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Apr 18, 2016 - ABSTRACT: This work reports on the astonishing high capacity of inverse supported ionic liquid absorbents, hereinafter denoted as “in...
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Novel Inverse Supported Ionic Liquid Absorbents for Acidic Gas Removal from Flue Gas D. S. Karousos,† E. Kouvelos,† A. Sapalidis,† K. Pohako-Esko,‡ M. Bahlmann,‡ P. S. Schulz,‡ P. Wasserscheid,*,‡ E. Siranidi,† O. Vangeli,† P. Falaras,† N. Kanellopoulos,† and G. Em. Romanos*,† †

Institute of Nanoscience and Nanotechnology (INN), NCSR “Demokritos”, 153 10 Aghia Paraskevi Attikis, Athens, Greece Lehrstuhl für Chemische Reaktionstechnik, Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany

‡

Ind. Eng. Chem. Res. 2016.55:5748-5762. Downloaded from pubs.acs.org by VANDERBILT UNIV on 01/13/19. For personal use only.

S Supporting Information *

ABSTRACT: This work reports on the astonishing high capacity of inverse supported ionic liquid absorbents, hereinafter denoted as “inverse SILPs” to remove acidic gases (SO2 and CO2) from flue gas streams. These nonconventional SILPs are easily prepared in the form of flowing powder via a phase inversion technique and consist of tiny ionic liquid (IL) droplets enclosed into an ultrathin, porous solid sleeve of pyrogenic silica nanoparticles. The CO2/N2 and SO2/CO2 separation performance and regeneration efficiency of inverse SILPs developed from six different ILs and two IL/chitosan ionogels was examined via gravimetric CO2, N2 absorption isotherms and via SO2, CO2, O2 breakthrough curves from gas mixtures in fixed beds. The involved ILs varied from chemisorbing ones, composed of alkyl- or alkanol-ammonium cations and amino acid anions, to physisorbing ones including ether functionalized anions and 1-alkyl-3methylimidazolium cations. It is noteworthy that the best performing inverse SILP consisted of a very common IL, the 1-butyl-3methylimidazolium chloride [BMIM][Cl], the absorption capacity of which was slightly enhanced by dissolving 5 wt % of chitosan to form the respective ionogel. The material’s performance was stable in repeated cycles of absorption and regeneration at 60 °C under helium flow, exhibiting SO2/CO2 selectivity of above 300, while the SO2 and CO2 absorption capacity was 1.6 and 0.6 mmol/g respectively at 25 °C, in a gas stream of 1 bar composed of 0.13 vol % SO2, 13 vol % CO2, 11.5 vol % O2 and N2 (balance).

1. INTRODUCTION Supported ionic liquid phase absorbents (SILPs) usually consist of thin layers of ionic liquids (ILs), stabilized on the internal (pore) and external surface of porous solids and nanoparticles. The benefits of using SILPs instead of bulk ILs include the solid form of the absorbents which promotes their application in technical, continuous flow processes for flue gas cleaning and the achievement of the highest possible gas/IL interface which enhances the absorption/desorption rate and makes possible the use of small quantities of a rather expensive type of absorbents such as ILs. A novel type of SILPs termed as “inverse” SILPs, was proposed by the authors in late 2014 as the inverse analogous of conventional SILPs.1 In these materials, tiny IL or IL/ionogel droplets are enveloped by SiO2 nanoparticles. Inverse SILPS exhibit several benefits over conventional SILPs. Importantly, controllable amounts of IL, up to 60 wt %, can be immobilized in the absorbent while in the case of conventional SILPs the amount of IL stabilized into the pores or the external surface depends on the pore size, the viscosity of the IL, and the chemical affinity between the IL and the solid surface. Moreover, the porous sleeve that envelops the IL droplet does not influence the absorption capacity of the inverse SILP. As a consequence of the above, the gas selectivity of the absorbent is primarily defined by the performance of the respective IL. To the contrary, in © 2016 American Chemical Society

conventional SILPs, the nanopore space which remains empty after deposition of the IL phase on the pore walls has the capacity to host significant amounts of all the gases contained in a mixture, thus attenuating the selectivity of the absorbent. There is also the benefit of limited resistance to gas diffusion. The gas molecules have to diffuse through pores of about 10 nm (the interparticle space of the silica nanoparticles covering each IL droplet) to reach the IL phase instead of penetrating into the narrow and tortuous channels of nanoporous solids with pore sizes of 2−5 nm.2,3 To make feasible the application of ILs in flue gas treatment, the challenge of very low acidic gas concentrations has to be confronted. Especially SO2 hardly exceeds the 1 vol % (10 mbar) and therefore the rate of absorption and the absorption capacity are of equivalent importance. Chemisorption with task specific ILs (TSILs) is much faster than physisorption. Given their stronger interaction with SO2 as compared to CO2, TSILs seem to hold advantage over physisorbing ILs in the SO2 capture from flue gas. However, chemisorption usually concludes to the significant Received: Revised: Accepted: Published: 5748

