Amine-grafted on lanthanide metal-organic frameworks: Three solid base catalysts for Knoevenagel condensation reaction

Article abstract of DOI:10.1016/S1872-2067(15)60945-7

A post-synthetic modification strategy has been used to prepare three solid base catalysts, including Er(btc)(ED)_0.75(H₂O)_0.25 (2, btc = 1,3,5-benzenetricarboxylates, ED = 1,2-ethanediamine), Er(btc)(PP)_0.55(H

Full text of DOI:10.1016/S1872-2067(15)60945-7

Chinese Journal of Catalysis 36 (2015) 1949–1956
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Amine‐grafted on lanthanide metal‐organic frameworks: Three
solid base catalysts for Knoevenagel condensation reaction
Yanwei Ren a,*, Jiaxian Lu a, Ou Jiang b, Xiaofei Cheng a, Jun Chen b
a School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, Guangdong, China
b Guangdong Winner Functional Materials Co., Ltd, Foshan 528521, Guangdong, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A post‐synthetic modification strategy has been used to prepare three solid base catalysts, in‐
cluding Er(btc)(ED)0.75(H2O)0.25 (2, btc = 1,3,5‐benzenetricarboxylates, ED = 1,2‐ethanediamine),
Er(btc)(PP)0.55(H2O)0.45 (3, PP = piperazine), and Er(btc)(DABCO)0.15(H2O)0.85 (4, DABCO = 1,4‐
diazabicyclo[2.2.2]octane), by grafting three different diamines onto the coordinatively unsatu‐
rated Er(III) ions into the channels of the desolvated lanthanide metal‐organic framework
(Er(btc)). The resulting metal‐organic frameworks were characterized by elemental analysis,
thermogravimetric analysis, powder X‐ray diffraction, and N2 adsorption. Based on its higher
loading ratio of the diamine, as well as its greater stability and porosity, catalyst 2 exhibited
higher catalytic activity and reusability than catalysts 3 and 4 for the Knoevenagel condensation
reaction. The catalytic mechanism of 2 has also been investigated using size‐selective catalysis
tests.
Received 29 May 2015
Accepted 3 July 2015
Published 20 November 2015
Keywords:
Lanthanide metal‐organic framework
Post‐synthetic modification
Solid base catalyst
Knoevenagel condensation reaction
Size‐selective catalysis
© 2015, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
Based on their interesting properties, several research groups,
including our own, have been working on the development of
Metal‐organic frameworks (MOFs) constructed from metal
ions (or metal ion clusters) and bridging organic linkers as a
rapidly growing class of new materials [1–5] have attracted
considerable interest because of their high porosity, tunable
pores, diverse structures, and potential applications. MOFs, in
particular, have shown great promise as heterogeneous cata‐
lysts [6–8] because they offer several advantages over conven‐
tional immobilized catalyst systems such as well‐defined
structure with uniformly accessible catalytic centers. Among
the MOFs reported to date, lanthanide MOFs (Ln‐MOFs) have
shown the greatest potential as heterogeneous catalysts be‐
cause of the flexible coordination sphere of the lanthanide ion,
which can create a coordinatively unsaturated metal center.
Furthermore, Ln‐MOFs generally exhibit high thermal stability.
new strategies for the crystal engineering of porous Ln‐MOFs
and their application in heterogeneous catalysis [9–16].
Post‐synthetic modification (PSM) is an efficient method for
the production of novel materials that contain the intrinsic
characteristics of MOFs, including open porosities, large surface
areas, and regular structures [17–19]. PSM can be achieved by
the formation of a covalent attachment to the organic linkers
[20–23] or by the grafting of organic molecules at coordina‐
tively unsaturated metal sites [24–27]. The latter of these two
strategies involves the use of a reagent to modify the compo‐
nents of the MOF (e.g., a secondary building unit or an organic
linker) by forming a new dative bond. PSM is particularly at‐
tractive for the production of materials to be used as catalysts
because it opens the possibility of employing a wide range of
* Corresponding author. Tel: +86‐20‐87112053; Fax: +86‐20‐87113735; E‐mail: renyw@scut.edu.cn
This work was supported by the National Natural Science Foundation of China (21372087).
DOI: 10.1016/S1872‐2067(15)60945‐7 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 36, No. 11, November 2015

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Yanwei Ren et al. / Chinese Journal of Catalysis 36 (2015) 1949–1956
catalytically active organic groups in MOFs, as well as other
attendant metal ions. There have been many reports in the
literature pertaining to the catalytic properties of MOFs pre‐
pared using a PSM strategy [23,24,28–30].
