(114d) Granulation of Soft Porous Crystal Particles

Metal-Organic frameworks (MOFs) are new porous crystals consisting of metal ions and organic ligands formed thorough the self-assembly process with coordination bond in between them [1]. Remarkable features of MOFs are a regular pore structure, a larger surface area, and tunable pore size by choosing the combinations of metal ions and organic ligands. In addition to these features, specific types of MOFs show a flexibility in their structure that leads to a structural transition in response to external gas pressure, and they are also referred to as soft porous crystals (SPCs) [2]. A typical adsorption behavior of SPCs shows a step-wise increase in adsorbed amount due to the structural transition from closed non-porous structure to an open porous one at a certain pressure, which is called a gate adsorption phenomenon. This peculiar adsorption behavior is promising for a variety of applications such as gas storages, gas separations, and molecular sensors. For these applications, it is of crucial importance to control gate adsorption phenomenon. As an interesting method, controlling the particle size is a simple approach for tuning the gate adsorption phenomenon [3]. Here, because the SPC particles are usually obtained with the size of a submicron order, a low handling performance is problematic, and accordingly establishing the granulation process of SPC particles is needed for the industrial applications.

In the present study, we focused on Zeolitic Imidazolate Framework-8 (ZIF-8) which is classified as an SPC. ZIF-8 consists of zinc ions and 2-methylimidazolate linkers, and exhibits intra-framework flexibility which stems from a rotation of linkers. We employed a high shear mixing granulator to establish the granulation process of ZIF-8 particles (650 nm) with water and hydroxypropyl cellulose (HPC) as a binder. As a result, we successfully obtained the granules of ZIF-8 with the size of ca. 200 µm under the condition of 5 wt% of HPC and 42 wt% of water. BET surface areas calculated from N₂ adsorption isotherms at 77 K were 1696 and 1540 m²/g for ZIF-8 particles and the granules, respectively. Also, the reposed angle and compressibility decreased from 50.2° to 37.8° and from 42% to 7 %. These results suggest that we obtained the granules with a less loss of the adsorption capacity and improvement of the fluidity. We also investigated the effect of the concentration of HPC and the size of ZIF-8 particles on the granulation process, and the control of the granule size and the granule size dependence of the gate adsorption will be discussed.

Reference

1. S. Kitagawa et al., Angew. Chem. Int. Ed. Engl., 43, 2334 (2004).

2. S. Horike et al., Nat. Chem., 1, 695 (2009).

3. Y. Sakata et al., Science, 339, 193 (2013).