Controlled growth of narrowly dispersed nanosize hexagonal MOF rods from Mn(iii)-porphyrin and In(NO3)3 and their application in olefin oxidation

Article abstract of DOI:10.1039/c2cc31075a

A new class of narrowly dispersed nanosize hexagonal MOF rods from Mn(iii)-porphyrin and In(iii) was obtained. The length of MOF rods was controlled by simple change of reaction times. Furthermore, the oxidation of styrene has been successfully demonstrated with Mn(iii)-porphyrin MOF rods and their reusability has been also tested.

Full text of DOI:10.1039/c2cc31075a

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COMMUNICATION
Controlled growth of narrowly dispersed nanosize hexagonal MOF rods
from Mn(III)–porphyrin and In(NO3)3 and their application in olefin
oxidationwz
Da Hee Lee,a Sundol Kim,a Min Young Hyun,b Jin-Yeon Hong,c Seong Huh,c Cheal Kim*b
and Suk Joong Lee*a
Received 14th February 2012, Accepted 11th April 2012
DOI: 10.1039/c2cc31075a
A new class of narrowly dispersed nanosize hexagonal MOF
rods from Mn(III)–porphyrin and In(III) was obtained. The length
of MOF rods was controlled by simple change of reaction times.
Furthermore, the oxidation of styrene has been successfully
demonstrated with Mn(III)–porphyrin MOF rods and their
reusability has been also tested.
such as molecular boxes, self-assembled arrays and coordi-
nation polymers, and MOFs with various applications in gas
storage,11 catalysis,12 and separation.13 In particular, Mn(III)
and Fe(III) mediated porphyrin derivatives are often used to
mimic the remarkable behavior of enzymes, that is the selec-
tive oxidation of olefins. Unfortunately, when used as homo-
geneous catalysts, they often suffer from quick degradation
such as ligand oxidation or m-oxo dimer formation.14 There-
fore, immobilization and site-isolation of homogeneous cata-
lysts on organic polymers, inorganic supports and MOFs to
screen out the side reactions has become one of the most
promising fields in current chemistry.15
Metal–organic frameworks (MOFs), resulting from the self-
assembly of metal complexes and organic linkers, have received
a great deal of attention due to their potential applications in
catalysis, gas storage, molecular recognition, separation and
optics.1 Recently, nano- and micro-scale MOFs (nMOFs or
mMOFs) have been reported with promising applications.2 They
provide various features like the advantages of nanomaterials in
addition to the properties of MOFs.3 Furthermore, they often
show novel and/or enhanced properties in applications such as
gas storage,4 separation,5 drug delivery,6 biosensing and imaging,7
and catalysis.8 However, the fabrication of nMOFs and mMOFs
is still a very challenging task because of the dearth of information
and knowledge regarding the critical factors required for the
precise control of these materials, even though a variety of nano-
and micro-scale MOFs showing different morphologies can be
successfully obtained by controlling the growth conditions of
crystals.9 Specially, the translation of their building blocks into
well-defined MOF particles, therefore, tuning their properties at
the building block level remains still quite challenging.10
In this spirit, we would like to describe herein the prepara-
tion and properties of length-controlled nano- and microsize
hexagonal MOF rods from Mn(III)–porphyrin biscarboxylic
acid and In(NO3)3, and their catalytic behaviour in olefin
oxidation as a heterogeneous catalyst.
Mn(III)–porphyrin bis-carboxylic acid 1 has been prepared
according to the modified literature procedure that is the
condensation reaction between dipyrrolemethane and aldehyde
followed by deprotection of ethyl groups and metallation with
MnCl2 (Scheme S1 in ESIz).16 In order to make the best use of
bis-carboxylic groups of the Mn(III)–porphyrin, In(NO3)3 was
chosen to assemble networks because In3+ is well known to
allow strong interactions with carboxylates to generate stable
coordinative networks.17 Therefore, using a high temperature
stirring method,18 a solution of 1 (3.134 mg) and In(NO3)3
(1.949 mg) in 0.45 mL of anhydrous DMF was vigorously
stirred at 120 1C for approximately 2, 5, 10, 30 and 60 min,
respectively, and translucent dark brown suspensions were
obtained after cooling. Analysis by scanning electron micro-
scopy (SEM) reveals these suspensions to be collections of fairly
uniform hexagonal rods which show systematically modulated
lengths by adjusting reaction times (Fig. 1). Presumably, when
the reaction is initiated in DMF at 120 1C, crystalline seeds are
generated and they continue to grow until they are held in a
solution phase. As they are cooled down, these nanocrystalline
rods become less soluble and precipitate from the DMF medium.
The lengths of the resulting rods are controlled by the ability of
the solution to support the particles at high temperature,
leading to narrowly dispersed Mn(III)–porphyrin MOF rods.19
As a good building block, metalloporphyrin often shows its
unique functions in a wide range of molecular architectures,
a Dept. of Chemistry, RINS, Korea University, 5 Anam-dong,
Sungbuk-gu, Seoul 136-701, Korea. E-mail: slee1@korea.ac.kr;
Tel: +82 2-924-3145
b Dept. of Fine Chemistry, Seoul National University of Science and
Technology, Seoul 139-743, Korea.
E-mail: chealkim@seoultech.ac.kr
c Dept. of Chemistry and Protein Research Center for Bio-Industry,
Hankuk University of Foreign Studies, Yongin 449-791, Korea
w This article is part of the ChemComm ‘Porphyrins and phthalocyanines’
web themed issue.
z Electronic supplementary information (ESI) available: Complete
experimental details of syntheses, compound characterization, catalysis
details, additional SEM images, EDX, PXRD patterns, TGA, and
adsorption–desorption isotherms. See DOI: 10.1039/c2cc31075a
c
5512 Chem. Commun., 2012, 48, 5512–5514
This journal is The Royal Society of Chemistry 2012

