Efficient absorption of SO₂ with low-partial pressures by environmentally benign functional deep eutectic solvents

Highlights

• Deep eutectic solvents (DESs) were designed with a function to absorb low-conc. SO₂.

• Betaine(Bet) and l-carnitine(L-car) with a functional group were used as H-bond acceptor.

• Bet + ethylene glycol (EG) DES and L-car + EG DES are environmentally benign.

• L-car + EG DES can absorb 0.644 mol SO₂ per mole L-car (0.37% SO₂).

• L-car + EG DES is a promising absorbent for SO₂ capture.

Abstract

Sulfur dioxide (SO₂) emitted from the burning of fossil fuels is one of the main air contaminants. In this work, we found that environmentally benign solvents, deep eutectic solvents (DESs) could be designed with a function to absorb low-partial pressure SO₂ from simulated flue gas. Two kinds of biodegradable functional DESs based on betaine (Bet) and l-carnitine (L-car) as hydrogen bond accepters (HBA) and ethylene glycol (EG) as a hydrogen bond donor (HBD) were prepared with mole ratios of HBA to HBD from 1:3 to 1:5, and they were investigated to absorb SO₂ with different partial pressures at various temperatures. The results showed that the two DESs could absorb low-partial pressure SO₂ efficiently. SO₂ absorption capacities of the DESs with HBA/HBD mole ratio of 1:3 were 0.332 mol SO₂/mol HBA for Bet + EG DES and 0.820 mol SO₂/mol HBA for L-car + EG DES at 40 °C with a SO₂ partial pressure of 0.02 atm. In addition, the regeneration experiments demonstrated that the absorption capacities of DESs did not change after five absorption and desorption cycles. Furthermore, the absorption mechanism of SO₂ by DESs was studied by FT-IR, ¹H NMR and ¹³C NMR spectra. It was found that there are strong acid–base interactions between SO₂ and −COO⁻ on HBA.

Introduction

The emission of sulfur dioxide (SO₂), which is mainly from the burning of fossil fuels, causes serious environmental problems such as acid rain and smog. Up to now, the technologies of flue gas desulfurization (FGD) play an important role on the removal of SO₂ [1], [2], [3]. Among them, limestone as an absorbent is widely used to control the emission of SO₂ over the past decades [4]. However, there are large amounts of by-product CaSO₄ and wastewater in the process, from which useful SO₂ is not recovered. Thus, the development of renewable and efficient absorbents for removal and recovery of SO₂ is important to our society.

Ionic liquids (ILs) are promising absorbents for SO₂ removal and recovery due to their benign properties, such as negligible vapor pressure, high thermal stability, and tunable structure. Functional ILs in particular could absorb SO₂ efficiently with low partial pressures. ILs have been attracting scientific and technological attention in the last decade. Han et al. [5] reported the first functional IL 1,1,3,3-tetramethylguanidinium lactate ([TMG][La]) for removing low-partial pressure SO₂, and the result showed that the mole ratio of SO₂ to IL could reach 0.98 at 40 °C with 8% SO₂ in N₂. Later, functional ILs based on quaternary-phosphonium[6], imidazolium [7], guanidinium [8], hydroxyl-ammonium [9] were studied by different research groups. The results showed that these functional ILs could capture SO₂ efficiently. However, the disadvantages of these functional ILs, such as toxicity, poor biodegradability, complicated preparation, and high expenses, may limit their large-scale applications in SO₂ removal.

