Shaping Hierarchical Porosity Beads from Metal–Organic Framework Powders Using Freeze Spherification

Abstract

Microporous powdered Metal–Organic Frameworks (MOFs) shaped by conventional mechanical densification methods (e.g., pelletization, granulation) present internal diffusion limitations and pressure drops that limit their technological leap to industrial applications. Therefore, the ongoing search for novel shaping methods that could manufacture controlled hierarchical porous structures with enhanced diffusion, high surface area, crystallinity, and mechanical robustness is needed. In this work, a freeze spherification method is developed to shape MOF-808 powder into spherical beads with hierarchical micro/macro porosity. The optimum spherical bead composition, in terms of suspension solids loading (10, 20, or 30 vol %) and hydroxypropyl methylcellulose (HPMC) binder content (30, 25, and 20 wt %), is determined by considering the bulk density, MOF surface accessibility, and the formation of a highly permeable porous structure in the final beads. The MOF-808 in powder and spherical bead forms is characterized by PXRD, IR, TGA, SEM, N2physisorption, and mercury porosimetry. The analyses confirm the formation of hierarchically ordered porosity with varying pore sizes, morphologies, and MOF coating degrees, which depend on both suspension solids loading and binder content. The combination of MOF-808 with the HPMC binder does not affect its crystallinity, but a reduction of the binder content of 30 vol % beads from 30 to 25 wt % is key to retain 86% of the MOF-808 surface area. Noteworthy, the beads processed under optimum conditions exhibit sufficient mechanical integrity and superior accessibility, highlighting freeze spherification as a promising shaping method to bring powdered MOF-based catalysts and adsorbents to real-world applications.