Dual template effect of supercritical CO2 in ionic liquid to fabricate a highly mesoporous cobalt metal-organic framework

Article abstract of DOI:10.1039/c5cc04009d

A hierarchical meso- and microporous metal-organic framework (MOF) was facilely fabricated in an ionic liquid (IL)/supercritical CO₂ (SC CO₂)/surfactant emulsion system. Notably, CO₂ exerts a dual effect during the synthes

Full text of DOI:10.1039/c5cc04009d

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DOI: 10.1039/C5CC04009D
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Dual template effect of Supercritical CO2 in Ionic Liquid to
Fabricate Highly Mesoporous Cobalt Metal-Organic Framework
Received 00th January 20xx,
Accepted 00th January 20xx
Huanan Yu, Dongdong Xu, Qun Xu*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Hierarchical meso- and microporous metal-organic framework Traditionally, amphiphilic surfactants can be utilized to synthesize
(MOF) was facilely fabricated in an ionic liquid (IL)/supercritical various interesting materials with controlled micro-morphologies
CO2 (SC CO2)/surfactant emulsion system. Notably, CO2 plays a such as micelles, vesicles, liquid crystals, etc.14,15 When the
dual effect during the synthesis, that is, CO2 droplets were utilized fabrication process is realized in CO2 or ILs, it will bring us more
as template for the cores of nanospheres while CO2-swollen possibilities.16 In particular, the fluorinated surfactants have an
micelles induce mesopores on nanospheres.
excellent CO2-responsive ability to dissolve much CO2 in micelles
and the amount of dissolved CO2 can be easily tuned by changing
CO2 pressure.17 To date, the formation of mesoporous materials
with assistance of IL or SC CO2 is very attractive due to their unusual
solvent properties.18 Zhang et al. reported that mesoporous
polymers with tunable thin mesopore walls and large mesopores
were synthesized through a CO2-wollen micelle templating route.19
Du et al. studied the fabrication of mesoporous silica nanospheres
by using a trisiloxane-tailed ionic liquid as a template.20 Fabricating
hierarchically meso- and microporous materials and further tuning
their porous structure efficiently is an interesting work, however,
still a big challenge.
In this work, we devised a reaction system to help build
surfactant-directed assembly of porous structure in a switchable
solution, in which it contains surfactant, ionic liquid and SC CO2. For
this system, it has bi-function, one is creating dispersed phase of
microemulsions containing SC CO2, and the other is formation of
continuous phase of ionic liquid. Here we choose Co-MOF as the
research object, so a series of Co-MOF with large BET surface area
and high mesoporosities are formed. Specifically, the Co-MOF is
made up of a microporous framework stemming from organic
ligand bridged by Co2+, and numerous mesopores in nanospheres
construct the molecular sieve-like structure. Moreover, the
As a new class of porous crystalline materials with diverse
fascinating topologies and permanent porosity after desolvation,
metal-organic frameworks (MOFs) have emerged as an attractive
material for a remarkably wide range of potential applications,
including gas separation, drug delivery, catalysis, and energy
storage.1-3 Nowadays, much effort has been devoted to the use of
MOFs as energy materials.4,5 Compared with conventional carbon
materials, MOFs are a new type of electrode material for their
intrinsic pseudo-capacitive redox centers.6 However, most of the
porous MOFs adopt microporous regime (<2 nm), which largely
restrict the diffusion and mass transfer as electrode materials.7,8
Therefore, researchers have continuously focused on designing
nanoscale MOFs with hierarchical meso- and micropores for their
advantages of providing dispersion of active sites at various length
scales of pores.9
In recent years, ionic liquids (ILs) and supercritical CO2 (SC CO2),
known as unconventional and green solvents, have received a lot of
attention in the synthesis of new materials.10 ILs can be designed to
satisfy various requirements of the applications owing to their
excellent solvency for both inorganic and organic compounds, high
chemical and thermal stability, negligible vapor pressures, wide
electrochemical windows and liquid temperature range.11 SC CO2,
which is also an interesting class of solvent with advantages of low
toxicity, low cost and nonflammability, has been widely used in
different fields as reaction media and material processing. More
importantly, the chemical and physical properties of SC CO2 can be
tuned easily by varying operating temperature and pressure.12,13
interconnected worm-like structure construct
a
series of
macropores to further enrich the hierarchical porosity of Co-MOF.
