3.1 Characterization
PXRD patterns of MIL-101(Cr) and IL/MIL-x were carried out. Figure 1a
indicates that the PXRD patterns of MIL-101(Cr) correspond well with
their simulated ones, confirming the successful synthesis28. Besides, the IL/MIL composites maintain similar
diffraction peaks to pristine MIL-101, indicating that the materials
retain the framework stability after loading IL. However, the
characteristic peaks of IL/MIL at 5-7° are weakened, which may be
ascribed to an alteration in electron density, morphology, as well as
crystallization 29-31. Then, the porosity of the
adsorbents was evaluated by analyzing the N2adsorption/desorption isotherms at 77 K. Figures 1b and 1c suggest that
the N2 uptake and the pore size of samples gradually
decrease with increasing the content of loaded IL. As listed in Table
S1, the BET surface area of the
sample decreases from 3111 m2·g-1 to
3 m2·g-1 and the total pore volume
decreased from 2.0 cm·g-1 to 0.14
cm3·g-1 with the increase of IL
content, which can be ascribed to the
pore blockage resulted by the IL
entering the pore of MIL-101(Cr). Furthermore, FT-IR spectra of
MIL-101(Cr) and IL/MIL-x are shown in Figure 1d. The asymmetric and
symmetric stretching vibrations of the dicarboxylate linker O–C–O can
be observed at 1624 and 1401 cm-132. In addition, other bands on the benzene ring of
the ligand can be found at 1507 cm-1 (C=C stretching)
and 1158, 1107, 882 and 748 cm-1 (C-H bending)31,33. These demonstrate the MOF framework is well
preserved after loading IL, indicating that the loaded IL does not
affect the structural integrity. Besides, new peaks were observed in
IL/MIL composites. For instance, the peak situated at 1579
cm-1 and the weak peaks at 2951 and 2852
cm−1 can be ascribed to the stretching vibration of
C-N 31,34 and the C-H stretching vibration of the
alkyl chain in IL32,35. The above
evidence indicates that IL is successfully loaded in MIL-101(Cr).
To further illustrate the existence of IL, the
XPS spectra and EDS mapping were
investigated by using IL/MIL-0.7 as a case study. Compared with that of
MIL-101(Cr) (Figure 2), the spectrum of IL/MIL-0.7 displays the
characteristic peaks corresponding to Cl and N elements. EDS mapping
under SEM mode of IL/MIL-0.7 shows the homogeneous distribution of N and
Cl elements (Figures S2c and S2d),
which shows the excellent dispersion
performance of IL in MIL-101(Cr). SEM images further confirmed the
maintenance of crystal morphology. IL/MIL-0.7 has similar particle sizes
(200–500 nm) and octahedral crystals typical of MIL-101(Cr) as shown in
Figures S2a and 2b. Additionally, TEM analysis (Figure 3) reveals a
layer of transparent substance with a non-uniform thickness (more than
15 nm) on the outside of IL/MIL-0.7 compared with pristine MIL-101(Cr),
suggesting that IL was deposited on the outer surface of MIL-101(Cr).
The above characterization data demonstrate the successful loading of
IL.
3.2 SO2adsorption performance
To evaluate the properties of the composites, the adsorption isotherms
of SO2 over four adsorbents were determined by the
volumetric method at 298 K and 1 bar. Figure 4a indicates that the
maximum adsorption uptake of IL/MIL-x decreases compared with
MIL-101(Cr) due to the reduction of pore volume and the different
adsorption sites caused by the introduction of
IL 9,19.
