Textural control of ionosilicas by ionic liquid templating

Abstract

Due to their unique self-assembly properties, ionic liquids (ILs) are versatile soft templates for the formation of mesoporous materials. Here, we report the use of ionic liquids as soft templates for the straightforward formation of mesoporous ionosilica phases. Ionosilicas are highly polyvalent functional materials that are constituted of ionic building blocks that are covalently immobilized within a silica hybrid matrix. Ionosilicas have attracted significant interest in the last few years due to their high potential for applications in water treatment and upgrading, separation and drug delivery. The straightforward and reproducible formation of mesoporous ionosilica phases is therefore highly desirable. In this context, we report the formation of mesoporous ionosilica phases via non-hydrolytic sol–gel procedures in the presence of ionic liquids. Ionic liquids appear as particularly versatile templates for mesoporous ionosilicas due to their high chemical similarity and affinity between ILs and silylated ionic precursors. We therefore studied the textures of the resulting ionosilica phases, after IL elimination, using nitrogen sorption, small angle X-ray scattering (SAXS) and transmission and scanning electron microscopies. All these techniques give concordant results and show that the textures of ionosilica scaffolds in terms of specific surface area, pore size, pore size distribution and connectivity can be efficiently controlled by the nature and the quantity of the ionic liquid that is used in the ionothermal sol–gel procedure.

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Introduction

Ionic liquids (ILs) have been widely investigated over the last few decades for their unique properties such as high ionic conductivity and chemical stability, low vapor pressure and low flammability. ILs are salts with a melting point below 100 °C.¹ They are known as “designer solvents” since one can tailor their properties choosing a distinct cation/anion combination that matches the requirements of a given application. ILs with specific properties have been developed for applications in various domains such as energy storage,^2,3 catalysis,^4,5 gas adsorption,⁶etc. Besides, ILs, and in particular imidazolium based ILs, show a very particular self-organization behavior.^7–10 On one side, ILs form structured phases in the liquid state via the formation of extended hydrogen bonded networks.¹¹ On the other side, IL phases display nanoscale segregation in polar and non-polar domains.¹² The more pronounced this segregation is the longer the alkyl chain of the IL is. ILs therefore display heterogeneities at the nanoscopic level that linearly scale with the length of the alkyl chain.¹² The nanoscale segregation increases with the length of the alkyl chain, resulting in liquid-crystalline (LC) behavior for long chain substituted ILs.¹³ Besides the length of the alkyl chain, the nature of the counter anion is also of prime importance for the formation of ionic liquid crystal phases. Whereas long-chain (>C12) substituted imidazolium halides form LC-phases, the corresponding bis-triflimides do not.^14,15 Altogether, ILs display a unique self-aggregation behavior at various levels, which results from their ability to form H-bonded networks, nanoscale segregation in polar/nonpolar domains or even highly organized and ordered lyotropic phases.¹⁶ Due to all these features, ILs should therefore be considered as ‘supramolecular’ solvents.¹⁷

This unique self-organizing behavior makes ILs efficient soft templates for the synthesis of mesoporous materials. Following the IUPAC nomenclature, mesoporous materials are porous compounds with a pore size between 2 and 50 nm¹⁸ that display some interesting properties such as a high specific surface area and high pore volume. These features ensure enhanced diffusivity of large molecules throughout the material, with diffusion being the dominant mass transport mechanism in mesopores.¹⁹ Due to their textural features including a high specific surface area, tunable pore size and high pore volume, mesoporous materials are potentially advantageous for applications in catalysis,²⁰ separation and energy storage.²¹

The first example of a mesoporous material synthesized in the presence of ILs goes back to the work of Dai et al. who reported the formation of a silica aerogel following a non-hydrolytic ionothermal sol–gel procedure.²² Several years later, Antonietti et al. reported the synthesis of mesoporous silica using the IL [C₄MIM]BF₄. The elimination of the confined IL from the host matrix via washing led to a mesoporous silica material displaying a worm-like architecture.²³ The authors postulated a cooperative hydrogen bonding–π–π stacking mechanism to be responsible for the generation of mesoporosity and the formation of moderately ordered phases within this material. Finally, long-chain substituted imidazolium salts were used for their lyotropic properties. These compounds have successfully been used in hydrolysis polycondensation processes following classical templating strategies.^24,25

