Simultaneous shaping and confinement of metal-organic polyhedra in alginate-SiO2spheres

Article abstract of DOI:10.1039/d0cc05619g

The simultaneous shaping and confinement of Cu-based MOP in alginate-SiO2 spheres significantly enhance the mechanical strength and leaching resistance of Cu-MOP. The resulting MOP-alginate-SiO2 is shown through chemical fixation of CO2 to exhibit improved product yield over the parent Cu-MOP and Cu-alginate-SiO2. This journal is

Full text of DOI:10.1039/d0cc05619g

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Simultaneous shaping and confinement of
metal–organic polyhedra in alginate-SiO2
spheres†
Cite this: Chem. Commun., 2020,
6, 14833
5
Received 18th August 2020,
Accepted 2nd November 2020
a
a
a
a
a
a
Zhuxiu Zhang, Yifan Lei, Jie Zhou, Mifen Cui, Xian Chen, Zhaoyang Fei,
a
ab
ab
Qing Liu, Jihai Tang
*
and Xu Qiao*
DOI: 10.1039/d0cc05619g
rsc.li/chemcomm
The simultaneous shaping and confinement of Cu-based MOP in polymerization shaping of MOPs was also found elsewhere in the
11
alginate-SiO
and leaching resistance of Cu-MOP. The resulting MOP-alginate-
SiO is shown through chemical fixation of CO to exhibit improved metal cations will form hydrogels with desired shape. The Cu-
product yield over the parent Cu-MOP and Cu-alginate-SiO alginate hydrogel has been used as a shapeable platform for the
2
spheres significantly enhance the mechanical strength literature.
It is well-known that the interaction between alginate and
1
2
2
2
2
.
1
3
in situ growth of Cu-based MOFs (HKUST-1). HKUST-1 and
Metal–organic materials including metal–organic polyhedra Cu-based MOP have the same paddle wheel-like molecular
MOPs) and metal–organic frameworks (MOFs) have been building blocks. It is anticipated that the MOP can be in situ
(
1
extensively studied for target-specific properties. The conver- formed in Cu-alginate hydrogel, but the macropores and meso-
sion of powdery porous materials into specific shaped bodies pores in Cu-alginate doesn’t confine the nanoscale Cu-MOP
while retaining their original properties is important for their molecules. We herein address this matter by investigating the
2
3
real-world applications. Since the seminal work of BASF,
simultaneous shaping and confinement of Cu-MOPs in
spheres. The interaction between alginate and
provides hierarchical pores to prevent in situ formed
researchers have developed various techniques to shape MOFs alginate-SiO
into beads, membranes, foams and other technical bodies.
2
4
5
6
7
SiO
2
In contrast, the shaping of MOPs is still in an embryonic Cu-MOPs leaching from the shaped body. This new shaping
stage. MOPs have the aggregation problem after the removal of approach is likely to be extended to other types of MOPs that
the guest solvent, which cannot be solved by common shaping contains divalent or trivalent metal cations.
8
techniques. The recently reported immerse-coating method
The dripping of alginate solution into copper nitrate solution
realizes the embedding of MOPs as fillers in polymeric membranes immediately formed sphere-like hydrogels. They were soaked in
9
in a well-distributed manner. This approach works for a variety of methanol solution of tetraethoxysilane (TEOS) overnight and
MOPs as long as they can be dissolved in certain solvents, but these then in distilled water for two hours. The condensation reaction
hybrid membranes suffer from the leaching of MOP molecules. between TEOS and alginate resulted in a composite hydrogel
1
4
Most recently, Chen, Zhang, and Zaworotko reported the simulta- with an interpenetrated network (Cu-alginate-SiO
2
).
The
neous shaping and immobilization of Zr-based MOPs via a poly- conversion of Cu-alginate-SiO to MOP-alginate-SiO was then
2
2
10
merization reaction among MOPs and other organic comonomers.
Leaching experiments show only a trace amount of MOPs leaching acid with a few drops of 2,6-lutidine (Fig. 1).
