A 2D metal-organic framework for selective adsorptions on organic dyes

Article abstract of DOI:10.1016/j.ica.2016.03.024

Removal of organic dyes has become an important issue from the biological and environmental standpoint. A metal-organic framework {[Zn₂(L)(H₂O)(DMA)]·DMA·H₂O}_n (1) (H₄L = 9-(3,5-Dicarboxy-benzyl)-9H-c

Full text of DOI:10.1016/j.ica.2016.03.024

Inorganica Chimica Acta 446 (2016) 198–202
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
A 2D metal–organic framework for selective adsorptions on organic dyes
a
Ji Xu a,b, Qidong Zhuo b, Renzhong Fu b, Hongjian Cheng b, Xiaoyan Tang b, Yunsheng Ma b, , Jimin Xie ,
⇑
Rongxin Yuan b,
⇑
a School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China
b School of Chemistry and Materials Engineering, Changshu Institute of Technology, Changshu 215500, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Removal of organic dyes has become an important issue from the biological and environmental
standpoint. A metal–organic framework {[Zn2(L)(H2O)(DMA)]ꢀDMAꢀH2O}n (1) (H4L = 9-(3,5-Dicarboxy-
benzyl)-9H-carbazole-3,6-dicarboxylic acid, DMA = N,N0-dimethylacetamide) has been synthesized
based on a multicarboxylate ligand through solvothermal reaction. Compound 1 has a 2D bilayered
structure and exhibits effective adsorption on organic dyes. The adsorbing capacity of MB and MO are
139.6 mg/g and 116.9 mg/g, respectively. The selective adsorption properties on dyes were studied.
Ó 2016 Elsevier B.V. All rights reserved.
Received 18 January 2016
Received in revised form 13 March 2016
Accepted 18 March 2016
Available online 25 March 2016
Keywords:
Metal–organic framework
Dye adsorption
Separation
Solvothermal
1. Introduction
MOFs have been considered as a promising adsorbent materials
for gas [14] and hazardous substance [15].
Air, water and soil pollutions are always going along with the
rapid development of industrialization [1]. Finding effective
methods to handle the heavy-metal, pesticide, printing and dying
waste in polluted water are hot research topics. The traditional
ways to treat these are biological oxidation [2], and advanced
oxidation [3]. Because of the simplicity, low cost and high
efficiency, removing contaminants through adsorption is deemed
as an adaptable method [4–5]. Generally, porous materials are
thought to be good materials in adsorption, so porous materials,
such as activated carbon and zeolites were studied in this area.
However, these materials are generally poor at selective removing
contaminants [6]. It is necessary to find new kind of porous
materials to solve this problem.
Considering many dyes have the conjugated system, we
convinced that a ligand with the large conjugated system, such
as carbazole derivatives, could stabilize the dyes through
p–p
interactions/stacking with dyes. In addition, variable interactions
between the framework and different dyes, could facilitate the
separation of dyes. Herein, a semi-rigid ligand 9-(3,5-Dicarboxy-
benzyl)-9H-carbazole-3,6-dicarboxylic acid (H4L, Scheme 1) that
has a large conjugation system has been designed to build a 2D
MOF {[Zn2(L)(H2O)(DMA)]ꢀDMAꢀH2O}n (1). The selective adsorp-
tion properties for dyes were studied.
2. Experimental
The keys to success in achieving ideal adsorbents for selectively
removing contaminants are the following: (1) A framework shows
stable and flexible properties, which are important for accommo-
dating guests [7]. (2) The framework could interact with guests
through some interactions, such as ionic interactions [8–9], hydro-
2.1. Materials and methods
3,6-Diacetyl-9H-carbazole [16] and 5-Bromomethyl-isophthalic
acid diethyl ester [17] were prepared according to the literature
method. All other materials were purchased as reagent grade and
used without any further purification. The IR spectra were
obtained as KBr disks on a NICOLET 380 spectrometer. Elemental
gen bonding [10],
p–p interactions [11]. Metal–organic frame-
works (MOFs) also known as porous coordination polymers
(PCPs) are one class of porous materials constructed from metal-
containing nodes and organic linkers [12]. Due to their variable
structures, ultrahigh porosity and large specific surface area [13],
analyses were performed on
a Perkin Elmer240C elemental
analyzer. The solution UV–Vis absorbance spectra were collected
on a Cary-300 UV–Vis spectrophotometer. Powder X-ray diffrac-
tion patterns (PXRD) were determined with Rigaku-D/Max-2200
X-ray diffractometer. The field-emission scanning electron micro-
scopy (SEM) images were obtained by using a JEOL JSM-6700F
⇑
Corresponding authors. Tel.: +86 51252251173.
E-mail addresses: myschem@hotmail.com (Y. Ma), yuanrx@cslg.edu.cn
(R. Yuan).
http://dx.doi.org/10.1016/j.ica.2016.03.024
0020-1693/Ó 2016 Elsevier B.V. All rights reserved.