The alkyl chain length of an IL has indeed a significant impact on the segregation in nano-domains within the IL and therefore also on the template effect in ionosilica synthesis. We consider that silica or ionosilica precursors, as polar components, have particular affinities toward the polar nano-domains of an IL, and that the non-polar domains particularly contribute to the formation of the texture of the materials. The size of the alkyl groups of an IL therefore has a direct influence on the textures of the materials formed in the presence of IL phases.^26,27

These examples highlight the templating behavior of ILs and clearly indicate that the structuration of ILs can be transcribed to solid phases. ILs are therefore not only structured but also structuring phases, with high potential in the area of materials chemistry and in particular for the elaboration of materials via bottom-up approaches.^28–32

Since several years, our special concern has been the elaboration of ionosilica phases with particular morphologies and architectures. We define ionosilicas as silica hybrid materials that are constituted of ionic building blocks.³³ Ionosilicas are highly modulable functional materials featuring a large range of surface and interface properties.^34,35 They found applications in water treatment and upgrading,^36,37 separation^38,39 and drug delivery.^40–42 Due to the high chemical similarity between ionosilicas and ILs and with the aim of studying the confinement of ILs within an ionosilica matrix, we recently focused on the formation of nanocomposites via hydrolysis–polycondensation reactions of ionosilica precursors in the presence of ILs.⁴³ For this purpose, we synthesized various ionosilica ionogels, and we could demonstrate particular confinement effects between the ionic host and guest. More specifically, we were able to show that an IL confined within an ionic host matrix forms two distinct populations: (i) the first one interacting with the pore walls via electrostatic interactions and (ii) the second one forming bulk-like IL domains.^44,45 As a particularity of these all-ionic composites, we could access free-standing and mechanically robust ionosilica ionogel monoliths containing up to 80 wt% of ILs by this approach which can be explained by the high affinity between the ionic host and the guest.

However, a systematic study of the textural properties of ionosilica scaffolds formed in the presence of ILs is missing so far. That is why we describe here how the textural properties of ionosilica scaffolds can be tuned as a function of the IL, i.e. via the amount of the used imidazolium bis-triflimide IL and the length of its cation alkyl chain. We demonstrate that ILs allow an efficient control of the texture of ionosilicas in terms of specific surface area, pore size and pore volume.

Experimental section

Chemicals

The ionosilica precursor tris(3-(trimethoxysilyl)propyl)amine (TTA) was synthesized following previously described protocols.³³ All imidazolium-based ionic liquids [C_xMIM]TFSI (x = 2, 4, 6, 8, 12 and 16) were purchased from IoLiTec. Formic acid (p.a.) was purchased from VWR. All chemicals were used as received.

Synthesis of ionosilica materials

The synthesis of various ionosilica ionogels was performed in one-pot synthesis involving the ionosilica precursor TTA, formic acid and ionic liquid. Various imidazolium TFSI ILs with variable alkyl chain lengths were used: [C_xMIM]TFSI (x = 2, 4, 6, 8, 12 and 16). The ionosilica matrices were formed via a non-hydrolytic sol–gel procedure from the tris(3-(trimethoxysilyl) propyl) amine precursor TTA in the presence of the IL. First, 3.0 g of the TTA precursor was mixed and vigorously stirred for three minutes with various molar amounts of IL in a Pyrex glass tube using a vortex apparatus. Then the polycondensation reaction was initiated by adding 2.0 mL of formic acid to the mixture. The exact quantities of the reactants in the ionosilica synthesis are given in Table 1. Gelation results from the transesterification–polycondensation reactions undergone by the trimethoxysilyl groups of the TTA precursor. Note that the protonation of the precursor occurs during the sol–gel process yielding the ionic building blocks of the ionosilica matrix. Colorless monoliths were obtained after 24 h at room temperature and kept under static conditions at 80 °C overnight. The ionosilica monoliths were obtained after the extraction of the IL with ethanol using a Soxhlet apparatus. Finally, the materials were dried at 80 °C under vacuum (0.01 mbar) for 24 h. The ionosilica monoliths were labelled as , where C_x is the length of the cation alkyl chain (x = 2, 4, 6, 8, 12, and 16) and z is the quantity of IL in mmoles.

