Photoactivity and chemical reactivity in titanium(iv)-organic frameworks

Resumen

This thesis targets the synthesis of homo and heterometallic Titanium-Organic Frameworks and the study of their stability, catalytic and photocatalytic properties. This dissertation highlights the importance of a careful examination the different factors in the synthesis to obtain stable and highly crystalline materials and the impact of their structure and chemical composition on their performance in applications of environmental relevance. Chapter 1 provides an overview of the different homometallic and heterometallic Titanium-Organic frameworks reported to date, their structural variety and the different synthetic strategies used to create them. Special attention is given to the chemical and physical properties that control their performance in photocatalytic processes as the H2 evolution reaction (HER) or CO2 reduction. Chapter 2 describes the systematic study of the different experimental variables relevant to the synthesis of this family of materials by using high-throughput (HT) methodologies. This approach is exemplified by the synthesis of a mesoporous Ti-MOF with the structure of the archetypical MIL-100 family. MIL-100(Ti) can be reproducibly synthesised with different Ti(IV) precursors with high crystallinity. It displays good stability in water and photocatalytic activity due to the presence of photoactive Ti3(µ3-O) nodes. Chapter 3 explores the chemistry of titanium frameworks beyond conventional carboxylate connectors. By using a similar synthetic approach, automated HT chemistry coupled to multiple metal precursors, we succeeded in isolating MUV-11 (MUV = Material of the Universitat de València). MUV-11 is a homometallic Ti-MOF built from siderophore-type hydroxamic acid linkers. It displays a layered chiral structure with excellent chemical stability due to the strong Ti-hydroxamate chelates. We also use a combination of photoelectrochemical and theoretical studies to establish the effect of this and other types of linkers on the electronic structure and the phenomenon of charge transfer in titanium frameworks, both relevant to their photocatalytic activity. Chapter 4 reports the synthesis and characterization of a new family of heterometallic titanium frameworks. The inorganic core of MUV-10 materials combines Ti(IV) with divalent metals for photoactive heterometallic Ti2M2 metal-oxo clusters with good water stability. Their heterometallic structure enables engineering of their photoactivity by metal doping rather than by linker functionalization. Compared to other methodologies based on the post-synthetic metallation of MOFs, our approach is well-fitted for controlling the positioning of dopants at an atomic level to gain more precise control over the band-gap and electronic properties of the porous solid. The introduction of open-shell metals like Mn(II), is used to reduce the band-gap of the solid and improve its photocatalytic activity under visible light. Chapter 5 describes the first family of heterometallic titanium frameworks that can be prepared by direct synthesis from metal precursors and trimesic acid. MUV-101 frameworks [TiM2(µ3-O)(O2CR)6X3] (M = Mg, Fe, Co, Ni; X = H2O, OH-, O2-) combine mesoporosity (1800-2200 m2·g-1) with good chemical stability. We use these materials to exemplify the advantages of controlling metal distribution across the framework in heterogeneous catalysis by exploring their activity toward the degradation of a nerve agent simulant of Sarin gas. By using an integrative experimental/computational approach we answer one of the key open questions in the area of mixed-metal MOFs: How can the metals in the inorganic node influence each other for tuning the performance of the material? To the best of our knowledge MUV-101(Fe) provides the first example of a dual metal transition state in heterometallic MOFs that enables clear understanding of the individual roles played by the metals combined and their mutual cooperation. We are confident our results represent an excellent platform to guide the design of other heterometallic frameworks and span the interest of this family of MOFs to a broad scope of cascade or tandem reactions in which synergetic catalysis might yield unprecedented boosts in performance.