Mixed-Metal Strategy on Metal-Organic Frameworks (MOFs) for Functionalities Expansion: Co Substitution Induces Aerobic Oxidation of Cyclohexene over Inactive Ni-MOF-74

Article abstract of DOI:10.1021/acs.inorgchem.5b01278

Different amounts of Co-substituted Ni-MOF-74 have been prepared via a post-synthetic metal exchange. Inductively coupled plasma mass spectrometry, powder X-ray diffraction (XRD), N₂ adsorption/desorption, and extended X-ray absorption fine structure (EXAFS) analyses indicated the successful metathesis between Co and Ni in Ni-MOF-74 to form the solid-solution-like mixed-metal Co/Ni-MOF-74. It was found that introduction of active Co into the Ni-MOF-74 framework enabled the inert Ni-MOF-74 to show activity for cyclohexene oxidation. Since Co was favorably substituted at positions more accessible to the substrate, the mixed-metal Co/Ni-MOF-74 showed superior catalytic performance, compared with pure Co-MOF-74 containing a similar amount of Co. This study provides a facile method to develop solid-solution-like MOFs for heterogeneous catalysis and highlights the great potential of this mixed-metal strategy in the development of MOFs with specific endowed functionalities. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.inorgchem.5b01278

Article
pubs.acs.org/IC
Mixed-Metal Strategy on Metal−Organic Frameworks (MOFs) for
Functionalities Expansion: Co Substitution Induces Aerobic
Oxidation of Cyclohexene over Inactive Ni-MOF-74
Dengrong Sun, Fangxiang Sun, Xiaoyu Deng, and Zhaohui Li*
Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou,
350002, People’s Republic of China
ABSTRACT: Different amounts of Co-substituted Ni-MOF-74 have been prepared
via a post-synthetic metal exchange. Inductively coupled plasma mass spectrometry,
powder X-ray diffraction (XRD), N adsorption/desorption, and extended X-ray
2
absorption fine structure (EXAFS) analyses indicated the successful metathesis
between Co and Ni in Ni-MOF-74 to form the solid-solution-like mixed-metal Co/Ni-
MOF-74. It was found that introduction of active Co into the Ni-MOF-74 framework
enabled the inert Ni-MOF-74 to show activity for cyclohexene oxidation. Since Co was
favorably substituted at positions more accessible to the substrate, the mixed-metal Co/
Ni-MOF-74 showed superior catalytic performance, compared with pure Co-MOF-74
containing a similar amount of Co. This study provides a facile method to develop
solid-solution-like MOFs for heterogeneous catalysis and highlights the great potential
of this mixed-metal strategy in the development of MOFs with specific endowed
functionalities.
INTRODUCTION
mixed-metal approach on MOFs provides great opportunities
■
to obtain MOFs with specific functionalities. However, studies
on the mixed-metal effect on the properties of these solid-
solution-like MOFs remained largely unexplored, especially on
the field of heterogeneous catalysis, which is believed to be one
of the most promising applications of MOF materials.
Metal−organic frameworks (MOFs) are a class of crystalline
micromesoporous hybrid materials constructed from metal or
metal clusters interconnected by organic linkers. Their
inherent high specific surface areas, and uniform but tunable
1
cavities, make them to show potentials in a variety of
2
M-MOF-74 is a three-dimensional honeycomb-like network
applications. The structural complexity of MOFs and their
constructed from divalent metal ions coordinated to 2,5-
adjustable characters, because of the versatile coordination
chemistry of the metal cations and the availability of different
organic linkers, enable MOFs to be an appealing platform to
integrate different functional building blocks into one MOF
framework for multifunctional applications. The recently
developed mixed-metal approach over MOFs allows for
incorporation of two different metals into a same framework
7
dihydroxyterephthalic acid (H DOBDC). Our previous work
4
has shown that Co-MOF-74 is active for cyclohexene oxidation8
while the isostructural Ni-MOF-74 is inactive for this reaction.
