Ultrathin permselective membranes: the latent way for efficient gas separation

Membrane gas separation has attracted the attention of chemical engineers for the selective separation of gases. Among the different types of membranes used, ultrathin membranes are recognized to break the trade-off between selectivity and permeance to provide ultimate separation. Such success has been associated with the ultrathin nature of the selective layer as well as their defect-free structure. These membrane features can be obtained from specific membrane preparation procedures used, in which the intrinsic properties of different nanostructured materials (e.g., polymers, zeolites, covalent-organic frameworks, metal-organic frameworks, and graphene and its derivatives) also play a crucial role. It is likely that such a concept of membranes will be explored in the coming years. Therefore, the goal of this review study is to give the latest insights into the use of ultrathin selective barriers, highlighting and describing the primary membrane preparation protocols applied, such as atomic layer deposition, in situ crystal formation, interfacial polymerization, Langmuir-Blodgett technique, facile filtration process, and gutter layer formation, to mention just a few. For this, the most recent approaches are addressed, with particular emphasis on the most relevant results in separating gas molecules. A brief overview of the fundamentals for the application of the techniques is given. Finally, by reviewing the ongoing development works, the concluding remarks and future trends are also provided.