Water vapour sorption properties of a family of square lattice topology porous coordination networks†

Porous coordination networks (PCNs) such as metal–organic frameworks are of topical interest thanks to their potential utility as sorbents for gas and vapour separations and/or storage. Interpenetrated PCNs, some of which offer promise for gas separations, remain relatively understudied in the context of water vapour sorption. Herein, we report an in-depth study of the water vapour sorption properties of a family of square lattice topology (sql) PCNs of general formula sql-[bipy,squa]-M-aqua (sql-M-aqua, M = Mn, Co, Ni, Zn, bipy = 4,4-bipyridine, squa = squarate). This family, several of which have been previously reported (Co, Ni, Mn), exist as rectangular grids that exhibit 2-fold inclined interpenetration, thereby forming ultramicroporous 3D supramolecular networks. Water vapour sorption studies of sql-M-aqua (M = Mn, Co, Ni, Zn) revealed S-shaped water vapour sorption isotherms with steps consistently below 10% relative humidity (RH) and little hysteresis. Such properties are pertinent to atmospheric water harvesting in arid regions (<30% RH). Water vapour humidity swing experiments (0–30% RH, 300 K) indicated hydrolytic stability for sql-M-aqua (M = Mn, Co, Ni, Zn) and retention of working capacity over 100 sorption/desorption cycles. sql-M-aqua (M = Mn, Co, Ni, Zn) also exhibit CO₂/N₂ selectivity.