CO2 Capture by Novel Supported Ionic Liquid Phase Systems

Article pubs.acs.org/JPCC

CO2 Capture by Novel Supported Ionic Liquid Phase Systems Consisting of Silica Nanoparticles Encapsulating AmineFunctionalized Ionic Liquids George Em. Romanos,*,† Peter S. Schulz,‡ Matthias Bahlmann,‡ Peter Wasserscheid,*,‡ Andreas Sapalidis,† Fotios K. Katsaros,† Chrysoula P. Athanasekou,† Konstantinos Beltsios,§ and N. K. Kanellopoulos† †

Department of Physical Chemistry, Institute for Advanced Materials, Physicochemical Processes, Nanotechnology and Microsystems (IAMPPNM), NCSR “Demokritos”, 153 10 Aghia Paraskevi Attikis, Athens, Greece ‡ Lehrstuhl für Chemische Reaktionstechnik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany § Department of Materials Science and Engineering, University of Ioannina, Ioannina 45110, Greece S Supporting Information *

ABSTRACT: We report novel supported ionic liquid (IL) phase systems, described as “inverse” SILPs, consisting of micron size IL droplets within an envelope of silica nanoparticles. These novel IL-in-air powders, produced by an easily scalable phase inversion process, are stable up to 60 °C and 30 bar and are proposed as a means to confront the major drawbacks of conventional SILPs for gas separation. SILPs are usually formed by filling the channels of nanoporous materials with the IL phase. In case the core space of the pores remains open, such conventional SILPs exhibit lack of gas absorption specificity, while complete pore filling leads to diffusivity that is very low compared to that for corresponding bulk ILs; the latter drop is largely due to the high tortuosity of the pore network of the support. The inverse SILPs prepared in this work exhibited promising CO2/N2 separation performance that had reached the value of 20 at absorption equilibrium and enhanced CO2 absorption capacity of 1.5−3 mmol g−1 at 1 bar and 40 °C. Moreover, the CO2 absorption kinetics were very fast compared to conventional SILP systems and to simultaneous N2 absorption; the CO2/N2 selectivity at the short times of the transient stage of absorption had reached values in excess of 200.

1. INTRODUCTION

through the core of the open pores. Hydroformylation of olefins5 and allylic alcohols,6 carbonylation of methanol,7 enzyme reactions,8 isopropylation of toluene,9 low temperature water gas shift reaction,10 and dimerization reactions of olefins,11 have already being successfully performed in supported ionic liquid phase catalysts. In contrast, there is a shortage of reports pertaining to the successful development of supported IL absorbents for gas separation applications,12,13 and most of them have been focused on the development of facilitated transport membranes.14 The difference here is that the existence of open pores, especially when these are of the nanopore variety, may be destructive for the separation efficiency of the SILP. Nanopores tend to accumulate large amounts of gaseous molecules existing in a stream with almost no specificity. As a consequence, the contribution of a very thin film of IL to the sorption separation performance of the entire SILP is

Imbibition and stabilization of ILs into the channels of various porous substrates and membranes is a topic of high interest for catalytic1,2 and gas separation processes.3,4 The morphology of the IL-modified pore structure in the supported ionic liquid phase materials and membranes (SILPs and SILMs) should vary markedly in accordance with the intended application and more importantly, must be tailor-made for achieving the required type of activity (either catalytic or separation type). The use of porous materials and membranes as supports for ILs, especially when the pore size is at the nanoscale level, might be thought as appropriate in view of the high surface area available for the attachment of ILs and the stability of the IL phase confined within the nanopores. Exploitation of the high surface area of the substrate is very attractive when for example the target is a catalytic reaction with the ILs being both the reaction media for the dispersion of the catalyst and the absorbent for the dissolution of the gaseous reactants. It is evident that in such a case, a very thin film of the catalystbearing IL phase must be formed onto the pore walls in order that the gas reactants have accessibility to the entire IL surface © XXXX American Chemical Society

