Stabilization effect of trivalent chromium on a Cu-based metal-organic framework: Strengthen the original O–Cu–O metal-ligand bond

Cu-BTC materials are widely used because of the high specific surface areas and special pore structures , but the water instability has also been proven. With the adsorption of trivalent chromium, the long-time stability of Cu-based metal-organic frameworks Cu-BTC was got improved in water treatments . A strategy to improve the framework stability in water by the adsorption of trivalent chromium ions into Cu-BTC has been developed. The results showed that the treated Cu-BTC-Cr material has much better water stability and alkaline resistance than the original Cu-BTC. Cr 3+ ions have empty 3d electron orbitals and strong affinity with O atoms, decreasing the electron density of BTC 3− and making the carboxylate softer (“indirect soft-soft” mechanism), which strength the bond between carboxylate (hard base) and Cu 2+ (soft acid), i.e., strength the O–Cu–O bond, and Cu-BTC is stabilized in water even tough conditions. • Utilizing Cr 3+ to improve the water stability and alkali resistance of Cu-BTC. • It is confirmed that Cr 3+ do not replace the position of Cu 2+ but stay in the pores of the material surface and strengthen the O-Cu-O bond, thus improving water stability. • We propose an “indirect soft-soft” mechanism for Cr 3+ stabilized the hard-soft MOFs structure and shed new light on improving the stability of MOFs.