Membrane technology has attracted great industrial interest in carbon capture and separation owing to the merits of energy-efficiency, environmental friendliness and low capital investment. Conventional polymeric membranes for CO₂ separation suffer from the trade-off between permeability and selectivity. Introducing porous fillers in polymers is one approach to enhance membrane separation performance. Metal-organic frameworks (MOFs), with ordered porous structure and diverse chemical functionalities, are promising fillers to prepare mixed matrix membranes (MMMs) for CO₂ separation. However, the main issue of MOF based MMMs in industry is their stability and processability. This review analyses recent work on stable and scalable MOF based MMMs for CO₂ separation. The typical stable MOFs, MOF-based MMMs and the scalable MOF synthesis are summarized. A large number of MOF-based MMM suffer from instability upon exposure to contaminants. For that reason, we also discuss the use of covalent organic frameworks (COFs) as an alternative to prepare MMMs for CO₂ separation, considering their excellent stability and good compatibility with polymers. Finally, a brief conclusion and current challenges on obtaining scalable and stable MMMs are outlined. This review may provide some guidance for designing high performance MMMs for industrial CO₂ capture and separation to help achieving carbon neutrality.

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Increasing helium use in research and production processes necessitates separation techniques to secure sufficient supply of this noble gas. Energy-efficient helium production from natural gas is still a big challenge. Membrane gas separation technology could play an important role. Herein, a novel poly(p-phenylene benzobisimidazole) (PBDI) polymeric membrane for helium extraction from natural gas with low He abundance is reported. The membranes were fabricated by a facile interfacial polymerization at room temperature. The thin and defect-free membrane structure was manipulated by the confined polymerization of monomers diffusing through the interface between two immiscible liquids. Both He/CH4 selectivity and He permeance are competitive over those of other commercial perfluoropolymers. Even at low He content of 1%, separation performance of the PBDI membrane transcended the current upper bound. The unprecedented selectivity (>1000) together with the excellent stability (∼360 h) endows PBDI membranes with a great potential for energy-efficient industrial recovery and production of this precious He resources from reservoirs with low abundance.@en

It is demonstrated that moderate oxygen vacancies can greatly improve the photo-responsive performance of Zinc oxide (ZnO) nanoparticles thin film/p-Si heterojunctions. The ZnO nanoparticles thin film/p-Si heterojunctions shows a stable, repeatable photo response at 365 nm or 900 nm. Under 900 nm of 0.1 mWcm −2 the sensitivity of device reaches an excellent value of 1.2 × 10 6 cm 2 /W, with outstanding detectivity of 3.6 × 10 12 cmHz 1/2 W −1 and ultrahigh responsivity of 3.5 AW -1 at −2 V, and a fast response speed with rising time (∼ms). The comprehensive properties of present device are significantly better than those reported ZnO thick film/Si heterojunctions and even compared with that of some new 2D nanomaterials/Si heterojunctions in all aspects of performance, achieving an optimal balance among all the critical features. The excellent performance of ZnO nanoparticles thin film/p-Si heterojunctions may be attributed to rising Fermi level of ZnO nanoparticles thin film due to moderate oxygen vacancies, the excellent optical absorption characteristics of ZnO nanoparticles thin film and the interface effect between ZnO nanoparticles thin film and Si. This high-performance heterojunctions photodetector should have important prospects for practical applications, and the integration of such metal oxide nanostructures with traditional Si semiconductor would open up great opportunities for next-generation optoelectronic devices. @en

