(403g) Electrochemical Synthesis and Polymorphism Control of Porphyrin-Based Zirconium (Zr) MOFs

Zirconium Metal-Organic Frameworks (Zr-MOFs) have widely been applied in the field of separations, storage, and a wide range of catalysis. These MOFs are mainly deployed as thin films for these applications while conserving their physical properties like porosity, surface area, size etc. In addition to the physical properties, it is also essential to optimize the film characteristics like thickness, grain boundaries, physical adhesion, and overall mechanical strength. Efforts in the past two decades have been made to develop various top-down and bottom-up approaches to synthesize thin films that suit the criteria for any desired field of application. While top-down approaches are straightforward, the synthesized films lack the necessary adhesion and moreover have limitations when it comes to scalability. Bottom-up approaches have proven to yield well-structured and strong substrate-bonded films. In this work, a novel electrochemical synthesis pathway of various Zr-MOFs (PCN, NU systems) has been developed. Even though these systems have a common Zr6 node and organic linker, the differences in the node coordination number lead to a wide range of polymorphs, especially in the PCN system with the Tetrakis(4-carboxyphenyl)porphyrin (TCPP) linker . The processing conditions (electrolyte composition, current density, temperature etc.) govern the regime in which the reaction is occurring i.e whether the reaction is thermodynamically controlled or kinetically controlled. Precise control of the synthesis conditions yields the pure phases of each polymorph. This study hypothesizes a reaction pathway leading to changes in Zr6 node coordination number which is further supported by a theoretical study on trends of free energies of deprotonation during the intermediate reactions involved (modulator addition & exchange, hydration and ligand addition). This combined approach gives us a clear insight into the reaction pathways leading to specific polymorph, which is a significant contribution to understanding these systems, which has been a major gap in this field.