Table 1 Used quantities for the synthesis of ionosilica ionogel materials

	TTA		[CxMIM]TFSI	HCOOH
	m (g)	n (mmol)	n (mmol)	V (mL)
	3.0	6.0	3.0	2.0
	3.0	6.0	10.0	2.0
	3.0	6.0	20.0	2.0
	3.0	6.0	41.0	2.0

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Nitrogen sorption. Isotherms were recorded at 77 K using a Micromeritics TriStar volumetric apparatus by adding doses of N₂ in a measurement cell after an outgassing treatment of the samples at 120 °C under vacuum overnight. The BET surface area of each sample was determined using the Brunauer–Emmett–Teller (BET) method⁴⁶ in the linear range of the isotherms typically at p/p₀ = 0.05–0.15. The pore size distribution was calculated using the Barret–Joyner–Halenda (BJH) method⁴⁷ from the adsorption isotherm. The total pore volume of the materials was determined at p/p₀ = 0.99.

Small angle X-ray scattering (SAXS). Experiments were performed using an in-house setup of the Laboratoire Charles Coulomb, Université de Montpellier, France. A high brightness low power X-ray tube, coupled with an aspheric multilayer optic (GeniX^3D from Xenocs), was employed. It delivers an ultralow divergent beam (0.5 mrad, λ = 0.15418 nm). Scatterless slits were used to give a clean 0.6 mm beam diameter with a flux of 35 Mphotons s⁻¹. We worked in a transmission configuration where the scattered intensity was measured using a 2D “Pilatus” 300 K pixel and detected using a Dectris detector (490 × 600 pixels) with a pixel size of 172 × 172 μm², placed at a 1.9 m distance from the sample. All intensities were corrected by transmission and the empty cell contribution (glass capillary) was subtracted.

Transmission electron microscopy (TEM). TEM measurements were performed with a FEG JEOL 2200 FS-200 KV electron microscope equipped with a CCD GATAN USC camera with 4092 × 4092 pixels and an electron gun of 200 kV. The probe diameter in STEM mode is about 1 nm with a resolution of up to 0.1 nm.

Scanning electron microscopy (SEM). Images were taken using a Hitachi S-4800 microscope, Japan. A 2 kV acceleration voltage was applied on the ground IL free samples.

Results and discussion

Nonhydrolytic sol–gel reactions of silylated precursors in the presence of ionic liquids result in the formation of silica based ionogels, where an ionic liquid is trapped within a silica matrix.²² Following this strategy, we recently reported ionosilica ionogels, a special case of all-ionic composite materials,⁴³ formed from ionosilica precursors. Although the confinement of the IL was investigated and strong interactions between the ionic host and guest could be demonstrated, the texture of the formed ionosilica scaffolds that are formed in this non-hydrolytic sol–gel approach has not been investigated so far. In this work, we therefore focused on the characteristics of the ionosilica scaffolds formed using the non-hydrolytic sol–gel approach, obtained after IL elimination by washing. We determined in particular the textural characteristics of the ionosilicas as a function of the reaction parameters, in particular of the quantity and the constitution of the used IL. We investigated the characteristics of the ionosilica materials as a function of two parameters: (1) the amount of ILs [C_xMIM]TFSI in the hydrolysis–polycondensation mixture and (2) the length of the alkyl chain on the imidazolium cation.

More specifically, the ionosilica materials were obtained from the tris-trialkoxysilylated amine precursor TTA (Fig. 1a) in the presence of various methyl-alkylimidazolium bis-triflimides (Fig. 1b) via non-hydrolytic sol–gel reactions in the presence of formic acid.⁴⁸ Ionosilica ionogel monoliths are formed at room temperature after several hours. After aging at 80 °C and IL elimination by washing, we obtained ionosilica phases that are labelled as , where z is the quantity of the IL in mmoles and C_x is the length of the cation alkyl chain (x = 2, 4, 6, 8, 12 and 16). Here, we focused particularly on the impact of two parameters on the texture of the formed ionosilicas: (i) the quantity of the IL and (ii) the length of the alkyl chain of the methyl–alkyl imidazolium cation, and we studied the textural properties of the obtained materials via nitrogen sorption, small-angle X-ray scattering (SAXS), and scanning and transmission electron microscopies (SEM and TEM).

