High Gas Selectivity Research Articles

Study on the selectivity difference of formaldehyde and ethanol induced by variation of energy gap in In2O3 hierarchical materials

Poor selectivity has been one of the most serious problems restricting the development of metal oxide gas sensors. Obtaining sensing materials with excellent selectivity and studying the dominate factors and mechanism affecting selectivity are of great significance to the application and development of gas sensors. Here, we have prepared a series of In2O3 hierarchical spheres (HS) samples which exhibited distinct sensing performance toward formaldehyde and ethanol by varying the solvent-urea. In particular, In2O3-0.05 HS have shown excellent selectivity to formaldehyde at low temperature (180 °C). With the increase of reaction solute-urea, the selectivity of the sample to formaldehyde decreases, but the sensitivity to ethanol increases. The UV–vis absorption spectrum test shows that the variation of urea changes the energy gap of samples, leading to different selectivity toward ethanol and formaldehyde. Especially, the response of In2O3-0.05 HS to 100 ppm formaldehyde is 10 times higher than the response to other gas. And the response of In2O3-0.8 HS to 100 ppm ethanol is as high as 24 at 300 °C, which is 3 times higher than that of In2O3-0.05 HS, suggesting an obvious link between energy gap and sensing properties. This experiment not only promotes the understanding of relationship between sensing performance and energy gap, but also provides a new method for designing high-selectivity gas sensors.

Molecular imprinted polymer modified terahertz metamaterial sensor for specific detection of gaseous hexanal

Gaseous hexanal is a biomarker for the diagnosis of human lung cancer. In this work, a molecular imprinted polymer (MIP) modified terahertz metamaterial sensor was demonstrated for specific detection of gaseous hexanal. The experimental results show that the transmission spectrum of the modified sensor has an obvious frequency shift under different concentration of gaseous hexanal. The modified sensor can detect gaseous hexanal in concentrations ranging from 100 PPM to 900 PPM with a maximum frequency shift of 12 GHz. In addition, the modified sensor has good repeatability and less response to interfering substances. This study provided a new idea for developing high selectivity gas sensors.

Diamond in the rough: Polishing waste polyethylene terephthalate into activated carbon for CO2 capture

The scientific community has believed the potential of waste PET plastics as an effective carbon precursor, however, developing PET-derived activated carbons (PETACs) for a specific application is still a challenge we are facing. To overcome the limitation, a whole chain from development method screening to experiments design, finally to sample optimization, for a sample with promising performance, is proposed in this work. By employing PETACs as CO2 adsorbents, the waste PET plastics, which we believed the “diamond in the rough”, have been polished successfully. Therewith the problems of plastic pollution and the greenhouse effect could be simultaneously solved. The first half part of this paper is a mini review: the PETACs development methods were reviewed and the most suitable solution to develop CO2 adsorbent, i.e., the two-step chemical activation method, was selected. In addition to that, the necessary procedure variables and their value range were determined. In the second half part, the central composite design method was applied for experiments design in which the procedure variables obtained were regarded as the independent indicators (factors here) while the performance indicators, including yield, CO2 adsorption uptake, and CO2 over N2 selectivity, were treated as the dependent indicators (responses here). The responses were obtained through the characterization of the samples developed and statistical analysis could be applied to reveal the relations between the factors and responses. A high-value PETAC, P600K600-1.5, with the highest gas selectivity (22.189) and decent CO2 adsorption uptake (3.933 mmol/g) was successfully designed.

Mixed-Linker Metal-Organic frameworks for carbon and hydrocarbons capture under moist conditions

