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Membrane technology has aroused great attention for gas separations due to its high energy efficiency, small capital investment, easy and continuous operation. Mixed matrix membranes (MMMs) constructed from a continuous polymer phase and a dispersed filler phase offer new opportunities to achieve a breakthrough in fabricating high-performance membranes. Metal–organic frameworks (MOFs) have been regarded as potential fillers to boost the MMM separation performance. However, MOFs occasionally exhibit limited compatibility with the polymer matrixes owing to their partially inorganic structure and tendency of agglomeration in the membranes. Here, an interfacial design strategy is demonstrated by coating size-selective MOF cores with covalent organic framework (COF) layers to construct MOF@COF hybrids as fillers in MMMs for enhanced polymer-filler compatibility. The pure organic COF layers exhibit high affinity to the polymer matrix, thereby preventing the formation of nonselective interfacial voids and alleviating filler agglomeration. With the incorporation of only 5 wt% of MOF@COF fillers, the resultant MMM exhibits 48% and 79% enhancements in CO2 permeability and CO2/CH4 selectivity, respectively, with better operational stability compared to that of the pure polymeric membrane. These results reveal a novel filler design strategy for the tailored synthesis of high-performance MMMs for natural gas and biogas purification.

Ultrathin mixed matrix membranes (MMMs) containing two-dimensional (2D) fillers are highly sought recently due to their good performance in gas separations. Here we report the preparation of PIM-1 based ultrathin MMMs containing 2D metal-organic framework (MOF) nanosheets using spin coating method. The lamellar structure of the 2D layered MOF filler (copper 1,4-benzenedicarboxylate nanosheets, CuBDC-ns) enlarges the contacting area between fillers and the polymer matrix, leading to the formation of dense MMMs. The centrifugal force generated during the spin coating process helps to horizontally align these CuBDC-ns, increasing the gas diffusion resistance across the membrane. The influence of membrane thickness and filler content on the separation performance of CO2/CH4 mixture is examined and discussed. Optimizing membrane thickness and filler content results in a 660 nm thick ultrathin MMM comprising 10 wt% CuBDC-ns, which exhibits a CO2/CH4 selectivity of 15.6 and CO2 permeance as high as 407 GPU, surpassing other PIM-1 based membranes. Besides, the long term stability test shows that the prepared ultrathin MMMs are stable up to 100 h. These results demonstrate that the ultrathin MMMs reported in this study are excellent candidates for CO2/CH4 separation, with potential applications in natural gas purification and biogas upgrading.

Thin film composite membranes (TFCMs) have displayed attractive CO2 capture performance, in which a critical gutter layer with small transport resistance is needed to prevent the penetration of the top separation layer into the bottom porous support for realizing high CO2 permeance. However, traditional and recently developed gutter layer materials face some issues such as physical aging, thickness-dependent permeability, instability, or poor compatibility. Herein, wholly organic, orientated, two-dimensional covalent organic framework (COF) films, for the first time, are used as the novel gutter layers due to the ideal pore size, intrinsic porosity, small transport resistance, good compatibility with polymer matrix, good anti-aging capability and high stability. Commercial or photo-initiated crosslinked polymer can be deposited compatiblely on the COF gutter layer with a thickness as thin as 55 nm, an ultrathin thickness that traditional gutter layers are challenging to attain. As a result, one optimized TFCM realizes high CO2 capture performance with CO2 permeance of 1843 gas permeation units (GPU) and CO2/N2 separation selectivity of 28.2, which displays unique advantages over other gutter layer-based TFCMs and meets the performance target for CO2 capture (CO2 permeance > 1000 GPU, CO2/N2 selectivity > 20).

Water-stable zirconium metal-organic frameworks (Zr-MOFs) such as the UiO-66 series have been recently identified as promising materials for polycrystalline membranes. However, the reliance of such membranes on rigid and expensive ceramic substrates with low packing density has greatly limited their wider applications. Herein, we report polycrystalline UiO-66(Zr)–NH2 membranes grown on cheap and flexible carbon cloth by in situ solvothermal synthesis. The membranes possess sub-micron thicknesses with a certain flexibility and angular bend tolerance up to 10°. In addition, these membranes exhibit great separation performance in organic solvent nanofiltration (OSN) because of their excellent solvent resistance, with above 99.8% dye rejection and high flux for dichloromethane (ca. 0.17 kg m−2 h−1 bar−1). Our study paves the way toward the development of polycrystalline Zr-MOF membranes for challenging liquid separations.

Because of their uniform yet tunable pore size and rich functionality, metal-organic frameworks (MOFs) have attracted huge interests as membrane materials. However, the relatively weak stability limits their applications in liquid separation. Herein, we report the fabrication of polycrystalline rare-earth MOF membranes (Sm-DOBDC) supported on alumina hollow fiber, exhibiting excellent water stability. Using these membranes for the dehydration of 5 wt% water/ethanol feed solution at 323 K, a total flux of 786.4 ± 33.7 g m−2 h−1 and a 99.8 ± 0.2 wt% water concentration in the permeate can be achieved, with a high separation factor of above 9481. In addition, in-situ healing ability of this membrane is demonstrated by treating with spent reaction solution, shedding light on the practical applications of polycrystalline MOF membranes in liquid separation.