Core-Shell Zeolitic Imidazolate Frameworks for Enhanced Hydrogen Storage

Abstract

Core-shell ZIF-8@ZIF-67- and ZIF-67@ZIF-8-based zeolitic imidazolate frameworks (ZIFs) were synthesized solvothermally using a seed-mediated methodology. Transmission electron microscopy-energy-dispersive X-ray spectrometry, line scan, elemental mapping, X-ray photoelectron spectroscopy, and inductively coupled plasma-atomic emission spectroscopy analyses were performed to confirm the formation of a core-shell structure with the controlled Co/Zn elemental composition of ∼0.50 for both the core-shell ZIFs. The synthesized core-shell ZIF-8@ZIF-67 and ZIF-67@ZIF-8 frameworks conferred enhanced H2 (2.03 and 1.69 wt %) storage properties at 77 K and 1 bar, which are ca. 41 and 18%, respectively, higher than that of the parent ZIF-8. Notably, the distinctly remarkable H2 storage properties shown by both the core-shell ZIFs over the bimetallic Co/Zn-ZIF and the physical mixture of ZIF-8 and ZIF-67 clearly evidenced their unique structural properties (confinement of porosity) and elemental heterogeneity due to the core-shell morphology of the outperforming core-shell ZIFs. Moreover, H2 adsorption isotherm data of these frameworks are best fitted with the Langmuir model (R2 ≥ 0.9999). Along with the remarkably enhanced H2 storage capacities, the core-shell ZIFs also displayed an improved CO2 capture behavior. Hence, we demonstrated here that the controlled structural features endorsed by the rationally designed porous materials may find high potential in H2 storage applications. © 2018 American Chemical Society.