• MOF-801-FA were synthesised via utilizing a mixed-ligand strategy. • Gas adsorption was boosted, e.g., a 2.5-fold increase for methane capture. • Prominent IAST selectivities were witnessed for CH 4 /N 2 and CO 2 /N 2 mixture. • MOF-801-FA reveal clean and sharp separations in a high relative humidity. The capture of carbon and hydrocarbons is significant to the utilization of valuable resources and the mitigation of global warming, but achieving high gas capacity and selectivity at the same time remains a challenge, especially in the presence of high humidity. In this regard, based on the building block of MOF-801, MOF-801-FA structures were synthesized with high quality and few defects through a mixed-ligand strategy, in which formate ligands were introduced into the framework apart from fumarate ligands. This brought about tremendous increases in gas adsorption capacity, e.g., ca. 2.5-fold and 1.1-fold enhancement occurred to the adsorption of CH 4 and CO 2 in MOF-801-Hf-FA than MOF-801-Hf at 298 K and 1 bar. Besides, prominent IAST selectivities were also witnessed in MOF-801-FA, higher than those in the pristine MOFs, for CH 4 /N 2 and CO 2 /N 2 mixtures under ambient conditions. We then confirmed the efficient performance of MOF-801-FA for real CH 4 /N 2 and CO 2 /N 2 mixtures using the breakthrough experiments and following cycling tests, even in a relative humidity of 90%. Thus, the mixed-linker strategy was demonstrated effective to tune the structures of MOFs, and boost their performance in gas adsorption and separation. The results also showed that, with high capacities for CH 4 and CO 2 , superior selectivities over N 2 , desirable water resistance and facile regeneration, MOF-801-M−FA (M = Zr or Hf) are promising candidates for capturing CH 4 and CO 2 from N 2 in industry.

Copper nanoparticles decorated N-doped mesoporous carbon with bimodal pores for selective gas separation and energy storage applications

• Mesoporous N & Cu doped carbon materials with enhanced physico-chemical features. • Materials display redox, conducting, basic, adsorption and porous characteristics. • N/Cu doped carons show ordered structure with a bimodal pores size distribution. • High gas adsorption capacity and selectivity in a mixture of CO 2 /H 2 and CO 2 /CH 4. • High values for supercapacitance with good cyclic ability and capacitance retention. We demonstrate a synthesis of copper nanoparticles decorated over nitrogen-doped mesoporous carbon with different N and Cu contents which exhibit conducting, redox, basic, adsorption, and excellent textural properties. These materials are prepared through a nanotemplating approach by simultaneously encapsulating sucrose, guanidine hydrochloride, and Cu(NO 3 ) 2 into the porous channels of mesoporous SBA-15 at a low carbonization temperature of 600 °C. The prepared materials exhibit an ordered mesoporous carbon framework with bimodal pores, decorated with nitrogen and Cu functionalities on the surface of the pores and in the wall structure. The presence of nitrogen functionalities in the porous carbon matrix not only helps to reduce the Cu ions but also stabilizes the nanoparticles and offers redox sites, which are beneficial for adsorption and electrochemical applications. The optimized sample exhibits the highest adsorption capacity of different gases such as CO 2 – 22.5 mmol/g at 273 K, H 2 −13.5 mmol/g at 77 K at 30 bar and CH 4 − 5 mmol/g at 298 K and 50 bar. We also demonstrate that the prepared material shows a high selectivity of adsorption towards CO 2 in a mixture of CO 2 /H 2 and CO 2 /CH 4 and it also registers a high supercapacitance of 209 F g −1 at a current density of 1 A g −1 with excellent cyclic stability.

Polybenzimidazole-derived carbon molecular sieve hollow fiber membranes with tailored oxygen selective transport

Polymer membrane carbonization is a promising strategy to obtain carbon molecular sieve (CMS) membrane materials with tailorable cavity size distributions for high gas selectivities beyond that attained from traditional polymer membrane materials. Despite their already demonstrated exceptional separation performance characteristics for several gas separation applications (e.g. CO2/CH4 and H2/CO2), further development of CMS membranes having high O2/N2 perm-selectivity for energy-efficient high-purity O2 production has proven challenging. Herein, we explore CMS hollow fiber membranes (HFMs) derived from highly rigid and tightly-packed polybenzimidazole (PBI) materials. Nearly defect-free PBI-derived CMS HFMs were fabricated and evaluated for O2/N2 separation performance for the first time. The micro-structural and O2 selective gas separation characteristics of PBI-CMS HFMs pyrolyzed at different pyrolysis conditions were intensively studied. With rigorous carbonization protocol optimization, we obtained highly O2 permselective PBI-CMS HFMs having O2/N2 selectivities approaching 14, enabling high purity O2 production. The PBI-CMS HFM O2/N2 separation performance dependence on operating conditions (e.g. temperature and feed pressure) are reported.

