MOFs beyond the framework: Investigating the role of polymeric guests, linker functionality, and structural disorder to enhance adsorption performance

Global water scarcity, intensified by natural and human-driven pollution, is among the greatest threats to human life in the coming years. Toxic ions such as arsenic, lead, and chromium demand effective purification strategies, with advanced adsorbent materials offering a promising solution for selective removal from contaminated water. Metal-organic frameworks (MOFs) are materials composed of metals or metal nodes which are interconnected by organic linkers through coordination bonds to form 2 or 3 dimensional structures. They have received a lot of research attention due to their structural diversity and exceptional tunability. In this thesis, we explore how the structural components of a MOF, particularly ligands, clusters, and the framework itself can be engineered to selectively capture contaminant ions from water. To this end, we investigate four different strategies to enhance removal performance: (I) use the framework as a porous host for redox-active polymers, (II) increase the acid stability of MOF/polymer composites by applying protective polymer coatings, (III) install ligands with functional groups that enable targeted ion binding, and (IV) engineer defects in MOFs. (Strategy I+II) In Chapters 2 and 3, we investigate strategies to polymerize monomeric units within MOF pores, with Chapter 3 further examining hydrophobic coatings to enhance stability under harsh conditions. Using electron microscopy combined with energy-dispersive X-ray spectroscopy (EDXS), we confirm that the polymers form within the MOF pores, while the coating remains on the exterior. In both chapters, we show that incorporating redox-active polymers enables the reduction or oxidation of pollutants, thereby improving the removal of targeted ions. Furthermore, the hydrophobic coating increases material stability to some extent, allowing functionality down to pH 2. (Strategy III) In Chapter 4, we use two flexible dicarboxylate ligandsâ succinic acid (SUC) and dimercaptosuccinic acid (DMSA), which differ only by two thiol groups, to investigate the influence of functional groups on targeted arsenic adsorption. Although we obtained highly crystalline materials with both linkers, the best-performing material for arsenic removal was a low particle size, amorphous Zr-DMSA MOF. X-ray absorption spectroscopy (XAS) and pair distribution function (PDF) analysis showed that rotatable dangling ligands allow the material to chelate arsenic, leading to high selectivity even when compared to phosphates. While the performance of this material was great, attempts to cycle the materials failed. Subsequently, in Chapter 5 we used the same material for the removal of lead. Investigation via X-ray photo-electron spectroscopy (XPS) revealed that Pb binds through one oxygen and one sulfur atom in the material. Compared to arsenic, the formed lead were reversible, which allowed us to study the material using different characterization methods such as PDF during cycling. (Strategy IV) Lastl