Fig. 1 (a) Molecular structure of the ionosilica precursor tris(3-(trimethoxysilyl)propyl)amine (TTA). (b) Molecular structure of the used imidazolium-based ionic liquids [C_xMIM]TFSI.

Nitrogen sorption measurements at 77 K allowed determining the textural properties of the materials in terms of specific surface area, S_BET, pore volume, V_p, and pore size, D_p. The results are summarized in Table 2, and a histogram of the specific surface area data of the materials is shown in Fig. 2. Histograms displaying the pore sizes and pore volumes are given in the ESI† (Fig. S1 and S2).

Table 2 BET surface areas, pore volumes and pore diameters of ionosilica monoliths synthesized in the presence of [C_xMIM]TFSI ionic liquids. The sets of materials that are discussed in detail in the text are in bold

z (mmol)	S BET (m^2 g^−1)				Total pore volume Vp^a (cm^3 g^−1)				Pore diameter Dp^b (Å)
	3	10	20	41	3	10	20	41	3	10	20	41
a Determined at p/p0 0.99. b Calculated following the Harkins and Jura equation.^49
	<1	76	444	358	n.d.	0.04	0.31	0.31	n.d.	23	30	36
	<1	310	463	306	n.d.	0.17	0.35	0.30	n.d.	24	31	34
	<1	306	463	353	n.d.	0.16	0.36	0.30	n.d.	24	32	35
	<1	402	525	500	n.d.	0.22	0.52	0.50	n.d.	25	39	39
	<1	516	577	520	n.d.	0.39	0.81	0.70	n.d.	32	55	52
	356	520	600	590	0.22	0.50	0.78	0.77	27	34	52	50

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Fig. 2 Histograms of the BET surface areas of the IL-free ionosilica ionogels , synthesized in the presence of different volumes of imidazolium ILs [C_xMIM]TFSI (x = 2, 4, 6, 8, 12 and 16).

At a first glance, the formed monoliths are moderately to highly porous materials, with specific surface areas in the range of 300–600 m² g⁻¹. The materials are mostly mesoporous with pore diameters in the range of 25–55 Å, except those that are synthesized with the lowest amount of IL, i.e., the series obtained with 3 mmoles of the IL. These latter materials are non-porous with specific surface areas S_BET < 1 m² g⁻¹. It should be mentioned that all materials (z = 10, 20 and 41) display a large pore size distribution (Fig. S3–S8, ESI†).

In order to highlight the impact of the two investigated parameters, i.e., the quantity of the IL and the substitution of the alkyl chain of the methyl–alkyl imidazolium cation, on the textures and architectures of the formed ionosilica phases, we discuss more in detail two sets of materials:

(i) The ionosilicas, , synthesized in the presence of variable amounts of the IL [C₄MIM]TFSI on the one side and.

(ii) The ionosilicas, , synthesized in the presence of 20 mmoles of different methyl–alkylimidazolium bis-triflimides with different alkyl chain lengths on the other side. The nitrogen sorption isotherms of both series of materials are shown in Fig. 3a and b.

Fig. 3 (a) Nitrogen sorption isotherms of monoliths synthesized in the presence of variable amounts of [C₄MIM]TFSI. (b) Nitrogen sorption isotherms of the ionosilica monoliths synthesized in the presence of 20 mmoles of methyl-alkylimidazolium bis-triflimides [C_xMIM]TFSI with variable alkyl chain lengths (x = 2, 4, 6, 8, 12 and 16).

In the series of ionosilicas, , formed in the presence of variable amounts of [C₄MIM]TFSI, our results indicate the low porosity of the material that is synthesized in the presence of the lowest quantity of the IL (3 mmoles). For the materials and , a significant increase of the specific surface area was observed, to reach a maximum value for the material . Finally, the material c, synthesized in the presence of the highest quantity of the IL, shows a slightly lower specific surface area compared to material . This general trend can be found in all other series of ILs, synthesized in the presence of variable amounts of a given IL.

The evolution of the pore volume in the series of the materials follows an identical trend. We found increasing values up to the material . This material shows the highest pore volume in this series. The pore volume slightly decreased when the IL quantity is further increased, i.e., in the case of the material .