MOF-based MMMs breaking the upper bounds of polymers for a large variety of gas separations

• Permeabilities of CO 2 , CH 4 , N 2 , O 2 , H 2 , and He gases were reported for 5599 MOFs. • 181,249 types of MOF/polymer MMMs were studied for 11 different gas separations. • A large number of MOF/polymer MMMs exceeded the upper bounds of polymers. • A guideline was provided for selection of the best MOF-polymer combinations. Mixed matrix membranes (MMMs) are a strong alternative to the conventional polymeric membranes which suffer from a trade-off between selectivity and gas permeability. Considering the existence of a large number of metal–organic frameworks (MOFs) and polymers, computational screening of MOF/polymer MMMs is strongly desired to explore their upper limits in various gas separation processes before experimental efforts. In this work, we computed permeability of CO 2 , CH 4 , N 2 , O 2 , H 2 , and He gases in 5599 metal organic frameworks (MOFs) using atomically-detailed simulations and then assessed performances of >180,000 different MOF/polymer MMMs for 11 different gas separations He/H 2 , He/N 2 , He/CH 4 , N 2 /CH 4 , H 2 /N 2 , H 2 /CH 4 O 2 /N 2 , CO 2 /N 2 , CO 2 /CH 4 , H 2 /CO 2 , He/CO 2 . Our results revealed that many MOF/polymer MMMs exceed the upper bounds due to their high gas permeability and/or selectivity. The impact of MOFs on the separation performances of polymers was examined to provide guidelines for the best MOF-polymer pairing for a variety of gas separations. Data showed that using MOFs as fillers significantly improves the permeability of CO 2 , CH 4 , N 2 , O 2 , H 2 , and He gases in 41 different types of polymers without changing their selectivities. Many MOFs offer a great opportunity for MMM applications by improving both the permeability and selectivity of polymers such as of Teflon AF-2400 for N 2 /CH 4 , PTMSP-co(95/5) for H 2 /CH 4 , PTMSDPA for O 2 /N 2 and PTMSP for CO 2 /CH 4 separations. The best MOF fillers leading to MMMs with exceptional selectivities were found to have narrow pores and low porosities. These results will contribute to directing the experimental efforts to the best MOF/polymer MMM materials for numerous industrially important gas separation applications.

Fabrication of Ag embedded−SnS films via the RF approach: First study on NO2 gas–sensing performance

We report on the synthesis of 5-nm thick-Ag embedded−SnS (Ag/SnS) films by the radio frequency (RF) sputtering method as an absorbent layer to detect NO2 gas. Structure, morphology, elements, and optical properties of the Ag/SnS films were studied by X − ray diffraction, scanning electron microscopy, energy−dispersive spectroscopy, Raman and photoluminescence techniques, respectively. We firstly investigated the sensitivity of 5 – 20 ppm concentration of the Ag/SnS films to NO2 gas at an operating temperature ranging 30 – 100 °C. It was indicated that the Ag/SnS film–based sensor got good response and recovery at an operating temperature of 80 °C for NO2 gas. The 100 nm thick SnS film−based sensor made a faster response time (40 s) and recovery time (1320 s) to 20 ppm NO2 gas at 30 °C than those of other SnS films−based sensors. The result shows that the Ag/SnS film might be suitable for improving NO2 sensor properties due to the enhanced hole mobility, and more defectiveness of the SnS monolayer (Sn–vacancy). Based on these findings, we propose the fabrication of nanometals on SnS film for the high sensitive and selective NO2 gas−sensing a lower detection limit and at room temperature.

Finely Tuning Tridentate Carboxylic Acids for the Construction of Rod Scandium Metal-Organic Frameworks with High Chemical Stability and Selective Gas Adsorption.

By systematic ligand tuning for control of the secondary building units, the use of tridentate carboxylic acid to construct the rod scandium metal-organic framework NTUniv-55 (NTUniv = Nantong University) with high chemical stability and interesting selective gas adsorption was reported.

Mesoporous Membrane Materials Based on Ultra-High-Molecular-Weight Polyethylene: From Synthesis to Applied Aspects.

