Ionic metal-organic frameworks (MOFs), a subgroup of MOF materials possessing cationic or anionic skeletons with counterions inside of pores, have been reported in many studies, yet people know few about how the charge of skeleton affects the entire properties of MOF materials.
In a study published in Angewandte Chemie International Edition, a research team led by Prof. CAO Rong and Prof. LIU Tianfu at Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences reported that the chemical stability of MOFs can be substantially improved through embedding electrostatic interactions in structure.
The researchers designed and synthesized two MOFs with identical components, space group, and topology, but with differently charged skeleton (cationic and neutral charges). The obtained cationic framework maintains crystallinity and porosity in extremely harsh condition including 12 M concentrated HCl (301 days), H2O2 (30 days) and even seawater (30 days) as well as aqua regia (86 days). This is far superior to the isoreticular neutral framework and has not yet been achieved in MOF field.
Computational studies suggested that the cationic skeleton repels the positively charged species in microenvironments, therefore limiting the access of competing agents to the most vulnerable dative bonds in MOFs.
The researchers further revealed that the dominant factor accounting for MOFs degradation in acidic solution is, counterintuitively, the strength of the corresponding conjugate base rather than the pKa value, providing important information for the researchers concerning MOFs’ practical applications.
Besides the robustness, the other inherent advantage of the charged frameworks is the prospect that the structure can be readily functionalized through a mild, convenient, and low-cost ion-exchange method, which was representatively demonstrated by the fabrication of MOF catalyst for formic acid dehydrogenation.
This study reveals that how backbone’s charge affects the stability and functionality of MOFs, which would broaden the horizon in design of MOFs and open a new pathway to regulate MOFs’ properties for diverse applications.
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