Controlled surface growth of iron metal-organic frameworks and electrochemical synthesis of metal organophosphonate and porous aromatic frameworks
Date
2021
Authors
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Publisher
University of Delaware
Abstract
Conventional solvothermal synthesis of metal organic frameworks (MOFs) typically involves reactions under high temperature and pressure. By introducing electrolysis into the system, shorter synthesis times and milder synthesis conditions can usually be approached. With the inspiration of two well-known electrolysis methods, namely anodic dissolution and reductive deprotonation, the formation of MOFs was facilitated by directly oxidizing dissolved metal cations in solution via constant potential electrolysis. In this manner, Fe2+ in DMF solvent was oxidized to Fe3+ near ITO electrode surfaces to form Fe-MIL-101 and Fe-MIL-101-NH2 thin layers within as little as 30 minutes at room temperature. The crystallinity and phase purity of the generated MOF films were further confirmed by powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM). The products from electrochemical synthesis also demonstrated a similar quality compared to materials generated through solvothermal methods in terms of surface areas assessed by nitrogen adsorption isotherms. ☐ Under solvothermal conditions, Fe–MIL–88 and Fe–MIL–101 are often generated as competing phases. However, when electrochemically synthesized via direct metal ion oxidation, the favored structure type of the products could be entirely switched between Fe-MIL-101 and Fe-MIL-88 by adding or eliminating water in the solvent. A series of Fe-MIL-88 materials made of the same structure containing but various lengths of ligands were synthesized electrochemically and characterized by PXRD, nitrogen adsorption measurement, XPS and SEM. The effect of the water content was also investigated, where a growth of the particle size was observed along with increasingamount of water under microscope. The feasibility of this approach to grow MOFs on other electrode substrates such as nickel films, graphite plates, and even more complex, carbon foam, was confirmed as well. ☐ Due to the more versatile binding modes, metal organophosphonates, compared to conventional carboxylate-based MOFs exhibit a remarkable structural diversity, while the challenges for construction of the framework structures were also more significant. Consequently, the electrochemical synthesis of metal organophosphonate materials was studied. A series of metal organophosphonates were successfully synthesized via anodic dissolution electrolysis, containing Cu2+, Zn2+, Ni2+ or Al3+ in their building units and benzene–1,4–diphosphonic acid (BDP), biphenyl–4,4’–diphosphonic acid (BPDP), 1,3,5–tris–(4–phosphonophenyl)benzene (TPPB) or 2,4,6–tri–(phenylene–4–phosphonic acid)–s–triazine (TPPT) as their organic linkers. Their structure features including crystallinity, porosity, stability and surface morphology were analyzed by physical techniques such as PXRD, gas adsorption, TGA, XPS and SEM. ☐ Another type of porous polymer networks synthesized in this work was porous aromatic frameworks (PAFs). The traditional synthesis usually comprises Yamamoto coupling reactions and requires stoichiometric amounts of bis(1,5-cyclooctadiene)nickel(0) (Ni(COD)2). An electrocatalytic synthesis route was proposed with catalytic amounts of Ni(COD)2 and PAF-1 with a Langmuir surface area of 1676 m2/g was successfully produced. The NiX2 species cogenerated during the disproportionation process in aryl-aryl coupling was recycled by applying a reductive potential to the reaction. It was verified by XPS that the reduced surface area of the product from electrocatalytic synthesis could be attributed to the residual terminal bromo groups in the framework. In spite of unreacted halogen content, this synthetic methodology is still a promising candidate for the preparation of different kinds of porous aromatic frameworks.
Description
Keywords
Electrochemical synthesis, Metal organic frameworks, Metal organophosphonates, MIL-101, MIL-88, Porous aromatic frameworks