Gas Adsorption Properties Research Articles

An ultramicroporous metal-organic framework based on octahedral-like cages showing high-selective methane purification from a six-component C1/C2/C3 hydrocarbons mixture

Given the crucial significance of using cleaner methane (CH4) to replace else fossil fuels in remitting energy consumption and preventing environmental degradation, developing prominent adsorbents to purify CH4 from multicomponent mixtures is fundamentally important but faces great challenges. Cage-based metal-organic frameworks (MOFs) bring about widespread attention in solving numerous separation problems due to their inherent structural preponderances. Herein, we constructed an octahedral-like cages-based MOF (NUM-18) that incorporates two different types of cages within the whole framework and bears abundant Lewis basic sites (naked N atoms) and nonpolar aromatic rings immobilized in the pore surface. Benefitting from its intriguing structural characteristics, the pure-component gas adsorption properties were systematically investigated and indicate that NUN-18a (activated NUM-18) has excellent adsorptive capacities with respect to C2-C3 hydrocarbons than CH4. The IAST adsorption selectivities for C2/CH4 are above 14.0, while the adsorption selectivities of C3/CH4 startlingly surpass 86.0, respectively, all of which forebode that NUM-18a can achieve efficient recovery of C2-C3 hydrocarbons associated with CH4 purification. Furthermore, the actual separation feasibility for an equimolar 6-component C1/C2/C3 hydrocarbons mixture was examined by simulated dynamic column breakthrough experiments. Finally, molecular simulation calculations were adopted to ascertain potential gas adsorption mechanisms. This work provided a new paradigm for developing novel porous crystalline MOFs materials to separate hyper-complex gas mixtures.

Density functional theory study on the adsorption properties of SO2 gas on graphene, N, Ti, and N–Ti doped graphene

Sulphur dioxide (SO2) is a corrosive and irritating gas that can cause serious damage to human health and natural environment. Therefore, designing effective gas sensors to detect and remove SO2 is urgent. The adsorption properties of SO2 on the graphene, N-doped, Ti-doped, and N–Ti co-doped graphene (N-G, Ti-G and N–Ti-G) are investigated. Our results based on the density functional theory show that SO2 molecules can stably chemically adsorb on Ti-G and N–Ti-G. The lowest adsorption energy (−2.836 eV) and remarkable charge transfer (0.735 e move from N–Ti-G to SO2) indicate the N–Ti-G is the optimal adsorbent. Biaxial strain can effectively adjust the adsorption characters and external electric field can enhance the stability of adsorption. Physical adsorption is formed between SO2 and graphene/N-G due to weak interactions. Our work provides a valuable theoretical guidance for the development of N–Ti-G substrate as promise gas sensor to detection and removal of SO2.

Adsorption application of tetraethylenepentamine (TEPA) modified SBA-15@MIL-101(Cr) in carbon capture and storage (CCS)

To design a better carbon capture material, a SBA-15@MIL-101(Cr) composite is prepared by an in situ synthesis method, and its adsorption performance is enhanced by TEPA modification. The SBA-15 with well-ordered mesopores acted as a carrier, which regulated the growth of MIL-101(Cr) crystals. The silicon hydroxyl group in SBA-15 stably binds to TEPA in the form of hydrogen bond, and the Lewis acid site (CUS) in MIL-101(Cr) makes it bind to TEPA in coordination bond. The unique pore cage structure of MIL-101(Cr) and the basic site provided by TEPA make the material contain both physical and chemical adsorption forces in the process of adsorption of CO2 molecules. The good thermal stability of SBA-15 improves the thermal stability of MIL-101(Cr) and TEPA in the SBA-15 channel. Compared with the original MIL-101(Cr), the composite and modified materials showed higher CO2 absorption capacity at 298K and 100 kPa. By fitting the Langmuir equation and Freundlich equation, it can be found that the adsorption behavior of the materials after TEPA loading has the non-uniformity of surface binding. The adsorption behavior is a typical exothermic process, so the adsorption capacity of the material is inversely proportional to the temperature. Compared with other similar materials, the materials designed in this work have good CO2 adsorption energy per unit area, and also have good reusability. Therefore, this study opens a new way for the design of graded porous amine-modified MOF-based materials with advanced gas adsorption properties.