III Elemental Analyzer. Infrared (IR) spectra were measured as
KBr pellets on a Nicolet Model Nexus 470 FT‐IR spectrometer
at wavenumbers in the range of 4000–400 cm–1. 1H NMR spec‐
tra were recorded on a Bruker Model AM‐400 (400 MHz) spec‐
trometer. Gas chromatography‐mass spectroscopy (GC‐MS)
was conducted on a Shimadzu GCMSQP5050A system equipped
with a 0.25 mm × 30 m DB‐WAX capillary column.
In an attempt to develop efficient new catalysts based on
Ln‐MOFs, we investigated the application of a PSM strategy to
the Ln‐MOF [Er(btc)(H2O)]·DMF0.7 (1), which has high porosity
and good stability (btc = 1,3,5‐benzenetricarboxylates). Three
different diamines, 1,2‐ethanediamine (ED), piperazine (PP),
and 1,4‐diazabicyclo[2.2.2]octane (DABCO), which represent a
variety of molecular size and basicity properties, were grafted
onto the coordinatively unsaturated Er(III) ions of desolvated
1. This grafting process resulted in the formation of several
new solid base catalysts, including Er(btc)(ED)0.75(H2O)0.25 (2),
Er(btc)(PP)0.55(H2O)0.45 (3), and Er(btc)(DABCO)0.15(H2O)0.85
(4) (Scheme 1). In these catalysts, one of the nitrogen atoms of
the diamine was coordinated to an Er(III) ion, whereas the
other nitrogen atom acted as a catalytically active site in chan‐
nels of the MOF. The catalytic properties of these three solid
catalysts were tested against the Knoevenagel condensation
reaction, and a plausible mechanism was proposed for catalyst
2 based on the results of a series of size‐selective catalysis tests.
2.2. Preparation of the catalysts
For preparation of [Er(btc)(H2O)]·DMF0.7 (1), pre‐function‐
alized MOF 1 was synthesized according to a previously re‐
ported procedure [31]. Er(NO3)3·6H2O (0.231 g, 0.5 mmol) and
H3btc (0.053 g, 0.25 mmol) were dispersed in a mixture of DMF
(4 mL) and H2O (4 mL), and the resulting mixture was heated at
105 °C for 24 h in a sealed Teflon reactor. The mixture was then
cooled to ambient temperature and purified by centrifugation
to give pure pink crystalline needles, which were dried at am‐
bient temperature. [Er(btc)(H2O)]·DMF0.7 (1): calcd. C 30.06, H
2.25, N 2.21; found: C 30.38, H 2.11, N 2.34. IR spectrum (cm–1):
3068 (w), 1628 (m), 1577 (w), 1445 (m), 1385 (s), 1213 (m),
1105 (w), 944 (w), 824 (w), 765 (m), 738 (m), 708 (s), 566 (m),
457 (w).
2. Experimental
For preparation of Er(btc)(ED)0.75(H2O)0.25 (2), after being
dehydrated under vacuum at 180 °C for 8 h, 0.5 g of desolvated
1 was suspended in 30 mL of anhydrous toluene, followed by
0.1 g (1.6 mmol) ED, and the resulting suspension was stirred
at room temperature over night. The mixture was then filtered,
and the resulting pink powder was washed sequentially with
anhydrous toluene and anhydrous diethyl ether (three times
each), and then dried at room temperature for 12 h.
Er(btc)(ED)0.75(H2O)0.25 (2): calcd. C 29.75, H 2.26, N 4.96;
found: C 28.96, H 2.52, N 5.17. IR spectrum (cm–1): 3363 (w),
3066 (w), 1613 (m), 1578 (w), 1451 (m), 1364 (s), 1210 (m),
1102 (w), 946 (w), 823 (w), 761 (m), 737 (m), 708 (s), 566 (w),
535 (w), 440 (w).
Er(btc)(PP)0.55(H2O)0.45 (3) was prepared in a similar man‐
ner to catalyst 2, except that ED was replaced with PP and the
suspension was heated to 40 °C to allow for the dissolution of
the solid amine. Er(btc)(PP)0.55(H2O)0.45 (3): calcd. C 31.29, H
2.20, N 3.58; found: C 30.71, H 2.80, N 3.59. IR spectrum (cm–1):
3063 (w), 1623 (m), 1562 (w), 1363 (s), 1209 (m), 1103 (w),
941 (w), 822 (w), 740 (m), 711 (s), 564 (w), 439 (w).
