Thermostable mesoporous silica nanospheres produced through the use of a trisiloxane-tailed ionic liquid as a template

Abstract

Mesoporous silica nanospheres can be prepared by a facile route using a new class of eco-friendly template: trisiloxane-tailed ionic liquids. The organosilicone component of the ionic liquids template contributes to the reaction during the formation of mesoporous material, which leads to a firm skeleton of pores resulting in excellent thermal stability.

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Mesoporous nanostructured materials with uniform pore size, high specific surface area and high pore volume have attracted much attention due to their applications in catalysis,¹ adsorption² and drug delivery.³ Well-ordered mesoporous materials are frequently prepared through the use of a self-assembling surfactant, which plays the role of a template. Recent results have encompassed the use of silicon-containing or organosilane surfactants as templates for mesoporous structures, in which the silicon components take part in the production of the material due to compatibility between the organosilane template and the tetraethyl orthosilicate (TEOS) reactant. The participation of the silicon group of these templates in the formation of mesoporous materials result in narrow pore widths and concomitantly thicker pore walls. Chen and colleagues⁴ synthesized ordered two dimensional hexagonal mesoporous silica with ultra-thick pore walls using a modified nonionic organosilicon surfactant as the template under acid condition. Furthermore, a facile synthetic route by which hydrophobic functional mesoporous silica with highly ordered pore channels can be produced using an integrated ABC copolymer silicone surfactant as the template was reported by Sun and colleagues.⁵

Ionic liquids (ILs) comprise both a hydrophilic ionic head-group and a hydrophobic organic chain. These systems exhibit a special templating behavior resulting in highly ordered pores.^6,7 ILs-based materials can possess novel pore structures and properties due to their diversity.⁸ Furthermore, the low vapor pressure and ease with which ILs may be recycled make such synthetic routes both environmentally friendly and simple to implement. Monolithic mesoporous silica with bicontinuous worm-like mesopore systems has been synthesized by Zhou and colleagues⁹ using [C₄mim]⁺BF₄⁻ as the template. Trewyn and colleagues¹⁰ also synthesized a series of mesoporous silica-based materials, which were produced by varying the ILs that was used as template. Hence, it is generally possible to use silicon-containing ILs, combining the advantages of organosilicon compounds with those inherent to ILs, as template and partial silicon source during the synthesis of mesoporous silica.

In this study we describe the fabrication of mesoporous silica nanospheres by a facile route through the use of the surface active ILs, trisiloxanepyridinium chloride ([Si(3)Py]Cl),¹¹ as template and tetraethyl orthosilicate (TEOS) as the silica source. Gratifyingly, the nanospheres obtained exhibited essentially identical morphology and pore structure before and after calcination, implying excellent thermal stability.

Silica nanospheres were prepared by combining 25 ml of [Si(3)Py]Cl (0.1 mol L⁻¹) and 2 ml of L-ascorbic acid (0.05 mol L⁻¹) within a 100 ml round-bottomed flask. This mixture was agitated vigorously and heated to 30 °C whereupon aqua ammonia (0.15 mmol L⁻¹) was incorporated in order to raise the pH to 9.0. Fifteen minutes later, 12.5 mmol TEOS was added to the solution dropwise and this mixture incubated for a further two hours. The products were rested at room temperature (25 °C) for 24 h, and the resulting white precipitate then extracted by filtration, washed several times with distilled water and anhydrous ethanol in turn, and dried at 80 °C for 12 h. Finally, this sample was calcined in air at 550 °C for 5 h to remove the organic components.

High resolution transmission electron micrographs of the silica nanospheres before (Fig. 1a) and after calcination (Fig. 1b) demonstrated that the sphere size remained essentially unchanged (80–100 nm) indicating good thermostability of the sample due to the presence of [Si(3)Py]Cl as both template and partial silica source. In addition, the clear white dots visible within the spherical particles, 4–6 nm in diameter, are consistent with the existence of the retention of pores in both heat treated and untreated samples.

Fig. 1 HRTEM images of spherical silica particles (a) before and (b) after calcination.

Prior to calcinations, small angle XRD (SAXRD) spectra exhibited a single peak at 2θ = 1.5° (Fig. 2a) corresponding to a spacing d = 58.8 Å by Bragg's equation (nλ = 2d sin θ). SAXRD patterns after calcination also exhibit only one peak (Fig. 2b), but at the slightly different 2θ of 1.8° with a corresponding spacing d = 49.1 Å. The small shift observed may be attributed to the removal of organic components retained from the template by the heat treatment leading to a small, but measureable, shrinkage of the pore to pore spacing. The hydrolyzed and condensed TEOS silica sources bind to the surface of the ILs micelles. During calcination, the extreme temperature will promote reactions between the pre-silicate species and the remaining silicon groups of the IL,⁴ resulting in slight shrinkage of the nanospheres.