Received: June 25, 2014 Revised: September 8, 2014

A

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significantly. However, there are concerns as regards the stability of the very thin IL film under repeated cycles of pressurization/depressurization and the capability to effectively regenerate the SILP during depressurization. Morphology C is the most appropriate for membrane technology applications, where regeneration steps are not necessary, but still the productivity (recovery) of the selectively permeating gas is moderate due to the need to avoid increased transmembrane pressures. Using nanoporous substrates with a pore size comparable to that of the IL’s ion pair, it is possible to achieve the formation of monomolecular layers of ILs, where the cations are oriented toward the pore walls due to Coulombic interactions with the usually negatively charged silica (Figure 1D). In such morphology, the anions, which correspond to the IL portion that usually interacts strongly with the gaseous molecules29 (i.e., CO2, SO2, H2S), are oriented toward the core of the pore and generate straight diffusion paths for the preferably absorbed gas. Diffusion proceeds via a hoping mechanism between the active sites (anions).30 Difficulties still remain in defining the most appropriate combination of solid substrate and IL, especially as regards the optimum relation between the nanopore shape and size and the IL ion pair shape and size. In order to circumvent the latter problem, the present authors have already developed alternative synthetic procedures for the formation of SILPs with the pore morphology D (Figure 1), where the cation of the IL is functionalized with an alkoxysilane and is chemically attached to the pore walls via condensation reactions with the silanol groups of the surface.31,20 Even with this kind of development the performance of the SILP was not quite satisfactory. Despite the significant enhancement of the diffusivity, the space available for gas adsorption is rather limited, and the material exhibited high separation performance and absorption rate but rather moderate absorption capacity. On the basis of our previous experience30,32 and having as the main target to confront the aforementioned limitations related to the development and performance of SILPs for gas separations, we present herein a very simple but highly controllable process for the synthesis of SILPs using low-cost, nonporous pyrogenic silica nanoparticles of primary diameter ∼20−30 nm with high surface area (in excess of 200 m2/g). We refer to this family of SILPs as “inverse” SILPs or dry ILs, in the sense that contrary to the usual trend (e.g., the porous or nonporous substrate is covered with the IL phase), in our case the IL becomes encapsulated into the solid matrix forming an IL-in-air powder (Figure 2) that can easily be handled and applied for gas separation in fixed bed processes. The concept is simple and inspired by the pioneer work of Binks and

negligible. For gas separation applications it seems that the preferable SILP morphology is the one with the pore structure completely filled with the IL phase. In this way, the inherent gas separation capacity of the IL is fully exploited; yet new problems, largely related to the high viscosity of the ILs and the concomitant low diffusivity of the gases into their bulk, arise.15−18 To add to this, nanoporous materials usually exhibit a very complex pore structure and except for the case of advanced, but rather expensive ordered substrates such as, carbon nanotubes,19 mesoporous ordered silicas (MCM, SBA),20 aluminophosphate zeolites (AlPO4),21 and zeolite imidazolate frameworks (ZIF),22,23 the high tortuosity is a drawback that cannot be avoided and has a negative impact on the rate of gas absorption. Phase changes of ILs confined within nanopores24−26 are also a source of concern; ordering and crystallization render the IL phase much more viscous and almost impermeable to the gases. As a consequence, conventional SILPs may exhibit a gas diffusivity lower than that of the corresponding bulk IL phases. Several strategies have been developed for overcoming the aforementioned problems. Especially the control of the depth of the IL imbibition into the pore length or the tendency to exploit orientation preference of the cation of IL toward the pore wall have already showed promising performance regarding the gas diffusivity in SILPs and the permeability properties of SILMs.24,27 The development of such SILPs with controlled depth and orientation of the IL phase necessitates costly delicate procedures when combined with the use of ordered nanoporous substrates. The difficulty here is that dissolution of the IL in a solvent (e.g., MeOH), which is the common way to prepare SILP catalysts, must be avoided for SILP absorbers since in that case the most probable morphology is that of pore walls covered with a very thin IL layer while the bulk of the pores can host, nonselectively, large amounts of molecules abstracted from the gas stream (Figure 1A).

Figure 1. Different SILP morphologies obtained with different methods of IL imbibition on substrates of various pore size.

