Properties of aqueous amine based protic ionic liquids and its application for CO₂ quick capture

Highlights

• The solubility of CO₂ in ethoxyacetate anion-based protic ionic liquids (PILs) with three different amine cations solution was measured.

• High absorption capacity, low cost, easy regeneration including fast absorption rate was found in these series of PILs solutions.

• The proposal mechanism of PILs solution for CO₂ capture was advised.

Abstract

Three kind of pure amines which have high solubility of CO₂ were used as absorbents hardly to be recovered. A series of aqueous ethoxyacetate anion-based protic ionic liquids (PILs) with three different amine cations were designed, and physical properties including measured density and viscosity were measured. Solubility of CO₂ in the three PILs solution, 90% [DMAPAH][EOAc], 90% [DEEDAH][EOAc] and 90% [DMEDAH][EOAc], were systematic determined at the range of 303.2–333.2 K under the pressure of 1.2 bar. Impressively, the solubility of CO₂ in 90% [DMAPAH][EOAc] reached up to 2.44 mol/kg, which showed more advantage than some traditional or functional ILs. The 90% [DMAPAH][EOAc] could be reused four times. Besides, the proposed mechanism was advised. Considering the advantages of high absorption capacity, low cost, easy regeneration including fast absorption rate, we believe the three PILs solution can be recommended as alternatives to some volatile organic amines to be applied in CO₂ capture.

Introduction

At least 97% of environmental scientists have an opinion that greenhouse gas emission (GHG) plays a key role for global climate change [1], [2]. Fuel use, food production and agriculture are all sources of GHG. According to recent report by B Jacob group, direct emission for advanced coal power plants contribute CO₂ 850–1000 g/kWh, NO_x 0.5–1.5 g/kWh, SO₂ 0.5–0.7 g/kWh and PM_2.5 g/kWh [3]. Hence, it is an urgent to find more efficient methods to reduce those GHG, especially CO₂ considering its huge emission. Currently, aqueous amines are common commercial absorbents to be widely used to remove CO₂ [4], [5], [6], [7], [8], [9]. Primary amines often have more special affinity to CO₂ because they have higher basicity relative to secondary amines and tertiary amines. The binding energy of CO₂ to different amines decreases in the order: primary amine > secondary amine > tertiary amine studied by P Ratnasamy [10]. It is clear that primary amine may have more superiority than secondary amine and tertiary amine. Ethanolamine (MEA) is the benchmark absorbent of primary amine because of its outstanding advantages such as low price, high absorption capacity of CO₂ and higher reaction speed [11], [12], [13]. In general, CO₂ could direct react with MEA to form the carbamate complex and hardly to be recovered for next CO₂ capture cycle comparing with some recyclable absorbents. However, some shortcomings such as easy degradation, volatile organic compound and corrosion activity, should overcome [14].

Ionic liquids (ILs) which is a type of room temperature melt point salts with designable cations and anions are recognized as an kind of environmental friendly absorbent of CO₂ for some highlighted properties, for instance, neglected vapor pressure, stable chemical property, tunable structures et al [15], [16]. Protic ionic liquids (PILs) are formed through the transfer of a proton from a BrØnsted acid to a BrØnsted base [17], [18]. Compared with pure amines, PILs have a proton to form hydrogen bond avoiding non-negligible vapor pressure and decomposition of absorbent. If PILs are synthesized by stronger acid and/or stronger base in the case of aqueous solutions, the proton-transfer process can be improved [19], [20]. D R MacFarlane et al. studied a series of diamino protic ILs, among that DEEDAH-formate and DMEDAH-acetate + water (1:1 by mol) could absorb 0.50 mol CO₂ of per mol ILs [21], [22]. Y T Wu group found that cyano-containing PILs showed highly selective absorption of SO₂ from CO₂, ethoxyacetate anions would play role to capture SO₂ rather than tertiary amine cations [23]. Inspired by those pioneering works, we design three different amine-containing ILs solutions which may combine virtue of amine cations and ethoxyacetate anions. These PILs solutions are assumed to be excellent absorbents for CO₂ capture. The structures of three kinds of pure amine-containing PILs are listed in Scheme 1. It is found that pure [DMAPAH][EOAc] and [DEEDAH][EOAc] are solid at room temperature because of stronger ion-ion interactions. The mass fractions of water in those pure PILs are all no more than 200 ppm. 10% (wt) water is added into those pure PILs to get PILs solution. To prove the potential application of 90% PILs solution uptake CO₂, some properties are determined including density, viscosity and solubility of CO₂.