22
4
5
6
Mo@COMOC-4
Mo@COMOC-4
Mo-AMP-CuBTC
7
7
2
21
81
90
29
29
8 mmol cyclohexene, 14.4 mmol
TBHP, 10 ml chloroform, 100
mg catalyst, 60 °C
This work
Conclusion
In summary, a novel catalyst through immobilization of aminopyridine groups was synthesized
followed by the attachment of the molybdenum complex using nanoporous CuBTC framework
as solid support. The resulting catalyst demonstrated high efficiency in the epoxidation reaction
compared to other molybdenum catalysts derived from different solid supports and different
catalysts immobilized on MOFs through PSM route. The Mo-AMP-CuBTC catalyst was found
22

to be catalytically active in the epoxidation reaction over five successive cycles without loss of
its activity or crystallinity.
Acknowledgments
The financial support from the University of Tehran is gratefully appreciated.
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