Growing ZIF-8 Seeds on Charged COF Substrates toward Efficient Propylene-Propane Separation Membranes.

We report a covalent organic framework (COF) induced seeding strategy to fabricate metal-organic framework (MOF) membranes. Contrary to graphene oxide nuclei-depositing substrate, COF substrate has uniform pore size, high microporosity and abundant functional groups. We designed a series of charged COF nanosheets to induce the formation of ZIF-8@COF nanosheet seeds with high aspect ratio over 150, which were readily processed into a compact and uniform seed layer. The resulting ZIF-8 membranes with thickness down to 100 nm exhibit an ultrahigh C3 H6 /C3 H8 separation performance and superior long-term stability. Our strategy is also validated by fabricating ultrathin ZIF-67 and UiO-66 membranes.

Organic molecular sieve membranes for chemical separations.

Molecular separations that enable selective transport of target molecules from gas and liquid molecular mixtures, such as CO2 capture, olefin/paraffin separations, and organic solvent nanofiltration, represent the most energy sensitive and significant demands. Membranes are favored for molecular separations owing to the advantages of energy efficiency, simplicity, scalability, and small environmental footprint. A number of emerging microporous organic materials have displayed great potential as building blocks of molecular separation membranes, which not only integrate the rigid, engineered pore structures and desirable stability of inorganic molecular sieve membranes, but also exhibit a high degree of freedom to create chemically rich combinations/sequences. To gain a deep insight into the intrinsic connections and characteristics of these microporous organic material-based membranes, in this review, for the first time, we propose the concept of organic molecular sieve membranes (OMSMs) with a focus on the precise construction of membrane structures and efficient intensification of membrane processes. The platform chemistries, designing principles, and assembly methods for the precise construction of OMSMs are elaborated. Conventional mass transport mechanisms are analyzed based on the interactions between OMSMs and penetrate(s). Particularly, the 'STEM' guidelines of OMSMs are highlighted to guide the precise construction of OMSM structures and efficient intensification of OMSM processes. Emerging mass transport mechanisms are elucidated inspired by the phenomena and principles of the mass transport processes in the biological realm. The representative applications of OMSMs in gas and liquid molecular mixture separations are highlighted. The major challenges and brief perspectives for the fundamental science and practical applications of OMSMs are tentatively identified.

Missing-linker Defects in Covalent Organic Framework Membranes for Efficient CO2 Separation.

Covalent organic framework (COF) membranes with tunable ordered channels and free organic groups hold great promise in molecular separations owing to the synergy of physical and chemical microenvironments. Herein, we develop a defect engineering strategy to fabricate COF membranes for efficient CO2 separation. Abundant amino groups are in situ generated on the COF nanosheets arising from the missing-linker defects during the reactive assembly of amine monomer and mixed aldehyde monomers. The COF nanosheets are assembled to fabricate COF membranes. Amino groups, as the CO2 facilitated transport carriers, along with ordered channels endow COF membrane with high CO2 permeances exceeding 300 GPU and excellent separation selectivity of 80 for CO2 /N2 , and 54 for CO2 /CH4 mixed gas under humidified state. Our defect engineering strategy offers a facile approach to generating free organic functional groups in COF membranes and other organic framework membranes for diverse chemical separations.

Oil-Water-Oil Triphase Synthesis of Ionic Covalent Organic Framework Nanosheets.

Ionic covalent organic framework nanosheets (iCOFNs) with long-range ordered and mono-dispersed ionic groups hold great potential in many advanced applications. Considering the inherent drawbacks of oil-water biphase method, herein, we explore an oil-water-oil triphase method based on phase engineering strategy for the bottom-up synthesis of iCOFNs. The middle water phase serves as a confined reaction region, and the two oil phases are reservoirs for storing and supplying monomers to the water phase. A large aqueous space and low monomer concentration lead to the anisotropic gradual growth of iCOFNs into few-layer thickness, large lateral size, and high crystallinity. Notably, the resulting three cationic and anionic iCOFNs exhibit ultra-high aspect ratios of up to 20,000. We further demonstrate their application potential by processing into ultrathin defect-free COF membranes for efficient biogas separation. Our triphase method may offer an alternative platform technology for the synthesis and innovative applications of iCOFNs.

Channel-facilitated molecule and ion transport across polymer composite membranes.

Biological membranes possess hierarchical channels and thus exhibit ultrahigh permselectivity for molecules and ions. Intrigued by the delicate structure and transport mechanisms in biochannels, polymer composite membranes with selective transport channels are successfully fabricated for diverse energy- and environment-related applications. This tutorial review aims to present the latest progress in the design and construction of selective molecule/ion transport channels within polymer composite membranes with emphasis on the regulation of the physical and chemical microenvironments of these channels. The concluding remarks are presented with a tentative perspective on the future research and development of highly efficient channel-facilitated molecule/ion transport across polymer composite membranes.

