Preparation of a bispyridine based porous organic polymer as a new platform for Cu(ii) catalyst and its use in heterogeneous olefin epoxidation

Article abstract of DOI:10.1039/c8nj02214c

A new type of bispyridine (bpy) incorporated POP was prepared via a cobalt-catalyzed acetylene trimerization. Subsequent immobilization of CuCl₂ gave POP-Cu(ii). This new heterogeneous catalyst displayed outstanding olefin oxidation activity compared to its homogeneous analogue, suggesting that the degradation of the homogeneous catalyst was successfully inhibited by site isolation.

Full text of DOI:10.1039/c8nj02214c

View Article Online
View Journal
NJC
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: J. Yi, H. M. Ahn, J.
H. Yoon, C. Kim and S. J. Lee, New J. Chem., 2018, DOI: 10.1039/C8NJ02214C.
This is an Accepted Manuscript, which has been through the
Volume 40 Number
1 January 2016 Pages 1–846
Royal Society of Chemistry peer review process and has been
accepted for publication.
NJC
Accepted Manuscripts are published online shortly after
New Journal of Chemistry
www.rsc.org/njc
A journal for new directions in chemistry
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
rsc.li/njc

Please do not adjust margins
New Journal of Chemistry
Page 1 of 5
View Article Online
DOI: 10.1039/C8NJ02214C
Journal Name
COMMUNICATION
Preparation of Bispyridine based Porous Organic Polymer as a
New Platform for Cu(II) Catalyst and Its Use in Heterogeneous
Olefin Epoxidation
Received 00th January 20xx,
Accepted 00th January 20xx
Jigyoung Yi,a Hye Mi Ahn,b Jong Ho Yoon,a Cheal Kim,*b and Suk Joong Lee*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
New class of bispyridine (bpy) incorporated POP was prepared via single-site catalytic reactions.10 Because immobilization of
cobalt-catalyzed acetylene trimerization. Subsequent molecular catalysts using traditional catalyst supports often
a
immobilization of CuCl2 gave POP-Cu(II). This new heterogeneous encounters difficulty in maximizing their potential,11 well-
catalyst displayed outstanding olefin oxidation activity compared controlled catalyst supports are essential for the development
to its homogeneous analogue, suggesting that degradation of of practical catalysts. Well-defined porous materials have
homogeneous catalysts was successfully inhibited by site recently considered as one of the most promising methods.12
isolation.
Particularly, POPs have become very attractive hybrid supports
for various applications despite they have no uniform pores
that make it difficult to analyze their structures via X-ray
crystallographic studies. This is because POPs have strong
covalent bonds, thus providing good chemical stability.13
Herein, we report the preparation of bispyridine (bpy)
containing POPs and immobilization of CuCl2 complex to
fabricate a new heterogeneous catalytic system incorporating
(bpy)CuCl2 units for oxidation of various olefins. Of various
transition metal-based catalytic systems, Cu based system is of
particular interest when taking economic considerations into
account, although it has inherently low catalytic activity.14
Preparation of epoxides via olefin oxidation has become one of
the most important research areas because the formation of a
carbon-oxygen bonding is a very critical process for the
production of various fine chemicals and industrial polymers.1
Although olefin oxidation using stoichiometric amount of toxic
reagents such as peracids has been one of the most widely
used synthetic methods, potentially explosive and toxic wastes
are inevitably produced.2 Therefore, research efforts have
been focused on using transition metal catalysts such as
vanadium,3 manganese,4 iron,5 and copper.6 Such transition
metal-based catalysts have demonstrated great potentials
with high selectivity and efficiency, but they usually suffer
from low stability and separation.7 Therefore, a new class of
catalyst such as heterogeneous system with excellent stability
and efficiency is urgently needed. Current research efforts are
focused on the development of efficient heterogeneous
catalysts.
Scheme 1. Preparation of POP-1 and POP-Cu(II)
.