It is demonstrated that moderate oxygen vacancies can greatly improve the photo-responsive performance of Zinc oxide (ZnO) nanoparticles thin film/p-Si heterojunctions. The ZnO nanoparticles thin film/p-Si heterojunctions shows a stable, repeatable photo response at 365 nm or 900 nm. Under 900 nm of 0.1 mWcm −2 the sensitivity of device reaches an excellent value of 1.2 × 10 6 cm 2 /W, with outstanding detectivity of 3.6 × 10 12 cmHz 1/2 W −1 and ultrahigh responsivity of 3.5 AW -1 at −2 V, and a fast response speed with rising time (∼ms). The comprehensive properties of present device are significantly better than those reported ZnO thick film/Si heterojunctions and even compared with that of some new 2D nanomaterials/Si heterojunctions in all aspects of performance, achieving an optimal balance among all the critical features. The excellent performance of ZnO nanoparticles thin film/p-Si heterojunctions may be attributed to rising Fermi level of ZnO nanoparticles thin film due to moderate oxygen vacancies, the excellent optical absorption characteristics of ZnO nanoparticles thin film and the interface effect between ZnO nanoparticles thin film and Si. This high-performance heterojunctions photodetector should have important prospects for practical applications, and the integration of such metal oxide nanostructures with traditional Si semiconductor would open up great opportunities for next-generation optoelectronic devices. @en

High permeable and selective membranes are expected to be a promising alternative for low cost and energy efficient CO₂ separation. However, the preparation and development of highly permeable and selective membranes are limited by the trade-off effect. Here, we prepared a stabilized high permeable and selective mixed matrix membrane based on the structural regulation of the NH₂-functionalized Co/ZIF-8 heterometallic nanoparticles (NH₂-Co/ZIF-8). The NH₂-Co/ZIF-8 can be homogeneously dispersed in the polymer via chelation of the ester group and the metal node in the methacrylate-terminated PEO and the acrylate-terminated PEO. The amphiphilic interaction between the polymer and NH₂-Co/ZIF-8 significantly increased the CO₂ solubility resulting in the boost of both CO₂ permeability and CO₂/N₂ selectivity. The best performance membrane exhibits a CO₂ permeability of 2916 Barrer and a CO₂/N₂ selectivity of 47, surpassing the Robeson's upper bond (2008) by a wide margin.

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Membranes with high selectivity and permeance are needed to reduce energy consumption in hydrogen purification and pre-combustion CO ₂ capture. Polybenzimidazole (PBI) is one of the leading membrane materials for this separation. In this study, we present superior novel supported PBI (poly(p-phenylene benzobisimidazole), PBDI) membranes prepared by a facile interfacial polymerization (IP) method. The effect of IP reaction duration, operating temperature and pressure on membrane separation performance was systematically investigated. The best performance was achieved for membranes prepared in a 2 h reaction time. The resulting membranes display an ultrahigh mixed-gas H ₂ /CO ₂ selectivity of 23 at 423 K together with an excellent H ₂ permeance of 241 GPU, surpassing the membrane performance of conventional polymers (the 2008 Robeson upper bound). These separation results, together with the facile manufacture, pressure resistance, long-term thermostability (>200 h) and economic analysis, recommend the PBDI membranes for industrial use in H ₂ purification and pre-combustion CO ₂ capture. Besides, PBDI membranes possess high selectivities towards H ₂ /N ₂ (up to 60) and H ₂ /CH ₄ (up to 48) mixtures, indicating their potential applications in ammonia synthesis and syngas production.

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The performance of mixed-matrix membranes (MMMs) based on Matrimid^® and benzimidazole-linked polymers (BILPs) have been investigated for the separation CO₂/N₂ and the dependency on the filler porosity. BILPs with two different porosities (BILP-101 and RT-BILP-101) were synthesized through controlling the initial polymerization rate and further characterized by several techniques (DRIFTs, ¹³C CP/MAS NMR, SEM, TEM, N₂ and CO₂ adsorption). To investigate the influence of porosity, the two types of fillers were incorporated into Matrimid^® to prepare MMMs at varied loadings (8, 16 and 24 wt%). SEM confirmed that both BILP-101 and RT-BILP-101 are well dispered, indicating their good compatibility with the polymeric matrix. The partial pore blockage in the membrane was verified by CO₂ adsorption isotherms on the prepared membranes. In the separation of CO₂ from a 15:85 CO₂:N₂ mixture at 308 K, the incorporation of both BILPs fillers resulted in an enhancement in gas permeability together with constant selectivity owing to the fast transport pathways introduced by the porous network. It was noteworthy that the initial porosity of the filler had a large impact in separation permeability. The best improvement was achieved by 24 wt% RT-BILP-101 MMMs, for which the CO₂ permeability increases up to 2.8-fold (from 9.6 to 27 Barrer) compared to the bare Matrimid^®.

