Ammonia capture from the gas phase by encapsulated ionic liquids (ENILs)

Abstract

Encapsulated ionic liquids (ENILs) based on carbonaceous submicrocapsules were designed, synthesized and applied to the sorption of NH₃ from gas streams. The ENILs were prepared using three different task-specific ILs with adequate properties for NH₃ capture: 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate (EtOHmimBF₄), choline bis(trifluoromethylsulfonyl)imide (CholineNTf₂) and tris(2-hydroxyethyl)methylammonium methylsulfate [(EtOH)₃MeNMeSO₄]. The ENILs synthesized were analyzed by different techniques to assess their morphology, chemical composition, porous structure and thermal stability. The capture of NH₃ was tested in fixed-bed experiments under atmospheric pressure. The influence of the type and load of IL, temperature (30, 45 and 60 °C) and NH₃ inlet concentration was analyzed. Desorption of NH₃ from the exhausted ENILs was also studied at atmospheric pressure and temperatures in the range of 150 to 200 °C. The ENILs prepared with task-specific ILs were found to be suitable for NH₃ capture in the fixed-bed operation. These systems can be a promising alternative to conventional absorption or adsorption due to: (i) high sorption capacity controlled by IL selection, (ii) remarkable mass transfer rate, (iii) low sensitiveness to high temperatures of the gas stream, (iv) fast and complete regeneration of the exhausted ENIL at mild conditions; and (v) recovery of NH₃.

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1. Introduction

Ammonia is present in gas streams from a diversity of industries and processes, including food processing, compost plants, fishery operations, rubber and leather manufacture, livestock crematoria, and wastewater treatment plants, among others.^1–3 NH₃ emissions contribute significantly to fine particulate matter formation, eutrophication of ecosystems, acidification of soils, alteration of the global greenhouse balance and odor problems.^4–7 Different approaches have been proposed to remove NH₃ from gas effluents. Dry adsorbents, such as activated carbon (AC), have attracted much attention due to their simple design and operation.⁸ On the other hand, in the past years ionic liquids (ILs) have generated interest for the development of a large array of engineering applications,^9,10 including their use as absorbents in gas separation processes.^11–13 The studies available reveal the great potential of ILs to absorb NH₃.^14–21 It has been demonstrated that the cation and anion can be successfully selected to improve both the thermodynamics and kinetics of NH₃ absorption.^22,23 In fact, recent works have evaluated the excellent performance of ILs as NH₃ absorbents in industrial processes of interest, such as absorption refrigeration cycles.^24–26 However, the transport properties of ILs represent still a main limitation for practical applications;²⁷ in particular, ILs generally exhibit much higher viscosity than conventional organic solvents.^28,29

In this sense, novel materials named Supported Ionic Liquid Phase (SILP),^30–35 consisting of the immobilization of ILs on a solid support, have been proposed as a technical solution to overcome the mass transfer limitations of ILs in gas–liquid separation processes.^36–40 SILP materials are commonly prepared by spreading a thin layer of IL onto the surface of a solid support, mostly consisting of materials with high specific surface area,³³ like TiO₂,⁴¹ SiO₂⁴² or AC.³⁹ It has been demonstrated that the IL immobilization on a porous support drastically increases the gas/liquid contact area, leading to much faster sorption rates than in the case of bulk IL.^43,44 However, one of the main drawback for the application of SILP materials in gas separation processes is the limited amount of IL uploaded onto the support (generally lower than 20% w/w,⁴⁵ reaching up to 50% w/w in some cases^40,46), that limits their application in gas separation processes. In this context, the new concept of encapsulated ionic liquids (ENILs) has arisen as an interesting alternative.^47–53 In a pioneer work, Deng et al. synthesized a highly dispersed IL material through the physical encapsulation of the IL in a silica-gel matrix through a sol–gel process, thus reaching IL contents from 8 to 53% w/w.⁴⁷ Nanocomposites of ILs confined in mesoporous silica gels were successfully applied to the selective adsorption of CO₂/N₂ streams.⁴⁸ We recently presented ENIL materials⁵³ consisting of hollow carbonaceous submicrocapsules (C_cap) that confine ILs.^54–56 The synthesis of ENILs is favored by the high affinity found between ILs and AC.^56–59 The novel material is characterized by a high amount of IL incorporated (70–85%, w) and capsule particle size ranging from 450 to 800 nm. This ENIL material implies a shift from a continuous to a discrete IL phase with submicron size units, thus drastically increasing the specific contact area with respect to the neat IL.⁵³ Moreover, ENIL show the additional advantage of their high versatility, since they can be prepared at different loads (up to 80% w/w) using ILs with optimized properties for a specific application. In a previous work,⁵³ the capture of NH₃ was preliminary tested at atmospheric pressure with ENILs prepared using the task-specific IL 1-2(-hydroxyethyl)-3-methylimidazolium tetrafluoroborate (EtOHmimBF₄), with a high NH₃ absorption capacity. Efforts to develop new sorbents based on ILs and to apply them in the removal of NH₃ from gas stream have also been recently reported by Kohler et al.⁶⁰ In their approach, a solution of metal complexes in IL was supported onto polymer-based spherical ACs, obtaining very high capacity by irreversible chemical absorption of NH₃.

The objective of this work is to evaluate the potential application of ENILs for the purification of gas streams contaminated with NH₃, with further recovery of this compound. The ENILs used have been designed to provide high NH₃ physical sorption capacities, high mass transfer rates, thermal stability and favorable regeneration. Hollow carbon submicrospheres with a thick porous wall were synthesized by templating method in order to increase their mechanical strength.⁵⁶ Then, the ENILs were prepared by introducing in the microcapsules different loads (33–80% w/w) of three selected task-specific ILs showing outstanding NH₃ absorption capacity and relatively high NH₃ diffusion coefficients:^22,23 1-2(-hydroxyethyl)-3-methylimidazolium tetrafluoroborate (EtOHmimBF₄), choline bis(trifluoromethylsulfonyl)imide (CholineNTf₂) and tris(2-hydroxyethyl)-methylammoniummethylsulfate [(EtOH)₃MeNMeSO₄] (Table 1). The ENILs were characterized by several techniques, including scanning and transmission electron microscopy (SEM and TEM), elemental analysis (EA), N₂ adsorption–desorption at −196 °C and thermogravimetric analysis (TGA). They were compared to the corresponding benchmark materials namely the empty carbon submicrocapsules (C_cap) and the neat ILs. The fixed-bed sorption experiments were carried out in fixed bed at temperatures ranging from 30 to 60 °C and inlet NH₃ concentrations from 500 to 2000 ppmv. The influence of the chemical nature and the load of IL in the ENILs was analyzed at different operating conditions. The regeneration of the exhausted ENIL beds was assessed by desorption experiment performed under N₂ flow at temperatures between 120 and 200 °C.

