A new heterogeneous catalyst for epoxidation of alkenes via one-step post-functionalization of IRMOF-3 with a manganese(ii) acetylacetonate complex

Article abstract of DOI:10.1039/c1cc00069a

A manganese(ii) acetylacetonate complex has been immobilized to the metal-organic framework IRMOF-3 through a one-step post-synthetic route for the first time, providing an effective and recyclable heterogeneous catalyst for epoxidation of alkenes.

Full text of DOI:10.1039/c1cc00069a

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COMMUNICATION
A new heterogeneous catalyst for epoxidation of alkenes via one-step
post-functionalization of IRMOF-3 with a manganese(II)
acetylacetonate complexw
Samiran Bhattacharjee, Da-Ae Yang and Wha-Seung Ahn*
Received 5th January 2011, Accepted 17th January 2011
DOI: 10.1039/c1cc00069a
A manganese(II) acetylacetonate complex has been immobilized
to the metal–organic framework IRMOF-3 through a one-step
post-synthetic route for the first time, providing an effective and
recyclable heterogeneous catalyst for epoxidation of alkenes.
the first report of the binding of a manganese(II) acetylaceto-
nate complex to IRMOF-3 through a one-step post-synthesis
functionalization route. We also demonstrate that the
resulting new MOF, IRMOF-3[Mn] containing Mn(acac)2
{acac = acetylacetonate}, is a highly selective, reusable, and
non-leaching catalyst for the epoxidation of several important
alkenes.
Metal–organic frameworks (MOFs), a new class of crystalline
porous materials, have been generating a great deal of interest
as a gas sorption medium owing to their very large surface
areas and large pore volumes.1 In recent years, numerous
studies have also focused on investigation of MOFs as active
heterogeneous catalysts.2 Previous works have revealed that
chemical functionalization of MOFs by a post-modification
technique plays a key role in generating catalytically active
sites.3 Rowsell and Yaghi4 reported an amine-functionalized
porous MOF, IRMOF-3, with a cubic structure having amino
groups that do not take part in the formation of a 3D frame-
work structure. Several successful attempts to chemically
transform the free amine in IRMOF-3 through various post-
modification techniques have been reported.3a,c,d,5 Rosseinsky’s
group, for example, described the transformation of IRMOF-3
to an imine and subsequent metallation using VO(acac)2, with
the resultant product showing low catalytic activity and
stability in the epoxidation of cyclohexene.3a
The post-synthetic functionalization route for IRMOF-3[Mn]
is shown in Scheme 1. IRMOF-3 was synthesized according to
a procedure reported in the literature,4b and kept immersed in
toluene for 24 h before functionalization (see ESIw, catalyst
preparation). IRMOF-3[Mn] was then prepared by dissolving
Mn(acac)2 in toluene and treating the resulting solution with
IRMOF-3 crystals at 55 1C for 20 h. The material is stable in
an inert atmosphere or in toluene for several months.
IRMOF-3 before and after functionalization was characterized
by X-ray powder diffraction, N2 adsorption–desorption
isotherms, thermogravimetry, FTIR, X-ray photoelectron
spectroscopy (XPS), inductively coupled plasma-MASS
(ICP-MS), and elemental analysis. The XRD patterns of both
IRMOF-3 and IRMOF-3[Mn] in Fig. 1 show well-resolved
identical peaks, indicating that the original MOF framework
structure was maintained with high phase purity even after the
post-synthetic reaction. Furthermore, no changes in the type I
N2 adsorption–desorption isotherms (Fig. 1 inset) were found
The transformation of alkenes to corresponding epoxides is
both an important industrial technology and a useful synthetic
method for a wide range of products including pharma-
ceuticals and agrochemicals.6 Manganese Schiff base
complexes are known to be good homogeneous epoxidation
catalysts due to their high activity and selectivity.7 However,
problems of catalyst separation and recyclability should be
addressed for these homogenous catalysts. To this end, several
strategies involving immobilization of the active catalytic
moiety, viz. to zeolites, polymer, silica, layered double
hydroxide, and amine-functionalized hexagonal mesoporous
silica, have been reported.8 In this communication, we present
Department of Chemical Engineering, Inha University,
Incheon 402-751, Korea. E-mail: whasahn@inha.ac.kr;
Fax: +82 32-872 0959; Tel: +82 32-866 0143
w Electronic supplementary information (ESI) available: Synthetic
details, thermogravimetric analysis, FTIR (fresh and reused catalyst),
1H NMR spectra, XPS spectra, X-ray powder diffraction (fresh and
reused catalyst) and catalytic reaction procedure. See DOI: 10.1039/
c1cc00069a
Scheme 1 Post-functionalization route of IRMOF-3 for a Mn(II)
complex.
c
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IRMOF-3[Mn] was in good agreement with the formula unit
(Zn4O)(C8H5NO4)2.71(MnC17H17NO5)0.21
.
