An Anion Metal-Organic Framework with Lewis Basic Sites-Rich toward Charge-Exclusive Cationic Dyes Separation and Size-Selective Catalytic Reaction

Article abstract of DOI:10.1021/acs.inorgchem.6b00019

Organic dye pollutants become a big headache due to their toxic nature to the environment, and it should be one of the best solutions if we can separate and reuse them. Here, we report the synthesis and characterization of a microporous anion metal-organi

Full text of DOI:10.1021/acs.inorgchem.6b00019

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An Anion Metal−Organic Framework with Lewis Basic Sites-Rich
toward Charge-Exclusive Cationic Dyes Separation and Size-Selective
Catalytic Reaction
Xu-Sheng Wang,†,‡ Jun Liang,† Lan Li,† Zu-Jin Lin,† Partha Pratim Bag,† Shui-Ying Gao,†
Yuan-Biao Huang,*,† and Rong Cao*,†
†State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou, Fujian, 350002, China
‡University of the Chinese Academy of Sciences, Beijing, 100049, China
S
* Supporting Information
ABSTRACT: Organic dye pollutants become a big headache due to their
toxic nature to the environment, and it should be one of the best solutions if
we can separate and reuse them. Here, we report the synthesis and
characterization of a microporous anion metal−organic framework (MOF)
with Lewis basic sites-rich based on TDPAT (2,4,6-tris(3,5-dicarboxyl-
phenylamino)-1,3,5-triazine) ligand, FJI-C2, which shows high adsorption
and separation of cationic dye based on the charge-exclusive effect.
Compared to other MOF materials, FJI-C2 shows the largest adsorption
amount of methylene blue (1323 mg/g) at room temperature due to the
nature of the anion frameworks and high surface area/pore volume.
Furthermore, motivated by the adsorption properties of large guest
molecules, we proceeded to investigate the catalytic behaviors of FJI-C2,
not only because the large pore facilitates the mass transfer of guest
molecules but also because the high density of Lewis basic sites can act as
effective catalytic sites. As expected, FJI-C2 exhibits excellent catalytic performance for size-selective Knoevenagel condensation
under mild conditions and can be reused several times without a significant decrease of the activity.
layer MOFs for separation of rhodamine B and methylene
blue.6d Most of the reported MOFs are used to separate organic
dyes based on their size exclusion.6a−g However, it is difficult to
effectively separate the dyes with similar sizes. Therefore, it is
highly desirable to develop porous MOFs to efficiently separate
mixed dyes with similar sizes. Porous ionic MOFs bearing a
positive/negative charge are potential candidates for selectively
separating organic dyes with similar sizes due to the space
charge effect between the dyes and MOFs. Recently, Su’s group
reported a unique cationic MOF as a platform for the highly
selective adsorption and separation of anionic organic dyes
through an ion-exchange process. Unfortunately, this cationic
MOF showed low adsorption capacity (2.6 wt %) and
adsorption rate (36 h).1h To the best of our knowledge, rare
ionic MOFs have been reported for rapidly separating organic
dyes with high capacity.1b,h,6i
INTRODUCTION
■
Organic dyes, as essential industrial materials, have been widely
applied to fibers, such as silk, wool, and nylon.1 However, they
should be removed or recovered to reuse because they are
unsustainable and environmentally unfriendly.2 So far, to solve
the problems, physical, chemical, and biological methods have
been developed to treat organic dye wastes.3 Among them,
adsorption is considered to be one of the best methods because
of its effectiveness and low cost. In this regard, many traditional
adsorbents, such as active carbon, zeolites, and polymeric
materials, have been used for removing organic dye pollutants.4
However, these adsorbents cannot effectively separate target
dyes to reuse, which leads to the waste of dyes. Therefore, it is
desirable to design and synthesize new adsorbent materials with
high adsorption capacity that can separate the dyes effectively.