The development of new porous polymeric materials with nanoscale pore dimensions and controlled morphology presents a challenging problem of modern materials and membrane science, which should be based on scientifically justified approaches with the emphasis on ecological issues. This work offers a facile and sustainable strategy allowing preparation of porous nanostructured materials based on ultra-high-molecular-weight polyethylene (UHMWPE) via the mechanism of environmental intercrystallite crazing and their detailed characterization by diverse physicochemical methods, including SEM, TEM, AFM, liquid and gas permeability, DSC, etc. The resultant porous UHMWPE materials are characterized by high porosity (up to ~45%), pore interconnectivity, nanoscale pore dimensions (below 10 nm), high water vapor permeability [1700 g/(m2 × day)] and high gas permeability (the Gurley number ~300 s), selectivity, and good mechanical properties. The applied benefits of the advanced UHMWPE mesoporous materials as efficient membranes, breathable, waterproof, and insulating materials, light-weight materials with reduced density, gas capture and storage systems, porous substrates and scaffolds are discussed.

Photo-enhanced acetone adsorption on δ-MnO2 nanoparticles: A step towards non-invasive detection of diabetes mellitus

δ-MnO2 nanoparticles were synthesized by one-pot route using precipitation method with and without poly-dimethyl-siloxane (PDMS) addition at room temperature. FE-SEM revealed that the nanoparticles have spherical morphology with porous nature. The addition of PDMS increased the rate of reaction for the formation of nanospheres. Their volatile organic compounds (VOCs) adsorption capability was investigated by measuring contact potential difference using scanning Kelvin probe system both in the dark and ultraviolet (UV) light illumination conditions. The pure δ-MnO2 nanoparticles showed high selective gas adsorption towards acetone vapours in comparison with other VOCs namely ethanol, n-Hexane and triethylamine under UV light illumination. Interestingly, δ-MnO2 nanoparticles prepared in the presence of PDMS had enhanced response of about 1.4 times towards acetone with respect to pure δ-MnO2 counterparts. Our results showed a potential pathway to develop low cost δ-MnO2 nanoparticles assisted by PDMS based acetone sensor for monitoring diseases such as diabetes mellitus.

Highly selective detection of acetone by TiO2-SnO2 heterostructures for environmental biomarkers of diabetes

The n-n TiO2-SnO2 heterostructures composed of SnO2 nanoparticles (NPs) topped with TiO2 NPs have been produced by a novel hybrid reactive magnetron sputtering and pulse laser deposition for sensing acetone (C3H6O) vapors in the environment. The gas sensing characteristics of the TiO2-SnO2 heterostructures were measured over a range of operating temperatures (200–500°C) and the optimal working temperature of 300 °C was fixed. Further, the adsorption of acetone gas on both (110) and (101) stoichiometric and reduced surfaces of TiO2-SnO2 are studied by density functional theory (DFT) calculations. The proposed TiO2-SnO2 sensor displayed an ultrasensitive response for detecting acetone vapors which is almost 12 times and a limit of detection of 0.02 ppm. DFT calculations showed that the adsorption of acetone gas on (110) and (101) facets is thermodynamically favorable. Moreover, TiO2-SnO2 heterostructures sensor displayed high gas response, selectivity, repeatability, and long-term stability. The developed sensor also demonstrated excellent resistance against humidity due to a good synergy between the component oxides, band bending, availability of chemisorbed oxygen, and adsorption-desorption mechanism at the TiO2-SnO2 heterojunction. This work provides a foundation for the development of advanced gas sensors for hazardous sensing of materials exposed to diabetic patients in the human environment.

Direct Chemical Vapor Deposition Synthesis of Porous Single-Layer Graphene Membranes with High Gas Permeances and Selectivities.

Single-layer graphene containing molecular-sized in-plane pores is regarded as a promising membrane material for high-performance gas separations due to its atomic thickness and low gas transport resistance. However, typical etching-based pore generation methods cannot decouple pore nucleation and pore growth, resulting in a trade-off between high areal pore density and high selectivity. In contrast, intrinsic pores in graphene formed during chemical vapor deposition are not created by etching. Therefore, intrinsically porous graphene can exhibit high pore density while maintaining its gas selectivity. In this work, the density of intrinsic graphene pores is systematically controlled for the first time, while appropriate pore sizes for gas sieving are precisely maintained. As a result, single-layer graphene membranes with the highest H2 /CH4 separation performances recorded to date (H2 permeance >4000 GPU and H2 /CH4 selectivity >2000) are fabricated by manipulating growth temperature, precursor concentration, and non-covalent decoration of the graphene surface. Moreover, it is identified that nanoscale molecular fouling of the graphene surface during gas separation where graphene pores are partially blocked by hydrocarbon contaminants under experimental conditions, controls both selectivity and temperature dependent permeance. Overall, the direct synthesis of porous single-layer graphene exploits its tremendous potential as high-performance gas-sieving membranes.