Table 1 Ionic liquid structures and abbreviations

Structure	Abbreviation	Name
	EtOHmimBF4	1-(2-Hydroxyethyl)-3-methylimidazolium tetrafluoroborate
	CholineNTf2	Choline bis(trifluoromethylsulfonyl)imide
	(EtOH)3MeNMeSO4	Tris(2-hydroxyethyl)methylammonium methylsulfate

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2. Materials and methods

2.1. Materials

Phenol (99%), paraformaldehyde (95–100%), aluminum trichloride (95–100%), ammonia (38%), absolute ethanol, hydrofluoric acid (48%) and sodium hydroxide was purchased from Panreac, and tetraethylorthosilicate (98%) (TEOS) and octadecyltrimethoxysilane (90%) (C18TMS) from Sigma-Aldrich. The ILs 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate (EtOHmimBF₄), choline bis(trifluoromethylsulfonyl)imide (CholineNTf₂) and tris(2-hydroxyethyl)-methylammonium methylsulfate [(EtOH)₃MeNMeSO₄] were supplied by Iolitec in the highest purity available (purity > 97–98%). They were used as received.

To prevent hydration, acetone and ILs were kept in their original tightly closed bottles in a desiccator before use, and they were manipulated inside a glovebox under dry nitrogen atmosphere.

The sorption experiments were carried out using a commercial mixture of N₂ and NH₃ (2000 ppmv) and N₂ (purity > 99.99%) supplied by Praxair.

2.2. Preparation of ENILs

Carbon submicrocapsules (C_cap) with a thick wall were synthesized following the template method, as detailed in previous works,^53,56 to achieve high mechanical resistance and porosity. The C_cap were filled by successive impregnation steps of 100 mg of hollow carbon capsules with 1 mL of IL–acetone solution.⁴⁶ The number of impregnation steps and the concentration of IL in the acetone solution were adjusted to obtain ENILs with different IL loadings. To ensure a homogeneous penetration of the IL solution into the pores, it was added dropwise over the C_cap, followed by vacuum evacuation of acetone at 60 °C and 10 mbar for 2 h. The ENILs synthesized contained from 33 to 80% of IL (w, ENIL basis), i.e. IL/carbon mass ratios between 0.5 and 4 (R_0.5–R₄). The amount of IL loaded was calculated as the difference between the weight of the final ENIL and the starting C_cap. Those measurements were validated by the N content determined by elemental analysis (EA), as established in a previous work.⁴⁶

All the ENILs prepared had the resemblance and texture of a powdered material. The ENILs were stored at 90 °C during 24 h prior to their use.

2.3. Characterization of ENILs

The morphology and microstructure of C_cap and ENILs were studied by scanning and transmission electron microscopy (SEM and TEM). SEM micrographs were obtained using a Hitachi S-3000N model and TEM images in a Philips 420 microscope. Elemental analyses (EA) of C_cap and ENILs were carried out in a Perkin-Elmer analyzer (210 CHN model) to obtain C, H and N content. The porous structure was characterized by means of nitrogen adsorption–desorption isotherms at −196 °C using a Micromeritics apparatus (Tristar II 3020 model). The samples were previously outgassed at 150 °C and 70 mTorr for 6 h. The BET equation was applied to determine the total surface area (A_BET), and the external or non-micropore area (A_s) was obtained using the t-method. The Dubinin–Radushkevich equation was applied for micropore volume estimation. The difference between the volume of N₂ adsorbed (as liquid) at 0.95 relative pressure and the micropore volume was taken as the mesopore volume, and the DFT method was used to obtain the pore size distribution.⁶¹ Thermogravimetric analyses (TGA) of ILs and ENILs were conducted in a Mettler Toledo Instrument (TGA/SDTA851e model) under nitrogen atmosphere. A dynamic method was used within the temperature range from 50 to 600 °C at a heating rate of 10 °C min⁻¹ under a nitrogen flow of 50 mL min⁻¹. The accuracy of temperature and mass measurements was 0.1 °C and 10⁻³ mg, respectively. Aluminum pans were used to hold an initial mass between 4 and 12 mg. The derivative curves (DTG) were also obtained to assess the temperature of IL decomposition in the ENILs.

2.4. NH₃ capture experiments

The sorption of ammonia was performed in a fixed bed with the ENIL (50–250 mg) placed in a quartz column (4 mm of internal diameter and 4–5 cm of length). N₂–NH₃ mixtures (500–2000 ppmv NH₃) were continuously passed at 20 mL min⁻¹ flow rate. Different temperatures within the 30–60 °C range were tested and the operating pressure was ∼1 atm. NH₃ was analyzed at the exit of the column with a quadrupole mass analyzer (OmniStar/ThermoStar).

The NH₃ uptake at saturation (q_s, mg g⁻¹) for C_cap and ENILs were calculated from the breakthrough curves as:

where Q is the gas flow rate (L min⁻¹); m is the mass of C_cap or ENIL in the column (g); C and C_o represent the exit and the inlet NH₃ concentration (mg L⁻¹), respectively, and t_s corresponds to the time at which C equals C_o. The comparison of the capacity of the different ENILs was made in terms of per unit mass of ENIL (q_s,ENIL; mg g_ENIL⁻¹), unit mass of IL (q_s,IL; mg g_IL⁻¹) or unit volume of the packed bed (q_s,V; mg cm⁻³). The breakthrough time (t_b) was defined as the time at which the C/C_o ratio reached a value of 0.05.

The length of the mass transfer zone (h_MTZ) was estimated from the breakthrough curves using the expression:

where h is the length of the entire sorbent bed in the column, t_0.95 and t_0.05 are the times at which the exit NH₃ concentration reaches 95% and 5% of the inlet value, respectively.

Desorption experiments were performed after the saturation of the ENIL beds. A N₂ flow (1 mL min⁻¹) was passed through the exhausted bed at 200 °C during 30 minutes. In the case of (EtOH)₃MeNMeSO₄, a regeneration temperature of 150 °C was used because of the lower thermal stability of this IL. The ENIL fixed beds were used in three sorption–desorption cycle tests.

