Nondoped blue fluorescent organic light-emitting diodes based on benzonitrile-anthracene derivative with 10.06% external quantum efficiency and low efficiency roll-off

Article abstract of DOI:10.1039/c8tc05707a

In this manuscript, by utilizing limited conjugation of carbazole and easily realizing triplet-triplet annihilation properties of 4-(anthracen-9-yl)benzonitrile, two new blue materials 4-(10-(9-phenyl-9H-carbazol-3-yl)anthracen-9-yl)benzonitrile (3CzAnBzt) and 4-(10-(4-(9H-carbazol-9-yl)phenyl)anthracen-9-yl)benzonitrile (pCzAnBzt) were designed and synthesized. In both cases, these compounds exhibited small energy gap and orbit overlapped between excited state of T_m and S_n, which would accelerate the triplet-triplet annihilation process. Additionally, when these two compounds were used as emitters in nondoped devices, the triplet energy was utilized efficiently through triplet-triplet annihilation, excellent blue electroluminescence performances were achieved. The maximum external quantum efficiency values were 10.06% and 9.23% for 3CzAnBzt and pCzAnBzt, respectively. The efficiency roll-off for 3CzAnBzt and pCzAnBzt were very low, the EQEs still remained 8.97% and 7.10% at the luminescence of 1000 cd m^-2. (0.14, 0.14) and (0.14, 0.10) are the Commission International de L'Eclairage coordinates for 3CzAnBzt and pCzAnBzt, respectively, at the voltage of 6 V. Up to now, the outstanding EL performances were the state-of-the-art compared to previously reported nondoped blue EL devices.

Full text of DOI:10.1039/c8tc05707a

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PAPER
Effect of metal–ligand ratio on the CO adsorption
2
properties of Cu–BTC metal–organic frameworks†
Cite this: RSC Adv., 2018, 8, 35551
a
b
a
Yuanyuan Liu,
Suqin Liu,
Alexandre A. S. Gonçalves
a
and Mietek Jaroniec
*
Initially, in the synthesis of Cu–BTC MOFs some fraction of Cu was expected to be replaced with Mg to
enhance its CO adsorption properties. Indeed, an enhancement in the specific surface area,
microporosity and CO adsorption capacity was observed; however, Mg was not detected. Therefore,
2
2
additional syntheses of Cu–BTC MOFs with the same Cu to BTC ratios were performed but in the
absence of Mg to explain the observed enhancement. It was found that the adjustment of the Cu–BTC
ratio to 1.09 : 1.0, which differs from that reported in the literature, resulted in a Cu–BTC MOF with
Received 19th September 2018
Accepted 10th October 2018
DOI: 10.1039/c8ra07774f
2
higher specific BET surface area, larger micropore volume, and consequently, superior CO adsorption
ꢀ1
ꢁ
rsc.li/rsc-advances
of 9.33 mmol g at 0 C and 1 bar.
Their pore size, surface area and ligand functionalities can be
easily tuned via selection of suitable metals clusters and organic
Introduction
1
0
11
Carbon dioxide (CO
gases. The increasing CO
one of the major factors causing global warming. In fact, the of their ultrahigh micropore volume and large specic surface
2
) is one of the most signicant greenhouse linkers, as well as through the post-synthetic modication.
concentration in the atmosphere is MOFs are desirable for gas adsorption and separation because
2
12
atmosphere CO
2
concentration was reported to continuously area. More than several thousands of different MOFs have
13
increase every year because of the continuous consumption of been reported so far, in which Cu–BTC is one of the most
1
fossil fuels. Therefore, the decrease of CO concentration in the widely studied framework. Cu–BTC, also known as HKUST-1,
2
14
atmosphere is urgently needed for the sustainable development was rst reported by Chui et al. in Science in 1999, and this
of our world. Recently, CO
sequestration has emerged as one of MOF and its derivatives were widely studied for gas adsorp-
the most promising technologies to defer global warming. As tion. The well-established Cu–BTC framework can be simply
2
2
15
a result, various novel strategies for CO
2
capture and storage assembled between copper cation and benzene-1,3,5-
have been reported, including membrane separation, adsorp- tricarboxylate (BTC) ligand into a microporous face centred-
3
4
5
2
tion, and cryogenic distillation. In particular, solid adsorbents cubic crystal network. As the efficient CO adsorbent, Cu–BTC
ꢀ
1
ꢁ
16,17
have been widely studied for CO sequestration because of their can adsorb CO
environmental friendliness and low energy consumption.
