Designing and understanding of extra-framework species in metal-organic frameworks

Abstract

As a new class of hybrid materials, metal-organic frameworks (MOFs) present advantages in various applications, such as gas separation, sensing, and catalysis. MOFs comprise metal nodes and organic linkers, which offer a wide range of structural diversities and chemical activities. MOFs are useful in many applications because of the widely tunable porous structures and customizable active sites. Notably, extra-framework species can be readily incorporated into MOFs to offer unique chemical properties via post-synthetic modification. For instance, the Lewis basic amino group of a Zr-based MOF (UiO-66-NH2) can be functionalized to stabilize Lewis acidic metal species. However, the characterization of the interactions between MOFs and extra-framework species, and the elucidation of the structure-activity relationship remain challenging. The thesis focuses on the fundamental and structural study of the interactions between MOFs and the extra-framework species and by primarily using X-ray absorption spectroscopy (XAS) and Rietveld refinement of high-resolution X-ray powder diffraction (PXRD) data. The structure-activity correlations of the MOF structures and corresponding applications have been established based on the comprehensive investigation. In the first part, based upon diffraction and computational evidence, the guest-anion­induced photoluminescence enhancement of UiO-66-NH2 is studied. The interactions between different anions and the photoluminescence property have been revealed. By carefully studying the refined crystal structures and structural parameters, it has been found that the guest carbonate and fluoride ions interact with four framework amino-functional groups through hydrogen bonding that ultimately forms a quaternary (­N(H))4...X- molecular bridge around the nodal center. In the second part, we have utilized UiO-66-NH2 to stabilize mid-to-late 3d transition metal ions to act as 'single-atom catalysts' (SACs). The geometric and electronic properties of the SACs within MOFs have been studied. A series of transition metals-SACs within the microporous cavity of UiO-66-NH2 have been prepared. The electronic and geometric properties of the SACs with respect to electrocatalytic CO2 reduction have been investigated. Despite only being demonstrated in selected SACs within UiO-66-NH2, this study sheds light on the rational engineering of molecular interactions with SACs for the sustainable provision of fine chemicals. At last, we have developed a new assembly approach to synthesize atom-precise metal­oxo clusters, such as binuclear Cu2-oxo and trinuclear Cu3-oxo supported on the nodal center of UiO-66-NH2. Approaching the atomic and molecular frontiers, the structure and surface sensitivities have been found to significantly affect the product selectivity in the photocatalytic formic acid reforming reaction. Through advanced structure elucidation and computational simulations, the interplay between the surface structures of the active sites, adsorption configuration, and catalytic reactivity of structurally related Cu 'single atoms', bi-, and trinuclear species has been revealed. We have demonstrated that the control in metal nuclearity can affect the product selectivity by altering the adsorption modes and the associated reaction energetics. In brief, this thesis describes a series of related structure-reactivity studies of extra-framework species within UiO-66-NH2. By our combined structural approach, the chemical activities, including optical and catalytic properties, induced by the introduction of extra-framework species can be investigated by evaluating the three-dimensional atomic and crystal structures.