Development of Multifunctional Metal-Organic Frameworks for Catalytic Applications

Metal-organic frameworks (MOFs) have emerged as a highly versatile and tunable platform for the design of structurally uniform solid catalysts. Highly active multifunctional MOF catalysts have been developed via a number of novel post-synthetic strategies, including functionalization at MOF linkers/ligands, functionalization at MOF SBUs, and entrapment of catalytically active species (e.g. metal nanoparticles, metal complexes, and organic molecules) in MOF pores, channels, and cavities. The resultant multifunctional MOF catalysts have been used to catalyze a broad scope of reactions. This dissertation focuses on the design and synthesis of multifunctional MOFs for several catalytic reactions, including reductive reactions, Lewis acid promoted reactions, and photocatalysis.The first part of this dissertation (Chapter 2 and Chapter 3) discusses MOF-promoted C-O bond cleavage. In Chapter 2, we showed that Ti8-BDC works as an excellent platform to support a single-site Ni-H catalyst for aryl ether hydrogenolysis. The C-O bond linkages of the model compounds for typical bonds in lignin were selectively cleaved by the Ni-H catalyst to produce aromatic molecules as well as cyclohexanol under mild conditions. In Chapter 3, we further developed a multifunctional aluminum MOF-based catalytic system (1-OTf-PdNP) for one-pot tandem C-O bond cleavage. This tandem catalysis overcomes the strong C-O binding energy and transforms biomass into valuable hydrocarbon fuels. The synergistic Lewis acids and Pd NPs in the MOF showed outstanding catalytic activities in the C-O bond cleavage of a broad scope of ethers, alcohols, and esters to exclusively generate deoxygenated saturated alkanes under relatively mild conditions. The second part of this dissertation (Chapter 4) focuses on photocatalytic organic synthesis. While the multifunctional catalytic system in Chapter 3 showed outstanding performance in tandem catalysis for C-O bond cleavage, we further utilized the same MOF platform to incorporate Ir-based photosensitizers (Ir-PSs) to construct a bioinspired MOF-based photocatalytic system. Strongly Lewis acidic sites and Ir-PSs sites were installed in 1-OTf-Ir for reductive cross-coupling reactions between N-hydroxyphthalimide (NHP) esters or aryl bromomethyl ketones with vinyl- or alkynyl-azaarenes to afford functionalized azaarene derivatives. The third part of this dissertation (Chapter 5 and Chapter 6) further discusses photocatalysis with MOFs, specifically photocatalytic hydrogen evolution reaction (HER), an important half reaction of water splitting. In Chapter 5, we hierarchically assembled photosensitizers and catalytic sites in Ti3-BPDC. The proximity of multiple photosensitizers to the catalytic Ti3(OH)2 SBU facilitates multielectron transfer upon photoexcitation, leading to 80-fold enhancement in HER activity over the corresponding homogeneous controls. In Chapter 6, we developed a two-dimensional Ce-based MOF in order to overcome the shortcomings of MOF materials in photocatalysis. The reduction of one dimension reduces light scattering and allows free diffusion of reactive species throughout the framework. The proximity of PSs to the catalytic Ce6 SBUs facilitated electron transfer upon photoexcitation, leading to 8.7- to 9.3-fold enhancement in HER activity over their homogeneous controls