Activation of molecular oxygen by a metal-organic framework with open 2,2′-bipyridine for selective oxidation of saturated hydrocarbons

Article abstract of DOI:10.1039/c2cc36733e

A metal-organic framework with open 2,2′-bipyridine sites can efficiently activate molecular oxygen for selective oxidation of a variety of saturated hydrocarbons with unprecedented activities and selectivities.

Full text of DOI:10.1039/c2cc36733e

View Online / Journal Homepage
Dynamic Article Links
ChemComm
Cite this: DOI: 10.1039/c2cc36733e
www.rsc.org/chemcomm
COMMUNICATION
Activation of molecular oxygen by a metal–organic framework with open
0
2
,2 -bipyridine for selective oxidation of saturated hydrocarbonsw
a
a
b
a
a
a
Jilan Long, Liming Wang, Xingfa Gao, Cuihua Bai, Huanfeng Jiang and Yingwei Li*
Received 16th September 2012, Accepted 30th October 2012
DOI: 10.1039/c2cc36733e
0
8
A metal–organic framework with open 2,2 -bipyridine sites can
materials, etc. Nevertheless, the attainment of high selectivity
for KA-oil at elevated conversion with a satisfactory rate
remains a great challenge because of the over-oxidation of
the products. To date, all the reported heterogeneous catalysts
perform with KA-oil yields of less than 20% in the liquid-
efficiently activate molecular oxygen for selective oxidation of a
variety of saturated hydrocarbons with unprecedented activities
and selectivities.
2
phase cyclohexane oxidation with O even with the presence of
Selective oxidation of carbon–hydrogen bonds is of crucial
importance in activating raw materials to form valuable
chemicals and intermediates in many important industrial
organic solvents and/or sacrificial promoters (Table S1, ESIw).
Recently, N-substituted porous carbon materials have been
shown as efficient heterogeneous catalysts for cyclohexane aerobic
1
processes. One class of raw materials is saturated alkanes;
8
oxidation. On the other hand, metal–organic frameworks
cyclohexane, for example, can be oxidized to cyclohexanone
and cyclohexanol (KA-oil), which are known as important
intermediates for the bulk production of polyamide and
plastics. However, the selective oxidation of saturated C–H
bonds also represents one of the greatest challenges in catalysis
because of the inertness of the C–H bonds and the selectivity
issues derived from the increasing reactivity of products as
(MOFs) are known as a new class of porous materials that have
exhibited interesting catalytic properties in a number of chemical
9
reactions. By altering the constituent organic linkers, the pore
surface properties of MOFs can be manipulated, affording unique
functionalities for specific catalysis transformations. Here we
1
0
0
report that a MOF (MOF-253) featuring open 2,2 -bipyridyl
2
(
bpy) moieties (Fig. S1, ESIw) can efficiently activate molecular
compared to starting materials. In particular, the use of
3
oxygen for the selective oxidation of sp C–H bonds. Cyclohexane
environmentally benign molecular oxygen as the oxidant in
2
these processes often results in overoxidation. As an example,
conversion as high as 60% was achieved with a good selectivity
(
>80%) for KA-oil within a short reaction time under mild,
the industrial production of KA-oil from aerobic oxidation of
cyclohexane has to be carefully controlled (at conversions
o5% with 80% selectivity for KA-oil) to avoid the formation
solvent-free conditions. The yield of KA-oil (48%) is ca. 10 times
greater than those of the industrial catalysts (with similar selectivity
for KA-oil) under similar conditions. Moreover, the catalyst is
recyclable and shows high compatibility with various substrates
including linear-chain saturated alkanes.
3
of excessive amounts of byproducts by over-oxidation.
2+
2+
Currently, homogeneous metal catalysts such as Co or Mn
complexes are mainly used for the aerobic oxidation of saturated
4
alkanes in the chemical industry. From an economic and environ-
The blank runs in the absence of catalyst gave no activity in the
solvent-free aerobic oxidation of cyclohexane at temperatures up to
mental viewpoint, heterogeneous catalysts are desirable because
often these materials can be readily separated from the reaction
mixture and reused. For the oxidation of cyclohexane with mole-
cular oxygen, a great deal of research efforts have been devoted to
finding a suitable oxidation catalyst, and a number of hetero-
geneous catalysts have been explored in this regard. These include
1
50 1C (Table 1, entry 1), as is also observed in the literature reports
under similar conditions. When MOF-253 was employed as the
catalyst, the reaction proceeded even at a temperature as low as
1
20 1C (entry 2). The conversion increased remarkably with
increasing the temperature, and the reaction furnished a conversion
of ca. 60% with a KA-oil selectivity of >80% at 150 1C (entry 3).
