Microporous assembly and shape control of a new Zn metal–organic framework: Morphology-dependent catalytic performance

Article abstract of DOI:10.1002/aoc.4097

A new metal–organic framework (MOF), [Zn(ATA)(bpd)]_∝ (1Zn) (ATA: 2-aminoterephthalic acid; bpd: 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene), exhibiting a three-dimensional extended porous structure was successfully assembled in a MeOH–H₂O solvent system. Under various controlled conditions, 1Zn was obtained in a variety of morphologies such as microspheres, microblocks, microsheets, microplates and microrods. The catalytic performance of the 1Zn microsized MOF was evaluated, and a possible catalytic mechanism was proposed. The flexibility of this MOF assembly strategy for shape control will certainly enhance new potential applications of micro?/nano?MOFs.

Full text of DOI:10.1002/aoc.4097

Received: 29 June 2017
DOI: 10.1002/aoc.4097
Revised: 16 August 2017
Accepted: 17 August 2017
F U L L P A P E R
Microporous assembly and shape control of a new Zn metal–
organic framework: Morphology‐dependent catalytic
performance
Jiajia Wang
| Yuqing Han | Haitao Xu
| Zhen‐Liang Xu
State Key Laboratory of Chemical
Engineering, East China University of
Science and Technology (ECUST),
Shanghai, China
A new metal–organic framework (MOF), [Zn(ATA)(bpd)]∝ (1Zn) (ATA:
2‐aminoterephthalic acid; bpd: 1,4‐bis(4‐pyridyl)‐2,3‐diaza‐1,3‐butadiene),
exhibiting a three‐dimensional extended porous structure was successfully
assembled in a MeOH–H2O solvent system. Under various controlled
conditions, 1Zn was obtained in a variety of morphologies such as micro-
spheres, microblocks, microsheets, microplates and microrods. The catalytic
performance of the 1Zn microsized MOF was evaluated, and a possible
catalytic mechanism was proposed. The flexibility of this MOF assembly
strategy for shape control will certainly enhance new potential applications of
micro−/nano−MOFs.
Correspondence
Haitao Xu, State Key Laboratory of
Chemical Engineering, Membrane Science
and Engineering R&D Lab, Chemical
Engineering Research Center, East China
University of Science and Technology
(ECUST), 130 Meilong Road, Shanghai
200237, China.
Email: xuhaitao@ecust.edu.cn
Funding information
KEYWORDS
National Natural Science Foundation of
China, Grant/Award Number: No.
21371058; East China University of Science
and Technology, Grant/Award Number:
YA0157117; Ministry of Education, Grant/
Award Number: A200‐C‐1101
catalytic performance, crystal structure, micro−/nanocrystals, microporous metal–organic framework
1 | INTRODUCTION
increases their new potential applications. Deeper
research is required for successfully integrating MOFs
Metal–organic frameworks (MOFs), formed by the self‐
assembly of metal ions and organic linkers, have
attracted extensive attention owing to their ultrahigh
porosity, enormous surface area, tunable pore size and
potential applications in areas such as gas storage,
separation, fluorescence, proton conductivity, catalysis
and magnetism.[1–6] Much research on MOFs remains
focused on elucidating the relation between their active
functions and topological structures. However, their
engineering remains a substantial challenge because of
subtle factors and weak interactions that ultimately play
decisive roles in determining their topologies. Many
structurally different MOFs have been reported in the
literature, but their multifunctional applications were
limited.[7] MOFs are also excellent candidates for the
fabrication of functional micro−/nanoparticles, which
into functional devices and for adequately regulating the
growth of such materials at the microscale and nanoscale.
Over the past few years, a wide range of materials
have been investigated for catalysis, such as Lewis acids,
cation‐exchanged zeolites and organometallic catalysts.
Most of them have significant drawbacks owing to the
use of hazardous and carcinogenic solvents for synthesis,
high catalyst loadings or non‐recoverable catalysts
containing toxic metals. MOFs have well‐ordered porous
structures, flexible and dynamic behaviors in response to
guest molecules and designable channel surface function-
alities. Therefore, there is a great need for MOFs to be
explored as environmentally friendly catalysts. To the best
of our knowledge, organic dyes are serious environmental
pollutants because of their high chemical stability
and versatility.[8] MOFs can be applied to catalyze the
Appl Organometal Chem. 2017;e4097.
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https://doi.org/10.1002/aoc.4097