Growth of Graphitic Carbon Nitride-Incorporated ZnO Nanorods on Silicon Pyramidal Substrates for Enhanced Hydrogen Sensing Applications

Monitoring the hydrogen gas (H2) level is highly important in a wide range of applications. Oxide-carbon hybrids have emerged as a promising material for the fabrication of gas sensors for this purpose. Here, for the first time, graphitic carbon nitride (g-C3N4)-doped zinc oxide nanorods (ZNRs) have been grown on silicon (Si) pyramid-shaped surfaces by the facile hydrothermal reaction method. The systematic material analyses have revealed that the g-C3N4 nanostructures (NS) have been consistently incorporated into the ZNRs on the pyramidal silicon (Py-Si) surface (g-C3N4-ZNRs/Py-Si). The combined properties of the present structure exhibit an excellent sensitivity (∼53%) under H2 gas exposure, better than that of bare ZNRs (12%). The results revealed that the fine incorporation of g-C3N4 into ZNRs on the Py-Si surface improves the H2 gas sensing properties when compared to that of the planar silicon (Pl-Si) surface. The doping of g-C3N4 into ZNRs increases the electrical conductivity through its graphene-like edges (due to the formation of delocalized bonds in g-C3N4 during carbon self-doping), as revealed by FESEM images. In addition, the presence of defects in g-C3N4 induces the gas adsorption properties of ZnO through its active sites. Moreover, the integration of the 1D structure (g-C3N4-ZNRs) into a 3D pyramidal structure opens up new opportunities for low-cost H2 gas sensing at room temperature. It is an easy way to enhance the gas sensing properties of ZNRs at room temperature, which is desirable for practical H2 sensor applications.

Experimental study on gas adsorption characteristics of coals under different Protodyakonov’s coefficient

The Protodyakonov’s coefficient of coal samples is a key factor affecting coal and gas outburst, so studying the gas adsorption pattern of coal samples with different Protodyakonov’s coefficients can effectively prevent coal and gas burst accidents. In order to study the influence of Protodyakonov’s coefficient on gas adsorption characteristics of coal, seven groups of coal samples with different Protodyakonov’s coefficients were selected for this study. Gas adsorption characteristics of coal bodies were investigated using gas adsorption tests with HCA high pressure adsorption devices. The gas adsorption properties of coal with different Protodyakonov’s coefficients were obtained and the effect of adsorption temperature and pressure on the gas adsorption properties were analyzed and further explored in depth. The results showed that the adsorption constant a showed an overall decreasing trend with the increase of the Protodyakonov’s coefficient. At constant temperature (30°C, 40°C, 50°C), the adsorption amount increased with the increase of relative pressure, and the increase was negatively correlated with the value of f. At constant pressure, the adsorption amount by and the adsorption constant a gradually decreased with the increase of temperature, and the adsorption constant decreased with the increase of temperature for the same coal sample under different temperature conditions. The results provide a theoretical basis for the prevention as well as the control of coal and gas burst accidents.

Sol-gel processing of a covalent organic framework for the generation of hierarchically porous monolithic adsorbents

Sol-gel processing of a covalent organic framework for the generation of hierarchically porous monolithic adsorbents

Combining molybdenum carbide with ceria overlayers to boost Mo/CeO2 catalyzed ammonia synthesis

AbstractThe design of an efficient non‐noble metal catalyst is of burgeoning interest for ammonia synthesis. Herein, we report a Mo2C/CeO2 catalyst that is superior in ammonia synthesis activity. In this catalyst, molybdenum carbide coexisted with the ceria overlayers which is from the ceria support as the strong metal–support interaction. There is a high proportion of low‐valent Mo species, as well as high concentration of Ce3+ and surface oxygen species. The presence of Mo2C and CeO2 overlayers not only leads to enhancement of hydrogen and nitrogen adsorption, but also facilitates the desorption and exchange of adsorbed species with the gaseous reagents. Compared with the Mo/CeO2 catalyst prepared without carbonization, the Mo2C/CeO2 catalyst is more than sevenfold higher in ammonia synthesis rate. This work not only presents an explicit example of designing Mo‐based catalyst that is highly efficient for ammonia synthesis by tuning the adsorption and desorption properties of the reactant gases, but opens a perspective for other elements in ammonia synthesis.