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The use of an azine-linked covalent organic framework (ACOF-1) as filler in mixed-matrix membranes (MMMs) has been studied for the separation of CO₂ from N₂. To better understand the mechanisms that govern separation in complex composites, MMMs were prepared with different loadings of ACOF-1 and three different polymers as continuous phase: low flux-mid selectivity Matrimid^®, mid flux-high selectivity Polyactive™ and high flux-low selectivity 6FDA:DAM. The homogeneous distribution of ACOF-1 together with the good adhesion between the ACOF-1 particles and the polymer matrices were confirmed by scanning electron microscopy. In mixed-gas CO₂/N₂ separation a clear influence of the polymer used was observed on the performance of the composite membranes. While for Matrimid^® and 6FDA:DAM an overall enhancement of the polymer's separation properties could be achieved, in case of Polyactive™ penetration of the more flexible polymer into the COF porosity resulted in a decreased membrane permeability. The best improvement was obtained for Matrimid^®-based MMMs, for which a selectivity increase from 29 to 35, together with an enhancement in permeability from 9.5 to 17.7 Barrer for 16 wt% COF loading, was observed. Our results demonstrate that the combination of the filler-polymeric matrix pair chosen is crucial. For a given filler the polymer performance improvement strongly depends on the polymeric matrix selected, where a good match between the discontinuous and continuous phase, both in the terms of compatibility and gas separation properties, is necessary to optimize membrane performance.

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The preparation and the performance of mixed matrix membranes based on metal-organic polyhedra (MOPs) are reported. MOP fillers can be dispersed as discrete molecular units (average 9 nm in diameter) when low filler cargos are used. In spite of the low doping amount (1.6 wt %), a large performance enhancement in permeability, aging resistance, and selectivity can be achieved. We rationalize this effect on the basis of the large surface to volume ratio of the filler, which leads to excellent dispersion at low concentrations and thus alters polymer packing. Although membranes based only on the polymer component age quickly with time, the performance of the resulting MOP-containing membranes meets the commercial target for postcombustion CO₂ capture for more than 100 days.

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We demonstrate that b-oriented MFI (Mobil Five) zeolite membranes can be manufactured by in situ crystallization using an intermediate amorphous SiO₂ layer. The improved in-plane growth by using a zeolite growth modifier leads to fusion of independent crystals and eliminates boundary gaps, giving good selectivity in the separation of CO₂/Xe mixtures. The fast diffusion of CO₂ dominates the overall membrane selectivity toward the CO₂/Xe mixture. Because of the straight and short [010] channels, the obtained CO₂ permeation fluxes are several orders of magnitude higher than those of carbon molecular sieving membranes and polymeric membranes, opening opportunities for Xe recovery from waste anesthetic gas.

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We demonstrate that b-oriented MFI (Mobil Five) zeolite membranes can be manufactured by in situ crystallization using an intermediate amorphous SiO₂ layer. The improved in-plane growth by using a zeolite growth modifier leads to fusion of independent crystals and eliminates boundary gaps, giving good selectivity in the separation of CO₂/Xe mixtures. The fast diffusion of CO₂ dominates the overall membrane selectivity toward the CO₂/Xe mixture. Because of the straight and short [010] channels, the obtained CO₂ permeation fluxes are several orders of magnitude higher than those of carbon molecular sieving membranes and polymeric membranes, opening opportunities for Xe recovery from waste anesthetic gas.

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During the last decade, the synthesis and application of metal-organic framework (MOF) nanosheets has received growing interest, showing unique performances for different technological applications. Despite the potential of this type of nanolamellar materials, the synthetic routes developed so far are restricted to MOFs possessing layered structures, limiting further development in this field. Here, a bottom-up surfactant-assisted synthetic approach is presented for the fabrication of nanosheets of various nonlayered MOFs, broadening the scope of MOF nanosheets application. Surfactant-assisted preorganization of the metallic precursor prior to MOF synthesis enables the manufacture of nonlayered Al-containing MOF lamellae. These MOF nanosheets are shown to exhibit a superior performance over other crystal morphologies for both chemical sensing and gas separation. As revealed by electron microscopy and diffraction, this superior performance arises from the shorter diffusion pathway in the MOF nanosheets, whose 1D channels are oriented along the shortest particle dimension.