1
+
1) Co2(CO)8
2) conc. HCl
CuCl2
Cl
Cl
N
Cu
N
N
N
N
2
N
Cl
Cu
N
N
In this regard, the immobilization of molecular catalysts
onto solid supports has attracted much attention,8 including
utilization of well-defined porous solids such as metal-organic
frameworks (MOFs), covalent organic frameworks (COFs), and
porous organic polymers (POPs) due to their high surface areas
and great structural versatility.9 In addition, these porous
materials may offer a highly tunable platform for various
Cl
POPꢀCu(II)
POPꢀ1
n
n
New POP incorporated with (bpy)CuCl2 units, POP-Cu(II)
can be prepared following modified literature procedure.15
cobalt-catalyzed acetylene trimerization copolymerizes 5,5′-
diethynyl-2,2′-bipyridine and tetra(4-ethynylphenyl)-
methane ( ) to form robust POP-1. Subsequent metalation
,
A
(1)
2
with CuCl2 gives POP-Cu(II) (Scheme 1, see Supporting
Information (ESI) section for detailed experiments and
characterizations). To optimize the copolymerization, various
reaction conditions were tested. Two-to-one molar ratio of
a.Dept. of Chemistry, Research Institute for Natural Sciences, Korea University,
Seoul 136-701, Republic of Korea, E-mail: slee1@korea.ac.kr.
b.Dept. of Fine Chemistry, Seoul National University of Science and Technology,
Seoul 136-743, Republic of Korea, E-mail: chealkim@seoultech.ac.kr.
†Electronic Supplementary
DOI: 10.1039/x0xx00000x
Information
(ESI)
available:
See
monomers
1 (69 mg) and 2 (70 mg) in a freshly distilled 1,4-
dioxane (15 mL) afforded POP-1 in high yield (96%), with a
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 2 of 5
View Article Online
DOI: 10.1039/C8NJ02214C
COMMUNICATION
Journal Name
surface area of about 812 m2/g and a mean pore diameter of
Liquid phase catalytic oxidation of various olefins was
5.3 nm. Inductively coupled plasma atomic emission carried out. Briefly, a solution of olefin (1.2 mmol), tert-butyl
spectroscopy (ICP-AES) study indicated that POP-1 showed hydroperoxide (TBHP, 70 % solution in water, 0.17 mL, 1.2
o
negligible amount of Co content (0.1 wt.%) and that cobalt mmol), and acetonitrile (1 mL) was stirred at 85 C with 2 mg
catalyst was successfully removed after washing with HCl of POP-Cu(II) (2.6 µmol of Cu) in powder form. The progression
solution. Interestingly, when the reaction concentration was of reaction was monitored by using a gas chromatography with
doubled by reducing the reaction solvent in half, POP-2 with a a flame ionization detector (GC-FID). Dodecane was used as an
surface area of 659 m2/g and a mean pore diameter of 6.1 nm internal standard and TBHP was chosen as an oxidant. Various
was quickly formed within a few minutes (Scheme S2 and olefins were tested as shown in Table 1.
Table S1 in ESI). Low solubility of oligomeric intermediates in
high reaction concentration might have quickly afforded highly
Table 1. Investigation of olefin epoxidation by POP-
Cu(II)a,b,c
branched polymers with low surface area. On the other hand,
when the reaction was conducted with low reaction
concentration by tripling the solvent amount, POP-3 was
afforded with very low product yield (5 %) with longer reaction
time (~72 h) required. It showed a surface area of 760 m2/g
and a mean pore diameter of 4.9 nm (Fig. S1 and Table S2 in
ESI).
