Stability of UiO-66 under acidic treatment: Opportunities and limitations for post-synthetic modifications

In chemical and functional modification of Metal-organic frameworks (MOFs), preserving their structure and topology is a challenging goal, due to the high sensitivity of these materials to moisture, chemical and thermal treatments. The porous material UiO-66 is one of the most stable MOFs, retaining its properties under a relative wide range of chemical and physical conditions. The unique robustness of UiO-66 derives from the 12-coordinated Zr-6-clusters that constitute the framework. Nevertheless, variation on the synthetic pathways may lead to different coordination of the Zirconium sites, influencing the stability and activity of this MOF during applications such as catalysis. Herein a survey of the stability of UiO-66 under acidic treatment is presented. The porous metal-organic framework has been synthesized according to two alternative procedures; these two solid materials and a commercially available UiO-66 sample have been suspended in several aqueous acids and have been characterized after the impregnation tests. These results lead to a better understanding of the matter of acidic stability of the selected MOF, clarifying whether it is possible to enhance its stability by choosing a dedicated synthesis route and providing essential information in order to proceed with a large number of post-synthetic modifications. Among these reactions, the sulfonation of UiO-66, which is an example of outstanding importance, has been investigated. The stability of UiO-66 versus mineral acids has been confirmed; however this MOF does not tolerate the use of electrophilic cationic species.