Fluids-dependent High-pressure Chemistry of ZIF-8 - Dr. Yongjae Lee
Shanghai Sept 8, 2016 — New work published in JACS (DOI: 10.1021/jacs.6b07374) by HPSTAR scientist, Dr. Yongjae Lee and Junhyuck Im of Yonsei University, Narae Yim and Jaheon Kim of Soongsil University, and Thomas Vogt of University of South Carolina, found clear fluids-dependent pressure-induced insertion in ZIF-8. This study suggests that the future studies of the high pressure chemistry of ZIFs and other MOFs should consider guest-host interactions under pressure.
Zeolitic-imidazolate frameworks (ZIFs) are members of a new class of MOFs (Metal-organic frameworks), a break discovery appeared in the early 2000s. The ZIF holds promise for the improved design of MOFs tailored specifically for carbon capture, or for the use of hydrogen and natural gas (methane) fuels. They exhibit high porosity and chemical functionality, with the advantages of an exceptional chemical stability and a large structural diversity, which are unusual for MOFs. ZIF-8 is one of the most studied prototypical and common ZIFs due to its high thermal stability and high surface area.
“It is desirable to have a way to tune porosity via controlling both the coordination-inclusion chemistry within and distortion/relaxation of the porosity. Pressure can be used as a clean and safe variable to extend both the structural and chemical diversity of MOFs materials are vulnerable to temperature-treatment”, said Yongjae.
Using diamond anvil cell to create just several gigapascals, the team probed the high-pressure chemistry of ZIF-8 in the different pressure medium.
They discovered PTM(pressure-transmitting medium)-dependent structural and chemical changes in ZIF-8 under pressure.
ZIF-8 framework expands and contracts under hydrostatic pressure depending on the solvent molecules used as PTM. When pressurized in water, ZIF-8 framework contracts gradually. In contrast, when the molecular size of PTM becomes larger to alcohols, the framework contraction becomes modulated and even undergoes abrupt expansions. During these processes, the pores of ZIF-8 are gradually filled by PTM molecules under pressure as modeled by the residual electron density from synchrotron X-ray powder diffraction data. In non-penetrating PTM (silicone oil), ZIF-8 becomes amorphous immediately upon pressurization.
“Our systematic experimental observations have significant implications when determining compressibilities and bulk moduli in high-pressure experiments of microporous materials as one has to assert the presence of a single high pressure phase in order to do so”, said Lee, a research scientist of HPSTAR and professor Yonsei Univ
“Our results will be contributing to understanding molecular diffusion within ZIF or MOF pores in different high pressure media, expanding the current status of the porous materials to high pressure regime”, Lee added.
Caption：pressure-induced insertion of guest molecules into the pores of ZIF-8 in water (W), methanol (M), and ethanol (E) PTM as pressure increase，courtesy of Yongjae Lee.