2.1. General information
All of the chemicals used in the current study were pur‐
chased from Alfa or TCI and used directly without purification.
Single crystal X‐ray diffraction analysis was performed on a
Rigaku Mercury CCD diffractometer operated at 90 kV and 50
mA using Mo Kα radiation (λ = 0.071073 nm) at room temper‐
ature. The structure was solved by direct methods, and all of
the non‐hydrogen atoms were refined anisotropically by least‐
squares on F2 using the SHELXTL program. Hydrogen atoms on
the organic ligands were generated using the riding mode.
Powder X‐ray diffraction (PXRD) patterns were collected on a
Bruker D8 powder diffractometer at 40 kV and 40 mA using Cu
Kα radiation (λ = 0.15406 nm), with a scan speed of 17.7 s/step
and a step size of 0.02° (2θ). Thermogravimetric analysis (TGA)
was performed on a Q600 SDT instrument under a steady flow
of N2 at a heating rate of 10 °C/min from ambient temperature
to 800 °C. Nitrogen adsorption measurements were carried out
at –196 °C on a micromeritics ASAP 2020 M instrument. Ele‐
mental analyses for C, H, and N were conducted on a Vario EL
Er(btc)(DABCO)0.15(H2O)0.85 (4) was prepared using similar
procedures to those described above for the preparation of 2,
except that ED was replaced with DABCO and the amine was
heated at 40 °C to allow for its dissolution.
Er(btc)(DABCO)0.15(H2O)0.85 (4): calcd. C 29.25, H 1.61, N 1.03;
found: C 28.58, H 1.86, N 0.95. IR spectrum (cm–1): 3060 (w),
1618 (m), 1561 (w), 1448 (m), 1363 (s), 1208 (m), 1102 (w),
940 (w), 824 (w), 752 (m), 710 (s), 564 (w), 534 (w), 442 (w).
H2N
R =
2
3
NH2
R
HN
NH
2.3. General procedure for Knoevenagel condensation reaction
N
4
N
The amine‐grafted MOF‐catalyzed Knoevenagel condensa‐
tion reactions of aldehydes and active methylene compounds
were conducted in magnetically stirred round bottom flasks
Scheme 1. The structure of catalysts 2, 3, and 4.

Yanwei Ren et al. / Chinese Journal of Catalysis 36 (2015) 1949–1956
1951
fitted with a reflux condenser. In a typical reaction, a mixture of
cyclohexane (10 mL), benzaldehyde (1 mmol), and malono‐
nitrile (1.1 mmol) was added to a 50 mL flask containing cata‐
lyst 2 (6 mg, 1 mol%). The concentration of the catalyst was
calculated with respect to the molar ratio of the terminal amino
group to the aldehyde. For comparison, the amounts of the
other catalysts were also calculated based on the molar
amounts of the amino groups available for the reaction (8 and
27 mg for 3 and 4, respectively). The resulting mixture was
stirred at 40 °C for 2 h. The catalyst was then separated from
the reaction mixture by centrifugation, and washed with ethyl
acetate (5 × 2 mL) to remove any physically adsorbed reagents.
The catalyst was then dried at room temperature for 6 h and
reused in the next run. The filtrate was concentrated under
reduced pressure to give the crude Knoevenagel product as a
residue, which was purified by flash column chromatography
over silica gel to give the pure product. The pure product was
1
2
3
4
100
90
80
70
60
50
0
100 200 300 400 500 600 700 800 900
Temperature (oC)
Fig. 1. TGA curves for Ln‐MOF (1) and catalysts 2, 3, and 4.
1
characterized by H NMR using 1,3,5‐trimethylbenzene as an
Although all of the coordinated water and DMF molecules in
1 could be removed under vacuum following at 180 °C for 8 h,
the coordinatively unsaturated Er(III) ions in the channels of
catalysts 2, 3, and 4 could not all be coordinated by the nitro‐
gen atoms of their respective diamines, because the simulta‐
neous grafting of the amines would be restricted in the limited
channels of 1. This process would therefore result in the partial
coordination of water molecules to the non‐grafted Er(III) ions
if the material was exposed to air. The volumes of the three
different diamines increased depending of the number of
–CH2CH2– groups between their two N atoms. The primary
amine ED had the smallest volume of the three amines tested in
the current study and could therefore move into the channels
much more readily to form a strong coordinate bond to the
coordinatively unsaturated Er(III) ions. For this reason, the
loading ratio of 2 reached 75% (i.e., about 75% of the Er(III)
ions were grafted to ED, with the other 25% being coordinated
with water molecules). Given that the volume of the secondary
amine PP is larger than that of ED, it would be more difficult for
the PP molecules to move into the channels to reach the Er(III)
ions. This difference in the volumes of the ED and PP amines
resulted in a lower loading ratio of about 55% for 3. Further‐
more, the volume of the tertiary amine DABCO is larger than
internal standard.