2+
Herein, we reported that the substitution of Ni in Ni-MOF-
2+
7
4 framework by active Co via a post-synthetic metal
exchange method can enable the inert Ni-MOF-74 to show
catalytic activity for cyclohexene oxidation. The performance
over the Co-substituted Ni-MOF-74 (Co/Ni-MOF-74) in-
3
to form solid-solution-like mixed-metal MOFs, which offers
MOFs with an additional degree of structural complexity and is
expected to endow them with new functionalities, since their
properties are dependent on the incorporated metal atoms.
Actually, some pioneering studies have already shown that the
mixed-metal MOFs did exhibit properties superior to their
parent MOFs and, in some cases, can even induce new
functionalities, because of the introduction of the second metal
ions. For example, Hill and co-workers have reported that
2+
creased with the amount of incorporated Co , indicating that
2+
the incorporated Co is responsible for the reaction. It is
interesting to determine that the mixed-metal Co/Ni-MOF-74
showed superior catalytic performance, compared with pure
2+
2+
Co-MOF-74 containing a similar amount of Co , because Co
was favorably substituted at positions more accessible to the
substrates.
substitution of Zr with Ti in NH −Uio-66(Zr) can improve its
2
4
EXPERIMENTAL SECTION
adsorption capacity toward CO . Our previous work showed
■
2
that the Ti moiety in the mixed-metal NH −Uio-66(Zr/Ti) can
Materials. 2,5-Dihydroxyterephthalic acid (H DOBDC) was
4
obtained from Alfa Aesar Co. Cyclohexene (GR, Aladdin Reagent
2
act as an electron mediator to facilitate the linker-to-metal
charge transfer, which resulted in improved photocatalytic
Co), cobaltous nitrate (Co(NO ) ·6H O), nickel nitrate (Ni(NO ) ·
3
2
2
3 2
5
performance. The introduction of reduced-metal cations such
as Ti3 and Fe into MOF-5 framework enabled redox
+
2+
Received: June 6, 2015
6
reactivity to inert MOF-5. These results suggest that the
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b01278
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Inorganic Chemistry
Article
Figure 1. (a) XRD patterns of the as-prepared Ni-MOF-74, Co/Ni-MOF-74-4, and Co-MOF-74 (inset shows enlarged regions of the XRD
patterns); (b) N adsorption/desorption isotherms of prepared Ni-MOF-74 and Co/Ni-MOF-74-4 (77 K).
2
6
H O), dimethylformamide (DMF), and ethanol (EtOH) were all
2
RESULTS AND DISCUSSION
■
used without further purification.
Ni-MOF-74 was prepared following the procedures reported
Synthesis of Ni-MOF-74 and Co-MOF-74. Ni-MOF-74 was
8
8
previously. The similar XRD patterns and comparable
prepared following the previous reported procedures. A solid mixture
of H DOBDC (0.2216 g, 1.12 mmol) and Ni(NO ) ·6H O (1.077 g,
3
2
−1
Langmuir specific surface area (860 m g ) of the as-prepared
Ni-MOF-74, compared with that previously reported, indicated
that Ni-MOF-74 with high quality has been obtained (Figure
4
3
2
2
.70 mmol) were added to a 75 mL solution of DMF, EtOH and H O
2
in 1:1:1 ratio. The above mixture was stirred at room temperature to
get a homogeneous solution and was transferred into a 100 mL Teflon
liner and heated at 120 °C for 24 h. After reaction, the solids were
separated from the solvent via centrifugation, and then washed with
fresh DMF and methanol. After that, the sample was treated under
vacuum at 180 °C for 5 h to obtain the final Ni-MOF-74. A similar
procedure was applied in the preparation of Co-MOF-74, except that
Ni(NO ) ·6H O was replaced with Co(NO ) ·6H O.
7
,9
1). Co-substituted Ni-MOF-74 (denoted as Co/Ni-MOF-74-
x, where x represents the incubation time) was prepared by
exposing Ni-MOF-74 to a DMF solution containing Co-
(
NO ) ·6H O for different times. The exchange process
3 2 2
proceeded slowly at room temperature, but the rate accelerated
when the temperature was increased to 80 °C, as evidenced
from a gradual color change of the solid from yellow-green to
brown (Figure 2, inset). In the meantime, the pink solution
3
2
2
3
2
2
Synthesis of Co/Ni-MOF-74. Co(NO ) ·6H O (0.118 g) was
3
2
2
dissolved in DMF (5 mL). Ni-MOF-74 (0.1 g) was dispersed in the
above solution and stirred for 30 min. After that, the obtained
suspension was sealed and incubated at 80 °C for different times to
obtained different amounts of Co-substituted Ni-MOF-74. After
reaction, the solids were separated from the solvent via centrifugation,
washed with fresh DMF and methanol, extracted by Soxhlet extractor
with deionized water overnight, and dried to obtain Co/Ni-MOF-74.