2
from 2.5 to 4.2 mmol g at 25 C and 1 bar.
2
6
In a general synthesis, Cu–BTC can be readily obtained by
Although numerous porous solids have been developed for mixing copper nitrate and BTC ligand in a ratio of 1.81 : 1.0. To
physical adsorption of CO
, many issues should be solved improve the CO adsorption capacity of MOFs, a wide range of
before their commercialization. Among them, the controlled strategies have been reported, such as ligands modication,
21,22
2
2
18
19
20
synthesis of adsorbents with desirable structural properties is of ion-doping, solvent activation, formation of composites,
17
great importance for efficient adsorption of CO
2
.
and optimization of the synthetic method. The successful
Recently, metal–organic frameworks (MOFs) have gained an strategies can be divided into two categories: (1) involving the
increasing attention for gas separations and adsorption owing incorporation of chemical active sites into MOFs, and (2)
to their tailorable chemical structures and versatile structural enhancing both the specic area and micropore volume of these
7,8
properties.
MOFs are microporous crystalline materials frameworks. It is of great interest to explore alternative
formed via coordination chemistry between metal ions/clusters approaches to increasethe CO
adsorption capacity of MOFs.
2
9
and organic spacers into a three dimensional periodic network.
In this work, we report the effect of metal–ligand ratio on the
assembly of Cu–BTC, to obtain materials with signicantly
a
higher CO
erties of Cu–BTC for CO
2
adsorption capacity. To enhance adsorption prop-
, our intention was to incorporate the
Department of Chemistry and Biochemistry, Kent State University, Kent, Ohio 44240,
USA. E-mail: jaroniec@kent.edu
b
2
2
+
School of Food Science and Technology & School of Chemical Engineering, Hubei basic sites such as Mg into this framework. However, our
University of Arts and Science, Xiangyang 441053, China
study found that Mg species are not present in the nal Cu–BTC
framework. A detailed analysis of synthesis conditions led to the
†
Electronic supplementary information (ESI) available: Three tables and nine
gures. See DOI: 10.1039/c8ra07774f

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conclusion that the adsorption properties of Cu–BTC can be Synthesis and characterization of Cu–BTC without Mg doping
well tuned by varying the metal–ligand ratio. A great improve-
To explain the aforementioned enhancement of the Cu–BTC
ment in the specic surface area, microporosity and CO2
samples obtained with Mg salt, we prepared the Cu–BTC samples
adsorption capacity was achieved by optimizing this ratio.
without Mg, but by varying the molar ratio of Cu to BTC to assure
the same composition as these samples obtained with Mg at the
Results and discussion
Synthesis and characterization of Cu–BTC with Mg doping
constant number of moles of both metals. Thus, pure Cu–BTC
MOFs with different molar ratios of Cu to BTC were optimized,
namely 1.45 : 1.0, 1.09 : 1.0 and 0.72 : 1.0, without the addition of
Mg salt, to achieve the same molar ratios of Cu to BTC as in the
Cu–BTC samples prepared with the addition of Mg.
In this study Cu–BTC MOF was prepared with a metal–ligand
ratio of 1.81 : 1.0 in a mixture of ethanol/water (1 : 1) as a refer-
23
ence sample. Through incorporation of the alkaline metal Mg
species, porous solids such as mesoporous silica show high
The powder XRD patterns of Cu–BTC frameworks with
controlled ratios of Cu to BTC are the same as that of the
reference Cu–BTC (1.81 : 1.0 molar ratio of Cu to BTC; see
Fig. 1). These XRD data are consistent with those on the
patterns obtained for the Cu–BTC samples with attempted
incorporation of Mg (see Fig. S1†) and for the reported Cu–
affinity to acidic CO
uptake. Additionally, Mg-MOF-74 is well known for the high
2
molecules, resulting in the increase of CO
2
24
ꢀ1
ꢁ
25
CO uptake (8.6 mmol g at 25 C and 1 bar) and selectivity,
2
which is attributed to the open Mg center in the MOF struc-
26,27
ture.