February April 18, April 18, April 18,

18, 2016 2016 2016 2016 DOI: 10.1021/acs.iecr.6b00664 Ind. Eng. Chem. Res. 2016, 55, 5748−5762

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Industrial & Engineering Chemistry Research

mixtures. In this work, we have first studied the capacity of inverse SILPs enclosing TSILs to separate CO2 from N2. Our further motivation was to conclude on whether chemisorbing or physisorbing ILs are better candidates for an efficient SO2/CO2 separation. For this purpose simultaneous SO2, CO2, and O2 breakthrough experiments have been performed in fixed beds of inverse SILPS that have been developed using different ILs. The ILs N,N-dimethyl-N,N-diethanolammonium prolinate [N1,1,2‑OH,2‑OH][Prol] N,N,N-trimethyl-N-ethylammonium prolinate [N1,1,1,2][Prol], N,N-dimethyl-N,N-diethanolammonium taurinate [N1,1,2‑OH,2‑OH][Tau] and N-methyl-N-ethyl-N,N-diethanolamonium β-alanate [N1,2,2‑OH,2‑OH][Ala] have been chosen as exceptional chemical absorbents for both CO223 and SO2. We have also used two physisorbing ILs. 1-Butyl-3-methyl imidazolium chloride [BMIM][Cl] was selected as one of the most widely investigated ILs capturing both SO2 and CO2 by physisorption and 1-butyl-3-methyl imidazolium methoxyethoxyethyl methyl phosphonate [EMIM][Me(EG)2(Me)PO3] was selected since ether groups have already shown high capacity for the absorption of SO2.8 Finally, exploiting the enhanced solvation properties of [BMIM][Cl] and [EMIM][Me(EG)2(Me)PO3] for biopolymers we have developed ionogel type inverse SILPS, with the ionogel being a gel-like material obtained by dissolving 5 wt % of chitosan in the IL phase. Chitosan was selected as the second most abundant biopolymer after cellulose owing to the existence of amino groups that can promote chemisorption of both SO2 and CO2, thus providing us with the opportunity to examine the effect of a combined physisorption/chemisorption activity on the performance of the ionogel containing inverse SILPs.

increase of the IL’s viscosity or even to IL solidification and the concomitant negative effect on the capture rate. Moreover a large amount of thermal energy is needed to break the chemical bonds between the IL and SO2. This is a disadvantage in terms of energy consumption and thermal liability of the TSILs, the latter posing limits to the maximum temperature that can be applied for regeneration. Using TSILs in the form of SILPs has the potential to overcome most of the aforementioned problems, achieving faster absorption/desorption rates and efficient transfer of thermal energy. It is also noteworthy that in practical terms inverse SILPs outmatch conventional SILPs since the IL droplets are protected by the solid nanoparticles. As a consequence, a fixed bed consisting of inverse SILPs will retain its powder form and porosity even if the IL phase becomes sticky or solid upon chemical interaction with the gas. It becomes evident that the choice of the IL to be enclosed in an inverse SILP largely defines the performance of these highly innovative absorbents for the desulfurization and carbon capture in flue gas streams. In this context, amine (amino acids, tetrazolates, guanidine), ether, and nitrile functionalities in the anion or cation of ILs enhance their capacity to absorb sulfur dioxide (SO2).4−12 For instance 1,1,3,3-tetramethyl-guanidium lactate [TMG][L] absorbs 0.98 mol per mol at 313 K and 80 mbar of partial SO2 pressure12 when the respective performance for a very common physisorbing IL such as [HMIM][Tf2N] at 298 K and 110 mbar is solely 0.15.13 Despite the high potential of TSILs to be used as the IL phase enclosed in the inverse SILP absorbents there are still problems to overcome which are related to the SO2/CO2 selectivity. It must be noted that the capture mechanism with amine functionalized TSILs is similar between SO2 and CO2 and that the conditions are more favorable for CO2 capture due to its much higher concentration in the flue gas as compared to SO2. Despite this, it could be expected that the stronger interaction with the more acidic SO2 will act competitively over CO2 leading to high separation performance. However, there are also cases of chemisorption, such as this on ILs with methanoate, ethanoate or acetate anions (e.g., 1-butyl-3-methyl imidazolium acetate [BMIM][Ac]), in which the mechanisms of CO2 and SO2 entrapment is different and the competitive absorption effect might be insignificant. [BMIM][Ac] adsorbs 0.23 mol per mol of CO2 at 298 K and 100 mbar based on its interaction with the carbene formed by the deprotonation of the imidazolium ring at position C(2).14 On the other hand, the chemisorption of SO2 by [BMIM][Ac] is presumed to result from displacement of acetate by sulfate and sulfite anions, resulting in the formation of acetic acid, [C4mim][HSO4] and [C4mim][HSO3].2 Thus, the respective loading capacity for SO2 at 50 mbars and 298 K is 0.497 (e.g., ideal SO2/CO2 selectivity of only 4.3).15 There is also the possibility to use common physisorbing ILs in order to achieve an efficient SO2/CO2 separation. Physisorbing ILs absorb from 1 to 2 mol of SO2 per mol at 298 K and 1 bar16,17 and present astonishing high ideal sorption (SO2/CO2) selectivity ranging between 15 and 30, with the SO2/N2 ideal selectivity being much higher. The latter were confirmed by one of the most concise reviews on the topic18 which unveiled also that there exists a very limited number (16) of studies concerning SO2 solubility in ILs as compared to the respective number of studies (140) for CO2. Similarly most of the studies (12 since 2007) on supported ILs for SO2 capture and separation deal with the use of supported IL membranes (SILMS)19,20 while solely four exist on the development of supported IL absorbents (SILPs), and among them,2,3,21,22 no one reports on SO2/CO2

