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.