3. Results and discussion
3.1. Catalyst characterization
The X‐ray diffraction pattern, thermogravimetric analysis
curve, and IR spectrum of the Ln‐MOF (1) material prepared in
the current study were in agreement with those reported in the
literature [31], except for the number of DMF molecules in the
channels. In 1, the Er(III) ion was coordinated to seven oxygen
atoms, with six oxygen atoms from the six carboxylate groups
of the six btc ligands and one oxygen atom from the terminal
water molecule in a distorted pentagonal‐bipyramidal coordi‐
nation sphere. Each btc ligand was bound to six different Er(III)
ions with each carboxylate oxygen connected to one Er(III) ion,
leading to the formation of a 1D channel (0.7 nm × 0.7 nm)
running along the c‐axis. The coordinated water molecules
pointing towards the channels and the guest DMF molecules in
the channels could be readily removed to expose the empty
coordination sites of the Er(III) ions, which could then be sub‐
jected to further PSM processes with the diamine ligand. More
importantly, this MOF material showed a very high thermal
stability of more than 500 °C (Fig. 1).
As shown in Fig. 1, the TGA curve for 2 revealed that this
material underwent an 11% mass loss from 25 to 300 °C, which
was attributed to the loss of the coordinated ED and water
molecules in the channels. In contrast, the TGA curve for 3
showed that the coordinated PP and water molecules were lost
before 250 °C, whilst almost all of the coordinated DABCO and
water molecules were lost from 4 before 100 °C. Furthermore,
the occurrence of PSM processes did not influence the stability
of the structural framework of 1, and catalysts 2, 3, and 4 were
found to be stable up to 500 °C. The PXRD patterns of 2, 3, and
4 were similar to that of the pristine sample of 1 (Fig. 2), which
indicated that the structural framework of the pre‐functional‐
ized MOF had not changed significantly following the PSM pro‐
cess. Indeed, all three catalysts were found to be stable in the
solid state under ambient conditions for 30 d.
2 after used 3 times
4
3
2
1
1 simulated
10
20
30
2(o )
Fig. 2. PXRD patterns of 1, 2, 3, and 4.
40
50
60

1952
Yanwei Ren et al. / Chinese Journal of Catalysis 36 (2015) 1949–1956
those of PP and ED, which resulted in 4 having the lowest
loading ratio among the three catalysts, with only 15% of the
Er(III) ions being grafted onto DABCO.
Table 1
Catalytic performance of the different catalystsa.
The surface area and pore volume of 1 were determined to
be 2000 m2/g and 0.75 cm3/g, respectively, based on its N2
adsorption isotherm at –196 °C (Fig. 3). The mean pore size of
1 was determined to be 0.65 nm using the Horvath‐Kawazoe
model. This value was consistent with the results of the single
crystal structure analysis. Compared with 1, catalyst 2 exhibit‐
ed a significant decrease in the amount of N2 adsorbed at p/p0 >
0.01, as well as a decrease in its surface area from 2000 to 650
m2/g after the grafting process. The pore size distribution
curves of the different catalysts revealed that the amine graft‐
ing process led to a significant decrease in their pore sizes.
Furthermore, catalysts 3 and 4 exhibited negligible N2 adsorp‐
tion capacities, most likely because the channels in the struc‐
tural frameworks of these catalysts were occupied by the larger
PP and DABCO bases.
Conversion b (%) (TOFc (min–1))
Catalyst
1st run
>99
2nd run
—
3rd run
ED
PP
DABCO
1
2
3
4
—
—
—
>99
—
>99
—
<3
—
—
>99 (1.33)
93 (1.30)
63 (0.88)
98 (1.26)
74 (0.99)
22 (0.28)
96 (1.24)
55 (0.77)
—
a All of reactions were performed with benzaldehyde (1 mmol) and
malononitrile (1.1 mmol) in 2 mL of cyclohexane using catalyst (1
mol%) at 40 °C for 2 h.
b Determined by 1H NMR using 1,3,5‐trimethylbenzene as an internal
standard.
c Turnover frequency calculated as moles of product formed per mole of
free N atoms in the grafted MOF per minute during the first hour of the
reaction.