Dramatic Enhancement of Rare-Earth Metal–Organic Framework Stability Via Metal Cluster Fluorination

Rare-earth polynuclear metal–organic frameworks (RE-MOFs) have demonstrated high durability for caustic acid gas adsorption and separation based on gas adsorption to the metal clusters. The metal clusters in the RE-MOFs traditionally contain RE metals bound by μ3–OH groups connected via organic linkers. Recent studies have suggested that these hydroxyl groups could be replaced by fluorine atoms during synthesis that includes a fluorine-containing modulator. Here, a combined modeling and experimental study was undertaken to elucidate the role of metal cluster fluorination on the thermodynamic stability, structure, and gas adsorption properties of RE-MOFs. Through systematic density-functional theory calculations, fluorinated clusters were found to be thermodynamically more stable than hydroxylated clusters by up to 8–16 kJ/mol per atom for 100% fluorination. The extent of fluorination in the metal clusters was validated through a 19F NMR characterization of 2,5-dihydroxyterepthalic acid (Y-DOBDC) MOF synthesized with a fluorine-containing modulator. 19F magic-angle spinning NMR identified two primary peaks in the isotropic chemical shift (δiso) spectra located at −64.2 and −69.6 ppm, matching calculated 19F NMR δiso peaks at −63.0 and −70.0 ppm for fluorinated systems. Calculations also indicate that fluorination of the Y-DOBDC MOF had negligible effects on the acid gas (SO2, NO2, H2O) binding energies, which decreased by only ∼4 kJ/mol for the 100% fluorinated structure relative to the hydroxylated structure. Additionally, fluorination did not change the relative gas binding strengths (SO2 > H2O > NO2). Therefore, for the first time the presence of fluorine in the metal clusters was found to significantly stabilize RE-MOFs without changing their acid-gas adsorption properties.

Antimicrobial and Gas Adsorption Properties of Electrospun Ferrocene-Polyurethane-Based Nanofibers Containing Silver Nitrate

Antimicrobial and Gas Adsorption Properties of Electrospun Ferrocene-Polyurethane-Based Nanofibers Containing Silver Nitrate

Modeling of diffusion kinetics during gas adsorption in a coal seam with a dimensionless inversion method

Modeling gas diffusion kinetics in a coal seam plays an essential role in predicting performance of both coalbed methane production and its enhanced recovery. Throughout the diffusion period, not only does there exist a large deviation between the analytical solution of the classical Fickian unipore diffusion model (UDM) and experimental measurements, but also constant diffusion coefficients calculated with the inversion of the simplified empirical/semi-empirical correlations are still not accurate enough. In this paper, we proposed a free gas density gradient model (FGDGM) to characterize the gas adsorption and transport properties in a coal seam. Experimentally, isothermal experiments were performed using a high-temperature and high-pressure gas adsorption analyzer with methane gas. Theoretically, a Langmuir gas concentration was derived and improved as a function of free gas density, and then we correlated a numerical inversion model to determine the effective diffusion coefficients by combining dimensionless variables and empirical correlations. Finally, the developed models were validated with the measured gas adsorption isotherms. The numerical solution curves based on the FGDGM are found to be basically consistent with the measured data in the entire time scale, while the developed inversion method is accurate as well as efficient in computational expenses. On the basis of the ideal gas assumption, the actual density gradient and diffusion coefficient will be underestimated and overestimated, respectively. By incorporating real gas behaviour into the improved FGDGM, both gas mass flux and effective diffusion coefficient can be determined more accurately to model gas diffusion kinetics and gas flow patterns in a coal seam.

Structural and textural control of high-pressure hydrogen adsorption on expandable and non-expandable clay minerals in geologic conditions

Both natural and anthropogenically-generated hydrogen gas occurs in sedimentary rocks and geotechnical barriers. Due to high-pressure conditions, a significant portion of H2 can be physisorbed in available micropores of clay minerals, which have been proven to control the gas adsorption properties of rocks. This study investigates H2 adsorption on clay minerals naturally occurring in rocks, by combining the high-pressure H2 experiments with sample characterization analyzed using low-pressure adsorption of N2 and CO2, and structural analysis using X-ray diffraction. We found that H2 adsorption depends strongly on the mineral texture, which is not related directly to its structure. Most of H2 is adsorbed in the micropores accessible to CO2. H2 intercalates in the smectitic interlayers with a basal spacing larger than 10.8 Å. The density of adsorbed H2 is about double that of free H2 gas under given pressure and temperature, effectively increasing the gas storage capacity of rocks.

