Selection of binder recipes for the formulation of MOFs into resistant pellets for molecular separations by fixed-bed adsorption

The formulation of metal-organic frameworks (MOFs) materials into resistant structures is a key step towards their use in adsorptive separation processes. In this study, ZIF-8 crystals have been formulated into resistant pellets with various binders by a simple extrusion-crushing-sieving (ECS) approach. 55 recipes were evaluated with a series of stability tests - mechanical stability upon friction, acid/base stability, hydrothermal stability and long-term stability for storage over several years - as well as with a set of adsorption and separation experiments. The stability tests have highlighted that PVF (polyvinylformal) is the most promising binder to generate resistant pellets with ZIF-8 crystals for the adsorptive recovery of n-butanol in the acetone-butanol-ethanol (ABE) fermentation process, while other binders, including methylcellulose, may find their use in gas/vapor adsorption processes. Adsorption of ethanol vapor has shown that the formulation results into a decrease of adsorption capacity, which is proportional to the binder fraction, while having no effect on the adsorption kinetics. On the other hand, packed bed experiments in liquid phase have indicated that ZIF-8 composite materials maintain the separation ability of the unformulated ZIF-8, but also that the binder results into an unfavorable additional mass transfer resistance. These results have been supported by complementary porosimetry measurements with Ar and Hg, indicating that the composite materials are very porous with narrow pore-size distributions. From this experimental work, various formulation recipes have been identified, which may lead to resistant MOF composite materials for adsorptive separations.