Precise Construction Of Covalent Organic Framework Membranes Toward Efficient Molecular Separations

Efficient molecular separation process is a significant demand and challenge in the field of energy and environment.Due to the highly similar physicochemical properties between molecules,it becomes a difficult and hot spot in the separation field.Membrane separation technology possesses the advantages of low energy consumption and environmental friendliness,becoming a new generation of molecular separation technology.High-performance membrane materials and precise membrane structure construction are crucial for efficient molecular separation.This study focuses on the emerging covalent organic frameworks（COFs）materials toward the major demand of molecular separation.The advanced film-forming processes have been developed to overcome the issue of the inherent difficulty of crystal material in film forming,weak interunit interactions and intergranular defects.The crystal plane structure,pore structure and carrier structure have been precisely manipulated.Based on the enhancement of physical-chemical multi-mechanism,the efficient molecular separation process is realized.This study provides theoretical and technical support to design and fabricate molecular separation membranes with high performance.The main contents are summarized as follows.（1）Blending method is adopted to overcome the issue of poor film-forming ability of COFs to prepare the facilitated transport membranes.Metal-ion loaded COF nanosheets（metal-ion@CONs）is used as the dispersed phase.The sorption selectivity and diffusion selectivity of the membrane are synergistically enhanced by regular channels of CONs and reversibleπcomplexation of metal ions.By tuning the CON microenvironments（e.g.the pore size,the number of carriers and the strength of the interaction）in the membrane,the high efficiency gasoline desulfurization process can be realized.The permeation flux of the facilitated membrane can reach 21.6 kg m^-2 h^-1,and the enrichment factor is 6.29.（2）The electrochemical method is used to overcome the issue of difficult interfacial reaction and poor controllability in the preparation of COF membranes,and the ultrathin COF membranes are efficiently prepared.Using the directional and controllable input of electrical energy,the interfacial reaction at the cathode is enhanced,and the polymerization-crystallization processes of COF membranes is accelerated based on kinetics and thermodynamics.The current density serves as a self-regulating factor,the overgrowth of COF membranes is restrained through the self-inhibitory behavior,and ultrathin COF layers as well as defect-free properties are obtained.The COF membrane is used in brine desalination,and the permeation flux can reach 92 kg m^-2 h^-1,while the salt rejection rate of 99.96%.（3）The adhesion method is explored to overcome the boundary defects and poor stability of COF membranes,and the COF membranes with ultra-high flux were prepared.Inspired by the adhesion and nesting process of sand worms,TpPa-SO₃H nanosheets are used as the bulk material of the membrane,and TpTTPA nanoribbons were used as“adhesive tapes”to prepare COF membranes towards seawater desalination.The permeation flux of the membrane can reach 267 kg m^-2 h^-1,which exceeds the upper limit reported and is 4-10 times higher than that of traditional polymer membranes.The long-term operational stability can reach 108 h.In addition,the adhesion method can also be applied to the construction of other two-dimensional material membranes,showing high universality.（4）The immersion-coating method is used to overcome the difficulties in processing and scale-up of COF membranes to process COF-based composite hollow fiber membranes,achieving the progress from flat membrane to hollow fiber membrane.Self-exfoliated TpDT nanosheets are used as the facilitated transport carriers of CO₂ in Pebax matrix to prepare hollow fiber composite membrane on PVDF by the immersion-coating method.The membrane module is also processed.The membranes are used in CO₂/CH₄ separation with a separation factor of 36 and CO₂ permeance of 142.3 GPU,which exceeds the 2008 Robeson upper bound.