S. Kim et al. / Inorganic Chemistry Communications 31 (2013) 29–32
31
1
00
Although Mn(III) and Fe(III)-porphyrin derivatives are often used
to mimic the enzymes [11], the degradation of catalyst is observed
during the homogeneous catalytic conditions due to ligand oxidation
or μ-oxo dimer formation [12], and many efforts are focused to immo-
bilize and site-isolate the catalysts on either organic polymers and in-
organic supports in order to prevent the degradations [13]. Recently,
the incorporation of active catalysts into MOF materials has become
one of the most emerging fields in current chemistry [14].
a
b
80
6
4
0
0
In this regard, the highly porous hexagonal plates and rods with
readily accessible Mn(III)-porphyrin sites have been evaluated for
their catalytic abilities towards epoxidation of various olefins with
20
0
2
-(tert-butylsulfonyl)iodosylbenzene as oxidant (Table 1). For the ini-
tial studies on the catalyst stability, we have compared styrene epoxida-
tion with hexagon-faced plate and its homogeneous counterpart,
Mn(III)-porphyrin bis-carboxyethylester. Dramatic changes in the con-
version (%) and catalyst lifetime have been observed as shown in Fig. 2,
suggesting that the deactivation of catalysts by oxo-bridged dimer for-
mation was effectively prevented by their incorporation into the PCP
material. Furthermore, the hexagonal plates obtained after the reaction
were separated and re-used for 4 times without losing their catalytic ac-
tivities significantly (Table S2 in SI) and retained their morphology after
the oxidation, confirmed by SEM analysis (Fig. S9 in SI). In addition, no
percolation of catalysts into the solution was verified by performing the
reaction with a solution prepared by filtration of hexagonal plates' sus-
pension in MC after shaking for 48 h.
To examine the generality of hexagonal plate and rods as heteroge-
neous catalysts for epoxidation, a wide range of olefins with different
steric effects was examined. As shown in Table 1, hexagonal plate and
rods are highly effective catalysts for oxidations of most of the olefins.
For example, the conversions of aromatic olefins (entries 7–10) are
ranged from 60% to 78%, while relatively less reactive linear olefins (en-
tries 1–6) range from 32% to 62%. Interestingly, the conversions are de-
creased with increasing the size of the hexagonal plate and rods in most
of the olefins, suggesting that most of the reactions are occurring on the
external surface of the materials which provides easy access for sub-
strates, and some degree of reactions takes place inside of cavities pro-
vided by the hexagonal plate and rods. Furthermore, the number of
easily accessible catalytic sites is highly dependent on the external sur-
face area which is inversely proportional to the size of crystallites [15].
When relatively bigger-sized substrate, (E)-stilbene (entry 10), was
employed, a significantly smaller conversion is observed compared
with other aromatic substrates while similar values are observed for
all cases of materials. This observation further supports that the hexag-
onal plates and rods provide relatively small size of cavities which do
not permit the big substrate such as (E)-stilbene to enter and allow
the reaction to take place only at the external surface.
0
200
400
600
Time (min)
Fig. 2. The comparison of conversions and catalytic stability of (a) the hexagonal
plate obtained without adding water and (b) homogeneous catalyst {5,15-bis(4-
carboxyethylphenyl)-10,20-bis[2,6-diethoxyphenyl]-porphyrinato}manganese(III) chlo-
ride (molar ratio of styrene:oxidant:catalyst = 2000:1000:1, CH Cl 6 mL). 2-t-
2 2
Butylsulfonyliodosylbenzene was used as oxidant and conversions were determined by
GC with hexadecane as an internal standard (see SI for detailed catalysis conditions).
wide range of length distributions, identical PXRD patterns show that
these objects are isostructural. Furthermore, the heterogeneous catalysis
has been successfully demonstrated, and their catalyst activity and life-
time were dramatically improved and the degradation of catalysts was
successfully inhibited by incorporating active catalysts into MOF mate-
rials. In addition, these heterogeneous catalysts show good re-usability
without losing their activity after 4 runs. Although these materials show
outstanding catalytic activity together with long lifetime, they allow
most of the reactions to occur on their external surfaces and provide
non-negligible cavities where smaller substrates may enter and react.
Acknowledgments
The authors acknowledge the financial support from the National
Research Foundation Program of Korea (KNRF, Nos. 20120002285,
2012001725 and 2012008875) and Priority Research Centers Pro-
gram (NRF20100020209) through the KNRF funded by the Ministry
of Education, Science and Technology (MEST).
Appendix A. Supplementary material
In summary, we have developed a new class of narrowly dispersed
nano- and micro-size hexagonal plate and rods based on Mn(III)-porphy-
rin biscarboxylate and In(III). The lengths of rods have been easily modu-
lated from nano- to micro-meter by the addition of water. Despite the
Complete compound characterizations, catalysis details, addition-
al SEM images, EDX, PXRD patterns, TGA, and adsorption/desorption
Table 1
a
Epoxidation of olefins using hexagonal plate and rods obtained from various reaction conditions.
Entry
1
2
3
4
5
6
7
8
9
10
Olefin
Cyclopentene
Cyclohexene
1-Hexene
1-Octene
cis-2-Octene
trans-2-Octene
Styrene
4-Methylstyrene
(Z)-Stilbene
(E)-Stilbene
0
1
3
5
7
μLb
μL
μL
μL
μL
62.4c (80.3)d
58.4 (73.9)
50.9 (77.8)
48.4 (75.1)
42.3 (77.7)
61.6 (100)
61.3 (100)
56.2 (100)
45.9 (100)
43.4 (100)
57.6 (77.8)
58.8 (67.4)
50.9 (64.5)
42.7 (72.7)
37.6 (74.5)
40.5 (100)
38.1 (97.2)
34.1 (97.4)
33.2 (100)
32.0 (100)
61.9 (100)
54.0 (100)
53.8 (100)
51.9 (100)
47.4 (100)
51.9 (100)
49.5 (94.8)
48.8 (98.7)
48.9 (100)
41.1 (100)
78.0 (69.0)
77.5 (72.3)
76.0 (67.5)
75.3 (68.9)
71.4 (65.7)
74.6 (100)
69.8 (100)
67.5 (100)
62.0 (100)
60.2 (100)
70.9 (100)
69.7 (100)
68.4 (100)
68.0 (100)
61.8 (100)
59.3 (83.2)
59.9 (81.9)
59.6 (93.0)
59.5 (84.9)
59.0 (87.1)
a
Reaction conditions; olefins: 35 μmol, oxidant (2-tert-butylsulfonyliodosylbenzene): 50 μmol, catalyst: 5 mg, CH
Hexagon-faced plate and rods with different lengths obtained from Mn(III)-porphyrin biscarboxylate and In(III) in freshly distilled PrOH at 80 °C with addition of water (0, 1, 3,
2
Cl
2
: 1 mL, reaction time: 30 min, room temperature.
b
5
and 7 μL).
c
Conversion (%) determined by GC with hexadecane as an internal standard.
Epoxide selectivity (%).
d