out from the membrane in certain solvents, but this method is only Direct mixing 1,3-bdc, Cu(NO and 2,6-lutidine in metha-
realized by the addition of the methanol solution of isophthalic
3 2
)
applicable for MOPs functionalized with amine groups. Similar nol generated a turquoise powder of which the PXRD pattern
matches well with the calculated pattern of prototypical Cu-
1
5
MOP with rhombihexahedral shape (Fig. S1, ESI†). The
presence of Cu-MOP in MOP-alginate-SiO was confirmed by
matrix-assisted laser desorption ionization/time of flight mass
a
State Key Laboratory of Materials-Oriented Chemical Engineering,
College of Chemical Engineering, Nanjing Tech University, No. 30 Puzhunan Road,
Nanjing 211816, China. E-mail: jhtang@njtech.edu.cn, qct@njtech.edu.cn
Jiangsu National Synergetic Innovation Centre for Advanced Materials (SICAM),
No. 5 Xinmofan Road, Nanjing 210009, China
2
spectrometry (MALDI-TOF-MS). The strong peak at m/z = 5464.1
b
+
matched with [Cu (C H O ) + H] of which Cu (C H O ) is
2
4
8
4
4 24
24
8
4 4 24
the formula of prototypal Cu-MOP. (Fig. 2a). In UV-Vis spectra,
the absorption band centered at 300 nm indicated the complete
deprotonation of the isophthalic acid (Fig. 2b). The band
†
Electronic supplementary information (ESI) available: Experimental section,
TEM image of Cu-alginate-SiO
2
, UV-Vis, PXRD, TEM and element mapping of
different MOP-alginate-SiO . See DOI: 10.1039/d0cc05619g
2
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analysis, in which the weight ratio between Cu-MOP and
‘‘MOP + alginate’’ is 51.6%. In comparison, the Cu-MOP con-
tent in MOP-alginate is only 35.1 wt% (Fig. S3, ESI†).
N sorption studies were conducted to study the confinement
2
of Cu-MOP in the alginate-SiO spheres. Cu-MOP exhibited neg-
2
2
ligible N sorption uptake because of the aggregation of Cu-MOP
20
molecules after the removal of the solvent. Cu-alginate exhibited
a type-IV sorption behaviour and a Brunauer–Emmett–Teller
2
À1
(
BET) surface area of 381 m g (Fig. 3a). Pore size distribution
analysis of Cu-alginate revealed a broad distribution of from
micropores to mesopores with an average pore size of 12 nm
(
Fig. 3b). In comparison, Cu-alginate-SiO exhibited a BET surface
2
2
À1
area of 466 m
alginate-SiO
g
. A pore size distribution analysis of Cu-
2
revealed a narrow pore size distribution centered
2
Fig. 1 Schematic representation of MOP-alginate-SiO synthesis.
at 4 nm and a relatively broad distribution from 5–11 nm centered
at 7 nm. The generation of Cu-MOP didn’t significantly increase
2
À1
2
the BET surface area of MOP-alginate-SiO (517 m g ). The
center of the broad pore size distribution of MOP-alginate-SiO
2
shifted from 7 to 5.5 nm, whereas the pore size of 4 nm remained
relatively unchanged.
Therefore, it would be fair to infer that Cu-MOP was con-
fined in cavities with the dimension of 5–11 nm. In addition,
the distribution at ca. 1.2 nm was consistent with the structure
of Cu-MOP that contained two types of pores with diameters of
11.5 Å and 8.4 Å. According to the literature, the appropriate
confinement of Cu-MOP in a porous matrix enables the obser-
vation of MOPs through TEM, which otherwise is difficult to
realize due to the high sensitivity of MOPs against the electron
2
1
beam. In this sense, we found several spherical nanoparticles
with the average filler size of ca. 3.3 nm in the TEM image of
MOP-alginate-SiO
2
(Fig. 3c). The particle size and morphology
Fig. 2 (a) MALDI-TOF-MS spectra of Cu-MOP in MOP-alginate-SiO
b) UV-Vis spectra and (c) PXRD pattern of MOP-alginate-SiO
Cu-alginate-SiO , alginate and MOP; (d) FT-IR spectra of MOP-alginate-
SiO , Cu-alginate-SiO , alginate and MOP.
2
;
were fairly comparable with those of Cu-MOP published in the
literature reports. Moreover, Cu-alginate-SiO only exhibited
2
the association of silica with the alginate matrix without any
nanoparticles (Fig. S4, ESI†). Therefore, it was suggested that
(
2
,
22
2
2
2
centered at 695 nm corresponded to the metal to ligand charge
transfer according to the semi-empirical ZINDO/S calculations
1
6
of Cu
2
(p-hydroxybenzoate)
4
.
