Hundreds of polymers have been evaluated as membrane materials for gas separations, but fewer than 10 have made it into current commercial applications, mainly due to the effects of physical aging and plasticization. Efforts to overcome these two problems are a significant focus in gas separation membrane research, in conjunction with improving membrane separation performance to surpass the Robeson upper bounds of selectivity versus permeability for commercially important gas pairs. While there has been extensive research, ranging from manipulating the chemistry of existing polymers (e.g., thermally rearranged or cross-linked polyimides) to synthesizing new polymers such as polymers of intrinsic microporosity (PIMs), there have been three major oversights that this review addresses: (1) the need to compare the approaches to achieving the best performance in order to identify their effectiveness in improving gas transport properties and in mitigating aging, (2) a common standardized aging protocol that allows rapid determination of the success (or not) of these approaches, and (3) standard techniques that can be used to characterize aging and plasticization across all studies to enable them to be robustly and equally compared. In this review, we also provide our perspectives on a few key aspects of research related to high free volume polymer membranes: (1) the importance of Robeson plots for membrane aging studies, (2) eliminating thermal history, (3) measurement and reporting of gas permeability and aging rate, (4) aging and storing conditions, and (5) promising approaches to mitigate aging.

Additive manufacturing, likewise known as 3-dimensional (3D) printing and rapid prototyping, has the ability to create almost any geometrically complex shape or feature in a range of materials across different scales. It has found its applications in various areas, such as medicine (bioprinting), art, manufacturing and engineering. On the other hand, its use in separation membrane engineering is relatively new. The use of additive manufacturing techniques could provide more control towards the design of separation membrane systems and offers novel membrane preparation techniques that are able to produce membranes of different shapes, types and designs which cannot be made using conventional techniques such as phase inversion or sintering. Here we provide key background information on 3D printing technologies and applications in membrane engineering; a discussion of the potential and limitations of current 3D printing technologies for membrane engineering and future aspects of the technology. Due to the potential benefits of 3D printing in membrane manufacturing, in particular the unprecedented control over membrane architecture the technique could allow, the use of 3D printing in membrane systems should see significant growth in the near future

The photoreduction of CO₂ on inorganic semiconductors has been researched for several decades, but the conversion efficiency is still low due to the recombination of photo-generated electron–hole pairs, low utilization efficiency of solar energy and weak adsorption of CO₂. Here we for the first time demonstrate that metal–organic frameworks such as ZIF-8 can effectively adsorb CO₂ dissolved in water, and promote photocatalytic activity of a semiconductor catalyst in CO₂ reduction into liquid fuels in an aqueous medium. In particular, Zn₂GeO₄/ZIF-8 hybrid nanorods were successfully synthesized by growing ZIF-8 nanoparticles on Zn₂GeO₄ nanorods. The Zn₂GeO₄/ZIF-8 nanocomposite inherits both high CO₂ adsorption capacity of ZIF-8 nanoparticles and high crystallinity of Zn₂GeO₄ nanorods. The Zn₂GeO₄/ZIF-8 hybrid nanorods containing 25 wt% ZIF-8 exhibit 3.8 times higher dissolved CO₂ adsorption capacity than the bare Zn₂GeO₄ nanorods, resulting in a 62% enhancement in photocatalytic conversion of CO₂ into liquid CH₃OH fuel. The strategy reported here is promising for developing more active photocatalysts for improving CO₂ conversion efficiency by taking advantage of excellent adsorption property of metal–organic frameworks in aqueous media.

A two-dimensional zeolitic imidazolate framework (ZIF), ZIF-L, with a unique anisotropic pore system makes it an excellent material for investigating ZIF membranes with crystallographically preferred orientation and their effects on gas separation properties. A b-oriented ZIF-L membrane was successfully prepared from a randomly oriented seed layer. In contrast, a c-oriented ZIF-L crystal layer was attached to a support through vacuum filtration in the presence of polyethyleneimine. After a short-time secondary growth, the oriented layer grew into a continuous membrane with a high degree of c-out-of-plane orientation. Gas separation studies demonstrated the orientation-dependent separation behavior. In single gas permeation experiments, the c-oriented ZIF-L membrane has higher ideal selectivities of 8.1 for H₂/N₂ and 24.3 for H₂/CO₂ compared to the corresponding values of 3.9 and 5.5 for the b-oriented ZIF-L membrane. The results of binary gas permeation showed decreased separation factors of the c-oriented membrane, but were still much higher than those of the b-oriented membrane. This work provides new insights into controlling ZIF membrane orientation for achieving desirable gas separation performance.

Ultrathin ZIF-8 membranes with a thickness of around 200 nm were prepared by chemical vapour modification of surface chemistry and nanopores of an asymmetric bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) substrate. The resulting ZIF-8 membranes exhibited exceptional H₂ permeance as high as 2.05 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ with high H₂/N₂ and H₂/CO₂ selectivities (9.7 and 12.8, respectively).