Combining Machine Learning and Molecular Simulations to Unlock Gas Separation Potentials of MOF Membranes and MOF/Polymer MMMs.

Due to the enormous increase in the number of metal-organic frameworks (MOFs), combining molecular simulations with machine learning (ML) would be a very useful approach for the accurate and rapid assessment of the separation performances of thousands of materials. In this work, we combined these two powerful approaches, molecular simulations and ML, to evaluate MOF membranes and MOF/polymer mixed matrix membranes (MMMs) for six different gas separations: He/H2, He/N2, He/CH4, H2/N2, H2/CH4, and N2/CH4. Single-component gas uptakes and diffusivities were computed by grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations, respectively, and these simulation results were used to assess gas permeabilities and selectivities of MOF membranes. Physical, chemical, and energetic features of MOFs were used as descriptors, and eight different ML models were developed to predict gas adsorption and diffusion properties of MOFs. Gas permeabilities and membrane selectivities of 5249 MOFs and 31,494 MOF/polymer MMMs were predicted using these ML models. To examine the transferability of the ML models, we also focused on computer-generated, hypothetical MOFs (hMOFs) and predicted the gas permeability and selectivity of 1000 hMOF/polymer MMMs. The ML models that we developed accurately predict the uptake and diffusion properties of He, H2, N2, and CH4 gases in MOFs and will significantly accelerate the assessment of separation performances of MOF membranes and MOF/polymer MMMs. These models will also be useful to direct the extensive experimental efforts and computationally demanding molecular simulations to the fabrication and analysis of membrane materials offering high performance for a target gas separation.

Metal-induced different structures and properties of two coordination polymers based on a semi-flexible carboxylic ligand

Pictogram: Two 3D coordination polymers (Cu/Ni-based) have been solvothermal synthesized and characterized. In addition, their magnetic properties, electrochemical properties and gas adsorption properties were also studied. • Two 3D coordination polymers (Cu/Ni-based) have been solvothermally synthesized and characterized. • The magnetic research showed that there are antiferromagnetic and ferromagnetic interactions between adjacent metal ions of 1 and 2, respectively. • In addition, CP 1 showed good electrochemical and gas adsorption properties. Two coordination polymers (CPs), namely, {[Cu 2 L(H 2 O) 3.5 ]·1.5H 2 O} n ( 1 ) and {[Ni 2 L(H 2 O) 5 ]·H 2 O} n ( 2 ) have been synthesized from the ligand of 3, 3′, 5, 5′-diphenyl ether tetracarboxylic acid(C 16 H 10 O 9 , H 4 L) and metal salts. CPs 1 and 2 were characterized by single crystal X-ray diffraction, infrared (IR) spectra, elemental analysis, powder X-ray diffraction (PXRD), and thermogravimetric (TG) analysis. Structure analysis indicates that both the compounds are composed of binuclear clusters linked by L 4- ligands, forming 3D framework with {4. 6 4 . 8} 2 {4 2 . 6 2 . 8 2 } and {6 6 } topology, respectively. The temperature dependence of the magnetic susceptibility of 1 reveals antiferromagnetic exchange between adjacent Cu(II) ions. In contrast, the magnetic behavior of 2 indicates that there is mainly strong ferromagnetic coupling between the two Ni(II) ions. In addition, due to the open metal site of Cu(II) ion in 1 , it showed good electrochemical performance. Furthermore, the adsorption properties of N 2 , CO 2 and CH 4 for 1 were studied. The results showed that the average heat of CO 2 adsorption is 11.41 kJ·mol −1 and the average selectivity coefficient of CO 2 /CH 4 (1:9, v/v) is 32.85 under 298 K and 1 bar.

