854
J. Huang et al. / Journal of Molecular Structure 1146 (2017) 853e860
catalysts, amine groups were embedded in metal centers or grafted
on the frameworks of MOFs by using homogeneous amine reagents
via postsynthetic modification [19]. Nevertheless, the base leaching
and poor catalyst recycling were demonstrated in the amino
functionalized MOFs [20,21]. On the other hand, basic MOFs could
be directly synthesized by utilization of aromatic carboxylic acids
attached amino groups or nitrogen heterocyclic carboxylic acid as
ligands. Up to now, a series of amino-tagged MOFs such as IRMOF-3
and MIL-101(Al) have been successfully prepared [22]. These
amino-MOFs suffered from low condensation reactivity due to their
low pKa and low electron density caused by the two adjacent
carboxylate groups which pull the electron density away from the
aromatic amino groups. In addition, nitrogen-rich MOFs may also
suffer from low basicity due to the competitive coordination of
nitrogen with metal ions [23].
2.2. Single crystal X-ray structure determination
Single crystal X-ray diffraction data of the as-synthesized
amino-functionalized terbium MOF were collected on a Bruker
SMART diffractometer equipped with the CCD camera and
graphite-monochromated MoK
quadrate crystal was mounted on a glass fiber and the data were
a
(
l
¼ 0.71073 Å) radiation. The
ꢁ
collected at 293 K under nitrogen (data collection: Du ¼ 0.60 ;
ꢁ
ꢁ
ꢁ
2q
¼ 1.73e25.01 ;
F
¼ 0, 85, 170 ;
c
¼ 54.77 ; t ¼ 45 s). The data
were integrated in the Saintþprogram package [26]. Empirical ab-
sorption correction was carried out by using the program SADABS
[27] and refined by SHELX [28] embedded in OLEX2 [29]. In the
structural model, all non-hydrogen atoms were refined anisotrop-
ically, while all hydrogen atoms were placed in calculated positions
and refined using a riding model and refined isotropically with ꢀ1.2
(ꢀ1.5 for H atoms in methyl groups) times the isotropic atomic
displacement value of the atoms to hydrogen bonds. The calcula-
tion of the accessible void space to the solvent molecules was
performed by PLATON program [30] using the obtained crystallo-
graphic information file (CIF).
Herein, we report an amino-derived lanthanide MOF using 2-
aminoterephthalic acid as ligand. Compared with amino-tagged
MOFs constructed by moisture-sensitive aluminum or zinc ions as
3
þ
nodes, lanthanum ions (Ln ) are featured by their tolerance to
water [24]. In addition, another characteristics of Ln-MOFs is the
higher coordination number and the flexible and irregular coordi-
nation geometry of Ln3 ions, which often results in a densely
packed structure with a reduced porosity due to multiple inter-
penetration of network [25]. On the other hand, the compact
structure of Ln-MOFs also renders a favorable dense distribution of
amino groups. Meanwhile, the strong multidentate coordination
þ
2.3. Material characterization
Powder X-ray diffraction (PXRD) analysis was performed on a
Bragg-Brentano diffractometer (Rigaku D/Max-2200) with mono-
chromatic CuK
chromator, 2 in 3e50 , the step length 0.02 and scan rate 4 /min.
Fourier transform infrared (FT-IR) spectra were collected at 2 cm
a
radiation (
l
¼ 1.5418 Å) of graphite curve mono-
between Ln3 ions and carboxylate groups in Ln-MOFs may
attenuate electron-withdrawing of carboxylate groups from the
amino group on aromatic rings. All these effects are helpful to a
robust basicity, catalytic activity and recycling of Ln-MOFs for base-
catalyzed condensation reactions in aqueous medium. It is believed
that with the highly ordered and crystalline structures, MOFs cat-
alysts are easily characterized by X-ray diffraction methods in
principle, which provides precise structure information of the
catalytic active sites, thus allowing elucidation of structure-
function relationships for the catalysts. With amino groups pre-
sented inside the micropores, the amino-tagged Ln-MOFs may have
promising application in base-catalyzed Knovenagel condensation
and Henry reactions in aqueous medium.