Porous metal−organic frameworks (MOFs) have been
receiving considerable attention in the research areas of gas
storage, separations, and catalysis owing to their tunable
structure, high surface area/pore volume, and controllable
chemical environment.5 So far, a handful of MOFs have been
used for organic dye adsorption and separation.1b,d,e,6 For
example, Xu et al. constructed a mesoporous MOF, which can
be employed as column-chromatographic fillers for separation
of bulk dye molecules.6f Su et al. synthesized a series of pillared-
In this work, a new microporous negatively charged MOF
based on the H6TDPAT ligand (2,4,6-tris(3,5-dicarboxyl-
phenylamino)-1,3,5-triazine), FJI-C2, was successfully synthe-
sized by a solvothermal method and structurally characterized
by a combination techniques such as single-crystal X-ray
Received: January 6, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b00019
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Inorganic Chemistry
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Figure 1. Coordination environments of Cd and TDPAT6− in FJI-C2: (a) node1, (b) node2, (c) node3, (d) node4, (e) node5. Color code: Cd,
yellow; O, red; N, blue; C, black. For clarity, all hydrogen atoms were omitted.
Dye Adsorption and Separation. To evaluate the accessibility of
dyes to the voids of the frameworks, both the dyes’ charge and size
need to be considered. The capture experiments of individual dye were
performed on FJI-C2 first. Typically, 10 mg of fresh crystalline
samples of FJI-C2 were put in the dyes solution (10 mL). The
adsorption abilities of FJI-C2 toward those dyes were determined by
UV−vis spectroscopy. Afterward, we arranged the organic dyes in two
groups. Dyes in the first group have different charges, but similar sizes
(methylene blue (MB+), methyl orange (MO−), acid red 2 (AR20)),
whereas dyes in the second group have the same charge, but different
sizes (methylene blue (MB+), rhodamine 6G (R6G+)). Typically, the
fresh crystalline samples of FJI-C2 (10 mg) were put in the mixed dyes
solutions (10 mL). UV−vis spectra were used to measure the rapid
and selective dyes adsorption ability of FJI-C2. All the concentration
of MB+, MO−, AR20, and R6G+ is 0.03 mmol/L.
diffraction (SCXRD), powder X-ray diffraction (PXRD),
thermogravimetric analysis (TGA), Fourier transform infrared
1
spectroscopy (FT-IR), H NMR, element analysis, and N2
sorption. FJI-C2 exhibits highly selective separation and release
of cationic dye through an ion-exchange process, which can
effectively separate organic dyes with similar sizes. The dye
adsorption experiments demonstrate that large reagents are
accessible to the active sites in the channels of FJI-C2. It is
highlighted that the Lewis basic sites are distributed uniformly
over the channels of the FJI-C2, which significantly improves
the catalytic activity of the Knoevenagel condensation reactions
with size-selectivity.
EXPERIMENTAL SECTION
■
The maximum adsorption capacity of MB+ was investigated by
adding 35 mg of as-synthesized FJI-C2 into of a 10 mmol/L DMF
solution (20 mL) for 25 h at room temperature. UV−vis spectra were
used to measure the maximum adsorption capacity of methylene blue
by FJI-C2.
Materials and Methods. All the reagents and solvents were
purchased from commercial sources and used as received, except for
H6TDPAT, which was synthesized according to the literature.1g
Synthesis of H6TDPAT. 5-Aminoisophthalic acid (0.084 mol),
NaOH (0.134 mol), and NaHCO3 (0.104 mol) were added into 140
mL of H2O in a round-bottom flask. The mixture was stirred at 0 °C
for 30 min; then, 70 mL of 1,4-dioxane containing 0.02 mol of
cyanuric chloride was slowly added with constant stirring. The final
mixture was refluxed at 110 °C for 24 h. After cooling to room
temperature, the solution was adjusted to pH = 1 with concentrated
HCl. A white solid (11 g) was collected by centrifugation, washed with
Dye Release. Dye-releasing experiments were performed on pure
DMF and NaCl-containing DMF solution, respectively. UV−vis
spectra were used to measure the release ability of FJI-C2.
Knoevenagel Condensation Reaction. A 50 mg portion of FJI-
C2 (3 mol % with respect to the amount of benzaldedyde) was added
to a mixture of benzaldehyde (or benzaldehyde derivative) (1 mmol)
and malononitrile (1.1 mmol) in toluene (5 mL). After the mixture
was stirred in a preheated oil bath (35 °C) in air for the appropriate
time, the resultant mixture was filtered. The filtrate was analyzed by
GC and GC−MS to identity the structure of the target product.