In contrast, the pore diameter progressively increases in this series of materials. This evolution can clearly be seen in the nitrogen sorption via the shape of the isotherms (Fig. 3a). Material displays low porosity, whereas the material gives rise to a type I isotherm, evidencing the presence of supermicropores (wide micropores of widths between 7 and 20 Å). The materials and show type IV isotherms with H2(a) type hysteresis loops, confirming the presence of narrow mesopores with diameters of 30 and 35 Å, respectively. The very steep decrease in the desorption branch at a relative pressure of ca. 0.4 is typical of cavitation-induced evaporation (the pore body empties while the pore neck remains filled) and suggests the presence of bottle-neck type mesopores.⁵⁰

Regarding the series of materials , synthesized in the presence of 20 mmoles of methyl-alkylimidazolium bis-triflimides, we observed an increase of the specific surface area, pore size and pore diameter for the materials with ethyl, butyl, hexyl, octyl and dodecyl groups (Fig. 3b). The specific surface area levels off for the material . In contrast, whereas the use of short-chain substituted ILs (C₂–C₆) results in the formation of materials with similar pore sizes, the use of long-chain substituted ILs (C₁₂/C₁₆) led to materials with significantly larger pores, as indicated by the different shape of the nitrogen sorption isotherms (Fig. 3b and Table 2 – pore diameter). This latter result indicates a particular templating behavior of long chain substituted ILs, namely of dodecyl- and hexadecyl-methylimidazolium bis-triflimide.

Our results therefore reveal a clear correlation between the nature and the amount of the IL on the textures of the formed materials. They indicate that the supramolecular aggregation of imidazolium based bis-triflimide ILs in hydrophilic and hydrophobic domains has a direct impact on the texture of the formed ionosilicas in terms of the specific surface area, pore size and pore volume. The porosity of the ionosilica matrix can be well tuned both via the amount of the confined IL and the length of the cation alkyl chain.

We then performed small angle X-ray scattering (SAXS) in order to obtain a more detailed insight into the pore architecture of the materials. Similar to the results of nitrogen sorption experiments, we discuss here more in detail the same sets of materials: (i) the ionosilicas , synthesized in the presence of variable quantities of the IL [C₄MIM]TFSI on the one side and (ii) the ionosilicas , synthesized in the presence of 20 mmoles of different methyl-alkylimidazolium bis-triflimides with different alkyl chain lengths on the other side. The SAXS patterns of the materials are shown in Fig. 4, and the patterns of the series are shown in Fig. S13 (ESI†).

Fig. 4 SAXS patterns of the monoliths synthesized in the presence of variable amounts of [C₄MIM]TFSI (z = 3, 10, 20 and 41).

Similar SAXS profiles were obtained for all the materials of the series (Fig. 4), synthesized in the presence of different quantities of ILs. However, the patterns show some significant evolutions namely in the high q (q = ∼0.5–1 nm⁻¹) and low q domains (q = ∼0.15–0.4 nm⁻¹). For q values in the range of ∼0.5–1 nm⁻¹, SAXS intensity profiles show a cross over, whose position clearly shifts toward lower q values as a function of the molar fraction z of the ionic liquid used to prepare the samples. This q region profile should be the characteristic of the form factor of pores in mesoporous solids. In the case of worm-like ink-bottle pores, we can estimate, from the q-position of the cross over (see dashed lines in Fig. 4), the order of magnitude of the average pore diameter d (nm) using the eqn (1):

d (nm) ∼ 2/q(nm⁻¹) (1)

The shift of the cross over toward lower q values as a function of z (Fig. 4), from 1.188 nm⁻¹ for to 0.420 nm⁻¹ for , clearly indicates an increase of the pore diameter values obtained by SAXS, from ∼17 to ∼48 Å, respectively (Table 3). The increase in the quantity of [C₄MIM]TFSI therefore promotes the formation of larger confined ionic liquid domains within the host matrix, resulting in the formation of larger pores within the ionosilica scaffold after IL elimination. Interestingly, the values of the pore diameters obtained from nitrogen sorption and SAXS measurements are in good agreement (Table 3). The slightly higher values obtained by SAXS compared to those obtained by nitrogen sorption can be explained by limited pore filling in the case of closed porosity or interconnected pores with too narrow openings in nitrogen sorption experiments. Furthermore, some pores could also not be completely filled with nitrogen, depending on the pore surface curvature distribution (convex/concave).⁵¹ Comparatively, the SAXS signal only depends on the real volumetric distribution of density fluctuations inside the monolith, regardless of the pore architecture and connectivity. Therefore, the pore sizes obtained by nitrogen sorption are then often underestimated compared to SAXS values, as also observed here.