To the best of our knowledge, such a high activity (with a good
selectivity at the same time) in the liquid-phase aerobic oxidation of
cyclohexane has not been reported to date. As shown in Table 1, a
further increase in the temperature did not improve the reaction
significantly (entry 4).
5
metal-incorporated molecular sieves, metal-substituted mole-
6
cular sieves, supported Au nanoparticles, and carbon-based
7
a
The Key Lab of Enhanced Heat Transfer and Energy Conservation,
Ministry of Education, School of Chemistry and Chemical
Engineering, South China University of Technology, Guangzhou
To identify the catalytic sites for aerobic cyclohexane oxidation,
510640, China. E-mail: liyw@scut.edu.cn
b
CAS Key Laboratory for Biomedical Effects of Nanomaterials and
Nanosafety, Institute of High Energy Physics, Chinese Academy of
Sciences, Beijing 100049, China
we examined the efficacy of this transformation in the presence of
0
0
the constituent ligand (i.e., 2,2 -bipyridine-5,5 -dicarboxylic acid,
bpydc) of MOF-253. Interestingly, when the same amount of
H
2
w Electronic supplementary information (ESI) available: Experimental
details, N adsorption data, XRD, IR, additional reaction results, and
proposed reaction mechanism. See DOI: 10.1039/c2cc36733e
H bpydc was employed to replace MOF-253, the reaction also
2
2
proceeded with a good selectivity for KA-oil (Table 1, entry 5),
This journal is c The Royal Society of Chemistry 2012
Chem. Commun.

View Online
a
2
6,11
Table 1 Oxidation of cyclohexane in the absence of solvent with O
been used in the liquid-phase oxidation of cyclohexane.
The
reaction with the addition of p-benzoquinone (a typical radical
scavenger) could demonstrate the involvement of radical
species in our system (Fig. S2, ESIw).
After the initiation period, the conversion increased rapidly
with time up to 4 h (Fig. S2, ESIw). The selectivity for KA-oil,
however, decreased apparently after 2.5 h due to the formation
of byproducts. Nevertheless, a KA-oil selectivity as high as 92%
could still be obtained at 46% conversion after 3 hours. It is
noteworthy that in the literature reports on liquid-phase cyclo-
hexane aerobic oxidation the conversion had to be controlled at
around 20% or even lower in order to achieve a KA-oil
c
Sel. (%)
7a,d,8b
Yield of
selectivity of >90%.
It has been reported that the product cyclohexanone could
b
Entry Catalyst
T (1C) Con. (%) CQO OH KA-oil (%)
8
a,12
1
2
3
4
5
6
7
8
9
1
1
—
150
o0.1
6.6
—
—
36.3
32.1 48.0
31.3 48.2
34.0 15.3
34.7 14.5
34.5
28.1 10.0
27.0 11.1
—
6.2
also catalyze the initiation of the oxidation process.
In our
MOF-253 120
MOF-253 150
MOF-253 160
57.8
48.1
47.9
53.2
49.8
57.9
55.5
53.5
—
case, we attribute the initiation of cyclohexane oxidation to the
catalytic role of bpy moieties because no radical initiators were
added in our system. We propose that the bpy moieties might
play a crucial role in the activation of molecular oxygen to
59.9
60.8
17.6
17.2
8.4
12.0
13.8
o0.1
o0.1
H
H
2
bpydc
bpydc
150
150
d
e
2
Bipyridine 150
Me bpydc 150
Phen
Biphenyl
Pyridine
7.8
ꢀ
ꢁ
form a superoxide radical anion ( O ), which further reacts
2
2
with cyclohexane to produce cyclohexyl hydroperoxide. To
test this suggestion, we pre-heated a mixture of MOF-253 in
acetonitrile. After 0.5 h, cyclohexane was injected to the
mixture. No apparent initiation period was observed and
the conversion increased rapidly to 30% (vs. 3% without the
pre-activation process) after 60 min (Fig. S2, ESIw). This result
strongly suggests the important role of MOF-253 in the
initiation of the oxidation reaction via oxygen activation.
Recently, we have shown that MOF-253 is highly efficient for
the direct arylation of unactivated arenes with aryl iodides/
bromides, in which aryl radical anions have been suggested as
150
150
150
0
1
—
—
—
—
—
a
Reaction conditions: cyclohexane (20 mL), catalyst (0.09 mol%), O
b
1.0 MPa), 4 h. Conversions were determined by GC-MS analysis
2
(
c
with anisole as an internal standard. CQO indicates cyclohexanone,
and OH denotes cyclohexanol. Al(acac) (0.09 mol%) was added.