3. Results and discussion

3.1. Preparation and characterization of ENILs

The selection of ILs for this work (EtOHmimBF₄, CholineNTf₂ and (EtOH)₃MeNMeSO₄) was conducted taking into consideration that ILs with acidic character and low Henry constants of NH₃ can lead to ENILs with high capacity for NH₃ capture. Previous works indicated that the inclusion of hydrogen-bond donor groups, such as hydroxyl group, in the IL structure significantly increases the interaction with NH₃, and thus its solubility.^22,23Table 2 collects the most relevant physical and thermodynamic properties of the task-specific ILs used in this work.

Table 2 Physical and thermodynamic properties of the IL studied^22,23

	M weight, (g mol^−1)	V molecule ^a, (nm^3)	Density, (g mL^−1)	Viscosity, (mPa s)	Henry constant^a, (MPa)
a NH3 absorption, calculated by COSMO-RS method at T = 25 °C.
EtOHmimBF4	214	0.266	1.36 (20 °C)	149 (20 °C)	0.013
CholineNTf2	368	0.427	1.52 (30 °C)	95 (30 °C)	<10^−4
(EtOH)3MeNMeSO4	275	0.374	1.34 (20 °C)	1492 (20 °C)	0.011

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A set of ENILs based on EtOHmimBF₄ and different IL loads (R_0.5, R₁, R₂ and R₄) were prepared with the view in analyzing their properties and behavior in NH₃ capture. Additional ENILs incorporating a high load (R₄) of CholineNTf₂ and (EtOH)₃MeNMeSO₄ were also prepared in order to also check the influence of the nature of the IL.

The SEM image of C_cap (Fig. 1A) showed regular spherical morphology and a mean diameter around 600 nm. A wall thickness ca. 80 nm can be measured from TEM image (Fig. 1B). The TEM image (Fig. 1C) of the ENIL–R₄ prepared with EtOHmimBF₄, shows successful incorporation of the IL into de C_cap structure, both in the hollow core and in the porous carbon wall. The EA results in Table 3 also confirm the adequate incorporation of IL into the ENILs.

Fig. 1 (A) SEM images of C_cap and TEM images of (B) C_cap and (C) ENIL–R₄ prepared with EtOHmimBF₄.

Table 3 Characterization of the C_cap and ENILs prepared with different load of IL

	Material	ENIL nominal IL load (% w/w)	EA				Textural properties				Thermal stability, TDTG (°C)
			%C	%H	%N	ENIL calculated IL load (%, w/w)	A BET (m^2 g^−1)	A s (m^2 g^−1)	V microp (cm^3 g^−1)	V mesop (cm^3 g^−1)
	Ccap	—	88.2	1.0	0.2	—	1503	701	0.32	0.53	—
ENIL–R0.5	EtOHmimBF4	33	70.0	2.7	4.5	34	519	297	0.19	0.22	—
ENIL–R1		50	60.1	3.4	6.9	56	197	108	0.06	0.08	385
ENIL–R2		67	52.9	3.8	8.7	66	13	12	0.00	0.01	397
ENIL–R4		80	46.3	4.4	10.5	85	1	1	0.00	0.00	405
ENIL–R4	CholineNTf2	80	36.1	3.1	5.8	80	2	2	0.00	0.00	392
ENIL–R4	(EtOH)3MeNMeSO4	80	48.4	5.6	4.4	85	3	3	0.00	0.00	242/318
IL	EtOHmimBF4	—	33.7	5.1	13.1	—	—	—	—	—	430
	CholineNTf2	—	21.9	3.6	7.3	—	—	—	—	—	416
	(EtOH)3MeNMeSO4	—	34.9	7.6	5.1	—	—	—	—	—	—

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Adsorption–desorption isotherms of N₂ at −196 °C were used to analyze the porous structure of C_cap and ENILs, being the results collected in Table 3. C_cap showed a highly developed porous structure (A_BET = 1503 m² g⁻¹), with high contributions of micro- and mesoporosity, as indicated by the high amount of nitrogen adsorbed in the whole relative pressure range. Fig. 2A shows the isotherms obtained for ENILs with different loads of EtOHmimBF₄. The porosity available for nitrogen adsorption decreases clearly as the load of IL increases. In fact, for ENIL–R_0.5 (33% of IL) a reduction of 65% in the BET surface area takes place (Table 3), whereas for ENIL–R₁ (50% of IL) only 13% of the initial BET surface area of the C_cap is still available for N₂ adsorption. The BET surface area decreases to negligible values for R₂, and particularly for R₄ (80% of IL). Fig. 2B shows that the loss of porosity takes place evenly in the whole range of porosity with the increase in the load of IL. Table 3 also summarizes the characteristics of the ENILs prepared with CholineNTf₂ and (EtOH)₃MeNMeSO₄, showing also nearly total loss of porosity (A_BET < 3 m² g⁻¹) for R₄ independently of the IL used.

Fig. 2 (A) Nitrogen adsorption–desorption isotherms at −196 °C and (B) pore size distribution (1 to 100 nm) of C_cap and ENILs prepared with increasing load of EtOHmimBF₄ (R_0.5–R₄).

Thermal stability is also an important factor to be considered for the application of ENILs. Fig. 3A depicts the TGA curves of the neat IL EtOHmimBF₄ and C_cap. As can be seen, EtOHmimBF₄ starts to decompose around 340 °C and it is decomposed almost completely at 460 °C. C_cap exhibits a loss of weight lower than 10% in the temperature range considered, which can be ascribed to the gradual loss of surface chemical groups. The thermal stability of the ENILs prepared with EtOHmimBF₄ can be compared with that of the neat IL in Fig. 3B (see also Fig. 1S in ESI† for complete TGA data). Previous studies^46,62 showed that the thermal stability of supported ILs can be remarkably affected by the nature of the support, with non-polar porous surfaces, such as activated carbon, leading more stable SILP than supports with acidic character, such as silica. Fig. 3B and Table 3 (see T_DTG values) show that the thermal stability of ENIL materials is slightly lower than that of the neat IL, in agreement with the low support effect observed with carbon surfaces containing low concentration of polar functional groups,⁴⁶ which is the case of C_cap. In addition, it can be observed that an increase of the IL load results in higher thermal stability of the ENIL material, with a shift in the DTG degradation peak (T_DTG) from 385 to 405 °C. This shift is probably related to the decreasing proportion of IL in contact with the support surface. Thus, ENIL–R₄ presents a thermal stability very close to that of the neat IL EtOHmimBF₄. Table 3 also summarizes the T_DTG for the ENIL–R₄ prepared with CholineNTf₂ and (EtOH)₃MeNMeSO₄ (complete TGA curves in Fig. 2S and 3S in ESI†). It can be concluded that ENILs, particularly R₄, present an adequate thermal stability to operate in the 30–200 °C range of sorption/desorption temperatures involved in the NH₃ capture and bed regeneration processes.