During the synthesis of nanospheres, CO2 takes on dual function.
One hand, CO2 droplets dispersed in IL can be utilized as template
to help form the cores of nanospheres; on the other hand, the CO2-
swollen micelles will induce numerous mesopores on the shell of
nanospheres. Both the size and mesopore properties of
nanospheres can be easily tuned by changing CO2 pressure.
Significantly, the as-synthesized hierarchically meso- and
microporous Co-MOF can be tested as electrode material, which
exhibits a high specific capacitance of 230.5 F g-1 at 0.5 A g-1.
a.College of Material Science and Engineering, Zhengzhou University, Zhengzhou
b.450001, P. R. China. E-mail: qunxu@zzu.edu.cn.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Fig. 2 N2 adsorption-desorption isotherm (a) and mesopore size
distribution curve (b) of Co-MOF synthesized in an IL/SC
CO2/surfactant emulsion system. The inset in (b) shows the
micropore size distribution curve.
condensation with pronounced adsorption-desorption hysteresis,
indicating that plenty of mesopores exist in the Co-MOF. The BET
surface area, the total specific pore volume and mesopore volume
are 720 m2 g-1, 0.49 cm3 g-1 and 0.131 cm3 g-1, respectively. The
mesopore size distribution curve in Fig. 2b shows a pore size
distribution centered around 3.2 nm, which is obtained from
Barrett-Joyner-Halenda method. This data is consistent with that
measured from the TEM images shown in Fig. 1c and d.
Furthermore, the Co-MOF has a high N2 adsorption capacity when
P/P0 (the relative pressure) is as low as 0.01, indicating the
mesoporous Co-MOF synthesized owns micropores structure. By
using the Horvath-Kawazoe analysis, the micropore size was
calculated to be 0.71 nm (inset in Fig. 2b) and the surface area of
micropores is 426 m2 g-1 (Smeso/Smicro = 0.51). The analysis above
confirms that hierarchical meso-micro porous structure of Co-MOF
was formed and as well as the mesopores walls are constructed
from a microporous framework.
To explore the formation process of mesoporous Co-MOF in the
IL/SC CO2/surfactant emulsion system, we prepared a series of
contrast tests. As shown in Fig. S5a and b. the pink Co-MOF in the
absence of IL but with SC CO2 has a lot of mesopores. However, the
single flaky structure shows no more multilevel pores. When
prepared in IL without SC CO2, the product also appears single flaky
structure but without mesopores, and the color turns white (Fig.
S5c and d), indicating that the reaction nearly cannot happen under
this condition. Only in the presence of both IL and SC CO2,
mesoporous Co-MOF nanospheres were successfully fabricated (Fig.
S5e and f). The reaction mechanism can be suggested as Fig. 3.
First, in the absence of CO2, surfactant molecules self-assemble
into cylindrical micelles through the hydrogen bonding interactions
between the NH group of N-EtFOSA and IL atoms. The fluorinated
tails of surfactant point to the inner, while the hydrophilic head
groups arrange towards the continuous IL phase, forming the “dry”
micelles with the empty cores. Second, SC CO2 dissolves in the IL
and enters into the micellar core to form a CO2-swollen micelle. At
the same time, CO2 droplets are dispersed in the IL continuous
phase with the aid of CO2-swollen cylindrical micelles of the
surfactant. Third, owing to the facile linkage property of Co2+ and
BDC, the CoII metal ions react with the deprotonated BDC in the IL
phase to form a crystalline microporous framework, which leaves
the cavities in micelles. At last, hierarchically meso- and
microporous Co-MOFs were formed after the removal of CO2, IL and
surfactant by degassing and washing with ethanol. During the
Fig. 1 TEM images (a-d) of the Co-MOF synthesized in an IL/SC
CO2/surfactant emulsion system at 16 MPa and 80 °C for 48 h.
