MOFs (Metal-Organic Frameworks) with anionic frameworks

• Applicant: Professor Dr. Uwe Ruschewitz
• Subject Area: Solid State and Surface Chemistry, Material Synthesis
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 555806684

The focus of the proposed project is the targeted synthesis of coordination polymers (CPs) and MOFs (metal-organic frameworks) with an anionic framework, abbreviated as anMOFs. Only systems based on aromatic carboxylate ligands analogous to the archetypal MOFs MOF-5 and HKUST-1 will be investigated. Based on database research, metal cations were identified (In3+, Ga3+, UO22+) that preferentially form anMOFs. Based on this, a "synthesis recipe" was developed that allows anMOFs to be obtained with a relatively high probability in a specific synthesis. In addition to the suitable metal cations, which should ideally be added as Lewis acids, this recipe includes the use of the solvents DMF, DEF or DMA - acidic hydrolysis provides the charge-compensating R2NH2+ cations - as well as the addition of a strong mineral acid such as HCl(aq). Based on this recipe, further anMOFs shall be synthesised and characterised. Furthermore, the concept is to be transferred to cations such as Mn2+ or Cd2+. Since the concept is still limited to only a few metal cations, linkers have been identified that also allow anMOFs to be obtained with other metal cations. These are mixed sulfonate-carboxylate ligands and linkers based on a tetraphenylborate framework. The synthesis of new anMOFs with these linkers will also be addressed in the project. An essential prerequisite for possible applications is the water stability of the anMOFs obtained. Here we were able to achieve very good results with UO22+-based anMOFs. Some of these show very promising cation exchange properties for radioactive 137Cs+. In addition to a high Kd value, fast exchange (< 5 min), high selectivity towards Na+ and K+ cations as well as good regenerability should also be mentioned here. In the proposed project, we want to optimise these properties further by modifying the linkers with suitable substituents. Our results to date indicate an electronic influence of the substituents (I vs. +M effect). These results with UO22+-based anMOFs are to be transferred to anMOFs with non-radioactive cations such as In3+ or Ga3+. The work is to be supplemented by investigations of Li+/Na+ ion conductivity. Preliminary investigations show that these cations can also be easily incorporated into the anMOFs by exchanging them for R2NH2+ cations. A methodological focus of the proposed project will be the localisation of cations in the pores of anMOFs, which is often hampered by disorder or a lack of long-range order. Here we want to use optimised methods of X-ray structure analysis (with synchrotron radiation complemented by calculations of Hirshfeld surfaces) and PDF analysis (pair distribution function). A suitable X-ray powder diffractometer for PDF analysis with Ag radiation will be available to us from the end of 2024. It has been approved via a large-scale equipment application.

• DFG Programme: Research Grants