The heterogeneous catalytic activity of IRMOF-3[Mn] was
tested for the epoxidation of alkenes using molecular oxygen
and aldehyde under identical conditions to those reported by
Mukaiyama et al.13 The reactions are expressed in Scheme S1
(ESIw) and the corresponding experimental results are listed in
Table 1. The products were identified by GC co-injection of
commercially available standards and GC-MS.
No catalytic activity was shown by the parent IRMOF-3,
and the catalytic activity of IRMOF-3[Mn] summarized in
Table 1 is indicative of the presence of the active metal
complex immobilized on IRMOF-3. The epoxidation of
cyclohexene over IRMOF-3[Mn] using molecular oxygen
(1 atm) with trimethylacetaldehyde as an oxidant precursor
at 40 1C formed cyclohexene oxide with excellent selectivity
(92%) and conversion (68%) (Table 1). The effect of the
reaction temperature on the epoxidation of cyclohexene was
investigated in a range of 25 to 55 1C (Fig. 2). The conversion
gradually increased with increasing temperature while the
selectivity to epoxide marginally decreased from 94 to 90%.
It was also found that non-catalytic thermal epoxidation takes
place in the presence of molecular oxygen and trimethyl-
acetaldehyde, leading to 11% cyclohexene conversion at
40 1C after 6 h.
Fig. 1 X-Ray powder patterns of (a) IRMOF-3 and (b) IRMOF-3[Mn]
(inset shows the N2 adsorption–desorption isotherms of (a) IRMOF-3
and (b) IRMOF-3[Mn]: ’ adsorption, & desorption).
after post-modification of IRMOF-3, again suggesting that the
structure of the parent material remained intact after functio-
nalization. The surface area (SLangmuir) and pore volume of
IRMOF-3[Mn] were 2115 m2 gÀ1 and 0.78 cm3 gÀ1, respec-
tively, and were reduced relative to the parent IRMOF-3
(SLangmuir = 2878 m2 gÀ1 and pore volume = 1.03 cm3 gÀ1
)
We then tested the catalytic activity of IRMOF-3[Mn] in the
epoxidation of larger molecule cyclooctene using molecular
oxygen and trimethylacetaldehyde. At 40 1C, cyclooctene was
converted to cyclooctene oxide with lower conversion (60%)
and slightly higher selectivity (96%) than cyclohexene
(Table 1) under identical reaction conditions.
as expected due to the added mass of Mn(acetylacetonate) to
IRMOF-3. Similar results have been observed in other MOFs
after post-modification.5b,9 The IRMOF-3[Mn] showed
identical thermal stability to IRMOF-3 (see Fig. S1, ESIw).
FTIR spectra of both IRMOF-3[Mn] and IRMOF-3 show
intense bands in the region 1660–1550 cmÀ1 (see Fig. S2, ESIw)
due to the coexistence of carboxylato and phenyl ring
vibrations.8c The spectrum of the functionalized MOF
exhibited a new band at 1523 cmÀ1, which may be attributed
to vibration of the acetylacetonate anion.10 The 1H NMR
spectrum of the digested IRMOF-3[Mn] (see Fig. S3, ESIw)
shows three new peaks at 2.06, 1.94 and 1.85 ppm due
to –CH3 groups,11 further confirming the presence of acac
and imine-functionalized acac ligand species attached to
IRMOF-3.
Styrene epoxidation was also carried out under the same
reaction conditions as applied for cyclohexene in order to
determine the preference for internal versus external double
bond epoxidation. At 40 1C, styrene was converted to the
corresponding epoxide with 52% conversion and 81%
selectivity. The reactivity of the double bond attached to
benzylic rings showed lower conversion and selectivity to
Table 1 Epoxidation of alkenes over IRMOF-3[Mn] using molecular
oxygena
Further insight into the Mn(II) coordination environment of
the post-modified MOF can be obtained from the XPS
spectra. The XPS Mn2p region of the IRMOF-3[Mn] showed
lower binding energies in comparison with the free Mn(acac)2
complex (see Fig. S4, ESIw). This suggests that a different
coordination environment12 is formed upon binding of Mn to
IRMOF-3, which is due to the Schiff base condensation
reaction between free amine groups of IRMOF-3 and the
carbonyl groups of the manganese(II) complex (Scheme 1).
