Molecular control of the functional and spatial interlayer environment of kaolinite by the grafting of selected pyridinium ionic liquids

Abstract

The synthesis and characterization of new kaolinite nanohybrid materials derived from pyridinium based ionic liquids with different functionalities located at the para position are described in this paper. Four ionic liquids (1-(2-hydroxyethyl)-4-methylpyridinium chloride (Mepyr), 1-(2-hydroxyethyl)-4-ethylpyridinium chloride (Ethpyr), 1-(2-hydroxyethyl)-4-(tert-butyl)pyridinium chloride (Tbutpyr) and 1-(2-hydroxyethyl)-4-benzylpyridinium chloride (Benzpyr)) were grafted on the aluminol interlayer surfaces of kaolinite by a melt intercalation method. The molecular structure of the grafted compounds was kept, as demonstrated by 13C CP-MAS NMR and FTIR spectroscopies. The grafted moieties have similar orientations in the interlayer space, as evidenced by a linear relationship between the observed d-value and the size of the ionic liquids (R2>0.99). This study highlights how it is possible to control, tune and functionalize the interlayer spaces of modified kaolinite. Pyridinium ionic liquids are particularly suitable for that purpose. Their substituents in the para position are the main agents of control of the rigidly fixed interlayer distance. Their choice permits also to tune the functionality of the interlayer space. Preliminary applications (adsorption of 2,4-dichlorophenoxyacetic acid(2,4-D) and electrochemical detection of thiocyanates ions) confirm the anion exchanger nature of these modified kaolinite. They confirm also that the spatial and functional controls of the interlayer environment drive the sequestration of the anions.