Fig. 3 (A) Dynamic thermogravimetric curve (TGA) of neat EtOHmimBF₄ and C_cap; (B) normalized derivative thermogravimetric curve (DTG) of ENILs prepared with EtOHmimBF₄ and neat EtOHmimBF₄ (nitrogen atmosphere and a heating rate of 10 °C min⁻¹).

3.2. Capture experiments in fixed-bed column

Fig. 4 depicts the NH₃ breakthrough curves obtained using a packed bed of C_cap at temperatures of 30, 45 and 60 °C. Table 4 collects the operating conditions and the main results extracted from the NH₃ capture experiments conducted, including the breakthrough time (t_b), the saturation capacity (q_s) calculated on different basis, and the length of mass transfer zone (h_MTZ). C_cap support presents a high NH₃ uptake per mass of sorbent (q_s,sorb = 6.0 mg g⁻¹), equivalent and even higher than those reported for physical adsorption with ACs.^63,64 The operating temperature has a remarkable effect on NH₃ adsorption by C_cap, with bed capacity decreasing by ∼40% when temperature increased from 30 to 60 °C. This change is in agreement with the results obtained for NH₃ adsorption with other ACs in bibliography.^63,64 The length of mass transfer zone values (e.g. h_MTZ = 1.5 cm at 30 °C) are close to the values reported for the adsorption of other solutes (chloromethanes, ethanol) on ACs.^65,66 This reveals fast and favorable NH₃ adsorption onto C_cap, ascribable to the large contact surface area and low size of the submicrocapsules.

Fig. 4 Breakthrough curves of NH₃ sorption by fixed bed of C_cap (2000 ppmv inlet NH₃ concentration, 20 mL min⁻¹ and a bed length of 5 cm).

Table 4 Operating conditions and results for the capture of NH₃ using packed beds of C_cap and ENILs (gas flow 20 mL min⁻¹, atmospheric pressure)

		T, (°C)	[NH3]0, (ppmv)	h, (cm)	m ENIL, (mg)	t b, (min)	q s,V, (mg cm^−3)	q s,IL, (mg gIL^−1)	q s,ENIL, (mg gENIL^−1)	h MTZ, (cm)
a Mixture of adsorption and absorption phenomena.^36 b q s,sorb (mg gCcap adsorbent^−1).
Ccap		30	2000	4.8	52.2	9.9	0.52	—	6.0^b	1.6
ENIL–R0.5	EtOHmimBF4	30	2000	4.9	77.0	8.6	0.48	—^a	3.8	2.3
ENIL–R1	EtOHmimBF4	30	2000	5.0	119.6	11.6	0.61	—^a	3.2	2.0
ENIL–R2	EtOHmimBF4	30	2000	5.1	164.7	12.8	0.77	—^a	3.0	1.7
ENIL–R4	EtOHmimBF4	30	2000	4.8	216.4	22.7	1.18	4.1	3.3	1.6
ENIL–R4	CholineNTf2	30	2000	4.7	236.0	8.9	0.48	1.5	1.2	1.5
ENIL–R4	(EtOH)3MeNMeSO4	30	2000	4.3	209.0	9.9	0.62	2.0	1.6	1.8
Ccap		45	2000	5.0	49.2	7.0	0.42	—	5.4	1.8
ENIL–R4	EtOHmimBF4	45	2000	4.8	214.0	21.9	1.10	3.9	3.1	1.3
ENIL–R4	CholineNTf2	45	2000	4.5	231.5	8.9	0.49	1.4	1.2	1.3
ENIL–R4	(EtOH)3MeNMeSO4	45	2000	4.0	207.7	7.1	0.50	1.5	1.2	1.8
Ccap		60	2000	5.0	50.0	4.9	0.30	—	3.8	2.1
ENIL–R4	EtOHmimBF4	60	2000	4.7	214.0	19.3	1.01	3.5	2.8	1.2
ENIL–R4	CholineNTf2	60	2000	4.5	231.2	8.1	0.45	1.3	1.1	1.2
ENIL–R4	(EtOH)3MeNMeSO4	60	2000	4.0	205.0	4.1	0.33	1.0	0.8	1.8
ENIL–R4	EtOHmimBF4	30	500	4.4	192.0	74.9	0.76	2.8	2.2	0.9
		30	800	4.3	195.4	44.9	1.08	3.7	3.0	1.3
		30	1200	4.5	198.0	35.7	1.12	3.9	3.2	1.4
		30	1500	4.4	194.0	33.0	1.12	3.9	3.2	1.4
ENIL–R4	CholineNTf2	30	500	4.3	224.5	28.8	0.29	1.1	0.7	1.3
		30	800	4.3	225.0	19.6	0.42	1.3	1.0	1.4
		30	1200	4.4	222.5	9.9	0.44	1.3	1.1	1.5
		30	1500	4.4	225.8	9.6	0.45	1.4	1.1	1.5
ENIL–R4	(EtOH)3MeNMeSO4	30	500	4.0	201.0	17.2	0.32	0.9	0.8	1.6
		30	800	4.0	197.0	19.9	0.51	1.7	1.3	1.7
		30	1200	4.0	200.5	15.8	0.56	1.8	1.4	1.7
		30	1500	4.0	200.9	13.9	0.60	1.9	1.5	1.8

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Fig. 5 presents the NH₃ breakthrough curves obtained in fixed bed experiments using ENIL–R_n prepared with different loads of EtOHmimBF₄, being the main results extracted from them and collected in Table 4. As can be seen, the incorporation of the IL into the C_cap support does not improve the sorption capacity per unit mass of sorbent (q_s,ENIL). However, due to filling of the pores of C_cap with IL, the NH₃ sorption capacity per bed volume unit (q_s,V; mg cm⁻³) evidences the advantages of ENILs in terms of bed efficiency when compared to C_cap. Thus, when the load of EtOHmimBF₄ in the ENILs is increased, the total amount of NH₃ captured by the bed was higher, with the breakthrough times increasing from 8.6 to 22.7 minutes for R_0.5 and R₄, respectively. These tests were carried out using approximately the same length of bed (4.8–5.1 cm), which implied different ENIL mass (77.0–216.4 mg) because of differences in density caused by the different load of IL in the ENILs. As a result of it the ENIL–R₄ prepared with EtOHmimBF₄ yielded much higher uptake of NH₃ per bed volume unit (q^{30 °C,2000 ppmv}_s,V = 1.18 mg cm⁻³) than the C_cap (q^{30 °C,2000 ppmv}_s,V = 0.52 mg cm⁻³). In other words, ENILs lead to a higher bed efficiency thank to the incorporation of the IL solvent into the void porous structure of the support, in good agreement with previous results.⁵³ Favorable conclusions are also achieved when comparing with other commercial NH₃ sorbents tested in fixed bed under similar operating conditions (see Table 4): BAHIACARBON AC, q_s,V = 0.97 mg cm⁻³ ([NH₃]₀ = 1200 ppmv, 40 °C, 1 atm),⁶⁴ HAYCARB AC q_s,V = 0.56 mg cm⁻³ ([NH₃]₀ = 1000 ppmv, 30 °C, 1 atm)⁶⁷ and MAST AC q_s,V = 1.8 mg cm⁻³ ([NH₃]₀ = 1000 ppmv, 23 °C, 1 atm).⁶⁸ It should be noted that increasing the surface acidity of these commercial adsorbents by chemical treatments may greatly enhance their NH₃ adsorption capacity,^67,68 but at the expense of lower mass transfer rates⁶⁷ and probably more difficult regeneration of exhausted adsorbent, due to strong acid–base chemical interactions between ammonia and the surface functional groups.⁶⁸

