Crystal engineering of a new platform of hybrid ultramicroporous materials and their C2H2/CO2 separation properties

Abstract

Hybrid ultramicroporous materials (HUMs) comprised of combinations of organic and inorganic linker ligands are a leading class of physisorbents for trace separations involving C1, C2 and C3 gases. First generation HUMs are modular in nature since they can be self-assembled from transition metal cations, ditopic linkers and inorganic “pillars”, as exemplified by the prototypal variant, SIFSIX-3-Zn (3 = pyrazine, SIFSIX = SiF₆²⁻). Conversely, HUMs that utilise chelating ligands such as ethylenediamine derivatives are yet to be explored as sorbents. Herein, we report the structures and sorption properties of two HUMs based upon the chelating ligand N¹,N²-bis(pyridin-4-ylmethyl)ethane-1,2-diamine (enmepy), [Zn(enmepy)(SiF₆)]_n (SIFSIX-24-Zn) and [Zn(enmepy)(SO₄)]_n (SOFOUR-2-Zn). These HUMs are isostructural and exhibit high C₂H₂ uptakes of 85 cm³ g⁻¹ (3.79 mmol g⁻¹) and 79 cm³ g⁻¹ (3.52 mmol g⁻¹), and C₂H₂/CO₂ IAST selectivities of 7.4 and 8.1 (1 bar, 1 : 1 mixture, 298 K), respectively. Dynamic column breakthrough experiments resulted in separation factors of 5.26 and 2.05, and CO₂ effluent purities of 99.991 and 99.989%, respectively. Temperature programmed desorption experiments at 60 °C resulted in rapid desorption of CO₂, followed by fuel grade C₂H₂ (>98%), affording productivities of 9.45 and 7.96 L kg⁻¹ and maximum C₂H₂ outlet purities of 99.92% and 99.66%, respectively. This study introduces the use of diamine chelating ligands in HUMs for gas separations through two parent sorbents that are prototypal for families of related materials, one of which, SOFOUR-2-Zn, uses the earth-friendly sulfate anion as a pillar.