3
d
e
0
0
5
,5 -bis(methoxycarbonyl)-2,2 -bipyridine.
although the conversion was remarkably reduced. It should be
3
+
mentioned that the constituent metal ion (i.e. Al ) in the MOF
1
0
3+
structure is fully coordinated. Thus, the Al ion in MOF-253
13
the key intermediates. MOF-253 exhibits remarkably higher
should not act as a catalytic site for the oxidation. In order to rule
reactivity than the H bpydc and bpy ligands in the arylations, as
2
3+
out the contribution of Al to the reactivity, a small amount of
also observed in this study on the cyclohexane oxidation. To gain a
Al(acac) was added with H bpydc in the reaction. The introduc-
2
fundamental understanding of the catalytic oxidation mechanism,
we performed a theoretical calculation at the DFT level of B3LYP/
6-31G(d,p), obtaining the HOMO and LUMO energies. Because
of the huge molecular structure of MOF-253, it is impossible to
include all the units for the calculations. Here, the smallest
representative structures with one, two, or three bpydc linkers
(denoted as monomer, dimer, and trimer, respectively) were calcu-
lated (Fig. 1). As shown in Table 2, the HOMO–LUMO (H–L)
gaps of the organic molecules decreased in the order: pyridine >
biphenyl > bipyridine > phen > monomer (i.e. bypdc). However,
the reactivities in cyclohexane oxidation followed a totally reverse
order (see Table 1). MOF-253 should have the lowest H–L gap
among the investigated catalysts as the calculations indicated that
the dimer and trimer have even lower H–L gaps as compared to the
3
3+
tion of Al did not affect the outcome obviously (entry 6). These
results suggest that the open bpy moiety in MOF-253 might be the
catalytic center for the oxidation transformation.
To further confirm this finding, we tested the reactivities of
0
some other bpy derivatives, e.g. 2,2 -bipyridine and 1,10-
phenanthroline (phen), in the reaction. As expected, these
molecules were also active for the aerobic oxidation of cyclo-
hexane under the investigated conditions (Table 1, entries
7
–9). However, the use of biphenyl or pyridine as the catalyst
did not give any products in the reaction (entries 10 and 11).
In another set of experiments, we investigated the effect of
possible metal impurities on the oxidation, because a radical
reaction could be induced by traces of metals in the reaction
system. The results (Tables S2 and S3, ESIw) revealed that
metal complexes were not involved (or at least not playing an
important role) in this oxidation system.
The reaction profile over an initial 12-hour reaction period
was investigated using MOF-253 as the catalyst (Fig. S2,
ESIw). The oxidation of cyclohexane clearly exhibited an
initiation period of ca. 20 min in which almost no conversion
was observed. Thereafter, the conversion increased gradually
to reach 3% after 1 h. The presence of such an induction time
was normally observed in previous studies, and to accelerate
the radical-involved oxidation process a radical initiator has
Fig. 1 The representative structures of MOF-253: dimer (left) and
trimer (right), fully optimized at the B3LYP/6-31G(d,p) level of
theory. Colour coding: Al, pink; C, grey; O, red; N, blue; H, white.
Chem. Commun.
This journal is c The Royal Society of Chemistry 2012

View Online
Table 2 HOMO and LUMO energies, and HOMO–LUMO gaps for
the catalysts, calculated at the (U)B3LYP/6-311+G(d,p)//B3LYP/
MOF-253 as the catalyst, furnishing the corresponding
ketones as main products (entry 5). Tertiary C–H bonds can
be oxidized to the corresponding alcohols (entry 6). Most
interestingly, MOF-253 was also active in the oxidation of
primary C–H bonds (e.g. toluene) with >99% selectivity for
benzaldehyde (entry 7), further demonstrating the general
applicability of this catalytic system for the selective oxidation
of saturated C–H bonds.
6-31G(d,p) level of theory (units in eV)
Species
HOMO
LUMO
H/L gap
Pyridine
Biphenyl
Bipyridine
Phen
Monomer
Dimer
Trimer
ꢁ6.87
ꢁ6.06
ꢁ6.37
ꢁ6.26
ꢁ7.39
ꢁ7.59
ꢁ7.58
ꢁ0.63
ꢁ0.69
ꢁ1.34
ꢁ1.43
ꢁ3.08
ꢁ3.47
ꢁ3.60
6.24
5.37
5.03
4.83
4.31
4.12
3.98
In conclusion, we have disclosed a novel catalytic system in
the presence of a MOF lined with bpy for the selective
oxidation of saturated (including primary, secondary, and
tertiary) C–H bonds with dioxygen. The DFT calculations
suggest that the H–L gap of bpy (or a molecule bearing the
bpy) could be significantly reduced by assembling the molecule
into a MOF structure, leading to remarkably enhanced
oxidation activity. The present methodology could also be
a
Table 3 Selective oxidation of various alkanes
Entry Substrate
1
Con (%) Selectivity of main products (%)
70.8
2
applied to the oxidation of sp and sp C–H bonds (actually we
have obtained some preliminary results in such types of
oxidation, Fig. S8, ESIw) which are relatively active compared
to sp C–H bonds.