J. Xu et al. / Inorganica Chimica Acta 446 (2016) 198–202
199
Table 1
Crystal data and structure refinements for compound 1.
Compound
1
Formula
Formula weight
T (K)
C
31H33N3O12Zn2
770.37
296(2)
Crystal system
Space group
a (Å)
monoclinic
P2(1)/n
11.8370(15)
14.6492(19)
21.388(3)
3676.0(8)
4
b (Å)
c (Å)
V (Å3)
Z
Dcalc (g cmꢁ3
)
1.202
1.350
Scheme 1. The chemical structure of the ligand H4L.
l
(mmꢁ1
)
F (000)
Rint
1352
0.0650
1.089
0.0677
0.1964
0.0863
0.2141
scanning electron microscope. 1H NMR spectrum was recorded on
a spectrometer (Bruker AVANCE DPX 300).
Goodness-of-fit (GOF) on F2
R1 [I > 2
r
(I)]a
wR2 [I > 2r
(I)]a
R1 (all data)a
2.2. Systhesis of 9-(3,5-Dicarboxy-benzyl)-9H-carbazole-3,6-dicar-
boxylic acid (H4L)
wR2 (all data)a
a
R1
=
R
(|Fo| ꢁ |Fc|)/
R
|Fo|, wR2 = [
R
w(|Fo| ꢁ |Fc|)2/
R .
w F2o]1/2
3,6-Diacetyl-9H-carbazole (0.75 g, 3 mmol) was dissolved in
50 mL acetone, then K2CO3 (0.42 g, 3 mmol) was added under
stirring. The mixture was refluxed for 1.5 h before dropping
5-bromomethyl-isophthalic acid diethyl ester (1.05 g, 3.3 mmol)
and kept at 60 °C overnight. Then, the mother solution was evapo-
rated to give the crude product, which was purified by column
chromatography (pet. ether: ethyl acetate = 1:1). 5-(3,6-Diacetyl-
carbazol-9-ylmethyl)-isophthalic acid (0.65 g, 1.5 mmol) was
obtained by hydrolyzed in NaOH solution and acidified with HCl.
Pure H4L was obtained through bromoform reaction according to
the literature method [18] (Scheme S1). IR (KBr, cmꢁ1): 3445.1
(m), 2981.9(w), 1693.4(s), 1629.6(s), 1598.5(s), 1485.2(m),
1418.2(s), 823.4(m), 769.0(s), 650.1(s), 543.0(s). 1H NMR
(400 MHz, DMSO-d6): 5.86–5.90 (2H, d, J = 16 Hz), 7.66–7.67 (4H,
dd, J = 4 Hz), 7.90–7.92 (2H, d, J = 8 Hz), 8.33 (1H, s), 8.54–8.61
(2H, d, J = 28 Hz).
2.3. Synthesis of compound {[Zn2(L)(H2O)(DMA)]ꢀDMAꢀH2O}n (1)
A mixture of Zn(NO3)2ꢀ6H2O (36 mg, 0.12 mmol), H4L (25.8 mg
0.06 mmol), DMA (3 mL), EtOH (6 mL) and H2O (6 mL) was placed
in a flask and stirred for 30 min at room temperature, then it was
transferred to a Teflon-lined autoclave (25 mL), the block crystal
was obtained after 3 days at 90 °C (yield, 40%, based on ligand).
Calc. for C31H33N3O12Zn2: C, 56.34; H, 4.28; N, 5.45. Found: C,
56.68; H, 4.86; N, 5.39%. IR (KBr, cmꢁ1): 3412.5(s), 1603.5(s),
1448.9(w), 1397.5(s), 1303.5(m), 1263.7(w), 1211.8(m), 1147.7
(m), 780.5(s), 724.8(m), 664.3(m), 599.1(w).
2.4. X-ray crystallography
Single crystal of compound 1 was tested on a Bruker APEX II
DUO CCD diffractometer equipped with graphite-monochroma-
tized Mo K
a radiation (k = 0.71073 Å) at room temperature, cell
parameters were refined on all observed reflections using the
Bruker SAINT (Bruker AXS, 2013). The collected data were reduced
by the program Bruker SAINT
, and an absorption correction
(multi-scan) was applied. The reflection data were also corrected
for Lorentz and polarization effects. The structures were solved
by direct methods and refined on F2 by full matrix least squares
using SHELXTL [19]. There were some guest solvent molecules were
chemically featureless to refine, to solve the issue, the SQUEEZE pro-
gram implemented PLATON was used to remove contribution of
these solvent electron densities, the void volume and void count
electrons are 581.4 and 211.7, which indicate the presence of one
Fig. 1. (a) Coordination environment of Zn1 and Zn2. (b) View of compound 1 along
the a-axis (the purple chain is the zigzag chain linked by carbazolyl). (c) Packing
diagram of compound 1 viewed along the b-axis. Symmetry codes: A = x, ꢁ1 + y, z;
B = x, 1 + y, z; C = 1/2 ꢁ x, ꢁ1/2 + y, 3/2 ꢁ z; D = 1/2 ꢁ x, 1/2 + y, 3/2 ꢁ z; E = 1 ꢁ x,
3 ꢁ y, 2 ꢁ z; F = ꢁ1/2 + x, 3/2 ꢁ y, ꢁ1/2 + z; G = ꢁ1/2 + x, 5/2 ꢁ y, ꢁ1/2 + z;
H = 1/2 + x, 3/2 ꢁ y, 1/2 + z; I = 1/2 + x, 5/2 ꢁ y, 1/2 + z.