Table 3 Small-angle X-ray scattering data of monoliths with different ratios of [C_xMIM]TFSI ionic liquids

n (mmol)	Pore diameter^a (Å)				Pore diameter^b (Å)
	3	10	20	41	3	10	20	41
a Determined by SAXS using relation (1). b Calculated using the Harkins and Jura equation from nitrogen sorption analysis.^49
	14	22	34	47	n.d.	23	30	36
	17	22	33	48	n.d.	24	31	34
	16	26	35	47	n.d.	24	32	35
	19	24	39	49	n.d.	25	39	39
	n.d	n.d	48	n.d	n.d.	32	55	52
	n.d	n.d	42	n.d	27	34	52	50

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At lower q values (in the ∼0.15–0.4 nm⁻¹q range), a second cross-over can be observed for all samples (Fig. 4). This type of SAXS profile suggests the aggregation of individual pores into primary pore clusters. Pores are interconnected by narrow paths to form larger pore domains in accordance with the “ink-bottle” shape deduced from the mentioned H2 type isotherms observed in Fig. 3a and b. Such primary porous clusters can be interconnected to form larger pore aggregates (at the 100–1000 nm scale), as suggested by the significant increase of the scattering intensity observed at lower q values as shown in Fig. 4 for all samples. A characteristic size d′ (nm) of the primary clusters can also be deduced from relation (1). A shift toward lower q values of the primary cluster cross-over as a function of z is observed in Fig. 4. It can be seen that d′ increases proportionally to the z value from ∼40 Å to ∼125 Å that corresponds to 2–3 times the size of the individual pore size.

Regarding the second series of materials , we also studied the effect of the length of the cation alkyl chain on the textural properties of the obtained material via SAXS measurements. As discussed earlier, the alkyl chains of the IL tend to aggregate into nonpolar domains whereas the imidazolium ring and the anion form polar domains.⁵² The increase of the length of the alkyl chain of the cation is expected to enlarge the nonpolar IL domains.¹⁵ The SAXS profiles (Fig. S13, ESI†) show the presence of two evolution domains as function of the length of the cation alkyl chain (C_x), similarly to the results discussed in Fig. 4 (vide supra). The average size of pores and pore primary-clusters clearly increases with the increase of the length of the cation alkyl chain for x > 8. SAXS experiments therefore indicate the same trend as observed in nitrogen sorption measurements and confirm that only the long-chain substituted imidazolium ILs (C12 and C16) led to an increase of the pore diameters, whereas no significant increase of the pore size could be observed for short chain substituted species (C < 10). These results therefore highlight a particular self-aggregation behavior of dodecyl- and hexadecyl-imidazolium bis-triflimides, as already described earlier.⁵³

Finally, the morphologies and textures of the materials were studied via scanning and transmission electron microscopies (SEM and TEM). Fig. 5 shows the SEM images of the materials , and , all synthesized in the presence of 20 mmoles of [C_xMIM]TFSI (x = 2, 8 and 16).

Fig. 5 Scanning electron microscopy (SEM) images of , and monoliths with 20 mmoles of (a) [C₂MIM]TFSI, (b) [C₈MIM]TFSI and (c) [C₁₆MIM]TFSI. Scale bar: 300 nm.

SEM images show that the materials consist of agglomerated primary particles of nanometric size (at the 10–100 nm scale). The diameter of the primary particles depends on the nature of the used ILs and increases with increasing alkyl chain length. Hence, the utilization of long chain (C₁₆) substituted imidazolium bis-triflimides led to the formation of a material displaying a widened architecture, larger pores and larger granulometry compared to the two other materials as shown in Fig. 5(a–c). All formed ionosilicas show interconnected pore architectures ensuring good diffusion throughout the whole material. SEM images seem to reveal the existence of macropores that cannot be detected neither by gas sorption nor by SAXS techniques.