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WANG ET AL.
degradation of such dyes.[9] As some of the most
important carbon–carbon double‐bond‐forming reactions,
Knoevenagel condensation reactions,[10] using solid base
catalysts including alkali or alkaline earth metal oxides
on a large scale,[11] are not environmentally friendly.
Fortunately, several reports have shown that MOFs
can be excellent and green catalysts for Knoevenagel
condensation.[12–15]
In the work reported herein, we developed a new
three‐dimensional microporous MOF, [Zn(ATA)(bpd)]∝
(1Zn), by combining 1,4‐bis(4‐pyridyl)‐2,3‐diaza‐1,3‐
butadiene (bpd),[16] 2‐aminoterephthalic acid (ATA) and
zinc ions in a MeOH–H2O solvent system. A series of
microcrystals with various morphologies was synthesized
via solvent‐dependent and surfactant‐templating methods.
1Zn exhibited high catalytic activity in both Knoevenagel
condensation and dye degradation reactions. Further-
more, morphology‐dependent catalytic performance of
1Zn was shown in the Knoevenagel condensation and
dye degradation reactions.
inner diameter = 0.25 mm, film thickness = 0.5 μm). Inlet
and detector temperatures were set constant at 260°C and
column temperature was 170°C.
2.3 | Dye degradation
The general degradation experiments were conducted as
follows. An amount of 20 mg of catalyst was mixed with
125 ml of dye solution with an initial concentration of
12 ppm, and then the solution was kept continuously
stirring at room temperature without the assistance of
irradiation. A 4 ml aliquot of liquid was taken from
the dye solution every 20 min and separated using a
disposable needle filter to obtain a clear solution. Then,
the clear solution was measured from the variation of
color in the reaction system by monitoring the absorbance
at the corresponding λmax with a UV–visible spectropho-
tometer (UV‐762).
3 | RESULTS AND DISCUSSION
2 | EXPERIMENTAL
3.1 | Infrared (IR), thermogravimetric
analysis (TGA), powder X‐ray diffraction
(PXRD) characterization
2.1 | Synthesis of [Zn(ATA)(bpd)] (1Zn)
Crystals suitable for single‐crystal X‐ray diffraction
studies were obtained via a slow diffusion reaction. A
mixture (4 ml) of sodium salt of ATA (0.15 mmol,
0.0272 g in 10 ml of water) and bpd (0.15 mmol,
0.0315 g in 10 ml of methanol) in methanol–water solu-
tion (20 ml) was carefully layered on an aqueous solution
(4 ml) containing Zn(NO3)2⋅6H2O (0.15 mmol, 0.0446 g)
dissolved in water (10 ml) through a buffer solution
(2 ml, methanol–water 1:1) in a test tube. After two
weeks, orange sheet crystals of 1Zn were obtained from
the junction of the layers. Microcrystals of 1Zn with
various morphologies were prepared in powder form by
direct mixing the metal solution and the ligand solution.
Elemental analysis results were as follows: calcd for
C21H19N5O5Zn (%): C, 51.81; H, 3.93; N, 14.38; found
(%): C, 49.52; H, 3.57; N, 14.43.
Absorption peaks in the IR spectrum at 3060, 2920 and
2980 cm−1 were assigned to the stretching vibration of
C―H bonds of 1Zn. The expected strong characteristic
peak at 1550 cm−1, indicating the stretching vibration of
C═N groups or the pyridine ring of bpd, was observed in
the spectrum of 1Zn. The TGA curve (Figure S5) showed
that 1Zn was stable up to approximately 312°C, exhibiting
a weight loss of 40.02% (calc. 43.21%) from 312 to 523°C
owing to its incomplete decomposition.
3.2 | Structural description of 1Zn
The 1Zn complex (Figure 1) crystallized in a monoclinic
system with the space group C2/c, with a = 24.53(2) Å,
b = 10.083(9) Å, c = 18.066(17) Å and β = 111.876(12)°.
It is constructed by combining one unique Zn(II) ion,
and two each of bpd and ATA ligands. Zn2+ ion is in a
six‐coordination environment in a distorted octahedron
and coordinated to four oxygen atoms from two ATA
ligands and two nitrogen atoms from two bpd ligands
(Figure 1a). Each Zn2+ ion is linked to two ATA
ligands and one bpd ligand to construct a six‐ring two‐
dimensional network (Figure 1b). The other bpd, whose
nitrogen atom is in the longitudinal direction of the
distorted octahedron, alternately connects the upper or
lower layers to furnish a micropore within pendant meth-
anol molecules (Figure 1c and Figure S9). The Zn―O
bond lengths range from 2.011 to 2.435 Å, and the
2.2 | Knoevenagel condensation
Knoevenagel condensation reaction was conducted in a
20 ml glass reactor. An amount of 2 mol% of 1Zn was
added to a mixture of malononitrile (1.1 mmol, 0.0727 g)
and benzaldehyde (1 mmol, 102 μl) in 5 ml of ethanol,
and n‐dodecane (140 μl) as an internal standard was
placed into the reactor. The resulting mixture was stirred
at room temperature for 4 h. The amount of products was
determined by GC 7900, which is equipped with a flame
ionization detector and a TM‐1 column (length = 30 m,

