Study on the efficiency of multiple amino groups in ionic liquids on their sorbents performance for low-temperature CO2 capture

https://doi.org/10.1016/j.cherd.2021.01.016Get rights and content

Highlights

  • Aminoethyl-3-methylimidazolium Lysine is synthesized and supported.

  • Mesoporous silica and PMMA polymer supports are characterized and compared.

  • CO2 capture capacities of sorbents depend on textural properties of the supports.

  • Loadings of amino acid ionic liquid affect sorbents’ CO2 capture capacities.

  • 50 wt% [AEMIM][Lys]-based PMMA sorbent exhibits the best CO2 capture capacity.

Abstract

The addition of more amino groups to ionic liquids is promising to enhance supported ionic liquid performance for low-temperature CO2 capture. In our previous studies, 1-ethyl-3-methylimidazolium lysine ([EMIM][Lys]) impregnated on mesoporous silica SBA-15 and commercial PMMA polymer supports showed moderately acceptable CO2 capture capacities. To improve sorbents performance, an amino acid ionic liquid (AAIL) (i.e., –1-aminoethyl-3-methylimidazolium lysine ([AEMIM][Lys]) with one additional amino group attached to the imidazole ring) was synthesized and immobilized into SBA-15 and PMMA with different loadings. The main purpose is to develop efficient AAIL-based sorbents with high CO2 capture capacities and to study the effect of an additional amino group on CO2 adsorption uptakes. 50 wt% [AEMIM][Lys]-immobilized on PMMA exhibited the best CO2 capture capacities of 1.5 mmol/g-sorb and 0.82 mol/mol-AAIL at adsorption-desorption conditions of 30 °C and 100 °C respectively, and under flue gas (CO2 composition of 15 %), when compared with previous results (1.06 mmol/g-sorb and 0.54 mol/mol-AAIL for 50 wt% [EMIM][Lys] supported on PMMA) under the same condition. This higher capacity demonstrated the efficiency of an additional amino group, though slightly far from the theoretical values (2.75 mmol/g-sorb and 1.5 mol/mol-AAIL). Cyclic performances were also conducted to assess the sorbents long-term stabilities.

Introduction

It is imperative for us to reduce carbon dioxide (CO2) emissions, as CO2 is generally recognized as the most one of the greenhouse gases that cause climate change. Large amount of global CO2 emissions was attributed to the burning of fossil fuels for electricity and heat production. Post-combustion is one technique that can capture CO2 from retrofitting existing power plants. Hitherto, plenty of CO2 separation methods from post-combustion fuel gases have been proposed. Traditional amine scrubbing has been employed in industry since 1950s and is considered the most advanced CO2 capture technique. Amines include monoethanolamine (MEA), diglycolamine (DGA), diethanolamine (DEA), diisopropanolamine (DIPA), methyldiethanolamine (MDEA), and mixed ones (Nielsen and Kohl, 1997). However, this process is hindered with some problems such as high regeneration energy consumption, equipment corrosion, and the requirement for a large absorber, due to amines’ intrinsic properties, including, high energy input for regeneration, corrosive nature, and high vapor pressure (Yu et al., 2012; Torralba-Calleja et al., 2013).

Thus, researchers have investigated alternative CO2 separation techniques, for instance, adsorption by solid sorbents, cryogenic separation, and membrane processes (Kumar Mondal et al., 2012). Krishna and va Baten (2012) compared two typical kinds of physical adsorbents — zeolites and metal organic frameworks (MOFs) — for the relative importance of CO2 selectivity and capacity in a pressure swing adsorption (PSA) unit and concluded that high capacities could dominantly affect the overall performance of PSA units. Ma et al. (2021) studied the CO2 capture performance using solid sorbents K2CO3/Al2O3 through chemical adsorption in a novel two-stage integrated bubbling-transport bed reactor. Hu et al. (2021) proposed that spherical CaO pellets derived from calcium laurate via a facile agar-assisted molding technique exhibited good CO2 sorption performance. Ho et al. (2017) synthesized high surface area MgO sorbents via an aerogel method and examined the CO2 capacity and stability of the sorbents.