The texture of the materials was then observed by transmission electron microscopy (TEM). The TEM images of the materials , and , all synthesized in the presence of 20 mmoles of [C_xMIM]TFSI (x = 2, 8 and 16), are displayed in Fig. 6.

Fig. 6 Transmission electron microscopy (TEM) images of the materials , and .

TEM images of the materials also show that the nature of the used IL has a direct impact on the texture and directly affects the morphology of the formed materials. The TEM image of the material shows a dense matrix indicating the presence of a low porous or mesoporous material (Fig. 6a), whereas the TEM images of and (Fig. 6b and c) reveal a macroporosity with increasing pore widths. These results confirm the meso–macroporous characteristics of these materials and an increase of both macropore and macropore sizes of the materials with the length of the cation alkyl chain of the ionic liquid. The interconnected macropores form pore domains of about 50 nm width for (Fig. 6c).

To sum up, nitrogen sorption and SAXS measurements of the ionosilica materials give a complete and concordant image of the mesoporous textures and architectures (at the 1–10 nm scale) of the ionosilica materials formed in the presence of ionic liquids (Fig. 7). Both investigated parameters, the quantity of the IL used for the sol–gel procedure on the one side (Fig. 7 – vertical) and the alkyl chain length of the imidazolium cation on the other side (Fig. 7 – horizontal), contribute to the evolution of the texture and morphology of the formed ionosilica materials. The increase in the amount of the IL leads to an increase of the specific surface area and enlarged mesopore diameter and primary clusters within the ionosilica scaffold. On the other side, the increase of the alkyl chain length of the cation also contributes to increasing specific surface areas. However, a significant increase of the pore diameters could only be observed with long-chain substituted ILs, i.e., imidazolium cations bearing dodecyl or hexadecyl groups. The texture of ionosilicas formed in non-hydrolytic sol–gel synthesis involving room-temperature ionic liquids can therefore efficiently be controlled by the two studied parameters.

Fig. 7 Scheme resuming the mesoporosity control via the number of moles of the confined IL and the cation alkyl chain length.

Conclusion

We report ionosilica materials obtained via a non-hydrolytic sol–gel procedure in the presence of various methyl-alkylimidazolium bis-triflimide ILs. The aim was to study the effect of the amount of the confined IL and the length of its cation alkyl chain on the morphological and textural properties of the ionosilica scaffold after the extraction of the IL. This study clearly shows a clear correlation between both the investigated parameters and the textural properties of the ionosilica scaffold. It also reveals, for our ionosilica monoliths, the existence of both macroporosity and mesoporosity. Electron microscopy analyses indicate that the ionosilica scaffolds show a tunable interconnected macroporosity. Nitrogen sorption and SAXS measurements give concordant results and show that by increasing the quantity of a given IL, an increase of the mesoporosity in terms of the specific surface area, pore volume and pore diameter could be observed, with a maximum for the materials synthesized in the presence of 20 mmoles of IL. Regarding materials obtained in the presence of a specific amount of the IL [C_xMIM]TFSI bearing different alkyl chain lengths (x = 2, 4, 6, 8, 12 and 16), an increase of the specific surface area could be detected. However, different substitution of the imidazolium cation leads to an increasing pore size only with long-chain substituted ILs (C > 8), thus highlighting a particular self-aggregation behavior of imidazolium bis-triflimides bearing dodecyl or hexadecyl groups. Our results indicate that the supramolecular aggregation of imidazolium based bis-triflimide ILs in hydrophilic and hydrophobic domains has a direct impact on the texture of the formed ionosilicas.

Author contributions

The manuscript was written through contributions of all the authors. All the authors gave approval to the final version of the manuscript.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

The authors are grateful for the excellent technical assistance of Didier Cot and Bertrand Rebière for SEM measurements and Franck Godiard for TEM measurements.

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Footnote

† Electronic supplementary information (ESI) available: Supplementary nitrogen sorption isotherms, BJH adsorption pore distribution, and supplementary SAXS diffractograms. See DOI: https://doi.org/10.1039/d2cp02524h

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