High Gas Selectivity Research Articles

Tailoring pores of AFN-related zeolites for enhanced separation of CO2/N2 and CO2/CH4

Zeolites with small-pores (8-ring) are widely used in separation of small-molecule gas, however, the precise tuning of aperture sizes to achieve the ideal selectivity still remains challenge due to their limited topology structures. In this work, SAPO-14 zeolite with AFN topology featuring by different 8-ring windows in three-dimension directions has been synthesized and studied in the separation of CO2/N2 or CO2/CH4. Aiming to further adjust pores and channels of the adsorbents, the composite zeolites of SAPO-14 and SAPO-34 (CHA) are prepared by seed-assisted method, and multi-cycle crystallization is adopted to form the interconnected pores of composites and reduce the crystal size. SAPO-14/SAPO-34 zeolites exhibit the comprehensive performance of high CO2 capacity of SAPO-34 and good selectivity of SAPO-14 due to their appropriate pore structure. At 298 K, the CO2 capacity of S-1 composite is 47.24 cm3 g−1, giving a high gas selectivity for CO2/N2 (68.64) and CO2/CH4 (21.61). The highly-efficiency and good reproducibility in gas separation are verified by the binary breakthrough experiments. The strategy of composite zeolite proposed by this work can effectively regulate the pores of adsorbent, thus endow them acceptable selectivity and capacity for target gas.

Innovative thermal crosslinked polyimide gas separation membrane with highly selective and resistance to physical aging base on phenyl ethynyl

In this study, a high performance phenyl ethynyl thermal crosslinked 6FDA-based polyimide (CR-4ETA-x) was developed, which has high gas permeability and selectivity, resistance to physical aging. By introducing 4,4′-(Acetylene-1,2-diyl) diphthalic anhydride (4ETA) monomer into the polyimide structure, the thermal crosslinking precursor membrane was formed. The precursor membrane was heat treated at 440℃ to obtain CR-4ETA-x. After heat treatment, we detected the olefin radical in the crosslinked membrane by electron paramagnetic resonance, which proved that the polyimide network was formed. This allows the crosslinked membrane to have greater d-spacing, higher rigidity, and BET surface area. The CO2 permeability of CR-4ETA-5% is 4114 barrer and the CO2/CH4 selectivity is 19.22. Compared to 6FDA-DAM, the CO2 permeability increased by 477% while the selectivity decreased by only 17%. CR-4ETA-20% shows the CO2 permeability of 3560 Barrer and CO2/CH4 selectivity of 21.1 in CO2/CH4 mixed-gas. After 60 days of aging the permeability maintenance of CR-4ETA-20% were 88%, 84%, 83% and 91% for O2, N2, CO2 and CH4. In addition, in the long-term stability test of more than 100 h, the separation performance of CR-4ETA-20% did not decrease significantly. However, CR-4ETA-x also has limitations, such as poor stability in alkaline environments and lower gas permeability than some advanced carbon molecular sieve membranes. In general, the method of phenyl ethynyl thermal crosslinking provides an effective way for the development of CO2 separation membranes with possible applications.

Carbon molecular sieve gas separation membranes derived from in-situ crosslinked polyimide containing bromine groups

The network polyimide (PI) precursor is a promising candidate to fabricate the advanced carbon molecular sieve (CMS) membrane since its robust crosslinked structure could prevent pore structure collapse during CMS formation. Exploring the relationship between network PI precursor structure and CMS membrane is highly significant. Herein, a novel brominated network PI precursor was fabricated via in-situ cross-linking technique firstly, and then the thermally induced bromination/debromination process was applied to tune the microporosity and gas transport properties of network PI precursor and its derived CMS membranes. After the bromination/debromination and carbonization at 550 °C, the resultant CMS membrane exhibited both high gas permeability and gas selectivities compared to its un-brominated network PI precursor derived CMS membrane. This is due to the bromination/debromination and carbonization enable the CMS membrane possesses large d-spacing values (11.06 Å), high BET surface areas (538.57 m2/g), and high percentage (∼71 %) of ultra-micropores. For instance, 6F-TA/TA-Br-550/30 membrane exhibits great CO2/CH4 separation ability, with a CO2 permeability of 6300 Barrer and a CO2/CH4 selectivity of 46.11, which is suppressed the latest 2019 trade-off curves. We anticipate that the present bromination/debromination and carbonization could provide a fresh perspective on the rational design of network PI precursor and its derived CMS membrane with excellent gas separation performance.

