Experimental and computational study on mechanism of dichromate adsorption by ionic liquid-bonded silica gel

The mechanism of adsorption of dichromate by imidazole ionic liquid-bonded silica gel was studied. Two different adsorbents for adsorption of dichromate ions were synthesized and characterized. By static adsorption experiments, when the pH was 5.6 and the initial concentration of Cr was 200 mu g/mL, the adsorption capacity of 1-methylimidazole ionic liquid-bonded silica gel (SilprMmimCl) and 1-ethylimidazole ionic liquid-bonded silica gel (SilprEmimCl) for Cr (VI) was 24.03 mg/g and 16.52 mg/g, respectively. To investigate the mechanism of adsorption on molecular scale, an simplified model of SilprMmimCl and SilprEmimCl have been modeled with [Mmim]Cl, [Emim]Cl, [Mmim][Cr2O7] and [Emim][Cr2O7]. The structures of related models were optimized at the B3LYP/6-311+G(d, p) level. The weak interaction between cations and anions of models was analyzed by vibration analysis and bond length analysis; the features of bonds at bond critical points and the variations of bond types was studied by Atoms In Molecules (AIM) theory. Simultaneously, noncovalent interactions were characterized and visualized by analysis of Reduced Density Gradient (RDG). Finally, The sequence of intra-molecular interaction energy is [Mmim][Cr2O7], [Emim][Cr2O7], [Mmim]Cl, [Emim]Cl from big to small, which demonstrates why SilprMmimCl and SilprEmimCl have a good adsorption capacity for dichromate and the adsorption capacity of SilprMmimCl is slightly more than that of SilprEmimCl.