Engineering pH-switchable UiO-66 via in-situ amino acid doping for highly selective adsorption of anionic dyes

The pH-switchable UiO-66 were engineered via in-situ amino acids (AA) doping for the highly selective adsorption of anionic dyes (i.e. sunset yellow FCF (SY)). Three AA doped samples, i.e. UiO-66-V (valine), UiO-66-T (threonine), and UiO-66-Y (tyrosine), have fabricated hierarchical structure and endowed –NH 2 surface in framework structure concurrently. These AA doped UiO-66s awarded highly selective adsorption and fast diffusivity towards the mentioned anionic dyes. More interestingly, enriched –NH 2 groups endowed AA doped UiO-66s a significant pH-responsive character for selectivity of anionic/cationic dyes. Textural characterization and adsorption measurement confirmed that high surface areas (>1070 m 2 /g) with defected pores and –NH 2 surface significantly enhanced adsorption uptake (352.8 mg/g) and diffusion rate (168 × 10 −3  min −1 ) for SY on AA doped UiO-66s, about 1.9 and 5.9 times respectively as that on pristine UiO-66. The AA doped UiO-66s exhibited one order of magnitude higher selectivity (>390) for SY against MB than that of original UiO-66. More importantly, the alteration in surface charge (–NH 2 H + ⇌ O H - -NH 3 + ) endorsed a great variation in the selectivity of AA doped UiO-66s to dyes when adjusting pH from 4 to 8, showing an excellent pH on-off switchable property. Also, it realized a stable self-adsorption/desorption recycling process via pH-switching for 6 consecutive cycles. With these features, the pH-switchable defected UiO-66 realized highly efficiency adsorption and non-thermal separation/regeneration process for anionic dyes.