Exceptional Mechanical Stability of Highly Porous Zirconium Metal-Organic Framework UiO-66 and Its Important Implications.

Metal-organic frameworks (MOFs) with high porosity usually exhibit weak mechanical stabilities, in particular, rather low stabilities against shear stress. This limitation remains one of the bottlenecks for certain applications of porous MOFs, such as gas storage or separation that requires dense packing of the MOF powders under mechanical compression without collapsing the pores. We found that UiO-66, a prototypical Zr-MOF with high porosity, exhibits unusually high shear stability. Its minimal shear modulus (Gmin = 13.7 GPa) is an order of magnitude higher than those of other benchmark highly porous MOFs (e.g., MOF-5, ZIF-8, HKUST-1), approaching that of zeolites. Our analysis clearly shows that the exceptional mechanical stability of UiO-66 is due to its high framework connections (i.e., the high degree of coordination of Zr-O metal centers to the organic linkers). Our work thus provides important guidelines for developing new porous MOFs targeting at high mechanical stabilities.