The morphology and microstructure of samples were examined by
SEM and TEM. As shown in Fig. 1a, the Co-MOF synthesized at 16
MPa for 48 h were uniform nanospheres with diameter of about 50
nm. These connective nanospheres with plenty of pores construct a
worm-like morphology in Fig. 1b. Moreover, the interconnected
worm-like structure construct a series of macropores to further
enrich the hierarchical porosity of Co-MOF (Fig. S1b). Fig. 1c and d
clearly shows numerous mesopores on nanospheres, which is
similar to the structure of molecular sieve. The pore size and wall
thickness were about 3 and 2 nm, respectively. When we shorten
the reaction time to 24 h, nanospheres with smaller size appear a
serious conglutination (Fig. S2).
Thermogravimetric analysis curve (Fig. S3a) shows the first part
of weight loss was 7.3 wt%, which is mainly from the removal of
absorbed H2O and ethanol. Then, the Co-MOF is stable up to about
°
275 C. In general, an obvious weight loss of 41.2 wt% is observed
°
from 275 to 445 C, which is likely corresponding to the thermal
decomposition of Co-MOF crystallites. Fourier transform infrared
spectra (Fig. S3b) revealed that the ligand KHBDC (Potassium acid
phthalate) has two major absorption bands of protonated BDC at
about 1566 cm-1 and 1290 cm-1. In sharp contrast, the Co-MOF
shows the strong characteristic absorption for the asymmetric and
symmetric vibration of BDC at about 1523 cm-1 and 1335 cm-1. The
wavenumber difference of asymmetric and symmetric vibration of
BDC anions for Co-MOF is narrowed. This indicates that both
carboxylate groups of the BDC are coordinated to CoII ions.21
Besides, the characteristic CH3 stretching vibration (2962-1872 cm-1)
and C-N stretching vibration (1230-1030 cm-1) for both IL and
surfactant in Co-MOF were not observed, demonstrating that the
product is free of N-EtFOSA and TMGA. It means that they could be
easily removed by degassing and washing with ethanol. The wide-
angel powder X-ray diffraction pattern (Fig. S3c) presents several
sharp peaks, which is consistent with the reported Co-MOF,22
indicating that the synthesized Co-MOF is well-crystalline. Based on
both EDX spectrum and XPS analysis in Fig. S4, Co, O and C
components are found in Co-MOF.
The porosity properties of the Co-MOF were determined by N2
°
adsorption-desorption method after the sample was dried at 60 C.
Fig. 2a shows a typical type IV isotherm and exhibits the pore
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synthesis of nanospheres, CO2 plays dual template role. One hand,
CO2 droplets were dispersed in IL to help form the cores of
nanospheres. On the other hand, these CO2-swollen cylindrical
micelles act as a template for the formation of mesoporeson the
shell of nanospheres. This differs a lot from the manuscript, in
which Zhao et al. reported the micelles’ orderly self-assembly
induced the formation of well-ordered mesopores.21 In this work, it
needs to address the effect of SC CO2. Because of the good solvency
of IL, the viscosity of reaction system can be reduced effectively by
the CO2 expansion. Thus this reaction for MOF formation between
the metal salt and organic linker could be accelerated, which is
reflected from the different color of the product. So it could be
concluded that the coexistence of IL and SC CO2 favors the
formation of the mesoporous framework.
