A (T–P) phase diagram for the adsorption/desorption of carbon dioxide and hydrogen in a Cu(II)-MOF

Abstract

Solvothermal synthesis of a highly porous metal–organic framework (PMOF) of Cu(II) had been achieved with a bent tetracarboxylate linker, L4− incorporating an –NH2 group in the middle. The design of the linker afforded paddle-wheel secondary building unit (SBU) with metal bound solvent molecules. Upon activation by heating, the overall integrity of the structure remained intact with the result of a PMOF embellished with unsaturated metal centers (UMCs) and free –NH2 groups. This activated PMOF showed adsorption of 6.6% by weight of H2 gas at 77 K and 62 bar and very high 60% by weight of CO2 gas at 298 K and 32 bar. The adsorption/desorption properties of this MOF has been probed theoretically to obtain additional insights into physisorption of these strategically important gases. The binding energy values reveal the willingness of the CO2 and H2 gas molecules to get absorbed in Cu(II)-MOF. Temperature–pressure (T–P) phase diagrams have been generated for both of the gas absorption processes at ωB97x-D/TZVP level of theory. Temperature and pressure regions have been identified where Gibbs free energy is negative that is the absorption process is spontaneous. The ab initio molecular dynamics study reveals that the gas saturated Cu(II)-MOFs are stable. Thus this Cu(II)-MOF can be used as an effective gas storage material.