Facile synthesis of a 3D flower-like SiO2-MOF architecture with copper oxide as a copper source for enantioselective capture

Article abstract of DOI:10.1039/c9nj04031e

A facile self-template synthetic approach for the facile synthesis of a 3D flower-like SiO₂-CuLBH architecture with copper oxide as a copper source has been demonstrated. The resulting composite as a sorbent showed a certain selective separation capability for phenyl methyl sulfoxide enantiomers (PMS) with an enantiomeric excess (ee) value of 31%.

Full text of DOI:10.1039/c9nj04031e

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Facile synthesis of a 3D flower-like SiO -MOF
2
architecture with copper oxide as a copper
Cite this: New J. Chem., 2019,
source for enantioselective capture†
4
3, 16123
Received 3rd August 2019,
Accepted 28th September 2019
a
a
a
a
a
Xinglin Li, Yu Gao, Cuijie Wang, Jiting Cui, Ajuan Yu * and
b
Shusheng Zhang
DOI: 10.1039/c9nj04031e
rsc.li/njc
13–18
3 4
A facile self-template synthetic approach for the facile synthesis of Fe O , ZnO, precious metal nanoparticles, alumina and silica.
a 3D flower-like SiO –CuLBH architecture with copper oxide as a MOF composites may have not only the outstanding perfor-
2
copper source has been demonstrated. The resulting composite as mance of MOFs, but also the synergistic effects generated
a sorbent showed a certain selective separation capability for by two kinds of additives. Therefore, MOF composites are
phenyl methyl sulfoxide enantiomers (PMS) with an enantiomeric considered to be good adsorbents and separation materials
excess (ee) value of 31%.
for high-efficiency removal of targets. Silica materials tend to
form functional composites mainly due to their high specific
1
9
Chirality is a unique property of chiral molecules, which plays surface area, rich oxygen-containing groups and surface
an essential role in aspects of the medicine industry, food modifiable properties. Traditionally, the metal sources of MOFs
1
chemistry and drug manufacture. Chiral compounds often are nitrate, acetate and other inorganic metallic salts, which
ꢀ
3ꢀ 20–22
exhibit different or opposite effects in terms of biological will produce by-product anions such as Cl and NO
interaction, pharmacology and toxicity
.
2
,3
because of the They may constitute a safety hazard in industrial use and are
difference in stereoscopic structure. Therefore, considering also unfriendly to the environment. Therefore, an appealing
the importance and challenge of chiral separation, consider- alternative is to use metal oxides or metal hydroxide as metal
able chiral recognition materials and techniques have been sources for MOF building from safety and economic stand-
4
–8
developed. Solid phase extraction relying on chiral adsorbent points. Recently, metal oxides and hydroxides have been reported
23–27
materials has been proved to be the direct approach in obtaining to act as sources of metal cations for MOF synthesis.
optical isomers. Thus, it is crucial to develop novel chiral adsorbing to the best of our knowledge, there is no report using metal oxides
materials for enantioselective adsorption. and metal hydroxides as metal sources for the synthesis of
In the last decade, metal–organic frameworks (MOFs) have chiral MOFs.
However,
emerged as promising materials by virtue of their structural
Herein, we adopt the strategy of a self-template synthetic
diversity, well-defined open channels, molecular-sized cavities approach for the facile synthesis of a 3D flower-like SiO -MOF
2
and so on. Apart from their potential applications in gas ([Cu(L-mal)(bpy)]ꢁH
2
O) (SiO
2
–CuLBH) architecture. First, silica
9
–11
storage, catalysis, separation and sensing,
MOFs are also spheres were prepared as a template to form a cupric oxide
1
2
ideally suited to serve as adsorbents and separators. In shell on their surface. Second, the cupric oxide shell served as
particular, chiral MOF materials show great potential in asym- the source of metal ions and was in situ converted to MOF,
metric catalysts and enantioselective separations because of the which made the composite present a structure similar to a
chiral environment in the open channels of the framework.
‘‘flower’’. Then the enantioselective performance of the resulting
Until now, composites have been proposed by combining MOF composite SiO –CuLBH was evaluated by enantioselective adsorp-
2
materials with other substrates, such as graphene oxide (GO), tion of racemic phenyl methyl sulfoxide (PMS).
