Zr-based metal-organic frameworks with tailored linkers: Immobilized liquids for sorption, extraction and ionic conduction
Final Report Abstract
Our work was devoted to Zr-based metal organic frameworks (Zr-based MOFs), especially to those with two interpenetrating frameworks of the UiO-type, the so-called PIZOFs. This family of MOFs was found to be highly variable in respect to the substituents at the linkers. It is also variable in respect to the length of the linkers, albeit in a rather narrow regime. With shorter diacids, a non-interpenetrated MOF with a UiO-type network is formed, with longer diacids, the degree of interpenetration increases to five. The PIZOFs that are most stable against water are those formed from PEPEP diacids. These PEPEP-PIZOFs stand out, even among the rather resilient Zr-based MOFs, with regard to their resistance to hydrolysis in water and drying from the water-wetted state - and thus fulfill an important requisite for application. A modular approach was set up for assembly of linear dicarboxylic acids differing in length from ca. 10-24 Å (distance between the carboxylic C atoms) and the type and number of substituents. Whereas the length primarily determines the pore size and the degree of interpenetration, and probably the tendency for defect formation, the type and number of substituents allow to tailor the pore interior for concrete applications: hydrophobic, hydrophilic, amphiphilic, fluorophilic, and more or less filled pores, pores with ligands for metal ions, pores with functional groups such as alkyne, amino, hydroxy, and sulfonic acid. The pore interior cannot only be tailored by substituents already attached to the building blocks that are used for MOF assembly, but as well by postsynthetic modification (PSM). We have shown that PIZOFs can be modified via Diels-Alder reaction, azide-alkyne cycloaddition, and nitrile oxide-alkyne cycloaddition. We regard the latter as a valuable extension of the PSM toolbox, considering the wide chemical variability of the nitrile oxide, the mild reaction conditions, and especially the simple purification of the modified MOF. This reaction is expected to be applicable to other hydrolytically stable MOFs. It has been possible to form a PIZOF from most of the PEPEP diacids tested. However, Zr source and type of modulator, reaction conditions and reaction processing had to be adjusted judiciously. The basis of the success was the application of the modulation approach which was developed within this priority program. This approach was studied intensely using commercially available diacids, especially fumaric acid, employing for example kinetic studies and NMR studies on synthesis batches after reaction. Whereas the general concept of coordination modulation could be supported, it was also shown that additional reactions like amide formation proceed within the synthesis solutions. Sorption measurements, accompanied by modeling studies, show that with increasing length of the side chains of PEPEP linkers, the pore volume becomes smaller until ca. 70% of the pore space is filled by the side chains which lends favour to the idea of an immobilized liquid. Water sorption studies proved the possibility to tailor the hydrophilicity of the pores; the kind of the side chains and their length influence the interaction with guest species and can be chosen according to the requirements of the desired application. Proton conducting ionic liquid (PIL) was introduced into the PIZOF pores. It could be demonstrated that the PIL-PIZOF system shows high proton conductivity under anhydrous conditions. Even more importantly, due to strong interactions with the side chains of the linkers, the PIL is not leaching out in the presence of water. Thus, we see interesting options for the application of PIL-PIZOF systems in fuel cells or other systems (e.g. sensors) requiring ionic conductivity.