Engineering HOF-Based Mixed-Matrix Membranes for Efficient CO2 Separation.

Hydrogen-bonded organic frameworks (HOFs) have emerged as a new class of crystalline porous materials, and their application in membrane technology needs to be explored. Herein, for the first time, we demonstrated the utilization of HOF-based mixed-matrix membrane for CO2 separation. HOF-21, a unique metallo-hydrogen-bonded organic framework material, was designed and processed into nanofillers via amine modulator, uniformly dispersing with Pebax polymer. Featured with the mix-bonded framework, HOF-21 possessed moderate pore size of 0.35 nm and displayed excellent stability under humid feed gas. The chemical functions of multiple binding sites and continuous hydrogen-bonded network jointly facilitated the mass transport of CO2. The resulting HOF-21 mixed-matrix membrane exhibited a permeability above 750 Barrer, a selectivity of ~ 40 for CO2/CH4 and ~ 60 for CO2/N2, surpassing the 2008 Robeson upper bound. This work enlarges the family of mixed-matrix membranes and lays the foundation for HOF membrane development.

Coordination-driven structure reconstruction in polymer of intrinsic microporosity membranes for efficient propylene/propane separation.

Polymers of intrinsic microporosity (PIMs), integrating unique microporous structure and solution-processability, are one class of the most promising membrane materials for energy-efficient gas separations. However, the micropores generated from inefficient chain packing often exhibit wide pore size distribution, making it very challenging to achieve efficient olefin/paraffin separations. Here, we propose a coordination-driven reconstruction (CDR) strategy, where metal ions are incorporated into amidoxime-functionalized PIM-1 (AO-PIM) to in situ generate coordination crosslinking networks. By varying the type and content of metal ions, the resulting crosslinking structures can be optimized, and the molecular sieving capability of PIM membranes can be dramatically enhanced. Particularly, the introduction of alkali or alkaline earth metals renders more precise micropores contributing to superior C3H6/C3H8 separation performance. K+ incorporated AO-PIM membranes exhibit a high ideal C3H6/C3H8 selectivity of 50, surpassing almost all the reported polymer membranes. Moreover, the coordination crosslinking structure significantly improves the membrane stability under higher pressure as well as the plasticization resistant performance. We envision that this straightforward and generic CDR strategy could potentially unlock the potentials of PIMs for olefin/paraffin separations and many other challenging gas separations.

MOF-COF "Alloy" Membranes for Efficient Propylene/Propane Separation.

Molecular-sieving membranes from metal-organic frameworks (MOFs) are promising candidates for separating olefin/paraffin mixtures, a critical demand in sustainable chemical processes and a grand challenge in molecular separation. Currently, the inherent lattice flexibility of MOFs severely compromises their precise sieving ability. Here, a proof-of-concept of "alloy" membranes (AMs), which are fabricated by incorporating quaternary ammonium (QA)-functionalized covalent organic frameworks (COFs) into a zeolitic imidazolate framework-8 (ZIF-8) matrix is demonstrated. The Coulomb force between the COFs and the ZIF-8 restricts the linker rotation of the ZIF-8, generating a distinct alloying effect, by which the lattice rigidity of ZIF-8 can be conveniently tuned through varying the content of the COFs, similar to the flexible-to-rigid transition in aluminum alloy manufacturing. Such an alloying effect confers the AM's superior propylene/propane separation performance, with a propylene/propane separation factor surpassing 200 and a propylene permeance of 168 GPU. Hopefully, the AMs concept and the concomitant alloying effect can update the connotation of mixed matrix membranes and stimulate the re-envisioning about the design paradigm and development of advanced membranes for energy-efficient separations.

Membrane-Based Olefin/Paraffin Separations.

Efficient olefin/paraffin separation is a grand challenge because of their similar molecular sizes and physical properties, and is also a priority in the modern chemical industry. Membrane separation technology has been demonstrated as a promising technology owing to its low energy consumption, mild operation conditions, tunability of membrane materials, as well as the integration of physical and chemical mechanisms. In this work, inspired by the physical mechanism of mass transport in channel proteins and the chemical mechanism of mass transport in carrier proteins, recent progress in channel-based and carrier-based membranes toward olefin/paraffin separations is summarized. Further, channel-based membranes are categorized into membranes with network structures and with framework structures according to the morphology of channels. The separation mechanisms, separation performance, and membrane stability in channel-based and carrier-based membranes are elaborated. Future perspectives toward membrane-based olefin/paraffin separation are proposed.