For attempted incorporation of Mg into Cu–BTC MOF,
28
BTC. The new Cu–BTC samples show octahedral morphology,
the total molar ratio of metals (Mg and Cu) to the BTC ligand
remained constant as 1.81 : 1.0, in which the molar ratio of Mg
was designed to be 20%, 40% and 60% of the total amount of
both metal ions. Using the same method as that for the fabri-
cation of pure Cu–BTC framework, the attempted incorporation
of Mg into Cu–BTC was carried out by varying the amount of Mg
salt during the synthesis. The resulting samples of MOF were
characterized by powder X-ray diffraction (XRD), scanning elec-
tron microscopy (SEM), thermogravimetric analysis (TGA) and
nitrogen adsorption (see ESI, Fig. S1–S4†). The corresponding
textural properties of these three Cu–BTC frameworks are
indicating that the variation of the metal–ligand ratio did not
alter the sample morphology. The surface morphology of these
Cu–BTC materials was examined by SEM imaging (see Fig. 2 and
S2†) to show octahedral morphology (ꢂ10 to 20 mm in width).
These SEM images provide further evidence about good crys-
tallinity of the MOF studied. The SEM images in Fig. 2 show that
2
summarized in Table S1.† The enhancement in the CO uptake is
observed with increasing amount of the Mg salt present during
synthesis (see Fig. S5 and Table S2†). Particularly, Cu–BTC
prepared with addition of 40% of Mg exhibited the highest CO
2
ꢀ
1
ꢁ
ꢁ
adsorption amount of 9.31 and 5.15 mmol g at 0 C and 25 C
(
4
1 bar), respectively. The CO
0% of Mg (9.31 mmol g at 0 C and 1 bar) is signicantly high
2
adsorption capacity of Cu–BTC with
ꢀ1
ꢁ
among various Cu–BTC derivatives reported so far (see Table 2).
Like in the case of incorporation of alkaline metals into
porous solids, we expected that Cu–BTC obtained with the
addition of Mg salt would provide MOF with Mg species. To Fig. 1 The powder XRD patterns of the Cu–BTC, Cu–BTC (80%), Cu–
BTC (60%), and Cu–BTC (40%) samples.
determine the role of Mg salt in the synthesis of Cu–BTC, we
performed the energy-dispersive X-ray spectroscopy (EDS) anal-
ysis (see Fig. S6†). Unexpectedly, the presence of Mg element was
not evidenced by EDS analysis of all three Mg (20%/40%/60%)-
Cu–BTC samples. In addition, the absence of Mg in all three
samples was also conrmed by atomic absorption spectroscopy
analysis (data not shown). These data indicate that Mg is not
involved in the formation of Cu–BTC framework, and is possibly
washed out at the end of synthesis. Therefore, the observed
improvements in the specic surface area, microporosity and
2
CO adsorption of the Cu–BTC samples synthesized with Mg salt
are not caused by incorporation of Mg into this framework.
Since Mg was not detected in the Cu–BTC MOFs assembled
in the presence of Mg salt, its addition during synthesis of Cu–
BTC is indeed equivalent with changing the molar ratio between
copper and BTC spacer. Therefore, we presume that the metal–
ligand ratio might be the leading cause for the observed
enhancement of the structural and adsorption properties.
Fig. 2 SEM images of (a) Cu–BTC, (b) Cu–BTC (80%), (c) Cu–BTC
(60%), and (d) Cu–BTC (40%) samples.
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the synthesis mixture. Interestingly, Cu–BTC (60%) shows high
2
ꢀ1
BET surface area of 1560 m g , which is ꢂ10% higher than
that of the pure Cu–BTC. Also, the surface area is slightly higher
2
ꢀ1
for Cu–BTC (40%) (1572 m g ). Moreover, both the total and
micropore volumes of these samples increase initially with
reduction of the copper amount, but then almost reach
a plateau. Specically, with addition of 60% of copper salt, the
total volume and micropore volume for this sample are higher
by 12% and 15% as compared to the reference sample. Pore size
distributions for all Cu–BTC samples exhibit two peaks, which
are shown in Fig. 4. When the ratio between metal and ligand
decreases, the pocket size became smaller as compared to that
of the parent MOF. Cu–BTC (80%) and Cu–BTC (40%) show
Fig. 3 Nitrogen sorption isotherms for Cu–BTC, Cu–BTC (80%), Cu– similar side pocket sizes (smaller than that of the parent Cu–
ꢁ
BTC (60%), and Cu–BTC (40%) samples at ꢀ196 C.