Apart from solid sorbents, ionic liquids (ILs) have attracted much attention recently as a promising alternative for low temperature CO2 capture. As organic salts, ionic liquids that contain poorly coordinated ions appear in the liquid phase at room temperature. ILs offer a few advantages due to their remarkable properties, including negligible vapor pressure, high thermal stability, tunability of ions for particular features, and they are unharmful to the environment (Fukumoto et al., 2005; Aghaie et al., 2018).

However, the solubility of CO2 in ionic liquids limits CO2 capture capacity by physical absorption. To improve this parameter, a few functional groups are introduced to adjust cations and anions in the ILs. One of the functional groups as anion is an amino acid (AA), which shows superior function due to carboxylic (-COOH) and amino (-NH2) groups in the structure. Especially active phase -NH2 in amino acids can capture CO2 through a chemical reaction, like amines. Thus, amino acids ionic liquids (AAILs) belong to amine-functionalized ILs (Aghaie et al., 2018; Zhou et al., 2017). 1-Ethyl-3-methylimidazolium lysine ([EMIM][Lys]) has been studied thoroughly (Uehara et al., 2018), with only two -NH2 groups in Lysine anion ([Lys-]) and without any same group in cation structure [EMIM+] as shown in Fig. 1 (Xing et al., 2013). The CO2 capture capacities of functionalized ILs are reported to depend closely on the numbers of amino groups in their molecular structures. A novel AAIL—1-aminoethyl-3-methylimidazolium lysine ([AEMIM][Lys]) has an extra amino group attached to the imidazole ring through aminoethyl group as illustrated in Fig. 2 (Zhou et al., 2017; Qian et al., 2017). Therefore, [AEMIM][Lys] is expected to absorb more CO2 when compared to [EMIM][Lys]. Due to more than two amino groups, this kind of AAILs is considered triamino- or multi amine- functionalized ILs.

Zwitterion mechanism is widely accepted to explain CO2 absorption by primary and secondary alkanolamine (Xu et al., 1996). In this case, 1 mol CO2 first reacts with 1 mol -NH2 group in AAIL to form an intermediate zwitterion and subsequently followed by a deprotonation reaction between 1 mol zwitterion with 1 mol NH2 that produces a carbamate. As a result, 1 mol CO2 requires 2 mols of -NH2 groups.R-NH2 + CO2 ⇌ R-NH2+COO-R-NH2+COO- + R-NH2 ⇌ R-NHCOO- + R-NH3+

Theoretically, one mole ionic liquid that has n amino groups may react with n/2 mol CO2. However, aqueous AAILs have a higher viscosity than alkanolamine solutions because of the complex chemical structures, synthesis, and purification processes, and then show very slow rates of absorption and desorption, and it is difficult for scaling up to the industrial applications (Aghaie et al., 2018; Ramdin et al., 2012a). Another disadvantage of ILs is the cost. The prices of most of ILs produced at the lab-scale range from $1/g–$10/g. Although the BASF company expects that the price can decrease by $40/kg–$100/kg when ILs are produced at larger scales, it is still much higher than conventional solvents (Aghaie et al., 2018; Ramdin et al., 2012b).