2. EXPERIMENTAL SECTION 2.1. Synthesis of the ILs. [BMIM]Cl was purchased from Solvent Innovation. Acetonitrile and chitosan (deacetylation 75−85%) were purchased from Sigma-Aldrich. [EMIM][Me(EG)2(Me)PO3] was synthesized using a method described previously.24 The amino acid anion ILs [N1,1,2‑OH,2‑OH][Prol], [N1,1,1,2][Prol], and [N1,1,2‑OH,2‑OH][Tau] were synthesized according to the procedure described in ref 1. For the synthesis of [N1,2,2‑OH,2‑OH][Ala], to a round-bottom flask containing N-methyldiethanolamine (3 mL, 26.1 mmol), bromoethane (2.3 mL, 30.8 mmol, 1.2 equiv) was added, and the mixture was left at room temperature and under stirring for 24 h to give a white solid N-ethyl-N-methyl-N,N diethanolammonium bromide. The solid was dissolved in 50 mL of water and extracted twice with dichloromethane (2 × 20 mL) to remove excess alkylation agent. The water was removed in a rotary evaporator to give a white solid 6.029 g. The bromide ionic liquid was dissolved in 10 mL of water and was transformed to the hydroxide ionic liquid by a strong basic ion-exchange resin. By the addition of 1 equiv of L-alanine the desired N-ethyl-N-methyl-N,N-diethanolammonium L-alaninate is obtained after the evaporation of solvent. All ILs were dried at about 10−2 mbar and 60 °C for at least 16 h until the water content determined by Karl Fischer coulometric titration (Metrohm, 756 KF Coulometer) was less than 1000 ppm. The optical microscope images were recorded with a Nikon eclipse 50i equipped with a DS-Fi1 camera and a linkam scientific Instruments LinkPad T95-PE heating table. 2.2. Development of Inverse SILPs. Preparation of Chitosan Ionogels from [BMIM]Cl and [EMIM][Me(EG)2(Me)PO3]. The IL (10 g) was preheated to 85 °C and chitosan powder (0.5 g) was added in small portions to the IL, the resulting mixture was stirred for 6 h. The solid was allowed to dissolve before each new addition. If chitosan did not dissolve the temperature was 5749

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Industrial & Engineering Chemistry Research increased (up to 110 °C). The dissolution process was carried out in inert atmosphere. Dissolution of chitosan was observed by the optical microscope. In Figure 1 we show the jelly like status of the ionogel produced after dissolution of chitosan in [BMIM][Cl].

Figure 3. Microscope images (5× magnification) of (a) silica; (b) chitosan; (c) inverse SILP, 60% [BMIM]Cl, 40% silica; (d) 60% ([EMIM][Me(EG)2(Me)PO3] + 5% chitosan), 40% silica.

Figure 1. Ionogel prepared by dissolving 5 wt % chitosan in the IL [BMIM][Cl].

with the pyrogenic silica in a round-bottom flask to obtain an inverse SILP-system with 40 wt % IL. The mixture is slurried with ethanol and the resulting suspension stirred for about 2 h and dried in vacuum. The samples developed and tested in this work and the respective sample codes are all included in Supporting Information, Table S1. 2.3. Characterization of the Gas Absorption Capacity and Separation Performance. 2.3.1. Gravimetric Setup. The gravimetric method was applied to elucidate the CO2 and N2 solubility in the inverse SILPs, using a force restoration full beam microbalance (IGA 02, for P < 2 MPa). In this method, the change in the weight of the SILP is recorded as a function of the gas pressure. IGA was also used to determine the absorption kinetics. A detailed description of the gravimetric setup and the applied procedure has already been presented in a previous work.1 Before measurement, all the inverse SILPs (100−150 mg) were outgassed at 333 K and high vacuum (10−6 mbar). The buoyancy correction was performed according to the following equation:

