Graphene oxide (GO) membranes have been attracting numerous attention due to their impressive performance in various applications, especially in water purification. However, because the swelling in water and polar organic solvents causes the increase of interlayer channels, GO membranes usually possess inferior rejection for subnanometer-sized molecules. How to control the transport channels of GO membranes at angstrom level is a significantly scientific and practical issue. Herein, a concept of external pressure regulation (EPR) is reported for restraining GO swelling and controlling its interlayer spacing precisely. Since anisotropic GO films only swell at vertical direction, the interlayer channels can be manipulated by externally unidirectional reverse force. Based on this concept, an EPR system with GO membranes is designed for water desalination by adjusting the external pressure that has high resolution. In cross-flow filtration, the compressed GO membranes show high KCl, NaCl, and CaCl rejections of 94%, 97%, and 98%, respectively, accompanied by large water permeance up to 25 L m h under low feed pressure of 2 bar, despite the fact that the semi-free spatial swelling of ultrathin GO layer above the substrate pores can deteriorate salt rejection. Our work provides a straightforward physical strategy to adjust the interlayer spacing of the membranes fabricated by two-dimensional nanosheets for achieving desired filtration capacity.

Metal-organic frameworks (MOFs) have been attracting intensive attention because of their commendable potential in many applications. Postsynthetic modification for redesigning chemical characteristics and pore structures can greatly improve performance and expand functionality of MOF materials. Here, we develop a versatile vapor-phase linker exchange (VPLE) methodology for MOF modification. Through solvent-free and environment-friendly VPLE processing, various linker analogs with functional groups but not for straightforward MOF crystallization are inserted into frameworks as daughter building blocks. Besides single exchange for preparing MOFs with dual linkers, VPLE can further be performed by multistage operations to obtain MOF materials with multiple linkers and functional groups. The halogen-incorporated ZIFs exhibit good porosity, tunable molecular affinity, and impressive CO2/N2 and CH4/N2 adsorption selectivities up to 31.1 and 10.8, respectively, which are two to six times higher than those of conventional adsorbents. Moreover, VPLE can substantially enhance the compatibility of MOFs and polymers.

Ultra-selective MOF membranes with controllable thickness down to 130 nm were prepared on discs and fibers by sol–gel asynchronous crystallization.

In this work, a versatile postmodification strategy based polydopamine (PDA) grafting is reported for improving CO2 separation performance of MOF membranes. Owning to the strong bioadhesion, PDA can be deposited on the UiO-66 membrane through a simple and mild process. Since PDA impregnation in invalid nanometer-sized pinholes and grain boundaries of the MOF membrane suppress nonselective gas transports, the modified PDA/UiO-66 membrane exhibits significantly enhanced CO2/N2 and CO2/CH4 selectivities of 51.6 and 28.9, respectively, which are 2–3 times higher than the reported MOF membranes with similar permeance. Meanwhile, because PDA modification do not change UiO-66 intrinsic pores and membrane thickness is submicrometer-sized, the CO2 permeance is 2–3 orders of magnitude larger than those membranes with similar selectivity, up to 3.7 × 10–7 mol m–2 s–1 Pa–1 (1115 GPU). Moreover, the PDA/UiO-66 membrane with good reproducibility has excellent long-term stability for CO2 capture under moist condition in 36 h measurement period.

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