3.2. Catalytic properties
substrates gave an NMR conversion of < 3% in the presence of
the non‐grafted‐amine MOF 1, which suggested that the pres‐
ence of a basic catalytic site was essential to the activity of
these catalysts. GC‐MS analysis of the filtrate following the fil‐
tration of catalyst 2 from the reaction mixture showed that it
did not contain any free ED. However, free PP and free DABCO
were detected in the filtrates of the reactions catalyzed by 3
and 4, respectively, which indicated that some of the amine
molecules grafted to catalysts 3 and 4 had leached from the
framework into the liquors. These results therefore suggested
that the poorer catalytic performance of 3 and 4 compared
with 2 could be attributed to the leaching of the amines from
the frameworks of these catalysts. The use of ketones in the
Knoevenagel condensation reaction instead of benzaldehydes
was also investigated using similar experimental conditions.
Unfortunately, however, ketones exhibited much weaker reac‐
tivity towards malononitrile, most likely because of the lower
reactivity of ketones.
The Knoevenagel condensation reaction was selected as a
suitable reaction to evaluate the heterogeneous catalytic be‐
havior for the three solid base catalysts 2, 3, and 4. The
Knoevenagel condensation reaction is not only well known as a
good model for weak base‐catalyzed reactions, but is also well
known in terms of its application for the synthesis of
α,β‐unsaturated carbonyl compounds from aldehydes/ketones
and active methylene compounds [32–34]. In this study, we
used a 1:1.1 molar ratio of benzaldehyde to malononitrile in
cyclohexane with a catalyst loading of 1 mol% without any
prior treatment. As a control experiment, all the free amines
(i.e., ED, PP, and DABCO) were evaluated in terms of their cata‐
lytic performance in the Knoevenagel reaction, and the results
revealed that they performed effectively. As shown in Table 1,
the Knoevenagel condensation of benzaldehyde and malono‐
nitrile in the presence of catalyst 2 gave an NMR conversion of
> 99%, while catalysts 3 and 4 exhibited lower catalytic activity
(93 and 63%, respectively). These results showed that the
three amine‐grafted catalysts exhibited efficient catalytic activ‐
ity for the Knoevenagel reaction of benzaldehyde and active
malononitrile. Notably, the Knoevenagel reaction of these two
The reusability of the three amine‐grafted catalysts was also
examined. Catalyst 2 was readily separated from the reaction
mixture by centrifugation following the first run. The catalyst
600
0.4
(a)
(b)
1
1
500
400
300
0.3
0.2
2
2
200
0.1
100
0
0.0
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.5
1.0
Pore diameter (nm)
Fig. 3. Nitrogen adsorption isotherms (a) and micropore size distributions (b) of 1 and 2.
1.5
2.0
2.5
p/p0

Yanwei Ren et al. / Chinese Journal of Catalysis 36 (2015) 1949–1956
1953
was then washed with ethyl acetate (5 × 2 mL) to remove any
physically adsorbed reagents and dried at room temperature
for 6 h before being used again in a subsequent run. Pleasingly,
recycled catalyst 2 could be reused up to two times without any
discernible reduction in its catalytic efficiency. For example,
recycled catalyst 2 gave conversions of 98 and 96% for its sec‐
ond and third runs, respectively (Table 1). The PXRD pattern of
recovered catalyst 2 showed that the structural integrity of the
MOF remained almost unchanged after the catalytic reaction
(Fig. 2). Recycling studies on catalysts 3 and 4 were also per‐
formed. However, recycled catalyst 3 gave a low conversion of
only 55% after its third reaction. Furthermore, recycled cata‐
lyst 4 gave a much lower conversion of 22% for its second run.
These results therefore revealed that catalysts 3 and 4 were
unstable and that their grafted‐amines were rapidly leaching
from the channels of their MOFs to the liquor phase during the
reaction, leading to a significant reduction in their amine‐
loading and catalytic efficiency.