Entry Substrate
Conversion (%)d
80.2 ± 6.3
92.5 ± 6.1
91.2 ± 6.7
78.8 ± 5.3
75.7 ± 6.2
96.7 ± 2.8
42.8 ± 5.4
33.4 ± 5.6
TONe,f
373 ± 20
431 ± 17
436 ± 23
365 ± 12
351 ± 18
447 ± 9
1
2
3
4
5
6
7
8
cyclooctene
cyclohexene
cyclopentane
1-octene
Infrared (IR) and solid state nuclear magnetic resonance
(NMR) spectroscopic analysis can clearly confirm the formation
of polymer.16 In IR analysis, stretches of terminal alkynes at
3300 cm-1 are strongly reduced compared to those of its
parent monomers, strongly suggesting that acetylene
trimerization has taken place (Fig. 1). In the 13C-CPMAS NMR
1-hexane
styrene
cis-stilbene
195 ± 12
trans-stilbene
154 ± 14
aReaction conditions; Olefin (1.2 mmol), POP-Cu(II) 2 mg
spectrum,
a
peak
for
quaternary
carbon
of
(2.6 μmol of Cu), TBHP 0.17 mL (70 % in H2O), acetonitrile
o
b
tetraphenylmethane moiety appeared at 64 ppm. Several
overlapping peaks between 120-150 ppm were associated
with aromatic carbons. No peak for the terminal alkyne was
observed around 80 ppm (Fig. S2 in ESI). Results of NMR along
with IR studies strongly support near-completion of acetylene
trimerization.17
1 mL, 85 C. Conversions were quantified by GC-FID and
dodecane was used as internal standard. cStandard
deviations are based on four runs. dConversion is based on
olefin consumption towards products. eTON = mmol
product/mmol catalyst. fThe products; 2-cyclohexene-1-
one (cyclohexene), 2-cyclohexene-1-ol (cyclohexene),
phenylacetaldehyde
(styrene),
phenylacetaldehyde
(styrene), benzaldehyde (styrene), 2-phenylacetophenone
(cis- and trans-stilbene), epoxides (all substrates) and
unidentified products.
POP-Cu(II) was found to be a good catalyst for most
olefins, generating oxide products. In the reaction of
cyclooctene (entry 1), the conversion rate after 24 h of
reaction period reached about ~80 % while cyclohexene (entry
2) and cyclopentane (entry 3) gave higher conversion rates of
93% and 91 %, respectively. Clearly, cyclohexene is more
reactive than other cycloolefins with POP-Cu(II) because of its
better diffusion profile.19 In cases of 1-octene (entry 4) and 1-
hexene (entry 5), the reaction showed conversion rates of
~79% and ~76 %, respectively, whereas oxidation of styrene
(entry 6) gave a conversion rate of ~97 %. Variation in
conversion rate might be due to conformation of double bond.
In cyclo-olefins, both double bond epoxidation and allylic
oxidation may occur simultaneously. Since cyclohexene
contains a double bond located in the plane of allylic carbons
(Fig. S4 in ESI), it might have facilitated oxidation at all
positions, resulting in high conversion. In case of cyclooctene,
the double bond is located at different plane from other
carbons20 which is presumably responsible for the relatively
lower conversion. For styrene, although the reaction would
Fig. 1. FT-IR spectra of 1, 2, POP -1, and POP-Cu(II)
.
Subsequent metalation of POP-1 with a solution of CuCl2 in
distilled DMF afforded POP-Cu(II) in high yield (83 %) with a
surface area of 462 m2/g. The small decrease in specific
surface area is consistent with the addition of CuCl2 by
metalation (Table S2 in ESI). ICP-AES analysis of digested POP-
Cu(II) indicated Cu content of 8.3 wt.%.
To investigate thermal stabilities of polymers,
thermogravimetric analysis (TGA) was performed. As shown in
Fig. S3 in ESI, they are thermally stable up to 300 oC with small
continuous weight loss at low temperature region due to free
solvent molecules. The thermal decomposition range is slightly
broadened due to collapse of polymer networks caused by
degradation of irregular network structures.18
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 3 of 5
View Article Online
DOI: 10.1039/C8NJ02214C
Journal Name
COMMUNICATION
take place primarily only at the double bond with low
conversion since there is no allylic hydrogen, it is highly
For a further control, molecular catalyst (bpy)CuCl2 was
activated in the presence of phenyl ring. In addition, highly prepared26 and its catalytic activity in styrene oxidation was
opened reaction site might have facilitated catalyst access, compared with POP-Cu(II) using identical reaction conditions.