WANG ET AL.
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FIGURE 1 (a) View of coordination environment around Zn(II) of 1Zn. (b) Perspective view showing a two‐dimensional layer of Zn ions,
two ATA ligands and one bpd ligand. (c) Perspective view showing the three‐dimensional network structure of 1Zn. (d) Space filling model
of 1Zn
Zn―N distances are 2.074 and 2.085 Å. Owing to the
large Zn⋅⋅⋅Zn distance, 1Zn has one‐dimensional chan-
nels of approximately 10.8149 Å × 15.2870 Å in cross
section (Figure 1d). PXRD patterns of the fresh micro-
crystals were in agreement with the pattern simulated
from the single‐crystal data, demonstrating the high
phase purity of the 1Zn crystals (Figure S7). Crystallo-
graphic data for 1Zn are listed in Tables S1–S3
(supporting information).
FIGURE
2
Different synthetic strategies toward 1Zn
microcrystals with controlled morphologies
3.3 | Scanning electron microscopy (SEM)
characterization
sheet‐like and rod‐like shapes (Figure 4), which are in
good agreement with SEM images. As shown in
Figure 3, spherical crystals of 1Zn were easier to obtain
in methanol than in other solvents. When surfactants
were introduced, the morphology of 1Zn showed rods
with a size of ca 3 μm in the presence of Tween‐80,
blocks with a size of 1–2 μm in the presence of CTAB
and sheets with a size of 1–2 μm in the presence of
PVP. As mentioned above, crystal growth kinetics can
explain the resulting morphologies: the growth rate dif-
fered along the various crystallographic directions.[17,18]
Solution species such as alcohols, Tween, OP‐10, PVP
and CTAB selectively adsorb onto different crystal
In our work, the architecture of 1Zn microcrystals
could be controllably constructed (Figure 2) using dif-
ferent synthetic conditions. SEM images revealed that
the products synthesized using these different solution
systems exhibited various morphologies such as
sphere‐like (Figure 3a,e), block‐like (Figure 3f), sheet‐
like (Figure 3g), plate‐like (Figure 3h) and rod‐like
(Figure 3i). Energy‐dispersive X‐ray spectroscopy (EDS)
elemental mapping showed that the elements C, O, N,
Zn of 1Zn are homogenously distributed throughout
the whole structure, and the distribution images of ele-
ments in 1Zn crystal exhibited sphere‐like, block‐like,