On the other hand, utilization of the pristine IL phase requires involved techniques; for example, the contact of the porous substrate with the IL must occur under vacuum and the IL imbibition into the pores must be assisted with the application of pressure. In the latter case, one needs a thorough experimental and theoretical study of the imbibition kinetics of the ILs28 in order to define the most appropriate surface chemistry of the substrate and the required pressure and duration of application in order to achieve the desired depth of imbibition. The vacuum/pressure-assisted imbibition of bulk ILs into the pores can lead to the morphologies depicted in Figure 1, panels B and C. In the first case (B) the IL fills the entire space of the pores and such a conformation leads to extremely slow diffusion of the preferably absorbed gas. In the second case (C) both diffusivity and selectivity are retained at high levels and, moreover, the absorption capacity increases

Figure 2. “Inverse” SILP morphology. B

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Murakami33 who achieved the phase inversion from silica particle stabilized air-in-water systems (foams) to water in air powders and vice versa either by variation of the silica-particle hydrophobicity [“transitional (or gradual) phase inversion”] or by variation of the air/water ratio at fixed particle wettability [“catastrophic (or discontinuous) phase inversion”]. Similar phenomena were observed for silica particles that have a size of a few nanometers and can be strongly attached to oil−water interfaces. In the latter case, by changing the oil/water ratio, a discontinuous phase inversion occurs that converts particle stabilized emulsion from oil-in-water to water-in-oil and vice versa. Shirato and Satoh34 were the first to apply the concept in discussion to the case of ionic liquids. Here, we have utilized both hydrophilic and slightly hydrophobic silica (50% silanol groups compared to hydrophilic which possesses 2 SiOH/ nm2). We have kept the silica/IL mass ratio constant at 1.5 and proceeded with a discontinuous phase inversion by progressively changing the solvent/IL mass ratio to zero where the polar solvent and IL act as the water and oil phase, respectively. The development procedure encompasses efficient mixing of the silica nanoparticles with the IL phase in appropriate solvent (ethanol), followed by solvent evaporation to obtain the final product. In such a SILP configuration (Figure 2), accessibility of the CO2 molecules to the silica-encapsulated tiny IL droplets of the micron size is achieved through the voids between the aggregated silica nanoparticles surrounding the IL phase. The size of these voids is sufficiently large (their maximum width will be approximately the one-third of the silica particle diameter (i.e. 6.7−10 nm) to avoid any significant gas diffusion hindering. Moreover, by achieving the formation of a monolayer of silica nanoparticles around the IL droplets, other drawbacks related to the high tortuosity of the pore structure are diminished (Figure 2). The major attractive feature is that the IL phase is highly dispersed, and the accessibility of the gas to the IL is achieved through a very high surface area. The prepared “inverse” SILPs exhibited very promising CO2 absorption capacity and fast kinetics, the later attributed to the high dispersion of the IL phase which was stabilized between the silica particles. On the other hand, the molar fraction of absorbed CO2 at 1 bar and 40 °C, calculated on the IL basis was above 0.35 and in one case (N,N,N-trimethyl-Npropylammonium prolinate) it reached the value of 0.63. Table 1 presents the CO2 solubility of the aminofunctionalized IL examined in this work. It should be noted that the 0.35−0.63 CO2 molar fraction absorbed at 40 °C and 1 bar represents one of the highest CO2 absorption efficiencies reported so far in the literature.35,36 A comparison between the CO2 absorption capacity of several ILs can be found in Table S1 of the Supporting Information.

2. EXPERIMENTAL SECTION 2.1. Synthesis of the Ionic Liquids and Inverse SILPs. Amine-functionalized ionic liquids were synthesized and tested as compounds in inverse SILP-systems applicable as particulate means for CO2 absorption. The procedure was as follows. In a 500 mL three-necked flask, 1 equiv of a tertiary amine, dissolved in water, is mixed with 1.2 equiv of a bromoalkane under rigorous stirring. The solution is refluxed for at least 12 h (see Scheme 1 for a general reaction scheme). Scheme 1. Alkylation of Tertiary Amines with Bromoalkanes

Excess alkylation agent is extracted twice with dichloromethane, and the solvent is removed in vacuum. A 0.05 M solution of the product (bromide ionic liquid) is transformed to a hydroxide ionic liquid by a strong basic ion-exchange resin (Dowex 1 × 8) (see Scheme 2). Scheme 2. Ion Exchange of the Bromide Group with the Hydroxide Group

