1. Introduction
Combustion of fossil fuels,
including coal, oil, and natural gas, produces sulfur dioxide
(SO2), which is an irritant, corrosive, and highly toxic
gas 1-5. With the growth of the world economy, the
emission of SO2 has increased significantly to meet the
increasing energy demand 6.
However, even trace
SO2 (e.g. 1000–3000 ppm) can cause severe
environmental problems (e.g. smog and acid rain). Besides, it can
irritate the lungs and induce cancer
once SO2 enters the respiratory tract of humans7. Therefore, the effective removal of
SO2 from the air has become an essential issue to ensure
human health and environmental safety. At present, various
desulfurization technologies have been developed to remove
SO2 from flue gas and natural gas, such as washing with
limestone slurry, ammonia and liquid adsorbents 8.
However, these technologies are accompanied by great challenges, for
example, low SO2 capture efficiency, high operation
cost, corroding pipelines and producing large numbers of secondary
pollutants 9-11. Although dry adsorption technologies
based on porous materials (including zeolite, activated carbon, and
metal oxides) can avoid solvent consumption, they have low adsorption
capacity, high energy consumption for regeneration, and poor durability12-13. In addition, trace SO2 will
permanently deactivate amines and reduce the efficacy of the
CO2 scrubbing process. Extremely poor absorption and low
selectivity of SO2 relative to N2 and
CO2 make the complete removal of trace
SO2 a formidable issue 14,15. Thus,
designing a new porous adsorbent to efficiently capture
SO2 from flue gas
and natural gas with high selectivity to achieve sustainable
development.
Metal-organic frameworks (MOFs), with large specific surface area, high
designability and tunability, have attracted extensive attention in
SO2 adsorption and separation 14.
Nevertheless, the causticity of SO2 may damage the
coordination bonds between metal clusters and organic ligands in MOFs6. For example,
Janiak et al. suggested that MOF-177 (BET = 4100
m2·g-1) has an ultra-high
SO2 adsorption capacity (25.7 mmol·g-1at 298 K and 1 bar). However, the formation of metal-sulfur bonds
disrupted the coordination bonds in MOFs which further induces its
structure to collapse 16. At present, some highly
stable MOFs with high adsorption
capacity for SO2 have been reported, such as DUT-67(Zr)
(9 mmol·g-1) and Zr-Fum (4.9
mmol·g-1) 17. Unfortunately, these
stable MOFs also suffered the low adsorption capacity of
SO2 6. It should be noted that
SO2 is often combined with several competitive gases
such as CO2 and N2 11.
However, only a few MOFs can selectively capture SO2from CO2 and N2, especially in the case
of deep desulfurization 16,18. For instance, Bao et
al. revealed the Mg-gallate showed a high IAST selectivity of 325 for
SO2/CO2 mix-gases 14.
Xing and co-workers reported that the adsorption capacity of
SIFSIX-2-Cu-i for SO2 can arrive at 6.9
mmol·g-1 and the selectivity of
SO2/CO2 is 87 19.
MFM-601 displayed a high SO2 capacity (12.3
mmol·g-1) at 298 K and 1 bar, while the selectivities
for SO2/CO2 and
SO2/N2 only reach 32 and 25520. Therefore, from a practical perspective, it is
essential to design a series of new adsorbents with high adsorption
capacity and high stability for SO2, and virtually
exclude CO2 and N2 adsorption. Recently,
ionic liquids (ILs) have been widely applied in SO2adsorption studies due to their unique physicochemical properties,
including low vapor pressure, high stability, and tunability21. Unfortunately, the high viscosity of ILs can cause
poor mass transfer and hamper their industrial application. To overcome
the difficulties, silica gel, porous carbon and silica have been used as
supports to disperse IL and change the adsorption capacity of
SO2 22. Compared with these materials,
MOFs with large surface area and tunable size have been used as
outstanding porous supports for the incorporation of ILs. More
importantly, the introduction of IL may provide additional adsorption
sites to enhance gas adsorption and separation performance of adsorbent23. For example, IL/MOF composites have been
extensively investigated on NH3, CO2,and H2O in our research group, exhibiting excellent
adsorption performances 24-26.
Herein,
a novel composite material IL/MIL
was designed and synthesized, as shown in Scheme 1. The introduction of
IL endows the adsorbent with unique adsorption sites (-OH, -Cl and the
long alkyl chain in IL) for SO2. Besides, the composite
of IL/MIL can provide adequate space to effectively capture
SO2, such as the free space originating from the alkyl
chain in IL and the inner surface of composites. Then, the experiment
indicated that IL/MIL-0.7 showed a high SO2 adsorption
capacity (13.18 mmol·g-1) at 298 K and 1 bar, almost
totally excluding
CO2 (0.27 mmol·g-1) and
N2 (0.07 mmol·g-1). Also, for the
mixture of 10% SO2 and 90% CO2, the
SO2/CO2 selectivity could reach
to11925 while the sorbent
selection parameter (S sp) could be as high as
472131, both of which are higher than those reported porous materials.
In addition, the excellent performance of the composite for the deep
removal of 2000 ppm SO2 also was confirmed by
breakthrough experiments with the
SO2/N2/CO2 mixed
gas. These reveal that the
IL/MIL-0.7 composite has the potential application to remove
SO2 efficiently from flue gas and natural gas.