(303f) Flowability Improvement of Metal-Organic Framework (MOF) Particles Via Pressure Swing Granulation with Small Amount of Binder

Soft metal-organic frameworks (soft-MOFs) are new class of porous crystals formed through coordination bonding of metal ions and organic ligands. Compared to conventional porous materials, soft-MOFs have regularly arranged micropores, large specific surface area, and structural flexibility. The most important feature of soft-MOFs is their adsorption behavior induced by the structural transition. The soft-MOFs show almost no adsorption at low pressure, while the amount of adsorption increases rapidly at a certain pressure. Taking advantage of this property, soft-MOFs are expected to have a wide range of applications, such as adsorbents, separators, catalysts, and sensors. However, the high adhesiveness and poor flowability of synthesized soft-MOFs hinder the industrial application.

One of approaches to overcome this problem is to establish a granulation process for soft-MOF particles. Granulation process is expected to improve flowability and reduce pressure loss in the particle packing layer. Our group has investigated the wet granulation with binder using agitator granulators and fluidized bed granulators. However, previous studies have suggested that the presence of binders in the granules reduced adsorbed amounts in soft-MOFs due to weight of binders. Also, the external mechanical force generated during the granulation process led to a significant decrease in adsorbed amounts. The formation of agglomerate structures with low bulk density, i.e., porous structures, through the binder-less granulation is required to address the above issues.

This study focused on the pressure swing granulation (PSG) method. In the PSG method, compressed gas is injected into the powder layer in the fluidized bed from above and below. The agglomerates then are formed by repeated crushing, fluidization, and consolidation. The van der Waals forces, which are universally possessed by fine powders, and electrostatic forces associated with particle charging should contribute to PSG. The PSG method utilizes the adhesion properties of fine powders, which allows the granulation of fine particles with a small amount of binder and few external mechanical forces. It is also expected that agglomerates can be formed while maintaining the adsorption performance of the original powder. The PSG method has a lot of operating parameters, such as cross-sectional flow velocity, consolidation pressure, crushing pressure, flow time, consolidation time, crushing time, and oscillating pressure. However, it is still insufficient to investigate which parameters affect granule properties.

In this study, therefore, as a preliminary study for soft-MOFs granulation experiments, inexpensive calcium carbonate particles was granulated with PSG method to investigate the effects of operating parameters on the PSG process. Then, utilizing the knowledge obtained from calcium carbonate granulations, ZIF-8, a typical soft-MOF, was granulated with PSG method to evaluate adsorption properties and flowability.

This study revealed that the flowability of the granules improved with an increase in consolidation pressure and the binder concentration. It was also suggested that the too high vibration pressure promoted the crushing of agglomerates, decreasing the flowability of the granules. The number of consolidation-crushing-flow cycles, was also a major factor to improve the flowability. Moreover, based on the operating conditions in calcium carbonate granulation, ZIF-8 particles were successfully granulated with PSG method while maintaining the adsorption amount.