In this work, two hydrostable Cr-based
metal–organic frameworks
(MOFs), MIL-101(Cr) and MIL-101(Cr)-SO<sub>3</sub>H, were successfully
synthesized and applied in the adsorption and separation of ionic
dye fluorescein sodium (FS) and cationic dye safranine T (ST). Interestingly,
MIL-101(Cr) can efficiently adsorb FS dye but hardly adsorbs ST dye,
and MIL-101(Cr)-SO<sub>3</sub>H exhibits the thoroughly opposite phenomenon.
More importantly, the reversed adsorption with high selectivity on
the two MOFs can also be attained in the mixed solutions of the dyes.
Finally, a mechanism analysis indicates that this significant reversal
in performance for the dyes is mainly attributed to the opposite surface
charges of the two MOFs caused by −SO<sub>3</sub>H groups