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

An unusual water-stable cationic metal-organic framework {[Cu(L)(bpe)(HO)](NO)•(HO)} (1) (HL = 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 pollutant ions via single-crystal to single-crystal (SCSC) coordination substitution process. The HCrO uptake capacity of 1 is high to 190 mg/g. Meaningfully, the structure of 1-HCrO ({[Cu(L)(bpe)(HCrO)]}) 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. This is the first example of a stable coordination between chromate and the framework during the capture process. The captured HCrO are not dissociated easily into the solution due to the coordination bond. This interaction makes the enrichment of HCrO more stable and the capture capacity excellent. Furthermore, the HCrO 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.