Abstract

Ultramicroporous materials can be highly effective at trace gas separations when they offer a high density of selective binding sites. Herein, we report that , a new variant of a previously reported ultramicroporous square lattice, , topology material, , can exist in two polymorphs. These polymorphs, () and (), exhibit AAAA and ABAB packing of the layers, respectively. Whereas () is isostructural with , each exhibiting intrinsic 1D channels, () has two types of channels, the intrinsic channels and extrinsic channels between the networks. Gas and temperature induced transformations of the two polymorphs of were 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 of resulted in properties with potential for selective CH/CH separation. Subsequent dynamic gas breakthrough measurements revealed exceptional experimental CH/CH selectivity (270) and a new benchmark for productivity (118 mmol g) of polymer grade CH (purity 99.99%) from a 1:99 CH/CH mixture. Structural analysis, gas sorption studies, and gas adsorption kinetics enabled us to determine that a binding "sweet spot" for CH 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 sites of CH and CH molecules 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.