resulting in high conversion.20 In case of linear olefins, However, oxidation with (bpy)CuCl2 did not proceed as
relatively low conversion is probably due to its steric hindrance effective as that with POP-Cu(II). Although the initial catalytic
arisen from long alkyl chain.21 Size-selectivity was also found activity of (bpy)CuCl2 was greater than that of POP-Cu(II), it
when bigger substrates were employed (entry 7 and 8). Access was slowly reduced after 5 h of the reaction time. No further
to metal centers of the larger olefins might have been less reaction took place after 8 h. This might be due to deactivation
facilitated due to structural disadvantage, resulting in smaller of homogeneous catalyst by the formation of less active dimer
conversion found in trans-stilbene.
and/or aggregated species27 and oxidation of ligand.28 Thus,
It is expected that the reaction would proceed via successful site isolation strategy via immobilization of catalysts
formation of copper-peroxo species using TBHP as an is effective for obtaining higher catalytic activity and stability.
oxidant.22 As shown in Fig. S5 in ESI, the activation of catalytic Immobilization of CuCl2 might be able to facilitate the
Cu-site by TBHP may follow Haber-Weiss-type decomposition accessibility of substrate to catalytically active centers while
process. Both t-BuO and t-BuOO are possible reactive species reducing the potential of catalyst deactivation process.29
responsible for olefin oxidations.23 Further study was carried
to investigate the function of oxidant. The oxidations were
carried out with the addition of diphenylamine as a radical
inhibitor (Table S3 in ESI). Conversions were decreased
significantly, supporting that t-BuO and t-BuOO might be
effectively consumed by radical inhibitor before catalytic
oxidation takes place.24
100
80
60
40
20
0
■
Homogeneous catalyst
Cu(II)ꢀPOPꢀ1
●
● POPꢀCu(II)
As a control, the reaction mixture was investigated by
removing all solid suspension after the reaction was completed
in order to verify the stability of POP-Cu(II) 25
No leaching of
.
catalysts into reaction solution was found after performing the
ICP-AES study, showing a negligible amount of Cu content
(0.02 wt.%). Encouraged by this result, catalyst reusability was
tested. After each reaction cycle, the catalyst was isolated,
washed three times with methanol in air, dried at room
temperature, and reused for the next run. Remarkably, it
showed no significant catalytic activity change even after five
times of re-use (Table 2). It also retained its morphology after
multiple catalytic reactions. This was confirmed by SEM
analysis (Fig. S6 in ESI).
0
10
20
30
40
Time (hour)
Fig. 2. Comparison of conversions and catalytic stability of
POP-Cu(II) (red) and its homogeneous analogue (bpy)CuCl2
(blue). Molar ratio of styrene:oxidant:catalyst = 670:670:1, 85
oC. tert-Butyl hydroperoxide was used as an oxidant.
In summary, we prepared a new class of heterogeneous
catalytic porous organic polymer, POP-Cu(II)
, based on
incorporation of (bpy)CuCl2 unit into POP-1 made of 5,5′-
diethynyl-2,2′-bipyridine
ethynylphenyl)methane
(
1
)
and
tetra(4-
(2) via cobalt-catalyzed acetylene
Table 2. Investigation of the recyclability of styrene
oxidationa,b,c
trimerization. The employment of heterogeneous catalyst,
POP-Cu(II), for oxidation of various olefins was successfully
demonstrated. Particularly, POP-Cu(II) showed outstanding
catalytic activity compared to its homogeneous analogue,
suggesting that degradation of catalysts via oxidation of ligand
and aggregation of active catalysts was successfully inhibited
or substantially reduced by effective site isolation via
immobilization of catalysts on POP based platform. Finally, this
heterogeneous catalyst showed good re-usability without
significant losing its activity.