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DOI: 10.1039/C5CC04009D
The Co-MOF with various pore structures was further
synthesized in an IL/SC CO2/surfactant system at different CO2
pressure. Both N2 adsorption-desorption isotherms in Fig. S7 exhibit
an intermediate mode between type I and type IV, which are
related to mesopores and micropores, respectively. The
experimental results show that all Co-MOFs present hierarchically
meso- and microporous structures. From 8 to 16 MPa, the average
diameter of nanospheres increases from 18 to 50 nm (Fig. S6), while
the mesopore diameter reduces from 6 to 3 nm (Table S1). Notably,
the higher pressure favors the formation of larger nanospheres and
smaller mesopores. According to the proposed mechanism above,
this phenomenon can be explained as follows. At higher pressure,
the surfactant is more CO2 soluble, thus the interface becomes less
curved about CO2 and the size of CO2 droplet is increased.23
Therefore, the nanospheres synthesized at higher pressure have
larger size because of the template effect of CO2 droplets. As for the
reduced mesopore size of the Co-MOF made at higher CO2 pressure,
it can be attributed to the decreasing amount of CO2 solubilized in
the micelles, a phenomenon which has been reported for CO2-in-
water micelles and CO2-in-IL micelles.24 The results indicate that
both the size and mesopore properties of nanospheres can be easily
tuned by controlling the CO2 pressure.
Fig. 4 (a) CV curves of the Co-MOF electrode at different scan
rates. (b) Galvanostatic charge-discharge profiles at different
current densities. (c) Specific capacitances of the Co-MOF
electrode at different current densities. (d) Cycling stability of
Co-MOF at a current density of 2 A g-1 and the corresponding
CV curves at a scan rate of 10 mV s-1.
4a, the representative CV curves show a pair of well-defined redox
peak, suggesting that the demonstrated capacitive property is
controlled by pseudocapacitance, rather than the appositively
charged electrical double layer. The appearance of nearly
symmetric potential-time curves in Fig. 4b indicates that the unique
electrode has
a low polarization and high charge-discharge
columbic efficiency. Fig. 4c shows the porous Co-MOF exhibits the
highest specific capacitance of 230.5 F g-1 at 0.5 A g-1, which is
almost two times capacitance of cobalt oxides.25,26 This outstanding
electrochemical performance should come from the novel structure
of Co-MOF. First, the unique molecular sieve-like structure with
high porosity increases the amount of electroactive sites, facilitate
the transport of electrolyte and boost the electrical conductivity.
Furthermore, the worm-like structure link to each other and serve
as ion reservoirs, which accelerate the process of ion diffusion by
diminishing the diffusion distance to the interior surfaces, so as to
enhance the electrochemical property (Fig. S8). The long-term
cycling performance of Co-MOF electrode was measured by
galvanostatic charge-discharge tests for 1000 cycles (Fig. 4d). From
the corresponding CV curves, the electrochemical redox switching is
reversible perfectly for a long run. The specific capacitances of Co-
MOF still retain 95.2% in capacitance after 1000 cycles, indicating
The unique structure of the Co-MOF inspired us to evaluate its
electrochemical performance for supercapacitor. As shown in Fig.
that the as-prepared Co-MOF not only represents
a high
capacitance, but also an excellent stability when used as electrode
material.
In summary, Co-MOF with novel hierarchical meso-micro pores
was simply fabricated by the dual template function of SC CO2 in IL.
Both the size and mesopore properties of nanospheres can be
easily tuned by changing CO2 pressure. By taking advantages of
interconnected nature of the hierarchically porous nanospheres,
the Co-MOF exhibits a high specific capacitance of 230.5 F g-1 at 0.5
A g-1, which is the highest data ever reported for Co-MOFs. We
believe that this simple and potentially universal design strategy
Fig. 3 Schematic illustration for the formation of mesoporous
Co-MOF in an IL/SC CO2/surfactant emulsion system.
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supplies a new way to fabricate more functional MOFs with novel
morphologies and porosities in the near future.
View Article Online
DOI: 10.1039/C5CC04009D
We are grateful for the National Natural Science Foundation of
China (No. 51173170, 50955010, and 20974102), the financial
support from the Innovation Talents Award of Henan Province
(114200510019), the Program for New Century Excellent Talents in
University (NCET) and the Key program of science and technology
(121PZDGG213) from Zhengzhou Bureau of science and technology.
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