Effects of Phase Purity and Pore Reinforcement on Mechanical Behavior of NU-1000 and Silica-Infiltrated NU-1000 Metal-Organic Frameworks

Metal-organic framework (MOF) materials have shown promise in many applications, ranging from gas storage to absorption and catalysis. Because of the high porosity and low density of many MOFs, densification methods such as pelletization and extrusion are needed for practical use and for commercialization of MOF materials. Therefore, it is important to elucidate the mechanical properties of MOFs and to develop methods of further enhancing their mechanical strength. Here, we demonstrate the influence of phase purity and the presence of a pore-reinforcing component on elastic modulus and yield stress of NU-1000 MOFs through nanoindentation methods and finite element simulation. Three types of NU-1000 single crystals were compared: phase-pure NU-1000 prepared with biphenyl-4-carboxylic acid as a modulator (NU-1000-bip), NU-1000 prepared with benzoic acid as a modulator (NU-1000-ben), which results in an additional, denser impurity phase of NU-901, and NU-1000-bip whose mesopores were infiltrated with silica (SiOx(OH)y@NU-1000) by nanocasting methods. By maintaining phase purity and minimizing defects, the elastic modulus could be enhanced by nearly an order of magnitude: phase-pure NU-1000-bip crystals exhibited an elastic modulus of 21 GPa, whereas the value for NU-1000-ben crystals was only 3 GPa. The introduction of silica into the mesopores of NU-1000-bip did not strongly affect the measured elastic modulus (19 GPa) but significantly increased the load at failure from 2000 μN to 3000-4000 μN.