Structure of a monolithic silica aerogel prepared from a short-chain ionic liquid

Use of an ionic liquid as a solvent to prepare silica aerogel via the sol–gel process has attracted considerable attention because a stable gel network can be formed during the aging process. One of the questions still remain unknown is the effect of the different molar ratios of ionic liquid to precursor on the resultant sol–gel framework structures of the synthesized silica gels; hence, such an effect will be explored in this article via   small-angle X-ray scattering (SAXS). Herein, two silica gels have been synthesized using a short-chain ionic liquid, 1-butyl-3-methyl-imidazolium-tetrafluoroborate (C4mim+BF4-), as the solvent and tetraethyl orthosilicate (TEOS) as a precursor via   an HCl-catalyzed sol–gel route. The molar ratio of C4mim+BF4- to TEOS (denoted by nIL/nTEOS) was controlled to be stoichiometric and 3. The sample drying proceeded through solvent extraction method with a vacuum to avoid the risky supercritical drying step. Transmission electron microscopy (TEM) and SAXS analyses revealed that a silica aerogel could be prepared with stoichiometric nIL/nTEOS and exhibited an open mass-fractal silica framework formed by the aggregation of the individual spherical silica particles. To our knowledge, such a structural characteristic of spherical primary particle had not been confirmed in the other silica aerogels using IL as a solvent via SAXS so far. Inversely, a silica xerogel born from nIL/nTEOS = 3 exhibited a collapsed sol–gel framework. The reasons leading to the structural differences derived from the different nIL/nTEOS will be explained with the sol pH values. Additionally, a two-level unified scattering function was also further adopted to fit the SAXS data to quantify the sizes of the mass-fractal clusters and their constituent silica particles.

Figure optionsDownload as PowerPoint slideHighlights
► A silica aerogel was yielded with stoichiometric ionic liquid-TEOS sol–gel route.
► Sample drying proceeded through solvent extraction method with a vacuum.
► The aerogel showed an open mass-fractal network composed of the spherical silicas.
► A pH-adjusted mechanism was presented to explain the aerogel formation.