Preparation of Inverse SILPs from [BMIM]Cl and [EMIM][Me(EG)2(Me)PO3]. The IL or the chitosan ionogel (3 g) was mixed with acetonitrile (ratio 2:1) to reduce the viscosity. Fumed silica (2 g) was added in portions to the IL or the chitosan ionogel mixture with acetonitrile and the resulting suspension was mixed with a Dremel variable speed rotary tool equipped with a homemade stirring device at 5−11 × 103 rpm for 15 min. Acetonitrile was removed by vacuum evaporation overnight at 50 °C. Several tests have been performed, and the optimal IL or ionogel content in the inverse SILPs was defined to be 40−60 wt %. At compositions lower than 40% the obtained inverse SILPs contained a lot of free silica while at compositions higher than 60% the obtained inverse SILP was not in the form of a freeflowing powder (Figure 2). The homogeneity and particle size of

mcor = mdis − mdis,init + Δm

with ms ρ (Ts , P) + ρs (Ts , P) g m + ∑ i ρg (Ts , P) − ∑ i = 1 ρi j=1

Δm =

Figure 2. Appearance of the inverse SILPs with increasing content of ionogel. At 40% content, excess silica particles can be distinguished on the walls of the glass vial. At 70% and 80% content the inverse SILP has lost the form of a flowing powder.

m1 ρ (Tc , P) ρ1 g mj ρ (Tc , P) ρj g

where mcor is the corrected mass uptake, mdis is the display of the balance during gas uptake, mdis,init is the initial display of the balance after outgassing, Δm is the correction for buoyancy at each pressure (P), ms/ρs is the volume of the sample at several pressures (P) up to 2 MPa and temperature Ts, m1/ρ1 is the volume (stable) of the components of the weighing section at the temperature of the air bath Tc, mi/ρi is the volume (stable) of the components of the weighing section at the temperature of the sample Ts, mj/ρj is the volume (stable) of the components of the counterweight section at the temperature of the air bath Tc,

the obtained materials were evaluated from optical microscope images (Figure 3). Preparation of Inverse SILPs from [N1,1,2‑OH,2‑OH][Prol], [N1,1,1,2][Prol], [N1,1,2‑OH,2‑OH][Tau], and [N1,2,2‑OH,2‑OH][Ala]. For the preparation of the inverse SILPs, the ionic liquid is mixed 5750

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Industrial & Engineering Chemistry Research and ρg (T, P) is the density of the gas phase at a specific temperature and pressure, which was calculated using the Peng−Robinson equation of state. The density ρs of the inverse SILPs (Table S1) was calculated taking into account the mass fractions and the respective densities of the three phases composing the SILP (silica, chitosan, and IL). The procedure for measuring the CO2 absorption capacity of the inverse SILPs with preadsorbed water has already been described in a previous work of the authors.25 2.3.2. Fixed Bed Setup for Breakthrough Experiments. The SO2, CO2, and O2 capture performance has been tested in a fixed bed reactor equipped with online gas specific analysers to measure breakthrough curves and therefore calculate the molar and gravimetric loadings. A Rapidox 3100AS CO2/SO2 analyzer (Cambridge Sensotec, 0−2000 ppm of SO2 and 0−100% CO2) and a Rapidox 3100 CO2/O2 analyzer (Cambridge Sensotec, 0−100% CO2 and 0−100% O2) were connected in series downstream of the fixed bed. Both analyzers had a response time of 10−20 s for reaching 90% of their full scale in a gas stream of 1 L/min. The fixed bed setup was fed with a gas mixture containing SO2, CO2, O2, and N2 via two mass flow controllers (MFC, Bronckhorst High Tech). The first one (MFC 1, 2−100 mL/min) received gas from a standard gas cylinder containing 0.15 vol % SO2 in N2. The second one (MFC 2, 20−1000 mL/min) received gas from a second standard gas cylinder containing 13 vol % CO2 and 11 vol % O2 in N2. The two gas streams were mixed and then either crossed the fixed bed or followed a bypass line. A mixture of 0.13 vol % SO2, 13 vol % CO2, 11.5 vol % O2 and N2 as the balance gas, was generated to simulate a typical CO2 off-gas concentration for postcombustion processes. At the beginning of each experiment the reactor was filled with the inverse SILP material and fixed with two layers of glass wool. The temperatures (25 to 120 °C) could be adjusted by a JUMO dTRON 316 heat controller. The regeneration was carried out at 60 and 85 °C with pure helium or vacuum over 16 h. After regeneration, MFC 2 was switched via a three-way valve, from the He gas cylinder to the gas cylinder containing the mixture of 13 vol % CO2, 11 vol % O2 and 76 vol % N2 while simultaneously, MFC 1 was triggered to deliver the required gas flow of the standard gas containing 0.15 vol % SO2 in N2. To ensure the higher possible accuracy in our measurements, the blank experiments were not performed by using the bypass line but rather by crossing the empty reactor tube containing solely the layers of glass wool. Most of the breakthrough experiments have been performed using 0.1 to 0.15 g of the inverse SILP and a total gas flow of 35 mL giving rise to space velocities and residence times of approximately 40−60 min−1 and 1 s, respectively (depending on the bulk density of the sample). The most promising samples have also been examined at higher flow rates (240 mL/min) and lower residence times (0.15 s) with the purpose to investigate their performance under the conditions required in a practical desulfurization application. 2.4. Raman Analysis. In the present Raman study, the CO2 capture by inverse SILPs encapsulating amine functionalized ionic liquids was examined. All spectra were acquired on a Renishaw inVia Reflex Raman microscope using a high power near-infrared (NIR) diode laser (λ = 785 nm) as excitation source. The laser beam was focused onto the samples by means of a ×50 long distance magnification lens, while analysis of the scattered beam was performed on a 250 mm focal length spectrometer along with a 1200 lines/mm diffraction grating and a high sensitivity CCD detector. In situ Raman measurements of the CO2 adsorption in ILs were performed at a temperature controlled freezing−heating