Characterizations. X-ray diffraction (XRD) patterns were
collected on a Model D8 Avance X-ray diffractometer (Bruker,
Germany) with Cu Kα radiation. Ultraviolet−visible (UV-vis) diffuse
reflectance spectra (UV-DRS) of the powders were obtained for the
dry-pressed disk samples using a Cary 500 Scan Spectrophotometer
(
Varian, USA). BaSO was used as a reflectance standard in the UV-
4
DRS experiment. Brunauer−Emmett−Teller (BET) surface area
Figure 2. (□) Amount of Co in Ni-MOF-74 and (▲) amount of Ni in
the solution determined by ICP-MS analyses after incubation for
different times; inset shows photographs of the solid and solution after
exchanging for different times. The reaction was carried out in DMF at
analyses were carried out on a Model ASAP2020 M apparatus
(
Micromeritics Instrument Corp., USA). For BET surface area
analyses, the samples were degassed under vacuum at 200 °C for 6
h and then measured at 77 K. Inductively coupled plasma mass
spectrometry (ICP-MS) analysis was performed on a XSERIES 2
system (ThermoFisher). Before the ICP-MS experiment, the solid
sample was digested in a mixture of HNO3 and Milli-Q water
8
0 °C for different times.
gradually faded with the reaction time, suggesting a metathesis
(
Millipore Corp., Bedford, MA) and the exchanged-out Ni moiety in
2+
2+
between the Co cations in solution and the Ni cations in the
MOF framework. ICP-MS analyses indicated that there was
∼4.7% of Co in the solid after 1 day incubation, while a
comparable amount of Ni (3.7%) was detected in the resultant
the solution was collected from the first residual solution. X-ray
absorption fine structure (XAFS) spectra were collected in Shanghai
Synchrotron Radiation Facility (SSRF) with a Si(111) double-crystal
monochromator in transmission mode. The source of BL14W1 is a 38-
pole wiggler device with a maximum magnetic field of 1.2 T and a
magnet period of 80 mm. The storage ring was operated at 3.5 GeV
with injection currents of 100 mA.
Catalytic Reaction. The oxidation of cyclohexene was carried out
in a glass reactor that was equipped with a reflux condenser and an
oxygen balloon. In a typical reaction, the catalyst (20 mg, 0.4 mmol)
2
+
solution, which was confirmation that part of the Ni cations in
Ni-MOF-74 had been substituted by Co2+ cations in the
solution (Figure 2). The exchange degree can be improved by
appropriately extending the incubation time. The amount of Co
substituted in the solid increased with the incubation time and
reached saturation with ca. 61% of Co incorporated after 4 days
of reaction. A prolonged reaction time (8 and 12 days) resulted
in only a slightly fluctuated incorporated amount of Co (59% in
Co/Ni-MOF-74-8 and 60% for Co/Ni-MOF-74-12). This
and cyclohexene (2 mL, 19.7 mmol) were heated at 80 °C in O under
2
atmospheric pressure. The products were analyzed on a Shimadzu gas
chromatograph that was equipped with a flame ionization detection
(
FID) device and a RTX-5 capillary column. Cyclohexene hydro-
2+
peroxide was treated with triphenylphosphine (PPh ) prior to the
indicated that not all of the Ni cations in Ni-MOF-74 can be
3
analyses.
replaced by Co2+ cations, probably because of the presence of
B
DOI: 10.1021/acs.inorgchem.5b01278
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
diffusional limitations in the honeycomb-like network of Ni-
corresponding to other Co species excluded the presence of
adsorbed Co or Co-based impurities. These results suggested
that Co/Ni-MOF-74 with different amounts of Co incorpo-
rated can be facilely obtained via a post-synthetic metal
exchange under mild conditions.
