Investigations of Electron Transport Properties in Metal-Organic Frameworks for Catalytic Applications

Metal-organic frameworks (MOFs) have attracted much attention in the past few decades due to their ordered, crystalline nature, synthetic tunability, and porosity. MOFs represent a class of hybrid inorganic-organic materials that have been investigated for their applications in areas such as gas sorption and separation, catalysis, drug delivery, and electron or proton conduction. It has been the goal of my graduate research to investigate MOFs for their ability to transport electrons and store and separate gases for ultimate catalytic applications in alternative energy generation. I aim to provide new insight into the design and development of stable MOFs for such applications.

We first investigated a cobalt(III) porphyrin based MOF comprised of Co(II)-carboxylate nodes, designated as CoPIZA, for its electron transport capabilities. Thin films of CoPIZA were formed solvothermally on conductive fluorine-doped tin oxide (FTO) substrates and used for electrochemical characterization. Electrochemistry coupled with spectroscopic analysis of the CoPIZA film revealed reversible reduction of the cobalt centers of the porphyrin linkers with maintenance of the overall framework structure. The mechanism of charge transport throughout the film was facilitated by redox hopping of electrons between the metal centers of the nodes and linkers.

The ability to incorporate desired properties, such as pore functionalities or open metal centers, into frameworks makes them attractive for applications in separation of gaseous mixtures, such as CO2/N2 from combustion power plants. To investigate the selective adsorption properties, we performed gas sorption measurements on bulk MOF materials to determine their affinity toward CO2. Two Zn-based MOFs containing 2,5-pyridine dicarboxylate linkers were prepared in our laboratory and contained unsaturated Zn(II) metal centers, which possess a binding site on the metal without an activation procedure to remove bound solvent molecules. These MOFs were compared to the well-known Zn-based MOF-69C containing 1,4-benzene dicarboxylate linkers. Thermodynamic analysis of the gas sorption data revealed that the mechanism of CO2 binding involved the coordinatively unsaturated Zn(II) center. The microporous MOF also demonstrated selectivity for CO2 over N2 under the same conditions. As these materials were able to uptake CO2, their ability to transport electrons was also investigated for ultimate applications in catalysis. Electrochemical impedance spectroscopy was performed on the bulk MOF powders and was coupled with solid-state nuclear magnetic resonance spectroscopy. These results determined that the conduction mechanism proceeded via solvent molecules within the pores of the framework.

The catalytic ability toward water oxidation of two MOFs was investigated electrochemically. Initial studies focused on a cobalt-based MOF comprised of 2-pyrimidinolate (pymo) linkers, designated as Co(pymo)2, which was prepared on FTO via drop-casting and used for electrochemical experiments. At applied anodic potentials, the CoII centers of Co(pymo)2 became oxidized to form a Co-oxide species on the electrode surface, which was found to be the active catalysis for water oxidation. Further investigations utilized a notably more stable Zr-based MOF with nickel(II) porphyrin linkers, designated as PCN-224-Ni. PCN-224-Ni was prepared solvothermally on FTO and used directly for electrochemical water oxidation. The mechanism of water oxidation at PCN-224-Ni proceeds via oxidation of the porphyrin macrocycle followed by binding of water to the Ni(II) center. Cooperative proton transfer to the Zr-oxo node facilitated water oxidation with the eventual release of O2. Thorough characterization revealed that PCN-224-Ni retained its structural integrity over the course of electrochemical catalysis.

These results have allowed us a deeper understanding of the mechanisms of electron transport and conduction throughout frameworks. Specifically, the incorporation of metalloporphyrin molecules with redox active metal centers coupled with the presence of redox active metal nodes resulted in redox hopping charge transport throughout the MOF. In addition, the presence of solvent molecules in the pores of the framework provided an extended network for charge transport. We have gained insight into the structure-function relationship of MOFs for applications in selective gas sorption, where an unsaturated metal center serves as the binding site for gas molecules. Finally, through selection of the components that comprise the framework, a stable metalloporphyrin MOF was found to be capable of electrochemically facilitating the water oxidation reaction. As a result, we have gained valuable insight into the properties of frameworks necessary for charge transport and stability, which will allow for further improvements in the smart design of MOFs for catalytic applications.