By addition of one equivalent of amino acid per hydroxide group (determined by titration) and subsequent solvent evaporation, the product is obtained as desired. All synthesized and tested ionic liquids are shown in Figure 3. For the preparation of the inverse SILPs, the ionic liquid is mixed 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 (Figure 4). The as-produced “inverse” SILPs are described in Table 2. 2.2. Experimental Setup for the Absorption Measurements. 2.2.1. Fixed Bed Reactor. The CO2 capture performance has been tested in a fixed bed reactor with online gas chromatography analytics to measure breakthrough curves and therefore calculate the molar and gravimetric loadings. The fixed bed setup (Figure 5a) is fed with N2 and CO2 via two mass flow controllers (MFC, Bronckhorst High Tech). A mixture of 84 vol % N2 and 14 vol % CO2 is used to simulate a typical CO2 off-gas concentration for postcombustion processes. The two gases are mixed and then either cross the reactor or follow the bypass. The off-gas is analyzed by a gas chromatograph (GC, Varian CP-3800) with a thermal conductivity detector. At the beginning of each experiment, the reactor is filled with the “inverse” SILP material and fixed with two layers of glass wool. The temperatures (40 or 100 °C) can be adjusted by a Eurotherm heat controller. Valves V-4 and V-8 are closed (gas mixture passes the bypass) until the GC shows a constant CO2 concentration. Afterward the gas stream is switched to the reactor (close valves V-3 and V-7, open V-4 and V-8) to start the absorption experiment. The experiment is completed when the CO2 concentration reaches the start value.

Table 1. CO2 Solubility in the ILs Synthesized in This Work ionic liquids N,N,N-trimethyl-N-ethylammonium prolinate N,N-dimethyl-N,Ndiethanolammonium prolinate N,N-dimethyl-N,Ndiethanolammonium taurinate N,N,N-trimethyl-N-propylammonium prolinate

T (K)

P (bar)

CO2 molar fraction

313.15

0.93

0.426

313.15

0.99

0.351

313.15

0.93

0.348

313.15

0.99

0.632

C

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concentration progress (bypass, reactor, regeneration) is shown in Figure 5b. Due to its porous structure, the pyrogenic silica affects the whole amount of adsorbed CO2 during the breakthrough experiments. The amount of CO2 adsorbed by the silica is determined as a function of the pressure in the test rig, and it is found that the amount of adsorbed CO2 rises with rising pressure drop (Figure 5c). For a known pressure drop during the experiments, the adsorbed CO2 by the silica and the absorbed CO2 by the ionic liquid may then be calculated. 2.2.2. Gravimetric Test Unit. The CO2 and N2 gas absorption measurements at 40 °C and several pressures up to 1 bar were performed with a gravimetric microbalance (IGA 001, Hiden Analytical). The masses of the sample and counterweight pans, the hooks, the counterweight material, and the hang chains of the microbalance assembly were on the order of one to three hundred milligrams per item and were defined with an accuracy of ±0.1%. The materials were appropriately selected to induce a symmetrical configuration to the balance setup to minimize buoyancy effects. The microbalance had a 0.1 μg stable resolution. The amount of the “inverse” SILP usually inserted was 100−150 mg. Before each measurement, the samples were degassed at 323 K and high vacuum (10−5 mbar). The densities of the gas bulk phase were calculated using the Benedict−Webb−Rubin equation of state for N2 and CO2.37 The uncertainty for this equation is on the order of 3%. The optimum regeneration conditions between successive measurements were 50 °C, vacuum 10−5 mbar, until no mass loss. The optimum regeneration conditions were defined by following the thermal stability of the inverse SILPs via repeated cycles of CO2 absorption/desorption and regeneration under high vacuum at increasing temperatures up to 60 °C; beyond the latter temperature, the mass loss becomes substantial. 2.3. Morphological and Structural Characterization. A Jeol JSM 7401F field emission scanning electron microscope equipped with Gentle Beam mode and the new r-filter was employed to characterize the surface morphology of the developed materials. Gentle Beam technology can reduce charging and improve resolution, signal-to-noise, and beam brightness, especially at low beam voltages (down to 100 V), without gold plating. In some cases, the samples were subjected to gold plating prior to the SEM imaging. FTIR spectra were collected on a Thermo Scientific Nicolet 6700 FTIR with a N2 purging system. Spectra were acquired using a single reflection ATR (attenuated total reflection) SmartOrbit accessory equipped with a single-bounce diamond crystal (Spectral range: 10000−55 cm−1, angle of incidence: 45°). A total of 32 scans were averaged for each sample, and the resolution was 4 cm−1. The spectra were obtained against a single-beam spectrum of the clean ATR crystal and converted into absorbance units. Data were collected in the range of 4000−400 cm−1. The corresponding spectra and peak assignments are included in Figure S2 and Table S2 of the Supporting Information. LN2 porosimetry (Autosorb-1MP, Quantachrome) provide a clue with regard to the size of the formed “IL beads” and the interparticle space of the silica nanoparticles of the IL phase envelopes. 2.4. Stability under High-Pressure Conditions. The stability of the IL phase was also investigated as a function of the gas pressure. Experiments were performed by placing the “inverse” SILP samples into glass test tubes of 6 mm OD and

Figure 3. Structures of the synthesized ionic liquids.