Besides,
the saturated adsorption capacity of SO2 changes with
the increase of loaded IL content. As shown in Figure 4a and Table S2,
the SO2 uptake capacity of IL/MIL-0.7 and IL/MIL-0.4 is
(13.18 and 11.36 mmol·g-1, respectively) lower than
that of IL/MIL-0.2 (13.51 mmol·g-1) at 298 K and 1
bar. Due to the BET surface area of the composite decreasing with the
increasing IL content, the adsorption capacity of the porous carrier
drops somewhat. Interestingly, the SO2 adsorption
capacity of IL/MIL-0.7 (13.18 mmol·g-1) is higher than
IL/MIL-0.4 (11.36 mmol·g-1), but the BET and pore
volume of the former material is smaller than those of the latter. This
indicates that the introduction of IL induces additional adsorption
sites for the adsorbent, prompting the capture of SO2.
Therefore, it can be concluded that the saturated adsorption capacity is
not only determined by the specific surface area and pore volume but
also depends on the action of IL. Although the adsorption capacity of
IL/MIL-0.7 is subequal to that of
MIL-101(Cr) at 298 K and 1 bar, its performance is better than some
other materials previously reported, including PI-COF-m10 (6.3
mmol·g-1) 36, and other typical
porous materials, such as SIFSIX-1-Cu (11.01 mmol·g-1)19, Ph-4MVIm-Br (8.12 mmol·g-1)18, MFM-300(In) (8.28 mmol·g-1)15, MIL-160 (7.2 mmol·g-1)16, ECUT-111(11.6
mmol·g-1)37, SIFSIX-2-Cu-I (6.90
mmol·g-1) 20, SIFIX-3-Ni (4.30
mmol·g-1) 19, CPL-1 (1.999
mmol·g-1) 38, ELM-12 (2.73
mmol·g-1) 39, NPC-1 (2.45
mmol·g-1) 40, and MFM-601 (12.3
mmol·g-1) 20 (Table S2). Notably,
the low-pressure adsorption capacity is increased for IL/MIL with the
increase of the loaded IL content, as shown in Figure 4b. The
SO2 adsorption capacity of IL/MIL-0.7 significantly
exceeds that of pristine MIL-101(Cr) at low partial pressures (below 0.1
bar). Moreover, the SO2 uptake of IL/MIL-0.7 (0.67
mmol·g-1) at 0.002 bar exceeds that of MFM-300(In)
(0.43 mmol·g-1) 15 and MFM-601 (0.24
mmol·g-1) [20], only lower than
[TMEDA][DES]@BN (0.82
mmol·g-1, 293 K) 41, and
P(Ph-4MVIm-Br) (1.55 mmol·g-1) 18.
However, these two kinds of materials exhibit lower adsorption capacity
compared to IL/MIL-0.7 at 1 bar. SO2 adsorption
performance is up to 1.68 mmol·g-1 when the partial
pressure reaches to 0.01 bar, which is more than 3.3 times as high as
MIL-101(Cr) (0.5 mmol·g-1). As the pressure further
increases to 0.1 bar, the adsorption amount of SO2 is up
to 4.7 mmol·g-1. Compared with the other reported
porous materials, this material has a much higher low-pressure
adsorption capability, including
P(Ph-4MVIm-Br) (4.14 mmol·g-1) 18,
SIFSIX-2-Cu-i (4.16mmol·g-1) 19, and
HNIP-TBMB-1(3.54 mmol·g-1) 42. These
suggest that IL/MIL-0.7 can provide excellent SO2capacity from low pressures to atmospheric pressures, which is of great
importance from a practical point of view.
