Ionic Liquid Mediated
Sol-Gel Synthesis in the Presence
of Water or Formic Acid: Which Synthesis for Which Material?
- Publication date
- Publisher
Abstract
Sol-gel syntheses involving either neutral water or formic
acid
as a reactant have been investigated (1) to determine the best conditions
to confine a maximum of ionic liquid (IL) inside silica-based matrixes
and (2) to reach the highest porosity after removing the IL from the
ion gels (washed gels). Several sets of ionogels were prepared from
various 1-butyl-3-methylimidazolium ILs and various silica or organosilica
sources. The study evidenced a critical effect of the anion on the
morphology (monolith, powder) and texture of the resulting washed
gels. Particularly, tetrafluoroborate anion led to monolith ionogels
by a simple hydrolytic method, affording highly condensed mesoporous
silicas with some fluorinated surface sites. Such sites have never
been reported before and were evidenced by <sup>19</sup>F NMR. On
the other hand, formic acid solvolysis turned out to be the only method
to get non-exuding, crack-free, and transparent monoliths from ILs
containing bis(trifluoromethylsulfonyl)imide [NTf<sub>2</sub>] anion,
with promising applications in photochemistry or photosensing. With
bulky imidazolium and pyridinium cations, removal of the IL led to
highly porous silicas with pore diameters and pore volumes as high
as 10–15 nm and 3 cm<sup>3</sup> g<sup>–1</sup>, respectively.
These silicas could find applications as supports for immobilizing
bulky molecules