pressure cell (THMS600PS Linkam) operating under variable temperature and pressure. The temperature inside the cell was set at 40 °C and the CO2 pressure was monitored externally by a pressure gauge in the range of 1−10 bar. Raman spectra were baseline-corrected by a polynomial fitting interpolation routine in order to subtract the background signal. Spectral deconvolution was carried out by nonlinear least-squares fitting of the Raman peaks to a mixture of Lorentzian and Gaussian line shapes, providing the peak position, width, and integrated intensity of each Raman band.

3. RESULTS AND DISCUSSION 3.1. Single Phase CO2, N2 Absorption (Gravimetric). Three of the inverse SILPs investigated in this work, the DD-Prol, TE-Prol, and DD-Taur, have already been examined in a previous study of the authors in terms of their single phase CO2 and N2 absorption capacity up to 1 bar and 40 °C.1 The mechanism of carbamate formation has already been concluded for the ILs with taurinate anion,26,27 while the almost double CO2 loading in N,N,N-trimethyl-N-ethylammonium prolinate (TE-Prol) as compared to N,N-dimethyl-N,N-diethanolammonium prolinate (DD-Prol) (Figure 4a) has been explained by the contribution of the cation to the 2:1 (mol/mol) mechanism of the CO2 capture by amines.1 It is also important to note that CO2 was reversibly absorbed in all the amino acid anion ILs (DD-Prol, TE-Prol, DD-Taur) and that the CO2/N2 ideal sorption selectivity was moderate with the highest separation performance obtained at the early times of the transient stage of absorption rather than at equilibrium (equilibrium points presented in Figure 4a,b). The CO2 absorption performance of one of the inverse SILPs, the DD-Taur, had also been examined in the presence of 7.3% relative humidity, with a procedure already developed by the authors to study the effect of water on the CO2 absorption by tricyanomethanide anion ILs.25 The results presented in Figure 4a show less than 10% drop of the CO2 absorption capacity at the low pressure region ( Cl-Chito > Cl. This means that the amine groups of the amino acids interact strongly and consequently faster with SO2 as compared to the interaction between SO2 and the amine groups of Chitosan. For that reason, upon saturation of the amino acid’s amine groups the breakthrough curve rises almost vertically to the x-axis (time). The strong binding between the amine groups of the prolinate and taurinate anions and SO2 is also proclaimed by the poor reversibility of the respective inverse SILPs. Indeed the SO2 loading efficiency of TE-Prol, DD-Prol, and DD-Taur drops by

with chitosan on the capture of SO2. The reason behind this is the highly active hydrogen atoms near the ether groups of the methoxyl−ethoxyethyl methyl phosphonate anion that form hydrogen bonds8 with the primary amine groups of chitosan leading to their deactivation in capturing SO2. A comparison of the SO2 capture performances of all samples during the first breakthrough experiment (fresh sample) shows that the inverse SILPs prepared with the amino acid anion ILs and among them, the TE-Prol, exhibited the higher SO2 loading capacity (Table S4) especially when this is expressed on the basis of the molar ratio of SO2 over the mol of the immobilized IL (TE-Prol, 0.80 mol·mol−1). Although the molar ratio and molar fraction can reflect the molecular interaction between the gas and the IL, we believe that with the target of a practical application other expressions of the SO2 solubility such as the molarity (mmol·g−1) and volume concentration (mmol·ml−1) can provide better insights on the applicability potential of a material. In this view, the inverse SILPs developed with the purely physisorbing IL [BMIM][Cl] and the respective ionogel with chitosan exhibited the highest absorption capacity and what is more, an astonishing high SO2/CO2 selectivity (200−400) accompanied by complete 5756

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Figure 9. SO2 breakthrough curves of the inverse SILPs embedding ILs with amino acid anions. The several lines correspond to successive runs after regeneration at the conditions described in Table S4. (a) TE-Prol. (b) DD-Taur. (c) DD-Prol. (d) MED-Al.