3.3 SO2 separationperformance
Since SO2 is usually
involved in many industrial gases as a contaminant, for example, in flue
gases (CO2 and N2)43, it is
crucial to explore the separation performance of SO2from gas mixtures. The CO2 and N2adsorption isotherms of IL/MIL-0.7 at 298 K were obtained to evaluate
the separation selectivity. Although both CO2 and
SO2 are acid gases, their adsorption isotherms on the
composites have different adsorption behaviors. As shown in
Figure 5a, the adsorption capacity of
SO2 (13.18 mmol·g-1) was significantly
higher than that of
CO2 (0.27
mmol·g-1) and N2 (0.07
mmol·g-1) in IL/MIL-0.7. This variation in the
adsorption capacity can be attributed to the introduction of IL which
induces unique adsorption sites for SO2. In detail, the
strong adsorption interaction between SO2 and IL/MIL-0.7
pushes the SO2 to overcome the large mass transfer
resistance of the IL layer to enter the material, while
CO2 and N2 cannot penetrate the
adsorbent due to their weak force between them. Therefore, IL/MIL-0.7
exhibits larger SO2 capacity and lower
N2 and CO2 uptake. Furthermore, from the
comparison of the SO2 adsorption capacity of IL/MIL-0.7
with reported porous materials in Figure 5b and Table S2, IL/MIL-0.7
shows superior SO2 adsorption capacity. More
importantly, the CO2 and N2 uptakes of
IL/MIL-0.7 are significantly lower
than other adsorbents reported.
To further evaluate the separation efficiency,
SO2/CO2 selectivity was calculated based
on the ideal adsorption solution theory (IAST) (Section 2 in SI).
Firstly, the dual-site Langmuir (DSL) model was used to fit the
adsorption isotherm (eq. S1), and then the separation selectivity was
calculated by eq. S2 from the single-component gas adsorption isotherms.
Fitted curves and fitted parameters can be found in Figure S3 and Table
S2, respectively. As expected, the composite exhibits an ultra-high
SO2/CO2selectivity over 104 (about 11925) for a mixture of
10% SO2 and 90% CO2 at 298 K and 1.0
bar (Figure 6a). The excellent adsorption selectivity towards
SO2 for IL/MIL-0.7 can be explained by a molecular-sieve
effect. The introduction of IL led to selective permeation, whereas
SO2 can be accessible to the interlayer spaces
IL/MIL-0.7 while the CO2 and N2molecules are excluded by their weak adsorption interaction. Then, the
SSO2/CO2 of IL/MIL-0.7 was compared with the reported
adsorbent. Figure 6b indicates that the separation performance of
IL/MIL-0.7 is significantly higher than those in the benchmark materials
(Figure 6b and Table S2), such as Mg-gallate (325) 14,
and P([allyl-TMG]Br-DVB)(452) 8. Furthermore,
for a pressure swing adsorption
(PSA) process, selectivity and working capacity are typically regarded
as two crucial criteria to identify a new adsorbent, while the sorbent
selection parameter (S sp), combining selectivity
and working capacity in a single parameter, can be used to better
evaluate the potential of adsorbent. Therefore, the value ofS sp of IL/MIL-0.7 for
SO2/CO2 is calculated by eq. S3 and
compared with the value of reported adsorbents. Figure 6c indicates that
IL/MIL-0.7 exhibits a higher value of S sp (about
472131) than the majority of other materials. To summarize, IL/MIL-0.7
has a good potential to capture trace SO2 with high
selectivity during the flue gas desulfurization (FGD) process.
It is well known that the content of SO2 in industrial
flue gas (such as coal combustion flue gas) is very low (about 2000 ppm)41,44. To confirm the practical ability of IL/MIL-0.7
to capture trace SO2 and the feasibility of composites
in the application process, the breakthrough experiment was carried out
on the simulated flue gas at 298 K and 1.0 bar, that is, a ternary gas
mixture SO2/CO2/N2 (2000
ppm/15%/84.8%, v/v/v) containing 2000 ppm SO2 with a
flow rate of 40
mL·min-1. Figure
7 indicates that CO2 and N2 rapidly
elute through the column at the beginning, while the breakthrough time
of SO2 on IL/MIL-0.7 could reach up to 130
min·g-1, which means that the material can realize the
deep and selective removal of trace SO2. The
breakthrough selectivity of SO2/CO2 in
such ternary gas mixture
SO2/CO2/N2 can reach up
to 668 calculated by eq. S4, which is at least 5–9 times higher than
those in other materials reported in the literature (Figure 7b).
