3.Al-Based Metal-Organic Framework MFM-300 and MIL-160 for SO₂ Capture: A Molecular Simulation Study

Abstract

SO₂ emission from fossil fuels combustion in the environment has led to various environmental and health hazards drawing the significant attention of the world to control. SO₂ capture through metal-organic frameworks (MOFs) as adsorbent is a promising environmental technology to eliminate the emission of SO₂. Many factors have been identified that influence the activity of SO₂ separations for flue gases by MOFs, but the precise mechanisms of selectivity underlying the interactions between host and guest molecules are still unclear. Moreover, the role of H₂O in flue gases needs to be considered since it is an essential factor in practical engineering applications. In this work, the MOFs of MIL-160 and MFM-300 are selected to capture SO₂ from flue gases with various components, which have been experimentally demonstrated to have great feature in flue gas desulfurization. Force-field-based grand canonical Monte Carlo (GCMC) simulations combined with density functional theory (DFT) are employed to predict the strength of host/guest interactions and the adsorption isotherms for all guests in flue gas in MIL-160 and MFM-300. The results show that MIL-160 has an outstanding SO₂/CO₂ selectivity up to 220 (298 K, 1bar), compared to MFM-300 with 53. CO₂ and SO₂ binding furanyl groups in MIL-160 are stronger than binding hydroxyl groups in MFM-300. We also found that the increasing weakens the performance of SO₂ capture due to the predominant H₂O adsorption in the separation process.

Introduction

The large quantities emission of CO₂ in flue gas from the combustion of fossil fuels has drawn worldwide attention because it is a significant source of global warming [1,2]. Amine-based processes for CO₂ post-combustion capture applications are considered as the most developed and adequate method for CO₂ utilization [2]. However, small amounts of SO₂ in flue gas can react with organo-amines used for CO₂ capture causing permanent loss of activity [3]. Accordingly, the development of new materials and processes for efficient, reversible, and economical capture of SO₂ is highly desired. Effective capture of SO₂ from flue gas requires strong physical or chemical absorption because of the relatively low partial pressure (e.g. 1000 ppm SO₂) in flue gas. Adsorption of SO₂ by porous materials such as zeolites [4], metal oxides [5], porous carbon [6] and porous organic polymers (POPs) [7] based on supramolecular host-guest interactions is a promising approach. However, these materials generally suffer from irreversible structural degradation when exposure to SO₂, leading to poor adsorption capacity and reusability.

Metal-organic frameworks (MOFs) have emerged as a new class of crystalline porous materials composed of self-assembled metallic species and organic linkers to form three-dimensional network structures. Due to their high surface area, tunable pore size, and tailorable surface property, MOFs have potential applications in separation, catalysis, and gas storage. Besides, MOFs can be synthesized with various combinations of organic linkers and metal centers, which provides a platform to design materials for specific applications [8]. A series of MOFs, such as DMOF-1 [9], MOF-74 [10], SIFSIX family [11] and KAUST family [12], have been reported as promising adsorbents in capture of SO₂ from gas mixtures, such as natural gas and flue gas. Among these MOFs, Al-based MFM-300 [13] and MIL-160 [14] were experimentally demonstrated to have great potential in flue gas desulfurization for their outstanding performances in SO₂ selectivity and high chemical and hydrothermal stability. Moreover, Al is more accessible and cheaper compared to other metals, and robust synthesis routes were developed [15,16]. These two MOFs are built from inorganic aluminum chains linked via bipheny l-3,3′,5,5′-tetracarboxylic acid (H₄BPTC) and 2,5-furan dicarboxylate, respectively, and contain the chains of corner-sharing (via μ₂-OH group) [AlO₄(OH)₂] octahedra being connected through ligand to form a three-dimensional framework possessing one-dimensional pores (Fig. 1). SO₂ adsorption and separation in these two MOFs have been studied through experimental techniques and density functional theory (DFT) calculation [13,17], in which the gas separation properties of MOFs were derived from the above breakthrough experiments or single gas adsorption isotherms. In this context, we emphasize to numerically reveal the mechanisms of SO₂ adsorption and separation performance in gas mixtures of MIL-160 and MFM-300.

Furthermore, the effects of H₂O in flue gases on SO₂ separation by these materials are still unclear, which are significant factors in practical engineering applications. Typically, adsorption behavior is likely to be further complicated by the presence of H₂O, which is known to co-adsorb alongside CO₂ and SO₂ under conditions of wet flue gas purification and reduce the adsorption capacity. The studies [1,18,19] showed that H₂O molecule impaired the adsorption capacity of MOFs with coordinatively unsaturated metal sites. It was also reported that H₂O has a minimal impact on CO₂ adsorption properties on MOFs, such as ZIF-68/69 [20], UIO-66 [21] and SGU-29 [22].

We here aim to understand the adsorption mechanisms of pure CO₂ and SO₂ as well as SO₂/CO₂ mixtures in MIL-160 and MFM-300. Grand canonical Monte Carlo (GCMC) simulations coupled with DFT calculation are used to predict adsorption isotherm and heat of adsorption. In addition, the SO₂/CO₂ selectivity is calculated based on ideal adsorbed solution theory (IAST), and the influence of H₂O in wet flue gas is probed to evaluate the actual performance of these two MOFs for post-combustion SO₂ capture.