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

An unusual water-stable cationic metal-organic framework {[Cu(L)0.5(bpe)(H2O)](NO3)•(H2O)0.5}n (1) (H4L = 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 HCrO4- pollutant ions via single-crystal to single-crystal (SCSC) coordination substitution process. The HCrO4- uptake capacity of 1 is high to 190 mg/g. Meaningfully, the structure of 1-HCrO4 ({[Cu(L)0.5(bpe)(HCrO4)]}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 Cu2+. This is the first example of a stable coordination between chromate and the framework during the capture process. The captured HCrO4- are not dissociated easily into the solution due to the coordination bond. This interaction makes the enrichment of HCrO4- more stable and the capture capacity excellent. Furthermore, the HCrO4- releasing process displays good regeneration in a single crystal state, which further elaborates the reversible SCSC 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.