Abstract
Although structures with the single functional constructions and micropores were demonstrated to capture many different molecules such as carbon dioxide, methane, and hydrogen with high capacities at low temperatures, their feeble interactions still limit practical applications at room temperature. Herein, we report in-situ growth observation of hierarchical pores in pomegranate metal-organic frameworks (pmg-MOFs) and their self-sequestering storage mechanism, not observed for pristine MOFs. Direct observation of hierarchical pores inside the pmg-MOF was evident by in-situ growth X-ray measurements while self-sequestering storage mechanism was revealed by in-situ gas sorption X-ray analysis and molecular dynamics simulations. The results show that meso/macropores are created at the early stage of crystal growth and then enclosed by micropore crystalline shells, where hierarchical pores are networking under self-sequestering mechanism to give enhanced gas storage. This pmg-MOF gives higher CO2 (39%) and CH4 (14%) storage capacity than pristine MOF at room temperature, in addition to fast kinetics with robust capacity retention during gas sorption cycles, thus giving the clue to control dynamic behaviors of gas adsorption.
Highlights
Chemical bonds to capture guest molecules, releasing guest molecules for practical applications requires a very high temperature and it suffers from slow kinetics attributed to its having high activation energy for breaking their strong bonds[14,15]
The low angle scattering that represents the existence of meso/macropores was getting stronger as the diffraction intensity for the micropore structure was increased from 80 to 100 minutes
The pomegranate MOF (pmg-MOF)-5 crystals are washed with organic solvents and dried for activation and their activated samples along with pristine MOF-5 and DBA were measured by the solid state NMR
Summary
Chemical bonds to capture guest molecules, releasing guest molecules for practical applications requires a very high temperature and it suffers from slow kinetics attributed to its having high activation energy for breaking their strong bonds[14,15]. We report a new finding that enables high capacities for gas storage using a pomegranate MOF (pmg-MOF) having hierarchical meso/macropores only in the core part and enclosed with the microporous crystalline body, in addition to their robust capacity retention with no hysteresis for reversible gas adsorption cycles, where the target molecules occupy first the micropores surrounding meso/macropores and are captured into these large pore spaces (Fig. 1) with fast kinetics. This so called “self-sequestering storage” mechanism was explored through combination of in-situ gas sorption X-ray measurements with molecular dynamics (MD) simulations. We have elucidated the in-situ crystal growth mechanism of the pmg-MOF-5 with the X-ray measurements
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