Design Principles Guiding Solvent Size Selection in ZIF-Based Type 3 Porous Liquids for Permanent Porosity

Porous liquids (PLs), which are solvent-based
systems that contain permanent porosity due to the incorporation of
a solid porous host, are of significant interest for the capture of
greenhouse gases, including CO2. Type 3 PLs formed by using
metal–organic frameworks (MOFs) as the nanoporous host provide
a high degree of chemical turnability for gas capture. However, pore
aperture fluctuation, such as gate-opening in zeolitic imidazole framework
(ZIF) MOFs, complicates the ability to keep the MOF pores available
for gas adsorption. Therefore, an understanding of the solvent molecular
size required to ensure exclusion from MOFs in ZIF-based Type 3 PLs
is needed. Through a combined computational and experimental approach,
the solvent-pore accessibility of exemplar MOF ZIF–8 was examined.
Density functional theory (DFT) calculations identified that the lowest-energy
solvent–ZIF interaction occurred at the pore aperture. Experimental
density measurements of ZIF–8 dispersed in various-sized solvents
showed that ZIF–8 adsorbed solvent molecules up to 2 Å
larger than the crystallographic pore aperture. Density analysis of
ZIF dispersions was further applied to a series of possible ZIF-based
PLs, including ZIF–67, −69, −71(RHO), and −71(SOD),
to examine the structure–property relationships governing solvent
exclusion, which identified eight new ZIF-based Type 3 PL compositions.
Solvent exclusion was driven by pore aperture expansion across all
ZIFs, and the degree of expansion, as well as water exclusion, was
influenced by ligand functionalization. Using these results, a design
principle was formulated to guide the formation of future ZIF-based
Type 3 PLs that ensures solvent-free pores and availability for gas
adsorption.