2
3
68.9
3
This work was supported by the NSFC (20803024,
2
0936001, 21073065), NSF of Guangdong Province
46.8
38.3
11.6
(
(
(
S2011020002397, 10351064101000000), and the FRFCU
2011ZG0009). XG was supported by the MOST 973 program
2012CB934001).
b
4
b
5
Notes and references
1
R. A. Sheldon and J. K. Kochi, Metal-Catalyzed Oxidations of
Organic Compounds, Academic Press, New York, 1981.
2 R. Bergman, Nature, 2007, 446, 391.
b
6
40.3
10.0
3
K. Weissermel and H. J. Horpe, Industrial Organic Chemistry,
VCH Press, Weinheim, 2nd ed., 1993.
4
S. Ciborowski, Dioxygen activation and homogenous catalytic
oxidation, Stud. Surf. Sci. Catal., ed. L. I. Simandi, Elsevier,
Amsterdam, 1991, vol. 66.
7
5
6
J. Li, Y. Shi, L. Xu and G. Lu, Ind. Eng. Chem. Res., 2010,
a
Substrate (10 mmol), MOF-253 (0.18 mol%), CH
b
3
CN (1 mL),
4
9, 5392.
(a) R. Raja, G. Sankar and J. M. Thomas, J. Am. Chem. Soc.,
999, 121, 11926; (b) H. X. Yuan, Q. H. Xia, H. J. Zhan, X. H. Lu
1
50 1C, O (1.0 MPa), 4 h. MOF-253 (0.7 mol%), CH CN (10 mL).
2 3
1
and K. X. Su, Appl. Catal., A, 2006, 304, 178.
monomer, and the H–L gap seemed to decrease gradually with
increasing the number of bypdc ligands in the structure. These
results strongly suggest that electron excitation has played an
important role in determining the reactivity of the catalysts. The
catalyst with a low H–L gap may facilitate the excitation of
electrons from HOMO to LUMO, where the excited electrons
7
(a) R. Zhao, D. Ji, G. Lv, G. Qian, L. Yan, X. Wang and J. Suo,
Chem. Commun., 2004, 904; (b) L. Li, C. Jin, X. Wang, W. Ji,
Y. Pan, T. V. D. Knaap, R. V. D. Stoel and C. Au, Catal. Lett.,
2009, 129, 303; (c) L. Xu, C. He, M. Zhu and S. Fang, Catal. Lett.,
2
007, 114, 202; (d) P. Wu, P. Bai, Z. Lei, K. P. Loh and X. Zhao,
Microporous Mesoporous Mater., 2011, 141, 222; (e) B. P. C.
Hereijgers and B. M. Weckhuysen, J. Catal., 2010, 270, 16.
8 (a) H. Yu, F. Peng, J. Tan, X. Hu, H. Wang, J. Yang and
W. Zheng, Angew. Chem., Int. Ed., 2011, 50, 3978; (b) X. H. Li,
J. S. Chen, X. Wang, J. Sun and M. Antonietti, J. Am. Chem. Soc.,
ꢀ
ꢁ
could reduce molecular oxygen to form O
ꢁ
2
(Fig. S6, ESIw). The
as-formed O2 on the surface of MOF-253 could then react
ꢀ
with adsorbed cyclohexane to produce cyclohexyl hydroperoxide
2
011, 133, 8074.
A. Corma, H. Garcı
110, 4606.
(Fig. S7, ESIw), which could react further to form cyclohexanol and
9
´
a and F. X. Xamena, Chem. Rev., 2010,
cyclohexanone.
1
0 E. Bloch, D. Britt, C. Lee, C. Doonan, F. Uribe-Romo,
H. Furukawa, J. R. Long and O. M. Yaghi, J. Am. Chem. Soc.,
The scope of saturated C–H bonds suitable for aerobic
oxidation catalyzed by this novel catalytic system was investi-
gated. MOF-253 could oxidize various secondary C–H bonds
to the corresponding ketones with excellent selectivities at
moderate to high conversions (Table 3, entries 1–4). n-Hexane
2010, 132, 14382.
1 G. J. Hutchings, S. Carrettin, P. Landon, J. K. Edwards,
D. Enache, D. W. Knight, Y. J. Xu and A. F. Carley, Top. Catal.,
1
2
006, 38, 223.
2 I. Hermans, P. Jacobs and J. Peeters, Chem.–Eur. J., 2006,
2, 4229.
3 H. Liu, B. Yin, Z. Gao, Y. W. Li and H. F. Jiang, Chem. Commun.,
2012, 48, 2033.
1
1
has the same secondary CH
2
groups, and has been reported
1
9b
difficult to oxidise. In the present study, we found that
n-hexane could also be efficiently activated with O
2
using
This journal is c The Royal Society of Chemistry 2012
Chem. Commun.