2
The morphology and structure of SiO –CuLBH was studied
a
by scanning electron microscopy (SEM). As displayed in Fig. 1a,
the prepared silica particles had a uniform size and good
dispersion. The SEM image of SiO @CuO showed that the
2
College of Chemistry, Key Laboratory of Molecular Sensing and Harmful
Substances Detection Technology, Zhengzhou University, Kexue Avenue 100,
Zhengzhou, Henan 450001, P. R. China. E-mail: yuajuan@zzu.edu.cn
Center of Advanced Analysis and Computational Science, Key Laboratory of
Molecular Sensing and Harmful Substances Detection Technology, Zhengzhou
University, Kexue Avenue 100, Zhengzhou, Henan 450001, P. R. China
Electronic supplementary information (ESI) available. See DOI: 10.1039/
b
shells of the silica spheres became coarse due to the deposition
of cupric oxide, although the distribution of copper oxide shell
on the SiO2 surface was not homogeneous (Fig. 2b). When
†
0
c9nj04031e
L-(ꢀ)-malic acid and 4,4 -bipyridyl were added into the
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2 2 2
Fig. 3 (a) EDS result of SiO –CuLBH; (b) FT-IR spectra of SiO , SiO @CuO,
CuLBH and SiO –CuLBH.
2
2
SiO –CuLBH, which implied that the CuO shell was completely
consumed and converted into CuLBH.
In addition, the energy dispersive X-ray (EDS) (Fig. 3a) of the
composite also revealed the presence of Cu, Si, C, N, and O. The
Fourier transform infrared (FT-IR) spectra are presented in
Fig. 1 SEM images of the as-prepared (a) 2.0 mm silica particles; (b) SiO
2
@CuO;
(
c) and (d) SiO –CuLBH.
2
Fig. 3b. In the FT-IR spectra of SiO
2
@CuO, the signals of
ꢀ
1
ꢀ1
water/methanol solution of SiO
2
@CuO, the color of the liquid Si–O–Si at 1099 cm , 947 cm and the stretching vibration
ꢀ
1
changed from black to blue gradually, indicating the consump- peaks of Cu–O at 587 cm were observed, confirming that the
tion of CuO and the formation of the CuLBH. The SEM of CuO was coated onto the SiO successfully. In the spectrum of
2
ꢀ
1
ꢀ1
ꢀ1
SiO
Compared with SiO
the silica spheres smooth in SiO
presented a beautiful architecture similar to 3D ‘‘flowers’’.
2
–CuLBH (Fig. 1c and d) indirectly exhibited this point. CuLBH, the peaks at 3345 cm , 2964 cm and 1605 cm
@CuO, the consumption of CuO shell made corresponded to the O–H stretching vibration, C–H asymmetric
–CuLBH and the composite stretching vibration and CQO asymmetric stretching vibration
of L-malic acid, respectively. The peak at 808 cm was the
The reported CuLBH was synthesized in Teflon-lined steel characteristic band of 4,4 -bipyridyl ligands. For SiO –CuLBH,
2
2
ꢀ
1
0
2
bombs without physical stirring, but it’s difficult to form it had both the characteristic absorption peak of SiO2 and
homogeneous composites under these conditions. To address CuLBH. Besides, the absence of Cu–O characteristic absorption
this problem, we modified the synthesis approach of CuLBH to bands indicated the complete conversion of CuO into CuLBH
use a round-bottomed flask in an oil bath with magnetic in situ. Therefore, it could be suggested that the composite was
stirring. Powder X-ray diffraction (PXRD) (Fig. 2a) demonstrated successfully prepared. The thermogravimetric curves provide
that the skeleton structure of the MOF synthesized with the information concerning the thermal stability of the composite.
modified method was in agreement with the reported homo- As shown in Fig. S1 (ESI†), the weight loss of the SiO –CuLBH
2
chiral CuLBH synthesized in a Teflon-lined steel bomb. To composite occurred in the range from 100 1C to 200 1C, which
demonstrate the successful synthesis of SiO –CuLBH, PXRD was a result of the loss of the solvent molecule DMA. The
2
analysis was recorded. Fig. 2b presents the PXRD patterns of decomposition of the organic ligand and the collapse of the
SiO , SiO @CuO, CuLBH and SiO –CuLBH, respectively. The structure occurred as the temperature increased up to 325 1C,
2
2
2
main diffraction peaks in the PXRD patterns with 2y at 32.51, which showed that the material has good thermal stability.
35.51, 38.61 and 48.61 of CuO verified the existence of the
Phenyl methyl sulfoxide (PMS) possesses two enantiomers of
CuO phase in the SiO
2
@CuO. Furthermore, the diffraction R-PMS and S-PMS, distributed in a racemic mixture. The
intensity of specific peaks of CuO completely decreased and enantioselective performance of SiO –CuLBH was evaluated
2
all of the pronounced diffraction peaks of CuLBH appeared in
Fig. 4 The adsorption result for phenyl methyl sulfoxide (PMS). Conditions:
Chiralpak IC column (4.6 ꢂ 250 mm, 5 mm); mobile phase, hexane–isopropanol
ꢀ1
Fig. 2 (a) The experimental powder X-ray diffractogram of the as-synthesised
CuLBH (inverted plot) and corresponding simulation result (normal plot);
(80/20, v/v); flow rate, 0.6 mL min ; detection wavelength, 254 nm; column
temperature, 25 1C. The peaks that appeared before 10 minutes were the peaks
of the extraction solvent (acetonitrile) and elution solvent (methanol).
