Ultramicroporous materials can be highly effective at
trace gas
separations when they offer a high density of selective binding sites.
Herein, we report thatsql-NbOFFIVE-bpe-Cu, a new variant
of a previously reported ultramicroporous square lattice,sql, topology material,sql-SIFSIX-bpe-Zn, can exist in
two polymorphs. These polymorphs,sql-NbOFFIVE-bpe-Cu-AA(AA) andsql-NbOFFIVE-bpe-Cu-AB(AB), exhibit AAAA and ABAB packing of thesqllayers, respectively. WhereasNbOFFIVE-bpe-Cu-AA(AA) is isostructural withsql-SIFSIX-bpe-Zn,
each exhibiting intrinsic 1D channels,sql-NbOFFIVE-bpe-Cu-AB(AB) has two types of channels, the intrinsic channels
and extrinsic channels between thesqlnetworks. Gas
and temperature induced transformations of the two polymorphs ofsql-NbOFFIVE-bpe-Cuwere investigated by pure gas sorption,
single-crystal X-ray diffraction (SCXRD), variable temperature powder
X-ray diffraction (VT-PXRD), and synchrotron PXRD. We observed that
the extrinsic pore structure ofABresulted in properties
with potential for selective C3H4/C3H6separation. Subsequent dynamic gas breakthrough measurements
revealed exceptional experimental C3H4/C3H6selectivity (270) and a new benchmark for productivity
(118 mmol g–1) of polymer grade C3H6(purity>99.99%) from a 1:99 C3H4/C3H6mixture. Structural analysis,
gas sorption studies, and gas adsorption kinetics enabled us to determine
that a binding “sweet spot” for C3H4in the extrinsic pores is behind the benchmark separation performance.
Density-functional theory (DFT) calculations and Canonical Monte Carlo
(CMC) simulations provided further insight into the binding sites
of C3H4and C3H6molecules
within these two hybrid ultramicroporous materials, HUMs. These results
highlight, to our knowledge for the first time, how pore engineering
through the study of packing polymorphism in layered materials can
dramatically change the separation performance of a physisorbent.