Fig. 5 Breakthrough curves of NH₃ capture by ENILs prepared with different load of EtOHmimBF₄ (2000 ppmv inlet NH₃ concentration, 20 mL min⁻¹ and 30 °C, bed length 5 cm).

It is noticeable that the NH₃ sorption capacity per unit mass of IL of the ENIL–R₄ prepared with EtOHmimBF₄ (q_s,IL = 4.1 mg g_IL⁻¹) is close to the absorption capacity of neat EtOHmimBF₄ (q^{30 °C}_s = 4.40 mg g_IL⁻¹; see Fig. S4 in ESI†) obtained by gravimetric measurements at identical temperature and NH₃ concentration. It indicates that the task-specific IL maintains its high NH₃ absorption capacity after encapsulation.

Fig. 6 compares the breakthrough curves obtained for ENIL–R₄ prepared with the different ILs (EtOHmimBF₄, CholineNTf₂ and (EtOH)₃MeNMeSO₄) at temperatures of 30, 45 and 60 °C. The NH₃ capacity per bed volume unit (q_s,V; mg cm⁻³, Table 4) at 30 °C increases in the order EtOHmimBF₄ > (EtOH)₃MeNMeSO₄ > CholineNTf₂, in good agreement with the absorption capacities of the corresponding neat ILs (see Fig. S4 in ESI†). This confirms that the selection of the task-specific IL also plays a crucial role in the NH₃ capture by ENIL in fixed-bed operation.

Fig. 6 Breakthrough curves of NH₃ sorption on ENIL–R₄ prepared with (A) EtOHmimBF₄ (B) CholineNTf₂ and (C) (EtOH)₃MeNMeSO₄ (2000 ppmv inlet NH₃ concentration, 20 mL min⁻¹, bed length 5 cm).

Increasing the temperature from 30 to 60 °C resulted in lower bed capacities, as can be seen from the shift of the breakthrough curves to shorter times and the parameters summarized in Table 4. It should be remarked that the NH₃ uptakes achieved with ENILs prepared with EtOHmimBF₄ and (EtOH)₃MeNMeSO₄ were less affected by temperature than in the case of C_cap (Fig. 4). The low influence of temperature may be ascribed to the low enthalpy associated to the physical absorption of NH₃ (ca. −5 kJ mol⁻¹ for IL EtOHmimBF₄,²²) in comparison with the physical adsorption of NH₃ on AC (ca. −25 kJ mol⁻¹,⁶³). Therefore, the bed efficiency (q_s,V in mg cm⁻³) is less affected by temperature in the case of these ENILs (∼15% variation in 30–60 °C temperature range) than for C_cap (∼40% variation in 30–60 °C temperature range) and more flexibility can be achieved for operation at different temperatures. This adverse temperature effect on NH₃ adsorption has been previously observed in commercial carbonaceous adsorbents: BAHIACARBON AC (∼50% variation in 40–80 °C temperature range)⁶⁴ and MERCK AC (∼50% variation in 25–55 °C temperature range).⁶³ The ENIL prepared with CholineNTf₂ is barely affected by the temperature in the range considered. This behavior is related to the fact that CholineNTf₂ melts at 30 °C ⁶⁹ and the solid state polymorphism can affect the capture experiments at surrounding temperatures.

Fig. 7 compares the NH₃ sorption capacity of ENILs in terms of the amount retained per g of IL (q_s,IL; mg g_IL⁻¹) and the amount retained per bed volume unit (q_s,V; mg cm⁻³). As can be seen, the relationship between these two parameters is almost linear, indicating that the absorption capacity of neat ILs can be considered as the main reference parameter for selecting ILs to prepare effective ENIL sorbents for a specific gas treatment. This linear relationship is also nearly independent of the IL used, since the three ones here considered have similar density (Table 2).

Fig. 7 NH₃ sorption capacity mg g_IL⁻¹versus sorption capacity in mg cm⁻³ for different ENIL–R₄ (30–60 °C, 2000 ppmv inlet NH₃ concentration, 20 mL min⁻¹).

Fig. 8 depicts the NH₃ sorption isotherms (q_s, mg g_ENIL⁻¹) at 30 °C for ENILs–R₄ prepared with EtOHmimBF₄, (EtOH)₃MeNMeSO₄ and CholineNTf₂, as calculated from the breakthrough curves obtained at different NH₃ inlet concentration (Fig. 5S of ESI†). The amount of ammonia retained generally increases with increasing inlet NH₃ partial pressures, in concordance with the absorption isotherms obtained for neat ILs.^22,23 The results show again the significantly higher NH₃ sorption capacity of the ENIL–R₄ prepared with EtOHmimBF₄.

Fig. 8 Experimental sorption isotherm obtained from breakthrough curves using different inlet NH₃ concentration and ENIL–R₄ prepared with (■) EtOHmimBF₄, (□) CholineNTf₂ and (●) (EtOH)₃MeNMeSO₄ (20 mL min⁻¹, and 30 °C).

The length of mass transfer zone (h_MTZ) is an additional parameter of interest to evaluate the performance of NH₃ capture by ENILs. The values obtained ENIL (see Table 4) indicate that, interestingly, the incorporation of IL into the C_cap has only minor effect on the slope of breakthrough curves and h_MTZ values at 30 °C, showing that the good mass transfer conditions are maintained. Increasing the sorption temperature evidences additional advantages of ENILs, since h_MTZ decreased for all ENIL sorbents prepared. This effect is assignable to the higher NH₃ diffusivity in ILs at higher temperatures.^22,23 On the contrary, h_MTZ increases with temperature for C_cap (see Table 4), which is the commonly reported effect of temperature on the adsorption of gases by porous carbons.⁶⁶ In fact, comparison to available data of NH₃ fixed bed adsorption using commercial AC (BAHIACARBON, [NH₃]₀ = 1200 ppmv, 1 atm)⁶⁴ leads to similar conclusions, with estimated h_MTZ values of 1.8 and 2.8 cm at 40 and 80 °C, respectively.

