Shaping of MOFs via freeze-casting method with hydrophilic polymers and their effect on textural properties

Abstract Shaping of metal-organic frameworks (MOFs) and tuning of their sorption properties is a very important and challenging aspect for the usage of MOFs for many applications. Both shaping and tuning can be addressed by composite preparation as an effective method. Herein, we present the shaping of the hydrothermally stable MOFs aluminum fumarate, MIL-160(Al) and MIL-101(Cr) using the freeze-casting method with the hydrophilic polymer binders polyacrylic acid (PAA), sodium polyacrylate (PAANa), polyethylene glycol (PEG), polyethylene imine (PEI), polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP). Furthermore, the effect of in-situ cross-linking below the freezing point (cryopolymerization) on the textural properties of monoliths was also investigated using MIL-101(Cr) and aluminum fumarate with the polymer PVA. The selected MOFs were chosen due to their outstanding stability, sorption properties and differing hydrophilicities. In the obtained mechanically stable monoliths, each MOF showed a different compatibility with the used polymers. From 21 different MOF@polymer composites with 80 wt% loading of MOF, 12 of them exhibited negligible pore blocking effects and very high nitrogen sorption properties in correspondence with the MOF mass fraction. On the other hand, water uptake is especially in cases of MOF@PVA and MOF@PVP in good agreement with the MOF fraction and can be enhanced by the contribution of the hydrophilic polymer to the composite. In particular, most MIL-101(Cr) composites showed a hydrophilic shift to lower relative pressure p·p0−1 compared to the rather hydrophobic neat MIL-101(Cr). IR spectroscopy indicated a clear interaction of PEI with the Al-MOFs aluminum fumarate and MIL-160(Al) resulting in MOF pore blocking. The molecular weight of the polymer is not significant for the freeze-casting method but plays an important role in the cryopolymerization.