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WANG ET AL.
FIGURE 3 SEM images of 1Zn microcrystals synthesized under various conditions: (a) methanol; (b) ethanol; (c) propanol; (d) ether; (e)
methanol–OP‐10; (f) methanol–CTAB; (g) methanol–PVP; (h) methanol–Tween‐40; (i) methanol–Tween‐80
facets, and thus impeding coordination interaction
between metal ion and organic linker. When the sur-
3.4 | Knoevenagel condensation studies
factants are added to the reaction solution, ATA and
bpd molecules are surrounded by the surfactant, and
thus restricting Zn2+ form interacting with the ligands
to form crystals. Compared with the 1Zn crystals using
OP‐10 or without additive, the nucleation rate of 1Zn
crystals slows down in the presence of PVP, CTAB or
Tween, which results in formation of particles with
better dispersion. At the same time, the surfactants
can also affect the growth rate of the facet leading to
the preparation of 1Zn with different morphologies.
The facet with the slowest growth rate determines
the final shape. Fast‐growing facets tend to grow out
of existence, whereas the slower‐growing facets tend
to survive.
To investigate the catalytic properties of 1Zn, a series
of condensation reactions was performed. Without a
catalyst, the reaction was very slow and a large amount
of reactants remained unreacted (Figure S10).
Using various solvents such as methanol, ethanol,
acetonitrile and dichloromethane for the Knoevenagel
condensation, the results indicated that higher conver-
sions were achieved in polar protic solvents, whereas the
reaction occurred with difficulty in solvents with lower
polarity, which showed a solvent effect for condensation
conversion (Table S5). The study was then extended to
the Knoevenagel condensation of various aldehydes with
malononitrile using 1Zn as catalyst. The results showed
that the reaction of benzaldehyde derivatives with

WANG ET AL.
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FIGURE 4 EDS elemental mapping of
1Zn crystal with four representative
morphologies: (a) microspheres;
(b) microblocks; (c) microsheets;
(d) microrods

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WANG ET AL.
electron‐withdrawing groups occurred faster than the
reaction of those with electron‐donating groups such as
methoxyl and hydroxyl (Table S6). The reactions of
4‐chlorobenzaldehyde and 4‐bromobenzaldehyde with
malononitrile proceeded readily to reach considerable
conversions after 4 h, with 96% conversion being
achieved for 4‐chlorobenzaldehyde and 95% for
4‐bromobenzaldehyde. The effect of catalyst concentra-
tion on reaction conversion was further investigated, with
2 mol% catalyst being enough to yield a conversion of
78% after 4 h (Figure S10). Interestingly, 1Zn showed
morphology‐dependent catalytic performance: the cata-
lytic activity for microblocks or microsheets was to some
extent higher than that for microplates or microrods.
The catalyst in the form of single crystals for Knoevenagel
condensation provided the worst catalytic performance
(Figure 5). Catalytic performances can be related to mor-
phologies and sizes by considering the number of exposed
catalytic active sites from the micro−/nanocrystals. 1Zn
could also be reused for at least five runs without a
significant decrease in its activity (Figure S11).
The PXRD pattern of 1Zn after catalysis showed the
same profile as the original samples, indicating the
integrity and stability of the MOF (Figure S7). Further-
more, Table S7 provides a comparison of the catalytic data
for 1Zn with those for catalysts reported in the literature
in terms of catalyst concentration, reaction temperature,
action time and conversion. As a result, 1Zn has the
advantages of less dosage, low temperature and high cata-
lytic activity in the Knoevenagel condensation reaction.
Based on previous studies,[19,20] a possible mechanism
for Knoevenagel condensation has been proposed
(Figure S12). The active sites for this reaction exist in
the amino groups of the ATA linkers. Owing to the basic
―NH2 site in 1Zn, the reaction between aromatic
aldehydes and active malononitrile could be promoted.
The first step during this reaction involves deprotonation
of the active malononitrile with assistance of ―NH2 site
of 1Zn to form a carbanion, followed by nucleophilic
attack on the carbonyl carbon of the aromatic aldehyde.
Then, the intermediate employs one proton from the
catalyst to create a β‐hydroxyl compound, and the catalyst
is regenerated. Finally, one molecule of water is eliminated
to form the final product.
3.5 | Dye degradation
To further explore the catalytic behavior of 1Zn, dye
degradation was also carried out. Common organic
pollutant targets methyl orange (MO), rhodamine B
(RhB), reactive red X‐3B (X‐3B), orange G (OG) and
malachite green (MG) were selected for evaluating the
catalytic activities of 1Zn. Crystals of 1Zn showed no
obvious degradation efficiency for OG, RhB and X‐3B;
however, the degradation values for MO and MG reached
90.2 and 18.2%, respectively (Figure 6), indicating the
selective catalytic properties of 1Zn.
Owing to their morphologies and sizes, the degrada-
tion catalyzed by micro−/nanocrystals was faster than
that catalyzed by single crystals (Figures S13 and S14),
and catalytic efficiency was enhanced with increasing
1Zn dosage (Figure S15). After catalytic degradation,
1Zn could be recovered, and the IR spectra and PXRD
patterns showed that the retrieved samples were identical
to the original samples (Figures S4 and S7). The degrada-
tion curve of recycled 1Zn single crystals was similar to
that of the original crystals (Figure S16), indicating the
stability of the catalysts during MO degradation. In
addition, the catalytic data for 1Zn and reported MOFs
are listed in Table S8. Most of these MOFs show catalytic
performance for MO under visible light or UV light, and
the amount of catalyst also far exceeds that of the sub-
strate. Considering the catalyst dosage, dye concentration,
reaction time and degradation efficiency, the Zn MOFs
synthesized in our work show greater degradation ability
for MO without the assistance of irradiation.
Only organic dyes containing N═N and C═C groups
could be catalytically degraded. RhB and X‐3B could not
be degraded because of their higher double‐bond energies
and steric hindrance. Furthermore, substituents linking to
the groups distinctly affect the degradation ability of 1Zn.
Unlike MO, which has two phenyl groups, OG has one
naphthyl group. The π conjugate effect of the naphthyl
group is greater than that of the phenyl group, and the
N═N group of OG is more stable and less easily oxidized
FIGURE
5
Conversion versus time for the Knoevenagel
condensation reaction of benzaldehyde with malononitrile in the
presence of catalyst in the form of single crystal, microsphere,
microblock, microsheet, microplate and microrod