Ab Initio Energetic Barriers of Gas Permeation across Nanoporous Graphene.

Realizing membranes of atomic thickness functioning reliably constitutes a giant leap forward for a plethora of applications where the efficient separation of fluid constituents at the molecular level is critical. Here, by employing density functional theory, we explore the energy landscape of typical gas molecules attempting permeation through graphene nanopores and determine the minimum energy permeation pathways, based on the precise knowledge of the related molecular level interactions. With this approach we investigate two basic permeation routes: direct permeation and surface-based transport. We find that for subnanometer pores, the diffusion barrier of direct and surface transport depends on the pore chemical functionalization, while the molecule pore permeation barrier is independent of the gas-pore approach due to the overlap of surface and direct diffusion paths over the pore center. The overall minimum energy permeation pathway of He, H2, CO2, and CH4 molecules, across nanopores of different dimensions and chemical functionalization, defines the pore diameter (∼1.2 nm) below which effusion theory is inaccurate, as well as the critical pore diameter (∼0.8 nm) required to achieve positive permeation barriers driving molecular sieving. We determine that achieving positive permeation barriers required for high selectivity gas separation is inseparably combined with postpermeation desorption barriers due to attractive van der Waals interactions. The discovered permeation energetics are pore-molecule-specific and are incorporated into an analytical model extending existing theory. Our results provide a scientific background for rational pore design in graphene membranes, which can lead to gas separation at a commercially relevant performance level.

Evaluating gas permeance through graphene nanopores and porous 2D-membranes: A generalized approach

Porous graphene and 2D-membranes are known for favorable nanofluidic properties, such as high gas permeability and selectivity, depending on energy barrier for the molecule translocated through the nanopore. Despite the immense demand for proper theoretical description of gas flow through nanopores, a formalism including the energy barrier and steric effects to translocation remains conspicuously absent. Here, using both analytical and first-principles calculations, we study the permeance properties of porous 2D membranes. In particular, we develop the general equation for the permeance (and selectivity) of gases through circular nanopores, including both energy barrier and additional steric effects. Our theory captures sensitivity to the pore size, yields easy estimates for permeance through nanopores of less regular shape, and corroborates experimental findings. For completeness of the analysis, we disclose the importance of the energy barrier against surface diffusion of the permeance and report that e.g. the energy barrier against surface diffusion of He (CH4) through a pore with effective size 0.55 nm is 32 meV (114 meV), which is four times larger than the barrier predicted by the classical force fields. Presented upgrades of the long-standing gas flow theory are critical for further development of nano-patterned 2D membranes, as needed for emergent gas separation and purification technology.

Synthesis and gas separation properties of polyimide membranes derived from oxygencyclic pseudo-Tröger's base

Two neoteric oxygencyclic pseudo-Tröger's base (TB) -derived naphthalene diamines characterized by large π conjugated naphthalene rings and half-open cavity structure unit, 16H-8,16-methanodinaphtho [2,1-d:1′,2′-g] (Yampolskii, 2012; Maqsood et al., 2014) [1,3]dioxocine-3,13-diamine (OTB) and its furan-based bicyclic analogue, 7a, 14c-dihydronaphtho [( Xu et al., 2014; Yampolskii, 2012) 2,12,1-b]naphth [1′,2':4,5]furo [( Maqsood et al., 2014; Xu et al., 2014) 3,23,2-d] furan-3,12- diamine (FB), were successfully synthesized. Their 6FDA-based polyimides (6FDA-OTB and 6FDA-FB) were obtained for membrane-based gas separation study. 6FDA-OTB and 6FDA-FB exhibited good solubility and excellent thermal stability due to the presence of large π conjugated aromatic rings and half-open cavity structure unit. The two polymer membranes showed high gas pair selectivity and modest gas permeability as well as good physical aging properties. The 6FDA-FB membrane performed a high selectivity of H2/CH4 (104.0) and CO2/CH4 (49.1), which arises from the larger permeation activation energy difference of different gases. The performances of 100 days aged 6FDA-OTB and 6FDA-FB membranes are almost overlapped with the 1991 Robeson upper bounds for CO2/CH4 and H2/CH4. In short, introducing the novel rigid oxygencyclic pseudo-Tröger's base into the polyimide chains could make attractive membrane materials for various gas separation applications.