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J. Xu et al. / Inorganica Chimica Acta 446 (2016) 198–202
DMA and one water molecule in the asymmetric unit. The crystal-
lographic and refinement details are listed in Table 1. Selected
bond lengths and angles are given in Table S1.
and O10 from water. Zn2 has tetrahedral geometry that was coor-
dinated by (O2, O4F) from two carboxylates of different carbazolyl
groups, O5C from carboxylate of phenyl and O9 from DMA. Zn1
and Zn2 atoms are bridged by three carboxylates into a dimer with
Zn1ꢀ ꢀ ꢀZn2 distance of 3.24 Å. These dimers are linked by carbazolyl
groups into a zigzag chain along the b-axis. Adjacent chains are
joined together through one phenyl carboxylate into one plane.
Adjacent planes are linked by the other phenyl carboxylate into
2.5. Dye adsorption experiments
In order to investigate the ability of compound 1 for dye adsorp-
tion, 10 mg of compound 1 was immersed in a vial containing
0.25 mg/mL methylene blue (MB) (5.6 mL) and methyl orange
(MO) (5.6 mL) respectively. Each of the solution was stirred for
a
double layer (Fig. 1b). Both carboxylates from carbazoyl
group adopt l2
g1 bidentate bridging coordination mode,
while one carboxylate group from the phenyl group adopts mono-
chelate coordination mode and the other one adopts l2
bidentate bridging coordination mode. stacking interactions
-
g1
,
4 h and precipitate was filtrated through centrifuge. 100 lL solu-
-
g1 g1
,
tion was taken out from the filtrate and diluted into 2 mL with
water. The abilities of the compound to adsorb dyes from aqueous
solution were determined through UV spectrophotometry.
p–p
were observed between the adjacent double layers with shortest
centroid to centroid distance of phenyl groups of 3.6 Å (Fig. S1),
which stabilizes the 3D structure.
3. Results and discussion
3.1. Structure description of compound 1
3.2. Dye adsorption of compound 1
Compound 1 crystallizes in the monoclinic space group P21/n
and has an asymmetry unit consisting of one L4ꢁ ligand, two Zn
(II) ions, one coordinated water and DMA molecule. Zn1 has dis-
torted trigonal bipyramid geometry that was coordinated by (O1I
and O3B) from different carboxylates of different carbazolyl
groups, (O6E and O7) from carboxylates of different phenyl groups
Adding compound 1 into the dye solution caused a large
adsorption (Fig. 2), which could see from the obviously lighted
solution color. The removal of MB and MO can reach 99.4% and
83.5% respectively (Table S2). Thus, the general formula of the
resulting precipitates can be proposed as 1ꢀ0.29MB and
1ꢀ0.28MO. The contrast test of ligand (10 mg) on the adsorption
Fig. 2. (a) UV–Vis absorption spectra of MB solution with the use of ligand (MB@L, black), compound 1 (MB@1, red) and the control experiment without compound 1 (blue).
(b) UV–Vis absorption spectra of MO solution with the use of ligand (MO@L, black), compound 1 (MO@1, red) and the control experiment without compound 1 (blue).
(c) UV–Vis absorption spectra of adsorption of MB–MO mixture solution with the use of compound 1 (red) and the control experiment without compound 1 (black). (For
interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

