π-Hole interaction: a theoretical insight into the mechanism of SO2 captured by [Et2NEMim][Tetz] ionic liquids

Abstract

The mechanism of SO2 capture by 1-(2-diethylaminoethyl)-3-methylimidazolium tetrazolate ([Et2NEMim][Tetz]) was investigated using B3LYP hybrid density functional methods at 6-31 + G(d,p) level. In order to find the origin of the high capacity of the subjected ionic liquids (IL) for SO2 capture, the 1: n (n = 1-5) complexes formed between [Et2NEMim][Tetz] and 1-5 SO2 molecules were optimized. Two interaction modes (π-hole interaction and hydrogen bond) were found in each 1: n (n = 1-5) complex; the second order perturbation stabilization energies, E(2)s, confirmed that the main interaction mode was a π-hole interaction. The calculated interaction energies indicated that the first SO2 absorption should be chemical absorption. The capture of the second and third SO2 should fall between chemical and physical interaction, and the fourth and fifth SO2 are incorporated by physical absorption. Thermodynamic analyses indicated that SO2 capture favors lower temperature and higher pressure. Owing to the interactions between SO2 and the [Tetz] anion or the [Et2NEMim] cation, the SOO asymmetric stretching frequency exhibits an obviously red shift in the complex. The strong absorptions of SOO asymmetric stretching in complex (1:5) appear at 1295 cm(-1) (interaction between SO2 and the [Tetz](-) anion) and 1247 cm(-1) (interaction between SO2 and the tertiary nitrogen on the cation). Graphical Abstract Geometric structures of the most stable [ET 2 NEMim][Tetz]ionic liquid (IL; left), and most stable SO2 complex (n = 1-5; right) optimized at the B3LYP/6-31+G (d,p) level (distances in angstroms).

An Overview of Flue Gas SO2 Capture Technology Based on Absorbent Evaluation and Process Intensification

“…[Tetraz]-based ILs, which have four electronegative N atoms, are typical examples of multisite ILs. 88 Chen et al used [P 66614 ][Tetraz] to capture SO 2 , and the absorption pathway is shown in Figure 8a. 89 According to this mechanism, 1 mol [P 66614 ][Tetz] should absorb 4 mol SO 2 molecules, but the experimental results showed that only 3.72 mol of SO 2 was absorbed, which is lower than the theoretical solubility.…”

Recent Advances in Ionic Liquid-Mediated SO2 Capture

“…53 As listed in Table S3 The main reason for these high capacities is that both the cation and the anion could capture SO 2 chemically. 58,59 Interestingly, [Et 2 NEmim][PF 6 ] is hydrophobic, indicating that the SO 2 absorption mechanism is different in the presence or absence of water, as shown in Scheme 10. 2,51 In the presence of water, SO 2 can dissolve in water to generate H 2 SO 3 , which is acidic and ionizes to H + and HSO 3 − .…”

“…What is more, [Et 2 NEmim]­[PF 6 ] can absorb 0.94 mol of SO 2 per mol of IL (30 °C, 3% SO 2 ), and [Et 2 NEMim]­[Tetz] can capture 0.47 g of SO 2 /g of IL at 0.1 bar SO 2 partial pressure. The main reason for these high capacities is that both the cation and the anion could capture SO 2 chemically. , Interestingly, [Et 2 NEmim]­[PF 6 ] is hydrophobic, indicating that the SO 2 absorption mechanism is different in the presence or absence of water, as shown in Scheme . , In the presence of water, SO 2 can dissolve in water to generate H 2 SO 3 , which is acidic and ionizes to H + and HSO 3 – . Subsequently, H + can react with the basic moieties of ILs, such as amine and anions, to form [ILH] + HSO 3 – (Scheme ).…”

“…Introducing SO 2 -philic groups, such as formyl or cyano groups on the basic anion of ILs, can effectively improve SO 2 solubility. As shown in Scheme , formyl-containing ILs ([P 66614 ]­[3-CHO-Indo], [P 66614 ]­[2-CHO-Pyro], [P 66614 ]­[4-CHO-Indo], [P 66614 ]­[4-CHO-PhO]) and cyano-containing ILs ([P 66614 ]­[2-CNPyro], [P 66614 ]­[4-CNC 6 H 4 O], [P 66614 ]­[4-CNC 6 H 4 COO]) were designed for SO 2 capture, and their SO 2 absorption capacities are listed in Table S4. Similar results can also be obtained by introducing cyano groups on cations .…”

Tuning Ionic Liquids with Functional Anions for SO2 Capture through Simultaneous Cooperation of N and O Chemical Active Sites with SO2

“…Wang et al presented a series of dual-functionalized ILs for SO 2 absorption through chemical absorption combined with physical absorption. Subsequently, other ILs with kinds of functional groups have been used for SO 2 capture with high efficiency, including azolate, − tertiary amine, − cyano, − carbonyl, carboxylate, , phenolate, , and ether. − It is proved that the electron-negative N and O atoms are main active sites in these ILs. Compared with sites in the cations, it is known that sites in the anions are more active and can be a key contributor to ILs for gas capture. , When a strong active site and a weak active site are both in one anion, the strong site will preferentially react with SO 2 .…”