Communications
Table 1: Catalytic properties of the amine-grafted MIL-101 and meso-
15).In this case, TOF values of the grafted MIL-101 were
calculated after subtracting the catalytic activity of MIL-101
from those of the grafted MIL-101.The higher activity of ED-
MIL-101 might be mainly attributed both to the easily
accessible amine functional groups and to its high surface
area.The lower activity of APS-SBA-15 can be ascribed to the
actual loss of catalytically active sites by the formation of H-
bonds between functional groups.[17] The recyclability test of
ED-MIL-101 clearly supports that it is easily isolated from the
reaction suspension by filtration and can be reused without
significant loss of activity in the third run (see the Supporting
Information, Figure S5).
Remarkably, ED-MIL-101 reveals the size dependence on
catalytic activities owing to the change of the substituent
groups of carbonyl compounds in the Knoevenagel conden-
sation.For example, with benzophenone, the condensation
reaction with malonitrile is hard to realize (see the Supporting
Information, Figure S6) because the formation of the quite
large product, 1,1-dicyano-2,2-diphenylethene, might be
occluded in the pores, indicating the transition state or
product shape-selectivity already known in microporous
zeolites.[18] The size-selective reactivity in ED-MIL-101
points out that the reaction essentially takes place in the
amine-grafted pores taking into consideration the guest-
selective properties of a 3D-porous coordination polymer
with amide groups in the base-catalyzed Knoevenagel con-
densation.[13b] This led us to tune the pore size of MIL-101 by
the type and shape of grafting agents leading to the pore
modification.
The current successful concept of amine grafting onto
CUSs has a very important consequence: the encapsulation of
metals, for which only few attempts have been made until
now.[6a,19] For example, Fischer and co-workers have per-
formed the loading of metals, such as palladium and copper,
onto MOF host lattices by the adsorption of metal organic
CVD precursors.[19a] Although the range of observed nano-
particle sizes are above the dimensions of the cage, the
corresponding solids exhibited distinct catalytic properties.
Paik Suh and co-workers have tried to generate nanoparticles
of silver and gold in situ within a flexible nickel-containing
MOF by the reduction of noble metals by the Ni2+ center of a
cyclam complex.[19b] The nickel-containing MOF network was
intact, whereas the generated nanoparticles were not incor-
porated between the layers.Therefore, inclusion of metallic
nanoparticles in MOFs still remains a challenge.In this work,
we propose another important way for the encapsulation of
noble metals, such as palladium, platinum, and gold over the
amine-grafted MIL-101 according to the procedure depicted
in Figure 1E.The encapsulation procedure (see the Support-
ing Information) comprises the neutralization of the surface
amine groups with an aqueous HCl solution, ionic reactions of
the positively charged surface ammonium groups with anionic
noble metal salts, that is, [PdCl4]2À, [PtCl6]2À, and [AuCl4]À, by
anionic exchange of the chloride anions, and finally the gentle
reduction of noble metals with NaBH4 at low temperature.
After the encapsulation of noble metals, there is no apparent
loss of crystallinity in X-ray diffraction patterns, and no
supplementary Bragg peaks appear, but the intensities of
those peaks of MIL-101 change specifically for each metal,
porous silica SBA-15 in the Knoevenagel condensation of benzaldehyde
and ethyl cyanoacetate.[a]
Catalyst
SBET
N content
Conv. Sel
[%][d]
TOF
[m2 gÀ1 [b]
]
[mmolgÀ1 cat.][c] [%]
[hÀ1
]
MIL-101
4230
3555
–
31.5[g] 99.4[g]
–
ED-MIL-101
2.07 (1.04)[f]
3.96 (1.98)[f]
3.03 (2.02)[f]
1.14
97.7[g] 99.1[g] 328
97.7[h] 99.3[h] 214
97.7[g] 99.3[g] 190
96.3[g] 99.3[g] 168
ED-MIL-101(D)[e] 3257
DETA-MIL-101
APS-MIL-101
SBA-15
3215
3306
780
–
2.89
2.6[g] 93.0[g]
74.8[i] 93.5[i]
–
32
APS-SBA-15
510
[a] Reaction was carried out with1 mmol of benzaldehyde, 1 mmol of
ethylcyanoacetate, and 20 mg of catalyst in 25 mL of cyclohexane at
353 K. [b] SBET BET surface areas obtained from N2 adsorption
:
isotherms. [c] Determined by elemental and thermogravimetric analyses.
[d] Sel: selectivity for trans-ethyl cyanocinnamate. [e] ED-MIL-101(D):
ED-grafted MIL-101 obtained by two times higher ED concentration than
that in ED-MIL-101. [f] Numbers in parentheses denote the content of
free amine group(s) available for the reaction. [g] The reaction time was
19 h. [h] 7h, and [i] 16 h. [j] TOF (turnover frequency): Moles of product
formed per mole of nitrogen in the grafted MIL-101 or the APS-SBA-15
per hour.
ium(III) CUSs are directed towards the center of the cages.
The thermal stability of the grafted amine groups is also an
important issue for further applications.Indeed, infrared
spectra recorded after outgassing of ED-MIL-101 with
increasing temperatures confirm the thermal stability of
amine species at least up to 473 K, as shown in the Supporting
Information, Figure S3.
The catalytic performance of dehydrated ED-MIL-101 in
base catalysis were measured by using the Knoevenagel
condensation as a base-catalyzed model reaction[4a] and its
activities compared to those of the APS-grafted mesoporous
silica SBA-15 (APS-SBA-15).Table 1 shows the catalytic
results of the condensation of benzaldehyde with cyanoethyl
acetate over various amine-grafted molecular sieves at 353 K.
The dehydrated mesoporous silica SBA-15 has only negligible
activity, whereas the dehydrated MIL-101 revealed low but
distinctive catalytic activity (31.5% conversion at 19 h). The
origin of activity in MIL-101 should be further clarified in
future work.Interestingly, for a small amount of catalyst
(20 mg, or 1.12m), catalytic activities of ED-MIL-101 are
noticeably better than those of APS-SBA-15, even though the
content of free amine groups in ED-MIL-101 (1.04 mmolgÀ1
)
is significantly lower than that of APS-SBA-15
(2.89 mmolgÀ1).However, the activity of ED-MIL-101(D) is
not much larger than that of ED-MIL-101, which is probably
due to the different reactivities of grafted amine species
(Table 1, and see the Supporting Information, Figure S4).For
the condensation of benzaldehyde into trans-ethyl cyanocin-
namate, the conversion for ED-MIL-101 is 97.1%, with high
selectivity (99.1%). By contrast, APS-SBA-15 exhibits only
74.8% conversion, with 93.5% selectivity. Moreover, in terms
of turnover frequency (TOF), ED-MIL-101 shows a remark-
ably superior activity (10 times higher than that of APS-SBA-
4146
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 4144 –4148