A theoretical study on screening ionic liquids for SO₂ capture under low SO₂ partial pressure and high temperature

Abstract

Most studies on SO₂ capture using ionic liquids (ILs) have been carried out under mild conditions (e.g., about 1 bar of SO₂ and 20–40 °C). However, these ILs only exhibit a fraction of their original SO₂ uptake under harsher conditions, i.e., much lower SO₂ partial pressure and evidently higher temperature (e.g., ≤0.2 mbar of SO₂ and ≥110 °C). In this work, we screen 127 reported ILs for multi-molar SO₂ absorption under the harsher conditions, covering nearly all the chemical space explored of the SO₂ capturing IL. A unique cooperative anion-SO₂ interaction mode with two forms (the insertion adduct and the ringed adduct) has been identified for branched anions, where SO₂ acts as both acid and base to attack different reactive sites on anions. The simulated Langmuir isotherms further help screen out potential sorbents (amino-based branched singly charged anions with an Al atom as the center and carboxyl-based divalent anions) with both high overall capacity and high per-cycle absorption capacity. The energies of the reacting orbitals of the anions are linearly correlated to their SO₂ binding energies. This work provides new insights for the design of task-specific ILs under harsh conditions.

Introduction

The emission of SO₂ mainly comes from the combustion of fossil fuels. Traditional desulfurization techniques use sorbents like limestone and ammonia to scrub flue gas to achieve direct SO₂ removal [1], [2], [3]. After desulfurization, however, the sorbents cannot be recovered and large amounts of wastes or valueless byproducts are left behind. To overcome these issues, novel reversible capture materials both in the forms of liquids (ionic liquids (ILs) [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56] and deep eutectic solvents (DESs) [57], [58], [59], [60]), and solids (metal-organic frameworks (MOFs) [61], [62], [63], [64] and two dimensional materials [65], [66], [67]) have been studied broadly in recent years. Most reversible SO₂ capture materials are liquids because solid materials are likely to undergo structural degradation due to the strong SO₂ bindings when exposed in humid SO₂ gas [68], [69], [70], [71].

ILs are especially promising because of their unique properties, including high thermal and chemical stability, low vapor pressure and virtually unlimited tunability, etc. In 2004, Han group reported the first example of chemisorption of SO₂ by employing the functionalized guanidinium-based ILs [4]. Later, ILs consisting of alkyl phosphonium cations and tetrazolide or imidazolide anions ([P₆₆₆₁₄][Tetz] and [P₆₆₆₁₄][Im]) were synthesized by Dai group in 2011, which achieved multiple-site SO₂ absorption for the first time (up to 4.80 mol SO₂ per mol IL at 1 bar of SO₂ and 20 °C) [17]. As the studies continue, two strategies to improve the absorption performance have been followed. One is mainly to optimize the absorption sites of ILs by tethering anions or cations with various functional groups comprising electronegative atoms like N, O, S, and halogens [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [47], [48], [49], [50], [51], [52], [54], [55]. The other is to increase the negative charges carried by an anion, from one to two and even to three by using multiple carboxylic acid groups [45], [46], [53], [72]. As a result, both high overall SO₂ uptake and high effective uptake per absorption-desorption cycle have been achieved.

In general, the reported multiple-site SO₂ ILs exhibit superior performance under mild conditions, i.e., at room temperature and near 1 bar partial pressure of SO₂, whereas, the direct capture of SO₂ from flue gas means much harsher conditions (>40 °C and 0.2−2 mbar of SO₂ in the absorber), which requires stronger chemical interactions between ILs and SO₂ [2]. Up to now, only a few works have focused on the IL-based SO₂ capture under low SO₂ partial pressure and/or at higher than room temperature [4], [23], [24], [31], [32], [45], [46], [73], [74], [75], [76]. Although relatively strong chemisorption is involved in these ILs, most of them still exhibit good reversibility. The first functionalized ILs by Han group maintains 1:1 IL to SO₂ molar ratio at 0.08 bar of SO₂ and 40 °C [4]. In 2013, Hu group reported a series of dicarboxylate-based ILs (e.g., [N₂₂₂₄][disuccinate]) with the potential advantage for direct flue gas desulfurization at even lower SO₂ partial pressure of 0.02 bar, which form hydrogen bonds with SO₂ to promote the absorption [24]. Later, a hydrophobic IL [Et₂NEmim][PF₆] was synthesized by Wu group, which exhibits 0.94 mol SO₂ per mol IL through a combination of chemisorption and physisorption under hydrous conditions at 30 °C and 0.03 bar of SO₂ [31]. Subsequently, Wang and Li group prepared a series of azole-based ILs and measured their SO₂ absorption at 20 °C and 2 mbar of SO₂. Among these ILs, [P₄₄₄₂][Triz] shows the highest SO₂ absorption capacity (1.22 mol SO₂ per mol IL) and its absorption enthalpy (−107.2 kJ/mol) is the most exothermic [32]. The temperature of flue gas is about 130−160 °C and is usually cooled to around 50 °C before entering the absorber [2]. Thus, it is desirable to absorb SO₂ at higher temperature to reduce the requirement for the cooling. In the meantime, the subsequent heating of desulfurized gas to meet the emission standard can also be mitigated. For this purpose, Wu group was the first to explore SO₂ capture at raised temperature. Their lactate-based ILs absorb over equimolar SO₂ at 1 bar of SO₂ and 110 °C [75].

However, up to date, few reversible sorbent has been shown to uptake multi-molar SO₂ under higher temperature (≥110 °C) and lower SO₂ concentration (≤0.2 mbar of SO₂) simultaneously, i.e., directly capturing SO₂ from flue gas. So far, over a hundred ILs have been experimentally or theoretically proved to effectively absorb SO₂ under mild conditions, many of which chemisorb SO₂. Usually, the binding strength of a IL-SO₂ complex is proportional to its absorption capacity. We naturally wonder whether some of the ILs may have appropriate binding strength to be able to trap SO₂ under these harsh conditions and at the same time can be regenerated to afford acceptable active capacity per sorption-desorption cycle.

Since the interaction between acidic SO₂ and absorbent is acid-base reaction in nature, the anionic part carrying negative charge acts as base and dominates the SO₂ uptake and the positively-charged cation portion is of secondary importance [17], [26], [51], [77]. Here, we mainly focus on the anions of the reported ILs. With the assistance of our homemade configuration search program coupled with semi-empirical Hartree-Fock and density functional theory (DFT) methods (Fig. S1, in the Supporting information), we first explore the configuration space comprised of one SO₂ molecule and the reactive sites in each of the reported 130 or so kinds of anions. Via the computed first SO₂ binding energies, we identify the most stable configurations of anion-SO₂ complexes. We then convert the theoretical binding energies into the reaction enthalpies according to experimentally available absorption isotherms. Next we simulate their Langmuir absorption isotherms under harsh conditions. After exhaustively surveying the existing SO₂ design space, we screen out the best anions for SO₂ capture. Later, we briefly discuss the cation effect and identify two descriptors that can qualitatively describe the difference in the reactivity among anions. Finally, we extend the study to investigate the possibility of multi-molar SO₂ capture per mole of IL under harsh conditions as well as the reversibility of the sorbents. We hope to find a promising route of designing SO₂ capture materials under harsh conditions.