3.3. Process reversibility

The last step in this study was the evaluation of the regeneration of the exhausted ENILs, since it is an important issue regarding the potential application to NH₃ capture. Fig. 9 shows the NH₃ capture capacities (q_{s, ENIL}, mg g_ENIL⁻¹) of the ENILs prepared with the three ILs (EtOHmimBF₄, CholineNTf₂ and (EtOH)₃MeNMeSO₄), calculated from breakthrough curves obtained in 3 successive capture-regeneration cycles. As can be seen, the sorption capacity of the ENILs remains nearly unaltered after the regeneration, thus showing the reusability of the bed. The effective regeneration is another main advantage of ENILs to conventional adsorbents, since the desorption of NH₃ from porous solid is a main concern.⁶³

Fig. 9 NH₃ capture by ENIL–R₄ prepared with the selected ILs in three consecutive capture/regeneration cycles (capture: 30 °C, 1 atm, 2000 ppmv, 20 mL min⁻¹; regeneration 150–200 °C, nitrogen atmosphere, 1 atm, 1 mL min⁻¹).

4. Conclusions

Encapsulated ionic liquids (ENILs) were specifically designed for NH₃ capture from gas effluent, with further recovery of this compound, and their performance was evaluated by means of fixed bed experiments. The confinement of task-specific ILs into the carbon submicrocapsules resulted in ENILs with high IL content (up to 85% w/w), high NH₃ uptake capacity and high thermal stability (this last very close to that of neat IL). The incorporation of the ILs into the carbon submicrocapsules hollow space increased the efficiency of the bed (capacity per bed volume unit), respect to the benchmark carbonaceous adsorbent (C_cap or commercial activated carbons). The large surface contact area of submicrocapsule particles promotes high mass transfer rates for NH₃ sorption in ENILs. As a result, the lengths of the mass transfer zone obtained in fixed bed experiments with ENILs are similar or even shorter than those obtained with common adsorbents, indicating an efficient use of ENILs in the fixed bed. The application of ENILs in fixed bed operations may be particularly advantageous at higher temperatures, from both thermodynamic and kinetic points of view, respect to carbonaceous adsorbents. In addition, ENILs were successfully regenerated under mild conditions and used in successive NH₃ capture-desorption cycles, without loss of operation efficiency. This a clear advantage respect to common gas adsorbents, which in general have problems of regeneration at mild conditions. The current results present ENILs as new separating agents with promising characteristics for future practical applications.

Acknowledgements

The authors are grateful to Comunidad de Madrid (project S2013-MAE-2800), Ministerio de Economia y Competitividad of Spain (Project CTQ2014-52288-R) and European Union (EXIL COST Action CM1206) for financial support.