BTC), but slightly larger than that of Cu–BTC (60%). Especially,
Cu–BTC (60%) is shown to have the side pockets (ꢂ0.55 nm)
and the main channel (ꢂ0.82 nm). Theoretically, the Cu–BTC
network has the square-type 0.90 nm channels and the tetra-
hedral side pockets of 0.50 nm. Therefore, the structure of Cu–
BTC (60%) closely agrees with that described in the theoretical
model.
the crystal size of Cu–BTC (80%) is smaller than that of the
parent MOF, and the Cu–BTC (40%) sample has the smallest
crystals among the samples studied. Meanwhile, the crystal size
of Cu–BTC (60%) is larger than that of the parent MOF.
The thermal stability of these samples was analyzed by TGA.
As can be seen in Fig. S7,† the TGA proles of the samples
studied are analogous. Thus, these samples show similar
31
CO adsorption properties of Cu–BTC
2
thermal stability as the parent MOF. The initial weight loss at As seen in Table 1, interestingly, all the properties including the
ꢁ
ꢂ65 C is due to the physically adsorbed molecules (moisture BET surface area, micropore volume and CO adsorption for
2
and solvent molecules) inside pores. Then a plateau in the pure Cu–BTC (80%/60%/40%) samples obtained by varying the
ꢁ
ꢁ
range between ꢂ150 C to ꢂ250 C indicates that the structure amount of BTC match the properties of the Cu–BTC samples
is stable in this temperature range. However, a sharp decrease prepared with addition of Mg salt (Tables S1 and S2†). The Cu–
ꢁ
in the weight seen at ꢂ300 C indicates the decomposition of BTC sample with the metal–ligand ratio of 1.09 : 1.0 shows
2
ꢀ1
the sample. In addition, the mass loss for all these samples a high specic BET surface area of 1560 m g and a large
ꢁ
ꢁ
3
ꢀ1
from 250 C to 800 C is high, which is ꢂ45.10%, 50.91%, micropore volume of 0.70 cm g . These structural properties
5
6.82% and 56.13% for the samples of Cu–BTC, Cu–BTC (80%), are very close to those of Cu–BTC prepared with Mg salt (40%).
ꢁ
Cu–BTC (60%), and Cu–BTC (40%), respectively.
In addition, the CO capture amount at 0 C and 1 bar is also
2
The specic surface area and porosity of these Cu–BTC similar to that of Cu–BTC fabricated with 40% of Mg salt.
samples were determined based on the nitrogen adsorption Through simple control of the metal–ligand ratio rather than
ꢁ
isotherms at ꢀ196 C (see Fig. 3). The reversible type I adsorp- the addition of Mg salt, we can also tune the specic BET
tion isotherms indicate microporous nature of these samples. surface area, micropore volume and CO adsorption capacity.
2
The corresponding textural properties, including BET surface Therefore, we conclude that the ratio of Cu to BTC is essential
area and micropore volume are summarized in Table 1. The for controlling the structural and adsorption properties of
BET surface areas of these Cu–BTC (80%/60%/40%) samples are MOFs.
higher than that of the parent Cu–BTC. Clearly, the BET surface
area increases initially with reducing the amount of copper in evaluated at both 0 C (see Fig. 5a) and 25 C (see Fig. 5b). The
The CO adsorption performance of all four materials was
2
ꢁ
ꢁ
Table 1 The corresponding textural properties of the Cu–BTC, Cu–BTC (80%), Cu–BTC (60%), and Cu–BTC (40%) samples and the CO
adsorption amount at 1 bar
2
a
CO
2
uptake
ꢀ
1
(mmol g
)
2
ꢀ1
3
ꢀ1
3
ꢀ1
ꢁ
ꢁ
Samples
S
BET (m g
)
V
total (cm g
)
V
micro (cm g
)
0 C
25 C
Cu/BTC ratio
Cu–BTC
1415
1506
1560
1572
0.66
0.69
0.74
0.72
0.61
0.67
0.70
0.70
8.59
9.06
9.33
9.07
4.74
5.05
5.15
5.08
1.81
1.45
1.09
0.72
Cu–BTC (80%)
Cu–BTC (60%)
Cu–BTC (40%)
a
S
BET: specic surface area was calculated by BET equation in the range of P/P
0
¼ 0.05–0.2; Vtotal: total pore volume was calculated based on the
nitrogen adsorption amount at P/P
NLDFT method up to pore width of 2 nm.
