(603a) Metal-Organic Frameworks Emerging As a Class of Well-Defined Solid Catalysts

Metal-organic frameworks (MOFs) are a huge, rapidly growing class of crystalline, porous materials that consist of inorganic nodes linked by organic struts. Some MOFs offer the advantages of thermal stability combined with high densities of accessible reactive sites and are therefore good candidate materials for catalytic applications. Such MOFs include those with nodes that are metal oxide clusters (e.g., Zr₆O₈, Zr₁₂O₂₂) and long rods (e.g., [Al(OH)]_n). These nano-structured metal oxides are often compared with bulk metal oxides—but they are in essence different, both in structure and—because of their uniform structures—in offering the prospect of deep understanding of catalytic properties that is barely attainable for most bulk metal oxides because of their surface heterogeneity. The prospect is being realized as it has become evident that adventitious components on MOF node surfaces—besides the linkers—are often dominant—and that they can be identified and tuned. These ligands arise from modulators, solvents, or products of solvent decomposition in MOF synthesis solutions. Hydroxyl groups on nodes are often regarded as native functional groups arising from water in syntheses, but they may barely be present, with common ligands instead being formate and acetate formed from the modulators formic acid and acetic acid and from dimethylformamide solvent. Replacement and control of the node ligands is being elucidated and facilitated by reactions (e.g., with alcohols or aqueous HCl/H₂SO₄ solutions)—to create catalytic sites such as Zr⁴⁺Lewis acid sites and OH groups. Adventitious node ligands can be (a) reaction inhibitors that block node active sites; (b) reaction intermediates (e.g., methoxy in methanol dehydration); or (c) active sites themselves. MOF node surface and catalytic chemistry is subtle and only recently being elucidated, and it is the topic of this presentation.