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[Display omitted] • Hollow ZIF-8 in micron accumulated continuously across membrane to construct transport highway. • The real CO 2 transport distance was reduced. • In-situ polymerization intensifies the interface affinity between ZIF-8 and PEO. The hollow filler has been verified to effectively enhance the CO 2 permeability of mixed matrix membrane (MMMs) by constructing low-resistance transport pathway. However, the in-continuity of hollow fillers across the membrane results in insufficient CO 2 permeability. In this work, the micron-sized hollow ZIF-8 (Mic-H-ZIF-8) were designed and proposed to accumulate continuously across the membrane to construct the transport highway for CO 2. Importantly, the in-situ polymerization was utilized to achieve continuous accumulation of the Mic-H-ZIF-8 fillers across the membrane. The Mic-H-ZIF-8 presents cavity size of 4 μm and shell thickness of 100 nm, and three or four Mic-H-ZIF-8 microspheres are aligned sequentially across the membrane, in which the effective gas transport distance is decreased from 13 μm to the range of 0.8–3 μm. Consequently, the CO 2 permeability of 5 wt% Mic-H-ZIF-8/PEO MMM is 224.6 % and 63.5 % higher than those of pure PEO membrane and nanosized hollow ZIF-8/PEO MMM at 0.1 MPa. Meanwhile, the CO 2 /N 2 selectivity is still as high as 70.0. This CO 2 separation performance is far exceeding the Robeson upper bound in 2008, and beyond the McKeown upper bound in 2019 as the feed pressure increasing, confirming the significance of continuous transport highway constructed by micron-sized hollow MOFs on improving CO 2 separation of MMMs. [ABSTRACT FROM AUTHOR]

Layered double hydroxide (LDH) bearing numerous hydroxyl groups shows high affinity with CO 2 and is promising in mixed matrix membranes (MMMs) based CO 2 capture. However, its non-porous structure limits the further improvement of CO 2 permeability. In this work, 3D hollow CoNi-LDH nanocages were developed to construct low-resistance and CO 2 -philic transport channel in MMM by in-situ etching of ZIF-67 template. The resultant material exhibits hollow structure with cavity size of about 500 nm, mesopores and micropores from LDH stacking and ZIF-67 template. The outer layered CoNi-LDH nanosheets can attract more CO 2 molecules by their abundant hydroxyl groups, resulting in an improvement in dissolution selectivity of CO 2 over N 2. Meanwhile, the tortuous gas diffusion paths originated from 2D nanosheet stacking can further enhance the CO 2 selectivity over N 2. Moreover, the hollow nanocages provide CO 2 molecules with fast transport path, resulting in high CO 2 permeability. As a result, compared with pure Pebax membrane and 2D LDH/Pebax MMM, all the 3D hollow CoNi-LDH/Pebax MMMs present enhanced CO 2 permeability. The 3D hollow CoNi-LDH-2h/Pebax MMM performs the highest CO 2 permeability of 135.49 Barrer, which are increased by 141.95% and 71.50% than those of the above two membranes, respectively. The selectivity of 3D hollow CoNi-LDH-2h/Pebax MMM is about 26.30% higher than that of the pure Pebax membrane. The present membrane presents comparable CO 2 capture capacity in contrast with most reported Pebax based MMMs and surpasses the Robeson upper bond. Together with the good long-term stability, the 3D hollow CoNi-LDH nanocages are believed to be a promising candidate for MMMs to improve CO 2 capture. [Display omitted] • MMM with low resistance and CO 2 -philic transport channel was constructed. • 3D Hollow CoNi-LDH nanocages were developed by etching ZIF-67 template. • Hollow and porous structure ensures the high CO 2 permeability. • Abundant hydroxyl groups and tortuous diffusion paths ensure high selectivity. [ABSTRACT FROM AUTHOR]

Carbon nanotubes (CNTs) with 1D gas transport channel has great prospect in MMMs based CO 2 separation. However, the insufficient compatibility with polymer and inner diameter of CNTs limit further improvement of their CO 2 separation performances. In this work, the unique ZIF-8 hollow nanotubes (HNTs) were proposed to construct high-speed gas transmission channel in MMMs by a three-step method (electrospinning-calcination-hydrothermal), aiming at concurrently improving the gas permeability and selectivity. The hollow inner cavity provides long-range and low-resistance gas transmission channels to ensure high gas permeability. At the same time, the ZIF-8 crystals composing the tube wall can not only screen for gas molecules pair, but also strengthen the affinity between the filler and polymer matrix by its organic ligands, thus the gas selectivity can be raised. Compared with ZIF-8 nanoparticles and ZnO HNTs, ZIF-8 HNTs based MMMs present competitive CO 2 separation. When the loading of ZIF-8 HNTs is 5 wt%, the gas separation performance shows the best, in which the CO 2 permeability is 147 Barrer and the CO 2 /N 2 selectivity is 68, approaching to the 2008 Robeson upper limit and surpassing most reported CNTs/Pebax based membranes. Therefore, the proposed ZIF-8 HNTs are promising for MMMs based CO 2 separation. [Display omitted] • ZIF-8 HNTs were designed by three steps of electrospinning-calcination- hydrothermal. • ZIF-8 HNTs were proposed to construct high-speed gas transmission channel in MMMs. • CO 2 permeability of ZIF-8 HNTs MMMs is 147 Barrer, 63% higher than that of pure ZIF-8. • CO 2 /N 2 selectivity of ZIF-8 HNTs MMMs is 68, 15% higher than that of pure ZIF-8. • CO 2 permeability and selectivity of ZIF-8 HNTs/Pebax surpassed most reported CNTs/Pebax. [ABSTRACT FROM AUTHOR]