(
b) PXRD patterns of SiO
2
, SiO
2
@CuO, CuLBH and SiO
2
–CuLBH.
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by PMS and the enantiomeric composition of the separated ultrasonicated for 15 minutes, and then the pH was adjusted to
ꢀ
1
enantiomer was analyzed by HPLC. As shown in Fig. 4, when 9.0–10.0 by 1.0 mol L
of NaOH aqueous solution under
acetonitrile and methanol were selected as the extraction vigorous stirring. The mixture was kept at 90 1C for 3 h. The
solvent and elution solvent, an ee value of 31% was achieved product was washed sequentially with ultrapure water and
for PMS. This result indicated that the stereo-selectivity of ethanol, and dried in a vacuum oven overnight to obtain
S-PMS was probably due to the most appropriate size and steric SiO
fit of SiO –CuLBH. To further demonstrate the superiority of SiO
the SiO
PMS, the selective adsorption ability of CuLBH which was and 4,4 -bipyridyl (0.27 g, 1.8 mmol). The mixture then was heated
synthesized in a Teflon-lined steel bomb, was also carried out in an oil-bath at 110 1C for 24 h. The product was washed
under the same conditions for comparison. It is obvious that sequentially with ultrapure water and methanol, and dried in a
the ee value of 31% for PMS achieved by the prepared SiO2– vacuum oven overnight.
2
@CuO.
2
2
–CuLBH. 0.4 g SiO @CuO was added into 23 mL water/
2
2
–CuLBH composite for enantioselective capture of methanol (1 : 1, v/v) solution of L-(ꢀ)-malic acid (0.46 g, 3.5 mmol)
0
CuLBH showed a much higher response than CuLBH (with an
ee value of 6%), which might be ascribed to the 3D flower-like
Chiral separation experiments
architecture of SiO
2
–CuLBH.
For extraction of PMS, racemic solution of PMS was added to
ꢀ1
10 mL acetonitrile (final concentration 0.05 mg mL ), and
then CuLBH (100 mg) or SiO –CuLBH (150 mg) was added. The
2
Conclusions
mixture was stirred for 24 h to realize even dispersion, and then
the mixture was centrifuged and the supernatants were
removed. The collected composite–enantiomer complexes were
mixed with 1 mL methanol and stirred for 12 h at room
temperature in order to retrieve the absorbed enantiomers.
Then the solution was collected via centrifugation and further
filtrated through a 200 nm filter membrane. The resulting
liquid was analyzed by HPLC with a Chiralpak IC column
In summary, we have described a self-template synthetic
approach for the facile preparation of a 3D flower-like SiO
2
–
CuLBH architecture, which served as a chiral adsorbent and
was utilized for ‘‘enantioselective capture’’ of enantiomers. The
results demonstrated that the prepared composite has a certain
enantioselective capability for PMS, which was much more
efficient than CuLBH conventionally synthesized in a Teflon-
lined steel bomb. This research not only demonstrates the
(
(
4.6 ꢂ 150 mm, 5 mm), from which the enantiomeric excess
ee) value was obtained.
2
utilization of SiO –CuLBH for enantioselective capture of PMS
racemate but also highlights the facile construction of a 3D
flower-like chiral MOF architecture for enantioseparation.
Conflicts of interest
There are no conflicts of interest to declare.
Experimental
Preparation of [Cu(L-mal)(bpy)]ꢁH
2
O (CuLBH) crystals
Acknowledgements
[
Cu(L-mal)(bpy)]ꢁH O was synthesized according to methods
2
2
8
Financial support from the National Natural Science Foundation
of China (21675144 and 21775140) is gratefully acknowledged.
from the literature. Cu(OAc) ꢁH O (0.090 g, 0.45 mmol),
2
2
L-malic acid (0.123 g, 0.9 mmol) and 4,4-bipyridyl (0.069 g,
.45 mmol) were added into 9.0 mL of water/methanol mixture
1 : 1, v/v). The mixed solution was transferred to a 25 mL
0
(
Notes and references
Teflon-lined stainless steel autoclave and heated at 100 1C for
4 h. The product was washed several times alternately with
methanol and water, and dried in a vacuum oven to obtain
Cu(L-mal)(bpy)]ꢁH O.
2
1
2
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