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The development of new membranes with high H₂ separation performance under industrially relevant conditions (high temperatures and pressures) is of primary importance. For instance, these membranes may facilitate the implementation of energy-efficient precombustion CO₂ capture or reduce energy intensity in other industrial processes such as ammonia synthesis. We report a facile synthetic protocol based on interfacial polymerization for the fabrication of supported benzimidazole-linked polymer membranes that display an unprecedented H₂/CO₂ selectivity (up to 40) at 423 K together with high-pressure resistance and long-term stability (>800 hours in the presence of water vapor).

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Membrane-based separation has become a promising alternative to traditional separation processes to capture CO2 owing to the great features such as energy efficiency and environmental friendliness. Polymers are easy to process and have been commercialized. However, most commercial polymer membranes suffer from a trade-off relation between gas permeability and selectivity, expressed as the Robeson upper bound1. Porous organic frameworks (POFs) are an emerging class of microporous polymers, which may have high CO2 permeability and selectivity when being processed into membranes due to their intrinsic porosity and strong CO2 adsorption ability. However, using POFs as membranes are still at the infancy stage due to their insolubility in most common solvents. Thus, this thesis focusses on the development of porous organic frameworks (POFs) membranes for various CO2 separation applications, including biogas upgrading, (Chapter 2), post-combustion CO2 capture (Chapter 3 and 4) and pre-combustion capture (Chapter 5). The fully organic nature together with the excellent thermal and chemical stabilities make POFs promising to be used as membranes for CO2 separation. @en

Recently various porous organic frameworks (POFs, crystalline or amorphous materials) have been discovered, and used for a wide range of applications, including molecular separations and catalysis. Silicon nanowires (SiNWs) have been extensively studied for diverse applications, including as transistors, solar cells, lithium ion batteries and sensors. Here we demonstrate the functionalization of SiNW surfaces with POFs and explore its effect on the electrical sensing properties of SiNW-based devices. The surface modification by POFs was easily achieved by polycondensation on amine-modified SiNWs. Platinum nanoparticles were formed in these POFs by impregnation with chloroplatinic acid followed by chemical reduction. The final hybrid system showed highly enhanced sensitivity for methanol vapour detection. We envisage that the integration of SiNWs with POF selector layers, loaded with different metal nanoparticles will open up new avenues, not only in chemical and biosensing, but also in separations and catalysis.

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Recently various porous organic frameworks (POFs, crystalline or amorphous materials) have been discovered, and used for a wide range of applications, including molecular separations and catalysis. Silicon nanowires (SiNWs) have been extensively studied for diverse applications, including as transistors, solar cells, lithium ion batteries and sensors. Here we demonstrate the functionalization of SiNW surfaces with POFs and explore its effect on the electrical sensing properties of SiNW-based devices. The surface modification by POFs was easily achieved by polycondensation on amine-modified SiNWs. Platinum nanoparticles were formed in these POFs by impregnation with chloroplatinic acid followed by chemical reduction. The final hybrid system showed highly enhanced sensitivity for methanol vapour detection. We envisage that the integration of SiNWs with POF selector layers, loaded with different metal nanoparticles will open up new avenues, not only in chemical and biosensing, but also in separations and catalysis.

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Mixed-matrix membranes (MMMs) comprising Matrimid and a microporous azine-linked covalent organic frameworks (ACOF-1) were prepared and tested in the separation of CO₂ from an equimolar CO₂/CH₄ mixture. The COF-based MMMs show a more than doubling of the CO₂ permeability upon 16 wt % ACOF-1 loading together with a slight increase in selectivity compared to the bare polymer. These results show the potential of COFs in the preparation of MMMs.

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