0
¼ 0.99; Vmicro: micropore volume was calculated from the cumulative pore volume curve obtained by the 2D-
29,30
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presence of small micropores (<0.7 nm) was reported to be
32
crucial for high CO uptake at ambient pressures. The size of
2
side pockets is in the same range, which determines the
sequence of the CO2 uptakes at 1 bar. Having the smallest
pocket size (ꢂ0.55 nm), Cu–BTC (60%) captures the highest
2
amount of CO into micropores at ambient pressure. To the best
of our knowledge, the CO
(
2
adsorption capacity of Cu–BTC (60%)
9.33 mmol g at 0 C and 1 bar) is at the top CO uptakes
ꢀ
1
ꢁ
2
among various Cu–BTC derivatives reported so far at ambient
conditions (see Table 2). In addition, the CO /N selectivity was
2
2
checked for both Cu–BTC and Cu–BTC (60%). The CO and N
2
2
ꢁ
adsorption isotherms measured at 0 C and ambient pressure
are displayed in Fig. S8.† As shown in Table S3,† Cu–BTC (60%)
shows higher selectivity (26.4) than the parent MOF. Recycla-
bility is another important issue for solid adsorbents. The
relevant data for Cu–BTC (60%) are shown in Fig. S9,† illus-
trating a good reusability of this sorbent over four adsorption–
desorption cycles.
Fig. 4 Pore sizes distributions for Cu–BTC, Cu–BTC (80%), Cu–BTC
(
60%), and Cu–BTC (40%) samples.
Currently, the porosity engineering to tune adsorption
properties of MOFs is of great interest and achievable by direct
synthesis and/or post-modication processes. These
approaches likely create defects in the form of missing organic
3
8–42
linkers and/or metal sites in the MOF structures.
example, organic ligand defects can increase the surface areas
For
43
and porosity of MOFs. In fact, the presence of defects in MOFs
ꢁ ꢁ
2
Fig. 5 CO adsorption isotherms at 0 C (a) and 25 C (b) measured on
the Cu–BTC, Cu–BTC (80%), Cu–BTC (60%), and Cu–BTC (40%)
samples.
39
is difficult to be quantitatively evaluated. Since the molar ratio
between copper and BTC spacer was intentionally tuned, an
increase in the BTC linker percentage in the synthesis mixture
likely resulted in the structural defects. Therefore, the smaller
corresponding CO
in Table 1. At either 0 C or 25 C, the CO
2
adsorption at 1 bar of these MOFs are listed Cu/BTC ratio (1.09 : 1.0) in the synthesis of Cu–BTC likely
ꢁ
ꢁ
adsorption shows generated more defects, which resulted in the enhancement of
2
a similar trend as the changes observed for the BET surface area porosity and adsorption properties of this sample in compar-
and microporosity. Clearly, the specic surface area and the ison to the parent MOF obtained with 1.81 : 1.0 ratio.
