Aliquat 336@SiO2 ionogels: Synthesis of, and insight into, iron(III) extraction mechanisms

Abstract

The immobilisation of ionic liquids (ILs) in porous solid matrices enables the design of ionogels, which are now regarded as a promising material in extraction science. Here, by the co-gelation of TMOS and MTMS in a commercially available ionic liquid, Aliquat 336 (A336Cl), a series of ionogels were synthesised with various levels of IL content and matrix hydrophobicity. Both of these factors were shown to have a small effect on Fe(III) extraction efficiency (57–70 %), while they strongly influenced the re-extraction efficiency (15–45 %) of the materials. The ionogels with the highest IL content (80 %) and a highly hydrophilic silica matrix showed the best extraction and re-extraction performance. A thorough characterisation of the ionogels confirmed the confinement of the IL in silica and revealed Fe(III) extraction mechanisms. It was shown that iron was extracted from the aqueous solutions by A336Cl@SiO2 ionogels in the form of FeCl4– ions typical of the extraction by pure A336Cl. Unexpectedly, the iron extraction by the ionogels resulted in the formation of Fe2Cl7– species that had not been observed earlier in the A336Cl-based extraction systems. Moreover, iron(III) directly bound to hydrophilic silica through Si–O–Fe bridges, and it was also found that, in the ionogels, the admixtures of alcohols could even reduce ferric ions to ferrous species. For the ionogels, both iron extraction and re-extraction followed pseudo-second order kinetics. Iron re-extraction from the ionogels with aqueous sulfuric acid solution resulted in the loss of recyclability, most probably due to the formation of FeSO4⋅H2O in the ionogels. The cycling performance of the ionogels can be improved by their conditioning in chloride-rich media after re-extraction stages.