a factor of 2−3 after the first breakthrough run (Figure 9a,b,c and Table S4) and remains stable at a value of about 0.6 mmol/g. To the contrary, the MEG and MEG-Chito samples presented a continuous decline in their SO2 absorption capacity, something that reveals a continuous degradation of the IL phase that should not be solely related to the irreversible occupancy of active sites. We speculate that the cause of this continuous degradation is the significant increase of the IL viscosity that makes the regeneration of the samples difficult. Since the parameters of the fixed bed used in this work were identical for all the tested absorbents, the SO2 breakthrough time could be used as a measure for defining the best among them. In that respect, sample Cl-Chito, exhibiting SO2 escape time of more than 1.5 h and constant performance after regeneration at 60 °C is the most promising candidate for desulfurization applications. It is also shown (see Table S5) that the SO2 capture performances of Cl-Chito and Cl go far beyond the performance of conventional SILPs that had been developed on porous silica, activated carbon and mesoporous ordered silica (MCM-41) supports.2,3,21,22 Moreover the regeneration in Cl-Chito was attainable from the temperature of 60 °C, with the material fully

recovering its initial SO2 absorptivity and SO2/CO2 selectivity, while other studies refer to the necessity for regeneration temperatures of up to 90−110 °C, in order to make possible the recovery of more than 95% of the SO2 absorption capacity.21 At this point we should emphasize how much important is to provide in full detail the characteristics of the fixed bed column and the experimental conditions in order to facilitate the comparison between the performances of several SILP absorbents. It can be seen that for some of the referred studies3 important parameters of the breakthrough process such as the residence time or the space velocity were not reported or it was impossible to be calculated from the provided information. High flow rates and low residence times are required in a practical desulfurization application. For that reason our best SILP material (Cl-Chito) underwent further breakthrough experiments at higher flow rates which resulted in residence times of approximately 0.15−0.52 s (Table S5). Despite the fact that the SO2 absorptivity of Cl-Chito remained unaffected (or even increased) at lower residence times (see Table S5), the ethyl-methylimidazolium acetate [EMIM][Ac] modified activated carbon of Severa et al.2 seems to be a SILP material of better performance. However, we should accentuate two important 5757

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Figure 10. Breakthrough curves of CO2 in a gas mixture containing 0.13 vol % SO2, 13 vol % CO2, 11.5 vol % O2 and N2 as the balance gas. The several lines correspond to successive runs after regeneration at the conditions described in Table S4. (a) MEG-Chito. (b) MEG. (c) Cl-Chito. (d) Cl.

of the two kinds of acid gases (SO2 and CO2) is the key procedure. It can be stated that the inclusion of amine groups in the IL phase of a SILP absorbent is not desirable for the cases where SO2 must be captured and recovered at the highest possible purity, since CO2 can compete with SO2 in the formation of the sulfurous acid amide. Therefore, even if the SO2 capture efficiency of amine functionalized ILs is high, thorough tests in the presence of the competitive gas (CO2) must be performed in order to define the purity of the recovered SO2 and decide on the best material. In relation to that, we present the CO2 breakthrough curves of all the samples (Figure 10, Figure 11) obtained together with SO2 (Figure 8, Figure 9) and O2 during the experiments with the gas mixture of 0.13 vol % SO2, 13 vol % CO2, 11.5 vol % O2 (N2 as the balance gas). The CO2 breakthrough plots and the respective absorption capacity results included in Table S4 indicate that the inverse SILPs prepared with the amino acid anion ILs (TE-Prol, DD-Taur) were the most efficient CO2 absorbents as was also concluded from the single phase gravimetric sorption experiments (Figure 4, Figure 5). It is however important to note that the variation of the SO2/CO2 selectivity within the repeated cycles of

issues when comparing this conventional SILP with our inverse SILP Cl-Chito. The first one has to do with missing information on the regeneration efficiency of the conventional SILP [EMIM][Ac] on activated carbon. Our sample was tested for seven cycles of SO2 absorption and regeneration with no loss of efficiency, while the regeneration efficiency of the chemically interacting [EMIM][Ac] is under contest.38 On the other hand the SO2 concentration in the work of Severa et al.2 is solely 15 ppm. In this context the SO2 flow rate per mass of the absorbent in our experiment of 240 mL/min (total flow rate) was 25.7 mL·min−1·g−1, while the respective one in the work of Severa et al.2 was 0.075 mL·min−1·g−1. Therefore, our material is exposed to 350 fold times higher SO2 amount per minute and gram while the breakthrough time is solely 4.2 times lower. 3.4. CO2, O2, Breakthrough Results. When the target is to further exploit the captured SO2, (i.e., SO2 might be processed to commercial grade sulfuric acid in the traditional catalytic oxidation process forming SO3 by oxidation with air), then the SO2 selectivity over the several gases present in an industrial stream is of paramount importance. As recently reported,16,39 N2 and O2 solubility in ILs are generally much lower than CO2 and SO2 solubility, which suggests that the effective separation 5758

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Figure 11. Breakthrough curves of CO2 in a gas mixture containing 0.13 vol % SO2, 13 vol % CO2, 11.5 vol % O2 and N2 as the balance gas. The several lines correspond to successive runs after regeneration at the conditions described in Table S4. (a) TE-Prol. (b) DD-Taur. (c) DD-Prol. (d) MED-Al.

