Notes Bull. Korean Chem. Soc. 2011, Vol. 32, No. 6    2073
DOI10.5012/bkcs.2011.32.6.2073 Effect of Diamine in Amine-Functionalized MIL-101 for Knoevenagel Condensation
Palraj Kasinathan,†,‡ You-Kyong Seo,† Kyu-Eun Shim,†,‡ Young Kyu Hwang,†,‡,* U-Hwang Lee,†,* Dong Won Hwang,†
Do-Young Hong,† Shiva B. Halligudi,† and Jong-San Chang†,‡
†Research Group for Nanocatalyst, Korea Research Institute of Chemical Technology, Daejeon 305-600, Korea
*E-mail: kr(Y. K. H); kr(U-H. L)
‡School of Science, University of Science and Technology (UST), Daejeon, 305-333, Korea
Received February 28, 2011, Accepted April 6, 2011
Key Words : MOF, MIL-101, Pore size, Coordinativ ely unsaturated metal sites (CUS), Knoev enagel
condensation
Crystalline Metal-Organic Frameworks (MOFs) are current-ly an important kind of advanced functional materials due to their novel coordination structures, diverse topologies, and potential applications.1-5 As one of topical M OFs, porous chromium terephthalate with giant pores labeled M IL-101(Cr) possesses several unique features such as hierar-chical pore structure6 including a mesoporous zeotype architecture, mesoporous cages and microporous windows, outstanding sorption properties, numerous unsaturated metal cation sites,and high hydrothermal and chemical stability. These properties have led to a number of application potential in catalysis, gas storage, drug delivery and adsorptive separation.7-10 One important challenge has to realize is funtionalization via incorporation of binding site or reactive centers for catalysis. The functionalization methods of metal organic frameworks (MOFs) in a wide range of applications are two possible approaches including pre- and post-modification with functional groups.7,9-15
The functionalized pores of MOFs can be used as active sites in base-catalyzed reactions.13,16-19 It was already described on an electrophilic surface functionalization of coordinatively unsaturated metal sites (CUS) with chelating agents or electron-rich molecules. M IL-101(Cr) may thus be finely tuned for catalytic applications in precise conditions. It is known that MIL-101(Cr) possesses two coordinatively
un-saturated metal sites (CUS) in a trimeric Cr(III) octahedral cluster (Scheme 1) with terminal water molecules which are removable from the framework under vacuum or in an inert gas flow at 423 K for 12 h. This CUS as Lewis acid sites in the structure was usable for the surface functionalization7 (Figure 1).
Here we further explore the preparation of the amine-
grafted MIL-101 as a heterogeneous base catalyst according
to a p K a value of the functional diamine group and their catalytic activities in Knoevenagel condensation of benz-
aldehyde and ethylcynoacetate. The diamine compounds
used for the functionalization include ethane-1,2-diamine
(ED), butane-1,4-diamine (BD), decane-1,10-diamine (DD)
and benzene-1,4-diamine (PD).
The XRD patterns of functionalized samples show de-
crease of intensity. These results provide that the diamines
were filled the pores of MIL-101. However, the patterns still
show the high-crystalline frameworks and porous structures
after functionalization. (Supplementary M aterials, Figure
S1) The structural details of the pores such as BET surface
area, pore volume and pore size are exhibited in Supple-
mentary Materials (Figure S2 and Table S1). Figure 1 shows
the N2 adsorption isotherms of as-prepared and function-
alized MIL-101. The as-prepared MIL-101 has BET surface
area of 4035 m2/g and pore volume of 1.96 cm3/g. After
functionalization, the surface areas and pore volumes are
decreased with change of the functional groups because the
different sizes of diamines are attached to the pore surfaces
and partially block the pores. It corresponds with the decrease
of adsorbed N2 amount gradually with the functionalization.
Scheme 1. Evolution of coordinatively unsaturated Cr(III) sites from chromium trimer after vacuum treatment 423 K for 12 h.Figure 1. Nitrogen adsorption isotherms of MIL-101 and amine-grafted MIL-101 at 77 K: (a) purified MIL-101, (b) ED-MIL-101, (c) PD-MIL-101, (d) BD-MIL-101, and (e) DD-MIL-101.
