Modeling Lewis acid catalyzed reactions in MOFs : how to solve the challenges?

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Modeling Lewis catalyzed reactions in Metal Organic Frameworks

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Modeling citronellal cyclization in Cu3BTC2 and UiO-66

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A breathing zirconium metal-organic framework with reversible loss of crystallinity by correlated nanodomain formation

The isoreticular analogue of the metal-organic framework UiO-66(Zr), synthesized with the flexible trans-1,4-cyclohexanedicarboxylic acid as linker, shows a peculiar breathing behavior by reversibly losing long-range crystalline order upon evacuation. The underlying flexibility is attributed to a concerted conformational contraction of up to two thirds of the linkers, which breaks the local lattice symmetry. X-ray scattering data are described well by a nanodomain model in which differently oriented tetragonal-type distortions propagate over about 7-10 unit cells

Open metal sites and cage confinement in adsorption and catalysis on metal-organic frameworks

Metal-organic frameworks (MOFs) were recently introduced as a novel class of highly crystalline, porous materials. Their crystal structure is formed through the coordination of metal nodes withpolyfunctional organic linkers. Consequently, the metal ions are maximally dispersed in a well-defined, highly porous framework. This feature, combined with the ease of functionalization, makes them highly interesting candidates to be used in catalysis and adsorption. Despite a rapidly increasing volume of research activities in the field, there is still much room for improvement inthe application of such materials in catalysis and adsorption. In this research project, it is shown how the intrinsic characteristics of MOFs allow us to tune their catalytic activity and to use them as highly selective materials in the separation of xylenes. In a first part, a new zirconium metal-organic framework, UiO-66(Zr), is shown to possess a highly selective catalytic activity in the cross-aldol condensation of benzaldehyde and heptanal. The highest activity and selectivity are found for the amino-substituted version. This is explained by the simultaneous activation of both reactants at respectively the open metal site and the NH2-group.In a second part a mild acid treatment is proposed to increase the catalytic activity. MOFs that are subjected to this mild acid treatment show a doubling in the number of Brønsted and Lewis acid sites. These materials display a better performance in the cyclization of citronellal and the Diels-Alder cyclization reaction, which is due to the presence of Brønsted acid groups in close vicinity of the open metal sites. In a third part, new methodologies are developed to increase the activity of MOFs, illustrated by the case of UiO-66(Zr). Analogously as found for homogeneous complexes, it is shown that the use of electron withdrawing substituents on the organic linker makes the open metal sites moreactive. A positive quantitative correlation was found between the rate constant and the electronic nature of the substituent. It is also shown that the catalytic activity of the material is due to linker deficiencies. Another way to improve the performance is to exert a precise control over the amount of defects in the crystal. A monocarboxylate modulator, added to the synthesis mixturecan be competitively incorporated in the structure, replacing the ligand. This creates extra defects and Lewis acid sites on the opposite cluster. These monocarboxylates can be removed during the thermal pretreatment. These activated materials have a higher activityin the cyclization of citronellal, demonstrating the importance of this method. In a final part, the first MOF with high para-selectivity in xylene separation is discovered. Due to the specific topology of the material, p-xylene is preferentially adsorbed in the material, with high separation factors for the industrially important p- vs. m-xylene separation. This demonstrates that MOFs can offer an alternative for the currently used zeolite KBaY.status: publishe

Metal-organic frameworks as catalysts: the role of metal active sites

In this perspective we first critically compare the use of metal-organic frameworks (MOFs) as supports for catalytic functions with the possibilities offered by other classes of porous materials. We then discuss the incidental or deliberate formation of active sites in MOF lattices, and review some strategies to control the number and activity of these sites, ultimately resulting in MOF catalysts with improved performance. © 2013 The Royal Society of Chemistry.status: publishe

An amino-modified Zr-terephthalate metal-organic framework as an acid-base catalyst for cross-aldol condensation

After controlled pretreatment, some Zr-terephthalate metal-organic frameworks are highly selective catalysts for the cross-aldol condensation between benzaldehyde and heptanal. The proximity of Lewis acid and base sites in the amino-functionalized UiO-66(NH(2)) material further raises the reaction yields.status: publishe