Reusability of FJI-C2 in the Knoevenagel Condensation
Reaction. A mixture of FJI-C2 (50 mg), 4-cyanobenzaldehyde (1
mmol), and malononitrile (1.1 mmol) in toluene (5 mL) was stirred at
35 °C in air for 6 h. After reaction, the mixture was filtered, and the
filtrate was analyzed by GC. After the reaction, the catalyst was
separated by simple centrifugation, and washed with toluene for three
times. The recovered catalyst was reused in further reaction under
identical conditions to those of the first run with aliquots analyzed by
GC.
1
distilled water and EtOH, and dried in air. H NMR (d6-DMSO, 400
MHz): δ = 8.12 (3H), 8.47 (6H), 9.68 (3H) ppm.
Synthesis of {Cd3.5(TDPAT)2[(CH3)2NH2]+ ·(H2O)2}·(H2O)20 (FJI-
5
C2). CdCl2 (0.02 mmol), H6TDPAT (0.01 mmol), and HBF4 (0.05
mL) were added into a mixed solvent (2 mL, DMF/MeOH = 1:1) in a
10 mL vial. The mixture was ultrasonicated for 20 min and heated at
80 °C for 3 days, and then cooled to room temperature. Colorless
rodlike crystals were obtained by filtration, washed with CH2Cl2
several times, and dried by vacuum at ambient temperature. The
compound is insoluble in common organic solvents such as methanol,
ethanol, acetonitrile, acetone, DMSO, and DMF. Colorless rodlike
crystals were collected in 79.5% yield based on H6TDPAT. IR (KBr,
cm−1, Figure S1): 3399 (s), 3311 (s), 3196 (s), 3116 (s), 1633 (s),
1535 (vs), 1416 (s), 1371 (vs), 1147 (w), 1113 (w), 1016 (w), 958
(w), 897 (w), 798 (m), 777 (m), 732 (m), 620 (m).
B
DOI: 10.1021/acs.inorgchem.6b00019
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Figure 2. Packing diagram of FJI-C2, showing the 1D opening channels as viewed along the c axis. (a) The channel with A cages along c axis, (b) 3D
packing diagram, (c) the distribution of B and C cages along c axis. Color code: Cd, green; O, red; N, blue; C, black; A cage, yellow ball; B cage, pink
ball; C cage, green ball. The diameters of cages A, B, and C are 15, 11, and 8 Å, respectively. The size of the 1D opening channels is 12.4 × 7.1 Å.
Both sizes of channels and cages are measured between opposite atoms without taking account of van der Waals radii of the atoms. For clarity, all
hydrogen atoms were omitted.
knowledge, this is an extraordinary topology which has not
been reported.
RESULTS AND DISCUSSION
■
Description of the Crystal Structure. The compound
Interestingly, large nanotubes with a size of 12.4 × 7.1 Å are
formed by Cd metals and TDPAT linkers viewed in the [110]
direction. There are three kinds of microporous cages (cage A,
cage B, and cage C) with different sizes in the range of 15, 11,
and 8 Å, respectively (Figure 2). The size distribution of FJI-C2
was also simulated by poreblazer v3.0 software developed by
Lev Sarkisov in 2012 (Figure S5), which is a little smaller than
that measured by X-seed software due to that the van der Waals
radii of the atoms is counted.7 It is worth noting that those
cages are connected with each other throughout the MOF.
The total accessible volume of FJI-C2 is 20201.4 Å3, which
corresponds to 69.6% empty volume (29014.5 Å3) by the
PLATON calculation. The accessible special surface area can
reach 2315 m2/g, which was calculated by poreblazer v3.0
software. However, the N2 adsorption experiment failed to give
an expected result to calculate the surface area, probably
because the long-range order structures of the frameworks were
lost under activation at high vacuum, which often happens in
some highly porous MOFs.1g,8 The unique structural features of
FJI-C2 (large aperture, pore volume and special surface area,
anion framework with counter cations, high density of Lewis
basic sites) indicate that it has the potential applications in dye
separation based on size-/charge-exclusion and size-selective
catalytic reaction.
Dye Adsorption and Separation Properties. On the
basis of the large aperture in the anion framework of FJI-C2,
the investigation of dye capture and separation in solution
seems to be promising to explore the porosity and its potential
application. To evaluate the accessibility of dyes to the voids of
the frameworks, both dyes’ charge and size need to be
considered. Thus, we arranged the organic dyes in two groups.