First-principles study on N2, H2, O2, NO, NO2, CO, CO2, and SO2 gas adsorption properties of the Sc2CF2 monolayer

In this work, we systematically investigated the gas adsorption properties of N2, H2, O2, NO, NO2, CO, CO2, and SO2 on the MXene Sc2CF2 monolayer using first-principles calculations. We determined structural geometries, adsorption energies, bandgap, charge transfer, density of states, and charge density difference of adsorption systems. We found that all N2, H2, O2, CO, CO2, SO2, NO, and NO2 molecules are physisorbed on the Sc2CF2 monolayer. The adsorption energies show that the Sc2CF2 monolayer exhibits low adsorption selectivity in respect of the studied gas molecules. Spin-polarized DOS indicates the transitions of non-magnetic pristine MXene Sc2CF2 to magnetic systems after O2, NO and NO2 adsorptions. Especially, we observed the semiconductor-to-semimetal transition of the Sc2CF2 monolayer by NO adsorption, while the adsorptions of CO, CO2, H2, N2, SO2, NO2 and O2 molecules do not change the conductive behavior of the Sc2CF2 monolayer. Our results suggest that the Sc2CF2 monolayer is a novel potential sensing nanomaterial for detecting NO gas.

Controlling the Flexibility of MIL-88A(Sc) Through Synthetic Optimisation and Postsynthetic Halogenation.

Breathing behaviour in metal‐organic frameworks (MOFs), the distinctive transformation between a porous phase and a less (or non) porous phase, often controls the uptake of guest molecules, endowing flexible MOFs with highly selective gas adsorptive properties. In highly flexible topologies, breathing can be tuned by linker modification, which is typically achieved pre‐synthetically using functionalised linkers. Herein, it was shown that MIL‐88A(Sc) exhibits the characteristic flexibility of its topology, which can be tuned by 1) modifying synthetic conditions to yield a formate‐buttressed analogue that is rigid and porous; and 2) postsynthetic bromination across the alkene functionality of the fumarate ligand, generating a product that is rigid but non‐porous. In addition to providing different methodologies for tuning the flexibility and breathing behaviour of this archetypal MOF, it was shown that bromination of the formate‐bridged analogue results in an identical material, representing a rare example of two different MOFs being postsynthetically converted to the same end product.

Molybdenum carbide phase effects in heterogeneous catalytic ammonia synthesis

The design of an efficient catalyst utilizing non-precious metals is a top priority in ammonia synthesis, and Mo is found to be inferior in ammonia synthesis due to strong nitrogen adsorption. The carburization of Mo species might be an ideal strategy for optimizing nitrogen adsorption energy and ammonia synthesis activity. In this work, we prepare two molybdenum carbides with different crystal structures (α-MoC and β-Mo2C). The result shows that there is a larger number of Mo-C bonds and lower Mo/C ratio in α-MoC, leading to the strong adsorption of hydrogen and ammonia species on α-MoC. In contrast, the desorption energies of hydrogen and ammonia are lower for β-Mo2C, and the spillover and migration of hydrogen species are boosted. The facile desorption of hydrogen and ammonia species also limits the accumulation of H adatoms and ammonia on the surface of β-Mo2C, releasing of active sites available for the subsequent activation of hydrogen and nitrogen as well as ammonia synthesis reaction. As a result, the ammonia synthesis activity of β-Mo2C catalyst is 2.5-fold higher than that of α-MoC, and a small negative value of NH3 reaction order also would be obtained for β-Mo2C. This work not only reports the effect of molybdenum carbide phases on the adsorption–desorption properties of reactants and catalytic activities for ammonia synthesis, but also opens a perspective for the design and development of efficient metal based heterogeneous catalysts by tuning the adsorption and desorption properties of the reactant gases. Keywords: Molybdenum Carbide; Phase effect; Adsorption–desorption property; Mo catalyst; Ammonia synthesis

Poly (Vinyl Alcohol) Composite Membrane with Polyamidoamine Dendrimers for Efficient Separation of CO2/H2 and CO2/N2