Cycle
Conversion (%)d
96.3 ± 3.8
TONe
1
2
3
4
5
446 ± 12
437 ± 10
438 ± 25
441 ± 9
94.0 ± 3.5
95.2 ± 6.2
95.8 ± 3.6
94.5 ± 3.5
438 ± 10
aReaction conditions; Styrene (0.14 mL,1.2 mmol), POP-
Cu(II) 2 mg (2.6 μmol of Cu), TBHP 0.17 mL (70 % in
H2O), acetonitrile 1 mL, 85 oC. bConversions were
quantified by GC-FID and dodecane was used as
internal standard. cStandard deviations are based on
three runs of POP-Cu(II). dConversion is based on olefin
Acknowledgements
J. Yi, J.Y. Yoon, S.J. Lee acknowledge financial support from the
Research Foundation Program (NRF-2016R1D1A1A09918637)
and Key Research Institute Program (NRF2010-0020209)
through the National Research Foundation of Korea (NRF) and
KU-FRG from Research Program of Korea University. H.M. Ahn,
consumption towards products. eTON
product/mmol catalyst.
fThe
=
mmol
products;
phenylacetaldehyde, benzaldehyde, epoxide, and
unidentified products.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 4 of 5
View Article Online
DOI: 10.1039/C8NJ02214C
COMMUNICATION
Journal Name
23 L. Saussine, E. Brazi, A. Robine, H. Mimoun, J. Fischer and R.
Weiss, J. Am. Chem. Soc., 1985, 107, 3534.
24 (a) U. Junghans, C. Suttkus, J. Lincke, D. Lassig, H. Krautscheid
and R. Glaser, Micropor. Mesopor. Mater., 2015, 216, 151.;
(b) F. Farzaneh, J. Taghavi, R. Malakooti and M. Ghandi, J.
Mol. Catal. A: Chem., 2006, 244, 252.
C. Kim acknowledge financial support from the Research
Foundation Program of Korea (NRF-2018R1A2B6001686).
Conflicts of interest
25 D. H. Lee, S. Kim, M. Y. Hyun, J.-Y. Hong, S. Huh, C. Kim and S.
J. Lee, Chem. Commun., 2012, 48, 5512.
There are no conflicts to declare.
26 D. A. Martins, L. R. Gouvea, D. Gama Jean Batista, P. B. Silva,
S. R. W. Louro, M. N. C. Soeiro and L. R. Teixeira, Biometals,
2012, 25, 951.
References
27 A. Schiller, R. Scopelliti and K. Severin, Dalton Trans., 2006,
3858.
28 S. Kim, K. Y. Lee, Y. S. Lee, H. G. Jang, J. K. Lee and S. J. Lee, J.
Porphyrins Phthalocyanines 2014, 18, 579.
29 K. Manna, T. Zhang and W. Lin, J. Am. Chem. Soc., 2014, 136
6566.
1
(a) K. C. Gupta and A. K. Sutar, Cood. Chem. Rev., 2008, 252,
,
1420.; (b) B. S. Lane and K. Burgess, Chem. Rev., 2003, 103
2457.
2
3
A. S. Sharma and H. Kaur, Appl. Catal. A Gen. 2017, 546, 136.
C. Cordelle, D. Agustin, J.-C. Daran and R. Poli, Inorganica
Chim. Acta 2010, 364, 144.
,
4
5
6
J. Brinksma, R. Hage, J. Kerschner and B. L. Feringa, Chem.
Commun., 2000, 537.
F. G. Gelalcha, B. Bitterlich, G. Anilkumar, M. K. Tse and M.
Beller, Angew. Chem. Int. Ed., 2007, 46, 7293.
R. Bera, C. Adhikary, S. Ianelli, S. Chaudhuri and S. Koner,
Polyhedron, 2010, 29, 2166.
7
8
L. Yin and L. Liebscher, J. Chem. Rev., 2007, 107, 133.
(a) K. Ariga, Y. Yamauchi, G. Rydzek, Q. Ji, Y. Yonamine, K. C.-
W. Wu and J. P. Hill, Chem. Lett., 2014, 43, 36.; (b) R. Cai, X.
Ye, Q. Sun, Q. He, Y. He, S. Ma and X. Shi, ACS Catal., 2017,
1087; (c) S. Fukuzumi, Y.-M. Lee, W. Nam, ChemCatChem,
2018, 10, 1686.
