Enhanced Performance Of Metal-organic Frameworks For Efficient Removal Of SO₂ From Flue Gas By Pore Modification Strategy

The continuous development of industrial technology has led to a significant increase in energy requirements,however,the environmental problems caused by the discharge of volume of pollutants emitted by power plants have become more serious in present day society.Sulfur dioxide（SO₂）is one of the main polluting gases in the flue gas emitted after coal combustion.It can combine with water vapor in the atmosphere to produce acid rain,thus posing a serious threat to the ecological environment and human health.In addition,the trace amount of sulfur dioxide（500-3000 ppm）in flue gas can also lead to catalyst poisoning and deactivation,which is not conducive to the catalytic conversion and utilization of C1 products in flue gas.Therefore,how to deep removal of trace sulfur dioxide in flue gas has become a research hotspot in the current environmental and industrial fields.The traditional wet desulfurization process will generate a mass of waste liquid and slag,resulting in secondary pollution.As a green and energy-saving separation method,adsorption separation technology is considered to be a promising desulfurization process because it does not require water consumption and does not produce low-grade by-products.However,the application of many adsorbents is limited due to the ultra-low concentration of sulfur dioxide and its corrosiveness.In this regard,this work will concentrate on investigating metal-organic frameworks（MOFs）with high stability.By virtue of the tunable pore structure of MOFs,the polar sites that strongly interact with sulfur dioxide are introduced or constructed into the channels of MOFs through pore modification strategy.Thus,several microporous adsorbents with suitable pore size and polar pore environment were prepared.The adsorption properties of these materials for sulfur dioxide and their ability to selectively remove trace sulfur dioxide from flue gas were researched,as follows:（1）We report a strategy of chemical immobilization of amino acids into robust MOF-808 to selectively remove SO₂ from flue gases.The histidine-modified MOF-808（MOF-808-His,His=L-histidine）has suitable pore size and abundant basic sites,which can selectively adsorb SO₂ in mixed gas.The single-component adsorption equilibrium experiment results show that MOF-808-His exhibited a top-ranking SO₂uptake(10.4 mmol g^-1)with an excellent SO₂/CO₂ selectivity（90.5）under ambient conditions.Dynamic breakthrough experiments verified the feasibility of separating SO₂ from water vapor and real flue-gas compositions.Computational simulations confirmed the vital role of immobilized amino acids in improving the SO₂ capture ability and selectivity.Furthermore,the post-modification strategy is universal,and five other natural amino acids were successfully immobilized into MOF-808 by the same strategy;all samples displayed improved adsorptive desulfurization performances under the low-pressure range.（2）A microporous metal-organic framework material containing dense unsaturated copper sites,Cu-ATC,was prepared for the efficient removal of trace SO₂from flue gas.The coordinated water could be detached from the Cu metal center after the proper degassing process,thus a large amount of metal sites were exposed on the pore surface.Impressively,Cu-ATC adsorbed a benchmark amount of SO₂(5.3 mmol g^-1)at 0.01 bar and 298 K with a record-high SO₂ storage density of 2.23 g cm^-3 at ambient conditions.DFT calculations revealed that its outstanding SO₂ trapping performance is due to the strong interaction between the high density of open metal sites with unique oppositely positions within the pore channels and the guest molecules.Under dry and wet conditions,dynamic breakthrough experiments of various mixed gases including trace SO₂ were carried out on Cu-ATC,which proved its ability to selectively remove trace SO₂.High stability and excellent recyclability make Cu-ATC a potential material for practical desulfurization processes.