Figure 4. Suspension of pyrogenic silica and ionic liquid in ethanol (left). Ready to-use inverse SILP-system (right).

Table 2. “Inverse” SILPs Produced in This Work and Abbreviated Codes for Their Further Reference sample no. 1 2 3 4 5 6 7 8

components HDK-T30 (hydrophilic) HDK-H20 (hydrophobic-50% SiOH) N,N-dimethyl-N,N-diethanolammonium prolinate/T30 N,N,N-trimethyl-N-ethylammonium prolinate/H20 N,N-dimethyl-N,N-diethanolammonium prolinate/H20 N,N-dimethyl-N,N-diethanolammonium taurinate/H20 N,N-dimethyl-N,N-diethanolammonium taurinate/T30 N,N,N-trimethyl-N-propylammonium prolinate/H20

sample code 1T30 2H20 3DD-Prol-T30 4TE-Prol-H20 5DD-Prol-H20 6DD-Taur-H20 7DD-Taur-T30 8TPE-Prol-H20

The regeneration is carried out at 100 °C with pure nitrogen over 16 h. After the regeneration, the gas stream is switched to the bypass, and CO2 is added to the gas stream and another experiment can be initiated. A scheme of a possible CO2 D

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Figure 5. (a) Scheme of the experimental setup which was used for the absorption measurements. (b) Characteristic CO2 breakthrough curves during an experimental sequence. (c) Amount of CO2 adsorbed on pristine silica vs the pressure drop in the fixed bed.

rather rough surface with particles of about 20−40 nm. In some cases, larger particle aggregates (∼100 nm) were apparent. On the other hand, SILPs samples (Figure 6, panels c and d) exhibited larger aggregates in the range of 0.5−1.5 μm due to the effective incorporation of ILs into silica particles. However, this trend is only limited to IL-rich areas, while the surface texture of the remaining sample part is similar to that of the raw silicas. It must be noted that due to poor electrical conductivity of the raw silicas, the samples were coated with a thin conductive film (gold) prior to their examination. On the other hand, SILPS samples, because of the enveloped aminofunctionalizedIL droplets, exhibited reduced charging effects, which in some cases enabled their observation without gold plating. 3.2. LN 2 Porosimetry. LN 2 porosimetry provided information as regards to (a) the size of the voids formed between the nanoparticles of the pristine hydrophilic (HDK-

exposes them to sequentially increasing pressures of CO2 (2.7, 4.2, 5, 6.6, 10, 20, 30, and 35 bar) in a thermostated autoclave. Depressurization was performed at a rate of 1 bar/min, and the samples were optically observed (Figure S1 of the Supporting Information). An IL-in-air powder to paste transformation was noticeable for all the “inverse” SILPs at pressures above 30 bar, independently of the IL and silica phase used for their development. This shows that the kind of IL and the hydrophilic versus hydrophobic character of the silica particles did not affect the stability of the formed “inverse” SILPs; the structure collapse of the latter is provoked by the swelling of the IL phase upon absorption of high CO2 amounts.

3. RESULTS AND DISCUSSION 3.3. SEM Analysis. The SEM images of both nonporous pyrogenic silica and inverse SILPs are presented in Figure 6. The raw silica samples (Figure 6, panels a and b) revealed a E

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Figure 6. SEM images of (a) hydrophilic silica HDK-T30, (b) hydrophobic silica HDK-H20, (c) 6DD-Taur-H20, and (d) 4TE-Prol-H20.

T30) and hydrophobic (HDK-H20) silica and (b) the size of the larger interbead cavities that are shaped between the silicaenveloped droplets of the ILs (beads) in the respective “inverse” SILP systems (Figure 7).