The comparison of the Zn/Mn ratios estimated by XPS and
ICP (see Tables S1 and S2, ESIw) suggested that Mn
complexes were preferentially bound to the NH2 of IRMOF-3
located at the external surface due to slow diffusion of
the Mn complex than its reaction rate.5a ICP-MS analysis
(see Table S1, ESIw) of IRMOF-3[Mn] revealed that the extent
of binding of the manganese complex led to a final weight
loading of 1.25% Mn in the MOFs, corresponding
to 0.23 mmol gÀ1 loading. An elemental analysis of
Product selectivity (%)
Epoxide
Others
8.0e
Cycle Substrate
Conversion (%)
1st
Cyclohexeneb 67.5
92.0
91.9
92.0
91.9
95.8
95.7
95.8
80.7
80.6
80.7
2nd
3rd
4th
1st
2nd
3rd
1st
67.3
67.4
67.4
60.2
60.3
60.3
52.3
52.1
52.2
Cyclooctenec
Styrened
4.2
19.3f
2nd
3rd
a
Reaction conditions: 1 mmol substrate, 2 mmol trimethylacetaldehyde,
0.015 g catalyst, 5 ml toluene, molecular oxygen (1 atm), temperature
b
40 1C and 6 h. Without catalyst (in the presence of molecular oxygen
and trimethylacetaldehyde) conversion = 10.9%. Conversion =
c
d
9.3%. Conversion = 9.8%. 2-Cyclohexen-ol (0.9%), 2-cyclohexen-
e
f
1-one (3.6%), 1,2-cyclohexenediol (3.5%). Benzaldehyde (6.5%),
benzyl alcohol (12.8%).
c
3638 Chem. Commun., 2011, 47, 3637–3639
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epoxidation of alkenes using molecular oxygen and aldehyde.
The catalyst did not suffer from a leaching problem during
catalysis and could be recycled several times without loss of
activity or selectivity. In addition, the high selectivity, stability,
and resistance to catalyst deactivation of IRMOF-3[Mn]
demonstrated in epoxidation reactions for several important
alkenes provide strong encouragement for future exploration
of functionalized MOFs as useful heterogeneous catalysts.
This work was supported by a National Research Founda-
tion of Korea (NRF) grant funded by the Korean government
(MEST) (No. 2010-0012257).
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epoxide than in the other two cyclic alkenes, cyclohexene and
cyclooctene.
a,
In order to asses the stability of IRMOF-3[Mn], epoxida-
tion reaction was tested by performing repeated reaction cycles
under the same reaction conditions. At the end of each
reaction cycle, the catalyst was recovered by simple
decantation of the solution mixture followed by washing with
a solvent (5 Â 10 ml). After being immersed in the solvent for
12 h and dried at 90 1C under vacuum for 12 h, the catalyst
was reused. As shown in Table 1, conversion and selectivity to
epoxide were almost identical irrespective of the number of
cycles performed. Further confirmation on the stability
of Mn in MOF was made through a hot filtering experiment
(see Fig. S6, ESIw). No manganese in the filtrate after the
reaction was detected by ICP measurement. The X-ray powder
diffraction pattern and FTIR spectra of the solid catalyst after
reuse were also indistinguishable from those of the fresh
catalyst (see Fig. S2 and S7, ESIw), suggesting that no
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The cyclohexene epoxidation was also carried out using free
Mn(acac)2 (0.0002 to 0.0010 mmol) at 40 1C under identical
reaction conditions. The conversion and selectivity to epoxide
were much lower [42% conversion and 66% epoxide selectivity
using 0.0010 mmol Mn(acac)2] than with the IRMOF-3[Mn].
Comparison of the performances by other Mn-containing
catalysts for cyclohexene epoxidation showed that the present
catalyst gave much higher conversion and selectivity to
epoxide than those reported for an alumina-supported
Mn(acac)2 complex.14a The present catalyst also provided
higher or comparable selectivity of epoxide than those
previously reported for a well-known Mn(III)–salen complex
immobilized to a layered double hydroxide or MCM-41.14b,c
In conclusion, a new and simple approach to one-step post-
functionalization of IRMOF-3 with a manganese(II) acetyl-
actonate complex was developed. The structure of IRMOF-3
was retained after post-functionalization. The new material,
IRMOF-3[Mn], showed high activity and selectivity in the
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This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 3637–3639 3639