A major problem for solid catalysts is the possibility that
some of their active sites can migrate from the solid support to
the liquid phase during the reactions and that these leached
species are actually responsible for a significant part of the
observed catalytic activity. With this in mind, we conducted
leaching tests to determine whether the grafted‐amino groups
in catalysts 2, 3, and 4 could leach from their MOF to the liquid
phase during the course of the reaction. Briefly, the liquid
phase of the reaction mixture was collected by centrifugation
when the reaction reached 65% conversion with respect to the
benzaldehyde (after 0.5 h, Fig. 4). The reaction solution was
then transferred to a new reactor vessel and stirred for an ad‐
ditional 1.5 h, with small samples being taken for analysis at
different time intervals. The results of this experiment showed
that the reaction over catalyst 2 did not proceed any further in
the absence of the catalyst, which excluded the presence of an
active catalytic species in the solution. In contrast, the results of
leaching experiments with catalysts 3 and 4 revealed that the
filtrates from these reactions continued to react in the absence
of the solid catalysts under similar reaction conditions. These
results were therefore in accordance with those of the reusa‐
bility tests with catalysts 3 and 4. Taken together with the re‐
sults for the PXRD patterns of these catalysts, we can conclude
that the ED‐grafted catalyst 2 was the only one of the three
catalysts to exhibited genuine heterogeneous behavior as a
catalyst towards the Knoevenagel condensation reaction.
To determine whether the catalytic reaction occurred inside
the pores of catalyst 2 or on its surface, we investigated the
Knoevenagel condensation reactions of a variety of different
aldehydes with different size and shape. The reactions of ben‐
zaldehyde and para‐substituted benzaldehydes proceeded
smoothly to give the desired products with very high yields
(Table 2, entries 1–3). However, the use of 9‐anthraldehyde as
a substrate led to a much lower conversion of only 59% (Table
2, entry 4). This reduction in the conversion was attributed to
the steric hindrance of the anthracene group, which would
have made it difficult for the carbanion intermediate to attack
the carbonyl group of the aldehyde. When larger nitrile com‐
pounds containing active methylene groups were used as sub‐
strates in the Knoevenagel reaction with benzaldehyde (Table
2, entries 5–7), there was a significant decrease in the conver‐
sions as the size of nitrile increased. For example, the use of
tert‐butyl cyanoacetate as a substrate failed to provide any of
the desired condensation product, presumably because of its
larger molecular volume. For comparison, we also conducted
the same reaction in the presence of the homogeneous organic
catalyst, ED (the molar amount of amine group was the same as
Table 2
Size‐selective catalytic performances of 2 a.
Conversionb (%)
(TOFc (min–1))
Entry
a
b
1
2
NC
NC
CN
CN
>99 (1.33)
O
H3C
O2N
>99 (1.32)
92 (0.84)
O
O
3
4
NC
NC
CN
CN
O
59 (0.75)
O
NC
NC
NC
NC
(1)
(3)
100
80
60
40
20
0
5
98 (1.30)
84 (1.08)
0 (0)
O
O
O
O
O
O
O
O
6
(4)
(2)
O
O
O
7
(5)
(6)
8 d
78 (1.00)
a All of these reactions were performed with aldehyde (1 mmol) and
nitrile (1.2 mmol) in 2 mL of cyclohexane using catalyst (1 mol%) at
40 °C for 2 h.
b Determined by 1H NMR using 1,3,5‐trimethylbenzene as an internal
standard.
c Turnover frequency calculated as moles of product formed per mole of
free N atoms in the grafted MOF per minute during the first hour of the
reaction.
0.0
0.5
1.0
1.5
2.0
Time (h)
Fig. 4. Leaching tests for catalysts 2, 3, and 4. Standard reactions with 2
(1), 3 (3), and 4 (5); Leaching tests with 2 (2), 3 (4), and 4 (6).
d Using ED (1 mol%) as catalyst.

1954
Yanwei Ren et al. / Chinese Journal of Catalysis 36 (2015) 1949–1956
4. Conclusions
O
NC
We have investigated the post‐synthetic modification of a
CN
robust lanthanide‐based MOF, which led to the production of a
series of solid base catalysts, whilst preserving the structural
integrity of the MOF. The ED‐grafted catalyst 2 had a high
loading of diamine and exhibited high thermal stability and
permanent porosity, making it an effective heterogeneous cat‐
alyst in the Knoevenagel condensation reaction. This catalyst
could also be readily recovered and recycled in subsequent
runs without any discernible decrease in its activity. In con‐
trast, catalysts 3 and 4, which were based on PP and DABCO,
respectively, were found to be unstable, with the graft‐
ed‐amines readily leaching from the channels of the MOFs into
the liquor phase during the catalytic reaction, making them
behave as homogeneous catalysts.