In contrast, Cu-alginate-SiO
2
exhibited a weak absorbance at ca. 800 nm, which presumably
was a metal d–d transition (Fig. 2b). In XPS spectra, the two
shakeup satellite lines at 944.1 and 963.0 eV of MOP-alginate-
2
+
SiO , corresponding to the paramagnetic chemical state of Cu
2
9
17
with the d configuration, further confirmed the coordination
between Cu and with O atoms of 1,3-bdc (Fig. S2, ESI†). There
were no powder X-ray diffraction peaks of MOP-alginate-SiO
2
+
2
compared to pure Cu-MOP crystals (Fig. 2c), which excluded
not only the periodic packing of Cu-MOP molecules, but also
the formation of isomers of Cu-MOP including a two dimen-
1
8
19
sional kagom ´e lattice and square grid lattice. In Fig. 2d, the
À1
À1
Si–O–Si stretching vibration bands at 1078 cm and 798 cm
Fig. 3 (a)
Cu-alginate, Cu-alginate-SiO
image of MOP-alginate-SiO
grams (collected over one hundred dots to reach more accurate measured
size) of MOP particles; (d) STEM images of MOP-alginate-SiO with the
N
2
sorption isotherms and (b) pore size distributions of
MOP-alginate-SiO and MOP; (c) TEM
and (inset) particle size distribution histo-
proved the presence of silica in MOP-alginate-SiO . The char-
acteristic peak at 1388 cm in the FT-IR spectrum of MOP and
2
À1
2
2
2
MOP-alginate-SiO
tion peaks of the two C–O bonds in COO . The loading of
2
belong to the symmetrical stretching vibra-
À
2
Cu-MOP in MOP-alginate-SiO
2
is 23.2 wt% according to the TGA corresponding EDX maps of Cu and Si.
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the Cu-MOP molecules were successfully confined in alginate- Table 1 Survey of catalysts for the cycloaddition of epoxides with CO
SiO spheres in a highly dispersed manner. The high dispersity
2
2
of Cu-MOP was also validated by elemental mapping in which
elements Cu and Si were detected (Fig. 3d).
The stability of MOP-alginate-SiO was then studied in terms of
b
2
Entry
Catalyst
Yield /%
both mechanical strength and leaching resistance. We prepared
MOP-alginate according to the same procedure of synthesizing
c
1
2
3
4
5
6
7
8
9
MOP-alginate-SiO
Null c
2
85.9
0
MOP-alginate-SiO
2
for comparison. The presence of MOP in MOP-
MOP
41.8
61.6
62.7
53.1
56.7
55.3
66.5
e
MOP-alginate
alginate was also verified by MALDI-TOF-MS (Fig. S5, ESI†). The
typical compressive stress–strain curves of the as-synthesized
composites at room temperature are shown in Fig. 4a. As
expected, the MOP-alginate composite exhibited a permanent
deformation and collapse at strain of 41.5%. Notably, the
d
Cu-alginate-SiO
2
c
M3S-0.1
d
MOPNT-2d
HKUST-1
d
Kagom´e
a
b
presence of SiO
alginate-SiO to increase from 65 to 98 kPa whereas the compres-
sive strain reduced from 41.5% to 31.5%. The result suggested
that SiO
served as the ‘‘backbone’’ of the alginate hydrogel to n(alginate in MOP-alginate).
resist deformation. The interpenetrated network formed by SiO
2
facilitated the compressive stress of MOP-
Tetrabutylammonium bromide is the cocatalyst. Conversion of
c
epoxide. n(MOP in MOP-alginate-SiO
2
) d
= n(MOP crystallites) =
2
n(MOP in M3S-0.1) = n(MOP in MOPNT-2) n(Cu in Cu-alginate-SiO
2
) =
) =
e
n(Cu in HKUST-1) = n(Cu in kagom ´e ). n(alginate in MOP-alginate-SiO
2
2
2
and alginate improved the strength–toughness of the MOP- experiments under the similar conditions revealed no conversion
alginte-SiO2 composites. In the aspect of leaching-resistance, of the substrate without a catalyst (Table 1, entry 2) whereas MOP
during the synthesis of MOP-alginate, the instant color change crystallites only afforded 41.8% yield (Table 1, entry 3). The low
of the reaction solution from colourless to turquoise blue after conversion exhibited by MOP crystallites was mainly due to aggre-
adding 1,3-bdc indicated the leaking of Cu-MOP molecules from gation of Cu-MOP molecules that blocked the unsaturated copper
Cu-alginate (Fig. 4c), which is consistent with the TG analysis. The sites. When MOP-alginate was used as the catalyst, only 26.3% of
UV-Vis spectra of the reaction solution and the MOP-alginate PC was obtained due to the low loading of MOP (Table 1, entry 4).
clearly showed the absorption of the copper paddle (Fig. 4b). In Cu-alginate-SiO
contrast, there was no color change in the solution after treating than MOP-alginate-SiO
the Cu-alginate-SiO with the 1,3-bdc solution whereas the color of dispersed copper paddle wheels are more active Lewis acid sites
exhibited a yield of 62.7% which was also lower
2
2
(Table 1, entry 5), because the highly
2
2+
Cu-alginate-SiO turned from transparent light blue to opaque than the Cu cation in Cu-alginate-SiO
2
for the epoxide adsorption.