Scheme 1a

a (a) The mechanism of SO2 capture in dialkyl-diethanolammonium cation/amino acid anion ILs towards the formation of alkylsulfites. (b) 2:1 (mol/mol) mechanism of CO2 capture in the secondary aminogroup of the amino acid anion.

cation of MED-Al is the main reason behind this behavior. As recently shown for N,N-dibutylundecanolamine40 SO2 can be chemically bound to the alcohol component as an alkylsulfite, which is then stabilized by the amine present in the anion (see Scheme 1a). Therefore, SO2 molecules mostly bind to the cation while CO2 binds to the anion toward the formation of carbamate with the 2:1 (mol amine/mol CO2) mechanism (see Scheme 1b). After irreversible consumption of a fraction of the SO2-specific (hydroxyls) and CO2-specific (aminogroup) active sites during the first breakthrough run, and since there is not competition

absorption/regeneration is quite different between the four samples prepared with the amino acid anion ILs. For instance, the SO2/CO2 selectivity of sample TE-Prol drops from the highest performance of the first run (125), to a lower (almost half, 60) but stable performance within the succeeding runs that followed regeneration, while in the case of DD-Prol, DD-Taur, and MED-Al the selectivity is almost stable varying over an average value. The presence of hydroxyl groups in the N,N-dimethyl-N,N-diethanolammonium cation of DD-Prol and DD-Taur and in the N-methyl-N-ethyl-N,N-diethanolamonium 5759

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Industrial & Engineering Chemistry Research

the ternary system as a function of the CO2 partial pressure in the gas phase. The results from the ternary system are plotted (Figure 12) in comparison with data of the binary (CO2/ [hmim][Tf2N])

between the two acidic gases for the same active sites, the SO2/CO2 selectivity remains the same within successive runs. This explains also the similar drop of the CO2 and SO2 absorption capacity between successive runs for the inverse SILPs DD-Prol, DD-Taur, and MED-Al (Table S4). Moreover we can state that the binding of SO2 with the hydroxyl group of the cation favors the reaction of CO2 with the amine group of the prolinate anion as explained in Scheme 2. Scheme 2a

a

(left) The acetate group in the prolinate anion receives the proton from the formed zwitterion facilitating the 1:1 capture mechanism. (right) Hydrogen bonding between the hydroxyl group in the alkanolammonium cation and the oxygen in the acetate group of the anion limits the capacity of acetate to abstract protons.

Figure 12. CO2 solubility data expressed as mole fraction in binary (CO2/[hmim][Tf2N]) and ternary (CO2/SO2/[hmim][Tf2N]) systems. Open marks are for the binary and filled marks are for the ternary system: open triangles and open circles, ref 42; open rectangles, ref 43; filled rectangles and filled diamonds, ref 13.

Because of the consumption of the cation’s hydroxyl groups toward the formation of alkylsulfites, the hydrogen bonding between the cation and anion is limited (H-bonding between the hydroxyl groups of the cation and the oxygen atom of the acetate group in the prolinate anion). Therefore, instead of sacrificing the amine group of a second prolinate anion (Scheme 2, right), acetate becomes a preferable proton abstractor that receives the proton from the carbamate zwitterion formed upon the capture of CO2 by the amine group (Scheme 2, left). Contrary in the case of TE-Prol the absence of hydroxyl groups in the cation leads to strong competence between CO2 and SO2 for their binding to the amine group of the prolinate anion. In this context, while the more reactive SO2 affects negatively the CO2 absorption capacity of TE-Prol, (see Table S6) it is also the gas which is mostly affected by the irreversibility of the reaction and as a consequence, the SO2/CO2 selectivity drops significantly (almost half) after the first breakthrough run. Having a general view of the CO2 absorption capacity of the inverse SILPs in the mixed gas experiments, it is important to comment on the discrepancies between the results of the breakthrough (multicomponent) and gravimetric (single phase) experiments. An examination of the results in Table S6 shows that the presence of SO2 had a beneficial effect on the CO2 absorption capacity of all inverse SILPs except for TE-Prol. As aforementioned, the beneficial (or not) effect for DD-Prol, DD-Taur, and MED-Al can be attributed to the promotion of the 1:1, instead of the typical 2:1, CO2 capture mechanism with amines toward the formation of carbamate. What was unexpected is the significant increase of the CO2 absorption capacity in the physisorbing ILs [BMIM][Cl], [EMIM][Me(EG)2(Me)PO3] and in the respective ionogels with chitosan. For the cases when CO2 is paired with gases of higher solubility (e.g., SO2, H2S) other gases can enhance the CO2 solubility.13,41−43 Regarding the latter, we should denote that the studies of Shiflett et al.13,41,42 are misleadingly reported by Lei et al.18 as a statement of CO2 solubility enhancement in the presence of SO2 and H2S, since Shiflett et al. referred to the CO2/SO2 selectivity in the gas phase. To prove this we have used the ternary VLE data of Shiflett et al.13 for the system CO2/SO2/[hmim][Tf2N] in order to reconstruct the gas absorption isotherm by plotting the CO2 molar fraction in