2
+
MOF-74, which restricts the access of Co cations in solution
2+
to Ni cations in the framework. The good agreement between
the XRD patterns of Co/Ni-MOF-74-4 and Ni-MOF-74
suggested that the framework of Ni-MOF-74 was retained
after the metalation process (Figure 1a). However, the peak
with 2θ value at ∼6.8° observed over Ni-MOF-74 shifted to
lower value after Co incorporation (Figure 1a, inset). Such a
Our previous study has revealed that the cycling of Co
between Co2+ and Co is essential for the oxidation of
3+
3
+
cyclohexene over Co-MOF-74, while the difficulty to form Ni
2+
8
shift is probably due to the substitution of smaller Ni (0.690
in Ni-MOF-74 makes it inert for this reaction. To study the
effect of substituted Co in the mixed Co/Ni-MOF-74 for this
Å) by larger Co2 (0.745 Å), which leads to the expanding of
+
2+
1
0
the crystal lattice. The absence of amorphous impurity in the
pores was confirmed by the nitrogen adsorption−desorption
measurement performed at 77 K (Figure 1b). Similar with Ni-
MOF-74, Co/Ni-MOF-74-4 also exhibited a Type I N2
isotherm with a high Langmuir specific surface area of 1085
reaction, the catalytic performance of the as-prepared Co/Ni-
MOF-74 for cyclohexene oxidation was investigated. As shown
in Table 2 (entries 1 and 2), although Ni-MOF-74 is inactive
for cyclohexene oxidation, because of the auto-oxidization of
cyclohexene, ∼37.2% of cyclohexene was converted over Ni-
MOF-74 after reacting for 20 h. This value is comparable to
that observed in the system without any catalyst (37.0%). The
detectable products from the auto-oxidization of cyclohexene
are cyclohexene oxide (A), 2-cyclohexen-1-ol (B), 2-cyclo-
hexen-1-one (C), and cyclohexene hydroperoxide (D); among
them, D, as the main product, had a yield of 18.7%. When Co/
Ni-MOF-74-1 (with 4.7% Co) was used as the catalyst,
although the conversion of the cyclohexene only increased
slightly from the original 37.0% to 39.2%, the products
distribution changed significantly (Table 2, entry 3). The
yield to D decreased from 18.7% over Ni-MOF-74 to 6.6% over
Co/Ni-MOF-74-1, with a concomitant increase of the yield to
B and C from the original 6.5% and 8.3% to 9.9% and 20.8%,
respectively. These observations suggest that the incorporation
of Co2+ in the framework facilitates the catalytic transformation
of D to B and C. The catalytic performance can be enhanced by
increasing the amount of Co incorporated in the Ni-MOF-74
2
−1
m
g . These observations clearly suggested that the
2
+
2+
incorporated Co cations has successfully replaced Ni
cations in the framework of Ni-MOF-74, instead of residing
in the pores. Since the atomic weight of Co (58.9 g mol ) is
comparable to that of Ni (58.7 g mol ), the higher specific
surface area of Co/Ni-MOF-74−4 is probably due to the larger
size of Co , compared with Ni .
−1
−1
2+
2+ 11,12
Actually, a similar
improvement in the specific surface area was also reported over
Co-substituted Zn-MOF-74, which showed ∼32% higher
12
Langmuir specific surface area than pure Zn-MOF-74. The
successful substitution of Ni by Co was also supported by
extended X-ray absorption fine structure (EXAFS) analyses
3
(
see Figure 3, and Table 1). The k -weighted Fourier
2
+
framework. For Co/Ni-MOF-74-4, in which 61% of Co was
incorporated, the conversion of cyclohexene reached 54.7%
under similar conditions and almost all D has been converted
to B (20.7%) and C (30.4%), showing a high total selectivity of
9
3.4% to B and C (Table 2, entry 4). After reacting for 20h,
Co/Ni-MOF-74-4 was removed from the reaction system and
the reaction continued for another 5 h. It was found that the
conversion of the cyclohexene only slightly increased from
3
Figure 3. Comparison of the k -weighted Fourier transforms between
5
4.7% to 55.7%, and most of the converted cyclohexene has
the Ni K-edge of Ni-MOF-74 and Co K-edge of Co/Ni-MOF-74-4.