J. Xu et al. / Inorganica Chimica Acta 446 (2016) 198–202
201
and Lꢀ0.04MO. The stability of MB@1 and MB@L was further
checked by immersing the samples in water and methanol. It is
found that both MB@1 and MB@L are stable in water. In the case
of methanol, the adsorbed MB could be released from MB@L com-
pletely, while seldom MB could be released from MB@1 (Fig. S2).
This result indicates that the interactions between MB and the
framework are strong. To explore the adsorption mechanism, we
examined the release performance of MB@1 and MB@L in water
under external stimulus, for example the salt response of the com-
posite. In this test, the loaded dye molecules are barely released in
water solution of LiNO3 [20], which exclude the guest cationic sub-
stitution-driven process. Thus we convinced the
p–p interactions
between dyes and framework play the key role in the adsorption
process. Relatively, MO@1 and MO@L are not stable in water and
methanol, which would release MO molecules readily (Fig. S2).
This may be explained by the fact that the MO that has the smaller
conjugated structure could form weaker p–p interactions with the
framework than MB do. The different adsorption ability of ligand
on MB and MO also indicates that the main factor of the adsorption
of compound 1 on dyes is p–p interaction.
To check the stability of compound 1, the SEM images of differ-
ent samples were investigated (Fig. 3), the crystal could maintain
their morphologies after different treatments, indicating the
framework is stable. Then the samples were further checked by
powder X-ray diffraction (PXRD) (Fig. 4). After being immersed in
water, the crystals still maintain their crystalline character with
peaks move to higher angles. This indicates that compound 1 is
stable and shrinked after exchanging of DMA and water molecules
in water. After dye adsorption, the peaks move to the lower angle
compared with that immersed in water one. This evidence means
that the dyes entered into the pores of crystals and expanded the
framework [21–23]. Because the different interactions between
MB/MO and frameworks, the selective adsorption was performed.
Compound
1 (10 mg) was immersed in a 13.4 mL MB–MO
mixed solution containing MB (0.25 mg/mL, 5.6 mL) and MO
(0.25 mg/mL, 2.8 mL) and the mixture was stirred for 4 h, the color
of the solution changed from indigo to orange. From Fig. 2c, the
peaks of MB disappeared after adsorption, while the peaks of MO
still maintain. Thus compound 1 has the selective adsorption
ability on the separation of MB–MO.
4. Conclusions
Fig. 3. The SEM of compound 1 after different treatments: (a) immersed in water
(b) MB adsorption (c) MO adsorption.
In this work, a tetracarboxylic acid bearing carbazolyl group
was designed to build a flexible bilayered compound, in which
p–p interactions were observed between adjacent layers. The com-
pound could function not only as absorbent materials for MB and
MO, but also as selective absorbent for MB from MB–MO solution.
Thus, we believe the introduction of bulky ligands with
p conjuga-
tion system can help to design many flexible MOFs that have dye
adsorption function. This study will widen the field of porous
nano-species science. Further work is in progress to construct
new MOFs with the combination of H4L and copper clusters.
Acknowledgment
This work is supported by NSF of Jiangsu Provence (Nos.
14KJA150001, 14KJB150001). The Six Talent Peaks project in
Jiangsu Province.
Fig. 4. The PXRD patterns for compound 1: (A) simulated, (B) experimental, (C)
immersed in water, (D) after adsorption on MO and (E) after adsorption on MB.
Appendix A. Supplementary material
of MB and MO were performed. It is found that the removal of MB
and MO can reach 98.7% and 22.2% respectively. Thus, the general
formula of the resulting precipitates can be proposed as Lꢀ0.16MB
CCDC 1444629 contains the supplementary crystallographic
data for compound 1. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via

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