A zwitterionic metal–organic framework with free carboxylic acid sites that exhibits enhanced hydrogen adsorption energies

Abstract

Metal–organic frameworks (MOFs) are permanently porous, crystalline materials and promising hydrogen sorbents. However, the H₂ heats of adsorption are generally too small to promote significant adsorption under desired storage conditions. Increasing the H₂–framework interaction energy is a prominent goal of current MOF design. Hydrogen binds to MOFs through a variety of interactions, such as dispersion, charge–induced-dipole, charge–quadrupole, and even chemical bonding. To date, these interactions have been enhanced by incorporating strongly charged groups into the MOF structures, but the effects tend to be short-range and only effective at low loadings. In this work we report the structures and H₂ storage properties of two zwitterionic MOFs. These structures feature zwitterionic characteristics arising from N-heterocyclic azolium groups in the linkers and negatively charged Zn₂(CO₂)₅ nodes. These groups exhibit net charges of +0.5 and −1.0, respectively, and interact strongly with the H₂ quadrupole. Isosteric heats of adsorption of up to 7.0 kJ mol⁻¹ are observed in these zwitterionic MOFs.