WANG ET AL.
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FIGURE 6 Degradation of five dyes using 1Zn: (a) MO, (b) MG, (c) OG, (d) RhB and (e) X‐3B. The initial concentration of all the dyes was
12 ppm
than that of MO. Thus, 1Zn cannot degrade OG, but
can degrade MO effectively. According to previous
studies,[21–23] a possible formation mechanism to explain
the catalytic activity of 1Zn for the degradation of MO
has been proposed (Figure S17). First, an oxygenated
intermediary species I is generated by the coordination
of oxygen to the Zn(II) center, whose active transition‐
state complex can catalytically oxidize the N═N or C═C
groups. The second step involves a nucleophilic attack of
the unsaturated substrate on species I, followed by
oxygen transfer, leading to the formation of species II.
Finally, the Zn―O bond is ruptured, oxidizing the dye
and regenerating the catalyst. As a result of the molecular
and geometric structures of MG and MO, the bond
energy of the C═C group is higher than that of the N═N
group, and the π conjugate effect of MG is higher than
that of MO. Thus, more energy is required to break
the C═C group in MG than the N═N group in MO,
which leads to the higher degradation efficiency of MO
compared to MG.
will certainly enhance the potential applications of micro
−/nano−MOFs.
ACKNOWLEDGEMENTS
We gratefully acknowledge the financial support from
the National Natural Science Foundation of China (no.
21371058) and the Ministry of Education (A200‐C‐1101).
This work was supported in part by East China University
of Science and Technology (YA0157117).
ORCID
Jiajia Wang http://orcid.org/0000-0002-1218-9862
Haitao Xu http://orcid.org/0000-0002-2785-2110
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Xu Z‐L. Microporous assembly and shape control of
a new Zn metal–organic framework: Morphology‐
dependent catalytic performance. Appl
Organometal Chem. 2017;e4097. https://doi.org/
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