Recently, a new type of solvents, whose characteristics and properties are similar to ILs, named deep eutectic solvents (DESs), has gained wide attention. DESs are typically mixed by a hydrogen bond acceptor (HBA) like quaternary ammonium salts and a hydrogen bond donor (HBD) like glycerol, and their melting point is lower than those of HBA and HBD because of the formation of hydrogen bond [10]. DESs are non-toxic, easy to prepare, free by-product in synthesis, cheaper than ILs, all of which make DESs promising for wider application. Han et al. [11] reported the pure SO₂ absorption capacity of choline chloride (ChCl)-glycerol (n_ChCl: n_glycerol = 1:1) DES could be 0.678 g/g at 20 °C and 1 atm. Though ChCl-glycerol DES could reach high absorption capacities of pure SO₂, the absorption process is physical and the solubility data conforms to Henry’s law, which led ChCl-glycerol DES unable to absorb low- partial pressure SO₂ efficiently. Then, Wei et al. [12] prepared ChCl-based DESs with four organic compounds (EG, malonic acid, urea, and thiourea), and they also demonstrated that the solubility of SO₂ in DESs is a physical process. Liu et al. [13] synthesized series of (CPL)-based DESs, and the solubility of SO₂ in these DESs were tested. The results showed that the pure SO₂ solubility in CPL-acetamide and CPL-imidazole are 0.497 g/g and 0.624 g/g at 30 °C and 1 atm, respectively. Same as ChCl based DESs, the absorption of SO₂ on CPL-acetamide is a physical process, which caused CPL-acetamide to not capture low-partial pressure SO₂ efficiently.

As shown in the previously discussed literatures, DESs for SO₂ absorption mostly exhibited a physical process and the solubility data accorded with Henry’s law, which makes difficult to apply to remove low-partial pressure SO₂ in the flue gas. So, it is necessary to design a class of environmentally benign functional DESs that can absorb low-partial pressure SO₂ in the flue gas.

In order to design DESs that can absorb low-partial pressure SO₂, we need to analyze the structure of existing absorbents for the absorption of low-partial pressure SO₂. As we know, functional ILs can absorb low-partial pressure SO₂ efficiently, which is believed to mainly rely on chemical interaction. From current studies, there are two classes of functional ILs. One class of ILs, such as 1-(2-diethyl-aminoethyl)-3-methylimidazolium hexafluorophosphate ([Et₂NEmim] [PF₆]), contains basic functional groups like amino, which makes the major contribution to the SO₂ absorption. Another class of functional ILs is protonated ILs (e.g., [TMG][La]), or aprotic ILs (e.g., [N₂₂₂₂][La]), whose anion is formed by organic acids, and the pKa of the organic acids (e.g., lactic acid, acetic acid) is larger than that of sulfurous acid [14]. During the absorption of SO₂, organic acid in functional ILs could be replaced by sulfurous acid [14]. However, ChCl typically used as HBA to synthesis DESs can be considered to form by neutralization of a strong acid (HCl) and a strong base (ChOH), obviously, the pKa of HCl is smaller than that of H₂SO₃, which means it cannot chemically absorb SO₂. So, if the HBA used to synthesis DESs is a zwitterionic structure that contains COO⁻ group, and the molecular structure is similar to the structure of protonated or aprotic functional ILs, then DESs with a functional to absorb low-partial pressure SO₂ could be developed.

Betaine (Bet) and l-carnitine (L-car) are inner salts with zwitterionic structure which is ionized by quaternary ammonium and carboxyl group, and the molecular structures are similar to the structure of aprotic functional ILs like [N₂₂₂₂][La], as shown in Fig. 1. Hence, it is possible to realize the functionalization of DESs based on Bet and L-car as HBA. In addition, Bet and L-car are natural compounds that are nontoxic, biodegradable and environmentally friendly. Ethylene glycol (EG) is an important chemical product with negligible vapor pressure. In this work, two kinds of environmentally benign DESs based on Bet and L-car as HBA and EG as HBD were prepared, and we investigated the solubility of SO₂ in DESs at different mole ratios of HBA to HBD, various temperatures and SO₂ partial pressures. The regeneration experiments of DES also were conducted. The results showed that the prepared DESs could absorb low-partial pressure SO₂ efficiently. Moreover, the absorption mechanism of SO₂ in the two DESs was investigated by FT-IR, ¹H NMR and ¹³C NMR.