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

Abstract

Fine tuning of the pore architecture and flexibility of zeolitic imidazolate frameworks (ZIFs) is highly crucial for realizing their applications in molecular gas separation. Mixed ligand frameworks (ZIF-7−8) synthesized by mixing 2-methylimidazole (2meIm) and benzimidazole (bIm) ligands show enhanced gas separation performance, attributable to pore and flexibility tuning. In the present study, positron annihilation lifetime spectroscopy (PALS) measurements under CO 2 pressure have been used to experimentally investigate the tuning of the pore architecture and flexibility of mixed ligand frameworks ZIF-7−8 having a ZIF-8 structure and similar morphology with varying bIm content up to 18.2%. The aperture and cavity of frameworks begin to open up with an increasing bIm ligand content followed by a decrease at a higher content. On the contrary, flexibility of the frameworks indexed from PALS measurements carried out under CO 2 pressure shows a decreasing trend followed by an increase. The present study shows that mixed ligand frameworks having a larger aperture size are less flexible as a result of inherent open configurations of ligands in the framework lattice. On the other hand, frameworks having a comparatively smaller aperture size show higher flexibility as a result of a possibility of twisting of the ligands under CO 2 pressure, resulting in aperture opening. The pore-opening phenomenon as a result of lattice flexibility under CO 2 pressure is observed to be fully reversible for ZIF-7−8.

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

“…These frameworks show distinct variations in the o -Ps lifetime corresponding to the aperture and cavity site, indicating the variation in the flexibility of these frameworks. This observation is consistent with the distinct flexibility or gate-opening behavior of ZIF-8 and ZIF-67 , as well as mixed linker frameworks . It is reported in the literature that the framework flexibility and gate-opening phenomenon observed in ZIFs is related to the gas–framework interaction as well as the applied gas pressure.…”

“…ZIFs have highly flexible frameworks. − The gas separation factor of ZIFs not only depends on the aperture size or grain boundary structure but also heavily depends on the flexibility of the frameworks. , As a result of framework flexibility, the pore sizes of ZIFs are expanded under gas pressure due to the gate-opening phenomenon, leading to a reduction in gas separation selectivity. − The flexibility of ZIFs can be effectively compared by determining the open volume at the pore sites in situ under gas pressure, which can be effectively measured by in situ PALS measurements under gas pressure. , In the present study, the PALS spectra of selected mixed-metal frameworks ( x = 0.10, 0.25, 0.75, and 0.90) were acquired in a CO 2 pressure range of 0 to ∼20 atm. Figure S5 (Supporting Information) shows the typical PALS spectra of ZIF-8(0.10) under three different CO 2 gas pressures.…”

“…Three o -Ps lifetime components were resolved in all frameworks, which are presented in Table S4 (Supporting Information) with the corresponding intensity values. In view of the SOD topology of the framework, the three o -Ps lifetime components with their increasing values are attributed to annihilation from the pore aperture (ap), central cavity (cv), and large-size intercrystalline pores (void) present in the framework. − The corresponding o -Ps intensities provide information about the pore surface area and pore interconnectivity in the frameworks. Figure a shows the o -Ps intensity corresponding to different types of pores (ap, cv, and void) along with the total o -Ps ( I total = I ap + I cv + I void ) intensity in the frameworks.…”

“…Because of the guest–host interaction-induced flexibility as well as the ultramicroporosity of ZIFs, gas-adsorption-based methods could not provide reliable pore sizes, pore interconnectivity, etc., to correlate with the physico-chemical properties of ZIFs. , As a result, researchers have mostly relied on theoretically determined pore sizes of ZIFs to correlate with their gas separation properties. Recently, it has been shown that positron annihilation lifetime spectroscopy (PALS) can be efficiently used to determine the pore architecture (different pore sizes in the framework, pore size distribution, and pore interconnectivity) in desolvated ZIFs and the framework flexibility under gas pressure. − In the present study, we synthesized highly crystalline mixed-metal (Zn/Co) ZIF-8 frameworks with varying Co contents and pure ZIF-8 and ZIF-67 at room temperature. The phase purity, morphology, and metal distribution in the frameworks were confirmed using X-ray diffraction, field-emission scanning electron microscopy (FE-SEM), and energy-dispersive X-ray spectroscopy coupled with SEM and transmission electron microscopy (TEM).…”

Interplay between the Pore Network Modifications and Dynamics of Confined Polyethyleneimine in Self-Assembled Polyethyleneimine–Silica Nanoparticle Microspheres

“…Due to the nonmonotonic jamming of silica nanoparticles as well as loading of PEI within the microspheres, the pore network of the microspheres may consist of a complex pore architecture. In order to investigate such complex pore networks of the microspheres at different length scales, we have used the positron annihilation lifetime spectroscopy (PALS) technique, which can be used to characterize from micropores to mesopores present in the microspheres, free volumes due to PEI chain packings, and closed pores, if any, not accessible from the outer surface to any probe molecules, e.g., N 2 , in gas adsorption methods. − …”