Flexible–robust MOFs/HOFs for challenging gas separations

The critical separation and purification industry accounts for almost half of industrial energy consumption. The use of porous solids for gas separation based on the molecular properties of gas mixture components, such as chemical affinity or molecular size, has attracted much attention as high-efficiency/low-energy alternatives to traditional technologies such as distillation. "Flexible-robust" metal–organic frameworks (MOFs) and hydrogen-bonded organic frameworks (HOFs) are new classes of porous solids that have caused serious progress in the field of gas separation due to their unique properties in their structure. In this critical review, we have focused on the detailed investigation of the emergence and evolution of this category of MOFs/HOFs for specific challenging gas separation targets. A detailed account of the structural and separation properties of flexible-robust MOFs/HOFs has been provided by comparison with both robust and flexible MOFs. Flexible-robust MOFs/HOFs are benchmark materials in many challenging gas separations as demonstrated via systematic analysis of the performance of this new class of porous materials. From the point of view of industrialization, the properties of this category are compared with other benchmarks, and issues related to separation performance, stability, and energy efficiency are highlighted.