available in the literature43 and in our previous work.44 It is quite clear that there is not any effect on the CO2 solubility from the presence of SO2. As a conclusion, the solubility mechanism and specific interaction between mixed gases and ILs are still debated by different authors, and therefore, further work should be addressed to provide deeper theoretical insight into the solubility of gas mixture in ILs. In this context we speculate that the astonishing enhancement of the CO2 absorptivity with coabsorbed SO2 in the inverse SILPs MEG and Cl is related to the strong tendency of the respective ILs to form hydrogen bonding. The strong association between the anion and cation through hydrogen bonding leads to reduced molar volume and consequently to reduced free volume available for the dissolution of the less reactive CO2.45 The results of Table S6 indicate that the inverse SILP prepared with the purely physisorbing [BMIM][Cl], is the one undergoing the higher enhancement of its CO2 absorptivity in the presence of SO2. Moreover, among the two ILs ([BMIM][Cl] and [EMIM][Me(EG)2(Me)PO3]) that we compare here in

Figure 13. Breakthrough curves of O2 through MEG and DD-Prol in comparison to the empty reactor (blank). The gas mixture contains 0.13 vol % SO2, 13 vol % CO2, 11.5 vol % O2 and N2 as the balance gas. 5760

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Industrial & Engineering Chemistry Research terms of the SO2 effect, [BMIM][Cl] is the one exhibiting the highest association degree between its anion and cation46 due to the small size of [Cl−]. In this context, the significant enhancement of the CO2 absorption capacity can be attributed to the attenuation of the association strength between the cation and anion of IL caused by the dissolution of SO2, which in turn generates a much higher free volume for the dissolution of CO2. Another possible explanation is the interaction of CO2 with the strongly Cl− coordinated SO2 molecule as recently evidenced by the structural characterization of 1,1,3,3-tetramethylguanidinium chloride ionic liquid by reversible SO2 gas absorption.47 Regarding the capacity of the inverse SILPs to coabsorb the O2 contained in the mixed gases stream, the breakthrough curves presented in Figure 13 indicate that this is negligible, and for that reason it cannot be calculated with the required accuracy. It is clear that in the case of O2 the blank-curve coincided with the breakthrough curves through the fixed bed of the several inverse SILPs examined in this work.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results has received funding from the European Community’s Seventh Framework Program FP7/ 2011−2014 under Grant Agreement No. 283077 (IOLICAP). We acknowledge also financial support from ESPA Greece−China bilateral cooperation 2012-2014, 12CHN290 (IOLIPURE).

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4. CONCLUSIONS Given the proved capacity of a very common IL, such as the [BMIM][Cl], to physisorb large quantities of SO2, in this work we further enhanced this capacity by finely dispersing the IL phase in the form of tiny droplets enclosed into porous sleeves of silica nanoparticles. The novel inverse SILP absorbent readily accelerates the rate of SO2 dissolution into the IL phase due to its high aspect ratio (surface area to volume). Moreover, exploiting the good solvation properties of [BMIM][Cl] for several biopolymers, we have prepared a novel inverse SILP with the silica nanoparticles enveloping droplets of a [BMIM][Cl]/chitosan ionogel. The ionogel-based SILP exhibited the highest SO2/CO2 separation efficiency (>300) and SO2 absorptivity (1.6 mmol/g) due to the significant contribution of the chitosan’s amino-groups. The sample showed also astonishing performance stability under repeated cycles of absorption/regeneration, with the latter performed at 60 °C in He flow or vacuum. The work shows that flue gas desulfurization can be successfully performed with conventional physisorbing ILs, avoiding complex synthetic procedures for the development of chemisorbing task specific ILs and the concomitant need of high energy for SO2 decomplexation. Other significant conclusions derived from this work include the reversibility of the SO2 capture on the amine groups of chitosan as compared to the primary and secondary amines existing in the prolinate, β-alaninate, and taurinate amino acids and the highly competitive effect of SO2 on the CO2 absorption of the ILs with the prolinate anion. Especially in the case of inverse SILPs made by amino acid anion ILs we found that the competition between CO2 and SO2 for the binding to the primary or secondary amino-groups can be limited through the inclusion of alcohol chains on the ammonium cation. In that case the hydroxyls preferably attract SO2 toward the formation of alkyl sulfites.

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the capture performance of inverse and conventional SILPs, comparison between the CO2 uptake of inverse SILPs in gravimetric and breakthrough experiments; Tables S1−S6 (PDF)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00664. The chemical structure and densities of the used ILs, the isosteric heats of CO2 sorption, the time constants of CO2 absorption vs pressure, the performance of the inverse SILPs in breakthrough experiments, comparison between 5761

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