In this way it was possible to roughly estimate both the size of the silica nanoparticles and the size of the IL droplets in the “inverse” SILPs and unveil morphological characteristics related to the degree of agglomeration of the silica aggregates that enveloped each IL droplet. The LN2 isotherms and the respective pore size distributions derived upon application of the Barrett−Joyner−Halenda (BJH) method are presented in Figure 7, whereas characteristic pore structure properties derived from the isotherms are included in Table 3. Regarding the pristine silica samples, it is obvious that the silanization of the hydrophilic silica (HDK-T30), aiming at the conversion of the latter to a hydrophobic version (HDK-H20), leads to significant reduction of the BET surface area due to the increase of the nanoparticle size upon grafting of the silane molecules. Moreover, the packing of nanoparticles becomes looser. By applying the rule of 1/3, holding for the size of voids between hexagonally packed particles over the particle size,38 the mean diameter of hydrophilic and hydrophobic silica nanoparticles is estimated at 26.7 and 29.1 nm, respectively, which is in agreement with the specifications of the silica provider (20−30 nm Wacker Chemie AG). The LN2 porosimetry of the “inverse” SILP samples (Figure 8, panels a and c), especially the BJH derived pore size distributions (PSD) presented in Figure 8 (panels b and d),

Figure 7. Illustrative representation of the pore structure characteristics assigned to the various pore morphologies.

Table 3. Pore Structure Characteristics as Derived from the LN2 Porosimetry 1T30 2H20 3DD-Prol-T30 4TE-Prol-H20 5DD-Prol-H20 6DD-Taur-H20 7DD-Taur-T30 a

BET (m2/g)

pore volume (mL/g)

interparticle pore size (nm)a

mean pore size BJH (nm)b

mean particle size (μm)c

305 195 72 61 49 70 63

0.681 0.471 0.457 0.27 0.363 0.35 0.496

8.9 9.7 72 47 69.3 75.5 61.2

6.2 6.2 6.2 5.6 6.2 6.2 6.2

0.0267 0.029 0.216 0.141 0.21 0.227 0.184

Calculated as [4(Pore volume)/BET]. bCorresponds to the interparticle space of silica. cCalculated as (3× interparticle pore size). F

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Figure 8. (a) LN2 porosimetry of the hydrophobic silica and the respective “inverse” SILPs. (b) PSDs of hydrophobic silica and the respective “inverse” SILPs. (c) LN2 porosimetry of the hydrophilic silica and the respective “inverse” SILPs. (d) PSDs of hydrophilic silica and the respective “inverse” SILPs.

droplets is larger than 0.15 μm, in good accordance with the size estimated from the SEM analysis. 3.3. Gravimetric CO2 and N2 Absorption. 3.3.1. Transients of Absorption. The transients of CO2 absorption for all the “inverse” SILPs are presented in Figure 9 (panels a−d) and in Figure S3 of the Supporting Information; the figures are indicative of the different mechanisms of CO2 capture occurring at the surface and in the bulk of ILs which are functionalized with primary and secondary amines.32 The ordinate of the plots [(mt − m0)/(m∞ − m0)] corresponds to the time-dependent amount of absorbed gas divided by the total amount absorbed within each pressure step. It can be seen that during the first pressure step on a fresh “inverse” SILP sample (e.g., fresh IL surface), the transient stage of absorption is very fast, while it is always followed by transients that become slower as the pressure increases. In a very recent study,32 the authors employed a combination of near-ambient pressure XPS analysis and atmospheric pressure infrared spectroscopy for the system CO2/N,Ndimethyl-N,N-diethanolammonium taurinate. It was concluded that at 310 K and a CO2 pressure of 0.9 mbar, the near-surface (7−9 nm depth) composition is in a quasi-equilibrium situation with the larger fraction (60%) of the CO2-capturing taurinate anion transformed into carbamic acid. On the other hand, the IR analysis of CO2 absorption in the bulk of this IL up to 1 bar indicated only the formation of

evidenced the formation of the silica nanoparticle-enveloped IL droplets that give rise to the targeted dry IL phase. The PSDs of the pristine silica samples 2H20 and 1T30 are very broad and characterized by a major peak at around 30 Å; the latter corresponds to the interparticle space of the primary nanoparticles, while the broadness of the distribution can be attributed to the existence of larger voids between the larger entities formed via aggregation and agglomeration. Contrary to that, in the case of “inverse” SILPs, the broadness of the PSDs at the small size region (