Crystal Size-Dependent Pore Architecture and Surface Chemical Characteristics of Desolvated ZIF-8 Investigated Using Positron Annihilation Spectroscopy

Abstract

Crystal downsizing of zeolitic imidazolate framework-8 (ZIF-8) has been shown to affect the flexibility of its framework, leading to the occurrence of “gate-opening” phenomenon at higher applied gas pressures compared to larger size crystals. In the present study, we have investigated the crystal size-dependent pore architecture of crystalline desolvated ZIF-8 samples which is expected to play the most deterministic role in its gas adsorption behavior. In order to avoid the pressurization or gas molecule–ZIF interaction, positron annihilation spectroscopy has been used to investigate the pores’ sizes, pores’ interconnectivity, and pores’ elemental surface characteristics of desolvated ZIF-8 samples. The studied ZIF-8 samples (size ∼14 nm −1.4 μm) have been synthesized at room temperature and systematically characterized for their phase purity, structural integrity, morphology, and thermal stability using complementary characterization techniques. Positronium annihilation from inter-crystal voids as a result of its diffusion through the crystals indicates that the pore network is highly interconnected in smaller size crystal samples. However, some interconnections from deep inside of larger crystals to the outer surface are observed to be blocked due to the imperfections produced in pore networking during crystal growth. The present study confirms that crystal downsizing results in modifications of average pore size corresponding to the aperture and central cavity. The average aperture size in nanometer size ZIF-8 crystals (size ∼14 and 45 nm) was measured to be smaller than the perfect lattice aperture (0.34 nm). We attribute this smaller aperture size in ZIF-8 nanocrystals to the occurrence of “gate-opening” phenomenon at higher applied gas pressure. The surface of nanocrystals is observed to be enriched by Zn compared to imidazole.

Gate Opening Induced High Pore Volume Expansion in Flexible Zeolitic Imidazole Frameworks during CO2 Adsorption: A Direct Observation Using Positron Annihilation Spectroscopy

“…Recently, it has been shown that PALS can efficiently probe the pore sizes of ZIFs that are true representation of average pore sizes from a bulk powder samples. − Typical PALS spectra of ZIF-67, ZIF-8, and ZIF-7­(np) acquired under vacuum (∼10 –3 mbar) are shown in Figure . PALS spectra clearly show long and distinct o -Ps lifetime components for ZIF-67, ZIF-8, and ZIF-7­(np).…”

Perspectives on the Influence of Crystal Size and Morphology on the Properties of Porous Framework Materials

“…In the case of ZIF-8 this shifts the gate-opening transition pressure towards higher pressures. In addition, surface blockage and pore network blockage of the interior are identified as contributing factors ( Sharma et al, 2020 ).…”

Flexibility of Mixed Ligand Zeolitic Imidazolate Frameworks (ZIF-7–8) under CO2 Pressure: An Investigation Using Positron Annihilation Lifetime Spectroscopy

“…In the present study, we investigated the pore architecture of fully crystalline ZIF-7 x –8 ( x = 6.6–18.2%) and their framework flexibility under a CO 2 pressure up to 10 kg cm –2 using positron annihilation lifetime spectroscopy (PALS). − Recently, it has been established that PALS-based methods are highly advantageous for pore architecture investigation of flexible ZIFs − and complementary to gas-adsorption-based methods. It is primarily because the PALS-based method is sensitive from ultramicropores to mesopores and the measurements are not affected by any guest–framework-interaction-induced flexibility, which is otherwise the case for gas-adsorption-based methods.…”