+
NC
CN
O
H2
N
NC
CN
NH2
NC
NC
H+
CN
CN
OH
Scheme 2. Catalytic mechanism of the Knoevenagel condensation reac‐
tion catalyzed by 2.
in 2). In this particular case, the conversion was 78% (Table 2,
entry 8). These results therefore suggest that the success of the
Knoevenagel condensation reactions catalyzed by 2 is highly
dependent on the pore size of the MOF, with large substrates
being excluded from the catalytic process because they cannot
diffuse into the channels of the MOF. These results indicate that
it is necessary for the nitrile substrate, but not the aldehyde, to
enter the channels of the MOF and trigger the catalytic reaction.
Based on these results, we have proposed a plausible catalytic
mechanism for the Knoevenagel condensation reaction cata‐
lyzed by 2, which is shown in Scheme 2. According to this
mechanism, the active methylene compound would initially
enter the channels of the MOF and interact with the grafted ED
to generate a carbanion intermediate. This carbanion interme‐
diate would then react with the carbonyl carbon atom of the
benzaldehyde substrate to produce the product. The success of
this reaction process is dependent on the active methylene
compound being able to access the channels of the MOF. The
volume of the active methylene compound must therefore be
small enough to access the channels of the MOF to allow the
reaction to proceed.
References
[1] O’Keeffe M, Yaghi O M. Chem Rev, 2012, 112: 675
[2] Li J R, Sculley J, Zhou H C. Chem Rev, 2012, 112: 869
[3] Ma L Q, Abney C, Lin W B. Chem Soc Rev, 2009, 38: 1248
[4] Cook T R, Zheng Y R, Stang P J. Chem Rev, 2013, 113: 734
[5] Cui Y J, Yue Y F, Qian G D, Chen B L. Chem Rev, 2012, 112: 1126
[6] Shi D B, Ren Y W, Jiang H F, Cai B W, Lu J X. Inorg Chem, 2012, 51:
6498
[7] Liu J W, Chen L F, Cui H, Zhang J Y, Zhang L, Su C Y. Chem Soc Rev,
2014, 43: 6011
[8] Sharma M K, Singh P P, Bharadwaj P K. J Mol Catal A, 2011,
342-343: 6
[9] Ren Y W, Liang J X, Lu J X, Cai B W, Shi D B, Qi C R, Jiang H F, Chen J,
Zheng D. Eur J Inorg Chem, 2011: 4369
[10] Cai B W, Ren Y W, Jiang H F, Zheng D, Shi D B, Qian Y Y, Hu H X.
Inorg Chem Commun, 2012, 15: 159
[11] Cai B W, Ren Y W, Jiang H F, Zheng D, Shi D B, Qian Y Y, Chen J.
CrystEngComm, 2012, 14: 5285
Table 3 shows a comparison of the work in this study with
the results in several different studies for the Knoevenagel
condensation reaction between benzaldehyde and ethyl cy‐
anoacetate in the presence of a MOF‐based solid catalyst. The
results of this comparison clearly show that catalyst 2 provided
the best performance of all of the MOF‐based catalysts tested in
terms of its high TOF value.
[12] Shi D B, Ren Y W, Jiang H F, Cai B W, Lu J X. Inorg Chem Commun,
2012, 24: 114
[13] Gustafsson M, Bartoszewicz A, Martín‐Matute B, Sun J L, Grins J,
Zhao T, Li Z Y, Zhu G S, Zou X D. Chem Mater, 2010, 22: 3316
[14] Zhang J, Huang J, Yang J, Chen H J. Inorg Chem Commun, 2012, 17:
163
[15] Lin Z J, Yang Z, Liu T F, Huang Y B, Cao R. Inorg Chem, 2012, 51:
Table 3
Comparison of the Knoevenagel condensation reaction of benzaldehyde and ethyl cyanoacetate in the presence of different MOF‐based catalysts.
Ph
H
CN
Catalyst
Ph
O
NC
CO2Et
+
CO2Et
Catalyst
Catalyst loadinga (%)
Temperature (°C)
Time (h)
Conversion (%)
TOFb (min–1)
1.30
Ref.
this work
[24]
[35]
[36]
Er(btc)(ED)0.75(H2O)0.25
ED grafted MIL‐101
IRMOF‐3
Zn2(tpt)2(2‐atp)I2
ZIF‐9 d
1.1
2.1
2.6
0.5
5
40
80
40
60
25
80
2
19
1
2
4
98
98
99
37
100
98
0.041
0.635
0.617
0.083
c
[37]
[38]
e
Fe‐MIL‐101‐NH2
15.1
24
0.0103
a The catalyst loading was calculated as the molar ratio of catalytically active groups to substrates.
b Turnover frequency calculated as moles of product formed per mole of catalytically active group per minute.
c tpt = tris(4‐pyridyl)triazine; 2‐atp = 2‐aminoterephthalate.
d Benzaldehyde and malononitrile were used as the substrates.
e The composition of this MOF was Fe3OCl(H2O)2(bdc‐NH2)3; H2bdc‐NH2 = 2‐aminoterephthalic acid.