7,
9
(a) S. Furukawa, J. Reboul, S. Diring, K. Sumida and S.
Kitagawa, Chem. Soc. Rev., 2014, 43, 5700.; (b) T. Yamada, K.
Otsubo, R. Makiura and H. Kitagawa, Chem. Soc. Rev., 2013,
42, 6655.; (c) A. Schneemann, B. Bon, I. Schwedler, I.
Senkovska, S. Kaskel and R. A. Fischer, Chem. Soc. Rev., 2014,
43, 6062.; (d) C. K. Brozek and M. Dincă, Chem. Soc. Rev.,
2014, 43, 5456.
10 A. Corma, H. García and F. X. L. Xamena, Chem. Rev., 2010,
110, 4606.
11 (a) L.-X. Dai, Angew. Chem. Int. Ed., 2004, 43, 5726.; (b) D. C.
Bailey and S. H. Langer, Chem. Rev., 1981, 81, 10.
12 (a) H. G. T. Nguyen, M. H. Weston, A. A. Sarjeant, D. M.
Gardner, Z. An, R. Carmieli, M. R. Wasielewski, O. K. Farha, J.
T. Hupp and S. T. Nguyen, Cryst. Growth Des., 2013, 13
,
3528.; (b) L. E. Kreno, K. Leong, O. K. Farha, M. Allendorf, R.
P. Van Duyne and J. T. Hupp, Chem. Rev., 2012, 112, 1105.
13 (a) P. Kaur, J. T. Hupp and S. T. Nguyen, ACS Catalysis, 2011,
1, 819.; (b) J. Weber, J. Schmidt, A. Thomas and W.
Böhlmann, Langmuir, 2010, 26, 15650.
14 S. E. Allen, R. R. Walvoord, R. P. Salinas and M. C. Kozlowski,
Chem. Rev., 2013, 113, 6234.
15 M. H. Weston, Y. J. Colón, Y.-S. Bae, S. J. Garibay, R. Q. Snurr,
O. K. Farha, J. T. Hupp and S. T. Nguyen, J. Mater. Chem. A,
2014, 2, 299.
16 Z. Xie, C. Wang, K. E. deKrafft and W. Lin, J. Am. Chem. Soc.,
2011, 133, 2056.
17 R. K. Totten, Y.-S. Kim, M. H. Weston, O. K. Farha, J. T. Hupp
and S. T. Nguyen, J. Am. Chem. Soc., 2013, 135, 11720.
18 A. Dani, V. Crocellà, C. Magistris, V. Santoro, J. Yuan and S.
Bordiga, J. Mater. Chem. A, 2017, 5, 372.
19 Q. Zhao, C. Bai, W. Zhang, Y. Li, G. Zhang, F. Zhang and X.
Fan, Ind. Eng. Chem. Res., 2014, 53, 4232.
20 M. Sarkheil and M. Lashanizadegan, Appl. Organometal.
Chem., 2017, 31, 3726.
21 M. Masteri-Farahani, F. Farzaneh and M. Ghandi, J. Mol.
Catal. A.: Chem., 2006, 248, 53.
22 J. Sun, Q. Kan, Z. Li, G. Yu, H. Liu, X. Yang, Q. Huo and J. Guan,
RSC Adv., 2014, 4, 2310.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 5
New Journal of Chemistry
View Article Online
DOI: 10.1039/C8NJ02214C
Preparation of Bispyridine based Porous Organic Polymer as a New Platform for
Cu(II) Catalyst and Its Use in Heterogeneous Olefin Epoxidation
Jigyoung Yi,a Hye Mi Ahn,b Jong Ho Yoon,a Cheal Kim,*b and Suk Joong Lee*a
[a] Dept. of Chemistry, Research Institute for Natural Sciences, Korea University, Seoul 136-701,
Republic of Korea, E-mail: slee1@korea.ac.kr, Tel: (+82-2) 3290-3145
[b] Dept. of Fine Chemistry, Seoul National University of Science and Technology, Seoul
136-743, Republic of Korea, E-mail: chealkim@seoultech.ac.kr
n
1