Fine-Tuning of the Pore Aperture
and Framework Flexibility
of Mixed-Metal (Zn/Co) Zeolitic Imidazolate Framework-8: An In Situ
Positron Annihilation Lifetime Spectroscopy Study under CO2 Gas Pressure
posted on 2023-07-12, 14:07authored byJaideep Mor, Renjith B. Nelliyil, Sandeep Kumar Sharma
The mixed-metal (Zn/Co) strategy has been used to enhance
the gas
separation selectivity of zeolitic imidazolate framework-8 (ZIF-8)-based
membranes. The enhancement in selectivity has been attributed to possible
modifications in the grain boundary structure, pore architecture,
and flexibility of the frameworks. In the present study, we used in
situ positron annihilation lifetime spectroscopy (PALS) under varying
CO2 pressure to investigate the tuning of the pore architecture
and framework flexibility of mixed-metal (Zn/Co) ZIF-8 frameworks
with varying Co contents. The random distribution of Zn and Co metal
nodes within the highly crystalline frameworks having an SOD topology
was established using electron microscopy, Fourier transform infrared
spectroscopy, and Raman spectroscopy. The inherent aperture as well
as cavity size of the frameworks, and the pore interconnectivity to
the outer surface, were observed to vary with the Co content in ZIF-8
due to the random distribution of Zn and Co metal nodes in the frameworks.
The aperture size is reduced with the incorporation of an additional
metal (Zn or Co) in ZIF-67 or ZIF-8, respectively. The aperture size
remains the smallest for a lower Co content (∼0.20) in ZIF-8.
The framework flexibility determined by in situ PALS measurements
under CO2 pressure continuously reduces with increasing
Co content in ZIF-8. A smaller aperture size as well as low flexibility
of ZIF-8 with a low Co content is seen to be directly correlated to
a higher separation selectivity of membranes prepared with this mixed-metal
composition.