Although polyvinyl alcohol (PVA) membranes are commonly used for CO2 separation, there is still large development space in mechanical properties and high selectivity of the gas separation process. In this study, the gas separation performance and mechanical properties of the (PVA/Cu2+) substrate membranes were improved by introducing polyamidoamine (PAMAM). PAMAM had an important effect on the gas adsorption and separation performance of the membrane. In addition, the gas adsorption and separation properties of the PVA/Cu2+/PAMAM membrane (PPCm) were analyzed and studied when the inlet gas pressure and the species of mixed gases were variable. The results showed that the crystallinity and mechanical properties of the membrane with the PAMAM had been significantly improved. Young’s modulus of PPCm with 30% PAMAM was 132% higher than that of the PVA/Cu2+ composite membrane without PAMAM. In addition, efficient separation efficiency and high selectivity of the gas separation process were observed. The separation factors of the PPCm for CO2/H2 and CO2/N2 were about three times higher than that of the PVA/Cu2+ substrate membranes. These results suggested that the introduction of PAMAM was promising for CO2 separation and permeance.

Effective oxygen reduction reaction and suppression of CO poisoning on Pt3Ni1/N-rGO electrocatalyst

The development of high-performance ORR catalysts that satisfy the requirements of low Pt content, high activity, and long-term durability is crucial. The adequately designed Pt3Ni1/N-rGO electrode, which was obtained by the regular dispersion and growth of uniform ≤ 5.0-nm-sized Pt3Ni1 alloy nanoparticles on an N-rGO support electrode by the solvothermal method, exhibited improved mass activity and specific surface activity (178% and 167%, respectively) compared to that of a commercial Pt/C electrode. The fabricated electrode showed almost unchanged ESCA and LSV polarization curves even after 5000 cyclic voltammetry tests. The Pt3Ni1/N-rGO electrode exhibited remarkable resistance to CO poisoning compared to that of the commercial Pt/C electrode. An electron-rich Pt–Ni–NC three-phase interface was created in the catalyst by introducing CN, which is more electron-rich than CC, to the rGO support, which increased its O2 gas-adsorption properties and provided adequate strength for OH group formation that eventually accelerated their conversion into H2O.

Investigation of isotherms and isosteric heat of adsorption for PW11@HKUST-1 composite

Impregnation of functional materials, such as polyoxometalates (POMs) into the pores of metal-organic frameworks (MOFs) is one approach to modifying their host-guest interactions. Although, the effect of this approach is well investigated in the fields of catalysis, luminescent materials, and biomedical imaging, still little is known about their gas adsorption properties. Isosteric heat of adsorption, (Qst) is one of the significant parameters in understanding the adsorption process for adsorbent materials. Here in we report the impregnation of a Keggin-type lacunary (PW11) nanoclusters into copper trimesic-based MOF of HKUST-1, to investigate the isotherms and isosteric heat of adsorption, for carbon dioxide calculated using the Clausius−Clapeyron equation. The PW11 brings π-accepting tendency and highly electronegative oxygen atoms on their surface to the HKUST-1 structure that selectively increases the affinity of HKUST-1 for CO2. This improves the Qst value for PW11@KUST-1 as compared with the pristine MOF. This study shows that the introduction of anionic metal-oxide of POMs can change the CO2 adsorption property of the MOF materials.

Enhanced hydrogen gas adsorption properties of B, N, and Au co-doped graphene in o-, m-, and p-configurations: DFT study

The adsorption of hydrogen gas molecules on boron, nitrogen, gold co-doped graphene has been investigated for different configurations using Density Functional Theory. Co-doped graphene systems outperform single doped graphene systems due to combined influence of hetero-atoms. Different configurations studied in present study include gold doped graphene (Au-G); boron‐gold (BAu); nitrogen‐gold (NAu); and boron‐nitrogen‐gold (BN-Au) co-doped graphene. In case of BN-Au co-doped graphene, the aspect of relative sites of boron and nitrogen has also been determined. Despite the fact that all configurations were confirmed to have interaction with hydrogen molecule, relative sites of boron and nitrogen resulted in distinct responses to hydrogen adsorption. The sensing performance has been evaluated by calculating adsorption energies and changes observed in electronic properties following geometry optimization. The primary goal of this research is to better interpret structure, stability, and prospective capability of hydrogen molecule detection by diverse combinations of boron, nitrogen, gold co-doped graphene geometries.