Metal-organic framework (MOF) materials containing oxometalate cluster-based nodes, such as oxozirconium or oxocerium nodes, can provide the basis for single-site heterogeneous catalysts, for example, if all nodes are uniformly modified with suitable metal species. However, the application of MOFs for catalysis at high temperatures, particularly in oxidizing environments, is limited, because the instability of the organic linkers under these conditions leads to loss of the MOF structure and aggregation/sintering of the catalytic clusters. Under those conditions, it may be necessary to stabilize the cluster sites. In this presentation, we will discuss methods of stabilizing MOF-derived functionalized and non-functionalized oxometalate clusters by nanocasting with silica, titania, or carbon scaffolds. A silica scaffold, for example, can prevent the oxozirconium clusters in the MOFs NU-1000 or UiO-66 from aggregating after the linkers are removed at temperatures between 500 and 600 degrees C. This is in contrast with bare NU-1000 or UiO-66, which shows sintering of the clusters after undergoing the same thermal treatment. The clusters in the nanocast material remain accessible to probe molecules and their Lewis acidity is maintained, keeping them active as catalysts in certain acid-catalyzed reactions. This nanocasting method can also be extended to other scaffolds to support cluster nodes in large-pore MOFs. We will consider synthesis parameters that permit uniform infiltration of the MOF with the scaffold precursors and correlate these to nanocast products that maintain discrete clusters after thermal treatment at 500 degrees C or higher, without formation of extended zirconia aggregates or aggregates of the modifying metal oxides. Accessibility of the stabilized clusters in the scaffolds is probed by spectroscopic and catalytic reaction methods. Nanocasting, combined with the exceptional tunability of the composition and structure of MOFs, provides a starting point for designing a new generation of highly active and stable heterogeneous catalysts.

Nanocasting in metal-organic framework materials

256th National Meeting (2018)

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