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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]

[Display omitted] • PSA modified hollow NH 2 -ZIF-8 (PHNZ) with high amino content was designed. • PSA hard template strategy was used to synthesis NH 2 -ZIF-8 rapidly. • The vesicle shape of PHNZ constructs low-resistance region to improve CO 2 permeability. • CO 2 /N 2 selectivity is improved by PSA layer and CO 2 affinity of amino groups. The amino functionalization of MOFs has been verified to be effective in improving the selective permeation of CO 2. However, the introduction of amino groups into ZIF-8 by mixed ligands strategy is hard to achieve due to the high steric hindrance of benzene ring in 2-aminobenzimidazole ligand, resulting in insufficient content of amino groups. Therefore, in this work, polystyrene-acrylate (PSA) template is proposed to provide abundant adsorption and reaction sites for the rapid synthesis of NH 2 -ZIF-8 with high content of 2-aminobenzimidazole. Furthermore, the incomplete etching strategy is used to construct PSA modified hollow NH 2 -ZIF-8 nanospheres (PHNZ). Importantly, the thickness of PSA layer and the hollow size are adjusted by the etching time, as revealed by the SEM and TEM images. In this PHNZ, the modified PSA can provide good interface compatibility between PHNZ and Pebax matrix to enhance the CO 2 selectivity, while the hollow structure can reduce the mass transfer resistance and improve the gas permeability. Moreover, the introduced amino groups can enhance the CO 2 affinity of PHNZ and further improve the CO 2 /N 2 selectivity of the PHNZ-based mixed matrix membranes (MMMs). Benefiting from the synergy of good interface compatibility and enhanced CO 2 affinity from the amino groups, the PHNZ/Pebax MMMs present greatly improved CO 2 /N 2 selectivity in contrast with pure Pebax and NH 2 -ZIF-8 based membranes. Besides, as the etching time increases, the PHNZ-2 and PHNZ-3 based MMMs present a higher CO 2 permeability than that of NH 2 -ZIF-8 based MMM, revealing the significance of the hollow structure. The PHNZ-2/Pebax MMM with filler loading of 10 wt% expresses the best gas separation performance with the CO 2 permeability of 121.9 Barrer and CO 2 /N 2 selectivity of 96.6, respectively, showing an increase of 54.7 % and 98.0 % in contrast with pure Pebax membrane. Meanwhile, this CO 2 separation performance is far beyond the Robeson upper bound in 2008 and close to the McKeown upper bound in 2019, demonstrating that the proposed PHNZ displays a promising and competitive potential in MMM based CO 2 capture. [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]