Yanwei Ren et al. / Chinese Journal of Catalysis 36 (2015) 1949–1956
1955
Graphical Abstract
Chin. J. Catal., 2015, 36: 1949–1956 doi: 10.1016/S1872‐2067(15)60945‐7
Amine‐grafted on lanthanide metal‐organic frameworks: Three solid base catalysts for Knoevenagel condensation reaction
Yanwei Ren *, Jiaxian Lu, Ou Jiang, Xiaofei Cheng, Jun Chen
South China University of Technology; Guangdong Winner Functional Materials Co., Ltd
O
NC
CN
+
NC
CN
O
H2
N
NC
CN
NH2
NC
NC
H+
CN
CN
OH
1,2‐Ethanediamine was grafted into the channels of a lanthanide metal‐organic framework (MOF) using a post‐synthetic modification
strategy, and the resulting material performed as a highly effective and recyclable heterogeneous catalyst in the Knoevenagel con‐
densation reaction.
1813
3: 675
[16] Li Y X, Xue M, Guo L J, Huang L, Chen S R, Qiu S L. Inorg Chem
Commun, 2013, 28: 25
[29] Vermoortele F, Ameloot R, Vimont A, Serre C, De Vos D. Chem
Commun, 2011, 47: 1521
[17] Wang Z Q, Cohen S M. Chem Soc Rev, 2009, 38: 1315
[18] Cohen S M. Chem Rev, 2012, 112: 970
[30] Banerjee M, Das S, Yoon M, Choi H J, Hyun M H, Park S M, Seo G,
Kim K. J Am Chem Soc, 2009, 131: 7524
[19] Corma A, García H, Llabrés i Xamena F X. Chem Rev, 2010, 110:
4606
[20] Wang Z Q, Cohen S M. J Am Chem Soc, 2007, 129: 12368
[21] Costa J S, Gamez P, Black C A, Roubeau O, Teat S J, Reedijk J. Eur J
Inorg Chem, 2008: 1551
[31] Jiang H L, Tsumori N, Xu Q. Inorg Chem, 2010, 49: 10001
[32] Angeletti E, Canepa C, Martinetti G, Venturello P. Tetrahedron Lett,
1988, 29: 2261
[33] Parida K M, Rath D. J Mol Catal A, 2009, 310: 93
[34] Tran U P N, Le K K A, Phan N T S. ACS Catal, 2011, 1: 120
[35] Gascon J, Aktay U, Hernandez‐Alonso M D, Van Klink G P M,
Kapteijn F. J Catal, 2009, 261: 75
[22] Ahnfeldt T, Gunzelmann D, Loiseau T, Hirsemann D, Senker J,
Férey G, Stock N. Inorg Chem, 2009, 48: 3057
[23] Garibay S J, Wang Z Q, Cohen S M. Inorg Chem, 2010, 49: 8086
[24] Hwang Y K, Hong D Y, Chang J S, Jhung S H, Seo Y K, Kim J,
Vimont A, Daturi M, Serre C, Férey G. Angew Chem Int Ed, 2008,
47: 4144
[25] Huang Y B, Zheng Z L, Liu T F, Lü J, Lin Z J, Li H F, Cao R. Catal
Commun, 2011, 14: 27
[36] Tan Y, Fu Z Y, Zhang J. Inorg Chem Commun, 2011, 14: 1966
[37] Nguyen L T L, Le K K A, Truong H X, Phan N T S. Catal Sci Technol,
2012, 2: 521
[38] Hartmann M, Fischer M. Microporous Mesoporous Mater, 2012,
164: 38
[26] Tanabe K K, Wang Z Q, Cohen S M. J Am Chem Soc, 2008, 130: 8508
[27] Demessence A, D’Alessandro D M, Foo M L, Long J R. J Am Chem
Soc, 2009, 131: 8784
Page numbers refer to the contents in the print version, which include
both the English version and extended Chinese abstract of the paper. The
online version only has the English version. The pages with the extended
Chinese abstract are only available in the print version.
[28] Canivet J, Aguado S, Daniel C, Farrusseng D. ChemCatChem, 2011,