References

1. T. H. Misselbrook, B. F. Pain and D. M. Headon, Estimates of ammonia emission from dairy cow collecting yards, J. Agric. Eng. Res., 1998, 71, 127–135.
2. T. H. Misselbrook, T. J. Van der Weerden, B. F. Pain, S. C. Jarvis, B. J. Chambers and K. A. Smith, Ammonia emission factors for UK agriculture, Atmos. Environ., 2000, 34, 871–880.
3. M. A. Sutton and D. Fowler, Introduction: fluxes and impacts of atmospheric ammonia on national, landscape and farm scales, Environ. Pollut., 2002, 119, 7–8.
4. T. H. Misselbrook, J. Webb and S. L. Gilhespy, Ammonia emissions from outdoor concrete yards used by livestock – quantification and mitigation, Atmos. Environ., 2006, 40, 6752–6763.
5. M. A. Sutton, J. W. Erisman, F. Dentener and D. Moeller, Ammonia in the environment: From ancient times to the present, Environ. Pollut., 2008, 156, 583–604.
6. J. Arogo, P. W. Westerman and A. J. Heber, A review of ammonia emissions from confined swine feeding operations, Trans. ASAE, 2003, 46, 805–817.
7. W. B. Faulkner and B. W. Shaw, Review of ammonia emission factors for United States animal agriculture, Atmos. Environ., 2008, 42, 6567–6574.
8. P. M. Ndegwa, A. N. Hristov, J. Arogo and R. E. Sheffield, A review of ammonia emission mitigation techniques for concentrated animal feeding operations, Biosyst. Eng., 2008, 100, 453–469.
9. D. R. Macfarlane, N. Tachikawa, M. Forsyth, J. M. Pringle, P. C. Howlett, G. D. Elliott, J. H. Davis, M. Watanabe, P. Simon and C. A. Angell, Energy applications of ionic liquids, Energy Environ. Sci., 2014, 7, 232–250.
10. C. Hardacre and V. Parvulescu, Catalysis in Ionic Liquids – From Catalyst Synthesis to Application, The Royal Society of Chemistry, 2014.
11. Z. Lei, C. Dai and B. Chen, Gas solubility in ionic liquids, Chem. Rev., 2014, 114, 1289–1326.
12. M. Hasib-ur-Rahman, M. Siaj and F. Larachi, Ionic liquids for CO₂ capture-Development and progress, Chem. Eng. Process., 2010, 49, 313–322.
13. M. Gonzalez-Miquel, J. Palomar and F. Rodriguez, Selection of Ionic Liquids for Enhancing the Gas Solubility of Volatile Organic Compounds, J. Phys. Chem. B, 2013, 117, 296–306.
14. A. Yokozeki and M. B. Shiflett, Ammonia solubilities in room-temperature ionic liquids, Ind. Eng. Chem. Res., 2007, 46, 1605–1610.
15. A. Yokozeki and M. B. Shiflett, Water Solubility in Ionic Liquids and Application to Absorption Cycles, Ind. Eng. Chem. Res., 2010, 49(19), 9496–9503.
16. S. Q. Liang, W. Chen, K. Cheng, Y. Guo and X. Gui. The latent application of ionic liquids in absorption refrigeration, Applications of Ionic Liquids in Science and Technology, ed. P. S. Handy, 2011.
17. F. Llovell, R. M. Marcos, N. MacDowell and L. F. Vega, Modeling the Absorption of Weak Electrolytes and Acid Gases with Ionic Liquids Using the Soft-SAFT Approach, J. Phys. Chem. B, 2012, 116, 7709–7718.
18. L. Chen, D. Huang, Y. Zhang, T. Dong, C. Zhou, S. Ren, Y. Chi and G. Chen, Ultrasensitive gaseous NH₃ sensor based on ionic liquid-mediated signal-on electrochemiluminescence, Analyst, 2012, 137, 3514–3519.
19. M. Khamooshi, K. Parham and U. Atikol, Overview of Ionic Liquids Used as Working Fluids in Absorption Cycles, Adv. Mech. Eng., 2013, 1–7.
20. M. Lorenzo, M. Vilas, P. Verdía, M. Villanueva, J. Salgado and E. Tojo, Long-term thermal stabilities of ammonium ionic liquids designed as potential absorbents of ammonia, RSC Adv., 2015, 5, 41278–41284.
21. Y. Shichao, Z. Jie, L. Zhenhua, B. Jie, Y. Yunfeng and P. Qin, Modeling and experimental studies of ammonia absorption in a spray tower, Korean J. Chem. Eng., 2016, 33, 63–72.
22. J. Palomar, M. Gonzalez-Miquel, J. Bedia, F. Rodriguez and J. J. Rodriguez, Task-specific ionic liquids for efficient ammonia absorption, Sep. Purif. Technol., 2011, 82, 43–52.
23. J. Bedia, J. Palomar, M. Gonzalez-Miquel, J. Bedia, F. Rodriguez and J. J. Rodriguez, Screening ionic liquids as suitable ammonia absorbents on the basis of thermodynamic and kinetic analysis, Sep. Purif. Technol., 2012, 95, 188–195.
24. E. Ruiz, V. R. Ferro, J. de Riva, D. Moreno and J. Palomar, Evaluation of ionic liquids as absorbents for ammonia absorption refrigeration cycles using COSMO-based process simulations, Appl. Energy, 2014, 123, 281–291.
25. Y. Chen, W. Han and H. Jin, Proposal and analysis of a novel heat-driven absorption–compression refrigeration system at low temperatures, Appl. Energy, 2015, 75, 560–565.
26. S. Popp, A. Bösmann, R. Wölfe and P. Wasserscheid, Screening of Ionic Liquid/H₂O Working Pairs for Application in Low Temperature Driven Sorption Heat Pump Systems, ACS Sustainable Chem. Eng., 2015, 3(4), 750–757.
27. Q. Gan, D. Rooney, M. Xue, G. Thompson and Y. Zou, An experimental study of gas transport and separation properties of ionic liquids supported on nanofiltration membranes, J. Membr. Sci., 2006, 280, 948–956.
28. P. Sun and D. W. Armstrong, Ionic liquids in analytical chemistry, Anal. Chim. Acta, 2010, 661, 1–16.
29. J. de Riva, V. R. Ferro, L. Del Olmo, E. Ruiz, R. Lopez and J. Palomar, Statistical refinement and fitting of experimental viscosity-to-temperature data in ionic liquids, Ind. Eng. Chem. Res., 2014, 53, 10475–10484.
30. A. Riisager, R. Fehrmann, M. Haumann and P. Wasserscheid, Supported Ionic Liquid Phase (SILP) catalysis: an innovative concept for homogeneous catalysis in continuous fixed-bed reactors, Eur. J. Inorg. Chem., 2006, 4, 695–706.
31. L. Vidal, M. Riekkola and A. Canals, Ionic liquid-modified materials for solid-phase extraction and separation: a review, Anal. Chim. Acta, 2012, 715, 19–41.
32. H. Li, P. S. Bhadury, B. A. Song and S. Yang, Immobilized functional ionic liquids: efficient, green, and reusable catalysts, RSC Adv., 2012, 2, 12525–12551.
33. T. Selvam, A. Machoke and W. Schwieger, Supported ionic liquids on non-porous and porous inorganic materials—a topical review, Appl. Catal., A, 2012, 445–446, 92–101.
34. M. J. Schneider, M. Haumann and P. Wasserscheid, Asymmetric hydrogenation of methyl pyruvate in the continuous gas phase using Supported Ionic Liquid Phase (SILP) catalysis, J. Mol. Catal. A: Chem., 2013, 376, 103–110.
35. M. J. Schneider, M. Lijewski, R. Woelfel, M. Haumann and P. Wasserscheid, Continuous Gas-Phase Hydroaminomethylation using Supported Ionic Liquid Phase Catalysts, Angew. Chem., Int. Ed., 2013, 52, 6996–6999.
36. F. T. U. Kohler, D. Roth, E. Kuhlmann, P. Wasserscheid and M. Haumann, Continuous gas-phase desulfurisation using supported ionic liquid phase (SILP) materials, Green Chem., 2010, 12, 979–984.
37. H. Kolding, R. Fehrmann and A. Riisager, CO₂ Capture technologies: Current status and new directions using supported ionic liquid phase (SILP) absorbers, Sci. China: Chem., 2012, 55, 1648–1656.
