Highly Robust MOF Polymeric Beads with a Controllable Size for Molecular Separations.

Abstract

Shaping metal-organic frameworks (MOFs) into robust particles with a controllable size is of large interest to the field of adsorption. Therefore, a method is presented here to produce robust MOF beads of different sizes, ranging from 250 μm to several millimeters, which, moreover, preserve the adsorption properties of the unformulated MOF. A simple, mild, and flexible method is demonstrated with the zeolitic imidazolate framework-8 (ZIF-8)/polyvinyl formal composite material. The properties of the composite material are determined via optical imaging, scanning electron microscopy, energy-dispersive X-ray spectroscopy, inductively coupled plasma mass spectrometry, X-ray diffraction, mercury intrusion, argon porosimetry and pycnometry as well as thermogravimetric analysis/differential scanning calorimetry, crush strength tests, and immersion experiments. The proposed method allows the production of resistant particles with a high MOF loading (up to 85 wt %) and remarkable structural and textural properties required for adsorptive separation processes, including a preserved ZIF-8 crystalline structure, microporosity, and a narrow macropore size distribution (1.27 μm average). The particles show a spherical shape with an average aspect ratio of 0.85. The stability tests demonstrated that the composite MOF material exhibits a high mechanical strength (3.09 N/Pc crushing strength) almost equivalent to that of a widely used commercial zeolite material. Furthermore, the material remains stable up to 200 °C and in most solvents. The adsorption properties are explored via static and dynamic experiments in the vapor and liquid phases. The results show that the adsorption capacities are only reduced in proportion to the binder content compared with the pristine material, indicating no binder intrusion in the ZIF-8 pores. Fixed-bed experiments demonstrate the remarkable separation performance in the vapor phase, whereas mass transfer limitations arise in the liquid phase with increasing flow rate. The mass transfer limitations are attributed to the diffusion in the macropores or through the ZIF-8 crystal outer layer.