micropore volume of these MOFs adsorbents play crucial role in
4
the adsorption of CO . The initial enhancement of the CO
2
2
Conclusions
uptake with decreasing amount of copper reaches a maximum
and next this uptake is reduced with further decrease of Cu. In summary, we report the signicance of the metal–ligand
Particularly, Cu–BTC (60%) exhibits the highest CO
2
uptakes of ratio in the synthesis of Cu–BTC MOFs, which can be used to
ꢀ
1
ꢀ1
ꢁ
ꢁ
9.33 mmol g and 5.15 mmol g under 1 bar at 0 C and 25 C, tune the textural and adsorption properties of this MOF as
respectively. As expected, an increase in the CO
2
2
adsorption evidenced by CO adsorption studies. The Cu–BTC sample ob-
amount with increasing pressure is also achieved (Fig. 5). We tained with a 1.09 : 1.0 metal–ligand ratio showed the highest
2
ꢀ1
presume that the side pockets are occupied rst, and the specic BET surface area of 1560 m
g
, large micropore
and smaller micropores (0.55 nm)
channels are subsequently lled with increasing pressure. The among the samples studied, and consequently, the highest CO2
3
ꢀ1
unoccupied positions around unsaturated Cu atoms and square volume of 0.70 cm
g
31
2
Table 2 Comparison of the CO adsorption uptakes at 1 bar for various Cu–BTC MOFs reported in literature
2
ꢀ1
3
ꢀ1
ꢀ1
ꢁ
Solvents
O/ethanol
DMF
DMF
S
BET (m g
)
V
total (cm g
)
CO
2
uptake (mmol g
)
T ( C)
Ref.
H
2
1382
1922
1611
892
1466
1542
1560
1560
0.57
0.80
0.76
0.43
0.60
0.65
0.74
0.74
6.49
7.23
3.35
2.32
4.16
2.50
9.33
5.15
0
0
33
34
35
4
36
37
25
25
25
27
0
H
H
2
O/ethanol/DMF
O/ethanol
2
H2O/ethanol
H
H
2
O/ethanol
O/ethanol
This work
This work
2
25
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ꢀ1
ꢁ
adsorption of 9.33 mmol g at 0 C and 1 bar. In addition, Cu– in water (12 mL) and 1,3,5-tricarboxybenzene was dissolved in
BTC (60%) exhibited high CO /N selectivity and good recycla- ethanol (12 mL). Next, the aqueous solution was added to
2
2
bility. We anticipated that these nding could be used for ethanol solution. Aer stirring for 20 min, the reaction mixture
ꢁ
further optimization of the synthesis conditions of already re- was transferred into autoclave and kept at 120 C for 12 h. The
ported MOFs. It is possible that their textural and adsorption resulting blue powder was obtained by centrifugation of the
ꢁ
properties can be further improved by controlling the metal– nal reaction mixture, which was further dried at 120 C under
ligand ratio and other experimental conditions. Further studies vacuum for 20 hours. When the mass of copper nitrate trihy-
of defects engineering in MOFs are in progress.
drate was 80%, 60% and 40% of 0.875 g, the product was
labelled as Cu–BTC (80%), Cu–BTC (60%), and Cu–BTC (40%),
respectively.
Experimental section
General materials
Characterization methods
All chemicals used in this work are commercially available:
copper nitrate trihydrate (99%, Acros Organics), magnesium
nitrate hexahydrate (99%, Sigma-Aldrich), 1,3,5-tricarbox-
ybenzene (95%, Strem Chemicals), ethanol (91%, Pharmco).
These chemicals were directly utilized in all experiments without
any further purication. Deionized water from the Milli-Q water
purication system was used in all experiments.
Powder X-ray diffraction (XRD) data of various nanomaterials
(Cu–BTC, Cu–BTC (80%), Cu–BTC (60%), and Cu–BTC (40%))
were obtained on a Bruker D8 Advance X-ray diffractometer using
ꢁ
ꢁ
Cu Ka radiation (0.154 nm) with scanning from 5.00 to 40.00
(
(
2q) (parameters: voltage (40 kV), current (35 mA), step size
0.020 ), and 0.8 min ). Powder X-ray diffraction measure-
ꢁ
ꢁ
ꢀ1
ments (XRD) obtained for the Cu–BTC samples (Mg 20%-Cu–
BTC, Mg 40%-Cu–BTC and Mg 60%-Cu–BTC) prepared in the
presence of Mg salt were performed on Geigerex (Rigaku, Inc.)
Synthesis of Cu–BTC MOF
ꢁ
diffractometer using Cu Ka radiation (0.154 nm) from 5.00 to
Cu–BTC samples were prepared using a slightly modied
ꢁ
23
40.00 (2q) (parameters: voltage (45 kV), current (35 mA), step size
method reported elsewhere. A copper solution obtained by
dissolving Cu(NO $3H O (0.875 g) in water (12 mL) was added
into a solution of 1,3,5-tricarboxybenzene (H BTC, 0.42 g) dis-
ꢁ ꢁ
ꢀ1
(
0.020 ), and 1.2 min ). A Hitachi S-2600 N scanning electron
3
)
2
2
microscope was utilized to obtain the scanning electron
microscopy (SEM) images and energy dispersive spectra (EDS).