38. L. A. Banu, D. Wang and R. E. Baltus, Effect of Ionic Liquid Confinement on Gas Separation Characteristics, Energy Fuels, 2013, 27, 4161–4166.
39. H. Klefer, D. Roth, I. Eckle, C. Schrage, B. Bohringer, M. Haumann and P. Wasserscheid, Continuous Purification of Gases Using SILP Materials, Chem. Ing. Tech., 2014, 85, 92–96.
40. N. A. Khan, Z. Hasan and S. H. Jhung, Ionic Liquids Supported on Metal-Organic Frameworks: Remarkable Adsorbents for Adsorptive Desulfurization, Chem.–Eur. J., 2014, 20, 376–380.
41. S. Y. Chen, C. C. Han, C. H. Tsai, J. Huang and Y. W. Chen-Yang, Effect of morphological properties of ionic liquid-templated mesoporous anatase TiO₂ on performance of PEMFC with Nafion/TiO₂ composite membrane at elevated temperature and low relative humidity, J. Power Sources, 2007, 171, 363–372.
42. M. Zhang, et al., One-pot synthesis of ordered mesoporous silica encapsulated polyoxometalate-based ionic liquids induced efficient desulfurization of organosulfur in fuel, RSC Adv., 2015, 5, 76048–76056.
43. I. H. Arellano, J. Huang and P. Pendleton, High capacity CO₂ sorbents based on zinc-functionalized ionic liquid confined in morphologically diverse porous matrices, RSC Adv., 2015, 5, 65074–65083.
44. V. Hiremath, A. H. Jadhav, H. Lee, S. Kwon and J. G. Seo, Highly reversible CO₂ capture using amino acid functionalized ionic liquids immobilized on mesoporous silica, Chem. Eng. J., 2016, 287, 602–617.
45. A. Erto, A. Silvestre-Albero, J. Silvestre-Albero, F. Rodriguez-Reinoso, M. Balsamo, A. Lancia and F. Montagnaro, Carbon-supported ionic liquids as innovative adsorbents for CO₂ separation from synthetic flue-gas, J. Colloid Interface Sci., 2015, 448, 41–50.
46. J. Lemus, J. Palomar, M. A. Gilarranz and J. J. Rodriguez, Characterization of Supported Ionic Liquid Phase (SILP) materials prepared from different supports, Adsorption, 2011, 17, 561–571.
47. F. Shi, Q. H. Zhang, D. M. Li and Y. Q. Deng, Silica-gel-confined ionic liquids: a new attempt for the development of supported nanoliquid catalysis, Chem.–Eur. J., 2005, 11, 5279–5288.
48. J. Zhang, Q. Zhang, X. Li, S. Liu, Y. Ma, F. Shi and Y. Deng, Nanocomposites of ionic liquids confined in mesoporous silica gels: preparation, characterization and performance, Phys. Chem. Chem. Phys., 2010, 12, 1971–1981.
49. L. C. Bradley and M. Gupta, Encapsulation of Ionic Liquids within Polymer Shells via Vapor Phase Deposition, Langmuir, 2012, 28, 10276–10280.
50. M. Buaki-Sogo, H. Garcia and C. Aprile, Imidazolium-based silica microreactors for the efficient conversion of carbon dioxide, Catal. Sci. Technol., 2015, 5, 1222–1230.
51. E. Weiss, D. Gertopski, M. K. Gupta and R. Abu-Reziq, Encapsulation of ionic liquid BMIm·PF6 within polyurea microspheres, React. Funct. Polym., 2015, 96, 32–38.
52. B. Yang, F. Zhou, S. Liu, P. Wang, A. S. Alshammaric and Y. Deng, Interaction between CO₂ and ionic liquids confined in the nanopores of SAPO-11, RSC Adv., 2015, 5, 48908–48915.
53. J. Palomar, J. Lemus, N. Alonso-Morales, J. Bedia, M. A. Gilarranz and J. J. Rodriguez, Encapsulated ionic liquids (ENILs): from continuous to discrete liquid phase, Chem. Commun., 2012, 48, 10046–10048.
54. S. B. Yoon, K. Sohn, J. Y. Kim, C. Shin, J. Yu and T. Hyeon, Fabrication of Carbon Capsules with Hollow Macroporous Core/Mesoporous Shell Structures, Adv. Mater., 2002, 14, 19–21.
55. P. Valle-Vigon, M. Sevilla and A. B. Fuertes, Synthesis of Uniform Mesoporous Carbon Capsules by Carbonization of Organosilica Nanospheres, Chem. Mater., 2010, 22, 2526–2533.
56. N. Alonso-Morales, M. A. Gilarranz, J. Palomar, J. Lemus, F. Heras and J. J. Rodriguez, Preparation of hollow submicrocapsules with a mesoporous carbon shell, Carbon, 2013, 59, 430–438.
57. J. Palomar, J. Lemus, M. A. Gilarranz and J. J. Rodriguez, Adsorption of ionic liquids from aqueous effluents by activated carbon, Carbon, 2009, 47, 1846–1856.
58. J. Lemus, J. Palomar, F. Heras, M. A. Gilarranz and J. J. Rodriguez, Developing criteria for the recovery of ionic liquids from aqueous phase by adsorption with activated carbon, Sep. Purif. Technol., 2012, 97, 11–19.
59. J. Lemus, J. Palomar, M. A. Gilarranz and J. J. Rodriguez, On the Kinetics of Ionic Liquid Adsorption onto Activated Carbons from Aqueous Solution, Ind. Eng. Chem. Res., 2013, 52, 2969–2976.
60. F. T. U. Kohler, S. Popp, H. Klefer, I. Eckle, C. Schrage, B. Böhringer, D. Roth, M. Haumann and P. Wasserscheid, Supported ionic liquid phase (SILP) materials for removal of hazardous gas compounds-efficient and irreversible NH₃ adsorption, Green Chem., 2014, 16, 3560–3568.
61. K. Sing, Adsorption methods for the characterization of porous materials, Adv. Colloid Interface Sci., 1998, 76, 3–11.
62. M. P. Singh, R. K. Singh and S. Chandra, Ionic liquids confined in porous matrices: physicochemical properties and applications, Prog. Mater. Sci., 2014, 64, 73–120.
63. J. Helminen, J. Helenius, E. Paatero and I. Turunen, Comparison of sorbents and isotherm models for NH₃-gas separation by adsorption, AIChE J., 2000, 46(8), 1541–1555.
64. C. C. Rodrigues, D. Moraes Jr, S. W. da Nobrega and M. G. Barboza, Ammonia adsorption in a fixed bed of activated carbon, Bioresour. Technol., 2007, 98, 886–891.
65. J. F. Vivo-Vilches, E. Bailón-García, A. F. Pérez-Cadenas, F. Carrasco-Marín and F. J. Maldonado-Hódar, Tailoring activated carbons for the development of specific adsorbents of gasoline vapors, J. Hazard. Mater., 2013, 263, 533–540.
66. J. Lemus, M. Martin-Martinez, J. Palomar, L. Gomez-Sainero, M. A. Gilarranz and J. J. Rodriguez, Removal of chlorinated organic volatile compounds by gas phase adsorption with activated carbon, Chem. Eng. J., 2012, 211–212, 246–254.
67. C. H. Huang, H. S. Li and C. H. Chen, Effect of surface acidic oxides of activated carbon on adsorption of ammonia, J. Hazard. Mater., 2008, 150, 523–527.
68. M. Goncales, L. Sanchez-Garcia, E. O. Jardim, J. Silvestre-Albero and F. Rodriguez-Reinoso, Ammonia removal using activated carbons: effect of the surface chemistry in dry and moist conditions, Environ. Sci. Technol., 2011, 45, 10605–10610.
69. P. Nockemann, K. Binnemans, B. Thijs, T. N. Parac-Vogt, K. Merz, A. V. Mudring, P. C. Menon, R. N. Rajesh, G. Cordoyiannis, J. Thoen, J. Leys and C. Glorieux, Temperature-driven mixing–demixing behavior of binary mixtures of the ionic liquid choline, bis(trifluoromethylsulfonyl)imide and water, J. Phys. Chem. B, 2009, 113, 1429–1437.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra11685j

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