been transformed to product D (Table 2, entry 5). This
phenomenon is similar to that observed in the system without
catalyst, indicating the heterogeneous nature of cyclohexene
oxidation over Co/Ni-MOF-74-4. Co/Ni-MOF-74-4 is also
stable during the catalytic reaction, as confirmed from the
similar XRD patterns between the fresh and used Co/Ni-MOF-
Table 1. EXAFS Fit Parameters for the Ni-MOF-74 and Co/
Ni-MOF-74-4
bond
coordination
number, CN
length, R Debye−Waller
goodness-of-fit
2
2
path
(Å)
factor, σ (Å ) index, Rfactor (%)
7
4-4 (Figure 4). The cycling results also showed that there was
Fit of Pristine Ni-MOF-74 (K Range = 3.74−12.51, R Range = 0.88−3.12)
no obvious loss of the catalytic activity over Co/Ni-MOF-74-4
after three reaction runs, which is more evidence that the
structure of the catalyst was well preserved during the reaction
Ni−O
Ni−Ni
5.59
2.04
0.0060
0
.09
2.40
3.02
0.0082
Fit of Co/Ni-MOF-74-4 (K Range = 3.79−10.32, R Range = 1.00−3.22)
(
Figure 5).
Co−O
Co−Ni
6.61
2.04
0.0096
0
.41
The above observations clearly suggest a strong correlation
2.16
3.04
0.0090
between the catalytic activity and the incorporated Co, which
indicates the important role of the substituted Co2+ in Co/Ni-
MOF-74 for the oxidation of cyclohexene. It is generally known
that paramagnetic O2 is favorable for the reaction with
transforms of the Ni K-edge EXAFS spectrum of Ni-MOF-74
revealed that Ni in Ni-MOF-74 was six-coordinated by O with
an average Ni−O bonds of 2.04 Å, in agreement with that in
paramagnetic metal ion and the activation of O by Co-
2
7
the previous report. A similar 6-fold coordination environment
containing homogeneous complexes has been well-docu-
13
around Co was also observed in Co/Ni-MOF-74-4, with an
average Co−O bond length of 2.04 Å, which is a reasonable
value. These indicated that Co has been successfully substituted
into the Ni-MOF-74 framework. A lack of observation of peaks
mented. Similar to that in homogeneous Co-containing
2+
complexes, O can also bind to the Co sites in Co/Ni-MOF-
2
74 to form O adduct due to the presence of open metal sites in
2
the MOF framework. Such Co−O adduct can react with
2
C
DOI: 10.1021/acs.inorgchem.5b01278
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Inorganic Chemistry
Article
a
Table 2. Aerobic Oxidation of Cyclohexene under Different Conditions
Yield (%)
entry
catalyst
incorporated amount of Co (%)
conversion (%)
A
B
C
D
1
2
3
4
37.0
37.2
39.2
54.7
55.7
3.5
3.5
1.9
3.0
3.2
6.5
6.5
8.3
8.4
18.7
18.8
6.6
Ni-MOF-74
0
Co/Ni-MOF-74-1
Co/Ni-MOF-74-4
4.7
61.0
9.9
20.8
30.4
30.0
20.7
21.0
0.6
b
5
1.5
a
b
Reaction conditions: 20 mg of catalyst, 2 mL cyclohexene, O balloon, 80 °C. After Co/Ni-MOF-74-4 was removed, the filtrate continued reacting
2
for another 5 h.
and C improved with the incorporated Co amount in Co/Ni-
MOF-74.
Although the role played by the substituted Co2+ in Co/Ni-
MOF-74 is similar to those in pure Co-MOF-74 reported in
8
our previous work, it is interesting to find that Co/Ni-MOF-74
exhibited higher catalytic performance for the oxidation of
cyclohexene than pure Co-MOF-74 which contains similar
amount of Co. As shown in Table 3, the conversion of
cyclohexene in 20 h over Co-MOF-74 was 50.0%, among which
1
5.4% was converted to B and 26.3% was converted to C,
showing a total selectivity of 83.4% to B and C. However, the
reaction performed over Co/Ni-MOF-74-12 with an equal
amount of Co showed a higher conversion of cyclohexene
Figure 4. XRD patterns of Co/Ni-MOF-74-4 before and after aerobic
cyclohexene oxidation.