SEM and EDS analyses were performed on a sample powder
previously dried and the sample surface was gold metallized.
A Q500 Thermogravimetric analyzer was used for the ther-
mogravimetric analysis (TGA). All TGA data were collected with
3
solved in ethanol (12 mL) at room temperature. The metal–
ligand molar ratio in this reaction was 1.81 : 1.0. Aer stirring
for 20 min at room temperature, the mixture was transferred
ꢁ
into an autoclave, and was kept at 120 C for another 12 h. A
blue powder product was obtained aer centrifugation. The
ꢁ
ꢁ
ꢀ1
ꢁ
heating up to 800 C (heating rate: 10 C min ) in owing air.
The nitrogen sorption was performed using ASAP 2010/2020
volumetric adsorption analyzers (Micromeritics Inc.) and
obtained powder was dried at 120 C under vacuum for 20 h.
The synthesis of Cu–BTC MOFs carried out in the presence of
Mg salt was similar to that provided above for the preparation of
Cu–BTC. However, the molar ratio of both metals (Mg and Cu)
to BTC ligand was kept the same (1.81 : 1.0) for a series of
samples obtained by using 20%, 40% and 60% of Mg in the total
number of moles of both metal components. Copper nitrate
trihydrate and magnesium nitrate hexahydrate were dissolved
in water (12 mL), and 1,3,5-tricarboxybenzene was dissolved in
ethanol (12 mL). The aqueous solution mixture was added to
the ethanol solution and stirred for 20 minutes. Then the
resulting reaction mixture was transferred to autoclave, and
ꢁ
ultrahigh-pure nitrogen at ꢀ196 C. Carbon dioxide adsorption at
ꢁ
ꢁ
0
C and 25 C for these samples was recorded on an ASAP 2020
volumetric adsorption analyzer. All samples were degassed under
ꢁ
vacuum at 200 C for 2 h before measurements. The specic
Brunauer–Emmett–Teller (BET) surface area for all samples were
evaluated from N adsorption isotherms in the pressure range of
2
0
.05–0.20. The total pore volumes (Vtotal) for the MOF studied
were determined based on the N amount adsorbed at P/P
.99. Pore size distribution (PSD) of microporous MOFs was ob-
2
0
¼
0
ꢁ
tained using a 2D-nonlocal density functional theory (2D-NLDFT)
method for heterogeneous surfaces with SAIEUS program
kept in oven at 120 C for 12 h. Finally, a blue powder product
was centrifuged from the reaction mixture and further dried at
29
ꢁ
provided by Micromeritics. The micropore volume (Vmicro) for
the MOFs studied were calculated from the cumulative pore
volume curve obtained by the 2D-NLDFT method up to pore
width of 2 nm. The CO /N selectivity was determined by the ideal
1
20 C under vacuum for 20 hours. When the molar ratio of Mg
was 20%, 40% and 60% of the total number of moles of both
metal components, the product was labeled as Mg X%-Cu–BTC
where X ¼ 20, 40 or 60.
2
2
44
adsorbed solution theory (IAST) model, based on the equation
of S ¼ n(CO )p(N )/[n(N )p(CO )] (S: the selectivity for CO /N
n(CO ): the CO adsorption amount at p(CO
) ¼ 0.15 bar; n(N
the N adsorption amount at p(N
) ¼ 0.85 bar).
Another series of Cu–BTC was synthesized without Mg salt by
using only 80%, 60% and 40% of Cu salt but keeping the same
2
2
2
2
2
2
;
2
2
2
2
):
3
amount of H BTC. Varying the amount of Cu at the same
2
2
amount of H BTC is equivalent with changing the molar ratio of
3
Cu to H BTC. This procedure is similar as that used for the
3
attempted incorporation of Mg into Cu–BTC but without addi-
tion of magnesium nitrate hexahydrate. Specically, copper
Conflicts of interest
nitrate trihydrate (80%, 60% and 40% of 0.875 g) was dissolved There are no nancial conicts to declare.
This journal is © The Royal Society of Chemistry 2018
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