Reversing the Dye Adsorption and Separation Performance of Metal–Organic Frameworks via Introduction of −SO₃H Groups

In this work, two hydrostable Cr-based metal–organic frameworks (MOFs), MIL-101­(Cr) and MIL-101­(Cr)-SO₃H, 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)-SO₃H 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 −SO₃H groups