Furthermore, when moderately diluted reactions (9 mL
instead of 4.5 mL of DMF) were carried out at 120 1C, the
lengths of MOF rods became greater than 5.5 mm despite the
variation in reaction times, except for the reaction that was
carried out for 2 min which gave a wide range of lengths from
50 nm to 400 nm (Fig. S5 in ESIz). Indeed the solubility of
porphyrin MOF rods becomes highly increased under this
diluted condition.
To investigate the thermal stability of MOF rods, thermo-
gravimetric analysis (TGA) of the samples obtained at various
reaction times at 120 1C was performed. The TGA curves
show an initial weight loss due to solvent liberation and a high
thermal stability up to 280 1C. They show continuous weight
loss in the range from 280 1C to 800 1C due to the decom-
position of frameworks (Fig. S7 in ESIz). Furthermore, the
sample of MOF rods obtained from 10 min of reaction at
120 1C was chosen to study their gas sorption properties. Its
volumetric gas sorption measurements were carried out using
N2, CO2, and H2 at 77 K, 196 K, and 77 K, respectively. As
shown in Fig. S8 in ESIz, the N2 adsorption isotherm reveals a
surface area of 77.7 m2 gꢁ1 and an uptake value of 85.1 cm3 gꢁ1
,
while the sorption of H2 and CO2 shows uptake values of 53.9
and 53.8 cm3 gꢁ1, respectively. Perhaps, the relatively smaller
surface area and gas uptakes are attributed to the framework
collapse during the CHCl3-exchange and/or evacuation processes.
This framework collapse was identified from the PXRD pattern
taken after the gas sorption measurements, showing an almost
complete loss of crystallinity. However, when an amorphous solid
is immersed in DMF, it changes to somewhat original crystalline
phase, which was detected from PXRD patterns (Fig. S9 in ESIz).
This flexible framework may provide pore structures that are
suitable for the given guest molecules.20
Fig. 1 SEM images of hexagonal MOF rods with different lengths
obtained from Mn(III)–porphyrin 1 and In(NO3)3 at 120 1C at various
reaction times; (a) 2, (b) 5, (c) 10, (d) 30, and (e) 60 min, and (f) a
magnified image of a MOF rod grown from 60 min of reaction at 120 1C.
Therefore, we have increased the reaction time to maximize the
solubility of porphyrin MOF rods in DMF during the crystal
growth. The longer reaction time may extend their retention
time in solution and hold the growth of particles to larger objects
while maintaining the narrow distribution. Upon allowing the
reaction for 60 min at 120 1C and cooling down to room
temperature, narrowly dispersed hexagonal MOF rods are
obtained with B3.6 mm in length (Fig. 1e). Remarkably, when
the reaction time was diversified, the lengths of hexagonal MOF
rods changed dramatically. As shown in Fig. 1, narrowly
dispersed MOF rods with lengths ranging from 0.5 ꢀ 0.2 to
3.6 ꢀ 1.2 mm can be obtained. However, the widths of these
MOF rods are also increased from B70 to B350 nm. Although
obtaining suitable single crystals for X-ray structure analysis has
not been successful, these materials exhibit remarkably the same
powder X-ray diffraction (PXRD) patterns, indicating the
identical unit cells despite their different lengths (Fig. S2 in ESIz).
Similar behaviors have been observed when the reactions
have been carried out at 100 1C and 80 1C, respectively. At 100 1C,
the lengths of hexagonal MOF rods ranging from 0.3 ꢀ 0.2 to
2.5 ꢀ 0.7 mm can be obtained over a reaction time range of 5 to
60 min (Fig. S3 in ESIz). However, no MOF rods were
observed for the 2 min reaction (Fig. S3a in ESIz). Similarly,
when the reactions were carried out at 80 1C, the lengths of
MOF rods ranged from 0.4 ꢀ 0.1 to 1.6 ꢀ 6.0 mm (Fig. S4 in
ESIz) and no MOF rods were observed for the 2 and 5 min
reactions, respectively (Fig. S4a and b in ESIz). Presumably,
crystalline seeds require longer reaction time to grow and
require high reaction temperature until they are retained in a
solution phase and grow into longer rods.
With the readily accessible Mn(III)–porphyrin catalyst on
board, the MOF rods grown from 10 min reaction at 120 1C
have been selected and tested for their catalytic activity in
oxidation of aromatic olefins. The catalyst was prepared by
immerging in CH2Cl2 for 24 h followed by a brief drying to
prevent complete decomposition of their framework. Styrene
epoxidation using this MOF rod catalyst and its homogeneous
counterpart Mn(III)–porphyrin bis-carboxyethylester 2 revealed
Fig. 2 Comparison of TONs and catalytic stability of the MOF rods
obtained from 10 min of reaction at 120 1C and homogeneous catalyst
{5,15-bis(4-carboxyethylphenyl)-10,20-bis[2,6-diethoxyphenyl]porphyri-
nato}manganese(III) chloride 2 (molar ratio of styrene : oxidant : catalyst =
2000 : 1000 : 1, CH2Cl2 5 mL). 2-tert-Butylsulfonyliodosylbenzene
was used as an oxidant (see ESIz for detailed catalysis conditions).
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 5512–5514 5513

