Highly Productive C3H4/C3H6 Trace Separation by a Packing Polymorph of a Layered Hybrid Ultramicroporous Material

Ultramicroporous materials can be highly effective attrace gasseparations when they offer a high density of selective binding sites.Herein, we report that sql-NbOFFIVE-bpe-Cu, a new variantof a previously reported ultramicroporous square lattice, sql, topology material, sql-SIFSIX-bpe-Zn, can exist intwo polymorphs. These polymorphs, sql-NbOFFIVE-bpe-Cu-AA (AA) and sql-NbOFFIVE-bpe-Cu-AB (AB), exhibit AAAA and ABAB packing of the sql layers, respectively. Whereas NbOFFIVE-bpe-Cu-AA (AA) is isostructural with sql-SIFSIX-bpe-Zn,each exhibiting intrinsic 1D channels, sql-NbOFFIVE-bpe-Cu-AB (AB) has two types of channels, the intrinsic channelsand extrinsic channels between the sql networks. Gasand temperature induced transformations of the two polymorphs of sql-NbOFFIVE-bpe-Cu were investigated by pure gas sorption,single-crystal X-ray diffraction (SCXRD), variable temperature powderX-ray diffraction (VT-PXRD), and synchrotron PXRD. We observed thatthe extrinsic pore structure of AB resulted in propertieswith potential for selective C3H4/C3H6 separation. Subsequent dynamic gas breakthrough measurementsrevealed exceptional experimental C3H4/C3H6 selectivity (270) and a new benchmark for productivity(118 mmol g(-1)) of polymer grade C3H6 (purity >99.99%) from a 1:99 C3H4/C3H6 mixture. Structural analysis,gas sorption studies, and gas adsorption kinetics enabled us to determinethat a binding "sweet spot" for C3H4 in 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 sitesof C3H4 and C3H6 moleculeswithin these two hybrid ultramicroporous materials, HUMs. These resultshighlight, to our knowledge for the first time, how pore engineeringthrough the study of packing polymorphism in layered materials candramatically change the separation performance of a physisorbent.