2074    Bull. Korean Chem. Soc. 2011, Vol. 32, No. 6Notes However, the shapes of the isotherm curves were not
changed after functionalization (Fig. 1). The XRD and BET
results revealed that the porous framework of M IL-101 is
very stable during the functionalization process. From pore
size distribution profiles calculated from adsorption branch
of N2-isotherms (Fig. S2), MIL-101 has two-types of meso-
pores centered in 1.9/1.4 nm, respectively. The pore sizes of
functionalized samples also decreased, as increased number of hydrocarbons (HC) of grafted amines. As a result, DD-MIL-101 has pores of 1.6/1.0 nm, respectively. On the other hand, small molecules functionalized samples as ED-MIL-101 are not shown noticeable change in pore size distribu-tion. Additionally, nitrogen contents obtained by elemental analysis (EA) were 2.1 mmol/g (ED-MIL-101), 1.9 mmol/g (BD-MIL-101), 1.4 mmol/g (PD-MIL-101) and 1.3 mmol/g (DD-MIL-101), respectively. Although the DD-MIL-101 has smallest number of nitrogen contents, it has small pores due to the long chain of DD filled the pores. PD-MIL-101 also has small number of nitrogen content because their weak electronegativity of phenyl amine group acted as to be impediment to amine functionalization at the CUS sites. In addition, this result could be main reason of PD-M IL-101 showing higher surface area than the BD-MIL-101 in spite of longer HC chain.
For the proof of amine-functionalization, the FT-IR spectra (Fig. S3) were measured at 433 K after the removal of solvent and water contents. The result shows a typical band of -NH (3150-3400 cm−1) and -
CH (2800-3000 cm−1) stretch-ing vibration modes as marked in Figure S3. However, the PD-MIL-101 does not possess -CH band and it shows a shift in the -NH band due to primary amine attached to aromatic ring of PD. (dash-circle) The peak of left-shifted -CH stretch-ing vibrations of BD- and ED-MIL-101 were appeared by neighboring aliphatic -CH of bonded amine to Lewis acid Cr sites. These results demonstrated the selective functionali-zation of diamines into the pores of MIL-101.15 The strong intensity of the -CH stretching band of DD-M IL-101 is appeared by long chain length of DD.
The catalytic activities of the amine functionalized MIL-101 in the Knoevenagel condensation of benzaldehyde and ethyl cynoacetate are shown in Figure 2. To confirm the effect of diamine on base catalysis, the reaction was carried out under less reactive condition using toluene as a solvent and lower temperature of 333 K. The samples have high selectivity (>99%) to trans-ethyl cyanocinnamate at the conversion ranges between 45 to 98% by different basicities as p K a values21 of functional group. The condensation reac-tions were carried out to probe the effect of basicity of the active sites which were created by diamines in the pores of M IL-101. The reactivity of the samples shows correlation with the p K a values of the diamines. BD-MIL-101 has the high reactivity as shown in Figure 2 and Table 1. The reac-tivity of BD-MIL-101 was both found to be high conversion of 98% and turn over frequency (TOF) of 5.7. The higher activity attributed to high p K a value of the functional gro
up. The reactivity of ED-MIL-101 and PD-MIL-101 with lower p K a values is lower than BD- and DD- functionalized samples. Although ED-M IL-101 still has large pore size and high surface area, it shows low conversion activity of 89% and TOF value of 3.5 because ED- is not high p K a value. The PD-M IL-101 has lower p K a value of 6.08 due to the delocalization of lone pair of electrons on nitrogen. Hence, the PD-M IL-101 has the lowest basicity and the lowest catalytic activity as conversion activity of 44.8% and TOF valve of 1.4. However, DD-M IL-101 with higher p K a of 11.00 shows lower conversion activity of 82%. The lower reactivity of DD-MIL-101 attributed to the less number of basic sites for the reaction as well as the longest chain length of hindered diffusion of the reactant molecules. Moreover, the amine of the long carbon chain might be has hydrogen bond with adjacent amines or the free amine group can graft with the neighboring CUS sites.22
In conclusion, we have demonstrated that amines with different basicities successfully functionalized into the pores of M IL-101 and amine functionalized chromium terephth-alate used as a base catalyst. The catalytic activity of amine functionalized M IL-101 in Knoevenagel condensation of ethylcyanoacetate and benzaldehyde depends on their basi-cities. The reactivity of these catalytic materials could be also affected by their pore size and/or surface area, which governs the facile diffusion of the molecules through the
Table 1. Catalytic activities of amine-g rafted MIL-101(Cr) in Knoevenagel condensation of benzaldehyde with ethyl cynoacetate a
C atalyst p K a Conv.
(%)b Sel.
(%)c
TOF
(h−1)d
ED-MIL-1018.668999  3.5 BD-MIL-10110.409899  5.7 DD-MIL-10111.008299  5.6 PD-MIL-101  6.084599  1.4
a The reaction was carried out at same condition as described in Figure 2.
b Conversion of benzaldehyde at 15 h.
c Selectivity towards trans-ethyl cyanoacetate.
d TOF (turn over frequency): moles of product formed per
mole of nitrogen in the grafted MIL-101 per hour (measured  at 2 h).
