Covalent organic frameworks (COFs) are a promising category of porous materials possessing extensive chemical tunability, high porosity, ordered arrangements at a molecular level, and considerable chemical stability. Despite these advantages, the application of COFs as membrane materials for gas separation is limited by their relatively large pore apertures (typically > 0.5 nm), which exceed the sieving requirements for most gases whose kinetic diameters are less than 0.4 nm. Herein, we report the fabrication of ultrathin two-dimensional (2D) membranes through Layer-by-Layer (LbL) assembly of two kinds of ionic covalent organic nanosheets (iCONs) with different pore sizes and opposite charges. Because of the staggered packing of iCONs with strong electrostatic interactions, the resultant membranes exhibit features of reduced aperture size, optimized stacking pattern, and compact dense structure without sacrificing thickness control, which are suitable for molecular sieving gas separation. One of the hybrid membranes, TpEBr@TpPa-SO3Na with a thickness of 41 nm, shows a H2 permeance of 2566 gas permeation units (GPUs) and a H2/CO2 separation factor of 22.6 at 423 K, surpassing the recent Robeson upper bound along with long-term hydrothermal stability. This strategy provides not only a high-performance H2 separation membrane candidate but also an inspiration for pore engineering of COF or 2D porous polymer membranes.

Ultrathin Two-Dimensional Membranes Assembled by Ionic Covalent Organic Nanosheets with Reduced Apertures for Gas Separation

Covalent organic frameworks (COFs) are a class of organic crystalline porous materials discovered in the early 21st century that have become an attractive class of emerging materials due to their high crystallinity, intrinsic porosity, structural regularity, diverse functionality, design flexibility, and outstanding stability. However, many chemical and physical properties strongly depend on the presence of metal ions in materials for advanced applications, but metal-free COFs do not have these properties and are therefore excluded from such applications. Metalated COFs formed by combining COFs with metal ions, while retaining the advantages of COFs, have additional intriguing properties and applications, and have attracted considerable attention over the past decade. This review presents all aspects of metalated COFs, from synthetic strategies to various applications, in the hope of promoting the continued development of this young field.

Metalated covalent organic frameworks: from synthetic strategies to diverse applications.

Developing highly effective salt-resistant solar evaporators for a long-term desalination with a high evaporation rate and water production rate remains a great challenge. Herein, we fabricated a three-dimensional printed hierarchical porous reduced graphene oxide/carbon black (3DP-HP rGO/CB) solar evaporator constructed with a thin layer of porous photothermal interface and a grid of hierarchical porous transport channel possessing a large-sized porous microstructure. The 3DP-HP rGO/CB solar evaporator demonstrates a tailored high-salt transport flux of up to 4.3 kg·m-2·h-1, which displays a highly effective salt-resistant performance at a high evaporation rate of 10.5 kg·m-2·h-1 during a desalination of 10 wt % NaCl brine under 8 kW·m-2 illumination. Experiments and theoretical calculations prove that the large porous microstructure with abundant and low-resistance salt ion channels endows solar evaporators with a high salt transport flux, therefore boosting salt resistance compared to traditional solar evaporators. A 10 d desalination experiment shows the long-term salt resistance of a 3DP-HP rGO/CB solar evaporator for a high-rate and stable evaporation and water production. Furthermore, the 3DP-HP rGO/CB evaporator can purify 10 wt % NaCl brine at an ultrafast water production rate of up to 5.6 L·m-2·h-1 under natural sunlight. This work demonstrates great potential for the practical implementation of solar desalination with high productivity.

Tailoring the Salt Transport Flux of Solar Evaporators for a Highly Effective Salt-Resistant Desalination with High Productivity.

Nanofluidics is promising in the construction of highly‐efficient osmotic energy generator, but it is still a challenge to develop large‐scale and high‐performance nanofluidic membranes. The emerging covalent organic frameworks (COFs) provide a desirable platform to create nanofluidic membranes with high ion selectivity and permeability towards effective osmotic energy conversion. Herein, an ultrathin self‐standing COF nanofluidic membranes based on terephthalaldehyde‐tetrakis(4‐aminophenyl)methane is developed to construct high‐efficiency nanofluidic osmotic energy generator. Benefiting from the nano‐confined channels (1.4 nm) and negative surface charges, the COF based nanofluidic membrane demonstrates both excellent cation selectivity and high ion conductance. Moreover, an ultrathin thickness of ≈1.5 µm significantly reduces the membrane resistance. Consequently, the nanofluidic osmotic energy generator based on COF membrane can deliver a high output power of 5.31 W m−2 under a 50‐fold salinity gradient simulating natural river/sea junction, which is superior to most reported systems and reaches the industrial level. More importantly, such a COF nanofluidic membrane exhibits excellent stability in response to various environmental factors, including wide saline solution concentration, temperature and pH ranges. This work is anticipated to highlight the great potential of 1D COF nanofluidic membranes toward highly‐efficient osmotic energy generators.

Ultrathin Self‐Standing Covalent Organic Frameworks toward Highly‐Efficient Nanofluidic Osmotic Energy Generator

A critical review of covalent organic frameworks-based sorbents in extraction methods.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/sstr.202100061

Jiaxing Guo

Papers

Covalent Organic Framework Membranes for Efficient Chemicals Separation