Dyes in the first group (methylene blue (MB+), methyl orange
(MO−), acid red 2 (AR20)) have similar sizes with different
charges, whereas another group of dyes (methylene blue
(MB+), rhodamine 6G (R6G+)) have the same charge, but
FJI-C2 was obtained by the reaction of H6TDPAT (H6L) and
CdCl2 in the mixture solvents of DMF/CH3OH (1:1, v/v) with
a little HBF4. The chemical formula of FJI-C2 was confirmed
by SCXRD, charge-balance considerations, H NMR (Figure
S2), TGA (Figure S3), and elemental analysis (EA). Single-
1
crystal X-ray diffraction revealed that FJI-C2 is a 3D anion
framework, which crystallizes in the acentric group P42 c (No.
̅
1
114) (Table S1). The negative charge is balanced by organic
cations ([(CH3)2NH2]+) that are generated in situ upon
solvent decomposition during the solvothermal synthesis.1e As
shown in Figure 1, the asymmetric unit of FJI-C2 contains
three and a half independent Cd(II) (Cd1, Cd2, Cd3, and
Cd4) nodes, two TDPAT6− (L1 and L2) linkers, and two water
molecules. The coordination environment is depicted elabo-
rately in the Supporting Information. The phase purity of the
bulk product was confirmed by the powder X-ray diffraction
1
(PXRD) analysis (Figure S8). H NMR reveals that only H2O
and [(CH3)2NH2]+ existed in the channel of FJI-C2. The ratio
of [(CH3)2NH2]+:TDPAT is 2.5:1, which is consistent with the
charge-balance calculation (calcd: 2.5:1). The TGA curve
reveals a weight loss of 17.82% (calcd: 17.34%) from room
temperature to 120 °C, which can be attributed to the loss of
uncoordinated water. Thereafter, a weight loss of 11.62%
(calcd: 12.82%) to 340 °C prior to decomposition was
observed, which corresponds to the loss of [(CH3)2NH2]+
cations and coordinated water. Elemental analysis (%) calcd for
+
{Cd3.5(TDPAT)2[(CH3)2NH2 ]5·(H2O)2}·(H2O)20: C
32.96%, H 5.07%, N 10.51%; found: C 36.95%, H 5.20%, N
11.46%.
To improve the understanding of the framework, a topology
method was applied to analyze FJI-C2 using the nodes
mentioned above. As shown in Figure S4, the nodes are
connected through metal−carboxylate bonds and it reveals an
unprecedented (4,4,4,6,8)-connected net with the Schlafli
̈
symbol {44.610.8}2{44.612.812}{44.62}4{46}. To the best of our
C
DOI: 10.1021/acs.inorgchem.6b00019
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Figure 3. Molecular structures of the organic dyes with a similar backbone used in this study.
Figure 4. UV−vis absorption curves of MB+ (a), MO− (b), AR20 (c), R6G+ (d). The inserted photographs show the colors of the solutions before
(left) and after (right) organic dyes absorption.
different sizes. The dimensions of organic dyes with different
charges and sizes are listed in Figure 3.
colorless. Moreover, compared to other absorbents using a few
days,2,5,8 the absorption rate of MB+ by FJI-C2 was really fast,
and 90% of dye molecules can be absorbed in 1 h. The high
adsorption rate is attributed to the large size of the cage
windows (12.4 × 7.1 Å) and negative charge framework.
Notably, the FJI-C2 has the highest adsorption amount of MB+
(1323 mg/g at room temperature) among all the other MOF
materials at higher MB+ concentration. What’s more, the
adsorption capacity is also higher than that of most of the other
The capture experiments of individual dye were performed
using FJI-C2. Typically, the fresh crystalline sample of FJI-C2
was immersed into the dye solution. The adsorption abilities of
FJI-C2 toward these dyes were determined by UV−vis
spectroscopy. As shown in Figure 4, nearly all the cationic
MB+ can be captured in 1 h. This can be further seen from the
color change of solution in which the blue gradually became
D
DOI: 10.1021/acs.inorgchem.6b00019
Inorg. Chem. XXXX, XXX, XXX−XXX