þ
q
ꢁ
ꢁ
ꢁ
ꢀ1
resolution on
a
Nicolet Magna 550 spectrometer (KBr,
ꢀ
1
3500e400 cm ). In situ variable-temperature XRD was carried out
ꢁ
by a Buhler furnace in the temperature range of 50e900 C under
air atmosphere. The sample was smeared on a platinum sample
plate and the customized temperature programming in steps of
ꢁ
ꢁ
50 C with a heating ramp of 10 C/min. Thermogravimetric and
differential thermal analysis (TG/DTA) were conducted on a Shi-
ꢁ
madzu DTG-60A instrument with a heating ramp of 10 C/min
under nitrogen and oxygen atmosphere, respectively. The surface
electronic states were analyzed by X-ray photoelectron spectros-
copy (XPS, Perkin-Elmer PHI 5000C ESCA). All the binding energy
values were calibrated by using C1s ¼ 284.6 eV as a reference. The
Tb and C, H, N contents were determined by an inductively coupled
plasma optical emission spectrometer (ICP, Varian VISTA-MPX) and
an Elementar Vario EL/micro cube (German), respectively.
2
. Experimental section
.1. Preparation of Tb (NH
The amino functionalized lanthanide MOF (denoted as NH
2
2
2
3
-bdc) (DMF)
4
(H
2
O)
2
2.4. Catalytic activity test
2
-Tb-
The Knoevenagel condensation reaction between aldehyde and
ethyl cyanoacetate (Reaction 1) and Henry reaction between alde-
hyde and nitromethane (Reaction 2) were used to evaluate the
MOF) was synthesized under the condition of microwave irradia-
tion. The chemicals were commercially obtained and used as
received. The starting reactants Tb(NO
3
)
3
$6H
2
O
(0.603 g,
basic catalytic performances of NH
ments, the catalyst was outgassed under vacuum at 150 C for 2 h.
The phase purity of NH -Tb-MOF catalyst was confirmed by com-
2
parison of the observed and calculated PXRD patterns.
2
-Tb-MOF. Prior to the experi-
ꢁ
1
2
3
.33 mmol) and 2-aminoterephthalic acid (NH -bdc, 0.363 g,
.00 mmol) were added to a Teflon-lined vessel filled with
0.0 mL N,N-dimethylformamide (DMF). The solution was stirred
2
for 10 min at ambient temperature. Then, the vessel was closed in a
polytetrafluoroethene autoclave, heated rapidly to 120 C by mi-
2
Knoevenagel condensation. The catalytic performance of NH -Tb-
ꢁ
MOF was tested using a series of aldehydes with ethyl cyanoacetate
in different solvents. As a model system, a catalyst equivalent to
0.076 mmol amino groups, 1.0 mmol ethyl cyanoacetate and
1.2 mmol aromatic aldehyde were added in a 10 mL round-
bottomed flask containing 4.0 mL distilled water and n-decane as
an internal standard. This reaction system was stirred for 3 h at
crowave and kept at this temperature for 24 h. The yellow single
crystals suitable for X-ray diffraction analysis were obtained. The
resulting small crystals were collected and washed with distilled
water and dried in air at room temperature, because they are stable
in air and insoluble in water and organic solvents such as ethanol,
acetonitrile, tetrahydrofuran, 1,2-dichloroethane, acetone, and
N,N’-dimethylformamide. For the purpose of catalysis application,
ꢁ
40 C. Then, the products were extracted by ethyl acetate, followed
by an analysis on a GC-17A gas chromatograph (SHIMADZU)
equipped with a JWDB-5, 95%-dimethyl-1-(5%)-diphenylpolysi-
2
the synthesis condition was optimized and the pure phased NH -
Tb-MOF was obtained with high yield 85% based on the above
preparation of single crystal.
loxane column and a FID detector. N
column temperature was programmed from 80 to 250 C at a speed
2
was used as carrier gas. The
ꢁ