Tunable Confined Aliphatic Pore Environment in Robust Metal–Organic Frameworks for Efficient Separation of Gases with a Similar Structure

The fine-tuning of the pore structure of metal–organic
frameworks
(MOFs) is of critical importance to developing energy-efficient processes
for the challenging separation of structurally similar molecules.
Herein, we demonstrate a strategy to realize a quasi-three-dimensional
refinement of the pore structure that utilizes the tunability of ring
size and number in polycycloalkane-dicarboxylate ligands. Two hydrolytically
stable MOFs with a confined aliphatic pore environment, ZUL-C1 and
ZUL-C2, were, for the first time, synthesized and applied in separating
low-concentration C2-C3 hydrocarbons from natural gas and ultralow-concentration
Xe from used nuclear fuel (UNF) off-gas. Validated by X-ray diffraction
and modeling, an expansion of the polycycloalkane moiety enables sub-angstrom
contraction in specific directions and forms a pore surface with more
alkyl sites, which affords stronger trapping of guest molecules with
relatively higher polarizability. The resultant material exhibits
record C2H6/CH4 and C3H8/CH4 selectivities coupled with a benchmark
low-pressure C2H6 capacity in alkane mixture
separation and also a benchmark Xe capacity at extremely diluted feed
concentration and record Kr productivity for the Xe/Kr (20:80, v/v)
mixture in Xe/Kr separation.