Progress in the development of flexible metal–organic frameworks for hydrogen storage and selective separation of its isotopes

Between 2018 and 2050, global energy usage is expected to increase by approximately 50% because of rapid technological advancements, high population growth rates, and urbanization. Various alternative energy sources have been proposed in response to the energy crisis, among which hydrogen is the most promising owing to its high gravimetric energy density and environmental benignity. Although deuterium is exceptionally useful in various applications such as nuclear-power production, 1H nuclear magnetic resonance spectroscopy, isotope tracing, and neutron-scattering experiments, its natural occurrence is only ∼0.0156%. However, hydrogen isotope mixtures are extremely challenging to separate because of their similar sizes and chemical properties. Metal–organic frameworks (MOFs) have been envisioned as cutting-edge materials for gas storage and isotope separation because of their tunable pore aperture, which is one of the most important criteria for high capacity and selectivity. Especially, flexible MOFs have reversible structural transformation and pore-shifting capabilities that provide better control in gate-opening phenomena. Herein, flexible MOFs are reviewed according to design and types, for the storage and separation of energy-efficient H2 from mixtures. Because hydrogen isotopes are difficult to separate, the fundamental concepts of hydrogen isotope separation are given careful consideration in the development of efficient materials. Flexible MOFs with gate-opening behavior exhibit excellent hydrogen isotope separation efficiency owing to dynamic pore shifting, which is one of the key requirements for isotope separation. We believe that this review will provide insights into the current trends and future aspects of flexible MOFs with respect to hydrogen storage and its isotope separation.