turquoise blue (Fig. 4d). These results apparently demonstrated We compared the catalytic performance of MOP-alginate-SiO with
the important role that silica plays in the enhancement of the other Cu-MOP-based composites and typical MOFs with the copper
stability of the MOP-alginate-SiO sphere.
paddle wheel. The mesoporous silica-supported MOP (M3S-0.1)
In order to investigate the advantage of highly dispersed MOP and the carbon nanotube-supported MOP (MOPNT-2) respectively
over pure Cu-MOP crystallite and Cu-alginate-SiO
, we decided to exhibited a PC yield of 53.1% and 56.7% (Table 1, entries 6 and 7).
2
2
2
2
evaluate the catalytic property of MOP-alginate-SiO in the chemical HKUST-1 with 3-dimensional structure only afforded a PC yield of
2
fixation of CO with different types of epoxides (Table 1). As shown 55.3% (Table 1, entry 8), whereas the two dimensional kagom´e
2
in entry 1, the formation of propylene carbonate (PC) was detected layer exhibited the PC yield of 66.5% (Table 1, entry 9).
2
with 85.9% yield over MOP-alginate-SiO . A series of control
The substrate scope of epoxide cycloaddition with CO
2
over
MOP-alginate-SiO was then investigated under the same con-
2
ditions (Table S1, ESI†). The CO cycloaddition of trifluoropro-
2
pane epoxide and epichlorohydrin with electron withdrawing
groups afforded only 62.1% and 69.1% yields respectively
(Table S1, ESI† entries 1 and 2). In contrast, the present
catalytic system was highly efficient for the conversion of 1,2-
epoxy-5-hexene and allyl glycidyl ether with the electron-
donating groups (Table S1, ESI† entries 3 and 4), giving rising
to a product yield over 80%. Notably, 2-(butoxymethyl)oxirane
was reported to be reluctant in CO cycloaddition because of
2
2
3
the large steric hindrance, but it could still be converted to
the corresponding product in good yield of 84.4% over MOP-
alginate-SiO
the ingress of the substrate and egress of the product (Table S1,
ESI† entry 7). The MOP-alginate-SiO catalyst could be reused
for at least five runs without significant reduction in catalytic
activity presumably due to the stability of the shaped body
(Fig. S6, ESI†). The catalyst after five catalytic cycles exhibited
2
thanks to its abundant porosities that facilitated
2
Fig. 4 (a) Compressive stress–strain curves of MOP-alginate and MOP-
alginate-SiO (b) UV-Vis spectra of MOP-alginate and its reaction
solution; the color changes of (c) MOP-alginate and (d) MOP-alginate-
SiO before and after ligand addition.
2
;
2
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We gratefully acknowledge the support of the National Nat-
ural Science Foundation of China (21808104, and 21676141), the
Natural Science Foundation of Jiangsu Province (BK20170989),
the National Key R&D Program of China (2017YFB0307304), and
the Natural Science Foundation of Jiangsu Higher Education
Institutions of China (17KJB530005).
Conflicts of interest
There are no conflicts to declare.
Fig. 5 (a) As-synthesized microsphere of Cu-alginate-SiO
2
, MOP-OH-
-alginate-SiO and
2
; MOP-alginate-SiO with shapes of (b) cylinder
Notes and references
alginate-SiO
2
,
MOP-NH
2
2 3 2
, MOP-CH -alginate-SiO ,
MOP-Br-alginate-SiO
and (c) membrane.
2
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prototypical Cu-MOP (Fig. 5a). The in situ generation of all
7
Cu-MOP derivatives in the alginate-SiO
UV-Vis and FT-IR spectra that were comparable to MOP-
alginate-SiO . The amorphism and dispersity of MOP mole-
cules were verified by the PXRD pattern, TEM and elemental
mapping (Fig. S14–S23, ESI†). In addition to the spherical
beads, MOP-alginate-SiO2 with rod (Fig. 5b) and membrane
2
was verified by the
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2
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2
1
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and to inhibit the leaching of MOPs particles. Highly-distributed
2
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The catalytic activity and recyclability of MOP-alginate-SiO sug-
2
gested that such shaping approach may overcome the limitation
of the current MOP shaping approaches in terms of structure
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2 2
able platform for other types of zero dimensional metal–organic
compounds.
14836 | Chem. Commun., 2020, 56, 14833--14836
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