Experimental and theoretical studies of sputter deposited pure SnO2 thin films for high selective and humidity-tolerant H2 gas sensor

Experimental and theoretical studies of sputter deposited pure SnO2 thin films for high selective and humidity-tolerant H2 gas sensor

Ultra-thin V2O5 nanowires: synthesis and gas sensing characteristics

This study presents the synthesis of vanadium oxide nanowires via a simple hydrothermal method and explores their potential as high-performance sensors for monitoring harmful gases, with a particular focus on NO2. The microstructure and morphology of the nanowires were characterized using scanning electron microscopy, powder x-ray diffraction, and Raman spectroscopy. The vanadium oxide nanowire material demonstrates outstanding NO2 gas sensing capabilities, detecting 5 ppm with a rapid response and high sensitivity at an optimal working temperature of 150 °C. It exhibits a relative resistance change of 70%, showcasing a sub-ppm detection limit. The V2O5 nanowires exhibited good stability and high gas selectivity for NO2 over other interfering gases (H2S, NH3, C2H4, and CO). The ultrathin structure of the nanowires holds promise for practical applications in developing NO2 gas sensors. The study sheds light on the superior sensitivity of the V2O5 gas sensor toward NO2 at low temperatures, emphasizing the influence of the 1D structure on the sensing mechanism.

Enhanced gas separation performance for H2 purification using MIL-68(ln)-nh2/PES mixed-matrix membranes

The growing demand for sustainable and efficient gas separation technologies has prompted the exploration of advanced materials to enhance the gas permeability and selectivity. Polyethersulfone (PES) membranes are widely used in gas separation, gas upgrading, and clean energy production owing to their environmental friendliness and low cost. However, their gas permeability and selectivity can be further improved for commercial application. This study explored the incorporation of 10 wt % of MIL-68(ln)-NH2 into PES membranes using a phase-inversion approach to enhance gas permeability and selectivity. The morphological, structural, and thermal properties of the resulting MOF/PES membrane were characterized using SEM, AFM, BET, XRD, FTIR, and TGA-DTG. Gas permeation experiments were conducted using different gases (CO2, N2, CH4, and H2) under different heating conditions (20–60 °C) to evaluate the gas permeability and selectivity of the MOF/PES membrane. The results showed that the incorporation of MOF into the mixed matrix membrane (MMMs) led to a 9% increase in porosity, 87% reduction in roughness, and 32% decrease in pore size compared to neat PES membranes. Significant changes in the morphology, crystallinity, and thermal stability were observed, with notable improvements of up to 22%. Moreover, the MOF/PES membrane exhibited high gas permeability (CO2 = 124656, N2 = 83650, CH4 = 159298, and H2 = 427075 Barrer) and selectivity (H2/N2 = 5.7, H2/CO2 = 4, CH4/N2 = 2, and CH4/CO2 = 1.7) for flammable gases. The optimal gas separation performance was observed at 20 °C and 60 °C for H2/N2 and H2/CO2 separation, respectively. These findings demonstrate the potential of MOF-based PES membranes for gas separation applications, particularly in H2 purification.

Large-Area Ultrathin Metal-Organic Framework Membranes Fabricated on Flexible Polymer Supports for Gas Separations.

