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

• 4-Methylimidazolium-modified silicas with BF4−, PF6− and Tf2N− were synthesized.
• Counter anions with good hydrophilicity benefit the adsorption of 2,4-DNP.
• Counter anions with simple geometry shape benefit the elution of 2,4-DNP.
• The adsorbent was regenerated and reused more than ten times by washing with HCl.

Three 4-methylimidazolium-modified silica adsorbents with different counter anions such as BF4−, PF6− and Tf2N− (named as SilprM4ImBF4, SilprM4ImPF6 and SilprM4ImTf2N, 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: SilprM4ImTf2N < SilprM4ImPF6 < SilprM4ImBF4, 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 BF4− and Tf2N− had similar recovery yields of 2,4-DNP, while regular octahedron such as PF6− had slightly low recovery yields.