Extending the Compositional Range of Nanocasting in the Oxozirconium Cluster-Based Metal-Organic Framework NU-1000 - A Comparative Structural Analysis

The process of nanocasting in metal-organic frameworks (MOFs) is a versatile approach to modify these porous materials by introducing supporting scaffolds. The nanocast scaffolds can stabilize metal-oxo clusters in MOFs at high temperatures and modulate their chemical environments. Here we demonstrate a range of nanocasting approaches in the MOF NU-1000, which contains hexanuclear oxozirconium clusters (denoted as Zr₆ clusters) that are suitable for modification with other metals. We developed methods for introducing SiO₂, TiO₂, polymeric, and carbon scaffolds into the NU-1000 structure. The responses of NU-1000 toward different scaffold precursors were studied, including the effects on morphology, precursor distribution, and porosity after nanocasting. Upon removal of organic linkers in the MOF by calcination/pyrolysis at 500 °C or above, the Zr₆ clusters remained accessible and maintained their Lewis acidity in SiO₂ nanocast samples, whereas additional treatment was necessary for Zr₆ clusters to become accessible to pyridine probe molecules in carbon nanocast samples. Aggregation of Zr₆ clusters was largely prevented with SiO₂ or carbon scaffolds even after thermal treatment at 500 °C or above. In the case of titania nanocasting, NU-1000 crystals underwent a pseudomorphic transformation, in which Zr₆ clusters reacted with titania to form small aggregates of a Zr/Ti mixed oxide with a local structure resembling that of ZrTi₂O₆. The ability to maintain high densities of discrete Lewis acidic Zr₆ clusters on SiO₂ or carbon supports at high temperatures provides a starting point for designing new thermally stable catalysts.