Fig. 2 Powder XRD patterns of mesoporous silica (a) before and (b) after calcination.

FT-IR was used to analyze the structure of our samples further. Prior to calcinations, bands at 1490 cm⁻¹, 760 cm⁻¹ and 684 cm⁻¹ can be assigned to the bone vibrations, C=C stretching and C–H bending vibrations in the pyridinium rings from the ILs, respectively (Fig. 3). Meanwhile, the band at 2793 cm⁻¹ in as-synthesized sample is attributed to –CH₂– and –CH₃ from the organic components present in the spheres. However, these bands disappeared after calcination of the sample. This result confirms the removal of the remaining organic components during the heating process.

Fig. 3 FT-IR spectra of products (a) before and (b) after calcination.

Nitrogen adsorption–desorption isotherms were carried out at 77 K (Fig. 4). The Brunauer–Emmett–Teller (BET) surface areas, pore volumes and average pore size were calculated using the Barett–Joyner–Halenda (BJH) method on the basis of the desorption branch and are listed in Table 1 together with the diameters of the nanospheres obtained by examination of transmission electron micrographs. Nanosphere diameters ranged between 80 and 100 nm, BET surface areas lay in the region of 460 m² g⁻¹, average pore volumes were 0.6734 cm³ g⁻¹ or 0.6390 cm³ g⁻¹ and average pore sizes were 5.56 nm or 5.92 nm (Fig. 4 and Table 1), the high degree of similarity providing further confirmation of the thermal stability of our nanospheres. The isotherms exhibited type IV curves with hysteresis loops, suggesting the existence of hierarchical mesoporous structure within the materials. Type H1 hysteresis loops and the narrow pore size distribution also imply that the corresponding pore sizes are uniform. These results are therefore in a good agreement with our results from XRD and TEM.

Fig. 4 Nitrogen adsorption–desorption isotherms and pore size distribution curves (insert) of the samples (a) before and (b) after calcination.

Table 1 Structural properties of silica nanospheres before and after calcination

Sample	Nanosphere diameters (nm)	SBET (m^2 g^−1)	Pore volume (cm^3 g^−1)	Average pore size (nm)
Before calcination	80–100	460.13	0.6734	5.56
After calcination	80–100	463.32	0.6390	5.92

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Although it is not unambiguously understood at present, on the basis of the above analysis, the possible synthesis mechanism of such nanospheres templated by trisiloxane-tailed ILs was proposed and shown in Scheme 1. Under basic conditions, the hydrolyzed silicate species from TEOS were adsorbed upon the surface of ILs micelles in water, and then condensed to form the skeleton of the mesoporous materials. The pore structures on the as-synthesized materials may be attributed to the hollow formed by ILs micelles due to the steric hindrance between trimethylsiane groups of trisiloxane-tailed ILs. Then, the removal of the organic components by calcination leads to larger pores, and meanwhile, the calcination can also promote the cross-linking process between the silica species and the organosilicone component of trisiloxane-tailed ILs. It is worth noting that the added ascorbic acid can promote the hydrolysis and condensation of TEOS. The mixture of ascorbic acid and ammonia plays a role of buffer solution, which could maintain an appropriate pH range (pH 8–10) for a long time.¹²

Scheme 1 Schematic illustrating the formation of mesoporous silica using silicone ILs as a template.

To summarise, we were able to successfully obtain thermally stable mesoporous silica nanospheres by a facile route through the use of a trisiloxane-tailed ionic liquid as a template. Our results indicated that our samples consist of spherical nanoparticles with a narrow range of diameters ranging from 80 to 100 nm with well defined mesopores in the region of 4 to 6 nm. Our nanospheres exhibited excellent thermal stability, which was confirmed by calcination at 550 °C. We suggest that this phenomenon may be ascribed to the specificity of the organosilicone ionic liquids template that contributes silicon to the formation of the mesoporous material, leading to firm pore structure. Our synthesis provides a convenient approach to produce ordered mesoporous spherical materials with a stable structure, which may have potential uses as sensors, nanoscale reactors, catalysts or catalyst carriers in high temperature processes.

This work was financially supported by National Science Foundation of China (no. 21073234 and no. 21103228) and the National Science & Technology Pillar Program during the Twelfth Five-year Plan Period (Grant no. 2014BAE03B03).

Notes and references

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