Ion exchange collaborating coordination substitution: more efficient Cr(VI) removal performance of a water-stable CuII-MOF material

Abstract An unusual water-stable cationic metal-organic framework {[Cu(L) 0.5 (bpe)(H 2 O)](NO 3 )•(H 2 O) 0.5 } n ( 1 ) (H 4 L = bis(3,5-dicarboxypyridinium)-p-xylylene) was synthesized, which was developed into an effective capture material for removal chromate from water. The results show that this material efficiently traps HCrO 4 − pollutant ions via single-crystal to single-crystal (SC SC) coordination substitution process. The HCrO 4 − uptake capacity of 1 is high to 190 mg/g. Meaningfully, the structure of 1- HCrO 4 ({[Cu(L) 0.5 (bpe)(HCrO 4 )]} n ) can be accurately obtained by single-crystal X-ray diffraction, where the chromate enter the framework to form stable coordination with central metal ions Cu 2+ . This is the first example of a stable coordination between chromate and the framework during the capture process. The captured HCrO 4 − are not dissociated easily into the solution due to the coordination bond. This interaction makes the enrichment of HCrO 4 − more stable and the capture capacity excellent. Furthermore, the HCrO 4 − releasing process displays good regeneration in a single crystal state, which further elaborates the reversible SC SC transformation. The mechanism of Cr(VI) removal was also confirmed by DFT calculation studies. This work provides a new way to design and develop efficient MOF capture materials.