Effects of counter anions on the adsorption properties of 4-methylimidazolium-modified silica materials

Three 4-methylimidazolium-modified silica adsorbents with different counter anions such as BF 4 − , PF 6 − and Tf 2 N − (named as SilprM 4 ImBF 4 , SilprM 4 ImPF 6 and SilprM 4 ImTf 2 N, respectively) were prepared and characterized. Three types of adsorbents were used to remove 2,4-dinitrophenol (2,4-DNP) from aqueous solutions under static and dynamic conditions. It was found that the adsorption capacity of 2,4-DNP increases according to the order: SilprM 4 ImTf 2 N   <   SilprM 4 ImPF 6   <   SilprM 4 ImBF 4 , which was opposite to the van der Waals volume of these counter anions and consistent with the hydrophilicity of these counter anions. The reason could be related that counter anions with large van der Waals volume would have large space steric hindrance and be unfavorable for the anion exchange reaction. Counter anions with good hydrophilicity would dissociate from these materials easily and the anion exchange reaction could easily take place. Under dynamic conditions, the geometry shape of counter anions could affect the recovery yields of 2,4-DNP. Tetrahedron such as BF 4 − and Tf 2 N − had similar recovery yields of 2,4-DNP, while regular octahedron such as PF 6 − had slightly low recovery yields.