Missing-node directed synthesis of hierarchical pores on a zirconium metal-organic framework with tunable porosity and enhanced surface acidity via a microdroplet flow reaction

A hierarchical porous zirconium metal-organic framework (UiO-66) was prepared continuously through a microdroplet flow reaction strategy for the first time. The existence of metal-node defects arising from incomplete coordination in UiO-66 was found to be the main reason for the formation of mesopores. The dimensions of mesopores could be facilely tuned by adjusting the residence time, and the portion of mesopores was linearly correlated with the missing-nodes. The surface acidity was enhanced due to a large amount of pendant coordinated-free carboxylate groups in pores. With increasing residence time, the missing-nodes among the frameworks were sequentially repaired by self-healing of coordination spheres. Also, these hierarchical porous MOFs demonstrate superior storage capacities for CO2 and CH4. The method of constructing mesopores and producing surface acids presented in this work may open up a new avenue for developing novel hierarchical porous MOFs with special functionalities.