(
(
54.2%) and almost all the product D has been converted to B
20.4%) and C (30.1%) with a total selectivity of 93.2% to B
and C under similar condition. Since Co/Ni-MOF-74 and Co-
MOF-74 is isostructural and comparable amount of Co was
used in these two catalytic systems, it is believed that, in
addition to the amount of Co, other factors related to the
substituted Co are responsible for their different catalytic
2
+
performance. The observation that only part of Ni in Ni-
MOF-74 can be substituted by Co2+ implied that the positions
where the metathesis occurs are more accessible to the
substrate than those unsubstituted sites. Therefore, the
substituted Co2 in Co/Ni-MOF-74 is more effective than
the overall Co2+ in pure Co-MOF-74, leading to a higher
catalytic performance of Co/Ni-MOF-74, compared with pure
Co-MOF-74 containing a similar amount of Co.
Figure 5. Cycling test on Co/MOF-74-4 for the oxidation of
cyclohexene.
+
3
+
cyclohexene to give the hydroperoxo complex (Co −OOH),
accompanied by the formation of cyclohexenyl radicals. The
3
+
fission of Co−O bond in Co −OOH would recover the Co/
•
Ni-MOF-74 and liberate the peroxy radical (HOO ), which can
CONCLUSIONS
react with the cyclohexenyl radicals to give the intermediate D
or react with the double bond to give A. Since allylic oxidation
of cyclohexene is much easier than its epoxidation, the
formation of D was observed as the main reaction, while
oxidation of cyclohexene to give A was a minor reaction in the
above-mentioned catalytic systems. The as-formed D can be
further transformed to form B and C via the Haber−Weiss
3+ 8,14
■
In summary, partial substitution of Ni2+ by Co2+ via a post-
synthetic metal exchange enabled the inert Ni-MOF-74 to show
catalytic activity for cyclohexene oxidation. The mixed-metal
Co/Ni-MOF-74 showed superior catalytic performance,
compared with pure Co-MOF-74 containing a similar amount
2+
2+
of Co , because Co was favorably substituted at positions
more accessible to the substrates. This study provides a facile
method to develop solid-solution-like MOFs for heterogeneous
2
+
process involving the cycling of Co between Co and Co ,
which was consistent with the observation that the yield to B
Table 3. Comparison of the Catalytic Performance for Cyclohexene Oxidation between Co-MOF-74 and Co/Ni-MOF-74-12
Yield (%)
catalyst
amount of catalyst used (mg)
conversion (%)
A
B
C
D
Co-MOF-74
Co/Ni-MOF-74-12 (60% of Co incorporated)
12
20
50.0
54.2
3.7
3.0
15.4
20.4
26.3
30.1
4.6
0.7
D
DOI: 10.1021/acs.inorgchem.5b01278
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Inorganic Chemistry
Article
catalysis and highlights the great potential of this mixed-metal
strategy in the development of MOFs with specific endowed
functionalities.
(10) Shannon, R. D. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr.,
Theor. Gen. Crystallogr. 1976, A32, 751−767.
(
11) Caskey, S. R.; Wong-Foy, A. G.; Matzger, A. J. J. Am. Chem. Soc.
2
008, 130, 10870−10871.
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(
́ ́
chez-Sanchez, M.; Orcajo, M. G. Int.
AUTHOR INFORMATION
Corresponding Author
E-mail: zhaohuili1969@yahoo.com.
■
́
́
*
(
1
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful to the Shanghai Synchrotron Radiation Facility
SSRF) of China for the XAFS spectra measurements at the
■
(
BL14W1 beamline. This work was supported by the 973
Program (No. 2014CB239303), the NSFC (No. 21273035),
the Specialized Research Fund for the Doctoral Program of
Higher Education (No. 20123514110002), and the Independ-
ent Research Project of State Key Laboratory of Photocatalysis
on Energy and Environment (No. 2014A03). Z.L. thanks the
Award Program for Minjiang Scholar Professorship for financial
support.
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DOI: 10.1021/acs.inorgchem.5b01278
Inorg. Chem. XXXX, XXX, XXX−XXX