Ultrathin continuous metal-organic framework (MOF) membranes have the potential to achieve high gas permeance and selectivity simultaneously for otherwise difficult gas separations, but with few exceptions for zeolitic-imidazolate frameworks (ZIF) membranes, current methods cannot conveniently realize practical large-area fabrication. Here, we propose a ligand back diffusion-assisted bipolymer-directed metal ion distribution strategy for preparing large-area ultrathin MOF membranes on flexible polymeric support layers. The bipolymer directs metal ions to form a cross-linked two-dimensional (2D) network with a uniform distribution of metal ions on support layers. Ligand back diffusion controls the feed of ligand molecules available for nuclei formation, resulting in the continuous growth of large-area ultrathin MOF membranes. We report the practical fabrication of three representative defect-free MOF membranes with areas larger than 2,400 cm2 and ultrathin selective layers (50-130 nm), including ZIFs and carboxylate-linker MOFs. Among these, the ZIF-8 membrane displays high gas permeance of 3,979 GPU for C3H6, with good mixed gas selectivity (43.88 for C3H6/C3H8). To illustrate its scale-up practicality, MOF membranes were prepared and incorporated into spiral-wound membrane modules with an active area of 4,800 cm2. The ZIF-8 membrane module presents high gas permeance (3,930 GPU for C3H6) with acceptable ideal gas selectivity (37.45 for C3H6/C3H8).

Large‐Area Ultrathin Metal–Organic Framework Membranes Fabricated on Flexible Polymer Supports for Gas Separations

AbstractUltrathin continuous metal–organic framework (MOF) membranes have the potential to achieve high gas permeance and selectivity simultaneously for otherwise difficult gas separations, but with few exceptions for zeolitic‐imidazolate frameworks (ZIF) membranes, current methods cannot conveniently realize practical large‐area fabrication. Here, we propose a ligand back diffusion‐assisted bipolymer‐directed metal ion distribution strategy for preparing large‐area ultrathin MOF membranes on flexible polymeric support layers. The bipolymer directs metal ions to form a cross‐linked two‐dimensional (2D) network with a uniform distribution of metal ions on support layers. Ligand back diffusion controls the feed of ligand molecules available for nuclei formation, resulting in the continuous growth of large‐area ultrathin MOF membranes. We report the practical fabrication of three representative defect‐free MOF membranes with areas larger than 2,400 cm2 and ultrathin selective layers (50–130 nm), including ZIFs and carboxylate‐linker MOFs. Among these, the ZIF‐8 membrane displays high gas permeance of 3,979 GPU for C3H6, with good mixed gas selectivity (43.88 for C3H6/C3H8). To illustrate its scale‐up practicality, MOF membranes were prepared and incorporated into spiral‐wound membrane modules with an active area of 4,800 cm2. The ZIF‐8 membrane module presents high gas permeance (3,930 GPU for C3H6) with acceptable ideal gas selectivity (37.45 for C3H6/C3H8).

Tunable Gas Admission via a "Molecular Trapdoor" Mechanism in a Flexible Cationic Metal-Organic Framework Featuring 1D Channels.

Achieving high gas selectivity is challenging when dealing with gas pairs of similar size and physiochemical properties. The "molecular trapdoor" mechanism discovered in zeolites holds promise for highly selective gas adsorption separation but faces limitations like constrained pore volume and slow adsorption kinetics. To address these challenges, for the first time, a flexible metal-organic framework (MOF) featuring 1D channels and functioning as a "molecular trapdoor" material is intoduced. Extra-framework anions act as "gate-keeping" groups at the narrowest points of channels, permitting gas admissions via gate opening induced by thermal/pressure stimuli and guest interactions. Different guest molecules induce varied energy barriers for anion movement, enabling gas separation based on distinct threshold temperatures for gas admission. The flexible framework of Pytpy MOFs, featuring swelling structure with rotatable pyridine rings, facilitates faster gas adsorption than zeolite. Analyzing anion properties of Pytpy MOFs reveals a guiding principle for selecting anions to tailor threshold gas admission. This study not only overcomes the kinetic limitations related to gas admission in the "molecular trapdoor" zeolites but also underscores the potential of developing MOFs as molecular trapdoor adsorbents, providing valuable insights for designing ionic MOFs tailored to diverse gas separation applications.

