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.