High capacity gas capture and selectivity properties of triazatruxene-based ultramicroporous hyper-crosslinked covalent polymer.

Tuning the selective sorption features of microporous organic networks is of great importance for subsequent applications in gas uptake and hiding, while it is more attractive in terms of being both time and cost effective to realize these optimizations without using functional groups in the core and linker. “Knitting” is one of the easiest and most used method to obtain a broad scope of hyper-crosslinked polymers on a large scale from aromatic structures that do not contain functional groups for polymerization. By the use of Knitting method, a hypercrosslinked covalent ultramicroporous organic polymer was obtained via stepwise process from using triazatruxene (TAT) as core -a planar indole trimer- through anhydrous FeCl3 catalyzed Friedel–Crafts alkylation using dimethoxybenzene as a linker. The resulting microporous polymer, namely TATHCCP was completely identified by analytical and spectral techniques after examined for gas properties (CO2, CH4, O2, CO, and H2) and selectivity (CO2/N2, CO2/O2, for CO2/CO and CO2/CH4) up to 1 bar and increased temperatures (273 K, 296 K and 320 K). Although it has a relatively low (Brunauer–Emmett–Teller) BET specific surface area around 557 m2/g, it was seen to have a high CO2 capture capacity approaching 10% wt. at 273 K. In accordance with (ideal adsorbed solution theory) IAST computations, it was revealed that interesting selectivity features hitting up to 60 for CO2/N2, 45 for CO2/O2, 35 for CO2/CO, 13 for CO2/CH4 at lower temperatures revealed that the material has much better selectivity values than many HCP (hyper-crosslinked polymer) derivatives in the literature even from its most similar analog dimethoxymethane derivative TATHCP, which has a surface area of 950 m2/g.

Metal-Oxide Based Ammonia Gas Sensors: A Review

This review paper encompasses a study of metal-oxide and their composite based gas sensors used for the detection of ammonia (NH3) gas. Metal-oxide has come into view as an encouraging choice in the gas sensor industry. This review paper focuses on the ammonia sensing principle of the metal oxides. It also includes various approaches adopted for increasing the gas sensitivity of metal-oxide sensors. Increasing the sensitivity of the ammonia gas sensor includes size effects and doping by metal or other metal oxides, which will change the microstructure and morphology of the metal oxides. Different parameters that affect the performances like sensitivity, stability and selectivity of gas sensors are discussed in this paper. Performances of the most operated metal oxides with strengths and limitations in ammonia gas sensing applications are reviewed. The challenges for the development of high sensitive and selective ammonia gas sensors are also discussed.

SnO2/SnSe2 Heterostructures for NO2 and H2 Sensing

SnO2/SnSe2 Heterostructures for NO2 and H2 Sensing

Green and simple approach for low-cost bioproducts preparation and CO2 capture

This study has demonstrated, for the first time, a simple, fast and flexible microwave processing method for the simultaneous preparation of bio-products (bio-oil, bio-gas and biochar) using a methodology that avoids any form of catalyst or chemical activation. The dielectric properties of biomass and physicochemical characterisation such as TGA, elemental and proximate analysis, XRD, SEM/EDX and textural properties, showed that 8 kJ g−1 of microwave energy can produce superior biochars for applications in CO2 capture. The maximum CO2 uptake capacity for biochar produced was 2.5 mmol g−1 and 2.0 mmol g−1 at 0 and 25 °C and 1 bar, which and also exhibited high gas selectivity compared with N2, fast kinetics of adsorption (<10 min) and desirable reusability (>95%) after 20 cycles. GC-MS analysis of generated bio-oil products revealed that higher microwave energies (>8 kJ g−1) significantly enhanced the amount of bio-oil produced (39%) and specifically the formation of levoglucosan, furfural and phenolics compounds, and bio-gas analysis identified trace levels of H2 and CH4. The results from this study confirm a green, inexpensive and efficient approach for biomass valorisation which can easily be embedded within bio-refinery process, and also demonstrates the potential of biochars for post-combustion CO2 uptake.