High selectivity and sensitivity gas sensors to n-butanol based on Ni-doped ultra-thin porous single-crystalline ZnO nanosheets

Ni doped porous single-crystalline (PSC) ZnO nanosheets were successfully synthesized through a facile wet-chemical method. The as-prepared products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectrum (EDS). It can be found that the Ni doped PSC ZnO nanosheets with a few micrometers in length and 8–10 nm in thickness. The gas sensing properties of the Ni doped PSC ZnO nanosheets were systematically investigated. And the results reveal that the 2 at% Ni doped PSC ZnO nanosheets shows dramatically high sensitivity and selectivity for the detection of n-butanol compared to the performance of pure ZnO nanosheets. The as-prepared products combined the advantages of porous structure, single-crystalline, and ultra-thin thickness, and Ni doping, which definitely resulted in high sensing performance and selectivity property in n-butanol detection. Therefore, the doping of Ni into PSC ZnO nanosheets may be a promising method to develop advanced nanomaterials for the practical application of n-butanol detection in complex gas atmosphere.

Temperature-modulated UV-Enhanced SnO2 Gas Sensor Cell with High-Stability and High-Selectivity Gas Discrimination

Temperature-modulated UV-Enhanced SnO₂ Gas Sensor Cell with High-Stability and High-Selectivity Gas Discrimination

3D ZnO/ZIF‐8 Hierarchical Nanostructure for Sensitive and Selective NO2Sensing at Room Temperature

Gas sensors based on semiconductor metal oxides (SMOs) have gained widespread attention for Internet of Things applications; however, high operating temperatures and low gas selectivity limit their applications. Recently, metal–organic frameworks (MOFs) have demonstrated potential in enhancing gas selectivity through the physical filtration of gas molecules based on their kinetic diameters. However, their application has been predominantly limited to simplistic nanostructured sensors. These sensors exhibit inherently inferior gas sensor performance compared to three‐dimensional nanostructure gas sensors. In this study, a highly periodic, 3D hierarchical ZnO/ZIF‐8 nanostructure is fabricated for photoactivated gas sensing at room temperature. Under UV illumination, the gas sensor exhibited a 17‐fold enhancement in gas response toward 0.1 ppm NO2compared to pristine ZnO. In addition, the ZIF‐8 coating selectively increased the NO2gas response compared to ethanol, acetone, and toluene gases, thereby improving the gas selectivity. The gas response improvement by the ZIF‐8 layer coating, which has not been achieved by previous studies, is based on enhanced photoactivation by the solid interaction between ZIF‐8 and ZnO. These results provide a systematic background for controlling the layer thickness of SMO‐MOF nanostructures and the catalytic role of MOF in photoactivated gas sensing.

Assembling ionic liquid into porous molecular filler of mixed matrix membrane to trigger high gas permeability, selectivity, and stability for CO2/CH4 separation

Assembling ionic liquid into porous molecular filler of mixed matrix membrane to trigger high gas permeability, selectivity, and stability for CO2/CH4 separation

Excellent gas separation performance of hybrid carbon molecular sieve membrane derived from polyimide/10X zeolite for hydrogen purification

Membrane-based gas separation is attracting more and more attention in hydrogen purification and CO2 separation. One critical challenging remaining for large-scale production is fabricating a high-performance membrane with both inexpensive and excellent separation performance. In this work, a novel hybrid-carbon molecular sieve (CMS) membranes were reported for hydrogen purification by pyrolysis of commercial polyimide precursor (PMDA-ODA) with incorporation of modified 10X zeolite. The addition of modified 10X zeolite provided additional gas transport channels to the CMS mixed matrix membrane (MMM), and contained a significant fraction of micropores within the 0.6–0.8 nm range, which greatly enhance hydrogen adsorption capacity. Interestingly, despite this, the selectivity for H2/N2 and H2/CH4 also increases with higher zeolite content. This unexpected phenomenon may be attributed to the partial blocking of zeolite pores by CMS strand, which narrows the 10X pore size to a dimension smaller than CH4 and N2, enabling differentiation between H2 and CH4 gas pairs. Therefore, the permeability and selectivity of H2 was significantly enhanced. The CMS/10X-7.5 (with 7.5 wt % 10X zeolite) exhibited an advanced H2 permeability (1709 Barrer) and high gas selectivity of 551 for H2/CH4 and 105 for H2/N2, respectively, which is higher than other samples and exceed the 2015 trade-off lines. Furthermore, the CMS/10X-7.5 delivered excellent stability after 20 cycles. Therefore, this hybrid CMS/10X-7.5 obtained by carbonization of zeolite-doped polymer precursors has great potential for larger industrial applications.