Therefore, the dispersed immobilization of AAILs into relatively cheap porous supports (e.g. PMMA ∼ CAD$0.271/g and silica gel ∼ CAD$0.473/g, quoted by Sigma-Aldrich) as solid sorbents have been proposed to enhance mass transfer, to improve CO2 sorption rates of AAILs in this approach, and to reduce cost (Zhang et al., 2006; Ren et al., 2012; Wang et al., 2013a; Wang et al., 2013b). Thus, the effects of textural properties of supports on CO2 adsorption by immobilized AAILs are considered important for better performance of [AEMIM][Lys]-based sorbents.In our previous studies, the impact of textural properties surface chemistry of silica-type supports on CO2 capture has been thoroughly examined (Uehara et al., 2019a). Among a series of mesoporous silicas and silica gel, the best results were achieved around 0.7 mol/mol-AAIL for the SBA-15 modified with remaining surfactant of P123 on the surface without calcination and pore expanding agent when compared with unmodified SBA-15 and PMMA polymeric support (0.54 mol/mol-AAIL).

In this work, for the first time, synthesized AEMIM ionic liquid was mobilized on two popular supports of mesoporous silica SBA-15 and poly(methyl methacrylate) (PMMA) microspheres in order to investigate the additional amino group effect on CO2 capture performance (Yu et al., 2012; Yan et al., 2011; Harlick and Sayari, 2007; Olea et al., 2013; Yue et al., 2006; Bo Yue et al., 2008; Heydari-Gorji et al., 2011). First, 1-aminoethyl-3-methylimidazolium Bromide ([AEMIM][Br]) was synthesized and passed through a column of anion exchange resin to obtain [AEMIM][OH] solution. [AEMIM][OH] reacted with Lysine to obtain 1-aminoethyl-3-methylimidazolium lysine ([AEMIM][Lys]) ionic liquid. Then, mesoporous silicas SBA-15, with different textural properties, were also prepared in order to compare the efficiency of supports. Finally, [AEMIM][Lys] was loaded on mesoporous silicas, and PMMA supports through an impregnation-vaporization method. Several [AEMIM][Lys]-based solid sorbents were characterized, and their CO2 capture capacities were compared.

Section snippets

1-Aminoethyl-3-methylimidazolium bromide ([AEMIM][Br])

The pure 1-methylimidazole (99%) and 2-bromoethylamine hydrobromide (≥98%) supplied from Sigma-Aldrich Canada Co. were used as received chemicals to synthesize [AEMIM][Br], according to procedures reported in the literature (Zhang et al., 2008; Zhou et al., 2009).

Typically, 1-methylimidazole (2.95 mL, 37 mmol) and acetonitrile (25 mL, ≥99%, EMPLURA®, Canada) were mixed in a two-necked flask immersed in an oil bath. 2-bromoethylamine hydrobromide (3.89 g, 19 mmol) was added to the solution

Characterization of pure supports and [AEMIM][Lys]-based sorbents

The structures of as-synthesized [AEMIM][Br] and [AEMIM][Lys] were confirmed using 1H and 13C NMR spectroscopy with unknown impurities. The 1H NMR spectrum (500 MHz) of synthesized [AEMIM][Br] is shown in Fig. 3. 1H and 13C NMR spectrum (500 MHz) of [AEMIM][Lys] are presented in Fig. 4(a) and (b) respectively. The standard chemical shift for 1H NMR of [AEMIM][Br] and 13C NMR of [AEMIM][Lys] were reported as δ = 3.12 (t), 3.90 (s), 4.28 (t), 7.46 (s), 7.51 (s), 8.76 (s), and δ = 176.86, 156.47,

Conclusion

An AAIL of [AEMIM][Lys], was synthesized and impregnated on mesoporous silica (SBA-15, SBA-15-SA, and PE-SBA-15) and PMMA support to fabricate [AEMIM][Lys]-based sorbents in order to study an additional amino efficiency on CO2 capture. Two different supports of mesoporous silica, unmodified and modified SBA-15 and PMMA, were used to achieve the best CO2 uptake. The thermal stabilities of pure [AEMIM][Lys] and mesoporous silicas were examined. The sorbents of 50 wt% [AEMIM][Lys] overall

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgements

The authors are grateful to the Natural Sciences and Engineering Research Council (NSERC) of Canadafor funding this study through its Industrial Research Chairs (IRC) program.

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