Figure 2. Conversion of benzaldehyde as a function of reaction
time over amine-g rafted MIL-101 in Knoevenag el condensation.
The reaction was carried out with 1 mmol of benzaldehyde, 1
mmol of ethylcynoacetate, 30 mg of catalyst and 25 mL of toluene
heated at 333 K.
Notes Bull. Korean Chem. Soc. 2011, Vol. 32, No. 6    2075
channels of the MIL-101. The present strategy ensures the development of new functionalities and lead to MOF appli-cations of practically useful heterogeneous base catalysts for chemical transformations.
Experimental Section
The MIL-101 was synthesized by a hydrothermal reaction of terephthalic acid with Cr(NO3)3·9H2O, HF, and H2O at 493 K in 8 h according to the procedure proposed by our previous report.7,9 The as-synthesized MIL-101 was further purified by a three-step process using double filtration, hot ethanol, and aqueous NH4F solutions.The purified MIL-101 (0.5 g) was dehydrated at 423 K for 12 h and then suspended in anhydrous toluene (30 mL). 1.5 mmol of ED, BD, DD and PD were added individually and the mixture was stirred under reflux for 12 h in nitrogen environment. The function-alized MIL-101s
or samples? were tested for their catalytic activities in the Knovenagel condensation reaction. The reaction was carried out with 1 mmol of benzaldehyde, 1 mmol of ethylcynoacetate, 30mg of catalyst and 25 mL of toluene heated at 333 K.
Acknowledgments. This research was supported by the Pioneer Research Center Program through the National Research Foundation of Korea funded by the M inistry of Education, Science and Technology (2011-0001667/2011-0001669) and the Korea Research Foundation Grant funded by the Korean Government (357-2008-1-C00070). The authors thank Prof. Gerard Ferey, and Dr. Christian Serre for their helpful discussion.
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Notes
Bull. Korean Chem. Soc . 2011, Vol. 32, No. 6    1
DOI 10.5012/bkcs.2011.32.6.
Effect of Diamine in Amine-Functionalized MIL-101 for Knoevenagel Condensation
Palraj Kasinathan,†,‡ You-Kyong Seo,† Kyu-Eun Shim,†,‡ Young Kyu Hwang,†,‡,* U-Hwang Lee,†,* Dong Won Hwang,†
Do-Young Hong,† Shiva B. Halligudi,† and Jong-San Chang †,‡
†
Research Group for Nanocatalyst, Korea Research Institute of Chemical Technology, Daejeon 305-600, Korea
*
E-mail: kr
‡
School of Science, University of Science and Technology (UST), Daejeon, 305-333, Korea
Received February 28, 2011, Accepted April 6, 2011
Table S1. Physicochemical properties of as-prepared and function-alized MIL-101 samples Sample a N content (mmolg −1)b
S BET (m 2g −1)c P.V (cm 3g −1)d PSD (nm)e MIL-101(Cr)-4035  1.96  1.9/1.4ED-MIL-101  2.13101 
1.49  1.8/1.3PD-MIL-101  1.42981  1.44  1.7/1.2BD-MIL-101  1.92603  1.21  1.6/1.1DD-MIL-101
1.3
2446
1.17
1.6/1.0
a
ED, BD, DD and PD are abbreviation of ethane-1,2-diamine, butane-1,4-diamine, decane-1,10-diamine and benzene-1,4-diamine, respec-tively. b Determined from Elemental analysis. c Surface area evaluated using the Brunauer-Emmett-Teller (BET) method from the N 2 adsorption isotherm. d Pore volume taken by a single point method at p/p 0 = 0.99.e
Pore size distribution values are obtained by the BJH equation.
Figure S1. XRD patte rns of (a) as-pre pare d Porous chromium te re phthalate  (MIL-101(Cr)) and (b-e )
amine s functionalize d samples; (b) ethane-1,2-diamine (ED-MIL-101) (c) butane-1,4-diamine (BD-MIL-101) (d) decane-1,10-diamine (DD-MIL-101)and (e) benzene-1,4-diamine (PD-MIL-101), respectively.
Figure S2. Pore-size distribution curves with adsorption volume and pore diameter. (a) as-prepared MIL-101, (b) ED-MIL-101, (c)PD-MIL-101, (d) BD-MIL-101 and (e ) DD-MIL-101, re spe c-reaction diffusion
tively.
Figure S3. FT-IR Spe ctra of amine  functionalize d MIL-101sample s at 473 K. (a) DD-MIL-101, (c) BD-MIL-101, (d) ED-MIL-101 and (e) PD-MIL-101.

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