This overview article discusses fundamental principles of gas sorption and transport in rubbery and glassy polymers and material selection guidelines for gas separation membranes. Comparisons between the performance of membrane‐based gas separation systems and more conventional technologies in key commercial applications are provided. Companion articles in this special edition focus on state‐of‐the‐art reviews and descriptions of theoretical and experimental developments important in the technology of gas separations using polymeric membranes.

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Biological fluoride ion channels are sub-1-nanometer protein pores with ultrahigh F − conductivity and selectivity over other halogen ions. Developing synthetic F − channels with biological-level selectivity is highly desirable for ion separations such as water defluoridation, but it remains a great challenge. Here we report synthetic F − channels fabricated from zirconium-based metal-organic frameworks (MOFs), UiO-66-X (X = H, NH 2 , and N + (CH 3 ) 3 ). These MOFs are comprised of nanometer-sized cavities connected by sub-1-nanometer-sized windows and have specific F − binding sites along the channels, sharing some features of biological F − channels. UiO-66-X channels consistently show ultrahigh F − conductivity up to ~10 S m −1 , and ultrahigh F − /Cl − selectivity, from ~13 to ~240. Molecular dynamics simulations reveal that the ultrahigh F − conductivity and selectivity can be ascribed mainly to the high F − concentration in the UiO-66 channels, arising from specific interactions between F − ions and F − binding sites in the MOF channels.

A model deueloped for multicomponent gas separation using hollow-fiber contactors permits simulation of cocurrent, countercurrent, and crossflow contacting patterns with permeate purging (or sweep