Inherent porosity of Zeolitic Imidazolate Framework-62 melt leading to formation of the porous melt-quenched glass

• Published in: Microporous and mesoporous materials 2025-01, Vol.382, p.113387, Article 113387
• Main Authors: Nelliyil, Renjith B., Mor, Jaideep, Liedke, Maciej Oskar, Butterling, Maik, Hirschmann, Eric, Wagner, Andreas, Jayakrishnan, V.B., Sharma, Sandeep Kumar
• Format: Article
• Language: English
• ISSN: 1387-1811
• DOI: 10.1016/j.micromeso.2024.113387

Porous glasses produced through melt-quenching of some selective metal organic frameworks like Zeolitic Imidazolate Framework-62 (ZIF-62) and ZIF-4 belong to the advanced functional materials because of their inherent porosity, ease of processing, high gas adsorption capacity and gas separation selectivity. We have delineated thermal induced modifications in the porosity features (pore size, size-distribution and pore density) of crystalline ZIF-62 from room temperature (RT) to its melt-state (> melting point, Tm) followed by its quenching, back to RT, carrying out the depth sensitive positron annihilation lifetime spectroscopy (PALS) measurements in-situ at varying temperatures (RT−Tm). On heating under vacuum, the pores' size as well as size-distribution of crystalline ZIF-62 increases up to ∼473 K as a consequence of removal of entrapped solvent molecules and nonuniform thermal expansion. At higher temperatures (∼473 −573 K), a reduction in pores’ size and size-distribution is observed due to the loss of long range ordering and volume collapse. On melting, ZIF-62 turns into a porous liquid having ∼1.4 times larger pores compared to its crystalline form. The quenching of this porous melt is fully irreversible, and results in the formation of a porous glass having the pores larger than its crystalline counterpart. The in-situ PALS investigation provides the first experimental evidence of inherent porosity in ZIF-62 melt existing at high temperature that has been predicted before through molecular dynamics simulation of the ZIFs-based melts. Zeolitic Imidazolate Framework-62 turns into porous liquid having large size pores which hinders its recrystallization on cooling and instead turns into a porous glass showing distinct Glass transition temperature. [Display omitted] •High quality crystalline ZIF-62 films have been deposited on Silicon crystals.•Depth dependent porosity characteristics are determined using depth dependent PALS measurements.•Pore size expansion along with broader size distribution confirmed the anisotropic thermal expansion.•Large size voids remain present in the melt-state of ZIF-62 as confirmed by high temperature PALS.•Inherent porosity of the ZIF-62 melt hinders the recrystallization leading to formation of porous melt-quenched glass.