To assess the binding energy between IL/MIL-0.7 and different gas
molecules, single-component adsorption isotherms were compared at
different temperatures. As shown in Figures 8a and 8b, the adsorption
capacity of both decreases significantly with the increase in
temperature, indicating an exothermic adsorption process. The heats of
adsorption (Qst ) of SO2 and
CO2 were calculated using the Clausius-Clapeyron
equation (eq. S5) (Figures 8c and 8d), which is from 20 to 40
kJ·mol-1 for SO2 while from 0 to 4
kJ·mol-1 for
CO2. This indicated that the affinity of IL/MIL-101 for
SO2 is strong than that for CO2. To
further explore the interaction between SO2 and
composites, FT-IR and XPS spectra were used to evaluate possible
SO2 adsorption sites in IL/MIL-0.7. Firstly, the
IL/MIL-0.7 before and after SO2 absorption was
characterized by FT-IR. Figure S4
demonstrates that the intensity of the hydroxyl peaks of IL/MIL-0.7
after adsorbing SO2 is weakened, indicating that
SO2 interacts with the hydroxyl groups45. No symmetric or asymmetric stretch band (between
1389 cm-1 and 1085 cm-1) was
observed for SO2, probably because of the overlap with
the strong vibrational band of IL/MIL-0.7. At the same time, the
characteristic peak of the S element appears in the XPS spectrum,
demonstrating that SO2 molecules were adsorbed in the
composites (Figure S5a). The binding energy of Cl increases with a
deviation of 0.14 eV after IL/MIL-0.7 adsorption of SO2(Figure S5b), which suggests that Cl plays a key role in the adsorption
of SO2 4,46,47. Acidic hydrogens in
[BOHmim]+ such as alkyl side chain C-H and 2-CH
can also form two hydrogen bonds with SO2:
H···O(SO2)-S(SO2)7,48,49. As a
result of the synergistic effect of multiple adsorption sites and the
inner surface of the composite, IL/MIL-0.7 has a high adsorption
capacity for SO2.
3.4
Stability and regeneration properties of composites
Considering the strong corrosive nature of SO2, the
regenerative prosperities and thermostability of IL/MIL-0.7 were
investigated. Firstly, the regeneration property of IL/MIL-0.7 was
evaluated. As shown in Figure 9a, 4 cycles of SO2adsorption were performed to test the reversibility of IL/MIL-0.7, which
still maintains good adsorption performance for the fourth time at 1
bar. In addition, FT-IR spectroscopy was employed to assess the
completion of SO2 desorption. Compared with the fresh
composites, the characteristic peaks of FT-IR spectra did not change
significantly (Figure 9b) in recycled IL/MIL-0.7, which means that the
material can completely desorb SO2. Furthermore, Figure
S6 suggests that the structure of IL/MIL-0.7 can maintain stability
until 450 K, demonstrating that IL/MIL-0.7 is stable enough for
industrial applications 14. All these results indicate
that the composites have good enough reversibility for
SO2 adsorption.
4. Conclusion
In conclusion, an IL/MOF composite with multiple adsorption sites has
been prepared and used as an efficient adsorbent for SO2capture. IL/MIL-0.7 exhibits a high SO2 capacity at 298
K and 1 bar, and an adsorption capacity of 4.7
mmol·g-1 at 298 K and 0.1 bar, which is higher than
most known adsorbents in the literature. Meanwhile, IL/MIL-0.7 excludes
N2 and shows high
SO2/CO2 selectivity, up to 11925 for the
10/99 mixture at 298 K and 1 bar. Finally, SO2,
CO2 and N2 molecular sieving can be
achieved. In addition, the breakthrough experiment of
SO2/CO2/N2 further
proves that IL/MIL-0.7 has excellent trace SO2 capture
capacity in its practical application. IL/MOF also exhibits good
structural stability and sufficient reversibility. Therefore, loading
ILs on MOFs to synthesize new adsorbents shows great potential for a
practical flue gas desulfurization process.