Stability trend of metal−organic frameworks with heterometal-modified hexanuclear Zr building units

Bimetallic metal−organic frameworks (MOFs) based on heterometal-modified hexanuclear Zr building units have emerged as promising materials for applications in energy-related fields such as heterogenous catalysis/electrocatalysis. However, their stability remains a challenge under some practical conditions, and the physical/chemical origin to the (in)stability is not well-understood. Herein, we selected three representative Zr-MOFs (MOF-808, NU-1000, and UiO-66) as platforms and incorporated different heterometals (Ti⁴⁺, V³⁺, V⁵⁺, Cr³⁺, Cr⁶⁺, Mn²⁺, Fe²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺) to form a series of bimetallic M/Zr-MOFs. Their stability was examined in aqueous solutions of various pH to define their stability windows and understand their stability trend. The stability of M/Zr-MOFs was found to be dictated by the oxidation states of incorporated heterometals and slightly affected by the supporting Zr-MOFs. Water-exchange rate constant, defined as the rate constant by replacing a coordinated water with a solution water, was proposed as the stability descriptor to explain the stability trend and guide the design and application of future stable MOFs.