Synthesis and characterization of a versatile titanium-based metal-organic framework for energy and environmental applications

Abstract

Innovative materials are crucial for addressing many current global sustainability challenges, such as transitioning to green fuels for renewable energy carriers, removing troublesome pollutants for clean water accessibility and minimizing frictional losses for efficient mechanical processes.Advancements in these demanding fields require materials science to move away from purely trialand-error approaches and focus on purposeful material development considering structurefunctionality relationships for targeted and rational materials design.Metal-organic frameworks (MOFs) have been established as next-generation materials due to many key features: they combine molecular organic and inorganic components, forming porous and crystalline three-dimensional structures. By integrating the synthetic precision of organic chemistry with the structural and compositional versatility of inorganic secondary building units (SBUs), they enable properties that surpass those of their individual components, such as high surface area,hybrid electronic structure and unparalleled tunability. Their excellent properties have commended MOFs as highly promising candidates for overcoming many limitations traditional materials (e.g.,oxides) are faced with in the challenging fields of adsorption, photocatalysis and even emerging applications, such as tribology. In this regard, titanium-based MOFs (Ti-MOFs) are particularly interesting because of their hydrolytic stability and especially due to their photo-responsive behavior, which promotes the formation of long-lived excitons. These can in turn be utilized for catalytic conversions, such as the hydrogen evolution reaction (HER), one of the half reactions for water splitting, or the degradation of organic pollutants. However, the reactivity of titanium precursors makes the synthesis of such Ti-MOFs extremely difficult, limiting the number of Ti-MOF structures described to date. Considering the emergence and enhanced properties of higher dimensional SBUs shown in recent Ti-MOF reports and aiming towards enlarging the set of such materials, this thesis focuses on the controlled synthesis of a 2D SBU Ti-MOF and evaluates it as a photocatalyst towards hydrogen evolution and dye degradation and finally as a friction reduction agent. Synthesis optimizationenables control over particle size and missing ligand defect content, directly influencing surface area and porosity. However, while the introduction of defects enhances porosity and accessibility, it also introduces recombination centers and decreases lifetimes, negatively impacting the photocatalytic activity, as discussed in Article 1. Herein, a small particle size, together with the lack of missing ligand defects are identified as key characteristics towards achieving topperforming hydrogen evolution rates under visible light irradiation. Article 2 investigates the material towards water remediation, showing selective adsorption and efficient photo-oxidation of persistent organic dyes, highlighting the importance of surface termination in adsorption-controlled processes. Finally, Article 3 explores this MOF’s unique layered structure as a solid lubricant for tribology, elucidating the amorphization of the material to form of a stable and friction reducing tribofilm, providing a pioneering in-depth study on MOFs in this field. Supported by experimental data and theoretical calculations, the articles also provide mechanistic insightsinto charge transfer, extraction kinetics and hydrolytic film formation, identifying dominant reaction pathways and offering a deeper understanding of this Ti-MOF’s behavior under reaction conditions, striving for a broader understanding of other MOF-based materials in similar applications. Overall, this thesis recognizes the intricate balance between particle size, defect content and surface termination as key parameters that will likely impact MOF design strategies in the respected fields of sustainability-related applications.