Ionogels as modular platform for metal extraction

Abstract

In this study, we have explored a strategy to develop new ionogels as materials for metal extraction. The methodology applied here consists, first, in confining a commercial salt tetrahexylammonium bromide [THN]Br in silica-based ionogels in order to determine their extracting properties. Anionic metathesis was then carried out in-situ on the [THN]Br ionogels with sodium dicyanamide (NaDca) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) to enhance their extracting efficiency or to bring selectivity to the extraction process. Treatment of [THN]Br ionogels with those salts ensures a quasi-quantitative replacement of bromide ions by Dca− and TFSI−. The replacement of bromide anions was confirmed by thermogravimetric analyses (TGA) and Infra-Red spectroscopy. Nitrogen adsorption–desorption isotherms measured before and after metathesis allow us to confirm that no major structural modification has been induced by the anionic exchange.[THN]Br ionogels were used for the extraction of Au(III), Pt(IV), Co(II), and Cr(VI). Au(III), Pt(IV) and Cr(VI) were fully extracted while the extraction of Co(II) did not exceed a few percent. Treatment of [THN]Br ionogels with NaDca improved the extraction capabilities of the material with respect to Co(II) while treatment with LiTFSI allowed the separation of Au(III) from Pt(IV), with separation factors higher than 1000. It also increased the separation of Au(III) from Cu(II) compared to the brominated ionogel. The extraction performances of each ionogel were compared to those of the pure similar ionic liquid. This work shows that, starting from the same ionogel, the extracting properties can be modulated as desired by simple anion exchange.