Postgraduate Seminar Presentation : Pelletization of Metal-Organic Frameworks for Natural Gas Storage

Speaker Bhuvankumar Bharatkumar Shah (Supervisor: Dr Zhao Dan)

Host Department of Chemical and Biomolecular Engineering

Date/Time 23 Nov - 23 Nov, 4.00pm

Venue E5-02-32 , Faculty of Engineering, National University of Singapore

Synopsis

MOFs are typically obtained in loose powder forms by conventional synthesis techniques, with low packing densities which pose major engineering challenges such as pressure drop through packed beds, dustiness, clogging, abrasion, mass loss, and handling issues. We have proposed pelletization as an easy shaping technique to solve this problem with minimum material loss. In this work, we studied pelletization and its effects on properties of MOFs for potential natural gas storage applications. While the rigid MOFs such as HKUST-1 and UiO-66-NH2 are easy to pelletize at very low compaction force (~1 kN), they undergo loss of crystallinity and porosity and are not ideal materials for high methane deliverable capacities (difference in uptake between 65 and 5.8 bar) owing to significant retention of gas at 5.8 bar during desorption. In comparison, functionalized flexible MOFs viz., MIL-53-X (X = NH2, OH, (OH)2) show high uptake at 65 bar and negligible retention at 5.8 bar because of gating behaviour but are difficult to pelletize at any compaction force between 1–80 kN. However, they retain their crystallinity and porosity even after 80 kN force. To solve this problem, different classes and compositions of binders ranging from colloidal silica to glassy polymers such as PMMA (poly (methyl methacrylate)) and PIM-1 (polymers of intrinsic microporosity) to rubbery polymers such as PEG (polyethylene glycol) to rigid MOFs such as UiO-66-NH2 were tested. While rubbery polymers are ineffective in binding the powder, mechanically strong tablets are achieved using colloidal silica and glassy polymers at a very low compaction force (~2kN) and 10 wt% concentration. And, rigid MOFs are only effective at very high concentrations (~40 wt%). However, the pellets, in case of colloidal silica and PMMA, become either nonporous due to blocking of pores by the binder molecules or disintegrate into powder during low pressure N2 sorption at 77K. The disintegration of pellets is attributed to the phase transitions (from NP to LP phase) associated with flexible MOFs. Although the MOF crystals are also covered with binder molecules in case of PIM-1, due to its porous nature, the pores of MOFs remain accessible to gaseous molecules. Thus, PIM-1 has been proves to be the right binder choice for pelletizing flexible MOFs. To effectively solve the problem, further optimization of pelletization parameters to enhance the mechanical stability, and flexible linkage between PIM-1 molecules to allow elastic expansion and contraction of the pellet without disintegration during sorption are proposed as future directions.

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