Bimetallic Active Sites In Metal-Organic Frameworks: Harnessing Bimetallic Cooperativity For Chemoselective Transformations

Abstract

Heterogeneous catalysts featuring multiple metal centers account for a significant fraction of US GDP, and new ways to enhance them would have enormous economic, scientific and technological impact. The availability of several modulating sites coupled with the ability to precisely tune the selectivity has enabled the usage of multi-metallic catalysts for a wide array of industrial transformations. Critical to the challenge in engineering such active sites, is to understand and predict how catalysts function at the atomic or molecular level. An emerging approach that facilitates the systematic study of such systems is the installation of molecular precursors onto three-dimensional crystalline supports such as metal-organic frameworks (MOFs). In this vein, a generalized solution-phase-deposition technique was developed wherein pre-assembled bimetallic precursors were deposited onto the MOF NU-1000, such that two metals of choice were delivered in a controlled manner. Rigorous spectroscopic and computational investigations confirmed that the bimetallic active sites were structurally well-defined and site isolated. An important outcome of this study was the identification of a rhodium-gallium (Rh–Ga) bimetallic catalyst for the chemoselective semihydrogenation of alkynes to alkenes. In the absence of the supporting Ga ion, over-hydrogenation to the alkane was observed by the Rh-only analogue. By quantitatively analyzing the initial rates, transition state properties, and thermodynamic profiles, it was concluded that Ga acts as an electronic and structural promoter. Finally, as an extension of this unique strategy, thermally robust Co–M (M = Al, Mn or Co) active sites, prepared by silica nanocasting of NU-1000, mediated industrially relevant chemical transformations such as oxidative dehydrogenation of propane to propylene and oxidation of alcohols. Overall, the design principles developed in this thesis provide compelling solutions to generate ‘multi-metallic single sites’ and add to the arsenal of existing metalation strategies.