Fiber optic room temperature ethanol sensor based on ZnSnO3/TiO2 with UV radiation sensitization

A fiber optic ethanol gas sensor with ZnSnO3 hollow cube/nano-TiO2 composite coating based on ultraviolet (UV) radiation sensitization was proposed, which can realize high sensitivity, high selectivity, and fast ethanol gas sensing at room temperature. The probe adopted a simple single-mode fiber (SMF)-multi-mode fiber-SMF structure. The ZnSnO3/TiO2 gas-sensitive film was coated on the sensing area by dip-coating method. Experiment results show that the probe sensitized by UV radiation has a linear sensitivity of 3.85 pm/ppm for ethanol gas at 0–250 ppm, which is about 6.6 times more sensitive than the probe without UV radiation (0.582 pm/ppm). In addition, the probe with UV radiation has good time measurement stability, a fast response time (response time is about 20 s, recovery time is about 60 s), and excellent ethanol gas selectivity. This study proves that UV radiation is an effective method to increase the sensitivity of gas sensing. This is also the first application of ZnSnO3/TiO2 composite material in fiber optic sensors. Combined with the advantages of fiber optic sensing, it shows anti-electromagnetic interference, micro size, high sensitivity, high selectivity, fast response, and remote sensing.

The Coordination Nanocages-Integrated Polymer Brush Networks for Flexible Microporous Membranes with Exceptional H2 /CO2 Separation Performance.

The emergence of polymers with intrinsic microporosity provides solutions for flexible gas separation membranes with both high gas permeability and selectivity. However, their applications are significantly hindered by the costly synthetic efforts, limited availability of chemical systems, and narrow window of microporosity sizes. Herein, flexible mixed matrix membranes with tunable intrinsic microporosity can be facilely fabricated from the coordination assembly of polymer brushes and coordination nanocages. Polymer brushes bearing isophthalic acid side groups can coordinate with Cu2+ to assemble into polymer networks crosslinked by 2nm nanocages. The semi-flexible feature of the polymer brush and the high crosslinking density of the network prevent the network from collapsing during solvent removal and the obtained aerogels demonstrate hierarchical structure with dual porosity from the crosslinked polymer network and coordination nanocage, respectively. The porosity can be facilely tuned via the amount of Cu2+ by regulating the network crosslinking density and nanocage loadings, and finally, optimized gas separation that surpasses Robeson upper bound for H2 /CO2 can be achieved. The coordination-driven assembly protocol paves a new avenue for the cost-effective synthesis of polymers with intrinsic microporosity and the fabrication of flexible gas separation membranes.

ZIF-8 @polycarbonate nanocomposite membranes for improved permeability and selectivity for hydrogen gas.

Through this work, we are reporting high-performance ZIF-8 @polycarbonate nanocomposite membranes with satisfactory structural stability for improving the gas separation performance. ZIF-8 nanoparticles were synthesised using the wet chemical route with cubic morphology and controlled size using CTAB as a surfactant. The membranes were prepared using the solution casting method by adding ZIF-8 filler at various concentrations. The synthesised filler material and MMMs were characterised through X-ray diffraction spectroscopy (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), andRAMAN spectroscopy techniques. The gas separation measurements were taken using H2, CO2, and N2 gas in the purest form. The SEM results confirm the formation of spherulite-like morphology with the addition of ZIF-8 due to the crystallisation of the polymer, which increased the membrane's free volume and opened up additional pathways for the transportation of the gas molecules. The gas separation results confirmed that the 15 wt% ZIF-8/PC nanocomposite membrane showed the maximum H2 permeability of 180,970 barrer with an increment of 316.03%, while H2/CO2 and H2/N2 selectivity showed the increments of 89.43% and 103.64%, respectively. Therefore, this PC/ZIF-8 system seems to be a promising approach to developing new H2 selective membranes with high gas permeability and gas selectivity values.