Table 1 Investigation of the recyclability of MOF rodsa
from nano- to micrometer by a simple change of reaction
times. PXRD patterns show that these particles have an
identical unit cell regardless of a wide range of length distribu-
tions. Furthermore, the oxidation of styrene has been success-
fully demonstrated with Mn(III)–porphyrin MOF rods. The
catalytic activity and lifetime were dramatically improved and
the self-degradation of catalysts was successfully blocked
by the immobilization of Mn(III)–porphyrin through MOF
formation. In addition, MOF rods show good recyclability
without losing their catalytic activity after 5 runs.
Number of catalytic cycles
1
2
3
4
5
TONc
888
900
893
891
885
a
Reaction conditions; styrene: 0.48 mmol, oxidant: 0.24 mmol, MOF
rods: 0.24 ꢂ 10ꢁ3 mmol (2000 : 1000 : 1), CH2Cl2: 1 mL, reaction time:
b
2 h, rt. Oxidant: 2-tert-butylsulfonyliodosylbenzene. Determined
c
by GC with undecane as an internal standard (2000 eq.).
The authors acknowledge the financial support from the
Korea Research Foundation Program (No. 20110004727) and
Priority Research Centers Program (NRF 2011-0018396)
through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education, Science and Technology
(MEST).
a dramatic change in the turnover number (TON). In addition,
the catalyst lifetime was dramatically extended from approxi-
mately 2 hours to greater than 6 hours because of the formation
of Mn(III)–porphyrin in the MOF (Fig. 2). Self-degradation of
catalysts by oxo-bridged dimer formation was successfully blocked
by the immobilization of catalysts through the MOF strategy.14
Furthermore, the MOF rods obtained after the reaction were
separated by centrifugation and reused 5 times. As shown in
Table 1, they show outstanding reusability without losing their
catalytic activity significantly and the morphology of MOF rods
has been retained after the oxidation (Fig. S10 in ESIz). Further-
more, the active catalyst species did not leach into solution which
was confirmed by performing the reaction with a solution pre-
pared by filtrating a MOF rod suspension in MC after shaking for
24 h. We next investigated the influence of the size of MOF rods
on the catalytic activities because the active catalytic sites may be
located only on the external surface which is affected by the size of
crystallites. As shown in Table 2, the TONs decreased upon
increasing the size of MOF rods. The same behaviors have been
observed when the relatively bigger substrates (Z)- and
(E)-stilbenes are used. These observations suggest that most of
the catalytic reactions occur on the external surface of MOF rods
and the number of active catalytic sites varies with the size of
crystallites. However, the size-selectivity has been also observed.
For example, the TONs of reactions with (E)-stilbene decreased
significantly compared to those with styrene cases, while the TONs
of the reactions with (Z)-stilbene decreased moderately. This
observation suggests that the MOF rods provide non-negligible
size of cavities which do not allow the bigger substrate to enter but
the smaller one, albeit in a significantly changed TONs. Although
the MOF rods show outstanding catalytic activity together with
long lifetime, they allow most of the reactions to occur on their
external surfaces and provide non-negligible size of cavities where
smaller substrates may enter and react.
Notes and references
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In summary, we have developed a new class of narrowly
dispersed nanosize hexagonal MOF rods from a Mn(III)–porphyrin
and In(III). The length of MOF rods has been easily controlled
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Table 2 TONs of various olefin oxidations with MOF rods obtained
at 120 1C at various reaction timesa
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2607.
2 min
5 min
10 min
30 min
60 min
Styrene
(Z)-Stilbene
(E)-Stilbene
1054
899
615
1003
881
614
957
861
596
926
842
563
905
821
528
a
Reaction conditions; olefin: 0.48 mmol, oxidant (2-tert-butylsulfo-
nyliodosylbenzene): 0.24 mmol, MOF rods: 0.24 ꢂ 10ꢁ3 mmol
(2000 : 1000 : 1), CH2Cl2: 1 mL, reaction time: 3 h, rt.
c
5514 Chem. Commun., 2012, 48, 5512–5514
This journal is The Royal Society of Chemistry 2012