Hydrothermal synthesis of CaFe2O4/Zn2SnO4 composite with its enhanced formaldehyde gas sensing property

Formaldehyde is a harmful volatile organic pollutant to the humans. Here, the CaFe2O4/Zn2SnO4 composite were synthesized by hydrothermal method and the crystal phase, morphologly structures and chemical composition et al. were characterised by XRD, SEM, TEM, BET and XPS. The CaFe2O4/Zn2SnO4 gas sensor displays a high gas response (52), excellent repeatability and selectivity to HCHO at 400 °C. This unique CaFe2O4/Zn2SnO4 composite is available as a potential gas-sensing material to monitor formaldehyde.

High-performance selective NO2 gas sensor based on In2O3–graphene–Cu nanocomposites

The control of atmosphere content and concentration of specific gases are important tasks in many industrial processes, agriculture, environmental and medical applications. Thus there is a high demand to develop new advanced materials with enhanced gas sensing characteristics including high gas selectivity. Herein we report the result of a study on the synthesis, characterization, and investigation of gas sensing properties of In2O3–graphene–Cu composite nanomaterials for sensing elements of single-electrode semiconductor gas sensors. The nanocomposite has a closely interconnected and highly defective structure, which is characterized by high sensitivity to various oxidizing and reducing gases and selectivity to NO2. The In2O3-based materials were obtained by sol–gel method, by adding 0–6 wt% of pre-synthesized graphene–Cu powder into In-containing gel before xerogel formation. The graphene–Cu flakes played the role of centers for In2O3 nucleation and then crystal growth terminators. This led to the formation of structural defects, influencing the surface energy state and concentration of free electrons. The concentration of defects increases with the increase of graphene–Cu content from 1 to 4 wt%, which also affects the gas-sensing properties of the nanocomposites. The sensors show a high sensing response to both oxidizing (NO2) and reducing (acetone, ethanol, methane) gases at an optimal working heating current of 91–161 mA (280–510 °C). The sensor with nanocomposite with 4 wt% of graphene–Cu additive showed the highest sensitivity to NO2 (46 ppm) in comparison with other tested gases with an absolute value of sensing response of (− ) 225 mV at a heating current of 131 mA (430 °C) and linear dependence of sensing response to NO2 concentration.

Green Synthesis of MOF-801(Zr/Ce/Hf) for CO2/N2 and CO2/CH4 Separation.

The purification of natural gas and the removal of carbon dioxide from flue gases are crucial to economize precious resources and effectively relieve a series of environmental problems caused by global warming. Metal-organic framework (MOF) materials have demonstrated remarkable performance and benefits in the area of gas separation; however, obtaining materials with high gas capacity and selectivity simultaneously remains difficult. In addition, harsh synthesis conditions and solvent toxicity have been restricted in large-scale production and industrial application. Therefore, MOF-801(Zr/Ce/Hf) was created based on the green synthesis of the MOF-801 construction unit by altering the kinds of metal salts, and the impact of three metal nodes on the performance of gas adsorption and separation was demonstrated by contrasting the three MOFs. The results showed that MOF-801(Ce) has the best CO2 adsorption capacity (3.3 mmol/g at 298 K), which also was demonstrated with in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results, CO2/CH4 (ideal adsorbed solution theory (IAST) = 13.28 at 298 K, 1 bar, CO2/CH4 = 1:1, v/v), and the separation performance of CO2/N2 (IAST = 57.46 at 298 K, 1 bar, CO2/N2 = 1:1, v/v) among the group. Green synthesis of MOF-801(Zr/Ce/Hf) is an ideal candidate for flue gas separation and methane purification because of its high regeneration capacity and strong cyclic stability.